We report herein on the possibility of using ionic liquids (ILs) as additives to conventional electrolyte solutions, based on alkyl carbonates and LiPF6 for attenuating thermal reactions in Li battery systems. As a model, a Li–Li0.5CoO2 system was used. The ionic liquids chosen included cations based on derivatives of pyrrolidinium and imidazolium, and the anions bioxalato borate (C4O8B−, BOB), (CH3SO2)2N− (TFSI), and PF3(C2S5)3− (FAP). The thermal behavior of solutions alone, solutions with Li metal, Li0.5CoO2 and Li metal + Li0.5CoO2 was studied. It was found that the presence of 10% of ILs, with derivatives of pyrrolidinium cations and FAP or TFSI anions in standard EC–DMC/LiPF6 solutions, improves considerably the thermal stability of Li0.5CoO2 in electrolyte solutions. The onset temperatures of the thermal reactions of Li0.5CoO2 with solution species are higher and their heat evolution is considerably lower, when they contain these ionic liquids as additives. This finding opens the door for further studies and optimization of the use of selected ILs as additives that may improve the safety features of Li-ion batteries.

We examined the possible use of the following ionic liquids all having the same anion, bis(trifluoromethylsulfonyl)imide (TFSI) and the following cations: 1-hexyl-3-methyl imidazolium (HMITFSI), 1-(2-methoxyethyl)-3-methyl imidazolium (MEMITFSI), N-ethyl-NN-dimethyl-2-methoxyethylammonium (EDMETFSI), 1-methyl-1-butylpyrrolidinium (BMPTFSI), and 1-methyl-1-propylpiperidinium (MPPpTFSI) solutions with LiTFSI (the source of Li ions), as electrolyte systems for 5V, rechargeable battery systems with Li metal anodes and LiMn1.5Ni0.5O4 spinel cathodes. Standard solution based on alkyl carbonates and LiPF6 was examined in this respect for comparison. The ionic liquids (ILs) based on derivatives of piperidinium and pyrrolidinium demonstrate a very wide electrochemical window (up to 5.5V) and they can be compatible with lithium metal anodes. At low potentials in the presence of Li ions in solutions (or on Li metal surfaces), TFSI anions are reduced to insoluble Li compounds which passivate Li, noble metal and graphite electrodes in the Li salt/IL solutions. The mechanism, kinetics and effectiveness of electrodes’ passivation in these systems depend on the nature of both IL and electrode used. It was possible to demonstrate reversible behavior of Li/LiMn1.5Ni0.5O4 cells (4.8V) with solutions based on BMPTFSI and MPPpTFSI. Possible parasitic anodic reactions upon charging at the high potentials are much lower in the ILs than in standard solutions.

We review herein several important aspects of surface chemistry in Li-ion batteries, and discuss the use of ionic liquids (ILs) for rechargeable Li batteries. We explored the suitability of ILs for 5V cathodes and Li-graphite anodes. Some advantages of the use of ILs to attenuate the thermal behavior of delithiated cathode materials are demonstrated. We also report briefly on a comparative study of the following cathode materials: LiNi0.5Mn0.5O2; LiNi0.33Mn0.33Co0.33O2; LiNi0.4Mn0.4Co0.2O2; LiNi0.8Co0.15Al0.05O2 and LiMnPO4, in standard electrolyte solutions based on mixtures of alkyl carbonates and LiPF6. We also discuss aging, rate capability, cycle life and surface chemistry of these cathode materials. The techniques applied included electrochemical measurements, e.g., XRD, HRTEM, Raman spectroscopy, XPS and FTIR spectroscopy. We found that ILs based on cyclic quaternary alkyl ammonium cations may provide much better electrolyte solutions for 5V cathodes than standard electrolyte solutions, while being quite suitable for Li-graphite electrodes. All the lithiated transition metal oxides studied (as mentioned above) develop unique surface chemistry during aging and cycling due to the acid-base and nucleophilic reactions of their surface oxygen anions. LiMn0.33Ni0.33Co0.33O2 has the highest rate capability compared to all the other above-mentioned cathode materials. Cathodes comprising nanometric size carbon-coated LiMnPO4 produced by HPL demonstrate a better rate capability than LiNi0.5Mn0.5O2 and LiNi0.8Co0.15Al0.05O2 cathodes. The former material seems to be the least surface reactive with alkyl carbonates/LiPF6 solutions, among all the cathode materials explored herein.

