JP6634648B2 - Lithium ion secondary battery - Google Patents

Description

  The present invention relates to a lithium ion secondary battery.

  In recent years, the realization of large-capacity and high-efficiency secondary batteries is expected. As one of the secondary batteries that can meet such demands, a lithium transition metal oxide is used as a positive electrode active material, and an electrolyte is used as an electrolyte. A lithium ion secondary battery using a non-aqueous electrolyte is known. This lithium ion secondary battery has excellent features such as a higher voltage, a higher energy density, and a higher Coulomb efficiency than other secondary batteries such as a lead storage battery.

The lithium ion secondary battery has a structure in which, for example, a positive electrode is disposed separately from a negative electrode via a separator inside a battery container containing a nonaqueous electrolyte in which a lithium salt such as LiPF ₆ is dissolved. ing. Each of the positive electrode and the negative electrode is configured such that a current collector is coated with an electrode active material (a positive electrode active material or a negative electrode active material).

As a positive electrode active material used for a positive electrode of a lithium ion secondary battery, lithium iron oxide such as lithium cobalt oxide (LiCoO ₂ ) and lithium manganate (LiMn ₂ O ₄ ) and lithium iron phosphate ( Lithium phosphate salts such as LiFePO ₄ ) have been put to practical use.
Further, a positive electrode active material composed of an element-substituted lithium transition metal composite oxide in which a part of a lithium transition metal oxide is substituted with another metal element has been put to practical use (for example, see Patent Document 1). .

As the lithium-transition metal composite oxide used for the positive electrode active material, for _{example, LiNi x Co y O 2 (} x + y = 1), LiNi x Co y Al z O 2 (x + y + z = 1) other, LiNi _{1 / 3} Co _1/3 Mn _1/3 O _{2 and the} like have been put to practical use. It is known that the capacity density (discharge capacity) of such a positive electrode active material comprising a lithium transition metal composite oxide is about 150 (mAh / g).

Japanese Patent No. 5451228

Here, lithium nickel oxide (LiNiO ₂ ) has a discharge capacity of 200 (mAh / g) or more. When used alone as a positive electrode active material, a large discharge capacity is obtained, but the crystal structure is unstable. Therefore, it has been difficult to put it to practical use. On the other hand, a part of nickel in lithium nickelate is, for example, an element-substituted positive electrode active material in which cobalt, aluminum or manganese is substituted, since the crystal structure is stable at a certain degree, the discharge capacity is several Ah or less. Although practically used for small batteries, practical use was difficult for large batteries having a discharge capacity of 10 Ah or more.

  In addition, as described above, the positive electrode active material made of a lithium nickelate-based material generally has a short life and has a high oxygen release rate and a large amount of release at high temperatures because it is difficult to stabilize the crystal structure. Therefore, there is a problem that safety is poor.

  The present invention has been made in view of the above problems, and provides a lithium ion secondary battery having a high discharge capacity applicable to a large battery, an excellent capacity retention rate, a long life, and excellent safety. The purpose is to:

  The present inventors have conducted intensive studies on the composition of the positive electrode active material used for the positive electrode of the lithium ion secondary battery in order to solve the above problems. As a result, in the lithium nickelate-based compound used for the positive electrode active material, it is essential that part of nickel is substituted with cobalt, aluminum, and niobium, and in particular, by adopting a configuration in which aluminum is a substitution element, the discharge capacity is increased. Was found to increase, and the capacity retention ratio was improved. Thereby, for example, it was found that excellent discharge characteristics were obtained in a large-sized lithium ion secondary battery having a discharge capacity of 10 Ah or more, and the present invention was completed.

That is, according to the lithium ion secondary battery of the present invention, at least a lithium ion secondary battery including a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and a non-aqueous electrolyte. the positive electrode, examples of the positive electrode active material, at least the general _{formula: LiNi w Co x Al y Nb} z O 2 ( where, in the formula, be 0 <w, 0 <x, 0 <y, 0 <z And w + x + y + z = 1) , and cobalt (Co), aluminum (Al), niobium (Nb), and cobalt (Co) in the lithium transition metal composite oxide. The mass ratio of nickel (Ni) is as follows: Co: 10 to 20% by mass, Nb: 0.5 to 1.5% by mass, Al: 3 to 10% by mass, Ni: balance (however, Co, Al, Nb and Ni 100% in total In a), characterized in der Rukoto.

  According to the lithium ion secondary battery having such a configuration, the discharge capacity can be reduced by using a lithium transition metal composite oxide in which part of Ni in the lithium nickelate-based compound is substituted by Co, Al, and Nb as the positive electrode active material. As the capacity increases, the capacity retention rate also increases. More specifically, first, the initial discharge capacity is increased by partially replacing Ni with Co and Nb. Further, the capacity retention ratio is improved by partially replacing Ni with Al. As described above, the detailed mechanism for improving the discharge characteristics of the lithium ion secondary battery is unknown, but the flexibility of the crystal structure can be improved by partially replacing Ni with the above metal elements (Co, Nb, etc.). It is considered that the initial capacity is increased by increasing the number of ions and facilitating insertion and desorption of Li ions. Further, by adding Al element as a substitution element, since the bonding force between Al element and oxygen is high, the crystal structure of the lithium transition metal composite oxide is more stabilized, and the collapse of the crystal structure is suppressed. It is considered that this is because the life of the positive electrode active material is improved. Further, since the crystal structure of the positive electrode active material is stabilized, the amount of oxygen released at a high temperature is suppressed, and the safety of the battery is improved.

Further, according to the lithium ion secondary battery having such a configuration, by setting the mass ratio of each metal element in the lithium transition metal composite oxide used for the positive electrode active material to the above range, the crystal structure as described above is obtained. A stable action can be obtained more effectively. Accordingly, the discharge characteristics are further improved, and the life characteristics and safety are further improved.

