JP5754606B2 - Non-aqueous electrolyte secondary battery positive electrode material, non-aqueous electrolyte secondary battery, and method for producing positive electrode material for non-aqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte secondary battery represented by a lithium ion secondary battery, a lithium secondary battery, a sodium secondary battery and the like.

  A lithium secondary battery and a lithium ion secondary battery, which are types of non-aqueous electrolyte secondary batteries, are batteries having a large charge / discharge capacity, and are mainly used as batteries for portable electronic devices. Lithium ion secondary batteries are also expected as batteries for electric vehicles. A sodium secondary battery and a sodium ion secondary battery, which are a kind of non-aqueous electrolyte secondary battery, have an advantage that they can be provided at low cost because they use easily available sodium instead of lithium. Sodium has a standard oxidation-reduction potential of 0.33 V lower than lithium and a density of about 80%, but it is considered that the entire cell can develop 70 to 80% of the performance of a lithium ion secondary battery.

  As the positive electrode active material of these non-aqueous electrolyte secondary batteries, those containing rare metals such as cobalt and nickel are generally used. However, since these metals have a small circulation amount and are expensive, in recent years, a positive electrode active material using a substance replacing these rare metals has been demanded.

  As a positive electrode active material of a lithium ion secondary battery, a technique using sulfur is known. By using sulfur as the positive electrode active material, the charge / discharge capacity of the lithium ion secondary battery can be increased. However, in a lithium ion secondary battery using elemental sulfur as the positive electrode active material, a compound of sulfur and lithium is generated during discharge. This compound of sulfur and lithium is soluble in a non-aqueous electrolyte solution (for example, ethylene carbonate, dimethyl carbonate, etc.) of a lithium ion secondary battery. For this reason, a lithium ion secondary battery using sulfur as a positive electrode active material has a problem that, when charging and discharging are repeated, it gradually deteriorates due to elution of sulfur into the electrolytic solution, and the battery capacity decreases. That is, a lithium ion secondary battery using sulfur as a positive electrode active material is inferior in cycle characteristics.

  As a result of intensive studies, the inventors of the present invention have invented a positive electrode active material obtained by heat-treating a mixture of polyacrylonitrile (PAN) and sulfur (see Patent Document 1). By using a sulfur-based positive electrode active material (hereinafter abbreviated as sulfur-modified PAN) made of PAN and sulfur as the positive electrode active material, the charge / discharge capacity of the lithium ion secondary battery is increased and the cycle characteristics are also improved. improves.

  By the way, although sulfur-modified PAN has a high charge / discharge capacity per mass, it is bulky because of its low mass density (bulk density). For this reason, in order to produce a battery having a high volumetric energy density (for example, 2 times or 3 times the conventional battery), a positive electrode mixture containing sulfur-modified PAN is coated on the current collector to a thickness of 150 μm or more. There is a need to. When the positive electrode material mixture is applied in such a thick manner, the electrode surface and the current collector surface are largely separated from each other, which may increase the electric resistance. If the electrical resistance is excessive, there is a concern about deterioration of battery characteristics.

  Therefore, a positive electrode material containing sulfur-modified PAN and having excellent conductivity is desired so that an increase in electric resistance can be suppressed even when the positive electrode mixture is applied thickly.

International Publication No. 2010/044437

  This invention is made | formed in view of the said situation, and it aims at providing the positive electrode material for nonaqueous electrolyte secondary batteries which contains sulfur modified PAN as a positive electrode active material, and is excellent in electroconductivity.

  As a result of extensive research, the inventors of the present invention have obtained sulfur-modified PAN particles having a small diameter by the presence of sulfur-modified PAN in the form of aggregates and by crushing the aggregated sulfur-modified PAN. It is possible to disperse the conductive aid in the positive electrode material by attaching the conductive aid to the surface of the particles, compared to the case where the conductive aid is directly attached to the aggregated sulfur-modified PAN. I found.

That is, the method for producing a non-aqueous electrolyte secondary battery of the present invention that solves the above-described problem is a solution for crushing a sulfur-based positive electrode active material comprising a carbon skeleton derived from polyacrylonitrile and the carbon skeleton and sulfur (S). Crushing process;
An auxiliary agent adding step of attaching a conductive auxiliary agent to the surface of the pulverized sulfur-based positive electrode active material;
A reaggregation step of reaggregating the sulfur-based positive electrode active material after the auxiliary agent addition step.

Moreover, the positive electrode material for a nonaqueous electrolyte secondary battery of the present invention obtained by such a method is a positive electrode material for a nonaqueous electrolyte secondary battery,
Using a carbon skeleton derived from polyacrylonitrile, a sulfur-based positive electrode active material comprising the carbon skeleton and sulfur (S), and a conductive additive,
In the particle size distribution of the sulfur-based positive electrode active material, 50% or more of the total particles are particles having a particle diameter of 2 μm or less.

  Moreover, the non-aqueous electrolyte secondary battery of the present invention that solves the above-described problems includes the positive electrode material for a non-aqueous electrolyte secondary battery of the present invention in a positive electrode.

  The nonaqueous electrolyte secondary battery of the present invention can be preferably used as a nonaqueous electrolyte secondary battery for vehicles.

  Hereinafter, unless otherwise specified, the positive electrode material for a nonaqueous electrolyte secondary battery of the present invention is simply abbreviated as the positive electrode material of the present invention, and the method for producing the positive electrode material for a nonaqueous electrolyte secondary battery of the present invention is simply referred to as the present invention. Abbreviated as manufacturing method.

  The sulfur-modified PAN exists in a state where a large number of sulfur-modified PAN particles are aggregated (secondary particles). For this reason, when a conductive additive is directly attached to such an aggregated sulfur-modified PAN, the conductive additive is attached only to the surface of the aggregated sulfur-modified PAN, and sulfur located at the center of the aggregate The modified PAN particles are greatly separated from the conductive assistant. For this reason, there is a problem that it is difficult to greatly improve the conductivity of the positive electrode material unless a large amount of conductive aid is added. According to the production method of the present invention, the conductive assistant is attached to the surface of each sulfur-modified PAN particle by attaching the conductive assistant to the sulfur-modified PAN after being crushed (that is, loosened into fine particles). Adhere to. For this reason, even if the sulfur-modified PAN particles are re-agglomerated, the conductive assistant is present inside the aggregate. In other words, the conductive assistant is dispersed at very small intervals in the positive electrode material, and the distance between the conductive assistant and the sulfur-modified PAN particles located inside the aggregate is small. Therefore, in the positive electrode material of the present invention obtained by such a production method of the present invention, the conductivity derived from the conductive additive is sufficiently exhibited. That is, according to the manufacturing method of the present invention, a positive electrode material having excellent conductivity can be manufactured.

