Method for the fabrication of an electroless-metal-plated sulfur nanocomposite, an electroless-metal-plated sulfur cathode which is made from the nanocomposite, and a battery that uses the cathode

Abstract

A method for the fabrication of an electroless-metal-plated sulfur nanocomposite, an electroless-metal-plated sulfur cathode which is made from the nanocomposite, and a battery that uses the cathode, where the method includes chemically plating a conductive metal nanoshell onto the surface of the insulating sulfur powder to improve the conductivity of the sulfur cathode material, where through enhancing the electrochemical reaction kinetics with metal catalysis capabilities, and performing physical and chemical adsorption of liquid polysulfides with metal activity, the electroless-metal-plated sulfur nanocomposite enables the battery to exhibit high electrochemical utilization and stable cyclability, such that the nanocomposite can achieve a high sulfur content and high metal content, and the cathode demonstrates a high sulfur loading with a low electrolyte-to-sulfur ratio, the lithium-sulfur battery with the cathode exhibiting a high discharge capacity along with high energy density, and maintaining stable and high reversible capacity after 200 cycles within a wide range of cycling rates.

Claims

1. A method for fabrication of an electroless-metal-plated sulfur nanocomposite, comprising the steps of: a. a sensitization step, wherein a sensitization solution is prepared by adding SnCl.sub.2 to HCl, sulfur powder is added to the sensitization solution and stirred, and the sensitization solution is filtered to collect a sensitized powder; b. an activation step, wherein an activation solution is prepared by adding PdCl.sub.2 to HCl, the sensitized powder is added to the activation solution and stirred, and the activation solution is filtered to collect a pre-treated powder; c. an acidic chemically plating step, wherein a solution 1 containing metal ions is prepared by dissolving sodium acetate and metallic salt in deionized water; a solution 2 is prepared by dissolving NaH.sub.2PO.sub.2.H.sub.2O in deionized water; solution 1 and solution 2 are preheated to the same temperature and mixed to form a mixture solution, the pre-treated powder is added to the mixture solution and stirred until the metal ions are reduced to metal atoms and form a resulting product, then the resulting product is filtered, dried and ground to form an electroless-metal-plated sulfur nanocomposite.

2. The method of claim 1, wherein the metallic salt is at least a substance selected from the group consisting of AuCl.sub.3, AgCl, CuCl.sub.2, SnCl.sub.4, NiCl.sub.2, CoCl.sub.2, and FeCl.sub.3.

3. The method of claim 1, wherein the sensitization solution with the sulfur powder is stirred for 10 to 180 minutes in the sensitization step, wherein the activation solution with the sensitized powder is stirred for 10 to 180 minutes in the activation step, and wherein the mixture solution with the pre-treated powder is stirred for 10 to 180 minutes in the acidic chemically plating step.

4. The method of claim 1, wherein the same temperature is 70 to 90° C.

5. The method of claim 1, wherein the electroless-metal-plated sulfur nanocomposite has a sulfur content of 60 to 95 wt %.

6. The method of claim 1, wherein the electroless-metal-plated sulfur nanocomposite has a metal content of 5 to 40 wt %.

7. An electroless-metal-plated sulfur cathode fabricated by drop-casting, comprising the steps of: dissolving the electroless-metal-plated sulfur nanocomposite fabricated by the method of claim 1 in an electrolyte, and drop-casting the dissolved electroless-metal-plated sulfur nanocomposite on a current collector to form an electroless-metal-plated sulfur cathode which contains sulfur loading.

8. The electroless-metal-plated sulfur cathode of claim 7, wherein the electrolyte has an electrolyte-to-sulfur ratio that is 7 to 20 μL/mg.

9. The electroless-metal-plated sulfur cathode of claim 7, wherein the sulfur loading is 2 to 14 mg/cm.sup.2.

10. A battery that utilizes the electroless-metal-plated sulfur cathode of claim 7.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] 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.

[0041] FIG. 1 shows (a) the process of fabricating the electroless-nickel-plated sulfur nanocomposite and the drop-casting process; the microstructural and elemental analysis (SEM) of (b) sulfur powder and (c) the electroless-nickel-plated sulfur nanocomposite; (d) the nitrogen adsorption-desorption isotherms of the electroless- nickel-plated sulfur nanocomposite (black) and the sulfur powder (red); (e) a porosity analysis of the electroless-nickel-plated sulfur nanocomposite (black) and the sulfur powder (red); the microstructural and elemental analysis (SEM) of the electroless-nickel-plated sulfur cathode (f) before and (g) after cycling for 200 cycles.

[0042] FIG. 2 shows the digital images and magnetic testing of the (a), (c) sulfur powder and (b), (d) electroless-nickel-plated sulfur nanocomposite.

[0043] FIG. 3 shows the energy-dispersive X-ray spectra of the (a) sulfur powder and (b) electroless-nickel-plated sulfur nanocomposite.

[0044] FIG. 4 shows the X-ray diffraction spectra of the sulfur powder (black) and electroless-nickel-plated sulfur nanocomposite (red).

[0045] FIG. 5 shows the pore-size distribution analysis of the electroless-nickel-plated sulfur nanocomposite (black) and sulfur powder (red) conducted with the (a) Barrett-Joyner-Halenda method, (b) density functional theory method, and (c) Horvath-Kawazoe method.

[0046] FIG. 6 shows the (a) Nyquist plot and (b) to (c) Bode plot of the electroless-nickel-plated sulfur nanocomposite (black) and sulfur powder (red); CV curves of electroless-nickel-plated sulfur cathode at the scan rates of (d) 0.020, (e) 0.025, (f) 0.030, (g) 0.035, and (h) 0.040 mV/s for 10 CV scannings; and (i) rate-dependent CV curves.

