A Mechanically Compliant Tin-Hard Carbon Anode for Sodium Solid-State Batteries

Abstract Sodium solid-state batteries (NaSSBs) with alloy anodes can increase cell-level energy density, but they suffer from chemo-mechanical degradation due to volume changes during sodiation and desodiation. Here, we develop a Sn hard carbon composite anode (SnHC), in which HC serves as a mechanically compliant scaffold that buffers alloying-induced stress and helps maintain electrode integrity. Meanwhile, Sn and Na-Sn phases provide coupled ionic and electronic transport pathways for Na+ diffusion and electron percolation. Morphology and pressure monitoring show that SnHC exhibits a 77% reduction in volume change and a 46% reduction in pressure fluctuations compared with Sn upon sodiation. This stress-buffering effect reduces structural degradation during cycling. As a result, SnHC delivers a reversible capacity of 588 mAh g−1 and retains 80% of its capacity over 1000 cycles when paired with NaNi1/3Fe1/3Mn1/3O2. Techno-economic analysis indicates that SnHC can reduce the projected manufacturing cost of NaSSBs by approximately 21% relative to HC counterparts.