  Further, as will be described later, the pulverized sulfur-modified PAN is a particle having a particle size of 50% or more of the total particle size and having a particle size of 2 μm or less. That is, the sulfur-modified PAN, which is a raw material of the positive electrode material of the present invention, has a particle size distribution in which 50% by frequency or more of the total particles are particles having a particle size of 2 μm or less. Thus, if a conductive support agent is made to adhere to small diameter sulfur modified PAN particle | grains, a conductive support agent will disperse | distribute highly in positive electrode material. For this reason, the distance from the conductive assistant to the center of the sulfur-modified PAN particles is reduced, and the distance between the conductive assistants in the positive electrode material is also reduced. If these distances are small, the conductivity of the positive electrode material inevitably improves, and an increase in electrical resistance can be suppressed even when a positive electrode mixture containing the positive electrode material is applied thickly.

It is a graph showing the result of having carried out X-ray diffraction of sulfur modified PAN sulfur modified PAN. It is a graph showing the result of the Raman spectrum analysis of sulfur-modified PAN. It is a graph. It is explanatory drawing which represents typically the reaction apparatus used with the manufacturing method of the sulfur modified PAN of an Example. It is explanatory drawing which represents typically the sulfur modification PAN used with the manufacturing method of the positive electrode material of an Example. It is explanatory drawing which represents typically the sulfur modified PAN particle | grains obtained at the crushing process in the manufacturing method of the positive electrode material of an Example. It is explanatory drawing which represents typically the water dispersion of the sulfur modified PAN particle | grains obtained at the auxiliary agent addition process in the manufacturing method of the positive electrode material of an Example, and a conductive support agent. It is explanatory drawing which represents typically the positive electrode material obtained at the re-aggregation process in the manufacturing method of the positive electrode material of an Example. It is explanatory drawing which represents typically the sulfur modification PAN used with the manufacturing method of the positive electrode material of a comparative example. It is explanatory drawing which represents typically the positive electrode material obtained with the manufacturing method of the positive electrode material of a comparative example. It is a graph and a table | surface showing the particle size distribution of the sulfur modified PAN before crushing. It is a graph and a table showing the particle size distribution of sulfur-modified PAN after crushing. It is a SEM image of sulfur-modified PAN before crushing. It is a SEM image of the positive electrode material of an Example. It is a graph showing the discharge characteristic of the nonaqueous electrolyte secondary battery of an Example. It is a graph showing the discharge characteristic of the nonaqueous electrolyte secondary battery of a comparative example.

(Positive electrode active material)
The positive electrode active material in the nonaqueous electrolyte secondary battery of the present invention is the above-described sulfur-modified PAN, which is composed of a carbon skeleton derived from PAN and sulfur (S).

The sulfur-modified PAN is the same as that disclosed in Patent Document 1 above. PAN as a material for sulfur-modified PAN is preferably in the form of powder, and preferably has a mass average molecular weight of about 10 ^{4 to} 3 × 10 ⁵ . Further, the particle size of PAN is preferably about 0.5 to 50 μm, more preferably about 1 to 10 μm, when observed with an electron microscope. If the molecular weight and particle size of PAN are within these ranges, the contact area between PAN and sulfur can be increased, and PAN and sulfur can be reacted with high reliability. For this reason, the elution of sulfur to the electrolytic solution can be more reliably suppressed.

  By using sulfur-modified PAN as the positive electrode active material of the non-aqueous electrolyte secondary battery, the high capacity inherent in sulfur can be maintained and the elution of sulfur into the electrolyte is suppressed, so that the cycle characteristics are greatly improved. . This is considered to be because sulfur does not exist as a simple substance in the sulfur-modified PAN but exists in a stable state taken into the structure of PAN. In the production method disclosed in Patent Document 1, sulfur is heat-treated together with PAN. When PAN is heated, it is considered that PAN is three-dimensionally cross-linked and closed while forming a condensed ring (mainly a six-membered ring). For this reason, it is considered that sulfur is present in the structure with the PAN that has advanced ring closure. In sulfur-modified PAN, sulfur exists in a stable state bonded to PAN, or although sulfur exists as a simple substance, it is trapped in the crosslinked structure when PAN is ring-closed by heating. It is difficult to contact, and it is considered that the reaction product is difficult to elute even if it contacts with the electrolyte. As a result, elution of sulfur into the electrolytic solution can be suppressed, and cycle characteristics can be improved.

By the way, a lithium ion secondary battery having an electrode using a single inorganic sulfur as an active material has a large initial capacity, but Li ₂ S _x soluble in the electrolyte is generated when charging and discharging are repeated. When this Li ₂ S _x elutes into the electrolytic solution, the battery performance represented by the cycle characteristics deteriorates rapidly.

Even when an organic sulfide in which sulfur is immobilized by a —C—S bond is used as an active material, Li ₂ S _x is generated and eluted into the electrolyte when the —C—S bond is cleaved. In addition, once cut, the -CS bond is hard to return. Therefore, also in this case, it is difficult to avoid deterioration of cycle characteristics.

Furthermore, even with sulfur-based active material with a fixed sulfur in the pores of the carbon material, the pores are left doorway of sulfur in order to easily contact the sulfur and the electrolyte solution, Li ₂ S _x is It elutes in the electrolyte.

  However, all lithium ion secondary batteries using sulfur-modified PAN have excellent cycle characteristics and maintain a high capacity even after repeated charge and discharge. This is considered to be an effect due to suppression of sulfur desorption from the positive electrode or the negative electrode. That is, in a lithium ion secondary battery using sulfur-modified PAN, it is considered that contact between sulfur and the electrolytic solution is suppressed.

