Electrochemical device

Abstract

An electrochemical device includes a positive electrode, a negative electrode, and a lithium ion-conductive electrolyte, the positive electrode includes a positive electrode current collector and a positive electrode mixture layer supported on the positive electrode current collector, the positive electrode mixture layer includes a positive electrode active material that reversibly dopes anions, 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 includes a negative electrode active material that reversibly dopes lithium ions, the negative electrode active material includes a non-graphitizable carbon, and a ratio of a mass Mp of the positive electrode active material supported per unit area on the positive electrode to a mass Mn of the negative electrode active material supported per unit area on the negative electrode: Mp / Mn is 1.1 or more and 2.5 or less.

Description

TECHNICAL FIELD

[0001] The present application relates to an electrochemical device. BACKGROUND

[0002] In recent years, an electrochemical device combining the power storage principle of a lithium ion secondary battery and that of an electric double layer capacitor has attracted attention. Such an electrochemical device generally uses a polarizable electrode as a positive electrode and a non-polarizable electrode as a negative electrode. As a result, the electrochemical device is expected to have both the high energy density of a lithium ion secondary battery and the high output characteristics of an electric double layer capacitor.

[0003] Patent Literature 1 proposes a lithium ion capacitor including a positive electrode, a negative electrode, and an electrolyte, which has a non-protic organic solvent electrolyte solution of a lithium salt as the electrolyte, wherein the positive electrode active material is a material capable of doping / dedoping lithium ions and / or anions, the negative electrode active material is a material capable of doping / dedoping lithium ions, lithium ions are doped in the negative electrode and / or the positive electrode in such a manner that the potential of the positive electrode after short-circuiting of the positive electrode and the negative electrode becomes 2 V or less (vs. Li / Li+), the positive electrode layer in the above positive electrode is formed on both surfaces of a current collector with the same thickness, and the total thickness of the positive electrode layer is 18 to 108 μm, and the total weight per unit area of the above positive electrode active material is 1.5 to 4.0 mg / cm 2 .

[0004] Patent Literature 2 proposes a lithium ion capacitor characterized by including a positive electrode, a negative electrode, and an electrolyte, which has a non-protic organic solvent electrolyte solution of a lithium salt as the electrolyte, wherein the positive electrode active material is a material capable of reversibly carrying lithium ions and / or anions, the negative electrode active material is a material capable of reversibly carrying lithium ions, lithium ions are doped in the negative electrode and / or the positive electrode before charging in such a manner that the potential of the positive electrode after short-circuiting of the positive electrode and the negative electrode becomes 2.0 V or less, and the above negative electrode active material is a heat-treated product of a carbon material precursor in the presence of a substance containing a transition metal.

[0005] Patent Literature 3 proposes an electrochemical capacitor including: an element, an electrolyte containing lithium ions, and a housing body that houses the above element and the above electrolyte, the above element being formed of: a negative electrode in which a negative electrode electrode layer containing a carbon material in which lithium ions are stored is formed on the surface of a current collector, a positive electrode in which a positive electrode electrode layer in which ions are adsorbed is formed on the surface of a current collector, and a separator sandwiched between the above negative electrode and the above positive electrode, a film containing lithium carbonate being formed on the surface of the above carbon material contained in the above negative electrode electrode layer.

[0006] PRIOR ART DOCUMENTS

[0007] PATENT LITERATURE

[0008] Patent Document 1: Japanese Patent No. 4971729

[0009] Patent Document 2: Japanese Patent Application Laid-Open No. 2006-310412

[0010] Patent Document 3: International Publication No. 2011 / 58748 SUMMARY

[0011] However, in the electrochemical device as described above, high capacity and high durability are in a trade-off relationship, and further improvement is required.

[0012] One aspect of the present application relates to an electrochemical device including a positive electrode, a negative electrode, and an electrolyte that conducts lithium ions, the positive electrode including a positive electrode current collector, and a positive electrode mixture layer supported on the positive electrode current collector, the positive electrode mixture layer including a positive electrode active material that reversibly dopes anions, the negative electrode including a negative electrode current collector, and a negative electrode mixture layer supported on the negative electrode current collector, the negative electrode mixture layer including a negative electrode active material that reversibly dopes lithium ions, the negative electrode active material including hard graphitized carbon, and a ratio of a mass Mp of the positive electrode active material supported per unit area of the positive electrode to a mass Mn of the negative electrode active material supported per unit area of the negative electrode, Mp / Mn, is 1.1 or more and 2.5 or less.

