Adriana Saldívar-Martínez: Building Solid Polymer Electrolytes From A Single Monomer (Almost)

Abstract

The current era is seeing a great increase in interest and application of batteries as a result of the world's transition to electrification. Today's exponential growth in battery demand is mostly driven by the transportation sector, particularly the need for electric vehicles.

The liquid electrolytes that are now utilized in lithium-ion batteries must be replaced with solid-state alternatives such as polymers. This replacement is primarily motivated by higher energy density and safety, but it also addresses sustainability and cost concerns, as well as a wider operating temperature range, which are usually noted as potential improvements. However, solid polymer electrolytes (SPEs) have lower ionic conductivity than liquid electrolytes, and enhancing conductivity usually comes at the expense of mechanical integrity.

We can improve the conductive characteristics of SPEs by altering the composition of their polymer hosts. First, in order to have a suitable polymer host, a copolymer with two distinct functionalities, ether and carbonates, was produced in Paper I. The synthesis was simplified by performing ring-opening polymerization in bulk of a single monomer, trimethylene carbonate. Poly(trimethylene carbonate-co-trimethylene ether) (PTMC-co-PTME) copolymers of various compositions were obtained. In Paper II, electrolytes of previously synthesized copolymers were produced using LiTFSI salt to assess their electrochemical properties. The ionic conductivity of these electrolytes was higher than that of pure PTMC electrolytes when a moderate concentration of ether groups was present in the main chain.

Finally, in Paper III, we investigated the use of PTMC-co-PTME electrolytes as the conductive phase in a triblock copolymer (BCP) system in which highly crystalline PLLA was added to act as a hard block, hence giving mechanical integrity. The morphology and electrochemical performance of BCP SPEs were assessed. Their ionic conductivity was discovered to be comparable to standard polymer electrolytes. The combination of a high melting temperature and good ionic conductivity in its semi-crystalline state creates prospects for using crystallinity to improve mechanical properties and broaden the operational temperature window of lithium-ion batteries.