JP2013089393A - Active material for secondary battery, and method for producing active material for secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: an active material for a secondary battery, with which a secondary battery having large discharge capacity can be obtained; a method for manufacturing the active material for a secondary battery; and a secondary battery manufactured by using the active material for a secondary battery.SOLUTION: An active material for a secondary battery includes lithium manganese phosphate. A primary particle of the lithium manganese phosphate has: a rod-like shape; a length in the short axis direction of 40 nm or less; and an aspect ratio (long axis length/short axis length) of 3 or more and 5 or less, the aspect ratio representing a ratio of a length in the long axis direction of the primary particle with respect to the length in the short axis direction thereof. Thus, the discharge capacity of a secondary battery manufactured by using the active material can be increased.

Description

  The present invention relates to an active material for a secondary battery and a method for producing an active material for a secondary battery.

  In recent years, non-aqueous electrolyte secondary batteries represented by lithium secondary batteries with high energy density and low self-discharge and good cycle performance have been used as power sources for mobile devices such as mobile phones and laptop computers, and electric vehicles. Attention has been paid.

The current mainstream of lithium secondary batteries is for consumer use, mainly for mobile phones of 2 Ah or less. Many positive electrode active materials for lithium secondary batteries have been proposed, but the most commonly known materials are lithium cobalt oxide (LiCoO ₂ ) and lithium nickel oxide whose operating voltage is around 4V. object is a (LiNiO _2), or lithium manganese oxide having a spinel structure lithium-containing transition metal oxide to basic configuration (LiMn ₂ O ₄₎ or the like. Among these, lithium cobalt oxide is widely used as a positive electrode active material for small-capacity lithium secondary batteries up to a battery capacity of 2 Ah because of its excellent charge / discharge performance and energy density.

  However, when considering the development of lithium secondary batteries for future medium-sized and large-sized, especially industrial applications where large demand is expected, safety is very important, so the specifications for current small batteries are not always sufficient. It cannot be said. One of the factors is the problem of thermal instability of lithium-containing transition metal compounds, and various countermeasures have been taken, but it is still not sufficient, and there is a need for batteries that are safe and have high energy density. It has been.

  Recently, polyanionic positive electrode active materials having excellent thermal stability have attracted attention. Since this polyanionic positive electrode active material is immobilized by covalently bonding oxygen to an element other than a transition metal, it does not release oxygen even at high temperatures, and can be used as a positive electrode active material to provide a lithium secondary battery. It is thought that the safety of can be dramatically improved.

As such a polyanionic positive electrode active material, lithium iron phosphate (LiFePO ₄ ) having an olivine structure has been actively studied. However, the theoretical capacity of LiFePO ₄ is limited to 170 mAh / g, and lithium is inserted and desorbed at a base potential of 3.4 V (vs. Li / Li ⁺ ). Compared with the energy density, the energy density is small.

Therefore, as a next-generation material of the positive electrode active material of a lithium ion secondary battery following the LiFePO _4, Study LiMnPO ₄ has been performed. Although this material has the same theoretical capacity as LiFePO ₄ , its potential shows 4.1 V (vs. Li ^/ Li ⁺ ), which is about 0.7 V higher, so theoretically high energy. May have a density. However, LiMnPO ₄ has a lower electronic conductivity than LiFePO _{4, and} it is very difficult to reach a theoretical capacity of 170 mAh / g. Thus, attempts have been made to obtain a large charge / discharge capacity close to the theoretical capacity even with LiMnPO ₄ having low electron conductivity by reducing the particle diameter of the particles.

Patent Document 1 states that “the method for producing a positive electrode active material for a lithium ion battery according to the present invention includes Li ₃ PO ₄ , or a Li source and a phosphate source, and a group of Fe source, Mn source, Co source, and Ni source. One or two or more selected are converted to LiMPO ₄ (where M is one or more selected from the group of Fe, Mn, Co and Ni) in terms of 0.5 mol / L or more and 1. A mixture containing 5 mol / L or less and water and a water-soluble organic solvent having a boiling point of 150 ° C. or higher is reacted under high temperature and high pressure, and LiMPO ₄ having an average primary particle size of 30 nm or more and 80 nm or less (provided that M is characterized in that it produces fine particles (one or more selected from the group consisting of Fe, Mn, Co and Ni). According to this technique, "narrow LiMPO ₄ particles having an average primary particle size of small and particle size distribution can be efficiently produced. Further, by changing the type and content of the water-soluble organic solvent, the LiMPO ₄ particles "The average primary particle size can be controlled." (Paragraph 0018), "According to the positive electrode active material for a lithium ion battery of the present invention, LiMPO ₄ (where M is a group of Fe, Mn, Co and Ni). (1 type or 2 types selected from the above) fine particles have an average primary particle diameter in the range of 30 nm to 80 nm and a narrow particle size distribution, so that the initial discharge capacity can be improved, and further high-speed charge can be achieved. The discharge characteristics can be improved "(paragraph 0019).

