JP2014035963A - Positive electrode material, lithium ion power storage device, and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode material used for forming a lithium ion power storage device having increased energy density without spoiling cycle characteristics, even under a high potential operation, a lithium ion power storage device using the positive electrode material, and a method of manufacturing the lithium ion power storage device.SOLUTION: There is provided a positive electrode material containing a high voltage positive electrode active material requiring impression of a voltage of 4.5 V or higher as a charging voltage, and a low potential plateau forming material forming a plateau at a potential of 3.8 V or lower or a low potential plateau retaining positive electrode active material having a plateau at a potential of 3.8 V or lower. There is also provided a lithium ion power storage device including a positive electrode containing the positive electrode material, a negative electrode containing a negative electrode active material, which lithium ions can be removed from and inserted into, and a nonaqueous electrolyte, in which a SEI coating having excellent lithium ion conductivity is formed in the negative electrode by impressing a voltage of 3.8 V or lower to the device during initial charging to float it for a prescribed time at the impression voltage. There is also provided a method of manufacturing the lithium ion power storage device.

Description

  The present invention relates to a positive electrode material lithium ion electricity storage device, particularly to a non-aqueous electrolyte lithium ion electricity storage device using a high voltage positive electrode active material, and a method for producing the same.

  In recent years, power storage devices such as lithium ion secondary batteries have been used as power sources for electric devices and the like, and are also being used as power sources for electric vehicles (EV, HEV, etc.). Power storage devices such as lithium ion secondary batteries further improve their characteristics, such as energy density (high capacity), output density (high output), and cycle characteristics (cycle life). High safety is desired.

  However, as the lithium ion secondary battery is repeatedly used, there is a possibility that the charge / discharge capacity retention rate is lowered, that is, the cycle characteristics are lowered. One reason for the deterioration of the cycle characteristics is oxidative decomposition of the electrolyte solution on the positive electrode surface.

  In an existing lithium ion secondary battery, there is a proposal of adding a predetermined additive to the electrolytic solution for the purpose of avoiding oxidative decomposition of the electrolytic solution and improving the cycle characteristics of the battery.

  For example, in Patent Document 1, by adding a phosphorus-containing compound to the electrolytic solution, a lithium ion conductive coating (SEI coating: solid electrolyte interface coating) is formed on the positive electrode surface, and the oxidative decomposition of the electrolytic solution at the positive electrode interface Is suppressed.

  Furthermore, Patent Document 2 shows that a film is formed on the positive electrode and is stably held by adding a disulfonic acid ester to the electrolytic solution and then performing charging at a predetermined temperature.

  In addition, due to the practical use of electric vehicles and the improvement in performance of other electric and electronic devices, the improvement of the performance of lithium ion secondary batteries has attracted great interest. As part of this, an increase in energy density is required.

  Since the energy density (Wh) corresponds to the product of the voltage (V) and the discharge capacity (Ah), in order to obtain a high energy density, the potential difference (battery electromotive force) between the positive and negative electrodes must be increased and Any means of performing cell design of discharge capacity can be taken.

  Among the above-mentioned means, in order to obtain a high electromotive force, use of a high-voltage positive electrode material that exhibits a charge / discharge capacity at a high potential has been studied. Since the high-voltage positive electrode material is a material that can withstand a higher voltage than the conventional positive electrode material, it is possible to increase the electromotive force of the battery and to improve the energy density.

  However, an electricity storage device using a high-voltage positive electrode material has a problem in reliability performance represented by deterioration of cycle characteristics. This is because, for example, by applying a high voltage to the high-voltage positive electrode material, the potential of the positive electrode at the time of full charge is increased, and the oxidative decomposition reaction of the electrolytic solution at the positive electrode is extremely likely to occur.

In this regard, according to Patent Document 3, a high-voltage active material is used as a positive electrode material, and a first electrolyte such as LiPF ₆ and a predetermined second electrolyte are added to an electrolytic solution, whereby an electricity storage device in repeated use for a plurality of cycles. It has been proposed to suppress an increase in resistance and suppress a decrease in discharge capacity.

JP 2009-266663 A JP 2006-351332 A JP2008-288049

  However, when the charging voltage is set to a high voltage such as 4.5V, even if an additive is added as described in Patent Documents 1 and 2, the decomposition / deterioration of the electrolyte is sufficiently suppressed, It is difficult to improve the cycle characteristics of the lithium ion secondary battery.

  Furthermore, in Patent Document 3, although a high-energy density lithium ion secondary battery is obtained using a high-voltage active material as a positive electrode material, the cost increases because a special compound is used as the second electrolyte. There are concerns.

  Further, in a high-voltage positive electrode material that is charged and discharged by applying a voltage of 4.5 V or more, when charging at 4.5 V or more is performed in the initial charge after the manufacture of a cell (storage cell) of a lithium ion storage device It has been pointed out that problems occur in battery performance.

  In other words, in the past, lithium ion electricity storage devices have formed an SEI film on the negative electrode surface, such as graphite, in the initial aging process after cell assembly, facilitating the desorption / insertion of lithium ions into the negative electrode, and increasing the capacity retention rate. It is common not to affect it.

  However, in a lithium ion storage cell including a high-voltage positive electrode material, when a charging voltage of 4.5 V or more is applied from the beginning, the negative electrode potential reaches a base potential sufficiently low for the formation of the SEI film. The potential reaches a high potential region where the oxidative decomposition reaction of the electrolyte proceeds.

  When the electrolytic solution is oxidatively decomposed, an oxidative decomposition product is deposited on the positive electrode, the internal resistance is increased, and lithium ion desorption / insertion is inhibited. The cycle characteristics of the lithium ion secondary battery are significantly deteriorated.

