SULFUR-BASED POSITIVE ELECTRODE ACTIVE MATERIAL FOR USE IN SOLID-STATE BATTERY, PREPARATION FOR MATERIAL, AND APPLICATIONS THEREOF

Abstract

The present invention provides a sulfur-based positive electrode active material for use in a solid-state battery, comprising: 30-80 wt % of Li.sub.2S, 10-40 wt % of one or more second lithium compounds selected from LiI, LiBr, LiNO.sub.3, and LiNO.sub.2, and 0-30 wt % of a conductive carbon material; a method for preparing the sulfur-based positive electrode active material, a positive electrode including the sulfur-based positive electrode active material, and a solid-state battery including the positive electrode. The sulfur-based positive electrode active material and the positive electrode provide a high specific capacity and an increased discharge voltage.

Claims

1. A sulfur-based positive electrode active material for a solid-state battery, comprising: 30-80 wt % of Li.sub.2S, 10-40 wt % of one or more second lithium compounds selected from LiI, LiBr, LiNO.sub.3, and LiNO.sub.2, and 0-30 wt % of a conductive carbon material.

2. The sulfur-based positive electrode active material according to claim 1, wherein the sulfur-based positive electrode active material comprises 30-70 wt %, preferably 60-70 wt %, for example 70 wt % of Li.sub.2S; preferably, the sulfur-based positive electrode active material comprises 15-20 wt % of a second lithium compound; more preferably, the second lithium compound is LiI and/or LiNO.sub.2; preferably, the sulfur-based positive electrode active material comprises 10-30 wt %, preferably 10-15 wt % of a conductive carbon material; preferably, the conductive carbon material is one or more selected from carbon black, carbon nanotubes, carbon nanofibers and graphene.

3. The sulfur-based positive electrode active material according to claim 1, wherein Li.sub.2S has a content of 60-70 wt %, and the second lithium compound has a content of 15-20 wt %; preferably, the weight ratio of Li.sub.2S, the second lithium compound, and the conductive carbon material in the sulfur-based positive electrode active material is 70:(15-20):(10-15).

4. A method for preparing the sulfur-based positive electrode active material according to claim 1, comprising the step of: mixing Li.sub.2S, the second lithium compound and the conductive carbon material via dry ball milling or wet ball milling.

5. The method according to claim 4, wherein the mixing via dry ball milling or wet ball milling is carried out under an inert atmosphere such as a nitrogen atmosphere or an argon atmosphere; preferably, the mixing via dry ball milling or wet ball milling is carried out at a rotation speed of 150-500 rpm, for example 300 rpm, for 1-24 hours, preferably 6-18 hours; preferably, in the mixing via wet ball milling, absolute ethyl alcohol is used as a solvent with an amount of preferably 5-10 wt % of the sulfur-based positive electrode active material; preferably, when using the mixing via wet ball milling, the method further comprises the step of: drying the mixture obtained by the mixing via wet ball milling under vacuum at 50-80° C.

6. A positive electrode for a solid-state battery, comprising a composition comprising 60-90 wt % of the sulfur-based positive electrode active material of claim 1, 0-20 wt % of a conductive additive, 0-40 wt % of a solid electrolyte, and 0-20 wt % of a binder.

7. The positive electrode according to claim 6, wherein the sulfur-based positive electrode active material has a content of 60-80 wt % in the composition; preferably, the conductive additive is one or more selected from carbon black, carbon nanotubes, carbon nanofibers and graphene; preferably, the conductive additive has a content of 10-20 wt % in the composition; preferably, the solid electrolyte is one or more selected from Li.sub.2S—P.sub.2S.sub.5—GeS.sub.2, Li.sub.1.5Al.sub.0.5Ti.sub.1.5(PO.sub.4).sub.3, and Li.sub.7La.sub.3Zr.sub.2O.sub.12; preferably, the solid electrolyte has a content of 10-30 wt %, preferably 10-20 wt % in the composition; preferably, the binder is one or more selected from polyvinylidene fluoride, polytetrafluoroethylene, sodium carboxymethylcellulose and styrene-butadiene rubber; preferably, the binder has a content of 0-10 wt % in the composition.