Non-Patent Document 1 discloses that a raw material solution containing each element of Li, Mn, and P is refluxed under conditions of 10.2 ≦ pH ≦ 10.7 and a temperature of 298 K, thereby passing through other substances. Techniques for synthesizing LiMnPO ₄ without doing so are disclosed. This technique describes that by increasing the concentration of each element in the raw material solution, the nucleation rate is increased, and LiMnPO ₄ having a small particle diameter of 100 nm or less can be synthesized.

JP 2010-251302 A

C. Delacourt et.al. Chem. Mater. 16, p. 93-99, 2004

In the above Patent Document 1, in addition to using an organic solvent, a pressure vessel is essential for performing a hydrothermal reaction, and it is necessary to heat at a high temperature of 180 ° C. for performing the reaction. There was a problem that the synthesis cost of the active material was increased.
Further, in the technique of Patent Document 1, “the reactant containing the above LiMPO ₄ microparticles is converted into LiMPO ₄ microparticles and Li-containing waste liquid (solution containing unreacted Li) by decantation, centrifugation, filter filtration, and the like. To separate.
Separated LiMPO ₄ microparticles, dryer, etc. and dried over 3 hours at 40 ° C. or higher with an average primary particle size can be obtained efficiently 30nm or more and 80nm or less of LiMPO ₄ particles. As can be seen from the description in paragraph 0035, the manufacturing process is complicated because several steps are required to separate the target active material for the secondary battery from the product after the hydrothermal reaction. In this respect, there was a problem that the synthesis cost was high.

In Non-Patent Document 1, in order to obtain LiMPO ₄ by coprecipitation, in addition to strictly controlling the pH of the raw material solution, the pH is controlled by LiOH as the Li source. Therefore, there is a problem that the synthesis cost becomes high because it is necessary to make the amount excessively larger than the above elements.

  This invention is made | formed in view of the said subject, and provides the secondary battery excellent in energy density by improving the discharge performance of the active material for secondary batteries containing lithium manganese phosphate. It is aimed.

  The configuration and effects of the present invention are as follows. However, the action mechanism described in this specification includes estimation, and its correctness does not limit the present invention.

The present invention is an active material for a secondary battery containing lithium manganese phosphate, wherein the lithium manganese phosphate has a primary particle having a rod-like shape, and the length of the primary particle in the minor axis direction is 40 nm or less. The aspect ratio (major axis length / minor axis length) representing the ratio of the length in the major axis direction to the length in the minor axis direction of the primary particles is 3 or more and 5 or less. Secondary battery active material.
The lithium manganese phosphate in the present invention is a compound represented by the general formula LiMn _(1-x) M _x PO ₄ (x <0.5). Here, M is at least one element selected from the group consisting of transition metals such as Fe, Co, and Ni, Mg, Na, K, Ca, B, Al, Zn, In, Sn, and Sr. Such a lithium manganese phosphate has a higher Li ⁺ insertion potential than lithium iron phosphate because 50% or more of manganese is contained in the compound.
The primary particles of the active material for a secondary battery of the present invention are the smallest particles constituting the active material powder and refer to particles having a crystal structure of lithium manganese phosphate.
By using a secondary battery active material containing primary particles having such a shape, the discharge performance of the secondary battery active material can be improved.

Further, the active material for a secondary battery of the present invention includes the primary particles of the lithium manganese phosphate or secondary particles in the order of μm in which a plurality of the primary particles are aggregated (aggregated), and further higher-ordered than those aggregated. It is preferable that carbon is provided on the surface of the particles.
Here, “comprising carbon on the surface of the particle” means a state in which carbon having electric conductivity higher than that of lithium manganese phosphate is in contact with at least a part of the surface of the particle.

  Moreover, this invention is the electrode for secondary batteries containing the said active material for secondary batteries.

  Furthermore, the present invention is a secondary battery comprising the secondary battery electrode, a counter electrode, and an electrolyte.

