Electrochemical Devices

Abstract

This electrochemical device is provided with: an electrode body that comprises a positive electrode, a negative electrode, and a separator that is interposed between the positive electrode and the negative electrode; and an electrolyte that contains a lithium salt. The negative electrode is provided with a negative electrode collector and a negative electrode mixture layer that is supported by the negative electrode collector; the negative electrode mixture layer contains a negative electrode active material which is reversibly doped with lithium ions; the negative electrode collector does not substantially have a through hole; the negative electrode mixture layer has a specific surface area of from 30 m2 / g to 60 m2 / g; and a good balance is able to be achieved between high capacity and high output at high levels by setting the potential of the negative electrode versus lithium reference electrode to 0.2 V or less in a discharged state.

Description

[Technical Field]

[0001] The present 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 as the positive electrode and a non-polarizable electrode as the negative electrode. As a result, electrochemical devices are expected to combine the high energy density of lithium-ion secondary batteries with the high output characteristics of electric double-layer capacitors.

[0003] Patent Document 1 describes a negative electrode coating for a lithium ion capacitor, which is formed by applying a coating composition for forming an electrode coating, which contains non-graphitizable carbon, a conductive additive, and a binder in an aqueous medium containing a dispersant, onto a metal foil, and then heating and drying the coating to form a film. The conductive additive is at least one of ketjen black, acetylene black, and graphite, and the particle size distribution of the constituent particles in the negative electrode coating is D 10 Particle size 0.5 μm or more, D 50 Particle size range: 1 to 4 μm, D 90 The particle diameter is 8 μm or less, and the specific surface area of ​​the negative electrode coating is 1.5 to 25 m 2 / g and the surface roughness of the negative electrode coating is in the range of 0.1 to 0.3 μm. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-98020 Summary of the Invention

[0005] However, in order to achieve both high capacity and high output characteristics at a higher level in the electrochemical devices described above, further improvements are required.

[0006] One aspect of the present invention Electrochemical devices related to The battery comprises an electrode assembly including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, and an electrolyte containing a lithium salt. do. The negative electrode comprises a negative electrode current collector and a negative electrode mixture layer carried on the negative electrode current collector. do. The negative electrode mixture layer contains a negative electrode active material that is reversibly doped with lithium ions, and the negative electrode current collector has substantially no through-holes. do not have. The specific surface area of ​​the negative electrode mixture layer is 30 m 2 / g or more, 60m 2 / g or less, and the potential of the negative electrode in a discharged state is 0.2 V or less relative to a Li counter electrode. do.

[0007] According to the present invention, it is possible to provide an electrochemical device that has both high capacity and high output characteristics. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a longitudinal sectional view of an electrochemical device according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0009] An electrochemical device according to an embodiment of the present invention includes an electrode assembly including a positive electrode, a negative electrode, and a separator interposed between the positive and negative electrodes, and an electrolyte containing a lithium salt. The electrode assembly may be configured, for example, as a columnar wound assembly by winding strip-shaped positive and negative electrodes with the separator interposed therebetween. Alternatively, the electrode assembly may be configured as a laminate by stacking plate-shaped positive and negative electrodes with the separator interposed therebetween.

[0010] The negative electrode includes 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 dopes lithium ions. The negative electrode exhibits capacity through a Faraday reaction in which lithium ions are reversibly absorbed and released. The doping of lithium ions into the negative electrode active material includes at least the phenomenon of lithium ions being absorbed 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.

[0011] The potential of the negative electrode increases with discharge. In the case of a typical lithium-ion secondary battery, in a fully discharged state, the potential of the negative electrode is, for example, 1 V relative to the Li counter electrode, exceeding 0.2 V. In contrast, in the electrochemical device of this embodiment, in a discharged state, the potential of the negative electrode is 0.2 V or less relative to the Li counter electrode. This means that sufficient lithium is doped into the negative electrode even in a discharged state. Such a negative electrode can be obtained by pre-doping lithium ions into the negative electrode before charging the device after manufacture. Using a pre-doped negative electrode can achieve high capacity.

[0012] Here, the discharge state is when the voltage is Charging status ( SOC ) 2 mA / cm per positive electrode area until the voltage corresponding to 95% or more (e.g., 3.8 V) is reached 2 The electrochemical device was charged at a constant current density of 2 mA / cm per positive electrode area until the voltage reached a voltage (e.g., 2.2 V) corresponding to a depth of discharge (DOD) of 95% or more in a 25°C environment. 2 The negative electrode potential is determined by disassembling the device after charging and discharging under the above conditions, assembling a half-cell with the removed negative electrode as the working electrode and Li metal foil as the counter electrode, and measuring the potential of the negative electrode relative to the Li metal foil.

[0013] Unlike conventional lithium-ion secondary batteries, electrochemical devices with a lithium-ion pre-doped negative electrode are capable of rapid charging and discharging, and are characterized by high output. To enable rapid charging and discharging and achieve high output, foils with through-holes are typically used as the negative electrode current collector of electrochemical devices to improve electrolyte circulation. However, the provision of through-holes increases the resistance of the current collector, which can prevent high output. When using foils with through-holes, there are limitations to improving output characteristics.

[0014] In addition, when a foil having through holes is used, the foil's strength decreases and it may break due to tensile stress. In particular, when a wound electrode body is used in which a positive electrode and a negative electrode are wound with a separator interposed therebetween, the wound body expands (and contracts) with charge and discharge, and tensile stress (and compressive stress) is applied to the current collector. As a result, the current collector having through holes is prone to breakage. To prevent breakage of the current collector having through holes, reduce resistance, and obtain high output, the current collector can be made thicker. However, if the thickness of the current collector is increased, the thickness of the negative electrode mixture layer will be correspondingly thinner, or if the thickness of the negative electrode mixture layer is unchanged, the area of ​​the negative electrode will be reduced. As a result, it is difficult to obtain high capacity.

[0015] In the electrochemical device of this embodiment, the negative electrode current collector is substantially free of through-holes. This prevents an increase in resistance, ensures high output, and prevents the wound body from breaking during charging and discharging. Furthermore, since the negative electrode current collector can be made thin while maintaining its strength, it is easy to increase the thickness of the negative electrode mixture layer or the area of ​​the negative electrode. This, combined with the use of a pre-doped negative electrode, allows for a high capacity to be easily achieved. Regarding the negative electrode current collector, "substantially free of through-holes" means that the aperture ratio of the negative electrode current collector is, for example, 1% or less. The aperture ratio of the negative electrode current collector refers to the ratio of the area of ​​the openings present within the main surface to the area of ​​the main surface of the negative electrode current collector.