Merck KGaA developed LiPF3(CF2CF3)(3), (LiFAP) as a new electrolyte that can replace the commonly used LiPF6 in Li-ion batteries. Vinylene carbonate and Li salicylato borate (Merck's AD25) were studied as additives for LiFAP solutions in mixtures of ethylene, dimethyl, and diethyl carbonates with composite graphite and LiMn2O4 electrodes. The tools for this study included voltammetry (fast and slow scan rates), chronopotentiometry, impedance spectroscopy, electron microscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. It was found that LiFAP solutions containing VC were superior for both graphite and LiMn2O4 (spinel) electrodes. The effect of additives on the electrodes' performance can be clearly attributed to their impact on the surface chemistry of these electrodes. (C) 2003 The Electrochemical Society.

The thermal stability of 1 M LiPF6 solutions in mixtures of ethylene carbonate, diethyl carbonate, and dimethyl carbonate in the temperature range of 40 to 350 degreesC was studied by accelerating rate calorimeters (ARC) and differential scanning calorimeters (DSC). Nuclear magnetic resonance (NMR) was used to analyze the condensed reaction products at different reaction stages. Studies by DSC and pressure measurements during ARC experiments with LiPF6 solutions detected a gas-releasing endothermic reaction starting at similar to170 degreesC, involving diethyl carbonate which occurs before a number of exothermic reactions, which follow as the temperature increases. Fluoride ions are released and react with the alkyl carbonate molecules both as bases and as nucleophiles. The bulk analysis by NMR shows that HO-CH2-CH2-OH, FCH2CH2-OH, F-CH2CH2-F, and polymer are major products. Gas analysis by NMR and Fourier transform infrared spectroscopy shows PF5, CO2, CH3F, CH3CH2F, and H2O as major gaseous products of the thermal reaction of these solutions. (C) 2003 The Electrochemical Society.

The thermal stability of 1M LiPF6, LiClO4, LiN(SO2CF2CF3)2 (LiBETI) and LiPF3(CF2CF3)3 (LiFAP) solutions in mixtures of ethylene carbonate, diethyl carbonate and dimethyl carbonate in the temperature range 40–350°C was studied by ARC and DSC. NMR was used to analyze the reaction products at different reaction stages. The least thermally stable are LiClO4 solutions. LiPF3(CF2CF3)3 solutions showed higher thermal stability than LiPF6 solutions. The highest thermal stability was found for LiN(SO2CF2CF3)2 solutions. Studies by DSC and pressure measurements during ARC experiments with LiPF6 and LiFAP solutions detected an endothermic reaction, which occurs before a number of exothermic reactions as the temperature increases. Fluoride ions are formed and react with the alkyl carbonate molecules both as bases and as nucleophiles.

LiPF3(CF2CF3)(3) from Merck KGaA (LiFAP) was tested as a new electrolyte for Li-ion batteries that can replace the commonly used LiPF6. The latter salt is known to be unstable, to decompose thermally to LiF and PF5, and to readily undergo hydrolysis with protic species to form HF contamination in solutions. The latter contamination may have a detrimental impact on the performance of both anodes and cathodes for Li-ion batteries. Solutions comprising LiFAP, LiPF6, and LiN(SO2CF2CF3)(2) (LiBETI) in mixtures of ethylene, dimethyl, and diethyl carbonates were tested with composite graphite and LiMn2O4 electrodes. The tools for this study included voltammetry (fast and slow scan rates), chronopotentiometry, impedance spectroscopy, Fourier transform infrared, and X-ray and photoelectron spectroscopies. It was found that LiFAP is superior to LiPF6 as an electrolyte for both graphite anodes and LiMn2O4 cathodes. This should be attributed to the different surface chemistry developed on these electrodes when LiPF6 is replaced by LiFAP. An important impact of such a replacement is probably the absence of possible pronounced HF contamination in LiFAP solutions. (C) 2003 The Electrochemical Society.

