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 PVDF-HFP, PVA, and PVDF as the polymer, and contains at least one of LiTFSI, LiFSI, LiBF4, and LiPF6 as the electrolyte, and the ionic conductivity of the polymer layer at 25°C is 0.60 mS / cm 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-porous fluoropolymer film containing at least one layer as a material used for a separator in a lithium-ion battery. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] International Publication No. 2011 / 070710 [Patent Document 2] Special Publication No. 2023-502035 [Overview of the Initiative] [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. [Means for solving the problem]

[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). A lithium-ion battery in which the ionic conductivity of the polymer layer described above at 25°C is 0.60 mS / cm or higher.

[0009] [2] The lithium-ion battery according to [1], wherein the ratio (by weight) of the electrolyte to the polymer in the polymer layer is 1.5 or more and 2.1 or less.

[0010] [3] A lithium-ion battery as described in [1] or [2], wherein the above-mentioned ionic conductivity is 1.00 mS / cm or less.

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

[0012] [5] The lithium-ion battery according to any one of [1] to [4], wherein the polymer layer contains at least the LiPF6 as the electrolyte.

Advantages of the Invention

[0013] In the present disclosure, an effect of good cycle characteristics is achieved.

Brief Description of the Drawings

[0014] [Figure 1] It is a schematic cross-sectional view illustrating the lithium-ion battery in the present disclosure.

Embodiments 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

[0016] ,

[0014] , , , , , , , [Figure 1] , , , ,

[0015] , T , , , , ,

[0017] In this case, it has in this order. In the lithium-ion battery 10, in particular, the polymer layer 2 contains at least one of PVDF-HFP, PVA, and PVDF as the polymer, and contains at least one of LiTFSI, LiFSI, LiBF4, and LiPF6 as the electrolyte, and the ionic conductivity of the polymer layer 2 at 25 °C is 0.60 mS / cm or more. Here, as shown in Fig. 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 Fig. 1(a) is charged, Li ions that have migrated 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 (Fig. 1(b)). The deposited Li layer may be formed over the entire surface of the negative electrode current collector facing the polymer layer, or may be formed partially.

[0018] According to the present disclosure, since the polymer layer contains a predetermined polymer and electrolyte and has a predetermined ionic conductivity, a lithium-ion battery with good cycle characteristics is obtained.

[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 (dendrite formation), 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 much to the discharge reaction, it becomes a factor in reducing 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, if the above-mentioned dendrites are formed, the surface area in contact with the electrolyte and lithium will increase, which may further promote the formation of lithium compounds. 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. Due to these factors, the battery's cycle characteristics may deteriorate.

[0020] In contrast, the battery in this disclosure has a polymer layer positioned between the negative electrode current collector and the separator. Since the Li deposited by charging is physically held down by the polymer layer, the dendritic growth of the deposited Li can be suppressed, thereby suppressing the formation of dendrites and the occurrence of delamination of Li. Furthermore, since the ionic conductivity of the polymer layer is 0.60 mS / cm or higher, Li can be uniformly deposited on the surface of the negative electrode current collector, suppressing localized growth of Li and inhibiting the formation of dendrites. Moreover, suppressing the formation of dendrites suppresses the increase in the surface area in contact with the electrolyte of the deposited Li, thereby suppressing the formation of the above-mentioned Li compounds. In addition, the polymer layer functions as a barrier layer, suppressing contact between the deposited Li and the electrolyte itself. As a result, the decrease in the discharge capacity of the battery can be suppressed, 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, it is preferable that the polymer layer 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, or it may contain other polymers. In the latter case, it is preferable that the proportion of PVDF-HFP, PVA, and PVDE in the total polymer in the polymer layer is 70% by weight or more.

[0023] 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.

[0024] The ratio (by weight) of the electrolyte to the polymer in the polymer layer is not particularly limited, but for example, it may be 1.3 or more, 1.5 or more, or 1.7 or more. On the other hand, for example, the ratio may be 2.3 or less, 2.1 or less, or 1.9 or less. If the ratio is too low, good ionic conductivity may not be obtained. Also, if the ratio is too high, sufficient strength may not be obtained in the polymer layer.

[0025] The ionic conductivity of the polymer layer in this disclosure at 25°C is 0.60 mS / cm or higher. The ionic conductivity may be 0.62 mS / cm or higher, 0.65 mS / cm or higher, 0.70 mS / cm or higher, or 0.75 mS / cm or higher. On the other hand, the ionic conductivity may be, for example, 1.00 mS / cm or lower, 0.90 mS / cm or lower, or 0.80 mS / cm or lower. The ionic conductivity can be adjusted, for example, by the type and ratio of the electrolyte described above. The ionic conductivity can be determined, for example, by impedance analysis.

