Ionic liquids (IL's) were proposed for use in Li-ion batteries (LIBs), in order to mitigate some of the well-known drawbacks of LiPF6/mixed organic carbonates solutions. However, their large cations seriously decrease lithium transference numbers and block lithium insertion sites at electrode-electrolyte interfaces, leading to poor LIB rate performance. Deep eutectic electrolytes (DEEs) (which share some of the advantages of ILs but possess only one cation, Li+), were then proposed, in order to overcome the difficulties associated with ILs. We report herein on the preparation, thermal properties (melting, crystallization, and glass transition temperatures), transport properties (specific conductivity and viscosity) and thermal stability of binary DEEs based on mixtures of lithium bis(trifluoromethane)sulfonimide or lithium bis(fluoro)sulfonimide salts with an alkyl sulfonamide solvent. Promise for LIB applications is demonstrated by chronoamperometry on Al current collectors, and cycling behavior of negative and positive electrodes. Residual current densities of 12 and 45 nA cm(-2) were observed at 5 V vs. Li/Li+ on aluminum, 1.5 and 16 nA cm(-2) at 4.5 V vs. Li/Li+, respectively for LiFSI and LiTFSI based DEEs. Capacities of 220, 130, and 175 mAh. g(-1) were observed at low (C/13 or C/10) rates, respectively for petroleum coke, LiMn1/3Ni1/3Co1/3O2 (a.k.a. NMC 111) and LiAl0.05Co0.15Ni0.8O2 (a.k.a. NCA). (C) 2016 Elsevier B.V. All rights reserved.

In this work we have studied the stability and performance of hard carbon in comparison with petroleum coke (soft carbon) as electrode materials for Li-ion batteries in an ethereal and alkyl carbonate based electrolyte solutions. 1 M bis(triflouromethane) sulfonimide lithium salt (LiTFSI) in diethylene glycol dimethyl ether (diglyme)) and a mixture of dimethyl carbonate (DMC)/mono-fluorinated ethylene carbonates (FEC) 4:1 (%v) with 1 M Lithium hexaflourophosphate (LiPF6) where chosen as representative solutions for this study. The motivation for this work is the potential importance of ethereal solutions for high energy density Li-S and Li-O-2 batteries and the possibility of using carbons as an alternative to Li metal anodes in these systems. An acceptable performance of hard carbon electrodes in the ether based solutions was demonstrated. In contrast, soft carbon electrodes which preform very well in alkyl carbonates solutions behave poorly in the ethereal solutions. Their failure mechanism was explored and is explained in this report. (C) 2015 The Electrochemical Society. All rights reserved.

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.

LiMnPO4 nanoparticles synthesized by the polyol method were examined as a cathode material for advanced Li-ion batteries. The structure, surface morphology, and performance were characterized by X-ray diffraction, high resolution scanning electron microscopy, high resolution transmission electron microscopy, Raman, Fourier transform IR, and photoelectron spectroscopies, and standard electrochemical techniques. A stable reversible capacity up to 145 mAh g(-1) could be measured at discharge potentials > 4 V vs Li/Li+, with a reasonable capacity retention during prolonged charge/discharge cycling. The rate capability of the LiMnPO4 electrodes studied herein was higher than that of LiNi0.5Mn0.5O2 and LiNi0.8Co0.15Al0.05O2 (NCA) in similar experiments and measurements. The active mass studied herein seems to be the least surface reactive in alkyl carbonate/LiPF6 solutions. We attribute the low surface activity of this material, compared to the lithiated transition-metal oxides that are examined and used as cathode materials for Li-ion batteries, to the relatively low basicity and nucleophilicity of the oxygen atoms in the olivine compounds. The thermal stability of the LiMnPO4 material in solutions (measured by differential scanning calorimetry) is much higher compared to that of transition-metal oxide cathodes. This is demonstrated herein by a comparison with NCA electrodes. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3125765] All rights reserved.

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.

In recent years, lithium bis(oxalato)borate, LiB(C2O4)2 (LiBOB) has been proposed as an alternative salt to the commonly used electrolyte, LiPF6. There is evidence of the enhanced stability of Li-ion battery electrodes in solutions of this salt, due to a unique surface chemistry developed in LiBOB solutions. The present study is aimed at further exploring the electrochemical and thermal properties of LiBOB solutions in mixtures of alkyl carbonates with non-active metal, graphite and lithium electrodes. FTIR spectroscopy, XPS, EQCM, in situ AFM imaging, and DSC were used in conjunction with standard electrochemical techniques. The study also included a comparison between LiBOB and LiPF6 solutions. The development of a favorable surface chemistry in LiBOB solutions that provides better passivation to Li and Li-graphite electrodes was clearly evident.

The thermal stability of different electrolytes—LiPF6, LiPF3(C2F5)3 (LiFAP), LiFAP solutions containing 5% vinylene carbonate, and LiFAP solutions containing 1% Li bisalicylato borate—in a mixture of (2:1:2 v/v/v) EC, DEC and DMC is investigated. The results of a comprehensive study of the thermal behavior of these solutions in contact with Li metal and lithiated graphite electrodes (synthetic flakes) using both accelerating rate (ARC) and differential scanning (DSC) calorimetry, are reported herein. The surface films formed at room temperature on Li and lithiated graphite surfaces in all the solutions studied are not stable at elevated temperatures. The main exothermic reactions of the solution thermal decomposition in contact with Li metal occur at temperatures lower than the Li melting point (180°C). The highest onset (at 165°C) for the thermal decomposition in the presence of Li, was observed for solutions containing both LiFAP and VC. LiPF6 solutions showed the lowest onset for thermal reaction (at 132°C).