That is, in sulfur-modified PAN, since PAN and sulfur coexist at the temperature at which the PAN ring-closing reaction occurs, sulfur is taken into the PAN cross-linked structure, and sulfur is trapped in the pores having no outlet. It is thought that there is. Therefore, direct contact between the electrolytic solution and sulfur is suppressed, and elution of Li ₂ S _x is prevented, so that a battery using sulfur-modified PAN is considered to have excellent cycle characteristics. Even if some sulfur can be in direct contact with the electrolytic solution, a stable charge / discharge capacity is maintained after the sulfur is eluted into the electrolytic solution as Li ₂ S _x during the first charge / discharge.

  Sulfur used for sulfur-modified PAN is preferably in the form of a powder, like PAN. Although it does not specifically limit about the particle size of sulfur, when classified using a sieve, those that do not pass through a sieve with a sieve opening of 40 μm and are within a size range that passes through a 150 μm sieve are preferable, It is more preferable that the mesh size does not pass through a 40 μm sieve and is within a size range that passes through a 100 μm sieve.

  The compounding ratio of the PAN powder and sulfur powder used for the sulfur-modified PAN is not particularly limited, but is preferably 1: 0.5 to 1:10 by mass ratio, and 1: 0.5 to 1: 7. More preferably, it is 1: 2 to 1: 5.

  Sulfur-modified PAN can be produced by the following method.

  A mixed raw material in which PAN powder and sulfur powder are mixed is heated (heat treatment step). What is necessary is just to mix a mixing raw material with common mixing apparatuses, such as a mortar and a ball mill. As the mixed raw material, a material obtained by simply mixing sulfur and PAN may be used. For example, the mixed raw material may be formed into a pellet shape. The mixed raw material may be composed only of PAN and sulfur, or may be blended with a general material (such as a conductive aid) that can be blended with the positive electrode active material.

  By heating the mixed raw material in the heat treatment step, sulfur contained in the mixed raw material is taken into the PAN structure. The heat treatment step may be performed in a closed system or an open system, but in order to suppress the dissipation of sulfur vapor, it is preferably performed in a closed system. The atmosphere in which the heat treatment process is performed is not particularly limited, but it is preferably performed in an atmosphere that does not prevent sulfur from being taken into PAN (for example, an atmosphere containing no hydrogen or a non-oxidizing atmosphere). For example, when hydrogen is present in the atmosphere, sulfur in the reaction system reacts with hydrogen to form hydrogen sulfide, so that sulfur in the reaction system may be lost. Further, it is considered that by performing heat treatment in a non-oxidizing atmosphere, vapor-state sulfur reacts with PAN simultaneously with the PAN ring-closing reaction, and PAN modified with sulfur is obtained. The non-oxidizing atmosphere referred to here includes a reduced pressure state in which the oxygen concentration is low enough not to cause an oxidation reaction, an inert gas atmosphere such as nitrogen or argon, a sulfur gas atmosphere, and the like.

  There is no particular limitation on the specific method for creating a non-oxidizing atmosphere in a sealed state. For example, the mixed raw material is placed in a container that is kept tight enough not to dissipate sulfur vapor, and the inside of the container is decompressed. Alternatively, heating may be performed in an inert gas atmosphere. In addition, the mixed raw material may be heated in a vacuum packaged state with a material that does not easily react with sulfur vapor (for example, an aluminum laminate film). In this case, in order to prevent the packaging material from being damaged by the generated sulfur vapor, for example, the packaged raw material is put in a pressure vessel such as an autoclave containing water and heated, and the generated steam is added from the outside of the packaging material. It is preferable to press. According to this method, since pressure is applied by water vapor from the outside of the packaging material, the packaging material is prevented from being swollen and damaged by sulfur vapor.

  What is necessary is just to set suitably the heating time of the mixed raw material in a heat processing process according to heating temperature, and it does not specifically limit it. The preferable heating temperature described above may be a temperature at which the incorporation of sulfur into the PAN proceeds and the conductive material does not deteriorate. Specifically, the heating temperature is preferably 250 to 500 ° C., more preferably 250 to 400 ° C., and further preferably 250 to 300 ° C.

  In the heat treatment step, sulfur is preferably refluxed. In this case, the mixed raw material may be heated so that a part of the mixed raw material becomes a gas and a part becomes a liquid. In other words, the temperature of the mixed raw material may be a temperature equal to or higher than the temperature at which sulfur is vaporized. Vaporization here refers to the phase change of sulfur from a liquid or solid to a gas, and may be any of boiling, evaporation, and sublimation. For reference, the melting point of α sulfur (orthogonal sulfur, which is the most stable structure near room temperature) is 112.8 ° C., the melting point of β sulfur (monoclinic sulfur) is 119.6 ° C., and γ sulfur (monoclinic sulfur). ) Has a melting point of 106.8 ° C. The boiling point of sulfur is 444.7 ° C. By the way, since the vapor pressure of sulfur is high, generation | occurrence | production of sulfur vapor | steam can also be confirmed visually when the temperature of a mixed raw material will be 150 degreeC or more. Therefore, if the temperature of the mixed raw material is 150 ° C. or higher, sulfur can be refluxed. In addition, what is necessary is just to recirculate | reflux sulfur using the reflux apparatus of a known structure, when recirculating | refluxing sulfur in a heat treatment process.

  Even when the amount of sulfur in the mixed raw material is excessive, a sufficient amount of sulfur can be taken into the PAN in the heat treatment step. For this reason, when adding sulfur excessively with respect to PAN, the bad influence by the elemental sulfur mentioned above can be suppressed by removing elemental sulfur from the to-be-processed body after a heat treatment process. Specifically, when the mixing ratio of PAN and sulfur in the mixed raw material is 1: 2 to 1:10 by mass ratio, the target object after the heat treatment step is heated at 200 ° C. to 250 ° C. while reducing the pressure. (Single element sulfur removal step) By taking a sufficient amount of sulfur into the PAN, it is possible to suppress adverse effects due to the remaining single sulfur. When the single sulfur removal step is not performed on the target object after the heat treatment step, this target object may be used as it is as the sulfur-modified PAN. Moreover, what is necessary is just to use the to-be-processed body after a single sulfur removal process as sulfur modified PAN, when performing the single-piece | unit sulfur removal process to the to-be-processed body after a heat treatment process. The time for the elemental sulfur removal step is not particularly limited, but is preferably about 1 to 6 hours.

