Abstract
As anode having an anode material, a current collector, a graphene-based material, and the graphene-based material covers the anode material.
Claims
1. An anode comprising: an anode material, a current collector, a graphene-based material, said graphene-based material covers said anode material.
2. The anode of claim 1 wherein said graphene-based material is nitrogen-doped graphene.
3. The anode of claim 2 wherein said nitrogen-doped graphene partially covers said anode material.
4. The anode of claim 2 wherein said nitrogen-doped graphene fully covers said anode material.
5. The anode of claim 1 wherein said graphene-based material includes a plurality of channels that promotes ion transfer.
6. The anode of claim 5 wherein said graphene-based material includes a plurality of doping sites made of at least one elemental dopant that forms channels that promote ion transfer.
7. The anode of claim 6 wherein said graphene-based material includes a plurality of N-doping sites that form channels that promote ion transfer.
8. The anode of claim 6 wherein said elemental dopants are N, S, B, and P, or combinations thereof.
9. The anode of claim 1 wherein said graphene-based material prevents polysulfide from contacting an electrolyte.
10. The anode of claim 9 further including a metal oxide on said graphene-based material.
11. The anode of claim 2 wherein said nitrogen-doped graphene prevents polysulfide from contacting an electrolyte.
12. The anode of claim 11 further including a metal on said nitrogen-doped graphene.
13. The anode of claim 12 further including a metal oxide on said nitrogen-doped graphene.
14. The anode of claim 13 wherein said metal oxide is Al.sub.2O.sub.3, ZrO.sub.2, TiO.sub.2. HfO.sub.2 and combination thereof.
15. The anode of claim 14 wherein said anode material Cu.sub.2S.
16. The anode of claim 15 wherein said nitrogen-doped graphene is in the form of a sheet.
17. The anode of claim 11 further including a coating on said nitrogen-doped graphene, said coating prevents SEI side reactions with the electrolyte decomposition.
18. The anode of claim 11 further including a coating on said nitrogen-doped graphene, said coating is chemically inert to electrolyte and prevents said anode material from reacting with electrolyte.
19. A method for fabricating a Cu.sub.2S/NGS composite electrode comprising the steps of: using ball milling to grind a Cu.sub.2S powder; mixing said ground powders with conductive agents that include carbon black and binders; and using nitrogen-doped graphene (NGS) to replace carbon black as the conductive additive.
Description
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0027] In the drawings, which are not necessarily drawn to scale, like numerals may describe substantially similar components throughout the several views. Like numerals having different letter suffixes may represent different instances of substantially similar components. The drawings illustrate generally, by way of example, but not by way of limitation, a detailed description of certain embodiments discussed in the present document.
[0028] FIG. 1A shows an as-received Cu.sub.2S powder has a particle size ranging from 10 to 50 μm for an embodiment of the present invention.
[0029] FIG. 1B is an SEM image of the electrode made from ball-milled Cu.sub.2S and SuperP (indicated as Cu.sub.2S/SuperP), showing significantly reduced particles with sizes in the nanoscale for an embodiment of the present invention.
[0030] FIG. 1C is an image of an electrode from ball-milled Cu.sub.2S and NGS (i.e., Cu.sub.2S/NGS), showing reduced particles with sizes mostly less than 2 μm for an embodiment of the present invention.
[0031] FIG. 1D shows an open-structure Cu.sub.2S@SuperP electrode where Cu.sub.2S particles are exposed to electrolyte for an embodiment of the present invention.
[0032] FIG. 1E shows a close-structure Cu.sub.2S@NG electrode where Cu.sub.2S particles are encapsulated by NG and isolated from the direct contact with electrolyte for an embodiment of the present invention.
[0033] FIG. 2A show a specific discharge capacity and Coulombic efficiency vs. cycles of Cu.sub.2S/SuperP and Cu.sub.2S/NGS electrodes in Na half cells tested under 0.01-3 V at a rate of 100 mA.Math.g.sup.−1.
[0034] FIG. 2B shows cyclic voltammetric profiles of a 5-cycle Na—Cu.sub.2S/SuperP cell
[0035] FIG. 2C shows cyclic voltammetric profiles of an 11-cycle Na—Cu.sub.2S/NGS cell.
[0036] FIG. 2D is a Nyquist plot from EIS measurements of a Na—Cu.sub.2S/SuperP cell during 5 cycles.
[0037] FIG. 2E is a Nyquist plot from EIS measurements and a Na—Cu.sub.2S/NGS cell during 50 cycles.
