JP6889751B2 - Dual ion power storage device - Google Patents

Description

The present invention relates to a dual ion power storage device.

In recent years, with the growing awareness of environmental issues, the shift from fossil fuel-based engine vehicles to electric vehicles that drive motors by obtaining electric power from power storage devices has been rapidly progressing. In the above conversion, the power storage device is a small lithium-ion secondary battery used in mobile devices such as mobile phones and digital cameras, and is enlarged while following the basic operating principle and structure. Is used.

In such a lithium ion secondary battery, a lithium cobalt composite oxide, a lithium nickel cobalt aluminum composite oxide, or the like is used as a positive electrode active material, and a carbonaceous material such as artificial graphite or natural graphite is mainly used as a negative electrode active material. _{A structure in which a non-aqueous electrolytic solution obtained by dissolving a lithium salt such as LiPF 6 having} a concentration of about 1 M in an aqueous solvent is contained, and a positive electrode containing a positive electrode active material and a negative electrode containing a negative electrode active material are separated by a separator such as polyolefin. At the time of charging, lithium ions are desorbed from the positive electrode active material and inserted into the negative electrode active material, and at the time of discharging, the lithium ions inserted into the negative electrode active material are desorbed by charging and returned to the positive electrode active material. It is a power storage device that is discharged.

However, such a lithium ion secondary battery has insufficient energy density.
As a power storage device with high energy density, a dual ion power storage device using a carbonaceous material as a positive electrode active material, a carbonaceous material as a negative electrode active material, and a non-aqueous electrolyte solution in which a lithium salt is dissolved in a non-aqueous solvent. Is being considered. In the dual ion power storage device, the anion separated from the electrolyte in the non-aqueous electrolyte solution is inserted into the positive electrode active material during charging, the cation is inserted into the negative electrode active material, and the anion inserted into the positive electrode active material and the negative electrode active material during discharging. , And the cation is desorbed into the electrolytic solution to perform charging and discharging.

In Patent Document 1, carbonaceous materials are used for both the positive electrode active material and the negative electrode active material, the initial charging voltage is 5.2 to 6.0 V, and the energy is charged by gradually increasing the voltage. Dual ion power storage devices that increase the density are listed.
Further, Patent Document 2 describes that a capacity can be obtained by using graphite as a positive electrode active material, using lithium metal or graphite as a negative electrode active material, and charging to 5.4 to 5.5 V based on lithium. Has been done.

Patent Document 3 discloses a dual ion storage device using a mixed solvent containing ethyl methyl carbonate (EMC) and cyclic carbonate as a non-aqueous solvent. In the examples of Patent Document 3, graphite is used as the positive electrode active material, lithium titanate (LTO) is used as the negative electrode active material, and lithium hexafluorophosphate (LiPF ₆ ) is 1.8 M as the electrolytic solution. A dual ion storage device using a mixed solvent containing lithium titanate (LiBF _{4) at a ratio of 0.2 M is described.}

JP-A-2017-204338 Japanese Unexamined Patent Publication No. 2005-251472 Japanese Unexamined Patent Publication No. 2018-41636

However, in the case of the dual ion power storage device using the carbonaceous material as the negative electrode active material shown in Patent Document 1, the potential of the positive electrode at the time of charging is 5.25 ^{V based on the lithium standard (V vs. Li / Li +).} As described above, for example, when exposed to a high temperature of 45 ° C. or higher, the electrolytic solution is rapidly decomposed, the battery is swollen by the gas generated by the decomposition of the electrolytic solution, the battery is deteriorated, and the capacity is reduced. is there. When graphite or lithium metal is used as the negative electrode active material shown in Patent Documents 1 and 2, lithium dendrite is deposited on the negative electrode, and this dendrite causes an internal short circuit, resulting in a lithium ion secondary battery. Is in danger of catching fire or exploding.

Further, the power storage device is generally required to have a high charge / discharge capacity and a long cycle life. In particular, power storage devices used outdoors, such as power storage devices for automobiles, are required to have small fluctuations in charge / discharge capacity and cycle life depending on the environmental temperature, but conventional dual-ion power storage devices are required to have a low temperature environment in winter. Under the circumstances, the movement of anions and cations was hindered due to an increase in the viscosity of the electrolytic solution, and the discharge capacity tended to decrease.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a dual ion power storage device having a large capacity, a low positive electrode potential, and a suppressed decrease in discharge capacity in a low temperature environment. To do.

The inventor of the present invention has expanded the capacity and increased the positive electrode potential of a dual ion storage device using graphite as the positive electrode active material and _{lithium titanate (LTO) represented by the general formula Li 4} Ti ₅ O _{12 as the negative electrode active material.} A study was conducted to suppress the decrease in discharge capacity in a low temperature environment while keeping it low. As a result, in the charging curve showing the relationship between the charging capacity per 1 g of graphite and the charging voltage when the dual ion power storage device is charged with a constant current of 1C in an environment of a temperature of 25 ° C., the charging capacity per 1 g of graphite is It has been found that it is effective to adjust the composition of the electrolytic solution so that an inflection point appears between 3.25 and 3.39 V at a charge voltage of 40 mAhg ^{-1 or more.} Further, as the positive electrode current collector, an aluminum foil in which a peak having a maximum peak value in the range of 74.8 eV or more and 76.7 eV is detected in the X-ray photoelectron spectroscopy measured by X-ray photoelectron spectroscopy is used. It was found that the positive potential can be suppressed to a lower level. Furthermore, it has been found that by using a predetermined lithium imide salt and lithium hexafluorophosphate as the electrolyte salt of the electrolytic solution, it is possible to further suppress the decrease in the discharge capacity in a low temperature environment.
Therefore, the present invention provides the following means for solving the above problems.

[1] The positive electrode active material has an electrolytic solution containing a non-aqueous solvent and an electrolyte dissolved in the non-aqueous solvent, a positive electrode current collector, and a positive electrode active material layer formed on the surface of the positive electrode current collector and the positive electrode current collector. The layer has a positive electrode containing a positive electrode active material capable of storing and releasing anions of the electrolyte, a negative electrode current collector, and a negative electrode active material layer formed on the surface of the negative electrode current collector, and the negative electrode active material layer has a negative electrode active material layer. A dual ion storage device including a negative electrode containing a negative electrode active material capable of storing and releasing cations of the electrolyte, wherein the positive electrode active material is graphite and the negative electrode active material is the general formula Li ₄ Ti ₅ O ₁₂ . The charge curve showing the relationship between the charge capacity per 1 g of the graphite and the charging voltage when the lithium titanate shown is charged at a constant current of 1 C in an environment of 25 ° C. is the charge per 1 g of the graphite. A dual ion power storage device having a capacity of 40 mAhg ^-1 or more and having a bending point between 3.25 and 3.39 V.

