JP2017073281A - Positive electrode material for nonaqueous electrolyte secondary battery, positive electrode for nonaqueous electrolyte secondary battery using the same, and nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a means capable of sufficiently suppressing the oxidative decomposition of an electrolyte and the self discharge of batteries in nonaqueous electrolyte secondary batteries.SOLUTION: A core-shell type positive electrode material for a nonaqueous electrolyte secondary battery is obtained by arranging, on a surface of a core part including a positive electrode active material particle, a shell part including a fluorinated compound in which 40-70% of hydrogen atoms at an R part in a compound that is represented by ROLi (where R represents a straight-chain alkyl group or a straight-chain acyl group) are substituted by a fluorine atom.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a positive electrode material for a nonaqueous electrolyte secondary battery, and a positive electrode for a nonaqueous electrolyte secondary battery and a nonaqueous electrolyte secondary battery using the same.

  Currently, non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries that are used for mobile devices such as mobile phones have been commercialized. A nonaqueous electrolyte secondary battery generally includes a positive electrode obtained by applying a positive electrode active material or the like to a current collector, and a negative electrode obtained by applying a negative electrode active material or the like to a current collector. It has the structure connected through the electrolyte layer holding electrolyte gel. Then, when ions such as lithium ions are occluded / released in the electrode active material, a charge / discharge reaction of the battery occurs.

  Incidentally, in recent years, it has been required to reduce the amount of carbon dioxide in order to cope with global warming. Thus, non-aqueous electrolyte secondary batteries with a low environmental load are being used not only for portable devices, but also for power supply devices for electric vehicles such as hybrid vehicles (HEV), electric vehicles (EV), and fuel cell vehicles. .

  Non-aqueous electrolyte secondary batteries intended for application to electric vehicles are required to have high output and high capacity. Here, as a method for improving the capacity of the battery, it is conceivable to increase the charging voltage. However, when the charging voltage is increased in the nonaqueous electrolyte secondary battery, oxidative decomposition of the electrolytic solution occurs. And when charging / discharging is repeated at such a high charging voltage, there is a problem that the capacity gradually decreases. In addition, nonaqueous electrolyte secondary batteries are also required to suppress self-discharge under high temperature storage.

As a technique for solving these problems, for example, Patent Document 1 discloses a technique using a fluorinated lithium transition metal composite oxide as a positive electrode active material. In Patent Document 1, more specifically, fluorine represented by R ₁ R ₂ R ₃ R ₄ NF (R ₁ , R ₂ , R ₃ and R ₄ represent H or an alkyl group having 1 to 4 carbon atoms). A method of immersing positive electrode active material particles in a solution of a compound is disclosed. Further, a method of directly reacting with F ₂ gas, a method of contacting with NF ₃ gas, a photochemical fluorination method using a perfluoro compound, and the like are also disclosed.

JP-A-8-213014

  According to the study by the present inventor, it has been found that the technique described in Patent Document 1 may not sufficiently suppress the oxidative decomposition of the electrolytic solution and the self-discharge of the battery.

  Therefore, an object of the present invention is to provide means capable of sufficiently suppressing oxidative decomposition of an electrolytic solution and self-discharge of a battery in a nonaqueous electrolyte secondary battery.

  The present inventor has accumulated extensive research. As a result, the above problem is solved by using a core-shell structure positive electrode material in which a shell portion made of a specific fluorinated compound containing lithium atoms is formed on the surface of the core portion made of positive electrode active material particles. As a result, the present invention has been completed.

  That is, according to one embodiment of the present invention, a core part including positive electrode active material particles and ROLi (R is a linear alkyl group or a linear acyl group) disposed on the surface of the core part are represented. And a shell part containing a fluorinated compound in which 40 to 70% of hydrogen atoms in the R portion in the compound are substituted with fluorine atoms.

  According to the positive electrode material for a nonaqueous electrolyte secondary battery of the present invention, the surface of the positive electrode active material is less likely to be in direct contact with the electrolyte solution by disposing the shell portion containing the predetermined fluorinated compound on the surface of the core portion. Thus, the oxidation resistance of the positive electrode active material is improved, and the diffusion resistance of lithium ions on the surface of the positive electrode active material is increased to some extent. As a result, it is possible to sufficiently suppress the oxidative decomposition and self-discharge of the electrolytic solution in the nonaqueous electrolyte secondary battery.

1 is a schematic cross-sectional view showing a basic configuration of a non-aqueous electrolyte lithium ion secondary battery that is not a flat (stacked) bipolar type, which is an embodiment of a nonaqueous electrolyte lithium ion secondary battery. It is a perspective view showing the appearance of a flat lithium ion secondary battery which is a typical embodiment of a secondary battery. It is the graph which plotted the result of having measured the diffusion resistance of the lithium ion in the surface of a positive electrode material with respect to the thickness of a shell part about the coin cell for evaluation produced in the Example and the comparative example.

  According to one aspect of the present invention, a core part including positive electrode active material particles and ROLi (R is a linear alkyl group or a linear acyl group) disposed on the surface of the core part are represented. There is provided a positive electrode material for a non-aqueous electrolyte secondary battery having a shell portion containing a fluorinated compound in which 40 to 70% of hydrogen atoms in the R portion of the compound are substituted with fluorine atoms.

  Hereinafter, as a preferred embodiment of the nonaqueous electrolyte secondary battery to which the positive electrode material according to the present embodiment is applied, a nonaqueous electrolyte lithium ion secondary battery will be described, but the present invention is not limited to the following embodiment. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted. In addition, the dimensional ratios in the drawings are exaggerated for convenience of explanation, and may be different from the actual ratios.

  FIG. 1 is a schematic cross-sectional view schematically showing a basic configuration of a non-aqueous electrolyte lithium ion secondary battery (hereinafter also simply referred to as “stacked battery”) that is not a flat (stacked) bipolar type. As shown in FIG. 1, the stacked battery 10 of the present embodiment has a structure in which a substantially rectangular power generation element 21 in which a charge / discharge reaction actually proceeds is sealed inside a battery exterior material 29 that is an exterior body. Have. Here, the power generation element 21 has a configuration in which a positive electrode, a separator 17, and a negative electrode are stacked. The separator 17 contains a nonaqueous electrolyte (for example, a liquid electrolyte). The positive electrode has a structure in which the positive electrode active material layers 15 are disposed on both surfaces of the positive electrode current collector 12. The negative electrode has a structure in which the negative electrode active material layer 13 is disposed on both surfaces of the negative electrode current collector 11. Specifically, the negative electrode, the electrolyte layer, and the positive electrode are laminated in this order so that one positive electrode active material layer 15 and the negative electrode active material layer 13 adjacent thereto face each other with a separator 17 therebetween. Thereby, the adjacent positive electrode, electrolyte layer, and negative electrode constitute one unit cell layer 19. Therefore, it can be said that the stacked battery 10 shown in FIG. 1 has a configuration in which a plurality of single battery layers 19 are stacked and electrically connected in parallel.

  In addition, although the negative electrode active material layer 13 is arrange | positioned only in the single side | surface at all the outermost layer positive electrode collectors located in both outermost layers of the electric power generation element 21, an active material layer may be provided in both surfaces. That is, instead of using a current collector dedicated to the outermost layer provided with an active material layer only on one side, a current collector having an active material layer on both sides may be used as it is as an outermost current collector. Further, by reversing the arrangement of the positive electrode and the negative electrode as compared with FIG. 1, the outermost positive electrode current collector is positioned on both outermost layers of the power generation element 21, and the outermost positive electrode current collector is disposed on one or both surfaces of the outermost layer positive electrode current collector. A positive electrode active material layer may be disposed.

  The positive electrode current collector 12 and the negative electrode current collector 11 are each provided with a positive electrode current collector plate (tab) 27 and a negative electrode current collector plate (tab) 25 that are electrically connected to the respective electrodes (positive electrode and negative electrode). It has the structure led out of the battery exterior material 29 so that it may be pinched | interposed into the edge part. The positive electrode current collector plate 27 and the negative electrode current collector plate 25 are ultrasonically welded to the positive electrode current collector 11 and the negative electrode current collector 12 of each electrode via a positive electrode lead and a negative electrode lead (not shown), respectively, as necessary. Or resistance welding or the like.

  Note that FIG. 1 shows a flat battery (stacked battery) that is not a bipolar battery, but a positive electrode active material layer that is electrically coupled to one surface of the current collector and the opposite side of the current collector. And a bipolar battery including a bipolar electrode having a negative electrode active material layer electrically coupled to the surface. In this case, one current collector also serves as a positive electrode current collector and a negative electrode current collector.

  Hereinafter, each member will be described in more detail.

≪Positive electrode≫
The positive electrode has a positive electrode current collector and a positive electrode active material layer formed on the surface of the positive electrode current collector.

