JP7087422B2 - Secondary battery - Google Patents

Description

The invention disclosed herein relates to a secondary battery.

Conventionally, as a secondary battery of this type, a lithium ion supply core portion containing an electrolyte and a current collector having a three-dimensional network structure formed by surrounding the outer surface of the core portion and coated with an internal electrode active material on the outer surface are used. A cable-type secondary battery including an internal electrode including an internal electrode and an external electrode formed by surrounding the outer surface of the internal electrode and including an external electrode active material layer has been proposed (see, for example, Patent Document 1). In this secondary battery, the electrolyte in the core portion easily penetrates into the active material of the electrode, and the capacity characteristics and cycle characteristics of the battery are excellent.

Japanese Patent Publication No. 2014-532277

By the way, in recent years, it has been desired to increase the capacity of lithium secondary batteries and increase the energy density per unit volume. For example, in order to increase the battery capacity of a secondary battery in which electrodes are laminated, it is necessary to increase the thickness of the active material layer, but if the electrodes are made thicker in this way, the thickness direction of the electrolytic solution The flow path of the battery becomes long, it becomes difficult to relax the concentration gradient of ions in the thickness direction, and the output may decrease. On the other hand, the above-mentioned lithium ion secondary battery of Patent Document 1 is said to have excellent capacity characteristics and cycle characteristics of the battery, but it is not yet sufficient, and it is hoped that the rate characteristics and the charge / discharge cycle characteristics will be further improved. It was rare.

The present disclosure has been made in view of such problems, and an object of the present disclosure is to provide a novel secondary battery capable of further improving the output characteristics and the charge / discharge cycle characteristics.

As a result of diligent research to achieve the above-mentioned object, the present inventors have obtained ionic conductivity and insulating property on the surface of a negative electrode using non-graphitizable carbon fiber having a predetermined volume change rate as a negative electrode active material. It has been found that by forming a positive electrode active material layer with a separating film having and binding a plurality of the negative electrodes, it is possible to provide a novel secondary battery capable of further improving output characteristics and charge / discharge cycle characteristics. The invention disclosed in the above is completed.

That is, the secondary battery disclosed in this specification is
A positive electrode with a positive electrode active material and a positive electrode
A linear negative electrode having a negative electrode active material of carbon fiber whose main component is non-graphitizable carbon whose volume change due to storage and release of ions is 1.0% by volume or less.
A separation membrane having ionic conductivity and insulating the positive electrode and the negative electrode,
It is equipped with.

The present disclosure can provide a novel secondary battery with further improved output characteristics and charge / discharge cycle characteristics. The reason why such an effect is obtained is presumed as follows. For example, by making the negative electrode of a secondary battery such as a lithium ion secondary battery linear and using carbon fibers, carrier ions can be stored and released from the entire circumference. The occlusion / release reaction from the entire circumference is expected to have the effect of improving the average reaction rate by increasing the positive / negative facing area to promote the reaction and reducing the amount of active material per facing area toward the deeper part (inner part). It can be done and high output can be achieved. It should be noted that the decrease in the amount of the active material in the deep part is considered to be suitable because the reaction is less likely to occur in the deeper part of the active material. Further, the improvement in the average reaction rate is based on the decrease in the average distance between the positive and negative active materials. Further, in the non-graphitizable carbon fiber having a volume change rate of 1.0% or less, cracks and cracks of the carbon fiber due to expansion and contraction are reduced, so that the durability of the charge / discharge cycle is further improved. It is inferred that. For these combined reasons, it is presumed that the secondary battery of the present disclosure can further improve the output characteristics, for example, the output characteristics such as rapid charge / discharge at a high capacity, and the charge / discharge cycle characteristics. To.

The schematic diagram which shows an example of a secondary battery 10. A cross-sectional view taken along the line AA of the secondary battery 10. Explanatory drawing which shows an example of the manufacturing process of a secondary battery 10. The relationship diagram of the positive electrode facing area and the cell energy density in a bundling structure and a laminated structure. The relationship diagram of the cell energy density and the electrode facing area with respect to the negative electrode fiber diameter or the film thickness of a negative electrode mixture. Explanatory drawing of evaluation cell 30.

The secondary battery described in the embodiment includes a positive electrode having a positive electrode active material, a linear negative electrode having a negative electrode active material of carbon fiber, and a separation film having ionic conductivity and insulating the positive electrode and the negative electrode. ing. This negative electrode has a negative electrode active material containing non-graphitizable carbon having a volume change of 1.0% by volume or less due to occlusion and release of ions as a main component. The positive electrode and the negative electrode may contain a conductive material and a binder in addition to the active material. The negative electrode may be linear, and its cross section may be circular or polygonal. Further, the positive electrode may be present around the negative electrode of the carbon fiber, or may be filled in the space between the negative electrodes. In this secondary battery, one or more of the positive electrode, the negative electrode and the separation membrane may contain an electrolytic solution. Alternatively, the separation membrane may be a solid electrolyte. Further, the secondary battery may have a structure in which a plurality of negative electrodes are bundled in a state of being adjacent to the positive electrode via a separation membrane. Further, the positive electrode and the negative electrode may be embedded with a current collector member such as a current collector, or may not be provided with the current collector member. Here, for convenience of explanation, a lithium secondary battery having a lithium ion as a carrier will be described below as a main example thereof.

