JP2020107601A - Lithium cobalt oxide positive electrode active material and secondary battery using the same - Google Patents

Abstract

To provide a positive electrode active material having a high capacity retention rate in high voltage charge/discharge (at high temperature).SOLUTION: A positive electrode active material for a secondary battery includes lithium cobalt oxide represented by the chemical formula: LixCoO2 (0.1<x<1.0), and the lithium cobalt oxide includes at least a Co-F bond and a Co-O bond as an atomic bonding group.SELECTED DRAWING: Figure 1

Description

The present invention relates to a lithium cobalt oxide positive electrode active material, particularly to a lithium cobalt oxide positive electrode active material having a specific atomic bonding group and a secondary battery using the same.

In recent years, with the development and spread of mobile tools and electric motors, a high-capacity energy source has been required, and a typical example thereof is a secondary battery. In particular, the lithium-ion secondary battery has a high average operating voltage and a high capacity, so it can be used not only for mobile and stationary personal computers (PCs), but also for large-scale power sources for storage batteries and high-output power sources for vehicles. The range is expanding rapidly.

In addition, with the expansion of the range of applications as secondary batteries for driving power sources, new active materials for secondary batteries with high energy density are being pioneered and developed, further improving energy density and improving output characteristics. The development of new positive electrode active materials for secondary batteries is being actively pursued.

The theoretical capacity of LiCoO ₂ that is a positive electrode active material for a secondary battery exhibits 274 mAh/g, but in actuality, the charging/discharging capacity is suppressed to about 160 mAh/g in order to prolong the charging/discharging cycle life. Thus, in the current oxide-based positive electrode active material, there is a margin in the redox amount that can be theoretically used, so by increasing the upper limit of the charging voltage and expanding the lithium insertion/desorption range, more capacity and Attempts have been made to ensure a higher average operating voltage and improve the energy density.

However, theoretically, it is possible to achieve a higher energy density by increasing the charging voltage, but considering the decomposition voltage of the electrolytic solution, it is difficult to ensure cycle life and safety in the conventional technology. In particular, rapid deterioration of the cycle has become a major issue for practical use.

Also, due to the widespread and diversified application range of the secondary battery, it has excellent temperature stability such that it can operate even under severe temperature environment such as high temperature environment above 40°C or minus temperature environment below 0°C. A secondary battery is required.

Conventionally, for the purpose of improving cycle characteristics and the like, a method has been proposed in which a surface layer of an oxide, a fluoride or the like is formed on a positive electrode material to suppress decomposition and oxidation of an electrolytic solution.

As a method of coating the positive electrode active material with an oxide, for example, Non-Patent Document 1 [Y. Akira, et al., J. Am. Electro. Chem. Soc. , 164 (1) A6116-A6122 (2017)], an electrochemically inactive compound such as aluminum oxide is used to form a LiAlO ₂ —LiCoO ₂ solid solution on the surface and rearranges the surface atoms. Means for suppressing oxygen deficiency and spinel transition have been proposed. However, even with this proposal, it is difficult for the high-voltage positive electrode active material to maintain stable drivability over a long-term cycle, and many problems remain for practical use.

In addition, as a technique of coating a positive electrode active material with a fluoride, for example, in Patent Document 1 (Japanese Patent Laid-Open No. 2014-53718), a forming solution containing NH ₄ F in an aqueous solution containing an aluminum salt is used. Methods have been proposed for coating materials to improve cycle characteristics. However, there are many problems to be solved such as elution of the positive electrode active material and a large amount of the forming solution in the coating step. Further, in Patent Document 2 (JP 2009-164139 A) and Patent Document 3 (JP 2006-514913 A), fluorine gas or hydrogen fluoride (HF) is used at room temperature or low temperature, and the surface metal is fluorine. The method of making it real is proposed.

However, attention must be paid to the handling and safety of fluoride (gas), and there is room for improvement in terms of effects on the human body, preservation of the surrounding environment, and work efficiency. Fluorine gas, which is a strong oxidant, can react with almost all atoms, but fluorine introduced by the direct fluorination method (gas) interacts with Li to some extent preferentially over transition metal. .. However, since the contribution of lithium bound to fluorine, which has an extremely high electronegativity, as an ion carrier disappears, in order to secure the energy density, the partner of fluorine that totally acts on the surface atoms must be an element other than lithium. Is preferred.

Therefore, research and development of new positive electrode active materials are still required from the viewpoints of ease of handling, safety, improvement of work efficiency, improvement of battery performance and the like.

Japanese Patent Publication No. 2014-531718 JP, 2009-164139, A Japanese Patent Publication No. 2006-514913

Y. Akira, et al., J. Am. Electro. Chem. Soc. , 164 (1) A6116-A6122 (2017)

The present invention has been made based on the finding that by providing a positive electrode active material with a specific atomic bond group, a high battery capacity, a high average operating voltage can be secured, and an energy density can be improved. Is.

