Secondary battery

Abstract

To provide a secondary battery capable of ensuring thermal stability even after low-temperature cycles.SOLUTION: A secondary battery according to an embodiment of the present invention includes a positive electrode, a negative electrode, and an electrolyte, and which is subjected to a low-temperature cycle followed by high-temperature storage or a room-temperature cycle in the secondary battery state, and in which the concentration of fluorine atoms on the surface of the negative electrode after the high-temperature storage or the room-temperature cycle based on X-ray photoelectron spectroscopy is higher than the concentration of oxygen atoms after the low-temperature cycle.SELECTED DRAWING: Figure 1A

Description

[Technical field]

[0001] The present invention relates to a secondary battery. [Background technology]

[0002] Secondary batteries that can be repeatedly charged and discharged have been used for various purposes. For example, secondary batteries are used as power sources for electronic devices such as smartphones and laptop computers.

[0003] A secondary battery has a structure in which an electrode assembly including a positive electrode, a negative electrode, and a separator disposed between the positive and negative electrodes, and an electrolyte are housed in an exterior body. The electrodes include a current collector and an electrode material layer provided on at least one of the main surfaces of the current collector. Specifically, the positive electrode includes a positive electrode current collector and a positive electrode material layer provided on at least one of the main surfaces of the positive electrode current collector. The negative electrode includes a negative electrode current collector and a negative electrode material layer provided on at least one of the main surfaces of the negative electrode current collector. An example of a secondary battery is a lithium secondary battery. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2005-222829 A Summary of the Invention [Problem to be solved by the invention]

[0005] Here, it is known that in a lithium secondary battery, when charging and discharging are repeatedly performed at a relatively low temperature (for example, −20° C. to 10° C.), i.e., when a low-temperature cycle is performed, the degree of absorption of lithium ions into the negative electrode material layer of the negative electrode may be reduced.

[0006] In this regard, carbonates are used as the electrolyte solvent and LiPF 6When lithium hexafluorophosphate (LiH2SO4) is used, lithium ions that are not accepted into the negative electrode layer during charging are combined with carbonate groups to form Li 2 CO 3 (lithium carbonate) can be formed.

[0007] In addition, the Li 2 CO 3 and LiPF 6 In the presence of these two substances, an exothermic reaction may occur at high temperatures (approximately 100° C. to approximately 180° C.), which may lead to a concern of a decrease in the thermal stability of the battery. In particular, if the temperature range that causes the above-mentioned exothermic reaction is reached during an overload test of the battery after the low-temperature cycle, there may be a risk of the battery exploding, and the decrease in the thermal stability of the battery may become apparent.

[0008] The present invention has been devised in view of the above circumstances. Specifically, an object of the present invention is to provide a secondary battery capable of ensuring thermal stability even after low-temperature cycles are performed. [Means for solving the problem]

[0009] In order to achieve the above object, in one embodiment of the present invention, A positive electrode, a negative electrode, and an electrolyte, In the secondary battery state, a low-temperature cycle is performed, followed by high-temperature storage or room temperature cycle. A secondary battery is provided in which the concentration of fluorine atoms is higher than the concentration of oxygen atoms at the surface of the negative electrode after the high temperature storage or room temperature cycle as determined by X-ray photoelectron spectroscopy compared to after the low temperature cycle. Effect of the Invention

[0010] According to the secondary battery according to one embodiment of the present invention, it is possible to ensure thermal stability even after low-temperature cycles are performed. [Brief description of the drawings]

[0011] [Figure 1A]1 is a graph showing the behavior of oxygen atom concentration and fluorine atom concentration on the negative electrode surface based on X-ray photoelectron spectroscopy after storage at 80° C. following a low-temperature cycle. [Figure 1B] 1 is a graph showing the behavior of oxygen atomic concentration and fluorine atomic concentration at the negative electrode surface based on X-ray photoelectron spectroscopy after performing a low-temperature cycle followed by a room temperature cycle. [Figure 1C] 1 is a graph showing the behavior of oxygen atom concentration and fluorine atom concentration on the negative electrode surface based on X-ray photoelectron spectroscopy after low-temperature cycling. [Figure 2A] 1 is a graph showing the behavior of battery temperature with respect to heating time during a heating test of a secondary battery. [Figure 2B] 1 is a graph showing the behavior of heat generation rate (dT / dt) versus heating temperature during a heating test of a secondary battery. [Diagram 3] 1 is a graph showing the behavior of the calorific value versus heating temperature for a powder mixture of LiPF6 and Li2CO3 and a powder mixture of LiPF6 and LiF. [Figure 4] FIG. 2 is a cross-sectional view showing a schematic structure of a positive electrode and a negative electrode. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, an electrode for a secondary battery according to an embodiment of the present invention will be described in detail with reference to the drawings. Various elements in the drawings are merely shown as schematic and illustrative elements for the understanding of the present invention, and the appearance, dimensional ratio, etc. may differ from the actual ones.