Next, the secondary battery disclosed in the present embodiment will be described with reference to the drawings. FIG. 1 is a schematic diagram showing an example of the secondary battery 10. FIG. 2 is a sectional view taken along the line AA of the secondary battery 10 of FIG. As shown in FIGS. 1 and 2, the secondary battery 10 includes a negative electrode 11, a negative electrode current collector 12, a positive electrode 16, a positive electrode current collector 17, and a separation film 21. The secondary battery 10 includes a negative electrode 11 using carbon fiber as a negative electrode active material, and a positive electrode 16 formed of a positive electrode active material layer formed around the negative electrode 11. It is assumed that the secondary battery 10 has a structure in which a plurality of negative electrodes 11 are bundled in a state of being adjacent to the positive electrode 16 via a separation film 21. Further, the secondary battery 10 may have a structure in which 50 or more negative electrodes 11 are bundled.

The negative electrode 11 may be a linear body made of a negative electrode active material of carbon fiber. The outer periphery of the negative electrode 11 other than the end face faces the positive electrode 16 via the separation membrane 21. For example, the negative electrode 11 may have a capacity of 1 / n of the negative electrode capacity of the entire cell, and n of them may be connected in parallel to the negative electrode current collector 12. The negative electrode 11 preferably has a cross-sectional length (diameter) perpendicular to the longitudinal direction in the range of 10 μm or more and 200 μm or less. When this length is 10 μm or more, the strength of the electrode structure can be ensured and stable charging / discharging can be performed. Further, when this length is 200 μm or less, the moving distance of the carrier ions does not become too long, and high output performance can be obtained. Moreover, in this length range, the energy density per unit volume can be further increased. Alternatively, in this range, the moving distance of the carrier ions can be made shorter, and charging / discharging can be performed with a larger current. The length of the carbon fiber in the longitudinal direction can be appropriately determined depending on the use of the secondary battery and the like, and may be, for example, in the range of 20 mm or more and 200 mm or less. When the length of the carbon fiber is 20 mm or more, the battery capacity can be further increased, and when it is 200 mm or less, the electric resistance of the negative electrode can be further reduced, which is preferable.

It is assumed that the carbon fiber is mainly composed of non-graphitizable carbon. This graphitizable carbon has an average interplanetary spacing of about 0.38 nm for d ₀₀₂ , which is larger than 0.335 nm for graphite (0.37 nm when Li is inserted), and is observed with a transmission electron microscope (TEM). Can be confirmed by. Here, the "main component" refers to a component having a larger proportion in the whole, for example, a component of 50% by mass or more, more preferably a component of 80% by mass or more, and further preferably a component of 90% by mass or more. good. It is assumed that the volume change rate of this carbon fiber due to occlusion and release of ions is 1.0% or less. When the volume change rate is 1.0% or less, cracks and cracks in the carbon fibers due to expansion and contraction are reduced, so that the durability of the charge / discharge cycle is further improved. Here, the volume change rate will be described. This volume change rate shall be obtained by the following method. First, the fully charged carbon fiber and the completely discharged carbon fiber are taken out of the cell in an unexposed state, and the diameter and length are determined by a high-resolution scanning electron microscope (SEM) in the unexposed state. By observing, the volume Vc of the carbon fiber at the time of full charge and the volume Vd of the carbon fiber at the time of complete discharge are obtained. Using the obtained value, it is assumed that the volume change rate (%) = (Vc-Vd) / Vd × 100 is used. The volume change rate is more preferably smaller, preferably 0.8% or less, more preferably 0.5% or less, still more preferably 0.3% or less.

Further, the carbon fiber preferably has a porosity of 6% by volume or less. When the porosity is 6% by volume or less, for example, the carbon fiber can be made difficult to contain the electrolytic solution. In this case, since the ion conduction in the negative electrode is overwhelmingly performed by the diffusion in the solid of carbon rather than through the electrolytic solution, it is possible to have better output characteristics such as rapid charge and discharge. can. Further, when the porosity is 6% by volume or less, the volume capacity of the negative electrode becomes higher, so that the energy density of the secondary battery can be further increased. The porosity is preferably lower, preferably 5% by volume or less, more preferably 3% by volume or less, still more preferably 2% by volume or less. This porosity is a value obtained by nano-CT using synchrotron radiation.

The carbon fiber preferably has a rate characteristic (%) of 80% or more, which is the ratio of the 60C rate charge capacity in the first cycle to the C / 4 rate charge capacity in the first cycle. This rate characteristic is preferably higher, more preferably 82% or more, and even more preferably 85% or more. For carbon fibers, it is preferable that the average charging voltage in the first cycle of C / 4 rate charging is 0.4 V or more at the Li reference potential. The average voltage is more preferably 0.45 V or more, and further preferably 0.50 V or more. It is preferable that the discharge capacity of the carbon fiber in the first cycle when charged and discharged at a 1C rate is 300 mAh / g or more. This discharge capacity is preferably higher. It is preferable that the carbon fiber is charged and discharged 30 times at a 1C rate, and the cycle maintenance rate obtained from the formula (capacity of the 30th cycle) / (capacity of the 1st cycle) × 100% is 70% or more. .. This cycle maintenance rate is preferably higher, more preferably 75% or more, and even more preferably 80% or more. The rate characteristics, average charge voltage, discharge capacity and cycle retention rate of the carbon fiber shall be the values measured by the carbon fiber in a half cell (see FIG. 6).

