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.

CO取代已被广泛用于改善钠离子电池（SIBS）阴极材料的电化学性能，但是CO的作用尚未得到充分了解。本文中，我们全面研究了Ni的CO取代对Na0.7MN0.7NI0.3-XCOXO2（X = 0，0.1，0.3）的NA0.7MN0.7NI0.3-XCOXO的结构和电化学性能的影响。与无共同样品相比，CO的替代可以极大地提高了高速功能和周期性能，并且首次提出了对CO作用的一些新见解。用CO（3+）代替Ni2+晶格参数A降低；然而，C增加了，并且已经扩大了钠离子扩散层的D间距，从而增强了“钠离子和阴极材料的高率能力的扩散系数。由于CO3+的尺寸小于Ni2+，TM-O（TM =过渡金属）和O-O的of ni2+的尺寸较小，这说明了厚度的减小和体积TMO6八面体。最后但并非最不重要的一点是，在电化学反应期间，二氧化碳+对Ni2+的亚价取代可以提高电子电导率，这也有利于提高高速率性能。这项研究不仅揭示了CO在提高层次NA0.7MN0.7NI0.3-XCOXO2阴极材料的高速功能和周期稳定性中的作用，而且还为设计高性能阴极材料提供了一些新的见解。