[0047] FIG. 7 shows a graph of the lithium-ion diffusion coefficient of the electroless-nickel-plated sulfur cathode.

[0048] FIG. 8 shows (a) the cyclability of the battery that uses the electroless-nickel-plated sulfur cathode at C/20, C/10, C/5, C/3, C/2 rates for 200 cycles; the voltage profiles at (b) C/20, (c) C/10, (d) C/5, (e) C/3, and (f) C/2 rates for 200 cycles.

[0049] FIG. 9 shows the areal capacity of the electroless-nickel-plated sulfur cathode at the cycling rates of C/20, C/10, C/5, C/3, C/2 (the 4.0 mAh/cm.sup.2 shown in the graph is the areal capacity required in an electric vehicle battery).

[0050] FIG. 10 shows the gravimetric capacity of the electroless-nickel-plated sulfur cathode at the cycling rates of C/20, C/10, C/5, C/3, C/2.

[0051] FIG. 11 shows the (a) upper-plateaus discharge capacity and (b) lower plateaus discharge capacity of the electroless-nickel-plated sulfur cathode at the cycling rates of C/20, C/10, C/5, C/3, C/2.

[0052] FIG. 12 shows the polarization analysis of the electroless-nickel-plated sulfur cathode at the cycling rates of C/20, C/10, C/5, C/3, C/2.

[0053] FIG. 13 shows the electrochemical characteristics of the pure sulfur cathode at the cycling rate of C/20.

[0054] FIG. 14 shows the morphological, microstructural and elemental analysis of the electroless-nickel-plated sulfur cathode before cycling ((a) to (b) are at low magnification, (c) to (d) are at high magnification).

[0055] FIG. 15 shows the morphological, microstructural and elemental analysis of the electroless-nickel-plated sulfur cathode after 200 cycles at the cycling rate of C/20 ((a) to (b) are at low magnification, (c) to (d) are at high magnification).

[0056] FIG. 16 shows the morphological, microstructural and elemental analysis of the electroless-nickel-plated sulfur cathode after 200 cycles at the cycling rate of C/10 ((a) to (b) are at low magnification, (c) to (d) are at high magnification).

[0057] FIG. 17 shows the morphological, microstructural and elemental analysis of the electroless-nickel-plated sulfur cathode after 200 cycles at the cycling rate of C/5 ((a) to (b) are at low magnification, (c) to (d) are at high magnification).

[0058] FIG. 18 shows the morphological, microstructural and elemental analysis of the electroless-nickel-plated sulfur cathode after 200 cycles at the cycling rate of C/3 ((a) to (b) are at low morphological, (c) to (d) are at high magnification).

[0059] FIG. 19 shows the form, microstructural and elemental analysis of the electroless-nickel-plated sulfur cathode after 200 cycles at the cycling rate of C/2 ((a) to (b) are at low magnification, (c) to (d) are at high magnification).

[0060] FIG. 20 shows the XRD data and component analysis data of the electroless-nickel-plated sulfur nanocomposite synthesized with a 0.1M NiCl.sub.2 plating solution at (a) 70° C., (b) 80° C., (c) 90° C., and with a 0.25 M NiCl.sub.2 plating solution at (d) 70° C., (e) 80° C., (f) 90° C.

[0061] FIG. 21 shows the thermogravimetric analysis data of the electroless-nickel-plated sulfur nanocomposite synthesized with a 0.1M NiCl.sub.2 plating solution at (a) 70° C., (b) 80° C., (c) 90° C., and with a 0.25 M NiCl.sub.2 plating solution at (d) 70° C., (e) 80° C., (f) 90° C.

[0062] FIG. 22 shows the performance test of a battery with sulfur contents of 60 wt %, 75 wt %, 85 wt %, 95 wt %, and 100 wt % (for reference), and with sulfur loadings of (a) 2mg/cm.sup.2, (b) 6 mg/cm.sup.2.

DETAILED DESCRIPTION

[0063] The following examples are given to describe the method for the fabrication of an electroless-metal-plated sulfur nanocomposite, an electroless-metal-plated sulfur cathode made from the nanocomposite, and a battery that uses the cathode of the present disclosure. It should be noted that the following examples are only given for illustrative purposes and are not intended to limit the scope of the present disclosure.

EXAMPLE 1

Preparing the Electroless-Nickel-Plated Sulfur Nanocomposite

[0064] The preparation of the electroless-nickel-plated sulfur nanocomposite begins with the sensitization of the materials. The sensitization solution is prepared as follows: dilute 4 ml HCl in 100 ml deionized water, and when evenly mixed, add 1 g SnCl.sub.2 and stir with a magnet. Next, add commercial pure sulfur powder to the sensitization solution and stir evenly for 1 hour until the sulfur particles adsorb the easily-oxidized Sn.sup.2+ ions. Then the sensitization solution is filtered to collect the sensitized powder, and the sensitized powder is then added to an activation solution.

[0065] The activation solution is prepared as follows: dilute 0.25 ml HCl in 100 ml deionized water, and when evenly mixed, add 0.025 g PdCl.sub.2 and mix evenly. Add the sensitization powder obtained earlier to the activation solution and mix evenly for 1 hour and rinse the surface of powder with deionized water, so that a highly active network is formed on the sulfur particle surface to obtain the pre-treated powder.

