PNNL

Abstract

Lithium ion batteries (LIBs) are attracting intensive interests worldwide because of their potential applications in transportation electrification and utility grid. The intercalation compounds used in LIBs electrochemically react with Li+ ions via single or multiple phase transitions depending on the nature of the material structure as well as the synthesis and testing conditions. It is generally accepted that for high voltage spinel LiNi0.5Mn1.5O4, a promising candidate for LIBs, there are two successive two-phase reactions occurring during the delithiation/lithiation processes, as reflected by the two flat plateaus with a small step in between. Here we demonstrate, experimentally and theoretically, that through elemental substitution, each of the two-phase transitions has been largely converted into a solid solution reaction during the electrochemical process. The latter favors fast Li+ diffusion due to the reduced number of phase boundaries that Li+ ions have to overcome, as well as the reduced shrinkage of unit cells after charge. This work clearly elaborates one of the critical functions for element doping/substitution that has been widely adopted in the materials synthesis for LIBs, whose fundamental mechanisms are still obscured.