JP5819782B2 - Method for producing positive electrode material for lithium secondary battery - Google Patents

Description

  The present invention relates to a positive electrode material for a lithium secondary battery, a method for producing the same, and a lithium secondary battery using the positive electrode material. Particularly, the positive electrode material used for a lithium secondary battery having a large discharge capacity and excellent cycle characteristics and the same It relates to a manufacturing method.

  Lithium secondary batteries are used as mobile terminal power supplies, electric vehicle power supplies, and backup power supplies because they have a high voltage of 3-4V, a high energy density, and a long cycle life.

This battery can be used as a secondary battery by the movement of lithium ions between the negative electrode and the positive electrode by the insertion / desorption or intercalation reaction of lithium ions at the positive electrode or the negative electrode. A conventional lithium secondary battery can express a high discharge voltage of 4 V class by using LiCoO ₂ (lithium cobaltate) as a positive electrode material, and has been widely put into practical use.

However, cobalt has a problem in that it is difficult to ensure safety because of its low resource amount and high price. Therefore, it is not desirable to use LiCoO ₂ in a large-sized lithium secondary battery used for an electric vehicle or the like that is expected to spread in the future. Therefore, a new positive electrode material replacing LiCoO ₂ is expected for the spread of large-sized lithium secondary batteries. In recent years, LiFePO ₄ (lithium iron phosphate) has attracted attention as an inexpensive alternative material with high safety. LiFePO ₄ exhibits a discharge voltage of about 3.5 V, is a positive electrode material that is excellent from the viewpoint of safety, and is being developed as a material suitable for a large-sized lithium secondary battery.

Despite that promising LiFePO _4, (often hydrogen flow under) a special atmosphere is not in the air due to the need for firing at, synthetic cost tended to rise.

On the other hand, as a similar iron-based material, a positive electrode material containing Fe ₂ (SO ₄ ) ₃ (iron sulfate) is reported as a material that can be synthesized by firing in air and can be expected to be low in cost ( Patent Document 1). As for the _voltage, the positive electrode material containing Fe 2 _{(SO 4) 3} has a high 3.6V voltage than in the case of LiFePO _4, it is promising as a positive electrode material (Non-Patent Document 1).

However, with regard to the charge / discharge cycle performance of Fe ₂ (SO ₄ ) ₃ , the irreversible capacity, which is the difference between the charge capacity and the discharge capacity, gradually increases, so that only about 20 cycles can be realized. This is considered to be mainly due to the low conductivity of Fe ₂ (SO ₄ ) ₃ .

  In general, as a method for improving the conductivity of such a low-conductivity positive electrode material, a method of mixing with a highly conductive material to form a composite, a method of coating the particle surface with carbon, and a nano-size of the particle are performed. A method of performing a ball mill by mixing a positive electrode material and a conductive material is known. Among them, a method of performing ball milling by mixing a positive electrode material and a conductive material is widely performed as a method for improving conductivity because it can be performed simply and at low cost.

Japanese Patent Laid-Open No. 6-119926

K. S. Nanjundaswami, A.N. K. Padhi, J .; B. Goodenough, S .; Okada, H .; Ohtsuka, H .; Arai, J. et al. Yamaki, Solid State Ionics 92 (1996) 1-10

However, mixing Fe ₂ (SO ₄ ) ₃ with a carbon conductive material is very difficult to mix compared to other materials because Fe ₂ (SO ₄ ) ₃ has a low bulk density. In particular, when mixed with a conductive material, Fe ₂ (SO ₄ ) ₃ particles and carbon particles are easily adsorbed, so that there is a problem that even when mixed, they are aggregated and cannot be uniformly dispersed. In addition, Fe ₂ (SO ₄ ) ₃ is a low-cost material, but has low electrical conductivity, and therefore has a problem that it has a large irreversible capacity during a charge / discharge cycle and it is difficult to discharge a large current.

The present invention has been made to solve the above-described conventional problems, and aims to reduce the irreversible capacity of Fe ₂ (SO ₄ ) ₃ and further improve the cycle characteristics.

The above-described problem can be overcome by using a technique peculiar to the present invention when combining Fe ₂ (SO ₄ ) ₃ particles with carbon, which is a conductive material, to form a composite.

  Specifically, a positive electrode material for a lithium secondary battery having improved conductivity effective for solving the problem can be obtained by using the following method.

