JP6939467B2 - Lithium metal rechargeable battery - Google Patents

Description

The present disclosure relates to a lithium metal secondary battery.

Japanese Unexamined Patent Publication No. 2016-207637 (Patent Document 1) discloses that lithium metal is deposited on carbon nanotubes protruding from the surface of a copper sheet.

Japanese Unexamined Patent Publication No. 2016-207637

Lithium metal secondary batteries are being considered. The “lithium metal secondary battery” refers to a secondary battery in which lithium (Li) metal is the negative electrode active material. That is, in the negative electrode of the lithium metal secondary battery, electrons are transferred by the dissolution reaction and the precipitation reaction of the Li metal. The lithium metal secondary battery is expected to have a higher energy density than the existing lithium ion secondary battery.

However, the lithium metal secondary battery has a problem in reversibility of charge and discharge. That is, the Li metal tends to grow like a dendrite (tree branch) at the time of precipitation. The dendrite-like Li metal is easily deactivated by a side reaction with the electrolytic solution. It is considered that the Li metal grown in a dendrite shape is very active. It is considered that the deactivated Li metal is difficult to be redissolved in the electrolytic solution.

Hereinafter, the growth of Li metal in the form of dendrites is also referred to as “dendrite growth”. The Li metal grown in the form of dendrite is also referred to as "dendrite Li".

Patent Document 1 states that dendrite growth is suppressed by depositing Li metal on carbon nanotubes protruding from the surface of a copper sheet. Patent Document 1 discloses the evaluation result of the discharge rate characteristic. However, Patent Document 1 does not disclose the capacity retention rate after the charge / discharge cycle. In the configuration of Patent Document 1, it is not clear whether dendrite growth is actually suppressed, that is, whether the capacity retention rate after the charge / discharge cycle is improved.

An object of the present disclosure is to improve the capacity retention rate after a charge / discharge cycle in a lithium metal secondary battery.

Hereinafter, the technical configuration and the action and effect 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.

[1] The lithium metal secondary battery of the present disclosure includes at least a positive electrode, a negative electrode, and an electrolyte. When the lithium metal secondary battery is fully charged, the negative electrode contains at least carbon fiber aggregates and lithium metal. The carbon fiber aggregate contains a plurality of carbon fibers. Each of the plurality of carbon fibers is bonded to each other. Each of the plurality of carbon fibers carries a lithium metal. The carbon fiber aggregate has a porosity of 70% or more and 90% or less.

In the lithium metal secondary battery of the present disclosure, the carbon fiber aggregate is used as a carrier of Li metal (negative electrode active material). That is, in the carbon fiber aggregate, each of the plurality of carbon fibers carries a Li metal. It is considered that a dissolution reaction and a precipitation reaction of Li metal occur on the surface of the carbon fiber.

In the carbon fiber aggregate, nucleation is considered to occur on the surfaces of the plurality of carbon fibers. That is, it is expected that the number of nucleations will increase. Further, when dendrite Li is generated inside the carbon fiber aggregate, it is considered that the dendrite Li is likely to come into contact with the surrounding carbon fibers. Carbon fibers are electron conductive. It is expected that the contact of dendrite Li with the surrounding carbon fibers will promote the flow of electrons from the dendrite Li to the carbon fibers during discharge. As a result, it is expected that dendrite Li will be redissolved.

Furthermore, in the carbon fiber aggregate, each carbon fiber can also be a host material for Li ions. That is, it is considered that some Li ions are occluded in the carbon fiber. It is expected that the carbon fiber occludes a part of Li ions, so that the nucleation and nucleation of the Li metal become uniform.

Due to the synergistic effect of the above actions, it is expected that the growth of dendrites will be suppressed in the lithium metal secondary battery of the present disclosure. That is, it is expected that the capacity retention rate after the charge / discharge cycle will be improved.