[One Embodiment of the Present Invention]
One aspect of the present invention is as follows.
[1] A positive electrode active material for a secondary battery,
A positive electrode active material for a secondary battery,
Chemical formula: Li _x CoO ₂ (0.1<x<1.0)
The lithium cobalt oxide has at least a Co—F bond and a Co—O bond as an atomic bonding group,
By X-ray photoelectron spectroscopy (XPS),
A Co2p3/2 peak for the Co—F bond appears at 780.5-779.5 eV, and
A positive electrode active material for a secondary battery, wherein a Co2p3/2 peak for the Co-O bond appears at 781-783eV.
[2] A peak area ratio S(CoF)/S(CoO) of the Co—O bond-derived Co2p3/2 peak to the Co—F bond-derived Co2p3/2 peak is 0.05 or more and 0.12 or less, The positive electrode active material for a secondary battery according to [1].
[3] As the atom-bonding group, one or a mixture of two or more kinds selected from the group consisting of Al-F bond, Al-O bond, and Al-Co bond is provided on the surface of the active material. ] Or the positive electrode active material for secondary batteries as described in [2].
[4] The Al atoms constituting the atomic bonding group are present in an amount of 0.03 mass% or more and 0.35 mass% or less with respect to the total mass of the positive electrode active material for a secondary battery, [3] 5. A positive electrode active material for a secondary battery according to.
[5] The atom bonding group according to any one of [1] to [4], which is derived from a constituent atom of the lithium cobalt oxide and/or a constituent atom of aluminum fluoride (AlF ₃ ). Positive electrode active material for secondary batteries.
[6] The positive electrode active material for a secondary battery according to any one of [1] to [5], wherein the Co atom in the Co—F bond is trivalent or divalent.
[7] The positive electrode active material for a secondary battery according to any one of [1] to [6], wherein the Co atom in the Co—F bond is trivalent.
[8] A positive electrode,
A positive electrode comprising a current collector and the positive electrode active material for a secondary battery according to any one of [1] to [7].
[9] A secondary battery,
A secondary battery comprising a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, wherein the positive electrode is as described in [8].
[10] A method for producing a positive electrode active material for a secondary battery according to any one of [1] to [7],
Aluminum fluoride (AlF ₃ ) and a polar solvent are mixed to prepare an aluminum fluoride sol composition,
Lithium cobalt oxide represented by the chemical formula is treated with the aluminum fluoride sol composition to prepare a positive electrode active material precursor for a secondary battery,
Firing or drying the positive electrode active material precursor for the secondary battery,
A production method comprising forming at least a Co—F bond and a Co—O bond as an atom bonding group on the surface of the lithium cobalt oxide.
[11] The positive electrode active material for a secondary battery according to any one of [1] to [7], which is obtained by the method for producing a positive electrode active material for a secondary battery according to [10].

According to the lithium cobalt oxide positive electrode active material having the specific atom-bonding group on the surface according to the present invention, the decomposition reaction of the electrolytic solution on the surface of the positive electrode active material is suppressed, and the cycle life can be significantly improved. In particular, due to the presence of a specific atom-bonding group on the surface of the positive electrode active material, it is extremely excellent for a system that is used at high temperature and is applied with a high voltage of 4.5 V or higher (particularly preferably 4.55 V or higher) based on lithium The secondary battery performance can be exhibited.

According to the production method of the present invention, by using aluminum fluoride having excellent specificity and safety and a polar solvent, in the step of forming an atomic bond group (treatment, coating), ensuring safety, workability, It becomes possible to form a specific atomic bonding group on the surface of the lithium cobalt oxide positive electrode material while ensuring the protection of the surrounding environment and the improvement of manufacturing efficiency.

FIG. 1 shows cycle characteristics at 25° C. for Example 1-1 and Comparative Example. FIG. 2 shows cycle characteristics at 45° C. for Examples 1-1 to 1-3 and Comparative Example. FIG. 3 shows changes in the capacity retention rate with respect to the AlF ₃ treatment amount at the 50th cycle. FIG. 4 shows cycle characteristics at 45° C. of Examples 2-1 to 2-3 and Comparative Example. FIG. 5 shows Co2p spectra of Example 1-1, Example 1-3, Example 2-2, and Comparative Example. FIG. 6 shows the result of peak separation of the Co2p3/2 spectrum for each component. FIG. 7 shows powder diffraction line patterns of AlF ₃ powder before (a) and after (b) with stirring and drying. FIG. 8 shows comparison of Al2p spectra of Example 1-1, Example 1-3, Example 2-2 and Comparative Example. FIG. 9 shows comparison of F1s spectra of Example 1-1, Example 1-3, Example 2-2 and Comparative Example.

[Definition]
(X-ray photoelectron spectroscopy)
X-ray Photoelectron Spectroscopy (XPS) analyzes a constituent element of a sample and its electronic state by irradiating a sample, particularly a sample surface with X-rays and measuring the energy of photoelectrons generated. Is the way. In particular, XPS can be used not only for qualitative analysis and quantitative analysis, but also for analyzing the chemical bond state that determines the characteristics of the sample, with respect to elements existing on the surface of several nm.

(Volume cumulative particle size distribution)
The volume cumulative particle size distribution is a particle size distribution obtained by assuming a set of one powder, and in the particle size distribution, when the total volume of the powder group is 100% and the cumulative curve is obtained, For example, the particle diameter at the point where the cumulative curve becomes minimum (min), 50%, and maximum (max) is expressed as D(min) diameter, D(50) diameter, D90(max) diameter (μm), respectively. Is. Actually, it can be obtained by measuring with Microtrac (laser diffraction/scattering method), assuming that the shape of the particles is spherical, and converting it into a number standard etc. using analysis software.

(Average particle size)
In the present invention, the average particle diameter is a volume average diameter (MV), and can be measured using a volume standard (volume distribution) for a granular substance, for example, Microtrac (laser diffraction/scattering method). The volume distribution can be measured using a method and apparatus such as ), and can be calculated using analysis software.

(Primary particles/Secondary particles)
In the present invention, the “primary particle” basically means one particle (basic particle). The average particle diameter of the “primary particles” is usually more than 0 and about 1000 nm or less, preferably about 100 nm or less. The “secondary particles” are formed as aggregates (or granules) formed by aggregating some “primary particles”, but they are particles.

[Lithium cobalt oxide positive electrode active material]
(Lithium cobaltate)
In the present invention, Li _x CoO ₂ (0.1<x<1.0) is used as the positive electrode active material. In the present invention, lithium cobalt oxide (LiCoO ₂ ) in which x is 1.0 in the above formula is preferable.

(Atomic bond group)
The positive electrode active material according to the present invention comprises one or more atomic bonding groups on the surface of the positive electrode active material. In the present invention, it is composed of the constituent atoms of lithium cobalt oxide, the constituent atoms of aluminum fluoride (AlF ₃ ), and the atomic bonding group with these constituent atoms. Therefore, the present invention comprises an atom-bonding group consisting of one or more atoms selected from the group consisting of Li, Co, O, Al, and F.

Due to the presence of the atomic bonding group, stacking fault and spinel transition, which occur when charging and discharging are repeated up to the high Co number state, can be suppressed, and as a result, Co elution into the electrolytic solution can be prevented, and further fluorinated bonding Due to the presence of the group, thermal and chemical stability and reduction resistance against side reactions on the surface of the positive electrode active material associated with charging and discharging are significantly improved, so that a stable charging and discharging cycle can be realized at a high level. it can.