That is, in the method for producing a positive electrode material for a lithium secondary battery of the present invention, Fe ₂ (SO ₄ ) ₃ (iron sulfate), a polymer, and carbon are mixed to obtain an Fe ₂ (SO ₄ ) ₃ / polymer / carbon mixture. And a step of heat-treating the obtained mixture. At this time, the polymer is selected from the group consisting of polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA), polyethylene glycol (PEG), polyvinyl chloride (PVC), and polyvinyl acetate (PVAc), and the molecular weight of the polymer is 50000 g. / Mol or less is preferable. _Further, Fe 2 _{(SO 4) 3} added amount of polymer to is preferably not less than 50% by weight 0.1% by weight. Furthermore, it is preferred that the mixing is carried out by ball mill (BM) treatment for at least 1 hour. Further, the heat treatment is preferably performed in a temperature range of 100 to 500 ° C. for a heat treatment time of 1 to 10 hours. Furthermore, it is preferred that the carbon in the mixture is at least 5% by weight.

Alternatively, the lithium secondary battery of the present invention uses a positive electrode containing a positive electrode material capable of inserting and removing lithium ions and a negative electrode or metallic lithium containing a negative electrode material capable of inserting and removing lithium ions as a negative electrode. , A lithium secondary battery in which a lithium ion conductive electrolyte is disposed between a positive electrode and a negative electrode, and obtained by the above-described method for producing a positive electrode material for a lithium secondary battery, Fe ₂ (SO ₄ ) ₃ / carbonation The polymer / carbon composite is contained in the positive electrode material.

Alternatively, the positive electrode material for a lithium secondary battery according to the present invention includes Fe ₂ (SO ₄ ) ₃ / carbonized polymer / carbon composite obtained by the above-described method for producing a positive electrode material for a lithium secondary battery. Features.

Thus, for example, by performing surface treatment of Fe ₂ (SO ₄ ) ₃ particles using a BM treatment in which a dry polymer is mixed as a simple technique, the diffusibility of lithium ions in the material is improved, and the irreversible capacity Is small, and excellent cycle characteristics can be realized.

  According to the present invention, for example, a positive electrode material for a lithium secondary battery having an excellent cycle characteristic and a large discharge capacity using a higher operating voltage of 3.6 V than a conventional olivine-type lithium iron phosphate-based positive electrode material. Can be provided.

(A) is a XRD pattern of _Fe 2 _{(SO 4) 3} powder without polymer treatment, and (b) is _Fe 2 _{(SO 4)} of Yes polymer treatment ₃ powder XRD patterns. (A) is a schematic sectional drawing showing the structure of the lithium secondary battery test cell used by the Example, (b) is a schematic sectional drawing showing the structure of the lithium secondary battery coin cell used by the comparative example. . It is a charging / discharging curve in the 3rd cycle of the sample which processed with (a) PVP non-addition (Table 1 No. 1) and (b) PVP 10 weight% addition sample (Table 1 No. 5). The vertical axis represents voltage, and the horizontal axis represents charge / discharge capacity. The results of the lithium secondary battery produced under the optimum conditions obtained in Examples 1 to 5 and the result of the lithium secondary battery of the comparative example are respectively referred to as “(present invention) iron sulfate” and “(comparative example)”. It is shown as cycle dependency of the discharge capacity up to 100 cycles as “iron phosphate”.

  The form for implementing this invention is demonstrated below.

The method for producing a positive electrode material for a lithium secondary battery according to the present invention includes a step of mixing Fe ₂ (SO ₄ ) ₃ (iron sulfate), a polymer, and carbon to obtain a Fe ₂ (SO ₄ ) ₃ / polymer / carbon mixture. including. For example, the Fe ₂ (SO ₄ ) ₃ obtained for improving the conductivity and carbon are mixed, and at the same time, the powdered polymer is mixed to obtain a uniform mixture of Fe ₂ (SO ₄ ) ₃ / polymer / carbon. Can do.

Fe ₂ (SO ₄ ) ₃ can be produced by heat-treating an iron sulfate compound at 400 to 700 ° C. using a method such as a solid phase reaction method, a sol-gel method, or a precipitation method. For example, in the solid phase reaction method, dehydration and decomposition reactions occur by heating ammonium iron (II) sulfate hexahydrate at a temperature of 400 to 700 ° C., and Fe ₂ (SO ₄ ) ₃ can be synthesized.

