JP4963675B2 - Lithium secondary battery, positive electrode active material thereof, and method of manufacturing the same - Google Patents

Description

  The present invention relates to a positive electrode active material for a lithium secondary battery, a method for producing the same, and a lithium secondary battery using the positive electrode active material.

  Due to recent rapid developments in the electronics field, electronic devices have become more sophisticated, smaller and more portable. There is an increasing demand for rechargeable high energy density secondary batteries used in these electronic devices. Conventionally, as secondary batteries mounted on these electronic devices, there are lead storage batteries, nickel-cadmium batteries, nickel-hydrogen batteries, etc., but those having higher energy density are required, and recently, lithium metal, lithium alloys, Alternatively, a lithium secondary battery combining a negative electrode using a carbon material that can electrochemically occlude and release lithium ions as a negative electrode active material and a positive electrode active material using a lithium-containing composite oxide or a chalcogen compound as a positive electrode active material. Secondary batteries have been researched and developed and put into practical use in part. This type of battery has a high battery voltage and a higher energy density per weight and volume than the above-described conventional secondary battery, and is regarded as a highly anticipated secondary battery in the future.

Conventionally, LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , and LiMn ₂ O ₄ are mainly used as positive electrode active materials for this type of battery, but cobalt and nickel used in these materials have a small reserve amount. It was produced only in a limited area. In the future, the positive electrode active material for lithium ion secondary batteries, for which demand is expected to increase further, has had problems in terms of both price and stable supply of raw materials. From the standpoint of safety, these active materials may be problematic due to their high reactivity, and manganese-based positive electrode active materials are relatively inexpensive materials, but the stability of cycle characteristics is problematic. There was something.

  As a result, lithium iron phosphate made from high-cost, inexpensive iron as a raw material or a material obtained by substituting part of iron in lithium iron phosphate with other elements should be used as the positive electrode active material for lithium secondary batteries. Has been proposed.

For example, represented by the composition formula, AyMPO ₄ [A is an alkali metal, M is a transition metal, 0 <y <2, where M is a phase containing only Fe, and AyFePO ₄ (0 <y <2) is excluded) It has been proposed to use a phosphoric acid compound as a positive electrode active material. (For example, Patent Document 1).

JP-A-9-134724 (Claim 1) In order to obtain a sufficient charge / discharge capacity, the general formula Li2Fe1-yXyPO4 (0≤y≤0.3, 0≤z≤1, X: magnesium) As a lithium secondary battery positive electrode active material of a lithium iron phosphate material that is electrically conductive and has a redox potential on the olivine-structured lithium iron phosphate material powder given by, cobalt, nickel, zinc) A positive electrode active material for a lithium secondary battery in which conductive fine particles having a potential no lower than the potential is supported is known. (For example, patent document 2).

JP, 2001-110414 (Claim 1) Further, a solution, dispersion or suspension containing a Li source, an Fe (III) source, a P source, a C source, and an O source in a high temperature atmosphere. There is a known method for producing an electrode material which is sprayed to form a precursor and heat-treated in a reducing atmosphere or an inert atmosphere. (For example, patent document 3).

JP-A-2005-116392 (Claim 1) Further, the particle surface of LiFePO4 obtained from a ferrous phosphate hydrate (Fe3 (PO4) 2.8H2O), lithium phosphate (Li3PO4) and a carbonaceous material precursor is used. Lithium iron phosphorus composite oxide carbon composite coated with a carbonaceous material, wherein the carbon composite has a mean particle size of 0.5 μm or less and is a lithium iron phosphorus composite oxide positive electrode for a lithium secondary battery Active materials are known.

JP2003-292309 (Claim 1)

However, the lithium phosphate M-based material having the olivine structure of prior art 1 (M is a transition element, the same applies hereinafter) has a slow lithium insertion / release reaction during battery charging / discharging, and LiCoO ₂ that has been conventionally used. Since the electric resistance is much larger than that of lithium metal oxide, there is a problem that when charging / discharging, resistance polarization increases, a sufficient discharge capacity cannot be obtained, and the rate characteristic is poor. In particular, when charging and discharging a large current, these problems are remarkable.

  As a method for solving these problems, the olivine type M lithium phosphate material particles are made finer to increase the reaction area, facilitate the diffusion of lithium ions, and the distance that electricity flows inside the M lithium phosphate material particles. It is considered to shorten. However, the fine particles of the olivine-type M lithium phosphate material are likely to cause secondary aggregation when mixed with a conductive material such as carbon black at the time of electrode preparation. Due to the point contact, a sufficient current collecting effect could not be obtained and the electrical resistance was very large. For this reason, the active material in the central part of the aggregated particles does not cause electron conduction even when the battery is charged / discharged, and the charge / discharge capacity decreases.

