WO2014181436A1

WO2014181436A1 - Positive electrode active material for secondary batteries and secondary battery using same

Info

Publication number: WO2014181436A1
Application number: PCT/JP2013/063073
Authority: WO
Inventors: 裕介浅利
Original assignee: 株式会社日立製作所
Priority date: 2013-05-09
Filing date: 2013-05-09
Publication date: 2014-11-13

Abstract

For the purpose of providing a positive electrode active material for secondary batteries, which has a crystal structure that comprises a pyrophosphate-type P₂O₇ structure having high thermal stability as the basic skeleton, and which has improved discharge capacity, and a secondary battery which uses the positive electrode active material for secondary batteries, a positive electrode active material for secondary batteries of the present invention, which contains Li_2-xM_A0.5M_B0.5P₂O₇ (wherein 0 ≤ x ≤ 2) as a main component, is configured so that M_A and M_B are transition metals, which are specifically (V and Ti), (V and Mn), (V and Fe), (Ni and Mn) or (V and Cu).

Description

Positive electrode active material for secondary battery and secondary battery using the same

The present invention relates to a positive electrode active material for a secondary battery (rechargeable battery) such as a lithium ion battery and a secondary battery using the same.

A non-aqueous electrolyte secondary battery using an alkali metal such as lithium or sodium, an alkaline earth metal such as magnesium, or an alloy or compound thereof as a negative electrode active material is used to insert or intercalate negative electrode metal ions into the positive electrode active material. To ensure the electric capacity and reversibility of charging. The positive electrode active material and the negative electrode active material are referred to as a host, and the movable metal ion that is inserted or intercalated into the host is referred to as a guest. A typical example of such a host / guest type non-aqueous electrolyte secondary battery is a lithium ion secondary battery.

Lithium ion secondary batteries have a higher energy density than conventional secondary batteries, and it is important to ensure the safety of the batteries. In particular, the thermal stability of the positive electrode active material is one of the factors that determine the safety of lithium ion secondary batteries. When the temperature rises due to external factors such as heating, crushing, and short circuit and exceeds the thermal decomposition temperature of the positive electrode active material, heat generation or oxygen release occurs. The released oxygen may react with the combustible organic electrolyte or the negative electrode active material, and the safety of the battery may be impaired.

The problem of thermal stability is particularly noticeable in the charged state. In the charged state, lithium ions are accumulated in the negative electrode active material, and the positive electrode active material is delithiated. The delithiated positive electrode active material is in a chemically high energy state, and the thermal decomposition temperature is lower than in the lithiated state. For this reason, the positive electrode active material is likely to be deteriorated during high temperature storage and may be thermally decomposed as the temperature rises.

In the layered rock salt type LiMO ₂ (where M is a transition metal) which is a metal oxide type positive electrode active material, for example, in LiCoO ₂ , the structural instability increases in an overcharged state, so the thermal decomposition temperature decreases, Specifically, a thermal decomposition reaction occurs at a temperature of 200 ° C. or higher, and oxygen may be released due to a phase transition to a more stable structure by self-heating. In the spinel type metal oxide LiMn ₂ O ₄ , manganese is eluted in the electrolyte during storage at high temperature, and the eluted manganese clogs the separator or forms a film on the negative electrode, resulting in battery resistance. May increase and the battery capacity may be reduced (see Patent Document 1). In a lithium ion secondary battery using the positive electrode group as described above, it is necessary to prevent overcharge by a protection circuit in order to ensure reliability and safety. Such protection circuits and mechanical mechanisms occupy a considerable volume of current battery packs.

The olivine type compound LiFePO ₄ is known to have high thermal stability. The charge phase of LiFePO ₄ , FePO ₄ (Heterosite), is extremely stable with respect to heating, and even when heated to 620 ° C. or higher, it only undergoes a phase transition to the Quartz phase, which is more thermodynamically stable. Does not release. The reason for exhibiting such high thermal stability is that the olivine type compound has a phosphate skeleton. In the phosphoric acid type structure, phosphorus (P) and oxygen (O) are connected by a strong covalent bond. That is, oxygen is fixed by phosphorus, oxygen release due to heat generation is difficult to occur, and heat stability is high.

Therefore, the covalent bond between phosphorus and oxygen is effective for ensuring the thermal stability of the positive electrode active material. A positive electrode active material containing a phosphoric acid type structure (P _x O _y ) is referred to as a polyanion positive electrode group. As polyanion positive electrode active materials, olivic acid compound LiMPO ₄ (for example, see Non-Patent Document 1), pyrophosphate compound Li ₂ MP ₂ O ₇ (for example, see Patent Document 2 and Non-Patent Document 2) and the like have been proposed. ing.

