Electrochemical devices

Abstract

The present disclosure provides an electrochemical device having a high output power; and this electrochemical device is provided with a positive electrode, a negative electrode and a separator that is interposed between the positive electrode and the negative electrode. With respect to this electrochemical device, the negative electrode has a negative electrode mixture layer; the negative electrode mixture layer contains a negative electrode active material which is reversibly doped with lithium ions, and a binder; the negative electrode active material contains hardly-graphitizable carbon; the binder has an average particle diameter of 140 nm or more; and the potential of the negative electrode versus an Li reference electrode is +0.2 V or less in a discharged state.

Description

[Technical Field]

[0001] This invention relates to an electrochemical device. [Background technology]

[0002] In recent years, electrochemical devices that combine the energy storage principles of lithium-ion secondary batteries and electric double-layer capacitors have attracted attention. Such electrochemical devices typically use a polarizable electrode for the positive electrode and a non-polarizable electrode for the negative electrode. As a result, these electrochemical devices are expected to possess both the high energy density of lithium-ion secondary batteries and the high power output characteristics of electric double-layer capacitors.

[0003] Patent Document 1 proposes an energy storage device having a positive electrode and a negative electrode having a negative electrode active material layer containing a fluorine-containing acrylic binder, wherein the density of the negative electrode active material layer is 0.75 g / cc or more and 1.10 g / cc or less. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] International Public Gazette WO2012 / 086340 [Overview of the project]

[0005] However, further improvements are needed in the electrochemical devices described above to achieve higher power output levels.

[0006] One aspect of the present invention is a device comprising a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, wherein the negative electrode has a negative electrode mixture layer, the negative electrode mixture layer comprises a negative electrode active material in which lithium ions are reversibly doped, and a binder, the negative electrode active material comprises non-graphitizable carbon, the average particle size of the binder is 140 nm or more, and in the discharge state, the potential of the negative electrode is equal to that of the Li counter electrode. electric potential This concerns electrochemical devices that are less than +0.2V relative to the given voltage.

[0007] According to the present invention, high-power electrochemical devices can be realized. [Brief explanation of the drawing]

[0008] [Figure 1] Figure 1 is a longitudinal cross-sectional view of an electrochemical device according to one embodiment of the present invention. [Modes for carrying out the invention]

[0009] An electrochemical device according to an embodiment of the present invention comprises a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode. The positive electrode, negative electrode, and separator can be formed, for example, by winding strip-shaped positive and negative electrodes with a separator in between to form a columnar wound body (electrode body). The electrode body may be constructed as a laminate by stacking plate-shaped positive and negative electrodes with a separator in between.

[0010] The negative electrode has a negative electrode mixture layer. The negative electrode mixture layer is supported, for example, on a negative electrode current collector. The negative electrode mixture layer is a layer of a mixture containing at least a negative electrode active material in which lithium ions are reversibly doped, and a binder. The negative electrode active material contains non-graphitizable carbon. The average particle size of the binder is 140 nm or larger.

[0011] At the negative electrode, the Faraday reaction proceeds, in which lithium ions are reversibly intercalated and released, thereby generating capacity. Doping the negative electrode active material with lithium ions is a concept that includes at least the phenomenon of intercalation of lithium ions into the negative electrode active material, and may also include the adsorption of lithium ions into the negative electrode active material and chemical interactions between the negative electrode active material and lithium ions.

[0012] The potential of the negative electrode increases with discharge. In a typical lithium-ion secondary battery, in a fully discharged state, the potential of the negative electrode is equal to that of the Li counter electrode. electric potential For example, it is +1V and exceeds +0.2V. In contrast, in the electrochemical device of this embodiment, even in the discharge state, the potential of the negative electrode is the same as the Li counter electrode. electric potentialThe voltage is less than +0.2V. This means that sufficient lithium is doped into the negative electrode even in the discharged state. Such a negative electrode can be obtained by pre-doping lithium ions into the negative electrode before charging the device after manufacturing. High capacity can be achieved by using a pre-doped negative electrode.

[0013] Here, the discharge state refers to a state where, under conditions of 25°C, the voltage reaches a voltage corresponding to a state of charge of 95% or higher (e.g., 3.8V), with a current flow rate of 2mA / cm² per positive electrode area. 2 An electrochemical device, charged with a constant current at a current density of 2 mA / cm² per positive electrode area, is discharged at 25°C until the voltage reaches a voltage corresponding to a depth of discharge (DOD) of 95% or more (e.g., 2.2V). 2 This refers to a state of constant current discharge at a given current density. The potential of the negative electrode can be determined by disassembling the device after charging and discharging under the above conditions, assembling a half-cell with the extracted negative electrode as the working electrode and the Li metal foil as the counter electrode, and measuring the potential of the negative electrode relative to the Li metal foil.

[0014] Unlike lithium-ion secondary batteries, electrochemical devices with lithium-ion pre-doped negative electrodes are capable of rapid charging and discharging, and have the advantage of high output. To enable rapid charging and discharging and obtain high output, non-graphitizable carbon (also called hard carbon) is preferably used as the negative electrode active material for electrochemical devices. By using non-graphitizable carbon, high cycle characteristics can be obtained even under charge and discharge conditions in which rapid charging and rapid discharging are repeated. Furthermore, by using non-graphitizable carbon, the internal resistance (DCR) is low even at low temperatures, enabling the realization of high-output electrochemical devices.

[0015] In addition, by setting the average particle diameter of the binder to 140 nm or more, when the content ratio of the binder in the negative electrode mixture layer is the same, the number of binder particles is reduced compared to the case of using a binder with a smaller particle diameter, and the area of the surface of the negative electrode active material covered by the binder, which is an insulator, is reduced. As a result, the internal resistance of the electrochemical device can be further reduced, and higher output can be achieved. On the other hand, from the viewpoint of maintaining the binding property between negative electrode active materials, the average particle diameter of the binder is, for example, 300 nm or less, and preferably 200 nm or less.

[0016] The average particle diameter of the binder may be 140 nm or more and 300 nm or less (alternatively, 180 nm or more and 300 nm or less), or may be 140 nm or more and 200 nm or less (alternatively, 180 nm or more and 200 nm or less).

