JP2015053252A - Positive electrode active material for lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode active material for lithium ion secondary battery containing a nickel manganese composite oxide of spinel crystal structure and a metal oxide of layered crystal structure, and excellent in durability.SOLUTION: In a positive electrode active material for lithium ion secondary battery, primary particles composing the positive electrode active material includes a phase consisting of a nickel manganese-containing composite oxide of spinel crystal structure containing at least nickel and manganese as a transition metal element, and a phase consisting of a metal oxide of layered crystal structure.

Description

  The present invention relates to a positive electrode active material for a lithium ion secondary battery and use thereof.

As part of improving the performance of non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries, higher energy density is required. The use of a positive electrode active material having a higher operating potential is one of effective means for increasing the energy density of a non-aqueous electrolyte secondary battery. As such a positive electrode active material, for example, a nickel-manganese-containing composite oxide having a spinel crystal structure that exhibits an operating potential of 4.5 V (vs. Li ⁺ / Li) or more based on metallic lithium (hereinafter referred to as “spinel structure oxidation”). Also referred to as "thing".

  In recent years, by using a positive electrode active material including a spinel structure oxide and a metal oxide having a layered rock salt type crystal structure (hereinafter also referred to as a “layered structure oxide”), a non-aqueous electrolyte secondary battery It has been proposed to improve performance (see Patent Document 3). For example, Patent Document 1 discloses a positive electrode in which a spinel structure oxide and a layered structure oxide are mixed as a positive electrode active material. Patent Document 2 discloses a positive electrode including a layered structure oxide in which a spinel structure oxide is deposited on a part of the surface in a non-solid solution state as a positive electrode active material. In addition, Patent Documents 4 and 5 are examples of conventional techniques in which a spinel structure oxide and a layered structure oxide are used in combination as the positive electrode active material. Patent Document 6 proposes to improve the cycle life by replacing part of the Mn site of the nickel manganese-containing spinel structure oxide with Fe and Ti.

JP 2013-51210 A JP 2011-171150 A Japanese Patent No. 4458232 JP 2008-091341 A JP 2004-006094 A JP 2013-143262 A

  However, the non-aqueous electrolyte secondary battery using the positive electrode active material disclosed in the above prior art can achieve higher energy density as compared with the case where only the layered structure oxide is included, but its performance is There was a tendency to deteriorate easily. For example, when charging / discharging is repeated under high temperature conditions (for example, about 60 ° C.) until the potential of the positive electrode is 4.5 V or higher with respect to metallic lithium, deterioration of the layered structure oxide proceeds, As the number of discharges increases, the battery resistance may increase significantly or the battery capacity may decrease significantly. For this reason, the durability improvement of the nonaqueous electrolyte secondary battery using a spinel structure oxide and a layered structure oxide was desired.

  The present invention was created to solve the above-described conventional problems, and its purpose is to provide a lithium ion secondary that is excellent in durability under high-temperature conditions while containing a spinel structure oxide and a layered structure oxide. It is to provide a positive electrode active material for a battery.

  In order to achieve the above object, the present invention provides a positive electrode active material for a lithium ion secondary battery. That is, in the positive electrode active material for a lithium ion secondary battery disclosed herein, the primary particles constituting the positive electrode active material have a spinel-type crystal structure and include at least nickel and manganese. And a first oxide phase composed of a complex oxide of lithium and lithium and a second oxide phase having a layered rock salt type crystal structure and composed of a complex oxide of a second transition metal and lithium.

In the positive electrode active material for a lithium ion secondary battery provided by the present invention, the primary particles constituting the positive electrode active material are a first oxide phase (hereinafter referred to as a nickel-manganese-containing lithium transition metal composite oxide having a spinel crystal structure). And a second oxide phase (hereinafter also referred to as “layered crystal phase”) made of a lithium transition metal composite oxide having a layered crystal structure. Preferably, the second transition metal is one or more transition metals containing at least manganese, selected from the first transition metals.
In a more preferred embodiment of the positive electrode active material, at least a part of the interface between the first oxide phase and the second oxide phase may be in a state of being aligned on the oxygen surface in each crystal structure. . In other words, the spinel crystal phase and the layered crystal phase are oriented in a predetermined crystal orientation, and can exist in a state where the oxygen position (O site) in the crystal structure of each other is shared and the oxygen position is shared. In other words, the oxide that constitutes the spinel crystal phase and the oxide that constitutes the layered crystal phase are bonded by oxygen that constitutes the oxygen plane of each other.

For this reason, in the lithium ion secondary battery constructed using the positive electrode active material, in the primary particles constituting the positive electrode active material, the contact area between the second oxide phase (layered structure oxide) and the electrolytic solution (electrolyte) Becomes smaller. As a result, under high temperature conditions (for example, about 60 ° C.), deterioration of the second oxide phase is suppressed even when charging / discharging is repeated until the potential of the positive electrode is 4.5 V or more based on metallic lithium. The
In addition, when oxygen is desorbed from the first oxide phase (spinel structure oxide) during the charge and discharge, a part of the electrolytic solution is oxidized and decomposed to generate an acid (for example, hydrogen fluoride (HF)). The transition metal element (for example, manganese) can be eluted from the first oxide phase into the electrolytic solution by this acid. However, since the second oxide phase in which the deterioration is suppressed effectively absorbs the acid, elution of the transition metal element from the first oxide phase is suppressed.
As described above, since the deterioration of the positive electrode active material is suppressed even under high temperature conditions, the lithium ion secondary battery constructed using the positive electrode active material is excellent in durability.
In addition, when the lithium ion secondary battery is charged and discharged under a high temperature condition until it reaches 4.5 V or more on the basis of lithium, lithium in the second oxide phase is activated and desorbed, and the amount of available lithium increases. To do. For this reason, lithium which has been deactivated on the negative electrode surface due to high temperature deterioration and can no longer be used can be supplemented, and capacity deterioration can be suppressed.

In another preferred embodiment of the positive electrode active material disclosed herein, when the total of the first oxide phase and the second oxide phase is 100 mol%, the proportion of the second oxide phase is 3 mol% or more and 8 mol%. It is as follows.
According to this configuration, since the ratio of the second oxide phase (layered structure oxide) is small, the contact area between the second oxide phase and the electrolytic solution is reduced. As a result, deterioration of the second oxide phase is suppressed, and acid (for example, HF) is sufficiently absorbed by the second oxide phase.

In a preferred embodiment of the positive electrode active material disclosed herein, at least a part of the second oxide phase is present in the first oxide phase.
According to this configuration, deterioration of the second oxide phase having a layered crystal structure is more effectively suppressed in the primary particles constituting the positive electrode active material. As a result, the lithium ion secondary battery constructed using the positive electrode active material is more excellent in durability.

In another preferred embodiment of the positive electrode active material disclosed herein, the first transition metal further contains at least one of titanium, iron, and cobalt.
According to this configuration, the binding energy between the transition metal element and the oxygen element in the first oxide phase (spinel structure oxide) is increased. For this reason, oxygen desorption from the spinel structure oxide is suppressed, and oxidative decomposition of the electrolyte and generation of acid can be suppressed.

In another preferred embodiment of the positive electrode active material disclosed herein, the first oxide phase is composed of an oxide represented by the following general formula (I).
LiMn _{2- (a1 + b1 + c1 + d1)} Ni _a1 Ti _b1 Fe _c1 Co _d1 O _4-δ (I)
In the formula, 0.3 ≦ a1 ≦ 0.6, 0 ≦ b1 ≦ 0.2, 0 ≦ c1 ≦ 0.2, 0 ≦ d1 ≦ 0.1, b1 + c1 + d1 ≠ 0, and 0 ≦ δ ≦ 1. .
According to such a configuration, oxygen desorption from the first oxide phase having a spinel crystal structure is suppressed, and oxidative decomposition of the electrolytic solution and generation of acid can be suppressed.

In another preferred embodiment of the positive electrode active material disclosed herein, the second transition metal includes at least manganese, and further includes at least one of nickel, titanium, iron, and cobalt.
This configuration is preferable because the effect of suppressing the capacity deterioration due to the first oxide phase and the capacity increasing effect due to the second oxide phase are realized in a well-balanced manner.

In another preferred embodiment of the positive electrode active material disclosed herein, the second oxide phase is composed of an oxide represented by the following general formula (II).
Li _3-x (Mn _{1- (a2 + b2 + c2 + d2)} Ni _a2 Ti _b2 Fe _c2 Co _d2 ) _x O _{3-δ ′} (II)
In the formula, 1 ≦ x ≦ 1.5, 0 ≦ a2 ≦ 0.2, 0 ≦ b2 ≦ 0.3, 0 ≦ c2 ≦ 0.2, 0 ≦ d2 ≦ 0.2, a2 + b2 + c2 + d2 ≠ 0 and 0 ≦ δ ′ ≦ 1.
The metal oxide having a layered crystal structure having such a structure is excellent in acid absorbability.

