Combinatorial printing of functionally graded solid-state electrolyte for high-voltage lithium metal batteries

Abstract

Described herein are functionally graded solid-state electrolytes and methods of making the same. In some embodiments, the electrolytes comprise a multi-polymer electrolyte gradient comprising poly(acrylonitrile) (PAN) polymer and poly(ethylene oxide) (PEO) polymer. In some embodiments, the methods of making such electrolytes comprise combinatory aerosol jet printing techniques using aerosolized ink streams comprising polymer electrolytes. The functionally graded solid-state electrolytes described herein have reduced resistance and improved lithium-ion transport and conductivity relative to conventional heterogeneous multilayered solid-state electrolytes. Also described herein are lithium-ion solid-state batteries comprising functionally graded solid-state electrolytes.

Description

[0001] COMBINATORIAL PRINTING OF FUNCTIONALLY GRADED SOLID-STATE ELECTROLYTE FOR HIGH-VOLTAGE LITHIUM METAL BATTERIES

[0002] CROSS-REFERENCE TO RELATED APPLICATIONS

[0003] This application claims priority to U.S. Provisional Patent Application No. 63 / 632,961 , filed on April 11 , 2024, which is incorporated by reference herein in its entirety.

[0004] FEDERALLY SPONSORED RESEARCH

[0005] This invention was made with government support under CMMI-1747685 and CBET- 2044386 awarded by the National Science Foundation. The government has certain rights in the invention.

[0006] BACKGROUND

[0007] High-voltage solid-state lithium metal batteries (SSLMBs) are advanced energy storage devices which have attracted extensive interests owing to the enhanced energy density and safety. It can meet the requirement of the electrification of transportation and the storage of intermittently produced solar and wind energy. Various types of lithium-conducting solid electrolytes have been explored as solid-state electrolytes (SSEs) in SSLMBs. Nonetheless, SSEs, whether made of ceramics, polymers, or composite materials, face challenges when it comes to facilitating the chemistries of both the anode and cathode. Some SSEs exhibit good stability when paired with a reducing Li metal anode, but their limited resistance to oxidation makes them incompatible with high-voltage cathodes, thereby restricting the energy density of SSLMBs. On the other hand, SSEs that are compatible with high-voltage cathodes often suffer from instability when used with a Li metal anode, significantly limiting their versatility and practicality. Considering that each SSE has its own advantages and drawbacks, it becomes a formidable task to identify a single SSE with the ability to withstand both reduction and oxidation simultaneously.

[0008] Instead of searching for a single SSE with a chemical composition capable of handling both reduction and oxidation simultaneously, heterogeneous multilayered solid-state electrolyte (HMSSE) strategy is introduced to solve this dilemma. For instance, researchers have applied this strategy via a dual-layered solid electrolyte consisting of polyethylene oxide (PEO) polymer contacting the lithium-metal anode and a poly(A / -methyl-malonic amide) (PMA) contacting the cathode. Others constructed a dual-layered SSE by leveraging the oxidation resistance of poly(acrylonitrile) (PAN) and reduction compatibility of poly(vinylidene fluoride) (PVDF) layer. Sandwich structure was also proposed in addition to the bilayer structure. Other workers further extended the electrochemical range of SSEs to 0-5 V by implementing a complex triple-layer SSE approach, using oxidation-resistant PAN and reduction-tolerant polyethylene glycol diacrylate (PEGDA) layers in contact with high-voltage cathodes and Li metal anodes, respectively, while incorporating a flexible PAN-Lii.4Alo.4Gei.6(P04)3 composite electrolyte as an intermediate layer to inhibit dendrite formation and ensure stable operation of high-voltage cathodes. Although the HMSSE strategy can potentially broaden the working voltage windows, the newly introduced interface between electrolyte layers, which is unavoidable in the HMSSE prepared by traditional manufacturing methods such as casting, has raised new challenges when considering the ion transport in the electrolyte. The ion transport resistance at the interface can be much higher than that from bulk electrolyte, leading to the inferior performance of the HMSSE. According to the in-depth electrochemical impedance spectroscopy (EIS) analysis and simulation results, others demonstrated that the interfacial resistance can be approximately 10-fold higher than that of the bulk ionic resistance. It was deduced that the electrolyte / electrolyte interfacial resistance can be 100 times of that of the bulk electrolyte in the PEO-LATP multilayer model. Thus, new manufacturing strategies are imperative to reduce the interfacial resistance and further improve the conductivity and the overall performance of the high-voltage SSLMBs.

[0009] Additive manufacturing (AM) has arisen as a versatile method for producing complex structures employing micro- and nanoscale building blocks. The capability to combine the design freedom of additive manufacturing with precise control over material composition at the local level holds the potential for producing complex materials that are not attainable through traditional manufacturing methods. Among the emerging AM approaches, aerosol jet printing has gained widespread attention due to its high resolution in material deposition and its wide applicability, encompassing materials such as polymers, ceramics, metals, semiconductors, adhesives, and biomaterials.

[0010] What is needed are functionally graded solid-state electrolytes for high-voltage solid-state batteries and methods of making the same.

[0011] SUMMARY

[0012] One embodiment described herein is a functionally graded solid-state electrolyte, comprising a multi-polymer electrolyte gradient extending between a first electrolyte end and a second electrolyte end, the first electrolyte end comprising a first polymer electrolyte content, the second electrolyte end comprising a second polymer electrolyte content, and the multi-polymer electrolyte gradient comprising a graded weight ratio of the first polymer electrolyte content to the second polymer electrolyte content of between 1 :0 and 0: 1 from the first electrolyte end to the second electrolyte end. In one aspect, the first polymer electrolyte content and the second polymer electrolyte content comprise a polynitrile polymer, a polyether polymer, a polyalcohol polymer, a polyethylene oxide polymer, a polyethylene glycol polymer, a polyacrylate polymer, a polyester polymer, a polyvinyl polymer, a polyphosphazene polymer, a polysulfone polymer, a polysiloxane polymer, a fluoropolymer, or combinations thereof. In another aspect, the electrolyte further comprises one or more ionic conducting materials evenly distributed throughout the electrolyte. In another aspect, the one or more ionic conducting materials comprises lithium b / s(trifluoromethanesulfonyl)imide (LiTFSI) salt, Li6.4La3Zr1.4Tao.6O12 (LLZTO) nanoparticles, succinonitrile (SN), or combinations thereof. In another aspect, the electrolyte comprises about 5 wt% to about 90 wt% of the one or more ionic conducting materials. In another aspect, the electrolyte comprises about 15 wt% to about 70 wt% of the one or more ionic conducting materials. In another aspect, the electrolyte has reduced resistance and improved lithium-ion transport and conductivity relative to a conventional heterogeneous multilayered solid-state electrolyte (HMSSE).

[0013] Another embodiment described herein is a lithium-ion solid-state battery, comprising: a positive electrode positioned on a positive electrode current collector; a negative electrode positioned on a negative electrode current collector; and a functionally graded solid-state electrolyte positioned between the positive electrode and the negative electrode, the functionally graded solid-state electrolyte comprising a multi-polymer electrolyte gradient extending between a first electrolyte end and a second electrolyte end, the first electrolyte end being in contact with the positive electrode and comprising a first polymer electrolyte content, the second electrolyte end being in contact with the negative electrode and comprising a second polymer electrolyte content, and the multi-polymer electrolyte gradient comprising a graded weight ratio of the first polymer electrolyte content to the second polymer electrolyte content of between 1 :0 and 0:1 from the first electrolyte end to the second electrolyte end. In one aspect, the positive electrode comprises a cathode active material comprising lithium nickel cobalt aluminum oxide (NCA), LiCoO2, LiMn2C>4, LiNiO2, LiFePCU, LiNio.5Mn1.5O4, or LiNio.6Mno.2Coo.2O2 (NCM622). In another aspect, the negative electrode comprises a lithium metal anode. In another aspect, the battery operates at voltages greater than 5.5 V. In another aspect, the battery maintains a discharge capacity of greater than 90 mAh g-1after 200 cycles. In another aspect, the battery has an improved rate performance relative to a battery having a conventional heterogeneous multilayered solid-state electrolyte (HMSSE). Another embodiment described herein is a method of making a functionally graded solid- state electrolyte, the method comprising: mixing a first aerosolized ink stream comprising a first polymer electrolyte and a second aerosolized ink stream comprising a second polymer electrolyte using a nitrogen carrier gas to form an aerosolized electrolyte mixture; and aerosol jet printing the aerosolized electrolyte mixture onto a substrate using a sheath gas such that the first aerosolized ink stream comprising the first polymer electrolyte is first deposited onto the substrate to form a first electrolyte end comprising the first polymer electrolyte; wherein flow rates of the first aerosolized ink stream and the second aerosolized ink stream are continuously adjusted to form a multi-polymer electrolyte gradient extending between the first electrolyte end and a second electrolyte end comprising the second polymer electrolyte, the multi-polymer electrolyte gradient comprising a graded weight ratio of the first polymer electrolyte to the second polymer electrolyte of between 1 :0 and 0:1 from the first electrolyte end to the second electrolyte end. In one aspect, the method is performed in an environment comprising argon gas. In another aspect, the substrate comprises a high-voltage cathode substrate. In another aspect, the substrate is maintained at a temperature of about 50 °C to about 70 °C. In another aspect, the method further comprises drying the functionally graded solid-state electrolyte to remove any solvent. In another aspect, the method further comprises adding a lithium metal anode to the second electrolyte end. In another aspect, the method comprises a sheath gas flow rate of about 50 seem to about 70 seem. In another aspect, the method comprises an aerosol jet print speed of about 1 mm / s to about 10 mm / s. In another aspect, the first polymer electrolyte and the second polymer electrolyte comprise a polynitrile polymer, a polyether polymer, a polyalcohol polymer, a polyethylene oxide polymer, a polyethylene glycol polymer, a polyacrylate polymer, a polyester polymer, a polyvinyl polymer, a polyphosphazene polymer, a polysulfone polymer, a polysiloxane polymer, a fluoropolymer, or combinations thereof. In another aspect, the first polymer electrolyte comprises poly(acrylonitrile) (PAN) polymer and the second polymer electrolyte comprises polyethylene oxide) (PEO) polymer. In another aspect, one or more of the first aerosolized ink stream and the second aerosolized ink stream further comprise one or more of lithium b / s(trifluoromethanesulfonyl)imide (LiTFSI) salt, Li6.4La3Zr1.4Tao.6O12 (LLZTO) nanoparticles, succinonitrile (SN), dimethylformamide (DMF), or / V-methylpyrrolidone (NMP).