[0017] The average particle diameter of the binder is obtained by taking out the negative electrode from the electrochemical device, drying the negative electrode at 25°C for 1 hour in an air atmosphere, and then measuring the surface of the obtained negative electrode mixture layer Scanning electron microscope ( SEM ) It is obtained by image analysis of the micrograph. The magnification in SEM is set to 30000 to 50000 times. Grain boundaries of the binder particles are determined from the SEM photograph, and the diameter of a circle having the same area as the occupied area of the particle specified from the grain boundaries is defined as the particle diameter of the particle. 30 or more binder particles are arbitrarily selected, their respective particle diameters are determined, and the average value thereof is taken as the average particle diameter of the binder.

[0018] The binder may contain an acrylic resin. This can suppress a decrease in the float characteristics of the electrochemical device.

[0019] The float characteristic is an index of the degree of deterioration of an electrochemical device when float charging for maintaining a constant voltage using an external DC power supply is performed. It can be said that the smaller the capacity decrease during float charging and the smaller the increase in internal resistance, the better the float characteristics.

[0020] Electrochemical devices are prone to a decrease in float characteristics. This is because the crystalline structure of non-graphitizable carbon is disordered, making it easy for lithium ions adsorbed by pre-doping to escape from the negative electrode active material. The escape of lithium ions increases the resistance of the negative electrode itself. In addition, compared to graphite, which exhibits a generally flat potential change in response to changes in the amount of accumulated charge, non-graphitizable carbon exhibits a large potential change in response to changes in the amount of accumulated charge. Therefore, under float charging conditions, where a constant voltage is continuously applied between the positive and negative electrodes, the potential of the negative electrode increases as lithium ions escape from the non-graphitizable carbon, and consequently, the potential of the positive electrode also increases. As a result, the positive electrode is more prone to degradation under float charging conditions, and its internal resistance tends to increase.

[0021] However, since the acrylic resin used as a binder has a tendency to swell in the electrolyte, it is thought that more electrolyte (lithium ions) is present on the surface of the negative electrode active material compared to when other binders are used, thus suppressing the desorption of lithium ions.

[0022] Acrylic resin refers to a polymer or copolymer containing units of acrylic monomers. As the acrylic monomer, at least one selected from the group consisting of acrylic acid, methacrylic acid, alkyl acrylate, and alkyl methacrylate may be used. Acrylic acid esters and methacrylic acid esters are collectively also called (meth)acrylic acid esters. Examples of (meth)acrylic acid alkyl esters include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, i-propyl (meth)acrylate, n-butyl (meth)acrylate, i-butyl (meth)acrylate, n-amyl (meth)acrylate, i-amyl (meth)acrylate, n-hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-octyl (meth)acrylate, n-nonyl (meth)acrylate, n-decyl (meth)acrylate, cyclohexyl (meth)acrylate, and cyclopentyl (meth)acrylate. Of the monomers constituting the acrylic resin, 50 mol% or more, or 80 mol% or more, may be the above-mentioned acrylic monomers. The acrylic monomers may also contain fluorine (F).

[0023] The negative electrode mixture layer may contain a thickening agent. The thickening agent may include carboxymethylcellulose (CMC). The carboxymethylcellulose may be a sodium salt, but an ammonium salt is more preferred.

[0024] If the thickening agent contains alkali metal ions (excluding lithium), such as sodium ions, it may inhibit the movement of lithium ions, which are also alkali metal ions, during the charging and discharging of electrochemical devices. Therefore, by using ammonium salt instead of sodium salt as carboxymethylcellulose, the movement of lithium ions is not inhibited, and the internal resistance can be reduced. Thus, high-power electrochemical devices can be easily realized. In addition, the increase in internal resistance after float charging is suppressed, and the float characteristics are also improved.

[0025] In particular, when the average particle size of the binder is set to a large particle size of 140 nm or more, as described above, the area covered by the insulating binder on the surface of the negative electrode active material decreases, and the area exposed to the negative electrode active material increases. Consequently, the movement of lithium ions by sodium ions is also more easily inhibited. However, by using ammonium salt of carboxymethylcellulose as a thickening agent, it is possible to realize an electrochemical device with low internal resistance, high output, and excellent float characteristics, even when using a large particle size binder.

[0026] The presence of ammonium carboxymethylcellulose (CMC) as a thickening agent in the negative electrode mixture layer can be detected by the following method: The negative electrode, obtained by disassembling an electrochemical device, is washed with propylene carbonate (PC). 10 g of the negative electrode active material layer is peeled off from the washed negative electrode, placed in 100 mL of water and boiled, then filtered to remove the active material and obtain a CMC solution. The water is evaporated, and the concentrated CMC solution is subjected to ion chromatography or 15 By performing N-NMR, ammonium ions (NH4) + ) detects.

[0027] The amount of binder contained in the negative electrode active material layer may be, for example, 0.5% by mass or more and 5% by mass or less of the total negative electrode active material layer. The amount of carboxymethylcellulose contained in the negative electrode active material layer may be 1.5% by mass or more and 7% by mass or less of the total negative electrode active material layer.

[0028] The surface layer of the negative electrode mixture layer may have a first layer (lithium carbonate-containing layer) containing lithium carbonate. The first layer is mainly formed on the surface of the negative electrode active material. By forming a first layer containing lithium carbonate, the degradation of the negative electrode is significantly suppressed.

[0029] The surface layer of the negative electrode mixture layer may further have a second layer (solid electrolyte layer) containing a solid electrolyte layer. The second layer may be formed to cover at least a portion of the surface of the first layer. The second layer has a different composition from the first layer and is distinguishable from the first layer. In electrochemical devices utilizing lithium ions, a solid electrolyte interface film (i.e., SEI film) is formed on the negative electrode mixture layer during charging and discharging. The second layer may be formed as an SEI film. The SEI film plays an important role in the charge-discharge reaction, but if the SEI film is formed to be excessively thick, the deterioration of the negative electrode will increase. In contrast, the first layer containing lithium carbonate promotes the formation of a good SEI film and helps maintain the SEI film in a good state when charging and discharging are repeated. That is, the first layer is interposed between the surface of the negative electrode active material and the second layer, becoming the underlayer of the second layer, so that the second layer is formed as a good SEI film. Therefore, by forming the first layer on the surface of the anode mixture layer, even when the specific surface area of ​​the anode mixture layer is increased to suppress the increase in low-temperature DCR, the degradation of the anode can be significantly suppressed.

[0030] The second layer may also contain lithium carbonate. If the second layer contains lithium carbonate, the amount of lithium carbonate in the second layer is less than the amount of lithium carbonate in the first layer. Using the first layer, which contains a large amount of lithium carbonate, as the base layer is a necessary condition for the second layer to form a good SEI coating.

