The Critical Size of Sn Nanograins for Achieving High Round-Trip Efficiency of Reversible Conversion Reaction in Lithiated SnO2 Nanocrystals
Authors: Hu Ren Zong , Zhu Min 
Published: February 2020

SnO2 has been considered one of the most promising anode materials for Li-ion batteries. However, the realization of the full capacity of SnO2 anode has been hindered mainly by the inferior reversibility of conversion (Sn+Li2O→SnO2+Li+), and especially the large initial capacity loss characterized by low initial Coulombic efficiency (ICE). In the past two decades, although many works have focused on enhancing the cycle performance of SnO2 based anodes, the achieved ICEs were still very low (<60%). Few works were devoted to understanding the fundamental reason and explore a strategy to solve this problem.
Recently, we have revealed that the coarsening of Sn in the Sn/Li2O mixture, mainly induced by recrystallization, is responsible for the limited conversion of Sn/Li2O back into SnO2 during Li extraction, and achieved highly reversible conversion between Li2O and SnO2 with ICEs >86.2%, by suppressing the Sn coarsening in Li2O matrix in a sputtered SnO2 film[1]. However, it was found that the reversibility declined as the cycles increased due mainly to the gradually Sn coarsening, which was induced by the cyclic electrochemical stress (σ) imposed on the electrode[2]. Furthermore, combining nanosized transition metal with SnO2 can help to reduce the surface/interface energy of Sn nanograins and dramatically suppress the Sn coarsening, which however also affect the Coulombic efficiency and energy efficiency of the Li-SnO2 cells[3-4]. Accordingly, uncovering the critical size of Sn nanograins in lithiated SnO2 nanocrystals is of great important for enabling high round-trip efficiency of reversible conversion reaction, ensuring large stable capacity in the electrode.
In this work, we report our finding that unlike the previous reports, even a pure SnO2 film consisting of nanocrystals with size around 15 nm, stable high capacity above 900mAh/g can be remained throughout 100cycles. The capacity decay in the initial dozens of cycles of SnO2 anode is mainly induced by the gradually degradation of reversible conversion reaction (Sn+Li2O←→SnO2) due to the thermal and tress driven Sn coarsening along cycles. Furthermore, as shown in Figure 1, the coarsening of Sn have reduced the reversible capacity, Coulombic efficiency, energy efficiency and Li+ ion diffusion kinetics of the SnO2 nanostructured film. The grain size of coarsening Sn (x) and the degree of irreversibility (y) monotonically increase along cycles, which is quantitatively expressed with a linear equation (y=0.0236x-0.266). It is determined that the Sn nanograins with diameters less than 11.3 nm will ensure fast interdiffusion kinetics among interfaces of Sn/Li2O for completely reversible conversion reactions, enabling full application of the high capacity of lithiated SnO2[5]. We firmly believe that these results will provide a valuable insight into designing new conversion-type electrode materials with high stable capacity for next generation rechargeable batteries.

Figure 1 (a) Degree of reversibility vs. cycle number for the SnO2 film electrodes cycled among potential ranges of 0.01–3.00 and 0.01–2.00 V, respectively. (b) Average grain size and degree of irreversibility vs. cycle number. (c) Relationship between degree of irreversibility and grain size of Sn. (d) Schematic of the partially reversible conversion reaction with different grain sizes of Sn.
References:
[1] R. Hu, D. Chen, G. Waller, Y. Ouyang, M. Zhu, M. Liu. Dramatically enhanced reversibility of Li2O in SnO2-based electrodes: the effect of nanostructure on reversible capacity Energy Environ. Sci., 2016, 9: 595.
[2] R. Hu, Y. Ouyang, D. Chen, H. Wang, M. Zhu, M. Liu. Inhibiting Sn coarsening and enhance the reversibility of conversion reaction in lithiated SnO2 anodes by application of super-elastic NiTi films Acta Mater., 2016, 109:248
[3] R. Hu, Y. Ouyang, T. Liang, H. Wang, J. Chen, M. Zhu. Stabilizing nanostrucutre of SnO2 anodes by transition metals: a route to achieve high initial Coulombic efficiency and stable capacities for lithium storage Adv. Mater., 2017, 29: 1605006
[4] R. Hu, Y. Ouyang, T. Liang, X. Tang, B. Yuan, J. Liu, L. Zhang, L. Yang, M. Zhu, Inhibiting grain coarsening and inducing oxygen vacancy: the roles of Mn in achieving highly reversible conversion reaction and long life SnO2-Mn-graphite ternary anode, Energy Environ.l Sci., 2017, 10:2017.
[5]R. Hu, H. Zhang, Z. Lu, J. Liu, M. Zeng, L. Yang, B. Yuan, M. Zhu*, Unveiling critical size of coarsened Sn nanograins for achieving high round-trip efficiency of reversible conversion reaction in lithiated SnO2 nanocrystals, Nano Energy, 2018, 45:255.

Figure 1