It is preferable that the carbon fiber has a conductive component formed on at least a part of the surface of the carbon fiber. Since non-graphitizable carbon is not highly crystalline, the graphene structure may not be developed and oriented, and the electron conductivity may not be sufficiently high. Therefore, by forming the conductive component to the extent that it does not cover the entire outer surface (side surface), the electron conductivity can be further enhanced. Considering the influence of the carrier ion on the occlusion / release characteristics, the conductive component is preferably formed at an area ratio of 10% or less, and more preferably 8% or less with respect to the entire outer surface. Further, from the viewpoint of improving electron conductivity, the conductive component is preferably formed at an area ratio of 2% or more, and more preferably at 4% or more. The coating thickness of the conductive component is, for example, preferably 2 nm or more, and more preferably 5 nm or more. Further, considering the reduction of the energy density of the negative electrode, the coating thickness is preferably 50 nm or less. The conductive component is not particularly limited as long as it is a highly conductive material, but is preferably, for example, a metal. As the metal, for example, a metal having high conductivity is more preferable, and examples thereof include precious metals such as Au, Pt and Ag, and transition metals such as Cu and Ni. The formation of the conductive component is preferably performed by a method capable of appropriately controlling the coating of the outer surface, and may be performed by, for example, thin-film deposition, sputtering, plating, or the like.

The negative electrode current collector 12 is a member having conductivity, and the end face of the negative electrode 11 is electrically connected. More than 50 negative electrodes 11 are connected in parallel to the negative electrode current collector 12. The negative electrode current collector 12 includes, for example, carbon paper, aluminum, copper, titanium, stainless steel, nickel, iron, platinum, calcined carbon, conductive polymer, conductive glass, and the like, as well as adhesiveness, conductivity, and acid resistance. For the purpose of improving the conversion (reducing) property, those whose surfaces such as aluminum and copper are treated with carbon, nickel, titanium, silver, platinum, gold and the like can also be used. The shape of the negative electrode current collector 12 is not particularly limited as long as a plurality of negative electrodes 11 can be connected, and for example, a plate shape, a foil shape, a film shape, a sheet shape, a net shape, a punched or expanded shape, or a lath. Examples include a body, a porous body, a foam, and a fiber group forming body.

The positive electrode 16 has a positive electrode active material and is formed on the outer periphery of the negative electrode 11 via a separation film 21. The positive electrode 16 may have a hexagonal outer shape in cross section and may include a columnar negative electrode 11. The positive electrode 16 may be filled between the negative electrodes 11, and the outer shape is not particularly limited to a hexagonal shape. The positive electrode 16 may have conductivity in itself, and the current collector member may be omitted. The positive electrode 16 may have its end face directly connected to the positive electrode current collector 17. The positive electrode 16 may be formed, for example, by forming a separation film 21 on the outer periphery of the negative electrode 11 and then applying the raw material of the positive electrode 16 to the outer periphery thereof.

The positive electrode 16 contains a positive electrode active material, but when the positive electrode active material does not have conductivity, for example, it may be formed by mixing a conductive material having conductivity. The positive electrode 16 may be formed by mixing, for example, a positive electrode active material, a conductive material, and a binder, if necessary. Examples of the positive electrode active material include a material capable of occluding and releasing lithium as a carrier. Examples of the positive electrode active material include compounds having lithium and a transition metal, for example, an oxide containing lithium and a transition metal element, and a phosphoric acid compound containing lithium and a transition metal element. Specifically, a lithium manganese composite oxide having a basic composition formula of Li _(1-x) MnO ₂ (0 ≦ x ≦ 1, etc., the same applies hereinafter) or Li _(1-x) Mn ₂ O ₄ or the like, basic composition. Lithium-cobalt composite oxide with formula Li _(1-x) CoO ₂ , etc., lithium nickel composite oxide with basic composition formula Li _(1-x) NiO ₂ , etc., basic composition formula Li _(1-x) Co _a Ni _b Mn _c O ₂ (a> 0, b> 0, c> 0, a + b + c = 1), Li _(1-x) Co _a Ni _b Mn _c O ₄ (0 <a <1, 0 <b) Lithium cobalt nickel-manganese composite oxide with <1, 1 ≦ c <2, a + b + c = 2), etc., lithium vanadium composite oxide with basic composition formula such as LiV ₂ O ₃ , basic composition formula V ₂ O ₅ , etc. A transition metal oxide or the like can be used. Further, a lithium iron phosphate compound or the like having a basic composition formula of LiFePO ₄ can be used as the positive electrode active material. Of these, lithium cobalt nickel-manganese composite oxides such as LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ and LiNi _0.4 Co _0.3 Mn _0.3 O ₂ are preferable. The "basic composition formula" means that other elements such as Al and Mg may be contained.

