In order to discover the effects of the inorganic particles on the polymer electrolytes, the composite polymer electrolytes(CPE) were prepared using poly(ethylene oxide)(PEO)(MW 600,000)/$LiCF_3SO_3$/particles. The inorganic particles used are alumina (5㎛ size), alumina(11nm size), and silica (11nm size).
We focused on two properties of CPE, that is, the bulk properties and the interfacial properties. Additionally, the effect of particle size, kind, and content are considered as variables. In the all experiments, the effect of particle kind experimented on the properties of CPE is not so much significant.
The ionic conductivity is maintained upto 20 wt% particle content and then decrease over 20 wt% in the all systems. The weight fraction of the amorphous region of PEO within CPE shows similar trend with that of ionic conductivity and thus is considered as dominant factor to determine the ion conduction behavior. The glass transition temperatures are not so much changed with particle contents. The heat of fusion of PEO was decreased with particle content in the micrometer-particle system, but was decreased and then increased with particle content in the nm-particle system. In the 11nm particle systems, considerable agglomeration of particles are observed at 25 wt%, which explain the increase of the crystallinity of PEO at 25 wt%. Contrarily, few agglomeration is found at 40 wt% particle in the micrometer-particle system, which illustrates well-dispersed particle as proved in the SEM.
The interfacial resistance(Ri) at the interface of Li/CPE is maintained with similar value as a function of the storage time under storage at 75℃ As particle content is increased, Ri is increased except for the CPEs containing small particle content, which have similar Ri values with polymer electrolytes of PEO/$LiCF_3SO_3$. The rate of increasing in Ri with particle content is larger in the nm-particle systems than that in the micrometer-one, due to the difference of surface area of particles. Thus, the Ri at 25 wt% nm-particle becomes very high, around 1000Ω, but the Ri at 52 wt% micrometer-particle shows only somewhat below 200Ω.
In order to observe the lithium deposition-stripping process, the cyclic voltammetry(CV) was experimented by using SS/CPE/Li for 20 cycles at 75℃. The anodic peak current corresponding to the deposition of Li on SS electrode around -0.7V and the cathodic one indicating the stripping of Li into electrolyte around 0.7V appeared. There are several characteristics observed ; a) maximum peak currents at low particle contents, 5 wt% in nm-particle system and 18 wt% in $\mu m$-particle system, indicating good filler effect to form the uniform interface at the electrode/CPE without blocking of the ion conduction. b) different maximum particle contents in nm-particle and $\mu m$-particle system each other, due to the different surface area of particles resulted from different particle size. c) different peak current value with particle size. The higher value of peak current is observed in the nm-particle system owing to the formation of the more uniform interface at the electrode/CPE.
In addition, the optical microscopic image at the surface of stainless steel electrode with CV experiment showed the generation of the more stable and uniform interface at the low particle content. These were also related with the lower increase of the interfacial resistance at low particle content after the CV is applied.