Further, in the lithium ion secondary battery having the above configuration, lithium difluorophosphate (LiPF ₂ O ₂ ) and lithium bis (oxalate) borate are added as additives to a non-aqueous electrolyte in which a lithium salt such as LiPF ₆ is dissolved. (Li [B (C ₂ O ₄ ) ₂ ]).

  According to the lithium ion secondary battery having such a configuration, first, since lithium difluorophosphate is added to the non-aqueous electrolyte, the additive and the negative electrode surface effectively react, and the negative electrode surface is stable. A film (SEI film: Solid Electrolyte Interface film) is formed. The formation of the SEI film facilitates the insertion / removal of Li ions in the negative electrode, suppresses the decomposition of the electrolytic solution itself on the negative electrode surface, increases the capacity retention ratio, and improves the battery life characteristics. . Further, by adding lithium bis (oxalate) borate to the non-aqueous electrolyte, a more stable SEI film is formed, and the life characteristics of the battery are improved. Therefore, the effect of improving the discharge characteristics and the life characteristics of the lithium ion secondary battery can be obtained.

  Further, in the lithium ion secondary battery having the above-described structure, if the thickness of the SEI film formed on the surface of the negative electrode is too small, the effect of suppressing the decomposition of the electrolytic solution becomes small. It is thought that there is an optimum addition amount for forming an optimum film thickness because this causes an increase. Therefore, the content of the lithium difluorophosphate in the nonaqueous electrolyte is 0.7 to 1.3% by mass, and the content of the lithium bis (oxalate) borate is 0.3 to 0%. More preferably, it is 0.7% by mass.

  According to the lithium ion secondary battery having such a configuration, by limiting the content of each additive in the nonaqueous electrolyte to the above range, the effect of improving the discharge characteristics and the life characteristics as described above is more remarkable. Is obtained.

According to the lithium ion secondary battery of the present invention, part of Ni in the lithium nickel oxide-based compound is substituted Co, the Al and Nb, the general _{formula: LiNi w Co x Al y Nb} z O 2 ( where formula Wherein 0 <w, 0 <x, 0 <y, 0 <z, and w + x + y + z = 1 are satisfied) as the positive electrode active material. And the energy density of the positive electrode active material is improved. As a result, the discharge capacity is increased, the capacity retention rate is also improved, and the safety of the battery is also improved, for example, the amount of oxygen released at high temperatures is suppressed. Therefore, for example, a lithium ion secondary battery applicable to a large battery having a discharge capacity of 10 Ah or more and having excellent discharge characteristics, life characteristics, and safety can be realized.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows typically an example of the lithium ion secondary battery which concerns on this invention, and is an exploded perspective view which shows a battery external appearance and component structure. FIG. 4 is a diagram illustrating an example of a lithium ion secondary battery according to the present invention, and is a graph showing the capacity density (discharge capacity) of the battery when the composition of a lithium transition metal composite oxide used for a positive electrode active material is changed. It is. It is a figure explaining the Example of the lithium ion secondary battery which concerns on this invention, and is a graph which shows the capacity | capacitance retention of a battery when the composition of the lithium transition metal composite oxide used for a positive electrode active material is changed. FIG. 4 is a diagram illustrating an example of a lithium ion secondary battery according to the present invention, and is a graph illustrating a capacity retention ratio of the battery when the composition of an additive added to a non-aqueous electrolyte is changed. FIG. 3 is a diagram illustrating an example of a lithium ion secondary battery according to the present invention, and shows a cycle capacity deterioration rate when an accelerated deterioration cycle test is performed by changing the composition of an additive added to a non-aqueous electrolyte. It is a graph.

  Hereinafter, a lithium ion secondary battery according to an embodiment of the present invention will be described in detail with reference to the drawings as appropriate. In the drawings used for the description, dimensions and scales of structures in the drawings may be different from actual structures in order to clearly show characteristic portions. In the present embodiment, the same components are denoted by the same reference numerals, and detailed description thereof may be omitted.

[Configuration of lithium ion secondary battery]
FIG. 1 is a perspective exploded view showing one embodiment of a lithium ion secondary battery.
The lithium ion secondary battery 1 illustrated in FIG. 1 includes a battery container 10 for storing a non-aqueous electrolyte containing a lithium supporting salt (non-aqueous electrolyte) such as LiPF ₆ therein. The battery container 10 is, for example, a hollow container made of aluminum. In the present embodiment, the outer shape is a substantially prismatic shape (a substantially rectangular parallelepiped shape). Further, the battery container 10 includes a cylindrical body 10a having an opening, and a lid 10b for closing the opening of the cylindrical body 10a.

  The lid 10 b provided in the battery container 10 is provided with electrode terminals 11 and 12 and an electrolyte inlet 13. In the present embodiment, the electrode terminal 11 is a positive terminal and the electrode terminal 12 is a negative terminal. Will be described. Further, inside the battery case 10, a plurality of electrodes 14 and 15 and a separator 16 arranged between the plurality of electrodes 14 and 15 are housed.

  The electrodes (positive electrode, negative electrode) 14 and 15 are usually made of a sheet-like current collector (conductor) such as a conductive foil or a conductive thin plate as a base material, and the surface of the base material is formed of an electrode active material corresponding to the type of the electrolytic solution. It is configured with a coating. The electrode 14 is a positive electrode in the present embodiment, and a positive electrode active material made of a lithium transition metal composite oxide in which a part of a metal element in a lithium nickelate-based compound is replaced with another metal element is a positive electrode made of aluminum. It is formed as a positive electrode active material layer on a current collector. The electrode 15 is a negative electrode in the present embodiment. For example, a negative electrode active material is provided on a negative electrode current collector made of a copper foil or the like.

