Patent Application
20240372150
This invention relates to electrochemical devices, such as lithium-sulfur batteries, and hybrid electrolytes that can be used in a lithium-sulfur battery.
Lithium-sulfur (Li-S) batteries are a promising next generation battery technology. With sulfur's high discharge capacity of 1,672 mAh/g coupled with a lithium metal anode, the battery has a significant theoretical energy density. Beyond the fact that Li-S offers significantly higher energy density than lithium-ion batteries, their benefits extend to better safety and lower cost. However, in a conventional Li-S battery with a liquid electrolyte, the battery suffers from unstable Li metal cycling and is plagued from the “polysulfide shuttle effect.” During the discharge process of sulfur, lithium polysulfide compounds form, solubilize in the electrolyte, and shuttle between electrodes leading to parasitic reactions, irrecoverable capacity loss, low Coulombic efficiency, and impedance growth. In an all-solid-state configuration with a solid electrolyte (SE), this polysulfide shuttle does not occur although forming good solid-solid contact at the cathode/solid electrolyte interface remains a challenge with resulting high impedances and sluggish kinetics. The addition of a liquid electrolyte (LE) at the solid electrolyte/cathode interface can simultaneously improve contact and ionic transport while the solid electrolyte acts as a physical barrier to protect the Li metal and isolate polysulfides to the cathode.
Electron pair donor (EPD) solvents represent a different class of solvents with large implications in the Li-S battery field. The pioneering work by Cuisinier et al. [Ref. 1.21] showed EPDs highly solvate polysulfide species and can deliver 24% more capacity than conventional glyme electrolytes with near full utilization of sulfur (1,575 mAh/g versus 1,300 mAh/g). This capacity increase is due to the S3·− radical stabilized in EPDs that acts as a redox mediator for further sulfur discharge. Unfortunately, EPD solvents are not stable with lithium. Li-S battery cells based on EPD solvents have low Coulombic efficiency and quickly short [Ref. 1.21].
Therefore, what is needed are compositions and methods for the formation of a stable solid electrolyte/liquid electrolyte interface for the successful implementation of hybrid electrolyte schemes to enable lithium-sulfur batteries.
The present disclosure meets the forgoing needs by providing hybrid electrolytes and methods for stabilizing an electron pair donor liquid electrolyte solvent and a solid state electrolyte in a lithium-sulfur battery.
In one aspect, this disclosure provides a hybrid electrolyte for an electrochemical device. The hybrid electrolyte comprises: (i) a first electrolyte having a first surface and an opposed second surface, wherein the first electrolyte comprises a solid state electrolyte material comprising an oxide, wherein the first surface is an acid-treated surface; and (ii) a second electrolyte comprising a liquid electrolyte, wherein the liquid electrolyte comprises an alkali metal salt and a solvent selected from the group consisting of electron pair donor solvents, and solvent mixtures including at least one electron pair donor solvent and at least one glyme solvent.
In one embodiment of the hybrid electrolyte, the solvent comprises an electron pair donor solvent having a donor number (DN) greater than 15 kcal/mol. In one embodiment of the hybrid electrolyte, the solvent comprises an electron pair donor solvent having a donor number (DN) greater than 20 kcal/mol. In one embodiment of the hybrid electrolyte, the solvent comprises an electron pair donor solvent having a donor number (DN) greater than 25 kcal/mol. In one embodiment of the hybrid electrolyte, the solvent comprises N,N-dimethylacetamide (DMA). In one embodiment of the hybrid electrolyte, the solvent comprises a solvent mixture including at least one electron pair donor solvent and at least one glyme solvent. In one embodiment of the hybrid electrolyte, the solvent comprises a solvent mixture including 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME).
In one embodiment of the hybrid electrolyte, the alkali metal salt is selected from the group consisting of lithium (halosulfonyl)imides, lithium (haloalkanesulfonyl)imides, lithium (halosulfonyl haloalkanesulfonyl)imides, and mixtures thereof. In one embodiment of the hybrid electrolyte, the alkali metal salt is selected from the group consisting of LiBF4, LiClO4, LiCF3SO3 (LiTf), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium (fluorosulfonyl trifluoromethanesulfonyl)imide (LiFTFSI), lithium bis(pentafluoroethanesulfonyl)imide (LiBETI), and mixtures thereof. In one embodiment of the hybrid electrolyte, the salt is lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).
In one embodiment of the hybrid electrolyte, the second electrolyte contacts the first surface of the first electrolyte. In one embodiment of the hybrid electrolyte, the second electrolyte is impregnated in a porous separator layer. In one embodiment of the hybrid electrolyte, the separator layer comprises a matrix comprising inorganic fibers. In one embodiment of the hybrid electrolyte, the separator layer comprises a polymeric material. In one embodiment of the hybrid electrolyte, a gasket is arranged adjacent a perimeter of the separator layer to contain the second electrolyte in the separator layer. In one embodiment of the hybrid electrolyte, the solid state electrolyte material is densified through conventional sintering or hot pressed.
In one embodiment of the hybrid electrolyte, the first surface of the first electrolyte is acid-treated using a mineral acid to remove surface impurities. In one embodiment of the hybrid electrolyte, the mineral acid is selected from the group consisting of H3PO4, HCl, HNO3, H2SO4, and mixtures thereof. In one embodiment of the hybrid electrolyte, the mineral acid is H3PO4. In one embodiment of the hybrid electrolyte, the mineral acid is HCl.
In one embodiment of the hybrid electrolyte, the solid state electrolyte material has a garnet phase.
In one embodiment of the hybrid electrolyte, an interfacial resistance of an interface of the first electrolyte and the second electrolyte is 100 Ωcm2 or less. In one embodiment of the hybrid electrolyte, an interfacial resistance of an interface of the first electrolyte and the second electrolyte is 50 Ωcm2 or less.
In one embodiment of the hybrid electrolyte, the solid state electrolyte material has the formula LiuRevMxAxOy, wherein
In one embodiment of the hybrid electrolyte, the solid state electrolyte material is Li6.5La3Zr1.5Ta0.5O12 (LLZTO). In one embodiment of the hybrid electrolyte, the solid state electrolyte material is Li7La3Zr2O12 (LLZO).
In one embodiment of the hybrid electrolyte, the first surface of the first electrolyte is acid-treated using H3PO4.
In one embodiment of the hybrid electrolyte, the first surface of the first electrolyte includes phosphate groups. In one embodiment of the hybrid electrolyte, the first surface of the first electrolyte includes Li3PO4. In one embodiment of the hybrid electrolyte, the first surface of the first electrolyte includes a phosphorylated layer. In one embodiment of the hybrid electrolyte, the phosphorylated layer has a thickness in a range of 1-30 nanometers.