The polymer is preferably selected from the group consisting of polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA), polyethylene glycol (PEG), polyvinyl chloride (PVC), and polyvinyl acetate (PVAc), and the molecular weight of the polymer is It is preferable that it is 50000 g / mol or less. _The amount of polymer to Fe 2 _{(SO 4) 3} is preferably not more than 50 wt% 0.1 wt%.

As the carbon, ketjen black or acetylene black can be used. In the examples shown below, ketjen black was used as carbon, but other carbon blacks, activated carbons, graphites and the like can be used as long as they are inert to iron sulfate. The particle diameter of the carbon, Fe 2 _{(SO 4)} is not particularly limited as long as the particle diameter suitable for conducting the sufficient securing between ₃ particles and the battery case. In general, the entire surface of the iron sulfate particles is preferably in contact with the carbon particles from the viewpoint of securing a conductive path, and the carbon particle diameter is preferably smaller than the Fe ₂ (SO ₄ ) ₃ particle diameter. A fine particle having a BET specific surface area of 1000 m ² / g or more is suitable.

  Preferably, the mixture is at least 5% by weight carbon. When the carbon content is less than 5% by weight, the resistance of iron sulfate having low conductivity is not lowered, and Li ions hardly enter and exit, so that there is a disadvantage that the capacity of the positive electrode material is reduced.

  The mixing is desirably performed for at least 1 hour. The mixing is preferably performed by BM treatment, but other mixing methods may be used.

  The method for producing a positive electrode material for a lithium secondary battery of the present invention further includes a step of heat-treating the obtained mixture.

The heat treatment is preferably performed in a temperature range of 100 to 500 ° C. for 1 to 10 hours. By this heat treatment, the polymer is thermally decomposed, and a polymer / carbon composite obtained by Fe ₂ (SO ₄ ) ₃ / carbonization can be obtained. The heating atmosphere is preferably an inert gas such as nitrogen, Ar gas, or reducing gas. Alternatively, heat treatment can be performed in hydrogen or an activated carbon carbon atmosphere. The activated carbon atmosphere can be obtained by placing the mixture in a small crucible and placing the crucible in a large crucible containing activated carbon pellets.

The lithium secondary battery according to the present invention uses a positive electrode containing a positive electrode material capable of inserting and removing lithium ions and a negative electrode or metallic lithium containing a negative electrode material capable of inserting and removing lithium ions as a negative electrode. A lithium ion conductive electrolyte is disposed between the electrode and the negative electrode. In this case, the polymer-added carbon-treated composite (Fe ₂ (SO ₄ ) ₃ / carbonized polymer / carbon powder) obtained by the method for producing a positive electrode material for a lithium secondary battery is included in the positive electrode material. .

The positive electrode material for a lithium secondary battery of the present invention is a polymer-added carbon-treated composite (Fe ₂ (SO ₄ )) obtained by heat-treating Fe ₂ (SO ₄ ) ₃ (iron sulfate), a polymer, and carbon. ₃ / carbonized polymer / carbon) and the binder in a generally known weight ratio, (Fe ₂ (SO ₄ ) ₃ / carbonized polymer: carbon): binder = 95: 5 is there.

  The binder is not particularly limited to those used in the examples shown below, and in addition to polytetrafluoroethylene (PTFE) used in the examples, polyethylene, polypropylene, polyvinylidene fluoride (PVDF), and the like. A binder used in a normal lithium secondary battery can be used. Furthermore, the nonaqueous electrolyte solution and separator used in the normal lithium ion secondary battery can be used for the nonaqueous electrolyte solution and separator used in the following examples, and are not particularly limited.

The positive electrode material for a lithium secondary battery obtained by the present invention is formed into a pellet shape or a sheet shape as in the case of the conventional positive electrode material, and the negative electrode is a metal lithium, a spinel type that is a lithium-containing substance with excess lithium. Using lithium titanate (Li ₇ Ti ₅ O ₁₂ ), cobalt nitride (Li _2.6 Co _0.4 N), etc., in combination with a known organic electrolyte, coin type, cylindrical type, square type, A lithium secondary battery having a sheet shape or the like can be manufactured.

