The thermal decomposition of lithium-ion battery electrolytes <mml:math display="inline"><mml:mrow><mml:mn>1.0</mml:mn><mml:mspace width="0.3em"></mml:mspace><mml:mi mathvariant="normal">M</mml:mi></mml:mrow></mml:math>1.0M <mml:math display="inline"><mml:mrow><mml:mi mathvariant="normal">Li</mml:mi><mml:mi mathvariant="normal">P</mml:mi><mml:msub><mml:mi mathvariant="normal">F</mml:mi><mml:mrow><mml:mn>6</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>LiPF6 in one or more carbonate solvents has been investigated. Electrolytes containing diethyl carbonate (DEC), ethylene carbonate (EC), a 1:1 mixture of EC/dimethyl carbonate (DMC), and a 1:1:1 mixture EC/DMC/DEC have been investigated by multinuclear nuclear magnetic spectroscopy, gas chromatography with mass selective detection, and size exclusion chromatography. Thermal decomposition affords products including: carbon dioxide <mml:math display="inline"><mml:mrow><mml:mo>(</mml:mo><mml:mi mathvariant="normal">C</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mo>)</mml:mo></mml:mrow></mml:math>(CO2) , ethylene <mml:math display="inline"><mml:mrow><mml:mo>(</mml:mo><mml:mi mathvariant="normal">C</mml:mi><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mi mathvariant="normal">C</mml:mi><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mo>)</mml:mo></mml:mrow></mml:math>(CH2CH2) , dialkylethers <mml:math display="inline"><mml:mrow><mml:mo>(</mml:mo><mml:msub><mml:mi mathvariant="normal">R</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mo>)</mml:mo></mml:mrow></mml:math>(R2O) , alkyl fluorides (RF), phosphorus oxyfluoride <mml:math display="inline"><mml:mrow><mml:mo>(</mml:mo><mml:mi mathvariant="normal">O</mml:mi><mml:mi mathvariant="normal">P</mml:mi><mml:msub><mml:mi mathvariant="normal">F</mml:mi><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub><mml:mo>)</mml:mo></mml:mrow></mml:math>(OPF3) , fluorophosphates <mml:math display="inline"><mml:mrow><mml:mo>[</mml:mo><mml:mi mathvariant="normal">O</mml:mi><mml:mi mathvariant="normal">P</mml:mi><mml:msub><mml:mi mathvariant="normal">F</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mi>OR</mml:mi><mml:mo>,</mml:mo><mml:mspace width="0.3em"></mml:mspace><mml:mi mathvariant="normal">O</mml:mi><mml:mi mathvariant="normal">P</mml:mi><mml:mi mathvariant="normal">F</mml:mi><mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mi>OR</mml:mi><mml:mo>)</mml:mo></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mo>]</mml:mo></mml:mrow></mml:math>[OPF2OR,OPF(OR)2] , fluorophosporic acids <mml:math display="inline"><mml:mrow><mml:mo>[</mml:mo><mml:mi mathvariant="normal">O</mml:mi><mml:mi mathvariant="normal">P</mml:mi><mml:msub><mml:mi mathvariant="normal">F</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mi mathvariant="normal">H</mml:mi><mml:mo>,</mml:mo><mml:mspace width="0.3em"></mml:mspace><mml:mi mathvariant="normal">O</mml:mi><mml:mi mathvariant="normal">P</mml:mi><mml:mi mathvariant="normal">F</mml:mi><mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mi mathvariant="normal">O</mml:mi><mml:mi mathvariant="normal">H</mml:mi><mml:mo>)</mml:mo></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mo>]</mml:mo></mml:mrow></mml:math>[OPF2OH,OPF(OH)2] , and oligoethylene oxides. The mechanism of decomposition is similar in all <mml:math display="inline"><mml:mrow><mml:mi mathvariant="normal">Li</mml:mi><mml:mi mathvariant="normal">P</mml:mi><mml:msub><mml:mi mathvariant="normal">F</mml:mi><mml:mrow><mml:mn>6</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>LiPF6 /carbonate electrolytes. Trace protic impurities lead to generation of <mml:math display="inline"><mml:mrow><mml:mi mathvariant="normal">O</mml:mi><mml:mi mathvariant="normal">P</mml:mi><mml:msub><mml:mi mathvariant="normal">F</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mi>OR</mml:mi></mml:mrow></mml:math>OPF2OR , which autocatalytically decomposes <mml:math display="inline"><mml:mrow><mml:mi mathvariant="normal">Li</mml:mi><mml:mi mathvariant="normal">P</mml:mi><mml:msub><mml:mi mathvariant="normal">F</mml:mi><mml:mrow><mml:mn>6</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>LiPF6 and carbonates. The presence of DEC leads to the generation of ethylene, while the presnce of EC leads to the generation of capped oligothylene oxides <mml:math display="inline"><mml:mrow><mml:mo>[</mml:mo><mml:mi mathvariant="normal">O</mml:mi><mml:mi mathvariant="normal">P</mml:mi><mml:msub><mml:mi mathvariant="normal">F</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mi mathvariant="normal">O</mml:mi><mml:mi mathvariant="normal">C</mml:mi><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mi mathvariant="normal">C</mml:mi><mml:msub><mml:mi mathvariant="normal">H</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mo>)</mml:mo></mml:mrow><mml:mi>n</mml:mi></mml:msub><mml:mi mathvariant="normal">F</mml:mi><mml:mo>]</mml:mo></mml:mrow></mml:math>[OPF2(OCH2CH2)nF] .