  The positive electrode (electrode) 14 is disposed so as to face the negative electrode (electrode) 15, and these electrodes 14 and 15 are disposed so as to be repeated in the direction facing each other. Separators 16 are provided between the electrodes 14 and 15, respectively, so that the electrodes 14 and 15 do not contact each other. As the separator 16, for example, an insulating material such as a porous polymer material film such as polyethylene or polypropylene, glass fiber, or a nonwoven fabric made of various polymer fibers is used.

An electrode tab 14a is formed at an end of the positive electrode 14 on the positive electrode terminal (electrode terminal) 11 side. Electrode tabs 14a of a plurality of positive electrodes 14 arranged repeatedly are collectively electrically connected to positive electrode terminal 11.
An electrode tab 15a is formed at an end of the negative electrode 15 on the negative electrode terminal (electrode terminal) 12 side. The electrode tabs 15a of the plurality of negative electrodes 15 arranged repeatedly are collectively electrically connected to the negative electrode terminal 12.

As described above, the positive electrode 14 includes the positive electrode current collector and the positive electrode active material layer (positive electrode active material) provided on the positive electrode current collector. In the lithium ion secondary battery 1 according to the present invention, the cathode active material layer constituting the positive electrode 14, at least the general _formula: in _{LiNi w Co x Al y Nb z} O 2 ( In the formula, 0 <w, (0 <x, 0 <y, 0 <z, and w + x + y + z = 1)) is used. The positive electrode active material layer of the positive electrode 14 is formed into a film by mixing, for example, a conductive additive powder and a binder in addition to the positive electrode active material described above.

The positive electrode active material provided as the positive electrode active material layer in the positive electrode 14 is, for example, part of a metal element in lithium nickel oxide (LiNiO ₂ ), specifically, part of Ni is replaced by Co, Al, and Nb. it is the general _{formula: LiNi w Co x Al y Nb} z O 2 ( where, in the formula, 0 <w, is 0 <x, 0 <y, 0 <z, and satisfies w + x + y + z = 1) in It is obtained as a lithium transition metal composite oxide represented. By adopting the above-mentioned lithium transition metal composite oxide as the positive electrode active material, it is essential that a part of Ni is replaced by Co, Al and Nb, and in particular, by adopting a structure in which Al is a substitution element, As the discharge capacity increases, the capacity retention rate also increases. More specifically, first, since a part of Ni in the above-mentioned lithium transition metal composite oxide is substituted by Co and Nb, the flexibility of the crystal structure is increased, and insertion and desorption of Li ions are facilitated. The effect of increasing the initial discharge capacity is obtained. Furthermore, since a part of Ni in the lithium transition metal composite oxide is substituted by Al, the bonding force between the Al element and oxygen is high, so that the crystal structure of the lithium transition metal composite oxide is more stabilized, It is considered that the effect of improving the capacity retention ratio is obtained by suppressing the collapse of the crystal structure.

  As described above, the detailed mechanism by which the discharge characteristics of the lithium ion secondary battery 1 are improved by employing the above lithium transition metal composite oxide as the positive electrode active material is unknown. However, the present inventors have conducted intensive studies and found that the crystal structure of the lithium-transition metal composite oxide is stabilized by partially replacing Ni with Co, Al and Nb, and the energy of the positive electrode active material is reduced. It is considered that the density is improved and the discharge characteristics of the lithium ion secondary battery 1 are improved.

  According to the lithium ion secondary battery 1 according to the present invention, the discharge capacity is increased by the above configuration, so that it can be applied to, for example, a large secondary battery having a discharge capacity of 10 Ah or more. In addition, the life characteristics are improved by increasing the capacity retention ratio, and the stability of the crystal structure of the positive electrode active material is suppressed, so that the amount of oxygen released at a high temperature is suppressed, thereby improving the safety of the battery. Therefore, it is possible to realize the lithium ion secondary battery 1 having excellent discharge characteristics and also excellent life characteristics and safety.

  Note that the positive electrode active material using the lithium transition metal composite oxide represented by the above general formula has a mass ratio of cobalt (Co), aluminum (Al), niobium (Nb), and nickel (Ni) of Co: 10 to 20% by mass, Nb: 0.5 to 5% by mass, Al: 3 to 10% by mass, Ni: The balance (however, the total of Co, Al, Nb and Ni is 100%). preferable. By setting the mass ratio of Co, Al, Nb and Ni in the lithium transition metal composite oxide within the above range, the crystal structure is more stabilized, the discharge capacity is increased, and the capacity retention rate is more effectively improved. As a result, the life characteristics and safety are also improved.

Co is an element whose crystal structure is stable and has a large effect on the battery life. To obtain such an effect, the lower limit of the mass ratio is preferably set to 10% by mass. On the other hand, since Co has a possibility of lowering the electric capacity as compared with Ni, it is considered that setting the upper limit of the mass ratio to 20% by mass is advantageous from the viewpoint of securing the discharge capacity.
Further, the mass ratio of Co is more preferably 12 to 18% by mass, and most preferably 14 to 16% by mass.

From the results of the cycle test (see the graph of FIG. 3) in the examples described later, Al has a small effect on the charge / discharge capacity, but has a large effect on the battery life characteristics due to the deterioration of the crystal structure. Conceivable. For this reason, the lower limit of the mass ratio is preferably set to 4% by mass from the viewpoint of suppressing deterioration of the crystal structure and improving battery life characteristics of Al. On the other hand, if the mass ratio of Al exceeds 10% by mass, the discharge capacity may be reduced. Therefore, it is preferable to set the upper limit.
Further, the mass ratio of Al is more preferably 3 to 6% by mass, and most preferably 3 to 5% by mass.

Nb is an element having a function of suppressing the amount of heat generated when the lithium transition metal composite oxide generates heat by adding a small amount thereof, and is considered to be effective in improving the safety of the secondary battery. In order to obtain the effect, the lower limit of the mass ratio is preferably set to 0.5% by mass. On the other hand, since Nb is a very expensive metal element, the upper limit of the mass ratio is preferably set to 5% by mass in consideration of cost.
Further, the mass ratio of Nb is more preferably 0.5 to 2% by mass, and most preferably 0.5 to 1.5% by mass.

