KR102229460B1 - Method for manufacturing iron phosphide - Google Patents

Abstract

Life Characteristics of a Lithium Secondary Battery In order to improve light discharge capacity, a method of manufacturing iron phosphide (FeP) applicable as a positive electrode additive of a lithium secondary battery is disclosed. The manufacturing method of iron phosphide includes the steps of: (1) heat-treating the iron hydrate to obtain an iron oxide represented by the following formula (1); And (2) heat-treating by mixing the obtained iron oxide and NaH ₂ PO ₂ ·H _{2 O.}
[Formula 1]
Fe _x O ₃ (however, 1.7 ≤ x <2)

Description

Manufacturing method of iron phosphide (FeP) {METHOD FOR MANUFACTURING IRON PHOSPHIDE}

The present invention relates to a method for producing iron phosphide (FeP) applicable as a positive electrode additive for a lithium secondary battery.

A secondary battery is an electrical storage device capable of continuous charging and discharging unlike a primary battery that can only be discharged once, and has established itself as an important electronic component of portable electronic devices since the 1990s. In particular, lithium-ion secondary batteries were commercialized by Sony in Japan in 1992, and have led the information age as a core component of portable electronic devices such as smartphones, digital cameras, and notebook computers.

In recent years, lithium-ion secondary batteries have expanded their application areas, and are used in medium-sized batteries to be used in fields such as vacuum cleaners, power tools, electric bicycles, electric scooters, electric vehicles (EVs), hybrid electric vehicles. vehicle; HEV), Plug-in hybrid electric vehicle (PHEV), various robots and large-capacity batteries used in fields such as Electric Storage System (ESS) at high speed. Demand is increasing.

However, the lithium secondary battery, which has the most excellent characteristics of the secondary batteries available to date, has several problems in being actively used in transportation devices such as electric vehicles and PHEVs, and the biggest problem among them is the limitation of capacity.

Lithium secondary batteries are basically composed of materials such as positive electrodes, electrolytes, and negative electrodes, and among them, positive and negative materials determine the capacity of the battery, so lithium-ion secondary batteries are due to the material limitations of the positive and negative electrodes. Limited by capacity. In particular, since a secondary battery to be used in applications such as electric vehicles and PHEVs must be able to be used for as long as possible after being charged once, the discharge capacity of the secondary battery is very important. The biggest limitation to the sales of electric vehicles is that the distance that can be driven after a single charge is much shorter than that of ordinary gasoline-engine cars.

The capacity limitation of such a lithium secondary battery is difficult to completely solve due to the structural and material limitations of the lithium secondary battery despite a lot of efforts. Therefore, in order to fundamentally solve the capacity problem of a lithium secondary battery, it is required to develop a new concept of a secondary battery that goes beyond the existing secondary battery concept.

The lithium-sulfur secondary battery exceeds the capacity limit determined by the intercalation/intercalation reaction of the layered metal oxide and graphite of lithium ions, which is the basic principle of the existing lithium ion secondary battery, and replaces transition metals and reduces costs. It is a new high-capacity, low-cost battery system that can bring back.

A lithium-sulfur secondary battery is a lithium ion and the sulfur conversion (conversion) reaction at the anode ^- the theoretical capacity resulting from ^{_{(S 8 + 16Li + + 16e}} → 8Li 2 S) reached 1,675 mAh / g anode is lithium metal (theoretical capacity: 3,860 mAh/g) can be used to increase the capacity of the battery system. In addition, since the discharge voltage is about 2.2 V, it theoretically shows an energy density of 2,600 Wh/kg based on the amount of positive and negative active materials. This is a value 6 to 7 times higher than the energy theoretical energy density of 400 Wh/kg of a commercial lithium secondary battery (LiCoO _{2 /graphite) using a layered metal oxide and graphite.}

Lithium-sulfur secondary batteries are attracting attention as new high-capacity, eco-friendly, and inexpensive lithium secondary batteries since it was known that the battery performance can be dramatically improved through the formation of nanocomposites around 2010. Currently, intensive research worldwide as a next-generation battery system Is being done.