In another aspect, this disclosure provides an electrochemical device comprising: a hybrid electrolyte of any embodiment of the present disclosure; a cathode facing the first surface of the first electrolyte of the hybrid electrolyte; an anode contacting the second surface of the first electrolyte of the hybrid electrolyte, wherein the anode comprises lithium metal. The cathode may comprise a cathode active material selected from sulfur containing materials. The cathode may comprise S8. In one embodiment of the electrochemical device, the cathode comprises 60-80 wt. % S8 and 20-40 wt. % of a carbon conductive additive based on total weight of the cathode.
In one embodiment of the electrochemical device, a discharge capacity of the electrochemical device is greater than 70% of theoretical discharge capacity at a tenth cycle. In one embodiment of the electrochemical device, a discharge capacity of the electrochemical device is greater than 80% of theoretical discharge capacity at a tenth cycle. In one embodiment of the electrochemical device, the solvent comprises an electron pair donor solvent. In one embodiment of the electrochemical device, the solvent comprises N,N-dimethylacetamide (DMA).
In one embodiment of the electrochemical device, a total interfacial resistance of a first interface of the first electrolyte and the second electrolyte and a second interface of the first electrolyte and the anode is 100 Ωcm2 or less. In one embodiment of the electrochemical device, a total interfacial resistance of a first interface of the first electrolyte and the second electrolyte and a second interface of the first electrolyte and the anode is 50 Ωcm2 or less.
In yet another aspect, this disclosure provides a method for stabilizing an electron pair donor liquid electrolyte solvent and a solid state doped or undoped LLZO electrolyte having a first surface and an opposed second surface in a lithium-sulfur battery having a sulfur-containing cathode facing the first surface of the LLZO electrolyte and a lithium metal anode contacting the second surface of the LLZO electrolyte. The method can comprise treating the first surface of the LLZO electrolyte with an acid before contacting the first surface of the LLZO electrolyte with a liquid electrolyte including a lithium metal salt and the electron pair donor liquid electrolyte solvent. The acid can be selected from the group consisting of H3PO4, HCl, HNO3, H2SO4, and mixtures thereof. The acid can be H3PO4. The acid can be HCl.
In one embodiment of the method, the electron pair donor liquid electrolyte solvent has a donor number (DN) greater than 15 kcal/mol. In one embodiment of the method, the electron pair donor liquid electrolyte solvent has a donor number (DN) greater than 20 kcal/mol. In one embodiment of the method, the electron pair donor liquid electrolyte solvent has a donor number (DN) greater than 25 kcal/mol. In one embodiment of the method, the electron pair donor liquid electrolyte solvent comprises N,N-dimethylacetamide (DMA).
In one embodiment of the method, the method further comprises including in the liquid electrolyte at least one glyme solvent. In one embodiment of the method, the electron pair donor liquid electrolyte solvent comprises a solvent mixture including 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME). In one embodiment of the method, the method further comprises including in the liquid electrolyte 1,2-dimethoxyethane (DME). In one embodiment of the method, the method further comprises impregnating the liquid electrolyte in a porous separator layer. In one embodiment of the method, the method further comprises arranging a gasket adjacent a perimeter of the separator layer to contain the liquid electrolyte in the separator layer.
In one embodiment of the method, an interfacial resistance of an interface of the solid state doped or undoped LLZO electrolyte and the liquid electrolyte is 100 Ωcm2 or less. In one embodiment of the method, an interfacial resistance of an interface of the solid state doped or undoped LLZO electrolyte and the liquid electrolyte is 50 Ωcm2 or less.
In one embodiment of the method, the treated first surface of the solid state doped or undoped electrolyte includes phosphate groups. In one embodiment of the method, the treated first surface of the solid state doped or undoped electrolyte includes Li3PO4. In one embodiment of the method, the treated first surface of the solid state doped or undoped electrolyte includes a phosphorylated layer.
In one embodiment of the method, the phosphorylated layer has a thickness in a range of 1-30 nanometers. In one embodiment of the method, the solid state electrolyte material is Li6.5La3Zr1.5Ta0.5O12 (LLZTO). In one embodiment of the method, the solid state electrolyte material is Li7La3Zr2O12 (LLZO).
The foregoing and other aspects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration example embodiments of the invention. Such embodiments do not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims and herein for interpreting the scope of the invention.
The invention will be better understood and features, aspects and advantages other than those set forth above will become apparent when consideration is given to the following detailed description thereof. Such detailed description makes reference to the drawings.
Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items.
The following discussion is presented to enable a person skilled in the art to make and use embodiments of the invention. Various modifications to the illustrated embodiments will be readily apparent to those skilled in the art, and the generic principles herein can be applied to other embodiments and applications without departing from embodiments of the invention. Thus, embodiments of the invention are not intended to be limited to embodiments shown, but are to be accorded the widest scope consistent with the principles and features disclosed herein. Skilled artisans will recognize the examples provided herein have many useful alternatives and fall within the scope of embodiments of the invention.
Although the systems and methods introduced herein are often described for use in an electrochemical cell or battery, one of skill in the art will appreciate that these teachings can be used for various applications (e.g. sensors, fuel cells).
This disclosure relates to designing a platform to stabilize and implement a garnet-electron pair donor hybrid (e.g., dual) electrolyte lithium-sulfur battery. In order to test the lithium-sulfur battery, this platform uses a perfluoro elastomer gasket to hermetically seal the lithium to protect it from the electron pair donor electrolyte and sulfur compounds. This gasket is an elastomer that: (1) is stable and does not solvent swell against liquid electrolytes used in lithium-sulfur batteries; and (2) does not interfere with the cathode/electrolyte interface. Furthermore, to stabilize the garnet-electron pair donor interactions in the presence of sulfur, this platform uses in one embodiment a phosphoric acid and ethanol wash of the garnet solid state electrolyte. This acid treatment removes garnet surfaces contaminations and forms a lithium phosphate protective film.
In one non-limiting example application, a hybrid electrolyte of the present disclosure can be used in a lithium-sulfur battery 110 as depicted in
The hybrid electrolyte of the battery 110 can comprise the solid-state electrolyte 116 and a liquid electrolyte. The liquid electrolyte may comprise a lithium salt in an organic solvent. The lithium salt may be selected from LiBF4, LiClO4, LiCF3SO3 (LiTf), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium (fluorosulfonyl trifluoromethanesulfonyl)imide (LiFTFSI),and lithium bis(pentafluoroethanesulfonyl)imide (LiBETI). The organic solvent may be selected from electron pair donor (EPD) solvents, glyme solvents (i.e., organic ethers with the molecular formula RO[CH2CH2O]nR, wherein R is an alkyl group and n is an integer), and mixtures thereof. Non-limiting example EPD solvents include N,N-dimethylacetamide (DMA), 1,3-dioxolane (DOL), dimethylsulfoxide (DMSO), and N,N-dimethylformamide (DMF). In one embodiment, the EPD solvent has a donor number (DN) of at least 15 kcal/mol, or at least 20 kcal/mol, or at least 25 kcal/mol. Non-limiting example glyme solvents include 1,2-dimethoxyethane (DME) (monoglyme) (wherein R is methyl and n=1), diglyme (R=methyl, n=2), triglyme (R=methyl, n=3), tetraglyme (R=methyl, n=4), ethyl glyme (R=ethyl, n=1), ethyl diglyme (R=ethyl, n=2), butyl glyme (R=butyl, n=1), and butyl diglyme (R=butyl, n=2).
