Layered and interfacially blended polyelectrolyte multi-walled carbon nanotube composites for enhanced ionic conductivity

To enhance the ionic conductivity in solid phase polyelectrolyte systems for lithium ion battery applications demands effective control of the phase properties. Here, we report on a strategy involving a layer-by-layer methodology of two polyelectrolytes, poly(ethylene oxide) (PEO) and poly(acrylic) acid (PAA) and carboxylic acid functionalized multi-walled carbon nanotubes (MWNTs). Optimization of the assembly strategy revealed that undoped and lithium-ion doped stacking of four layers provides excellent film growth and improvement of the ionic conductivity of up to 10− 5 S cm− 1, which exceeds conventional assemblies of lithium-ion doped [PEO/PAA] by up to two orders of magnitude. Although ionic conductivity was most effectively enhanced for ultrathin films (< 100 nm), [PEO/PAA/PEO/(PAA + MWNT)] stacking still provides an ionic conductivity of > 10− 6 S cm− 1 for thick films (> 2 μm). The improvement of ionic conductivity was attributed to (i) interfacial phase mixing (blending) of the two polyelectrolytes, (ii) the MWNT contribution in the interfacial region, and (iii) the preferential adsorption of lithium-ions along the carbon nanotubes. This study involved a series of scanning probe methods including lateral force microscopy, and electrostatic force microscopy.