The detail regarding the said phosphoric acid type positive electrode active material is demonstrated below. The olivic acid compound LiMPO ₄ is known as a part of a series of positive electrode active material groups represented by a polyanion (chemical formula (XO ₄ ) ^y− ). The olivic acid compound is X = P (phosphorus). M = Fe, Mn, Co, Ni or the like is used for the chemical composition formula LiMPO ₄ . Among them, olivine-type lithium-containing iron phosphate (Li _x FePO ₄ , 0 ≦ x ≦ 1, hereinafter olivine Fe) contains one atom of lithium per chemical composition formula and has a theoretical electric capacity of 160 mAh / g. In experiments, almost all of the theoretical electric capacity can be used (see Non-Patent Document 1).

Pyrophosphate compound Li ₂ MP ₂ O ₇ (see Non-Patent Document 2) is a positive electrode active material using Fe, Mn, Co or the like as M. Although synthesized with M = Mn, it is known that charging and discharging are almost impossible (see Non-Patent Document 3). The first charge / discharge was measured with M = Fe (see Non-Patent Document 2). M = Fe has better charge / discharge characteristics than M = Mn, and the experimental capacitance value reaches 80 to 110 mAh / g (see Non-Patent Documents 2 and 4). However, this value is a capacity corresponding to a one-electron reaction, meaning that only one lithium ion in the chemical composition formula Li ₂ MP ₂ O ₇ can be used. The electric capacity when the pyrophosphate compound can use all the lithium ions for charging and discharging is called the theoretical electric capacity, which is 220 mAh / g.

From the above, the electrical capacity is summarized as follows. The theoretical electric capacity of the olivine type positive electrode is 160 mAh / g, and the capacity can be used even in experiments. On the other hand, the theoretical electric capacity of the pyrophosphate-type positive electrode is 220 mAh / g, but only 110 mAh / g, which is half of that, can be used in the experiment. The larger the electric capacity, the smaller and lighter the lithium ion battery can be. Therefore, if the capacity of the pyrophosphate-type positive electrode can be maximized, the positive electrode active material can exceed the olivine-type positive electrode.

First, an electrochemical reaction for realizing a high capacity will be described with an olivine-type positive electrode active material LiFePO ₄ as an example. When delithiation reaction is performed by applying a voltage to olivine-type LiFePO ₄ , FePO ₄ is formed. Due to the electrical neutral principle, the system needs to maintain a state in which the sum of formal charges of all ionic species is zero through such a lithium elimination reaction. Consider the formal charge of each ion. Lithium is ionized and the formal charge is a monovalent cation. The phosphate group PO ₄ is a trivalent anion. Therefore, Fe in LiFePO ₄ is a divalent cation. On the other hand, Fe in FePO ₄ in a delithiated state is a trivalent cation. In other words, it can be seen that the Fe ion is transitioned from a divalent cation to a trivalent cation by delithiation. In this way, ions that change their valence in order to maintain the electrical neutral principle associated with lithium desorption are called redox centers and serve as a charge compensation mechanism in lithium batteries.

Next, an electrochemical reaction related to the case where the pyrophosphate-type positive electrode active material Li ₂ FeP ₂ O ₇ is delithiated will be described. In this active material, lithium is ionized and becomes a cation having a formal charge. The P ₂ O ₇ group is a tetravalent anion. Therefore, Fe is a divalent cation and has the same formal valence as Fe in the olivine-type positive electrode active material. When 1 mol of lithium is desorbed from this state, a state represented by the chemical formula LiFeP ₂ O ₇ is obtained. In LiFeP ₂ O ₇ , Fe is a trivalent cation due to lithium elimination. This electrochemical reaction is equivalent to the change in the valence of Fe in the olivine-type positive electrode active material, and confirms that the pyrophosphate-type positive electrode active material has a one-electron reaction. In Non-Patent Document 2, it is experimentally observed that Fe is trivalent by analyzing the Mossbauer spectrum of Fe ions in LiFeP ₂ O ₇ .

Next, an electrochemical reaction in the case where one more mole of lithium can be eliminated from LiFeP ₂ O ₇ will be described. By this lithium desorption, the chemical formula of the positive electrode active material becomes FeP ₂ O ₇ . Since the P ₂ O ₇ group is a tetravalent anion, it can be seen from the electrical neutral principle that the valence of Fe in this state is a tetravalent cation. However, it is known that Fe is difficult to be a tetravalent cation. Currently known substances in which Fe ⁴⁺ appears are peroxidase which is a biological enzyme, cytochrome which is a hydroxylase, iron oxoporphyrin complex, and perovskite type metal oxide (SrFeO ₃ or CaFeO ₃ ) in solid. Yes and very limited. For this reason, the tetravalent state in Fe is called an abnormal valence and does not appear easily. Actually, 1 mol or more of lithium desorption from the pyrophosphate-type positive electrode active material has not been confirmed so far.