[0016] In addition, the electrochemical device of this embodiment has a specific surface area of ​​30 m 2 / g or more. This allows for even higher output. The specific surface area of ​​the negative electrode mixture layer is 40m 2 On the other hand, from the viewpoint of suppressing an increase in side reactions and maintaining high cycle characteristics, the specific surface area of ​​the negative electrode mixture layer is preferably 60 m 2 / g or less, and 50m 2 / g or less is more preferable.

[0017] The specific surface area of ​​the negative electrode mixture layer can be adjusted to a desired value by changing the materials and properties (e.g., particle size, specific surface area, etc.) of the negative electrode active material, the binder and the conductive additive that may be contained in the negative electrode active material, and the blending ratio. 2 / g or more. This makes it easy to form a negative electrode mixture layer having a desired specific surface area. The specific surface area of ​​the conductive additive is 800 m 2 / g or more 1500m 2 / g or less is more preferable.

[0018] The specific surface area of ​​the negative electrode mixture layer is the BET specific surface area determined using a measuring device conforming to JIS Z8830 (e.g., a Tristar II3020 manufactured by Shimadzu Corporation). Specifically, the electrochemical device is disassembled and the negative electrode is removed. A half cell is assembled using this negative electrode as the working electrode and Li metal foil as the counter electrode, and Li is dedoped from the negative electrode until the negative electrode potential reaches 1.5 V. Next, the Li-dedoped negative electrode is washed with dimethyl carbonate (DMC) and dried. The negative electrode mixture layer is then peeled off from the negative electrode current collector, and approximately 0.5 g of a sample of the negative electrode mixture layer is collected.

[0019] Next, the collected sample is heated at 150°C for 12 hours under a reduced pressure of 95 kPa or less, and then nitrogen gas is adsorbed onto a sample of known mass to obtain an adsorption isotherm over a relative pressure range of 0 to 1. The surface area of ​​the sample is then calculated from the monolayer adsorption amount of gas obtained from the adsorption isotherm. Here, the specific surface area is calculated using the BET single-point method (relative pressure 0.3) using the BET equation below.

[0020] P / V(P0-P)=(1 / VmC)+{(C-1) / VmC}(P / P0)...(1)

[0021] S = kVm (2)

[0022] P0: Saturated vapor pressure P: adsorption equilibrium pressure V: adsorption amount at adsorption equilibrium pressure P Vm: monolayer adsorption amount C: Parameters related to heat of adsorption, etc. S: Specific surface area k: Nitrogen monomolecular area 0.162nm 2

[0023] When the negative electrode mixture layer contains a conductive additive, the content of the conductive additive in the negative electrode mixture layer is 3 mass % or less. % Preferably, the content is 10% by mass or more and 15% by mass or less, and more preferably 5% by mass or more and 10% by mass or less. When the content of the conductive additive is 3% by mass or more, the resistance of the negative electrode mixture layer is reduced and the current collection performance is improved. This allows for even higher output. On the other hand, when the content of the conductive additive is too high, the proportion of the negative electrode active material in the negative electrode mixture layer decreases, and high capacity may not be obtained. Furthermore, an increase in the specific surface area of ​​the negative electrode mixture layer may increase side reactions, resulting in a deterioration in cycle characteristics. From the viewpoint of maintaining high capacity and high cycle characteristics, the content of the conductive additive is preferably 15% by mass or less or 10% by mass or less.

[0024] The content of the conductive additive is determined for the conductive additive separated from the negative electrode mixture layer by the following method. The electrochemical device is disassembled to remove the negative electrode, and a portion of the negative electrode mixture layer is peeled off from the negative electrode from which Li has been dedoped by the method described above. The negative electrode mixture layer is washed with water to remove the binder and other materials, and then the conductive additive is separated by centrifugation. The content of the conductive additive is the ratio of the mass of the conductive additive after separation to the mass of the negative electrode mixture layer before washing with water.

[0025] The specific surface area of ​​the conductive additive separated by the above method can be determined by the BET method in the same manner as in determining the specific surface area of ​​the negative electrode mixture layer.

[0026] The negative electrode active material preferably contains non-graphitizable carbon. Non-graphitizable carbon is also called hard carbon. By using hard carbon, high cycle characteristics can be obtained even under charge-discharge conditions in which rapid charging and rapid discharging are repeated. In this case, the negative electrode mixture layer preferably contains carbon black as a conductive additive. Carbon black has a large specific surface area and can easily increase the specific surface area of ​​the negative electrode mixture layer. In addition, carbon black can easily coat hard carbon, which can easily improve the binding between the negative electrode active materials.

[0027] The thickness of the negative electrode current collector may be 15 μm or less. As described above, in the electrochemical device of this embodiment, since the aperture ratio of the negative electrode current collector is small, the strength can be maintained even if the thickness of the negative electrode current collector is made thin. do Increasing the thickness of the negative electrode mixture layer can achieve high capacity. The thickness of the negative electrode current collector is preferably 10 μm or less, more preferably 8 μm or less. On the other hand, reducing the thickness of the negative electrode current collector increases resistance, and high output may not be obtained. To maintain high output, the thickness of the negative electrode current collector is preferably 3 μm or more, more preferably 4 μm or more. The upper and lower limits of the thickness can be combined arbitrarily.

[0028] The thickness of the negative electrode mixture layer may be, for example, 25 μm or more, 30 μm or more, or 32 μm or more. In the above, when the negative electrode mixture layer is formed on both sides of the negative electrode current collector, the thickness of the negative electrode mixture layer means the thickness on one side.

[0029] 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 contains a positive electrode active material that reversibly dopes anions. When anions are adsorbed onto the positive electrode active material, an electric double layer is formed, thereby generating capacitance. The positive electrode may be a polarizable electrode, or an electrode that has the properties of a polarizable electrode but also contributes to capacitance through a faradaic reaction.

[0030] The positive electrode active material may be a carbon material or a conductive polymer. The doping of the positive electrode active material with anions includes at least the adsorption of anions onto the positive electrode active material, and may also include the occlusion of anions by the positive electrode active material and chemical interaction between the positive electrode active material and the anions.

