WO2021251543A1

WO2021251543A1 - Negative electrode active material for secondary battery, method for preparing same, and secondary battery comprising same

Info

Publication number: WO2021251543A1
Application number: PCT/KR2020/008882
Authority: WO
Inventors: 김광범; 나자리안-사마니마수드; 하기가트-시샤바니사파; 나자리안-사마니마부베; 이건우
Original assignee: 연세대학교 산학협력단
Priority date: 2020-06-11
Filing date: 2020-07-08
Publication date: 2021-12-16
Also published as: KR102294471B1; KR102294471B9

Abstract

The present technology relates to a negative electrode active material for a secondary battery, a method for preparing same, and a secondary battery comprising same. BiPS4 having a tunnel structure, which is the negative electrode active material according to the present invention, and a BiPS3-CNT composite using same can be prepared through a simple process, and exhibit excellent charge/discharge properties and electrical properties as compared to existing negative electrode active materials, and thus can be effectively used as a negative electrode material of not only lithium secondary batteries but also sodium secondary batteries or potassium secondary batteries.

Description

Anode active material for secondary battery, manufacturing method thereof, and secondary battery comprising same

The present technology relates to an anode active material for a secondary battery, a method for manufacturing the same, and a secondary battery including the same.

Recently, the demand for a secondary cell as a power device for a personal portable terminal device such as a mobile phone, a smart phone, and a tablet PC or an electric vehicle such as a hybrid electric vehicle or a plug-in electric vehicle is increasing significantly. In particular, lithium secondary batteries are currently the most common among secondary batteries, and since these lithium secondary cells use a lot of rare metals such as cobalt (Co), nickel (Ni), and lithium (Li), large There is concern about the supply of rare metals according to the increase in demand for secondary batteries.

In this regard, there is a demand for the development of sodium secondary batteries and potassium secondary batteries that exhibit high efficiency and high stability as substitutes for lithium secondary batteries. Sodium secondary batteries and potassium secondary batteries are attracting attention as next-generation secondary batteries that can overcome the limitations of current lithium secondary batteries as energy storage and conversion devices because of the advantages of easy resource supply and demand and low manufacturing cost. However, there is a problem in that the negative active material mainly used in lithium secondary batteries is not suitable in terms of battery performance and energy characteristics to be directly applied to sodium secondary batteries and potassium secondary batteries.

One object of the present technology is to provide an anode active material for a secondary battery having improved charge/discharge characteristics and electrical characteristics.

Another object of the present technology is to provide a secondary battery including the negative active material for secondary batteries.

Another object of the present technology is to provide a method of manufacturing the negative active material for a secondary battery.

In order to achieve the above object,

The present technology provides an anode active material for a secondary battery including a compound having an orthorhombic crystal structure and having a chemical formula of BiPS _{4 .}

In addition, the present technology provides a secondary battery including the negative active material.

In addition, the present technology, (a) mixing bismuth (Bi), phosphorus (P) and sulfur (S) in a molar ratio of 1: 0.8 to 1.2: 3.8 to 4.2; And (b) ball milling the mixture to obtain a compound having a _{chemical formula of BiPS 4 having an orthorhombic crystal structure;}

_{BiPS 4} having a tunnel structure, which is an anode active material according to the present technology, _{and BiPS 4} -CNT composite using the same can be manufactured through a simple process, and exhibit excellent charge/discharge characteristics and electrical characteristics compared to conventional anode active materials, so that not only lithium secondary batteries However, it can be usefully used as an anode material for a sodium secondary battery or a potassium secondary battery. _{BiPS 4} having a tunnel structure, which is an anode active material according to the present technology, _{and BiPS 4} -CNT composite using the same can be manufactured through a simple process, and exhibit excellent charge/discharge characteristics and electrical characteristics compared to conventional anode active materials, so that not only lithium secondary batteries However, it can be usefully used as an anode material for a sodium secondary battery or a potassium secondary battery.

1 shows the structure of _{BiPS 4} according to an embodiment of the present technology.

2 shows the XRD measurement results of the negative active material for a secondary battery according to an embodiment of the present technology.

3 is a view showing a measurement result of a selective region electron diffraction pattern of a negative active material for a secondary battery according to an embodiment of the present technology.

4 shows the results of X-ray photoelectron spectroscopy analysis of a negative active material for a secondary battery according to an embodiment of the present technology.

