JP2016207316A - Positive electrode active material for nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode active material having a low output resistance even when used in a nonaqueous secondary battery of such use as performing charge and discharge at high rate, and capable of maintaining excellent battery characteristics of good cycle characteristics.SOLUTION: A positive electrode active material containing active material particles composed of a lithium transition metal oxide having a layer crystal structure, and constituted of a first region on the center side in the radial direction and substantially not containing an additive element, and a second region on the outside of the first region and containing an additive element, is provided. This active material particles contains at least one kind of additive element selected from silicon (Si), phosphor (P) and germanium (Ge). In the second region, the content ratio of the additive element increases gradually in a range of 0-5 mass%, from the radial inside toward outside, assuming that the total of transition metal element in the lithium metal transition oxide is 100 mass%.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a positive electrode active material for a non-aqueous electrolyte secondary battery.

  Non-aqueous electrolyte secondary batteries represented by lithium secondary batteries are lighter and have higher energy density than existing batteries. Therefore, so-called portable power sources such as personal computers and portable terminals, vehicle drive power sources, and housing It is used as a power storage device. Particularly in recent years, non-aqueous electrolyte secondary batteries are used for driving vehicles such as electric vehicles (EV), hybrid vehicles (HV), and plug-in hybrid vehicles (PHV) that charge and discharge at a high capacity and high rate. It is preferably used as a high output power source.

  In this type of non-aqueous electrolyte secondary battery, a positive electrode including a positive electrode active material and a negative electrode including a negative electrode active material are disposed to face each other via a separator, and are accommodated in a battery case together with a non-aqueous electrolyte. Charging and discharging can be realized by inserting and extracting charge carriers (lithium ions in the case of a lithium secondary battery) between the positive electrode active material and the negative electrode active material. The battery performance of the secondary battery largely depends on various characteristics of the active material provided in the positive electrode and the negative electrode, and development of an active material that realizes appropriate characteristics is being promoted depending on the use of the battery.

JP 2008-016232 A International Publication No. 2007/114557

  For example, Patent Document 1 includes a coating layer made of a lithium transition metal composite oxide containing nickel and / or manganese on the surface of lithium transition metal composite oxide particles containing at least cobalt, and at least a part of the coating layer. Discloses a positive electrode active material having a surface layer containing silicon. Patent Document 1 describes that the use of this positive electrode active material improves the capacity and charge / discharge cycle characteristics of a non-aqueous electrolyte secondary battery. However, when the positive electrode active material disclosed in the technology of Patent Document 1 is used for a non-aqueous electrolyte secondary battery for charging / discharging at a high rate, there is a drawback that it is difficult to obtain sufficient battery characteristics. In addition, the cycle characteristics are not so good.

  The present invention has been made in view of such circumstances, and the purpose thereof is, for example, a low output resistance even when used in a non-aqueous electrolyte secondary battery for use in charging / discharging at a high rate, and the cycle. An object of the present invention is to provide a positive electrode active material having good characteristics and capable of maintaining excellent battery characteristics.

  In order to achieve the above object, the present invention provides a positive electrode active material for a non-aqueous electrolyte secondary battery. This positive electrode active material includes active material particles made of a lithium transition metal oxide having a layered crystal structure. The active material particles (primary particles) contain at least one additive element selected from the group consisting of silicon (Si), phosphorus (P), and germanium (Ge). The active material particles are a region on the center side in the radial direction of the active material particles, a first region substantially not containing the additive element, and a region outside the first region, And a second region containing an additive element. Here, in the positive electrode active material disclosed herein, in the second region, when the content ratio of the additive element is 100% by mass of the total transition metal elements in the lithium transition metal oxide, the radial direction It is characterized by a gradual increase in the range from 0% to 5% by mass from the inside to the outside.

  According to the positive electrode active material disclosed herein, the active material particles contain an additive element while maintaining a layered crystal structure as a whole. This increases the interlayer distance at the end of the layered crystal structure and improves the acceptability of charge carriers. In addition, since the additive element is contained only in the second region with the concentration and concentration distribution mode being limited, the internal resistance of the battery can be reduced. With these features, a positive electrode active material that realizes a secondary battery with both high-rate charge / discharge characteristics and cycle characteristics is realized.

In addition, although the present inventors suppressed sintering between lithium composite oxide particles by the surface layer containing silicon about the positive electrode active material disclosed by patent document 1, the battery which charges / discharges at high rate It is known that the resistance of the surface layer is extremely increased by silicon as a resistance component. It has also been found that when charging and discharging at a high rate are repeated, the crystal structure of the surface layer and the coating layer are different, so that the crystal stability at the interface is lowered and the cycle characteristics are impaired.
Further, in Patent Document 2, in lithium transition metal oxide particles having different types of transition metal elements in the internal bulk portion and the external bulk portion, the boundary surface between the external bulk portion and the internal bulk portion is directed toward the active material surface. A positive electrode active material in which the metal composition exists in a continuous concentration gradient is disclosed. However, Patent Document 2 does not improve the acceptability of charge carriers by making the entire lithium transition metal oxide particles have a layered crystal structure and further modifying the crystal structure.
In these respects, the present invention can be completely distinguished from the techniques disclosed in Patent Documents 1 and 2.

