JP2022140180A - Positive electrode and storage battery - Google Patents

Abstract

To provide a positive electrode active material that has an oxidation-reduction potential equivalent to that of the positive electrode active material of a current lithium-ion battery and a storage capacity that is several times that of the current lithium-ion battery in order to increase the energy density of the positive electrode active material by solving a problem in which a lithium-ion battery widely used as a storage battery for a mobile phone lacks energy density as a storage battery for an electric vehicle, and cannot meet the driving distance per charge required for a private automobile, such that high energy density in particular, a positive electrode active material is required to have high energy density.SOLUTION: A high energy density positive electrode active material that satisfies a condition of high capacity and high voltage by inserting and desorbing charge carrier ions, which are well known in the current technology to provide gaps in a compound with redox potential, and absorbing and storing charge carrier ions in the gaps.SELECTED DRAWING: Figure 1

Description

The present invention relates to positive electrode active materials for storage batteries. Moreover, it is related with the storage battery comprised by the positive electrode active material.

Lithium-ion storage batteries, which were commercialized by Sony Corporation in 1991, have high energy density and are widely used in laptop computers, mobile phones, and the like, and are recently attracting attention as batteries for electric vehicles. However, gasoline cars can run more than 1000km with a full tank of gasoline, but the mileage of one charge of the Nissan Leaf, an electric car currently on the market, is 400km in the catalog value. Insufficient distance. Therefore, it is desired to develop an innovative battery that can run 1000 km on one charge, that is, that has twice or more the energy density.

Layered metal oxides, into which lithium ions are intercalated and deintercalated during charging and discharging, are used as positive electrode active materials for lithium-ion batteries. Experimental results showing higher capacities have been published. This greatly obtained capacity is called positive electrode excess capacity, and many studies are being conducted with the goal of making it more than double the current cobalt-nickel-manganese ternary layered metal oxide of 160 mAh/g. It is believed that the cause of the excess capacity is that the oxygen atoms forming the crystal have a valence of minus 2, which is the normal valence of minus 2, in the peroxide state. However, the oxygen in the peroxide state is weak in bonding force and escapes from the crystal, destroying the structure and preventing a sufficient charge-discharge cycle as a storage battery. In addition, the occupied positions in the crystal of lithium ions excessively incorporated into the crystal have not been clarified.

In a dual carbon battery that uses insertion type carbon for both electrodes, the pores of the positive and negative electrode active materials, which have a porous structure, are drawn in a cylindrical shape as shown in FIG. The surfaces of both electrodes are covered by an SEI (solid electrolyte interface) formed by reaction with the electrolyte or a "sieve membrane" that allows the passage of charge carrier ions such as lithium ions but blocks the passage of solvents such as ethylene carbonate. , and there is no solvent in the pores. Electrons are injected into the negative electrode active material by charging, and electron deficiencies are injected into the positive electrode active material. Negative ions are stored and charged in the pores. The reaction formula is as follows.
Positive electrode reaction: C + (X ^- solvent) ⇔ CX ^- + (solvent) + (electron deficiency) ⁺
Anode reaction: C + (L _i ⁺ solvent) ⇔ CL _i ⁺ + (solvent) + e ^-
Full cell reaction: 2C + (X ⁻ solvent) + (L _i ⁺ solvent) ⇔ CX ⁻ +CL _i ⁺ +2 (solvent)
After charging, there are almost no positive and negative ions in the electrolyte, and the amount of positive and negative ions present in the electrolyte limits the amount of charging and discharging of the battery. Since the positive and negative ions present in the electrolyte are obtained by dissolving the electrolyte salt in a solvent, it is clear that it cannot surpass current lithium-ion batteries that store and charge positive ions in solid active materials. .