[0038] FIG. 3A is an SEM of the morphology of pristine Cu.sub.2S/SuperP.
[0039] FIG. 3B is an SEM of the morphology of the Cu.sub.2S/SuperP shown in FIG. 3A after 1.sup.st discharge.
[0040] FIG. 3C is an SEM of the morphology of morphology of the Cu.sub.2S/SuperP shown in FIG. 3A after 1.sup.st charge.
[0041] FIG. 3D is an SEM of the morphology of pristine Cu.sub.2S/NGS electrodes.
[0042] FIG. 3E is an SEM of the morphology of the Cu.sub.2S/NGS electrodes shown in FIG. 3D after 1st discharge.
[0043] FIG. 3F is an SEM of the morphology of morphology of the Cu.sub.2S/NGS electrodes shown in FIG. 3D after 1st charge.
[0044] FIG. 4A shows selected voltage plates on galvanostatic profile at 50 mA.Math.g.sup.−1 in 0.01-3.0 V for ex-situ XRD tests.
[0045] FIG. 4B show ex situ XRD patterns of pristine Cu.sub.2S/NGS electrode and the electrodes after cycling at the potentials of 0.7 V (discharge), 0.3 V (discharge), 0.1 V (discharge), 0.01 V (discharge), 2 V (charge), and 3 V (charge) versus Na/Na.sup.+.
[0046] FIG. 4C is a comparison of the normalized Cu K-edge XAS spectra for Cu.sub.2S Cu.sub.2S/NGS electrodes discharged to 0.01V and charged to 3V. The dot-dash lines show the near-edge spectra of Cu and Cu.sub.2S powder as references for Cu and Cu.sup.+ valency state. The inset shows the radical distance to first neighboring atoms.
[0047] FIGS. 5A, 5B, 5C and 5D are comparisons between Cu.sub.2S/SuperP and Cu.sub.2S/NGS electrodes for (a) (c) cycle performance with cutoff voltages of 0.01, 0.2, 0.4 V versus Na/Na.sup.+ at 100 mA.Math.g.sup.−1 and (b) (d) rate performance with a cutoff voltage of 0.2 V versus Na/Na.sup.+ at current densities of 100 mA.Math.g.sup.−1, 0.5, 1, 2, 5, and 10 C, respectively.
[0048] FIGS. 6A, 6B, 6C, 6D, 6E and 6F are Nyquist plots from EIS measurements during 50 cycles of a Na—Cu.sub.2S/SuperP cell under (a) 0.2-3 V and (b) 0.4-3 V versus Na/Na.sup.+. (c) R.sub.SEI film resistance in Na—Cu.sub.2S/SuperP cell shows as functions of either cutoff voltage or cycle number. Nyquist plot from EIS measurements during 50 cycles of a Na—Cu.sub.2S/NGS cell under (d) 0.2-3 V and (e) 0.4-3 V versus Na/Na.sup.+. (f) R.sub.SEI film resistance in Na—Cu.sub.2S/NGS cell shows as functions of either cutoff voltage or cycle number.
[0049] FIGS. 7A, 7B, 7C, 7D, 7E and 7F are comparisons of specific discharge capacity vs. cycles for pristine Cu.sub.2S/NGS electrodes and the electrodes with ALD Al.sub.2O.sub.3 coating of 1, 2, 4, 6, and 8 nm. Nyquist plot from EIS measurements during 50 cycles of a Na-ALD/Cu.sub.2S/NGS cell with the Al.sub.2O.sub.3 coating of (b 1 nm, (c) 2 nm, (d) 4 nm, (e) 6 nm, and (f) 8 nm at the cutoff voltage of 0.01-3 V versus Na/Na.sup.+.
DETAILED DESCRIPTION OF THE INVENTION
[0050] Detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed method, structure, or system. Further, the terms and phrases used herein are not intended to be limiting, but rather to provide an understandable description of the invention.
[0051] In a preferred embodiment, the present invention concerns systems and methods for creating anodes for use with sodium-ion batteries. In another preferred embodiment, the present invention provides a method of ball milling to accomplish superior Cu.sub.2S anodes for sodium-ion batteries.
[0052] First, ball milling may be used to grind commercial micro-sized Cu.sub.2S powders and mixing the powders with conductive agents and binders. Second, nitrogen-doped graphene (NGS) is used to replace traditional carbon black as conductive additive. This embodiment has multiple benefits such as suppressing formation of solid electrolyte interphase, inhibiting sulfur shuttling, and improving conductivity.
