WO2020079819A1

WO2020079819A1 - Lithium secondary battery

Info

Publication number: WO2020079819A1
Application number: PCT/JP2018/038899
Authority: WO
Inventors: 大祐飯田; 英二中島; 雄樹藤田
Original assignee: 日本碍子株式会社
Priority date: 2018-10-18
Filing date: 2018-10-18
Publication date: 2020-04-23

Abstract

Provided is a lithium secondary battery in which reaction resistance is significantly reduced. This lithium secondary battery comprises: a positive electrode layer that includes a lithium composite oxide; an electroconductive positive electrode current collector; a negative electrode layer that includes carbon; an electroconductive negative electrode current collector; a separator that is interposed between the positive electrode layer and the negative electrode layer; and an electrolytic solution that includes lithium bis(fluorosulfonyl)imide (LiFSI) and lithiumdifluoro(oxalate)borate (LiDFOB) in an organic solvent. At least one of the positive electrode current collector and the negative electrode current collector includes aluminum.

Description

Lithium secondary battery

The present invention relates to a lithium secondary battery.

In recent years, smart cards with built-in batteries are being put to practical use. An example of a smart card with a built-in primary battery is a credit card with a one-time password display function. An example of a smart card with a built-in secondary battery is a card with a fingerprint authentication / wireless communication function, which includes a wireless communication IC, a fingerprint analysis ASIC, and a fingerprint sensor. A smart card battery is generally required to have characteristics such as a thickness of less than 0.45 mm, a high capacity and a low resistance, bending resistance, and resistance to a process temperature.

Meanwhile, lithium bis (fluorosulfonyl) imide (LiFSI) is known as an electrolyte for lithium batteries. LiFSI has a higher ionic conductivity and a higher thermal decomposition temperature than LiPF ₆ which is a commonly used electrolyte. Therefore, when used in combination with LiPF ₆ , cycle life and high temperature storage characteristics can be improved.

It has been proposed to use LiFSI in combination with lithium difluoro (oxalato) borate (LiDFOB). For example, in Non-Patent Document 1, as compared with a LiCoO ₂ -based battery using a LiPF ₆ -based electrolyte, cycle stability of a LiCoO ₂ -based battery using a LiFSI _0.7 -LiDFOB _0.3 -based electrolyte and Although disclosed as having excellent rate performance, the ratings presented in this document were made with respect to half-cells containing LiCoO ₂ cathodes.

Several reports have been made on LiDFOB. For example, in Non-Patent Document 2, LiDFOB was used as a salt-type additive to enhance the interfacial stability of a high-voltage Li-rich cathode and a graphite anode, and the LiDFOB additive effectively reduces the cycle performance of the electrode. It is disclosed that it was suppressed to. Non-Patent Document 3 describes LiDFOB with respect to capacity and impedance characteristics of a cell having a Li _1.2 Ni _0.15 Mn _0.55 Co _0.1 O ₂ -based positive electrode, a graphite-based negative electrode, and a LiPF ₆ -based electrolyte. The effect of electrolyte additives has been evaluated using a combination of electrochemical cycling and surface analysis techniques. It is also described in this document that LiDFOB acts as a bifunctional additive and reacts at both electrodes to reduce both cell capacitance loss and impedance.

Patent No. 5587052 International Publication No. 2017/146088

In a lithium secondary battery for use in a smart card with a built-in battery, suppression of voltage drop due to pulse discharge is required, and further reduction of bulk resistance and reaction resistance is desired. Therefore, it is desired to use an electrolytic solution that reduces the reaction resistance. Therefore, as described above, it is possible to improve the cycle life and the high temperature storage characteristics by using LiFSI having higher ionic conductivity and thermal decomposition temperature than LiPF ₆ . However, when LiFSI is used as a single electrolyte in a lithium secondary battery including an aluminum current collector, the aluminum of the current collector is corroded, and as a result, the reaction resistance of the battery increases due to the corrosion of aluminum. There is a problem with inviting.

The present inventors have recently proposed a lithium secondary battery including a positive electrode layer containing a lithium composite oxide, a negative electrode layer containing carbon, a separator, and a current collector made of aluminum in a lithium bis (fluoro) compound in an organic solvent. It was found that the reaction resistance of the battery is significantly reduced by adopting the electrolytic solution containing sulfonyl) imide (LiFSI) and lithium difluoro (oxalato) borate (LiDFOB).

Therefore, an object of the present invention is to provide a lithium secondary battery whose reaction resistance is significantly reduced.

According to one embodiment of the present invention, a positive electrode layer containing a lithium composite oxide,
A positive electrode current collector having conductivity,
A negative electrode layer containing carbon,
A negative electrode current collector having conductivity,
A separator interposed between the positive electrode layer and the negative electrode layer,
An electrolytic solution containing lithium bis (fluorosulfonyl) imide (LiFSI) and lithium difluoro (oxalato) borate (LiDFOB) in an organic solvent,
Equipped with
A lithium secondary battery is provided in which at least one of the positive electrode current collector and the negative electrode current collector contains aluminum.

It is a schematic cross section of an example of the lithium secondary battery of the present invention. It is a figure which shows the first half of an example of the manufacturing process of a lithium secondary battery. FIG. 2B is a diagram illustrating a second half of the example of the manufacturing process of the lithium secondary battery and showing a process following the process shown in FIG. A photograph of the film-clad battery is included at the right end of FIG. 2B. 3 is an SEM image showing an example of a cross section perpendicular to the plate surface of the oriented positive electrode plate. 4 is an EBSD image in a cross section of the oriented positive electrode plate shown in FIG. 3. 5 is a histogram showing an area-based distribution of orientation angles of primary particles in the EBSD image of FIG. 4. 5 is a graph showing the resistance at 10 Hz measured by an AC impedance method at a battery voltage of 3.8 V for the lithium secondary batteries manufactured in Example 1 and Comparative Examples 1 and 2. 3 is a Cole-Cole plot measured by an AC impedance method at a battery voltage of 3.8 V for the lithium secondary batteries manufactured in Example 1 and Comparative Examples 1 and 2.

