JP2013175374A - Positive electrode active material for secondary battery, active material particle, and positive electrode and secondary battery using those - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode active material for a secondary battery, allowing improvement in discharge characteristics of a battery.SOLUTION: A positive electrode active material for a secondary battery comprises elements of lithium, manganese, phosphorus, oxygen and vanadium and contains a compound having an olivine structure, where the ratio of the vanadium atomic number relative to the sum of atomic numbers of manganese and vanadium is 20% or less. With this constitution, the discharge capacity maintenance rate of a battery in high rate discharge can be large compared with the case of not comprising vanadium.

Description

  The present invention relates to a positive electrode active material for a secondary battery.

As a positive electrode active material for a lithium secondary battery, an active material having an olivine type crystal structure (hereinafter simply referred to as “olivine type active material”) is being developed. The olivine-type active material has a general formula of LiMPO ₄ (M is a transition metal such as Fe, Mn, Ni, and Co) and releases oxygen even at high temperatures because oxygen is immobilized by a covalent bond with phosphorus. It is difficult to resist and exhibits excellent thermal stability.

Among olivine-type active materials, lithium iron phosphate (LiFePO ₄ ) is known as an excellent positive electrode material because of its high electron / ion conductivity and large capacity density as an olivine-type active material. ing. However, the lithium iron phosphate has a redox potential associated with insertion / extraction of Li ions of 3.4 V with respect to the lithium electrode, and is about 4 V for a general positive electrode material for a lithium ion battery such as LiCoO _2. There is a problem that the potential is low. On the other hand, lithium manganese phosphate (LiMnPO ₄ ) has a high redox potential of 4.1 V due to insertion / extraction of Li ions, but it is sufficiently discharged when used in a battery due to its poor electronic conductivity. There was a problem that characteristics could not be obtained.

As a technique for improving the characteristics of lithium manganese phosphate, Patent Document 1 includes a general formula Li _x Mn _y A _1-y PO ₄ (0 <x ≦ 2, 0 <y <1, A represents Ti, Zn, Mg , A metal element selected from Co.), and by replacing a part of Mn of lithium manganese phosphate with each of the above metal elements, Mn ³⁺ in a charged state is disclosed. It is described that the redox generation of Mn becomes possible by diluting the yarn-teller effect caused by the above.

Patent Document 2 describes a mixture of various metals including Mn as M in the composition formula LiMPO ₄ . However, a mixture of two or more metals is not specifically evaluated.

Patent Documents 3 to 5 describe Li _x MnPO _{4 in which a part} of Mn (0 <x <2) is substituted with various metals, and specifically, a part of Mn is converted to Fe. , Fe and Ti, Fe and Co, Co and Ti, Ni and Ti, and those substituted with Co and Ni have been evaluated.

JP 2001-307731 A International Publication No. 07/034821 International Publication No. 07/034823 International Publication No. 08/018633 JP 2009-532323 A

  Despite various development efforts as described in each of the above patent documents, no battery using a lithium manganese phosphate-based active material has been put to practical use. The present invention has been made in consideration of the above points, and an object of the present invention is to provide an active material capable of improving the discharge characteristics of a battery for a positive electrode active material for a secondary battery such as a lithium secondary battery. . It is another object of the present invention to provide a positive electrode and a battery using such an active material.

  The positive electrode active material for a secondary battery of the present invention contains lithium, manganese, phosphorus, oxygen, and vanadium elements and contains a compound having an olivine structure, and the number of vanadium atoms relative to the sum of the number of manganese and vanadium atoms The ratio is 20% or less. With this configuration, it is possible to increase the discharge capacity maintenance rate at the time of high rate discharge of the battery as compared with the case where vanadium is not included.

  Preferably, in the positive electrode active material for a secondary battery, the ratio of the number of vanadium atoms to the sum of the number of manganese and vanadium atoms is 1% or more and 15% or less. Thereby, compared with the case where vanadium is not included, the discharge capacity and discharge capacity maintenance rate at the time of high rate discharge of a battery can be enlarged.