This invention is a manufacturing method of the active material for secondary batteries including the process of synthesize | combining lithium manganese phosphate by making the aqueous solution containing a lithium source, a manganese source, and a phosphorus source react at 110 degrees C or less.
By adopting such a synthesis method, the primary particles of the lithium manganese phosphate can be used as an active material for a secondary battery having the rod shape.

Moreover, this invention is a manufacturing method of the active material for secondary batteries including the process of synthesize | combining the said lithium manganese phosphate by making the said aqueous solution react for 6 hours or more.
Furthermore, this invention is a manufacturing method of the active material for secondary batteries including the process of synthesize | combining the said lithium manganese phosphate by making the said aqueous solution react at normal pressure vicinity.
Here, the vicinity of normal pressure in the present invention refers to a range of 0.5 to 1.5 atm.
By adopting such a synthesis method, the primary particles of the lithium manganese phosphate have the rod-like shape, and the active material for a secondary battery in which the short axis growth of the primary particles is suppressed is used. Is possible.

  ADVANTAGE OF THE INVENTION According to this invention, the secondary battery active material excellent in discharge performance and a secondary battery with high energy density using the same can be provided.

The lithium manganese phosphate has a crystal structure belonging to the orthorhombic space group Pnmb. This lithium manganese phosphate is known to have a one-dimensional lithium ion diffusion path in the solid phase, and an oxide-based active material having a two-dimensional or three-dimensional lithium ion diffusion path. In comparison, it is one of the factors inferior in discharge performance.
In the present invention, as shown in a TEM photograph to be described later, the primary particles of lithium manganese phosphate have a rod shape. Detailed examination (analysis) of this rod-like shape revealed that the b-axis direction of the lithium manganese phosphate crystal lattice corresponds to the minor axis direction of the primary particles. The lithium ion diffusion path of lithium manganese phosphate exists in the b-axis direction, that is, lithium ions are diffused in one dimension in the b-axis direction. Therefore, as in the present invention, by using rod-shaped primary particles whose b-axis direction is the short axis, the distance of the lithium ion diffusion path is shortened, so that the discharge performance of lithium manganese phosphate is improved. It is thought that there is. Furthermore, in the secondary battery active material of the present invention, the primary particles of lithium manganese phosphate contained are rod-shaped having a specific aspect ratio. Thereby, in the secondary particles formed by aggregation of the primary particles, the primary particles in the secondary particles are easily aligned in a certain direction, that is, the direction of the lithium ion diffusion path of the primary particles is in a certain direction. Since it becomes easy to arrange, it is thought that the diffusion rate of lithium ions in the secondary particles is improved, and it is assumed that it contributes to the improvement of the discharge performance of the active material for secondary battery of the present invention.
Therefore, the short axis of the primary particles in the active material for a secondary battery of the present invention is preferably short, and the length is preferably 40 nm or less. As for the long axis, as the nature of lithium manganese phosphate, the crystal lattice volume changes by about 10% with the insertion / extraction of lithium ions due to charging / discharging of the battery, so if the aspect ratio is too large, There is a possibility that the discharge performance and the charge / discharge cycle performance of lithium manganese phosphate may deteriorate due to the particles becoming unable to adapt to the change in crystal lattice volume and cracking the particles. Therefore, the aspect ratio is preferably 3 or more and 5 or less. In addition, the specific major axis length of the primary particles is preferably 50 nm or more and 150 nm or less.

  It can be confirmed that the active material for a secondary battery contains a lithium atom, a manganese atom, a phosphorus atom, and the like, and the amount thereof by high frequency inductively coupled plasma (ICP) emission spectroscopy. The orthorhombic crystal structure can be confirmed by powder X-ray diffraction analysis (XRD). In addition, transmission electron microscope observation (TEM), energy dispersive X-ray spectroscopy (EDX), scanning electron microscope X-ray analysis (EPMA), high resolution electron microscope analysis (HRAEM), electron energy loss spectroscopy (EELS), etc. It is possible to perform detailed analysis by using together the analytical instrument.