Lithium Secondary Battery The lithium secondary battery 10 shown in FIG. 1 includes a positive electrode current collector 14, a positive electrode layer 16, a separator 18, a negative electrode layer 20, a negative electrode current collector 22, and an electrolytic solution 24. The positive electrode layer 16 contains a lithium composite oxide. The negative electrode layer 20 contains carbon. The separator 18 is interposed between the positive electrode layer 16 and the negative electrode layer 20. The electrolytic solution 24 contains lithium bis (fluorosulfonyl) imide (LiFSI) and lithium difluoro (oxalato) borate (LiDFOB) in an organic solvent. The positive electrode current collector 14 and the negative electrode current collector 22 each have conductivity, and at least one of the positive electrode current collector 14 and the negative electrode current collector 22 contains aluminum. As described above, in the configuration of the lithium secondary battery including the positive electrode layer 16 including the lithium composite oxide, the negative electrode layer 20 including carbon, the separator 18, and the current collector made of aluminum, LiFSI and LiDFOB are included in the organic solvent. By adopting the electrolytic solution, the reaction resistance of the battery is significantly reduced.

As described above, it is conceivable to use LiFSI, which has higher ionic conductivity and thermal decomposition temperature than LiPF ₆ , to improve cycle life and high temperature storage characteristics. However, when LiFSI is used as a single electrolyte in a lithium secondary battery including an aluminum current collector, the aluminum of the current collector is corroded, and as a result, the reaction resistance of the battery increases due to the corrosion of aluminum. There is a problem with inviting. In this respect, in the present invention, the above problem can be solved by adopting an electrolytic solution containing LiFSI and LiDFOB in an organic solvent.

Therefore, the electrolytic solution 24 contains LiFSI and LiDFOB in the organic solvent. LiFSI is lithium bis (fluorosulfonyl) imide and is represented by Li [N (SO ₂ F) ₂ ]. LiDFOB (sometimes referred to as LiODFB) is lithium difluoro (oxalato) borate and is represented by LiF ₂ BC ₂ O ₄ . A preferred organic solvent is a mixed solvent of ethylene carbonate (EC) and methyl ethyl carbonate (MEC), a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC), or a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC). A mixed solvent is included, and a particularly preferable organic solvent includes a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC). The proportion of LiFSI in the total number of moles of LiFSI and LiDFOB in the electrolytic solution is preferably 20% or more, more preferably 50% or more, and the upper limit is 70%. In other words, the ratio of the molar concentrations of LiFSI and LiDFOB in the electrolytic solution is preferably 7: 3 to 2: 8.

The electrolytic solution 24 preferably further contains vinylene carbonate (VC) and / or fluoroethylene carbonate (FEC) and / or vinyl ethylene carbonate (VEC) as an additive. Both VC and FEC have excellent heat resistance. Therefore, by including such an additive in the electrolytic solution 24, an SEI film having excellent heat resistance can be formed on the surface of the negative electrode layer 20.

The lithium secondary battery 10 preferably has low internal resistance (particularly reaction resistance). In particular, in the lithium secondary battery 10 of the present invention, by using the above-described electrolytic solution 24, it is possible to realize a desired low internal resistance (in particular, reaction resistance) despite including the aluminum current collector. The reaction resistance referred to here is the diameter of the arc component in the real axis direction in the Cole-Cole plot obtained from the AC impedance test, and this value matches the resistance value at approximately 10 Hz. For example, the lithium secondary battery 10 has a resistance at 10 Hz measured by an AC impedance method of 9.5 Ω · cm ² or less, more preferably 9.0 Ω · cm ² or less, and further preferably 8. It is 1 Ω · cm ² or less. The lower limit of resistance at 10 Hz is not particularly limited, but is typically 2.0 Ω · cm ² or more.

The positive electrode layer 16 is preferably a lithium composite oxide sintered body plate. The fact that the positive electrode layer 16 is a sintered body plate means that the positive electrode layer 16 does not contain a binder or a conductive additive. This is because even if the green sheet contains the binder, the binder disappears or burns out during firing. Further, since the positive electrode layer 16 does not contain a binder, there is an advantage that deterioration of the positive electrode due to the electrolytic solution 24 can be avoided. The lithium composite oxide forming the sintered plate is particularly preferably lithium cobalt oxide (typically LiCoO ₂ (hereinafter sometimes abbreviated as LCO)). Various lithium complex oxide sintered body plates or LCO sintered body plates are known, and are disclosed, for example, in Patent Document 1 (Japanese Patent No. 5587052) and Patent Document 2 (International Publication No. 2017/146088). Things can be used.

According to a preferred embodiment of the present invention, the positive electrode layer 16, that is, the lithium composite oxide sintered body plate includes a plurality of primary particles composed of a lithium composite oxide, and the plurality of primary particles are on the plate surface of the positive electrode plate. In contrast, the oriented positive electrode plate is oriented at an average orientation angle of more than 0 ° and 30 ° or less. 3 shows an example of a cross-sectional SEM image perpendicular to the plate surface of the oriented positive electrode plate 16, while FIG. 4 shows an electron backscatter diffraction (EBSD) image in a cross section perpendicular to the plate surface of the oriented positive electrode plate 16. Indicates. Further, FIG. 5 shows a histogram showing the distribution of the orientation angle of the primary particles 11 in the EBSD image of FIG. 4 on an area basis. In the EBSD image shown in FIG. 4, discontinuity of crystal orientation can be observed. In FIG. 4, the orientation angle of each primary particle 11 is shown by the shade of color, and the darker the color, the smaller the orientation angle. The orientation angle is an inclination angle formed by the (003) plane of each primary particle 11 with respect to the plate surface direction. It should be noted that, in FIGS. 3 and 4, the black-displayed portions inside the oriented positive electrode plate 16 are pores.

