WO2015079546A1

WO2015079546A1 - Positive electrode for lithium ion secondary battery, and lithium ion secondary battery

Info

Publication number: WO2015079546A1
Application number: PCT/JP2013/082119
Authority: WO
Inventors: 小西　宏明; 小林　満; 豊隆湯浅; 所　久人; 秀一高野; 崇中林; 章軍司; 達哉遠山; 孝亮馮; 翔古月
Original assignee: 株式会社日立製作所
Priority date: 2013-11-29
Filing date: 2013-11-29
Publication date: 2015-06-04

Abstract

The present invention pertains to a positive electrode for a lithium ion secondary battery and to a lithium ion secondary battery containing the same, the present invention addressing the problem of providing a lithium ion secondary battery having high energy density and high output. The above-mentioned problem can be resolved in a positive electrode for a lithium ion secondary battery provided with a positive electrode binder layer containing a positive electrode material and a current collector having the positive electrode binder layer formed on the surface thereof by adopting a configuration in which the positive electrode material contains a first positive electrode active material made of a specific laminar solid solution and a specific second positive electrode active material that reacts at the potential at the end period of electric discharging of the layer solid solution, the positive electrode binder layer in a region in contact with the current collector having a greater proportion of the second positive electrode active material than the first positive electrode active material, and the surface of the positive electrode binder layer opposite to the surface in contact with the current collector having a greater proportion of the first positive electrode active material than the second positive electrode active material.

Description

Positive electrode for lithium ion secondary battery and lithium ion secondary battery

The present invention relates to a positive electrode for a lithium ion secondary battery and a lithium ion secondary battery including the same.

The problem with electric vehicles is that the energy density of the driving battery is low and the travel distance for one charge is short. Therefore, there is a demand for an inexpensive secondary battery with high energy density.

Lithium ion secondary batteries have a higher energy density per weight than secondary batteries such as nickel hydrogen batteries and lead batteries. Therefore, the application to an electric vehicle and a power storage system is expected. However, in order to meet the demand for electric vehicles, it is necessary to further increase the energy density. In order to realize high energy density, it is necessary to increase the energy density of the positive electrode and the negative electrode.

Patent Document 1, the discharge capacity is large, in order to obtain the initial charge-discharge efficiency nonaqueous electrolyte secondary battery excellent, is constituted by a general formula _{_{_{_{Li a Co x Ni y Mn z}}}} O 2 (a + x + y + z = 2), The molar ratio Li / Me (a / (x + y + z)) of Li to the total transition metal element Me is 1.25 to 1.40, and the molar ratio Co / Me (x / (x + y + z)) is 0.020 to 0 A positive electrode active material is described which is characterized by having a molar ratio of Mn / Me (z / (x + y + z)) of 0.625 to 0.719.

Patent Document 2 describes a composition formula Li _a Mn _b M _c O _z (M is more than Ni, Co, Al and F) in order to provide a battery having high capacity, high temperature storage performance, and excellent cycle performance. Lithium manganese-containing oxide represented by one or more elements selected from the group consisting of 0 ≦ a ≦ 2.5, 0 <b ≦ 1, 0 ≦ c ≦ 1, 2 ≦ z ≦ 3), and an olivine structure And a positive electrode containing an Fe-containing phosphorus compound.

JP 2012-151084 A JP, 2012-033507, A

Although the positive electrode active material of the composition shown by patent document 1 can obtain high energy density, since resistance is high, the subject that a high output is not obtained occurs. In particular, at low SOC (State of Charge) where the potential is low and the resistance increases, further improvement of the output is desired.

Also for the positive electrode material shown in Patent Document 2, it is desirable to improve the resistance at the end of discharge for further improvement of the output.

An object of the present invention is to provide a lithium ion secondary battery having a high energy density and a high output.

The positive electrode for a lithium ion secondary battery according to the present invention is a positive electrode including a positive electrode mixture layer containing a positive electrode material and a current collector having a positive electrode mixture layer formed on the surface, and the positive electrode material is Li And a lithium transition metal oxide containing a metal element, containing at least Ni and Mn as the metal element, and the atomic ratio of Li to the metal element is 1.15 <Li / metal element <1.5 A first positive electrode active material having an atomic ratio of Ni to Mn of 0.334 <Ni / Mn ≦ 1.0, and a composition formula LiFe _1-y M ′ _y PO ₄ (0 ≦ y ≦ 0.2, M 'Includes a second positive electrode active material represented by at least one element of Mn, Co, Ni, V, Mg, Mo, W, Al, Nb, Ti, and Cu), and is in contact with the current collector In the region of the positive electrode mixture layer, the ratio of the second positive electrode active material is higher than that of the first positive electrode active material. Surface of the surface in contact with the body opposite the positive electrode mixture layer is characterized by than the second cathode active material is the ratio of the first positive electrode active material higher.