The conductive material contained in the positive electrode is not particularly limited as long as it is an electronically conductive material that does not adversely affect the battery performance, and for example, graphite such as natural graphite (scaly graphite, scaly graphite) or artificial graphite, acetylene black, or carbon. One or a mixture of black, Ketjen black, carbon whisker, graphite coke, carbon fiber, metal (copper, nickel, aluminum, silver, gold, etc.) can be used. The binder serves to hold the active material particles and the conductive material particles together to maintain a predetermined shape, and contains, for example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), fluororubber, and the like. Fluororesin, thermoplastic resin such as polypropylene and polyethylene, ethylene propylene diene monomer (EPDM) rubber, sulfonated EPDM rubber, natural butyl rubber (NBR) and the like can be used alone or as a mixture of two or more. Further, an aqueous dispersion of cellulose-based binder or styrene-butadiene rubber (SBR), which is an aqueous binder, can also be used.

In the positive electrode 16, the content of the positive electrode active material is preferably larger, preferably 70% by mass or more, and more preferably 80% by mass or more with respect to the total mass of the positive electrode 16. The content of the conductive material is preferably in the range of 0% by mass or more and 20% by mass or less, and more preferably in the range of 0% by mass or more and 10% by mass or less with respect to the total mass of the positive electrode 16. In such a range, it is possible to suppress a decrease in battery capacity and sufficiently impart conductivity. The content of the binder is preferably in the range of 0.1% by mass or more and 5% by mass or less, and in the range of 0.2% by mass or more and 3% by mass or less with respect to the total mass of the positive electrode 16. Is more preferable.

The positive electrode current collector 17 is a conductive member and is electrically connected to the positive electrode 16. The end faces of 50 or more positive electrodes 16 are connected in parallel to the positive electrode current collector 17. The positive electrode current collector 17 may be the same member as the negative electrode current collector 12.

The separation membrane 21 has ionic conductivity of carriers (for example, lithium ions) and insulates the negative electrode 11 and the positive electrode 16. The separation film 21 is formed on the entire outer peripheral surface of the negative electrode 11 facing the positive electrode 16 and the entire outer peripheral surface of the positive electrode 16 facing the negative electrode 11 to prevent a short circuit between the negative electrode 11 and the positive electrode 16. .. The separation membrane 21 is preferably a polymer having ionic conductivity and insulating properties. Examples of the separation film 21 include a copolymer of polyvinylidene fluoride (PVdF) and hexafluoropropylene (HFP), methyl polymethacrylate (PMMA), and a copolymer of PMMA and an acrylic polymer. .. For example, in a copolymer of PVdF and HFP, a part of the electrolytic solution swells and gels this membrane to become an ionic conduction membrane. The thickness t of the separation membrane 21 is, for example, preferably 0.5 μm or more, more preferably 2 μm or more, and may be 5 μm or more. When the thickness t is 0.5 μm or more, it is preferable in order to secure the insulating property. The thickness t of the separation membrane 21 is preferably 20 μm or less, and more preferably 10 μm or less. When the thickness t is 20 μm or less, it is preferable in that the decrease in ionic conductivity can be suppressed. In the range of thickness t in the range of 0.5 to 20 μm, ionic conductivity and insulating property are suitable. The separation film 21 may be formed, for example, by immersing the negative electrode 11 in a solution containing a raw material and coating the surface thereof.

The separation membrane 21 may contain an electrolytic solution that conducts ions as carriers. Examples of this electrolytic solution include non-aqueous solvents. Examples of the solvent of the electrolytic solution include a solvent of a non-aqueous electrolytic solution. Examples of the solvent include carbonates, esters, ethers, nitriles, furans, sulfolanes, dioxolanes and the like, and these can be used alone or in combination. Specifically, as carbonates, cyclic carbonates such as ethylene carbonate (EC), propylene carbonate, vinylene carbonate, butylene carbonate, and chloroethylene carbonate, dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), diethyl carbonate, and ethyl are used. Chain carbonates such as -n-butyl carbonate, methyl-t-butyl carbonate, di-i-propyl carbonate, t-butyl-i-propyl carbonate, cyclic esters such as γ-butyl lactone and γ-valerolactone, Chain esters such as methyl formate, methyl acetate, ethyl acetate, methyl butyrate, ethers such as dimethoxyethane, ethoxymethoxy ethane, diethoxyethane, nitriles such as acetonitrile and benzonitrile, furan such as tetrahydrofuran and methyl tetrahydrofuran, etc. Classes, sulfolanes such as sulfolane and tetramethylsulfolane, and dioxolanes such as 1,3-dioxolane and methyldioxolan. In this electrolytic solution, a supporting salt containing ions, which is a carrier of the secondary battery 10, may be dissolved. Examples of the supporting salt include LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiSbF ₆ , LiSiF ₆ , LiAlF ₄ , LiSCN, and the like. Examples thereof include LiClO ₄ , LiCl, LiF, LiBr, LiI, and LiAlCl ₄ . Of these, 1 selected from the group consisting of inorganic salts such as LiPF ₆ , LiBF ₄ , and LiClO ₄ , and organic salts such as LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , and LiC (CF ₃ SO ₂ ) ₃ . It is preferable to use a seed or a combination of two or more kinds of salts from the viewpoint of electrical characteristics. The concentration of this supporting salt in the electrolytic solution is preferably 0.1 mol / L or more and 5 mol / L or less, and more preferably 0.5 mol / L or more and 2 mol / L or less.