The oriented positive electrode plate 16 is an oriented sintered body composed of a plurality of primary particles 11 bonded to each other. Each of the primary particles 11 is mainly in the form of a plate, but may be in the form of a rectangular parallelepiped, a cube or a sphere. The cross-sectional shape of each primary particle 11 is not particularly limited, and may be a rectangle, a polygon other than a rectangle, a circle, an ellipse, or a complicated shape other than these.

Each primary particle 11 is composed of a lithium composite oxide. The lithium composite oxide means Li _x MO ₂ (0.05 <x <1.10, M is at least one kind of transition metal, and M is typically one or more kinds of Co, Ni and Mn. Is included). The lithium composite oxide has a layered rock salt structure. The layered rock salt structure is a crystal structure in which a lithium layer and a transition metal layer other than lithium are alternately stacked with an oxygen layer in between, that is, a transition metal ion layer and a lithium single layer are alternately provided via oxide ions. It refers to a laminated crystal structure (typically an α-NaFeO ₂ type structure, that is, a structure in which a transition metal and lithium are regularly arranged in the [111] axis direction of a cubic rock salt type structure). Examples of the lithium composite oxide include Li _x CoO ₂ (lithium cobaltate), Li _x NiO ₂ (lithium nickelate), Li _x MnO ₂ (lithium manganate), Li _x NiMnO ₂ (lithium nickel manganate). , Li _x NiCoO ₂ (nickel / lithium cobalt oxide), Li _x CoNiMnO ₂ (cobalt / nickel / lithium manganate), Li _x CoMnO ₂ (lithium / cobalt manganate), and the like, particularly preferably Li _x CoO ₂ (Lithium cobaltate, typically LiCoO ₂ ). The lithium composite oxide includes Mg, Al, Si, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, Ag, Sn, Sb, Te, Ba. , Bi, and W may be contained in one or more kinds of elements.

As shown in FIGS. 4 and 5, the average value of the orientation angle of each primary particle 11, that is, the average orientation angle is more than 0 ° and 30 ° or less. This brings various advantages as follows. Firstly, since the respective primary particles 11 lie in a direction inclined with respect to the thickness direction, it is possible to improve the adhesion between the respective primary particles. As a result, lithium ion conductivity between a certain primary particle 11 and another primary particle 11 adjacent to both sides in the longitudinal direction of the primary particle 11 can be improved, and thus rate characteristics can be improved. Secondly, the rate characteristic can be further improved. This is because, as described above, when lithium ions move in and out, the oriented positive electrode plate 16 is more liable to expand and contract in the thickness direction than in the plate surface direction, so that the oriented positive electrode plate 16 expands and contracts smoothly. This is because the lithium ions can move in and out smoothly.

The average orientation angle of the primary particles 11 is obtained by the following method. First, in an EBSD image obtained by observing a rectangular region of 95 μm × 125 μm at a magnification of 1000 as shown in FIG. 4, three horizontal lines that divide the oriented positive electrode plate 16 into four equal parts in the thickness direction and the oriented positive electrode plate 16 are formed. And draw three vertical lines that divide it in the board direction. Next, the average orientation angle of the primary particles 11 is obtained by arithmetically averaging the orientation angles of all the primary particles 11 intersecting at least one of the three horizontal lines and the three vertical lines. The average orientation angle of the primary particles 11 is preferably 30 ° or less, and more preferably 25 ° or less from the viewpoint of further improving the rate characteristics. From the viewpoint of further improving the rate characteristics, the average orientation angle of the primary particles 11 is preferably 2 ° or more, more preferably 5 ° or more.

As shown in FIG. 5, the orientation angle of each primary particle 11 may be widely distributed from 0 ° to 90 °, but most of them are distributed in the region of more than 0 ° and 30 ° or less. Is preferred. That is, when the cross section of the oriented sintered body that constitutes the oriented positive electrode plate 16 is analyzed by EBSD, the orientation angle of the primary particles 11 included in the analyzed cross section with respect to the plate surface of the oriented positive electrode plate 16 is 0 °. The total area of the primary particles 11 (hereinafter, referred to as low-angle primary particles) that is less than 30 ° is less than 30 °, and the total area of the primary particles 11 (specifically, the 30 primary particles 11 used to calculate the average orientation angle) The total area is preferably 70% or more, more preferably 80% or more. As a result, the proportion of the primary particles 11 having high mutual adhesion can be increased, so that the rate characteristics can be further improved. The total area of the low-angle primary particles having an orientation angle of 20 ° or less is more preferably 50% or more with respect to the total area of the 30 primary particles 11 used to calculate the average orientation angle. . Furthermore, the total area of the low-angle primary particles having an orientation angle of 10 ° or less is more preferably 15% or more with respect to the total area of the 30 primary particles 11 used for calculating the average orientation angle. .

Since each primary particle 11 is mainly plate-shaped, as shown in FIGS. 3 and 4, the cross section of each primary particle 11 extends in a predetermined direction, and typically has a substantially rectangular shape. That is, when the cross section of the oriented sintered body is analyzed by EBSD, the total area of the primary particles 11 having an aspect ratio of 4 or more among the primary particles 11 included in the analyzed cross section is the primary area included in the cross section. The total area of the particles 11 (specifically, 30 primary particles 11 used for calculating the average orientation angle) is preferably 70% or more, more preferably 80% or more. Specifically, in the EBSD image as shown in FIG. 4, this can further improve the mutual adhesiveness between the primary particles 11 and, as a result, further improve the rate characteristics. The aspect ratio of the primary particles 11 is a value obtained by dividing the maximum Feret diameter of the primary particles 11 by the minimum Feret diameter. The maximum Feret diameter is the maximum distance between the straight lines when the primary particles 11 are sandwiched by two parallel straight lines on the EBSD image when the cross-section is observed. The minimum Feret diameter is the minimum distance between the straight lines when the primary particles 11 are sandwiched by two parallel straight lines on the EBSD image.

