Graphite particles for lithium ion secondary batteries, electrodes for lithium ion secondary batteries, and methods for producing graphite particles

Abstract

The problem to be solved by the present invention is to provide a graphite particle for lithium ion secondary battery, which can suppress the increase of internal resistance during repeated charge and discharge cycles and has excellent durability to charge and discharge cycles. In order to solve the above problems, the present invention provides a graphite particle for lithium ion secondary battery, which has a structure in which a high dielectric inorganic solid is integrated inside the graphite particle. The high dielectric inorganic solid preferably has at least one ion conductivity among Li ion conductivity, Na ion conductivity and Mg ion conductivity, and the ion conductivity is 10 ‑7 S / cm or above, and the relative dielectric constant of the powder is 10 or above.

Description

Technical Field

[0001] The present invention relates to graphite particles for lithium ion secondary batteries, electrodes for lithium ion secondary batteries, and a method for producing the graphite particles. Background Art

[0002] In the past, many lithium-ion secondary batteries using lithium-ion conductive solid electrolytes have been proposed. For example, there is a known lithium-ion secondary battery that contains an active material in the positive electrode or the negative electrode, and the active material is coated with a coating layer containing a conductive additive and a lithium-ion conductive solid electrolyte (for example, see Patent Document 1).

[0003] According to the lithium-ion secondary battery described in Patent Document 1, since the active material in the positive electrode or the negative electrode is coated with a coating layer containing a conductive additive and a lithium-ion conductive solid electrolyte, the internal resistance can be reduced. It is also generally believed that the deformation of the active material during charge and discharge can be suppressed, thereby preventing a decrease in charge and discharge cycle characteristics and high-rate discharge characteristics.

[0004] [Prior Art Literature]

[0005] (Patent Document)

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 2003-59492 Summary of the Invention

[0007] [Problems to be solved by the invention]

[0008] In the lithium ion secondary battery described in Patent Document 1, although the aforementioned effects can be well obtained in the initial stage of the charge and discharge cycle, there is a disadvantage that the durability to charge and discharge decreases rapidly during use.

[0009] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide graphite particles for lithium ion secondary batteries that can suppress an increase in internal resistance even when repeated charge and discharge cycles occur, thereby realizing a lithium ion secondary battery having excellent durability against charge and discharge cycles.

[0010] [Technical means to solve the problem]

[0011] (1) The present invention relates to graphite particles for lithium-ion secondary batteries having a structure in which a high-dielectric inorganic solid is integrated within the graphite particles.

[0012] According to the invention (1), it is possible to provide graphite particles for a lithium ion secondary battery that can suppress an increase in internal resistance even when charge and discharge cycles are repeated and realize a lithium ion secondary battery having excellent durability against charge and discharge cycles.

[0013] (2) The graphite particles for lithium ion secondary batteries according to (1), wherein the high dielectric inorganic solid has at least one of Li ion conductivity, Na ion conductivity, and Mg ion conductivity.

[0014] According to the invention of (2), since a pseudo-solvation state is formed by capturing the free solvent in the electrolyte, a stabilization effect of the solvent can be obtained, thereby suppressing the amount of decomposition of the electrolyte and suppressing the capacity reduction of the secondary battery.

[0015] (3) The graphite particles for lithium ion secondary batteries according to (1) or (2), wherein the high dielectric inorganic solid powder has a relative dielectric constant of 10 or more.

[0016] According to the invention of (3), since the high-dielectric inorganic solid is polarized, fluorine-based anions or acids generated by solvent decomposition can be captured on the surface of the graphite particles. Therefore, corrosion of the positive electrode active material can be suppressed, and the cracking of the positive electrode active material and metal precipitation associated with charge and discharge can be suppressed. As a result, the resistance increase of the secondary battery associated with charge and discharge cycles can be suppressed.

[0017] (4) The graphite particles for lithium ion secondary batteries according to (2), wherein the ion conductivity is 10 -7 S / cm or more.

[0018] According to the invention of (4), a more preferable stabilizing effect of the solvent can be obtained, thereby suppressing the amount of decomposition of the electrolyte solution and suppressing the decrease in the capacity of the secondary battery.

[0019] (5) The graphite particles for lithium ion secondary batteries according to (1), wherein the weight ratio of the high dielectric inorganic solid to the graphite particles is 0.01 wt% or more and 0.5 wt% or less.

[0020] According to the invention of (5), a lithium ion secondary battery having excellent durability against charge and discharge cycles can be realized.

