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 1.5 Ni 0.5 O 4 , 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 depthprofiling 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.

Understanding the phase transition mechanisms of active materials inside Li-ion batteries is critical for rechargability 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 (LiMn1.5Ni0.5O4, LMNO) cathode. The structure of active material which resists complete delithiation is studied, towards 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.

The core principles and nanoscale mechanisms of ion deintercalation in battery cathode materials remain poorly understood, in particular, the relationship between crystallographic defects (dislocations, small angle grain boundaries, vacancies, etc.) and microscopic features of Li deintercalation. Here, we used operando scanning X-ray diffraction microscopy (SXDM) to investigate the local strain and lattice tilt inhomogeneities inside Li1−xMn1.5Ni0.5O4 cathode crystals (diameter from 1 to 2 µm) during electrochemical delithiation and lithiation. The technique has been combined to operando multi crystal X-ray diffraction (MCXD) to differentiate between inter- and intra-particle heterogeneity in the sample. Operando SXDM revealed three distinct domains within the crystal that displayed metastable angular rotations of the lattice during both of the phase transitions. These rotations, reaching up to 0.4°, likely arise due to a dynamic lattice mismatch between phases with different unit cell parameters coexisting within the particle. The persistent location of tilt boundaries implies the presence of inherent structural defects locally facilitating the defect formation. Residual misorientations were observed in the particle even after the full discharge suggesting an irreversible change of the lattice structure. Tilt boundaries, affecting ionic conductivity, may impede the rate capability and lead to localized strain, stress, and potential capacity fade due to cracking. However, the observed self-healing angular lattice reorganization could enable the coexistence of two phases in a single crystal. Understanding this phenomenon can optimize cathode material microstructure.