Electrochemical Alkali-Ion Exchange in Electrode Materials for Secondary Batteries

Room-temperature Na-ion batteries currently attract increasing interest in research and industry, and could have the potential to replace the widely applied Li-ion batteries [1]. However, the energy density of Na-ion batteries is inherently reduced when compared to their respective Li-analogues and extensive research on novel high capacity anode and cathode materials is carried out to compensate for these disadvantages of the sodium chemistry [2]. Electrochemical alkali-ion exchange techniques are powerful tools to synthesize novel active materials for secondary batteries or to study Li/Na/K analogous intercalation compounds [3]. Herein, we report comprehensive investigations on the electrochemically conducted exchange of Li-ions by Na-ions within the host lattice of layered LiCoO2. Linear sweep and cyclic voltammetry experiments are applied to analyze the Li-Na-substitution behavior and any impact on the electrochemical properties in sodium-ion batteries. The electrochemical measurements are complemented by in-operando X-ray diffraction (XRD) analysis and comprehensive ex-situ material characterization including scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and electron backscatter diffraction (EBSD). We discuss the mechanisms of Li-Na-substitution in LiCoO2 with respect to the different properties of Li and Na-ions and their interactions with the host lattice. The combination of electrochemical investigations and in-operando materials characterization reveals that the Li-Na substitution in a potential range of 2.0 - 4.0 V vs Na/Na+ leads to the formation of a Li-Na-mixed intercalation compound with the formal stoichiometry Li0.5Na0.4CoO2. The different ionic radii of Li and Na-ions and the corresponding CoO2-layer distances forbid Li-Na mixing in the lattice, resulting in the emergence of Na- and Li-rich domains during Na-ion insertion. Charging and discharging of the Li-Na-mixed intercalation compound is dominated by Na-insertion and extraction, while Li-ions remain in the lattice until it is completely depleted of Na-ions. These novel insights concerning competitive kinetics, thermodynamics and phase evolution behavior can be highly useful to understand similarities and differences of Na- and Li-insertion chemistries to develop materials for future Na-ion batteries based on the comprehensive scientific knowledge in Li-ion technology.

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