According to the present invention, it is possible to provide a lithium ion secondary battery having a high energy density and a high output.

It is a figure which shows SOC of layered solid solution, and the relation of output. It is a graph which shows the discharge curve of layered solid solution. It is a figure which shows typically arrangement | positioning of the collector in a positive electrode, a 1st positive electrode active material, and a 2nd positive electrode active material. It is sectional drawing which shows the structure of a lithium ion secondary battery typically. It is a graph which shows the charging / discharging curve of Example 5 and Comparative Example 1.

<Positive material>
When using a lithium ion secondary battery for an electric vehicle, it is expected that the traveling distance per charge is long. In order to increase the travel distance per charge, the battery is required to have a high energy density and to be able to use the battery in a wider range of SOC.

A layered solid solution represented by Li ₂ MO ₃ -LiM′O ₂ is expected as a high energy density positive electrode active material. The layered solid solution can be expected to increase the energy density of the positive electrode because a high capacity can be obtained. The layered solid solution can also be represented by the composition formula Li _{1 + x} M _1-x O ₂ . Here, the layered solid solution is a lithium transition metal oxide having a rock salt type layered structure, and is a material containing an excess of Li relative to the transition metal and having a composition ratio of 50% or more of Mn in the transition metal. Indicates

A lithium ion secondary battery using a layered solid solution as a positive electrode active material can be expected to have a high energy density but has high resistance at the end of discharge. There is a problem that a sufficient output can not be obtained because the potential is also low at the end of discharge. Figure 1 shows the relationship between SOC and output of layered solid solution. In FIG. 1, the vertical axis is the power density, and the horizontal axis is the SOC. It can be seen that the output is low in the low SOC region. In a car-mounted lithium ion secondary battery requiring high output, a low output region can not be used, so the range of usable SOC is narrowed, and as a result, the battery capacity is reduced. Therefore, in order to improve the output, it is necessary to reduce the resistance at the end of discharge and to improve the potential.

As a result of intensive investigations by the inventors, it is possible to maintain high capacity and reduce resistance at the discharge end by using a positive electrode material containing a layered solid solution and an active material that reacts at the discharge end potential of the layer solid solution. I found it. As a result, a lithium ion secondary battery with high energy density and high output can be provided.

In the layered solid solution, a reaction in which a transition metal participates in redox occurs in the early stage of charging, and a redox reaction in which oxygen participates in the late stage of charging. On the other hand, at the beginning of the discharge, a reaction involving the transition metal occurs, and at the end of the discharge, a redox reaction involving the oxygen occurs. Reactions involving transition metals are at high potential while reactions involving oxygen are at low potential and high resistance. By reducing the atomic ratio of Li to the transition metal element in the layered solid solution, the reaction region of the transition metal can be increased, and the capacity is increased in the region where the potential is high.

FIG. 2 shows discharge curves of LiNi _0.35 Mn _0.45 O _{2 + δ} and Li _1.2 Ni _0.2 Mn _0.6 O _{2 + δ} . In FIG. 2, the discharge curve of LiNi _0.35 Mn _0.45 O _{2 + δ} is indicated by a solid line, and the discharge curve of Li _1.2 Ni _0.2 Mn _0.6 O _{2 + δ} is indicated by a dotted line. In the region where the potential of 3.3 V or more is high, both capacities are equal, but Li ₁ Ni _0.35 Mn _0.45 O _{2 + δ} has a higher potential than Li _1.2 Ni _0.2 Mn _0.6 O _{2 + δ} Thus, it can be seen that a high energy density can be obtained. Therefore, by reducing the atomic ratio of Li in the metal element in the layered solid solution, the capacity becomes high at a high potential.

In addition, the resistance can be reduced by mixing an active material that reacts at 2.5 to 3.5 V, which is the potential at the end of discharge of LiNi _0.35 Mn _0.45 O _{2 + δ} . The Fe-containing phosphorus compound having an olivine structure represented by LiFePO ₄ has a reaction potential of about 3.5 V, and has good electron conductivity by coating carbon. Therefore, the resistance at the end of the discharge can be reduced by mixing the layered solid solution having a small atomic ratio of Li to the transition metal element and the Fe-containing phosphorus compound having an olivine structure. As a result, the output can be improved and the range of usable SOC can be expanded.

