Understanding the Formation and Evolution of Lithium-Ion Battery Solid Electrolyte Interphase

Abstract

Lithium ion batteries are widely used as energy storage devices in a variety of products such as smartphones, tablets, laptops and other portable electronics. Thanks to their high energy density and cyclability, they are currently being used by and developed for electric vehicles. There is a growing need for cost reduction; increase in energy density; wider operating temperature range; and improved safety characteristics of the batteries. Organic carbonates are the primary solvents used in lithium-ion battery electrolytes along with electrolyte additives. The reversibility of current lithium-ion batteries is dependent upon the electrolyte used in the batteries. During the initial charging cycles of the cell, a solid electrolyte interface (SEI) is formed by reduction of organic carbonates, electrolyte salts and/or electrolyte additives on the surface of the graphitic anode in lithium-ion batteries. The generation of a stable anode SEI prevents continuous electrolyte reduction on the surface of the anode. The SEI functions as a Li ion conductor but an electrical insulator. The reduction reactions of the electrolytes on the graphitic anode surface have been investigated for many years and it been proposed to contain a complicated mixture of products including lithium oxalate, lithium alkoxides, and lithium oxide from the carbonate solvents and LiF and lithium fluorophosphates from the reduction of LiPF6. Similar ambiguity exists about the components of SEI formed from electrolyte additives and other electrolyte salts. Despite the extensive investigations, the structure, formation mechanisms and evolution of the SEI are poorly understood. Understanding the mechanisms of the reduction reactions of organic carbonates, electrolyte salts and electrolyte additives along with the products of the reactions which result in the generation of the SEI is essential for the development of safer lithium-ion batteries with wider operating temperature range. Lithium naphthalenide has been investigated as a one electron reducing agent for organic carbonates solvents, some of the most robust additives and salts used in lithium ion battery electrolytes. The reaction precipitates have been analyzed by IR-ATR, XPS and solution NMR spectroscopy. The evolved gases and the volatile components have been analyzed by GC-MS. The reduction products of ethylene carbonate and propylene carbonate are lithium ethylene dicarbonate (LEDC) and ethylene and lithium propylene dicarbonate (LPDC) and propylene, respectively. The reduction products of diethyl and dimethyl carbonate are lithium ethyl carbonate (LEC) and ethane and lithium methyl carbonate(LMC) and methane, respectively. Electrolyte additives, FEC and VC reductively decompose to HCO2Li, Li2C2O4, Li2CO3, and polymerized VC.