It is preferable that the average particle diameter of the plurality of primary particles constituting the oriented sintered body is 5 μm or more. Specifically, the average particle size of the 30 primary particles 11 used to calculate the average orientation angle is preferably 5 μm or more, more preferably 7 μm or more, and further preferably 12 μm or more. As a result, the number of grain boundaries between the primary particles 11 in the direction in which lithium ions are conducted is reduced and the lithium ion conductivity as a whole is improved, so that the rate characteristics can be further improved. The average particle size of the primary particles 11 is a value obtained by arithmetically averaging the equivalent circle diameters of the primary particles 11. The equivalent circle diameter is the diameter of a circle having the same area as each primary particle 11 on the EBSD image.

The porosity of the lithium composite oxide sintered body plate is preferably 3 to 40%, more preferably 5 to 38%, further preferably 10 to 36%, and particularly preferably 20 to 35%. The stress relief effect by the pores and the increase in capacity can be expected, and the mutual adhesion between the primary particles 11 can be further improved, so that the rate characteristics can be further improved. The porosity of the sintered body is calculated by polishing the cross section of the positive electrode plate by CP (Cross Section Polisher) polishing and then observing with a SEM at 1000 magnifications, and binarizing the obtained SEM image. The average equivalent circle diameter of each pore formed inside the oriented sintered body is not particularly limited, but is preferably 8 μm or less. The smaller the average equivalent circle diameter of each pore, the more the mutual adhesion between the primary particles 11 can be further improved, and as a result, the rate characteristics can be further improved. The average equivalent circle diameter of pores is a value obtained by arithmetically averaging the equivalent circle diameters of 10 pores on the EBSD image. The equivalent circle diameter is the diameter of a circle having the same area as each pore on the EBSD image. The pores formed inside the oriented sintered body may be open pores connected to the outside of the positive electrode layer 16, but preferably do not penetrate the positive electrode layer 16. Each pore may be a closed pore.

The average pore diameter of the lithium composite oxide sintered body plate is preferably 15 μm or less, more preferably 12 μm or less, and further preferably 10 μm or less. It is possible to suppress the occurrence of stress concentration locally in the large pores, and to easily release the stress uniformly in the sintered body. The lower limit of the average pore diameter is not particularly limited, but the average pore diameter is preferably 0.1 μm or more, more preferably 0.3 μm or more, from the viewpoint of the stress releasing effect of the pores.

The thickness of the positive electrode layer 16 is 70 to 120 μm, preferably 80 to 100 μm, more preferably 80 to 95 μm, and particularly preferably 85 to 95 μm. Within such a range, the active material capacity per unit area is increased to improve the energy density of the lithium secondary battery 10, and the deterioration of battery characteristics (especially increase in resistance value) due to repeated charging / discharging. Can be suppressed.

The negative electrode layer 20 contains carbon as a negative electrode active material. Examples of carbon include graphite, hard carbon, soft carbon, pyrolytic carbon, coke, resin fired bodies, mesophase spherules, mesophase pitch, and the like, and preferably graphite. The graphite may be either natural graphite or artificial graphite. Moreover, you may mix and use several types among these carbons. The negative electrode layer 20 preferably further contains a binder. Examples of the binder include styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE) and the like, and preferably styrene-butadiene rubber (SBR) or polyvinylidene fluoride (PVDF).

The thickness of the negative electrode layer 20 is 90 to 200 μm, preferably 95 to 160 μm, and more preferably 100 to 150 μm. Within such a range, the active material capacity per unit area can be increased and the energy density of the lithium secondary battery 10 can be improved. The density of the negative electrode layer 20 is preferably 1.15 to 1.70 g / cm ³ , more preferably 1.20 to 1.48 g / cm ³ , and even more preferably 1.25 to 1.45 g / cm ³ . It is ³ . Within such a range, the active material capacity per unit area can be increased and the energy density of the lithium secondary battery 10 can be improved.

The separator 18 is preferably a polyolefin, polyimide, or cellulose separator. Examples of polyolefins include polypropylene (PP), polyethylene (PE), and combinations thereof. From the viewpoint of being inexpensive, a polyolefin or cellulose separator is preferable. Further, the surface of the separator 18 may be covered with ceramics such as alumina (Al ₂ O ₃ ), magnesia (MgO), silica (SiO ₂ ).

Preferably, the lithium secondary battery 10 further includes a pair of exterior films 26, the outer peripheral edges of the exterior films 26 are sealed to each other to form an internal space, and the battery element 12 and the electrolytic solution 24 are housed in the internal space. To do. That is, as shown in FIG. 1, the battery element 12 and the electrolytic solution 24, which are the contents of the lithium secondary battery 10, are packaged and sealed with a pair of exterior films 26, and as a result, the lithium secondary battery The battery 10 is in the form of a so-called film-clad battery. Here, the battery element 12 is defined as including the positive electrode layer 16, the separator 18, and the negative electrode layer 20, and typically further includes the positive electrode current collector 14 and the negative electrode current collector 22. The positive electrode current collector 14 and the negative electrode current collector 22 are not particularly limited as long as they have conductivity, but are preferably metal foils such as copper foil and aluminum foil. At least one of the positive electrode current collector 14 and the negative electrode current collector 22 contains aluminum, and is typically an aluminum foil. Particularly preferably, the positive electrode current collector 14 is an aluminum foil and the negative electrode current collector 22 is a copper foil. The positive electrode current collector is preferably interposed between the positive electrode layer 16 and the exterior film 26, and the negative electrode current collector is preferably interposed between the negative electrode layer 20 and the exterior film 26. Further, the positive electrode current collector is preferably provided with a positive electrode terminal extending from the positive electrode current collector, and the negative electrode current collector is provided with a negative electrode terminal extending from the negative electrode current collector. preferable. The outer edge of the lithium secondary battery 10 is preferably sealed by heat-sealing the exterior films 26. The sealing by heat fusion is preferably performed using a heat bar (also referred to as a heating bar) which is generally used for heat sealing. Typically, it is a quadrilateral shape of the lithium secondary battery 10, and it is preferable that the outer peripheral edges of the pair of exterior films 26 are sealed over all four outer peripheries.

