Sodium‐ and Potassium‐Hydrate Melts Containing Asymmetric Imide Anions for High‐Voltage Aqueous Batteries

Abstract

Aqueous Na‐ or K‐ion batteries could virtually eliminate the safety and cost concerns raised from Li‐ion batteries, but their widespread applications have generally suffered from narrow electrochemical potential window (ca. 1.23 V) of aqueous electrolytes that leads to low energy density. Herein, by exploring optimized eutectic systems of Na and K salts with asymmetric imide anions, we discovered, for the first time, room‐temperature hydrate melts for Na and K systems, which are the second and third alkali metal hydrate melts reported since the first discovery of Li hydrate melt by our group in 2016. The newly discovered Na‐ and K‐ hydrate melts could significantly extend the potential window up to 2.7 and 2.5 V (at Pt electrode), respectively, owing to the merit that almost all water molecules participate in the Na+ or K+ hydration shells. As a proof‐of‐concept, a prototype Na3V2(PO4)2F3|NaTi2(PO4)3 aqueous Na‐ion full‐cell with the Na‐hydrate‐melt electrolyte delivers an average discharge voltage of 1.75 V, that is among the highest value ever reported for all aqueous Na‐ion batteries.

A High‐Energy Aqueous Manganese–Metal Hydride Hybrid Battery

“…On the basis of the total mass of hydrogen storage alloy and 5.5 m MnCl 2 aqueous solution, the as-assembled Mn-MH battery can achieve an impressive gravimetric energy density of about 240 Wh kg −1 , which is much higher than that of most reported aqueous rechargeable batteries, including aqueous Li-ion batteries (100-130 Wh kg −1 ), [40,41] aqueous Na-ion batteries (≈78 Wh kg −1 ), [42] aqueous Zn-MnO 2 batteries (150-170 Wh kg −1 ), [16,43,44] aqueous K-ion batteries, [12] and other typical aqueous batteries (Figure 6c and Table S3, Supporting Information). [3,13,27,30,[45][46][47][48] Furthermore, the volumetric energy density of the as-assembled Mn-MH battery can reach about 557 Wh L −1 , which is superior to that of lead-acid batteries (80-90 Wh L −1 ), [3] Ni-MH batteries (509 Wh L −1 ), [4] Mn-H battery (263 Wh L −1 ), [24] and vanadium redox flow battery (50 Wh L −1 ).…”

Recent Progress in “Water-in-Salt” Electrolytes Toward Non-lithium Based Rechargeable Batteries

“…Sodium, as one an alkali metal, is closely located with lithium in the periodic table and has a relatively low electrochemical potential (−2.71 V vs. SHE). Typically, SIBs share many chemical properties with LIBs (Kim et al, 2012;Li et al, 2013;Boyd and Augustyn, 2018;Zheng et al, 2019). The high concentrated WIS electrolytes produce ASIBs with better cycling stability.…”

“…However, the easy crystallization of highly concentrated electrolytes at room temperature will seriously limit their practical application, and even damage the batteries (Wu et al, 2015 ; Reber et al, 2019 ; Zhang et al, 2020 ). Currently, the hydrate melts or bisalt, especially the adoption of asymmetric imide anions (such as FTFSI and PTFSI), are proved to be effective for reducing the viscosity and density as well as restraining crystallization by breaking the water structure and/or changing the probability of solvation structures with ion aggregations (Marcus, 2009 ; Brini et al, 2017 ; Suo et al, 2017 ), which thus results in the high solubility of salt anions (Suo et al, 2016 ; Zheng et al, 2019 ).…”

“…Kühnel and co-workers obtained an ultra-high-concentration (up to 37 M) sodium bis(fluorosulfonyl)imide (NaFSI) in water by rapid solidification of the entire supersaturated solution, offering a stable electrochemical window of 2.6 V. Strikingly, an aqueous NaTi 2 (PO 4 ) 3 //Na 3 (VOPO 4 ) 2 F sodium-ion battery with an electrolyte of 35 m NaFSI shows electrochemically reversible behaviors within an electrochemical window over 2.0 V (Kühnel et al, 2017 ). The NaFSI electrolytes with different concentrations are also shown to effectively broaden the voltage windows of ASIBs (Zheng et al, 2019 ). Another well-used electrolyte in ASIBs is NaClO 4 solution.…”

“…With the introduced stable asymmetric anion (i.e., PTFSI − ), the K(PTFSI) 0.12 (TFSI) 0.08 (OTf) 0.8 ·2H 2 O as the alkali melts exhibits excellent water solubility and an expanded operating window of ~2.5 V (~2.14–4.65 V vs. K + /K), but does not suffer from the vulnerable S-F bond. Moreover, the ionic conductivity of the K(PTFSI) 0.12 (TFSI) 0.08 (OTf) 0.8 2H 2 O is maintained at ~34.6 mS cm −1 , much higher than other typical non-aqueous electrolytes (~10 mS cm −1 ) (Zheng et al, 2019 ).…”