JP6798371B2 - Positive electrode for lithium ion secondary battery - Google Patents

Description

The present disclosure relates to a positive electrode for a lithium ion secondary battery.

Japanese Unexamined Patent Publication No. 2010-129430 (Patent Document 1) discloses that a positive electrode containing a lithium compound containing Fe or Mn as a metal element as a constituent element as a positive electrode active material contains zeolite in the positive electrode active material. ing.

JP-A-2010-129430

In Patent Document 1, zeolite is contained in the positive electrode active material in order to suppress capacity deterioration when the lithium ion secondary battery is used at a high temperature. Zeolite is a general term for aluminosilicates having fine pores in crystals, and has a three-dimensional network structure in which a Si—O tetrahedron and an Al—O tetrahedron share an O atom at the apex. Typical zeolites include ZSM-5 type zeolite, faujasite type zeolite, mordenite type zeolite, L type zeolite, A type zeolite, X type zeolite, Y type zeolite and the like.

It is thought that the reason why zeolite is used as the material contained in the positive electrode active material is that zeolite adsorbs metal ions, which prevents metal ions from reaching the negative electrode, and as a result, suppresses deterioration of battery capacity. This is because it has been done.

In Patent Document 1, by containing zeolite in the positive electrode active material, Fe ions that elute from the positive electrode active material at high temperature and deteriorate the negative electrode are adsorbed, and the capacity of the battery when a lithium ion secondary battery is used at high temperature. Deterioration is suppressed. However, at the time of high-rate discharge, the lithium ion concentration in the vicinity of the positive electrode current collector may decrease and the battery resistance may increase. That is, there is room for improvement in reducing battery resistance during high-rate discharge. The term "high-rate discharge" as used herein means discharge at a current value that is somewhat larger than the rated capacity of the battery. For example, discharge at a current value that is 1.5 times or more that of normal discharge. To say.

An object of the present disclosure is to provide a positive electrode for a lithium ion secondary battery whose battery resistance can be reduced at the time of high-rate discharge.

Hereinafter, the technical configuration and the mechanism of action of the present disclosure will be described. However, the mechanism of action of the present disclosure includes estimation. The scope of claims should not be limited by the correctness of the mechanism of action.

The positive electrode for a lithium ion secondary battery includes a current collector, a zeolite layer arranged on the current collector, and a positive electrode mixture layer arranged on the zeolite layer. The zeolite layer contains a lithium ion-containing zeolite and a polymer containing a COONa group, and has a thickness of 0.5 μm or more and 10 μm or less.

When the lithium ion concentration on the current collector side of the positive electrode mixture layer decreases, lithium ions are released from the pores of the lithium ion-containing zeolite contained in the zeolite layer, and sodium ions derived from the polymer containing the COONa group are adsorbed. It is considered that ion exchange occurs. It is expected that the lithium ions desorbed from the lithium ion-containing zeolite are supplied to the positive electrode mixture layer, and the decrease in the lithium ion concentration in the positive electrode mixture layer is suppressed. That is, it is expected that a positive electrode for a lithium ion secondary battery capable of reducing battery resistance will be provided. In addition, since the decrease in the lithium ion concentration in the positive electrode mixture layer is suppressed, electrons are transferred between the current collector and the positive electrode mixture layer, the battery reaction proceeds, and the decrease in battery capacity is suppressed. It is also expected to be done.

FIG. 1 is a conceptual diagram showing the configuration of a positive electrode for a lithium ion secondary battery according to the embodiment of the present disclosure.