[0013] According to the present application, an electrochemical device that balances high capacity and high durability can be provided. BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1 is a perspective view of a part of the electrochemical device of one embodiment of the present application after a cutout is made. DETAILED DESCRIPTION

[0015] The electrochemical device of one embodiment of the present application includes a positive electrode, a negative electrode, and an electrolyte that conducts lithium ions. Generally, the positive electrode and the negative electrode, together with a separator interposed therebetween, form an electrode body. The electrode body is formed in a cylindrical shape, for example, in which the positive electrode and the negative electrode each in a strip shape are wound with the separator interposed therebetween. Alternatively, the electrode body can be formed in a laminate shape in which the positive electrode and the negative electrode each in a sheet shape are stacked with the separator interposed therebetween.

[0016] The positive electrode includes a positive electrode current collector and a positive electrode mixture layer supported on the positive electrode current collector. The positive electrode mixture layer includes a positive electrode active material that reversibly dopes anions. If anions are adsorbed to the positive electrode active material, an electric double layer is formed, and capacity is exhibited. The positive electrode can be a polarizable electrode, or an electrode that has the properties of a polarizable electrode and in which a Faradaic reaction also contributes to capacity.

[0017] The positive electrode active material can be a carbon material or a conductive polymer. The doping of anions into the positive electrode active material refers to at least the concept of adsorption of anions into the positive electrode active material, and can further include the concept of occlusion of anions into the positive electrode active material, chemical interaction between the positive electrode active material and anions, and the like.

[0018] 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, and the negative electrode active material contains a non-graphitizable carbon.

[0019] The non-graphitizable carbon exhibits a capacity by a Faraday reaction in which lithium ions are reversibly occluded and released. The doping of lithium ions into the negative electrode active material refers to at least the concept of occlusion of lithium ions into the negative electrode active material, and can further include the concept of adsorption of lithium ions into the negative electrode active material, chemical interaction between the negative electrode active material and lithium ions, and the like.

[0020] Hereinafter, the positive electrode and the negative electrode are sometimes collectively referred to as an electrode. In addition, the positive electrode current collector and the negative electrode current collector are sometimes collectively referred to as a current collector (or an electrode current collector). In addition, the positive electrode mixture layer and the negative electrode mixture layer are sometimes collectively referred to as a mixture layer (or an electrode mixture layer). In addition, the positive electrode active material and the negative electrode active material are sometimes collectively referred to as an active material (or an electrode active material).

[0021] The ratio of the mass Mp of the positive electrode active material supported per unit area of the positive electrode to the mass Mn of the negative electrode active material supported per unit area of the negative electrode: Mp / Mn is 1.1 or greater and 2.5 or less, preferably 1.4 or greater and 1.8 or less, and more preferably 1.5 or greater and 1.8 or less. The above-described electrochemical device having the above-described Mp / Mn ratio can achieve a high capacity. When the Mp / Mn ratio is less than 1.1, a decrease in the static capacity of the electrochemical device becomes significant. On the other hand, when the Mp / Mn ratio is 1.1 or greater, further 1.4 or greater, and in particular 1.5 or greater, a high static capacity can be obtained. However, when the Mp / Mn ratio exceeds 2.5, the resistance (DCR) of the electrochemical device at low temperatures (hereinafter referred to as low-temperature DCR) excessively increases. On the other hand, when the Mp / Mn is 2.5 or less, further 1.8 or less, a high static capacity can be obtained, and an excessive increase in the low-temperature DCR can be suppressed, and an electrochemical device having a balance of characteristics that is excellent can be obtained.

[0022] The mass Mp and the mass Mn of the electrode active material supported per unit area of the electrode are each represented by the following formula.