One of the major problems of the lithium-sulfur secondary battery that has been discovered so far is that the electrical conductivity of sulfur is about 5.0 x 10 ^-14 S/cm, which is close to a non-conductor, making it difficult to conduct electrochemical reactions at the electrode. The voltage is far below theory. Early researchers tried to improve the performance by mechanical ball milling of sulfur and carbon or surface coating using carbon, but there was no significant effect.

In order to effectively solve the problem that the electrochemical reaction is limited by the electrical conductivity, as in the example _{of LiFePO 4} ^{, one of the other positive electrode active materials (electrical conductivity: 10 -9} to 10 ^-10 S/cm), the size of the particle is set to several tens of nanometers It is necessary to reduce the size to the following and surface treatment with a conductive material.To this end, various chemicals (melt impregnation into nano-sized porous carbon nanostructures or metal oxide structures), and physical methods (high energy ball milling) have been reported. Has become.

Another major problem associated with lithium-sulfur secondary batteries is the dissolution of lithium polysulfide, an intermediate product of sulfur generated during discharge, into an electrolyte. As the discharge proceeds, sulfur (S ₈ ) continuously reacts with lithium ions, resulting in the phase of S ₈ → Li ₂ S ₈ → (Li ₂ S ₆ ) → Li ₂ S ₄ → Li ₂ S ₂ → Li ₂ S, etc. The (phase) changes continuously. Among them, Li ₂ S ₈ and Li ₂ S ₄ (lithium polysulfide), which are long chains of sulfur, are easily soluble in general electrolytes used in lithium ion batteries. When such a reaction occurs, not only the reversible anode capacity is greatly reduced, but also dissolved lithium polysulfide diffuses into the cathode, causing various side reactions.

Lithium polysulfide in particular causes a shuttle reaction during the charging process, which causes the charging capacity to continue to increase, and the charging and discharging efficiency rapidly decreases. Recently, various methods have been proposed to solve this problem, and can be largely divided into a method of improving the electrolyte, a method of improving the surface of the negative electrode, and a method of improving the characteristics of the positive electrode.

The method of improving the electrolyte is to suppress the dissolution of polysulfide into the electrolyte by using a new electrolyte such as a functional liquid electrolyte, a polymer electrolyte, or an ionic liquid of a new composition, or the dispersion rate to the negative electrode by controlling the viscosity. It is a method to suppress the shuttle reaction as much as possible by controlling it.

Research is being actively conducted to control the shuttle reaction by improving the characteristics of SEI formed on the surface of the cathode. Typically, an oxide film such as _{Li x} NO _y and Li _x SO _y is added to the surface of the lithium anode by introducing an electrolyte additive such _{as LiNO 3.} There is a method of forming and improving the structure, a method of forming a thick functional SEI layer on the surface of lithium metal, and the like.

Finally, methods of improving the characteristics of the positive electrode include forming a coating layer on the surface of the positive electrode particles to prevent the dissolution of polysulfide, or adding a porous material that can catch the dissolved polysulfide. Typical conductive polymers include sulfur particles. A method of coating the surface of a positive electrode structure containing lithium ions, a method of coating the surface of a positive electrode structure with a metal oxide through which lithium ions are conducted, and a porous metal oxide having a large specific surface area and large pores capable of absorbing a large amount of lithium polysulfide as a positive electrode. A method of adding to the carbon structure, a method of attaching a functional group capable of adsorbing lithium polysulfide on the surface of a carbon structure, a method of enclosing sulfur particles using graphene or graphene oxide, and the like have been suggested.

Although such efforts are being made, there is a problem in that this method is somewhat complicated and the amount of sulfur, which is an active material, can be limited. Therefore, it is necessary to develop a new technology to solve these problems in a complex manner and improve the performance of a lithium-sulfur battery.

