JP2024062054A - Lithium-ion secondary battery - Google Patents

Abstract

[Problem] To provide a means for improving the charge/discharge capacity at low temperatures in a lithium-ion secondary battery using a lithium-rich manganese positive electrode material. [Solution] The lithium-ion secondary battery is provided with a power generating element including a positive electrode including a positive electrode active material layer containing a lithium transition metal composite oxide having a composition represented by the following composition formula (1): Li1.5[NiaCobMnc[Li]d]O3(1) (wherein a, b, c, and d satisfy the relationships of 0<a<1.4, 0≦b<1.4, 0<c<1.4, 0.1<d≦0.4, a+b+c+d=1.5, 1.1≦a+b+c<1.4) as a positive electrode active material, a negative electrode, and an electrolyte layer including an electrolyte, wherein the ratio DLi15/DLi80 of the lithium diffusion coefficient (DLi15) at SOC 15% to the lithium diffusion coefficient (DLi80) at SOC 80% at 25°C is 0.05 or more. [Selected figure] Figure 3

Description

The present invention relates to a lithium-ion secondary battery.

In recent years, the spread of various electric vehicles is expected to help solve environmental and energy problems. Secondary batteries are being developed as on-board power sources for driving motors and other purposes, which hold the key to the spread of these electric vehicles. As secondary batteries, attention has been focused on non-aqueous electrolyte secondary batteries such as lithium-ion secondary batteries, which are expected to have high energy density and high output.

Lithium ion secondary batteries intended for application to electric vehicles are required to have high output and high capacity. As a positive electrode active material used in the positive electrode of such lithium ion secondary batteries, a solid solution positive electrode active material containing lithium and a transition metal such as manganese, for example, a lithium-rich manganese positive electrode material such as a solid solution of Li ₂ MnO ₃ -LiMO ₂ (M=Ni, Co, Mn), is known.

Such lithium-rich manganese positive electrode materials have been attracting attention as next-generation positive electrode materials because they have high energy density and can reduce the amount of cobalt used, but their insufficient charge-discharge cycle durability is an issue. In response to this issue, Patent Document 1 discloses that charge-discharge cycle durability can be improved by a specific positive electrode activation protocol. Specifically, the battery is initially charged to a voltage of about 4.3 V or less, the battery is maintained in an open circuit for a rest period of at least about 12 hours after the initial charge is completed, and the battery is then charged a second time to a voltage of at least about 4.35 V after the rest period is completed. This allows the crystal structure change of the positive electrode active material that occurs around 4.3 V during the initial charge to proceed slowly, making the structure after the change more stable, which is thought to lead to an improvement in the charge-discharge cycle.

JP 2013-524413 A

However, the inventors' investigations revealed that even batteries using the technology described in Patent Document 1 may have significantly low charge/discharge capacities under low-temperature conditions.

The present invention aims to provide a means for improving the charge/discharge capacity at low temperatures in lithium-ion secondary batteries that use lithium-rich manganese positive electrode materials.

The inventors conducted extensive research to solve the above problems. In the process, they discovered that the decrease in charge-discharge cycle durability under low-temperature conditions is due to a decrease in the diffusibility of lithium ions in the low state of charge (SOC) region. Based on this knowledge, the inventors discovered that the diffusibility of lithium ions in the low SOC region can be improved by activating the positive electrode under specified conditions. They also discovered that it is possible to suppress the decrease in charge-discharge capacity under low-temperature conditions by controlling the diffusion coefficient of lithium ions in the low SOC region, which led to the completion of the present invention.

That is, one embodiment of the present invention provides a positive electrode active material represented by the following composition formula (1):
_Li1.5 [ _{NiCobMnc [} Li] _d ] _O3 ( ₁ )
(wherein a, b, c and d satisfy the relationships 0<a<1.4, 0≦b<1.4, 0<c<1.4, 0.1<d≦0.4, a+b+c+d=1.5, 1.1≦a+b+c<1.4), a negative electrode, and an electrolyte layer containing an electrolyte solution, wherein the ratio D _Li15 /D _Li80 of the lithium diffusion coefficient (D _Li15 ) at SOC 15% to the lithium diffusion coefficient (D _Li80 ) at SOC 80% at 25° C. is 0.05 or more.

According to the present invention, the charge/discharge capacity at low temperatures can be improved in a lithium-ion secondary battery using a lithium-rich manganese positive electrode material.

FIG. 1 is a schematic cross-sectional view showing a schematic overall structure of a flat (laminated) non-bipolar (internal parallel connection type) secondary battery (laminated secondary battery) according to one embodiment of the present invention. FIG. 2 is a cross-sectional view showing a schematic diagram of a bipolar secondary battery according to another embodiment of the present invention. FIG. 1 is a graph showing the change in Li diffusion coefficient at 25° C. versus SOC for the batteries produced in Example 1 and Comparative Example 4.

One aspect of the present invention is a positive electrode active material having the following composition formula (1):
_Li1.5 [ _{NiCobMnc [} Li] _d ] _O3 ( ₁ )
(wherein a, b, c and d satisfy the relationships 0<a<1.4, 0≦b<1.4, 0<c<1.4, 0.1<d≦0.4, a+b+c+d=1.5, 1.1≦a+b+c<1.4), a negative electrode, and an electrolyte layer containing an electrolyte solution, the lithium ion secondary battery including a power generating element including a positive electrode including a positive electrode active material layer containing a lithium transition metal composite oxide having a composition represented by the formula: (wherein a, b, c and d satisfy the relationships 0<a<1.4, 0≦b<1.4, 0<c<1.4, 0.1<d≦0.4, a+b+c+d=1.5, 1.1≦a+b+c<1.4), wherein the ratio D _Li15 /D _Li80 of the lithium diffusion coefficient (D _Li15 ) at SOC ₁₅ % to the lithium diffusion coefficient (D Li80 ) at SOC 80% at 25° C. is 0.05 or more. This lithium ion secondary battery can improve the charge/discharge capacity at low temperatures.

