Understanding the Lithiation of the Sn Anode for High-Performance Li-Ion Batteries with Exploration of Novel Li-Sn Compounds at Ambient and Moderately High Pressure.

Abstract

Volume expansion and elastic softening of the Sn anode on lithiation result in mechanical degradation and pulverization of Sn, affecting the overall performance of Li-Sn batteries. It can, however, be overcome with the help of void space engineering by using a LixSn phase as the prelithiated anode, where an optimal value for x is desired. Currently, Li4.25Sn is known as the most lithiated Li-Sn compound, but recent studies have shown that at high pressure, several exotic and unusual stoichiometries can be obtained that may even survive decompression from high-to-ambient pressure with improved mechanical properties. With a belief that hydrostatic pressure may help in realizing Li-richer (x > 4.25) Li-Sn compounds as well, we performed extensive calculations using an evolutionary algorithm and density functional theory to explore all stable and low-energy metastable Li-Sn compositions at pressures ranging from 1 atm to 20 GPa. This not only helped us in enriching the chemistry of a Li-Sn system, in general, but also in improving our understanding of the reaction mechanism in Li-Sn batteries, in particular, and guiding a route to improve the performance of Li-ion batteries through synthesis of Li-rich phases. Besides the experimentally known Li-Sn compounds, our study reveals the existence of five unreported stoichiometries (Li8Sn3, Li3Sn1, Li4Sn1, Li5Sn1, and Li7Sn1) and their associated crystal structures at ambient and high pressure. Although Li8Sn3 has been identified as one of the most stable Li-Sn compound in the entire pressure range (1 atm-20 GPa) with R3̅m symmetry, the Li-rich compounds like Li3Sn1-P2/m, Li4Sn1-R3̅m, Li5Sn1-C2/m, and Li7Sn1-C2/m are predicted to be metastable at ambient pressure and found to get thermodynamically stable at high pressure. Here, the discovery of Li5Sn1 and Li7Sn1 opens up the possibility to integrate them as a prelithiated anode for efficiently preventing electrochemical pulverization, as compared to the experimentally known highest lithiated compound, Li17Sn4.