Hereinafter, embodiments of the present disclosure (hereinafter referred to as “the present embodiment”) will be described. However, the following description does not limit the scope of claims.
<Positive electrode for lithium-ion secondary battery>
FIG. 1 is a conceptual diagram showing a configuration of a positive electrode for a lithium ion secondary battery (hereinafter, also simply referred to as “positive electrode”) according to the embodiment of the present disclosure. The positive electrode 100 includes a current collector 10, a zeolite layer 20, and a positive electrode mixture layer 30. That is, the positive electrode 100 includes a current collector 10, a zeolite layer 20 arranged on the current collector 10, and a positive electrode mixture layer 30 arranged on the zeolite layer 20. The current collector 10 may be, for example, an aluminum (Al) foil or the like. The Al foil may be a pure Al foil or an Al alloy foil. The current collector 10 may have a thickness of, for example, 10 to 30 μm.
《Zeolite layer》
The zeolite layer 20 is arranged on the surface of the current collector 10. The zeolite layer 20 may be arranged on the entire surface of the current collector 10, for example, or may be arranged on a part of the surface of the current collector 10. The zeolite layer 20 may be arranged on both the front and back surfaces of the current collector 10. The zeolite layer 20 contains a lithium ion-containing zeolite and a polymer containing a COONa group. The zeolite layer 20 has a thickness of 0.5 μm or more and 10 μm or less, may have a thickness of 0.5 μm or more and 5 μm or less, or may have a thickness of 5 μm or more and 10 μm or less. However, it is desirable to have a thickness of 1 μm or more and 10 μm or less. When the thickness of the zeolite layer 20 exceeds 10 μm, it is considered that the diffusion resistance of lithium ions diffused from the zeolite layer 20 increases and the battery resistance increases. When the thickness of the zeolite layer 20 is less than 0.5 μm, it is considered that the amount of lithium ions contained in the zeolite layer 20 is insufficient and the increase in battery resistance cannot be sufficiently suppressed. The zeolite layer 20 may further contain other substances such as a conductive material and a binder as long as it contains a lithium ion-containing zeolite and a polymer containing a COONa group. The conductive material may be, for example, acetylene black (AB), furnace black, vapor-grown carbon fiber (VGCF), graphite or the like. The binder may be, for example, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), or the like.

The zeolite layer 20 is formed by using, for example, a slurry for forming the zeolite layer 20 (hereinafter, referred to as “zeolite slurry”). That is, a coating film is formed by applying the zeolite slurry to the surface of the current collector 10, and the zeolite layer 20 is formed by drying the coating film. Zeolite slurry can be prepared, for example, by mixing a lithium ion-containing zeolite, a polymer containing a COONa group, and the above other substances with a solvent. The solvent may be, for example, water or the like.
(Lithium ion-containing zeolite)
The lithium ion-containing zeolite is a zeolite in which at least a part of the metal ions contained in the metal ion-containing zeolite is replaced with lithium ions. For example, when a Na ion-containing zeolite is immersed in a lithium salt aqueous solution, Na ions are exchanged for lithium ions. As a result, a lithium ion-containing zeolite is produced. As long as at least a portion of the Na ions have been exchanged for lithium ions, it is considered to be a lithium ion containing zeolite. By using the lithium ion-containing zeolite having a high lithium ion content in the zeolite layer 20, the amount of the lithium ion-containing zeolite contained in the zeolite layer 20 can be reduced. It is considered that this can suppress the increase in battery resistance. It is desirable that, for example, 50 to 100 mL of the metal ions in the zeolite are exchanged for lithium ions. The lithium salt aqueous solution may be, for example, a LiCl aqueous solution or the like.

It is considered that the lithium ion-containing zeolite adsorbs sodium ions caused by a polymer containing a COONa group contained in the zeolite layer 20 and releases lithium ions instead when the lithium ion concentration in the vicinity of the current collector 10 decreases. Be done. Since the lithium ions released from the lithium ion-containing zeolite are supplied to the positive electrode mixture layer 30, the decrease in the lithium ion concentration on the current collector 10 side of the positive electrode mixture layer 30 is suppressed, and the increase in battery resistance is suppressed. It is expected that In addition, since the decrease in the lithium ion concentration of the positive electrode mixture layer 30 is suppressed, electrons are exchanged between the current collector 10 and the positive electrode mixture layer 30, the battery reaction proceeds, and the battery capacity decreases. Is also expected to be suppressed.

The content of the lithium ion-containing zeolite is preferably 10 parts by weight or more and 60 parts by weight or less when the zeolite slurry is 100 parts by weight. When the content of lithium ion-containing zeolite in the zeolite slurry is less than 10 parts by weight, it is considered that the amount of lithium ion contained in the zeolite layer 20 is insufficient, and the content of lithium ion-containing zeolite in the zeolite slurry is 60 weight by weight. If it exceeds the portion, it is considered that the coatability deteriorates and the battery resistance increases. The zeolite layer 20 is considered to have the same composition as that obtained by removing the solvent from the zeolite slurry.