[0013] Before specifically describing the electrode for a secondary battery according to one embodiment of the present invention, the basic structure of a secondary battery will be described. In this specification, the term "secondary battery" refers to a battery capable of repeated charging and discharging. The term "secondary battery" is not limited to its name, and may include, for example, an "electricity storage device". In this specification, the term "planar view" refers to a state in which an object is viewed from above or below along the thickness direction based on the stacking direction of the electrode materials constituting the secondary battery. In addition, in this specification, the term "cross-sectional view" refers to a state in which an object is viewed from a direction approximately perpendicular to the thickness direction based on the stacking direction of the electrode materials constituting the secondary battery. The "upper and lower directions" and "left and right directions" used directly or indirectly in this specification correspond to the upper and lower directions and left and right directions in the drawings, respectively. Unless otherwise specified, the same reference numerals or symbols indicate the same members or parts or the same meanings. In a preferred embodiment, the vertical downward direction (i.e., the direction in which gravity acts) corresponds to the "downward direction", and the opposite direction corresponds to the "upward direction".

[0014] Various numerical ranges mentioned in this specification are intended to include the lower and upper limit values ​​themselves. That is, for example, a numerical range such as 1 to 10 can be interpreted as including the lower limit "1" and the upper limit "10".

[0015] [Basic structure of secondary batteries] A secondary battery has a structure in which an electrode assembly and an electrolyte are housed and sealed inside an exterior body. The electrode assembly may include a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode. The electrode assembly may be a stacked electrode assembly or a wound (jelly roll) electrode assembly. The stacked electrode assembly is one in which a plurality of electrode constituent layers including a positive electrode, a negative electrode, and a separator are stacked. The wound electrode assembly is one in which an electrode constituent layer including a positive electrode, a negative electrode, and a separator is wound. In addition, for example, the electrode assembly may have a so-called stack and folding structure in which a positive electrode, a separator, and a negative electrode are stacked on a long film and then folded.

[0016] The positive electrode 10A is composed of at least a positive electrode collector 11A and a positive electrode material layer 12A (see FIG. 4), and a positive electrode material layer 12A is provided on at least one side of the positive electrode collector 11A. A positive electrode side lead-out tab is located at a portion of the positive electrode collector 11A where the positive electrode material layer 12A is not provided, i.e., at an end of the positive electrode collector 11A. The positive electrode material layer 12A contains a positive electrode active material as an electrode active material. The negative electrode 10B is composed of at least a negative electrode collector 11B and a negative electrode material layer 12B (see FIG. 4), and a negative electrode material layer 12B is provided on at least one side of the negative electrode collector 11B. A negative electrode side lead-out tab is located at a portion of the negative electrode collector 11B where the negative electrode material layer 12B is not provided, i.e., at an end of the negative electrode collector 11B. The negative electrode material layer 12B contains a negative electrode active material as an electrode active material.

[0017] The positive electrode active material contained in the positive electrode layer 12A and the negative electrode active material contained in the negative electrode layer 12B are materials that are directly involved in the transfer of electrons in the secondary battery, and are the main materials of the positive and negative electrodes that are responsible for charging and discharging, i.e., the battery reaction. More specifically, ions are brought to the electrolyte due to the "positive electrode active material contained in the positive electrode layer 12A" and the "negative electrode active material contained in the negative electrode layer 12B", and these ions move between the positive electrode 10A and the negative electrode 10B to transfer electrons and charge and discharge. It is preferable that the positive electrode layer 12A and the negative electrode layer 12B are layers that can absorb and release lithium ions in particular. In other words, a secondary battery in which lithium ions move between the positive electrode 10A and the negative electrode 10B via the electrolyte to charge and discharge the battery is preferable. When lithium ions are involved in charging and discharging, the secondary battery corresponds to a so-called "lithium ion battery".