[0031] The first layer is formed on the surface of the negative electrode mixture layer before assembling the electrochemical device. In the electrochemical device assembled using this negative electrode, a second layer (SEI coating) of a uniform and appropriate thickness is formed on the surface of the negative electrode active material through subsequent charging and discharging. The SEI coating is formed, for example, by the reaction of the electrolyte and the negative electrode within the electrochemical device. Since the electrolyte can pass through both the second and first layers, the entire surface area including both the first and second layers may be referred to as the SEI coating; however, for convenience, in this specification, only the second layer is referred to as the SEI coating to distinguish it from the first layer.

[0032] The presence of regions containing lithium carbonate, such as the first layer, can be confirmed, for example, by analyzing the surface layer using X-ray photoelectron spectroscopy (XPS). However, the analytical method is not limited to XPS.

[0033] The thickness of the first layer can be, for example, 1 nm or more. If a longer-term effect is expected, it can be 5 nm or more, and if a more reliable effect is expected, it may be 10 nm or more. However, if the thickness of the first layer exceeds 50 nm, the first layer itself may become a resistive component. Therefore, the thickness of the first layer may be 50 nm or less, or 30 nm or less.

[0034] The thickness of the second layer can be, for example, 1 nm or more, 3 nm or more, or 5 nm or more. However, if the thickness of the second layer exceeds 20 nm, the second layer itself may become a resistive component. Therefore, the thickness of the second layer may be 20 nm or less, or 10 nm or less.

[0035] The ratio of the thickness A of the first layer to the thickness B of the second layer, A / B, is preferably 1 or less from the viewpoint of reducing the initial low-temperature DCR. In this case, the thickness of the second layer is preferably 20 nm or less, and may be 10 nm or less. However, from the viewpoint of forming a second layer of good quality, it is desirable that A / B be 0.1 or more, and for example, the A / B ratio may be 0.2 or more.

[0036] The thicknesses of the first and second layers are measured by analyzing the surface layer of the negative electrode mixture layer at multiple locations (at least five locations). The average of the thicknesses of the first or second layer obtained at multiple locations can then be used as the thickness of the first or second layer. The negative electrode mixture layer used as the measurement sample may be peeled off from the negative electrode current collector. In this case, the coating formed on the surface of the negative electrode active material that constituted the vicinity of the surface layer of the negative electrode mixture layer should be analyzed. Specifically, the negative electrode active material covered with the coating should be collected from the region of the negative electrode mixture layer located on the side opposite to the side that was in contact with the negative electrode current collector and used for analysis.

[0037] XPS analysis of the surface layer of the negative electrode mixture layer is performed, for example, by irradiating the surface layer or a coating formed on the surface of the negative electrode active material with an argon beam in the chamber of an X-ray photoelectron spectrometer, and observing and recording the changes in the spectra attributed to C1s, O1s electrons, etc., with respect to irradiation time. At this time, from the viewpoint of avoiding analytical errors, the spectrum of the outermost surface of the surface layer may be ignored. The thickness of the region in which the peak attributed to lithium carbonate is stably observed corresponds to the thickness of the first layer.

[0038] In the case of a negative electrode removed from an electrochemical device that has been completed and undergone a predetermined aging process or at least one charge-discharge cycle, the surface layer of the negative electrode mixture layer has a SEI coating (i.e., a second layer) containing a solid electrolyte. The thickness of the region where peaks attributable to the bonds of the compounds contained in the SEI coating are stably observed corresponds to the thickness of the SEI coating (i.e., the thickness of the second layer).

[0039] The compounds included in the SEI coating should be compounds containing elements that can serve as labels for the second layer. These elements could be, for example, elements found in the electrolyte but substantially absent in the first layer (e.g., F). A compound containing these elements could be, for example, LiF.

[0040] When the second layer contains LiF, X-ray photoelectron spectroscopy of the second layer reveals a substantial F1s peak attributed to the LiF bond. In this case, the thickness of the region where the peak attributed to the LiF bond is stably observed corresponds to the thickness of the second layer.

[0041] On the other hand, the first layer typically does not contain LiF, and even when the first layer is measured by X-ray photoelectron spectroscopy, no substantial F1s peak attributed to LiF bonding is observed. Therefore, the thickness of the region where peaks attributed to LiF bonding are not stably observed may be used as the thickness of the first layer.

[0042] O1s peaks attributed to lithium carbonate may also be observed in SEI coatings. However, SEI coatings formed within electrochemical devices have a different composition from the pre-formed first layer, making them distinguishable. For example, XPS analysis of SEI coatings shows F1s peaks attributed to LiF bonds, while virtually no F1s peaks attributed to LiF bonds are observed in the first layer. Furthermore, the amount of lithium carbonate contained in SEI coatings is trace. Note that Li1s peaks originating from compounds such as ROCO2Li and ROLi may also be detected.

[0043] When the first layer is analyzed by XPS, in addition to the first peak of O1s attributed to the C=O bond, a second peak of O1s attributed to the Li-O bond may also be observed. The region of the coating near the surface of the negative electrode active material may contain small amounts of LiOH or Li2O.

[0044] Specifically, when analyzing the first layer constituting the surface of the negative electrode mixture layer in the depth direction, a first region may be observed where a first peak (O1s attributed to the C=O bond) and a second peak (O1s attributed to the Li-O bond) are observed, and the intensity of the first peak is greater than that of the second peak, in order of increasing distance from the outermost surface of the surface layer. A second region may also be observed where both the first and second peaks are observed, and the intensity of the second peak is greater than that of the first peak. Furthermore, a third region may exist where the distance from the outermost surface of the surface layer is closer than that of the first region, and where the first peak is observed but the second peak is not. The third region is more likely to be observed when the thickness of the lithium carbonate-containing region is large. The magnitude of the peak intensity can be determined by the height of the peak from the baseline.

[0045] In the center of the first layer in the thickness direction, the C1s peak, which is usually attributed to C-C bonds, is either virtually absent or, if observed, is less than half the intensity of the peak attributed to C=O bonds.

[0046] Next, a method for forming a first layer containing lithium carbonate on the surface of the negative electrode mixture layer will be described. The process of forming the first layer can be carried out by, for example, a gas phase method, a coating method, a transfer method, etc.

