JP2024050027A - Coating-type silicon negative electrode for all-solid-state lithium secondary battery and all-solid-state lithium secondary battery using the same - Google Patents

Abstract

[Problem] To provide a coated silicon negative electrode for an all-solid-state lithium secondary battery that has good cycle characteristics and does not cause the negative electrode active material (silicon) to become fine or isolated. [Solution] A negative electrode active material layer containing silicon particles with an average particle size of 0.8 to 2.0 μm is formed on a metal foil, and the porosity in the negative electrode active material layer is controlled to preferably 50% or more, and the size of the voids in the negative electrode active material layer is controlled to preferably 0.5 μm or less. [Selected Figure] Figure 3

Description

The present invention relates to a coated silicon negative electrode for an all-solid-state lithium secondary battery. The present invention also relates to an all-solid-state lithium secondary battery using the same.

Lithium ion secondary batteries (hereinafter sometimes abbreviated as "LiB") are widely used because they have a high energy density among secondary batteries. In lithium ion secondary batteries, a positive electrode containing an active material such as lithium cobalt oxide (LiCoO ₂ ) or lithium iron phosphate (LiFePO ₄ ) and a negative electrode containing an active material such as graphite capable of absorbing and releasing lithium ions are arranged through a separator, and non-aqueous electrolyte LiBs are well known, which are composed of an organic solvent such as ethylene carbonate and a non-aqueous electrolyte in which an electrolyte made of a lithium salt such as LiBF ₄ is dissolved as the electrolyte. Charging and discharging of non-aqueous electrolyte LiBs are performed by the lithium ions in the secondary battery moving between the positive electrode and the negative electrode through the non-aqueous electrolyte, and the lithium ions being inserted and removed from the active materials of the positive electrode and the negative electrode. However, such non-aqueous electrolyte LiBs have the risk of leakage of the electrolyte and the possibility of ignition due to the organic solvent used in the non-aqueous electrolyte, and there is a demand for improvements in terms of safety.

As the use of batteries expands, attention is being paid to large batteries such as automobile batteries and stationary batteries. Ensuring safety is more important for large batteries than for small batteries, and safe and highly reliable all-solid-state batteries have been proposed, which use the electrolyte described above, but by replacing the electrolyte with an inorganic solid electrolyte, it is easier to ensure safety even when the battery is large compared to LiBs that use electrolytes.

In addition, carbon-based materials such as graphite have been used as negative electrode active materials for LiB in the past. From the viewpoint of increasing capacity, the use of silicon as a negative electrode active material is being considered. Silicon has a theoretical capacity density of 4200 mAh/g (volume capacity density of 2370 mAh/cm ³ ) as a negative electrode active material, which is about 11 times higher in capacity/weight ratio and about 3 times higher in capacity/volume ratio compared to carbon materials, and is expected to dramatically increase the capacity of secondary batteries.

However, silicon undergoes a very large volume change of about 400% during charging and discharging, and as a result, the silicon deteriorates and becomes pulverized and isolated during repeated charging and discharging. As a result, the charge-discharge efficiency and cycle life characteristics are poor, and it is considered difficult to put LiBs using silicon as the negative electrode active material to practical use, especially in large batteries for electric vehicles that are intended for long-term use.

In particular, in conventional lithium ion secondary batteries using a liquid ion conductor such as an electrolyte solution, an ionic liquid, or a gel polymer electrolyte, the following problems arise when silicon is used as the negative electrode active material.
(1) The expansion and contraction of silicon during charging and discharging causes the electrode structure to collapse, resulting in deterioration of battery performance.
(2) The deterioration of battery performance is caused by the pulverization of silicon particles due to expansion and contraction, and the adhesion of decomposition products of the liquid ionic conductor to the surface of these pulverized particles, which reduces the ionic conductivity at the interface between the negative electrode active material and the ionic conductor and cuts off the electronic conductive path.
(3) In order to maintain electronic conductivity even after pulverization, it is necessary to add a large amount of an electronic conductive additive such as carbon. However, since carbon can also act as an anode active material, problems such as competition between carbon and silicon, an increase in irreversible capacity due to carbon, and localization of lithium occur.
(4) The decomposition products of the liquid ionic conductor adhere to the interface between the silicon negative electrode active material and the ionic conductor, increasing the electrical resistance at this interface, making it difficult to increase the current density during charging and discharging, and making it difficult to improve the battery performance. Furthermore, the formation of these decomposition products consumes electrochemical capacity, increasing the irreversible capacity and causing a decrease in the electrochemical capacity of the negative electrode active material.
(5) This phenomenon causes the silicon powder to become isolated or to fall off the current collector, making it unable to participate in the next charge/discharge. If further charge/discharge is performed in this state, the amount of lithium inserted into the silicon particles that remain in contact with the current collector will increase. This will cause the silicon particles to expand and contract more, leading to further miniaturization. By repeating this process, the number of silicon particles involved in charge/discharge will decrease, and the charge/discharge capacity will drop rapidly. In other words, the cycle characteristics will deteriorate due to electrode collapse.