In one embodiment, the hybrid electrolyte has an interfacial resistance of an interface of the first electrolyte and the second electrolyte of 100 Ωcm2 or less. In one embodiment, the hybrid electrolyte has an interfacial resistance of an interface of the first electrolyte and the second electrolyte of 50 Ωcm2 or less. In one embodiment, the hybrid electrolyte has an interfacial resistance of an interface of the first electrolyte and the second electrolyte of 30 Ωcm2 or less.
In some embodiments, the solid state electrolyte 116 can comprise a solid state electrolyte material having the formula LiuRevMwAxOy, wherein
The solid-state electrolyte 116 may be formed by (a) casting a slurry of a solid-state electrolyte material on a surface to form a layer; and (b) sintering the layer to form the solid-state electrolyte. In this method, the layer may be sintered at a temperature in a range of 600° C. to 1250° C. to achieve the necessary electrochemical properties. The layer can have a thickness in a range of 10 to 200 microns. The solid state electrolyte material can also be heat-treated under inert atmosphere to remove surface impurities. The solid state electrolyte material can be heat-treated in a temperature range of 350° C. to 700° C., or in a temperature range of 375° C. to 425° C.
The solid-state electrolyte 116 may alternatively be formed by a hot pressing technique comprising combining dry powders having the desired cations for the final solid-state electrolyte to form a mixture; cold-pressing and calcining the mixed dry powders at temperatures between 500-1000 degrees Celsius for 2-8 hours; and applying simultaneous heat and pressure to the mixture to form the solid-state electrolyte. The hot-pressing technique may use at least one of induction heating, indirect resistance heating, or direct hot-pressing. Heat can be applied at a temperature at or below 1250 degrees Celsius. Pressure can be applied at between 5 and 80 MPa. The solid state electrolyte material can also be heat-treated under inert atmosphere to remove surface impurities. The solid state electrolyte material can be heat-treated in a temperature range of 350° C. to 700° C., or in a temperature range of 375° C. to 425° C.
The first surface 123 of the formed solid-state electrolyte 116 can be acid-treated using a mineral acid to remove surface impurities. The mineral acid can be selected from the group consisting of H3PO4, HCl, HNO3, H2SO4, and mixtures thereof. The mineral acid can be H3PO4. The mineral acid can be HCl.
A suitable material for the cathode 114 of the lithium-sulfur battery 110 is a sulfur containing material. The cathode 114 may include some form of solid elemental sulfur, including crystalline sulfur, amorphous sulfur, precipitated sulfur, and sulfur solidified from the melt. Elemental sulfur includes the various polyatomic molecules of sulfur, especially the octasulfur allotrope characterized as cyclo-S8 ring, and polymorphs thereof such as α-octasulfur, β-octasulfur, and γ-octasulfur. For example, elemental sulfur (in the form of sulfur particulates including nano-sized sulfur particles) may be incorporated as a material component of the cathode, wherein, e.g., the sulfur may be admixed with high surface area or activated carbon particles and an appropriate binder (PVDF) for adhering the material components in a suitable liquid carrier for formulating a slurry to be coated onto or impregnated into a porous matrix structure. In some embodiments, the cathode 114 may include a conductive additive such as graphite, carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, conductive fibers, metallic powders, conductive whiskers, conductive metal oxides, and mixtures thereof. In one embodiment, the cathode 114 may include 50-90 wt. % S8 and 10-50 wt. % of a carbon conductive additive, or 60-80 wt. % S8 and 20-40 wt. % of a carbon conductive additive based on total weight of the cathode.
A suitable active material for the anode 118 of the lithium-sulfur battery 110 comprises lithium metal. In other embodiments, an example anode 118 material consists essentially of lithium metal, or consists of lithium metal.
The first current collector 112 and the second current collector 122 can comprise a conductive metal or any suitable conductive material. In some embodiments, the first current collector 112 and the second current collector 122 comprise aluminum, nickel, copper, combinations and alloys thereof. In other embodiments, the first current collector 112 and the second current collector 122 have a thickness of 0.1 microns or greater. It is to be appreciated that the thicknesses depicted in
The present invention is not limited to lithium-sulfur batteries with a lithium metal anode. The electrochemical cell may be a lithium-ion cell wherein the anode may comprise a lithium host material selected from the group consisting of graphite, activated carbon, carbon black, lithium titanate, graphene, tin-cobalt alloys, and silicon, and the cathode is a lithium host material capable of storing and subsequently releasing lithium ions. An example cathode active material is a lithium metal oxide wherein the metal is one or more aluminum, cobalt, iron, manganese, nickel and vanadium. Non-limiting example lithium metal oxides are LiCoO2 (LCO), LiFeO2, LiMnO2 (LMO), LiMn2O4, LiNiO2 (LNO), LiNixCoyO2 (x+y=1), LiMnxCoyO2 (x+y=1), Li(NixMny)O4 (x+y=1), LiMnxNiyO2 (x+y=1), LiMnxNiyO4 (x+y=1), LiNixCoyAlzO2 (x+y+z=1), LiNi1/3Mn1/3Co1/3O2 and others. Another example of cathode active materials is a lithium-containing phosphate having a general formula LiMPO4 wherein M is one or more of cobalt, iron, manganese, and nickel, such as LiFePO4, LiCoPO4, Li(CoxFeyNiz)PO4 (x+y+z=1), and lithium iron fluorophosphates. Another example of a cathode active material is V2O5. Many different elements, e.g., Co, Mn, Ni, Cr, Al, or Li, may be substituted or additionally added into the structure to influence electronic conductivity, ordering of the layer, stability on delithiation and cycling performance of the cathode materials. The cathode active material can be selected from the group consisting of cathode material particles having a formula LiNixMnyCozO2, wherein x+y+z=1 and x:y:z=1:1:1 (NMC 111), x:y:z=4:3:3 (NMC 433), x:y:z=5:2:2 (NMC 522), x:y:z=5:3:2 (NMC 532), x:y:z=6:2:2 (NMC 622), or x:y:z=8:1:1 (NMC 811). The cathode active material can be a mixture of any number of these cathode active materials.
In alternative embodiments, a suitable anode can comprise sodium in which case a suitable Na ion conducting solid electrolyte is needed. For example, the solid electrolyte could be: sodium-β-alumina, sodium-β″-alumina or the family of compounds refer to NaSICON of nominal formula Na3Zr2Si2PO12.