[2] The dual ion power storage device according to [1], wherein the upper limit of the charging voltage of the dual ion power storage device is 3.5 V.
[3] The dual ion power storage device according to [1] or [2], wherein the surface spacing of the (002) planes of the graphite is 3.360 nm or less.
[4] The electrolyte anion contains one or more selected from _{PF 6} ⁻ , BF ₄ ⁻ , TFSI ⁻ ((CF ₃ SO ₂ ) ₂ N ⁻ ), and FSI ⁻ ((FSO ₂ ) ₂ N ^−). The dual ion power storage device according to any one of [1] to [3].
[5] The dual ion storage device according to any one of [1] to [4], wherein the cation of the electrolyte ^{contains Li +.}
[6] The dual ion storage device according to any one of [1] to [5], wherein the non-aqueous solvent contains a chain carbonate.
[7] The non-aqueous solvent is one selected from ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC). The dual ion power storage device according to any one of [1] to [5], which comprises the above.
[8] At least one of the positive electrode active material layer and the negative electrode active material layer is selected from carbon nanotubes, carbon fibers, vapor phase carbon fibers, graphite, amorphous carbon, and carbon blacks. The dual ion power storage device according to any one of [1] to [7], which comprises an agent.
[9] The dual ion according to any one of [1] to [7], wherein at least one of the positive electrode active material layer and the negative electrode active material layer contains carbon nanotubes and carbon blacks. Power storage device.
[10] The dual ion storage device according to [8] or [9], wherein the carbon blacks are at least one selected from acetylene black, ketjen black, and furnace black.
[11] The electrolyte is at least one lithium imide salt selected from the group consisting of lithium bis (fluorosulfonyl) imide (LiFSI) and lithium bis (trifluoromethanesulfonyl) imide (LiTFSI), and phosphoric acid hexafluorophosphate. The positive electrode current collector contains lithium (LiPF ₆ ) and has a peak having a maximum peak value in the range of 74.8 eV or more and 76.7 eV in the X-ray photoelectron spectroscopy spectrum measured by X-ray photoelectron spectroscopy. The dual ion power storage device according to any one of [1] to [10], which comprises a detected aluminum foil.
[12] The dual ion storage device according to [11], wherein the lithium imide salt is contained in a range of 0.5 mol or more and 10 mol or less with respect to 1 mol of the lithium hexafluorophosphate.
[13] The electrolyte further contains lithium tetrafluoroborate (LiBF ₄ ) in a range of 0.05 mol or more and 10 mol or less with respect to 1 mol of lithium hexafluorophosphate [11] or [ 12] The dual ion power storage device.

According to the present invention, it is possible to provide a dual ion power storage device having a large capacity, a low positive electrode potential, and a suppressed decrease in discharge capacity in a low temperature environment.

Charging that plots the relationship between the charging capacity per 1 g of positive electrode active material and the charging voltage when the coin-type dual-ion power storage device produced in Example 1 is charged at a constant current of 1 C in an environment of a temperature of 25 ° C. It is a curve. Charging that plots the relationship between the charging capacity per 1 g of positive electrode active material and the charging voltage when the coin-type dual-ion power storage device manufactured in Comparative Example 1 is charged at a constant current of 1 C in an environment of a temperature of 25 ° C. It is a curve. It is a measurement result of cyclic voltammetry (CV) of the coin type evaluation cell prepared in the preliminary experiment 1. It is a measurement result of cyclic voltammetry (CV) of the coin type evaluation cell prepared in the preliminary experiment 2. It is a measurement result of cyclic voltammetry (CV) of the coin type evaluation cell prepared in the preliminary experiment 3. It is a measurement result of cyclic voltammetry (CV) of the coin type evaluation cell prepared in the preliminary experiment 4. It is an X-ray photoelectron spectroscopic spectrum of the aluminum foil of the coin type evaluation cell which performed the CV measurement in the preliminary experiment 2. It is an X-ray photoelectron spectroscopic spectrum of the aluminum foil of the coin type evaluation cell which did not perform CV measurement in the preliminary experiment 2. It is a cyclic voltammetry (CV) measurement result of the coin type evaluation cell prepared in the preliminary experiment 5.

Hereinafter, the dual ion power storage device according to the embodiment of the present invention will be described. The materials, dimensions, etc. exemplified in the following description are examples, and the present invention is not limited thereto, and the present invention can be appropriately modified without changing the gist thereof.

The dual ion power storage device according to the present embodiment includes an electrolytic solution containing a non-aqueous solvent and an electrolyte dissolved in the non-aqueous solvent, a positive electrode current collector, and a positive electrode active material layer formed on the surface of the positive electrode current collector. The positive electrode active material layer has a positive electrode containing a positive electrode active material capable of storing and releasing anions of an electrolyte, a negative electrode current collector, and a negative electrode active material layer formed on the surface of the negative electrode current collector. The negative electrode active material layer includes a negative electrode containing a negative electrode active material capable of storing and releasing the cation of the electrolyte.

<Positive electrode>
The positive electrode has a positive electrode current collector and a positive electrode active material layer formed on the surface of the positive electrode current collector.

[Positive current collector]
Positive current collectors include aluminum foil, aluminum alloy foil, stainless steel foil, nickel foil, titanium foil, carbon sheet, etc., as well as carbon, diamond dry carbon, nickel on the surface of aluminum foil, aluminum alloy foil, or stainless steel foil. , A composite to which a surface treatment film such as titanium or silver is attached, and the like, but the present invention is not limited to such examples. Among the materials constituting the current collector, aluminum and an aluminum alloy are preferable. The thickness of the foil as the positive electrode current collector is not particularly limited, but is preferably in the range of 5 μm or more and 40 μm or less, and more preferably in the range of 10 μm or more and 30 μm or less. If the thickness of the aluminum foil is less than 5 μm, the strength of the positive electrode current collector may decrease. On the other hand, if the thickness of the aluminum foil exceeds 40 μm, it is necessary to make the thickness of the positive electrode active material layer relatively thin in order to accommodate it in the battery case having a predetermined shape, and the capacity of the dual ion power storage device decreases. There is a risk of

In the present embodiment, when the electrolyte contains a lithium imide salt and the aluminum foil is used for the positive electrode current collector, it is preferable to coexist with _{LiPF 6.} This is because when the lithium imide salt is used alone, if an aluminum foil is used for the positive electrode current collector, the aluminum foil may be corroded and eluted by the anion of the lithium imide salt depending on the conditions in which the power storage device is used. .. _{PF 6} ^−, which is an anion of LiPF ₆ , reacts preferentially with the surface of the aluminum foil at the time of initial charging to form a film (passivation film) of a compound having an Al−F bond, and is a lithium imide salt. This is because it has an effect of suppressing the corrosion reaction between the anion and the aluminum foil.

_{The content of LiPF 6} in the electrolytic solution is preferably in the range of 0.01 mol or more and 15 mol or less, and in the range of 0.1 mol or more and 10.0 mol or less, with respect to 1 mol of the lithium imide salt. It is particularly preferable to be in.

When the electrolyte contains a lithium imide salt and LiPF ₆ coexists, the aluminum foil is in the range of 74.8 eV or more and 76.7 eV in the X-ray photoelectron spectroscopy spectrum measured by X-ray photoelectron spectroscopy (XPS). The peak having the maximum peak value is detected. This peak is considered to indicate an Al—F bond. That is, it is considered that the aluminum foil in which this peak is detected has a film (passivation film) of a compound having an Al—F bond formed on its surface.

The aluminum foil does not have to have the above peak immediately after assembling the dual ion power storage device. This is because, as described above, _{by using the lithium imide salt and LiPF 6} together as the electrolyte, a film (passivation film) of a compound having an Al—F bond is formed on the surface of the aluminum foil at the time of initial charging. .. As the aluminum foil used in the manufacture of the dual ion power storage device, various aluminum foils used as current collectors for batteries can be used. The purity of the aluminum foil is preferably 99.3% by mass or more, more preferably 99.85% by mass or more.