[0021] (6) An electrode for a lithium ion secondary battery comprising the graphite particles for a lithium ion secondary battery according to any one of (1) to (5).

[0022] According to the invention of (6), a lithium ion secondary battery having excellent durability against charge and discharge cycles can be realized.

[0023] (7) The present invention also relates to a method for producing graphite particles for lithium-ion secondary batteries, comprising the steps of: dispersing graphite particles in a solution containing a highly dielectric inorganic solid having ion conductivity and a solvent; and removing the solvent.

[0024] According to the invention of (7), it is possible to produce graphite particles for lithium ion secondary batteries having a structure in which a high dielectric inorganic solid is integrated inside the graphite particles. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a cross-sectional view of the lithium ion secondary battery of this embodiment.

[0026] Figure 2 This is a schematic diagram illustrating an active material for a lithium-ion secondary battery according to this embodiment.

[0027] Figure 3 This is an electron probe microanalyzer (EPMA) reflection electron composition image of graphite particles in the example.

[0028] Figure 4 This is an EPMA reflection electron composition image of graphite particles produced by a conventional method. DETAILED DESCRIPTION

[0029] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. However, the content of the present invention is not limited to the description of the following embodiment.

[0030] <Lithium-ion secondary battery>

[0031] The graphite particles of this embodiment are used as, for example, active materials for lithium ion secondary batteries. Figure 1 As shown, the lithium-ion secondary battery 1 of this embodiment includes: a positive electrode 4, which is formed by forming a positive electrode mixture layer 3 on a positive electrode collector 2; a negative electrode 7, which is formed by forming a negative electrode mixture layer 6 on a negative electrode collector 5; a separator 8, which electrically insulates the positive electrode 4 and the negative electrode 7; an electrolyte 9; and a container 10.

[0032] (Current Collector)

[0033] As materials for the positive electrode collector 2 and the negative electrode collector 5, copper, aluminum, nickel, titanium, stainless steel foil or plate, carbon sheet, carbon nanotube sheet, etc. can be used. The above materials can be used alone, or a metal foil composed of two or more materials can be used as needed. The thickness of the positive electrode collector 2 and the negative electrode collector 5 is not particularly limited, for example, it can be set to a thickness in the range of 5 to 100 μm. From the perspective of structural and performance improvement, the thickness of the positive electrode collector 2 and the negative electrode collector 5 is preferably set to a thickness in the range of 7 to 20 μm.

[0034] (Electrode mixture layer)

[0035] The positive electrode mixture layer 3 is composed of a positive electrode active material, a conductive additive, and a binder. The negative electrode mixture layer 6 is composed of a negative electrode active material 11, a conductive additive, and a binder.

[0036] [Active substance]

[0037] As the positive electrode active material, lithium composite oxide (LiNi x Co y Mn z O2(x+y+z=1), LiNi x CoyAl z O2 (x+y+z=1)), lithium iron phosphate (LiFePO4 (LFP)), etc. These may be used alone or in combination of two or more.

[0038] Graphite particles are used as negative electrode active material 11. Examples of graphite particles include graphitizable carbon (easily graphitizable carbon), hard carbon (hardly graphitizable carbon), and graphite. One or more of these materials may be used. The details of negative electrode active material 11 will be described in detail below.

[0039] [Conductive additive]

[0040] Examples of conductive additives used in the positive electrode mixture layer 3 or the negative electrode mixture layer 6 include carbon black such as acetylene black (AB) and Ketchen black (KB); carbon materials such as graphite powder; and conductive metal powders such as nickel powder. These additives may be used alone or in combination of two or more.

[0041] [Binder]

[0042] Examples of the binder used in the positive electrode mixture layer 3 or the negative electrode mixture layer 6 include cellulose polymers, fluorine resins, vinyl acetate copolymers, and rubbers. Specifically, when a solvent-based dispersion medium is used, examples of the binder include polyvinylidene fluoride (PVdF), polyimide (PI), polyvinylidene chloride (PVdC), and polyethylene oxide (PEO). When an aqueous dispersion medium is used, examples of the binder include styrene butadiene rubber (SBR), acrylic-modified SBR resin (SBR latex), carboxymethylcellulose (CMC), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), hydroxypropyl methylcellulose (HPMC), and tetrafluoroethylene-hexafluoropropylene copolymer (FEP). These may be used alone or in combination of two or more.