An electrochemical device of the invention may comprise: a hybrid electrolyte of the present disclosure; a cathode facing the first surface of the first electrolyte of the hybrid electrolyte; an anode contacting the second surface of the first electrolyte of the hybrid electrolyte, wherein the anode comprises lithium metal. The cathode may comprise a cathode active material selected from sulfur containing materials. The cathode may comprises S8. In the electrochemical device, a total interfacial resistance of a first interface of the first electrolyte and the second electrolyte and a second interface of the first electrolyte and the anode can be 100 Ωcm2 or less. In the electrochemical device, a total interfacial resistance of a first interface of the first electrolyte and the second electrolyte and a second interface of the first electrolyte and the anode can be 50 Ωcm2 or less. In one embodiment of the electrochemical device, a discharge capacity of the electrochemical device is greater than 70% of theoretical discharge capacity at a tenth cycle. In one embodiment of the electrochemical device, a discharge capacity of the electrochemical device is greater than 80% of theoretical discharge capacity at a tenth cycle.
The invention also provides a method for stabilizing an electron pair donor liquid electrolyte solvent and a solid state doped or undoped LLZO electrolyte having a first surface and an opposed second surface in a lithium-sulfur battery having a sulfur-containing cathode facing the first surface of the LLZO electrolyte and a lithium metal anode contacting the second surface of the LLZO electrolyte. The method can comprise treating the first surface of the LLZO electrolyte with an acid before contacting the first surface of the LLZO electrolyte with a liquid electrolyte including a lithium metal salt and the electron pair donor liquid electrolyte solvent. The acid can be selected from the group consisting of H3PO4, HCl, HNO3, H2SO4, and mixtures thereof. The acid can be H3PO4. The acid can be HCl.
In the method, the electron pair donor liquid electrolyte solvent can have a donor number (DN) greater than 15 kcal/mol, or a donor number (DN) greater than 20 kcal/mol, or a donor number (DN) greater than 25 kcal/mol. The electron pair donor liquid electrolyte solvent can comprise N,N-dimethylacetamide (DMA). The method may further comprise including in the liquid electrolyte at least one glyme solvent. The electron pair donor liquid electrolyte solvent can comprise a solvent mixture including 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME). The method may further comprise impregnating the liquid electrolyte in a porous separator layer. The method may further comprise arranging a gasket adjacent a perimeter of the separator layer to contain the liquid electrolyte in the separator layer. In the method, an interfacial resistance of an interface of the LLZO electrolyte and the liquid electrolyte can be 100 Ωcm2 or less. In the method, an interfacial resistance of an interface of the LLZO electrolyte and the liquid electrolyte can be 50 Ωcm2 or less, or 30 Ωcm2 or less. The treated first surface of the solid state doped or undoped electrolyte can include phosphate groups. The treated first surface of the solid state doped or undoped electrolyte can include Li3PO4. The treated first surface of the solid state doped or undoped electrolyte can include a phosphorylated layer. The phosphorylated layer can have a thickness in a range of 1-30 nanometers, or in a range of 1-20 nanometers, or in a range of 5-15 nanometers.
The following Examples are provided in order to demonstrate and further illustrate certain embodiments and aspects of the present invention and are not to be construed as limiting the scope of the invention. The statements provided in the Examples are presented without being bound by theory.
Lithium-sulfur (Li—S) batteries are a promising next generation battery technology. With sulfur's high discharge capacity of 1,672 mAh/g coupled with a lithium metal anode, the battery has a significant theoretical energy density. Beyond the fact that Li—S offers significantly higher energy density than lithium-ion batteries, their benefits extend to better safety and lower cost. However, in a conventional Li—S battery with a liquid electrolyte, the battery suffers from unstable Li metal cycling and is plagued from the “polysulfide shuttle effect.” During the discharge process of sulfur, lithium polysulfide compounds form, solubilize in the electrolyte, and shuttle between electrodes leading to parasitic reactions, irrecoverable capacity loss, low Coulombic efficiency, and impedance growth. In an all-solid-state configuration with a solid electrolyte (SE), this polysulfide shuttle does not occur although forming good solid-solid contact at the cathode/solid electrolyte interface remains a challenge with resulting high impedances and sluggish kinetics. The addition of a liquid electrolyte (LE) at the solid electrolyte/cathode interface can simultaneously improve contact and ionic transport while the solid electrolyte acts as a physical barrier to protect the Li metal and isolate polysulfides to the cathode. Several authors have demonstrated this dual electrolyte configuration to successfully block the polysulfides and improve Li—S performance.
Recently, studies have probed the interactions between various solid and liquid electrolytes. Busche et al. first demonstrated that a solid-liquid electrolyte interphase (SLEI) spontaneously forms on the Nasicon-type materials and LiPON due to the decomposition of the solvent and salt. These SLEIs interfacial impedance vary significantly over time and strongly depend on the solvent and salt. The interfacial impedance can be several hundred to several thousand Ωcm2 and lead to high activation energy for ion transport. In general, 1,3-dioxolane (DOL), an electron pair donor (EPD) solvent which has a donor number (DN) of 21.3 kcal/mol, forms a much lower interfacial impedance with LiPON and Nasicon-type materials than 1,2-dimethoxyethane (DME), a glyme solvent which has a donor number (DN) of 20 kcal/mol. The DOL/LiPON SLEI stabilizes to an interfacial impedance of ˜100 Ωcm2 after several hours while DME/LiPON SLEI increases to 1,500 Ωcm2 after 6 days without plateauing. This large interfacial impedance will lead to a significant overpotential and energy loss in commercially relevant cells [Ref. 1.11, 1.14, 1.19]. Following studies elucidated similar findings that an SLEI forms on garnet LLZO with a variety of liquid electrolytes. Fu et al. and Naguib et al. went further showing that LLZO and sulfur are thermodynamically incompatible and react to yield a sulfate/sulfide SLEI in glyme electrolytes.
These studies into the SLEI have only investigated some glymes, carbonates, and acetonitrile. Electron pair donor (EPD) solvents represent a different class of solvents with large implications in the Li—S field. The pioneering work by Cuisinier et al. showed EPDs highly solvate polysulfide species and can deliver 24% more capacity than conventional glyme electrolytes with near full utilization of sulfur (1,575 mAh/g versus 1,300 mAh/g) [Ref. 1.21]. This capacity increase is due to the S3·− radical stabilized in EPDs that acts as a redox mediator for further sulfur discharge. Unfortunately, EPD solvents are not stable with lithium. Li—S cells based on EPD solvents have low Coulombic efficiency and quickly short [Ref. 1.21].