  In order to solve such a problem, in Japanese Patent Application Laid-Open No. 2001-110414 of Prior Art 2, fine particles of silver, carbon, platinum, palladium, etc. that are conductive and nobler than the redox potential are formed on the fine particles of the lithium iron phosphate material. Although it has been proposed to support the metal particles, there is a problem that the use of a reducing agent is necessary because of the metal particle support, and further, ball milling or bead milling is necessary, and the process becomes complicated. Furthermore, since it is a metal, it is susceptible to chemical changes and has a problem in stability, and since the particles are connected to each other, the above-described problem of current collection cannot be sufficiently solved.

In addition, the method of introducing a suspension of carbon black or an organic compound as a carbon source of the prior art 3 at the time of synthesizing the lithium iron phosphate material and uniformly dispersing the suspension on the surface of the LiFePO ₄ particles has a dispersion effect. Insufficient current collection effect could not be achieved.

  Furthermore, the lithium iron phosphorus-based composite oxide carbon composite obtained by coating the particle surface of the prior art 4 with a carbonaceous material has a problem that it is difficult to control because it requires advanced particle size control as a battery active material. there were.

The surface of the particles of the olivine-type lithium M phosphate material is considered to be in an active amorphous form because it has lower crystallinity than the bulk. For this reason, a divalent metal is easily oxidized by being left in the air, and is converted into an oxide such as M ₂ O ₃ or trivalent phosphoric acid M having higher resistance. As a result, a large polarization is generated in the initial charge, so that there are problems that activation is complicated and resistance components remain if the leaving conditions are severe. Furthermore, trivalent M easily elutes into the electrolyte solution in the battery, and migrates to the negative electrode surface by electrophoresis during battery operation, causing the solid electrolyte film (SEI) to break down and deteriorating the battery life performance. It was.

  Accordingly, the present invention is intended to obtain a positive electrode active material for a lithium secondary battery using an inexpensive lithium iron phosphate material, improving discharge characteristics at high current and high temperature characteristics, and suppressing capacity decrease during standing. It is.

  In the present invention, an olivine-type M lithium phosphate (M is a transition metal, the same shall apply hereinafter) obtained by a conventional method or an olivine-type M lithium compounded with carbon is added to a reducing acid or acid. A method for producing a positive electrode active material for a lithium secondary battery characterized by washing with an aqueous solution (Claim 1), an olivine-type M lithium obtained by a conventional method or an olivine-type M lithium compounded with carbon being reduced. A lithium secondary battery positive electrode active material washed with an acid having acid or an aqueous solution obtained by adding a reducing agent to the acid (Claim 2), and the transition metal of the olivine-type M lithium phosphate is iron, cobalt, manganese, or a part thereof The method for producing a positive electrode active material for a lithium secondary battery according to claim 1, wherein the compound is a compound or a mixture thereof substituted with another element (Claim 3), The lithium secondary battery positive electrode active material according to claim 1, wherein the transfer metal is iron, cobalt, manganese, or a compound thereof partially substituted with other elements or a mixture thereof. A lithium secondary battery using the positive electrode active material according to 1 or 4 (Claim 5) and the positive electrode active material according to Claim 1 or 4 are dried without being dried. A lithium secondary battery obtained by applying a kneaded paste with a dispersant added to the current collector and applying the paste to a current collector, followed by drying and removing the dispersant (Claim 6).

According to the present invention, olivine-type M lithium phosphate (M is a divalent metal) particles are washed with a reducing acid or an aqueous solution in which a reducing agent is added to the acid, and the surface of the lithium M phosphate particles has high resistance. Iron phosphate or lithium phosphate, which is a rare impurity, is removed to reduce the electrical resistance between the active materials and between the active material and the conductive agent, improving the current collection effect and improving the high rate discharge It can be set as the lithium secondary battery using the olivine type | mold M lithium phosphate positive electrode active material. Further, according to the present invention, since surface impurities are dissolved in advance with an acid, the elution of iron in the battery is suppressed, and it is possible to suppress a decrease in capacity during operation at high temperature or during standing.