However, the carbon fiber aggregate has a porosity of 70% or more and 90% or less. If the porosity is less than 70%, the capacity retention rate may decrease. This is probably because there is little space inside the carbon fiber aggregate. If there is little space inside the carbon fiber aggregate, Li metal may precipitate on the outer surface of the carbon fiber aggregate. When dendrite growth occurs from the outer surface of the carbon fiber aggregate to the outside, re-dissolution of dendrite Li cannot be expected. This is because dendrite Li does not come into contact with carbon fibers. Even if the porosity exceeds 90%, the capacity retention rate may decrease. This is thought to be because the surface area of the precipitation carrier is reduced, so that local current concentration is likely to occur.

In the lithium metal secondary battery of the present disclosure, the carbon fiber aggregate itself functions as a current collector for the negative electrode. Further, in the carbon fiber aggregate, since a plurality of carbon fibers are bonded to each other, it is considered that the carbon fiber aggregate can stand on its own even without a support. Therefore, in the lithium metal secondary battery of the present disclosure, it is considered that the negative electrode does not have to include a conductive support (copper foil or the like).

[2] The negative electrode may further contain a polymer material. The polymer material covers the surface of the carbon fiber. The polymer material has ionic conductivity.

Since the surface of the carbon fiber is coated with the ionic conductive polymer material, the formation of a non-uniform film can be suppressed on the surface of the precipitated Li metal. This is expected to suppress the growth of dendrites. The polymeric material itself may have ionic conductivity. The polymer material may have ionic conductivity by absorbing a liquid electrolyte (electrolyte or ionic liquid).

[3] The polymer material may be a polyvinylidene fluoride-hexafluoropropene copolymer (PVDF-HFP).

PVDF-HFP tends to have a high retention capacity of liquid electrolyte. It is expected that PVDF-HFP will exhibit high ionic conductivity by absorbing the liquid electrolyte. It is expected that the high ionic conductivity of the polymer material suppresses the current concentration of the precipitated Li metal on the tip.

FIG. 1 is a first schematic view showing an example of the configuration of the lithium metal secondary battery of the present embodiment. FIG. 2 is a second schematic view showing an example of the configuration of the lithium metal secondary battery of the present embodiment. FIG. 3 is a cross-sectional conceptual diagram showing the configuration of the negative electrode of the present embodiment. FIG. 4 is a cross-sectional conceptual diagram showing the configuration of the negative electrode of the reference form. FIG. 5 is an SEM image showing carbon fiber aggregates. FIG. 6 is an SEM image showing the negative electrode after charging.

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. Hereinafter, the lithium metal secondary battery may be abbreviated as "battery".

<Lithium metal secondary battery>
FIG. 1 is a first schematic view showing an example of the configuration of the lithium metal secondary battery of the present embodiment.
The battery 100 includes an exterior material 50. The exterior material 50 is made of an aluminum laminated film. That is, the battery 100 is a laminated battery. However, in the present embodiment, the type and type of the battery 100 should not be particularly limited. The battery 100 may be, for example, a square battery. The battery 100 may be, for example, a cylindrical battery. The positive electrode tab 51 and the negative electrode tab 52 communicate with each other inside and outside the exterior material 50. The positive electrode tab 51 is, for example, an aluminum (Al) thin plate. The negative electrode tab 52 is, for example, a copper (Cu) thin plate.

FIG. 2 is a second schematic view showing an example of the configuration of the lithium metal secondary battery of the present embodiment.
The exterior material 50 houses the electrode group 40 and the electrolyte. The electrode group 40 is a stacked type. However, the electrode group 40 may be of a winding type. The electrode group 40 includes a positive electrode 10, a negative electrode 20, and a separator 30. That is, the battery 100 includes at least a positive electrode 10, a negative electrode 20, and an electrolyte.

The electrode group 40 is formed by laminating the positive electrode 10 and the negative electrode 20. The electrode group 40 may be formed by alternately stacking one or more layers of the positive electrode 10 and the negative electrode 20. A separator 30 is arranged between each of the positive electrode 10 and the negative electrode 20. The positive electrode tab 51 is joined to the positive electrode 10. The negative electrode tab 52 is joined to the negative electrode 20.