[0043] (diaphragm)

[0044] The separator 8 is not particularly limited, and examples thereof include porous resin sheets (films, nonwoven fabrics, etc.) made of resins such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide.

[0045] (Electrolyte)

[0046] An electrolyte solution composed of a non-aqueous solvent and an electrolyte can be used as the electrolyte solution 9. The concentration of the electrolyte is preferably in the range of 0.1 to 10 mol / L.

[0047] [Non-aqueous solvent]

[0048] The non-aqueous solvent contained in the electrolytic solution 9 is not particularly limited, and examples thereof include aprotic solvents such as carbonates, esters, ethers, nitriles, sulfones, and lactones. Specifically, the following may be mentioned: ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE), tetrahydrofuran (THF), 2-methyltetrahydrofuran, dioxane, 1,3-dioxolane, diethylene glycol dimethyl ether, ethylene glycol dimethyl ether, acetonitrile (AN), propionitrile, nitromethane, N,N-dimethylformamide (DMF), dimethyl sulfoxide, sulfolane, γ-butyrolactone, and the like.

[0049] [Electrolytes]

[0050] Examples of the electrolyte contained in the electrolyte solution 9 include LiPF6, LiBF4, LiClO4, LiN(SO2CF3), LiN(SO2C2F5)2, LiCF3SO3, LiC4F9SO3, LiC(SO2CF3)3, LiF, LiCl, LiI, Li2S, Li3N, Li3P, Li 10 GeP2S 12 (LGPS), Li3PS4, Li6PS5Cl, Li7P2S8I, Li x PO y N z (x=2y+3z-5, LiPON), Li7La3Zr2O 12 (LLZO), Li 3x La 2 / 3-x TiO3(LLTO), Li 1+x Al x Ti 2-x (PO4)3(0≤x≤1, LATP), Li 1.5 Al 0.5 Ge 1.5 (PO4)3(LAGP), Li 1+x-y Al x Ti 2-x PcqI 3-y O12 、Li 1+x+y Al x (Ti, Ge) 2-x PcqI 3-y O 12 、Li 4-2x Zn x GeO4 (LISICON), etc. Among them, LiPF6, LiBF4 or a mixture thereof is preferably used as the electrolyte.

[0051] In addition to the above, the electrolyte 9 may also be an ionic liquid, or a liquid containing an ionic liquid and a polymer containing an aliphatic chain, such as a copolymer of polyethylene oxide (PEO) and polyvinylidene fluoride (PVdF). By including such an ionic liquid in the electrolyte 9, the electrolyte 9 can flexibly cover the surface of the positive electrode active material and the negative electrode active material, thereby preferably forming a contact area between the electrolyte 9 and the positive electrode active material and the negative electrode active material.

[0052] The electrolyte 9 fills the gaps between the positive electrode mixture layer 3 and the negative electrode mixture layer 6, and the pores of the separator 8. The electrolyte 9 is stored at the bottom of the container 10. The mass of the electrolyte 9 stored at the bottom of the container 10 can be set to be in the range of 3 to 25 mass % relative to the mass of the electrolyte 9 filling the gaps between the positive electrode mixture layer 3 and the negative electrode mixture layer 6, and the pores of the separator 8. The mass of the electrolyte 9 filling the gaps between the positive electrode mixture layer 3 and the negative electrode mixture layer 6, and the pores of the separator 8 can be calculated, for example, based on the total volume of the gaps between the positive electrode mixture layer 3 and the negative electrode mixture layer 6, and the pores of the separator 8, as measured using a mercury porosimeter, and the specific gravity of the electrolyte 9. Alternatively, the total volume of the gaps between the positive electrode mixture layer 3 and the negative electrode mixture layer 6, and the pores of the separator 8 can be calculated based on the density of the positive electrode mixture layer 3 and the negative electrode mixture layer 6, the density of the materials constituting each mixture layer, and the porosity of the separator 8.

[0053] By storing the electrolyte 9 in the container 10 and in contact with the separator 8 , when the electrolyte 9 is consumed, the electrolyte 9 can be replenished to the positive electrode mixture layer 3 and the negative electrode mixture layer 6 through the separator 8 .

[0054] Container 10 houses positive electrode 4, negative electrode 7, separator 8, and electrolyte 9. Within container 10, positive electrode mixture layer 3 and negative electrode mixture layer 6 face each other with separator 8 interposed therebetween. Electrolyte 9 is stored beneath the positive electrode mixture layer 3 and negative electrode mixture layer 6. Furthermore, the ends of separator 8 are immersed in electrolyte 9. The structure of container 10 is not particularly limited; any known container used in secondary batteries can be used.