JP 2010-23001 A Special Table 2006-523930

In light of the above situation, the present inventors have a problem, but as a positive electrode active material excellent in thermal stability, a polyanion positive electrode group containing a phosphoric acid type structure (P _x O _y ) is optimal. In particular, a method for making M tetravalent in the pyrophosphate compound Li ₂ MP ₂ O ₇ , which is still expected to improve the discharge capacity, was studied.

In order to solve this problem, it is conceivable to attempt the elimination of 2 mol of lithium by substituting Fe with another element. For example, Non-Patent Document 4 discloses a case where Fe is replaced with Mn. It is known that Mn is a multivalent ion. Specifically, the olivine-type positive electrode active material LiMnPO ₄ is in a divalent cation state, the spinel structure Mn ₃ O ₄ is in a trivalent cation state, and the manganese dioxide MnO ₂ is in a tetravalent state. It is in the state of cations. Therefore, Mn becomes a redox center in the elimination of lithium and is expected to be responsible for electrochemical reactions from divalent to tetravalent. However, in Non-Patent Document 4, as a result of experiments, Mn is not oxidized at all, and lithium is hardly desorbed. The reason why Mn does not work as a redox center is not known.

If the transition metal element can change the valence from 2 to 4, the charge / discharge capacity can be increased. If a low-cost transition metal element can be used as the redox center, it can be a positive electrode material with high price competitiveness. The pyrophosphate-type positive electrode active material can be charged and discharged using particles having a larger particle diameter (1 μm) than the olivine-type positive electrode material. That is, the micronization process can be omitted, and the restriction of the surface modification treatment is greatly relaxed, leading to a reduction in battery cost, ease of process management, and elimination of performance hindrance factors. In addition, since surface modification treatment with a conductive material such as graphite, which was essential for the olivine positive electrode active material, is unnecessary, there are many advantages such as cost reduction, ease of process, and ease of electrode binding. is there. From the above, it can be said that the significance of putting a pyrophosphate-type positive electrode active material capable of detaching 1 mol or more of lithium into practical use is great.

From the above examination results by the present inventors, the conditions for the positive electrode active material that satisfies the requirements for safety and electric capacity are as follows: (1) Positive electrode active material having a pyrophosphoric acid type crystal structure with potentially large electric capacity (2) having a skeleton based on phosphoric acid with high thermal stability, and (3) being capable of detaching 1 mol or more of lithium. However, pyrophosphate-type positive electrode active materials having these characteristics have not been realized yet.

The present invention has been proposed to improve the discharge capacity of a pyrophosphate-type positive electrode active material, and has an object of having a crystal structure having a pyroskeleton-type P ₂ O ₇ structure having a high thermal stability as a basic skeleton. It is another object of the present invention to provide a non-aqueous electrolyte positive electrode active material for a secondary battery with improved discharge capacity and a secondary battery using the same.

As an embodiment for achieving the above object, the chemical composition formula is Li _2-x M _A0.5 M _B0.5 P ₂ O ₇ , and M _A and M _B are each a transition metal element, The combination is (V, Ti), (V, Mn), (V, Fe), (Ni, Mn), or (V, Cu), and x is the main component of a compound in the range of 0 ≦ x ≦ 2. It is set as the positive electrode active material for secondary batteries characterized by these.

Also, a secondary battery is characterized in that the positive electrode active material for a secondary battery is used in a positive electrode molded body.

According to the present invention, it is possible to provide a positive electrode active material for a secondary battery that has high thermal stability and can improve the discharge capacity, and a secondary battery using the same.

It is a diagram showing a pyrophosphate type _Li 2 _MP 2 _{O 7} crystal structure. It is a diagram illustrating a geometrical structure created by the transition metal site and coordinated oxygen atom thereof in pyrophosphate-type Li _₂ MP ₂ O ₇ crystal structure. Is a diagram showing the C layer lithium ion diffusion network in pyrophosphate-type _Li 2 _MP 2 _{O 7} crystal structure. It is a diagram showing the presence of a lone pair with oxygen in the pyrophosphate-type Li _₂ MP ₂ O ₇ crystal structure. It is sectional drawing of the coin-type battery structure which is an example of embodiment. It is a table | surface of the stabilization energy of the mixed type pyrophosphate type positive electrode active material evaluated by the first principle calculation. It is a table | surface which shows the result of having evaluated the valence change of the transition metal element.

In order to achieve the above-mentioned object, the present inventor has repeatedly examined the lithium desorption structure and the valence change of the redox center associated with the charge / discharge reaction of the pyrophosphate-type positive electrode active material. It was found that 1 mol or more of lithium can be eliminated by constructing a compound with the use of. For example, to replace the combination of 0.5 mole of 0.5 mole and _{M B} of 1 mole of two kinds of elements _{M A} transition metal element M. The present invention was born based on this new knowledge, has a highly safe pyrophosphate-type crystal structure, and a positive electrode for a lithium ion secondary battery having an electric capacity higher than 110 mAh / g. An active material can be provided. Details of the positive electrode active material design that leads to an improvement in charge / discharge capacity will be described below.

