INTERFACE PROTECTION FOR ALL-SOLID-STATE BATTERIES

Abstract

An interfacial protective coating layer of LTO is effective in preventing unwanted interfacial reactions between the solid-state electrolyte and cathode electrodes from occurring. Incorporation of the inventive coating into sodium-based all-solid-state batteries allows for room temperature operation, high voltage, and long cycle life.

Claims

1. A method for improving cycling stability of a sodium all-solid-state battery, comprising: applying a LTO coating to a cathode of the battery.

2. The method of claim 1, wherein the cathode is NLNMO.

3. The method of claim 1, wherein the cathode is Na.sub.0.8[Li.sub.0.12Ni.sub.0.22Mn.sub.0.66]O.sub.2.

4. The method of claim 1, wherein the LTO coating is applied to particles of cathode material prior to formation of the cathode.

5. The method of claim 1, wherein the LTO coating is Li.sub.4Ti.sub.5O.sub.12.

6. A coating for improving cycling stability of a sodium all-solid state battery, the coating comprising LTO applied to a cathode of the battery.

7. The coating of claim 6, wherein the cathode is NLNMO.

8. The coating of claim 6, wherein the cathode is Na.sub.0.8[Li.sub.0.12Ni.sub.0.22Mn.sub.0.66]O.sub.2.

9. The coating of claim 6, wherein the LTO coating is applied to particles of cathode material and thermally processed prior to formation of the cathode.

10. A sodium all-solid-state battery comprising: a Na—Sn negative electrode; a NLNMO positive electrode having a LTO coating incorporated therein; and a NPS solid state electrolyte disposed between the positive electrode and the negative electrode.

11. The battery of claim 10, further comprising a carbon conductive additive disposed between the NPS solid electrolyte and the NLNMO positive electrode.

12. The battery of claim 10, wherein the cathode is Na.sub.0.8[Li.sub.0.12Ni.sub.0.22Mn.sub.0.66]O.sub.2.

13. The battery of claim 10, wherein the LTO coating is applied to particles of positive electrode material and thermally processed prior to formation of the positive electrode.

14. The battery of claim 10, wherein the LTO coating is Li.sub.4Ti.sub.5O.sub.12.

15. The battery of claim 10, wherein the NPS solid electrolyte is Na.sub.3PS.sub.4.

16. A method for fabricating a sodium all-solid state battery, the method comprising: disposing within a mold, a composition comprising layers of: an electrode powder comprising a metallic sodium alloy; a cathode powder comprising particles having a LTO coating thereon; a solid electrolyte powder; and a carbon conductive additive; and compressing the layers to form a cell.

17. The method of claim 16, wherein the composition comprises 10 weight ratio of cathode powder, 16 weight ratio of electrolyte powder, 1 weight ratio of carbon conductive additive, and an excess of electrode powder.

18. The method of claim 16, wherein the metallic sodium alloy comprises Na—Sn.

19. The method of claim 16, wherein the cathode powder is Na.sub.0.8[Li.sub.0.12Ni.sub.0.22Mn.sub.0.66]O.sub.2.

20. The method of claim 16, wherein the LTO coating is Li.sub.4Ti.sub.5O.sub.12.

21. The method of claim 16, wherein the solid electrolyte is Na.sub.3PS.sub.4.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] FIG. 1A is a schematic of solid-state cell setup in accordance with an embodiment of the invention; FIG. 1B plots voltage profile comparisons of NLNMO in liquid and solid electrolyte with and without protective coating layers. First cycle Coulombic efficiencies are displayed for reference.

[0016] FIGS. 2A-2C are plots of computational density functional theory-based calculations to evaluate interfacial stability of the solid-electrolyte and cathode electrode interfacial with and without the protective coating layer, where FIG. 2A is a reactivity phase diagram, FIG. 2B plots electrochemical stability of LTO vs Na/Na.sup.+ redox; and FIG. 2C plots electrochemical stability of NPS vs Na/Na.sup.+ redox.

[0017] FIGS. 3A-3D illustrate the X-ray Photoelectron Spectroscopy (XPS) (FIG. 3A), X-ray Diffraction (XRD) (FIGS. 3B, 3C) and lattice parameters (FIG. 3D) of chemical and electrochemical degradation between the Na.sub.3PS.sub.4 solid-state electrolyte and the NLNMO cathode in the absence of the coating material.