[0066] Subsequently, the acidic chemically plating process comprises the following steps: prepare a Watts bath plating solution by dissolving 5 g sodium acetate (NaAc) and 3 g NiCl.sub.2 in 50 ml deionized water, and separately, dissolve 1 g NaH.sub.2PO.sub.2.H.sub.2O in 50 ml deionized water. Preheat the two solutions to 80° C., combine, add the activated pre-treated powder, and stir evenly with a glass rod for 1 hour, allowing the activated sulfur surface to adsorb the Ni.sup.2+ ions in the solution. With the help of NaH.sub.2PO.sub.2.H.sub.2O as the reducing agent and a high temperature, the Ni.sup.2+ ions disproportionate and reduce to Ni atoms. The reaction rate of nickel salts and reducing agents depend largely on the concentration of the reducing agent and temperature. The higher the concentration of the reducing agent, the stronger the reducing capability and faster the reaction. Upon completion of the reaction, rinse the surface 3 times with deionized water, filter, and collect the composite powder. Dry the composite powder in a 50° C. convection oven and grind to form the electroless-nickel-plated sulfur nanocomposite. The nanocomposite has a sulfur content of 74 wt %.

Preparing the Electroless-Nickel-Plated Sulfur Cathode

[0067] The electroless-nickel-plated sulfur cathode is formed with a drop-casting process, wherein the electroless-nickel-plated sulfur nanocomposite is dissolved in a suitable amount of electrolyte and drop-cast on a current collector, with the electrolyte-to-sulfur ratio (E/S) being 7 μL/mg. The electrolyte is prepared by mixing 1.85 M Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), 0.2 M LiNO.sub.3 and a Dimethoxyethane/Dioxolane (DME/DOL) solution with a volume ratio of 55:40, i.e., dissolving 50.5 mg LiTFSI and 1.3 mg LiNO.sub.3 in 55 ml DME and 40 ml DOL.

Preparing the Battery

[0068] A battery is assembled in the order of the aforementioned cathode, a separator, and a lithium anode. Each component is sealed into a CR2032 button cell battery in a glove box in an argon environment, wherein the cathode has a sulfur loading of 14 mg/cm.sup.2.

EXAMPLE 2

Preparing the Electroless-Nickel-Plated Sulfur Nanocomposite

[0069] The preparation of the electroless-nickel-plated sulfur nanocomposite begins with the sensitization of the materials. The sensitization solution is prepared as follows: dilute 0.4 ml HCl in 10 ml deionized water, and when evenly mixed, add 0.1 g SnCl.sub.2 and stir with a magnet. Next, add commercial pure sulfur powder to the sensitization solution and stir evenly for 1 hour until the sulfur particles adsorb the easily-oxidized Sn.sup.2+ ions. Then the sensitization solution is filtered to collect the sensitized powder, and the sensitized powder is then added to an activation solution.

[0070] The activation solution is prepared as follows: dilute 0.025 ml HCl in 10 ml deionized water, and when evenly mixed, add 0.0025 g PdCl.sub.2 and mix evenly. Add the sensitization powder obtained earlier to the activation solution and mix evenly for 1 hour and rinse the surface of powder with deionized water, so that a highly active network is formed on the sulfur particle surface to obtain the pre-treated powder.

[0071] Subsequently, the acidic chemically plating process comprises the following steps: prepare 0.1 M NiCl.sub.2 plating solution (a Watts bath) by dissolving sodium acetate (NaAc) and NiCl.sub.2 in 5 ml deionized water, and separately, dissolve NaH.sub.2PO.sub.2.H.sub.2O in 5 ml deionized water, wherein the weight of the NaH.sub.2PO.sub.2.H.sub.2O and NiCl.sub.2 powder are determined based on an ion ratio Ni.sup.2+:H.sub.2PO.sub.2.sup.2− of 0.5. Preheat the two solutions to 70° C., combine, and add the activated pre-treated powder, and stir evenly with a glass rod for 1 hour, allowing the activated sulfur surface to adsorb the Ni.sup.2+ ions in the solution. With the help of NaH.sub.2PO.sub.2.H.sub.2O as the reducing agent and a high temperature, the Ni.sup.2+ ions disproportionate and reduce to Ni atoms. The reaction rate of nickel salts and reducing agents depend largely on the concentration of the reducing agent and temperature. The higher the concentration of the reducing agent, the stronger the reducing capability and faster the reaction. Upon completion of the reaction, rinse the surface 3 times with deionized water, filter, and collect the composite powder. Dry the composite powder in a 50° C. convection oven and grind to form the electroless-nickel-plated sulfur nanocomposite. The nanocomposite has a sulfur content of 95 wt %.

Preparing the Electroless-Nickel-Plated Sulfur Cathode

[0072] The electroless-nickel-plated sulfur cathode is formed with a drop-casting process, wherein the electroless-nickel-plated sulfur nanocomposite is dissolved in a suitable amount of electrolyte and drop-cast on a current collector, with the electrolyte-to-sulfur ratio being 20 μL/mg. The electrolyte is prepared by mixing 1.85 M LiTFSI, 0.2 M LiNO.sub.3 and a DME/DOL solution with a volume ratio of 55:40, i.e., dissolving 50.5 mg LiTFSI and 1.3 mg LiNO.sub.3 in 55 ml DME and 40 ml DOL.

Preparing the Battery

[0073] A battery is assembled in the order of the aforementioned cathode, a separator, and a lithium anode. Each component is sealed into a CR2032 button cell battery in a glove box in an argon environment, wherein the cathode has a sulfur loading of 2 mg/cm.sup.2.

EXAMPLE 3

[0074] Except for adjusting the concentration of the NiCl.sub.2 plating solution to 0.25M in the preparing the electroless-nickel-plated sulfur nanocomposite step, the electroless-nickel-plated sulfur nanocomposite, the electroless-nickel-plated sulfur cathode and the battery are prepared as in Example 2.