First, the crystal structure of the pyrophosphate-type positive electrode active material is shown in FIG. The crystal structure consists of an alternating layered structure of lithium layers and transition metal layers along the bc plane. The transition metal M has an MO _x polyhedral structure with oxygen atoms coordinated around it. The dotted polyhedron 6 in FIG. 1 has an MO _x structure. Reference numeral 1 denotes a Li1 site, reference numeral 2 denotes a Li2 site, reference numeral 3 denotes a Li3 site, reference numeral 4 denotes a Li4 site, reference numeral 5 denotes a phosphor polyhedron, and reference numeral 7 denotes a unit cell. There are two types of MO _x polyhedral structures, MO ₆ and MO ₅ . The detailed structure is shown in FIG. The transition metal element M (21, 22) is located at the center of the polyhedron, and oxygen 23 is coordinated around it. A metal site coordinated with six oxygens 23 is M1, and a metal site coordinated with five oxygens is M2. The hexacoordinate polyhedron M1O ₆ and the pentacoordinate polyhedron M2O ₅ form a cluster by covalently bonding edges. Reference numeral 24 indicates a chemical bond between the transition metal and oxygen. As shown in FIG. 4, a phosphoric acid structure P ₂ O ₇ is arranged between polyhedral clusters in which MO ₆ and MO ₅ are bonded, and the clusters are joined to each other. For this reason, the transition metal layer can maintain a layered structure during charging and discharging. Reference numeral 41 denotes a lone electron paired oxygen ion, reference numeral 42 denotes an oxygen ion, reference numeral 43 denotes a phosphoric acid polyhedron, and reference numeral 44 denotes an iron oxide polyhedron (transition metal oxide polyhedron). Lithium forms a two-dimensional network structure as shown in FIG. 3, and can be desorbed and inserted from the active material through this network during charging and discharging. Reference numeral 31 denotes a Li3 site, reference numeral 32 denotes a Li4 site, and reference numeral 33 denotes a unit cell.

Next, the MO _x polyhedron structure will be described. In general, transition metal oxides often have an MO ₆ structure in which oxygen is six-coordinated. For example, in a rock salt type oxide structure, the transition metal has an octahedral structure in which oxygen is coordinated to six. The structure is similar to the six-coordinate structure in the pyrophosphate-type positive electrode active material, but the symmetry is lower in the pyrophosphate-type positive electrode active material, and the bond length between the transition metal and oxygen varies. Therefore, even if it has the same topology as the six-coordinate structure, the stability of the transition metal is considered to be different. As for the MO ₅ polyhedral structure, there are few cases where the transition metal oxide is pentacoordinated, but the α-phase V ₂ O ₅ is known to have a 5-coordinated MO ₅ structure. Therefore, the stable coordination number and the stable coordination state are generally not unique depending on the type of transition metal. In the pyrophosphate-type positive electrode active material Li ₂ MP ₂ O ₇ , since the same transition metal element M is assigned to the transition metal sites M1 and M2 having different coordination numbers, the two sites It is considered that the stability of M is different, and the ease of valence change is also different. That is, it is considered that there are combinations of transition metal sites and transition metal species that are difficult to change in valence due to differences in local structure.

As the valence of the oxidation-reduction center changes, the ionic radius of the transition metal element that becomes the oxidation-reduction center increases and decreases, so that the bond distance between the transition metal and oxygen ions increases and decreases, and the polyhedral structure changes. On the other hand, the two types of polyhedrons MO ₆ and MO ₅ have edge covalent bonds, and the shape of the polyhedron is strongly bound to each other's geometric structure. In the case of having a polyhedral structure with such a strong constraint, it is considered that the structural change cannot sufficiently follow the valence change. That is, the present inventors considered that it is difficult for the MO _x polyhedron to sufficiently relax the structure, and as a result, the valence cannot be changed with respect to lithium desorption.

Therefore, the present inventors considered that there exist elements that are best suited to the respective coordination structures for the two types of transition metal sites, that is, M1 and M2, and that enable desired redox. Therefore, first, a crystal structure in which a transition metal element is applied to a transition metal site of (M1, M2) was searched by computer simulation. Specifically, six types of transition metals, Ti, V, Mn, Fe, Ni, and Cu, are considered and applied to the brute force (M1, M2) sites to improve the stability of the crystal structure with high accuracy. Theoretical prediction was made by using the one-principles density functional method. The reason why the above six kinds of transition metals are selected is that the weight energy density is not disadvantageous when used as the positive electrode material of the lithium ion battery, so avoid heavy transition metal elements after Y, That is, among Sc to Zn, except for Sc that is not tetravalent, except for Cr that is expected to have a cost corresponding to environmental load, except for Co that is inferior in price competitiveness, and excluding Zn that is not expected to be redox with a stable element. is there.