  In addition, the range of Ni in the above mass ratio is the remainder when the total of Co, Al, Nb and Ni is 100%.

In addition, as the conductive additive powder added to the positive electrode active material layer (positive electrode active material), for example, a carbonaceous material such as carbon black, acetylene black, graphite, and carbon fiber can be used.
As the binder, for example, polyvinylidene fluoride, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, tetrafluoroethylene-hexafluoroethylene copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroethylene- Perfluoroalkyl vinyl ether copolymer, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene copolymer, ethylene-tetrafluoroethylene copolymer, and the like. Species or a combination of two or more can be used.

  Further, a conductive polymer material such as polyaniline, polypyrrole, polythiophene, or polyimidazole may be added to the positive electrode active material layer. Since these conductive polymer materials are electrochemically stable and have excellent electron conductivity, the effect of reducing the internal resistance of the positive electrode active material layer can be obtained.

  The positive electrode current collector used for the positive electrode 14 is not limited to the above-described aluminum, and may be a metal foil, a metal net, an expanded metal, or the like made of another metal material. May be used.

  The negative electrode 15 includes a current collector made of a copper foil or the like, and a negative electrode active material layer formed on the current collector. The negative electrode active material layer includes, for example, a negative electrode active material powder such as graphite that electrochemically deintercalates lithium, a binder such as polyvinylidene fluoride, and an additive that forms an SEI film on the surface of the negative electrode active material layer. Are mixed and formed. In addition, part or all of the additives react to form an SEI film on the surface of the negative electrode active material. Note that a conductive auxiliary powder such as carbon black may be added to the negative electrode layer.

  As the negative electrode active material, in addition to graphite, various carbon materials such as coke, pyrolytic carbon, glassy carbon, amorphous carbon, graphitized carbon fiber, fired bodies of various polymer materials, mesocarbon microbeads, activated carbon, etc. Can be used. Alternatively, an alloy of Si, Sn, In, or the like, which can be charged and discharged at a relatively low potential, or a metal oxide / nitride may be used.

The binder used in the negative electrode active material layer is not particularly limited, and examples thereof include polymers such as polyvinylidene difluoride (PVDF), carboxymethyl cellulose, hydroxypropyl cellulose, regenerated cellulose, diacetyl cellulose, and sulfonated EPDM. And these can be used alone or in combination of two or more.
In addition, the additive is not particularly limited as long as it can form an SEI film on the surface of the negative electrode active material layer. For example, in addition to vinylene carbonate, a conventionally known material can be used without any limitation. is there.

A non-aqueous electrolyte (non-aqueous electrolyte) is accommodated in the battery container 10 so as to be in contact with each of the electrodes 14 and 15. As the non-aqueous electrolyte, for example, a lithium salt such as lithium hexafluorophosphate (LiPF ₆ ) or lithium tetrafluoroborate (LiBF ₄ ) is dissolved in an organic solvent such as ethylene carbonate, diethyl carbonate, or propyl carbonate. And the like.

  Further, the non-aqueous electrolyte is not limited to the above, and for example, one having one or both of a cyclic carbonate and a chain carbonate and further containing a solute is employed. It is possible. In particular, those obtained by dissolving a solute (electrolyte) in an organic solvent in which both the cyclic carbonate and the chain carbonate are mixed are preferable.

Examples of the cyclic carbonate include conventionally known materials such as propylene carbonate, and examples of the chain carbonate include known materials such as dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC). Further, the cyclic carbonate and the chain carbonate are not limited to those listed above, but may be any as long as the solute and the quaternary ammonium salt described below can be dissociated.
In addition, as the organic solvent used for the non-aqueous electrolyte, a mixed solvent of two or more organic solvents can be used.

In addition, as a solute (electrolyte), LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiCF ₃ CO ₂ , Li ₂ C ₂ F ₄ (SO ₃ ) ₂ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiN (RfSO ₂ ) ₂ , LiC (RfSO ₂ ) ₃ , Examples thereof include lithium salts such as LiCnF ₂ n ⁺¹ SO ₃ (n ≧ 2) and LiN (RfOSO ₂ ) ₂ [where Rf is a fluoroalkyl group].

Further, in the lithium ion secondary battery 1 according to the present invention, lithium difluorophosphate (LiPF ₂ O ₂ ) represented by the following chemical formula 1 and lithium bismuth represented by the following chemical formula 2 are added to the nonaqueous electrolyte as additives. More preferably, (oxalate) borate (Li [B (C ₂ O ₄ ) ₂ ]) is contained.

  In the lithium ion secondary battery 1 according to the present invention, first, since lithium difluorophosphate is added to the nonaqueous electrolyte, the additive effectively reacts with the surface of the negative electrode active material layer, A film (SEI film: Solid Electrolyte Interface film) is formed on the surface of the negative electrode active material layer in the negative electrode 15. The formation of this SEI film facilitates the insertion and removal of Li ions in the negative electrode active material layer, increases the capacity retention ratio, and improves the battery life characteristics. Further, by adding lithium bis (oxalate) borate to the non-aqueous electrolyte, a more stable SEI film is formed, and the life characteristics of the battery are improved. Therefore, the effect of improving the discharge characteristics and the life characteristics of the lithium ion secondary battery 1 can be obtained.

  The content of the additive contained in the non-aqueous electrolyte is not particularly limited, but in the non-aqueous electrolyte, if the thickness of the SEI film formed on the negative electrode surface is too thin, the effect of suppressing the decomposition of the electrolyte is reduced, Even if the thickness of the SEI film is too large, the internal resistance increases, so it is considered that there is an optimum addition amount for forming an optimum film thickness. From this, in the non-aqueous electrolyte, the content of lithium difluorophosphate is 0.7 to 1.3% by mass, and the content of lithium bis (oxalate) borate is 0.3 to 0.7%. It is preferable that the content is mass%. By setting the content of each additive in the non-aqueous electrolyte within the above range, the effect of improving the discharge characteristics and the life characteristics as described above is more remarkably obtained.

