Li-ion batteries are widely used at room temperature because of their high specific energy and energy density, long cycle life, low self-discharge, and long shelf life. 1-3 For certain defense and space applications they are required to be used at low temperatures (<Ϫ30ЊC). In general they exhibit poor low-temperature performance. For example, it has been reported that the capacity of Li-ion cells at Ϫ40ЊC is 12% of the room temperature value. 4 It has been suggested that the poor performance of Li-ion batteries at low temperatures is due to the electrolyte, which determines Li-ion mobility between the electrodes and the transport properties of the passivation film on the carbon anode. 4-8 Most previous studies have focused on the development of low-temperature electrolytes. However, recent preliminary studies by Huang et al. suggest that the primary cause of the poor Li-ion cell performance at low temperature is related to Liion diffusion in the carbon anode and not in the electrolyte or passivating film. 9 It is the purpose of this paper to confirm that the poor Li-ion cell performance at low temperature (<Ϫ30ЊC) is controlled by slow Liion diffusion in the carbon anode and not in the electrolyte or in the passivating film on the carbon anode by evaluating the following at low temperature: (i) electrolytes, (ii) cathode performance, (iii) solid electrolyte interphase (SEI), (iv) anode performance, and (v) effect of the electrode active material particle size. Experimental Electrolyte.-The electrolytes evaluated in this study were chosen because of previous low-temperature Li-ion battery studies of the Jet Propulsion Laboratory (JPL). 10 Four carbonate-based solvent electrolytes were tested: (i) 1.0 M LiPF 6 ethylene carbonate (EC):diethyl carbonate (DEC):dimethyl carbonate (DMC) (1:1:1 vol %); (ii) 0.9 M LiPF 6 EC:DEC:DMC:propylene carbonate (PC) (3:3:3:1); (iii) 0.8 M LiPF 6 EC:DEC:DMC:ethyl methyl carbonate (EMC) (3:5:4:1), and (iv) 0.8 M LiPF 6 EC:DEC:DMC:EMC (3:5:4:2). The electrolytes, purchased from Mitsubishi Chemical Company, were sealed in glass tubes in a glove box. The samples were put into a low-temperature chamber, and the temperature was lowered first to Ϫ20, then to Ϫ30, Ϫ35, Ϫ40, and finally to Ϫ42.5ЊC. The samples were kept at each temperature for 4 days. The electrolyte liquidus range at low temperatures was evaluated through visual inspection. In the case of 1.0 M LiPF 6 EC:DEC:DMC (1:1:1), the solid phase that formed at Ϫ35ЊC was separated and analyzed using a gas chromatograph coupled with a mass spectrometer.Electrode.-The electrode materials investigated were: mesocarbon microbead (MCMB) graphite, coke, and LiCo 0.2 Ni 0.8 O 2 . MCMB graphite and coke were chosen because of their potential as anodes in Li-ion batteries as a result of their high specific capacity and long cycle life. LiCo 0.2 Ni 0.8 O 2 was investigated because of its potential as a cathode in Li-ion batteries as a result of its relatively high specific capacity and more importantly, its high Li diffusion coefficient.To ev...

The electrolyte composition plays a strong role in determining the low temperature performance of lithium-ion cells, both in terms of ionic mobility in the electrolyte solution, as well as forming suitable surface films on the electrode surfaces. A series of ester solvents was chosen for incorporation into multicomponent electrolyte formulations due to their favorable physiochemical properties (i.e., low viscosity, low melting point, and high permittivity), as well as good compatibility with carbonaceous anodes and mixed metal cathodes (i.e., LiCoO2 and LiNiCoO2false). In addition to determining the relative facility of lithium intercalation and deintercalation in Li-carbon cells as a function of temperature, a number of conventional electrochemical methods were employed to further enhance the understanding of the nature of the electrode surface films in these ester-based electrolytes, including dc polarization and ac impedance measurements. A distinct trend was observed with respect to the stability of the surface films formed. In solutions containing low molecular weight cosolvents (i.e., methyl acetate and ethyl acetate) the surface films appear resistive and inadequately protective, whereas electrolytes containing higher molecular weight esters resulted in surface films with more desirable attributes. Promising electrolyte formulations were further evaluated in prototype lithium-ion cells (AA-size) and fully characterized in terms of their low temperature discharge performance. © 2002 The Electrochemical Society. All rights reserved.

