Investigation of non-transition metal elements substitution in layered oxides as cathode materials for sodium ion batteries

Abstract

Lithium ion batteries (LIBs) exhibit outstanding performance and have been applied in smart phones, electric vehicles, etc. Since successful commercialization of LIBs, price of lithium resources have been increasing as well and post-LIB systems have drawn significant attention of researchers. Among many potential alternatives, sodium ion batteries (SIBs) have gained much attention due to natural abundance and uniform, world-wide distribution of sodium resource, which are especially attractive feature for large-scale energy storage application. Transition metal oxides ($NaMO_2$, M=single or mixture of transition metal elements) are promising candidate for electrode materials for SIBs due to high theoretical capacity, simple and scalable synthesis method, and appropriate operating voltage. In order to achieve develop high performance TM oxides, it is critical to understand the crystal structure, phase transformation behavior, and ion intercalation properties within the structure. One of the common approaches for improving electrochemical properties of transition metal oxides is varying the composition of transition metal elements. Another approach is substitution of electrochemically inactive, non-transition metal. In this work, non-transition metals, $Ca^{2+}$ and $Li^+$, were substituted on Fe/Mn-based transition metal oxides and their effects on crystal structure, phase transformation behavior, and Na intercalation were studied by electrochemical analysis, Rietveld refinement of synchrotron and neutron diffraction patterns, etc. $Ca^{2+}$ was substituted on alkali site and its effect on the structure and electrochemical performance strongly depended on layered structure. The substitution site of $Li^+$ was controlled by controlling synthesis condition. Rietveld refinement of diffraction patterns and theoretical calculation revealed that substitution of small $Li^+$ ion at alkali site assists intercalation of $Na^+$ and stabilized the structure. When $Li^+$ was substituted on P2/O3 composite, electrochemical performance indicated additional capacity other than TM redox was attributed to Li substitution at P2/O3 biphasic structure. Synergetic effect of Li substitution in layered structure and P2/O3 biphasic structure resulted high capacity beyond theoretical limit, stable cycle life, and superior rate capability.