Patent Application
20240154182
The present invention relates to a lithium secondary battery.
In recent years, in order to fight global warming, there is a strong need for reduction of the amount of carbon dioxide emissions. In the automobile industry, there are increasing expectations for a reduction of carbon dioxide emissions by introduction of electric vehicles (EV) and hybrid electric vehicles (HEV), and development of non-aqueous electrolyte secondary batteries such as secondary batteries for motor driving, which are key to practical application of such vehicles, has been actively conducted.
A secondary battery for motor driving is required to have extremely high output characteristics and high energy as compared with a lithium ion secondary battery for consumer use used in a mobile phone, a notebook computer, and the like. Therefore, a lithium ion secondary battery having the highest theoretical energy among all practical batteries has attracted attention, and is currently being rapidly developed.
Here, lithium ion secondary batteries that are currently widespread use a combustible organic electrolyte solution as an electrolyte. In such liquid-based lithium ion secondary batteries, safety measures against liquid leakage, short circuit, overcharge, and the like are more strictly required than other batteries.
Therefore, in recent years, research and development on an all-solid lithium secondary battery using an oxide-based or sulfide-based solid electrolyte as an electrolyte have been actively conducted. The solid electrolyte is a material mainly made of an ion conductor that enables ion conduction in a solid. For this reason, in an all-solid lithium secondary battery, in principle, various problems caused by combustible organic electrolyte solution do not occur unlike the conventional liquid-based lithium ion secondary battery. In general, use of a high-potential and large-capacity positive electrode material and a large-capacity negative electrode material can achieve significant improvement in output density and energy density of a battery.
In the related art, as one type of all-solid lithium secondary battery, a so-called lithium-deposition-type has been known in which lithium metal is deposited on a negative electrode current collector in a charging process (see, for example, JP 2019-61867 A). In the charging process of the lithium-deposition-type all-solid lithium secondary battery, the lithium metal is deposited between a solid electrolyte layer and the negative electrode current collector. In the lithium secondary battery described in JP 2019-61867 A, an electrolyte layer interposed between a positive electrode containing lithium and a negative electrode containing lithium is configured to include an electrolyte layer including a first electrolyte and a second electrolyte containing iodine provided between the electrolyte layer and the negative electrode, the ion conductivity of the second electrolyte being smaller than the ion conductivity of the first electrolyte. According to JP 2019-61867 A, with such a configuration, even when the interface between the electrolyte layer and the negative electrode is not flat, ununiform deposition of lithium is eliminated, leading to suppression of dendrite formation. As a result, variations in internal resistance and a decrease in discharging capacity due to the growth of lithium dendrites are reduced, and it is possible to provide a lithium battery having excellent charge and discharge characteristics.
However, according to the study of the present inventors, it has been found that a sufficient charge and discharge efficiency may still not be achieved even when the technique described in JP 2019-61867 A is used.
Therefore, an object of the present invention is to provide a means capable of further improving a charge and discharge efficiency in a lithium-deposition-type lithium secondary battery.
The present inventors have carried out a diligent study to solve the problems described above. As a result, the present inventors have found that in a secondary battery including a lithium-deposition-type power-generating element and a pressurizing member that pressurizes the power-generating element in a direction of lamination thereof, a positive electrode active material layer is formed into a size which is one size smaller than a solid electrolyte layer, and a predetermined functional layer is provided on at least a part of a principal surface of the solid electrolyte layer facing a negative electrode current collector and at least a part of a side surface of the solid electrolyte layer, and thus the above-described problems can be solved. Therefore, they have completed the present invention.
That is, one embodiment of the present invention relates to a lithium secondary battery including: a power-generating element that includes a positive electrode in which a positive electrode active material layer containing a positive electrode active material capable of absorbing and desorbing lithium ions is disposed on a surface of a positive electrode current collector, a negative electrode having a negative electrode current collector, where lithium metal is deposited on the negative electrode current collector during charging, and a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte; and a pressurizing member that pressurizes the power-generating element at a predetermined pressure in a direction of lamination thereof. In the lithium secondary battery, at least a part of the outer peripheral end of the positive electrode active material layer is located inside the outer peripheral end of the solid electrolyte layer, when the power-generating element is viewed in in planar view. The present invention is also characterized in that a first functional layer is provided on at least a part of a principal surface of the solid electrolyte layer facing the negative electrode current collector and at least a part of a side surface of the solid electrolyte layer, the first functional layer having electronic insulation properties and lithium ion conductivity and being more stable in reductive decomposition due to contact with lithium metal than the solid electrolyte.
According to the present invention, it is possible to further improve a charge and discharge efficiency in a lithium-deposition-type lithium secondary battery.
