Application of the Hybrid-Ion Battery Concept to Selected Oxide Systems

Abstract

The success of Li-ion batteries led to a steady growth of applications that rely on this energy storage technology. However, the non-abundancy of necessary resources requires the establishment of successor technologies such as Na- or Mg-ion batteries. Both are still missing a breakthrough due to a variety of issues. Recently, the concept of hybrid batteries, which is based on a combined operation of two kinds of cations, has been proposed, bridging the gap between lithium and post-lithium energy storage [1]. Its main idea is to restrict chemical processes of one cation type to only one electrode while the other cation type takes part in processes on the other electrode. This results in the replacement or significant reduction of lithium amounts in a battery. Focusing on Li+, Na+ and Mg2+ ions as charge carriers, three types of hybrid batteries can be considered as Mg-Li, Mg-Na and Na-Li batteries. Currently, the main challenges are the search of a suitable electrolyte system compatible with both anodes and cathodes, as well as obtaining acceptable long-term cycling behavior, which is influenced strongly by many factors such as different SEI compositions. We studied different types of hybrid batteries with non-aqueous dual salt electrolytes using well-known electrode materials like Li4Ti5O12, TiNb2O7, LiFePO4 and LiV3O8 with the aim to evaluate issues concerning individual materials as well as general correlations referring to the certain type of a hybrid system. For example, we investigated LiV3O8 cathodes in mixed Na-Li electrolytes and found structural changes and the electrochemical behavior to be strongly influenced by the Li/Na ratio (Fig. 1). By obtaining higher capacities than in pure Na-ion batteries and better cycling behavior than in Li-ion batteries, we demonstrated that this concept represents a good compromise between these battery types. However, observed lithium plating on sodium anodes seems to be a general weakness of the hybrid Na-Li battery for the given electrolyte compositions, which has to be overcome [2]. Mg-Li batteries were studied using TiNb2O7 and Li4Ti5O12 operating at intermediate potentials that yielded promising results (Fig.2). A careful examination of structural evolution and the composition in a subsurface region revealed Li+ selectivity for bulk intercalation while Mg2+ is restricted to the anodic reaction. When aiming at higher voltages, electrolytes capable of depositing and stripping Mg become either unstable or reactive towards non-inert current collectors, but the applicability of LiFePO4 and LiV3O8 in dual salt electrolytes using special setups could be demonstrated. For the Mg-Na combination, we studied layered sodium oxides in a broad voltage range, and showed capability of desodiation of the layered structures and anodical deposition of Mg. However, the reverse reaction occurs at significantly lower discharge potentials with a large polarization. Figure 1