Lithium-ion battery

Abstract

The primary objective of this disclosure is to provide a lithium-ion battery with good cycle characteristics. [Solution] The present disclosure provides a lithium-ion battery having a negative electrode current collector, a polymer layer, a separator, and a positive electrode active material layer in this order in the thickness direction, wherein the polymer layer contains at least one of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), polyvinyl alcohol (PVA), and polyvinylidene fluoride (PVDF) as the polymer, and contains at least one of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium tetrafluoroborate (LiBF4), and lithium hexafluorophosphate (LiPF6) as the electrolyte, the thickness of the polymer layer is 20 μm or more and 30 μm or less, and the crush strength of the polymer layer is 325 N or more, thereby solving the above problem.

Description

[Technical Field]

[0001] This disclosure relates to lithium-ion batteries. [Background technology]

[0002] In recent years, there has been a great deal of activity in the development of batteries, such as lithium-ion batteries. For example, in the automotive industry, development is progressing on batteries used in electric vehicles (BEVs), plug-in hybrid vehicles (PHEVs), and hybrid electric vehicles (HEVs). Generally, a battery has a positive electrode, a negative electrode, and a separator placed between the positive and negative electrodes.

[0003] For example, Patent Document 1 discloses a non-aqueous electrolyte secondary battery having a positive electrode, a negative electrode, and a separator, wherein the separator has a main layer and a plurality of thin films, each of the plurality of thin films having a thinner film thickness than the main layer and having a lower ion transmittance than the main layer, and the plurality of thin films having different ion transmittances.

[0004] Furthermore, Patent Document 2 discloses a non-aqueous secondary battery equipped with a current collector that elastically deforms to be able to adhere closely to an electrode (sintered body). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] International Publication No. 2011 / 070710 [Patent Document 2] Japanese Patent Publication No. 2000-195523 [Overview of the project] [Problems that the invention aims to solve]

[0006] In lithium-ion batteries, there are known batteries that utilize metallic Li (pure Li or Li alloy) deposited on the negative electrode side during charging as the negative electrode active material during discharge. From the viewpoint of improving the performance of lithium-ion batteries, it is necessary to effectively react the deposited metallic Li during discharge to improve cycle characteristics.

[0007] This disclosure is made in view of the above circumstances and primarily aims to provide a lithium-ion battery with good cycle characteristics.

[0008] [1] It is a lithium-ion battery, The negative electrode current collector, polymer layer, separator, and positive electrode active material layer are arranged in this order in the thickness direction. The polymer layer contains, as a polymer, at least one of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), polyvinyl alcohol (PVA), and polyvinylidene fluoride (PVDF), and as an electrolyte, at least one of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium tetrafluoroborate (LiBF4), and lithium hexafluorophosphate (LiPF6). The thickness of the above polymer layer is 20 μm or more and 30 μm or less. A lithium-ion battery in which the crushing strength of the polymer layer described above is 325N or higher.

[0009] [2] The lithium-ion battery described in [1], wherein the crush strength is 1000 N or less.

[0010] [3] The lithium-ion battery according to [1] or [2], wherein the polymer layer contains at least the PVDF-HFP as the polymer.

[0011] [4] The lithium ion battery according to any one of [1] to [3], wherein the polymer layer contains at least the LiPF6 as the electrolyte.

[0012] [5] The lithium ion battery according to any one of [1] to [4], wherein the thickness of the negative electrode current collector is 5 μm or more and 20 μm or less. [Advantages of the Invention]

[0013] In the present disclosure, there is an effect that the cycle characteristics are good. [Brief Description of the Drawings]

[0014] [Figure 1] It is a schematic cross-sectional view illustrating the lithium ion battery in the present disclosure. [Figure 2] It is a diagram for explaining the compression fracture test in the examples. [Figure 3] It is a graph showing the results of the compression fracture test in Examples 1, 4 and Comparative Example 3. [Modes for Carrying Out the Invention]

[0015] Hereinafter, the lithium ion battery in the present disclosure will be described in detail. The drawings shown below are schematic, and the sizes and shapes of each part are exaggerated as appropriate for easy understanding. In the present disclosure, the lithium ion battery may be simply referred to as a "battery".