In the lithium ion secondary battery of this embodiment, the ratio D _Li15 /D _Li80 of the lithium diffusion coefficient (D _Li15 ) at SOC 15% to the lithium diffusion coefficient (D _Li80 ) at SOC 80% at 25 ° C. is 0.05 or more. The lower the SOC, the higher the battery resistance and the lower the charge/discharge characteristics tend to be, but this tendency is more pronounced under low temperature conditions. Therefore, it is considered that a high reversible capacity can be obtained even at low temperatures by showing a predetermined value of the lithium diffusion coefficient in the low SOC region, which has a large effect on the charge/discharge characteristics at low temperatures. When D _Li15 /D _Li80 is below 0.05, the reversible capacity at low temperatures is significantly reduced. When D _Li15 /D _Li80 is below 0.05, for example, the discharge capacity at -20 ° C. may be 1/10 or less of the discharge capacity at 25 ° C.

The value of D _Li15 /D _Li80 is preferably 0.08 or more, more preferably 0.085 or more. When it is in the above range, the effect of improving low-temperature characteristics can be more significantly obtained. In addition, cycle durability can be further improved. The upper limit of D _Li15 /D _Li80 is not particularly limited, but is, for example, 1.2 or less, preferably 1.0 or less.

In a preferred embodiment, at 25° C., the ratio D _Li15 /D _{Li60 of the lithium diffusion coefficient (D Li15 )} at SOC 15% to the lithium diffusion coefficient (D _Li60 ) at SOC 60% is 0.1 or more. This makes it possible to obtain the effects of the present invention more significantly. In addition, a battery having excellent battery performance in the range of practical use can be obtained. The value of D _Li15 /D _Li60 is more preferably 0.2 or more, and even more preferably 0.3 or more. When it is in the above range, the effect of improving low-temperature characteristics can be obtained more significantly. In addition, cycle durability can be further improved. The upper limit value of D _Li15 /D _Li80 is not particularly limited, but is, for example, 1.2 or less, preferably 1.0 or less.

In a preferred embodiment, the lithium diffusion coefficient (D _Li15 ) at 25° C. and SOC 15% is 1×10 ⁻¹⁶ cm ² /s or more. This makes it possible to obtain the effects of the present invention more significantly. The value of _{D Li15} is more preferably 2×10 ⁻¹⁶ cm ² /s or more, and further preferably 5×10 ⁻¹⁶ cm ² /s or more. The upper limit of _{D Li15} is not particularly limited, but is, for example, 1.2×10 ⁻¹⁵ cm ² /s or less, and preferably 1×10 ⁻¹⁵ cm ² /s or less.

The lithium diffusion coefficient D _Li is a value obtained by constant current intermittent titration method (GITT method), and can be measured by the method described in the Examples.

Below, an embodiment of the present invention will be described with reference to the drawings. However, the technical scope of the present invention should be determined based on the claims and is not limited to the following forms. The dimensional ratios in the drawings have been exaggerated for the convenience of explanation and may differ from the actual ratios. In this specification, the range "X to Y" means "X or more and Y or less." Furthermore, unless otherwise specified, operations and measurements of physical properties are performed at room temperature (20 to 25°C) and a relative humidity of 40 to 50% RH.

Figure 1 is a schematic cross-sectional view showing the overall structure of a flat (stacked) non-bipolar (internal parallel connection) secondary battery (hereinafter also referred to simply as a "stacked secondary battery") according to one embodiment of the present invention.

As shown in FIG. 1, the stacked secondary battery 10a of this embodiment has a structure in which a roughly rectangular power generating element 21, where the charge/discharge reaction actually proceeds, is sealed inside a laminate film 29. Here, the power generating element 21 has a structure in which a positive electrode in which a positive electrode active material layer 13 is arranged on both sides of a positive electrode collector 11', an electrolyte layer 17 made of a separator containing an electrolytic solution, and a negative electrode in which a negative electrode active material layer 15 is arranged on both sides of a negative electrode collector 11" are stacked. Specifically, the negative electrode, electrolyte layer, and positive electrode are stacked in this order, with one positive electrode active material layer 13 and the adjacent negative electrode active material layer 15 facing each other through the electrolyte layer 17.

As a result, the adjacent positive electrode, electrolyte layer, and negative electrode constitute one single cell layer 19. Therefore, the stacked secondary battery 10a shown in FIG. 1 can be said to have a configuration in which a plurality of single cell layers 19 are stacked and electrically connected in parallel. The positive electrode collectors of the outermost layers located on both outermost layers of the power generation element 21 each have the positive electrode active material layer 13 disposed on only one side, but active material layers may be disposed on both sides. That is, instead of using a collector dedicated to the outermost layer with an active material layer disposed on only one side, a collector having active material layers on both sides may be used as the outermost layer collector as it is. Also, by reversing the arrangement of the positive electrode and negative electrode from FIG. 1, the outermost negative electrode collectors may be positioned on both outermost layers of the power generation element 21, and the negative electrode active material layers may be disposed on one or both sides of the outermost negative electrode collectors.

The positive electrode collector 11' and the negative electrode collector 11" are respectively attached with a positive electrode collector 25 and a negative electrode collector 27 that are electrically connected to each electrode (positive electrode and negative electrode), and are structured so as to be sandwiched between the ends of the laminate film 29 and led out to the outside of the laminate film 29. The positive electrode collector 25 and the negative electrode collector 27 may be attached to the positive electrode collector 11' and the negative electrode collector 11" of each electrode by ultrasonic welding, resistance welding, or the like, via a positive electrode terminal lead and a negative electrode terminal lead (not shown), respectively, as necessary.

Figure 2 is a cross-sectional view showing a bipolar secondary battery according to another embodiment of the present invention. The bipolar secondary battery 10b shown in Figure 2 has a structure in which a substantially rectangular power generating element 21, in which the actual charge/discharge reaction proceeds, is sealed inside a laminate film 29, which is the battery exterior. In this specification, the bipolar lithium ion secondary battery may also be referred to simply as a "bipolar secondary battery", and the electrode for the bipolar lithium ion secondary battery may also be referred to simply as a "bipolar electrode".