Japanese Patent Laid-Open Publication No. WO2012-098960 (2012.07.26), "Anode active material and its manufacturing method and secondary battery" Chinese Patent Application Publication No. 106129375 (2016.11.16), "A method of a kind of composite lithium salt modified electrode material"

The present inventors conducted various studies to solve the above problems, and as a result _{of mixing and reacting heat-treated iron hydrate and NaH 2} PO ₂ ·H ₂ O, selectively high-purity iron phosphide by controlling the heat treatment temperature and mixing ratio. It was confirmed that can be prepared.

Accordingly, an object of the present invention is to provide a method for producing high purity iron phosphide through a simple process.

In order to achieve the above object, the present invention, (1) heat treatment of iron hydrate to obtain an iron oxide represented by the following formula (1); And (2) heat-treating by mixing the obtained iron oxide and NaH ₂ PO ₂ ·H ₂ O; provides a method for producing iron phosphide (FeP) comprising.

[Formula 1]

Fe _x O ₃ (however, 1.7 ≤ x <2)

According to the present invention, high-purity iron phosphide can be prepared by a simple process including the step of reacting by mixing the heat-treated iron hydrate and NaH ₂ PO ₂ ·H _{2 O.} By mixing the heat-treated iron hydrate and NaH ₂ PO ₂ ·H ₂ O and controlling the heat treatment temperature and mixing ratio during the reaction, the shape and purity of the produced iron phosphide can be controlled. In addition, when the produced iron phosphide is applied as a positive electrode additive for a lithium secondary battery, in particular, a lithium-sulfur battery, the life characteristics and discharge capacity of the battery may be increased.

1 shows a scanning electron microscope (SEM) image of iron oxide according to the present invention.
2 shows a scanning electron microscope (SEM) image of iron phosphide (FeP) according to the present invention.
3 shows the X-ray diffraction analysis (XRD) results of _{Fe 1.766} O ₃ according to the present invention.
4 shows the results of X-ray diffraction analysis (XRD) of iron phosphide (FeP) according to the present invention.
5 is a graph showing a change in chromaticity of a lithium polysulfide adsorption reaction according to Examples and Comparative Examples of the present invention as a result of measuring UV absorbance.
6 shows a result of measuring a discharge capacity of a lithium-sulfur battery including a positive electrode according to an exemplary embodiment of the present invention.
7 shows the results of measuring the life characteristics of a lithium-sulfur battery including a positive electrode according to an exemplary embodiment of the present invention.

Hereinafter, the present invention will be described in detail.

The method for producing iron phosphide (FeP) according to the present invention includes the steps of (1) heat-treating the iron hydrate to obtain an iron oxide represented by the following formula (1), and (2) the obtained iron oxide and NaH ₂ PO ₂ · It _{includes the step of heat treatment by mixing H 2} O, and in this case, it is applied as a positive electrode additive for lithium secondary batteries, in particular, lithium-sulfur batteries, to improve the discharge capacity and life characteristics of the battery. Eggplant has the advantage of being able to manufacture iron phosphide.

[Formula 1]

Fe _x O ₃ (however, 1.7 ≤ x <2)

The iron hydrate may be in the form of an aqueous solution, and may be 1.0 to 2.0 M. If it is less than 1.0 M, the production yield of iron phosphide may be lowered, and if it is more than 2.0 M, the physical properties of the produced iron phosphide may not be suitable for application as a positive electrode additive of a lithium secondary battery. As such an iron hydrate, Fe(NO ₃ ) ₃ ·9H ₂ O may be exemplified.