  However, the present invention does not have the above-described disadvantages of the prior art, and does not impair cycle characteristics even under high voltage operation, and does not impair cycle characteristics, and a positive electrode material used to configure a lithium ion electricity storage device with increased energy density, It is an object of the present invention to provide a lithium ion electricity storage device using the same, and a manufacturing method thereof.

  In order to achieve the above object, the present inventors have developed a high-voltage positive electrode active material that requires a voltage application of 4.5 V or more as a charging voltage and a low-potential plateau forming material that forms a plateau at a potential of 3.8 V or less. The positive electrode material containing the low potential plateau holding positive electrode active material which has a plateau in the electric potential of 3.8 V or less was discovered.

  By applying this positive electrode material to a lithium ion electricity storage device, in the initial charge of the electricity storage device, the electrolyte solution is reduced and decomposed on the negative electrode side before the oxidative decomposition of the electrolyte solution starts on the positive electrode side, thereby building a good SEI coating Therefore, an electricity storage device having excellent cycle characteristics is configured.

Furthermore, in order to achieve the above object, a positive electrode including the above positive electrode material;
A negative electrode containing a negative electrode active material capable of removing and inserting lithium ions;
A non-aqueous electrolyte,
A method for producing a lithium ion electricity storage device comprising:
By applying a voltage of 3.8 V or less during initial charging and allowing the applied voltage to float for a predetermined time, an SEI film is formed on the negative electrode before the positive electrode potential reaches the decomposition potential of the non-aqueous electrolyte. The manufacturing method of the lithium ion electrical storage device characterized by these was discovered.

  In the lithium ion electricity storage device of the present invention, a low potential plateau was intentionally introduced to the positive electrode using the high voltage positive electrode active material. During the first charge, a rapid rise in the positive electrode potential is suppressed by going through a state where lithium ions are released from the low potential plateau, so that a time margin is obtained before the positive electrode potential reaches the decomposition potential of the electrolyte. . As a result, the SEI film is formed on the negative electrode before the electrolytic solution is oxidatively decomposed, and a highly reliable lithium ion storage device can be provided.

  Further, in subsequent use of the lithium ion electricity storage device, charging and discharging at a high voltage corresponding to the high voltage positive electrode material which is the main component of the positive electrode material is performed, and a high energy density is ensured. Furthermore, since the SEI film is already formed on the negative electrode, the negative electrode operates stably even under high voltage operation, and the cycle characteristics of the electricity storage device are maintained.

It is a schematic sectional drawing which shows an example of embodiment of the electrical storage device (lithium ion secondary battery) of this invention. It is a schematic sectional drawing which shows another example of embodiment of the electrical storage device (lithium ion secondary battery) of this invention. It is a graph which shows the charging / discharging curve of the sample using the positive electrode material which concerns on embodiment of this invention, and the positive electrode material for a comparison. It is a graph which shows the initial stage charge / discharge curve of the lithium ion secondary battery which concerns on embodiment of this invention, and the lithium ion secondary battery for a comparison. It is a graph which shows the cycle characteristic of the lithium ion secondary battery which concerns on embodiment of this invention, and the lithium ion secondary battery for a comparison. It is a graph which shows the XPS analysis result of the SEI membrane | film | coat formed in the negative electrode in the lithium ion secondary battery which concerns on embodiment of this invention, and the lithium ion secondary battery for a comparison.

  Hereinafter, embodiments of the present invention will be described in detail.

  The positive electrode material of the present invention includes (A) a high voltage positive electrode material that requires a voltage application of 4.5 V or more as a charging voltage, and (B-1) a low potential plateau forming material that forms a plateau at a potential of 3.8 V or less. Or (B-2) a positive electrode active material having a low potential plateau having a plateau at a potential of 3.8 V or less.

Further, the lithium ion electricity storage device of the present invention (simply also referred to as electricity storage device),
(A) A high-voltage positive electrode active material that requires a voltage application of 4.5 V or more as a charging voltage, and (B-1) a low-potential plateau forming material that forms a plateau at a potential of 3.8 V or less, or (B-2 ) A positive electrode material comprising a positive electrode active material having a low potential plateau having a plateau at a potential of 3.8 V or less,
A negative electrode containing a negative electrode active material capable of removing and inserting lithium ions; and a non-aqueous electrolyte.

  That is, in the present invention, (A) In addition to the high-voltage positive electrode material that requires voltage application of 4.5 V or more as a charging voltage, (B-1) low potential plateau formation that forms a plateau at a potential of 3.8 V or less By using the material, it is possible to utilize a low potential capacity that does not appear in the oxidation-reduction reaction process of the high potential positive electrode material itself as a low potential plateau on the positive electrode side.

  Instead of this, in addition to the high-voltage positive electrode material (A) that requires a voltage application of 4.5 V or more as the charging voltage, (B-2) a low potential plateau possessing positive electrode active having a plateau at a potential of 3.8 V or less Use substances. Thereby, not the high potential positive electrode material itself but the low potential capacity of the positive electrode active material having a low potential plateau added separately is used as the low potential plateau.

  The low potential plateau of the present invention is formed at a potential lower than the average discharge potential of the high voltage positive electrode material, 3.8 V or less, preferably 2.0 V to 3.8 V.

  Thus, at the initial charge of the electricity storage device including the positive electrode material of the present invention to which the low potential plateau is added to the high voltage positive electrode material (first charge after the completion of the cell for the electricity storage device), the voltage is 3.8 V or less. Aging is performed at a low potential by applying a voltage and allowing it to float for a predetermined time.

  Aging is a voltage corresponding to the potential of the plateau formed by the low potential plateau forming material or the positive electrode active material having the low potential plateau at the initial charge, for example, 2.0 V to 3.8 V, that is, the voltage of the battery, 0 V to 4 A voltage of 0.0 V is applied for 1 to 24 hours, preferably 12 to 24 hours to float (float charging), and a good SEI film is formed on the negative electrode surface (aging process).

  Here, the float charging is easily performed by low current low voltage charging (CC-CV charging).

