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        Carbonyl-coordinating polymers for high-voltage solid-state lithium batteries: Solid polymer electrolytes
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        Carbonyl-coordinating polymers for high-voltage solid-state lithium batteries: Solid polymer electrolytes
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Abstract

Solid polymer electrolytes are a crucial class of compounds in the next-generation solid-state lithium batteries featured by high safety and extraordinary energy density. This review highlights the importance of carbonyl-coordinating polymer-based solid polymer electrolytes in next-generation safe and high–energy density lithium metal batteries, unraveling their synthesis, sustainability, and electrochemical performance.

With the massive consumption of fossil fuel in vehicles nowadays, the resulted air pollution and greenhouse gases issue have now aroused the global interest on the replacement of the internal combustion engines with engine systems using renewable energy. Thus, the commercial electric vehicle market is growing fast. As the requirement for longer driving distances and higher safety in commercial electric vehicles becomes more demanding, great endeavors have been devoted to developing the next-generation solid-state lithium metal batteries using high-voltage cathode materials, e.g., high nickel (Ni) ternary active materials, LiCoO2, and spinel LiNi0.5Mn1.5O4. However, the most extensively investigated solid polymer electrolytes (SPEs) are based on polyether-based polymers, especially the archetypal poly(ethylene oxide), which are still suffering from low ionic conductivity (10−7 to 10−6 S/cm at room temperature), limited lithium ion transference number (<0.2), and narrow electrochemical stability window (<3.9 V), restricting this type of SPEs from realizing their full potential for the next-generation lithium-based energy storage technologies. As a promising class of alternative polymer hosts for SPEs, carbonyl-coordinating polymers have been extensively researched, exhibiting unique and promising electrochemical properties. Herein, the synthesis, sustainability, and electrochemical performance of carbonyl-coordinating SPEs for high-voltage solid-state lithium batteries will be reviewed.

DISCUSSION POINTS

  • Carbonyl-coordinating polymer-based SPEs exhibit unique electrochemical properties compared with their polyether-based analogues, making them promising for the next-generation solid-state high–energy density lithium batteries.

  • What makes the carbonyl-coordinating polymers outperform their polyether-based analogues in terms of electrochemical properties?

  • The bio-based synthesis approach and biodegradable characteristics endow the carbonyl-coordinating polymers with increasing attention from both industry and academia.

Introduction

Why solid polymer electrolytes

The world overall energy consumption has increased tremendously over the past several decades.Reference Mohanty, Li, Nagpure, Wood and Daniel1 Take the USA as an example, it consumed 101.3 quadrillion Btu (British thermal unit, equals approximately 1.1 × 1018 Joule) of energy in 2018, of which 80% was based on fossil fuels (petroleum, natural gas, and coal) and 28% was used in the transportation systems.2 Such massive utilization of fossil fuel gives rise to the world severe challenges of air pollution and greenhouse gases issues (mainly carbon dioxide and methane).Reference Lewis3 To tackle these issues and to replace the heavy dependence on fossil fuel, renewable energy techniques, i.e., the classical lithium ion batteries, have been advanced dramatically and are considered as one of the most successful energy conservation systems in human history.Reference Goodenough and Kim4 However, the conventional lithium ion batteries based on liquid electrolyte systems are virtually impossible to satisfy the stringent and booming demand for higher energy density and safety from 3 C electronics, auto industry, and grid energy conservation systems. Developing lithium ion batteries with lower cost, higher energy density, longer calendar life, and more satisfying safety level is still in desperate need.

Many leading economies, i.e., China, Germany, Japan, and the USA, have set up their own ambitious milestones toward the next-generation high safety and high–energy density lithium battery.Reference Bresser, Hosoi, Howell, Li, Zeisel, Amine and Passerini5 The state-of-art lithium ion battery industry is built on the liquid electrolyte systems with highly volatile and flammable solvents, which also suffer from inadequate electrochemical and thermal stabilities and low ion selectivity.Reference Tarascon and Armand6,Reference Quartarone and Mustarelli7 To solve these problems, the replacement of liquid electrolytes with solid-state electrolytes, and the employment of high–energy density anode and cathode has been regarded as the best choice.Reference Goodenough and Kim4,Reference Goodenough and Park8Reference Goodenough10 As one of the most promising solid-state electrolytes, solid polymer electrolytes (SPEs) show proximity with respect to device integration, processing cost, and mechanical properties over inorganic solid electrolytes.Reference Manthiram, Yu and Wang11 The first successful landmark of commercial EV applications of SPE was made by the launch of pure electric vehicles (Bluecar) provided by Bolloré in 2011 in Paris.Reference Osada, de Vries, Scrosati and Passerini12,Reference Sarabi, Kefsi, Merdassi and Robyns13 The battery using polyethylene oxide (PEO)-based SPE and LiFePO4 cathode (working plateau at 3.5 V) coupled with lithium metal anode delivers a competitive energy density of 180–200 Wh/kg over liquid electrolyte-based automotive batteries. As an encouraging industrial demonstration of the realistic application of solid-state batteries, SPEs are exhibiting more than just potentials toward the next-generation lithium batteries.

Why carbonyl-coordinating polymers

The dawn of the investigation on SPEs was from the work of P.V. Wright, who studied the ionic conductivity of PEO doped with sodium and potassium salts,Reference Wright14 and was boosted by the seminal work of M. Armand, who used this type of material in electrochemical systems, especially in lithium batteries.Reference Armand15 Since then, tremendous efforts from both academia and industry have been dedicated to the polyether-related polymers, especially PEO, owing to its comparatively low T g (≈ −60 °C) and excellent solvation of various types of lithium salts.Reference Wang, Zhang, Li, Chai, Dong and Cui16 PEO is an excellent complexing agent for Li+, even though the dielectric constant is rather low (≈5).Reference Fish and Smid17Reference Wu, Rao, Cheng, Yuan, Li and Huang19 Besides, polyethers are generally recognized as one of the most (electro)chemically stable polymer hosts against lithium metal anode, and the solid electrolyte interface (SEI) layer formed on the anode side is originated from the lithium salt decomposition and water traces.Reference Xu, Sun, Gustafsson, Edström, Brandell and Hahlin20,Reference Wei, Chen, Wang, Wang, Zhang, Yao, Deng and Xu21 However, polyether-based SPEs are hardly to realize the full potentials of solid-state batteries due to several shortcomings, of which primarily are low ambient-temperature ionic conductivity (<10−6 S/cm), low lithium ion transference number (<0.2), and narrow electrochemical window (<3.8 V), as reviewed in many publications.Reference Quartarone and Mustarelli7,Reference Di Noto, Lavina, Giffin, Negro and Scrosati22Reference Mindemark, Lacey, Bowden and Brandell28

As a promising class of alternative SPE hosts, carbonyl-coordinating polymers, primarily aliphatic polycarbonates and polyesters, endow the SPEs with superior electrochemical properties owing to their fast chain segmental motions, high dielectric constant, and unique lithium ion complexation and transport mechanism.Reference Zhang, Yang, Dong, Zhang, Chai, Dong, Wu, Zhou and Cui29 Their aromatic analogues generally show very poor ionic conductivity because of the low local segmental motion of the rigid polymer chain and high degree of crystallinity. The electrochemical properties, i.e., ionic conductivity, lithium ion transference number, and electrochemical stability, are critically important when one benchmarking an SPE. The unique properties of carbonyl-coordinating polymers will be discussed in the following sections. To the best of our knowledge, polyketones (PKs) and polyanhydrides were rarely reported as SPE hosts, despite they are also in the family of carbonyl-coordinating polymers. This is most likely due to their chemical instability, high crystallinity, poor solubility, and the resulted extremely low ionic conductivity.

In the following sections, the three main indexes in measuring electrochemical performance, i.e., ionic conductivity, lithium ion transference number, and electrochemical stability window (ESW), are compared between polyether-based SPEs and carbonyl-based SPEs.

Ionic conductivity

The ionic conductivity represents the capability of an SPE that shuttles ions. With respect to PEO-based SPE, the conduction of Li+ is realized by solvating and coordinating lithium ions with the ether oxygen on PEO segments and transporting in terms of dynamically breaking/forming lithium–oxygen (Li–O) bonds by intrachain or interchain hooping in the PEO segments. Owing to such dynamical displacement of the ligands for the solvation of lithium ions, the continuous polymer chain segmental motion (local chain motion) results in a long-range motion of lithium ions.Reference Xue, He and Xie27,Reference Manuel Stephan30,Reference Xu31 The dynamical rearrangement of this complexation in ionic transport can be well simulated by the dynamic bond percolation model.Reference Druger, Nitzan and Ratner32,Reference Webb, Savoie, Wang and Miller33 Typically, the ionic conductivity of a polyether-based SPE at room temperature is in the order of 10−7 to 10−6 S/cm, which is consequentially lower than the generally recognized sufficient value of 10−4 S/cm toward usable solid-state batteries for realistic applications.Reference Song, Wang and Wan34,Reference Wang, Wang, Wang, Hu, Cui, Li, Dong, Zhou and Cui35 Compared with other ligand candidates such as carbonates and esters, PEO solvates and coordinates lithium ions with a much higher complexation strength.Reference Xu36 A good example is that in the electrolyte system consisting of ≈90 mol% of propylene carbonate (PC) and ≈10 mol% of tetraethylene glycol dimethyl ether (TEGDME), most lithium ions chelate to ether units of TEGDME.Reference Fish and Smid17 However, such good complexation not only facilitates high salt dissolution but also highly restricts the breaking of Li+/ether oxygen complexation by local segmental motions. Thus, the complexation renders the polymer chain a very stable coordination structure and the ions therefore inferiorly mobile, despite a low T g (≈ −60 °C). Besides, the ion–dipole interactions between Li+/ether oxygen have significant impacts on the microscopic morphology of SPEs, and in most cases, they act as intermolecular cross-linker in SPEs to increase T g.Reference Zhou, Ma, Dong, Li and Cui37,Reference Zaheer, Xu, Wang, Li and Deng38 Hence, the favorable complexation of Li+/ether oxygen that makes PEO dissolve lithium salt well indeed also serves to restrict its segmental motion. Furthermore, PEO is a highly crystalline polymer, with a high degree of crystallization of approximately 80% at room temperature.Reference Takahashi and Tadokoro39 Substantial evidences suggested that the dominant lithium ions transportation takes place in the amorphous region,Reference Quartarone and Mustarelli7 due to the higher polymer chain dynamics on the amorphous nature. Although some reports presented the superior ionic conductivity of Li+ in the crystal lattice of PEO,Reference Gadjourova, Andreev, Tunstall and Bruce40 prevailingly PEO crystallization is generally considered to be detrimental to lithium ions transportation due to the sacrificed polymer chain dynamics.Reference Cheng, Smith and Li41