2, the power generating element 21 of the bipolar secondary battery 10b of this embodiment has a plurality of bipolar electrodes 23 in which a positive electrode active material layer 13 electrically connected to one side of the current collector 11 is formed, and a negative electrode active material layer 15 electrically connected to the opposite side of the current collector 11 is formed. Each bipolar electrode 23 is stacked via an electrolyte layer 17 to form the power generating element 21. At this time, each bipolar electrode 23 and the electrolyte layer 17 are alternately stacked so that the positive electrode active material layer 13 of one bipolar electrode 23 and the negative electrode active material layer 15 of the other bipolar electrode 23 adjacent to the one bipolar electrode 23 face each other via the electrolyte layer 17. That is, the electrolyte layer 17 is sandwiched and arranged between the positive electrode active material layer 13 of one bipolar electrode 23 and the negative electrode active material layer 15 of the other bipolar electrode 23 adjacent to the one bipolar electrode 23.

Adjacent positive electrode active material layer 13, electrolyte layer 17, and negative electrode active material layer 15 constitute one single cell layer 19. Therefore, it can be said that the bipolar secondary battery 10b has a configuration in which the single cell layers 19 are stacked. In addition, a seal portion (insulating layer) 31 is arranged on the outer periphery of the single cell layer 19. This prevents liquid junctions due to leakage of the electrolyte from the electrolyte layer 17, prevents contact between adjacent current collectors 11 in the battery, and prevents short circuits caused by slight misalignment of the ends of the single cell layers 19 in the power generation element 21. The positive electrode active material layer 13 is formed on only one side of the outermost current collector 11a on the positive electrode side, which is located in the outermost layer of the power generation element 21. The negative electrode active material layer 15 is formed on only one side of the outermost current collector 11b on the negative electrode side, which is located in the outermost layer of the power generation element 21.

Furthermore, in the bipolar secondary battery 10b shown in FIG. 2, a positive electrode current collector (positive electrode tab) 25 is disposed adjacent to the outermost current collector 11a on the positive electrode side, and is extended to lead out from the laminate film 29, which is the battery outer casing. On the other hand, a negative electrode current collector (negative electrode tab) 27 is disposed adjacent to the outermost current collector 11b on the negative electrode side, and is similarly extended to lead out from the laminate film 29.

The number of times the cell layers 19 are stacked is adjusted according to the desired voltage. In addition, in the bipolar secondary battery 10b, the number of times the cell layers 19 are stacked may be reduced if sufficient output can be ensured even if the thickness of the battery is made as thin as possible. In order to prevent external impacts and environmental deterioration during use, the bipolar secondary battery 10b is also preferably structured such that the power generating element 21 is vacuum sealed in the laminate film 29, which is the battery exterior, and the positive electrode current collector 25 and the negative electrode current collector 27 are exposed to the outside of the laminate film 29.

The main components of the lithium-ion secondary battery according to this embodiment are described below.

[Current collector]
The current collector has a function of mediating the movement of electrons from the positive electrode active material layer and the negative electrode active material layer described later. There is no particular limitation on the material constituting the current collector. For example, metals and conductive resins can be used as the material constituting the current collector.

Specific examples of metals include aluminum, nickel, iron, stainless steel, titanium, and copper. In addition, clad materials of nickel and aluminum, clad materials of copper and aluminum, and the like may be used. Also, a foil in which a metal surface is coated with aluminum may be used. Among these, aluminum, stainless steel, copper, and nickel are preferred from the viewpoints of electronic conductivity, battery operating potential, and adhesion of the negative electrode active material to the current collector by sputtering. In addition, examples of resins having electrical conductivity include resins in which a conductive filler is added to a non-conductive polymer material.

The current collector may be a single layer structure made of a single material, or may be a laminate structure made of a suitable combination of layers made of these materials. From the viewpoint of reducing the weight of the current collector, it is preferable that the current collector contains at least a conductive resin layer made of a resin having conductivity. In addition, from the viewpoint of blocking the movement of lithium ions between the single cell layers, a metal layer may be provided on a part of the current collector. Furthermore, if the positive electrode active material layer and the negative electrode active material layer described later are conductive and can perform a current collecting function by themselves, it is not necessary to use a current collector as a member separate from these electrode active material layers. In such a form, the positive electrode active material layer described later constitutes the positive electrode as it is, and the negative electrode active material layer described later constitutes the negative electrode as it is.

[Positive electrode active material layer]
The positive electrode active material layer contains a positive electrode active material and may further contain a binder, a conductive assistant, an electrolytic solution (liquid electrolyte), and the like.

(Positive Electrode Active Material)
The lithium ion secondary battery of this embodiment uses a lithium transition metal composite oxide represented by the following composition formula (1) as a positive electrode active material:
_Li1.5 [ _{NiCobMnc [} Li] _d ] _O3 ( ₁ )
In the formula, a, b, c and d satisfy 0<a<1.4, 0≦b<1.4, 0<c<1.4, 0.1<d≦0.4, a+b+c+d=1.5, and 1.1≦a+b+c<1.4.

The positive electrode active material represented by the above composition formula (1) is a layered _lithium -containing transition metal oxide consisting of a solid solution of electrochemically inactive layered _Li2MnO3 and electrochemically active layered _LiMO2 (wherein M is a transition metal selected from Co, Ni and Mn). This layered lithium-containing transition metal composite oxide has a portion that changes to a spinel structure by being subjected to charging or charging and discharging in a predetermined potential range, and it is known that after the battery is completed, it is in a state where it partially has a spinel phase. After the charge-discharge cycle treatment, this lithium-containing transition metal composite oxide is useful as a material that can obtain a large discharge capacity exceeding 200 mAh/g.

In the above composition formula (1), a represents the atomic ratio of Ni, b represents the atomic ratio of Co, c represents the atomic ratio of Mn, and d represents the atomic ratio of Li. The structure of the solid solution can be stabilized when a, b, c, and d satisfy 0<a<1.4, 0≦b<1.4, 0<c<1.4, 0.1<d≦0.4, a+b+c+d=1.5, and 1.1≦a+b+c<1.4.

As a preferred embodiment, in the composition formula (1), 0.1≦a≦0.75, 0≦b≦0.6, 0.2≦c≦0.9, and 0.15≦d≦0.35. The above ranges can further stabilize the crystal structure of the positive electrode active material. In addition, a positive electrode active material with high charge/discharge characteristics and high capacity retention can be obtained. From the viewpoint of obtaining a higher energy density, it is more preferable that in the general formula (1), 0.1≦a≦0.4, 0≦b≦0.3, 0.5≦c≦0.9, and 0.2≦d≦0.3, and it is even more preferable that 0.1≦a≦0.4, 0.05≦b≦0.25, 0.6≦c≦0.9, and 0.2≦d≦0.3. The composition of each element can be measured, for example, by plasma (ICP) emission spectrometry.

