JP6839385B2 - How to manufacture a secondary battery - Google Patents

Description

The present invention relates to a method for manufacturing a secondary battery.

The electrodes of the secondary battery typically include a current collector and an active material layer anchored onto the current collector. For such an electrode, for example, an active material and other optional components (for example, a conductive auxiliary agent, a binder, etc.) are mixed in a solvent to prepare a paste, and the paste is applied onto a current collector. , Produced by drying and removing solvent components. Non-aqueous solvents (organic solvents such as N-methyl-2-pyrrolidone) have been widely used as solvents for preparing pastes, but in recent years, aqueous solvents (water) have been used from the viewpoint of reducing cost and environmental load. There is an increasing demand for the use of a solvent (typically water) as the main component.

Japanese Unexamined Patent Publication No. 2016-122550 Japanese Unexamined Patent Publication No. 2015-0862868 Japanese Unexamined Patent Publication No. 2014-157653 Japanese Unexamined Patent Publication No. 2014-067629

However, according to the study of the present inventor, a positive electrode prepared using an aqueous solvent tends to have a higher reaction resistance than a positive electrode prepared using, for example, a non-aqueous solvent.
As an example, when a lithium manganese-containing composite oxide is used as a positive electrode active material, when the positive electrode active material comes into contact with the water content of an aqueous solvent, the exchange reaction between ^{Li + in the positive electrode active material and H +} in the water content occurs. Occurs. ^{As a result, H +} is adsorbed on the surface of the positive electrode active material. In such a state, when the positive electrode paste is exposed to a high temperature by, for example, heating and drying, oxygen in the positive electrode active material is desorbed together with ^{H + on the surface of the positive electrode active material.} As a result, the positive electrode active material becomes oxygen-deficient, and the valence of Mn in the positive electrode active material decreases from 4+ to 3+ due to charge compensation. Therefore, in the positive electrode produced by using an aqueous solvent, the proportion of high resistance Mn3 + increases, and the reaction resistance increases.

The present invention has been made in view of the above points, and an object of the present invention is to produce a low resistance positive electrode using an aqueous solvent and to provide a low resistance secondary battery provided with the positive electrode. It is in.

According to the present invention, _{a step of compounding a conductive auxiliary agent and Li 5} FeO ₄ by a mechanochemical treatment to granulate composite particles; a lithium manganese-containing composite oxide as a positive electrode active material, the composite particles, and an aqueous solvent. And to prepare a positive electrode paste; a step of applying the positive electrode paste onto a positive electrode current collector to prepare a positive electrode; the positive electrode, the negative electrode, and an electrolytic solution are housed in a battery case. , A method for manufacturing a secondary battery including a step of constructing a battery assembly; a step of initial charging the battery assembly;

Upon charging, Li ₅ FeO ₄ undergoes the following reaction: _{2Li 5} FeO ₄ = 5Li ₂ O + Fe ₂ O ₃ ; to produce Li ₂ O. By mixing Li ₅ FeO ₄ in the positive electrode, even if the valence of Mn in the positive electrode active material is reduced to 3+ due to heating and drying, etc., it positively becomes the positive electrode active material via _{Li 2 O during charging.} Oxygen can be supplied. As a result, the regeneration of Mn4 + is promoted, and the abundance of high-resistance Mn3 + can be reduced.
Further, by compounding the conductive auxiliary agent and Li ₅ FeO ₄ before the preparation of the positive electrode paste, a hydrophilic group is imparted to the surface of the conductive auxiliary agent in advance. As a result, the dispersibility of the conductive auxiliary agent is improved in the positive electrode paste, and aggregates (lumps) of the conductive auxiliary agent are less likely to occur. As a result, the conductive auxiliary agent can be evenly arranged around the positive electrode active material, and the conductivity of the positive electrode active material can be effectively enhanced.
Combined with these effects, according to the above-mentioned production method, a secondary battery having low resistance can be realized even when a positive electrode is produced using an aqueous solvent.

As a conventional technique for producing a positive electrode using an aqueous solvent, for example, Patent Document 1 can be mentioned. Patent Document 1 describes at least of phosphoric acid monoester and phosphoric acid diester when preparing a positive electrode paste by mixing lithium manganese-containing composite oxide particles as a positive electrode active material, a conductive auxiliary agent, and an aqueous solvent. A technique for adding a dispersant consisting of one of them is disclosed.

