WO2015146423A1

WO2015146423A1 - Positive electrode active material, positive electrode for secondary batteries, secondary battery and method for producing positive electrode active material

Info

Publication number: WO2015146423A1
Application number: PCT/JP2015/055130
Authority: WO
Inventors: 吉則風間; 可織関根; 真也敷島; 英今井; 耕二幡谷
Original assignee: 古河電気工業株式会社
Priority date: 2014-03-27
Filing date: 2015-02-24
Publication date: 2015-10-01
Also published as: CN105144441A; TW201542459A; JP6046259B2; JPWO2015146423A1; KR20150135245A

Abstract

To provide a lithium iron manganese silicate-based positive electrode active material having a stable cation exchange structure, and the like. A positive electrode active material which is characterized by being represented by general formula Li_XFe_YMn_(1-Y)SiO₄ (0 < X ≤ 2.5, 0 < Y ≤ 1), by having a space group P2₁/n or a space group Pmn2₁ and by having a cation exchange structure wherein Li is introduced into an Fe/Mn site and Fe/Mn or Li is introduced into an Li site is obtained without using an electrochemical method. According to the present invention, even lithium iron manganese silicate which contains Mn that is amorphousized by charging and discharging is able to obtain a cation exchange structure. Since the cation exchange structure maintains a stable crystal structure during the subsequent charge and discharge, a secondary battery having excellent cycle characteristics is able to be achieved.

Description

Positive electrode active material, positive electrode for secondary battery, secondary battery, and method for producing positive electrode active material

The present invention relates to an iron manganese silicate lithium-based positive electrode active material used for a secondary battery.

In recent years, with the increasing mobility and functionality of electronic devices, secondary batteries, which are driving power sources, have become one of the most important components. In particular, lithium ion secondary batteries replace the conventional NiCd batteries and Ni hydrogen batteries due to the high energy density obtained from the high voltage of the positive electrode active material and the negative electrode active material used, and the position of the mainstream of secondary batteries It has come to occupy. However, the lithium ion secondary battery using a combination of a lithium cobalt oxide (LiCoO ₂ ) positive electrode active material and a graphite-based carbon negative electrode active material, which is used as a standard in current lithium ion batteries, has a high performance in recent years. The power consumption of high-load electronic components cannot be sufficiently supplied, and the required performance cannot be satisfied as a portable power source.

Furthermore, since lithium cobaltate uses cobalt, which is a rare metal, resource constraints are large, it is expensive, and there is a problem in price stability. Further, since lithium cobaltate releases a large amount of oxygen at a high temperature of 180 ° C. or higher, there is a possibility that explosion occurs during abnormal heat generation or short-circuiting of the battery.

Therefore, polyanionic lithium silicate transition metals such as lithium iron silicate (Li ₂ FeSiO ₄ ) and lithium manganese silicate (Li ₂ MnSiO ₄ ), which have better thermal stability than lithium cobaltate, are a resource. It is attracting attention as a material that satisfies the requirements of cost, cost and safety. The lithium silicate transition metal lithium has two Li in the composition formula, and is a material that can be expected to have a high capacity by a two-electron reaction.

Lithium iron silicate as a positive electrode material is known to be able to remove and insert only one Li when charge and discharge are performed after synthesis, and it is difficult to realize a high capacity for two Li ( For example, Non-Patent Document 1). This is because the reaction potential of the second electron is as high as 4.8 V (Non-patent Document 2), and when the battery cell is actually charged and discharged, the electrolytic solution is decomposed at a high potential of 4.5 V or higher. This is due to the inability to charge and discharge. On the other hand, it is known that the crystal structure of lithium iron silicate that reacts only with one electron changes during the first charge (for example, Patent Document 1, Non-Patent Documents 1 and 3). When lithium iron silicate is charged, Li is desorbed from some Li sites. At this time, Fe atoms move to the Li sites where Li atoms originally existed. As a result, at the time of discharge, Li is inserted into the conventional Fe site, and after such a cation exchange structure is obtained, the Li insertion and removal is stably performed by charge and discharge.

On the other hand, lithium manganese silicate as the positive electrode material is charged and discharged after synthesis, and the reaction potential is 4.5 V or less for both the first and second electrons. It is known that the material can achieve a high capacity. However, the lithium manganese silicate has an amorphous crystal structure due to the first charge, and the two-electron reaction cannot be performed with good cycle characteristics (for example, Non-Patent Document 4).

Japanese Patent No. 5298286

Patent Document 1 describes lithium iron silicate and lithium manganese silicate, and also discloses an XRD pattern resulting from the crystal structure after charging. However, it is known that manganese-manganese silicate and manganese-manganese silicate containing manganese are amorphous during the first charge and do not retain the crystal structure. A good quality cation exchange structure of lithium manganese silicate cannot be stably formed.

The present invention has been made in view of the above-described problems, and an object thereof is to provide a lithium iron silicate lithium-based positive electrode active material having a stable cation exchange structure.

In order to achieve the above-described object, the first invention is represented by the general formula Li _X Fe _Y Mn _(1-Y) SiO ₄ (0 <X ≦ 2.5, 0 <Y ≦ 1), and the space group P2 ₁ / n or Pmn2 ₁ having a crystal structure, a Li atom in part of the Fe / Mn site, and a Fe atom or Mn atom in part of the Li site It is a positive electrode active material characterized by having an exchange structure. The crystal structure of at least one of the space group P2 ₁ / n or Pmn2 ₁ described above includes a crystal structure having the space group P2 ₁ / n or Pmn2 ₁ as a base structure.

Here, the space group _P2 1 / n or Pmn2 ₁ the base material structure, and the space group _P2 1 / n, such as Non-Patent Document 5, is a crystal structure with a space group Pmn2 ₁ such as Non-Patent Document 6 It does not mean that, but describes only the relative positional relationship of the atoms based on their crystal structure.
Journal of the American chemical society, 2011, 133, 1263-1265. Electrochemistry Communications, 7, 156, 2005.