[0018] FIG. 4A is a TEM image of an exemplary interface coating; FIG. 4B shows an image generated from STEM-EDX mapping of the coating; FIG. 4C compares the binding energies of Ti 2p region for bare LTO, LTO coated NLNMO and bare NLNMO.

[0019] FIG. 5 provides a comparison of the cell level impedance for the inventive LTO coating versus bare material.

[0020] FIGS. 6A-6C provide plots of performance measurements of the LTO coated NLNMO battery, in contrast with the performance without the coating material. FIG. 6A plots XPS S 2p region binding energies of NPS and LTO-NLNMO at charged states; FIG. 6B shows the improved rate capability of LTO-NLNMO compared to bare NLNMO; and FIG. 6C show the extended cell cycling performance of LTO-coated NLNMO.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0021] Using a representative sodium solid-state battery designs comprising a metallic sodium-tin (Na—Sn) alloy, sulfide solid electrolyte (Na.sub.3PS.sub.4) and sodium transition metal oxide cathode Na.sub.0.8[Li.sub.0.12Ni.sub.0.22Mn.sub.0.66]O.sub.2 (NLNMO) as the starting point, evaluation of the protective material is performed using both computational and experimental methods using characterization tools as well as electrochemical measurements. Such evaluations were probed using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) based off a computationally-guided protective coating (an additional oxide layer). STEM revealed that coating layer is amorphous and about 5 nm thick. After optimizing the coating process, the electrochemical performance of the cell dramatically improved, achieving a specific capacity comparable to that of the liquid cell while exhibiting 80% capacity retention after 300 cycles. This coating method can be an effective strategy for achieving higher electrochemical performance in room temperature all-solid-state Na-ion batteries.

[0022] The inventive method is applicable to a wide variety of sodium solid-state battery chemistries including those using: Na.sub.2Sx.P.sub.2S.sub.5y, NaSbS.sub.4, Na.sub.3PS.sub.4, Na.sub.xP.sub.zSi.sub.zS.sub.4, Na.sub.3PS.sub.4—Na.sub.4SiS.sub.4, sulfide based solid electrolytes, Na.sub.0.7CoO.sub.2+x, NaFePO.sub.4, NaFe.sub.xMn.sub.yO.sub.4, NaMnPO.sub.4, Na.sub.3V.sub.2(PO.sub.4).sub.3, Na.sub.xNi.sub.0.5Mn.sub.0.5O.sub.2, Na.sub.0.7MnO.sub.2+x based cathode materials, carbon, sodium alloy, or sodium metal based anode materials.

[0023] A battery was fabricated using a metallic sodium alloy (Na—Sn), a Na.sub.3PS.sub.4 (NPS) glass ceramic as the solid-state electrolyte, and a NLNMO cathode to demonstrate the technical concept in an ASSB. The average protecting coating thickness of LTO used is 5 nm on the surface of the cathode material. The coating material is applied to the cathode particles via sol-gel method, as is known in the art. First, stoichiometric amounts of sodium ethioxide and titanium isopropoxide are dispersed in anhydrous ethanol at 2-11 wt % relative to the sodium cathode amount. The solution is then mixed with the cathode material and dried under vacuum and at 60° C. to remove the solvent. Next, the coated cathode particles are annealed under heat treatment at 450° C. for 1 hour in ambient conditions.

[0024] An ASSB according to the present invention includes a NLNMO positive electrode, a Na—Sn negative electrode, and the above-described Na.sub.3PS.sub.4 solid state electrolyte interposed between the positive electrode and the negative electrode. In the exemplary embodiment, the ASSB is manufactured through a dry compression process, in which electrode powder and solid electrolyte powder are manufactured, introduced into a predetermined mold, and pressed in a composition of (10:16:1), in which 10 weight ratio of cathode electrode is used, 16 weight ratio of solid electrolyte is used, and 1 weight ratio of carbon conductive additive is used. An excess of Na—Sn alloy is used at the negative electrode. As will be apparent to those of skill in the art, any other well-known fabrication method may be used. In the exemplary process, the solid electrolyte is disposed between the positive electrode and the negative electrode and the layers are compressed at 370 MPa in order to assemble a cell. The assembled cell is encapsulated in a case of aluminum or stainless steel, or a prismatic metal container that can appropriately hold the cell. The cell is then electrochemically cycled and compared against an equivalent cell with and without the coating material to study its effects. After cycling, the cell was also removed for characterization studies to evaluate the effectiveness of the protecting coating material in preventing long term interfacial reactions.