In addition, even if it mixes the layered solid solution with low electric potential at the end of discharge as represented by the composition formula Li _1.2 Ni _0.2 Mn _0.6 O _{2 + δ} with the Fe-containing phosphorus compound having an olivine structure, the resistance at the discharge end Is not reduced. This is because the potential in the high resistance region at the end of discharge of the layered solid solution is lower than the reaction potential of the Fe-containing phosphorus compound having an olivine structure.

As a result of the above examination, the positive electrode material for a lithium ion secondary battery according to the present invention includes a first positive electrode active material and a second positive electrode active material. The first positive electrode active material has a composition formula Li _x Ni _a Mn _b M _c O _{2 + δ} (0.95 ≦ x <1.2, 0.2 <a ≦ 0.4, 0.4 ≦ b <0 .6, 0 ≦ c ≦ 0.02, a + b + c = 0.8, −1 ≦ δ ≦ 1). In the above composition formula, it is difficult to specify the composition ratio of oxygen. Therefore, the first positive electrode active material is made of a lithium transition metal oxide containing Li and a metal element, the metal element contains at least Ni and Mn, and the atomic ratio of Li, Ni, and Mn is It is characterized in that 1.15 <Li / metal element <1.5 and 0.334 <Ni / Mn ≦ 1 are satisfied.

The second positive electrode active material has a composition formula LiFe _1-y M ′ _y PO ₄ (0 ≦ y ≦ 0.2, where M ′ is Mn, Co, Ni, V, Mg, Mo, W, Al, Nb, And at least one element of Ti and Cu).

In the first positive electrode active material, when Li / (Ni + Mn) is less than 1.15, the amount of Li contributing to the reaction is reduced, and a high capacity can not be obtained. On the other hand, when it is larger than 1.5, the crystal lattice becomes unstable and the discharge capacity is reduced. When Ni / Mn is lower than 0.334, the discharge potential decreases, and a large difference occurs between the reaction end potential and the reaction potential of the second positive electrode active material, and the second positive electrode active material improves the resistance at the discharge end Can not. When Ni / Mn is larger than 1, almost no charge / discharge reaction involving oxygen occurs, and the capacity decreases.

In addition, the metal element may further contain an additive element M. However, the atomic ratio of Ni and Mn to the metal element is preferably 0.975 ≦ (Ni + Mn) / metal element ≦ 1.0. The additive element M is an additive or an impurity added within a range not affecting the present invention, and at least one element selected from Co, V, Mo, W, Zr, Nb, Ti, Cu, Al, Fe It is.

In order to maintain the high energy density and reduce the resistance at the end of discharge, the first positive electrode active material is prepared by the composition formula Li _x Ni _a Mn _b M _c O _{2 + δ} (0.95 ≦ x ≦ 1 It is preferable that the following conditions be satisfied: .1, 0.3 <a <0.4, 0.4 <b <0.5, 0 ≦ c ≦ 0.02, a + b + c = 0.8, −1 ≦ δ ≦ 1). That is, it is preferable to satisfy 1.15 <Li / metal element <1.4 and 0.334 <Ni / Mn <0.8.

In the second positive electrode active material, y represents the content ratio (the mass ratio of substance) of M ′. M ′ is an element to be added appropriately, and the addition amount thereof needs to be suppressed to the range of 0 ≦ y ≦ 0.2 so that the effect of the present invention is not suppressed. From the viewpoint of conductivity, the second positive electrode active material is preferably coated with a carbon material.

The content of the second positive electrode active material in the positive electrode material is preferably 2% by mass or more and 15% by mass or less. If the content of the second positive electrode active material exceeds 15% by mass, the proportion of the first positive electrode active material having a high energy density is reduced, and a high energy density can not be obtained.

Furthermore, in order to maintain high energy density and reduce resistance at the end of discharge, the content of the second positive electrode active material is preferably 10% by mass or less.

The positive electrode material according to the present invention can be produced by a method generally used in the technical field to which the present invention belongs.

The first positive electrode active material can be produced, for example, by mixing and firing compounds containing Li, Ni, and Mn in appropriate proportions. The composition of the positive electrode material can be appropriately adjusted by changing the ratio of the compounds to be mixed. Examples of the compound containing Li include lithium acetate, lithium nitrate, lithium carbonate, lithium hydroxide, lithium oxide and the like. Examples of the compound containing Ni include nickel acetate, nickel nitrate, nickel carbonate, nickel sulfate, nickel hydroxide and the like. As a compound containing Mn, manganese acetate, manganese nitrate, manganese carbonate, manganese sulfate, manganese oxide etc. can be mentioned, for example.

