March 2017

Lithium-ion-batteries (LIBs) have become indispensable in many areas in our lives. LIBs exhibit both high energy density and high power density, which make them particularly suitable for electric vehicle applications. Regarding the battery components, LiNi1-x-yCoxMnyO2 (NCM) is frequently employed as the cathode active material. Interestingly, it was recently proposed that the nickel ions could be reduced to the divalent state when the Li ions are being shuttled between the electrodes during cycling. Given that this effect would hinder the electrochemical performance, it is therefore important to gain a better understanding of such mechanisms occurring during battery operation. By using synchrotron X-ray diffraction, it is possible to obtain very high-resolution structural data (e.g., pair distribution functions), which provide very precise information about the local crystal structure. Once the local structure is elucidated, the underlying mechanisms for the cation mixing become clearer, thereby enabling new development strategies toward mitigating the associated electrochemical issues. The figure shows an exemplary pair distribution function of NCM and two of the representative probability densities for atomic distances within the unit cell. (Picture submitted by Jan Binder.)