KR20150135245A - Anode active material, anode for secondary battery, secondary battery, and method for manufacturing the anode active material - Google Patents

Abstract

A lithium-based manganese iron-based positive electrode active material having a stable cation exchange structure, and the like.
Regardless of the electrochemical method, having the general formula _{Li X Fe Y Mn (1-} Y) SiO 4 (0 <X≤2.5, 0 <Y≤1) is represented by the space group P2 ₁ / n or ₁ Pmn2 , A positive electrode active material characterized in that Li enters the Fe / Mn site and has a cation exchange structure in which one of Fe / Mn and Li is contained in the Li site. In the present invention, a cation exchange structure can also be obtained in lithium silicate manganese lithium containing Mn which is amorphized by charge and discharge. Such a cation exchange structure maintains a stable crystal structure even in subsequent charge and discharge, and thus a secondary battery excellent in cycle characteristics can be obtained.

Description

TECHNICAL FIELD The present invention relates to a positive electrode active material, a positive electrode for a secondary battery, a secondary battery, and a method for manufacturing a positive electrode active material. BACKGROUND OF THE INVENTION 1. Field of the Invention [0001]

The present invention relates to a lithium-based manganese lithium-based cathode active material used for a secondary battery.

2. Description of the Related Art [0002] In recent years, as electronic devices have become more mobile and more sophisticated, secondary batteries, which are driving power sources, have become one of the most important components. In particular, the lithium ion secondary battery has replaced the conventional NiCd battery or the Ni hydrogen battery to occupy the mainstream position of the secondary battery from the height of the energy density obtained from the high voltage of the used positive electrode active material and negative electrode active material. However, the lithium ion secondary battery by the combination of the lithium cobalt oxide (LiCoO ₂ ) cathode active material which is used for the current lithium ion battery and the carbon-based anode active material which is the main body of the graphite, consumes the high functional high- The power can not be supplied sufficiently and the required performance can not be satisfied as a portable power source.

Lithium cobalt oxide uses cobalt, which is a rare metal, and therefore, there are problems in resource stability, high price, and price stability. Further, since lithium cobalt oxide releases a large amount of oxygen at a high temperature of 180 DEG C or higher, there is a possibility that explosion may occur during abnormal heat generation or short circuit of the battery.

Therefore, a polyanionic silicate transition metal lithium, including lithium iron silicate (Li ₂ FeSiO ₄ ) or lithium manganese lithium (Li ₂ MnSiO ₄ ), which is superior in thermal stability to lithium cobalt oxide, As a material that meets the requirements. The lithium silicate transition metal lithium has two Li in the composition formula and is a material which can expect a high capacity by two electron reactions.

It is known that lithium silicate ferrite as a cathode material can be inserted and inserted only by one Li after carrying out charging and discharging after synthesis, and it is difficult to realize a high capacity of Li 2 pieces (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 electrolyte is decomposed at a high potential of 4.5 V or higher, . On the other hand, it is known that the crystal structure of lithium silicate which reacts only with one electron changes during the first charging (for example, Patent Document 1, Non-Patent Documents 1 and 3). When Li silicate is charged, Li is separated from some Li sites, but the Fe atoms migrate to Li sites originally having Li atoms. As a result, at the time of discharging, Li is inserted into the conventional Fe site. After this cation exchange structure is formed, release and insertion of Li is stably performed by charging and discharging.

On the other hand, lithium manganese lithium as a positive electrode material is charge / discharge after synthesis, and its reaction potential is 4.5 V or less for the first electron and the second electron, so that Li can be released and inserted into two portions and a material capable of realizing a high capacity Is known. However, the lithium manganese lithium can not be satisfactorily subjected to a two-electron reaction with good cycle characteristics because the crystal structure is amorphized by the first charging (see Non-Patent Document 4, for example).

Patent Document 1: Japanese Patent No. 5298286

Non-Patent Document 1: Journal of Electrochemical Society, 159 (5) A525-A531 (2012) Non-Patent Document 2: Electrochemical Communications 8 (2006) 1292-1298 Non-Patent Document 3: Journal of The American Chemical Society 2011, 133, 13031-13035 Non-Patent Document 4: Chemistry of Materials 2010, 22, 5754-5761

In Patent Document 1, lithium iron silicate and lithium manganese lithium are described, and an XRD pattern due to the crystal structure after filling is also disclosed. However, it has been known that lithium manganese silicate containing manganese or lithium manganese iron manganese is amorphized at the time of initial charging and does not maintain the crystal structure. Actually, it has been known by the method of Patent Document 1 that a lithium manganese- It is impossible to stably form a cation exchange structure.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a lithium-based manganese iron-based cathode active material having a stable cation exchange structure.

In order to achieve the above 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), space group (群) P2 ₁ / n or Pmn2 ₁ , and also has a cation exchange structure in which a Li atom is contained in a part of the Fe / Mn site and either a Fe atom or a Mn atom is contained in a part of the Li site As a cathode active material. The crystal structure of at least one of the above-described space group P2 ₁ / n or Pmn2 ₁ also includes a crystal structure having a space group P2 ₁ / n or Pmn2 ₁ as a matrix structure.