[0016] FIG. 1 is a schematic cross-sectional view illustrating the lithium ion battery in the present disclosure. Specifically, FIG. 1(a) is a schematic cross-sectional view illustrating the battery before the first charge, and FIG. 1(b) is a schematic cross-sectional view illustrating the battery after the first charge.

[0017] As shown in FIGS. 1(a) and (b), the lithium ion battery 10 includes a negative electrode current collector 1, a polymer layer 2, a separator 3, a positive electrode active material layer 4, and a positive electrode current collector 5 in the thickness direction D TIn this order, the lithium-ion battery 10 has the following components. In particular, the polymer layer 2 contains at least one of PVDF-HFP, PVA, and PVDF as a polymer, and at least one of LiTFSI, LiFSI, LiBF4, and LiPF6 as an electrolyte. The thickness of the polymer layer 2 is 20 μm or more and 30 μm or less, and the crush strength is 325 N or more. Here, as shown in Figure 1(a), in the lithium-ion battery 10 before the first charge, the entire polymer layer 2 is in contact with the negative electrode current collector. On the other hand, when the lithium-ion battery 10 shown in Figure 1(a) is charged, Li ions that have moved from the positive electrode active material layer are deposited on the negative electrode current collector, and a deposited Li layer (negative electrode active material layer) 6 is formed (Figure 1(b)). The deposited Li layer may be formed on the entire surface of the negative electrode current collector facing the polymer layer, or it may be formed on a part of it.

[0018] According to this disclosure, the polymer layer contains a predetermined polymer and electrolyte, and has a predetermined thickness and crush strength, resulting in a lithium-ion battery with good cycle characteristics.

[0019] Batteries in which metallic lithium is deposited as the negative electrode active material during charging (batteries that utilize the deposition and dissolution reaction of metallic lithium as the negative electrode reaction) have the advantage of easily improving energy density. On the other hand, if the deposited lithium grows in a dendritic structure during charging (when dendrites are formed), there is a risk that some of the deposited lithium will be cut during discharge, resulting in depleted lithium with broken electronic connections. Since depleted lithium does not contribute to the discharge reaction, it becomes a factor in reducing the discharge capacity. Furthermore, if depleted lithium continues to accumulate on the negative electrode side due to repeated charging and discharging, it will become a factor in increasing resistance. In addition, if the deposited lithium comes into contact with the electrolyte, there is a risk that a film (Li compound; Solid Electrolyte Interphase: SEI) will be formed on the surface of the deposited lithium due to the decomposition reaction of the electrolyte. In that case, lithium will be consumed in the formation of SEI, and the amount of lithium consumed during discharge may decrease. In particular, when the above-mentioned dendrites are formed, the surface area in contact with the electrolyte and lithium increases, so there is a risk that the formation of lithium compounds will be further promoted. Furthermore, Li compounds do not contribute much to the discharge reaction, and Li compounds that are not consumed during discharge continue to accumulate on the negative electrode side as charging and discharging are repeated, which may result in increased resistance. These factors may lead to a decrease in the battery's cycle characteristics.