  The lithium manganese phosphate according to the present invention is a method of synthesizing a precursor aqueous solution prepared by mixing an aqueous solution in which a lithium source, a manganese source, and a phosphorus source are dissolved in water, respectively, by holding at 110 ° C. or lower for a predetermined time. Can be obtained. The holding time is preferably 6 hours or more and 30 hours or less. By setting the holding time to 6 hours or longer, the synthesis time required for the production reaction of lithium manganese phosphate can be secured, so that lithium manganese phosphate having excellent performance can be synthesized. Therefore, in the production method of the present invention, in order to synthesize lithium manganese phosphate, the holding time is preferably 6 hours or more. In addition, by setting the reaction time to 30 hours or less, the lithium manganese phosphate particles once generated are repeatedly subjected to the dissolution-precipitation process, whereby the growth reaction of the primary particles is promoted and the primary particles are coarsened. A decrease in discharge performance can be suppressed. Therefore, the holding time is preferably 30 hours or less.

Furthermore, the lithium source, manganese source, and phosphorus source are preferably water-soluble. When the raw materials of these elements are sparingly water-soluble, the production reaction rate of lithium manganese phosphate is remarkably slow and the holding time is required for a long time, so it cannot be said that it is preferable from the viewpoint of synthesis cost. Examples of water-soluble lithium sources include lithium hydroxide, lithium sulfate, and lithium acetate; manganese sources such as manganese sulfate and manganese chloride; and phosphorus sources such as ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and phosphoric acid. Etc. The aqueous precursor solution is preferably neutral or alkaline. This is because when the precursor aqueous solution is acidic, the produced lithium manganese phosphate may be dissolved. In view of this point, the lithium source is preferably lithium hydroxide that is water-soluble and exhibits alkalinity after being dissolved in water. Of the manganese sources, those with high solubility in water often show acidity after dissolution, and phosphorus sources often show acidity to neutrality. This is because if the lithium source is neutral after being dissolved in water, the precursor aqueous solution becomes acidic. However, when the precursor aqueous solution is strongly alkaline, Li ₃ PO ₄ tends to remain in the product after synthesis. Therefore, the pH of the precursor aqueous solution preferably does not exceed 14, and further, the pH is 12 The following is more preferable.

  Moreover, when preparing the precursor aqueous solution, it is preferable to work in an inert atmosphere. This is to suppress the generation of impurities by preventing manganese from being oxidized from divalent to trivalent in the precursor aqueous solution. Therefore, it is preferable that the atmosphere in the reaction vessel is an inert atmosphere also in the step of holding the precursor aqueous solution at 110 ° C. or lower for a predetermined time. However, in this step, it is possible to synthesize lithium manganese phosphate even under a normal air atmosphere.

  Further, it is preferable that a carbon source is allowed to coexist in the aqueous precursor solution during the synthesis of lithium manganese phosphate. In this case, since a uniform carbon precursor film is formed on the surface of the generated lithium manganese phosphate particles by coexisting a carbon source during the synthesis of lithium manganese phosphate, it is particularly preferable. In addition, as a carbon source coexisting at the time of a synthesis | combination, saccharides, such as organic acid compounds, such as ascorbic acid, and sucrose are mentioned.

  Further, in the present invention, carbon generated by thermal decomposition of organic substances such as polyvinyl alcohol, sucrose, and ascorbic acid is formed on the surface of the positive electrode active material particles for the secondary battery in order to supplement the electronic conductivity of lithium manganese phosphate. It is preferable to provide. The temperature at which the pyrolysis step performed to provide carbon on the surface of the positive electrode active material particles for secondary batteries is required to be higher than the temperature at which the organic matter used undergoes pyrolysis, but the performance of lithium manganese phosphate is reduced. In order to reduce the risk of this, the temperature of the pyrolysis step is preferably 300 ° C to 900 ° C. More preferably, it is 500 degreeC-800 degreeC.

In the present invention, the active material for secondary battery provided with carbon is preferably a powder having a secondary particle average particle size of 100 μm or less. In particular, the average particle diameter of the secondary particles is more preferably 0.1 to 50 μm. Moreover, the BET specific surface area by the flow method nitrogen gas adsorption method of powder particle | grains is good in order to improve the high rate charge / discharge characteristic of a positive electrode, and 1-100 m < ² > / g is preferable. More preferably, it is 5-100 m < ² > / g. In order to obtain the powder in a predetermined shape, a pulverizer or a classifier can be used. For example, a mortar, a ball mill, a sand mill, a vibrating ball mill, a planetary ball mill, a jet mill, a counter jet mill, a swirling air flow type jet mill, a sieve, or the like can be used. At the time of pulverization, wet pulverization in which water or an organic solvent such as alcohol or hexane coexists may be used. The classification method is not particularly limited, and a sieve, an air classifier, or the like can be used dry or wet as necessary.

