The importance of heat treatment for cathode active materials

 

Introduction:

The synthesis of cathode active material (CAM) for electric vehicle (EV) batteries involves several process steps, including a heat treatment step which plays a crucial role in determining the final quality and performance of the battery. Increasing the quality CAM, has become significantly more important for the development of high-quality lithium-ion batteries, especially during a global market shift towards EVs as sustainable means of transportation. 

A lithium-ion battery will have its properties, such as energy density, cycle life, and safety measures heavily influenced by the active material in the cathode. CAM is composed of a mixture of lithium, and other metals such as cobalt, manganese, and nickel in various chemistries, two of the more popular ones being Lithium Nickel Manganese Cobalt Oxide (NMC) and Lithium Iron Phosphate (LFP).

The choice of active materials and their synthesis process will directly impact the battery’s overall performance.

Heat Treatment in CAM Production

The heat treatment step involved in the synthesis of CAM is called calcination. Calcination is a heat treatment process in a controlled environment, aiming to change the physical constitution or chemical composition of a material, through the removal of water, separation of volatile compounds, decomposition/synthesis, or oxidation/reduction of the material. This is a critical step in the synthesis of CAM and involves heating the material to high temperatures.

The basic electrochemical principle applied for lithium battery cathode materials, is that the molecular structure formed by lithium and other additive atoms can allow electrons and ions to easily move in and out. The aim of calcination consists in forming this stable crystal structure and enhancing structural integrity, which is essential for consistent performance and longevity of the battery. 

Consequently, proper, and controlled heat treatment ensures optimal electronic and conductivity and lithium-ion diffusion rates within the CAM, which then also affects the battery’s charge and discharge efficiency.

Furthermore, during the heat treatment of CAM, unwanted impurities and reaction byproducts are released from the material, and need to be eliminated or evacuated as they can degrade the overall performance and life of a battery.

Key Considerations in Heat Treatment

As mentioned previously, the heat treatment parameters must be carefully selected and the requirements must be met, in order to obtain the desired molecular structure. The first process parameter that needs to be taken into consideration is the temperature. The maximum temperature at which the reaction occurs, as well as the heating gradient to reach that temperature need to be carefully controlled, as overheating can lead to a breakdown of the material’s structure or unwanted phase transition, while insufficient heating might not allow the formation of the desired crystal structure or unprocessed CAM powder.

Another parameter that needs to be considered is the atmosphere in which the process takes place. 

The atmospheric composition during heating, whether oxidizing or reducing, can impact the chemical properties of the CAM. Certain battery powder chemistries require very high purity protective gas, to avoid any reducing agents during and oxidation reaction, and vice versa. Additionally, excess moisture, reaction byproducts generated during the firing process, need to be removed from the atmosphere during the heating process to prevent any unwanted gases from accumulating.

Post heat treatment, the cooling process is equally important. The cooling needs to be executed in a controlled manner. Excessively high cooling rates can lead to cracks and structural weaknesses, while slow cooling might lead to unwanted phase transformations.

Impact on EV Battery Quality

As explained beforehand the quality of CAM produced during the heat treatment process directly influences the EV battery’s performance. A well-executed heat treatment process results in a CAM that offers firstly higher energy densities, with batteries leading to longer driving range EVs. They will additionally have a better cycle life, enhancing the overall lifespan of the battery, as well as improved safety, with reduced risk of thermal runaway and battery failures.

Conclusion

In summary, the heat treatment of cathode active materials is a pivotal step in the production of high-quality EV batteries. The precision in temperature control, atmosphere management, and cooling rates during this process plays a significant role in enhancing the performance, safety, and durability of EV batteries. In the battery production industry, the roller hearth kiln (RHK), amongst other furnace types, has been established to be the most used and standard equipment technology for the heat treatment process of both cathode active material and anode active material. As the EV market continues to grow, the innovation in CAM processing, especially in heat treatment techniques, will be a key factor to meeting the rising demands for efficient and reliable electric vehicles.