HK1091036B

HK1091036B - Cathode material for secondary battery, method for producing same, and secondary battery

Info

Publication number: HK1091036B
Application number: HK06111542.4A
Authority: HK
Inventors: 八田直树; 稻叶俊和; 内山泉
Original assignee: 三井造船株式会社; 财团法人地球环境产业技术研究机构
Priority date: 2003-01-31
Filing date: 2004-01-03
Publication date: 2012-05-25

Description

Cathode material for secondary battery, production method and secondary battery

Technical Field

The present invention relates to a cathode material for a secondary battery, a method for producing the cathode material, and a secondary battery using the cathode material. More particularly, the present invention relates to a cathode material for a lithium secondary battery for use in electric vehicles and hybrid electric vehicles and portable devices such as mobile phones, a method for producing the cathode material, and a secondary battery using the cathode material.

Background

Lithium iron phosphate LiFePO₄The cathode material used in a secondary battery such as a lithium metal battery, a lithium ion battery or a lithium polymer battery undergoes electrode oxidation/reduction accompanied by lithium doping/dedoping during charge and discharge. Lithium iron phosphate LiFePO₄Because of having a considerable theoretical capacity (170mAh/g) and being capable of generating a comparatively high electromotive force (about 3.4 to 3.5V at a Li/Li anode), and because of being generated from abundant resources of iron and phosphorus, it is considered to be produced at a low cost, and thus is expected to be a highly potential next-generation cathode material. LiFePO₄The cathode system has an olivine-type crystal structure, and many currently available cathode systems such as lithium cobaltate [ LiCoO ]₂]The cathode system is in a two-phase equilibrium state, in which only the reduced form (discharged state) LiFe (II) PO is present₄Fe (III) PO as a first phase and in oxidized form (charged state) in which Li has been sufficiently inserted₄As a second phase from which Li has been extracted completely [ that is, without an intermediate phase, e.g. Li_0.5(Fe²⁺ _0.5Fe³⁺ _0.5)PO₄Has not been formed during the electrode oxidation/reduction process. As a result, the cathode system has a charge/discharge voltageAn interesting property that it always remains constant and thus its charge/discharge state is easily controlled. In any case, the oxidized form (discharged state) LiFe (II) PO₄And Li extracted redox form (charged state) Fe (III) PO₄Having very low conductivity, Li⁺The ions cannot move rapidly in the cathode material (these two features are assumed to be related to each other as described later). Therefore, even when a battery is fabricated using Li or the like at the anode, only a small-scale effective power, poor rate characteristics, and poor cycle characteristics can be obtained.

As a method for enhancing the surface conductivity of the cathode material, the method disclosed is useful for the surface treatment of the cathode material represented by the formula A_aM_mZ_zO_oN_nF_f(wherein A represents an alkali metal atom, M represents Fe, Mn, V, Ti, Mo, Nb, W or other transition metal atom, and Z represents S, Se, P, As, Si, Ge, B, Sn or other non-metal atom) represents a method for depositing carbon on the surface of a complex oxide (containing an oxyacid salt such As a sulfate, phosphate or silicate) particle. When the composite material is used in a battery electrode system, the interface around the electric field of the complex oxide particles, the current collector (conductivity imparting) material and the electrolyte may be uniform and stable and the efficiency in the electrode oxidation/reduction process may be improved (see document 1). To deposit carbon on the surface of the complex oxide particles, an organic substance (polymer, monomer, or low molecular weight compound) or carbon monoxide which forms carbon by pyrolysis is added to the complex oxide and thermally decomposed (the composite material of the complex oxide and the surface-coated carbon can be obtained by thermally reacting the organic substance and the components of the complex oxide under reducing conditions). According to document 1, improvement in surface conductivity of complex oxide particles can be achieved by the method, and high electrode performance such as high discharge capacity can be achieved by using a metal oxide in a cathode material such as LiFePO₄Is obtained when a composite material prepared by depositing carbon on the surface of the particles of (a) is used to produce a Li polymer battery.

Also disclosed is a method for producing a cathode active material comprising mixing and milling a mixture of a compound of the formula LixFePO₄(wherein x is more than 0 and less than or equal to 1)The composition of the represented compound, and firing the mixture under an atmosphere having an oxygen content of 1012ppm (by volume) or less, wherein an amorphous carbon material such as acetylene black is added at any point of the process (see document 2).

The above techniques are used to improve cathode performance based on phosphate cathode materials such as LiFePO₄Low conductivity and slow movement of Li ions in the cathode material. Basically, the technology tries to avoid these difficulties by depositing conductive substances, such as carbon, on the surface of the cathode material or adding conductive substances to the cathode material and reducing the particle size of the cathode material as much as possible to limit the distance of ion diffusion.

Attempts have been made to enhance LiFePO by replacing some Li or Fe in the cathode material with different metallic elements₄The conductivity of the cathode material, or mixing or doping some Li or Fe in the cathode material with different metal elements to improve the cathode performance (see, for example, documents 3 and 4).

Document 3 discloses that when Al, Ca, Ni or Mg is introduced into LiFePO₄When the cathode material is used, the capacity can be improved. For example, it is reported that LiFePO without the above elements is used₄Lithium metal batteries of cathode material showed a discharge capacity of 117mAh/g at the first cycle and a rapid discharge capacity decrease as the cycle proceeded, however using LiFePO by replacing it with Mg₄Some Fe-derived LiMg in cathode materials_0.05Fe_0.95PO₄The stack of cathode materials showed a discharge capacity of about 120 to 125mAh/g and less degradation with cycling (although there was no objective evidence showing that iron was replaced with Mg in the cathode material).

Document 4 discloses a cathode material to which Mg, Al, Ti, Zr, Nb and W elements are doped, respectively, by adding Mg²⁺、Al³⁺、Ti⁴⁺、Zr⁴⁺、Nb⁵⁺And W⁶⁺To LiFePO (Mg as oxalate, Nb as metal phenoxide, and others as metal alkoxides) separately₄The composition of the cathode material and calcining the mixture. Can be used forThe materials in the literature are believed to have some of their Li replaced by elements and have Li as a substituent_1-xM_xFePO₄Exist in the form of (1). The conductivity of the metal ion doped cathode material is also reported to be 10^-1To 10^-2S/cm, which is about 10 higher than the undoped cathode material at room temperature⁸And a lithium metal battery using the cathode material doped with the metal ions having such high conductivity has excellent rate characteristics and a long cycle life. According to document 4, one such lithium metal battery exhibits a discharge capacity of slightly more than 140mAh/g at a low charge/discharge rate C/10 (although the discharge capacity is referred to as about 150mAh/g in this document, as long as it is close to 140mAh/g as can be seen with reference to the drawings), and is capable of being stably and periodically charged and discharged at very high rates of 21.5C and 40 ℃, exhibiting reduced discharge capacities of slightly less than 70mAh/g and about 30mAh/g, respectively (C/n is the rate at which the battery is charged or discharged at a constant current, where n is the number of hours the battery is fully charged or discharged.

It is considered that in document 4, a small amount (less than 1 mol% in terms of iron element proportion) of polyvalent ions enters reduced form life (ii) PO of the cathode material₄And its Li-depleted oxidized form Fe (III) PO₄In the crystal structure of (A) Li⁺The position of the ion, a small amount of Fe is generated in the reduction phase and the oxidation phase respectively³⁺And Fe²⁺To produce Fe²⁺And Fe³⁺The coexisting oxidation states, and thus the P-type and N-type semi-conductivities, appear in the reduced and oxidized phases, respectively, and provide conductivity improvements. It is also reported when LiFePO is used₄The conductivity of the cathode material is also improved when the cathode material is calcined with any of the above-mentioned compounds containing divalent to hexavalent ions (since the transition metal elements Ti, Zr, Nb and W may be present in the form of stable positive ions of different valences, the valence of the positive ion in the obtained cathode material may be different from the valence of the compound used for doping).

Document 1: JP-A2001-15111

Document 2: JP-A2002-110163

Document 3: "future project research, Tatsumisago research project: preparation and application of newly designed solid electrolyte (Japanese society for science promotion: Research project No. JPS-RFTF 96PO010) "Research for the Future Program, Tatsumago Research project: preparation and Application of New Designed solid electrolytes (Japan Society for the motion of Science: Research project No. JSPS-RFTF96PO010) [ http: html/chem.sci.hyogo-u.ac.jp/ndse/index (updated 6/21/2000)

Document 4: nature Materials Vol.1, pp.123-128(October, 2002)

In any case, the methods disclosed in documents 3 and 4 do not currently provide satisfactory results. The charge/discharge capacity achieved by the former method was at most 120 to 125 mAh/g. Furthermore, although the latter method is remarkably adaptable to high rate charge/discharge, even at low C/10 rates only much less than 170mAh/g (slightly more than 140mAh/g) of theoretical capacity of the cathode material can be obtained despite the fact that LiFePO₄The conductivity of the cathode material is improved. In addition, the rise/fall of the voltage at a constant current in the final stage of charging or discharging is not very steep in the battery capacity-voltage characteristic curve in spite of the high rate characteristic. According to the data shown in document 4, there is a gentle rise/fall of the C/10 proportional voltage from a point of about 80% of the charge and discharge limit (depth). In the battery pack having a small internal resistance and a high rate characteristic, the rise/fall of the voltage should be steep such as 90 degrees in any case. It has been shown that the types of elements that may be combined or doped and the methods of mixing or doping are not entirely suitable.

Detailed Description

An object of the present invention is to provide a cathode material comprising lithium iron phosphate as a cathode active material and having a large charge/discharge capacity, a high rate adaptability and good charge/discharge cycle characteristics, a simple method of producing the cathode material, and a secondary battery using the cathode material.

As a result of earnest studies for achieving the object, the present inventors have found that LiFePO is a cathode active material mixed with molybdenum (hereinafter referred to as "Mo")₄The obtained cathode material has strongly improved charge/discharge characteristics. In addition, when the conductive carbon is deposited on the surface of the Mo composite cathode material, an effective capacity close to the theoretical capacity of a cathode system of 170mAh/g and good charge/discharge cycle characteristics can be obtained.

A first aspect of the present invention is a cathode material for secondary batteries comprising, as a main component, a material represented by the general formula Li_nFePO₄(wherein n represents a number of 0 to 1, and the same will apply hereinafter) and molybdenum (Mo). Containing Li_nFePO₄The cathode material, which is a main component of the cathode active material and Mo, has a large charge/discharge capacity, high-rate adaptability, and good charge/discharge cycle characteristics, which have not been achieved before, as shown in the description of examples later.

A second aspect of the invention is the cathode material for a secondary battery according to the first aspect, wherein the content of molybdenum (Mo) is in the range of 0.1 to 5 mol% in terms of elemental proportion based on iron of the cathode active material. When the content of Mo is within the above range, excellent charge/discharge properties can be obtained.

A third aspect of the present invention is a secondary battery cathode material having an olivine-type crystal structure, comprising lithium ions (Li) as a main component⁺) Iron (II) ion (Fe)²⁺) And phosphate ion (PO)₄ ^3-) And molybdenum (Mo) in an amount of 0.1 to 5 mol% based on P.

The cathode material for a secondary battery has a large capacity and exhibits excellent cathode characteristics.

A fourth aspect of the invention is the battery cathode material according to the third aspect, wherein the content of lithium or iron, or the total content of lithium and iron, is smaller than the corresponding content in olivine-type lithium iron phosphate with a stoichiometric ratio of lithium, iron and phosphorus of 1: 1, the difference being at most a number of moles corresponding to the content of molybdenum (Mo).

When the amount of Li is relatively reduced, an excellent cathode material for secondary batteries having good cycle characteristics can be obtained. When the amount of Fe is relatively reduced, an excellent cathode material for secondary batteries, in which the internal resistance of the battery pack is reduced, can be obtained.

A fifth aspect of the invention is a battery cathode material according to the third or fourth aspect, substantially free of Fe (II)₂Mo(IV)₃O₈. The battery cathode material has the same function as the cathode material of the fourth aspect.