[0053] In other embodiments, atomic layer deposition may be used to improve the performance of Cu.sub.2S anodes.
[0054] In yet other embodiments, the Cu.sub.2S anodes for sodium-ion batteries (SIBs) may be further improved through coupling with a SIB cathode.
[0055] Using a ball-milling method to fabricate nanostructured Cu.sub.2S wrapped by ultra-thin nitrogen-graphene sheets (NGS) produces Cu.sub.2S/NGS composite electrodes that exhibit superior rate capability and long-term stable cyclability in comparison with Cu.sub.2S/SuperP electrodes. Moreover, a uniform and conformal Al.sub.2O.sub.3 coating via atomic layer deposition (ALD) further improves the performance of Cu.sub.2S/NGS electrodes.
[0056] In one embodiment, Copper(I) sulfide (Cu.sub.2S) was mixed with a conductive additive (either SuperP or NGS (ACS Material)) and a PVDF binder (MTI Corporation) in 1-Methyl-2-pyrrolidone (NMP, Sigma-Aldrich) solvent in a weight ratio of 8:1:1. The mixtures were then ball-milled at 870 rpm for 6 hours using a planetary system (MSE Supplies LLC.) to grind down the Cu.sub.2S particle size. The resultant slurries were subsequently cast on Cu foil with a thickness of 200 μm by a doctor blade. The electrode films were dried in an ambient environment and were heated at 100° C. in a vacuum for 8 hours. Al.sub.2O.sub.3 coatings were performed using atomic layer deposition (ALD, Savannah 200, Veeco) for different thicknesses at 100° C. The ALD precursors are trimethylaluminum (TMA) and H.sub.2O.
[0057] Electrochemical Tests
[0058] The electrochemical performance of Cu.sub.2S electrodes made in accordance with the present invention were evaluated in half coin cells (CR2032 stainless steel coin cells), using sodium metal as the counter electrode. Cu.sub.2S electrodes were punched into circular disks with a 7/16-inch diameter, and the loading of Cu.sub.2S active material is in the range of 1.5-2 mg cm.sup.−2. Celgard polypropylene/polyethylene membranes (25 μm thick, MTI Corporation) were used as separators. Sodium metal (Millipore Sigma) was roll-pressed to sodium foil and then punched into 7/16-inch circular disks as counter electrodes. The liquid electrolyte was 1 M NaPF.sub.6 (Sigma-Aldrich) in DEGDME (Sigma-Aldrich). The coin cells were then assembled and pressed under a hand-operated hydraulic crimping machine (MTI Corporation) at 1000 psi within an Ar-filled glove box with H.sub.2O and 02 levels <0.01 ppm.
[0059] Galvanostatic cycling tests were performed on a Neware Battery Testing System at room temperature. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were both performed on an SP-200 potentiostat (Bio-logic). CV applied a scan rate of 0.1 mV s.sup.−1 in the voltage range of 0.01-3 V vs. Na/Na.sup.+. EIS were measured at the frequency range from 0.01 Hz to 100 kHz with an AC signal amplitude of 5 mV.
[0060] Characterizations
[0061] Materials morphologies were characterized using a scanning electron microscopy (SEM, NanoSEM 450). Elemental mapping of elements was conducted using energy-dispersive X-ray (EDX, NanoSEM 450) analysis. The phase structure and valence state of materials were determined using synchrotron-based X-ray diffraction (XRD, 11 ID-D beamline) and X-ray adsorption spectroscopy (XAS, 20 BM-B beamline) at Advanced Photon Source (APS) at Argonne National Laboratory, respectively. Ex-situ XRD and ex-situ XAS characterizations were performed at the beamlines 11 ID-D (0.1173 Å wavelength) and 20 BM-B (Cu K edge) at APS, respectively.
[0062] FIG. 1A shows that the as-received Cu.sub.2S powder has a particle size ranging from 10 to 50 μm. In order to facilitate Na-ion diffusion into Cu.sub.2S lattice during electrochemical cycling, the micro-sized Cu.sub.2S particles were ball-milled at 870 rpm for 6 hrs. FIG. 1B is the SEM image of the electrode made from ball-milled Cu.sub.2S and SuperP (indicated as Cu.sub.2S/SuperP), showing significantly reduced particles with sizes in the nanoscale. In comparison, FIG. 1C is the image of the electrode from ball-milled Cu.sub.2S and NGS (i.e., Cu.sub.2S/NGS), showing reduced particles with sizes mostly less than 2 μm. The bigger particle sizes in Cu.sub.2S/NGS electrodes are believed due to the porous structure of NGS and its high mechanical strength against the particle grinding. It is also noteworthy that the two electrodes have distinct morphologies. As illustrated in FIG. 1B, the Cu.sub.2S/SuperP electrode has numerous cracks and voids fully accessible to the electrolyte.