The second positive electrode active material is prepared by mixing compounds containing Li, Fe, and P in appropriate proportions, and then mixing with a carbon source such as polyvinyl alcohol to ensure conductivity, and firing. can do.

Examples of the compound containing Li include lithium acetate, lithium nitrate, lithium carbonate, lithium hydroxide, lithium oxide and the like. Examples of the compound containing Fe include iron oxalate, iron nitrate, iron sulfide and iron chloride. Examples of the compound containing P include ammonium dihydrogen phosphate, phosphoric acid and the like.

The composition of the positive electrode material can be determined, for example, by elemental analysis using inductively coupled plasma (ICP) or the like.

<Positive electrode>
The positive electrode for a lithium ion secondary battery according to the present invention comprises a positive electrode mixture layer containing the above-described positive electrode material and a current collector, and a positive electrode mixture layer is formed on the surface of the current collector. The ratio of the second positive electrode active material in the positive electrode mixture layer in the region in contact with the current collector is higher than that of the first positive electrode active material, and the surface of the positive electrode mixture layer opposite to the surface in contact with the current collector is And a ratio of the first positive electrode active material is higher than that of the second positive electrode active material. The output can react for about a few seconds. Therefore, it is preferable to dispose a second positive electrode active material having high electron conductivity in the vicinity of the current collector. By arranging the second positive electrode active material closer to the current collector than the first positive electrode active material, the second positive electrode active material can be brought into contact with the current collector, and the second positive electrode active material has high electron conductivity. Can contribute to the flow of electrons. On the other hand, when the first positive electrode active material is disposed closer to the current collector than the second positive electrode active material, the second positive electrode active material can not be in direct contact with the current collector, and the first positive electrode has high resistance. Since the contact is made via the active material, the performance of the second positive electrode active material can not be sufficiently exhibited.

FIG. 3 is a view schematically showing the arrangement of the current collector, the first positive electrode active material, and the second positive electrode active material in the positive electrode. In FIG. 3, 1 is a current collector, 2 is a mixture layer containing a second positive electrode active material, 3 is a mixture layer containing a first positive electrode active material. When manufacturing the positive electrode, the second positive electrode active material is collected by applying a slurry containing the second positive electrode active material to the current collector and applying a slurry containing the first positive electrode active material thereon. A large amount of the first positive electrode active material can be distributed on the side opposite to the current collector. Note that there is no limitation on the method of manufacturing the positive electrode, and in the positive electrode mixture layer, the proportion of the second positive electrode active material is higher in the vicinity of the current collector than the first positive electrode active material, The proportion of the first positive electrode active material may be higher on the side than the second positive electrode active material.

<Lithium ion secondary battery>
A lithium ion secondary battery according to the present invention includes the above-described positive electrode. By using the above positive electrode, it is possible to provide a lithium ion secondary battery having a high energy density and a high output. Furthermore, since the resistance at the end of discharge can be reduced, the range of usable SOC is expanded. Therefore, the lithium ion secondary battery according to the present invention can be preferably used for an electric vehicle.

The positive electrode active material occludes and releases lithium ions by charge and discharge. Since not all lithium ions released from the positive electrode active material return to the positive electrode, the composition of the positive electrode active material after charge and discharge is expected to be different from that before charge and discharge. For example, the positive electrode active material of a layered oxide represented by Li _0.8 M _0.8 O _1.6 has a composition ratio of Li in a full discharge state (2.5 V) when used in a potential range of 2.5 to 4.3 V Is known to be about 0.75. In the same way as in the case of the layered oxide, the amount of Li after charge and discharge of the layered solid solution is also estimated to be reduced by about 15 to 30% in the fully discharged state as compared to that before the charge and discharge. Therefore, when a lithium secondary battery is manufactured using the positive electrode active material according to the present invention and charged and discharged at 4.6 to 2.5 V, the composition of the layered solid solution which is the first positive electrode active material in the full discharge state Is Li _x Ni _a Mn _b M _c O _{2+ δ} (0.75 ≦ x ≦ 1.2, 0.2 ≦ a ≦ 0.4, 0.4 ≦ b ≦ 0.8, 0 <c ≦ 0 It can be assumed that .02, a + b + c = 0.8, -1 ≦ δ ≦ 1). That is, the atomic ratio of Li to the metal element of the first positive electrode active material after charge and discharge is 0.90 <Li / metal element <1.5.

The lithium ion secondary battery is composed of a positive electrode containing a positive electrode material, a negative electrode containing a negative electrode material, a separator, an electrolytic solution, an electrolyte and the like.

