JP2012059527A - Positive electrode for lithium ion battery and lithium ion battery using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode for a lithium ion battery which has high durability and improved capacity density per unit volume.SOLUTION: A positive electrode for a lithium ion battery includes a positive electrode material which is formed by mixing a solid solution which comprises Li[LiMn]Oand LiM1O, (where M1 is at least one transition metals), and has total metal valence of 4, with a secondary active material represented by LiM2O, (where M2 is at least one transition metals having a total valence of 3), as a positive electrode active material.

Description

  The present invention relates to a positive electrode for a lithium ion battery and a lithium ion battery using the same. More specifically, the present invention relates to an improvement for improving capacity density per volume while maintaining high durability of the battery.

  In recent years, in order to cope with global warming, reduction of the amount of carbon dioxide is eagerly desired. In the automobile industry, there is a great expectation for the reduction of carbon dioxide emissions by the introduction of electric vehicles and hybrid electric vehicles, and motor drive batteries that hold the key to commercialization of these are actively being developed.

  As a battery for driving a motor, a lithium ion battery having a relatively high theoretical energy is attracting attention, and is currently being developed rapidly. In general, a lithium ion battery includes a positive electrode in which a positive electrode active material or the like is applied to both surfaces of a positive electrode current collector using a binder, and a negative electrode in which a negative electrode active material or the like is applied to both surfaces of the negative electrode current collector using a binder. It is connected via an electrolyte layer and is configured to be stored in a battery case.

  In order for electric vehicles equipped with such a lithium ion battery to be widely used, it is necessary to make the lithium ion battery have high performance. In particular, for electric vehicles, the travel distance per charge needs to be close to the travel distance per refueling of a gasoline engine vehicle, and a battery with higher energy density is desired. In order to make the battery have a high energy density, it is necessary to increase the electric capacity per unit mass of the positive electrode and the negative electrode.

A lithium manganese composite oxide having a layered structure has been proposed as a positive electrode material that can meet this demand. Among them, the solid solution of the electrochemically inactive layered Li ₂ MnO ₃ and the electrochemically active layered LiMO ₂ (where M is a transition metal such as Co, Mn, Ni) is 200 mAh / g. It is expected as a candidate for a high-capacity positive electrode material capable of exhibiting a large electric capacity that exceeds. For example, in Patent Document 1 below, xLi [Mn _1/2 Ni _1/2 ] O ₂ .yLiCoO ₂ .zLi [Li _1/3 Mn _2/3 ] O ₂ (x + y + z = 1, which is a pseudo ternary solid solution, 0 <x <1, 0 ≦ y <0.5, 0 <z <1) is disclosed.

JP 2007-287445 A

  The pseudo ternary solid solution described above has a high capacity density per weight and high durability when used as a positive electrode active material. On the other hand, since the packing density (tap density) is low, there is a problem that the capacity density per volume is low.

  Then, an object of this invention is to provide the positive electrode for lithium ion batteries with which the capacity density per volume was improved, maintaining high durability.

The present inventors have intensively studied to solve the above problems. As a result, as the positive electrode active _material, a solid solution of Li 2 MnO ₃ and LiM1O _2, by mixing LiM2O ₂ is a high subsidiary active substance filling density, found that the above problems can be solved, the present invention Completed.

That is, the present invention is a solid solution of _{Li [Li 1/3 Mn 2/3] O} 2 and LiM1O ₂ (M1 is one or more transition metals), the sum of the metal valence is 4, solid solutions, LiM2O 2 _(M2 is one or more transition metals total valence is 3) comprising a positive electrode material and subsidiary active substance represented by is formed by mixing a positive electrode active material, a lithium ion battery This is a positive electrode for use.

  According to the positive electrode for a lithium ion battery of the present invention, the tap density is improved by mixing the secondary active material into the solid solution. Moreover, even when the secondary active material is mixed, high durability of the solid solution is maintained. For this reason, the capacity density per volume can be improved while maintaining the high durability of the battery.

1 is a schematic diagram showing a basic configuration of a flat (stacked) non-bipolar lithium ion secondary battery, which is a representative embodiment of the present invention. Li is a schematic diagram showing the relationship between the crystal structure of _{[Li 1/3} Mn _{2/3] O} 2 with (a) LiM1O ₂ and (b). 1 is a perspective view schematically showing an appearance of a stacked battery which is an embodiment of the present invention. Is a diagram representing the composition of _{Li 2 MnO} 3 -LiM1O 2 based solid solution used in Example. About the battery using the positive electrode obtained by the Example and the comparative example, content (mass%) of the secondary active material with respect to the total mass of a positive electrode active material, and the capacity density (volume capacity density) per volume of a positive electrode active material It is a graph which shows the relationship. It is a graph which shows the relationship between content (mass%) of the subactive material with respect to the total mass of a positive electrode active material, and a capacity | capacitance retention rate about the battery using the positive electrode obtained by the Example and the comparative example. It is a graph which shows the relationship between the capacity retention of the battery using the positive electrode obtained by the Example and the comparative example, and the capacity density per volume.

One exemplary embodiment of the present invention is a solid solution of Li [Li _1/3 Mn _2/3 ] O ₂ and LiM 1 O ₂ (M 1 is one or more transition metals) having a metal valence of A positive electrode material obtained by mixing a solid solution having a total of 4 and a secondary active material represented by LiM ₂ O ₂ (M ₂ is one or more transition metals having a total valence of 3) A positive electrode for a lithium ion battery.