By an ordinary method, olivine-type M lithium compounded with olivine-type M lithium phosphate or carbon is produced. As an example of the olivine-type M lithium phosphate, for example, a method for synthesizing lithium iron phosphate by a hydrothermal method will be described as follows. 4.63 g of lithium phosphate and 7.95 g of divalent iron chloride tetrahydrate as a divalent iron oxide were placed in a pressure vessel together with 200 ml of distilled water, and this was sealed with argon gas. This pressure vessel was placed in a 180 ° C. oil bath and allowed to react for 48 hours. Then, after cooling to room temperature, the contents were taken out from the pressure vessel and washed with water, and dried at 100 ° C. to obtain 6.47 g of a powder sample. When the obtained powder was examined by X-ray diffraction, it was lithium iron phosphate. Moreover, it was confirmed by scanning microscope (SEM) observation that it has a particle size of 20 nm to 200 nm.
Further, in the method of preparing olivine-type M lithium combined with carbon, ammonia water is added to an aqueous solution of 2.07 g of lithium nitrate and 10.26 g of ferrous nitrate nonahydrate until neutral, The mixed precipitate of iron and lithium phosphate is collected by filtration. To this, 2.25 g of sucrose is added, and this precursor is pre-baked in argon gas at 350 ° C. for 10 hours, followed by main baking at 600 ° C. for 16 hours to obtain lithium iron phosphate complexed with carbon.

  The thus obtained olivine-type lithium phosphate M or lithium iron phosphate powder complexed with carbon is washed with a reducing acid or an aqueous solution obtained by adding a reducing agent to the acid. As the acid having reducibility, ascorbic acid, dithionite, oxalic acid, formic acid and the like can be used. In the combination of an acid and a reducing agent, weak organic acids such as acetic acid, citric acid and lactic acid can be used as the acid, and aldehydes such as formaldehyde and acetaldehyde, hydrazine and hydroquinone can be used as the reducing agent. The concentration of the treatment liquid is preferably about 0.01 to 0.1N. The treatment temperature may be around room temperature. By treating with this treatment solution, high-resistance trivalent iron present on the surface of lithium iron phosphate particles is reduced to divalent, and impurities such as lithium phosphate and iron phosphate are removed by elution into the acidic solution. The As a result, it is possible to improve the current collection efficiency by removing the high resistance layer and to suppress the elution of iron in the battery in a high temperature environment by removing the acid elution in advance.

  In the process of making the positive electrode material fine particles by the hydrothermal synthesis method and drying to obtain the active material powder, electrostatic aggregation is caused, and it is difficult to sufficiently disperse the aggregated particles even using a high-speed disper mixer. As a result, the aggregated particles become nuclei and cause lumps in the paste. Therefore, in the present invention, a conductive agent, a binder and a thickener are added without drying the olivine-type lithium M phosphate, and a paste kneaded with a dispersant is applied to the current collector. As a result, it is possible to suppress lumps in the paste due to electrostatic aggregation.

Example 1
(Hydrothermal synthesis method)
Lithium iron phosphate was synthesized by hydrothermal synthesis. 46.3 g of lithium phosphate and 79.5 g of divalent iron chloride tetrahydrate as a divalent iron compound were placed in a pressure vessel together with 200 ml of distilled water, and the inside was replaced with argon gas and then sealed. The pressure vessel was reacted in an oil bath at 180 ° C. for 48 hours. Thereafter, the pressure vessel was naturally allowed to cool to room temperature, the contents were taken out, and dried at 100 ° C. to obtain 64.7 g of a powder sample. A small amount of the powder obtained here was collected and X-ray diffractometrically confirmed to be lithium iron phosphate. Furthermore, when observed with a scanning electron microscope (SEM), it was confirmed that it had a particle size of 20-200 nm.

  Separately, 20 ml of a 0.1N ascorbic acid aqueous solution as a reducing acid was prepared, put into a stainless steel container, and charged with 6.47 g of the lithium iron phosphate obtained above, and this was added at room temperature for 30 minutes. After stirring, the mixture was filtered, further thoroughly washed with water, and vacuum dried at 60 ° C. for 2 hours.

Next, acetylene black as a conductive agent was mixed with lithium iron phosphate subjected to the ascorbic acid treatment so as to have a total carbon amount of 10%. This mixed powder and polyvinylidene fluoride (PVF) as a binder are mixed at a weight ratio of 95: 5, N-methyl-2-pyrrolidone (NMP) is added, and a bead mill with a bead diameter of 1 mm is sufficient. A positive electrode slurry was obtained by mixing. Next, this positive electrode slurry was applied to an aluminum foil current collector having a thickness of 20 μm and dried at 120 ° C. for 30 minutes. Then, this was rolled with a roll press and punched into a 2 cm ² disk shape to obtain a positive electrode.