EXAMPLE 4

[0075] Except for adjusting the preheated temperature of the two solutions to 80° C. and adjusting the sulfur content to 85 wt % in the preparing the electroless-nickel-plated sulfur nanocomposite step, the electroless-nickel-plated sulfur nanocomposite, the electroless-nickel-plated sulfur cathode and the battery are prepared as in Example 2.

EXAMPLE 5

[0076] Except for adjusting the NiCl.sub.2 plating solution to 0.25M, adjusting the preheated temperature of the two solutions to 80° C., and adjusting the sulfur content to 75 wt % in the preparing the electroless-nickel-plated sulfur nanocomposite step, the electroless-nickel-plated sulfur nanocomposite, the electroless-nickel-plated sulfur cathode and the battery are prepared as in Example 2.

EXAMPLE 6

[0077] Except for adjusting the preheated temperature of the two solutions to 90° C., and adjusting the sulfur content to 75 wt % in the preparing the electroless-nickel-plated sulfur nanocomposite step, the electroless-nickel-plated sulfur nanocomposite, the electroless-nickel-plated sulfur cathode and the battery are prepared as in Example 2.

EXAMPLE 7

[0078] Except for adjusting the NiCl.sub.2 plating solution to 0.25M, adjusting the preheated temperature of the two solutions to 90° C., and adjusting the sulfur content to 60 wt % in the preparing the electroless-nickel-plated sulfur nanocomposite step, the electroless-nickel-plated sulfur nanocomposite, the electroless-nickel-plated sulfur cathode and the battery are prepared as in Example 2.

EXAMPLES 8 to 13

[0079] In Examples 8 to 13, except for adjusting the sulfur loading of Examples 2 to 7 to 6 mg/cm.sup.2 in the preparing the battery step, the electroless-nickel-plated sulfur nanocomposite, the electroless-nickel-plated sulfur cathode and the battery are prepared as in Example 2 to 7.

COMPARATIVE EXAMPLES 1 to 6

Preparing the Pure Sulfur Cathode

[0080] Mix 75 mg of pure sulfur powder with 25 mg of SuperP conductive carbon black, and set the mixed powder in a convection oven at 50° C. to form a 75 wt % pure sulfur powder with 25 wt % conductive carbon. The 75 wt % pure sulfur cathode is formed with a drop-casting process, wherein the 75 wt % sulfur powder is dissolved in a suitable amount of electrolyte and drop-cast on a current collector, with the electrolyte-to-sulfur ratio being 7 μL/mg. Repeat the above process to form the 75 wt % pure sulfur cathode in Comparative Examples 1 to 6.

Preparing the Battery

[0081] A battery is assembled in the order of the aforementioned 75 wt % pure sulfur cathode, a separator, electrolyte and a lithium anode. Each component is sealed into a CR2032 button cell battery in a glove box in an argon environment. The electrolyte is prepared by mixing 1.85 M LiTFSI, 0.2 M LiNO.sub.3 and a DME/DOL solution with a volume ratio of 55:40, i.e., dissolving 50.5 mg LiTFSI and 1.3 mg LiNO.sub.3 in 55 ml DME and 40 ml DOL.

[0082] Further description and analysis on the electroless-nickel-plated sulfur nanocomposite, the electroless-nickel-plated sulfur cathode and the battery in Example 1 are given as follows:

The Electroless-Nickel-Plated Sulfur Nanocomposite

[0083] FIG. 1(a) shows the method of synthesizing the electroless-nickel-plated sulfur nanocomposite, wherein sulfur powder (sulfur particles) serves as the raw substrate for the nickel-plating process. To improve coating homogeneity, SnCl.sub.2 and PbCl.sub.2 are used as a sensitizer and activator respectively to generate catalytic active sites on the sulfur particles. The sensitization process makes the sulfur surface adsorb the easily-oxidized Sn.sup.2+ ions. During the subsequent activation process, the Sn.sup.2+ ions are oxidized and generate an activated layer on the sulfur surface as the crystalline core, which accelerates the reduction of nickel ions and promotes uniform coating of nickel. Electroless-nickel plating is an autocatalysis reaction involving the reduction of nickel ions from a nickel salt on the surface of a reducing agent, as follows: H.sub.2PO.sub.2.sup.−+Ni.sup.2++H.sub.2O⇄H.sub.2PO.sub.3.sup.−+Ni.sup.0+2H.sup.+.

[0084] The stirring time in the sensitization and activation process in Example 1 lasts for 1 hour each. This timeframe allows for even pre-treatment of the sulfur particles on the self-redox reaction sites and helps form a uniform coating. As a result, the electroless-nickel-plated sulfur nanocomposite contains high nickel content.

[0085] The electroless-nickel plating is conducted using a nickel salt and a strong reducing agent, which deposits metallic nickel ions on the sulfur surface. As shown in FIG. 2, the resulting electroless-nickel-plated sulfur nanocomposite changes in color from yellow to yellowish-grey after deposition, and becomes magnetic. The change of its physical properties shows that the electroless-nickel-plated sulfur nanocomposite is formed with the formation of the nickel-coating layer on the sulfur-based nanocomposite.

[0086] The microstructure and elemental composition of the pure sulfur powder and electroless-nickel-plated sulfur nanocomposite are compared by scanning electron microscopy (SEM). As shown in FIG. 1(b), 1(c), the pure sulfur powder shows a rough surface and a particle size of 10 to 20 μm. Meanwhile, the electroless-nickel-plated sulfur nanocomposite features a thin layer of nickel uniformly coated on the sulfur particles, which maintains almost the same particle sizes as the pure sulfur powder. Further elemental analysis indicates that the chemical composition of the electroless-nickel-plated sulfur nanocomposite is 74 wt % sulfur, 24 wt % nickel, and 2 wt % phosphorus (impurities).

