JP2017195129A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the decomposition of an ionic liquid in a lithium secondary battery which uses the ionic liquid.SOLUTION: A lithium secondary battery 1 comprises: a negative electrode 3; a positive electrode 4; and an electrolyte layer 5 disposed between the negative electrode 3 and the positive electrode 4. The electrolyte layer 5 has a porous separator 51. The separator 51 holds an electrolyte solution 50 including an ionic liquid in pores. The positive electrode 4 is formed by a sintered compact. In the sintered compact, a surface in contact with the ionic liquid is dense. Because of this, the decomposition of the ionic liquid owing to a local rise of lithium ion concentration in the electrolyte solution 50 when charging a lithium secondary battery 1 can be suppressed and the lithium secondary battery can be charged with stability.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a lithium secondary battery.

  Conventionally, a lithium secondary battery (lithium ion secondary battery) using an organic solvent as an electrolyte solvent has been used. Patent Documents 1 to 3 propose lithium secondary batteries using an ionic liquid as an electrolyte solvent. The ionic liquid has higher flame retardancy and heat resistance than the organic solvent. Therefore, in the lithium secondary battery using the ionic liquid, improvement in safety and stable operation in a high temperature environment are realized. For example, in a lithium secondary battery using an organic solvent, the upper limit of the environmental temperature is 60 ° C., whereas in a lithium secondary battery using an ionic liquid, the upper limit of the environmental temperature is 120 ° C.

  Patent Document 4 discloses a positive electrode active material film of a lithium secondary battery having a layered rock salt structure, and the active material film is a fired body in which a surface other than (003) is oriented in the plate surface direction of particles. Formed as a film.

Japanese Patent No. 5726707 Japanese Patent Laying-Open No. 2015-201437 Japanese Patent No. 5865672 Japanese Patent No. 5043203

  By the way, the electrolytic solution using the ionic liquid has lower lithium ion conductivity (diffusibility) than the electrolytic solution using the organic solvent, and the ionic liquid has a higher viscosity when the lithium ion concentration is higher. . In the lithium secondary batteries of Patent Documents 1 to 3, the density of the positive electrode is low, and the electrolyte is held in the pores (pores) in the positive electrode. Therefore, when the lithium secondary battery is charged, the lithium ion concentration of the electrolytic solution locally increases in the pores. As a result, an overvoltage due to an increase in viscosity occurs, and the ionic liquid contained in the electrolytic solution is decomposed. In order to avoid decomposition of the ionic liquid, charging may be performed at a low rate, or the pore density may be increased by lowering the relative density of the positive electrode, but the current value that can be taken out in the lithium secondary battery is reduced, The energy density of the battery is reduced.

  The present invention has been made in view of the above problems, and an object of the present invention is to stably charge a lithium secondary battery using an ionic liquid by suppressing the decomposition of the ionic liquid.

  A lithium secondary battery according to the present invention includes a negative electrode, a positive electrode formed of a sintered body and including lithium, an ionic liquid disposed between the negative electrode and the positive electrode, and in contact with the surface of the sintered body. An electrolyte layer, and at least the surface of the sintered body is dense.

  In one preferred embodiment of the present invention, the relative density of the sintered body is 90% or more.

  In another preferred embodiment of the present invention, the relative density of the sintered body is 80% or more, and the ratio of closed pores to all pores in the sintered body is 90% or more.

  In one aspect of the present invention, the sintered body is an oriented sintered plate including a plurality of oriented lithium cobalt oxide crystal grains. Preferably, the oriented sintered plate has a thickness of 40 micrometers or more.

  Typically, in the ionic liquid, the viscosity increases as the lithium ion concentration increases.

  In another aspect of the present invention, in the negative electrode, a surface in contact with the ionic liquid is dense.

  In still another aspect of the present invention, both of the positive electrode and the negative electrode are plate-shaped, and the surface of the positive electrode and the surface of the negative electrode facing each other through the electrolyte layer are parallel, and the positive electrode When viewed along a direction perpendicular to the surface, the surface of the negative electrode has a portion that extends outward from the outer edge of the surface of the positive electrode.

  When the electrolyte layer includes a separator, it is preferable that the thickness of the separator is 0.5 mm or less and the porosity of the separator is 50% or more.

  ADVANTAGE OF THE INVENTION According to this invention, in the lithium secondary battery using an ionic liquid, decomposition | disassembly of an ionic liquid can be suppressed and it can charge stably.

It is sectional drawing which shows the structure of a lithium secondary battery. It is a figure which shows a negative electrode and a positive electrode. It is sectional drawing which shows the structure of the lithium secondary battery of a comparative example. It is a figure which expands and shows the positive electrode of the lithium secondary battery of a comparative example. It is a figure which expands and shows a part of positive electrode of the lithium secondary battery of a comparative example. It is sectional drawing which shows the other example of a lithium secondary battery.