FIG. 1 is a perspective view schematically showing the outer shape of a nonaqueous electrolyte secondary battery according to an embodiment of the present invention. FIG. 2 is a sectional view taken along line II-II in FIG. FIG. 3 is a transmission electron microscope (TEM) image of the positive electrode active material according to an embodiment of the present invention. FIG. 4 is an enlarged view of (a1), (b1), and (c1), and (a2), (b2), and (c2) Fast Fourier Transform (FFT) diagrams of the TEM images in FIG. (B3) (c3) The simulation results and (a4) (b4) (c4) crystal structure models.

  Hereinafter, preferred embodiments of the present invention will be described. It should be noted that matters other than matters specifically mentioned in the present specification and necessary for carrying out the present invention can be grasped as design matters of those skilled in the art based on the prior art in this field. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field.

  The positive electrode of the lithium ion secondary battery disclosed herein can have the same configuration as the conventional one except that the positive electrode active material characterizing the present invention is provided. Such a positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector. And this positive electrode active material layer contains the positive electrode active material disclosed here.

[Positive electrode active material]
The positive electrode active material disclosed here is a positive electrode active material for a lithium ion secondary battery. The primary particles constituting the positive electrode active material have a spinel crystal structure, and a first oxide phase (spinel crystal phase) composed of a composite oxide of lithium and a first transition metal containing at least nickel and manganese. ) And a second oxide phase (layered crystal phase) having a layered rock salt type crystal structure and comprising a complex oxide of a second transition metal and lithium. That is, the spinel crystal phase and the layered crystal phase are configured to be inseparable from each other.
In the present specification, “primary particles” refers to the minimum unit of particles constituting the positive electrode active material, and specifically refers to the minimum unit determined from the apparent geometric form. Such an aggregate of primary particles is called a secondary particle.

  In such a positive electrode active material, at least a part of the interface between the first oxide phase and the second oxide phase is aligned on the oxygen surface in each crystal structure. The state of matching between the first oxide phase and the second oxide phase will be described in detail by giving a specific example below. Essentially, the crystal of the oxide constituting the first oxide phase An arbitrary oxygen surface in the structure (for example, the first oxygen surface) and an arbitrary oxygen surface (for example, the second oxygen surface) in the crystal structure of the oxide constituting the second oxide phase are substantially They are parallel and share oxygen positions at their interfaces, so that they are connected substantially continuously, in other words, in a substantially flush state. That is, the first oxide phase constituting the spinel crystal phase and the second oxide phase constituting the layered crystal phase are bonded by oxygen constituting the mutual oxygen surface. Therefore, there is no crystal grain boundary where impurities, other precipitates, etc. are present at the matching interface. In the present specification, the alignment of the oxygen surface described above is, for example, by observation with a transmission electron microscope (TEM) or the like, if it can be confirmed that the oxygen surfaces of both phases as a whole are substantially continuous. It is permissible to have differences (so-called mismatches).

The first oxide phase having the spinel crystal structure is composed of a composite oxide of lithium and a first transition metal containing at least nickel and manganese. Such a first oxide phase may be a compound that is recognized as a so-called spinel-type nickel-manganese composite oxide in lithium transition metal composite oxides that are widely used as positive electrode active materials for lithium ion secondary batteries. Such an oxide may typically be one in which a part of the Mn site of the compound represented by the general formula: LiMn ₂ O ₄ is substituted with Ni. Further, a part of the Mn site may be further substituted with another transition metal element. With this configuration of the first oxide phase, the driving voltage of the lithium ion secondary battery using this positive electrode active material is set to 4.5 V or higher (for example, so-called 5 V class) on a lithium basis under high temperature conditions. It becomes possible.

  Here, the first transition metal element is not particularly limited. For example, various transition metal elements belonging to Groups 3 to 11 of the periodic table can be considered. Essentially, manganese (Mn) and nickel (Ni) are considered as the transition metal element. In the configuration including the first transition metal element other than Mn and Ni, as the other first transition metal element, specifically, for example, scandium (Sc), yttrium (Y), titanium (Ti), iron (Fe), cobalt (Co), zinc (Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), molybdenum (Mo), tungsten (W), copper Examples thereof include using one kind from (Cu), silver (Ag), gold (Au) or the like alone or in combination of two or more kinds. Especially, it is more preferable that it is 1 type, or 2 or more types among Ti, Fe, and Co. The composite oxide constituting the first oxide phase is allowed to contain an element other than the transition metal element (for example, aluminum (Al), magnesium (Mg), etc.).

Here, the composite oxide constituting the first oxide phase is not necessarily limited thereto, but more preferably can be represented by the following general formula (I).
LiMn _{2- (a1 + b1 + c1 + d1)} Ni _a1 Ti _b1 Fe _c1 Co _d1 O _4-δ (I)
Here, in the formula (I), a1, b1, c1, d1, and δ satisfy the following relationship. That is, 0.3 ≦ a1 ≦ 0.6, 0 ≦ b1 ≦ 0.2, 0 ≦ c1 ≦ 0.2, 0 ≦ d1 ≦ 0.1, b1 + c1 + d1 ≠ 0, and 0 ≦ δ ≦ 1. Although these values a1, b1, c1, d1, and δ can vary depending on the ratio of the mutual elements, they cannot be generally stated, but as an approximate guideline, for example, the following conditions are more preferably satisfied.

That is, the a1 indicates the Ni content in the composite oxide constituting the first oxide phase, more preferably 0.35 ≦ a1, and more preferably a1 ≦ 0.5. B1 represents the Ti content. When b1 is too larger than 0.2, the electron conductivity of the positive electrode active material is lowered, and the upper limit operating potential of the positive electrode is 4.5 V or more based on metal lithium. Battery resistance may increase in the ion secondary battery. Therefore, it is more preferable that 0.01 ≦ b1, and it is more preferable that b1 ≦ 0.15. The above c1 represents the Fe content. When c1 is too larger than 0.2, the battery capacity that operates stably when the upper limit operating potential of the positive electrode is 4.5 V or more and 5 V or less with respect to metallic lithium is reduced. There is a fear. Therefore, it is more preferable that 0.01 ≦ c1, and it is more preferable that c1 ≦ 0.1. Said d1 shows Co content, It is more preferable that it is 0.01 <= d1, and it is more preferable that it is d1 <= 0.07 (for example, d1 <= 0.05). Of the other first transition metal elements described above, Ti, Fe, and Co are not necessarily essential elements, but it is preferable that any one or more of them be contained in the first oxide phase. More preferably, 3 are included. Examples of a preferable composite oxide constituting the first oxide phase include LiNi _0.45 Fe _0.05 Mn _1.45 Ti _0.05 O ₄ and LiNi _0.42 Fe _0.05 Mn _1.45. Ti _0.05 Co _0.03 O ₄ etc. are mentioned.

  Note that δ indicates the amount of oxygen defects in the above spinel crystal structure, and may vary depending on environmental conditions and the like in addition to the type of substitution atoms and the substitution ratio in the crystal structure, so that it is difficult to display accurately. Therefore, δ, which is a variable for determining the number of oxygen atoms, is a positive number not exceeding 4, and typically, for example, 0 ≦ δ ≦ 1 can be adopted. However, in the present specification, δ may be omitted for the sake of convenience as illustrated in the preferred composite oxide, but even in such a case, a different compound is not shown. That is, (4-δ) in the above general formula (I) is not intended to limit the technical scope of the present invention.

The second oxide phase having the layered crystal structure is composed of a composite oxide of a second transition metal and lithium. The composition of the composite oxide constituting the second oxide phase is not particularly limited, but in a lithium transition metal composite oxide widely used as a positive electrode active material of a lithium ion secondary battery, a so-called layered rock salt type manganese is used. It may be a compound recognized as a system complex oxide or a nickel manganese system composite oxide. Such an oxide is typically a compound represented by the general formula: LiMnO ₂ , and more specifically, a so-called lithium rich type (lithium-excess type) compound typically represented by Li ₂ MnO ₃ . It can be. Further, a part of the Mn site may be substituted with Ni. Further, a part of the Mn site may be substituted with Ni. Further, a part of the Mn site may be further substituted with another transition metal element. With such a configuration of the second oxide phase, the lithium ion secondary battery using this positive electrode active material, for example, suitably absorbs the acid generated on the surface of the first oxide phase, and the first oxide phase The elution of the transition metal element from the phase can be suppressed.

  Here, the second transition metal element is not particularly limited, and for example, the same thing as the first transition metal element can be considered. That is, manganese (Mn) is essentially considered as the second transition metal element. In the configuration including the second transition metal element other than Mn, as the other first transition metal element, specifically, for example, nickel (Ni), scandium (Sc), yttrium (Y), titanium (Ti ), Iron (Fe), cobalt (Co), zinc (Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), molybdenum (Mo), tungsten (W ), Copper (Cu), silver (Ag), gold (Au), etc., one type is used alone, or two or more types are used in combination. Especially, it is more preferable that it is 1 type, or 2 or more types among Ni, Ti, Fe, and Co. The composite oxide constituting the second oxide phase is allowed to contain an element other than the transition metal element (for example, aluminum (Al), magnesium (Mg), etc.).

