Post-Cycling Surface Analysis of Li2s-FeS Composite Positive Electrode
Authors: Yamauchi Erina , Takeuchi Tomonari , Sakaebe Hikari 
Published: February 2020

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 Li2S-based materials (Li8FeS5), and found that Li8FeS5 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 Li8FeS5 positive electrode [2]. In particular, a capacity decay related to the sulfur redox were suppressed. This indicates that the surface reaction on the Li8FeS5 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 Li8FeS5 positive electrode after cycle test to clarify the effect of coating on the cycle stability. The Li8FeS5 was prepared based on the previously reported method [1]. Because of the sensitivity of Li2S to moisture, all the material synthesis procedures were carried out in an argon atmosphere. The obtained Li8FeS5 and commercially available titanium tetrachloride (TiCl4) 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 Li8FeS5-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 Li8FeS5 without coating exhibited a gradually capacity decay on the whole range of cycle test. On the other hand, Li8FeS5-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 cycle performance. The Secondary ion images of Li8FeS5 electrodes surface were shown in Fig.2. The formation of Li2O particle was observed after the first charge at electrode surface without coating. Furthermore, the particle size increased after the first discharge, until eventually the electrode surface was fully covered by Li2O. Even though the Li2O formation was observed at Li8FeS5-Ti electrode surface after first charge, the particle size was significantly smaller than uncoated samples. In addition, the particle size didn’t increase after 10 cycles. It seemed that the formation of Li2O at the surface of positive electrode might be one of the causes of rapid capacity decay which was observed on uncoated electrode. Also it was revealed that the sulfite compounds formed uniformly only at the Li8FeS5-Ti electrode surface after the first charge and discharge. However the surface concentration of sulfite compound decreased after 10 cycles, and then the degradation products from LiPF6 covered the whole area of electrode surface. This could indicate that certain side reactions of the surface sulfur layer with the electrolyte occurred at first cycle. However, this undesirable reaction was suppressed in the subsequent cycles and therefore, the electrode surface could be stabilized. We have carried out an in-depth analysis of the electrode surface, and the results are presented in this conference. The work was financially supported by RISING2 Project of NEDO. References: [1] T. Takeuchi, et al., J. Electrochem. Soc. 162(9), A1745 (2015). [2] E. Yamauchi, et al., Abstract of the 19th International Meeting on Lithium Batteries. [3] Z. W. Seh, et al., Nature Communication, 5, 5017 (2014), DOI:10. 1038/ncomms6017

Figure 1