Opposite Effects of Removing Ethylene Carbonate from the Electrolyte on Ni-Rich Layered Oxide and Mn-Rich Olivine Phosphate Cathodes

Ethylene carbonate (EC)-free electrolytes are promising for high-voltage lithium-ion batteries. However, their benefits have been demonstrated primarily for layered oxide cathodes, while their effects on olivine phosphates remain unclear. Here, we show that EC removal accelerates the degradation of LiMn 0.6 Fe 0.4 PO 4 (LMFP64), in contrast to LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811). Experiments and first-principles calculations suggest that these opposite responses reflect distinct degradation pathways associated with transition-metal–oxygen (TM–O) covalency. On NCM811, strongly hybridized O 2 p states near the Fermi level favor EC dehydrogenation, initiating degradation; EC removal eliminates this pathway. In contrast, olivine models exhibit weaker TM 3d–O 2 p hybridization, more ionic TM–O bonding, and stronger PF 6 − adsorption at TM sites. Without EC, experiments on LMFP64 reveal salt decomposition before cycling and accelerated degradation; EC otherwise limits anion access to TM sites. These findings show that the effect of EC removal is cathode-dependent and can be rationalized by TM–O covalency.