One embodiment of the present invention is a lithium secondary battery including: a power-generating element that includes a positive electrode in which a positive electrode active material layer containing a positive electrode active material capable of absorbing and desorbing lithium ions is disposed on a surface of a positive electrode current collector, a negative electrode having a negative electrode current collector, where lithium metal is deposited on the negative electrode current collector during charging, and a solid electrolyte layer interposed between the positive electrode and the negative electrode and containing a solid electrolyte; and a pressurizing member that pressurizes the power-generating element at a predetermined pressure in a direction of lamination thereof, in which when the power-generating element is viewed in planar view, at least a part of an outer peripheral end of the positive electrode active material layer is located inside an outer peripheral end of the solid electrolyte layer, and
Hereinafter, the present embodiment will be described with reference to the drawings, but the technical scope of the present invention should be determined based on the description of the claims, and is not limited to the following embodiments. The dimensional ratios of the drawings are exaggerated for convenience of description and may differ from the actual ratios.
As illustrated in
The negative electrode current collector 11′ and the positive electrode current collector 11″ have a structure in which a negative electrode current collecting plate 25 and a positive electrode current collecting plate 27 which are electrically connected to the respective electrodes (the negative electrode and the positive electrode) are respectively attached to the negative electrode current collector 11′ and the positive electrode current collector 11″ and are led to an outside of the laminate film 29 so as to be sandwiched between ends of the laminate film 29. The negative electrode current collecting plate 25 and the positive electrode current collecting plate 27 may be attached to the negative electrode current collector 11′ and the positive electrode current collector 11″ of the respective electrodes with a negative electrode terminal lead and a positive electrode terminal lead (not illustrated) interposed therebetween, respectively by ultrasonic welding, resistance welding, or the like as necessary.
In the embodiment illustrated in
In addition, in the embodiment illustrated in
As illustrated in
The lower limit of the load applied to the power-generating element 21 (confining pressure in the direction of lamination of the power-generating element) is, for example, 0.1 MPa or more, preferably 1 MPa or more, more preferably 3 MPa or more, and still more preferably 5 MPa or more. The upper limit of the confining pressure in the direction of lamination of the power-generating element is, for example, 100 MPa or less, preferably 70 MPa or less, more preferably 40 MPa or less, and still more preferably 10 MPa or less.
Hereinafter, main components of the laminate type secondary battery 10a described above will be described.
[Positive Electrode Current Collector]
The positive electrode current collector is a conductive member that functions as a flow path for electrons emitted from a positive electrode toward an external load or flowing from a power source toward the positive electrode along with the progression of the battery reaction (charge and discharge reaction). A material constituting the positive electrode current collector is not particularly limited. As the material constituting the positive electrode current collector, for example, a metal or a resin having conductivity can be adopted. The thickness of the positive electrode current collector is not particularly limited, but is, for example, from 10 to 100 μm.
[Positive Electrode Active Material Layer]
A positive electrode constituting the lithium secondary battery according to the present embodiment has a positive electrode active material layer containing a positive electrode active material capable of absorbing and desorbing lithium ions. The positive electrode active material layer 15 is disposed on the surface of the positive electrode current collector 11″ as illustrated in
The positive electrode active material is not particularly limited as long as it is a material that can desorb lithium ions in the process of charging the secondary battery and can absorb lithium ions in the process of discharging the secondary battery. An example of the positive electrode active material includes a positive electrode active material containing an M1 element and an O element, in which the M1 element includes at least one element selected from the group consisting of Li, Mn, Ni, Co, Cr, Fe, and P. Examples of the positive electrode active material include layered rock salt-type active materials such as LiCoO2, LiMnO2, LiNiO2, and Li(Ni—Mn—Co)O2, spinel-type active materials such as LiMn2O4 and LiNi0.5Mn1.5O4, olivine-type active materials such as LiFePO4 and LiMnPO4, and Si-containing active materials such as Li2FeSiO4 and Li2MnSiO4. Examples of the oxide active material other than those described above include Li4Ti5O12 and LiVO2. In some cases, two or more kinds of positive electrode active materials may be used in combination. Needless to say, a positive electrode active material other than the above-described positive electrode active materials may be used. In a preferred embodiment, the positive electrode active material layer 15 constituting the lithium secondary battery according to the present embodiment includes a layered rock salt-type active material (e.g., Li (Ni—Mn—Co)O2) containing lithium and cobalt as positive electrode active materials, from the viewpoint of output characteristics.
The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but for example, is preferably within a range of 30 to 99 mass %, more preferably within a range of 40 to 90 mass %, and still more preferably within a range of 45 to 80 mass %.