General _{formula: ALI} represented by _{[Li 1/3 Mn 2/3] O 2} · (1-a) LiM1O 2 (0 <a <1), cathode material of the so-called solid solution system is expected as a high-capacity material Yes. In the formula, M1 is one or more transition metals, and is selected so that the total valence of all the metals contained in the solid solution is 4. That is, it is selected so that the total valence of all atoms except oxygen is 4. Here, for example, considering Li (I) [Li (I) _1/3 Mn (IV) _2/3 ] O ₂ , the total metal valence is 4, and the valence of oxygen (−2) × Consistent with 2. Further, for example, if the total metal valence is 4 in LiM1O ₂ , it matches the oxygen valence (−2) × 2. _Thus, ALI in solid solution represented by _{[Li 1/3 Mn 2/3] O 2} · (1-a) LiM1O 2 (0 <a <1), the valence sum of all the metal contained in the solid solution If it is 4, it becomes consistent with the valence of oxygen. Preferably, M1 is one or more transition metals having a total valence of 3, for example, Mn, Ni, Co, Al, Cu, Ti, Fe, etc. may be preferably used. The “total metal valence” indicates an average oxidation state of the constituent metals, and is calculated from the molar amount and valence of the constituent metals. For example, when composed of 50% Ni (II) and 50% Mn (IV) on a molar basis, the total metal valence is (0.5) · (+2) + (0.5) (+4) = + 3

Specifically, as described in Patent Document 1, Li ₂ MnO ₃ , Li [Ni _0.5 Mn _0.5 ] O ₂ , Li [Ni _1/3 Co _1/3 Mn _1/3 Investigations have been made on solid solutions between O ₂ , LiCoO ₂ and the like. Note that since Li [Li _1/3 Mn _2/3 ] O ₂ can also be expressed as Li ₂ MnO ₃ , in this specification, the general formula: aLi [Li _1/3 Mn _2/3 ] O _2. the solid solution represented by a) LiM1O _2, also referred to as Li ₂ MnO 3 -LiM1O ₂ based solid solution.

  Such a solid solution positive electrode material has a high capacity density per weight and high durability, but has a problem that the capacity density per volume is poor because the packing density (tap density) is low.

In contrast, in the positive electrode of the present _embodiment, Li in ₂ MnO 3 -LiM1O ₂ based solid solution, which comprises using a mixed cathode material having a high packing density LiM2O ₂ as subsidiary active material as a positive electrode active material. By using such a secondary active material, the packing density can be improved and the capacity density per volume can be improved. Therefore, in a lithium ion battery, a high capacity and high durability battery can be obtained by using the positive electrode for a lithium ion battery of the present embodiment.

LiM2O 2 of electrochemically active layers _(M2 is one or more transition metals total valence is 3) has a high packing density. However in the case of using only LiM2O ₂ as a positive electrode material, charged to a high potential, an excessive withdrawal of the lithium from the crystal structure, durability has been known to be significantly reduced. Therefore, when mixed with the solid solution LiM2O ₂ as subsidiary active material, the durability as the content increases the subsidiary active substance is expected to linearly decrease. However, in the positive electrode of the present embodiment, such a linear decrease in durability is not observed, and it is clear that a positive electrode having an improved capacity density per volume while maintaining high durability can be obtained. became.

One of the reasons why high durability can be obtained with the positive electrode is that the Li ₂ MnO ₃ —LiM ₁ O ₂ -based solid solution easily reacts in a high potential region, so that the electrochemical load on LiM ₂ O ₂ can be reduced. Conceivable. However, it is not necessarily limited to the form in which the positive electrode characteristics are improved by such a mechanism.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. However, the present invention is not limited only to the following embodiments. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted. The dimensional ratios in the drawings are exaggerated for convenience of explanation, and may differ from actual ratios.

  First, a basic configuration of a lithium battery to which the positive electrode material of the present embodiment can be applied will be described with reference to the drawings.

[Battery overall structure]
In this invention, a lithium ion battery should just be formed using the positive electrode for lithium ion batteries of this embodiment, and it does not restrict | limit in particular regarding other structural requirements.

  For example, when the lithium ion battery is distinguished by its form / structure, it can be applied to any conventionally known form / structure, such as a stacked (flat) battery or a wound (cylindrical) battery. is there. By adopting a stacked (flat) battery structure, long-term reliability can be secured by a sealing technique such as simple thermocompression bonding, which is advantageous in terms of cost and workability.

  Moreover, when viewed in terms of electrical connection form (electrode structure) in a lithium ion battery, it can be applied to both non-bipolar (internal parallel connection type) batteries and bipolar (internal series connection type) batteries. is there. Here, in a non-bipolar battery, a positive electrode active material, a negative electrode active material, or the like is applied to a positive electrode current collector or a negative electrode current collector using a binder or the like to constitute an electrode (positive electrode or negative electrode). In the case of a bipolar battery, a positive electrode active material or the like is applied to one surface of the current collector, and a positive electrode active material layer is applied to the opposite surface, and a negative electrode active material layer is stacked. Thus, a bipolar electrode is formed.

  In the following description, as a representative embodiment, a case of a non-bipolar (internal parallel connection type) lithium ion secondary battery using a positive electrode material for a lithium ion battery will be described as an example. However, the technical scope of the present invention is not limited to the following forms.

  FIG. 1 is a schematic diagram showing a basic configuration of a flat (stacked) non-bipolar lithium ion secondary battery (hereinafter also simply referred to as “stacked battery”) according to an embodiment of the present invention. As shown in FIG. 1, the stacked battery 10 of the present embodiment has a structure in which a substantially rectangular power generation element 21 in which a charge / discharge reaction actually proceeds is sealed inside a laminate sheet 29 that is an exterior body. . Here, in the power generation element 21, the negative electrode in which the negative electrode active material layer 13 is disposed on both surfaces of the negative electrode current collector 11, the electrolyte layer 17, and the positive electrode active material layer 15 are disposed on both surfaces of the positive electrode current collector 12. It has a configuration in which a positive electrode is laminated. Specifically, the negative electrode, the electrolyte layer, and the positive electrode are stacked in this order so that one negative electrode active material layer 13 and the positive electrode active material layer 15 adjacent thereto face each other with the electrolyte layer 17 therebetween. .