[0018] The positive electrode active material of the positive electrode layer 12A is preferably made of granular material, and a binder is preferably included in the positive electrode layer 12A to ensure sufficient contact between the particles and shape retention. Furthermore, a conductive assistant may be included in the positive electrode layer 12A to facilitate the transfer of electrons that promote the battery reaction. Similarly, the negative electrode active material of the negative electrode layer 12B is preferably made of granular material, and a binder is preferably included in the positive electrode layer 12A to ensure sufficient contact between the particles and shape retention, and a conductive assistant may be included in the negative electrode layer 12B to facilitate the transfer of electrons that promote the battery reaction. Thus, because of the form in which a plurality of components are contained, the positive electrode layer 12A and the negative electrode layer 12B can also be referred to as a "positive electrode mixture layer" and a "negative electrode mixture layer", respectively.

[0019] The positive electrode active material is preferably a material that contributes to the absorption and release of lithium ions. From this perspective, the positive electrode active material is preferably, for example, a lithium-containing composite oxide. More specifically, the positive electrode active material is preferably a lithium transition metal composite oxide containing lithium and at least one transition metal selected from the group consisting of cobalt, nickel, manganese, and iron. In other words, in the positive electrode layer 12A of the secondary battery, such a lithium transition metal composite oxide is preferably contained as the positive electrode active material. For example, the positive electrode active material may be lithium cobalt oxide, lithium nickel oxide, lithium manganate, lithium iron phosphate, or a material in which a part of the transition metal is replaced with another metal. Such a positive electrode active material may be contained as a single type, or may be contained in a combination of two or more types. In a more preferred embodiment, the positive electrode active material contained in the positive electrode layer 12A is lithium cobalt oxide.

[0020] The binder that may be included in the positive electrode material layer 12A is not particularly limited, but may be at least one selected from the group consisting of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, polytetrafluoroethylene, etc. The conductive assistant that may be included in the positive electrode material layer 12A is not particularly limited, but may be at least one selected from carbon black such as thermal black, furnace black, channel black, ketjen black, and acetylene black, carbon fibers such as graphite, carbon nanotubes, and vapor-grown carbon fibers, metal powders such as copper, nickel, aluminum, and silver, and polyphenylene derivatives. For example, the binder of the positive electrode material layer 12A may be polyvinylidene fluoride. By way of example only, the conductive assistant of the positive electrode material layer 12A is carbon black. Furthermore, the binder and conductive additive of the positive electrode layer 12A may be a combination of polyvinylidene fluoride and carbon black.

[0021] The negative electrode active material is preferably a material that contributes to the absorption and release of lithium ions, and from this viewpoint, the negative electrode active material is preferably, for example, various carbon materials, oxides, lithium alloys, or the like.

[0022] Examples of various carbon materials for the negative electrode active material include graphite (natural graphite, artificial graphite), soft carbon, hard carbon, diamond-like carbon, and the like. In particular, graphite is preferable because of its high electronic conductivity and excellent adhesion to the negative electrode current collector 11B. Examples of oxides for the negative electrode active material include at least one selected from the group consisting of silicon oxide, tin oxide, indium oxide, zinc oxide, and lithium oxide. The lithium alloy for the negative electrode active material may be any metal that can form an alloy with lithium, and may be, for example, a binary, ternary, or higher alloy of lithium and a metal such as Al, Si, Pb, Sn, In, Bi, Ag, Ba, Ca, Hg, Pd, Pt, Te, Zn, or La. It is preferable that such oxides have an amorphous structure. This is because deterioration caused by non-uniformity such as grain boundaries or defects is less likely to occur. Although this is merely an example, the negative electrode active material for the negative electrode material layer 12B may be artificial graphite.

[0023] The binder that may be contained in the negative electrode material layer 12B is not particularly limited, but may be at least one selected from the group consisting of styrene butadiene rubber, polyacrylic acid, polyvinylidene fluoride, polyimide resin, and polyamide-imide resin. For example, the binder contained in the negative electrode material layer 12B may be styrene butadiene rubber. The conductive assistant that may be contained in the negative electrode material layer 12B is not particularly limited, but may be at least one selected from carbon black such as thermal black, furnace black, channel black, ketjen black, and acetylene black, carbon fibers such as graphite, carbon nanotubes, and vapor-grown carbon fibers, metal powders such as copper, nickel, aluminum, and silver, and polyphenylene derivatives. The negative electrode material layer 12B may contain a component resulting from a thickener component (e.g., carboxymethyl cellulose) used during battery production.

[0024] By way of example only, the negative electrode active material and binder in the negative electrode material layer 12B may be a combination of artificial graphite and styrene-butadiene rubber.