It is sectional drawing which showed typically the structure of the positive electrode active material particle which concerns on one Embodiment. (A) It is the figure which showed typically the general crystal structure of layer type lithium transition metal oxide, (B) The figure which showed typically the crystal structure of the lithium transition metal oxide disclosed here is there. It is a figure which shows concentration distribution of the additive element in the depth direction from the surface of the positive electrode active material particle which concerns on one Embodiment. It is a figure which shows concentration distribution of the additive element in the depth direction from the surface of the conventional positive electrode active material particle.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings as appropriate. Note that matters other than matters specifically mentioned in the present specification and necessary for the implementation of the present invention (for example, the configuration and operation of a non-aqueous electrolyte secondary battery that does not characterize the present invention) It can be grasped as a design matter of a person skilled in the art based on the prior art in the field. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field. Moreover, 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 or simplified. The dimensional relationship (length, width, thickness, etc.) in each figure does not necessarily reflect the actual dimensional relationship.

The positive electrode active material for a non-aqueous electrolyte secondary battery disclosed herein essentially includes active material particles made of a lithium transition metal oxide having a layered crystal structure. The lithium transition metal oxide having a layered crystal structure constituting the active material particles is mainly composed of a composite oxide of a transition metal and lithium. The composition of the lithium transition metal oxide is not strictly limited. For example, in lithium transition metal oxides widely used as positive electrode active materials for non-aqueous electrolyte secondary batteries, so-called zigzag layered manganese-based lithium composite oxides or layered rock-salt-type cobalt-based lithium composite oxides or nickel-based lithium composites It can be a compound recognized as an oxide or the like. This oxide is typically a compound represented by the general formula: LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , Li (Ni, Mn, Co) O ₂ , and typically, Li ₂ MnO. ₃ may be a so-called lithium rich type (excess lithium type) compound. In addition, these compounds may be obtained by substituting a part of each transition metal site with one or more other transition metal elements or typical elements. Moreover, both may form a solid solution. When the lithium transition metal oxide has such a chemical composition, the positive electrode active material can have, for example, high capacity and high stability of the layered crystal structure against heat and battery chemical reaction.

As schematically shown in FIG. 1, the active material particles disclosed herein are regions on the center side in the radial direction of the active material particles, and the first region does not substantially contain an additive element; A region outside the first region, and a second region containing an additive element. In other words, when the active material particles are divided into a first region on the center side and a second region outside the first region in the radial direction, the first region does not substantially contain an additive element. In the second region on the surface layer side, an additive element is included. Typically, the second region has essentially the same chemical composition as the first region except that the additive element is included (the metal element other than the additive element is the same as the first region). The same stoichiometric composition).
In this specification, “radial direction” means a direction passing through the center of gravity of the active material particles, “center side” in the radial direction means a side closer to the center of gravity of the active material particles, and “outside” means active side. The side farther from the center of gravity of the substance particle and closer to the surface is meant. The active material particles are not limited to a spherical shape, and may be an elliptical shape or an indefinite shape having an uneven surface.
Further, “substantially does not contain” an additive element means that the additive element is not intentionally contained. For example, the content of the additive element may be 3% by mass or less, more specifically 2% by mass or less, for example, 1% by mass or less.

  In a layered lithium transition metal oxide, typically, a surface in which oxygen atoms are closely packed is stacked, and lithium (Li) and a transition metal element are regularly arranged in octahedral sites of the close-packed gap. Thus, as schematically shown in FIG. 2A, a surface made of only lithium (hereinafter sometimes simply referred to as a lithium layer) and a surface composed of a transition metal element (hereinafter simply referred to as a transition metal layer). Are alternately stacked to form a crystal. In the lithium layer, lithium ions can move relatively smoothly in two dimensions. Then, the lithium ions are released from the end of the lithium layer to the outside of the crystal structure or inserted (occluded) into the surface from the outside, whereby the battery is charged and discharged.

  Here, as shown in Table 1 below, the transition metal element (M) forms the surrounding six oxygen atoms (O) and the MO6 octahedron as a structural unit. Here, the present inventors have introduced an element constituting an MO4 tetrahedron having a shorter bond distance with oxygen than the MO6 octahedron into the layer made of transition metal, whereby the distance between oxygen atoms, that is, the transition metal layer. It was conceived that an effect of reducing the thickness of the film can be obtained. This relatively increases the thickness of the lithium layer and can increase the interlayer distance of the transition metal layer. Therefore, by introducing this MO4 tetrahedron locally on the surface of the lithium transition metal oxide particle (that may be the end of the crystal structure), for example, as shown in FIG. The thickness of the lithium layer on the surface (edge) can be increased. As a result, the insertion and extraction of lithium ions from the surface of the active material particles can be smoothly realized with low resistance. In particular, the significant improvement in lithium ion acceptance characteristics is significant in that it greatly contributes to the improvement of the high-rate characteristics of the secondary battery.