(Positive electrode current collector)
There is no particular limitation on the material constituting the positive electrode current collector, but a metal is preferably used. Specifically, examples of the metal include aluminum, nickel, iron, stainless steel, titanium, copper, and other alloys. In addition to these, a clad material of nickel and aluminum, a clad material of copper and aluminum, or a plating material of a combination of these metals can be preferably used. Moreover, the foil by which aluminum is coat | covered on the metal surface may be sufficient. Of these, aluminum, stainless steel, and copper are preferable from the viewpoints of electronic conductivity and battery operating potential.

  The size of the current collector is determined according to the intended use of the battery. For example, if it is used for a large battery that requires a high energy density, a current collector having a large area is used. There is no particular limitation on the thickness of the current collector. The thickness of the current collector is usually about 1 to 100 μm.

(Positive electrode active material layer)
The positive electrode active material layer includes a positive electrode active material. In this embodiment, the positive electrode active material essentially includes a predetermined core-shell type positive electrode material. Specifically, the positive electrode material according to the present embodiment is a core part including positive electrode active material particles and ROLi (R is a linear alkyl group or a linear acyl group) disposed on the surface of the core part. And a shell portion containing a fluorinated compound in which 40 to 70% of hydrogen atoms in the R portion in the compound represented are substituted with fluorine atoms. Hereinafter, the core-shell type positive electrode material will be described in more detail.

[Core]
The core portion includes positive electrode active material particles. There is no restriction | limiting in particular about the specific form of the positive electrode active material which comprises the positive electrode active material particle contained in a core part, A conventionally well-known positive electrode active material can be used. Examples of such a positive electrode active material include lithium nickel composite oxide and spinel lithium manganese composite oxide. Among these, from the viewpoint of increasing the capacity of the battery, the positive electrode active material particles constituting the core part preferably include a lithium nickel-based composite oxide,
-Lithium nickel complex oxide The composition of lithium nickel complex oxide is not specifically limited as long as it is a complex oxide containing lithium and nickel. A typical example of a composite oxide containing lithium and nickel is lithium nickel composite oxide (LiNiO ₂ ). However, a composite oxide in which some of the nickel atoms of the lithium nickel composite oxide are substituted with other metal atoms is more preferable. As a preferable example, a lithium-nickel-manganese-cobalt composite oxide (hereinafter simply referred to as “NMC composite”) is preferable. (Also referred to as “oxide”) has a layered crystal structure in which a lithium atomic layer and a transition metal (Mn, Ni, and Co are arranged in order) are alternately stacked via an oxygen atomic layer. One Li atom is contained per atom, and the amount of Li that can be taken out is twice that of the spinel-based lithium-manganese composite oxide, that is, the supply capacity is doubled, and a high capacity can be obtained. In addition, since it has higher thermal stability than LiNiO ₂ , it is particularly advantageous among the nickel-based composite oxides used as the positive electrode active material.

  In the present specification, the NMC composite oxide includes a composite oxide in which a part of the transition metal element is substituted with another metal element. Other elements in that case include Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, Cr, Fe, B, Ga, In, Si, Mo, Y, Sn, V, Cu , Ag, Zn, etc., preferably Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, Cr, more preferably Ti, Zr, P, Al, Mg, From the viewpoint of improving cycle characteristics, Ti, Zr, Al, Mg, and Cr are more preferable.

Since the NMC composite oxide has a high theoretical discharge capacity, it is preferable that the general formula (1): Li _a Ni _b Mn _c Co _d M _x O ₂ (where a, b, c, d, x Satisfies 0.9 ≦ a ≦ 1.2, 0 <b <1, 0 <c ≦ 0.5, 0 <d ≦ 0.5, 0 ≦ x ≦ 0.3, and b + c + d = 1. And at least one element selected from Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, and Cr. Here, a represents the atomic ratio of Li, b represents the atomic ratio of Ni, c represents the atomic ratio of Mn, d represents the atomic ratio of Co, and x represents the atomic ratio of M. Represents. From the viewpoint of cycle characteristics, it is preferable that 0.4 ≦ b ≦ 0.6 in the general formula (1). The composition of each element can be measured by, for example, inductively coupled plasma (ICP) emission spectrometry.

  In general, nickel (Ni), cobalt (Co), and manganese (Mn) are known to contribute to capacity and output characteristics from the viewpoint of improving the purity of materials and improving electronic conductivity. Ti or the like partially replaces the transition metal in the crystal lattice. From the viewpoint of cycle characteristics, it is preferable that a part of the transition element is substituted with another metal element, and it is particularly preferable that 0 <x ≦ 0.3 in the general formula (1). Since at least one selected from the group consisting of Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, and Cr is dissolved, the crystal structure is stabilized. It is considered that the battery capacity can be prevented from decreasing even if the above is repeated, and that excellent cycle characteristics can be realized.

In the NMC composite oxide, for example, when the metal composition of nickel, manganese and cobalt is not uniform, such as LiNi _0.5 Mn _0.3 Co _0.2 O ₂ , The effect of strain / cracking is increased. This is presumably because the stress applied to the inside of the particles during expansion and contraction is distorted and cracks are more likely to occur in the composite oxide due to the non-uniform metal composition. Therefore, for example, a complex oxide having a rich Ni abundance ratio (for example, LiNi _0.8 Mn _0.1 Co _0.1 O ₂ ) or a complex oxide having a uniform ratio of Ni, Mn, and Co. Compared with (for example, LiNi _0.3 Mn _0.3 Co _0.3 O ₂ ), the deterioration of long-term cycle characteristics (decrease in capacity accompanying an increase in the number of cycles) becomes significant. On the other hand, with the configuration according to this embodiment, cycle characteristics can be improved even in a complex oxide having a non-uniform metal composition such as LiNi _0.5 Mn _0.3 Co _0.2 O ₂ .

  Therefore, in the general formula (1), composite oxides in which b, c and d are 0.44 ≦ b ≦ 0.51, 0.27 ≦ c ≦ 0.31, 0.19 ≦ d ≦ 0.26 The positive electrode active material is preferable because a battery having excellent long-term cycle characteristics can be provided.

  Lithium nickel composite oxides such as NMC composite oxides can be prepared by selecting various known methods such as coprecipitation method and spray drying method. The coprecipitation method is preferably used because the complex oxide according to this embodiment is easy to prepare. Specifically, as a method for synthesizing the NMC composite oxide, for example, a nickel-cobalt-manganese composite oxide is produced by a coprecipitation method as in the method described in JP2011-105588A, and then nickel-cobalt. -It can be obtained by mixing and firing a manganese composite oxide and a lithium compound. This will be specifically described below.

  A raw material compound of the composite oxide, for example, a Ni compound, a Mn compound, and a Co compound is dissolved in an appropriate solvent such as water so as to have a desired composition of the active material. Examples of the Ni compound, Mn compound, and Co compound include sulfates, nitrates, carbonates, acetates, oxalates, oxides, hydroxides, and halides of the metal elements. Specific examples of the Ni compound, Mn compound, and Co compound include, but are not limited to, nickel sulfate, cobalt sulfate, manganese sulfate, nickel acetate, cobalt acetate, and manganese acetate. In this process, as necessary, for example, Ti, Zr, Nb as a metal element that substitutes a part of the layered lithium metal composite oxide constituting the active material so as to have a desired active material composition. , W, P, Al, Mg, V, Ca, Sr, and a compound containing at least one metal element such as Cr may be further mixed.

  A coprecipitation reaction can be performed by a neutralization and precipitation reaction using the raw material compound and an alkaline solution. Thereby, the metal composite hydroxide and metal composite carbonate containing the metal contained in the said raw material compound are obtained. As the alkaline solution, for example, an aqueous solution of sodium hydroxide, potassium hydroxide, sodium carbonate, ammonia or the like can be used, but sodium hydroxide, sodium carbonate or a mixed solution thereof is preferably used for the neutralization reaction. . In addition, an aqueous ammonia solution or an ammonium salt is preferably used for the complex reaction.

  The addition amount of the alkaline solution used for the neutralization reaction may be an equivalent ratio of 1.0 with respect to the neutralized amount of all the metal salts contained, but it is preferable to add the alkali excess together for pH adjustment.

  As for the addition amount of the aqueous ammonia solution or ammonium salt used for the complex reaction, the ammonia concentration in the reaction solution is preferably added in the range of 0.01 to 2.00 mol / L. The pH of the reaction solution is preferably controlled in the range of 10.0 to 13.0. The reaction temperature is preferably 30 ° C. or higher, more preferably 30 to 60 ° C.

  The composite hydroxide obtained by the coprecipitation reaction is then preferably suction filtered, washed with water and dried. The particle size of the composite hydroxide can be controlled by adjusting the conditions (stirring time, alkali concentration, etc.) for carrying out the coprecipitation reaction, which is the secondary electrode of the positive electrode active material finally obtained. Affects the average particle size of the particles.

  Next, the nickel-cobalt-manganese composite hydroxide is mixed with a lithium compound and fired to obtain a lithium-nickel-manganese-cobalt composite oxide. Examples of the Li compound include lithium hydroxide or a hydrate thereof, lithium peroxide, lithium nitrate, and lithium carbonate.

