JP2021086790A - Lithium secondary battery - Google Patents

Abstract

To suppress the resistance degradation.SOLUTION: A lithium secondary battery 1 comprises: a positive electrode 2; a separator 4 disposed on the positive electrode in a predetermined superposing direction; a negative electrode 3 disposed on the separator 4 on a side opposite to the positive electrode 2 in the superposing direction; an electrolyte solution 5 impregnated into the positive electrode 2, the negative electrode 3 and the separator 4; and an outer packaging body 6 which covers the positive electrode 2 and the negative electrode 3 from opposing sides in the superposing direction. The outer packaging body 6 houses the positive electrode 2, the separator 4, the negative electrode 3 and the electrolyte solution 5 therein. The electrolyte solution 5 contains lithium difluorophosphate (LiDFP) and phosphoric acid tris(trimethylsilyl) (TMSP). In the electrolyte solution 5, the content of LiDFP is 0.1 mass% or more and 5.0 mass% or less. In the electrolyte solution 5, the content of TMSP is 0.1 mass% or more and 5.0 mass% or less. According to this, the resistance degradation can be suppressed in the lithium secondary battery 1.SELECTED DRAWING: Figure 1

Description

The present invention relates to a lithium secondary battery.

Conventionally, in a lithium secondary battery (also referred to as a lithium ion secondary battery), as an electrolytic solution impregnated in a positive electrode, a negative electrode and a separator, for example, an electrolyte such as a lithium salt is dissolved in a solvent, and an additive is further added. The solution is used. As the additive, for example, lithium difluorophosphate (LiDFP) or the like is used.

In Patent Document 1, for the purpose of improving the output characteristics of a lithium secondary battery, electrolysis containing cyclic sulfate ester such as 1,3,2-dioxathiolane-2,2-dioxide (DTD) and lithium difluorophosphate. It has been proposed to use liquids.

JP-A-2018-133284

By the way, the use of a lithium secondary battery as a power supply source for a smart card is being studied. In this case, the lithium secondary battery is required to have a low resistance characteristic corresponding to a large current pulse discharge. Further, the low resistance characteristic needs to be maintained not only at the initial stage of use of the lithium secondary battery but also for a long period of time. That is, in a lithium secondary battery, it is required to suppress an increase in resistance (that is, resistance deterioration) with time.

The present invention is directed to a lithium secondary battery, and an object of the present invention is to suppress resistance deterioration.

The lithium secondary battery according to one preferred embodiment of the present invention has a positive electrode, a separator arranged on the positive electrode in a predetermined stacking direction, and arranged on the opposite side of the separator from the positive electrode in the stacking direction. The negative electrode to be formed, the negative electrode, the electrolytic solution impregnated in the negative electrode and the separator, and the positive electrode and the negative electrode are coated from both sides in the stacking direction, and the positive electrode, the separator, the negative electrode and the electrolytic solution are coated. It includes an exterior body to be housed inside. The electrolyte contains lithium difluorophosphate and tris (trimethylsilyl) phosphate. The content of lithium difluorophosphate in the electrolytic solution is 0.1% by mass or more and 5.0% by mass or less. The content of tris (trimethylsilyl) phosphate in the electrolytic solution is 0.1% by mass or more and 5.0% by mass or less.

The lithium secondary battery according to another preferred embodiment of the present invention has a positive electrode, a separator arranged on the positive electrode in a predetermined stacking direction, and arranged on the opposite side of the separator from the positive electrode in the stacking direction. The negative electrode to be formed, the negative electrode, the electrolytic solution impregnated in the negative electrode and the separator, and the positive electrode and the negative electrode are coated from both sides in the stacking direction, and the positive electrode, the separator, the negative electrode and the electrolytic solution are coated. It includes an exterior body to be housed inside. The electrolyte contains 1,3,2-dioxathiolane-2,2-dioxide and tris (trimethylsilyl) phosphate. The content of 1,3,2-dioxathiolane-2,2-dioxide in the electrolytic solution is 0.1% by mass or more and 5.0% by mass or less. The content of tris (trimethylsilyl) phosphate in the electrolytic solution is 0.1% by mass or more and 5.0% by mass or less.

The lithium secondary battery according to another preferred embodiment of the present invention has a positive electrode, a separator arranged on the positive electrode in a predetermined stacking direction, and arranged on the opposite side of the separator from the positive electrode in the stacking direction. The negative electrode to be formed, the negative electrode, the electrolytic solution impregnated in the negative electrode and the separator, and the positive electrode and the negative electrode are coated from both sides in the overlapping direction, and the positive electrode, the separator, the negative electrode and the electrolytic solution are coated. It includes an exterior body to be housed inside. The electrolytic solution contains lithium difluorophosphate and 1,3,2-dioxathiolane-2,2-dioxide. The content of lithium difluorophosphate in the electrolytic solution is 0.1% by mass or more and 5.0% by mass or less. The content of 1,3,2-dioxathiolane-2,2-dioxide in the electrolytic solution is 0.1% by mass or more and 5.0% by mass or less.

Preferably, the positive electrode includes a positive electrode active material plate which is a plate-shaped ceramic sintered body containing a lithium composite oxide.

In the present invention, resistance deterioration can be suppressed.

It is sectional drawing of the lithium secondary battery which concerns on one Embodiment. It is a top view of the lithium secondary battery. It is a figure which shows the flow of manufacturing of a lithium secondary battery. It is a figure which shows the flow of manufacturing of a lithium secondary battery.

FIG. 1 is a cross-sectional view showing the configuration of a lithium secondary battery 1 according to an embodiment of the present invention. FIG. 2 is a plan view of the lithium secondary battery 1. In FIG. 1, the lithium secondary battery 1 and its configuration are drawn thicker than they actually are in order to facilitate the understanding of the figure. Further, in FIG. 1, the widths of the positive electrode 2, the separator 4, and the negative electrode 3 described later in the left-right direction are smaller than the actual widths, and the joint portions of the exterior body 6 (that is, both ends in the left-right direction in FIG. 1) are in the left-right direction. The width is drawn larger than it actually is. In addition, in FIG. 1, a part of the structure on the front side and the back side of the cross section is also shown.

The lithium secondary battery 1 is a small and thin battery. The shape of the lithium secondary battery 1 in a plan view is, for example, a substantially rectangular shape. For example, the length of the lithium secondary battery 1 in the plan view in the vertical direction is 10 mm to 46 mm, and the length in the horizontal direction is 10 mm to 46 mm. The thickness of the lithium secondary battery 1 (that is, the thickness in the vertical direction in FIG. 1) is, for example, 0.30 mm to 0.45 mm, preferably 0.40 mm to 0.45 mm. The lithium secondary battery 1 is a sheet-shaped or flexible thin plate-shaped member. The sheet-shaped member is a thin member that is easily deformed by a relatively small force, and is also called a film-shaped member. The same applies to the following description.

The lithium secondary battery 1 is mounted on, for example, a sheet-shaped device or a flexible device and used as a power supply source. The sheet-like device is a thin device that is easily deformed by a relatively small force, and is also called a film-like device. In the present embodiment, the lithium secondary battery 1 is built in, for example, a smart card having an arithmetic processing function, and is used as a power supply source in the smart card. A smart card is a card-type flexible device. The smart card is used, for example, as a wireless communication IC, an ASIC for fingerprint analysis, a card with a fingerprint authentication / wireless communication function provided with a fingerprint sensor, and the like. In the following description, a target device such as a smart card in which the lithium secondary battery 1 is used as a power supply source is also referred to as a “target device”.

The lithium secondary battery 1 is mounted on the smart card by, for example, cold laminating that pressurizes at room temperature or hot laminating that pressurizes while heating. The processing temperature in hot laminating is, for example, 110 ° C to 260 ° C. The upper limit of the processing temperature is preferably less than 240 ° C, more preferably less than 220 ° C, still more preferably less than 200 ° C, and most preferably 150 ° C or less. The processing pressure in hot laminating is, for example, 0.1 MPa (megapascal) to 6 MPa, and the processing time (that is, heating / pressurizing time) is, for example, 10 minutes to 20 minutes.