  Thereby, the adjacent negative electrode, electrolyte layer, and positive electrode constitute one unit cell layer 19. Therefore, it can be said that the stacked battery 10 of the present embodiment has a configuration in which a plurality of the single battery layers 19 are stacked and electrically connected in parallel. In addition, although the negative electrode active material layer 13 is arrange | positioned only in the single side | surface at all the outermost layer negative electrode collectors located in both outermost layers of the electric power generation element 21, an active material layer may be provided in both surfaces. That is, instead of using a current collector dedicated to the outermost layer provided with an active material layer only on one side, a current collector having an active material layer on both sides may be used as it is as an outermost current collector. Further, the arrangement of the positive electrode and the negative electrode is reversed from that in FIG. 1 so that the outermost positive electrode current collector is positioned on both outermost layers of the power generation element 21, and the outermost positive electrode current collector is disposed on one or both surfaces of the outermost layer positive electrode current collector. A positive electrode active material layer may be disposed.

  The negative electrode current collector 11 and the positive electrode current collector 12 are attached to a negative electrode current collector plate 25 and a positive electrode current collector plate 27 that are electrically connected to the respective electrodes (negative electrode and positive electrode), and are sandwiched between end portions of the laminate sheet 29. Thus, it has a structure led out of the laminate sheet 29. The negative electrode current collector plate 25 and the positive electrode current collector plate 27 are ultrasonically welded to the negative electrode current collector 11 and the positive electrode current collector 12 of each electrode via a negative electrode lead and a positive electrode lead (not shown), respectively, as necessary. Or resistance welding or the like.

  Hereinafter, members constituting the battery of this embodiment will be described in detail.

(Positive electrode for lithium ion battery)
In the present embodiment, the general formula (1):
aLi [Li _1/3 Mn _2/3 ] O _2. (1-a) LiM 1 O ₂ (1)
(Wherein M1 is one or more transition metals, 0 <a <1, and the total metal valence of the solid solution is 4)
It used a solid solution represented in the LiM2O 2 _(M2 total valence is one or more transition metals is 3) positive electrode material and subsidiary active substance represented by is formed by mixing a positive electrode active material It is characterized by that.

The above Li ₂ MnO ₃ —LiM ₁ O ₂ solid solution has a space group of C2 / m (monoclinic structure) at least in the initial state. Such a structure can be confirmed from electron diffraction or X-ray diffraction (broad peak appearing at 20-25 ° at 2θ) of the active material.

FIG. 2 is a schematic diagram showing the relationship between the crystal structures of Li [Li _1/3 Mn _2/3 ] O ₂ (a) and LiM 1 O ₂ (b). For each of FIG. 2A and FIG. 2B, the diagram on the right side shows the arrangement of atoms as viewed from the direction of the arrow on the left side structure and the arrangement of atoms in the lattice adjacent thereto. As shown in FIG. 2A, the crystal structure of Li [Li _1/3 Mn _2/3 ] O ₂ includes a metal layer composed of a transition metal (Mn) and lithium (Li). In the metal layer, as shown below, lithium is regularly arranged every third (three-fold period) in the a-axis direction and the b-axis direction to form a two-dimensional plane.

The lithium (Li) regularly arranged in the metal layer is caused by Li _1/3 of [Li _1/3 Mn _2/3 ] O ₂ . Such a periodic arrangement structure of lithium can be confirmed from electron beam diffraction data.

On the other hand, in the crystal structure of LiM ₁ O ₂ , no three-fold arrangement of lithium in the metal layer occurs.

A in the general formula (1) may be a number satisfying 0 <a <1. Preferably, 0.4 <a <0.9, more preferably 0.5 ≦ a ≦ 0.8, and still more preferably 0.6 ≦ a ≦ 0.75. If the value of a is larger than 0.4, the ratio of the Li ₂ MnO ₃ component in the crystal becomes large, and a large capacity can be expressed. If the value of a is less than 0.9, sufficient reactivity can be obtained and a large capacity can be obtained. The charge / discharge reaction mechanism has not yet been elucidated, so it is still unclear how much lithium can be used, but the reason why a large capacity can be expected when there are many Li ₂ MnO ₃ components in the crystal is This is because if all the lithium in the Li layer in the crystal can contribute to the reversible capacity, the capacity is 344 mAh / g, and if all the Li in the composition formula can contribute to the reversible capacity, the value is as large as 459 mAh / g. In comparison, LiM1O ₂ has a theoretical capacity of about 275 mAh / g.

  In the general formula (1), M1 is one or more transition metals, and is selected so that the total metal valence of the solid solution is 4. As the element species of M1, for example, one or more selected from the group consisting of Mn, Ni, Co, Al, Cu, Ti, and Fe can be used. When a solid solution containing at least one of the above as M1 is used, high capacity and high durability can be obtained. Among these, it is preferable to contain Mn, Ni, and Co.

In the secondary active material LiM ₂ O ₂ , M ₂ is one or more transition metals having a total valence of 3. As the element species of M2, for example, one or more selected from the group consisting of Mn, Ni, Co, Al, Ti, and Fe can be used. When M2 contains at least one of the above, the secondary active material exhibits high performance, and high battery performance can be obtained. Preferably, the _secondary active material is Li [Ni _1/3 Co _1/3 Mn _1/3 ] O ₂ , LiCoO ₂ , LiNiO ₂ , LiNi _1/2 Mn _1/2 O ₂ , particularly preferably Li [Ni _1/3 Co _1/3 Mn _1/3 ] O ₂ .

Next, the general _{formula (1):} aLi As a manufacturing method of a solid solution represented by _{[Li 1/3 Mn 2/3] O 2} · (1-a) LiM1O 2 is not particularly limited, conventionally known The method can be carried out appropriately. For example, the complex carbonate method can be used as follows.

