JP7525834B2 - All-solid-state battery - Google Patents

Description

The present invention relates to an all-solid-state battery.

Lithium-ion batteries have a higher energy density than other secondary batteries and are used in a wide range of fields, including portable devices and mobility. In the future, there will be an increasing demand for longer use of portable devices, higher power consumption, and increased driving range for mobility. This has created a demand for secondary batteries that are both high energy density and safe, and expectations are high for next-generation batteries to replace lithium-ion batteries.

Among next-generation batteries, all-solid-state batteries use an inorganic solid electrolyte, whereas lithium-ion batteries use a liquid organic electrolyte. All-solid-state batteries are highly safe because they use non-flammable and chemically stable inorganic ceramics as the electrolyte, and because they do not use an organic electrolyte, they can be used at high temperatures, can be operated at high potentials, and can increase energy density.

Regarding the negative electrode used in an all-solid-state battery, a method is known in which a solid electrolyte with lithium ion conductivity is contained in the negative electrode to construct a conductive path for lithium ions in the negative electrode. In addition, a configuration is known in which an ion conductive resin is contained in the negative electrode as a configuration for constructing a conductive path for lithium ions without containing a solid electrolyte in the negative electrode (for example, Patent Document 1). Both methods have in common the fact that an ion conductive material must be mixed in advance to prepare the negative electrode. However, it is difficult to sufficiently form a good interface between the negative electrode active material and the solid electrolyte, making it difficult to reduce the interface resistance.

JP 2019-186212 A

In view of the above problems, the present invention aims to provide an all-solid-state battery that can reduce the interfacial resistance between the negative electrode active material and the solid electrolyte.

In order to achieve the above object, the present invention provides an all-solid-state battery including a positive electrode layer, a negative electrode layer, and a solid electrolyte layer disposed between the positive electrode layer and the negative electrode layer, the negative electrode layer containing a compound represented by M( _BH4 ) ₂ (M is one or more elements selected from the group consisting of Mg, Ca, Mn, and Zn).

As described above, in the all-solid-state battery according to the present invention, the compound represented by M( _BH4 ) ₂ (wherein M is one or more elements selected from the group consisting of Mg, Ca, Mn, and Zn) contained in the negative electrode layer generates an ion-conductive material during the discharge process, and the gaps between the particles of the negative electrode active material and the solid electrolyte are filled, improving the adhesion and reducing the interface resistance.

FIG. 1 is a schematic diagram showing one embodiment of an all-solid-state battery according to the present invention. FIG. 2 is a schematic diagram showing another embodiment of an all-solid-state battery according to the present invention. FIG. 2 is a schematic diagram showing the configuration of a cell for evaluating battery characteristics used in the examples. 1 is a graph showing the results of a charge/discharge test in Example 1. 1 is a graph showing the results of a charge/discharge test in Example 2. 1 is a graph showing the reversible capacity of Examples 1 and 2 at cycles 1 to 15. 1 is a graph showing the diffraction patterns of the negative electrode layer before and after discharge in Example 1.

The following describes an embodiment of the all-solid-state battery according to the present invention with reference to the attached drawings.

As shown in FIG. 1, the all-solid-state battery 10 of the first embodiment includes a positive electrode layer 11, a negative electrode layer 13, and a solid electrolyte layer 12 located between the positive electrode layer 11 and the negative electrode layer 13.

The positive electrode layer 11 is a layer containing a positive electrode active material. Examples of the positive electrode active material include LiCoO ₂ , LiNi _x Co _1-x O ₂ (0<x<1), LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiMnO ₂ , heteroelement-substituted Li-Mn spinel (LiMn _1.5 Ni _0.5 O ₄ , LiMn _1.5 Al _0.5 O ₄ , LiMn _1.5 Mg _0.5 O ₄ , LiMn _1.5 Co _0.5 O ₄ , LiMn _1.5 Fe _0.5 O ₄ , LiMn _1.5 Zn _0.5 O ₄ ), lithium transition metal phosphate (LiFePO ₄ , LiMnPO ₄ Examples of the lithium-based materials include lithium-based materials such as LiCoPO ₄ and LiNiPO ₄ .