  Examples of a positive electrode material for a lithium secondary battery according to the present invention, a method for producing the same, and a lithium secondary battery using the positive electrode material will be described in detail below.

  In addition, this invention is not limited to the following Example, In the range which does not change the summary, it can implement by a change and a combination suitably.

[Example 1]
Fe ₂ (SO ₄ ) ₃ was synthesized in accordance with the chemical reaction shown below using Fe (NH ₄ ) ₂ (SO ₄ ) ₂ .6H ₂ O as a starting material.

Fe (NH ₄ ) ₂ (SO ₄ ) ₂ .6H ₂ O → 1 / 2Fe ₂ (SO ₄ ) ₃ + 2NH ₃ + 7H ₂ O + 1 / 2SO ₂

For the synthesis of iron sulfate, the hydrate was subjected to heat treatment (500 ° C., 5 hours, in air). The obtained powder has a diffraction pattern of Fe ₂ (SO ₄ ) ₃ having a known NASICON structure shown at the bottom, as shown in an XRD (X-ray diffraction) pattern without polymer treatment in FIG. (The database number is consistent with PDF No. 33-0679), and it was confirmed that Fe ₂ (SO ₄ ) ₃ can be synthesized by this method.

Polypyrrole pyrrolidone (molecular weight: 40000 g / mol, hereinafter referred to as PVP) was added to Fe ₂ (SO ₄ ) ₃ synthesized above and mixed with ketjen black (hereinafter referred to as KB) as a carbon material, and a ball mill (BM for 24 hours) was added. ) Processing. The addition amount (% by weight) of PVP was not added (0), 0.05, 0.1, 5, 10, 50, and 75% by weight. The experimental conditions of Example 1 are shown in Table 1. In addition, Example 1 No. No. 5, “Examples 1, 2 and 3” are described in No. 5 below. This means that a positive electrode material having a polymer component and weight of 5 is used not only in Example 1 but also in Examples 2 and 3.

By heating the BM-treated Fe ₂ (SO ₄ ) ₃ / polymer / carbon mixture in an activated carbon carbon atmosphere at 300 ° C. for 5 hours, the polymer is decomposed, and the polymer-added carbon-treated iron sulfate composite (Fe ₂ ( SO _{4) 3} / carbonized polymer / carbon) was obtained. _{Incidentally, Fe 2 (SO 4) 3} / carbonized the weight ratio of polymer / carbon used in the embodiments described _{below, Fe 2 (SO 4) 3} : Polymer: Carbon = 67: 6.7: 26. 3. The XRD (X-ray diffraction) pattern of this polymer-added carbon-treated iron sulfate composite (Fe ₂ (SO ₄ ) ₃ / carbonized polymer / carbon) powder is shown in FIG. This indicates that the sample has a single-layer NASICON structure, and it was confirmed that the Fe ₂ (SO ₄ ) ₃ was not modified by the BM treatment.

This polymer-added carbon-treated composite (Fe ₂ (SO ₄ ) ₃ / carbonized polymer / carbon) powder is further mixed with polytetrafluoroethylene (hereinafter referred to as PTFE) as a binder, and the pellets are formed by roll pressing. It produced and was set as the positive electrode. At this time, the mixing ratio of the polymer-added carbon-treated Fe ₂ (SO ₄ ) ₃ and the carbon conductive material was 95: 5 by weight. In addition, using lithium metal as a negative electrode, a 2320-size coin cell (hereinafter referred to as a test cell) was manufactured as a lithium secondary battery, and the electrode characteristics were evaluated.

  FIG. 2A shows a schematic cross-sectional view of the test cell used in this example. Hereinafter, a detailed manufacturing method of the test cell will be described.

First, a polymer-added carbon-treated iron sulfate composite (Fe ₂ (SO ₄ ) ₃ / carbonized polymer / carbon) powder and PTFE serving as a binder were pulverized and mixed with a roughing machine, and then rolled by a roll press. The sheet was rolled into a thickness of 5 mm.

The obtained sheet was punched into a circular shape having a diameter of 15 mm to produce a positive electrode pellet 2. The positive electrode pellet 2 was vacuum-dried overnight. The composition of the positive electrode pellet 2 was, as a weight ratio, a polymer-added carbon-treated iron sulfate composite (Fe ₂ (SO ₄ ) ₃ / carbonized polymer / carbon) powder: PTFE = 95: 5.