  Therefore, in the present invention, as the additive element, an element that can be dissolved in a transition metal layer having a layered crystal structure of a lithium transition metal oxide and can stably form an MO4 tetrahedron with oxygen atoms. Adopted. According to the inventors' diligent research, in order to stably form the above-mentioned MO4 tetrahedron, it is an element that can take a valence of 4 or more, and further has an ionic radius ratio of 2.42 to oxygen ions. The knowledge that it is necessary to be 4.44 or less is obtained. As an element that is stable as an additive element of the positive electrode active material and satisfies the above conditions, specifically, for example, silicon (Si), phosphorus (P), and germanium (Ge) can be considered. Any one of these elements may be added alone, or two or more of these elements may be added in combination.

  In addition, the said effect can be acquired by introduce | transducing an additional element even a little in a transition metal layer. As the proportion of the additive element increases, the thickness of the lithium layer increases, and the acceptability of charge carriers is improved. However, when a portion in which the ratio of the additive element exceeds 5% by mass when the total of transition metal elements is 100% by mass, the structure of the active material particles (including the layered crystal structure) is liable to collapse. If the layered crystal structure is disturbed, such an additive element acts as a resistance component of the battery, and the battery characteristics are deteriorated. Moreover, the movement of lithium ions in the lithium layer can be performed smoothly. Therefore, in the invention disclosed herein, the additive element is included at the highest ratio (concentration) of 5% by mass on the surface of the active material particles that can contribute most to the lithium ion acceptance characteristics.

  Further, when the content ratio of the additive element is rapidly changed, the stability of the structure of the active material particles is remarkably lowered. For example, if the content ratio of the additive element in the radial direction changes suddenly or exists locally, cracking or peeling occurs at the interface between the region containing a large amount of the additive element and the region containing a small amount of the additive element due to repeated charge and discharge. It may occur and the capacity degradation of the active material may easily occur. From this point of view, the content of the additive element is gradually and gradually decreased from the surface toward the center (center of gravity). In other words, the additive element gradually increases in the second region in the range of 0% by mass to 5% by mass or less in the radial direction.

  Note that the expansion of the lithium layer by the additive element as described above has little contribution even if it is realized inside the active material particles, and may cause the inclusion of an excessive additive element. From such a viewpoint, although not necessarily limited thereto, the thickness (depth from the surface) of the second region containing the additive element can be limited to, for example, 700 nm or less, and preferably 600 nm or less. More preferably, the thickness is 500 nm or less.

Further, “gradual increase” with respect to the content ratio of the additive element in the present specification means that, for example, when the second region is divided into five equal parts by the depth from the surface as shown in FIG. When the content of the additive element in the regions A to E is X (A) mass% to X (E) mass%, and the content in the first region is X (1) mass%, these X (A) to X X (E) and X (1) can be confirmed by satisfying the conditions of the following formulas (1) and (2).
Formula (1): 0 = X (1) ≦ X (E) ≦ X (D) ≦ X (C) ≦ X (B) ≦ X (A), 0 <X (A) ≦ 5
Formula (2): X (A) −X (B) ≦ 2, X (B) −X (C) ≦ 2, X (C) −X (D) ≦ 2, X (D) −X (E) ≦ 2, X (E) −X (1) ≦ 2

By satisfying the formula (1), it is confirmed that the content (concentration) of the additive element is increased in the range of 0% by mass to 5% by mass from the inner side to the outer side in the radial direction of the active material particles. can do. Further, by satisfying the formula (2), the concentration (content) difference between the adjacent regions becomes 2% by mass or less, and the degree of increase (concentration distribution) changes gradually and gradually (gradually). You can confirm that you are doing.
The content of the additive element in such a fine region of the active material particles can be measured using a known appropriate analysis means. As such analysis means, for example, X-ray photoelectron spectroscopy (XPS), time of flight secondary ion mass spectrometry (TOF-SIMS) or the like is used. can do. In addition, when investigating the transition of the content of the additive element in the depth direction, it is possible to use a known technique such as ion beam etching (ion milling) while appropriately shaving the surface of the active material particles with a predetermined thickness. What is necessary is just to measure content of the additive element.

  Further, whether or not the active material particles maintain the layered structure as a whole can be confirmed by X-ray diffraction (XRD) analysis or appropriate electron microscope observation. For example, heterogeneous phases with different crystal structures cannot be identified by X-ray diffraction, or an electron diffraction pattern obtained by observation with a transmission electron microscope (TEM) is analyzed to form a layered crystal structure on the surface of active material particles. Can be ascertained by observing the crystal structure with a high-resolution bright-field image of TEM or the like.

