The present invention is directed to organic solvents and electrolytes for electrochemical cells, particularly lithium ion cells, and to electrochemical cells exhibiting good low temperature performance.
State-of-the-art lithium ion cells typically include a carbon (e.g., coke or graphite) anode intercalated with lithium ions to form LixC; an electrolyte consisting of a lithium salt dissolved in one or more organic solvents; and a cathode made of an electrochemically active material, typically an insertion compound, such as LiCoO2. During cell discharge, lithium ions pass from the carbon anode, through the electrolyte to the cathode, where the ions are taken up with the simultaneous release of electrical energy. During cell recharge, lithium ions are transferred back to the anode, where they reintercalate into the carbon matrix.
Lithium ion rechargeable batteries have the demonstrated characteristics of high energy density, high voltage, and excellent cycle life, making them more attractive than competing systems such as Nixe2x80x94Cd and Nixe2x80x94H2 batteries. However, few state-of-the-art lithium ion cells perform well at low temperatures making them unsuitable for many terrestrial and extra-terrestrial applications. Many scheduled NASA missions demand good low temperature battery performancexe2x80x94without sacrificing such properties as light weight, high specific energy, long cycle life, and moderate cost. The Mars Exploration Program, for example, requires rechargeable batteries capable of delivering 300 cycles with high specific energy, and the ability to operate over a broad range of temperatures, including the extremely low temperatures encountered on and beneath the surface of Mars. Mars Rovers and Landers require batteries that can operate at temperatures as low as xe2x88x9240xc2x0 C. Mars Penetrators, which will penetrate deep into the Martian surface, require operation at temperatures lower than xe2x88x9260xc2x0 C.
To be used on the Mars missions and in low earth orbit (LEO) and geostationary earth orbit (GEO) satellites, as well as in terrestrial applications, lithium ion rechargeable batteries should exhibit high specific energy (60-80 Wh/Kg) and long cycle life (e.g.,  less than 500 cycles).
Unfortunately, state-of-the-art lithium ion cells typically exhibit poor capacities below 0xc2x0 C. This is primarily due to limitations of the electrolyte solutions, which become very viscous and freeze at low temperatures, resulting in poor ionic conductivity. In addition, the surface film (i.e., solid electrolyte interphase, SEI) that forms on the electrodes either builds up over the course of repeated charge/discharge cycling or becomes highly resistive at lower temperatures. Ideally, the SEI layer on the carbon anode needs to be protective toward electrolyte reduction and yet conductive to lithium ions to facilitate lithium ion intercalation, even at low temperature.
A number of factors can influence the low temperature performance of lithium ion cells, including (a) the physical properties of the electrolyte (i.e., conductivity, melting point, viscosity, etc.), (b) the electrode type, (c) the nature of the SEI layers that can form on the electrode surfaces, (d) cell design, (e) electrode thickness, separator porosity and separator wetting properties. Of these, the electrolyte properties have the predominant impact upon low temperature performance, as sufficient electrolyte conductivity is a necessary condition for good performance at low temperatures. Ideally, a good low temperature performance electrolyte should possess a combination of several critical properties, including high dielectric constant, low viscosity, adequate Lewis acid-base coordination behavior, as well as appropriate liquid ranges and salt solubilities in the medium.
Conventional electrolytes employed in state-of-the-art lithium ion cells have typically consisted of binary mixtures of organic solvents, for example high proportions of ethylene carbonate, propylene carbonate or dimethyl carbonate, within which is dispersed a lithium salt, such as LiPF6. Examples include 1.0M LiPF6 in a 50:50 mixture of ethylene carbonate/dimethyl carbonate, or ethylene carbonate/diethyl carbonate. Such electrolytes do not perform well at low temperature because they become highly viscous and/or freeze.
It can be seen, therefore, that a clear need exists for improved organic solvents, electrolytes, and electrochemical cells capable of performing well at low and moderate temperatures, with high specific energy and high cycle lives.