The iron hydrate may be further subjected to a pretreatment step of drying at 70 to 90° C. for 4 to 12 hours prior to heat treatment for manufacturing iron phosphide. If the temperature is less than the above temperature or the drying time is short, an excessive amount of moisture in the iron hydrate as a reactant may remain, and the moisture may be evaporated unevenly through a heat treatment process. In this case, the distribution of particles may become non-uniform through the heat treatment process, and the iron oxide represented by Chemical Formula 1 may not be synthesized. In addition, when the above temperature is exceeded or the drying time is long, all of the moisture of the iron hydrate as a reactant is evaporated, and then an oxidation reaction by heat treatment may partially proceed. In this case, a non-uniform oxidation reaction may occur through the heat treatment process, and the iron oxide may not be synthesized.

The iron hydrate may be subjected to a drying pretreatment process and then heat treated at 140 to 160° C. for 12 to 24 hours to generate the iron oxide represented by Formula 1 above. If the heat treatment temperature is less than 140 °C or shorter than the heat treatment time, a reaction residue may remain after heat treatment. In addition, when the heat treatment temperature exceeds 160° C. or is longer than the heat treatment time, the size of the generated particles may increase and may be expressed in a clustered form, and a stable material such as _{Fe 2} O _{3 may be generated.} Therefore, even after step (2), it may be difficult to synthesize the desired iron phosphide, so it is appropriately adjusted within the above range of temperature and time. The drying pretreatment and heat treatment may be performed in air using a convection oven.

The iron hydrate (Fe(NO ₃ ) ₃ ·9H ₂ O) undergoes the heat treatment step while HNO ₃ (g) is degassed to generate the iron oxide. In Formula 1, the oxidation number of iron may have various oxidation numbers depending on the heat treatment time and temperature, and preferably x may be 1.7≦x<1.9, more preferably 1.7≦x<1.8.

The iron oxide represented by Chemical Formula 1 after step (1) may be _{mixed with NaH 2} PO ₂ ·H ₂ O and then subjected to a heat treatment process to prepare iron phosphide (Step (2)). The iron oxide and NaH ₂ PO ₂ ·H ₂ O may be mixed in a weight ratio of 1:1 to 1:2, and if the ratio of NaH ₂ PO ₂ ·H ₂ O is lower than this, the raw material of phosphorus (P) Due to the lack of, pure phase iron phosphide (FeP) may not be synthesized. If the ratio is higher than the above ratio, there is a fear that excessive impurities may remain.

The heat treatment in step (2) may be performed at 200 to 300 °C, preferably 220 to 280 °C, more preferably 240 to 260 °C. If the temperature is lower than the heat treatment temperature, NaH ₂ PO ₂ ·H ₂ O is not decomposed and iron phosphide may not be synthesized.If the temperature is higher than the above temperature, the size of the produced iron phosphide particles increases, making it not suitable as a positive electrode additive for lithium secondary batteries. Otherwise, iron phosphide of a phase other than FeP may be prepared. The heat treatment in step (2) may be performed for 1 to 3 hours, preferably 1.5 to 2.5 hours, and if it is less than the above time, the reaction time may not be sufficient, so that the desired phase iron phosphide may not be produced. If it is long, the size of the synthesized iron phosphide particles increases and may not be suitable for use as a positive electrode additive for lithium secondary batteries. In addition, the heat treatment in step (2) may be performed by adjusting the heating rate in the range of 5 to 10° C. per minute. If the rate is higher than the above rate, the reaction may occur rapidly and a non-uniform reaction may proceed. When the rate is slower than the above rate, the temperature increase time may be lengthened, and the size of the iron phosphide particles may increase.

The heat treatment in step (2) may be performed in an inert gas atmosphere. The inert gas atmosphere may be (i) under an inert gas atmosphere in which the gas inside the reactor is replaced with an inert gas, or (ii) in a state in which the inert gas is continuously introduced to continuously replace the gas inside the reactor. . In the case of (ii), for example, the flow rate of the inert gas may be 1 to 500 mL/min, specifically 10 to 200 mL/min, and more specifically 50 to 100 mL/min. Here, the inert gas may be selected from the group consisting of nitrogen, argon, helium, and mixtures thereof.

