Metal–Ligand Spin-Lock Strategy for Inhibiting Anion Dimerization in Li-Rich Cathode Materials

Anion dimerization poses a significant challenge for the application of Li-rich cathode materials (LCMs) in high-energy-density Li-ion batteries because of its deleterious effects, including rapid capacity and voltage decay, sluggish reaction kinetics, and large voltage hysteresis. Herein, we propose a metal-ligand spin-lock strategy to inhibit anion dimerization, which involves introducing an Fe-Ni couple having antiferromagnetic superexchange interaction into the LCM to lock the spin orientations of the unpaired electrons in the anions in the same direction. As proof of concept, we applied this strategy to intralayer disordered Li₂TiS₃ (ID-LTS) to inhibit S-S dimerization. Electrochemical characterization using the galvanostatic charge/discharge and intermittent titration technique demonstrated the considerably enhanced anionic redox activity, reduced voltage hysteresis, and improved kinetics of the Fe-Ni-couple-incorporated ID-LTS. Fe L_2,3-edge X-ray absorption spectroscopy and magnetic susceptibility measurements revealed that the metal-ligand spin-lock effect and consequent suppression of anion dimerization involve ligand-to-metal charge transfer between S and Fe. Further electrochemical tests on a Fe-Ni-couple-incorporated Li-rich layered oxide (Li_0.7Li_0.1Fe_0.2Ni_0.1Mn_0.6O₂) indicated the importance of the π backbond in enhancing ligand-to-metal charge transfer from S to Fe. These findings demonstrate the potential application of our metal-ligand spin-lock strategy in the development of high-performance LCMs.