First, as a starting material, a predetermined amount of each of the metal sulfates, metal nitrates (eg, nickel sulfate, cobalt sulfate, manganese sulfate, etc.) of each transition metal element in the general formula (1) is weighed and mixed. Prepare the solution. Ammonia water is added dropwise to this until pH 7 is reached, and then a Na ₂ CO ₃ solution is further added dropwise. Mn-M1 composite carbonate (however, the type of the composite carbonate differs depending on the combination of M1 in the above formula) ). Then, after suction filtration, it is washed with water and dried at a predetermined temperature for a predetermined time (for example, 120 ° C. for 5 hours). The obtained dried product is calcined at a predetermined temperature for a predetermined time (for example, at 500 ° C. for 5 hours). A small excess of LiOH.H ₂ O is added thereto and mixed in an automatic mortar for a predetermined time (for example, 30 minutes). Then, by carrying out main firing at a predetermined temperature for a predetermined time (for example, at 800 to 900 ° C. for 12 hours), the above general formula (1): aLi [Li _1/3 Mn _2/3 ] O _2. (1-a) can be prepared a solid solution represented by LiM1O _2. After the main calcination, quenching with liquid nitrogen or the like is preferable because the reactivity and cycle stability are improved and a very beautiful solid solution state can be obtained.

  The solid solution can be identified using electron diffraction, X-ray diffraction (XRD), or inductively coupled plasma (ICP) elemental analysis.

  The method for producing the secondary active material is not particularly limited, and any conventionally known method can be used as appropriate. For example, the composite carbonate method similar to the above can be used.

Preferably, a positive electrode material obtained by mixing the Li ₂ MnO ₃ —LiM ₁ O ₂ solid solution and the secondary active material is used as a main positive electrode material. The mixing ratio of the solid solution and the secondary active material is not particularly limited. For example, the content of the secondary active material with respect to the total mass of the solid solution and the secondary active material is, for example, 90% by mass or less, preferably 70% by mass or less, and more preferably 60% by mass or less. More preferably, it is 50 mass% or less. When the content of the secondary active material is 90% by mass or less, high durability can be obtained. The lower limit of the content of the secondary active material is not particularly limited, but is, for example, 5% by mass or more, and preferably 10% by mass or more. If it is the said range, the effect of improving the capacity density per volume will be high.

The positive electrode material obtained above is desirably subjected to an oxidation treatment. The method for the oxidation treatment is not particularly limited. For example,
(1) Charging or charging / discharging in a predetermined potential range, specifically charging or charging / discharging in a low potential region that does not cause a significant change in the solid solution positive electrode crystal structure from the beginning;
(2) Oxidation with an oxidizing agent (for example, halogen such as bromine or chlorine) corresponding to charging;
(3) Oxidation treatment using a redox mediator;

  Here, the oxidation treatment method (1) is not particularly limited, but in a state where the battery is configured, or in an electrode or an electrode-corresponding configuration so as not to exceed a predetermined maximum potential, Charging or charging / discharging (= charging / discharging pretreatment with regulated potential) is effective. This is because a positive electrode material for a lithium ion battery having high capacity and good cycle durability, and thus a high energy density battery using the positive electrode material can be manufactured.

  As the charge / discharge pretreatment method with regulated potential, the highest potential in the predetermined potential range with respect to the lithium metal counter electrode (the upper limit potential of charge / discharge converted to lithium metal or lithium metal) is preferably 3.9 V or more and less than 4.6 V, More preferably, it is desirable to perform charging and discharging for 1 to 30 cycles under the condition of 4.4V or more and less than 4.6V. By performing oxidation treatment by charging / discharging within the above range, a positive electrode for a lithium ion battery having high capacity and good cycle durability, and thus a battery having a high energy density can be produced. In particular, when charging or charging / discharging at a maximum potential of about 4.8 V in order to obtain a high capacity after the oxidation treatment (pre-charge / discharge pretreatment with regulated potential), particularly remarkable effects such as cycle durability can be obtained. It can be expressed effectively. Furthermore, in this case, it is preferable from the viewpoint of improving durability that the upper limit potential is gradually (stepwise) increased after charging / discharging at the initial predetermined upper limit potential. Note that the potential converted to lithium metal or lithium metal corresponds to a potential based on the potential exhibited by the lithium metal in the electrolytic solution in which 1 mol of lithium ion is dissolved.

  Further, it is desirable to further increase the maximum potential in the predetermined potential range for charging and discharging stepwise after 1 to 30 cycles of charging and discharging in the predetermined potential range with respect to the lithium metal counter electrode. In particular, 4.7V, 4.8Vvs. When using up to the high potential capacity of Li (use of high capacity), the maximum potential of the charge / discharge potential in the oxidation process is increased step by step, so that the oxidation process can be performed in a short time (pre-charge / discharge pretreatment) However, the durability of the electrode can be improved.

  The number of cycles required for charging and discharging at each stage when raising the maximum potential (upper limit potential) in a predetermined potential range of charging and discharging is not particularly limited, but a range of 1 to 10 is effective. . In addition, the total number of charge / discharge cycles in the oxidation process when increasing the maximum potential (upper limit potential) in the predetermined potential range of charge / discharge (the total number of cycles required for charge / discharge in each stage) ) Is not particularly limited, but a range of 4 to 20 times is effective.

  There is no particular limitation on the range of increase (increase) in potential when increasing the maximum potential (upper limit potential) in a predetermined potential range of charge and discharge in stages, but 0.05V to 0.1V is effective. is there.

  It is effective that the final maximum potential (end maximum potential) when increasing the maximum potential (upper limit potential) in a predetermined potential range of charge / discharge stepwise is 4.6V to 4.9V. . However, the present invention is not limited to the above range, and oxidation treatment (charge / discharge pretreatment with regulated potential) may be performed up to a higher termination maximum potential as long as the above effects can be achieved.

  The minimum potential in the predetermined potential range is not particularly limited, and is 2 V or more and less than 3.5 V, more preferably 2 V or more and less than 3 V with respect to the lithium metal counter electrode. By performing oxidation treatment (charging / discharging pretreatment with regulated potential) by charging or charging / discharging within the above range, a positive electrode for a lithium ion battery having a high capacity and good cycle durability and a battery having a high energy density can be produced. The charge / discharge potential (V) is a potential per unit cell (unit cell).