[0031] The pre-doping of lithium ions into the negative electrode may be performed by contacting the negative electrode with an electrolyte having lithium ion conductivity before assembling the device. As the lithium ion source used for pre-doping, for example, metallic lithium may be used. For example, a working electrode (e.g., stainless steel( SUS ) Pre-doping can be performed by placing a separator between the negative electrode and the working electrode (a metal plate made of lithium ion conductive material) in a battery container filled with an electrolyte having lithium ion conductivity, and applying a voltage between the positive electrode and the negative electrode with the working electrode as the positive electrode. The voltage application can be performed, for example, under conditions where a predetermined constant current flows between the positive electrode and the negative electrode. The voltage application time is, for example, 1 to 75 hours.

[0032] Metallic lithium as a lithium ion source may be attached to the surface of the negative electrode mixture layer in advance, and the negative electrode with metallic lithium attached thereto may be placed in a battery container, and a voltage may be applied between the negative electrode and the working electrode to perform pre-doping. The deposition of metallic lithium on the surface of the negative electrode mixture layer can be carried out by, for example, a vapor phase method, transfer, etc. 。 Examples of the gas phase method include chemical vapor deposition, physical vapor deposition, and sputtering. For example, a film of metallic lithium may be formed on the surface of the negative electrode mixture layer using a vacuum deposition device. The pressure in the chamber of the device 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.

[0033] By exposing a negative electrode having a negative electrode mixture layer to which metallic lithium is attached to a carbon dioxide gas atmosphere, a layer containing lithium carbonate (first layer) can be formed on the surface of the negative electrode mixture layer. As will be described later, this layer containing lithium carbonate suppresses deterioration of the negative electrode and improves the low-temperature operation of the electrochemical device. DC resistance ( It can have the effect of suppressing an increase in the DCR. The carbon dioxide atmosphere is desirably a dry atmosphere that does not contain moisture, and for example, the dew point may be -40°C or lower or -50°C or lower. The carbon dioxide atmosphere may contain gases other than carbon dioxide, but the molar fraction of carbon dioxide is desirably 80% or higher, and more desirably 95% or higher. The carbon dioxide atmosphere desirably does not contain oxidizing gases, and the molar fraction of oxygen may be 0.1% or lower. The partial pressure of carbon dioxide in the carbon dioxide atmosphere is, for example, 0.5 atmospheres (5.05 x 10 4 Pa) is more efficient than 1 atmosphere (1.01×10 5 Pa) or more.

[0034] By contacting the negative electrode with an electrolyte having lithium ion conductivity, lithium ions are pre-doped, and a layer (second layer) containing a solid electrolyte can be formed on the surface of the negative electrode. The second layer acts as a solid electrolyte interfacial coating (i.e., SEI coating). When the first layer is formed on the surface of the negative electrode mixture layer, the second layer can be formed so as to cover at least a portion of the first layer. The first layer containing lithium carbonate promotes the formation of a good SEI coating and maintains the SEI coating in a good condition during repeated charge and discharge.

[0035] Hereinafter, the positive electrode and the negative electrode may be collectively referred to as electrodes. The positive electrode current collector and the negative electrode current collector may be collectively referred to as current collectors (or electrode current collectors). The positive electrode mixture layer and the negative electrode mixture layer may be collectively referred to as mixture layers (or electrode mixture layers). The positive electrode active material and the negative electrode active material may be collectively referred to as active materials (or electrode active materials).

[0036] FIG. 1 schematically illustrates the configuration of an electrochemical device 200 according to one embodiment of the present invention. The electrochemical device 200 includes an electrode assembly 100, a nonaqueous electrolyte (not shown), a metal cell case 210 with a bottom that accommodates the electrode assembly 100 and the nonaqueous electrolyte, and a sealing plate 220 that seals the opening of the cell case 210. A gasket 221 is disposed around the periphery of the sealing plate 220, and the interior of the cell case 210 is sealed by crimping the open edge of the cell case 210 to the gasket 221. A positive current collector 13 having a through-hole 13h in its center is welded to a positive current collector exposed portion 11x. One end of a tab lead 15 is connected to the positive current collector 13, and the other end is connected to the inner surface of the sealing plate 220. Thus, the sealing plate 220 functions as an external positive terminal. Meanwhile, a negative current collector 23 is welded to a negative current collector exposed portion 21x. 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.

[0037] Each component of the electrochemical device according to the embodiment of the present invention will now be described in more detail.

[0038] (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 is reversibly doped with 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.

[0039] The negative electrode current collector is made of a sheet-like metal material that is substantially free of through-holes. Examples of the sheet-like metal material include metal foil. Examples of the metal material include copper, copper alloy, nickel, and stainless steel. The aperture ratio of the negative electrode current collector may be 1% or less.

[0040] The negative electrode current collector is a generally disk-shaped metal plate. The material of the negative electrode current collector is, for example, copper, copper alloy, nickel, stainless steel, etc. The material of the negative electrode current collector may be the same as the material of the negative electrode current collector.

[0041] The non-graphitizable carbon may have a (002) plane spacing (i.e., the spacing between carbon layers) d002 of 3.8 Å or more as measured by X-ray diffraction. The theoretical capacity of the non-graphitizable carbon is preferably, for example, 150 mAh / g or more. The use of non-graphitizable carbon facilitates the production of a negative electrode with a small low-temperature DCR and small expansion and contraction during charge and discharge. The non-graphitizable carbon preferably accounts for 50% by mass or more, more preferably 80% by mass or more, and even 95% by mass or more of the negative electrode active material. The non-graphitizable carbon preferably accounts for 40% by mass or more, more preferably 70% by mass or more, and even 90% by mass or more of the negative electrode mixture layer.

[0042] As the negative electrode active material, non-graphitizable carbon and a material other than non-graphitizable carbon may be used in combination. Examples of materials other than non-graphitizable carbon that can be used as the negative electrode active material include 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, tin alloy, etc.

[0043] From the viewpoint of improving the packing property of the negative electrode active material in the negative electrode and easily suppressing side reactions with the electrolyte, the average particle size of the negative electrode active material (particularly non-graphitizable carbon) is preferably 1 μm to 20 μm, and more preferably 2 μm to 15 μm.

[0044] In this specification, the average particle size refers to the volume-based median diameter (D 50 ) means

[0045] The negative electrode mixture layer contains a negative electrode active material as an essential component, and optionally contains a conductive additive, a binder, etc. Examples of the conductive additive include carbon black and carbon fiber. The conductive additive preferably contains carbon black. Examples of the binder include fluororesin, acrylic resin, rubber material, and cellulose derivative.

[0046] The negative electrode mixture layer is formed, for example, by mixing a negative electrode active material, a conductive agent, a binder, and the like 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 the slurry.

