Unveiling critical size of coarsened Sn nanograins for achieving high round-trip efficiency of reversible conversion reaction in lithiated SnO2 nanocrystals

A pure SnO2 film consisting of SnO2 nanocrystals with a size of similar to 15 nm can deliver a stable and high capacity > 900 mAhg(-1) for 100 cycles. Unlike the previous perception, the capacity decay in the initial cycles of the SnO2 anode is mainly induced by the gradual degradation of the reversible conversion reaction (Sn + Li2O <--> SnO2) at a potential> 1.0 V due to Sn coarsening with cycling. The coarsening of Sn has a significant impact on the reversible capacity, Coulombic efficiency, energy efficiency and Li+ ion diffusion kinetics of the SnO2 electrode. The grain size of coarsening Sn and the degree of irreversibility monotonically increase with cycling, which is quantitatively expressed with a linear equation. It is extrapolated that if the Sn grains remain with diameters < 11 nm, the fast interdiffusion kinetics among the interfaces of Sn/Li2O will enable complete reversible conversion reactions in lithiated SnO2 electrodes. Since the stability of nanostructured interfaces in metal (M)/LinX (X = O, F, S) compounds is also crucial for the reversible conversion reactions in binary M-X compounds, we firmly believe that these results will provide valuable insight into the design of new conversion-type electrode materials with high and stable capacities for next-generation rechargeable batteries.