[0023] Mp = (mass of the positive electrode - mass of the positive electrode current collector) x mass ratio of the positive electrode active material ÷ positive electrode area

[0024] Mn = (mass of negative electrode - mass of negative electrode current collector) x mass ratio of negative electrode active material ÷ negative electrode area

[0025] Here, the mass ratio of the positive electrode active material refers to the ratio of the mass of the positive electrode active material contained in the positive electrode mixture layer when the mass of the positive electrode mixture layer is taken as 1. Similarly, the mass ratio of the negative electrode active material refers to the ratio of the mass of the negative electrode active material contained in the negative electrode mixture layer when the mass of the negative electrode mixture layer is taken as 1. In addition, the positive electrode area refers to the area of the projection drawing when the positive electrode is orthogonally projected from the main surface side of the positive electrode, and the negative electrode area refers to the area of the projection drawing when the negative electrode is orthogonally projected from the main surface side of the negative electrode.

[0026] Note that, as the samples of the positive electrode and the negative electrode for calculating Mp and Mn, portions that are uniform in the thickness direction of the electrode are cut out from the electrode and used, respectively. For example, an electrode portion having a portion of the current collector exposed is not used as a sample. In addition, an electrode portion in which portions provided with the electrode mixture layer on both surfaces and one surface of the current collector are mixed is not used as a sample.

[0027] From the viewpoint of obtaining an electrochemical device having a high capacity density, the mass Mp of the positive electrode active material supported per unit area of the positive electrode is, for example, 3.6 mg / cm 2 or more and 4.5 mg / cm 2 or more, it can also be 3.9 mg / cm 2 or more and 4.2 mg / cm 2 or more. From the same viewpoint, the mass Mn of the negative electrode active material supported per unit area of the negative electrode is, for example, 1.8 mg / cm 2 or more and 3.2 mg / cm 2 or more, it can also be 2.3 mg / cm 2 or more and 2.8 mg / cm 2 or more. Note that, in the case where the electrode mixture layer is provided on both surfaces of the current collector, the mass of the active material supported per unit area of the electrode is calculated from the total amount of the active material on both surfaces of the current collector having a size of the unit area, in the manner derived from the definition of the electrode area.

[0028] Next, the specific surface area of the negative electrode mixture layer can be, for example, 10 m 2 / g or more and 70 m 2 / g or less. There is a tendency that the larger Mp / Mn is, the larger the low-temperature DCR becomes, and by setting the specific surface area of the negative electrode mixture layer to 10 m 2 / g or more, further 25 m 2 / g or more, the increase in the low-temperature DCR can be significantly suppressed. That is, by setting the specific surface area to 10 m2 / g or more, and further 25 m 2 / g or more, so that a large Mp / Mn ratio is easily selected, and a high electrostatic capacity can be easily achieved. In addition, in a case where the specific surface area of the negative electrode mixture layer is set to 70 m 2 / g or less, and further 50 m 2 / g or less, the deterioration of the negative electrode is easily suppressed, and the durability of the electrochemical device is improved. Here, as for the deterioration of the negative electrode, typically, an increase rate of a low-temperature DCR of the electrochemical device at the time of float charging in which a constant voltage is applied to the electrochemical device using an external direct-current power source at a high temperature can be evaluated. The increase rate of the low-temperature DCR refers to a proportion of a difference (ΔDCR) between the initial and the low-temperature DCR after the float charging with respect to the initial low-temperature DCR. It is considered that the smaller the increase rate of the low-temperature DCR, the smaller the deterioration of the negative electrode.

[0029] The specific surface area of the negative electrode mixture layer is a BET specific surface area obtained using a measuring device according to JIS Z8830 (for example, TriStar II 3020 manufactured by Shimadzu Corporation). Specifically, the electrochemical device is disassembled, and the negative electrode is taken out. Using the negative electrode as a working electrode, and using a Li metal foil as a counter electrode, a half-cell is assembled, and Li is de-doped in the negative electrode until the potential of the negative electrode reaches 1.5 V. Next, the negative electrode after the de-doping of Li is washed with dimethyl carbonate (DMC), and is dried. Then, the negative electrode mixture layer is peeled from the negative electrode current collector, and a sample of about 0.5 g of the negative electrode mixture layer is collected.