[Positive electrode active material layer]
The positive electrode active material layer preferably contains a positive electrode active material, a binder, and a conductive auxiliary agent. The content of the positive electrode active material, the binder and the conductive auxiliary agent in the positive electrode active material layer is preferably in the range of 70% by mass or more and 99.5% by mass or less for the positive electrode active material, and 0.1% by mass or more for the binder. It is in the range of 25% by mass or less, and the conductive auxiliary agent is in the range of 0.1% by mass or more and 25% by mass or less.

The positive electrode active material used in this embodiment contains graphite. The graphite may be natural graphite or artificial graphite. One type of graphite may be used alone, or two or more types may be used in combination. As the natural graphite, scaly natural graphite, lump graphite, earthy natural graphite and the like can be used. As the artificial graphite, scaly artificial graphite, massive artificial graphite, spherical artificial graphite, expanded graphite, carbon fiber and the like can be used. Further, graphite may be coated with an anion-permeable carbonaceous material or the like.

The crystal structure of graphite is roughly divided into hexagonal crystals having a layered structure made of ABAB and rhombohedral crystals having a layered structure made of ABCABC. In this embodiment, both graphite having hexagonal crystals and graphite having rhombohedral crystals can be used.
The interplanar spacing of the (002) planes of graphite, d (002), is not particularly limited as long as the object of the present invention can be achieved, but is preferably 0.3365 nm or less, more preferably 0.3363 nm or less, and further preferably 0.3360 nm or less. preferable. The lower limit of d (002) is 0.3354 nm.

As the binder, for example, a fluororesin, a rubber, an acrylic resin, an olefin resin, a carboxymethyl cellulose (CMC) resin, or a natural polymer resin can be used. Examples of the fluororesin include polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE). Examples of rubber include fluororubber, ethylene propylene diene rubber, and styrene butadiene rubber. Examples of natural polymer resins include gelatin, chitosan, alginic acid and the like. These binders may be used alone or in combination of two or more.

As the conductive auxiliary agent, for example, carbon nanotubes, carbon fibers, vapor-phase carbon fibers, graphite, amorphous carbon, and carbon blacks can be used. The carbon nanotubes may be single-walled carbon nanotubes or multi-walled carbon nanotubes. Examples of carbon blacks include acetylene black, ketjen black, furnace black and the like. These conductive auxiliaries may be used alone or in combination of two or more. As the conductive auxiliary agent in which two or more kinds are combined, it is preferable to use a combination of two kinds of single-walled carbon nanotubes and acetylene black.

[Manufacturing method of positive electrode]
The positive electrode can be produced, for example, by applying a paste for forming a positive electrode active material layer in which a positive electrode active material, a binder, and a conductive additive are dispersed in a solvent to the surface of a positive electrode current collector and drying it. As the solvent of the paste for forming the positive electrode active material layer, a polar organic solvent such as N-methylpyrrolidone or water can be used. The method for applying the paste for forming the positive electrode active material layer is not particularly limited, and for example, it can be applied using a coating device such as a doctor blade, a die coater, a bar coater, or a roll coater.
The thickness of the active material layer of the positive electrode is 5 to 100 μm, more preferably 10 to 70 μm, and even more preferably 15 to 35 μm.

<Negative electrode>
The negative electrode preferably has a negative electrode current collector and a negative electrode active material layer formed on the surface of the negative electrode current collector.

[Negative electrode current collector]
As the negative electrode current collector, for example, aluminum, aluminum alloy, copper, copper alloy, stainless steel, nickel, titanium and the like can be used. The shape of the negative electrode current collector may be a foil, a sheet, a film, a mesh, or the like. The thickness of the negative electrode current collector is not particularly limited, but is preferably in the range of 5 μm or more and 40 μm or less, and more preferably in the range of 8 μm or more and 30 μm or less.

[Negative electrode active material layer]
The negative electrode active material layer preferably contains a negative electrode active material, a binder, and a conductive auxiliary agent. The content of the negative electrode active material, the binder and the conductive auxiliary agent in the negative electrode active material layer is preferably in the range of 70% by mass or more and 99.5% by mass or less for the negative electrode active material, and 0.1% or more and 25% by mass for the binder. It is in the range of mass% or less, and the conductive auxiliary agent is in the range of 0.1% by mass or more and 25% by mass or less.
The negative electrode active material used in this embodiment includes an LTO whose _{general formula is represented by Li 4} Ti ₅ O _12.

As the binder, for example, a fluororesin, a rubber, an acrylic resin, an olefin resin, a carboxymethyl cellulose (CMC) resin, or a natural polymer resin can be used. Examples of fluororesin, rubber, and natural polymer resin are the same as in the case of the binder of the positive electrode.

As the conductive auxiliary agent, for example, carbon nanotubes, carbon fibers, vapor-phase carbon fibers, graphite, amorphous carbon, and carbon blacks can be used. The carbon nanotubes may be single-walled carbon nanotubes or multi-walled carbon nanotubes. Examples of carbon blacks include acetylene black, ketjen black, furnace black and the like. These conductive auxiliaries may be used alone or in combination of two or more. As the conductive auxiliary agent in which two or more kinds are combined, it is preferable to use a combination of two kinds of single-walled carbon nanotubes and acetylene black.

[Manufacturing method of negative electrode]
The negative electrode can be manufactured, for example, by applying a paste for forming a negative electrode active material layer in which a negative electrode active material, a binder, and a conductive auxiliary agent are dispersed in a solvent to the surface of a negative electrode current collector and drying the negative electrode. The example of the solvent of the paste for forming the negative electrode active material layer is the same as that of the paste for forming the positive electrode active material layer. The example of the method of applying the paste for forming the negative electrode active material layer is the same as that of the paste for forming the positive electrode active material layer.
The thickness of the active material layer of the negative electrode is 5 to 100 μm, more preferably 10 to 70 μm, and even more preferably 15 to 35 μm.

<Electrolytic solution>
The electrolytic solution contains a non-aqueous solvent and an electrolyte. In the dual ion power storage device of the present embodiment, the anion of the electrolyte in the electrolytic solution is stored in the positive electrode active material during charging, the cation of the electrolyte in the electrolytic solution is stored in the negative electrode active material, and the cation of the electrolyte in the electrolytic solution is stored in the negative electrode active material during discharging. Anions are released into the electrolytic solution, and cations in the negative electrode active material are released into the electrolytic solution.

[Non-aqueous solvent]
As the non-aqueous solvent, an organic solvent capable of dissolving the electrolyte is used. The non-aqueous solvent preferably contains at least one compound selected from the group consisting of cyclic carbonates and chain carbonates. As the cyclic carbonate, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) and the like can be used. As the chain carbonate, ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC) and the like can be used. These cyclic carbonates and chain carbonates may be used alone or in combination of two or more.

The non-aqueous solvent is preferably one kind or two or more kinds of chain carbonates, or a mixture of chain carbonates and cyclic carbonates, and particularly preferably one kind of chain carbonates. One type of chain carbonate is preferably EMC. The mixture of the chain carbonate and the cyclic carbonate is preferably in the range of 50:50 to 99: 1 (former: latter) in volume ratio.

