Polymer Electrolyte-Cathode Surface Bonding Suppresses Oxygen Evolution and Cation Mixing in Ni-Rich Layered Oxides...

Although lithium nickel manganese cobalt oxides (NMC-811) exhibit high practical capacity and voltage, they face severe structural degradation at high states of delithiation, including oxygen evolution, Li/Ni mixing, transition-metal (TM) dissolution, etc. These phenomena are dependent on the interactions with the electrolyte. Here, we systematically studied interfacial O-TM bonding between three distinct solid polymer electrolytes and the NMC-811 surface using density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations. The polymers considered include poly(ethylene oxide) (PEO), poly-ε-caprolactone (PCL), and poly(trimethylene carbonate) (PTMC). The DFT results indicate that PTMC exhibits the strongest Ni-O_polymer bonding strength due to the highly negative charge of its carbonyl oxygen. This strong PTMC-NMC bond further enhances the Ni-O_NMC bonding strength, and both the Ni-O_polymer and Ni-O_NMC bonds are strengthened after delithiation. Consequently, PTMC can stabilize undercoordinated surface Ni ions. The AIMD simulations support this observation, showing the fewest oxygen-evolution and Li/Ni-mixing events in the PTMC-based system. We propose that strong hybridization between the donor oxygen and surface Ni facilitates Ni²⁺ oxidation and reduces the redox activity of lattice oxygen. Thereby, this study highlights that the structural stability of NMC can be tuned by selecting polymer functional groups.