Non-Aqueous M Salomon, MaxPower, Inc., Harleysville, PA, USA & 2009 Elsevier B.V. All rights reserved. Introduction Modern research on nonaqueous batteries is focusing on lithium-based systems, which require electrolyte solu- tions stable in the presence of metallic lithium and other anodes such as LiC 6 , which have potentials very close to the Li/Li þ reversible potential. For this reason, this article reviews the state-of-the-art of aprotic solvents, and reviews on protic solvents such as alcohols can be found in Further Reading. The selection of the aprotic electrolyte solution for a given battery system will have a major influence on performance such as power capability, operational temperature range, and in some cases on the maximum specific energy and energy density based on the cell chemistry. For practical systems, power and en- ergy are highly dependent upon transport properties, particularly on conductivity and ion transpor t. Thermodynamic Fundamentals Standard Electrode Potentials and Reference Electrodes In determining thermodynamic properties of both full- cell and half-cell properties of lithium-based systems, it is essential to have a reference electrode that is ther- modynamically relevant to the system of interest. For lithium-based cells and batteries, this is the reversible Li/ Li þ electrode in an aprotic solvent with a lithium-based electrolyte such as lithium perchlorate (LiClO 4 ), lithium hexaflurophosphate (LiPF 6 ), and lithium bisper- fluoroethyl sulfonyl imide (LiN(SO 2 C 2 F 5 ) 2 , and for which the reversible reaction for the lithium electrode is Li þ þ e À $Li ½I Reference electrodes such as a silver or alumium wire immersed in the electrolyte solution containing only a lithium salt have no thermodynamic significance but are often used in solutions where metallic lithium slowly reacts with the solvent (e.g., in many ionic liquids). The possibility of the reaction of metallic lithium with an electrolyte solution can be avoided by using a reversible Li-based reference electrode that is more positive and that can be thermodynamically related to the reversible Li/Li þ electrode potential. One example of a highly reversible lithium reference electrode that is precisely defined in reference to the Li/Li þ electrode is based on lithium titrate for which the electroche mical reaction versus Li is Li 4 Ti 5 O 12 þ 3Li $ Li 7 Ti 5 O 12 ; E ¼ 1:55 V ½II where E is the all potential. Early studies on nonaqueous high-energy-density batteries focused on metallic Li anodes and a metallic chloride cathode, such as silver chloride (AgCl) and copper(II) chloride (CuCl 2 ). The overall cell reaction for li/AgCl cell can be represented as Li þ AgCl $ Li þ þ Cl À þ Ag ½III The reaction products involve Li þ and Cl À ions, and to determine the potential of this cell, both E1 and activity coefficient (g) I data are required as input to the Nernst equation E ¼ E1 À 2RT F lnfmg u 7 g½1 where R is the gas constant F the Faraday constant, and I the temperature. The E values for cell reactions such as eqn [III] can be easily determined provided E1 and g 7 values are known, whereas for aprotic solvents these data are most often not available in the literature. Other problems exist such as the dissolution of the cathode due to the reaction product of Cl À ions, which form complex anions such as AgCl À 2 and CuCl 2À 4 . Modern R&D on batteries involves cells in which the cell potentials are independent of the nature of the electrolyte solution because initial reactants and products of the cell reaction are all solid materials as indicated by the example cell reactions given in Table 1. As the potentials of these cells are independent of the nature of the electrolyte solution, maximum specific energies and energy densities can be calculated from Gibbs energies of formation (DG o f ) through the relations DG1 ¼ X DG o f ðproductsÞÀ X DG o f ðreactantsÞ½2 DG1 ¼ÀnFE1 ½3 where n is the number of electrons involved in the overall cell reaction. In the absence of literature data for DG o f , one can often rely upon experimental open-circuit voltage (OCV), E, values for highly reversible reactions (e.g., for lithium-ion cells) as they are usually very close to E1 values and independent of the nature of the elec- trolyte solution. In this case, the experimental OCV can 160 . Non-Aqueous M Salomon, MaxPower, Inc., Harleysville, PA, USA & 2009 Elsevier B.V. All rights