[Electrolytes]
In the present embodiment, the electrolyte is one or more anions selected from PF ₆ ⁻ , BF ₄ ⁻ , TFSI ⁻ ((CF ₃ SO ₂ ) ₂ N ⁻ ), and FSI ⁻ ((FSO ₂ ) ₂ N ⁻ ). The cation must contain ^{Li +.} From such a combination of anions and cations, examples of the electrolyte include, but are not limited to _{, LiPF 6} , LiBF _{4, LiFSI, and LiTFSI.}
The concentration of the electrolyte in the electrolytic solution is in the range of 2M or more and 5M or less.

In the present embodiment, in the charging curve showing the relationship between the charging capacity per 1 g of graphite and the charging voltage when the composition of the electrolytic solution is charged at a constant current of 1 C in an environment of a temperature of 25 ° C., per 1 g of graphite. The charging capacity is adjusted to 40 mAhg ^-1 or more, and the charging voltage is adjusted so that an inflection point appears between 3.25 and 3.39 V. The inflection point is that the slope of the charging curve (= charging voltage / charging capacity per 1 g of graphite) decreases.

As a result of detailed examination of the relationship between the discharge capacity of the dual ion power storage device and the charge / discharge curve, the present inventor of the dual ion power storage device exhibiting an inflection point in the above range in the charge curve. I found that there was a tendency. Then, as a result of further studies, as is clarified in Examples described later, charging is performed according to the composition of the electrolytic solution (combination of anion and cation of electrolyte, type and combination of non-aqueous solvent, electrolyte concentration). It was found that the position where the inflection point of the curve appears may fluctuate, or the inflection point may not appear. The reason why the inflection point appears on the charge curve is considered as follows.

Generally, when the electrolyte dissolves in a non-aqueous solvent, the electrolyte ionizes into anions and cations. The ionized anions and cations are solvated with non-aqueous solvent molecules, respectively, to stabilize them. The number of non-aqueous solvent molecules solvated with anions varies depending on the type of anion, the type of non-aqueous solvent, and the combination. In addition, the number of non-aqueous solvent molecules solvated may change depending on the concentration of the electrolyte. That is, the number of non-aqueous solvent molecules solvated with the anion changes depending on the composition of the electrolytic solution.

On the other hand, when the anion is inserted into the graphite of the positive electrode active material, some or all of the solvated non-aqueous solvent molecules are desorbed from the anion, and the desorbed anion is inserted between the layers of the graphite. It is considered that the energy required for the non-aqueous solvent molecule to desorb from the anion determines the charging voltage. Therefore, in order to reduce the charging voltage, it is necessary to adjust the composition of the electrolytic solution so that the energy required for desorbing the non-aqueous solvent molecule from the anion does not increase.

Further, the anion inserted into the graphite diffuses between the graphite layers to the inside of the graphite particles to form an intercalation compound. It is known that this interlayer compound forms a stage structure, and the stage with the smallest number of anion insertions is stage IV, and changes from stage III to stage II to stage I as the number of anion insertions increases. (See References: Jeffery A. Read, J. Phys. Chem., C2015, 119, 8438.).
It is considered that the inflection point appears on the charge curve because the energy required for the non-aqueous solvent molecule to desorb from the anion is reduced due to the transition of the stage of the intercalation compound formed on graphite. Be done. Then, it is considered that due to the transition of the stage of the intercalation compound, an increase in capacity occurs as a result.
The position where the inflection point appears may fluctuate depending on the composition of the electrolytic solution, or the inflection point may not appear because the stage transition of the intercalation compound formed on graphite occurs depending on the composition of the electrolytic solution. This is thought to be due to changes in ease.

Further, the present inventor has found that the lithium imide salt has an effect of suppressing a decrease in the discharge capacity of the dual ion power storage device in a low temperature environment. The reason why the lithium imide salt has the effect of suppressing the decrease in the discharge capacity of the dual ion storage device in a low temperature environment is not always clear, but it is not always clear, but the solvent of the non-aqueous solvent molecule in the electrolytic solution and the anion or / and the cation. It is considered that this is because the sum form is different from the case where the lithium imide salt is not contained and the dissociation degree of the lithium imide salt is different, so that the effect of suppressing the deterioration of the discharge characteristics at low temperature is exhibited. In order to obtain the effect of this lithium imide salt, the content of the lithium imide salt in the electrolytic solution is such that the position of the inflection point of the charging curve described above is the charging voltage and the charging capacity per 1 g of the positive electrode active material (graphite). Is not particularly limited as long as the amount is within the above range, but it is preferably within the range of 0.01 M or more and 4.0 M or less.

When using a lithium imide salt, LiPF ₆ needs to coexist. The reason for this will be described later. _{The content of LiPF 6} in the electrolytic solution is not particularly limited as long as the position of the inflection point of the charging curve is within the above-mentioned range of the charging voltage and the charging capacity per 1 g of the positive electrode active material. However, it is preferably in the range of 0.01 M or more and 4.0 M or less.

Further, LiBF ₄ may be allowed to coexist with the lithium imide salt. By _{coexisting LiBF 4} with a lithium imide salt, there is an effect of suppressing a decrease in the discharge capacity of the dual ion storage device in a low temperature environment. The reason for this is not necessarily clear, but _{it is considered that the presence of LiBF 4} reduces the interface between the electrolytic solution and the electrode and the charge transfer resistance. _{The content of LiBF 4} in the electrolytic solution is not particularly limited as long as the position of the inflection point of the charging curve is within the above-mentioned range of the charging voltage and the charging capacity per 1 g of the positive electrode active material. However, it is preferably in the range of 0.01 M or more and 4.0 M or less.

[Electrolyte additive]
Additives such as VC (vinylene carbonate), LiBOB (lithium bisoxalate borate), and LiDFOB (lithium difluorooxalate borate) may coexist in the electrolytic solution as long as the characteristics of the present invention are not impaired.

<Configuration of dual-ion power storage device>
The configuration of the dual ion power storage device of the present embodiment is not particularly limited, and may be a known configuration adopted as a configuration of a power storage device such as a coin type, a button type, a sheet type, a cylindrical type, or a square type. ..

In the coin-type, button-type, and laminate-type dual ion power storage devices, a laminate in which a positive electrode and a negative electrode are laminated via a separator so that the positive electrode active material layer and the negative electrode active material layer face each other is formed together with an electrolytic solution. , A power storage device having a configuration in which it is housed in a battery case and sealed. As the separator, for example, a porous membrane, a non-woven fabric, or paper can be used. As the porous membrane, a porous membrane made of polyolefin such as polyethylene or polypropylene or a porous membrane made of fluororesin can be used. As the non-woven fabric, a polypropylene non-woven fabric, a cellulose non-woven fabric, or a glass fiber non-woven fabric can be used.

In the cylindrical and square dual ion power storage devices, a wound body in which a positive electrode and a negative electrode are wound with a separator so that the positive electrode active material layer and the negative electrode active material layer face each other is wound together with an electrolytic solution. It is a power storage device that is housed in a battery case and sealed. Further, the wound body may be put in a laminated mold. As the separator, a porous membrane, a non-woven fabric, or paper can be used.

