Studies on Li-ion cells with and without Li/Li ϩ reference electrodes confirm that the limiting factor for low-temperature performance is the carbon anode. The cycling behavior of MCMB ͑2528͒-LiCoO 2 cells were studied over the temperature range of Ϫ40°C to room temperature, and at all temperatures it is the polarization of the anode which limits performance. Even at modest ͑C/5͒ to low ͑C/10͒ rates of charge and discharge, deposition of metallic lithium at the anode for temperatures рϪ20°C causes permanent capacity loss. This loss is not attributed to electrolyte conductivities, which are relatively high over this temperature range, but rather to continual growth of the solid electrolyte interphase resulting from solvent/electrolyte reduction at freshly deposited metallic lithium. The studies also reveal that at low temperatures, lithium cannot be completely removed or inserted into the coexistence range of dilute stage 1 and stage 4. This may be a simple electrokinetic phenomenon or, more likely, the limiting solid-state diffusion of lithium in carbon.A number of recent papers have focused on the poor performance of Li-ion cells at low temperatures. 1-5 The poor performance of Li-ion cells at low temperatures has been attributed to poor solution transport properties ͑e.g., conductivities and transference of lithium ions 1,2,4 ͒, deposition of metallic lithium upon charge with subsequent growth of the solid electrolyte interphase ͑SEI͒, 3 and Li diffusivity into graphite. 5 In all likelihood, all these factors contribute to poor low-temperature performance with one exception; the conductivities of lithium salt solutions which, in reality, are high at low temperatures. For example, mixtures of various solvents such as alkyl and cyclic carbonates with low viscosity solvents such as esters or hydrocarbons have been designed with specific conductivities between 0.0003 and 0.001 S/cm at temperatures as low as Ϫ40°C.The studies reported in this paper were undertaken to expand on these concepts by employing a practical configuration of a Li-ion cell with and without a metallic Li reference electrode. ExperimentalAll cells used in this study are represented by 4 MCMB-2528 ͉ PVdF-based electrolyte ͉ LiCoO 2 ͓1͔Anode compositions were generally 80% MCMB, 4% Super P, and the remainder PVdF, and the typical thickness of this anode was 0.012 cm ͑4.7 mil͒ as discussed in Ref. 4. Cathode compositions were generally 80% LiCoO 2 ͑FMC͒, 7% Super P, and the remainder poly͑vinylidene difluoride͒ ͑PVdF͒, and the typical thickness of this cathode was 0.021 cm ͑8.3 mil͒. 4 All cells were anode limited, having a cathode/anode capacity ratio between 1.2 and 1.3. The PVdFbased electrolyte was prepared with Bellcore technology 6 with 1 mol dm Ϫ3 LiPF 6 in 1:3 wt % EC/EMC electrolyte. Sources and purity of materials and other cell details can be found in an earlier publication. 4 Cell 1 was used for studies on capacity retention during cycling at various temperatures. For more detailed studies, a metallic Li reference electrode was sandwic...

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Improvement of the cycling behavior of the Li/Li=CoO2 cell has been brought about by the use of esters and alkyl carbonate-based mixed solvents, and by physical modifications of the cathode structure. Binary solvent mixtures of methyl formate with either dimethyl carbonate, diethyl carbonate, or CO2 and LiAsF6 electrolyte exhibit superior resistance towards solvent oxidation as well as improved lithium cycling efficiencies. Use of these mixed solvents with newly developed thin cathodes has resulted in stable Li/Li=CoO2 cells which can be cycled more than 100 times at current densities of 1-10 mA cm -2, and at an average cell discharge voltage of between 3.9 and 3.7V. Due to the high rate of solid-state diffusion of Li § the LixCoO2 cathode can be charged at rates of up to 2 mAcm -2 over at least 100 deep charge and discharge cycles.