  In the present invention, the “initial charge” means the first (first) charge after the manufacture of the lithium ion storage cell, particularly in the aging process or the first charge.

  The potential of 3.8 V or less corresponding to the plateau applied by the low potential plateau forming material (B-1) or the low potential plateau holding positive electrode active material (B-2) is a potential region where the electrolytic solution is not oxidatively decomposed on the positive electrode side. It hits.

  When the aging is performed at a potential of 3.8 V or less with respect to the electricity storage device including the positive electrode material having such a low potential plateau, the positive electrode potential is polarized to 3.8 V or more, and the oxidative decomposition reaction of the electrolytic solution is excessive. Before proceeding, the potential of the negative electrode can be polarized to a sufficiently low potential for SEI film formation, and a good SEI film can be formed on the surface of the negative electrode.

  As a result, since the positive and negative electrodes are kept in a good state, capacity deterioration and cycle performance deterioration of the lithium ion electricity storage device are suppressed.

  In addition, by introducing into the positive electrode material a low potential plateau forming material that gives a discharge capacity to a low potential or a positive electrode active material having a low potential plateau, the average discharge potential of the positive electrode active material is theoretically the average discharge of only the high voltage positive electrode active material. It drops below the potential.

  However, since the capacity corresponding to the negative electrode irreversible capacity is operated as the low potential plateau capacity in the use stage of the lithium ion electricity storage device of the present invention, the discharge capacity corresponding to the low potential plateau in the positive electrode material is usually involved in charging and discharging. Without being practically negligible.

  However, if a capacity that can participate in charging / discharging is applied in the low potential plateau region, a decrease in the average discharge potential may not be negligible.

  That is, the negative electrode material generally has a predetermined capacity that cannot be released by normal discharge even when lithium ions are loaded (the irreversible capacity of the negative electrode). And since the lithium ions that are not released cannot participate in charging / discharging, the positive electrode from which this amount of lithium ions has been released has a capacity that cannot receive lithium ions (a positive electrode active material that cannot participate in charging / discharging). become.

In addition, when the design is made so that the irreversible capacity of the negative electrode per unit electrode capacity (mAh / cm ² ) matches the low potential plateau capacity formed on the positive electrode, an energy storage device with high energy density can be manufactured.

  In the lithium ion electricity storage device of the present invention, the voltage applied at the time of initial charging is a voltage (plus 3.8 V) corresponding to the average potential of the plateau formed by the low potential plateau forming material or the positive electrode active material having a low potential plateau. Or less than that is preferable.

  By suppressing oxidative decomposition of the electrolytic solution accompanying excessive polarization of the positive electrode, a good SEI film can be formed on the negative electrode.

  Hereinafter, an example of a lithium ion secondary battery will be described as an example of an embodiment of a lithium ion electricity storage device of the present invention with reference to the drawings.

  FIG. 1 is a schematic cross-sectional view showing an example of an embodiment of a lithium ion secondary battery according to the present invention. As shown in the figure, in the lithium ion secondary battery 20, a positive electrode 21 and a negative electrode 22 are disposed to face each other with a separator 23 interposed therebetween.

  The positive electrode 21 includes a positive electrode mixture layer 21a containing the positive electrode material of the present invention and a positive electrode current collector 21b. The positive electrode mixture layer 21a is formed on the separator 23 side surface of the positive electrode current collector 21b. The negative electrode 22 includes a negative electrode mixture layer 22a and a negative electrode current collector 22b.

  The negative electrode mixture layer 22a is formed on the separator 23 side surface of the negative electrode current collector 22b. The positive electrode 21, the negative electrode 22, and the separator 23 are sealed in an outer container (not shown), and the outer container is filled with a non-aqueous electrolyte. Examples of the exterior material include a battery can and a laminate film. Further, leads (not shown) for connecting external terminals are respectively connected to the positive electrode current collector 21b and the negative electrode current collector 22b as necessary.

  Next, FIG. 2 is a schematic sectional view showing another example of the embodiment of the lithium ion secondary battery according to the present invention. As shown in the figure, the lithium ion secondary battery 30 includes an electrode unit 34 in which a plurality of positive electrodes 31 and negative electrodes 32 are alternately stacked via separators 33. The positive electrode 31 is configured by providing a positive electrode mixture layer 31a on both surfaces of a positive electrode current collector 31b. The negative electrode 32 is configured such that the negative electrode mixture layer 32a is provided on both surfaces of the negative electrode current collector 32b (however, for the uppermost and lowermost negative electrodes 32, the negative electrode mixture layer 32a is only on one side).

  Further, although not shown, the positive electrode current collector 31b has a protruding portion, the protruding portions of the plurality of positive electrode current collectors 31b are overlapped, and the lead 36 is welded to the overlapped portion. Similarly, the negative electrode current collector 32b has a protruding portion, and a lead 37 is welded to a portion where the protruding portions of the plurality of negative electrode current collectors 32b are overlapped. The lithium ion secondary battery 30 is configured by enclosing an electrode unit 34 and a non-aqueous electrolyte in an exterior container such as a laminate film (not shown). The leads 36 and 37 are exposed to the outside of the outer container for connection with an external device.

  In addition, the lithium ion secondary battery 30 has the negative electrode disposed at the uppermost part and the lowermost part. However, the present invention is not limited thereto, and the positive electrode may be disposed at the uppermost part and the lowermost part.

  The lithium ion secondary battery of the present invention can be obtained by applying a voltage of 3.8 V or less during initial charging of a cell for a lithium ion secondary battery having the above-described configuration and allowing the cell to float at the applied voltage for a predetermined time. can get.

[Production of positive electrode]
In the present invention, a high-voltage positive electrode active material that requires a voltage application of 4.5 V or more as a charging voltage is used as the positive electrode active material. Examples of the high-voltage positive electrode active material are a solid solution, a spinel nickel manganese-based material, a polyanion-based material, a silicate-based material, or an olivine-based material, and preferably a solid solution or a spinel nickel manganese-based material is used.