  Next, a titanium mesh with a diameter of 15 mm is welded to the inside of the positive electrode case 1 of the test cell, the positive electrode pellet 2 is lightly crimped on the mesh, and the positive electrode pellet 2 is covered with a titanium mesh with a diameter of 17 mm. After being welded to the positive electrode case 1, the positive electrode pellet 2 was fixed to the positive electrode case 1 by further pressing.

  On the other hand, a nickel mesh was welded inside the negative electrode case 5, a metal lithium sheet (thickness 0.5 mm) having a diameter of 17 mm was pressure-bonded thereon, and the gasket 3 was set on the outer edge of the negative electrode case 5.

Next, about 2 mL of 1 mol / L LiPF ₆ / ethylene carbonate (EC) -dimethyl carbonate (DMC) was poured as a non-aqueous electrolyte solution 7 into the positive electrode case 1 to which the positive electrode pellet 2 was fixed, and a polyethylene separator 6 was immersed therein. Later, the negative electrode case 5 to which the gasket 3 was attached was covered from above the positive electrode case 1, and the whole was caulked to produce a test cell.

Using the prepared test cell, a charge / discharge test was performed under the conditions of a current density of 1 mA / cm ² and a voltage range of 3 to 4.2 V.

  The charge / discharge curve in the third cycle is shown in FIG. 3A is a charge / discharge curve of a sample treated with a PVP non-added sample (Table 1 No. 1), and FIG. As shown in FIG. 3, the charge / discharge voltage of these iron sulfate materials is a high voltage of 3.6 V or higher with respect to the lithium negative electrode, and thus is a promising material as a positive electrode material of a lithium secondary battery.

  The results of the charge / discharge cycle test of the above samples are shown in Table 1. The sample without PVP (Table 1 No. 1) showed a 5% capacity decrease (capacity decreased to 95%) at the 20th cycle, but the sample with PVP added (Table 1 No. 2-7). ) Was found to require at least 50 cycles or more for a 5% capacity reduction. Among them, in the case of addition of 10% by weight of PVP (Table 1 No. 5), the capacity decreased by 5% in 100 cycles.

  Thus, compared with the sample without PVP (No. 1 in Table 1), the discharge capacity of the sample with PVP added is large, the capacity is less deteriorated, and the number of cycles until the capacity reduction reaches 5% increases. It can be seen that the cycle stability is improved. However, in the case of addition of 75% by weight of PVP (No. 7 in Table 1), it can be seen that the cycle stability decreases, and it is considered that the amount of polymer added has an appropriate range. This is considered to suggest that a residual portion of the polymer that cannot be decomposed by the heat treatment remains on the surface of the iron sulfate particle due to the amount of the polymer. When the amount of polymer added is large, it is desirable to further optimize the heat treatment conditions.

  Such improvement in characteristics is considered to be achieved by heat treatment after the BM treatment, whereby iron sulfate particles and carbon are combined, and improvement in cycle characteristics is achieved with an increase in capacity due to an increase in conductivity on the surface of the positive electrode material.

  As shown in Table 1, irreversible capacity was reduced by performing the treatment under the predetermined conditions, which led to improvement of the charge / discharge cycle. In particular, Table 1 In the case of 5, the capacity retention rate at the 100th cycle showed the highest value of 95% or more. However, it was found that a sample deviating from the polymer addition condition (polymer addition amount (x): 0.1 wt% ≦ x ≦ 50 wt%) has a low effect of improving the capacity and cycle characteristics.

[Example 2]
In Example 2, the influence on the BM treatment time after the preparation of the Fe ₂ (SO ₄ ) ₃ / polymer / carbon mixture was examined. The mixture is shown in Table 1. BM treatment was performed using a polymer having the same component as component 5 and mixing. The BM treatment time was 0.5 hour, 1 hour, 5 hours, 24 hours (Example 1), and 48 hours. Thereafter, the mixture was heat-treated. The heat treatment was performed for 5 hours under the same conditions as in Example 1.