[0055] [Negative electrode active material (graphite particles)]

[0056] like Figure 2As shown, the graphite particles serving as the negative electrode active material 11 have a structure in which a high-dielectric inorganic solid 12 is integrated. In the negative electrode 7 densely filled with the negative electrode active material 11, the electrolyte 9 does not easily penetrate the negative electrode 7, so the impregnation of the negative electrode active material 11 with the electrolyte 9 sometimes becomes uneven. The surface of the negative electrode active material 11, which is less impregnated with the electrolyte 9, has high internal resistance to the release and injection of lithium ions. Repeated charge and discharge in this state increases the potential variation within the negative electrode active material 11. In this state, the solvent in the electrolyte 9 may decompose on the surface of the negative electrode active material 11, causing the electrolyte 9 to become depleted.

[0057] High Dielectric Inorganic Solids

[0058] The high dielectric inorganic solid 12 reduces the surface potential of the negative electrode active material 11 caused by the electrolyte 9. As a result, the interfacial resistance of lithium ions between the negative electrode active material 11 and the high dielectric inorganic solid 12 can be reduced, and the resistance to the movement of lithium ions can be reduced. Therefore, the increase in internal resistance of the lithium ion secondary battery 1 during repeated charge and discharge cycles can be suppressed, and the decomposition of the solvent of the electrolyte 9 on the surface of the negative electrode active material 11 can be suppressed. In addition, due to the effect of suppressing solvent decomposition brought about by the interaction with the electrolyte 9, the growth of the solid electrolyte interphase (SEI) film formed on the surface of the negative electrode active material 11 is suppressed, and due to the effect of capturing the decomposition products of the electrolyte, acid corrosion of the positive electrode active material is prevented. In the past, high dielectric inorganic solids could not physically penetrate into the interior of the graphite particles, but the electrolyte would penetrate, so the effect of suppressing electrolyte decomposition brought about by the high dielectric inorganic solid inside the graphite particles could not be obtained. However, in this embodiment, by allowing the precursor or dissolved product of the high dielectric inorganic solid to penetrate into the graphite particles and integrate them, the high dielectric inorganic solid can penetrate into the graphite particles, thereby achieving the effect of suppressing electrolyte decomposition also within the graphite particles.

[0059] The internal voids of the graphite particles serving as the negative electrode active material 11 are mostly less than 100 nm in diameter, and the path for the high dielectric inorganic solid 12 to penetrate into the interior is also long. In addition, most of the particle sizes of the high dielectric inorganic solid 12 are above 100 nm, so even if the high dielectric inorganic solid 12 is mixed and dispersed using a general method, it is difficult to configure it inside the graphite particles. However, the graphite particles of this embodiment have a structure in which the high dielectric inorganic solid 12 is already integrated internally. Thus, for the electrolyte 9 that penetrates into the interior of the graphite particles, the effect of suppressing the decomposition of the above-mentioned solvent can also be obtained. In addition, the so-called internal integration, in this specification, refers to the physical integration of the high dielectric inorganic solid 12 into the interior of the graphite particles.

[0060] The high-dielectric inorganic solid 12 has high dielectric properties. The dielectric constant of the solid particles obtained by crushing the crystalline solid is lower than the dielectric constant of the original crystalline solid. Therefore, the high-dielectric inorganic solid of this embodiment is preferably crushed while maintaining the high dielectric constant as much as possible.

[0061] The high dielectric inorganic solid 12 preferably has a powder relative dielectric constant of 10 or more. Thus, since the high dielectric inorganic solid 12 is strongly polarized, the acid generated by the decomposition of fluorine-based anions such as PF6- or solvents can be captured on the surface of the graphite particles. When acid is generated in the lithium-ion secondary battery 1, it sometimes corrodes the positive electrode active material, causing the positive electrode active material to rupture or metal precipitation. By making the powder relative dielectric constant of the above-mentioned high dielectric inorganic solid 12 be 10 or more, the rupture or metal precipitation of the above-mentioned positive electrode active material can be suppressed, thereby suppressing the resistance increase of the lithium-ion secondary battery 1 accompanying the charge and discharge cycle. The powder relative dielectric constant of the high dielectric inorganic solid 12 is more preferably 20 or more.