[0030] Next, the collected sample is heated at 150°C for 12 hours under a reduced pressure of 95 kPa or less, and then nitrogen is adsorbed to the sample of which the mass is known, and an adsorption isotherm is obtained in a range of a relative pressure of 0 to 1. Then, the surface area of the sample is calculated from the monolayer adsorption amount of gas obtained from the adsorption isotherm. Here, the specific surface area is obtained by the BET single-point method (relative pressure 0.3) and according to the following BET equation.

[0031] P / V (P0-P) = (1 / VmC) + {(C-1) / VmC} (P / P0) · · (1)

[0032] S = kVm · · (2)

[0033] P0: Saturated vapor pressure

[0034] P: Adsorption equilibrium pressure

[0035] V: Adsorption amount at adsorption equilibrium pressure P

[0036] Vm: Monolayer adsorption amount

[0037] C: Parameter related to adsorption heat or the like

[0038] S: specific surface area

[0039] k: nitrogen monolayer occupation area 0.162 nm 2

[0040] Next, the surface layer portion of the negative electrode mixture layer can have a first layer containing lithium carbonate as a constituent element of the coating film. The first layer is mainly formed on the surface of the negative electrode active material. The more the specific surface area of the negative electrode mixture layer is increased, the more the negative electrode is likely to deteriorate, and by forming the first layer, the deterioration of the negative electrode can be significantly inhibited.

[0041] The surface layer portion of the negative electrode can have a second layer containing a solid electrolyte as a constituent element of the coating film. The second layer has a different composition from the first layer, and the second layer can be distinguished from the first layer. In an electrochemical device using lithium ions, a solid electrolyte interface film (i.e., SEI film) is formed on the negative electrode mixture layer at the time of charge and discharge. The second layer can be formed in the form of an SEI film. The SEI film plays an important role in the charge and discharge reaction, but if the SEI film is formed in an excessively thick thickness, the deterioration of the negative electrode becomes large. In contrast, the first layer containing lithium carbonate promotes the formation of a good SEI film, and has the effect of maintaining the SEI film in a good state even in the case where charge and discharge are repeatedly performed. Thus, by forming the first layer on the surface layer portion of the negative electrode mixture layer, even in the case where the specific surface area of the negative electrode mixture layer is increased in order to inhibit the increase in low-temperature DCR, the deterioration of the negative electrode can be significantly inhibited.

[0042] In the case where the coating film has the first layer and the second layer, at least a part of the second layer covers at least a part of the surface of the negative electrode active material with the first layer interposed therebetween. That is, at least a part 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 becomes a base layer of the second layer. By making the first layer a base layer, the second layer is formed in the form of a good SEI film.

[0043] The second layer can also contain lithium carbonate. In the case where the second layer contains lithium carbonate, the content of lithium carbonate contained in the second layer is less than the content of lithium carbonate contained in the first layer. The first layer containing a large amount of lithium carbonate as a base layer is a necessary condition for the second layer to be formed in the form of a good SEI film.

[0044] The first layer is formed on the surface layer portion of the negative electrode mixture layer before assembling the electrochemical device. In the electrochemical device assembled using the negative electrode, a second layer (SEI film) that is homogeneous and has a moderate thickness is formed on the surface of the negative electrode active material through the subsequent charge and discharge. The SEI film is formed, for example, as a result of a reaction between the electrolyte and the negative electrode in the electrochemical device. The electrolyte can pass not only through the second layer but also through the first layer, and thus the surface layer portion including the first layer and the second layer as a whole can be referred to as the SEI film, but in the present specification, the second layer is referred to as the SEI film in order to distinguish it from the first layer for convenience.

[0045] The presence of the first layer, which is a region containing lithium carbonate, can be confirmed, for example, by analysis of the surface layer portion based on X-ray photoelectron spectroscopy (XPS). However, the analysis method is not limited to XPS.

[0046] The thickness of the first layer is, for example, 1 nm or more, and in the case where a longer-term effect is expected, 5 nm or more, and in the case where a more reliable effect is expected, 10 nm or more. However, when the thickness of the first layer exceeds 50 nm, the first layer itself becomes a resistance component. Thus, the thickness of the first layer can be 50 nm or less, and can be 30 nm or less.

