JP2019091638A - Positive electrode and lithium ion secondary battery - Google Patents

Abstract

To provide a technology capable of improving thermal stability of a lithium ion secondary battery while using a lithium nickel composite oxide.SOLUTION: A positive electrode includes, in a positive electrode active material layer: a composite particle obtained by coating a particle of a lithium nickel composite oxide with a particle of a first material obtained by carbon-coating of an olivine structure LiMPO(where, M represents at least one element selected from among Mn, Fe, Co, Ni, Cu, Mg, Zn, V, Ca, Sr, Ba, Ti, Al, Si, B, Te and Mo, and 0<h<2), the lithium nickel composite oxide being represented by general formula: LiNiCoAlDO(where, 0.2≤a≤2, 0.6≤b, b+c+d+e=1, 0≤e<1, D represents at least one element selected from among W, Mo, Re, Pd, Ba, Cr, B, Sb, Sr, Pb, Ga, Mn, Nb, Mg, Ta, Ti, La, Zr, Cu, Ca, Ir, Hf, Rh, Fe, Ge, Zn, Ru, Sc, Sn, In, Y, Bi, S, Si, Na, K, P and V, and 1.7≤f≤3); and a particle of a second material obtained by carbon-coating an olivine structure LiMPO(where M represents at least one element selected from among Mn, Fe, Co, Ni, Cu, Mg, Zn, V, Ca, Sr, Ba, Ti, Al, Si, B, Te and Mo, and 0<h<2). When the positive electrode active material layer is 100 pts.mass, the first material and the second material are included by 20 pts.mass or more and 30 pts.mass or less. When the amount of the second material is 100 pts.mass, the amount of the first material included in the composite particle is 20 pts.mass or more and 100 pts.mass or less.SELECTED DRAWING: None

Description

  The present invention relates to a positive electrode and a lithium ion secondary battery.

Lithium metal composite oxides such as LiCoO ₂ and LiNi _1/3 Co _1/3 Mn _1/3 O ₂ are known as high-capacity positive electrode active materials for secondary batteries.
This type of lithium metal composite oxide is considered to be low in thermal stability while having high capacity. In order to provide the positive electrode with high capacity and excellent thermal stability, a technique in which a plurality of positive electrode active materials are used in combination has been proposed.

For example, Patent Document 1 discloses a positive electrode active material in which LiCoO ₂ and LiFePO ₄ are used in combination. In Patent Document 1, LiCoO ₂ is one of the first lithium-containing metal complex oxides, and LiFePO ₄ is one of the second lithium-containing metal complex oxides. And the said patent document 1 makes resistance of a 2nd lithium containing metal complex oxide higher than a 1st lithium containing metal complex oxide, and makes it a lithium pair electric potential (Li / Li ^<+> ) is low. The second lithium-containing metal composite oxide is a so-called olivine positive electrode active material, and is known to be a positive electrode active material excellent in thermal stability.
According to Patent Document 1, the second lithium-containing metal complex oxide is coated on the entire surface of the particles of the first lithium-containing metal complex oxide, whereby it is transmitted from the negative electrode to the positive electrode during an internal short circuit of the secondary battery. It is described that the transfer speed of a large amount of lithium ions and electrons can be relaxed to prevent heat generation due to the generation of a momentary overcurrent, and as a result, the thermal safety of the secondary battery can be enhanced.

  If lithium metal complex oxide is coated with a positive electrode active material of an olivine structure and used as a positive electrode of a lithium ion secondary battery based on the technology of Patent Document 1 described above, high capacity and excellent thermal stability for lithium ion secondary battery There is a possibility of giving a sex.

By the way, in recent years, further increase in capacity is desired for lithium ion secondary batteries. For this reason, it is considered to use a higher capacity lithium metal composite oxide. Among lithium metal composite oxides, lithium nickel composite oxide containing a large amount of nickel is considered to have a high capacity. For example, Patent Document 2, as a positive electrode active material, the general formula LiNi _c M1 _d O ₂ (M1 are, Mg, Co, Mn, at least one metallic element selected from Al, 0.4 <c <0.95, c + d = 1) is mentioned. In Examples in Patent Document 2, as a positive electrode active _{material, LiNi 0.8 Co 0.15 Al 0.05 O} 2 is used.

  While this type of lithium nickel composite oxide has a higher capacity than a lithium metal composite oxide having a low nickel content, it has further inferior thermal stability. Therefore, a technique using the lithium nickel composite oxide and capable of imparting excellent thermal stability to a lithium ion secondary battery is desired.

Japanese Patent Publication No. 2010-517218 JP, 2009-54318, A

  The present invention has been made in view of such circumstances, and an object thereof is to provide a technology capable of improving the thermal stability of a lithium ion secondary battery using a lithium nickel composite oxide as a positive electrode.

The positive electrode of the present invention is
LiM _h PO _{4 of} olivine structure (M is at least one element selected from Mn, Fe, Co, Ni, Cu, Mg, Zn, V, Ca, Sr, Ba, Ti, Al, Si, B, Te and Mo , 0 <h <2) with particles of a first material which is carbon-coated, the general _{formula: Li a Ni b Co c Al} d D e O f (0.2 ≦ a ≦ 2,0.6 ≦ b, b + c + d + e = 1: 0 ≦ e <1, D is W, Mo, Re, Pd, Ba, Cr, B, Sb, Sr, Pb, Ga, Mn, Nb, Mg, Ta, Ti, La, Zr, Cu, Ca, At least one element selected from Ir, Hf, Rh, Fe, Ge, Zn, Ru, Sc, Sn, In, Y, Bi, S, Si, Na, K, P, V, 1.7 ≦ f ≦ 3 Composite particles coated with particles of lithium nickel composite oxide represented by
LiM _h PO _{4 of} olivine structure (M is at least one element selected from Mn, Fe, Co, Ni, Cu, Mg, Zn, V, Ca, Sr, Ba, Ti, Al, Si, B, Te and Mo , Particles of the second material carbon-coated with 0 <h <2) in the positive electrode active material layer,
When the positive electrode active material layer is 100 parts by mass, it contains 20 parts by mass or more and 30 parts by mass or less of the first material and the second material,
When the amount of the second material is 100 parts by mass, the amount of the first material contained in the composite particles is 20 parts by mass or more and 100 parts by mass or less.

  The positive electrode of the present invention uses a lithium nickel composite oxide and can improve the thermal stability of a lithium ion secondary battery.

  Hereinafter, the lithium ion secondary battery of the present invention having the positive electrode of the present invention and the positive electrode of the present invention will be described in detail.

  In addition, unless otherwise indicated, the numerical range “x to y” described in the present specification includes the lower limit x and the upper limit y in the range. And, new numerical ranges can be configured by arbitrarily combining these upper and lower limits and the numerical values listed in the examples. Furthermore, numerical values arbitrarily selected from any of the above numerical ranges can be used as the upper and lower numerical values of the new numerical range.

