Revisiting Phase Transformation Mechanisms in LiNi_0.5Mn_1.5O₄ High Voltage Cathodes with Operando Microdiffraction

Understanding the phase transition mechanisms of active materials inside Li-ion batteries is critical for rechargeability and optimizing the power/energy density of devices. In this work, high-energy microfocused X-ray diffraction is used to measure in operando the state-of-charge heterogeneities inside a high-voltage spinel (LiMn_1.5Ni_0.5O₄, LMNO) cathode. The structure of an active material which resists complete delithiation is studied to move toward unlocking the full storage capacity of ion-conductive spinels. High-precision diffraction also reveals nonlinear coupling between strain and lithiation state inside the cathode at high voltages, which suggests the phase diagram of this material is more complex than previously assumed. X-ray diffraction depth-profiling shows that large lithiation heterogeneities through the cross-section of the electrode are formed even at low currents and that decoupling these gradients are necessary to study the phase transitions in detail.