In this secondary battery 10, the positive / negative electrode capacity ratio (negative electrode capacity / positive electrode capacity), which is the ratio of the capacity of the negative electrode active material to the capacity of the positive electrode active material, is preferably in the range of 1.0 or more and 1.5 or less. , More preferably in the range of 1.2 or less. The formation thickness X of the positive electrode (see FIG. 4) is appropriately set according to the diameter of the negative electrode and the capacity ratio of the positive and negative electrodes, but may be, for example, in the range of 5 μm or more and 50 μm or less. The formed thickness of the positive electrode is, for example, the maximum thickness of the portions formed on the negative electrode.

Next, a method for manufacturing the secondary battery will be described. This manufacturing method may include a heat treatment step, a separation film forming step, a positive electrode active material forming step, and a binding step. In the heat treatment step, the carbon raw material is heat-treated to obtain carbon fibers. In addition, carbon fiber may be prepared separately and this step may be omitted. In the separation film forming step, a separation film having ionic conductivity and insulating property is formed on the surface of the negative electrode, which is a linear body having the negative electrode active material of carbon fiber. In the positive electrode active material forming step, a positive electrode mixture containing the positive electrode active material is formed on the carbon fibers on which the separation film is formed. After this positive electrode active material forming step, a conductive material addition step of additionally forming a conductive material may be performed. In the binding step, a plurality of negative electrodes are bound in a state adjacent to the positive electrode having the positive electrode active material via the formed separation membrane. Here, as a specific example, the manufacturing process of the secondary battery 10 will be described. 3A and 3B are explanatory views showing an example of a manufacturing process of the secondary battery 10. FIG. 3A is a heat treatment step, FIG. 3B is a separation film forming step, and FIG. 3C is a positive electrode active material forming. A step, FIG. 3 (d) is a conductive material addition step, and FIG. 3 (e) is a bundling step. As the material used for the secondary battery, any of the above can be appropriately used.

In the heat treatment step, the raw material of the carbon material is heat-treated to produce carbon fibers (FIG. 3 (a)). In this step, the raw material of the carbon material may be spun and then heat-treated. As the raw material of the carbon fiber, one that becomes non-graphitizable carbon such as amorphous carbon can be used. The heat treatment may be performed in an inert atmosphere at a temperature of 800 ° C. or higher and 1200 ° C. or lower. The heat treatment temperature is preferably in the range of 900 ° C. or higher and 1100 ° C. or lower. Examples of the inert atmosphere include nitrogen and noble gas, and of these, the argon atmosphere is preferable. In this step, it is assumed that the treatment is performed so that the carbon fiber has a volume change rate of 1.0% or less. Further, in this step, it is preferable to treat the carbon fibers so that the porosity is less than 6% by volume. Further, it is preferable to form carbon fibers with a diameter in the range of 10 μm or more and 200 μm or less. The porosity can be adjusted by adjusting the particle size of the raw material of the carbon material, the molding pressure, and the like. The length (diameter) of the cross section perpendicular to the longitudinal direction can be adjusted according to the conditions at the time of spinning. Further, a conductive component may be formed on the surface of the obtained carbon fiber. As described above, the conductive component is formed in an area ratio of 10% or less, more preferably 8% or less, further 2% or more, and more preferably 4% or more with respect to the entire surface. The conductive component can be formed by a vapor deposition method, a sputtering method, a plating method, or the like.

Next, in the separation membrane forming step, a separation membrane is formed on the outer surface of the carbon fiber (FIG. 3 (b)). In this step, the raw material of the separation membrane described above may be applied onto the carbon fibers and dried. Alternatively, in this step, the carbon fibers may be immersed in the raw material solution of the separation membrane. Next, in the positive electrode active material forming step, the positive electrode active material is formed on the separation membrane (FIG. 3 (c)). The positive electrode active material may be, for example, a material in which a conductive material or a binder is attached to fine particles of the active material. A positive electrode mixture in which such positive electrode active material particles are made into a slurry may be prepared and applied onto the separation membrane. A conductive material is added to the positive electrode mixture as needed (FIG. 3 (d)). As the conductive material, a carbon material or metal particles (for example, Cu, Ni, Al, etc.) may be used. Then, in the binding step, a plurality of the carbon fibers of the negative electrode produced above are arranged and bound (FIG. 3 (e)). In this step, pressure may be applied to the bound body. In this way, the secondary battery 10 can be manufactured.