[0014] DESCRIPTION OF THE DRAWINGS

[0015] The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. FIG. 1A-E show schematic illustrations and SEM images of an exemplary functionally graded solid-state electrolyte (FGSSE) as described herein. FIG. 1A shows a schematic illustration of an exemplary combinatorial aerosol jet printing method of FGSSE using aerosolized inks. FIG. 1 B shows a schematic illustration of the FGSSE with enhanced ion transport capability. FIG. 1C shows a cross-sectional SEM image of the FGSSE. FIG. 1 D shows a schematic illustration of an HMSSE with impeded ion transport capability. FIG. 1 E shows a cross-sectional SEM image of the HMSSE.

[0016] FIG. 2 shows an image of PEG and PAN inks for use in the combinatorial aerosol jet printing methods described herein.

[0017] FIG. 3 shows the relationship between the printed layer number and electrolyte film thickness. The average thickness of one layer was ~1.6 pm. The total thickness of the printed electrolyte film can be precisely controlled by the printed layer number.

[0018] FIG. 4 shows an SEM image of the surface of a gradient electrolyte film. A smooth and crack-free surface of the gradient electrolyte film can be obtained after optimizing the printing process which can afford the good contact between an electrode and the electrolyte.

[0019] FIG. 5A-D show energy dispersive spectroscopy (EDS) results for FGSSE. FIG. 5A shows an EDS of N and O signals of FGSSE. FIG. 5B shows an EDS of Zr signal of FGSSE. FIG. 50 shows a Raman spectra at every 5 pm along the Z-axis of the FGSSE. FIG. 5D shows the normalized intensity of C N peak at different locations for varied PAN content.

[0020] FIG. 6A-B show an SEM (FIG. 6A) and EDS (FIG. 6B) image of an HMSSE film. The EDS N and O distributions show that there is a composition change, which will lead to a sharp interface between polymers causing the discontinuous transport of ions in the film. The diffusion of the PEG ink led to some PEG entry into the PAN film, but this had limited effect on diminishing the interface.

[0021] FIG. 7 shows a Raman spectra of the gradient electrolyte film at different locations.

[0022] FIG. 8 shows the relative intensity of Raman peak at 2244.7 cm-1of gradient electrolyte film at different locations. The peak at 2244.7 cm-1belongs to the C=N bond. This does not overlap with other bonds, and therefore its intensity was used to estimate the relative content of PAN in order to understand the spatial distribution of PAN. The intensity at 2244.7 cm-1of the spectrum at 30 pm was 201.9, which was similar to the baseline. So, this value was taken as baseline when normalizing the intensity at different locations.

[0023] FIG. 9 shows Raman spectra for LiTFSI salt and PEO-based and PAN-based inks. The TFSI’ peak at 749.5 cm-1from LiTFSI is very sensitive to the complex environment and shifts to 744.5 cm-1and 742.6 cm-1for the TFSI’ in PAN and PEG electrolytes. FIG. 10A-B show the FT-IR spectra of ink components. FIG. 10A shows the spectra, and FIG. 10B shows an expanded view with assignments. FTIR shows the vibrational shifts of LiTFSI in the electrolyte. The asymmetric S-N-S stretching [va(SNS)] at 1063 cm-1shifts to 1057 cm-1. The va(CF3) shifts from 1200 cm-1to 1186-1190 cm-1.

[0024] FIG. 11A-B show XRD data. FIG. 11 A shows XRD patterns of PEG, PEG ink, and LLZTO nanoparticles. FIG. 11 B shows XRD patterns of PAN, PAN ink, and LLZTO nanoparticles. The XRD measurement manifested the interaction between polymer electrolytes and the LLZTO nanoparticles. The disappearance of characteristic peaks (19.5° and 23.5° for PEG and 16.9° for PAN) after the introduce of LLZTO nanoparticles indicates that the incorporation of LLZTO nanoparticles efficaciously decreases the crystallinity of the PEO and PAN matrix, and greatly enhance the movement of the polymer chain which is beneficial to the ion transport.

[0025] FIG. 12A-D show measured parameters for FGSSE. FIG. 12A shows real conductivity as a function of frequency and temperature for FGSSE. FIG. 12B shows temperature-dependent conductivity of FGSSE and HMSSE. FIG. 12C shows impedance spectra and DC polarization curve of Li / FGSSE / Li cell for Li+transference number test. FIG. 12D shows linear scan voltammetry of Li / FGSSE / SS. Inset is the zoom-in image of the onset of the oxidation process.

[0026] FIG. 13 shows real conductivity as a function of frequency and temperature for HMSSE. The value at the plateau was extracted as the bulk conductivity.

[0027] FIG. 14 shows impedance spectra and DC polarization curve of Li / HMSSE / Li for Li+transference number tests.

[0028] FIG. 15 shows linear scan voltammogram of Li / PEO electrolyte / SS to determine oxidative stability.

[0029] FIG. 16 shows linear scan voltammogram of Li / HMSSE / SS to determine oxidative stability. Inset is the zoom-in image of the onset of the oxidation process.

[0030] FIG. 17A-D shows data for Li / HMSSE / NCM622 cells. FIG. 17A shows Nyquist plots of the Li / FGSSE / NCM622 and Li / HMSSE / NCM622 cells. FIG. 17B shows the equivalent circuit model used to fit the EIS spectra. FIG. 17C shows CV curves of the Li / FGSSE / NCM622 cell. FIG. 17D shows rate performance of the Li / FGSSE / NCM622 and Li / HMSSE / NCM622 cells. All measurements were conducted at 60 °C.

[0031] FIG. 18A-D show data for Li / FGSSE / NCM622 and Li / HMSSE / NCM622 cells. FIG. 18A shows long-term cycling performance of Li / FGSSE / NCM622 and Li / HMSSE / NCM622 cells at 1 C at 60°C. FIG. 18B shows SEM image of the Li anode before cycling. FIG. 18C-D show SEM images of the Li anode after 200 cycles with FGSSE (FIG. 18C) and HMSSE electrolytes (FIG. 18D). FIG. 19 shows Nyquist plots of the LI / FGSSE / NCM622 cell at different cycles.

[0032] DETAILED DESCRIPTION

[0033] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. For example, any nomenclatures used in connection with, and techniques of materials science and chemistry described herein are well known and commonly used in the art. In case of conflict, the present disclosure, including definitions, will control. Exemplary methods and materials are described below, although methods and materials similar or equivalent to those described herein can be used in practice or testing of the embodiments and aspects described herein.

[0034] As used herein, terms such as “include,” “including,” “contain,” “containing,” “having,” and the like mean “comprising.” The present disclosure also contemplates other embodiments “comprising,” “consisting essentially of,” and “consisting of’ the embodiments or elements presented herein, whether explicitly set forth or not. As used herein, “comprising,” is an “open- ended” term that does not exclude additional, unrecited elements or method steps. As used herein, “consisting essentially of” limits the scope of a claim to the specified materials or steps and those that do not materially affect the basic and novel characteristics of the claimed invention. As used herein, “consisting of” excludes any element, step, or ingredient not specified in the claim.

