The reaction processes in Li‐ion batteries can be highly heterogeneous at the electrode scale, leading to local deviations in the lithium content or local degradation phenomena. To access the distribution of lithiated phases throughout a high energy density silicon‐graphite composite anode, correlative operando SAXS and WAXS tomography are applied. In‐plane and out‐of‐plane inhomogeneities are resolved during cycling at moderate rates, as well as during relaxation steps performed at open circuit voltage at given states of charge. Lithium concentration gradients in the silicon phase are formed during cycling, with regions close to the current collector being less lithiated when charging. In relaxing conditions, the multi‐phase and multi‐scale heterogeneities vanish to equilibrate the chemical potential. In particular, Li‐poor silicon regions pump lithium ions from both lithiated graphite and Li‐rich silicon regions. This charge redistribution between active materials is governed by distinct potential homogenization throughout the electrode and hysteretic behaviors. Such intrinsic concentration gradients and out‐of‐equilibrium charge dynamics, which depend on electrode and cell state of charge, must be considered to model the durability of high capacity Li‐ion batteries.

High‐resolution, surface sensitive soft X‐ray photoemission electron microscopy (XPEEM) reveals the fine interplay between oxygen and transition metal (TM) redox activities on the surface of a Li_1.17(Ni_0.22Co_0.12Mn_0.66)_0.83O₂ (Li‐rich NCM) electrode. We demonstrate that the oxidation of oxygen in the lattice is accompanied by TM reduction already at 4.47 V vs. Li⁺/Li, as a result of oxygen loss from the surface, the latter process being enhanced at 4.8 V where oxygen gas reaches a maximum release rate. Simultaneously, we find evidence for the chemical oxidation of the electrolyte solvents above 4.8 V induced by the released oxygen, leading to the formation of a carbonate by‐products layer that covers homogeneously the particles of the counter electrode Li₄Ti₅O₁₂ (LTO). The latter observation demonstrates that migration‐diffusion of such oxidized solvent by‐products occurs only when the solvents are chemically oxidized. We showed also that the Li‐rich NCM is susceptible to oxygen loss during soaking in the electrolyte, which causes TMs reduction and the poisoning of the counter electrode.

Ni-rich layered oxides LiNi_1-x-yMn_xCo_yO₂ (NMC811, x = 0.1 and y = 0.1) are considered promising cathode materials in lithium-ion batteries (LiBs) due to their high energy density. However, those suffer a severe capacity loss upon cycling at high delithiated states. The loss of performance over time can be retarded by Zr doping. Herein, a small amount of Zr is added to NMC811 material via two alternative pathways: during the formation of the transition metal (TM) hydroxide precursor at the co-precipitation step (0.1%-Zr-cp) and during the lithiation at the solid-state synthesis step (0.1%-Zr-ss). In this work, the crystallographic Zr uptake in both 0.1%-Zr-ss and 0.1%-Zr-cp is determined and quantified through synchrotron X-ray diffraction and X-ray absorption spectroscopy. We prove that the inclusion of Zr in the TM site for 0.1%-Zr-cp leads to an improvement of both specific capacity (156 vs 149 mAh/g) and capacity retention (85 vs 82%) upon 100 cycles compared to 0.1%-Zr-ss where the Zr does not diffuse into the active material and forms only an extra phase separated from the NMC811 particles.