The lithium secondary battery 1 includes a positive electrode 2, a negative electrode 3, a separator 4, an electrolytic solution 5, an exterior body 6, and two terminals 7. The positive electrode 2, the separator 4, and the negative electrode 3 are superposed in a predetermined superposition direction. In the example shown in FIG. 1, the positive electrode 2, the separator 4, and the negative electrode 3 are laminated in the vertical direction in the drawing. In the following description, the upper side and the lower side in FIG. 1 are simply referred to as "upper side" and "lower side". Further, the vertical direction in FIG. 1 is simply referred to as "vertical direction" and also referred to as "superimposition direction". The vertical direction in FIG. 1 does not have to coincide with the actual vertical direction when the lithium secondary battery 1 is mounted on a target device such as a smart card.

In the example shown in FIG. 1, the separator 4 is arranged on the upper surface of the positive electrode 2 in the vertical direction (that is, the overlapping direction). The negative electrode 3 is arranged on the upper surface of the separator 4 in the vertical direction. In other words, the negative electrode 3 is arranged on the side opposite to the positive electrode 2 of the separator 4 in the vertical direction. Each of the positive electrode 2, the separator 4, and the negative electrode 3 has a substantially rectangular shape in a plan view, for example. The positive electrode 2, the separator 4, and the negative electrode 3 have substantially the same shape (that is, approximately the same shape and approximately the same size) in a plan view.

The exterior body 6 is a sheet-shaped and bag-shaped member. The exterior body 6 is substantially rectangular in a plan view. The exterior body 6 includes two layers of seat portions 65 and 66 that overlap in the vertical direction. In the following description, the sheet portion 65 located on the lower side of the positive electrode 2 is referred to as the “first sheet portion 65”, and the sheet portion 66 located on the upper side of the negative electrode 3 is referred to as the “second sheet portion 66”. The outer peripheral edge of the first sheet portion 65 and the outer peripheral edge of the second sheet portion 66 are joined by, for example, heat fusion (so-called heat sealing). The first sheet portion 65 and the second sheet portion 66 of the exterior body 6 are each made of, for example, a laminated film in which a metal foil 61 formed of a metal such as aluminum (Al) and an insulating resin layer 62 are laminated. It is formed. In the first sheet portion 65 and the second sheet portion 66, the resin layer 62 is located inside the metal foil 61.

The exterior body 6 covers the positive electrode 2 and the negative electrode 3 from both sides in the vertical direction. The exterior body 6 houses the positive electrode 2, the separator 4, the negative electrode 3, and the electrolytic solution 5 inside. The electrolytic solution 5 is continuously present around the positive electrode 2, the separator 4, and the negative electrode 3. In other words, the electrolytic solution 5 is interposed between the positive electrode 2 and the negative electrode 3. The electrolytic solution 5 is impregnated in the positive electrode 2, the separator 4, and the negative electrode 3. The two terminals 7 extend from the inside of the exterior body 6 to the outside. Inside the exterior body 6, one terminal 7 is electrically connected to the positive electrode 2, and the other terminal 7 is electrically connected to the negative electrode 3.

The positive electrode 2 includes a positive electrode current collector 21, a positive electrode active material plate 22, and a conductive bonding layer 23. The positive electrode current collector 21 is a sheet-like member having conductivity. The lower surface of the positive electrode current collector 21 is bonded to the resin layer 62 of the exterior body 6 via the positive electrode bonding layer 63. The positive electrode bonding layer 63 is formed of, for example, a mixed resin of an acid-modified polyolefin resin and an epoxy resin. The positive electrode bonding layer 63 may be formed of various other materials. The thickness of the positive electrode bonding layer 63 is, for example, 0.5 μm to 10 μm.

The positive electrode current collector 21 includes, for example, a metal foil formed of a metal such as aluminum, and a conductive carbon layer laminated on the upper surface of the metal foil. In other words, the main surface of the positive electrode current collector 21 facing the positive electrode active material plate 22 is covered with a conductive carbon layer. The above-mentioned metal foil is formed of various metals other than aluminum (for example, copper, nickel, silver, gold, chromium, iron, tin, lead, tungsten, molybdenum, titanium, zinc, or alloys containing these). You may. Further, the conductive carbon layer may be omitted from the positive electrode current collector 21.

The positive electrode active material plate 22 (that is, the active material plate of the positive electrode 2) is a relatively thin plate-shaped ceramic sintered body containing a lithium composite oxide. Preferably, the positive electrode active material plate 22 is substantially composed of only the lithium composite oxide. The positive electrode active material plate 22 is bonded onto the upper surface of the positive electrode current collector 21 via the conductive bonding layer 23. The positive electrode active material plate 22 faces the separator 4 in the vertical direction. The upper surface of the positive electrode active material plate 22 comes into contact with the lower surface of the separator 4. Gold (Au) or the like may be sputtered on the positive electrode active material plate 22 as a current collector assisting agent.

The positive electrode active material plate 22 has a structure in which a plurality of (that is, a large number of) primary particles are bonded. The primary particles are composed of a lithium composite oxide having a layered rock salt structure. Lithium composite oxide, a composite oxide of lithium and a transition metal element M (general formula: in _Li p MO ₂ (wherein, 0.05 <p <1.10)) in, there partially with another metal element It is substituted with a substituted metal element. The transition metal element M contains, for example, one or more selected from cobalt (Co), nickel (Ni) and manganese (Mn). The transition metal element M is a main metal other than lithium contained in the lithium composite oxide, and is also hereinafter referred to as a “main transition metal element”.

Preferred examples of the above-mentioned composite oxide of lithium and the main transition metal element are lithium cobaltate (Li _p CoO ₂ (in the formula, 1 ≦ p ≦ 1.1), lithium nickelate (LiNiO ₂ ), lithium manganate). (Li ₂ MnO ₃ ), lithium nickel manganate (Li _p (Ni _0.5 , Mn _0.5 ) O ₂ ), general formula: Li _p (Co _x , Ni _y , Mn _z ) O ₂ (in the formula, 0.97 ≦ p ≦ 1.07, x + y + z = 1) with solid solution _{represented, Li p (Co x, Ni} y, in Al z) O2 (wherein, 0.97 ≦ p ≦ 1.07, x + y + z = 1 , 0 <x ≦ 0.25, 0.6 ≦ y ≦ 0.9 and 0 <z ≦ 0.1), or Li ₂ MnO ₃ and _{Li MO 2} (in the formula, M is Co, a solid solution of transition metal) of Ni, and the like. particularly preferred are composite oxides of lithium and the main transition metal element is an lithium cobaltate Li p _CoO 2 _(wherein, 1 ≦ p ≦ 1.1) For example, LiCoO ₂ (LCO).

The above-mentioned substituted metal elements include, for example, magnesium (Mg), aluminum (Al), silicon (Si), calcium (Ca), titanium (Ti), vanadium (V), chromium (Cr), iron (Fe), and copper. (Cu), zinc (Zn), gallium (Ga), germanium (Ge), strontium (Sr), ittrium (Y), zirconia (Zr), niobium (Nb), molybdenum (Mo), silver (Ag), tin It is one or more selected from (Sn), antimony (Sb), tellurium (Te), barium (Ba), and bismuth (Bi). Preferably, the substituted metal element is Ti or Nb. The substituted metal element is replaced with a part of the main transition metal element in the above-mentioned composite oxide of lithium and the main transition metal element, for example.

In the positive electrode active material plate 22, in the above-mentioned lithium composite oxide, a part of the main transition metal element does not necessarily have to be replaced with the substituted metal element. For example, the lithium composite oxide does not contain a substituted metal element, and the lithium composite oxide may be substantially composed of lithium and a main transition metal element.

The above-mentioned layered rock salt structure is a crystal structure in which a lithium layer and a metal layer other than lithium are alternately laminated with an oxygen layer interposed therebetween. That is, the layered rock salt structure is a crystal structure in which metal ion layers other than lithium and a lithium single layer are alternately laminated via oxide ions (typically, α-NaFeO _{type 2} structure: cubic rock salt type structure). [111] A structure in which a metal other than lithium and lithium are regularly arranged in the axial direction).

