Unveiling the Role of Co in Improving the High-Rate Capability and Cycling Performance of Layered Na0.7Mn0.7Ni0.3-xCoxO2 Cathode Materials for Sodium-Ion Batteries

Co substitution has been extensively used to improve the electrochemical performances of cathode materials for sodium-ion batteries (SIBs), but the role of Co has not been well understood. Herein, we have comprehensively investigated the effects of Co substitution for Ni on the structure and electrochemical performances of Na0.7Mn0.7Ni0.3-xCoxO2 (x = 0, 0.1, 0.3) as cathode materials for SIBs. In comparison with the Co-free sample, the high-rate capability and cycle performance have been greatly improved by the substitution of Co, and some new insights into the role of Co have been proposed for the first time. With the substitution of Co(3+)for Ni2+ the lattice parameter a decreases; however, c increases, and the d-spacing of the sodium-ion diffusion layer has been enlarged, which enhances the diffusion coefficient of "the sodium ion and the high-rate capability of cathode materials. In addition, Co substitution shortens the bond lengths of TM-O (TM = transition metal) and O-O due to the smaller size of Co3+ than Ni2+, which accounts for the decreased thickness and volume of the TMO6 octahedron. The contraction of TM-O and O-O bond lengths" and the shrinkage of the TMO6 octahedron improve the structure stability and the cycle performance. Last but not least, the aliovalent substitution of Co3+ for Ni2+ can improve the electronic conductivity during the electrochemical reaction, which is also favorable to enhance the high-rate performance. This study not only unveils the role of Co in improving the high-rate capability and the cycle stability of layered Na0.7Mn0.7Ni0.3-xCoxO2 cathode materials but also offers some new insights into designing high performance cathode materials for SIBs.