[0025] The positive electrode current collector 11A and the negative electrode current collector 11B used in the positive electrode 10A and the negative electrode 10B are members that contribute to collecting and supplying electrons generated in the active material due to the battery reaction. Such a current collector may be a sheet-like metal member and may have a porous or perforated form. For example, the current collector may be a metal foil, a punched metal, a net, an expanded metal, or the like. The positive electrode current collector 11A used in the positive electrode 10A is preferably made of a metal foil containing at least one selected from the group consisting of aluminum, stainless steel, nickel, and the like, and may be, for example, an aluminum foil. On the other hand, the negative electrode current collector 11B used in the negative electrode 10B is preferably made of a metal foil containing at least one selected from the group consisting of copper, stainless steel, nickel, and the like, and may be, for example, a copper foil.

[0026] The separator 50 is a member provided from the viewpoint of preventing short circuit due to contact between the positive and negative electrodes and maintaining electrolyte. In other words, the separator 50 is a member that allows ions to pass while preventing electronic contact between the positive electrode 10A and the negative electrode 10B. Preferably, the separator 50 is a porous or microporous insulating member, and has a membrane form due to its small thickness. Although it is merely an example, a microporous membrane made of polyolefin may be used as the separator. In this regard, the microporous membrane used as the separator 50 may contain, for example, only polyethylene (PE) or only polypropylene (PP) as polyolefin. Furthermore, the separator 50 may be a laminate composed of a "microporous membrane made of PE" and a "microporous membrane made of PP". The surface of the separator 50 may be covered with an inorganic particle coating layer and / or an adhesive layer, etc. The surface of the separator may have adhesive properties.

[0027] The separator 50 should not be limited by its name, and may be a solid electrolyte, a gel electrolyte, insulating inorganic particles, or the like having the same function. From the viewpoint of further improving the handling of the electrodes, it is preferable that the separator 50 and the electrodes (positive electrode 10A / negative electrode 10B) are bonded together. The adhesion between the separator 50 and the electrodes can be achieved by using an adhesive separator as the separator 50, applying an adhesive binder to the electrode material layer (positive electrode material layer 12A / negative electrode material layer 12B) and / or by thermocompression bonding, or the like. Examples of the adhesive binder material that provides adhesiveness to the separator 50 or the electrode material layer include polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene polymer, acrylic resin, and the like. The thickness of the adhesive layer formed by applying an adhesive binder, or the like, may be 0.5 μm or more and 5 μm or less.

[0028] When the positive electrode 10A and the negative electrode 10B have layers capable of absorbing and releasing lithium ions, the electrolyte is preferably a "non-aqueous" electrolyte such as an organic electrolyte and / or an organic solvent (i.e., the electrolyte is preferably a non-aqueous electrolyte). Metal ions released from the electrodes (positive electrode 10A and negative electrode 10B) are present in the electrolyte, and therefore the electrolyte assists the transfer of metal ions in the battery reaction.

[0029] The non-aqueous electrolyte is an electrolyte containing a solvent and a solute. A specific solvent for the non-aqueous electrolyte is preferably one containing at least a carbonate. Such a carbonate may be a cyclic carbonate and / or a chain carbonate. Although not particularly limited, the cyclic carbonate may be at least one selected from the group consisting of propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC) and vinylene carbonate (VC). The chain carbonate may be at least one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC) and dipropyl carbonate (DPC). Although merely illustrative, a combination of a cyclic carbonate and a chain carbonate may be used as the non-aqueous electrolyte, for example, a mixture of ethylene carbonate and diethyl carbonate may be used. A specific example of the solute of the non-aqueous electrolyte is preferably LiPF 6 , LiBF 4 A specific example of the solute of the non-aqueous electrolyte is preferably LiPF 6 and / or LiBF 4 Li salts such as the above are used.

[0030] As the positive electrode current collector lead and the negative electrode current collector lead, any current collector lead used in the field of secondary batteries can be used. Such current collector leads may be made of any material that can achieve the movement of electrons, and may be made of conductive materials such as aluminum, nickel, iron, copper, and stainless steel. The positive electrode current collector lead is preferably made of aluminum, and the negative electrode current collector lead is preferably made of nickel. The form of the positive electrode current collector lead and the negative electrode current collector lead is not particularly limited, and may be, for example, in the form of a wire or a plate.