The present invention provides novel organic solvent systems, electrolytes, and electrochemical cells characterized by improved low temperature performance, including high conductivity, good cycle life, good discharge characteristics, good stability and self-discharge characteristics, and excellent compatibility with the cell components, as well as excellent room temperature and elevated temperature performance. Lithium ion cells containing the new electrolytes are ideal for use in portable electronic products, space vehicles, and other applications.
In one embodiment of the invention, an organic solvent system comprises a ternary mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC), preferably an equal volume mixture of each. Fluorinated analogsxe2x80x94especially perfluorinated analogsxe2x80x94can be used in place of one or more of the solvents.
In a second embodiment of the invention, an organic solvent system comprises a mixture of organic carbonates and at least one aliphatic ester, preferably an alkyl or fluoroalkyl ester. Thus, ternary, quaternary, and higher solvent systems are provided. One such solvent system comprises a ternary mixture of EC, DMC, and MA (methyl acetate). Another solvent system comprises a quaternary or higher mixture of EC, DMC, DEC and at least one alkyl or fluoroalkyl ester.
In still another embodiment, an improved organic solvent system includes two or more alkyl carbonates, preferably EC, DMC, DEC, and/or PC (propylene carbonate), and an asymmetric alkyl carbonate.
In yet another embodiment, the solvent system includes a compound having the formula LiOX (where X is xe2x80x94R, xe2x80x94COOR, or xe2x80x94COR, where R is alkyl or fluoroalkyl) or another basic species that can effectively catalyze the described disproportionation reactions. Fluorinated (especially perfluorinated) analogs of one or more of the co-solvents or additives can also be used. Where the organic solvent system includes an asymmetric alkyl carbonate, such as ethyl methyl carbonate, it can be added directly to the solvent system or generated in situ by including a lithium alkoxide or similar basic species in the mixture. Thus, the present invention also provides a method for making organic solvent systems for electrochemical cells in which asymmetric alkyl carbonates can be produced.
The addition of lithium methoxide or a related basic species also has the observed benefit of improving the SEI formation characteristics of carbonate-based (and other) electrolytes, which contributes to improved cell performance, especially at low temperature, due to low electrode polarization behavior. Lithium methoxide has previously been detected as a by-product formed from electrolyte reactions in lithium-ion cells which have been cycled or exposed to lithiated graphite. In fact, the addition of alkoxides and related basic compounds to lithium ion cell electrolytes has now been found to have a beneficial effect, irrespective of whether the additive(s) can facilitate disporportionation/exchange reactions resulting in asymmetric carbonate formation. The present invention, therefore, includes a method of making an improved electrolyte by adding to an electrolyte solvent system a small amount of a compound of the formula LiOX (as described herein), or a similar basic species.
In addition to novel organic solvent systems, the invention also provides a variety of improved electrolytes, comprising a lithium salt having high ionic mobility, dispersed in an organic solvent system. A preferred salt is LiPF6. The electrolytes are particularly well-suited for use in lithium ion cells, especially where low temperature performance is required.
In another embodiment of the invention, an improved, reversible electrochemical cell comprises an anode, a cathode, and an electrolyte as described herein. Preferably, the anode and cathode are of the insertion type, i.e., a coke, graphite, or modified carbon anode intercalated with lithium ions, and the cathode is a lithiated metal oxide, such as LiCoO2, or a similar material. In operation of the cell, lithium ions pass from the carbon through the electrolyte to the cathode, where they are taken up. During recharge, lithium ions are transferred back to the anode, where they reintercalate into the carbon matrix.
Lithium ion cells prepared in accordance with this invention have high specific energy, energy density, operating voltage, and coulombic efficiency, and low self-discharge tendencies. Cell performance at low temperatures ( less than xe2x88x9220xc2x0 C.)is notably improved over comparable, state-of-the-art systems. When used in upcoming space missions, the invention promises the additional benefits of improved reliability, and broader operating range.