In the lithium secondary battery according to the present embodiment, it is preferable that the positive electrode active material layer 15 further contains a solid electrolyte. Examples of the solid electrolyte include a sulfide solid electrolyte, a resin solid electrolyte, and an oxide solid electrolyte. As the solid electrolyte, a material having a desired volume modulus can be appropriately selected according to the degree of volume expansion accompanying charge and discharge of the electrode active material to be used.
In a preferred embodiment of the secondary battery according to the present embodiment, the solid electrolyte preferably includes a resin solid electrolyte from the viewpoint of following the volume change of the electrode active material associated with charging and discharging. Examples of the resin solid electrolyte include fluorine resins, polyethylene oxide, polyacrylonitrile, polyacrylates, derivatives of these materials, and copolymers of these materials. Examples of the fluorine resins include fluorine resins containing vinylidene fluoride (VdF), hexafluoropropylene (HFP), tetrafluoroethylene (TFE), derivatives of these compounds, and the like as constituent units. Specific examples thereof include homopolymers such as polyvinylidene fluoride (PVdF), polyhexafluoropropylene (PHFP), and polytetrafluoroethylene (PTFE), and a binary copolymer such as a copolymer of VdF and HFP.
In another preferred embodiment of the secondary battery according to the present embodiment, the solid electrolyte is preferably a sulfide solid electrolyte containing an S element, more preferably a sulfide solid electrolyte containing a Li element, an M element, and an S element, the M element is a sulfide solid electrolyte containing at least one element selected from the group consisting of P, Si, Ge, Sn, Ti, Zr, Nb, Al, Sb, Br, Cl, and I, and still more preferably a sulfide solid electrolyte containing an S element, a Li element, and a P element, from the viewpoint of exhibiting excellent lithium ion conductivity and following the volume change of the electrode active material associated with charging and discharging. The sulfide solid electrolyte may have a Li3PS4 skeleton, a Li4P2S7 skeleton, or a Li4P2S6 skeleton. Examples of the sulfide solid electrolyte having a Li3PS4 skeleton include LiI—Li3PS4, LiI—LiBr—Li3PS4, and Li3PS4. Examples of the sulfide solid electrolyte having a Li4P2S7 skeleton include a Li—P—S-based solid electrolyte called LPS. As the sulfide solid electrolyte, for example, LGPS expressed by Li(4-x)Ge(1-x)PxS4 (x satisfies 0<x<1) or the like may be used. More specifically, examples of the sulfide solid electrolyte include LPS(Li2S—P2S5), Li7P3S11, Li3.2P0.96S, Li3.25Ge0.25P0.75S4, Li10GeP2Si2, Li6PS5X (where X is Cl, Br, or I), and the like. The description of “Li2S—P2S5” means a sulfide solid electrolyte obtained by using a raw material composition containing Li2S and P2S5, and the same applies to other descriptions. Above all, the sulfide solid electrolyte has high ion conductivity and low volume modulus, and thus is preferably selected from the group consisting of LPS (Li2S—P2S5), Li6PS5X (wherein X is Cl, Br, or I), Li7P3S11, Li32P0.96S, and Li3PS4 from the viewpoint of following the volume change of the electrode active material associated with charging and discharging.
The content of the solid electrolyte in the positive electrode active material layer is not particularly limited, but is, for example, preferably within a range of 1 to 70 mass %, more preferably within a range of 10 to 60 mass %, and still more preferably within a range of 20 to 55 mass %.
The positive electrode active material layer may further contain at least one of a conductive aid and a binder in addition to the positive electrode active material and the solid electrolyte. The thickness of the positive electrode active material layer varies depending on the configuration of the intended lithium secondary battery, but is, for example, preferably within the range of 0.1 to 1000 μm, and more preferably within a range of 40 to 100 μm.
[Solid Electrolyte Layer]
The solid electrolyte layer is a layer interposed between the positive electrode active material layer and the negative electrode current collector during discharging, and contains a solid electrolyte (usually as a main component). Since the specific form of the solid electrolyte contained in the solid electrolyte layer is the same as that described above, the detailed description thereof is omitted here.
The content of the solid electrolyte in the solid electrolyte layer is preferably, for example, within a range of 10 to 100 mass %, more preferably within a range of 50 to 100 mass %, and still more preferably within a range of 90 to 100 mass % with respect to the total mass of the solid electrolyte layer. The solid electrolyte layer may further contain a binder in addition to the solid electrolyte described above. The thickness of the solid electrolyte layer varies depending on the configuration of the intended lithium secondary battery, but is, for example, preferably within a range of 0.1 to 1000 μm, and more preferably within a range of 10 to 40 μm.