Here, assuming that the space group P2 ₁ / n or Pmn2 ₁ is a matrix structure does not mean a crystal structure having a space group P _{2 1} / n as in non-patent document 5 or a space group Pmn _{2 1} such as non-patent document 6 , Based on their crystal structure and describing only the relative positional relationship of the atoms.

Non-Patent Document 5: Journal of the American Chemical Society, 2011, 133, 1263-1265.

Non-Patent Document 6: 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 in a very close relationship. These crystal structures have a stable crystal structure which is usually produced by producing 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) It is referred to as a normal structure in this specification. Here, as in the case of non-patent document 5, P2 / n becomes ₁ / n when Y is close to ₁ and Pmn2 ₁ when Y is close to 0 at any firing temperature, and P2 ₁ / n and Pmn2 ₁ Coexisted state, or solid solution state ((Non-Patent Document 7) Journal of Materials Chemistry 2011, 21, 17823-17831).

However, since an equilibrium state can not always be obtained in an actual material, and the difference in crystal structure is also small, it is difficult to identify it by X-ray diffraction, so that the crystal structure is uniquely determined in relation to composition It is difficult to do. In the case of providing the cathode active material for practical use, any crystal structure can be used as long as it has the above-mentioned composition. Therefore, in the present invention, at least the space group P2 ₁ / n or Pmn2 ₁ It is defined as having one of the crystal structures.

On the other hand, for these conventional structures, a crystal structure in which a Fe atom or a Mn atom and a Li atom are substituted at a position of a general structure is called a cation exchange structure. Here, the Fe / Mn site refers to a position where a Fe atom or a Mn atom exists in a normal structure. The Li site is a position where a Li atom exists in a normal structure.

At least one of Co and Ni may be substituted for Fe and / or a part of Mn.

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

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. Therefore, the present invention can be applied to lithium silicate-manganese lithium which can not be transferred to a cation exchange structure because it becomes amorphous by an electrochemical method. Further, the lithium silicate manganese lithium having a cation exchange structure is not amorphized even after charging and discharging, and can maintain the crystal structure.

Since the cation exchange structure of Mn-containing lithium silicate manganese lithium can be produced in this manner, a two-electron reaction can be obtained with satisfactory cycle characteristics, and an active material with increased capacity and increased energy density can be obtained.

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

Further, by adding any one of Mg, Ca, Ti, V, Cr, Zn and Mo in place of a part of Fe and / or Mn, the same effect as the above- The crystal structure can be stabilized and the cycle characteristics can be improved.

Further, by substituting at least one of Mg, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, Zr and Mo in place of a part of Li, , The effect of increasing the energy density, stabilizing the crystal structure, and improving the cycle characteristics can be obtained.

Further, in such a cation exchange structure, Fe or Mn has a valence of +2.5 to +3.5. It can be seen that Si or O does not change in the valence state and maintains the stable structure due to the change in the number of Fe or Mn.

A second aspect of the present invention is a positive electrode for a secondary battery, comprising a current collector and a positive electrode active material layer comprising a positive electrode active material according to the first aspect of the invention on at least one side of the current collector.

According to a third aspect of the present invention, there is provided a secondary battery comprising a positive electrode for a secondary battery according to the second invention, a negative electrode capable of absorbing and storing lithium ions, and a separator disposed between the positive electrode and the negative electrode, Wherein the negative electrode and the separator are provided.

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

A fourth aspect of the present invention is a process for producing a lithium secondary battery comprising the steps of synthesizing an active material of lithium-manganese iron manganese at least using a lithium source, an iron source, a manganese source, and a silicon source; a step of removing a part of lithium from the active material; And a step of transferring a portion of the lithium site and a portion of the iron site to a cation exchange structure.

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

According to the present invention, it is possible to provide a cathode active material of lithium iron silicate having a stable cation exchange structure.

1 is a view showing a nonaqueous electrolyte secondary battery 30;
2 is a schematic view showing the particulate production apparatus 1. Fig.
3 is a diagram showing a unit grid of space group Pmn2 ₁ ;
4 (a) is a diagram showing a crystal structure having a space group Pmn2 _1, and showing a typical structure thereof.
Fig. 4 (b) is a diagram showing a crystal structure having a space group Pmn2 ₁ , which shows a cation exchange structure. Fig.
Fig. 5 (a) is a conceptual diagram showing a crystal structure 20a immediately after synthesis. Fig.
5 (b) is a conceptual diagram showing a crystal structure 20b in a state in which a part of Li is removed.
Fig. 6 (a) is a diagram showing a cation exchange structure, and is a conceptual diagram showing a crystal structure 20c of one electron-charged state. Fig.
Fig. 6 (b) is a diagram showing a cation exchange structure, and is a conceptual diagram showing a crystal structure 20d in a discharge state. Fig.
7 is a view showing a peak in X-ray diffraction measurement.
8 is a view showing a peak in X-ray diffraction measurement.
9 (a) is a result of X-ray absorption near-edge structure (XANES) of the Fe-K end in a normal structure and a cation exchange structure.
FIG. 9 (b) is the mantissa of Fe estimated in the XANES spectrum of FIG. 9 (a).
10 is a result of X-ray photoelectron spectroscopy (XPS) in a normal structure and a cation exchange structure.
11 (a) is a result of XANES of the Mn-K stage in a normal structure and a cation exchange structure.
11 (b) is a mantissa of Mn estimated in the XANES spectrum of Fig. 11 (a).
12 shows the results of XPS in a conventional structure and in a cation exchange structure.