[0020] In contrast, the battery in this disclosure has a polymer layer positioned between the negative electrode current collector and the separator. The polymer layer functions as a barrier layer, suppressing contact between deposited Li and the electrolyte, thereby suppressing the formation of dendrites. During charging, the battery tends to expand due to the deposition of metallic Li, and during discharge, it tends to contract due to the dissolution of metallic Li. If the polymer layer ruptures due to such expansion and contraction, there is a risk that contact between deposited Li and the electrolyte may not be sufficiently suppressed, especially in batteries that have undergone cycles. In contrast, the polymer layer in this disclosure has a thickness of 20 μm or more and 30 μm or less, and a crush strength of 325 N or more, so it is considered to have high resistance to rupture. Therefore, even in batteries that have undergone cycles, rupture of the polymer layer is suppressed, and contact between deposited Li and the electrolyte can be sufficiently suppressed. Furthermore, since the Li deposited by charging is physically held down by the polymer layer, the dendritic growth of deposited Li can be suppressed, and the formation of dendrites and the occurrence of detached Li can be suppressed. These measures suppress the decrease in battery discharge capacity, resulting in a battery with good cycle characteristics.

[0021] 1.Negative electrode current collector Examples of materials for the negative electrode current collector include SUS, copper, and nickel. Examples of shapes for the negative electrode current collector include foil. The thickness of the negative electrode current collector is not particularly limited, but may be, for example, 1 μm or more, 5 μm or more, or 10 μm or more. On the other hand, the thickness of the negative electrode current collector may be, for example, 500 μm or less, 300 μm or less, 100 μm or less, 50 μm or less, 30 μm or less, or 20 μm or less. Furthermore, the thickness of the negative electrode current collector relative to the thickness of the polymer layer, which will be described later, is not particularly limited, but may be, for example, 0.4 to 1.5.

[0022] 2. Polymer layer The polymer layer in this disclosure contains a polymer. Examples of polymers include PVDF-HFP, PVA, and PVDF. Among these, the polymer layer preferably contains at least PVDF-HFP. The polymer layer may contain only one of the above polymers, or it may contain two or more. The polymer layer may also contain materials other than those mentioned above, or it may not contain any. In other words, the polymer layer may contain only at least one of PVDF-HFP, PVA, and PVDF as a polymer, or it may contain other polymers. In the latter case, it is preferable that the proportion of PVDF-HFP, PVA, and PVDF in the total polymer in the polymer layer is 70% by weight or more.

[0023] The proportion (by weight) of polymer in the polymer layer is not particularly limited, but for example, it is between 20% and 70% by weight. If the proportion of polymer is too low, sufficient crush strength may not be obtained. Conversely, if the proportion of polymer is too high, the proportion of electrolyte will decrease relatively, and the ionic conductivity of the polymer layer may decrease too much.

[0024] The polymer layer in this disclosure contains an electrolyte. Examples of electrolytes include LiTFSI, LiFSI, LiBF4, and LiPF6. Among these, the polymer layer preferably contains at least LiPF6. The polymer layer may contain only one of the above electrolytes, or it may contain two or more. The polymer layer may also contain materials other than those mentioned above as electrolytes, or it may not contain any. In other words, the polymer layer may contain only at least one of LiTFSI, LiFSI, LiBF4, and LiPF6 as an electrolyte, or it may contain other electrolytes. In the latter case, it is preferable that the proportion of LiTFSI, LiFSI, LiBF4, and LiPF6 in the total electrolyte in the polymer layer is 70% by weight or more.

[0025] The proportion (by weight) of the electrolyte in the polymer layer is not particularly limited, but for example, it is between 20% and 70% by weight. If the proportion of electrolyte is too low, the ionic conductivity of the polymer layer may decrease too much. Conversely, if the proportion of electrolyte is too high, the relative proportion of polymer will decrease, and sufficient tensile strength may not be obtained.

[0026] In this disclosure, the thickness of the polymer layer is 20 μm or more and 30 μm or less. The thickness of the polymer layer may be 21 μm or more, or 23 μm or more. On the other hand, the thickness of the polymer layer may be 27 μm or less, or 25 μm or less.

[0027] The crushing strength of the polymer layer in this disclosure is 325 N or more. The crushing strength may be 350 N or more, 450 N or more, or 600 N or more. On the other hand, the crushing strength may be, for example, 1000 N or less, 900 N or less, 800 N or less, or 700 N or less. The crushing strength can be measured by a compression crushing test as described in the examples.

