Principles of SEI formation

Lithium-ion batteries occupy a large and increasing share of the rechargeable-battery market as a result of their excellent performance in terms of cycle life, energy density, power density and charge rate. However, for the successful use of carbon electrodes in secondary lithium-ion batteries, much work, such as the selection of high reversible and low irreversible capacity carbons, as well as understanding the complex mechanism of lithium-ion intercalation into lithium, has still to be done. 

At the electrode surface there is a competition among many reduction reactions of salts, solvents and impurities, the rates of which depend on i and T for each process and on the catalytic properties of the carbon surface. The products of reduction of salt anions are typically inorganic compounds like LiF, 

which precipitate on the electrode surface. Reduction of solvents is followed by the formation of both insoluble SEI components like Li COj and partially soluble semicarbonates and polymers. The voltage at which the SEI is formed depends on the type of carbon, the catalytic properties of its surface (ash content, type of crystallographic plane, basal-to-edge plane ratio), temperature, concentration and types of solvents, salts and impurities, and on the current density. Eor lithium-ion battery electrolytes, is typically in the range 1.7-0.5 V vs Li reference electrode, but the SEI continues to form down to 0 V. In some cases, p is less than 100% during the first few cycles. This means that the completion of SEI formation may take several charge-discharge cycles. 

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