[0047] The negative electrode mixture layer is pre-doped with lithium ions. This reduces the potential of the negative electrode, increasing the potential difference (i.e., voltage) between the positive electrode and the negative electrode, thereby improving the energy density of the electrochemical device. The amount of pre-doped lithium may be, for example, approximately 50% to 95% of the maximum amount that can be absorbed into the negative electrode mixture layer.

[0048] The capacitance per unit mass of the negative electrode active material may be, for example, 1000 F / g or more. From the viewpoint of increasing the capacity density of the electrochemical device, the capacitance per unit mass of the negative electrode active material may be, for example, 30000 F / g or less. The capacitance per unit mass of the negative electrode active material is usually larger than the capacitance per unit mass of the positive electrode active material, for example, 20 to 800 times the capacitance per unit mass of the positive electrode active material. The capacitance per unit mass of the negative electrode active material can be measured by the following method.

[0049] First, a negative electrode for evaluation was prepared by cutting it to a size of 31 mm × 41 mm. A 100 μm thick piece of lithium metal foil was prepared as a counter electrode for the negative electrode by cutting it to a size of 40 mm × 50 mm. A 25 μm thick cellulose paper (e.g., product number TF4425) manufactured by Nippon Kodoshi Kogyo Co., Ltd. was placed between the negative electrode mixture layer and the lithium metal foil as a separator to form an electrode assembly. The electrode assembly was then immersed in the electrolyte of Example 1 described below to form a cell.

[0050] Charge at a constant current (CC) of 0.5 mA until the cell voltage reaches 0.01 V, then charge at a constant voltage (CV) for 1 hour, and then discharge at 0.5 mA until the cell voltage reaches 1.5 V. Calculate the capacitance per unit mass of the negative electrode active material from the discharge time required for the negative electrode potential to change by 0.1 V from the potential of the negative electrode 1 minute after the start of discharge.

[0051] The surface layer portion of the negative electrode mixture layer may have a first layer containing lithium carbonate as a coating component. The first layer is formed mainly on the surface of the negative electrode active material. The larger the specific surface area of ​​the negative electrode mixture layer, the more likely the negative electrode is to deteriorate. However, by forming the first layer, deterioration of the negative electrode is significantly suppressed. Here, deterioration of the negative electrode is typically evaluated as the rate of increase in the low-temperature DCR of the electrochemical device when float charging is performed at high temperature, in which a constant voltage is applied to the electrochemical device using an external DC power source.

[0052] The surface portion of the negative electrode may have a second layer containing a solid electrolyte as a component of the coating. The second layer has a different composition from the first layer and is distinguishable from the first layer. In electrochemical devices that use lithium ions, a solid electrolyte interfacial coating (i.e., SEI coating) is formed on the negative electrode mixture layer during charge and discharge. The second layer may be formed as an SEI coating. The SEI coating plays an important role in charge and discharge reactions, but if the SEI coating is formed too thick, negative electrode deterioration will increase. In contrast, the first layer containing lithium carbonate promotes the formation of a good SEI coating and maintains the SEI coating in good condition even during repeated charge and discharge. Therefore, by forming the first layer on the surface portion of the negative electrode mixture layer, negative electrode deterioration can be significantly suppressed even when the specific surface area of ​​the negative electrode mixture layer is increased to obtain high output.

[0053] When the coating has a first layer and a second layer, at least a portion of the second layer covers at least a portion of the surface of the negative electrode active material via the first layer. That is, at least a portion of the first layer is covered by the second layer. The first layer is interposed between the surface of the negative electrode active material and the second layer, and serves as an underlayer for the second layer. With the first layer serving as an underlayer, the second layer is formed as a good-condition SEI coating.

[0054] The second layer may also contain lithium carbonate. When the second layer contains lithium carbonate, the lithium carbonate content in the second layer is less than the lithium carbonate content in the first layer. The first layer, which contains a large amount of lithium carbonate, as the underlayer is a necessary condition for the second layer to be formed as a good SEI coating.

[0055] The first layer is formed on the surface of the negative electrode mixture layer before assembling the electrochemical device. In an electrochemical device assembled using the negative electrode, a second layer (SEI film) that is homogeneous and has an appropriate thickness is formed on the surface of the negative electrode active material through subsequent charge and discharge. The SEI film is formed, for example, by a reaction between an electrolyte and a negative electrode in an electrochemical device. Because the electrolyte can pass through not only the second layer but also the first layer, the entire surface including the first and second layers may be referred to as the SEI film. However, for convenience, the second layer will be referred to as the SEI film in this specification to distinguish it from the first layer.

[0056] The presence of a region containing lithium carbonate such as the first layer can be confirmed by analyzing the surface layer by, for example, X-ray photoelectron spectroscopy (XPS), although the analysis method is not limited to XPS.

[0057] The thickness of the first layer may be, for example, 1 nm or more, or 5 nm or more if a longer-term effect is desired, or 10 nm or more if a more reliable effect is desired. However, if the thickness of the first layer exceeds 50 nm, the first layer itself may become a resistance component. Therefore, the thickness of the first layer may be 50 nm or less, or may be 30 nm or less.

[0058] The thickness of the second layer may 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 resistance component. Therefore, the thickness of the second layer may be 20 nm or less, or may be 10 nm or less.

[0059] The ratio A / B of the thickness A of the first layer to the thickness B of the second layer 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 in good condition, it is desirable that A / B be 0.1 or more, and for example, the A / B ratio may be 0.2 or more.

[0060] The thicknesses of the first and second layers are measured by analyzing the surface portion of the negative electrode mixture layer at multiple locations (at least five locations). The average of the thicknesses of the first or second layer measured at multiple locations is taken 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 constituting the vicinity of the surface portion of the negative electrode mixture layer is analyzed. Specifically, the negative electrode active material covered with the coating is collected from the region of the negative electrode mixture layer disposed on the side opposite to the surface bonded to the negative electrode current collector and used for analysis.

[0061] XPS analysis of the surface portion of the negative electrode mixture layer is performed, for example, by irradiating an argon beam onto the surface portion or a coating formed on the surface of the negative electrode active material in the chamber of an X-ray photoelectron spectrometer, and observing and recording the changes in the spectra attributable to C1s, O1s electrons, etc. versus irradiation time. To avoid analytical errors, the spectra of the outermost surface of the surface portion may be ignored. The thickness of the region where peaks attributable to lithium carbonate are stably observed corresponds to the thickness of the first layer.

