New chemistry of Sn-based negative electrodes for Na-ion batteries

Abstract

Lithium-ion batteries (LIBs) dominate the market due to their high power and energy density. However, concerns regarding the scarcity of raw materials have prompted the scientific community to explore alternatives. Sodium-ion batteries (NIBs) have emerged as a promising alternative, owing to their abundance and cost-effectiveness. Tin and tin-based materials are particularly attractive for NIBs, given their high specific capacity (847 mAh g^-1 for Na₁₅Sn₄), natural abundance, and low operating voltage. Despite advances in understanding the mechanisms and developing advanced materials for next-generation NIBs, challenges such as poor electronic conductivity, significant volume expansion, and low initial coulombic efficiency (ICE) persist.

This work aims to develop a comprehensive understanding of tin’s electrochemical properties through a comparative study of in-house synthesised tin and commercial micron-sized tin. Tin particles, ranging from ≈50 nm to ≈1 µm, were synthesised via a chemical reduction method and examined under two electrolytes: 1M NaPF₆ in EC:DEC and diglyme, given the known degradation of carbonates in the presence of tin metal. Ex-situ SEM and EDS analyses were employed to assess the impact of these electrolytes on cycling performance, revealing key issues of significant volume expansion (up to 430%) due to NaSn to Na₁₅Sn₄ alloy formation and incompatibility with carbonate-based electrolytes.

To address these issues, the introduction of buffer matrices such as Na₄P₂O₇ or Na₃(PO₄) was explored. It was found that the in-situ carbon-composite SnP₂O₇ delivered the best electrochemical performance, achieving a reversible capacity of 300 mAh g^-1 over 200 cycles, Additionally, the poor ICE was mitigated by using 1M Na-biphenyl, which improved the ICE to ≈100%.

Further investigation into NaSn₂(PO₄)₃, a NASICON-type material with high Na-ion conductivity, demonstrate that amorphous NaSn₂(PO₄)₃ materials effectively accommodated volume expansion and exhibited enhanced cycle life compared to their pristine and carbon-coated counterparts, underscoring its potential as a viable electrode material for NIBs.

Date of Award	3 Jul 2025
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Robert Armstrong (Supervisor) & John Thomas Sirr Irvine (Supervisor)