The solid electrolyte was created by a research team at the Graduate School of Engineering at Osaka Metropolitan University under the direction of Professor Akitoshi Hayashi. It is extremely deformable and can have a sizable contact surface with an electrode. In this study, scientists constructed both electrodes for a bulk-type all-solid-state capacitor using a composite made from the same highly malleable solid electrolyte and carbon. This capacitor makes the first bulk-type, all-solid-state capacitors in the world by enabling high current densities and high-capacity charging and discharging at temperatures of 200–300 °C. As a result of these technical constraints, researchers anticipate that their capacitor will be utilised to advance high-temperature environment technologies.
Due to the charge adsorption and desorption characteristics at the electrode-electrolyte interface, capacitors are energy storage devices with two electrodes and an electrolyte that enable quick charging and discharging. Capacitors are useful for power levelling for renewable energy that needs repeated charging at high currents, regenerative braking energy for trains and electric or hybrid cars, as well as instantaneous voltage drop compensation devices that prevent equipment failure due to lightning strikes. However, because capacitors’ energy storage does not involve chemical reactions, their storage capacity is lower than that of lithium-ion batteries. In the near future, they are also anticipated to be utilised to store energy for wearable technology.
The liquid electrolyte that is used in the majority of capacitors has a low boiling point and can only be used at temperatures below 80°C. As a result of their lesser storage capacity compared to liquid electrolyte capacitors, ceramic capacitors that use solid inorganic materials as a dielectric can only be used in electronic circuits at temperatures over 80°C. Large contact areas between the electrode and the electrolyte are required to maximise the energy storage capacity of capacitors. Solid electrolytes make it challenging to create a wide contact area, therefore the development of a capacitor with a large storage capacity and high operating temperature has long been desired.
“The key to realizing this capacitor was to take the solid oxide electrolytes that we have been developing for all-solid-state lithium batteries—which combine excellent deformability and lithium-ion conductivity—and apply them to capacitors,” explained Professor Hayashi.
By regulating the chemical reaction between a solid electrolyte and carbon and combining it with positive electrode materials used in lithium-ion batteries, the researchers expect to build all-solid-state hybrid capacitors with even better energy densities in the future.