  FIG. 1 is a cross-sectional view showing a configuration of a lithium secondary battery 1 according to an embodiment of the present invention. The lithium secondary battery 1 is a coin-type secondary battery, for example, and includes a negative electrode case 21, a negative electrode 3, a positive electrode case 22, a positive electrode 4, and a separator 51. The negative electrode 3 has a rectangular plate shape and is accommodated in a covered and bottomless negative electrode case 21. The positive electrode 4 has a rectangular plate shape, and is accommodated in a positive electrode case 22 having no lid and a bottom. The negative electrode case 21 is disposed in the positive electrode case 22, and a thin film separator 51 is provided between the negative electrode 3 and the positive electrode 4. The negative electrode case 21 and the positive electrode case 22 are caulked through an insulating gasket 23. As described above, the negative electrode case 21, the positive electrode case 22, and the gasket 23 mainly constitute the case 2 that accommodates the negative electrode 3, the positive electrode 4, and the separator 51. The case 2 is filled with the electrolytic solution 50 and held. The lithium secondary battery 1 may be a chip-type secondary battery or a cylindrical secondary battery in addition to the coin-type secondary battery. The negative electrode 3, the positive electrode 4, the case 2, etc. It is not limited.

  A spring 25 and a spacer 24 are provided between the top plate portion 211 and the negative electrode 3 in the negative electrode case 21. The spacer 24 is in contact with the surface of the negative electrode 3 opposite to the separator 51. The spring 25 is disposed between the spacer 24 and the top plate portion 211 and is in contact with both. The negative electrode case 21, the spring 25 and the spacer 24 are made of a conductive material, and the negative electrode case 21 and the negative electrode 3 are electrically connected via the spring 25 and the spacer 24. A spacer 26 is provided between the bottom plate portion 221 in the positive electrode case 22 and the positive electrode 4. The spacer 26 is in contact with both the bottom plate portion 221 and the positive electrode 4. The positive electrode case 22 and the spacer 26 are formed of a conductive material, and the positive electrode case 22 and the positive electrode 4 are electrically connected via the spacer 26.

  The separator 51 has a porous film shape. A large number of pores are formed inside the separator 51, and the electrolytic solution 50 is held in the pores. That is, the separator 51 is a holding layer for the electrolytic solution 50. In the lithium secondary battery 1 of FIG. 1, the electrolyte layer 5 disposed between the negative electrode 3 and the positive electrode 4 is realized by the separator 51 and the electrolytic solution 50 held in the separator 51. The thickness of the separator 51 is constant throughout. The thickness of the separator 51 is preferably 0.5 millimeters (mm) or less, more preferably 0.1 mm or less. The thickness of the separator 51 is, for example, 0.01 mm or more. Further, the porosity of the separator 51 is preferably 50% or more, and more preferably 60% or more. The porosity of the separator 51 is, for example, 90% or less. The porosity is a ratio of the volume of all the pores to the total volume of the separator 51 including the pores. By reducing the thickness of the separator 51 and increasing the porosity, the ionic conductivity between the negative electrode 3 and the positive electrode 4 is increased.

  The surface 31 of the negative electrode 3 and the surface 41 of the positive electrode 4 that face each other with the electrolyte layer 5 (separator 51) therebetween extend along the separator 51, and both are substantially parallel. In the following description, the surface 31 of the negative electrode 3 is referred to as “negative electrode facing surface 31”, and the surface 41 of the positive electrode 4 is referred to as “positive electrode facing surface 41”. The negative electrode facing surface 31 and the positive electrode facing surface 41 are substantially in contact with the separator 51. A minute gap may be provided between the negative electrode facing surface 31 and the separator 51 and between the positive electrode facing surface 41 and the separator 51.

  FIG. 2 is a diagram showing the negative electrode 3 and the positive electrode 4. 2, the positive electrode 4 and the negative electrode 3 viewed from the bottom plate portion 221 side to the top plate portion 211 side in FIG. 1 are shown, and the illustration of the separator 51 is omitted. As described above, both the positive electrode 4 and the negative electrode 3 have a rectangular plate shape. The entire positive electrode facing surface 41 overlaps the negative electrode facing surface 31, and the area of the positive electrode facing surface 41 is smaller than the area of the negative electrode facing surface 31. Actually, the entire outer edge of the positive electrode facing surface 41 exists inside the outer edge of the negative electrode facing surface 31. In other words, when viewed along a direction perpendicular to the positive electrode facing surface 41, the negative electrode facing surface 31 has a portion that extends outward from the outer edge of the positive electrode facing surface 41.

Next, details of the positive electrode 4, the negative electrode 3, and the electrolytic solution 50 will be described in order. The positive electrode 4 is formed of a sintered body and contains lithium. In the present embodiment, the positive electrode 4 is formed of an oriented sintered plate (hereinafter referred to as “LCO oriented sintered plate”) including a plurality of oriented lithium cobalt oxide (LiCoO ₂ ) crystal grains. . In the lithium secondary battery 1 of FIG. 1, one plate surface (main surface) perpendicular to the thickness direction of the LCO oriented sintered plate is the positive electrode facing surface 41. In the LCO oriented sintered plate, the thickness in the direction perpendicular to the positive electrode facing surface 41 is preferably 40 micrometers (μm) or more, and more preferably 50 μm or more. By increasing the thickness of the LCO oriented sintered plate, the energy density in the lithium secondary battery 1 can be easily improved. The thickness of the LCO oriented sintered plate is, for example, 300 μm or less. The meaning of orientation in the LCO oriented sintered plate will be described later.