As described above with reference to
[Negative Electrode Current Collector]
The negative electrode current collector is a conductive member that functions as a flow path for electrons emitted from a negative electrode toward a power source or flowing from an external load toward the negative electrode with the progression of the battery reaction (charge and discharge reaction). A material constituting the negative electrode current collector is not particularly limited. As the material constituting the negative electrode current collector, for example, a metal or a resin having conductivity can be adopted. The thickness of the negative electrode current collector is not particularly limited, but is, for example, from 10 to 100 μm.
[Negative Electrode Active Material Layer]
The lithium secondary battery according to the present embodiment is a so-called lithium-deposition-type lithium secondary battery in which lithium metal is deposited on a negative electrode current collector in a charging process. A layer made of the lithium metal deposited on the negative electrode current collector in this charging process is a negative electrode active material layer of the lithium secondary battery according to the present embodiment. Therefore, the thickness of the negative electrode active material layer increases with the progress of the charging process, and the thickness of the negative electrode active material layer decreases with the progress of the discharging process. The negative electrode active material layer need not be present when the battery has been completely discharged. However, in some cases, a negative electrode active material layer made of a certain amount of lithium metal may be disposed when the battery has been completely discharged. The thickness of the negative electrode active material layer (lithium metal layer) when the battery has been completely discharged is not particularly limited, but is usually from 0.1 to 1000 μm.
[First Functional Layer]
In the lithium secondary battery according to the present embodiment, the first functional layer is provided on at least a part of a principal surface of the solid electrolyte layer facing the negative electrode current collector (preferably, the entire principal surface) and at least a part of a side surface of the solid electrolyte layer (preferably, the entire side surface). The first functional layer is a layer having electronic insulation properties and lithium ion conductivity. Further, the first functional layer is desired to be more stable in reductive decomposition due to contact with lithium metal than the solid electrolyte. Here, the term “to be more stable in reductive decomposition due to contact with lithium metal than the solid electrolyte” means that when a tendency of the solid electrolyte constituting the solid electrolyte layer to undergo reductive decomposition due to contact with lithium metal is compared with a tendency of the material constituting the first functional layer to undergo reductive decomposition due to contact with lithium metal, the latter tendency is smaller. Whether or not the material constituting the first functional layer satisfies this condition can be determined by confirming whether or not the current flowing through the first functional layer is smaller than the current flowing through the solid electrolyte layer when a voltage is swept near 0 V [vs.Li/Li+] by a cyclic voltammetry method using each of the solid electrolyte layer and the first functional layer as a working electrode and using lithium metal as a counter electrode.
In a case where the first functional layer is also disposed on the side surface of the solid electrolyte layer, even when lithium metal deposited on the surface of the negative electrode current collector during charging is pushed out from the outer peripheral end of the solid electrolyte layer due to confining pressure from the pressurizing member, contact between the solid electrolyte layer and the negative electrode active material layer is prevented, resulting in suppression of deterioration of the solid electrolyte layer due to reductive decomposition. Further, the effective area of the lithium metal facing the positive electrode active material layer with the first functional layer and the solid electrolyte layer interposed therebetween is further increased, which leads an advantage that the charge and discharge efficiency can be further improved. Here, whether or not the first functional layer of the lithium secondary battery according to the present embodiment is disposed can be determined, for example, by confirming whether or not a layer corresponding to the first functional layer is present on the principal surface and the side surface of the solid electrolyte layer by SEM-EDX observation of a cross section of the lithium secondary battery, and then analyzing the composition by elemental analysis or the like. Further, in a case where it is difficult to make a determination by the above method because the first functional layer is thin or the like, it is also possible to make a determination by analyzing a layer corresponding to the first functional layer while performing etching by the XPS method.
Note that the material constituting the first functional layer as described above is not particularly limited, and any material that satisfies the above-described conditions can be suitably used. For example, the first functional layer preferably contains one or more materials selected from the group consisting of lithium halide (lithium fluoride (LiF), lithium chloride (LiCl), lithium bromide (LiBr), lithium iodide (LiI)), a lithium ion conductive polymer, a composite metal oxide represented by Li-M-O (M is one or more metal elements selected from the group consisting of Mg, Au, Al, Sn, and Zn), and a Li—Ba—TiO3 composite oxide. All of these materials are particularly stable in reductive decomposition due to contact with lithium metal, and thus are suitable as materials constituting the first functional layer. In particular, when the first functional layer contains lithium halide, the rate characteristics of the battery can be improved. This is considered to be because the activation barrier when lithium ions diffuse through the solid electrolyte layer and the first functional layer during charging and discharging is lowered, and thus the diffusion rate of lithium ions at the interface is improved, and the area of contact between the first functional layer and the negative electrode active material layer (metal lithium layer) is sufficiently secured.