(Positive electrode)
The positive electrode of the present invention includes composite particles and particles of the second material in a positive electrode active material layer.
Among these, the composite particles are obtained by coating the particles of the lithium nickel composite oxide with particles of the first material. The first material is LiM _h PO _{4 of} olivine structure (M is Mn, Fe, Co, Ni, Cu, Mg, Zn, V, Ca, Sr, Ba, Ti, Al, Si, B, Te, and Mo At least one element to be selected, 0 <h <2) is coated with carbon.
Further, the lithium nickel composite oxide represented by the general _{formula: Li a Ni b Co c Al} d D e O f (0.2 ≦ a ≦ 2,0.6 ≦ b, b + c + d + e = 1,0 ≦ e <1, D is W, Mo, Pd, Ba, Cr, B, Sb, Sr, Pb, Ga, Mn, Nb, Mg, Ta, Ti, La, Zr, Cu, Ca, Ir, Hf, Rh, Fe, At least one element selected from Ge, Zn, Ru, Sc, Sn, In, Y, Bi, S, Si, Na, K, P, V, and a compound represented by 1.7 ≦ f ≦ 3) .
Further, the second material is LiM _h PO _{4 of} olivine structure (M is Mn, Fe, Co, Ni, Cu, Mg, Zn, V, Ca, Sr, Ba, Ti, Al, Si, B, Te and Carbon coated with at least one element selected from Mo, 0 <h <2).

In addition, the 1st material and the 2nd material in the positive electrode of the present invention mean what carried out carbon coating of LiM _h PO ₄ of olivine structure. As described later, in the positive electrode of the present invention, since the relationship between the amount of the first material contained in the composite particles and the amount of the second material is defined, in the present specification, for convenience, Both were distinguished from the 1st material and the 2nd material. Therefore, in the positive electrode of the present invention, the first material and the second material may have the same composition, or the particles of the first material and the particles of the second material may have the same composition and the same shape. .

Both the particles of the first material and the particles of the second material in the positive electrode of the present invention are carbon-coated particles of LiM _h PO ₄ of olivine structure. Therefore, the positive electrode active material of the present invention contains a plurality of carbon-coated particles of LiM _h PO ₄ of olivine structure, and some of the particles cover the particles of lithium nickel composite oxide to constitute composite particles, The other particles can be said to be included in the positive electrode active material layer together with the composite particles. Among the plurality of carbon-coated particles of LiM _h PO ₄ having an olivine structure, the particles constituting the composite particles are particles of the first material, and the particles constituting the positive electrode active material layer together with the composite particles are It is a particle of the second material.

In the positive electrode of the present invention, the particle coated with LiM _h PO ₄ having an olivine structure on carbon is divided into two types, an aspect of constituting composite particles and an aspect of being contained in the positive electrode active material layer together with the composite particles. It can also be said to be present in the positive electrode active material layer.

  In addition, the positive electrode of the present invention includes the first material and the second material in an amount of 20 parts by mass or more and 30 parts by mass or less, based on 100 parts by mass of the positive electrode active material layer. Furthermore, in the positive electrode of the present invention, when the amount of the second material is 100 parts by mass, the amount of the first material contained in the composite particles is 20 parts by mass or more and 100 parts by mass or less.

  By configuring the positive electrode including the lithium nickel composite oxide in the positive electrode active material layer as described above, a positive electrode using the lithium nickel composite oxide and having improved thermal stability can be obtained. Although the reason is not clear, it is considered that by configuring the positive electrode in this way, the battery resistance of the lithium ion secondary battery can be contained in a suitable range that is neither too low nor too high.

If the battery resistance of the lithium ion secondary battery is too high, the capacity of the lithium ion secondary battery may be reduced. In addition, if the battery resistance of the lithium ion secondary battery is too low, the positive electrode is inferior in thermal stability, and as a result, the thermal stability of the lithium ion secondary battery may be deteriorated.
In the positive electrode of the present invention, by setting the amount of the first material and the amount of the second material within the above ranges, the battery resistance of the lithium ion secondary battery is within the preferable range which is neither too low nor too high. be able to. Therefore, the positive electrode of the present invention exhibits excellent thermal stability despite the use of the lithium-nickel composite oxide in the positive electrode active material layer. As a matter of course, by having the lithium nickel composite oxide in the positive electrode active material layer, the positive electrode of the present invention has a large capacity.

In addition, in the positive electrode of the present invention, particles obtained by carbon-coating LiM _h PO ₄ having an olivine structure with particles of the first material constituting the composite particles, and the second material contained in the positive electrode active material layer together with the composite particles The particles are present in the positive electrode active material layer in two ways. This is considered to make it easier to control the battery resistance of the lithium ion secondary battery, as compared to the case where only the particles of the first material or the particles of the second material are included in the positive electrode active material layer.

  Hereinafter, if necessary, the first material and the second material may be collectively referred to as an olivine material. Also, the particles of the first material and the particles of the second material may be collectively referred to as olivine particles.

The lithium nickel composite oxide contained in the composite particles, the general _{formula: Li a Ni b Co c Al} d D e O f (0.2 ≦ a ≦ 2,0.6 ≦ b, b + c + d + e = 1,0 ≦ e <1, D is W, Mo, Re, Pd, Ba, Cr, B, Sb, Sr, Pb, Ga, Mn, Nb, Mg, Ta, Ti, La, Zr, Cu, Ca, Ir, Hf, At least one element selected from Rh, Fe, Ge, Zn, Ru, Sc, Sn, In, Y, Bi, S, Si, Na, K, P, V, and represented by 1.7 ≦ f ≦ 3) Are used.
From the viewpoint of high capacity, of the general formula of the layered rock salt _{structure: Li a Ni b Co c Al} d D e O f (0.2 ≦ a ≦ 1.7,0.6 ≦ b, b + c + d + e = 1, 0 ≦ e <1, D is at least one element selected from Li, Fe, Cr, Cu, Zn, Ca, Mg, Zr, S, Si, Na, K, and Mn, 1.7 ≦ f ≦ 2.1 The compounds represented by) are preferred.

The values of b, c, d and e are not particularly limited as long as the above conditions are satisfied, but for b and c, 0.6 ≦ b <1 and 0 <c <0.4. Is good. For d and e, it is preferable that 0 <d <0.4 and / or 0 <e <0.4.
Among these, for b, the range of 0.6 ≦ b ≦ 0.97, 0.7 ≦ b ≦ 0.95, 0.75 ≦ b ≦ 0.92, 0.8 ≦ b ≦ 0.9 It is more preferable that it is inside.
For c, it is preferable that 0.05 <c <0.3, 0.08 <c <0.25, 0.085 <c <0.2, or 0.09 <c <0.17. It is more preferable that
As for d, it is in the range of 0 ≦ d <0.3, 0.005 <d <0.05, 0.01 <d <0.04, 0.015 <d <0.032. Is more preferred.
As for e, it is in the range of 0 ≦ e <0.3, 0.005 <e <0.05, 0.01 <e <0.04, 0.015 <e <0.032. Is more preferred.

  The values of a and f may be numerical values within the range defined by the general formula, and a is preferably in the range of 0.5 ≦ a ≦ 1.5, and is in the range of 0.7 ≦ a ≦ 1.3. The range of 0.9 ≦ a ≦ 1.2 is more preferable. About f, f = 2 can be illustrated.

Specific examples of the lithium nickel composite oxide having a layered rock salt _{structure, LiNi 0.6 Co 0.2 Mn 0.2 O} 2, LiNi 0.75 Co 0.1 Mn 0.15 O 2, LiNi 0.8 Co _0.15 Al _0.05 O ₂ , LiNi _0.82 Co _0.15 Al _0.03 O ₂ , LiNi _0.83 Co _0.154 Al _0.016 O ₂ , LiNi _0.887 Co _0.095 Al _{0 Examples include 018} O ₂ , LiNi _0.825 Co _0.145 Al _0.029 O ₂ , and LiNi _0.874 Co _0.097 Al _0.029 O ₂ .