  Furthermore, the active material for a secondary battery of the present invention may intentionally coexist with impurities or different elements for the purpose of improving its performance, and in such a case, the effect of the present invention is lost. Absent.

  When the active material for a secondary battery of the present invention is used in a non-aqueous electrolyte, it is preferable that the amount of water contained in the positive electrode is small, specifically, less than 1000 ppm. As a means for reducing the amount of moisture, a method of drying the electrode in a high temperature / depressurized environment and a method of electrochemically decomposing moisture contained in the electrode are suitable.

  In producing a positive electrode for a secondary battery using the active material for a secondary battery of the present invention, in addition to the active material for a secondary battery, polyvinylidene fluoride, silicon butadiene rubber, polytetrafluoroethylene, carboxymethylcellulose, etc. Well-known binders and well-known conductive aids such as acetylene black, ketjen black, and carbon nanofiber can be used in a well-known formulation.

  In addition, the thickness of the electrode mixture layer is preferably 10 μm or more and 500 μm or less in consideration of the energy density of the battery.

The negative electrode of the battery of the present invention is not limited in any way, but lithium metal, lithium alloy (lithium metal such as lithium-aluminum, lithium-lead, lithium-tin, lithium-aluminum-tin, lithium-gallium, and wood alloy) Alloys), alloys capable of inserting and extracting lithium, carbon materials (eg, graphite, hard carbon, low-temperature fired carbon, amorphous carbon, etc.), metal oxides, lithium metal oxides (Li ₄ Ti ₅ O ₁₂₎ Etc.), polyphosphoric acid compounds and the like. These can be used according to the kind of electrolyte used for the secondary battery. Among these, graphite is preferable as a negative electrode material because it has an operating potential very close to that of metallic lithium and can realize charge and discharge at a high operating voltage. For example, artificial graphite and natural graphite are preferable. In particular, graphite in which the surface of the negative electrode active material particles is modified with amorphous carbon or the like is desirable because it generates less gas during charging.

  In general, the form of the lithium secondary battery is composed of a positive electrode, a negative electrode, and an electrolyte containing an electrolyte salt in a solvent. Generally, a separator and an outer packaging for packaging them between the positive electrode and the negative electrode. A body is provided.

  Solvents include cyclic carbonates such as propylene carbonate and ethylene carbonate; cyclic esters such as γ-butyrolactone and γ-valerolactone; chain carbonates such as dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate; formic acid Chain esters such as methyl, methyl acetate and methyl butyrate; tetrahydrofuran or derivatives thereof; 1,3-dioxane, 1,4-dioxane, 1,2-dimethoxyethane, 1,4-dibutoxyethane, methyldiglyme, etc. Ethers such as acetonitrile, nitriles such as benzonitrile, dioxolane or derivatives thereof, and non-aqueous solvents and water consisting of ethylene sulfide, sulfolane, sultone or derivatives thereof alone or a mixture of two or more thereof You can It is not limited.

Examples of the electrolyte salt include ionic compounds such as LiBF ₄ , LiPF ₆ , LiClO ₄ , LiN (C ₂ F ₅ SO ₂ ) ₂ , and LiN (CF ₃ SO ₂ ) _2. It can be used alone or in combination of two or more. The concentration of the electrolyte salt in the electrolyte is preferably 0.5 mol / l or more and 5 mol / l or less, more preferably 1 mol / l or more and 2.5 mol / l, in order to reliably obtain a secondary battery having high battery characteristics. It is as follows.

  Hereinafter, the present invention will be described in more detail by way of examples. However, the present invention is not limited to the following embodiments.