In addition, the composite oxide constituting the second oxide phase is not necessarily limited thereto, but more preferably can be represented by the following general formula (II).
Li _3-x (Mn _{1- (a2 + b2 + c2 + d2)} Ni _a2 Ti _b2 Fe _c2 Co _d2 ) _x O _{3-δ ′} (II)
Here, in the formula (II), x, a2, b2, c2, d2, and δ ′ satisfy the following relationship. That is, 1 ≦ x ≦ 1.5, 0 ≦ a2 ≦ 0.2, 0 ≦ b2 ≦ 0.3, 0 ≦ c2 ≦ 0.2, 0 ≦ d2 ≦ 0.2, a2 + b2 + c2 + d2 ≠ 0 and 0 ≦ δ ′ ≦ 1. Although these values a2, b2, c2, d2, and δ ′ cannot be generally described because their values may vary depending on the ratio of mutual elements, as an approximate guideline, for example, satisfying the following is shown as a more preferable example. .

  That is, the above x represents the content of the transition metal element in the composite oxide constituting the second oxide phase, and thus the amount of Li. Depending on the type of other constituent elements, the substitution ratio, environmental conditions, etc. It can take a value of about 1.5. Said a2 shows Ni content, It is more preferable that it is 0.05 <= a2, and it is more preferable that it is a2 <= 0.15. Said b2 shows Ti content, It is more preferable that it is 0.01 <= b2, and it is more preferable that it is b2 <0.3, for example, b2 <= 0.25 (for example, b2 <= 0.1). Said c2 shows Fe content, it is more preferable that it is 0.001 <= c2, and it is more preferable that it is c2 <= 0.1. The d2 represents a Co content, more preferably 0.01 ≦ d2, and more preferably d2 <0.2, for example, d2 ≦ 0.15 (for example, d2 ≦ 0.1). Of the second transition metal elements, Ni is preferably contained together with Mn. Further, with respect to the other second transition metal elements, Ti, Fe, and Co are not necessarily essential elements, but they contain any two or more types (for example, Ti and Co) in the second oxide phase. It is more preferable that three types are further included.

Note that δ ′, like δ, indicates the amount of oxygen defects in the spinel crystal structure and can be accurately displayed because it can vary depending on the type of substitution atoms and the substitution ratio in the crystal structure as well as environmental conditions. It is difficult. Therefore, δ ′, which is a variable for determining the number of oxygen atoms, is a positive number not exceeding 3, and typically, for example, 0 ≦ δ ′ ≦ 1 can be adopted. However, for convenience, δ ′ may be omitted, but even in such a case, a different compound is not shown. That is, (3-δ) in the general formula (II) is not intended to limit the technical scope of the present invention.
Moreover, as above-mentioned, it is a preferable aspect that the 1st oxide phase and the 2nd oxide phase are in agreement. Therefore, the second transition metal element in the composite oxide constituting the second oxide phase may be a part of or all of the elements used as the first transition metal element.

Here, the matching state of the first oxide phase and the second oxide phase in the primary particles of the positive electrode active material disclosed herein will be described.
The first oxide phase is composed of a lithium transition metal composite oxide having a spinel crystal structure. As a representative example of the first oxide phase, for example, LiNi _0.45 Fe _0.05 Mn _1.45 Ti _0.05 O ₄ will be described as an example. The second oxide phase is composed of a lithium transition metal composite oxide having a layered rock salt type crystal structure. As a typical example of such a second oxide phase, Li ₂ MnO ₃ will be described as an example.

FIG. 3 is a transmission electron microscope (TEM) image of the positive electrode active material according to an embodiment. As shown in FIG. 3, in the positive electrode active material disclosed herein, the primary particles are the first oxide phase (LiNi _0.45 Fe _0.05 Mn _{1.45) having} a spinel crystal structure shown as “a”. Ti _0.05 O ₄ ; brighter region) and two layered crystal structure second oxide phases (Li ₂ MnO ₃ ; darker region) shown as “b” and “c”. Although there is a slight contrast difference between these three phases and at the phase interface, no clear contrast is seen. Moreover, it turns out that the positive electrode active material disclosed here does not have a crystal grain boundary containing impurities or the like at the boundary between the first oxide phase and the second oxide phase.

FIG. 4 shows (a1), (b1), (c1) high-resolution TEM images, and (a2), (b2), and (c2) Fast Fourier Transform (FFT) diagrams for the respective areas a to c. (A3) (b3) (c3) The simulation results and (a4) (b4) (c4) crystal structure model (basic unit cell) are shown. (A1) to (c1) are images obtained when the inclination angle of the sample in TEM observation is observed as α: + 25.95 ° and β: + 21.94 °.
FIG. 4 (a2) is an FFT image of the first oxide phase (which may be the parent phase) shown as “spinel” in the drawing, and the spinel crystal structure of LiNi _0. It was confirmed to match the incident pattern in the [110] direction of ₄₅ Fe _0.05 Mn _1.45 Ti _0.05 O ₄ . FIG. 4 (b2) is an FFT image of the first second oxide phase (which may be a precipitated phase) indicated as “layer 1” in the figure, and is a layered type as shown in (b3). It was confirmed that it coincided with the incident pattern in the [0-10] direction of Li ₂ MnO _{3 having} a crystal structure. Further, (c2) is an FFT image of the second second oxide phase (which may be a precipitated phase) shown as “layer 2”, and [101] of Li ₂ MnO ₃ as shown in (c3). ] Coincident with the incident pattern in the direction. As shown in (c3), the diffraction spot of (11-1) 0.37 nm cannot appear in a layered crystal structure such as LiNi _1/3 Mn _1/3 Co _1/3 O ₂ , for example. Is.

  That is, the [110] direction of the first oxide phase of (a), the [0-10] direction of the second oxide phase of (b), and the [101] direction of the second oxide phase of (c) are all parallel. It can be seen that it is. Further, as shown in the FFT image, the {−111} plane of the first oxide phase of (a) and the {001} plane of the second oxide phase of (b) substantially coincide, and both phases are oxygen ( O) It can be seen that lattice matching occurs at the site. Then, every two cycles of the {1-11} plane of the first oxide phase of (a), the {001} plane of the second oxide phase of (c) substantially matches, and both phases are at the oxygen (O) site. It can be seen that the lattice is matched.

Although not specifically shown, when the same sample is observed with a sample inclination angle α: -18.23 ° and β: -11.93 °, the FFT image of the first oxide phase described above. Shows the incident pattern of LiNi _0.45 Fe _0.05 Mn _1.45 Ti _0.05 O _{4 in} the [211] direction, and the FFT images of the first and second second oxide phases are respectively Li ₂ MnO. ₃ coincident with the incident patterns in the [011] direction and the [110] direction. Further, even when the sample tilt angles α: −0.77 ° and β: −1.43 ° are observed, the FFT image of the first oxide phase is LiNi _0.45 Fe _0.05 Mn _{1. In} the incident pattern of ₄₅ Ti _0.05 O _{4 in} the [111] direction, the FFT images of the first and second second oxide phases are incident in the [103] direction and [103] direction of Li ₂ MnO ₃ , respectively. It was confirmed to match the pattern. Also from these facts, it can be confirmed that in the positive electrode active material shown in FIG. 4, the first oxide phase is in an aligned state with crystal orientations different from the two second oxide phases.

In general, a lithium transition metal oxide having a spinel crystal structure is configured such that a surface in which oxygen is closest packed in the <111> direction is laminated, and lithium and a transition metal are arranged in the gap. Are known. Then, the lithium can move in a zigzag manner from the tetrahedral 8a site to the adjacent 8a site via the occupying octahedral 16c site, so that the battery reaction can proceed. Such a surface containing oxygen can be seen in various directions other than the <111> direction.
In addition, the layered rock salt type crystal structure also has a surface in which oxygen is closely packed in the <003> direction, and lithium and transition metals are regularly arranged in the octahedral sites of the gap, so that It is known that the surfaces composed only of transition metals are alternately laminated. And in the plane which consists only of this lithium, a battery reaction can advance because lithium moves two-dimensionally. In such a surface containing oxygen, the abundance of oxygen varies. It can be seen in various directions other than the <003> direction.

In the first oxide phase and the second oxide phase disclosed herein, at least a part of the interface has such a plane containing oxygen in a substantially parallel positional relationship, and further, the plane containing oxygen is the interface. Is continuous. In other words, the first oxide phase and the second oxide phase are oriented in a predetermined crystal orientation at the interface, and can exist in a state where the oxygen position (O site) in the crystal structure of each other is shared. In this specification, such an interface state is expressed as being matched on the oxygen surface.
In addition, the alignment in the oxygen-containing surface in the first oxide phase and the second oxide phase is not limited to the specific oxygen-containing surface, and may be realized in a plurality of oxygen-containing surfaces. In addition, the oxygen-containing surfaces in the first oxide phase and the second oxide phase may not completely constitute the same plane. For example, in TEM observation or the like, if it is recognized that the regular arrangement of oxygen atoms in both phases (that is, the oxygen-containing surface) is continuous as a whole, even if there is a slight difference in the spacing between the oxygen-containing surfaces in both phases Permissible. That is, some lattice mismatch is allowed.