[0063] In another aspect, the present invention provides an open-structure Cu.sub.2S@SuperP electrode 100 on Cu foil 101 comprised of Cu.sub.2S particles 102A-102H and SuperP particles 103A-103 as shown in FIG. 1D. As shown are SEI 104A-104C, Na.sup.+ diffusion 105A-105C and polysulfide shuffling 106A-106C.
[0064] In comparison, FIG. 1E shows a close-structure anode comprising an anode material which may be Cu.sub.2S particles 202A-202E. Also provide is a current collector 201 which may be a copper foil. The embodiment further includes a graphene-based material 204A-204H. The graphene-based material covers the anode material.
[0065] In other aspects, the graphene-based material either partially or fully covers the anode material. In another embodiment, the graphene-based material prevents polysulfide from contacting electrolyte 206.
[0066] In yet other embodiments, the graphene-based material includes a plurality of channels that promotes ion transfer. In another embodiment, the graphene-based material includes a plurality of doping sites made of at least one elemental dopant that forms channels that promote ion transfer. The elemental dopants may be N, S, B, and P, or combinations thereof. FIG. 1E also shows SEI 207A-207C, Na.sup.+ diffusion 215A-21C and polysulfide shuffling 216A-216B.
[0067] The electrochemical behaviors of Cu.sub.2S/SuperP and Cu.sub.2S/NGS electrodes were comparatively investigated. FIG. 2A shows the cyclability and Coulombic efficiency (CE) of the electrodes at a current density of 100 mA g.sup.−1 in the voltage range of 0.01-3 V. The Cu.sub.2S/SuperP electrode faded in capacity and CE quickly in 10 cycles. CE here is the ratio of discharge capacity over charge capacity. The low CE (<50%) and the almost unchanged discharge capacity (˜300 mAh g.sup.−1) for Cu.sub.2S/SuperP electrode after 3 cycles reveal that there has an abnormally long charging phenomenon. This might have been caused by a polysulfide shuttling effect, as reported in Na—S batteries. In comparison, Cu.sub.2S/NGS electrode delivered an initial discharge capacity of ˜450 mAh g.sup.−1 and then >300 mAh g.sup.−1 in 200 cycles with CE of 99%. Thus, the Cu.sub.2S/NGS electrode has improved performance electrochemically in cyclability, efficiency, and sustainable high capacity. This also suggests that NGS replacing SuperP inhibits the polysulfide shuttling issue through isolating Cu.sub.2S from direct contact with the electrolyte as shown in FIG. 1E. NGS is a physical barrier that reduces or inhibits S shuttling. The gradually fading capacity of Cu.sub.2S/NGS after 80 cycles might be due to the reconstruction of Cu.sub.2S/NGS during electrochemical cycling.
[0068] The CV profiles are shown in FIGS. 2B and 2C for the Cu.sub.2S/SuperP and Cu.sub.2S/NGS electrode, respectively. In the first discharge, both the electrodes show five reduction peaks at 0.41, 0.80, 1.16, 1.50, and 1.89 V commonly, ascribed to Cu.sub.2S intercalation and conversion reactions. Two additional reduction peaks at low potentials of 0.13 and 0.01 V might be the side reactions between the electrodes and the electrolyte with the formation of solid electrolyte interphase (SEI). In the first charge, there are three common oxidation peaks at 1.59, 1.88, and 2.13 V for the two electrodes. After the first cycle, both electrodes underwent some distinct evolutions at each peak, indicating that electrochemical behaviors of the two electrodes are dynamically changing. In addition, regarding the polysulfide shutting issue in Cu.sub.2S/SuperP electrode after 3 cycles, FIG. 2B shows many side reactions in the charge processes, likely related to polysulfide shuttling. In comparison, FIG. 2C shows fewer reactions during the charge processes.
[0069] The electrochemical stability was also investigated via EIS tests. the Cu.sub.2S/SuperP cell shows a suddenly increasing impedance at the 5.sup.th cycle (FIG. 2D), probably due to the unstable SEI formation and the polysulfide shuttling. Similarly, the Cu.sub.2S/NGS cell has an increasing impedance in the first 5 cycles (FIG. 2E), probably due to the reconstruction of Cu.sub.2S/NGS materials. However, the Cu.sub.2S/NGS cell shows a decreasing impedance after 5 cycles, indicating some optimization by the reconstruction.