[0047] The thickness of the second layer is, for example, 1 nm or more, and can be 3 nm or more, and 5 nm or more is sufficient. However, when the thickness of the second layer exceeds 20 nm, the second layer itself becomes a resistance component. Thus, the thickness of the second layer can be 20 nm or less, and can be 10 nm or less.

[0048] From the viewpoint of reducing the initial low-temperature DCR, 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. At this time, the thickness of the second layer is preferably 20 nm or less, and can be 10 nm or less. However, from the viewpoint of forming a second layer in a good state, A / B is preferably 0.1 or more, and for example, the ratio A / B can be 0.2 or more.

[0049] The thicknesses of the first layer and the second layer are measured by analyzing the surface layer portion of the negative electrode mixture layer at a plurality of portions (at least 5 portions) of the negative electrode mixture layer. Furthermore, the average of the thicknesses of the first layer or the second layer obtained at the plurality of portions can be the thickness of the first layer or the second layer. Note that the negative electrode mixture layer for the measurement sample can be peeled from the negative electrode current collector. In this case, the film formed on the surface of the negative electrode active material in the vicinity of the surface layer portion constituting the negative electrode mixture layer can be analyzed. Specifically, the negative electrode active material covered with the film can be collected from a region of the negative electrode mixture layer disposed on the side opposite to the side bonded to the negative electrode current collector and used for analysis.

[0050] For the XPS analysis of the surface layer portion of the negative electrode mixture layer, for example, a film formed on the surface layer portion or the surface of the negative electrode active material is irradiated with an argon beam in the chamber of an X-ray photoelectron spectrometer, and the change in each spectrum attributed to C1s, O1s electrons, etc. with respect to the irradiation time is observed and recorded. At this time, from the viewpoint of avoiding analysis errors, the spectrum of the outermost surface of the surface layer portion can be ignored. The thickness of the region in which the peak attributed to the lithium carbonate bond is stably observed corresponds to the thickness of the first layer.

[0051] In the case of the negative electrode taken out from the electrochemical device after a prescribed aging or at least one charge-discharge after completion, the surface layer portion of the negative electrode mixture layer has an SEI film (i.e., the second layer) containing a solid electrolyte. The thickness of the region in which the peak attributed to the bond possessed by the compound contained in the SEI film is stably observed corresponds to the thickness of the SEI film (i.e., the thickness of the second layer).

[0052] As the compound contained in the SEI film, a compound containing an element capable of being a marker of the second layer is selected. The element capable of being a marker of the second layer can be selected, for example, from an element contained in the electrolyte and substantially not contained in the first layer (e.g., F). As the compound containing an element capable of being a marker of the second layer, for example, LiF can be selected.

[0053] When the second layer contains LiF, if the second layer is measured by X-ray photoelectron spectroscopy, a peak attributed to the substantial F1s of the LiF bond is observed. In this case, the thickness of the region in which the peak attributed to the LiF bond is stably observed corresponds to the thickness of the second layer.

[0054] On the other hand, the first layer generally does not contain LiF, and even if the first layer is measured by X-ray photoelectron spectroscopy, a peak attributed to the substantial F1s of the LiF bond cannot be observed. Thus, the thickness of the region in which the peak attributed to the LiF bond cannot be stably observed can be taken as the thickness of the first layer.

[0055] The O1s peak attributed to lithium carbonate can also be observed in the SEI film. However, the SEI film generated in the electrochemical device has a different composition from the first layer formed in advance, and thus, the two can be distinguished. For example, in the XPS analysis of the SEI film, the F1s peak attributed to the LiF bond is observed, but the substantial F1s peak attributed to the LiF bond cannot be observed in the first layer. In addition, the lithium carbonate contained in the SEI film is in a trace amount. Note that, as the Li1s peak, for example, a peak from a compound such as ROCO2Li or ROLi can be detected.

[0056] When the first layer is analyzed by XPS, a second peak of Ols attributable to a Li-O bond can be observed in addition to a first peak of Ols attributable to a C=O bond. The region of the film present in the vicinity of the surface of the negative electrode active material can contain a small amount of LiOH or Li2O.