  Such a matching state between the first oxide phase and the second oxide phase is not particularly limited in the manufacturing method. However, in order to create such a lattice-matched state in the granular material (within the primary particles), typically, the positive electrode active material disclosed herein has the first oxide phase as a parent phase and the second oxide phase. Can be suitably produced in a form in which a second phase composed of is precipitated, that is, in a form involving so-called two-phase separation. The first oxide phase and the second oxide phase formed by such two-phase separation can be precipitated in a form in which at least a part of the second oxide phase is present in the first oxide phase. In such a configuration, in the lithium ion secondary battery using such a positive electrode active material, the second oxide phase itself is preferable in that the influence of deterioration due to a high-potential electrolyte is reduced. In addition, the second oxide phase formed by such two-phase separation is such that the type of the second transition metal element contained therein is a part of the first transition metal element contained in the first oxide phase or It can be all. Such a configuration is also preferable in that both the first oxide phase and the second oxide phase are precipitated with a thermodynamically relatively stable composition.

In the primary particles constituting the positive electrode active material, the ratio of the first oxide phase having a spinel crystal structure and the second oxide phase having a layered crystal structure is appropriately selected according to the purpose of the present structure. For example, when the total of the first oxide phase and the second oxide phase is 100 mol%, the ratio of the second oxide phase is preferably 2 mol% or more and 9 mol% or less. In the case where the second oxide phase is contained in two or more parts in the primary particles, the total ratio thereof is preferably in the above range. When the ratio of the layered structure oxide is less than 2 mol%, the acid absorption effect by the layered structure oxide is small, and the deterioration of the spinel structure oxide by the acid may not be sufficiently suppressed. In addition, when it charges / discharges until it becomes 4.5V or more (vs.Li ^<+ > / Li) on high temperature conditions, the replenishment effect of lithium is small. When the ratio of the layered structure oxide is larger than 9 mol%, the contact area between the layered structure oxide and the electrolytic solution (electrolyte) becomes large, and thus there is a possibility that deterioration of the layered structure oxide cannot be sufficiently suppressed. From this viewpoint, the proportion of the second oxide phase is preferably 9 mol% or less, more preferably 8 mol% or less. In addition, the ratio of the second oxide phase is preferably 5 mol% or more, more preferably 6 mol% or more, from the viewpoint of suppressing a decrease in battery capacity. That is, in the primary particles constituting the positive electrode active material, the molar ratio of the first oxide phase to the second oxide phase is preferably 92: 8 to 97: 3, more preferably 92: 8 to 95: 5 (more preferably 94: 6 to 95: 5).

In addition, about the composition of complex oxide which comprises said 1st oxide phase and 2nd oxide phase, and the ratio of a 1st oxide phase and a 2nd oxide phase, it uses various well-known analysis techniques Can be confirmed. For example, the composition and ratio of each phase can be confirmed by, for example, X-ray diffraction (XRD) analysis or TEM observation.
More specifically, the ratio of the first oxide phase to the second oxide phase is, for example, in the XRD pattern for the positive electrode active material, with respect to the peak of the first oxide phase having a spinel crystal structure, By measuring the intensity of the peak derived from the second oxide phase having a layered crystal structure, the abundance ratio of both can be grasped. To grasp the abundance ratio of the second oxide phase, for example, from the intensity of the peak derived from the second oxide phase having a layered crystal structure near 2θ = 36.8 ° of the XRD diffraction pattern, and the abundance ratio thereof, A calibration curve may be prepared. The abundance ratio between the first oxide phase and the second oxide phase can be obtained, for example, by pattern fitting an XRD diffraction pattern based on the Rietveld analysis method.

  The average particle diameter of the primary particles constituting the positive electrode active material is preferably 0.01 μm or more and 30 μm or less (for example, 0.01 μm or more and 10 μm or less). Here, the “average particle diameter” refers to a median diameter (D50) in a volume-based particle size distribution obtained by a general laser scattering / diffraction particle size distribution measuring apparatus.

  Hereinafter, the manufacturing method of the positive electrode active material containing said 1st oxide phase and 2nd oxide phase is demonstrated. The manufacturing method disclosed herein includes a step of preparing a precursor of a first oxide phase and a second oxide phase, and a firing step.

<Precursor preparation step>
In the precursor preparation step disclosed herein, a precursor compound containing the first and second transition metal elements contained in the first oxide phase and the second oxide phase is prepared. Hereinafter, for example, a case where a positive electrode active material including a first oxide phase and a second oxide phase represented by the above formula (I) and the above formula (II) is manufactured will be described. That is, a basic aqueous solution having a pH of 11 to 14 is added to an aqueous solution containing a salt (or a salt containing two or more metal elements) containing each transition metal element alone in the above formula (I) and the above formula (II). By stirring and liquid phase reaction in water, the general formula: Mn _{2− (a1 + b1 + c1 + d1)} Ni _a1 Ti _b1 Fe _c1 Co _d1 (OH) _{4 + m} (where m is 0 ≦ m ≦ 0.5). A precursor represented by is prepared. The meanings of a1, b1, c1 and d1 are the same as in the above formula (I).

  The aqueous solution in the precursor preparation step can be prepared by, for example, dissolving predetermined amounts of a desired nickel salt, manganese salt, titanium salt, iron salt and the like in an aqueous solvent. The order in which these salts are added to the aqueous solvent is not particularly limited. Moreover, you may prepare by mixing the aqueous solution of each salt. What is necessary is just to select the anion of these metal salts (the said nickel salt, manganese salt, etc.) so that each said salt may become desired water solubility. For example, it may be sulfate ion, nitrate ion, chloride ion, carbonate ion and the like. That is, the metal salts may be sulfates such as nickel and manganese, nitrates, hydrochlorides, carbonates, and the like. All or part of these metal salt anions may be the same or different from each other. For example, nickel sulfate and manganese carbonate can be used in combination. These salts may be solvates such as hydrates. The order of addition of these metal salts is not particularly limited.

  The basic aqueous solution contains a strong base (alkali metal hydroxide or the like) and a weak base (ammonia or the like), the pH at a liquid temperature of 25 ° C. is maintained at about 11 to 14, and the precursor (A1 And those that do not inhibit the production of (B1) can be preferably used. For example, a mixed solution of an aqueous sodium hydroxide solution and aqueous ammonia is used.

<Baking process>
In the firing step disclosed here, a mixture of the precursor (C) and the lithium salt is fired to synthesize a positive electrode active material containing the spinel crystal phase and the layered crystal phase. Lithium salt addition amount (for example, when the total amount of transition metals in the precursor is 2 mol, Li is 1.1 mol or more (typically about 1.1 to 1.3 mol, for example, about 1.2 mol) By adjusting the lithium excess ratio), a spinel crystal phase having a desired composition and a layered crystal phase having a desired composition can be obtained. As said lithium salt, the general lithium salt used for formation of the conventional lithium complex oxide can be especially used without a restriction | limiting. Specific examples include lithium carbonate and lithium hydroxide. These lithium salts can be used alone or in combination of two or more.

  The firing temperature is preferably in the range of about 700 to 1000 ° C. Firing may be performed at the same temperature at a time, or may be performed stepwise at different temperatures. The firing time can be appropriately selected. For example, it may be baked at about 800 to 1000 ° C. for about 2 to 24 hours, or after baking at about 700 to 800 ° C. for about 1 to 12 hours and then at about 800 to 1000 ° C. for about 2 to 24 hours. Also good. Moreover, the positive electrode active material of a desired property can be prepared by applying processes, such as crushing and sieving, as needed. Thereby, the positive electrode active material disclosed here is provided.

[Positive electrode]
Moreover, the positive electrode of the lithium ion secondary battery disclosed here has a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector. The positive electrode active material layer can typically include the positive electrode active material disclosed herein and a binder. As the positive electrode current collector, a conductive member made of a metal having good conductivity is preferably used, like the current collector used in the positive electrode of a conventional lithium ion secondary battery. For example, aluminum or an alloy containing aluminum as a main component can be used. The shape of the positive electrode current collector can vary depending on the shape of the lithium ion secondary battery and the like, and thus is not particularly limited, and may be various forms such as a rod shape, a plate shape, a sheet shape, and a foil shape.

The positive electrode active material layer may contain an optional component such as a conductive material and a binder (binder) in addition to the positive electrode active material.
As the conductive material, a carbon material such as carbon black (for example, acetylene black or ketjen black) can be adopted. As the binder, various polymer materials such as polyvinylidene fluoride (PVDF) and polyethylene oxide (PEO) can be adopted.

[Lithium ion secondary battery]
The negative electrode of the lithium ion secondary battery disclosed here has a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector. The negative electrode active material layer typically includes a negative electrode active material, a binder, a thickener, and the like. As the negative electrode current collector, a conductive material made of a metal having good conductivity (for example, copper) can be suitably used. As the negative electrode active material, a carbon material such as graphite (graphite), non-graphitizable carbon (hard carbon), graphitizable carbon (soft carbon), or the like can be used, and among them, graphite can be preferably used. As the binder, various polymer materials such as styrene butadiene rubber (SBR) can be adopted. As the thickener, various polymer materials such as carboxymethyl cellulose (CMC) can be employed.