《Negative electrode》
FIG. 3 is a cross-sectional conceptual diagram showing the configuration of the negative electrode of the present embodiment.
The negative electrode 20 is in the form of a sheet. In the fully charged state of the battery 100, the negative electrode 20 contains at least the carbon fiber aggregate 21 and the Li metal 22. The carbon fiber assembly 21 contains a plurality of carbon fibers. Each of the plurality of carbon fibers is bonded to each other. Each of the plurality of carbon fibers carries Li metal 22. A plurality of pores 23 are formed inside the carbon fiber assembly 21. The Li metal 22 also grows in the pores 23.

In the carbon fiber assembly 21, substantially all carbon fibers may carry the Li metal 22. In the carbon fiber aggregate 21, as long as a plurality of (two or more) carbon fibers support the Li metal 22, some carbon fibers may not support the Li metal 22.

The use of the carbon fiber aggregate 21 as a carrier is expected to suppress the growth of dendrites. That is, it is expected that the capacity retention rate after the charge / discharge cycle will be improved.

FIG. 4 is a cross-sectional conceptual diagram showing the configuration of the negative electrode of the reference form.
In the negative electrode 200, the copper foil 201 is the base material. Li metal 202 is deposited on the surface of the copper foil 201. In this configuration, it is considered that the Li metal 202 grows like a dendrite.

(Carbon fiber aggregate)
The carbon fiber assembly 21 is the base material of the negative electrode 20. The carbon fiber assembly 21 may be in the form of a sheet, for example. The carbon fiber assembly 21 may have a thickness of, for example, 50 μm or more and 500 μm or less. The thickness of the carbon fiber assembly 21 is measured by, for example, a micrometer or the like. The thickness is measured at at least 3 points. The arithmetic mean of at least three locations is the thickness of the carbon fiber assembly 21.

The carbon fibers constitute the carbon fiber aggregate 21. The carbon fibers may be, for example, PAN-based carbon fibers, pitch-based carbon fibers, cellulose-based carbon fibers, vapor-grown carbon fibers, or the like. The PAN-based carbon fiber represents a carbon fiber made from polyacrylonitrile (PAN) as a raw material. The pitch-based carbon fiber refers to carbon fiber made from, for example, petroleum pitch. The cellulosic carbon fiber refers to a carbon fiber made from, for example, viscose rayon or the like.

It is desirable that the carbon fibers are graphitized. Since the carbon fibers are graphitized, it is expected that Li ions are easily occluded in the carbon fibers. It is expected that the nucleation of Li metal 22 becomes uniform due to the occlusion of Li ions in the carbon fibers.

In the carbon fiber assembly 21, the plurality of carbon fibers are bonded to each other. The carbon fibers can be bonded, for example, by the following methods. A mixture is prepared by mixing a plurality of carbon fibers and binders. The carbon fibers and binders are graphitized by heating the mixture in an inert atmosphere. This allows the plurality of carbon fibers to be bonded to each other. The binder may be, for example, coal tar, petroleum pitch, phenol resin, epoxy resin or the like.

Since the plurality of carbon fibers are bonded to each other, it is expected that the carbon fiber aggregate 21 has a self-sustaining strength. Further, it is expected that electron conduction becomes active in the in-plane direction of the carbon fiber assembly 21. The in-plane direction indicates a direction orthogonal to the thickness direction of the carbon fiber aggregate 21. The carbon fiber assembly 21 may have a resistivity of, for example, 1 mΩ · cm or more and 10 mΩ · cm or less in the in-plane direction.

The carbon fibers may have, for example, an average diameter of 1 μm or more and 50 μm or less. The average diameter may be, for example, the average value of 100 or more carbon fibers. The carbon fibers may have, for example, a number average fiber length of 1 mm or more and 50 mm or less. The number average fiber length may be, for example, an average value of 100 or more carbon fibers.

(Porosity)
The carbon fiber aggregate 21 has a porosity of 70% or more and 90% or less. If the porosity is less than 70%, the capacity retention rate may decrease. This is probably because there is little space inside the carbon fiber assembly 21. Even if the porosity exceeds 90%, the capacity retention rate may decrease. This is thought to be because the surface area of the precipitation carrier is reduced, so that local current concentration is likely to occur.