The negative electrode material is not particularly limited as long as it is a substance capable of inserting and extracting lithium ions. Materials generally used in lithium ion secondary batteries can be used as the negative electrode material. For example, graphite, lithium alloy and the like can be exemplified.

As a separator, those generally used in lithium ion secondary batteries can be used. For example, a microporous film or non-woven fabric made of polyolefin such as polypropylene, polyethylene, and a copolymer of propylene and ethylene can be exemplified.

As the electrolytic solution and the electrolyte, those generally used in lithium ion secondary batteries can be used. For example, as the electrolytic solution, diethyl carbonate, dimethyl carbonate, ethylene carbonate, propylene carbonate, vinylene carbonate, methyl acetate, ethyl methyl carbonate, methyl propyl carbonate, dimethoxyethane and the like can be exemplified. Further, as the _{_{electrolyte, LiClO 4, LiPF 6, LiBF}} 4, LiAsF 6, LiSbF 6, LiCF 3 SO 3, LiC 4 F 9 SO 3, LiCF 3 CO 2, Li 2 C 2 F 4 (SO 3) 2, LiN _{_{_{(CF 3 SO 2) 2,}}} LiC (CF 3 SO 2) can be exemplified ₃ or the like.

One embodiment of the structure of a lithium ion secondary battery according to the present invention will be described with reference to FIG. The lithium ion secondary battery 4 includes an electrode group including a positive electrode 5 having a positive electrode material coated on both sides of a current collector, a negative electrode 6 having a negative electrode material coated on both sides of the current collector, and a separator 7. The positive electrode 5 and the negative electrode 6 are wound via the separator 7 to form a wound electrode group. The wound body is inserted into the battery can 8.

The negative electrode 6 is electrically connected to the battery can 8 via the negative electrode lead piece 10. A sealing lid 11 is attached to the battery can 8 via a packing 12. The positive electrode 5 is electrically connected to the sealing lid 11 through the positive electrode lead piece 9. The wound body is insulated by the insulating plate 13.

The electrode group may not be a wound body shown in FIG. 4, and may be a laminate in which the positive electrode 5 and the negative electrode 6 are stacked via the separator 7.

Hereinafter, the present invention will be described in more detail using Examples and Comparative Examples, but the technical scope of the present invention is not limited thereto.

<Production of positive electrode material>
The first positive electrode active material was produced by the following method. Lithium carbonate, nickel carbonate and manganese carbonate were mixed in a ball mill to obtain a precursor. The obtained precursor was calcined at 500 ° C. for 12 hours in the air to obtain a lithium transition metal oxide. The obtained lithium transition metal oxide was pelletized and then fired at 850 to 1050 ° C. for 12 hours in the air. The fired pellet was ground in an agate mortar and classified with a 45 μm sieve to obtain a first positive electrode active material represented by the composition formula Li _x Ni _a Mn _b O _{2 + δ} .

The second positive electrode active material was produced by the following method. Lithium carbonate, iron oxalate, and ammonium dihydrogen phosphate were mixed in a ball mill to obtain a precursor. The obtained precursor was calcined at 300 ° C. for 8 hours in argon. Then, after mixing the material after temporary baking with a polyvinyl alcohol by a ball mill, the main baking was performed at 700 ° C. for 8 hours in argon. By this firing, a material in which the positive electrode active material represented by the composition formula LiFePO ₄ was covered with the carbon material was obtained. This was used as a second positive electrode active material.

<Fabrication of positive electrode>
The positive electrodes of Examples 1 to 10 and Comparative Examples 1 to 7 were produced using the positive electrode active material produced as described above.

In Examples 1 to 10 and Comparative Examples 2 to 4 and 6, first, the second positive electrode active material, the conductive agent, and the binder were uniformly mixed to prepare a slurry containing the second positive electrode active material. The slurry containing the second positive electrode active material was applied onto a 20 μm thick aluminum current collector foil and dried at 120 ° C. Thereafter, the first positive electrode active material, the conductive agent, and the binder were uniformly mixed to prepare a slurry containing the first positive electrode active material. The slurry containing the first positive electrode active material was applied over the second positive electrode active material and dried at 120 ° C. After drying, compression molding was performed with a press so that the electrode density was 2.2 g / cm ³ , to obtain an electrode plate. Thereafter, the electrode plate was punched into a disk shape having a diameter of 15 mm to produce a positive electrode.

In Comparative Examples 1 and 5, the first positive electrode active material, the conductive agent, and the binder were uniformly mixed to prepare a positive electrode slurry containing the first positive electrode active material. The positive electrode slurry was applied onto a 20 μm thick aluminum current collector foil and dried at 120 ° C. After drying, compression molding was performed with a press so that the electrode density was 2.2 g / cm ³ , to obtain an electrode plate. Thereafter, the electrode plate was punched into a disk shape having a diameter of 15 mm to produce a positive electrode.