Further, Patent Documents 2 to 4 disclose a secondary battery containing _{Li 5} FeO _{4 in the positive electrode.} However, all of these techniques differ from the present invention in the purpose, use, and combination with the active material in which _{Li 5} FeO _{4 is used, and there is no commonality between the present invention and the technical idea.}

It is a flowchart which shows the manufacturing method of the secondary battery which concerns on one Embodiment. It is a flowchart which shows the preparation method of the positive electrode paste which concerns on one Embodiment. It is a flowchart which shows the preparation method of the positive electrode paste which concerns on a reference example. It is a graph which shows the relationship between the addition ratio of Li ₅ FeO _{4 and the reaction resistivity.}

Hereinafter, preferred embodiments of the manufacturing method disclosed herein will be described. Matters other than those specifically mentioned in the present specification and necessary for carrying out the present invention can be grasped as design matters of those skilled in the art based on the prior art in the art. The present invention can be carried out based on the contents disclosed in the present specification and common general technical knowledge in the art.
In addition, when the numerical value range is described as A to B (where A and B are arbitrary numerical values) in this specification, it means A or more and B or less.

FIG. 1 is a flowchart showing a method of manufacturing a secondary battery. FIG. 2 is a flowchart showing a method for preparing a positive electrode paste. The method for manufacturing the secondary battery according to the present embodiment includes the following five steps: (step S1) granulation step of composite particles; (step S2) preparation step of positive electrode paste; (step S3) preparation step of positive electrode; (step S3) Step S4) Containing the process of constructing the battery assembly; (Step S5) the initial charging step. Hereinafter, each step will be described in detail.

<(Step S1) Granulation step of composite particles>
In this step, first, a conductive auxiliary agent and Li ₅ FeO ₄ as an additive are prepared.
As the conductive auxiliary agent, for example, a carbon material such as carbon black (typically acetylene black), activated carbon, graphite, or carbon fiber can be preferably used.
Li ₅ FeO ₄ may be purchased as a commercially available product, or may be synthesized by a conventionally known method. Li ₅ FeO ₄ has a redox potential near 4.0 V (vs. Li / Li ⁺ ), and when the positive electrode potential becomes 4.0 V (vs. Li / Li ⁺ ) by charging, the next reaction: 2 Li ₅ FeO ₄ = 5Li ₂ O + Fe ₂ O ₃ ; is generated to generate _{Li 2 O.}

In this step, the conductive auxiliary agent and Li ₅ FeO ₄ are then combined by mechanochemical treatment. For example, the conductive auxiliary agent weighed at a predetermined ratio and Li ₅ FeO ₄ are put into a mixing device to perform mechanochemical treatment. The mixing device is not particularly limited, and for example, a planetary ball mill, a bead mill, a jet mill, a dispa, a planetary mixer, a homogenizer, or the like can be preferably used. Here, the "mechanochemical treatment" refers to a treatment in which mechanical energy such as compressive force, shearing force, and frictional force is applied to a powdered material to physically (mechanically) bond the materials to each other. .. As a result, the conductive auxiliary agent and Li ₅ FeO ₄ are integrated into particles. Then, a hydrophilic group is imparted to the surface of the conductive auxiliary agent.

The mixing ratio of the conductive auxiliary agent and Li ₅ FeO ₄ is not particularly limited, but the mass of the conductive auxiliary agent is typically larger than _{the mass of Li 5} FeO _4. For example, the ratio (B / A) of the mass (B) of _{Li 5} FeO ₄ to the mass (A) of the conductive auxiliary agent may be approximately 0.04 to 0.4, preferably 0.1 to 0.14. .. As a result, the hydrophilic groups can be adhered to the surface of the conductive auxiliary agent in a well-balanced manner. As a result, the resistance of the positive electrode can be better reduced.
As described above, composite particles can be granulated.

<(Step S2) Preparation step of positive electrode paste>
In this step, first, the positive electrode active material, the composite particles granulated in step S1 above, and an aqueous solvent are prepared. Examples of the aqueous solvent include water or a mixed solvent mainly composed of water. Of these, the use of ion-exchanged water or distilled water is preferable.