For example, the Al-O bond suppresses surface metal pitting and electrolytic solution reaction, and particularly contributes to the stabilization of the LiCoO ₂ structure in the Co high-value state, and the Co-F bond and the Al-F bond. By coexisting with etc. on the surface of the positive electrode active material (particles), it becomes possible to impart high voltage resistance over a long-term cycle.

In the present invention, the atom-bonding group is, for example, one or two selected from the group consisting of Co—F bond, Co—O bond, Al—F bond, Al—O bond, and Al—Co bond. A mixture of one or more kinds can be mentioned, and preferably one or a mixture of two or more kinds selected from the group consisting of Co—F bond, Co—O bond, and Al—O bond can be mentioned. In the present invention, at least a Co—F bond and a Co—O bond are provided.

In the present invention, Al may be present in an amount of 0.03% by mass or more and 0.35% by mass or less based on the total mass (100% by mass) of the positive electrode active material.

<Atomic bond group formation>
In the present invention, the “atom-bonding group” means that the constituent atoms of lithium cobalt oxide and the constituent atoms of aluminum fluoride (AlF ₃ ) are solid-solubilized or ion-bonded on the surface of lithium cobalt oxide. Is formed. More specifically, as will be described later, it can be formed by chemically bonding (forming, treating or coating) lithium cobalt oxide with aluminum fluoride.

In the present invention, in the meaning of the above-mentioned formation treatment, the site where the atomic bonding group exists may be referred to as a functional layer, a surface layer, a treatment layer, or a coating layer.

(peak)
In the positive electrode active material according to the present invention, the atomic bonding group has a CoF peak appearing at 780.5-779.5 eV and 781-783 eV, which are obtained by an X-ray photoelectron spectroscopy (hereinafter referred to as “XPS”) analysis. And a CoO peak appearing in.

(Peak area ratio S)
The area ratio S(CoF)/S(CoO) between the CoF peak and the CoO peak is preferably 0.05 or more and 0.12 or less.

In the present invention, the lithium cobalt oxide positive electrode active material is preferably a powder. The lithium cobalt oxide positive electrode active material is preferably composed of secondary particles formed by aggregating primary particles or a mixture of primary particles and secondary particles. In this case, the average particle diameter of each of the primary particles and the secondary particles is 2 μm or more and 20 μm or less.

By setting the average particle diameter to the above lower limit or more, application to the current collector becomes easy, and the surface reaction with the electrolytic solution during charge/discharge can be suppressed, and cycle deterioration can be significantly prevented. .. Further, by setting the average particle diameter to be equal to or less than the above upper limit, it is possible to significantly prevent the generation of voids at the time of filling the positive electrode and improve the filling efficiency. It is also possible to increase the filling rate by mixing powders of different particle sizes.

[Method for producing positive electrode active material]
(Raw material preparation)
In the present invention, lithium cobalt oxide, aluminum fluoride (AlF ₃ ) and a polar solvent are prepared.

<Cathode active material>
As the positive electrode active material, the lithium cobalt oxide Li _x CoO ₂ (0.1<x<1.0) is used as described above.

<Aluminum fluoride (AlF _3)>
In the present invention, aluminum fluoride may be prepared by a conventional production method, or a commercially available product may be used as a raw material. As a commercially available product, for example, AlF ₃ (Fuji Film Wako Pure Chemical Industries, Ltd.; density 2.882 g/cm ³ ) can be used. In the present invention, preferably, Co can be selectively fluorinated by undergoing an atomic bond forming step using an intermediate product AlF ₃ sol, which can be expected to react slowly with lithium cobalt oxide.

In the present invention, considering that it is prepared as a sol composition described later, it is preferable to use aluminum fluoride containing few impurities. In the present invention, the aluminum content of aluminum fluoride is 25% by mass or more and 40% by mass or less, preferably 32% by mass or less, based on the total mass (100%) of aluminum fluoride. Further, the average particle size of aluminum fluoride may be any as long as it is within a range in which it can be dissolved in a polar solvent, but in consideration of solubility, it is preferably 10 μm or less.

The content of the aluminum fluoride sol forming the atomic bonding group is 0.05% by mass or more and 5% by mass or less, as AlF ₃ solid content, when the lithium cobalt oxide positive electrode active material 100 (mass%) is used, Is 0.3% by mass or more and 3% by mass or less, and more preferably 0.1% by mass or more and 1% by mass or less. When the content of the lithium cobalt oxide positive electrode active material is within the above numerical range, it becomes possible to form a desired atomic bond group on the surface of the lithium cobalt oxide positive electrode active material.

<Polar solvent>
The polar solvent is used in preparing the sol composition. As the polar solvent, water (pure water, distilled water, ion-exchanged water, non-ionized water), alcohol (lower alcohol having 5 or less carbon atoms, preferably methanol, ethanol, propanol), ketone (acetone), lower diol ( One kind or a mixture of two or more kinds such as ethylene glycol) can be used.

(Preparation of aluminum fluoride sol composition)
In the present invention, the aluminum fluoride sol composition is prepared by mixing the aluminum fluoride (AlF ₃ ), the polar solvent, and any additive (as required). The mixing method may be an ordinary method, a mixer, a stirrer, or the like may be used, and heating may be performed as necessary.

If the content of aluminum fluoride (AlF ₃ ) is about 1.0 mass% or less when the total mass of the polar solvent is 100 (mass %), a desired aluminum fluoride sol composition can be obtained. ..

(Atom bond forming process)
In the present invention, a lithium cobalt oxide positive electrode active material is mixed with the aluminum fluoride sol composition.

In the present invention, the mixture of the aluminum fluoride sol composition and the lithium cobalt oxide positive electrode active material (hereinafter, simply referred to as “mixture”) is stirred and mixed, or the mixture is allowed to stand. Mixing or stirring can be performed by using a mixer or a stirrer, and preferably by the method described in the examples. The standing is usually about 1 to 2 days, although it depends on the amount of the mixture.