[0070] The morphological changes of the two Cu.sub.2S electrodes during the first cycle were examined using SEM. Compared to the surface of a pristine electrode in FIG. 3A, the surface of Cu.sub.2S/SuperP electrode (FIG. 3B) is covered by an additional dense layer after sodiation. This additional layer may be an SEI layer. In contrast, the sodiated Cu.sub.2S/NGS electrode (FIG. 3E) has little difference with the pristine electrode (FIG. 3D), implying less SEI formation than that with the Cu.sub.2S/SuperP electrode. After desodiation (FIG. 3F), the Cu.sub.2S/NGS electrode almost recovers back to its pristine morphology (FIG. 3a), but the Cu.sub.2S particles become smaller. The smaller Cu.sub.2S particles provide direct evidence for the reconstruction of the Cu.sub.2S/NGS electrode. In contrast, the desodiated Cu.sub.2S/SuperP electrode (FIG. 3C) becomes more porous, compared to its pristine electrode in FIG. 3A. Compared to FIG. 3B, FIG. 3C implies that the SEI layer is unstable and can cause continuous electrolyte decomposition and increased impedance.
[0071] FIG. 4A shows the selected voltage for performing ex-situ synchrotron-based XRD analyses, including the voltages of 0.7, 0.3, 0.1, and 0.01 V during the discharge, and 2.0 and 3.0 V during the charge of the first cycle. XRD results in FIG. 4b show that the pristine Cu.sub.2S/NGS electrode contains mixed phases of Cu.sub.2S, Cu.sub.1.8S, and Cu.sub.1.92S. Discharging to 0.7 V, the XRD patterns of Cu.sub.2S phases became left-shifted, suggesting the enlarged volume of Cu.sub.2S lattice by Na intercalation. Discharging to 0.3V, the shifted Cu.sub.2S peaks have diminished in intensity, while simultaneously Cu and Na.sub.2S peaks arise. This indicates that Cu.sub.2S may have experienced a conversion reaction and converted into Na.sub.2S and Cu between 0.7 and 0.3 voltage. Further discharging to 0.1 V, the Cu.sub.2S features nearly have disappeared while Cu and Na.sub.2S peaks become much evident. When fully discharging to 0.01 V, no significant phase changed for Cu and Na.sub.2S patterns in comparison with that at 0.1 V. This may suggest that the electrochemistry of Cu.sub.2S via sodiation may have accomplished above 0.01 V. On the other hand, the desodiated electrode at 2.0 V showed two major Cu and Na.sub.2S patterns while some minor Cu.sub.xS patterns. As desodation to 3.0 V, Cu.sub.2S patterns are reversibly recovered and there is no Cu peak found. In addition, synchrotron-based XAS in FIG. 4C confirmed the reversible electrochemistry by tracking the valency state of the Cu element based on its K edge. The electrode discharged to 0.01 V exhibits similar Cu.sup.0 valency state and first-neighboring-atom distance to the standard Cu foil, suggesting the complete conversion from Cu.sub.2S to Cu metal. As charging to 3 V, the valency state of Cu recovered to Cu.sup.1+ with the close first-neighboring-atom distance to pristine Cu.sub.2S powder. Thus, XRD and XAS strongly evidence that Na and Cu.sub.2S undergo a highly reversible electrochemical reactions, including intercalation and conversion.