"Porosity" indicates the ratio of the pore volume in the carbon fiber aggregate 21. Porosity is measured by a common mercury porosity meter. Porosity is measured at least 3 times. The arithmetic mean of at least three times is the porosity of the carbon fiber assembly 21. The carbon fiber assembly 21 may have a porosity of, for example, 80% or more and 90%. In this range, improvement of capacity retention rate is expected.

(Polymer material)
The negative electrode 20 may further contain a polymer material. The polymer material covers the surface of the carbon fiber. The polymer material has ionic conductivity. Since the surface of the carbon fiber is covered with an ionic conductive polymer material, it is expected that dendrite growth will be suppressed. The coating thickness of the polymer material may be, for example, 1 μm or more and 20 μm or less.

The polymer material may be, for example, PVDF-HFP, polyvinylidene fluoride (PVDF), polyethylene oxide (PEO), polyvinyl alcohol (PVA), polyacrylic acid (PAA), polymethyl methacrylate (PMMA), or the like. .. One type of polymeric material may be used alone. Two or more kinds of polymer materials may be used in combination. The polymeric material may be crosslinked.

The polymeric material itself may have ionic conductivity. The polymer material may have ionic conductivity by absorbing the liquid electrolyte. For example, the polymer material may be PVDF-HFP. PVDF-HFP is expected to exhibit high ionic conductivity by absorbing the liquid electrolyte. Since the polymer material has high ionic conductivity, it is expected that the current concentration on the tip of the precipitated Li metal 22 is suppressed.

《Positive electrode》
The positive electrode 10 has a sheet shape. The positive electrode 10 includes, for example, a positive electrode current collector 11 and a positive electrode mixture layer 12. The positive electrode current collector 11 may be, for example, an Al foil or the like. The positive electrode current collector 11 may have a thickness of, for example, 10 μm or more and 50 μm or less.

The positive electrode mixture layer 12 is formed on the surface of the positive electrode current collector 11. The positive electrode mixture layer 12 may be formed on both the front and back surfaces of the positive electrode current collector 11. The positive electrode mixture layer 12 may have a thickness of, for example, 10 μm or more and 200 μm or less. The positive electrode mixture layer 12 contains at least the positive electrode active material.

The positive electrode active material should not be particularly limited. The positive electrode active material is, for example, LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , Li (Ni, Co, Mn) O ₂ (for example, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ), It _{may be LiFePO 4} or the like. One kind of positive electrode active material may be used alone. Two or more kinds of positive electrode active materials may be used in combination.

The positive electrode mixture layer 12 may further contain a conductive material and a binder. The conductive material may be, for example, carbon black or the like. The content of the conductive material may be, for example, 1 part by mass or more and 10 parts by mass or less with respect to 100 parts by mass of the positive electrode active material. Binders should not be particularly limited either. The binder may be, for example, PVDF or the like. The content of the binder may be, for example, 1 part by mass or more and 10 parts by mass or less with respect to 100 parts by mass of the positive electrode active material.

《Separator》
The separator 30 is a porous film. The separator 30 may have a thickness of, for example, 10 μm or more and 50 μm or less. The separator 30 may be made of, for example, polyolefin. The separator 30 may have a single layer structure. The separator 30 may have a multi-layer structure.

"Electrolytes"
The electrolyte is typically a liquid electrolyte. The liquid electrolyte may be an electrolytic solution, an ionic liquid, or the like. The electrolyte contains a Li salt and a solvent. The Li salt may be, for example, LiPF ₆ , LiBF _4, LiN (SO ₂ F) _2, or the like. The electrolytic solution may contain, for example, a Li salt of 0.5 mL / l or more and 2 mL / l or less. The electrolytic solution may contain, for example, a Li salt of 3 mL / l or more and 5 mL / l or less.

The solvent is, for example, ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), acetonitrile (AN), N, N-dimethylformamide (DMF). , 1,2-Dimethoxyethane (DME), dimethyl sulfoxide (DMSO) and the like. One solvent may be used alone. Two or more kinds of solvents may be used in combination.