On the other hand, the problems with silicon anode materials in all-solid-state batteries using solid electrolytes are thought to be as follows.
(1) During charging, i.e., when the lithium insertion reaction into silicon occurs, the active material expands. At this time, since there is no mechanism to ease this volume change within the electrode layer, where the solid electrolyte and active material are densely packed, stress is generated in the solid electrolyte layer formed on the electrode layer and also in the positive electrode layer, which is the counter electrode. This causes the solid electrolyte layer to break down, the negative electrode layer to short-circuit the positive electrode layer, and other problems, impairing the function of the battery.
(2) Furthermore, during discharge, i.e., when the lithium extraction reaction occurs from the negative electrode active material, the volume of the active material shrinks. At this time, tensile stress is applied to the solid electrolyte layer, and the distance between the solid electrolyte particles increases. This causes the ion conduction path to the active material to be cut off. Furthermore, the contact between the active material and the carbon-based material added to the collector and electrode layer as an electronic conductive additive is also dissociated due to the contraction of the active material. This causes the electronic conductive path to be cut off. As a result, the silicon particles are removed from the electrochemical reaction system, becoming isolated, and the electrochemical capacity rapidly deteriorates. Even if there is no problem with the expansion during charging, the contraction of the active material during discharge causes deterioration of the battery performance.

For this reason, although lithium ion secondary batteries using silicon as the negative electrode active material are expected to have a high capacity density, it has been difficult to put them into practical use. Various technical proposals have been made from the viewpoint of suppressing electrode collapse due to the expansion and contraction of silicon during charging and discharging. Patent Document 1 (JP 2003-109590 A) discloses a negative electrode material in which volume change is mitigated by doping silicon with phosphorus, boron, or aluminum. Patent Document 2 (JP 2005-11699 A) proposes a battery structure in which the density of the negative electrode and the size of the gap in the battery are controlled to absorb the volume change of the negative electrode and reduce the effect of the volume change. However, the secondary batteries described in these patent documents use a nonaqueous electrolyte, which is not preferable from the viewpoint of safety as mentioned above.

In addition, Patent Document 3 (JP 2021-68706 A) gives an example of silicon as one of the negative electrode materials for all-solid-state batteries, but gives no examples of its use, and therefore does not mention the problem of silicon becoming finer and more isolated due to repeated charging and discharging, nor does it suggest a solution to this problem. In addition, although it is described as all-solid-state, it uses an ionic liquid, so it is not a solid-state battery in the strict sense, and the risk of liquid leakage has not been eliminated.

Capacity loss due to silicon microparticulation is an issue for both non-aqueous electrolyte LiBs and all-solid-state batteries, but the causes are different. The main cause of capacity loss in all-solid-state batteries is poor contact between the solid electrolyte or electronic conductive material or current collector and silicon, which is the electrode active material. For this reason, the all-solid-state battery manufacturing process includes a step in which a relatively large molding pressure is applied. Furthermore, even when the battery is in operation, a pressure called confining pressure may be applied to maintain contact current. For this reason, it is said that capacity loss due to silicon microparticulation is less likely to occur in all-solid-state batteries than in non-aqueous electrolyte LiBs.

The use of silicon in all-solid-state LiBs is also progressing, but in Patent Documents 4 to 6, the charge/discharge capacity is low at about 1200 mAh/g, and the capacity retention rate remains at about 50%. In Patent Document 7, the capacity retention rate is improved to 100% by controlling the amorphous rate of silicon, but the capacity itself remains at about 1200 mAh/g. Patent Documents 8 and 9 describe suppressing the increase in the confining pressure by the manufacturing method and charge/discharge conditions, but do not mention the charge/discharge capacity. Furthermore, Patent Document 10 specifies the silicon particle size and the porosity in the negative electrode layer, but describes that it is preferable to suppress the porosity because the battery characteristics decrease with an increase in voids. Furthermore, in all of Patent Documents 4 to 10, there is no description regarding the size of the voids. Furthermore, in all of Patent Documents 4 to 10, the negative electrode layer contains a solid electrolyte as an ion conductor and an electronic conductive agent as additives other than the active material, and there is no description regarding a negative electrode layer that does not substantially contain a solid electrolyte or an electronic conductive agent.

In addition, in all-solid-state batteries, the negative electrode active material layer is generally formed by mixing fine carbon particles such as acetylene black or metal powder as an electronic conductive material to impart electronic conductivity, and/or a solid electrolyte as an ion conductivity imparting agent. When charging and discharging are performed under a confining pressure, cracks and the like occur in the solid electrolyte as the silicon expands and contracts, and the silicon gradually becomes isolated, resulting in a decrease in capacity. Furthermore, adding a large amount of electronic conductive material and ion conductivity imparting agent reduces the relative volume of silicon in the negative electrode, which is not desirable from the viewpoint of improving capacity.

JP 2003-109590 A JP 2005-11699 A JP 2021-68706 A JP 2013-069416 A JP 2013-222530 A JP 2014-192093 A JP 2017-059534 A JP 2019-091547 A JP 2019-140042 A JP 2019-185897 A

As described above, from the viewpoint of safety and capacity improvement, there is a high demand for realizing an all-solid-state lithium secondary battery using silicon as the negative electrode active material, but there are many technical problems. In order to solve these problems, the inventors have conducted extensive research and found that by using silicon having a specific particle size as the negative electrode active material, the porosity in the negative electrode layer can be set to 50% by volume or more, and the size of the voids in the active negative electrode layer can be controlled to 0.5 μm or less, and many of the technical problems inherent to all-solid-state lithium secondary batteries can be solved, leading to the completion of the present invention. In other words, the present invention aims to provide an all-solid-state lithium secondary battery that does not have fine or isolated negative electrode active material (silicon) and has good cycle characteristics.

In order to solve such problems, the present invention encompasses the following.
(1) A coated silicon negative electrode for an all-solid-state lithium secondary battery, in which a negative electrode active material layer containing silicon particles having an average particle size of 0.8 to 2.0 μm is formed by coating a metal foil.