(Cathode active material)

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The lithium silicate lithium manganese is represented by the general formula Li _x Fe _Y Mn _(1-Y) SiO ₄ (0 <X? 2.5, 0 < _{Y? 1} ). Normally, the range is 0 < It is preferable that 0 < Y < 1.

In addition, a part of Fe and / or Mn may be substituted with at least one of Co and Ni which can be expected to increase the energy density due to the increase of the actual capacity or the average potential, 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 and the cycle life can be improved. In addition, the energy density can be expected to be increased by increasing the actual capacity and the potential. Details of the crystal structure of lithium manganese silicide lithium will be described later.

The particles of the lithium silicate iron manganese lithium are present in the range of 10 to 200 nm when the particle diameter distribution of the primary particles is obtained by measuring the particle diameter by a transmission electron microscope (TEM) observation, and the average particle diameter is 25 to 100 nm Is preferably present. It is more preferable that the particle diameter distribution is in the range of 10 to 150 nm and the average particle diameter is in the range of 25 to 80 nm. The presence of the particle diameter distribution in the range of 10 to 200 nm means that the obtained particle diameter distribution does not need to span the entire range of 10 to 200 nm and that the lower limit of the obtained particle diameter distribution is 10 nm or more and the upper limit is 200 nm or less do. That is, the obtained particle diameter distribution may be 10 to 100 nm or 50 to 150 nm.

In addition, a part of SiO ₄ may be replaced by another anion. For example, a transition metal acid 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 ). By substituting a part of the silicate ion by these anion species, it contributes to inhibition and stabilization of crystal structure change due to repetition of separation and insertion of Li ions and improves cycle life. Further, since these anionic species are difficult to release oxygen even at high temperatures, they can be safely used because they do not lead to ignition.

The positive electrode active material preferably has a carbon coating on the surface. In addition, it is preferable that the powder conductivity of the cathode active material having a carbon coating is 10 ^-3 S / cm or more. When the powder conductivity of the positive electrode active material is 10 &lt; ^-3 &gt; S / cm or more, sufficient conductivity can be obtained when used for the positive electrode. Further, it is preferable that the carbon content in the positive electrode active material having carbon coating is 1.5 wt% or more. When the content of carbon is 1.5% by weight or more, the powder conductivity is also high, and when the positive electrode active material is used as the positive electrode, sufficient conductivity can be obtained.

(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 forming the positive electrode for a nonaqueous electrolyte secondary battery by using the positive electrode active material, the positive electrode active material powder may further contain a conductive additive such as carbon black, if necessary, and at the same time, a polytetrafluoroethylene or polyvinylidene fluoride, , A dispersing agent such as butadiene rubber, a thickening agent such as carboxymethyl cellulose and other cellulose derivatives, and an aqueous solvent is put in an organic solvent to form a slurry. The slurry is coated on a collector such as an aluminum alloy foil containing 95 wt% Coated on both sides or both sides, and fired to volatilize and dry the solvent. Thereby, a positive electrode for a nonaqueous electrolyte secondary battery having an active material layer containing a positive electrode active material on the current collector is obtained.

When the particle size of the positive electrode active material is small, the positive electrode active material may be granulated with a carbon source or the like by a spray-drying method in order to increase the applicability of the slurry, the adhesion between the current collector and the active material layer, and the current collecting property. The mass of the granulated secondary particles becomes a large mass of about 1 to 20 mu m, but the slurry application performance is improved, and the characteristics and the life of the battery electrode are also improved. The slurry used in the spray-drying method may be either an aqueous solvent or a non-aqueous solvent.

Further, in the case of a positive electrode in which a slurry containing a positive electrode active material is coated on a current collector such as an aluminum alloy foil, a ten-point average roughness Rz defined in Japanese Industrial Standard (JIS B 0601-1994) as a collector surface roughness of the active material layer- And more preferably 0.5 mu m or more. The adhesion between the formed active material layer and the current collector is excellent, the electron conductivity accompanying the insertion and desorption of Li ions increases, and the current collecting ability to the collector increases, and the cycle life of charging and discharging is improved.

(Non-aqueous electrolyte secondary battery)

In order to obtain a secondary battery of a high capacity using the anode of the present embodiment, various materials such as a negative electrode using an anode active material known in the art, an electrolytic solution, a separator, and a battery case can be used without particular limitation.

1 is a sectional view showing a nonaqueous electrolyte secondary battery 30. The nonaqueous electrolyte secondary battery 30 of the present embodiment is constituted by a positive electrode 33, a negative electrode 35 capable of absorbing and storing lithium ions, and a separator 37. The anode 33, the cathode 35 and the separator 37 are stacked in the order of the separator 37, the cathode 35, the separator 37 and the anode 33 in this order. Further, the positive electrode 33 is wound so as to be inwardly to constitute the electrode plate group (electrode plate group) and inserted into the battery can 41. The positive electrode 33 is connected to the positive electrode terminal 47 via the positive electrode lead 43 and the negative electrode 35 is connected to the battery can 41 via the negative electrode lead 45. As a result, the chemical energy generated inside the nonaqueous electrolyte secondary battery 30 can be taken out to the outside as electric energy. In the battery can 41, the electrolyte 31 having lithium ion conductivity is filled so as to cover the electrode plate group. A sealing member 39 is attached to an upper end (opening portion) of the battery can 41 via an annular insulating gasket. The sealing member 39 is constituted by a circular lid plate and a positive electrode terminal 47 thereon, and a safety valve mechanism is incorporated therein. Thus, the nonaqueous electrolyte secondary battery 30 is manufactured.