<Activation>
In the dual ion power storage device of the present embodiment, the laminate of the electrolytic solution and the electrode / separator is housed in a battery case, sealed, and then charged to be activated. The final charge voltage is 3.40V or more and 3.75V or less, preferably 3.40V or more and 3.55V or less, and more preferably 3.40V or more and 3.50V or less. This is because if the charging voltage is higher than this range, there is a risk of decomposition of non-aqueous solvents and additives such as LiBOB in the electrolytic solution, and if the charging voltage is lower than this range, anions are not inserted into graphite. In the present embodiment, the inflection point appears on the charging curve in the dual ion storage device after activation, and even if the inflection point appears on the charging curve in the charging for activation. It may or may not be expressed.

If the current value during charging becomes too high, the structure of the interlayer compound generated by inserting ions into graphite may be destroyed, and the voltage may increase to decompose the electrolytic solution. For this reason, it is preferable that the current value at the time of charging is small, but if the current value becomes too small, it takes time to charge. Therefore, the current value during charging is 0.01C to 1C, preferably 0.05C to 0.5C, and more preferably 0.1C to 0.3C. Further, the current value may be changed during charging or discharging.

When the lithium imide salt and LiPF ₆ coexist, the positive electrode current collector (aluminum foil) is in the range of 74.8 eV or more and 76.7 eV in the X-ray photoelectron spectroscopy measured by X-ray photoelectron spectroscopy. The first charge is performed so that the peak having the maximum peak value is detected. The charging method is the same as above.

According to the dual ion storage device in the present embodiment, when a predetermined lithiumimide salt and LiPF ₆ coexist as the electrolyte, the positive electrode current collector is X-ray photoelectron spectroscopy measured by X-ray photoelectron spectroscopy. Since the aluminum foil is an aluminum foil in which a peak having a maximum peak value is detected in the range of 74.8 eV or more and 76.7 eV in the spectrum, a decrease in discharge capacity in a low temperature environment is suppressed. In particular, _{the content of LiPF 6} is preferably in the range of 0.01 mol or more and 15 mol or less, and is contained in the range of 0.1 mol or more and 10.0 mol or less with respect to 1 mol of the lithium imide salt. As a result, the effect of suppressing the decrease in discharge capacity due to the lithium imide salt in a low temperature environment can be surely obtained.

<Example 1>
[Preparation of positive electrode]
93.0 parts by mass of artificial graphite powder (HPG-6 manufactured by Hitachi Kasei Co., Ltd., d (002) = 0.3358 nm) as a positive electrode active material, 3.0% by mass of acetylene black as a conductive auxiliary agent, and alginic acid as a binder. The system binder and carboxymethyl cellulose were mixed at a ratio of 2.0% by mass in total and 2.0% by mass of the acrylic resin. Water was added to the obtained mixture, and the mixture was stirred and mixed to prepare a paste for forming a positive electrode active material layer.

An aluminum foil (thickness: 15 μm) was prepared as a positive electrode current collector. The above-mentioned paste for forming a positive electrode active material layer was applied to the surface of the aluminum foil by a die coating method and dried to prepare a positive electrode having a positive electrode active material layer formed on the surface of a positive electrode current collector. The thickness of the obtained positive electrode active material layer was measured using a micrometer and found to be 35 μm.

[Preparation of negative electrode]
LTO was mixed at a ratio of 93.0% by mass as a negative electrode active material, acetylene black at a ratio of 3.0% by mass as a conductive auxiliary agent, and PVDF at a ratio of 4.0% by mass as a binder. N-Methylpyrrolidone was added to the obtained mixture, and the mixture was stirred and mixed to prepare a paste for forming a negative electrode active material layer. In LTO, lithium carbonate and anatase-type titanium oxide are mixed at a ratio of lithium and titanium in a chemical quantity theory ratio of 4: 5, and then the obtained mixture is heated at a temperature rising rate of 10 using a muffle furnace. The temperature was raised to 800 ° C. at ° C./min, the temperature was maintained at this temperature for 5 hours, and then the mixture was naturally cooled to room temperature for synthesis.

An aluminum foil (thickness: 15 μm) was prepared as a negative electrode current collector. The above-mentioned paste for forming a negative electrode active material layer was applied to the surface of the aluminum foil by a die coating method and dried to prepare a negative electrode having a negative electrode active material layer formed on the surface of a negative electrode current collector. The thickness of the obtained negative electrode active material layer was measured using a micrometer and found to be 15 μm.

[Preparation of electrolyte]
EMC was prepared as a non-aqueous solvent. _{An electrolytic solution was prepared by dissolving LiPF 6} as an electrolyte in this EMC so as to have a concentration of 3M. The electrolytic solution was prepared in a dry room having a dew point of −40 ° C. or lower at room temperature.

[Manufacturing of coin-type dual-ion power storage device]
The positive electrode prepared above was punched into a disk shape having a diameter of 14 mm and the negative electrode prepared above was punched into a disk shape having a diameter of 12 mm, and the mixture was transferred to a dry room having a dew point of −40 ° C. or lower and vacuum dried at 150 ° C. for 24 hours. Then, the positive electrode and the negative electrode after vacuum drying were laminated via a paper separator (trade name: TF4425) manufactured by Nippon Kodoshi Paper Industry Co., Ltd. so that the positive electrode active material layer and the negative electrode active material layer face each other. The obtained laminate and the above electrolytic solution were housed in a 2032 coin cell made of SUS316L clad with aluminum and sealed to prepare a coin-type dual ion power storage device.

[Activation of coin-type dual-ion power storage device]
The coin-type dual-ion power storage device was activated by charging it in a constant temperature bath at 25 ° C. with a current value corresponding to 0.3 C up to 3.5 V in CC mode (constant current mode). After that, the mode was switched to CV mode (constant voltage mode), and charging was continued for 30 minutes. Further, the electric discharge was performed up to 2 V with a current value corresponding to 0.3 C.

[Charge / discharge evaluation of coin-type dual-ion power storage device]
The activated coin-type dual ion power storage device was charged in CC mode to 3.5 V with a current value corresponding to 1 C in a constant temperature bath at 25 ° C., then switched to CV mode and continued charging for 30 minutes. After the charging was completed, the current value corresponding to 1C was discharged to 2V. The discharge capacity obtained by this discharge was used as the initial discharge capacity.

FIG. 1 shows a charging curve that plots the relationship between the charging capacity and the charging voltage per 1 g of the positive electrode active material when the coin-type dual ion power storage device is charged. In addition, Table 1 shows the measurement results of the charging capacity and charging voltage at the inflection point and the initial discharge capacity.
From the charging curve of FIG. 1, it was confirmed that ^{the inflection point occurred at a charging capacity of 50 mAhg -1} per 1 g of the positive electrode active material and a cell voltage of 3.32 V, and the initial discharge capacity was 100 mAhg ^-1.

<Examples 2 to 6>
A coin-type dual-ion power storage device was produced in the same manner as in Example 1 with the combination of the electrolyte and the non-aqueous solvent shown in Table 1, and after activating the obtained coin-type dual-ion power storage device, charge / discharge evaluation was performed. It was. The results are shown in Table 1.

<Comparative Examples 1 to 5>
A coin-type dual-ion power storage device was produced in the same manner as in Example 1 with the combination of the electrolyte and the non-aqueous solvent shown in Table 1, and after activating the obtained coin-type dual-ion power storage device, charge / discharge evaluation was performed. It was. FIG. 2 shows a charging curve plotting the relationship between the charging voltage when the coin-type dual ion power storage device produced in Comparative Example 1 is charged and the charging capacity per 1 g of the positive electrode active material. Table 1 shows the measurement results of the charging voltage, the charging capacity, and the initial discharge capacity at the inflection point measured by the coin-type dual ion power storage devices of Comparative Examples 1 to 5.