As described above, the crystal structure of the chemical formula Li ₂ M _A0.5 M _B0.5 P ₂ O ₇ prepared by applying the transition metal elements M _A and M _B to (M1 and M2), respectively, is theoretically predicted, and the result is obtained. Let the total energy given be E _AB . Further, the crystal structure of a single type of transition metal element _{M A} alone were prepared by applying the formula _Li 2 _M

A

_P 2 _{O 7} with respect to (M1, M2) and theoretical predictions, the resulting total energy It is referred to as _{E a.} Further, (M1, M2) with respect to only the crystal structure of Formula _Li 2 _M

B

_P 2 _{O 7} created by applying the theory predicts single type of transition metal element _{M B,} the resulting total energy Is _EB . In order to evaluate the stability of Li ₂ M _A0.5 M _B0.5 P ₂ O ₇ , the mixing energy is defined as E _mix = E _AB − (E _A + E _B ) / 2. The mixing energy defined in this way is obtained when Li ₂ M _A P ₂ O ₇ and Li ₂ M _B P ₂ O ₇ are separated and when Li ₂ M _A0.5 M _B0.5 P ₂ O ₇ is present. It is a comparison of the energy when it exists. If E _mix > 0, it indicates that it is more stable if it exists separately, and if E _mix <0, it indicates that it is more stable if it is mixed. .

The result of the theoretically calculated mixing energy E _mix is shown in FIG. The combination (M1, M2) in which the value of E _mix is negative indicates that it can be stabilized by mixing to form a compound. Therefore, (M1, M2) = (Ti, Cu), (V, Ti), (V, Mn), (V, Fe), (Fe, Mn), (Ni, Mn), (Ni, Fe), ( It can be seen that a combination of (Cu, Ti) and (Cu, V) can be formed.

Next, the present inventor investigated whether 1 mol or more of lithium can be extracted from the positive electrode material by a multi-electron reaction. Specifically, changes in the magnetic moment of transition metals were investigated using a high-accuracy first-principles density functional method. The reason why a multi-electron reaction is possible by this method is that the magnetic moment varies depending on the valence of the transition metal. An oxidation reaction from Fe ²⁺ to Fe ³⁺ will be described as an example. Fe has 8 valence electrons, and its electron configuration is (4s) ² (3d) ⁶ . When two electrons are desorbed from Fe to form a divalent cation, the electron configuration is (3d) ⁶ . If the iron is in the high spin state, four of the five d orbitals magnetic moment generated by the spin magnetic moments of the resulting 4 [mu] _B occurs. When iron is further oxidized to become Fe ^3+, since electron configuration becomes (3d) ^5, the magnetic moment of the case 5 [mu] _B of high-spin state occurs. From the above, if the valence change from divalent iron to trivalent occurs, since the magnetic moment of the iron is changed to 5 [mu] _B from 4 [mu] _B, valence change of iron by measuring the magnetic moment It is possible to determine whether it has occurred.

Using this method, the valence change of the transition metal element was examined with respect to the combination (V, Fe). First, the magnetic moment in the lithiated state Li ₂ V _0.5 Fe _0.5 P ₂ O ₇ was examined. Then V is 3μ _B, Fe was found to have become 4μ _B. Since electron configuration of V is neutral state is ^{^{(2s) 2 (3d) 3}} , electron configuration by a divalent cation ^{V 2+} ^{(3d) 3,} and the that the magnetic moment is 3.mu. _B I understand. Therefore, in this state, V is a divalent cation. From the above discussion, it can be seen that Fe is a divalent cation like V. Next, a structure V _0.5 Fe _0.5 P ₂ O ₇ in which all 2 mol of lithium was desorbed from this positive electrode material was prepared, and the magnetic moment was examined. Then V is 0μ _B, Fe was found to have become 5μ _B. V of 0Myu _B has means that all of the valence are eliminated, a pentavalent cation V ^5+. It can also be seen that Fe is a trivalent cation Fe ³⁺ from the magnetic moment.

Summarizing the above valence change, 0.5 mol of V changes from divalent to pentavalent, and 0.5 mol of Fe changes from divalent to trivalent with 2 mol elimination of lithium. The valence changes. Therefore, V and Fe emit a total of 2 moles of electrons, and the charge is compensated so that the electrical neutral condition of the positive electrode active material is maintained as lithium is desorbed. In this way, an excellent positive electrode active material Li ₂ V _0.5 Fe _0.5 P ₂ O ₇ that can utilize all lithium for charging and discharging can be realized.