Specific examples of these are Li ₂ MnO ₃ —LiMO ₂ (M = Co, Ni, Mn, one or more than Al) as solid solution cathode materials,
As a spinel nickel manganese positive electrode material, LiMn _1.5 Ni _0.5 O ₄ ,
As the fluorinated olivine-based positive electrode material, (LiMPO ₄ F (M = V, Mn, Fe, Ni),
As silicate-based positive electrode materials, (Li ₂ MSiO ₄ (M = V, Mn, Fe, Ni),
As an olivine-based positive electrode material (LiMPO ₄ (M = V, Mn, Ni),
Is mentioned.

  Moreover, the material which doped the different element with respect to the said material is also included.

  In the present invention, other positive electrode active materials that require voltage application of 4.5 V or more as the charging voltage can also be used.

  In addition, (B-1) a low potential plateau forming material or (B-2) a low potential plateau holding positive electrode active material is used in combination with (A) a high voltage positive electrode active material.

  The (B-1) low potential plateau forming material and (B-2) the positive electrode active material having a low potential plateau both have a discharge potential of 3.8 V or less, preferably a potential in the range of 2.0 to 3.8 V. A material that forms and retains a plateau is preferably used.

  By using the (B-1) low potential plateau forming material and the (B-2) positive electrode active material having a low potential plateau, the potential for charging and discharging with the high potential active material has a sufficient potential difference. A plateau (low potential plateau) is formed on the low potential side of the positive electrode active material.

  (B-1) The low-potential plateau forming material is a material for pre-doping lithium ions into a high-voltage positive electrode active material, preferably a powdery or foil-like lithium metal, and the powdery lithium metal is supported on the substrate. It is possible to use it in the form.

  With respect to the high-voltage positive electrode active material, powdered lithium metal is mixed with other necessary materials to form a positive electrode mixture. The foil-like lithium metal is used in a state of being attached to a positive electrode mixture containing a high-voltage positive electrode active material, and the lithium metal powder carried on the substrate is once attached to the positive electrode mixture, and then the substrate It is used by peeling.

  The combination of (A) a high voltage positive electrode active material and (B-1) a low potential plateau forming material is used, for example, when a spinel nickel manganese-based material is used as the high voltage active material.

Spinel nickel manganese-based materials have a redox response of Mn ^{+ 3 / + 4} near 3.0 V in addition to the redox capacity of Ni ^{+ 2 / + 4} near 4.8 V, which is generally used as charge / discharge capacity. It has a charge / discharge capacity corresponding to. By utilizing this Mn ^{+ 3 / + 4} redox site, a low potential plateau can be formed.

In addition, (B-2) a positive electrode active material having a low potential plateau,
A material in which lithium ions are pre-doped with (i) LiFePO ₄ or (ii) V ₂ O ₅ or MnO ₂ is used.

As a material for pre-doping lithium ions with respect to V ₂ O ₅ or MnO ₂ , the same material as the specific example of the low potential plateau forming material described above is used in the same manner.

In the positive electrode active material (B-2) having a low potential plateau, in particular, V ₂ O ₅ or MnO ₂ is a material capable of occluding a large amount of lithium ions, so that a large amount of lithium can be efficiently taken in. Therefore, the addition amount of the positive electrode active material having a low potential plateau can be reduced, whereby the content of the high voltage positive electrode active material in the positive electrode material can be relatively increased.

  The positive electrode active material (B-2) having a low potential plateau is in an amount corresponding to 0.1% by volume ≦ X ≦ 30% by volume, preferably 1% by volume ≦ X ≦ 30% by volume of the discharge capacity (X) of the negative electrode. It is preferable to mix in the positive electrode material.

  By using the positive electrode active material having a low potential plateau in the above proportion, a favorable SEI film can be reliably formed on the negative electrode, and the average potential of the negative electrode can be lowered to 0.5 V or less. Furthermore, the average discharge potential of the positive electrode material is not significantly reduced.

  In addition to the above materials, a binder, a conductive additive and a solvent are used for forming the positive electrode mixture layer.

  As the binder used for forming the positive electrode mixture layer, for example, a fluorine-containing resin such as polyvinylidene fluoride, an acrylic binder, a rubber binder such as SBR, a thermoplastic resin such as polypropylene or polyethylene, carboxymethylcellulose, or the like can be used. . The binder is preferably a fluorine-containing resin or a thermoplastic resin that is chemically and electrochemically stable with respect to the non-aqueous electrolyte used in the electricity storage device of the present invention, and particularly preferably a fluorine-containing resin. As the fluorine-containing resin, in addition to polyvinylidene fluoride, polytetrafluoroethylene, vinylidene fluoride-3 fluoroethylene copolymer, ethylene-4 fluoroethylene copolymer, propylene-4 fluoroethylene copolymer, etc. Can be mentioned. As for the compounding quantity of a binder, 0.5-20 mass% is preferable with respect to the said positive electrode compound material amount.

  Examples of the conductive assistant used for forming the positive electrode mixture layer include conductive carbon such as ketjen black, metals such as copper, iron, silver, nickel, palladium, gold, platinum, indium and tungsten, indium oxide and tin oxide. Conductive metal oxides such as can be used. As for the compounding quantity of an electrically conductive material, 1-30 mass% is preferable with respect to the said positive electrode active material.

  As a solvent used for forming the positive electrode mixture layer, water, isopropyl alcohol, N-methylpyrrolidone, dimethylformamide, or the like can be used.

  A positive electrode slurry in which a mixture containing the high-voltage positive electrode active material, the low-potential plateau-forming material or the positive-electrode active material having a low-potential plateau, a binder, and a conductive additive is dispersed in a solvent is applied onto a positive electrode current collector and dried. The positive electrode mixture layer is formed by a process including You may press-press etc. after a drying process. As a result, the positive electrode mixture layer is uniformly and firmly pressed onto the current collector.