From the results in Table 1, the coin-type dual-ion power storage devices of Examples 1 to 6 have an inflection point of 45 mAhg ^-1 or more per 1 g of the positive electrode active material and a charging voltage of 3.28 to 3.34 V. It was found that the initial discharge capacity was in the range and showed a high value of ^{83 mAhg -1 or more.}
On the other hand, in the coin-type dual-ion power storage device of Comparative Example 1, as shown in FIG. 2, the inflection point is that the charging capacity per 1 g of the positive electrode active material is 36 mAhg ^-1, which is lower than the range of the present invention, and the charging voltage is high. It occurred at 3.40 V at a position higher than the range of the present invention, and the initial discharge capacity was as low as ^{52 mAhg -1.} In the case of Comparative Examples 2 to 5, no inflection point was observed. In addition, the initial discharge capacity was 61 mAhg ^{-1 at the} maximum, which was smaller than that of Examples 1 to 6. It is considered that the inflection point was not observed because the stage transition of the intercalation compound formed on graphite did not proceed sufficiently. That is, from this result, it was confirmed that the likelihood of stage transition of the intercalation compound formed on graphite changes depending on the composition of the electrolytic solution.

<Examples 7 to 27, Comparative Examples 6 to 7>
A coin-type dual-ion power storage device was produced in the same manner as in Example 1 with the combination of the electrolyte and the non-aqueous solvent shown in Table 2, and after activating the obtained coin-type dual-ion power storage device, charge / discharge evaluation was performed. It was. The results are shown in Table 2.

From Table 2, the coin-type dual-ion power storage devices obtained in Examples 7 to 27 all had an inflection point in the range of 3.33 to 3.39 V, and the charge capacity per 1 g of the positive electrode active material was in 44MAhg ^-1 or higher, the initial discharge capacity was confirmed that more than 62mAhg ^-1. On the other hand, in Comparative Examples 6 to 7, it was confirmed that no inflection point was observed and the initial discharge capacity was small.

<Preliminary experiment 1>
[Preparation of electrolyte]
EMC was prepared as a non-aqueous solvent. An electrolytic solution was prepared by dissolving LiTFSI as an electrolyte in this EMC so as to have a concentration of 3M.

[Creation of coin-type evaluation cell]
A disk-shaped aluminum foil having a diameter of 12 mm and a thickness of 15 μm and a disk-shaped lithium foil having a diameter of 14 mm and a thickness of 100 μm were laminated via a polyethylene separator so as to face each other. The obtained laminate and the above electrolytic solution were housed in a 2032 coin cell made of SUS316L clad with aluminum and sealed to prepare a coin-type evaluation cell. In this case, the aluminum foil was the SUS316L coin-shaped cell case side clad with aluminum. The coin-shaped evaluation cell was prepared in a dry room where the dew point was adjusted to −40 ° C. or lower.

[Cyclic voltammetry (CV) measurement]
Cyclic voltammetry (CV) measurement was performed under the conditions that the voltage scanning range was 3.0 to 6.0 V at the lithium reference voltage and the scanning speed was 1.0 mV / sec. The measurement temperature was 25 ° C. The result is shown in FIG. In FIG. 3, the horizontal axis represents the potential (V) with respect to the metal Li, and the vertical axis represents the response current (mA).
From the results shown in FIG. 3, it was considered that a large oxidation response current in the 100 μA range flowed in the first cycle of the CV measurement, and the corrosion of the foil was progressing.

<Preliminary experiment 2>
[Preparation of electrolyte]
EMC was prepared as a non-aqueous solvent. An electrolytic solution was prepared by dissolving _{LIPF 6} as an electrolyte in this EMC so as to have a concentration of 1 M, LiTFSI at a concentration of 2 M, and LiBF _{4 at a concentration of 0.4 M.}

[Creation of coin-type evaluation cell]
A coin-type evaluation cell was prepared in the same manner as in Preliminary Experiment 1 except that the above electrolytic solution was used.

[Cyclic voltammetry (CV) measurement]
The obtained coin-shaped evaluation cell was subjected to cyclic voltammetry (CV) measurement for 5 cycles in the same manner as in Preliminary Experiment 1. The result is shown in FIG.
From the results of FIG. 4, in the first cycle of CV measurement, the response current starts to flow from about 3.6V, the slope of the current increase decreases at 4.0V, and reaches 6.0V while maintaining the slope as it is. did. However, the oxidation current value was in the 1 μA range, and the double-digit current value was lower than that in the preliminary experiment 1. Furthermore, as the cycles were repeated, the oxidation current value decreased with each cycle and became almost zero. It is considered that this is because the passivation film is formed on the surface of the aluminum foil by _{LiPF 6} as described above.

<Preliminary experiment 3>
An electrolytic solution was prepared by dissolving _{LIPF 6} as an electrolyte in EMC so as to have a concentration of 1 M, LiTFSI at a concentration of 1 M, and LiBF _{4 at a concentration of 0.4 M.} Using this electrolytic solution, CV measurement was carried out for one cycle in the same manner as in Preliminary Experiment 1. The result is shown in FIG. In FIG. 5, it was found that the current value was the same as that in the first cycle of the preliminary experiment 1 and was not affected by the LiTFSI concentration.

<Preliminary experiment 4>
_{An electrolytic solution was prepared by dissolving LIPF 6} as an electrolyte in EMC so as to have a concentration of 1 M and LiTFSI at a concentration of 2 M. Using this electrolytic solution, CV measurement was carried out for one cycle in the same manner as in Preliminary Experiment 1. The result is shown in FIG. In FIG. 6, it was found that the current value was the same as that in the first cycle of the preliminary experiment 1 and was not affected by the presence or absence of _{LiBF 4.}

From the above results, it was found that the passivation film on the surface of the aluminum foil _{was formed by the presence of LiPF 6.} It was also found that an aluminum foil can be used as the lithium imide salt in _{the coexistence of LiPF 6.}

<Preliminary experiment 5>
[X-ray photoelectron spectroscopy spectrum measurement of aluminum foil]
The X-ray photoelectron spectroscopic spectra of the aluminum foil of the coin-shaped evaluation cell in which the above CV measurement was performed and the aluminum foil of the coin-type evaluation cell in which the CV measurement was not performed were measured as follows.

The coin-shaped evaluation cell was disassembled in a glove box having an argon gas atmosphere, and the aluminum foil taken out was washed with EMC and vacuum dried. The X-ray photoelectron spectroscopy spectrum of the vacuum-dried aluminum foil was measured by X-ray photoelectron spectroscopy (XPS). The measurement conditions of the X-ray photoelectron spectroscopic spectrum are as follows.
Measuring device: Composite electron spectroscopic analyzer (PHI, ESCA-5800)
X-ray source: Monochromated Al Kα ray (1486.6 eV) 300 W
Photoelectron extraction angle: 45 ° (measurement depth: approx. 4 nm)
Measurement area: 2 mm x 0.8 mm

FIG. 7 shows an X-ray photoelectron spectroscopic spectrum of the aluminum foil taken out after disassembling the coin-shaped evaluation cell in which the CV measurement was performed in the preliminary experiment 2, and FIG. 8 shows the CV measurement. It is an X-ray photoelectron spectroscopic spectrum of an aluminum foil of a coin-shaped evaluation cell.