  The relative density of the LCO oriented sintered plate is preferably 90% or more, more preferably 95% or more. The relative density of the LCO oriented sintered plate is 100% or less. Here, the relative density is the ratio of the density of the actual sintered body (sintered body with pores) to the density estimated when there are no pores in the sintered body. In the LCO oriented sintered plate having a high relative density, that is, a high density, the internal pores are not connected to the outside of the LCO oriented sintered plate in principle, and almost all of the pores contained in the LCO oriented sintered plate. Becomes closed pores. Therefore, in the positive electrode 4 constituted by the LCO oriented sintered plate, liquid does not permeate between the positive electrode facing surface 41 and the surface opposite to the positive electrode facing surface 41 (surface in contact with the spacer 24). In addition, the electrolytic solution 50 is hardly present inside the positive electrode 4.

LiCoO ₂ contained in the LCO oriented sintered plate has a layered rock salt type crystal structure. In the crystal grains of LiCoO ₂ , there are crystal planes (surfaces other than the (003) plane, for example, the (101) plane and the (104) plane) where lithium ions can enter and exit satisfactorily. In the LCO oriented sintered plate, a large number of crystal planes in which lithium ions can enter and exit satisfactorily exist in parallel with the plate surface, and this state is called an oriented state here. The positive electrode facing surface 41 which is the plate surface of the LCO oriented sintered plate is mainly composed of crystal planes other than the (003) plane.

  Whether or not the sintered plate is oriented can be confirmed by XRD (X-ray diffraction) measurement. Specifically, from the XRD profile obtained by XRD measurement on the plate surface of the sintered plate, the diffraction intensity I [003] by the (003) plane with respect to the diffraction intensity (peak height) I [104] by the (104) plane. When the ratio I [003] / I [104] is obtained and the value is smaller than 1.0, it is confirmed that the sintered plate is oriented. A method for producing the LCO oriented sintered plate will be described later.

  In the LCO oriented sintered plate, Mg, Al, Si, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, Ag, Sn, Sb, Te , Ba, Bi, Ni, Mn, etc. contain only a trace amount of one or more elements in a doping or equivalent form (for example, partial solid solution, segregation, coating or adhesion of crystal grains to the surface layer) Also good.

  The negative electrode 3 is formed of a material capable of inserting and extracting lithium ions, for example, metallic lithium, In, Al, Sn, Sb, Bi, Si, or the like, or an alloy containing any of these. The negative electrode 3 in FIG. 1 is a film or foil formed of the material, and is a dense electrode like the positive electrode 4. In the negative electrode 3, the pores are not substantially included, and the electrolyte solution 50 does not exist inside the negative electrode 3 as in the positive electrode 4. Depending on the design of the lithium secondary battery 1, the negative electrode 3 may be formed of a carbon-based material, an oxide-based material, or the like.

  The electrolyte solution 50 in the lithium secondary battery 1 contains an ionic liquid as a solvent. The ionic liquid has higher flame retardancy and heat resistance than the organic solvent. Therefore, in the lithium secondary battery 1 using an ionic liquid, improvement in safety and stable operation in a high temperature environment are realized. Here, the ionic liquid is a substance composed of only cations and anions (each ion may be a complex ion) and becomes liquid at 100 ° C. or lower. Ionic liquids are roughly classified into the following three types depending on the constituent substances.

1) Organic ionic liquid The organic ionic liquid is a salt in which one or both of a cation (cation) and an anion (anion) constituting the ionic liquid are organic substances. In many cases, the cation is an organic substance, and the melting point is lowered by molecular design of the cation so that the interaction between ions is weakened (for example, making it bulky or making it into a cyclic structure).

2) Inorganic molten salt An inorganic molten salt is a salt which melts at 100 ° C. or less among salts of inorganic cations and anions.

3) Solvated ionic liquid Normally, when a salt that does not melt at 100 ° C. or lower and a solvent are mixed, the ion and the solvent contained in the salt form a complex, and the substance is liquefied along with this complex formation. Since the solvent is used to form complex ions, the system is considered free of solvent.

  The electrolytic solution 50 in the present embodiment includes an ionic liquid whose viscosity increases as the lithium ion concentration increases, and includes, for example, the organic ionic liquid. In an example of the electrolytic solution 50, as the cation of the ionic liquid, 1-ethyl-3-methylimidazolium cation (EMI), 1-methyl-1-propylpyrrolidinium cation (P13), and 1-methyl-1- At least one kind of propylpiperidinium cation (PP13) (including these derivatives, the same applies hereinafter) is included. As the anion of the ionic liquid, at least one kind of bis (trifluoromethylsulfonyl) imide anion (TFSI) and bis (fluorosulfonyl) imide anion (FSI) is included. In addition, the electrolyte includes at least one kind of bis (trifluoromethylsulfonyl) imide lithium salt (LiTFSI) and bis (fluorosulfonyl) imide lithium salt (LiFSI). The electrolytic solution 50 may include other types of ionic liquids that have a positive correlation between the lithium ion concentration and the viscosity. Moreover, the electrolytic solution 50 may include other electrolytes including lithium.

In the lithium secondary battery 1, lithium ions are released from the positive electrode 4 when charging is performed. At this time, since the electrolyte solution 50 does not exist in the positive electrode 4, lithium ions move between the LiCoO ₂ particles, and lithium ions are released into the electrolyte solution 50 from the entire surface of the positive electrode facing surface 41. . Therefore, a local and excessive increase in the lithium ion concentration of the electrolytic solution 50 does not occur. Lithium ions in the electrolytic solution 50 are taken into the negative electrode 3 through the negative electrode facing surface 31.