Further, in the case where a material in which lithium ions are pre-doped using a material pre-doped with lithium ions with respect to V ₂ O ₅ or MnO ₂ is used as the low-potential plateau-carrying positive electrode active material, a high-voltage positive electrode active material, V A mixture containing ₂ O ₅ or MnO ₂ , a binder, and a conductive additive is dispersed in a solvent to form a positive electrode slurry, and a positive electrode mixture layer is formed and dried on the positive electrode current collector by the same procedure as described above, and further, A material for pre-doping lithium ions, for example, a lithium metal foil can be attached to the positive electrode mixture layer and V ₂ O ₅ or MnO ₂ can be pre-doped with lithium ions.

  The thickness of the positive electrode mixture layer is 10 to 200 μm, preferably 20 to 100 μm.

  The positive electrode current collector need only be a conductive substrate whose surface in contact with the positive electrode mixture layer exhibits conductivity. For example, a conductive substrate formed of a conductive material such as metal, conductive metal oxide, or conductive carbon. In addition, a conductive foil or a non-conductive base body covered with the conductive material can be used. As the conductive material, copper, gold, aluminum, an alloy thereof, or conductive carbon is preferable.

  As the positive electrode current collector, an expanded metal, a punching metal, a foil, a net, a foam, or the like of the above materials can be used. The shape and number of through holes in the case of a porous body are not particularly limited, and can be appropriately set within a range that does not inhibit the movement of lithium ions.

  In the present invention, the pre-doping means that the positive electrode material is mixed with a low potential plateau forming material or a low potential plateau holding positive electrode active material in advance, and the positive electrode containing the low potential plateau forming material or the low potential plateau holding positive electrode active material. In the lithium ion electricity storage device using a material, it means performing the above-mentioned aging treatment.

Moreover, in this invention, the outstanding cycling characteristics can be acquired because the fabric weight of a positive electrode compound-material layer shall be 4 mg / cm < ² > or more and 25 mg / cm < ² > or less. When the basis weight is less than 4 mg / cm ² or more than 25 mg / cm ² , cycle deterioration occurs. Note that the larger the basis weight, the higher the capacity. The basis weight of the positive electrode mixture layer is more preferably 10 mg / cm ² or more and 20 mg / cm ² or less. Note that the basis weight here means the basis weight of the positive electrode mixture layer on one surface side of the positive electrode current collector. In the case where the positive electrode mixture layer is formed on both surfaces of the positive electrode current collector, the positive electrode mixture layer on one surface and the other surface is formed so as to be included in the above ranges.

[Manufacture of negative electrode]
In the present invention, the negative electrode is not particularly limited, and can be produced using a known material.

  Any material can be used as the negative electrode active material as long as it is a material capable of removing and inserting lithium ions, and examples thereof include a carbon-based material, a silicon-based material, a tin-based material, and a polyacene-based material.

More specifically, as the carbon-based material, for example, lithium intercalation carbon material (substance capable of reversibly removing and inserting lithium ions) such as graphite, non-graphitizable carbon, and graphitizable carbon,
Examples of silicon-based materials include silicon and silicon alloys.
Examples of tin-based materials include tin and tin alloys.
Examples of lithium metal materials include lithium alloys (Li-Al alloys and the like) and lithium compounds (lithium nitride and the like)
Examples of the polyacene organic semiconductor include insoluble and infusible PAS having a polyacene skeleton.

  Of these, carbon-based materials, silicon-based materials, and tin-based materials are preferably used in the present invention.

  These negative electrode active materials are excellent in the ability to occlude lithium ions, and can constitute a lithium ion electricity storage device having a large charge / discharge capacity. The negative electrode active material may be composed of one material or a mixture of two or more materials.

  In the production of the negative electrode in the present invention, a negative electrode mixture layer is formed by applying a negative electrode slurry in which a mixture containing the negative electrode active material and the binder described above is dispersed in a solvent and drying the mixture on a negative electrode current collector. Note that the same binder, solvent, and current collector as in the case of the positive electrode described above can be used.

  The thickness of the negative electrode mixture layer is generally 10 to 200 μm, preferably 20 to 100 μm.

  In the present invention, the basis weight of the negative electrode mixture layer is appropriately designed in accordance with the basis weight of the positive electrode mixture layer. Usually, a lithium ion secondary battery is designed so that the capacity (mAh) of the positive electrode and the negative electrode is approximately the same from the viewpoint of capacity balance and energy density of the positive and negative electrodes. Therefore, the basis weight of the negative electrode mixture layer is set based on the type of the negative electrode active material, the capacity of the positive electrode, and the like.

[Non-aqueous electrolyte]
There is no restriction | limiting in particular in the non-aqueous electrolyte in this invention, A well-known material can be used. For example, an electrolytic solution in which a general lithium salt is used as an electrolyte and is dissolved in an organic solvent can be used from the viewpoint that electrolysis does not occur even at a high voltage and that lithium ions can exist stably.

Examples of the electrolyte include CF ₃ SO ₃ Li, C ₄ F ₉ SO ₈ Li, (CF ₃ SO ₂ ) ₂ NLi, (CF ₃ SO ₂ ) ₃ CLi, LiBF ₄ , LiPF ₆ , LiClO _{4, and the} like. The mixture of 2 or more types is mentioned.

As the organic solvent, for example, C ₁ -C ₁₀ alkylene carbonate, preferably C ₁ -C ₆ alkylene carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, C ₁ -C ₁₀ alkyl carbonate, preferably C ₁ -C ₆ Alkyl carbonates such as dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and fluoroethylene carbonate, vinyl carbonate, trifluoromethyl propylene carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, tetrahydrofuran, 1 , 3-dioxolane, 4-methyl-1,3-dioxolane, diethyl ether, sulfolane, methylsulfolane, acetonitrile, and propiotolyl.

