Rational design of anti-freezing electrolyte concentrations via freeze concentration process

Abstract

Electrolyte concentration is crucial for low-temperature aqueous batteries (LTABs) as it directly dictates electrolyte freezing point. However, the conventional approach for identifying suitable concentrations relies on determining freezing points of a large number of concentration combinations in the given H₂O–solute system, which is inefficient, particularly for multiple-solute systems. Here, we propose an approach to efficiently and rationally design anti-freezing electrolyte concentrations via the freeze concentration process. Freeze concentration is a process of concentrating dilute solution by precipitating ice or hydrates at target low temperatures (T_t). For single-solute systems, the frozen concentrated electrolyte (FCE) extracted from the liquid–ice mixture has, by nature, the lowest concentration (therefore the lowest cost) that remains unfrozen at T_t. For multiple-solute systems, instead of testing a large number of concentration combinations, the proposed approach can directly determine suitable concentrations via one freeze concentration experiment at T_t. As a demonstration, we successfully designed FCEs in H₂O–LiCl, H₂O–NaClO₄, and H₂O–NaClO₄–NaCF₃COO systems, and demonstrated superior performance in Li-based LiMn₂O₄//3,4,9,10-perylenetetracarboxylic diimide (PTCDI) and Na-based Na_1.4Co[Fe(CN)₆]_0.84·2.5H₂O//PTCDI full cells. This work provides a universal and efficient strategy to design electrolyte concentrations for LTABs.