In the positive electrode active material plate 22, the primary particle size, which is the average particle size of the plurality of primary particles, is, for example, 20 μm or less, preferably 15 μm or less. The primary particle size is, for example, 0.2 μm or more, preferably 0.4 μm or more. The primary particle size can be measured by analyzing an SEM (scanning electron microscope) image of a cross section of the positive electrode active material plate 22. Specifically, for example, the positive electrode active material plate 22 is processed with a cross section polisher (CP) to expose the polished cross section, and the polished cross section is subjected to a predetermined magnification (for example, 1000 times) and a predetermined field of view (for example, 125 μm). Observe by SEM at × 125 μm). At this time, the field of view is set so that 20 or more primary particles are present in the field of view. The diameter of the circumscribed circle when the circumscribed circle is drawn for all the primary particles in the obtained SEM image is obtained, and the average value of these is taken as the primary particle size.

In the positive electrode active material plate 22, the average inclination angle (that is, the average orientation angle) of the plurality of primary particles is preferably larger than 0 ° and 30 ° or less. The average inclination angle is more preferably 5 ° or more and 28 ° or less, and further preferably 10 ° or more and 25 ° or less. The average inclination angle is an average value of the angles formed by the (003) surface of the plurality of primary particles and the main surface of the positive electrode active material plate 22 (for example, the lower surface of the positive electrode active material plate 22).

The inclination angle of the primary particles (that is, the angle formed by the (003) surface of the primary particles and the main surface of the positive electrode active material plate 22) is determined by analyzing the cross section of the positive electrode active material plate 22 by electron backscatter diffraction (EBSD). It can be measured by doing. Specifically, for example, the positive electrode active material plate 22 is processed with a cross section polisher to expose the polished cross section, and the polished cross section is subjected to a predetermined magnification (for example, 1000 times) and a predetermined field of view (for example, 125 μm × 125 μm). Analyze by EBSD. In the obtained EBSD image, the inclination angle of each primary particle is represented by the shade of color, and the darker the color, the smaller the inclination angle. Then, the average value of the inclination angles of the plurality of primary particles obtained from the EBSD image is taken as the above-mentioned average inclination angle.

Among the primary particles constituting the positive electrode active material plate 22, the proportion of the primary particles having an inclination angle of more than 0 ° and 30 ° or less is preferably 60% or more, more preferably 80% or more. More preferably, it is 90% or more. The upper limit of the ratio is not particularly limited and may be 100%. The ratio can be obtained by obtaining the total area of the primary particles having an inclination angle of more than 0 ° and not more than 30 ° in the above-mentioned EBSD image, and dividing the total area of the primary particles by the total particle area. it can.

The porosity of the positive electrode active material plate 22 is, for example, 25% to 45%. The porosity of the positive electrode active material plate 22 is a volume ratio of pores (including open pores and closed pores) in the positive electrode active material plate 22. The porosity can be measured by analyzing an SEM image of a cross section of the positive electrode active material plate 22. For example, the positive electrode active material plate 22 is processed with a cross section polisher to expose the polished cross section. The polished cross section is observed by SEM at a predetermined magnification (for example, 1000 times) and a predetermined field of view (for example, 125 μm × 125 μm). The obtained SEM image is analyzed, the area of all pores in the visual field is divided by the area (cross-sectional area) of the positive electrode active material plate 22 in the visual field, and the obtained value is multiplied by 100 to obtain the porosity (porosity). %) Is obtained.

The average pore diameter, which is the average value of the pore diameters contained in the positive electrode active material plate 22, is, for example, 15 μm or less, preferably 12 μm or less, and more preferably 10 μm or less. The average pore diameter is, for example, 0.1 μm or more, preferably 0.3 μm or more. The diameter of the above-mentioned pore is typically the diameter in the sphere, assuming that the pore is a sphere having the same volume or the same cross-sectional area. The average pore diameter is calculated by calculating the average value of the diameters of a plurality of pores on the basis of the number of pores. The average pore diameter can be determined by a well-known method such as analysis of a cross-sectional SEM image or a mercury intrusion method. Preferably, the average pore size is measured by a mercury intrusion method using a mercury porosimeter.

In the example shown in FIG. 1, the positive electrode active material plate 22 is one plate-shaped member, but may be divided into a plurality of plate-shaped members (hereinafter, referred to as “active material plate elements”). In this case, each of the plurality of active material plate elements is bonded to the positive electrode current collector 21 via the conductive bonding layer 23. The plurality of active material plate elements are arranged in a matrix (that is, in a grid pattern) on the positive electrode current collector 21, for example. The shape of each active material plate element in a plan view is, for example, a substantially rectangular shape. The plurality of active material plate elements may have substantially the same shape (that is, substantially the same shape and substantially the same size) in a plan view, or may have different shapes. The plurality of active material plate elements are arranged apart from each other in a plan view.

The conductive bonding layer 23 contains a conductive powder and a binder. The conductive powder is, for example, a powder such as acetylene black, scaly natural graphite, carbon nanotubes, carbon nanofibers, carbon nanotube derivatives, or carbon nanofiber derivatives. The binder contains, for example, a polyimide amide resin. The polyimide amide resin contained in the binder may be one type or two or more types. Further, the binder may contain a resin other than the polyimide amide resin. In the conductive bonding layer 23, the above-mentioned conductive powder and binder, and a liquid or paste-like adhesive containing a solvent are applied to the positive electrode current collector 21 or the positive electrode active material plate 22 to apply the positive electrode current collector 21 or the positive electrode current collector 21. It is formed by the solvent evaporating and solidifying between the positive electrode active material plate 22 and the positive electrode active material plate 22.

The thickness of the positive electrode current collector 21 is, for example, 9 μm to 50 μm, preferably 9 μm to 20 μm, and more preferably 9 μm to 15 μm. The thickness of the positive electrode active material plate 22 is, for example, 15 μm to 200 μm, preferably 30 μm to 150 μm, and more preferably 50 μm to 100 μm. By increasing the thickness of the positive electrode active material plate 22, the active material capacity per unit area can be increased, and the energy density of the lithium secondary battery 1 can be increased. By thinning the positive electrode active material plate 22, deterioration of battery characteristics (particularly, increase in resistance value) due to repeated charging and discharging can be suppressed. The thickness of the conductive bonding layer 23 is, for example, 3 μm to 28 μm, preferably 5 μm to 25 μm.

The negative electrode 3 includes a negative electrode current collector 31 and a negative electrode active material layer 32. The negative electrode current collector 31 is a sheet-like member having conductivity. The upper surface of the negative electrode current collector 31 is bonded to the resin layer 62 of the exterior body 6 via the negative electrode bonding layer 64. The negative electrode bonding layer 64 is formed of, for example, a mixed resin of an acid-modified polyolefin resin and an epoxy resin. The negative electrode bonding layer 64 may be formed of various other materials. The thickness of the negative electrode bonding layer 64 is, for example, 0.5 μm to 10 μm.

The negative electrode current collector 31 is a metal foil formed of, for example, a metal such as copper. The metal foil is made of various metals other than copper (for example, stainless steel, nickel, aluminum, silver, gold, chromium, iron, tin, lead, tungsten, molybdenum, titanium, zinc, or alloys containing these). It may be formed.

The negative electrode active material layer 32 contains a binder containing a resin as a main component and a carbonaceous material which is a negative electrode active material. The negative electrode active material layer 32 is applied on the lower surface of the negative electrode current collector 31. That is, the negative electrode 3 is a so-called coating electrode. The negative electrode active material layer 32 faces the separator 4 in the vertical direction. The lower surface of the negative electrode active material layer 32 comes into contact with the upper surface of the separator 4. In the negative electrode active material layer 32, the above-mentioned carbonaceous material is, for example, graphite (natural graphite or artificial graphite), pyrolytic carbon, coke, a calcined resin body, a mesophase microsphere, a mesophase pitch, or the like. In the negative electrode 3, a lithium occlusion material may be used as the negative electrode active material instead of the carbonaceous material. The lithium occlusion substance is, for example, silicon, aluminum, tin, iron, iridium, or an alloy containing these, an oxide, a fluoride, or the like. The binder is, for example, styrene butadiene rubber (SBR), polyvinylidene fluoride (PVDF) or a mixture thereof.

