Understanding the electrode – electrolyte interphase of high voltage positive electrode Na4Co3(PO4)2P2O7 for rechargeable sodium-ion batteries

Highlights: • Electrode – electrolyte interphase study of high voltage Na₄Co₃(PO₄)₂P₂O₇ electrode. • Interphase study with conventional and high energy X-ray photoelectron spectroscopy. • Bilayer composition of interphase at full Na⁺ extracted state. • The interphase is composed by additional inorganic layer at Na⁺inserted state. -- Abstract: Sodium-ion batteries (SIBs) have been postulated as a potential solution for large-scale stationary applications and light electromobility. Among positive electrode materials for SIBs, Na₄Co₃(PO₄)₂P₂O₇ attracted significant attention due to its high voltage and good specific capacity even at very high current densities. However, details of the formed electrode – electrolyte interphase (EEI) are still uncertain, being this of extreme importance considering that the high operating voltage of this electrode material is around the stability edge of most of the conventional electrolytes which, in some cases, already display side reactions above 3.0 V vs. Na/Na⁺. In this work, the EEI of Na₄Co₃(PO₄)₂P₂O₇ is analyzed in half-cell configuration using 1 M NaPF₆ in EC:DEC as electrolyte. Conventional and high energy X-ray photoelectron spectroscopy (XPS) has been used so as to understand the stability and the chemical composition of the EEI. The results reveal that a bilayer EEI is formed at full Na⁺ extracted state of charge (SOC - 4.7 V vs. Na/Na⁺), with semi-organic-rich species found in the subsurface region close to the electrode while more organic species are formed in the outermost surface region close to the electrolyte. Meanwhile, after full Na⁺ insertion (SOC - 3.0 V vs. Na/Na⁺) an additional outermost inorganic overlayer is formed which is composed of sodium carbonate and sodium fluorophosphate. Additionally, this inorganic-rich EEI is dissolving upon oxidation / charge process - affecting the outermost ~10 nm of the EEI. Despite this dynamic behavior of the EEI, the Na₄Co₃(PO₄)₂P₂O₇ positive electrode delivers excellent cyclability (94% capacity retention after 100 cycles at 0.2C), proving that it can be a good candidate as positive electrode material for SIBs.