Electrolyte solutions comprising a mixture of LiPF6 and LiPF3(CF2CF3)3 (LiFAP) in alkyl carbonates (ethylene, dimethyl and diethyl carbonate) were found to be superior to single salt LiFAP or LiPF6 solutions for lithium–graphite anodes at elevated temperatures. Graphite electrodes could be cycled (Li insertion–deinsertion) more than hundred times at 80 °C with high and stable capacity in the two-salt solutions, while in the single-salt solutions this was impossible. Preliminary studies by voltammetry and impedance spectroscopy indicate that the combination of the two salts in solution has a unique influence on the electrodes surface (not yet defined). Thermal studies by accelerating rate and differential scanning calorimetry show that thermal decomposition of LiFAP solutions has a higher onset, but very high heat and pressure developing rates, compared to LiPF6 solutions. The presence of LiPF6 in LiFAP solutions decreased their self-heating and pressure-developing rates pronouncedly. From product analysis of the thermal reactions by NMR, FTIR and MS, we can suggest possible unique bulk reactions that occur in LiPF6–LiFAP solutions. One of these is a nucleophilic reaction between F− and PF3(CF2CF3)3−, which may neutralize the effect of trace HF in solutions (thus forming new P–F bonds and HCF2CF3). Such a reaction should have a positive effect on both the performance of the Li–graphite electrodes and the thermal behavior of the solutions.

Lithium fluoroalkyl phosphates (LiFAP) are described as a new class of electrolyte salts with remarkable properties for applications in Li cells. While LiFAP electrolytes possess high ionic conductivities close to established LiPF6-based systems, the striking benefits of LiFAP are excellent electrochemical stability in combination with superior stability towards hydrolysis.

In this work, we studied the impact of some factors on the behavior of practical electrodes of Li-ion batteries. These included elevated temperatures (45–80°C), prolonged storage of Li-ion cells, and additives in the electrolyte solution. The Li-ion battery systems studied included negative electrodes (anodes) comprising of mesocarbon microbeads (MCMB) and mesocarbon fibers (MCF), and LixCoO2 positive electrodes (cathodes) in an ethylene carbonate (EC)/ethyl-methyl carbonate (EMC) (1:2)/LiPF6 1M solution. Vinylene carbonate (VC) and a Li-organo-borate complex (Li-OBC) were tested as additives. It is shown that the electrochemical response of Li–C negative electrodes depends on the structure of the surface films controlling their behavior, which change upon storage, temperature, and cycling. We established that impedance of these electrodes increased with storage time due to the enrichment of the surface films by LiF and other fluorine-containing species. The capacity fading of the LixCoO2 electrodes in cycling/storage processes at elevated temperatures relates mostly to surface phenomena, whereas the bulk structural characteristics of the electrodes do not change.

EC-DMC-DEC solutions comprising LiPF3(CF2CF3)(3) (LiFAP), LiPF6 and LiN(SO2CF2CF3)(2) (LiBETI) were tested with graphite and LiMn2O4 electrodes. Cyclic voltammetry (CV, fast and slow scan rates), chronopotentiometry, impedance spectroscopy, surface sensitive FTIR and XPS were used for this study. It was found that the new salt LiFAP is a promising candidate for use in rechargeable Li-ion batteries. The thermal behavior of these electrolyte solutions was also studied using accelerating rate calorimetry (ARC). It was found that LiFAP solutions are more stable than LiPF6 solutions while LiBETI solutions have the highest thermal stability. (C) 2003 Elsevier Science B.V. All rights reserved.

The effort to develop improved electrolytes that satisfy the requirements of lithium rechargeable batteries has intensified the search for new and advanced electrolytes. The challenges are the optimization of parameters such as long-term cycling stability, rate capability, low temperature performance and safety features. At least the first two aspects are mainly influenced by the electrolyte, Consequently, with the aim to improve the overall performance of lithium ion batteries, one has to address surface film formation and bulk properties of the electrolyte. With the special quality of vinylene carbonate (VC), and the new salt, lithium fluoroalkylphosphate Li[(C2F5)(3)PF3], we discuss the properties of the new compounds for advanced electrolytes used in high-energy lithium ion batteries. Results of the electrochemical studies in comparison to standard vinylene carbonate with respect to LiPF6 are reported. In addition, the influence of Li[(C2F5)(3)PF3] on the flammability of non-aqueous electrolytes is demonstrated. (C) 2002 Elsevier Science B.V. All rights reserved.