A sixth aspect of the invention is the battery cathode material according to any one of the first to fifth aspects, further comprising conductive carbon deposited on a surface thereof. In addition, when conductive carbon is deposited on the surface of the Mo-containing cathode material, the conductivity of the cathode material can be further enhanced, and a near Li can be obtained as shown in the following examples_nFePO₄Effective capacity of theoretical capacity of the cathode system and good charge/discharge cycle characteristics.

The seventh aspect of the present invention is a method for producing a cathode material for a secondary battery, comprising mixing a cathode active material Li_nFePO₄And a step of containing a compound of molybdenum (Mo) to obtain a calcined precursor and calcining the calcined precursor to composite the cathode active material with Mo. The cathode material of the first aspect can be conveniently obtained by compounding a cathode active material with Mo.

An eighth aspect of the invention is the method for producing a cathode material for a secondary battery according to the seventh aspect, wherein the molybdenum (Mo) containing compound is added so that the content of molybdenum (Mo) in the molybdenum containing compound is based on introduction of phosphate ions (PO)₄ ^3-) The P content in the component (A) is 0.1 to 5 mol%. According to the eighth aspect, the cathode material of the third aspect can be easily obtained.

The ninth aspect of the invention is a method for producing a secondary battery cathode material according to the seventh or eighth aspect, wherein a cathode active material Li is introduced_nFePO₄(wherein n represents a number of 0 to 1) such thatThe lithium content in the lithium-incorporating component, the iron content in the iron-incorporating component or the total amount thereof is less than the corresponding content in the olivine-type lithium iron phosphate in which the stoichiometric ratio of lithium, iron and phosphorus is 1: 1, the difference being at most a number of moles corresponding to the molybdenum (Mo) content. According to the ninth aspect, the cathode material of the fourth aspect can be easily obtained.

A tenth aspect of the invention is the method for producing a cathode material for a secondary battery according to any one of the seventh to ninth aspects, wherein the calcination step has a first stage at a temperature ranging from room temperature to 300-450 ℃ and a second stage at a temperature ranging from room temperature to the calcination completion temperature, and wherein the second stage of the calcination step is conducted after adding a substance from which conductive carbon is formed by pyrolysis to the product of the first stage of the calcination step. According to this feature, by adding a substance from which conductive carbon is formed by pyrolysis after the first stage of the calcination step, a cathode material in which conductive carbon is uniformly deposited can be obtained. When the action of carbon deposition is combined with the action of Mo recombination, a cathode material exhibiting excellent charge/discharge properties is easily obtained.

An eleventh aspect of the invention is the method for producing a cathode material for a secondary battery according to the tenth aspect, wherein the substance from which the conductive carbon is formed by pyrolysis is pitch or a saccharide. The pitch and saccharides are converted into conductive carbon by pyrolysis and impart conductivity to the cathode material. Specifically, pitch such as refined coal pitch, which is very inexpensive, is melted and uniformly spread on the surfaces of component particles during calcination, and is converted into carbon deposits having high electrical conductivity by thermal decomposition through calcination at a relatively low temperature. When a saccharide is used, the cathode material produced by the action of a plurality of hydroxyl groups contained in the saccharide on the surface of the component particles strongly prevents crystal growth of the cathode material. Therefore, by using the saccharide, an excellent crystal-growth inhibiting effect and an effect of imparting conductivity can be provided.

A twelfth aspect of the invention is a secondary battery, comprising the cathode material according to any one of the first to sixth aspects as a constituent element. According to this feature, the same effects as those in any of the first to sixth aspects can be obtained in the secondary battery.

Brief Description of Drawings

FIG. 1 is a schematic diagram for explaining the charge and discharge performance of a secondary battery;

FIG. 2 is a diagram illustrating a two-dimensional hypothetical model of the vicinity of a cathode material particle;

FIG. 3 is a graph showing the results of X-ray diffraction analysis of the Mo composite cathode material obtained in example 1;

fig. 4 is a graph showing charge/discharge capacity and voltage characteristics of the secondary battery obtained in example 1;

fig. 5 is a graph showing the cycle charge/discharge characteristics of the secondary batteries obtained in example 1 and comparative example 1;

FIG. 6 is a graph showing the difference in battery discharge capacity produced by adding different amounts of Mo at a fixed calcination temperature of 675 deg.C;

FIG. 7 is a graph showing the difference in battery discharge capacity produced by adding the same amount of Mo at different calcination temperatures;

FIG. 8 is a graph showing the results of X-ray diffraction analysis of the Mo composite cathode material obtained in example 4;

fig. 9 is a graph showing charge/discharge capacity and voltage characteristics of the secondary battery obtained in example 4;

fig. 10 is a graph showing the charge/discharge capacity and voltage characteristics of the secondary battery obtained in example 4 at the third and tenth cycles;

fig. 11 is a graph showing the cycle charge/discharge characteristics of the secondary batteries obtained in example 4 and comparative example 11;

fig. 12 is a graph showing the difference in battery discharge capacity produced with a fixed amount of added Mo and a fixed amount of conductive carbon deposits at different firing temperatures;

fig. 13 is a graph showing charge/discharge capacity and voltage characteristics at the third cycle of a secondary battery produced at a final calcination temperature of 725 ℃;

fig. 14 is a graph showing charge/discharge capacity and voltage characteristics at the third and tenth cycles of a secondary battery produced at a final calcination temperature of 725 ℃; and

fig. 15 is a graph showing the results of powder X-ray diffraction analysis of samples a to D obtained in example 6.

Best mode for carrying out the invention

Hereinafter, specific embodiments of the present invention are described in detail in the following order: (A) a battery cathode material, (B) a component, (C) a method of producing a battery cathode material, and (D) a battery.

(A) Cathode material for secondary battery

The cathode material of the storage battery comprises a material represented by the general formula Li_nFePO₄A cathode active material represented by Li as a main component and Mo_nFePO₄Mo is used for compounding (the material is hereinafter referred to as "composite cathode material"). It is not shown what state Mo is in the composite cathode material. It is believed that Mo has been replaced by some Li or Fe and crystallized as a solid solution such as (Li)_1-yMo_y)FePO₄Or Li (Li)_1-zMo_z)PO₄(wherein y and z are numbers satisfying the stoichiometric condition) in the form of an olivine-type LiFePO existing in a single phase₄Or as another conjugated compound which can donate electrons or holes. It is also considered that not an olivine-type single crystal phase but an impurity by-product Fe (II) is formed depending on the mixing ratio of the components at the time of Mo addition₂Mo(IV)₃O₈(molynite) coexists.

In the present invention, the terms "complexed" and "complexing" are used in a broad sense to encompass both solid solution forms and conjugated forms.

Due to Li as the main active material of the composite cathode material of the invention_nFePO₄Having a crystal structure [ point group Pnma (olivine type) or Pbnm, both of which can be used as cathode active materials but the former is more general]The structure does not undergo any substantial change when subjected to electrochemical oxidation-reduction, Li_nFePO₄Can be used as an alkali metal secondary battery cathode material that can be repeatedly charged and discharged. As a cathode material, the substance is in its own state, corresponding to a discharged state, and when oxidation of the central metal element occurs at the interface of its electrolyte by electrochemical oxidation with dedoping of the alkali metal Li, the cathode material is restored to a charged state. When the cathode material is electrochemically reduced in the charged state, reduction of the central metal element occurs with heavy doping of the alkali metal Li and the cathode material returns to the initial state, i.e., to the discharged state.

The Mo content of the composite cathode material is preferably 0.1 to 5 mol%, more preferably 0.5 to 5 mol%, in terms of element ratio, based on iron (or phosphorus) of the cathode active material. In some cases, in order to control the charge/discharge characteristics of the resulting cathode, as shown in example 6 described later, the components of the cathode active material are preferably incorporated so that the molar amount of Li or Fe or the total molar amount of Li and iron in the resulting cathode active material may be less than the molar amount of up to the later molybdenum (Mo).

In a preferred embodiment of the invention, the cathode material has conductive carbon deposited on its surface.

The conductive carbon is deposited on the surface of the cathode material by adding a substance from which conductive carbon is formed by pyrolysis during the calcination process (hereinafter referred to as "conductive carbon precursor") as described later.

(B) Composition (I)

Li of a general olivine-type structure will be described below_nFePO₄The cathode active material is described. For use in Li having an olivine-type structure_nFePO₄Suitable examples of the substance for introducing lithium in the composition include hydroxides such as LiOH, carbonates and bicarbonates such as Li₂CO₃The halides include chlorides such as LiCl, nitrates such as LiNO₃And other degradable volatile compounds containing Li, wherein only Li remains in the resulting cathode material, such as an organic acid salt of Li. Phosphates and hydrogenphosphates such as Li₃PO₄、LiH₂PO₄And Li₂HPO₄May also be used.

Suitable examples of materials for the introduction of iron include hydroxides, carbonates and bicarbonates, halides such as chlorides, nitrates of iron, and other iron-containing degradable volatile compounds in which only iron remains in the resulting cathode material (e.g., organic acid salts of iron, such as oxalates and acetates of iron, and organic complexes such as iron acetylacetonate complexes and metallocene complexes). Iron phosphates and hydrogen phosphates may also be used.

Suitable examples of the substance for introducing phosphoric acid include phosphoric anhydride P₂O₅Phosphoric acid H₃PO₄Degradable volatile phosphates and hydrogenphosphates, in which only phosphate ions remain in the resulting cathode material [ e.g. ammonium salts, e.g. (NH) ]₄)₂HPO₄、NH₄H₂PO₄And (NH)₄)₃PO₄In]。

When a component containing an undesirable element or substance remains in the resulting cathode material, the element or substance should be decomposed or vaporized during calcination. Non-volatile salts of oxyacids other than phosphate ions should of course not be used. Hydrates of the above compounds [ e.g. LiOH. H ] may also be used₂O、Fe₃(PO₄)₂·8H₂O]Although not shown here.

< case where metallic iron is used as the constituent for introducing iron >

As the component for introducing iron, inexpensive and easily available metallic iron can be used as a raw material in place of the above compound. The metallic iron used is present in the form of particles having a diameter of 200 μm or less, preferably 100 μm or less. In this case, metallic iron, a compound that releases phosphate ions in a solution, a lithium source compound, and water may be used as components of the cathode material.

Examples of "phosphate ion-releasing compounds in solution" usable with metallic iron include phosphoric acid H₃PO₄Phosphorus pentoxide P₂O₅Ammonium dihydrogen phosphate NH₄H₂PO₄And diammonium hydrogen phosphate (NH)₄)₂HPO₄. Among them, phosphoric acid, phosphorus pentoxide, ammonium dihydrogen phosphate are preferable because iron can be kept in a relatively strongly acidic condition during the dissolution process. Although commercially available reagents can be used as these compounds, when phosphoric acid is used, it is preferable to precisely measure the purity thereof by titration and calculation factors in advance to the stoichiometric accuracy.

As the "lithium source compound" usable together with metallic iron, it is preferable to select a compound (Li-containing degradable volatile compound) in which only lithium remains in the resulting cathode material after calcination. Suitable examples of such compounds include hydroxides such as lithium hydroxide LiOH, carbonates such as lithium carbonate Li₂CO₃Organic acid salt of Li and hydrate thereof (LiOH. H)₂O, etc.).