Figure 5 shows the CV curve measurement result of the BPSC electrode when using _{the KPF 6} solute according to an embodiment of the present technology.

6 shows the CV curve measurement results obtained by measuring the BPSC electrode at different scan speeds when _{KPF 6} solute is used according to an embodiment of the present technology.

7 shows the results of analyzing the potentials at 10 points on the first charge/discharge curve of the BPSC electrode using HRXRD when _{KPF 6} solute is used according to an embodiment of the present technology.

FIG. 8 is an HR-TEM micrograph at 0.01 V, which is the first discharge voltage of the BPSC electrode when _{KPF 6} solute is used according to an embodiment of the present technology.

9 shows the SAED pattern at 0.01V, which is the first discharge voltage of the BPSC electrode, when _{the KPF 6} solute is used according to an embodiment of the present technology.

10 is an HR-TEM micrograph at 3.0V, which is the first charging voltage of the BPSC electrode when _{KPF 6} solute is used according to an embodiment of the present technology.

11 shows the SAED pattern at 3.0V, which is the first charging voltage of the BPSC electrode, when _{the KPF 6} solute is used according to an embodiment of the present technology.

12 shows the CV curve measurement result of the BPSC electrode when the KFSI solute is used according to an embodiment of the present technology.

13 shows the CV curve measurement results obtained by measuring the BPSC electrode at different scan speeds when KFSI solute is used according to an embodiment of the present technology.

14 shows the results of analyzing the potentials at 10 points on the first charge/discharge curve of the BPSC electrode using HRXRD when KFSI solute is used according to an embodiment of the present technology.

15 is an HR-TEM micrograph at 0.01 V, which is the first discharge voltage of the BPSC electrode when KFSI solute is used according to an embodiment of the present technology.

FIG. 16 shows the SAED pattern at 0.01V, which is the first discharge voltage of the BPSC electrode, when the KFSI solute is used according to an embodiment of the present technology.

17 is an HR-TEM micrograph at 3.0V, which is the first charging voltage of the BPSC electrode when KFSI solute is used according to an embodiment of the present technology.

18 shows the SAED pattern at 3.0V, which is the first charging voltage of the BPSC electrode, when the KFSI solute is used according to an embodiment of the present technology.

19 shows a result of measuring the initial charge/discharge capacity of an electrode according to an embodiment of the present technology.

20 shows the results of measuring the initial Coulombic efficiency of the electrode according to an embodiment of the present technology.

21 shows the results of measuring the speed performance of the electrode according to an embodiment of the present technology.

22 shows the results of measuring ^{electron and K +} ion transport performance of an electrode according to an embodiment of the present technology through electrochemical impedance spectroscopy (EIS) analysis.

The technology to be described below may have various changes and may have various embodiments, and specific embodiments will be illustrated in the drawings and described in detail. However, this is not intended to limit the technology described below to specific embodiments, and it should be understood to include all modifications, equivalents, and substitutes included in the spirit and scope of the technology described below.

The present technology provides an anode active material for a secondary battery, including a compound having a _{chemical formula of BiPS 4} having an orthorhombic crystal structure.

The compound may have a tunnel structure. In this structure, Bi atoms are placed at the (0 ¼ z), (0 ¾ -z), (½ ¾ z), and (½ ¼ z) positions, where z = 0.1414 and z = 0.3986. The P atoms are (x 0 ¼), (-x 0 ¾), (x ½ ¾), (-x ½ ¼), (¼ y 0), (¾ y 0), (¾ -y ½) and (¼ -y ½), where x = 0.035 and y = 0.221. The S atoms are located at the edge-shared tetrahedral positions and the other tetrahedral positions are filled with P atoms, _{resulting in a PS 4} tetrahedral structure with angles between 104.9 and 115.6 and edge lengths ranging from 3.26 to 3.39 Å.

The negative active material may further include a _{carbon nanotube, and the weight ratio of the compound having the Chemical Formula BiPS 4} to the carbon nanotube (CNT) may be 6 to 8:3.

In another aspect, the present technology provides a secondary battery including the negative active material for secondary batteries.

The secondary battery may be a sodium secondary battery, a potassium secondary battery, or a lithium secondary battery, preferably a sodium secondary battery or a potassium secondary battery.

