A series of polyetheresters(PES) containing the different length of ethylene oxide(EO) units or methylene units in the polymer backbone was synthesized by solution polycondensation and was characterized by $^1H$＆$^{13}C$ NMR, DSC, GPC, X-ray diffraction, polarizing microscopy experiments. Most of polyetheresters synthesized had a relatively flexible chain to assist the ion transport, and the degree of crystallinity was significantly reduced by alternating introduction of methylene units into EO chains, as compared with that of the poly(ethylene oxide)(PEO) homopolymer. The polyetheresters were complexed with lithium salts to obtain a novel class of solid polymer electrolytes, and the relationships between the structures and properties of these polymer electrolytes were investigated. These polymer electrolytes showed the ionic conductivities up to the range of $10^{-5}~10^{-4}$S/cm at 25℃. The enhanced conductivities of these polymer electrolytes are attributed to the reduced crystallinity of a matrix polymer. The $^7Li$ NMR spin-spin relaxation studies were carried out to investigate the local environments and dynamics of ions in the polymer electrolytes. The $^7Li$ NMR relaxation results revealed the coexistence of two separate components with substantially different spin-spin relaxation times($T_2$), indicating that the presence of mobile lithium species as well as tightly bound lithium species. From these results, the relative ionic mobility and the fraction of free mobile ions contributing to the ionic conductivity could be monitored. The combination of the ionic conductivity and $^7Li$ NMR relaxation data of the polyetherester complexes containing a $LiClO_4$ salt provided the evidence that the controlling factor for ionic conductivity in these systems was a solvating capacity caused by the difference in the density of polar groups. An increase of conductivity with temperature proved to be mainly attributed to the increase of the mobility of the charge carriers by dielectric relaxation studies.
The crosslinking of the linear polyetherester with an amorphous poly(propylene glycol)(PPG) triol using hexamethylene diisocyanate provided the networks exhibiting both good mechanical properties and low melting point without compromising the chain flexibility. The crosslinked polyetherester complexed with a $LiClO_4$ salt showed an ionic conductivity as high as $2×10^{-5}$ S/cm at room temperature. From the $^7Li$ NMR relaxation results, conductivity differences between linear PES complex and crosslinked PES complex were confirmed to be caused by the mobility differences rather than the differences in the number of charge carriers.
Blend-based polymer electrolytes composed of PEO, poly(oligo[oxyethylene] oxysebacoyl)(PES) and lithium salts have been prepared. The characteristics of these polymer electrolytes were investigated in terms of blend composition, the salt species and its concentration. The addition of PEO to PES-based polymer electrolyte significantly improved the dimensional stability with a slight decrease in the latter's ionic conductivity. The PES(60,0.10,$LiClO_4$) polymer electrolytem exhibited an ionic conductivity of $3×10^{-5}$ S/cm at 25℃, and was an elastomeric material with dimensional stability.
The interfacial characteristics of the blended polymer electrolytes composed of PES, PEO and lithium salts were investigated in terms of the blend composition, the salt used and its concentration. The lithium passivation appeared to take place in all the blended-based polymer electrolytes studied. Based on the activation energy of the ionic conduction process in the passivating layer, LiOH and $Li_2O$ are thought to be the major components in the passivating film. The kinetics and the degree of passivation varied as a function of blend composition, the salt used and its concentration, and eventually PES(60,0.10,$LiClO_4$) electrolyte system proved to be the most satisfactory system to maximize the ionic conductivity and to assure acceptable stability of the lithium electrode in the polymer electrolytes. This system has proved to be electrochemically stable up to 4.9 V vs. Li and the transport number in this system was determined to be about 0.37 at 40℃
The flat-type lithium polymer battery prototype comprising Li|PES(60,0.10,$LiClO_4$)|CC(CC : composite cathode consisting of polymer electrolyte, $V_6O_{13}$ and carbon black) was assembled, and its discharge characteristics was investigated. The poor discharge performances observed in this cell proved to be mainly associated with the loss of electrical contact between active material particles in the composite electrode as a result of the volume expansion/contraction during the lithium intercalation/deintercalation processes. It is thought that the morphology and composition of the composite cathode should be optimized to achieve the final goal of improving the battery performance.