According to an embodiment of the present invention, after the iron oxide represented by Formula 1 _{reacts with NaH 2} PO ₂ ·H ₂ O, it is converted to 80% or more iron phosphide (FeP) through a heat treatment step in an inert gas atmosphere. It can be, preferably 90% or more can be converted to iron phosphide. According to the XRD analysis result of FIG. 3, _{it can be confirmed that Fe 1.766} O ₃ (iron oxide), which is an embodiment of the present invention, is synthesized, and as a result of XRD analysis of FIG. 4, it can be confirmed that iron phosphide (FeP) is prepared.

The prepared iron phosphide may be formed by agglomeration of spherical primary particles to form secondary particles. At this time, the primary particles of the iron phosphide may have a particle diameter of 50 to 300 nm, preferably 100 to 150 nm. The secondary particles formed by the agglomeration of the primary particles of iron phosphide may have a spherical shape and a particle diameter of 0.5 to 15 µm, preferably 1 to 10 µm, more preferably 2 to 5 µm. The shape of the prepared iron phosphide can be adjusted as needed by controlling the reaction time, and all of these can be applied as a cathode material for a lithium secondary battery. As the particle diameter of the secondary particles decreases within the above range, it is suitable as a cathode material for a lithium secondary battery, and when the particle diameter of the secondary particles exceeds the above range, the particle size is large and may not be suitable as a positive electrode additive for a lithium secondary battery.

When the iron phosphide manufactured by the method for manufacturing iron phosphide as described above is applied to a lithium secondary battery, in particular, a lithium-sulfur battery, it can adsorb lithium polysulfide that is eluted during charging and discharging of the battery, thereby improving the performance of the battery. Can be improved. In addition, the method of manufacturing iron phosphide according to the present invention has a characteristic that simple and safe iron phosphide can be manufactured because it does not use phosphine _{(PH 3, phosphine), which is a toxic gas in the existing iron (Fe) oxide.} In addition _{, there is an advantage in that other phases such as Fe 2} P, Fe ₃ P, etc. are not generated, and an iron phosphide of a target phase can be selectively prepared.

Hereinafter, the present invention will be described in more detail through examples and the like, but the scope and contents of the present invention are reduced or limited by the following examples, and thus cannot be interpreted. In addition, if based on the disclosure of the present invention including the following examples, it is obvious that the present invention for which no specific experimental results are presented can be easily carried out by a person skilled in the art. It is natural to fall within the scope of the claims.

[Production Example 1] Preparation of iron oxide (Fe _1.766 O ₃ )

2.0 M Fe(NO ₃ ) ₃ ·9H ₂ O (Sigma-Aldrich) aqueous solution was dried in a convection oven at 80° C. for 6 hours. _(Pre- treatment process) Thereafter, heat treatment was performed at 155° C. for 18 hours in a convection oven to prepare Fe 1.766 O _3.

[Production Example 2] Preparation of iron oxide (Fe ₂ O ₃ )

A 2.0 M FeCl ₃ ·6H ₂ O (Sigma-Aldrich) aqueous solution was dried in a convection oven at 80° C. for 6 hours. (Pre-treatment process) Thereafter, heat treatment was performed at 155° C. for 18 hours in a convection oven to prepare _{Fe 2} O _3.

[Production Example 3] Preparation of iron oxide (Fe ₂ O ₃ )

A 2.0 M FeSO ₄ ·7H ₂ O (Sigma-Aldrich company) aqueous solution was dried in a convection oven at 80° C. for 6 hours. (Pre-treatment process) Thereafter, heat treatment was performed at 155° C. for 18 hours in a convection oven to prepare _{Fe 2} O _3.

[Example 1] Preparation of iron phosphide (FeP)

_{After reacting by mixing the iron oxide (Fe 1.766} O ₃ ) prepared in Preparation Example 1 and NaH ₂ PO ₂ ·H ₂ O at a mass ratio of 1:1.5, nitrogen gas of a flow rate of 100 mL/min was flowed at 250° C. Heat treatment was performed at for 2 hours to prepare iron phosphide (FeP). At this time, the rate of temperature increase for the heat treatment was 10°C per minute.