< Mo-containing Compound >

Various compounds can be used as the Mo-containing compound to be added to the composition of the cathode material. Examples of Mo-containing compounds include halides and oxyhalides, e.g., chlorides, bromides, iodides, and fluorides of Mo (e.g., MoCl, molybdenum pentachloride)₅) Organic acid salts such as O-oxalates, acetates and naphthenates of Mo, hydroxides and oxyhydroxides of Mo, alkoxides and phenoxides of Mo, and complexes of Mo such as acetylacetonates of MoAromatic complexes and carbonyl complexes. These examples are discussed in more detail below (hydrates of the compounds may also be used, although not shown here): examples of halides and oxyhalides other than MoCl₅In addition to MoCl₃、MOBr₃、MoI₂、MoF₆、MoOCl₄And MoO₂Cl₂. Examples of the oxygen-oxalate salt include MoOC₂O₄And MoO₂(C₂O₄)₂. Examples of the acetate salt include [ Mo (CH)₃COO)₂]₂. Examples of hydroxides and oxyhydroxides include Mo (OH)₃And MoO (OH)₃. Examples of alkoxides include Mo (C)₂H₅)₅And Mo (i-C)₃H₇)₅. Examples of acetylacetonate complexes include MoO₂(C₆H₇O₂). Examples of the aromatic compound include Mo (C)₆H₆)₂And Mo (C)₅H₅)₂X₃(wherein X represents a halogen atom). Examples of carbonyl complexes include Mo (CO)₆. Among them, the use of a halide such as a chloride is preferable from the viewpoint of improving the performance of the cathode. These compounds may be added to the constituents of the cathode material either alone or together with a solvent or dispersion medium such as alcohol, ketone, acetonitrile, cyclic ether or water. The resulting mixture is stirred and milled to obtain a calcined precursor.

The Mo-containing compound is added in an amount such that the Mo content may be about 0.1 to 5 mol%, preferably about 0.5 to 5 mol%, based on the central metal element Fe (or P) in the composition of the cathode material. In some cases, in order to control the charge/discharge characteristics of the resulting cathode, as described previously, the components of the cathode article material are preferably incorporated so that the molar amount of Li or Fe or the total molar amount of Li and iron in the resulting cathode article material may be less than the molar amount of molybdenum (Mo) added at most. Since halides and oxyhalides have high reactivity, when they are added to the components of the cathode material together with water or alcohol, they are converted into molybdenum hydroxide and molybdenum alkoxide, respectively, before being recombined by other components. In some cases, when a reducing agent such as carbon or hydrogen, an oxidizing agent such as oxygen, and/or a third component such as chlorine or phosgene is added to a calcined precursor to which a cathode material containing a Mo compound has been added before calcination, depending on the type of the Mo compound, the Mo composite cathode material can be produced under optimum conditions. When the preparation of the calcined precursor or the pre-calcination is performed under conditions that produce a compound that allows the cathode material to be composited with Mo while being mixed with another substance, metallic Mo or an oxide of Mo may be used as a component of the Mo composite material.

< conductive carbon precursor >

Examples of the conductive carbon precursor include pitch (known as asphalt; including pitch obtained from coal or petroleum residue), saccharides, styrene-divinylbenzene copolymer, ABS resin, phenol resin and crosslinked polymer containing aromatic group. Among them, pitch (in particular, so-called refined coal pitch) and saccharides are preferable. The pitch and saccharides are converted into conductive carbon by pyrolysis and impart conductivity to the cathode material. In particular, refining coal pitch is very inexpensive. Also, the refined coal pitch is melted and uniformly spread on the surfaces of the component particles during calcination, and is converted into a carbon deposit having high conductivity by thermal decomposition through calcination at a relatively low temperature (650 to 800 ℃). Also, since the conductive carbon deposit has an effect of suppressing fusion of the cathode material particles by sintering, the particle diameter of the resulting cathode material particles can be favorably small. When a saccharide is used, the cathode material produced by the action of a plurality of hydroxyl groups contained in the saccharide on the surface of the component particles strongly prevents crystal growth of the cathode material. Therefore, the use of the saccharide can provide an excellent crystal growth-inhibiting effect and an effect of imparting conductivity.

In particular, coal pitch suitably used has a softening point of 80 to 350 ℃ and a pyrolysis weight-loss initiation temperature of 350 to 450 ℃ and is capable of forming conductive carbon by pyrolysis and calcination at a temperature of not less than 500 ℃ and not more than 800 ℃. In order to further improve the performance of the cathode, it is more preferable to use refined coal pitch having a softening point ranging from 200 to 300 ℃. There is no doubt that the refined coal pitch will contain impurities that will not adversely affect the cathode performance, and it is particularly preferred to use refined coal pitches having an ash content of not higher than 5000 ppm.

Particularly preferred as the saccharide is a saccharide which decomposes at a temperature range of not less than 250 ℃ and less than 500 ℃ and at least partially melts at least once during heating from 150 ℃ up to the above temperature range, and from which conductive carbon is formed by pyrolysis and calcination at a temperature of not less than 500 ℃ and not more than 800 ℃. This is because the saccharides having the above-mentioned specific properties are melted under heating during the reaction and sufficiently coat the surface of the cathode material particles, and are converted into conductive carbon properly deposited on the surface of the resulting cathode material particles by pyrolysis, and because it can prevent crystal growth during the aforementioned process. Furthermore, at least 15% by weight, preferably at least 20% by weight, of the conductive carbon is preferably formed by calcining the saccharide, based on the dry weight of the saccharide before calcination. This makes it easy to control the amount of the conductive carbon obtained. Examples of the saccharides having the above-mentioned properties include oligosaccharides such as dextrin and high-molecular saccharides such as soluble starch, and slightly crosslinked starch (e.g., starch containing 50% or more of amylose) which tends to melt when heated.

(C) Method for producing cathode material for secondary battery

< brief description of production method >

The cathode material for secondary batteries of the present invention is obtained by mixing the cathode active material Li in a prescribed environment and at a prescribed temperature by calcination_nFePO₄And a Mo-containing compound for a prescribed time.

The carbon-deposited composite cathode material obtained by depositing conductive carbon on the Mo composite cathode material exhibits better charge/discharge characteristics than the cathode material without carbon deposition. The carbon deposition composite cathode material is produced by the following steps: preparing a calcined precursor by adding a Mo-containing compound to the ingredients of the cathode active material and stirring and grinding the mixture in the same manner as previously described, performing a first-stage calcination (precalcination) of the calcined precursor at 300 to 450 ℃ for several hours (e.g., five hours), adding a prescribed amount of a conductive carbon precursor (pitch such as coal pitch or saccharide such as dextrin) to the precalcined product and grinding and stirring the mixture, and performing a second-stage calcination (final calcination) under a prescribed environment for several hours to one day.

A carbon-deposited composite cathode material having relatively good charge/discharge characteristics can be obtained by calcining a calcined precursor prepared by adding a conductive carbon precursor and a Mo-containing compound (which are not added to a pre-calcined product) to the components of a cathode active material and grinding and stirring the mixture (in this case, it is preferable to perform a two-stage calcination process and grinding the pre-calcined product).

The above two methods differ in the time of addition of the conductive carbon precursor, and the former (in which the conductive carbon precursor is added after the pre-calcination) is preferable because a carbon-deposited composite cathode material having better charge/discharge characteristics can be obtained. Thus, the former method is mainly described below. In any case, in the latter method (in which the conductive carbon precursor is added before the precalcination), the preparation of the calcined precursor and the selection of the calcination conditions may also be carried out in the same manner as in the former method.

< preparation of calcined precursor >

The calcined precursor can be prepared by adding a Mo-containing compound to the dry ingredients of the cathode active material and grinding and stirring the mixture in a planetary ball mill or the like from one hour to one day. An organic solvent such as an alcohol, ketone or tetrahydrofuran or water may be added to the mixture to complete the milling and stirring of the mixture under wet conditions. At this time, when water or alcohol is added to a compound having high reactivity with water or alcohol, such as molybdenum chloride, so that the milling and stirring of the mixture are performed under wet conditions, a reaction to generate molybdenum hydroxide or molybdenum alkoxide occurs during the process.

When metallic iron is used as a component of the cathode active material, the calcined precursor is prepared by mixing a compound that releases phosphate ions in a solution, water, and metallic iron, adding a degradable volatile compound containing Li such as lithium carbonate, lithium hydroxide, or a hydrate thereof to the mixture, adding a Mo-containing compound to the reaction product, and grinding and stirring the resulting mixture in the same manner as described above under wet conditions. Upon mixing the ingredients, phosphate ion releasing compounds such as phosphoric acid, metallic iron and water in solution are first mixed and milled to dissolve and interact. Grinding is intended to apply a shear force to the metallic iron in the solution to renew its surface. The yield of cathode material can thereby be improved. The milling is preferably carried out in an automatic mill, a ball mill, a bead mill, etc. for about 30 minutes to 10 hours, depending on the efficiency of the milling apparatus. Ultrasonic irradiation is also effective for completing the dissolution reaction of metallic iron. In milling the iron, a volatile acid such as oxalic acid or hydrochloric acid may be added to increase the acid concentration, or a volatile oxidizing agent such as oxygen (air), hydrogen peroxide, halogen (bromine, chlorine, etc.), or oxyhalide such as hypochlorous acid or bleaching powder may be added. The addition of nitric acid, which is a volatile acid that is oxidizing and acidic, is also effective. The reaction proceeds efficiently when the reactants are heated to about 50 to 80 ℃. The above volatile acids and oxidizing agents are preferably used in amounts equal to or less than that required for the oxidation of iron from its metallic state to iron (II) ions. As a result, the dissolution of metallic iron into a phosphoric acid solution or the like can be accelerated, and volatile acids, oxidizing agents, and the like are removed by the calcination process and do not remain in the cathode material. Then, lithium hydroxide or the like is added to the solution as a lithium source after grinding. After addition of the lithium source, if necessary, pulverization or grinding is preferred. When the milling and stirring are performed after the addition of the Mo-containing compound, a calcined precursor is prepared.

< outline of calcination >

The calcined precursor obtained by mixing the components of the cathode material and the Mo-containing compound as described above is calcined. The calcination is carried out under calcination conditions at a suitable temperature range, typically using from 300 to 900 ℃, and a suitable treatment time. The calcination is preferably conducted under oxygen-free conditions to avoid generating oxidant impurities and to promote reduction of remaining oxidant impurities.

In the production method of the present invention, although calcination may be carried out in a single stage including heating and subsequent temperature maintenance, the calcination process is preferably divided into two stages, i.e., a first calcination stage at a lower range of temperature (typically, a temperature range from room temperature to 300-450 ℃; hereinafter may be simply referred to as "pre-calcination") and a second calcination stage at a higher range of temperature (typically, from room temperature to a calcination completion temperature (approximately 500 to 800 ℃); hereinafter may be simply referred to as "final calcination").

During precalcination, the constituents of the cathode material are heated and reacted into an intermediate phase before being converted into the final cathode material. At this time, a pyrolysis gas is generated in most cases. As the temperature at which the precalcination should be ended, a temperature at which gas generation has almost been completed but the reaction of the cathode material as the final product has not completely proceeded is selected (in other words, the constituent elements of the cathode material have room for re-diffusion and homogenization in the final calcination in the high temperature range of the second stage).

In the final calcination after the pre-calcination, the temperature is raised and maintained in a range in which the re-diffusion and homogenization of the constituent elements occur, the reaction of the cathode material is completed, and furthermore, the crystal growth by sintering or the like can be prevented as much as possible.

When the second-stage calcination is performed after the conductive carbon precursor has been added to the first-stage calcined product, the properties of the resulting cathode material can be further improved if the aforementioned carbon-deposited composite cathode material is produced. When the conductive carbon precursor, particularly coal pitch or glucide melted by heating is used, it is preferable to carry out final calcination after adding it to the ingredients after the pre-calcination (in an almost completed mesophase generating gas from the ingredients), although it may be added to the ingredients before the pre-calcination (even in this case, the cathode performance may be considerably improved). This means that there is a step of adding a conductive carbon precursor to the composition between the precalcination and the final calcination of the calcination process. This makes it possible to prevent the conductive carbon precursor, such as coal pitch or saccharides, which is melted and pyrolyzed by heating, from being foamed due to the gas emitted from the components, so that the melted carbon precursor can be more uniformly spread on the surface of the cathode material, allowing pyrolytic carbon to be more uniformly deposited.

This is attributed to the following reasons.

