Meaning
Polymerized compounds on the surfaces of lithium-ion anodes form a flexible barrier that accommodates mechanical expansion during cycling. Each organic sei component consists of alkyl carbonates, polymeric structures, and semi-carbonates generated through the partial reduction of the electrolyte solvents. This flexible outer layer helps to maintain the integrity of the protective interface during the volumetric changes of the electrode.
Molecular Formation
Chemical reactions occurring during the first few charging cycles determine the composition of the protective interface. Solvent reduction yields various carbon-containing molecules, and each organic sei component adds to the polymeric network on the graphite surface. This soft outer region prevents further solvent penetration while allowing the passivated electrode to remain electrically insulated.
Mechanical Function
High-capacity anodes undergo substantial changes in volume when lithium is inserted and extracted. Unlike the brittle inner inorganic compounds, the organic sei component possesses the elasticity required to stretch without fracturing during these dynamic shifts. This flexibility is essential for preventing the exposure of fresh graphite to the aggressive electrolyte.
Degradation Path
High operating temperatures and continuous cycling can cause the organic species in the interface to dissolve or decompose. When the organic sei component degrades, it exposes the active material, leading to the formation of additional passivation products and the loss of active lithium. Sourcing decisions for electrolyte additives aim to create highly stable organic compounds that can resist thermal dissolution.
This chemical stability ensures that the battery retains its high energy efficiency and experiences lower rates of capacity fade over its operational life.
