Phase Evolution of Tin Nanocrystals in Lithium Ion Batteries

Sn-based nanostructures have emerged as promising alternative materials for commercial lithium–graphite anodes in lithium ion batteries (LIBs). However, there is limited information on their phase evolution during the discharge/charge cycles. In the present work, we comparatively investigated how the phases of Sn, tin sulfide (SnS), and tin oxide (SnO₂) nanocrystals (NCs) changed during repeated lithiation/delithiation processes. All NCs were synthesized by a convenient gas-phase photolysis of tetramethyl tin. They showed excellent cycling performance with reversible capacities of 700 mAh/g for Sn, 880 mAh/g for SnS, and 540 mAh/g for SnO₂ after 70 cycles. Tetragonal-phase Sn (β-Sn) was produced upon lithiation of SnS and SnO₂ NCs. Remarkably, a cubic phase of diamond-type Sn (α-Sn) coexisting with β-Sn was produced by lithiation for all NCs. As the cycle number increased, α-Sn became the dominant phase. First-principles calculations of the Li intercalation energy of α-Sn (Sn₈) and β-Sn (Sn₄) indicate that Sn₄Li_<i>x</i> (<i>x</i> ≤ 3) is thermodynamically more stable than Sn₈Li_<i>x</i> (<i>x</i> ≤ 6) when both have the same composition. α-Sn maintains its crystalline form, while β-Sn becomes amorphous upon lithiation. Based on these results, we suggest that once α-Sn is produced, it can retain its crystallinity over the repeated cycles, contributing to the excellent cycling performance