The method for preparing the positive electrode active material represented by the above composition formula (1) is not particularly limited, and any conventionally known method can be used as appropriate.

In this embodiment, the positive electrode active material represented by the composition formula (1) may have an oxide coating on the surface of the positive electrode active material. Examples of such oxides include Al ₂ O ₃ , CeO ₂ , ZrO ₂ , ZnO, and SiO _2. The oxide coating may cover at least a part of the surface of the positive electrode active material. The mass of the oxide coating is preferably 5% by mass or less in terms of oxide with respect to the total amount of the positive electrode active material including the oxide coating. The thickness of the oxide coating is, for example, 1 to 20 nm. The method of forming the oxide coating on the surface of the positive electrode active material is not particularly limited, and a conventionally known method may be appropriately adopted.

In the lithium ion secondary battery according to this embodiment, the positive electrode active material may further include a positive electrode active material other than the positive electrode active material represented by the composition formula (1). However, in this embodiment, the content of the positive electrode active material represented by the composition formula (1) in 100% by mass of the positive electrode active material is, for example, 50% by mass or more, preferably more than 50% by mass, more preferably 70% by mass or more, even more preferably 80% by mass or more, even more preferably 90% by mass or more, particularly preferably 95% by mass or more, and most preferably 100% by mass.

The average particle size of the positive electrode active material contained in the positive electrode active material layer is not particularly limited, but from the viewpoint of achieving high output, it is preferably 1 to 20 μm, and more preferably 2 to 10 μm. The average particle size is measured using a particle size distribution measuring device using the laser diffraction/scattering method.

The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but is preferably within the range of 60 to 99 mass%, and more preferably within the range of 80 to 98 mass%.

(Conductive assistant)
The conductive assistant has a function of forming an electronic conductive path (conductive passage) in the positive electrode active material layer. When such an electronic conductive path is formed in the positive electrode active material layer, the internal resistance of the battery can be reduced and the output characteristics at a high rate can be improved.

Examples of conductive assistants include particulate carbon materials such as acetylene black, carbon black, channel black, thermal black, and Ketjen Black (registered trademark), and fibrous carbon materials such as carbon nanotubes (single-walled carbon nanotubes and multi-walled carbon nanotubes), carbon nanofibers, vapor-grown carbon fibers, electrospun carbon fibers, polyacrylonitrile-based carbon fibers, and pitch-based carbon fibers. Only one type of conductive assistant may be used alone, or two or more types may be used in combination.

The content of the conductive assistant that can be contained in the positive electrode active material layer (the total amount when two or more types are contained) is not particularly limited, but is preferably 0.5 to 10 mass % and more preferably 1 to 5 mass % relative to 100 mass % of the total solid content of the positive electrode active material layer.

The optional binder used in the positive electrode active material layer is not particularly limited, but examples thereof include the following materials:
Thermoplastic polymers such as polybutylene terephthalate, polyethylene terephthalate, polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are replaced by other halogen elements), polyethylene, polypropylene, polymethylpentene, polybutene, polyether nitrile, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR), ethylene-propylene-diene copolymer, styrene-butadiene-styrene block copolymer and hydrogenated products thereof, styrene-isoprene-styrene block copolymer and hydrogenated products thereof, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene Examples of the fluororesin include fluororubbers such as vinylidene fluoride-hexafluoropropylene-based fluororubber (VDF-HFP-based fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-HFP-TFE-based fluororubber), vinylidene fluoride-pentafluoropropylene-based fluororubber (VDF-PFP-based fluororubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluororubber (VDF-PFP-TFE-based fluororubber), vinylidene fluoride-perfluoromethylvinyl ether-tetrafluoroethylene-based fluororubber (VDF-PFMVE-TFE-based fluororubber), and vinylidene fluoride-chlorotrifluoroethylene-based fluororubber (VDF-CTFE-based fluororubber); and epoxy resins. Among these, polyvinylidene fluoride (PVDF), polyimide, styrene-butadiene rubber, carboxymethyl cellulose, polypropylene, polytetrafluoroethylene, polyacrylonitrile, and polyamide are preferable.

The amount of binder contained in the positive electrode active material layer (the total amount when two or more types are contained) is not particularly limited, but is preferably 0.5 to 10 mass % and more preferably 1 to 5 mass % based on the total solid content of the positive electrode active material layer.

The thickness of the positive electrode active material layer is not particularly limited, and conventionally known knowledge about batteries can be referred to as appropriate. For example, the thickness of the positive electrode active material layer is usually about 1 to 1000 μm, preferably 20 to 800 μm, more preferably 30 to 500 μm, and even more preferably 40 to 200 μm. The thicker the positive electrode active material layer is, the more positive electrode active material can be retained to achieve sufficient capacity (energy density). On the other hand, the thinner the positive electrode active material layer is, the more the discharge rate characteristics can be improved.

[Negative electrode active material layer]
(Negative Electrode Active Material)
The negative electrode active material has a function of releasing ions such as lithium ions during discharge and absorbing ions such as lithium ions during charge.

Examples of the negative electrode active material include carbon materials such as graphite, soft carbon, and hard carbon, lithium-transition metal composite oxides (e.g., Li ₄ Ti ₅ O ₁₂ ), metal materials (tin, silicon), silicon-containing alloy-based negative electrode materials (e.g., Si ₆₀ Sn ₁₀ Ti ₃₀ ), and lithium alloy-based negative electrode materials (e.g., lithium-tin alloy, lithium-silicon alloy, lithium-aluminum alloy, lithium-aluminum-manganese alloy, etc.). In some cases, two or more negative electrode active materials may be used in combination. Preferably, from the viewpoint of capacity and output characteristics, silicon-containing alloy-based negative electrode materials, carbon materials, lithium-transition metal composite oxides, and lithium alloy-based negative electrode materials are preferably used as the negative electrode active material. Of course, negative electrode active materials other than the above may be used.

