Post-Cycling Surface Analysis of Li 2 s-FeS Composite Positive Electrode

High energy density batteries have been required for the next generation EV applications, triggering the search for sulfur-based positive electrode materials with high capacity. Recently, we have developed the Fe-containing Li 2 S-based materials (Li 8 FeS 5 ), and found that Li 8 FeS 5 cell showed relatively high initial discharge capacity (> 700 mAh g -1 ) [1]. However, a rapid capacity degradation was observed during the initial several cycles. To address this issue, we have focused on improving cycle performance and it was revealed that a surface coating by Titanium oxide was effective for suppressing the capacity degradation of Li 8 FeS 5 positive electrode [2]. In particular, a capacity decay related to the sulfur redox were suppressed. This indicates that the surface reaction on the Li 8 FeS 5 particles is one of the causes of capacity degradation and the surface reaction might be related to sulfur redox. In the present work, we analyzed the surface of Li 8 FeS 5 positive electrode after cycle test to clarify the effect of coating on the cycle stability. The Li 8 FeS 5 was prepared based on the previously reported method [1]. Because of the sensitivity of Li 2 S to moisture, all the material synthesis procedures were carried out in an argon atmosphere. The obtained Li 8 FeS 5 and commercially available titanium tetrachloride (TiCl 4 ) were mixed in solvent and filtered after stirring, and washed by dimethyl carbonate, according to a previously reported procedure [3]. Then this intermediate product was heated at 400 ºC to obtain the sample (hereafter noted as Li 8 FeS 5 -Ti). The electrochemical charge/discharge tests were carried out with 1 mol dm -3 lithium hexafluorophosphate in 10:90 (v/v) ethylene carbonate and propylene carbonate electrolyte at a current density of 98.6 mA g −1 (corresponding to 0.1 C) and between 2.6 and 1.0 V vs. Li/Li + . The surface of positive electrodes were characterized by time-of-flight secondary ion mass spectrometry (TOF-SIMS) with Bi as the primary ion source. The cycle performance of discharge capacity related to sulfur redox were shown in Fig.1. The Li 8 FeS 5 without coating exhibited a gradually capacity decay on the whole range of cycle test. On the other hand, Li 8 FeS 5 -Ti showed an almost constant discharge capacity after rapid decreasing at initial charge-discharge. The TOF-SIMS measurement was carried out after the first charge, the first discharge and after 10 cycles to determine the cause of different c