  Next, a coin cell was produced from the pellet in the same manner as in Example 1 using the powder produced in Example 2 with a different BM treatment time, and charge / discharge cycle measurement was performed. The results are shown in Table 2. In addition, Example 2 No. The description of “Examples 1, 2, 3, 4” in No. 4 is “No. This means that the BM processing time of 4 is applied not only in the second embodiment but also in the first, third, and fourth embodiments.

  From Table 2, it was confirmed that samples other than samples subjected to BM treatment for 0.5 hours showed a relatively large discharge capacity of 150 mAh / g. It was found that the effect of improving the capacity and improving the cycle characteristics is small in the short BM treatment of 0.5 hours. This is presumably because the polymers aggregate when they deviate from the conditions of the present invention, making it difficult to form a uniform state, and there is a limit to the improvement in conductivity.

  Further, when the BM treatment for 1 hour or more was performed, the discharge capacity was improved by 40% as compared with the case where PVP was not added. However, even if the treatment was performed for 24 hours or longer, the improvement effect was low. Therefore, considering the cost, the preferred processing time was set to 24 hours under the conditions of Example 2.

These properties improve, by performing the processing of the appropriate _time, Fe 2 _{(SO 4) 3} and polymer are mixed in a high dispersion degree, _Fe 2 _{(SO 4)} after heat treatment ₃ / carbonized polymer / carbon Since the dispersity of the composite is also improved, the conductive path of Fe ₂ (SO ₄ ) ₃ and carbon increases, and as a result, insertion / extraction of lithium ions in the charge / discharge cycle can proceed more efficiently. This is probably because of this.

[Example 3]
Next, the heat treatment conditions after the BM treatment were examined. First, the influence of the heat treatment temperature (50, 100, 300, 500, 600 ° C.) was examined.

Using the heat treated Fe ₂ (SO ₄ ) ₃ / carbonized polymer / carbon composite, a coin cell was prepared in the same manner as in Examples 1 and 2, and a charge / discharge cycle test was performed. The results are summarized in Table 3. In addition, Example 3 No. No. 5, “Examples 1, 2, 3 and 4” are described in No. 5. 5 means that the temperature, heating time, and atmosphere of 5 are used not only in Example 3, but also in Examples 1, 2, and 4.

In the heat treatment process, the polymer is thermally decomposed by the heat treatment of the iron sulfate (Fe ₂ (SO ₄ ) ₃ / polymer / carbon) mixture in which the polymer and the conductive material carbon are mixed, and the residual carbon and the conductive carbon KB are Fe ₂ ( SO ₄ ) ₃ remains on the surface of the particle or in the vicinity of the particle and contributes to the improvement of conductivity, and as a result, the battery characteristics are considered to be improved.

  First, the heat treatment time was fixed at 5 hours, and the influence of the heat treatment temperature was examined.

  From Table 3, it was found that the discharge capacity was reduced when it was performed at a low temperature of 80 ° C., compared with the case where PVP was not added. When the treatment is performed at such a low temperature, it is considered that the added polymer is not sufficiently decomposed and the conductivity exists because the polymer exists as an insulator.

On the other hand, a decrease in capacity was also confirmed when heated at a high temperature of 500 ° C. or higher. This is because Fe ₂ (SO ₄ ) ₃ is oxidized and decomposed by high-temperature heat treatment, and iron oxide is mixed. When the powder after heat treatment was actually evaluated by XRD, an iron oxide peak was confirmed.

It has been found that 300 ° C. is optimal as the heat treatment temperature. This is because 300 ° C. is near the decomposition temperature of the polymer, a carbon film is efficiently formed in the vicinity of the Fe ₂ (SO ₄ ) ₃ particles, there is no decomposition of Fe ₂ (SO ₄ ) ₃ , This is thought to be due to the achievement of improvement.

Next, the heat treatment temperature was fixed at the optimum temperature of 300 ° C., and the influence of the treatment time (1, 5, 10, 15 hours) was examined. The results are shown in Table 3. Although the cycle characteristics were improved up to the heating time of 5 hours, there was no change at 10 hours, and a slight decrease in the cycle characteristics was seen at 15 hours. This is considered to be due to the modification of Fe ₂ (SO ₄ ) ₃ or carbon by the heat treatment for a long time.