The crystal structure represented by the space group P2 ₁ / n and the crystal structure represented by the space group Pmn2 ₁ are very close to each other. These crystal structures are obtained by manufacturing a material having a composition represented by the general formula Li _X Fe _Y Mn _(1-Y) SiO ₄ (0 <X ≦ 2.5, 0 <Y ≦ 1) by firing. In the present specification, it is called a normal structure because it is a stable crystal structure that is normally generated. Here, as in Non-Patent Document 5, when Y is close to 1 at a certain firing temperature, P2 ₁ / n is obtained, and when Y is close to 0, Pmn2 ₁ is present, and P2 ₁ / n and Pmn2 ₁ coexist in the meantime. It is known that it is common to be in a state or a solid solution state. ((Non-Patent Document 7) Journal of Materials Chemistry 2011, 21, 17823-17831)

However, since an equilibrium state is not always obtained with an actual material and the difference in crystal structure is slight, identification by X-ray diffraction is difficult. Therefore, the crystal structure is uniquely determined in relation to the composition. difficult. Further, when practically used as a positive electrode active material, any crystal structure can be used in the same manner as long as the composition is in the above range. Therefore, in the present invention, these are not strictly distinguished, and the space group P2 ₁ It is defined as having a crystal structure of at least _one of / n or Pmn21.

On the other hand, a crystal structure in which an Fe atom or Mn atom and a Li atom are replaced from the position of the normal structure with respect to these normal structures is called a cation exchange structure. Here, the Fe / Mn site is a position where Fe atoms or Mn atoms are present in the normal structure. The Li site is a position where a Li atom exists in a normal structure.

In place of a part of Fe and / or Mn, at least one of Co or Ni may be substituted.

Instead of a part of Fe and / or Mn, at least one of Mg, Ca, Ti, V, Cr, Cu, Zn, Sr, Zr, and Mo may be substituted.

In the cation exchange structure, Fe or Mn has a valence of +2.5 to +3.5.

According to the first invention, a cation exchange structure can be obtained by a synthetic chemical method without using a conventional electrochemical method. For this reason, the present invention can also be applied to lithium iron manganese silicate that could not be transferred to the cation exchange structure because it was made amorphous by an electrochemical method. Further, the lithium iron manganese silicate having a cation exchange structure does not become amorphous during subsequent charge and discharge, and can maintain the crystal structure.

Thus, since a cation exchange structure of Mn-containing lithium manganese manganese silicate can be produced, a two-electron reaction can be obtained with good cycle characteristics, and an active material with increased capacity and increased energy density can be obtained. Can do.

Further, the same effect can be obtained even when at least one of Co and Ni is substituted in place of a part of Fe and / or Mn.

Further, in place of a part of Fe and / or Mn, any one of Mg, Ca, Ti, V, Cr, Zn, and Mo is added to increase the capacity and increase the energy density. In addition to the effects, the crystal structure can be stabilized and cycle characteristics can be improved.

Further, in place of a part of Li, at least one of Mg, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, Zr, and Mo is substituted. In addition, effects such as increased capacity, increased energy density, stabilized crystal structure, and improved cycle characteristics can be obtained.

In addition, in such a cation exchange structure, it is understood that Fe or Mn has a valence of +2.5 to +3.5, and Si or O changes in valence by changing the valence of Fe or Mn. It can be seen that the stable structure is maintained.

A second invention is a positive electrode for a secondary battery comprising: a current collector; and a positive electrode active material layer containing the positive electrode active material according to the first invention on at least one surface of the current collector. is there.

The third invention includes a positive electrode for a secondary battery according to the second invention, a negative electrode capable of inserting and extracting lithium ions, and a separator disposed between the positive electrode and the negative electrode, The secondary battery is characterized in that the positive electrode, the negative electrode, and the separator are provided in an electrolyte having lithium ion conductivity.

According to the second and third inventions, a positive electrode for a secondary battery and a secondary battery excellent in cycle characteristics can be obtained.

A fourth invention includes a step of synthesizing an iron manganese silicate lithium-based active material using at least a lithium source, an iron source, a manganese source, and a silicon source, and a step of desorbing a part of lithium from the active material And a step of heating the active material so that a part of the lithium site and a part of the iron site are exchanged and transferred to a cation exchange structure. .

According to the fourth invention, a positive electrode active material having a cation exchange structure can be obtained without using an electrochemical technique.

According to the present invention, a lithium iron manganese silicate positive electrode active material having a stable cation exchange structure can be provided.

The figure which shows the nonaqueous electrolyte secondary battery. 1 is a schematic view showing a fine particle manufacturing apparatus 1. FIG. It shows a unit cell of space group Pmn2 _1. The figure which shows the crystal structure with space group Pmn2 _1, and is a figure which shows a normal structure. The figure which shows the crystal structure with space group Pmn2 _1, and is a figure which shows a cation exchange structure. The conceptual diagram which shows the crystal structure 20a immediately after a synthesis | combination. The conceptual diagram which shows the crystal structure 20b of the state which remove | eliminated a part of Li. The figure which shows a cation exchange structure, and is the conceptual diagram which shows the crystal structure 20c of a one-electron charge state. It is a figure which shows a cation exchange structure, and is a conceptual diagram which shows the crystal structure 20d of a discharge state. The figure which shows the peak in a X-ray-diffraction measurement. The figure which shows the peak in a X-ray-diffraction measurement. Results of XANES (X-ray Absorption Near Edge Structure) of Fe-K end in normal structure and cation exchange structure. The valence of Fe estimated from the XANES spectrum of FIG. The result of XPS (X-ray Photoelectron Spectroscopy) in the normal structure and the cation exchange structure. XANES results for Mn-K ends in normal and cation exchange structures. The valence of Mn estimated from the XANES spectrum of FIG. XPS results for normal and cation exchange structures.

