The surface of the layered transition metal oxide cathode plays an important role in its function and degradation. Modification of the surface structure and chemistry is often necessary to overcome the debilitating effect of the native surface. Here, we employ a chemical reduction method using CaI2 to modify the native surface of single-crystalline layered transition metal oxide cathode particles. High-resolution transmission electron microscopy shows the formation of a conformal cubic phase at the particle surface, where the outmost layer is enriched with Ca. The modified surface significantly improves the long-term capacity retention at low rates of cycling, yet the rate capability is compromised by the impeded interfacial kinetics at high voltages. The lack of oxygen vacancy generation in the chemically induced surface phase transformation likely results in a dense surface layer that accounts for the improved electrochemical stability and impeded Li-ion diffusion. This work highlights the strong dependence of the electrode’s (electro)chemical stability and intercalation kinetics on the surface structure and chemistry, which can be further tailored by the chemical reduction method.
Powder XRD patterns for SX-NMC, Br-NMC, Ca-NMC, and Rietveld refined structural parameters; operando XRD patterns for the first charge-discharge of SX-NMC and the refined lattice parameters; EDX spectrum for Ca-NMC; SEM images of SX-NMC, Br-NMC, and Ca-NMC powders; EIS for SX-NMC and Ca-NMC; rate testing results for SX-NMC and Ca-NMC; electrochemistry for Mg-modified single crystal NMC and Mg-, Ca-, Al-modified polycrystalline NMC811.