Review—Solid Electrolytes in Rechargeable Electrochemical Cells

Abstract

Solid electrolytes may be used where the electrode reactants are gaseous as in a rechargeable oxide fuel cell or are liquids as in a cell of a rechargeable Na-S battery; porous polymer/oxide composites may be used as a separator where one electrode is a flow-through liquid containing a soluble redox molecule. Where two solid electrodes interact reversibly across an electrolyte, a solid-electrolyte may be used (i) as a separator in a dual liquid-electrolyte cell, (ii) in an all-solid-state battery, or (iii) in contact with an alkali-metal anode on one side and a liquid electrolyte on the other side. A solid-electrolyte interface with the anode would also block from reaching the anode soluble intermediate species of a solid-sulfur cathode reaction in a liquid catholyte. We discuss strategies for realizing the different strategies and introduce the concept of water-solvated glass/amorphous solid cation electrolytes/dielectrics. Rechargeable electrochemical cells convert chemical energy into electric power on discharge and store electric power as chemical energy on charge. The chemical reaction between a reductant at the anode and an oxidant at the cathode has an electronic and an ionic component. The electrolyte of a cell conducts inside the cell the working ion of the reaction and forces the electronic component to traverse a circuit outside the cell to deliver a discharge electric power P dis = I dis V dis for a time t dis , where I dis is the electronic current delivered at a voltage V dis = V(q) dis ; the cell voltage is constant with q for two-phase electrode reactions, but for a single-phase reaction in one electrode it decreases with the state of charge q of the cell. The chemical reaction is completed after a time t dis , where t dis depends on I dis .In a rechargeable cell, the chemical reaction is reversed by the application of a charging power P ch = I ch V ch . The voltages are:where the open-circuit voltage V oc = (μ A − μ C )/e is the difference between the electrochemical potentials of the electrons in the two electrodes divided by the magnitude of the electronic charge e. The η ch and η dis are called, respectively, the overvoltage and the polarization. The η(q) = IR cell depend on the resistances R cell = R el + R ct ; R el is the resistance to the ionic conductivity σ i = n i q i μ i in the electrolyte and R ct is the resistance to ionic transport across any electrode/electrolyte interfaces. The R ct at the anode and the cathode interface with the electrolyte are different from one another and the charge transport across an interface is also different between charge and discharge, so η ch = η dis . The efficiency of storage of electric power in a rechargeable battery is 100 P dis /P ch %.The capacity of a battery cell is the amount of charge per unit weight or volume passed between the electrodes on a complete cell reaction at a constant current I = dq/dt:An irreversible capacity loss in a charge/discharge cycle, i.e. a t dis (n+1) < t dis (n) where (n + 1) and n are cell cycle numbers, represents...

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