[0031] As the external terminal, any external terminal used in the field of secondary batteries can be used. Such an external terminal may be made of a material that can achieve the transfer of electrons, and is usually made of a conductive material such as aluminum, nickel, iron, copper, or stainless steel. The external terminal may be electrically and directly connected to the substrate, or may be electrically and indirectly connected to the substrate via another device. However, this is not limited to this, and a positive electrode current collector lead connected to each of the multiple positive electrodes may have the function of a positive electrode external terminal, and a negative electrode current collector lead connected to each of the multiple negative electrodes may have the function of a negative electrode external terminal.

[0032] The exterior body may take the form of a conductive hard case or a flexible case (pouch, etc.). When the exterior body is in the form of a flexible case (pouch, etc.), each of the multiple positive electrodes is connected to a positive electrode external terminal via a positive electrode current collecting lead. The positive electrode external terminal is fixed to the exterior body by a seal part, and the seal part prevents leakage of the electrolyte. Similarly, each of the multiple negative electrodes is connected to a negative electrode external terminal via a negative electrode current collecting lead. The negative electrode external terminal is fixed to the exterior body by a seal part, and the seal part prevents leakage of the electrolyte. However, this is not limited to this, and the positive electrode current collecting lead connected to each of the multiple positive electrodes may have the function of a positive electrode external terminal, and the negative electrode current collecting lead connected to each of the multiple negative electrodes may have the function of a negative electrode external terminal. When the exterior body is in the form of a conductive hard case, each of the multiple positive electrodes is connected to a positive electrode external terminal via a positive electrode current collecting lead. The positive electrode external terminal is fixed to the exterior body by a seal portion, and the seal portion prevents leakage of the electrolyte.

[0033] The conductive hard case is composed of a main body and a lid. The main body is composed of a bottom and a side portion that constitute the bottom surface of the exterior body. The main body and the lid are sealed after the electrode assembly, the electrolyte, the current collecting lead, and the external terminal are housed. The sealing method is not particularly limited, and may be, for example, a laser irradiation method. As the material for the main body and the lid, any material that can constitute a hard case type exterior body in the field of secondary batteries can be used. Such a material may be any material that can achieve the movement of electrons, and may be, for example, a conductive material such as aluminum, nickel, iron, copper, or stainless steel. The dimensions of the main body and the lid are determined mainly according to the dimensions of the electrode assembly, and it is preferable that the dimensions are such that the movement (displacement) of the electrode assembly within the exterior body is prevented when the electrode assembly is housed. By preventing the movement of the electrode assembly, the destruction of the electrode assembly is prevented, and the safety of the secondary battery is improved.

[0034] The flexible case is composed of a soft sheet. The soft sheet may have enough flexibility to achieve the folding of the seal portion, and is preferably a plastic sheet. The plastic sheet is a sheet that has a property of maintaining the deformation caused by an external force when the external force is applied and then removed, and for example, a so-called laminate film can be used. A flexible pouch made of a laminate film can be produced, for example, by overlapping two laminate films and heat sealing the periphery. A typical laminate film is a film in which a metal foil and a polymer film are laminated, and specifically, a three-layer structure consisting of an outer layer polymer film / metal foil / inner layer polymer film is exemplified. The outer layer polymer film is intended to prevent the permeation of moisture and the like and damage to the metal foil due to contact, etc., and polymers such as polyamide and polyester can be suitably used. The metal foil is intended to prevent the permeation of moisture and gas, and foils such as copper, aluminum, and stainless steel can be suitably used. The inner layer polymer film serves to protect the metal foil from the electrolyte contained therein and also to melt-seal the opening during heat sealing, and is preferably made of polyolefin or acid-modified polyolefin.

[0035] [Characteristics of the present invention] The present inventors have conducted extensive research into solutions for maintaining the thermal stability of secondary batteries, and as a result have devised the present invention having the following characteristics.

[0036] Specifically, the secondary battery according to one embodiment of the present invention is subjected to a low-temperature cycle followed by high-temperature storage or a room temperature cycle.

[0037] As used herein, "low temperature cycle" refers to 100 cycles of charge and discharge at low temperature (-20°C or higher and 10°C or lower). As used herein, "room temperature cycle" refers to 100 cycles of charge and discharge at room temperature (approximately 25°C). As used herein, "high temperature storage" refers to storing a battery at high temperature (approximately 50°C or higher and 90°C or lower) for a specified period of time.

