Solid polymer electrolytes applied in lithium ion batteries

5 Solid polymer electrolytes applied in lithium ion batteries 

One of the typical preparation procedures of the solid polymer electrolytes is as follows. The polymer and lithium salts are first dissolved in a selective solvent under dry atmosphere to form a homogeneous solution. Then, the mixture is coated onto a PTFE plate and dried in vacuo for several days to give a polymer electrolyte membrane. 

The ion conducting mechanism is shown in Fig. 14.5. The lithium salts dissolve and disassociate in the polymer matrix to set off free ions. The ion conduction mainly takes place in the amorphous phase, and the lithium ion can coordinate with oxygen atoms in the PEO chain in a fixed ratio. These complexed ions move with the motion of a segment of polymer chain. Consequently, when the electric field is applied towards the polymer electrolyte, the cations move freely from one electrode to another (Fontenella et al, 1983 Song et al, 1999). 

As discussed above, a polymer electrolyte for rechargeable lithium ion batteries should satisfy at least four requirements for practical applications. However, among those requirements the ion conductivity is the most vital and should be put first. The following discussion focuses on how to enhance the ion conductivity of solid polymer electrolytes. As a typical polymer 

When lithium salts dissolve, not ah the salts can dissociate and become free ions. On the contrary, there exist at least three states of lithium salts in a polymer matrix, i.e. free ions, ion pairs and aggregations. The hrst requirement for hthium salts is their solubihty in the polymer matrix. Traditional lithium salts used in the polymer electrolytes are L1PF4, LiC104, L1BF4, etc. Recently lithium salts with a large anion such as LiCFsSOs and hthium trifluoromethanesulfonimide (LiTFSI) have attracted much attention, because a larger anion radius promotes ion conductitivity as it is easier to dissociate in the polymer matrix and set off free hthium cations that increase the ion conductivity. 

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