[0047] Examples of vapor phase methods include chemical vapor deposition, physical vapor deposition, and sputtering. For example, lithium carbonate can be deposited onto the surface of the negative electrode mixture layer using a vacuum deposition apparatus. The pressure inside the apparatus chamber during deposition is, for example, 10 -2 ~10 -5 The temperature can be set to Pa, the lithium carbonate evaporation source temperature can be 400-600°C, and the negative electrode mixture layer temperature can be -20-80°C.

[0048] As a coating method, a solution or dispersion containing lithium carbonate can be applied to the surface of the negative electrode, for example, using a microgravure coater, and then dried to form the first layer. The lithium carbonate content in the solution or dispersion is, for example, 0.3 to 2% by mass, and if a solution is used, the concentration should be below the solubility (for example, about 0.9 to 1.3% by mass for an aqueous solution at room temperature).

[0049] Furthermore, a negative electrode can be obtained by performing a step of forming a second layer containing a solid electrolyte so as to cover at least a portion of the first layer. The surface layer of the obtained negative electrode mixture layer has a first layer and a second layer. The second layer is formed such that at least a portion of it covers at least a portion (preferably the whole) of the surface of the negative electrode active material via the first layer (i.e., with the first layer as the base layer).

[0050] The process of forming the second layer is carried out with the negative electrode mixture layer and the electrolyte in contact, and therefore may also serve as at least part of the lithium ion pre-doping process for the negative electrode mixture layer. For example, metallic lithium may be used as the lithium ion source for pre-doping.

[0051] Lithium metal may be attached to the surface of the negative electrode mixture layer. Furthermore, by exposing a negative electrode having a negative electrode mixture layer with lithium metal attached thereto to a carbon dioxide gas atmosphere, for example, a first layer containing lithium carbonate having a thickness of 1 nm or more and 50 nm or less can also be formed.

[0052] The step of attaching lithium metal to the surface of the negative electrode mixture layer can be performed by, for example, a vapor phase method, transfer, or the like. Examples of the vapor phase method include methods such as chemical vapor deposition, physical vapor deposition, and sputtering. For example, lithium metal may be formed into a film on the surface of the negative electrode mixture layer using a vacuum deposition apparatus. The pressure inside the apparatus chamber during deposition is, for example, 10 -2 ~10 -5 Pa, the temperature of the lithium evaporation source may be 400 to 600°C, and the temperature of the negative electrode mixture layer may be -20 to 80°C.

[0053] The carbon dioxide gas atmosphere is desirably a dry atmosphere that does not contain moisture, and for example, may have a dew point of -40°C or lower or -50°C or lower. The carbon dioxide gas atmosphere may contain gases other than carbon dioxide, but it is desirable that the mole fraction of carbon dioxide is 80% or more, and more desirably 95% or more. It is desirable that no oxidizing gas is contained, and the mole fraction of oxygen may be 0.1% or less.

[0054] To form a thicker first layer, it is efficient to set the partial pressure of carbon dioxide to be higher than, for example, 0.5 atm (5.05×10 4 Pa), and it may be 1 atm (1.01×10 5 Pa) or higher.

[0055] The temperature of the negative electrode exposed to the carbon dioxide gas atmosphere may be, for example, in the range of 15°C to 120°C. The higher the temperature, the thicker the first layer becomes.

[0056] By changing the time for which the negative electrode is exposed to the carbon dioxide gas atmosphere, the thickness of the first layer can be easily controlled. The exposure time may be, for example, 12 hours or more and less than 10 days.

[0057] While it is preferable to perform the step of forming the first layer before constructing the electrode body, this does not preclude performing it afterward. That is, a positive electrode may be prepared, a negative electrode having a negative electrode mixture layer with metallic lithium attached may be prepared, an electrode body may be formed by interposing a separator between the positive and negative electrodes, and the electrode body may be exposed to a carbon dioxide atmosphere to form the first layer on the surface of the negative electrode mixture layer.

[0058] The lithium ion pre-doping process for the negative electrode mixture layer can be further advanced by, for example, bringing the negative electrode mixture layer into contact with the electrolyte and then leaving it for a predetermined time to complete the process. Such a process may involve forming a second layer so as to cover at least a portion of the first layer. For example, the second layer can be formed in the negative electrode mixture layer and the pre-doping of lithium ions into the negative electrode can be completed by performing at least one charge-discharge cycle on the electrochemical device. Alternatively, the pre-doping of lithium ions into the negative electrode can also be completed by applying a predetermined charging voltage (e.g., 3.4-4.0V) between the terminals of the positive and negative electrodes for a predetermined time (e.g., 1-75 hours).

[0059] Pre-doping of the negative electrode with lithium ions may be performed by bringing the negative electrode into contact with a lithium-ion conductive electrolyte before assembling the device. For example, metallic lithium may be used as the lithium ion source for pre-doping. For example, the working electrode to which the negative electrode and lithium ion source are attached (e.g., stainless steel( SUS ) Pre-doping can be performed by placing a metal plate (made of lithium-ion material) into a battery case filled with a lithium-ion conductive electrolyte, with a separator interposed between the negative electrode and the working electrode, and applying a voltage between the positive and negative electrodes, with the working electrode acting as the positive electrode. The voltage can be applied, for example, under conditions where a predetermined constant current flows between the positive and negative electrodes. The voltage application time is, for example, 1 to 75 hours.

[0060] The lithium metal, used as a lithium ion source, may be pre-adhered to the surface of the negative electrode mixture layer, the negative electrode with the attached lithium metal may be placed in the electrolytic cell, and a voltage may be applied between the negative electrode and the working electrode to perform pre-doping.

[0061] Hereinafter, the positive electrode and negative electrode may be collectively referred to as electrodes. The positive electrode current collector and negative electrode current collector may be collectively referred to as current collectors (or electrode current collectors). The positive electrode mixture layer and negative electrode mixture layer may be collectively referred to as mixture layers (or electrode mixture layers). The positive electrode active material and negative electrode active material may be collectively referred to as active material (or electrode active material).

