1688 Journal of The Electrochemical Society, 147 (5) 1688-1694 (2000) S0013-4651(99)08-117-03 CCC: $7.00 © The Electrochemical Society, Inc. The continued research and development of recent years in the area of lithium batteries has established a number of organic car- bonates as the most suitable solvents for making the nonaqueous electrolytes. 1 These carbonates can be divided, according to their molecular structure, into cyclic and noncyclic groups. EC and PC are two well-known examples of the cyclic carbonates, and the non- cyclic carbonates include DMC, EMC, and diethyl carbonate (DEC). While most of these carbonates (except DEC 2 ) have good electro- chemical stability with lithium, their other properties vary. Some properties differ according to their cyclicity, such as dielectric con- stant, viscosity, and boiling point, the values of which are high for the cyclic and low for the noncyclic carbonates. Others change with- out an obvious correlation with the cyclicity, such as melting tem- perature (see Table I). A high dielectric constant of a solvent is good for dissolving a salt and separating the ions, and a low viscosity is good for conducting the ions. As a consequence, no solvent alone of either a cyclic or a noncyclic carbonate can best satisfy the require- ments for producing an electrolyte of a high electrolytic conductivi- ty. Fortunately, it has been found that these organic carbonates are sufficiently unstructured in their liquid states to form binary solvents approaching ideal mixtures. As a result, the dielectric constant of a binary mixture,  mix , is related to  1 and  2 of the two components by the equation  mix = (1 - y) 1 + y 2 , where y is the volume frac- tion of the second component. The corresponding relationship for viscosity, on the other hand, has been found to be best described by the equation  mix = 1 1-x  x 2 , where  denotes viscosity and x the mole fraction of the second component. 3 Because of these monoto- nous relationships between the composition of a binary solvent sys- tem and its dielectric constant and viscosity, these latter properties can be optimized for a binary carbonate by choosing for its compo- nents a cyclic and a noncyclic carbonate and by adjusting the rela- tive proportions of the two components. For these reasons, many of the widely used electrolyte solvents for lithium batteries, such as DMC-EC and EMC-EC, are binary carbonates composed of a cyclic and a noncyclic carbonate in nearly equal proportions. But, in addi- tion to the conductivity consideration, the change of liquid range with composition of such a binary solvent is another piece of essen- tial knowledge for its proper use. 4 This change, however, is usually Liquid-Solid Phase Diagrams of Binary Carbonates for Lithium Batteries Michael S. Ding, z, * Kang Xu,** and T. Richard Jow* Army Research Laboratory, Adelphi, Maryland 20783-1197, USA We present the liquid-solid phase diagrams that we mapped with a differential scanning calorimeter (DSC) for the following seven binary carbonates: dimethyl carbonate (DMC)-ethylene carbonate (EC), ethyl methyl carbonate (EMC)-EC, EMC-propylene car- bonate (PC), EMC-dimethyl ethylene carbonate (DMEC), EMC-isobutylene carbonate (iBC), PC-EC, and EMC-DMC. Many of these are among the most frequently used solvent systems for making the nonaqueous electrolytes for lithium batteries. The phase diagrams of these carbonate systems are all of the simple eutectic type but with vastly different particular features. Comparison of these phase diagrams shows that to expand the liquid region of a carbonate system toward low temperature, the two components of the system need to have comparable melting temperatures and compatible molecular structures. These results are consistent with thermodynamic considerations and have significant practical implications. © 2000 The Electrochemical Society. S0013-4651(99)08-117-3. All rights reserved. Manuscript submitted August 30, 1999; revised manuscript received January 8, 2000. ** Electrochemical Society Active Member. ** Electrochemical Society Student Member. * z E-mail: michaelsding@yahoo.com Table I. Structural formulas and relevant properties of some carbonate solvents. Carbonate EC a PC a DMEC iBC DMC b EMC Structural formula Cyclicity Cyclic Cyclic Cyclic Cyclic Noncyclic Noncyclic Melting point (°C) 36.4 -48.8 -14.0 -2.2 4.6 -53 Boiling point (°C) 248 242 244 >287 91 109 Viscosity/cP at 25°C 1.93 c 2.53 — 11 0.59 0.66 c Dielectric constant at 20°C 89.78 c 66.14 57 48 3.12 3 Density (g cm -3 ) at 25°C 1.3214 d 1.2047 e 1.125 1.172 1.071 1.007 f Molecular weight 88.06 102.09 116.12 116.12 90.08 104.11 a Ref. 5. b Ref. 1. c At 40°C. d At 39°C. e At 20°C. f Ref. 2. ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 158.12.18.118 Downloaded on 2015-08-06 to IP