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

60 boost stability and reliability. The charged polymeric binder builds a dynamically charge-directed network on the active materials with high versatility, and efficiently dissipates the electrode stress. The polyethylene glycol (PEG) in the charged binder offers a fast lithium-ion conduction pathway that can form a thick silicon oxide-based electrode (about 10.2 mAh/cm 2 ) without compromising the reversible specific capacity and promoting effective charge interaction as a mechanical modulator. The challenge is that the volume expansion of high-capacity anode materials during the reactionwith lithium poses a threat to battery performance and stability. Tomitigate this issue, the researchers have been investigating polymer binders that can effectively control the volumetric expansion. Research to date has focused on the chemical cross-linking of the polymer and hydrogen bonding. Chemical cross-linking involves covalent bonding between binder molecules, making them solid but has a flaw: once broken, the bonds cannot be restored. On the other hand, hydrogen bonding is a reversible secondary bonding betweenmolecules based on electro- negativity differences, but its strength (10-65 kJ/mol) is relatively weak. The polymer developed by the research teamuses hydrogen bonding but also takes advantage of Coulombic forces that are the result of the attraction between the positive and negative charges. These forces have a strength of 250 kJ/mol, much higher than that for hydrogen bonding, yet they are reversible, making it easy to control the swelling of thematerial. The surface of high-capacity anodematerials is mostly negatively charged, and the layering charged polymers are arrayed alternately with positive and negative charges to effectively bind with the anode. Introducing PEG to regulate the physical properties and boost the lithium-ion diffusion results in the high-capacity electrode and maximum energy density in a cell. “The research holds the potential to significantly increase the energy density of lithium-ion batteries by incorporating high-capacity anode materials, thereby extending the driving range of EVs,” says Professor Soojin Park at Postech, who led the work. “Silicon-based anode materials could potentially increase driving range at least tenfold.” Vanadium anode Researchers in Japan have developed an anode material that is particularly stable in solid-state battery cells. The material is Li 8 /7Ti 2 /7V 4 /7O 2 , a binary system composed of optimised portions of lithium titanate (Li 2 TiO 3 ) and lithium vanadium dioxide (LiVO 2 ). When milled down to particles nanometres in diameter, it offers a high capacity thanks to its large quantity of lithium ions that can be reversibly inserted and extracted during the charge/discharge process. Unlike other positive-electrode materials, it has nearly the same volume when fully charged as when fully discharged. That is the result of a fine balance between two independent phenomena that occur when lithium ions are inserted or extracted from the crystal. The removal of the ions, called delithiation, causes an increase in free volume in the crystal, making it shrink. Some vanadium ions thenmigrate from their original position to the spaces left behind, acquiring a higher oxidation state in the process. This causes a repulsion with oxygen, which in turn produces an expansion of the crystal lattice. “When shrinkage and expansion are well-balanced, dimensional stability is retained while the battery is charged or discharged – that is, during cycling,” says Professor Naoaki Yabuuchi of Yokohama National University, who led the work. “We anticipate that a truly dimensionally invariablematerial – one that retains its volume upon electrochemical cycling – could be developed by further optimising the chemical composition of the electrolyte.” The research team tested this new material in an all-solid-state cell by combining it with a solid electrolyte and a negative electrode. It had a capacity of 300 mAh/g with no degradation over 400 charge/discharge cycles. “The absence of capacity fading over 400 cycles clearly indicates the superior performance of this material compared with those reported for conventional all- solid-state cells with layered materials,” says Associate Professor Neeraj Sharma from UNSW Sydney, Australia, who also worked on the project. “This finding could drastically reduce battery costs.” Deep insight | Optimising battery materials Researchers have developed a bilayer membrane to prevent short-circuits between lithium-ion electrodes (Courtesy of Shinshu University) May/June 2023 | E-Mobility Engineering