Film-forming Mechanism of Sulfur-containing Additives on Graphite Negative Electrodes and Their Low-temperature Performances

Abstract

Lithium ion batteries (LIB) have been applied as a major power source for portable electric devices due to high energy density and long life cycles. Lately, they are the most promising candidate for hybrid electric vehicles (HEVs) and pure electric vehicles (EVs). There are, however, a series of technical barriers in LIB must first be overcome, one of which is the drastic decrease in various aspects of performance in cold condition. It is well known that graphite, which represents the preferred negative electrode for LIB, delivers poor electrochemical performance at low-temperatures. The performance indicators of graphite electrode, like irreversible capacity, rate capability, cycleability and safety are highly dependent on characteristics of solid electrolyte interphase (SEI), which inevitably passivates the surface of graphite. Film-forming additives can be used as one way to optimize the chemical composition and physio-chemical properties of SEI, leading to improved performances of LIB. Two additive types are studied in this thesis, elemental sulfur as an electrode additive and allyl sulfide as an electrolyte additive. Both additives improve the low-temperature performance of the graphite electrode. Of the two, the latter type has an advantage of stability in cell operation. In the first half of the thesis, the film-forming mechanism and the low-temperature performance of elemental sulfur additive are studied. In the first lithiation step, the elemental sulfur is electrochemically reduced to be lithium polysulfide (Li2S8), which is soluble in the working solvent (carbonate-based). Organic thiocarbonates are generated by the chemical reaction between the lithium polysulfide and carbonate solvents. The as-generated thiocarbonates are then electrochemically decomposed to form the sulfur-containing surface film. The sulfur-added graphite shows better reversible capacity at low-temperature, also, Li plating is suppressed. The superior low-temperature performance of the sulfur-added graphite is thus attributed to the presence of sulfur-enriched surface film with less inorganic species, which seems to facilitate the charge transfer reaction between the graphite and lithium. Secondly, allyl sulfide (AS) additive is examined along the same lines. In 12 hr rest period before cycle, allyl sulfide additive is oxidized spontaneously and forms film on the surface of graphite. This pre-formed film is reduced and develops sulfur and carbon rich inner film onto the graphite. The low-temperature performance of the AS-added graphite is also superior to the control one, in reversible capacity and prevention of Li plating. This is attributed to the chemical aspects of surface film, like in case of elemental sulfur additive. Since AS additive does not involve side reactions during film formation, it has an advantage of stability in cell operation over elemental sulfur.