  As a result of elemental analysis, sulfur-modified PAN contains carbon, nitrogen, and sulfur, and may contain small amounts of oxygen and hydrogen. Further, as shown in FIG. 1, as a result of X-ray diffraction of sulfur-modified PAN with CuKα rays, only a broad peak having a peak position in the vicinity of 25 ° was confirmed in the diffraction angle (2θ) range of 20-30 °. It was. For reference, X-ray diffraction was measured by X-ray diffraction using CuKα rays with a powder X-ray diffractometer (manufactured by MAC Science, model number: M06XCE). The measurement conditions were voltage: 40 kV, current: 100 mA, scan speed: 4 ° / min, sampling: 0.02 °, number of integrations: 1, measurement range: diffraction angle (2θ) 10 ° -60 °.

Furthermore, the mass loss by thermogravimetric analysis when sulfur-modified PAN is heated from room temperature to 900 ° C. at a rate of temperature increase of 20 ° C./min is 10% or less at 400 ° C. On the other hand, when a mixture of sulfur powder and PAN powder is heated under the same conditions, a mass decrease is observed from around 120 ° C., and when the temperature is 200 ° C. or higher, a large mass loss due to the disappearance of sulfur is recognized.
That is, in the sulfur-modified PAN, it is considered that sulfur does not exist as a simple substance but is present in a state of being taken into the PAN that has progressed in the ring closure.

An example of the Raman spectrum of sulfur-modified PAN is shown in FIG. In the Raman spectrum shown in FIG. 2, there are major peak near 1331cm ^-1 of Raman shift, ^{and, 1548cm} -1 in the range of ^{200cm -1 ~1800cm -1, 939cm -1,} 479cm -1, 381cm -1, There is a peak near 317 cm ⁻¹ . The above-described Raman shift peak is observed at the same position even when the ratio of elemental sulfur to PAN is changed. Thus, these peaks characterize sulfur-modified PAN. Each of the peaks described above exists in a range of approximately ± 8 cm ⁻¹ with the above peak position as the center. In the present specification, the “main peak” refers to a peak having the maximum peak height among all peaks appearing in the Raman spectrum.

For reference, the Raman shift described above was measured with RMP-320 (excitation wavelength λ = 532 nm, grating: 1800 gr / mm, resolution: 3 cm ⁻¹ ) manufactured by JASCO Corporation. Note that the number of Raman spectrum peaks may change or the position of the peak top may be shifted depending on the wavelength of incident light or the difference in resolution. Therefore, when the Raman spectrum of the positive electrode of the present invention using sulfur-modified PAN as the positive electrode active material is measured, the same peak as the above peak, or a peak slightly different from the above peak in number and peak top position is confirmed. The

(Positive electrode material for non-aqueous electrolyte secondary battery)
The positive electrode material for a nonaqueous electrolyte secondary battery of the present invention contains sulfur-modified PAN as a positive electrode active material and a conductive additive. The sulfur-modified PAN has been described above.

  Examples of the conductive assistant include vapor grown carbon fiber (VGCF), carbon powder, carbon black (CB), acetylene black (AB), ketjen black (KB), graphite, positive electrodes such as aluminum and titanium. Examples thereof include fine metal powders stable in potential.

The particle size of the conductive assistant is preferably 0.005 μm to 1 μm, and more preferably 0.01 μm to 0.1 μm. This is because when the blending amount of the conductive assistant is the same, a smaller conductive assistant particle size can form many conductive paths in the positive electrode.
(Method for producing positive electrode material)
The manufacturing method of the present invention for manufacturing the positive electrode material described above includes a crushing step, an auxiliary agent adding step, and a reaggregation step.

[Crushing process]
The crushing step is a step of crushing the sulfur-modified PAN. As will be described later, the sulfur-modified PAN before crushing is generally present as an aggregate having a particle size (median diameter) of about 10 μm. In the crushing step, the aggregate is crushed to obtain a small-diameter sulfur-modified PAN. The crushing step may be any known method as long as the aggregated sulfur-modified PAN can be loosened. For example, a device such as a ball mill, a bead mill, an air jet mill, or a submerged collision device may be used, or mortar may be ground. As described above, the sulfur-modified PAN particles after pulverization may be “50 frequency% or more of all particles having a particle diameter of 2 μm or less”. The particle size of the sulfur-modified PAN particles can be measured by a laser diffraction scattering method particle size distribution analyzer. Furthermore, the median diameter of the sulfur-modified PAN particles measured by a laser diffraction / scattering particle size distribution analyzer is preferably 4 μm or less, and more preferably 2 μm or less. The crushing step may be performed in the atmosphere or under an inert gas atmosphere, or may be performed in a liquid medium. In order to prevent re-aggregation of the crushed sulfur-modified PAN particles, the crushing process is performed in a liquid medium. Is preferred. Further, a dispersant such as Tween 20 (polyoxyethylene sorbitan monolaurate) may be used. As the liquid medium, a known medium such as water or an organic solvent may be used.

[Auxiliary agent addition process]
The auxiliary agent adding step is a step of attaching a conductive auxiliary agent to the surface of the crushed sulfur-modified PAN. In the auxiliary agent adding step, the conductive auxiliary agent may be attached to the surface of the sulfur-modified PAN particles by a known method. For example, the sulfur-modified PAN particles may be simply mixed with the conductive auxiliary agent, or the sulfur-modified PAN may be mixed. And the conductive assistant may be bound by a binder resin or the like. In any case, in order to adhere the conductive assistant to the surface of the pulverized sulfur-modified PAN particles while preventing re-aggregation, it is preferable to carry out in a liquid medium as in the pulverization step. In addition, an auxiliary agent addition process may be performed after the crushing process mentioned above, and may be performed simultaneously with a crushing process. Further, only a part of the conductive auxiliary agent may be added during the crushing step, and the remainder may be added during the production of the positive electrode mixture described later.

