The Rate Equation of Decomposition for Electrolytes with LiPF₆in Li-Ion Cells at Elevated Temperatures

Abstract

Organic compounds that are unstable at high temperatures are used as electrolytes in Li-ion cells. Therefore, the heat generation by chemical reactions of these organic compounds within the cells is an important factor to be considered. The thermal stabilities of 1M LiPF 6 /PC and 1M LiPF 6 /EC+DMC (1:1 in vol.) electrolytes used in lithium cells were measured by differential scanning calorimetry (DSC) using airtight containers. Rate equations, which explain the heat generation, were studied. The salt LiPF 6 is in equilibrium with LiF and PF 5 in the electrolyte solutions. As a mechanism of electrolyte decomposition, Sloop et al. showed that the PF 5 reacts with solvents, generating heat. Based on Sloop's mechanism and Wang's rate equation for thermal LiPF 6 decomposition, rate equations were developed, and the rate of heat generation as a function of temperature was calculated using the rate equations at DSC scan rate of 3, 7, 10, 12, 15, 17, and 20 • C min −1 . Unfortunately, these calculated curves did not fit the experimental data well. Therefore, an additional side reaction of PF 5 , which did not contribute to heat generation, was assumed. With the inclusion of this side reaction, the calculated curves showed good agreement with the experimental data. Li-ion cells are widely used as power sources for portable electric devices and electric vehicles (EV). However, the safety of Li-ion cells remains an important issue where improvements can be made. Unstable organic compounds are used as electrolytes of Li-ion cells, and these compounds may undergo exothermic reactions at elevated temperatures. Therefore, the generation of heat by chemical decomposition and chemical reactions in the cells is an important factor to be considered. Safety testing of Li-ion cells has been reported by some laboratories. [1][2][3][4][5] In addition, the thermal stability or thermal behavior of Li-ion cells has been investigated by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and accelerating rate calorimetry (ARC). 1,[6][7][8][9][10] Because the electrolytes themselves decompose and cause exothermic reactions even without a cathode and an anode, the thermal stability of the electrolytes themselves has been investigated. [11][12][13][14] Lithium hexafluourophosphate (LiPF 6 ) is in equilibrium with LiF and PF 5 in organic electrolyte solutions. Sloop et al. [15][16] showed that the product from the equilibrium, PF 5 , reacts with ethylene carbonate (EC) and dimethyl carbonate (DMC) solvents at room temperature. Wang et al.17 studied the kinetic properties of thermal decomposition reaction of LiPF 6 .To ensure the safety of the batteries it is very important to avoid the risk of fire. Mathematical simulation, 18-25 which describes the heat production in a cell, is a good approach. The simulations predict the thermal behavior of a cell. However, the heat production in the simulation is not described by a rate equation. Therefore, the risk of fire predicted from the simulation is unreliable, because the r...

Effect of Hexafluorophosphate and Fluoroethylene Carbonate on Electrochemical Performance and the Surface Layer of Hard Carbon for Sodium‐Ion Batteries

“…In Equation (3), NaPF 6 is in equilibrium with NaF and PF 5 in the electrolyte solutions, and in Equation (4) NaPF 6 is decomposed into a gaseous product, PF 5 , and NaF as a solid product. In addition, strong Lewis acid PF 5 acts as a strong reducing agent with PC and EC solvents, leading to further electrolyte decomposition . These reactions are consistent with the HAXPES results, which show that the surface layer formed in the NaPF 6 ‐based electrolyte is different from that of the NaClO 4 ‐based electrolyte.…”

“…In addition, strong Lewis acid PF 5 acts as as trong reducing agent with PC and EC solvents, leading to further electrolyte decomposition. [38] These reactions are consistentw ith the HAXPES results, which show that the surfacel ayer formed in the NaPF 6based electrolyte is different from that of the NaClO 4 -based electrolyte.…”

“…Hence, intensified peaks of NaF, Na x PF y , and Na x PO y F z for EC/PC electrolyte indicate that NaPF 6 is further decomposed at the hard‐carbon surface. According to the decomposition scheme from LiPF 6 in the Li‐ion electrolyte, the decomposition mechanism of NaPF 6 in the electrolyte is proposed to follow the reactions given by Equations (1)—: …”

“…The P1ss pectrum can be deconvolutedi nto at least three components:r esidual NaPF 6 and its decomposed products, Na x PF y and Na x PO y F z at 2150.1 and2 151.6 eV,r espectively.H ence, intensified peaks of NaF,N a x PF y ,a nd Na x PO y F z for EC/PC electrolyte indicate that NaPF 6 is furtherd ecomposed at the hard-carbon surface. According to the decomposition scheme from LiPF 6 in the Li-ion electrolyte, [2,[36][37][38] the decomposition mechanism of NaPF 6 in the electrolyte is proposedt of ollow the reactions given by Equations (1)-(4): [31]…”

Ionic Liquid–Organic Carbonate Electrolyte Blends To Stabilize Silicon Electrodes for Extending Lithium Ion Battery Operability to 100 °C

“…1,2 Moreover, when these solvents are used with Li salts, such as lithium hexafluorophosphate (LiPF 6 ), a resistive film forms on the electrode surface affording poor cycle life. 3,4 These side reactions become more dominating at higher temperatures as the rate of chemical reaction between the dissolved lithium salt and electrolyte solvent increases. 3−5 Thus, there is an unmet need for alternative electrolytes with superior thermal and chemical stability to expand the use of LIBs to a wider working temperature range without compromising the electrochemical performance.…”