The thickness of the negative electrode current collector 31 is, for example, 5 μm to 25 μm, preferably 8 μm to 20 μm, and more preferably 8 μm to 15 μm. The thickness of the negative electrode active material layer 32 is, for example, 20 μm to 300 μm, preferably 30 μm to 250 μm, and more preferably 30 μm to 150 μm. By thickening the negative electrode active material layer 32, the active material capacity per unit area can be increased, and the energy density of the lithium secondary battery 1 can be increased. By thinning the negative electrode active material layer 32, deterioration of battery characteristics (particularly, increase in resistance value) due to repeated charging and discharging can be suppressed.

The separator 4 is a sheet-shaped or thin plate-shaped insulating member. The separator 4 is, for example, a single-layer separator formed of a resin. The resin is, for example, polyimide, polyester (for example, polyethylene terephthalate (PET)) or the like. In this embodiment, the separator 4 is a polyimide porous membrane (for example, a three-dimensional porous structure (3DOM)). Polyimide is superior in heat resistance to polyethylene and polypropylene. Therefore, the heat resistance of the lithium secondary battery 1 can be improved by using the polyimide separator 4.

The thickness of the separator 4 is, for example, 15 μm or more, preferably 18 μm or more, and more preferably 20 μm or more. The thickness of the separator 4 is, for example, 31 μm or less, preferably 28 μm or less, and more preferably 26 μm or less. By making the separator 4 thicker, it is possible to suppress a short circuit between the positive electrode 2 and the negative electrode 3 due to the lithium dendrite even when lithium dendrite (lithium dendritic crystals) is precipitated. Further, by making the separator 4 thin, it is possible to facilitate the permeation of the electrolytic solution 5 and the lithium ion separator 4, and reduce the internal resistance of the lithium secondary battery 1.

The structure of the separator 4 may be changed in various ways. For example, the separator 4 may be a two-layer separator in which a resin layer is laminated on a ceramic substrate, or may be a two-layer separator in which a ceramic is coated on a resin substrate. Alternatively, the separator 4 may be a three-layer separator in which resin layers are laminated on the upper surface and the lower surface of the ceramic substrate, or may have a multilayer structure of three or more layers. The separator 4 does not necessarily have to include a resin layer, and may be a microporous film formed only of ceramic. The ceramic is, for example, at least one selected from _{MgO, Al 2} O ₃ , ZrO ₂ , SiC, Si ₃ N ₄ , Al N and cordierite, preferably selected from _{MgO, Al 2} O ₃ and ZrO _2. At least one of them.

The electrolytic solution 5 is a liquid obtained by adding an electrolyte and an additive to a solvent. The electrolyte (ie, solute) is, for example, lithium hexafluorophosphate (LiPF ₆ ). _{The concentration of LiPF 6} in the electrolytic solution 5 is, for example, 0.5 mol / L to 2.0 mol / L, preferably 0.75 mol / L to 1.5 mol / L, and more preferably 1.0 mol / L to 1.0 mol / L. It is 1.25 mol / L. The electrolyte may be variously modified and may be, for example, lithium borofluoride (LiBF ₄ ).

The additives include lithium difluorophosphate (LiDFP) and tris phosphate (also called trimethylsilyl) (TMSP). The content of LiDFP in the electrolytic solution 5 is, for example, 0.1% by mass to 5.0% by mass, preferably 0.5% by mass to 2.0% by mass, and more preferably 1.0% by mass to 1.0% by mass. It is 1.5% by mass. The content of TMSP in the electrolytic solution 5 is, for example, 0.1% by mass to 5.0% by mass, preferably 0.5% by mass to 2.0% by mass, and more preferably 1.0% by mass to 1.0% by mass. It is 1.5% by mass. The mass% ratio of LiDFP: TMSP in the electrolytic solution 5 is, for example, 1: 0.02 to 1:50, preferably 1: 0.5 to 1: 2. The electrolytic solution 5 may contain other additives in addition to LiDFP and trimethyl. In this embodiment, the electrolytic solution 5 is substantially free of additives other than LiDFP and TMSP.

The solvent of the electrolytic solution 5 is, for example, ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), γ-butyrolactone (GBL), methyl butyrate. It is a non-aqueous solvent such as (MB) and propyl acetate (PA). The solvent may be, for example, a solvent containing EC and EMC. High electrical conductivity can be obtained by using a mixed solvent of EC having a high dielectric constant and EMC having a low viscosity. The volume ratio of EC: EMC in the non-aqueous solvent is, for example, 1: 0.43 to 1: 2.3 (that is, the EMC ratio is 30% to 70% by volume), preferably 1: 1 to 1: 2. .3 (EMC ratio 50% by volume to 70% by volume). The solvent of the electrolytic solution 5 may be changed in various ways.

In the lithium secondary battery 1, the heat resistance of the lithium secondary battery 1 is improved because _{the electrolytic solution 5 contains LiPF 6 having a high decomposition temperature.} Further, when the electrolytic solution 5 contains LiDFP and TMSP, an SEI (Solid Electrolyte Interface) film is formed on the surface of the negative electrode 3. Since the SEI film is a stable film, the surface of the negative electrode 3 is covered with the SEI film, so that the reductive decomposition of the electrolytic solution 5 is suppressed and the resistance deterioration of the lithium secondary battery 1 is suppressed.

In the lithium secondary battery 1, since the electrolytic solution 5 contains LiDFP and TMSP, the surface of the positive electrode 2 (specifically, the surface of the positive electrode active material plate 22) is further covered with the SEI film. As a result, deterioration of the crystallinity of the positive electrode active material on the surface of the positive electrode 2 over time is suppressed. As a result, the resistance deterioration of the lithium secondary battery 1 is further suppressed.

Next, an example of the manufacturing flow of the lithium secondary battery 1 will be described with reference to FIGS. 3A and 3B. First, as the first sheet portion 65 and the second sheet portion 66 of the exterior body 6, two aluminum laminated films (manufactured by Showa Denko Packaging, thickness 61 μm, three-layer structure of polypropylene film / aluminum foil / nylon film) are formed. Be prepared. Further, the positive electrode active material plate 22 is prepared. In the examples shown in FIGS. 3A and 3B, the main transition metal element of the lithium composite oxide of the positive electrode active material plate 22 is Co, and the substituted metal element is Nb, similarly to the above. In the example shown in FIG. 3A, the positive electrode active material plate 22 is a single plate-shaped member, but even when the positive electrode active material plate 22 has a plurality of active material plate elements, the following manufacturing method is omitted. It is the same.

The positive electrode active material plate 22 is produced, for example, by a so-called green sheet process. First, a mixed powder of an oxide of lithium and an oxide of a main transition metal element (ie, Co) is prepared. _{Specifically, Co 3} O ₄ powder (manufactured by Shodo Chemical Industry Co., Ltd.) and Li ₂ CO ₃ powder (manufactured by Honjo Chemical Co., Ltd.) weighed so that the molar ratio of Li / Co is 1.02. After mixing, the mixed powder is held at 800 ° C. for 5 hours. The above-mentioned Li / Co molar ratio may be appropriately changed, for example, in the range of 1.00 to 1.05. The holding temperature and holding time of the mixed powder may also be changed as appropriate.

_{Subsequently, Nb 2} O ₅ powder (manufactured by Mitsui Mining & Smelting Co., Ltd.) is added to the mixed powder, and the powder is pulverized and crushed by a pot mill so that the volume-based D50 particle size has a particle size distribution of 0.8 μm. As a result, a raw material powder can be obtained. The particle size distribution can be measured by Microtrack MT3000II (manufactured by Microtrack Bell Co., Ltd.). _{The addition ratio of the Nb 2} O ₅ powder to the raw material powder is 0.03% by mass. Incidentally, from the atomic weight ratio of the constituent elements of _Nb 2 _{O 5,} about 70% of the added _Nb 2 _{O 5} is equivalent to Nb element. Therefore, the content of Nb in the raw material powder is 0.021% by mass. The volume-based D50 particle size of the raw material powder may be appropriately changed, for example, in the range of 0.2 μm to 10 μm. The content of Nb in the raw material powder may also be appropriately changed, for example, in the range of 0.1% by mass to 2.0% by mass.