[0062] In the case of a negative electrode removed from a completed electrochemical device after a predetermined aging period or at least one charge / discharge cycle, the surface portion of the negative electrode mixture layer has an SEI coating (i.e., second layer) containing a solid electrolyte. The thickness of the region where peaks attributable to 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).

[0063] The compound contained in the SEI coating is selected to contain an element that can be a label for the second layer. The element that can be a label for the second layer may be, for example, an element (e.g., F) that is contained in the electrolyte and is not substantially contained in the first layer. LiF, for example, may be selected as the compound that contains an element that can be a label for the second layer.

[0064] When the second layer contains LiF, a substantial F1s peak attributed to LiF bonds is observed when the second layer is measured by X-ray photoelectron spectroscopy. In this case, the thickness of the region where the peak attributed to LiF bonds is stably observed corresponds to the thickness of the second layer.

[0065] On the other hand, the first layer usually does not contain LiF, and when the first layer is measured by X-ray photoelectron spectroscopy, a substantial F1s peak attributable to LiF bonds is not observed. Therefore, the thickness of the region where a peak attributable to LiF bonds is not stably observed may be taken as the thickness of the first layer.

[0066] An O1s peak attributed to lithium carbonate can also be observed in the SEI coating. However, the SEI coating formed in the electrochemical device has a different composition from the pre-formed first layer, making it possible to distinguish between the two. For example, in XPS analysis of the SEI coating, an F1s peak attributed to LiF bonds is observed, but a substantial F1s peak attributed to LiF bonds is not observed in the first layer. Furthermore, the amount of lithium carbonate contained in the SEI coating is very small. Note that peaks attributed to compounds such as ROCO2Li and ROLi can be detected as Li1s peaks.

[0067] When the first layer is analyzed by XPS, a second O1s peak attributed to a Li-O bond may be observed in addition to a first O1s peak attributed to a C=O bond. The region of the coating near the surface of the negative electrode active material may contain a small amount of LiOH or Li2O.

[0068] Specifically, when the first layer constituting the surface layer portion of the negative electrode mixture layer is analyzed in the depth direction, a first peak (O1s attributed to a C=O bond) and a second peak (O1s attributed to a Li-O bond) may be observed in order of increasing distance from the outermost surface of the surface layer portion, and a first region in which the first peak intensity is greater than the second peak intensity, and a second region in which the first peak and the second peak are observed and the second peak intensity is greater than the first peak intensity may be observed. Furthermore, a third region may also be present that is closer to the outermost surface of the surface layer portion than the first region, and in which the first peak is observed but the second peak is not observed. The third region is more likely to be observed when the lithium carbonate-containing region is thick.

[0069] The magnitude of the peak intensity can be determined by the height of the peak from the baseline.

[0070] At the center of the first layer in the thickness direction, the C1s peak attributed to a C═C bond is generally not observed, or even if observed, its intensity is less than half of the peak intensity attributed to a C═O bond.

[0071] Next, a method for forming the first layer containing lithium carbonate on the surface layer of the negative electrode mixture layer will be described. The step of forming the first layer can be carried out by, for example, a vapor phase method, a coating method, or a transfer method.

[0072] Examples of gas phase methods include chemical vapor deposition, physical vapor deposition, and sputtering. For example, lithium carbonate can be deposited on the surface of the negative electrode mixture layer using a vacuum deposition device. The pressure in the device chamber during deposition is, for example, 10 -2 ~10 -5 Pa, the temperature of the lithium carbonate evaporation source may be 400 to 600°C, and the temperature of the negative electrode mixture layer may be -20 to 80°C.

[0073] The first layer can be formed by applying a solution or dispersion containing lithium carbonate to the surface of the negative electrode using, for example, a microgravure coater, followed by drying. The lithium carbonate content in the solution or dispersion is, for example, 0.3 to 2 mass %, and when a solution is used, the concentration may be below the solubility (for example, about 0.9 to 1.3 mass % in an aqueous solution at room temperature).

[0074] Furthermore, a step of forming a second layer containing a solid electrolyte so as to cover at least a portion of the first layer is carried out to obtain a negative electrode. The surface layer portion of the obtained negative electrode mixture layer has a first layer and a second layer. The second layer is formed so that at least a portion of it covers at least a portion (preferably the entire) of the surface of the negative electrode active material through the first layer (i.e., with the first layer as a base layer).

[0075] The step of forming the second layer proceeds by contacting the negative electrode mixture layer with an electrolyte and then leaving it for a predetermined period of time to complete. For example, the second layer may be formed on the negative electrode mixture layer by charging and discharging the electrochemical device at least once. The step of forming the second layer may also serve as at least a part of the step of pre-doping the negative electrode mixture layer with lithium ions.

[0076] The step of forming the first layer is preferably carried out before the electrode body is constructed, but this does not exclude the case where it is carried out after the electrode body is constructed.

[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, and the positive electrode mixture layer contains a positive electrode active material that reversibly dopes anions, such as a carbon material, a conductive polymer, etc. The thickness of the positive electrode mixture layer is, for example, 10 to 300 μm per side of the positive electrode current collector.

[0078] The positive electrode current collector is made of a sheet-like metal material. The sheet-like metal material may be a metal foil, a porous metal, an etched metal, or the like. Examples of the metal material that can be used include aluminum, an aluminum alloy, nickel, and titanium. As with the negative electrode current collector, the positive electrode current collector is preferably made of a sheet material that is substantially free of through-holes.

[0079] The positive electrode current collector is a generally disk-shaped metal plate. A through-hole is preferably formed in the center of the positive electrode current collector to serve as a passage for the non-aqueous electrolyte. The material of the positive electrode current collector is, for example, aluminum, an aluminum alloy, titanium, stainless steel, or the like. The material of the positive electrode current collector may be the same as the material of the positive electrode current collector.

[0080] The carbon material used as the positive electrode active material is preferably a porous carbon material, such as activated carbon or the carbon materials exemplified as the negative electrode active material (e.g., non-graphitizable carbon). Examples of raw materials for activated carbon include wood, coconut shells, coal, pitch, and phenolic resin. The activated carbon is preferably activated.

[0081] 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.

[0082] The specific surface area of ​​the positive electrode mixture layer generally reflects the specific surface area of ​​the positive electrode active material. 2 / g or more, 4000m 2 / g or less is sufficient, and 2 / g or more, 3000m 2 / g or less is desirable. The specific surface area of ​​the positive electrode mixture layer is the BET specific surface area determined using a measuring device conforming to JIS Z8830 (e.g., a Tristar II3020 manufactured by Shimadzu Corporation). Specifically, the electrochemical device is disassembled, and the positive electrode is removed. Next, the positive electrode is washed with DMC and dried. Thereafter, the positive electrode mixture layer is peeled from the positive electrode current collector, and approximately 0.5 g of a sample of the positive electrode mixture layer is collected. Next, the specific surface area of ​​the collected sample is determined in accordance with the method for measuring the specific surface area of ​​the negative electrode mixture layer already described.