[0062] Figure 1 schematically shows the configuration of an electrochemical device 200 according to one embodiment of the present invention. The electrochemical device 200 comprises an electrode body 100, a non-aqueous electrolyte (not shown), a metal cell case 210 with a closed bottom that houses the electrode body 100 and the non-aqueous electrolyte, and a sealing plate 220 that seals the opening of the cell case 210. A gasket 221 is placed on the periphery of the sealing plate 220, and the inside of the cell case 210 is sealed by crimping the open end of the cell case 210 to the gasket 221. A positive electrode current collector plate 13 having a through hole 13h in the center is welded to the exposed portion 11x of the positive electrode current collector. The other end of a tab lead 15, one end of which is connected to the positive electrode current collector plate 13, is connected to the inner surface of the sealing plate 220. Therefore, the sealing plate 220 functions as an external positive electrode terminal. On the other hand, a negative electrode current collector plate 23 is welded to the exposed portion 21x of the negative electrode current collector. The negative electrode current collector plate 23 is directly welded to a welding member provided on the inner bottom surface of the cell case 210. Therefore, the cell case 210 functions as an external negative electrode terminal.

[0063] The components of the electrochemical device according to the embodiment of the present invention will be described in more detail below.

[0064] (Negative electrode) The negative electrode comprises a negative electrode current collector and a negative electrode mixture layer supported on the negative electrode current collector. The negative electrode mixture layer contains a negative electrode active material that reversibly dops lithium ions, and the negative electrode active material contains non-graphitizable carbon (i.e., hard carbon). The thickness of the negative electrode mixture layer is, for example, 10 to 300 μm per side of the negative electrode current collector. The thickness of the negative electrode mixture layer may be 25 μm or more per side of the negative electrode current collector.

[0065] A sheet-like metal material is used for the negative electrode current collector. Examples of sheet-like metal materials include metal foil. Suitable metal materials include copper, copper alloys, nickel, and stainless steel. The aperture ratio of the negative electrode current collector may be 1% or less.

[0066] The negative electrode current collector plate is generally a disc-shaped metal plate. The material of the negative electrode current collector plate may be, for example, copper, copper alloy, nickel, or stainless steel. The material of the negative electrode current collector plate may be the same as the material of the negative electrode current collector body.

[0067] The non-graphitizable carbon may have a (002) plane interplanar spacing (i.e., interplanar spacing between carbon layers) d002 measured by X-ray diffraction of 3.8 Å or more. The theoretical capacity of the non-graphitizable carbon is preferably, for example, 150 mAh / g or more. Using non-graphitizable carbon makes it easier to obtain a negative electrode with a low low-temperature DCR and small expansion and contraction during charging and discharging. It is desirable that the non-graphitizable carbon accounts for 50% by mass or more, more preferably 80% by mass or more, and more preferably 95% by mass or more of the negative electrode active material. Furthermore, it is desirable that the non-graphitizable carbon accounts for 40% by mass or more, more preferably 70% by mass or more, and more preferably 90% by mass or more of the negative electrode mixture layer.

[0068] As the negative electrode active material, a combination of non-graphitizable carbon and materials other than non-graphitizable carbon may be used. Examples of materials other than non-graphitizable carbon that can be used as the negative electrode active material include easily graphitizable carbon (soft carbon), graphite (natural graphite, artificial graphite, etc.), lithium titanium oxide (spinel-type lithium titanium oxide, etc.), silicon oxide, silicon alloy, tin oxide, and tin alloy.

[0069] From the viewpoint of ensuring high packing efficiency of the negative electrode active material and easily suppressing side reactions with the electrolyte, the average particle size of the negative electrode active material (especially non-graphitizable carbon) is preferably 1 μm to 20 μm, and more preferably 2 μm to 15 μm.

[0070] In this specification, the average particle size refers to the median diameter (D) on a volume basis in the particle size distribution obtained by laser diffraction particle size distribution measurement. 50 ) means.

[0071] The negative electrode mixture layer contains a negative electrode active material and a binder as essential components, and optionally contains conductive additives, thickeners, etc. The average particle size of the binder is 140 nm or larger. Examples of conductive additives include carbon black and carbon fibers. It is preferable that the conductive additive contains carbon black. Examples of binders include fluororesins, acrylic resins, rubber materials, and cellulose derivatives.

[0072] The negative electrode mixture layer is formed, for example, by mixing a negative electrode active material with a conductive agent, a binder, and a thickener together with a dispersion medium to prepare a negative electrode mixture slurry, applying the negative electrode mixture slurry to a negative electrode current collector, and then drying it.

[0073] The negative electrode mixture layer is pre-doped with lithium ions. This lowers the potential of the negative electrode, increasing the potential difference (i.e., voltage) between the positive and negative electrodes, and improving the energy density of the electrochemical device. The amount of lithium pre-doped should be, for example, about 50% to 95% of the maximum amount that can be absorbed into the negative electrode mixture layer.

[0074] The capacitance per unit mass of the negative electrode active material should be, for example, 1000 F / g or more. Furthermore, from the viewpoint of increasing the capacitance density of the electrochemical device, the capacitance per unit mass of the negative electrode active material should be, for example, 30000 F / g or less. The capacitance per unit mass of the negative electrode active material is usually larger than that of the positive electrode active material, for example, 20 to 800 times greater. The capacitance per unit mass of the negative electrode active material can be measured by the following method.

[0075] First, prepare an evaluation negative electrode cut to a size of 31 mm x 41 mm. Prepare a 100 μm thick metallic lithium foil cut to a size of 40 mm x 50 mm as the counter electrode to the negative electrode. Use a 25 μm thick cellulose paper manufactured by Nippon Kodo Paper Industry Co., Ltd. (for example, product number TF4425) as a separator to place the negative electrode mixture layer and the metallic lithium foil opposite each other to form an electrode body, and immerse the electrode body in the electrolyte of Example 1 described later to assemble a cell.

[0076] The cell is charged with a constant current (CC) of 0.5 mA until the cell voltage reaches 0.01 V, then charged with a constant voltage (CV) for 1 hour, and then discharged with 0.5 mA until the cell voltage reaches 1.5 V. The capacitance per unit mass of the negative electrode active material is determined from the discharge time during which the potential changes by 0.1 V from the potential of the negative electrode 1 minute after the start of discharge.

[0077] (positive electrode) The positive electrode comprises a positive electrode current collector and a positive electrode mixture layer supported on the positive electrode current collector, the positive electrode mixture layer containing a positive electrode active material to which anions are reversibly doped. When anions are adsorbed onto the positive electrode active material, an electric double layer is formed, and capacitance is exhibited. The positive electrode may be a polarizable electrode, or it may be an electrode that has the properties of a polarizable electrode while also having a Faraday reaction that contributes to its capacitance. Examples of positive electrode active materials include carbon materials and conductive polymers. Doping of the positive electrode active material with anions includes at least the phenomenon of anion adsorption onto the positive electrode active material, and may also include anion intercalation by the positive electrode active material and chemical interactions between the positive electrode active material and anions.