As the positive electrode active material, at least a lithium manganese-containing composite oxide is prepared. Examples of the lithium manganese-containing composite oxide include a lithium manganese composite oxide having a spinel structure (LiMn ₂ O ₄ ) and a lithium nickel manganese manganese composite oxide (LiNi _x Mn _2-x O ₄ ; x is 0 <x <. 2. Preferably 0 <x <1.). From the viewpoint of high energy density, a so-called 5V class lithium manganese-containing composite oxide having an operating potential of 4.3V or more, preferably 4.5V or more based on metallic lithium is preferable.
The positive electrode active material may be composed of only a lithium manganese-containing composite oxide, or in addition to the lithium manganese-containing composite oxide, a conventionally known type of positive electrode active material may be used, for example, a lithium manganese-containing composite oxide. It may be further contained in a smaller mass, preferably in a proportion of 10% by mass or less of the total positive electrode active material.
The positive electrode active material is in the form of particles. The average particle size of the positive electrode active material (laser diffraction light scattering method 50 vol% particle diameter based on _{(D 50} particle size)) is typically 1 to 20 [mu] m, for example when there is about 3 to 10 [mu] m.

In this step, in addition to the above-mentioned positive electrode active material, composite particles, and aqueous solvent, an optional component may be further prepared if necessary. Examples of the optional component include a binder, a thickener, an acid consuming agent, a pH adjuster, and the like.

As the binder, for example, at least of acrylic acid and methacrylic acid such as polyacrylic acid (PAA), polyacrylic acid salt, polyacrylic acid ester, polymethacrylic acid, polymethacrylate, polymethacrylic acid ester and the like. Acrylic polymers containing repeating units derived from one can be preferably used. The binder may be a fluorine-based resin such as polytetrafluoroethylene or rubbers such as a styrene-butadiene copolymer. As the thickener, for example, celluloses such as carboxymethyl cellulose and hydroxypropyl methyl cellulose can be preferably used. As the acid consuming agent, for example, lithium phosphate (LPO), lithium pyrophosphate and the like can be preferably used. As the pH adjuster, for example, an acidic substance such as phosphoric acid can be preferably used.

Next, in the present embodiment, the various components prepared above are mixed as shown in the flowchart shown in FIG. That is, first, the composite particles of the conductive auxiliary agent granulated in step S1 and Li ₅ FeO ₄ are dry-mixed with the positive electrode active material as powder to prepare a mixed powder. Separately from this, a thickener is added to an aqueous solvent and mixed to prepare a homogeneous liquid (gel). Next, the liquid thickener is added to the mixed powder and mixed. A binder is added thereto, and the mixture is further mixed. This makes it possible to suitably prepare a homogeneous positive electrode paste with less aggregates (lumps) of the conductive auxiliary agent.

The ratio of the positive electrode active material to the total solid content (100% by mass) of the positive electrode paste is generally 50 to 95% by mass, for example, 80 to 90% by mass. The ratio of the conductive auxiliary agent to the total solid content of the positive electrode paste is generally 1 to 10% by mass, for example, 3 to 5% by mass. _{The ratio of Li 5} FeO ₄ to the total solid content of the positive electrode paste is generally 0.1 to 2% by mass, for example, 0.2 to 1.5% by mass, preferably 0.5 to 0.7% by mass. .. When _{the ratio of Li 5} FeO ₄ is equal to or higher than a predetermined value, the regeneration of Mn4 + is better promoted, and the high resistance Mn3 + can be more preferably reduced. Further, when _{the ratio of Li 5} FeO ₄ is not more than a predetermined value, the pH of the positive electrode paste is suppressed from being raised by the decomposition product of _{Li 5} FeO _4. As a result, the positive electrode current collector is less likely to be corroded, and the durability of the positive electrode can be improved. When a binder, a thickener, an acid consuming agent, etc. are included as optional components, the proportion of the positive electrode paste in the total solid content is approximately 0.1 to 5% by mass, for example, 1 to 3% by mass. It is good to do.

