Decomposition of LiPF[sub 6] and Stability of PF[sub 5] in Li-Ion Battery Electrolytes

Abstract

The decomposition of LiPF 6 and the stability of PF 5 in organic solvents, diethyl carbonate ͑DEC͒, dimethyl carbonate ͑DMC͒, ␥-butyrolactone ͑GBL͒, and ethylene carbonate ͑EC͒, have been investigated through density functional theory ͑DFT͒ calculations, in which solvent was modeled as a dielectric continuum, and also by molecular dynamics ͑MD͒ simulations which treated solvents explicitly. Both calculations showed a similar trend in which the decomposition was further promoted in more polar solvents, yet the DFT calculations predicted an endothermic decomposition, while the MD simulations indicated exothermic. This sharp contrast in the results suggests strong solute-solvent interactions, especially for PF 5 , which were not accounted for in the DFT calculations. The specific interaction between PF 5 and solvent was further investigated by DFT calculations for adduct models and also by the MD simulations for solutions. Both calculations suggest a stable formation of a PF 5 -solvent adduct in solution and its stability depends on the solvent. It was found that PF 5 is more stabilized in polar and sterically compact solvents such as EC and GBL than in less polar and bulky, linear carbonates such as DMC and DEC. The reactivity of PF 5 with organic solvents and the difference in the stability of LiPF 6 between organic and aqueous solution are also discussed.

The mechanism of HF formation in LiPF6 based organic carbonate electrolytes

“…[8,11] Furthermore, quantum chemical calculations demonstrate the influence of the highly positive lithium ion on the solvent coordination sphere of the complex anion. [7] As a consequence, a lowered stability of LiPF 6 in organic carbonate electrolyte is observed, when compared with other alkali metals hexafluorophosphates. The decomposition is not only catalyzed by the elevated temperature nor by the presence of moisture , it is also activated by the release of protons upon formation of LiF either within the Solid electrolyte interphase (SEI) at the negative electrode or within the passivation layer at the positive electrode.…”

“…[1][2][3][4][5] However, despite the relatively high thermal stability in dry inert atmosphere up to 107°C where only solid lithium fluoride and PF 5 are formed [6], this salt suffers from degradation upon exposure to traces of water, moisture or alcohols [7]. In literature it is proposed that the reaction starts with the decomposition of LiPF 6 [7][8][9][10]: (1) to form phosphorous pentafluoride which can react with water to form HF and POF 3 .…”

Thermal stability of LiPF6 salt and Li-ion battery electrolytes containing LiPF6

“…The thermal stability and interaction of this salt with the organic solvents in the electrolytes have been studied experimentally [1][2][3] by Thermogravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC) and Accelerating Rate Calorimetry (ARC), and computationally by Density Functional Theory (DFT) and Molecular Dynamics (MD) methods 4 . However, all the thermal analysis techniques used so far test thermal stability based on macroscopic quantities such as mass loss, heat flow, or self-heating rate measured as a function of temperature.…”

Hydrolysis in the system LiPF6—propylene carbonate—dimethyl carbonate—H2O

“…The products formed, especially HF, have a negative influence on the performance of lithium batteries. There are a number of publications dedicated to the problem of hydrolytic stability of the electrolytes containing lithium hexafluorophosphate, various aprotic solvents and water as well as to the use of water-binding additives [5][6][7][8]. At the same time, the reaction mechanism of the hydrolysis of LiPF 6 in the systems studied is not clear.…”