The Role of Li Doping in Layered/Layered Na x Li y Ni 0.4 Fe 0.2 Mn 0.4 O 2 Intergrowth Electrodes for Sodium Ion Batteries

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Layered NaTMO 2 (TM = Ni, Fe, Mn) materials with the O3-type structure are attractive positive electrodes for sodium ion batteries because of their high theoretical capacity. Li doping in these materials offers substantial enhancements to their electrochemical properties by promoting the formation of intergrowth structures that 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 x Li y Ni 0.4 Fe 0.2 Mn 0.4 O 2 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. 7 Li and 23 Na 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 Na 2 O. The mechanically linked Na-O3/Li-O’3 structure at higher Li content is associated with improved capacity retention in the initial cycles because it suppresses the P3 to OP2 phase transition during charge. However, high Li content increases the rate of parasitic side reactions and reduces long-term cycling stability. These side reactions are connected to the instability of the cathode-electrolyte interphase but 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 can provide structural stability and suppress undesired phase transformations but is overwhelmed by the increased reactivity of the surface if not protected by surface coating.

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