Since most of the gas generated from the decomposition of the components is released during the pre-calcination and substantially no gas is generated during the final calcination, the addition of the conductive carbon precursor after the pre-calcination can achieve uniform deposition of the conductive carbon. As a result, the obtained cathode material has high surface conductivity, and the particles of the cathode material are firmly and stably bonded together. When the conductive carbon precursor is added to the composition as described before the precalcination, a carbon-deposited composite cathode material having relatively good charge/discharge characteristics can be obtained. However, the performance of cathode materials produced in this way is not as good as cathode materials produced by the addition of conductive carbon precursors after pre-calcination. This is considered to be because the gas vigorously emitted from the composition during the precalcination foams the conductive carbon precursor in a molten and incompletely thermally decomposed state, inhibiting uniform deposition of carbon, and thus adversely affecting the composition of Mo.

The calcination may be performed while a predetermined amount of hydrogen or water (water, steam, etc.) is continuously fed into the furnace together with an inert gas. Then, a carbon-deposited composite cathode material having better charge/discharge characteristics than the carbon-deposited composite cathode material produced without supplying hydrogen or water is obtained. In this case, hydrogen or water may be added at the whole stage of the calcination process, or particularly when the temperature is in the range of not more than 500 ℃ to the calcination completion temperature, preferably not more than 400 ℃ to the calcination completion temperature, more preferably not more than 300 ℃ to the calcination completion temperature. "adding" gaseous hydrogen or water vapor includes calcining in the presence of hydrogen (under a hydrogen atmosphere or the like).

< calcination conditions (in the case where deposition of conductive carbon is not included) >

The conditions under which the calcined precursor is calcined (specifically, the calcination temperature and the calcination period) should be set carefully.

The higher the calcination temperature, the better the reaction of the components of the composite cathode material is completed and stabilized. However, when deposition of conductive carbon is not included, an excessively high calcination temperature may cause too much sintering and crystal growth, resulting in significant deterioration of charge/discharge ratio characteristics (see experimental example 1 described later). Thus, the calcination temperature is in the range of about 600 to 700 deg.C, preferably about 650 to 700 deg.C, and the calcination is carried out in an inert gas such as N₂Or under Ar. When hydrogen (including water for producing hydrogen by pyrolysis) is added at this time as described previously, the performance of the resulting cathode material can be improved.

The calcination period ranges from hours to three days. When the calcination temperature is about 650 to 700 c, if the calcination period is about 10 hours or less, the uniformity of the Mo solid solution in the resulting cathode material may not be sufficient. If so, abnormal charge/discharge occurs, and the performance deteriorates rapidly after a dozen charge and discharge cycles. Therefore, the calcination period is preferably one to two days (24 to 48 hours). The abnormal charge/discharge is an abnormal behavior in which the internal resistance of the battery increases as the cycle progresses, and the relationship of the charge/discharge capacity and the voltage shows an intermittent two-stage curve, the cause of which has not been found yet. At present, it is considered that the bonding or phase separation of the localized chemical species of the composite element Mo is by Li⁺The movement of ions during charging and discharging is induced, while Li⁺The movement of the ions is suppressed.

When the calcination temperature is 700 c or more, sintering and crystal growth of the cathode material are promoted and good battery performance cannot be obtained although such behavior is not observed. Therefore, a suitable period shorter than 10 hours should be selected as the calcination period. LiFePO with metallic Li anode and Mo recombination produced under good conditions₄The battery of the cathode material showed large charge/discharge capacity at room temperature (at a charge/discharge current density of 0.5mA/cm for coin-type secondary batteries)²About 150mAh/g at 0.5C and about 135mAh/g at ratio 2C for a battery with a thin film cathode (about 120mAh/g at ratio 2C), and good charge/discharge cycle characteristics as shown in the examples described later.

In order to obtain a cathode material with good uniformity, it is preferable to completely pulverize and stir the precalcined product between the first and second stages of calcination (precalcination and final calcination) and to perform the second stage calcination (final calcination) at the aforementioned prescribed temperature.

< calcination conditions (in the case including conductive carbon deposition) >

The final calcination temperature is also very important when conductive carbon deposition is involved. The final calcination temperature is preferably higher than that in the case where the conductive carbon deposition is not included (e.g., 750 to 800 ℃). When the calcination temperature is high, the uniformity of Mo distribution may be insufficient. Therefore, the calcination period is selected to be 10 hours or less. The composite cathode material is produced by depositing electrically conductive pyrolytic carbon derived from pitch such as coal pitch or sugars such as dextrin on Mo-composite LiFePO₄On the cathode material, if the final calcination temperature is not higher than 750 ℃, the resulting cathode material shows the same abnormal behavior during charge/discharge cycles as the Mo composite cathode material without carbon deposition. That is, the internal resistance of the battery increases as the cycle progresses, and the relationship between the charge/discharge capacity and the voltage shows an intermittent two-stage curve and the performance deteriorates (see experimental example 2 described later). In the case of carbon deposited Mo composite cathode materials, such abnormal charge/discharge is often found at an earlier stage, i.e. within a few cycles of charge/discharge.

However, carbon deposited composite cathode materials that undergo final calcination at temperatures above about 750 ℃, e.g., 775 ℃, and in an inert gas, do not exhibit this abnormal behavior. This is assumed to be because, by using a relatively high final calcination temperature, the distribution of Mo is uniform and stable. As shown in the examples described later, it has been found that there is a metallic Li anode and that the Mo/carbon/LiFePO thus obtained is used₄The battery of composite cathode material showed a charge/discharge capacity close to the theoretical capacity (170mAh/g) at room temperature (0.5 mA/cm at charge/discharge current density for coin-type batteries²About 160mAh/g or higher, and about 155mAh/g at a ratio of 0.5C (about 140mAh/g at a ratio of 2C) for a battery having a thin film cathode, and has a long cycle life and good rate characteristics. In the case of the carbon-deposited composite cathode material, unlike the cathode material in which carbon is not deposited, deterioration of performance such as capacity decrease does not occur even when calcination is performed at a high temperature of 775 ℃. This is considered to be because the conductivity of the cathode material is improved by compounding Mo and depositing conductive carbon, and because Li ions can easily move in the cathode material particles due to the deposited carbon suppressing sintering and crystal growth to prevent an increase in the size of the cathode material particles. Therefore, the composite cathode material deposited with carbon under the above conditions has very high performance and stability.

The amount of conductive carbon deposits is preferably in the range of about 0.5 to 5% by weight based on the total amount of Mo composite cathode material and conductive carbon, depending on the size of the Mo composite cathode material crystal particles. Preferably, the amount of the conductive carbon deposit is about 1 to 2% by weight when the crystal particle size is about 50 to 100nm, and 2.5 to 5% by weight when the crystal particle size is about 150 to 300 nm. When the amount of carbon deposit is less than the above range, the electroconductivity effect is low. When the amount of carbon deposit is too large, depositing carbon inhibits Li⁺The movement of ions over the surface of the crystal particles of the cathode material. In both cases, the charge/discharge performance tends to decline. In order to deposit an appropriate amount of carbon, the amount of pitch such as coal pitch and/or saccharide such as dextrin as the carbon precursor to be added is preferably determined according to the weight loss ratio of the carbon precursor obtained in advance at the time of pyrolysis carbonization.

(D) Storage battery

Examples of secondary batteries using the cathode material of the present invention obtained as described above include lithium metal batteries, lithium ion batteries, and lithium polymer batteries.

The basic structure of the secondary battery is described below, taking a lithium ion battery as an example. Lithium ion batteries are secondary batteries, characterized by Li⁺Ions reciprocate between the anode active material and the cathode active material upon charge and discharge (see fig. 1), and are generally called "rocker type" or "shuttlecock shuttle type". In fig. 1, the anode is designated 10, the electrolyte is designated 20, the cathode is designated 30, the external circuit is designated 40 (power/load), C is in the charged state, and D is in the discharged state.

During charging, Li⁺Ions are intercalated into the anode (carbon, e.g. graphite is used in currently-available batteries) to form intercalation compounds (when Li is used at this point⁺Anode carbon is reduced when the discharged cathode is oxidized). During discharge, Li⁺Ions are inserted into the anode to form an iron compound-lithium complex (when Li is used at this time)⁺The discharged anode is oxidized and the iron of the cathode is reduced when returned to graphite or the like). During charging and discharging, Li⁺The ions move back and force through the electrolyte to transport charge. As the electrolyte, a solution obtained by adding an electrolyte salt such as LiPF₆、LiCF₃SO₃Or LiClO₄A liquid electrolyte prepared by dissolving in a cyclic organic solvent such as ethylene carbonate, propylene carbonate or γ -butyrolactone and a chain organic solvent such as dimethyl carbonate or ethyl methyl carbonate; a gel electrolyte prepared by impregnating the above electrolyte into a polymer gel substance; or a solid polymer electrolyte prepared by impregnating the above liquid electrolyte into a partially crosslinked polyethylene oxide. When a liquid electrolyte is used, the cathode and the anode must be insulated from each other by interposing a porous separation film (separator) made of polyolefin or the like therebetween to prevent them from being short-circuited. The cathode and the anode are produced by adding a predetermined amount of an electrically conductive material such as carbon black and a binder such as a synthetic resin such as polytetrafluoroethylene, polyvinylidene fluoride or fluorine resin, or a synthetic rubber such as ethylene-propylene rubber, respectively, to the cathode or anode material, kneading the mixture with or without a polar organic solvent and forming the kneaded mixture into a film. Then, a metal foil is usedOr metal screens for current collection to construct a battery pack. When metallic lithium is used in the anode, Li (O) and Li occur when the anode is charged and discharged⁺And thereby forming a battery pack.

As the structure of the secondary battery, a coin-type lithium secondary battery formed by combining a pellet-type cathode and a closed case in a coin-type battery case, and a lithium secondary battery including a plated thin-layer cathode can be employed as shown in the examples described below.

The inventors of the present invention contained Mo⁵⁺、Mg²⁺、Al³⁺、Zr⁴⁺、Ti⁴⁺、V⁵⁺、Sn²⁺、Cr³⁺、Cu²⁺Etc. are added to the lithium iron phosphate component and the mixture is calcined to obtain a cathode material having the elements compounded, and the charge/discharge behavior of the material is examined. As a result, Mo was found to be most effective in improving charge/discharge properties. Although these elements cannot be handled in the same manner because different types of compounds are used as components of the elements to be compounded, the order of the effects of improving charge/discharge capacity is as follows (which will also be shown in comparative examples described later).

[ order of action ]

Mo＞＞CrCuVSn is not less than (additive-free) NbZrTi≥Mg

Despite the mechanism of Mo recombination acting on the cathode materialIt is not yet understood that it is possible to act Mo as a dopant on the cathode material and improve the reduced form of LiFePO₄And oxidized form of FePO₄Is used for the electrical conductivity of (1). In addition to the effect of electrostatic charge compensation, it is possible to use Li as cathode material_nFePO₄/FePO₄Central metal element of (4) Fe²⁺/Fe³⁺And a redox couple of the Mo ion type (which may be in a largely oxidized form). For example, it is possible that Mo (which may exist in multiple valence states) may be present in a lithium battery cathode material such as LiFePO for use in electromotive force of 3 to 4V₄Has one or more oxidation-reduction potentials (e.g., electrode potential Mo) near the oxidation-reduction potential of the central metal element⁵⁺/Mo⁶⁺And/or Mo⁴⁺/Mo⁵⁺) And they act as mediators of oxidation/reduction of Fe during charge and discharge to produce a state that can easily provide conduction electrons or holes to the cathode material.

As described previously, in the evaluation of the inventors, the elements disclosed in documents 3 and 4 have no effect, but elements other than, for example, V, Sn, Cr and Cu have effects, and particularly Mo has a prominent effect. It is believed that these elements may be close to Fe of the lithium iron phosphate cathode at the potential²⁺/Fe³⁺The redox potential of (a) forms a stable redox ion pair, and this is consistent with the above judgment.

The relationship between olivine-type lithium iron (II) phosphate and Li-emitting oxidized form iron (III) phosphate and the redox of the counter electrode and Li will be described below⁺The main hypothesis of the relationship between ion mobility behavior is described.