  In the lithium secondary battery 1, lithium ions are released from the negative electrode 3 when discharging is performed. At this time, the electrolytic solution 50 does not exist inside the negative electrode 3, and lithium ions are released into the electrolytic solution 50 from the entire surface of the negative electrode facing surface 31. Therefore, a local and excessive increase in the lithium ion concentration of the electrolytic solution 50 does not occur. Lithium ions in the electrolytic solution 50 are taken into the positive electrode 4 through the positive electrode facing surface 41.

Next, a method for producing the LCO oriented sintered plate of the positive electrode 4 will be described. The LCO oriented sintered plate was prepared by (a) preparing a green sheet containing Co ₃ O ₄ particles, (b) firing the green sheet at 900 to 1450 ° C., and producing a sintered intermediate; (c) The firing intermediate is cooled to prepare a Co ₃ O ₄ oriented sintered plate containing a Co ₃ O ₄ phase, and (d) lithium is introduced into the Co ₃ O ₄ oriented sintered plate. Hereinafter, the detail of the said process is demonstrated.

(A) Preparation of Green Sheet In this step (a), a green sheet containing Co ₃ O ₄ particles is prepared. The green sheet preferably further contains bismuth oxide (typically Bi ₂ O ₃ particles) as a grain growth promoter. Green sheets, a raw material containing bismuth oxide may be made by molding into a sheet by Co ₃ O ₄ particles and desired. The amount of Bi ₂ O ₃ particles added is not particularly limited, but is preferably 0.1 to 30% by weight, more preferably 1 to 30% by weight based on the total amount of Co ₃ O ₄ particles and Bi ₂ O ₃ particles. 20% by weight, more preferably 3 to 10% by weight. The volume-based D50 particle size of the Co ₃ O ₄ particles is preferably 0.1 to 2.0 μm, more preferably 0.3 to 1.2 μm. The volume-based D50 particle size of Bi ₂ O ₃ particles is preferably 0.1 to 1.0 μm, more preferably 0.2 to 0.5 μm.

Further, the thickness of the green sheet is, for example, 300 μm or less. The thickness of the green sheet is preferably 40 μm or more, more preferably 50 μm or more. The green sheet may include CoO particles and / or Co (OH) ₂ particles in place of all or part of the Co ₃ O ₄ particles. In this case, the green sheet is subjected to the firing in the step (b). Thus, as described later, it is possible to obtain a CoO fired intermediate body in which the (h00) plane is oriented parallel to the sheet surface. As a result, an LCO oriented sintered plate can be produced as in the case of using a green sheet containing Co ₃ O ₄ particles.

  Examples of a method for forming a green sheet include (i) a doctor blade method using a slurry containing raw material particles, and (ii) applying a slurry containing the raw material onto a heated drum and drying it with a scraper. (Iii) A method using a drum dryer, (iii) A slurry is applied to a heated disk surface, dried and scraped with a scraper, (iv) A clay containing raw material particles is removed. Examples include the extrusion molding method used. A particularly preferable sheet forming method is a doctor blade method. When using the doctor blade method, the slurry is applied to a flexible plate (for example, an organic polymer plate such as a PET film), and the applied slurry is dried and solidified to form a molded body, and the molded body and the board are peeled off. Thus, a green sheet may be produced. When preparing a slurry or clay before molding, inorganic particles may be dispersed in a dispersion medium, and a binder, a plasticizer, or the like may be added as appropriate. Moreover, it is preferable to prepare a slurry so that a viscosity may be 500-4000 cP (centipoise), and it is preferable to deaerate under reduced pressure.

(B) Preparation of firing intermediate (firing step)
In this step (b), the green sheet is fired at 900 to 1450 ° C., and all or a part (preferably all) of the Co ₃ O ₄ particles have a (h00) plane (h is an arbitrary integer, h = 2)) is produced, which is changed to CoO oriented parallel to the sheet surface. That is, at 900 ° C. or higher (eg, 920 ° C. or higher), the Co oxide undergoes a phase transformation from a spinel structure represented by Co ₃ O ₄ at room temperature to a CoO rock salt structure. By this firing, all or a part of Co ₃ O ₄ is reduced and transformed into CoO, and the sheet is densified. The Co ₃ O ₄ particles before firing have an isotropic form, and therefore the green sheet does not initially have an orientation, but the Co ₃ O ₄ particles undergo phase transformation to CoO and undergo grain growth upon firing. Orientation occurs (hereinafter referred to as CoO oriented grain growth). In particular, in the presence of bismuth oxide (typically Bi ₂ O ₃ ), oriented grain growth of CoO is promoted. However, when the green sheet contains bismuth oxide, bismuth volatilizes and is removed from the sheet during firing. The firing temperature of the green sheet is 900 to 1450 ° C, preferably 1000 to 1300 ° C, and more preferably 1100 to 1300 ° C. The green sheet is preferably fired at the firing temperature for 1 to 20 hours, more preferably 2 to 10 hours.