Thus, a dual electrolyte configuration will not only physically block polysulfides to protect Li but will separate EPD solvents to protect them from the Li. Overall, the SE/LE interface remains a complicated component of dual electrolyte-based batteries. For dual electrolytes to be on par with Li-ion electrolyte/electrode interfaces, the SE/LE interface must be lower than 55 Ωcm2. Therefore, there is a need to minimize the SE/LE interfacial impedance and investigate the compatibility between EPD solvents and solid electrolytes and minimize the EPD/SE interfacial impedance. In this Example 1, we use LLZO due to its stability with Li metal and compare its stability in EPD and glyme electrolytes. 1 M LiTFSI in N,N-dimethylacetamide (DMA), an electron pair donor (EPD) solvent which has a donor number (DN) of 27.8 kcal/mol, was the representative EPD solvent due to the work by Cuisinier et al., and 1 M LiTFSI in DOL:DME (1:1 v/v) the representative glyme due to its prevalence in the literature. We used a heat treatment, HCl acid treatment, and H3PO4 acid treatment of LLZO to minimize the LLZO/LE interface and explore its effects on the LLZO surface chemistry. We show that LLZO and EPD electrolytes are incompatible in the presence of sulfur. A simple H3PO4 acid treatment creates a buffer layer on the LLZO surface to stabilize the electrolytes yielding a LLZO/DMA interfacial impedance of 87 Ωcm2. No deleterious effects are observed in glyme electrolytes.
Li6.5La3Ta0.5Zr1.5O12 powder was synthesized through a solid-state route and then densified into billets using rapid-induction hot pressing (RIHP). The LLZO billet was then cut into pellets and polished to a 0.1 μm finish. To clean the surface of LLZO, the polished pellets were heat treated (HT) at 400° C. for 3 hours in an argon-filled glovebox [Ref. 1.18]. For HCl treatment, the LLZO pellets were submerged in 1 M HCl for 30 seconds, then rinsed with acetone, dried with a heat gun, and then quickly transferred to a glovebox. For the H3PO4 treatment, the LLZO pellets were submerged in 85% H3PO4 for 5 minutes under sonication to ensure better reaction owing to the high viscosity of the acid. The pellets were then rinsed with acetone, dried with a heat gun and then quickly transferred to a glovebox. These will be referred to as HT-LLZO, HCl-LLZO, and HPO-LLZO.
A S/LE/LLZO/LE/S symmetric cell was designed to investigate the LLZO/LE interface. Sulfur was chosen as the electrode for two reasons: first, a sulfur electrode is more realistic for catholyte studies than Li, and second, Li is not stable in contact with EPDs.
XPS analysis was performed on a Thermo ESCALAB 250 instrument using a non-monochromatic Al Kα source. The samples were transported to the spectrometer under an Ar atmosphere and transferred into the chamber anaerobically. All spectra were fitted with the Gaussian-Lorentzian functions and a Shirley-type background. The binding energy values were all calibrated using the C 1s peak at 284.8 eV. S 2p and P 2p peaks were fitted by sulfur doublets with the area ratio of 2:1 and the peak splitting of 1.18 eV and 0.86 eV respectively. Zr 3d peaks were fitted with the 3d5/2-3d3/2 doublet with the area ratio of 3:2 and the splitting of 2.43 eV. The La 3d XPS was fitted with the two sets of 3d5/2-3d3/2 doublets (main peaks and satellites). The full width at half maximum (FWHM) was identical for both spin-orbit coupling components of the peaks. For sputtering experiments, a X-ray spot size of 400 μm was sputtered using a 3 kV Ar+ ion beam over an area of about 1.5 mm×1 mm at a rate of approximately 1 nm·min−1.
After heat treatment of the LLZO, 750 μm thick lithium foil ( 5/16″, Alfa Aesar) was attached to one side of the LLZO in an argon-filled glovebox. Prior to attachment the lithium foil was scraped to remove surface contaminants until visually shiny and uniform. The Li/LLZO was then compressed and heated to 170° C. for at least 4 hours to form homogenous contact and low Li/LLZO interfacial resistance. Li/LLZO/Li symmetric cells confirmed the low interfacial resistance and a critical current density of at least 1 mA/cm2. The Li/LLZO was assembled into a similar Swagelok union and compressed under 3.4 MPa to seal the lithium side from the LE/S which can be seen in
Upon transfer to a glovebox, 20 μL of 1 M LiTFSI in DOL:DME or DMA was dropped on top of the LLZO and a sulfur cathode (NEI Corporation, 3.36 mg S/cm2) was added. A layer of carbon paper was included between the LLZO/S to assist with electrical connection of the cathode. The Li—S dual electrolyte cell was cycled galvanostatically between 1.7-2.9 V vs Li/Li+ at a 0.1 mA/cm2 current density at room temperature. EIS was performed between each cycle to assess impedance evolution.
After extracting RLLZO/LE from our model, it can be seen that right after initial SE/LE contact, the DOL:DME/HT-LLZO forms a moderately low interphase with an average areal resistance of 68 Ωcm2. After 3 days, the areal resistance negligibly increased to an average value of 72 Ωcm2. This behavior indicates a thermodynamically stable interphase between DOL:DME/HT-LLZO and differs from the behavior between HT-LLZO/acetonitrile and ethylene carbonate/dimethyl carbonate [Ref. 1.18,1.19]. The HT-LLZO/DOL:DME has a characteristic frequency of ca. 1.3 kHz, in good agreement with the literature for SLEIs. In contrast, DMA/HT-LLZO has radically different results. Immediately after SE/LE contact the areal resistance is >1,250 Ωcm2, already drastically higher than the DOL:DME/HT-LLZO interphase after 3 days. The DMA/HT-LLZO shows rapid growth and within 1 hour after contact the areal resistance increased to >1,750 Ωcm2. Extended testing of this symmetric cell indicates uncontrollable growth with the areal resistance approaching 20,000 Ωcm2 after 3 days. Postmortem analysis of the DMA/HT-LLZO symmetric cell revealed the DMA electrolyte turned into a deep red-purple color,
It is known that the surface of LLZO is contaminated with Li2CO3 and LiOH [Ref. 1.23]. Heat treatment of LLZO reduces the contamination to satisfactory levels (>90%) for Li/LLZO interface compatibility, although the heat treatment does not completely clean the surface. To elucidate the influence of these surface groups, Li2CO3 and LiOH were separately added to DMA solutions with excess sulfur. The Li2CO3 solution turned a faint blue hue while the LiOH solution turned into the same deep red/purple color of the HT-LLZO/DMA solution (
Recently, Huo et al. used a simple acid treatment to clean the surface of LLZO [Ref. 1.26]. The acid treatment was confirmed to have completely removed the resistive LLZO surface of contaminants and improved the Li/LLZO interface. To neutralize surface hydroxides and stabilize LLZO/DMA, we employed two different acid treatments of LLZO. After heat treatment, LLZO pellets were submerged in either 1 M HCl for 30 seconds and washed with acetone or submerged in H3PO4 and washed ethanol.
After acid washing, the LLZO pellets were immediately transferred into a symmetric Swagelok cell to test their compatibility with DMA in the presence of sulfur through EIS. Right after cell assembly it can be seen in
As the HCl acid wash of LLZO cleans the LLZO surface more efficiently than a heat treatment, we investigated the HCl acid treatment on the LLZO/DOL:DME interface. The Nyquist plots of the HT-LLZO and HCl-LLZO against DOL:DME are shown in
To gauge the performance of the acid-treated dual electrolyte, a full Li—S cell was studied.