The average particle size (D ₅₀ ) of the negative electrode active material is not particularly limited, but from the viewpoint of achieving high output, it is preferably 1 to 100 μm, more preferably 1 to 20 μm.

The content of the negative electrode active material in the negative electrode active material layer is, for example, 60% by mass or more and less than 100% by mass, preferably 80% by mass or more and 99.5% by mass or less, more preferably more than 95% by mass or more and 99.0% by mass or less, and even more preferably 97% by mass or more and 98.5% by mass or less, relative to 100% by mass of the total solid content. If the content of the negative electrode active material is within the above range, it is possible to achieve both battery capacity and output characteristics.

The negative electrode active material layer may further contain other additives, such as a conductive assistant and a binder, as described above for the positive electrode active material layer, as necessary.

The thickness of the negative electrode active material layer is not particularly limited, and the same thickness as that described above for the positive electrode active material layer can be used.

[Electrolyte layer]
The electrolyte layer contains an electrolytic solution (liquid electrolyte) and preferably has a configuration in which a separator is impregnated with the electrolytic solution.

(Electrolyte)
The electrolyte has a function as a carrier of lithium ions. The electrolyte has a form in which a lithium salt is dissolved in a non-aqueous solvent. Preferably, the electrolyte is obtained by further adding a fluorine-containing carbonate to the non-aqueous solvent in which a lithium salt is dissolved.

As the non-aqueous solvent, one that easily dissolves lithium salts is preferred, for example, chain carbonates such as dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), and methyl ethyl carbonate (MEC); fluorine-containing chain carbonates in which some of the hydrogen atoms of these chain carbonates are replaced with fluorine atoms; ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and the like. cyclic carbonates such as carbonate (BC); fluorine-containing cyclic carbonates in which some of the hydrogen atoms of these cyclic carbonates are replaced with fluorine atoms; methyl propionate (MP), methyl acetate (MA), methyl formate (MF), 4-methyldioxolane (4MeDOL), dioxolane (DOL), 2-methyltetrahydrofuran (2MeTHF), tetrahydrofuran (THF), dimethoxyethane (DME), dimethyl sulfoxide (DMSO), and γ-butyrolactone (GBL).

In particular, from the viewpoint of further improving the rapid charging characteristics and output characteristics, it is preferable that the non-aqueous solvent contains a chain carbonate, and more preferably contains at least one selected from the group consisting of diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC).

Examples _of lithium salts include Li( _FSO2 ) _2N (lithium bis(fluorosulfonyl)imide; _LiFSI ), Li( _C2F5SO2 ) _2N , _LiPF6 , _LiBF4 , _LiClO4 , _LiAsF6 , _{and LiCF3SO3} .

The concentration of the lithium salt in the non-aqueous solvent is preferably 0.1 to 3.0 mol/L, and more preferably 0.8 to 2.2 mol/L.

In addition, it is preferable that the electrolyte further contains a fluorine-containing carbonate such as a fluorine-containing cyclic carbonate or a fluorine-containing chain carbonate. In this way, the battery can have excellent durability even when operated at a high voltage. In addition, these fluorine-containing carbonates form a protective film on the surface of the positive electrode active material, and can increase the voltage resistance of the positive electrode active material. In this case, as the fluorine-containing carbonate, fluorine-containing cyclic carbonates such as fluoroethylene carbonate (FEC), difluoroethylene carbonate, and 4-fluoropropylene carbonate; fluorine-containing chain carbonates such as ethyl trifluoromethyl carbonate, 2,2,2-trifluoroethyl methyl carbonate, and bis(2,2,2-trifluoroethyl) carbonate can be preferably used. The content of the fluorine-containing carbonate is not particularly limited. In a preferred embodiment, the electrolyte contains 0.5 to 10 mass% of a fluorine-containing carbonate, particularly fluoroethylene carbonate, with respect to the total amount of the finally obtained electrolyte. This makes it possible to obtain the above-mentioned effect more significantly. If the electrolyte contains two or more types of fluorine-containing carbonates, the total amount is preferably within the above range.

The electrolyte may further contain additives other than the above-mentioned components. Specific examples of such compounds include, for example, ethylene carbonate, vinylene carbonate, methylvinylene carbonate, dimethylvinylene carbonate, phenylvinylene carbonate, diphenylvinylene carbonate, ethylvinylene carbonate, diethylvinylene carbonate, vinylethylene carbonate, 1,2-divinylethylene carbonate, 1-methyl-1-vinylethylene carbonate, 1-methyl-2-vinylethylene carbonate, 1-ethyl-1-vinylethylene carbonate, 1-ethyl-2-vinylethylene carbonate, and 1-ethyl-2-vinylethylene carbonate. Examples of the additives include ethylene carbonate, vinyl vinylene carbonate, allyl ethylene carbonate, vinyloxymethyl ethylene carbonate, allyloxymethyl ethylene carbonate, acryloxymethyl ethylene carbonate, methacryloxymethyl ethylene carbonate, ethynyl ethylene carbonate, propargyl ethylene carbonate, ethynyloxymethyl ethylene carbonate, propargyloxyethylene carbonate, methylene ethylene carbonate, and 1,1-dimethyl-2-methylene ethylene carbonate. These additives may be used alone or in combination of two or more. In addition, the amount of additive used in the electrolyte can be appropriately adjusted.

(Separator)
The separator constituting the electrolyte layer has a function of retaining the electrolyte to ensure lithium ion conductivity between the positive electrode and the negative electrode, and also a function of acting as a partition between the positive electrode and the negative electrode.

Examples of the separator's form include a porous sheet separator made of a polymer or fiber that absorbs and retains the electrolyte, and a nonwoven fabric separator.

As a separator of a porous sheet made of a polymer or fiber, for example, a microporous sheet (microporous membrane) can be used. Specific forms of the porous sheet made of the polymer or fiber include, for example, polyolefins such as polyethylene (PE) and polypropylene (PP); laminates of multiple layers of these (for example, a laminate with a three-layer structure of PP/PE/PP), polyimide, aramid, hydrocarbon resins such as polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), glass fibers, etc., and microporous (microporous membrane) separators.

Nonwoven fabric separators may be made of any of the conventional materials known in the art, such as cotton, rayon, acetate, nylon, polyester, polyolefins such as PP and PE, polyimide, and aramid, either alone or in combination.