  Finally, the influence of the atmosphere during the heat treatment was examined. Optimized temperature and time conditions of 300 degrees and 5 hours for 5 types of atmosphere (nitrogen and Ar as inert gas, hydrogen (diluted to 5% with nitrogen) and activated carbon (activated carbon atmosphere, carbothermal) Also referred to as reduction method) and air). As shown in Table 3, no significant change was observed in the treatment of the inert gases nitrogen and Ar and air. On the other hand, it was found that the sample treated with hydrogen and activated carbon showed a more stable cycle. Comparing the cases of reducing atmospheres of hydrogen and activated carbon (No. 8 and No. 5), the capacity and cycle characteristics were almost the same. In view of cost, the carbothermal reduction method is considered more desirable.

  As shown in Table 3, it was found that temperature was the most important factor for the heat treatment conditions. In the case of the PVP used this time, 300 ° C. was the optimum condition. Thus, it was confirmed that the battery characteristics can be improved by performing the heat treatment under conditions close to the polymer decomposition temperature.

[Example 4]
Next, the type and molecular weight of the polymer were examined. First, the optimum conditions obtained in Examples 1 to 3 were fixed, and the difference depending on the polymer type was examined. Specifically, the polymer addition amount was fixed at 10% by weight, the BM treatment time was 24 hours, and the heat treatment conditions were fixed at 300 ° C. for 5 hours.

Next, in the same manner as in Examples 1 to 3, using the obtained composite (polymer added carbon-treated iron sulfate: Fe ₂ (SO ₄ ) ₃ / carbonized polymer / KB composite) powder, And a charge / discharge cycle test was conducted.

  The results are shown in Table 4. In addition, Example 4 No. No. 3 “Examples 1, 2, 3 and 4” are described in No. 3. This means that a polymer with a molecular weight of 3 is used not only in Example 4 but also in Examples 1, 2 and 3.

  As shown in Table 4, the effect of improving capacity and cycle characteristics differed depending on the type of polymer. Compared to the sample with no polymer added, the capacity was improved by at least 20%. In addition, the cycle characteristics were improved by at least four times in the number of cycles when the capacity was reduced by 5%.

  Furthermore, when the polymerization degree was different with the same polymer, that is, the influence of molecular weight was investigated. As a result, as shown in Table 4, both PVP and PVA exhibited excellent performance in both discharge capacity and cycle characteristics when the degree of polymerization was low, that is, when the molecular weight was low.

  This is thought to be because, in the case of a high molecular weight, the polymer is difficult to decompose, and a part of the polymer remains in the powder as an insulator. Therefore, it was found that in order to obtain excellent battery characteristics, it is desirable to use one having a lower molecular weight (low polymerization degree) although it varies depending on the polymer type used.

[Example 5]
Next, using various polymers, the optimum processing conditions were examined. As the polymer, among the polymers examined in Example 4, PEG, PVAc, PVA, and PVP having relatively low molecular weight were used, and the addition amount of the polymer, BM treatment time, heat treatment temperature, and time were examined.

Using the obtained Fe ₂ (SO ₄ ) ₃ / carbonized polymer / carbon composite powder, coin cells were produced in the same manner as in Examples 1 to 4, and a charge / discharge cycle test was performed.

  The results are shown in Tables 5 (a) to (d) for each polymer.

  As shown in Table 5, the capacity and cycle characteristics were improved differently depending on the type of polymer. Among the examined conditions, the heat treatment temperature has the most influence on the charge / discharge characteristics. This is considered to be because the decomposition temperature differs depending on the type of polymer. When heat treatment is performed at a temperature lower than the decomposition temperature of the polymer, the polymer does not decompose and remains in the composite as it is. These unreacted polymers are considered to be due to the loss of conductivity by being present as insulators, as in Example 3.

On the other hand, a decrease in capacity was also confirmed when heated at a temperature higher than the decomposition temperature of the polymer. This is presumably because the carbon film obtained by the decomposition of the polymer becomes thick or particles, so that the mixing with the conductive carbon KB becomes non-uniform. The optimum temperatures for PEG200000, PVAc170000, PVA2000, and PVP8000 were found to be 400 ° C, 250 ° C, 300 ° C, and 350 ° C, respectively. Furthermore, when heat treatment is performed at a temperature of 500 ° C. or higher, as described in Example 3, Fe ₂ (SO ₄ ) ₃ is oxidatively decomposed by high-temperature heat treatment and iron oxide is mixed. When the powder after heat treatment was actually evaluated by XRD, an iron oxide peak was confirmed.