Similarly, the valence change of the transition metal element was calculated for other combinations. As a result, (V, Ti) was found to be capable of detaching 2 moles of lithium as well. Regarding (V, Mn), (Ni, Mn), and (Cu, V), it was found that 1.5 mol of lithium can be eliminated. For (Ti, Cu) and (Fe, Mn), (Ni, Fe), only 1.0 mol of lithium can be desorbed and does not exceed the electric capacity of Li ₂ FeP ₂ O ₇ . From the above, the change in the valence of the transition metal element is large and the value exceeding the electric capacity of Li ₂ FeP ₂ O ₇ is indicated by “◯”, and the other value is indicated by “X” in FIG. That _is, the combination of _{M A} and _{M B} in _{_{_{Li 2 M A0.5 M B0.5 P 2}}} O 7, (V, Ti), (V, Mn), (V, Fe), (Ni, Mn) , (Cu, V) is expected to be promising.

The compound which is the positive electrode active material can be produced using a known general method, and various methods can be adopted as the method. Specifically, for example, in the case of Li ₂ V _0.5 Fe _0.5 P ₂ O ₇ , iron oxide (Fe ₂ O ₃ ), a lithium phosphate compound, and vanadium oxide (V ₂ O ₅ ) are mixed, Synthesized by firing in an inert gas atmosphere such as argon. The lithium phosphate compound is one selected from the group consisting of Li ₃ PO ₄ , Li ₄ P ₂ O ₇ and LiPO ₃ , for example.

When producing a positive electrode for a non-aqueous electrolyte secondary battery using the positive electrode active material, the active material may be usually used in the form of powder, and the average particle size may be about 0.1 to 1 μm. The average particle diameter is a value measured by, for example, a laser diffraction particle size distribution measuring apparatus. Moreover, what is necessary is just to set suitably content of the said active material in a positive electrode according to the kind of active material to be used, a binder (binder), the usage-amount of a electrically conductive agent, etc. Further, in the production of the positive electrode, as long as a predetermined positive electrode characteristic is obtained as the positive electrode active material, the above active material alone or a mixture with other conventionally known positive electrode active materials may be used.

When preparing the positive electrode using the positive electrode active material, a known positive electrode preparation method may be used except that the positive electrode active material is used. For example, a powder of the above active material may be added to a known binder (polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl chloride, ethylene propylene diene polymer, styrene butadiene rubber, acrylonitrile butadiene rubber, fluoro rubber, polyvinyl acetate as necessary. , Polymethyl methacrylate, polyethylene, nitrocellulose, etc.), and if necessary, mixed with known conductive materials (acetylene black, carbon, graphite, natural graphite, artificial graphite, needle coke, carbon nanotube, carbon nanohorn, graphene nanosheet, etc.) After that, the obtained mixed powder may be pressure-formed on a support made of stainless steel or filled in a metal container. Alternatively, for example, the above mixed powder is mixed with an organic solvent (N-methylpyrrolidone, toluene, cyclohexane, dimethylformamide, dimethylacetamide, methyl ethyl ketone, methyl acetate, methyl acrylate, diethyltriamine, NN-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran. Etc.) can also be produced by a method such as applying a slurry obtained by mixing with a metal substrate such as aluminum, nickel, stainless steel or copper.

The negative electrode is formed by applying a negative electrode mixture to a current collector made of copper or the like. The negative electrode mixture includes an active material, a conductive material, a binder, and the like. As the active material of the negative electrode, metallic lithium, a carbon material, a material capable of inserting lithium or forming a compound can be used, and a carbon material is particularly preferable. Examples of the carbon material include graphites such as natural graphite and artificial graphite, and amorphous carbon such as coal-based coke, coal-based pitch carbide, petroleum-based coke, petroleum-based pitch carbide, and pitch-coke carbide. Preferably, these carbon materials are subjected to various surface treatments. These carbon materials can be used not only in one kind but also in combination of two or more kinds. Examples of the material capable of inserting lithium or forming a compound include metals such as aluminum, tin, silicon, indium, gallium, and magnesium, alloys containing these elements, and metal oxides containing tin, silicon, and the like. . Furthermore, the composite material of the above-mentioned metal, an alloy, a metal oxide, and the carbon material of a graphite type or an amorphous carbon is mentioned.

FIG. 5 is a longitudinal sectional view of a coin-type lithium secondary battery which is an example of a battery using the positive electrode active material. Here, a battery having a diameter of 6.8 mm and a thickness of 2.1 mm was produced. In FIG. 5, a positive electrode can 51 also serves as a positive electrode terminal and is made of stainless steel having excellent corrosion resistance. The negative electrode can 52 also serves as a negative electrode terminal and is made of stainless steel made of the same material as the positive electrode can 51. The gasket 53 insulates the positive electrode can 51 and the negative electrode can 52 and is made of polypropylene. Pitch is applied to the contact surface between the positive electrode can 51 and the gasket 53 and the contact surface between the negative electrode can 52 and the gasket 53. A separator 55 made of a nonwoven fabric made of polypropylene is disposed between the positive electrode molded body (pellet) 54 and the negative electrode molded body (pellet) 56. The electrolyte solution is infiltrated when the separator 55 is installed.