(2) The coated silicon negative electrode for an all-solid-state lithium secondary battery described in (1), in which the negative electrode active material layer contains 5 parts by mass or less of a carbon-based electronic conductive material and 20 parts by mass or less of a binder per 100 parts by mass of the silicon particles.

(3) A coated silicon negative electrode for an all-solid-state lithium secondary battery according to (1) or (2), in which the porosity in the negative electrode active material layer is 50% by volume or more, and the size of the voids in the negative electrode active material layer is 0.5 μm or less.

(4) A coated silicon anode for an all-solid-state lithium secondary battery according to any one of (1) to (3), in which the anode active material layer does not contain a solid electrolyte.

(5) An all-solid-state lithium secondary battery using a negative electrode described in any one of (1) to (4) above.

The porosity in the negative electrode active material layer in the above (3) refers to a value calculated by the following procedure.
(a) The film thickness of the negative electrode active material layer is obtained from an image taken with a scanning electron microscope, and the volume V0 of the negative electrode active material layer per unit area is calculated.
(b) The volume V1 of the silicon itself is calculated by dividing the mass of silicon per unit area in the negative electrode active material layer by the density of silicon.
(c) When the negative electrode active material layer contains an electronic conductive material or an ionic conductive assistant, the combined volume of the electronic conductive material and the ionic conductive assistant contained in the negative electrode active material layer per unit area is defined as V2.
(d) The porosity of the negative electrode active material layer is calculated as (V0-V1-V2)/V0×100(%).
The size of the voids in the negative electrode active material layer in the above (3) means the median diameter of the maximum diameter of 100 or more independent voids observed in a scanning electron microscope photograph of the cross section of the negative electrode active material layer.

In the present invention, a silicon coating film formed by coating silicon particles having a specific particle size on a metal foil, which is a negative electrode current collector, is used as the negative electrode of an all-solid-state lithium secondary battery. When an all-solid-state lithium secondary battery is assembled using this silicon coating film as the negative electrode and lithium is inserted (charged) into silicon, which is the negative electrode active material, a part or all of the silicon becomes amorphous and expands in volume, the silicon particles into which lithium has been inserted coalesce, and the negative electrode active material layer becomes dense. This expansion of silicon reaches 200 to 400% of the original volume, but the voids generated during the formation of the silicon coating film mitigate this volume expansion, and the expansion of the negative electrode active material layer as a whole is suppressed to about 1.5 to 2 times in the film thickness direction. This is due to the large porosity compared to conventional silicon coating films. In addition, when the lithium absorbed in the silicon is extracted (discharged), the negative electrode active material layer shrinks in the film thickness direction and vertical cracks occur in the negative electrode active material layer. In this case, if a roughened metal foil is used as the current collector, the adhesion between the negative electrode active material layer and the negative electrode current collector is good due to the anchor effect, and the negative electrode active material layer is less likely to peel off from the negative electrode current collector, so it is preferable to use a roughened metal foil as the current collector. In addition, a film structure with vertical cracks occurs in the negative electrode active material layer. At this time, the film thickness remains at about 130 to 200% of the initial coating film. In subsequent charge/discharge cycles, such film thickness changes are repeated. In other words, except for the first charge/discharge, the amount of expansion and contraction in the film thickness direction during charging and discharging is kept relatively small, so the charge/discharge cycle characteristics are stable and good characteristics are maintained.

As described above, during discharge, the negative electrode active material layer shrinks in the film thickness direction and vertical cracks occur within the layer. The vertical cracks are formed into which the solid electrolyte penetrates from the solid electrolyte layer, and this penetration structure functions as an ion conduction path within the negative electrode active material layer. Furthermore, since the above-mentioned negative electrode active material layer is in close contact with the negative electrode current collector and the solid electrolyte layer, an electrode film that functions sufficiently as a negative electrode can be produced without using an electronic conductive material or an ion conductive auxiliary. As a result, an all-solid-state battery that has good battery charge/discharge cycle characteristics and can increase the current density during charge and discharge can be obtained.

FIG. 1 is a cross-sectional view of an exemplary all-solid-state lithium secondary battery. 4 is a scanning electron microscope photograph showing the upper surface of the negative electrode active material layer after the formation of a silicon coating film. 4 is a scanning electron microscope photograph showing a cross section of a negative electrode active material layer after a silicon coating film is formed. 1 is a graph showing the relationship between the average particle size of silicon when a negative electrode active material layer is produced, and the porosity and size of voids in the negative electrode active material layer. 1 is a scanning electron microscope photograph showing a cross section of a negative electrode active material layer after the first discharge (after lithium is extracted). 1 is a cross-sectional photograph of a negative electrode active material layer after the first discharge (after lithium extraction), showing how the solid electrolyte penetrates into vertical cracks that have formed. 1 shows the charge/discharge curves of an all-solid-state half-cell using a silicon negative electrode.

The following describes an embodiment of the present invention. First, silicon particles, which are the negative electrode active material used in the negative electrode of an all-solid-state lithium secondary battery, are described. Then, a composition for forming a negative electrode for an all-solid-state lithium secondary battery containing the particles, a negative electrode for an all-solid-state lithium secondary battery obtained using the composition, and an all-solid-state lithium secondary battery containing the negative electrode are described. In this specification, an all-solid-state lithium secondary battery refers to a battery that does not contain a liquid such as a non-aqueous electrolyte or an ionic liquid as an electrolyte.

