The present disclosure relates to relates to heat-resistant gel electrolyte materials and their uses, for example, in electrochromic devices such as electrochromic windows. In certain embodiments, the disclosure provides an electrolyte material including a polymer of ethyleneimine, optionally at least partially crosslinked (e.g., with an epoxide crosslinker such as the diglycidyl ether of bisphenol A); a lithium salt (e.g., lithium perchlorate); and a high-boiling solvent (e.g., DMSO). The electrolyte materials can be used in electrochromic devices, such as electrochromic windows, e.g., for use as automobile sunroofs.