Example 1
First, 10.07 g of lithium hydroxide monohydrate (LiOH.H ₂ O) (Nacalai Tesque, Inc.) and diammonium hydrogen phosphate ((NH ₄ ) ₂ HPO ₄ ) (Nacalai Tesque, Inc.) After weighing 06 g and dissolving in 80 and 20 ml of ion-exchanged water, both solutions were mixed for 2 hours with stirring. Next, 19.28 g of manganese sulfate pentahydrate (MnSO ₄ .5H ₂ O) (Nacalai Tesque) was dissolved in 60 ml of ion-exchanged water in which 0.36 g of ascorbic acid (Nacalai Tesque, Inc.) was dissolved. I let you. This solution was added to the mixed solution of LiOH.H ₂ O and (NH ₄ ) ₂ HPO ₄ to obtain an aqueous precursor solution. The pH of this aqueous precursor solution was measured (pH meter D-51, manufactured by Horiba, Ltd.), and the pH was 7.92. Incidentally, all operations up to this point, in order to prevent oxidation of Mn ³⁺ in the Mn ²⁺ in aqueous solution, were carried out in a simple glove box was thoroughly purged with nitrogen. The obtained precursor aqueous solution was transferred to a two-necked separable flask (500 ml), and a reflux tube was installed in one of the ports, and a thermocouple and a nitrogen gas introduction tube were installed in the remaining ports. The separable flask was placed on a hot plate, heated until the precursor aqueous solution boiled, and then the synthesis reaction was performed by maintaining the heated state. The synthesis reaction time (time for maintaining the heating state) was 8 hours. Then, it cooled to room temperature by natural cooling. During this time, nitrogen gas was continuously introduced from the nitrogen gas introduction pipe. The aqueous solution in the separable flask after the synthesis reaction is filtered, washed with ion-exchanged water and acetone, and then vacuum-dried at 120 ° C. for 5 hours for a secondary battery containing lithium manganese phosphate. An active material was prepared. This is designated as an active material a1 for secondary battery of the present invention.

(Example 2)
A secondary battery active material was synthesized in the same manner as in Example 1 except that the synthesis reaction time was 6 hours. This is designated as an active material a2 for secondary battery of the present invention.

(Example 3)
The precursor aqueous solution was transferred to a polytetrafluoroethylene container (internal volume: 500 cm ³ ), and this was installed in a hydrothermal reaction container (manufactured by Pressure Glass Industrial Co., Ltd., TVS-N2). The inside of the reaction vessel was sufficiently replaced with nitrogen gas and sealed, and the precursor aqueous solution was heated to 100 ° C., and then kept at that temperature to carry out the synthesis reaction. The synthesis reaction time (time for keeping the precursor aqueous solution at 100 ° C.) was 6 hours. The temperature increase rate was 100 ° C./hr, the temperature decrease was naturally allowed to cool, and the precursor aqueous solution was stirred at 200 rpm from the start of the temperature increase until the temperature decrease ended. Further, in this example, a hydrothermal reaction vessel was used, but since the temperature of the precursor aqueous solution was 100 ° C., the pressure in the hydrothermal reaction vessel was 1.2 atm. A secondary battery active material was synthesized in the same manner as in Example 1 except that the above synthesis reaction was performed. This is designated as an active material a3 for secondary battery of the present invention.

(Comparative Example 1)
The precursor aqueous solution was transferred to a polytetrafluoroethylene container (internal volume: 500 cm ³ ), and this was installed in a hydrothermal reaction container (manufactured by Pressure Glass Industrial Co., Ltd., TVS-N2). The inside of the reaction vessel was sufficiently substituted with nitrogen gas and sealed, and the precursor aqueous solution was heated to 170 ° C., and then kept at that temperature to carry out the synthesis reaction. The synthesis reaction time (time for keeping the precursor aqueous solution at 170 ° C.) was 6 hours. The temperature increase rate was 100 ° C./hr, the temperature decrease was naturally allowed to cool, and the precursor aqueous solution was stirred at 200 rpm from the start of the temperature increase until the temperature decrease ended. In this comparative example, the pressure in the hydrothermal reaction vessel was 1.2 atm. A secondary battery active material was synthesized in the same manner as in Example 1 except that the above synthesis reaction was performed. This is referred to as a comparative active material b1.

(Comparative Example 2)
In synthesizing the active material for a secondary battery, lithium phosphate (Li ₃ PO ₄ ) (Nacalai Tesque) was used instead of lithium hydroxide monohydrate (LiOH.H ₂ O) as a lithium source. A secondary battery active material was synthesized in the same manner as in Example 1 except that .26 g was weighed and suspended in 100 ml of ion exchange water. This is referred to as a comparative active material b2.

(TEM observation)
The present invention active material a1 and comparative active material b1 were subjected to field emission analytical transmission electron microscope (FE-TEM) (EB002BF manufactured by Topcon Technohouse Co., Ltd.). An image of the active material a1 of the present invention obtained in this observation is shown in FIG. 1, and an image of the comparative active material b1 is shown in FIG.