Hereinafter, although one form of the lithium ion secondary battery constructed | assembled using the said positive electrode and the said negative electrode is demonstrated referring drawings, it is not intending limiting this invention to this embodiment. That is, as long as the positive electrode and the negative electrode are employed, the shape (outer shape and size) of the lithium ion secondary battery to be constructed is not particularly limited. In the following embodiment, a lithium ion secondary battery having a configuration in which a wound electrode body and an electrolytic solution are housed in a rectangular battery case will be described as an example.
In addition, in the following drawings, the same code | symbol is attached | subjected to the member and site | part which show | plays the same effect | action, and the overlapping description may be abbreviate | omitted. Moreover, the dimensional relationship (length, width, thickness, etc.) in each drawing does not necessarily reflect the actual dimensional relationship.

  As shown in FIGS. 1 and 2, the lithium ion secondary battery 10 of this embodiment includes a battery case 15. The battery case 15 of the present embodiment is a battery case made of metal (for example, aluminum) and has a bottomed flat box shape (typically a rectangular parallelepiped shape) with an open upper end (external case). ) 30 and a lid body 25 that closes the opening 20 of the case main body 30. On the upper surface (that is, the lid body 25) of the battery case 15, a positive electrode terminal 60 that is electrically connected to the positive electrode sheet 64 of the wound electrode body 100 and a negative electrode terminal that is electrically connected to the negative electrode sheet 84 of the wound electrode body 100. 80 is provided. Further, the lid body 25 is formed with an inlet (not shown) for injecting a non-aqueous electrolyte described later into the case body 30 (battery case 15) in which the wound electrode body 100 is accommodated. . The inlet is sealed with a sealing plug after the nonaqueous electrolyte is injected. Further, as in the case of the conventional lithium ion secondary battery, the lid 25 is provided with a safety valve 40 for discharging the gas generated inside the battery case 15 to the outside of the battery case 15 when the battery is abnormal. ing. The wound electrode body 100 is placed in a case in a posture in which the wound axis of the wound electrode body 100 is laid down (that is, the opening 20 is formed in the normal direction of the wound axis of the wound electrode body 100). Housed in the main body 30. Thereafter, the opening 20 of the case body 30 is sealed with the lid body 25, thereby producing the lithium ion secondary battery (nonaqueous electrolyte secondary battery) 10. The lid 25 and the case body 30 are joined by welding or the like.

  The wound electrode body 100 is manufactured by winding a long positive electrode sheet 64 and a long negative electrode sheet 84 in the longitudinal direction in a state where a total of two long separator sheets 90 are interposed. . At the time of the above lamination, the positive electrode active material layer non-formed portion of the positive electrode sheet 64 (that is, the portion where the positive electrode current collector 62 is exposed without forming the positive electrode mixture layer 66) 63 and the negative electrode active material of the negative electrode sheet 84 The positive electrode sheet 64 and the negative electrode sheet are so formed that the layer non-formed portion (that is, the portion where the negative electrode current collector layer 82 is not formed and the negative electrode current collector 82 is exposed) 83 protrudes from both sides of the separator sheet 90 in the width direction. 84 are slightly shifted in the width direction and overlapped. As a result, a laminated portion in which the positive electrode sheet 64, the negative electrode sheet 84, and the separator sheet 90 are laminated and wound is formed in the central portion of the wound electrode body 100 in the winding axis direction.

  As shown in FIG. 2, a positive electrode terminal 60 (for example, made of aluminum) is joined to the positive electrode active material layer non-formed portion 63 of the wound electrode body 100 via a positive electrode current collector plate 61. The positive electrode sheet 64 and the positive electrode terminal 60 are electrically connected. Similarly, a negative electrode terminal 80 (for example, made of nickel) is joined to the negative electrode active material layer non-forming portion 83 via the negative electrode current collector plate 81 to electrically connect the negative electrode sheet 84 and the negative electrode terminal 80. The positive and negative electrode terminals 60 and 80 and the positive and negative electrode active material layer non-forming portions 63 and 83 (typically, the positive and negative electrode current collectors 62 and 82) are joined by, for example, ultrasonic welding or resistance welding. be able to.

  The separator disclosed here can use a conventionally well-known thing without a restriction | limiting in particular. For example, a porous sheet made of resin (a microporous resin sheet) can be preferably used. A porous polyolefin resin sheet such as polyethylene (PE) or polypropylene (PP) is preferred. For example, a PE single layer sheet, a PP single layer sheet, a two layer structure (PE / PP structure) in which a PE layer and a PP layer are laminated, and a three layer structure in which a PP layer is laminated on both sides of the PE layer. A sheet of (PP / PE / PP structure) or the like can be preferably used.

As the non-aqueous electrolyte solution disclosed here, typically, an organic solvent (non-aqueous solvent) containing a supporting salt is used. As the supporting salt, a lithium salt, a sodium salt or the like can be used, and among them, a lithium salt such as LiPF ₆ or LiBF ₄ can be preferably used. As the organic solvent, aprotic solvents such as carbonates, esters and ethers can be used. Among these, for example, carbonates such as ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC), and fluorine such as methyl 2,2,2-trifluoroethyl carbonate (MTFEC). Fluorinated cyclic carbonates such as fluorinated chain carbonate, monofluoroethylene carbonate (MFEC), and difluoroethylene carbonate (DFEC) can be preferably used.

  In the lithium ion secondary battery constructed using the positive electrode active material disclosed herein, the contact area between the second oxide phase (layered structure oxide) and the non-aqueous electrolyte in the primary particles constituting the positive electrode active material Is small. As a result, even when charging / discharging is repeated under a high temperature condition of about 60 ° C. until the potential of the positive electrode is 4.5 V or more based on the metallic lithium, the deterioration of the second oxide phase is suppressed. . In addition, during the charge / discharge, a part of the non-aqueous electrolyte is oxidatively decomposed to generate an acid (for example, hydrogen fluoride (HF)), and this acid causes a transition metal from the first oxide phase (spinel structure oxide). Elements (eg, manganese) can elute into the non-aqueous electrolyte. However, the second oxide phase in which the deterioration is suppressed effectively absorbs the acid. For this reason, even if an acid generate | occur | produces, the elution of the transition metal element from a 1st entering oxide phase is suppressed. From the above, even under a high temperature condition of about 60 ° C., deterioration of the oxide of each phase of the primary particles constituting the positive electrode active material is suppressed. For this reason, the lithium ion secondary battery constructed | assembled using the said positive electrode active material becomes a thing excellent in durability.

  EXAMPLES Examples relating to the present invention will be described below, but the present invention is not intended to be limited to those shown in the examples.

<Experimental example 1>
[Production of lithium ion secondary battery]
<Example 1>
The precursor according to Example 1 is prepared by dissolving nickel sulfate and manganese sulfate in water so that the molar ratio of metal elements Ni: Mn is 0.5: 1.5, and stirring while adding sodium hydroxide. Obtained. The precursor according to Example 1 and a predetermined amount of lithium carbonate were mixed, calcined at 900 ° C. for 15 hours in an air atmosphere, and then pulverized by a ball mill to form LiNi _{0 having} a spinel crystal structure with an average particle size of 10 μm. _.5 Mn _1.5 O ₄ was obtained. The obtained LiNi _0.5 Mn _1.5 O ₄ was used as the positive electrode active material according to Example 1.

  The positive electrode active material according to Example 1, acetylene black as a conductive material, and PVdF as a binder, N-methyl-2-pyrrolidone (NMP) so that the mass ratio of these materials is 87: 10: 3 To prepare a paste-like composition for forming a positive electrode active material layer. This composition was applied to an aluminum foil (positive electrode current collector) having a thickness of 15 μm. The coated material was dried and pressed to prepare a positive electrode sheet in which a positive electrode active material layer was formed on the positive electrode current collector.

  Natural graphite having an average particle diameter of 20 μm as a negative electrode active material, styrene-butadiene copolymer (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener, and a mass ratio of these materials is 98: A paste-like composition for forming a negative electrode active material layer was prepared by mixing with water so as to be 1: 1. This composition was applied to a copper foil (negative electrode current collector) having a thickness of 15 μm. The coated material was dried and pressed to prepare a negative electrode sheet in which a negative electrode active material layer was formed on the negative electrode current collector.

The prepared positive electrode sheet and negative electrode sheet are disposed opposite to each other with a separator sheet (polypropylene / polyethylene two-layer porous film) interposed therebetween (laminated), and this is accommodated in a laminate type case (laminate film) together with a non-aqueous electrolyte. Thus, a lithium ion secondary battery according to Example 1 was produced. As a non-aqueous electrolyte, 1 mol / L LiPF ₆ was dissolved in a mixed solvent of methyl 2,2,2-trifluoroethyl carbonate (MTFEC) and monofluoroethylene carbonate (MFEC) in a volume ratio of 1: 1. I used something.

<Example 2>
Nickel sulfate, iron sulfate, manganese sulfate and titanium sulfate are dissolved in water so that the molar ratio of metal elements Ni: Fe: Mn: Ti is 0.45: 0.05: 1.45: 0.05, The precursor according to Example 2 was obtained by stirring while adding sodium hydroxide. A spinel crystal structure LiNi _0.45 Fe _0.05 Mn _1.45 Ti _0.05 O ₄ having an average particle diameter of 10 μm was obtained in the same manner as in Example 1 except that the precursor according to Example 2 was used. . The obtained LiNi _0.45 Fe _0.05 Mn _1.45 Ti _0.05 O ₄ was used as the positive electrode active material according to Example 2. A lithium ion secondary battery according to Example 2 was produced in the same manner as Example 1 except that the positive electrode active material according to Example 2 was used.