(Positive electrode active material)
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Lithium iron manganese silicate is represented by the general formula Li _X Fe _Y Mn _(1-Y) SiO ₄ (0 <X ≦ 2.5, 0 <Y ≦ 1). In general, the range is 0 <X ≦ 2. Moreover, it is desirable that 0 <Y <1.

Further, a part of Fe and / or Mn may be replaced with at least one of Co and Ni that can be expected to improve the energy density by increasing the practical amount and improving the average potential, and Fe and / or Mn A part of may be substituted with at least one of Mg, Ca, Ti, V, Cr, Zn, and Mo. By adding such an element, the crystal structure can be stabilized, the cycle life can be expected to be increased, and the energy density can be expected to be increased by increasing the actual capacity and the potential. Details of the crystal structure of lithium iron manganese silicate will be described later.

The particles of lithium iron manganese silicate of the present invention are present in the range of 10 to 200 nm when the particle size distribution of primary particles is determined by measuring the particle size by observation with a transmission electron microscope (TEM). Is preferably present at 25 to 100 nm. The particle size distribution is more preferably in the range of 10 to 150 nm and the average particle size in the range of 25 to 80 nm. Note that the existence of the particle size distribution in the range of 10 to 200 nm does not require the obtained particle size distribution to cover the entire range of 10 to 200 nm, the lower limit of the obtained particle size distribution is 10 nm or more, and the upper limit is It means 200 nm or less. That is, the obtained particle size distribution may be 10 to 100 nm or 50 to 150 nm.

Moreover, a part of SiO ₄ can be substituted with other anions. For example, transition metal acids such as titanic acid (TiO ₄ ), chromic acid (CrO ₄ ), vanadic acid (VO ₄ , V ₂ O ₇ ), zirconic acid (ZrO ₄ ), molybdic acid (MoO ₄ , Mo ₇₎ O ₂₄ ), tungstic acid (WO ₄ ), etc., or substitution with boric acid (BO ₃ ) or phosphoric acid (PO ₄ ). Replacing a part of the silicate ions with these anion species contributes to the suppression and stabilization of the crystal structure change due to repeated desorption and insertion of Li ions, and improves the cycle life. In addition, these anionic species are less likely to release oxygen even at high temperatures, and can be used safely without causing ignition.

The positive electrode active material preferably has a carbon coating on the surface. Furthermore, the positive electrode active material having a carbon coating preferably has a powder conductivity of 10 ⁻³ S / cm or more. When the powder conductivity of the positive electrode active material is 10 ⁻³ S / cm or more, sufficient conductivity can be obtained when used for the positive electrode. The carbon content in the positive electrode active material having a carbon coating is preferably 1.5% by weight or more. When the carbon content is 1.5% by weight or more, the powder conductivity is increased, and sufficient conductivity can be obtained when the positive electrode active material is used for the positive electrode.

(Positive electrode for non-aqueous electrolyte secondary battery)
The positive electrode active material can be used as a positive electrode active material used for a positive electrode for a nonaqueous electrolyte secondary battery. In order to form a positive electrode for a non-aqueous electrolyte secondary battery using a positive electrode active material, a conductive additive such as carbon black is further added to the powder of the positive electrode active material as necessary, and polytetrafluoroethylene or 95% by weight of aluminum is used as a slurry by adding a binder such as polyvinylidene fluoride and polyimide, a dispersant such as butadiene rubber, and a thickener such as carboxymethylcellulose and cellulose derivatives in an aqueous or organic solvent. On the current collector such as an aluminum alloy foil as described above, it is applied on one or both sides and baked to evaporate and dry the solvent. Thereby, the positive electrode for nonaqueous electrolyte secondary batteries which has an active material layer containing a positive electrode active material on a collector is obtained.

When the particle size of the positive electrode active material is small, the positive electrode active material is granulated with a carbon source or the like by a spray drying method in order to improve the slurry coating property, the adhesion between the current collector and the active material layer, and the current collecting property. May be. The granulated secondary particle lump becomes a large lump of about 1 to 20 μm, which improves the slurry coating property and further improves the characteristics and life of the battery electrode. As the slurry used for the spray drying method, either an aqueous solvent or a non-aqueous solvent can be used.

Furthermore, in a positive electrode in which a slurry containing a positive electrode active material is applied and formed on a current collector such as an aluminum alloy foil, the current collector surface roughness of the active material layer forming surface conforms to Japanese Industrial Standard (JIS B 0601-1994). The specified ten-point average roughness Rz is desirably 0.5 μm or more. The adhesiveness between the formed active material layer and the current collector is excellent, the electron conductivity accompanying the insertion and release of Li ions and the current collecting power to the current collector are increased, and the cycle life of charge / discharge is improved.

(Non-aqueous electrolyte secondary battery)
In order to obtain a high-capacity secondary battery using the positive electrode of the present embodiment, various materials such as a negative electrode, an electrolytic solution, a separator, and a battery case using a conventionally known negative electrode active material should be used without particular limitation. Can do.

FIG. 1 is a cross-sectional view showing a non-aqueous electrolyte secondary battery 30. The nonaqueous electrolyte secondary battery 30 according to the present embodiment includes a positive electrode 33, a negative electrode 35 that can occlude and release lithium ions, and a separator 37. The positive electrode 33, the negative electrode 35, and the separator 37 are stacked in the order of separator 37-negative electrode 35-separator 37-positive electrode 33. Further, the positive electrode 33 is wound so as to be on the inner side to constitute an electrode plate group, and is inserted into the battery can 41. The positive electrode 33 is connected to the positive electrode terminal 47 through the positive electrode lead 43, and the negative electrode 35 is connected to the battery can 41 through the negative electrode lead 45. As described above, the chemical energy generated inside the nonaqueous electrolyte secondary battery 30 can be taken out as electric energy. The battery can 41 is filled with an electrolyte 31 having lithium ion conductivity so as to cover the electrode plate group. A sealing body 39 is attached to the upper end (opening) of the battery can 41 via an annular insulating gasket. The sealing body 39 is composed of a circular lid plate and a positive electrode terminal 47 on the upper portion thereof, and a safety valve mechanism is built therein. Thus, the nonaqueous electrolyte secondary battery 30 is manufactured.