  More preferably, in the positive electrode active material for a secondary battery, the ratio of the number of vanadium atoms to the sum of the number of manganese and vanadium atoms is 5% or more and 10% or less. Thereby, compared with the case where vanadium is not included, it is possible to increase the discharge capacity and the discharge capacity maintenance ratio at the time of high rate discharge while maintaining the discharge capacity at the time of 0.1 CmA discharge of the battery at the same level or higher.

  Further, the positive electrode active material for a secondary battery has 2θ = 23.4 °, 34.2 ° in addition to a diffraction peak derived from an olivine type crystal structure in powder X-ray diffraction measurement using Cu—Kα ray. And a diffraction peak appears in the vicinity of 36.7 °.

  The positive electrode active material particles for a secondary battery of the present invention may further include carbon in any of the particles of the positive electrode active material for a secondary battery described above. The particles of the positive electrode active material for secondary batteries refer to primary particles, secondary particles, or higher order particles of the positive electrode active material for secondary batteries. Carbon refers to a conductive carbonaceous compound mainly composed of the element C. Here, carbon is provided on the surface and / or inside of the particles of the positive electrode active material for a secondary battery in the form of adhesion, coating or the like. Thereby, the electron conductivity of the active material can be supplemented, and the utilization factor of the active material can be increased during charging and discharging.

  The positive electrode of the present invention uses any one of the positive electrode active materials for secondary batteries described above.

  The secondary battery of the present invention uses any one of the positive electrode active materials for secondary batteries described above.

  According to the present invention, a secondary battery with improved discharge capacity maintenance rate during high rate discharge can be obtained.

It is a powder X-ray-diffraction profile of the positive electrode active material particle of an Example and a comparative example. It is a figure which shows the initial stage discharge capacity of the simple cell using the positive electrode active material of an Example and a comparative example. It is a figure which shows the discharge capacity at the time of the high rate discharge of the simple cell using the positive electrode active material of an Example and a comparative example.

  First, an embodiment of the positive electrode active material for a secondary battery of the present invention will be described below.

The positive electrode active material for a secondary battery of this embodiment mainly contains a phosphate having an olivine structure. According to the profile of the X-ray diffraction (XRD) measurement, it is considered that most of vanadium is present at the Mn site of LiMnPO ₄ , and the main component of the active material for the secondary battery of this embodiment has an average composition of Li _w. It is considered that Mn _1-x V _x PO4 (0 <w ≦ 1, 0 <x ≦ 0.20). Here, the amount of Li (value of w) increases or decreases as the battery is charged / discharged. Moreover, as long as the effects of the present invention are not lost, impurities and different elements may be intentionally allowed to coexist in the active material for the purpose of improving its performance.

  It can be confirmed that the active material of the present embodiment contains a lithium atom, a manganese atom, a vanadium atom, a phosphorus atom, and the like, and the amount thereof by high frequency inductively coupled plasma (ICP) emission spectroscopy. The crystal structure of the active material 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), and electron energy loss spectroscopy (EELS) A detailed analysis can be performed by using an analytical instrument such as the above.

  In the present invention, carbon may be provided on the surface and / or inside of the particles in order to supplement the electron conductivity of the positive electrode active material for secondary batteries. It is preferable to provide carbon in the particles of the positive electrode active material for a secondary battery of the present invention because the utilization factor of the active material can be increased during charging and discharging.