[0087] FIGS. 3 and 4 are the energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) spectra of the pure sulfur powder and the electroless-nickel-plated sulfur nanocomposite, respectively. These figures also confirm the existence of the nickel coating. Four point resistivity measurement system is used for measurement, and the results show that the conductivity of the electroless-nickel-plated sulfur nanocomposite increases from purely insulating to 0.03 S/cm, and its surface area and pore size distribution are shown in FIG. 1(d), 1(e). The nitrogen adsorption-desorption isotherms indicate that the material features a similar surface adsorption-desorption behavior before and after the electroless-nickel-plating process. The pore-size analysis in FIG. 5 shows that the material has a low surface area (less than 10 m.sup.2/g) and a total pore volume of 0.01 cm.sup.3/g both before and after the electroless-nickel-plating process, suggesting that both powders are non-nanoporous materials (Surface area less than 10 m.sup.2/g is defined as non-nanoporous material). The detailed physiochemical properties of the materials are summarized below in Table 1. Such properties indicate that the electroless-nickel-plating process only adds a nickel nanoshell layer (nickel coating) onto the sulfur particles with no obvious changes in the composition or structure of the sulfur substrate.

TABLE-US-00001 TABLE 1 Microporosity Surface Pore Surface Pore Particle area volume area volume size Electrical Sample (m.sup.2/g) (cm.sup.3/g) (m.sup.2/g) (cm.sup.3/g) (μm) conductivity Electroless- 4.82 0.01 0 0 10 to 20 3 × 10.sup.−2 nickel-plated sulfur nanocomposite Pure sulfur powder 8.56 0.01 0 0 10 to 20 10.sup.−30

[0088] Since metallic nickel has a high conductivity of over 0.1 S/cm, the present disclosure applies nickel in the sulfur cathode to improve the chemical properties of Li—S batteries, including decreasing cathode resistance, increasing polysulfide retention, and facilitating the redox reaction of the solid active materials. To form the cathode, the electroless-nickel-plated sulfur nanocomposite is dispersed in liquid electrolyte, and drop-cast onto a current collector, as shown in FIG. 1(a). The electroless-nickel-plated sulfur nanocomposite of the present disclosure does not require additional conductive carbon or binders, nor does it require additional process steps such as ball milling, coating, or dry pressing. It can be mixed directly with electrolyte to form a slurry, thus ensuring an even mix of the electrolyte and the electroless-nickel-plated sulfur nanocomposite and allowing the electrolyte to sufficiently wet the electroless-nickel-plated sulfur nanocomposite. As a result, the slurry can be directly drop-cast onto the current collector and form a cathode, which exhibits good electronic conductivity due to the metallic nickel coating, good ionic conductivity due to electrolyte wetting, and good dispersibility of the electroless-nickel-plated sulfur nanocomposite. Thus, the cathode can then demonstrate its great performance with high weight percentage of active substances (high sulfur content) while simultaneously increasing the nickel content of the electroless-nickel-plated sulfur nanocomposite so that the battery that uses the cathode exhibits high capacity, high energy density, and cycle stability.

[0089] As shown in FIG. 1(f) to 1(g), the elemental analysis shows that the electroless-nickel-plated sulfur cathode has an apparent nickel coating tightly attached on the sulfur particles. According to Table 1, the nickel coating improves the conductivity of the electroless-nickel-plated sulfur nanocomposite and forms a long-range electron transfer network, thus decreasing the cathode impedance. Moreover, the conductive network can be uniformly applied to each sulfur particle in order to trap and electrocatalyze the polysulfide species during cycling. Thus, as shown in FIG. 1(g), even though the cycled electroless-nickel-plated sulfur cathode is directly exposed to the liquid electrolyte, the solid sulfur-based active material still exhibits strong elemental sulfur signals in the cathode, proving the enhanced polysulfide retention and limited active-material loss. The surface of the cycled electroless-nickel-plated sulfur nanocomposite has muddy deposition, resulting from the stabilized sulfur species converted from the trapped polysulfides. The muddy deposits mainly concentrate at the electrocatalytic nickel spots and form small particles rather than huge clusters, suggesting they remain high electrochemical activity.

[0090] Electrochemical characteristics of the electroless-nickel-plated sulfur cathode and battery:

[0091] FIG. 6 shows the electrochemical characterization of the electroless-nickel-plated sulfur cathode and battery. The battery resistance and reaction kinetics of the battery is analyzed by electrochemical impedance spectroscopy (EIS), as shown in FIG. 6(a) to (c). The Nyquist plots of the electroless-nickel-plated sulfur cathode and the reference sulfur cathode demonstrate a low bulk resistance of 7 and 13 Ohm, respectively, at the real part of the impedance. The semicircles arising from the charge-transfer impedance indicate that the conductive nickel coating decreases the charge-transfer resistance from 210 to 52 Ohm, which would benefit the reaction capability of the electroless-nickel-plated sulfur cathode, allowing it to attain high electrochemical utilization and rate capability. The inclined line at the low-frequency region denotes the Warburg impedance, which presents a lower diffusion impedance in the electroless-nickel-plated sulfur cathode. The main enhancement of the electroless-nickel-plated sulfur nanocomposite in reducing the charge-transfer and diffusion impedances is reaffirmed by the Bode plots.