  The thickness of the green sheet contributes to the growth of oriented grains of CoO. That is, in a relatively thin green sheet (for example, having a thickness of 300 μm or less), the amount of material existing in the thickness direction is extremely small compared to the in-plane direction (the direction perpendicular to the thickness direction). For this reason, in the initial stage where there are a plurality of grains in the thickness direction, grains grow in random directions. On the other hand, when the grain growth proceeds and the material in the thickness direction is consumed, the grain growth direction is limited to a two-dimensional direction in the sheet surface (hereinafter referred to as a plane direction). This reliably promotes grain growth in the surface direction. Even when the green sheet is relatively thick, by promoting grain growth as much as possible, grain growth in the plane direction can be surely promoted. Thus, during firing, only particles having a crystal plane with the lowest surface energy in the plane of the green sheet selectively grow in a flat shape (plate shape) in the plane direction. As a result, by firing the green sheet, CoO plate-like crystal grains having a large aspect ratio and oriented so that the (h00) plane is parallel to the plate face of the grains are oriented with the (h00) plane parallel to the sheet plane. And the baking intermediate body couple | bonded with a surface direction in a grain boundary part is obtained.

(C) Preparation of oriented sintered plate (cooling process)
This step (c) is a temperature lowering step performed subsequent to the firing in the step (b) (that is, from the firing temperature). In the step (c), the temperature is lowered from the firing temperature in the step (b) so as to return the sintered intermediate to Co ₃ O ₄ , thereby obtaining a Co ₃ O ₄ oriented sintered plate containing a Co ₃ O ₄ phase. The Co ₃ O ₄ oriented sintered plate may contain CoO remaining partially. The temperature lowering rate after firing is preferably 10 to 200 ° C./h, more preferably 20 to 100 ° C./h.

In this step (c), CoO is oxidized to Co ₃ O ₄ in the process of lowering the temperature of the calcined intermediate. At that time, since the alignment direction of CoO is taken over in the _Co 3 _{O 4,} (h00) face is oriented _Co 3 _{O 4} crystal particles are obtained in parallel with the plate surface of the particles. As a result, an independent plate-like sheet including a large number of Co ₃ O ₄ particles oriented so that the (h00) plane is parallel to the sheet surface is formed. An “independent” sheet refers to a sheet that can be handled as a single unit independently of other supports after firing. That is, the “independent” sheet does not include a sheet that is fixed to another support (substrate or the like) by firing and integrated with the support (unseparable or difficult to separate). In this way, a self-supporting oriented sintered plate is obtained in which a large number of grains oriented so that the (h00) plane is parallel to the grain plate face are bonded. This self-supporting plate becomes a dense ceramic sheet in which a large number of particles as described above are bonded without gaps. Depending on the design of the lithium secondary battery 1, the sheet may be fixed to another support.

(D) Introduction of lithium In this step (d), lithium is introduced into the Co ₃ O ₄ oriented sintered plate to form an LCO oriented sintered plate. The introduction of lithium is preferably performed by reacting a Co ₃ O ₄ oriented sintered plate with a lithium compound. Examples of lithium compounds for introducing lithium include (i) lithium hydroxide, (ii) various lithium salts such as lithium carbonate, lithium nitrate, lithium acetate, lithium chloride, lithium oxalate, and lithium citrate, (iii) Examples include lithium alkoxides such as lithium methoxide and lithium ethoxide, and lithium carbonate and lithium hydroxide are particularly preferable.

Conditions for introducing lithium, for example, the mixing ratio, heating temperature, heating time, atmosphere, and the like may be appropriately set in consideration of the melting point, decomposition temperature, reactivity, etc. of the material used as the lithium source, and are not particularly limited. For example, lithium can be introduced into the Co ₃ O ₄ particles by placing a predetermined amount of lithium carbonate on a (h00) oriented Co ₃ O ₄ oriented sintered plate and heating. Lithium carbonate may be placed by placing a lithium-containing sheet containing lithium carbonate on a Co ₃ O ₄ oriented sintered plate, but the Co ₃ O ₄ oriented sintered plate contains lithium from above and below. It is particularly preferable that sandwiching between sheets is sufficient in that lithium can be sufficiently introduced when a thick oriented sintered plate is produced.

The lithium-containing sheet is preferably obtained by slurrying lithium carbonate and subjecting it to tape molding, and the tape molding method is the same as the method described in the step (a) described above. Good. What is necessary is just to determine suitably the thickness of a lithium containing sheet | seat so that the lithium carbonate of the quantity which the below-mentioned Li / Co ratio may become a desired value may be given, for example, it is 20-60 micrometers. Alternatively, as another technique, a predetermined amount of slurry in which LiOH powder is dispersed is applied to a (h00) -oriented Co ₃ O ₄ oriented sintered plate, dried, and then heated to form Co ₃ O ₄ particles. Lithium may be introduced. In any method, the heating temperature is preferably 700 to 900 ° C., and it is preferable to perform heating at a temperature within this range for 2 to 30 hours.

