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The Use of Accelerating Rate Calorimetry (ARC) for the Study of the Thermal Reactions of Li-Ion Battery Electrolyte Solutions J.S

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The Use of Accelerating Rate Calorimetry (ARC) for the Study of the Thermal Reactions of Li-Ion Battery Electrolyte Solutions J.S Journal of Power Sources 119–121 (2003) 794–798 The use of accelerating rate calorimetry (ARC) for the study of the thermal reactions of Li-ion battery electrolyte solutions J.S. Gnanaraja, E. Zinigrada, L. Asrafa, H.E. Gottlieba, M. Sprechera, D. Aurbacha,*, M. Schmidtb aDepartment of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel bMerck KGaA, D-64293 Darmstadt, Germany Abstract The thermal stability of 1M LiPF6, LiClO4, LiN(SO2CF2CF3)2 (LiBETI) and LiPF3(CF2CF3)3 (LiFAP) solutions in mixtures of ethylene carbonate, diethyl carbonate and dimethyl carbonate in the temperature range 40–350 8C was studied by ARC and DSC. NMR was used to analyze the reaction products at different reaction stages. The least thermally stable are LiClO4 solutions. LiPF3(CF2CF3)3 solutions showed higher thermal stability than LiPF6 solutions. The highest thermal stability was found for LiN(SO2CF2CF3)2 solutions. Studies by DSC and pressure measurements during ARC experiments with LiPF6 and LiFAP solutions detected an endothermic reaction, which occurs before a number of exothermic reactions as the temperature increases. Fluoride ions are formed and react with the alkyl carbonate molecules both as bases and as nucleophiles. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Accelerating rate calorimetry (ARC); Differential scanning calorimetry (DSC); Thermal stability; Alkyl carbonate solutions 1. Introduction 2. Experimental Accelerating rate calorimetry (ARC) is an important One molar LiPF6, LiClO4, LiN(SO2CF2CF3)2 and method for studying the thermal behavior of materials LiPF3(CF2CF3)3 solutions in mixture of ethylene carbonate [1,2]. LiPF6 solutions in alkyl carbonates are widely used (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC) in commercial Li-ion batteries in spite of their relatively low (2:1:2 v/v/v) were obtained from Merck KGaA (highly pure, thermal stability. Recently, Merck KGaA introduced a new Li battery grade). salt LiPF3(CF2CF3)3 (LiFAP) as an alternative electrolyte Both an accelerating rate calorimeter (ARC, Arthur D to LiPF6. We explored the possibility of replacing LiPF6 Little Inc. Model 2000) and a differential scanning calori- solutions by LiFAP solutions for Li ion batteries. In the meter (DSC, Mettler Toledo Inc. Model DSC 25) were used. present study ARC was used to explore endothermic In the ARC tests the solutions were heated between 40 and exothermic reactions of Li ion battery electrolyte solu- and 350 8C with 5 8C increments at the rate of 2 8C/min in tions by monitoring both gas pressure and temperature the search for self-heating at the sensitivity threshold of responses. Differential scanning calorimetry (DSC) was 0.02 8C/min. The controller was programmed to wait 15 min also used to complement the thermal data obtained from for the sample and calorimeter temperatures to equilibrate, the studies by ARC. Nuclear magnetic resonance (NMR) and then to search 20 min for a temperature increase of was used to analyze the products of the thermal reactions of 0.02 8C/min. After ARC experiments the bomb was cooled the solutions. with liquid nitrogen till the pressure was slightly above the In addition to the above two salts solutions, solutions atmospheric pressure. The gas was released through a high- containing LiClO4 and LiN(SO2CF2CF3)2 (LiBETI) were pressure valve, specially designed for this purpose. DSC tests also studied. were carried out at a heating rate of 2 8C/min in the tem- perature range 30–400 8C. We have used 1H, 13C, 31P and 19F * Corresponding author. Tel.: þ972-3-5318317; fax: þ972-3-5351250. NMR and GCMS to analyze the reaction products at different E-mail address: [email protected] (D. Aurbach). reaction stages. Two milliliters of a solvent mixture or a Li 0378-7753/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0378-7753(03)00255-6 J.S. Gnanaraj et al. / Journal of Power Sources 119–121 (2003) 794–798 795 salt solution were placed in a titanium spherical bomb (8 ml higher and the maximal pressure is 200 psi lower for the volume) in an argon filled glove box and were transferred to LiFAP solution than for the LiPF6 solution. the ARC under a highly pure Ar atmosphere. High-pressure A small exotherm between 325 and 350 8C and one gold plated stainless crucibles, 30 ml in volume, were used in process with a pressure development between 250 and the DSC tests. The crucibles were filled by three micro liters 350 8C were detected upon heating the solvent mixture of a sample and were closed in the glove box. (Fig. 