In this Example 1, we showed for the first time that LLZO and electron pair donor catholytes are inherently unstable for a Li—S battery. Instability arises from both residual hydroxides on the garnet surface and reactivity of the La3+ within the bulk garnet where each can attack dissolved sulfur. We developed a rapid H3PO4 acid wash of LLZO that forms a spontaneous Li3PO4 surface film to act as a buffer layer. This HPO-LLZO remains stable in electron pair donor/sulfur contact to form an interphase with a low resistance of 87 Ωcm2. A similar HCl acid wash readily cleans the LLZO surface to enable an ultralow 27 Ωcm2 interfacial resistance with DOL:DME.
Conventional Li—S batteries rely on liquid electrolytes based on LiNO3/DOL/DME mixtures that produce a quasistable interface with the lithium anode. Electron pair donor (EPD) solvents, also known as high donor number solvents, provide much higher polysulfide solubility and close-to-ideal sulfur utilization, making them solvents of choice for lean electrolyte Li—S cells. However, their instability to reduction requires incorporation of an ion-conductive membrane that is stable with Li, such as garnet LLZO and also stable with sulfur/polysulfides. In this Example 2, we report that even trace amounts of LiOH on a LLTZO surface trigger a complex reaction with sulfur dissolved in typical EPD solvents (i.e., N,N-dimethylacetamide, DMA) to produce a highly resistive impedance layer that quickly grows with time from 1000 to 10,000 Ωcm2 over a few hours, thus impeding Li+ transport across the interface. Decorating the LLZO with protective phosphate groups to produce a modified surface provides a very low charge-transfer resistance of 40 Ωcm2 that is maintained over time and inhibits the reaction of LiOH and dissolved sulfur. Hybrid liquid-solid electrolyte cells constructed on this concept result in a high sulfur utilization of 1400 mAh g−1 which is 85% of theoretical and remains constant over cycling even with conventional, unoptimized carbon/sulfur cathodes.
Li-ion batteries are today's electrochemical energy storage technology of choice for portable devices, electric vehicles, and even storage grid applications [Ref. 2.1, 2.2], but they are approaching their energy density limitations based on the intercalation chemistry that lies at their foundation. This, together with factors of cost and inorganic mineral/metal resources, is prompting accelerated research in alternative battery systems that go beyond the state-of-the-art Li-ion. Li—S batteries are prominent among these. They benefit from a high theoretical specific energy density of 2560 Wh kg−1 and a potential full cell energy density of 600 Wh kg−1 owing to the multielectron redox reactions of the sulfur cathode [Ref. 2.3].
It is now widely understood that for a Li—S cell to achieve even 500 Wh kg−1, a low electrolyte to sulfur (E/S) ratio of approximately 1 μL·mg−1 is required [Ref. 2.4]. Operating Li—S cells at such low E/S ratios poses a massive challenge because conventional DOL-DME electrolytes have limited polysulfide (PS) solubility. The resulting PS saturation in the electrolyte leads to increased viscosity and altered electrolyte transport properties that ultimately dictate sluggish kinetics for both the charge and discharge processes [Ref. 2.5]. Furthermore, the aggravated precipitation of Li2S under such conditions creates thick Li-ion-blocking layers on the cathode surface, shutting down the redox processes in the cell.
Highly solvating electrolytes based on electron pair donor (EPD) solvents, also termed high donor number solvents, were proposed long ago because they dissolve higher concentrations of PS and can provide better transport properties [Ref. 2.6]. They also sustain unique polysulfide speciation equilibria on redox as a result of stabilization of the trisulfide radical (S3·−). These solvents include dimethyl sulfoxide (DMSO), N,N-dimethylacetamide (DMA), 2-methylimidazole (MeIM), 2,6-dimethyl imidazolidinone (DMI), and tetramethyl urea (TMU), which can potentially dissolve Li2S8 at high concentrations (DMSO (14.3 M), DMA (32 M), MeIM (16M), DMI (36 M), and TMU (10.5M) [Ref. 2.7-2.9]) compared to DOL-DME, which reaches a limit at about 7 M [Ref. 2.10] based on sulfur. Previously, some of us reported on a Li—S cell using 1 M LiTFSI in DMA as the electrolyte with LATP serving as a second solid electrolyte to prevent reduction of the DMA at the Li surface. We initially observed that with a typical sulfur cathode, almost 25% more capacity was observed with DMA than TEGDME [Ref. 2.11]. The primary factor responsible for the higher capacity lies in the intrinsic property of DMA (and other EPD solvents) in stabilizing the trisulfide radical (S3-). The formation and consumption of this species during redox were directly monitored via operando XANES studies, owing to its characteristic pre-edge spectral feature. These studies showed that the trisulfide radical reacts with other PS species through disproportionation reactions to maintain PS species in complex solution equilibria, thus delaying precipitation of Li2S on discharge or S8 on charge. Its function as an internal redox mediator leads to increased active material utilization. While EPD solvents promise higher sulfur utilization and better electrochemical performance, two major challenges still exist and need to be overcome: (1) increased PS solubility results in aggravated polysulfide shuttle leading to sulfur loss, anode corrosion, and low Coulombic efficiency; and (2) all EPD solvents are highly reactive with Li metal and cannot form a favorable SEI on the lithium metal anode.
Ta-substituted garnet, Li6.5La3Ta0.5Zr1.5O12 (LLTZO), is a promising solid electrolyte as a physical barrier to inhibit EPD reduction and mitigate polysulfide crossover. Research has overwhelmingly shown that LLZO is stable with lithium metal [Ref. 2.12, 2.13], can be “interface-engineered” to enhance charge transfer [Ref. 2.14], and capable of forming an ultralow interfacial resistance interface (<2 Ωcm2) [Ref. 2.15]. Implementing a solid physical barrier in a liquid lithium-sulfur cell leads to a symbiotic dual electrolyte configuration [Ref. 2.16-2.21]. The physical barrier inhibits solvent reduction and any crosstalk arising from PS diffusion in the cell, and the liquid electrolyte (LE) simultaneously can ease typical sluggish solid-solid kinetics and reduce high solid-solid interfacial resistance. Such a dual electrolyte configuration aims to achieve the benefits of both solid and liquid electrolytes. However, other studies have shown that in this case, the formation of a resistive solid-liquid electrolyte interphase (SLEI) is thermodynamically favored [Ref. 2.22, 2.23]. Gupta et al. demonstrated that the choice of lithium-conducting salt greatly influences the SLEI resistance [Ref. 2.24]. They showed that in a symmetric cell with graphite electrodes in the same solvent LiTFSI easily forms a stable interface with LLZO, whereas LiBOB and LiPF6 do not. Other work has investigated SLEI formation between LLZO and 1 M LiTFSI in DOL-DME. Fu et al. showed a sulfur-rich SLEI comprised of Li2S and Li2SO4 which was formed on LLZO after it was soaked in Li2S8 solution [Ref. 2.25]. Naguib et al. similarly demonstrated the formation of a thick (75-90 nm) Li and S-rich SLEI that exhibited a continuously increasing interfacial resistance of several thousand Ωcm2 during rest at OCV and upon cycling [Ref. 2.26]. This high resistance interphase was attributed to decomposition of the LiTFSI salt. These large SLEI resistances are not sustainable for Li—S batteries. For hybrid Li—S cells to compete with cells with liquid electrolytes, the SLEI resistance needs to be minimized to compensate for the voltage drop observed during high-current density cell operation and ensure sufficient Li+ transport across the interface [Ref. 2.27].