[0078] The thickness of the positive electrode mixture layer is, for example, 10 to 300 μm per side of the positive electrode current collector.

[0079] A sheet-like metal material is used for the positive electrode current collector. This sheet-like metal material can be metal foil, porous metal, etched metal, etc. Suitable metal materials include aluminum, aluminum alloys, nickel, and titanium. Similar to the negative electrode current collector, a sheet material that substantially lacks through-holes is preferred for the positive electrode current collector.

[0080] The positive electrode current collector plate is generally a disc-shaped metal plate. Preferably, a through-hole is formed in the center of the positive electrode current collector plate to serve as a passage for the non-aqueous electrolyte. The material of the positive electrode current collector plate may be, for example, aluminum, aluminum alloy, titanium, or stainless steel. The material of the positive electrode current collector plate may be the same as the material of the positive electrode current collector body.

[0081] As the carbon material used as the positive electrode active material, porous carbon materials are preferred, such as activated carbon and the carbon materials exemplified as negative electrode active materials (e.g., non-graphitizable carbon). Examples of raw materials for activated carbon include wood, coconut shells, coal, pitch, and phenolic resin. It is preferable that the activated carbon has been activated.

[0082] The average particle size of the activated carbon is not particularly limited, but is preferably 20 μm or less, and more preferably 3 μm to 15 μm.

[0083] The specific surface area of ​​the positive electrode mixture layer generally reflects the specific surface area of ​​the positive electrode active material. For example, the specific surface area of ​​the positive electrode mixture layer is 600 m². 2 / g or more, 4000m 2 It is sufficient if it is less than / g, 800m 2 / g or more, 3000m 2A value of less than / g is desirable. The specific surface area of ​​the positive electrode mixture layer is the BET specific surface area determined using a measuring device compliant with JIS Z8830 (for example, TriStar II3020 manufactured by Shimadzu Corporation). Specifically, the electrochemical device is disassembled and the positive electrode is removed. Next, the positive electrode is cleaned with DMC and dried. After that, the positive electrode mixture layer is peeled off from the positive electrode current collector, and a sample of about 0.5g of the positive electrode mixture layer is taken. Next, the specific surface area of ​​the taken sample is determined in accordance with the measurement method for the specific surface area of ​​the negative electrode mixture layer described above.

[0084] It is desirable that activated carbon constitute 50% or more by mass of the positive electrode active material, more preferably 80% or more by mass, and more preferably 95% or more by mass. Furthermore, it is desirable that activated carbon constitute 40% or more by mass of the positive electrode mixture layer, more preferably 70% or more by mass, and more preferably 90% or more by mass.

[0085] The positive electrode mixture layer contains a positive electrode active material as an essential component, and optional components such as conductive additives and binders. Examples of conductive additives include carbon black and carbon fibers. Examples of binders include fluororesins, acrylic resins, rubber materials, and cellulose derivatives.

[0086] The positive electrode mixture layer is formed, for example, by mixing a positive electrode active material with a conductive agent and a binder, etc., together with a dispersion medium to prepare a positive electrode mixture slurry, applying the positive electrode mixture slurry to a positive electrode current collector, and then drying it.

[0087] As the conductive polymer used as the positive electrode active material, π-conjugated polymers are preferred. Examples of π-conjugated polymers include polypyrrole, polythiophene, polyfuran, polyaniline, polythiophene vinylene, polypyridine, or derivatives thereof. These may be used individually or in combination of two or more. The weight-average molecular weight of the conductive polymer is, for example, 1,000 to 1,000,000. A derivative of a π-conjugated polymer refers to a polymer whose basic skeleton is a π-conjugated polymer such as polypyrrole, polythiophene, polyfuran, polyaniline, polythiophene vinylene, or polypyridine. For example, polythiophene derivatives include poly(3,4-ethylenedioxythiophene) (PEDOT).

[0088] Conductive polymers are formed, for example, by immersing a positive electrode current collector having a carbon layer in a reaction solution containing raw material monomers for the conductive polymer, and electrolytically polymerizing the raw material monomers in the presence of the positive electrode current collector. In electrolytic polymerization, the positive electrode current collector and the counter electrode are immersed in a reaction solution containing raw material monomers, and a current is passed between them with the positive electrode current collector as the anode. Conductive polymers may also be formed by methods other than electrolytic polymerization. For example, conductive polymers may be formed by chemical polymerization of raw material monomers. In chemical polymerization, the raw material monomers are polymerized with an oxidizing agent or the like in the presence of the positive electrode current collector.

[0089] The raw material monomers used in electropolymerization or chemical polymerization can be polymerizable compounds capable of producing conductive polymers through polymerization. The raw material monomers may also include oligomers. Examples of raw material monomers include aniline, pyrrole, thiophene, furan, thiophenevinylene, pyridine, or derivatives thereof. These may be used individually or in combination of two or more. Among these, aniline is particularly well-suited for growth on the surface of carbon layers through electropolymerization.

[0090] Electrolytic polymerization or chemical polymerization can be carried out using a reaction solution containing anions (dopants). Excellent conductivity can be achieved by doping π-electron conjugated polymers with dopants. Examples of dopants include sulfate ions, nitrate ions, phosphate ions, borate ions, benzenesulfonate ions, naphthalenesulfonate ions, toluenesulfonate ions, methanesulfonate ions, perchlorate ions, tetrafluoroborate ions, hexafluorophosphate ions, and fluorosulfate ions. Dopants may also be polymer ions. Examples of polymer ions include polyvinylsulfonic acid, polystyrenesulfonic acid, polyallylsulfonic acid, polyacrylic sulfonic acid, polymethacrylatesulfonic acid, poly(2-acrylamido-2-methylpropanesulfonic acid), polyisoprenesulfonic acid, and polyacrylic acid.

[0091] (Separator) As the separator, nonwoven fabrics made of cellulose fibers, nonwoven fabrics made of glass fibers, microporous membranes made of polyolefin, woven fabrics or nonwoven fabrics can be used. The thickness of the separator is, for example, 8 to 300 μm, and preferably 8 to 40 μm.