The mixture is dried by heating with stirring (100° C. or higher and 180° C. or lower, preferably 110° C. or higher and 150° C. or lower). Further, the above-mentioned mixture left to stand is filtered and dried. The filtration may be carried out by a usual method, and may be solid-liquid separation filtration because of its simplicity and economy. After filtration, it may be dried in a normal state, the drying temperature is preferably 100° C. or higher, and more preferably 120° C. is dried under vacuum. As a result, a powdery mixture can be obtained. In the present invention, a lithium cobalt oxide positive electrode active material having a desired atomic bond group may be obtained by this drying step. Further, by this drying step, a lithium cobalt oxide positive electrode active material precursor having an atom-bonding group may be subjected to the subsequent firing step.

(Roasting process)
In the present invention, the dried mixture is fired at 400° C. or higher and 600° C. or lower, but it is more preferable to heat (fire) at about 500° C. By this firing step, a lithium cobalt oxide positive electrode active material having a more desired atomic bonding group can be obtained.

Preferably, the positive electrode active material is a fluoride-coated positive electrode, aluminum fluoride (AlF ₃ ), a polar solvent, and a positive electrode active material are prepared, and the aluminum fluoride (AlF ₃ ) and the polar solvent are mixed. , An aluminum fluoride sol composition is prepared, the aluminum fluoride sol composition is heated and mixed with the positive electrode active material, or the mixture is allowed to stand and then filtered and dried, and if necessary, 400° C. or higher. It is possible to propose a lithium cobalt oxide positive electrode active material having an atom-bonding group, which is obtained by a production method including firing at a temperature of 600° C. or lower.

[Positive electrode]
In the present invention, a positive electrode for a secondary battery including the positive electrode active material according to the present invention can be proposed. The positive electrode can be prepared by preparing a positive electrode composition to which the positive electrode active material (powder) according to the present invention, a conductive agent, a binder, a thickener or the like is added, and forming it on the positive electrode current collector. is there.

<Current collector>
The current collector may be any as long as it can be used electrochemically stably at the positive electrode charging voltage, for example, copper, aluminum, stainless steel, titanium, silver, palladium, nickel, alloys thereof, And any one kind or a mixture of two or more kinds selected from the group consisting of these combinations. The stainless steel may be surface-treated with carbon, nickel, titanium, or silver, and an aluminum-cadmium alloy can be preferably used as the alloy. In addition, as the current collector, fired carbon, a non-conductive polymer surface-treated with a conductive material, a conductive polymer, or the like can also be used.

<Conductive agent>
The conductive material imparts conductivity to the electrodes, and can be used without particular limitation as long as it has electronic conductivity without causing a chemical change in the battery. Specific examples of the conductive material include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, fanace black, lamp black, thermal black, and carbon fiber; Metal powder or metal fiber such as copper, nickel, aluminum, silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives Examples include one kind or a mixture of two or more kinds selected from the group. The content of the conductive agent may be 0.3% by mass or more and 7% by mass or less, preferably 1.0% by mass or more, and 4% by mass or less, based on the total mass of the positive electrode.

<Binder>
The binder plays a role of improving adhesion between particles of the positive electrode active material and the adhesion between the positive electrode active material and the current collector. Specific examples of the binder include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, Hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof. Examples include one kind or a mixture of two or more kinds selected from the group consisting of coalescence. The binder may be contained in an amount of 1% by weight or more and 30% by weight or less based on the total weight of the positive electrode active material layer.

[Secondary battery]
In the present invention, a secondary battery (preferably a lithium secondary battery) composed of a positive electrode containing the positive electrode active material according to the present invention, a negative electrode, and a separator can be proposed.

According to the present invention, since the positive electrode active material according to the present invention is adopted, it is possible to propose a secondary battery having excellent cycle characteristics, particularly in charging/discharging with a high charging voltage. Specifically, according to the secondary battery of the present invention, 1 atmosphere and a high temperature of 40° C., preferably a high temperature of 45° C., preferably in a temperature range of 25° C. or higher and 60° C. or lower, preferably 40° C. It can be carried out in a temperature range (high temperature) of 60° C. or higher and 60° C. or lower, more preferably 45° C. Further, according to the present invention, a secondary battery having stable cycle characteristics, which can be executed at a cutoff voltage (3V to 4.55V, preferably 4.5V to 4.55V, most preferably 4.55V). Can be provided.

(Positive electrode)
As the positive electrode, the one according to the present invention described above is used.

(Negative electrode)
The negative electrode can be prepared by preparing a negative electrode composition containing a negative electrode active material, a conductive agent, a binder, a thickener, etc., and forming the composition on the negative electrode current collector.

<Negative electrode active material>
The negative electrode active material may be any material as long as it is a compound capable of reversibly inserting and releasing lithium. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys. , Sn alloys, Al alloys, and other metallic compounds that can be alloyed with lithium; SiO _x (0<x<2), SnO ₂ , vanadium oxide, lithium vanadium oxide, such as lithium doping and dedoping A metal oxide that can be formed; or a complex including the metallic compound and a carbonaceous material, such as a Si—C complex or a Sn—C complex, and any one of these Alternatively, a mixture of two or more may be used. Furthermore, a metal lithium thin film may be used as the negative electrode active material. Further, as the carbon material, low crystalline carbon, high crystalline carbon and the like may all be used.

<Current collector>
The current collector may be any one that does not induce a chemical change in the secondary battery and has high conductivity. For example, surface treatment of copper, stainless steel, aluminum, nickel, titanium, sintered carbon, aluminum or stainless steel surface with one or a mixture of two or more selected from the group consisting of carbon, nickel, titanium and silver. Those that have been used are used.

<Conductive agent>
It is preferable that the conductive agent does not induce a chemical change in the secondary battery. For example, graphite such as natural graphite and artificial graphite; carbon black, acetylene black, Ketjen black (trade name), graphene, carbon nanotube, carbon nano Examples thereof include fibers, carbon black such as channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber. The content of the conductive agent may be more than 0% by mass and 5% by mass or less, preferably 0.5% by mass or more and 3% by mass or less, based on the total mass of the negative electrode.

