Cathode Powder – The calcination process
The positive electrode in the battery is often referred to as the “cathode”. In the conventional lithium ion batteries, lithium cobalt oxide is used as the cathode. In the last few years, however, many alternative material systems have been developed and used.
In most cases, however, lithium and oxygen are still an essential part of the system. Only the metallic element cobalt is often completely or only partially replaced by other metallic elements such as nickel and manganese. For this reason, most lithium ion batteries can be described as a socalled lithium metal oxide cathode.
Lithium metal oxides are produced as solid powders. The microstructure, morphology, particle size and degree and type of possible contamination in the powder play a decisive role in the selection of the powder as a suitable material for use as a cathode in a lithium ion battery (LiB). These influence the electrochemical characteristics of the battery, which is subsequently produced from it. Especially the energy density, which is of great importance e.g. for the distance electric vehicles can drive, is influenced by the above mentioned microstructural parameters.
The microstructure of the cathode must therefore be precisely adjusted. On the one hand this can be achieved by the right choice of raw materials and on the other hand by a controlled production process of cathode powders. As the term implies, a lithium metal oxide is a mixed crystal of lithium oxide and oxides of other metals. These mixed crystals are formed by thermal treatment of a mixture of the individual oxides at high temperatures; typically between 800-1000 °C under certain atmospheric conditions. The individual oxides, in turn, are provided by the addition of various raw materials to the mixture. The starting raw materials are often hydroxides or carbonates of lithium and the other respective metallic elements.
By heat treatment of these starting materials water (H2O) or carbon dioxide (CO2) is released at temperatures of 600-800 °C; the remaining oxides participate later by further treatments in a mixed crystal.
Basically, in the manufacturing process of the cathode, various oxides are extracted from the respective hydroxides or carbonates of the same elements in the first step and then, in a second step, the desired mixed crystal is produced from these oxides.
The first step, in which two solids react together to form a third solid and gases are released, is called calcination. The second step is called sintering or solid diffusion. Calcination occurs almost independently of time as soon as the temperatures and starting materials required for the beginning of the reaction are available. Consequently, the first steps of thermal treatment in the production of the cathode take place relatively quickly. On the other hand, the diffusion processes to form the mixed crystals are very time-dependent and last much longer. For some manufacturers, the two steps are carried out one after the other in one process; for others, however, the calcination products are first cooled and later sintered in a separate thermal process.