As a promising analogue, carbonyl-coordinating polymer–based SPEs generally exhibit relatively high ionic conductivity at ambient temperature.Reference Zhou, Zhang and Cui42 In carbonyl-coordinating polymer–based SPEs, the lithium ions are coordinated by the carbonyl group oxygen (primarily) and the alkoxy oxygen adjacent to the carbonyl group ether oxygen atoms (partly) on polymer segments in a similar fashion to the polyether-based SPEs.Reference Mindemark, Lacey, Bowden and Brandell28,Reference Matsubara, Kaneuchi and Maekita43Reference Bogle, Vazquez, Greenbaum, Cresce and Xu45 With the processes of dynamically breaking/forming lithium–oxygen bonds, the lithium ions transport is realized by the intrachain or interchain hooping in the carbonyl-coordinating polymer segments. Owing to the continuous displacement of the ligands for the solvation of Li+ by the segmental rearrangement of glass transitions, the long-range conduction of lithium ions is thus occurred. Compared with polyether-based SPEs, carbonyl-coordinating polymer–based SPEs show significantly faster lithium ion transport, attributed to their fast “decoupling” during ion transport and segmental dynamics.Reference Tominaga and Yamazaki46 Generally, the ionic conductivity of carbonyl-coordinating polymer–based SPEs closely correlates to T g and reaches a maximum value at high lithium salt loadings (polymer-in-salt systems), where the T g is significantly depressed owing to the plasticizing effect.Reference Tominaga and Yamazaki46Reference Okumura and Nishimura48 This may enable the carbonyl-coordinating SPEs baring with high lithium salt loadings to achieve high ionic conductivity at ambient temperature and realize the practical applications. A comparison of the ionic conductivity as a function of the salt concentration for polyether-based (PEO) SPE and carbonyl-coordinating polymer–based (polyethylene carbonate, PEC) SPE is presented in Fig. 1.

Figure 1. Comparison of the ionic conductivity as a function of the salt concentration for (a) polyether-based (PEO) SPE and (b) carbonyl-coordinating polymer–based (polyethylene carbonate, PEC) SPE. Reproduced with permission.Reference Tominaga and Yamazaki46 Copyright 2014, Royal Society of Chemistry.

Lithium ion transference number

Lithium ion transference number $\left( {{t_{{\rm{L}}{{\rm{i}}^ &#x002B; }}}} \right)$ is another fundamental electrochemical parameter.Reference Doyle, Fuller and Newman49 A high ${t_{{\rm{L}}{{\rm{i}}^ &#x002B; }}}$ implies that the conductivity primarily depends on the lithium ion transport rather than on the anionic mobility. As only the mobility of lithium ions is the effective migration, which takes part in the electrochemical redox reaction in electrodes, a high ${t_{{\rm{L}}{{\rm{i}}^ &#x002B; }}}$ (ideally, a unity of ${t_{{\rm{L}}{{\rm{i}}^ &#x002B; }}}$) is desired in lithium batteries to avoid local depletion and high internal resistance caused by gradient polarization from anion migrations. Besides, it was found that an SPE with high ${t_{{\rm{L}}{{\rm{i}}^ &#x002B; }}}$ is effective in the utilization of active electrode materials and the suppression of lithium dendrite growth.Reference Thomas, Sloop, Kerr and Newman50Reference Brissot, Rosso, Chazalviel and Lascaud52 Unfortunately, a low value of ≈0.2 was typically reported on the PEO/LiTFSI systems,Reference Gorecki, Jeannin, Belorizky, Roux and Armand53 implying the high anion dynamics. The reason is that the anion mobility is less restricted by the polymer segmental motions, while the lithium cations are dominantly complexed with ether oxygen as mentioned above.Reference Borodin and Smith54,Reference Mao, Saboungi, Price, Armand and Howells55

To date, all the carbonyl-coordinating polymer–based SPEs exhibited significantly higher ${t_{{\rm{L}}{{\rm{i}}^ &#x002B; }}}$ than the PEO/LiTFSI systems. As discussed above, the lithium ion shuttle in PEO-based SPE system is realized by the dynamically breaking/forming lithium–oxygen (Li–O) bonds of intrachain or interchain hooping and assisted by the polymer chain local segmental motions. Due to the high donor numbers of ethers, the dynamic coordination cage formed by the polymer chains in polyether-based SPE systems is quite stable and hard to break.Reference Kim and Oh56 The lower donor numbers of carbonyl-coordinating polymers translate into an enhanced dissociation ability and favorable dynamic dislocation of lithium ions. The typical DC polarization and AC impedance results of both the PEO-based SPE and the PEC-based SPE at 60 °C are presented in Fig. 2, showing the much higher ${t_{{\rm{L}}{{\rm{i}}^ &#x002B; }}}$ of the PEC/LiFSI SPE system.

Figure 2. Comparison of the DC polarization (left) and AC impedance (right) experiment results of the (a) PEO-based SPE and (b) PEC-based SPE at 60 °C. Reproduced with permission.Reference Tominaga and Yamazaki46 Copyright 2014, Royal Society of Chemistry.

Electrochemical stability window

A sufficiently wide ESW is another prerequisite for SPEs toward high-voltage lithium batteries, as it determines the cycle life of lithium batteries.Reference Goodenough and Kim4,Reference Goodenough and Park8,Reference Goodenough and Singh9,Reference Agrawal and Pandey24,Reference Xu36,Reference Mindemark, Sun, Törmä and Brandell58,Reference Manuel Stephan and Nahm59 It should be noted that the morphological and chemical aspects of the polymer matrix and its complex interactions with lithium salts have obvious impacts on the ESW of SPEs, which should be considered when evaluating an SPE host.Reference Chen, Venkatram, Kim, Batra, Chandrasekaran and Ramprasad57 In general, a lithium battery often operates to 4.3 V versus Li/Li+ and sometimes approaching 5 V in the case of using high-voltage cathode materials, which is a tough challenge for SPEs. Even though the polyether-based SPEs are (electro)chemically stable with lithium metal anode, they are generally unable to sustain an operating voltage higher than 3.9 V.Reference Sun, Liu, Gong, Wilkinson and Zhang60,Reference Cheng, Hou, Zhang, Peng, Zhao, Huang and Zhang61 This highly restricts the further improvement toward the high–energy density systems, with the employment of high-voltage cathodes, e.g., LiNi1−xyCoxMnyO2, LiCoO2, LiNi0.5Mn1.5O4, and Li-rich Mn-based oxides.Reference Yang, Li, Dong, Ma, Yin, Wu, Xu, Ma, Fan and Sun62 Up to date, LiFePO4 seems to be the only successful cathode candidate for the utilization of PEO electrolyte.

Numerous publications on carbonyl-coordinating SPEs claimed the significant enhancement of high-voltage compatibility, with a typical onset of oxidation in the range of 4.5–5 V versus Li/Li+.Reference Sun, Mindemark, Edstrom and Brandell63Reference Barbosa, Rodrigues, Silva and Smith65 However, there are still limited reports on underlying the mechanism of high-voltage compatibility of SPEs, even for the most investigated PEO-based SPEs.Reference Zhou, Ma, Dong, Li and Cui37 Ramprasad et al. presented the theoretical estimation of ESW for polyethers [PEO and polypropylene oxide (PPO)] and carbonyl-coordinating SPEs [polycaprolactone (PCL), polyketone (PK), polymethyl methacrylate (PMMA), and polyethyl acrylate (PEA)] using first-principles density functional theory computations,Reference Chen, Venkatram, Kim, Batra, Chandrasekaran and Ramprasad57 as shown in Fig. 3. The carbonyl-coordinating SPEs showed improved high-voltage compatibility because the valence band minimum (VBM) is lower than the electrochemical potential of cathode (μC). In a similar fashion but involved the impacts of SEI and cathode electrolyte interface (CEI), Zhou et al. implied that the higher LUMO values of carbonyl-coordinating SPEs [PMMA, polypropylene carbonate (PPC) and polyvinyl carbonate (PVCA)] compared to their polyether-based SPE (PEO) analogues were responsible for their better high-voltage compatibility.Reference Zhou, Ma, Dong, Li and Cui37

Figure 3. Energy diagram of the electrolyte interface with anode and cathode. ESW of the SPE is determined by its reduction and oxidation potentials, controlled by the conduction band maximum (CBM) and the VBM, respectively. μA and μC are the electrochemical potentials of the anode and the cathode, respectively. Adapted with permission. Reference Chen, Venkatram, Kim, Batra, Chandrasekaran and Ramprasad57 Copyright 2019, John Wiley and Sons.

