Structure modification of P2-Na0.67Ni0.15Fe0.2Mn0.65O2 cathode by incorporation of cerium for high-performance sodium-ion batteries

The P2-type transition metal oxides hold promise for the cathode of sodium-ion batteries (SIBs). However, the prevailing irreversible phase transition and Jahn-Teller effects of this kind of metal oxides lead to low capacity and poor cycling stability of SIBs. To address these challenges, in this study, the rare-earth metal element cerium (Ce) is successfully introduced into P2-Na_0.67Ni_0.15Fe_0.2Mn_0.65O₂ (NFM), achieving a synergistic bulk doping and surface modification for NFM. Upon incorporating larger Ce³⁺ into the lattice, the interlayer spacing of P2- NFM is increased, facilitating the diffusion of Na⁺. The substitution of Ce for Mn sites suppresses the Jahn-Teller effect caused by Mn³⁺ in P2-NFM. The concurrently formed CeO₂ on the surface effectively inhibits the corrosion and degradation of P2-NFM by the electrolyte. Benefitting from the dual-modification, P2-Na_0.67Ni_0.15Fe_0.2Mn_0.61Ce_0.04O₂ (NFMC-0.04) exhibits an increased discharge capacity of 171.6 mAh g^-1 at 0.1C as compared to that of NFM (142.9 mAh g^-1), and a higher capacity retention of 57.40 % after 200 cycles than that of NFM (37.97 %). Impressively, it can deliver a capacity retention of 58.41 % after 250 cycles at 1C, while the discharge capacity of un-modified NFM is close to zero. In-situ XRD analysis reveals that the successful doping of Ce into the bulk phase of NFM suppresses the structural distortion of NFM from P2 to OP4 phase. Density Functional Theory calculations disclose that the substitution of Mn sites by Ce elements is energetically most favorable in NFM. Ce doping not only improves the electronic conductivity of NFM by enhancing the degree of electron localization, but also suppresses the Jahn-Teller effect by modulating the electronic structure of Mn. This study provides a new approach for engineering of layered transition metal oxides toward the high-performance cathode of SIBs.