While there has been early research on the interactions between LLZO and common glyme or carbonate electrolytes [Ref. 2.24-2.27], there is yet to be any studies on EPD solvents. Here, we explore the compatibility between LLZO and EPD solvent DMA and compare it to a conventional DOL-DME electrolyte. However, LLTZO is thought to be chemically stable in organic electrolytes; we find that LLTZO is incompatible with sulfur cathodes in DMA due to trace amounts of LiOH on the LLTZO surface, even when the garnet is heated to 400° C. to remove the residue. In DMA, surprisingly, the surface hydroxide reduces sulfur from the cathode that is dissolved in solution, thereby saturating the electrolyte with polysulfide species and leading to a remarkable and rapid impedance increase and hence poor electrochemical behavior. Furthermore, La in LLTZO reacts with dissolved polysulfides to form a lanthanum oxysulfide layer at the surface of the garnet, which also increases the cell impedance and negatively affects electrochemical performance. Herein, we report that a rapid H3PO4 acid treatment phosphorylates the garnet surface, significantly reducing the interfacial resistance and leading to greatly improved cell performance. It forms a Li3PO4 layer that results in a low interfacial interface of 40 Ωcm2 and inhibits the reaction of surface impurities with sulfur in an EPD solvent such as DMA. The hybrid Li—S cells constructed with phosphorylated garnet and a LiTFSI/DMA electrolyte deliver a high capacity of 1410 mAh g−1 and reversibly maintain a capacity retention of almost 100% over cycling, thus providing a promising avenue to implement EPD solvents as electrolytes in Li—S batteries.
The EIS of the symmetric cells with S8/C electrodes was measured at 25° C. in the frequency range between 1 MHz and 0.5 Hz with a 10 mV amplitude. The impedance data was fitted with the equivalent circuit shown in
Li6.5La3Ta0.5Zr1.5O12 powder was synthesized through a solid-state route and then densified into billets by using rapid-induction hot pressing (RIHP). The LLZO billet was then cut into pellets and polished to a 0.1 μm finish. To clean the surface of LLZO, the polished pellets were heat treated at 400° C. for 3 hours in an argon-filled glovebox.
The LLZO pellets were immersed into 1 M HCl or 85% H3PO4 and allowed to react for 30 seconds or 5 minutes, respectively. They were then washed multiple times with acetone, dried with a heat gun, and then quickly transferred to an Ar-filled glovebox for further analysis. Due to the high viscosity of H3PO4, the vials with LLZO and H3PO4 were sonicated to achieve better mixing and avoid bubble formation.
A sulfur composite with 70 wt. % sulfur and 30 wt. % Ketjen Black carbon was prepared through melt diffusion at 155° C. This composite was then mixed with Super P and PTFE (60 wt. % dispersion in water, Sigma-Aldrich) in the weight ratio of 77:10:13 in isopropanol. The obtained mixture was then rollcast directly onto a carbon coated Al foil. After drying overnight at 60° C., the foil were punched into 8 mm discs to obtain the sulfur cathodes with a nominal loading of 3 mg·cm−2.
LLZO pellets were placed against an FFKM O-ring in a 12′ Swagelok union. A glass-fiber separator (GFS) flooded with the electrolyte of choice (1 M LiTFSI in DMA or 1 M LiTFSI in DOL-DME) was placed on each side of the LLZO for sufficient solid-liquid contact. A sulfur cathode was then placed on each exposed glass-fiber separator face and compressed to form a hermetic seal. PEIS was performed from 7 MHz to 1 Hz with a 10 mV perturbation. The first impedance measurement was taken within 5 minutes of cell assembly.
After heat treatment of the LLZO, 750 μm thick lithium foil ( 5/16 in. Alfa Aesar) was attached to one side of the LLZO in an argon-filled glovebox. Prior to attachment, the lithium foil was scraped to remove surface contaminants until it was shiny and uniform. Li/LLZO was then compressed and heated to 170° C. for at least 4 hours to form a homogeneous contact and low Li/LLZO interfacial resistance. Li/LLZO/Li symmetric cells confirmed the low interfacial resistance and a critical current density of at least 1 mA/cm2. Li/LLZO was assembled into a similar Swagelok union and compressed under 3.4 MPa to seal the lithium side from the liquid electrolyte/S8. The Li—S hybrid electrolyte cell was galvanostatically cycled between 1.8 and 2.9 V versus Li/Li+ at a 0.1 mA/cm2 current density at room temperature. A layer of P50 carbon paper soaked with electrolyte (held constant at 12 uL/mg S) was used to enhance the electrical wiring.
UV-vis measurements were carried out on a Thermo Scientific Evolution 201 UV-visible spectrophotometer. Samples were loaded in quartz cuvettes with a path length of 1 mm inside a glovebox and then analyzed within a wavelength range of 400-800 nm.
X-ray photoelectron spectroscopy (XPS) analysis was performed on a Thermo ESCALAB 250 instrument. Monochromatic and nonmonochromatic Al Kα sources were used for conductive composite and nonconductive materials, respectively. The samples were transported to the spectrometer under an Ar atmosphere and transferred to the chamber anaerobically. All spectra were fitted with Gaussian-Lorentzian functions and a Shirley-type background. The binding energy values were all calibrated using the C 1s peak at 284.8 eV. S 2p, and P 2p peaks were fitted by sulfur doublets with an area ratio of 2:1 and a peak splitting of 1.18 and 0.86 eV, respectively. Zr 3d peaks were fitted with the 3d5/2-3 d3/2 doublet with an area ratio of 3:2 and a splitting of 2.43 eV. The La 3d spectrum was fitted with two sets of 3d5/2-3 d3/2 doublets (main peaks and satellites). The full width at half-maximum (fwhm) was identical for both spin-orbit coupling components of the peaks. For sputtering experiments, an X-ray spot size of 400 μm was sputtered using a 3 kV Ar+ ion beam over an area of about 1.5 mm×1 mm at a rate of approximately 1 nm·min−1.
1H NMR measurements were obtained on a Bruker Avance II spectrometer at 7.05 T. The 1H spectra were recorded at 300 MHz in a 0.5 mm OD tube with no solvent lock.