[0028] Furthermore, it is preferable that the polymer layer in this disclosure has good ionic conductivity. The ionic conductivity at 25°C is, for example, 0.30 mS / cm or more and 1.00 mS / cm or less. The ionic conductivity can be determined, for example, by impedance spectroscopy.

[0029] The polymer layer can be formed, for example, by coating a substrate with a slurry containing the polymer and electrolyte and drying it. Examples of organic solvents in the slurry include N,N-dimethylacetamide and N-methylpyrrolidone. The substrate may be the negative electrode current collector or a metal foil for transfer.

[0030] 3. Separator The separator is usually impregnated with an electrolyte. The electrolyte preferably contains a supporting salt and a solvent. Examples of supporting salts (lithium salts) for lithium-ion conductive electrolytes include inorganic lithium salts such as LiPF6, LiBF4, LiClO4, and LiAsF6, and organic lithium salts such as LiCF3SO3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(FSO2)2, and LiC(CF3SO2)3. Examples of solvents used in the electrolyte include cyclic esters (cyclic carbonates) such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and fluoroethylene carbonate (FEC), linear esters (linear carbonates) such as dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC), and ether-based solvents such as hydrofluoroethers (HFCs).

[0031] The separator material may be organic or inorganic. Specifically, examples include porous membranes such as polyethylene (PE), polypropylene (PP), cellulose, polyvinylidene fluoride, polyamide, and polyimide; nonwoven fabrics such as resin nonwoven fabrics and glass fiber nonwoven fabrics; and porous ceramic membranes. Furthermore, the separator may have a single-layer structure or a laminated structure.

[0032] The thickness of the separator is, for example, between 0.1 μm and 1000 μm.

[0033] 4.Cathode active material layer The positive electrode active material layer contains at least positive electrode active material.

[0034] Examples of positive electrode active materials include oxide active materials. Examples of oxide active materials include LiCoO2, LiMnO2, LiNiO2, LiVO2, and LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 Examples of rock salt layered active materials include O2. Other examples of oxide active materials include LiMn2O4 and Li4Ti5O 12 and Li(Ni0.5 Mn 1.5 Examples of spinel-type active materials include O4. Other examples of oxide-type active materials include olivine-type active materials such as LiFePO4, LiMnPO4, LiNiPO4, and LiCoPO4.

[0035] The positive electrode active material layer may contain at least one of an electrolyte, a binder, and a conductive material, as needed. The electrolyte is as described above. Examples of binders include carbon materials, metal particles, and conductive polymers. Examples of carbon materials include particulate carbon materials such as acetylene black (AB) and fibrous carbon materials such as carbon nanotubes (CNT). Examples of binders include rubber-based binders such as butadiene rubber (BR) and fluorine-containing binders such as polyvinylidene fluoride (PVDF).

[0036] The thickness of the positive electrode active material layer is, for example, 0.1 μm or more and 1000 μm or less.

[0037] 5. Other components (1) Negative electrode active material layer A charged battery typically has a negative electrode active material layer (deposited Li layer) between the negative electrode current collector and the polymer layer. If the battery before charging has a metal layer (described later), the negative electrode active material layer may be located between the metal layer and the polymer layer. The negative electrode active material layer contains at least metallic Li. The metallic Li may be pure Li or a Li alloy. Examples of Li alloys include Li-Au, Li-Mg, Li-Sn, Li-Al, Li-B, Li-C, Li-Ca, Li-Ga, Li-Ge, Li-As, Li-Se, Li-Ru, Li-Rh, Li-Pd, Li-Ag, Li-Cd, Li-In, Li-Sb, Li-Ir, Li-Pt, Li-Hg, Li-Pb, Li-Bi, Li-Zn, Li-Tl, Li-Te, and Li-At. The lithium alloy may be one type or two or more types.

