Understanding structural changes in NMC Li-ion cells by in situ neutron diffraction

A combination of the in-situ high-resolution neutron powder diffraction along with electrochemical analysis was applied to study commercial NMC cells (18650-type) based on a Li_x(Ni_0.5Mn_0.3Co_0.2)O₂(NMC) cathode and a graphitic anode. The structural changes of the electrode materials during cell charge/discharge have been analyzed using Rietveld refinement and single profile decomposition techniques. A different behavior of electrochemically-driven lattice distortion was observed for NMC material in comparison to Li_xCoO₂ which influence the overall cell performance. Detailed analysis of the structural changes in the Li_x(Ni_0.5Mn_0.3Co_0.2)O₂ cathode material revealed reversible Li/Ni cation mixing (5.6(8) %), which is state-of-charge independent below 1600 mAh and disappearing above 1800 mAh (~0.8 Q_max). The cation mixing behavior was traced in detail using the neutron diffraction and it is considered as one of the reasons of the nonlinear change of the cathode material lattice parameters. A transformation of graphite to LiC₁₂ and LiC₆ including the formation of higher ordered lithium intercalated carbons was observed, where the evolution of the anode at higher charge states of the cell is reflected in a change of the phase fractions of LiC₆ and LiC₁₂ due to further graphite lithiation.