[0035] As used herein, the term “a,” “an,” “the” and similar terms used in the context of the disclosure (especially in the context of the claims) are to be construed to cover both the singular and plural unless otherwise indicated herein or clearly contradicted by the context. In addition, “a,” “an,” or “the” means “one or more” unless otherwise specified.

[0036] As used herein, the term “or” can be conjunctive or disjunctive.

[0037] As used herein, the term “and / or” refers to both the conjunctive and disjunctive.

[0038] As used herein, the term “substantially” means to a great or significant extent, but not completely.

[0039] As used herein, the term “about” or “approximately” as applied to one or more values of interest, refers to a value that is similar to a stated reference value, or within an acceptable error range for the particular value as determined by one of ordinary skill in the art, which will depend in part on how the value is measured or determined, such as the limitations of the measurement system. In one aspect, the term “about” refers to any values, including both integers and fractional components that are within a variation of up to ± 10% of the value modified by the term “about.” Alternatively, “about” can mean within 3 or more standard deviations, per the practice in the art. Alternatively, such as with respect to biological systems or processes, the term “about” can mean within an order of magnitude, in some embodiments within 5-fold, and in some embodiments within 2-fold, of a value. As used herein, the symbol means “about” or “approximately.”

[0040] All ranges disclosed herein include both end points as discrete values as well as all integers and fractions specified within the range. For example, a range of 0.1-2.0 includes 0.1 , 0.2, 0.3, 0.4 . . . 2.0. If the end points are modified by the term “about,” the range specified is expanded by a variation of up to ±10% of any value within the range or within 3 or more standard deviations, including the end points, or as described above in the definition of “about.”

[0041] As used herein, the terms “room temperature,” “RT,” or “ambient temperature” refer to the typical temperature in an indoor laboratory setting. In one aspect, the laboratory setting is climate controlled to maintain the temperature at a substantially uniform temperature or with a specific range of temperatures. In one aspect, “room temperature” refers a temperature of about 15-30 °C, including all integers and endpoints within the specified range. In another aspect, “room temperature” refers a temperature of about 15-30 °C; about 20-30 °C; about 22-30 °C; about 25-30 °C; about 27-30 °C; about 15-22 °C; about 15-25 °C; about 15-27 °C; about 20-22 °C; about 20-25 °C; about 20-27 °C; about 22-25 °C; about 22-27 °C; about 25-27 °C; about 15 °C ± 10%; about 20 °C ± 10%; about 22 °C ± 10%; about 25 °C ± 10%; about 27 °C ± 10%; ~20 °C, ~22 °C, ~25 °C, or ~27 °C, at standard atmospheric pressure.

[0042] As used herein, the terms “control,” or “reference” are used herein interchangeably. A “reference” or “control” level may be a predetermined value or range, which is employed as a baseline or benchmark against which to assess a measured result. “Control” also refers to control experiments.

[0043] Heterogeneous multilayered solid-state electrolyte (HMSSE) has been widely explored for its broadened working voltage range and compatibility with electrodes. However, due to the limitations of traditional manufacturing methods such as casting, the interface between electrolyte layers in HMSSE can severely decrease the ionic conductivity. As disclosed herein, a combinatory aerosol jet printing (CAJP) method is introduced to fabricate functionally graded solid-state electrolytes (FGSSE) without any sharp interface between different polymer electrolyte / electrolyte layers. Owing to CAJP’s unique ability of in-situ mixing and instantaneous tuning of the mixing ratios, an FGSSE with smooth microscale compositional gradation is achieved. Electrochemical tests showed that FGSSE has excellent oxidative stability exceeding 5.5 V and improved conductivity (>7 times of an analogous HMSSE). By decoupling the total resistance, the resistance from the electrolyte / electrolyte interface of HMSSE was found to be 5.7-times the total resistance of FGSSE. Further, a Li / FGSSE / NCM622 cell can be stably run for more than 200 cycles with improved rate performance. In some embodiments described herein, a multi-polymer FGSSE comprises a multipolymer electrolyte gradient extending between a first electrolyte end and a second electrolyte end, where the multi-polymer electrolyte gradient comprises a first polymer electrolyte content and a second polymer electrolyte content, the first and second polymer electrolyte contents being distinct from one another, where a compositional ratio of the first polymer electrolyte content to the second polymer electrolyte content gradually changes from 1 :0 to 0: 1 from the first electrolyte end to the second electrolyte end.

[0044] In certain aspects, the first polymer electrolyte content and the second polymer electrolyte content include, but are not limited to, a polynitrile polymer, a polyether polymer, a polyalcohol polymer, a polyester polymer, or a fluoropolymer. Non-limiting exemplary polymer electrolytes include polyolefins (e.g., polyethylenes, poly(butene-l), poly(n-pentene-2), polypropylene, polytetrafluoroethylene), polyamines (e.g., polyethylene imine) and polypropylene imine (PPI)); polyamides (e.g., polyamide (Nylon), poly(E-caprolactam) (Nylon 6), poly(hexamethylene adipamide) (Nylon 66)), polyimides (e.g., polyimide, polynitrile, poly(acrylonitrile) (PAN), and poly(pyromellitimide-1 ,4-diphenyl ether) (Kapton®) (NOMEX®) (KEVLAR®)); polyether ether ketone (PEEK); vinyl polymers (e.g., polyacrylamide, poly(2-vinyl pyridine), poly(N- vinylpyrrolidone), poly(methylcyanoacrylate), poly(ethylcyanoacrylate), poly(butylcyanoacrylate), poly(isobutylcyanoacrylate), poly(vinyl acetate), poly (vinyl alcohol), poly(vinyl chloride), poly(vinyl fluoride), poly(-vinyl pyridine), vinyl polymer, polychlorotrifluoro ethylene, and poly(isohexylcynaoacrylate)); polyacetals; polyesters (e.g., polycarbonate, polybutylene terephthalate, polyhydroxybutyrate); polyethers (polyethylene oxide) (PEO), polypropylene oxide) (PPO), poly(tetramethylene oxide) (PTMO)); vinylidene polymers (e.g., polyisobutylene, poly(methyl styrene), poly(methylmethacrylate) (PMMA), poly(vinylidene chloride), and poly(vinylidene fluoride)); polyaramides (e.g., poly(imino-1 ,3-phenylene iminoisophthaloyl) and poly(imino-1 ,4-phenylene iminoterephthaloyl)); polyheteroaromatic compounds (e.g., polybenzimidazole (PBI), polybenzobisoxazole (PBO) and polybenzobisthiazole (PBT)); polyheterocyclic compounds (e.g., polypyrrole); polyurethanes; phenolic polymers (e.g., phenolformaldehyde); polyalkynes (e.g., polyacetylene); polydienes (e.g., 1 ,2-polybutadiene, cis or trans-1 ,4-polybutadiene); polysiloxanes (e.g., poly(dimethylsiloxane) (PDMS), poly(diethylsiloxane) (PDES), polydiphenylsiloxane (PDPS), and polymethylphenylsiloxane (PMPS)); and inorganic polymers (e.g., polyphosphazene, polyphosphonate, polysilanes, polysilazanes). In some embodiments, the first polymer electrolyte content comprises poly(acrylonitrile) (PAN) polymer and the second polymer electrolyte content comprises polyethylene oxide) (PEO) polymer. In some embodiments, the multi-polymer FGSSEs described herein may further comprise one or more ionic conducting materials evenly distributed throughout the electrolyte. For example, the FGSSE may further comprise an ionic conducting salt, including, but not limited to, lithium b / s(trifluoromethanesulfonyl)imide (LiTFSI) and lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(oxalato)borate (LiBOB), lithium Difluro(oxalato)borate (LiDFOB), LiSCN, LiBr, Lil, LiCIO4, LiAsF6, LiSO3CF3, LiSO3CH3, LiBF4, LiB(Ph)4, LiPF6, LiC(SO2CF3)3, LiN(SO2CF3)2), LiNO3, sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) and sodium bis(fluorosulfonyl)imide (NaFSI), sodium bis(oxalato)borate (NaBOB) Sodium-difluoro(oxalato)borate (NaDFOB), NaSCN, NaBr, Nal, NaAsF6, NaSO3CF3, NaSO3CH3, NaBF4, NaPF6, NaN(SO2F)2, NaCIO4, NaN(SO2CF3)2, NaNO3, magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSI)2) and magnesium bis(fluorosulfonyl)imide (Mg(FSI)2), magnesium bis(oxalato)borate (Mg(BOB)2), magnesium Difluro(oxalato)borate (Mg(DFOB)2), Mg(SCN)2, MgBr2, Mgl2, Mg(CIO4)2, Mg(AsF6)2, Mg(SO3CF3)2, Mg(SO3CH3)2, Mg(BF4)2, Mg(PF6)2, Mg(NO3)2, Mg(CH3COOH)2, potassium bis(trifluoromethanesulfonyl)imide (KTFSI) and potassium bis(fluorosulfonyl)imide (KFSI), potassium bis(oxalato) borate (KBOB), potassium difluro(oxalato)borate (KDFOB), KSCN, KBr, KI, KCIO4, KASF6, KSO3CF3, KSO3CH3, KBF4, KB(Ph)4, KPF6, KC(SO2CF3)3, KN(SO2CF3)2), KNO3, AI(NO3)2, AICI3, AI2(SO4)3, AIBr3, All3, AIN, AISCN, or AI(CIO4)3. As another example, the FGSSE may further comprise oxide-based solid-state electrolytes including, but not limited to, Li?La3Zr20i2(LLZO), LATP, LAGP, LLTO, LiPON, LiBON, lithium borate, or Li6.4La3Zri.4Tao.50i2(LLZTO) nanoparticles. LLZTO is a tantalum-doped version of LLZO. LLZTO nanoparticles have high ionic conductivity and are chemically stable with lithium metal. In addition, the FGSSEs described herein may comprise one or more plasticizers, binders, or fillers. Plasticizers help to facilitate lithium-ion transport and conductivity within polymer electrolyte materials. Non-limiting exemplary plasticizers include plastic- crystal plasticizers such as succinonitrile (SN). In some embodiments, the one or more ionic conducting materials comprises lithium b / s(trifluoromethanesulfonyl)imide (LiTFSI) salt, Li6.4La3Zri.4Tao.60i2(LLZTO) nanoparticles, succinonitrile (SN), or combinations thereof. Non-limiting exemplary binders include polyvinylidene fluoride (PVDF), polyacrylic acid, carboxymethyl cellulose, styrene-butadiene rubber, sodium alginate, and the like.