(Negative electrode active material particles for all-solid-state lithium secondary batteries)
The active material particles used in the negative electrode for an all-solid-state lithium secondary battery according to the present invention are made of silicon having an average particle size of 0.8 to 2.0 μm. When the silicon particles are used to form a negative electrode active material layer on a roughened metal foil serving as a negative electrode current collector, the porosity and pore size in the negative electrode active material layer are optimized, and the volume change accompanying the expansion and contraction of the negative electrode active material during charging and discharging is mitigated, improving cycle characteristics.

In an all-solid-state lithium secondary battery using a negative electrode in which the porosity and size of the voids in the negative electrode active material layer are controlled, when lithium is inserted (charged) into the silicon active material, some or all of the silicon powder becomes amorphous, and the powder particles coalesce and become dense. When the lithium ions absorbed in the silicon are extracted (discharged), a vertical crack structure is formed in the dense negative electrode active material layer. Due to this effect, the expansion and contraction in the film thickness direction during the volume expansion and contraction of the silicon active material associated with charging and discharging is mitigated.

The silicon may be in any state, such as polycrystalline, single crystal, or amorphous. Therefore, the negative electrode active material particles of the present invention may be polycrystalline silicon powder, single crystal silicon powder, amorphous silicon, or a mixture of these.

The average particle diameter of the silicon particles is 0.8 to 2.0 μm from the viewpoint of optimizing the porosity and pore size of the negative electrode active material layer. The average particle diameter means the 50% cumulative diameter (D50) in the particle size distribution measurement results by the laser scattering method.

The silicon particles preferably have the following properties:
The specific surface area of the particles is in the range of 3 to 50 m ² /g, and more preferably 10 to 25 m ² /g, and is determined by gas adsorption measurement using a constant volume method.
In addition, when the silicon particles are crystalline, the average crystallite size of silicon is in the range of 30 to 110 nm, and more preferably 50 to 90 nm. The average crystallite size can be analyzed from an X-ray diffraction pattern by a method such as the Scherrer method, the Willamson-Hall method, or the Halder-Wagner method.

The particle shape is irregular when obtained by pulverization, but other shapes such as spherical shapes may also be used without any particular limitations.
In order to improve the adhesion between the silicon particles and a binder component described below, the silicon particles may be subjected to a surface treatment.

The purity of silicon is not particularly limited, but in the present invention, the purity of silicon is preferably 90% or more. However, doped silicon can be used in the present invention because it is thought that it may improve the chemical stability against sulfide-based solid electrolytes or improve the diffusibility of lithium.

(Composition for forming negative electrode for all-solid-state lithium secondary battery)
The negative electrode active material is mixed with components constituting the negative electrode to form a composition for forming a negative electrode, and a negative electrode active material layer is formed on a current collector to obtain a negative electrode. When a negative electrode is formed from a composition containing the negative electrode active material, the porosity of the negative electrode is controlled to an appropriate range, and the size of the voids in the negative electrode is also within an appropriate range. This reduces the volume change associated with the expansion and contraction of the negative electrode active material during charging and discharging, improving cycle characteristics and output characteristics. In addition, since structural changes in the negative electrode are suppressed, the adhesion between the negative electrode active material and the current collector and solid electrolyte is maintained, and an electrode film that functions sufficiently as a negative electrode can be prepared without using an electronic conductive material or an ion conductive agent.

The composition for forming the negative electrode of an all-solid-state lithium secondary battery contains the above-mentioned negative electrode active material particles, and the content of the carbon-based electronic conductive material is preferably 5 parts by mass or less per 100 parts by mass of the negative electrode active material particles. According to the present invention, the proportion of the carbon-based electronic conductive material is reduced, so that the relative amount of the active material particles can be increased, which contributes to improving the capacity. The content of the carbon-based electronic conductive material in the composition for forming the negative electrode of an all-solid-state battery is preferably 3% by mass or less, more preferably 1% by mass or less, and more preferably substantially free of the carbon-based electronic conductive material.

The composition for forming the negative electrode of the all-solid-state lithium secondary battery can contain an ion-conductivity imparting agent to improve ion conductivity. As such an ion-conductivity imparting agent, a solid electrolyte as described below can be used. If the amount of the ion-conductivity imparting agent is too large, the relative amount of the active material particles decreases and the cycle characteristics also decrease, so the content of the ion-conductivity imparting agent in the composition for forming the negative electrode of the all-solid-state lithium secondary battery is preferably 30% by mass or less, more preferably 20% by mass or less, and more preferably substantially free of the ion-conductivity imparting agent.

The composition for forming the negative electrode for an all-solid-state lithium secondary battery may contain a binder. The amount of the binder is preferably 20 parts by mass or less, and more preferably 15 parts by mass or less, per 100 parts by mass of the negative electrode active material particles. If the amount of the binder is too large, the amount of active material in the negative electrode active material layer will relatively decrease, which is not preferable in terms of increasing the battery capacity.

Examples of binders include thermosetting resins such as thermosetting polyimide, thermosetting polyamideimide, phenolic resin, epoxy resin, urea resin, melamine resin, unsaturated polyester resin, and polyurethane; cellulose derivatives such as carboxymethyl cellulose, hydroxypropyl methyl cellulose, hydroxypropyl cellulose, ethyl cellulose, hydroxyethyl cellulose, and hydroxymethyl cellulose, and water-soluble polymers such as polyvinyl alcohol; polycarbonate resins such as polypropylene carbonate; polyvinylidene fluoride; styrene-butadiene copolymers (so-called SBR rubber-based), styrene-propylene copolymers, and styrene-ethylene-propylene copolymers (so-called SES-based and SEPS-based).