The secondary battery using the positive electrode according to the present embodiment has high capacity and good electrode characteristics. However, when a non-aqueous solvent containing fluorine is used or added to the electrolyte solution using the non-aqueous solvent constituting the secondary battery, The capacity is hardly lowered and the life is long.

For example, in the case of using a negative electrode containing a high-capacity negative active material of silicon, in order to suppress large expansion and contraction due to doping and de-doping of Li ions, it is preferable to use an electrolyte containing fluorine in the electrolyte or a nonaqueous solvent containing fluorine as a substituent It is preferable to use an electrolytic solution. The fluorine-containing solvent relaxes the volumetric expansion of the silicone film due to the alloying with Li ions during charging, particularly during the initial charging treatment, and thus the capacity decrease due to charge and discharge can be suppressed. Fluorinated ethylene carbonate or fluorinated chain-like carbonate can be used as the fluorine-containing nonaqueous solvent. Mono-tetra-fluoroethylene carbonate (4-fluoro-1,3-dioxolan-2-one, FEC) is used for the fluorinated ethylene carbonate and methyl 2,2,2-trifluoroethyl carbonate , Ethyl 2, 2, 2-trifluoroethyl carbonate, and the like, and they can be used singly or in a combination of two or more in an electrolytic solution. Since the fluorine group is easy to bond with silicon and is firm, it is believed that the fluorine group can stabilize the coating even when inflated by charging alloyed with Li ion, thereby contributing to suppression of expansion.

(Method for producing a positive electrode active material according to this embodiment)

First, the precursor of lithium manganese silicide is baked. The precursor of lithium manganese iron lithium is synthesized by a manufacturing 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. The amorphous compound contained in the precursor particles or the mixture of the oxide form is changed to a compound of crystalline form of lithium manganese iron manganese oxide by firing, and a cathode active material is obtained.

Further, 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 under an atmosphere of hydrocarbon gas.

(Method for producing precursor particles by spray combustion)

An example of a particulate production apparatus 1 for producing precursor particles by a spray combustion method is shown in Fig. The reaction vessel 11 is provided with a particulate synthesis nozzle 9 to which a combustion gas supply unit 5, a delayed gas supply unit 7 and a raw material solution supply unit 3 are connected. Flammable gas, air, raw material solution and the like are supplied from the combustion gas supply part 5, the retarding gas supply part 7 and the raw material solution supply part 3 to the flames generated from the particulate synthesis nozzle 9, respectively. In addition, the precursor particles 15 in the exhaust produced in the reaction vessel 11 are recovered by the filter 13.

The spray combustion method is a method in which a constituent material is fed into a flame together with a retarding gas and a combustible gas by a method of supplying a raw material gas such as chloride or a method of supplying a raw material liquid or a raw material solution through a vaporizer, This is a method for obtaining a target substance. As a spray combustion method, a Vapor-Phase Axial Deposition (VAD) method or the like is a very suitable example. The temperature of these flames generally varies depending on the mixing ratio of the flammable gas and the retarding gas, and further depends on the addition ratio of the constituent materials, but is usually in the range of 1000 to 3000 캜, particularly preferably in the range of 1500 to 2500 캜, Deg.] C. If the flame temperature is low, there is a possibility that the particulates may come out of the flame before the reaction in the flame is completed. In addition, if the flame temperature is high, the crystallinity of the produced fine particles becomes excessively high, and an image which is stable (stable phase) in the subsequent firing step, but is undesirable for the positive electrode active material is likely to be generated.

In addition, the flame hydrolysis method is a method in which a constituent material is hydrolyzed in a flame. In the flame hydrolysis method, an oxyhydrogen flame is generally used as a flame. A solution containing a constituent material of a cathode active material and a flame raw material (oxygen gas and hydrogen gas) are simultaneously supplied from a nozzle to a center of a flame in which hydrogen gas as a flammable gas and oxygen gas as a delayed gas are supplied, Synthesized. In the flame hydrolysis method, fine particles of a target material composed mainly of amorphous nanoscale minuscule in an inert gas filled atmosphere can be obtained.

The thermal oxidation method is a method in which the constituent materials are thermally oxidized in the flame. In the thermal oxidation method, hydrocarbon flame is generally used as a flame. A target material is synthesized by simultaneously supplying a constituent material and a flame raw material (for example, propane gas and oxygen gas) from a nozzle to a center of a flame in which hydrocarbon gas as a combustible gas is supplied with air as a retarding gas. As the hydrocarbon-based gas, paraffinic hydrocarbon gases such as methane, ethane, propane and butane, and olefinic hydrocarbon gases such as ethylene, propylene and butylene can be used.

(Constituent material for obtaining precursor particles)

The constituent 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. Further, the additive materials of other elements may be used if necessary. When the raw material is a solid, it is supplied as a powder, dispersed in a liquid or dissolved in a solvent to form a solution, and supplied to the flame through a vaporizer. When the raw material is a liquid, it is also possible to vaporize and supply the vapor by raising the vapor pressure by heating, depressurizing and bubbling before the supply nozzle, in addition to passing through the vaporizer. Particularly, 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 mu m or less.