[0045] In some embodiments, a lithium metal solid-state battery is described herein, the battery comprising: a positive electrode positioned on a positive electrode current collector; a negative electrode positioned on a negative electrode current collector; and a functionally graded solid- state electrolyte positioned between the positive electrode and the negative electrode, the functionally graded solid-state electrolyte comprising a multi-polymer electrolyte gradient extending between a first electrolyte end adjacent to the positive electrode and a second electrolyte end adjacent to the negative electrode, the multi-polymer electrolyte gradient comprising a first polymer electrolyte content and a second polymer electrolyte content, wherein a compositional ratio of the first polymer electrolyte content to the second polymer electrolyte content gradually changes from 1 :0 to 0:1 from the first electrolyte end to the second electrolyte end.

[0046] In some embodiments, the positive electrode (i.e., cathode) comprises a cathode active material comprising lithium nickel cobalt aluminum oxide (NCA), LiCoO2, LiMn2O4, LiNiO2, LiFePO4, LiNio.5Mn1.5O4, or LiNio.6Mno.2Coo.202(NCM622).

[0047] In some embodiments, the negative electrode (i.e., anode) comprises a lithium (Li) metal anode material. For example, the anode may incorporate dense Li metal or a Li metal alloy. In other aspects, an active anode material may include, for example, lithium powder, titanium oxide, silicon, tin oxide, germanium, antimony, silicon oxide, iron oxide, cobalt oxide, ruthenium oxide, molybdenum oxide, molybdenum sulfide, chromium oxide, nickel oxide, manganese oxide, carbon-based materials (hard carbons, soft carbons, graphene, graphite, carbon nanofibers, carbon nanotubes, etc.), or a combination thereof.

[0048] In certain non-limiting embodiments, the positive electrode current collector comprises aluminum foil, which can optionally be surface treated (e.g., carbon coating). In certain nonlimiting embodiments, the negative electrode current collector comprises copper foil, which can optionally be surface treated.

[0049] One embodiment described herein is a functionally graded solid-state electrolyte, comprising a multi-polymer electrolyte gradient extending between a first electrolyte end and a second electrolyte end, the first electrolyte end comprising a first polymer electrolyte content, the second electrolyte end comprising a second polymer electrolyte content, and the multi-polymer electrolyte gradient comprising a graded weight ratio of the first polymer electrolyte content to the second polymer electrolyte content of between 1 :0 and 0:1 from the first electrolyte end to the second electrolyte end. In one aspect, the first polymer electrolyte content and the second polymer electrolyte content comprise a polynitrile polymer, a polyether polymer, a polyalcohol polymer, a polyethylene oxide polymer, a polyethylene glycol polymer, a polyacrylate polymer, a polyester polymer, a polyvinyl polymer, a polyphosphazene polymer, a polysulfone polymer, a polysiloxane polymer, a fluoropolymer, or combinations thereof. In another aspect, the electrolyte further comprises one or more ionic conducting materials evenly distributed throughout the electrolyte. In another aspect, the one or more ionic conducting materials comprises lithium b / s(trifluoromethanesulfonyl)imide (LiTFSI) salt, Li6.4La3Zr1.4Tao.6O12 (LLZTO) nanoparticles, succinonitrile (SN), or combinations thereof. In another aspect, the electrolyte comprises about 5 wt% to about 90 wt% of the one or more ionic conducting materials. In another aspect, the electrolyte comprises about 15 wt% to about 70 wt% of the one or more ionic conducting materials. In another aspect, the electrolyte has reduced resistance and improved lithium-ion transport and conductivity relative to a conventional heterogeneous multilayered solid-state electrolyte (HMSSE).

[0050] Another embodiment described herein is a lithium-ion solid-state battery, comprising: a positive electrode positioned on a positive electrode current collector; a negative electrode positioned on a negative electrode current collector; and a functionally graded solid-state electrolyte positioned between the positive electrode and the negative electrode, the functionally graded solid-state electrolyte comprising a multi-polymer electrolyte gradient extending between a first electrolyte end and a second electrolyte end, the first electrolyte end being in contact with the positive electrode and comprising a first polymer electrolyte content, the second electrolyte end being in contact with the negative electrode and comprising of a second polymer electrolyte content, and the multi-polymer electrolyte gradient comprising a graded weight ratio of the first polymer electrolyte content to the second polymer electrolyte content of between 1 :0 and 0:1 from the first electrolyte end to the second electrolyte end. In one aspect, the positive electrode comprises a cathode active material comprising lithium nickel cobalt aluminum oxide (NCA), LiCoCh, LiM C , LiNiC>2, LiFePCU, LiNio.5Mn1.5O4, or LiNi0.6Mn0.2Co0.2O2 (NCM622). In another aspect, the negative electrode comprises a lithium metal anode. In another aspect, the battery operates at voltages greater than 5.5 V. In another aspect, the battery maintains a discharge capacity of greater than 90 mAh g-1after 200 cycles. In another aspect, the battery has an improved rate performance relative to a battery having a conventional heterogeneous multilayered solid-state electrolyte (HMSSE).

[0051] Another embodiment described herein is a method of making a functionally graded solid- state electrolyte, the method comprising: mixing a first aerosolized ink stream comprising a first polymer electrolyte and a second aerosolized ink stream comprising a second polymer electrolyte using a nitrogen carrier gas to form an aerosolized electrolyte mixture; and aerosol jet printing the aerosolized electrolyte mixture onto a substrate using a sheath gas such that the first aerosolized ink stream comprising the first polymer electrolyte is first deposited onto the substrate to form a first electrolyte end comprising of the first polymer electrolyte; wherein flow rates of the first aerosolized ink stream and the second aerosolized ink stream are continuously adjusted to form a multi-polymer electrolyte gradient extending between the first electrolyte end and a second electrolyte end comprising of the second polymer electrolyte, the multi-polymer electrolyte gradient comprising a graded weight ratio of the first polymer electrolyte to the second polymer electrolyte of between 1:0 and 0:1 from the first electrolyte end to the second electrolyte end. In one aspect, the method is performed in an environment comprising argon gas. In another aspect, the substrate comprises a high-voltage cathode substrate. In another aspect, the substrate is maintained at a temperature of about 50 °C to about 70 °C. In another aspect, the method further comprises drying the functionally graded solid-state electrolyte to remove any solvent. In another aspect, the method further comprises adding a lithium metal anode to the second electrolyte end. In another aspect, the method comprises a sheath gas flow rate of about 50 seem to about 70 seem. In another aspect, the method comprises an aerosol jet print speed of about 1 mm / s to about 10 mm / s. In another aspect, the first polymer electrolyte and the second polymer electrolyte comprise a polynitrile polymer, a polyether polymer, a polyalcohol polymer, a polyethylene oxide polymer, a polyethylene glycol polymer, a polyacrylate polymer, a polyester polymer, a polyvinyl polymer, a polyphosphazene polymer, a polysulfone polymer, a polysiloxane polymer, a fluoropolymer, or combinations thereof. In another aspect, the first polymer electrolyte comprises poly(acrylonitrile) (PAN) polymer and the second polymer electrolyte comprises polyethylene oxide) (PEO) polymer. In another aspect, one or more of the first aerosolized ink stream and the second aerosolized ink stream further comprise one or more of lithium b / s(trifluoromethanesulfonyl)imide (LiTFSI) salt, Li6.4La3Zr1.4Tao.6O12 (LLZTO) nanoparticles, succinonitrile (SN), dimethylformamide (DMF), or / V-methylpyrrolidone (NMP).

