Meaning
Movement of lithium ions through the crystalline lattice of electrode active materials governs the rate at which a battery can be charged and discharged at various temperatures. This solid state lithium diffusion is often the rate limiting step in high power energy storage systems. It governs the concentration gradients within the electrode particles and the resulting mechanical stress.
The term applies to the transport of ions within the bulk of the solid phase and ends at the interface with the electrolyte.
Transport Kinetic
Fick’s laws of diffusion describe the flux of ions in response to the concentration gradient established during the electrochemical reaction. This solid state lithium diffusion depends on the availability of vacant sites and the energy barriers between these sites. The crystal structure of the material, such as the layered oxide or the olivine framework, defines the paths available for ion movement.
One dimensional or two dimensional diffusion paths can limit the rate of transport compared to three dimensional networks. The diffusion coefficient is a fundamental property that characterizes the mobility of the ions. Faster diffusion allows for higher power density and shorter charging times.
Temperature Dependency
Mobility of the ions is highly sensitive to the operating temperature of the battery cell. This solid state lithium diffusion follows an Arrhenius relationship where the rate increases exponentially with temperature. At low temperatures, the diffusion becomes very slow which leads to a significant loss of power and the risk of lithium plating.
Self heating of the cell during high current operation can temporarily improve the diffusion rate. However, excessive heat can also lead to the degradation of the material structure. Thermal management systems are designed to keep the battery within a range that balances performance and safety.
Diffusion Barrier
Structural defects and grain boundaries can impede the smooth flow of ions through the electrode material. This solid state lithium diffusion is also affected by the phase transitions that occur during the insertion and extraction process. The formation of a new phase can create a barrier that slows down the movement of ions.
Particle size reduction is a common strategy to shorten the diffusion distance and improve the rate capability. Coating the particles with ionically conductive materials can also enhance the transport at the surface. Researchers use electrochemical impedance spectroscopy to measure the diffusion resistance in different battery states.
The final performance of the battery is limited by the speed of these internal transport processes.