[Comparative Example 1] Preparation of iron phosphide (FeP)

Iron oxide (FeP) was prepared in the same manner as in Example 1, except that the iron oxide was _changed from Fe 1.766 O _{3 to Fe 2} O ₃ of Preparation Example 2.

[Comparative Example 2] Preparation of iron phosphide (FeP)

An iron oxide Fe In _1.766 O _3, except that a change to the preparation 3 of the Fe ₂ O _3, the same way as in Example 1 to prepare a printing iron (FeP).

[Experimental Example 1] SEM (scanning electron microscope) analysis

SEM analysis (S-4800 FE-SEM of Hitachi Corporation) was performed on the iron oxides prepared in Preparation Examples 1 to 3, respectively, and the iron phosphides prepared in Example 1 and Comparative Examples 1 and 2, respectively.

1 is an SEM image of each iron oxide prepared in Preparation Example, and FIG. 2 is an SEM image of iron phosphide prepared in Example 1 and Comparative Examples 1 and 2.

As a result of performing SEM analysis with a magnification of 50k, the iron oxide (Fe _1.766 O ₃ ) of Preparation Example 1 was synthesized into particles of 10 to 100 nm, and they aggregate to form secondary particles of 0.5 to 5 μm, Preparation Example The iron phosphide of Example 1 using the iron oxide of 1 was produced as spherical primary particles having a level of 50 to 300 nm, and spherical primary particles were aggregated to form secondary particles of 1 to 10 µm.

In addition, the iron oxide (Fe ₂ O ₃ ) of Preparation Example 2 is synthesized into particles of 10 to 100 nm, and they aggregate to form secondary particles of 5 to 20 μm, and Comparative Example 1 using the iron oxide of Preparation Example 2 Phosphorus of iron was produced as spherical primary particles at a level of 100 to 200 nm, and spherical primary particles were aggregated to form secondary particles of 5 to 20 µm.

In addition, the iron oxide (Fe ₂ O ₃ ) of Preparation Example 3 is synthesized into particles of 100 to 300 nm, and they aggregate to form secondary particles of 1 to 10 μm, and Comparative Example 2 using the iron oxide of Preparation Example 3 It was confirmed that the iron phosphide of was produced as spherical primary particles of 100 to 300 nm level, and spherical primary particles were aggregated to form secondary particles of 10 to 50 μm.

[Experimental Example 2] XRD (X-ray Diffraction) analysis

The iron oxide (Fe _1.766 O ₃ ) prepared in Preparation Example 1 and the iron phosphide prepared in Example 1 were subjected to XRD analysis (D4 Endeavor of Bruker Corporation).

_{3 is an XRD analysis result for Fe 1.766} O 3 prepared in Preparation Example, and FIG. 4 is a graph showing an XRD analysis result for iron phosphide prepared in Example.

_{It was confirmed through FIG. 3 that Fe 1.766} O ₃ was prepared with a greater amount of oxygen than _{Fe 2} O ₃ in the stable phase, and the crystalline iron phosphide (FeP) of the pure phase was confirmed by confirming the XRD peak (FIG. 4) of the Example. It was found that it was selectively manufactured.

[Experimental Example 3] Evaluation of polysulfide adsorption ability

The absorption ability of the iron oxide (Fe _1.766 O ₃ ) prepared in Preparation Example 1 and the lithium polysulfide of the iron phosphide prepared in Example 1 was analyzed by ultraviolet (UV, Agilent 8453 UV-visible spectrophotometer) absorbance analysis. It was confirmed through, and the results are shown in FIG. 5.