8. The positive electrode according to claim 6, wherein the positive electrode further comprises a current collector such as an aluminum foil.

9. A solid-state battery comprising the positive electrode according to claim 6, a solid electrolyte sheet, and a negative electrode.

10. The solid-state battery according to claim 9, wherein the solid electrolyte sheet is composed of one or more selected from Li.sub.2S—P.sub.2S.sub.5—GeS.sub.2, Li.sub.1.5Al.sub.0.5Ti.sub.1.5(PO.sub.4).sub.3, and Li.sub.7La.sub.3Zr.sub.2O.sub.12; preferably, the solid-state battery is a solid-state lithium secondary battery.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0046] The embodiments of the present invention will be described below in conjunction with the accompanying drawings, where:

[0047] FIG. 1 shows a voltage-specific capacity graph of a solid-state battery using the sulfur-based positive electrode active material of Example 1 during charging-discharging for the first time and charging-discharging for the second time;

[0048] FIG. 2 shows a cyclic graph of a solid-state battery using the sulfur-based positive electrode active material of Example 1;

[0049] FIG. 3 shows a voltage-specific capacity graph of a solid-state battery using the positive electrode active material of Comparative Example 1 during charging-discharging for the first time and charging-discharging for the second time;

[0050] FIG. 4 shows a voltage-specific capacity graph of a solid-state battery using the sulfur-based positive electrode active material of Example 9 during charging-discharging for the first time and charging-discharging for the second time;

[0051] FIG. 5 shows a lateral scanning electron microscope photograph of a solid-state battery prepared using the sulfur-based positive electrode active material of Example 1, before charge/discharge and after two charge/discharge cycles; and

[0052] FIG. 6 shows a voltage-specific capacity graph of a solid-state battery using the positive electrode active material of Comparative Example 2 during a first charge/discharge cycle and a second charge/discharge cycle.

DETAILED DESCRIPTION OF THE INVENTION

[0053] The present invention will be further described in detail below in conjunction with specific embodiments, and the examples given are only to illustrate the present invention, but not to limit the scope of the present invention.

Examples 1-9

[0054] The preparation method of the sulfur-based positive electrode active materials includes the following steps:

[0055] (1) Li.sub.2S, a second lithium compound and a conductive carbon material were mixed at a certain mass ratio under an argon atmosphere with a moisture content lower than 0.5 ppm. The obtained mixture was mixed with ball milling beads, then transferred into a ball milling tank, and dry ball milling or wet ball milling was carried out under the argon atmosphere with a moisture content lower than 0.5 ppm. The parameters of dry ball milling and wet ball milling are as follows: a rotating speed of 300 revolutions per minute, a time of 12 hours, and absolute ethyl alcohol accounting for 5 wt % of the sulfur-based positive electrode active material was added into the wet ball milling to serve as a solvent.

[0056] (2) A sample was taken out under an argon atmosphere with a moisture content lower than 0.5 ppm, followed by filtering through a screen to remove ball milling beads, and mixing by adopting dry ball milling to obtain a sulfur-based positive electrode active material. During the process of mixing via wet ball milling, the obtained mixture material was filtered through a screen, and then was subjected to vacuum drying for 12 hours at 60° C. to obtain a sulfur-based positive electrode active material.

[0057] The starting materials used in Examples 1-9 were as follows:

TABLE-US-00001 Carbon nanotubes Available from Nanjing XFNANO, Industrial grade carbon nanotubes Graphene Available from Nanjing XFNANO, Chemical process Graphene Carbon nanofiber Available from Toray, Japan under the trade name KCF-100

[0058] The composition and process of the sulfur-based positive electrode active materials prepared in Examples 1-9 were shown in Table 1.

Comparative Example 1

[0059] A positive electrode active material without a second lithium compound was prepared in the same manner as in Example 1, and the composition and related process thereof were shown in Table 1.