Benefitted from the unique structural characteristics of the carbonyl groups, the carbonyl-coordinating polymer–based SPEs present more appealing properties and surpass PEO-based analogues with respect to ionic conductivity, lithium ion transference number, and ESW, as summarized in Table 1. The differences are rooted in the different donor numbers of the functionalities, the disparate dielectric constants, the diverse polymer segmental motion abilities and the distinct Li+/oxygen coupling energy.Reference Zhang, Yang, Dong, Zhang, Chai, Dong, Wu, Zhou and Cui29

Table 1. A brief comparison of electrochemical properties between carbonyl-coordinating polymer–based SPEs and polyether-based SPEs.

σ: ionic conductivity; ${t_{{\rm{L}}{{\rm{i}}^ &#x002B; }}}$: lithium ion transference number; ESW: electrochemical stability window.

These advantages bring carbonyl-coordinating polymer–based SPEs even closer toward the implementation in commercial devices. As summarized in Fig. 4, carbonyl-coordinating polymer–based SPEs exhibit several potential advantages: (i) high ionic conductivity, (ii) excellent lithium ion transference number, (iii) superior electrochemical stability, (iv) versatile chemistry approach, and (v) environmental benignity.

Figure 4. Structural characteristics of carbonyl-coordinating polymer–based SPEs (polycarbonates and polyesters) and their unique advantages.

Synthesis and sustainability

Carbonyl-coordinating polymers (primarily aliphatic polycarbonate and polyester) have long been recognized as biodegradable and sustainable polymers. This is because the ester- and the carbonate-linked polymers are among one of the largest families of natural biomacromolecules and are capable to undergo environmentally friendly biodegradation with appropriate structures.Reference Kobayashi66 A vast number of review articles on the synthesis, characterization, properties, application, and degradation of aliphatic polyesters and polycarbonates have been reported in the literature.Reference Artham and Doble67Reference Taherimehr and Pescarmona85 Herein, a brief summary of the synthesis strategies and sustainability of aliphatic polyesters and polycarbonates, which can potentially be involved in the application of SPEs, will be discussed in the context below.

Synthesis approach toward polycarbonates

Traditionally, there are primarily three synthesis methods toward polycarbonates, i.e., polycondensation, ring-opening polymerization (ROP), and CO2–epoxide coupling reaction, as summarized in Scheme 1. Recently, Tamura et al. presented a synthesis approach for versatile polycarbonates from various diols and CO2 directly, which was promising in terms of the simple and environmentally friendly chemistry.Reference Tamura, Ito, Honda, Nakagawa, Sugimoto and Tomishige86 Aliphatic polycarbonates were initially prepared via polycondensation involved aliphatic phosgene or derivatives, which was not a fine chemistry, and the obtained polymers generally were with low molecular weight and high polydispersity.Reference Carothers, Dorough and Natta87 By using the melt polycondensation and post transesterification techniques, high–molecular weight aliphatic polycarbonates were obtained via the polycondensation from dialkyl carbonates and aliphatic diols.Reference Bendler88Reference Park, Jeon, Lee, Jang, Varghese and Lee90 In addition, the polycondensation technique enables one to prepare various structured polycarbonates straightforward from different diol-functionalized aliphatic oligomers.

Scheme 1. Synthesis strategies for aliphatic polycarbonates. (a) Polycondensation, (b) ROP, (c) CO2–epoxide coupling reaction, and (d) CO2–diol coupling reaction.

As an alternative, ROP of cyclic carbonates is a more effective strategy to synthesize polycarbonates with high molecular weight and narrow polydispersity without the formation and removal of any by-products as in polycondensation. The most commonly used monomers for ROP were the five- and six-membered rings of cyclic carbonates, e.g., trimethylene carbonate (TMC), a well-known biodegradable material.Reference Mespouille, Coulembier, Kawalec, Dove and Dubois91 The thermodynamic analysis of ROP showed that the high angular strain (Bayer’s strain) of the small (three- or four-membered) size rings had a favorable enthalpy effect, while the medium (six-membered) and larger size rings were primarily driven by entropy.Reference Möller, Hedrick, Degée, Dubois, Buschow, Cahn, Flemings, Ilschner, Kramer, Mahajan and Veyssière92Reference Paul, Zhu, Romain, Brooks, Saini and Williams94 Various types of catalysts such as transition metal catalysts, alkyl halides, basic and acidic organocatalysts, as well as enzyme catalysts were reported to be effective in catalyzing the ROP of cyclic carbonates.Reference Xu, Feng and Song95 Depending on the selection of catalyst, the ROP could be conducted in different mechanisms including cationic, anionic, coordination–insertion, monomer activation, monomer and initiator dual activation, and enzymatic activation, in solution or in bulk.Reference Xu, Feng and Song95 Notably, enzyme-catalyzed ROP was also powerful for synthesizing polycarbonates, with respect to the unique advantages of enzymes over conventional catalysts due to their milder reaction conditions, higher tolerance for functional groups, and higher selectivity. However, the catalyzing efficiency and the control on molecular weight and polydispersity were rather restricted.Reference Varma, Albertsson, Rajkhowa and Srivastava77,Reference Gross, Kalra and Kumar84

As one of the most promising and sustainable strategies of green chemistry, synthesizing polycarbonates from carbon dioxide (CO2) has aroused extensive research interest to tackle the greenhouse gas pollution issue and to alleviate the shortage in conventional petroleum fuel supplies.Reference D’Alessandro, Smit and Long96Reference Sugimoto and Inoue99 Ever since the first report of copolymerizing CO2 with epoxide by Inoue et al. in 1969,Reference Inoue, Koinuma and Tsuruta100 this copolymerization strategy has now turned into one of the most innovative techniques involved in the utilization of CO2 in industry. The main challenge of this synthesis strategy is searching for a highly efficient and selective catalyst while suppressing the side reactions, e.g., polyether formation from epoxide, backbiting reactions leading to cyclic carbonates.Reference Xu, Feng and Song95 A bunch of organometal catalysts have been developed, e.g., zinc-containing catalysts, aluminum complex, and transition metal complex.Reference Sugimoto and Inoue99 The mechanism of the alternating copolymerization of CO2 with epoxide catalyzed by metal coordination complexes has been revealed in depth in several excellent reviews.Reference Darensbourg, Mackiewicz, Phelps and Billodeaux98,Reference Lu, Ren and Wu101,Reference Coates and Moore102

Synthesis strategies toward polyesters

Despite the very similar chemical structure to polycarbonates, the synthesis strategies for polyesters are slightly different. Aliphatic polyesters were primarily prepared via polycondensation and ROP, as shown in Scheme 2.Reference Kobayashi66,Reference Albertsson and Varma103

Scheme 2. Synthesis strategies toward aliphatic polyesters. (a) Polycondensation, including AA + BB type and AB type, and (b) ROP.

The most traditional way of synthesizing polyesters involves the step-growth mechanism (polycondensation) from dicarboxylic acid (or dicarboxylic acid derivatives, such as diacid chloride) and diols, or from an oxyacid or its ester derivatives. The study of polycondensation of aliphatic polyesters was pioneered by Carothers et al. Reference Carothers, Dorough and Natta87 to extract the fundamental knowledge related to the step-growth polymerization and the relationship between molecular weight and extent of reaction, the stoichiometric imbalance of functionalities. However, polycondensation suffer from some major drawbacks, e.g., high reaction temperature, long reaction time, and removal of reaction by-products. Polyesters with sufficiently high molecular weight could only be achieved in the case of very high conversion and precise stoichiometry.Reference Albertsson and Varma103 Oligomers with molecular weight of several thousand (g/mol) were easily obtained, which could be further improved by chain extension treatment.Reference Park, Cho, Kim and Yoon104Reference Zhao, Wu and Yang106

To tackle these challenges, the ROP approach was developed to achieve a high molecular weight at mild reaction conditions. The monomers used in the ROP involve cyclic esters, e.g., lactones, cyclic diesters (lactides and glycolides), and cyclic ketene acetals. In sharp contrast to polycondensation, ROP can be carried out with complete monomer conversion and virtually free from side reactions, providing very good control over molecular structures, e.g., molecular weight, polydispersity, and end-group functionalization. In most cases, both the synthesis approaches for aliphatic polyesters were carried out in the presence of catalysts, e.g., protonic acid (HCl, RCO2H, etc.), Lewis acids (AlCl3, BF3, etc.), and organometallic compounds (stannous octoate, etc.),Reference Löfgren, Albertsson, Dubois and Jérôme107 even though some lactones polymerized spontaneously on heating. In principle, most catalysts utilized in the ROP of cyclic carbonates are also effective for the ROP of cyclic esters owing to the chemical structural similarity. The mechanism of ROP is highly dependent on the selection of catalysts and monomers, including cationic, anionic, free radical, active hydrogen, zwitterionic, and coordination.Reference Löfgren, Albertsson, Dubois and Jérôme107 Like the enzyme-catalyzed synthesis of polycarbonates, enzymatic synthesis of polyesters via both the polycondensation and the ROP techniques has been studied extensively. However, enzymatic polymerization generally yielded product with low molecular weight (<10,000 g/mol) and broad molecular weight distribution,Reference Kobayashi66,Reference Varma, Albertsson, Rajkhowa and Srivastava77,Reference Douka, Vouyiouka, Papaspyridi and Papaspyrides79,Reference Gross, Kalra and Kumar84 which is in need for further improvement.