  The shape of the particles of the lithium nickel composite oxide is not particularly limited, but in terms of average particle diameter, it is preferably 100 μm or less, more preferably 0.1 μm to 50 μm, still more preferably 1 μm to 20 μm, and 5 μm The thickness is preferably 15 μm or less. If the thickness is less than 0.1 μm, problems may occur such as adhesion to the current collector is easily impaired when the electrode is manufactured. If it exceeds 100 μm, the size of the electrode may be affected, or the separator of the secondary battery may be damaged. In addition, the average particle diameter in this specification means the value of D50 at the time of measuring by wet measurement with a common laser diffraction type particle size distribution measuring apparatus. If it is a sample for which wet measurement is difficult, dry measurement may be performed.

  The volume resistivity of the lithium nickel composite oxide particles is preferably larger than that of the olivine particles used in combination. However, particles of lithium nickel composite oxide having an excessive volume resistivity may not be preferable in terms of balance with the volume resistivity of olivine particles. Therefore, it is considered that the volume resistivity of the lithium-nickel composite oxide particles also has a suitable range. Specifically, the volume resistivity of the lithium nickel composite oxide particles is preferably 3000 Ω · cm or less, more preferably 2000 Ω · cm or less, and still more preferably 1000 Ω · cm or less. It is more preferably 500 Ω · m or less, and particularly preferably 100 Ω · m or less. The lower limit of the volume resistivity of the particles of the lithium nickel composite oxide is not particularly limited, but if it is strong, 5 Ω · cm or more can be exemplified as the lower limit. The method of measuring the volume resistivity will be described in detail in the section of Examples.

The olivine particles are selected from LiM _h PO _{4 of} olivine structure (M is Mn, Fe, Co, Ni, Cu, Mg, Zn, V, Ca, Sr, Ba, Ti, Al, Si, B, Te, and Mo At least a part of at least one element, 0 <h <2) is coated with carbon and has a particulate form.

M of LiM _h PO ₄ is at least one element selected from Mn, Fe, Co, Ni, Mg, V, and Te, and preferably 0.6 <h <1.1. In addition, the M is Mn and / or Fe, more preferably 0.9 <h <1.1, and still more preferably h = 1. The M is Mn and Fe, more preferably 0.9 <h <1.1, and still more preferably h = 1.
The _LiM h PO ₄ of olivine structure, for _{example, LiFePO 4, LiCoPO 4, LiNiPO} 4, LiMnPO 4, LiVPO 4, LiTePO 4, LiV 2/3 PO 4, LiFe 2/3 PO 4, LiMn 7/8 Fe 1 _{/ 8 PO 4, LiMn 7/10 Fe} 3/10 PO 4, LiMn 0.68 Fe 0.27 PO 4 and the like. Among these, LiFePO ₄ is preferable from the point of heat stability. The reason is presumed as follows. LiFePO ₄ exhibits a relatively flat discharge curve during discharge. Then, even if the positive electrode and the negative electrode of the lithium ion secondary battery are short-circuited and a sudden discharge occurs, a rapid potential difference associated with the discharge is unlikely to occur at the location where LiFePO ₄ is present. Therefore, it is difficult to induce charge transfer from another place in the electrode, and generation of an overcurrent can be suppressed. As a result, heat generation of the secondary battery can be suitably suppressed.
_{Incidentally, LiFe x Mn 1-x PO} 4 where a part of Fe of LiFePO ₄ was replaced with Mn may also be used likewise preferably as LiFePO _4.

The olivine particles may be those in which at least a portion of the olivine structured LiM _h PO ₄ is coated with carbon as described above, and at least a portion of the surface is composed of carbon. Carbon may constitute only a part of the surface of olivine particles, or may constitute the entire surface. Also, part of the carbon may penetrate into the interior of the olivine particle. By forming part of the surface of the olivine particles with carbon, olivine particles having low volume resistivity and excellent conductivity can be obtained.

As a method of coating the olivine structured LiM _h PO ₄ with carbon to obtain olivine particles, a conventional carbon coating method can be used. By appropriately selecting and using an appropriate method from among various conventional carbon coating methods, olivine particles of any volume resistivity can be obtained. For example, JP-A-2014-194879, JP-A-2012-204079, JP-A-2014-179176, JP-A-2014-029863, International Publication 2013/018758, JP-A 2012-216473, Various carbon coating methods are disclosed in Haruo Watanabe "Lithium Ion Secondary Battery", pages 278-280.

  As the olivine particles, those having a small volume resistivity are preferably used. Specifically, the volume resistivity of the olivine particles is preferably less than 5 Ω · cm, more preferably 3 Ω · cm or less, still more preferably 1 Ω · cm or less, and 0.5 Ω · cm or less Particular preference is given to The lower limit of the volume resistivity of the olivine particles is not particularly limited, but if it is strong, 0.05 Ω · cm or more can be exemplified as the lower limit.

The shapes of the carbon-coated olivine structured LiM _h PO ₄ and the olivine particles are not particularly limited, but in terms of the average particle diameter, they are preferably smaller than the average particle diameter of the lithium nickel composite oxide particles. The average particle diameter of the LiM _h PO ₄ and olivine particles is preferably less than 100 μm, more preferably 0.1 μm to 50 μm, still more preferably 0.5 μm to 15 μm, and particularly preferably 1 μm to 10 μm.

  The method for coating the particles of the lithium-nickel composite oxide with olivine particles is not particularly limited, and for example, known methods such as a granulation method and a coating method can be used. As an example, particles of lithium nickel composite oxide and olivine particles may be placed in a mixing apparatus and mixed. The mixing apparatus is not particularly limited, and may be, for example, a mortar or a known mixer. As a mixer, a stirring mixer such as a V-type mixer, a W-type mixer, a ribbon type mixer, or a drum mixer may be used, or a ball mill, jet mill, hammer mill, pin mill, disc mill, turbo mill, etc. A grinding mixer may be used. Alternatively, a mixing device may be used to mix and combine particles of lithium nickel composite oxide and olivine particles while applying a compressive force or a shear force.

  The mixing speed and mixing time may be appropriately determined appropriately. However, since it is not appropriate to mix excessively until particles of lithium nickel composite oxide and olivine particles disappear, the presence of lithium nickel composite oxide and olivine material using an analyzer such as a powder X-ray diffractometer Preferably, the mixing speed and mixing time are determined while confirming.

  In the positive electrode of the present invention, when the amount of the second material is 100 parts by mass, the amount of the first material contained in the composite particles is 20 parts by mass or more and 100 parts by mass or less. The amount of the first material with respect to 100 parts by mass of the second material may be within the above range, and for example, 22 parts by mass or more as the amount of the first material with respect to 100 parts by mass of the second material The ranges of 100 parts by mass or less, 25 parts by mass or more and 75 parts by mass or less, and 30 parts by mass or more and 70 parts by mass or less can be mentioned.

  The positive electrode of the present invention contains the first material and the second material in an amount of 20 parts by mass or more and 30 parts by mass or less, based on 100 parts by mass of the positive electrode active material layer. The positive electrode of the present invention can be rephrased as containing 20 parts by mass or more and 30 parts by mass or less of the olivine material with respect to 100 parts by mass of the positive electrode active material layer. The mass of the olivine material relative to 100 parts by mass of the positive electrode active material layer is more preferably more than 20% by mass, still more preferably 21 parts by mass or more, and still more preferably 22 parts by mass or more. Further, the mass part of the olivine material is more preferably less than 30 parts by mass, still more preferably 29 parts by mass or less, and particularly preferably 28% by mass or less.

