The role of Li doping in layered/layered NaxLiyNi0.4Fe0.2Mn0.4O2 intergrowth electrodes for sodium ion batteries

The layered NaTMO₂ (TM = Ni, Fe, Mn) materials with the O3-type structure are attractive as positive electrodes for sodium ion batteries because of their high theoretical capacity. Additionally, Li doping in these materials has been shown to offer substantial enhancements to their electrochemical properties by promoting the formation of intergrowth structures, which intimately integrate the substituent phases. However, the influence of the specific Li content on the structural and electrochemical properties of the intergrowth materials requires investigation. Systematic variation of Li content in Na_xLi_yNi_0.4Fe_0.2Mn_0.4O₂ (NFM-Li_y) was conducted to identify the role of Li in modification of the intergrowth structure and electrochemical performance. Li contents of 0.15 and greater generate a layered/layered Na-O3/Li-O’3 intergrowth structure. ⁷Li and ²³Na solid-state nuclear magnetic resonance and x-ray absorption spectroscopy identify that when the total solubility for alkali ions in the layered structure is exceeded, Li continues to form the Li-O’3 phase while the excess Na forms residual sodium compounds such as Na₂O. Higher Li content is associated with improved capacity retention in the initial cycles from the superior stability of the mechanically linked Na-O3/Li-O’3 structure that suppresses the P3 to OP2 phase transition during charge. However, high Li contents are associated with increased rates of parasitic side reactions that reduce long-term cycling stability. These side reactions are connected to the instability of the cathode-electrolyte interphase, which can be partially mitigated by atomic layer deposition (ALD) coating with alumina, which significantly enhances the capacity retention and Coulombic efficiency. Overall, we find that the layered/layered Na-O3/Li-O’3 intergrowth structure is able to provide structural stability and suppress undesired phase transformations but is overwhelmed by the increased reactivity of the surface if not protected by surface coating.