The lithium ion-containing zeolite may have, for example, an average particle size of 0.1 to 10 μm (typically 0.5 to 2 μm). The "average particle size" in the present specification indicates a cumulative 50% particle size from the fine particle side in a volume-based particle size distribution measured by a laser diffraction / scattering method. The lithium ion-containing zeolite may have, for example, an average pore diameter of 0.1 to 10 nm (typically 0.1 to 1 nm). The average pore size can be measured by the gas adsorption method. For example, nitrogen gas is used as the medium of the gas adsorption method.

The zeolite serving as the skeleton of the lithium ion-containing zeolite contained in the zeolite layer 20 may be a natural zeolite or a synthetic zeolite. Examples of the zeolite include A-type zeolite, X-type zeolite, Y-type zeolite, L-type zeolite, ZSM-5, and mordenite. However, A-type zeolite or X-type zeolite is preferable, and A-type zeolite is used. It is even more desirable to have.
(Polymer containing COONa group)
Examples of the polymer containing a COONa group contained in the zeolite layer 20 include sodium carboxymethyl cellulose (CMC-Na) and sodium polyacrylate (PAA-Na). The polymers containing these COONa groups may be used alone or in combination of two or more. The "COONa group" in the present specification is one in which H contained in the carboxyl group (-COOH) is replaced with Na by, for example, a neutralization treatment.

The content of the polymer containing the COONa group is not particularly limited as long as a coating film of the zeolite slurry can be formed on the surface of the current collector 10, and when the zeolite slurry is 100 parts by weight, for example, 0.5. It may be more than 20 parts by weight and less than 2.0 parts by weight, less than 0.5 parts by weight, and more than 2.0 parts by weight.
<< Positive electrode mixture layer >>
The positive electrode mixture layer 30 contains a positive electrode active material, a conductive material, and a binder. The positive electrode mixture layer 30 is, for example, a positive electrode active material of 80 parts by weight or more and 98 parts by weight or less and a conductive material of 1 part by weight or more and 15 parts by weight or less and 1 part by weight when the positive electrode mixture layer 30 is 100 parts by weight. A binder of 5 parts by weight or less may be included. The positive electrode mixture layer 30 preferably has a thickness of 20 μm or more and 100 μm or less, and more preferably 60 μm or more and 80 μm or less.
(Positive electrode active material, conductive material and binder)
The positive electrode active material, the conductive material and the binder should not be particularly limited. The positive electrode active material may be, for example, LiCoO ₂ , LiNiO ₂ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (NCM), LiMnO ₂ , LiMn ₂ O ₄ , LiFePO _4, or the like. The conductive material may be, for example, acetylene black (AB), furnace black, vapor-grown carbon fiber (VGCF), graphite or the like. The binder may be, for example, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), or the like.
<Negative electrode for lithium ion secondary battery>
The negative electrode for a lithium ion secondary battery (hereinafter, also simply referred to as “negative electrode”) includes a negative electrode mixture layer and a negative electrode current collector. The negative electrode current collector may be, for example, copper (Cu) foil or the like. The negative electrode current collector may have a thickness of, for example, about 5 to 20 μm. The negative electrode mixture layer is formed on the surface of the negative electrode current collector. The negative electrode mixture layer may have a thickness of, for example, about 10 to 150 μm. The negative electrode mixture layer contains a negative electrode active material, a binder material, and the like. The negative electrode mixture layer contains, for example, 95 to 99% by mass of the negative electrode active material and 1 to 5% by mass of the binder.