  The method for synthesizing the active material of the present embodiment is not particularly limited. Specific examples include a solid phase method, a liquid phase method, a sol-gel method, and a hydrothermal method. The carbon layer on the surface of the active material particles can be formed by thermally decomposing organic substances such as polyvinyl alcohol, sucrose, and ascorbic acid. In particular, it is preferable to coexist a carbon source during active material synthesis. In the case of the hydrothermal method, a uniform carbon precursor film is formed on the surface of the generated lithium phosphate (manganese / vanadium) lithium particles by coexisting a carbon source during synthesis. At this time, it is particularly preferable to use polyvinyl alcohol as the carbon source because a carbon protrusion is formed on the particle surface by thermal decomposition. The temperature at which the pyrolysis is performed needs to be higher than the temperature at which the organic substance used as the carbon source is pyrolyzed. However, if the temperature is too high, the performance of lithium phosphate (manganese / vanadium) is lowered. It is preferable that it is 500-800 degreeC.

  In order to obtain the active material 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 an organic solvent such as water or 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.

  The size of the active material particles of the present embodiment is not particularly limited, but the primary particles preferably have a particle size of 1 to 500 nm, and the secondary particles preferably have an average particle size of 0.1 to 50 μm. When the primary and secondary particle sizes are too large, the utilization factor of the active material is lowered. On the other hand, when the primary and secondary particle sizes are too small, the workability of the positive electrode paste preparation and the coating process deteriorates, and the work efficiency of electrode preparation decreases.

Although the specific surface area of the active material particle of the present embodiment is not particularly limited, it is preferable that the BET specific surface area due to the flow process the nitrogen gas adsorption method of the powder is 1 to 100 m ² / g, is 5 to 50 m ² / g Is more preferable, and it is still more preferable that it is 5-30 m < ² > / g. This is because the higher the specific surface area, the higher rate charge / discharge characteristics are improved. On the other hand, if the specific surface area is too large, the physical strength becomes too small.

  The positive electrode and secondary battery of the present invention can be produced by using the positive electrode active material of the present invention and using known materials and methods.

  Next, the effect of vanadium in the present invention will be described. By using the positive electrode active material for a secondary battery according to the present invention, the discharge capacity during high rate discharge increases as will be described later in the evaluation results of the examples. The mechanism of action is not clear, but the following mechanism is presumed, and one or both of the following mechanisms may be acting.

  When vanadium replaces manganese in lithium manganese phosphate, the electronic conductivity of the compound is improved by replacing manganese with vanadium, which has higher conductivity, or with vanadium having a different ionic radius. As a result, the lithium ion conduction path is changed and the lithium ion conductivity is considered to be improved.

  In addition, when vanadium exists as a phase separate from lithium manganese phosphate, the compound (vanadium compound) produced as a separate phase has excellent electronic conductivity, and this coexists in the secondary particles. It is considered that the utilization rate of lithium manganese phosphate is improved. In this case, as will be described later in Examples, the primary particles of lithium manganese phosphate and the vanadium compound are brought into close contact with each other by synthesizing the lithium manganese phosphate raw material and the vanadium source in the precursor aqueous solution. Therefore, it is considered that a large effect can be obtained even if the ratio of the number of vanadium atoms to the sum of the numbers of manganese and vanadium atoms is small.

Example 1
Weighs 6.74 g of lithium hydroxide monohydrate (LiOH.H ₂ O) (Nacalai Tesque, Inc.) and 10.60 g of diammonium hydrogen phosphate ((NH ₄ ) ₂ HPO ₄ ) (Nacalai Tesque, Inc.) After being dissolved in 80 and 20 mL of ion-exchanged water, both solutions were mixed with stirring, and the mixed solution was bubbled with N ₂ gas for about 3 minutes. Thereafter, 0.09 g of ammonium vanadate (NH ₄ VO ₃ ) (Nacalai Tesque, Inc.) was weighed out and added to the above mixed solution and mixed. Next, 19.09 g of manganese sulfate pentahydrate (MnSO ₄ .5H ₂ O) (Nacalai Tesque) was dissolved in 60 mL of ion exchange water in which 0.36 g of ascorbic acid (Nacalai Tesque, Inc.) was dissolved. The solution was bubbled with N ₂ gas for about 3 minutes. This precursor solution was added to the mixed solution of LiOH.H ₂ O, (NH ₄ ) ₂ HPO ₄ and NH ₄ VO ₃ to obtain an aqueous precursor solution. 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 precursor aqueous solution was transferred to a polytetrafluoroethylene container (internal volume 500 cm ³ ), and this was installed in a hydrothermal reaction container (Pressure Glass Industry 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 of the precursor aqueous solution was agitated by rotating the stirring blade at 200 rpm from the start of the temperature increase until the temperature decrease was completed. The pressure in the hydrothermal reaction vessel during the synthesis reaction was about 1.2 atmospheres. After completion of the synthesis reaction, the aqueous solution in the polytetrafluoroethylene container is filtered, washed with ion-exchanged water and acetone, and then vacuum-dried at 120 ° C. for 5 hours to obtain phosphorus containing 1 atomic% of vanadium. Lithium manganese oxide (LiMn _0.99 V _0.01 PO ₄ ) was produced.