<Example 3>
The precursor according to Example 3 is obtained by dissolving lithium acetate dihydrate and manganese (II) acetate tetrahydrate in water and stirring while adding glycolic acid (C ₂ H ₄ O ₃ ). It was. The precursor according to Example 3 was calcined at 800 ° C. for 10 hours in an air atmosphere, and then pulverized with a ball mill to obtain a layered crystal structure of Li ₂ MnO ₃ (lithium manganese oxide) having an average particle size of 5 μm. . The molar ratio of the obtained Li ₂ MnO ₃ and the positive electrode active material according to Example 2 (Li ₂ MnO ₃ : LiNi _0.45 Fe _0.05 Mn _1.45 Ti _0.05 O ₄ ) was 0.05: 0. A positive electrode active material according to Example 3 was obtained by mixing so as to be 95. When the total amount of the positive electrode active material was 100 mol%, the ratio of Li ₂ MnO ₃ was 5 mol%. A lithium ion secondary battery according to Example 3 was fabricated in the same manner as in Example 1 except that the positive electrode active material according to Example 3 was used.

<Example 4>
Nickel sulfate, iron sulfate, manganese sulfate and titanium sulfate are dissolved in water so that the molar ratio of metal elements Ni: Fe: Mn: Ti is 0.45: 0.05: 1.45: 0.05, The precursor according to Example 4 was obtained by stirring while adding sodium hydroxide, further dissolving manganese sulfate in water, and stirring while adding sodium hydroxide. The precursor according to Example 4 and a predetermined amount of lithium carbonate were mixed, calcined at 900 ° C. for 15 hours in an air atmosphere, and then pulverized with a ball mill to obtain LiNi _0.45 Fe _{0. 05} Mn _1.45 Ti _0.05 O ₄ —Li ₂ MnO ₃ (hereinafter sometimes referred to as “LiNiFeMnTiO—LiMnO”) was obtained. The obtained LiNiFeMnTiO—LiMnO was used as the positive electrode active material according to Example 4. In addition, as a result of analyzing the X-ray diffraction profile based on the Rietveld method, primary particles constituting LiNiFeMnTiO—LiMnO were obtained from LiNi _0.45 Fe _0.05 Mn _1.45 Ti _0.05 O _{4 having} a spinel crystal structure. And a phase composed of Li ₂ MnO ₃ having a layered crystal structure. These phases have a molar ratio (LiNi _0.45 Fe _0.05 Mn _1.45 Ti _0.05 O ₄ : Li ₂ MnO ₃ ) of 0.95: 0.05 (that is, the total amount of the positive electrode active material). It was confirmed that the ratio of Li ₂ MnO ₃ was 5 mol%). A lithium ion secondary battery according to Example 4 was produced in the same manner as in Example 1 except that the positive electrode active material according to Example 4 was used.

[Initial capacity measurement]
For the lithium ion secondary batteries according to Examples 1 to 4 prepared above, after charging at a constant current up to 4.9 V at a current value (charge rate) of C / 5, the current value during constant voltage charging is The battery was fully charged (SOC 100%) by performing constant voltage charging up to a point where C / 50 was reached. Thereafter, the discharge capacity (initial capacity) when discharging at a constant current up to 3.5 V with a current value of C / 5 under a temperature condition of 25 ° C. was measured. Here, 1C means a current value that can charge the battery capacity (Ah) predicted from the theoretical capacity of the positive electrode in one hour.

[High-temperature charge / discharge cycle test]
Charging / discharging was repeated 200 cycles for each of the lithium ion secondary batteries of Examples 1 to 4 after the initial capacity measurement, and the discharge capacity after 200 cycles was measured in the same manner as the initial capacity measurement. The charge / discharge condition of one cycle is a constant current charge to a voltage of 4.9V at a charge rate of 2C under a temperature condition of 60 ° C., and then a constant current discharge to a voltage of 3.5V at a discharge rate of 2C. It was. The rate of reduction in capacity after 200 cycles with respect to the initial capacity ((initial capacity−discharge capacity after 200 cycles) / initial capacity × 100 (%)) was calculated. Table 1 shows the capacity reduction ratio of each battery when the capacity reduction ratio of the lithium ion secondary battery according to Example 1 is used as a reference.

As shown in Table 1, LiNi _0.45 Fe _0.05 Mn _{1.45 in} which a part of Mn was replaced with 0.05 mol of Ti in addition to a part of Ni was replaced with 0.05 mol of Fe. In the lithium ion secondary battery according to Example 2 provided with Ti _0.05 O ₄ , it was confirmed that the capacity reduction was suppressed as compared with the lithium ion secondary battery according to Example 1. Lithium ion secondary according to Example 3 mixed such that the molar ratio of LiNi _0.45 Fe _0.05 Mn _1.45 Ti _0.05 O ₄ and Li ₂ MnO ₃ is 0.95: 0.05. In the battery, although the spinel structure oxide was substituted with Fe and Ti, it was confirmed that the decrease in capacity was worsened by adding Li ₂ MnO ₃ . On the other hand, as shown in Example 4, primary particles containing a phase composed of spinel structure LiNi _0.45 Fe _0.05 Mn _1.45 Ti _0.05 O ₄ and a phase composed of layered structure Li ₂ MnO ₃ In the provided lithium ion secondary battery, it was confirmed that the capacity | capacitance fall was further suppressed compared with the lithium ion secondary battery which concerns on Example 2. FIG.

In addition, about the positive electrode active material (LiNiFeMnTiO-LiMnO) produced in Example 4, the fine structure was observed using TEM, and the obtained TEM image was shown in FIG. As a result, as shown in FIG. 3, it was confirmed that the positive electrode active material had three regions, a brighter region and two darker regions. In addition, the presence of precipitates or the like could not be confirmed at the interface of each region. Further, the darker region is present in a state where at least a part thereof is surrounded so as to be deposited in a brighter region. The Fast Fourier Transform (FFT) image about each area | region was shown to Fig.4 (a)-(c). FIG. 4A is an FFT image of the parent phase, which matches the incident pattern in the [110] direction of LiNi _0.45 Fe _0.05 Mn _1.45 Ti _0.05 O ₄ having a spinel crystal structure. Was confirmed. FIG. 4B is an FFT image of the precipitated phase indicated as “layer 1” in the drawing, and the incident pattern in the [0-10] direction of Li ₂ MnO ₃ having a layered crystal structure is represented by (c ) Is an FFT image of the precipitated phase shown as “Layer 2”, and it was confirmed that each coincided with the incident pattern in the [101] direction of Li ₂ MnO ₃ . When these patterns were compared, it was confirmed that O sites in specific crystal planes overlapped.

Therefore, this positive electrode active material has a first oxide phase (LiNi _0.45 Fe _0.05 Mn _1.45 Ti _0.05 O ₄ ; brighter region) having a spinel crystal structure as a parent phase. It was confirmed that the primary particles coexisted with the second oxide phase (Li ₂ MnO ₃ ; darker region) having a layered crystal structure shown as “layer 1” and “layer 2”. These (a) first oxide phase and two second oxide phases; (b) layered 1 and (c) layered 2 are aligned in a specific crystal orientation, and (a) first The oxide phase and the (b) layered second oxide phase, and (a) the first oxide phase and (c) the layered second oxide phase share an O site at the interface, and It was confirmed that lattice matching was found on the oxygen-containing surface. It can be considered that the characteristic crystal structure of Example 4 contributes to the above-described effect of suppressing the decrease in capacity.

<Experimental example 2>
[Production of lithium ion secondary battery]
<Example 5 to Example 12>
For LiNiFeMnTiO-LiMnO the above example _4, the ratio of Li 2 MnO ₃ prepared so that as shown in Table 2, were positive active material according to Example 7 to Example 12. That is, in the preparation of the positive electrode active material, the amount of lithium carbonate is adjusted and mixed so that Li becomes 1.1 mol to 1.3 mol when the total of the four transition metal elements in the raw material compound is 2 mol. did. Then, in the positive electrode active material thus prepared, LiMnO (second oxide phase) in LiNiFeMnTiO (first oxide phase) is shown in Table 2 by XRD analysis and accompanying Rietveld analysis. It was confirmed that the content was 3 mol% to 10 mol%.
Other conditions were the same as in Example 4, and lithium ion secondary batteries according to Examples 7 to 12 were produced. The lithium ion secondary battery according to Example 5 was manufactured in the same manner as the lithium ion secondary battery according to Example 1, and the lithium ion secondary battery according to Example 6 was the same as the lithium ion secondary battery according to Example 2. Made.

