Correlating the Voltage Hysteresis in Li- and Mn-Rich Layered Oxides to Reversible Structural Changes by Using X-ray and Neutron Powder Diffraction

Abstract

Li- and Mn-rich layered oxides (LMR-NCMs) are promising cathode active materials (CAMs) in future lithium-ion batteries (LIBs) due to their high energy density. However, the material undergoes a unique open circuit voltage (OCV) hysteresis between charge and discharge after activation, which compromises its roundtrip energy efficiency and affects the thermal management requirements for a LIB system. The hysteresis is believed to be caused by transition metal (TM) migration and/or by oxygen redox activities. Using in-situ X-ray powder diffraction (XPD), we monitor the lattice parameters of over-lithiated NCMs during the initial cycles and show that also the lattice parameters feature a distinct path dependence. When correlated to the OCV instead of the state of charge (SOC), this hysteresis vanishes for the unit cell volume and gives a linear correlation that is identical for different degrees of over-lithiation. We further aimed at elucidating the role of TM migration on the hysteresis phenomena by applying joint Rietveld refinements to a series of ex-situ XPD and neutron powder diffraction (NPD) samples. We critically discuss the limitations of this approach and compare the results with DFT simulations, showing that the quantification of TM migration in LMR-NCMs by diffraction is not as straightforward as often believed.