[0038] (2) Metal layer In a battery before charging, a metal layer containing a metal alloyable with Li may be present between the negative electrode current collector and the polymer layer. The metal contained in the metal layer alloys with Li to form the negative electrode active material layer (a negative electrode active material layer containing a Li alloy). The metal layer can disperse the deposition starting points of metallic lithium, thereby further suppressing dendrite growth. Examples of metal alloyable with Li include the metals described above for Li alloys. The metal layer may contain only one type of metal, or two or more types.

[0039] In this case, after charging, the entire metal layer may be replaced by the negative electrode active material layer, or only a portion of it may be replaced. In other words, the metal layer may remain in the battery after charging. If the metal layer remains, the negative electrode active material layer is usually located between the metal layer and the polymer layer.

[0040] (3) Positive electrode current collector The battery in this disclosure typically has a positive electrode current collector that collects electrons from the positive electrode active material layer. Examples of materials for the positive electrode current collector include SUS, aluminum, nickel, iron, titanium, and carbon. The thickness of the positive electrode current collector is, for example, 1 μm or more and 500 μm or less.

[0041] 6. Lithium-ion batteries The lithium-ion battery in this disclosure is typically a liquid-type battery containing an electrolyte. The electrolyte is as described above.

[0042] The use of the battery is not particularly limited, and examples include a power source for a vehicle. Examples of the vehicle include a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), a battery electric vehicle (BEV), a gasoline vehicle, and a diesel vehicle. In particular, it is preferably used as a driving power source for a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), or a battery electric vehicle (BEV). Further, the lithium ion battery in the present disclosure may be used as a power source for a moving body other than a vehicle (for example, a railway, a ship, and an aircraft). Further, the lithium ion battery in the present disclosure may be used as a power source for an electric product such as an information processing device.

[0043] Note that the present disclosure is not limited to the above-described embodiments. The above-described embodiments are examples, and any configuration that has substantially the same configuration as the technical idea described in the claims in the present disclosure and exhibits the same operational effects is included in the technical scope of the present disclosure.

Example

[0044] [Example 1] (Production of polymer layer) 7.2 g of a solvent (N,N-dimethylacetamide: DMA), 0.8 g of a polymer (PVDF-HFP), and 1.0 g of an electrolyte (LiTFSI) were placed in a container and stirred at 70° C. for 1 hour using a stirrer. Thereby, a slurry was obtained. The obtained slurry was applied onto a negative electrode current collector (copper foil; 10 μm) using a doctor blade so as to have a uniform thickness (20 μm). This was dried on a hot plate. Thereby, a polymer layer was produced on the negative electrode current collector.

[0045] (Production of evaluation battery) Positive electrode active material (LiNi 1 / 3 Co 1 / 3 Mn 1 / 3A positive electrode slurry was obtained by stirring O2, a conductive material (acetylene black), and a binder (a binder solution containing 5% PVDF) at a ratio that resulted in a non-volatility value of 65. The positive electrode slurry was coated onto a positive electrode current collector (Al foil; 16 μm thick) using a doctor blade and dried. The coating amount was 23 mg / cm² after drying. 2 The material was adjusted to achieve the desired result. After drying, it was roll-pressed to densify it and cut to the specified size. This resulted in obtaining a positive electrode having a positive electrode active material layer and a positive electrode current collector. In addition, a separator having a laminated structure of PP and PE layers was prepared.

[0046] In a coin cell, a negative electrode current collector, polymer layer, separator, positive electrode active material layer, and positive electrode current collector were stacked in that order, and the cell was filled with electrolyte. The cell was left to stand overnight to allow the electrolyte to permeate the separator. This prepared an evaluation battery. The electrolyte used was a solution of LITFSI dissolved at a concentration of 1 M in a mixed solvent containing propylene carbonate (PC) and fluoroethylene carbonate (FEC) in a volume ratio of 70:30. This obtained an evaluation battery.

[0047] [Examples 2-5 and Comparative Examples 1-6] As shown in Table 1, an evaluation battery was prepared in the same manner as in Example 1, except that the polymer layer was fabricated by changing at least one of the following: polymer amount, solvent amount, electrolyte amount, and polymer layer thickness.