<binder>
Examples of the binder include water-based and solvent-based binders. Examples of the water-based binder include styrene-butadiene rubber (SBR), polyacrylic acid, polyimide, polyvinylidene fluoride, polyacrylonitrile, polyvinylidene fluoride-hexafluoropropylene copolymer, and polyfluorine. Vinylidene fluoride-trichloroethylene, polyvinylidene fluoride-chlorotrifluoroethylene, polymethyl methacrylate, polyvinyl acetate, ethylene vinyl acetate copolymer, polyethylene oxide, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethyl pullulan, cyanoethyl polyvinyl From alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, carboxymethyl cellulose (CMC), acrylonitrile-styrene-butadiene copolymer, alkyl-modified carboxyl group-containing copolymer, copolymer of vinyl ester and ethylenically unsaturated carboxylic acid ester Preference is given to one or a mixture of two or more selected from the group consisting of:

(Separator)
The separator is an insulating thin film interposed between the positive electrode and the negative electrode and having high ion permeability and mechanical strength. Generally, the separator has a pore diameter of 0.01 to 10 μm and a thickness of 5 to 300 μm. As such a separator, for example, an olefin polymer such as polypropylene having chemical resistance and hydrophobicity, a sheet or a non-woven fabric made of polyimide, glass fiber, polyethylene, or the like is used, and in order to enhance safety, There may be an oxide layer of alumina, titania, silica or the like on the surface. When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte can also serve as the separator.

Further, in the present invention, the separator is, for example, selected from the group consisting of ethylene homopolymer, propylene homopolymer, ethylene-butene copolymer, ethylene-hexene copolymer and ethylene-methacrylate copolymer. A porous polymer substrate having a polyolefin-based polymer; selected from the group consisting of polyester, polyacetal, polyamide, polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenylene oxide, polyphenylene sulfide and polyethylene naphthalate. Porous polymer substrate provided with a polymer; porous substrate provided with a mixture of organic or inorganic particles (preferred) and binder polymer; or on at least one surface of the porous polymer substrate A porous substrate layer having a porous coating layer formed from a mixture of organic particles or inorganic particles (preferred) and a binder polymer can be used.

In the case of a porous substrate (layer), in the porous coating layer formed from a mixture of organic particles or inorganic particles (preferred) and a binder polymer, organic particles or inorganic particles (preferred) particles are bound to each other. In order to maintain the above state, the binder polymer adheres them to each other (that is, the binder polymer connects and fixes the organic particles or the inorganic particles to each other), and the porous coating layer is a polymer. The binder maintains a state of being bound to the porous polymer substrate. The organic particles or the inorganic particles of the porous coating layer have a close-packed structure in a state of being substantially in contact with each other, and an interstitial volume (interstitial volume) generated when the inorganic particles are in contact with each other. ) Becomes pores of the porous coating layer.

According to a preferred embodiment of the present invention, in order for lithium ions to be easily transferred to an external electrode, the polyester, polyacetal, polyamide, polycarbonate, polyimide, polyether ether ketone, polyether sulfone, polyphenylene oxide, polyphenylene sulfide can be used. It is possible to use a separator made of a non-woven fabric material corresponding to a porous polymer substrate provided with a polymer selected from the group consisting of and polyethylene naphthalate.

[Non-aqueous electrolyte]
In the present invention, the non-aqueous electrolyte may contain, for example, a cyclic carbonate and/or a chain carbonate. Examples of cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), γ-butyrolactone (GBL), fluoroethylene carbonate (FEC), and the like. As an example of the chain carbonate, one or a mixture of two or more selected from the group consisting of diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC) and methylpropyl carbonate (MPC) is preferable. However, the present invention is not limited thereto, and one or a mixture of two or more selected from the group consisting of γ-butyl lactone (γ-BL), tetrahydrofuran (THF), acetonitrile and its derivatives, and an ionic liquid is used. May be.

In the present invention, the cyclic carbonate and the chain carbonate are mixed and used at an arbitrary ratio. Generally, a cyclic carbonate has a high dielectric constant and high viscosity, and a chain carbonate has a low dielectric constant and low viscosity. Therefore, by appropriately mixing these, a lithium salt described later can be dissolved well while having an appropriate viscosity. However, a non-aqueous electrolyte having high ionic conductivity can be obtained.

The content of the other non-aqueous electrolyte solvent component is 5% by mass or more, preferably 10% by mass or more, more preferably 20% by mass or less, based on the total mass (100% by mass) of the non-aqueous electrolyte. It is at least mass% and the upper limit is at most 94 mass%, preferably at most 92 mass%, more preferably at most 90 mass%. By setting the content within the above numerical range, the viscosity and the ionic conductivity of the non-aqueous electrolyte can be set to appropriate ranges, and it becomes easy to cope with the large current charge/discharge characteristics of the secondary battery.

<Lithium salt>
The non-aqueous electrolyte can contain a lithium salt. Examples of the lithium salt, LiCl, LiBr, LiI, LiClO 4, LiBF 4, LiB 10 Cl 10, LiPF 6, LiCF 3 SO 3, LiCF 3 CO 2, LiAsF 6, LiSbF 6, LiAlCl 4, CH 3 SO 3 Li, CF ₃ SO ₃ Li, (CF ₃ SO ₂ ) ₂ NLi, lithium chloroborate, lower aliphatic lithium carboxylate, and a mixture of two or more selected from the group consisting of lithium tetraphenylborate. To be

The content of the lithium salt is not particularly limited as long as it does not impair the effects of the present invention, but the ionic conductivity of the electrolyte is in a good range, from the viewpoint of ensuring good secondary battery characteristics, a non-aqueous system. Based on the total mass of the electrolyte (100 mass %), the lower limit value is 4 mass% or more, preferably 6 mass% or more, more preferably 8 mass% or more, and the upper limit value is 30 mass% or less. %, preferably 25 mass% or less, more preferably 20 mass% or less. By setting the content of the lithium salt within the above range, a high ionic conductivity non-aqueous electrolyte having an appropriate viscosity can be constructed, and good secondary battery characteristics can be obtained.

<Other non-aqueous electrolyte additives>
In the present invention, one or a mixture of two or more selected from the group consisting of vinylene carbonate, biphenyl, propane sultone, and diphenyl disulfide may be added as a non-aqueous electrolyte additive.