In FIGS. 7 and 8, the horizontal axis represents the binding energy (eV) and the vertical axis represents the strength. In FIGS. 7 and 8, the round solid line is the measured value, and the × solid line is the fitting (the sum of the components obtained by separating the waveforms, and the closer the fitting and the measured value are, the higher the accuracy). The broken line, the alternate long and short dash line, and the alternate long and short dash line represent the metallic aluminum, the Al—O bond, and the Al−F bond obtained by waveform separation, respectively.

Comparing the X-ray photoelectron spectroscopic spectra of FIGS. 7 and 8, the aluminum foil of the coin-shaped evaluation cell on which the CV measurement was performed has a large peak corresponding to the Al—F bond. From this, in the coin-type evaluation cell of the preliminary experiment 2, the response current was greatly reduced by the reaction of forming a film of the compound having an Al—F bond on the surface of the aluminum foil in the first cycle of the CV measurement. The decrease in the response current after the cycle is considered to be due to the formation of more Al-F bonds.

<Preliminary experiment 6>
[Creation of coin-type evaluation cell]
One surface of the aluminum foil (thickness: 15 μm) was coated with a coat layer (thickness: 2 μm) containing graphite. Next, the coated aluminum foil was punched into a disk shape.
The above-mentioned disk-shaped coated aluminum foil was used instead of the aluminum foil, and the coin mold was used in the same manner as in the preliminary experiment 2 except that the surface of the coated aluminum foil coated layer containing graphite was arranged so as to be in contact with the separator. An evaluation cell was prepared.

Cyclic voltammetry (CV) was measured on the produced coin-shaped evaluation cell in the same manner as in Preliminary Experiment 1. The result is shown in FIG.
From the result of FIG. 9, the response current value in the first cycle was significantly larger than that in the preliminary experiment 2. In addition, although the response current value gradually decreased after the second cycle, the response current was 2 to 3 orders of magnitude larger than that of the preliminary experiment 2, and the behavior was clearly different from that of the preliminary experiment 2.

Further, when the X-ray photoelectron spectroscopic spectrum of the aluminum foil of the coin-shaped evaluation cell in which the CV measurement was performed was measured in the same manner as in the preliminary experiment 1, a peak corresponding to the Al-F bond could not be confirmed on the surface. ..

From the above results, when a lithium imide salt is used as the electrolyte, _{it is desirable that LiPF 6} and LiBF ₄ be used in combination, and that the aluminum foil used as the current collector does not have a coat layer like a carbonaceous material. Was confirmed.

<Example 31>
As the positive electrode active material, artificial graphite powder (HPG-6 manufactured by Hitachi Kasei Co., Ltd., d (002) = 0.3358 nm) was 93.0% by mass, and as a conductive auxiliary agent, single-walled carbon nanotube (CNT) was 0.2. Mass% and acetylene black 2.8% by mass, and as a binder, an alginate-based binder and carboxymethyl cellulose were mixed at a ratio of 2.0% by mass in total and 2.0% by mass of acrylic resin. Water was added to the obtained mixture, and the mixture was stirred and mixed to prepare a paste for forming a positive electrode active material layer.

An aluminum foil (thickness: 15 μm) was prepared as a positive electrode current collector. The above-mentioned paste for forming a positive electrode active material layer was applied to the surface of the aluminum foil by a die coating method and dried to prepare a positive electrode having a positive electrode active material layer formed on the surface of a positive electrode current collector. The thickness of the obtained positive electrode active material layer was measured using a micrometer and found to be 35 μm.

[Preparation of negative electrode]
LTO was mixed at a ratio of 92.8% by mass as a negative electrode active material, CNT at 0.4% by mass and acetylene black at a ratio of 2.8% by mass as a conductive auxiliary agent, and PVDF at a ratio of 4.0% by mass as a binder. N-Methylpyrrolidone was added to the obtained mixture, and the mixture was stirred and mixed to prepare a paste for forming a negative electrode active material layer. In LTO, lithium carbonate and anatase-type titanium oxide are mixed at a ratio of lithium and titanium in a chemical quantity theory ratio of 4: 5, and then the obtained mixture is heated at a temperature rising rate of 10 using a muffle furnace. The temperature was raised to 800 ° C. at ° C./min, the temperature was maintained at this temperature for 5 hours, and then the mixture was naturally cooled to room temperature for synthesis.

An aluminum foil (thickness: 15 μm) was prepared as a negative electrode current collector. The above-mentioned paste for forming a negative electrode active material layer was applied to the surface of the aluminum foil by a die coating method and dried to prepare a negative electrode having a negative electrode active material layer formed on the surface of a negative electrode current collector. The thickness of the obtained negative electrode active material layer was measured using a micrometer and found to be 15 μm.

[Preparation of electrolyte]
EMC was prepared as a non-aqueous solvent. _{An electrolytic solution was prepared by dissolving LiPF 6} as an electrolyte in this EMC so as to have a concentration of 1 M and LiTFSI at a concentration of 1 M.

[Manufacturing of coin-type dual-ion power storage device]
The obtained positive electrode was punched into a disk shape having a diameter of 14 mm and the obtained negative electrode having a diameter of 12 mm, and the mixture was transferred to a dry room having a dew point of −40 ° C. or lower and vacuum dried at 150 ° C. for 24 hours. The dried positive electrode and negative electrode were laminated via a paper separator (trade name: TF4425) manufactured by Nippon Kodoshi Paper Industry Co., Ltd. so that the positive electrode active material layer and the negative electrode active material layer face each other. The obtained laminate and the above electrolytic solution were housed in a 2032 coin cell made of SUS316L clad with aluminum and sealed to prepare a coin-type dual ion power storage device.

<Example 32>
A coin-type dual ion power storage device was produced in the same manner as in Example 31 except that the concentration of LiPF _{6 in the electrolytic solution was 1.5 M and the concentration of LiTFSI was 1.5 M.}

<Example 33>
A coin-type dual-ion power storage device was produced in the same manner as in Example 31 except that _{LiBF 4} was further dissolved in the electrolytic solution so as to have a concentration of 0.4 M.

<Example 34>
The same _{as in Example 31 except that the concentration of LiPF 6 in} the electrolytic solution was 1.5 M, the concentration of LiTFSI was 1.5 M, and LiBF ₄ was dissolved so as to have a concentration of 0.5 M. A coin-type dual-ion power storage device was manufactured.

<Example 35>
A coin-type dual ion power storage device was produced in the same manner as in Example 31 except that a mixed solvent containing 90% by volume of EMC and 10% by volume of PC was used as the non-aqueous solvent of the electrolytic solution.

<Example 36>
As a non-aqueous solvent for the electrolytic solution, a mixed solvent containing 90% by volume of EMC and 10% by volume of PC was used, _{except that the concentration of LiPF 6} was 1.5M and the concentration of LiTFSI was 1.5M. , A coin-type dual ion power storage device was produced in the same manner as in Example 31.