Next, 100 parts by weight of the raw material powder, 32 parts by weight of the dispersion medium (2-ethylhexanol), 8 parts by weight of a binder (product number BLS, manufactured by Sekisui Chemical Co., Ltd.), and a plasticizer (DOP: di (phthalate) 2-Ethylhexyl), manufactured by Kurokin Kasei Co., Ltd.) 3.2 parts by weight and a dispersant (product number Marialim SC0505K, manufactured by Nichiyu Co., Ltd.) 2.5 parts by weight are mixed by a 3-roll mill and a self-rotating mixer. To. A slurry is prepared by stirring the obtained mixture under reduced pressure to defoam and adjusting the viscosity to 4000 cP to 10000 cP. The composition and type of the various raw materials in the slurry may be changed as appropriate. The viscosity can be measured with an LVT type viscometer manufactured by Brookfield.

_{A lithium compound other than LiCoO 2} (for example, lithium carbonate) is excessively added to the slurry in an amount of about 0.5 mol% to 30 mol% for the purpose of promoting grain growth and compensating for volatile matter during the firing step described later. May be done. Further, it is desirable that no pore-forming material is added to the slurry.

Then, the slurry is formed into a sheet on a polyethylene terephthalate (PET) film by the doctor blade method to form a green sheet. The thickness of the green sheet after drying is, for example, about 105 μm. The thickness of the green sheet may be appropriately changed, for example, within the range in which the thickness of the positive electrode active material plate 22 after firing is in the above-mentioned range of 15 μm to 200 μm.

Next, the green sheet peeled off from the PET film is cut out to a predetermined size (for example, 68 mm square) by a cutter and placed in the center of the lower setter (for example, dimension 90 mm square, height 1 mm). In addition, the upper setter is placed on the green sheet on the lower setter. The upper setter and lower setter are made of ceramics, preferably zirconia or magnesia. When the setter is made of magnesia, the pores of the positive electrode active material plate 22 tend to be small. The upper setter may have a porous structure or a honeycomb structure, or may have a dense structure. When the upper setter is dense, the pores of the positive electrode active material plate 22 tend to be small and the number of pores tends to be large.

The green sheet is placed in, for example, a 120 mm square alumina sheath (manufactured by Nikkato Corporation) in a state of being sandwiched between setters. At this time, the alumina sheath is not sealed, and the lid is closed with a gap of a predetermined size (for example, 0.5 mm).

The laminate of the green sheet and the setter is fired, for example, by raising the temperature to 900 ° C. at a heating rate of 100 ° C./h and holding it for 15 hours. Then, the fired body cooled to room temperature is taken out from the alumina sheath to obtain a positive electrode active material plate 22 which is a sintered body plate. The thickness of the positive electrode active material plate 22 is, for example, about 100 μm. The sizes of the positive electrode active material plate 22, the setter, and the like may be changed as appropriate. The firing conditions of the positive electrode active material plate 22 may also be changed as appropriate. The firing step may be performed in two steps or once. When firing in two steps, it is preferable that the firing temperature of the first firing is lower than the firing temperature of the second firing. When the positive electrode active material plate 22 has a plurality of active material plate elements, the sintered body plate is cut to a predetermined size by a laser processing machine or the like to form a plurality of active material plate elements.

The average orientation angle of the primary particles on the positive electrode active material plate 22 is, for example, 16 °. The average orientation angle can be measured as follows. First, the positive electrode active material plate 22 is polished with a cross section polisher (IB-15000CP, manufactured by JEOL Ltd.), and the obtained cross section (that is, a cross section substantially perpendicular to the main surface of the positive electrode active material plate 22) is multiplied by 1000. EBSD measurement is performed in the field of view (125 μm × 125 μm) to obtain an EBSD image. The EBSD measurement is performed using, for example, a Schottky field emission scanning electron microscope (model JSM-7800F, manufactured by JEOL Ltd.). Then, for all the particles specified in the obtained EBSD image, _{the angle formed by the (003) plane of the primary particles and the main plane of the LiCoO 2} sintered body plate (that is, the inclination of the crystal orientation from (003)). Is obtained as the inclination angle, and the average value of these angles is taken as the average orientation angle of the primary particles.

The thickness of the positive electrode active material plate 22 is determined by polishing the positive positive active material plate 22 with a cross section polisher (IB-15000CP, manufactured by JEOL Ltd.) and observing the above-mentioned cross section obtained by SEM observation (JSM6390LA, JEOL Ltd.). It can be measured by manufacturing). The thickness of the green sheet after drying can be measured in the same manner.

The porosity of the positive electrode active material plate 22 is, for example, 30%. The porosity can be measured as follows. First, the positive electrode active material plate 22 is polished with a cross section polisher (IB-15000CP, manufactured by JEOL Ltd.), and the above-mentioned cross section obtained is observed by SEM with a 1000-fold field view (125 μm × 125 μm) (manufactured by JEOL Ltd., JSM6390LA). The obtained SEM image is image-analyzed, the area of all pores is divided by the area of the positive electrode active material plate 22, and the obtained value is multiplied by 100 to calculate the porosity (%).

The average pore diameter of the positive electrode active material plate 22 is, for example, 0.8 μm. The average pore diameter can be measured by a mercury intrusion method using a mercury porosimeter (manufactured by Shimadzu Corporation, Autopore IV9510).

In the above-mentioned production of the positive electrode active material plate 22, the green sheet is sandwiched between the upper and lower setters, but the green sheet placed on the lower setter is temporarily placed before the upper setter is placed on the green sheet. It may be fired. The temperature and time of the preliminary firing are, for example, 600 ° C. to 850 ° C. and 1 hour to 10 hours. Further, the green sheet may be degreased before the temporary firing. The upper setter is installed on the calcined body formed by calcining.

In the production of the positive electrode active material plate 22 described above, a raw material powder is obtained by adding _{Nb 2} O _{5 powder to a mixed powder of Co 3} O ₄ powder and Li ₂ CO ₃ powder and pulverizing the mixture. May be obtained by other methods. For example, first, a powder of a composite oxide of lithium and a main transition metal element (ie, LiCoO ₂ ) is prepared. Specifically, for example, Co ₃ O ₄ powder and Li ₂ CO ₃ powder are mixed and fired at 500 ° C. to 900 ° C. for 1 hour to 20 hours to synthesize _{LiCoO 2 powder.} The LiCoO ₂ powder preferably contains plate particles (ie, LiCoO ₂ plate particles). The volume-based D50 particle size of the LiCoO ₂ powder is, for example, 0.3 μm to 30 μm.

LiCoO ₂ powder is _{a plate-like method such as a method of growing green sheets formed by using LiCoO 2} powder slurry and then crushing them, a flux method, hydrothermal synthesis, single crystal growth using a melt, a sol-gel method, etc. It can be obtained by various methods for synthesizing crystals. Like the above LiCoO _{2 particles, the LiCoO 2} particles obtained by these methods are in a state of being easily cleaved along the cleavage plane. Therefore, the plate-shaped LiCoO ₂ particles can be cleaved by crushing. It can be easily obtained.

Subsequently, _Nb 2 _{O 5} powder LiCoO ₂ powder was added 0.1 wt% to 2.0 wt%, as in a pot mill, the volume-based D50 particle size becomes a particle size distribution of 0.2μm~10μm grinding And by crushing, a raw material powder is obtained. In the raw material powder, plate-shaped LiCoO ₂ particles capable of conducting lithium ions parallel to the plate surface can be obtained by the above-mentioned pulverization or the like. Nb added before pulverization or the like is attached to the surface of the LiCoO _{2 particles as Nb 2} O ₅ or in another state.

