Sodium solid electrolytes

Difficulties with the Na-S system arise in part from the ceramic nature of the alumina separator the specific /S-alumina is expensive to prepare, and the material is brittle and quite fragile, Separator failure is the leading cause of early cell failure. Cell failure may also be related to performance problems caused by polarization at the sodium/solid electrolyte interface. Lastly, seal leakage can be a determinant of cycle life. In spite of these pioblems, howevei, the safety and reliability of the Na—S system has progressed to the point where pilot plant production of these batteries is anticipated for EV and aerospace applications. 

This paper restricts itself to sodium-sulfur cells and batteries which use solid electrolyte cell dividers and provides a current picture of the state of scientific knowledge and technological achievement with respect to sodium-solid electrolyte-sulfur batteries. The references cited should not be construed as a complete review but should instead be viewed as an introduction to the relevant literature. 

Basic Properties of Sodium -Solid Electrolyte-Sulfur Batteries... 

W. E. Cowley, G. Thwaite, G. Waine, The Selective Recovery Of Sodium From Mmalgam Using fd-Mlumina, Associated Octel Co. Ltd. 1978, presented at the Second International Meeting of Solid Electrolytes, University of St. Andrews, Scotiand. 

Conventional batteries consist of a liquid electrolyte separating two solid electrodes. In the Na/S battery this is inverted a solid electrolyte separates two liquid electrodes a ceramic tube made from the solid electrolyte sodium /5-alumina (p. 249) separates an inner pool of molten. sodium (mp 98°) from an outer bath of molten sulfur (mp 119°) and allows Na" " ions to pass through. The whole system is sealed and is encased in a stainless steel canister which also serves as the sulfur-electrode current collector. Within the battery, the current is passed by Na+ ions which pass through the solid electrolyte and react with the sulfur. The cell reaction can be written formally as... 

Typical dimensions for the /5-alumina electrolyte tube are 380 mm long, with an outer diameter of 28 mm, and a wall thickness of 1.5 mm. A typical battery for automotive power might contain 980 of such cells (20 modules each of 49 cells) and have an open-circuit voltage of lOOV. Capacity exceeds. 50 kWh. The cells operate at an optimum temperature of 300-350°C (to ensure that the sodium polysulfides remain molten and that the /5-alumina solid electrolyte has an adequate Na" " ion conductivity). This means that the cells must be thermally insulated to reduce wasteful loss of heat atjd to maintain the electrodes molten even when not in operation. Such a system is about one-fifth of the weight of an equivalent lead-acid traction battery and has a similar life ( 1000 cycles). 

Sodium-Sulfur Batteries. The sodium-sulfur battery consists of molten sodium at the anode, molten sulfur at the cathode, and a solid electrolyte of a material that allows for the passage of sodium only. For the solid electrolyte to be sufficiently conductive and to keep the sodium and sulfur in a liquid state, sodium-sulfur cells must operate at 300°C to 350°C (570°F to 660°F). There has been great interest in this technology because sodium and sulfur are widely available and inexpensive, and each cell can deliver up to 2.3 volts. 

Zinc is electrodeposited from the sodium zincate electrolyte during charge. As in the zinc/bromine battery, two separate electrolytes loops ("posilyte" and "nega-lyte") are required. The only difference is the quality of the separator The zinc/ bromine system works with a microporous foil made from sintered polymer powder, but the zinc/ferricyanide battery needs a cation exchange membrane in order to obtain acceptable coulombic efficiencies. The occasional transfer of solid sodium ferrocya-nide from the negative to the positive tank, to correct for the slow transport of complex cyanide through the membrane, is proposed [54],... 

The power of the ZEBRA cell depends on the resistance of the cell during discharge. The resistance of the ZEBRA cell rises with increasing depth of discharge (DOD). There is a contribution to the resistance from the fixed values of the solid metal components and of the/ "-alumina solid electrolyte. The variable parts of the resistance arc the sodium electrode and the positive electrode. The increase in internal resistance during discharge is almost entirely due to the positive electrode, as can be seen from Fig. 4. 

In the Na/S system the sulfur can react with sodium yielding various reaction products, i.e. sodium polysulfides with a composition ranging from Na2S to Na2S5. Because of the violent chemical reaction between sodium and sulfur, the two reactants have to be separated by a solid electrolyte which must be a sodium-ion conductor. / " -Alumina is used at present as the electrolyte material because of its high sodium-ion conductivity. 

Today, the term solid electrolyte or fast ionic conductor or, sometimes, superionic conductor is used to describe solid materials whose conductivity is wholly due to ionic displacement. Mixed conductors exhibit both ionic and electronic conductivity. Solid electrolytes range from hard, refractory materials, such as 8 mol% Y2C>3-stabilized Zr02(YSZ) or sodium fT-AbCb (NaAluOn), to soft proton-exchange polymeric membranes such as Du Pont s Nafion and include compounds that are stoichiometric (Agl), non-stoichiometric (sodium J3"-A12C>3) or doped (YSZ). The preparation, properties, and some applications of solid electrolytes have been discussed in a number of books2 5 and reviews.6,7 The main commercial application of solid electrolytes is in gas sensors.8,9 Another emerging application is in solid oxide fuel cells.4,5,1, n... 

Reversible sodium backspillover as the origin of electrochemical promotion when using Na+ conductors, such as P"-A1203, as the solid electrolyte has been confirmed by the in situ XPS work of Lambert and coworkers.56 61... 

The same reversible appearance and disappearance of the Pt(lll)-(12xl2)-Na overlayer is shown in Figure 5.51, together with the corresponding two-dimensional Fourier-transform spectra and also in Fig. 5.52, which shows smaller areas of the sodium-free and sodium doped Pt(lll) surface. The reversible electrochemically controlled spillover/backspillover of sodium between the solid electrolyte and the Pt(lll) surface is clearly proven. 

As already analysed in Chapter 5, once the backspillover species originating from the solid electrolyte have migrated at the metal/gas interface, then they act as normal (chemical) promoters for catalytic reactions. For example, Lambert and coworkers via elegant use of XPS18 have shown that the state of sodium introduced via evaporation on a Pt surface interfaced with P"-A1203 is indistinguishable from Na5+ introduced on the same Pt surface via negative (cathodic) potential application. 

Nasicon solid electrolyte electrochemical promotion with, 440 sodium ion conductor, 440 NEMCA, see electrochemical promotion NEMCA coefficient, 152,319... 

A sodium-sulfur cell is one of the more startling batteries (Fig. 12.23). It has liquid reactants (sodium and sulfur) and a solid electrolyte (a porous aluminum oxide ceramic) it must operate at a temperature of about 320°C and it is highly dangerous in case of breakage. Because sodium has a low density, these cells have a very high specific energy. Their most common application is to power electric... 

Rechargeable cells with a sulfur cathode, a sodium anode, and a solid electrolyte such as yS-alumina have been much investigated [8[. They are operated at about 618 K, temperature at which sodium and sulfur are both liquid. 

Sodium )S-alumina has received considerable attention, triggered by the discovery of Weber Kummer (1967) that it can be used as a solid electrolyte in the... 

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