Hemalatha K, Jayakumar M, Prakash A S
CSIR-Central Electrochemical Research Institute-Chennai Unit, CSIR Madras Complex, Taramani, Chennai 600113, India.
Dalton Trans. 2018 Jan 23;47(4):1223-1232. doi: 10.1039/c7dt04372d.
The resurgence of sodium-ion batteries in recent years is due to their potential ability to form intercalation compounds possessing a high specific capacity and energy density comparable to existing lithium systems. To comprehend the role of cobalt substitution in the structure and electrochemical performance of NaMnO, the solid solutions of P2-NaMnCoO (x = 0.25, 0.5, 0.75) are synthesized and characterized. The XRD-Rietveld analysis revealed that the Co-substitution in NaMnO decreases lattice parameters 'a' and 'c' resulting in the contraction of MO octahedra and the enlargement of inter-layer 'd' spacing. XPS indicates that the isovalent cobalt substitution in NaMnO results in the partial/complete replacement of Jahn-Teller active trivalent manganese to form low-spin complexes of better structural stability. The Na-ion diffusion coefficient, D, derived from cyclic voltammetry and impedance spectroscopy, confirmed the enhanced mass transport in Co-rich phases compared to Mn-rich phases. Furthermore, higher diffusion coefficient values are observed for Co/Co than for their Mn/Mn redox processes. In addition, Co-rich phases exhibit a high structural stability and superior capacity retention, whereas Mn-rich phases discharge higher capacities.
近年来钠离子电池的复兴归因于其潜在能力，即能够形成具有高比容量和能量密度的插层化合物，这与现有的锂系统相当。为了理解钴取代在NaMnO结构和电化学性能中的作用，合成并表征了P2-NaMnCoO（x = 0.25、0.5、0.75）的固溶体。XRD-Rietveld分析表明，NaMnO中的钴取代降低了晶格参数“a”和“c”，导致MO八面体收缩和层间“d”间距增大。XPS表明，NaMnO中的等价钴取代导致Jahn-Teller活性三价锰部分/完全被取代，形成结构稳定性更好的低自旋配合物。由循环伏安法和阻抗谱得出的钠离子扩散系数D证实，与富锰相相比，富钴相中的质量传输增强。此外，观察到Co/Co的扩散系数值高于其Mn/Mn氧化还原过程的扩散系数值。此外，富钴相表现出高结构稳定性和优异的容量保持率，而富锰相放电容量更高。