Examples of the lithium source include lithium inorganic salts such as lithium chloride, lithium hydroxide, lithium carbonate, lithium acetate, lithium bromide, lithium phosphate and lithium sulfate, lithium organic acid salts such as lithium oxalate, lithium acetate and lithium naphthenate, An organic lithium compound such as an alkoxide, a? -Diketonato 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 components are the carboxylic acid compounds 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. Further, organic metal salts of iron such as stearic acid, dimethyldithiocarbamic acid, acetylacetonate, oleic acid, linoleic acid and linolenic acid, iron oxide and the like are also used depending on the conditions.

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

Examples of the silicon source include silicon tetrachloride, octamethylcyclotetrasiloxane (OMCTS), silicon dioxide or silicon monoxide, hydrates of these silicon oxides, condensation silicic acids such as orthosilicic acid and metasilicic acid and meta-2 silicic acid, tetraethylorthosilicate (Methoxysilane, ethoxysilane, TEOS), tetramethylorthosilicate (tetramethoxysilane, TMOS), methyltrimethoxysilane (MTMS), methyltriethoxysilane (MTES), hexamethyldisiloxane (TMDSO), tetramethylcyclotetrasiloxane (TMCTS), octamethyltrisiloxane (OMTSO), tetra-n-butoxysilane, and the like.

When a part of silicic acid of lithium manganese silicide lithium is replaced by another anion, a raw material of a transition metal oxide, boric acid and phosphoric acid is added as an anion source.

Examples thereof include metal salts such as titanium oxide, metal titanate such as iron (II) titanate or manganese octanate, zinc titanate, magnesium titanate, titanate such as barium titanate, vanadium oxide, ammonium metavanadate, chromium oxide Zirconium oxide, zirconate, molybdenum oxide, molybdate, tungsten oxide, tungstate, boric acid or trioxide disodium, sodium metaborate or sodium tetraborate, borax Phosphoric acid such as orthophosphoric acid or metaphosphoric acid, ammonium dihydrogenphosphate, ammonium hydrogenphosphate such as dihydrogenphosphate, and the like can be used depending on the synthesis conditions with each desired anion source.

These raw materials are supplied to the same reaction system (system) together with the flame raw materials to synthesize precursor particles. The resulting precursor particles can be recovered as a filter in the exhaust. It may also be formed around the core rod as follows. A silicon source or a silicon-based core rod (also called a seed rod) is installed in the reactor and a lithium source, a copper source, a manganese source, and a silicon source are supplied together with a flame raw material in an oxyhydrogen flame or a propane flame, When decomposition or oxidation reaction is carried out, fine particles of about nanometers are mainly formed on the surface of the core rod. These produced fine particles are recovered, and if necessary, filtered or sieved to remove impurities or coarse particles. The precursor particles thus obtained are composed of fine particles having a nanoscale extremely minute particle diameter and mainly amorphous.

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

In the present invention, the positive electrode active material having a normal structure can be obtained by mixing precursor particles with a reducing agent and firing. The precursor in the present embodiment is a material capable of obtaining crystals of lithium manganese iron lithium by firing. Particularly, the precursor in the present embodiment is amorphous and has a valence of iron or manganese. However, when the iron or manganese is mixed with a reducing agent and fired, the valence of iron or manganese changes from trivalent to two. The composition of the precursor particles preferably satisfies the stoichiometric composition.

The shape of the precursor particles is substantially spherical, and the average aspect ratio (long diameter / short diameter) of the particles is 1.5 or less, preferably 1.2 or less, and more preferably 1.1 or less. The term &quot; substantially spherical &quot; means that the particle shape is not limited to a geometrically rigid spherical or elliptical spherical shape, and even if there is a minute protruding portion, the surface of the particle may be formed of a substantially smooth curved surface.

When these powder precursor particles are measured by powder X-ray diffractometry in the range of 2? = 10 to 60 °, they show little or no diffraction peak, and a small diffraction peak and a broad diffraction angle. That is, the precursor particles are composed of polycrystalline microparticles in which crystallites are small or small single crystals are aggregated, or in the microcrystalline form in which amorphous components exist around these microparticles.

In the spray combustion method of the present embodiment, since carbon is burned in the flame, the resulting precursor particles do not contain carbon. Even if the carbon content is mixed, it is very small in amount and not enough to be used as an auxiliary agent for use in an anode.

(Production of cathode active material)

The precursor particles obtained by the spray combustion method are further mixed with a carbon source and then fired in an atmosphere filled with an inert gas. At this time, an amorphous compound or an oxide-type mixture contained in the precursor particles is changed into a compound of a crystal form of predominantly polyanionic iron silicic iron-manganese lithium by firing.

Further, under an atmosphere filled with an inert gas, it is possible to prevent the carbon source from burning at the time of firing and to prevent the cathode active material from being oxidized. As the inert gas, a nitrogen gas, an argon gas, a neon gas, a helium gas, a carbon dioxide gas, or the like can be used. In order to enhance the conductivity of the product after firing, a conductive carbon such as a polyhydric alcohol such as polyvinyl alcohol, a polymer such as polyvinylpyrrolidone, carboxymethylcellulose, acetylcellulose, a sugar such as sugar, or carbon black may be used as a carbon source The precursor particles are added and calcined. The polyvinyl alcohol is particularly preferable because it plays a role as a binder of the precursor particles before firing and is capable of satisfactorily reducing iron or manganese during firing.

