Non-Newtonian fluid electrode slides through gravity-induced flow cell
Flow batteries could mean big improvements in energy storage. That storage is needed for developing renewable energy sources that rely on natural phenomena beyond our control: weather and time of day cause power fluctuations and the banked energy can be released during low-output periods. However, the cost of energy storage presents an ongoing challenge. An inexpensive, durable, and high-capacity battery is needed.
“The world of battery design has historically had two limiting-cases: Stationary batteries with no moving parts, and flow batteries with actively pumped fluids,” says Yet-Ming Chiang, leader of a team of materials scientists and mechanical engineers at the Massachusetts Institute of Technology (MIT). “We recognized that there was a huge unexplored design space between these two extremes.” Under the US Department of Energy’s Joint Center for Energy Storage, the MIT team developed a (mechanically) simple flow battery that uses gravity to drive flow of a liquid electrode, and an appropriate combination of cell geometry, surface engineering, and gas-flow control to control flow rate. The researchers published their results in Energy & Environmental Science.
Redox batteries store energy in the chemical states of their components. Electrons flow from one material undergoing oxidation (electron loss) to another material undergoing reduction (electron gain). In commercial batteries, electrically charged ions travel through a fluid medium from one solid electrode to the other and, in the process, generate electric current. The reliance on solid electrodes makes them vulnerable to structural degradation and reliant on media that can transfer ions efficiently without breaking down over time. Chiang’s team saw room for improvement: “We were motivated firstly by the high cost of existing rechargeable batteries such as lithium ion, which arises from both an excess of non-energy-storing components used in the battery as well as complex, expensive, capital-intensive manufacturing processes. Thus we were seeking battery designs that could be manufactured simply and with a minimum of unit operations.”
The team developed a cell around a novel liquid electrode. Though some flow battery schemes include complicated pump and tank systems to actively move the liquid electrode, those systems “suffered from having too many mechanical points of failure as well as mechanical and electrical efficiency losses,” Chiang says. The liquid electrode flow occurs under gravity and the researchers call their device a gravity-induced flow cell (GIFcell). The anode (positive electrode) of the GIFcell is a liquid mixture principally composed of three materials. The compound gaining electrons is lithium sulfide; as the cell is discharged, the ratio of lithium to sulfur in the material shifts from soluble Li2S8 through a variety of ratios to ultimately form small, insoluble particles of Li2S. The lithium sulfide is suspended in viscous liquid glyme (glycol ether), a solvent also used in common lithium ion batteries. Electrons, however, cannot move freely in this combination, which would limit the current the device produced. To solve this challenge, the scientists added carbon nanotubes to the liquid electrode, significantly increasing its conductivity.
When this liquid catholyte flows over the solid anode, electrons travel between them and steel collectors on opposite sides of the device collect the resulting current. The current generation changes based on how quickly the flow occurs. Though this means high tunability, the nature of the liquid electrode proved challenging. Much like ketchup, the liquid electrode is a non-Newtonian fluid: it transitions from flowing quickly under pressure to coagulating when pressure is removed. The electrolyte would barely move, then suddenly rush out, like ice cubes in the bottom of a glass. If the channel was too wide or the device angle was suddenly changed, the electrode flowed unevenly. Testing a variety of channel sizes and materials, the research team characterized flow in high- and low-slip conditions.
“Flow batteries are electrochemical as well as mechanical devices,” Chiang says; managing the ill-behaved catholyte called for a mechanical engineering solution. The team constructed a prototype GIFcell by incorporating the flow channel, tanks, and current collectors into a three-dimensional-printed polymer case. By adding a small valve, the flow rate could be controlled by releasing air trapped in the bottom tank back into the top tank. Like an hourglass, flipping the GIFcell allowed the process to be repeated until the power was exhausted.
The cell ran for several charging cycles before sulfides infiltrated the solid lithium electrode and caused resistance in the device to significantly increase. The solution to the degradation issue will come naturally from the scientists’ next step: a GIFcell in which both anode and cathode materials are fluids. “Although we demonstrated the GIFcell with the lithium-polysulfide couple, the approach is in principle applicable to any flow battery chemistry,” Chiang says. The design is flexible and adaptable.
Robert Baker, an Assistant Professor of Chemistry at the Ohio State University who studies electrode-electrolyte interfaces, says, “Short-term energy storage is important for making better use of many renewable energy sources like wind and solar. The idea to use gravity to drive flow in a redox flow battery is simple, yet novel, and can remove a lot of the complexities associated with these short-term energy storage devices.” With the device architecture developed, there is a potential for new fluid electrode materials that could demonstrate the full capabilities of GIFcells.