The average thickness of the first functional layer is not particularly limited, and the first functional layer may be disposed to have a thickness that exhibits the above-described function. However, when the average thickness of the first functional layer is too large, the internal resistance is increased, and this causes a decrease in charge and discharge efficiency. Accordingly, it is preferable that the average thickness of the first functional layer is smaller than the average thickness of the solid electrolyte layer. Meanwhile, when the average thickness of the first functional layer is too small, there is a possibility that a sufficient protective effect by providing the first functional layer cannot be obtained. From these viewpoints, the average thickness of the first functional layer is preferably from 0.5 nm to 20 μm, and more preferably from 5 nm to 10 μm. The “average thickness” of the first functional layer means a value calculated as an arithmetic average value of the thicknesses measured at several to several tens of different portions of the first functional layer constituting the lithium secondary battery. In addition, from the viewpoint of further improving the rate characteristics of the battery, the arithmetic average roughness (Ra; measured in accordance with JIS B 0601: 2013) of the principal surface of the first functional layer facing the negative electrode current collector is preferably less than 1 μm, more preferably 100 nm or less, still more preferably 50 nm or less, particularly preferably 20 nm or less, and most preferably 10 nm or less. Meanwhile, the lower limit of Ra is not particularly limited, but is actually 1 nm or more.
[Insulating Layer]
In the lithium secondary battery according to the present embodiment, as illustrated in
Note that the material constituting the insulating layer as described above is not particularly limited, and any material that satisfies the above-described conditions can be suitably used. An example of the material constituting the insulating layer includes a material in which an inorganic powder such as aluminum oxide, zirconium oxide, silicon oxide, or S—B—Na-based glass frit is dispersed in a solid electrolyte constituting the solid electrolyte layer. However, the material constituting the insulating layer is preferably a resin material or a rubber material. Since these materials have high durability and elasticity, for example, even when internal stress is generated in a region where the insulating layer is formed, the insulating layer extends without being broken. Thus, occurrence of a short circuit can be effectively prevented. Examples of the resin materials include thermoplastic resins such as polyethylene (e.g., low density polyethylene, high density polyethylene, and the like), a polyolefin resin such as polypropylene, a polyester resin such as polyethylene terephthalate (PET), a polyvinyl chloride resin, an acrylic resin, a methacrylic resin, an acrylonitrile-butadiene-styrene resin, a vinyl acetate resin, an ethylene-vinyl acetate resin, and a styrene-butadiene resin; and thermosetting resins such as a silicone resin, a urethane resin, a melamine resin, a thermosetting acrylic resin, a urea resin, a phenol resin, a resorcin resin, an alkyl resorcin resin, an epoxy resin, and thermosetting polyester. Examples of the rubber material include latex rubber, chloroprene rubber (CR), styrene-butadiene rubber (SBR), ethylene-propylene-diene rubber (EPDM), and acrylonitrile-butadiene rubber (NBR).
[Second Functional Layer]
In the lithium secondary battery according to the present embodiment, as illustrated in
Although the case where the secondary battery according to the present embodiment is an all-solid lithium secondary battery is described as an example, the lithium secondary battery according to the present embodiment need not be an all solid type. Hence, the solid electrolyte layer may further contain a conventionally known liquid electrolyte (electrolyte solution). The amount of the liquid electrolyte (electrolyte solution) that can be contained in the solid electrolyte layer is not particularly limited, but is preferably such an amount that the shape of the solid electrolyte layer formed by the solid electrolyte is maintained and liquid leakage of the liquid electrolyte (electrolyte solution) does not occur. As the liquid electrolyte (electrolyte solution), a solution containing a known lithium salt dissolved in a known organic solvent is used. The liquid electrolyte (electrolyte solution) may further contain an additive other than the organic solvent and the lithium salt. These additives may be used singly or in combination of two or more kinds thereof. The amount of the additive used in the electrolyte solution can be appropriately adjusted.
Hereinbelow, the present invention will be described in more detail with reference to Examples. However, the technical scope of the present invention is not limited to the following Examples. Hereinafter, the operation was performed in a glove box. Further, the instruments, devices, and the like used in the glove box were sufficiently dried in advance.