Although the secondary battery using the positive electrode according to the present embodiment has a high capacity and good electrode characteristics, the non-aqueous solvent containing fluorine is used in the electrolytic solution using the non-aqueous solvent constituting the secondary battery. When or is added, the capacity is hardly lowered even after repeated charging and discharging, resulting in a long life.
For example, in particular, when using a negative electrode containing a silicon-based high-capacity negative electrode active material, in order to suppress large expansion and contraction due to Li ion doping / dedoping, the electrolyte contains fluorine, It is desirable to use an electrolytic solution containing a non-aqueous solvent as a substituent. Since the fluorine-containing solvent relaxes the volume expansion of the silicon-based film due to alloying with Li ions during charging, particularly during the first charging process, it is possible to suppress a decrease in capacity due to charging and discharging. As the fluorine-containing non-aqueous solvent, fluorinated ethylene carbonate, fluorinated chain carbonate, or the like can be used. Mono-tetra-fluoroethylene carbonate (4-fluoro-1,3-dioxolan-2-one, FEC) is used for fluorinated ethylene carbonate, and

methyl

2,2,2-trifluoroethyl carbonate is used for fluorinated chain carbonate.

Ethyl

2,2,2-trifluoroethyl carbonate, etc., and these can be used alone or in combination with a plurality of electrolytes. Since the fluorine group is easy to bond with silicon and is strong, it is considered that the film can be stabilized and contribute to suppression of expansion even when it is expanded by charging alloy with Li ion.

(Method for producing positive electrode active material according to the present embodiment)
First, a precursor of lithium manganese manganese silicate is fired. The precursor of lithium manganese manganese silicate is synthesized by a production method including a reaction process such as flame hydrolysis or thermal oxidation, for example, a spray combustion method.

Next, the obtained precursor is mixed with a carbon source and fired in an inert gas atmosphere. A mixture of an amorphous compound or an oxide form contained in the precursor particles is changed to a crystalline iron-manganese silicate-based compound by firing to obtain a positive electrode active material.

Furthermore, since it is preferable to coat the surface of the positive electrode active material with carbon, it is preferable to anneal the positive electrode active material in an atmosphere of hydrocarbon gas.

(Precursor particle production method by spray combustion method)
An example of a fine particle production apparatus 1 that produces precursor particles by spray combustion is shown in FIG. A fine particle synthesis nozzle 9 is disposed in the reaction vessel 11, and a combustion gas supply unit 5, a combustion support gas supply unit 7, and a raw material solution supply unit 3 are connected to the reaction vessel 11. Combustible gas, air, raw material solution, and the like are supplied from the combustion gas supply unit 5, the combustion-supporting gas supply unit 7, and the raw material solution supply unit 3 to the flame generated from the fine particle synthesis nozzle 9. Further, the precursor particles 15 in the exhaust gas generated in the reaction vessel 11 are collected by the filter 13.

The spray combustion method consists of supplying raw materials into the flame together with the combustion-supporting gas and the combustible gas by supplying a raw material gas such as chloride or supplying a raw material liquid or raw material solution through a vaporizer. In this method, raw materials are reacted to obtain a target substance. As a spray combustion method, a VAD (Vapor-phase Axial Deposition) method or the like can be cited as a suitable example. The temperature of these flames varies depending on the mixing ratio of the flammable gas and the combustion-supporting gas and the addition ratio of the constituent raw materials, but is usually between 1000 and 3000 ° C., particularly about 1500 to 2500 ° C. It is more preferable that the temperature is about 1500 to 2000 ° C. When the flame temperature is low, there is a possibility that the fine particles may come out of the flame before the reaction in the flame is completed. Further, if the flame temperature is high, the crystallinity of the generated fine particles becomes too high, and a phase that is a stable phase but is not preferable as a positive electrode active material tends to be generated in the subsequent firing step.

Further, the flame hydrolysis method is a method in which constituent raw materials are hydrolyzed in a flame. In the flame hydrolysis method, an oxyhydrogen flame is generally used as a flame. A solution containing the constituent material of the positive electrode active material under the flame supplied with hydrogen gas as a combustible gas and oxygen gas as a combustible gas, and a flame raw material (oxygen gas and hydrogen gas) are simultaneously supplied from the nozzle. Synthesize the target substance. In the flame hydrolysis method, nanoscale ultrafine, mainly amorphous particles of the target substance can be obtained in an inert gas-filled atmosphere.

Also, the thermal oxidation method is a method in which constituent raw materials are thermally oxidized in a flame. In the thermal oxidation method, a hydrocarbon flame is generally used as the flame. A target material is synthesized while simultaneously supplying constituent raw materials and flame raw materials (for example, propane gas and oxygen gas) from a nozzle to a flame in which hydrocarbon gas is supplied as combustible gas and air is supplied as combustible gas. To do. As the hydrocarbon-based gas, paraffin-based hydrocarbon gases such as methane, ethane, propane, and butane, and olefin-based hydrocarbon gases such as ethylene, propylene, and butylene can be used.

(Constituent material for obtaining precursor particles)
The constituent raw materials for obtaining the precursor particles of the present embodiment are at least a lithium source, an iron source, a manganese source, and a silicon source. Furthermore, you may use the addition raw material of another element as needed. When the raw material is solid, it is supplied as a powder, dispersed in a liquid, or dissolved in a solvent to form a solution, which is supplied to a flame through a vaporizer. When the raw material is liquid, in addition to passing through the vaporizer, it can be vaporized and supplied by increasing the vapor pressure by heating or pressure reduction and bubbling before the supply nozzle. In particular, it is preferable to supply a mixed solution of a lithium source, an iron source, a manganese source, and a silicon source as mist droplets having a diameter of 20 μm or less.