As the exterior film 26, a commercially available exterior film may be used. The thickness of the exterior film 26 is preferably 50 to 80 μm per sheet, more preferably 55 to 70 μm, and further preferably 55 to 65 μm. The preferable exterior film 26 is a laminate film containing a resin film and a metal foil, and more preferably an aluminum laminate film containing a resin film and an aluminum foil. The laminate film is preferably provided with resin films on both sides of a metal foil such as an aluminum foil. In this case, the resin film on one side of the metal foil (hereinafter referred to as the surface protective film) is composed of a material having excellent reinforcing properties such as nylon, polyamide, polyethylene terephthalate, polyimide, polytetrafluoroethylene, polychlorotrifluoroethylene. It is preferable that the resin film on the other side of the metal foil is made of a heat seal material such as polypropylene.

Typically, the negative electrode layer 20 has a size larger than the size of the positive electrode layer 16, while the separator 18 has a size larger than the sizes of the positive electrode layer 16 and the negative electrode layer 20. Then, the outer peripheral portion of the separator 18 is in close contact with at least the outer peripheral edge of the exterior film 26 on the positive electrode layer 16 side or the peripheral region in the vicinity thereof to separate the compartment containing the positive electrode layer 16 from the compartment containing the negative electrode layer 20. ing. Further, the outer peripheral portion of the separator 18 may be in close contact with the outer peripheral edge of the exterior film 26 on the negative electrode layer 20 side or the peripheral area in the vicinity thereof.

The lithium secondary battery 10 is preferably a thin secondary battery that can be embedded in a card, and more preferably a thin secondary battery that is embedded in a resin base material to form a card. That is, according to another preferred embodiment of the present invention, there is provided a battery-embedded card including a resin base material and a lithium secondary battery embedded in the resin base material. Such a card with a built-in battery typically includes a pair of resin films and a lithium secondary battery sandwiched between the pair of resin films, and the resin films are attached to each other with an adhesive or heated. It is preferable that the resin films are heat-sealed together by a press.

Preferably, the lithium secondary battery 10 is a small and thin lithium secondary battery with high energy density. Specifically, the energy density of the lithium secondary battery 10 is 200 to 300 mWh / cm ³ , preferably 210 to 300 mWh / cm ³ , more preferably 225 to 295 mWh / cm ³ , and further preferably 240 to 280 mWh / cm ³ . It is cm ³ . The thickness of the lithium secondary battery 10 is 350 to 500 μm, preferably 380 to 450 μm, and more preferably 400 to 430 μm. Further, the lithium secondary battery 10 has a rectangular flat plate shape with each side having a length of 20 to 55 mm. If the thickness and size are within such a range, the lithium secondary battery 10 can be incorporated in a thin device such as a smart card. It will be extremely advantageous.

Production Method The lithium composite oxide sintered body plate of the present invention may be produced by any method, but preferably, (a) production of a lithium composite oxide-containing green sheet, (b) optional production. Excess lithium source-containing green sheet is produced, and (c) the green sheet is laminated and fired.

(A) Preparation of Green Sheet Containing Lithium Composite Oxide First, a raw material powder composed of a lithium composite oxide is prepared. This powder preferably contains synthesized plate-like particles (for example, LiCoO ₂ plate-like particles) having a composition of LiMO ₂ (M is as described above). The volume-based D50 particle size of the raw material powder is preferably 0.3 to 30 μm. For example, the method for producing LiCoO ₂ plate-like particles can be performed as follows. First, the Co ₃ O ₄ raw material powder and the Li ₂ CO ₃ raw material powder are mixed and fired (500 to 900 ° C., 1 to 20 hours) to synthesize the LiCoO ₂ powder. The obtained LiCoO ₂ powder is pulverized with a pot mill to a volume-based D50 particle size of 0.2 μm to 10 μm, whereby plate-shaped LiCoO ₂ particles capable of conducting lithium ions in parallel with the plate surface are obtained. Such LiCoO ₂ particles are formed into a plate shape such as a method in which a green sheet using a LiCoO ₂ powder slurry is grown and then crushed, a flux method, hydrothermal synthesis, single crystal growth using a melt, a sol-gel method, or the like. It can also be obtained by a method of synthesizing a crystal. The obtained LiCoO ₂ particles are in a state of being easily cleaved along the cleavage plane. Be to cleave by crushing the LiCoO ₂ particles, it can be produced LiCoO ₂ plate-like particles.

The above plate-like particles may be used alone as a raw material powder, or a mixed powder of the above plate-like powder and another raw material powder (for example, Co ₃ O ₄ particles) may be used as a raw material powder. In the latter case, it is preferable that the plate-like powder functions as template particles for imparting orientation, and the other raw material powder (for example, Co ₃ O ₄ particles) functions as matrix particles that can grow along the template particles. In this case, it is preferable that the raw material powder is a powder in which template particles and matrix particles are mixed at 100: 0 to 3:97. When the Co ₃ O ₄ raw material powder is used as matrix particles, the volume-based D50 particle diameter of the Co ₃ O ₄ raw material powder is not particularly limited and may be, for example, 0.1 to 1.0 μm, but LiCoO ₂ template particles It is preferable that the particle size is smaller than the volume-based D50 particle size. The matrix particles can also be obtained by heat treating a Co (OH) ₂ raw material at 500 ° C. to 800 ° C. for 1 to 10 hours. In addition to Co ₃ O ₄ , Co (OH) ₂ particles may be used as the matrix particles, or LiCoO ₂ particles may be used.

When the raw material powder is composed of 100% LiCoO ₂ template particles or when LiCoO ₂ particles are used as matrix particles, a large-sized (eg 90 mm × 90 mm square) and flat LiCoO ₂ sintered body plate is obtained by firing. You can The mechanism is not clear, but since LiCoO ₂ is not synthesized in the firing process, it is expected that a volume change during firing or local unevenness is unlikely to occur.