  In the present invention, any one of these or a mixed solvent of two or more thereof is used.

Among these, in the present invention, in particular, it is preferable to include one or more of C ₁ -C ₁₀ alkyl carbonate and C ₁ -C ₁₀ alkylene carbonate, and by using these carbonates in the electrolyte, lithium ion conductive A good SEI film can be produced.

  The electrolyte concentration in the non-aqueous electrolyte is preferably 0.1 to 5.0 mol / L, and more preferably 0.5 to 3.0 mol / L.

  The non-aqueous electrolyte may be liquid, or may be a solid electrolyte or polymer gel electrolyte mixed with a plasticizer or a polymer.

[Separator]
The separator used in the present invention is not particularly limited, and a known separator can be used. For example, a porous body having durability against an electrolytic solution, a positive electrode active material, and a negative electrode active material and having continuous air holes and having no electronic conductivity can be preferably used. Examples of such a porous body include woven fabric, non-woven fabric, synthetic resin microporous membrane, and glass fiber. A synthetic resin microporous membrane is preferably used, and a polyolefin microporous membrane such as polyethylene or polypropylene is particularly preferable in terms of thickness, membrane strength, and membrane resistance.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to this embodiment.

I. Production of positive electrode material [Example 1] Use example of positive electrode active material (LFP) possessing low potential plateau

(1) Production of positive electrode A positive electrode mixture was prepared from the following materials.
Spinel nickel manganese lithium (LiNi _0.5 Mn _1.5 O ₄ ) 80 parts by mass LiFePO ₄ (LFP) 5 parts by mass Polyvinylidene fluoride (PVdF) 5 parts by mass Carbon black 10 parts by mass N-methyl 2-pyrrolidone (NMP) 100 parts by mass High The positive electrode active material (LiNi _0.5 Mn _1.5 O ₄ ) and the remaining materials were mixed to obtain a positive electrode mixture slurry.

The positive electrode mixture slurry was applied to a positive electrode current collector made of aluminum foil (thickness 10 μm) at a thickness of 400 μm by the doctor blade method and dried to form a positive electrode mixture layer on the positive electrode current collector. The basis weight of the positive electrode mixture layer was 20 mg / cm ² (per one side). Thereby, the positive electrode material of the present invention was obtained.

[Example 2] Use example of low potential plateau forming material (1) Production of positive electrode (1-1) Production of positive electrode mixture layer A positive electrode mixture was prepared from the following materials.
Spinel nickel manganese lithium (LiNi _0.5 Mn _1.5 O ₄ ) 85 parts by mass Polyvinylidene fluoride (PVdF) 5 parts by mass Carbon black 10 parts by mass N-methyl 2-pyrrolidone (NMP) 100 parts by mass High-voltage positive electrode active material (LiNi _0.5 Mn _1.5 O ₄ ) and the remaining materials were mixed to obtain a positive electrode mixture slurry.

The positive electrode mixture slurry was applied to a positive electrode current collector made of aluminum foil (thickness 10 μm) at a thickness of 400 μm by the doctor blade method and dried to form a positive electrode mixture layer on the positive electrode current collector. The basis weight of the positive electrode mixture layer was 20 mg / cm ² (per one side).

(1-2) Addition of Low Potential Plateau Forming Material to Positive Electrode Composite Layer On the surface of the positive electrode composite layer obtained in this manner, LMCF (SLMP (Li fine powder: Stabilized Lithium Metal Powder) manufactured by FMC of the United States was used. A sheet coated on the transfer film) was affixed, and Li fine powder was transferred to the positive electrode mixture layer. Thereby, the positive electrode material of the present invention was obtained.

[Comparative Example 1]
The same operation as in Example 1 was repeated except that LiFePO ₄ was not used. Thereby, a positive electrode material for comparison was obtained.

II. Production of lithium ion secondary battery [Example 3]
A: Production of lithium ion secondary battery cell (before aging) (1) Positive electrode material The positive electrode material of Example 1 was used.
(2) Production of negative electrode Graphite 90 parts by mass Styrene butadiene rubber (SBR) 3 parts by mass Carboxymethylcellulose (CMC) 2 parts by mass Carbon black 5 parts by mass Distilled water 100 parts by mass Negative electrode active material (graphite) and the remaining materials were mixed. Thus, a negative electrode mixture slurry was obtained.

The negative electrode mixture slurry was applied to a copper foil (thickness: 15 μm) negative electrode current collector with a thickness of 120 μm by a doctor blade method and dried to form a negative electrode mixture layer on the negative electrode current collector. The basis weight of the negative electrode mixture was 8 mg / cm ² (100% by volume with respect to the positive electrode mixture).

(3) Preparation of electrolyte solution For 30 parts by weight of ethylene carbonate (EC), 25 parts by weight of diethyl carbonate (DEC), 24 parts by weight of ethyl methyl carbonate (EMC), and 5 parts by weight of fluoroethylene carbonate (FEC) An electrolytic solution in which lithium hexafluorophosphate (Li-PF ₆ ) was dissolved at 1.2 M was produced.

(4) Production of Coin Cell for Lithium Ion Secondary Battery A lithium ion secondary battery as shown in the embodiment of FIG. 1 was produced using one positive electrode and one negative electrode produced as described above. Specifically, the positive electrode and the negative electrode were laminated via a polyethylene separator.

  The non-aqueous electrolyte was poured from above, and the electrolyte was permeated into the electrode by vacuum impregnation, and then the coin cell was sealed.

  Thereby, the cell (coin cell) for lithium ion secondary batteries containing the positive electrode material of this invention was obtained.

