JP6500920B2 - Lithium secondary battery - Google Patents

Description

  The present invention relates to a lithium secondary battery.

  Lithium secondary batteries can realize high capacity, and are widely used from mobile batteries such as mobile phones and laptop computers to automobile batteries and large-sized power storage batteries.

  The lithium secondary battery charges and discharges using an electrolyte serving as a medium for the movement of ions. Liquid electrolytes have problems such as liquid leakage, and liquid leakage causes ignition. Therefore, studies are underway to make the electrolyte flame-retardant and improve safety.

  For example, Patent Document 1 describes a lithium secondary battery using an ionic liquid as a non-aqueous electrolyte. An ionic liquid is attracting attention as an electrolyte material because it is non-volatile, has a high decomposition temperature, and is excellent in chemical stability.

JP-A-9-259929

  However, although the safety is enhanced by using the ionic liquid, there is a problem that the performance of the positive electrode can not be sufficiently exhibited.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a lithium secondary battery capable of satisfying the theoretical capacity of a positive electrode even when using an ionic liquid excellent in safety.

The inventors of the present invention have found that the electrolyte containing the ionic liquid can not penetrate to the inside of the positive electrode active material layer because the ionic liquid is higher in viscosity than the organic solvent. That is, it has been found that the performance of the positive electrode can not be utilized to the maximum unless the positive electrode is set according to the ionic liquid. As a result, it was found that high capacity of the lithium secondary battery could not be realized.
That is, in order to solve the above-mentioned subject, the following means are provided.

(1) A lithium secondary battery according to the first aspect includes a positive electrode, a negative electrode, and an electrolytic solution containing an ionic liquid and a lithium salt, wherein the electrolytic solution has a viscosity of more than 10 mPa · s, and the positive electrode is And a positive electrode active material layer having a density of 2.0 g / cm ³ or more and 3.5 g / cm ³ or less.

(2) In the lithium secondary battery according to the above aspect, the loading amount of the positive electrode active material layer in the positive electrode may be 25 mg / cm ² or less.

(3) In the lithium secondary battery according to the above aspect, the positive electrode active material layer may have at least one or more of the positive electrode active materials represented by the following general formula (1).
Li _x M _{1 y} M 2 _1-y O ₂ (1)
In the general formula (1), M1 is at least one selected from the group consisting of Ni and Co, M2 is at least one selected from the group consisting of Al, Mg and Mn, and x is 0 .05 ≦ x ≦ 1.1 and y satisfies 0.3 ≦ y ≦ 1.

  According to the lithium secondary battery according to the above aspect, it is possible to provide a lithium secondary battery capable of satisfying the theoretical capacity of the positive electrode even when the ionic liquid having excellent safety is used.

It is a cross-sectional schematic diagram of the lithium secondary battery concerning this embodiment. It is the cross-sectional schematic diagram which expanded positive electrode active material layer vicinity of the lithium secondary battery concerning this embodiment. It is the graph which put in order the relationship between the molecular weight of an ionic liquid, and the viscosity of the electrolyte solution containing an ionic liquid for every kind of ionic liquid. It is a graph which shows the relationship between the density of the positive electrode active material layer of the lithium secondary battery concerning this embodiment, and the implement | achieved capacity ratio with respect to the theoretical capacity of a lithium secondary battery. The relationship of the measurement capacity of a lithium secondary battery with respect to the loading amount of the positive electrode active material layer of the lithium secondary battery concerning this embodiment is shown. The battery capacity of the lithium secondary battery of Example 5-1 to Example 5-4 is shown.

  Hereinafter, the present embodiment will be described in detail with reference to the drawings as appropriate. The drawings used in the following description may show enlarged features for convenience for the purpose of clarifying the features of the present invention, and the dimensional ratio of each component may be different from the actual one. is there. The materials, dimensions, and the like exemplified in the following description are merely examples, and the present invention is not limited to them, and can be appropriately changed and implemented without changing the gist of the invention.

[Lithium secondary battery]
FIG. 1 is a schematic cross-sectional view of a lithium secondary battery according to the present embodiment. The lithium secondary battery 100 shown in FIG. 1 mainly includes a stack 40, a case 50 that houses the stack 40 in a sealed state, and a pair of leads 60 and 62 connected to the stack 40. Although not shown, the electrolytic solution is accommodated in the case 50 together with the laminate 40.

  In the laminate 40, the positive electrode 20 and the negative electrode 30 are disposed to face each other with the separator 10 interposed therebetween. The positive electrode 20 is obtained by providing a positive electrode active material layer 24 on a plate-like (film-like) positive electrode current collector 22. The negative electrode 30 is one in which a negative electrode active material layer 34 is provided on a plate-like (film-like) negative electrode current collector 32.