[Production of Evaluation Cell]
(Production of Positive Electrode)n
First, LiNi0.8Mn0.1Co0.1O2 as a positive electrode active material, acetylene black as a conductive aid, and a sulfide solid electrolyte (LPS(Li2S—P2S5)) were weighed to give a mass ratio of 50:30:20, these materials were mixed in a glove box using an agate mortar, and then the resultant mixture was further mixed and stirred in a planetary ball mill. 2 parts by mass of styrene-butadiene rubber (SBR) was added to 100 parts by mass of the resultant mixed powder, and a solvent: mesitylene was added thereto to prepare a positive electrode active material slurry. Next, the positive electrode active material slurry prepared above was applied to a surface of aluminum foil as a positive electrode current collector, and the surface was dried and subjected to a pressing process to form a positive electrode active material layer (thickness: 50 μm). Thus, a positive electrode was produced.
(Production of Solid Electrolyte Layer and First Functional Layer)
2 parts by mass of styrene-butadiene rubber (SBR) was added to 100 parts by mass of sulfide solid electrolyte (LPS(Li2S—P2S5)), and a solvent: mesitylene was added thereto to prepare a solid electrolyte slurry. Then, a surface of stainless steel foil as a support was coated with the solid electrolyte slurry prepared above and dried to form a solid electrolyte layer (thickness: 30 μm) as a free standing film. Thereafter, a first functional layer (thickness: 20 nm) made of lithium fluoride (LiF) was formed by sputtering over one principal surface and side surfaces of the obtained solid electrolyte layer.
(Production of Evaluation Cell)
The solid electrolyte layer having the first functional layer produced as described above was transferred by cold isostatic pressing (CIP) to the side of the positive electrode active material layer of the positive electrode produced as described above such that the exposed surface of the solid electrolyte layer faced the positive electrode active material layer. At this time, the pressure during the CIP was controlled such that the outer peripheral edge portion of the solid electrolyte layer was extended to the middle of the side surface of the positive electrode active material layer over the entire periphery. Finally, stainless steel foil as a negative electrode current collector was laminated on the exposed surface of the first functional layer to produce an evaluation cell (lithium-deposition-type all-solid lithium secondary battery) in the same form as in
[Evaluation of Evaluation Cell (Measurement of Charge and Discharge Efficiency)]
A positive electrode lead and a negative electrode lead were connected to the positive electrode current collector and the negative electrode current collector of the evaluation cell produced above, respectively. Then, the evaluation cell was charged and discharged for 2 cycles according to the following charge and discharge test conditions. At this time, the charge and discharge test described below was performed while applying a confining pressure of 5 [MPa] in the direction of lamination of the evaluation cell using the pressurizing member.
(Charge and Discharge Test Conditions)
The evaluation cell was charged from 3.0 V to 4.3 V at 0.2 C in a constant current/constant voltage (CCCV) mode in a charging process (lithium metal was deposited on the negative electrode current collector) in a thermostatic bath set at the above evaluation temperature using a charge and discharge tester (0.01 C cut-off). Thereafter, in the discharging process (lithium metal on the negative electrode current collector was dissolved), a constant current (CC) mode was used, and the evaluation cell was discharged from 4.3 V to 3.0 V at 0.2 C. Here, 1 C is a current value at which the cell is fully charged (100% charged) when charged at the current value for 1 hour. Here, the charging capacity (capacity of the cell during charging) and the discharging capacity (capacity of the cell during discharging) were each measured during charging and discharging processes of the evaluation cell. Then, the charge and discharge efficiency (coulombic efficiency) was calculated as a proportion of the battery capacity during discharging to the battery capacity during second cycle charging.
As a result, the charge and discharge efficiency in this example was 99%.
An evaluation cell in the same form as in
An evaluation cell in the same form as in
The solid electrolyte layer before formation of the first functional layer was transferred to the side of the positive electrode active material layer of the positive electrode by cold isostatic pressing (CIP). At this time, the transfer conditions were controlled such that the solid electrolyte layer on the side surface of the positive electrode active material layer widened toward the positive electrode current collector side while covering the entire exposed surface and side surface of the positive electrode active material layer as illustrated in
An evaluation cell in the same form as in
For each of Examples 1 to 5 described above, the thickness of the first functional layer was changed from 20 nm to 5 nm, 100 nm, 250 nm, 1 μm, 5 μm, or 15 μm, such evaluation cells as described above were produced, and the charge and discharge efficiency was measured. As a result, it was observed that the same results as those described above were obtained in each of the cases.
An evaluation cell in the same form as in
An evaluation cell in the same form as in
An evaluation cell in the same form as in
An evaluation cell in the same form as in
An evaluation cell was produced in the same manner as in Example 1 described above, except that the thickness of the first functional layer was changed to 40 nm.
An evaluation cell was produced in the same manner as in Example 6 described above, except that the material constituting the first functional layer was changed from lithium fluoride to lithium bromide (LiBr).