In the present invention, the non-aqueous electrolyte that conducts lithium ions may be either a liquid or a solid, and is preferably a liquid. In the case of a solid (including a layered form), for example, a gel type polyelectrolyte using PEO, PVdF, PVdF-HFP, PMMA, PAN, or PVAc; or PEO, polypropylene oxide (PPO), polyether It can be manufactured as a solid non-aqueous electrolyte using imine (PEI), polyethylene sulfide (PES), or polyvinyl acetate (PVAc).

(Production method)
The secondary battery according to the present invention is manufactured by inserting a porous separator between the positive electrode and the negative electrode and charging an electrolyte by a usual method. The secondary battery according to the present invention may be used as a cylindrical battery, a prismatic battery, a pouch battery, or the like regardless of the external shape. The secondary battery may be a single battery or a plurality of secondary batteries.

The contents of the present invention will be described using the following examples, but the scope of the present invention should not be construed as being limited to these examples. In addition, various aspects of the present invention disclosed in this specification can be easily implemented by those skilled in the art from this embodiment.

(Preparation of positive electrode active material 1)
(1) With respect to the positive electrode active material (LiCoO ₂ ), 0.28% by weight or 0.1% by weight of aluminum fluoride (AlF ₃ ) was added with 0.18 l/g of ion-exchanged water to fluorinate. An aluminum sol composition was prepared.
(2) A positive electrode active material (LiCoO ₂ ) was added to each of the obtained aluminum fluoride sol compositions, and the mixture was heated in a temperature range of 110 to 150° C., and was stirred, heated and dried until the ion-exchanged water was evaporated. The obtained mixture (powder) was heated under nitrogen reflux at 500° C. for 2 hours to obtain a fluoride-coated positive electrode active material. This fluoride-coated positive electrode active material was prepared with 0.2 wt% AlF ₃ sol composition in Example 1-1 and 0.1 wt% AlF ₃ sol composition was prepared in Example 1-2. Was used as the positive electrode of the secondary battery.
(3) Aluminum fluoride sol composition is prepared by adding 0.18 l/g of ion-exchanged water to 0.2% by weight of aluminum fluoride (AlF ₃ ) with respect to the positive electrode active material (LiCoO ₂ ). did. After cooling and standing in a refrigerator (9° C.) for 1 day (24 hours), the mixture was filtered. The obtained mixture (powder) was heated (baked) at 500° C. for 2 hours under nitrogen reflux to obtain a fluoride-coated lithium cobalt oxide positive electrode active material. This fluoride-coated lithium cobalt oxide positive electrode active material was used as the positive electrode of the secondary battery of Example 1-3.

(Preparation of positive electrode active material 2)
(1) Aluminum fluoride sol composition obtained by adding 0.18 l/g of ion-exchanged water to 0.2% by weight or 1.0% by weight of aluminum fluoride based on the positive electrode active material (LiCoO ₂ ). Was prepared.
(2) A positive electrode active material (LiCoO ₂ ) was added to each of the obtained aluminum fluoride sol compositions, and the mixture was heated in a temperature range of 110 to 150° C., and was stirred, heated and dried until the ion-exchanged water was evaporated. The obtained mixture (powder) was heated under vacuum at 120° C. for 12 hours to obtain a fluoride-coated positive electrode active material. This fluoride-coated positive electrode active material was prepared from the 0.2% by weight AlF ₃ sol composition in Example 2-1 and 1.0% by weight in the AlF ₃ sol composition. Was used as the positive electrode of the secondary battery.
(3) An aluminum fluoride sol composition was prepared by adding 0.18 l/g of ion-exchanged water to 0.2% by weight of aluminum fluoride based on the positive electrode active material (LiCoO ₂ ). After cooling and standing in a refrigerator (9° C.) for 1 day (24 hours), the mixture was filtered. The obtained mixture (powder) was heated under vacuum at 120° C. for 12 hours to obtain a fluoride-coated lithium cobalt oxide positive electrode active material. This fluoride-coated lithium cobalt oxide positive electrode active material was used as the positive electrode of the secondary battery of Example 2-3.

(Preparation of battery)
<Preparation of positive electrode>
96% by weight of each of the coated positive electrode active materials prepared above was mixed with 2% by weight of carbon black (trade name: SuperP: IMERYS) as a conductive material and 2% by weight of polyvinylidene fluoride as a binder, and N-methyl was added. 2-Pyrrolidone was used as a slurry, and this slurry was applied to an aluminum foil having a thickness of 20 μm, dried at 130° C., pressed to a thickness of about 60 μm, and then punched into a circle having a diameter of 13 mm to obtain an electrode density. A positive electrode of 3.8 to 3.9 g/cc was produced.
<Preparation of negative electrode>
A counter electrode was prepared by punching metal lithium having a thickness of 0.3 mm into a circle having a diameter of 14 mm.
<Electrolytes>
An electrolyte (liquid) in which ethylene carbonate:dimethyl carbonate:diethyl carbonate was mixed at a ratio of 1:2:1 and 1 mol of LiPF ₆ was dissolved was used.
<Preparation>
The electrolytic solution was put between the positive electrode and the negative electrode via a polymer separator to produce 2016 type coin cells, which were batteries of Examples 1-1 to 2-3, respectively.

[Comparative example]
A battery was prepared by a configuration and procedure according to Example 1-1, except that a positive electrode active material (LiCoO ₂ ) which was not subjected to the positive electrode active material preparation 1 or 2 was used, and was used as a comparative battery.

[Evaluation test 1: Capacity maintenance evaluation]
(1) Regarding the coin batteries of Examples and Comparative Examples, in a constant temperature bath kept at a temperature of 25° C. and 45° C., a voltage range was set to 4.55V-3V, a current rate of charging 0.5C and discharging 1C. The charging/discharging was repeated 50 times. The C rate was defined as 1C=190 mAh. The results were as described in [Table 1] and [Fig. 1] to [Fig. 4].
(2) Using the comparative example (uncoated) as a reference (100), the initial discharge capacity, the initial efficiency, and the 2C/0.1C rate characteristic ratio in the 25°C charge/discharge measurement of each example were measured. The results were as described in [Table 2].
(3) In Example 1-1, the amount of treatment with AlF ₃ was 0.1, 0.2, 0.3, and 0.4% by weight with respect to LiCoO ₂ , and the charge and discharge cycle of (1) above was used. The test was conducted. The results were as described in [Fig. 3].