Examples of lithium sources include lithium inorganic acid salts such as lithium chloride, lithium hydroxide, lithium carbonate, lithium nitrate, lithium bromide, lithium phosphate, and lithium sulfate, and lithium organic acids such as lithium oxalate, lithium acetate, and lithium naphthenate. A salt, a lithium alkoxide such as lithium ethoxide, an organic lithium compound such as a β-diketonate compound of lithium, lithium oxide, lithium peroxide, or the like can be used. Naphthenic acid is a mixture of different carboxylic acids mainly mixed with a plurality of acidic substances in petroleum, and the main component is a carboxylic acid compound of cyclopentane and cyclohexane.

As the iron source, ferric chloride, iron oxalate, iron acetate, ferrous sulfate, iron nitrate, iron hydroxide, ferric 2-ethylhexanoate, iron naphthenate and the like can be used. Furthermore, iron organic metal salts such as stearic acid, dimethyldithiocarbamic acid, acetylacetonate, oleic acid, linoleic acid, and linolenic acid, and iron oxide are also used depending on conditions.

As the manganese source, manganese chloride, manganese oxalate, manganese acetate, manganese sulfate, manganese nitrate, manganese oxyhydroxide, manganese 2-ethylhexanoate, manganese naphthenate, hexoate manganese and the like can be used. Furthermore, organic metal salts of manganese such as stearic acid, dimethyldithiocarbamic acid, acetylacetonate, oleic acid, linoleic acid, and linolenic acid, and manganese oxide are also used depending on conditions.

Silicon sources include silicon tetrachloride, octamethylcyclotetrasiloxane (OMCTS), silicon dioxide, silicon monoxide or hydrates of these silicon oxides, condensed silicic acid such as orthosilicic acid, metasilicic acid, metadisilicic acid, tetraethyl Orthosilicate (tetraethoxysilane, TEOS), tetramethylorthosilicate (tetramethoxysilane, TMOS), methyltrimethoxysilane (MTMS), methyltriethoxysilane (MTES), hexamethyldisiloxane (HMDSO), tetramethyldisiloxane (TMDSO), tetramethylcyclotetrasiloxane (TMCTS), octamethyltrisiloxane (OMTSO), tetra-n-butoxysilane, and the like can be used.

In addition, when a part of silicic acid of lithium iron manganese silicate is replaced with other anions, transition metal oxides, boric acid, and phosphoric acid raw materials are added as anion sources.

For example, titanium oxide, metal titanates such as iron titanate and manganese titanate, titanates such as zinc titanate, magnesium titanate, barium titanate, vanadium oxide, ammonium metavanadate, chromium oxide, chromium Acid salts and dichromates, manganese oxides, permanganates and manganates, cobaltates, zirconium oxides, zirconates, molybdenum oxides, molybdates, tungsten oxides, tungstates, boric acid and trioxides Various borates such as diboron, sodium metaborate, sodium tetraborate, borax, phosphoric acid, phosphoric acid such as orthophosphoric acid and metaphosphoric acid, phosphoric acid such as diammonium hydrogen phosphate, ammonium dihydrogen phosphate Use ammonium hydrogen salt, etc., depending on the desired anion source and synthesis conditions. Can.

These raw materials are supplied to the same reaction system together with the flame raw material to synthesize precursor particles. The produced precursor particles can be recovered from the exhaust gas with a filter. It can also be generated around the core rod as follows. A silica or silicon-based core rod (also called a seed rod) is installed in the reactor, and the lithium source, iron source, manganese source, and silicon source together with the flame raw material in the oxyhydrogen flame or propane flame sprayed on it. Is supplied and hydrolyzed or oxidized to produce fine particles of nanometer order mainly on the surface of the core rod. These generated fine particles are collected and, if necessary, passed through a filter or sieve to remove impurities and agglomerated parts. Precursor particles obtained in this manner are composed of fine particles having a nanoscale ultrafine particle size and mainly amorphous.

In the spray combustion method, which is a method for producing precursor particles according to the present embodiment, the precursor particles that can be produced are amorphous and the size of the particles is small. Furthermore, in the spray combustion method, a large amount of synthesis is possible in a short time compared to the conventional hydrothermal synthesis method and solid phase method, and homogeneous precursor particles can be obtained at low cost.

In the present invention, a positive electrode active material having a normal structure can be obtained by mixing the precursor particles with a reducing agent and baking. The precursor in this embodiment is a material capable of obtaining crystals of lithium iron manganese silicate by firing. In particular, the precursor in this embodiment is amorphous in which the valence of iron or manganese is trivalent, but the valence of iron or manganese is changed from trivalent to divalent by baking with a reducing agent. To do. It is desirable that the composition of the precursor particles satisfies the stoichiometric composition.

Further, the shape of the precursor particles is substantially spherical, and the average aspect ratio (major axis / minor axis) of the particles is 1.5 or less, preferably 1.2 or less, more preferably 1.1 or less. It should be noted that the fact that the particle is substantially spherical does not mean that the particle shape is a geometrically strict spherical or elliptical sphere, and the surface of the particle is generally a smooth curved surface even if there are a few protrusions. It only has to be configured.

When these precursor particles are measured by powder method X-ray diffraction in the range of 2θ = 10 to 60 °, they have little or no diffraction peaks, and even if they have, they show a wide diffraction angle. That is, the precursor particles are composed of fine particles having small crystallites or polycrystalline fine particles in which small single crystals are gathered, or are in a microcrystalline form in which an amorphous component exists around these fine particles.

In the spray combustion method of the present embodiment, carbon burns in the flame, so the obtained precursor particles do not contain carbon. Even if a carbon component is mixed, the amount is very small and is not so large as to be a conductive aid when used for the positive electrode.

(Manufacture of positive electrode active material)
The precursor particles obtained by the spray combustion method are further mixed with a carbon source, and then fired in an inert gas-filled atmosphere. At this time, the amorphous compound or oxide mixture contained in the precursor particles is changed into a polyanion-based iron manganese lithium silicate-based crystal form compound by firing.