The fact that lithium metal can be used as the negative electrode and graphite, which is the current negative electrode, can be discharged as the positive electrode is well known and put into practical use as a technique called "pre-doping" in which lithium ions are inserted in advance for SEI formation processing of the carbon electrode. This reaction is reversible, and a lithium metal deposition reaction occurs at the negative electrode, and a lithium ion insertion/extraction reaction occurs at the positive electrode. The reaction formula is as follows.
Cathode reaction: nC + {L _i ⁺ (solvent)} + e ⁻ ⇔ LiC _n + (solvent)
Anode reaction: L _i (metal) + (solvent) ⇔ {L _i ⁺ (solvent)} + e ⁻
Whole cell reaction: Li ₍ metal) + nC ⇔ LiC _n
When a battery is constructed with predoped carbon as both positive and negative electrodes, the carbon electrode with a higher potential becomes the positive electrode and the carbon electrode with a lower potential becomes the negative electrode. FIG. 3 schematically shows the reaction mechanism. The reaction formula is as follows.
Cathode reaction: mC + {L _i ⁺ (solvent)} + e ⁻ ⇔ L _i C _m + (solvent)
Anode reaction: 2L _i C _n + (solvent) + (m-2n) C
⇔ L _i C _m + {L _i ⁺ (solvent)} + e ⁻
Whole cell reaction: L _i C _n + (mn) C ⇔ L _i C _m (n<m)
With reference to Patent Document 1 and Patent Document 2 of the prior art documents, if a sieve membrane that allows lithium ions to pass through but not the solvent and negative ions to pass through is used, the negative electrode can insert and store up to the carbon number n = 2, that is, LiC ₂ . The positive electrode can be charged to a state where lithium ions are not inserted, and can be discharged to an intermediate state. The theoretical capacity of current graphite is 372 mAh. The capacity is about three times that of graphite, or 1000 mAh, and the capacity per volume is about two times, or 700 mAh, so that the targeted high-capacity positive electrode twice that of the current one can be realized. However, since the deinsertion reaction is the same at both electrodes, the potential difference between the positive and negative electrodes, that is, the storage battery voltage, is only about half that of the current lithium ion storage battery, and the energy density cannot be improved.

FIG. 4 was drawn with reference to Non-Patent Document 4. FIG. During charging, lithium ions move from the positive electrode lithium cobaltate to the negative electrode current collector Pt in the solid electrolyte LATSPO. An existing distribution area can be created. Although it is drawn discontinuously in the figure, the concentration changes continuously and the concentration gradient becomes like an exponential function. It was named "in-situ formation negative electrode" because lithium ions in the region of 700 nm distributed in high concentration increase and decrease with charging and discharging.

Since the charging and discharging is not the deposition potential of lithium metal, but the lithium ions are inserted into and detached from the solid electrolyte, storage and storage of lithium ions based on the "cantilever theory" demonstrated by computational chemistry in Non-Patent Documents 1 and 2 is. The left diagram of FIG. 5 is a state diagram showing nano-sized voids, crevices, and cracks formed in the solid electrolyte due to strain in the crystal due to the insertion of lithium ions during the initial charge. According to the presenter's explanation, the presence of voids can be observed by observation with a transmission microscope after charging and discharging, but the existence of lithium ions in the voids cannot be measured due to insufficient measurement resolution. . Lithium ions are charged and discharged by inserting/deintercalating them into and out of the voids. The "cantilever theory" demonstrated by computational chemistry described in Non-Patent Documents 1, 2 and 3 is not limited to carbon nanotubes, but qualitatively To establish. The right view of FIG. 5 is an enlarged view of the void, and lithium ions are arranged at regular intervals along the wall of the void. They are not sandwiched in the center of the gap, nor are they clustered together. The intercalation of lithium ions during the initial charge causes strain in the crystal, forming voids. The carbon nanotubes in Non-Patent Documents 1 and 2 are replaced by solid electrolytes exhibiting "voids" and electronic conductivity. The reaction formula is as follows. Charging and discharging also follow the same reaction formula.
Negative charge (formation) reaction: Li ⁺ + <void> + e ^- ⇔ <void Li ⁺ e ^- >
The positive and negative electrodes are determined by which side has the higher potential, so in the case of a reversible reaction, depending on the partner, it can be either the positive electrode or the negative electrode. The "in-situ negative electrode" also becomes an "in-situ positive electrode" if the mating electrode is metallic lithium.

Japanese Patent No. 5062989

Japanese Patent No. 5134254

Abstracts of the 50th Battery Symposium Lecture No. 3A21 Shigeru Sano, Michiko Kusunoki, Akechi Tachibana

Abstracts of the 51st Battery Symposium Lecture No. 2B14 Shigeru Sano, Michiko Kusunoki, Akitomo Tachibana

Information Organization Co., Ltd. Website Lecturer Column Shigeru Sano

Electrochemistry Communications 20 (2012) 113-116

The stored energy of a lithium-ion secondary battery is the product of the amount of stored electricity and the voltage. Conventional theory and technology can be used to increase the capacity, but there was no theory or technology that could obtain a voltage of 3.6 V, which is equivalent to that of current lithium-ion batteries, using the same materials.

In order to achieve high capacity and high voltage, a new battery theory is required for the storage method, and the present invention provides the theory and means.

分割したｎ部は並列に接続されているので、負極に対しｎ部の充放電（閉路）電位は１～ｎで全て同一である。ｎ部の電子抵抗、ｎ部のイオン移動抵抗、ｎ部の電荷移動過電圧、ｎ部の活物質不均一性のＳＯＣ調整電圧、ｎ部の反応面イオン濃度補正電圧はｎ部で全て異なるが、ｎ部の充放電（閉路）電位が等しくなるようにｎ部の電流が変化するので、各部の各構成要素を外部電圧には現れない。Divide the positive electrode into n elements. Since the n divisions are made virtually, they do not have to be in a specific place, they do not have to be even, they may be arbitrary, and the number of n may be arbitrary or infinite. The electrical characteristics of each n part can be written as follows.