In addition, the composition for forming a negative electrode for an all-solid-state lithium secondary battery may contain a dispersion medium for making into a paint. The dispersion medium is appropriately selected from alcohols, aldehydes, ketones, ethers, esters, amides, imides, aliphatic hydrocarbons, alicyclic hydrocarbons, aromatic hydrocarbons, heterocycles, and the like, and examples thereof include methanol, ethanol, normal propyl alcohol, isopropyl alcohol, normal butyl alcohol, isobutyl alcohol, pentyl alcohol, hexyl alcohol, heptyl alcohol, octyl alcohol, 2-ethylhexyl alcohol, benzyl alcohol, ethylene glycol monomethyl ether, diethylene glycol monomethyl ether, triethylene glycol monomethyl ether, ethylene glycol monoisopropyl ether, diethylene glycol monoisopropyl ether, triethylene glycol monoisopropyl ether, ethylene glycol monobutyl ether, diethylene glycol butyl ether, triethylene glycol monobutyl ether, propylene glycol monomethyl ether, dipropylene glycol monomethyl ether, ethyl acetate, normal propyl acetate, isopropyl acetate, normal butyl acetate, isobutyl acetate, hexyl acetate, ethylene glycol monomethyl ether acetate, ether acetate, diethylene glycol monomethyl ether acetate, triethylene glycol monomethyl ether acetate, ethylene glycol monoisopropyl ether acetate, diethylene glycol monoisopropyl ether acetate, triethylene glycol monoisopropyl ether acetate, ethylene glycol monobutyl ether acetate, diethylene glycol monobutyl ether acetate, triethylene glycol monobutyl ether acetate, propylene glycol monomethyl ether acetate, dipropylene glycol monomethyl ether acetate, hexane, nona Examples of the solvent include benzene, decane, isodecane, dodecane, isododecane, terpene, naphthenic solvents, isoparaffinic solvents, cyclohexane, methylcyclohexane, dimethylcyclohexane, ethylcyclohexane, alkylcyclohexane, toluene, xylene, aromatic high boiling point solvents, acetone, methyl ethyl ketone, methyl pentyl ketone, methyl isobutyl ketone, cyclohexanone, terpineol, dihydroterpineol, dihydroterpineol acetate, NMP (N-methylpyrrolidone), methoxybenzene, diethyl ether, dipropyl ether, dibutyl ether, etc. These may be used alone or in combination. The amount of the dispersion medium used may be an amount that allows the viscosity of the negative electrode forming composition for all-solid-state lithium secondary batteries to be thickly coated. The dispersion medium can be removed by drying after coating the negative electrode forming composition for all-solid-state lithium secondary batteries.

(Coated silicon anode for all-solid-state lithium secondary batteries)
The negative electrode active material layer made of the composition for forming a negative electrode for an all-solid-state lithium secondary battery is applied onto a current collector to obtain a coated silicon negative electrode for an all-solid-state lithium secondary battery. The negative electrode is one of the components that make up a battery, and when the battery is assembled and charged and discharged, charging and discharging are performed by the absorption and release of lithium ions by the active material silicon.

When the negative electrode active material layer made of the above-mentioned composition for forming a negative electrode for an all-solid-state lithium secondary battery is formed on a current collector, the size of the voids present in the negative electrode active material layer is preferably 0.5 μm or less, more preferably in the range of 0.2 to 0.5 μm, and particularly preferably 0.25 to 0.4 μm. The porosity in the negative electrode active material layer is preferably 50 volume % or more, more preferably in the range of 50 to 65 volume %. The size and porosity of the voids in the negative electrode are mainly controlled by the average particle diameter of the active material particles. The porosity in the negative electrode active material layer means the ratio of the volume of the voids obtained by subtracting the volumes of the negative electrode active material, the electronic conductive material, and the ion-conductivity imparting agent from the volume of the negative electrode active material layer.

To form an anode active material layer made of a composition for forming an anode for an all-solid-state lithium secondary battery on a current collector, the composition for forming an anode for an all-solid-state lithium secondary battery may be applied to the current collector and dried. The application method is not particularly limited, and may be, for example, application using a doctor blade, or general application such as gravure coating, screen coating, die coating, bar coating, spin coating, or nip coating. Drying may be performed at a temperature at which the dispersion medium used is sufficiently volatilized.

There is no particular limit to the thickness of the negative electrode active material layer, but if it is too thin, the battery capacity will be low, and if it is too thick, the electronic conductivity and ionic conductivity may decrease. Therefore, the thickness of the negative electrode active material layer is usually 5 to 40 μm, and preferably 10 to 30 μm.

The current collector used for the negative electrode is generally copper foil, nickel foil, iron foil or SUS foil, but other conductive metal foils may also be used. The surface of the current collector may be roughened. The method of roughening is not particularly limited, and known surface treatments may be applied, such as mechanical surface processing, etching, chemical conversion treatment, anodization, wash primer, corona discharge, and glow discharge. The roughness (arithmetic mean roughness Ra) of the roughened surface is preferably 1.2 to 2.5. The current collector may be electrolytic copper that has been subjected to anti-rust treatment. The thickness of the current collector is not particularly limited, but from the viewpoint of the miniaturization of the battery and handling properties, a thickness of 3 μm to 100 μm is usually used, and when a roll-to-roll manufacturing method is used, a thickness of 5 μm to 50 μm is preferably used. The shape of the current collector may be a sheet without holes, or a sheet with holes such as a two-dimensional mesh, a three-dimensional net, or a punched metal.