As previously mentioned, the volume ratio of the reduced form of lithium iron phosphate and the oxidized form of iron phosphate that Li emits, coexisting at both sides of the single crystal interface, changes during charge and discharge. Upon full charge, the conversion of the oxidized form of Li ejection is complete. Upon full discharge, the conversion of the reduced form of Li insertion is complete.

To simplify this phenomenon, a two-dimensional model of the vicinity of the cathode material particles as shown in fig. 2 is useful. FIGS. 2a to 2c illustrate the charging process, respectivelyInitial, intermediate and final stages of the discharge process (electrode oxidation for Li ejection), while fig. 2d to 2f illustrate initial, intermediate and final stages of the discharge process (reduction for Li insertion electrode), respectively. The elements of the cathode material particles are in contact along the X-axis with one side of the elements in contact with one side of a current collector material (which corresponds to a conductive attachment comprising conductive carbon deposited on the cathode material) positioned on the Y-axis. The other three sides of the cathode material element are in contact with the electrolyte and an electric field is applied in the x-axis direction. When the cathode material has low conductivity as in this cathode system, it is considered that, at the initial stage of charging shown in fig. 2a, electrode reduction is started at the corner where the three phases of the current collector material, the cathode material and the electrolyte meet, and Li as a first phase has been completely inserted into it in a reduced form of LiFePO₄And as a second phase an oxidized form FePO in which Li has been completely expelled₄The interface therebetween moves in the x-axis direction as charging progresses. At this time, Li⁺FePO with Li-penetrating ion discharge₄And Li-inserted LiFePO₄Is difficult. Therefore, it is likely that Li⁺Ions move into the electrolyte along the interface between the two phases as shown (as in LiFePO)₄In which Li is lost and in FePO₄With Li remaining sites, some Li⁺Ions can pass through them to rearrange the positions). On the other hand, the electrons inevitably go to the external circuit through the oxidized form of FePO₄And a current collector material. At steady state during constant current charging, the reduction occurs at a point on the interface to satisfy electroneutrality. When one Li⁺Ions moving along the interface, Li⁺The partial velocities of the ions in the x and y directions are equal, but are generated simultaneously and cross over the FePO₄The partial velocities of the electrons of (2) in the x and y directions, respectively, are opposite (the velocity vectors are shown by the arrows in fig. 2). Therefore, when Li⁺The local moving velocity vectors of ions and electrons are integrated at all interfaces, Li⁺The ions and electrons generally move in opposite directions along the X-axis. At this time, if Li-extracted oxidized form FePO₄Is low, electrode oxidation and Li⁺The movement of the ions is suppressed. In particular, it is believed that FePO is in an oxidized form due to Li-abstraction₄Have to be as inThe intermediate and final stages of charging shown in fig. 2b and 2c move long distances, creating large polarization to increase the voltage. If the Li-excreted oxidized form is FePO₄Is highly insulating and the final stage as shown in figure 2c cannot be reached and charging must be done with very low utilization of the active material.

During discharge, the exact opposite process occurs as shown in fig. 2d to 2 f. That is, electrode reduction by Li insertion starts from the corner where the three phases of the current collector material, cathode material and electrolyte meet, and the interface moves along the x-axis direction as discharge progresses. Then, in the intermediate and final stages of the discharge shown in fig. 2e and 2f, LiFePO in reduced form due to the fact that electrons have to be inserted in Li₄The medium and long distance movement causes large polarization to lower the discharge voltage. These represent the true change in voltage of the battery using the cathode system during charging and discharging at a constant current.

For the reasons mentioned above, in this cathode system, it is believed that it is significantly advantageous to increase the reduced form LiFePO of Li insertion₄And Li-depleted oxidized form FePO₄To promote electrode redox and Li⁺The ejection/insertion of ions improves the utilization rate (charge/discharge capacity) of the active material and reduces polarization to achieve good rate characteristics.

The recombination of Mo of the present invention has a large effect on this and suppresses the increase of polarization in the intermediate and final stages of charge shown in fig. 2b and 2c and the intermediate and final stages of discharge shown in fig. 2e and 2 f. Thus, the charge/discharge voltage curve can be flat for a large charge/discharge limit range and a high utilization rate of the active material (about 75% at a 1C ratio) can be achieved. The deposition of suitable conductive carbon in combination with the recombination of Mo in the present invention corresponds to contacting the other three sides of the cathode material particle elements with the current collector material as shown in fig. 2. Then, it is considered that the effect of Mo recombination is synergistically enhanced due to an increase in three-phase interfaces at which the current collector material, the cathode material, and the electrolyte meet. As described above, it is assumed that when Mo recombination and deposition of conductive carbon are combined, high utilization of the active material (about 88% at a 1C ratio) can be achieved, and the battery capacity-voltage characteristic curve shows a voltage surge or decrease after sufficient current equivalent to a charge/discharge capacity close to the theoretical capacity has been supplied.

The following examples further describe the invention in more detail. However, the present invention should not be limited to these examples.

Example 1

(1) Preparation of cathode materials

LiFePO compounded with Mo₄The cathode material was synthesized by the following method.

4.4975g FeC₂O₄.2H₂O (product of Wako Pure Chemical Industries Ltd.), 3.3015g (NH)₄)₂HPO₄(Special grade; product of Wako Pure Chemical Industries Ltd.) and 1.0423g LiOH₂A mixture of O (Special grade; product of Wako Pure Chemical Industries Ltd.) was mixed with ethanol in an amount of about 1.5 times the volume of the mixture. The resulting mixture was pulverized and stirred for 1.5 hours with 2mm zirconia beads and zirconia pots on a planetary ball mill and dried at 50 ℃ under reduced pressure. The dried mixture was used 0.1366g (based on FeC)₂O₄·2H₂Elemental ratio of Fe in O, corresponding to 2 mol%) molybdenum pentachloride MoCl₅(product of Wako Pure Chemical Industries Ltd.) and the resulting mixture was ground in an automatic agate mortar and stirred for 1.5 hours to obtain a calcined precursor. The calcined precursor was pre-calcined in an alumina crucible at 400 ℃ for five hours while feeding pure N at a flow rate of 200l/min₂And (4) qi. The pre-calcined product was ground in an agate mortar for 15 minutes and subjected to final calcination at 675 deg.c for 24 hours under the same atmosphere (gas was supplied before heating and gas was continuously supplied during calcination until after the calcined product was cooled). According to the results of powder X-ray diffraction analysis, the cathode material thus obtained showed a structure similar to that of LiFePO having an olivine-type crystal structure₄The same peak, and no observation ofCrystal diffraction peaks due to impurities. The results of the X-ray diffraction analysis are shown in fig. 3.

Elemental analysis of the cathode material by ICP emission spectrometry showed that the composition was (Li: Fe: Mo: P: O) ═ 0.98: 1.02: 0.017: 1: 4.73 (molar ratio relative to phosphorus (P) element; amount of oxygen (O) was calculated). For convenience, the amount of added elements such as Mo is hereinafter not expressed by actual content but by mole percentage based on Fe (or P). As described previously, if the final calcination period is insufficient in producing the Mo composite cathode material, an abnormal phenomenon in which the charge/discharge voltage shows a two-stage curve may occur and its performance deteriorates as the charge/discharge cycle progresses (which often, but not always, occurs at about 10 hours in the final calcination period). This phenomenon can be achieved by completely pulverizing and stirring MoCl₅And is avoided with a sufficiently long final calcination period.

(2) Production of storage battery

Cathode material, acetylene Black [ Denka Black (registered trademark), Denki Kagaku kogyo k.k. product; 50% compressed product ] as a material causing conductivity and unsintered PTFE (polytetrafluoroethylene) powder as a binder were mixed and kneaded in a weight ratio of 70: 25: 5. The kneaded mixture was rolled into a sheet having a thickness of 0.6mm, and the sheet was punched out into a disk having a diameter of 1.0cm to form pellets as a cathode.

The metallic titanium and nickel screens were welded by spot welding into cathode and anode current collectors, respectively, to form a coin-type battery case made of stainless steel (model No. CR 2032). The cathode and anode composed of metallic lithium foil were mounted in a battery case having a porous polyethylene separation film (E-25, product of Tonen Chemical Corp) interposed between the cathode and the anode. Battery box full of 1MLiPF₆A solution of dimethyl carbonate and ethylene carbonate mixed solvent 1: 1 (product of Tomiyama pure chemical Industries Ltd.) was used as an electrolyte solution, and then sealed to fabricate a coin type lithium secondary battery. All the process of assembling cathode and anode, separation membrane, and electrolyte into the battery was performed in a dry argon-purged handAnd (4) performing in a box.

Batteries with cathode materials produced as described above at a current density of 0.5mA/cm per apparent area of cathode pellet at constant current²Is repeatedly charged and discharged at 25 c in an operating voltage range of 3.0 to 4.0V. The discharge capacities for the first, tenth and twentieth cycles are shown in table 1. The charge/discharge capacity and voltage characteristics of the third cycle are shown in FIG. 4 (at a current density of 1.6 mA/cm)²Also shown in fig. 4). The cycle charge/discharge characteristics of the battery are shown in fig. 5. In the following examples, comparative examples and experimental examples, capacity values were calibrated by the dry weight of the cathode active material including an added element such as molybdenum, except carbon (the weight of conductive carbon deposition was calibrated).

As shown in Table 1 and FIGS. 4 and 5, when the Mo-composited lithium iron phosphate cathode material of the present invention was used, the current density at charge/discharge was 0.5mA/cm²A large initial capacity of up to 153 mAh/g for the cathode system is obtained. Also, although a slight capacity drop was observed, a relatively stable cyclic charge/discharge characteristic was obtained.

Comparative example 1

The same procedure as in example 1 was repeated, except that MoCl was not added₅To the dried mixture to prepare LiFePO without additives such as Mo₄The cathode material was the opposite of the 2 mol% Mo composite cathode material of example 1. A coin-type battery was fabricated in the same manner as in example 1 using a cathode material, and the characteristics of the battery were evaluated. The discharge capacities of the batteries at the first, tenth and twentieth cycles are shown in table 1, and the cycle charge/discharge characteristics of the batteries are shown in fig. 5.

TABLE 1

As shown in Table 1 and FIG. 5, the coin-type cell using the Mo composite cathode material of example 1 hadA large initial discharge capacity and showed less cycle deterioration than the additive-free cathode material of comparative example 1. This is assumed to be due to the reduced form of LiFePO of Li-insertion₄And Li-depleted oxidized form FePO₄The conductivity of (2) is improved as the cathode material is compounded by Mo.

Example 2

The components were mixed in the same ratio as in example 1 and synthesized into a Mo composite cathode material by the same method as in example 1. The cathode film-coated sheet was more practical than the above-described spherical cathode, was prepared by using a composite cathode material by the following method, and the characteristics of the lithium secondary battery were evaluated.

The particle size of 2 mol% Mo composite cathode material powder is adjusted by a 45 mu m sieve. Cathode material, acetylene Black [ Denka Black (registered trademark), Denki Kagaku Kogyo k.k. product; 50% compressed product]As the conductive material and a 12% polyvinylidene fluoride PVDF/N-methyl pyrrolidone (NMP) solution (product of Kureha Chemical Industry co. ltd) were mixed in a weight ratio of 85: 5: 10 (as PVDF). N-methylpyrrolidone (NMP) (water content: less than 50ppm, product of Wako Pure Chemical Industries Ltd.) was added to adjust the viscosity of the mixture, and the mixture was stirred in a defoaming mixer to prepare a cathode mixture coating ink. The ink was uniformly distributed to an aluminum foil having a thickness of 20 μm and air-dried at 80 ℃, and the aluminum foil was rolled to obtain a cathode film-coated sheet having a coating thickness of 80 μm. The aforementioned porous polyolefin separation membrane E-25, excess metal Li foil anode, and cathode membrane plating plate were incorporated into a stainless steel analyzable small-sized cell (insulating portion made of polyvinyl fluoride). Battery case full of 1M LiPF₆A solution of dimethyl carbonate and ethylene carbonate mixed solvent 1: 1 (product of Tomiyama Pure Chemical industries Ltd.) was used as an electrolyte solution, and then sealed to fabricate a lithium secondary battery. The storage battery was subjected to a cyclic charge/discharge test to evaluate the characteristics of the storage battery. The accumulator is alternately charged at a constant current in a specified current density (ratio) in an operating voltage range of 3.0 to 4.0V at 25 DEG CAnd discharging. The results are summarized in Table 2.

