(Invited) Direct Fabrication of Cathode Layer Onto Oxide-Based Solid Electrolyte with Good Interface

Abstract

All-solid-state lithium batteries with oxide-based solid electrolytes are expected as one of the next generation batteries with larger energy density and higher safety than conventional lithium-ion batteries. Among the oxide-based solid electrolytes, Li7La3Zr2O12 (LLZO) is one of the leading candidates for fabrication of the all-solid-state lithium batteries, because of high ionic conductivity of ~10−4 S cm−1 and high stability against lithium metal.[1] However, it is difficult to form an interface between the oxide-based solid electrolytes and cathode particles. In order to apply the all-solid-state lithium batteries, simple techniques for the interface formation are strongly required. We focus on two approaches. One of them is aerosol deposition (AD) method.[2] Cathode layer can be directly deposited onto the oxide-based solid electrolytes by AD method. We deposited a LiCoO2-Li3BO3 composite particles onto an Al-doped LLZO (Li6.25Al0.25La3Zr2O12, Al-LLZO) pellet. Li3BO3 is plastic material and it plays a role of binder for formation of layer. We succeeded the formation of LiCoO2-Li3BO3 layer on Al-LLZO. A test cell with LiCoO2-Li3BO3 / Al-LLZO / Li construction worked as a secondary battery. In addition, interestingly, better battery performances was obtained by annealing of the cell consisting of Al-LLZO pellet and LiCoO2-Li3BO3 cathode layer. Charge and discharge curves are shown in Figure 1a, which was measured at 0.1 C at 60 °C. The initial cycle showed great charge and discharge performance. Although the capacity gradually decreased during 100 cycles. Another technique is a quasi-solid cathode. It was prepared by mixing a conventional cathode slurry and a lithium ion conductive liquid. The slurry was directly painted onto Al-LLZO pellet. After drying, an interfacial resistance was significantly decreased. Namely, the interface between Al-LLZO and cathode layer was formed by addition of the lithium ion conductive liquid. Since the amount of lithium ion conductive liquid was small relative to the amount of cathode particles, the quasi-solid cathode did not have fluidity. We tested several kinds of lithium ion conductive liquids, i.e. conventional liquid electrolytes, solvated ionic liquids and ionic liquids with lithium salts. As a result, ionic liquid was the best additive for the quasi-solid cathode. Furthermore, we assembled a test cell consisting of LiCoO2-based quasi-solid cathode / Al-LLZO / Li. The cell worked for 100 cycles at 0.1 C under 60 °C with good cycling performance, as shown in Figure 1b. Development of simple techniques for the interface formation is very important to fabricate all solid state batteries. Both methods proposed in this study are not so difficult. It can be said that they are promising techniques to establish the practical application of all-solid-state lithium batteries. [1] R. Murugan, V. Thangadurai, W. Weppner, Angew. Chem. Int. Ed., 2007, 46, 7778–7781. [2] S. Iwasaki, T. Hamanaka, T. Yamakawa, W. C. West, K. Yamamoto, M. Motoyama, T. Hirayama, Y. Iriyama, J. Power Sources, 2014, 272, 1086–1090. Figure 1