  The temperature of the electrode (material) to be charged / discharged as the oxidation treatment (pre-charge / discharge pretreatment with regulated potential) can be arbitrarily set as long as it does not impair the effects of the present invention. From the economical point of view, it is desirable to carry out at room temperature that does not require special heating and cooling. On the other hand, it is desirable to carry out at a temperature higher than room temperature from the viewpoint that a larger capacity can be expressed and the cycle durability can be improved by a short charge / discharge treatment.

  The step (time) for applying the oxidation treatment (pre-charge / discharge pretreatment with regulated potential) method is not particularly limited. For example, as described above, the oxidation treatment can be performed in a state where a battery is configured, or in a configuration corresponding to an electrode or an electrode. That is, any of application in the state of positive electrode active material powder, application by constituting an electrode, and application after assembling a battery together with the negative electrode may be used. Application to a battery can be carried out by applying an oxidation treatment condition (condition for pre-charge / discharge with regulated potential) in consideration of the potential profile of the capacitance of the negative electrode to be combined. Here, in the case where the battery is configured, it is superior in that the oxidation treatment of a large number of electrodes can be performed at a time, rather than the individual electrodes or the respective configurations corresponding to the electrodes. On the other hand, when it is performed for each individual electrode or each electrode-corresponding configuration, it is easier to control conditions such as the oxidation potential than the state in which the battery is configured, and variations in the degree of oxidation to individual electrodes occur. It is excellent in a difficult point.

  The oxidizing agent used in the oxidation method (2) is not particularly limited, and for example, halogen such as bromine and chlorine can be used. These oxidizing agents may be used alone or in combination. Oxidation with an oxidizing agent can be gradually oxidized by, for example, dispersing solid solution fine particles in a solvent in which the solid solution positive electrode material does not dissolve, and blowing and dissolving the oxidizing agent into the dispersion solution.

  The above is the description regarding the characteristic configuration requirements of the lithium ion battery of the present embodiment, and the other configuration requirements are not particularly limited. Therefore, in the following, other structural requirements other than the characteristic structural requirements of the lithium ion battery will be described below with a focus on the respective structural requirements of the stacked battery 10 described above. However, it goes without saying that the constituent elements of batteries other than the stacked battery, for example, bipolar batteries, can be configured by appropriately using the same constituent elements.

[Current collector]
As the current collectors (the negative electrode current collector 11 and the positive electrode current collector 12), any member conventionally used as a current collector material for a battery can be appropriately employed. As an example, examples of the positive electrode current collector and the negative electrode current collector include aluminum, nickel, iron, stainless steel (SUS), titanium, and copper. Among these, from the viewpoints of electron conductivity and battery operating potential, aluminum is preferable as the positive electrode current collector, and copper is preferable as the negative electrode current collector. A typical thickness of the current collector is 10 to 20 μm. However, a current collector having a thickness outside this range may be used. The current collector plate can also be formed of the same material as the current collector.

[Active material layer]
The active material layers (the negative electrode active material layer 13 and the positive electrode active material layer 15) are configured to include an active material (negative electrode active material, positive electrode active material). Furthermore, these active material layers include a binder, a conductive agent for increasing electrical conductivity, an electrolyte (polymer matrix, ion-conductive polymer, electrolytic solution, etc.), a lithium salt for increasing ionic conductivity, if necessary. including.

(A) Active material The material (material) of the positive electrode active material and the negative electrode active material is not particularly limited as long as it has the characteristic configuration requirements of the lithium ion battery of the present embodiment. What is necessary is just to select suitably according to the kind of battery.

  Specifically, as the positive electrode active material, the above positive electrode material is used as the main active material of the positive electrode. As the positive electrode active material, the above-described positive electrode material may be used alone, or, if necessary, other conventionally known positive electrode active materials may be used in combination. In order to exert the effect of the present invention remarkably, the positive electrode material described above is preferably contained in the active material by 50% by mass or more, more preferably by 80% by mass or more, and further preferably by 90% by mass or more.

The negative electrode active material is not particularly limited as long as it can reversibly occlude and release lithium, and any conventionally known negative electrode active material can be used. For example, high crystalline carbon graphite (natural graphite, artificial graphite, etc.), low crystalline carbon (soft carbon, hard carbon), carbon black (Ketjen black, acetylene black, channel black, lamp black, oil furnace black, Carbon materials such as thermal black), fullerene, carbon nanotube, carbon nanofiber, carbon nanohorn, carbon fibril; Si, Ge, Sn, Pb, Al, In, Zn, H, Ca, Sr, Ba, Ru, Rh, Ir, Pd, Pt, Ag, Au, Cd, Hg, Ga, Tl, C, N, Sb, Bi, O, S, Se, Te, Cl, and the like, and simple elements of these elements alloyed with lithium oxide containing (silicon monoxide (SiO), SiO _x 0 <x <2), tin _{(SnO 2} dioxide), SnO x (0 <x <2), etc. SnSiO ₃₎ and carbides (such as silicon carbide (SiC)) or the like; a metal material of a lithium metal or the like; lithium - titanium Lithium-transition metal composite oxides such as composite oxides (lithium titanate: Li ₄ Ti ₅ O ₁₂ ); and other conventionally known negative electrode active materials can be used. The negative electrode active material may be used alone or in the form of a mixture of two or more. As an example of a particularly preferable negative electrode material for producing a high-capacity battery, there is, for example, a type of graphite having high crystallinity, high orientation, charge / discharge capacity close to the theoretical capacity of 372 mAh / g, and small initial irreversible capacity. .

  The average particle diameter of each active material contained in each active material layer (13, 15) is not particularly limited, but is usually about 0.1 to 100 μm from the viewpoint of high capacity, reactivity, and cycle durability. The thickness is preferably 1 to 20 μm.

  The compounding ratio of the components contained in each active material layer (13, 15) is not particularly limited, and can be adjusted by appropriately referring to known knowledge about lithium ion secondary batteries or lithium ion batteries. Moreover, there is no restriction | limiting in particular also about the thickness of an active material layer, The conventionally well-known knowledge about a lithium ion secondary battery or a lithium ion battery can be referred suitably. For example, the thickness of the active material layer is about 2 to 100 μm.