[Evaluation result 1-1]
Table 1, 25 ^th or 20 ^th, and shows the capacity retention rate at 50 ^th or 40 ^th. In the comparative example, when the 25 ^th or 20 ^th equivalent to 50% of all cycles and the 50 ^th or 40 ^th maintenance rate decrease rate at the end of the cycle are compared, it is confirmed that the cycle deterioration is accelerated after 25 ^th. It was Further, in the high temperature 45° C. test, it was confirmed that the deterioration degree was significantly advanced in the comparative example. On the other hand, in the examples, the discharge capacity exhibited stable cycle characteristics, and the capacity retention ratios at the end of the cycle were all close to 90% in a series of tests.

[Evaluation result 1-2]
Table 2 shows the initial discharge capacity, the initial efficiency, and the 2C/0.1C rate characteristic ratio in the 25° C. charge/discharge measurement of each example with Comparative Example 100. In all items of the initial discharge capacity, the initial efficiency, and the 2C/0.1C rate characteristic, the values were almost the same as those of the comparative example. From this, by using the atomic bond group formation step described in the present invention, while suppressing the effect on the resistance increase and the redox reaction field decrease due to the atomic bond group, while suppressing the electrolyte and the surface reaction and stable cycle characteristics. Can be obtained.

[Evaluation result 1-3]
FIG. 1 shows the cycle characteristics of Example 1-1 and Comparative Example when charge and discharge were repeated 50 times at a cutoff of 4.55-3.0 V at 25° C. In addition, as a reference example, the result of repeating charging and discharging at a charge upper limit of 4.50 V is also shown in FIG. 1 as a reference example. In Example 1-1, the capacity retention ratio at the 50th cycle was 96.0%, whereas in the comparative example, it was 83.5%, which was clearly low.
In Example 1-1, stable cycle characteristics in which the discharge capacity gradually decreased monotonically were obtained, but in Comparative Example, the cycle rapidly deteriorated after the 35th cycle. Further, in the 4.50 V charge cycle of the reference example described together, even in the example in which the atom bonding group was not exhibited, the cycle characteristics of 96% were exhibited at the 50th cycle. From this fact, there is an intrinsic factor that causes cycle deterioration, such as that the reaction between the electrolytic solution and the surface of the active material becomes dominant over redox over 4.50 to 4.55 V, and it is probable that the electrolyte and the surface of the positive electrode active material are It is considered that since the reaction proceeded, deposits were accumulated on the surface of the positive electrode or the negative electrode to increase the surface resistance. From this, it is considered that in the positive electrode active material according to the present invention, the reaction between the surface and the electrolyte is predominantly suppressed, and as a result, the cycle characteristics are improved.

[Evaluation result 1-4]
FIG. 2 shows cycle characteristics at 45° C. for Examples 1-1 to 1-3 and Comparative Example.
In Examples 1-1 to 1-3, the capacity retention rates at the 50th cycle were 92.5%, 89.8%, and 89.8%, respectively, and Example 1-1 exhibited the highest cycle characteristics. Indicated. In addition, in these examples, the unstable cycle deterioration behavior observed in the comparative example was not observed, and it was confirmed that stable charge/discharge was repeated throughout the cycle.
On the other hand, in Comparative Example, significant cycle deterioration was observed even when compared with the 25° C. cycle test, and the capacity retention ratio at the 50th cycle was 77.9%, which was extremely low compared to the Examples. .. In addition, the obtained charge capacity value did not vary in each cycle and did not simply deteriorate, and unstable cycle behavior and cycle deterioration behavior were clearly confirmed.

[Evaluation result 1-5]
FIG. 3 shows changes in capacity retention rate with respect to the amount of AlF ₃ treatment in the 50th cycle. The capacity retention rate at the 50th cycle obtained was improved by 10% or more under all conditions, compared to the capacity retention rate of 77.9% of the comparative example. Among them, 0.2% treatment amount of AlF ₃ was adopted. The obtained Example 1-1 showed the highest value.

[Evaluation result 1-6]
FIG. 4 shows cycle characteristics at 45° C. of Examples 2-1 to 2-3 and Comparative Example. In Examples 2-1 to 2-3, the capacity retention rates at the 50th cycle were 90.5%, 90.8%, and 88.2%, respectively, and after the sintering process at 500° C. for 2 hours. Among the non-existing examples, Example 2-3 showed the highest value.

[Evaluation test 2: Surface analysis]
The prepared positive electrode active material was measured using XPS (manufactured by Kratos, Axis-NOVA X light source Monochromated-Al-Kα (1486.6 eV); measurement depth: 5 nm), and Examples 1-1 and 1- The surface states of Co2p, Al2p, and F1s of the positive electrode active materials were measured by XPS using 3, Example 2-2, and Comparative Example.

[Evaluation test 3: Peak area ratio S]
A semi-quantitative value was calculated from the CoF peak appearing at 780.5-779.5 eV of the obtained Co 2 P3/2 spectrum and the peak area ratio S(CoF)/S(CoO) of CoO appearing at 781-783 eV.

[Evaluation result 2-1 and evaluation result 3: Co2p]
FIG. 5 shows a Co2p spectrum comparison of the surface states of the positive electrode active materials in Example 1-1, Example 1-3, Example 2-2 and Comparative Example. FIG. 6 shows the result of separating the Co2p2/3 spectrum by each component. Table 3 shows the abundance ratios for the respective peak components.