[0038] In addition, as shown in Figures 1A and 1B, in comparison with Figure 1C, the present invention has a technical feature in that the concentration of fluorine atoms is higher than the concentration of oxygen atoms on the surface of the negative electrode of the component after high-temperature storage or room-temperature cycle based on X-ray photoelectron spectroscopy, as compared with the concentration after low-temperature cycle.

[0039] In contrast, as shown in FIG. 1C, the fluorine atomic concentration on the surface of the negative electrode after the low-temperature cycle is lower than the oxygen atomic concentration based on X-ray photoelectron spectroscopy.

[0040] In the present invention, the electrolyte is a carbonate (solvent) and LiPF 6 In this case, during the low-temperature cycle, the lithium ions that were not accepted into the negative electrode layer during charging are combined with the carbonate groups to form Li 2 CO 3 can be formed.

[0041] In the present invention, the oxygen atoms that may be present on the surface of the negative electrode are at least the Li 2 CO 3 The fluorine atoms present on the surface of the negative electrode may be derived from at least the LiPF 6 It may be of origin.

[0042] Here, compounds containing fluorine atoms generally have thermal stability, and in the present invention, the concentration of fluorine atoms is higher than the concentration of oxygen atoms on the surface of the negative electrode after a low-temperature cycle followed by high-temperature storage or a room-temperature cycle.

[0043] The increase in the fluorine atom concentration after high-temperature storage or room-temperature cycling was due to the presence of LiPF 6 This utilizes the property that it can decompose at room temperature or high temperatures relatively compared to low temperatures.

[0044] Based on the above, in the present invention, after high temperature storage or room temperature cycle, Li 2 CO 3 The fluorine-containing material has a relatively higher concentration than the oxygen atoms derived from the negative electrode and has thermal stability, and is present on the surface of the negative electrode. That is, since the fluorine is more concentrated than the oxygen, it is possible for a heat-resistant coating derived from the fluorine-containing material to be present on the surface of the negative electrode. The fluorine-containing material is not particularly limited, but for example, LiF, POF 3 , and C.H. 3 It may contain at least one selected from the group consisting of F.

[0045] Another technical feature of the present invention is that the concentration of fluorine atoms is higher at least on the outermost surface of the negative electrode after high-temperature storage as shown in FIG. 1A or after a room-temperature cycle as shown in FIG. 1B than the concentration of fluorine atoms after a low-temperature cycle as shown in FIG. 1C.

[0046] Specifically, at the outermost surface of the negative electrode, the ratio of the concentration of fluorine atoms after high-temperature storage or room-temperature cycle to the concentration of fluorine atoms after low-temperature cycle can be 1.5 or more.

[0047] The presence of such a high concentration of fluorine-containing material having thermal stability on the surface of the negative electrode after high-temperature storage or room temperature cycling provides the following advantages.

[0048] Specifically, as shown in Figures 2A and 2B, the heat generation start time and heat generation start temperature can be shifted later after high temperature storage is performed after a low temperature cycle compared to after a low temperature cycle (without high temperature storage).

[0049] Furthermore, the temperature of the heated cell can be reduced to that of the uncharged or undischarged state, and the rate of heat generation in the temperature range (approximately 100°C or higher and 150°C or lower) when an overload test is performed can be reduced.

[0050] From the above, the present invention makes it possible to restore or recover the thermal stability of a battery, which may decrease after the low-temperature cycle, to the state before the decrease in thermal stability or to a state close to that state. As a result, the thermal stability of a secondary battery once subjected to a low-temperature cycle can be maintained at a predetermined level. In other words, the thermal stability of the secondary battery can be suitably secured even after the low-temperature cycle.

[0051] As shown in Figure 3, the Li 2 CO 3 and LiPF 6 In the presence of LiPF, an exothermic reaction can occur at temperatures between about 100°C and about 180°C. Therefore, in the present invention, LiPF is used in a temperature range below this range. 6 It should be noted that it is necessary to select a temperature range within which the compound can decompose. EXAMPLES

[0052] Examples 1 to 3 will be described below.

[0053] Example 1 In this example, a secondary battery having the following components was prepared, and the secondary battery was subjected to a low-temperature cycle, followed by high-temperature storage or a room temperature cycle.

[0054] ■ Secondary battery configuration Electrolytes: Solvent: Ethyl methyl carbonate (EMC) and solute: LiPF 6 Contains Negative electrode: Mainly carbon Positive electrode: Lithium cobalt oxide Separator: PE ■ Cycle conditions Condition 1: Low temperature cycle only Charge / discharge at -5℃ for 100 cycles (1C charge / 2.5C discharge / 2.5V cut) Condition 2: A low-temperature cycle was performed, followed by high-temperature storage (80°C, 12 hours). The battery after the low-temperature cycle was subjected to high-temperature storage. Condition 3: A low-temperature cycle was performed, followed by a room temperature cycle. After the low-temperature cycle, 100 cycles of charge / discharge (2C charge / 2.5C discharge / 2.5V cut) were performed at room temperature.