The firing conditions can be suitably obtained in combination with a temperature of 300 to 900 DEG C and a treatment time of 0.5 to 10 hours to obtain a fired product having desired crystallinity and particle diameter. Excessive heat load due to firing at a high temperature or for a long time may cause coarse crystal grains to be formed. Therefore, it is necessary to avoid the size of the crystallite under heating conditions to such an extent that desired crystalline or microcrystalline lithium manganese- It is preferable that the firing condition is as small as possible. The firing temperature is preferably about 400 to 700 占 폚.

(Annealing by hydrocarbon gas)

A cathode active material is formed by firing and then annealed with a hydrocarbon gas to form a carbon coating on the surface of the cathode active material.

It is preferable that the temperature at annealing is 600 占 폚 to 750 占 폚. If the annealing temperature is too low, the deposition of carbon from the hydrocarbon gas will be delayed. If the annealing temperature is too high, the crystals will grow excessively.

The hydrocarbon gas is preferably at least one selected from the group consisting of methane, ethane, propane and butane. The hydrocarbon gas is also reducible, but a reducing gas may be mixed and supplied for further reduction.

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

By the annealing, the hydrocarbon gas reacts with the particles containing iron or iron carbide, and the hydrocarbon gas is decomposed and bonded to cover the surface of the cathode active material with carbon.

Further, since the obtained positive electrode active material is often aggregated in the sintering step or the annealing step, it can be re-granulated by hanging it on a sphere or ball mill or other grinding means.

4 (a) is a diagram showing a crystal structure having a space group Pmn2 ₁ of fine particles formed as described above. Fig. 4 shows a crystal structure having a space group Pmn2 ₁ , but the following description is also applicable to a crystal structure having a space group P2 ₁ / n. The crystal structure having the space group Pmn2 ₁ is a crystal structure having an orthorhombic unitary lattice and 16 atoms in the unit lattice.

The unit grid is shown in a perspective view as shown in Fig. 3, and the sides of a, b, and c are orthogonal. The lengths (lattice constants) of a, b, and c in the system of Li _x Fe _Y Mn _(1-Y) SiO ₄ (0 <X? 2.5, 0 < _{Y? 1} ) are 6.3 angstroms, 5.3 angstroms, It has a value of about 5.0 angstroms and may vary by about 1% depending on the composition.

When the unit lattice is repeatedly arranged, as shown in Fig. 4 (a), the atoms represented by A in Fig. 4 (a) are Fe atoms or Mn atoms. Similarly, the atom represented by B is a Si atom. The atom represented by C is a Li atom. The atom represented by O is O atom.

The crystal structure having a space group of P2 ₁ / n means that a crystal structure having a space group of P2 ₁ / n includes O atoms (atoms) surrounding atoms of a column parallel to the a axis formed by the Fe / Mn site of Pmn2 ₁ and the Si site of FIG. And the direction of the tetrahedron is periodically changed. Therefore, it can be seen that the unit lattice of P2 ₁ / n is different from the quadratic quadrature shown in Fig. 3, and the axes have different monoclinic and long period structures, but have a very close relationship with the arrangement of atoms.

The crystal structure shown in Fig. 4 (a) is called a normal structure. This conventional structure has a structure in which a tetrahedron (represented by a broken line in FIG. 4) by a Si-O bond and a tetrahedron by a Fe / Mn-O bond And a tetrahedron (not shown in Fig. 4) is connected. In Fig. 4 (a), the position where the Fe atom or the Mn atom exists is referred to as the Li site, and the position where the Fe / Mn site and the Li atom exist is referred to as Li site. That is, in the normal structure, Fe atoms or Mn atoms are contained in the Fe / Mn site and Li atoms are contained in the Li site. In addition, for each of the tetrahedrons formed by atoms and O atoms in Fig. 4, only tetrahedra with Si-O bonds is described in order to clarify the explanation in the drawings.

Fig. 5A is a conceptual diagram showing a crystal structure 20a shown in a two-dimensional manner, with the general structure of Fig. 4A being simplified. In reality, Si and Fe are combined with oxygen to form a tetrahedron, but the illustration is omitted. In a typical structure, a Fe atom or a Mn atom is contained in the Fe / Mn site and a Li atom is contained in the Li site, and is usually used as a cathode 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 carried out. By carrying out this treatment, Li atoms can be removed from a part of the Li site as in the crystal structure 20b shown in Fig. 5 (b). That is, a part of the Li site becomes an empty hole.

From this state, if it is heat-treated at a predetermined temperature in an inert gas atmosphere, it is considered that Fe atoms in the Fe / Mn site or a part of Mn atoms migrate to Li sites in which vacancies are formed.

6 (a) is a diagram showing a crystal structure 20c in which Fe atoms or Mn atoms migrate to Li sites and vacancies are formed in Fe / Mn sites. This state is a crystal structure of the cathode active material having a cation exchange structure.