B: Manufacture of lithium ion secondary battery (after aging) The obtained cell for lithium ion secondary batteries was aged. That is, using a charging / discharging device, the cell was charged at a constant current and a constant voltage for 12 hours to a current of 0.1 C and a voltage of 3.0 V. The voltage became 3.0V about 60 minutes after the start of charging. In this state, the voltage rise was suppressed and the remaining 11 hours were left floating.

  Thereby, the lithium ion secondary battery of the present invention was obtained.

[Example 4]
A lithium ion secondary battery containing the positive electrode material of the present invention was manufactured by repeating the same operation as in Example 3 except that the positive electrode material described in Example 2 was used.

[Comparative Example 2]
The same operation as in Example 3 was repeated except that the positive electrode material described in Comparative Example 1 was used. Thereby, the lithium ion secondary battery containing the positive electrode material for a comparison was obtained.

[Example 5]
A: Production of lithium ion secondary battery cell (before aging) (1) Positive electrode material The positive electrode material of Example 2 was used.
(2) Production of negative electrode 90 parts by mass of silicon-containing graphite (83 parts by weight of graphite, 7 parts by weight of silicon)
Styrene butadiene rubber (SBR) 3 parts by weight Carboxymethyl cellulose (CMC) 2 parts by weight Carbon black 5 parts by weight Distilled water 100 parts by weight A negative electrode active material (silicon-containing graphite) and the remaining materials are mixed to obtain a negative electrode mixture slurry. It was.

The negative electrode mixture slurry was applied to a negative electrode current collector made of copper foil (thickness: 15 μm) at a thickness of 70 μm by a doctor blade method and dried to form a negative electrode mixture layer on the negative electrode current collector. The basis weight of the negative electrode mixture was 4 mg / cm ² (100% by volume with respect to the positive electrode mixture).

(3) Preparation of electrolyte solution For 30 parts by weight of ethylene carbonate (EC), 25 parts by weight of diethyl carbonate (DEC), 24 parts by weight of ethyl methyl carbonate (EMC), and 5 parts by weight of fluoroethylene carbonate (FEC) An electrolytic solution in which lithium hexafluorophosphate (Li-PF ₆ ) was dissolved at 1.2 M was produced.

(4) Production of Coin Cell for Lithium Ion Secondary Battery A lithium ion secondary battery as shown in the embodiment of FIG. 1 was produced using one positive electrode and one negative electrode produced as described above. Specifically, the positive electrode and the negative electrode were laminated via a polyethylene separator.

  The non-aqueous electrolyte was poured from above, and the electrolyte was permeated into the electrode by vacuum impregnation, and then the coin cell was sealed.

  Thereby, the cell (coin cell) for lithium ion secondary batteries containing the positive electrode material of this invention was obtained.

B: Manufacture of lithium ion secondary battery (after aging) The obtained cell for lithium ion secondary batteries was aged. That is, using a charging / discharging device, the cell was charged at a constant current and a constant voltage for 12 hours to a current of 0.1 C and a voltage of 3.0 V. The voltage became 3.0V about 60 minutes after the start of charging. In this state, the voltage rise was suppressed and the remaining 11 hours were left floating.

  Thereby, the lithium ion secondary battery of the present invention was obtained.

[Comparative Example 3]
The same operation as in Example 5 was repeated except that the positive electrode material described in Comparative Example 1 was used (the same negative electrode as in Example 5 was used). Thereby, the lithium ion secondary battery containing the positive electrode material for a comparison was obtained.

III. Characteristic Confirmation Test of Positive Electrode Material Using the positive electrode material of Examples 1 and 2 and Comparative Example 1 and a metal lithium foil (negative electrode material) having a thickness of 120 μm, the other conditions were the same as in Example 3 and A lithium ion secondary battery cell (coin cell) containing a positive electrode material was produced. The coin cells including the positive electrode materials of Examples 1 and 2 and Comparative Example 1 are referred to as Sample 1, Sample 2, and Comparative Sample 1 in this order.

  Samples 1 and 2 and Comparative Sample 1 were charged with constant current and constant voltage up to a current of 0.1 C and a voltage of 4.9 V for 24 hours using a charging / discharging device, and a constant current discharge of a current of 0.2 C was applied to the voltage. Until the voltage dropped to 3V (1 cycle). Furthermore, for these samples, a constant current and constant voltage charge up to a current of 1 C and a voltage of 4.9 V is performed for 24 hours, and a constant current discharge of a current of 1 C is performed until the voltage drops to 3 V, up to 200 cycles. Repeated (2 to 200 cycles).

  During this time, the change in charge / discharge capacity in Samples 1 and 2 and Comparative Sample 1 was monitored using a charge / discharge device, and the potential having the discharge capacity was confirmed for each sample. The result is shown in FIG.

  From FIG. 3, in samples 1 and 2 of the present invention (using the positive electrodes of Examples 1 and 2), a low potential plateau of 3.8 V or less that cannot be confirmed in the comparative sample (using the positive electrode of Comparative Example 1). You can see that

  When voltage is applied to the electricity storage device, charging is performed according to the charging / discharging curve. Due to the presence of this plateau, the time until the positive electrode reaches a high voltage and the oxidative decomposition reaction of the electrolyte is started is delayed. It is obvious. That is, it can be said that the low-potential plateau-carrying positive electrode active material or the low-potential plateau forming material added to the high-voltage positive electrode material provides a time margin until the start of oxidative decomposition of the electrolytic solution in the vicinity of the positive electrode.

  This is consistent with the results of the following battery performance test.

IV. Battery performance test [Measurement of initial capacity]
The lithium ion secondary batteries obtained in Examples 3 and 4 and Comparative Example 2 were charged at a constant current and a constant voltage up to a current of 0.1 C and a voltage of 4.8 V using a charging / discharging device in 24 hours. A constant current discharge of 0.2 C was performed until the voltage dropped to 3 V (one cycle).