The shape of the secondary battery is not limited to the coin type, but may be a cylindrical shape obtained by winding an electrode, for example, an 18650 type. Alternatively, the electrodes may be stacked to form a square shape.

Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples. In the examples, the battery was manufactured and measured in a dry box under an argon atmosphere. The battery started from discharging for the first time, and then charged and discharged.

In this example, lithium carbonate (Li ₂ CO ₃ ), ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ), iron oxide Fe ₂ O ₃ , and vanadium oxide (V ₂ O ₅ ) are used as raw materials in a ratio of 2: 2. 1: 1: Mix at a predetermined molar ratio, and add citric acid as a chelating agent and mix. Thereafter, the water is evaporated while heating and stirring. After evaporation of the water, the remaining material was recovered as a precursor, and this precursor was heat-treated in a firing atmosphere at 800 ° C. for 4 hours using an atmosphere furnace (argon gas stream) to obtain a pyrophosphate positive electrode active material (Li ₂ V producing _{_{_{_{0.5 Fe 0.5 P 2 O 7)}}}} .

In place of citric acid, other organic acids such as malic acid, tartaric acid, succinic acid and the like can be used. The organic acid may be a mixture of a plurality of organic acids among citric acid, malic acid, tartaric acid, succinic acid, and the like.

The fired sample was pulverized for 1 hour using a meteor type ball mill (FRITSCH, Planetary micromill pulverisette 7). Thereafter, coarse particles of 50 μm or more are removed by sieving.

Using an automatic X-ray diffractometer (RINT-Ultima III, manufactured by Rigaku Corporation), in the so-called 2θ / θ measurement, an X-ray diffraction profile was measured with an X-ray source: CuXα and an output of 40 kV × 40 mA. A characteristic diffraction peak is obtained for the pyrophosphate-type positive electrode, and Li ₂ V _0.5 Fe _0.5 P ₂ O ₇ can be confirmed.

When a charge / discharge test is performed using this positive electrode active material with a cut-off potential of 4.8 V or 1.0 V, a discharge capacity of 180 mAh / g can be confirmed. This discharge capacity corresponds to a capacity increase of 63% from the conventionally confirmed discharge capacity of 110 mAh / g. By using this positive electrode active material for a positive electrode molded body, a secondary battery having a large electric capacity can be produced.

In addition, in the combination of (Fe, Mn) and (Ni, Fe), the performance exceeding the discharge capacity of 110 mAh / g could not be obtained.

As described above, according to the present example, the positive electrode active material for a secondary battery of a nonaqueous electrolyte having a crystal structure having a pyroskeleton-type P ₂ O ₇ structure with high thermal stability as a basic skeleton and improved discharge capacity, and A secondary battery using the same can be provided.

In this example, Li ₃ PO ₄ , copper oxide (CuO), and vanadium oxide (V ₂ O ₅ ) are used as raw materials for preparing the positive electrode active material. Weigh so that Li: Cu: V: P is 4: 1: 1: 4 in the raw material ratio, and wet pulverize and mix with a pulverizer. The powder is dried and fired at 650 ° C. under an argon stream. It can be confirmed that the obtained sample is Li ₂ V _0.5 Cu _0.5 P ₂ O ₇ .

As a result of examining the magnetic moment using the first-principles density functional method capable of predicting physical properties with high accuracy, V is trivalent in the lithiated state Li ₂ V _0.5 Cu _0.5 P ₂ O ₇ . It was found to be a cation, and Cu was a monovalent cation. Further, in V _0.5 Cu _0.5 P ₂ O ₇ from which 2 mol of lithium has been eliminated from this crystal, V is a pentavalent cation, and Cu is a divalent cation. I understood. That is, it is expected that the capacity will be equivalent to 1.5 mol since charge compensation is possible up to 1.5 mol of electrons with 2 mol of lithium desorption.

When a charge / discharge test is performed using this positive electrode active material with a cutoff potential of 4.7 V or 1.0 V, a discharge capacity of 150 mAh / g can be confirmed. This discharge capacity corresponds to a 36% increase in capacity from the conventionally confirmed discharge capacity of 110 mAh / g. By using this positive electrode active material for a positive electrode molded body, a secondary battery having a large electric capacity can be produced.