In addition, the present technology is another aspect, (a) mixing bismuth (Bi), phosphorus (P) and sulfur (S) in a molar ratio of 1: 0.8 to 1.2: 3.8 to 4.2; And (b) ball milling the mixture to obtain a compound having a _{chemical formula of BiPS 4 having an orthorhombic crystal structure;}

The ball milling of step (b) may be performed by mixing the balls and the mixture in a weight ratio of 30 to 50: 1, and performing at a speed of 300 to 500 rpm for 16 to 24 hours.

Step (b) may be performed in an inert gas atmosphere. The inert gas is a gaseous material corresponding to Group 18 on the periodic table and refers to a material that does not react with other elements. The inert gas may be helium (He), neon (Ne), argon (Ar), or the like, preferably argon (Ar).

In addition, the method for manufacturing a negative active material for a secondary battery according to the present technology comprises the steps of (c) mixing _{a compound having the chemical formula BiPS 4} and carbon nanotubes (CNT) in a weight ratio of 6 to 8: 3; and (d) ball milling the mixture to obtain a _{BiPS 4 -CNT composite.}

The ball milling in step (d) may be performed by mixing the balls and the mixture in a weight ratio of 10 to 30: 1, and performing for 40 to 60 hours at a speed of 200 to 400 rpm.

Hereinafter, the present technology will be described in more detail with reference to the accompanying drawings in order to help the understanding of the present technology. However, the following examples are only provided for easier understanding of the present technology, and the content of the present technology is not limited by the following examples.

실시예 1: 음극활물질의 제조Example 1: Preparation of negative electrode active material

Bismuth (Bi, Sigma Aldrich, 99.50 %), amorphous red P, Alfa Aesar, 98.90 %) and sulfur (S, Sigma Aldrich, 99.98 %) (1 : 1 : 4 and A total weight of 2 g) was put into a WC vial together with a WC ball and ball milled to synthesize _{BiPS 4 .} The weight ratio of the WC balls and these reactants was set to 40:1. Ball milling was performed for 20 hours at a speed of 400 rpm with a 20-minute rest interval after 1 hour. The synthesized BiPS ₄ is hereinafter referred to as BPS.

As shown in FIG. 1 , the BiPS ₄ represents a tunnel structure in which empty spaces are formed along the y and z axes in an orthorhombic shape.

Next, the synthesized BPS and multi-walled carbon nanotubes (CNT, Carbon Nanomaterials Technology Co. Ltd., Korea) were mixed in a weight ratio of 7: 3 (total weight 1 g), and the mixed powder was placed inside the glove box. placed in a vial. The weight ratio of the powder mixed with the WC balls was 20: 1. Ball milling was performed at a speed of 300 rpm for 50 hours. The finally obtained BiPS ₄ -CNT complex was named BPSC.

실시예 2: 음극활물질 구조 분석Example 2: Structural Analysis of Anode Active Material

2-1. XRD analysis

The crystal structures of BPS and BPSC prepared according to Example 1 were analyzed through powder X-ray diffraction (XRD) analysis.

X-ray diffraction patterns were measured on an Ultima IV Rigaku diffractometer at 40 kV and 40 mA using Cu Ka radiation. High-resolution XRD (HRXRD) patterns were measured through SmartLab Rigaku at 45 kV and 200 mA using Cu Kα radiation. The results are shown in FIGS. 2 to 4 .

As shown in FIG. 2 , as a result of XRD measurement, it was confirmed that the BPS prepared according to Example 1 had a complete polycrystalline structure. In addition, it was confirmed _{that all XRD peaks of BiPS 4} were consistent with the information of Powder Diffraction File (PDF) #01-071-0364 of ICDD (The International Center for Diffraction Data). Accordingly, it can be confirmed that the BPS according to the present technology was manufactured according to the method of Example 1.

In addition, according to the XRD pattern, it was confirmed that the properties of the BPS crystalline remained unchanged even after being prepared as _{a BiPS 4 -CNT composite (BPSC) using the same.} This can also be confirmed by comparing the selected area electron diffraction (SAED) patterns of BPS and BPSC, as shown in FIG. 3 . The transparent rings in the SAED patterns of BPS and BPSC mean that BiPS ₄ has polycrystalline properties in BPS and BPSC.

2-2. XPS analysis

X-ray photoelectron spectroscopy (XPS) of the BPS and BPSC prepared according to Example 1 above with monochromatic Al Kα radiation was performed using an ESCA 2000, VG Microtech spectrometer. The results are shown in FIG. 4 .