In Comparative Example 7, first, the first positive electrode active material, the conductive agent, and the binder were uniformly mixed to prepare a slurry containing the first positive electrode active material. The slurry containing the first positive electrode active material was applied onto a 20 μm thick aluminum current collector foil and dried at 120 ° C. Thereafter, the second positive electrode active material, the conductive agent, and the binder were uniformly mixed to prepare a slurry containing the second positive electrode active material. The slurry containing the second positive electrode active material was applied over the first positive electrode active material and dried at 120 ° C. After drying, compression molding was performed with a press so that the electrode density was 2.2 g / cm ³ , to obtain an electrode plate. Thereafter, the electrode plate was punched into a disk shape having a diameter of 15 mm to produce a positive electrode.

The composition of the first positive electrode active material and the content of the second positive electrode active material used in the positive electrode of each of Examples 1 to 10 and Comparative Examples 1 to 7 are shown in Table 1.

<Creating a trial battery>
Prototype batteries were produced using the positive electrodes of Examples 1 to 10 and Comparative Examples 1 to 7.

The negative electrode was produced using metallic lithium. As a non-aqueous electrolytic solution, one in which LiPF ₆ was dissolved at a concentration of 1.0 mol / L in a mixed solvent of ethylene carbonate and dimethyl carbonate at a volume ratio of 1: 2 was used.

<Charge / discharge test>
The charge / discharge test was performed on the 17 types of trial-produced batteries manufactured as described above, using the positive electrode of each example and comparative example. For the prototype battery, charge and discharge tests were performed with a current equivalent to 0.05C and an upper limit voltage of 4.6V at a current equivalent to 0.05C and a current equivalent to 0.5C and a lower limit voltage of 2.5V, and the discharge capacity at the second cycle As the rated capacity. Table 1 shows discharge capacities in the range of 4.6 to 3.3 V at which high output can be obtained in each of the examples and the comparative examples.

<Measurement of DC resistance>
Using the positive electrode of each of the examples and the comparative examples, the direct current resistance at an SOC of 20% was determined for the 17 types of trial batteries produced as described above.

For the prototype battery, charge and discharge tests were performed with a current of 0.05C equivalent and an upper limit voltage of 4.6V, and a discharge of 0.05C equivalent current and a lower limit voltage of 2.5V, and the discharge capacity at the second cycle As the rated capacity. The prototype battery after 2 cycles was charged to 4.6 V and then discharged to a capacity of 80% of the rated capacity. Thereafter, a current of 1.2 mA was applied for 10 seconds to measure the direct current resistance. The value obtained by dividing the potential difference before and after applying the current for 10 seconds by the applied current value (1.2 mA) was defined as the DC resistance value. The values of DC resistance are shown in Table 1.

FIG. 5 shows discharge curves of Example 5 and Comparative Example 1 at 4.6 to 3.3V. In FIG. 5, the discharge curve of Example 5 is indicated by a solid line, and the discharge curve of Comparative Example 1 is indicated by a dotted line. In the region of 4.6 to 3.3 V, Example 5 has a higher capacity than Comparative Example 1. Further, Example 5 has a higher potential than the Comparative Example 1 when the discharge capacity is the same. Also in Examples 1 to 4 and 6 to 10, the same discharge curves as in Example 5 were obtained. Accordingly, it was found that in Examples 1 to 10, high discharge capacity was obtained in the region where the potential was high.

As shown in Table 2, in Examples 1 to 10, the discharge capacity in the region of 4.6 to 3.3 V is as high as 140 Ah / kg or more, and the direct current resistance is as low as 40 Ω or less. On the other hand, Comparative Example 1 has a smaller discharge capacity and higher DC resistance than the example. In the positive electrode material of Comparative Example 1, the composition of the first positive electrode active material does not satisfy the relationship of 1.15 <Li / (Ni + Mn) <1.5 and 0.334 <Ni / Mn ≦ 1, 3.3 V The above discharge capacity is small, and the DC resistance is high because the second positive electrode active material is not included.