_{As a result of adsorbing lithium polysulfide by Fe 1.766} O ₃ and iron phosphide in the range of 200 to 1000 nm wavelength, it was confirmed that the intensity of ultraviolet absorbance decreased, as shown in FIG. 5, and Fe _1.766 O according to the preparation example _{Compared to 3} , it was found that the iron phosphide of Example had more excellent adsorption capacity of lithium polysulfide.

[Example 2] Preparation of lithium secondary battery including positive electrode to which iron phosphide was added

First, the iron phosphide prepared in Example 1 was added and dissolved in an amount of 10 parts by weight based on 100 parts by weight of the base solid content in water as a solvent. Thereafter, with respect to the obtained solution, a total of 100 parts by weight of a base solid including an active material, a conductive material and a binder, that is, 90 parts by weight of a sulfur-carbon composite (S/C 7:3) as an active material, and 5 parts by weight of Denka Black as a conductive material. Part and 5 parts by weight of styrene butadiene rubber/carboxymethyl cellulose (SBR/CMC 7:3) as a binder were added and mixed to prepare a positive electrode slurry composition.

Subsequently, the prepared slurry composition was coated on a current collector (Al Foil) and dried at 50° C. for 12 hours to prepare a positive electrode. At this time, the loading amount was 3.5mAh/cm ² , and the porosity of the electrode was 60%.

Thereafter, the prepared positive electrode, negative electrode, separator, and electrolyte were applied as follows to prepare a lithium secondary battery in the form of a coin cell (the positive electrode was punched and used as a 14phi circular electrode, and the polyethylene (PE) separator was 19phi, 150um lithium. Metal is used by punching with 16phi as the cathode).

[Comparative Example 3] Preparation of a lithium secondary battery including a positive electrode to which iron phosphide was not added

A lithium secondary battery was manufactured in the same manner as in Example 2, except that iron phosphide was not added to the positive electrode.

[Comparative Example 4] Preparation of a lithium secondary battery including a positive electrode to which iron oxide is added

A lithium secondary battery was prepared in the same manner as in Example 2, except that the iron oxide (Fe _1.766 O ₃ ) prepared in Preparation Example 1 was added in an amount of 10 parts by weight based on 100 parts by weight of the base solid content instead of iron phosphide. Was prepared.

[Comparative Example 5] Preparation of a lithium secondary battery including a positive electrode to which iron phosphide was added

A lithium secondary battery in the form of a coin cell was manufactured in the same manner as in Example 2, except that the iron phosphide prepared in Comparative Example 1 was used instead of the iron phosphide prepared in Example 1 in water as a solvent. .

[Comparative Example 6] Preparation of a lithium secondary battery including a positive electrode to which iron phosphide was added

A lithium secondary battery in the form of a coin cell was manufactured in the same manner as in Example 2, except that the iron phosphide prepared in Comparative Example 2 was used instead of the iron phosphide prepared in Example 1 in water as a solvent. .

[Experimental Example 4] Evaluation of discharge capacity of lithium secondary battery

To test the discharge capacity of a lithium secondary battery (lithium-sulfur battery) according to the type of cathode material, the discharge capacity of the lithium secondary battery (lithium-sulfur battery) prepared in Example 2 and Comparative Examples 3 to 6 was measured, respectively. And the results are shown in FIG. 6. At this time, the measurement current was 0.1 C and the voltage range was 1.8 to 2.6 V, and Table 1 below shows the configuration of the positive electrode of the lithium secondary battery prepared from Example 2 and Comparative Examples 3 to 6.

