A new super-concentrated aqueous electrolyte is proposed by introducing a second lithium salt. The resultant ultra-high concentration of 28 m led to more effective formation of a protective 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 TiO2 as the anode material, and a 2.5 V aqueous Li-ion cell based on LiMn2 O4 and carbon-coated TiO2 delivered the unprecedented energy density of 100 Wh kg(-1) for rechargeable aqueous Li-ion cells, along with excellent cycling stability and high coulombic efficiency. It has been demonstrated that the introduction of a second 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.
Despite its importance in dictating electrochemical reversibility and cell chemistry kinetics, the solid electrolyte interphase (SEI) on graphitic anodes remains the least understood component in Li ion batteries due to its trace presence, delicate chemical nature, heterogeneity in morphology, elusive formation mechanism, and lack of reliable in situ quantitative tools to characterize it. This work summarizes our systematic approach to understand SEI live formation, via in situ electrochemical atomic force microscopy, which provides topographic images and quantitative information about the structure, hierarchy, and thickness of interphases as function of electrolyte composition. Complemented by an ex situ chemical analysis, a comprehensive and dynamic picture of interphase formation during the first lithiation cycle of the graphitic anode is described. This combined approach provides an in situ and quantitative tool to conduct quality control of formed interphases.
Suppressing lithium (Li) dendrite growth is one of the most critical challenges for the development of Li metal batteries. Here, we report for the first time the growth of dendrite-free lithium films with a self-aligned and highly compacted nanorod structure when the film was deposited in the electrolyte consisting of 1.0 M LiPF6 in propylene carbonate with 0.05 M CsPF6 as an additive. Evolution of both the surface and the cross-sectional morphologies of the Li films during repeated Li deposition/stripping processes were systematically investigated. It is found that the formation of the compact Li nanorod structure is preceded by a solid electrolyte interphase (SEI) layer formed on the surface of the substrate. Electrochemical analysis indicates that an initial reduction process occurred at ∼ 2.05 V vs Li/Li(+) before Li deposition is responsible for the formation of the initial SEI, while the X-ray photoelectron spectroscopy indicates that the presence of CsPF6 additive can largely enhance the formation of LiF in this initial SEI. Hence, the smooth Li deposition in Cs(+)-containing electrolyte is the result of a synergistic effect of Cs(+) additive and preformed SEI layer. A fundamental understanding on the composition, internal structure, and evolution of Li metal films may lead to new approaches to stabilize the long-term cycling stability of Li metal and other metal anodes for energy storage applications.
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...
Sodium ion batteries are on the cusp of being a commercially available technology.
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