The lithium transition metal oxide constituting the first region is not necessarily limited to this, but can be represented by, for example, the following general formula (I).
_{Li 3-x (Mn 1- (} a + b + c) Ni a Co b M 1 c) x O 3-δ (I)
Here, in the formula (I), M represents at least one of any transition metal element and metal element other than Mn, Ni, Co, Si, Ge, and P. Moreover, it is preferable that x, a, b, c, and δ satisfy the following relationship. That is, 1 ≦ x ≦ 1.5, 0 ≦ a <1, 0 ≦ b <1, 0 ≦ c ≦ 0.2, a + b + c <1 and 0 ≦ δ ≦ 1.5. These values a, b, c, and δ cannot be unequivocally described because their values may vary depending on the ratio of the mutual elements. However, as an approximate guideline, for example, it is more preferable to satisfy the following.

  That is, the above x represents the content of the metal element in the composite oxide constituting the layered lithium transition metal oxide, and thus the ratio of Li as (1-x), the type of other constituent elements, the substitution ratio In addition, it may take a value of about 1 to 1.5 depending on environmental conditions. Said a shows Ni content, It is more preferable that it is 0.05 <= a, and it is more preferable that it is a <= 0.35. Said b shows Co content, It is more preferable that it is 0.01 <= b, It is more preferable that it is b <= 0.5, for example, b <= 0.35. The above c represents the content of transition metal elements other than Mn, Ni and Co and typical elements other than Si, Ge and P, more preferably 0.01 ≦ c, and c ≦ 0.3. Is more preferable. [1- (a + b + c)] represents the content of Mn, and is preferably larger than 0. That is, the lithium transition metal oxide preferably contains Mn. The δ indicates the amount of oxygen defects in the layered crystal structure, and may vary depending on environmental conditions and the like in addition to the type of substitution atom and the substitution ratio in the crystal structure, so that it is difficult to display accurately. For this reason, δ which is a variable for determining the number of oxygen atoms may be a positive number not exceeding 3, and typically 0 ≦ δ ≦ 1.5, for example, 0 ≦ δ ≦ 1 may be employed.

The transition metal element and the metal element that can be adopted as M ¹ are not particularly limited. For example, specifically, scandium (Sc), yttrium (Y), titanium (Ti), iron (Fe), zinc ( Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), molybdenum (Mo), tungsten (W), copper (Cu), silver (Ag), gold ( Examples thereof include use of one kind from Au), aluminum (Al), magnesium (Mg), or the like alone or in combination of two or more kinds. Especially, it is more preferable that it is any one or both of Ti and Fe.

The lithium transition metal oxide constituting the second region of the above, but are not necessarily limited to, for example, Si of Mn in the formula (I), Ni, Co, and M ^1, as an additive element , Ge and P can be substituted. As described above, since the concentration of the additive element is changed in the second region, the chemical composition is not shown by a general formula.

Such active material particles can be typically prepared in a powder state and can be used as a positive electrode active material for a non-aqueous electrolyte secondary battery. The average particle size of the powdered positive electrode active material can be appropriately adjusted depending on the application, but can be, for example, 1 μm or more and 25 μm or less, and typically 2 μm or more and 20 μm or less, for example, 6 μm or more. The thickness is preferably 15 μm or less. In the present specification, the “average particle diameter” means a particle diameter (D ₅₀ ) corresponding to a cumulative 50% in a volume-based particle size distribution based on a general laser diffraction / light scattering method. Therefore, the active material particles may exist in the form of primary particles, or may form secondary particles.

  Although the manufacturing method of the positive electrode active material and the like are not strictly limited, as described above, since the additive element is included in the continuous concentration distribution only in the second region on the surface layer side of the active material particles, for example, A method in which a predetermined additive element is solid-dissolved from the surface of the active material particles using, as a starting material, active material particles composed of only one region (which may be in the form of a plurality of particles or a powder). It is preferable to manufacture by. The production method of the active material particles having a layered crystal structure as a starting material is not particularly limited. For example, as an example, relatively high quality active material particles can be produced by a coprecipitation method (crystallization method) using an alkaline aqueous solution as shown in the examples described later. When the predetermined additive element is dissolved, for example, the surface of the active material particles as the starting material is coated with the compound containing the additive element, and the temperature is, for example, about 1100 ° C. to 1300 ° C. for 0.25 hours. It is exemplified that the target additive element is gradually diffused into the crystal structure from the surface of the active material particle toward the center by a relatively high-temperature and short-time heat treatment of heating for about 3 hours. Although it does not restrict | limit especially as a compound containing an additional element, For example, the various salts of Si, Ge, and P can be used suitably. Specifically, for example, various silicates, germanates, phosphates and hydrates thereof can be used.

