WITHDRAWN

Sodium-ion batteries (NIBs) offer a more sustainable and cost-effective alternative to lithium-ion batteries (LIBs), but challenges related to the formation process and the impact of electrolyte additives on the solid electrolyte interphase (SEI) remain underexplored. Moreover, current SEI diagnostic tools are often prohibitively expensive, limiting broader adoption. This work investigates operando, non-invasive acoustic techniques - combining passive acoustic emission (AE) and active ultrasonic testing (UT) - to monitor SEI formation in NaMn₀.₃₉Fe₀.₃₁Ni₀.₂₂Zn₀.₀₈O₂/hard carbon (HC) pouch cells using four electrolyte formulations: a baseline of 1 M NaPF₆ in 1:1 EC:DMC, and three with additives (5 wt% fluoroethylene carbonate (FEC), vinylene carbonate (VC), or both). Identical formation protocols were applied, with SEI evolution monitored via AE and UT, and correlated with a range of characterisation techniques including operando gas volume measurements, three-electrode dQ/dV analysis, and X-ray CT. Bespoke machine learning algorithms were developed to interpret acoustic signals. Results revealed incomplete SEI formation across all formulations, evidenced by ongoing gas evolution and high irreversible capacity loss after three formation cycles. The behaviour in formation was reflected in the long-term cycling results: FEC delivered the best performance, VC alone was less effective, and cells without additives performed worst. These findings highlight the importance of tailored additive selection when using NaPF₆ in NIBs. This work demonstrates that AE and UT provide a viable, low-cost solution for real-time SEI monitoring, offering mechanistic insight previously accessible only through more complex and costly techniques.