[Re-aggregation process]
The reaggregation step is a step of reaggregating the sulfur-modified PAN (that is, the sulfur-modified PAN with the conductive auxiliary agent attached) after the auxiliary agent addition step described above. As described above, since the conductive auxiliary agent adheres to the surface of the sulfur-modified PAN particles in the auxiliary agent adding step, the conductive auxiliary agent exists not only on the surface but also inside the aggregate obtained in the re-aggregating step. . The particle size of the aggregate obtained in the reaggregation step is not particularly limited, but is preferably about 1 to 10 μm (median diameter). Moreover, it is preferable that the particle size of the aggregate obtained in the reaggregation step is uniform. In order to obtain an aggregate having a uniform or substantially uniform particle size, it is desirable to reaggregate the sulfur-modified PAN by a method capable of controlling the particle size of the aggregate, such as spray drying or fluidized bed granulation. In addition, when not using a method that can disintegrate in a liquid medium and simultaneously perform drying and agglomeration such as spray drying in the reaggregation step, sulfur modification after the auxiliary agent addition step is performed before the reaggregation step. A step of drying PAN is required. This drying process may be a known method according to the liquid medium, such as heat drying or vacuum drying.

(Non-aqueous electrolyte secondary battery)
Hereinafter, the configuration of the nonaqueous electrolyte secondary battery of the present invention will be described.

[Positive electrode]
The positive electrode in the nonaqueous electrolyte secondary battery of the present invention can have the same structure as a general positive electrode for a nonaqueous electrolyte secondary battery except for the positive electrode material. For example, the positive electrode in the nonaqueous electrolyte secondary battery of the present invention can be manufactured by applying a positive electrode mixture in which the above-described positive electrode material, binder, and solvent are mixed to a current collector. As another method, the positive electrode material and binder mixture described above are kneaded with a mortar or a press and formed into a film, and the film-like mixture is pressure-bonded to a current collector with a press or the like to produce a positive electrode. You can also

  As the binder, polyvinylidene difluoride (PVDF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyimide (PI), polyamideimide (PAI), carboxymethylcellulose (CMC), polychlorinated Examples include vinyl (PVC), methacrylic resin (PMA), polyacrylonitrile (PAN), modified polyphenylene oxide (PPO), polyethylene oxide (PEO), polyethylene (PE), and polypropylene (PP).

  Examples of the solvent include N-methyl-2-pyrrolidone, N, N-dimethylformaldehyde, alcohol, water and the like. These conductive assistants, binders and solvents may be used as a mixture of plural kinds. The blending amount of these materials is not particularly limited. For example, it is preferable to blend about 5 to 100 parts by mass of a conductive additive and about 5 to 20 parts by mass of a binder with respect to 100 parts by mass of sulfur-modified PAN.

  What is necessary is just to use what is generally used for the positive electrode for nonaqueous electrolyte secondary batteries as a collector. For example, current collectors include aluminum foil, aluminum mesh, punched aluminum sheet, aluminum expanded sheet, stainless steel foil, stainless steel mesh, punched stainless steel sheet, stainless steel expanded sheet, foamed nickel, nickel nonwoven fabric, copper foil, copper mesh Punching copper sheet, copper expanded sheet, titanium foil, titanium mesh, carbon paper (nonwoven fabric / woven fabric) and the like are exemplified. Among these, a carbon paper current collector made of carbon having a high degree of graphitization does not contain hydrogen and has low reactivity with sulfur, and is therefore suitable as a current collector for a positive electrode in the nonaqueous electrolyte secondary battery of the present invention. It is. As a raw material for carbon fiber having a high degree of graphitization, various pitches (that is, by-products such as petroleum, coal, coal tar, etc.), PAN fibers, etc., which are carbon fiber materials can be used.

[Negative electrode]
As the negative electrode active material for the non-aqueous electrolyte secondary battery of the present invention, an element compound capable of occluding and releasing lithium ions and having an element that can be alloyed with lithium and / or an element that can be alloyed with lithium is used. Is preferably used. Or what consists of an element compound which can occlude / release sodium ion and has an element which can be alloyed with sodium and / or an element which can be alloyed with atrium is used preferably. For example, elements that can be alloyed with lithium include Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Ti, Ag, Zn, Cd, Al, Ga, In, Si, Examples thereof include at least one selected from Ge, Sn, Pb, Sb, and Bi. Among these, it is particularly preferable to use silicon (Si) or tin (Sn). The elemental compound having an element capable of alloying with lithium is preferably a silicon compound or a tin compound. Examples of the silicon compound include SiO _x (0.5 ≦ x ≦ 1.5). Examples of the tin compound include a tin alloy (Cu—Sn alloy, Co—Sn alloy, etc.), a tin alloy (Cu—Sn alloy, Co—Sn alloy, etc.), and the like. In addition, known carbon-based materials such as metallic lithium, metallic sodium, and graphite can also be used. When metallic lithium is used for the negative electrode active material, the nonaqueous electrolyte secondary battery of the present invention is a lithium secondary battery. When metallic sodium is used for the negative electrode active material, the nonaqueous electrolyte secondary battery of the present invention is a sodium secondary battery (so-called sodium sulfur battery). When a negative electrode material that does not contain lithium, for example, a carbon-based material, a silicon-based material, an alloy-based material, etc., among the negative electrode materials described above, short-circuiting between the positive and negative electrodes due to the generation of dendrites is unlikely to occur. This is advantageous. However, when these negative electrode materials not containing lithium are used in combination with the positive electrode material of the present invention, neither the positive electrode nor the negative electrode contains lithium. For this reason, a lithium pre-doping process in which lithium is inserted in advance into either one or both of the negative electrode and the positive electrode is necessary. The lithium pre-doping method may be a known method. For example, when lithium is doped into the negative electrode, a half-cell is assembled using metallic lithium as the counter electrode, and lithium is inserted by an electrolytic doping method in which lithium is electrochemically doped, or a metallic lithium foil is attached to the electrode After that, there is a method in which lithium is inserted by a pasting pre-doping method in which it is left in an electrolytic solution and doped using diffusion of lithium to the electrode. Also, when the positive electrode is predoped with lithium, the above-described electrolytic doping method can be used.

  As the negative electrode material that does not contain lithium, a silicon-based material that is a high-capacity negative electrode material is particularly preferable, and among these, thin-film silicon that has a thin electrode thickness and is advantageous in terms of capacity per volume is more preferable.

  As the current collector for the negative electrode, aluminum foil, aluminum mesh, punched aluminum sheet, aluminum expanded sheet, stainless steel foil, stainless steel mesh, punched stainless steel sheet, stainless steel expanded sheet, nickel foam, nickel non-woven fabric, copper foil, Examples include copper mesh, punched copper sheet, copper expanded sheet, titanium foil, titanium mesh, carbon paper (nonwoven fabric / woven fabric) and the like.