  The positive electrode active material layer 24 and the negative electrode active material layer 34 are in contact with both sides of the separator 10 respectively. Leads 62 and 60 are connected to ends of the positive electrode current collector 22 and the negative electrode current collector 32, respectively, and the ends of the leads 60 and 62 extend to the outside of the case 50. Although FIG. 1 exemplifies the case where one laminate 40 is provided in the case 50, a plurality of the laminates 40 may be stacked.

  FIG. 2 is a schematic cross-sectional view enlarging the vicinity of the positive electrode active material layer of the lithium secondary battery according to the present embodiment. As shown in FIG. 2, the positive electrode active material layer 24 has a positive electrode active material 26, a conductive material 28, and a positive electrode binder (not shown). An electrolytic solution is present in the gap K formed between the positive electrode active material 26 and the conductive material 28.

(Electrolyte solution)
The electrolyte contains an ionic liquid and a lithium salt. An ionic liquid is a salt which is liquid at less than 100 ° C. obtained by the combination of a cation and an anion. Since the ionic liquid is a liquid consisting only of ions, it has strong electrostatic interaction, and is characterized as being non-volatile and non-combustible. A lithium secondary battery using an ionic liquid as an electrolytic solution is excellent in safety.

  There are various types of ionic liquids depending on combinations of cations and anions. Examples thereof include nitrogen-based ionic liquids such as imidazolium salts, pyrrolidinium salts, piperidinium salts, pyridinium salts and ammonium salts, phosphorus-based ionic liquids such as phosphonium salts, and sulfur-based ionic liquids such as sulfonium salts. Nitrogen-based ionic liquids can be divided into cyclic ammonium salts and chained ammonium salts.

  As cations of ionic liquids, those of nitrogen type, phosphorus type, sulfur type and the like have been reported. Nitrogen-based cations are excellent in availability of raw materials, diversity, safety, operability, price and the like. Among nitrogen-based cations, imidazolium-based, ammonium-based and pyridinium-based cations are relatively inexpensive and readily available.

As the anion of the ionic liquid, AlCl ₄ ⁻ , NO ₂ ⁻ , NO ₃ ⁻ , I ⁻ , BF ₄ ⁻ , PF ₆ ⁻ , AsF ₆ ⁻ , SbF ₆ ⁻ , NbF ₆ ⁻ , TaF ₆ ⁻ , F (HF) − ^{_{2.3 -, p-CH 3 PhSO}} 3 -, CH 3 CO 2 -, CF 3 CO 2 -, CH 3 SO 3 -, CF 3 SO 3 -, (CF 3 SO 2) 3 C -, C 3 F ₇ CO ₂ , C ₄ F ₉ SO ₃ ⁻ , (CF ₃ SO ₂ ) ₂ N ⁻ , (C ₂ F ₅ SO ₂ ) ₂ N ⁻ , (CF ₃ SO ₂ ) (CF ₃ CO) N ⁻ , (CN ₂ ) N ^- etc. are mentioned. When the anion is (CF ₃ SO ₂ ) ₂ N ⁻ (hereinafter referred to as TFSI), the viscosity of the ionic liquid tends to be low regardless of the cationic species.

As lithium salts, inorganic acid anion salts such as LiPF ₆ , LiBF ₄ , LiBOB, etc., organic acid anion salts such as LiCF ₃ SO ₃ , (CF ₃ SO ₂ ) ₂ NLi, (FSO ₂ ) ₂ NLi, etc. are used. be able to.

  An ionic liquid has a strong electrostatic interaction between a cation and an anion, and thus has a high solution viscosity as compared to a molecular liquid such as water or an organic solvent. Generally, the viscosity of the ionic liquid is at least 10 times the viscosity of the organic electrolyte.

  The viscosity of the ionic liquid depends on the combination of the cation and the anion. When the molecular weight of the ionic liquid is large, the viscosity of the ionic liquid tends to increase. On the other hand, the viscosity of the ionic liquid can not be determined only by the molecular weight, and varies depending on the type of ionic liquid.

  FIG. 3 is a graph in which the relationship between the molecular weight and the viscosity of the ionic liquid is arranged for each type of ionic liquid. As shown in FIG. 3, when compared at the same molecular weight, the viscosities of the imidazolium-based and pyrrolidinium-based ionic liquids are lower than the viscosity of the ammonium-based and piperidinium-based ionic liquids. In the same compound group, the alkyl chain of the cation moiety is long, and when branching occurs, the viscosity tends to increase.

