All-Solid-State Lithium Secondary Battery Using Plastic Crystal As Solid Electrolyte

Introduction

All-solid-state lithium ion batteries (ASS-LIBs), which consist of nonflammable solid electrolyte, are expected as high capacity and safety secondary batteries for vehicle and industrial use.  Bulk-type ASS-LIBs, whose positive and negative electrodes are composed of an active material, a conductive agent, a binder and a solid electrolyte, are more desirable than film type electrode, in terms of capacity and energy density.  A main issue of the bulk-type ASS-LIBs is reduction of internal resistance of the electrodes, which originated from a poor contact between the electrolyte and the active materials to form the micro void. Plastic crystalline fast ion conductors have been studied as the solid-state electrolyte for ASS-LIBs [1, 2]. As these materials readily deform above relatively low melting point, for example 1-ethyl-1-methyl pyrolidinium bis(trifluoromethylsulfonyl imide (12PyrTFSI) melt at 90 °C, it will form a favorable active material-electrolyte solid-solid interface. In this study, to reduce the internal resistance of the electrode, lithium ion conductive plastic crystals which show thermoplasticity was used as solid electrolyte.

Experimental

In this research, a mixture of 12PyrTFSI and lihium sulfonyl imide (LiTFSI) was used as lithium conductive plastic crystal. We developed a negative electrodes consisting of lithium titanium oxide (LTO), acetylene black, polyvinylidene fluoride, the mixture of 12PyrTFSI, LiTFSI. The negative electrode was obtained by coating the dispersion slurry containing the above mentioned materials and N-methylpyrrolidone (NMP), and thereby drying NMP solvent. Drying temperature was selected more than 71 °C that is a melting point of the mixture of 12PyrTFSI and LiTFSI [3]. The negative electrodes were structurally characterized by SEM-EDX. The anode half cell was prepared by assembling the anode on a polyethylene oxide (PEO) polymer sheet and Li foil as reference counter electrode to measure the charge and discharge property of the negative electrodes. At 50 °C, the charge and discharge were operated in the galvanostatic mode at 5.57 μA¥cm^-2 (0.02C). The internal resistance of the negative electrode after charge was obtained using an impedance analyzer over a frequency range of 0.1 to 10⁶ Hz and amplitude of 10 mV at 50 °C. The cells were cycled between 1 to 1.35 V vs. Li/Li⁺.

Fig.1 illustrates the charge and discharge curves of the LTO negative electrode with a plastic crystal. The discharge capacity at the first cycle was 82 mA¥g^-1 at 0.02 C, which is only 47 % of theoretical capacity. The cross sectional SEM images showed that the plastic crystal was not uniformly distributed in the electrode.  Conceivably, the non-uniform distribution resulted in the formation of heterogeneous lithium ion conduction path over the negative electrode, deteriorating the discharge capacity. Fig. 2 shows Nyquist plots of the negative cell after first charging. The interfacial resistance between lithium metal and PEO polymer sheet and internal resistance of the negative electrode forms an arc. It shows the small resistance of the negative cell 55 Ωcm². Evidently, partial ion conduction path comprising of 12PyrTFSI and LiTFSI contributed to the low internal resistance of the negative cell. It was concluded that the formation of the homogeneous ion conduction path was needed to increase the capacity of the negative electrode.

References

[1] D. R. Macfarlane, J. H. Forsyth, M. Forsyth, Nature, 402, 792, (1999)

[2] J. M. Pringle, Phys. Chem. Chem. Phys., 15, 1339, (2013)

[3] W. A. Henderson, S. Passerini, Chem Mater., 16, 2882, (2004)