Raw material powder is mixed with a dispersion medium and various additives (binder, plasticizer, dispersant, etc.) to form a slurry. A lithium compound other than LiMO ₂ (for example, lithium carbonate) may be added to the slurry in an excess of about 0.5 to 30 mol% for the purpose of promoting grain growth during the firing step described later or compensating for volatile components. It is desirable not to add a pore former to the slurry. The slurry is preferably degassed by stirring under reduced pressure, and the viscosity is preferably adjusted to 4000 to 10000 cP. The obtained slurry is shaped into a sheet to obtain a lithium composite oxide-containing green sheet. The green sheet thus obtained is an independent sheet-shaped molded body. An independent sheet (sometimes referred to as a "free-standing film") refers to a sheet that can be handled as a single unit independently of other supports (including a thin piece having an aspect ratio of 5 or more). That is, the independent sheet does not include a sheet fixed to another support (substrate or the like) and integrated with the support (which cannot be separated or becomes difficult to separate). Sheet molding is preferably performed using a molding method capable of applying a shearing force to the plate-like particles (eg template particles) in the raw material powder. By doing so, the average inclination angle of the primary particles can be set to more than 0 ° and 30 ° or less with respect to the plate surface. The doctor blade method is suitable as a molding method capable of applying a shearing force to the plate-like particles. The thickness of the lithium composite oxide-containing green sheet may be appropriately set so as to be the desired thickness described above after firing.

(B) Preparation of green sheet containing excess lithium source (optional step)
If desired, an excess lithium source-containing green sheet is prepared separately from the lithium composite oxide-containing green sheet. This excess lithium source is preferably a lithium compound other than LiMO ₂ such that components other than Li disappear by firing. Lithium carbonate is mentioned as a preferable example of such a lithium compound (excess lithium source). The excess lithium source is preferably in powder form, and the volume-based D50 particle size of the excess lithium source powder is preferably 0.1 to 20 μm, more preferably 0.3 to 10 μm. Then, the lithium source powder is mixed with a dispersion medium and various additives (binder, plasticizer, dispersant, etc.) to form a slurry. It is preferable to stir the obtained slurry under reduced pressure to defoam and adjust the viscosity to 1000 to 20000 cP. The obtained slurry is shaped into a sheet to obtain an excess lithium source-containing green sheet. The green sheet thus obtained is also an independent sheet-shaped molded body. The sheet can be formed by various known methods, but the doctor blade method is preferable. The thickness of the excess lithium source-containing green sheet is preferably such that the molar ratio (Li / Co ratio) of the Li content in the excess lithium source-containing green sheet to the Co content in the lithium composite oxide-containing green sheet is 0.1 or more. It is preferable to set the thickness so that it can be 0.1 to 1.1.

(C) Lamination and Firing of Green Sheets A lithium composite oxide-containing green sheet (for example, LiCoO ₂ green sheet) and, if desired, an excess lithium source-containing green sheet (for example, Li ₂ CO ₃ green sheet) are sequentially placed on the lower setter. Then, place the upper setter on it. The upper and lower setters are made of ceramics, preferably zirconia or magnesia. If the setter is made of magnesia, the pores tend to be small. The upper setter may have a porous structure, a honeycomb structure, or a dense structure. If the upper setter is dense, the pores in the sintered plate tend to be small and the number of pores tends to increase. If necessary, the excess lithium source-containing green sheet preferably has a molar ratio (Li / Co ratio) of the Li content in the excess lithium source-containing green sheet to the Co content in the lithium composite oxide-containing green sheet is preferably 0. It is preferable to cut into a size of 1 or more, more preferably 0.1 to 1.1 before use.

When a lithium composite oxide-containing green sheet (eg, LiCoO ₂ green sheet) is placed on the lower setter, the green sheet may be degreased if desired, and then calcined at 600 to 850 ° C. for 1 to 10 hours. . In this case, the excess lithium source-containing green sheet (for example, Li ₂ CO ₃ green sheet) and the upper setter may be sequentially placed on the obtained calcined plate.

Then, the green sheet and / or the calcined plate is sandwiched between setters, and after degreasing as desired, heat treatment (calcination) is performed at a calcining temperature in a medium temperature range (for example, 700 to 1000 ° C.) to obtain a lithium composite oxide. A sintered body plate is obtained. This firing step may be performed twice or once. When firing is performed twice, it is preferable that the first firing temperature be lower than the second firing temperature. The sintered plate thus obtained is also in the form of an independent sheet.

The present invention will be described more specifically by the following examples.

Example 1
(1) Preparation of Positive Electrode Plate (1a) Preparation of LiCoO ₂ Green Sheet First, Co ₃ O ₄ powder (manufactured by Shodo Chemical Co., Ltd.) weighed so that the Li / Co molar ratio was 1.01. Li ₂ CO ₃ powder (manufactured by Honjo Chemical Co., Ltd.) was mixed, held at 780 ° C. for 5 hours, and the obtained powder was crushed and crushed in a pot mill so that the volume-based D50 was 0.4 μm. A powder consisting of the obtained LiCoO ₂ plate-like particles was obtained. 100 parts by weight of the obtained LiCoO ₂ powder, 100 parts by weight of a dispersion medium (toluene: isopropanol = 1: 1), 10 parts by weight of a binder (polyvinyl butyral: product number BM-2, manufactured by Sekisui Chemical Co., Ltd.), plastic 4 parts by weight of an agent (DOP: Di (2-ethylhexyl) phthalate, manufactured by Kurogane Kasei Co., Ltd.) and 2 parts by weight of a dispersant (product name: Leodol SP-O30, manufactured by Kao Corporation) were mixed. A LiCoO ₂ slurry was prepared by stirring the resulting mixture under reduced pressure for defoaming and adjusting the viscosity to 4000 cP. The viscosity was measured by Brookfield LVT viscometer. The slurry thus prepared was formed into a sheet on a PET film by a doctor blade method to form a LiCoO ₂ green sheet. The thickness of the LiCoO ₂ green sheet after drying was 98 μm.

