Development of All-Solid-State Lithium Battery Based on Quasi-Solidified Room Temperature Ionic Liquid-Nanoparticles-Composite As Hybrid Electrolytes (II) –Application of Quasi-Solidified Electrolytes to Bipolar-Type All-Solid-State Lithium Battery–

Bipolar-type all-solid-state lithium batteries are expected to maintain higher energy density than that of the conventional stacked lithium-ion battery using organic liquid electrolytes. The main problem of the conventional stacked lithium-ion battery is the leakage of organic liquid electrolytes. Bipolar-type all-solid-state battery is composed of solid-state electrolytes as shown in Figure 1, it is possible to supply a high voltage, avoiding the aforementioned weak point of the conventional stacked lithium-ion battery. Presently, the cases of solid-state electrolytes having sufficient lithium ion conductivity and high stability in the voltage range for battery operation are limited. We reported the fabrication and characterization of quasi-solidified hybrid electrolytes composed of room temperature ionic liquids (RTILs) and 7 nm fumed-silica in our previous study. On the basis of our previous works, the quasi-solidified hybrid electrolytes can be treated as solid-state electrolytes, and it possess liquid-like Li-ion transport properties with high ionic conductivity and high stability. Therefore, the quasi-solid-state hybrid electrolyte is considered as a potential candidate for bipolar-type all-solid-state lithium-ion batteries. In this study, we assembled the bipolar-type all-solid-state Li battery using a quasi-solid-state hybrid electrolyte and the battery performance was investigated.

Lithium bis(trifluoromethaesulfonyl)amide (LiTFSA) was dissolved into tetraethylene glycol dimethyl ether (G4) in equimolar ratio to prepare RTIL-like complex solution. The quasi-solid-state electrolyte (QSE) powder was fabricated according to our previously reported method [1]. In this study, we prepared the QSE powder containing 80 vol% of G4/LiTFSA liquid, and a transparent quasi-solid-state hybrid electrolyte sheet was obtained by mixing polytetrafluoroethylene (PTFE) powder with the QSE powder in 5 wt%. The cathode was made by mixing LiFePO₄ (theoretical capacity of 170 mAh g^-1), AB, QSE powder and PTFE, with a weight ratio of 35:10:45:10. The composite cathode, the QSE sheet and Li metal anode was assembled as one-unit cell. Two and three unit-cells, connected in series, were prepared. The charge-discharge measurements of the one-, two- and three-unit bipolar all-solid-state cells were performed at 35 ^oC and a rate of 0.1C.

The quasi-solid-state hybrid electrolyte was a white powder and the quasi-solid-state electrolyte sheet of 200 mm thickness was prepared [1] and used to assemble all-solid-state cells. The bipolar-type all-solid-state lithium batteries with different stack structures of one-, two- and three-unit cells connected in series in one package were assembled and the battery performances were investigated. The discharge capacity of the one-unit cell reached 161 mAh g^-1 at the initial cycle with coulombic efficiency of 99 %. For such one-unit cell, discharge capacity was still maintained at 160 mAh g^-1 even after 50 cycles, and the capacity retention was kept 99 %. The discharge capacities of the batteries with two- and three-unit cells were 160 mAh g^-1 and 165 mAh g^-1, respectively, which were as high as that of one-unit cell. These results implied the cathode utilization ratios of all the bipolar batteries up to 94 %. The coulombic efficiencies for two- and three-unit cells reached 98 % and 99 %, respectively. The discharge plateaus of two- and three-unit cells were observed in the voltage range of 6.7-6.8 V and 10.0-10.2 V, which were twice and three times to the one-unit cell voltage. In summary, these results suggest that our quasi-solid-state electrolytes were successfully applied to the bipolar-type all-solid-state lithium battery.

Reference

 [1] A. Unemoto, T. Matsuo, H. Ogawa, Y. Gambe and I. Honma, J. Power Sources, 244, 354 (2013).