Li-Ion Conductive Poly(ethylene carbonate)-Based Electrolytes and Composites for Novel Battery Materials

Polymer electrolytes have attracted much attention as ion-conductive soft materials for novel battery systems, because of their safety compared with liquid electrolytes, their flexibility and their lightweight.  Ionic conduction in poly(ethylene oxide) (PEO)-metal salt complexes was reported in 1973, and there have since been many studies of the macromolecular design of PEO-based polymers as electrolyte materials that addressed reducing degrees of crystallinity as well as the exhibition of good conductivity and superior salt solubility.  However, the conductivity of typical PEO-based electrolytes is limited to approximately 10^-5 S/cm at room temperature, and it is extremely difficult to enhance the lithium (cation) transference number.  We believe that the development of novel polymers without PEO chains is one solution for improving the conductivity.

We shall consider polycarbonates that can be obtained by the alternating copolymerization of carbon dioxide with epoxides and comprise novel polymer candidates for electrolytes.  Following the first report of the CO₂/epoxide copolymer by Japanese group, there has been considerable development of highly active catalysts for effective polymerization.  This copolymer is a remarkable macromolecule because it utilizes CO₂ as a raw material and has excellent properties; it is biodegradable, easily processed and colored, and has high transparency and low oxygen permeability.  We have focused on the chemical structure of the copolymer, which has one alternating carbonate group (-O-(C=O)-O-) in each repeating unit of the main chain.  The carbonate group has a large dipole moment, and it can dissolve many types of salts.  Carbonate-based organic solvents, such as dimethyl carbonate, are used as the electrolyte solution in Li-ion batteries because of their high dielectric constant.  Therefore, the carbonate group provides a suitable structure for the polymer framework.  Only two polycarbonate-based electrolytes using a poly(vinylene carbonate) and a poly(trimethylene carbonate) have previously been reported.  In 2010, we synthesized glycidyl ether-based polycarbonates and reported the ionic conductivities of these electrolytes.  Last year, we reported PEC-based electrolytes including typical Li salts.

In this study, we found that the use of LiFSI as a salt for PEC is very effective for improving the ionic conductivity.  Here, we first report that the addition of TiO₂ nanoparticles to the PEC-based electrolytes can enhance lithium transference number, t₊, using an electrochemical combination method of DC polarization and AC impedance measurements for Li│electrolyte│Li cells.  Moreover, we undertook lithium-7 and fluoride-19 NMR spectroscopic and pulsed field gradient (pfg) diffusion measurements on these electrolytes.  The values of self-diffusion coefficients of the lithium cation (D_Li) and the estimated t₊ values for PEC-LiFSI-TiO₂ are more than 10^-7 cm²/s and 0.8 at 60 ^oC.  The Li-ion conductivities (s at 60 ^oC × t₊ estimated from the electrochemical method) of samples PEO₂₀LiFSI, PEC_0.53LiFSI and PEC_0.53LiFSI-TiO₂ (1 wt%) were calculated to be 5.6×10^-5, 2.2×10^-4 and 4.3×10^-4 S/cm respectively.  We conclude that the polycarbonate is superior as a structure to polyether as an electrolyte for flexible batteries.