2.29 g of polyvinyl alcohol (Wako Pure Chemical Industries, Ltd., polymerization degree 1500) is added to 2.0 g of the synthesized vanadium-containing lithium manganese phosphate powder, mixed using an agate mortar, and further heated to 60 ° C. A small amount of the water was added and mixed and kneaded again in a mortar to obtain a gum-like paste. The mixture was put 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. 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. In this way, vanadium-containing lithium manganese phosphate having carbon on the surface of the particles was produced.

(Examples 2-6 and comparative examples)
Vanadium-containing lithium manganese phosphate with carbon on the particle surface as Examples 2 to 6 in the same manner as Example 1 except that the amounts of manganese sulfate pentahydrate and ammonium vanadate used as raw materials were different, and comparison As an example, lithium manganese phosphate having carbon on the particle surface was prepared. Table 1 shows the amounts of raw materials used in the production of all examples and comparative examples. In Table 1, V addition amount (mol%) is the ratio of the number of V atoms to the sum of the number of Mn and V atoms. The same meaning is used in the following charts.

(X-ray diffraction measurement)
The vanadium-containing and non-containing lithium manganese phosphate powder with carbon on the surface is subjected to X-ray diffraction (XRD) measurement by performing an X-ray diffractometer using a CuKα radiation source (manufactured by Rigaku, model name: MiniFlex II). Went. 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 °.

FIG. 1 shows XRD profiles of the active material particles of Examples and Comparative Examples. In FIG. 1, in the comparative example not containing vanadium, only the diffraction peak attributed to the olivine type crystal structure was observed. On the other hand, even when a part of manganese was replaced with vanadium in Examples 1 to 6, the XRD profile derived from the olivine type crystal structure was maintained until Example 6 in which the V addition amount reached 20 mol% was maintained. Has been. Therefore, also in Example 1-6, most vanadium present in Mn site of LiMnPO _4, considered _{LiMn 1-x} V _x PO ₄ having an olivine-type crystal structure was the main component.

  On the other hand, in Examples 1 to 6, peaks not attributed to the olivine type crystal structure were observed in the vicinity of 2θ = 23.4 °, 34.2 °, and 36.7 °. Therefore, a part of vanadium may form a phase different from the phase having the olivine type crystal structure. However, details are unknown.

  Next, simple cells were prepared using the positive electrode active materials of Examples 1 to 6 and comparative examples made of vanadium-containing and non-containing lithium manganese phosphate having carbon on the surface, and the battery performance was evaluated.

(Preparation of positive electrode)
A positive electrode containing N-methyl-2-pyrrolidone (NMP) as a solvent, containing a positive electrode active material, acetylene black as a conductive additive, and polyvinylidene fluoride as a binder in a mass ratio of (80: 8: 12). A paste 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 was pressed so that the thickness after bending was 400 μm, to obtain a positive electrode. The application area of the active material after bending was 2.25 cm ² , and the application mass was 0.07 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 (JIS symbol: SUS316) mesh current collector attached with a stainless steel (JIS symbol: SUS316) mesh current collector is bonded to both sides of a 300 μm thick lithium metal foil and pressed. did.