〔Electrolytes〕
As the electrolyte used for the nonaqueous electrolyte secondary battery, an electrolyte obtained by dissolving an alkali metal salt as an electrolyte in an organic solvent can be used. As the organic solvent, it is preferable to use at least one selected from non-aqueous solvents such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dimethyl ether, isopropyl methyl carbonate, vinylene carbonate, γ-butyrolactone, and acetonitrile. . As the electrolyte, LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiI, LiClO ₄ or the like can be used when lithium is used as the negative electrode active material. When sodium is used as the negative electrode active material, it is selected from NaPF ₆ , NaBF ₄ , NaClO ₄ , NaAsF ₆ , NaSbF ₆ , NaCF ₃ SO ₃ , NaN (SO ₂ CF ₃ ) ₂ , lower fatty acid sodium salt, NaAlCl _{4 and the} like. One kind or a plurality of kinds can be used. Among them, LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , NaPF ₆ , NaBF ₄ , NaAsF ₆ , NaSbF ₆ , NaCF ₃ SO ₃ , NaN (SO ₂ CF ₃ ) _{2 and the} like contain fluorine (F). Therefore, it is preferably used. The concentration of the electrolyte may be about 0.5 mol / L to 1.7 mol / L. Note that the electrolyte is not limited to liquid, and may be solid (for example, a polymer gel).

[Others]
The nonaqueous electrolyte secondary battery may include a member such as a separator in addition to the above-described negative electrode, positive electrode, and electrolyte. The separator is interposed between the positive electrode and the negative electrode, allows ions to move between the positive electrode and the negative electrode, and prevents an internal short circuit between the positive electrode and the negative electrode. If the non-aqueous electrolyte secondary battery is a sealed type, the separator is also required to have a function of holding an electrolytic solution. As the separator, it is preferable to use a thin, microporous or non-woven membrane made of polyethylene, polypropylene, PAN, aramid, polyimide, cellulose, glass or the like. The shape of the nonaqueous electrolyte secondary battery is not particularly limited, and can be various shapes such as a cylindrical shape, a stacked shape, and a coin shape.
Hereinafter, the nonaqueous electrolyte secondary battery of the present invention and the manufacturing method thereof will be specifically described.

(Example)
The nonaqueous electrolyte secondary battery of an Example is a lithium ion secondary battery. The non-aqueous electrolyte secondary battery of the example was produced as follows.

<Sulfur modified PAN>
[1] Mixed raw material A sulfur powder having a particle size of 50 μm or less when classified using a sieve was prepared. A PAN powder having a particle size in the range of 0.2 μm to 2 μm when prepared with an electron microscope was prepared. 5 parts by mass of sulfur powder and 1 part by mass of PAN powder were mixed and pulverized in a mortar to obtain a mixed raw material.

[2] Apparatus As shown in FIG. 3, the reaction apparatus 1 includes a reaction vessel 2, a lid 3, a thermocouple 4, an alumina protective tube 40, two alumina tubes (gas introduction tube 5, gas discharge tube 6), argon gas It has a pipe 50, a gas tank 51 containing argon gas, a trap pipe 60, a trap tank 62 containing a sodium hydroxide aqueous solution 61, an electric furnace 7, and a temperature controller 70 connected to the electric furnace.

  As the reaction vessel 2, a bottomed cylindrical glass tube (made of quartz glass) was used. In the heat treatment step described later, the mixed raw material 9 was accommodated in the reaction vessel 2. The opening of the reaction vessel 2 was closed with a glass lid 3 having three through holes. An alumina protective tube 40 (alumina SSA-S, manufactured by Nikkato Co., Ltd.) accommodating the thermocouple 4 was attached to one of the through holes. A gas introduction pipe 5 (alumina SSA-S, manufactured by Nikkato Corporation) was attached to the other one of the through holes. A gas exhaust pipe 6 (alumina SSA-S, manufactured by Nikkato Corporation) was attached to the remaining one of the through holes. The reaction vessel 2 had an outer diameter of 60 mm, an inner diameter of 50 mm, and a length of 300 mm. The alumina protective tube 40 had an outer diameter of 4 mm, an inner diameter of 2 mm, and a length of 250 mm. The gas introduction pipe 5 and the gas discharge pipe 6 had an outer diameter of 6 mm, an inner diameter of 4 mm, and a length of 150 mm. The tips of the gas introduction pipe 5 and the gas discharge pipe 6 were exposed to the outside of the lid 3 (inside the reaction vessel 2). The length of this exposed portion was 3 mm. The tips of the gas introduction pipe 5 and the gas discharge pipe 6 become approximately 100 ° C. or less in a heat treatment process described later. For this reason, the sulfur vapor generated in the heat treatment step does not flow out of the gas introduction pipe 5 and the gas discharge pipe 6 but is returned (refluxed) to the reaction vessel 2.

  The tip of the thermocouple 4 placed in the alumina protective tube 40 indirectly measured the temperature of the mixed raw material 9 in the reaction vessel 2. The temperature measured by the thermocouple 4 was fed back to the temperature controller 70 of the electric furnace 7.

  An argon gas pipe 50 was connected to the gas introduction pipe 5. The argon gas pipe 50 was connected to a gas tank 51 containing argon gas. One end of a trap pipe 60 was connected to the gas discharge pipe 6. The other end of the trap pipe 60 was inserted into the sodium hydroxide aqueous solution 61 in the trap tank 62. The trap pipe 60 and the trap tank 62 are traps for hydrogen sulfide gas generated in a heat treatment process to be described later.

[3] Heat treatment step The reaction vessel 2 containing the mixed raw material 9 was placed in an electric furnace 7 (crucible furnace, opening width φ80 mm, heating height 100 mm). At this time, argon was introduced into the reaction vessel 2 through the gas introduction tube 5. The flow rate of argon gas at this time was 100 ml / min. Ten minutes after the start of the introduction of the argon gas, heating of the mixed raw material 9 in the reaction vessel 2 was started while continuing the introduction of the argon gas. The temperature rising rate at this time was 5 ° C./min. When the mixed raw material 9 reached 100 ° C., the introduction of argon gas was stopped while continuing to heat the mixed raw material 9. Gas was generated when the mixed raw material 9 reached about 200 ° C. The heating was stopped when the mixed raw material 9 reached 360 ° C. After stopping the heating, the temperature of the mixed raw material 9 increased to 400 ° C. and then decreased. Therefore, in this heat treatment step, the mixed raw material 9 was heated to 400 ° C. Thereafter, the mixed raw material 9 was naturally cooled, and when the mixed raw material 9 was cooled to room temperature (about 25 ° C.), the product (that is, the object to be treated after the heat treatment step) was taken out from the reaction vessel 2. The heating time at this time was about 10 minutes at 400 ° C., and sulfur was refluxed.