In general, the peak around 779.5 eV is derived from Co ³⁺ bound to oxygen, and the peak around 780.3 eV is derived from Co ²⁺ bound to oxygen. A separation was performed. From the past knowledge, Co ³⁺ can be derived from LiCoO ₂ and Co ²⁺ can be derived from CoO.
As a result, it was found that about 28.39% of Co on the surface of the positive electrode active material in the comparative example was present as Co ³⁺ and about 71.61% was present as Co ²⁺ . In Example 1-1, a peak derived from the F—Co ³⁺ bond was confirmed near 782.0 eV, and the abundance ratio was 10.30%. On the other hand, the ratio of Co ³⁺ derived from LiCoO ₂ is remarkably reduced at 5.33%, but this is because the oxygen bonded to the Co octahedron forming LiCoO ₂ on the surface is partially replaced by fluorine, This is because the -F bond is formed. In comparison, the proportion of Co ²⁺ component increased to 84.36%. On the surfaces of the positive electrode active materials in Examples 1-3 and 2-2, 782.6 eV derived from F—Co ²⁺ was present, and the ratios thereof were 5.39% and 7.12%. On the other hand, the proportion of Co ³⁺ derived from LiCoO ₂ decreased to 16.09% and 22.28%, but the reduction amount was small as compared with Example 1-1, and the peak attributed to F-Co ²⁺ was obtained. From the fact that is observed, it can be considered that O in the Co—O bond derived from CoO is replaced by fluorine and at the same time, oxygen in LiCoO ₂ is partially replaced by fluorine. From this, it is considered that fluorine substitution is possible also in Examples 1-3 and 2-2, but the atom-bonding group is formed in Example 1-1 in the most preferable state.

As reference data, FIG. 7 shows a powder diffraction line pattern of the AlF ₃ powder before (a) before and (b) at 110° C. to 150° C. with stirring. Regarding the AlF ₃ powder before and after stirring to dryness at 110° C. to 150° C., the AlF ₃ diffraction line pattern did not change, and no diffraction line corresponding to aluminum oxide could be confirmed. Therefore, although aluminum fluoride does not release fluorine by simple heating or sol formation at 110°C to 150°C, by mixing LiCoO ₂ with AlF ₃ sol and stirring at 110°C to 150°C, F It is considered that and Co react with each other to form a Co—F bond.

[Evaluation result 2-2: Al2p)
FIG. 8 shows comparison of Al2p spectra of Example 1-1, Example 1-3, Example 2-2 and Comparative Example. The Al compound names and binding energies shown in the figures are those extracted from generally known databases, such as NIST X-ray Photoelectron Spectroscopy Database. The peak around 71.0 eV confirmed in the Al2p spectra of all the samples can be assigned to the Co3p satellite component. On the other hand, a broad peak was confirmed at 75-71 eV. Considering the spread and symmetry of the peaks, it is predicted that a plurality of peaks are mixed. First, the peak derived from Al in Al ₂ O ₃ is Al ₂ O ₃ has a polymorphism, the Al2p spectrum over existing wide range of 73.0-74.0EV, largely by foundation and base for mounting Chemical shift. Further, since the peaks derived from the M (metal)-Al bond can be confirmed within the same range, it can be inferred that the peaks are attributed to Al-O bond, Al-Co bond, and Al-Li bond.

Further, the peak of 74.3 eV, which is specifically observed in the Al2p spectrum of Example 2-2, can be derived from the AlF ₃ peak. Therefore, it is considered that AlF ₃ is present as an atomic bonding group in Example 2-2 that has not undergone the sintering step. Further, the broad peaks observed from 73 eV to 76 eV can be inferred to be AlOOH and Al halides from the values in the database. This is because when a part of fluorine is bonded to Co at the time of stirring at 110 to 150° C., a pair of lost Al has a bond different from AlF ₃ (AlOF _(x) , Al _x F _y ), or a part of it. It can be considered that it is due to transfer to boehmite (AlOOH).

[Evaluation result 2-3: F1s]
FIG. 9 shows comparison of F1s spectra of Example 1-1, Example 1-3, Example 2-2 and Comparative Example. The peak near 687.1 eV in the F1s spectrum of Example 2-2 can be assigned to the AF component derived from AlF ₃ . This result is a result that is correlated with the Al2p peak described above. On the other hand, in the attribution of the F1s peak near 685.0 eV, there was one peak due to the symmetry of the F1s peak, and it was confirmed from the peak separation result of the Co2p spectrum that Co—F composition was obtained. The peak at about 685.0 eV in Example 1-3 and Example 2-2 is considered to be derived from F bound to Co.

Claims (10)

A positive electrode active material for a secondary battery,
Chemical formula: Li _x CoO ₂ (0.1<x<1.0)
The lithium cobalt oxide has at least a Co—F bond and a Co—O bond as an atomic bonding group,
By X-ray photoelectron spectroscopy (XPS),
A Co2p3/2 peak for the Co-F bond appears at 780.5-779.5 eV, and
A positive electrode active material for a secondary battery, wherein a Co2p3/2 peak for the Co-O bond appears at 781-783eV. 2. The peak area ratio S(CoF)/S(CoO) of the Co—O bond-derived Co2p3/2 peak to the Co—F bond-derived Co2p3/2 peak is 0.05 or more and 0.12 or less. 5. A positive electrode active material for a secondary battery according to. The atomic bond group according to claim 1 or 2, comprising one or more kinds selected from the group consisting of Al-F bond, Al-O bond, and Al-Co bond. Positive active material for secondary battery. The Al atom constituting the atomic bonding group is present in an amount of 0.03% by mass or more and 0.35% by mass or less based on the total mass of the positive electrode active material for a secondary battery. Positive electrode active material for secondary batteries. The positive electrode for a secondary battery according to claim 1, wherein the atomic bonding group is derived from a constituent atom of the lithium cobalt oxide and/or a constituent atom of aluminum fluoride (AlF ₃ ). Active material. The positive electrode active material for a secondary battery according to claim 1, wherein the Co atom in the Co—F bond is trivalent or divalent. The positive electrode active material for a secondary battery according to claim 1, wherein the Co atom in the Co—F bond is trivalent. The positive electrode,
A positive electrode comprising a current collector and the positive electrode active material for a secondary battery according to claim 1. A secondary battery,
A positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode,
A secondary battery in which the positive electrode is as defined in claim 8. A method for producing a positive electrode active material for a secondary battery according to any one of claims 1 to 7,
Aluminum fluoride (AlF ₃ ) and a polar solvent are mixed to prepare an aluminum fluoride sol composition,
Lithium cobalt oxide represented by the chemical formula is treated with the aluminum fluoride sol composition to prepare a positive electrode active material precursor for a secondary battery,
Firing or drying the positive electrode active material precursor for the secondary battery,
A production method comprising forming at least a Co—F bond and a Co—O bond as an atomic bonding group on the surface of the lithium cobalt oxide.