(1b) Preparation of LiCoO ₂ Sintered Body Plate The LiCoO ₂ green sheet peeled from the PET film was cut into 50 mm square with a cutter and placed in the center of a magnesia setter (dimension 90 mm square, height 1 mm) as a lower setter. . A porous magnesia setter as an upper setter was placed on the LiCoO ₂ sheet. The LiCoO ₂ sheet was sandwiched between setters and placed in a 120 mm square alumina sheath (manufactured by Nikkato Co., Ltd.). At this time, the alumina sheath was not sealed, and the lid was opened with a gap of 0.5 mm. The obtained laminate was heated to 600 ° C. at a heating rate of 200 ° C./h and degreased for 3 hours, then heated to 870 ° C. at 200 ° C./h and held for 20 hours for firing. After firing, the temperature was lowered to room temperature, and then the fired body was taken out from the alumina sheath. In this way, a 90 μm thick LiCoO ₂ sintered body plate was obtained as a positive electrode plate. The obtained positive electrode plate was cut into a rectangular shape of 10.5 mm × 9.5 mm square by a laser processing machine to obtain a plurality of chip-shaped positive electrode layers 16.

(2) Preparation of Lithium Secondary Battery A lithium secondary battery 10 in the form of a film-clad battery as schematically shown in FIG. 1 was prepared by the procedure as shown in FIGS. 2A and 2B. Specifically, it is as follows.

Two aluminum laminate films (Showa Denko Packaging, thickness 61 μm, three-layer structure of polypropylene film / aluminum foil / nylon film) were prepared as the exterior film 26. Further, as the positive electrode current collector 14, one aluminum foil (thickness 9 μm) was prepared. Acetylene black was mixed with a solution in which polyamideimide (PAI) was dissolved in N-methylpyrrolidone to prepare a slurry, and 2 μL of this slurry was dropped on an aluminum foil, and then the positive electrode layer 16 was placed and dried. Thereafter, as shown in FIG. 2A, a composite of the positive electrode current collector 14 and the plurality of chip-shaped positive electrode plates 16 was laminated on one outer film 26 to obtain a positive electrode assembly 17. At this time, the positive electrode current collector 14 was fixed to the exterior film 26 with an adhesive. A positive electrode terminal 15 is fixed to the positive electrode current collector 14 by welding so as to extend from the positive electrode current collector 14. On the other hand, the negative electrode layer 20 (carbon layer having a thickness of 130 μm) was laminated on the other exterior film 26 via the negative electrode current collector 22 (copper foil having a thickness of 10 μm) to obtain a negative electrode assembly 19. . At this time, the negative electrode current collector 22 was fixed to the exterior film 26 with an adhesive. A negative electrode terminal 23 is fixed to the negative electrode current collector 22 by welding so as to extend from the negative electrode current collector 22. The carbon layer as the negative electrode layer 20 was a coating film containing a mixture of graphite as an active material and polyvinylidene fluoride (PVDF) as a binder.

A porous polyolefin membrane (Celguard # 2500) was prepared as the separator 18. As shown in FIG. 2A, the positive electrode assembly 17, the separator 18, and the negative electrode assembly 19 are sequentially laminated so that the positive electrode layer 16 and the negative electrode layer 20 face the separator 18, and both surfaces are covered with the exterior film 26. A laminated body 28 was obtained in which the outer peripheral portion of the exterior film 26 protruded from the outer edge of the battery element 12. The thickness of the battery element 12 (the positive electrode current collector 14, the positive electrode layer 16, the separator 18, the negative electrode layer 20, and the negative electrode current collector 22) thus constructed in the laminated body 28 is 0.33 mm, and the shape and size thereof. Was a square of 2.3 cm × 3.2 cm.

As shown in FIG. 2A, the three sides A of the obtained laminated body 28 were sealed. This sealing is performed by heating and pressing the outer peripheral portion of the laminated body 28 at 200 ° C. and 1.5 MPa for 10 seconds by using a padding jig whose sealing width is adjusted to 2 mm, and the outer peripheral film 26 ( The aluminum laminated films) were heat-sealed together. After sealing the three sides A, the laminated body 28 was put into the vacuum dryer 34 to remove water and dry the adhesive.

As shown in FIG. 2B, in the glove box 38, a gap is formed between the pair of exterior films 26 on the remaining unsealed one side B of the laminated body 28 whose outer edges 3 sides A are sealed. The injection device 36 was inserted into the gap to inject the electrolytic solution 24, and the side B was temporarily sealed using a simple sealer under a reduced pressure atmosphere with an absolute pressure of 5 kPa. As the electrolytic solution, a mixed solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3: 7, and LiFSI and LiDFOB at concentrations of 0.7 mol / L and 0.3 mol / L, respectively. The solution was dissolved so that vinylene carbonate (VC) was further dissolved to have a concentration of 2% by weight. In this way, the laminated body in which the side B was temporarily sealed was subjected to initial charging and aged for 7 days. Lastly, the outer peripheral portion of the remaining one side B (the end portion not including the battery element) was cut off to perform degassing.

As shown in FIG. 2B, in the glove box 38, the side B ′ generated by the excision of the temporary sealing was sealed under a reduced pressure atmosphere with an absolute pressure of 5 kPa. This sealing was also performed by heating and pressing the outer peripheral portion of the laminate 28 at 200 ° C. and 1.5 MPa for 10 seconds to heat-bond the exterior films 26 (aluminum laminate films) to each other at the outer peripheral portion. In this way, the side B'is sealed with a pair of exterior films 26 to obtain a lithium secondary battery 10 in the form of a film exterior battery. The lithium secondary battery 10 was taken out from the glove box 38, and an unnecessary portion on the outer periphery of the exterior film 26 was cut off to adjust the shape of the lithium secondary battery 10. Thus, the lithium secondary battery 10 in which the four outer edges of the battery element 12 were sealed with the pair of exterior films 26 and the electrolytic solution 24 was injected was obtained. The obtained lithium secondary battery 10 was a rectangle having a size of 38 mm × 27 mm, a thickness of 0.45 mm or less, and a capacity of 30 mAh.