More specifically, when the content of lithium difluorophosphate contained in the nonaqueous electrolyte is in the above range, an SEI film is effectively formed on the surface of the negative electrode active material layer, and the life characteristics are improved. The effect is obtained. When the content of lithium difluorophosphate in the non-aqueous electrolyte is less than 0.7% by mass, the effect of improving the life is reduced. On the other hand, even when the content exceeds 1.3% by mass, the effect is not changed. .
Further, the content of lithium difluorophosphate contained in the nonaqueous electrolyte is more preferably 0.9 to 1.2% by mass, and most preferably 1.0 to 1.2% by mass. According to the study of the present inventors, the optimum content at which lithium difluorophosphate effectively reacts with the surface of the negative electrode active material to form an SEI film is about 1% by mass. It is possible (see also Examples below and the graph in Table 5).

Further, when the content of lithium bis (oxalate) borate contained in the nonaqueous electrolyte is in the above range, the deterioration of the crystal structure in the electrode material can be effectively suppressed. When the content of lithium bis (oxalate) borate in the non-aqueous electrolyte is less than 0.3% by mass, the effect of suppressing the deterioration of the crystal structure in the above-described electrode material becomes small, and the content exceeds 0.7% by mass. The same applies to the case where it is added.
Further, the content of lithium bis (oxalate) borate contained in the nonaqueous electrolyte is more preferably 0.4 to 0.7% by mass, and further preferably 0.5 to 0.6% by mass. From this, it is considered that the optimum content of lithium bis (oxalate) borate contained in the non-aqueous electrolyte is about 0.5% by mass (see also Examples described later and the graph of FIG. 5).

  In the present invention, a solid electrolyte (non-aqueous electrolyte) can be used instead of the above-mentioned non-aqueous electrolyte. Examples of such a solid electrolyte include a gel electrolyte obtained by impregnating a polymer matrix such as polyethylene oxide, polypropylene oxide, polyvinylidene fluoride, and polyacrylonitrile with the above nonaqueous electrolyte. When a solid electrolyte is used, the separator 16 can be omitted.

[Method of manufacturing lithium ion secondary battery]
Hereinafter, an example of a method for manufacturing a lithium ion secondary battery according to the present invention will be described with reference to FIG.
In manufacturing the lithium ion secondary battery 1 according to the present invention, first, a positive electrode active material is manufactured by the procedure described below, and then the positive electrode active material is applied on a positive electrode current collector. The positive electrode 14 is manufactured. Further, as described later, the lithium ion secondary battery 1 is manufactured by preparing and preparing a negative electrode, a non-aqueous electrolyte, and the like, respectively.

First, M (OH) ₂ (M = Ni, Co, Nb, Al) and LiOH are prepared and mixed as raw materials of the positive electrode active material. Specifically, nickel hydroxide, cobalt hydroxide, niobium hydroxide, and aluminum hydroxide, which are raw materials of the positive electrode active material, are mixed so that Ni, Co, Al, and Nb have a predetermined metal weight ratio. Mix with lithium hydroxide (LiOH).

Then, by baking the above mixture in an oxygen atmosphere at 700 to 800 ° C., a part of Ni derived from nickel hydroxide is replaced with Co, Al and Nb. Thus, the general _{formula: LiNi w Co x Al y Nb} z O 2 ( where, in the formula, 0 <w, is 0 <x, 0 <y, 0 <z, and satisfies w + x + y + z = 1) in A powdered positive electrode active material composed of the represented lithium transition metal composite oxide is obtained.

Further, the conductive active material powder and the binder are mixed with the positive electrode active material obtained by the above procedure to prepare a mixture slurry.
The conductive assistant is not particularly limited, and a material having conductivity such as the above-described carbon black can be appropriately selected and employed.
Further, as the binder, a fluoropolymer such as polyvinylidene fluoride as described above can be appropriately employed.

Next, the mixture of the positive electrode active material, the conductive additive, and the binder is further added to a dispersion medium such as NMP in which polyaniline is dissolved to form a slurry. Thereafter, the slurry is applied to the positive electrode current collector by a doctor blade method or the like, and then the dispersion medium is removed by heating for a predetermined time and dried. Then, the positive electrode 14 is manufactured by compressing the dried mixture by a press or the like. The heating temperature at this time is a temperature range equal to or higher than the boiling point of the dispersion medium contained in the slurry, or drying under reduced pressure.
According to such a procedure, the positive electrode 14 is manufactured.

Next, in the present embodiment, the negative electrode 15 can be formed by a conventionally known forming material or forming method appropriately selected. That is, for example, a graphite-based material is used as a negative electrode material, and an aqueous slurry in which the graphite-based material and the binder are mixed is prepared, and then this slurry is applied to the surface of a negative electrode current collector made of copper foil or the like. I do. Then, similarly to the case of the positive electrode 14, the negative electrode 15 is manufactured by drying at a predetermined temperature for a predetermined time.
And the positive electrode 14, the separator 16, and the negative electrode 15 are laminated | stacked sequentially, and these laminated bodies are formed.

Next, a cylindrical body 10a and a lid 10b that constitute the battery container 10 are prepared, and the stacked body including the electrodes 14, 15 and the separator 16 is accommodated in the cylindrical body 10a. The lid 10b is provided with electrode terminals 11, 12 and an inlet 13 in advance. Then, the plurality of electrode tabs 14a in the laminate are collectively electrically connected to the electrode terminals 11, and the plurality of electrode tabs 15a in the laminate are collectively electrically connected to the electrode terminals 12. Then, the lid 10b is attached to the tubular body 10a and welded.
Then, a nonaqueous electrolyte prepared with a predetermined composition is injected into the inside of the battery container 10 from the injection port 13, and then the injection port 13 is sealed.