The amount of the lithium compound attached to the Co ₃ O ₄ oriented sintered plate is the Li / Co ratio (that is, the molar ratio of the amount of Li contained in the lithium compound to the amount of Co contained in the Co ₃ O ₄ oriented sintered plate). It is preferably 1.0 or more, more preferably 1.0 to 4.0, and still more preferably 1.2 to 3.0. Even when there is too much Li, there is no problem since the excess Li volatilizes and disappears with heating. In order to increase the flatness of the LCO oriented sintered plate (for example, to suppress the degree of unevenness of the plate surface of the oriented sintered plate to be small), the Co ₃ O ₄ oriented sintered plate may be fired under a load. Good. In order to sufficiently supply oxygen necessary for the synthesis to the plate surface of the Co ₃ O ₄ oriented sintered plate, the oxygen may be applied with a porous setter or a setter having a hole (for example, a honeycomb setter). When the LCO oriented sintered plate is relatively thick, the Li raw material to be attached becomes bulky, and a part of the Li raw material melted during heating flows out without being used for the synthesis, which tends to cause poor synthesis. In such a case, a process of attaching a Li raw material and performing a heat treatment (that is, a process of introducing Li) may be repeated.

The LCO oriented sintered plate thus obtained has an orientation such that the (101) plane and (104) plane of the LiCoO ₂ particles are parallel to the plate plane. That is, among the plurality of crystal planes, the (101) plane and the (104) plane where lithium ions can be satisfactorily entered and exited are oriented in parallel with the plane of the LCO oriented sintered plate. For this reason, in the lithium secondary battery 1 using the LCO oriented sintered plate as the positive electrode 4 (positive electrode active material), the exposure or contact of the surface with respect to the electrolyte layer 5 is increased, and the surface of the LCO oriented sintered plate is The exposure ratio of the (003) plane (surface not suitable for entering and exiting lithium ions) is extremely low. As a result, the lithium secondary battery 1 can simultaneously achieve high capacity and high rate characteristics.

  In an example of an LCO oriented sintered plate produced according to the above production method, an XRD apparatus (manufactured by Rigaku Corporation, Geiger Flex RAD-IB) is used to obtain I [003 from an XRD profile when the plate surface is irradiated with X-rays. ] / I [104] was determined to be 0.3. On the other hand, after the same LCO oriented sintered plate was sufficiently pulverized in a mortar to form a powder and the profile of the powder XRD was measured, the value of I [003] / I [104] was 1.3. . From this, in the LCO oriented sintered plate, it was confirmed that there are a large number of crystal grains having the (104) plane parallel to the plate surface, that is, the LCO oriented sintered plate is actually oriented. . In this case, the orientation was confirmed using the (104) plane as a reference, but the peak of the (101) plane also appears, and the (101) plane can also be used as a reference. When the thickness of the LCO oriented sintered plate is increased, the orientation of the (101) plane becomes dominant.

Here, a lithium secondary battery of a comparative example will be described. FIG. 3 is a cross-sectional view showing a configuration of a lithium secondary battery 9 of a comparative example. In the lithium secondary battery 9 of the comparative example, the structures of the positive electrode 92 and the negative electrode 91 are different from the lithium secondary battery 1 of FIG. Specifically, as shown in FIG. 4, the positive electrode 92 is obtained by mixing LiCoO ₂ powder 921 and a conductive additive 922 and pressing them together. The positive electrode 92 has low density, and most of the pores 923 included in the positive electrode 92 are open pores. As shown in FIG. 5 in which a part of the positive electrode 92 in FIG. 4 is enlarged, in the lithium secondary battery 9 of the comparative example, the electrolytic solution 50 enters and is held in the pores 923 in the positive electrode 92. When the lithium secondary battery 9 is charged, lithium ions move through the electrolytic solution 50 in the pores 923 inside the positive electrode 92. At this time, if the current density is too high (charging speed is too fast), the lithium ion concentration relaxation cannot catch up, and the lithium ion concentration of the electrolytic solution 50 locally increases in the pores 923. As a result, an overvoltage due to an increase in viscosity (resistance increase) of the ionic liquid is generated, and the ionic liquid contained in the electrolytic solution 50 is decomposed. For example, when the lithium ion concentration in the electrolytic solution 50 is changed from 0.4 M (molar) to 0.8 M, the viscosity is approximately doubled.

  Further, in the lithium secondary battery 9 of the comparative example, the negative electrode 91 is obtained by mixing and compacting a carbon powder and a conductive additive. The negative electrode 91 is also low in density, and most of the pores included in the negative electrode 91 are open pores. When the lithium secondary battery 9 is discharged, lithium ions move through the electrolyte 50 in the pores inside the negative electrode 91. Accordingly, the lithium ion concentration of the electrolytic solution 50 locally increases in the pores of the negative electrode 91. As a result, an overvoltage is generated due to an increase in the viscosity of the ionic liquid, and the ionic liquid contained in the electrolytic solution 50 is decomposed.

  In contrast to the lithium secondary battery 9 of the comparative example, in the lithium secondary battery 1 of FIG. 1, the positive electrode 4 is formed of a dense sintered body, and almost all of the pores included in the positive electrode 4 are closed pores. When the lithium secondary battery 1 is charged, lithium ions move between the particles of the dense sintered body inside the positive electrode 4. Thereby, decomposition | disassembly of the ionic liquid resulting from the local raise of the lithium ion concentration of the electrolyte solution 50 can be suppressed, and it can charge stably, without generating voltage abnormality. Moreover, the energy density in the lithium secondary battery 1 can be improved because the sintered body is an oriented sintered plate including a plurality of oriented lithium cobalt oxide crystal grains.