The negative electrode was prepared by mixing artificial graphite (average particle size 5 μm, d ₀₀₂ = 0.337 nm, Lc = 58 nm) and polyvinylidene fluoride in a weight ratio of 95: 5, and mixing it with N-methyl- 2-Pyrrolidone was added and sufficiently kneaded to obtain a negative electrode paste. Next, this negative electrode paste was applied to a copper foil current collector having a thickness of 20 μm, dried naturally at a room temperature of 25 ° C., and further dried at 130 ° C. under reduced pressure for 12 hours. Then, this was rolled with a roll press and punched into a 2 cm ² disk shape to obtain a negative electrode.

The electrolytic solution was prepared by dissolving LiPF ₆ at a concentration of 1M in a mixed solvent in which ethylene carbonate and diethylene carbonate were mixed at a volume ratio of 1: 1. The amount of water in the electrolytic solution before introducing LiPF ₆ was less than 15 ppm.

  A coin-type lithium secondary battery was produced using the battery member described above. The production atmosphere was a dew point of −50 ° C. or lower. Each electrode was used by being crimped to a battery case with a current collector. A coin-type lithium secondary battery having a diameter of 25 mm and a thickness of 1.6 mm was produced using the above positive electrode, negative electrode, electrolyte and separator.

(Example 2)
A coin-type lithium secondary battery having a diameter of 25 mm and a thickness of 1.6 mm was produced in the same manner as in Example 1 except that 20 ml of a 0.1N aqueous solution was prepared using acetaldehyde as the reducing agent and acetic acid as the acid.

(Example 3)
46.3 g of lithium phosphate and 79.5 g of divalent iron chloride tetrahydrate as a divalent iron compound were placed in a pressure vessel together with 200 ml of distilled water, and the inside was replaced with argon gas and then sealed. The pressure vessel was reacted in an oil bath at 180 ° C. for 48 hours. Thereafter, the pressure vessel was naturally allowed to cool to room temperature, and then the contents were taken out to obtain 64.7 g of a reaction product in a cake state. A small amount of the reaction product obtained here was collected and vacuum-dried at 70 ° C. for 2 hours to obtain a powder. This was confirmed by X-ray diffraction to be lithium iron phosphate. Furthermore, when observed with a scanning electron microscope (SEM), it was confirmed that it had a particle size of 20-200 nm.

  Separately, 20 ml of 0.1N ascorbic acid aqueous solution as a reducing acid was prepared and placed in a stainless steel container, and 6.47 g of the cake-like lithium iron phosphate obtained above was added thereto, After stirring at room temperature for 30 minutes, the mixture was filtered, thoroughly washed with water, and vacuum-dried at 60 ° C. for 2 hours. Otherwise, a coin-type lithium secondary battery having a diameter of 25 mm and a thickness of 1.6 mm was produced in the same manner as in Example 1.

Example 4
(Solid-phase synthesis method)
Lithium iron phosphate was synthesized by solid phase synthesis. The method of combining with carbon is to put ammonia water into an aqueous solution of 20.7 g of lithium nitrate and 102.6 g of ferrous nitrate nonahydrate until mixed, and a mixed precipitate of ferrous phosphate and lithium phosphate After being collected by filtration, 22.5 g of sucrose was added thereto. This precursor was pre-baked in argon gas at 350 ° C. for 10 hours, and then subjected to main baking at 600 ° C. for 16 hours to obtain lithium iron phosphate combined with carbon. As a result of X-ray diffraction, this powder was confirmed to be lithium iron phosphate, and the particle size was found to be 50 to 500 nm by observation with a scanning electron microscope. As a result of elemental analysis using a plasma emission spectrometer (ICP), the carbon content contained in the powder was 6.5%.

Separately, 20 ml of a 0.1N ascorbic acid aqueous solution as a reducing acid was prepared and placed in a stainless steel container, and 6.47 g of the lithium iron phosphate obtained above was added thereto, and this was added at room temperature for 30 minutes. After stirring for 5 minutes, the mixture was filtered, washed thoroughly with water, and vacuum-dried at 60 ° C. for 2 hours. Otherwise, a coin-type lithium secondary battery having a diameter of 25 mm and a thickness of 1.6 mm was produced in the same manner as in Example 1.