The secondary battery 10 and the method for manufacturing the secondary battery 10 described in detail above can provide a novel battery having further improved output characteristics and charge / discharge cycle characteristics. The reason why such an effect is obtained is presumed as follows. For example, by using carbon fiber as the negative electrode of a secondary battery such as a lithium ion secondary battery, carrier ions can be stored and released from the entire circumference. The occlusion / release reaction from the entire circumference is expected to have the effect of improving the average reaction rate by increasing the positive / negative facing area to promote the reaction and reducing the amount of active material per facing area toward the deeper part (inner part). It can be done and high output can be achieved. It should be noted that the decrease in the amount of the active material in the deep part is considered to be suitable because the reaction is less likely to occur in the deeper part of the active material. Further, the improvement in the average reaction rate is based on the decrease in the average distance between the positive and negative active materials. Further, in the non-graphitizable carbon fiber having a volume change rate of 1.0% or less, cracks and cracks of the carbon fiber due to expansion and contraction are reduced, so that the durability of the charge / discharge cycle is further improved. It is inferred that. For these combined reasons, the secondary battery 10 and its manufacturing method can further improve the output characteristics, for example, the output characteristics such as rapid charge / discharge at a high capacity, and the charge / discharge cycle characteristics. It is inferred. Further, according to this manufacturing method, the secondary battery 10 can be manufactured by a relatively simple step of forming a separation film and a positive electrode active material layer on the surface of carbon fibers and binding them.

Further, for example, in the conventional electrode structure in which an active material is formed on a current collector of a metal foil and laminated via a separator, when an attempt is made to increase the energy density, the coating amount and density of the electrode mixture on the current collector foil are increased. Must be increased, which may cause adverse effects such as a decrease in ionic conductivity. On the other hand, in the secondary battery 10 of the present disclosure, the conduction distance of ions can be further shortened by adopting a structure in which carbon fiber electrodes are bundled. Further, in the secondary battery 10 of the present disclosure, it is not necessary to provide a foil-shaped current collector inside the structure, and further, the space occupancy rate by the active material is increased by changing the separator or the like to a separation film to make it thinner. Can be further enhanced. Therefore, the energy density can be further increased.

It should be noted that the present disclosure is not limited to the above-described embodiment, and it goes without saying that the present disclosure can be carried out in various embodiments as long as it belongs to the technical scope of the present disclosure.

For example, in the above-described embodiment, in the secondary battery 10, the negative electrode and the positive electrode have been described as having no collecting wire, but the present invention is not particularly limited to this, and each electrode may have a collecting wire embedded therein. ..

Further, in the above-described embodiment, the carrier of the secondary battery is lithium ion, but the carrier is not particularly limited to this, and as an alkali metal ion such as sodium ion or potassium ion, or as a group 2 element ion such as calcium ion or magnesium ion. May be good. Further, the positive electrode active material may contain carrier ions. Further, although the electrolytic solution is a non-aqueous electrolytic solution, an aqueous solution-based electrolytic solution may be used.

In the above-described embodiment, the example in which the carbon fiber has a cylindrical shape has been described, but the carbon fiber is not particularly limited to this, and may have a shape such as a quadrangular column or a hexagonal column.

Hereinafter, an example in which the above-mentioned secondary battery is specifically manufactured will be described as an example. First, the result of considering the structure of the secondary battery will be described as an experimental example.

FIG. 4 is a relationship diagram obtained by calculation of the diameter and thickness of the positive and negative electrode mixture, the positive and negative electrode facing areas, and the cell energy density in the carbon fiber binding structure and the electrode foil laminated structure in the secondary battery 10. FIG. 5 is a diagram showing the relationship between the cell energy density and the electrode facing area with respect to the negative electrode fiber diameter or the film thickness of the negative electrode mixture in the carbon fiber binding structure and the electrode foil laminated structure. Experimental Examples 1 to 3 were calculated using the diameter A of the negative electrode, the thickness X of the positive electrode, the thickness t of the separation film, and the like shown in FIG. In Experimental Example 6, a conventional electrode having a laminated structure was used as a model, and in Experimental Example 4, a high-energy type electrode obtained by thickening the electrode mixture of the conventional electrode was used. In addition, the positive and negative electrode capacity ratio is set to 1.0 because the Li movement distance is short in the bound structure (Experimental Examples 1 to 3), and the excess negative electrode is unnecessary, and the conventional laminated structure (Experimental Examples 4 to 6) is the same as before. It was set to 1.2. The cell efficiency was calculated as 70% for the laminated structure because the Li movement distance is long, and 80% for the bound structure. As shown in FIGS. 4 and 5, when the binding structure of the secondary battery 10 is adopted, the diameter of the negative electrode is 20 to 100 μm, the thickness of the positive electrode is 5 to 15 μm, and the cell energy density is 650 Wh / L or more. It was found that the facing area of the positive and negative electrodes can be 100 cm ² or more. As described above, the electrode structure shown in FIG. 1 uses a positive electrode active material, a negative electrode active material, and an organic electrolytic solution used for a Li battery, and has an energy density of 600 Wh / L suitable for an EV vehicle. It was found that the body integration rate of the electrode mixture can be improved to about 88%). On the other hand, it was found that it is difficult for the conventional laminated structure to have an electrode facing area of 100 cm ² or more and an energy density of 700 Wh / L or more. Further, it was found that it is difficult to improve the output characteristics and obtain a high energy density because the moving distance of the carrier ions becomes long when the battery capacity is further increased.

