Low-Temperature Characteristics and Film-Forming Mechanism of Elemental Sulfur Additive on Graphite Negative Electrode

Abstract

We report here that the low-temperature performance of graphite electrode is improved by adding a small amount of elemental sulfur (S 8 ) into the graphite negative electrode. The reversible capacity at −30 • C is much larger for the sulfur-added graphite electrode. The origin of this beneficial feature is examined through impedance analysis, which illustrates that the charge transfer resistance is much smaller in the sulfur-added graphite electrode at low temperatures. In the first lithiation step, the elemental sulfur is electrochemically reduced to be lithium polysulfide (Li 2 S 8 ), which is soluble in the working solvent (carbonate-based ones). 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 superior low-temperature performance of the sulfur-added graphite is thus attributed to the presence of sulfur-enriched surface film, which seems to facilitate the charge transfer reaction between the graphite and lithium. Increases in oil prices and greenhouse gas emissions have increased the need for hybrid electric vehicles (HEVs) and pure electric vehicles (EVs). Lithium-ion batteries (LIBs) are the most promising candidate for these applications due to their high energy and power density relative to other power sources. To market HEVs and EVs, a series of technical barriers in LIBs must first be overcome, one of which is the drastic decrease in reversible capacity at low temperatures.1 According to the U.S. Department of Energy, LIBs in power-assisted HEVs should function at −30• C and survive at −46 • C. 2 It has been reported that a commercial 18650 LIB delivers an energy density of only 5% at −40• C compared to the value at 20 • C. 3 It is well known that graphite, which represents the preferred negative electrode for LIBs, delivers poor electrochemical performance below −20• C. [4][5][6] The poor performance at low temperatures has been attributed to several factors; (i) increased viscosity and reduced Li + conductivity in electrolytes, (ii) sluggish Li + ion mobility in the surface films called solid electrolyte interphase (SEI), (iii) increased charge transfer resistances at the electrode/electrolyte interface, and (iv) reduced solid-state Li + diffusivity within the graphene sheets. Several approaches have been pursued to overcome these limitations, 4,6-8 one of which is the use of propylene carbonate (PC) instead of ethylene carbonate (EC) as the solvent. With PC, the conductivity problem can be somewhat overcome because the freezing point is lower for PC. Unfortunately, however, PC is not compatible with graphite electrode because of exfoliation problem. 9Electrolyte decomposition and film deposition are unavoidable in graphite negative electrodes because their working voltage is beyond the thermodynamic stability window of common organic electrolytes. The SEI film, which is formed at electrode/electrolyte interface, pas...

Designed Synergetic Effect of Electrolyte Additives to Improve Interfacial Chemistry of MCMB Electrode in Propylene Carbonate-Based Electrolyte for Enhanced Low and Room Temperature Performance

“…23,26,27 In addition, sulfur derived SEI and chlorine atom containing additive are reported to improve low temperature performance of graphite electrode. 13,14,27 Based on the considerations and requirements of PC-based electrolyte, 4chloromethyl-1,3,2-dioxathiolane 2-oxide (CMDO), a sulfur-containing compound with similar structure to sultones, seems to be a reasonable additive and the material has not been used and studied for this purpose. CMDO has sulfite functional group and chlorine substituent on the structure and is designated to utilize the advantages of sulfite functional group and chlorine atom.…”

Atomic thermodynamics and microkinetics of the reduction mechanism of electrolyte additives to facilitate the formation of solid electrolyte interphases in lithium-ion batteries

“…6, the continuous electron reduction of the secondary radical with the addition of Li + brings about the formation of an inorganic compound LiCl in 26 and 27 with a respective DG of À147.4 and À136.8 kcal mol À1 (relative to 23 and Li + with SMD-B3PW91/6-311++G(d,p)). Complex 27, consisting of free LiCl and PC-solvated LiSO 3 CH 2 CH]CH 2 , is less stable by approximately 10 kcal mol À1 than 26, in which LiCl is still coordinated to LiSO 3 R. The further electron reduction of the primary radical 25 results in a diradical intermediate (28) that has only a rather small barrier ($1 kcal mol À1 ) through a transition state (29) to produce inorganic compound Li 2 SO 3 by 16308 | RSC Adv., 2020, 10, 9 . As shown in Fig.…”

“…16 It has been shown that the resistance of the sulfur-derived SEI layer is small. [27][28][29][30] In addition, sulfur-and chlorine-containing additives have been able to improve the performance of the LIB at low temperatures. 27,28 Although there are many theoretical and experimental efforts in this regard, the SEI is still considered the least understood component in LIBs, yet it presents the most critical problems.…”

Low-Temperature Performance Improvement of Graphite Electrode by Allyl Sulfide Additive and Its Film-Forming Mechanism

“…Third, Li 2 S being generated in the first lithiation disappears upon the forthcoming delithiation, revealing that Li 2 S is electrochemically oxidized in the de-lithiation step. 16 The appearance of sulfur compounds (sulfides or disulfides) without etching in the first de-lithiated sample (iii) manifests itself that the film components populated on the topmost layer are dissolved in the 1 st de-lithiation period, therefore S 2p photoelectrons can be emitted without etching. Note that the similar film dissolution upon de-lithiation has been reported elsewhere.…”

“…[12][13][14] At this point, film-forming additives can be used to modify the chemical and physical properties of SEI, leading to better performances of graphite electrodes at low-temperature. 15 Our approach to improve the low-temperature performance involves the addition of sulfur-containing additives, such as elemental sulfur, 16 as a film-forming agent. We have demonstrated that adding a small amount of elemental sulfur (S 8 ) into the graphite electrode enhances low-temperature performances by virtue of the sulfur-containing surface film.…”