[0062] The relative dielectric constant of the powder of the high dielectric inorganic solid 12 can be determined as follows. The powder is introduced into a tablet forming device with a diameter (R) of 38 mm for measurement, and compressed using a hydraulic press so that the thickness (d) becomes 1 to 2 mm to form a powder compact. The compacting conditions are set to the relative density (D powder ) = weight density of the compact / true specific gravity of the dielectric × 100 is 40% or more. For this compact, the capacitance C at 1 kHz at 25°C is measured using an inductance capacitance resistance (LCR) meter and an automatic balancing bridge method. total , calculate the relative dielectric constant ε of the powder compact total In order to obtain the dielectric constant ε of the actual volume part from the obtained relative dielectric constant of the powder compact, power , the dielectric constant ε0 of vacuum can be set to 8.854×10 -12 , the relative dielectric constant ε of air air Assuming 1, the powder relative dielectric constant ε is calculated using the following formulas (1) to (3): power ”.

[0063] The contact area between the powder compact and the electrode is A = (R / 2) 2 *π (1)

[0064] C total =ε total ×ε0×(A / d) (2)

[0065] ε total =ε powder ×Dpowder +ε air ×(1-D powder ) (3)

[0066] From the perspective of increasing the electrode volume packing density of the active material, the particle size of the high dielectric inorganic solid 12 is preferably 1 / 5 or less of the particle size of the negative electrode active material 11, and more preferably in the range of 0.02 to 1 μm. If the particle size of the high dielectric inorganic solid 12 is 0.02 μm or less, high dielectric properties may not be maintained, and the effect of suppressing resistance increase may not be achieved.

[0067] The high dielectric inorganic solid 12 preferably has ion conductivity, and more preferably has at least one of Li ion conductivity, Na ion conductivity, and Mg ion conductivity. By making the high dielectric inorganic solid 12 have the above ion conductivity, it is possible to capture the free solvent present in the electrolyte 9 and form a pseudo-solvation state. As a result, the solvent in the electrolyte 9 can be stabilized, thereby suppressing the decomposition of the solvent. From this point of view, the above ion conductivity is preferably 10 -7 S / cm or more.

[0068] Here, the "ion conductivity" in this specification refers to a value determined as follows.

[0069] [Measurement method of ionic conductivity]

[0070] A tablet forming machine is used to form a sintered body or powder of a high dielectric inorganic solid 12, and Au is sputtered on both sides of the thus obtained pressed powder molded body to produce electrodes. Using the produced electrodes, an AC two-terminal method is used with an applied voltage of 50 mV and a temperature of 25°C until the frequency is 1 to 10 to the sixth power of Hz. The real number of the point where the imaginary component of the impedance becomes 0 is obtained, and thus the ion conductivity is calculated from the resistance value. As a measuring device, for example, Solartron 1260 / 1287 (manufactured by Solartron Analytical) can be used. The ion conductivity k is expressed by the following formula (4) using the Au area A' and the thickness 1 of the high dielectric inorganic solid 12.

[0071] k=1 / (Ri×A′)(S / cm) (4)

[0072] The weight ratio of the high dielectric inorganic solid 12 to the graphite particles is preferably 0.01 wt% to 0.5 wt%, and more preferably 0.05 wt% to 0.5 wt%.

[0073] As the high dielectric inorganic solid 12, for example, Na 3+x (Sb 1-x , Snx )S4(0≤X≤0.1),Na 3-x Sb 1- x W x S4 (0≤X≤1). Specifically, Na3SbS4, Na2WS4, Na 2.88 Sb 0.88 W 0.12 S4 et al.

[0074] In the lithium ion secondary battery 1 , the negative electrode active material 11 in the negative electrode mixture layer 6 is described above as including the high dielectric inorganic solid 12 . However, the high dielectric inorganic solid 12 may be included in the positive electrode active material in the positive electrode mixture layer 3 .

[0075] <Method for Producing Graphite Particles>

[0076] The method for producing graphite particles used as the negative electrode active material 11 of the lithium ion secondary battery 1 of this embodiment includes the steps of dispersing the graphite particles in a solution containing a high dielectric inorganic solid 12 and a solvent, and removing the solvent.

[0077] Ion-exchanged water or the like can be used as a solvent for dissolving the high dielectric inorganic solid 12. The step of dispersing the graphite particles in a solution of the high dielectric inorganic solid 12 dissolved in the above-mentioned solvent is not particularly limited and can be performed by mixing and stirring the solution and the graphite particles using a known stirrer or the like. Stirring conditions can be, for example, a temperature of 60 to 80°C and a stirring time of 1 to 10 hours.