  In addition to the above heat treatment temperature, heat treatment time was examined. However, unlike the heat treatment temperature, it was confirmed that the conditions for the heat treatment time were not significantly different from the study with PVP performed in Example 3.

  Next, the time of BM treatment used at the time of mixing was examined. It was also confirmed that the BM treatment time was not greatly related to the type of polymer. When a high molecular weight polymer was used, it was found that mixing by BM treatment was difficult when the molecular size of the polymer was relatively large (PVA molecular weight 96000 g / mol). This is presumably because iron sulfate and carbon, which are active material materials, were adsorbed on the polymer surface and a uniform mixture could not be obtained.

[Comparative example]
As a comparative example, a coin cell type lithium secondary battery was produced under the same conditions as in Example 1 except that olivine type lithium iron phosphate was used for the positive electrode. Specifically, the positive electrode pellet 20 was prepared so that the olivine-type lithium iron phosphate, acetylene black, and PTFE were in a weight ratio of LiFePO ₄ : acetylene black: PTFE = 70: 25: 5, and the electrolytic solution, the negative electrode In the same manner as in Example 1, a coin cell type lithium secondary battery was produced.

  FIG. 2B shows a schematic cross-sectional view of a lithium secondary battery in a comparative example. In FIG. 2B, the same members as those in FIG. 2A are denoted by the same reference numerals, and the description thereof is omitted.

  Table 6 shows the results of a charge / discharge test of 100 cycles under the same conditions as in Example 1 for the manufactured coin cell type lithium secondary battery.

  Further, the results of the lithium secondary battery produced under the optimum conditions obtained in Examples 1 to 5 and the result of the lithium secondary battery of the comparative example are respectively represented by “(present invention) iron sulfate” and “(comparison). FIG. 4 shows the cycle dependency of the discharge capacity up to 100 cycles as “example) iron phosphate”.

  As shown in Table 6, the lithium secondary battery manufactured under the optimum conditions showed an operating voltage 0.2 V higher than that of the olivine type lithium iron phosphate, and excellent cycle characteristics (capacity after 200 cycles). The maintenance rate was 95%).

As described above, the positive electrode including the Fe ₂ (SO ₄ ) ₃ / carbonized polymer / carbon composite according to the present invention has an improved discharge capacity and higher operating voltage than the conventional olivine-type iron phosphate-based positive electrode material. Further, it was confirmed that the positive electrode material was capable of realizing a large-capacity lithium secondary battery excellent in cycle characteristics.

1: Positive electrode case 2: Positive electrode pellet containing iron sulfate 3: Gasket 4: Metal lithium 5: Negative electrode case 6: Separator 7: Nonaqueous electrolyte 20: Positive electrode pellet containing olivine type lithium iron phosphate

Claims (6)

Mixing Fe ₂ (SO ₄ ) ₃ (iron sulfate), polymer, and carbon to obtain a Fe ₂ (SO ₄ ) ₃ / polymer / carbon mixture;
Heat-treating the resulting mixture,
A method for producing a positive electrode material for a lithium secondary battery, wherein the heat treatment step is a heat treatment in which a polymer is pyrolyzed to carbonize.   The polymer is selected from the group consisting of polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA), polyethylene glycol (PEG), polyvinyl chloride (PVC), and polyvinyl acetate (PVAc), and the molecular weight of the polymer is 50000 g / mol or less The manufacturing method of the positive electrode material for lithium secondary batteries of Claim 1 characterized by the above-mentioned. The method for producing a positive electrode material for a lithium secondary battery according to claim 1 or 2, wherein the amount of the polymer added to Fe ₂ (SO ₄ ) ₃ is 0.1 wt% or more and 50 wt% or less.   Mixing is performed by a ball mill process for at least 1 hour, The manufacturing method of the positive electrode material for lithium secondary batteries as described in any one of Claims 1-3 characterized by the above-mentioned.   The method for producing a positive electrode material for a lithium secondary battery according to any one of claims 1 to 4, wherein the heat treatment is performed in a temperature range of 100 to 500 ° C for a heat treatment time of 1 to 10 hours.   The method for producing a positive electrode material for a lithium secondary battery according to any one of claims 1 to 5, wherein the mixture contains at least 5% by weight of carbon.

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