  In a lithium ion secondary battery including lithium nickel composite oxide particles and olivine particles in a positive electrode active material layer, when current flows through the current collector to the positive electrode active material layer, the current is particles of lithium nickel composite oxide and olivine Flow to both sides with particles. Therefore, a mixed electrode containing particles of lithium nickel composite oxide and olivine particles can be represented by a parallel circuit of particles of lithium nickel composite oxide and olivine particles.

As described above, the olivine structure LiM _h PO ₄ is superior in thermal stability to lithium nickel composite oxide. Therefore, if the amount of current flowing to the olivine particles excellent in thermal stability can be made larger than the amount of current flowing to the particles of the lithium-nickel composite oxide, it is considered that the thermal stability of the entire lithium ion secondary battery is enhanced.

Here, a large amount of current flows in the low resistance circuit. In parallel circuits, the amount of current is inversely proportional to the volume resistivity. Therefore, it is considered that, if olivine particles having a volume resistivity smaller than that of the lithium nickel composite oxide particles are used, more current can flow to the olivine material which is excellent in heat stability than the lithium nickel composite oxide. By doing this, it is possible to prevent an excessive current from flowing in the lithium-nickel composite oxide even when, for example, a conductive foreign matter causes a short circuit between the positive and negative electrodes of the lithium ion secondary battery. It is believed that the thermal stability of the lithium ion secondary battery can be improved.
That is, in a positive electrode in which particles of lithium nickel composite oxide and olivine particles are used in combination in the positive electrode active material layer, lithium ion can be obtained by setting the volume resistivity of olivine particles smaller than the volume resistivity of particles of lithium nickel composite oxide. It is believed that the thermal stability of the entire secondary battery is improved.

In general, the volume resistivity of the lithium nickel composite oxide is lower than that of the olivine structure LiM _h PO ₄ . However, the olivine particles used in the positive electrode of the present invention are carbon-coated with the LiM _h PO ₄ . For this reason, the volume resistivity of olivine particles is lower than the volume resistivity of mere LiM _h PO ₄ .
The volume resistivity of the olivine particles is adjusted to the volume resistivity of the particles of the lithium nickel composite oxide by appropriately adjusting the thickness of the carbon coating, or by appropriately adjusting the combination of LiM _h PO ₄ and the lithium nickel composite oxide. The volume resistivity of the olivine particles may be divided by the volume resistivity of the particles of the lithium-nickel composite oxide to obtain a desired value. Hereinafter, a value obtained by dividing the volume resistivity of the olivine particles by the volume resistivity of the particles of the lithium nickel composite oxide is referred to as an LP / LO volume resistivity ratio.

Volume resistivity is also referred to as resistivity or resistivity, and can be represented by electrical resistivity ρ.
The electrical resistivity ρ is represented by ρ = (RA) / L, where R is the electrical resistance, L is the length of the conductor, and A is the cross-sectional area of the conductor. The unit of the electrical resistivity ρ is Ω · cm.
If the above-mentioned LP / LO volume resistance ratio is less than 1, it can be said that the volume resistivity of the lithium nickel composite oxide particles is larger than the volume resistivity of the olivine particles.

In the positive electrode, in order to make the amount of current flowing to the olivine particles excellent in thermal stability larger than the amount of current flowing to the particles of the lithium nickel composite oxide, the LP / LO volume resistance ratio may be less than 1. As a preferable range of the LP / LO volume resistance ratio, each of less than 1, 0.5 or less, 0.2 or less, 0.1 or less, 0.05 or less, 0.034 or less, 0.02 or less, or 0.017 or less A range can be mentioned. The lower limit value of the LP / LO volume resistance ratio is not particularly limited and may be more than 0. Furthermore, the LP / LO volume resistance ratio is preferably 0.01 or more.
As the LP / LO volume resistivity ratio, a value obtained by dividing the volume resistivity of the particles of the first material by the volume resistivity of the particles of the lithium nickel composite oxide, and the volume resistivity of the particles of the second material are lithium The value is divided by the volume resistivity of the particles of the nickel composite oxide. If the particles of the first material and the particles of the second material are different, the LP / LO volume resistance ratio may be in the above preferred range for both the particles of the first material and the particles of the second material.

A known method can be used as a method for setting the LP / LO volume resistance ratio within the above range. For example, the LP / LO volume resistance ratio can be adjusted by selection of the type of lithium nickel composite oxide, selection of the type of LiM _h PO ₄ of olivine structure, adjustment of carbon coverage and coverage of olivine particles, and the like. For example, Japanese Patent Application Laid-Open No. 2014-194879 shows that the volume resistivity of olivine particles decreases as the amount of carbon coating increases.

It is known that there are various volume resistivities as LiM _h PO ₄ of the olivine structure. For example, in paragraph 0043 and paragraph 0057 (Table 1) of JP-A-2014-179176, paragraph 0187 of JP-A-2014-029863, and paragraph 0057 of JP-A-2012-204079, there are various types of volume resistivity. The LiM _h PO ₄ is specifically disclosed.

  In the lithium ion secondary battery of the present invention, the positive electrode active material layer may further contain an additive such as a conductive aid, a binder, and a dispersant, in addition to the above-described positive electrode active material.

  A conductive aid is added to enhance the conductivity of the electrode. Therefore, the conductive additive may be optionally added when the conductivity of the electrode is insufficient, and may not be added when the conductivity of the electrode is sufficiently excellent. When the conductive support agent is added to the positive electrode, the current easily flows to both the particles of the lithium nickel composite oxide and the olivine particles. Therefore, in the conventional positive electrode and lithium ion secondary battery, if a short circuit occurs, an overcurrent may flow even to particles of lithium nickel composite oxide which can not be said to be excellent in thermal stability. is there.

  However, also in such a case, in the positive electrode and the lithium ion secondary battery of the present invention, particles of the lithium nickel composite oxide are used as composite particles covered with particles of the first material which is olivine particles. Further, in the positive electrode active material layer, the composite particles and particles of the second material which is olivine particles coexist. Therefore, the overcurrent at the time of the short circuit is likely to flow to the olivine particles, and the supply of the overcurrent to the particles of the lithium nickel composite oxide is considered to be suppressed. In fact, the positive electrode and the lithium ion secondary battery of the present invention are excellent in thermal stability even if they contain a lithium-nickel composite oxide and contain a conductive additive.

  The conductive support agent may be any chemically active high electron conductor, and carbon black fine particles such as carbon black, graphite, vapor grown carbon fiber (vapor grown carbon fiber), and various metal particles are exemplified. . Examples of carbon black include acetylene black, ketjen black (registered trademark), furnace black, channel black and the like. These conductive assistants can be added to the positive electrode active material layer singly or in combination of two or more.

  The shape of the conductive aid is not particularly limited, but in view of its role, it is preferable that the average particle size of the conductive aid be smaller. 10 micrometers or less are illustrated as a preferable average particle diameter of a conductive support agent, and the range of 0.01-1 micrometer is illustrated as a more preferable average particle diameter.

  The compounding amount of the conductive aid is not particularly limited, but if the compounding amount of the conductive aid in the positive electrode active material layer is mentioned, it is preferably in the range of 0.5 to 10% by mass and in the range of 1 to 7% by mass Preferably, the range of 2 to 5% by mass is particularly preferable.