Sustainability

In the past several decades, environmental concerns have spurred tremendous research interest in renewable resources and green chemistry. As a promising class of sustainable alternatives to commodity plastics,Reference Kobayashi66,Reference Taherimehr and Pescarmona85 the synthesis, characterization, properties, and application of bio-based and biodegradable polycarbonates and polyesters have been studied extensively.Reference Artham and Doble67,Reference Douka, Vouyiouka, Papaspyridi and Papaspyrides79,Reference Taherimehr and Pescarmona85,Reference Albertsson and Varma103,Reference Webb, Yang and Ameer108Reference Hilf and Frey112 The sustainable and eco-friendly synthesis approaches for polycarbonates and polyesters have been summarized above, among which enzyme-catalyzed polymerizations are promising with respect to their unique advantages of low requirement of reaction conditions, high tolerance for functional groups, and high selectivity. However, enzyme-catalyzed polymerizations also show drawbacks, which must be improved by further study. Notably, polycarbonates can be synthesized effectively in large scale by copolymerization of CO2 with epoxides. As one of the most abundant and renewable carbon resources, the industrial scale utilization of CO2 and the selective transformation of CO2 into degradable polycarbonates are regarded as the most successful green and sustainable chemistries.Reference Xu, Feng and Song95

The carbonate and ester functionalities are naturally formed, thus can go easy naturally breaking under certain conditions and appreciate backbone structures. As an example, Li et al. reported the enzyme-catalyzed biodegradation of poly(trimethylene carbonate) (PTMC) in the presence of lipases from Candida antarctica.Reference Liu, Yang, Fan, Li, Kasperczyk and Dobrzynski113 It was found that the PTMC degraded rapidly by Candida antarctica lipase, with 98% mass loss after 9 days, which was promising in terms of green recycling of battery materials. To date, the well-studied example of biodegradable material is the bio-based polylactide (PLA), which is currently manufactured on a large scale by the ROP of lactide, a cyclic ester from the fermentation product based on corn or wheat, as shown in Fig. 5.Reference Williams80 PLA has been utilized in a variety of applications, e.g., sutures, stents, dental implants, vascular grafts, bone screws, and pins, because of its biocompatibility. Under certain conditions, such as enzyme, PLA is capable of undergoing enzyme-catalyzed biodegradation into lactic acid and further into CO2 and H2O. This great success of PLA highlights the commercial and environmental potential for plastics sourced from plants, with renewable life cycles and which are carbon neutral. The mentioned green aspects of enzyme catalysis are expected to contribute to solving the environmental problems.

Figure 5. The cycle life of PLA. Adapted with permission.Reference Williams80 Copyright 2007, Royal Society of Chemistry.

The synthesis, properties, application, and recycling of polyesters and polycarbonates have been explored extensively over the years. As a promising class of high-voltage SPEs, aliphatic polyester and polycarbonate-based SPEs for lithium batteries have also been studied. In this review, we will summarize the recent key advances in polycarbonate- and polyester-based SPEs. The primary focus will be on the electrochemical property, lithium ion transport mechanism, and battery performance (if available). Based on our understanding and to the best of our knowledge, we tentatively suggest some challenges and perspective for aliphatic polycarbonate- and polyester-based SPEs in the last part of this review. We hope this review may give some helpful hints on understanding the correlation between structure and electrochemical performance and advancing the carbonyl-coordinating polymer-based SPEs.

Polycarbonate-based SPEs

Side chain polycarbonate-based SPEs

To date, conventional liquid electrolytes are still the dominant player in the field of lithium batteries, of which the majority are based on the carbonate solvent families, such as ethylene carbonate (EC), vinylene carbonate (VC), PC, and dimethyl carbonate (DMC).Reference Xu36 From a chemical perspective, the polycarbonates are essentially the linearly polymerized carbonates with high molecular weights either in the main chain or in the side chain, thus can solvate, coordinate, and conduct lithium ions in a similar fashion. The strategy of polymerizing carbonates endows polycarbonate-based SPEs with unique properties in terms of lithium ion conductivity, lithium ion transference number, ESW, and mechanical properties, etc. The molecular structures of polymer hosts and electrochemical properties of some typical polycarbonate-based SPEs are summarized in Table 2.

Table 2. Molecular structure and electrochemical characteristics of side chain polycarbonate-based SPEs.

σ: ionic conductivity; ESW: electrochemical stability window; LiCF3SO3: lithium trifluoromethanesulfonate; LiDFOB: lithium difluoro(oxalate) borate; LiTFSI: lithium bis(trifluoromethanesulfonyl)imide; LiPF6: lithium hexafluorophosphate.

The earlier work of applying polycarbonates for SPEs was provided by Holt et al.Reference Kaplan, Rietman, Cava, Holt and Chandross114 using poly(vinylene carbonate) (PVIC) as the polymer host (Entry 1 in Table 2). By integrating with crown ether (12-crown-4) and LiCF3SO3, such polycarbonate-based SPE showed considerably high ionic conductivity of 2.5 × 10−4 S/cm at ambient temperature. Shriver and Wei also reported the SPEs based on PVIC or poly(1,3-dioxolan-2-one-4,5-diyloxalate) (PVICOX) with LiCF3SO3,Reference Wei and Shriver115 delivering ionic conductivities of 1 × 10−6 S/cm (PVIC) and 1 × 10−4 S/cm (PVICOX) at 60 °C, respectively. The enhanced ionic conductivity of PVICOX/LiCF3SO3 SPE system was attributed to the larger free volume of PVICOX over the PVIC analogue. Besides, the crystalline behavior did not appear to suppress the ion conduction in such highly rigid chain SPE, which was in contrast with polyether-based SPEs.Reference Xue, He and Xie27 The ion transport was decoupled from the polymer chain segmental motion in these SPEs. By replacing the backbone of poly(vinylethylene carbonate) with highly flexible polysilanes, the polycarbosilanes, i.e., polySBMC and polySBDC, delivered impressively high ionic conductivities of 6.1 × 10−5 and 1.5 × 10−4 S/cm at 30 °C, respectively, and at high LiTFSI loading (Entry 2 in Table 2).Reference Matsumoto, Kakehashi, Ouchi, Yuasa and Endo116 As the result, the mechanical properties were severely compromised by such high lithium salt concentration.

Recently, Chai et al. reported the in situ polymerized poly(vinylene carbonate)/lithium difluoro(oxalate)borate (PVC/LiDFOB)-integrated SPE for high-voltage lithium metal battery, as shown in Fig. 6 and Entry 3 of Table 2.Reference Chai, Liu, Ma, Wang, Liu, Liu, Zhang, Cui and Chen117 A high ionic conductivity of 9.8 × 10−5 S/cm, high ${t_{{\rm{L}}{{\rm{i}}^ &#x002B; }}}$ of 0.57 and wide electrochemical window of 4.5 V were obtained at 50 °C, ensuring the outstanding electrochemical performance of LiCoO2/Li cell. The in situ technique intrinsically endowed the cell with superior interfacial stability and structural integrity, thus the high-voltage LiCoO2/Li cell delivered satisfactory rate capability (0.5 C, 73 mAh/g) and long-term cycling stability (84% after 150 cycles at 0.1 C). However, the abnormally high ionic conductivity is more likely because of plasticizing effect from the unreacted VC monomers rather than the contribution from the polymer host.

Figure 6. (a) In situ polymerization of PVC-based SPE and (b) self-standing SPE film. (c–f) Battery cycling performance of LiCoO2/Li cells with PVC/LiDFOB-integrated electrolyte. Reproduced with permission under CC BY-NC-ND 4.0,Reference Chai, Liu, Ma, Wang, Liu, Liu, Zhang, Cui and Chen117 Copyright 2016, the Authors.

Compared with their homopolymer-based analogues, SPEs from block copolymer are capable of exhibiting combined properties from each block,Reference Xu, Bijleveld, Hedge and Dingemans118 thus to achieve balanced properties with respect to ionic conductivity, electrochemical stability, and mechanical strength.Reference Soo, Huang, Jang, Chiang, Sadoway and Mayes119 Itoh et al. reported the integrated SPE from block copolymers of poly(AcIM/VC) and LiTFSI, exhibiting a significantly higher ionic conductivity value (7 × 10−4 S/cm at 80 °C) compared with poly(AcIM) homopolymer–based SPE (8.5 × 10−5 S/cm), as shown in Entry 4 of Table 2.Reference Mitsuda, Uno, Kubo and Itoh120 The block copolymers with high VC content (>75 mol%) were mechanically brittle, failing to perform the electrochemical measurements.