[0072] The ex-situ XRD results in FIG. 4B also confirmed two side reactions at 0.13 and 0.01 V, as illustrated in the CV profiles of FIG. 2C. The side reactions should be responsible for SEI formation. The stability of SEI is related to the performance of both the Cu.sub.2S/SuperP and Cu.sub.2S/NGS electrodes. A stable SEI is beneficial for achieving high performance of Cu.sub.2S electrodes. Otherwise, an unstable SEI will degrade the performance of Cu.sub.2S electrodes quickly. The SEI formation is always associated with electrolyte deposition and lithium consumption. The electrochemical performance (including cyclability, stability, rate capability and impedance) was investigated due to different discharge cutoff voltages (0.01, 0.2, and 0.4 V) on both the electrodes of Cu.sub.2S/SuperP and Cu.sub.2S/NGS. FIG. 5A shows the cyclability and CE of the Cu.sub.2S/SuperP electrode, while FIG. 5C shows the cyclability and CE of the Cu.sub.2S/NGS electrode. Apparently, the 0.2 V discharge cutoff voltage dramatically improved the cyclability and stability of Cu.sub.2S/SuperP electrode (302 mAh g.sup.−1 and 96% CE at 120 cycles) and Cu.sub.2S/NGS (313 mAh g.sup.−1 and 98% CE at 450 cycles). A higher discharge cutoff voltage further improved the cyclability and stability but sacrificed some capacity. EIS tests in FIGS. 6A through 6F also validated the SEI-suppressing effect of high discharge cutoff voltage on the SEI formation. The resistance of SEI layer calculated from those Nyquist plots exhibited a remarkable decrease as applying a cutoff voltage of >0.2 V. It validates that discharge cutoff voltage could significantly suppress the formation of an unstable SEI layer. In addition, rate capacity was also tested for both electrodes. FIGS. 5B and 5D show excellent capacity retention at high rates up to 10 C for both electrodes. However, FIG. 5B reveals that Cu.sub.2S/SuperP electrode suffers some low CE at high rates of >5 C using a 0.2 V discharge cutoff voltage, possibly due to the polysulfide shutting issue. In comparison, the Cu.sub.2S/NGS electrode exhibits an extremely stable high-rate performance (especially at 10 C) using a 0.2 V cutoff voltage, as illustrated in FIG. 5D.
[0073] In order to further suppress the formation of the SEI layer, as shown in FIG. 1E, a coating 300 may be applied to graphene-based material 204A-204G. The coating may be chemically inert to electrolyte and prevents the anode material from reacting with electrolyte. Coating 300 may be applied by ALD.
[0074] In a preferred embodiment, coating 300 is a metal or a metal oxide. In another preferred embodiment the metal oxide may be Al.sub.2O.sub.3 is Al.sub.2O.sub.3, ZrO.sub.2, TiO.sub.2, HfO.sub.2 and combination thereof.
[0075] FIG. 7A compares the pristine Cu.sub.2S/NGS electrode and the electrodes coated with Al.sub.2O.sub.3 coatings of 1, 2, 4, 6, and 8 nm in thickness. Apparently, Al.sub.2O.sub.3 coating thicker than 2 nm can effectively hinder the capacity fading of Cu.sub.2S/NGS electrodes. Among all electrodes, the electrode with a 6-nm Al.sub.2O.sub.3 coating delivers the highest specific capacity after 100 cycles. However, a coating thicker than 6 nm resulted in a lower capacity than that of the electrode with a 6-nm thick coating. This may be due to some limited diffusion of Na.sup.+ to Cu.sub.2S with a thick coating of 8 nm. In addition, the effect of ALD coating on the interfacial state was also examined by measuring EIS. Compared to the pristine electrode in FIG. 2e, the impedances versus cycle with Al.sub.2O.sub.3 coatings were significantly reduced as the coating thickness thicker than 4 nm, as shown in FIG. 7b-f. However, no more decrease in impedance when further increasing the coating thickness in FIG. 7D-7F. To summarize, a 6-nm thick Al.sub.2O.sub.3 coating via ALD shows the optimal performance in boosting the performance of the Cu.sub.2S/NGS electrode.
[0076] The embodiments of the present invention deploy a technical route via ball milling to make superior Cu.sub.2S anodes, in which NGS is used to replace SuperP as the conductive agents. In addition, NGS may serve as a physical barrier to inhibit S shuttling and SEI formation. Compared to the NGS/SuperP electrode, the Cu.sub.2S/NGS electrode enables superior performance, including long-term cyclability, high efficiency, and high sustainable capacity. This may be ascribed to the multiple roles played by NGS. A cutoff voltage at 0.2 V also helped Cu.sub.2S/NGS electrode to accomplish better performance, i.e., a sustainable capacity of ˜300 mAh g.sup.−1 and 98% CE for over 450 cycles at 100 mA g.sup.−1. It also helps to achieve high-rate capability up to 10 C (equivalent to 3.37 A g.sup.−1). Moreover, a conformal and uniform Al.sub.2O.sub.3 coating via ALD further suppresses SEI formation and improves Cu.sub.2S performance, enabling better electrochemical performance using the voltage window of 0.01-3 V.
[0077] While the foregoing written description enables one of ordinary skill to make and use what is considered presently to be the best mode thereof, those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific embodiment, method, and examples herein. The disclosure should therefore not be limited by the above-described embodiments, methods, and examples, but by all embodiments and methods within the scope and spirit of the disclosure.