[0055] ■Measurements based on X-ray photoelectron spectroscopy After each cycle was performed, the negative electrode was removed from the cell in a glove box filled with argon gas, and the negative electrode was washed with dimethyl carbonate. Then, the negative electrode surface was analyzed by X-ray photoelectron spectroscopy, and the surface sputtering was performed and the exposed surface was analyzed under the following conditions. The results are shown in Figures 1A to 1C. The conditions for the X-rays and other equipment used were as follows: ·X-ray source ··· Monochromatic Al-Kα (1486.6eV) X-ray spot diameter: 100μmφ ·Ar + Sputtering conditions: 1 KV, 1 mm x 1 mm, approximately 7.7 nm / min Analytical equipment used for X-ray photoelectron spectroscopy (ULVAC-PHI Corporation, Quantera SXM scanning X-ray photoelectron spectrometer)

[0056] (Measurement results) Figure 1A shows the behavior of the oxygen and fluorine atomic concentrations on the negative electrode surface after low-temperature cycling followed by storage at 80°C, as measured by X-ray photoelectron spectroscopy. Figure 1B shows the behavior of the oxygen and fluorine atomic concentrations on the negative electrode surface after low-temperature cycling followed by room temperature cycling, as measured by X-ray photoelectron spectroscopy. Figure 1C shows the behavior of the oxygen and fluorine atomic concentrations on the negative electrode surface after low-temperature cycling, as measured by X-ray photoelectron spectroscopy.

[0057] It was found that the concentration of fluorine atoms after high-temperature storage shown in Fig. 1A and room-temperature cycle shown in Fig. 1B was higher than the concentration of fluorine atoms after low-temperature cycle shown in Fig. 1C. Since compounds containing fluorine atoms are thermally stable, it was found that fluorine-containing substances can be present on the surface of the negative electrode after high-temperature storage / room-temperature cycle, which allows a heat-resistant coating derived from the fluorine-containing substances to exist on the surface of the negative electrode.

[0058] Example 2 ■ Heating tests for secondary batteries After performing cycle conditions 1 and 2, the secondary battery was placed in an oven and the temperature was measured with a thermocouple. Starting from room temperature, the battery was heated at a temperature increase rate of about 2.5°C / min. Figure 2A shows the behavior of the battery temperature versus heating time. Figure 2B also shows the behavior of the heat generation rate (dT / dt) versus heating temperature during this heating test.

[0059] 2A and 2B, it was found that the heat generation start time and heat generation start temperature can be shifted later after high temperature storage is performed after the low temperature cycle compared to after the low temperature cycle (without high temperature storage). Furthermore, it was found that the cell temperature that generated heat can be reduced to the state before charging and discharging, and the heat generation rate in the temperature range for overload testing (approximately 100°C to 150°C) can be reduced.

[0060] Therefore, it was found that it is possible to restore the thermal stability of the battery, which has decreased after the low-temperature cycle, to the state before the decrease in thermal stability or to a state close to that state.

[0061] Example 3 Li that may occur in the battery during low-temperature cycling 2 CO 3 and LiPF as the solute in the electrolyte. 6 The behavior of the heat generation when Li and Li coexist is 2 CO 3 There is no LiPF as a solute in the electrolyte. 6 The behavior of the calorific value was compared when the fluorine-containing material LiF coexists with the fluorine-containing material LiF.

[0062] The behavior of the calorific value was measured using a differential scanning calorimeter (Hitachi High-Tech Science Corporation) under the following conditions. The results are shown in FIG. Temperature range: 20~380℃ Heating rate: 10℃ / min Atmosphere: Sealed in a dry air environment using a SUS airtight container

[0063] As shown in Figure 3, Li 2 CO 3 and LiPF 6 In the presence of Li, an exothermic reaction occurs at about 100°C to about 180°C. 2 CO 3 and LiPF 6 This provides evidence that there is a concern that the thermal stability of a battery may decrease when using a battery in which these two materials coexist.

[0064] The present invention may take the following forms.

