Polyphosphate-based copolymers as Solid Polymer Electrolytes in Lithium-Metal betteries

Abstract

Lithium batteries are now present in our everyday life, powering portable electronics, power tools, sustainable vehicles such as hybrids and electric, as well as back-up devices and electrochemical storage systems in renewable energy plants. Lithium-metal is the best choice of anode material, since it provides the lowest reduction potential (∼ -3.0 V versus SHE) as well as the lowest density (0.534 g.cm-3), which is responsible for high specific capacity (3.86 Ah.g-1) and energy density (1470 WhK.g-1). Lithium-metal based batteries (LMBs) seem thus to be the most promising technology for the implementation of high energy density storage devices. However, before their practical application, LMBs must face the issue of lithium dendrites growth, which is the main cause of internal short circuits and thermal run-away reactions. Today, the most widespread solution to face this problem is based on the replacement of conventional liquid electrolytes with solid-state electrolytes (SSEs). This work is focused on solid polymer electrolytes (SPE), which are lightweight materials that provide flexibility, easy handling, long lifespan, wide electrochemical stability window as well as safety, by eliminating lithium dendrites growth. Hence, the first part of the thesis is dedicated to the synthesis of a triblock copolymer for solid-polymer electrolytes (SPEs) application in lithium-metal batteries. This triblock copolymer PPE-b-PEO-b-PPE covalently associates a poly(ethylene oxide) block that ensures ionic conduction with two poly(phosphate) side blocks. Since the poly(phosphate) blocks are characterised by a very low Tg (about -70°C), they will contribute to increase the mobility of PEO block and its amorphous phase, affording SPEs with enhanced ionic conductivity. The mechanical properties of the SPE synthesised were assessed by conducting tensile experiments, which showed a maximum Young modulus of 26 MPa. Cyclic voltammetry experiments displayed an electrochemical stability window ranging from 0 V to 5 V, which is in good agreement with general requirements. Moreover, thermogravimetric analysis showed that triblock copolymer is stable until ~220°C; the solid polymer electrolyte proposed can thus be safely implemented in batteries, in a wide temperature range.