Table 1 below shows specific combinations of cations and anions, molecular weights, and viscosities of some of the ionic liquids. Here, the viscosities in Table 1 are prepared by dissolving (FSO ₂ ) ₂ NLi as a lithium salt so as to be 1 mol / L.

  In the lithium secondary battery according to the present embodiment, an electrolyte containing an ionic liquid and a lithium salt has a viscosity of greater than 10 mPa · s. The viscosity of the electrolytic solution containing the ionic liquid and the lithium salt is preferably more than 10 mPa · s and 200 mPa · s or less, and more preferably 70 mPa · s or more and 160 mPa · s or less.

  In the lithium secondary battery according to this embodiment, the molecular weight of the ionic liquid is preferably 200 or more and 700 or less, and more preferably 300 or more and 500 or less. The molecular weight of the ionic liquid is a parameter that affects the viscosity of the ionic liquid. Further, even in the case of the same viscosity, the smaller the molecular weight, the smaller the molecular size, and the easier the flow of the ionic liquid in the space K becomes.

"Positive"
(Positive electrode active material layer)
The density of the positive electrode active material layer 24 of the lithium secondary battery 100 according to the present embodiment is 2.0 to 3.5 g / cm ³ . Here, the density of the positive electrode active material layer 24 means the density including the positive electrode active material 26, the conductive material 28 and the binder. Since the amounts of the conductive material 28 and the binder in the positive electrode active material layer 24 are smaller than those of the positive electrode active material 26, they may be converted as the density of the positive electrode active material 26. The density of the positive electrode active material layer 24 differs strictly between the charged state and the discharged state. The positive electrode active material layer 24 here is the density at the time of assembling the lithium secondary battery 100, and corresponds to the density of the discharge state.

  When the density of the positive electrode active material layer 24 is high, the area occupied by the voids K in the positive electrode active material layer 24 is reduced. The ionic liquid has a viscosity higher than that of the organic electrolytic solution, so it is difficult for the ionic liquid to be impregnated in the space K. The lithium secondary battery 100 exchanges Li ions through an electrolytic solution containing an ionic liquid and a lithium salt. If the electrolytic solution is not sufficiently impregnated in the air gap K, the conduction path of the lithium secondary battery through the electrolytic solution is limited, and the performance of the positive electrode can not be maximized. That is, the theoretical capacity of the positive electrode can not be satisfied, and the realization of a high capacity lithium secondary battery is hindered.

  On the other hand, when the density of the positive electrode active material layer 24 is low, the conductivity between the positive electrode active material 26 and the positive electrode current collector 22 is reduced. The positive electrode active material layer 24 is produced by applying a paint containing the positive electrode active material 26, the conductive material 28 and the binder onto the positive electrode current collector 22, and pressing it. When the pressing pressure is small, the positive electrode active material layer 24 has a low density. When the pressing pressure is small, the amount of the positive electrode active material 26 that can penetrate the oxide film coated on the surface of the positive electrode current collector 22 decreases. As a result, the conductivity of the positive electrode active material 26 and the positive electrode current collector 22 is reduced.

The loading amount of the positive electrode active material layer 24 is is preferable, more preferably 10 mg / cm ² or more 25 mg / cm ² or less, 15 mg / cm ² or more 25 mg / cm ² or less is 25 mg / cm ² or less Is more preferred. The loading amount of the positive electrode active material layer 24 is an index indicating the total amount of the positive electrode active material, the conductive material, and the binder carried on the surface of the positive electrode current collector 22 per unit area.

  Generally, when the loading amount increases, the amount of the positive electrode active material that functions as a positive electrode increases. Therefore, the capacity of the lithium secondary battery 100 is increased. However, when an ionic liquid is used, when the loading amount is too large, the capacity tends to decrease. On the other hand, when the loading amount of the positive electrode active material 26 is too small, the absolute amount of the positive electrode active material functioning as the positive electrode decreases, so the capacity of the lithium secondary battery decreases.

The positive electrode active material 26 used for the positive electrode active material layer 24 is lithium ion absorption and release, lithium ion desorption and insertion (intercalation), or lithium ion and lithium ion counter anion (for example, PF ₆ ⁻ ) An electrode active material capable of reversibly advancing doping and de-doping can be used.