  Moreover, in the lithium secondary battery 1, the negative electrode 3 is formed of a dense member. During discharge, lithium ions move inside the negative electrode 3 through the dense member. Thereby, decomposition | disassembly of the ionic liquid resulting from the local raise of the lithium ion density | concentration of the electrolyte solution 50 can be suppressed, and it can discharge stably, without generating voltage abnormality.

  By the way, in the charging in which lithium ions move from the positive electrode 4 to the negative electrode 3, if there is a portion where the current density is high (lithium ions flow in a concentrated manner) in the positive electrode 4, decomposition of the ionic liquid is promoted. End up. For example, in the negative electrode 3 and the positive electrode 4 of FIG. 2, if the positive electrode facing surface 41 is larger than the negative electrode facing surface 31, the current density increases in the vicinity of the portion of the positive electrode 4 that faces the outer edge of the negative electrode facing surface 31. Decomposition of the ionic liquid may occur.

  On the other hand, in the lithium secondary battery 1, when viewed along a direction perpendicular to the positive electrode facing surface 41, the negative electrode facing surface 31 has a portion that extends outward from the outer edge of the positive electrode facing surface 41. Thereby, it can suppress that an electric current density becomes high in some positive electrodes 4, and can further suppress decomposition | disassembly of the ionic liquid in the positive electrode 4. FIG.

  In the lithium secondary battery 1, the lithium ion concentration increases near the positive electrode facing surface 41 during charging, and the lithium ion concentration increases near the negative electrode facing surface 31 during discharging. Such an increase in lithium ion concentration may also contribute to the decomposition of the ionic liquid. Therefore, in the lithium secondary battery 1, the thickness of the separator 51 is 0.5 mm or less, and the porosity of the separator 51 is 50% or more. Thereby, the ionic conductivity between the positive electrode facing surface 41 and the negative electrode facing surface 31 can be increased, and an increase in the lithium ion concentration in the vicinity of the positive electrode facing surface 41 and the negative electrode facing surface 31 during charging and discharging is suppressed. It becomes possible to do.

  Next, experimental examples will be described. Here, lithium secondary batteries having the same structure as in FIG. 1 were manufactured under the conditions of numbers 1 to 4 shown in Table 1, and the presence or absence of voltage abnormality when charging / discharging was examined.

In the lithium secondary batteries of Nos. 1 and 2, an LCO oriented sintered plate having a thickness of 25 μm and an LCO oriented sintered plate having a thickness of 50 μm were used as positive electrodes, respectively, and an electrolytic solution containing an ionic liquid (in Table 1, “ The case was filled with “ionic liquid electrolyte”. In the ionic liquid electrolytic solution, P13 was used as a cation of the ionic liquid as a solvent, FSI was used as an anion, and LiFSI as an electrolyte was mixed so as to have a concentration of 0.4M. In the lithium secondary batteries of Nos. 3 and 4, similarly to the lithium secondary battery 9 of the comparative example of FIG. 3, an electrode obtained by mixing and compacting LiCoO ₂ powder and a conductive additive (in Table 1, “powder combination” ”) Was used as the positive electrode. The compounding ratio of the LiCoO ₂ powder, the conductive additive and the binder in the powder mixture electrode is 90: 5: 5. The thickness of the powder mixture electrode is 60 μm, and the density is 3.6 g / cc. In addition, all the positive electrodes of the battery 1-4 were used in a size of 8 mm × 8 mm. Further, in the lithium secondary battery of No. 3, the above ionic liquid electrolyte is filled inside, and in the lithium secondary battery of No. 4, an electrolyte using an organic solvent (in Table 1, “organic electrolyte” The inside was filled. In the organic electrolytic solution, a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) as an organic solvent in a ratio of 1: 1 was used, and lithium hexafluorophosphate (LiPF6) was mixed as an electrolyte so as to have a concentration of 1M. . The separators in the lithium secondary batteries of Nos. 1 to 4 were polyolefin nonwoven fabric (thickness 0.1 mm, porosity 50%), and the negative electrode was metallic lithium (size 9 mm × 9 mm × 0.2 mmt). In this experiment, charging / discharging was performed at a charging / discharging rate of 0.2 mA / cm ² .

  Of the lithium secondary batteries of Nos. 3 and 4 using the powder mixture electrode as the positive electrode, the lithium secondary battery of No. 4 using the organic electrolyte operates normally, but the lithium secondary battery of No. 3 using the ionic liquid electrolyte is used. In the secondary battery, during charging, a voltage abnormality that does not increase due to the vibration of the voltage near a certain value occurred, and charging was not completed. This voltage abnormality is considered to be due to the ionic liquid being decomposed due to a local increase in the lithium ion concentration of the electrolytic solution (an increase in the pores of the positive electrode). On the other hand, the lithium secondary batteries of Nos. 1 and 2 using the LCO oriented sintered plate as the positive electrode operated normally in the same manner even though the ionic liquid was used (no voltage abnormality occurred).

  In the lithium secondary battery 1 of FIG. 1, an LCO oriented sintered plate that is dense as a whole is used. However, as shown in FIG. 6, an LCO oriented sintered plate that is dense only on the surface may be used. In FIG. 6, by making the interval between the parallel oblique lines attached to the outer edge portion of the positive electrode 4 smaller than the interval between the parallel oblique lines attached to the inside, the surface vicinity of the positive electrode 4 is dense and the inside is rougher than the surface vicinity. It shows that there is. The relative density of the LCO oriented sintered plate in the positive electrode 4 of FIG. 6 is, for example, 80% or more and 90% or less. In the LCO oriented sintered plate, the ratio (volume ratio) of closed pores to all pores is preferably 90% or more, and more preferably 95% or more. Further, the ratio of closed pores is, for example, 100% or less.

