Ideal electrolyte batteries

An ideal electrolyte solute for ambient rechargeable lithium batteries should meet the following minimal requirements (1) It should be able to completely dissolve and dissociate in the nonaqueous media, and the solvated ions (especially lithium cation) should be able to move in the media with high mobility. (2) The anion should be stable against oxidative decomposition at the cathode. (3) The anion should be inert to electrolyte solvents. (4) Both the anion and the cation should remain inert toward the other cell components such as separator, electrode substrate. 

Initial measurements carried out on PEO-alkali metal salt complexes indicated that the observed conductivities were mostly ionic with little contribution from electrons. It should be noted that the ideal electrolyte for lithium rechargeable batteries is a purely ionic conductor and, furthermore, should only conduct lithium ions. Contributions to the conductivity from electrons reduces the battery performance and causes self-discharge on storage. Salts with large bulky anions are used in order to reduce ion mobility, since contributions to the conductivity from anions produces a concentration gradient that adds an additional component to the resistance of the electrolyte. 

For graphitic carbon, as mentioned in Section Z4.2.2, PC is usually not regarded as an ideal electrolyte solvent therefore, EC-based electrol5de solution is commonly used. However, the melting point of PC (-49°C) is lower than that of EC (38°C) [1]. In order to broaden the application fields of liquid electrolyte-based lithium-ion batteries, one should try to improve their performance at low temperatures such as -30°C. One may achieve this by coating with additional carbon. There is still some room for the enhancement of the reversible capacity of carbon negative electrode materials, and coating with additional carbon is one possible choice. 

In solid polymer electrolyte-composite cathode types, solid polymer electrolytes, a more recently developed material, have this required conductivity and are seen as ideal electrolytes for solid state batteries. 

Like any electrochemical device, a lithium battery uses two electrodes (anode and cathode) and an electrolyte it is thus obvious that the choice of electrolyte components is dictated by the electrode materials in use. In other words the chemistry of the two electrode-electrolyte interfaces involved in the battery ultimately determines the optimum electrolyte. In principle, however, one may choose to define an ideal electrolyte (which is usually only a wish list ) that would have the following properties (1) a large window of phase stability, i.e., no vaporization or crystallization, (2) non-flammability, (3) a wide electrochemical stability window, (4) non-toxicity, (5) abundant availability, (6) nfui-corrosive to battery components, (7) environmentally friendly, (8) robust against various abuses, such as electrical, mechanical, and thermal ones, and (9) good wetting properties at the electrolyte-electrode interface. 

The vast amount of work published on lithium batteries shows that an ideal electrolyte does not exist. What one hopes to achieve is a workable electrolyte which has enough combination of desirable properties for an acceptable commercial battery. The literature on the electrolytes for lithium batteries is extremely vast, covering perhaps well over a thousand papers and reviews. No attempt is made here to survey all these publications. The approach adopted is to give a brief synopsis of the main points by giving reference and literature entries to some key papers, especially a few critical reviews that survey the huge amount of literature. A most excellent review was published by Xu in 2004 [1] and this chapter draws heavily upon this publication for the earlier work. More recent work, especially on electrolytes involving ionic liquids is drawn from original publications and our own recent review [2]. 

The ideal nonaqueous electrolyte for practical batteries would possess the following properties ... 

For a battery to give a reasonable power output, the ionic conductivity of the electrolyte must be substantial. Historically, this was achieved by the use of liquid electrolytes. However, over the last quarter of a century there has been increasing emphasis on the production of batteries and related devices employing solid electrolytes. These are sturdy and ideal for applications where liquid electrolytes pose problems. The primary technical problem to overcome is that of achieving high ionic conductivity across the solid. 

The )5-aluminas are described in some detail in Chapter 2, only a few specific features are noted here. In the "-aluminas, the spinel blocks are stacked in such a way that the energetically equivalent sites occupied by Na" ions are ideally just half-filled in the -aluminas the spinel blocks are stacked so as to distinguish two types of Na" -ion sites of different potential energy, the Beevers-Ross (BR) and anti-Beevers-Ross (aBR) sites. In the Na-O planes, the shortest bottleneck distance 2.7 A is just a little greater than the sum of the ionic radii, 2.4 A, at room temperature, so a small value of AH can be anticipated. The discovery of fast Na -ion conductivity in the Na j5-aluminas (Yao and Kummer, 1967 Kummer and Weber, 1967) led to the invention of the Na/S battery that triggered extensive interest in the solid-electrolyte problem. 

A fuel cell is an electrochemical conversion device. It produces electricity from fuel and an oxidant, which react in the presence of an electrolyte. The reactants flow into the cell, and the reaction products flow out of it, while the electrolyte remains within it. Fuel cells are different from electrochemical cell batteries in that they consume reactant, which must be replenished, whereas batteries store electrical energy chemically in a closed system. The chemical energy of the fuel is released in the form of an electrical energy instead of heat when the fuel is oxidized in an ideal electrochemical cell. Energy conversion by a fuel cell depends largely... 

Various materials have been used as separators in zinc—bromine cells. Ideally a material is needed which allows the transport of zinc and bromide ions but does not allow the transport of aqueous bromine, polybromide ions, or complex phase structures. Ion selective membranes are more efficient at blocking transport then nonselective membranes.These membranes, however, are more expensive, less durable, and more difficult to handle then microporous membranes (e.g., Daramic membranes).The use of ion selective membranes can also produce problems with the balance of water between the positive and negative electrolyte flow loops. Thus, battery developers have only used nonselective microporous materials for the separator. 

The ideal battery separator would be infinitesimally thin, offer no resistance to ionic transport in electrolytes, provide infinite resistance to electronic conductivity for isolation of electrodes, be highly tortuous to prevent dendritic growths, and be inert to chemical reactions. Unfortunately, in the real world the ideal case does not exist. Real world separators are electronically insulating membranes whose ionic resistivity is brought to the desired range by manipulating the membranes thickness and porosity. 

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