(B) Binder The binder is added for the purpose of maintaining the electrode structure by binding the active materials or the active material and the current collector.

  As such a binder, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl acetate, polyimide (PI), polyamide (PA), polyvinyl chloride (PVC), polymethyl acrylate (PMA), Thermosetting resins such as polymethyl methacrylate (PMMA), polyether nitrile (PEN), polyethylene (PE), polypropylene (PP) and polyacrylonitrile (PAN), epoxy resins, polyurethane resins, and urea resins And rubber-based materials such as styrene butadiene rubber (SBR).

(C) Conductive agent A conductive agent means the conductive additive mix | blended in order to improve electroconductivity. The conductive agent that can be used in the present embodiment is not particularly limited, and a conventionally known one can be used. Examples thereof include carbon materials such as carbon black such as acetylene black, graphite, and carbon fiber. When the conductive agent is included, an electronic network inside the active material layer is effectively formed, which can contribute to improvement in reliability by improving output characteristics of the battery and improving liquid retention of the electrolyte.

(D) Electrolyte As the electrolyte, a liquid electrolyte, a gel polymer electrolyte, and an intrinsic polymer electrolyte described in the section of [Electrolyte layer] described later can be used without particular limitation. Specific forms of the liquid electrolyte, the gel polymer electrolyte, and the intrinsic polymer electrolyte will be described later in the section of (electrolyte layer), and the details are omitted here. These electrolytes may be used alone or in combination of two or more. Moreover, an electrolyte different from the electrolyte used for the electrolyte layer described later may be used, or the same electrolyte may be used.

[Electrolyte layer]
The electrolyte layer is a layer containing a non-aqueous electrolyte. A nonaqueous electrolyte (specifically, a lithium salt) contained in the electrolyte layer has a function as a carrier of lithium ions that moves between the positive and negative electrodes during charge and discharge. The nonaqueous electrolyte is not particularly limited as long as it can exhibit such a function, but a liquid electrolyte or a polymer electrolyte may be used.

The liquid electrolyte has a form in which a lithium salt is dissolved in an organic solvent. Examples of the organic solvent include carbonates such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), and diethyl carbonate (DEC). As the lithium _{salt, Li (CF 3 SO 2)} 2 N, Li (C 2 F 5 SO 2) 2 N, LiPF 6, LiBF 4, LiAsF 6, LiTaF 6, LiClO 4, LiCF 3 SO 3 , etc. A compound that can be added to the active material layer of the electrode can be similarly employed.

  On the other hand, the polymer electrolyte is classified into a gel polymer electrolyte containing an electrolytic solution (gel electrolyte) and an intrinsic polymer electrolyte containing no electrolytic solution.

  The gel polymer electrolyte has a configuration in which the liquid electrolyte is injected into a matrix polymer (host polymer) made of an ion conductive polymer. Using a gel polymer electrolyte as the electrolyte is superior in that the fluidity of the electrolyte is lost and it is easy to block the ion conductivity between the layers. The ion conductive polymer used as the matrix polymer (host polymer) is not particularly limited. For example, polyethylene oxide (PEO), polypropylene oxide (PPO), polyvinylidene fluoride (PVDF), a copolymer of polyvinylidene fluoride and hexafluoropropylene (PVDF-HFP), polyethylene glycol (PEG), polyacrylonitrile (PAN), Examples thereof include polymethyl methacrylate (PMMA) and copolymers thereof. Here, the ion conductive polymer may be the same as or different from the ion conductive polymer used as the electrolyte in the active material layer, but is preferably the same. The type of the electrolytic solution (lithium salt and organic solvent) is not particularly limited, and organic solvents such as the lithium salt and carbonates exemplified above can be used.

  The intrinsic polymer electrolyte has a structure in which a lithium salt is dissolved in the above matrix polymer and does not contain an organic solvent. Therefore, by using an intrinsic polymer electrolyte as the electrolyte, there is no fear of liquid leakage from the battery, and the battery reliability can be improved.

  The matrix polymer of gel polymer electrolyte or intrinsic polymer electrolyte can express excellent mechanical strength by forming a crosslinked structure. In order to form a crosslinked structure, thermal polymerization, ultraviolet polymerization, radiation polymerization, electron beam polymerization, etc. are performed on a polymerizable polymer (for example, PEO or PPO) for forming a polymer electrolyte using an appropriate polymerization initiator. A polymerization treatment may be performed.

  The non-aqueous electrolyte contained in these electrolyte layers may be one kind alone, or two or more kinds.

  In addition, when an electrolyte layer is comprised from a liquid electrolyte or a gel polymer electrolyte, a separator is used for the electrolyte layer. Specific examples of the separator include a microporous film made of polyolefin such as polyethylene or polypropylene.

  It can be said that the thinner the electrolyte layer, the better to reduce the internal resistance. The thickness of the electrolyte layer is usually 1 to 100 μm, preferably 5 to 50 μm.

[Exterior body]
In a lithium ion secondary battery, it is desirable to accommodate the entire power generating element in an exterior body in order to prevent external impact and environmental degradation during use. As the exterior body, a conventionally known metal can case can be used, and a bag-like case that can cover a power generation element using a laminate film containing aluminum can be used. For example, a laminate film having a three-layer structure in which PP, aluminum, and nylon are laminated in this order can be used as the laminate film, but the laminate film is not limited thereto.

[Battery appearance]
FIG. 3 is a perspective view schematically showing the appearance of the stacked battery which is one embodiment of the present invention. As shown in FIG. 3, the stacked battery 10 has a rectangular flat shape, and a negative electrode current collector plate 25 and a positive electrode current collector plate 27 for taking out electric power are drawn out from both sides thereof. Yes. The power generation element 21 is encased in an outer package 29 of the battery 10 and the periphery thereof is heat-sealed. The power generation element 21 is sealed with the negative electrode current collector plate 25 and the positive electrode current collector plate 27 drawn out. Here, the power generation element 21 corresponds to the power generation element 21 of the stacked battery 10 shown in FIG. 1, and is composed of a negative electrode (negative electrode active material layer) 13, an electrolyte layer 17, and a positive electrode (positive electrode active material layer) 15. A plurality of battery layers (single cells) 19 are stacked.