Lithium-sulfur battery (anode) Example 2 Sulfur-carbon composite + conductive material + binder + iron phosphide of Example 1
(Weight ratio-90:5:5:10,) Comparative Example 3 Sulfur-carbon composite + conductive material + binder
(Weight ratio-90:5:5) Comparative Example 4 Sulfur-carbon composite + conductive material + binder + Fe _1.766 O _{3 of Preparation Example 1}
(Weight ratio-90:5:5:10) Comparative Example 5 Sulfur-carbon composite + conductive material + binder + iron phosphide of Comparative Example 1
(Weight ratio-90:5:5:10,) Comparative Example 6 Sulfur-carbon composite + conductive material + binder + iron phosphide of Comparative Example 2
(Weight ratio-90:5:5:10,)

As shown in FIG. 6, it was confirmed that the initial discharge capacity of Comparative Example 4 to which _{iron oxide (Fe 1.766} O ₃ ) of Preparation Example 1 was added was higher than that of Comparative Example 3 in which iron phosphide or iron oxide was not added. In the case of Example 2 to which iron phosphide was added, it was confirmed that the initial discharge capacity was further increased compared to Comparative Example 4 to which iron oxide was applied. In addition, Example 2 using iron oxide made of iron oxide (Fe _1.766 O ₃ ) of Preparation Example 1, using iron phosphide made of iron oxide (Fe ₂ O ₃ ) of Preparation Example 2 or Preparation Example 3 It was confirmed that the initial discharge capacity was superior even compared to Comparative Examples 5 and 6. That is, through these results, it can be seen that iron phosphide (FeP), especially iron _{phosphide made of a specific iron oxide (Fe 1.766} O ₃ ), is effective in increasing the initial discharge capacity of a lithium-sulfur battery.

[Experimental Example 5] Evaluation of life characteristics of lithium secondary battery

In order to test the life characteristics of lithium secondary batteries (lithium-sulfur batteries) according to the type of cathode material, the discharge capacity of lithium secondary batteries (lithium-sulfur batteries) prepared from Example 2 and Comparative Examples 3 to 6 were measured, respectively. And the results are shown in FIG. 7. At this time, the measurement was repeated at 0.1C/0.1C (charge/discharge) 2.5 cycles, 0.2C/0.2C 3 cycles and then 0.3C/0.5C.

As shown in FIG. 7, the lithium-sulfur battery of Example 2 had higher discharge capacities in 0.1C, 0.2C and 0.5C sections compared to the lithium-sulfur batteries of Comparative Examples 3 to 6, and the life characteristics were also improved. I could see that. From these results, when iron phosphide (FeP), especially iron _{phosphide made of a specific iron oxide (Fe 1.766} O ₃ ), is added to the positive electrode of a lithium-sulfur battery, the discharge capacity effect is excellent and the life-impairing factor does not occur. I could confirm that.

Claims (11)

(1) heat-treating the iron hydrate at 140 to 160° C. for 12 to 24 hours to obtain an iron oxide represented by the following formula (1); And
(2) heat-treating by mixing the obtained iron oxide and NaH ₂ PO ₂ ·H ₂ O; Method for producing iron phosphide (FeP) comprising.
[Formula 1]
Fe _x O ₃ (however, 1.7 ≤ x <2) The method of claim 1, wherein the iron hydrate is Fe(NO ₃ ) ₃ ·9H ₂ O. The method of claim 1, further comprising a pretreatment step of drying the iron hydrate at 70 to 90° C. for 4 to 12 hours before heat treatment. delete _{The method of claim 1, wherein the mixing of the iron oxide and NaH 2} PO ₂ ·H ₂ O in the step (2) is performed in a weight ratio of 1:1 to 1:2. The method of claim 1, wherein the heat treatment in step (2) is performed at 200 to 300° C. for 1 to 3 hours. The method of claim 1, wherein the heat treatment in step (2) is performed under an inert gas atmosphere or in a state in which the inert gas is continuously introduced. The method according to claim 1, wherein the heat treatment in step (2) is characterized in that the heating rate is controlled within a range of 5 to 10° C. per minute. The method of claim 1, wherein the iron phosphide is characterized in that spherical primary particles are aggregated to form secondary particles. The method of claim 9, wherein the primary particles of the iron phosphide have a particle diameter of 50 to 300 nm. The method of claim 9, wherein the secondary particles of iron phosphide have a particle diameter of 0.5 to 15 µm.

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