  The binder plays a role of fixing the positive electrode active material and the conductive additive to the surface of the current collector. Examples of the binder include fluorine-containing resins such as polyvinylidene fluoride, polytetrafluoroethylene and fluororubber, thermoplastic resins such as polypropylene and polyethylene, imide resins such as polyimide and polyamideimide, and alkoxysilyl group-containing resins. be able to. Moreover, you may employ | adopt the polymer which has a hydrophilic group as a binding agent. As a hydrophilic group of the polymer which has a hydrophilic group, a carboxyl group, a sulfo group, a silanol group, an amino group, a hydroxyl group, and a phosphoric acid group are illustrated. Specific examples of the polymer having a hydrophilic group include polyacrylic acid, carboxymethylcellulose, polymethacrylic acid and poly (p-styrenesulfonic acid).

  The compounding amount of the binder is not particularly limited, but if the compounding amount of the binder in the positive electrode active material layer is mentioned, it is preferably in the range of 0.5 to 20% by mass, and preferably in the range of 1 to 15% by mass. More preferably, the range of 5 to 10% by mass is particularly preferable. If the amount of the binder is too small, the formability of the positive electrode active material layer may be reduced. In addition, when the blending amount of the binder is too large, the amount of the positive electrode active material in the positive electrode active material layer relatively decreases, which is not preferable.

  Well-known additives can be adopted as additives such as a dispersing agent other than the conductive aid and the binder.

  The current collector refers to a chemically inactive electron conductor for keeping current flowing to the electrode during discharge or charge of the lithium ion secondary battery. As the current collector, at least one selected from silver, copper, gold, aluminum, magnesium, tungsten, cobalt, zinc, nickel, iron, platinum, tin, indium, titanium, ruthenium, tantalum, chromium, molybdenum, and stainless steel Etc. can be exemplified.

  When the potential of the positive electrode is 4 V or more based on lithium, it is preferable to use aluminum as a current collector.

  Specifically, it is preferable to use an aluminum or an aluminum alloy as the positive electrode current collector. Here, aluminum refers to pure aluminum, and aluminum having a purity of 99.0% or more is referred to as pure aluminum. An alloy obtained by adding various elements to pure aluminum is called an aluminum alloy. Examples of aluminum alloys include Al-Cu-based, Al-Mn-based, Al-Fe-based, Al-Si-based, Al-Mg-based, Al-Mg-Si-based and Al-Zn-Mg-based.

  As aluminum or aluminum alloy, specifically, for example, A1000 series alloys (pure aluminum series) such as JIS A1085, A1N30, A3000 series alloys such as JIS A3003, A3004 (Al-Mn series), JIS A8079, A8021 etc. A 8000 series alloy (Al-Fe series) is mentioned.

  The current collector may be coated with a known protective layer. What processed the surface of a collector by a well-known method may be used as a collector.

  The current collector can take the form of a foil, a sheet, a film, a line, a rod, a mesh or the like. Therefore, as the current collector, for example, metal foils such as copper foil, nickel foil, aluminum foil, and stainless steel foil can be suitably used. When the current collector is in the form of a foil, a sheet or a film, the thickness is preferably in the range of 1 μm to 100 μm.

  In order to form the positive electrode active material layer on the surface of the current collector, a collection of conventionally known methods such as roll coating method, die coating method, dip coating method, doctor blade method, spray coating method and curtain coating method is performed. The positive electrode active material may be applied to the surface of the current collector. Specifically, the active material, the solvent, and, if necessary, the binder and the conductive auxiliary agent are mixed to form a slurry, and the slurry is applied to the surface of the current collector and then dried. Examples of the solvent include N-methyl-2-pyrrolidone, methanol, methyl isobutyl ketone and water. The dried one may be compressed to increase the electrode density.

(Lithium ion secondary battery)
The lithium ion secondary battery of the present invention includes, as battery components, a positive electrode, a separator, a negative electrode and an electrolyte. The positive electrode has already been described.

  The separator separates the positive electrode and the negative electrode to prevent a short circuit due to the contact of the two electrodes, and provides a storage space and a passage for the electrolytic solution. As the separator, a known one may be employed, and polytetrafluoroethylene, polypropylene, polyethylene, polyimide, polyamide, polyaramid (Aromatic polyamide), polyester, synthetic resin such as polyacrylonitrile, polysaccharide such as cellulose, amylose, fibroin Examples thereof include porous materials, non-woven fabrics, and woven fabrics using one or more kinds of natural polymers such as keratin, lignin and suberin, and electrically insulating materials such as ceramics. In addition, the separator may have a multilayer structure.

  The negative electrode has a current collector and a negative electrode active material layer formed on the surface of the current collector. The negative electrode active material layer contains a negative electrode active material, and, if necessary, a binder and / or a conductive auxiliary. As the current collector and the conductive additive, those described for the positive electrode may be employed.

The negative electrode active material is not particularly limited as long as it is a single body, an alloy or a compound capable of inserting and extracting lithium ions. For example, Li as a negative electrode active material, a Group 14 element such as carbon, silicon, germanium, tin, a Group 13 element such as aluminum or indium, a Group 12 element such as zinc or cadmium, a Group 15 element such as antimony or bismuth, magnesium And alkaline earth metals such as calcium, and Group 11 elements such as silver and gold may be used alone. Specific examples of the alloy or compound include tin-based materials such as Ag-Sn alloys, Cu-Sn alloys, Co-Sn alloys, carbon-based materials such as various types of graphite, and SiO _x (disproportionate into silicon alone and silicon dioxide) Examples thereof include silicon-based materials such as 0.3 ≦ x ≦ 1.6), a simple substance of silicon, or a composite obtained by combining a silicon-based material and a carbon-based material. In addition, as the negative electrode active material, oxides such as Nb ₂ O ₅ , TiO ₂ , Li ₄ Ti ₅ O ₁₂ , WO ₂ , MoO ₂ , Fe ₂ O ₃ or Li _3-x M _x N (M = A nitride represented by Co, Ni, Cu) may be employed. One or more of these can be used as the negative electrode active material.

It is also possible to use the silicon material disclosed in WO 2014/080608 as a kind of silicon-based material. The silicon material has a structure in which a plurality of plate-like silicon bodies are stacked in the thickness direction. For example, a silicon material may be reacted with CaSi ₂ and an acid to synthesize a layered silicon compound containing polysilane as a main component, and further, the layered silicon compound may be heated at 300 ° C. or higher to release hydrogen. It is manufactured.

An ideal reaction formula in the case of using hydrogen chloride as an acid is as follows when a method of manufacturing a silicon material is used.
3CaSi ₂ +6 HCl → Si ₆ H ₆ +3 CaCl ₂
Si ₆ H ₆ → 6Si + 3H ₂ ↑

However, in the upper reaction for synthesizing Si ₆ H ₆ which is polysilane, water is generally used as a reaction solvent from the viewpoint of removal of by-products and impurities. And, since Si ₆ H ₆ can react with water, in the process of synthesizing the layered silicon compound including the reaction in the upper stage, the layered silicon compound is hardly produced as one containing only Si ₆ H ₆ , and the layered silicon compound is layered The silicon compound is represented by Si ₆ H _s (OH) _t X _u (X is an element or group derived from the anion of an acid, s + t + u = 6, 0 <s <6, 0 <t <6, 0 <u <6) Manufactured as In the above chemical formula, unavoidable impurities such as remaining Ca are not taken into consideration. And the silicon material obtained by heating the said layered silicon compound also contains the element derived from the anion of oxygen or an acid.

