Conventional polymer electrolytes

Because the conductivity of polymer electrolytes is generally low, thin batteries are assembled (50-200 pm) with electrolyte thickness ranging from 20 to 50 pm. Conventional polymer electrolytes based on PEO and lithium salts, owing to their high crystallinity, reach useful conductivity values only at temperatures above 60 °C, i.e., above the melting temperature of the crystalline phase. If the low conductivity at room temperature prevents their application for consumer electronics, this does not represent an obstacle for electric vehicles, for which an operating temperature higher than that of the transition in the amorphous phase is expected. 

For the purpose of the discussion here, highly conductive polymer electrolytes are defined as those which have conductivities of greater than or equal to 10" Scm at room temperature. They may be broadly classified as (i) conventional polymer electrolytes and (ii) non-conventional polymer electrolytes. It is to be noted that while I will draw ample examples from the literature to illustrate the topics of discussion, no attempt will be made to present a comprehensive list of highly conductive polymer electrolytes developed to date. 

It is perhaps not coincidental that the conventional polymer electrolyte which reportedly exhibits a conductivity of about 10 Scm at room temperature is a LiN(S02CF3)2 complex. Le Nest et al. [19] prepared this electrolyte from a new polymer network composed of PEO-polyurethane... 

We have prepared [17] electrolytes from LiN(S02Cp3)2 and both MEEP and MEEP/PEO mixtures vide infra). These electrolytes exhibited a room temperature conductivity of about 6x 10" Scm" one of the highest for conventional polymer electrolytes. 

Dimensional stability of higly conductive conventional polymer electrolytes... 

LiN(S02CF3)2. Not only was the electrolyte film of 55 wt % MEEP 45 wt % PEO-[LiN(S02CF3)2]o.i3 dimensionally stable, but it also exhibited a conductivity of 6.7 x 10 Scm" at 25°C, among the highest reported to date for a dimensionally stable conventional polymer electrolyte. The log conductivity versus 1/T plot for this electrolyte given in Figure 3.4 has a profile expected for an amorphous material showing the VTF behaviour. 

Very recently, new solid ionic conductors, so-called polymer in-salt materials, have been reported, in which lithimn salts are mixed with small quantities of the polymers PEO and PPO, while conventional polymer electrolytes ( salt-in-polymer ) contain only one Li per about ten repeat units of ether. The reported conductivity in the AlCl3-LiBr-LiC104-PP0 system is as high as 0.02 S cm at room temperature. 

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