As shown in Figure 4, XPS measurement results for both BPS and BPSC Bi 4f _7/2 and Bi 4f _5/2 peaks and S 2p _3/2 and S 2p _1/2 peaks, as well as P 2p _3/2 and The P 2p _1/2 peak was clearly shown. It can be seen that the peak of BPSC moves to a higher binding energy and becomes wider than that of BPS. This means that stable Bi-OC, SOC and POC chemical bonds were formed between _{BiPS 4 and the CNT network.}

From the above results, it can be seen that the BPS and BPSC according to the present technology form a very stable structure.

실시예 3: 전극 제조Example 3: Electrode Preparation

An electrode (cathode) was prepared using the BPS and BPSC prepared according to Example 1. First, BPS and BPSC were mixed with carbon black and a binder in a weight ratio of 70: 15: 15, respectively, and then the formed slurry was cast on copper foil and the slurry loading of 1.30 - 1.50 mg was carried out at 150 ° C. It was dried under vacuum for 3 hours. As a binder, a mixture of polyacrylic acid (PAA, 3 wt% in water) and sodium carboxyl methyl cellulose (NaCMC, 1 wt%) in a weight ratio of 1: 1 was used.

실시예 4: 전기화학적 특성 측정Example 4: Measurement of electrochemical properties

Electrochemical properties of the BPS and BPSC electrodes prepared according to Example 3 were measured. This measurement was carried out inside a glove box filled with argon (Ar), and lithium (Li) or potassium (K) as a counter electrode. This was done using a CR2032 coin-type half cell using metal.

In a lithium ion battery (LIB), ethylene carbonate (EC), dimethyl carbonate (DMC) and diethyl carbonate (DEC) are mixed at 2: 2: 1 vol%, An electrolyte containing 1 M LiPF6 was used in a solution to which 10 vol% of fluoroethylene carbonate (FEC) was added.

In a potassium ion battery (KIB), bis (fluorosulfonyl) imide (Either bis (fluorosulfonyl) imide , KFSI, 1 M) or _{an electrolyte containing potassium hexafluorophosphate (KPF 6} , 0.8 M) was used.

Galvanostatic cycling, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) were performed using an electrochemical measuring instrument (potentiostat/galvanostat, VMP2, Princeton Applied Research, USA). It was carried out in a voltage range of 0.01 - 3.00 V with respect to Li ⁺ /Li or K ^{+ /K at room temperature.}

EIS was performed in the charged state before and after cycling by applying a perturbation voltage of 5 mV at a frequency of 1 to 105 Hz. The digested LIB and KIB cells were washed with DMC for 24 hours and then dried overnight in a glove box.

5 is an ethylene carbonate (EC) / diethyl carbonate (DEC) solvent using an electrolyte using _{0.8M potassium hexafluorophosphate (KPF 6} ^{) solute at a scan rate of 0.05 mV s -1} to 0.01 - 3.00 V The CV curve of the BPSC electrode measured in the voltage range is shown.

As shown in FIG. 5 , as a result of the initial cathode scan measurement, a broad and irreversible peak starting at about 1.40 V and a large and clear peak at 0.15 V were measured. The peak at about 1.40 V is the insertion of K ⁺ _{into BiPS 4} of the tunnel structure and multi-step conversion/alloying reaction in which K-Bi, KP and KS phases are formed, as well as solid electrolyte interphase (SEI). ) indicates that a layer is formed. Then, four anodic peaks were detected at 0.27, 0.63, 1.29 and 2.29 V, indicating de-potassium as well as de-alloying and reverse conversion reactions of ^{K + ions.}

In addition, measurements were made by varying the scan speed from 0.05 mV·s ⁻¹ to a maximum of 3.00 mV·s ^{−1 .} As shown in FIG. 6 , as the scan rate increased, a significant positive peak shifted to a higher potential, and a negative peak shifted to a lower potential even with a lower potential. This indicates that a large amount of polarization occurs between the anodes.

To measure the phase change during the potassiumization/depotassation process, HRXRD was used to analyze the potential at 10 points on the first charge/discharge curve of the BPSC electrode. The results are shown in FIG. 7 .