[0083] The activated carbon preferably accounts for 50% by mass or more, more preferably 80% by mass or more, and even 95% by mass or more of the positive electrode active material, and more preferably 40% by mass or more, more preferably 70% by mass or more, and even more preferably 90% by mass or more of the positive electrode mixture layer.

[0084] The positive electrode mixture layer contains a positive electrode active material as an essential component, and optionally contains a conductive additive, a binder, etc. Examples of the conductive additive include carbon black and carbon fiber. Examples of the binder include fluororesin, acrylic resin, rubber material, and cellulose derivative.

[0085] The positive electrode mixture layer is formed, for example, by mixing a positive electrode active material, a conductive agent, a binder, and the like 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 the slurry.

[0086] The conductive polymer used as the positive electrode active material is preferably a π-conjugated polymer. Examples of the π-conjugated polymer include polypyrrole, polythiophene, polyfuran, polyaniline, polythiophene vinylene, polypyridine, and derivatives thereof. These may be used alone or in combination of two or more. The weight-average molecular weight of the conductive polymer is, for example, 1,000 to 100,000. The derivative of a π-conjugated polymer refers to a polymer having a π-conjugated polymer skeleton such as polypyrrole, polythiophene, polyfuran, polyaniline, polythiophene vinylene, or polypyridine. For example, polythiophene derivatives include poly(3,4-ethylenedioxythiophene) (PEDOT).

[0087] Conductive polymers are formed, for example, by immersing a positive electrode current collector with a carbon layer in a reaction solution containing raw material monomers of the conductive polymer and electrolytically polymerizing the raw material monomers in the presence of the positive electrode current collector. In electrolytic polymerization, a positive electrode current collector and a counter electrode are immersed in a reaction solution containing the raw material monomers, and a current is passed between them using the positive electrode current collector as the anode. Conductive polymers may also be formed by methods other than electrolytic polymerization. For example, a conductive polymer may be formed by chemical polymerization of raw material monomers. In chemical polymerization, raw material monomers are polymerized using an oxidizing agent or the like in the presence of the positive electrode current collector.

[0088] The raw material monomer used in electropolymerization or chemical polymerization may be any polymerizable compound capable of producing a conductive polymer by polymerization. The raw material monomer may include an oligomer. Examples of the raw material monomer include aniline, pyrrole, thiophene, furan, thiophene vinylene, pyridine, or derivatives thereof. These may be used alone or in combination of two or more. Among these, aniline is particularly easy to grow on the surface of the carbon layer by electropolymerization.

[0089] Electropolymerization or chemical polymerization can be carried out using a reaction solution containing an anion (dopant). By doping a π-electron conjugated polymer with a dopant, excellent conductivity can be achieved. 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. The dopant may also be a polymeric ion. Examples of polymeric ions include ions of polyvinylsulfonic acid, polystyrenesulfonic acid, polyallylsulfonic acid, polyacrylicsulfonic acid, polymethacrylic acid, poly(2-acrylamido-2-methylpropanesulfonic acid), polyisoprenesulfonic acid, and polyacrylic acid.

[0090] (separator) The separator may be a nonwoven fabric made of cellulose fiber, a nonwoven fabric made of glass fiber, a microporous membrane made of polyolefin, a woven fabric or a nonwoven fabric, etc. The thickness of the separator is, for example, 8 to 300 μm, and preferably 8 to 40 μm.

[0091] (electrolyte) The electrolyte has lithium ion conductivity and contains, for example, a lithium salt and a solvent for dissolving the lithium salt. The anions of the lithium salt are reversibly doped and dedoped into the positive electrode. The lithium ions derived from the lithium salt are reversibly absorbed into and released from the negative electrode.

[0092] Examples of lithium salts include LiClO4, LiBF4, LiPF6, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiFSO3, LiCF3CO2, LiAsF6, and LiB 10 Cl 10 , LiCl, LiBr, LiI, LiBCl4, LiN(FSO2)2, LiN(CF3SO2)2, etc. These may be used alone or in combination of two or more. Among these, salts having a fluorine-containing anion are preferred, and it is particularly preferred to use lithium bis(fluorosulfonyl)imide, i.e., LiN(SO2F)2. The concentration of the lithium salt in the electrolyte in the charged state (state of charge (SOC) 90 to 100%) is, for example, 0.2 to 5 mol / L. Hereinafter, LiN(SO2F)2 will be referred to as LiFSI. For example, 80 mass % or more of the lithium salt may be LiFSI.

[0093] 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 positive and negative electrode active materials. Among salts with fluorine-containing anions, the FSI anion is highly stable, so it is thought to be less likely to produce by-products and contribute to smooth charge and discharge without damaging the surface of the active material. In particular, when increasing the capacity of the positive electrode and 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 impact of by-products on each active material, becomes significant.

[0094] Examples of solvents that can be used include cyclic carbonates such as ethylene carbonate, propylene carbonate, and butylene carbonate, chain 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, chain 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, methyl sulfolane, and 1,3-propane sultone. These may be used alone or in combination of two or more.

[0095] The electrolyte may contain various additives as needed. For example, unsaturated carbonates such as vinylene carbonate, vinylethylene carbonate, and divinylethylene carbonate may be added as additives that form a lithium ion conductive coating on the surface of the negative electrode.

[0096] [Example] 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 outline of the configuration of each device fabricated below.

[0097] (Device A1) (1) Preparation of the positive electrode A 30 μm thick aluminum foil (positive electrode current collector) was prepared. Separately, 88 parts by mass of activated carbon (average particle size 5.5 μm) as the positive electrode active material, 6 parts by mass of polytetrafluoroethylene as the binder, and 6 parts by mass of acetylene black as the conductive material were dispersed in water to prepare a positive electrode mixture slurry. The obtained positive electrode mixture slurry was applied to both sides of aluminum foil, and the coating was dried and rolled to form a positive electrode mixture layer, thereby obtaining a positive electrode. A 10 mm wide positive electrode current collector exposed portion was formed at the longitudinal end of the positive electrode current collector.