  The firing process may be performed in one stage, but is preferably performed in two stages (temporary firing and main firing). A composite oxide can be obtained efficiently by two-stage firing. The pre-baking conditions are not particularly limited, and differ depending on the lithium raw material, so that it is difficult to uniquely define them. Here, the factors for controlling the average primary particle size and crystallite size are particularly important as the firing temperature and firing time during firing (temporary firing and main firing in the case of two stages). By adjusting based on the following tendency, the average primary particle diameter and crystallite diameter can be controlled. That is, when the firing time is lengthened, the average primary particle diameter and crystallite diameter increase. Further, when the firing temperature is increased, the average primary particle size and crystallite size are increased. In addition, it is preferable that a temperature increase rate is 1-20 degrees C / min from room temperature. The atmosphere is preferably in air or in an oxygen atmosphere. Here, in the case of synthesizing the NMC composite oxide using lithium carbonate as the Li raw material, the pre-baking temperature is preferably 500 to 900 ° C, more preferably 600 to 800 ° C, and further preferably 650. ~ 750 ° C. Furthermore, the pre-baking time is preferably 0.5 to 10 hours, and more preferably 4 to 6 hours. On the other hand, the conditions for the main firing are not particularly limited, but the rate of temperature rise is preferably from room temperature to 1 to 20 ° C./min. The atmosphere is preferably in air or in an oxygen atmosphere. In the case of synthesizing an NMC composite oxide using lithium carbonate as a Li raw material, the firing temperature is preferably 800 to 1200 ° C, more preferably 850 to 1100 ° C, and still more preferably 900 to 1050. ° C. Furthermore, baking time becomes like this. Preferably it is 1 to 20 hours, More preferably, it is 8 to 12 hours.

  If necessary, when adding a trace amount of a metal element that replaces a part of the layered lithium metal composite oxide constituting the active material, as the method, a method of previously mixing with nickel, cobalt, manganate, Any means such as a method of adding nickel, cobalt and manganate simultaneously, a method of adding to the reaction solution during the reaction, a method of adding to the nickel-cobalt-manganese composite oxide together with the Li compound may be used.

  The lithium nickel composite oxide can be produced by appropriately adjusting reaction conditions such as pH of the reaction solution, reaction temperature, reaction concentration, addition rate, and stirring time.

Spinel-based lithium manganese composite oxide Spinel-based lithium manganese composite oxide is a composite oxide that typically has a composition of LiMn ₂ O ₄ and has a spinel structure and essentially contains lithium and manganese. Conventionally known knowledge can be appropriately referred to for the specific configuration and manufacturing method.

  The positive electrode active material particles constituting the core portion usually have a secondary particle configuration in which primary particles are aggregated. And the average particle diameter (average primary particle diameter) of the said primary particle becomes like this. Preferably it is 0.9 micrometer or less, More preferably, it is 0.20-0.6 micrometer, More preferably, it is 0.25-0.5 micrometer. is there. Moreover, the average particle diameter (average secondary particle diameter (D50)) of the secondary particles is preferably 3 to 20 μm, more preferably 5 to 15 μm. Furthermore, the value of these ratios (average secondary particle size / average primary particle size) is preferably greater than 11, more preferably 15 to 50, and even more preferably 25 to 40. Here, in this specification, the values of the average primary particle diameter and the average secondary particle diameter of the positive electrode active material particles constituting the core part can be measured by a known method, but particularly about the average secondary particle diameter. The value measured by the laser diffraction method shall be adopted.

[Shell part (fluorinated compound)]
The shell part is characterized in that it is disposed on the surface of the core part including the positive electrode active material particles described above and contains a predetermined fluorinated compound. Specifically, the shell portion in the positive electrode material according to the present embodiment is such that 40 to 70% of hydrogen atoms in the compound represented by ROLi (R is a linear alkyl group or a linear acyl group) are substituted with fluorine atoms. It is characterized in that it contains a modified fluorinated compound.

  Here, R in the fluorinated compound (ROLi) is a linear alkyl group or a linear acyl group. The number of carbon atoms of these groups is not particularly limited, but from the viewpoint of sufficiently exerting the effects of the present invention, the number of carbon atoms of the linear alkyl group is preferably 1 to 7, more preferably 1 ~ 5. Moreover, the number of carbon atoms of the linear acyl group is preferably 2 to 7, and more preferably 2 to 5.

Specific examples of the linear alkyl group include, for example, a methyl group (CH ₃ —), an ethyl group (C ₂ H ₅ —), an n-propyl group (C ₃ H ₇ —), an n-butyl group (C ₄ H). ₉ -), n-pentyl group _{(C 5 H 11 -),} n- hexyl group _{(C 6 H 13 -),} n- heptyl _{(C 7 H 15} -), and the like. Specific examples of the linear acyl group include, for example, an acetyl group (CH ₃ CO—), an n-propanoyl group (C ₂ H ₅ CO—), an n-butanoyl group (C ₃ H ₇ CO—), n - pentanoyl group _{(C 4 H 9 CO -)} , n- hexanoyl group _{(C 5 H 11 CO -)} , n- heptanoyl _{(C 6} H 13 CO-), and the like.

And the fluorinated compound contained in a shell part in the positive electrode material which concerns on this form is a compound by which 40 to 70% of the hydrogen atoms of R part in ROLi mentioned above were substituted by the fluorine atom. The said ratio (fluorination rate) becomes like this. Preferably it is 50 to 70%, More preferably, it is 60 to 70%. Here, specific examples of the fluorinated compound when R is a linear alkyl group include, for example, CHF ₂ OLi, C ₂ H ₃ F ₂ OLi, C ₂ H ₂ F ₃ OLi, and C ₃ H ₄ F _{3. OLi, C 3 H 3 F 4} OLi, C 4 H 5 F 4 OLi, C 4 H 4 F 5 OLi, C 4 H 3 F 6 OLi, C 5 H 6 F 5 OLi, C 5 H 5 F 6 OLi, _{C 5 H 4 F 7 OLi,} C 6 H 7 F 6 OLi, C 6 H 6 F 7 OLi, C 6 H 5 F 8 OLi, C 6 H 4 F 9 OLi, C 7 H 9 F 6 OLi, C 7 _{H 8 F 7 OLi, C 7} H 7 F 8 OLi, C 7 H 6 F 9 OLi, include C _{7 H 5} F 10 OLi. Similarly, specific examples of the fluorinated compound when R is a linear acyl group include, for example, CHF ₂ COOLi, C ₂ H ₃ F ₂ COOLi, C ₂ H ₂ F ₃ COOLi, and C ₃ H ₄ F _{3. COOLi, C 3 H 3 F 4} COOLi, C 4 H 5 F 4 COOLi, C 4 H 4 F 5 COOLi, C 4 H 3 F 6 COOLi, C 5 H 6 F 5 COOLi, C 5 H 5 F 6 COOLi, _{C 5 H 4 F 7 COOLi,} C 6 H 7 F 6 COOLi, C 6 H 6 F 7 COOLi, C 6 H 5 F 8 COOLi, include C ₆ H ₄ F 9 COOLi. Among these, from the viewpoint of excellent effects of suppressing oxidative decomposition of the electrolyte and self-discharge of the battery, R in ROLi corresponding to the fluorinated compound is preferably a linear acyl group, and has 2 to 7 carbon atoms. Is more preferably a linear acyl group having 2 to 5 carbon atoms, particularly preferably a linear acyl group having 2 to 3 carbon atoms, Most preferably, it is a straight-chain acyl group (acetyl group) of formula 2. In other words, as the fluorinated compound according to the present invention, lithium difluoroacetate (CHF ₂ COOLi) is most preferable. The reason why the lower limit of the value of the fluorination rate is set to 40% or more is that a compound having a too low fluorination rate is thermally decomposed at a high temperature and cannot sufficiently exhibit the above-described effects. On the other hand, the upper limit of the value of the fluorination rate is set to 70% or less because a compound having a too high fluorination rate is dissolved in the solvent constituting the electrolytic solution, and the above-described effects can be sufficiently exhibited. This is because it cannot be done. Moreover, the structure of the R portion in the above-mentioned ROLi is limited to a straight-chain structure because the dispersibility in the solution for forming the shell portion on the surface of the core portion is poor and a uniform shell portion is produced. It is because it cannot be done.