[4] Elemental sulfur removal step In order to remove elemental sulfur (free sulfur) remaining in the object to be treated after the heat treatment step, the following steps were performed.

  The object to be treated after the heat treatment step was pulverized in a mortar. 2 g of the pulverized product was placed in a glass tube oven and heated at 200 ° C. for 3 hours while being vacuumed. The temperature elevation temperature at this time was 10 ° C./min. By this process, the elemental sulfur remaining in the object to be treated after the heat treatment process was evaporated and removed, and a sulfur-modified PAN not including (or substantially not including) elemental sulfur was obtained.

<Positive electrode material>
[1] Crushing step As shown in FIG. 4A, the sulfur-modified PAN 80 obtained in the above-described step forms an aggregate in which primary particles 81 of the sulfur-modified PAN particles are aggregated. In the crushing step, the aggregated sulfur-modified PAN80 is dispersed in water and subjected to submerged collision crushing treatment using a Starburst Lab HJP-25005 (oblique collision chamber, 245 MPa pressure) manufactured by Sugino Machine. The aggregated sulfur-modified PAN80 collided with each other in water. As shown in FIG. 4B, through this step, sulfur-modified PAN particles 82 in which aggregated sulfur-modified PAN 80 was crushed were obtained.

[2] Auxiliary agent adding step After the crushing step was completed for one cycle, acetylene black (AB) as the conductive auxiliary agent 83 was added to the sample tank of the crushing apparatus. The compounding amount of AB was 6 parts by mass with respect to 100 parts by mass of the sulfur-modified PAN. The particle diameter of this AB was 0.02 μm (median value). As shown in FIG. 4C, after adding the conductive auxiliary agent 83, the conductive auxiliary agent 83 was attached to the surface of the sulfur-modified PAN particle 82 by repeating the crushing cycle for four cycles.

[3] Re-aggregation step The aqueous dispersion of the sulfur-modified PAN particles 82 and the conductive auxiliary agent 83 obtained in the auxiliary agent addition step was spray-dried using a spray drying apparatus MDL-050B manufactured by Fujisaki Electric. By this step, water was evaporated and an aggregate having a substantially constant particle size was obtained. This aggregate is the positive electrode material 84 of the example. As shown in FIG. 4D, in the positive electrode material 84 of the example, the conductive additive 83 is present on the surface and inside of the positive electrode material 84.

  <Lithium secondary battery>

[1] Positive electrode 80 parts by mass of the positive electrode material obtained in the above step, 20 parts by mass of polyimide (PI), and N-methyl-2-pyrrolidone (NMP) were mixed to prepare a slurry.

  On the other hand, a current collector obtained by punching carbon paper (manufactured by Toray Industries Inc .; TGP-H-030) to a diameter of φ11 mm was prepared, and after filling the slurry, dried at 200 ° C. under reduced pressure for 2 hours to produce a positive electrode. .

[2] Negative electrode As the negative electrode, a metal lithium foil punched into a diameter of φ11 mm was used.

[3] Electrolytic Solution As the electrolytic solution, a nonaqueous electrolyte in which LiPF ₆ was dissolved in propylene carbonate was used. The concentration of LiPF ₆ in the electrolytic solution was 1.0 mol / L.

[4] Battery A coin battery was manufactured using the positive electrode, the negative electrode and the electrolytic solution obtained in [1], [2] and [3] above. Specifically, in a dry room, a separator made of a polypropylene microporous membrane having a thickness of 25 μm (“Celgard 2400” manufactured by Celgard) and a glass nonwoven fabric filter having a thickness of 500 μm are sandwiched between a positive electrode and a negative electrode. Thus, an electrode body battery was obtained. This electrode body battery was accommodated in a battery case (CR2032-type coin battery member, manufactured by Hosen Co., Ltd.) made of a stainless steel container. The electrolyte solution obtained in [3] was injected into the battery case. The battery case was sealed with a caulking machine to obtain the nonaqueous electrolyte secondary battery of the example.

(Comparative example)
The positive electrode material for the nonaqueous electrolyte secondary battery of the comparative example is the same as the positive electrode material for the nonaqueous electrolyte secondary battery of the example, except that the conductive auxiliary was directly attached to the sulfur-modified PAN aggregate before crushing. Is. The positive electrode material for a non-aqueous electrolyte secondary battery of the comparative example was obtained by mixing the sulfur-modified PAN aggregate before crushing and the conductive additive in an agate mortar. As shown in FIG. 5A, the sulfur-modified PAN 80 used in the comparative example is in the form of an aggregate. When the conductive auxiliary agent 83 is directly attached to the aggregated sulfur-modified PAN 80, the conductive auxiliary agent 83 is attached only to the surface of the aggregated sulfur-modified PAN 80 as shown in FIG. 5B. The nonaqueous electrolyte secondary battery of the comparative example is the same as the nonaqueous electrolyte secondary battery of the example except for the positive electrode material.

<Particle size distribution of sulfur-modified PAN>
The particle size distribution of sulfur-modified PAN before pulverization and sulfur-modified PAN after pulverization was measured. As the sulfur-modified PAN after pulverization, the one obtained by separating the sulfur-modified PAN aqueous dispersion after the same pulverization step as that of the example from the sample tank of the pulverizer was used.

  The particle size distribution of each sulfur-modified PAN was measured by a laser diffraction scattering method particle size distribution method using a particle size distribution measuring apparatus LA-910W manufactured by Horiba. The measurement results are shown in FIGS. Specifically, FIG. 6 shows the particle size distribution of sulfur-modified PAN before crushing, and FIG. 7 shows the particle size distribution of sulfur-modified PAN after crushing.