(Carbon coat)
Add 2.29 g of polyvinyl alcohol (Wako Pure Chemical Industries, Ltd., degree of polymerization 1500) to each 2.0 g of the powders of the active materials a1 to a3 for the secondary battery of the present invention and the comparative active material b1, and use an agate mortar. After mixing, a small amount of water heated to 60 ° C. was added and mixed and kneaded again in a mortar to obtain a gum-like paste. The mixture was placed in an alumina bowl (outside dimensions 90 × 90 × 50 mm), and the atmosphere-replacement-type firing furnace (a tabletop vacuum gas replacement furnace KDF-75 manufactured by Denken Co., Ltd., internal volume 2400 cm ³ ) was used for nitrogen. Heating was performed under gas flow (flow rate 0.5 l / min). The heating temperature was 700 ° C., and the heating time (time for maintaining the heating temperature) was 1 hour. The rate of temperature rise was 10 ° C./min, and the temperature was naturally cooled. Thus, the active material for secondary batteries provided with carbon on the surface of each active material particle was produced.

(XRD measurement)
The active material a1 for the secondary battery of the present invention and the comparative active material b2 were subjected to X-ray diffraction measurement using an X-ray diffraction apparatus (manufactured by Rigaku, model name: MiniFlex II) using a CuKα radiation source. The measurement was performed under the conditions of a tube voltage of 30 kV, a tube current of 15 mA, a scan speed of 4 ° / s, and a scan step of 0.02 °. The measurement results are shown in FIG.

(Preparation of positive electrode)
Containing an active material a1 for a secondary battery of the present invention including carbon, acetylene black as a conductive additive, and polyvinylidene fluoride (PVdF) as a binder in a mass ratio of (80: 8: 12), N -A positive electrode paste using methyl-2-pyrrolidone (NMP) as a solvent was prepared. The solid content concentration of the positive electrode paste was adjusted to 30% by mass. The positive electrode paste is applied to both sides of the half of the long side of an aluminum mesh current collector (6 cm × 1.5 cm) with an aluminum terminal attached, NMP is removed at 80 ° C., and then the applied portions are overlapped in layers. Then, it was bent so that the projected area of the coated part was halved, and press working was performed so that the thickness after bending was 400 μm, to obtain a positive electrode. The application area of the active material after bending is 2.25 cm ² and the application mass is 0.065 g. The positive electrode was dried under reduced pressure at 150 ° C. for 5 hours or more to remove moisture in the electrode plate.

(Preparation of negative electrode)
A stainless steel (product name: SUS316) mesh current collector attached with a stainless steel (product name: SUS316) terminal was bonded to both sides of a 300 μm-thick lithium metal foil and pressed to form a negative electrode.

(Production of reference electrode)
A reference electrode was prepared by attaching a lithium metal piece to the tip of a current collector rod made of stainless steel (product name: SUS316).

(Preparation of electrolyte)
In a mixed solvent in which ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate are mixed at a volume ratio of 1: 1: 1, LiPF ₆ which is a fluorine-containing electrolyte salt is dissolved at a concentration of 1.0 mol / l, and a non-aqueous electrolyte is obtained. Was made. The amount of water in the non-aqueous electrolyte was less than 50 ppm.

(Battery assembly)
A glass lithium ion secondary battery was assembled in an Ar box having a dew point of −40 ° C. or lower. Each of the positive electrode, the negative electrode, and the reference electrode was sandwiched between gold-plated clips whose conductors were previously fixed to the lid portion of the container, and then fixed so that the positive and negative electrodes were opposed to each other. The reference electrode was fixed at a position on the back side of the positive electrode when viewed from the negative electrode. Next, a polypropylene cup containing a certain amount of electrolyte was placed in a glass container, and a battery was assembled by covering the positive electrode, the negative electrode, and the reference electrode so as to be immersed therein. This battery is referred to as a battery A1 of the present invention.

  Similarly, a positive electrode was prepared using each of the active materials for secondary battery active materials a2 and a3 of the present invention provided with carbon and a comparative active material b1 provided with carbon, and the lithium secondary material was subjected to the above procedure. I assembled the battery. The lithium secondary battery using the positive electrode using the active materials a2 and a3 for the secondary battery of the present invention including carbon is used as the lithium secondary battery of the present invention battery A2 and A3, and the positive electrode using the comparative active material b1 including the carbon. The used lithium secondary battery is referred to as comparative battery B1.

(Charge / discharge test)
The inventive batteries A1 to A3 and comparative battery B1 produced as described above were subjected to a charge / discharge process in which charge / discharge of one cycle was performed at a temperature of 20 ° C. The charging conditions were a current of 0.1 CmA (about 10 hours rate), a voltage of 4.3 V, and a constant current constant voltage charging of 15 hours. The discharging conditions were a current of 0.1 CmA (about 10 hours rate) and a final voltage of 2.0 V. Constant current discharge. Table 1 shows the results of the obtained discharge capacity.