  Here, the mechanism that the effect of suppressing the oxidative decomposition of the electrolytic solution and the self-discharge of the battery is exhibited by forming the shell portion containing the above-mentioned predetermined fluorinated compound on the surface of the core portion containing the positive electrode active material particles. Although not completely clear, the following mechanism is presumed. That is, when the shell portion containing the predetermined fluorinated compound is disposed on the surface of the core portion, the surface of the positive electrode active material is less likely to be in direct contact with the electrolytic solution, and the oxidation resistance of the positive electrode active material is improved. That is, it is considered that the progress of the oxidative decomposition reaction due to the contact between the exposed surface of the positive electrode active material and the electrolytic solution is sufficiently suppressed by decreasing the contact probability. Moreover, the diffusion resistance of lithium ions on the surface of the positive electrode active material is increased by disposing the shell portion containing the predetermined fluorinated compound on the surface of the core portion. As a result, it is difficult for lithium ions occluded in the negative electrode to pass through the inside of the battery and return to the positive electrode active material in the charged state (this is a cause of self-discharge). As a result, it is considered that self-discharge in the nonaqueous electrolyte secondary battery is sufficiently suppressed. Here, when the diffusion resistance of lithium ions increases on the surface of the positive electrode active material particles, it seems that the internal resistance of the battery also increases. It has been found that the diffusion resistance of lithium ions increases only by about 20% even when a shell portion that is sufficiently thick (for example, a thickness of about 30 nm) is formed. This means that the present invention provides an extremely excellent technique because the internal resistance of the battery is not significantly increased while sufficiently exhibiting the various effects described above. ing.

  There is no particular limitation on the thickness of the shell portion disposed on the surface of the core portion (when there is a portion where the shell portion is not formed, the average thickness at the portion where the shell portion is formed), From the viewpoint of higher output and improved safety, the thickness is preferably 5 to 40 nm, more preferably 10 to 35 nm, still more preferably 15 to 35 nm, and particularly preferably 15 to 30 nm. In addition, the value measured by observation using a scanning electron microscope (SEM) shall be employ | adopted for the value of the thickness of a shell part.

  Further, the area ratio (coverage) of the portion where the shell portion is disposed on the surface of the core portion is not particularly limited, but in the positive electrode material according to the present embodiment, the larger the coverage, It has been found that the effect of suppressing the self-discharge of the battery is high. For this reason, the coverage of the core part by the shell part is preferably 50% or more, more preferably 70% or more, still more preferably 90% or more, and particularly preferably 95% or more. The upper limit value of the coverage is not particularly limited, but is theoretically 100%. Here, in the method described in Patent Document 1, the oxidation resistance of the surface of the active material particles is improved by fluorinating the hydroxy group (—OH group) present on the surface of the active material particles. In the fluorination by such a technique, only the site where the hydroxy group exists is fluorinated, so that fluorination cannot be achieved over the entire surface of the active material particles.

  There is no restriction | limiting in particular about the method of forming a shell part in the surface of a core part, A conventionally well-known knowledge can be referred suitably. As an example of the method of forming the shell portion on the surface of the core portion, for example, a method of forming the shell portion on the surface of the core portion using physical treatment or electrochemical treatment can be given. That is, according to another aspect of the present invention, there is provided a method for manufacturing a positive electrode material according to one aspect of the present invention described above, wherein a shell part is formed on a surface of a core part using physical treatment or electrochemical treatment. A manufacturing method is provided. According to these methods, it is possible to produce a positive electrode material having a shell portion on the surface of the core portion by a simple technique.

As a method for forming the shell portion by physical treatment, for example, a method in which positive electrode active material particles are brought into contact with a solution in which a fluorinated compound is dissolved in a solvent can be mentioned. In this case, first, a solution in which the fluorinated compound is dissolved in an appropriate solvent is prepared. Next, the fluorinated compound is attached to the surface of the core part (positive electrode active material particles) by bringing the positive electrode active material particles constituting the core part into contact with the solution. Thereafter, the core part (positive electrode active material particles) having a fluorinated compound attached to the surface is separated from the solution and dried to obtain the core-shell type positive electrode material according to the present invention. At this time, as a solvent constituting the solution to be brought into contact with the core part (positive electrode active material particles), methyl difluoroacetate (CHF ₂ COOCH ₃ ), ethyl difluoroacetate (CHF ₂ COOCH ₂ CH ₃ ), and the like, propyl difluoroacetate Etc. can be used. Moreover, there is no restriction | limiting in particular about the density | concentration of the fluorinated compound in the said solution, As an example, the solution which dissolved about 5-15 mass% of fluorinated compounds with respect to 100 mass% of said solvents may be used. Moreover, there is no restriction | limiting in particular also about the temperature at the time of making the said solution contact with a core part (positive electrode active material particle), and making it contact, If it is an expert, it can select suitably. In addition, the coverage of the core part by a shell part and the thickness of a shell part can be enlarged, so that the density | concentration of the fluorinated compound in the said solution is large and contact time is long.

  In addition, as a method for forming the shell portion by physical treatment, for example, a sol-gel method, a sputtering method, a spin coating method, an electrostatic spray volume method, a pulse laser method, or the like can be used.

On the other hand, when the shell part is formed by electrochemical treatment, the core part (positive electrode active material particles) in which the shell part is not formed is included in the positive electrode active material layer as a positive electrode material. On the other hand, a solvent such as methyl difluoroacetate (CHF ₂ COOCH ₃ ) or ethyl difluoroacetate (CHF ₂ COOCH ₃ ) is included in the electrolytic solution as a precursor of the fluorinated compound. At this time, the amount of the precursor to be contained in the electrolytic solution is not particularly limited. Usually, the precursor is about 10 to 35% by volume with respect to 100% by volume of the electrolytic solution (solvent + lithium salt). Should be included. Thus, during charging / discharging (mainly initial discharge) of the battery, the shell portion made of the corresponding fluorinated compound (in the case of using the above precursor, lithium difluoroacetate (CHF ₂ COOLi)) is changed to the core portion. It can be formed on the surface. Here, there is no restriction | limiting in particular about the conditions of charging / discharging for forming a shell part, The charging / discharging conditions conventionally used for charging / discharging of nonaqueous electrolyte secondary batteries, such as a lithium ion secondary battery (for example, 4. In the voltage range of 2 to 3.0 V, a charging current value of 0.2 C is applicable as it is. As an example, according to the study of the present inventors, 20% by volume of methyl difluoroacetate (CHF ₂ COOCH ₃ ) is added to 100% by volume (solvent + lithium salt) of the electrolytic solution, and the charge exemplified above is used. When first charge / discharge was performed under the discharge conditions, it was confirmed that a shell portion made of lithium difluoroacetate (CHF ₂ COOLi) having a thickness of about 20 nm was formed.

-Other components In addition to the positive electrode material described above, the positive electrode active material layer includes a conductive additive, a binder, an electrolyte (polymer matrix, ion conductive polymer, electrolyte, etc.), and lithium for enhancing ion conductivity as necessary. It further contains other additives such as salts (excluding the above fluorinated compounds). However, the content of a material that can function as an active material in the positive electrode active material layer and the negative electrode active material layer described later is preferably 85 to 99.5% by mass.

(binder)
Although it does not specifically limit as a binder used for a positive electrode active material layer, For example, the following materials are mentioned. Polyethylene, polypropylene, polyethylene terephthalate (PET), polyether nitrile, polyacrylonitrile, polyimide, polyamide, cellulose, carboxymethyl cellulose (CMC) and its salts, ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR) ), Isoprene rubber, butadiene rubber, ethylene / propylene rubber, ethylene / propylene / diene copolymer, styrene / butadiene / styrene block copolymer and hydrogenated product thereof, styrene / isoprene / styrene block copolymer and hydrogenated product thereof Thermoplastic polymers such as products, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (F P), tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer (PFA), ethylene / tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene / chlorotrifluoroethylene copolymer (ECTFE) ), Fluororesin such as polyvinyl fluoride (PVF), vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP) -TFE fluorine rubber), vinylidene fluoride-pentafluoropropylene fluorine rubber (VDF-PFP fluorine rubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene Fluorine rubber (VDF-PFP-TFE fluorine rubber), vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene fluorine rubber (VDF-PFMVE-TFE fluorine rubber), vinylidene fluoride-chlorotrifluoroethylene fluorine Examples thereof include vinylidene fluoride fluorine rubber such as rubber (VDF-CTFE fluorine rubber), epoxy resin, and the like. These binders may be used independently and may use 2 or more types together.

  The amount of the binder contained in the positive electrode active material layer is not particularly limited as long as it is an amount capable of binding the active material, but is preferably 0.5 to 15% by mass with respect to the active material layer. More preferably, it is 1-10 mass%.

  The positive electrode active material layer further includes other additives such as a conductive additive, an electrolyte (polymer matrix, ion conductive polymer, electrolytic solution, etc.), and a lithium salt for enhancing ion conductivity, as necessary.

  The conductive assistant refers to an additive that is blended in order to improve the conductivity of the positive electrode active material layer or the negative electrode active material layer. Examples of the conductive aid include carbon materials such as carbon black such as ketjen black and acetylene black, and carbon fibers. When the active material layer contains a conductive additive, an electronic network inside the active material layer is effectively formed, which can contribute to improvement of the output characteristics of the battery.

Examples of the electrolyte salt (lithium salt) include Li (C ₂ F ₅ SO ₂ ) ₂ N, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO _{3 and the} like.

  Examples of the ion conductive polymer include polyethylene oxide (PEO) -based and polypropylene oxide (PPO) -based polymers.