[0092] The redox reaction of the electroless-nickel-plated sulfur cathode is analyzed by cyclic voltammetry (CV), as shown in FIG. 6(d) to (h). In FIG. 6(d), the reversible CV scanning curves at 0.020 mV/s depict the stable redox reactions between sulfur and lithium sulfide during the lithiation/delithiation processes. From 3.0 to 1.5 V of the CV scanning, the two-step reduction reaction of sulfur involves two distinguishable cathodic peaks (cathodic 1 peak and cathodic 2 peak). The cathodic 1 and cathodic 2 peaks represent the reduction reaction of solid-state sulfur to liquid-state polysulfides and to solid-sate sulfides, respectively. From 1.5 to 3.0 V, the two-step oxidation reaction displays a broad anodic peak, corresponding to the oxidation reaction from sulfides to polysulfides and sulfur. The CV curves display no obvious changes in potential and current density during the successive 10 scans, demonstrating the constant low polarization and reversible electrochemical reactions, respectively. In addition, as shown in FIG. 6(d) to (h), as the scanning rate increases from 0.020 to 0.040 mV/s, the cathodic and anodic reactions only show a slight increase of the polarization, display no obvious changes in potential and current density during the successive 10 scans, and generally exhibit the same superior electrochemical reversibility and stability, demonstrating excellent rate capability. As shown in FIG. 6(i), the rate-dependent CV scans based on the second CV curves from 0.020 to 0.040 mV/s are collected for calculating the lithium-ion diffusion coefficient by the relationship of the peak current and the CV scanning rate.

[0093] As shown in FIG. 7, the calculated lithium-ion diffusion coefficients of the cathodic 1, cathodic 2, and anodic peaks are 4.16×10.sup.−8, 4.44×10.sup.7, and 1.57×10.sup.−7 cm.sup.2/s, respectively. The high diffusion coefficients demonstrate the smooth lithium-ion diffusion in the electroless-nickel-plated sulfur cathode and its high electrochemical utilization. According to the EIS and CV analyses, the smooth electron and ion transports within the electroless-nickel-plated sulfur nanocomposite should ensure its capability as a high-performance cathode that can host and normally cycle a high weight of sulfur in a lean-electrolyte battery.

[0094] The performance of the battery which uses the electroless-nickel-plated sulfur cathode:

[0095] FIG. 8 shows the long-term rate cyclability and discharge/charge voltage profiles of the Li—S battery of Example 1. As shown in FIG. 8(a), the battery attains high areal capacities of 1,008, 901, 674, 550, and 460 mA.Math.h/g at broad cycling rates of C/20, C/10, C/5, C/3, and C/2, respectively. After 200 cycles, the reversible capacity of the battery exhibits good capacity retention rates (50 to 70%) and capacity-fade rates as low as 0.05% per cycle. The Coulombic efficiency remains high at over 98 to 99%.

[0096] At the different cycling rates of C/20, C/10, C/5, C/3, and C/2, the battery can reach a long-term cyclability of 200 cycles, and retain capacities of 48%, 51%, 67%, 70%, and 64%. Calculation of the ratio between the discharge capacities after 100 cycles and the second cycle shows the retention rates of 76%, 81%, 94%, 94%, and 95%. The details are summarized in Table 2 below.

TABLE-US-00002 TABLE 2 gravimetric Areal Energy Capacity at Capacity at Capacity at Cycling capacity capacity density second cycle 100.sup.th cycle 200.sup.th cycle rates (mA .Math. h/g) (mA .Math. h/cm.sup.2) (mW .Math. h/cm.sup.2) (mA .Math. h/g) (mA .Math. h/g) (mA .Math. h/g) C/20 1008 14 28 980 742 482 C/10 901 13 25 867 700 461 C/5 674 9 19 668 630 451 C/3 550 8 15 555 519 382 C/2 460 7 13 351 335 293 *For practicality, areal capacity must reach 2 to 4 mA .Math. h/cm.sup.2 or above, energy density must reach 10 mW .Math. h/cm.sup.2.

[0097] The above confirms the excellent rate capability, high electrochemical stability and efficiency of the electroless-nickel-plated sulfur nanocomposite.

[0098] As shown in FIG. 9, the battery that utilizes the electroless-nickel-plated sulfur cathode demonstrates high areal capacities of 14 to 7 mA.Math.h/cm.sup.2 at various cycling rates from C/20 (slow) to C/2 (fast), and the corresponding energy density values approach 28 to 13 mW.Math.h/cm.sup.2. In contrast, the cathodes of commercial lithium-ion battery have the capacity of 2 to 4 mA.Math.h/cm.sup.2 and the energy density of 10 to 14 mW.Math.h/cm.sup.2.

[0099] Moreover, due to its high sulfur content, the gravimetric capacity of the cathode may reach as high as 744 mA.Math.h/g, calculated based on the total mass of the cathode, as shown in FIG. 10. The excellent electrochemical characteristics and promising performance show that the electroless-nickel-plated sulfur battery can function as a high-performance cathode system simultaneously possessing excellent electrochemical characteristics: high utilization, retention, and reaction kinetics, outstanding practical cathode property, and increased weight and content of the active material at a lean-electrolyte condition.

