Preparation of Li2s-FeSx-C Composite Positive Electrode Materials and Their Electrochemical Properties with Pre-Cycling Treatment

1. Introduction

                  Lithium sulfide (Li₂S) is one of the promising cathode active materials for high-energy rechargeable lithium batteries, because it has a high theoretical capacity (ca. 1170 mAhg^-1) and has an advantage that a variety of anode materials without lithium sources are applicable in the practical battery system [1]. However, this material is both electronically and ionically resistive, which gives rise to low active material utilization in the cells [2]. Several attempts have been performed to enhance the conductivity of Li₂S, such as forming composites with several metals (Li₂S-Fe, Li₂S-Cu) [1,2] or carbon [3-5]. Recently, we have tried to prepare the composite with FeS₂ (Li₂S : FeS₂ = 1 : 1 molar ratio) using the spark-plasma-sintering (SPS) process and found that the cells showed the initial charge capacity of ca. 330 mAhg^-1 [6]. For improving the charge capacity, preparation of the composite with Li₂S-rich composition is required.

                  In the present work, we tried to prepare the Li₂S-FeS_x-C (x = 1, 2) composites with a Li₂S : FeS_x = 4 : 1 molar ratio using the combination processes of the SPS and the mechanical milling, and examined their electrochemical properties.

2. Experimental

                  Li₂S and FeS₂ (or FeS) with a molar ratio of 4 : 1 were mixed thoroughly, and the mixture was then treated by the SPS process [6] at 600^oC. The resulting pellet was ground and mixed with carbon (acetylene black) powder with a weight ratio of Li₂S-FeS_x : C = 9 : 1, and then it was mechanically milled for 8 h to yield the Li₂S-FeS_x-C composites. All the procedures were carried out in an argon atmosphere, because Li₂S is very sensitive to atmospheric moisture.

                  The obtained composite samples were characterized by XRD, as well as Fe and S K-edge XAFS measurements. Electrochemical lithium insertion / extraction reactions were carried out using lithium coin-type cells with 1M LiPF₆ / (EC + DMC) electrolyte at a current density of 46.7 mAg^-1 (corresponding to 0.04C) initially with charging. For improving the electrochemical performances, we applied pre-cycling treatment to the present Li₂S-FeS_x-C composite sample cells, whereby cycle capability of some oxide cathode active materials (such as Li(NiCoMn)O₂) was successfully improved [7].

3. Results and Discussion

The obtained Li₂S-FeS_x-C composite samples were black in color, and their XRD patterns showed that they consisted of Li₂S (antifluorite structure) with low crystallinity. The atomic ratio, estimated by ICP and atomic absorption spectroscopy, was Li : Fe : S = 7.8 : 1.0 : 4.8 for the Li₂S-FeS_x-C composite sample with Li₂S : FeS = 1 : 4. Since the initial composition was Li : Fe : S = 8 : 1 : 5, this result indicates that negligibly slight amounts of Li and S (ca. 3 – 4%) were lost during the SPS and the following milling processes.

                  The electrochemical tests for the Li₂S-FeS_x-C sample cells showed that the initial charge capacity was ca. 600 mAhg^-1, which was much higher than the previously reported Li₂S-FeS₂ (1 : 1 molar ratio) sample cells (ca. 330 mAhg^-1) [6]. The discharge capacity was ca. 330 mAhg^-1 with the normal electrochemical cycling, and it was enlarged to ca. 730 mAhg^-1 after the pre-cycling treatment, Fig. 1. This result indicates that the pre-cycling treatment was effective for improving the electrochemical performances of the Li₂S-FeS_x-C composite cells. Ex-situ XRD measurements showed that the peaks ascribed to Li₂S disappeared after the normal electrochemical cycling, while they were still observable after the pre-cycling treatment. Also, ex-situ S K-XAFS measurements showed the reversible changes of the valence states of S atom with the pre-cycling treatment. Therefore, the pre-cycling treatment of the Li₂S-FeS_x-C sample was effective for maintaining the antifluorite structure, as well as for improving the reversibility of the valence states of S atom, which would be responsible for improving the electrochemical performances of the Li₂S-FeS_x-C composite sample cells.

References

[1] A. Hayashi et al., J. Power Sources, 183, 422 (2008).

[2] M. N. Obrovac and J. R. Dahn, Electrochem. Solid-State Lett., 5, A70 (2002).

[3] T. Takeuchi et al., J. Power Sources, 195, 2928 (2010).

[4] J. Hassoun et al., J. Power Sources, 196, 343 (2011).

[5] M. Nagao et al., J. Mater. Chem., 22, 10015 (2012).

[6] T. Takeuchi et al., J. Electrochem. Soc., 159, A75 (2012).

[7] A. Ito et al., J. Power Sources, 183, 344 (2008).

Acknowledgment

                  This work was financially supported by R&D project for lithium batteries (RISING Project) by METI and NEDO.