A high energy-density laminate cell with high safety and durability was realized. The energy-density of over 250 Wh/kg was obtained using high nickel-content cathode material of NCM811. Sufficient robustness against nail penetration test was also obtained in 60 Ah cell adopting a high heat-resistant separator and a heat-suppressing electrolyte (HSE). Moreover, a long durability of over 1000 charge/discharge cycles has been obtained by optimizing the surface conditions of the NCM811-based cathode electrodes with the S-based electrolyte additives.
1. Introduction
Recently, the needs of high energy-density LIBs for EV have increased. Ensuring the safety characteristics and long duration are strongly required for high energy-density LIBs. For the next generation of EV, long cruising range is one of the most important elements in the market demand. Other than EV, there is expectation of high energy-density cells for realizing compactness of the battery system.High energy-density LIBs of around 250 Wh/kg have already been developed using cylindrical cells of lower than 5 Ah in cell-capacity in recent years [1]. It is considered that the cylindrical cells are suitable for higher energy-density but difficult to realize larger capacity. And its durability and safety characteristics have not been mentioned in detail. On the other hand, the laminate cells have been considered that they are suitable for larger capacity because of the flexibility of cell-design and its higher heat radiation. However, as the cell-capacity becomes larger, the safety generally becomes more disadvantageous. Furthermore, the thermal instability of higher nickel-content cathode becomes more serious for securing the safety [2]. Therefore, there has been an obstacle to realize the laminate LIBs for large cell-capacity with high energy-density. In this study, the method that can achieve high energy-density and high safety is presented. The characteristics of 60 Ah laminate cells using this method are also shown.
2. Results and Discussion
60 Ah laminate cells with high energy-density of over 250 Wh/kg, keeping high safety, were realized. Such a high energy-density cell was obtained featuring a high nickel-content cathode material based on NCM811. The nail penetration tests under the penetrating-speed of 10 mm/s with the nails of 3 mm in diameter were carried out. The results were no-fire and no-smoke even when the cell-capacity was up to 60 Ah (The photo inset in Fig.1). It was caused by the combination of a high heat-resistant separator and an HSE. It was found that the hard-short area was not increased, because of the small shrinkage of the separator near the penetrated nail. These effects enable us to secure sufficient robustness against the nail penetration test for 60 Ah laminate cells with high energy-density of over 250 Wh/kg. The safety tests except for nail penetration tests were also acceptable for the demand of application.Cycle performances were tested for the obtained cells. The result showed 90% of capacity retention after 1000 cycles at 25 deg. C. (Fig. 1). This result was obtained by optimizing the surface conditions of the cathode electrodes using the S-based electrolyte additives. The cell resistance was not increased after 1000 cycles. By impedance analysis, it was found that the charge-transfer resistances for the cathode electrodes were not increased. This result suggests that the specific S-based electrolyte additives effectively prevent the surface deterioration of NCM811 by forming surface film on cathodes, as well as anodes.
3. Conclusion
In conclusion, 60 Ah laminate cells with high energy-density of over 250 Wh/kg were successfully realized, featuring NCM811-based cathode, optimized surface conditions for electrodes, and the combination of the heat-resistant separator and the HSE. The cell-series using such novel materials and process integration technologies can be applicable for wide-range applications, such as large-capacity EV, ESS, and also high-current power-supply devices attached to E-assisted vehicles, UPS, robots, and drones.
References
[1] V. Muenzel, A. F. Hollenkamp, A. I. Bhatt, J. D. Hoog, M. Brazil, D. A. Thomas, and I. Mareels, J. Electrochem. Soc., 162, A1592-A1600 (2015)
[2] G. Kimand and J. R. Dahn, J. Electrochem. Soc., 161, A1394-A1398 (2014)