Further, in Comparative Examples 2 to 6, it was not possible to simultaneously achieve high capacity and low resistance. In Comparative Example 2, the positive electrode containing the first positive electrode active material and the Fe-containing phosphorus compound having an olivine structure is mixed. However, since the composition of the first positive electrode active material is Li / (Ni + Mn) = 1.50 and Ni / Mn = 0.33, the first positive electrode active material already has a high resistance at the end of discharge. The reaction of the Fe-containing phosphorus compound having an olivine structure, which is the second positive electrode active material, is completed. Therefore, since the first positive electrode active material having high resistance reacts at the end of discharge, the resistance at the end of discharge can not be improved. Therefore, in Comparative Example 2, the resistance of the second positive electrode active material was not completed before reaching the SOC of 20% in the discharge process, so that the resistance could not be reduced.

In Comparative Example 3, since the Ni / Mn of the first positive electrode active material was as low as 0.33, a high capacity was not obtained in the region of 3.3 V or more.

In Comparative Example 4, a high capacity was not obtained because Li / (Ni + Mn) was as large as 1.50.

In Comparative Example 5, since the second positive electrode active material is not contained, the resistance is high.

In Comparative Example 6, the content of the second positive electrode active material was large, and the content of the second positive electrode active material capable of obtaining a high capacity was small, so a high capacity was not obtained.

In Comparative Example 7, since the second positive electrode active material was disposed on the side far from the current collector, the resistance could not be sufficiently reduced.

From the above, in Examples 1 to 10, the first positive electrode in which the atomic weight ratio of Li, Ni, and Mn satisfies the relationships 1.15 <Li / (Ni + Mn) <1.5 and 0.334 <Ni / Mn ≦ 1 is satisfied. Active material and composition formula LiFe _1-y M ′ _y PO ₄ (0 ≦ y ≦ 0.2, M ′ is at least any of Mn, Co, Ni, V, Mg, Mo, W, Al, Nb, Ti, and Cu Higher discharge capacity because the second positive electrode active material is contained in the vicinity of the current collector and the second positive electrode active material is disposed in a larger amount than the first positive electrode active material. The resistance at the end of discharge is small. As a result, a lithium ion secondary battery with high energy density and high output can be provided.

In addition, in Examples 1 to 5 and 7 to 9, the discharge capacity is particularly high. This is because the composition of the first positive electrode active material satisfies 0.334 <Ni / Mn <0.8 and the content of the second positive electrode active material is 10% by mass or less.

DESCRIPTION OF SYMBOLS 1 current collector 2 second positive electrode active material layer 3 first positive electrode active material layer 4 lithium ion secondary battery 5 positive electrode 6 negative electrode 7 separator 8 battery can 9 positive electrode lead piece 10 negative electrode lead piece 11 sealing lid 12 packing 13 insulation Board

