DESC:TECHNICAL FIELD
[0001] The present disclosure relates to lithium-sulfur Li–S) batteries, and more specifically, to a Lithium–sulfur (Li–S) battery with a cathode electrode enabling the direct utilization of commercial sulfur powder.

BACKGROUND
[0002] Recently, the need for high capacity batteries is emerging as portable electronic devices, electric vehicles, and large-capacity power storage systems and the like have been developed. Lithium-sulfur batteries is using a sulfur-based material having a positive electrode active material, and a secondary battery using lithium metal as an anode active material, and the main material of the positive electrode active material, the sulfur resource is very rich, toxicity no, it is advantageous to have a low weight per atom. However, a lithium-sulfur battery has not yet been widely used commercially because it has a poor lifetime characteristic compared to currently commercially available lithium-ion batteries.

[0003] Sulfur is a promising cathode for lithium batteries due to its high theoretical specific capacity (1673 mAh /g), low cost and environmental friendliness. With the very high theoretical energy density (~2600 W h kg?1), Lithium–sulfur (Li–S) batteries are considered as one of most shouting alternatives to commercial lithium-ion batteries, hold great potential for next-generation high energy storage system. However, wide-scale commercial use is so far limited because of some key challenges, such as sluggish reaction kinetics of sulfur. Thus, Li–S batteries suffer from poor practical capacity. Furthermore, owing to gradual dissolution of active material and the intermediate products into the liquid electrolyte, Li-S batteries face the most important challenge of cycling stability.

[0004] To resolve the issues related to sluggish kinetics and gradual dissolution of active material into electrolyte, a multifunctional sulfur-based electrode made of sulfur nanoparticles and polar additives are used in the Li-S batteries. The use of sulfur nanoparticles reduces the ion-diffusion length and enhances the reaction kinetics in Li-S batteries; while, the polar additives trap and restrict the gradual dissolution of active material from electrode through chemical interaction. However, considering the complicated and very expensive approach to use sulfur nanoparticles in Li-S batteries, use of sulfur nanoparticles limits the bulk production Li-S batteries.

[0005] Thus, there is a need to develop a cathode scaffold for Li-S batteries enabling the direct utilization of commercial sulfur powder.

OBJECT OF THE INVENTION
[0006] It is the primary object of the present disclosure to provide a method of fabricating a cathode electrode for Lithium–sulfur (Li–S) batteries utilizing commercial sulfur powder.

[0007] It is another object of the present disclosure to provide a method of fabricating a Lithium–sulfur (Li–S) battery with a cathode electrode utilizing commercial sulfur powder.

SUMMARY
[0008] In an aspect of the present disclosure, a method of fabricating a cathode electrode for Lithium–sulfur (Li–S) batteries is disclosed. The method comprises the step of synthesizing lithium aluminate (LiAlO2) nanoflakes, preparing a lithium aluminate (LiAlO2)/sulfur composite by mixing ultrathin lithium aluminate (LiAlO2) nanoflakes and commercial sulfur powder, and milling the resultant mixture, heating the lithium aluminate (LiAlO2)/sulfur composite to embed and uniformly distribute the LiAlO2 nanoflakes into the sulfur particles and fabricating the cathode electrode for a Lithium–sulfur (Li–S) battery.

[0009] The ultrathin lithium aluminate (LiAlO2) nanoflakes are synthesized by preparing a first mixture of lithium hydroxide, aluminium iso-propoxide and cetyltrimethylammonium bromide, dissolving the prepared first mixture into deionized water and performing a hydrothermal synthesis of the dissolved first mixture. The cathode electrode for a Lithium–sulfur (Li–S) battery is fabricated by preparing a second mixture of lithium aluminate (LiAlO2)/sulfur composite, carbon black and polyethylene oxide, dispersing the second mixture in acetonitrile and casting the second mixture on aluminium foil followed by a vacuum drying process.

[0010] In another aspect of the present disclosure, a method of fabricating a Lithium–sulfur (Li–S) battery with the novel cathode electrode is disclosed. The method comprises the steps of fabricating a cathode electrode by using the method as discussed in the previous embodiment. The method of fabricating a Lithium–sulfur (Li–S) battery further comprises the steps of placing the fabricated cathode electrode against a lithium anode placed in an electrolyte and placing polypropylene membrane between the anode and the cathode. The electrolyte comprises of 1M Bis(trifluoromethane)sulfonimide lithium salt and 0.1 M lithium nitrate in 1:1 (v/v) mixture of 1,2-dimethoxy ethane and 1,3-dioxolane solvents.