The coated silicon negative electrode for the all-solid-state lithium secondary battery has the above-mentioned negative electrode active material layer on the current collector, and a solid electrolyte layer may be further formed on the negative electrode active material layer. The solid electrolyte is not particularly limited, but examples include sulfide-based solid electrolytes and oxide-based solid electrolytes that are widely used. Sulfide-based solid electrolytes are advantageous in that they have high lithium ion conductivity. Oxide-based solid electrolytes are relatively chemically stable and are advantageous in terms of high voltage resistance. When an oxide-based solid electrolyte is used for the solid electrolyte layer, a general-purpose ion conductive material may be used in combination as necessary to improve lithium ion conductivity.

The sulfide-based solid electrolyte contains, for example, lithium, phosphorus, and sulfur, and may further contain elements such as O, Al, B, Si, Ge, I, Br, and Cl. Specifically, amorphous Li ₃ PS ₄ , amorphous 40LiI-60Li ₃ PS ₄ (mol %), Li ₁₀ P ₃ S ₁₂ Br, Li ₇ P ₂ S ₈ I, β-Li ₃ PS ₄ , α-Li ₃ PS ₄ , and Li ₇ P ₃ S ₁₁ crystals may be used. A so-called LGPS-based solid electrolyte represented by Li ₁₀ GeP ₂ S ₁₂ or an argyrodite-based solid electrolyte represented by Li ₆ PS ₅ X (X=I, Br, Cl) may also be used.

Examples of oxide-based solid electrolytes include Li5 _{+xLa3 ( Zrx} , _A2-x ) _O12 (wherein A is one or more elements selected from the group consisting of Sc, Ti, C, Y, Nb, Hf, Ta, Al, Si, Ga, Ge, and Sn, and X is 1.4≦X≦2), Li1 _{+ xAlxTi2 -x} ( _PO4 ) ₃ (wherein X is 0≦X≦1), and _{Li3xLa2 /3-xTiO3 (} wherein X is 0≦X≦2/3). These have high ionic conductivity at room temperature and high electrochemical stability.

From the viewpoint of electrochemical stability, the oxide-based solid electrolyte may further contain insulating particles such as silica (SiO ₂ ) particles, γ-alumina (Al ₂ O ₃ ) particles, ceria (CeO ₂ ) particles, zirconia (ZrO ₂ ) particles, etc. Also, other known metal oxide particles may be used.

The solid electrolyte preferably has a Young's modulus (25°C) of 10 to 70 GPa, more preferably 15 to 30 GPa, because the solid electrolyte can easily penetrate into the gaps between the negative electrode active materials and can maintain high ion conductivity. Examples of the solid electrolyte having a Young's modulus (25°C) of 10 to 70 GPa include amorphous Li ₃ PS ₄ , LiX-Li ₃ PS ₄ (X = I, Br, Cl)-based glass, β-Li ₃ PS ₄ , α-Li ₃ PS ₄ , LI ₇ P ₃ S ₁₁ crystal, LGPS crystal-based solid electrolytes represented by Li ₁₀ GeP ₂ S ₁₂ , and argyrodite crystals represented by Li ₆ PS ₅ X (X = I, Br, Cl).

There are no particular limitations on the thickness of the solid electrolyte layer, but it is preferable that it be 10 μm or more in order to maintain the strength required to withstand the expansion and contraction of the silicon negative electrode during charging and discharging.

When silicon is used as the negative electrode active material, the theoretical maximum charge capacity of a silicon negative electrode is approximately 4200 mAhg ^-1 , but the practical charge/discharge range is approximately 1000 to 3000 mAhg ^-1 .

Furthermore, when conventional silicon is used as the negative electrode active material, the current density during charging and discharging of the all-solid-state battery is practically used in the range of 0.1 to 0.4 mA/cm ^-2 because an increase in current density leads to a decrease in charge and discharge capacity. In contrast, in the present invention, even when the current density is increased to 0.6 mA/cm ^-2 or more, no significant capacity decrease is observed and good cycle characteristics are maintained. That is, one of the reasons why the all-solid-state lithium secondary battery according to the present invention exhibits stable battery performance with good cycle characteristics for a long period of time is thought to be that the volume change of the negative electrode active material is mitigated by the voids, and the interface between the negative electrode active material layer and the solid electrolyte is stable.

(All-solid-state lithium secondary battery)
The all-solid-state lithium secondary battery of the present invention has a positive electrode, a negative electrode, and a solid electrolyte layer. The negative electrode is made of the above-mentioned coated silicon negative electrode for all-solid-state lithium secondary batteries. The solid electrolyte layer is made of the above-mentioned solid electrolyte. In the all-solid-state lithium secondary battery of the present invention, the configuration other than the negative electrode can be the same as that of a known all-solid-state battery and is not particularly limited. The positive electrode is made of a positive electrode active material layer and a positive electrode current collector, and a known positive electrode active material and current collector may be used. The configuration of the all-solid-state battery is shown in FIG. 1.

The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

(average silicon particle size)
The average particle size and standard deviation of silicon were determined from the laser diffraction/scattered light intensity using an LA-950S2 (manufactured by Horiba, Ltd.).

(specific surface area of silicon)
The specific surface area of the silicon particles was determined by gas adsorption measurement by a constant volume method using a BELSORP-miniX (manufactured by MicrotracBEL).

(Crystalline diameter of silicon)
The crystallite size of silicon was determined from the X-ray diffraction profile by the Halder-Wagner method using Smart Lab (manufactured by Rigaku Corporation).