  As can be seen from Table 1, the battery A1 of the present invention has a larger discharge capacity than the comparative battery B1. This difference in battery performance is considered to be derived from the active material. As can be seen from the TEM images in FIG. 1 and FIG. 2, the shape of the primary particles of the active material a1 for the secondary battery of the present invention is different from the cubic shape of the primary particles of the comparative active material b1, and has a rod-like shape. In addition, the length of the minor axis is 40 nm or less, and the aspect ratio is in the range of 3 to 5. Although not shown here, it has been confirmed by TEM observation that the length of the short axis is at most 40 nm. On the other hand, the shape of the primary particles of the comparative active material b1 is not a rod shape but close to a cube. Therefore, as described above, the active material for a secondary battery of the present invention has a specific aspect ratio in addition to the distance of the lithium ion diffusion path being shortened. It is thought that the discharge performance is improved because the direction of the lithium ion diffusion path is easily aligned in a certain direction. Although not shown here, it has been confirmed that the secondary battery active materials a2 and a3 of the present invention have similar primary particle shapes. Therefore, the discharge capacity is superior to that of the comparative battery B1 using the comparative active material b1 as the active material.

Further, as can be seen from FIG. 3, when the active material a1 for the secondary battery of the present invention and the comparative active material b2 are compared, the active material a1 for the secondary battery of the present invention is produced mainly with lithium manganese phosphate. but it is known, the comparison active material b2, Li ₃ PO ₄ is present impurities peaks mainly, it can be seen that Li ₃ PO ₄ was used as the raw material is remained unreacted. This difference is considered to be caused by the raw material of the Li source. When lithium hydroxide is used as the Li source like the active material a1 for the secondary battery of the present invention, an aqueous solution in which lithium hydroxide is dissolved in advance is used when preparing the precursor aqueous solution. Even if Li ₃ PO ₄ is produced in the body aqueous solution, it is very fine particles, so that it proceeds smoothly without hindering the synthesis reaction of lithium manganese phosphate, whereas the comparative active material b2 When Li ₃ PO ₄ is used as the Li source, since Li ₃ PO ₄ is poorly water-soluble, the reaction rate between the giant Li ₃ PO ₄ particles and the manganese source in the precursor aqueous solution is slow, It is presumed that the formation reaction of lithium manganese phosphate did not proceed sufficiently in the same synthesis reaction time. That is, when Li ₃ PO ₄ is used as the Li source, a longer synthesis reaction time is required, which is not preferable from the viewpoint of synthesis cost. Therefore, in the synthesis method of the present invention, it is preferable to use a water-soluble Li source as the Li source, and it is particularly preferable to use lithium hydroxide.

  Since the secondary battery active material of the present invention has a large discharge capacity, a secondary battery having a high energy density can be provided. Since the secondary battery of the present invention has a high energy density, the secondary battery is suitable for application to fields requiring a higher capacity in industrial batteries such as electric vehicles that are expected to be developed in the future. The availability is extremely large.

The TEM image of this invention active material a1. TEM image of comparative active material b1. The XRD profile of this invention active material a1 and the comparison active material b2.

Claims (7)

An active material for a secondary battery containing lithium manganese phosphate, wherein the lithium manganese phosphate has a primary particle having a rod-like shape, and a length in a minor axis direction of the primary particle is 40 nm or less, A secondary battery having an aspect ratio (major axis length / minor axis length) representing a ratio of a major axis length to a minor axis length of primary particles of 3 or more and 5 or less. Active material. 2. The active material for a secondary battery according to claim 1, wherein carbon is provided on a surface of the primary particles or higher-order particles higher than the secondary particles. The electrode for secondary batteries containing the active material for secondary batteries of Claim 1 or 2. A secondary battery comprising the electrode according to claim 3, a counter electrode, and an electrolyte. The manufacturing method of the active material for secondary batteries including the process of synthesize | combining lithium manganese phosphate by making the aqueous solution containing a lithium source, a manganese source, and a phosphorus source react at 110 degrees C or less. The manufacturing method of the active material for secondary batteries of Claim 5 including the process of synthesize | combining the said lithium manganese phosphate by making the said aqueous solution react for 6 hours or more. The manufacturing method of the active material for secondary batteries of Claim 5 including the process of synthesize | combining the said lithium manganese phosphate by making the said aqueous solution react at normal pressure vicinity.