Comparative Example 2

[0060] A positive electrode active material using lithium iron phosphate as a second lithium compound was prepared in the same manner as in Example 1, and the composition and related process thereof were shown in Table 1.

TABLE-US-00002 TABLE 1 Composition, ratio and mixing mode of the positive electrode active material Li.sub.2S/second lithium compound/conductive carbon Second lithium Conductive carbon material (weight ratio) compound material Mixing mode Example 1 70:20:10 Lithium iodide Carbon nanotubes Dry ball milling Example 2 70:20:10 Lithium iodide Carbon nanotubes Dry ball milling Example 3 70:20:10 Lithium iodide Carbon nanotubes Dry ball milling Example 4 70:30:0 Lithium iodide Carbon nanotubes Dry ball milling Example 5 60:30:10 Lithium iodide Graphene Dry ball milling Example 6 70:15:15 Lithium iodide Carbon nanotubes Wet ball milling Example 7 70:20:10 Lithium iodide Carbon nanotubes Dry ball milling Example 8 30:40:30 Lithium iodide Carbon nanotubes Dry ball milling Example 9 70:20:10 Lithium nitrite Carbon nanotubes Dry ball milling Comparative 90:0:10 Lithium iodide Carbon nanofibers Dry ball Example 1 milling Comparative 70:20:10 Lithium iron Carbon nanotubes Dry ball Example 2 phosphate milling

[0061] Preparation and Performance Test of Solid-State Battery

[0062] 1. Preparation of Positive Electrode

[0063] The sulfur-based positive electrode active materials prepared in Examples 1-9 and the positive electrode active materials prepared in Comparative Examples 1-2 were used as positive electrode active materials, respectively, to prepare positive electrode materials. The positive electrode active material, solid electrolyte and conductive additive were weighed and mixed at a weight ratio of 7:2:1 under an argon atmosphere with a moisture content of less than 0.5 ppm, and the obtained mixture was mixed with ball milling beads, then transferred to a ball milling tank, and dry ball milling was carried out under an argon atmosphere with a moisture content lower than 0.5 ppm to obtain a positive electrode material. The dry ball milling parameters were as follows: a rotation speed was 150 rpm and time was 2 hours.

[0064] The obtained positive electrode material was molded under a pressure of 20 MPa to prepare a positive electrode sheet with a weight of 10 mg and a diameter of 4 mm.

[0065] The starting materials for preparing the positive electrode were as follows:

TABLE-US-00003 Conductive additives Source Carbon nanotubes Available from Nanjing XFNANO, Industrial grade carbon nanotubes Graphene Available from Nanjing XFNANO, Chemical process Graphene Carbon nanofiber Available from Toray, Japan under the trade name KCF-100

[0066] 2. Assembling Battery

[0067] The battery was assembled in an argon glove box with a moisture content of less than 0.5 ppm.

[0068] A solid electrolyte sheet with a thickness of 300 μm was molded under a pressure of 20 MPa.

[0069] A secondary solid-state battery was prepared by cold-pressing and assembling a positive sheet with a diameter of 4 mm, a solid electrolyte plate with a thickness of 300 μm and a metal lithium foil with a thickness of 80 μm under a pressure of 20 MPa using a mould.

[0070] 3. Performance Test

[0071] The secondary solid-state battery was charged and discharged at constant current using a CT2001A type charge and discharge tester available from Wuhan LAND Electronic Co., Ltd., the cycle test was carried out at a rate of 0.05 C under a test temperature of 60° C. The voltage range was 1.5-3.6 V (vs. Li/Li.sup.+), and the results were shown in Table 2.

