We have synthesized the reduction products of fluoroethylene carbonate (FEC) and vinylene carbonate (VC) via lithium naphthalenide reduction. By analyzing the resulting solid precipitates and gas evolution, our results confirm that both FEC and VC decomposition products include HCO 2 Li, Li 2 C 2 O 4 , Li 2 CO 3 , and polymerized VC. For FEC, our experimental data supports a reduction mechanism where FEC reduces to form VC and LiF, followed by subsequent VC reduction. In the FEC reduction product, HCO 2 Li, Li 2 C 2 O 4 , and Li 2 CO 3 were found in smaller quantities than in the VC reduction product, with no additional fluorine environments being detected by solid-state nuclear magnetic resonance or X-ray photoelectron spectroscopy analysis. With these additives being practically used in higher (FEC) and lower (VC) concentrations in the base electrolytes of lithium-ion batteries, our results suggest that the di↵erent relative ratios of the inorganic and organic reduction products formed by their decomposition may be relevant to the chemical composition and morphology of the solid electrolyte interphase formed in their presence.

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 strong 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 gases 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.

The reduction products of common lithium salts for lithium ion battery electrolytes, LiPF 6 , LiBF 4 , lithium bisoxalato borate (LiBOB), lithium difluorooxalato borate (LiDFOB), and lithium trifluorosulfonylimide (LiTFSI), have been investigated. The solution phase reduction of different lithium salts via reaction with the one electron reducing agent, lithium naphthalenide, results in near quantitative reactions. Analysis of the solution phase and head space gasses suggests that all of the reduction products are precipitated as insoluble solids. The solids obtained through reduction were analyzed with solution NMR, IR-ATR and XPS. All fluorine containing salts generate LiF upon reduction while all oxalate containing salts generate lithium oxalate. In addition, depending upon the salt other species including, Li x PF y O z , Li x BF y , oligomeric borates, and lithium bis [N-(trifluoromethylsulfonylimino) A typical lithium-ion battery contains a graphite anode, a lithiated transition metal oxide cathode, and an electrolyte solution composed of inorganic lithium salts dissolved in a mixture of organic carbonate solvents which frequently includes electrolyte additives.1 The longterm cyclability of the lithium-ion battery is dependent upon the anode solid electrolyte interphase (SEI), formed due to the electrochemical reduction of the electrolyte solution.2 Understanding the mechanisms of the reduction reactions along with the products of the reactions is essential for the development of better lithium-ion batteries. The SEI has been proposed to contain lithium alkyl carbonates, lithium carbonate, lithium oxalate, lithium alkoxides, and lithium oxide from the carbonate solvents and LiF, lithium fluorophosphates, lithium fluoroborates, and lithium oxalate from the reduction of electrolyte salts, depending upon the salt utilitzed.3-21 Electrolyte additives have also been used to tailor the properties of the SEI through preferential reduction on anode.1 Despite significant effort over the last two decades, the formation mechanism of the SEI is not well understood. One difficulty in understanding the composition of the SEI is that the SEI is a complicated mixture of compounds, which results from multiple simultaneous and competing reduction reactions. In addition, since the SEI is very thin (∼ 50 nm) and unstable in the presence of oxygen or water, characterization is very difficult. We have reported a detailed analysis of binder free graphitic anodes cycled in simplified electrolytes which suggest that the initial reduction reaction of the carbonates generate lithium alkyl carbonates and LiF as the predominant components of the anode SEI.20,21 Synthesis of initial SEI components from carbonate solvents in high yield through reduction of the solvents with lithium naphthalenide has been reported. The unique advantage of this reduction technique is the generation and isolation of SEI constituents from individual electrolyte components in high yield without competing reduction reactions. Reduction of ethylene carbonate re...

No abstract

Thermal behavior of the solid electrolyte interphase (SEI) on a silicon electrode for lithium ion batteries has been investigated by TGA. In order to provide a better understanding of the thermal decomposition of the SEI on silicon, the thermal decomposition behavior of independently synthesized lithium ethylene dicarbonate (LEDC) was investigated as a model SEI. The model SEI (LEDC) has three stages of thermal decomposition. Over the temperature range of 50–300 °C, LEDC decomposes to evolve CO2 and C2H4 gases leaving lithium propionate (CH3CH2CO2Li) and Li2CO3 as solid residues. The lithium propionate decomposes over the temperature range of 300–600 °C to evolve pentanone leaving Li2CO3 as a residual solid. Finally, the Li2CO3 decomposes over 600 °C to evolve CO2 leaving Li2O as a residual solid. A very similar thermal decomposition process is observed for the SEI generated on cycled silicon electrodes. However, two additional thermal decomposition reactions were observed characteristic of Li x PO y F z at 300 °C and the polyimide binder at 550 °C. TGA measurements of Si electrodes after various numbers of cycles suggest that the LEDC on Si electrodes thermally decomposes during cycling to form lithium propionate and Li2CO3, resulting in increased complexity of the SEI.