[0100] FIG. 8(b) to (f) are the discharge and charge voltage profiles of the electroless-nickel-plated sulfur cathode at various cycling rates which explores its redox-conversion capability and stability. As shown in FIG. 6, the charge/discharge curves nearly overlap and correlate with the anodic/cathodic peaks in the CV curves, confirming the typical lithium-sulfur redox conversions. Neither irreversible capacity loss nor additional decomposition reactions of the metal plating layer are observed. From 3.0 to 1.5 V, the overlapping upper and lower discharge plateaus at different cycling rates demonstrate the superior discharge reversibility and cyclability of the electroless-nickel-plated sulfur cathode. As shown in FIG. 11, the stable discharge capacities of both upper and lower discharge plateaus of the electroless-nickel-plated sulfur cathode indicate the complete conversion reaction, which agrees with the discharge curves in FIG. 8(b) to (f). As shown in FIG. 11(a), the discharge capacities of the upper plateaus show similar trends at various rates due to the fast conversion reaction from sulfur to highly active polysulfides. The electrochemically active polysulfides anchor on the nickel coating and become catholytes, limiting the loss of the active material and enhancing the redox conversion. Meanwhile, the capacities of the lower plateaus at various rates display the challenge of high-loading cathodes in pursuit of a high discharge rate. The sluggish liquid-to-solid conversion from highly active polysulfides to insulating sulfides has a more comprehensive reduction reaction at a slow rate than at a fast rate. As shown in FIG. 11(b), although the discharge capacity of the lower plateau is influenced by the cycling rates, the electroless-nickel-plated sulfur cathode still has good cyclability for 200 cycles from slow to fast rates due to the high polysulfide-trapping and activation capabilities of the conductive nickel coating. As shown in FIG. 8, from 1.5 to 3.0 V, the overlapping charge plateaus indicate the successful oxidation of the high amount of sulfide generated at the discharge reaction. As the battery voltage approaches 3.0 V, the almost vertical voltage increase suggests that batteries employing the electroless-nickel-plated sulfur cathodes can be completely charged, demonstrating the high conductivity and electrocatalytic behavior of the nickel coating in promoting the oxidation reaction.

[0101] Subsequently, data are collected from the discharge and charge plateaus of the electroless-nickel-plated sulfur cathode to study the polarization issue intrinsically encountered in high sulfur loading cathodes with a low electrolyte-to-sulfur ratio. As shown in FIG. 12, the electroless-nickel-plated sulfur cathode keeps a low polarization from C/20 to C/3 rates, with no increase in the polarization after 200 cycles. However, at the C/2 rate, the polarization increases due to the sluggish liquid-to-solid reduction of the lower discharge plateau, thus no apparent impact on the electrochemical reaction is found.

[0102] In contrast, FIG. 13 shows the performance of the battery which uses the 75 wt % control pure sulfur cathodes in Comparative Examples 1 to 6. The battery in Example 1 and Comparative Examples 1 to 6 have an electrolyte-to-sulfur ratio of 7 μL/mg, and a sulfur loading of 14 mg/cm.sup.2. The 75 wt % pure sulfur cathode in Comparative Examples 1 to 6 have a sulfur content of 75 wt %, and the electroless-nickel-plated sulfur cathode in Example 1 has a sulfur content of 74 wt %. As they have almost equal sulfur content, the 75 wt % pure sulfur cathode and the electroless-nickel-plated sulfur cathode are made under the same conditions.

[0103] According to FIGS. 8(a) and 13, the performances of the 75 wt % pure sulfur cathode and the electroless-nickel-plated sulfur cathode are compared as follows:

[0104] a. The electroless-nickel-plated sulfur cathode can reach 200 cycles at the cycling rates of C/20 to C/2, whereas the 75 wt % pure sulfur cathode can only be cycled at the slow rate of C/20 and cannot reach 200 cycles (only 10 to 20 cycles), exhibiting extreme instability;

[0105] b. The electroless-nickel-plated sulfur cathode can reach a high Coulombic efficiency of over 95% at various cycling rates, whereas the 75 wt % pure sulfur cathode shows unstable Coulombic efficiency and difficulty in reaching above 90%;

[0106] c. At the slow cycling rate of C/20, the electroless-nickel-plated sulfur cathode can reach a capacity of 1,008 mA.Math.h/g, whereas the 75 wt % pure sulfur cathode can only reach a capacity of 700 mA.Math.h/g at most.

[0107] According to the above, a battery assembled with the 75 wt % pure sulfur cathode of Comparative Examples 1 to 6 cannot be cycled normally and suffers a low discharge capacity, which is resulted from the lack of the nickel coating that could offer a conductive network to improve the electrochemical utilization of the electrodes. Moreover, the lack of the nickel coating also prevents the polysulfide from staying in the cathode and functioning as catholyte to activate the unreacted sulfur, resulting in unstable Coulombic efficiency.

[0108] FIGS. 14 to 19 are the morphological, microstructural and elemental analyses of the electroless-nickel-plated sulfur cathode of the battery before and after 200 cycles at the cycling rates of C/20 to C/2. As shown in the figures, the electroless-nickel-plated sulfur cathode exhibits almost the same cathode morphology before and after cycling through broad observation at a low magnification and in-depth inspection at high magnification. The almost unchanged microstructures and elemental mapping results also illustrate how the nickel coating facilitates the activation and redox reaction of sulfur as well as the stabilization of polysulfides within the cathode as the catholyte. These results agree with the electrochemical characteristics and battery performance, proving the advantages of the electroless-nickel-plated sulfur nanocomposite in boosting the cathode conductivity, electrocatalytic activity and preventing the formation of inactive deposits on the cycled cathode.

