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Phase diagram of the binary ionic liquid 1-ethyl-3- methylimidazolium hexafluorophosphate ([emim][PF6]) - 1- ethyl-2-methylpyrazolium hexafluorophosphate ([empz][PF6]), was determined. The binary system shows a eutectic at 23oC, (which is 38oC below the melting point of the lower melting salt [emim][PF6]), and at the molar composition [emim][PF6]: [empz][PF6] ≈ 47:53. Therefore using the eutectic method a low melting ionic liquid was formed with single salts that contain only small alkyl groups on the cations. At ambient temperature the ionic conductivity of the eutectic mixture is comparable to that of a low melting single salt such as, 1-n-butyl-3- methylimidazolium hexafluorophosphate ([bmim][PF6]), but at higher temperature (e.g. 90oC) the conductivity of the eutectic mixture is 20% higher. This method allows the preparation of purer ionic liquids since the two single salts can be purified by recrystallization, before they are mixed to form the binary system which is a liquid at ambient temperature. Hence the ionic liquids produced by the eutectic method have the potential for finding wider range of applications compared with low melting ionic liquids (single salts) containing longer alkyl groups prepared by the conventional method.

The cell Na/β"-alumina/S(IV) in AlCl 3 -NaCl melt had been operated almost exclusively in the temperature range 180 o to 250 o C. Lower temperature operation was explored using the acidic region of the AlCl 3 -NaCl phase equilibrium, which has a eutectic at 108 o C, and the first step in the S(IV) discharge process (OCV > 4.2 V). When the commonly used low surface area current collector was used in the cathode, cell operation below 150 o C led to only partial utilization of cell capacity. However, using a cathode constructed with a high surface area current collector (RVC) and operated exclusively in acidic melts, the cell was cycled down to 120 o C. It was operated with essentially full capacity and produced ≈100% coulombic efficiency. Energy efficiency at 120 o was 87%. Cell operation at and below 115 o C led to incomplete and irreproducible discharges. Low temperature operation of this cell can minimize or eliminate corrosion of cathode seal components and extend cell life-time. Because of these favorable properties this cell can find applications in energy storage as well as in downhole oilfield tools.

At this time there is no rechargeable battery available which can deliver required currents in a wide operating temperature range, such as from ambient or low temperature to higher temperatures (e.g. 150°C). In this paper a CC size rechargeable Li-ion molten salt battery which can operate in the temperature range from room temperature to 130°C is presented. The cell has open circuit voltage of 2.3V for fully charged cell. It exhibits high coulombic efficiency. The cell can operate at low currents as well as at high currents; however, at very high currents coulombic efficiency decreases. It can operate at temperatures greater than 130°C but its cycle life is shorter. The battery demonstrates very good safety features. Short-circuiting, overcharge, deep discharge, or reverse charge does not result in violent reaction. These properties make this cell a very good candidate for application in drilling and logging industry. In addition, these cells could help in extending the drilling capabilities of tools.