  Note that the lithium ion battery of this embodiment is not limited to a flat shape (stacked type) as shown in FIG. For example, the wound type lithium ion battery may have a cylindrical shape, or may have a shape that is a flattened rectangular shape by deforming such a cylindrical shape. . In the cylindrical shape, a laminate sheet or a conventional cylindrical can (metal can) may be used as the exterior material, and there is no particular limitation.

  Further, the removal of the current collector plates 25 and 27 shown in FIG. 3 is not particularly limited, and the negative electrode current collector plate 25 and the positive electrode current collector plate 27 may be drawn out from the same side or the negative electrode current collector plate 25. The positive electrode current collector plate 27 may be divided into a plurality of pieces and taken out from each side. Further, in a wound bipolar secondary battery, a terminal may be formed using, for example, a cylindrical can (metal can) instead of the current collector plate.

  According to this embodiment, since a positive electrode material that is highly durable and has an improved capacity density per volume is used as the main positive electrode active material, a lithium ion battery with high capacity density and durability can be provided. The lithium ion battery of this embodiment is a power source for driving a vehicle or an auxiliary power source that requires high volume energy density and high volume output density as a large capacity power source for an electric vehicle, a hybrid electric vehicle, a fuel cell vehicle, a hybrid fuel cell vehicle, etc. Can be suitably used.

  The effects of the present invention will be described using the following examples and comparative examples. However, the technical scope of the present invention is not limited only to the following examples.

1. Preparation of positive electrode active material (1) Synthesis of Li ₂ MnO ₃ —LiM 1 O ₂ solid solution Li [Ni _0.183 Li _0.200 Co _0.033 Mn _0.583 ] O ₂ (this is 0.6Li [ Li _1/3 Mn _2/3 ] O ₂ .0.4Li [Ni _0.4575 Co _0.0825 Mn _0.4575 ] O ₂ ) can be expressed using the complex carbonate method as follows: Prepared.

As a starting material, nickel sulfate, cobalt sulfate and manganese sulfate are weighed in a predetermined amount, and a mixed solution thereof is prepared. Ammonia water is added dropwise until pH 7 is reached, and then a Na ₂ CO ₃ solution is further added dropwise. Ni—Co—Mn complex carbonate was precipitated. While the Na ₂ CO ₃ solution was dropped, pH 7 was maintained with aqueous ammonia. Then, it filtered by suction, washed with water, and dried at 120 degreeC for 5 hours. This was calcined at 500 ° C. for 5 hours. A small excess of LiOH.H ₂ O was added thereto and mixed for 30 minutes in an automatic mortar. The addition amount of LiOH.H ₂ O was 1.2 to 1.3 while the stoichiometric amount was 1.2.

(2) Synthesis of side active material Li [Ni _1/3 Co _1/3 Mn _1/3 ] O _{2 as} a side active material was produced by the same method as in the above (1).

2. Analysis of synthetic sample (1) Elemental analysis: The composition ratio of the obtained solid solution and by-active material was confirmed using inductively coupled plasma (ICP) elemental analysis, and the obtained sample had the above composition. It was confirmed. FIG. 4 shows the composition of the solid solution obtained.

  (2) Powder X-ray diffraction: The crystal structure of the obtained solid solution and by-active material was examined by a powder X-ray diffraction method. The solid solution can be assigned to the space group R3M, and a diffraction line showing a superlattice structure with 2θ of 20 to 23 ° appears, confirming that the target compound was obtained.

3. Preparation of Positive Electrode Material The Li ₂ MnO ₃ —LiM ₁ O ₂ -based solid solution obtained above and a secondary active material were mixed at a mixing ratio shown in Table 1 below to obtain positive electrode active materials (Examples 1 to 4). In Comparative Example 1, only the solid solution was used as the positive electrode active material, and in Comparative Example 2, only the secondary active material was used as the positive electrode active material.

4). Evaluation (1) Production of electrode and evaluation cell An evaluation cell was produced by the following procedure using the positive electrode active material of Table 1. First, the positive electrode active material and the conductive binder TAB-2 were mixed at a positive electrode active material: TAB-2 = 60: 40 (mass ratio) and diluted with NMP to prepare a positive electrode active material slurry. This positive electrode active material slurry was applied onto an Al foil having a diameter of 15 mm, which was a positive electrode current collector, and dried in a dryer at 120 ° C. for 4 hours to produce a positive electrode. Here, TAB-2 is a mixture of acetylene black and polytetrafluoroethylene having a mass ratio of 1: 1. The amount of the positive electrode active material per unit area of the electrode was set to 10 mg / cm ² .

Metallic lithium was used as the negative electrode. The positive electrode and the negative electrode obtained above were opposed to each other through two 20 μm-thick polypropylene porous membranes as separators, and were placed on the bottom of the coin cell. Subsequently, after mounting a gasket for maintaining insulation between the positive electrode and the negative electrode, an electrolyte was injected using a syringe, and a spring and a spacer were laminated. Then, the upper part of the coin cell was overlapped and caulked to obtain an evaluation cell. In addition, as electrolyte solution, electrolyte solution of ethylene carbonate (EC): diethyl carbonate (DEC) = 1: 2 (volume ratio) of 1M LiPF ₆ was used.

(2) Charge / discharge test (pre-charge / discharge treatment)
About each evaluation cell produced by said method, the pre-charging / discharging process was performed as follows. Each evaluation cell was connected to a charging / discharging device, charged at a constant current rate of 1/12 C until the potential difference became 4.5 V at room temperature, and then discharged until this potential difference became 2.0 V. . This operation was repeated twice. Further, in the same manner, charging / discharging in the range of 4.6V to 2.0V, charging / discharging in the range of 4.7V to 2.0V, and charging / discharging in the range of 4.8V to 2.0V were performed twice each. It was.