2). The exothermic reaction of the LiClO4 solution with its onset at 220 8C(Fig. 2c) leads to an explosion. The highest thermal stability was found with the LiBETI solu- 3. Results and discussion tions, which have the highest exothermic onset at 230 8C and the lowest self-heating rate (about of 1 8C/min). Exothermic We measured both pressure and temperature rise during decomposition of the LiFAP solution starts at temperatures the decomposition reactions of the electrolyte solution. We only 10 8C higher than that of LiPF6 solutions however the identified both endothermic and exothermic reactions. The self-heating rate of the LiFAP solutions is considerably endothermic reactions could be identified by gas evolution higher in the temperature range 210–290 8C(Fig. 2a and b). with no exotherms in the ARC measurements and by DSC. NMR measurements of the thermal reaction products indi- Fig. 1 shows the change in pressure measured when a cate higher thermal stability of the P–F bonds in LiFAP solvent mixture and solutions containing different lithium solutions than the P–F bonds of LiPF6 solutions. DSC salts were heated between 40 and 350 8C. The minor pres- measurements (Fig. 3) correlate well with both the NMR sure increase, which starts near 60 8C(Fig. 1), cannot be and the ARC measurements showing higher thermal stability related to any chemical interaction between salts and the of LiFAP solutions than of LiPF6 solutions. The DSC mea- solvents because solvents without salt show exactly the same surements also showed that the LiBETI solutions have the behavior (Fig. 1). The pure solvents mixture shows a sig- highest thermal stability. nificant pressure increase near 250 8C while some electro- The lowest onset (200 8C) for an exothermic reaction was lyte solutions show a remarkable change in pressure below observed for LiPF6 solutions. Fig. 2a shows a self-heating 200 8C. We suggest that all salts react with the solvents rate (SHR) with a maximum value of 5.4 8C/min and a leading to the formation of gaseous products. pressure development rate (PDR) with a maximum value of Explosion was observed with lithium perchlorate solution 32 psi/min when LiPF6 solutions are heated during ARC near 220 8C, resulting in the formation of small black tests. It is clear from Figs. 1 and 2 that while the exothermic particles inside the pipes of the pressure measuring system. reactions of LiPF6 solutions start at 200 8C there are two The smallest pressure increase (about 110 psi) was mea- processes in which pressure is developed: the process which sured with the LiBETI solution. starts at 170 8C and during the exothermic reaction which Solutions containing LiPF6 developed the highest pres- starts above 200 8C. sure (about 1700 psi) during the heating tests by ARC and The pressure rise at 170 8C can be attributed not only to À the lowest temperature onset, around 170 8C, for the first the release of PF5 from the PF6 (Eq. (1)) but also to the process, which develops pressure, when compared to other production of ethylene from DEC, in an elimination reaction electrolyte solutions. The pressure versus T plot for the with fluoride ion acting as a strong base in aprotic solvent LiFAP solution is similar to that for the LiPF6 solution. (Eq. (2)). The occurrence of this elimination explains the However, the onset of the pressure development is 30 8C loss of condensed material during the reaction as well as the significantly lower stability of DEC solutions when com- pared to EC or DMC solutions [3–5]. LiPF6 ! LiF þ PF5 ðgÞ (1) À CH3CH2ÀOÀCOÀOÀCH2CH3 þ F À ! CH3CH2ÀOÀCOÀO þ HF þ CH2¼CH2 (2) RÀOÀCOÀOÀR þ FÀ ! RÀOÀCOÀOÀ þ RÀF (3) À À RÀOÀCOÀO ! RÀO þ CO2 (4) À À RÀO þ PF5 ! RÀOÀPF4 þ F (5) Concentrating on the LiPF6 solutions, the pressure build up starts at 170 8C(Figs. 1 and 2a) and the pressure develop- ment rate (PDR) versus T plot shows a peak at ca. 210 8C (Figs. 1 and 2a). The SHR versus T plots however show no peak before 190 8C. It is clear from the NMR results Fig. 1. Pressure vs. temperature profiles for 1M LiPF6, LiPF3(CF2CF3)3, LiClO4 and LiN(SO2CF2CF3)2 solutions in a mixture of EC:DEC:DMC that most of the species with P–F bonds still exist and are (2:1:2 v/v/v) and for the solvents mixture without Li salt. stable at temperatures below 180 8C. At 180 8C, there is a 796 J.S. Gnanaraj et al. / Journal of Power Sources 119–121 (2003) 794–798 Fig. 2. Self-heating rate and pressure developing rate profiles for 1M electrolyte solutions in EC:DEC:DMC (2:1:2 v/v/v): (a) LiPF6; (b) LiPF3(CF2CF3)3; (c) LiClO4; (d) LiN(SO2CF2CF3)2; (e) the solvents mixture with no salt added. transesterification of dimethyl and diethyl carbonates lead- remain in the condensed phase and none of the original ing to the formation of methyl ethyl carbonate. Traces of solvent molecules survive in the sample.
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