  Although the positive electrode active material disclosed here can be used for secondary batteries for various applications, the positive electrode active material for secondary batteries used in applications for charging / discharging at a high rate, in particular, from the above configuration. It can be suitably used. Here, the high rate means charging or discharging at a current value of 5C or more, for example, a large current at 10C or more, further 20C or more, such as 25C or more, 30C or more, 35C or more, 40C or more. The charge / discharge can be targeted. Therefore, taking advantage of such characteristics, the non-aqueous electrolyte secondary battery using the positive electrode active material disclosed herein is, for example, as a power source for driving a vehicle mounted in a plug-in hybrid vehicle, a hybrid vehicle, an electric vehicle, etc. It can be suitably used.

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

In this test example, in producing a lithium ion secondary battery, first, the positive electrode active materials of Examples 1 to 15 were prepared by the following procedures 1 to 3.
[Procedure 1]
First, using nickel acetate II (Ni (CH ₃ COO) ₂ ), cobalt acetate (Co (CH ₃ COO) ₂ ) and manganese acetate (Mn (CH ₃ COO) ₂ ), the molar ratio of Ni, Co and Mn is A NiCoMn aqueous solution was prepared by dissolving in water to a predetermined ratio (here, 1: 1: 1). By dropping this NiCoMn aqueous solution into a basic aqueous solution containing sodium hydroxide and ammonia having a pH of 12.0 (liquid temperature 25 ° C.) prepared in the coprecipitation reaction vessel while maintaining the pH, and mixing them, NiCoMn hydroxide was crystallized (co-precipitated). The crystallized product was filtered, and the alkali component was washed and dried to obtain a precursor mainly composed of particulate NiCoMn composite hydroxide.

Carbon dioxide so that the molar ratio of lithium to M (Li / M) is 1.0, where M is the total number of moles of all transition metals (ie, Ni, Co, Mn) in the precursor. Lithium (Li ₂ CO ₃ ) was weighed and uniformly mixed with the precursor, and then fired at 800 ° C. for 24 hours in the air to obtain a fired product. The process so far corresponds to a method for producing a positive electrode active material made of a lithium transition metal oxide by a known coprecipitation method.
Next, the calcined product and lithium metasilicate (Li ₂ SiO ₃ , average particle size 1 μm to 50 μm) are changed in several ways within a mass ratio of lithium metasilicate to the calcined product in the range of 0.01 to 0.001. A positive electrode active material comprising a powder having an average particle size of about 5 μm by firing in the atmosphere while changing the firing temperature and firing time in the range of 1100 ° C. to 1300 ° C. and 0.25 h to 3 h. Examples 1 to 8) were obtained.

Note that, by using lithium germanate (Li ₂ GeO ₃ ) or lithium phosphate (Li ₃ PO ₄ ) in place of the above-described lithium metasilicate, a positive electrode active material made of powder having an average particle diameter of about 5 μm (example) 9-10) were obtained.

[Procedure 2]
The fired product obtained in the above procedure 1 was directly used as the positive electrode active material of Example 12 in powder form. The average particle diameter of this positive electrode active material is about 5 μm, and is a conventional general positive electrode active material that does not contain additive elements such as Si.
[Procedure 3]
In the above procedure 1, the mass ratio of the fired product and lithium metasilicate (Li ₂ SiO ₃ ) is 1: 0.005 (Example 13) and 1: 0.05 (Example 14). The positive electrode active material which consists of a powder with an average particle diameter of about 5 micrometers was obtained by mixing and baking for 6 to 12 hours at 600 degreeC in air | atmosphere.

[Procedure 4]
With respect to the NiCoMn aqueous solution of Procedure 1, when the total of each element of Ni, Co, and Mn is 100% by mass, the amount of Si contained in lithium metasilicate (Li ₂ SiO ₃ ) is 5% by mass, Lithium metasilicate was dissolved. Using this aqueous solution, a powdery fired product was obtained through the precursor in the same manner as in Procedure 1. This fired product was used as the positive electrode active material of Example 14. The average particle diameter of this positive electrode active material is about 5 μm, and Si as an additive element is contained substantially uniformly throughout the particles constituting the positive electrode active material.

[Procedure 5]
2N metasilicate aqueous solution was prepared by dissolving sodium metasilicate (Na ₂ SiO ₃ ) in ion-exchanged water. Then, the pH is maintained in the basic aqueous solution containing sodium hydroxide / ammonia containing the NiCoMn aqueous solution of Procedure 1 and the aqueous metasilicic acid solution prepared in the coprecipitation reaction vessel and having a pH of 12.0 (liquid temperature 25 ° C.). The NiCoMnSi hydroxide was crystallized (coprecipitated) by dropping and mixing at a predetermined ratio. Subsequently, by increasing the ratio of the aqueous metasilicic acid solution and supplying the aqueous metasilicic acid solution and the NiCoMn aqueous solution to the reactor, a new precipitate is formed so that the Si concentration gradually increases on the surface of the previously precipitated precipitate. A layer of material was formed. The mixing ratio of the metasilicic acid aqueous solution and the NiCoMn aqueous solution was changed in mass ratio until the final Si: (Ni + Co + Mn) was changed from 0: 100 to 5: 100. The precursor thus obtained and lithium carbonate (Li ₂ CO ₃ ) are uniformly mixed and then calcined in the air at 800 ° C. for 24 hours, thereby calcining with an average particle diameter of about 5 μm. I got a thing. This fired product was used as the positive electrode active material of Example 15.