For example, lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganese spinel (LiMn ₂ O ₄ ), and the general formula: LiNi _x Co _y Mn _z M a ₂ (x + y + z + a = 1, 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ z ≦ 1, 0 ≦ a ≦ 1, M is a composite metal represented by one or more elements selected from Al, Mg, Nb, Ti, Cu, Zn, Cr Oxide, lithium vanadium compound (LiV ₂ O ₅ ), olivine type LiMPO ₄ (wherein M represents one or more elements selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al and Zr, or VO shown), lithium titanate _{(Li 4 Ti 5 O 12)} , LiNi x Co y Al z O 2 ( composite metal oxide such as 0.9 <x + y + z < 1.1) can be mentioned.

The positive electrode active material 26 used for the positive electrode active material layer 24 preferably has at least one or more of the positive electrode active materials represented by the following general formula (1).
Li _x M _{1 y} M 2 _1-y O ₂ (1)
In the general formula (1), M1 is at least one selected from the group consisting of Ni and Co, M2 is at least one selected from the group consisting of Al, Mg and Mn, and x is 0 .05 ≦ x ≦ 1.1 and y satisfies 0.3 ≦ y ≦ 1.

  Specific examples of the positive electrode active material represented by the general formula (1) include nickel-cobalt-lithium aluminate (NCA), lithium cobaltate (LCO), nickel-cobalt-lithium manganate (NCM) and the like. .

  The positive electrode active material 26 represented by General Formula (1) has a high theoretical capacity, and contributes to the increase in capacity of the lithium secondary battery 100.

  The composition ratio of the positive electrode active material 26 in the positive electrode active material layer 24 is preferably 80% to 90% by mass. The composition ratio of the conductive material 28 in the positive electrode active material layer 24 is preferably 0.5% to 10% in mass ratio, and the binder composition ratio in the positive electrode active material layer 24 is 0.5 in mass ratio % Or more and 10% or less.

  In general, the main constituent material of the positive electrode active material layer 24 is the positive electrode active material 26. With respect to the positive electrode active material 26, the conductive material 28 and the binder differ in particle diameter, mass, and the like. Therefore, when the ratio of the conductive material 28 and the binder is increased, the filling degree of the constituent material occupying the inside of the positive electrode active material layer 24 is changed. The degree of filling of the positive electrode active material layer 24 affects the ease with which the electrolytic solution containing the ionic liquid and the lithium salt is impregnated into the positive electrode active material 24. If each material constituting the positive electrode active material layer 24 is present within the above range, the lithium secondary can be ensured while securing the ease of impregnation of the electrolytic solution containing the ionic liquid and the lithium salt into the positive electrode active material layer 24. The performance of the battery 100 can be secured.

(Conductive material)
Examples of the conductive material 28 include carbon powders such as carbon blacks, carbon nanotubes, carbon materials, metal fine powders such as copper, nickel, stainless steel and iron, mixtures of carbon materials and metal fine powders, and conductive oxides such as ITO. Be When sufficient conductivity can be ensured only by the positive electrode active material 26, the lithium secondary battery 100 may not contain the conductive material 28.

(Positive electrode binder)
The binder bonds the active materials to one another and bonds the active material to the positive electrode current collector 22. The binder may be any one capable of the above-mentioned bonding, for example, polyvinylidene fluoride (PVDF), polyether sulfone (PESU), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer ( FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE) And fluorine resins such as polyvinyl fluoride (PVF).

  In addition to the above, as a binder, for example, vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFPTFE-based) Fluororubber), vinylidene fluoride-pentafluoropropylene fluororubber (VDF-PFP fluororubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene fluororubber (VDF-PFP-TFE fluororubber), vinylidene fluoro Lido-perfluoromethylvinylether-tetrafluoroethylene-based fluororubber (VDF-PFMVE-TFE-based fluororubber), vinylidene fluoride-chlorotrifluoroethylene-based fluorocarbon It may be used vinylidene fluoride-based fluorine rubbers such as rubber (VDF-CTFE-based fluorine rubber).

  In addition, an electron conductive conductive polymer or an ion conductive conductive polymer may be used as the binder. Examples of the electron conductive conductive polymer include polyacetylene and the like. In this case, since the binder also exhibits the function of the conductive material, the conductive material may not be added. As the ion conductive conductive polymer, for example, a polymer compound such as polyethylene oxide and polypropylene oxide complexed with a lithium salt or an alkali metal salt mainly composed of lithium can be mentioned.

  The binder adheres to the surface of the positive electrode active material 26. The space K which is impregnated with the electrolytic solution containing the ionic liquid and the lithium salt is surrounded by the positive electrode active material 26. That is, the binder species affects the ease of impregnation of the ionic liquid into the voids K. Among these binders, polyvinylidene fluoride (PVDF) and polyether sulfone (PESU) are preferable when selected in this respect.