TABLE 2

As shown in FIG. 2, the secondary battery using 2 mol% Mo composite cathode film plating sheet has a capacity comparable to that of the coin-type battery of the ball-type cathode of example 1 and can be operated at a 1C ratio (2 mA/cm)²) Or higher high current densities. The cycle characteristics were slightly better than those of the ball-type cathode cell. The discharge capacities at the 2.0C and 0.5C ratios in table 2 were larger than the corresponding values of the data shown in document 4.

The Mo composite cathode film plated sheet of example 2, when combined with a carbon film anode of graphite, Meso Carbon Microbeads (MCMB), etc., can form a practical, high performance lithium ion battery.

Example 3

The same procedure as in example 1 was repeated except for the addition of MoCl₅The amount of (A) was reduced to 0.0683g, which is half the amount used in example 1, and the final calcination period was shortened to 10 hours to prepare 1 mol% Mo composite LiFePO₄The cathode material was the opposite of the 2 mol% Mo composite cathode material of example 1. A coin type battery was fabricated and evaluated for characteristics using the cathode material in the same manner as in example 1.

The current density of the battery per the surface area of the cathode sphere is 0.5mA/cm²Is repeatedly charged and discharged at 25 c in an operating voltage range of 3.0 to 4.0V. The discharge capacities of the batteries at the first, tenth and twentieth cycles are shown in table 3. In this example, as described above, no two-phase curve of the charge/discharge voltage was observed as the cycle proceeded.

In the following comparative examples 2 to 10, characteristics in the case where Mo was not added and the case where an element other than Mo was used were evaluated to compare the effects of Mo with other elements in example 3.

Comparative example 2

The same process as in comparative example 1 was repeated except that the final calcination period was shortened to 10 hours to obtain LiFePO containing no additives such as Mo₄A cathode material. A coin-type secondary battery was produced using the cathode material in the same manner as in example 1, and the characteristics of the secondary battery were evaluated. The measurement results of the discharge capacity are shown in table 3.

Comparative example 3

The same procedure as in example 3 was repeated, except that 0.0146g of magnesium hydroxide Mg (OH) was added₂(particle diameter: 0.6 μm, purity: 97%; product of Wako Pure Chemical Industries Ltd.) in place of MoCl₅Preparation of 1 mol% Mg composite LiFePO₄The cathode material was the opposite of the 1 mol% Mo composite cathode material of example 3. LiFePO composited with Mg₄A coin-type secondary battery was produced from the cathode material in the same manner as in example 1, and the characteristics of the secondary battery were evaluated. The measurement results of the discharge capacity are shown in table 3.

Comparative example 4

The same procedure as in example 3 was repeated, except that 0.0851g of the titanium butoxide monomer Ti [ O (CH)₂)₃CH₃]₄(product of Wako Pure Chemical Industries Ltd.) instead of MoCl₅Preparation of 1 mol% Ti composite LiFePO₄The cathode material was the opposite of the 1 mol% Mo composite cathode material of example 3. LiFePO composited with Ti₄A coin-type secondary battery was produced from the cathode material in the same manner as in example 1, and the characteristics of the secondary battery were evaluated. The measurement results of the discharge capacity are shown in table 3.

Comparative example 5

The same procedure as in example 3 was repeated, except that 0.0796g of niobium ethoxide Nb (OC) was added₂H₅)₅(product of Wako Pure Chemical Industries Ltd.) instead of MoCl₅Preparation of 1 mol% Nb composite LiFePO₄The cathode material was the opposite of the 1 mol% Mo composite cathode material of example 3. LiFePO composited with Nb₄A coin-type secondary battery was produced from the cathode material in the same manner as in example 1, and the characteristics of the secondary battery were evaluated. The measurement results of the discharge capacity are shown in table 3.

Comparative example 6

The same procedure as in example 3 was repeated, except that 0.1128g of an 85% butanol solution of zirconium butoxide (product of Wako pure chemical Industries Ltd.) was added in place of MoCl₅Preparation of 1 mol% Zr composite LiFePO₄The cathode material was the opposite of the 1 mol% Mo composite cathode material of example 3. LiFePO composited with Zr₄A coin-type secondary battery was produced from the cathode material in the same manner as in example 1, and the characteristics of the secondary battery were evaluated. The measurement results of the discharge capacity are shown in table 3.

Comparative example 7

The same procedure as in example 3 was repeated, except that 0.0328g of vanadyl oxalate n-hydrate VOC was added₂O₄.nH₂O (assuming that the number of hydrated molecules added is 2; product of Wako Pure chemical industries Ltd.) instead of MoCl₅Preparation of 1 mol% V composite LiFePO₄The cathode material was the opposite of the 1 mol% Mo composite cathode material of example 3. LiFePO using V recombination₄A coin-type secondary battery was produced from the cathode material in the same manner as in example 1, and the characteristics of the secondary battery were evaluated. The measurement results of the discharge capacity are shown in table 3.

Comparative example 8

The same procedure as in example 3 was repeated, except that 0.0499g of copper acetate 1 hydrate Cu (CH)₃COO)₂.H₂O (product of Wako Pure Chemical Industries Ltd.) instead of MoCl₅Preparation of 1 mol% Cu composite LiFePO₄The cathode material was the opposite of the 1 mol% Mo composite cathode material of example 3. LiFePO composited with Cu₄A coin-type secondary battery was produced from the cathode material in the same manner as in example 1, and the characteristics of the secondary battery were evaluated. The measurement results of the discharge capacity are shown in table 3.

Comparative example 9

The same procedure as in example 3 was repeated, except that 0.0517g of SnC (stannous oxalate) were added₂O₄(product of Wako Pure Chemical Industries Ltd.) instead of MoCl₅Preparation of 1 mol% Sn composite LiFePO₄The cathode material was the opposite of the 1 mol% Mo composite cathode material of example 3. LiFePO composited with Sn₄A coin-type secondary battery was produced from the cathode material in the same manner as in example 1, and the characteristics of the secondary battery were evaluated. The measurement results of the discharge capacity are shown in table 3.

Comparative example 10

The same procedure as in example 3 was repeated, except that 0.0278g of chromium acetate Cr (CH)₃COO)₃(product of Wako Pure Chemical Industries Ltd.) instead of MoCl₅Preparation of 1 mol% Cr composite LiFePO₄The cathode material was the opposite of the 1 mol% Mo composite cathode material of example 3. LiFePO composited with Cr₄A coin-type secondary battery was produced from the cathode material in the same manner as in example 1, and the characteristics of the secondary battery were evaluated. The measurement results of the discharge capacity are shown in table 3.

TABLE 3

As shown in table 3, the battery capacity increasing effect of Mo is significantly higher than that of other elements, and none of the elements other than Mo has a significant effect. However, Cu, Cr, V and Sn appear to have a small effect of increasing the capacity of the battery. On the other hand, Mg, Ti, Zr and Nb have no effect, or stated otherwise, the results of cathode materials using these elements are inferior to those of cathode materials without additives.

Experimental example 1

Study of Mo composite conditions

To reveal the conditions for producing the preferred Mo composite lithium iron phosphate cathode material, the effect of the amount of Mo added and the final calcination temperature on the Mo composite cathode discharge capacity was investigated. In substantially the same manner as in example 1 (Current Density: 0.5 mA/cm)²) The discharge capacity was measured.

Fig. 6 is a graph showing the difference in discharge capacity of coin type batteries produced using different amounts of Mo added at a fixed final calcination temperature of 675 deg.c. As shown in fig. 6, when there is only 0.1 mol% Mo in terms of element ratio based on the added Fe, the battery capacity is greater than that of the battery using the cathode material without the additive. When about 0.5 to 3 mol% Mo is added, the battery capacity is a maximum value, gradually decreasing as the amount of Mo increases. However, even when 5 mol% of Mo was added, no large capacity decrease was observed.

Fig. 7 is a graph showing the difference in discharge capacity of coin-type secondary batteries produced using 2 mol% Mo added based on Fe in the composition at a final calcination temperature of 775 ℃.

As shown in fig. 7, the battery discharge gradually increases from the point where the calcination temperature is about 575 c, reaching a maximum value when the calcination temperature is about 625 to 675 c. When the calcination temperature is 725 deg.c or more, the battery discharge capacity sharply decreases. The reason why the capacity rapidly decreases around the calcination temperature of about 700 ℃ is considered as follows: since sintering and growth of the crystalline cathode material are accelerated in this temperature range and the particle size thereof is increased, movement of Li ions in the cathode material crystals is suppressed. The preferred temperature range is not applicable to the Mo composite material having conductive carbon deposition with the effect of suppressing the increase in particle diameter as described below.

Example 4

Conductive carbon deposited Mo composite LiFePO₄Preparation of cathode materials

4.4975g FeC according to the same method and conditions as in example 1₂O₄.2H₂O (product of Wako pure chemical Industries Ltd.), 3.3015g (NH)₄)₂HPO₄(Special grade; product of Wako pure chemical Industries Ltd.) and 1.0423g of LiOH₂A mixture of O (Special grade, product of Wako pure chemical Industries Ltd.) was mixed with ethanol in an amount of about 1.5 times the volume of the mixture. The resulting mixture was pulverized and stirred in a planetary ball mill with 2mm zirconia beads and zirconia pots for 1.5 hours and dried at 50 ℃ under reduced pressure. The dried mixture was used at 0.0683g (based on Fe in FeC)₂O₄.2H₂Fe element in O corresponding to 1 mol%) of molybdenum pentachloride MoCl₅(product of Wakopure Chemical Industries Ltd.) and the resulting mixture was ground in an automatic agate mortar and stirred for 1.5 hours to obtain a calcined precursor. The calcined precursor was pre-calcined in an alumina crucible at 400 ℃ for five hours while feeding pure N at a flow rate of 200ml/min₂And (4) qi. 0.0979 g of refined coal pitch (MCP-250; Adchemco Corp. product) having a softening temperature of 250 ℃ was added to 1.9075g of the pre-calcined product. The mixture was ground in an agate mortar for 15 minutes and subjected to final calcination at 775 ℃ for 10 hours under the same atmosphere (after the gas was blown before heating and the gas was continuously supplied during calcination until the calcined product was cooled). According to the results of powder X-ray diffraction analysis, the cathode material thus obtained showed a crystal structure similar to that of LiFePO having an olivine-type crystal structure₄The same peak, and no crystal diffraction peak due to impurities was observed. The results of the X-ray diffraction analysis are shown in fig. 8.

The elemental analysis result showed that 3.92% by weight of carbon generated by pyrolysis of refined coal pitch was contained, but no diffraction peak corresponding to graphite crystal was observed, assuming that a composite material having amorphous carbon was formed. Elemental analysis of the cathode material by ICP emission spectrometry showed a composition (Li: Fe: Mo: P: O) ═ 1.03: 1.08: 0.0089: 1: 4.44) (elemental molar ratio relative to phosphorus (P); amount of oxygen (O) was calculated).

(2) Production of storage battery

A coin-type lithium secondary battery was manufactured in the same manner as in example 1. The coin-type secondary battery is operated at a constant current with a current density of 0.5mA/cm per cathode ball surface area²Is repeatedly charged and discharged at 25 c in an operating voltage range of 3.0 to 4.0V. The discharge capacities for the first, tenth and twentieth cycles are shown in table 4. The charge/discharge capacity and voltage characteristics of the third cycle are shown in FIG. 9 (at a current density of 1.6 mA/cm)²Also shown in fig. 9). An enlarged view of the characteristic curves at the third and tenth cycles is shown in table 10, and the battery cycle charge/discharge characteristics are shown in fig. 11.

