ELECTROLYTES docx

ELECTROLYTES Contents Overview Gel Ionic Liquids Non-Aqueous Polymer Solid: Mixed Ionic-Electronic Conductors Solid: Oxygen Ions Solid: Protons Solid: Sodium Ions Overview M Salomon, MaxPower, Inc., Harleysville, PA, USA & 2009 Elsevier B.V. All rights reserved. Introduction The electrolyte solution is a major component of all battery systems and as such will greatly influence energy densities (thermodynamic properties such as Gibbs en- ergy, enthalpy, and ionic activity) and power densities (nonthermodynamic properties such as viscosity, con- ductivity, and transference). The electrolyte solution consists of a liquid or solid phase containing at least one component, e.g., water, which is called the solvent, and an ionizable substance, e.g., a salt or an acid, which is called the electrolyte. The fundamental thermodynamic and nonthermodynamic concepts that are of importance to batteries are discussed below. Thermodynamic Fundamentals Standard Electrode Potentials A battery is defined as a string of individual Galvanic cells, and the basic cell consists of two electrodes (half-cells) immersed in a solution containing an electrolyte com- posed of ions reversible to each of the half-cells. The cell reaction is Li þAgCl $ Li þ þ Cl À þ Ag ½I In aqueous solution, the two half-cell reactions can be written in terms of standard potentials as Li$Li þ þ e À ðoxidation e:m:f:Þ; E o ¼þ3:045 V Li þ þ e À $Li ðreduction e:m:f:Þ; E o ¼À3:045 V AgCl þe À $Cl À þ Ag; E o ¼þ0:2223 V where the E o values are standard potentials (in units of volts) as indicated by the superscript and thus refer to STP (standard temperature (298.15 K) and pressure (101.33 kPa)), and the concentration or activity of the electrolyte is unity. Under these conditions, the cell po- tential (e.m.f.) for reaction [I] is written as E o ¼ E o ðoxidationÞþE o ðreductionÞ and using the half-cell e.m.f.’s given above, the e.m.f. of cell [I] is E o ¼3.045 þ0.2223 ¼3.267 V. For many half-cells, standard e.m.f.’s, particularly for aqueous solutions, have been tabulated in various text books on thermodynamics and electrolytes. The value of 2.823 V is the standard e.m.f. for cell [I], but in practice, the observed e.m.f. for this cell will be dependent upon the concentration of the electro- lyte in the electrolyte solution, or, more accurately, upon the activity of the electrolyte in the electrolyte solution. Provided the activities of all ionic species in solution are known or can be calculated from theoretical expressions, the e.m.f., E, of the Galvanic cell can be calculated from the Nernst equation E ¼ E o À RT nF ln Pa products Pa reactants &' ½1 where R is the g as constant, T the absolute temperature, n the number of electrons involved in the electrochemical reaction, and a the activities of initial reactants and final products as indicated in eqn [I]. A discussion on activities and concentrations is given below. There are numerous instances where tabulated e.m.f. values are not available, but it is still possible to calculate 134 . ELECTROLYTES Contents Overview Gel Ionic Liquids Non-Aqueous Polymer Solid: Mixed Ionic-Electronic. particularly for aqueous solutions, have been tabulated in various text books on thermodynamics and electrolytes. The value of 2.823 V is the standard e.m.f. for cell [I], but in practice, the observed