[0048] [evaluation] (Compression crush test) The crushing strength of the polymer layer was measured by a compression crushing test. The compression crushing test is explained using Figure 2. As shown in Figure 2, the sample was placed on a SUS base so that the base side would be the negative electrode current collector. The sample was cut to a size of 30 mm x 30 mm. Copper foil was placed on the polymer layer, and the resistance value (Ω) between the copper foils was measured while applying a load with a pressure jig (made of SUS; Φ20 mm). Although not shown in the figure, a polypropylene sheet with the center cut out was placed between the copper foil on the pressure jig side and the polymer layer for fixing. The size of the pressure jig can be adjusted as appropriate according to the size of the sample, but for example, Φ15 mm or more and Φ25 mm or less is preferred.

[0049] As shown in Figure 3, described later, the polymer layer has a high resistance value at the beginning of the load application, but as the load is applied, the polymer layer thins and the resistance value decreases. When the polymer layer ruptures and the copper foils come into contact with each other, the resistance value (Ω) drops to near zero. In the compression crush test, it was determined that the polymer layer had ruptured when the resistance value fell below 50 mΩ (0.05 Ω), and the load applied when the resistance value reached 50 mΩ was obtained as the crush strength (N). The results are shown in Table 1. In addition, as representative results, graphs summarizing the changes in resistance value, load, and time are shown for Examples 1 and 4 and Comparative Example 3 (Figures 3(a) to (c)).

[0050] (Cycle testing) The following cycle, "CCCV charge → 1st rest → CCCV discharge → 2nd rest," was considered one cycle, and this was repeated 20 times. The capacity retention rate (%) was calculated from the discharge capacity after 1 cycle and the discharge capacity after 20 cycles. The results are shown in Table 1. CCCV charging: 1 / 4C, 4.3V (ending current 1 / 100C) First rest: 5 minutes CCCV discharge: 1 / 4C, 3.0V Second rest: 5 minutes

[0051] [Table 1]

[0052] As shown in Table 1, Examples 1-5 showed better capacity retention after 20 cycles compared to Comparative Examples 1-6, confirming that the lithium-ion battery in this disclosure has good cycle characteristics. This is thought to be because, as shown in Table 1 and Figures 3(a)-(c), the polymer layer in this disclosure exhibited good crush strength, suppressing cracking of the polymer layer due to the expansion and contraction of the battery, and suppressing contact between deposited Li and the electrolyte even at the end of the cycle. [Explanation of symbols]

[0053] 1...Negative electrode current collector 2…polymer layer 3 ... Separator 4...Cathode active material layer 5...Positive electrode current collector 6...Precipitated Li layer (negative electrode active material layer) 10…Lithium-ion battery

Claims

1. It is a lithium-ion battery, The negative electrode current collector, polymer layer, separator, and positive electrode active material layer are arranged in this order in the thickness direction. The polymer layer contains, as a polymer, at least one of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), polyvinyl alcohol (PVA), and polyvinylidene fluoride (PVDF), and as an electrolyte, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium tetrafluoroborate (LiBF ４ ) and lithium hexafluoride phosphate (LiPF) ６ ) contains at least one of the following, The thickness of the polymer layer is 20 μm or more and 30 μm or less. A lithium-ion battery in which the crushing strength of the polymer layer is 325 N or more.

2. The lithium-ion battery according to claim 1, wherein the crush strength is 1000 N or less.

3. The lithium-ion battery according to claim 1, wherein the polymer layer contains at least the PVDF-HFP as the polymer.

4. The polymer layer contains at least the LiPF as the electrolyte. ６ A lithium-ion battery according to claim 1, comprising the same components.

5. The lithium-ion battery according to claim 1, wherein the thickness of the negative electrode current collector is 5 μm or more and 20 μm or less.

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