  The compounding ratio of the components contained in the positive electrode active material layer and the later-described negative electrode active material layer is not particularly limited. The blending ratio can be adjusted by appropriately referring to known knowledge about lithium ion secondary batteries. The thickness of each active material layer is not particularly limited, and conventionally known knowledge about the battery can be appropriately referred to. For example, the thickness of each active material layer is about 2 to 100 μm.

≪Negative electrode≫
Similar to the positive electrode, the negative electrode has a structure in which a negative electrode active material layer is formed on the surface of the negative electrode current collector. Here, the negative electrode current collector can have the same configuration as the positive electrode current collector.

[Negative electrode active material layer]
The negative electrode active material layer contains a negative electrode active material and, if necessary, other materials such as a conductive aid, a binder, an electrolyte (polymer matrix, ion conductive polymer, electrolyte, etc.), and a lithium salt for increasing ion conductivity. Further includes an additive. Other additives such as conductive assistants, binders, electrolytes (polymer matrix, ion conductive polymers, electrolytes, etc.) and lithium salts for improving ion conductivity are those described in the above positive electrode active material layer column. It is the same.

Examples of the negative electrode active material include carbon materials such as graphite (graphite), soft carbon, and hard carbon, lithium-transition metal composite oxides (for example, Li ₄ Ti ₅ O ₁₂ ), metal materials, lithium alloy negative electrode materials, and the like. Is mentioned. In some cases, two or more negative electrode active materials may be used in combination. Preferably, from the viewpoint of capacity and output characteristics, a carbon material or a lithium-transition metal composite oxide is used as the negative electrode active material. Of course, negative electrode active materials other than those described above may be used.

  The average particle diameter of the negative electrode active material is not particularly limited, but is preferably 1 to 100 μm, more preferably 1 to 20 μm from the viewpoint of increasing the output.

  The negative electrode active material layer preferably contains at least an aqueous binder. A water-based binder has a high binding power. In addition, it is easy to procure water as a raw material, and since steam is generated at the time of drying, the capital investment in the production line can be greatly suppressed, and the environmental load can be reduced. There is.

  The water-based binder refers to a binder using water as a solvent or a dispersion medium, and specifically includes a thermoplastic resin, a polymer having rubber elasticity, a water-soluble polymer, or a mixture thereof. Here, the binder using water as a dispersion medium refers to a polymer that includes all expressed as latex or emulsion and is emulsified or suspended in water. For example, a polymer latex that is emulsion-polymerized in a system that self-emulsifies. Kind.

  Specific examples of water-based binders include styrene polymers (styrene-butadiene rubber, styrene-vinyl acetate copolymer, styrene-acrylic copolymer, etc.), acrylonitrile-butadiene rubber, methyl methacrylate-butadiene rubber, (meta ) Acrylic polymers (polyethyl acrylate, polyethyl methacrylate, polypropyl acrylate, polymethyl methacrylate (methyl methacrylate rubber), polypropyl methacrylate, polyisopropyl acrylate, polyisopropyl methacrylate, polybutyl acrylate, polybutyl methacrylate, polyhexyl acrylate , Polyhexyl methacrylate, polyethylhexyl acrylate, polyethylhexyl methacrylate, polylauryl acrylate, polylauryl methacrylate Relate, etc.), polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene copolymer, polybutadiene, butyl rubber, fluororubber, polyethylene oxide, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, polystyrene, ethylene-propylene-diene copolymer , Polyvinyl pyridine, chlorosulfonated polyethylene, polyester resin, phenol resin, epoxy resin; polyvinyl alcohol (average polymerization degree is preferably 200 to 4000, more preferably 1000 to 3000, and saponification degree is preferably 80. Mol% or more, more preferably 90 mol% or more) and a modified product thereof (saponified product of 1 to 80 mol% of vinyl acetate units of a copolymer of ethylene / vinyl acetate = 2/98 to 30/70 mol ratio) , Poly Nyl alcohol 1-50 mol% partially acetalized product), starch and modified products thereof (oxidized starch, phosphate esterified starch, cationized starch, etc.), cellulose derivatives (carboxymethylcellulose, methylcellulose, hydroxypropylcellulose, hydroxyethylcellulose, And their salts), polyvinylpyrrolidone, polyacrylic acid (salt), polyethylene glycol, (meth) acrylamide and / or (meth) acrylate copolymer [(meth) acrylamide polymer, (meth) acrylamide- (Meth) acrylate copolymer, alkyl (meth) acrylate (1 to 4 carbon atoms) ester- (meth) acrylate copolymer, etc.], styrene-maleate copolymer, polyacrylamide Mannich Denatured body Formalin condensation type resin (urea-formalin resin, melamine-formalin resin, etc.), polyamide polyamine or dialkylamine-epichlorohydrin copolymer, polyethyleneimine, casein, soybean protein, synthetic protein, and water-soluble polymers such as galactomannan derivatives, etc. Is mentioned. These aqueous binders may be used alone or in combination of two or more.

  The aqueous binder may contain at least one rubber binder selected from the group consisting of styrene-butadiene rubber, acrylonitrile-butadiene rubber, methyl methacrylate-butadiene rubber, and methyl methacrylate rubber from the viewpoint of binding properties. preferable. Furthermore, it is preferable that the water-based binder contains a styrene-butadiene rubber because the binding property is good.

  When styrene-butadiene rubber is used as the water-based binder, it is preferable to use the water-soluble polymer in combination from the viewpoint of improving coatability. Water-soluble polymers suitable for use in combination with styrene-butadiene rubber include polyvinyl alcohol and modified products thereof, starch and modified products thereof, cellulose derivatives (carboxymethyl cellulose, methyl cellulose, hydroxyethyl cellulose, and salts thereof), polyvinyl Examples include pyrrolidone, polyacrylic acid (salt), or polyethylene glycol. Among them, it is preferable to combine styrene-butadiene rubber and carboxymethyl cellulose (salt) as a binder. The content mass ratio of the styrene-butadiene rubber and the water-soluble polymer is not particularly limited, but is preferably styrene-butadiene rubber: water-soluble polymer = 1: 0.1-10, 0 More preferably, it is 5-2.

  Among the binders used in the negative electrode active material layer, the content of the aqueous binder is preferably 80 to 100% by mass, preferably 90 to 100% by mass, and preferably 100% by mass.

≪Separator (electrolyte layer) ≫
The separator has a function of holding an electrolyte and ensuring lithium ion conductivity between the positive electrode and the negative electrode, and a function as a partition wall between the positive electrode and the negative electrode.

  Examples of the separator include a porous sheet separator or a nonwoven fabric separator made of a polymer or fiber that absorbs and holds the electrolyte.

  As a separator for a porous sheet made of a polymer or fiber, for example, a microporous (microporous film) can be used. Specific examples of the porous sheet made of the polymer or fiber include polyolefins such as polyethylene (PE) and polypropylene (PP); a laminate in which a plurality of these are laminated (for example, three layers of PP / PE / PP) And a microporous (microporous membrane) separator made of a hydrocarbon resin such as polyimide, aramid, polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP), glass fiber, and the like.

  The thickness of the microporous (microporous membrane) separator cannot be uniquely defined because it varies depending on the intended use. For example, in applications such as secondary batteries for driving motors such as electric vehicles (EV), hybrid electric vehicles (HEV), and fuel cell vehicles (FCV), it is 4 to 60 μm in a single layer or multiple layers. Is desirable. The fine pore diameter of the microporous (microporous membrane) separator is desirably 1 μm or less (usually a pore diameter of about several tens of nm).

As the nonwoven fabric separator, cotton, rayon, acetate, nylon, polyester; polyolefins such as PP and PE; conventionally known ones such as polyimide and aramid are used alone or in combination. The bulk density of the nonwoven fabric is not particularly limited as long as sufficient battery characteristics can be obtained by the impregnated polymer gel electrolyte. Furthermore, the thickness of the nonwoven fabric separator may be the same as that of the electrolyte layer, preferably 5 to 200 μm, particularly preferably 10 to 100 μm.

  In addition, as described above, the separator includes an electrolyte. The electrolyte is not particularly limited as long as it can exhibit such a function, but a liquid electrolyte or a gel polymer electrolyte is used. By using the gel polymer electrolyte, the distance between the electrodes is stabilized, the occurrence of polarization is suppressed, and the durability (cycle characteristics) is improved.