(3) Evaluation The LiCoO ₂ sintered body plate (positive electrode plate) synthesized in (1b) above and the battery produced in (2) above were evaluated in various ways as shown below.

<Average orientation angle of primary particles>
A LiCoO ₂ sintered body plate was polished with a cross section polisher (CP) (IB-15000CP, manufactured by JEOL Ltd.), and the cross section of the obtained positive electrode plate (cross section perpendicular to the plate surface of the positive electrode plate) was viewed at 1000 times. (125 μm × 125 μm), EBSD measurement was performed to obtain an EBSD image. The EBSD measurement was performed using a Schottky field emission scanning electron microscope (JSM-7800F, manufactured by JEOL Ltd.). For all the particles specified in the obtained EBSD image, the angle formed by the (003) plane of the primary particles and the plate surface of the positive electrode plate (that is, the inclination of the crystal orientation from (003)) was determined as the tilt angle, and The average value of the angles was taken as the average orientation angle of the primary particles. As a result, the average orientation angle was 16 °.

<Plate thickness>
The LiCoO ₂ sintered body plate was polished by a cross section polisher (CP) (manufactured by JEOL Ltd., IB-15000CP), and the cross section of the obtained positive electrode plate was observed by SEM (JEOL, JSM6390LA) to measure the thickness of the positive electrode plate. Was measured. The thickness of the dried LiCoO ₂ green sheet described above in regard to step (1a) is also measured in the same manner as above. As a result, the thickness of the positive electrode plate was 90 μm.

<Porosity>
A LiCoO ₂ sintered body plate was polished by a cross section polisher (CP) (IB-15000CP, manufactured by JEOL Ltd.), and the cross section of the obtained positive electrode plate was observed by SEM with a field of view (125 μm × 125 μm) of 1000 times. Manufactured by JSM6390LA). The obtained SEM image was subjected to image analysis, the area of all pores was divided by the area of the positive electrode, and the obtained value was multiplied by 100 to calculate the porosity (%). As a result, the porosity was 30%.

<Average pore size>
Using a mercury porosimeter (manufactured by Shimadzu Corp., Autopore IV9510), the average pore diameter of the LiCoO ₂ sintered body plate was measured by the mercury injection method. As a result, the average pore diameter was 0.8 μm.

<10Hz resistance>
Cole obtained by measuring the internal resistance of the lithium secondary battery 10 manufactured in (2) above using a VMP-300 manufactured by Bio-Logic at a battery voltage of 3.8 V by an AC impedance method. The resistance value (Ω · cm ² ) at 10 Hz was read from the −Cole plot. This measurement was performed with an amplitude of 2 mV and a measurement frequency range of 250 kHz to 200 mHz. The results were as shown in FIGS. 6 and 7 and Table 1.

Comparative Example 1
As an electrolytic solution, LiPF ₆ was dissolved in a mixed solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3: 7 so as to have a concentration of 1.0 mol / L, and vinylene carbonate was further added. A lithium secondary battery was prepared and evaluated in the same manner as in Example 1 except that the one obtained by dissolving (VC) in a concentration of 2% by weight was used. The results were as shown in FIGS. 3 and 4 and Table 1.

Comparative example 2
As an electrolytic solution, a mixed solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3: 7 was used, and LiPF ₆ and LiFSI were respectively added at concentrations of 0.8 mol / L and 0.2 mol / L. A lithium secondary battery was prepared and evaluated in the same manner as in Example 1 except that a solution obtained by dissolving vinylene carbonate (VC) at a concentration of 2% by weight was used. It was The results were as shown in FIGS. 3 and 4 and Table 1.

Claims

A positive electrode layer containing a lithium composite oxide,
A positive electrode current collector having conductivity,
A negative electrode layer containing carbon,
A negative electrode current collector having conductivity,
A separator interposed between the positive electrode layer and the negative electrode layer,
An electrolytic solution containing lithium bis (fluorosulfonyl) imide (LiFSI) and lithium difluoro (oxalato) borate (LiDFOB) in an organic solvent,
Equipped with
A lithium secondary battery in which at least one of the positive electrode current collector and the negative electrode current collector contains aluminum.
The organic solvent is a mixed solvent of ethylene carbonate (EC) and methyl ethyl carbonate (MEC), a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC), or ethylene carbonate (EC) and ethyl methyl carbonate (EMC). The lithium secondary battery according to claim 1, comprising a mixed solvent.
The lithium secondary battery according to claim 1, wherein the organic solvent contains a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC).
The lithium according to any one of claims 1 to 3, wherein the electrolytic solution further contains vinylene carbonate (VC) and / or fluoroethylene carbonate (FEC) and / or vinyl ethylene carbonate (VEC) as an additive. Secondary battery.
The lithium secondary battery according to any one of claims 1 to 4, wherein a ratio of molar concentrations of LiFSI and LiDFOB in the electrolytic solution is 7: 3 to 2: 8.
The lithium secondary battery according to any one of claims 1 to 5, wherein the lithium composite oxide is lithium cobalt oxide.
The lithium secondary battery according to any one of claims 1 to 6, wherein the positive electrode layer is a lithium composite oxide sintered body plate.
The lithium secondary battery according to claim 6 or 7, wherein the lithium composite oxide sintered body plate has a porosity of 3 to 40%.
The lithium secondary battery according to any one of claims 6 to 8, wherein the average pore diameter of the lithium composite oxide sintered body plate is 15 µm or less.
The lithium composite oxide sintered body plate includes a plurality of primary particles composed of a lithium composite oxide, and the plurality of primary particles have an average orientation of more than 0 ° and 30 ° or less with respect to the plate surface of the positive electrode plate. The lithium secondary battery according to any one of claims 6 to 9, which is an oriented positive electrode plate that is oriented at an angle.
The lithium secondary battery according to any one of claims 1 to 10, which has a resistance at 10 Hz measured by an AC impedance method of 9.5 Ω · cm 2 or less.