(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 (JIS symbol: 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 non-aqueous An electrolyte was prepared. 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 that they were completely immersed therein.

(Charge / discharge test)
The battery produced as described above was subjected to a charge / discharge process in which one cycle of charge / discharge 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.

(High rate discharge test)
Following the above charge / discharge test, a constant current / constant voltage charge of current 0.1 CmA (approximately 10 hour rate), voltage 4.3 V, and 15 hours was performed, and then discharge current 2 CmA (approximately 0.5 hour rate), discharge A constant current discharge with a final voltage of 2.0 V was performed. The ratio of the discharge capacity obtained during the high rate discharge test to the discharge capacity of the charge / discharge test was recorded as “discharge capacity maintenance rate (%)”.

(Results of charge / discharge test and high rate discharge test)
Table 2 shows the test results of simple cells using the positive electrode active materials of Examples 1 to 6 and Comparative Example. FIG. 2 shows the discharge capacity at a current of 0.1 CmA, and FIG. 3 shows the discharge capacity at a current of 2 CmA.

  In Table 2, FIG. 2, and FIG. 3, compared with the comparative example which does not contain vanadium, in all the samples which added vanadium, the discharge capacity maintenance factor is large (Examples 1-6). The discharge capacity and the discharge capacity retention rate at a current of 2 CmA increase as the amount of V added is increased (Examples 1 to 4). However, when the amount of V added exceeds 10 mol%, the discharge capacity at a current of 2 CmA tends to decrease. It was observed. When the amount of addition of V exceeds 15 mol%, the discharge capacity at a current of 2 CmA was almost the same as that of the comparative example, and a tendency that the discharge capacity retention rate decreased was observed. Therefore, it can be seen that the V addition amount in the present invention is preferably 1 mol% to 20 mol%. Moreover, since the discharge capacity at the time of 2 CmA discharge is large, 1 mol%-15 mol% are more preferable, and since the discharge capacity at the time of 0.1 CmA discharge is large, 5 mol%-10 mol% are especially preferable. Since the active material for a secondary battery of the present invention is excellent in high rate discharge characteristics, it is advantageous in applications where high rate discharge is required, for example, a hybrid vehicle power source.

  In addition, this invention is not limited to the above embodiment or Example, A various deformation | transformation is possible within the range of the technical idea of this invention. For example, the positive electrode active material for a secondary battery of the present invention can be used for secondary batteries other than lithium secondary batteries because sodium and magnesium can be inserted and removed.

Claims (7)

Contains lithium, manganese, phosphorus, oxygen and vanadium elements, and
Contains a compound of the olivine structure,
A positive electrode active material for a secondary battery, wherein the ratio of the number of vanadium atoms to the sum of the number of manganese and vanadium atoms is 20% or less. 2. The positive electrode active material for a secondary battery according to claim 1, wherein the ratio of the number of vanadium atoms to the sum of the number of manganese and vanadium atoms is 1% or more and 15% or less. The positive electrode active material for a secondary battery according to claim 2, wherein the ratio of the number of vanadium atoms to the sum of the number of manganese and vanadium atoms is 5% or more and 10% or less. In the X-ray diffraction measurement using Cu-Kα rays, in addition to the diffraction peak derived from the olivine type crystal structure, diffraction peaks appear in the vicinity of 2θ = 23.4 °, 34.2 ° and 36.7 °.
The positive electrode active material for secondary batteries as described in any one of Claims 1-3.   The positive electrode active material particle for secondary batteries which further provided carbon to the particle | grain of the positive electrode active material as described in any one of Claims 1-4.   The positive electrode using the positive electrode active material for secondary batteries as described in any one of Claims 1-5.   The secondary battery using the positive electrode active material for secondary batteries as described in any one of Claims 1-5.

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