[0052] It will be apparent to one of ordinary skill in the relevant art that suitable modifications and adaptations to the compositions, formulations, methods, processes, and applications described herein can be made without departing from the scope of any embodiments or aspects thereof. The compositions and methods provided are exemplary and are not intended to limit the scope of any of the specified embodiments. All of the various embodiments, aspects, and options disclosed herein can be combined in any variations or iterations. The scope of the compositions, formulations, methods, and processes described herein include all actual or potential combinations of embodiments, aspects, options, examples, and preferences herein described. The exemplary compositions and formulations described herein may omit any component, substitute any component disclosed herein, or include any component disclosed elsewhere herein. The ratios of the mass of any component of any of the compositions or formulations disclosed herein to the mass of any other component in the formulation or to the total mass of the other components in the formulation are hereby disclosed as if they were expressly disclosed. Should the meaning of any terms in any of the patents or publications incorporated by reference conflict with the meaning of the terms used in this disclosure, the meanings of the terms or phrases in this disclosure are controlling. Furthermore, the foregoing discussion discloses and describes merely exemplary embodiments. All patents and publications cited herein are incorporated by reference herein for the specific teachings thereof.

[0053] Various embodiments and aspects of the inventions described herein are summarized by the following clauses:

[0054] Clause 1 . A functionally graded solid-state electrolyte, comprising a multi-polymer electrolyte gradient extending between a first electrolyte end and a second electrolyte end, the first electrolyte end comprising a first polymer electrolyte content, the second electrolyte end comprising a second polymer electrolyte content, and the multi-polymer electrolyte gradient comprising a graded weight ratio of the first polymer electrolyte content to the second polymer electrolyte content of between 1 :0 and 0:1 from the first electrolyte end to the second electrolyte end.

[0055] Clause 2. The electrolyte of clause 1, wherein the first polymer electrolyte content and the second polymer electrolyte content comprise a polynitrile polymer, a polyether polymer, a polyalcohol polymer, a polyethylene oxide polymer, a polyethylene glycol polymer, a polyacrylate polymer, a polyester polymer, a polyvinyl polymer, a polyphosphazene polymer, a polysulfone polymer, a polysiloxane polymer, a fluoropolymer, or combinations thereof.

[0056] Clause 3. The electrolyte of clause 1 or 2, wherein the first polymer electrolyte content comprises poly(acrylonitrile) (PAN) polymer and the second polymer electrolyte content comprises poly(ethylene oxide) (PEO) polymer.

[0057] Clause 4. The electrolyte of any one of clauses 1-3, further comprising one or more ionic conducting materials evenly distributed throughout the electrolyte.

[0058] Clause 5. The electrolyte of clause any one of clauses 1-4, wherein the one or more ionic conducting materials comprises lithium b / s(trifluoromethanesulfonyl)imide (LiTFSI) salt, Li6.4La3Zr1.4Tao6O12 (LLZTO) nanoparticles, succinonitrile (SN), or combinations thereof.

[0059] Clause 6. The electrolyte of clause any one of clauses 1-4, wherein the electrolyte comprises about 5 wt% to about 90 wt% of the one or more ionic conducting materials.

[0060] Clause 7. The electrolyte of clause any one of clauses 1-4, wherein the electrolyte comprises about 15 wt% to about 70 wt% of the one or more ionic conducting materials.

[0061] Clause 8. The electrolyte of any one of clauses 1-7, wherein the electrolyte comprises a thickness of about 20 pm to about 40 pm. Clause 9. The electrolyte of any one of clauses 1-8, wherein the electrolyte has reduced resistance and improved lithium-ion transport and conductivity relative to a conventional heterogeneous multilayered solid-state electrolyte (HMSSE).

[0062] Clause 10. A lithium-ion solid-state battery, comprising: a positive electrode positioned on a positive electrode current collector; a negative electrode positioned on a negative electrode current collector; and a functionally graded solid-state electrolyte positioned between the positive electrode and the negative electrode, the functionally graded solid-state electrolyte comprising a multi-polymer electrolyte gradient extending between a first electrolyte end and a second electrolyte end, the first electrolyte end being in contact with the positive electrode and comprising a first polymer electrolyte content, the second electrolyte end being in contact with the negative electrode and comprising a second polymer electrolyte content, and the multi-polymer electrolyte gradient comprising a graded weight ratio of the first polymer electrolyte content to the second polymer electrolyte content of between 1 :0 and 0:1 from the first electrolyte end to the second electrolyte end.

[0063] Clause 11. The battery of clause 10, wherein the positive electrode comprises a cathode active material comprising lithium nickel cobalt aluminum oxide (NCA), LiCoO2, LiMn2O4, LiNiC>2, LiFePC , LiNio.5Mn1.5O4, or LiNi06Mn0.2Co0.2O2 (NCM622).

[0064] Clause 12. The battery of clause 10 or 11 , wherein the negative electrode comprises a lithium metal anode.

[0065] Clause 13. The battery of any one of clauses 10-12, wherein the battery operates at voltages greater than 5.5 V.

[0066] Clause 14. The battery of any one of clauses 10-13, wherein the battery maintains a discharge capacity of greater than 90 mAh g-1after 200 cycles.

[0067] Clause 15. The battery of any one of clauses 10-14, wherein the battery has an improved rate performance relative to a battery having a conventional heterogeneous multilayered solid- state electrolyte (HMSSE).

[0068] Clause 16. A method of making a functionally graded solid-state electrolyte, the method comprising: mixing a first aerosolized ink stream comprising a first polymer electrolyte and a second aerosolized ink stream comprising a second polymer electrolyte using a nitrogen carrier gas to form an aerosolized electrolyte mixture; and aerosol jet printing the aerosolized electrolyte mixture onto a substrate using a sheath gas such that the first aerosolized ink stream comprising the first polymer electrolyte is first deposited onto the substrate to form a first electrolyte end comprising the first polymer electrolyte; wherein flow rates of the first aerosolized ink stream and the second aerosolized ink stream are continuously adjusted to form a multi-polymer electrolyte gradient extending between the first electrolyte end and a second electrolyte end comprising the second polymer electrolyte, the multi-polymer electrolyte gradient comprising a graded weight ratio of the first polymer electrolyte to the second polymer electrolyte of between 1 :0 and 0:1 from the first electrolyte end to the second electrolyte end.

[0069] Clause 17. The method of clause 16, wherein the method is performed in an environment comprising argon gas.

[0070] Clause 18. The method of clause 16 or 17, wherein the substrate comprises a high-voltage cathode substrate.

[0071] Clause 19. The method of any one of clauses 16-18, wherein the substrate is maintained at a temperature of about 50 °C to about 70 °C.

[0072] Clause 20. The method of any one of clauses 16-19, further comprising drying the functionally graded solid-state electrolyte to remove any solvent.

[0073] Clause 21. The method of any one of clauses 16-20, further comprising adding a lithium metal anode to the second electrolyte end.

[0074] Clause 22. The method of any one of clauses 16-21 , wherein the method comprises a sheath gas flow rate of about 50 seem to about 70 seem.

[0075] Clause 23. The method of any one of clauses 16-22, wherein the method comprises an aerosol jet print speed of about 1 mm / s to about 10 mm / s.

[0076] Clause 24. The method of any one of clauses 16-23, wherein the first polymer electrolyte and the second polymer electrolyte comprise a polynitrile polymer, a polyether polymer, a polyalcohol polymer, a polyethylene oxide polymer, a polyethylene glycol polymer, a polyacrylate polymer, a polyester polymer, a polyvinyl polymer, a polyphosphazene polymer, a polysulfone polymer, a polysiloxane polymer, a fluoropolymer, or combinations thereof.

[0077] Clause 25. The method of any one of clauses 16-24, wherein the first polymer electrolyte comprises poly(acrylonitrile) (PAN) polymer and the second polymer electrolyte comprises polyethylene oxide) (PEO) polymer. Clause 26. The method of any one of clauses 16-25, wherein one or more of the first aerosolized ink stream and the second aerosolized ink stream further comprise one or more of lithium b / s(trifluoromethanesulfonyl)imide (LiTFSI) salt, Li6.4La3Zr1.4Tao.5O12 (LLZTO) nanoparticles, succinonitrile (SN), dimethylformamide (DMF), or / V- methylpyrrolidone (NMP).

[0078] EXAMPLES

[0079] Materials

[0080] Poly(acrylonitrile) (PAN, MW = 150,000 g mol-1), polyethylene oxide) (PEO, MW = 100,000 g mol-1), dimethylformamide (DMF), / V-methylpyrrolidone (NMP), and succinonitrile (SN) were obtained from Sigma Aldrich. Lithium b / s(trifluoromethanesulfonyl)imide (LiTFSI) was obtained from TCI chemicals. LLZTO (300 nm) was obtained from Neware. Lithium chips, LiNi06Mn02Co02O2 (NCM622), and Super P were obtained from MSE supplies. All chemicals were used without further purification.