Hereinafter, examples of the present disclosure will be described. However, the following description does not limit the scope of claims.

<Battery manufacturing>
<< Example 1 >>
1. 1. Preparation of carbon fiber aggregate A carbon fiber aggregate (sheet-like, thickness 110 μm, porosity 80%) was prepared as a base material for the negative electrode 20. The carbon fiber assembly 21 was cut to a predetermined size.

2. Production of Positive Electrode A positive electrode mixture layer 12 was formed by applying a paste to the surface of the positive electrode current collector 11 (Al foil). As a result, the positive electrode 10 was manufactured. The positive electrode 10 was cut to a predetermined size. The positive electrode mixture layer 12 has a basis weight ^{of 16 mg / cm 2 on one side.} The positive electrode mixture layer 12 contains a positive electrode active material [Li (Ni, Co, Mn) O ₂ ], a conductive material (carbon black), and a binder (PVDF).

3. 3. Assembly The carbon fiber assembly 21, the separator 30 and the positive electrode 10 were laminated in this order. As a result, the electrode group 40 was formed. The separator 30 is a polyethylene porous film (thickness 20 μm).

The electrode group 40 was housed in the exterior material 50. The electrolytic solution was injected into the exterior material 50. The electrolytic solution contains the following components. The exterior material 50 was sealed. From the above, the battery 100 was assembled.

Li salt: LiPF ₆ (1 mol / l)
Solvent: [EC: DMC: EMC = 3: 4: 3 (volume ratio)]

4. Initial charge / discharge Battery 100 was charged to 4.2 V. That is, the battery 100 is fully charged. By charging, Li metal 22 was deposited on the surface of the carbon fibers in the carbon fiber aggregate 21. That is, the negative electrode 20 containing the carbon fiber aggregate 21 and the Li metal 22 was formed. It is considered that a plurality of carbon fibers each support the Li metal 22 in the negative electrode 20.

FIG. 5 is an SEM image showing carbon fiber aggregates. FIG. 6 is an SEM image showing the negative electrode after charging. It is considered that the Li metal 22 is deposited substantially uniformly in the carbon fiber aggregate 21. It is considered that carbon fibers are evenly present around the Li metal 22. After that, the battery 100 was discharged to 3V. From the above, the battery 100 was manufactured.

<< Example 2 >>
PVDF-HFP was dissolved in N-methyl-2-pyrrolidone (NMP). As a result, a polymer solution was prepared. In the polymer solution, the content of PVDF-HFP is 5% by mass. The carbon fiber aggregate 21 was immersed in the polymer solution. After immersion, the carbon fiber aggregate 21 was pulled out of the polymer solution. The carbon fiber assembly 21 was dried. The carbon fiber assembly 21 was analyzed by a scanning electron microscope (SEM) and an energy dispersive X-ray analyzer (EDX). This confirmed that PVDF-HFP covered the surface of the carbon fiber. The coating thickness was about several μm. The battery 100 was manufactured in the same manner as in Example 1 except that the coated carbon fiber assembly 21 was used.

<< Examples 3 and 4 >>
The battery 100 was manufactured in the same manner as in Example 1 except that the carbon fiber aggregate 21 having the porosity shown in Table 1 below was used.

<< Comparative Example 1 >>
The battery 100 was manufactured in the same manner as in Example 1 except that the copper foil was used as the base material of the negative electrode 20 instead of the carbon fiber assembly 21.

<< Comparative Example 2 >>
The surface of the copper foil was coated with PVDF-HFP by the same method as in Example 2. The battery 100 was manufactured in the same manner as in Comparative Example 1 except that the coated copper foil was used as the base material of the negative electrode 20.

<< Comparative Example 3 >>
A copper porous body (product name "Celmet") manufactured by Sumitomo Electric Industries, Ltd. was prepared. The copper porous body has a porosity of 96%. The copper porous body was compressed by a flat plate press. The compressed copper porous material has a porosity of 80%. The battery 100 was manufactured in the same manner as in Example 1 except that the compressed copper porous body was used as the base material of the negative electrode 20.