[0092] (electrolyte) The electrolyte is lithium ion conductive and, for example, comprises a lithium salt and a solvent for dissolving the lithium salt. The anions of the lithium salt reversibly dope and dedope the positive electrode. Lithium ions originating from the lithium salt are reversibly intercalated and released at the negative electrode.

[0093] Examples of lithium salts include LiClO4, LiBF4, LiPF6, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiFSO3, LiCF3CO2, LiAsF6, LiB 10 Cl 10Examples include LiCl, LiBr, LiI, LiBCl4, LiN(FSO2)2, and LiN(CF3SO2)2. These may be used individually or in combination of two or more. Among these, salts containing a fluorine-containing anion are preferred, and lithium bis(fluorosulfonyl)imide, i.e., LiN(SO2F)2, is particularly preferred. The concentration of the lithium salt in the electrolyte in the charged state (state of charge (SOC) 90-100%) is, for example, 0.2-5 mol / L. Hereinafter, LiN(SO2F)2 will be referred to as LiFSI. For example, 80% by mass or more of the lithium salt may be LiFSI.

[0094] The use of LiFSI tends to significantly reduce the rate of increase in low-temperature DCR. LiFSI is thought to have the effect of reducing the degradation of both the positive and negative electrode active materials. Among salts containing fluorine-containing anions, FSI anions have excellent stability, so they are less likely to produce by-products and are thought to contribute to smooth charging and discharging without damaging the surface of the active materials. In particular, when increasing the capacity of the positive electrode and increasing the specific surface area of ​​the negative electrode mixture layer, the effect of suppressing degradation (the effect of suppressing the increase in low-temperature DCR) by using LiFSI, which significantly reduces the influence of by-products on each active material, becomes more pronounced.

[0095] Suitable solvents include cyclic carbonates such as ethylene carbonate, propylene carbonate, and butylene carbonate; linear carbonates such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; aliphatic carboxylic acid esters such as methyl formate, methyl acetate, methyl propionate, and ethyl propionate; lactones such as γ-butyrolactone and γ-valerolactone; linear ethers such as 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE), and ethoxymethoxyethane (EME); cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran; dimethyl sulfoxide, 1,3-dioxolane, formamide, acetamide, dimethylformamide, dioxolane, acetonitrile, propionitrile, nitromethane, ethyl monoglyme, trimethoxymethane, sulfolane, methylsulfolane, and 1,3-propanesaltone. These may be used individually or in combination of two or more.

[0096] Various additives may be added to the electrolyte as needed. For example, unsaturated carbonates such as vinylene carbonate, vinylethylene carbonate, and divinylethylene carbonate may be added as additives to form a lithium-ion conductive film on the negative electrode surface.

[0097] [Examples] The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples. Table 1 shows an overview of the configuration of each device fabricated below.

[0098] (Device A1) (1) Preparation of the positive electrode A 20 μm thick aluminum foil (positive electrode current collector) was prepared. Meanwhile, 88 parts by mass of activated carbon (average particle size 5.5 μm), which is the positive electrode active material, 2 parts by mass of polytetrafluoroethylene (PTFE), which is the binder, 4 parts by mass of carboxymethylcellulose (CMC), and 6 parts by mass of acetylene black, which is the conductive agent, were dispersed in water to prepare a positive electrode mixture slurry. The obtained positive electrode mixture slurry was applied to both sides of the aluminum foil, the coating was dried, and the foil was rolled to form a positive electrode mixture layer and obtain a positive electrode. A 10 mm wide positive electrode current collector exposure section was formed at the end along the longitudinal direction of the positive electrode current collector.

[0099] (2) Fabrication of the negative electrode A copper foil with a thickness of 8 μm (negative electrode current collector) was prepared. Meanwhile, 80 parts by mass of non-graphitizable carbon (average particle size 5 μm), 4 parts by mass of carboxymethylcellulose (CMC), 10 parts by mass of Ketjenblack, and 6 parts by mass of styrene-butadiene rubber (SBR) (average particle size 140 nm) as a binder were dispersed in water to prepare a negative electrode mixture slurry. The obtained negative electrode mixture slurry was applied to both sides of the copper foil, the coating was dried, and the foil was rolled to form a negative electrode mixture layer and obtain the negative electrode. The thickness of the negative electrode mixture layer was 32 μm on one side. Sodium salt was used for the carboxymethylcellulose.

[0100] Subsequently, a thin film of metallic lithium for pre-doping was formed on the entire surface of the negative electrode mixture layer by vacuum deposition. The amount of lithium to be pre-doped was set so that the negative electrode potential in the non-aqueous electrolyte after pre-doping was 0.2V or less relative to the metallic lithium.

[0101] Subsequently, the chamber of the apparatus was purged with carbon dioxide to create a carbon dioxide atmosphere, thereby forming a first layer containing lithium carbonate on the surface of the negative electrode mixture layer. The dew point of the carbon dioxide atmosphere was -40°C, the mole fraction of carbon dioxide was 100%, and the pressure inside the chamber was 1 atmosphere (1.01 × 10⁻⁶). 5 The pressure was set to Pa. The temperature of the negative electrode exposed to a carbon dioxide atmosphere at 1 atmosphere was set to 25°C. The exposure time of the negative electrode to the carbon dioxide atmosphere was set to 22 hours.

[0102] (3) Fabrication of electrodes An electrode body was formed by winding a positive electrode and a negative electrode in a columnar shape with a cellulose nonwoven fabric separator (25 μm thick) in between. At this time, the exposed portion of the positive electrode current collector protruded from one end face of the winding body, and the exposed portion of the negative electrode current collector protruded from the other end face of the electrode body. Disc-shaped positive electrode current collector plates and negative electrode current collector plates were welded to the exposed portions of the positive electrode current collector and negative electrode current collector, respectively.

[0103] (4) Preparation of non-aqueous electrolyte Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 3:5:2, and vinylene carbonate (VC) was added at a concentration of 0.2% by mass to prepare a solvent. LiPF6 was dissolved in the obtained solvent at a concentration of 1.0 mol / L as a lithium salt to prepare a non-aqueous electrolyte.

[0104] (5) Assembly of electrochemical devices An electrode assembly was housed in a bottomed cell case with an opening. Tab leads connected to the positive electrode current collector plate were connected to the inner surface of a sealing plate, and the negative electrode current collector plate was welded to the inner bottom surface of the cell case. After placing a non-aqueous electrolyte inside the cell case, the opening of the cell case was sealed with a sealing plate, and an electrochemical device as shown in Figure 1 was assembled.