As described above, the silicon material has a structure in which a plurality of plate-like silicon bodies are stacked in the thickness direction. The plate-like silicon body preferably has a thickness in the range of 10 nm to 100 nm, and more preferably in the range of 20 nm to 50 nm in order to efficiently store and release charge carriers such as lithium ions. The length in the longitudinal direction of the plate-like silicon body is preferably in the range of 0.1 μm to 50 μm. The plate-like silicon body preferably has a (longitudinal length) / (thickness) in the range of 2 to 1000. The layered structure of the plate-like silicon body can be confirmed by observation with a scanning electron microscope or the like. Moreover, this laminated structure is considered to be a remnant of the Si layer in the raw material CaSi ₂ .

  The silicon material preferably includes amorphous silicon and / or silicon crystallites. In particular, in the plate-like silicon body, it is preferable that amorphous silicon be a matrix and silicon crystallites be scattered in the matrix. The size of the silicon crystallite is preferably in the range of 0.5 nm to 300 nm, more preferably in the range of 1 nm to 100 nm, still more preferably in the range of 1 nm to 50 nm, and particularly preferably in the range of 1 nm to 10 nm. The size of the silicon crystallite is calculated from the Scheller equation using the half width of the diffraction peak of the Si (111) plane of the obtained X-ray diffraction chart by performing X-ray diffraction measurement on the silicon material. .

  The amount and size of the plate-like silicon body, amorphous silicon and silicon crystallite contained in the silicon material mainly depend on the heating temperature and heating time. The heating temperature is preferably in the range of 350 ° C. to 950 ° C., and more preferably in the range of 400 ° C. to 900 ° C.

  The Si-containing negative electrode active material containing silicon such as silicon oxide or silicon material may be coated with carbon. Silicon coated with carbon is excellent in conductivity.

  As the binder for the negative electrode, those mentioned in the column of the above-mentioned positive electrode can be used. In addition, as a binder for the negative electrode, a binder is a crosslinked polymer in which a carboxyl group-containing polymer such as polyacrylic acid or polymethacrylic acid is crosslinked with a polyamine such as diamine, disclosed in WO 2016/063882. It may be used as

Examples of the diamine used for the cross-linked polymer include alkylene diamines such as ethylene diamine, propylene diamine and hexamethylene diamine, 1,4-diaminocyclohexane, 1,3-diaminocyclohexane, isophorone diamine, bis (4-aminocyclohexyl) methane and the like. Saturated carbocyclic ring diamine, m-phenylenediamine, p-phenylenediamine, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenylether, bis (4-aminophenyl) sulfone, benzidine, o-tolidine, 2,4- Aromatic diamines such as tolylene diamine, 2,6-tolylene diamine, xylylene diamine and naphthalene diamine can be mentioned.
Well-known additives can be adopted as additives such as a dispersing agent other than the conductive aid and the binder.

  The blending ratio of the binder in the negative electrode active material layer is preferably, in mass ratio, negative electrode active material: binder = 1: 0.005 to 1: 0.3. When the amount of the binder is too small, the formability of the electrode decreases, and when the amount of the binder is too large, the energy density of the electrode decreases.

  The electrolytic solution contains a non-aqueous solvent and an electrolyte dissolved in the non-aqueous solvent.

  As the non-aqueous solvent, cyclic esters, linear esters, ethers and the like can be used. Examples of cyclic esters include ethylene carbonate, propylene carbonate, butylene carbonate, gamma butyrolactone, vinylene carbonate, 2-methyl-gamma butyrolactone, acetyl-gamma butyrolactone, and gamma valerolactone. Examples of the chain ester include dimethyl carbonate, diethyl carbonate, dibutyl carbonate, dipropyl carbonate, ethyl methyl carbonate, alkyl propionic acid ester, malonic acid dialkyl ester, acetic acid alkyl ester and the like. As the ethers, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,2-dibutoxyethane can be exemplified. For the electrolytic solution, these non-aqueous solvents may be used alone or in combination of two or more.

Examples of the electrolyte include lithium salts such as LiClO ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ and LiN (FSO ₂ ) ₂ .

As an electrolytic solution, nonaqueous solvents containing cyclic carbonate or linear carbonate such as ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, propylene carbonate, diethyl carbonate, LiClO ₄ , LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , A solution in which a lithium salt such as LiN (CF ₃ SO ₂ ) ₂ or LiN (FSO ₂ ) ₂ is dissolved can be exemplified.
The preferable range of the concentration of the lithium salt is 0.5 mol / L to 3.0 mol / L, 1.5 mol / L to 3.0 mol / L, and 1.8 mol / L to 3.0 mol / L. Each range of L may be mentioned.

  In order to manufacture a lithium ion secondary battery, for example, a separator is interposed between a positive electrode and a negative electrode to form an electrode body. The electrode body may be any of a laminated type in which the positive electrode, the separator and the negative electrode are stacked, or a wound type in which the positive electrode, the separator and the negative electrode are wound. After connecting the current collector of the positive electrode and the current collector of the negative electrode to the positive electrode terminal leading to the outside and the negative electrode terminal using a current collection lead or the like, an electrolytic solution is added to the electrode body to prepare a lithium ion secondary battery You should do it.

  The lithium ion secondary battery can be mounted on a vehicle. The lithium ion secondary battery maintains a large charge and discharge capacity and has excellent cycle performance, so a vehicle equipped with the lithium ion secondary battery is a high performance vehicle.

  Any vehicle may be used as long as it uses electric energy from batteries for all or part of the power source. For example, electric vehicles, hybrid vehicles, plug-in hybrid vehicles, hybrid railway vehicles, electric forklifts, electric wheelchairs, electric assists There are bicycles and electric motorcycles.

  As mentioned above, although the positive electrode and lithium ion secondary battery of this invention were demonstrated, this invention is not limited to the said embodiment. In the range which does not deviate from the summary of the present invention, it can carry out with various forms which gave change, improvement, etc. which a person skilled in the art can make.

Example 1
(Positive electrode active material)
[Particles of lithium nickel composite oxide]
As a positive electrode active material, particles of lithium nickel composite oxide and olivine particles were used.
As particles of lithium nickel complex oxide, LiNi _0.83 Co _0.154 Al _0.016 O ₂ having an average particle diameter of about 9 to 12 μm was used.
[Olivine particles (particles of first material, particles of second material)]
As olivine particles, carbon-coated LiM _h PO ₄ with an olivine structure with an average particle diameter of about 2 to 6 μm was used. As the LiM _h PO ₄ , LiMn _7/10 Fe _3/10 PO ₄ containing Fe and Mn was used. In addition, olivine particles are observed as aggregates of primary particles having a diameter of about several hundred nm under an SEM (scanning electron microscope).
The olivine particles were produced by further carbon-coating a commercially available olivine particle in which LiM _h PO ₄ having an olivine structure was carbon-coated by a chemical vapor deposition (CVD) method using propane gas as a carbon source. The commercially available olivine particles as the raw material are referred to as raw material olivine particles, and are distinguished from particles of the first material and olivine particles as particles of the second material.
While supplying the mixed gas to a rotary kiln containing 100 g of raw material olivine particles, the rotary kiln was rotated at 1 rpm and heated at 860 ° C. for 5 minutes. As mixed gas, what mixed propane gas and argon gas by volume ratio 1: 1 was used, and the supply rate of mixed gas was 2 L / min.
In the above steps, olivine particles which are particles of the first material and particles of the second material are obtained. The average particle size of the olivine particles was about 2 to 6 μm.

