What is SEI Affect Lithium ion Battery Effective?
Graphite (including natural and artificial graphite) is the main anode material for lithium-ion batteries used in electric vehicles. The aging effect of graphite is mainly due to the change of electrode/electrolyte interface. The growth of SEI and the deposition of lithium are the main degradation mechanisms. In addition, mechanical degradation such as stress expansion can also be observed in both positive and negative electrodes and SEI films.
First of all, SEI film, negative graphite and electrolyte interface through the interface reaction, these reactions include electrolyte solvent reduction and lithium salt decomposition. The resulting reduction products form a passive film on the surface of the anodic active material, which is called the solid electrolyte interface (SEI). The composition and structure of SEI depend largely on the active substances, solvents, lithium salts, electrolyte additives, and the formation process. The decomposition products of typical carbonate electrolytes (e.g. EC, EMC, DMC) with LiPF 6 as lithium salts are Li2CO3, ROCO2Li, (CH2OCO2) Li2 and LiF. SEI based on organic reaction products is often described as a two-layer structure: a dense inner layer, a soft outer layer containing inorganic salts.
The ideal SEI has low conductivity and allows the transmission of lithium ions, but not other electrolyte components. Therefore, SEI inhibited further electrolyte breakdown and negative electrode degradation. In addition, the ideal SEI has uniform morphology and chemical composition to ensure uniform current distribution. The formation and growth of SEI consumes recyclable lithium, leading to irreversible capacity degradation, especially during battery formation and the first few cycles. In the long run, SEI penetrates into the pores of the electrode and may also penetrate into the pores of the diaphragm, which may result in a decrease in the active surface area of the electrode, a decrease in the capacity of the battery and an increase in internal resistance. The growth of SEI is also affected by the positive aging reaction; for example, the dissolution of transition metals, the corresponding mechanism will be described in the next section on the degradation of positive active materials.
becuase SEI is one of the main reasons affect lithium ion battery effect, some manufacturer take advatage of reducing seperator film so as to reduce SEI effect, therefore, battery could get much more effective factor with lithium, energy could be probably increase by about 10%.