[0078] The solvent removal step may be performed by vaporizing the solvent by at least one of heating and reducing pressure, or by adding a poor solvent having low solubility in the high dielectric inorganic solid 12 to precipitate the high dielectric inorganic solid 12 and then removing the solvent. Examples of the poor solvent include acetone.

[0079] As mentioned above, although the preferred embodiment of the present invention was described, the content of the present invention is not limited to the above-mentioned embodiment, and can be modified appropriately.

[0080] [Example]

[0081] Hereinafter, the present invention will be described in more detail based on examples. However, the present invention is not limited to the description of the following examples.

[0082] <Synthesis of High Dielectric Inorganic Solids>

[0083] (Synthesis of Na3SbS4)

[0084] Na3SbS4(NSS) was synthesized using the following method. 70.4g of Na2S, 75g of Sb2S3, and 21g of S were dissolved in 2210ml of ion-exchanged water and stirred at 70°C for 5 hours. Thereafter, the mixture was cooled to 25°C and undissolved matter was removed. 1400ml of acetone was then added and stirred for 5 hours, followed by standing for 12 hours. Na3SbS4 was obtained by drying under reduced pressure at 200°C. The obtained sample was subjected to X-ray diffraction (XRD) measurement, confirming that it was a crystalline phase of Na3SbS4(H2O)9.

[0085] (Synthesis of Na2WS4)

[0086] Na2WS4 (NWS) was synthesized using the following method. 17.66 g of NaOH and 153.74 g of (NH4)2WS4 were dissolved in 2110 ml of ion-exchanged water, stirred at 70°C for 5 hours, and allowed to stand for 12 hours. The resulting solid was then dried under reduced pressure at 150°C. The resulting powder was heated at 275°C in an Ar atmosphere to obtain Na2WS4.

[0087] (Na 2.88 Sb 0.88 W 0.12 Synthesis of S4)

[0088] Na was synthesized using the following method 2.88 Sb 0.88 W 0.12 S4 (NSWS). 123.95 g of the above NSS and 18.97 g of the above NWS were dissolved in ion exchange water at 50°C, and the water in the solution was removed at 70°C. The obtained solid was then dried under reduced pressure at 150°C. The obtained powder was heated at 275°C in an Ar atmosphere to obtain Na 2.88 Sb 0.88 W 0.12 S4.

[0089] (Li3PO4)

[0090] As Li3PO4 (LPO), one having a particle size D50 of 0.8 μm was used.

[0091] The ion conductivity and relative dielectric constant of the powders of the NSS, NWS, NSWS, and LPO obtained above were measured. The results are shown in Table 1.

[0092] [Table 1]

[0093] High dielectric inorganic solids Abbreviation Ionic conductivity (S / cm) Powder relative dielectric constant <![CDATA[Na3SbS4]]> NSS <![CDATA[1.0×10 -3 ]]> 44 <![CDATA[Na2WS4]]> NWS <![CDATA[1.0×10- 7 ]]> 30 <![CDATA[Na 2.88 Sat 0.88 IN 0.12 S4]]> NSWS <![CDATA[4.0×10 -3 ]]> 50 <![CDATA[Li3PO4]]> LPO <![CDATA[1.0×10 -7 ]]> 28

[0094] <Production of Graphite Particles>

[0095] (Example 1)

[0096] 199.8 g of graphite particles (96.4% by weight in the negative electrode composition) and 0.2 g of the high-dielectric inorganic solid NSS (0.1% by weight in the negative electrode composition) obtained above were mixed in 200 ml of ion-exchanged water. The mixture was heated to 50°C and stirred for 5 hours. The water was then removed at 70°C and dried under reduced pressure at 120°C to obtain the graphite particles of Example 1.

[0097] (Examples 2 to 7, Comparative Example 1)

[0098] Graphite particles for Examples 2 to 7 were prepared in the same manner as in Example 1, except that the weight ratios of graphite particles and high-dielectric inorganic solids in the negative electrode composition and the types of high-dielectric inorganic solids were as shown in Table 2. In Comparative Example 1, no high-dielectric inorganic solids were added. In Comparative Example 2, a negative electrode was prepared in the same manner as in Example 1, except that LPO, which is insoluble in the solvent, was prepared at the ratios shown in Table 2.