[Composite particles]
350 g of particles of lithium nickel composite oxide and 50 g of olivine particles were placed in a mixing apparatus and mixed for 3 minutes at 4250 rpm. As a mixing apparatus, Nobilta (R) NOB-130 manufactured by Hosokawa Micron Corporation was used. The mixing device is a device that mixes while applying compressive force, shear force and impact force to an object to be mixed.
Composite particles were obtained through the above steps. The average particle size of the composite particles was about 9 to 13 μm.

Volume resistivity was measured for the particles of the lithium-nickel composite oxide, olivine particles and composite particles described above. Specifically, 2 g of each sample was placed in a cylindrical tube having a diameter of 2 cm, each component was compressed with a load of 20 kN, and volume resistivity was determined with a resistance measuring device manufactured by Mitsubishi Chemical Analytech (trade name MCP-PD51). As a result, the volume resistivity of particles of lithium nickel composite oxide was 29.8 Ω · cm, and the volume resistivity of olivine particles was 0.35 to 0.45 Ω · cm.
The LP / LO volume resistivity ratio which divided the volume resistivity of olivine particles by the volume resistivity of particles of lithium nickel complex oxide was 0.012-0.015.

(Lithium ion secondary battery)
[Positive electrode]
80 parts by mass of the above composite particles as a positive electrode active material, 10 parts by mass of the above olivine particles as a positive electrode active material, 3 parts by mass of acetylene black having an average particle diameter of 0.05 to 0.1 μm as a conductive additive Seven parts by mass of polyvinylidene fluoride as an adhesive, and an appropriate amount of N-methyl-2-pyrrolidone as a solvent were weighed, respectively, and these were mixed using a planetary stirring degassing apparatus to produce a slurry. An aluminum foil with a thickness of 20 μm was prepared as a positive electrode current collector. The slurry was applied in the form of a film on the surface of the aluminum foil using a doctor blade. The slurry-coated copper foil was dried to remove N-methyl-2-pyrrolidone. Thereafter, the aluminum foil was pressed to obtain a bonded product. The obtained bonded product was dried by heating at 120 ° C. for 6 hours using a drier, and cut to obtain a positive electrode made of an aluminum foil on which a positive electrode active material layer was formed. The positive electrode of Example 1 was obtained through the above steps. The weight of the positive electrode was 20.2 mg / cm ² , and the density of the positive electrode was 2.86 g / cm ³ .

[Negative electrode]
CaSi ₂ was added to a 36% by weight aqueous solution of HCl at 10 ° C. in an argon atmosphere and stirred. The reaction solution was filtered, the residue was washed with distilled water and ethanol, and further dried under reduced pressure to separate a layered silicon compound containing polysilane. The layered silicon compound was heated at 900 ° C. for 1 hour in an argon gas atmosphere to obtain a silicon material. The ground material of the silicon material was coated with carbon and used as the following negative electrode active material.

  70.3 parts by mass of a silicon material as a negative electrode active material, 13.5 parts by mass of acetylene black as a conductive additive, 9 parts by mass of a mixture of polyacrylic acid and 4,4'-diaminodiphenylmethane as a binder, and an appropriate amount The slurry was prepared by mixing N-methyl-2-pyrrolidone. A copper foil was prepared as a current collector for the negative electrode. The above slurry was applied in a film form on the surface of this copper foil using a doctor blade. The copper foil coated with the slurry was dried to remove N-methyl-2-pyrrolidone. Thereafter, the copper foil was pressed to obtain a joint. The obtained bonded product was dried by heating with a vacuum dryer to produce a negative electrode made of copper foil on which a negative electrode active material layer was formed. The mixture of polyacrylic acid and 4,4'-diaminodiphenylmethane used as the binding agent was dehydrated by the above-mentioned heating and drying, and the polyacrylic acid was crosslinked with 4,4'-diaminodiphenylmethane. It changes to a crosslinked polymer.

[Others]
As a separator, a 25 μm thick polyethylene porous membrane having a single layer structure was prepared. The said separator was pinched | interposed with said positive electrode and negative electrode, and it was set as the electrode group. The electrode plate group was covered with a pair of laminate films, the three sides were sealed, and then an electrolytic solution was injected into the bag-like laminate film. As an electrolytic solution, a solution in which 1 mol / L of LiN (FSO ₂ ) ₂ and 1 mol / L of LiPF ₆ are dissolved in a solvent in which 1,2-dimethoxyethane: dimethyl carbonate is mixed at a volume ratio of 7: 3 Was used. Thereafter, by sealing the other side, the lithium ion secondary battery of Example 1 was obtained in which the four sides were airtightly sealed and the electrode plate group and the electrolytic solution were sealed.
The compositions of the positive electrode and the lithium ion secondary battery of Example 1 are shown in Table 1 described later together with Examples 2 to 4 and Comparative Examples 1 to 3 below.

(Example 2)
A lithium ion secondary battery of Example 2 was manufactured in the same manner as Example 1 except for the positive electrode.
As the positive electrode active material, the same particles of lithium nickel composite oxide as in Example 1 and the same olivine particles as in Example 1 were used.
369 g of particles of lithium nickel composite oxide and 31 g of olivine particles were placed in the same mixing apparatus as in Example 1 and mixed under the same conditions as in Example 1 to obtain composite particles. The average particle size of the composite particles was about 9 to 13 μm.
The positive electrode of Example 2 was obtained in the same manner as in Example 1 except that 65 parts by mass of the above-mentioned composite particles were used as a positive electrode active material and 25 parts by mass of the above olivine particles as a positive electrode active material. In the positive electrode of Example 2, the basis weight and the density were the same as in Example 1. A lithium ion secondary battery of Example 2 was obtained in the same manner as Example 1 except that the positive electrode of Example 2 was used.

(Example 3)
A lithium ion secondary battery of Example 3 was manufactured in the same manner as Example 1 except for the positive electrode.
As the positive electrode active material, the same particles of lithium nickel composite oxide as in Example 1 and the same olivine particles as in Example 1 were used.
343 g of lithium nickel composite oxide particles and 57 g of olivine particles were placed in the same mixing apparatus as in Example 1 and mixed under the same conditions as in Example 1 to obtain composite particles. The average particle size of the composite particles was about 9 to 13 μm.
The positive electrode of Example 3 was obtained in the same manner as in Example 1 except that 70 parts by mass of the above composite particles were used as a positive electrode active material and 20 parts by mass of the above olivine particles were used as a positive electrode active material. Also in the positive electrode of Example 3, the basis weight and the density were the same as in Example 1. A lithium ion secondary battery of Example 3 was obtained in the same manner as Example 1 except that the positive electrode of Example 2 was used.

(Example 4)
A lithium ion secondary battery of Example 4 was manufactured in the same manner as Example 1 except for the positive electrode.
As the positive electrode active material, the same particles of lithium nickel composite oxide as in Example 1 and the same olivine particles as in Example 1 were used.
320 g of particles of lithium nickel composite oxide and 80 g of olivine particles were placed in the same mixing apparatus as in Example 1 and mixed under the same conditions as in Example 1 to obtain composite particles. The average particle size of the composite particles was about 9 to 13 μm.
The positive electrode of Example 4 was obtained in the same manner as in Example 1 except that 75 parts by mass of the above-mentioned composite particles as the positive electrode active material and 15 parts by mass of the above olivine particles as the positive electrode active material were used. Also in the positive electrode of Example 4, the basis weight and the density were the same as in Example 1. A lithium ion secondary battery of Example 4 was obtained in the same manner as Example 1 except that the positive electrode of Example 4 was used.