  When the initial charge / discharge capacities of the positive electrodes in the lithium ion secondary batteries of Examples 3 and 4 and Comparative Example 2 were measured using a charge / discharge device, they were 114 mAh / g, 125 mAh / g, and 50 mAh / g in order. Was requested. Furthermore, the charging / discharging electrode wire in this measurement is shown in FIG.

  Thus, comparing the lithium ion secondary batteries of Examples 3 and 4 and Comparative Example 2, it can be seen that the initial capacity of the lithium ion secondary battery of the present invention has increased more than twice.

  FIG. 4 also shows that a charging plateau appears in the region of 3.8 V or less in Examples 3 and 4.

  Since the lithium ion secondary battery of the present invention has been subjected to aging treatment at a voltage corresponding to this low potential plateau, it is considered that SEI having good lithium ion conductivity was formed on the negative electrode side by aging treatment. It is done.

  Furthermore, regarding the positive electrode, during the aging treatment at a low voltage of the negative electrode, it is expected that the oxidative decomposition of the electrolytic solution is prevented by suppressing excessive polarization of the positive electrode.

[Measurement of capacity maintenance ratio]
The lithium ion secondary batteries obtained in Examples 3 and 4 and Comparative Example 2 were charged with a constant current and a constant voltage up to a current of 0.7 C and a voltage of 4.8 V using a charging / discharging device in 3 hours. A constant current discharge of 0.5 C was performed until the voltage dropped to 3 V (one cycle). This charging / discharging was repeated 50 cycles.

  In the meantime, the change of the charge / discharge capacity in the lithium ion secondary batteries of Examples 3 and 4 and Comparative Example 2 was monitored using a charge / discharge device, and the capacity retention rate with respect to the charge / discharge capacity after one cycle was determined. The result is shown in FIG.

  As shown in FIG. 5, in the lithium ion secondary battery according to Comparative Example 2, the capacity maintenance rate decreased to 20% or less in 50 cycles, whereas in the lithium ion secondary batteries of Examples 4 and 5, the capacity maintenance rate was reduced. Is improved to 50% or more.

V. Analysis of negative electrode SEI coating [XPS analysis]
The result of having analyzed the negative electrode in the lithium ion secondary battery obtained by Example 5 and the comparative example 3 by X-ray photoelectron spectroscopy (XPS: X-ray photoelectron spectroscopy) is shown in FIG.
In the analysis result of Example 5 in FIG. 6, Li · F, which is the SEI coating component derived from the electrolyte, is observed over almost the entire region in the negative electrode depth direction (0 nm to over 300 nm). Since almost no Si is detected, it can be seen that an SEI film having a sufficient film thickness is formed. This SEI coating is considered to have a thickness sufficient to suppress continuous reductive decomposition of the electrolyte.

  On the other hand, according to the analysis result of the negative electrode in Comparative Example 3 shown in FIG. 6, the ratio of Si constituting the negative electrode is large in almost all ranges in the depth direction of the negative electrode, and the SEI coating having a sufficient film thickness is formed. I understand that there is no.

  The present invention is not limited to the configurations and examples of the above-described embodiment, and various modifications are possible within the scope of the gist of the invention.

20, 30 Lithium ion secondary battery 21, 31 Positive electrode 21a, 31a Positive electrode mixture layer 21b, 31b Positive electrode collector 22, 32 Negative electrode 22a, 32a Negative electrode mixture layer 22b, 32b Negative electrode collector 23, 33 Separator 34 Electrode Unit 36, 37 lead

Claims (11)

A high-voltage positive electrode active material that requires a voltage application of 4.5 V or more as a charging voltage and a low-potential plateau forming material that forms a plateau at a potential of 3.8 V or less, or a low potential that has a plateau at a potential of 3.8 V or less A plateau-owned positive electrode active material,
A positive electrode material comprising:   The positive electrode material according to claim 1, wherein the high-voltage positive electrode active material is a solid solution or a spinel nickel manganese-based material.   The positive electrode material according to claim 1, wherein the low potential plateau forming material is a material in which lithium ions are pre-doped into a high voltage positive electrode active material. The positive electrode material according to claim 1, wherein the positive electrode active material having a low potential plateau is LiFePO ₄ . The positive electrode active material having a low potential plateau is a positive electrode active material pre-doped with lithium ions using a material pre-doped with lithium ions with respect to V ₂ O ₅ or MnO ₂ . The positive electrode material described.   6. The positive electrode material according to claim 3, wherein the material predoped with lithium ions is a lithium metal in a powder form or a foil form. A lithium ion electricity storage device comprising: a positive electrode comprising the positive electrode material according to claim 1; a negative electrode comprising a negative electrode active material capable of removing and inserting lithium ions; and a non-aqueous electrolyte.   The low potential plateau forming material or the positive electrode active material having a low potential plateau is contained in an amount corresponding to 0.1% by volume ≦ X ≦ 30% by volume of the discharge capacity (X) of the negative electrode. 8. The lithium ion electricity storage device according to 7.   The lithium ion storage device according to claim 7 or 8, wherein the negative electrode active material is at least one material selected from a carbon-based material, a silicon-based material, and a tin-based material. The non-aqueous electrolyte is, C ₁ -C ₁₀ lithium ion electric storage device according to any one of claims 7-9, characterized in that it comprises one or more alkyl carbonates, and C ₁ -C ₁₀ alkylene carbonate . A positive electrode comprising the positive electrode material according to claim 1;
A negative electrode containing a negative electrode active material capable of removing and inserting lithium ions;
A non-aqueous electrolyte,
A method for producing a lithium ion electricity storage device comprising:
During initial charging, a voltage of 3.8 V or less is applied, and the applied voltage is allowed to float for a predetermined time, thereby forming an SEI film on the negative electrode before the positive electrode potential reaches the oxidative decomposition potential of the non-aqueous electrolyte. A method for producing a lithium ion electricity storage device.

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