Toward Safe and Sustainable Batteries: Na₄Fe₃(PO₄)₂P₂O₇ as a Low-Cost Cathode for Rechargeable Aqueous Na-Ion Batteries

The electrochemical properties of Na₄Fe₃(PO₄)₂P₂O₇ in aqueous and organic electrolyte are compared under similar conditions. Na₄Fe₃(PO₄)₂P₂O₇ is able to deliver almost the same capacity in both types of electrolytes despite the smaller electrochemical window entailed by the aqueous one. As shown by electrochemical impedance spectroscopy (EIS), this is possible thanks to the lower overpotential that this material exhibits in aqueous electrolyte. It is shown here that the main contribution to overpotential in organic electrolyte mainly originates from a SPI (Solid Permeable Interphase) layer formed below 3.5 V vs Na⁺/Na that acts as a blocking layer and hinders Na⁺ diffusion and that is absent in aqueous electrolyte. Overall, the obtained results highlight the positive attributes of using low-cost and environmentally friendly aqueous electrolytes and the challenges to be overcome in terms of air and moisture stability of the studied material

Composition and Evolution of the Solid-Electrolyte Interphase in Na₂Ti₃O₇ Electrodes for Na-Ion Batteries: XPS and Auger Parameter Analysis

Na₂Ti₃O₇ is considered a promising negative electrode for Na-ion batteries; however, poor capacity retention has been reported and the stability of the solid-electrolyte interphase (SEI) could be one of the main actors of this underperformance. The composition and evolution of the SEI in Na₂Ti₃O₇ electrodes is hereby studied by means of X-ray photoelectron spectroscopy (XPS). To overcome typical XPS limitations in the photoelectron energy assignments, the analysis of the Auger parameter is here proposed for the first time in battery materials characterization. We have found that the electrode/electrolyte interface formed upon discharge, mostly composed by carbonates and semicarbonates (Na₂CO₃, NaCO₃R), fluorides (NaF), chlorides (NaCl) and poly­(ethylene oxide)­s, is unstable upon electrochemical cycling. Additionally, solid state nuclear magnetic resonance (NMR) studies prove the reaction of the polyvinylidene difluoride (PVdF) binder with sodium. The powerful approach used in this work, namely Auger parameter study, enables us to correctly determine the composition of the electrode surface layer without any interference from surface charging or absolute binding energy calibration effects. As a result, the suitability for Na-ion batteries of binders and electrolytes widely used for Li-ion batteries is questioned here

Important Impact of the Slurry Mixing Speed on Water-Processed Li₄Ti₅O₁₂ Lithium-Ion Anodes in the Presence of H₃PO₄ as the Processing Additive

The aqueous processing of lithium transition metal oxides into battery electrodes is attracting a lot of attention as it would allow for avoiding the use of harmful N-methyl-2-pyrrolidone (NMP) from the cell fabrication process and, thus, render it more sustainable. The addition of slurry additives, for instance phosphoric acid (PA), has been proven to be highly effective for overcoming the corresponding challenges such as aluminum current collector corrosion and stabilization of the active material particle. Herein, a comprehensive investigation of the effect of the ball-milling speed on the effectiveness of PA as a slurry additive is reported using Li4Ti5O12 (LTO) as an exemplary lithium transition metal oxide. Interestingly, at elevated ball-milling speeds, rod-shaped lithium phosphate particles are formed, which remain absent at lower ball-milling speeds. A detailed surface characterization by means of SEM, EDX, HRTEM, STEM-EDX, XPS, and EIS revealed that in the latter case, a thin protective phosphate layer is formed on the LTO particles, leading to an improved electrochemical performance. As a result, the corresponding lithium-ion cells comprising LTO anodes and LiNi0.5Mn0.3Co0.2O2 (NMC532) cathodes reveal greater long-term cycling stability and higher capacity retention after more than 800 cycles. This superior performance originates from the less resistive electrode–electrolyte interphase evolving upon cycling, owing to the interface-stabilizing effect of the lithium phosphate coating formed during electrode preparation. The results highlight the importance of commonly neglectedfrequently not even reportedelectrode preparation parameters