Addressing Bottlenecks to Achieve High-Energy Sodium-Ion Cells Using Tin Anodes or Anode-Free

Sodium-ion batteries (NIBs), as a complementary energy storage device for Li-ion batteries, are swiftly making their way into high-power applications market. However, further progress in NIBs requires high energy density. This necessitates shifting from the commonly used hard carbon (HC) anodes to alloy anodes such as Bi/Sn/Sb, etc., while overcoming the problems these materials pose with regard to volume changes and interfacial reactivity. This article focuses on issues related to the chemistry of Na3V2(PO4)2F3 (NVPF)|Sn-HC based cells using 1M NaPF6 in diglyme as electrolyte and proposes solutions. Through operando, ex-situ, and post-mortem (electro)chemical characterizations, we demonstrate that glyme electrolytes remain stable under reduction. However, their low oxidative stability gives rise to parasitic species that interfere with and poison the Sn-HC electrode. This results in poor cycling stability, preventing their use in Naion cells. To mitigate this problem, we propose various solutions, such as (i) using a low-voltage cathode, (ii) coating Sn with a protective layer, and (iii) introducing chemical traps such as Nametal, Na15Sn4, or NaxC between the separators. Of all these solutions, the traps are the most effective, completely suppressing cross-talk poisoning, enabling 100% capacity retention of NVPF|Sn-HC cells over 200 cycles at C/5. Furthermore, we demonstrate that this concept can be extended to anode-free configurations, achieving 91% capacity retention after 80 cycles at C/10. This presents a practical design principle for Na-ion cells with energy densities approaching those of Li-ion systems, and can be adapted to other chemistries susceptible to cross-talk poisoning.