Li-Ion Coordination and Transport Properties in Gel Polymer Electrolytes Based on Amphiphilic Block Copolymers

Lithium-ion batteries (LIBs) are widely used energy storage systems in the sector of electric devices and are already utilized in electric vehicles (EVs) and hybrid electric vehicles (HEVs). Especially in the latter case, safety plays a crucial role. For this reason, in the last decades, extensive efforts have been made to reduce the safety concerns related to the use of organic carbonate-based liquid electrolytes. State-of-the-art liquid electrolytes show a high conductivity and good electrochemical performance but they also display low flash and boiling points and are prone to leakage. As alternative solid polymer electrolytes (SPEs)¹ were introduced. Consisting of a polymer matrix containing a lithium salt without solvents, these electrolytes avoid the danger of leakage and show a high mechanical stability, which allows their simultaneous use as separator. However, they display a low conductivity at room temperature.

A possibility to combine the advantages of the two above-mentioned systems is the use of gel polymer electrolytes (GPEs)². Consisting of a polymer matrix in which the liquid electrolyte is immobilized, GPEs show conductivities in the mS cm^-1 range while having a sufficient mechanical stability to work as separator. Furthermore, in the absence of “free solvent” the danger of leakage can be avoided.

In this paper, we report on GPEs based on non-commercial amphiphilic block copolymers with a norbornene backbone. The use of monomers with different side chains and a living polymerization method (ring opening metathesis polymerization, ROMP)³ allow the tailoring of the properties of the polymer matrix according to the desired application. Additionally, the obtained polymers show a low PDI. When gelled with the liquid electrolyte 1 M LiPF₆ in EC:DMC 1:1 (w:w) the resulting GPEs display conductivities up to 2.5 mS cm ^-1 and a broad electrochemical stability window comparable to that of the liquid electrolytes. Within the investigations of the GPEs, particular attention was laid on the interactions between the lithium ions and the other components of the GPE system as well as on the lithium ion transport properties. For these investigations Raman spectroscopy⁴ and pulsed field gradient (PFG)-NMR were used⁵. Besides the influence of the side chains of diblock copolymer-based host systems, the difference between GPEs based on diblock copolymers and triblock copolymers comprising an additional block containing cyclic carbonate moieties was studied.

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