(Positive current collector)
The positive electrode current collector 22 may be a conductive plate, and for example, a thin metal plate of aluminum, copper, or nickel foil can be used.

"Negative electrode"
(Anode active material layer)
The negative electrode has a negative electrode active material layer. The negative electrode active material layer contains a negative electrode active material, and further contains a negative electrode binder and a conductive material as necessary.

(Anode active material)
The negative electrode active material may be any compound that can occlude and release lithium ions, and known negative electrode active materials for lithium secondary batteries can be used. Examples of the negative electrode active material include metallic lithium, graphite capable of absorbing and desorbing lithium ions (natural graphite, artificial graphite), carbon nanotubes, carbon materials such as non-graphitizable carbon, graphitizable carbon, low temperature calcined carbon, etc. Metals that can be combined with lithium such as aluminum, silicon and tin, SiO _x (0 <x <2), amorphous compounds mainly composed of oxides such as tin dioxide, lithium titanate (Li ₄ Ti ₅ The particle | grains containing O < ₁₂ > etc. are mentioned.

(Negative current collector)
The negative electrode current collector 32 may be any conductive plate, and for example, a thin metal plate of aluminum, copper, or nickel foil can be used.

(Anode conductive material)
Examples of the conductive material include carbon powders such as carbon blacks, carbon nanotubes, carbon materials, metal fine powders such as copper, nickel, stainless steel and iron, mixtures of carbon materials and metal fine powders, and conductive oxides such as ITO. Be

(Negative electrode binder)
The binder used for the negative electrode can be the same as the positive electrode. In addition to these, as a binder, for example, cellulose, styrene butadiene rubber, ethylene propylene rubber, polyimide resin, polyamide imide resin, acrylic resin, etc. may be used.

In addition, an electron conductive conductive polymer or an ion conductive conductive polymer may be used as the binder. Examples of the electron conductive conductive polymer include polyacetylene and the like. In this case, since the binder also exhibits the function of the conductive auxiliary particles, the conductive auxiliary may not be added. As the ion conductive conductive polymer, for example, one having conductivity of an ion such as lithium ion can be used, and for example, a polymer compound (polyether type polymer compound such as polyethylene oxide or polypropylene oxide) And compounds obtained by complexing monomers such as polyphosphazenes) and lithium salts such as LiClO ₄ , LiBF ₄ and LiPF ₆ or alkali metal salts mainly containing lithium. As a polymerization initiator used for compounding, the photoinitiator or thermal polymerization initiator compatible with said monomer is mentioned, for example.

  The contents of the negative electrode active material, the conductive material, and the binder in the negative electrode active material layer 34 are not particularly limited. The composition ratio of the negative electrode active material 26 in the negative electrode active material layer 34 is preferably 70% to 98% by mass. The composition ratio of the conductive material in the negative electrode active material layer 34 is preferably 1% to 20% by mass, and the composition ratio of the binder in the negative electrode active material layer 34 is 1% to 10% by mass Is preferred.

  By making content of a negative electrode active material and a binder into the said range, in the obtained negative electrode active material layer 34, the quantity of a binder is too small and it can suppress the tendency which can not form a firm negative electrode active material layer. In addition, the amount of the binder that does not contribute to the electric capacity is increased, and the tendency that it is difficult to obtain a sufficient volume energy density can also be suppressed.

"Separator"
The separator 10 may be formed of an electrically insulating porous structure, for example, a single layer of a film made of polyethylene, polypropylene or polyolefin, a stretched film of a laminate or a mixture of the above resins, cellulose, polyester, A non-woven fabric made of at least one component selected from the group consisting of polypropylene can be mentioned.

"Case"
The case 50 seals the laminate 40 and the electrolytic solution inside. The case 50 is not particularly limited as long as it can prevent leakage of the electrolytic solution to the outside, intrusion of moisture and the like into the lithium secondary battery 100 from the outside, and the like.

  For example, as shown in FIG. 1, as the case 50, a metal laminate film in which the metal foil 52 is coated from both sides with the polymer film 54 can be used. For example, an aluminum foil can be used as the metal foil 52, and a film such as polypropylene can be used as the polymer film 54. For example, as the material of the outer polymer film 54, a polymer having a high melting point, for example, polyethylene terephthalate (PET), polyamide, etc. is preferable, and as the material of the inner polymer film 54, polyethylene (PE), polypropylene (PP) Etc. is preferred.