  As shown in FIGS. 6 and 7 and Table 1, small-sized sulfur-modified PAN particles having a median diameter of 2 μm or less can be obtained from a large-sized sulfur-modified PAN aggregate having a median diameter of 10 μm or more by the crushing step. Further, sulfur-modified PAN particles having a substantially uniform particle diameter can be obtained by the crushing step. In the sulfur-modified PAN aggregate before the crushing step, particles having a particle size of 2 μm or less are less than 1 frequency% (about 0.6 frequency%), whereas in the sulfur-modified PAN particles after the crushing step, Particles having a particle size of 2 μm or less are 50% or more (about 70%) of the whole.

In the positive electrode material of the example in which the conductive auxiliary agent is adhered to the surface of such sulfur-modified PAN particles, the conductive auxiliary agent is dispersed and arranged at intervals of about the particle size of the sulfur-modified PAN particles. As described above, in the particle size distribution of the sulfur-modified PAN particles after pulverization, particles having a particle size of 2 μm or less account for about 70 frequency% of the total. For this reason, in the positive electrode material of an Example, the area | region where the conductive support agent does not exist in the range of 2 micrometers or more becomes less than 30% of the whole positive electrode material. That is, when the positive electrode material is analyzed and the region where the conductive auxiliary agent is not present in the range of 2 μm or more is less than 30% of the total positive electrode material, the positive electrode material of the example (that is, the positive electrode material of the present invention) It can be judged that there is. In other words, if the region where the adjacent conductive assistants are separated by 2 μm or more is less than 30% of the entire positive electrode material, it can be determined that the positive electrode material of the present invention.
<Observation of positive electrode material by SEM>

  The surface of the positive electrode material of Example and the sulfur-modified PAN before crushing was observed with a scanning electron microscope (SEM). The acceleration voltage at this time was 5 kV, and the magnification was 5000 times. FIG. 8 shows an SEM image of the sulfur-modified PAN before crushing, and FIG. 9 shows an SEM image of the positive electrode material of the example.

  As shown in FIGS. 8 and 9, the positive electrode material of the example has a smaller diameter than the sulfur-modified PAN before crushing. Moreover, the particle | grains of the acetylene black which is a conductive support agent can be observed on the surface of sulfur-modified PAN reduced in diameter.

<Charge / discharge characteristics>
The charge / discharge characteristics of the nonaqueous electrolyte secondary batteries of Examples and Comparative Examples were measured. In detail, charging / discharging was repeatedly performed with each nonaqueous electrolyte secondary battery. At this time, the discharge rate was 0.1 C, and the cut-off voltage was 3.0 V to 1.0 V. The temperature was 25-30 ° C. Of the charge / discharge characteristic test, the results of the discharge test in the second cycle are shown in FIGS. 10 relates to the non-aqueous electrolyte secondary battery of the example, and FIG. 11 relates to the non-aqueous electrolyte secondary battery of the comparative example.

In the nonaqueous electrolyte secondary battery of the comparative example (FIG. 11), the battery voltage immediately after the start of discharge was 2.53 V. In contrast, in the nonaqueous electrolyte secondary battery (FIG. 10) of the example, the battery voltage immediately after the start of discharge was 2.68 V. From the results of the charge / discharge characteristic measurement test shown in FIG. 10 and FIG. 11, it can be seen that the non-aqueous electrolyte secondary battery of the example has a lower internal resistance than the non-aqueous electrolyte secondary battery of the comparative example. This has shown that the positive electrode material of an Example is more excellent in electroconductivity than the positive electrode material of a comparative example. That is, when the conductive assistant is attached to the pulverized sulfur-modified PAN, the conductivity is increased even with the same amount of conductive assistant as compared to the case where the conductive assistant is attached to the sulfur-modified PAN before pulverization. It can be said that it can improve.

1: Reactor 2: Reaction vessel 3: Lid 4: Thermocouple 5: Gas introduction pipe 6: Gas discharge pipe 7: Electric furnace 80: Aggregated sulfur-modified PAN 81: Primary particles 82 of sulfur-modified PAN particles: Sulfur Modified PAN particles 83: Conductive aid 84: Positive electrode material

Claims (9)

A crushing step of crushing a sulfur-based positive electrode active material comprising a carbon skeleton derived from polyacrylonitrile and the carbon skeleton and sulfur (S);
An auxiliary agent adding step of attaching a conductive auxiliary agent to the surface of the pulverized sulfur-based positive electrode active material;
A reaggregation step of reaggregating the sulfur-based positive electrode active material after the auxiliary agent addition step;
A method for producing a positive electrode material for a non-aqueous electrolyte secondary battery.   The method for producing a positive electrode material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the crushing step is performed in a liquid medium.   The method for producing a positive electrode material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the auxiliary agent adding step is performed in a liquid medium.   The said auxiliary agent addition process is a manufacturing method of the positive electrode material for nonaqueous electrolyte secondary batteries as described in any one of Claims 1-3 performed simultaneously with the said crushing process.   The said re-aggregation process is a manufacturing method of the positive electrode material for nonaqueous electrolyte secondary batteries of Claim 3 or Claim 4 performed by spray-drying or fluidized-bed granulation. A positive electrode material manufactured by the method for manufacturing a non-aqueous electrolyte secondary battery positive electrode material according to any one of claims 1 to 5,
Using a carbon skeleton derived from polyacrylonitrile, a sulfur-based positive electrode active material comprising the carbon skeleton and sulfur (S), and a conductive additive,
A positive electrode material for a non-aqueous electrolyte secondary battery, wherein, in the particle size distribution of the sulfur-based positive electrode active material, 50 frequency% or more of the total particles are particles having a particle diameter of 2 μm or less. In the Raman spectrum of the sulfur-based positive active material, there is a main peak near 1331cm ^-1 of Raman shift, ^{and, 1548cm} -1 in the range of ^{200cm -1 ~1800cm -1, 939cm -1,} 479cm -1, The positive electrode material for a nonaqueous electrolyte secondary battery according to claim 6, wherein peaks exist in the vicinity of 381 cm ⁻¹ and 317 cm ⁻¹ , respectively.   8. The positive electrode material for a nonaqueous electrolyte secondary battery according to claim 6, wherein in the particle size distribution of the sulfur-based positive electrode active material, 70% by frequency or more of the total particles are particles having a particle diameter of 2 μm or less.   A nonaqueous electrolyte secondary battery comprising the positive electrode material for a nonaqueous electrolyte secondary battery according to any one of claims 6 to 8 in a positive electrode.

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