(Evaluation)
The evaluation cell after the above charge / discharge pretreatment is charged at 25 ° C. at a constant current rate (1 / 12C rate) until the upper limit voltage becomes 4.8 V, and then the lower limit voltage becomes 2.0 V. Until discharged. This charge / discharge process was defined as one cycle, and the discharge capacity at the first cycle was defined as the initial discharge capacity. Thereafter, 10 cycles of charge and discharge were performed, and the discharge capacity at the 10th cycle was measured. The ratio of the discharge capacity at the 10th cycle to the initial discharge capacity was defined as the capacity retention rate.

FIG. 5 shows the contents (mass%) of the secondary active material with respect to the total mass of the positive electrode active material and the capacity per volume of the positive electrode active material for the batteries using the positive electrodes of Examples 1 to 4 and Comparative Examples 1 and 2. The relationship with density is shown. The capacity density per volume (mAh / cm ³ ) is the initial discharge capacity (mAh / g) obtained on the basis of the mass of the positive electrode active material of the battery produced in each example and comparative example. It is the value calculated by multiplying the density (g / cm ³ ). Here, the density of the solid solution was 1.9 g / cm ³ , and the density of the side active material was 3.0 g / cm ³ . From the result of FIG. 5, the capacity of the battery per unit volume of the positive electrode active material is improved by mixing the _secondary active material Li [Ni _1/3 Co _1/3 Mn _1/3 ] O ₂ into the solid solution. I understand that. This is because Li [Ni _1/3 Co _1/3 Mn _1/3 ] O ₂ has a relatively high capacity although it is 240 mAh / g, which is lower than 280 mAh / g of the solid solution, when the upper limit voltage is 4.8V. This is probably because the tap density is high.

  FIG. 6 shows the contents (% by mass) of the secondary active material with respect to the total mass of the positive electrode active material and the capacity after 10 cycles for the batteries using the positive electrodes obtained in Examples 1 to 4 and Comparative Examples 1 and 2. The graph which shows the relationship with a retention rate is shown. In addition, the predicted value of the capacity retention rate of the mixed positive electrode material estimated from the capacity retention rate when the solid solution and the secondary active material are used alone is indicated by a dotted line. FIG. 6 shows that mixing the secondary active material does not cause the durability to decrease linearly in proportion to the content of the secondary active material, but maintains the durability of the solid solution. When the content of the secondary active material is 70% by mass or less, particularly 50% by mass or less, the high durability is hardly impaired in the solid solution. This is thought to be due to the preferential reaction of the solid solution in the high-potential region by mixing the solid solution, although the durability decreases when the secondary active material is charged to a high potential and lithium is excessively extracted from the crystal structure. It is done. In such a mixed positive electrode material, for example, in the low potential region up to about 3.8 V, the secondary active material reacts preferentially and greatly contributes to the capacity. However, in the higher potential region, the solid solution reacts more. It is easy and the components of the solid solution greatly contribute to the capacity. Therefore, the electrochemical load on the secondary active material in the high potential region can be reduced, the progress of the destruction of the crystal structure of the secondary active material when charged to a high potential can be mitigated and suppressed, and high durability can be obtained. Conceivable.

  The above results are summarized in Table 1.

  As shown in Table 1, when each of the solid solution and the secondary active material is used alone as the positive electrode active material, a battery having both excellent capacity density per volume and durability cannot be obtained. When mixed and used, a battery having both excellent capacity density per volume and durability can be obtained.

FIG. 7 is a graph showing the relationship between the capacity retention rate of the battery using the positive electrode obtained in the example and the comparative example and the capacity density per volume. From FIG. 7, the point corresponding to the results of Examples 1 to 4 has a larger capacity density per volume than the straight line connecting the points corresponding to the results of Comparative Example 1 and Comparative Example 2, and the capacity retention after 10 cycles. It can be seen that exists in a large area. That is, when a positive electrode material obtained by mixing a Li ₂ MnO ₃ —LiM ₁ O ₂ solid solution and a secondary active material is used as a positive electrode active material, a Li ₂ MnO ₃ —LiM ₁ O ₂ solid solution or a secondary active material is used alone. It was confirmed that better battery performance can be obtained.

10 stacked battery,
11 negative electrode current collector,
12 positive electrode current collector,
13 negative electrode active material layer (negative electrode),
15 positive electrode active material layer (positive electrode),
17 electrolyte layer,
19 Single battery layer (single cell),
21 power generation elements,
25 negative current collector,
27 positive current collector,
29 Exterior body (laminate sheet).

Claims (7)

_{Li [Li 1/3 Mn 2/3] O} 2 and LiM1O ₂ (M1 is a is one or more transition metals) a solid solution of the total of the metal valence is 4, and the solid solution, LiM2O ₂ A positive electrode for a lithium ion battery, comprising a positive electrode material obtained by mixing a secondary active material represented by (M2 is one or more transition metals having a total valence of 3).   2. The positive electrode for a lithium ion battery according to claim 1, wherein a content of the secondary active material with respect to a total mass of the solid solution and the secondary active material is 90% by mass or less.   2. The positive electrode for a lithium ion battery according to claim 1, wherein a content of the secondary active material with respect to a total mass of the solid solution and the secondary active material is 50% by mass or less. The lithium ion battery according to any one of claims 1 to 3, wherein a composition ratio a of Li [Li _1/3 Mn _2/3 ] O _{2 in} the solid solution is 0.4 <a <0.9. Positive electrode.   The lithium ion according to any one of claims 1 to 4, wherein the element species of M1 in the solid solution is one or more selected from the group consisting of Mn, Ni, Co, Al, Cu, Ti, and Fe. Battery positive electrode.   The lithium ion according to any one of claims 1 to 5, wherein the element species of M2 in the secondary active material is one or more selected from the group consisting of Mn, Ni, Co, Al, Ti, and Fe. Battery positive electrode.   The lithium ion battery which uses the positive electrode for lithium ion batteries of any one of Claims 1-6.