The challenge for a sustainable energy supply is a suitable storage technology. Currently, the lithium-ion battery has the largest application as energy storage device. Due to increasing number of applications and subsequent scarcity of Li resources, the interest of research and industry in alternative storage systems is growing. Na-ion batteries (SIB) could be an alternative to the established Li-ion batteries . Nevertheless, the properties of the Na-ion (e. g. higher ionic radii) lead to lower energy density in comparison to Li and intense research on novel anode and cathode materials is implemented to offset the disadvantages of the Na-Ion battery .
Electrochemical alkali-ion exchange is a powerful tool to synthesize novel active materials for secondary batteries or to study Li/Na/K analogous intercalation compounds, hybrid materials and Na/K doped Li-materials [3,4]. Herein, we report extensive studies on the electrochemically Li-Na-substitution in the spinel structured LiMn2O4. Linear sweep, cyclic voltammetry and galvanostatic charge discharge experiments are applied to characterize the Li-Na-substitution behavior and the electrochemical performance as cathode material in sodium-ion batteries. With the support of ex-situ and operando X-ray diffraction (XRD) analysis, we analyzed the mechanism of Li-Na-substitution in LiMn2O4.
The combination of electrochemical investigations and materials characterization demonstrate that the Li-Na substitution in a potential range of 2.0 – 4.0 V vs Na/Na+ leads to a structural transformation. The spinel structure of NaMn2O4 is not stable and transforms into layered NaMnO2. This transformation requires several sodiation and desodiation cycles to change region by region from spinel to layered structure. Finally, NaMnO2 is formed with small Li spinel domains that stabilize the layered structure even for very low Na-content. Accordingly, this Li-spinel doped NaMnO2 shows high reversible capacity about 200 mAh g-1 (C/6), being a highly promising cathode material for Na-ion batteries. Moreover, the insights gained from electrochemical alkali-ion substitution studies can be used to identify differences and similarities of Na- and Li-intercalation chemistry and can thus form the basis for the development of new materials for advanced sodium-ion batteries.
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