The solid content (NV) of the positive electrode paste is not particularly limited, but is typically 70% by mass or more, for example, 70 to 85% by mass in consideration of the dry removal property of the aqueous solvent. The pH of the positive electrode paste is not particularly limited, but it may be in the neutral range (for example, about 7 to 11) from the viewpoint of suppressing corrosion of the current collector. The average particle size of the positive electrode paste is not particularly limited, but from the viewpoint of achieving a homogeneous state with less aggregation, it is generally equivalent to the average particle size of the positive electrode active material, typically (the average particle size of the positive electrode active material). It is preferable that it is × 1.3) or less, for example, (average particle size of positive electrode active material × 1.25) or less.
As described above, the positive electrode paste can be prepared.

<(Step S3) Positive electrode manufacturing process>
In this step, first, a positive electrode current collector is prepared. As the positive electrode current collector, a conductive member made of a metal having good conductivity (for example, aluminum) can be preferably used.
Next, the positive electrode paste prepared in step S2 is applied onto the positive electrode current collector. The method for producing the positive electrode is the same as the conventional method and is not particularly limited. For example, first, a suitable coating device is used to apply the positive electrode paste to the surface of the positive electrode current collector. Next, the positive electrode current collector to which the positive electrode paste is attached is dried to remove the aqueous solvent from the positive electrode paste. The drying treatment can be performed by, for example, heat drying at 100 ° C. or higher or vacuum drying.
As described above, a positive electrode having a positive electrode active material layer on the positive electrode current collector can be produced.

<(Step S4) Battery assembly construction process>
In this step, the positive electrode, the negative electrode, and the electrolytic solution are housed in a battery case to construct a battery assembly. For example, first, a negative electrode is prepared. The negative electrode is the same as the conventional one and is not particularly limited. The negative electrode typically includes a negative electrode current collector and a negative electrode active material layer fixed on the negative electrode current collector. As the negative electrode current collector, a conductive material made of a metal having good conductivity (for example, copper) is suitable. The negative electrode active material layer contains a negative electrode active material. As the negative electrode active material, for example, a carbon material such as natural graphite, artificial graphite, or amorphous coated graphite is suitable.

Next, the positive electrode produced in step S3 and the negative electrode prepared above are laminated in an insulated state to produce an electrode body. For the insulation between the positive electrode and the negative electrode, for example, a resin separator such as polyethylene (PE) or polypropylene (PP) can be preferably used.

Next, an electrolytic solution is prepared. The electrolytic solution is the same as the conventional one and is not particularly limited. The electrolytic solution is typically a non-aqueous electrolytic solution containing a supporting salt and a non-aqueous solvent. The supporting salt dissociates in a non-aqueous solvent to produce a charge carrier. As the supporting salt, typically a lithium salt, for example, a fluorinated lithium salt such as _{LiPF 6} or LiBF _{4 can be preferably used.} As the non-aqueous solvent, for example, non-fluorinated or fluorinated carbonate can be preferably used. As a preferred example, cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC) and monofluoroethylene carbonate (FEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methyl-2,2,2 -Chain carbonates such as trifluoroethyl carbonate (MTFEC) can be mentioned.

Next, the prepared electrode body and the prepared electrolytic solution are housed in a battery case. As the material of the battery case, a relatively lightweight metal (for example, aluminum) is suitable.
This makes it possible to construct a battery assembly.

<(Step S5) Initial charging process>
In this step, the battery assembly constructed in step S4 is initially charged. For example, the battery assembly is charged at a predetermined charging rate (eg 0.1-1C). This step is preferably performed until the potential of the positive electrode (vs. Li / Li ⁺ ) becomes equal to or higher than the redox potential of _{Li 5 FeO 4.} In one preferred example, the battery assembly is charged until the potential of the positive electrode is 4.0 V (vs. Li / Li ^{+) or higher.} In another preferred example, the battery assembly is charged until the voltage between the positive and negative electrodes is approximately 4.1 V or higher, for example 4.3 to 5.0 V.

It should be noted that charging may be performed once, and for example, charging may be repeated twice or more with a discharge in between. Further, in this step, it is possible to hold (age) for a predetermined period while maintaining the charged state. In aging, it is preferable to hold the battery assembly for about 1 hour or more, typically 10 to 40 hours, for example 20 to 30 hours, in a high temperature range of about 40 ° C. or higher, for example 40 to 60 ° C.
As a result, the battery assembly can be initially charged.