In addition to the LiCoO ₂ powder, _{other powders such as Co 3} O ₄ particles may be mixed with the raw material powder. In this case, the plate-shaped LiCoO ₂ particles function as template particles for imparting orientation, and the other powder (for example, Co ₃ O ₄ particles) functions as matrix particles that can grow along the template particles. It is preferable to do so. The mixing ratio of the template particles and the matrix particles is preferably 100: 0 to 3:97.

When Co ₃ O ₄ particles are used as matrix particles, the volume-based D50 particle size of the matrix particles is not particularly limited, but is, for example, 0.1 μm to 1.0 μm, which is smaller than the volume-based D50 particle size of the template particles. Is preferable. Co ₃ O ₄ particles can also be obtained by heat-treating the Co (OH) ₂ raw material at 500 ° C. to 800 ° C. for 1 hour to 10 hours. Further, as the matrix particles, in addition to the Co ₃ O ₄ particles, Co (OH) ₂ particles may be used, or LiCo _{O 2} particles may be used.

When LiCoO ₂ particles are used as the matrix particles, or when only the template particles (LiCoO ₂ particles) are used without the matrix particles, the LiCoO is large-sized (for example, 90 mm square) and flat by firing. ₂ Sintered plate can be obtained.

When the positive electrode active material plate 22 is prepared, the conductive bonding layer 23 (see FIG. 1) is formed on the positive electrode current collector 21 (for example, an aluminum foil having a thickness of 9 mm). The conductive bonding layer 23 is formed, for example, by dropping 2 μL (microliter) of a slurry in which acetylene black is mixed with a solution of polyamide-imide (PAI) in N-methylpyrrolidone onto the positive electrode current collector 21. Will be done.

Subsequently, the positive electrode active material plate 22 is placed on the conductive bonding layer 23 and dried. The positive electrode active material plate 22 is bonded to the positive electrode current collector 21 via the conductive bonding layer 23. After that, as shown in FIG. 3A, the composite of the positive electrode current collector 21 and the positive electrode active material plate 22 is laminated on the first sheet portion 65, and the first sheet is interposed via the positive electrode bonding layer 63 (see FIG. 1). By joining to the portion 65, the positive electrode assembly 20 is formed. One end of one terminal 7 is preliminarily fixed to the positive electrode current collector 21 by welding.

On the other hand, a negative electrode active material layer 32 (for example, a thickness of 130 μm), which is a carbon layer, is coated on the negative electrode current collector 31 (for example, a copper foil having a thickness of 10 μm). The negative electrode active material layer 32 is a carbon coating film containing a mixture of graphite as an active material and PVDF as a binder. The thickness and material of the negative electrode current collector 31 and the thickness and raw material of the negative electrode active material layer 32 may be appropriately changed.

Subsequently, the composite of the negative electrode current collector 31 and the negative electrode active material layer 32 is laminated on the second sheet portion 66 and bonded to the second sheet portion 66 via the negative electrode bonding layer 64 (see FIG. 1). As a result, the negative electrode assembly 30 is formed. One end of one terminal 7 is preliminarily fixed to the negative electrode current collector 31 by welding.

As the separator 4, for example, a polyolefin-based porous membrane (manufactured by JNC Corporation, S115) is prepared. Then, the positive electrode assembly 20, the separator 4, and the negative electrode assembly 30 are laminated in this order so that the positive electrode active material plate 22 and the negative electrode active material layer 32 face the separator 4, and the intermediate laminate 10 is formed. In the intermediate laminated body 10, both upper and lower surfaces of the laminated body of the positive electrode 2, the separator 4, and the negative electrode 3 (hereinafter, also referred to as “battery element”) are the exterior body 6 (that is, the first sheet portion 65 and the second sheet portion 66). ), And the first sheet portion 65 and the second sheet portion 66 extend around the battery element. The vertical thickness of the battery element is, for example, 0.33 mm. The shape of the battery element in a plan view is, for example, a substantially rectangular shape of 2.3 cm × 3.2 cm.

Subsequently, three of the four sides of the substantially rectangular intermediate laminate 10 are sealed by heat fusion bonding. In the example shown in FIG. 3A, three sides other than the upper one side in the figure are sealed. The three sides include one side on which the two terminals 7 project. In the sealing of the three sides, for example, a patch jig adjusted so that the sealing width is 2 mm is used, and the outer peripheral portion of the intermediate laminate 10 is 10 at 200 ° C. and 1.5 MPa (megapascal). Heated and pressurized for seconds. As a result, the first seat portion 65 and the second seat portion 66 of the exterior body 6 are heat-sealed. After sealing the three sides, the intermediate laminate 10 is housed in a vacuum dryer 81 to remove moisture and dry the adhesive (that is, the positive electrode bonding layer 63, the negative electrode bonding layer 64, and the conductive bonding layer 23). Is done.

Next, as shown in FIG. 3B, the intermediate laminate 10 is housed in the glove box 82. Then, on one unsealed side of the intermediate laminate 10, the injection instrument 83 is inserted between the first sheet portion 65 and the second sheet portion 66, and the electrolytic solution 5 (see FIG. 1) is inserted through the injection instrument 83. ) Is injected into the intermediate laminate 10. _{In the electrolytic solution 5, for example, LiPF 6} which is an electrolyte is dissolved in a mixed solvent containing EC and EMC at a volume ratio of 3: 7 so as to have a concentration of 1 mol / L, and LiDFP and TMSP are added as additives, respectively. , A liquid added so as to be 1.0% by volume. The solvent of the electrolytic solution 5, the type of the electrolyte, and the like may be variously changed as described above. Moreover, the addition amount of each of the additives LiDFP and TMSP may be changed as appropriate.

When the injection of the electrolytic solution 5 is completed, one of the unsealed sides is temporarily sealed (that is, vacuum sealed) by a simple sealer under a reduced pressure atmosphere (for example, an absolute pressure of 5 kPa) in the glove box 82. To. Subsequently, the intermediate laminate 10 is initially charged and aged for a predetermined period (for example, 7 days). When the aging is completed, the portion of the first sheet portion 65 and the second sheet portion 66 near the outer edge of one temporarily sealed side (that is, the end portion that does not overlap with the battery element) is excised, and the aging causes the aging. The generated gas containing water and the like is removed (that is, degassing is performed).

When the degassing is completed, the glove box 82 is sealed by heat-sealing joining the sides formed by the above-mentioned cutting under a reduced pressure atmosphere (for example, an absolute pressure of 5 kPa). In the sealing, for example, a patch jig adjusted so that the sealing width is 2 mm is used as in the sealing of the above-mentioned three sides, and the first sheet portion 65 and the second sheet portion 66 are formed. It is heated and pressurized at 200 ° C. and 1.5 MPa for 10 seconds. As a result, the first seat portion 65 and the second seat portion 66 of the exterior body 6 are heat-sealed to form the lithium secondary battery 1. After that, the excess portion on the outer peripheral portion of the exterior body 6 is cut off, and the shape of the lithium secondary battery 1 is adjusted. For example, the shape of the lithium secondary battery 1 in a plan view is a rectangle of 38 mm × 27 mm, the thickness is 0.45 mm or less, and the capacity is 30 mAh.

As described above, the substitution metal element of the positive electrode active material plate 22 is not limited to Nb, and may be, for example, Ti. Hereinafter, an example of a method for manufacturing the positive electrode active material plate 22 when the substituted metal element is Ti will be described. First, a mixed powder of an oxide of lithium and an oxide of a main transition metal element (that is, Co) is prepared and held at 800 ° C. for 5 hours, as in the case where the substituted metal element is Nb.