That is, conventionally, the crystal structure 20c may be formed by filling the crystal structure 20a in this form. However, in the present invention, a cation exchange structure can be obtained without using such an electrochemical method. Therefore, even if lithium manganese iron manganese containing manganese is amorphized by charging from the state of the crystal structure 20a, a cation exchange structure can be synthesized chemically by this method.

When the discharge is performed in this state, it becomes the crystal structure 20d shown in Fig. 6 (b). When this is expressed in three dimensions, it is as shown in Fig. 4 (b). In the figure, AC indicates that Fe atoms or both Mn atoms and Li atoms can be arranged. That is, Li atoms are inserted into vacancies formed in the Fe / Mn site. Subsequently, even if charging and discharging are repeated, the change of the crystal structures 20c and 20d is repeated while maintaining the cation exchange structure.

Next, the crystal structure of the cathode active material obtained by the above method was evaluated by X-ray diffraction measurement using CuK alpha rays. Figures 7 and 8 show the measurement results.

In Fig. 7, D is the measurement result of the normal structure (crystal structure 20a) of Li _x FeSiO ₄ , and the measurement result including the space group P _{2 1} / n or the possibility that the crystal structure of Pmn _{2 1} is partially contained to be. E is a cation exchange structure of _{Li x FeSiO 4 (0 <X≤2.5} ) measurement results (crystal structure (20d)), F is Li _x (Fe _0.75 Mn _0.25) cation exchange of SiO ₄ (0 <X≤2.5) Structure (crystal structure 20d).

It can be seen that none of the Li _x FeSiO ₄ and Li _x (Fe _0.75 Mn _0.25 ) SiO ₄ has at least one of the cation exchange structures of the space group P _{2 1} / n or Pmn _{2 1} obtained synthetically, not electrochemically. In particular, two peaks near 22.2 ° and 23.0 ° 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 transition to a cation exchange structure without substantial amorphization.

The positive electrode active material thus obtained was mixed with 10% by weight of a conductive auxiliary agent (carbon black), 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 in a weight ratio of 95: 5, and N-methyl-2-pyrrolidone (NMP) was added thereto and sufficiently kneaded to obtain a positive electrode slurry.

A positive electrode slurry was applied to an aluminum foil current collector having a thickness of 15 탆 at a coating amount of 50 g / m ² and dried at 120 캜 for 30 minutes. Thereafter, the sheet was rolled so as to have a density of 2.0 g / cm &lt; ³ &gt; using a roll press, and punched out in the form of a disk of ² cm &lt; ² &gt;

A lithium secondary battery was produced by dissolving LiPF ₆ in a concentration of 1 M in a mixed solvent obtained by mixing these positive electrodes, metallic lithium to the negative electrode, and ethylene carbonate and diethyl carbonate to the electrolyte in a volume ratio of 1: 1. In addition, the atmosphere of the dew point was set to -50 DEG C or less. Each pole was used by squeezing it into a rolling can with a collector. A coin-type lithium secondary battery having a diameter of 25 mm and a thickness of 1.6 mm was obtained by using the positive electrode, negative electrode, electrolyte and separator.

Next, the electrode characteristics of the positive electrode active material were tested and evaluated by the coin type lithium secondary battery as follows.

(Pair Li / Li +) by a CC-CV method (constant current constant voltage) at a test temperature of 25 ° C or 60 ° C and a current rate of 0.1C. After the current rate dropped to 0.01C, Stopped. Thereafter, discharge was carried out at 0.1 C rate up to 1.5 V (same as above) by the CC method (constant current), and the charge-discharge capacity and the cycle life were measured.

Table 1 shows the results of the first discharge capacity. Table 2 shows the discharge capacity after 30 cycles. An embodiment of the present invention is represented by a "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 represented by "normal structure" in the table, and the crystal structure before the first charge (discharge state) is a normal structure. As can be seen from Tables 1 and 2, by carrying out charge and discharge using the positive electrode active material having a cation exchange structure from the beginning of charging for the first time, it is possible to obtain a cycle characteristic superior in a high capacity and in a normal structure .

[Table 1]

(0 <

[Table 2]

(0 <

9 (a) shows the typical structure of the Li _x FeSiO ₄ and Li _x (Fe _0.75 Mn _0.25 ) SiO ₄ cathode active materials prepared by the above-described method and Fe-K in the cation exchange structure X-ray Absorption Near Edge Structure (XANES). 9 (b), with reference to the measurement method disclosed in the non-patent document 8, the energy at 90% of the normalized XANES spectrum intensity is regarded as the absorption edge rising portion, and the correlation between the valence of Fe and the rising edge position of the absorption edge Estimated. From these results, it can be seen that in the normal structure, Fe having +2 is converted into a cation exchange structure and oxidized, and +3 is changed to the vicinity. At this time, the valence of Fe in the estimated cation exchange structure was 3.1 to 3.2.

(Non-Patent Document 8) Electrochimica Acta 55 (2010) 8876.

Fig. 10 is a graph showing the relationship between the positive electrode active material Li _x FeSiO ₄ And Li _x (Fe _0.75 Mn _0.25) The results of XPS (X-ray Photoelectron Spectroscopy) of Fe2p _3/2 in the normal structure and a cation exchange structure of SiO _4. As a result of the normal structure, it can be seen that Fe has a vicinity of +2 because Fe exhibits a wide peak (difference with respect to the comparison object) structure in the vicinity of 710 eV and 716 eV in the same manner as the standard sample of +2 valence. On the other hand, since these peak structures are not seen in the cation exchange structure, it is suggested that Fe has +3 near the valence. From the results of XANES and XPS, it can be confirmed that the valence of Fe in the cation exchange structure is generally higher than the valence of Fe in the structure.