The thickness of the separator should be the same as that of the electrolyte layer, and is preferably 5 to 200 μm, and particularly preferably 10 to 100 μm.

In addition, a separator in which a heat-resistant insulating layer is laminated on a porous substrate (separator with heat-resistant insulating layer) can be used as the separator. The heat-resistant insulating layer is a ceramic layer containing inorganic particles and a binder. The separator with heat-resistant insulating layer is one with high heat resistance, with a melting point or thermal softening point of 150°C or higher, preferably 200°C or higher. By having a heat-resistant insulating layer, the internal stress of the separator that increases when the temperature rises is alleviated, so that a heat shrinkage suppression effect can be obtained. As a result, it is possible to prevent the induction of short circuits between the electrodes of the battery, resulting in a battery configuration in which performance deterioration due to temperature rise is unlikely to occur. In addition, by having a heat-resistant insulating layer, the mechanical strength of the separator with heat-resistant insulating layer is improved, and the separator is unlikely to break. Furthermore, due to the heat shrinkage suppression effect and high mechanical strength, the separator is unlikely to curl during the battery manufacturing process.

[Positive and negative current collector plates]
The material constituting the current collector plate (25, 27) is not particularly limited, and a known highly conductive material conventionally used as a current collector plate for a lithium ion secondary battery can be used. As the material constituting the current collector plate, for example, a metal material such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof is preferable. From the viewpoint of light weight, corrosion resistance, and high conductivity, aluminum and copper are more preferable, and aluminum is particularly preferable. In addition, the same material may be used for the positive electrode current collector plate 25 and the negative electrode current collector plate 27, or different materials may be used.

[Sealing section]
The sealing portion is a member specific to bipolar secondary batteries (series stacked batteries) and has the function of preventing leakage of the electrolyte from the electrolyte layer. In addition, it can also prevent contact between adjacent current collectors in the battery and short circuits caused by slight unevenness at the ends of the stacked electrodes. The constituent material of the sealing portion is not particularly limited, but polyolefin resins such as polyethylene and polypropylene, epoxy resins, rubber, polyimide, etc. can be used. Among these, it is preferable to use polyolefin resins from the viewpoints of corrosion resistance, chemical resistance, film formability, and economic efficiency.

[Positive and negative electrode leads]
Although not shown, the current collector and the current collector plate may be electrically connected via a positive electrode lead or a negative electrode lead. As the constituent materials of the positive electrode and the negative electrode lead, materials used in known lithium ion secondary batteries may be similarly adopted. Note that the part removed from the exterior is preferably covered with a heat-resistant insulating heat shrink tube or the like so as not to come into contact with peripheral devices or wiring, causing leakage and affecting products (e.g., automobile parts, particularly electronic devices, etc.).

[Battery exterior body]
As the battery exterior body, a known metal can case can be used, and a bag-shaped case using a laminate film 29 containing aluminum that can cover the power generating element as shown in Figures 1 and 2 can be used. The laminate film can be, for example, a three-layer laminate film formed by laminating PP, aluminum, and nylon in this order, but is not limited thereto. A laminate film is preferable from the viewpoint of being excellent in high output and cooling performance and being suitable for use in batteries for large equipment for EVs and HEVs. In addition, a laminate film containing aluminum is more preferable as the exterior body because it can easily adjust the group pressure applied to the power generating element from the outside.

The lithium ion secondary battery according to the present embodiment has excellent charge/discharge capacity at low temperatures and excellent cycle durability. Therefore, the lithium ion secondary battery according to the present embodiment is suitable for use as a power source for driving EVs and HEVs.

[Method of manufacturing lithium ion secondary battery]
The lithium ion secondary battery according to the present embodiment is not particularly limited, but may be manufactured by a method including a step of performing an initial activation charge on a power generating element including a positive electrode having a predetermined positive electrode active material at a temperature of −20° C. or higher and 10° C. or lower under conditions in which the positive electrode upper limit potential is 4.5 to 4.8 V vs. Li ⁺ /Li, and a step of performing an open circuit pause for 1 hour or more at a temperature of −20° C. or higher and 25° C. or lower after the initial activation charge.

That is, one embodiment of the present invention is a positive electrode active material comprising a compound represented by the following formula (1):
_Li1.5 [ _{NiCobMnc [} Li] _d ] _O3 ( ₁ )
(in formula (1), a, b, c, and d satisfy the relationships 0<a<1.4, 0≦b<1.4, 0<c<1.4, 0.1<d≦0.4, a+b+c+d=1.5, 1.1≦a+b+c<1.4), a negative electrode, and an electrolyte layer containing an electrolytic solution. The method for producing a lithium ion secondary battery is provided with a power generating element including a positive electrode including a positive electrode active material layer containing a lithium transition metal composite oxide having a composition represented by the following formula (1): (in formula (1), a, b, c, and d satisfy the relationships 0<a<1.4, 0≦b<1.4, 0<c<1.4, 0.1<d≦0.4, a+b+c+d=1.5, 1.1≦a+b+c<1.4), a negative electrode, and an electrolyte layer containing an electrolytic solution. The method includes the steps of: performing an initial activation charge at a temperature of −20° C. or higher and 10° C. or lower under conditions such that the positive electrode upper limit potential is 4.5 to 4.8 V vs. Li ⁺ /Li; and performing an open circuit pause at a temperature of −20° C. or higher and 25° C. or lower for 1 hour or more after the initial activation charge.

It is believed that the lithium-rich manganese positive electrode material can be activated and exhibit high capacity by performing the initial charge at around 4.5 V vs. Li ⁺ /Li. This is thought to be because oxygen in the crystal is oxidized and lithium ions are released from the crystal during charging, which causes a significant change in the crystal structure. In the positive electrode activation protocol described in the above Patent Document 1, such a change in the crystal structure is allowed to proceed slowly, thereby achieving a more stable arrangement of the constituent elements and a gradual transition to a stable structure. It is believed that by doing so, the structure after the change becomes a more stable structure, and the charge-discharge cycle durability is improved.