"Lead"
The leads 60 and 62 are formed of a conductive material such as aluminum. The leads 60 and 62 are welded to the positive electrode current collector 22 and the negative electrode current collector 32, respectively, and the separator 10 is sandwiched between the positive electrode active material layer 24 of the positive electrode 20 and the negative electrode active material layer 34 of the negative electrode 30; It is inserted into the case 50 together with the electrolytic solution, and the inlet of the case 50 is sealed.

[Method of manufacturing lithium secondary battery]
A method of manufacturing the lithium secondary battery 100 according to the present embodiment will be described.

  First, the positive electrode active material 26, the binder and the solvent are mixed. A conductive material 28 may be further added as needed. As the solvent, for example, water, N-methyl-2-pyrrolidone and the like can be used. The composition ratio of the positive electrode active material 26, the conductive material 28, and the binder is preferably 80 wt% to 98 wt%: 0.1 wt% to 10 wt%: 0.1 wt% to 10 wt% in mass ratio. These mass ratios are adjusted to be 100 wt% in total.

  The method of mixing the components constituting the paint is not particularly limited, and the order of mixing is also not particularly limited. The paint is applied to the positive electrode current collector 22. There is no restriction | limiting in particular as a coating method, The method employ | adopted when producing an electrode normally can be used. For example, a slit die coating method or a doctor blade method may be mentioned. The paint is similarly applied on the negative electrode current collector 32 for the negative electrode.

  Subsequently, the solvent in the paint applied on the positive electrode current collector 22 and the negative electrode current collector 32 is removed. The removal method is not particularly limited. For example, the positive electrode current collector 22 and the negative electrode current collector 32 to which the paint is applied may be dried in an atmosphere at 80 ° C. to 150 ° C.

  Then, the electrode on which the positive electrode active material layer 24 and the negative electrode active material layer 34 are formed as described above is subjected to a pressing process using a roll press device or the like as necessary. Although the linear pressure of a roll press changes with materials to be used, it adjusts so that the density of the positive electrode active material layer 24 becomes a predetermined value. The relationship between the density of the positive electrode active material layer 24 and the linear pressure can be obtained by preliminary examination based on the relationship with the material ratio constituting the positive electrode active material layer 24.

  Next, the positive electrode 20 having the positive electrode active material layer 24, the negative electrode 30 having the negative electrode active material layer 34, the separator 10 interposed between the positive electrode and the negative electrode, and the electrolytic solution are sealed in the case 50.

  For example, the positive electrode 20, the negative electrode 30, and the separator 10 are laminated, and the laminate 40 is placed in a bag-shaped case 50 prepared in advance.

  Finally, the electrolytic solution is injected into the case 50, and the case 50 is vacuum sealed to fabricate a lithium secondary battery. In addition, you may impregnate the laminated body 40 to electrolyte solution instead of inject | pouring electrolyte solution into a case. Since the positive electrode active material layer 24 is adjusted to a predetermined density, the electrolytic solution is sufficiently impregnated in the voids K formed in the positive electrode active material layer 24.

  As described above, in the lithium secondary battery according to the present embodiment, the positive electrode active material layer 24 has a predetermined density. Therefore, even when using an electrolytic solution containing a high viscosity ionic liquid and a lithium salt, an electrolytic solution containing an ionic liquid and a lithium salt can be sufficiently supplied into the space K formed in the positive electrode active material layer 24. . That is, the performance of the positive electrode can be extracted to the extent close to the theoretical capacity of the positive electrode theoretically required, and high capacity of the lithium secondary battery can be realized.

  The embodiments of the present invention have been described in detail with reference to the drawings, but the respective configurations and the combinations thereof and the like in the respective embodiments are merely examples, and additions and omissions of configurations are possible within the scope of the present invention. , Permutations, and other modifications are possible.

"Example 1"
First, as Example 1, the battery capacity actually obtained was determined with respect to the theoretical capacity of the positive electrode of the obtained lithium secondary battery while changing each element constituting the lithium secondary battery.

Example 1-1
NCA as a positive electrode active material (composition _{formula: Li 1.0 Ni 0.78 Co 0.19 Al} 0.03 O 2), carbon black as a conductive material was prepared PVDF as a binder. These were mixed in a solvent to prepare a paint, which was applied on a positive electrode current collector made of aluminum foil. The mass ratio of the positive electrode active material, the conductive material, and the binder was 95: 2: 3. After application, the solvent was removed.

  A metal lithium was used as a negative electrode active material, and a metal lithium foil with a thickness of 100 μm was attached to a negative electrode current collector made of copper foil.