[0099] <Production of positive electrode>

[0100] Acetylene black (AB) as an electronic conductive material and polyvinylidene fluoride (PVdF) as a binder were pre-mixed in N-methyl-2-pyrrolidone (NMP) as a dispersion solvent and wet-mixed using a rotary mixer to obtain a pre-mixed slurry. 0.6 Co 0.2 Mn 0.2 O2 (NCM622) is mixed with the obtained premixed slurry and dispersed using a planetary mixer to obtain a positive electrode slurry. The mass ratio of each component in the positive electrode slurry is set to NCM622:AB:PVdF=94:4.2:1.8. The median particle size of NCM622 is 12μm. Next, the obtained positive electrode slurry is applied to an aluminum positive electrode collector and dried. After being pressed by roller pressing, it is dried in a vacuum at 120°C to form a positive electrode plate having a positive electrode mixture layer. The obtained positive electrode plate is punched into a size of 30mm×40mm to form a positive electrode.

[0101] <Production of negative electrode>

[0102] A planetary mixer is used to pre-mix a carboxymethyl cellulose (CMC) aqueous solution as a binder and acetylene black (AB) as an electron conductive material. Subsequently, the graphite particles (MGr) of the above-mentioned examples and comparative examples as negative electrode active materials are mixed and further pre-mixed using a planetary mixer. Thereafter, water as a dispersion solvent and styrene butadiene rubber (SBR) as a binder are added and dispersed using a planetary mixer to obtain a negative electrode slurry. The mass ratio of each component in the negative electrode slurry is set to MGr:AB:CMC:SBR=96.5:0.1:1.0:1.0:1.5. The median particle size of natural graphite is 12μm. Next, the obtained negative electrode slurry is applied to a copper negative electrode collector and dried. After being pressed by roller pressing, it is dried in a vacuum at 130°C to form a negative electrode plate having a negative electrode mixture layer. The obtained negative electrode plate was punched into a size of 34 mm×44 mm to prepare a negative electrode.

[0103] (Manufacturing of lithium-ion secondary batteries)

[0104] A secondary battery aluminum laminate (manufactured by Dai Nippon Printing Co., Ltd.) was heat-sealed and processed into a bag shape. Into the container thus formed, a laminated body formed by sandwiching a separator between the positive and negative electrodes prepared above was introduced, and an electrolyte was injected into each electrode interface. After that, the container was depressurized to -95kPa and sealed, thereby producing a lithium-ion secondary battery. As a separator, a polyethylene microporous membrane with a single surface coated with aluminum oxide particles of about 5μm was used. In addition, as an electrolyte, an electrolyte prepared as follows was used: LiPF6 was dissolved as an electrolyte salt at a concentration of 1.2mol / L in a mixed solvent of ethylene carbonate, ethyl methyl carbonate and dimethyl carbonate in a volume ratio of 30:30:40.

[0105] <Evaluation>

[0106] The lithium ion secondary batteries produced using the graphite particles of Examples 1 to 7 and Comparative Example 1 were evaluated as follows.

[0107] [Initial performance (discharge capacity)]

[0108] The prepared lithium-ion secondary battery was placed at a measurement temperature (25°C) for 1 hour, charged at a constant current of 8.4 mA to 4.2 V, then charged at a constant voltage of 4.2 V for 1 hour. After 30 minutes, it was discharged at a constant current of 8.4 mA to 2.5 V. The above process was repeated 5 times, and the discharge capacity at the fifth discharge was defined as the initial discharge capacity (mAh). The results are shown in Table 2. In addition, the current value that can complete the discharge in 1 time for the obtained discharge capacity is defined as 1C.

[0109] [Initial performance (initial battery resistance)]

[0110] After the initial discharge capacity of the lithium-ion secondary battery is measured, it is placed at the measurement temperature (25°C) for 1 hour, charged at 0.2C, adjusted to a charge level (State of Charge (SOC)) of 50%, and placed for 10 minutes. Next, the C rate is set to 0.5C for a 10-second pulse discharge, and the voltage at the time of discharge for 10 seconds is measured. Then, the horizontal axis is the current value and the vertical axis is the voltage, and the voltage at the time of discharge for 10 seconds relative to the current at 0.5C is plotted. Then, after being placed for 10 minutes, it is supplemented with electricity to restore the SOC to 50%, and then placed for another 10 minutes. The above operation is performed for each C rate of 1.0C, 1.5C, 2.0C, 2.5C, and 3.0C, and the voltage at the time of discharge for 10 seconds relative to the current value at each C rate is plotted. Then, the slope of the approximate straight line based on the least squares method obtained from each drawing is set as the internal resistance value (Ω) of the lithium-ion secondary battery obtained in this embodiment. The results are shown in Table 2.