By copolymerizing poly(VC) with a series of poly(methoxy oligo(ethyleneoxy)ethyl vinyl ether) with different chain lengths, the poly(1a-g-alt-VC) alternating copolymer–based SPEs showed a significant increase in ionic conduction, as shown in Entry 5 of Table 2.Reference Itoh, Fujita, Inoue, Iwama, Kondoh, Uno and Kubo121 The highest ionic conductivity reached 1.2 × 10−4 S/cm at 30 °C due to the incorporation of large amount of flexible oligoether side chains. As a compromise, the electrochemical stability and lithium ion transference number were consequentially sacrificed, indicating that the ion conduction is mainly coupled with the ether segmental motions indeed. Wang et al. recently reported the block copolymer of poly(vinylene carbonate-acrylonitrile) [P(VC-AN)], aiming to achieve high-voltage cathode material (LiNi0.5Mn1.5O4) compatibility by taking complementary advantages of EC and nitrile functionality, as shown in Entry 6 of Table 2.Reference Wang, Chai, Zhang, Zhang, Ma, Xu, Du, Liu, Li and Cui122 However, the SPE most likely shows low ionic conductivity (not presented in the paper) due to the highly rigid polymer chain and possible crystallinity. Gel polymer electrolyte was prepared to perform all the electrochemical experiments and battery cycling tests. To avoid using such a highly rigid backbone structure, preparing flexible block copolymers with pendant carbonate moieties seems to be a reasonable choice. To this end, Wegner et al. synthesized block copolymers of polyacrylate and polymethacrylate with flexible carbonate functionalities at the side chain, as shown in Entry 7 of Table 2.Reference Britz, Meyer and Wegner123 The ionic conductivity was still insufficient for the electrochemical application at room temperature. Preparing gel electrolyte using PC was able to reach an order of 10−4 in ionic conductivity.

Main chain polycarbonate-based SPEs

Typically, aromatic polycarbonates exhibit high T g and poor segmental motions due to the rigid backbone of the polymer chain, which are detrimental for lithium ion transportation. The exploration of applying aromatic polycarbonates as SPE hosts has been reported in the early stage. Spiegel et al. reported the solvation behavior of lithium salts using poly(dimethyl siloxane)/poly(bisphenol-A carbonate) block copolymers with LiCF3SO3 (Entry 1 in Table 3).Reference Spiegel, Adamic, Williams and Sammells125 Even though the ionic conductivity could be improved significantly from 7 × 10−9 S/cm to 8.1 × 10−7 S/cm (both at room temperature) by the induction of soft poly(dimethyl siloxane) block, such ionic conductivity was still far below the essential value (10−4 S/cm). Attempts to improve the ionic conductivity of aromatic polycarbonate-based SPEs using oligo(ethylene oxide) side chains were effective (Entry 2 in Table 3); however, the electrochemical stability and lithium ion transference number were compromised.Reference Matsumoto, Uno, Kubo and Itoh126 This is easy to be understood that the ion conduction is dominated by the ether segmental motions.Reference Itoh, Fujita, Inoue, Iwama, Kondoh, Uno and Kubo121 All these results further confirm the failure of using aromatic polycarbonates as SPE host. Nevertheless, aromatic polycarbonates are potentially useful in strengthening the mechanical properties of SPEs via preparing block copolymer strategy.

Table 3. Molecular structures and electrochemical characteristics of main chain polyester-based SPEs.

σ: ionic conductivity; ESW: electrochemical stability window; LiCF3SO3: lithium trifluoromethanesulfonate; LiTFSI: lithium bis(trifluoromethanesulfonyl)imide; LiClO4: lithium perchlorate; LiFSI: lithium bis(fluorosulfonyl)amide; LiBF4: lithium tetrafluoroborate; LiPF6: lithium hexafluorophosphate; LiSbF6: lithium hexafluoroantimonate; LiDFOB: lithium difluoro(oxalate) borate.

As a contrast, aliphatic polycarbonates undergo polymer chain segmental motions much easier due to the higher molecular flexibility of aliphatic backbones. The simplest aliphatic polycarbonate used as SPE host was the principal ring-opening products from cyclic carbonates (EC) with the characteristic backbone structure of –O–(C=O)–O–, as shown in Entry 3 of Table 3. Practically, this polymer was more often synthesized via the green chemistry of the copolymerization of carbon dioxide with epoxides,Reference Taherimehr and Pescarmona85 rather than the ring-opening reaction from EC owing to its thermodynamic stability of the five-membered ring structure of EC.Reference Abdul-Karim, Hameed and Malik127 Besides, the ROP on EC is generally associated with the emission of carbon dioxide, resulting in polymer backbones with ether junctions alongside with carbonate linkages.Reference Lee and Litt128 Novakov and colleagues investigated the Li+-carbonyl group coordination behavior in PEC/LiClO4 SPE systems using Fourier Transform Infrared (FTIR).Reference Dukhanin, Dumler, Sablin and Novakov129 Tominaga et al. extensively studied the lithium ion conduction behavior of PEC-based SPE using various types of lithium salts.Reference Tominaga and Yamazaki46,Reference Kimura, Motomatsu and Tominaga47,Reference Tominaga124,Reference Tominaga, Nanthana and Tohyama130Reference Kimura and Tominaga140 Some lithium salts, e.g., LiBF4, LiFSI, and LiTFSI, have significant plasticizing effects on PEC-based SPE systems at high concentrations due to the declined intermolecular interactions by highly saturated ion pairs and aggregated ions and suppressed intramolecular interactions between carbonyl and CH2 arising from ion–dipole interaction.Reference Motomatsu, Kodama, Furukawa and Tominaga133 Distinctly, the PEC/LiCF3SO3 and PEC/LiClO4 SPE systems behaved more similarly to their polyether-based analogues, with respect to the dependencies of ionic conductivity and T g on lithium salt loadings.Reference Tominaga, Nanthana and Tohyama130 Besides, the PEC-based SPEs showed uniquely high ${t_{{\rm{L}}{{\rm{i}}^ &#x002B; }}}$ value (>0.5) despite high salt concentration,Reference Tominaga, Yamazaki and Nanthana134 clearly outperforming the polyether-based SPEs, which exhibited a drastically decreased ${t_{{\rm{L}}{{\rm{i}}^ &#x002B; }}}$ at elevated salt concentrations.Reference Munshi, Owens and Nguyen141 Especially, the ionic conductivity of PEC/LiFSI-integrated SPE was estimated to be higher than 10−4 S/cm at room temperature, with a ${t_{{\rm{L}}{{\rm{i}}^ &#x002B; }}}$ value of 0.63.Reference Tominaga, Yamazaki and Nanthana134 Even better electrochemical properties in terms of ionic conductivity (4 × 7 × 10−4 S/cm) and ${t_{{\rm{L}}{{\rm{i}}^ &#x002B; }}}$ (0.88) at 20 °C were reported by Okumura et al. based on the PEC/LiTFSI-integrated SPE systems.Reference Okumura and Nishimura48 These unique properties made PEC a promising SPE host, which was well summarized in a focused review by Tominaga.Reference Tominaga124 Even though high ionic conductivity and ${t_{{\rm{L}}{{\rm{i}}^ &#x002B; }}}$ are achievable at high lithium salt concentrations, the plasticization is accompanied by a notable suppression of T g and deterioration of mechanical properties, as depicted in Fig. 7.

Figure 7. (a) T g and storage elastic modulus as a function of LiTFSI molar ratio versus monomer unit of PEC. Reprinted with permission.Reference Okumura and Nishimura48 Copyright 2014, Elsevier B.V. (b) Ionic conductivity of PEC/LiTFSI SPE systems as a function of temperature and various lithium salt loadings. Reprinted with permission.Reference Tominaga124 Copyright 2016, Japan Science and Technology.

While the PEC-based SPEs surpassed their PEO-based analogues regarding the electrochemical properties, the versatile chemistry of the synthesis of PEC enabled one to explore various functional PEC-derivatives toward higher ionic conductivity.Reference Tominaga124 Several different side groups functionalized PECs were prepared by Tominaga et al. via the green chemistry approach starting from CO2 and a set of epoxides as electrolyte materials, as summarized in Entry 4 of Table 3.Reference Tominaga124,Reference Morioka, Ota and Tominaga137,Reference Tominaga, Shimomura and Nakamura142,Reference Nakamura and Tominaga143 The side functionalities used were methyl, ethyl, n-propyl, methylene phenyl, etc. Depending on the selection of the substituents, the T g’s of polymers were tunable within the temperature range 3–45 °C. The bulky groups (such as methylene phenyl) substituted PEC SPEs showed high T g (≈45 °C) and low ionic conductivities with a typical order of 10−8 S/cm at 80 °C, and the flexible side groups (such as ethyl and n-propyl) suppressed T g (<13 °C) and brought much higher ionic conductivity in an order of 10−5 S/cm at room temperature.Reference Tominaga124 This is clearly because the bulky side substitutes impede segmental motions, whereas the inclusion of flexible side groups increases the free volume and lowers T g. Aside from the coordination and conduction effects of lithium ions by carbonyl groups, the embodiment of ether-based side groups has considerable enhancement on lithium ion transport. Hitherto, the ether-based substituents, e.g., ethyl, i-propyl, n-butyl, t-butyl, phenyl, and oxyethylene groups, have been investigated by Tominaga et al.Reference Morioka, Ota and Tominaga137 However, the electrochemical stability and ${t_{{\rm{L}}{{\rm{i}}^ &#x002B; }}}$ were found to be deteriorated with the involvement of ether-based side groups due to the lower stability of ethers toward electrochemical oxidation and stronger chelation of lithium ion with ether oxygen compared with carbonyl groups.