_{Subsequently, TiO 2} powder (manufactured by Ishihara Sangyo Co., Ltd.) is added to the mixed powder, and the powder is pulverized and crushed by a pot mill so that the volume standard D50 particle size has a particle size distribution of 0.8 μm. Raw material powder is obtained. The particle size distribution can be measured by Microtrack MT3000II (manufactured by Microtrack Bell Co., Ltd.). The addition ratio of the TiO ₂ powder to the raw material powder is 0.4% by mass. Incidentally, from the atomic weight ratio of the constituent elements of TiO _2, about 60% of TiO ₂ added it is equivalent to Ti element. Therefore, the content of Ti in the raw material powder is 0.24% by mass. The volume-based D50 particle size of the raw material powder may be appropriately changed, for example, in the range of 0.2 μm to 10 μm. The content of Ti in the raw material powder may also be appropriately changed, for example, in the range of 0.1% by mass to 2.0% by mass.

Next, 100 parts by weight of the raw material powder, 32 parts by weight of the dispersion medium (2-ethylhexanol), 8 parts by weight of a binder (product number BLS, manufactured by Sekisui Chemical Co., Ltd.), and a plasticizer (DOP: di (phthalate) 2-Ethylhexyl), manufactured by Kurokin Kasei Co., Ltd.) 3.2 parts by weight and a dispersant (product number Marialim SC0505K, manufactured by Nichiyu Co., Ltd.) 2.5 parts by weight are mixed by a 3-roll mill and a self-rotating mixer. To. A slurry is prepared by stirring the obtained mixture under reduced pressure to defoam and adjusting the viscosity to 4000 cP to 10000 cP. The composition and type of the various raw materials in the slurry may be changed as appropriate. The viscosity can be measured with an LVT type viscometer manufactured by Brookfield.

Similar to the case where the substituted metal element is Nb, the slurry contains 0.5 mol of a lithium compound other than _{LiCoO 2 (for example, lithium carbonate) for the purpose of promoting grain growth and compensating for volatile matter during the firing step.} It may be added excessively in an amount of about% to 30 mol%. Further, it is desirable that no pore-forming material is added to the slurry.

Hereinafter, the positive electrode active material plate 22 containing Ti as the substituted metal element is produced by the same method and production conditions (for example, firing conditions) as in the case where the substituted metal element is Nb. Specifically, first, the slurry is formed into a sheet on a PET film by the doctor blade method to form a green sheet. Subsequently, the green sheet peeled off from the PET film is cut out to a predetermined size by a cutter. Then, the positive electrode active material plate 22 is obtained by firing the green sheet.

Further, in the lithium secondary battery 1, the positive electrode 2 does not necessarily have to be a sintered plate electrode having a positive electrode active material plate 22 which is a plate-shaped ceramic sintered body, but is a coated electrode such as a negative electrode 3. May be good. In this case, the positive electrode 2 includes the above-mentioned positive electrode current collector 21 and a positive electrode active material layer coated on the positive electrode current collector 21. The positive electrode active material layer contains the above-mentioned lithium composite oxide which is a positive electrode active material and a binder containing a resin as a main component. As described above, the lithium composite oxide is partially substituted with a substituted metal element (for example, Nb, Ti) in the composite oxide of lithium and a main transition metal element (for example, Co, Ni, Mn). It is a thing. The binder is, for example, styrene butadiene rubber (SBR), polyvinylidene fluoride (PVDF) or a mixture thereof.

Hereinafter, an example of a method for manufacturing the positive electrode 2 when the positive electrode 2 is a coating electrode will be described. In the following description, the case where the main transition metal element and the substituted metal element are Co and Nb, respectively, will be described. First, a mixed powder of an oxide of lithium and an oxide of a main transition metal element (that is, Co) is prepared as in the case where the positive electrode 2 includes the positive electrode active material plate 22. The mixed powder is held at 900 ° C. for 10 hours.

Subsequently, as in the case where the positive electrode 2 includes the positive electrode active material plate 22, Nb ₂ O ₅ powder (manufactured by Mitsui Mining & Smelting Co., Ltd.) is added to the mixed powder, and the volume standard D50 particle size is 0 in a pot mill. A raw material powder is obtained by pulverizing and crushing so as to have a particle size distribution of .8 μm. _{The addition rate of the Nb 2} O ₅ powder to the raw material powder is 0.03% by mass, and the content of Nb in the raw material powder is 0.021% by mass. The volume-based D50 particle size of the raw material powder may be appropriately changed, for example, in the range of 0.2 μm to 10 μm. The content of Nb in the raw material powder may also be appropriately changed, for example, in the range of 0.1% by mass to 2.0% by mass.

Next, 91% by mass of the raw material powder, 5% by mass of acetylene black, 4% by mass of polyvinylidene fluoride (PVdF), and an N-methylpyrrolidone (NMP) solution are mixed to prepare a slurry. Then, the slurry is applied onto the positive electrode current collector 21 (for example, an aluminum foil having a thickness of 10 mm) and dried. Then, the dried coating layer is pressed to produce a positive electrode 2 which is a coating electrode.

As described above, the lithium secondary battery 1 includes a positive electrode 2, a separator 4, a negative electrode 3, an electrolytic solution 5, and an exterior body 6. The separator 4 is arranged on the positive electrode 2 in a predetermined superposition direction. The negative electrode 3 is arranged on the opposite side of the separator 4 from the positive electrode 2 in the stacking direction. The electrolytic solution 5 is impregnated into the positive electrode 2, the negative electrode 3, and the separator 4. The exterior body 6 covers the positive electrode 2 and the negative electrode 3 from both sides in the stacking direction. The exterior body 6 houses the positive electrode 2, the separator 4, the negative electrode 3, and the electrolytic solution 5 inside.

The electrolyte 5 contains lithium difluorophosphate (LiDFP) and tris (trimethylsilyl) phosphate (TMSP). The content of LiDFP in the electrolytic solution 5 is 0.1% by mass or more and 5.0% by mass or less. The content of TMSP in the electrolytic solution 5 is 0.1% by mass or more and 5.0% by mass or less. As a result, in the lithium secondary battery 1, a film is formed on the surface of the positive electrode 2 and the surface is covered. As a result, the resistance deterioration rate of the lithium secondary battery 1 can be reduced. In other words, in the lithium secondary battery 1, resistance deterioration (that is, increase in resistance with time) can be suppressed. Therefore, the lithium secondary battery 1 is particularly suitable as a power supply source in a thin device, that is, a sheet-like device or a flexible device (for example, a smart card).

As described above, the positive electrode 2 preferably includes a positive electrode active material plate 22 which is a plate-shaped ceramic sintered body containing the lithium composite oxide. Unlike the positive electrode active material layer of the coated electrode, the positive electrode active material plate 22 does not substantially contain a binder. Therefore, since substantially the entire active surface of the positive electrode active material is in contact with the electrolytic solution 5, the exposed portion of the positive electrode active material tends to deteriorate easily. Since the surface of the positive electrode 2 is coated in the lithium secondary battery 1 as described above, the structure of the lithium secondary battery 1 includes a positive electrode active material plate 22 in which the positive electrode active material is relatively easily exposed to the surface. Especially suitable for secondary batteries.

In the lithium secondary battery 1, the additive of the electrolytic solution 5 is not limited to LiDFP and TMSP. For example, the electrolyte 5 contains 1,3,2-dioxathiolane-2,2-dioxide (DTD) and tris (trimethylsilyl) phosphate (TMSP). The content of DTD in the electrolytic solution 5 is 0.1% by mass or more and 5.0% by mass or less. The content of TMSP in the electrolytic solution 5 is 0.1% by mass or more and 5.0% by mass or less. The electrolytic solution 5 may contain other additives in addition to DTD and TMSP, and may not substantially contain additives other than DTD and TMSP. The method for manufacturing the lithium secondary battery 1 is, for example, substantially the same as that shown in FIGS. 3A and 3B described above.

By adding DTD and TMSP to the electrolytic solution 5, the surface of the positive electrode 2 of the lithium secondary battery 1 is coated in substantially the same manner as described above. As a result, the resistance deterioration rate of the lithium secondary battery 1 can be reduced. In other words, in the lithium secondary battery 1, resistance deterioration (that is, increase in resistance with time) can be suppressed. Therefore, the lithium secondary battery 1 is particularly suitable as a power supply source in a thin device, that is, a sheet-like device or a flexible device (for example, a smart card). Further, the structure of the lithium secondary battery 1 in which the surface of the positive electrode 2 is coated is particularly suitable for a lithium secondary battery provided with a positive electrode active material plate 22 in which the positive electrode active material is relatively easily exposed on the surface.