In addition to the positive electrode active material, the positive electrode layer 11 may contain, for example, a conductive assistant, a binder, and a solid electrolyte. Examples of the conductive assistant include highly conductive carbon black such as acetylene black (AB) and ketjen black, graphite, coke, and metal powders such as Ni powder, Cu powder, and Ag powder. Examples of the binder include fluorine-based binders such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), and rubber-based binders such as styrene butadiene rubber (SBR). The content of the positive electrode active material in the positive electrode layer 11 is preferably in the range of, for example, 70% by weight to 95% by weight. The thickness of the positive electrode layer 11 is preferably in the range of, for example, 2 μm to 100 μm.

The solid electrolyte layer 12 is a layer containing solid electrolyte particles 1. Examples of solid electrolytes include hydride-based solid electrolytes, sulfide-based solid electrolyte materials, and oxide-based solid electrolyte materials, among which hydride-based solid electrolytes are particularly preferred. By using a hydride-based solid electrolyte that is the same as the ion-conductive material generated in the anode layer 13 during the discharge process, which will be described later in detail, the interface resistance between the anode active material and the solid electrolyte can be further reduced. Examples of hydride-based solid electrolytes include LiBH ₄ (lithium borohydride), Li ₂ B ₁₂ H ₁₂ , Li ₄ (BH ₄ ) ₃ I, and Li ₂ B ₁₂ H ₁₂ , among which LiBH ₄ is particularly preferred. By using LiBH ₄ , which is the same compound as the ion-conductive material generated in the anode layer 13 during the discharge process, which will be described later in detail, the interface resistance between the anode active material and the solid electrolyte can be further reduced. Examples of sulfide-based solid electrolyte materials include _{Li9.54Si1.74P1.44S11.7Cl0.3 and Li10GePS12} , _and examples of oxide-based solid electrolyte materials include _{Li1 + xAlxTi2 -x ( PO4 ) 3 and Li1.5Al0.5Ge1.5} ( _PO4 ) _{3 .}

The average particle diameter of the solid electrolyte particles 1 in the solid electrolyte layer 12 is preferably in the range of, for example, 50 μm to 100 μm. In this specification, the term "average particle diameter" refers to the median diameter (50% particle diameter) in the volume-based particle size distribution measured using a laser diffraction particle size distribution measuring device. The thickness of the solid electrolyte layer 12 is preferably in the range of, for example, 300 μm to 500 μm.

The negative electrode layer 13 is a layer containing particles 2 of a compound represented by M(BH ₄ ) ₂ (M is one or more elements selected from the group consisting of Mg, Ca, Mn, and Zn) as a negative electrode active material. By using a compound represented by M(BH ₄ ) ₂ as a negative electrode active material, an ion conductive material can be self-generated in the negative electrode layer 13 during discharge as shown in the following reaction formula 1, so that the gap between the particles 2 of the negative electrode active material and the particles 1 of the solid electrolyte can be filled, and the adhesion can be improved to reduce the interface resistance. Note that the following reaction formula is for the case where a lithium-based material is used as the positive electrode active material.
M( _BH4 ) ₂ + 2Li ^{++ 2e-} →M+ _2LiBH4 ... (Reaction formula 1)

In addition, since an ion conductive material such as LiBH ₄ is generated in the anode layer 13 during the discharge process, it is not necessary to previously include solid electrolyte particles 1 in the anode layer 13 as an ion conductive material. In a method in which solid electrolyte particles 1 similar to the solid electrolyte layer 22 are previously mixed as an ion conductive material in the anode layer 23 as in the all-solid-state battery 20 shown in FIG. 2 described later, the amount of solid electrolyte particles 1 is mixed in the anode layer 23 in a larger amount than the amount of ion conductive material originally required, so that the amount of negative electrode active material particles 2 is relatively small, and there is a risk of reducing the actual negative electrode capacity. Therefore, since the negative electrode layer 13 does not contain the solid electrolyte particles 1, it is possible to relatively hold a large amount of negative electrode active material particles 2, and the actual negative electrode capacity can be increased.