Further, under an inert gas filled atmosphere, it is possible to prevent the carbon source from burning during firing and the positive electrode active material from being oxidized. Nitrogen gas, argon gas, neon gas, helium gas, carbon dioxide gas, etc. can be used as the inert gas. In order to increase the conductivity of the product after firing, a polyhydric alcohol such as polyvinyl alcohol, a polymer such as polyvinyl pyrrolidone, carboxymethyl cellulose, and acetyl cellulose, a saccharide such as sucrose, and a conductive carbon such as carbon black are used as a carbon source. In addition to the precursor particles, firing is performed before firing. Polyvinyl alcohol is particularly preferable because it plays a role as a binder for the precursor particles before firing, and can favorably reduce iron and manganese during firing.

Calcination conditions can be suitably obtained by combining a temperature of 300 to 900 ° C. and a treatment time of 0.5 to 10 hours to obtain a fired product having desired crystallinity and particle size. Excessive heat load due to high temperature or prolonged firing can generate coarse crystal grains, and should be avoided, under heating conditions such that the desired crystalline or microcrystalline lithium iron manganese silicate is obtained, Firing conditions that can suppress the crystallite size as small as possible are desirable. The firing temperature is preferably about 400 to 700 ° C.

(Annealing with hydrocarbon gas)
A positive electrode active material is formed by firing, and then annealed with a hydrocarbon gas to form a carbon coating on the surface of the positive electrode active material.

The temperature during annealing is preferably 600 ° C. to 750 ° C. This is because if the annealing temperature is too low, the deposition of carbon from the hydrocarbon gas is slow, and if it is too high, the crystal grows excessively.

The hydrocarbon gas is preferably one or more selected from methane, ethane, propane, and butane. The hydrocarbon gas also has a reducing property, but a reducing gas may be mixed and supplied for further reduction.

The reducing gas is preferably one or more selected from hydrogen, acetylene, carbon monoxide, hydrogen sulfide, sulfur dioxide, and formaldehyde.

By annealing, the hydrocarbon gas reacts with iron or particles containing iron carbide, the hydrocarbon gas is decomposed and combined, and the surface of the positive electrode active material can be coated with carbon.

In addition, since the obtained positive electrode active material is often agglomerated in the firing step or the annealing step, it can be made into fine particles again by applying to a mortar, a ball mill or other pulverizing means.

FIG. 4 (a), the fine particles formed as described above, is a diagram showing a crystal structure having a space group Pmn2 _1. FIG. 4 shows a crystal structure having the space group Pmn2 ₁ , but the following description is the same for the crystal structure having the space group P2 ₁ / n. The crystal structure having the space group Pmn2 ₁ is a crystal structure having an orthorhombic crystal as a unit cell and 16 atoms in the unit cell.
When the unit cell is represented by a perspective view, it is as shown in FIG. 3, and each side of a, b, and c is orthogonal. In the system of Li _X Fe _Y Mn _(1-Y) SiO ₄ (0 <X ≦ 2.5, 0 <Y ≦ 1), the lengths (lattice constants) of a, b, and c are each 6.3 angstroms. It has values of about 5.3 angstroms and 5.0 angstroms, and can vary by about 1% depending on the composition.

When the unit cells are arranged repeatedly, the result is as shown in FIG. 4A, but the atom indicated by A in FIG. 4A is an Fe atom or Mn atom. Similarly, the atom shown by B is a Si atom. The atom represented by C is a Li atom. An atom represented by O is an O atom.

In addition, the crystal structure having a space group of P2 ₁ / n refers to the four faces of O atoms surrounding atoms in a row parallel to the a axis formed by the Fe / Mn site and Si site of Pmn2 _{1 in} FIG. It is a structure in which the body orientation changes periodically. Therefore, unlike the orthorhombic crystal shown in FIG. 3, the unit cell of P2 ₁ / n is a monoclinic crystal having a different axis and has a long-period structure, but it is understood that the atomic arrangement is very close.

The crystal structure shown in FIG. 4 (a) is called a normal structure. This normal structure is composed of a chain portion in which a tetrahedron formed by Si—O bonds (shown by a broken line in FIG. 4) and a tetrahedron formed by Fe / Mn—O bonds (not shown in FIG. 4), and a tetrahedron formed by Li—O bonds. It is composed of chain parts that are connected to each other (not shown in FIG. 4). In FIG. 4A, a position where an Fe atom or Mn atom exists is called an Fe / Mn site, and a position where a Li atom exists is called a Li site. That is, the normal structure has a structure in which Fe atoms or Mn atoms are contained in the Fe / Mn site and Li atoms are contained in the Li site. Note that the tetrahedrons formed by the respective atoms and O atoms in FIG. 4 are shown only for the tetrahedrons with Si—O bonds for the sake of clarity in the drawing.

FIG. 5 (a) is a conceptual diagram showing a crystal structure 20a that is two-dimensionally expressed by simplifying the normal structure shown in FIG. 4 (a). In practice, Si and Fe combine with oxygen to form a tetrahedron, but illustration thereof is omitted. The normal structure takes a structure in which Fe atoms or Mn atoms are contained in the Fe / Mn site and Li atoms are contained in the Li site, and is usually used as a positive electrode active material in this state.

In the present invention, further chemical treatment is performed from this state. For example, acid treatment with hydrochloric acid or immersion in water is performed. By performing this treatment, Li atoms can be desorbed from a part of the Li site as in the crystal structure 20b shown in FIG. That is, a part of the Li site becomes a hole.

When heat treatment is performed at a predetermined temperature in an inert gas atmosphere from this state, it is considered that Fe atoms or part of Mn atoms in the Fe / Mn sites move to Li sites that are vacant.
FIG. 6A is a diagram showing the crystal structure 20c in a state where Fe atoms or Mn atoms move to the Li site and vacancies are formed in the Fe / Mn site. This state becomes the crystal structure of the positive electrode active material having a cation exchange structure.

