Crystalline polymer electrolyte structure

There is increasing interest in sodium-based batteries, because of the greater abundance and lower costs of sodium compared with lithium. Recently the first examples of crystalline polymer electrolytes that support Na" " conduction have been reported.Potassium and rubidium based crystalline polymer electrolytes have also been described. " This result is significant not only because these are the first Na, K" ", Rb, crystalline polymer electrolytes but also because they are the first example with a crystal structure that differs from the PEOg LiXF6 complexes. In the former case, the ratio of ether oxygens to cations is 8 1 rather than 6 1 and only one polymer chain wraps around the cations (Figure 1.15). 

There are two classes of materials which may be used as electrolytes in all-solid-state cells polymer electrolytes, materials in which metal salts are dissolved in high molar mass coordinating macromolecules or are incorporated in a polymer gel, and ceramic crystalline or vitreous phases which have an electrical conductance wholly due to ionic motion within a lattice structure. The former were described in Chapter 7 in this... 

Various methods have been employed to find out about the structure of polymer electrolytes. These include thermal methods such as differential scanning calorimetry (DSC), differential thermal analysis (DTA), X-ray methods such as X-ray diffraction and X-ray absorption fine structure (XAFS), solid state NMR methods particularlyusing7LiNMR,andvibrationalspectroscopicmethodssuch as infrared and Raman [27]. The objective of these various studies is to establish the structural identity of the polymer electrolyte at the macroscopic as well as the molecular levels. Thus the points of interest are the crystallinity or the amorphous nature of materials, the glass transition temperatures, the nature and extent of interaction between the added metal ion and the polymer, the formation of ion pairs etc. Ultimately the objective is to understand how the structure (macroscopic and molecular) of the polymer electrolyte is related to its behavior particularly in terms of ionic conductivity. Most of the studies have been carried out, quite understandably, on PEO-metal salt complexes. In comparison, there has been no attention on the structural aspects of the other polymers particularly at the molecular level. 

Wieczorek, W. et al.. Modifications of crystalline structure of PEO polymer electrolytes with ceramic additives. Solid State Ionics, 1989. 36(3) 255-257. 

The WLF formula shows that the ionic conductivity of the polymer electrolyte is shown in the temperature range higher than Tg. Ionic conductivity decreases rapidly if its temperature goes below that of Tg. The EO unit is recognized as the most excellent structure from the ionic dissociation viewpoint. The ion is transported coupled with the oxyethylene chain motion in amorphous polymer domain. However, oxyethylene structure easily becomes crystalline. Therefore, in order to accelerate the quick molecular motion of the polymer chain and quick ion diffusion, it is important to lower the crystallization of polymer matrixes. The methods for inhibiting the crystallization of the polymer are, for example, to introduce the polyethylene oxide chain into the low Tg polymer such as polysiloxane and phosp-hazene, or to introduce the asymmetric units such as ethylene oxide/propylene oxide (EO/PO) into polymer main chain. 

As two important copolymers of PVDF, the P(VDF-HFP) [4] and P(VDF-CTFE) [23] had been developed for gel polymer electrolyte in LIBs. The introduction of copolymer components was to reduce the crystallinity of the PVDF chain. The reduction of crystallinity could increase the ionic conductivity. Electrospun P(VDF-HFP) and P(VDF-CTFE) fibrous membranes had been proved to show high ionic conductivities in the range of several mS cm which was attributed to the easy transportation of the liquid electrolyte through the fully interconnected pore structure of the membrane. For example, the electrospun P(VDF-HFP) fibrous membrane had high ionic conductivities in the range of 4.59 mS cm", high electrolyte uptake of 425 % at room temperature, and good electrochemical stability with a potential of over 4.5 V versus Li/Li+ [29]. 

Currently, most polymer hosts as well as the polymer electrolytes are crystalline, which is one reason why the ionic conductivity of the polymer electrolyte is not comparable to that of fhe liquid electrolyte. The crystals formed by a polymer are mostly spherulite, which also contains amorphous areas. It is generally believed that ionic conduction occurs mainly in amorphous areas. Understanding the crystal structure of the polymer is therefore beneficial for the understanding of its ionic conductive behavior, although several factors can affect ionic conductivity. 

The PE0-LiCp3S03 has a helical crystal structure. The Li+ ions are located at the centerline of the helix, and the anions are located on the outer side of the helix. Therefore, anions can also be transferred in this polymer electrolyte, leading to self-discharge of the battery. In Section 9.5.2, the syntheses of several new lithium salts were discussed. They are expected to reduce the crystallinity of PEO and the transference number of the anions, increase the ionic conductivity, and reduce self-discharge. 

The structure of PVDF is symmetrical and regular, and it crystallizes easily, which is not favorable for ion conduchon. The copolymer P(VDF-HFP) has a lower crystallinity and a better gel-forming ability than PVDF. The ionic conductivity of the gel copolymer is higher than that of PVDF (around 10 S/cm), and it is mechanically strong. Hence, most research on F-containing gel polymer electrolytes has been done with P(VDF-HFP). 

C- Li REDOR experiments applied to polymer electrolytes have been conducted by Reichert et who were able to determine carbon-lithium distances in crystalline PEOeLiPFg. In another study, Wickham et al have used the REDOR approach to investigate the microscopic structure of the crystalline domains in PE02oLiTf.The interatomic distances obtained from solid state NMR were reported to be in excellent agreement with the values from X-ray diffraction. 

Multivalent polymer electrolytes that use PEO as the polymer base are usually multiphase systems consisting of salt-rich crystalline phases, another crystalline phase of pure polymer, and amorphous phases with dissolved salts. Conductivity is thus often affected by factors such as slow crystallisation and salt redistribution processes between the phases, which result in values dependent on the thermal history, preparation methods, etc. The morphological and crystallographic structures of these polymeric systems are presented, and the influence of factors such as crystallisation of the pure polymer on ionic conduction, and the factors that influence them in turn, are highlighted. Discussion of conduction takes into account the two possible mechanisms of mobility of charged ionic clusters and exchange of... 

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