Composite polymer electrolytes in lithium ion batteries

Evidently, both SPEs and GPEs are not yet perfect, since, for example, the former suffers from low ion conductivity and the latter is impaired by its liquid components. However, recent studies have revealed that the addition of ceramic fillers into the SPE can improve the ion conductivity of polymer hosts and their interfacial properties in contact with the lithium electrode. The rise in the ion conductivity is explained by means of enhancing the amorphous phase of the polymer or hindering recrystaUization. In all the cases the particle size and characteristics of the ceramic fillers are the key factors to improve the electrochemical properties of the electrolytes. In a pioneering work,Weston and Steele (1982) first demonstrated the effectiveness of incorporating inert filler (a-alumina) into the PEG system. The mechanical strength and ion conductivity are significantly enhanced upon 

Li et al (2001, 2003) prepared a kind of hyperbranched solid polymer electrolyte with a low ion conductivity of about 10 S/cm at room temperature. Later, they added 10 wt% BaTiOs as ceramic fillers into this SPE, making the ion conductivity rise to 1.4 x 10 S/cm. 

Based on the GPEs formed from a PVCrPMMA blend with micropore structure, some researchers added silica to increase the uptake of the Uquid electrolyte. The ion conductivity increased with increasing PMMA content in the blend and a room temperature ion conductivity based on PVCrPMMA (5 5, w w) blend of 1.1 x 10 S/cm was reached. 

Features of ceramic particles such as the type, morphology, particle size, particle content in the CPE and so on are important in improving its ion conductivity. In general, ceramic fillers used for the polymer electrolyte matrix are classified into active and passive species. The active one participates in the ion conduction process, e.g. li2N and LiAl203,while the inactive ones, such as AI2O3, Si02, MgO, are not involved in the lithium ion transport process. The selection of filler between active and passive components is quite arbitrary. 

It was reported that the ion conductivities measured at various temperatures for PVdF-HFP/Ti02 (rutile) show slightly lower values than those of using T1O2 (anatase), but the mechanism is not clear (Stephan and Nahm, 2006). 

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