Polymer electrolytes solvent requirements

Development efforts are under way to displace the use of microporous membranes as battery separators and instead use gel electrolytes or polymer electrolytes. Polymer electrolytes, in particular, promise enhanced safety by eliminating organic volatile solvents. The next two sections are devoted to solid polymer and gel polymer type lithium-ion cells with focus on their separator/electrolyte requirements. 

Abraham et al. were the first ones to propose saturating commercially available microporous polyolefin separators (e.g., Celgard) with a solution of lithium salt in a photopolymerizable monomer and a nonvolatile electrolyte solvent. The resulting batteries exhibited a low discharge rate capability due to the significant occlusion of the pores with the polymer binder and the low ionic conductivity of this plasticized electrolyte system. Dasgupta and Ja-cobs patented several variants of the process for the fabrication of bonded-electrode lithium-ion batteries, in which a microporous separator and electrode were coated with a liquid electrolyte solution, such as ethylene—propylenediene (EPDM) copolymer, and then bonded under elevated temperature and pressure conditions. This method required that the whole cell assembling process be carried out under scrupulously anhydrous conditions, which made it very difficult and expensive. 

Solvent-free polymer-electrolyte-based batteries are still developmental products. A great deal has been learned about the mechanisms of ion conductivity in polymers since the discovery of the phenomenon by Feuillade et al. in 1973 [41], and numerous books have been written on the subject. In most cases, mobility of the polymer backbone is required to facilitate cation transport. The polymer, acting as the solvent, is locally free to undergo thermal vibrational and translational motion. Associated cations are dependent on these backbone fluctuations to permit their diffusion down concentration and electrochemical gradients. The necessity of polymer backbone mobility implies that noncrystalline, i.e., amorphous, polymers will afford the most highly conductive media. Crystalline polymers studied to date cannot support ion fluxes adequate for commercial applications. Unfortunately, even the fluxes sustainable by amorphous polymers discovered to date are of marginal value at room temperature. Neat polymer electrolytes, such as those based on poly(ethyleneoxide) (PEO), are only capable of providing viable current densities at elevated temperatures, e.g., >60°C. 

The strategy of hybrid and gel electrolytes is very promising for lithium-ion batteries, but it seems less viable for lithium-metal batteries due to the reactivity of lithium metal with the encapsulated solvent. In fact, high conductivity is not the only parameter in selecting a successful polymer electrolyte for the development of lithium batteries a low interface resistance and a high interface stability over time are also required to assure good cyclability and long life. 

Solid electrolytes are not usually solutions of a conducting solute in a solvent matrix. Liquid electrolyte solutions are often sufficiently dilute (1-10 millimolar) to be described by the textbook theories of Debye-Hiickel or Onsager and oppositely charged ions are sufficiently dispersed for interaction between anions and cations to be minimized. By contrast, molten salts are very concentrated (typically 2-20 molar), ion-ion interactions are pronounced, and alternative theories such as that of Fuoss [105] are required. Polymer electrolytes typically have [repeat unit] [cation] ratios, n, in the range 8 to 30, corresponding to 0.7 to 2.5 molar for PEOn LiC104 [106], and ion clustering is an important feature of their behaviour. To account for both the ion-polymer and ion-cluster interactions, Ratner and Nitzan have developed dynamic percolation theory [107]. 

Once the coexistence of different processes during electropolymerization is detected, the final composition and properties of the electrogenerated polypyrroles can be related to the chosen parameters of synthesis. The reverse reasoning is always true and fundamental from a technological point of view on defining a property, specific conditions of synthesis can be selected in order to optimize it. As the electrochemical polymerization of pyrrole involves many experimental variables, adequate control of the polymer synthesis will require analysis of the effects of the individual parameters (electrode, solvent, electrolyte, pH of the solution, temperature, and potential of synthesis) and their interdependence. 

The polymer electrolytes discussed so far suffer from a number of disadvantages. Firstly, they exhibit low conductivities in comparison with liquid or solid (crystalline or glassy) electrolytes at or below room temperature. The best all-amorphous systems have conductivities less than 10 S cm at room temperature. These ambient temperature conductivities may be insufficient in some cases for the power required by a lithium battery. Secondly, the interfacial impedances present at both the lithium anode (passivation) and composite cathode (passivation, contact) are in addition to the ohmic losses in the electrolyte. Thirdly, the lowness of cation transference number, although similar to the values in liquid systems, is a major issue since the total conductivity is lower and could limit the use of solvent-free polymer electrolytes except in the form of extremely thin films or above room temperature. 

This discussion shows that the gel electrolyte must match the use of the battery, requiring optimization of the composition of the gel polymer electrolyte, the supporting salt and its concentration, and the solvent. PAN gel electrolytes made using different solvents, lithium salts, and composition will display different behaviors with respect to the ionic conductivity, lithium-ion transference number, electrochemical window, cyclic voltam-metric behavior, and compatibility with electrodes. Table 11.1 lists the ionic conductivity at room temperature of some gel electrolytes based on PAN. Because the PAN chain contains highly polar -CN groups, which exhibit poor compatibility with lithium metal electrodes, the passivation of the interface between the gel electrolyte and lithium metal electrode is crucial. At the same time, PAN has a high crystallization tendency. At elevated temperatures, the liquid electrolyte and plasticizer will separate therefore, the polymer is modified, mainly by copolymerization and cross-linking. 

This book opens with an exhaustively complete chapter by Aurbach on the role of surface films in the stability and operation of lithium-ion batteries. His discussion lays the groundwork for the rest of the book because it puts many of the required properties of anode, cathode, solvent, salt, or polymer electrolyte into perspective in regards to their reactivity and passivation. Development of new electrolytes, anodes, and cathodes must account for this reactivity and indeed some new and promising electrode materials may continuously lose capacity due to their inability to passivate with the electrolytes employed. 

For this reason, battery manufacturers are reluctant to fabricate batteries based on lithium metal. The only way to safely use lithium as an electrode is by coupling it with a stable electrolyte. The most common examples are sol-vent-free, polymer membranes formed by the combination of a poly(ethylene oxide) (PEO) matrix and a lithium salt, LiX [5,6]. The excess of negative charge on the oxygen in the PEO chains coordinates by coulombic attraction of the Li+ ions, thus separating them from the anions. By this process, the lithium salt is dissolved in the PEO matrix, analogous to the process of salt dissolution in liquid solvents [5]. The main difference is that while the ions can move with their solvation shell in liquids, this is not possible in the PEO complexes due to the large size and encumbrance of the chains. Therefore, ion transport in the polymer electrolytes requires flexibility of the PEO chains so... 

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