Next, in the present embodiment, initial charge and discharge are performed. Specifically, charging control is performed by electrically connecting the electrode terminals 11 and 12 to a charger or the like and applying a voltage between the positive electrode 14 and the negative electrode 15.
As a charging method, for example, it is preferable to adopt a constant current / constant voltage method. That is, charging is performed at a constant current until the battery voltage reaches the charging upper limit voltage (constant current charging). After reaching the charging upper limit voltage, a current is supplied for a predetermined charging time while maintaining the upper limit voltage (constant charging). Voltage charging). The upper limit charging voltage at the time of constant current / constant voltage charging is preferably in the range of 4.1 to 4.2 V, though it depends on the positive and negative electrode configurations of the battery. Further, the charging current at the time of constant current charging depends on the battery size, but is 0.1 to 2 C (1 C is defined as a current value at which the whole battery capacity can be discharged or charged in one hour. In this case, 10 hours are required for charging). Further, the charging time during the constant voltage charging depends on the battery size, but is preferably within 10 hours.

  In addition, as a discharging method, it is preferable to employ constant current discharging. That is, the battery is discharged at a constant current until the battery voltage reaches the discharge lower limit voltage (constant current discharge). The discharge lower limit voltage is preferably in the range of 2.7 to 3.5 V, depending on the configuration of the positive and negative electrodes of the battery. Also, the discharge current is preferably in the range of 0.1 C to 2 C, depending on the battery size.

By performing the initial charge and discharge as described above, a chemical reaction occurs in the positive electrode active material of the positive electrode 14. When this reaction occurs, the positive electrode 14 fulfills its function.
Further, at the time of initial charge and discharge, a chemical reaction of the negative electrode active material occurs also in the negative electrode 15, and the negative electrode 15 performs its function as in the positive electrode 14.

[Effects]
As described above, according to the lithium ion secondary battery 1 according to the present invention, part of Ni in the lithium nickelate compound is substituted Co, the Al and Nb, the general _formula: LiNi w _Co x Al _y A lithium transition metal composite oxide represented by Nb _z O ₂ (where 0 <w, 0 <x, 0 <y, 0 <z, and w + x + y + z = 1 is satisfied) is used as a positive electrode active material. By using the material, the crystal structure is stabilized, and the energy density of the positive electrode active material is improved. As a result, the discharge capacity is increased, the capacity retention rate is also improved, and the safety of the battery is also improved, for example, the amount of oxygen released at high temperatures is suppressed. Therefore, for example, a lithium ion secondary battery applicable to a large battery having a discharge capacity of 10 Ah or more and having excellent discharge characteristics, life characteristics, and safety can be realized.

In addition, in the lithium ion secondary battery 1 described in the present embodiment, in addition to employing the above-described lithium transition metal composite oxide as the positive electrode active material, the lithium ion secondary battery 1 is further added to a non-aqueous electrolyte (non-aqueous electrolyte). By adopting a composition containing lithium difluorophosphate (LiPF ₂ O ₂ ) and lithium bis (oxalate) borate (LiBOB) as the agent, the capacity retention rate is further increased, and the effect of improving the battery life characteristics is obtained. can get.

  Note that the technical scope of the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the gist of the present invention. For example, in the above-described embodiment, the external shape is described by taking a lithium ion secondary battery having a substantially rectangular parallelepiped shape as an example, but a laminate type, a coin type, a cylindrical type, and a sheet type lithium ion secondary battery are also described. The present invention can be applied.

  Hereinafter, the lithium ion secondary battery according to the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

[Example 1]
In Example 1, first, a positive electrode active material was prepared according to the following procedure and conditions, and further, a positive electrode, a negative electrode, and a nonaqueous electrolyte (nonaqueous electrolyte) provided with the positive electrode active material were prepared. Then, by laminating each of these electrodes and the non-aqueous electrolyte in a laminate cell container, a laminate cell of a lithium ion secondary battery was completed, and an evaluation test described below was performed.

(Preparation of positive electrode active material and positive electrode)
First, nickel hydroxide, cobalt hydroxide, niobium hydroxide, aluminum hydroxide, and lithium hydroxide as raw materials of a positive electrode active material are shown in Table 1 below using a “planetary ball mill (P6 manufactured by Fritsch)”. The mixture was charged into a ball mill while adjusting the respective amounts so as to have a metal weight ratio, and mixed by dry stirring to prepare a preliminary mixture. At this time, the rotation speed of the ball mill was set to about 150 rpm, and stirring was performed for 3 hours.

Next, the preliminary mixture obtained by the above procedure is calcined in an oxygen atmosphere at 700 to 800 ° C., whereby a part of Ni derived from nickel hydroxide is replaced with Co, Al, and Nb, whereby lithium transition is performed. A powdery positive electrode active material comprising a metal composite oxide was obtained. This positive electrode active material is composed of LiNi _0.764 Co _0.143 Al _0.083 Nb _0.01 O ₂ in which the metal weight ratio of Co, Nb, Al and Ni is in the above range.

Next, acetylene black was added to the positive electrode active material as a conductive additive powder, and polyvinylidene fluoride was added as a binder to prepare a positive electrode mixture. Then, this positive electrode mixture was added to NMP as a dispersion medium to prepare a slurry, and then this slurry was applied on a positive electrode current collector made of an aluminum foil by using a doctor blade method. Thereafter, the dispersion medium contained in the slurry was removed by heating at a temperature of 120 ° C. for a certain period of time, and the positive electrode mixture was dried.
And the positive electrode was produced by press-compressing the dry positive electrode mixture laminated on the positive electrode current collector.