Further, in the lithium secondary battery 1, for example, the electrolytic solution 5 contains lithium difluorophosphate (LiDFP) and 1,3,2-dioxathiolane-2,2-dioxide (DTD). The content of LiDFP in the electrolytic solution 5 is 0.1% by mass or more and 5.0% by mass or less. The content of DTD in the electrolytic solution 5 is 0.1% by mass or more and 5.0% by mass or less. The electrolytic solution 5 may contain other additives in addition to LiDFP and DTD, and may not substantially contain additives other than LiDFP and DTD. The method for manufacturing the lithium secondary battery 1 is, for example, substantially the same as that shown in FIGS. 3A and 3B described above.

By adding LiDFP and DTD to the electrolytic solution 5, the surface of the positive electrode 2 of the lithium secondary battery 1 is coated in substantially the same manner as described above. As a result, the resistance deterioration rate of the lithium secondary battery 1 can be reduced. In other words, in the lithium secondary battery 1, resistance deterioration (that is, increase in resistance with time) can be suppressed. Therefore, the lithium secondary battery 1 is particularly suitable as a power supply source in a thin device, that is, a sheet-like device or a flexible device (for example, a smart card). Further, the structure of the lithium secondary battery 1 in which the surface of the positive electrode 2 is coated is particularly suitable for a lithium secondary battery provided with a positive electrode active material plate 22 in which the positive electrode active material is relatively easily exposed on the surface.

Further, in the lithium secondary battery 1, for example, the electrolytic solution 5 contains fluoroethylene carbonate (FEC) and tris (trimethylsilyl) phosphate (TMSP). The content of FEC in the electrolytic solution 5 is 0.1% by mass or more and 5.0% by mass or less. The content of TMSP in the electrolytic solution 5 is 0.1% by mass or more and 5.0% by mass or less. The electrolytic solution 5 may contain other additives in addition to FEC and TMSP, and may not substantially contain additives other than FEC and TMSP.

By adding FEC and trimethyl to the electrolytic solution 5, the surface of the positive electrode 2 of the lithium secondary battery 1 is coated in substantially the same manner as described above. As a result, the resistance deterioration rate of the lithium secondary battery 1 can be reduced. In other words, in the lithium secondary battery 1, resistance deterioration (that is, increase in resistance with time) can be suppressed. Therefore, the lithium secondary battery 1 is particularly suitable as a power supply source in a thin device, that is, a sheet-like device or a flexible device (for example, a smart card). Further, the structure of the lithium secondary battery 1 in which the surface of the positive electrode 2 is coated is particularly suitable for a lithium secondary battery provided with a positive electrode active material plate 22 in which the positive electrode active material is relatively easily exposed on the surface.

In the above-mentioned lithium secondary battery 1, various changes can be made.

For example, the structure of the positive electrode active material plate 22 of the positive electrode 2 may be changed in various ways. For example, in the positive electrode active material plate 22, the average inclination angle of the plurality of primary particles having a layered rock salt structure may be larger than 30 ° or 0 °. Alternatively, the structure of the plurality of primary particles may be a structure other than the layered rock salt structure.

The positive electrode active material plate 22 does not necessarily have to be produced by the above-mentioned green sheet process, and may be produced by various other methods. For example, the surface of the primary particles of the composite oxide of lithium and the main transition metal element is coated with the substituted metal element by using the sol-gel method (sol-gel coating) and then fired to obtain the above-mentioned positive electrode active material plate 22. It may be made.

In the lithium secondary battery 1, instead of the negative electrode 3 which is a coating electrode, a negative electrode having substantially the same structure as the positive electrode 2 shown in FIG. 1 (that is, a sintered plate electrode) may be provided. Specifically, the negative electrode includes a negative electrode current collector, a negative electrode active material plate, and a conductive bonding layer. The negative electrode current collector is a conductive sheet-like member. The negative electrode active material plate is a relatively thin plate-shaped ceramic sintered body containing a lithium composite oxide (for example, lithium titanium oxide (LTO)). The negative electrode active material plate is bonded to the lower surface of the negative electrode current collector via the conductive bonding layer which is substantially the same as the conductive bonding layer 23 of the positive electrode 2. Like the positive electrode active material plate 22, the negative electrode active material plate may be a single plate-shaped member, or may be divided into a plurality of plate-shaped members.

The lithium secondary battery 1 is used in a flexible device other than a smart card (for example, a card-type device) or a sheet-like device (for example, a wearable device provided on clothes or the like, or a body-attachable device). It may be used as a power supply source. Further, the lithium secondary battery 1 may be used as a power supply source for various objects (for example, IoT modules) other than the above-mentioned devices.

The above-described embodiment and the configurations in each modification may be appropriately combined as long as they do not conflict with each other.

The lithium secondary battery of the present invention can be used in various fields in which the lithium secondary battery is used, for example, as a power supply source for a smart card having an arithmetic processing function.

1 Lithium secondary battery 2 Positive electrode 3 Negative electrode 4 Separator 5 Electrolyte 6 Exterior body 22 Positive electrode Active material plate

Claims (4)

It ’s a lithium secondary battery,
With the positive electrode
A separator arranged on the positive electrode in a predetermined superposition direction,
A negative electrode arranged on the side of the separator opposite to the positive electrode in the stacking direction,
The electrolytic solution impregnated in the positive electrode, the negative electrode and the separator,
An exterior body that covers the positive electrode and the negative electrode from both sides in the stacking direction and internally accommodates the positive electrode, the separator, the negative electrode, and the electrolytic solution.
With
The electrolyte contains lithium difluorophosphate and tris phosphate (trimethylsilyl).
The content of lithium difluorophosphate in the electrolytic solution is 0.1% by mass or more and 5.0% by mass or less.
A lithium secondary battery characterized in that the content of tris (trimethylsilyl) phosphate in the electrolytic solution is 0.1% by mass or more and 5.0% by mass or less. It ’s a lithium secondary battery,
With the positive electrode
A separator arranged on the positive electrode in a predetermined superposition direction,
A negative electrode arranged on the side of the separator opposite to the positive electrode in the stacking direction,
The electrolytic solution impregnated in the positive electrode, the negative electrode and the separator,
An exterior body that covers the positive electrode and the negative electrode from both sides in the stacking direction and internally accommodates the positive electrode, the separator, the negative electrode, and the electrolytic solution.
With
The electrolyte contains 1,3,2-dioxathiolane-2,2-dioxide and tris phosphate (trimethylsilyl).
The content of 1,3,2-dioxathiolane-2,2-dioxide in the electrolytic solution is 0.1% by mass or more and 5.0% by mass or less.
A lithium secondary battery characterized in that the content of tris (trimethylsilyl) phosphate in the electrolytic solution is 0.1% by mass or more and 5.0% by mass or less. It ’s a lithium secondary battery,
With the positive electrode
A separator arranged on the positive electrode in a predetermined superposition direction,
A negative electrode arranged on the side of the separator opposite to the positive electrode in the stacking direction,
The electrolytic solution impregnated in the positive electrode, the negative electrode and the separator,
An exterior body that covers the positive electrode and the negative electrode from both sides in the stacking direction and internally accommodates the positive electrode, the separator, the negative electrode, and the electrolytic solution.
With
The electrolytic solution contains lithium difluorophosphate and 1,3,2-dioxathiolane-2,2-dioxide.
The content of lithium difluorophosphate in the electrolytic solution is 0.1% by mass or more and 5.0% by mass or less.
A lithium secondary battery characterized in that the content of 1,3,2-dioxathiolane-2,2-dioxide in the electrolytic solution is 0.1% by mass or more and 5.0% by mass or less. The lithium secondary battery according to any one of claims 1 to 3.
The positive electrode is a lithium secondary battery including a positive electrode active material plate which is a plate-shaped ceramic sintered body containing a lithium composite oxide.

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