During formation several processes take place that lead to significant changes in the cell properties. These processes include:
- solid electrolyte interphase (SEI) formation
- cathode electrolyte interphase (CEI) formation
- structural changes in the active material
- copper corrosion
- aluminium dissolution
Based on the Arrhenius equation, temperature affects the reaction kinetic. Thus having a significant impact on the resultant properties. The papers and results reviewed by Schomburg et al [1] show that elevated formation temperatures:
- accelerate SEI formation, resulting in a reduction in formation time
- SEI formation at 40 °C results in a layer consisting of compact, inorganic components instead of a less compact, organic structure
- formation at 60°C resulted in a more homogeneous layer (compared to 25°C) due to higher diffusion rates
- formation at 50 °C resulted in lower total internal resistances during and after formation in comparison to a formation at 25 °C
- as temperature increases, the over-potentials decrease and the formation time decreases
- higher ambient temperatures:
- increase the effective conductivity of the electrolyte in the separator
- improve the solid diffusivity in the active material
- reduce the charge transfer resistance
- increases the reaction rate
- reduces the internal resistance
Overall the review shows that both negative effects due capacity loss and positive effects due to improved interfacial homogeneity have been observed.
References
- Felix Schomburg, Thomas Roth, Bastian Heidrich, Michael Kurrat, Sarah Wennemar, Robin Drees, Heiner Heimes,c Andreas Jossen, Martin Winter, JunYoung Cheong and Fridolin Roder, Lithium-ion battery cell formation: status and future directions towards a knowledge-based process design, Energy & Environmental Science, RSC
See also: Fast Formation Protocols