[0081] Characterization

[0082] X-ray diffraction (XRD) patterns were performed by D8 Discover, Bruker (40 kV, 40 mA) with the scan angle from 10° to 60°. The morphologies and energy-dispersive spectral (EDS) images of the SPEs were investigated with a scanning electron microscope (SEM) Helios G4 L)X. Fourier transform infrared spectroscopy (FT-IR) spectra was obtained in a Bruker Tenor 27 from 4000 to 500 cm-1with a diamond lens attenuated total reflectance (ATR) module. The spatial distribution of organic functionalities in the electrolyte was checked by a Raman microscope (NRS-5100, Jasco). Ionic conductivity measurements were conducted on a Novocontrol Broadband Dielectric spectrometer equipped with an alpha-A high performance frequency analyzer and Quatro temperature control system with a cryostat. Data was collected in a frequency range from 1 * 106Hz to 0.1 Hz at an AC voltage amplitude of 0.1 V from 25 to 85 °C at intervals of 15 °C. The temperature was ramped at 5 °C / min with 5 min of stabilization time at each measurement temperature. Cycling tests of the coin cell were conducted by a Neware battery test system with a voltage range of 2.8-4.3 V. Cyclic voltammetry (CV) was conducted using a Parstat, AMETEK potentiostat / galvanostat with a scanning rate of 0.1 mV s-1and a voltage range of 2.8-4.3 V. Electrochemical impedance spectroscopy (EIS) measurement was conducted using a Gamry Interface 1010E Potentiostat with frequency range from 1 * 106Hz to 0.1 Hz at an AC voltage amplitude of 0.1 V. Linear sweep voltammetry (LSV) measurements were conducted using a Parstat, AMETEK potentiostat / galvanostat with a scanning rate of 0.2 mV s-1. All the electrochemical tests were conducted at 60 °C.

[0083] Preparation of Inks

[0084] For PEO-based ink, 20 mg of PEO, 20 mg of LLZTO, and 10 mg of LiTFSI were added into a glass vial with 2 mL DMF. The glass vial was sonicated for 1 h in a bath sonicator before use. For PAN-based ink, the same procedure was followed as the preparation of PEO ink with the following changes: 20 mg of PAN was used to replace the PEO, 5 mg SN was added (SN can work as a plasticizer and help facilitate the ion transport within the PAN), and 2 mL solvent mixture (NMP: DMF = 1 :2) was used to improve the printability of the PAN ink. FIG. 2 shows an image of the PEO and PAN inks.

[0085] Printing of Electrolyte Films

[0086] For the gradient structure electrolyte films, 6 layers of pure PAN ink were deposited onto the cathode discs. Then, 10 intermediate layers were deposited. When printing the intermediate layer, the ink ratio of PAN:PEO changed from 1 :0 to 0:1 gradually. For each layer, the ratio between PAN ink and PEO ink was fixed which means that the composition of each layer was the same. After that, 6 layers of pure PEO ink were deposited onto the top. For the heterogeneous multilayered solid-state electrolyte (HMSSE) film, 11 layers of pure PAN ink were printed on the cathode discs followed by 11 layers of pure PEO ink. The inks used for the printing of HMSSE were the same as the inks used for the printing of functionally graded solid-state electrolyte (FGSSE). The overall composition of the HMSSE was the same as FGSSE. The difference between FGSSE and HMSSE is that the FGSSE had a functionally graded layer within the electrolyte. The thickness of FGSSE was 32 pm and the thickness of the HMSSE was 33 pm. All the electrolyte films were dried for 48 hours in a vacuum oven at 65 °C to remove the solvent. The specific parameters of the aerosol jet printing method are provided in Table 1 .

[0087] Table 1. Aerosol Jet Printing Parameters of the Electrolyte

[0088] Parameters Values

[0089] Nozzle Diameter (pm) 233

[0090] Nozzle Outlet Diameter (pm) 20

[0091] Sheath Gas Fow Rate (seem) 60

[0092] Platen Temperature (°C) 65

[0093] Print Speed (mm / s) 4 FIG. 3 shows the relationship between the printed layer number and electrolyte film thickness. The average thickness of one layer was ~1.6 pm / layer. The total thickness of the printed electrolyte film can be precisely controlled by the printed layer number.

[0094] FIG. 4 shows an SEM image of the surface of a gradient electrolyte film. A smooth and crack-free surface of the gradient electrolyte film can be obtained after optimizing the printing process which can afford the good contact between an electrode and the electrolyte.

[0095] Electrode Preparation and Cell Assembly

[0096] The cathode was prepared by casting onto a carbon coated aluminum current collector from a NMP slurry. The NMP slurry was prepared by mixing and stirring the LiNi0.6Mn0.2Co0.2O2 (NCM622) active material (70%), conductive carbon (Super P) (15%), PVDF binder (15%) and NMP in a vial overnight. The electrode was vacuum dried at 120 °C for 24 h. The mass loading of the cathode material was ~2 mg cm-2. Then, the cathode was cut into small squares with length of ~6 mm and cold pressed at ~10 MPa. The obtained cathodes were used as the substrate during the printing of electrolyte. 2032 coin cells were assembled in an argon-filled glovebox with Li metal anode, NMC622 cathode, and the in-situ printed solid-state electrolyte.

[0097] Measurement and Calculation of Li+Transference Number

[0098] To measure the Li+transference number, solid electrolytes were printed onto thin copper film (~6 * 6 mm). After printing, the thin copper films with electrolyte were dried in a vacuum oven at 65 °C for 48 h. Then, the Cu / electrolyte / Li cell was obtained by assembling the copper film with lithium metal into 2032-coin cells. With this asymmetric cell, Li was then plated onto the Cu electrode with current of 0.2 mA cm-2for 2.5 h to create the Li / electrolyte / Li configuration in-situ. After resting for 10 h, EIS and a DC polarization (10 mV) experiment were performed. The Li+transference number can be deduced from the Bruce-Vincent-Evans equation as follows, t+ =Iss -IfM Li+IO( -ISSRSS) ’ where A V (10 mV) is the voltage applied to the cell, Zo and Ro are the initial current and impedance of the cell before polarization, respectively, and Zssand Rssare the steady-state current and impedance of the cell after the polarization, respectively.

[0099] Described herein is a novel combinatory aerosol jet printing (CAJP) method that was introduced to fabricate a functionally graded solid-state electrolyte (FGSSE). As shown in FIG. 1A, oxidation-tolerant PAN and reduction-resistant PEG were separately dissolved and used as two inks in the CAJP. Lithium b / s(trifluoromethane)sulfonimide (LiTFSI) salt and nano-size Li6.4 a3Zr1.4Tao.6O12 (LLZTO) particles as inorganic filler, which can promote the transport of Li+, were also added into both inks. In CAJP, two ultrasonic atomizers were used to convert liquid inks into aerosols containing microscale ink droplets. The two aerosolized ink streams were transported by nitrogen (N2) carrier gas and then mixed within a single nozzle. The resulting mixture was directed and focused by a co-flowing sheath gas to achieve high spatial resolution before being deposited on a substrate. A unique advantage of aerosol-based ink deposition is the ability to realize in-situ mixing of multiple aerosolized inks and quickly change the mixing ratio due to the extremely low viscous drag of the aerosols compared with liquid or solid feedstock materials. Owing to the instantaneous mixing and tuning of the mixing ratio of two aerosols, the composition could be finely tuned along with the thickness direction of the printed materials to realize the FGSSE. The generated FGSSE displayed an excellent oxidative stability exceeding 5.5 V and improved ionic conductivity (over 7 times that of HMSSE). A high-voltage lithium metal cell with the FGSSE was found to stably run for more than 200 cycles.

[0100] To print the FGSSE with Z-axis gradient structure, 100% PAN-LLZTO ink was first deposited onto high-voltage cathode discs. As the FGSSE was continuously printed with increasing thicknesses, the flow rate of PAN-LLZTO ink was gradually decreased from 100% to 0 and the flow rate of PEO-LLZTO ink was gradually increased from 0 to 100%. Finally, a gradient structure electrolyte with pure PAN-LLZTO at the bottom and pure PEO-LLZTO on the top was obtained. The overall composition and thickness of FGSSE and HMSSE was similar (FGSSE: 32 pm, HMSSE: 33 pm). As shown in FIG. 1 B-C, the pure PAN-LLZTO electrolyte contacts the high-voltage cathode which avoids the oxidation of PEO electrolyte when the batteries are run at high-voltage. And the pure PEO-LLZTO contacts the lithium metal which can separate the PAN from lithium metal avoiding the side reaction between PAN and the lithium metal. More importantly, the compositional ratio between PAN and PEO was gradually changed from 1 :0 to 0:1 during the printing process along the Z-axis of the FGSSE. The gradual and smooth compositional modulations avoid the sharp compositional changes and render the electrolyte merging very well between different layers of printing. In this FGSSE, the Li+can transfer efficiently across the whole electrolyte. In contrast, the HMSSE with pure PAN-LLZTO at the bottom and pure PEO-LLZTO on the top showed a sharp interface between the PAN-LLZTO and the PEO-LLZTO (FIG. 1 D-E) layers, which acts as a barrier hindering the Li+transport across the electrolyte and reducing the Li+conductivity. FIG. 6A-B show an SEM (FIG. 6A) and EDS (FIG. 6B) image of the HMSSE film. The EDS N and O distributions show that there is a composition change, which will lead to a sharp interface between polymers causing the discontinuous transport of ions in the film. The diffusion of the PEO ink led to some PEO entry into the PAN film but this had limited effect on diminishing the interface.