[0026] The thickness of the polymer layer is not particularly limited, but for example it may be 15 μm or more, 17 μm or more, or 20 μm or more. On the other hand, the thickness may be 30 μm or less, 25 μm or less, or 23 μm or less. If the polymer layer is too thin, the force holding down the precipitated Li may become too small, and dendrite growth may not be sufficiently suppressed.

[0027] 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.

[0028] 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).

[0029] 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.

[0030] The thickness of the separator is, for example, between 0.1 μm and 1000 μm.

[0031] 4.Cathode active material layer The positive electrode active material layer contains at least positive electrode active material.

[0032] 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(Ni 0.5 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.

[0033] 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).

[0034] The thickness of the positive electrode active material layer is, for example, 0.1 μm or more and 1000 μm or less.

[0035] 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.

[0036] (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.

[0037] 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.

[0038] (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.

[0039] 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.

[0040] The applications of the battery are not particularly limited, but one example is that it can be used as a power source for vehicles. Examples of vehicles include hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), battery electric vehicles (BEVs), gasoline cars, and diesel cars. It is particularly preferable that the battery be used as a power source for hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), or battery electric vehicles (BEVs). The lithium-ion battery in this disclosure may also be used as a power source for mobile devices other than vehicles (e.g., railways, ships, and aircraft). Furthermore, the lithium-ion battery in this disclosure may also be used as a power source for electrical products such as information processing devices.

[0041] This disclosure is not limited to the embodiments described above. The embodiments described above are illustrative, and any configuration that is substantially identical to the technical idea described in the claims of this disclosure and achieves similar effects is included within the technical scope of this disclosure. [Examples]

[0042] [Example 1] (Preparation of polymer layer) 7.2 g of a solvent (N,N-dimethylacetamide: DMA), 0.8 g of a polymer (PVDF-HFP), and 1.3 g of an electrolyte (LiTFSI) were placed in a container and stirred at 70 °C for 1 hour using a stirrer. As a result, a slurry was obtained. The obtained slurry was applied onto a negative electrode current collector (copper foil; 10 μm) using a doctor blade to a uniform thickness (22 μm). This was dried on a hot plate. Thereby, a polymer layer was formed on the negative electrode current collector.

[0043] (Preparation of Evaluation Battery) A positive electrode active material (LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2), a conductive material (acetylene black), and a binder (a binder solution containing 5% PVDF) were stirred at a ratio such that the non-volatility value was 65 to obtain a positive electrode slurry. The positive electrode slurry was applied onto a positive electrode current collector (Al foil; thickness 16 μm) using a doctor blade and dried. The coating amount was adjusted so that the basis weight after drying was 23 mg / cm 2 . After drying, roll pressing was performed for densification and it was cut into a predetermined size. Thereby, a positive electrode having a positive electrode active material layer and a positive electrode current collector was obtained. Also, a separator having a laminated structure of a PP layer and a PE layer was prepared.

[0044] In a coin cell, a negative electrode current collector, a polymer layer, a separator, a positive electrode active material layer, and a positive electrode current collector were laminated in this order and filled with an electrolyte. It was left standing overnight to impregnate the separator with the electrolyte. Thereby, an evaluation battery was prepared. The electrolyte used was a solution in which LITFSI was dissolved at a concentration of 1 M in a mixed solvent containing propylene carbonate (PC) and fluoroethylene carbonate (FEC) at a volume ratio of 70:30. Thereby, an evaluation battery was obtained.

[0045] [Examples 2 to 6 and Comparative Examples 1 to 6] As shown in Table 1, an evaluation battery was prepared in the same manner as in Example 1, except that a polymer layer was prepared by changing at least one of the polymer amount, solvent amount, electrolyte amount, and thickness of the polymer layer.

[0046] [evaluation] (Measurement of ionic conductivity) The resistance of the battery cell was measured by impedance measurement, and the resistance of the polymer layer was calculated by subtracting the previously measured resistance of the separator from this value. Based on the obtained resistance value, the ionic conductivity of the polymer layer was calculated. The results are shown in Table 1. A device manufactured by Solartron was used for impedance measurement, and the measurement conditions were frequency: 0.1 Hz to 10 Hz. 6 The frequency was set to Hz and the amplitude to 10mV.

[0047] (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

[0048] [Table 1]

[0049] As shown in Table 1, Examples 1-6 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. [Explanation of Symbols]

[0050] 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, A lithium-ion battery in which the ionic conductivity of the polymer layer at 25°C is 0.60 mS / cm or higher.

2. The lithium-ion battery according to claim 1, wherein the ratio (by weight) of the electrolyte to the polymer in the polymer layer is 1.5 or more and 2.1 or less.

3. The lithium-ion battery according to claim 1, wherein the ionic conductivity is 1.00 mS / cm or less.

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

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

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