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 (beta-Sn) was produced upon lithiation of SnS and SnO2 NCs. Remarkably, a cubic phase of diamond-type Sn (alpha-Sn) coexisting with beta-Sn was produced by lithiation for all NCs. As the cycle number increased, alpha-Sn became the dominant phase. First-principles calculations of the Li intercalation energy of alpha-Sn (Sn-8) and beta-Sn (Sn-4) indicate that Sn4Lix (x <= 3) is thermodynamically more stable than Sn8Lix (x <= 6) when both have the same composition. alpha-Sn maintains its crystalline form, while alpha-Sn becomes amorphous upon lithiation. Based on these results, we suggest that once alpha-Sn is produced, it can retain its crystallinity over the repeated cycles, contributing to the excellent cycling performance.