The results of a comprehensive study of the thermal stability of the salt LiPF6, using both accelerating rate (ARC) and differential scanning (DSC) calorimetry, are presented. Pressure monitoring during ARC experiments permits also the study of endothermic processes. The origins of apparently inconsistent results and conflicting interpretations in previous reports in the literature are explicated. In a confined volume, LiPF6(s) melts reversibly at 467K with a heat of melting of 2.0±0.2kJmol−1. Reversible decomposition to PF5(g) and LiF(s) commences with melting, but the autogenic development of PF5(g) pressure makes the temperature profile of decomposition a function of volume and sample size. The heat of this reaction at constant volume, ΔUr, as determined by a variety of methods is in the range 60±5kJmol−1, and is approximately temperature independent in range 490–580K.

This paper reviews approaches to the design of advanced electrolyte solutions for Li and Li-ion batteries. Important challenges are wide electrochemical windows, a wide temperature range of operation, acceptable safety features, and most important, appropriate surface reactions on the electrodes that induce efficient passivation, but not on the account of low impedance. We describe research tools, quick tests, and discuss some selected examples and strategies for R&D of solutions of improved performance.

Li deposits, which are formed in practical rechargeable Li-Li0.3MnO2 batteries, were studied. The relationship of surface morphology, grain size, microstructure, and texture to the operation conditions was characterized by scanning electron microscopy and X-ray diffraction. Lithium grains consist of relatively large textured crystallites. Two directions (110) and (211) are preferred in the crystal growth process. X-ray and element analysis were used for investigating the composition of the Li surface layer. Our results show that the surface films that appear on the Li deposits formed at low current density, contain a smaller amount of LiF as compared to surface films on Li deposits formed at high current density. Accumulation of insoluble products, formed as a result of reduction of solution species by the active metal, takes place more quickly as the current density is higher. The major cause of the failure of these batteries is depletion of the solvent (by reactions with the Li deposits) and not the salt. (C) 2003 The Electrochemical Society.

Li deposits, which are formed in practical rechargeable Li-Li 0.3 MnO 2 batteries, were studied. The relationship of surface morphology, grain size, microstructure, and texture to the operation conditions was characterized by scanning electron microscopy and X-ray diffraction. Lithium grains consist of relatively large textured crystallites. Two directions (110) and (211) are preferred in the crystal growth process. X-ray and element analysis were used for investigating the composition of the Li surface layer. Our results show that the surface films that appear on the Li deposits formed at low current density, contain a smaller amount of LiF as compared to surface films on Li deposits formed at high current density. Accumulation of insoluble products, formed as a result of reduction of solution species by the active metal, takes place more quickly as the current density is higher. The major cause of the failure of these batteries is depletion of the solvent (by reactions with the Li deposits) and not the salt.

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.

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.

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.

Li electrodes in any relevant electrolyte solution (i.e., polar aprotic) are covered by surface films of a very complicated structure. It was found that even in cases where the surface films formed on lithium contain elastomers, or where the lithium metal reactivity is reduced by doping with elements such as N, As, Al, Mg, Ca, etc., it is impossible to achieve sufficient passivation with lithium electrodes and liquid solutions. Passivation is considerably worsened when Li electrodes are operated at high rates (especially at high charging, Li deposition rates). Thus, there is no way that rechargeable Li batteries can compete with Li-ion batteries in any application that requires high charging rates (e.g., in powering portable electronic devices). The electrochemical behavior of lithiated graphite electrodes also depends on passivation phenomena. The surface films formed on lithiated graphite are similar to those formed on Li metal in the same solutions. The volume changes of graphite electrodes during Li insertion–deinsertion are small enough to enable their reasonable passivation in a variety of electrolyte solutions. A critical factor that determines the stability of graphite electrodes is their morphology. It was found that the shape of graphite particles plays a key role in their application as active mass in anodes for Li-ion batteries.

In this work we studied properties of modified lithium electrodes in an attempt to improve the high rate performance of rechargeable Li (metal) batteries containing liquid electrolyte solutions. Li (metal)-Li0.3MnO2 AA batteries with solutions containing 1,3-dioxolane (DN), LiAsF6, and a basic stabilizer became commercial several years ago but failed to compete with Li-ion battery technology because of a very limited cycle life at high charging rates. The problem relates to intensive reactions between Li deposited at high rates and the electrolyte solutions, which dry the batteries. The lithium-solution reactivity was modified through several approaches. Li anodes doped by Li3N, Al, and Mg were tested, as well as solutions containing derivatives of DN that are expected to be less reactive toward lithium than DN. It was concluded that reduction of the Li anode-solution reactivity by these approaches cannot solve the problem, because it is impossible to modify the rough morphology, high surface of lithium electrodes when charging (Li deposition) rates are high (>1 mA/cm(2)). Since there is no hermetic passivation of any Li surface in liquid electrolyte solutions, the high-surface-area Li deposits react with solution components. Therefore, upon charge-discharge cycling of practical Li (metal) batteries, the electrolyte solution is consumed in these reactions. Hence, the future of Li (metal) rechargeable batteries lies either in the use of solid electrolyte matrices instead of the liquid solutions, or in applications where low charging rates are tolerable. (C) 2002 The Electrochemical Society.