Since the divided n parts are connected in parallel, the charging/discharging (closed circuit) potentials of the n parts with respect to the negative electrode are all the same from 1 to n. The electronic resistance of the n part, the ion transfer resistance of the n part, the charge transfer overvoltage of the n part, the SOC adjustment voltage of the active material non-uniformity of the n part, and the reaction surface ion concentration correction voltage of the n part are all different in the n part, Since the current in the n section changes so that the charging and discharging (closed circuit) potentials of the n section become equal, each component of each section does not appear in the external voltage.

ｎ部の正極電位、平衡電位、ｎ部活物質不均一性のＳＯＣ調整電圧、ｎ部反応面イオン濃度補正電圧は、同一ではなく、各ｎ部の固有値であるが、その各部が並列に接続しているので、正極電位は負極を基準にして、最も大きな電位差となる電位になる。つまり、電極内に異なる平衡電位、つまり熱力学的内部エネルギーの異なる部分があったとしても、正極電位は負極に対し最も高い電位しか現れない。この電位についての考察・提案は新規な理論であり、本発明を構成する上で非常に重要である。When the current is cut off, the current becomes "0", so the value obtained by multiplying the electronic resistance of the n part and the ionic transfer resistance of the n part by the current of the n part immediately becomes "0", and the charge, which is a logarithmic function of the current The moving overvoltage also becomes "0" after several seconds, and the positive electrode potential of the n part is expressed by the following equation.

The positive electrode potential, the equilibrium potential, the n-part active material heterogeneity SOC adjustment voltage, and the n-part reaction surface ion concentration correction voltage are not the same, but are peculiar values of each n part, but each part is connected in parallel. Therefore, the potential of the positive electrode becomes the potential with the largest potential difference with respect to the negative electrode. That is, even if there are different equilibrium potentials in the electrodes, that is, parts with different thermodynamic internal energies, the positive electrode potential appears only the highest potential with respect to the negative electrode. Considerations and proposals regarding this potential are novel theories, and are very important in constructing the present invention.

Even if there are components with different potentials, a charge transfer reaction occurs, ie contact with the electrolyte is necessary to indicate the electrode potential. Even if there are different components, the electrode potential of the electrochemical reaction on the surface in contact with the electrolyte, that is, the cell voltage appears. If the m-th potential in contact with the electrolytic solution is the highest in the n-part, the potential of the positive electrode is the m-th potential regardless of the potentials of other locations.

As shown in FIG. 1, there are voids in the layered compound with redox potential produced in the discharged state, and when the first discharge, that is, the injection of electrons, the charge carrier ions such as lithium ions in the layered compound are generated by electrostatic force. It is stored and stored in the void. Electricity is stored and charged by the "cantilever principle" in equilibrium with the injected electrons at a position separated by several nanometers along the wall of the cavity. Charge carrier ions in the electrolytic solution or solid electrolyte are intercalated into the layered compound by oxidation-reduction reactions so as to replenish the charge carrier ions depleted in the layered compound. Any reaction that has an oxidation-reduction potential, that is, an electrode reaction potential, by insertion may be used. For example, lithium cobalt oxide used in current lithium-ion batteries has the following reaction formula.
L _i ⁺ +2 L _{i0.05 Co O 2} + e ⁻ ⇔ 2 L _i Co _{O 2}
The storage reaction in the void is represented by the following reaction formula.
xLi ⁺ + <gaps> + xe ^- ⇔ <gaps xLi ⁺ xe ^- >
The value of x is determined by the size of the voids, and according to the results of computational chemistry in Non-Patent Documents 1 and 2, a diameter of 4 nm or more is preferable, but since they are arranged along the walls, if they are too large, the central portion becomes redundant. Since it becomes a large space, the volumetric efficiency becomes worse. Even with x=1, the energy density is doubled that of the current positive electrode material, and the positive electrode potential is the oxidation-reduction potential of lithium cobaltate, so the energy density is at least doubled, satisfying the requirements for electric vehicle batteries. can do Each discharge after the first discharge results in the same reaction.