Negative electrode active materials and binders should not be particularly limited. The negative electrode active material may be, for example, graphite, graphitizable carbon, non-graphitizable carbon, silicon, silicon oxide, tin, tin oxide or the like. The binder may be, for example, carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), or the like.
<Separator>
The separator is an electrically insulating porous membrane. The separator electrically separates the positive electrode and the negative electrode. The separator may have a thickness of, for example, 5 to 30 μm. The separator 30 may be made of, for example, a porous polyethylene (PE) film, a porous polypropylene (PP) film, or the like. The separator may include a multi-layer structure. For example, the separator may be composed of a porous PP film, a porous PE film, and a porous PP film laminated in this order. The separator may include a heat-resistant layer on its surface. The heat-resistant layer contains a heat-resistant material. Examples of the heat-resistant material include metal oxide particles such as alumina and a refractory resin such as polyimide.
<Electrolytic solution>
The electrolyte contains a lithium salt and a solvent. The solvent is aprotic. The solvent may be, for example, a mixture of cyclic carbonate and chain carbonate. The mixing ratio of the cyclic carbonate and the chain carbonate may be a volume ratio, for example, cyclic carbonate: chain carbonate = 1: 9 to 5: 5. The cyclic carbonate may be, for example, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) or the like. The chain carbonate may be, for example, dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC) or the like. The electrolytic solution contains, for example, 0.5 to 2.0 mol / L lithium salt. The lithium salt may be, for example, LiPF ₆ , LiBF ₄ , Li [N (FSO ₂ ) ₂ ], Li [N (CF ₃ SO ₂ ) ₂ ], or the like.
<Battery case>
The battery case may be, for example, square (flat rectangular parallelepiped) or cylindrical. For example, a metal such as aluminum (Al) or Al alloy constitutes a battery case. However, as long as the battery case has a predetermined airtightness, for example, a composite material of metal and resin may form the battery case. Examples of the metal and resin composite material include an aluminum laminated film and the like. The battery case may include an external terminal, a liquid injection hole, a gas discharge valve, a current cutoff mechanism (CID), and the like.
<Lithium-ion secondary battery>
The lithium ion secondary battery includes an electrode body having a positive electrode, a negative electrode, and a separator arranged between the positive electrode and the negative electrode, and the electrode body is arranged in a battery case together with an electrolytic solution. The electrode body is configured by winding or laminating a positive electrode and a negative electrode with a separator interposed therebetween.
<Use>
The lithium ion secondary battery including the positive electrode 100 is expected to reduce the battery resistance at the time of high-rate discharge. Examples of applications of the lithium ion secondary battery provided with the positive electrode 100 include a power source for a hybrid vehicle (HV), a plug-in hybrid vehicle (PHV), an electric vehicle (EV), and the like.

However, the use of the lithium ion secondary battery provided with the positive electrode 100 should not be limited to such in-vehicle use. The lithium ion secondary battery provided with the positive electrode 100 can be applied to all applications.

Hereinafter, examples of the present disclosure will be described. However, the following examples do not limit the scope of claims.
<Example 1>
1. 1. Preparation of Lithium Ion-Containing Zeolite The following materials were prepared.

Zeolite: A-type zeolite containing Na ions (particle size: about 1 μm, containing Na ⁺ ions as cations for ion exchange, ion exchange capacity: about 5.5 meq / g)
Lithium salt aqueous solution: LiCl aqueous solution (lithium salt concentration: 5.0 mol / l)
The Na ion-containing A-type zeolite is adjusted by adjusting the ratio of the Na ion-containing A-type zeolite to the LiCl aqueous solution so that 90 mol% of the Na ions in the Na ion-containing A-type zeolite is exchanged with lithium ions. A lithium ion-containing zeolite was prepared by immersing the contained A-type zeolite in an aqueous lithium salt solution.
2. 2. Formation of zeolite layer The following materials were prepared.

Zeolite: Lithium ion-containing zeolite obtained above Conductive material: AB
Polymer containing COONa group: CMC-Na
Binder: PTFE
Solvent: Water A planetary mixer mixed lithium ion-containing zeolite, AB, CMC-Na, PTFE and water. As a result, a zeolite slurry was prepared. The composition of the zeolite slurry was 80 parts by weight of lithium ion-containing zeolite, 10 parts by weight of AB, 1 part by weight of CMC-Na, 1 part by weight of PTFE, and 8 parts by weight of water when the zeolite slurry was 100 parts by weight. The prepared zeolite slurry was applied to both sides of the Al foil as a current collector, and then the applied zeolite slurry was dried to form a zeolite layer having a thickness of 5 μm on the Al foil. The zeolite layer is considered to have the same composition as that obtained by removing water from the zeolite slurry.
3. 3. Preparation of positive electrode mixture The following materials were prepared.