[0011] In yet another aspect of the present disclosure, a Lithium–sulfur (Li–S) battery is disclosed. The Lithium–sulfur (Li–S) battery comprises a cathode electrode fabricated by using the method as discussed in the first embodiment. The Lithium–sulfur (Li–S) battery further comprises an anode electrode made up of Lithium, an electrolyte comprising 1M lithium bis(trifluoro methane)-sulfonamide and 0.1 M lithium nitrate in 1:1 (v/v) mixture of 1,2-dimethoxy ethane and 1,3-dioxolane solvents and a polypropylene membrane placed between the anode electrode and the cathode electrode. The cathode electrode is placed against the lithium anode electrode placed in the electrolyte

BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The detailed description is described with reference to the accompanying figures.
[0013] Figure 1, illustrates a flow chart for a method of fabricating a cathode electrode for Lithium–sulfur (Li–S) batteries in an embodiment of the present disclosure.
[0014] Figure 2, illustrates a Scanning electron microscopy (SEM) images of ultrathin lithium aluminate (LiAlO2)/sulfur composite.
[0015] Figure 3(a), illustrates cross-sectional SEM image of Sulfur/LiAlO2 cathode showing the thickness of active material layer casted on aluminum foil
[0016] Figures 3(b)–(f), illustrate EDS layered image and elemental mapping acquired on Sulfur/LiAlO2 cathode.
[0017] Figure 4, illustrates a long-term cycling performance of Sulfur/LiAlO2 cathode at the current rates of 2C (300 cycles) and 3C (500 cycles), respectively.

DETAILED DESCRIPTION OF THE INVENTION

[0018] In the present invention, a method of fabricating a cathode electrode for Lithium–sulfur (Li–S) batteries and a method of fabricating a Lithium–sulfur (Li–S) battery employing the cathode electrode are disclosed. In the present disclosure, ultrathin lithium aluminate (LiAlO2) nanoflakes are used as cathode-additive with excellent Li+ ion conductivity to achieve the long-term cyclability of the commercial sulfur-based cathodes.

[0019] In an embodiment of the present disclosure, a method of fabricating a cathode electrode for Lithium–sulfur (Li–S) batteries is disclosed. The method comprises the step of synthesizing ultrathin lithium aluminate (LiAlO2) nanoflakes, preparing a lithium aluminate (LiAlO2)/sulfur composite and heating the lithium aluminate (LiAlO2)/sulfur composite and fabricating the cathode electrode for a Lithium–sulfur (Li–S) battery. Referring to figure 1, illustrated is a flow chart depicting the method of fabricating a cathode electrode for Lithium–sulfur (Li–S) batteries.

[0020] The ultrathin lithium aluminate (LiAlO2) nanoflakes are synthesized by using the following steps: A first mixture of lithium hydroxide, aluminium iso-propoxide and cetyltrimethylammonium bromide at a mole ratio of 42:42:16 is prepared and the prepared first mixture is dissolved into deionized water. In an example, 0.25 mmol lithium hydroxide, 0.25 mmol aluminium iso-propoxide and 0.1 mmol cetyltrimethylammonium bromide is used for preparing the first mixture. A hydrothermal synthesis of the dissolved first mixture is performed at a temperature of 150 ? ± 1 ? for a period of 10 hours ± 15 minutes.

[0021] 20 wt% of ultrathin lithium aluminate (LiAlO2) nanoflakes and 80 wt% of commercial sulfur powder are mixed and ball-milled for 1 hour to prepare the lithium aluminate (LiAlO2)/sulfur composite. The ball-milling process may be performed for a period in a range of 1 hour ± 5 minutes. Then, the lithium aluminate (LiAlO2)/sulfur composite is the heated at a temperature of 150 ? ± 2 ? for a period of 2 hours ± 5 minutes. Referring to figure 2, illustrated is a Scanning electron microscopy (SEM) images of ultrathin lithium aluminate (LiAlO2)/sulfur composite.