The low-temperature performance of lithium-ion cells is mainly limited by the electrolyte solution, which not only determines the ionic mobility between electrodes but also strongly affects the nature of surface films formed on the carbonaceous anode. The surface films provide kinetic stability to the electrode (toward electrolyte) and permit charge (electron) transfer across them, which in turn determine the cycle life and rate capability of lithium-ion cells. Aiming at enhancing low-temperature cell performance, we have studied electrolyte solutions based on different ratios of alkyl carbonate solvent mixtures, i.e., ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC), in terms of electrolyte conductivity, film resistance, film stability, and kinetics of lithium intercalation and deintercalation, at various temperatures. Electrolytes based on the ternary mixtures of EC, DEC, and DMC emerged as preferred combination compared to the binary analogues both in terms of conductivity and surface film characteristics, especially at low temperatures. These studies are further corroborated in sealed AA cells, which showed a synergistic effect of high durability from the DMC-based solutions and improved low-temperature performance from the DEC-based electrolytes.

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Fuel cells that can operate directly on fuels such as methanol are attractive for low to medium power applications in view of their low weight and volume relative to other power sources.A liquid feed direct methanol fuel cell has been developed based on a proton exchange membrane electrolyte and Pt/Ru and Pt catalyzed fuel and air/O2 electrodes respectively.The cell has been shown to deliver significant power outputs at temperatures of 60 to 90* C. The cell voltage is near 0.5 V at 300 mA/cm 2 current density and an operating temperature of 90* C. A deterrent to performance appears to be methanol crossover through the membrane to the oxygen electrode.Further improvemerits in performance appear possible by minimizing the methanol crossover rate.

Nonaqueous electrolyte solutions in current lithium-ion cells achieve stability toward the graphite anode (negative electrode) via the formation of passive surface films on the anode surface. These films are composed of reaction products resulting from electrolyte reduction and some reduced lithium. These films reportedly contain various lithium compounds, such as lithium carbonate (Li 2 CO 3 ), lithium oxide (Li 2 O), lithium hydroxide (LiOH), lithium alkoxides, lithium fluoride (LiF), as well as electrolyte salt reduction products that are still to be accurately characterized. 1 Relative amounts of these constituents are also equally uncertain. In addition to the expense of lithium for such surface films, termed solid electrolyte interphase (SEI), 2 a portion of lithium might be "trapped" in the anode material and is "kinetically inaccessible." Consequently, a differential exists between the intercalated lithium (charge capacity) and deintercalated lithium (discharge capacity), which is loosely termed irreversible capacity. This irreversible capacity depends not only on the rate of lithiation during the formation cycles and the temperature, but also on the extent of charge-discharge cycling, during which the surface film may grow. Irreversible capacity is typically estimated as the cumulative differential in the capacity after five cycles (one cycle in some reports), when the charge capacity/discharge capacity ratio approaches unity.It is difficult to separate the irreversible capacity into a component involving SEI formation and a component involving capacity loss due to kinetic effects, unless one of the components is estimated by a nonelectrochemical method. In this work, we attempted such a study on SEI formation on graphite in different electrolyte solutions using solid-state 7 Li nuclear magnetic resonance (NMR). Solid-state 7 Li NMR has been commonly used for qualitative detection and characterization of lithium intercalation in (and the SEI formation on) graphite and disordered carbons. 3-8 However, this technique has not been used extensively for quantitative determinations. In this work, the surface films were also examined ex situ by transmission electron microscopy (TEM) for elucidating the microstructure of film-covered graphite electrodes. These studies were further complemented by electrochemical impedance spectroscopy (EIS) measurements to understand the surface film characteristics of graphite anodes in different electrolytes.Improving low-temperature performance of lithium-ion cells remains a formidable technical challenge. The present investigation is an outgrowth of our previous studies 9 of novel electrolyte formulations for low-temperature performance. Similar efforts to improve the properties of electrolytes at low temperatures are being made elsewhere. 10-14 Our recent results have shown that a ternary mixture of alkyl carbonates, i.e., 1:1:1 (vol. %) of EC (ethylene carbonate):DEC (diethyl carbonate):DMC (dimethyl carbonate) with 1 M LiPF 6 exhibits favorable electrochemical characteris...

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