That is, conventionally, in order to obtain this form, the crystal structure 20c can be obtained by charging from the crystal structure 20a. However, in the present invention, a cation exchange structure can be obtained without using such an electrochemical technique. Therefore, a cation exchange structure can be obtained synthetically and chemically by this method even for lithium iron manganese silicate containing manganese that becomes amorphous when charged from the state of the crystal structure 20a.

When discharging from this state, the crystal structure 20d shown in FIG. 6B is obtained. This is shown three-dimensionally as shown in FIG. In the figure, AC indicates that both Fe atom or Mn atom and Li atom can be arranged. That is, Li atoms are inserted into vacancies formed at the Fe / Mn site. After this, even if charging / discharging is repeated, the change of the crystal structures 20c and 20d is repeated while maintaining the cation exchange structure.

Next, the crystal structure of the positive electrode active material obtained by the above method was evaluated by X-ray diffraction measurement using CuKα rays. 7 and 8 show the measurement results.

In FIG. 7, D is a measurement result of the normal structure (crystal structure 20a) of Li _x FeSiO ₄ , and it is possible that the space group P2 ₁ / n is included or a part of the crystal structure of Pmn2 ₁ is included. It is a measurement result to have. E is the measurement result of the cation exchange structure (crystal structure 20d) of Li _x FeSiO ₄ (0 <X ≦ 2.5), and F is Li _x (Fe _0.75 Mn _0.25 ) SiO ₄ (0 <X It is a measurement result of the cation exchange structure (crystal structure 20d) of ≦ 2.5).

In both Li _x FeSiO ₄ and Li _x (Fe _0.75 Mn _0.25 ) SiO ₄ , at least one of the cation exchange structures of the space group P2 ₁ / n or Pmn2 ₁ is not electrochemical but synthetic chemistry. You can see that it was obtained. In particular, two peaks near 22.2 degrees and 23.0 degrees are characteristic of the cation exchange structure in this composition. These have the same peak as the cation exchange structure shown in Non-Patent Document 1 or Non-Patent Document 2. That is, even Li ₂ (Fe _0.75 Mn _0.25 ) SiO ₄ containing Mn could be transferred to a cation exchange structure without being substantially amorphous.

The positive electrode active material obtained above was mixed with a conductive assistant (carbon black) at 10% by weight, and further mixed for 5 hours using a ball mill in which the inside was replaced with nitrogen. The mixed powder and polyvinylidene fluoride (PVdF) as a binder were mixed at a weight ratio of 95: 5, and N-methyl-2-pyrrolidone (NMP) was added and kneaded sufficiently to obtain a positive electrode slurry.

The positive electrode slurry was applied to an aluminum foil current collector with a thickness of 15 μm at a coating amount of 50 g / m ² and dried at 120 ° C. for 30 minutes. Thereafter, it was rolled to a density of 2.0 g / cm ³ with a roll press, punched into a 2 cm ² disk shape, and used as a positive electrode.

Using these positive electrodes, metallic lithium for the negative electrode, and a mixed solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 1: 1 in the electrolyte solution, LiPF ₆ was dissolved at a concentration of 1M, and a lithium secondary battery was used. Produced. 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 formed using the positive electrode, the negative electrode, the electrolyte, and the separator.

Next, with the coin-type lithium secondary battery, test evaluation of the electrode characteristics of the positive electrode active material,
It carried out as follows.
Charging to 4.5 V (vs. Li / Li +) by CC-CV method (constant current constant voltage) at a test temperature of 25 ° C. or 60 ° C. and a current rate of 0.1 C, and then a current rate of 0.01 C The charging was stopped after it dropped to. Thereafter, the battery was discharged at a rate of 0.1 C to 1.5 V (same as above) by the CC method (constant current), and the charge / discharge capacity and cycle life were measured.

Table 1 shows the results of the initial discharge capacity. Table 2 shows the discharge capacity after 30 cycles. Examples of the present invention are indicated as “cation exchange structure” in the table, and the crystal structure before the first charge (discharge state) is a cation exchange structure. The comparative example is indicated as “normal structure” in the table, and the crystal structure before the first charge (discharge state) is the normal structure. As can be seen from Tables 1 and 2, by using a positive electrode active material having a cation exchange structure before the first charge and charging and discharging, a high capacity and a normal structure (before the first charge) It can be seen that better cycle characteristics are obtained.

In FIG. 9A, in the normal structure and the cation exchange structure of the positive electrode active materials Li _x FeSiO ₄ and Li _x (Fe _0.75 Mn _0.25 ) SiO _{4 in} the uncharged / discharged state prepared by the above method, The result of XANES (X-ray Absorption Near Edge Structure) of Fe-K end is shown. 9B, with reference to the measurement method disclosed in Non-Patent Document 8, the energy at the 90% position of the normalized XANES spectral intensity is defined as the absorption edge rising, and the valence of Fe and the absorption edge rising position are measured. The correlation was estimated. From these, it can be seen that in the normal structure, Fe having +2 valence is oxidized by becoming a cation exchange structure and is changed to around +3 valence. The valence of Fe in the cation exchange structure estimated at this time was 3.1 to 3.2.
(Non-patent Document 8) Electrochimica Acta 55 (2010) 8876.

FIG. 10 shows Fe2p _3/2 in the normal structure and the cation exchange structure of the positive and negative electrode active materials Li _x FeSiO ₄ and Li _x (Fe _0.75 Mn _0.25 ) SiO ₄ prepared by the above method. Results of XPS (X-ray Photoelectron Spectroscopy) are shown. Looking at the results of the normal structure, it can be seen that a broad peak (deviation from the comparison target) structure is observed in the vicinity of 710 eV and 716 eV in the same manner as the standard sample in which Fe is +2 valent. I understand. On the other hand, in the cation exchange structure, since these peak structures are not seen, it is suggested that Fe has +3 valence vicinity. From the results of XANES and XPS, it was confirmed that the valence of Fe having a cation exchange structure was increased from the valence of Fe having a normal structure.

