Probing Reaction Heterogeneity in Thick Electrodes: LiFePO₄ Versus LiNi_0.6Mn_0.2Co_0.2O₂

Introducing thick electrodes is a sound practice for increasing the energy density of lithium-ion batteries by minimizing the ratio of inactive components at device level. However, the inferior rate performance of the thick electrodes, which is believed to be caused by the inhomogeneous reaction, limits their further applications. In this study, we investigate the reaction distribution in LiFePO 4 and LiNi 0.6 Mn 0.2 Co 0.2 O 2 thick electrodes with combined scanning X-ray fluorescence (XRF) microscopy and X-ray absorption near edge structure (XANES) spectroscopy. 2D XRF-XANES imaging shows discharge reaction is highly non-uniform in the LFP electrode’s depth direction, the degree of which increases with the discharge rate. In contrast, NMC retains a uniform SOC distribution up to 1C (or 1.5C) rate. The XRF-XANES observations are verified by ToF-SIMS measurements. The different reaction distributions observed in LFP and NMC is explained by P2D modeling and has a thermodynamic origin Figure 1