[0111] [Performance after endurance (discharge capacity)]

[0112] As a charge and discharge cycle durability test, the battery was charged to 4.2V at a constant current rate of 1C in a constant temperature bath at 45°C, and then discharged to 2.5V at a constant current rate of 2C as one cycle, and the above operation was repeated 500 times. After 500 cycles, the battery was placed in a constant temperature bath at 25°C for 24 hours, then charged to 4.2V at a constant current rate of 0.2C, and then charged to a constant voltage of 4.2V for 1 hour. After being placed for 30 minutes, the battery was discharged to 2.5V at a constant current rate of 0.2C, and the discharge capacity (mAh) after durability was measured. The results are shown in Table 2.

[0113] [Battery resistance after endurance]

[0114] Similar to the initial battery resistance measurement, the lithium-ion secondary battery after the endurance discharge capacity measurement was charged to a state of charge (SOC) of 50%. The endurance battery resistance (Ω) was then determined using the same method as the initial battery resistance measurement. The results are shown in Table 2.

[0115] [Capacity retention rate after endurance test]

[0116] The ratio of the discharge capacity (mAh) after endurance test to the initial discharge capacity (mAh) was calculated and defined as the capacity retention rate (%) after endurance test.

[0117] [Resistance increase rate after durability]

[0118] The ratio of the battery resistance value after endurance test to the initial battery resistance value (Ω) was calculated as the battery resistance increase rate (%). The results are shown in Table 2.

[0119] [EPMA measurement]

[0120] The cross-section of the graphite particles of Example 5 and Comparative Example 2 was imaged using EPMA (JXA-8500F manufactured by JEOL Ltd.). The EPMA image of Example 5 is shown in FIG. Figure 3 The EPMA image of Comparative Example 2 is shown in Figure 4 .exist Figure 3 and Figure 4 In the graph, the whitest part represents the high dielectric inorganic solid, the gray part represents the graphite particles, and the blackest part represents the voids. Figure 3 and Figure 4 It was found that the high dielectric inorganic solid was integrated inside the graphite particles of Example 5. On the other hand, in the graphite particles of Comparative Example 2, it was found that the high dielectric inorganic solid was not arranged inside the graphite particles.

[0121]

[0122] The results in Table 2 confirm that the lithium-ion secondary batteries of each example have a higher capacity retention rate and a lower resistance increase rate after endurance compared to the lithium-ion secondary batteries of the comparative examples. In other words, it is confirmed that the lithium-ion secondary batteries of each example have excellent durability against charge and discharge cycles.

[0123] Reference numerals

[0124] 1: Lithium-ion secondary battery

[0125] 11: Negative electrode active material (graphite particles)

[0126] 12: High dielectric inorganic solids

Claims

1. A graphite particle for a lithium-ion secondary battery having a structure in which a high-dielectric inorganic solid having a relative dielectric constant of 10 or more is integrated within the graphite particle. The aforementioned high dielectric inorganic solid is Na 2.88 Sb 0.88 W 0.12 S4 or Na2WS4.

2. The graphite particles for lithium ion secondary batteries according to claim 1, wherein The high dielectric inorganic solid has at least one ion conductivity among Li ion conductivity, Na ion conductivity, and Mg ion conductivity.

3. The graphite particles for lithium ion secondary batteries according to claim 2, wherein The aforementioned ion conductivity is 10 -7 S / cm or more.

4. The graphite particles for lithium ion secondary batteries according to claim 1, wherein The weight ratio of the high dielectric inorganic solid to the graphite particles is 0.01 wt % or more and 0.5 wt % or less. 5 . An electrode for a lithium ion secondary battery, comprising the graphite particles for a lithium ion secondary battery according to claim 1 .

6. A method for producing graphite particles for lithium-ion secondary batteries, comprising the following steps: Dispersing graphite particles in a solution containing a high dielectric inorganic solid having an ion-conductive powder with a relative dielectric constant of 10 or more and a solvent; and Remove the aforementioned solvent, The aforementioned high dielectric inorganic solid is Na 2.88 Sb 0.88 W 0.12 S4.

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