Anion-redox chemistry has emerged as a groundbreaking strategy to unlock higher energy densities in lithium-ion batteries by utilizing oxygen redox alongside traditional transition metal redox reactions. Materials like Li-rich layered oxides (e.g., Li1.2Ni0.13Co0.13Mn0.54O2) exhibit reversible oxygen redox at voltages above 4.5 V vs Li/Li+, delivering capacities exceeding 300 mAh/g—nearly double that of conventional cathodes like NMC811 (~180 mAh/g). However,the irreversible oxygen evolution remains a significant challenge,causing voltage fade and capacity loss over cycling.To mitigate this,surface coatings such as AlF3 have been employed,thereby reducing oxygen loss by ~50% after100 cycles while maintaining >90% capacity retention.The development of anion-redox cathodes is thus pivotal for achieving next-generation batteries with energy densities beyond400 Wh/kg.
The mechanism of anion redox involves the formation of peroxo-like (O2)n− species during charging,a process that can be probed using advanced characterization techniques like resonant inelastic X-ray scattering(RIXS).Recent studies on Li2RuO3 have revealed that the formation of stable O-O dimers is key to achieving reversible anion redox.This understanding has guided the design of new materials such as Li2IrO3 which exhibits negligible voltage fade after500 cycles due to its robust O-O bonding network.Furthermore,the integration of anion-redox-active elements like Ru and Ir into layered oxides has enabled capacities up to350 mAh/g,making these materials highly attractive for electric vehicle applications.
Despite their promise,the high cost and scarcity of Ru and Ir pose significant barriers to large-scale adoption.To address this researchers are exploring earth-abundant alternatives such as Mn-based disordered rocksalt oxides(e.g.,Li1+ xMn2−xO4).These materials exhibit reversible anion redox at ~4V vs Li/Li+ with capacities reaching250 mAh/g.Moreover,the disordered structure facilitates rapid Li+ diffusion with ionic conductivities on the order10−4 S/cm comparable to liquid electrolytes.This combination of high capacity and fast kinetics makes disordered rocksalt oxides a viable candidate for low-cost high-performance batteries.
To further enhance the reversibility of anion-redox reactions surface engineering strategies such as atomic layer deposition(ALD)of protective coatings(e.g.,Al2O3)have been employed.For instance ALD-coated Li-rich cathodes demonstrate >95%capacity retention after200 cycles compared to <80%for uncoated counterparts.This improvement is attributed to the suppression of surface degradation reactions including electrolyte decomposition and transition metal dissolution.Future research will focus on optimizing coating thicknesses(~1-2 nm)and compositions to maximize performance while minimizing additional manufacturing costs.
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