[0022] A second mixture of 75 wt% of lithium aluminate (LiAlO2)/sulfur composite, 10 wt% of carbon black and 15 wt% of polyethylene oxide is dispersed in acetonitrile. Further, the second mixture is casted on aluminium foil followed by vacuum drying at a temperature of 60 ? for 48 hours to fabricate the cathode electrode for Lithium–sulfur (Li–S) batteries. The vacuum drying is performed at a temperature in a range of 60 ? ± 5 ? and for a period of 48 hours ± 15 minutes. Referring to figure 3(a), illustrated is a Cross-sectional SEM image of Sulfur/LiAlO2 cathode showing the thickness of active material layer casted on aluminum foil. Referring to figures 3(b) – 3(f), illustrated are EDS layered image and elemental mapping acquired on Sulfur/LiAlO2 cathode.

[0023] As disclosed above, the commercial sulfur powder is mixed with high surface area, ionically conductive lithium aluminate (LiAlO2) nanoflakes for preparing the composite, followed by a heat treatment for the fabrication. After heat treatment, the LiAlO2 nanoflakes are uniformly distributed as well as embedded on/into the sulfur particles. The uniform distribution of LiAlO2 nanoflakes on sulfur particles act as a barrier against gradual dissolution of active material and thus could effectively enhance the longevity of Li-S batteries. Due to high Li+ ion conductivity, the embedded LiAlO2 nanoflakes, on other hand, improve the overall reaction kinetics in lithium-sulfur batteries. The enhancement in reaction kinetics results in higher utilization of active material, leading to higher practical cell capacity.

[0024] In another embodiment of the present disclosure, a method of fabricating a Lithium–sulfur (Li–S) battery with the novel cathode electrode is disclosed. The method comprises the steps of fabricating a cathode electrode by using the method as discussed in the previous embodiment. The method of fabricating a Lithium–sulfur (Li–S) battery further comprises the steps of placing the fabricated cathode electrode against a lithium anode placed in an electrolyte and placing polypropylene membrane between the anode and the cathode. The electrolyte comprises of 1M lithium bis(trifluoro methane)-sulfonamide and 0.1 M lithium nitrate in 1:1 (v/v) mixture of 1,2-dimethoxy ethane and 1,3-dioxolane solvents. The polypropylene membrane is used as a separator between the anode and cathode. The lithium bis(trifluoro methane)-sulfonamide solution is used a main salt and lithium nitrate is used an additive salt.

[0025] In yet another embodiment of the present disclosure, a Lithium–sulfur (Li–S) battery with the novel cathode electrode is disclosed. The Lithium–sulfur (Li–S) battery comprises an anode made up of Lithium, an electrolyte comprises of 1M lithium bis(trifluoro methane)-sulfonamide and 0.1 M lithium nitrate in 1:1 (v/v) mixture of 1,2-dimethoxy ethane and 1,3-dioxolane solvents and a cathode prepared from ultrathin lithium aluminate (LiAlO2) nanoflakes and commercial sulfur as discussed in the above method disclosed in the first embodiment. The Lithium–sulfur (Li–S) battery further comprises a polypropylene membrane between the anode and the cathode.

[0026] The present invention provides a fabrication of sulfur-based cathode with ultrathin lithium aluminate (LiAlO2) nanoflakes as cathode-additive to achieve adequate capacity with long-term cyclability of the Lithium–sulfur (Li–S) batteries. Having excellent Li+ ion conductivity, the ultrathin lithium aluminate (LiAlO2) nanoflakes, not only immobilizes soluble intermediates, but also promote their utilization. The ultrathin LiAlO2 nanoflakes inlaid sulfur cathode, containing high amount of active material (sulfur loading = 5.2 mg cm-2), exhibits stable cycling, retaining a high areal capacity of 3.34 mA h cm–2 after 300 cycles with an extremely low capacity decay rate of 0.017 % per cycle. At high current rate of 3C, the cathode retains a high areal capacity of 2.4 mA h cm–2 after 500 cycles, with an extremely low capacity decay rate of 0.02 % per cycle. Referring to figure 4, illustrated is a long-term cycling performance of Sulfur/LiAlO2 cathode at the current rates of 2C (300 cycles) and 3C (500 cycles), respectively.

[0027] Being a waste material from petroleum industries and other industries, the direct utilization of commercial sulfur powder in rechargeable lithium batteries drastically cuts down the production cost of the fabrication method. Thus, the present invention is advantageous with the use of cost-effective sulfur as a material and also the overall cost for preparing an effective cathode material for rechargeable lithium batteries is considerably reduced. The use of sulfur nanoparticles in Li-S batteries can impede their bulk production; while the direct utilization of commercial sulfur powder might support the bulk production of Li-S batteries. Considering the facile, cost-effective disclosed method to prepare the cathode material and its stable cycling performance with adequate practical cell capacity, the disclosed fabrication process is easily scalable to the industrial requirements of bulk production of Li-S batteries.