FIG. 11A shows XANES at the Mn—K end in the normal structure and the cation exchange structure of the uncharged / discharged positive electrode active material Li _x (Fe _0.75 Mn _0.25 ) SiO ₄ produced by the above-described method. The results are shown. In FIG. 11B, referring to the measurement method disclosed in Non-Patent Document 8, the energy at the 90% position of the normalized XANES spectral intensity is defined as the absorption edge rising, and the Mn valence and the absorption edge rising are illustrated. The position correlation was estimated. From these, it can be seen that Mn having +2 valence in the normal structure is oxidized by becoming a cation exchange structure and approaches the +3 valence direction. The valence estimated at this time was 2.6.

FIG. 12 shows the XPS results of Mn2p _3/2 in the normal structure and cation exchange structure of the positive electrode active material Li ₂ (Fe _0.5 Mn _0.5 ) SiO ₄ produced by the above method. When the result of the normal structure is seen, a broad peak (deviation from the comparison target) structure is observed in the vicinity of 647 eV as in the case of the standard sample having Mn of +2, indicating that Mn has a vicinity of +2. On the other hand, in the cation exchange structure, since this peak structure is not seen, it is suggested that Mn is approaching in the +3 valence direction. From the results of XANES and XPS, it was confirmed that the valence of Mn having a cation exchange structure was increased from the valence of Mn having a normal structure.

From the above, according to the present invention, the composition Li _X Fe _{Y in} an uncharged / discharged state (before use as a battery) is obtained by changing the lithium silicate transition metal to a cation exchange structure regardless of the electrochemical method. In Mn _(1-Y) SiO ₄ (0.5 <X ≦ 1.5, 0 <Y ≦ 1), it can be seen that Fe or Mn has a valence of +2.5 to +3.5. It can be seen that when Fe or Mn changes in valence, Si or O does not change in valence and maintains a stable structure. For this reason, since the stable crystal structure is maintained also in subsequent charging / discharging, the secondary battery excellent in cycling characteristics can be obtained.

FIG. 8 shows a change in crystal structure when Li _x (Fe _0.75 Mn _0.25 ) SiO ₄ is repeatedly charged and discharged. G is the result before charging / discharging, H is the result after one cycle of charging / discharging, and I is the result after five cycles of charging / discharging.

From the result, even if Li _x (Fe _0.75 Mn _0.25 ) SiO ₄ containing Mn was maintained, the cation exchange structure was maintained without being amorphized.

As described above, according to the present invention, lithium metal silicate can be changed into a cation exchange structure regardless of an electrochemical method. For this reason, in the past, even with lithium manganese silicate containing Mn that becomes amorphous by charge / discharge, a stable cation exchange structure is obtained, and a stable crystal structure is maintained during subsequent charge / discharge. A secondary battery can be obtained.

The preferred embodiments of the present invention have been described above with reference to the accompanying drawings, but the present invention is not limited to such examples. It will be apparent to those skilled in the art that various changes and modifications can be made within the scope of the technical idea disclosed in the present application, and these are naturally within the technical scope of the present invention. Understood.
In the figure, “O” means a normal structure, and “CM” means a cation exchange structure.

DESCRIPTION OF SYMBOLS 1 ......... Particle production apparatus 3 ......... Raw material solution supply part 5 ......... Combustion gas supply part 7 ......... Air supply part 9 ......... Particle synthesis nozzle 11 ......... Reaction vessel 13 ......... Filter 15 ... …… Precursor fine particles 20a, 20b, 20c, 20d ……… Crystal structure 30 ………… Non-aqueous electrolyte secondary battery 31 ………… Electrolyte 33 ……… Positive electrode 35 ……… Negative electrode 37 ……… Separator 39 …… ... Sealing body 41 ... ... Battery can 43 ... ... Positive electrode lead 45 ... ... Negative electrode lead 47 ... ... Positive electrode terminal

Claims

Represented by the general formula Li X Fe Y Mn (1-Y) SiO 4 (0 <X ≦ 2.5, 0 <Y ≦ 1),
It has a crystal structure of at least one of space group P2 1 / n or Pmn2 1 , and further Li atoms enter part of the Fe / Mn site, and either Fe atoms or Mn atoms enter part of the Li site. A positive electrode active material characterized by having a cation exchange structure.
2. The positive electrode active material according to claim 1, wherein at least one of Co and Ni is substituted in place of a part of Fe and / or Mn.
3. Instead of a part of Fe and / or Mn, at least one of Mg, Ca, Ti, V, Cr, Cu, Zn, Sr, Zr, and Mo is substituted. The positive electrode active material of any one of Claims.
The at least one of Mg, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, Zr, and Mo is substituted for a part of Li. The positive electrode active material according to any one of 1 to 3.
A current collector,
The positive electrode active material layer containing the positive electrode active material according to any one of claims 1 to 3, on at least one surface of the current collector;
A positive electrode for a secondary battery, comprising:
A positive electrode for a secondary battery according to claim 4,
A negative electrode capable of inserting and extracting lithium ions;
Having a separator disposed between the positive electrode and the negative electrode;
A secondary battery, wherein the positive electrode, the negative electrode, and the separator are provided in an electrolyte having lithium ion conductivity.
Using at least a lithium source, an iron source, a manganese source and a silicon source to synthesize an iron manganese silicate lithium-based active material;
Desorbing a portion of lithium from the active material;
The active material is heated to replace a part of the Li site and a part of the Fe / Mn site,
Transferring to a cation exchange structure;
The manufacturing method of the positive electrode active material characterized by comprising.
Represented by the general formula Li X Fe Y Mn (1-Y) SiO 4 (0.5 ≦ X <1.5, 0 <Y ≦ 1),
It has a crystal structure of at least one of space group P2 1 / n or Pmn2 1 , and further Li atoms enter part of the Fe / Mn site, and either Fe atoms or Mn atoms enter part of the Li site. A positive electrode active material having a cation exchange structure in which the valence of Fe or Mn is +2.5 to +3.5.