Insights into the Function of Electrode and Electrolyte Materials in a Hybrid Lithium–Sodium Ion Cell

Hybrid Li–Na ion batteries (HLSIBs) have become attractive because they combine the high energy density of lithium-ion batteries with the safety and low cost of sodium-ion batteries. However, the performance of HLSIBs is still far from the desired. The present study aims to probe the function of electrodes and the electrolyte in model half and full hybrid Li–Na cells by combining experimental and computational methods. As a positive electrode, sodium-deficient nickel manganese oxide, Na2/3Ni1/2Mn1/2O2, with a three-layered structure is used, while spinel Li4Ti5O12 serves as a negative electrode. Two types of conventional LiPF6- and NaPF6-based electrolyte solutions are used. The structure and surface changes in the oxide electrodes after cell cycling are monitored by ex situ transmission electron microscopy and X-ray photoelectron spectroscopy analyses. The competitive solvation/desolvation of Li+ and Na+ by ethylene carbonate (EC) molecules is modeled as binuclear heterocomplexes, Li+Na+(EC)n (1 ≤ n ≤ 8), in the framework of density functional theory computations. The hybrid-ion cell operates by a dual intercalation of Li+ and Na+ in the oxide electrodes, which provokes after few cycles a generation of a mixed Li–Na electrolyte. In the mixed electrolyte, each Li+ and Na+ ion is solvated as much as possible without any competition between them. At the electrode–electrolyte interface, the competition between Li+ and Na+ ions leads to the formation of polynuclear cationic associates which are strongly coupled with electrolyte molecules. The desolvation of the latter is further complicated by the presence of surface-deposited F– ions, resulting in the accumulation of Li+ and Na+ ions at the surface of both the oxide electrodes. Based on the extracted correlations, an outlook of further steps for optimization of the HLSIBs performance is suggested.