The negative electrode layer 13 may contain particles 3 of a conductive assistant, if necessary, in addition to particles 2 of a compound represented by M(BH ₄ ) _2. Examples of conductive assistants include highly conductive carbon black such as acetylene black (AB) and ketjen black, graphite, coke, and metal powders such as Ni powder, Cu powder, and Ag powder. The content of the compound represented by M(BH ₄ ) ₂ in the negative electrode layer 13 is preferably 40% by weight or more, more preferably 60% by weight or more, and even more preferably 70% by weight or more. The upper limit of the content of the compound represented by M(BH ₄ ) ₂ in the negative electrode layer 13 is not particularly limited, but is preferably 85% by weight or less, and more preferably 70% by weight or less. The content of the conductive assistant in the negative electrode layer 13 is preferably, for example, in the range of 15% by weight to 40% by weight, and more preferably in the range of 25% by weight to 35% by weight. The negative electrode layer 13 does not contain any solid electrolyte particles 1 .

The average particle size of the particles 2 of the compound represented by M( _BH4 ) ₂ in the negative electrode layer 13 is preferably in the range of, for example, 5 μm to 30 μm, and more preferably in the range of 5 μm to 20 μm. The average particle size of the particles 3 of the conductive assistant in the negative electrode layer 13 is preferably in the range of, for example, 1 μm to 10 μm, and more preferably in the range of 1 μm to 5 μm. The thickness of the negative electrode layer 13 is preferably in the range of, for example, 2 μm to 100 μm.

The all-solid-state battery 10 may, if necessary, include a positive electrode current collector (not shown) in the positive electrode layer 11 and a negative electrode current collector (not shown) in the negative electrode layer 13. Materials for the positive electrode current collector and the negative electrode current collector may be, for example, platinum, copper, stainless steel, nickel, titanium, aluminum, etc.

As shown in FIG. 2, the all-solid-state battery 20 of the second embodiment includes a positive electrode layer 21, a negative electrode layer 23, and a solid electrolyte layer 22 located between the positive electrode layer 21 and the negative electrode layer 23. The all-solid-state battery 20 has a configuration similar to that of the all-solid-state battery 10 of the first embodiment, except for the configuration of the negative electrode layer 23. As shown in FIG. 2, the negative electrode layer 23 includes particles 2 of a compound represented by M(BH ₄ ) ₂ (M is one or more elements selected from the group consisting of Mg, Ca, Mn, and Zn), particles 3 of a conductive assistant, and particles 1 of a solid electrolyte. These particles 1 to 3 are the same as those of the first embodiment, and therefore will not be described here.

The content of the compound represented by M( _BH4 ) ₂ in the negative electrode layer 23 is preferably in the range of 30% by weight to 70% by weight, more preferably in the range of 35% by weight to 60% by weight. The content of the conductive assistant in the negative electrode layer 23 is preferably in the range of 15% by weight to 40% by weight, more preferably in the range of 25% by weight to 35% by weight. The content of the solid electrolyte in the negative electrode layer 23 is preferably in the range of 15% by weight to 40% by weight, more preferably in the range of 25% by weight to 35% by weight.

In this way, the negative electrode layer 23 contains the same solid electrolyte particles 1 as the solid electrolyte layer 22 as an ion conductive material in advance. However, not all of the solid electrolyte particles 1 in the negative electrode layer 23 contribute to the electrode reaction during charging and discharging, and the solid electrolyte particles 1 that do not contribute to the electrode reaction are also scattered in the negative electrode layer 23, so that a larger amount of solid electrolyte particles 1 than the amount of ion conductive material originally required is mixed in the negative electrode layer 23. Therefore, although the amount of the negative electrode active material particles 2 is relatively smaller than that of the negative electrode layer 13 of the all-solid-state battery 10 of the first embodiment and the actual negative electrode capacity is reduced, the present invention does not completely exclude the embodiment in which the solid electrolyte particles 1 are already contained in the negative electrode layer 23.

[1. Preparation of cells for evaluating battery characteristics]
A cell 30 for evaluating battery characteristics shown in FIG. 3 was fabricated. The cell 30 for evaluating battery characteristics includes a counter electrode layer 31, an anode layer 33, and a solid electrolyte layer 32 disposed between the counter electrode layer and the anode layer. The cell 30 for evaluating battery characteristics was restrained in the stacking direction by a stainless steel restraining frame 34 with a constant pressure. Both ends of the upper frame plate 34e and the lower frame plate 34b of the restraining frame 34 were connected via vertical frame members 34b and 34c, respectively. Each member 34b to 34e of the restraining frame was connected via an O-ring 35. The cell 30 for evaluating battery characteristics was sandwiched between an adjustment plate 34a and a lower frame plate 34b, which were located in the restraining frame 34 and provided on the upper frame plate 34e via a spring 36.