[0098] (2) Preparation of the negative electrode An 8 μm thick copper foil (negative electrode current collector) was prepared. The copper foil prepared had no through holes. 94 parts by mass of non-graphitizable carbon (average particle size 5 μm), 1 part by mass of carboxycellulose, and 5 parts by mass of carbon black were dispersed in water to prepare a negative electrode mixture slurry. The carbon black had a BET specific surface area of ​​800 m 2 The resulting negative electrode mixture slurry was applied to both sides of a copper foil, and the coating was dried and rolled to form a negative electrode mixture layer, resulting in a negative electrode. The thickness of the negative electrode mixture layer was 32 μm on each side.

[0099] The BET surface area of ​​the negative electrode mixture layer was measured using the method described above. 2 / g.

[0100] The negative electrode was then placed in a battery container filled with a lithium-ion conductive electrolyte. Similarly, a SUS metal plate carrying metallic lithium was placed in the battery container as the working electrode. A separator was placed between the negative electrode and the working electrode, and a voltage was applied between the negative electrode and the working electrode. The working electrode was used as the positive electrode, and the cell was charged at a constant current (CC) of 0.1 mA until the cell voltage reached 0.01 V. It was then charged at a constant voltage (CV) for 5 hours to perform pre-doping. The electrolyte solvent was a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a volume ratio of 1:2:7, to which 1% by mass of vinylene carbonate (VC) was added. LiPF6 was added as a lithium salt to the mixed solvent at a concentration of 1.2 mol / L to form the electrolyte. After pre-doping was completed, the negative electrode was washed with dimethyl carbonate (DEC) to obtain a negative electrode pre-doped with lithium ions.

[0101] (3) Preparation of the electrode body The positive and negative electrodes were wound into a cylindrical shape with a cellulose nonwoven separator (25 μm thick) sandwiched between them to form an electrode assembly. The exposed portion of the positive electrode current collector protruded from one end of the wound assembly, and the exposed portion of the negative electrode current collector protruded from the other end of the electrode assembly. A disk-shaped positive electrode current collector and a disk-shaped negative electrode current collector were welded to the exposed portion of the positive electrode current collector and the exposed portion of the negative electrode current collector, respectively.

[0102] (4) Preparation of non-aqueous electrolyte A solvent was prepared by adding 0.2 mass% of vinylene carbonate to a 1:1 volumetric mixture of propylene carbonate and dimethyl carbonate. LiFSI (a lithium salt) was dissolved in the solvent to a concentration of 1.2 mol / L to prepare a nonaqueous electrolyte.

[0103] (5) Assembly of electrochemical devices The electrode assembly was housed in a cell case with an opening and a bottom, and a tab lead connected to the positive current collector was connected to the inner surface of the sealing plate. Furthermore, a negative current collector was welded to the inner bottom surface of the cell case. After pouring a nonaqueous electrolyte into the cell case, the opening of the cell case was sealed with a sealing plate to assemble an electrochemical device as shown in Figure 1.

[0104] Thereafter, aging was performed at 60° C. while a charging voltage of 3.8 V was applied between the terminals of the positive electrode and the negative electrode to complete pre-doping of lithium ions into the negative electrode.

[0105] (6) Evaluation [Evaluation 1: Measurement of capacity (energy density) of electrochemical devices] The initial charge and discharge was carried out under the following conditions. Immediately after aging, the electrochemical device was subjected to a current of 2 mA / cm per positive electrode area in an environment of 25°C until the voltage reached 3.8 V. 2 After that, a constant current charge of 3.8 V was applied and maintained for 10 minutes. Thereafter, the battery was charged at a constant current density of 2 mA / cm per positive electrode area until the voltage reached 2.2 V in a 25°C environment. 2 A constant current discharge was performed at a current density of 1000 kJ / s. During the discharge, the time t (sec) required for the voltage to drop from 3.3 V to 3.0 V was measured. The measured time t was used to calculate the initial capacity C1 of the electrochemical device according to the following formula (A).

[0106] Capacity C1=Id×t / V (A)

[0107] In the formula (A), Id is the current value during discharge (current density per positive electrode area: 2 mA / cm 2 × positive electrode area), and V is the value obtained by subtracting 3.0 V from 3.3 V (0.3 V).

[0108] Based on the capacity C1, the energy density E (Wh / L) of the electrochemical device was evaluated using the following formula (B): In formula (B), V1 is the upper limit voltage (3.8 V) during charge / discharge, and V2 is the lower limit voltage (2.2 V). CELL is the volume occupied by the device. E=(1 / 2)×C1×(V1 2 -V2 2 ) / (3600×V CELL ) (B)

[0109] [Evaluation 2: Measurement of power density of electrochemical devices] Next, using the discharge curve (vertical axis: discharge voltage, horizontal axis: discharge time) obtained from the above discharge, a linear approximation line was calculated for the range of 0.5 to 2 seconds after the start of discharge of the discharge curve, and the voltage at the intercept of the approximation line (voltage at 0 seconds after the start of discharge) Vs was calculated. The value (V0-Vs) obtained by subtracting the voltage Vs from the voltage V0 at the start of discharge (0 seconds after the start of discharge) was used as the voltage drop ΔV at the start of discharge. ΔV and the current value during discharge (current density per positive electrode area: 2 mA / cm 2 × positive electrode area) to obtain the following formula ( C The internal resistance (DCR) R1 (Ω) of the electrochemical device was calculated from the above data.

[0110] Internal resistance R1=ΔV / Id ( C )

[0111] Based on R1, the power density P (kW / L) of the electrochemical device is calculated using the following formula: D ) was evaluated. P = (V1-V2) × V2 / (1000 × R1 × V CELL ) ··( D )

[0112] [Evaluation 3: Measurement of cycle characteristics of electrochemical devices] Charging as below - 1 and discharge - 1 was repeated 100,000 times. 100,000th discharge - The capacity C after the cycle test of the electrochemical device was calculated based on the above formula (A) in the same manner as in deriving the initial capacity C1 from the time change of the voltage at 1. 100000 asked for.

[0113] (Charge-1) For electrochemical devices, the current is 2mA / cm per positive electrode area until the voltage reaches 3.8V in an environment of 25°C. 2 Constant current charging was performed at a current density of It After that, a voltage of 3.8 V was applied and maintained for 10 minutes. did . (discharge-1) Then, in an environment of 25°C, the current was 2 mA / cm per positive electrode area until the voltage reached 2.2 V. 2A constant current discharge was performed at a current density of It .