(Preparation of secondary battery structure (laminate cell))
Next, in this example, using the positive electrode obtained by the above procedure, in a conventionally known method, a laminate in which a positive electrode, a separator, and a negative electrode were sequentially laminated was housed and sealed in a laminate cell container. Then, a laminate cell having a secondary battery structure was produced.

The dimension of the positive electrode used in this case in plan view was 97 × 130 mm, and was provided with the positive electrode active material produced according to the above procedure and conditions.
The dimension of the negative electrode in plan view is 100 × 134 mm. In the present embodiment, using a conventionally known graphite-based negative electrode material, after preparing an aqueous slurry in which a graphite-based material and a binder are mixed, the slurry is applied to the surface of a negative electrode current collector made of copper foil, A negative electrode was manufactured by drying at a drying temperature and time conventionally applied to the preparation of the negative electrode.

Further, the electrolyte solution contains 1 M LiPF ₆ as a solute, a mixed solvent of ethylene carbonate (EC) / dimethyl carbonate (DMC) / ethyl methyl carbonate (EMC) as an organic solvent, and lithium difluorophosphate as an additive. LiPF ₂ O ₂ ) was prepared and used at 1% by mass. The mixing ratio of the organic solvent at this time was [(33.3 vol% (EC) /33.3 vol% (DMC) /33.4 vol% (EMC)].
A microporous polypropylene (Celgard) was used as a separator.

  After fabricating the above secondary battery structure, initial charge / discharge was performed at a current of 0.2 C up to 4.3 V to complete the lithium ion secondary battery of this example.

(Measurement of charge / discharge characteristics)
The charge / discharge characteristics of the lithium ion secondary battery of this example obtained according to the above procedure and conditions were measured and evaluated.
At this time, “ASCA charge / discharge system device ACD-01” was used as a measuring device, the measuring temperature was 25 ° C., the voltage range was 3.0 to 4.3 V, and the current density was 0.2 mA / cm _2. And

More specifically, first, the lithium ion secondary battery of this example was installed in a constant temperature bath set to a 55 ° C. atmosphere, and charging and discharging were repeated at a current of 1 C using a charging and discharging device ( Cycle test). At this time, the upper limit voltage is set to 4.3 V, the lower limit voltage is set to 3.0 V, and at the time of charging, CC-CV charging (charging with a current amount of 1 C, and after reaching 4.3 V, the current is reduced to a predetermined current value. At the time of the decrease, the charge was completed), and at the time of the discharge, the discharge was defined as CC discharge (discharge was performed at a current of 1 C, and the discharge was completed when the voltage became 3.0 V).
The measurement results of the initial discharge capacity (Ah / kg) are shown in the graph of FIG. 2, and the measurement results of the capacity retention ratio (%) with respect to the initial discharge capacity are shown in the graph of FIG.

[Comparative Example 1]
In Comparative Example 1, lithium hydroxide having a metal weight ratio shown in Table 1 above was used without using aluminum hydroxide as a raw material of the positive electrode active material and using only nickel hydroxide, cobalt hydroxide, niobium hydroxide, and lithium hydroxide. A laminate cell of a lithium-ion secondary battery was prepared in the same procedure and under the same conditions as in Example 1 except that a positive electrode active material was prepared so as to be a transition metal composite oxide, and a similar evaluation test was performed. .

That is, in Comparative Example 1, a lithium transition metal composite oxide represented by LiNi _0.8 Co _0.150 Nb _0.050 O ₂ was prepared as a positive electrode active material, and lithium having a positive electrode using the same was used. A laminate cell of an ion secondary battery was prepared and evaluated.
As in the above, the measurement results of the initial discharge capacity (Ah / kg) are shown in the graph of FIG. 2, and the measurement results of the capacity retention ratio (%) with respect to the initial discharge capacity are shown in the graph of FIG.

[Comparative Example 2]
Also in Comparative Example 2, aluminum hydroxide was not used as a raw material of the positive electrode active material, and only nickel hydroxide, cobalt hydroxide, niobium hydroxide, and lithium hydroxide were used. A laminate cell of a lithium-ion secondary battery was prepared in the same procedure and under the same conditions as in Example 1 except that a positive electrode active material was prepared so as to be a lithium transition metal composite oxide, and a similar evaluation test was performed. Was.

That is, in Comparative Example 2, a lithium transition metal composite oxide represented by LiNi _0.821 Co _0.149 Nb _0.030 O ₂ was prepared as a positive electrode active material, and a lithium including a positive electrode using the same was prepared. A laminate cell of an ion secondary battery was prepared and evaluated.
As in the above, the measurement results of the initial discharge capacity (Ah / kg) are shown in the graph of FIG. 2, and the measurement results of the capacity retention ratio (%) with respect to the initial discharge capacity are shown in the graph of FIG.

[Example 2]
In Example 2, as an additive in the non-aqueous electrolyte, lithium difluorophosphate was added to 1% by mass, and lithium bis (oxalate) borate (Li [B (C ₂ O ₄ ) ₂ ]) was further added. A laminate cell of a lithium ion secondary battery was prepared and evaluated in the same procedure and under the same conditions as in Example 1 except that the addition was performed at 0.5% by mass.
The measurement results of the capacity retention ratio (%) with respect to the initial discharge capacity in Example 2 are shown in the graph of FIG.

