E-Mobility Engineering 019 | In conversation: Stephen Lambert l WAE EVR l Battery case materials focus l Quality control insight l Clipper Automotive Clipper Cab digest l Optimising battery chemistries insight l Powertrain testing focus

Solid polymer electrolytes Lithiummetal batteries (LMBs) have the potential to double the amount of energy stored in a single charge compared to current lithium-ion batteries (LIBs), but lithium dendrite growth and electrolyte consumption in current LMB technologies are hindering battery performance. Substrates for solid polymer electrolytes (SPEs) can help but require optimisation before being used in all- solid-state LMB (ASSLMB) systems. Scientists from Shinshu University and Kyoto University, in Japan, and Sungkyunkwan University, in South Korea, have therefore used a mechanical pressing method to develop a bilayer, non-woven polyethylene terephthalate (PET) microfibre/polyvinylidene fluoride (PVDF) nanofibre membrane. This acts as a separator for LIB systems to prevent short-circuits between electrodes. The separator demonstrated improved wettability (the ability of the lithium ion-containing electrolyte liquid to come into contact with the electrodes) and thermal stability of the battery system. The bilayer membrane can also be used in lithiummetal batteries to prevent detrimental lithiumdendrite growth and structural failure. A similar bilayer, non-woven PET/PVDF (nPPV) substrate was developed using an electrospinning method to prevent the formation of voids and folds between the two layers that reduce the longevity of the PVDF layer. The study characterised the nPPV- reinforced solid polymer electrolytes (nPPV-SPEs) for their mechanical, thermal and electrochemical properties. The tests confirmed that the substrate significantly improved the performance of ASSLMB systems. “Considering that the poor cycling performance of SPEs stems from low mechanical and thermal properties, this project focused on the fabrication of SPEs reinforced by a bilayer substrate consisting of a layer of PET non-woven fabric and a layer of PVDF nanofibres to improve the structural stability and thus the cycle performance of SPEs,” says Ick Soo Kim, professor at the Nano Fusion Technology Research Group in the Institute for Fibre Engineering (IFES) at Shinshu University. SPEs made up of polymer matrices and lithium salts demonstrate properties such as flexibility and processability that are compatible with LMB electrodes. The electrospinning method also eliminates the folds and voids generated by the pressing method between the PET and PVDF layers, providing a simple, easy and adaptable manufacturing method for nanofibre membranes. “The nanofibres, produced using industrial-scale electrospinning equipment by Lemon Co, ensure small and uniform pore size with high porosity, thus accommodating polymeric materials and lithium salts without affecting ion diffusion and improving electrochemical oxidation stability,” says Prof Kim. For this study, PVDF nanofibres were electrospun directly onto the microfibre PET layer to produce a more robust bilayer material. Improved structural stability allows SPEs to tolerate the chemical reactions occurring in the system during longer-term charging and recharging cycles. “The bilayer substrate significantly improves the mechanical and thermal properties of solid polymer electrolytes, which enables the cell to operate over 2000 hours,” says Prof Kim. The high tensile strength of the material suppresses lithium dendrite growth, one of the significant challenges of LMB systems. However, more work is required to increase the performance of SSEs such as SPEs. “The improved structure stability of SPEs ensures a long lifespan and safe use of lithium batteries, but the rate performance and lithium mobility of SPEs are still inferior to liquid electrolytes in lithium-ion batteries,” Prof Kim says. “The next step is to improve the ionic conductivity to meet the requirements of fast charge and discharge.” Lithium-air SSBs Researchers at the Illinois Institute of Technology and the US Department of Energy’s Argonne National Laboratory have developed a lithium- air battery using a solid electrolyte that could provide an energy density of 1200 Wh/kg – four times that of today’s batteries. “The lithium-air battery has the highest projected energy density of any battery technology being considered for the next generation of batteries beyond 61 Using a layer of solid electrolyte (CPE) could give a lithium-air cell four times the performance of today’s batteries (Courtesy of Argonne National Laboratory) May/June 2023 | E-Mobility Engineering