[Measurement of concentration distribution of additive elements in active material]
About the positive electrode active material of Examples 1-15 prepared above, the solid solution state of the additive element was confirmed by investigating the concentration distribution of the additive element in the particles constituting the active material. In this embodiment, the concentration distribution of the additive element was measured in the following three steps.
S1: XRD analysis S2: TEM observation S3: XPS quantitative analysis

(S1: XRD analysis) Specifically, first, an X-ray diffraction (XRD) analysis is performed on the positive electrode active material of each example, whereby an additive element compound used for adding an additive element is obtained. It is confirmed whether or not it remains in the positive electrode active material.
(S2: TEM observation) Next, using TEM, observe the electron diffraction of the surface of the positive electrode active material of each example, and whether the positive electrode active material has a crystal structure belonging to a layered type (in this case, a layered rock salt type), Or it is confirmed whether it has the crystal structure of the raw material compound used when adding an additional element.
From these steps, if the additive element compound does not remain in S1 and the crystal surface of the powder constituting the positive electrode active material maintains a layered structure in S2, the target additive element is a layered rock salt type crystal structure. It can be considered that it is in solid solution. When it is determined in S1 and S2 that the target additive element is dissolved in the layered rock salt type crystal structure, the process proceeds to the next S3.

(S3: XPS quantitative analysis) That is, in S3, the chemical composition near the surface layer of the positive electrode active material of each example is analyzed in the depth direction using XPS. Specifically, as shown in FIG. 2, for the positive electrode active material particles of each example, a region of, for example, 500 nm from the surface is set as the second region, and the inner side of the second region (that is, 500 nm from the surface in the radial direction). (Center side) is defined as the first region. Further, the second region is further divided into five in order from the surface into layered regions A to E each having a thickness of 100 nm. Then, the concentration of the additive element (Si, Ge, or P) is measured by XPS analysis for each of the regions A to E in the second region and the first region. In the composition analysis in the depth direction, every time the XPS analysis in one surface region of the positive electrode active material particles is finished, the surface is irradiated with a rare gas ion to etch the surface layer with a thickness of 100 nm, thereby being exposed. It can be carried out by repeating the deeper region as the next surface region (new measurement surface) of the positive electrode active material.

Also in this embodiment, the concentration of the additive element was measured according to S1 to S3. In addition, argon (Ar) ion is used as a rare gas ion in the XPS measurement, and the relative relationship between the additive element (Si, Ge or P) and the transition metal (Mn, Ni, Co) constituting the positive electrode active material particle is used. The concentration of the additive element was calculated from the appropriate quantitative ratio. Specifically, the total of transition metal elements was 100% by mass, and the ratio of the additive element based on the amount of the transition metal element was taken as the concentration of the additive element. The results are shown in Table 2 below.
In Table 2, for example, in the positive electrode active material of Example 4, the concentration from region A to first region is region A: 5% by mass, region B: 4% by mass, region C: 3% by mass, region D. : 2% by mass, region E: 1% by mass, and first region: 0% by mass.

In addition, when the concentration (mass%) of the additive element in each region measured based on the above steps is X (A) to X (E) and X (1), respectively, these are the following formulas (1) and ( In the case where 2) is satisfied, it can be determined that the additive element is “continuously increasing gradually from the inner side toward the outer side” in the second region.
Therefore, the case where the additive element concentration in the positive electrode active material of each example satisfies the following expressions (1) and (2) is “gradual increase”, and in other cases, the case where the expression (2) is not satisfied is “rapid increase”, The case where the formula (2) is satisfied but the formula (1) is not satisfied is defined as “uniform”, and the case where neither is true is indicated as “−” in the column of “concentration distribution in the radial direction of the additive element” in Table 2. .

<< Formula (1) >> 0 = X (1) ≦ X (E) ≦ X (D) ≦ X (C) ≦ X (B) ≦ X (A), 0 <X (A) ≦ 5
<< Formula (2) >> X (A) −X (B) ≦ 2, X (B) −X (C) ≦ 2, X (C) −X (D) ≦ 2, X (D) −X (E ) ≦ 2, X (E) −X (1) ≦ 2

[Construction of lithium ion secondary battery]
Using the positive electrode active materials of Examples 1 to 15 prepared above, the mass ratio of acetylene black (AB) as the conductive material and polyvinylidene fluoride (PVdF) as the binder is 90: 8: 2. A slurry-like composition was prepared by mixing with N-methylpyrrolidone (NMP). This composition was applied to a long aluminum foil (positive electrode current collector) having a width of about 130 mm and a thickness of about 20 μm so as to have a width of 110 mm from one side and a basis weight of 15 mg / cm ² to form a positive electrode active material layer. Formed. The obtained positive electrode was dried and pressed to produce a sheet-like positive electrode (positive electrode sheet).