The liquid electrolyte functions as a lithium ion carrier. The liquid electrolyte constituting the electrolytic solution layer has a form in which a lithium salt as a supporting salt is dissolved in an organic solvent as a plasticizer. Examples of the organic solvent used include carbonates such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate. As the lithium _{salt, Li (CF 3 SO 2)} 2 N, Li (C 2 F 5 SO 2) 2 N, LiPF 6, LiBF 4, LiClO 4, LiAsF 6, LiTaF such 6, LiCF ₃ SO ₃ A compound that can be added to the active material layer of the electrode can be similarly employed. The liquid electrolyte may further contain additives other than the components described above. Specific examples of such compounds include, for example, vinylene carbonate, methyl vinylene carbonate, dimethyl vinylene carbonate, phenyl vinylene carbonate, diphenyl vinylene carbonate, ethyl vinylene carbonate, diethyl vinylene carbonate, vinyl ethylene carbonate, 1,2-divinyl ethylene carbonate. 1-methyl-1-vinylethylene carbonate, 1-methyl-2-vinylethylene carbonate, 1-ethyl-1-vinylethylene carbonate, 1-ethyl-2-vinylethylene carbonate, vinylvinylene carbonate, allylethylene carbonate, vinyl Oxymethyl ethylene carbonate, allyloxymethyl ethylene carbonate, acryloxymethyl ethylene carbonate, methacrylate Oxy methylethylene carbonate, ethynyl ethylene carbonate, propargyl carbonate, ethynyloxy methylethylene carbonate, propargyloxy ethylene carbonate, methylene carbonate, etc. 1,1-dimethyl-2-methylene-ethylene carbonate. Among these, vinylene carbonate, methyl vinylene carbonate, and vinyl ethylene carbonate are preferable, and vinylene carbonate and vinyl ethylene carbonate are more preferable. These cyclic carbonates may be used alone or in combination of two or more.

  The gel polymer electrolyte has a configuration in which the liquid electrolyte is injected into a matrix polymer (host polymer) made of an ion conductive polymer. The use of a gel polymer electrolyte as the electrolyte is superior in that the fluidity of the electrolyte is lost and the ion conductivity between the layers is easily cut off. Examples of the ion conductive polymer used as the matrix polymer (host polymer) include polyethylene oxide (PEO), polypropylene oxide (PPO), polyethylene glycol (PEG), polyacrylonitrile (PAN), polyvinylidene fluoride-hexafluoropropylene ( PVdF-HEP), poly (methyl methacrylate (PMMA), and copolymers thereof.

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

  The separator is preferably a separator (heat-resistant insulating layer-attached separator) in which a heat-resistant insulating layer is laminated on a porous substrate. The heat-resistant insulating layer is a ceramic layer containing inorganic particles and a binder. As the separator with a heat-resistant insulating layer, a highly heat-resistant separator having a melting point or a heat softening point of 150 ° C. or higher, preferably 200 ° C. or higher is used. By having the heat-resistant insulating layer, the internal stress of the separator that increases when the temperature rises is relieved, so that the effect of suppressing thermal shrinkage can be obtained. As a result, it is possible to prevent the induction of a short circuit between the electrodes of the battery, so that the battery configuration is unlikely to deteriorate in performance due to a temperature rise. Moreover, by having a heat-resistant insulating layer, the mechanical strength of the separator with a heat-resistant insulating layer is improved, and it is difficult for the separator to break. Furthermore, the separator is less likely to curl in the battery manufacturing process due to the effect of suppressing thermal shrinkage and high mechanical strength.

The inorganic particles in the heat resistant insulating layer contribute to the mechanical strength and heat shrinkage suppressing effect of the heat resistant insulating layer. The material used as the inorganic particles is not particularly limited. Examples thereof include silicon, aluminum, zirconium, titanium oxides (SiO ₂ , Al ₂ O ₃ , ZrO ₂ , TiO ₂ ), hydroxides and nitrides, and composites thereof. These inorganic particles may be derived from mineral resources such as boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine and mica, or may be artificially produced. Moreover, only 1 type may be used individually for these inorganic particles, and 2 or more types may be used together. Of these, silica (SiO ₂ ) or alumina (Al ₂ O ₃ ) is preferably used, and alumina (Al ₂ O ₃ ) is more preferably used from the viewpoint of cost.

The basis weight of the heat-resistant particles is not particularly limited, but is preferably 5 to 15 g / m ² . If it is this range, sufficient ion conductivity will be acquired and it is preferable at the point which maintains heat resistant strength.

  The binder in the heat-resistant insulating layer has a role of bonding the inorganic particles or the inorganic particles and the resin porous substrate layer. With the binder, the heat-resistant insulating layer is stably formed, and peeling between the porous substrate layer and the heat-resistant insulating layer is prevented.

  The binder used for the heat-resistant insulating layer is not particularly limited. For example, carboxymethyl cellulose (CMC), polyacrylonitrile, cellulose, ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR), isoprene rubber A compound such as butadiene rubber, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl fluoride (PVF), or methyl acrylate can be used as a binder. Of these, carboxymethylcellulose (CMC), methyl acrylate, or polyvinylidene fluoride (PVdF) is preferably used. As for these compounds, only 1 type may be used independently and 2 or more types may be used together.

  It is preferable that content of the binder in a heat resistant insulating layer is 2-20 mass% with respect to 100 mass% of heat resistant insulating layers. When the binder content is 2% by mass or more, the peel strength between the heat-resistant insulating layer and the porous substrate layer can be increased, and the vibration resistance of the separator can be improved. On the other hand, when the binder content is 20% by mass or less, the gaps between the inorganic particles are appropriately maintained, so that sufficient lithium ion conductivity can be ensured.

The thermal contraction rate of the separator with a heat-resistant insulating layer is preferably 10% or less for both MD and TD after holding for 1 hour at 150 ° C. and ² gf / cm ² . By using such a material having high heat resistance, it is possible to effectively prevent the separator from contracting even if the positive electrode heat generation amount increases and the battery internal temperature reaches 150 ° C. As a result, it is possible to prevent the induction of a short circuit between the electrodes of the battery, so that the battery configuration is unlikely to deteriorate in performance due to a temperature rise.

≪Positive electrode collector plate and negative electrode collector plate≫
The material which comprises a current collector plate (25, 27) is not restrict | limited in particular, The well-known highly electroconductive material conventionally used as a current collector plate for lithium ion secondary batteries can be used. As a constituent material of the current collector plate, for example, metal materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof are preferable. From the viewpoint of light weight, corrosion resistance, and high conductivity, aluminum and copper are more preferable, and aluminum is particularly preferable. In addition, the same material may be used for the positive electrode current collecting plate 27 and the negative electrode current collecting plate 25, and different materials may be used.

≪Positive lead and negative lead≫
Moreover, although illustration is abbreviate | omitted, you may electrically connect between the collector 11 and the current collector plates (25, 27) via a positive electrode lead or a negative electrode lead. As a constituent material of the positive electrode and the negative electrode lead, materials used in known lithium ion secondary batteries can be similarly employed. In addition, heat-shrinkable heat-shrinkable parts are removed from the exterior so that they do not affect products (for example, automobile parts, especially electronic devices) by touching peripheral devices or wiring and causing leakage. It is preferable to coat with a tube or the like.

≪Battery exterior body≫
As the battery outer case 29, a known metal can case can be used, and a bag-like case using a laminate film containing aluminum that can cover the power generation element can be used. For example, a laminate film having a three-layer structure in which PP, aluminum, and nylon are laminated in this order can be used as the laminate film, but the laminate film is not limited thereto. A laminate film is desirable from the viewpoint that it is excellent in high output and cooling performance, and can be suitably used for a battery for large equipment for EV and HEV. Moreover, since the group pressure to the electric power generation element applied from the outside can be adjusted easily and it is easy to adjust to the desired electrolyte layer thickness, the exterior body is more preferably an aluminate laminate.

≪Cell size≫
FIG. 2 is a perspective view showing the appearance of a flat lithium ion secondary battery which is a typical embodiment of the secondary battery. Like this lithium ion secondary battery, according to a preferred embodiment of the present invention, there is provided a flat laminated battery having a structure in which the power generation element is enclosed in a battery outer package made of a laminate film containing aluminum. The

  As shown in FIG. 2, the flat lithium ion secondary battery 50 has a rectangular flat shape, and a positive electrode tab 58 and a negative electrode tab 59 for taking out electric power are drawn out from both sides thereof. Yes. The power generation element 57 is encased by the battery outer packaging material 52 of the lithium ion secondary battery 50, and the periphery thereof is heat-sealed. The power generation element 57 is sealed with the positive electrode tab 58 and the negative electrode tab 59 pulled out to the outside. Has been. Here, the power generation element 57 corresponds to the power generation element 21 of the lithium ion secondary battery 10 shown in FIG. 1 described above. The power generation element 57 is formed by laminating a plurality of single battery layers (single cells) 19 composed of a positive electrode (positive electrode active material layer) 15, an electrolyte layer 17, and a negative electrode (negative electrode active material layer) 13.

  The lithium ion secondary battery is not limited to a stacked flat shape. The wound lithium ion secondary battery may have a cylindrical shape, or may have a shape that is a flattened rectangular shape by deforming such a cylindrical shape. There is no particular limitation. In the said cylindrical shape thing, a laminate film may be used for the exterior material, and the conventional cylindrical can (metal can) may be used, for example, It does not restrict | limit. Preferably, the power generation element is covered with an aluminum laminate film. With this configuration, weight reduction can be achieved.

