All-Solid-State Lithium Batteries with Wide Operating Temperature Range

Lithium ion secondary batteries have a high voltage and a high energy density, as shown in Fig. 1, and are widely used in mobile devices such as cell phones, notebook PCs and PDAs. However, since lithium ion secondary batteries use a flammable organic liquid electrolyte, there is a risk of explosion or fire. Fire accidents can also occur due to contamination during production or from overcharging. To reduce such risk, a protection circuit is added to the battery. However, since there is a trade-off between energy density and the risk of accidents, it is difficult to meet today’s growing demand for high energy density. When the battery is larger, for automotive or industrial equipment for example, ensuring safety is increasingly important. On the other hand, in terms of performance, the battery capacity is significantly reduced or the battery becomes unusable when the electrolyte freezes in cold climates. This inconvenience is primarily due to the organic electrolyte. Moreover, high-capacity active materials, such as the sulfur cathode and lithium metal anode, cannot be used with liquid electrolytes due to dissolution or dendrite problems, respectively. This makes it difficult to increase the battery’s energy density. Because of these issues, all-solid-state lithium batteries have been proposed as a fundamental solution (Fig. 2)(2)-(4). All-solid-state batteries never use any liquid cell components. Instead of using organic electrolyte, a lithium ion conductive ceramic, known as a solid electrolyte, is used. This is expected to principally resolve various problems caused by the liquid electrolyte. Previously, all-solid-state batteries were significantly inferior to liquid batteries because of high internal resistance. However, active development has proceeded recently due to the discovery of a solid electrolyte with high lithium ion conductivity and reduction in the interfacial resistance by forming a buffer layer between the solid electrolyte and cathode material(5), (6).