[0065] <1> A positive electrode, a negative electrode, and an electrolyte, In the secondary battery state, a low-temperature cycle is performed, followed by high-temperature storage or room temperature cycle. a secondary battery in which the concentration of fluorine atoms is higher than the concentration of oxygen atoms on the surface of the negative electrode after the high-temperature storage or room-temperature cycle as determined by X-ray photoelectron spectroscopy compared to after the low-temperature cycle. <2> the concentration of fluorine atoms is higher at least on the outermost surface of the negative electrode after the high-temperature storage or room-temperature cycle than the concentration of fluorine atoms after the low-temperature cycle; <1> The secondary battery according to claim 1. <3> a ratio of the concentration of fluorine atoms after the high-temperature storage or room-temperature cycle to the concentration of fluorine atoms after the low-temperature cycle on the outermost surface of the negative electrode is 1.5 or more; <2> The secondary battery according to claim 1. <4> The negative electrode includes a fluorine-containing material on the surface. <1> ~ <3> 13. The secondary battery according to claim 12, <5> The negative electrode is capable of absorbing and releasing lithium ions, and the fluorine-containing material is LiF, POF 3 , and C.H. 3 F, <4> The secondary battery according to claim 1. <6> The fluorine-containing material has heat resistance. <4> or <5> The secondary battery according to claim 1. <7> The electrolyte is a carbonate and LiPF 6 Including, <1> ~ <6> 13. The secondary battery according to claim 12, <8> The oxygen atoms are at least Li generated after the low-temperature cycle. 2 CO 3 The origin is, <1> ~ <7> 13. The secondary battery according to claim 12, <9> The fluorine atoms are at least LiPF 6 The origin is, <7> The secondary battery according to claim 1. [Industrial Applicability]

[0066] The secondary battery according to one embodiment of the present invention can be used in various fields where power storage is expected. By way of example only, the secondary battery according to one embodiment of the present invention, particularly the nonaqueous electrolyte secondary battery, can be used in the electrical, information, and communication fields where mobile devices are used (e.g., mobile devices such as mobile phones, smartphones, notebook computers, digital cameras, activity meters, arm computers, and electronic paper), household and small industrial applications (e.g., power tools, golf carts, household, nursing, and industrial robots), large industrial applications (e.g., forklifts, elevators, and port cranes), transportation systems (e.g., hybrid cars, electric cars, buses, trains, electrically assisted bicycles, and electric motorcycles), power system applications (e.g., various power generation, road conditioners, smart grids, and general household installation-type power storage systems), medical applications (medical devices such as earphones and hearing aids), medical applications (medical applications such as medication management systems), IoT, and space and deep sea applications (e.g., space probes, submersible research vessels, and the like). [Explanation of symbols]

[0067] 10A positive electrode 10B negative electrode 11A positive electrode current collector 11B Negative electrode current collector 12A Cathode material layer 12B Negative electrode material layer

Claims

1. A positive electrode, a negative electrode, and an electrolyte, In the secondary battery state, a low-temperature cycle is performed, followed by high-temperature storage or room temperature cycle. a concentration of fluorine atoms is higher than a concentration of oxygen atoms on the surface of the negative electrode after the high-temperature storage or room-temperature cycle, as determined by X-ray photoelectron spectroscopy, compared to a concentration of fluorine atoms after the low-temperature cycle.

2. 2. The secondary battery according to claim 1, wherein at least on the outermost surface of the negative electrode, the concentration of fluorine atoms is higher after the high-temperature storage or room-temperature cycle than after the low-temperature cycle.

3. 3. The secondary battery according to claim 2, wherein, on the outermost surface of the negative electrode, a ratio of a concentration of fluorine atoms after the high-temperature storage or room-temperature cycle to a concentration of fluorine atoms after the low-temperature cycle is 1.5 or more.

4. 2. The secondary battery of claim 1, wherein the negative electrode comprises a fluorine-containing material on the surface.

5. The negative electrode is capable of absorbing and releasing lithium ions, and the fluorine-containing material is LiF, POF ３ , and C.H. ３ The secondary battery according to claim 4 , further comprising at least one selected from the group consisting of F.

6. The secondary battery according to claim 4 , wherein the fluorine-containing material has heat resistance.

7. The electrolyte is a carbonate and LiPF ６ The secondary battery according to claim 1 .

8. The oxygen atoms are at least Li generated after the low-temperature cycle. ２ CO ３ The secondary battery according to claim 1 ,

9. The fluorine atoms are at least LiPF ６ The secondary battery according to claim 7 ,

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