  Further, the removal of the tabs 58 and 59 shown in FIG. 2 is not particularly limited. The positive electrode tab 58 and the negative electrode tab 59 may be drawn out from the same side, or the positive electrode tab 58 and the negative electrode tab 59 may be divided into a plurality of parts and taken out from each side, as shown in FIG. It is not limited to. Further, in a wound type lithium ion battery, instead of a tab, for example, a terminal may be formed using a cylindrical can (metal can).

  In a general electric vehicle, the battery storage space is about 170L. Since auxiliary devices such as cells and charge / discharge control devices are stored in this space, the storage efficiency of a normal cell is about 50%. The efficiency of loading cells into this space is a factor that governs the cruising range of electric vehicles. If the size of the single cell is reduced, the loading efficiency is impaired, so that the cruising distance cannot be secured.

  Therefore, in the present invention, the battery structure in which the power generation element is covered with the exterior body is preferably large. Specifically, the length of the short side of the laminated cell battery is preferably 100 mm or more. Such a large battery can be used for vehicle applications. Here, the length of the short side of the laminated cell battery refers to the side having the shortest length. The upper limit of the short side length is not particularly limited, but is usually 400 mm or less.

≪Volume energy density and rated capacity (rated discharge capacity) ≫
In a general electric vehicle, a travel distance (cruising range) by a single charge is 100 km. Considering such a cruising distance, the volume energy density of the battery is preferably 157 Wh / L or more, and the rated capacity is preferably 20 Wh or more.

Here, the non-aqueous electrolyte secondary battery in which the positive electrode according to this embodiment is used as a viewpoint of a large-sized battery, which is different from the viewpoint of the physical size of the electrode, is large in size due to the battery area and battery capacity. Is defined. Specifically, the nonaqueous electrolyte secondary battery according to this embodiment is a flat laminated battery, and the ratio of the battery area to the rated capacity (projected area of the battery including the battery outer package) is 5 cm ² / It is preferably Ah or more and the rated capacity is 3 Ah or more. In such a large-area and large-capacity battery, since the problem of electrode deterioration due to an increase in the amount of heat generated in the positive electrode active material layer as described above can be manifested more significantly, the configuration according to the present invention is adopted. The effect by is obtained more significantly.

  Furthermore, the aspect ratio of the rectangular electrode is preferably 1 to 3, and more preferably 1 to 2. The electrode aspect ratio is defined as the aspect ratio of the rectangular positive electrode active material layer. By setting the aspect ratio in such a range, there is an advantage that both required vehicle performance and mounting space can be achieved.

≪Battery battery≫
The assembled battery is configured by connecting a plurality of batteries. Specifically, at least two or more are used, and are configured by serialization, parallelization, or both. Capacitance and voltage can be freely adjusted by paralleling in series.

  A plurality of batteries can be connected in series or in parallel to form a small assembled battery that can be attached and detached. Then, a plurality of small assembled batteries that can be attached and detached are connected in series or in parallel to provide a large capacity and large capacity suitable for vehicle drive power supplies and auxiliary power supplies that require high volume energy density and high volume output density. An assembled battery having an output can also be formed. How many batteries are connected to make an assembled battery, and how many small assembled batteries are stacked to make a large-capacity assembled battery depends on the battery capacity of the mounted vehicle (electric vehicle) It may be determined according to the output.

≪Vehicle≫
The nonaqueous electrolyte secondary battery of the present invention maintains a discharge capacity even when used for a long period of time, and has good cycle characteristics. Furthermore, the volume energy density is high. Vehicle applications such as electric vehicles, hybrid electric vehicles, fuel cell vehicles, and hybrid fuel cell vehicles require higher capacity, larger size, and longer life than electric and portable electronic devices. . Therefore, the nonaqueous electrolyte secondary battery can be suitably used as a vehicle power source, for example, a vehicle driving power source or an auxiliary power source.

  Specifically, a battery or an assembled battery formed by combining a plurality of these batteries can be mounted on a vehicle. In the present invention, since a battery having a long life with excellent long-term reliability and output characteristics can be configured, a plug-in hybrid electric vehicle having a long EV mileage or an electric vehicle having a long charge mileage can be formed by mounting such a battery. . For example, in the case of a car, a hybrid car, a fuel cell car, an electric car (four-wheeled vehicles (passenger cars, trucks, buses, commercial vehicles, light cars, etc.) This is because it can be used for motorcycles (including motorcycles) and tricycles) to provide a long-life and highly reliable automobile. However, the application is not limited to automobiles. For example, it can be applied to various power sources for moving vehicles such as other vehicles, for example, trains, and power sources for mounting such as uninterruptible power supplies. It is also possible to use as.

  Hereinafter, although it demonstrates still in detail using an Example and a comparative example, this invention is not necessarily limited to the following Examples at all.

≪Preparation of positive electrode≫
[Comparative Example 1]
LiNi _0.5 Mn _0.3 Co _0.2 O ₂ (average secondary particle diameter (D50) = 7 μm) 92% by mass as a positive electrode active material without forming a shell part, Ketjen Black as a conductive auxiliary 3% by mass, 5% by mass of polyvinylidene fluoride (PVdF) as a binder, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) as a slurry viscosity adjusting solvent were mixed to prepare a positive electrode active material slurry. The obtained positive electrode active material slurry was applied to one side of an aluminum foil (thickness: 10 μm) as a current collector and dried. Then, the press process was performed and the positive electrode which has a positive electrode active material layer on one side was produced.

[Example 1]
(Formation of shell on the surface of positive electrode active material)
The positive electrode active material (LiNi _0.5 Mn _0.3 Co _0.2 O ₂ ) used in Comparative Example 1 is used as a core portion, and lithium difluorophosphate which is a fluorinated compound according to the present invention is formed on the surface of the core portion. A shell portion made of (CHF ₂ COOLi) was formed by physical treatment. Specifically, methyl difluoroacetate (CHF ₂ COOCH ₃ ) having a volume 30 times the volume of the positive electrode active material (LiNi _0.5 Mn _0.3 Co _0.2 O ₂ ) particles in the core is prepared as a solvent. Then, 5% by mass of lithium difluorophosphate (CHF ₂ COOLi) was dissolved in 100% by mass of the solvent to prepare a coating solution. Next, positive electrode active material (LiNi _0.5 Mn _0.3 Co _0.2 O ₂ ) particles were put into the obtained coating solution, and stirred in a constant temperature bath at 40 ° C. for 12 hours. Then, solid content was taken out and vacuum-dried at 100 degreeC for 8 hours, and the positive electrode material which has a core-shell structure was obtained. In addition, when the thickness of the shell part in the site | part in which the shell part was formed on the surface of the core part was observed using the scanning electron microscope (SEM), it was 20 nm. Moreover, when the area ratio (coverage) of the site | part in which the shell part was formed on the surface of the core part was observed visually by the SEM-EDX method, it was 95%.

The same as in Comparative Example 1 described above except that the positive electrode material obtained in this way was used instead of LiNi _0.5 Mn _0.3 Co _0.2 O ₂ in which the shell part was not formed. A positive electrode was produced by the method.

[Example 2]
The core is prepared in the same manner as in Example 1 except that the amount of lithium difluorophosphate (CHF ₂ COOLi) dissolved in the solvent when preparing the coating solution is reduced from 5% by mass to 3% by mass. -A positive electrode material having a shell structure was obtained. In addition, when the thickness of the shell part in the site | part in which the shell part was formed on the surface of the core part was observed using the scanning electron microscope (SEM), it was 15 nm. Moreover, it was 50% when the area ratio (coverage) of the site | part in which the shell part was formed on the surface of the core part was observed visually by the SEM-EDX method.

The same as in Comparative Example 1 described above except that the positive electrode material obtained in this way was used instead of LiNi _0.5 Mn _0.3 Co _0.2 O ₂ in which the shell part was not formed. A positive electrode was produced by the method.

≪Positive electrode evaluation (withstand voltage confirmation test and self-discharge rate measurement) ≫
[Production of coin cell for evaluation]
In a glove box under an argon atmosphere, the positive electrode produced in Comparative Example 1 and Examples 1 and 2 was punched into a disk shape having a diameter of 14 mm to obtain a positive electrode for a coin cell. On the other hand, a negative electrode in which metallic lithium was punched into a disk shape having a diameter of 15 mm was used. Further, as an electrolyte, a mixed solvent (volume ratio 1: 1) of a 1.0 M LiPF ₆ ethylene carbonate (EC) and dimethyl carbonate (DMC) dissolved solution was prepared in the. A positive electrode and a negative electrode were laminated via a separator (material: polypropylene, thickness: 25 μm), placed in a coin cell container, injected with an electrolytic solution, and covered with an upper lid to produce an evaluation coin cell.

[Withstand voltage test]
About each evaluation coin cell produced above, it is 6.0V vs. 25 degreeC temperature conditions. The voltage was raised to Li / Li ⁺ and the current value was confirmed. 6.0V vs. The current value was measured 0.5 hours, 1 hour and 2 hours after the start of holding at Li / Li ⁺ . The results are shown in Table 1 below.