_{As described above, in the production method of the present embodiment, the conductive auxiliary agent and Li 5} FeO ₄ are combined in step S1, that is, prior to the preparation of the positive electrode paste (step S2). This imparts a hydrophilic group to the surface of the conductive auxiliary agent. Then, the dispersibility of the conductive auxiliary agent is improved in the positive electrode paste, and agglomerates of the conductive auxiliary agent are less likely to be formed. As a result, for example, the conductive auxiliary agent is evenly arranged around the positive electrode active material without using a separate dispersant, and the conductivity of the positive electrode active material is effectively enhanced.
Further, in the production method of the present embodiment, _{Li 2} O is produced from Li ₅ FeO _{4 in step S5.} As a result, _{oxygen derived from Li 5} FeO ₄ (typically, an ionic form such as an oxide ion or a peroxide ion) is supplied to the positive electrode active material, and the regeneration of Mn4 + is promoted. As a result, the abundance of Mn3 + in the positive electrode active material is reduced.
Combined with these effects, according to the manufacturing method disclosed herein, a secondary battery having low resistance can be realized even when a positive electrode is manufactured using an aqueous solvent.

Hereinafter, test examples relating to the present invention will be described, but the present invention is not intended to be limited to those shown in such specific examples.

(Examples 1 to 7)
In the examples, a total of 7 types of positive electrode pastes having different amounts of _{Li 5} FeO _{4 added were prepared according to the flowchart of FIG.}
That is, first, acetylene black (AB) as a conductive auxiliary agent and Li ₅ FeO ₄ are added to a planetary ball mill, and a zirconia ball having a diameter of 5 mm is used as a pulverizing medium and treated with mechanochemical treatment at a rotation speed of 300 rpm for 1 hour. Was done. As a result, composite particles were obtained.
_{Next, powdered LiNi 0.5} Mn _1.5 O ₄ (spinnel-structured lithium nickel-manganese composite oxide, average particle size 5 μm) as a positive electrode active material, the above composite particles, and phosphorus as an acid consuming agent. Lithium acid (LPO) was dry-mixed in a dairy pot for 5 minutes to prepare a mixed powder.
Separately, carboxymethyl cellulose (CMC) as a thickener was added to ion-exchanged water and mixed at a rotation speed of 4000 rpm for 30 minutes using a dispa to prepare a liquid.
Next, the liquid thickener was added to the mixed powder so that the final solid content (NV) was 70% or more, and the mixture was mixed at a rotation speed of 4000 rpm for 30 minutes using a dispa.
Polyacrylic acid (PAA: viaduct water-absorbent resin particles, pH = 10.5) as a binder was added thereto, and the mixture was further mixed for 5 minutes using a stirring rod.
Finally, phosphoric acid was added to adjust the pH of the paste to the neutral range (7-11).

The solid content of the positive electrode paste prepared above _{, except for Li 5} FeO _{4 , was LiNi 0.5} Mn _1.5 O ₄ : AB: PAA: LPO = 90: 5: 2.2 in terms of mass ratio. : 2.8. On the other hand, _{the proportions of Li 5} FeO ₄ are 0.2% by mass, 0.4% by mass, 0.6% by mass, and 0.8% by mass, respectively, when the total solid content of the positive electrode paste is 100% by mass. , 1.0% by mass, 1.5% by mass, and 2.0% by mass.

Next, the positive electrode paste prepared above was applied to the surface of an aluminum foil (positive electrode current collector) using a comma coater, and then heated and dried at 140 ° C. for 30 seconds to prepare a positive electrode.
Further, a negative electrode (commercially available) containing natural graphite as a negative electrode active material and the above-mentioned positive electrode were laminated via a resin separator to prepare an electrode body. _{Further, as a non-aqueous electrolytic solution, a solution prepared by dissolving LiPF 6} as a lithium salt in a mixed solvent containing a cyclic carbonate and a chain carbonate so as to have a concentration of 1 mol / L was prepared.
Then, the electrode body and the non-aqueous electrolyte solution were housed in a laminated battery case to construct a total of seven types of lithium ion secondary battery assemblies.