  Then, a positive electrode on which the positive electrode active material layer was applied, a separator, and a negative electrode in which a metal lithium foil was attached to a negative electrode current collector were sequentially laminated to prepare a laminate. In addition, the number of laminated layers of the positive electrode and the negative electrode was one.

The density of the positive electrode after forming the laminate was 2.0 g / cm ³ , and the loading amount of the positive electrode active material layer supported by the positive electrode current collector per unit area was 15 mg / cm ² .

The obtained laminate was impregnated in an electrolyte and then enclosed in a case. The electrolyte used contained 1-ethyl-3-methylimidazolium as the cation of the ionic liquid, bis (fluorosulfonyl) imide as the anion of the ionic liquid, (FSO ₂ ) ₂ NLi as the lithium salt, and the viscosity was 15 mPa · s. . The lithium salt was dissolved in the ionic liquid so as to be 1 mol / L to adjust the electrolyte.

The obtained lithium secondary battery was operated at a charge voltage of 4.3 V, a discharge voltage of 3 V, a charge / discharge rate of 0.1 C, and the capacity of the battery was measured. As a result, the capacity of the lithium secondary battery of Example 1-1 was 2.6 mAh / cm ² and showed a performance of 90% of the theoretical capacity.

(Example 1-2)
Example 1-2 was different from Example 1-1 only in that the pressing pressure was changed and the density of the positive electrode was 2.5 g / cm ^3, and the other configuration was the same as that of Example 1-1.
The capacity of the obtained lithium secondary battery was 2.9 mAh / cm ² and showed a performance of 100% of the theoretical capacity.

(Example 1-3)
Example 1-3 was different from Example 1-1 only in that the pressing pressure was changed and the density of the positive electrode was 3.0 g / cm ^3, and the other configuration was the same as that of Example 1-1.
The capacity of the obtained lithium secondary battery was 2.9 mAh / cm ² and showed a performance of 100% of the theoretical capacity.

(Example 1-4)
Example 1-4 was different from Example 1-1 only in that the pressing pressure was changed and the density of the positive electrode was 3.5 g / cm ^3, and the other configuration was the same as that of Example 1-1.
The capacity of the obtained lithium secondary battery was 2.7 mAh / cm ² and showed a performance of 95% of the theoretical capacity.

(Comparative Example 1-1)
Comparative Example 1-1 was different from Example 1-1 only in that the pressing pressure was changed and the density of the positive electrode was 1.6 g / cm ^3, and the other configuration was the same as that of Example 1-1.
The capacity of the obtained lithium secondary battery was 2.0 mAh / cm ² and showed a performance of 70% of the theoretical capacity.

(Comparative example 1-2)
Comparative Example 1-2 was different from Example 1-1 only in that the pressing pressure was changed and the density of the positive electrode was 4.0 g / cm ^3, and the other configuration was the same as that of Example 1-1.
The capacity of the obtained lithium secondary battery was 2.0 mAh / cm ² and showed a performance of 70% of the theoretical capacity.

(Example 1-5-Example 1-8, Comparative Example 1-3, and Comparative Example 1-4)
Example 1-5-Example 1-8, Comparative Example 1-3, and Comparative Example 1-4 differ only in the point which changed the viscosity of electrolyte solution. Ionic liquid cation is N- methyl -N- propyl pyrrolidinium, anion using bis (fluorosulfonyl) imide, lithium salt using _{(FSO 2)} 2 NLi, and the viscosity and 60 mPa · s. The lithium salt was dissolved in an ionic liquid and adjusted to 1 mol / L. The density of each of Example 1-5 to Example 1-8, Comparative Example 1-3, and Comparative Example 1-4 is the same as that of Example 1-1 to Example 1-4, Comparative Example 1-1, and Comparative Example 1. Table 2 shows the correspondence and the battery characteristics of each of the lithium secondary batteries, corresponding to -2.

FIG. 4 shows the relationship between the density of the positive electrode active material layer and the capacity ratio of the battery capacity obtained in an actual lithium secondary battery to the theoretical capacity of the lithium secondary battery. As shown in FIG. 4, when the density of the positive electrode active material layer is 2.0 g / cm ³ or more and 3.5 g / cm ³ or less, the capacity ratio realized to the theoretical capacity is 80% or more, Even when an ionic liquid is used, the performance of the positive electrode is sufficiently extracted.

"Examples 2 to 4"
In Examples 2 to 4, the battery capacity of the lithium secondary battery was measured by changing the density and the loading amount of the positive electrode active material layer.

