A Comparative Study of Sulphur-Containing Electrolyte Additives on the Reactivity Between Electrolytes and Charged Electrodes Using Accelerating Rate Calorimetry (ARC)

Abstract

Introduction Recently, electrolyte additives have attracted much attention because they can help improve the lifetime of lithium-ion batteries1,2. Among them, sulphur-containing electrolyte additives are very promising. For example, 1,3,2-dioxathiolane-2,2-dioxide (DTD)1 and prop-1-ene-1,3-sultone (PES)2 can help increase coulombic efficiency (CE) while 1,3,2-dioxathiane 2,2-dioxide (TMS)1 can limit gas production during formation. However, electrolyte additives may have an effect on Li-ion battery safety because these additives modify the interfaces between the electrode materials and the electrolyte. In this work, the reactivity of lithiated graphite or delithiated Li(Ni1/3Mn1/3Co1/3)O2 (NMC) with control electrolytes (1M LiPF6 ethylene carbonate (EC):ethyl methyl carbonate (EMC) 3:7 wt% ratio) or electrolytes containing several sulphur-containing additives was studied, respectively, using accelerating rate calorimetry (ARC). Experimental The ARC sample preparation process was similar to that reported before3. 2325 coin type pellet cells were made using control electrolyte and charged to 4.2 V for the NMC electrodes or discharged to 0.0 V for the graphite electrodes using the protocol described in Reference 3. The ratios between charged electrode materials and electrolyte were 94 mg:30 mg and 140 mg:140 mg for NMC electrodes and graphite electrodes, respectively. The single-point BET surface areas of the graphite and NMC powders were measured with a Micromeritics Flowsorb 2300 instrument. Results and Discussion Table 1 shows the specific surface area results for the NMC and graphite materials used in this experiment.Figure 1 shows the molecular structures of the electrolyte additives were used in this experiment.Figure 2 shows the self-heating rate (SHR) versus temperature for the reaction of lithiated graphite or delithiated NMC with different electrolyte additives. Although there is a short-lived exothermic observation at 50°C, 5% DTD decreases the SHR for lithiated graphite. TMS causes a small exothermic peak at around 75°C but the SHR is very small. Both 5% DTD and 5% TMS help eliminate the exothermic peak at around 100°C resulting from the decomposition of metastable solid electrolyte interface (SEI)4. Furthermore, 2% PES does not dramatically increase the reactivity of the delithiated NMC with electrolyte from the starting temperature to around 250°C and PES helps decrease the SHR compared with the control electrolyte after 250°C. Further experiments showing the impact of these additives in combination with other additives like VC will be reported.In summary, some sulphur-containing electrolyte additives which show good electrochemical performance, such as DTD, TMS etc., should not compromise the safety of lithium-ion batteries.