Intricacies of Oxide Cathode Chemistry in Lithium-Ion and Sodium-Ion Batteries

Electrical energy storage is at the center of green energy revolution with respect to renewable energy use and vehicle electrification. Batteries are the most viable option for electrical energy storage. Efficient and economical adoption of battery technologies for these applications requires an optimization of several parameters, including cost, energy, power, cycle life, safety, and environmental impact, which are linked to severe materials challenges. Among them, cost along with sustainability and supply-chain challenges will be the dominant factor as the battery industry is expanding rapidly.

Cathodes play the dominant role in determining the cost and energy density of both lithium-ion and sodium-ion batteries. With respect to energy density, layered oxides are the favorites for both lithium-ion and sodium-ion batteries. The layered oxide cathode cost can be reduced by eliminating or reducing the amount of cobalt in lithium-ion batteries. The cathode cost can further be reduced by replacing lithium by sodium as a working ion and reducing the nickel content significantly while completely eliminating cobalt in sodium-ion batteries. However, optimization of the cathode compositions to be cost-effective, while adequately maintaining energy density and safety, requires a firm fundamental understanding of the intricacies of the oxide cathode chemistry.

This presentation will focus first on the intrinsic structural, chemical, lattice, and surface instabilities that are largely controlled by the electronic configuration of the transition-metal ions involved. This understanding will help design optimal cathode compositions with a trade-off among different performance factors. Often, the cathode behavior and their cycle life and safety features are drastically influenced by the electrolyte compositions, so the presentation will then focus on cathode-electrolyte interactions. As the cathodes involved have highly oxidized ions like Ni^3+/4+ and Co^3+/4+, the positions of the highest occupied molecular orbital (HOMO) of the electrolyte relative to the redox energies of the Ni^3+/4+ and Co^3+/4+ play a critical role in controlling the cathode surface reorganization pathway and the cathode-electrolyte interphase (CEI) formation and characteristics.

Finally, although Na lies below Li in the same group in the Periodic Table with a single valence electron, oxide cathodes for sodium-ion cells differ from those for lithium-ion cells in some aspects, while also exhibiting some similarities. More importantly, the electrolytes used in the cells display significant differences with respect to their interaction with the surfaces of oxide cathodes in lithium-ion and sodium-ion batteries. The presentation will, therefore, finally provide a highlight of the differences with some specific examples. The importance of the use of advanced analytical techniques, such as in-situ XRD, SEM, XPS, and TOF-SIMS, in developing the understanding will be discussed.