Advanced High‐Voltage Aqueous Lithium‐Ion Battery Enabled by “Water‐in‐Bisalt” Electrolyte

Abstract

An ew super-concentrated aqueous electrolyte is proposed by introducing as econd lithium salt. The resultant ultra-high concentration of 28 ml ed to more effective formation of aprotective interphase on the anode along with further suppression of water activities at both anode and cathode surfaces.The improved electrochemical stability allows the use of TiO 2 as the anode material, and a2.5 Vaqueous Li-ion cell based on LiMn 2 O 4 and carbon-coated TiO 2 delivered the unprecedented energy density of 100 Wh kg À1 for rechargeable aqueous Li-ion cells,along with excellent cycling stability and high coulombic efficiency.I th as been demonstrated that the introduction of as econd salts into the "water-in-salt" electrolyte further pushed the energy densities of aqueous Li-ion cells closer to those of the state-of-the-art Li-ion batteries.Lithium-ion batteries (LIB) overwhelmingly dominate the portable electronics market (ca. 50 Wh) with their superior energy densities. [1] To withstand the high voltages (> 3.0 V) generated by the highly energetic electrochemical couples, flammable and toxic non-aqueous electrolytes have to be used, causing safety and environmental concerns that will worsen by orders of magnitude in large-scale applications, such as automotive (ca. 10 3 Wh) and grid-storage (ca. 10 6 Wh). Aqueous electrolytes that are intrinsically nonflammable and green would have provided ideal solutions. However,t heir narrow electrochemical stability window (1.23 V), imposed by hydrogen and oxygen evolution, [2] restricted the voltage output of such aqueous LIB under 1.50 Vand resulted in severely compromised energy densities. Thus,e xpanding the electrochemical stability window of aqueous electrolytes becomes an issue of fundamental importance that would not only determine the practicality of aqueous LIB,b ut in ab roader context, general aqueous electrochemistry.U nfortunately,n os uch effort has been reported given the significant difficulty of suppressing water decomposition reactions,inparticular the reduction of water leading to hydrogen evolution, until recently when we successfully demonstrated a3 .0 Vs tability window when an ew class of "water-in-salt" electrolyte was formulated. In such as uper-concentrated electrolyte,t he decomposition of salt anion occurs preferentially on the anode before hydrogen evolution occurs,l eading to the formation of ad ense solid electrolyte interphase (SEI) primarily consisting of LiF. [3] A 2.3 Vaqueous Li-ion cell based on the electrochemical couple of LiMn 2 O 4 and Mo 6 S 8 was supported by such an electrolyte to provide an unprecedented energy density of 84 Wh kg À1 based on total electrode weight, which should be over 100 Wh kg À1 if the irreversible loss associated with SEI formation could be eliminated. [3a] Apparently,t he efficiency of forming aS EI in aqueous electrolytes depends on the salt concentration, whose increase indicates ad ecrease in water molecules in the solvation sphere of Li + and areduction in the electrochemical activity of water. However, th...

Unique aqueous Li-ion/sulfur chemistry with high energy density and reversibility

“…WiBS electrolyte was obtained by dissolving two Li salts, lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) and lithium trifluoromethane sulfonate (LiOTf) at 21 and 7 mol·kg −1 , respectively, in water at 25°C. Using two lithium salts allows us to circumvent the solubility limits of each single lithium salt in water, hence reaching the highest salt concentration possible (13,18). The extremely high concentration therein provides an expanded stability window thanks to the formation of a dense and protective solid electrolyte interphase (SEI) and the reduced water activity (13); it is also a key to manage stable phase-separation of liquid reaction intermediate and liquid electrolyte, which will be fully discussed below.…”

“…Using two lithium salts allows us to circumvent the solubility limits of each single lithium salt in water, hence reaching the highest salt concentration possible (13,18). The extremely high concentration therein provides an expanded stability window thanks to the formation of a dense and protective solid electrolyte interphase (SEI) and the reduced water activity (13); it is also a key to manage stable phase-separation of liquid reaction intermediate and liquid electrolyte, which will be fully discussed below. Reversible lithiation/delithiation reaction of sulfur in this 28-mol·kg −1 electrolyte was observed at 2.46 and 2.65 V, respectively (Fig.…”

“…Remarkably, at a slow rate of 0.2C (discharge/ charge of full theoretical capacity in 5 h), the cell being cycled between 2.2 and ∼0.5 V exhibited a single discharge plateau with an average voltage of 1.60 V, delivering a discharge capacity of 84.40 mAh·g of total electrode mass (i.e., 1,327 mAh·g −1 of sulfur mass) or an areal capacity of 10.6 mAh·cm −2 . The energy density was conservatively estimated to be ∼135 Wh/(kg of total electrode mass), which represents a marked improvement not only over all conventional aqueous Li-ion systems (<75 Wh·kg −1 ) (9, 11), but in particular also over what was achieved by the highly concentrated aqueous electrolytes (12,13,18). At a rate five times higher (1C), the capacity dropped only slightly, to 68.24 mAh·g −1 , reflecting the fast kinetics of the cell reactions.…”

“…In particular, wide electrochemical stability windows (>3.0 V) comparable to those of nonaqueous electrolytes have been recently demonstrated by water-in-salt (WiS) and waterin-bisalt (WiBS) electrolytes. With water molecules still far outnumbering that of Li salts, this class of completely nonflammable WiS and WiBS electrolytes realized a dramatic improvement in safety, while placing many Li-ion and even beyond Li-ion chemistries within the reach of aqueous electrolytes (12,13). Historically, it has been very challenging to use elemental sulfur as the cathode material in aqueous electrolytes, primarily because of the high solubility of short-chain lithium polysulfide (Li 2 S x , x < 6) and Li 2 S in aqueous media, and the strong parasitic shuttling reaction occurring thereafter (14).…”

In situ surface protection for enhancing stability and performance of conversion-type cathodes

“…5(b) and 5(c)]. 55,84,85,89 Note that the larger aggregates where anion is polarized by two Li + are predicted to reduce at higher potentials than the contact ion pairs, as shown in Fig. 5(c) using LiFSI as an example.…”