As the counter electrode layer 31, lithium metal (Li) foil (diameter 10 mm, thickness 150 μm) was used. As the material of the solid electrolyte layer 32, 30 mg of LiBH ₄ particles (manufactured by Sigma-Aldrich, average particle size 300 μm) was used. As the material of the negative electrode layer 33, a total of 6 mg of Mg (BH ₄ ) ₂ particles (manufactured by Sigma-Aldrich, average particle size 30 μm) and acetylene black (AB) particles (manufactured by Denka, primary particle size 36 nm) were used in a weight ratio of 7:3, and ball mill mixing was performed at 400 rpm for 2 hours. Then, the solid electrolyte layer 32 was formed into a pellet shape by uniaxially pressing the above material with a load of 20 kN. Moreover, as the negative electrode layer 33, the above mixed material was formed into a pellet shape by uniaxially pressing the above mixed material with a load of 30 kN. These operations were performed with a press machine in a glove box. The above-mentioned Li foil was attached to these two-layer pellets to prepare a cell for evaluating battery characteristics (Example 1).

[2. Charge/Discharge Measurement]
A charge-discharge test was performed on the battery characteristic evaluation cell 30 prepared above using a charge-discharge tester (Nagano Corporation, product number: BTS2004H) under conditions of a temperature of 120° C., a current density of 0.5 mA/cm ⁻² , and a voltage range of 0.3 V-1.5 V vs. Li ⁺ /Li. The results are shown in FIG. 4. In addition, in order to verify the results, a charge-discharge test was also performed on a battery characteristic evaluation cell (Example ₂ ) prepared in the same manner as in Example 1, except _that LiBH ₄ particles were further added as the material for the negative electrode layer 33, and Mg(BH 4 ) 2 :LiBH ₄ :AB was mixed in a weight ratio of 4:3:3. The results are shown in FIG. 5.

As shown in Figures 4 and 5, in both Example 1 and Example 2, a flat discharge plateau of about 0.9 V was obtained at a large current of 0.5 mA cm ^-2 . In Example 1, the discharge reaction proceeded in the same manner as in Example 2 even if no ion conductive material was mixed into the negative electrode layer. In addition, Figure 6 shows the reversible capacity from 1 to 15 cycles in Examples 1 and 2. Charge and discharge proceeded even if LiBH ₄ was not present in the negative electrode layer, and the reversible capacity was larger up to the third cycle and from the 12th cycle onwards when LiBH ₄ was not present.

From this result, it is presumed that _LiBH4 is self-generated as an ion conductive material on the negative electrode layer side by discharging, as shown in the above reaction formula 1.

In addition, the diffraction patterns of the negative electrode layer side before and after discharge in Example 1 were obtained using an X-ray diffraction measuring device (Rigaku Corporation, product number: MiniFlex 600) and are shown in Figure 7. The above results could be confirmed by observing peaks of _LiBH4 and Mg after discharge.

In the above example, Mg( _BH4 ) ₂ was used as the negative electrode active material. However, since the structure is similar and the reaction is considered to be similar, it is presumed that _LiBH4 will be self-generated as an ion conductive material even if Ca( _BH4 ) ₂ , Mn( _BH4 ) ₂ , or Zn( _BH4 ) ₂ is used as the negative electrode active material.

10 All-solid-state battery 20 All-solid-state battery (comparative example)
30 Cell for evaluating battery characteristics 11, 21 Positive electrode layer 12, 22, 32 Solid electrolyte layer 13, 23, 33 Negative electrode layer 31 Counter electrode layer 34 Restraint frame

Claims (3)

An all-solid-state lithium ion battery comprising a positive electrode layer, a negative electrode layer, and a solid electrolyte layer disposed between the positive electrode layer and the negative electrode layer, wherein the negative electrode layer contains a compound represented by M( _BH4 ) ₂ (M is one or more elements selected from the group consisting of Mg, Ca, Mn , and Zn). 2. The all-solid-state lithium ion battery according to claim 1, wherein the solid electrolyte layer comprises a hydride-based solid electrolyte. 3. The all-solid-state lithium ion battery of claim 2, wherein the hydride-based solid electrolyte comprises _LiBH4 .