By increasing the number of flexible methylene unit of PEC, the developed PTMC emerged as a promising SPE host candidate due to its low T g (≈ −15 °C) and amorphous nature (Entry 5 in Table 3). Smith et al. investigated the influence of various lithium salts, e.g., LiCF3SO3, LiClO4, LiBF4, LiSbF6, and LiPF6, on the ionic conduction of PTMC-based SPEs.Reference Silva, Barros, Smith and MacCallum64,Reference Barbosa, Rodrigues, Silva and Smith65,Reference Smith, Silva, Cerqueira and MacCallum144,Reference Manuela Silva, Barbosa, Evans and Smith145 The lithium salt–dependent ionic conductivity (<10−4 S/cm at 80 °C) and electrochemical stability (>4.5 V) are summarized in Entry 5 of Table 3. The PTMC-based SPE systems were also comprehensively studied using LiTFSI as the lithium salt by Brandell et al.Reference Sun, Mindemark, Edstrom and Brandell63,Reference Sun, Mindemark, Edström and Brandell146Reference Sun, Mindemark, Morozov, Costa, Bergman, Johansson, Fang, Furó and Brandell148 The flexible PTMC/LiTFSI-integrated SPE delivered high electrochemical stability (>5 V) and high ${t_{{\rm{L}}{{\rm{i}}^ &#x002B; }}}$ (0.8 at 60 °C), with a moderate ionic conductivity (10−7 S/cm at 60 °C). Interestingly, even though the PTMC had a much lower T g (−15 °C)Reference Sun, Mindemark, Edstrom and Brandell63 than PEC (13 °C),Reference Tominaga124 the ionic conductivity did not outperform PEC. This is because of the lack of lithium salt plasticizing effect in the PTMC-based systems at high lithium salt loadings, which plays a decisive role in the high ionic conductivity of PEC/LiFSI or PEC/LiTFSI systems as mentioned above. The PTMC/LiTFSI systems behaved more like PEO/LiTFSI analogues, showing increased T g with increasing LiTFSI concentration, and a maximum value of ionic conductivity at a moderate LiTFSI loading could be obtained.Reference Sun, Mindemark, Edstrom and Brandell63 By further increasing the number of methylene units, Mecerreyes et al. reported the synthesis and properties of a series of polycarbonates containing 4–12 methylene units by polycondensation of aliphatic diols and DMCs, showing low T g of approximately −40 °C.Reference Meabe, Lago, Rubatat, Li, Müller, Sardon, Armand and Mecerreyes149 High ionic conductivity (1 × 10−4 S/cm at room temperature) and high electrochemical stability (>4 V) were achieved for poly(dodecamethylene carbonate) at high LiTFSI concentration (80 wt%), as shown in Entry 6 of Table 3. By using the same polycondensation approach using diethylene glycol or triethylene glycol with DMCs, He et al. reported the synthesis of poly(carbonate-ether)s (PEDC and PTEC) incorporating with ether units, as shown in Entry 7 of Table 3.Reference He, Cui, Liu, Cui, Chai, Zhou, Liu and Cui150 Compared with PEC, the polycarbonates with ether units showed significantly superior electrochemical properties, i.e., an optimized ionic conductivity of 1.1 × 10−5 S/cm at 25 °C, a decent ${t_{{\rm{L}}{{\rm{i}}^ &#x002B; }}}$ of 0.39, and a broad electrochemical window of approximately 4.5 V. The LiFePO4//SPE//Li cell with PTEC-based SPE was stably cycled at low C rates and at room temperature. The high-voltage cathode material of LiFe0.2Mn0.8PO4 (4.35 V) was also used, exhibiting decent rate capability and good cycling performance at ambient temperature using PTEC-based SPE. The rate capability and cycling performance of the LiFePO4//PTEC-LiTFSI//Li and LiFe0.2Mn0.8PO4//PTEC-LiTFSI//Li cells at 25 °C are displayed in Fig. 8. Through the in situ polymerization strategy, Liu et al. provided an interpenetrating network IPN-PDEC SPE using acrylate functionalized poly(diethylene glycol carbonate), shown in Entry 8 of Table 3.Reference Liu, Ding, Zhou, Li, He, Chai, Pang, Liu and Cui151 The as-prepared IPN-PDEC-LiTFSI exhibited a decent ionic conductivity of 1.6 × 10−4 S/cm at 25 °C and a high electrochemical oxidation voltage of 4.5 V. The LiFe0.2Mn0.8PO4//IPN-PDEC–LiDFOB//Li cells were capable of being cycled up to 4.35 V and showed a capacity retention of 96% (140 mAh/g) at 0.1 C after 100 cycles. Kim et al. also reported the electrochemical performance of a cross-linked poly(carbonate-ether)-based SPE room temperature for solid-state batteries using an in situ cross-linking strategy (Entry 9 of Table 3).Reference Jung, Park, Kim, Ue, Eftekhari and Kim152 The LiNi0.6Co0.2Mn0.2O2//PEEC-based SPE//Li cells delivered an ionic conductivity value of 4 × 10−5 S/cm at 25 °C and a broad electrochemical window (4.9 V), capable of being reversibly cycled at ambient temperature, delivering an initial discharge capacity of 141.4 mAh/g and a good capacity retention with high Coulombic efficiency.

Figure 8. (a) The charge/discharge curves and (b) rate capability of LiFePO4//PTEC-LiTFSI//Li cells with varied C-rates at 25 °C. (c) The charge/discharge curves and (d) cycling performance of LiFe0.2Mn0.8PO4//PTEC-LiTFSI//Li cells at 25 °C. Reproduced with permission.Reference He, Cui, Liu, Cui, Chai, Zhou, Liu and Cui150 Copyright 2017, Elsevier B.V.

To further improve the ionic conductivity, polycarbonate–polyether block copolymer was investigated toward the one-component polymer hosts for high-performance SPEs, as summarized in Entries 10–16 of Table 3.Reference Morioka, Nakano and Tominaga138,Reference Melchiors, Keul and Höcker153Reference Deng, Wang, Ren, Han, Xiao and Meng157 Höcker et al. first reported the preparation of SPE based on poly(2,2-dimethyltrimethylene carbonate)-b-poly(ethylene oxide)-b-poly(2,2-dimethyltrimethylene carbonate) (PDTC-b-PEO-PDTC) triblock copolymer and LiCF3SO3, showing a moderate ionic conductivity value at the order of 10−4 S/cm at 60 °C.Reference Melchiors, Keul and Höcker153 Tominaga et al. also reported the high ionic conductivity (4.8 × 10−4 S/cm at 60 °C) and high ${t_{{\rm{L}}{{\rm{i}}^ &#x002B; }}}$ (0.66) obtained from the random block copolymer of poly(ethylene carbonate/ethylene oxide)-based SPE.Reference Morioka, Nakano and Tominaga138 By cross-linking, the ionic conductivity of the poly(ethylene carbonate/ethylene oxide) was severely lowered to an order of magnitude of 10−6 S/cm at 20 °C due to the hindered polymer chain segmental motions.Reference Elmér and Jannasch154 FTIR experiment proved that the Li+ had no preference in chelating with carbonyl groups or ether units, which was in sharp contrast with the PEC systems containing oligoether side moieties.Reference Morioka, Ota and Tominaga137 However, the electrochemical stability of the polycarbonates is most likely compromised from the induction of ether units. Single-ion conducting polymer electrolyte was prepared based on block copolymer of poly(carbonate-ether) by Deng et al., Reference Deng, Wang, Ren, Han, Xiao and Meng157 with a high ionic conductivity of 1.6 × 10−4 S/cm at 80 °C and very high ${t_{{\rm{L}}{{\rm{i}}^ &#x002B; }}}$ (0.86). This work highlighted a facile, efficient, and eco-friendly way of preparing single ion–conducting polymer electrolyte with high ionic conductivity. A similar single ion–conducting polymer electrolytes from poly(ethylene oxide carbonate) was reported by David et al., Reference Meabe, Goujon, Li, Armand, Forsyth and Mecerreyes158 delivering a high ionic conductivity value of 1.2 × 10−4 S/cm at 70 °C and a high ${t_{{\rm{L}}{{\rm{i}}^ &#x002B; }}}$ (0.89).

Apart from the polymer backbone design, several ways have been explored in enhancing the ionic conductivity of polycarbonate-based SPEs. Soaking with liquid electrolytes together with preparing gel polymer electrolytes seems to be the most effective way in improving ionic conductivity, reaching an order of magnitude of 10−4 to 10−2 S/cm at 25 °C. Different polycarbonates (Entries 17–19 of Table 3) have been used as the polymer hosts, including polybutadiene/PPC interpenetrating network,Reference Huang, Huang, Wu, Yu, Gao, Luo and Hu159 poly(vinylidene fluoride)/PPC,Reference Huang, Zeng, Liu, He, Sun, Xu, Yu, Luo, Zhou and Wu160 cellulose-supported poly(propylene carbonate),Reference Zhao, Zhang, Hu, Ma, Wang, Yue, Xu, Qin, Liu, Zhou and Cui161 cross-linked poly(propylene carbonate maleate).Reference Yu, Xiao, Wang, Han and Meng155 However, such gel polymer electrolytes sacrificed the inherently high security and reliability of SPEs. Ionic liquid has long been regarded as an efficient additive in plasticizing SPEs.Reference Shin, Henderson and Passerini162Reference Wu, Xu, Ren, Liu, Engelhard, Ding, El-Khoury, Zhang, Li, Xu, Wang, Zhang and Xu164 Tominaga et al. reported the PEC-based composite SPE with ionic liquid (N-n-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide, labeled as Pyr14TFSI) and LiTFSI, showing a low ionic conductivity in the order of 10−7 S/cm at ambient temperature,Reference Kimura, Hassoun, Panero, Scrosati and Tominaga131,Reference Kimura, Matsumoto, Hassoun, Panero, Scrosati and Tominaga132 which was the reason for its poor electrochemical performance in solid-state batteries. Much better electrochemical performance of the ionic liquid–containing SPE was reported by Lu et al., using PPC as the matrix, LiClO4 as the lithium salt, and 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM+BF4) as the additive, as shown in Entry 20 of Table 3.Reference Zhou, Zhou, Chen, Yee, Kong, Ding and Lu165