(Comparative Example 1)
A coin-type lithium secondary battery having a diameter of 25 mm and a thickness of 1.6 mm was produced in the same manner as in Example 1 except that the surface treatment with a reducing agent was not performed.

(Comparative Example 2)
A coin-type lithium secondary battery having a diameter of 25 mm and a thickness of 1.6 mm was produced in the same manner as in Example 4 except that the surface treatment with a reducing agent was not performed.

Batteries using lithium iron phosphate as the positive electrode are known to have a capacity reduction due to iron eluting into the electrolyte due to float charge or discharge at high temperature, which deposits on the negative electrode and destroys the solid electrolyte film. It has been. Therefore, in order to confirm the elution of iron into the electrolyte, 5 g of lithium iron phosphate prepared in various ways was put in the electrolyte used for the coin-type lithium battery, and this was left at 80 ° C. for 10 days. Thereafter, the iron concentration eluted in the electrolyte was measured by atomic absorption spectrometry. The results are shown in Table 1.

  As shown in Table 1, in the methods according to the present invention 1 to 3, the amount of trivalent iron on the surface of the lithium iron phosphate particles is small, and the components that are soluble in an acid are removed in advance, compared with Comparative Examples 1 and 2. It can be seen that the elution amount of iron can be sufficiently suppressed. In addition, the hydrothermal synthesis method and the solid phase synthesis method are compared, and the reason why the solid phase synthesis method has a smaller amount of Fe elution is presumed to be because the surface exposure was suppressed by the combination with carbon. These facts show that a battery that is stable for a long time even at high temperatures can be obtained by suppressing the amount of Fe elution.

(Battery test of lithium secondary battery)
A cycle test was performed on the coin-type lithium secondary batteries of Examples 1 to 4 and Comparative Examples 1 and 2. This cycle test is performed at a low rate of 0.1 C up to the 10th cycle (charging condition: constant current and constant voltage charging at a current of 0.1 C and a voltage of 4.5 V, discharging condition: current of 0.1 C and a final voltage of 2.0 V. Constant current discharge), eleventh cycle 5.0C high rate discharge (charge condition: current 0.1C, constant voltage constant voltage 4.5V, discharge condition: current 5.0C, end voltage 2.0V) Constant current discharge). All the temperatures were adjusted using a thermostatic bath so that the ambient temperature was 25 ° C.

Table 2 shows the 0.1 C discharge capacity at the 10th cycle and the 5.0 C discharge capacity at the 11th cycle. In addition, each capacity | capacitance was made into the capacity | capacitance per 1g of charged lithium iron phosphate.

  As shown in Table 2, the present inventions 1 to 3 subjected to the surface treatment had better discharge performance than Comparative Examples 1 and 2 where the surface treatment was not performed. In particular, it can be seen that the difference in the high rate discharge capacity of 5 CA is significant. This is presumably because the surface treatment shown in the first to fourth aspects of the present invention resulted in the removal of the high resistance layer on the surface of the lithium iron phosphate particles and the improvement of the current collection rate. In contrast, Comparative Examples 1 and 2 are presumed to have inferior discharge characteristics due to the remaining high resistance layer on the surface generated during synthesis adjustment.

Claims (6)

  The olivine-type M lithium phosphate (M is a transition metal, the same shall apply hereinafter) obtained by a conventional method or the olivine-type M lithium compounded with carbon is washed with a reducing acid or an aqueous solution obtained by adding a reducing agent to an acid. The manufacturing method of the positive electrode active material of a lithium secondary battery characterized by the above-mentioned.   A positive electrode active material for a lithium secondary battery obtained by washing olivine-type M lithium combined with olivine-type M lithium phosphate or carbon obtained by a conventional method with a reducing acid or an aqueous solution in which a reducing agent is added to the acid.   The lithium secondary battery positive electrode active material according to claim 1, wherein the transition metal of the olivine-type lithium M phosphate is iron, cobalt, manganese, or a compound thereof partially substituted with another element or a mixture thereof. Manufacturing method. The lithium secondary battery positive electrode active material according to claim 2, wherein the transition metal of the olivine-type lithium phosphate M is iron, cobalt, manganese, or a compound thereof partially substituted with another element or a mixture thereof. . A lithium secondary battery using the positive electrode active material according to claim 2 . Without drying the positive electrode active material according to claim 2 or 4, a conductive material, a binder, a thickener, a dispersant, and a kneaded paste are applied to a current collector and then dispersed. Lithium secondary battery obtained by drying and removing the agent.