(Porosity of negative electrode active material layer)
The porosity of the negative electrode active material layer before charge and discharge was measured as follows.
(a) The film thickness of the negative electrode active material layer is obtained from an image taken with a scanning electron microscope, and the volume V0 of the negative electrode active material layer per unit area is calculated.
(b) The volume V1 of the silicon itself is calculated by dividing the mass of silicon per unit area in the negative electrode active material layer by the density of silicon.
(c) When the negative electrode active material layer contains an electronic conductive material or an ionic conductive assistant, the combined volume of the electronic conductive material and the ionic conductive assistant contained in the negative electrode active material layer per unit area is defined as V2.
(d) The porosity of the negative electrode active material layer is calculated as (V0-V1-V2)/V0×100(%).
In the examples shown below, no electronically conductive material or ionically conductive assistant is included.

(Size of voids in negative electrode active material layer)
The size of the voids in the negative electrode active material layer before charging and discharging was measured as follows.
(1) The cross section of the silicon coating film was observed using an image taken with a scanning electron microscope, and the size of the voids (axial diameter in the longitudinal direction) was measured from the image.
The measurement in (2) was carried out for 100 or more voids, and the median diameter was calculated.

(Cycle characteristics)
The charge and discharge test was performed using a BTS-2004H (manufactured by Nagano Corporation) at a constant current density of 0.1 mAcm ^-2 . The measurement temperature was 25°C, and only the initial charge amount was 3000 mAhg ^-1 , but the initial discharge amount was 2000 mAhg ^-1 . From the second time onwards, the state after the initial discharge was set to 0 mAhg ^-1 , and the charge amount was set to 2000 mAhg ^-1 and the discharge amount was set to 2000 mAhg ^-1 . The charge and discharge of 2000 mAhg ^-1 was repeated, and the change in charge and discharge capacity was monitored for each cycle.

During charging and discharging, if the potential of the negative electrode reaches +0.02 V, which is the lower limit voltage during charging, or +1.00 V, which is the upper limit voltage during discharging, relative to the metallic Li electrode before the set capacity is reached, it is determined that the capacity limit has been reached, and the charging or discharging operation is temporarily interrupted. If the charging or discharging capacity at this time is 1800 mAhg ^-1 or more (capacity maintenance rate 90%) relative to the set capacity of 2000 mAhg ^- 1, the next discharging or charging operation is resumed to continue the cycle test.

In the charge/discharge test, the test was terminated when the number of cycles reached 100 or when the capacity retention rate fell below 90%. The superiority or inferiority of the battery performance was evaluated based on the number of cycles at which the capacity started to decrease and the number of cycles at which the test was terminated.

(Battery materials)
A half cell was assembled using the materials listed below, and the structure and characteristics of the negative electrode were evaluated.
Counter electrode: Lithium (Li) foil: 0.1 mm thick (manufactured by Honjo Metals Co., Ltd.)
Indium (In) foil: thickness 0.127 mm (manufactured by Aldrich)
Negative electrode Negative electrode current collector: CF-T7F-35 (manufactured by Fukuda Metal Foil & Powder Co., Ltd., arithmetic mean roughness Ra: 2.07)
Binder: A thermosetting polyimide resin (DreamBond (trade name) manufactured by IST Co., Ltd.) was used, and NMP was used as the solvent.
Carbon-based electronic conductive material Solid electrolyte: a-40LiI·60Li ₃ PS ₃ (prepared by mechanical milling) Polycrystalline silicon manufactured by Tokuyama Corporation was used as silicon powder and pulverized in a planetary mill to prepare the following powders.

(Examples 1 to 4, Comparative Examples 1 to 3)
(Production of negative electrode)
360 mg of the polycrystalline silicon powder and the polyimide solution were mixed in an amount of about 40 mg of solid content, and NMP (N-methylpyrrolidone) was further added to obtain a composition for forming an anode for an all-solid-state lithium secondary battery. The composition was stirred for 2 hours (rotation 1056 rpm, revolution 1600 rpm) and degassed for 6 minutes (rotation 290 rpm, revolution 1360 rpm). In Example 4 and Comparative Example 3, an electronic conductive material was also blended.

The resulting coating solution was applied onto the negative electrode current collector using a doctor blade (feed rate 1.9 mm/sec, blade gap 12.5 μm). After drying at room temperature for more than half a day, the polyimide was cured by heating with a heater under vacuum (250-300°C, 30 minutes) to obtain a negative electrode. The porosity of the negative electrode was measured. Figure 2A shows a scanning electron microscope photograph of the top surface of the negative electrode active material layer, and Figure 2B shows a cross-sectional photograph of the negative electrode active material layer. The thickness of the active material layer was in the range of 11-14 μm. The size of the voids present in the negative electrode active material layer was measured from the scanning electron microscope photograph. The results are shown in Table 2.

(Control of porosity and pore size of negative electrode active material layer by particle size of silicon particles)
FIG. 3 shows the average particle size of crystalline silicon and the change in the porosity and size of the voids in the negative electrode active material layer when the negative electrode active material layer is formed using the crystalline silicon. As the Si particle size increases, the porosity in the coating film increases and becomes almost constant when the particle size is around 1.0 μm. The voids contribute to the relaxation of the expansion of silicon. On the other hand, the median diameter indicating the size of the voids increases with the increase in the Si particle size, but the larger the size of each void, the more difficult it becomes to effectively relax the expansion. In this way, the porosity and size of the voids in the coating film can be changed by the Si particle size.