Positive electrode active material: NCM
Conductive material: AB
Binder: PVdF
Solvent: N-methyl-2pyrrolidone (NMP)
NCM, AB, PVdF and NMP were mixed by a planetary mixer. As a result, a paste-like positive electrode mixture (hereinafter, referred to as “positive electrode mixture paste”) was prepared. The solid content composition of the positive electrode mixture paste was 93 parts by weight of NCM, 4 parts by weight of AB, and 3 parts by weight of PVdF when the solid content of the positive electrode mixture paste was 100 parts by weight.
4. Formation of Positive Electrode for Lithium Ion Secondary Battery A positive electrode mixture paste was applied onto the zeolite layer formed on the Al foil obtained above and dried. As a result, a positive electrode mixture layer having a thickness of 60 μm was formed on the zeolite layer. From the above, a positive electrode for a lithium ion secondary battery was formed. The positive electrode mixture layer is considered to have the same composition as the solid content composition of the positive electrode mixture paste.
5. Manufacture of Lithium Ion Secondary Battery A band-shaped positive electrode, a band-shaped negative electrode, and a band-shaped separator were prepared, respectively. The positive electrode, the separator, the negative electrode, and the separator were laminated in this order so that the positive electrode and the negative electrode faced each other with the separator interposed therebetween, and further wound in a spiral shape. This formed a group of electrodes. The terminals were connected to the positive electrode and the negative electrode, respectively. The electrode group was housed in the battery case. The electrolyte was injected into the battery case. The battery case was sealed. From the above, a lithium ion secondary battery provided with a positive electrode was manufactured. Hereinafter, the lithium ion secondary battery may be abbreviated as "battery".
<Examples 2, 3 and 8>
As shown in Table 1 below, a positive electrode was manufactured in the same manner as in Example 1 except that the thickness of the zeolite layer was different, and a battery having the positive electrode was manufactured.
<Example 4>
As shown in Table 1 below, a positive electrode was manufactured in the same manner as in Example 1 except that a Na ion-containing X-type zeolite was used as the zeolite, and a battery having the positive electrode was manufactured.
<Examples 5 and 6>
As shown in Table 1 below, a positive electrode was manufactured in the same manner as in Example 1 except that the content of the polymer containing a COONa group was different, and a battery having the positive electrode was manufactured.
<Example 7>
As shown in Table 1 below, a battery in which a positive electrode is manufactured as in Example 1 and the positive electrode is provided, except that PAA-Na is used instead of CMC-Na as the polymer containing a COONa group. Was manufactured.
<Comparative example 1>
As shown in Table 1 below, a positive electrode was manufactured in the same manner as in Example 1 except that the thickness of the zeolite layer was different, and a battery having the positive electrode was manufactured.
<Comparative example 2>
As shown in Table 1 below, the zeolite layer was not formed between the current collector and the positive electrode mixture layer, and 7 parts by weight of lithium ion-containing zeolite was formed with respect to 100 parts by weight of the solid content of the positive electrode mixture paste. And 0.1 part by weight of CMC-Na were added and mixed to prepare a slurry. The slurry is applied to both sides of the Al foil, and then the applied slurry is dried to form a positive electrode mixture layer in which lithium ion-containing zeolite and CMC-Na are mixed on the Al foil to form a positive electrode. A positive electrode was manufactured in the same manner as in Example 1 except that the positive electrode was manufactured, and a battery having the positive electrode was manufactured.
<Comparative example 3>
As shown in Table 1 below, a positive electrode was manufactured in the same manner as in Example 1 except that the zeolite layer was arranged on the positive electrode mixture layer, and a battery having the positive electrode was manufactured.
<Comparative example 4>
As shown in Table 1 below, a positive electrode was manufactured in the same manner as in Example 1 except that CMC-NH ₄ , which is a polymer containing ₄ COONH groups, was used instead of the polymer containing COONa groups. A battery including the positive electrode was manufactured.
<Comparative example 5>
As shown in Table 1 below, a positive electrode is manufactured and provided with the positive electrode in the same manner as in Example 1 except that the zeolite layer is not formed and the positive electrode mixture layer is arranged on the current collector. The battery was manufactured.
<Evaluation>
1. 1. Measurement of Battery Resistance of Battery At 25 ° C, the battery was charged to 3.7V. The battery was then discharged for 10 seconds at a current value of I = 350A and a voltage drop ΔV was measured. The electrical resistance R was calculated by calculating R = ΔV / I. The results are shown in the "Electrical resistance" column of Table 1 below. The smaller the value, the lower the electrical resistance.