[0028] The above description along with the accompanying drawings is intended to disclose and describe the preferred embodiments of the invention in sufficient detail to enable those skilled in the art to practice the invention. It should not be interpreted as limiting the scope of the invention. Those skilled in the art to which the invention relates will appreciate that many variations of the exemplary implementations and other implementations exist within the scope of the claimed invention. Various changes in the form and detail may be made therein without departing from its spirit and scope. Similarly, various aspects of the present invention may be advantageously practiced by incorporating all features or certain sub-combinations of the features.
,CLAIMS:We claim:

1. A method of fabricating a cathode electrode for Lithium–sulfur (Li–S) batteries, the method comprising the steps of:
(a) synthesizing ultrathin lithium aluminate (LiAlO2) nanoflakes by:
preparing a first mixture of lithium hydroxide, aluminium iso-propoxide and cetyltrimethylammonium bromide;
dissolving the prepared first mixture into deionized water; and
performing a hydrothermal synthesis of the dissolved first mixture;
(b) preparing a lithium aluminate (LiAlO2)/sulfur composite by mixing ultrathin lithium aluminate (LiAlO2) nanoflakes and commercial sulfur powder, and milling the resultant mixture;
(c) heating the lithium aluminate (LiAlO2)/sulfur composite to embed and uniformly distribute the LiAlO2 nanoflakes into the sulfur particles;
(d) fabricating the cathode electrode for a Lithium–sulfur (Li–S) battery by:
preparing a second mixture of lithium aluminate (LiAlO2)/sulfur composite, carbon black and polyethylene oxide;
dispersing the second mixture in acetonitrile; and
casting the second mixture on aluminium foil followed by a drying process.

2. The method as claimed in claim 1, wherein the first mixture comprises lithium hydroxide, aluminium iso-propoxide and cetyltrimethylammonium bromide at a mole ratio of 42:42:16.

3. The method as claimed in claim 1, wherein the hydrothermal synthesis of the dissolved first mixture is performed at a temperature of 150 ? for 10 hours.

4. The method as claimed in claim 1, wherein 20 wt% of ultrathin lithium aluminate (LiAlO2) nanoflakes and 80 wt% of commercial sulfur powder are mixed for preparing the lithium aluminate (LiAlO2)/sulfur composite.

5. The method as claimed in claim 1, wherein milling the resultant mixture comprises ball-milling the mixture of ultrathin lithium aluminate (LiAlO2) nanoflakes and commercial sulfur powder for 1 hour.

6. The method as claimed in claim 1, wherein the lithium aluminate (LiAlO2)/sulfur composite is the heated at a temperature of 150 ? for 2 hours.

7. The method as claimed in claim 1, wherein the second mixture comprises 75 wt% of lithium aluminate (LiAlO2)/sulfur composite, 10 wt% of carbon black and 15 wt% of polyethylene oxide.

8. The method as claimed in claim 1, wherein the drying process comprises vacuum drying at a temperature of 60 ? for 48 hours.

9. A method of fabricating a Lithium–sulfur (Li–S) battery, the Lithium–sulfur (Li–S) battery comprises a cathode electrode fabricated using the steps as claimed in claim 1, the method comprising the steps of:
placing the fabricated cathode electrode against a lithium anode electrode placed in an electrolyte; and
placing a polypropylene membrane between the anode electrode and the cathode electrode;
wherein electrolyte comprises 1M lithium bis(trifluoromethane)-sulfonimide and 0.1 M lithium nitrate in 1:1 (v/v) mixture of 1,2-dimethoxy ethane and 1,3-dioxolane solvents.

10. A Lithium–sulfur (Li–S) battery, the Lithium–sulfur (Li–S) battery comprising a cathode electrode fabricated using the steps as claimed in claim 1, the Lithium–sulfur (Li–S) battery further comprising:
an anode electrode made up of Lithium;
an electrolyte comprising 1M lithium bis(trifluoro methane)-sulfonamide and 0.1 M lithium nitrate in 1:1 (v/v) mixture of 1,2-dimethoxy ethane and 1,3-dioxolane solvents; and
a polypropylene membrane placed between the anode electrode and the cathode electrode;
wherein the cathode electrode is placed against the lithium anode electrode placed in the electrolyte.