Claims

A positive electrode for a lithium ion battery, comprising: a positive electrode mixture layer containing a positive electrode material; and a current collector having the positive electrode mixture layer formed on the surface,
The positive electrode material includes a first positive electrode active material and a second positive electrode active material,
The first positive electrode active material is made of a lithium transition metal oxide containing Li and a metal element, contains at least Ni and Mn as the metal element, and the atomic ratio of Li to the metal element is And 1.15 <Li / metal element <1.5, and the atomic ratio of the Ni to the Mn is 0.334 <Ni / Mn ≦ 1.0,
The second positive electrode active material has a composition formula LiFe 1-y M ′ y PO 4 (0 ≦ y ≦ 0.2, where M ′ is Mn, Co, Ni, V, Mg, Mo, W, Al, Nb, And at least one element of Ti and Cu),
The ratio of the second positive electrode active material in the positive electrode mixture layer in the region in contact with the current collector is higher than that of the first positive electrode active material,
The surface of the positive electrode mixture layer opposite to the surface in contact with the current collector has a ratio of the first positive electrode active material higher than that of the second positive electrode active material. For positive electrode.
It is a positive electrode for lithium ion secondary batteries of Claim 1, Comprising:
The first positive electrode active material contains an additive element M as the metal element, and the M is at least one element selected from Co, Al, V, Mo, W, Zr, Nb, Ti, and Fe. And the atomic ratio of the Ni and the Mn to the metal element is 0.975 ≦ (Ni + Mn) / metal element ≦ 1.0.
It is a positive electrode for lithium ion secondary batteries of Claim 1, Comprising:
Content of the said 2nd positive electrode active material with respect to the said positive electrode material is 2 mass% or more and 15 mass% or less, The positive electrode for lithium ion secondary batteries characterized by the above-mentioned.
It is a positive electrode for lithium ion secondary batteries of Claim 3, Comprising: Content of said 2nd positive electrode active material 2 with respect to the said positive electrode material is 2 mass% or more and 10 mass% or less, The lithium ion characterized by the above-mentioned Positive electrode for secondary battery.
It is a positive electrode for lithium ion secondary batteries of Claim 1, Comprising:
An atomic ratio of the Ni to the Mn of the first positive electrode active material is 0.334 <Ni / Mn <0.8. A positive electrode for a lithium ion secondary battery.
It is a positive electrode for lithium ion secondary batteries of Claim 1, Comprising:
The first positive electrode active material has a composition formula Li x Ni a Mn b M c O 2 + δ (0.95 ≦ x <1.2, 0.2 <a ≦ 0.4, 0.4 ≦ b < A positive electrode for a lithium ion secondary battery characterized by being represented by 0.6, 0 ≦ c ≦ 0.02, a + b + c = 0.8, −1 ≦ δ ≦ 1).
It is a positive electrode for lithium ion secondary batteries of Claim 6, Comprising:
The first positive electrode active material has a composition formula Li x Ni a Mn b M c O 2 + δ (0.95 ≦ x ≦ 1.1, 0.3 <a <0.4, 0.4 <b < 0.5, 0 ≦ c ≦ 0.02, a + b + c = 0.8, −1 ≦ δ ≦ 1) A positive electrode for a lithium ion secondary battery.
It is a manufacturing method of the positive electrode for lithium ion secondary batteries in any one of Claims 1 thru | or 7, Comprising:
Applying a slurry containing the second positive electrode active material to the current collector;
Applying the slurry containing the first positive electrode active material onto the slurry containing the second positive electrode active material; and a method of manufacturing a positive electrode for a lithium ion secondary battery.
A lithium ion secondary battery comprising a positive electrode comprising a positive electrode mixture layer containing a positive electrode material, a positive electrode current collector having the positive electrode mixture layer on the surface, and a negative electrode comprising a negative electrode material,
The positive electrode material includes a first positive electrode active material and a second positive electrode active material,
The first positive electrode active material is made of a lithium transition metal oxide containing Li and a metal element, contains at least Ni and Mn as the metal element, and the atomic ratio of Li to the metal element is 0.90 <Li / metal element <1.5, and the atomic ratio of the Ni to the Mn is 0.334 <Ni / Mn ≦ 1.0,
The second positive electrode active material has a composition formula LiFe 1-y M ′ y PO 4 (0 ≦ y ≦ 0.2, where M ′ is Mn, Co, Ni, V, Mg, Mo, W, Al, Nb, And at least one element of Ti and Cu),
The ratio of the second positive electrode active material in the positive electrode mixture layer in the region in contact with the current collector is higher than that of the first positive electrode active material,
The surface of the positive electrode mixture layer opposite to the surface in contact with the current collector has a ratio of the first positive electrode active material higher than that of the second positive electrode active material. .
It is a positive electrode active material for a lithium ion secondary battery according to claim 9,
The first positive electrode active material contains an additive element M as the metal element, and the M is at least one element selected from Co, Al, V, Mo, W, Zr, Nb, Ti, and Fe. The atomic ratio of the Ni and the Mn to the metal element is 0975 ≦ (Ni + Mn) / metal element ≦ 1.0.
It is a lithium ion secondary battery according to claim 9,
Content of said 2nd positive electrode active material is 2 mass% or more and 15 mass% or less with respect to said positive electrode material, The lithium ion secondary battery characterized by the above-mentioned.
12. The lithium ion secondary battery according to claim 11, wherein
Content of said 2nd positive electrode active material is 2 mass% or more and 10 mass% or less with respect to said positive electrode material, The lithium ion secondary battery characterized by the above-mentioned.
It is a lithium ion secondary battery according to claim 9,
The atomic ratio of Ni to Mn of the first positive electrode active material is 0.334 <Ni / Mn <0.8.
It is a lithium ion secondary battery according to claim 9,
The first positive electrode active material has a composition formula Li x Ni a Mn b M c O 2+ δ (0.75 ≦ x <1.2, 0.2 <a ≦ 0.4, 0.4 ≦ b < A lithium ion secondary battery characterized by being represented by 0.6, 0 ≦ c ≦ 0.02, a + b + c = 0.8, −1 ≦ δ ≦ 1).
It is a lithium ion secondary battery according to claim 14,
The first positive electrode active material has a composition formula Li x Ni a Mn b M c O 2+ δ (0.75 ≦ x ≦ 1.1, 0.3 <a <0.4, 0.4 <b < A lithium ion secondary battery characterized by being represented by 0.5, 0 ≦ c ≦ 0.02, a + b + c = 0.8, −1 ≦ δ ≦ 1).
A lithium ion secondary battery according to any one of claims 9 to 15, wherein
The lower limit electric potential at the time of using it is 3.0 V or more, The lithium ion secondary battery characterized by the above-mentioned.