Solid Polymer Electrolytes (SPEs) are promising candidates for all-solid-state lithium batteries. Light, flexible and non-flammable, they could overtake many limits of classical Li-ion batteries. Moreover, polymer electrolytes allow for the use of Li metal as negative electrode without safety issues of dendrites formation. But the current state-of-the-art on SPEs generally reports low ionic conductivity at room temperature (RT), poor lithium transport properties, and large interfacial resistance. All these limitations explain the difficulties of SPEs to be used in practical applications at RT.

Gel Polymer Electrolytes (GPEs), composed of a polymer matrix swelled by a liquid electrolyte, are a first step to approach solid state electrolytes. Preliminary results confirm that GPEs exhibit high ionic conductivity and good interfacial contact. In addition, preparation via in-situ polymerization is compatible with the conventional Li-ion fabrication equipment simplifying the battery assembly process.

A GPE based on poly(ethylene glycol diacrylate) (PEGDA) as polymer matrix was synthetized using in-situ thermal cross-linking. Coupled with a non-flammable liquid phase, this GPE shows good performance in terms of ionic conductivity, electrochemical stability and safety. The mechanical properties can be tuned by changing the polymer-to-liquid weight ratio. As a proof of its suitability for Li-ion batteries, NMC and graphite electrodes with high loading were used to prepare 100 mAh pouch cells. After studying polymer distribution in the electrode, electrochemical testing revealed a very good capacity retention (>80% after 350 cycles).