Electrolyte-driven interphase stabilization in high-voltage sodium-ion full cells

Abstract

Sodium-ion batteries with a high-voltage O3-type layered oxide cathode paired with a hard carbon anode can offer high energy density; however, significant interfacial instabilities driven by electrode/electrolyte reactions limit a broader industrial adoption. Localized high concentration electrolytes (LHCEs) are a rational choice as they promote salt decomposition over solvent, forming stable, inorganic-rich electrode–electrolyte interphases (EEIs). We present here a comparison of high-voltage (4.2 V) hard carbon | NaNi_1/3Fe_1/3Mn_1/3O₂ pouch cells in LHCEs and in a standard carbonate-based electrolyte by (i) examining the influence of diluent choice on the electrochemical performance of LHCEs and (ii) investigating how the electrolyte chemistry affects the composition and structure of EEIs formed. Importantly, LHCEs demonstrate superior electrochemical performance, achieving 37 % higher capacity after 200 cycles (119 vs. 87 mA h g⁻¹) compared to the carbonate-based electrolyte. The enhanced stabilization provided by LHCEs at the interface with high-voltage sodium layered oxide cathode is revealed by gas evolution measurements obtained through online electrochemical mass spectrometry (OEMS). Time-of-flight secondary ion mass spectrometry paired with focused ion beam and advanced statistical analyses reveal that the superior performance of LHCE stems from a robust, thin cathode electrolyte interphase formed on the sodium layered oxide cathode and a homogeneous solid electrolyte interphase formation on the hard carbon anode. This study highlights the critical importance of electrolyte design in interphase stabilization, which plays a key role in advancing sodium-ion batteries toward commercial viability.