Ionically-conductive and stretchable hydrogels are ideally suited for the synthesis of flexible electronic devices. However, conventional hydrogels undergo dehydration at ambient conditions and freeze at subzero temperatures, limiting their functions. As an alternative to counteract these limitations, we propose double network hydrogels that are easily synthesized by a one-step acrylamide (AM) polymerization in the presence of cellulose nanofibrils (CNF) and LiCl. Following molecular dynamics simulation, thermogravimetric and spectroscopic (Raman and low-field nuclear magnetic resonance) analyses, we show that LiCl increases the interactions between the colloidal phase and water molecules, ensuring water holding capability at atmospheric conditions and endowing the hydrogels with freezing tolerance over a wide range of temperatures, from -80 to 25 °C. The synergy between CNF and LiCl is critical in maintaining the mechanical strength of the system, which simultaneously displays high stretchability (∼748 %) and ionic conductivity (2.25 S/m) at low temperatures (-40 °C). As a proof of concept, a flexible supercapacitor comprising the proposed electrolyte hydrogel is demonstrated as a reliable, low-temperature electrochemical device. Our results provide the basis for simple and universally applicable systems that fulfill the requirements of flexible electronics under extreme cold conditions.