(Example 2-1)
Example 2-1 corresponds to Example 1-5.

(Example 2-2 to Example 2-6)
In Examples 2-2 to 2-6, the loading amount of the positive electrode active material layer was changed. The loading amount was controlled by changing the mass of the positive electrode active material used when forming the positive electrode active material layer. The loading amounts and the characteristics of the lithium secondary battery in each of Example 2-2 to Example 2-6 are shown in Table 3.

Example 3-1
Example 3-1 corresponds to Example 1-7. The density of the positive electrode active material layer is different from that of Example 2-1.

(Example 3-2 to Example 3-6)
In Examples 3-2 to 3-6, the loading amount of the positive electrode active material layer was changed. The loading amount was controlled by changing the mass of the positive electrode active material used when forming the positive electrode active material layer. The loading amounts in each of Examples 3-2 to 3-6 and the characteristics of the lithium secondary battery are shown in Table 4.

Example 4-1
Example 4-1 corresponds to Example 1-8. The density of the positive electrode active material layer is different from that of Example 2-1.

(Example 4-2 to Example 4-6)
In Example 4-2 to Example 4-6, the loading amount of the positive electrode active material layer was changed. The loading amount was controlled by changing the mass of the positive electrode active material used when forming the positive electrode active material layer. The loading amounts and the characteristics of the lithium secondary battery in each of Example 4-2 to Example 4-6 are shown in Table 5.

Further, FIG. 5 shows the relationship between the measured battery capacity of the lithium secondary battery and the loading amount of the positive electrode active material layer. As shown in FIG. 5, when the loading amount is in the range of 25 mg / cm ² or less, as the loading amount increases, the battery capacity of the actually measured lithium secondary battery increases. On the other hand, when the loading amount was too large, the battery capacity of the lithium secondary battery began to decrease. This is considered to be due to the fact that the ionic liquid is not sufficiently impregnated due to the increased thickness of the space K.

"Example 5"
In Example 5, the material type of the positive electrode active material layer was changed, and the battery capacity of the lithium secondary battery was measured.

(Example 5-1)
In Example 5-1, NCA was used as a positive electrode active material. Example 5-1 corresponds to Example 1-7 and Example 3-1.

(Example 5-2)
Example 5-2 as a positive active material, LCO (composition _{formula: Li} 1.0 CoO ₂₎ was used. The other conditions were the same as in Example 5-1.

Example 5-3
Example 5-3 as a positive active material, NCM (composition _{formula: Li 1.0 Ni 0.33 Co 0.33 Mn} 0.33 O 2) was used. The other conditions were the same as in Example 5-1.

Example 5-4
Example 5-4 as a positive active material, LMO (composition _formula: Li 1.0 _Mn 2 _{O 4)} was used. The other conditions were the same as in Example 5-1.

  The conditions of Example 5-1 to Example 5-4 are summarized in Table 6 below.

  The battery capacity of the lithium secondary battery of Example 5-1 to Example 5-4 is shown in FIG. In Examples 5-1 to 5-3 using other materials than Example 5-4 using LMO as a positive electrode active material, the battery capacity was excellent.

DESCRIPTION OF SYMBOLS 10 ... Separator, 20 ... positive electrode, 22 ... positive electrode collector, 24 ... positive electrode active material layer, 26 ... positive electrode active material, 28 ... conductive material, 30 ... negative electrode, 32 ... negative electrode collector, 34 ... negative electrode active material layer , 40: laminate, 50: case, 60, 62: lead, 100: lithium secondary battery, K: air gap

Claims (2)

A positive electrode, a negative electrode, and an electrolytic solution containing an ionic liquid and a lithium salt,
The electrolyte has a viscosity of more than 10 mPa · s and 200 mPa · s or less,
The ionic liquid is such that the cation is N-methyl-N-propylpyrrolidinium and the anion is bis (fluorosulfonyl) imide.
The lithium salt is (FSO ₂ ) ₂ NLi,
The positive electrode, the positive electrode active material density have a positive electrode active material layer of 2.5 g / cm ³ or more 3.0 g / cm ³ or less, the positive electrode active material layer, which is denoted by the following general formula (1) that having a, lithium secondary battery.
_{Li x M1 y M2 1-y} O 2 ··· (1)
In the general formula (1), M1 is Ni and Co, M2 is Al, x satisfies 0.05 ≦ x ≦ 1.1, and y satisfies 0.3 ≦ y ≦ 1. ^2. The lithium secondary battery according to claim 1, wherein a loading amount of the positive electrode active material layer in the positive electrode is 25 mg / cm ² or less.

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