(Comparative example 1)
A lithium ion secondary battery of Comparative Example 1 was manufactured in the same manner as Example 1 except for the positive electrode.
As the positive electrode active material, the same particles of lithium nickel composite oxide as in Example 1 and the same olivine particles as in Example 1 were used. In Comparative Example 1, particles of the lithium nickel composite oxide were used as they were, not composite particles.
The positive electrode of Comparative Example 1 was prepared in the same manner as in Example 1 except that 70 parts by mass of the above lithium nickel composite oxide particles were used as the positive electrode active material and 20 parts by mass of the above olivine particles as the positive electrode active material. Obtained. Also in the positive electrode of Comparative Example 1, the basis weight and the density were the same as in Example 1. A lithium ion secondary battery of Comparative Example 1 was obtained in the same manner as Example 1 except that the positive electrode of Comparative Example 1 was used.

(Comparative example 2)
A lithium ion secondary battery of Comparative Example 2 was manufactured in the same manner as Example 1 except for the positive electrode.
As the positive electrode active material, the same particles of lithium nickel composite oxide as in Example 1 and the same olivine particles as in Example 1 were used.
300 g of particles of lithium nickel composite oxide and 100 g of olivine particles were placed in the same mixing apparatus as in Example 1 and mixed under the same conditions as in Example 1 to obtain composite particles. The average particle size of the composite particles was about 9 to 13 μm.
The positive electrode of Comparative Example 2 was obtained in the same manner as in Example 1 except that 80 parts by mass of the above composite particles were used as a positive electrode active material and 10 parts by mass of the above olivine particles were used as a positive electrode active material. Also in the positive electrode of Comparative Example 2, the basis weight and the density were the same as in Example 1. A lithium ion secondary battery of Comparative Example 2 was obtained in the same manner as Example 1 except that the positive electrode of Comparative Example 2 was used.

(Comparative example 3)
A lithium ion secondary battery of Comparative Example 3 was manufactured in the same manner as Example 1 except for the positive electrode.
As the positive electrode active material, the same particles of lithium nickel composite oxide as in Example 1 and the same olivine particles as in Example 1 were used.
333 g of particles of lithium nickel composite oxide and 67 g of olivine particles were placed in the same mixing apparatus as in Example 1 and mixed under the same conditions as in Example 1 to obtain composite particles. The average particle size of the composite particles was about 9 to 13 μm.
The positive electrode of Comparative Example 3 was obtained in the same manner as in Example 1 except that 60 parts by mass of the above-mentioned composite particles as the positive electrode active material and 30 parts by mass of the above olivine particles as the positive electrode active material were used. Also in the positive electrode of Comparative Example 3, the basis weight and the density were the same as in Example 1. Further, a lithium ion secondary battery of Comparative Example 3 was obtained in the same manner as Example 1 except that the positive electrode of Comparative Example 3 was used.

(Evaluation 1 10 seconds discharge resistance)
The lithium ion secondary batteries of Examples 1 to 4 and Comparative Examples 1 to 3 adjusted to an SOC of 15% were discharged for 10 seconds at a constant current of 25C and a 1 C rate. From the voltage change amount and current value before and after discharge, the direct current resistance at the time of discharge was calculated according to Ohm's law. The results are shown in Table 2. Table 2 also shows the results of evaluation 2 nail penetration test described later.

(Evaluation 2 nail penetration test)
A nailing test as a forced short circuit test was performed on each of the lithium ion secondary batteries of Example 1 and Comparative Example 1 by the following method.
Constant voltage charging was performed on the lithium ion secondary battery until it stabilized at a potential of 4.5 V. The charged lithium ion secondary battery (the discharge capacity is expected to be about 4 Ah) was placed on a restraint plate having a hole with a diameter of 20 mm. The restraint plate was placed on a press machine with nails attached to the top. The nail penetrates the battery on the restraint plate and the nail is placed 20 mm / sec from the top to the bottom until the tip of the nail is located inside the hole in the restraint plate. It moved at the speed of The temperature of the thermocouple provided inside the nail was measured over time. Among the measured surface temperatures, the maximum temperature is shown in Table 2.

  In the lithium ion secondary battery of Comparative Example 1, while the temperature of the thermocouple inside the nail became high at 343.3 ° C., in the lithium ion secondary battery of Example 1, the thermocouple inside the nail was The temperature of the mixture remained at 198.8.degree. From the results of this nail penetration test, it can be said that the lithium ion secondary battery of Example 1 is superior in thermal stability to the lithium ion secondary battery of Comparative Example 1.

  Considering the results of the 10-second discharge resistance shown in Table 2 and the results of the nail penetration test, in order to improve the thermal stability, as in the lithium ion secondary battery of Example 1, the 10-second discharge resistance is 3.%. It is considered better to exceed 0 Ω. On the other hand, when the 10-second discharge resistance exceeds 4.5 Ω, it is considered that the output of the lithium ion secondary battery decreases. From these facts, it is considered that by setting the 10-second discharge resistance in the range of more than 3.0 Ω to 4.5 Ω or less, both of the excellent thermal stability and the output can be imparted to the lithium ion secondary battery. Then, in order to keep the 10-second discharge resistance in the range of more than 3.0 Ω to 4.5 Ω or less, when the positive electrode is (1) 100 parts by mass of the positive electrode active material layer, the first material and the second material When the amount of the second material is 100 parts by mass, and the amount of the first material contained in the composite particles is 20 parts by mass or more and 100 parts by mass or less. It is good to have both.

Claims (3)

LiM _h PO _{4 of} olivine structure (M is at least one element selected from Mn, Fe, Co, Ni, Cu, Mg, Zn, V, Ca, Sr, Ba, Ti, Al, Si, B, Te and Mo , 0 <h <2) with particles of a first material which is carbon-coated, the general _{formula: Li a Ni b Co c Al} d D e O f (0.2 ≦ a ≦ 2,0.6 ≦ b, b + c + d + e = 1: 0 ≦ e <1, D is W, Mo, Re, Pd, Ba, Cr, B, Sb, Sr, Pb, Ga, Mn, Nb, Mg, Ta, Ti, La, Zr, Cu, Ca, At least one element selected from Ir, Hf, Rh, Fe, Ge, Zn, Ru, Sc, Sn, In, Y, Bi, S, Si, Na, K, P, V, 1.7 ≦ f ≦ 3 Composite particles coated with particles of lithium nickel composite oxide represented by
LiM _h PO _{4 of} olivine structure (M is at least one element selected from Mn, Fe, Co, Ni, Cu, Mg, Zn, V, Ca, Sr, Ba, Ti, Al, Si, B, Te and Mo , Particles of the second material carbon-coated with 0 <h <2) in the positive electrode active material layer,
When the positive electrode active material layer is 100 parts by mass, it contains 20 parts by mass or more and 30 parts by mass or less of the first material and the second material,
The amount of the first material contained in the composite particles is 20 parts by mass or more and 100 parts by mass or less when the amount of the second material is 100 parts by mass. A value obtained by dividing volume resistivity of particles of the first material by volume resistivity of particles of the lithium-nickel composite oxide,
The positive electrode according to claim 1, wherein a value obtained by dividing the volume resistivity of the particles of the second material by the volume resistivity of the particles of the lithium nickel composite oxide is 0.034 or less. A positive electrode according to claim 1 or claim 2, a negative electrode, and an electrolyte solution,
The lithium ion secondary battery, wherein the electrolyte contains an electrolyte containing a lithium salt and a non-aqueous solvent containing a chain carbonate, and the concentration of the electrolyte in the electrolyte is 1.8 to 3 mol / L.