Electrochemical Performance of a Large Size All-Solid-State Lithium-Ion Battery 2

An all-solid-state lithium-ion battery (SSB) with non-flammable solid electrolytes is one of the promissing candidates for next-generation high performance power source, because of its low safety concerns. Recently, sulfide based electrolytes are of considerable practical interest due to its high ionic conductivity in the room temperature range.^1,2 However, there are still some serious issues to overcome before the realization of a SSB for practical applications. Since the ionic and electric conduction passes are given by the contact of the solid particles, the cell impedance is much higher than those of conventional liquid electrolyte lithium-ion batteries. Consequently, most of the SSBs need an external pressure during the charging and discharging processes.³ Also, several papers have suggested the necessity of a thin buffer layer on the cathode surface to prevent the mutual diffusion of atoms between the metal oxide and sulfide electrolyte.^3,4 In order to create a SSB of practical size, we adopted Li₂O-ZrO₂ (LZO) thin layer coating on the cathode material for reducing the solid-solid interface resistance.^5,6

In our previous report⁵, fabrication and analyses of large-size SSBs test cells consisted of LZO coated NCM based cathode, graphite anode were reported. An improved amorphous 80Li₂S-20P₂S₅sulfide was used as the solid electrolyte. The electrodes and solid-electrolyte layer were prepared by a printing method from the composite slurries containing polymer binders. The batteries had attained the capacities of 125 mAh for the single cell and 1 Ah for the stacked cell.

In this work, we have improved the large size SSBs test cell performance by optimization of the components and fabrication processes. The new large size test cell is shown in Figure 1. The test cells showed 250 mAh (single cell) and 2Ah class cell with the power density of 175Wh/kg. Figure 2 is the discharge curve of a representative test cell. The major change from the previous cells is the optimization of the coating process of LZO. This has significantly improved the ionic conductivities of the solid electrolyte. Thickness of the solid electrolyte layer could be also decreased to 50 micrometers to reduced the cell resistance.

We also examined the cycle performance of the new test cells. The stable charge-discharge cycle was obtained without an artificial external pressure. Figure 3 shows the cycle performance of 125 and 250 mAh class cells. They retained the capacities  above 80% after 60 cycle . Furthermore, a safety test was performed, and the result and its implication will be discussed in the presentation. 

In this work, we demonstrated the applicability of the sulfide based electrolyte for a practical size SSB. Although further development is still needed, SSBs hold a great promise as next generation energy storage.

1. F. Mizuno, A. Hayashi, K. Tadanaga, and M. Tatsumisago, Advanced Materials, 17, 918 (2005).
2. N. Kamaya, K. Homma, Y. Yamakawa, M. Hirayama, R. Kanno, M. Yonemura, T. Kamiyama, Y. Kato, S. Hama, K. Kawamoto, and A. Mitsui, Nature Materials, 10, 682 (2011).
3. N. Machida, J. Kashiwagi, M. Naito, and T. Shigematsu, Solid State Ionics, 225, 354 (2012).
4.  N. Ohta, K. Takada, L.-Q. Zhang, R.-Z. Ma, M. Osada, and T. Sasaki, Advanced Materials, 18, 2226 (2006).
5. Satoshi Fujiki, Yuichi Aihara, Takanobu Yamada, Seitaro Ito, ECS
6. Seitaro Ito, Satoshi Fujiki, Takanobu Yamada, Yuichi Aihara et.al J. Power Sources 248 (2014) 943-950

Figure 1. The 2Ah 175Wh/kg class Cell. The cell consists of four parallel stacks of the single cells. 

Figure 2. 0.2mAcm^-2Discharge curve at 60 ˚C.

Figure 3. Cycle performance of  125mAh class cell(Gray), and 250mAh-class single cell(Black) at 60 °C.