An LCO oriented sintered plate having a low relative density and a high ratio of closed pores is produced, for example, in the same manner as the above-described method for producing an LCO oriented sintered plate. Specifically, first, a Co ₃ O ₄ oriented sintered plate having a relatively low relative density is produced by steps (a) to (c). In such a Co ₃ O ₄ oriented sintered plate, the ratio of open pores to all pores is high. Then, in the step (d), lithium is introduced into the Co ₃ O ₄ oriented sintered plate. At this time, lithium is introduced from the surface of the Co ₃ O ₄ oriented sintered plate, and a portion near the surface of the oriented sintered plate expands due to the introduction of lithium. As a result, a part of the open pores are closed on the surface of the oriented sintered plate to become closed pores. That is, an LCO oriented sintered plate having a dense surface only is produced. Actually, the relative density of the Co ₃ O ₄ oriented sintered plate produced by the steps (a) to (c) and the surface introduced into the Co ₃ O ₄ oriented sintered plate from the surface by the step (d). By adjusting the amount of lithium ions and the like, the relative density and the ratio of closed pores in the LCO oriented sintered plate can be changed. Also in the lithium secondary battery 1 of FIG. 6, since the electrolytic solution 50 is hardly present inside the positive electrode 4, the local and excessive increase in the lithium ion concentration of the electrolytic solution 50 does not occur, and the decomposition of the ionic liquid is suppressed. The

  As described above, in the lithium secondary battery 1 in which the electrolyte layer 5 contains the ionic liquid, the decomposition of the ionic liquid caused by the local increase in the lithium ion concentration is suppressed, and charging is performed stably. In the LCO oriented sintered plate of the positive electrode 4, it is important that the surface in contact with the ionic liquid is a wetted surface and at least the wetted surface is dense. Similarly, from the viewpoint of stably discharging, it is preferable that the surface of the negative electrode 3 in contact with the ionic liquid is dense.

  The lithium secondary battery 1 can be variously modified.

  Depending on the energy density required for the lithium secondary battery 1, a non-oriented sintered body may be used as the positive electrode 4. The positive electrode 4 may be formed of a material other than lithium cobaltate (a material containing lithium). Furthermore, the positive electrode 4 may be cylindrical or rod-like (the same applies to the negative electrode 3).

  A sintered body having only a dense surface may be produced by processing to fill the pores on the surface (sealing treatment) or adjusting various conditions such as the firing temperature for a sintered body having many open pores. .

  The above-described method for making the wetted surface at least dense in the positive electrode 4 may be employed in a lithium secondary battery including a solid electrolyte. In this case, the electrolytic solution held between the solid electrolyte and the positive electrode 4 includes an ionic liquid.

  The configurations in the above-described embodiments and modifications may be combined as appropriate as long as they do not contradict each other.

DESCRIPTION OF SYMBOLS 1 Lithium secondary battery 3 Negative electrode 4 Positive electrode 5 Electrolyte layer 31 Negative electrode opposing surface 41 Positive electrode opposing surface 51 Separator

Claims (9)

A lithium secondary battery,
A negative electrode,
A positive electrode formed of a sintered body and containing lithium;
An electrolyte layer disposed between the negative electrode and the positive electrode and containing an ionic liquid in contact with the surface of the sintered body;
With
A lithium secondary battery, wherein at least the surface of the sintered body is dense. The lithium secondary battery according to claim 1,
A lithium secondary battery, wherein the sintered body has a relative density of 90% or more. The lithium secondary battery according to claim 1,
The lithium secondary battery characterized in that a relative density of the sintered body is 80% or more, and a ratio of closed pores to all pores in the sintered body is 90% or more. The lithium secondary battery according to any one of claims 1 to 3,
The lithium secondary battery, wherein the sintered body is an oriented sintered plate including a plurality of oriented lithium cobalt oxide crystal grains. The lithium secondary battery according to claim 4,
A lithium secondary battery, wherein the oriented sintered plate has a thickness of 40 micrometers or more. A lithium secondary battery according to any one of claims 1 to 5,
The lithium secondary battery, wherein the viscosity of the ionic liquid increases as the lithium ion concentration increases. The lithium secondary battery according to any one of claims 1 to 6,
The lithium secondary battery, wherein the negative electrode has a dense surface in contact with the ionic liquid. A lithium secondary battery according to any one of claims 1 to 7,
Both of the positive electrode and the negative electrode are plate-like, and the surface of the positive electrode and the surface of the negative electrode facing each other through the electrolyte layer are parallel,
The lithium secondary battery, wherein when viewed along a direction perpendicular to the surface of the positive electrode, the surface of the negative electrode has a portion extending outward from an outer edge of the surface of the positive electrode. A lithium secondary battery according to any one of claims 1 to 8,
The electrolyte layer has a separator;
A lithium secondary battery, wherein the separator has a thickness of 0.5 mm or less and the separator has a porosity of 50% or more.

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