New method to produce layered lithium cobalt oxide cathode materials at low temperatures; hydroflux process

Traditionally, the synthesis of layered lithium cobalt oxide cathode materials for Li-ion batteries requires temperatures above 800 °C and takes 10 to 20 hours to complete. Now a team of researchers at Hokkaido University and Kobe University, led by Professor Masaki Matsui at Hokkaido University’s Faculty of Science, have developed a new method to synthesize lithium cobalt oxide at temperatures as low as 300°C and durations as short as 30 minutes.

Their findings were published in the ACS journal Inorganic Chemistry.

Reaction pathway of the hydroflux process to form layered lithium cobalt oxide (LiCoO₂) at 300 °C. (Illustration: Masaki Matsui)

Using cobalt hydroxide and lithium hydroxide as starting materials, with sodium or potassium hydroxide as an additive, the team conducted a series of experiments under varying conditions to synthesize layered LiCoO₂ crystals. The process was called the hydroflux process. They were also able to determine the reaction pathway that led to the formation of the layered crystals.

Obvious crystal growth of LiCoO₂ synthesized via hydroflux process compared with solid-state process at 300 °C by (a) XRD and (b) SEM observation. (Rannosuke Maeda, et al. Inorganic Chemistry.)

Molten mixed hydroxide-containing water molecules significantly accelerated the formation of LiCoO₂, which showed a highly reversible capacity of 120 mAh g^–1 without postannealing. The reaction mechanism study showed fast growth of LiCoO₂ crystals, suggesting that the excess molten hydroxides containing water dissolve the cobalt species of HCoO₂^–. Consequently, the accelerated LiCoO₂ formation suppresses the competing reaction of Co₃O₄ formation, leading to spinel LiCoO₂ formation at low temperatures.

Excess water in the starting materials further accelerated the crystal growth of LiCoO₂, forming large particles (>1 μm). Moreover, the layered LiCoO₂ began to form at 150 °C.

This study is the first experimental demonstration that proves the thermodynamic stability of layered LiCoO₂ at low temperatures (150–300 °C) under ambient pressure. This novel process offers significant energy savings in the production process of LiCoO₂ and other ceramics materials.

—Maeda et al.

The team also measured the electrochemical properties of the layered LiCoO₂, showing that they were only marginally inferior to that of commercially available LiCoO₂ synthesized by the traditional high temperature method.

Resources

• Rannosuke Maeda, Ryo Nakanishi, Minoru Mizuhata, and Masaki Matsui (2023) “Kinetically Enhanced Reaction Pathway to Form Highly Crystalline Layered LiCoO₂ at Low Temperatures Below 300 °C” Inorganic Chemistry doi: 10.1021/acs.inorgchem.3c01704