As shown in Table 4 and FIGS. 9 to 11, when the Mo composite lithium iron phosphate cathode material of the invention was used, the current density at charge/discharge was 0.5mA/cm²A large capacity of 164mAh/g is obtained, which is close to LiFePO₄Theoretical capacity of the cathode system (170mAh/g), and very stable cyclic charge/discharge characteristics were obtained. As shown in fig. 9 and 10, the voltage is very flat almost throughout the charging and discharging process and shows the ideal voltage profile of the cathode of the battery, where sharp rises and falls occur at the end of the charging and discharging process. As can be understood from fig. 10 and 11, the discharge capacity slightly increases from the start of the cycle charge/discharge to about the tenth cycle. This is a phenomenon specific to cathode materials on which conductive carbon is deposited.

Comparative example 11

The same procedure as in example 4 was repeated, except that MoCl was not added₅To the dried mixture to obtain LiFePO with conductive carbon deposited₄The cathode material (without Mo) was the opposite of the 1 mol% Mo composite cathode material of example 4. The carbon content in the cathode material was 3.67% by weight. A coin-type secondary battery was produced using the cathode material in the same manner as in example 4, and the characteristics of the secondary battery were evaluated. The discharge capacities of the batteries at the first, tenth and twentieth cycles are shown in Table 4, and the stored electric powerThe characteristics of the cell cycle charge/discharge are shown in fig. 11.

As shown in table 4 and fig. 11, it is apparent that the Mo composite material LiFePO of example 4 using conductive carbon deposition₄The coin type cell of the cathode material had a significantly larger initial discharge capacity and better cycle charge/discharge characteristics, which can be considered to have high performance in general, as compared with the cell using the conductive carbon-deposited cathode material (containing no Mo) of comparative example 11. This is assumed to be because the interface where the cathode active material, the electrolyte and the current collector material meet (where cathode redox starts) is drastically increased by the deposition of conductive carbon, and the active material utilization is improved, and LiFePO is obtained by the Mo composite material₄The cathode material itself is improved in conductivity and the charge/discharge characteristics are improved.

TABLE 4

Example 5

The components were mixed in the same ratio as in example 4 and synthesized into conductive carbon deposited Mo composite LiFePO by the same method as in example 4₄In the cathode material. The cathode film plated sheet was more practical than the ball-type cathode, and was prepared by using the composite cathode material in the same manner as in example 2, and the characteristics of the lithium secondary battery were evaluated. The results are summarized in Table 5.

As shown in Table 5, the secondary battery using 2 mol% Mo composite film sheet as the cathode had a capacity comparable to that of the coin-type secondary battery of the ball-type cathode of example 1 and could be operated at a 1C ratio (2 mA/cm)²) Or higher, without causing any problems, and has good cycle characteristics. The discharge capacities at all ratios in table 5 were larger than the corresponding values of the data shown in document 4.

A cathode using the Mo composite membrane sheet of example 5, when combined with a graphite carbon-containing membrane anode, meso-carbon microbeads (MCMB), etc., can form a practical, high performance lithium ion battery. The battery is suitable for use in power systems including electric vehicles and hybrid electric vehicles as well as portable devices such as mobile phones because of its large capacity and good rate characteristics.

TABLE 5

Experimental example 2

Investigation of carbon deposition conditions and Mo-mixing conditions

To reveal production of the preferred conductive carbon deposited Mo composite LiFePO₄The conditions of the cathode material will be described as the effect of the final calcination temperature on the composite cathode discharge capacity. The measurement was performed under substantially the same conditions as in example 1.

FIG. 12 shows coin cells (current density 0.5 mA/cm) in which the amount of Mo added and the amount of deposited conductive carbon were fixed to 1 mol% based on the iron in the composition and to about 4% by weight, respectively, but the calcination temperature was changed²) Difference in discharge capacity.

As shown in fig. 12, the battery discharge gradually increases from the point where the calcination temperature is about 575 deg.c, and continues to increase even when the calcination temperature is higher than about 625 to 6750 c. When the calcination temperature is 775 ℃, a cathode material having very high performance can be obtained. This is very different from the case where the conductive carbon is not deposited, and shows that when the conductive carbon is deposited on the surface of the cathode active material, sintering and crystal growth are suppressed even though calcination is performed at a temperature higher than 700 ℃ and Li⁺The state in which ions can smoothly move in the active material particles is maintained.

As described previously, when the final calcination is performed at a temperature of not higher than 750 ℃ in the production of a conductive carbon-deposited Mo composite cathode material, the charge/discharge voltage characteristics tend to show abnormal behavior (in particular, this behavior frequently occurs during discharge) with the progress of charge/discharge cycles. As a typical example, the charge/discharge characteristics of the cathode material subjected to the final calcination at 725 ℃ for 10 hours are shown in fig. 13 and 14. Fig. 13 and 14 show data in the case where the amount of Mo added was 1 mol% based on iron in the composition and the amount of conductive carbon deposit was 3.87% by weight.

The charge/discharge curve for the third cycle of the cathode material calcined at 725 c shown in fig. 13 shows good cathode characteristics similar to the cathode material calcined at 775 c shown in fig. 9, except for a slightly smaller capacity, a slightly larger polarization during charge/discharge and a slightly poorer charge/discharge coulombic efficiency. At the tenth cycle (current density 0.5 mA/cm)²) However, the polarization during charge/discharge was significantly large and an abnormal voltage step was observed on the discharge side shown in fig. 14. In the cycle after that, the situation gradually deteriorates and the performance deteriorates in most cases. The cause of the abnormal behavior is not known yet, but it is considered to be due to the occurrence of phase separation and precipitation of Mo chemical by charge and discharge and the suppression of Li⁺The movement of the ions. It is understood that this problem can be avoided when the final calcination is carried out at not less than 750 c, for example 775 c, because the structure of Mo is unified by annealing at the time of high-temperature calcination. The final calcination is preferably carried out at a temperature in the range of about 775 to 800 c, because the active material LiFePO when the final calcination is carried out at a temperature of not less than about 850 c₄Is thermally decomposed to cause a change in composition and sintering inevitably occurs.

Example 6

To determine Mo in Mo composite material LiFePO₄The presence in the cathode material of a fixed quantity of molybdenum pentachloride MoCl₅With varying amounts of Li containing ions⁺Iron ion Fe²⁺Or phosphate radical ion PO₄ ^3-The cathode material was produced, and experiments were conducted to evaluate the influence on the crystal structure and the charge/discharge behavior of the battery. In the experiments, relatively large amounts of Mo (5 mol% based on Li, Fe or P) were added without considering optimization of the cathode material properties, so that structural changes and effects on charge/discharge behavior were apparent. No conductive carbon precursor was added.

The components were introduced so that the molar ratio of the components of Li, Fe, phosphate ion and Mo as essential components was 1: 0.05, and a cathode material (which will be hereinafter referred to simply as "sample a") was produced.

3.5979g FeC₂O₄.2H₂O (product of Wako Pure Chemical Industries Ltd.), 2.3006g (NH)₄)₂HPO₄(Special grade; product of Wako Pure Chemical Industries Ltd.) and 0.8393g LiOH₂A mixture of O (product of Wako Pure Chemical Industries Ltd.) was mixed with isopropyl alcohol in an amount of about 1.5 times the volume of the mixture. The resulting mixture was pulverized and stirred for 1.5 hours with 2mm zirconia beads and zirconia pots on a planetary ball mill and dried at 50 ℃ under reduced pressure. The dried mixture was taken up in 0.2732g (which is based on NH)₄H₂PO₄Elemental ratio of P in (5 mol%) molybdenum pentachloride MoCl₅(product of Wako Pure Chemical Industries Ltd.) and the resulting mixture was ground in an automatic agate mortar and stirred for 1.5 hours to obtain a calcined precursor. The calcined precursor was pre-calcined in an alumina crucible at 400 ℃ for 5 hours while feeding pure N at a flow rate of 200ml/min₂And (4) qi. The pre-calcined product was ground in an agate mortar for 15 minutes and subjected to a final calcination at 675 deg.c for 10 hours under the same atmosphere (gas was supplied before heating and gas was continuously supplied during calcination until after the calcined product was cooled).

Elemental analysis of sample A by ICP emission spectrometry showed a composition (Li: Fe: Mo: P: O) ═ 1.01: 0.045: 1: 3.94) (molar ratio relative to phosphorus (P) element; amount of oxygen (O) was calculated). According to powder X-ray diffractionAs a result of the analysis by sputtering, most of the samples A showed LiFePO having an olivine-type crystal structure₄The same peak. Although no diffraction peak due to impurities was observed, only a small amount of Fe (II) was suggested₂Mo(IV)₃O₈(molyolitite) present [ in this experiment, measurement was carried out using a higher sensitivity apparatus than examples 1 and 4 (automated X-ray diffraction System RINT2000/PC, product of Regaku Corporation)]. The results of the X-ray diffraction analysis are shown in fig. 15.

The components were prepared so that the molar ratios of Li, Fe, phosphate ions, and Mo components as essential components were as follows, and cathode materials B, C and D were produced. That is, samples B, C and D were produced by the same method as for preparing sample A, except that LiOH₂O (sample B), FeC₂O₄.2H₂O (sample C) or NH₄H₂PO₄(sample D) corresponds to 0.95 times of sample A, respectively.

Sample B Li: Fe: P: Mo ═ 0.95: 1: 0.05

Sample C Li, Fe, P, Mo 1: 0.95: 1: 0.05

Sample D Li: Fe: P: Mo ═ 1: 0.95: 0.05

Elemental analysis of the samples by ICP emission spectrometry showed that the compositions of samples B, C and D were (Li: Fe: Mo: P: O) ═ 0.95: 1.01: 0.044: 1: 3.96), (Li: Fe: Mo: P: O) ═ 0.99: 0.95: 0.046: 1: 3.95), and (Li: Fe: Mo: P: O) ═ 1.05: 1.048: 1: 3.96, respectively (molar ratio relative to phosphorus (P) elements; the amount of oxygen (O) is the calculated value).

The results of powder X-ray diffraction analysis of the samples are also shown in fig. 15. Samples B and C showed LiFePO with an olivine-type crystal structure₄The same peak, and no diffraction peak by impurity was observed. Sample D shows LiFePO with an olivine-type crystal structure₄Identical peaks, and the clear peaks correspond to Fe (II)₂Mo(IV)₃O₈(molyianite), which is suggested to be present in sample a. This indicates thatThe molybdenite phase-separated apparently as an impurity.

Coin-type lithium secondary batteries having metallic lithium anodes were fabricated in the same manner as in example 1 using samples, and at a temperature of 25 ℃ and a charge/discharge current density of 0.5mA/cm²A cyclic charge/discharge test was conducted under the conditions of (1). The discharge capacities of the batteries for the second, tenth and twentieth cycles are shown in table 6. The internal resistance of the coin-type lithium secondary battery charged to 50% of the actual capacity obtained from the voltage difference during charge and discharge is also shown in table 6.

TABLE 6

It is presumed from table 6 that the state of Mo in the olivine-type lithium iron phosphate cathode material samples a to D compounded with 5 mol% of Mo and the effect of Mo on the cathode function are as described in the following (i) to (iii).

(i) Sample B

Assuming that when the molar amount of Li decreases in sample B as the molar amount of Mo increases, the cathode material has a single-phase olivine-type LiFePO after calcination₄The Mo, which increases in structure, enters the octahedral sites that are normally occupied by Li instead of it in the olivine-type crystal structure (the occupied sites cannot be determined exactly, however). The secondary battery produced using the Mo composite cathode and the metallic Li anode in this state (the anode capacity is excessive for the cathode capacity) showed an intermediate level of internal resistance at the initial stage of charge/discharge (second cycle) in the four samples and had a low discharge capacity because the amount of Li was reduced.

However, the capacity increased with the progress of the number of cycles (became larger than sample a at the twentieth cycle), and finally showed good cycle characteristics with a small decrease in capacity.

Likewise, unlike samples A, C and D, the internal resistance of the cell decreased significantly as cycling progressed (became lower than sample C at the twentieth cycle).