<< Comparative Examples 4 and 5 >>
The battery 100 was manufactured in the same manner as in Example 1 except that the carbon fiber aggregate 21 having the porosity shown in Table 1 below was used.

<Evaluation>
<< Initial charge / discharge efficiency >>
The initial charge / discharge efficiency was calculated by dividing the initial discharge capacity by the initial charge capacity. The results are shown in Table 1 below.

<< Capacity maintenance rate after 10 cycles >>
In a 25 ° C. environment, charging and discharging were carried out for 10 cycles under the following conditions. The capacity retention rate after 10 cycles was calculated by dividing the discharge capacity at the 10th cycle by the discharge capacity at the 1st cycle. The results are shown in Table 1 below.

Charging: Constant current method, charging voltage 4.2V, current density 1mA / cm ²
Discharge: Constant current method, discharge voltage 3.0V, current density 1mA / cm ²

<Result>
As shown in Table 1 above, Examples 1 to 4 in which the carbon fiber aggregate 21 is the base material of the negative electrode 20 have a capacity after 10 cycles as compared with Comparative Examples 1 to 3 in which the other materials are the base material. The maintenance rate is improving. It is considered that in Example 1, nucleation of Li metal 22 is likely to occur uniformly during charging, and dendrite Li is likely to be dissolved during discharging.

When the porosity of the carbon fiber aggregate 21 exceeds 90%, the capacity retention rate decreases (Comparative Example 4). This is thought to be because the surface area of the precipitation carrier is reduced, so that local current concentration is likely to occur.

When the porosity of the carbon fiber aggregate 21 is less than 70%, the capacity retention rate is lowered (Comparative Example 5). It is considered that the Li metal 22 is deposited on the outside of the carbon fiber assembly 21 because there is little space inside the carbon fiber assembly 21.

It is considered that the copper porous body of Comparative Example 3 has an internal structure similar to that of the carbon fiber assembly 21. Comparative Example 3 has a porosity equivalent to that of Example 1. Nevertheless, Comparative Example 3 has a low capacity retention rate. From this result, it is considered that the carbon fiber aggregate 21 has a special action as a carrier for precipitating the Li metal 22.

By coating the surface of the carbon fiber with PVDF-HFP, the capacity retention rate is improved (Example 2). It is considered that PVDF-HFP (ionic conductive polymer material) suppresses dendrite growth.

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

10 Positive electrode, 11 Positive electrode current collector, 12 Positive electrode mixture layer, 20,200 Negative electrode, 21 Carbon fiber aggregate, 22,202 Lithium metal, 23 Pore, 30 Separator, 40 Electrode group, 50 Exterior material, 51 Positive electrode tab , 52 Negative electrode tab, 100 battery (lithium metal secondary battery), 201 copper foil.

Claims (4)

Lithium metal rechargeable battery
Contains at least positive, negative and electrolyte
In the fully charged state of the lithium metal secondary battery, the negative electrode contains at least a carbon fiber aggregate and a lithium metal.
The carbon fiber aggregate contains a plurality of carbon fibers and contains a plurality of carbon fibers.
The plurality of carbon fibers are each bonded to each other, and the plurality of carbon fibers are bonded to each other.
The plurality of carbon fibers each support the lithium metal, and the plurality of carbon fibers each support the lithium metal.
The carbon fiber aggregate has a porosity of 70% or more and 90% or less.
Lithium metal secondary battery. Electrons are transferred by the dissolution reaction and precipitation reaction of the lithium metal on the surfaces of the plurality of carbon fibers.
The lithium metal secondary battery according to claim 1. The negative electrode further contains a polymer material and contains
The polymer material covers the surfaces of the plurality of carbon fibers.
The polymer material has ionic conductivity.
The lithium metal secondary battery according to claim 1 or 2. The polymer material is a polyvinylidene fluoride-hexafluoropropene copolymer.
The lithium metal secondary battery according to claim 3.

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