[0101] To characterize the compositional distribution of PAN and PEO in FGSSE, EDS and Raman measurements were conducted to reveal the element and the organic functionality distributions across the FGSSE. FIG. 5A shows N and O distributions along the cross-section of FGSSE film. At the bottom of the film, there was a dense layer of N signal indicating the bottom layer was composed of PAN. Right above the PAN layer, the N signal decreased, and the O signal increased gradually along the Z axis from the bottom to the top, which indicates the content of PAN decreased and the content of PEO increased gradually. This smooth compositional change can be attributed to the in-situ mixing and accurate modulation of the ink mixing ratio during deposition. At the top of the film, the O signal became more obvious demonstrating that the PEO dominates the FGSSE. Few N signals were detected. This might be the N signal from high boiling point solvent (DMF and NMP) residue, which showed negligible impact on the electrochemical performance. FIG. 5B shows the homogeneous distribution of Zr element, indicating the good dispersion of the LLZTO nanoparticles within the polymer electrolyte. The confocal Raman microscope can distinguish different polymers and their spatial distribution by revealing the organic functionalities information. So, Raman spectra were collected at every 5 pm along the Z-axis of the electrolyte film to verify the spatial distribution of the polymers. In FIG. 5C, the peak at 848 cm-1can be assigned as C-0 stretching from PEO and the peak at 2244.7 cm-1can be assigned as C=N peak from PAN, respectively. The intensity of C=N peak becomes weaker, and the C-0 peak becomes stronger from the bottom to the top. Since the C=N peak located at 2244.7 cm-1did not overlap with any other peaks, its intensity was used to estimate the relative content of PAN to understand the spatial distribution of PAN. FIG. 5D shows the normalized intensity of the C=N peak. The peak intensity decreased along the film thickness direction revealing the composition transition from PAN to PEO across the electrolyte film. The relative interactions between polymer, LiTFSI, and LLZTO additives were also investigated by Raman, FTIR, and XRD (FIG. 9-11). FIG. 7 shows a Raman spectra of the gradient electrolyte film at different locations. FIG. 8 shows the relative intensity of Raman peak at 2244.7 cm-1of gradient electrolyte film at different locations. The peak at 2244.7 cm-1belongs to the C=N bond. This does not overlap with other bonds, and therefore its intensity was used to estimate the relative content of PAN in order to understand the spatial distribution of PAN. The intensity at 2244.7 cm-1of the spectrum at 30 pm was 201.9, which was similar to the baseline. So, this value was taken as baseline when normalizing the intensity at different locations. The ionic conductivity of the FGSSE and HMSSE was systematically studied by dielectric spectroscopy at different temperatures from 25 to 85°C. As shown in FIG. 12A and FIG. 13, the ionic conductivity increased with the increase of the temperature for both electrolytes. The conductivities at different temperatures are summarized and compared in FIG. 12B. The FGSSE displayed a conductivity of 2.0 x 10-5S cm-1at 25 °C which is over 7 times of that of HMSSE (2.8 x 1Q-6 s cm-1at 25 °C). This revealed that the FGSSE has an improved ion transport capability compared with the HMSSE. The activation energy (Ea) of conduction for each electrolyte was challenging to obtain because of the complex composition of electrolytes, rendering the use of a single model (Vogel-Tammann-Fulcher (VTF) equation or Arrhenius equation) inapplicable. FIG. 12C and FIG. 14 show the EIS and DC polarization experimental results of symmetric lithium cells with FGSSE and HMSSE for Li+transference measurements. Based on these results and according to the Bruce-Vincent-Evans equation, the Li+transference number (tLi+) of FGSSE was estimated to be 0.32 and the tLi+ of the HMSSE was about 0.28. These values were within the range of tLi+ of PEO, PAN, and LLZTO-containing composite electrolytes.

[0102] The electrochemical stability window is an important property that determines whether an electrolyte is suitable for high-voltage Li metal batteries. The electrochemical window of the electrolyte was investigated by linear sweep voltammetry (LSV) test in Li / electrolyte / stainless steel (SS) cells at a scan rate of 0.2 mV s-1. Adverse reactions started to occur for the PEO electrolyte when the voltage went above 3.8 V, indicating its poor antioxidation performance (FIG. 15). For FGSSE (FIG. 12D), the excellent anti-oxidation capability was observed as with HMSSE (FIG. 16). No clear onset of current was observed until 5.5 V versus Li / Li+, indicating its exceptional high-voltage stability.

[0103] The performance of the printed electrolytes for energy storage application was evaluated by coin cell batteries with lithium anode and NCM622 cathode. FIG. 17A presents the EIS spectra recorded at 60 °C for the cells Li / FGSSE / NCM622 and Li / HMSSE / NCM622. The spectra can be divided into high-frequency (HF, the first semicircle), medium-frequency (MF, the second semicircle) and low-frequency domains (LF, linear tail). The HF, MF, and LF contributions were, respectively, assigned to the bulk electrolyte response, the interface response comprising both the polymer / polymer electrolyte interface and electrode / electrolyte interfaces, and the diffusion process. The total polarization resistance (Rp) of the cell was the sum of bulk electrolyte (migration, Rb), interfacial charge transfer (Rcf), and diffusion resistance RM , i.e. , Rp= Rb+ Ret + Rdtf. The Rb, Ret, and RM are well decoupled in frequencies, thus the Rct from different cells could be extracted from the EIS and compared directly. For cells with FGSSE, the Rct included Li / PEO and PAN / cathode interfaces. The Rct of cells with HMSSE was composed of Li / PEO, PAN / cathode, and the polymer / polymer electrolyte interfaces. According to the difference methodology, the polymer / polymer interface resistance in the HMSSE can be deduced by subtracting the Rct of FGSSE from the Rct of HMSSE. To obtain the Rct from each electrolyte, the equivalent electrical circuit presented in FIG. 17B was applied to fit the impedance spectra. The equivalent circuit is composed of the cable contribution [resistance Rc) and inductance (Lc)] in series with the bulk electrolyte and charge transfer response (modeled by F?b / / CPEband F?Cf / / CPEcf), and the Warburg impedance (Zw). According to the fitting result, the Rct from HMSSE was 397.4 ± 4.2 Qcm2, while the Rctfrom FGSSE was 64.3 ± 0.4 Qcm2. Thus, polymer / polymer interface resistance in the HMSSE was calculated to be 333.1 ± 4.2 Qcm2, which is 5.7 times the total resistance of FGSSE (58.6 Qcm2). This revealed that the significant charge transfer resistance arising from polymer / polymer interface can be dramatically reduced via the CAJP process described herein.

[0104] Cyclic voltammetry (CV) measurements of the cells showed a typical NCM622 oxidization peak at 3.9 V and a reduction peak at 3.58 (FIG. 17C). In the first cycle, the oxidation peak shifted to 4.06 V because of the formation of interphase layer. No other redox peak could be found, which also confirmed the high stability of gradient electrolyte when it was paired with high- voltage cathode. FIG. 17D shows the performance of the cells at various cycling rates from 0.2- 5 C. The cell with FGSSE displayed discharge capacities of 161 , 153, 136, 110, and 44 mAh g-1at current rates of 0.2, 0.5, 1 , 2, and 5 C, respectively. In contrast, the cell with HMSSE displayed discharge capacities of 160, 136, 103, 43, and 0.3 mAh g-1at corresponding rates. After the end of cycling at a high current rate of 5 C, the discharge capacity of FGSSE cell could still be boosted to 155 mAh g-1when the current rate returned to 0.2 C, reaching 96% of the initial discharge capacity at 0.2 C. While cells with different electrolytes exhibit similar capacity at relatively low currents, the cell with HMSSE displayed significantly lower capacity once the current exceeded 1 C. Especially, when the current was increased to 5 C, the cell with HMSSE showed almost no capacity while the cell with FGSSE could still run and deliver a capacity of 44 mAh g-1. This demonstrated the battery with FGSSE had much better rate performance.