An evaluation cell was produced in the same manner as in Example 6 described above, except that the material constituting the first functional layer was changed from lithium fluoride to lithium chloride (LiCl).
An evaluation cell was produced in the same manner as in Example 6 described above, except that the material constituting the first functional layer was changed from lithium fluoride to lithium iodide (LiI).
An evaluation cell was produced in the same manner as in Example 6 described above, except that the material constituting the first functional layer was changed from lithium fluoride to lithium carbonate (Li2CO3).
An evaluation cell was produced in the same manner as in Example 6 described above, except that the material constituting the first functional layer was changed from lithium fluoride to lithium oxide (Li2O).
An evaluation cell was produced in the same manner as in Example 8 described above, except that the thickness of the first functional layer was changed to 10 μm.
A powder of lithium chloride was dispersed in an appropriate amount of mesitylene, SBR was added thereto in an amount of 1 mass % relative to lithium chloride, and the resultant mixture was mixed to prepare a slurry. An evaluation cell was produced in the same manner as in Example 8 described above, except that the slurry was applied and dried to form the first functional layer (thickness: 2 μm).
[Measurement of Arithmetic Average Roughness (Ra) of Surface of First Functional Layer]
An arithmetic average roughness (Ra) of a principal surface of the first functional layer, the surface facing the negative electrode current collector, was measured in accordance with JIS B 0601: 2013. The results are shown in Table 1 below.
[Evaluation of Evaluation Cell (Measurement of Charge Rate Characteristics)]
A positive electrode lead and a negative electrode lead were connected to the positive electrode current collector and the negative electrode current collector of each of the evaluation cells produced in Examples 6 to 13, respectively, and the evaluation cells were charged and discharged at 1.0 C or 0.2 C for 3 cycles under the same charge and discharge test conditions as described above, except that the evaluation temperature was changed to 333 K (60° C.). Then, the percentage [%] of the charging capacity (0.2 C) at the third cycle to the charging capacity (1.0 C) at the third cycle was calculated and used as charge rate characteristics. The results are shown in Table 1 below.
*1) Charging and discharging at 0.2 C were performed without any trouble, but a short circuit occurred during charging and discharging at 1.0 C.
The results in Table 1 show that Examples 6 to 9 in which the first functional layer contains lithium halide exhibit excellent charge rate characteristics as compared with Examples 10 and 11 in which the first functional layer is made of other materials. In particular, it is also found that the first functional layer is more preferably made of LiBr, LiCl, or LiI. Comparison among Examples 8, 12, and 13 shows that when the arithmetic average roughness (Ra) of the principal surface of the first functional layer facing the negative electrode current collector is less than 1 μm (preferably 20 nm or less, more preferably 10 nm or less), a lithium secondary battery having more excellent charge rate characteristics is obtained.
A solution obtained by dissolving polyethylene glycol (PEG (polyethylene oxide; PEO); number average molecular weight; 200000) in an appropriate amount of water was applied and drying was carried out to form the first functional layer (thickness: 2 μm). At this time, the size of the first functional layer was controlled such that the outer peripheral edge portion of the first functional layer entirely covered the side surface of the solid electrolyte layer and the side surface of the positive electrode active material layer over the entire periphery. Except for the above, an evaluation cell in the same form as in
An evaluation cell was produced in the same manner as in Example 14 described above, except that no first functional layer was formed.
An evaluation cell was produced in the same manner as in Example 14 described above, except that no insulating layer was formed.
A solution obtained by dissolving polyethylene glycol (PEG (polyethylene oxide; PEO); number average molecular weight; 200000) in an appropriate amount of water was applied and drying was carried out to form the first functional layer (thickness: 2 μm). At this time, the size of the first functional layer was controlled such that the first functional layer was disposed at the position illustrated in
The material constituting the first functional layer was changed from polyethylene glycol to an aluminum metal film (thickness: 20 nm). At this time, the first functional layer made of the aluminum metal film was formed by transferring the solid electrolyte layer such that the exposed surface of the solid electrolyte layer faced the positive electrode active material layer and performing sputtering before disposing the negative electrode current collector. Except for the above, an evaluation cell was produced in the same manner as in Example 14 described above. The Young's modulus of the first functional layer measured by the sphere indentation test was 70 GPa.
[Evaluation of Evaluation Cell (Measurement of Charge and Discharge Cycle Characteristics)]
A positive electrode lead and a negative electrode lead were connected to the positive electrode current collector and the negative electrode current collector of each of the evaluation cells produced in Examples 14 to 17 and Comparative Example 5, respectively, and the evaluation cells were repeatedly charged and discharged under the same charge and discharge test conditions as the measurement of the charge and discharge efficiency described above, except that the evaluation temperature was changed to 333 K (60° C.) and the voltage range was changed to a range of 2.5 to 4.3 V. Then, the number of cycles in which charging and discharging can be carried out until a short circuit occurred was measured. The results are shown in Table 2 below.