As shown in FIG. 7 , when KPF ₆ solute is used, the final discharge products are K ₃ Bi, K ₄ P ₃ and K ₂ S. And at the end of the charging phase, BiPS ₄ was not restored. Instead, it was found that not only amorphous P but also crystalline Bi and S were detected, confirming that BiPS ₄ was irreversibly consumed in the _{KPF 6 electrolyte.}

As shown in FIGS. 8 to 11 , this was confirmed through HR-TEM micrographs and SAED patterns. 8 is an HR-TEM micrograph at a first discharge voltage of 0.01V. FIG. 9 shows the SAED pattern in FIG. 8 . Through this, it was possible to reconfirm the reactants identified in FIG. 7 . 10 is an HR-TEM micrograph at a first charging voltage of 3.0V. FIG. 11 shows the SAED pattern in FIG. 10 . Through this, it was reconfirmed that _{BiPS 4} _{was irreversibly consumed in the KPF 6} electrolyte confirmed in FIG. 7 .

In contrast, as shown in FIGS. 12 and 13 , the measurement was performed using an electrolyte using 1M potassium bis(fluorosulfonyl)imide (KFSI) solute in an ethylene carbonate (EC)/diethyl carbonate (DEC) solvent. In the CV curve of one BPSC electrode, the redox peak showed negligible change even as the sweep speed increased.

As shown in Figure 14, in the case of using the KFSI solute final discharge product K ₃ Bi, K ₄ and P _3, KP, K ₂ S, and a part of unreacted sulfur, the end of the charge phase BiPS ₄ is seen that reversibly Recovery can

\*62In addition, as shown in FIGS. 15 to 18, this was confirmed through HR-TEM micrographs and SAED patterns.

15 is an HR-TEM micrograph at a first discharge of 0.01 V; FIG. 16 shows the SAED pattern in FIG. 15 . Through this, it was possible to reconfirm the reactants identified in FIG. 14 . 17 is an HR-TEM micrograph at a first charge of 3.0V. 18 shows the SAED pattern in FIG. 17 . Through this, it was reconfirmed that the charging step confirmed in FIG. 14 was reversibly restored upon completion. The electrochemical properties of BPS and BPSC electrodes applied to KIB and LIB were analyzed with coin-type 2032 cells within potential windows of 0.01 and 3.00 V. As shown in FIGS. 19 and 20, the BPSC electrode (BPSC-KFSI) using an electrolyte containing KFSI had an initial charge/discharge capacity of 523.26/648.39 mAh ^{g -1 at} ^{a current density of 0.05 A g -1} . was measured. As a result of the measurement, it was found that the initial Coulombic efficiency (ICE) was 80.70%. The initial discharge capacity at a current density of 0.2 A·g ^-1 ^{was 438.98 mAh g -1} , and was maintained at 387.62 mAh g ^-1 after 300 cycles (88.30% capacity retention rate). Similarly, after 3 cycles at 0.20 mAh g ^-1 , the CE value increased rapidly from 94.50% to 99.96% or more, and it was found to be steadily maintained for more than 300 cycles.

BPS electrode (BPS-KFSI) using an electrolyte containing KFSI is measured with 0.05 A · g ^-1 initial charge / discharge capacity of the 523.73 / 992.21 mAh · g ^-1 at a current density of: was (ICE 52.78%). As a result of the measurement, the initial coulombic efficiency (ICE) was found to be 52.78%. 0.20 The first discharge capacity at a current density of A · g ^-1 was measured as 344.78 mAhA · g ^-1 tended to gradually decrease to 300 cycles, the discharge capacity at the final cycle is measured as the 171.23 mAh · g ^-1 became In addition, the CE value increased to 99.02% after about 20 cycles at a current density of 0.20 A·g ^{−1 and showed stable behavior for 300 cycles.}

The rate performance of BPS and BPSC electrodes using KFSI and KPF _{6-containing electrolytes, respectively, was measured.} The results are shown in FIG. 21 .

As shown in FIG. 21, the BPSC electrode (BPSC-KFSI) using an electrolyte containing KFSI has current densities of 0.05, 0.10, 0.20, 0.30, 0.40, 0.50, 0.70, 1.00, 1.50 and 2.00 A g ^-1 The discharge capacities were measured as 653.49, 493.14, 433.78, 420.20, 415.21, 408.19, 399.65, 384.64, 359.65 and 348.66 mAh g ^-1 , respectively, which significantly superior rate performance (0.1 - 2) than BPS-KFSI and BPSC-KPF _{6 .} 70.70% capacity retention at A·g ^{-1) was confirmed.} In addition, when the current density is ^{switched back to 0.05A·g -1} (519.24mAh·g ^-1 ) corresponding to 79.46% of the initial capacity, the capacity is increased.