Next, a carbon fiber suitable for a secondary battery having a binding structure in which a positive electrode mixture was formed on the outer periphery of the negative electrode was specifically examined. The performance of the secondary battery is generally evaluated by a full cell in which a positive electrode and a negative electrode are combined, but the battery performance of the full cell is affected by the finished state of the positive electrode and the negative electrode. Here, the characteristics of the carbon fiber were evaluated in detail by using a half cell (see FIG. 6) in which the counter electrode was a Li metal.

[Example 1]
The graphitizable carbon fiber obtained by ejecting a graphitizable pitch material, which is a raw material for graphitization, from a nozzle having a diameter of 50 μm and spinning it, and then treating it at a high temperature of 2000 ° C. for 5 minutes in an inert atmosphere (Ar). Was used as the negative electrode active material. The carbon fiber had a diameter of 14 μm, a length of 32 mm, and a pore ratio of 5% by volume measured by nano-CT using synchrotron radiation. This carbon fiber was attached to an evaluation cell (half cell) for measuring carbon fiber, and the charge / discharge characteristics were measured. 6A and 6B are explanatory views of the evaluation cell 30, FIG. 6A is a side view of the evaluation cell 30, and FIG. 6B is a sectional view taken along the line AA of FIG. 6A. The evaluation cell 30 includes a support current collector 32, a counter electrode 33, a current collector 34, a separator 35, a holding member 36, an electrolytic solution 37, and a housing member 38. The carbon fiber 31 is a negative electrode active material to be measured. The support current collector 32 is a conductive member that grips one end of the carbon fiber 31, and is formed of a thin metal plate. The support current collector 32 is also provided on the other end side of the carbon fiber 31, and the electric resistance of the carbon fiber 31 can also be measured. A tab made of Ni is arranged at the tip of the support current collector 32. The counter electrode 33 was a Li metal having a width of 21 mm. The current collector 34 is connected to the counter electrode 33, and a tab made of Ni is arranged at the tip thereof. The separator 35 had a thickness of 15 μm and a porosity of 50% by volume, and was made of polyethylene for high-rate charging / discharging. The holding member 36 is a member made of polyetheretherketone resin having a thickness of 8 mm. The holding member 36 sandwiches and fixes the support current collector 32, the separator 35, and the counter electrode 33 on both side surfaces of the carbon fiber 31. The electrolytic solution 37 was prepared by dissolving LiPF ₆ at a concentration of 1.0 M in a mixed solvent in which ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate were mixed at a volume ratio of 30/40/30. The accommodating member 38 is a sealed bag made of Al laminate. The carbon fiber 31 to be measured was heat-treated at 450 ° C. for 2 hours in an air atmosphere, and after removing the oil and fat on the surface, about 2 mg (about 50 fibers) cut into 32 mm lengths was used.

[Examples 2 and 3]
Those obtained through the same steps as in Example 1 except that the high temperature treatment was performed at 1000 ° C. were used as the non-graphitizable carbon fibers of Example 2. The non-graphitizable carbon fiber of Example 2 had a diameter of 12 μm and a porosity of 3% by volume. The evaluation cell prepared in the same manner as in Example 1 was used as the evaluation cell of Example 2 except that the non-graphitizable carbon fiber was used. Example 3 was an evaluation cell prepared in the same manner as in Example 2 except that 5% of the outer surface of the non-graphitizable carbon fiber of Example 2 was vapor-deposited with Au as a conductive component.

[Comparative Examples 1 to 4]
Same as Example 1 except that carbon fiber (XN-60-60S manufactured by Nippon Graphite Fiber Corporation) having a diameter of 10 μm obtained by spinning the graphitizable pitch material and then treating it at high temperature in an inert atmosphere was used. The evaluation cell prepared in 1 was designated as Comparative Example 1. Same as Example 1 except that carbon fiber (XN-60-90S manufactured by Nippon Graphite Fiber Corporation) having a diameter of 11 μm obtained by spinning the graphitizable pitch material and then treating it at high temperature in an inert atmosphere was used. The evaluation cell prepared in 1 was designated as Comparative Example 2. An evaluation cell produced in the same manner as in Example 1 except that a carbon fiber (K63712 manufactured by Mitsubishi Chemical Corporation) having a diameter of 10 μm obtained by spinning a graphitizable pitch material and then treating it at a high temperature in an inert atmosphere was used. Was referred to as Comparative Example 3. An evaluation cell produced in the same manner as in Example 1 except that a carbon fiber (K13916 manufactured by Mitsubishi Chemical Corporation) having a diameter of 11 μm obtained by spinning a graphitizable pitch material and then treating it at a high temperature in an inert atmosphere was used. Was referred to as Comparative Example 4.

(Measurement of volume change)
Using the above evaluation cell, fully charged carbon fiber and completely discharged carbon fiber are taken out from the evaluation cell in an air-unexposed state, and a high-resolution scanning electron microscope (SEM: manufactured by Hitachi High-Technology Co., Ltd.) is taken out in the air-unexposed state. The diameter and length were observed by S-5500), and the volume Vc of the carbon fiber at the time of full charge and the volume Vd of the carbon fiber at the time of complete discharge were obtained. Using the volumes Vc and Vd, the volume change rate (%) was obtained from the formula (Vc-Vd) / Vd × 100. The number of carbon fibers measured was 10, and the volume change rate was the average value.

