|Cycle number and discharge capacity for the new cathode (ZrO2-coated) vs. conventional cathode. Click to enlarge.|
Nikkei. Researchers at the Central Research Institute of Electric Power Industry (CRIEPI) in Japan have devised a new type of cathode that improves the performance of lithium-ion batteries that use an ionic liquid as the electrolyte.
Unlike the organic solvents usually used as the electrolyte in a li-ion battery, room-temperature ionic liquids (RTIL) are nonflammable and nonvolatile. However, the RTILs lack electrochemical stability up to the reduction potential of lithium.
Batteries designed with the new cathode have around 30% higher charge storage capacity and can be recharged up to three times as often than other lithium-ion batteries with the ionic liquid electrolyte, according to CRIEPI.
The researchers used a mix of ammonium cations and imide anions in the fluid. A separator seeped in this liquid is sandwiched between the cathode and the anode. During charging, lithium ions dissolve off the cathode and migrate to the anode via the separator.
To make the new cathode, the team applied a nanolayer of zirconium oxide (ZrO2) on a lithium cobalt oxide (LiCoO2) cathode. In previous versions of the cathode, the particles of lithium cobalt oxide would release oxygen when subjected to high voltages during recharging, oxidizing and degrading the cations in the ionic liquid.
By coating the particles, the institute improved the stability of the cathode material, thereby allowing the battery to be recharged using a higher voltage.
The result is a battery with higher charge storage capacity and the ability to handle two to three times as many recharges as its predecessors. Even though the newly-developed battery loses 15% of its charge storage capacity after 100 recharges, it represents a step forward in the effort to develop a practical lithium-ion battery based on an ionic fluid electrolyte.
“Highly reversible lithium metal secondary battery using a room temperature ionic liquid/lithium salt mixture and a surface-coated cathode active material”; Shiro Seki, Yo Kobayashi, Hajime Miyashiro, Yasutaka Ohno, Akira Usami, Yuichi Mita, Masayoshi Watanabe and Nobuyuki Terada; Chem. Commun., 2006, 544 - 545, DOI: 10.1039/b514681j
“Lithium Secondary batteries Using Modified-Imidazoliium Room-Temperature Ionic Liquid”; Shiro Seki,Yo Kobayashi, Hajime Miyashiro, Yasutaka Ohno, Akira Usami, Yuichi Mita, Nobuo Kihira, Masayoshi Watanabe, and Nobuyuki Terada; Phys. Chem. B, 110 (21), 10228 -10230, 2006. 10.1021/jp0620872