In this embodiment, the initial charge (initial activation charge) for activating the lithium-rich manganese positive electrode material is performed at a low temperature of -20°C or higher and 10°C or lower. This can significantly improve lithium diffusivity in the low SOC region. It is believed that the initial charge at a low temperature suppresses the release of oxygen in the crystals and the occurrence of cation mixing between lithium ions and transition metal ions that occurs during activation by charging, compared to when activation is performed at room temperature. Therefore, it is presumed that a crystal structure with high lithium diffusivity is formed even in the low SOC region where the lithium ion concentration is relatively high, compared to when the initial activation charge is performed at room temperature.

In this embodiment, if the temperature during the initial activation charge exceeds 10°C, it is difficult to form a crystal structure with high lithium diffusivity in the low SOC region, and excellent charge/discharge capacity cannot be obtained under low temperature conditions. On the other hand, if the temperature falls below -20°C, the lithium conductivity of the electrolyte may decrease, making it difficult to perform activation efficiently.

The upper limit potential of the initial activation charge may be 4.5 to 4.8 V vs. Li ⁺ /Li, but is preferably 4.6 to 4.7 V vs. Li ⁺ /Li because the charge/discharge characteristics and cycle durability at low temperatures can be improved more significantly. On the other hand, if the upper limit potential is below 4.5 V vs. Li ⁺ /Li, the activation of the positive electrode material does not proceed sufficiently, so that a high capacity cannot be obtained. Also, if it exceeds 4.8 V vs. Li ⁺ /Li, the crystal structure becomes unstable and the electrolyte is easily decomposed, so that good battery performance cannot be obtained.

The conditions for the initial activation charge are not particularly limited as long as the temperature and upper limit potential are within the above ranges. For example, the CCCV mode or the CC mode may be used. The charge rate during the initial activation charge is also not particularly limited, but is, for example, 0.5 C or less, and preferably 0.01 to 0.1 C.

The initial activation charge should be performed at least once, but may be performed multiple times.

After the initial activation charge is completed, an open circuit rest is performed for at least 1 hour at a temperature of -20°C or higher and 25°C or lower. The open circuit rest refers to maintaining the battery in an open circuit at a specified temperature for a specified time after charging is completed. This allows the change in the crystal structure to proceed slowly, resulting in a more stabilized structure after the change, which is thought to further improve the charge/discharge cycle durability. The temperature conditions during the open circuit rest are preferably -20°C or higher and 10°C or lower, as this provides better charge/discharge capacity and cycle durability at low temperatures. In addition, the time for the open circuit rest is preferably 1 to 30 hours, as this provides better charge/discharge capacity and cycle durability at low temperatures. After the open circuit rest is performed, discharge can be performed as appropriate, and then a charge/discharge cycle can be performed. There are no particular restrictions on the conditions for the discharge at this time and the subsequent charge/discharge cycle.

The present invention will be explained in more detail below using examples and comparative examples, but the present invention is not limited to the following examples.

Making a Battery
[Example 1]
<Preparation of Positive Electrode>
As the positive electrode active material, _a lithium transition metal composite oxide having a composition represented by _the following composition formula: _Li1.5 [ _{Ni0.25Co0.1Mn0.85} [Li] _0.3 ] _O3 (a compound represented by the above composition formula (1) where a = 0.25, b = 0.1, c = 0.85, d = 0.3, a + b + c + d = 1.5, and a + b + c = 1.2) was used.

A solid content consisting of 93 parts by mass of the positive electrode active material prepared above, 3 parts by mass of carbon black as a conductive assistant, and 4 parts by mass of polyvinylidene fluoride (PVDF) as a binder was prepared. An appropriate amount of N-methyl-2-pyrrolidone (NMP) as a slurry viscosity adjusting solvent was added to this solid content and mixed to prepare a positive electrode active material slurry. Next, the obtained positive electrode active material slurry was applied to one side of an aluminum foil (thickness 20 μm) as a current collector using a doctor blade, and the coating film was dried on a hot plate at 80 ° C for 1 hour. Thereafter, the coating film was pressed, placed in a vacuum dryer, and dried under vacuum conditions at 130 ° C for 8 hours to obtain a positive electrode in which a positive electrode active material layer with a thickness of 70 μm and a porosity of 25% was formed on the current collector.

<Production of lithium-ion secondary battery (coin cell)>
The positive electrode and the counter electrode Li prepared above were placed opposite each other, and a polypropylene separator was placed between them. Next, a laminate of the positive electrode, the separator, and the counter electrode (Li metal) was placed on the bottom side of a coin cell (CR2032, material: stainless steel (SUS316)). Furthermore, a gasket was attached to maintain the insulation between the electrodes, the electrolyte was injected with a syringe, a spring and a spacer were laminated, and the upper side of the coin cell was overlapped and sealed by crimping to prepare a lithium ion secondary battery (coin cell). As the electrolyte, an electrolyte obtained by dissolving LiPF ₆ 1 mol/L in a mixed solvent (volume ratio 30:70) of ethylene carbonate (EC) and diethyl carbonate (DEC) and mixing fluoroethylene carbonate (FEC) so that the amount of the electrolyte finally obtained was 1 mass % with respect to the total amount of the electrolyte was used.

<Initial activation charge>
The cell prepared above was placed in a thermostatic chamber set at −20° C., and after the cell temperature became constant, constant-current/constant-voltage charging was performed at −20° C. from 2.0 V vs. Li ⁺ /Li to an upper limit potential of 4.7 V vs. Li ⁺ /Li at a rate of 0.1 C.

<Open circuit suspension>
Next, aging was performed for 12 hours in an open circuit state at −20° C. After that, discharging was performed to 2.0 V at a rate of 0.1 C in CC mode.

[Examples 2 to 6, Comparative Examples 1 to 9]
A lithium ion secondary battery was produced in the same manner as in Example 1 described above, except that the temperature and upper limit potential of the initial activation charge, and the temperature of open circuit rest were changed as shown in Table 1 below.

Battery evaluation
<Measurement of lithium diffusion coefficient>
The lithium diffusion coefficient at 25° C. was measured for the cells produced in the above examples and comparative examples. The lithium diffusion coefficient was measured by Galvano static intermittent titration technique (GITT method) using an electrochemical measurement device manufactured by Bio-Logic.