<Example 37>
Examples except that a mixed solvent containing 90% by volume of EMC and 10% by volume of PC was used as the non-aqueous solvent of the electrolytic solution, and LiBF ₄ was further dissolved so as to have a concentration of 0.4M. A coin-type dual ion power storage device was manufactured in the same manner as in 31.

<Example 38>
As a non-aqueous solvent for the electrolytic solution, a mixed solvent containing 90% by volume of EMC and 10% by volume of PC was used, _{the concentration of LiPF 6} was 1.5M, the concentration of LiTFSI was 1.5M, and further. , LiBF ₄ was dissolved so as to have a concentration of 0.5 M, and a coin-type dual ion power storage device was produced in the same manner as in Example 31.

<Comparative Example 31>
A coin type as in Example 31 except that the concentration of _{LiPF 6 was 3.5 M and LiBF 4} was dissolved so as to have a concentration of 0.5 M without using LiTFSI as the electrolytic solution. A dual ion power storage device was manufactured.

<Comparative Example 32>
As the non-aqueous solvent of the electrolytic solution, a mixed solvent containing 90% by volume of EMC and 10% by volume of PC was used, and the concentration of _{LiPF 6 was 3.5M without using LiTFSI in the electrolytic solution.} , LiBF ₄ was dissolved so as to have a concentration of 0.5 M, and a coin-type dual ion storage device was produced in the same manner as in Example 31.

<Evaluation>
The coin-type dual-ion power storage device was CC-charged to 3.5 V with a current value of 0.3 C in a constant temperature bath at 25 ° C. After that, the mode was switched to CV mode and charging was continued for 30 minutes. Then, discharge was performed up to 2.0 V with a current value of 0.3 C. This operation was performed twice more. Next, the current value was changed to 1C and the same operation was performed, and the discharge capacity obtained at the time of 1C discharge was used as the initial capacity.

The low temperature capacity retention rate was measured by CC charging up to 3.5 V with a current value of 1 C in a constant temperature bath at 25 ° C. After that, the mode was switched to CV mode and charging was continued for 30 minutes. The temperature of the constant temperature bath was switched to −30 ° C., held for 2 hours after reaching −30 ° C., and then discharged to 2.0 V at 1 C. The low temperature capacity retention rate at -30 was calculated using the following formula.
(Discharge capacity at -30 ° C / Discharge capacity at 25 ° C) x 100 (%)

The results are shown in Table 3. The coin-type dual-ion power storage devices of Examples 31 to 38 in which the electrolyte contains LiTFSI and LiPF ₆ have a temperature of −30 ° C. as compared with the coin-type dual-ion power storage devices of Comparative Examples 31 and 32 in which the electrolyte does not contain LiTFSI. The low temperature capacity maintenance rate has increased. In particular, in _{Examples 33, 34, 37, and 38 further containing LiBF 4} , the low temperature capacity retention rate was high.

<Comparative Example 33>
A coin-type dual ion power storage device was produced in the same manner as in Example 31 except that the thickness of the active material layer of the positive electrode was 60 μm and that of the negative electrode was 40 μm, and the low temperature characteristics were evaluated.
The results are also shown in Table 3. As a result of comparing Example 31 and Comparative Example 33, in Example 31, by making both electrodes thinner, the low temperature capacity retention rate of -30 ° C is higher than that of Comparative Example 33, and it is effective to make both electrodes thin. It was confirmed that there was.

<Comparative Example 34>
A coin-type dual ion power storage device was produced in the same manner as in Example 31 except that the positive electrode was 3.0% by mass of acetylene black without using CNT and the negative electrode was 3.0% by mass of acetylene black without using CNT, and the temperature was low. The characteristics were evaluated.
The results are also shown in Table 3. As a result of comparing Example 31 and Comparative Example 34, by adding CNT to the electrode, the low temperature capacity retention rate of -30 ° C was higher than that of Comparative Example 34, and it was found that the addition of CNT was effective. confirmed.

Claims (13)

It has an electrolytic solution containing a non-aqueous solvent and an electrolyte dissolved in the non-aqueous solvent, a positive electrode current collector, and a positive electrode active material layer formed on the surface of the positive electrode current collector, and the positive electrode active material layer is the positive electrode active material layer. It has a positive electrode containing a positive electrode active material capable of storing and releasing anions of an electrolyte, a negative electrode current collector, and a negative electrode active material layer formed on the surface of the negative electrode current collector, and the negative electrode active material layer is made of the electrolyte. A dual-ion storage device including a negative electrode containing a negative electrode active material capable of storing and releasing cations.
The positive electrode active material is graphite.
The negative electrode active material is lithium titanate _{represented by the general formula Li 4} Ti ₅ O _12.
The charging curve showing the relationship between the charging capacity per 1 g of graphite and the charging voltage when charged at a constant current of 1 C in an environment of a temperature of 25 ° C. shows that the charging capacity per 1 g of graphite is 40 mAhg ^-1 or more. , A dual ion power storage device characterized in that the charging voltage has an inflection point between 3.25 and 3.39 V. The dual ion power storage device according to claim 1, wherein the upper limit of the charging voltage of the dual ion power storage device is 3.5 V. The dual ion power storage device according to claim 1 or 2, wherein the surface spacing of the (002) planes of graphite is 3.360 nm or less. The dual ion storage device according to any one of claims 1 to 3, wherein the anion of the electrolyte _{contains one or more selected from PF 6} ⁻ , BF ₄ ⁻ , TFSI ⁻ , and FSI ^−. The dual ion storage device according to any one of claims 1 to 4, wherein the cation of the electrolyte ^{contains Li +.} The dual ion storage device according to any one of claims 1 to 5, wherein the non-aqueous solvent contains a chain carbonate. The non-aqueous solvent according to any one of claims 1 to 5, wherein the non-aqueous solvent contains at least one selected from ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, ethylene carbonate, propylene carbonate, and butylene carbonate. Dual ion power storage device. At least one of the positive electrode active material layer and the negative electrode active material layer contains at least one conductive auxiliary agent selected from carbon nanotubes, carbon fibers, vapor phase carbon fibers, graphite, amorphous carbon, and carbon blacks. The dual ion power storage device according to any one of claims 1 to 7, wherein the dual ion power storage device is characterized. The dual ion power storage device according to any one of claims 1 to 7, wherein at least one of the positive electrode active material layer and the negative electrode active material layer contains carbon nanotubes and carbon blacks. The dual ion power storage device according to claim 8 or 9, wherein the carbon blacks are at least one selected from acetylene black, ketjen black, and furnace black. The electrolyte contains at least one lithium imide salt selected from the group consisting of lithium bis (fluorosulfonyl) imide and lithium bis (trifluoromethanesulfonyl) imide, and lithium hexafluorophosphate.
The positive electrode current collector includes an aluminum foil in which a peak having a maximum peak value in the range of 74.8 eV or more and 76.7 eV is detected in the X-ray photoelectron spectroscopy spectrum measured by X-ray photoelectron spectroscopy. The dual ion power storage device according to any one of claims 1 to 10. The dual ion storage device according to claim 11, wherein the lithium imide salt is contained in a range of 0.5 mol or more and 10 mol or less with respect to 1 mol of lithium hexafluorophosphate. The dual ion storage device according to claim 11 or 12, wherein the electrolyte further contains lithium tetrafluoroborate in a range of 0.05 mol or more and 10 mol or less with respect to 1 mol of lithium hexafluorophosphate. ..

Images