Cathode vs Electrolyte: Decoupling the Origins of Degradation in High-Voltage Ni-Rich Lithium-Ion

Raising the upper cutoff voltage and deploying Ni-rich layered oxides (e.g., NMC811) are promising routes to higher lithium-ion cell energy density. However, the origin of degradation remains unclear at high-voltage operation. We propose using a cycle-relevant scan rate (1.0 mV s−1) cyclic voltammetry in a three-electrode cell, combined with post-cycled 1H NMR, ex-situ XRD, SEM, and XRF to decouple intrinsic cathode and electrolyte degradation contributions. Comparing to controls (glassy-carbon working electrode; 0.5M LiTFSI in PC electrolyte), we show that conventional electrolyte LiPF6/EC/DMC undergoes oxidative decomposition at ≥4.2 V vs Li/Li+ on an inert electrode, producing characteristic decomposition products that are reproduced when NMC811 is cycled in the same electrolyte. By contrast, NMC811 in LiTFSI/PC shows only minor solvent oxidation to 4.3 V but rapid surface reconstruction and rock-salt formation by 4.5 V. Critically, NMC811 coupled with LiPF6/EC/DMC at 4.3V exhibits rapid electrochemical decay (oxidation-peak current loss 68 ± 13% after 20 cycles) and accelerated appearance of intermediate superlattice phases by XRD. Together, these results identify electrolyte oxidative instability as a primary trigger for degradation near 4.3 V and implicate synergistic cathode-electrolyte mechanisms at higher voltages, motivating the design of higher-voltage stability electrolytes and cathode surface protection for durable Ni-rich cathode batteries.