Next, scaly natural graphite as a negative electrode active material, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener have a mass ratio of 98.3: 1.0: A slurry-like composition was prepared by mixing with ion-exchanged water so as to be 0.7. This composition was applied to a long copper foil (negative electrode current collector) having a width of about 135 mm and a thickness of about 10 μm so that the basis weight was 15 mg / cm ² to form a negative electrode active material layer. The obtained negative electrode was dried and pressed to prepare a sheet-like negative electrode (negative electrode sheet).

Next, the positive electrode sheet and the negative electrode sheet produced as described above are wound with being overlapped via a polyethylene (PE) separator having a width of 120 mm and a thickness of 25 μm, and the obtained wound electrode body is crushed from the side surface direction. Molded into a flat shape. And the edge part of the positive electrode collector of this winding electrode body and the edge part of a negative electrode collector were joined to the connection terminal of the positive electrode and negative electrode which were integrated with the lid | cover of the battery case, respectively.
This electrode body was accommodated in a rectangular battery case having a width of 120 mm, a height of 75 mm, a depth of 15 mm, and a case thickness of 1 mm, and a non-aqueous electrolyte was injected. As the non-aqueous electrolyte, a mixed solvent containing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 3: 5: 2 is mixed with LiPF ₆ as an electrolyte. Those dissolved at a concentration of 1 mol / L were used. And the lithium ion secondary battery of Examples 1-15 was constructed | assembled by attaching a cover body to the opening part of a battery case, and welding and joining. In addition, the rated capacity of the lithium ion secondary battery produced in this embodiment is 5 Ah.

[Conditioning] The lithium ion secondary batteries of Examples 1 to 15 prepared were discharged to 4.2 V at 1 C (5 A) at 25 ° C., and then provided with a 5 minute rest period and charged to 3.0 V. A 5-minute rest period was then provided. Then, CC-CV charge (4.1V, rate 1C, 0.01C cut) that completes the charge when the current value becomes 0.01C after constant current (CC) charge to 4.1V at 1C, After a constant current (CC) discharge at 1 C to 3.0 V, CC-CV discharge was completed to complete the discharge when the current value reached 0.01 C. And the capacity | capacitance at the time of this discharge was made into the initial stage capacity | capacitance.

[Measurement of Battery Resistance] The output IV resistance of each battery of Examples 1 to 15 after the above conditioning was measured. That is, after adjusting to SOC 60% at 25 ° C., the voltage drop 10 seconds after the discharge when discharging at each rate of 5C, 10C, and 20C was measured. Then, an output IV resistance was obtained by calculating a slope R from a current-voltage straight line obtained by plotting the measured voltage drop (V) against current. The obtained results are shown in Table 2.

[Evaluation of cycle characteristics] The cycle characteristics of the batteries of Examples 1 to 15 after the above conditioning were evaluated. That is, charge and discharge in a voltage range of 4.2 V to 3.0 V at a rate of 2C (10 A) at 25 ° C. was repeated 1000 cycles, and the post-cycle capacity was measured in the same manner as the above initial capacity. The capacity retention rate (%) was calculated as the ratio of the capacity after the cycle to the initial capacity ((capacity after the cycle / initial capacity) × 100 (%)). The obtained capacity retention ratio is also shown in the column of cycle characteristics in Table 2.

[Evaluation] From the analysis results of the particle surface crystal structure and element concentration distribution of the positive electrode active material of each example according to S1 to S3 above, all of the materials other than the conventional positive electrode active material of Example 11 containing no additive element are Si, It was confirmed that the additive element of Ge or P was dissolved in the lithium transition metal oxide.
FIG. 3 illustrates the concentration distribution of Si element in the positive electrode active material particles of Example 4. As shown in FIG. 3, the positive electrode active material particles of Examples 1 to 10 have an inclined structure in which the concentration of the additive element in the second region continuously decreases from the outer surface to the inner side of the active material particle. It was confirmed that In addition, the gradient structure due to the solid solution of the additive element is observed in a layered region (second region) having a thickness of about 1/10 or less (here, 500 nm or less) from the surface with respect to the diameter of the positive electrode active material particles. Further, no solid solution of the additive element in the central region (first region) was confirmed. This is considered to be due to the production method of Procedure 1.

  And the positive electrode active material of Examples 12-14 has confirmed that the density | concentration of the additive element in a 2nd area | region was uniform. FIG. 4 illustrates the concentration distribution of the Si element in the positive electrode active material particles of Example 13. As shown in FIG. 4, it was confirmed that in the positive electrode active materials of Examples 12 and 13, the concentration of the additive element changed rapidly in the vicinity of the interface between the first region and the second region. On the other hand, for the positive electrode active material of Example 14, it was confirmed that the concentration of the additive element was uniform not only in the second region but also in the entire active material particles including the first region. Further, it was confirmed that the active material of Example 15 had an inclined structure in which the concentration of the additive element in the entire active material particles continuously decreased from the outer surface toward the inner side.