[0114] Capacity C 100000 The change ΔC from the initial capacitance C1 is calculated using the following formula ( E The cycle characteristics were evaluated by the following method. The evaluation results are shown in Table 1. ΔC=((C 100000 / C1)-1)×100 ( E )

[0115] (Devices A2 to A7, B1, B2) The thickness of the copper foil serving as the negative electrode current collector, the content of the conductive additive (carbon black) in the negative electrode mixture slurry, and the specific surface area and thickness per side of the negative electrode mixture layer were changed as shown in Table 1. When the content of the conductive additive in the negative electrode mixture slurry was changed, the content of the carboxycellulose binder was kept constant, and the content of the non-graphitizable carbon negative electrode active material was decreased (increased) as the content of the conductive additive increased (decreased). Other than this, devices A2 to A7, B1, and B2 were assembled in the same manner as device A1 and evaluated in the same manner. The results are shown in Table 1.

[0116] (Devices B3 and B4) As a negative electrode current collector, a perforated copper foil (opening ratio: 23%) with openings having a diameter of 0.075 mm was prepared.

[0117] In producing the electrode body, the positive electrode and the negative electrode that had not been pre-doped were wound into a columnar shape with a separator (thickness: 25 μm) made of cellulose nonwoven fabric interposed therebetween to form the electrode body. The electrode assembly was housed in a bottomed cell case with an opening, and the electrode assembly, along with lithium pieces, was housed therein. A tab lead connected to the positive electrode current collector was connected to the inner surface of the sealing plate, and a negative electrode current collector was welded to the inner bottom surface of the cell case. After placing a non-aqueous electrolyte in the cell case, the opening of the cell case was sealed with a sealing plate, and an electrochemical device was assembled as shown in Figure 1. 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 0.2 V or less relative to metallic lithium.

[0118] Devices B3 and B4 were assembled and evaluated in the same manner as Device A1, except that the thickness of the negative electrode current collector in Device B3 was 8 μm, and the thickness of the negative electrode current collector in Device B4 was 20 μm. The evaluation results are shown in Table 1.

[0119] For each of Devices A1 to A7 and B1 to B4, the device was disassembled after the initial charge / discharge cycle to remove the negative electrode. A half-cell was then prepared using a lithium metal foil reference electrode and the negative electrode, which was then brought into contact with a non-aqueous electrolyte to measure the potential of the negative electrode relative to the reference voltage. As a result, the potential of the negative electrode was 0.2 V or less for all of Devices A1 to A7 and B1 to B4.

[0120] [Table 1]

[0121] Comparing devices A1 to A7, B1, and B2, a negative electrode current collector without through-holes was used, and the specific surface area of ​​the negative electrode mixture layer was set to 30 m 2 / g or more 60m 2 In the devices A1 to A7, in which the valence ratio was in the range of 0.1 to 0.2g or less, high energy density and high power density were maintained, and the cycle characteristics were also high.

[0122] In Devices B3 and B4, the strength of the negative electrode current collector was low due to the use of perforated foil as the negative electrode current collector. In Device B3, where the negative electrode current collector was 8 μm thick, the negative electrode broke during the winding process, making it impossible to obtain an electrode assembly. In Device B3, where the negative electrode current collector was 20 μm thick, it was possible to fabricate an electrode assembly, but both the energy density and high power density were reduced.

[0123] An electrochemical device was also fabricated in the same manner as Device A1, except that graphite was used as the negative electrode active material instead of non-graphitizable carbon (hard carbon). In this case, the expansion and contraction during charge-discharge cycles was greater than that of hard carbon, resulting in reduced reliability. [Industrial Applicability]

[0124] The electrochemical device according to the present invention is suitable for use in a vehicle, for example. [Explanation of symbols]

[0125] 100: Electrode body 10: Positive electrode 11x: Positive electrode current collector exposed part 13: Positive 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. An electrode assembly including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode. and, an electrolyte including a lithium salt; the negative electrode includes a negative electrode current collector and a negative electrode mixture layer supported on the negative electrode current collector, the negative electrode mixture layer contains non-graphitizable carbon as a negative electrode active material that is reversibly doped with lithium ions, the negative electrode current collector is substantially free of through holes, The specific surface area of ​​the negative electrode mixture layer is 30 m ２ / g or more, 60m ２ / g or less, In a discharged state, the potential of the negative electrode is 0.2 V or less relative to a Li counter electrode; an electrochemical device, wherein a surface layer portion of the negative electrode mixture layer has a first layer containing lithium carbonate and a second layer containing a solid electrolyte that is a reaction product of the electrolyte, and at least a portion of the first layer is covered with the second layer.

2. the second layer comprises lithium carbonate; 2. The electrochemical device according to claim 1, wherein the second layer contains less lithium carbonate than the first layer.

3. the thickness of the first layer is 1 nm or more and 50 nm or less; 3. The electrochemical device according to claim 1, wherein the second layer has a thickness of 1 nm or more and 20 nm or less.

4. 4. The electrochemical device according to claim 1, wherein a ratio A / B of a thickness A of the first layer to a thickness B of the second layer is 0.1 or more and 1 or less.

5. When the surface layer portion was analyzed in the depth direction by X-ray photoelectron spectroscopy, the first layer has, in an O1s spectrum, a first region in which a peak intensity of a first peak attributed to a C═O bond is greater than a peak intensity of a second peak attributed to a Li—O bond, and a second region in which the peak intensity of the first peak is smaller than the peak intensity of the second peak; 5. The electrochemical device according to claim 1, wherein the second region is located deeper than the first region.

6. 6. The electrochemical device according to claim 5, further comprising a third region in which the first peak is observed but the second peak is not observed, the third region being closer to the outermost surface of the surface layer portion than the first region.

7. the negative electrode mixture layer contains a conductive additive, 7. The electrochemical device according to claim 1, wherein the content of the conductive additive in the negative electrode mixture layer is 3% by mass or more and 15% by mass or less.

8. the negative electrode mixture layer contains a conductive additive, The specific surface area of ​​the conductive additive is 800 m ２ The electrochemical device according to any one of claims 1 to 7, wherein the surface area of ​​the electrochemical device is 1 / g or more.

9. The electrochemical device according to claim 7 or 8, wherein the conductive additive contains carbon black. vinegar.

10. The thickness of the negative electrode current collector is 15 μm or less, The electrode according to any one of claims 1 to 9, wherein the thickness of the negative electrode mixture layer is 25 µm or more. Gas-chemical devices.

11. The electrode body is formed by winding the strip-shaped positive electrode and the strip-shaped negative electrode with the separator interposed therebetween. The electrochemical device according to any one of claims 1 to 10, which is a columnar wound body.

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