TABLE-US-00004 TABLE 2 Assembly and performance of secondary solid-state batteries Discharge specific capacity Conductive of positive electrode EXAMPLES additives Solid state electrolyte (mAh/g) Example 1 Carbon nanotubes Li.sub.2S—P.sub.2S.sub.5—GeS.sub.2 883 Example 2 Carbon nanotubes Li.sub.2S—P.sub.2S.sub.5—GeS.sub.2 822 Example 3 Carbon nanotubes Li.sub.7La.sub.3Zr.sub.2O.sub.12 782 Example 4 Carbon nanotubes Li.sub.2S—P.sub.2S.sub.5—GeS.sub.2 106 Example 5 Graphene Li.sub.2S—P.sub.2S.sub.5—GeS.sub.2 462 Example 6 Carbon nanotubes Li.sub.2S—P.sub.2S.sub.5—GeS.sub.2 865 Example 7 Carbon nanotubes Li.sub.1.5Al.sub.0.5Ti.sub.1.5(PO.sub.4).sub.3 512 Example 8 Carbon nanotubes Li.sub.2S—P.sub.2S.sub.5—GeS.sub.2 412 Example 9 Carbon nanotubes Li.sub.2S—P.sub.2S.sub.5—GeS.sub.2 762 Comparative Carbon nanofibers Li.sub.2S—P.sub.2S.sub.5—GeS.sub.2 469 Example 1 Comparative Carbon nanotubes Li.sub.2S—P.sub.2S.sub.5—GeS.sub.2 351 Example 2

[0072] FIG. 1 showed a charge/discharge curve of a solid-state battery using the sulfur-based positive electrode active material of Example 1. As can be seen from FIG. 1, during a first charge cycle, lithium sulfide decomposed firstly and then lithium iodide decomposed, and the decomposition voltage plateau of lithium iodide was high. During a first discharge cycle, there was no discharge plateau of lithium iodide, but the discharge plateau of lithium sulfide was increased; during a second charge/discharge cycle, there was also a high decomposition voltage plateau of lithium iodide, indicating the electron-withdrawing effect of highly electronegative iodine ions. Meanwhile, as can be seen from FIG. 1, the first discharge plateau of the solid-state battery using the sulfur-based positive electrode active material of Example 1 was located at 2.31-2.42 V (vs. Li/Li.sup.+), which is significantly higher than that of the Li—S battery (see FIG. 3).

[0073] FIG. 2 showed a 223-cycle curve of a solid-state battery using the sulfur-based positive electrode active material of Example 1. As can be seen from FIG. 2, the solid-state battery using the sulfur-based positive electrode active material of Example 1 had excellent cycle performance.

[0074] FIG. 3 showed a voltage-specific capacity graph of a solid-state battery using the positive electrode active material of Comparative Example 1 during charging-discharging for the first time and charging-discharging for the second time. As can be seen from FIG. 3, the discharge plateau of the Li—S battery using the positive electrode active material of Comparative Example 1 was located at 1.71-2.13 V (vs. Li/Li.sup.+).

[0075] FIG. 4 showed a charge/discharge curve of a solid-state battery using the sulfur-based positive electrode active material of Example 9. As can be seen from FIG. 4, the solid-state battery using the sulfur-based positive electrode active material of Example 9 also had an elevated first discharge plateau located at 2.26-2.35 V (vs. Li/Li.sup.+).

[0076] FIG. 5 showed a volumetric deformation of a positive electrode of a solid-state battery using the sulfur-based positive electrode active material of Example 1 after two charge/discharge cycles. As can be seen from FIG. 5, during the use of the solid-state battery positive electrode including the lithium-sulfur-based positive electrode active material, the positive electrode was subjected to lithium removal when the battery was charged, the solid electrode did not generate volume expansion that causes electrode deformation but formed pores, and the structure was recovered during discharge. A positive electrode using the sulfur-based positive electrode active material of Example 1 had a volume change rate of only 6.42% after two cycles.

[0077] Further, FIG. 6 showed a voltage-specific capacity graph of the solid-state battery using the positive electrode active material of Comparative Example 2 during a first charge/discharge cycle and a second charge/discharge cycle. As can be seen from FIG. 6, the discharge plateau of the battery to which lithium iron phosphate was added as a second lithium compound was located at 1.81-2.11 V (vs. Li/Li.sup.+), and there was no elevation compared to Comparative Example 1, which means that simply mixing the positive electrode material having a high discharge plateau with lithium sulfide did not significantly improve the performance of the solid-state battery, especially the discharge plateau of the solid-state lithium sulfur battery.