Brine Refrigerants for Low‐cost, Safe Aqueous Supercapacitors with Ultra‐long Stable Operation at Low Temperatures

Abstract

Traditional aqueous energy storage devices are difficult to operate at low temperatures owing to the poor ionic conductivity and sluggish interfacial dynamics in frozen electrolytes. Herein, the low-cost brine refrigerants for food freezing and preservation as electrolytes, and unexpectedly realize high ionic conductivity and stable operation of an aqueous storage device at low temperatures are demonstrated. A CaCl 2 brine refrigerant electrolyte (BRE) with a low freezing point −55 °C and high ionic conductivity (10.1 mS cm −1 at −50 °C) is developed for supercapacitors (SCs), which retains 80% of the room temperature capacity at −50 °C and exhibits ultra-long cycle life with excellent capacity retention of 92% over 98,500 cycles, outperforming the other SCs which can be operated below −40 °C in literature. Moreover, the SCs with MgCl 2 and NaCl BREs can also be operated successfully with excellent cycle stability and high-capacity retention at low temperatures of −30 and −20 °C, respectively. Fundamental correlation between various cations and their effect on the freezing point reduction of aqueous electrolytes is revealed via Raman investigation and molecular dynamics simulations. This study provides a rational design strategy for green, inexpensive, and safe low-temperature aqueous electrolytes for energy storage devices.

Low-Cost “Water-in-Salt” Hydrogel Electrolyte Enabled Flexible Supercapacitors with 2.7 V Voltage and −40 °C Adaptability

“…Generally, hydrogen bonding can be split into strong and weak hydrogen bonding. For the Raman spectra of HE-NaNO 3 and HE-LiNO 3 , Raman peaks at 3230 and 3450 cm –1 belong to O–H stretching vibration, which represents the strong and weak hydrogen bonding, respectively . The peak intensity ratio I 3230 / I 3450 can represent the hydrogen-bonding ratio of hydrogel electrolytes (Figure d,e), and it can be seen from Figure S6a,b of these two salts that I 3230 / I 3450 both decrease with increasing salt concentration, indicating that NaNO 3 and LiNO 3 broken the strong hydrogen bonding among water molecules.…”

“…The conductivity of HE-NaClO 4 -15 at low temperatures is superior to that of hydrogel electrolyte without the addition of an antifreezing cosolvent. 44 The ESW of hydrogel electrolytes is crucial because it directly affects the energy density of energy storage devices. The oxygen evolution potentials of positive potential, which determine the voltage window of a supercapacitor, were studied by linear sweep voltammetry (LSV).…”

Understanding Pseudocapacitance Mechanisms by Synchrotron X‐ray Analytical Techniques

“…Hence, the characterization of the electrolyte via synchrotron radiation techniques may yield some vital information that is not available from conventional laboratory characterization. The design of in situ XAS reaction cells under extreme conditions: the failure mechanisms of pseudocapacitor operating under extreme conditions (e.g., very high and very low temperatures, humidity, high voltage, and strong magnetic fields) are also of interest. [ 157–159 ] However, there are few reports related to in situ XAS under extreme conditions; thus we call for more attention to the development of in situ XAS reaction cells for pseudocapacitive materials to meet the testing requirements of samples under extreme conditions. XAFS contributes to the search for novel pseudocapacitance mechanisms: besides the three well‐known pseudo‐capacitance mechanisms we discussed above, are there other surface redox mechanisms? For example, Li et al.…”

An inexpensive electrolyte with double-site hydrogen bonding and a regulated Zn2+ solvation structure for aqueous Zn-ion batteries capable of high-rate and ultra-long low-temperature operation

“…[19][20][21][22][23] Among them, cations, as typical hydrogen bonding acceptors, can form typical solvated structures via the solvation effect coordinated with water, thus effectively breaking the initial hydrogen bonding networks between the water molecules. [24][25][26][27] However, this strategy always requires significant amounts of cations to fully interact with substantial quantities of water, resulting in aqueous electrolytes with certain concentrations of salt or even ''salt-in-water'', which leads to increased costs and viscosity, drastically affecting the diffusion and storage of ions at low temperatures. 25,[28][29][30] On the other hand, by adding organic solutions to the water to form co-solvents, the hybrid aqueous electrolyte can also extend the low temperature applicability of the electrolyte.…”