After charging the cell to a fully charged state, a constant current pulse was applied to the electrode, and the change in potential over time after the current was cut off was measured to determine the lithium diffusion coefficient at states of charge (SOC) of 10 to 90%. Here, state of charge (SOC) is an index that indicates the ratio of charge to the capacity when the battery is fully charged, with the SOC at a fully charged state being 100% and the SOC at a fully discharged state being 0%. The SOC is calculated by first determining the capacity (current x time) that can be extracted from a fully charged state, and then subtracting (or adding) from this the current value multiplied by the time used for discharging and charging, and the remaining capacity can be estimated.

3 shows the lithium diffusion coefficient D _Li at 10 to 90% state of charge (SOC) for the cells produced in Example 1 and Comparative Example 4. Table 1 also shows the ratio D Li15 /D Li80 of the lithium diffusion coefficient at SOC 15% to the lithium diffusion coefficient at SOC 80%, the ratio D _Li15 /D _Li60 of the lithium diffusion coefficient at SOC 15% to the lithium diffusion coefficient at SOC 60%, and the lithium diffusion coefficient D _Li15 at SOC 15%, for the cells produced in each _{Example and} Comparative Example.

As shown in Fig. 3, in the high SOC region of 60% or more, the lithium diffusion coefficient D Li of the battery of Example 1, which was subjected to the initial activation charge and open circuit rest under the specified conditions, and the battery of Comparative Example 4, which does not satisfy the specified conditions, do not differ significantly. However, in the low SOC region of 40% or less, it can be seen that the lithium diffusion coefficient D _Li is increased by performing the initial activation charge and open circuit rest under the specified conditions as in Example 1, and the _lithium diffusibility is significantly improved. From Table 1, it can be seen that the lithium diffusibility in the low SOC region is also improved in Examples 2 to 6 compared to Comparative Examples 1 to 9.

<Measurement of cycle durability>
A charge-discharge cycle durability test was carried out at 25° C. for each of the batteries of the examples and comparative examples prepared above. Specifically, 100 cycles of charge-discharge were carried out in the range of 2.0 to 4.6 V at a rate of 0.33 C. Then, the ratio of the discharge capacity at the 100th cycle to the discharge capacity at the 1st cycle was calculated as the cycle capacity retention rate (%). The results are shown in Table 1 below.

<Measurement of low-temperature characteristics>
For the batteries of each of the examples and comparative examples prepared above, a charge/discharge test was carried out at -20°C and 25°C, in which the batteries were fully charged by charging to 4.6V in CCCV mode at a rate of 0.33C, and then discharged to 2.0V in CC mode at a rate of 1C. The ratio of the discharge capacity at 1C rate at -20°C to the discharge capacity at 1C rate at 25°C was calculated and taken as the discharge capacity ratio (%). The results are shown in Table 1 below.

From the results shown in Table 1, it was found that the batteries of Examples 1 to 6, which use a predetermined positive electrode active material and have a D _Li15 /D _Li80 of 0.05 or more, have a suppressed capacity decrease at low temperatures compared to the capacity at room temperature. They also have excellent cycle durability. In contrast, it was found that the batteries of Comparative Examples 1 to 9, which have a D _Li15 /D _Li80 of less than 0.05, have a large capacity decrease at low temperatures.

10a: stacked secondary battery;
10b Bipolar secondary battery 11 Current collector,
11a: outermost current collector on the positive electrode side;
11b: outermost current collector on the negative electrode side;
11' Positive electrode current collector 11" Negative electrode current collector 13 Positive electrode active material layer,
15 negative electrode active material layer,
17 electrolyte layer,
19 cell layer,
21 power generating element,
23 bipolar electrodes,
25 Positive electrode current collector (positive electrode tab),
27 negative electrode current collector (negative electrode tab),
29 Laminate film,
31 Sealing portion.

Claims (7)

The positive electrode active material is a material having the following composition formula (1):
_Li1.5 [ _{NiCobMnc [} Li] _d ] _O3 ( ₁ )
(wherein a, b, c and d satisfy the relationships 0<a<1.4, 0≦b<1.4, 0<c<1.4, 0.1<d≦0.4, a+b+c+d=1.5, 1.1≦a+b+c<1.4), a negative electrode, and an electrolyte layer containing an electrolyte solution,
A lithium ion secondary battery, in which the ratio D _Li15 /D _Li80 of the lithium diffusion coefficient (D _Li15 ) at an SOC of 15% to the lithium diffusion coefficient (D _Li80 ) at an SOC of 80% at 25° C. is 0.05 or more. 2. The lithium ion secondary battery according to claim 1, wherein the ratio (D _Li15 /D _Li60 ) of the lithium diffusion coefficient (D _Li15 ) at SOC 15% to the lithium diffusion coefficient (D _Li60 ) at SOC 60% at 25° C. is 0.1 or more. 3. The lithium ion secondary battery according to claim 1, wherein the lithium diffusion coefficient (D _Li15 ) at 25° C. and SOC 15% is 1×10 ⁻¹⁶ cm ² /s or more. The lithium ion secondary battery according to claim 1 or 2, wherein the electrolyte solution contains a non-aqueous solvent, a lithium salt, and a fluorine-containing carbonate. The lithium ion secondary battery according to claim 4, wherein the electrolyte contains 0.5 to 10 mass % of fluoroethylene carbonate as the fluorine-containing carbonate. As a positive electrode active material, the following formula (1):
_Li1.5 [ _{NiCobMnc [} Li] _d ] _O3 ( ₁ )
(wherein a, b, c and d satisfy the relationships 0<a<1.4, 0≦b<1.4, 0<c<1.4, 0.1<d≦0.4, a+b+c+d=1.5, 1.1≦a+b+c<1.4), a negative electrode, and an electrolyte layer containing an electrolyte solution,
A step of performing an initial activation charge under conditions where the upper limit potential of the positive electrode is 4.5 to 4.8 V vs. Li ⁺ /Li at a temperature of −20° C. or higher and 10° C. or lower;
and performing an open circuit rest at a temperature of -20°C or higher and 25°C or lower for 1 hour or more after the initial activation charge. The method according to claim 6, wherein the lithium ion secondary battery has a ratio D _Li15 /D _{Li80 of the lithium diffusion coefficient (D Li15 )} at SOC 15% to the lithium diffusion coefficient (D _Li80 ) at SOC 80% at 25°C of 0.05 or more.