During charging, as shown in the lower diagram of FIG. 1, the charge carrier ions present in the layered compound are desorbed from the layered compound into the electrolytic solution or solid electrolyte by oxidation-reduction reactions. Any reaction that has an oxidation-reduction potential, that is, an electrode reaction potential, may be used.
2L _i CoO ₂ ⇔ L _i ⁺ +2L _{i0.05 CoO 2} + _e ⁻
In parallel with the charging reaction of the above formula, electron deficiencies are injected around the voids of the layered compound, and the charge carrier ions in the voids are pushed out by electrostatic repulsion, replenishing the decrease in the charge carrier ions in the layered compound. Charge carrier ions are inserted into the layered compound so as to The storage reaction in the void is represented by the following reaction formula.
<Void xLi ⁺ xe ⁻ >+x (electron deficiency) ⁺
⇔<Void x (electron deficiency) ⁺ >+xLi ⁺ +e ^-

According to the present invention, voids are provided in existing layered compounds such as lithium cobalt oxide, lithium nickel-cobalt-manganese oxide, lithium nickel manganese oxide, lithium iron phosphate, and the like, as illustrated in FIG. Charge carrier ions, such as lithium ions, can be stored inside the cavity, and the charge carrier ions in the voids come into contact with the electrolyte or solid electrolyte via the layered compound and undergo a charge transfer reaction. Since it becomes the redox potential of the compound, the energy density can be increased. By optimizing the voids, it is possible to store more than twice as much electric charge as current layered metal oxides.

According to the present invention, the surface of the porous structure having voids facing the electrolytic solution or solid electrolyte is coated with an existing layered compound, resulting in intercalation/deintercalation as illustrated in FIG. Charge carrier ions can be incorporated into the porous structure at the potential of the layered compound having an oxidation-reduction potential of . Charge carrier ions can be stored in the voids of the porous body structure with electron storage properties such as hard carbon and soft carbon, so it can store much more charge than the current layered metal oxide such as lithium cobalt oxide. can do Although the potential of the charge carrier ions in the voids estimated from the calculation results of the stabilization energy in Non-Patent Documents 1 and 2 is very low, it passes through the layered compound, so it becomes the oxidation-reduction reaction potential of the layered compound. .

Schematic diagram of charging/discharging mechanism of layered metal oxide with voids Schematic diagram explaining charge/discharge mechanism of dual carbon storage battery with sieve membrane Bipolar Carbon Lithium Ion Battery Charging and Discharging Mechanism Explanation Schematic Diagram Schematic diagram of in-situ formed negative electrode Enlarged schematic diagram of in-situ formed negative electrode Schematic diagram of layered metal oxide-coated electronically conductive porous structure

As shown in the schematic diagram of FIG. 1, with the intercalation and deintercalation of lithium ions, charges such as lithium ions are generated in the voids formed inside the layered compound such as layered metal oxides or layered metal phosphates having oxidation-reduction reaction potentials. It is a positive electrode active material that occludes and stores carrier ions. and a storage battery composed of the positive electrode active material. Prior to use as a storage battery, charge carrier ions are stored in the positive electrode active material voids by initial discharge. The initial discharge may be performed for the positive electrode active material alone before incorporating the positive electrode into the storage battery. Further, the layered compound may contain a conductive aid.

The voids capable of storing and accumulating charge carrier ions shown in the schematic diagram of FIG. 1 in the present invention can be created by insufficient sintering of conventional layered metal oxides. Furthermore, it can be formed by controlling the temperature and time so that only the surface is completely sintered when the sintered body is pulverized and refired to form granules. Amorphization by quenching, which tends to create voids, can also be applied.

The voids capable of storing and accumulating charge carrier ions shown in the schematic diagram of FIG. By controlling , voids can be formed relatively easily, and voids can be formed inside the polycrystalline grains by completely sintering only the grain surfaces.

In order to store more charge carrier ions than the method of providing voids in the layered metal oxide according to the present invention, charges such as lithium ions are added to the voids of the electronically conductive porous material such as hard carbon or soft carbon. There is a method of storing and accumulating carrier ions. In this case, as illustrated in FIG. 6, the perimeter/surface of the porous body structure can be coated with a layered compound having an oxidation-reduction potential upon insertion/desorption reaction by a known method.

Since the present invention can store two to several times as many charge carrier ions at the same potential as the current layered metal oxide, it is optimal as a battery for electric vehicles, and can extend the current one-charge driving distance by more than twice. , the performance of which is equivalent to that of gasoline-powered vehicles, which will greatly contribute to the electrification of automobiles and contribute to the realization of a decarbonized society.

Since the present invention can store two to several times as many charge carrier ions at the same potential as the current layered metal oxide, it can be applied to a stationary power storage system, the number of storage batteries in the system can be halved, and the system cost of the storage batteries can be halved. Therefore, it can greatly contribute to the spread of stationary power storage systems that are installed alongside renewable energy power generation. As a result, it will support the spread of renewable energy and contribute to the realization of a decarbonized society.

Claims (1)

1. A storage battery comprising a cathode active material surrounded by a compound that is oxidized and reduced by the insertion and desorption of charge carrier ions and having voids through which the charge carrier ions can be inserted and desorbed.

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