As can be seen from the results shown in Table 1, in the coin cell of Example 1 (coverage 95%), 6.0 V vs. Up to 0.01 C or less after 2 hours after increasing the voltage to Li / Li ⁺ (the positive electrode active material LiNi _0.5 Mn _0.3 Co _0.2 O _{2 has} a 4.2 V charge capacity (mAh) of 1 C) The current value has converged. In the coin cell of Example 2 (coverage 50%), the current value after 2 hours was 0.03C. In contrast, a current value of 0.05 C was confirmed in the coin cell of Comparative Example 1. From this, in the high voltage environment of 6.0 V vs. Li / Li ⁺ , in the coin cell using the positive electrode of the comparative example, the oxidative decomposition of the electrolyte occurs, whereas the core-shell type according to the present invention It was suggested that the oxidative decomposition of the electrolyte was suppressed in the coin cell using the positive electrode material. It was also confirmed that the effect of suppressing the oxidative decomposition of the electrolytic solution increases as the coverage increases.

[Measurement of self-discharge rate]
About each coin cell for evaluation of Example 1 and Comparative Example 1 produced above, the self-discharge rate when left at a cell voltage of 4.2 V for 24 hours was measured by the following method. The results are shown in Table 2 below.

  As can be seen from the results shown in Table 2, in the coin cell of Example 1, the self-discharge rate was suppressed to 4%. On the other hand, in the coin cell of Comparative Example 1, the self-discharge rate was increased to 16%. From this, by constituting the non-aqueous electrolyte secondary battery using the positive electrode material according to the present invention, it is possible to suppress the above-described oxidative decomposition of the electrolytic solution and to suppress the self-discharge of the battery. It was confirmed.

[Example 3]
When the coating solution was prepared, the core-shell was formed in the same manner as in Example 1 except that the amount of lithium difluorophosphate (CHF ₂ COOLi) dissolved in the solvent was changed to 1.5% by mass. A positive electrode material having a structure was obtained. In addition, it was 10 nm when the thickness of the shell part in the site | part in which the shell part was formed on the surface of the core part was observed using the scanning electron microscope (SEM). Moreover, it was 35% when the area ratio (coverage) of the site | part in which the shell part was formed on the surface of the core part was observed visually by the SEM-EDX method.

The same as in Comparative Example 1 described above except that the positive electrode material obtained in this way was used instead of LiNi _0.5 Mn _0.3 Co _0.2 O ₂ in which the shell part was not formed. A positive electrode was produced by the method.

[Example 4]
A core-shell structure was prepared in the same manner as in Example 1 except that the amount of lithium difluorophosphate (CHF ₂ COOLi) dissolved in the solvent was changed to 9% by mass when preparing the coating solution. A positive electrode material was obtained. In addition, when the thickness of the shell part in the site | part in which the shell part was formed on the surface of the core part was observed using the scanning electron microscope (SEM), it was 30 nm. Moreover, when the area ratio (coverage) of the site | part in which the shell part was formed on the surface of the core part was observed visually by the SEM-EDX method, it was 95%.

The same as in Comparative Example 1 described above except that the positive electrode material obtained in this way was used instead of LiNi _0.5 Mn _0.3 Co _0.2 O ₂ in which the shell part was not formed. A positive electrode was produced by the method.

[Example 5]
A core-shell structure was prepared in the same manner as in Example 1 except that the amount of lithium difluorophosphate (CHF ₂ COOLi) dissolved in the solvent was changed to 12% by mass when preparing the coating solution. A positive electrode material was obtained. In addition, when the thickness of the shell part in the site | part in which the shell part was formed on the surface of the core part was observed using the scanning electron microscope (SEM), it was 40 nm. Moreover, when the area ratio (coverage) of the site | part in which the shell part was formed on the surface of the core part was observed visually by the SEM-EDX method, it was 95%.

The same as in Comparative Example 1 described above except that the positive electrode material obtained in this way was used instead of LiNi _0.5 Mn _0.3 Co _0.2 O ₂ in which the shell part was not formed. A positive electrode was produced by the method.

≪Evaluation of positive electrode (measurement of lithium ion diffusion coefficient on the surface of positive electrode material) ≫
[Production of coin cell for evaluation]
In a glove box under an argon atmosphere, the positive electrode produced in Comparative Example 1 and Examples 1 and 2 was punched into a disk shape having a diameter of 14 mm to obtain a positive electrode for a coin cell. On the other hand, a negative electrode in which metallic lithium was punched into a disk shape having a diameter of 15 mm was used. Further, as an electrolyte, a mixed solvent (volume ratio 1: 1) of a 1.0 M LiPF ₆ ethylene carbonate (EC) and dimethyl carbonate (DMC) dissolved solution was prepared in the. A positive electrode and a negative electrode were laminated via a separator (material: polypropylene, thickness: 25 μm), placed in a coin cell container, injected with an electrolytic solution, and covered with an upper lid to produce an evaluation coin cell.

[Measurement of lithium ion diffusion coefficient on the surface of positive electrode material]
About each coin cell for evaluation produced above, it is charged to 4.2V by a constant current constant voltage method (CCCV, current: 1C) under a temperature condition of 25 ° C., and Cottrell's formula (voltage displacement is 4.2V → 4.4). 19V) was used to measure the lithium ion diffusion coefficient on the surface of the positive electrode material. The results are shown in FIG.

  As can be seen from the results shown in FIG. 3, it was confirmed that the lithium ion diffusion coefficient on the surface of the positive electrode material decreases as the thickness of the shell portion increases. However, even when the thickness of the shell portion is as comparatively large as about 30 nm (Example 4), as compared with the case of using the positive electrode active material in which the shell portion is not formed (Comparative Example 1). It was also found that the decrease in the lithium ion diffusion coefficient was suppressed to about 20%.

10, 50 lithium ion secondary battery,
11 negative electrode current collector,
12 positive electrode current collector,
13 negative electrode active material layer,
15 positive electrode active material layer,
17 separator,
19 cell layer,
21, 57 power generation element,
25 negative current collector,
27 positive current collector,
29, 52 Battery exterior material,
58 positive electrode tab,
59 Negative electrode tab.

Claims (14)

A core containing positive electrode active material particles;
In the compound represented by ROLi (R is a linear alkyl group or a linear acyl group) arranged on the surface of the core portion, 40 to 70% of hydrogen atoms in the R portion are substituted with fluorine atoms. A shell portion containing a fluorinated compound;
A positive electrode material for a nonaqueous electrolyte secondary battery.   2. The positive electrode material for a nonaqueous electrolyte secondary battery according to claim 1, wherein R is a linear alkyl group having 1 to 7 carbon atoms or a linear acyl group having 2 to 7 carbon atoms.   The positive electrode material for a nonaqueous electrolyte secondary battery according to claim 1, wherein R is a linear acyl group.   The positive electrode material for a nonaqueous electrolyte secondary battery according to claim 3, wherein the fluorinated compound is lithium difluoroacetate.   The positive electrode material for a nonaqueous electrolyte secondary battery according to any one of claims 1 to 4, wherein the shell portion has a thickness of 15 to 35 nm.   The positive electrode material for a non-aqueous electrolyte secondary battery according to claim 1, wherein a coverage of the core portion by the shell portion is 95% or more. A positive electrode current collector;
A positive electrode active material layer formed on a surface of the positive electrode current collector and including a positive electrode active material;
Have
The positive electrode for nonaqueous electrolyte secondary batteries in which the said positive electrode active material layer contains the positive electrode material for nonaqueous electrolyte secondary batteries of any one of Claims 1-6.   The non-aqueous electrolyte secondary battery according to claim 7, wherein the positive electrode active material layer is rectangular, and an aspect ratio of the electrode defined as an aspect ratio of the rectangular positive electrode active material layer is 1 to 3. Positive electrode. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 7 or 8,
A negative electrode in which a negative electrode active material layer is formed on the surface of the negative electrode current collector;
A separator;
A non-aqueous electrolyte secondary battery having a power generation element comprising: The nonaqueous electrolyte secondary battery according to claim 9, wherein a value of a ratio of a battery area to a rated capacity (projected area of a battery including a battery outer casing) is 5 cm ² / Ah or more, and a rated capacity is 3 Ah or more. .   The nonaqueous electrolyte secondary battery according to claim 9 or 10, wherein the separator is a separator with a heat-resistant insulating layer.   The nonaqueous electrolyte secondary battery according to any one of claims 9 to 11, which is a flat laminated laminate battery having a configuration in which the power generation element is enclosed in a battery outer package made of a laminate film containing aluminum.   The nonaqueous electrolyte secondary battery according to any one of claims 9 to 12, which is a lithium ion secondary battery. It is a manufacturing method of the positive electrode material of any one of Claims 1-6,
The manufacturing method including forming the shell portion on the surface of the core portion using a physical treatment or an electrochemical treatment.