The interfacial compatibility between LLZO and liquid electrolytes (LEs) was explored using electrochemical impedance spectroscopy (EIS) of symmetric cells. We employed simple sulfur/carbon electrodes (S:Ketjen black; 70:30 by weight) in a Swagelok-type cell modified with an O-ring seal to prevent electrolyte leakage (
LLZO was heat treated at 400° C. to clean the surface prior to the EIS measurements [Ref. 2.10] (denoted HT-LLZO). A representative Nyquist plot of the symmetric cell with HT-LLZO and 1 M LiTFSI/DOL-DME (
The contribution of Rgb to the total impedance of LLZO is ˜5%. The LLZO/LE interface is represented by two depressed semicircles (R1 and R2) that lie between 50 kHz and 10 Hz. The impedance of a single LLZO/LE interface (Rint) was calculated as the algebraic average between R1 and R2. The capacitive contribution of the S8-C blocking electrodes (the tail below 10 Hz) was fitted by a transmission line model (TLM) with a blocking boundary condition (see
The notable difference in the interfacial stability of the symmetric S8/C cells in DOL-DME vs. that in DMA is illustrated in
Surface Chemistry. UV-vis spectroscopy was performed on the burgundy electrolyte obtained from the S8-C/HT-LLZO/DMA cell to identify the possible sulfidic species that are present. As shown in
The garnet surface in contact with the electrolyte was also analyzed by X-ray photoelectron spectroscopy (XPS;
LiOH and Li2CO3 are known contaminants on the LLZO surface that exist in trace quantities even after heat treatment [Ref. 2.15]. To probe the reactivity of these surface impurities, small amounts of LiOH and Li2CO3 were separately added to colorless solutions of elemental sulfur dissolved in DMA. In the case of LiOH, the resultant deep burgundy solution yielded a UV-vis spectrum identical to
Reactive intermediate species such as the superoxide radical in the Li—O2 battery or the trisulfide radical in Li—S batteries are known to initiate solvent degradation [Ref. 2.11, 2.31]. The deep burgundy liquid obtained from the symmetric cell and a control solution obtained from the reaction of LiOH, sulfur, and DMA were thus subjected to 1H NMR analysis to examine possible reactivity.
Based on the observed results, we propose Scheme 1 (see
Since the reaction pathway involves the residual surface LiOH present on HT-LLZO, we employed a rapid acid treatment (RAT) of LLZO with 1 M HCl (denoted as HCl-LLZO). This method was proposed by Huo et al. to remove Li2CO3 from the LLZO surface [Ref. 2.33]. HCl-RAT results in a 2.5-fold reduction in impedance from 50 Ωcm2 (HT-LLZO) to 15 Ωcm2 (HCl-LLZO) in Rint of the sulfur symmetric cells with DOL-DME (
The O 1s spectrum of HCl-LLZO and HT-LLZO reveals three major peaks, which are assigned to Li2CO3 (532.6 eV), LiOH (531.3 eV), and the LLZO lattice oxygen (529.2 eV) (
In the case of HCl-RAT, LiOH is the only component of the HCl-LLZO surface since Li2CO3 is completely removed by HCl. The concentration of LiOH exceeds 60% after 20 minutes of sputtering, which signifies the growth of a LiOH passivation layer during HCl-RAT. We attribute this to the competing Li+/H+ ion exchange reaction: HCl initially removes LiOH from the surface, but it is regenerated as Li+ from the lattice exchanges with H+. Thus, HCl-RAT ensures the removal of Li2CO3 but, at the same time, generates more LiOH at the surface, which results in the instability of the symmetric cells with S8-C electrodes and DMA.
Reacting the native layer of LLZO with H3PO4 is a better method to modify the surface (denoted as HPO-LLZO) by effectively phosphorylating it. This approach has been demonstrated to improve the lithiophilicity of LLZO via converting the surface contaminants to Li3PO4 [Ref. 2.35, 2.36]. The significant improvement in the stability of HPO-LLZO with DMA is demonstrated by a ˜7-fold decrease in the Rint of sulfur symmetric cells from 300 to 40 Ωcm2 (
The protective role of the H3PO4 RAT of LLZO is furthermore evidenced by La 3d XPS studies. HCl-LLZO (
Li—S Cells with Protected LLZO. To gauge the performance of the HCl and HPO acid-treated garnets, we assembled Li∥S8-C cells without any LiNO3 in the liquid electrolyte. The performance of a cell with HCl-LLZO/DOL-DME cycled at 0.1 mA·cm−2 is shown in
In Example 2, we report that native LLZO and electron pair donor (i.e., high donor number) solvent electrolytes are inherently incompatible with a Li—S battery. Reactivity arises from residual hydroxide on the garnet surface that dissolves and attacks sulfur in solution to form various sulfur species, including insoluble, insulating thiosulfates and polythionates that build up on the garnet surface. In turn, these sulfur species react with lanthanum in the LLZO to form La—S and/or La—O—S surface layers, dramatically increasing interfacial impedance and leading to high overpotential and poor capacity upon cycling. We developed a rapid H3PO4 acid wash treatment of LLZO that forms a Li3PO4 surface film to act as a buffer layer. This HPO-LLZO surface remains stable in electron pair donor solvent/sulfur contact to form an interphase with a low resistance of 40 Ωcm2. An HCl acid wash is insufficient to modify the LLZO surface and exacerbates the parasitic reactions in DMA. Nevertheless, HCl sufficiently removes lithium carbonate from the LLZO surface to enable an ultralow 20 Ωcm2 interfacial resistance with DOL-DME. This Example 2 shows that the symbiotic relationship between the LLZO and DMA in a hybrid configuration enables EPD electrolytes for Li—S batteries.
The citation of any document is not to be construed as an admission that it is prior art with respect to the present invention.
Thus, the invention provides hybrid electrolytes and methods for stabilizing an electron pair donor liquid electrolyte solvent and a solid state electrolyte in a lithium-sulfur battery.
In light of the principles and example embodiments described and illustrated herein, it will be recognized that the example embodiments can be modified in arrangement and detail without departing from such principles. Also, the foregoing discussion has focused on particular embodiments, but other configurations are also contemplated. In particular, even though expressions such as “in one embodiment”, “in another embodiment”, “in some embodiments”, or the like are used herein, these phrases are meant to generally reference embodiment possibilities, and are not intended to limit the invention to particular embodiment configurations. As used herein, these terms may reference the same or different embodiments that are combinable into other embodiments. As a rule, any embodiment referenced herein is freely combinable with any one or more of the other embodiments referenced herein, and any number of features of different embodiments are combinable with one another, unless indicated otherwise.
Although the invention has been described in considerable detail with reference to certain embodiments, one skilled in the art will appreciate that the present invention can be practiced by other than the described embodiments, which have been presented for purposes of illustration and not of limitation. Therefore, the scope of the appended claims should not be limited to the description of the embodiments contained herein. Various features and advantages of the invention are set forth in the following claims.
This application is based on, claims benefit of, and claims priority to U.S. Application No. 63/463,417 filed on May 2, 2023, which is hereby incorporated by reference herein in its entirety for all purposes.
This invention was made with government support under DE-EE 0008855 awarded by the Department of Energy. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 63463417 | May 2023 | US |