Next, the lithium ion secondary battery assembly was charged and discharged under a temperature environment of 25 ° C. at a voltage of 0 to 4.75 V and a charge / discharge rate of 0.3 C. Next, the lithium ion secondary battery assembly was adjusted to a state of 100% SOC, and high-temperature aging was performed at 60 ° C. for 20 hours. As a result, lithium ion secondary batteries (Examples 1 to 7) were produced.

(Reference Examples 1 to 7, Comparative Example)
In the reference example, a total of 7 types of positive electrode pastes having different amounts of _{Li 5} FeO _{4 added were prepared according to the flowchart of FIG.} That is, without compounding the conductive auxiliary agent and Li ₅ FeO _4, and once dry mixing the positive electrode active material and conductive additive and Li ₅ FeO _4, except that a mixed powder was prepared in Example 1 The positive electrode pastes of Reference Examples 1 to 7 were prepared in the same manner as in 1 to 7. Then, a lithium ion secondary battery (Reference Examples 1 to 7) was produced in the same manner as in Examples 1 to 7.
At the same time, a lithium ion secondary battery (comparative example) containing no _{Li 5} FeO _{4 was produced for comparison.}

<Measurement of particle size distribution of positive electrode paste>
50 vol% particle diameter based on laser diffraction light scattering method of the positive electrode paste prepared above to (D ₅₀ particle size) were measured. The results are shown in Table 1.

<Resistance measurement>
The AC impedance of the lithium ion secondary battery produced above was measured at 25 ° C. The diameter of the semicircle was read from the Nyquist plot of the obtained impedance and used as the reaction resistance (Ω).
Then, _{the reaction resistance of the comparative example not containing Li 5} FeO ₄ was used as the reference (1), and the relative ratio (reaction resistance ratio) to the reference was calculated for each example. The results are shown in Table 1 and FIG.

As shown in Table 1, the conductive additive and _Li 5 FeO ₄ and the complexed with Examples 1-7 of the positive electrode paste, Reference Examples 4 and 6 Ya the conductive auxiliary agent and _Li 5 FeO ₄ were not complexed , Li ₅ FeO ₄ was not included in the positive electrode paste of the comparative example, and the average particle size of the positive electrode paste was relatively small. It is considered that this is an effect that the aggregation of the conductive auxiliary agent is reduced by combining the _{conductive auxiliary agent and Li 5 FeO 4 to form a positive electrode paste.}

As shown in Table 1 and FIG. 4, in _{Examples 1 to 7 and Reference Examples 2 to 7 in which Li 5} FeO ₄ was contained in the positive electrode, the reaction resistance was relatively reduced as compared with the comparative example. It is considered that this is an effect that _{oxygen derived from Li 5} FeO ₄ is supplied to the positive electrode active material at the time of initial charging, and the high resistance Mn3 + is restored to the low resistance Mn4 +.
In Example 1-7 a conductive aid and the _Li 5 FeO ₄ complexed, the conductive auxiliary agent and _Li 5 FeO ₄ as compared to Reference Examples 1 to 7 did not complexed, relatively reaction resistance Was reduced. This is considered to be the effect of reducing the aggregation of the conductive auxiliary agent.
_{Further, in this test example, the amount of Li 5} FeO ₄ added to the total solid content in the positive electrode paste is preferably 0.2 to 2.0% by mass, and is preferably 0.6 ± 1% by mass. It was suggested that it was particularly preferable.

Although the present invention has been described in detail above, the above-described embodiments and examples are merely examples, and the inventions disclosed herein include various modifications and modifications of the above-mentioned specific examples.

S1 Composite particle granulation step S2 Positive electrode paste preparation step S3 Positive electrode fabrication step S4 Battery assembly construction step S5 Initial charging step

Claims (1)

A step of compounding a conductive auxiliary agent and Li ₅ FeO ₄ by mechanochemical treatment to granulate composite particles , wherein the mass of the conductive auxiliary agent is larger than the mass of the _{Li 5} FeO _4.
A step of preparing a positive electrode paste by mixing a lithium manganese-containing composite oxide as a positive electrode active material, the composite particles, and an aqueous solvent.
A step of applying the positive electrode paste onto a positive electrode current collector to prepare a positive electrode.
A step of accommodating the positive electrode, the negative electrode, and the electrolytic solution in a battery case to construct a battery assembly.
The process of initial charging the battery assembly,
A method for manufacturing a secondary battery, including the above.

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