Rational analysis of layered oxide power performance limitations in a lithium battery application
Résumé
The purpose of this paper is to rationalize material geometry contribution on the electrochemical performances of four model LiNi1/3Mn1/3Co1/3O2 materials. A methodology combining the exhaustive microstructural characterization and the careful study of each component of the electrochemical process is established to discuss the limiting factors of power performances. Intuitions based on the exhaustive microstructural characterization are first confronted with the study, by cyclic voltammetry, of the rate-limiting step of the electrochemical process. Depending on the microstructure, diffusion controlled electrochemical behavior is observed, which is expected in Li-ion battery, and also charge-transfer limitation even at extremely high scan rates. This second behavior surprisingly occurs for high surface area materials. Possible electronic limitations in these materials are explored using broadband dielectric spectroscopy. This unique technique shows that flake-shaped, highly anisotropic, crystallites facilitate electronic motion at all scale levels compared to cuboidal crystallites. Charge-transfer limitations are not electronic, but come from the material interface contribution to the electrochemical process. Numerical simulations allow quantifying the actual electroactive surface area. Between 15% and 30% of the BET surface area, corresponding to the thickness of the crystallites, are actually active.