Vinylene carbonate (VC) was tested as an additive to electrolyte solutions for Li-ion batteries. For the model electrodes, synthetic graphite was chosen as the anode material, while LiMn2O4 spinel and LiNiO2 were chosen as the cathode materials. The test solution was 1 M LiAsF6 in a 1:1 mixture of ethylene and dimethyl carbonates (EC–DMC). Cyclic voltammetry (CV), chronopotentiometry, impedance spectroscopy, electrochemical quartz crystal microbalance (EQCM), FTIR and X-ray photoelectron spectroscopies have been used in this study. It was found that VC is a reactive additive that reacts on both the anode and the cathode surfaces. The influence of this additive on the behavior of Li–graphite anodes is very positive, since it improves their cyclability, especially at elevated temperatures, and reduces the irreversible capacity. The spectroscopic studies indicate that VC polymerizes on the lithiated graphite surfaces, thus forming poly alkyl Li-carbonate species that suppress both solvent and salt anion reduction. The presence of VC in solutions reduces the impedance of the LiMn2O4 and LiNiO2 cathodes at room temperature. However, we have not yet found any pronounced impact of VC on the cycling behavior of the cathodes, either at room temperature or at elevated temperatures. Thus, VC can be considered as a desirable additive for the anode side in Li-ion batteries, one which has no adverse effect on the cathode side.

N,N-Bis(trifluoromethyl)perfluoroalkanesulfonamides can be synthesized via electrochemical fluorination in anhydrous hydrogen fluoride (Simons process). These compounds are convenient starting materials for the generation of N-(CF3)(2) anion. The anodic substitution reactions of organic compounds with N-(CF3)(2) anion were studied.

The effort to develop improved electrolytes that satisfy the requirements of lithium rechargeable batteries has intensified the search for new conducting salts having an improved chemical and electrochemical stability. With lithium fluoroalkylphosphates, we introduce a new class of conducting salts for electrolytes for high energy lithium-ion batteries. The results of electrochemical studies of Li[(C2F5)3PF3] in organic carbonates in comparison to LiPF6 including electrochemical stability and charge–discharge efficiency are reported. In addition, the influence of perfluorinated alkyl groups on stability towards hydrolysis is demonstrated.

The electroanalytical behavior of LixNiO2 and LixCo0.2Ni0.8O2 was studied by simultaneous application of slow-scan rate cyclic voltammetry (SSCV), potentiostatic and galvanostatic intermittent titration (PITT and GITT), and electrochemical impedance spectroscopy (EIS). Application of a finite-space diffusion model for treating the results obtained by these techniques allowed us to calculate the diffusion coefficient of Li ions (D) and the differential (incremental) capacity (Cint) as functions of the electrode’s potential. Our final purpose was to compare D versus E and Cint versus E plots for both the electrodes, in order to correlate the observed difference in their electroanalytical behavior with the clear distinction in the related Li-insertion mechanisms deduced from XRD studies. While Li insertion into LixCo0.2Ni0.8O2 exhibits a single-phase reaction upon charge in the 3.0–4.08V (versus Li/Li+) range, Li intercalation into LixNiO2 undergoes two-phase transitions in the same potential range. The shape of both plots, D versus E and Cint versus E for these electrodes, is discussed in the framework of a finite-space diffusion model and Li-insertion processes that can be described by Frumkin-type intercalation isotherms with short-range attraction interactions among intercalation sites.

The authors report herein on the comparative study of LiNiO{sub 2} and LiMn{sub 2}O{sub 4} electrodes in three salt solutions, namely, LiAsF{sub 6}, LiPF{sub 6}, and LiC(SO{sub 2}CF{sub 3}){sub 3} in a mixture of the commonly used ethylene and dimethyl carbonates. The surface chemistry of the electrodes in these solutions was studied by surface-sensitive Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and energy-dispersive X-ray analysis, and their electrochemical behavior was studied by variable-scan-rate voltammetry and impedance spectroscopy. It was found that the electrochemical behavior of these electrodes is strongly dependent on their surface chemistry. Complicated reactions between the active mass and solution components, which include the solvents, the salt anions, and unavoidable contaminants such as HF and perhaps, HSO{sub 3}CF{sub 3}, lead to the precipitation of surface films through which the Li ion has to migrate in order to reach the active mass. The impedance spectroscopy of these electrodes clearly reflects their surface chemistry. It demonstrates the serial nature of the Li insertion-deinsertion processes, which includes, in addition to solid-state diffusion and accumulation, Li-ion migration through surface films and their charge transfer across the surface film/active mass interface, which strongly depends on the chemical composition of the surface films andmore » hence, the solution chosen. LiNiO{sub 2} is considerably more reactive with these solutions than LiMn{sub 2}O{sub 4}, probably due to its stronger nucleophilic nature. In addition, in LiPF{sub 6} solutions, the electrodes' impedance is higher due to precipitation of films comprising LiF, which is highly relative to Li ion transport (probably produced by reactions of the Li{sub x}Mo{sub y} active mass with trace HF).« less