Dispersing inorganic fillers, e.g., TiO2,Reference Tominaga and Yamazaki46 SiO2,Reference Kimura, Matsumoto, Hassoun, Panero, Scrosati and Tominaga132 LLZTO,Reference Zhang, Zang, Wen, Dong, Chai, Li, Chen, Zhao, Dong, Ma, Yue, Liu, Guo, Cui and Chen166,Reference He, Chen, Zhang, Liu and Fan167 and POSS,Reference Kwon, Kim, Shim, Lee, Baik and Lee156 in polycarbonate-based SPEs has been regarded as an effective way to improve their electrochemical performance, especially the ionic conductivity (Entries 14 and 21 of Table 3). Traditionally, such enhancement is attributed to the reduction of the crystallinity and lower T g; however, the real case is more complex. A Lewis acid–base model of the interactions between inorganic fillers and polymer hosts was recently proposed, which provided an explanation of the strengthening effect of ionic conductivity from a unique perspective.Reference Zhou, Ma, Dong, Li and Cui37 As an example, the fabrication of PEOEC/POSS hybrid semi-IPN SPEs is shown in Fig. 9.

Figure 9. Fabrication of PEOEC/POSS hybrid semi-IPN SPEs. Reproduced with permission.Reference Kwon, Kim, Shim, Lee, Baik and Lee156 Copyright 2014, Elsevier B.V.

Polyester-based SPEs

Side chain polyester-based SPEs

The SPEs based on side chain–type polyesters with pendant ester moieties attached to the backbone have also been developed, of which the majority are acrylates and methyl acrylates. To date, most side chain polyester-based SPEs are in the realm of gel polymer electrolytes and are beyond the scope of this review.Reference Manuel Stephan30,Reference Zhu, Wu, Wang, Song, Long, Siyal, Yang and Sui169 Florjańczyk et al. performed a systematic work on the SPEs with polymer matrices of poly(acrylonitrile-co-butyl acrylate), poly(methyl methacrylate), and poly(butyl acrylate).Reference Florjańczyk, Zygadło-Monikowska, Wieczorek, Ryszawy, Tomaszewska, Fredman, Golodnitsky, Peled and Scrosati170Reference Florjańczyk, Zygadło-Monikowska, Affek, Tomaszewska, Łasińska, Marzantowicz, Dygas and Krok172 A considerably high ionic conductivity in the order of 10−4 S/cm at 30 °C was obtained for poly(butyl acrylate) containing 58 wt% LiI, due to its very low T g (−55 °C) and the plasticizing effect of the concentrated lithium salt. Recently, a grafted copolymer composed of polyrotaxane backbone and PCL grafted side chains was utilized as SPE host,Reference Imholt, Dörr, Zhang, Ibing, Cekic-Laskovic, Winter and Brunklaus168 as shown in Fig. 10. The polyrotaxanes were prepared via self-assembly of cyclodextrin (CD) host molecules threading onto polyethylenoxide (PEO) chains, and the CD severed as the initiator for the ROP of CL to afford pendant PCL side chains with a few repeating units. Extraordinary ionic conductivity (10−3 S/cm at 60 °C and 10−4 S/cm at 25 °C), high ${t_{{\rm{L}}{{\rm{i}}^ &#x002B; }}}$ (0.6 at 60 °C), and superior electrochemical oxidative voltage (up to 4.7 V) were obtained for this SPE.

Figure 10. (a) Grafted polyrotaxane structure with cyclodextrin in blue, PEO backbone in green, and PCL side chains in orange. (b) General synthesis scheme for the synthesis of GPR with (1) functionalization of PEO ends, (2) complexation with CD, and (3) end capping and simultaneous grafting. (c) ROP of ε-caprolactone initiated by CD. Reprinted with permission.Reference Imholt, Dörr, Zhang, Ibing, Cekic-Laskovic, Winter and Brunklaus168 Copyright 2019, Elsevier B.V.

Main chain polyesters

Esters were used as electrolyte solvents mainly due to their high electrochemical stability against anodic decomposition at cathode surfaces.Reference Xu36 Sharing the similar skeleton of esters but in the polymeric form, main chain–type polyesters used in SPEs are generally obtained via the ROP of cyclic esters or polycondensation of a diacid derivative and a diol, as discussed above. Similar to polycarbonates, FTIR confirmed that the coordination and Li+ transport behavior in polyester-based SPEs primarily occurred between Li+ and carbonyl groups, rather than alcohol residue ester oxygen.Reference Wu and Chang173

Some typical polyester-based SPEs explored by pioneers are summarized in Table 4. To date, the well-studied main chain–type polyester-based SPE system is based on sustainable PCL, with various lithium salts like LiSCN Reference Ravi, Song, Gu, Tang and Zhang174,Reference Ravi, Song, Gu, Tang and Zhang175 and LiClO4.Reference Polo Fonseca and Neves176Reference Lin and Wu178 The ion transport was rather limited due to its semicrystalline nature, which resulted in low ionic conductivity (10−6 S/cm at room temperature, for both LiSCN and LiClO4) at low lithium salt loadings of 10–15 wt% (Entry 1 in Table 4). Similar systems of PPL/LiClO4 exhibited a moderate ionic conductivity of 3.5 × 10−5 S/cm at 70 °C.Reference Watanabe, Togo, Sanui, Ogata, Kobayashi and Ohtaki179 Surprisingly, quenching of the electrolyte system increased the ionic conductivity by 1–3 orders of magnitude, achieving 3.7 × 10−4 S/cm, as shown in Entry 2 of Table 4. This is most likely due to the decrease in crystallinity by quenching, because mostly the lithium ion coordination and conduction occur in amorphous regions. The restricted lithium ion conductions were also reported in other semicrystalline polyester-based SPE systems with limited ionic conductivities (Entries 3 and 4 in Table 4), e.g., PEA/LiClO4 (1.06 × 10−5 S/cm at 30 °C),Reference Lin and Wu178 PBA/LiClO4 (9 × 10−7 S/cm at 30 °C),Reference Lin and Wu178 PHA/LiClO4 (5 × 10−6 S/cm at 30 °C),Reference Lin and Wu178 PE-2,4/LiClO4 (10−5 S/cm at 90 °C),Reference Watanabe, Rikukawa, Sanui, Ogata, Kato, Kobayashi and Ohtaki180,Reference Watanabe, Rikukawa, Sanui and Ogata181 PE-2,4/LiBF4 (3.4 × 10−6 S/cm at 65 °C),Reference Dupon, Papke, Ratner and Shriver182 PE-2,4/LiSCN (4.9 × 10−7 S/cm at 87 °C),Reference Watanabe, Rikukawa, Sanui and Ogata181 and PE-2,10/LiSCN (2.2 × 10−8 S/cm at 53 °C).Reference Watanabe, Rikukawa, Sanui and Ogata181 It is clear that the ionic conductivity of an SPE is highly dependent on carbonyl groups distributions, T g, polymer chain rigidity, crystallinity, electron donation strength of carbonyl group, lithium salt, etc. Shriver et al. also reported a moderate ionic conductivity (1.6 × 10−6 S/cm at 25 °C) of a noncrystalline polyester-based SPE system of PEM/LiCF3SO3,Reference Lee, Ratner and Shriver183 as shown in Entry 5 of Table 4. All the investigations were performed at low lithium salt concentrations (≈10 wt%). As discussed above, high-concentration imide-type lithium salts, e.g., LiFSI and LiTFSI, have significant plasticizing effect and potentially lower T g, achieving higher ionic conductivity for polycarbonate-based SPE systems.Reference Tominaga124 Considering the similarity in the skeletons of these two polymers, it is reasonable to assume that such plasticizing mechanism is also valid for polyesters with imide-type lithium salt systems.

Table 4. Molecular structures and electrochemical characteristics of side chain polyester-based SPEs.

σ: ionic conductivity; ESW: electrochemical stability window; LiSCN: lithium thiocyanate; LiClO4: lithium perchlorate; LiBF4: lithium tetrafluoroborate; LiCF3SO3: lithium trifluoromethanesulfonate; LiTFSI: lithium bis(trifluoromethanesulfonyl)imide.

Better electrochemical performance was reported for two series of systematically studied ether side chain–modified polyester-based SPE systems, as summarized in Table 4. By doping with LiTFSI (Entry 6 in Table 4), the ionic conductivity fell in the range of 1.4 × 10−4 to 3.0 × 10−4 S/cm at 90 °C, except that of 1b polymer was approximately one order of magnitude lower (10−5 S/cm) owing to its higher T g and slower segmental motion.Reference Pesko, Jung, Hasan, Webb, Coates, Miller and Balsara184,Reference Webb, Jung, Pesko, Savoie, Yamamoto, Coates, Balsara, Wang and Miller185 Most of the SPEs showed low mechanical properties at room temperature, which was due to their low molecular weight (<10,000 g/mol). By using the long-timescale molecular dynamics (MD) simulations, the lithium ion solvation and diffusion mechanisms in the above-mentioned polymers (as well as PEO) were analyzed, showing that the intrachain hopping of Li+ took place in PEO only. The Li+ transport in the polyesters primarily depended on the interchain hopping behavior, which was intrinsically slow.

