In-situ XRD study of a Chromium doped LiNi0.5Mn1.5O4 cathode for Li-ion battery

This paper deals with structural (in-situ XRD) and electrochemical characterization of high-voltage lithium-ion cathode materials LiMn2O4 (LMO), LiNi0.5Mn1.5O4 (LNMO), and LiCr0.1Ni0.4Mn1.5O4 (LCNMO) prepared by solid-state synthesis. Structural in-situ X-ray diffraction spectra were measured by an affordable Rigaku diffractometer. Our synthesis route produced the samples with similar morphologies where the average particle sizes were 1.11 µm and 1.46 µm for LNMO and LCNMO respectively. Results of the Rietveld analysis brought detailed insight into two-phase structure transitions for LMO and three-phase transitions for LNMO and LCNMO. XRD study revealed differences in the structural behavior of LMO and LNMO prepared by solid-state synthesis compared to the results of other authors using the sol-gel synthesis route. In the case of chromium-doped LNMO, our results indicate ability of the chromium metal to effectively reduce Mn3+ content while the ordering of the structure increases. Chromium doping also promotes a larger lattice parameter in a fully delithiated state than in the case of undoped LNMO. Therefore, lowering of the volume changes was observed and faster phase II/III transition taking place, when Ni3+/Ni4+ redox pair was oxidized, was also identified. Cr doping of LNMO also promotes the reaching the lattice parameters of phases in both phase transitions and thus could reduce the internal stress of active material under high C-rate cycling. Results thus suggested that chromium doping can improve the stability of the inner structure and performance at higher charging C-rates even though the structure goes through a three-phase region during charging as undoped LNMO. The evaluation of diffusion coefficients of Cr-doped LNMO revealed increased diffusivity in a full discharge state and as the cathode underwent the cycling the differences in diffusivity seemed to be more pronounced.