The reaction of an electrolyte (1 M in ethylene carbonate/dimethyl carbonate/diethyl carbonate, 1:1:1) with the surface of at various voltages (4.0–5.3 V vs Li) was investigated. Coin cells stored at low voltage (4.0–4.5 V vs Li) show low residual current, and ex situ analysis of the surface by X-ray photoelectron spectroscopy (XPS) and IR spectroscopy suggests low concentrations of electrolyte decomposition products. Coin cells stored at high voltage (4.7–5.3 V vs Li) show increased residual current, and ex situ analysis of the cathode surface by XPS and IR supports the presence of poly(ethylenecarbonate).

Developing electrolytes that enable commercially viable lithium metal anodes for rechargeable lithium batteries remains challenging, despite recent exhaustive efforts. Electrolytes of similar composition, yet different structure, have been investigated to understand key mechanisms for improving the cycling performance of lithium metal anodes. Specifically, the electrolytes investigated include LiPF₆, LiBF₄, lithium bis(oxalato)borate (LiBOB), and lithium difluoro(oxalato)borate (LiDFOB) dissolved in a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC). There is a remarkable difference in the cycling performance of 1.2 M LiDFOB in EC : EMC (3 : 7) compared to 0.6 M LiBF₄ + 0.6 M LiBOB in EC : EMC (3 : 7), despite the effectively equivalent chemical composition. The LiDFOB electrolyte has significantly better cycling performance. Furthermore, the chemical compositions of the SEI generated on the lithium metal electrode from the two electrolytes are very similar, especially after the 1st plating, suggesting that the chemical composition of the SEI may not be the primary source for the difference in cycling performance. Ex situ transmission electron microscopy (TEM) reveals that the difference in cycling performance can be traced to the presence of nanostructured LiF particles in the SEI from the LiDFOB electrolyte. It is proposed that the capping ability of the oxalate moiety from LiDFOB, in combination with simultaneous generation of LiF, leads to generation of uniform and evenly distributed nanostructured LiF particles. The presence of nanostructured LiF in the SEI results in uniform diffusion field gradients on the lithium electrode which leads to improved cycling performance. The proposed mechanism not only provides insight for improving lithium metal anodes for batteries, but also expands upon the understanding of the role of LiF in the SEI on graphite electrodes in commercial lithium ion batteries. A superior understanding of the structure and function of the SEI will facilitate the development of next-generation energy storage systems.

The anode solid electrolyte interface (SEI) on the anode of lithium ion batteries contains lithium carbonate (Li2CO3), lithium methyl carbonate (LMC), and lithium ethylene dicarbonate (LEDC). The development of a strong physical understanding of the properties of the SEI requires a thorough understanding of the evolution of the SEI composition over extended timeframes. The thermal stability of Li2CO3, LMC, and LEDC in the presence of LiPF6 in dimethyl carbonate (DMC), a common salt and solvent, respectively, in lithium ion battery electrolytes, has been investigated to afford a better understanding of the evolution of the SEI. The residual solids from the reaction mixtures have been characterized by a combination of X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy with attenuated total reflectance (IR-ATR) while the solution and evolved gasses have been investigated by nuclear magnetic resonance (NMR) spectroscopy and gas chromatography with mass selective detection (GC-MS). The thermal decomposition of Li2CO3 and LiPF6 in DMC yields CO2, LiF, and F2PO2Li. The thermal decomposition of LMC and LEDC with LiPF6 in DMC results in the generation of a complicated mixture including CO2, LiF, ethers, phosphates, and fluorophosphates.