In this example, Li ₃ PO ₄ , titanium oxide (TiO ₂ ), and vanadium oxide (V ₂ O ₅ ) are used as raw materials for preparing the positive electrode active material. Weigh so that Li: Ti: V: P is 4: 1: 1: 4 in the raw material ratio, and wet pulverize and mix with a pulverizer. The powder is dried and fired at 700 ° C. under an argon stream. It can be confirmed that the obtained sample is Li ₂ V _0.5 Ti _0.5 P ₂ O ₇ . When a charge / discharge test is performed using this active material, a discharge capacity of 170 mAh / g can be confirmed. By using this positive electrode active material for a positive electrode molded body, a secondary battery having a large electric capacity can be produced.

In this example, Li ₃ PO ₄ , manganese (III) (Mn ₂ O ₃ ), and vanadium oxide (V ₂ O ₅ ) are used as raw materials for preparing the positive electrode active material. Weigh so that Li: Mn: V: P is 4: 1: 1: 4 in the raw material ratio, and wet pulverize and mix with a pulverizer. The powder is dried and fired at 650 ° C. under an argon stream. It can be confirmed that the obtained sample is Li ₂ V _0.5 Mn _0.5 P ₂ O ₇ . When a charge / discharge test is performed using this active material, a discharge capacity of 130 mAh / g can be confirmed. By using this positive electrode active material for a positive electrode molded body, a secondary battery having a large electric capacity can be produced.

In this example, Li ₃ PO ₄ , nickel oxide, and manganese (III) oxide (Mn ₂ O ₃ ) are used as raw materials for preparing the positive electrode active material. Weigh so that Li: Ni: Mn: P is 4: 1: 1: 4 in the raw material ratio, and wet pulverize and mix with a pulverizer. The powder is dried and fired at 700 ° C. under an argon stream. It can be confirmed that the obtained sample is Li ₂ Ni _0.5 Mn _0.5 P ₂ O ₇ . When a charge / discharge test is performed using this active material, a discharge capacity of 130 mAh / g can be confirmed. By using this positive electrode active material for a positive electrode molded body, a secondary battery having a large electric capacity can be produced.

1: Li1 site,
2: Li2 site,
3: Li3 site,
4: Li4 site,
5: phosphate polyhedron,
6: Iron oxide polyhedron,
7: Unit cell,
21: Transition metal 1 (M1) site,
22: Transition metal 2 (M2) site,
23: oxygen site,
24: chemical bond between transition metal and oxygen,
31: Li3 site,
32: Li4 site,
33: Unit cell,
41: oxygen ion with lone electron pair,
42: oxygen ions,
43: Phosphoric polyhedron,
44: Iron oxide polyhedron
51: Positive electrode can,
52: negative electrode can,
53: Gasket,
54: Positive electrode molded body,
55: separator,
56: Negative electrode molded body.

Claims

The chemical composition formula is Li 2-x M A0.5 M B0.5 P 2 O 7 , where M A and M B are transition metal elements, and the combinations thereof are (V, Ti), (V, Mn ), (V, Fe), (Ni, Mn), or (V, Cu), and x is a positive electrode for a secondary battery comprising a compound in the range of 0 ≦ x ≦ 2 as a main component Active material.
The positive electrode active material for a secondary battery according to claim 1, wherein the combination of M A and M B is (V, Fe).
Wherein M combinations of A and M B are positive electrode active material for a secondary battery according to claim 1, characterized in that the (V, Cu).
Wherein M combinations of A and M B are positive electrode active material for a secondary battery according to claim 1, characterized in that the (V, Ti).
Wherein M combinations of A and M B are positive electrode active material for a secondary battery according to claim 1, characterized in that the (V, Mn).
Wherein M combinations of A and M B are, (Ni, Mn) positive electrode active material for a secondary battery according to claim 1, characterized in that a.
A secondary battery, wherein the positive electrode active material for a secondary battery according to claim 1 is used in a positive electrode molded body.
A secondary battery, wherein the positive electrode active material for a secondary battery according to claim 2 is used in a positive electrode molded body.
A secondary battery, wherein the positive electrode active material for a secondary battery according to claim 3 is used for a positive electrode molded body.
A secondary battery, wherein the positive electrode active material for a secondary battery according to claim 4 is used for a positive electrode molded body.
A secondary battery, wherein the positive electrode active material for a secondary battery according to claim 5 is used in a positive electrode molded body.
A secondary battery, wherein the positive electrode active material for a secondary battery according to claim 6 is used in a positive electrode molded body.
A positive electrode forming body, a negative electrode forming body, a separator disposed between the positive electrode forming body and the negative electrode forming body, a positive electrode connected to the positive electrode forming body, and a negative electrode connected to the negative electrode forming body. In a non-aqueous electrolyte secondary battery having
The positive electrode forming body has a chemical composition formula of Li 2-x M A0.5 M B0.5 P 2 O 7 , M A and M B are transition metal elements, and the combination thereof is (V, Ti) , (V, Mn), (V, Fe), (Ni, Mn), or (V, Cu), wherein x is a main component of a compound in the range of 0 ≦ x ≦ 2. Secondary battery.