It is believed that as Li is supplied from the anode, a change in charge/discharge characteristics occurs due to some rearrangement of Li ions, Fe ions, and Mo ions in the crystalline phase cathode active material of sample B during charge/discharge (e.g., some ions migrate between their sites) changing physical characteristics until the conductivity and movement of Li ions are enhanced to reduce cathode polarization. Although the conductive carbon was not deposited, sample B can be suitably used as a cathode material because the cycle characteristics are excellent.

(ii) Sample C

Assuming that when the molar amount of Fe in sample C decreases the same as the molar amount of Mo increases, the cathode material has a single-phase olivine-type LiFePO after calcination₄The structure, and the added Mo enters the octahedral sites that are normally occupied by Fe instead of it in the olivine-type crystal structure (the occupied sites cannot be determined exactly, however). The secondary battery produced using the Mo composite cathode and the anode with an excess of metallic Li in this state showed the lowest internal resistance in the initial stage of charge/discharge (second cycle) among the four samples, indicating that the cathode polarization was small. However, since the amount of Fe as the redox center is reduced, the discharge capacity is small.

The capacity of the battery using sample C gradually decreased with repeated cycles of charge/discharge, and the stability of the capacity was significantly inferior to that of the battery using sample B with the progress of cycles. Likewise, the internal resistance of the cell increases slightly as the cycle progresses. It is believed that the reduction in capacity with cycling is generally due to the degradation often observed in such cathode systems, i.e., it results from an increase in contact resistance between the cathode active material particles caused by the repeated expansion and contraction of the cathode lattice. Sample C is advantageous in that the internal resistance of the battery can be small from the early stage of charge/discharge, and its cycle characteristics can be improved by depositing conductive carbon thereon. Thus, it can be used as a cathode material.

(iii) Samples A and D

It is considered that sample A prepared with Mo addition without reduction in the amounts of Li and Fe does not have a single-phase olivine-type structure precisely, but contains a small amount of Fe (II)₂Mo(IV)₃O₈(molypterite). It is considered that, as the composition indicated by ferromolybdenum ore, added Mo is mainly replaced by Fe, excess Mo which cannot be replaced with Fe forms a complex oxide, and discharged Fe is released and precipitated. Therefore, it is considered that when Mo is added without reducing the amount of Li or Fe, Mo tends to enter octahedral sites normally occupied by Fe (however, occupied sites cannot be accurately determined).

The secondary battery produced using the Mo composite cathode and the anode with an excess of metallic Li in this state exhibited a slightly larger internal resistance than the battery using sample B at the start stage of charge/discharge (second cycle). It should be noted that it was shown that the initial capacity was close to the theoretical capacity of 170mAh/g (about 150mAh/g, which was also observed in experimental example 1), which is greater than the initial capacity of the batteries using B, C and D, despite the addition of Mo in an amount as large as 5 mol%. This shows that most of the constituent elements Li, Fe, and P of lithium iron phosphate can also function as a cathode active material even when 5 mol% Mo is added.

As the charge/discharge cycles were repeated, the capacity of the battery using sample a gradually decreased and the internal resistance of the battery gradually increased, as with the battery using sample C. When Mo was added in sample a without reducing the amounts of Li and Fe, abnormal discharge and abnormal increase in internal resistance (cathode polarization component thereof) as described in example 1 sometimes occurred. This trend was also observed when using sample a.

It is believed that the reduction in discharge capacity with cycling is due to an increase in contact resistance and an abnormal increase in polarization at the particle interface of the cathode material. Since Mo was added without adjusting the amounts of Li and Fe at the time of cathode material calcination, there may be a correlation between the fact that the composition of sample a exceeded the stability limit of the single olivine-type crystalline phase (and thus the molybdenite was separated) and the occurrence of abnormal discharge and increase in cathode polarization.

However, sample a is advantageous in terms of its large initial capacity, and its cycle characteristics can be improved by depositing conductive carbon thereon. Thus, it can be used as a cathode material.

In sample D produced by reducing the amount of the phosphate component (P) by adding Mo, a considerable amount of ferromolybdenum ore was phase-separated and precipitated. The lithium metal secondary battery using sample D has a small initial capacity because the cathode active material is reduced, and its capacity stability is significantly reduced as the cycle progresses. It is considered that another cause of the deterioration of the cycle characteristics of samples D and a appears to be that the deposition of ferromolybdenum ore on the surface of the cathode active material adversely affects the activity of the cathode active material.

Therefore, in producing the Mo composite cathode, when Mo is added, it is preferable to use a slightly larger amount of the phosphate component than in sample a (in other words, to use a slightly smaller amount of the lithium component and/or the iron component than in the phosphate component so as not to generate by-products such as molybdenite.

As described above, lithium ion (Li)⁺) Iron (II) ion (Fe)²⁺) And phosphate ion (PO)₄ ^3-) As an essential component, and 0.1 to 5 mol%, preferably 0.5 to 5 mol% Mo based on P, of a cathode material having an olivine-type crystal structure, has a large capacity and provides excellent cathode performance. In addition, when the amount of Li and/or Fe in the Mo-containing cathode material is reduced to such an extent that by-products such as Fe (H) are not produced₂Mo(IV)₃O₈(molyinteite) to the extent that higher cathode performance can be expected.

In the case where the amount of Li and/or iron is reduced, when the amount of reduction of Li is relatively larger than that of iron, a cathode material having improved cycle characteristics can be obtained, and when the amount of reduction of iron is relatively larger than that of Li, a cathode material having small cathode polarization from the early stage of charge/discharge cycles can be obtained. At this time, it is preferable that the total molar amount of reduction of Li and/or Fe does not exceed the molar amount of Mo addition.

As described above, the cathode characteristics can be controlled by adjusting the amounts of Li, Fe, and phosphate components to be introduced and increasing the amount of Mo.

Document 3 reports that lithium iron phosphate anions complexed with Mg, Cl, Al, Ni, etc., prepared using metals added in less than stoichiometric amounts based on P (phosphate ion) and Fe, have very improved cycle characteristics. However, as for the recombination of Mo, the recombination mechanism and its effect are very complicated, as shown in example 6.

In document 4, Nb, Ti, Zr, Al, or Mg in an amount of 1 mol% based on P (phosphate ion) and Fe in an amount of 1 mol% less than the stoichiometric amount are added to prepare lithium iron phosphate doped with elements (the molar ratio of the constituent elements is Li: Fe: P: doping metal: 1: 0.99: 1: 0.01). It is reported that, according to the results of X-ray diffraction analysis of the substance, LiPO corresponding to the impurity lithium phosphate was observed in addition to the peak corresponding to the main component having an olivine-type structure₄A peak of crystals, and lithium iron phosphate doped with a metal element prepared using a reduced amount of Li without reducing the amount of Fe (molar ratio of constituent elements Li: Fe: P: doping metal 0.99: 1: 0.01) showed no peak corresponding to impurities. In the literature, one reason is that the additive elements replace Fe instead of Li. Samples B and C prepared by adding 5 mol% Mo and reducing the same amount of Fe or Li are different from document 4 in that they do not show a peak corresponding to impurity crystallization, but have a single-phase olivine-type structure as shown in fig. 15.

While the invention has been described in terms of preferred embodiments, it is to be understood that the invention is not limited to the specific embodiments described above, but is applicable to other embodiments described within the scope of the patent claims.

For example, lithium iron phosphate LiFePO complexed with Mo in reduced form, except for the conductive carbon deposited thereon₄Cathode material and reduced form of Mo composite cathode material from which an oxidized form of iron phosphate [ FePO ] is produced by a battery charging reaction or chemical oxidation₄]Mo composites also as identical Mo composite cathode materials and carbon depositsCathode materials are included within the scope of the present invention.

As already described in detail above, containing Li_nFePO₄The cathode material, which is a main component of the cathode active material and Mo, is a cathode material having good charge/discharge characteristics that have not been achieved before. The cathode material can be easily prepared by compounding a cathode active material with Mo. In addition, the cathode material obtained by depositing conductive carbon on the above cathode material exhibits better charge/discharge characteristics.

Industrial applicability

The cathode material produced by the method of the present invention can be used as a cathode material for secondary batteries such as lithium metal batteries, lithium ion batteries and lithium polymer batteries. In addition, a secondary battery using the cathode material is expected to be used as a large current power source to drive movable objects such as hybrid electric vehicles and portable telephones.

Claims

1. A cathode material for a secondary battery comprising a cathode active material as a main component and molybdenum (Mo),

wherein the cathode active material is represented by the general formula Li_nFePO₄Represents, wherein n represents a number from 0 to 1,

compounding the cathode active material with molybdenum (Mo) by:

mixed cathode active material Li_nFePO₄And a component containing a molybdenum (Mo) compound to obtain a calcined precursor, and

calcining the calcined precursor to composite the cathode active material with molybdenum (Mo),

the content of molybdenum (Mo) is in the range of 0.1 to 5 mol% based on the elemental proportion of iron in the cathode active material.

2. A cathode material for a secondary battery, comprising a cathode active material having an olivine-type crystal structure and molybdenum (Mo),

wherein the cathode active material is made of lithium ion (Li)⁺) Divalent iron ion (Fe)²⁺) And phosphate ion (PO)₄ ^3-) Is composed of, and

wherein the content of molybdenum (Mo) is 0.1 to 5 mol% based on the content of P in the cathode active material,

compounding the cathode active material with molybdenum (Mo) by:

the calcined precursor is calcined to composite the cathode active material with molybdenum (Mo).

3. The battery cathode material according to claim 2, wherein the molar content of lithium, the molar content of iron, or the molar sum of lithium and iron is smaller than the molar content of lithium, the molar content of iron, and the molar sum of lithium and iron, respectively, in the olivine-type lithium iron phosphate having a stoichiometric ratio of lithium, iron, and phosphorus of 1: 1, the molar difference corresponding to a molar amount of molybdenum (Mo) or less.

4. The battery cathode material according to one of claims 1 to 3, further comprising conductive carbon deposited on the surface thereof.

5. The battery cathode material according to claim 4, wherein the deposited conductive carbon has a crystal growth inhibiting effect which inhibits crystal growth during calcination of the cathode material containing molybdenum (Mo).

6. Method for producing cathode material of storage batteryA method of preparing a cathode material for a secondary battery comprising a cathode active material as a main component and molybdenum (Mo), wherein the cathode active material is represented by the general formula Li_nFePO₄Represents, wherein n represents a number from 0 to 1,

the method includes mixing a cathode active material Li_nFePO₄And a component containing a molybdenum (Mo) compound to obtain a calcined precursor and calcining the calcined precursor to composite the cathode active material with molybdenum (Mo),

wherein the molybdenum-containing compound is added so that the content of molybdenum in the molybdenum-containing compound is such that phosphate ions (PO) are introduced based on P₄ ^3-) The content of (A) is 0.1 to 5 mol%.

7. The method for producing a cathode material for secondary batteries according to claim 6, wherein the cathode active material Li is introduced_nFePO₄Wherein n represents a number from 0 to 1 so that the molar content of lithium in the lithium-introduced component, the molar content of iron in the iron-introduced component or the molar total amount of lithium and iron are smaller than the molar content of lithium, the molar content of iron and the molar total amount of lithium and iron, respectively, in an olivine-type lithium iron phosphate having a stoichiometric ratio of lithium, iron and phosphorus of 1: 1, the molar difference corresponding to the molar amount of molybdenum (Mo) or less.

8. The method for producing a cathode material for a secondary battery according to claim 6 or 7, wherein the calcination step has a first stage at a temperature ranging from room temperature to 300-450 ℃ and a second stage at a temperature ranging from room temperature to the calcination completion temperature, and wherein the second stage of the calcination step is conducted after adding a substance from which conductive carbon is formed by pyrolysis to the product of the first stage of the calcination step.

9. The method for producing a cathode material for secondary batteries according to claim 8, wherein the substance from which the conductive carbon is formed by pyrolysis is pitch or saccharides.

10. Storage battery comprising as a constituent element a cathode material according to one of claims 1 to 5.