The results in Table 2 show that an insulating layer is disposed on a surface of the positive electrode current collector on the side of the solid electrolyte layer, where the positive electrode active material layer is not disposed on the surface, the outer peripheral edge portion of the solid electrolyte layer is extended to the side surface of the positive electrode active material layer, and the outer periphery of the positive electrode active material layer is covered with the solid electrolyte layer, the first functional layer, or the insulating layer, as a result of which the charge and discharge cycle characteristics are improved. Further, it is found that the first functional layer is made of a lithium ion conductive polymer (e.g., polyethylene glycol or the like) or the like having a small Young's modulus (specifically, it is less than 100 MPa), as a result of which charge and discharge cycle characteristics are improved.
The material constituting the first functional layer was changed from lithium fluoride to lithium chloride, and the thickness of the first functional layer was changed to 100 nm. A second functional layer made of silver was formed on the entire surface of the negative electrode current collector on the side of the first functional layer by sputtering, before stainless steel foil as the negative electrode current collector was laminated on the exposed surface of the first functional layer. Except for the above, an evaluation cell in the same form as in
An evaluation cell was produced in the same manner as in Example 18 described above, except that the second functional layer was formed by spray-coating the entire surface of the negative electrode current collector on the side of the first functional layer with a dispersion liquid obtained by dispersing a silver powder in an appropriate amount of mesitylene and drying the spray-coated surface.
An evaluation cell was produced in the same manner as in Example 18 described above, except that the second functional layer was formed by spray-coating a region facing the negative electrode current collector, on the surface of the first functional layer on the side of the negative electrode current collector, with a dispersion liquid obtained by dispersing a silver powder in an appropriate amount of mesitylene and drying the dispersion liquid. In the obtained evaluation cell, the silver particles constituting the second functional layer were present not only between the first functional layer and the negative electrode current collector but also inside the first functional layer.
An evaluation cell was produced in the same manner as in Example 18 described above, except that the second functional layer made of silver was formed on the entire surface of the first functional layer on the side of the negative electrode current collector by sputtering before the solid electrolyte layer having the first functional layer was transferred to the side of the positive electrode active material layer of the positive electrode.
An evaluation cell was produced in the same manner as in Example 19 described above, except that the powder of silver was changed to a powder of magnesium in the production of the second functional layer.
An evaluation cell was produced in the same manner as in Example 19 described above, except that the powder of silver was changed to a powder of zinc in the production of the second functional layer.
An evaluation cell was produced in the same manner as in Example 4 described above, except that before the solid electrolyte layer having the first functional layer was transferred to the side of the positive electrode active material layer of the positive electrode, the entire surface of the first functional layer on the side of the negative electrode current collector was spray-coated with a dispersion liquid obtained by dispersing a silver powder in an appropriate amount of mesitylene and the spray-coated surface was dried to form a second functional layer made of silver on the entire surface of the first functional layer on the side of the negative electrode current collector.
[Evaluation of Evaluation Cell (Measurement of Charge and Discharge Cycle Characteristics)]
A positive electrode lead and a negative electrode lead were connected to the positive electrode current collector and the negative electrode current collector of each of the evaluation cells produced in the above-mentioned Examples 18 to 24 and Example 1 described above, respectively, and the evaluation cells were charged and discharged for 30 cycles under the same charge and discharge test conditions as those of the measurement of the charge and discharge efficiency described above, except that the evaluation temperature was changed to 333 K (60° C.) and the charge and discharge rate was changed to 1.0 C. Then, the percentage [%] of the discharging capacity at the thirtieth cycle to the discharging capacity at the first cycle was calculated and used as a capacity retention rate in charge and discharge cycles. The results are shown in Table 3 below.
The results in Table 3 show that the second functional layer containing a simple substance of an element capable of forming an alloy with lithium or a compound containing the element is further provided on the principal surface of the first functional layer facing the negative electrode current collector, as a result of which charge and discharge cycle characteristics are improved.
This application is based on Japanese Patent Application No. 2021-055645 filed on Mar. 29, 2021 and Japanese Patent Application No. 2022-032266 filed on Mar. 3, 2022, the contents of which are incorporated herein by reference in their entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-055645 | Mar 2021 | JP | national |
| 2022-032266 | Mar 2022 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2022/013655 | 3/23/2022 | WO |
