Phase Equilibria and Lithiation of Industrially Produced Heterogeneous Tin Sulphide Anode Materials for Lithium Ion Batteries

Abstract

Due to its theoretical capacity of 960 mAh/g, which is approximately two and a half times that of carbon (372 mAh/g), as well as its natural abundance, Sn has been considered as an interesting anode active material to replace the currently used graphite in lithium ion batteries. Despite their high theoretical capacities, Sn-based anodes have not yet been commercialized because they suffer from prohibitively large volume expansions during lithiation via formation of LixSn alloys (>185 % for full lithiation to Li17Sn4). These volume changes lead to extensive crack formation, loss of electrical contact between particles, pulverization of the electrode during cycling, lithium trapping, and the growth of an unstable SEI. One way to take advantage of the high capacity of Sn and simultaneously overcome the detrimental effects associated with its volume expansion is to use Sn-based chalcogenide compounds as anode active materials. In this respect, tin sulfides such as SnS, Sn2S3 and SnS2 are an interesting class of materials due to the formation of a rigid, Li2S matrix phase upon discharge, which can serve to restrict the volume expansion of Sn during subsequent alloying reactions. During the conversion reaction, which can be written as 2b∙Li+SnaSb → b∙Li2S + a∙Sn and occurs at potentials between 1 and 1.6 V vs Li+/Li, a composite electrode structure consisting of Sn particles distributed in a rigid Li2S matrix is produced in-situ. Once pure Sn is formed, additional lithium can be stored via alloying reactions to produce Li17Sn4 encapsulated in the Li2S matrix. According to the mechanism, the capacities of the alloying reactions are 782, 706, and 644 mAh/g for SnS, Sn2S3 and SnS2, respectively. Heterogeneous (mixed-phase) tin sulfides can be readily produced industrially via the thermal reaction of tin and sulphur in closed vessels. However, additional processing steps may be required to produce phase pure materials on the industrial scale. Since these steps may be costly, thereby driving up the price of electrode active material production, it is worthwhile to consider heterogeneous tin sulfides as anode active materials. This is further rooted in the fact that unlike intercalation reactions, where a stable host structure is required for the reversible intercalation/extraction of lithium, the tin sulfide crystal structure is destroyed in the first conversion reaction and does not readily form again on subsequent cycling if the voltage cutoff is limited to below 2V vs Li+/Li. Therefore, in this work, industrially produced heterogeneous tin sulfides were investigated in order to assess their structure-property-performance relationships as anode active materials for lithium ion batteries (LiBs). Firstly, the heterogeneous tin sulfides were subjected to an extensive physio-chemical characterization to determine their chemical compositions (XRF), phase compositions (powder X-ray diffraction), specific surface areas (BET method), particle size distributions (laser diffraction), and morphologies (SEM). Following this, galvanostatic cycling under potential limitation (GCPL), cyclic-voltammetry (CV), and galvanostatic titration intermittent technique (GITT) experiments vs Li+/Li were conducted in order to correlate the electrochemical reaction mechanisms to the initial phase and chemical compositions of the heterogeneous tin sulfide electrode materials. In addition to the physico-chemical characterization and electrochemical investigations, a thermodynamic modelling and simulation approach was also selected to elucidate the complex reaction mechanisms of the heterogeneous tin sulfides. In this regard, calculated phase and property diagrams are essential tools, since they can be used to predict the compositional changes and phase transformations of the electrode materials during lithiation/de-lithiation. Therefore, a CALPHAD (CALculation of PHAse Diagrams)-based thermodynamic description for the multi-component Li-Sn-S system was developed using analytic Gibbs free energy expressions for the phases in the Li-Sn1, Sn-S2, and Li-S3 binary systems which are available in the literature. The ternary Li-Sn-S description was then used to simulate the phase formation and coulometric titration curves for the pure and heterogeneous tin sulphides. Additionally, ambient temperature phase diagrams were calculated, and all phase and property diagrams were used in conjunction with the electrochemical results to elucidate the electrochemical reaction mechanisms for the lithiation of the heterogeneous tin sulphides.