[Initial resistance measurement test]
About the lithium ion secondary battery which concerns on the produced said Example 5-Example 12, the measurement similar to the measurement of the initial stage capacity | capacitance performed with respect to the lithium ion secondary battery which concerns on the said Example 1- Example 4 was performed. Then, initial resistance was measured about each lithium ion secondary battery. That is, after adjusting to a SOC of 50% with a current value of 1 C under a temperature condition of 25 ° C., a constant current discharge was performed for 10 seconds at a current value of 5 C under a temperature condition of 25 ° C. The battery voltage ΔV dropped for 10 seconds was measured, and the initial resistance was obtained from Ohm's law from the voltage ΔV and the discharge current. Table 2 shows the initial resistance ratio of each battery when the initial resistance of the lithium ion secondary battery according to Example 5 is used as a reference.

[High-temperature charge / discharge cycle test]
About the lithium ion secondary battery which concerns on the said Example 5-Example 12 produced, the test similar to the charging / discharging cycle test performed with respect to the lithium ion secondary battery which concerns on the said Example 1- Example 4 was done. That is, for the lithium ion secondary batteries according to Examples 5 to 12, the capacity decrease rate after 200 cycles with respect to the initial capacity was calculated. Table 2 shows the capacity reduction ratio of each battery when the capacity reduction ratio of the lithium ion secondary battery according to Example 5 is used as a reference.

[Resistance measurement test after cycle test]
For the lithium ion secondary batteries according to Examples 5 to 12 after the charge / discharge cycle test, the resistance after the charge / discharge cycle test (post-cycle resistance) was measured. The measurement conditions are the same as in the initial resistance test. Table 2 shows the resistance ratio after cycling of each battery when the initial resistance of the lithium ion secondary battery according to Example 5 is used as a reference.

As shown in Table 2, when the positive electrode active material in which the proportion of Li ₂ MnO ₃ in Example 7 is 1 mol% and the positive electrode active material not containing Li ₂ MnO ₃ in Example 6 are compared, there is a particular difference in the suppression of capacity reduction. It was confirmed that there was no. On the other hand, comparing the positive electrode active material having a Li ₂ MnO ₃ ratio of 10 mol% in Example 12 and the positive electrode active material not containing Li ₂ MnO ₃ in Example 6, the capacity decrease is slightly suppressed in the positive electrode active material in Example 12. It was confirmed that In the positive electrode active materials in which the proportion of Li ₂ MnO _{3 in} Examples 8 to 11 was 3 mol% to 8 mol%, it was confirmed that the capacity decrease was suppressed as the proportion of Li ₂ MnO ₃ increased.
Regarding the initial resistance, the positive electrode active material of Example 7 having a Li ₂ MnO ₃ ratio of 1 mol% and the positive electrode active material of Example 12 having a Li ₂ MnO ₃ ratio of 10 mol% In comparison, it was confirmed to be smaller. On the other hand, in the positive electrode active materials of Examples 8 to 11, it was confirmed that the initial resistance was greatly reduced as compared with the positive electrode active material of Example 6.

  Moreover, about the post-cycle resistance, it was confirmed that the positive electrode active material of Example 7 was smaller than the positive electrode active material of Example 6. On the other hand, it was confirmed that the positive electrode active material of Example 12 had increased post-cycle resistance compared to the positive electrode active material of Example 6. This is considered to be due to the deterioration of the layered oxide due to the large contact area between the layered oxide and the non-aqueous electrolyte. In the positive electrode active materials of Examples 8 to 11, it was confirmed that the resistance after cycling was greatly reduced as compared with the positive electrode active material of Example 6. In particular, the positive electrode active material of Example 9 was confirmed to have the lowest post-cycle resistance.

<Experimental example 3>
[Production of lithium ion secondary battery]
<Example 13 to Example 21>
In accordance with the LiNiFeMnTiO—LiMnO preparation procedure of Example 4 above, a positive electrode active material having a changed composition was prepared. That is, using manganese sulfate, nickel sulfate, titanium sulfate, iron sulfate and cobalt sulfate as raw material compounds, the molar ratio of each transition metal element is the numerical ratio shown in the column of “first oxide phase” in Table 3 below. The amount of precursor charged was adjusted so that For example, in preparing the positive electrode active material of Example 18, the molar ratio of transition metal elements, Mn: Ni: Fe: Ti: Co is 1.45: 0.42: 0.05: 0.05: 0.03. Thus, each raw material compound was blended to prepare a precursor. Then, when the total amount of transition metal elements was 2 mol when the total amount of lithium carbonate was 2 mol, Li was mixed at a ratio of 1.2 mol, and baked, whereby positive electrode active materials according to Examples 15 to 21 were obtained. In addition, in the positive electrode active material adjusted in this way, the second oxide phase is included so as to be about 5 mol% of the whole (total of the first oxide phase and the second oxide phase) by XRD analysis. It was confirmed. Further, the composition of each phase was determined from XRD analysis and TEM observation and analysis based on these, and are shown in Table 3. In each of these positive electrode active materials, it was confirmed that the first oxide phase and the second oxide phase were lattice-matched on the oxygen-containing surface.
Incidentally, in the same manner as the positive electrode active material according to Example 1 above, composition was prepared the positive electrode active material according to Example 13 represented by _LiMn 1.5 _Ni 0.5 _{O 4.} In the same manner as the positive electrode active material according to Example 3 above, Li ₂ MnO ₃ was mixed with LiMn _1.5 Ni _0.5 O ₄ of Example 13 so that the proportion was 5 mol% of the whole, A positive electrode active material according to No. 14 was prepared.
Using these positive electrode active materials of Examples 13 to 21, lithium ion secondary batteries according to Examples 13 to 21 were prepared in the same manner as in Example 1 above.

[Initial capacity measurement]
About the lithium ion secondary battery which concerns on the produced said Example 13-Example 21, the measurement similar to the measurement of the initial stage capacity | capacitance performed with respect to the lithium ion secondary battery which concerns on the said Example 1- Example 4 was performed. The obtained initial capacity values are shown in Table 3.

[High-temperature charge / discharge cycle test]
About the lithium ion secondary battery which concerns on the produced said Example 13-Example 21, charging / discharging was repeated 200 cycles, and the discharge capacity after 50 cycles and after 200 cycles was measured with the method similar to initial stage capacity | capacitance measurement. The charge / discharge cycle conditions were the same as in Examples 1 to 4.
From these results, capacity reduction rates after 50 cycles and 200 cycles with respect to the initial capacity were calculated for the lithium ion secondary batteries according to Examples 13 to 21. And the result was shown in Table 3 as ratio when the capacity | capacitance reduction rate after 200 cycles of the lithium ion secondary battery which concerns on Example 13 is made into a reference | standard (100).

In Experimental Example 3, as shown in the above formula (II), the amount of Li is excessive (2 or more) as compared with Li ₂ MnO ₃ and the Mn site is shown in Table 3 above as the second oxide phase. Thus, a composite oxide substituted with Ni, Ti, Fe, and Co is obtained.
The positive electrode active material of Example 13 has a configuration containing only the first oxide phase. In the lithium ion secondary battery of Example 13, the capacity after the cycle is greatly reduced.
The positive electrode active material of Example 14 has a configuration in which a lithium transition metal oxide containing excessive lithium is mixed as the second oxide phase with the positive electrode active material of Example 13. In the lithium ion secondary battery of Example 14, the capacity decrease rate after the first 50 cycles was greatly reduced as compared with the lithium ion secondary battery of Example 13, but the capacity decrease during the subsequent 50 to 200 cycles. The rate was almost the same, and it was found that there was not much improvement effect of durability against long-term cycle.

On the other hand, about the lithium ion secondary battery which concerns on Examples 15-21, it turned out that all the capacity | capacitance fall rates after 50 cycles and 200 cycles are improved greatly.
In the lithium ion secondary battery according to Example 15, the Mn and Ni sites of the first oxide phase that is the parent phase are replaced with Ti and Fe, and the so-called lithium-rich second oxide phase containing Ti and Fe as the precipitated phase The positive electrode active material of the structure containing is used. As compared with the lithium ion secondary battery according to Example 14, such a lithium ion secondary battery was confirmed to have greatly improved capacity reduction rate after 50 cycles and after 200 cycles. This is because the substitution of the first oxide phase with Ti and Fe strengthens the bond between the oxygen (O) atom and the transition metal atom in the crystal structure, and changes the valence of Mn. This is considered to be due to the decrease in the phase separation region.

  In the lithium ion secondary battery according to Example 16, the Mn and Ni sites of the first oxide phase which is the parent phase are replaced with Ti and Co, and the so-called lithium-rich second oxide phase containing Ti and Co as the precipitated phase The positive electrode active material of the structure containing is used. In comparison with the lithium ion secondary battery according to Example 15, this lithium ion secondary battery has a greatly improved capacity reduction rate after 50 cycles. This is thought to be due to the fact that the second oxide phase of the positive electrode active material of Example 16 was also replaced with Co, thereby promoting the separation of Li and oxygen (O) and increasing the amount of Li to be replenished. It is done.

  In the lithium ion secondary battery according to Example 17, the Mn and Ni sites of the first oxide phase that is the parent phase are replaced with Fe and Co, and the so-called lithium-rich second oxide containing Fe and Co as the precipitated phase A positive electrode active material including a phase is used. In this lithium ion secondary battery, as in Example 16, it is considered that the capacity reduction rate after 50 cycles was greatly improved by including Co in the second oxide phase. In addition, it is thought that the capacity | capacitance reduction rate after 200 cycles increased a little from Example 15 by the influence which Ti is not contained in the 1st oxide phase.