<Result>
As shown in Table 1 above, in the examples, the battery resistance is reduced as compared with Comparative Examples 1 to 5. In the embodiment, during high-rate discharge, lithium ions are desorbed from the skeleton of the zeolite from the pores of the lithium ion-containing zeolite, and the lithium ions are supplied to the positive electrode mixture layer in the positive electrode mixture layer on the current collector side. It is considered that the decrease in lithium ion concentration was suppressed. In addition, since the decrease in the lithium ion concentration in the positive electrode mixture layer is suppressed, electrons are transferred between the current collector and the positive electrode mixture layer, the battery reaction proceeds, and the decrease in battery capacity is suppressed. It is also expected to be done. Further, it is considered that after the desorption of lithium ions, sodium ions, which are more easily adsorbed, are adsorbed by zeolite and become electrically stable.

From the results of Examples 1 to 3, 8 and Comparative Example 1, it can be seen that the battery resistance tends to be reduced by setting the thickness of the zeolite layer to 0.5 μm or more and 10 μm or less. By setting the thickness of the zeolite layer within the above range, the zeolite layer contains a sufficient amount of lithium ions to suppress a decrease in the concentration of lithium ions in the positive electrode mixture layer on the current collector side, and diffuses from the zeolite layer 20. It is considered that the increase in the diffusion resistance of the lithium ion produced could be suppressed.

From the results of Examples 1 and 4, it can be seen that the battery resistance is further reduced by using the A-type zeolite as the zeolite species.

From the results of Example 1 and Comparative Examples 2 and 3, it can be seen that the battery resistance is reduced by arranging the zeolite layer between the current collector and the positive electrode mixture layer. When lithium ion-containing zeolite or the like is mixed in the positive electrode mixture layer as in Comparative Example 2, it is considered that the lithium ion-containing zeolite is evenly distributed in the positive electrode mixture layer. Therefore, since there are cases where the lithium ion-containing zeolite and the current collector are separated from each other, the diffusion resistance of lithium ions increases, and the decrease in the lithium ion concentration in the vicinity of the current collector can be sufficiently alleviated. It is probable that it was not. Further, when the zeolite layer is arranged on the positive electrode mixture layer as in Comparative Example 3, since the distance between the lithium ion-containing zeolite and the current collector is large, the diffusion resistance of lithium ions becomes large. It is probable that the decrease in lithium ion concentration near the current collector could not be alleviated.

From the results of Example 1 and Comparative Example 4, when a polymer containing ₄ COONH groups was used instead of the polymer containing COONa groups, the battery resistance was not sufficiently reduced. It is considered that this is because the ion exchange between ammonium ion and lithium ion was not sufficiently performed and the amount of lithium ion contained in the zeolite layer was insufficient.

From the results of Example 1 and Comparative Example 5, it can be seen that the battery resistance is reduced by arranging the zeolite layer between the current collector and the positive electrode mixture layer.

The above embodiments and examples are exemplary in all respects and are not restrictive. The technical scope defined by the scope of claims includes all changes within the meaning and scope equivalent to the scope of claims.

100 positive electrode, 10 current collector, 20 zeolite layer, 30 positive electrode mixture layer.

Claims (1)

Current collector,
A zeolite layer arranged on the current collector and a positive electrode mixture layer arranged on the zeolite layer are included.
The zeolite layer contains a lithium ion-containing zeolite and a polymer containing a COONa group.
The zeolite layer has a thickness of 0.5 μm or more and 10 μm or less.
Positive electrode for lithium-ion secondary batteries.

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