Energy Storage and the Role of Materials

The ongoing electrification of various areas of life from electric vehicles to portable electronics is continuously driving research and development in the field of energy storage. Lithium-ion batteries have emerged as the (provisionally) dominant storage medium due to their high energy density and long service life. The performance and reliability of these energy storage systems are largely determined by the properties of their components, particularly the active materials used in the anodes and cathodes.

The cathode active materials (CAM), such as lithium iron phosphate (LFP) or lithium nickel manganese cobalt oxides (NMC), play a decisive role in determining a battery’s capacity, energy density, and performance. Anode materials such as graphite (C), on the other hand, are crucial for charging speed, lifetime, and safety. Depending on the materials used, different types of lithium-ion batteries can be distinguished.

The manufacturing process is highly complex and involves a series of critical steps from powder synthesis to final electrode fabrication. One of the most crucial stages, which has a profound impact on the microstructure and thus the electrochemical properties of the active material, is the thermal treatment during the calcination and sintering process. This step is essential for obtaining a high-quality and pure cathode powder.

The Sintering Process – A Key to Material Quality

During the sintering of the cathode material, fine powder particles are heated to temperatures below their melting point, causing them to fuse into a dense, solid body. Throughout this process, atoms diffuse across grain boundaries, which binds the particles together, reduces porosity and increases the material density. For cathode active materials, sintering between 700 °C and 1.100 °C is essential to:

Adjusting crystal structure and phase composition:
The sintering temperature and atmosphere have a significant impact on the formation of desired crystal phases and the degree of crystallinity, which directly affect ionic and electronic conductivity as well as material stability.

Controlling particle size and morphology:
The sintering conditions determine grain growth and the formation of specific particle shapes, which in turn influence the electrode’s packing density and electrolyte accessibility.

Improving mechanical stability:
Through grain boundary fusion and material densification, the mechanical integrity of the particles increases, which is an essential factor for long-term electrode stability under the stress of charge/discharge cycles.

Optimizing electrical conductivity:
Proper sintering improves particle-to-particle contact and reduces internal electrical resistance, which is crucial for efficient current flow.

Precise control of sintering parameters such as temperature, dwell time, heating/cooling rates, and gas atmosphere is therefore vital to achieve optimal cathode material properties for high-performance batteries. Suboptimal sintering can prevent the active material from attaining the desired crystal structure, particle morphology, and chemical stability. This results in reduced capacity, increased internal resistance, and lower charge/discharge performance. Moreover, defects and unstable phases accelerate cathode degradation, significantly reducing the lifetime and reliability of the entire battery.

In contrast to cathode active materials, most anode materials, especially graphite (C), are not conventionally sintered. In synthetic graphite, a high-temperature treatment (graphitization at 2.500 - 3.000 °C) is carried out primarily to form an ordered carbon structure rather than to bind grains, as in ceramic processes. Only special anode materials, such as lithium titanate (Li₄Ti₅O₁₂) or certain silicon composites, also undergo calcination or sintering processes to ensure their crystalline structure and electrochemical stability.

Monitoring the Sintering Process with Process Temperature Control Rings

To ensure the quality of the sintering process and sintered products, Process Temperature Control Rings (PTCR) record the exact heat input at the point in the furnace where they are placed. These small pyrometric rings (20 mm outer diameter) shrink in proportion to the amount of absorbed heat, allowing for precise determination of the so-called Ring Temperature (RT). This enables a reliable assessment of the thermal load inside the furnace - independent of the displayed furnace temperature. The rings can be placed directly in or next to the charge containers and pass through the sintering process together with the cathode material - for example, in a roller hearth or pusher plate kiln. The PTCR should not come into direct contact with the sintered material. For precise control and monitoring of the firing process during the production of cathode active materials, two PTCR variants are primarily used, depending on the process:

• PTCR UTH (660 - 900 °C) and
• PTCR ETH (850 - 1.100 °C)

Both ring types contain organic components (binder and food colouring) and must therefore undergo a ring-specific debinding process in air before use. When selecting the appropriate ring type, the firing/sintering temperature should ideally be near the midpoint of the temperature range covered by that ring type. If the maximum process temperature lies near the boundary between two types, the choice depends on process conditions such as dwell time and atmosphere, as both strongly influence ring shrinkage. In total, eight different PTCR ring types are available, covering a temperature range from 560 °C to 1.750 °C.