Toyota develops method to observe behavior of Li ions in electrolyte; expected contributions to battery performance & durability
24 November 2016
Toyota Motor’s Central R&D Labs, along with Nippon Soken and four universities (Hokkaido, Tohoku, Kyoto, and Ritsumeikan) has developed the first method for observing the real-time behavior of lithium ions (Li-ions) in an electrolyte as a Li-ion battery charges and discharges. The researchers will present a paper on this method, results and the implications for battery design at the 57th Battery Symposium in Japan next week.
Toyota believes that this method—which combines high-intensity X-ray (synchrotron radiation) for high speed measurement at high resolution with a special, easy-to-observe electrolyte—will provide essential guidelines for R&D that aims to improve the performance and durability of batteries, which would lead to longer battery life, as well as longer driving ranges for plug-in hybrid vehicles (PHVs) and electric vehicles (EVs).
Current Li-ion batteries use a metal oxide in the cathode, a carbon material in the anode, and an organic electrolyte for the electrolyte. Lithium ions flow in the electrolyte from the cathode to the anode when the battery is charging, and from the anode to the cathode when the battery is discharging, which results in an electric current flow. (The Li ions move by binding with ions containing phosphorous in the electrolyte.) The lithium ions thus play an essential role within the electrolyte during battery charging and discharging.
Li-ion deviation occurs in the electrodes and electrolyte as a result of charging and discharging, and such deviation has been believed to limit the usage area of batteries—one factor that reduces the area in which the maximum performance of the battery can be achieved.
When investigating the mechanism of Li-ion deviation, however, confirming the behavior of Li-ions in the electrolyte under the same environment and conditions as when it is being used in related products was not possible using existing methods.
The two main features of the observation method that Toyota developed to help solve this problem are:
The Toyota Beamline in the SPring-83 synchrotron radiation facility produces high-intensity X-rays that are approximately 1 billion times more powerful than those generated by an X-ray equipment. This makes it possible to create 0.65 micron/pixel high-resolution and 100 ms/frame high-speed measurements.
In place of the electrolyte with phosphorous that is used in many Li-ion batteries, a new electrolyte with heavy elements is being utilized instead, thus replacing the phosphorous-containing ions that the Li-ions bind to as they move in the electrolyte with heavy element-containing ions.
As the heavy elements with large atomic numbers have more electrons than phosphorous, they transmit less of the X-rays than phosphorous, and the shadows on the images taken after the X-rays pass through are darker. By observing the behavior of the heavy elements, it is possible to observe the deviation behavior of the Li-ions which are bound to them in the electrolyte.
By using the method described above, and a battery that is similar to that of related products such as laminated cell under the environment and conditions that resemble those of actual battery use, it is possible to observe in real-time, the process of Li-ion deviation that occurs in electrolytes during the course of battery charging and discharging.
Going forward, Toyota will observe the behavior of Li-ions caused by differences in the materials and structures of cathodes, anodes, separators, and electrolytes, as well as the differences in battery control. Analyzing the mechanisms that cause deterioration of battery performance will lead to R&D that can help to improve the performance and durability of batteries, to bring about longer battery life and longer driving ranges.