  In the lithium ion secondary battery according to Example 18, the Mn and Ni sites of the first oxide phase that is the parent phase are replaced with Ti, Fe, and Co, and so-called lithium-rich Ti, Fe, and Co are used as the precipitated phase. The positive electrode active material of the structure containing the 2nd oxide phase containing is used. In this lithium ion secondary battery, the capacity increasing effect due to the inclusion of Co in the second oxide phase and the capacity deterioration suppressing effect due to the inclusion of Ti and Fe in the first oxide phase are realized in a well-balanced manner. It can be said that it is preferable.

  The lithium ion secondary battery according to Example 19 uses a positive electrode active material in which the substitution amount of Ti is increased as compared with Example 18. The lithium ion secondary battery according to Example 20 uses a positive electrode active material that does not contain Fe and further increases the substitution amount of Ti as compared with Example 18. It can be seen that these lithium ion secondary batteries have a larger capacity reduction rate after 50 cycles than in Example 18, and the capacity has decreased. This is considered due to the fact that the separation of Li and O was suppressed due to the increase in the Ti content of the second oxide phase. However, in both cases, the capacity deterioration rate after the high-temperature cycle is suppressed to be lower than those of the lithium ion secondary batteries of Examples 13 and 14.

The lithium ion secondary battery according to Example 21 uses a positive electrode active material with an increased amount of Co substitution than in Example 18. In this lithium ion secondary battery, the capacity decrease rate after 50 cycles was further suppressed as compared with Example 18, but the capacity decrease rate after 200 cycles was higher than Example 18. This is considered to be because the Co content of the second oxide phase increased, Li and O were separated excessively, and the crystal structure became unstable. In addition, the Co content of the first oxide phase slightly increased, and the initial capacity tended to decrease.
For these reasons, in both the first oxide phase and the second oxide phase, the transition metal is not limited to Mn and Ni, but can be substituted with other transition metal elements such as Ti, Fe and Co. It can be said that it is preferable. It can be said that it is preferable that the number of such substitution elements is two or more, for example three, rather than one, in order to stably improve the high-temperature cycle characteristics of the lithium ion secondary battery over a long period of time.

<Experimental example 4>
[Production of lithium ion secondary battery]
<Example 22 to Example 25>
Similar to the procedure for preparing LiNiFeMnTiO—LiMnO in Example 4 above, a precursor having a transition metal element molar ratio of Mn: Ni: Ti: Fe of 1.45: 0.45: 0.05.45: 0.05 was prepared. did. Then, lithium carbonate is mixed with this precursor at a ratio of Li to 1.1 to 1.3 mol when the total amount of transition metal elements is 2 mol, and calcined. A positive electrode active material was obtained. In addition, in the positive electrode active materials of Examples 23 to 25 prepared as described above, the XRD analysis showed that the second oxide phase was 1 mol% of the total (total of the first oxide phase and the second oxide phase), It was confirmed that they were contained to be 5 mol% and 10 mol%, respectively. Moreover, the composition of each phase was calculated | required from XRD analysis and TEM observation, and the analysis based on these, and it showed in Table 4. In each of these positive electrode active materials, it was confirmed that the first oxide phase and the second oxide phase were lattice-matched on the oxygen-containing surface.
Incidentally, according to the positive active material according to Example 1 above, composition was prepared the positive electrode active material according to Example 22 represented by _{LiMn 1.45 Ni 0.45 Ti 0.05 Fe 0.05} O 4.
Using the positive electrode active materials of Examples 22 to 25, the other conditions were the same as in Example 1 above, and lithium ion secondary batteries according to Examples 22 to 25 were produced.

[Initial capacity measurement]
About the lithium ion secondary battery which concerns on the produced said Example 22-Example 25, the measurement similar to the measurement of the initial stage capacity | capacitance performed with respect to the lithium ion secondary battery which concerns on the said Example 1- Example 4 was performed. The obtained initial capacity values are shown in Table 4.

[High-temperature charge / discharge cycle test]
About the lithium ion secondary battery which concerns on the produced said Example 22-Example 25, it carried out similarly to Example 13-Example 24, and performed the high temperature charging / discharging cycle test. That is, charging / discharging was repeated 200 cycles, and the discharge capacity after 50 cycles and after 200 cycles was measured by the same method as the initial capacity measurement. The charge / discharge cycle conditions were the same as in Examples 1 to 4.
From these results, for the lithium ion secondary batteries according to Examples 22 to 25, the capacity decrease rate after 50 cycles and 200 cycles with respect to the initial capacity was calculated. And the result was shown in Table 4 as ratio when the capacity | capacitance reduction rate after 200 cycles of the lithium ion secondary battery which concerns on Example 22 is made into a reference | standard (100).

The lithium ion secondary batteries according to Examples 23 to 25 were found to have no significant difference from Example 22 although the initial capacity was slightly reduced. However, for the lithium ion secondary battery of Example 25 in which the ratio of the second oxide phase was 10 mol%, the initial capacity was reduced. This is thought to be due to the decrease in the proportion of the first oxide phase.
However, it was confirmed that the capacity reduction rate after 50 cycles was significantly improved as the proportion of the second oxide phase increased. Further, regarding the high-temperature cycle characteristics after 200 cycles, the case where the ratio of the second oxide phase was 5 mol was minimized, but it was found that the capacity reduction was generally suppressed. However, from the results of the initial capacity and the capacity decrease ratio after 200 cycles, it can be said that the ratio of the second oxide phase is preferably suppressed to less than 10 mol%.
From these facts, in both the first oxide phase and the second oxide phase, the transition metal is preferably limited to other transition metal elements such as Ti and Fe rather than being limited to Mn and Ni only. I can say that. In addition to Mn and Ni, it is also possible to add two or more kinds of other transition metal elements, such as three kinds as shown in Example 3 above, in addition to Mn and Ni. It can be said that it is preferable for stably improving the high-temperature cycle characteristics of the battery over a long period of time.

  As mentioned above, although the specific example of this invention was demonstrated in detail, these are only illustrations and do not limit a claim. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.

DESCRIPTION OF SYMBOLS 10 Lithium ion secondary battery 15 Battery case 25 Cover body 30 Case main body 60 Positive electrode terminal 62 Positive electrode current collector 64 Positive electrode sheet 80 Negative electrode terminal 82 Negative electrode current collector 84 Negative electrode sheet 90 Separator sheet 100 Winding electrode body

Claims (11)

A positive electrode active material for a lithium ion secondary battery,
The primary particles constituting the positive electrode active material are:
A first oxide phase having a spinel crystal structure and comprising a composite oxide of lithium and a first transition metal containing at least nickel and manganese;
A positive electrode active material for a lithium ion secondary battery, which has a layered rock salt type crystal structure and includes a second oxide phase made of a composite oxide of a second transition metal and lithium.   2. The positive electrode active material according to claim 1, wherein the second transition metal is one or more transition metals containing at least manganese and selected from the first transition metal.   The positive electrode active material according to claim 1, wherein at least a part of an interface between the first oxide phase and the second oxide phase is aligned on an oxygen surface in each crystal structure.   The ratio of said 2nd oxide phase is 3 mol% or more and 8 mol% or less, when the sum total of said 1st oxide phase and said 2nd oxide phase is 100 mol%, Any one of Claim 1 to 3 The positive electrode active material described in 1.   5. The positive electrode active material according to claim 1, wherein at least part of the second oxide phase is present in the first oxide phase. 6.   The positive electrode active material according to any one of claims 1 to 5, wherein the first transition metal further includes at least one of titanium, iron, and cobalt. The first oxide phase has the following general formula (I):
LiMn _{2- (a1 + b1 + c1 + d1)} Ni _a1 Ti _b1 Fe _c1 Co _d1 O _4-δ (I)
(Where 0.3 ≦ a1 ≦ 0.6, 0 ≦ b1 ≦ 0.2, 0 ≦ c1 ≦ 0.2, 0 ≦ d1 ≦ 0.1, b1 + c1 + d1 ≠ 0 and 0 ≦ δ ≦ 1) ;
The positive electrode active material of Claim 6 which consists of an oxide shown by these.   The positive electrode active material according to any one of claims 1 to 7, wherein the second transition metal includes at least manganese and further includes at least one of nickel, titanium, iron, and cobalt. The second oxide phase has the following general formula (II):
Li _3-x (Mn _{1- (a2 + b2 + c2 + d2)} Ni _a2 Ti _b2 Fe _c2 Co _d2 ) _x O _{3-δ ′} (II)
(Where 1 ≦ x ≦ 1.5, 0 ≦ a2 ≦ 0.2, 0 ≦ b2 ≦ 0.3, 0 ≦ c2 ≦ 0.2, 0 ≦ d2 ≦ 0.2, a2 + b2 + c2 + d2 ≠ 0 and 0 ≦ δ ′ ≦ 1);
The positive electrode active material of Claim 8 comprised from the oxide shown by these.   The positive electrode for lithium ion secondary batteries provided with the positive electrode active material as described in any one of Claim 1 to 9.   A lithium ion secondary battery comprising the positive electrode for a lithium ion secondary battery according to claim 10.