  Here, for the positive electrode active materials of Examples 4 to 10, the concentration of the additive element in the region A on the outermost surface side is 5% by mass or less, and the output IV resistance is significantly higher than that of the conventional positive electrode active material of Example 11. It was confirmed that the battery output characteristics were improved. Such an effect could be confirmed in any case where the additive element was Si, Ge, or P. It is understood that the presence of the additive element on the outermost surface of the active material particle can increase the width of the lithium layer at the end of the crystal structure and greatly improve the lithium ion acceptability. it can. The output IV resistance is particularly greatly reduced when the concentration of the additive element in the region A is as high as 5% by mass (Examples 4, 5, 9, 10), and the effect is reduced as the concentration of the additive element in the region A decreases. Was found to be small (Examples 6 to 8). Further, from the comparison between Example 4 and Example 5, if the additive element is sufficiently present in the region A on the outermost surface and the concentration of the additive element in the regions B to E on the inner side does not rapidly decrease, the region B to Even if the additive element concentration in E is low, the influence on the output IV resistance is considered to be low. Moreover, about the positive electrode active material of Examples 4-10, it was confirmed that favorable cycling characteristics can be maintained, although an additional element is partially contained in the 2nd field.

  However, for the positive electrode active materials of Examples 1 to 3 in which the concentration of the additive element in the region A exceeds 5% by mass, the concentration of the additive element is low when the total additive amount of the additive element is small (Example 3). It was found that the output IR resistance was significantly increased as compared with Example 11 even when the value was reduced (Examples 1 and 2). This can be understood to mean that an excessive additive element disrupts the structure (including the crystal structure) of the active material particles, preventing normal occlusion and release of lithium ions. When the content of the additive element on the surface is particularly large (Example 1), even if the additive element is present in a solid solution state, it is the same as the increase in the resistance component, and the resistance becomes particularly high. It was confirmed. In addition, when the concentration of the additive element was changing rapidly (Example 3), it was confirmed that the cycle characteristics were significantly deteriorated even when the total amount of the additive element was small. This is considered to be due to the fact that a large change in the volume expansion coefficient of the active material occurred at a location where the concentration of the additive element was suddenly changed, and a large defect such as cracking of the active material occurred.

  In Example 12 and Example 13, the additive element is contained in the second region in a relatively small amount and uniformly at 5% by mass or 3% by mass. Even in such an embodiment, the width of the lithium layer at the end in the crystal structure can be increased, and thus the output IV resistance is reduced as compared with Example 11. However, since the additional elements are excessively contained in the regions A to E inside the region A, it has been found that the effect of reducing the overall output IV resistance is suppressed as compared with Examples 4, 5, 7, and the like. . And in Example 12 and Example 13, since the concentration of the additive element changes abruptly at the interface between the second region and the first region, the active material layer cracks or the second region peels off at the interface, It was confirmed that the cycle characteristics deteriorated remarkably.

In Example 14, the concentration of the additive element is kept uniform throughout the active material particles including the first region. In Example 14, on the contrary to Example 13, the cycle characteristics were very good because the concentration of the additive element did not change at the interface between the second region and the first region. Since the additive element is included as a resistance component, almost no effect of reducing the output IV resistance was observed.
In addition, although the output IV resistance of the positive electrode active material of Example 15 was reduced to the same level as in Examples 6 to 7, the cycle characteristics were remarkably deteriorated. The positive electrode active material of Example 15 could not be produced by, for example, a technique of dissolving an additive element in order to gradually reduce the concentration to the center of the active material particles. Therefore, a method was adopted in which the additive element was coprecipitated with the transition metal and the ratio was increased stepwise. However, according to such a manufacturing method, a difference occurs in crystal growth at the stage of changing the ratio of the additive element, and a crystal interface is formed. It was considered that the cycle characteristics were significantly deteriorated by the presence of such an interface.

  From the above, according to the positive electrode active material disclosed herein, it was confirmed that the output IV resistance was reduced and the input / output characteristics could be greatly improved. Moreover, it was confirmed that such good input / output could be maintained even after repeated charge / discharge cycles, and that the cycle characteristics were also excellent. 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.

Claims (1)

A positive electrode active material for a non-aqueous electrolyte secondary battery,
Including active material particles made of a lithium transition metal oxide having a layered crystal structure,
The active material particles include at least one additive element selected from the group consisting of silicon (Si), phosphorus (P), and germanium (Ge),
A first region in the radial direction center side of the active material particles, which does not substantially contain the additive element;
A region outside the first region, the second region containing the additive element;
Consists of
In the second region, the content ratio of the additive element is 0% by mass to 5% as it goes from the inner side to the outer side in the radial direction when the total amount of transition metal elements in the lithium transition metal oxide is 100% by mass. A positive electrode active material that gradually increases within a range of less than or equal to%.

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