[0105] The long-term cycling performance of the cells is presented in FIG. 18A in terms of the discharge capacity and Coulombic efficiency as a function of cycle number. The battery with FGSSE delivered a capacity of 142 mAh g-1after a few cycles of activation and maintained a capacity of 97 mAh g-1after 200 cycles, while the capacity of the cell with HMSSE was about 113 mAh g-1after activation and just 40 mAh g-1after 200 cycles. The cell with FGSSE upheld a stable Coulombic efficiency throughout the cycling test at a level of 98.6-100% after the initial activation. The EIS of the cell with FGSSE electrolyte at the 0, 25th, 50th, and 200thcycles were compared (FIG. 19). The interface resistances extracted from the EIS data show a small increase (~20%) after 200 cycles compared to the pristine cell, demonstrating the good stability of the electrolyte against electrodes. After cycling, the cells were disassembled to observe the surface morphology evolution of Li metal anodes. A flat and smooth surface without a porous or dendritic structure could be clearly observed for the Li metal from the cell with FGSSE after 200 cycles (FIG. 18C). In sharp contrast, cracks and dendrite Li apparently existed in Li metal anode from the cell with HMSSE after cycling (FIG. 18D). The growth of Li dendrites can be ascribed to the severe polarization due to the limited ionic diffusion and low tLi+ caused by the solid-state electrolyte / electrolyte interface of the HMSSE.

[0106] In summary, the FGSSE described herein with an expanded electrochemical window and an improved ionic conductivity was successfully prepared via an innovative CAJP method for the first time. Owing to the unique capability of modulating the ink mixing ratio on the fly, FGSSE was printed with oxidation-tolerant PAN deposited in contact with the cathode followed by a gradual and smooth transition to reduction-resistant PEO. In this way, the side reactions among the electrodes and electrolyte could be avoided. And the electrochemical window of the electrolyte was expanded to 0-5.5 V. In addition, the continuous compositional gradation of the electrolyte within the FGSSE improved the conductivity by avoiding the sharp compositional changes and the associated interfacial barrier for ion transport. Thus, the overall conductivity of the FGSSE was over 7-times that of HMSSE at 25 °C. The EIS analysis revealed the resistance from the electrolyte / electrolyte interface in the HMSSE was 5.7-times the total resistance of FGSSE. By coupling with lithium anode and NCM622 cathode, the cell with FGSSE displayed a capacity of 142 mAh g-1at 1 C and could stably run for more than 200 cycles with an improved rate performance. This innovative CAJP method opens new opportunities to produce FGSSEs with enhanced properties and facilitates the development of high-performance solid-state batteries. The possibility of scaling up and integrating the FGSSEs with roll-to-roll processing for large-scale Li-ion battery production will be explored.

Claims

CLAIMSWhat is claimed:

1. A functionally graded solid-state electrolyte, comprising a multi-polymer electrolyte gradient extending between a first electrolyte end and a second electrolyte end, the first electrolyte end comprising a first polymer electrolyte content, the second electrolyte end comprising a second polymer electrolyte content, and the multi-polymer electrolyte gradient comprising a graded weight ratio of the first polymer electrolyte content to the second polymer electrolyte content of between 1 :0 and 0:1 from the first electrolyte end to the second electrolyte end.

2. The electrolyte of claim 1 , wherein the first polymer electrolyte content and the second polymer electrolyte content comprise a polynitrile polymer, a polyether polymer, a polyalcohol polymer, a polyethylene oxide polymer, a polyethylene glycol polymer, a polyacrylate polymer, a polyester polymer, a polyvinyl polymer, a polyphosphazene polymer, a polysulfone polymer, a polysiloxane polymer, a fluoropolymer, or combinations thereof.

3. The electrolyte of claim 1 , wherein the first polymer electrolyte content comprises poly(acrylonitrile) (PAN) polymer and the second polymer electrolyte content comprises polyethylene oxide) (PEO) polymer.

4. The electrolyte of claim 1 , further comprising one or more ionic conducting materials evenly distributed throughout the electrolyte.

5. The electrolyte of claim 4, wherein the one or more ionic conducting materials comprises lithium b / s(trifluoromethanesulfonyl)imide (LiTFSI) salt, Li6.4La3Zr1.4Tao.6O12 (LLZTO) nanoparticles, succinonitrile (SN), or combinations thereof.

6. The electrolyte of claim 4, wherein the electrolyte comprises about 5 wt% to about 90 wt% of the one or more ionic conducting materials.

7. The electrolyte of claim 4, wherein the electrolyte comprises about 15 wt% to about 70 wt% of the one or more ionic conducting materials.

8. The electrolyte of claim 1, wherein the electrolyte comprises a thickness of about 20 pm to about 40 pm.

9. The electrolyte of claim 1, wherein the electrolyte has reduced resistance and improved lithium-ion transport and conductivity relative to a conventional heterogeneous multilayered solid-state electrolyte (HMSSE).

10. A lithium-ion solid-state battery, comprising: a positive electrode positioned on a positive electrode current collector; a negative electrode positioned on a negative electrode current collector; and a functionally graded solid-state electrolyte positioned between the positive electrode and the negative electrode, the functionally graded solid-state electrolyte comprising a multi-polymer electrolyte gradient extending between a first electrolyte end and a second electrolyte end, the first electrolyte end being in contact with the positive electrode and comprising a first polymer electrolyte content, the second electrolyte end being in contact with the negative electrode and comprising a second polymer electrolyte content, and the multi-polymer electrolyte gradient comprising a graded weight ratio of the first polymer electrolyte content to the second polymer electrolyte content of between 1 :0 and 0:1 from the first electrolyte end to the second electrolyte end.

11. The battery of claim 10, wherein the positive electrode comprises a cathode active material comprising lithium nickel cobalt aluminum oxide (NCA), LiCoO2, LiMn2O4, LiNiC>2, LiFePO4, LiNio5Mn1.5O4, or LiNi0.6Mn0.2Co0.2O2 (NCM622).

12. The battery of claim 10, wherein the negative electrode comprises a lithium metal anode.

13. The battery of claim 10, wherein the battery operates at voltages greater than 5.5 V.

14. The battery of claim 10, wherein the battery maintains a discharge capacity of greater than 90 mAh g-1after 200 cycles.

15. The battery of claim 10, wherein the battery has an improved rate performance relative to a battery having a conventional heterogeneous multilayered solid-state electrolyte (HMSSE).

16. A method of making a functionally graded solid-state electrolyte, the method comprising: mixing a first aerosolized ink stream comprising a first polymer electrolyte and a second aerosolized ink stream comprising a second polymer electrolyte using a nitrogen carrier gas to form an aerosolized electrolyte mixture; and aerosol jet printing the aerosolized electrolyte mixture onto a substrate using a sheath gas such that the first aerosolized ink stream comprising the first polymer electrolyte is first deposited onto the substrate to form a first electrolyte end comprising the first polymer electrolyte; wherein flow rates of the first aerosolized ink stream and the second aerosolized ink stream are continuously adjusted to form a multi-polymer electrolyte gradient extending between the first electrolyte end and a second electrolyte end comprising the second polymer electrolyte, the multi-polymer electrolyte gradient comprising a graded weight ratio of the first polymer electrolyte to the second polymer electrolyte of between 1 :0 and 0:1 from the first electrolyte end to the second electrolyte end.

17. The method of claim 16, wherein the method is performed in an environment comprising argon gas.

18. The method of claim 16, wherein the substrate comprises a high-voltage cathode substrate.

19. The method of claim 16, wherein the substrate is maintained at a temperature of about 50 °C to about 70 °C.

20. The method of claim 16, further comprising drying the functionally graded solid-state electrolyte to remove any solvent.

21. The method of claim 16, further comprising adding a lithium metal anode to the second electrolyte end.

22. The method of claim 16, wherein the method comprises a sheath gas flow rate of about 50 seem to about 70 seem.

23. The method of claim 16, wherein the method comprises an aerosol jet print speed of about 1 mm / s to about 10 mm / s.

24. The method of claim 16, wherein the first polymer electrolyte and the second polymer electrolyte comprise a polynitrile polymer, a polyether polymer, a polyalcohol polymer, a polyethylene oxide polymer, a polyethylene glycol polymer, a polyacrylate polymer, a polyester polymer, a polyvinyl polymer, a polyphosphazene polymer, a polysulfone polymer, a polysiloxane polymer, a fluoropolymer, or combinations thereof.

25. The method of claim 16, wherein the first polymer electrolyte comprises poly(acrylonitrile) (PAN) polymer and the second polymer electrolyte comprises polyethylene oxide) (PEO) polymer.

26. The method of claim 16, wherein one or more of the first aerosolized ink stream and the second aerosolized ink stream further comprise one or more of lithium b / s(trifluoromethanesulfonyl)imide (LiTFSI) salt, Li6.4La3Zr1.4Tao6O12 (LLZTO) nanoparticles, succinonitrile (SN), dimethylformamide (DMF), or A / -methylpyrrolidone (NMP).

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