Inorganic Electrolyte for Low‐Temperature Aqueous Sodium Ion Batteries

Abstract

Aqueous sodium ion batteries have received widespread attention due to their great application potential and high safety. However, the serious capacity fading under low temperature dramatically restricts their practical application. Compared to flammable and toxic organic antifreezing additives, addition of common cheap inorganic inert additives to improve low‐temperature performance is of interest scientifically. Herein, low‐cost calcium chloride is served as antifreezing additive in 1 m NaClO4 aqueous electrolyte due to its strong interaction with water molecules. The freezing point of the optimized electrolyte is significantly reduced to below −50 °C with an ultrahigh ionic conductivity (7.13 mS cm−1) at −50 °C. All pure inorganic composition of the full battery delivers a high capacity of 74.5 mAh g−1 under 1 C (1 C = 150 mA g−1) at −30 °C. More importantly, when tested under 10 C at −30 °C, the battery can achieve an ultralong cycling stability of 6000 cycles with no obvious capacity decay, indicating fast Na+ transport under low temperature. Significantly, this work provides an easy‐to‐operate strategy by adding cheap inorganic salt to develop high‐performance low‐temperature aqueous batteries.

Design strategies for low temperature aqueous electrolytes

“…Some of reported aqueous electrolytes contain multicomponent salts [23], co-solvents [24] and polymers [25], whose phase diagrams are more complicated than the binary phase diagram. However, their phase diagrams are similar to the binary phase diagram when the multicomponent substances are regarded as one whole component.…”

Optimization Strategies of Electrolytes for Low‐Temperature Aqueous Batteries

“…À as a supporting salt and adding chloride ions as an additive, such an electrolyte achieves a good antifreeze effect at low temperatures. [55] In addition, BF 4 À has also been suitable for low temperatures. The F atoms in the four BÀ F bonds theoretically provide many H bond sites.…”

“…Likewise, Cl ‐ is also considered to be a chaotropic salt. Using the chaotropic salt ClO 4 − as a supporting salt and adding chloride ions as an additive, such an electrolyte achieves a good antifreeze effect at low temperatures [55] …”

Tailoring Pure Inorganic Electrolyte for Aqueous Sodium‐Ion Batteries Operating at −60 °C

“…Although the most frequently ASIBs electrolyte based on NaClO 4 salt has the lowest freezing point of −37 °C when its concentration is ∼8.85 m (m: mol kg −1 ), the lower‐temperature limit is still higher than that of many regions during winter on earth [8] . More interestingly, introducing inorganic salt CaCl 2 as inert and anti‐freezing additive for NaClO 4 based electrolyte can reduce its freezing point below −50 °C and the ASIBs can efficiently work at −30 °C [7a] . However, the improvement degree was still undesirable.…”