The thermal decomposition of lithium-ion battery electrolytes 1.0M LiPnormalF6 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 (CnormalO2) , ethylene (CnormalH2CnormalH2) , dialkylethers (normalR2O) , alkyl fluorides (RF), phosphorus oxyfluoride (OPnormalF3) , fluorophosphates [OPnormalF2OR,OPF(OR)2] , fluorophosporic acids [OPnormalF2OH,OPF(OH)2] , and oligoethylene oxides. The mechanism of decomposition is similar in all LiPnormalF6 /carbonate electrolytes. Trace protic impurities lead to generation of OPnormalF2OR , which autocatalytically decomposes LiPnormalF6 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 [OPnormalF2(OCnormalH2CnormalH2)nF] .

A solid electrolyte interphase (SEI) is generated on the anode of lithium-ion batteries during the first few charging cycles. The SEI provides a passivation layer on the anode surface, which inhibits further electrolyte decomposition and affords the long calendar life required for many applications. However, the SEI remains poorly understood. Recent investigations of the structure of the initial SEI, along with changes which occur to the SEI upon aging, have been conducted. The investigations provide significant new insight into the structure and evolution of the anode SEI. The initial reduction products of ethylene carbonate (EC) are lithium ethylene dicarbonate (LEDC) and ethylene. However, the instability of LEDC generates an intricate mixture of compounds, which greatly complicates the composition of the SEI. Mechanisms for the generation of the complicated mixture of products are presented along with the differences in the SEI structure in the presence of electrolyte additives.

The surface reactions of electrolytes with the graphitic anode of lithium ion batteries have been investigated. The investigation utilizes two novel techniques, which are enabled by the use of binder-free graphite anodes. The first method, transmission electron microscopy (TEM) with energy dispersive X-ray spectroscopy, allows straightforward analysis of the graphite solid electrolyte interphase (SEI). The second method utilizes multi-nuclear magnetic resonance (NMR) spectroscopy of D 2 O extracts from the cycled anodes. The TEM and NMR data are complemented by XPS and FTIR data, which are routinely used for SEI studies. Cells were cycled with LiPF 6 and ethylene carbonate (EC), ethyl methyl carbonate (EMC), and EC/EMC blends. This unique combination of techniques establishes that for EC/LiPF 6 electrolytes, the graphite SEI is ∼50 nm thick after the first full lithiation cycle, and predominantly contains lithium ethylene dicarbonate (LEDC) and LiF. In cells containing EMC/LiPF 6 electrolytes, the graphite SEI is nonuniform, ∼10−20 nm thick, and contains lithium ethyl carbonate (LEC), lithium methyl carbonate (LMC), and LiF. In cells containing EC/EMC/LiPF 6 electrolytes, the graphite SEI is ∼50 nm thick, and predominantly contains LEDC, LMC, and LiF.

There is significant interest in the development of higher energy lithium-ion batteries for electric vehicles, hybrid electric vehicles, and aerospace applications. One method of improving the energy density of lithium-ion batteries is to increase the operating voltage of the cells by increasing the working potentials of positive electrode employing, for example, lithium nickel manganese spinel ͓LiNi 0.5 Mn 1.5 O 4 ͑LNMS͔͒ as the active material.1 However, cycling of lithium-ion cells to high voltages ͓ϳ5.0 V vs lithium reference electrode ͑LRE͔͒ proceeds with relatively low ͑99% and less͒ coulombic efficiency.2 Among the primary contributors to the poor cycling efficiency are the electrochemical oxidation reactions of the electrolytes at the high positive potentials of the positive electrode. 4,6 The surface-modified cathodes have superior cyclability compared to the uncoated cathode, suggesting that the coatings form a stable passivation layer or a solid electrolyte interface ͑SEI͒ on the cathode. An alternative approach using sacrificial electrolyte additives for in situ formation of a cathode SEI has also been reported to improve cycling to 4.9 V vs LRE. 7 There have been several investigations of the structure of cathode surface films formed under standard cycling ͑Ͻ4.5 V vs LRE͒ and aging conditions. 8-10 However, there have not been any detailed investigations of the cathode surface film structure as a function of electrode potential, especially at high voltage ͑Ͼ4.5 V vs LRE͒. Problems associated with reactions of the electrolyte on the surface of high voltage cathode materials have been reported to limit the application of these interesting materials.2,4 We present here an investigation of the voltagedependent electrochemical reactions in 1 M LiPF 6 in ethylene carbonate ͑EC͒/diethyl carbonate ͑DEC͒/dimethyl carbonate ͑DMC͒ ͑1/ 1/1 vol͒ on an LNMS-based electrode and the characterization of the surface species by XPS and Fourier transform infrared spectroscopy with attenuated total reflectance ͑FTIR-ATR͒. Our results suggest that poly͑ethylenecarbonate͒ ͑PEC͒ derived from the oxidative polymerization of EC is the primary component of the cathode SEI upon high voltage cycling. ExperimentalCoin-type cells with mixed metal oxide-based positive and lithium-metal negative electrodes and filled with 1 M solution of LiPF 6 in EC/DMC/DEC ͑1/1/1 vol͒ were used in the experiments. Two tests were undertaken. In the first one, a lithium-free material, nickel manganese mixed oxide ͑Ni 0.5 Mn 1.5 O 3 ͒, as the positive electrode mimicking the LNMS material was employed in two-electrode half-cells. The binder in the first set was based on poly͑vinylidene flouride͒ ͑PVDF, Kureha 1910͒. In the second experiment, the cathode was based on an LMNS adhered by a fluorine-free binder, ethylene propylene diene monomer ͑EPDM, Vistalon 2504͒ rubber, dissolved in toluene. Both experiments used two-electrode half-cells with aluminum current collectors. The electrodes contained 81% of active material and increased fraction ͑8%͒ of ...

The solution structures of organic carbonate solvents (ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), and diethyl carbonate (DEC)) as electrolyte solutions of LiPF 6 were investigated with FTIR and NMR spectroscopy and DFT computational methods. Both coordinated and uncoordinated solvents are observed by IR spectroscopy, allowing the determination of solvent coordination numbers, which a range from 2 to 5. The predominant species in solution changes as a function of LiPF 6 concentration. At low salt concentrations (<1.2 M), the predominant species is a solvent-separated ion pair, whereas at high salt concentrations (>2.0 M) the predominant species in solution is the contact ion pair. In mixed solvent systems (PC−DMC, PC−DEC, EC−DMC, or EC−DEC), the mixed solvated cations are observed in the presence of high concentrations of uncoordinated cyclic carbonate despite the much larger dielectric constant of the cyclic carbonates than dielectric constant of linear carbonate.