(Half-cell manufacturing)
The negative electrode sheet was punched to 9 mmΦ and placed in an insulating die, and 65 mg of solid electrolyte powder was loaded from the top of the negative electrode sheet, and uniaxial pressing was performed at a molding pressure of 560 MPa. The thickness of the solid electrolyte layer after pressing was 300 to 400 μm. After removing the upper punch once, a counter electrode made of In and Li metal foils punched to 6 mmΦ stacked in the order of In/Li/In was placed on the top of the solid electrolyte layer, and uniaxial pressing was performed again at a pressure of about 50 MPa to produce an all-solid-state half cell (half cell). The assembly of the above half cell was performed in an argon atmosphere in a glove box that was blocked from the outside air in order to eliminate the influence of oxygen, nitrogen, moisture, etc. The above battery evaluation was performed on the obtained half cell. Scanning electron microscope photographs of the cross section of the negative electrode active material layer after the first discharge are shown in FIG. 4 and FIG. 5. In addition, the charge and discharge curve of the half cell shown in Table 2 as Example 2 is shown in FIG. 6.

As is clear from FIG. 4 and FIG. 5, a vertical crack structure is formed in the negative electrode active material layer after discharge. Due to this effect, the expansion and contraction in the film thickness direction during volume expansion and contraction of the silicon active material accompanying charge and discharge are alleviated. In addition, a portion where the solid electrolyte penetrates from the solid electrolyte layer is formed in a part of the vertical crack, and this penetration structure functions as an ion conduction path in the negative electrode active material layer. Furthermore, since the negative electrode active material layer having a vertical crack structure is in close contact with the negative electrode current collector and the solid electrolyte layer, high electronic conductivity and ion conductivity can be achieved without using an electronic conductive material or an ion conductive auxiliary. Due to these effects, as shown in FIG. 6, even when charged and discharged at a current density of 1.0 mA/cm ² , the electrochemical capacity of 2,000 mAh/g is maintained, and the battery charge/discharge cycle characteristics are good, and the current density during charging and discharging can be increased, and the silicon negative electrode for an all-solid-state lithium secondary battery functions as such.

As examples, Table 2 shows the characteristics of silicon negative electrodes made using silicon particles with average particle sizes of 0.8, 1.0, and 1.6 μm, and a silicon negative electrode containing silicon particles with an average particle size of 1.0 μm and 1.1 parts by mass of electronic conductive material. Table 2 also shows the characteristics of silicon negative electrodes made using silicon particles with average particle sizes of 0.5 and 3.9 μm, and a silicon negative electrode containing silicon particles with an average particle size of 1.0 μm and 5.9 parts by mass of electronic conductive material, as comparative examples. The amount of electronic conductive material and binder added in the table indicates the ratio to 100 parts by mass of silicon particles.

A: Charge/discharge capacity after 100 cycles: 2,000 mAh/g (capacity maintenance rate: 100%)
F1: The initial charge amount did not reach 3,000 mAh/g, and the discharge amount was 1,300 mAh/g, so the test was terminated. F2: The charge/discharge capacity decreased at 20 cycles, and the capacity retention rate became less than 90% at the 50th cycle, so the test was terminated. F3: The charge/discharge capacity decreased at 25 cycles, and the capacity retention rate became less than 90% at the 50th cycle, so the test was terminated.

REFERENCE SIGNS LIST 1 All-solid-state battery 11 Negative electrode current collector 12 Negative electrode active material layer 13 Solid electrolyte layer 14 Positive electrode active material layer 15 Positive electrode current collector 21 Vertical crack formed in negative electrode active material layer 31 Solid electrolyte penetrating vertical crack in negative electrode active material layer 32 Void remaining in vertical crack in negative electrode active material layer

Claims (5)

A coated silicon anode for all-solid-state lithium secondary batteries, in which a layer of anode active material containing silicon particles with an average particle size of 0.8 to 2.0 μm is coated onto a metal foil. The coated silicon anode for an all-solid-state lithium secondary battery according to claim 1, wherein the anode active material layer contains 5 parts by mass or less of a carbon-based electronic conductive material and 20 parts by mass or less of a binder per 100 parts by mass of the silicon particles. 2. The coated silicon negative electrode for an all-solid-state lithium secondary battery according to claim 1, wherein the negative electrode active material layer has a porosity of 50% or more and a void size of 0.5 μm or less.
The porosity in the negative electrode active material layer refers to a value calculated by the following procedure.
(a) The film thickness of the negative electrode active material layer is obtained from an image taken with a scanning electron microscope, and the volume V0 of the negative electrode active material layer per unit area is calculated.
(b) The volume V1 of the silicon itself is calculated by dividing the mass of silicon per unit area in the negative electrode active material layer by the density of silicon.
(c) When the negative electrode active material layer contains an electronic conductive assistant or an ionic conductive assistant, the combined volume of the electronic conductive assistant and the ionic conductive assistant contained in the negative electrode active material layer per unit area is defined as V2.
(d) The porosity of the negative electrode active material layer is calculated as (V0-V1-V2)/V0×100(%).
The size of the voids in the negative electrode active material layer refers to the median diameter of the maximum diameter of 100 or more independent voids observed in a scanning electron micrograph of the cross section of the negative electrode active material layer. The coated silicon anode for an all-solid-state lithium secondary battery according to claim 1, wherein the anode active material layer does not contain a solid electrolyte. An all-solid-state lithium secondary battery using the negative electrode according to any one of claims 1 to 4.

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