[0025] FIG. 1A provides a schematic of solid-state cell setup according to an embodiment of the invention. This cell was manufactured using the same procedure described in the preceding section. FIG. 1B is a plot of voltage profile comparisons of NLNMO in liquid and solid electrolyte with and without protective coating layers. First cycle Coulombic efficiencies are displayed for reference. It can be seen that significant electrochemical improvements are achieved after incorporation of the coating material, where lower cell polarization, higher capacities, and high efficiencies are observed. This illustrates the effectiveness of the protective coating layer.

[0026] Computational density functional theory-based calculations were performed to evaluate interfacial stability of the solid-electrolyte and cathode electrode interfacial with and without the protective coating layer. FIG. 2A is a reactivity phase diagram between NPS-LTO and NPS-NLNMO. The free energies of reactions indicate the thermodynamic favorability of reaction between each material. As shown, reactivities between NPS-LTO are much less than NPS-NLNMO, indicating a higher interfacial stability can be expected after incorporation of the coating material between the solid-state electrolyte and the cathode electrode.

[0027] FIG. 2B plots electrochemical stability of LTO vs Na/Na.sup.+ redox. This plot shows the intrinsically high thermodynamic stability of the LTO, which allows continuous long-term utility of the coating material to protect the interface between solid-state electrolyte and cathode electrode. FIG. 2C shows electrochemical stability of NPS vs Na/Na.sup.+ redox. Unlike other oxide based solid-state electrolytes, the selected material Na.sub.3PS.sub.4 readily undergoes electrochemical decomposition when exposed to oxidative environment under high voltage. However, the computationally predicted products of P.sub.2S.sub.5 and S, both insulating compounds, prevent further degradation from occurring continuously and thus passivate the interface and allow continuous long-term cyclability of the battery.

[0028] FIGS. 3A-3D illustrate the X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD) and lattice parameters of chemical and electrochemical degradation between the Na.sub.3PS.sub.4 solid-state electrolyte and the NLNMO cathode in the absence of the coating material. These plots describe the reaction mechanisms of interfacial reactions in the absence of a protecting coating and can be compared against the extent of such reactions when the coating material is applied.

[0029] FIG. 3A plots the XPS S 2p region binding energies of Na.sub.3PS.sub.4 and NLNMO mixtures at bare and charged states. Upon mixture of Na.sub.3PS.sub.4 with uncoated NLNMO cathode material, spontaneous chemical reactions start to occur, producing unwanted side products such as Na.sub.2S. Notably, the mixture was heated to 60° C. to increase the kinetics of any reactions in order to amplify them for XPS. Upon mixture of Na.sub.3PS.sub.4 with uncoated but charged NLNMO cathode material, more aggressive spontaneous chemical reactions start to occur, producing unwanted Na.sub.2SO.sub.3 side products. When the mixtures of Na.sub.3PS.sub.4 with uncoated NLNMO cathode material are charged together, typical of a normal cell, side products are also observed—Na.sub.2SO.sub.3 and S. These results indicate that Na.sub.3PS.sub.4 with uncoated NLNMO cathode material are intrinsically unstable with each other, with both chemical and electrochemical reactions occurring at their interface.

[0030] FIGS. 3B and 3C show enlarged XRD patterns of NPS and NLNMO mixtures at bare and charged states comparing their relative shifts of the (002) and (011) peaks, respectively. Upon charging of NLNMO alone, we can expect characteristic shifts in the (002) and (011) peaks due to lattice changes as a result of sodiation. However, when charged NLNMO is mixed with Na.sub.3PS.sub.4 according to the inventive method, the extent of this shift is significantly reduced, indicating that side reactions have occurred. The relative positions of each peak are illustrated in FIG. 3D, which provides a, b, and c lattice parameter comparisons of NPS and NLNMO mixtures at bare and charged states.

