Application of Natural Polymer Binder for Nonaqueous Slurry to FeF3-Vox-Based Lib’s Positive
Authors: Soeda Kazunari , Matsui Keitaro , Shikano Masahiro , Sakaebe Hikari , Ishikawa Masashi 
Published: May 2020

Iron fluoride (FeF3) for LIB is expected as a positive electrode material that contributes to the realization of innovative secondary battery of 500Wh/kg class from the viewpoint of high theoretical capacity (712 mAh / g) and superiority in element strategy (not including rare earth). However, the entire reaction uses not only an intercalation reaction process that is common to conventional lithium ion batteries but also a conversion reaction process. Therefore, it is difficult for conventional binders (such as PVdF) to deal with volume changes associated with charge / discharge reactions that are characteristics of this material, to disperse conductive carbon, and to produce thick-coated electrodes. Previously we have reported a natural polymer as a binder suitable both for LIB positive and for negative electrodes in a nonaqueous slurry [1]. In this report, we examined the applicability of our binder to FeF3 cathode materials and vanadate glass (VOx), which is a candidate cathode material for innovative storage batteries. A slurry was prepared using a non-aqueous natural polymer binder (NP binder) with a positive electrode active material obtained by mechanical milling of iron fluoride and carbon conductive material. PVdF was used as an electrode binder for comparison. For the peel strength test, the 90 ° peeling method was performed in 15 mm of the tape adhesive surfaces . LiPF6 / EC + PC was mainly used as the electrolyte. A coin-type cell (CR2032) was fabricated with the prepared electrodes, separator and lithium foil. The characteristics were evaluated by various electrochemical measurements. The cells with vanadate glass and composite systems were also prepared and evaluated. All electrode fabrication steps were performed in a dry room with a dew point of −55 ° C or lower, and all cells were constructed in a glove box under an argon atmosphere with a dew point of −60 ° C or lower.

Figure 1 shows the charge-discharge curves of an electrode with FeF3/NP binder. The cell using NP binder did not deteriorate rapidly during the first 5 cycles, and stable charge / discharge characteristics were obtained. The cycle evaluation will be continued as the multi-cycle performance. In order to solve the rapid deterioration of the capacity of FeF3, combined use of it with another active material possessing excellent cycle performance has been proposed. Here, we focus on the selection of an appropriate binder for the VOx, another positive electrode active material, which is expected as a combination candidate. Figure 2 shows the cycle characteristics evaluation using NP binder suitable for the VOx system. The cycle performance and coulomb efficiency are quite stable compared with the FeF3-based cathode material. These results suggest that the new natural polymer binder can be applied to FeF3-VOx cathode. However, the capacity development is lower than the theoretical capacity of 500 mAh/g. Thus, optimization of the amount of binder and conductive additive would be necessary. In the poster session, we discuss the binder optimization conditions for these materials, and report the correlation between the peel strength and the characteristics of the composite cathode. This work was financially supported by RISING2 project by NEDO and METI.

[1] K. Soeda, M. Ishikawa, The 19th International Meeting on Lithium Batteries (IMLB2018), Kyoto, Japan, 17-22 June 2018.