The BPSC electrode using an electrolyte containing _{KPF 6} _{(BPSC-KPF 6} ) exhibited similar or higher capacity (up to 0.20 A g ^-1 ) to the BPSC electrode (BPSC-KFSI) using the KFSI electrolyte, 2A · g ^-1 was in the charge / discharge capacity of the 133.51 / 144.69 mAh · g ^-1, when the current density is again changed to 0.05 a · g ^-1 charge / discharge capacity is increased to 415.52 / 451.11 mAh · g ^-1 appeared to do

(With BPS-KFSI) BPS electrode using an electrolyte comprising a KFSI is 0.05 A · g ^-1 in the charge / discharge capacity at the initial cycle was specified by the ^{523.95 / 1002.33 mAh · g -1,} 2.00 A g · g - ^It was measured to be 147.62/152.85 mAh·g ^{-1 at 1 .}

^{The electron and K +} ion transport performance of BPS and BPSC were measured to confirm their potential to be used as an anode material for a potassium secondary battery. For this purpose, electrochemical impedance spectroscopy (EIS) analysis was performed. The results are shown in FIG. 22 .

As shown in FIG. 22, after 1, 10, and 50 cycles in a fully charged state, BPS-KFSI (FIG. 22A), BPSC-KPF ₆ (FIG. 22B) and BPSC-KFSI (FIG. 22C) electrodes Comparing the Nyquist plots of , three resistors were considered consistent with the circuit. In particular, it was confirmed that the BPSC electrode (BPSC-KFSI) using an electrolyte containing KFSI showed the minimum resistance in various cycles.

Through the measurement results of this embodiment, it can be confirmed that the negative electrode active material according to the present technology has excellent charge/discharge characteristics and electron and ion transport performance, and thus can be usefully used as a negative electrode material for a secondary battery.

The description of the present technology stated above is for illustration, and those of ordinary skill in the art to which this technology pertains can understand that it can be easily modified into other specific forms without changing the technical spirit or essential characteristics of the present technology. There will be. Therefore, it should be understood that the embodiments described above are illustrative in all respects and not restrictive.

The negative active material for a secondary battery according to the present technology exhibits excellent charge/discharge characteristics and electrical characteristics compared to the existing negative electrode active material, so that it can be usefully used as an anode material for a lithium secondary battery as well as a sodium secondary battery or a potassium secondary battery.

Claims

A negative active material for a secondary battery, comprising a compound having a chemical formula BiPS 4 having an orthorhombic crystal structure.
The method of claim 1,

The compound is characterized in that the tunnel structure, a negative active material for secondary batteries.
The method of claim 1,

Further comprising carbon nanotubes, wherein the weight ratio of the compound having the formula BiPS 4 and carbon nanotube (CNT) is 6 to 8: 3, a negative electrode active material for a secondary battery.
The method of claim 1,

A secondary battery comprising the negative active material for the secondary battery.
5. The method of claim 4,

The secondary battery is a sodium secondary battery or a potassium secondary battery, characterized in that the secondary battery.
(a) mixing bismuth (Bi), phosphorus (P) and sulfur (S) in a molar ratio of 1: 0.8 to 1.2: 3.8 to 4.2; and

(B) ball milling the mixture to obtain a compound having the formula BiPS 4 having an orthorhombic crystal structure; comprising, a method for producing a negative active material for a secondary battery.
7. The method of claim 6,

The ball milling of step (b) is performed by mixing the balls and the mixture in a weight ratio of 30 to 50: 1, and performing at a speed of 300 to 500 rpm for 16 to 24 hours, Preparation of a negative active material for secondary batteries Way.
7. The method of claim 6,

The step (b) is a method of manufacturing a negative active material for a secondary battery, characterized in that it is performed in an inert gas atmosphere.
7. The method of claim 6,

(c) mixing a compound having the chemical formula BiPS 4 and carbon nanotubes (CNT) in a weight ratio of 6 to 8: 3; and

(d) ball milling the mixture to obtain a BiPS 4 -CNT composite; further comprising, a method for producing a negative active material for a secondary battery.
10. The method of claim 9,

The ball milling of step (d) is characterized in that the balls and the mixture are mixed in a weight ratio of 10 to 30: 1, and carried out at a speed of 200 to 400 rpm for 40 to 60 hours, Preparation of a negative active material for secondary batteries Way.