This technology is significant as it allows the thermally durable Li-ion battery to be used in a wider variety of applications. Because this technology does not require the cooling system common in conventional Li-ion batteries, Hitachi expects it to lead to the development of more compact battery systems and to reduce overall costs.

This research was part of a collaborative project between Hitachi and AIMR has developed the new technology to reduce the internal resistance that is a factor of deterioration of charge-discharge performance.

A conventional Li-ion battery consists of a separator, a positive electrode (cathode), and a negative electrode (anode). The battery is filled with organic electrolyte solution in which lithium ions move between the two electrode layers during the charge and discharge process. One issue with conventional Li-ion batteries is the thermal durability of the organic electrolyte solution.

The upper operating temperature is limited to around 60 ˚C owing to the volatility of the organic electrolyte. Consequently, it is difficult to use the conventional Li-ion battery in a high temperature environment without a cooling system.

While solid electrolytes with no volatility have been developed for use in a high temperature environment, the lithium ion conductivity of the solid electrolyte, is lower than that of the organic electrolyte solution. The internal resistance of all-solid-state Li-ion battery needs to be reduced for its commercialization.

Schematic illustration of conventional Li-ion battery (left) and new solid-state battery (right). Source: Hitachi. Click to enlarge.

Details of the new battery technology include:

1. Composite positive electrode layer to suppress the decomposition of active materials at interface. One potential issue is that Li-ion conductivity will be inhibited by the decomposition of the cathode material, which is reduced by LiBH₄-based complex hydrides. To solve this issue, a Li-B-Ti-O-based oxide material was developed to form a dense composite positive electrode with the active materials. The Li-B-Ti-O in the electrode effectively protected the active materials, and suppressed the increment of internal resistance caused by the decomposition. In consequence, the discharge capacity of the battery was improved from 0 to 50% of theoretical value.

2. Adhesive layer for reducing the interface resistance between solid electrolyte and composite positive electrode layer.Another issue is that the composite cathode material and the metal hydride complex solid electrolyte layer were delaminated due to the volume change of the active materials during charge-discharge reaction. This causes incremental interfacial resistance by poor lithium ion conduction at the delaminated interface. To prevent delamination of the interface, the team developed an amide-added metal hydride complex with a low melting point for use as an adhesive layer.

   As a result, the internal resistance of the all-solid-state Li-ion battery was successfully reduced to 1/100 of the value compared to that of a battery with no adhesive layer.

The discharge capacity was improved up to 90% at 150 ˚C by applying the two technologies. In addition, the degradation of the discharge capacity during charge-discharge cycles was effectively suppressed and the stable charge-discharge of the all-solid-state Li-ion battery was confirmed.

The researchers said that now that they have verified the fundamental operation of a thermally durable all-solid-state Li-ion battery, they intend to look into further improving battery capacity, energy density and charge-discharge duration.

This research was part of a project between Hitachi and AIMR called “Collaborative Research for Next Generation Innovative Battery”. The findings of this research were partially presented on 13 November at The 56th Battery Symposium in Japan.