[0109] In conclusion, the electroless-nickel-plated sulfur nanocomposite provides a new research direction for developing advanced Li—S batteries with high-performance sulfur cathodes. The electroless-nickel-plated sulfur nanocomposite of the present disclosure is mixed evenly with the electrolyte, does not require addition of conductive carbon or binders, and can exhibit good cathode functionality only using the drop-casting process. Moreover, the nickel coating creates a thin, lightweight nanocoating onto the sulfur particles which decreases insulation, enhances electrochemical reaction kinetics with metal catalysis capability, and facilitates physical and chemical adsorption of liquid polysulfides with metal activity, demonstrating high electrochemical utilization and stable cyclability. As a result, the electroless-nickel-plated sulfur cathode that utilizes the electroless-nickel-plated sulfur nanocomposite attains a high sulfur loading of 14 mg/cm.sup.2 and high sulfur content of 74 wt % in a battery of low electrolyte-to-sulfur ratio of 7 μL/mg, which is significantly superior to prior art (low sulfur loading of 2 mg/cm.sup.2, low sulfur content of 30 to 60 wt %, and an excessive amount of electrolyte that is over 20 μL/mg). The battery performance of the present disclosure can reach a high charge-storage capacity of 1,008 mA.Math.h/g, high areal capacity of 7 to 14 mA.Math.h/cm.sup.2, and a high energy density of 13 to 28 mW.Math.h/cm.sup.2, which is superior to the energy density of commercial oxide electrodes (around 10 mW.Math.h/cm.sup.2). It also demonstrates long-term cycle stability for 200 cycles while maintaining high capacity at the large range of cycling rates of C/20 to C/2. The battery that utilizes the electroless-nickel-plated sulfur cathode exhibits excellent electrochemical and battery performances: high utilization, high retention of capacity, and reaction kinetics, and outstanding practical cathode property that enables increased sulfur loading and increased content of the active material at a lean-electrolyte condition.

[0110] Further description and analysis of the electroless-nickel-plated sulfur nanocomposite as prepared in Examples 2 to 7 are given as follows:

[0111] According to FIG. 20, the XRD data show that the electroless-nickel-plated sulfur nanocomposite of the present disclosure is mainly pure sulfur powder, and that the component analysis data show that in Examples 2 to 7, the nickel content are 1.8 wt %, 3.0 wt %, 10.0 wt %, 22.8 wt %, 20.2 wt %, 34.2 wt %; the sulfur content are 96.5%, 94.9 wt %, 86.4 wt %, 75.0 wt %, 77.7 wt %, 60.2 wt %; the impurities are 1.6 wt %, 2.0 wt %, 3.5 wt %, 2.1 wt %, 2.0 wt %, 5.5 wt % (all below 5.5 wt %), indicating that the long term (60 minutes) high temperature acidic chemically plating of 70 to 90° C. employed in the present disclosure helps to keep the alloy impurities only under 1 to 5.5 wt %, thus achieving high nickel content.

[0112] According to the thermal analysis data of FIG. 21, the electroless-nickel-plated sulfur nanocomposite of Examples 2 to 7 have a sulfur content of 60 to 95 wt %, which is identical to the XRD data. The alloy impurities are only under 1 to 5.5 wt %, thus achieving high nickel content.

[0113] Further description and analysis of the battery as prepared in Examples 2 to 13 are given as follows:

[0114] As shown in FIG. 22, with different sulfur contents of 60 wt %, 75 wt %, 85 wt %, 95 wt %, and 100% (reference), and with different sulfur loadings of 2 mg/cm.sup.2, 6 mg/cm.sup.2, the battery can achieve long term cycles.

[0115] Further analysis and conclusion of the electroless-nickel-plated sulfur nanocomposite, the electroless-nickel-plated sulfur cathode, and the battery are given as follows:

[0116] When applied in a battery cathode, the electroless-nickel-plated sulfur nanocomposite of the present disclosure achieves high nickel content (10 to 40 wt %) while simultaneously retaining high sulfur content (60 to 95 wt %), showing enhanced electrochemical characteristics compared to prior art. It also achieves high capacity of 7 to 14 mA.Math.h/cm.sup.2 and high energy density of 13 to 28 mW.Math.h/cm.sup.2, which is significantly superior to the 2 to 4 mA.Math.h/cm.sup.2 capacity and 10 to 14 mW.Math.h/cm.sup.2 energy density of current lithium-ion battery technology.

Measuring Instrument

[0117] The instrument and measuring methods of the present disclosure are described in detail below:

[0118] Morphological, microstructural, and elemental analyses were conducted with a scanning electron microscope (SEM, SU-1510, Hitachi) with an energy dispersive X-ray spectrometer (EDS,)(Flash, Bruker) for collecting the elemental mapping signals and analyzing the elemental signals.

[0119] The resistivity and sheet resistance of the electrode material is measured with a four point resistivity measurement system (Pro-4, Lucas).

[0120] Structure and porosity were analyzed with an automated gas sorption analyzer (AutoSorb iQ, Anton Paar) at 77 K. The resulting nitrogen adsorption-desorption isotherms were calculated with Brunauer-Emmett-Teller for investigating the surface area and Barrett-Joyner-Halenda, Horvath-Kawazoe, density functional theory, and T-plot methods for studying the pore characteristics. Impedance spectroscopy was conducted with a research grade potentiostat/galavanostat (SP-150, Biologic) in a frequency range of 1 MHz to 100 mHz with an AC voltage amplitude of 5 mV.

[0121] Cyclic voltammetry and the lithium-ion diffusion analysis were conducted with a research grade potentiostat/galavanostat (SP-150, Biologic) in a scan voltage range of 1.5 to 3.0 V at various scan rates of 0.020, 0.025, 0.030, 0.035, and 0.040 mV/s. Voltage profiles, cycling performance, and polarization were collected with a programmable battery cycler (BCS-800 series, Biologic) in a voltage window of 1.5 to 3.0 V at C/20, C/10, C/5, C/3, and C/2 rates.