(Measurement of carbon fiber porosity)
For the porosity, a three-dimensional cross-sectional image is constructed using nano CT in SPring-8, and the volume Pa of the carbon region and the volume Pp of the pore region are obtained from this three-dimensional image, and the porosity (%). It was calculated from the formula of = Pp / (Pa + Pp) × 100.

(Charge / discharge evaluation)
Using the evaluation cell produced above, under a temperature environment of 25 ° C., the process of charging (reducing) to 1.5 V with CC and the process of discharging (oxidizing) to 0.01 V with CC CV are performed at various rates. , C / 4 rate average charge voltage is calculated, and the rate characteristic is the ratio of the charge capacity at 60C rate to the charge capacity at C / 4 rate (60C rate charge capacity / (C / 4 rate charge capacity) ×. 100%) was calculated. Further, charging and discharging were repeated 30 times at a 1C rate, and the cycle retention rate was calculated from the formula (capacity of the 30th cycle) / (capacity of the 1st cycle) × 100%.

(Results and discussion)
Carbon fiber raw material, diameter (μm), average charge voltage (V) in C / 4 rate charging, discharge capacity (mAh / g) at 1C rate, rate characteristics (%), cycle maintenance rate (%), volume change The rates (%) are summarized in Table 1. Although the range of suitable fiber diameters was grasped from the above-mentioned experimental examples, in this example, measurements were made with the fiber diameters of Examples 1 to 3 above, and the influence of charge and discharge depending on the carbon fiber material was examined. did. As shown in Table 1, it was found that the discharge capacity, rate characteristics, cycle maintenance rate, etc. differed depending on the type of carbon fiber. For example, in a secondary battery for an electric vehicle (EV), it is desired that the rate characteristic is high (quick chargeability) and the cycle maintenance rate is high (high durability). In Examples 1 to 3, the rate characteristic showed 80% or more, which is a practical range, and the cycle maintenance rate also showed 70% or more, which is a practical range. Further, it was found that the non-graphitizable carbon fibers having a volume change rate of 1.0% or less are carbon fibers having high rate characteristics and cycle maintenance rate. Further, it was presumed that the carbon fiber having a porosity of 6.0% by volume or less was within the practical range of the rate characteristics and the cycle maintenance rate.

Further, as shown in Example 3, it was found that when a metal as a conductive component is formed in a film shape on a part of the outer surface of the carbon fiber, the rate characteristics and the cycle maintenance rate can be further improved. It was presumed that this is because the conductivity of the non-graphitizable carbon fiber having relatively low conductivity can be further enhanced. Considering the influence of carrier ions on the occlusion and release characteristics, it was presumed that the formation of this conductive component is preferably performed at an area ratio of 10% or less with respect to the entire outer surface.

From the above results, for a secondary battery (see FIG. 1) having a binding structure in which a separation film is formed on the outer periphery of a linear negative electrode using carbon fiber as a negative electrode active material and a positive electrode mixture is filled between the negative electrodes. It was found that it is preferable to use a non-graphitizable carbon fiber having a volume change rate of 1.0% or less.

It should be noted that the present disclosure is not limited to the above-mentioned examples, and it goes without saying that the present disclosure can be carried out in various embodiments as long as it belongs to the technical scope of the present disclosure.

10 secondary battery, 11 negative electrode, 12 negative electrode current collector, 16 positive electrode, 17 positive electrode current collector, 21 separation membrane, 30 evaluation cell, 31 carbon fiber, 32 support current collector, 33 counter electrode, 34 current collector, 35 Separator, 36 holding member, 37 electrolyte, 38 accommodating member.

Claims (7)

A positive electrode with a positive electrode active material and a positive electrode
A linear negative electrode having a negative electrode active material of carbon fiber whose main component is non-graphitizable carbon having a volume change rate of 1.0% or less due to occlusion and release of ions.
A separation membrane having ionic conductivity and insulating the positive electrode and the negative electrode is provided .
The carbon fiber is a secondary battery having an average charging voltage of 0.4 V or more at a Li reference potential in C / 4 rate charging . The secondary battery according to claim 1, wherein the carbon fiber has a conductive component formed on at least a part of the surface thereof. The secondary battery according to claim 2, wherein the conductive component is a metal. The secondary battery according to any one of claims 1 to 3, wherein the carbon fiber has a porosity of 6% by volume or less. The secondary battery according to any one of claims 1 to 4, wherein the carbon fiber has a rate characteristic (%) of 80% or more, which is a ratio of a 60C rate charging capacity to a C / 4 rate charging capacity. The positive electrode is formed by an active material layer containing the positive electrode active material formed around the negative electrode.
The separation film is formed on an outer peripheral surface other than the end surface of the negative electrode.
The secondary battery according to any one of claims 1 to 5 , wherein the secondary battery has a structure in which a plurality of the negative electrodes are bundled in a state of being adjacent to the positive electrode via the separation membrane. The secondary battery according to claim 6 , wherein 50 or more of the negative electrodes are bundled.

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