Fig. 11 (a) shows the results of XANES of the Mn-K phase in the cation exchange structure and the normal structure of the Li _x (Fe _0.75 Mn _0.25 ) SiO ₄ cathode active material in the insecticidal state produced by the above-mentioned method. 11 (b), with reference to the measurement method disclosed in the non-patent document 8, the energy at 90% of the normalized XANES spectrum intensity is used as the absorption edge rising portion, and the correlation between the mantle of Mn and the position of the rising edge of the absorption edge Estimated. In view of these, in the normal structure, Mn having +2 is converted to a cation exchange structure and oxidized, and it can be seen that it is approaching +3 direction. The estimated singer number was 2.6.

Figure 12, produced by the method described above positive electrode active material Li ₂ (Fe _0.5 Mn _0.5) The results of Mn2p of the _3/2 XPS in the normal structure and a cation exchange structure of SiO _4. As a result of the normal structure, Mn is in the vicinity of 647 eV in the vicinity of 647 eV as in the case of the standard sample of Mn +2, and thus Mn is in the vicinity of +2. On the other hand, this peak structure is not seen in the cation exchange structure, suggesting that Mn approaches +3. From the results of XANES and XPS, it was confirmed that the number of manganese in the cation exchange structure is generally larger than the number of manganese in the structure.

According to the present invention, according to the present invention, the lithium silicate transition metal lithium is changed to the cation exchange structure without using the electrochemical method and the composition Li _x Fe _Y Mn _{(1-Y) of the} insecticidal discharge state In the case of SiO ₄ (0.5 <X? 1.5, 0 <Y? 1), Fe or Mn has a valence of +2.5 to +3.5, and Fe or Mn is changed to a valence, The structure is maintained. Therefore, a secondary battery having excellent cycle characteristics can be obtained because a stable crystal structure is maintained even in subsequent charge and discharge.

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 1 charge / discharge cycle, and I is the result after 5 cycles of charging / discharging.

From these results, Li _x (Fe _0.75 Mn _0.25 ) SiO ₄ containing Mn was not amorphized and maintained a cation exchange structure.

As described above, according to the present invention, lithium silicate transition metal lithium can be changed to a cation exchange structure without using an electrochemical method. Therefore, conventionally, a cation exchange structure is obtained even in lithium manganese iron manganese containing manganese which is amorphized by charge / discharge, so that a stable crystal structure is maintained even in the subsequent charging and discharging. Thus, have.

While the preferred embodiments of the present invention have been described with reference to the accompanying drawings, the present invention is not limited to these examples. It will be apparent to those skilled in the art that various changes or modifications can be made within the scope of the technical idea disclosed in the present application and they are obviously also within the technical scope of the present invention.

In the figures, "O" means a normal structure and "CM" means a cation exchange structure.

1: Particle preparation device
3: raw material solution supply part
5: Combustion gas supply part
7: Air supply
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: anode
35: cathode
37: Separator
39:
41: Battery cans
43: positive lead
45: cathode lead
47: positive terminal

Claims (8)

Is represented by the general formula Li _X Fe _Y Mn _(1-Y) SiO ₄ (0 <X? 2.5, 0 < _{Y? 1} )
Mn, at least one of a space group P2 ₁ / n and Pmn2 ₁ , a Li atom is contained in a part of the Fe / Mn site and a Fe atom or a Mn atom is contained in a part of the Li site, Structure. &Lt; / RTI &gt; The method according to claim 1,
Wherein at least one of Co and Ni is substituted for Fe and / or a part of Mn. 3. The method according to claim 1 or 2,
Wherein at least one of Mg, Ca, Ti, V, Cr, Cu, Zn, Sr, Zr and Mo is substituted in place of a part of Fe and / or Mn. 4. The method according to any one of claims 1 to 3,
Wherein 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 whole house,
A positive electrode active material layer comprising the positive electrode active material according to any one of claims 1 to 3 on at least one surface of the current collector
And a positive electrode for a secondary battery. A positive electrode for a secondary battery according to claim 4,
A negative electrode capable of absorbing and storing lithium ions,
And a separator disposed between the anode and the cathode,
Wherein the positive electrode, the negative electrode and the separator are provided in an electrolyte having lithium ion conductivity. A step of synthesizing a lithium-based active material between iron silicic acid and manganese oxide using at least a lithium source, an iron source, a manganese source, and a silicon source;
Removing a part of lithium from the active material;
A step of heating the active material to convert a part of the Li site and a part of the Fe / Mn site to a cation exchange structure
And forming a cathode active material layer on the cathode active material layer. Is expressed by the general formula Li _X Fe _Y Mn _(1-Y) SiO ₄ (0.5? X <1.5, 0 < _{Y? 1} )
At least one of the space group P2 ₁ / n and Pmn2 ₁ has a crystal structure, Li atoms are incorporated into a part of the Fe / Mn site, and either a Fe atom or a Mn atom is contained in a part of the Li site, And the cation of Mn has a cation exchange structure of +2.5 to +3.5.

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