[0105] Subsequently, the electrochemical device A1 was completed by aging it at 60°C while applying a charging voltage of 3.8V between the positive and negative terminals to complete the pre-doping of the lithium ion into the negative electrode.

[0106] (6) Evaluation (Measurement of internal resistance of electrochemical devices) For an electrochemical device immediately after aging, under conditions of -30°C, the voltage is increased to 3.8V until the current is 2mA / cm² per positive electrode area. 2 After constant current charging at the specified current density, a voltage of 3.8V was applied and maintained for 10 minutes. Subsequently, under conditions of -30°C, the current was increased to 2mA / cm² per positive electrode area until the voltage reached 2.2V. 2 Constant current discharge was performed at the specified current density.

[0107] In the discharge curve obtained from the above discharge (vertical axis: discharge voltage, horizontal axis: discharge time), a first-order approximation line was found in the range from 0.5 seconds to 2 seconds after the start of discharge, and the voltage VS at the intercept of this approximation line was determined. The value obtained by subtracting the voltage VS from the voltage V0 at the start of discharge (0 seconds after the start of discharge) (V0-VS) was calculated as ΔV. ΔV(V) and the discharge current value (current density per positive electrode area: 2mA / cm²) were used. 2 Using the positive electrode area, the internal resistance (DCR) R1 (Ω) of the electrochemical device was determined from the following equation (B). The evaluation results are shown in Table 1.

[0108] Internal resistance R1=ΔV / Id (B)

[0109] (Float testing of electrochemical devices) Next, a float test was performed on the electrochemical device under a constant voltage of 3.8V for 450 hours in an environment of 85°C. Afterward, the internal resistance (DCR) R2 (Ω) of the electrochemical device was determined in the same manner as R1 under an environment of -30°C. The evaluation results are shown in Table 1.

[0110] (Device A2) In the fabrication of the negative electrode, a styrene-butadiene rubber with an average particle size of 180 nm was used as a binder to form the negative electrode mixture layer. Except for this, electrochemical device A2 was fabricated and evaluated in the same manner as electrochemical device A1.

[0111] (Device A3) In the fabrication of the negative electrode, an acrylic resin with an average particle size of 200 nm was used as a binder to form the negative electrode mixture layer. Otherwise, electrochemical device A3 was fabricated and evaluated in the same manner as electrochemical device A1.

[0112] (Device A4) In the fabrication of the negative electrode, ammonium carboxymethylcellulose was used as a thickening agent instead of sodium carboxymethylcellulose. Otherwise, electrochemical device A4 was fabricated and evaluated in the same manner as electrochemical device A2.

[0113] (Device A5) In the fabrication of the negative electrode, ammonium carboxymethylcellulose was used as a thickening agent instead of sodium carboxymethylcellulose. Otherwise, electrochemical device A5 was fabricated and evaluated in the same manner as electrochemical device A3.

[0114] (Device B1) In the fabrication of the negative electrode, a styrene-butadiene rubber with an average particle size of 95 nm was used as a binder to form the negative electrode mixture layer. Otherwise, electrochemical device B1 was fabricated and evaluated in the same manner as electrochemical device A1.

[0115] (Device B2) In the fabrication of the negative electrode, ammonium carboxymethylcellulose was used as a thickening agent instead of sodium carboxymethylcellulose. Otherwise, electrochemical device B2 was fabricated and evaluated in the same manner as electrochemical device B1.

[0116] Table 1 shows the evaluation results of the initial internal resistance R1 and the internal resistance R2 after the float test for electrochemical devices A1-A5, B1, and B2. In Table 1, R1 and R2 are shown as relative values ​​with the internal resistance of device B1 set to 100. Electrochemical devices A1-A3 are examples, and B1 is a comparative example. Table 1 also shows the composition (type and average particle size) of the binder used to form the negative electrode mixture layer of each electrochemical device, along with the evaluation results of the internal resistance.

[0117] [Table 1]

[0118] Table 1 shows that devices A1 to A3, in which the average particle size of the binder in the negative electrode mixture layer is 140 nm or larger, exhibit a lower initial internal resistance and can achieve higher output than device B1. Furthermore, device A3, which uses acrylic resin as the binder, shows a significant reduction in internal resistance after the float test.

[0119] Comparing devices B1 and B2, changing the carboxymethylcellulose thickener from sodium salt to ammonium salt resulted in a reduction of approximately 2% in initial internal resistance and approximately 3% in internal resistance after the float test.

[0120] In contrast, comparing devices A2 and A4, changing the carboxymethylcellulose thickener from sodium salt to ammonium salt resulted in an approximately 8% reduction in initial internal resistance and an approximately 13% reduction in internal resistance after the float test, demonstrating a significant reduction in both initial and post-float internal resistance. Similarly, comparing devices A3 and A5, changing the carboxymethylcellulose thickener from sodium salt to ammonium salt resulted in an approximately 8% reduction in initial internal resistance and an approximately 13% reduction in internal resistance after the float test, demonstrating a significant reduction in both initial and post-float internal resistance. [Industrial applicability]

[0121] The electrochemical device according to the present invention is suitable, for example, for automotive applications. [Explanation of Symbols]

[0122] 100: Electrode body 10: Positive electrode 11x: Positive electrode current collector exposed part 13: Positive electrode current collector plate 15: Tabreed 20: Negative electrode 21x: Negative electrode current collector exposed part 23: Negative electrode current collector plate 30: Separator 200: Electrochemical devices 210: Cell case 220: Sealing board 221: Gasket

Claims

1. A polarizing electrode comprising a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, The negative electrode has a negative electrode mixture layer, The negative electrode mixture layer comprises a negative electrode active material to which lithium ions are reversibly doped, a binder, and a thickener. The negative electrode active material contains non-graphitizable carbon, The binder contains an acrylic resin, and the average particle size of the binder is 140 nm or larger. The thickening agent comprises an ammonium salt of carboxymethylcellulose, An electrochemical device in which, in a discharge state, the potential of the negative electrode is +0.2V or less relative to the potential of the Li counter electrode.

2. The electrochemical device according to claim 1, wherein the surface layer of the negative electrode mixture layer has a first layer containing lithium carbonate.

3. The surface layer of the negative electrode mixture layer has a second layer containing a solid electrolyte, The electrochemical device according to claim 2, wherein the second layer covers at least a portion of the surface of the first layer.

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