SPEs from polyester/polycarbonate copolymers

As a sustainable material, biodegradable PCL has been studied thoroughly as SPE host for high–energy density solid-state lithium batteries. However, the PCL-based SPEs show restricted lithium ion diffusion (with an ionic conductivity in an order of 10−6 S/cm at room temperature) due to the semicrystalline nature. In polymer science, preparing block copolymers with two distinct repeating units has been regarded as an effective strategy to reduce the chain regular arrangement and thus to suppress crystallinity.Reference Van Horn, Steffen and O’Connor186 To this end, the PCL backbone with randomly distributed carbonate repeating units (TMC) was prepared by Mindemark et al. via bulk ROP at a TMC/CL ratio of 2:8.Reference Mindemark, Sun, Törmä and Brandell58,Reference Sun, Mindemark, Morozov, Costa, Bergman, Johansson, Fang, Furó and Brandell148 The as-prepared PTMC/PCL block copolymer-based SPE delivered a high ionic conductivity of 4.1 × 10−5 S/cm at 25 °C and a high ${t_{{\rm{L}}{{\rm{i}}^ &#x002B; }}}$ (>0.6). The corresponding cell could be operated at temperatures down to as low as 23 °C, dramatically surpassing their polyether-based SPE analogues. The rate capability of the cells using the PTMC/PCL block copolymer–based SPE is presented in Fig. 11. The enhanced ionic conductivity was attributed to the higher lithium ion mobilities by the random copolymerization with carbonate units in polyesters. FTIR experiments indicated that the Li+ preferentially coordinated with ester carbonyl group rather than with carbonate carbonyl moiety, which was in consistency with the structure–dynamics model provided by MD simulations. However, by increasing the TMC content to 60%, the highest ionic conductivity achievable was significantly depressed, with a value of 7.9 × 10−7 S/cm at 25 °C, despite the totally amorphous nature.Reference Mindemark, Törmä, Sun and Brandell187 The reduced lithium ion mobility further confirmed the FTIR results of the preferential coordination of Li+ with ester carbonyls.Reference Sun, Mindemark, Morozov, Costa, Bergman, Johansson, Fang, Furó and Brandell148

Figure 11. (a) Rate performance and coulombic efficiencies for a Li/SPE/LiFePO4 cell at room temperature and (b) initial charge/discharge voltage curves cycled at different temperatures and C-rates (right). Reprinted with permission.Reference Mindemark, Sun, Törmä and Brandell58 Copyright 2015, Elsevier B.V.

Summary and perspective

As one of the most successful energy storage systems in history, liquid electrolyte–based lithium ion batteries have been used massively, despite their inherent limitations of high volatilities and flammability. To realize the application of lithium ion batteries with high energy density, low cost, and excellent safety, using solid-state lithium batteries with SPEs seems to be promising and probably an inevitable choice. Taking the inherent advantages of SPEs, solid-state lithium batteries can alleviate the low energy density and safety concerns and endow the energy storage system with improved flexibility and processability. Given the lower donor number and weaker complexation strength of Li+ with carbonyl groups in polycarbonates and polyesters than with ether units in polyethers, the carbonyl-coordinating polymers deliver superior electrochemical properties with respect to ionic conductivity and ${t_{{\rm{L}}{{\rm{i}}^ &#x002B; }}}$. A unique property regarding the ionic conductivity in carbonyl-coordinating polymers is that the maximum value is generally obtained at high lithium salt concentrations due to the plasticizing effect, while the high lithium salt loading in polyethers ordinarily causes inter-/intrachain cross-linking and polymer chain stiffening. Besides, considering the electrochemical oxidation mechanism of SPEs with high-voltage cathode materials, the carbonyl-coordinating polymers exhibit broader ESW compared with their polyether-based analogues.

Carbonyl-coordinating polymer–based SPEs have emerged as a promising class of material for electrochemical applications, in the sense that this type of material exhibits several major advantages over the polyether-based analogues, including higher ionic conductivity, higher ${t_{{\rm{L}}{{\rm{i}}^ &#x002B; }}}$ and wider electrochemical window. However, such superior electrochemical properties are generally obtained at high lithium salt concentrations or in polymer-in-salt systems, arousing a series of challenges. For example, how can we solve the contradiction between ionic conductivity and mechanical strength? Is there any difference in terms of the lithium ion conduction mechanism between salt-in-polymer and polymer-in-salt systems? What is the special role of polymer dynamics at such high salt concentrations? What is the influence of high lithium salt concentrations on the ESW? How can we further improve the ESW of carbonyl-coordinating SPEs to higher than 4.5 V or even 5 V? What is the electrochemical decomposition mechanism of the carbonyl-coordinating SPEs at high lithium salt concentrations and high voltage (>4.5 V)? How can we improve the interfacial stability of SPE/cathode or SPE/anode at such high lithium salt concentrations and very high voltage? All these questions remain mystery, and much more efforts are still needed to further study and refine the carbonyl-coordinating polymer–based SPEs for high-voltage electrochemical applications.

In this review, we summarized and highlighted the synthesis, sustainability, and ionic conduction of carbonyl-coordinating polymers and the SPEs thereof. To the best of our knowledge and our understanding on SPEs, we tentatively suggest the following challenges and perspective:

  1. (1) Further improvement of ionic conductivity. The transport of lithium ions highly depends on the lithium ion dynamic complexation with the polymer chains and the segmental motions. Given this mechanism, the ionic conductivity of carbonyl-coordinating polymer–based SPEs is possible to be enhanced. Strategies on improving the ionic conductivity of polyether-based SPEs have been explored extensively, including the fabrication of organic/inorganic hybrid solid electrolyte, advanced polymer architecture modification, and incorporation of novel lithium salts, which are seldom reported for carbonyl-coordinating SPEs.

  2. (2) Balance between mechanical strength and ionic conduction. As the Li+ transport is highly relied on the polymer chain segmental motions, while the mechanical strength is in the retro-correlation with the segmental movements. To tackle this predicament, preparing block copolymers containing both the hard block and the soft ion conductive block seems to a reasonable strategy. The self-assembled nanostructures composed of the hard block and soft block enable the SPE thereof with both satisfied mechanical strength and unacted high ionic conductivity. As a bonus, the lithium dendrite growth problem is solved by this approach. Taking the advantage of the interaction between polymer and other additives in SPE, e.g., plastic crystal and inorganic particles, the ionic conductivity is improved by enhancing the lithium salt dissociation and suppression of the polymer crystallinity without sacrificing the mechanical strength.

  3. (3) Appropriate evaluation of cathodic stability with high-voltage cathode materials. Despite most of the carbonyl-coordinating polymers show satisfying results from linear sweep and cyclic voltammetry measurements, very few work investigated the long-term cycling and electro(chemical) decomposition of such SPEs in contact with high-voltage cathode materials (such as LiMn0.5Fe0.5PO4, lithium nickel cobalt manganese oxide (NCM), or lithium nickel cobalt aluminium oxide (NCA)), not to mention the building up of the practical energy storage devices. A technique that can simulate the realistic high-voltage lithium battery internal environment should be developed to evaluate the realistic cathodic stability of the SPEs. It is noteworthy that most of the high-voltage cathode materials, e.g., NCM and NCA, are strong base, which may cause base-catalyzed degradation on the carbonyl-coordinating polymers. Tailoring the SPE/cathode interface or coating the cathode active materials with high chemical stability layers should be helpful.

  4. (4) Amelioration of the lithium metal compatibility. In theory, a healthy battery requires both the good anodic and the cathodic stabilities with SPE. Due to the extremely high reactivity, the chemical and electrochemical stability and compatibility of carbonyl-coordinating polymer–based SPEs with lithium metal anode should be accessed comprehensively. Preparing a dual-layered SPE constructed with both anodic electrolyte (such as polyethers) and cathodic electrolyte with high cathodic stability (such as polycarbonates) should be promising.

  5. (5) Adopting advanced characterization technologies. The interfacial properties of the SPE with electrodes are critical for the healthy long-term cycling of a lithium battery. Using advanced in situ techniques, such as in situ FTIR, Raman, XRD, and cryo-TEM, to study the interfacial layer formation, development and compositions, and lithium dendrite growth during long-term cycling is meaningful.

  6. (6) Recycling of lithium battery materials. With the current massive application of lithium batteries in electrical vehicles, millions of tons of lithium battery will be retired in the coming several years, and the recycling of lithium battery material is increasingly urgent. The recycling of lithium battery is seriously challenging due to the versatile battery components and rich chemistry. While most attentions on battery material recycling are focused on valuable anodic and cathodic materials, the liquid electrolytes in current lithium ion batteries contain hazardous organic materials and need to be treated very carefully. Developing the alternative solid electrolytes for lithium batteries from bio-based and biodegradable polymers emerges as an important and attractive strategy in terms of environmental benignity, lower cost, and higher operability.

Acknowledgments

The authors gratefully acknowledge the financial support from National Key R&D Program of China (2018YFB0104300), Shenzhen Key Laboratory of Solid State Batteries (ZDSYS201802081843465), and Guangdong Provincial Key Laboratory of Energy Materials for Electric Power (2018B030322001).

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