[Experimental Examples 1-4]
Examples 1 to 4 were the same as Examples 1 to 4 except that the content of additives (lithium difluorophosphate, lithium bis (oxalate) borate) in the non-aqueous electrolyte was changed at the following content. A laminate cell of a lithium ion secondary battery was manufactured in the same procedure and under the same conditions.
(1) Experimental example 1: lithium difluorophosphate: 0.7% by mass + lithium bis (oxalate) borate: 0.5% by mass
(2) Experimental Example 2: lithium difluorophosphate: 1.2% by mass + lithium bis (oxalate) borate: 0.5% by mass
(3) Experimental Example 3: Lithium difluorophosphate: 1% by mass + lithium bis (oxalate) borate: 0.3% by mass
(4) Experimental example 4; lithium difluorophosphate: 1% by mass + lithium bis (oxalate) borate: 0.7% by mass

Then, a capacity deterioration rate per cycle (% / cycle) was measured by performing an accelerated deterioration test by performing a cycle test (charge and discharge characteristics) in which charge and discharge were repeated in the same manner as in Example 1. The results are shown in the graph of FIG.
In this experimental example, the accelerated deterioration test was also performed on the laminated cells of the lithium ion secondary batteries manufactured in Examples 1 and 2 above, and the capacity deterioration rate per cycle (% / cycle) was measured. The results are shown in the graph of FIG.

[Evaluation results]
As shown in the graph of FIG. 2, a positive electrode active material having a composition defined by the present invention, that is, a lithium transition metal composite oxide represented by LiNi _0.764 Co _0.143 Al _0.083 Nb _0.01 O ₂ The lithium ion secondary battery of Example 1 provided with the positive electrode using has a discharge capacity of about 190 (Ah / kg), which is almost equivalent to Comparative Examples 1 and 2 using the positive electrode active material of the conventional composition. Was. However, according to the result of the cycle test by repeating the charge and discharge shown in the graph of FIG. 3, the lithium ion secondary battery of Example 1 has a larger number of charge and discharge times than the lithium ion secondary battery of the conventional configuration. It was revealed that the composition exhibited an excellent capacity retention rate. As shown in FIG. 3, the lithium ion secondary batteries of Examples have a capacity retention ratio of about 92% even when charge / discharge cycles are repeated 100 times, indicating that Comparative Example 1 (about 54%) and Comparative Example 2 ( (Approximately 66%). This is because, unlike the positive electrode active materials of Comparative Examples 1 and 2, the positive electrode active material of Example 1 has a structure in which a part of Ni is replaced by Al, so that the crystal structure is stable. It is considered that the discharge characteristics are improved.

  Further, as shown in the graph of FIG. 4, Example 2 in which both lithium difluorophosphate and lithium bis (oxalate) borate were added to the nonaqueous electrolytic solution was compared with Example 1 in which only lithium difluorophosphate was added. It can be seen that the capacity retention rate was greatly increased. As in Example 2, by adding both lithium difluorophosphate and lithium bis (oxalate) borate to the non-aqueous electrolyte, the negative electrode is compared with the SEI film formed when lithium difluorophosphate alone is added. It is considered that the formation of the SEI film that suppresses the decomposition of the electrolytic solution on the surface significantly increases the capacity retention ratio and improves the life characteristics.

  Further, as shown in the graph of FIG. 5, Experimental Example 3 in which lithium difluorophosphate: 1% by mass and lithium bis (oxalate) borate: 0.3% by mass were added as additives to the non-aqueous electrolyte, and Experimental Example 4 in which lithium difluorophosphate: 1% by mass and lithium bis (oxalate) borate: 0.7% by mass was added, compared to Experimental Examples 1 and 2 and Examples 1 and 2, capacity degradation per cycle. It was confirmed that the rate was low. From this result, after employing the lithium transition metal composite oxide having the composition according to the present invention as the positive electrode active material, further, as additives in the non-aqueous electrolyte, lithium difluorophosphate and lithium bis (oxalate) borate were used. It has been clarified that by adding both in appropriate amounts, capacity deterioration can be significantly suppressed. First, by adding an appropriate amount of lithium difluorophosphate to a non-aqueous electrolyte, an SEI film is effectively formed on the surface of the negative electrode active material layer, thereby improving discharge characteristics and life characteristics, and improving lithium bisulfate characteristics. It is considered that by adding (oxalate) borate in an appropriate amount, it is possible to more effectively suppress the decomposition of the electrolytic solution and form an SEI film having an optimum thickness without increasing the internal resistance.

  According to the lithium ion secondary battery according to the present invention as described above, the above configuration increases the discharge capacity, improves the capacity retention rate, and further enhances the safety of the battery. Therefore, for example, it is expected that the present invention is applied to a large-sized lithium ion secondary battery having a discharge capacity of 10 Ah or more, such as a mobile power supply system and a stationary power storage system.

1: Lithium ion secondary battery,
10 ... battery container,
10a ... cylindrical body,
10b ... lid,
11, 12, ... electrode terminals,
13 ... inlet,
14 ... Positive electrode (electrode),
14a: electrode tab,
15 ... negative electrode (electrode),
15a: electrode tab,
16 ... Separator

Claims (3)

At least a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, a lithium ion secondary battery including a non-aqueous electrolyte,
The positive electrode, examples of the positive electrode active material, at least the general _{formula: LiNi w Co x Al y Nb} z O 2 ( where, in the formula, a 0 <w, 0 <x, 0 <y, 0 <z, And w + x + y + z = 1) , and cobalt (Co), aluminum (Al), niobium (Nb) and nickel in the lithium transition metal composite oxide. The mass ratio of (Ni) is as follows: Co: 10 to 20% by mass, Nb: 0.5 to 1.5% by mass, Al: 3 to 10% by mass, Ni: balance (however, Co, Al, Nb and Ni A total of 100%) . The nonaqueous electrolyte, as an additive, according to claim characterized in that it comprises a lithium difluorophosphate (LiPF ₂ O ₂₎ and lithium bis (oxalato) borate _{(Li [B (C 2 O} 4) 2]) 1 lithium ion secondary battery according to. The content of the lithium difluorophosphate in the nonaqueous electrolyte is 0.7 to 1.3% by mass, and the content of the lithium bis (oxalate) borate is 0.3 to 0.7% by mass. The lithium ion secondary battery according to claim 2 , wherein:

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