[0031] FIGS. 4A-4C illustrate that a uniform and conformal 5 nm protective coating layer can be applied onto the surface of the sodium cathode material, protecting its interface against the Na.sub.3PS.sub.4 solid-state electrolyte. FIG. 4A is high resolution Transmission Electron Microscopy (TEM) image of amorphous LTO coated onto a NLNMO particle surface. The coating was applied via the sol-gel method described in the previous section. FIG. 4B is a Scanning Transmission Electron Microscopy (STEM)-Energy Dispersive X-ray mapping of Ti from LTO coated NLNMO is conducted. To differentiate the coating layer from the transition metal oxide layer, Ti is imaged as it is the primary component of the LTO coating easily distinguishable from the NLNMO cathode material, which does not contain any Ti. Thus, the coating indeed appears on the surface of the particle.

[0032] FIG. 4C plots XPS binding energies of Ti 2p region for bare LTO, LTO coated NLNMO and bare NLNMO. The inset at the vertical center of the plot is a schematic of the particle with coating. To ensure that the LTO coating material on the NLNMO cathode indeed matches the chemical signatures of pristine LTO, their binding energies (indicative of their relative oxidation states) are compared. It can be seen that the Ti 2p signals from the LTO coated NLNMO matches with the bare LTO material.

[0033] In FIG. 5, the LTO coating layer is shown to reduce cell level impedance compared to the bare material. This is reflected by lower polarization when the cell is cycled. The Nyquist plots (insets) show the comparative impedance measurements of an uncoated NLNMO battery with acetylene black carbon additive, a LTO-coated NLNMO battery with acetylene black carbon additive, and a LTO-coated NLNMO battery with vapor grown carbon fiber additive. Each measurement was taken after one full charge cycle, followed by allowing the cell to relax. These results illustrate that once the LTO coating is applied in the cell, the impedance growth is significantly reduced.

[0034] FIGS. 6A-6C provide performance measurements of the LTO coated NLNMO battery, in contrast with the performance without the coating material. FIG. 6A plots XPS S 2p region binding energies of NPS and LTO-NLNMO at charged states, showing absence of interfacial products. This is in large contrast with FIG. 3A, where significantly higher quantities of interfacial products are found.

[0035] FIG. 6B shows the improved rate capability of LTO-NLNMO compared to bare NLNMO. It can be seen that all discharge conditions of the LTO-coated NLNMO exhibit lower polarization and higher capacity compared to the uncoated cathode. FIG. 6C plots extended cell cycling performance of LTO-coated NLNMO, demonstrating good stability and long term cycle life of the LTO coated NLNMO ASSB.

[0036] Table 1 provides a list that compares the performance between the LTO-coated cathode according to the present invention and other various sodium transition metal-based cathode materials. As indicated, the NLNMO cathode battery described herein exhibits 60% retention after 300 cycles.

TABLE-US-00001 TABLE 1 Voltage/ Capacity Retention Temp/ Cathode Anode Solid Electrolyte V (mAh/g) (cycle no) ° C. NaCrO.sub.2 Na-Sn Na.sub.3SbS.sub.4 3.1 106 57% (20)  RT NaCrO.sub.2 Na-Sn Na.sub.3PS.sub.4 3.1 90 66% (20)  RT Na.sub.2+2δFe.sub.2−δ(SO.sub.4).sub.3 Na.sub.2Ti.sub.3O.sub.7 Na.sub.3.1Sn.sub.0.1 P.sub.0.9S.sub.4 3.1 114 21% (100) RT NLNMO Na-Sn Na.sub.3PS.sub.4 4.1 78 60% (300) RT

[0037] The improved all solid-state batteries incorporating the novel interface protection provide an important solution for low-cost, safe and robust energy storage capable of operating under any climate. Scalable, sustainable designs can thus be enabled, in addition to being fully recyclable. This can be a solution for homes, the grid, and a variety of distributed energy storage needs.

REFERENCES (INCORPORATED HEREIN BY REFERENCE)

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