[0057] Specifically, when the first layer constituting the surface layer portion of the negative electrode active material layer is analyzed in the depth direction, in order of increasing distance from the surface of the surface layer portion, a first region in which a first peak (Ols attributable to a C=O bond) and a second peak (Ols attributable to a Li-O bond) are observed and 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 can be observed. In addition, a third region can further exist, the third region being closer to the surface of the surface layer portion than the first region, and the first peak being observed and the second peak not being observed in the third region. The third region is easily observed in the case where the thickness of the lithium carbonate-containing region is large.

[0058] Note that the magnitude of the peak intensity can be determined based on the height of the peak from the baseline.

[0059] In the center of the thickness direction of the first layer, the Cis peak attributable to a C-C bond is not substantially observed, and even if observed, the peak intensity is less than half of that attributable to a C=O bond.

[0060] Next, a method of forming the first layer containing lithium carbonate in the surface layer portion of the negative electrode active material layer will be described. The step of forming the first layer can be performed, for example, by a vapor phase method, a coating method, transfer, or the like.

[0061] As the vapor phase method, a chemical vapor deposition method, a physical vapor deposition method, a sputtering method, or the like can be given. For example, lithium carbonate can be attached to the surface of the negative electrode active material layer by a vacuum deposition device. The pressure in the chamber of the device at the time of deposition can be, for example, 10 -2 ~ 10 -5 Pa, the temperature of the lithium carbonate evaporation source can be 400 to 600°C, and the temperature of the negative electrode active material layer can be -20 to 80°C.

[0062] As the coating method, a solution or dispersion liquid containing lithium carbonate can be coated on the surface of the negative electrode and dried by using, for example, a micro gravure coater, whereby the first layer can be formed. The content of lithium carbonate in the solution or dispersion liquid can be, for example, 0.3 to 2 mass%, and in the case of using a solution, the concentration can be below the solubility (for example, about 0.9 to 1.3 mass% if it is an aqueous solution at room temperature).

[0063] Furthermore, a negative electrode can be obtained by performing a process of forming a second layer containing a solid electrolyte in such a way that it covers at least a portion of the first layer. The surface portion of the resulting 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 entire surface) of the negative electrode active material, with the first layer in between (i.e., the first layer serving as a substrate layer).

[0064] The process of forming the second layer is carried out while the negative electrode binder layer is in contact with the electrolyte; therefore, it can also serve as at least part of the pre-doping process of lithium ions in the negative electrode binder layer. For example, metallic lithium can be used as the lithium ion source for pre-doping.

[0065] Lithium metal can be attached to the surface of the negative electrode flux layer. It should be noted that by exposing the negative electrode with the lithium metal flux layer to a carbon dioxide atmosphere, a first layer containing lithium carbonate with a thickness of, for example, 1 nm or more and 50 nm or less can also be formed.

[0066] The process of attaching metallic lithium to the surface of the negative electrode binder layer can be performed, for example, by vapor phase deposition or transfer. Examples of vapor phase deposition methods include chemical vapor deposition, physical vapor deposition, and sputtering. For instance, metallic lithium can be formed as a film on the surface of the negative electrode binder layer using a vacuum vapor deposition apparatus. The pressure within the apparatus chamber during vapor deposition is, for example, set to 10... -2 ~10 -5 Pa is sufficient, the temperature of the lithium evaporation source is 400-600℃, and the temperature of the negative electrode binder layer is -20-80℃.

[0067] The carbon dioxide atmosphere is preferably a dry atmosphere free of moisture, for example, with a dew point below -40°C or -50°C. The carbon dioxide atmosphere may contain gases other than carbon dioxide, but the mole fraction of carbon dioxide is preferably 80% or more, more preferably 95% or more. Preferably, it contains no oxidizing gases, and the mole fraction of oxygen is set to 0.1% or less.

[0068] In order to form a thicker first layer, the partial pressure of carbon dioxide needs to be greater than, for example, 0.5 atmospheres (5.05 × 10⁻⁶). 4 Pa) is effective and can be applied to 1 atmosphere (1.01 × 10⁻⁶). 5 Pa) or above.

[0069] The temperature of the negative electrode exposed to the carbon dioxide atmosphere can be, for example, in the range of 15°C to 120°C. The higher the temperature, the thicker the first layer becomes.

[0070] The thickness of the first layer can be easily controlled by varying the exposure time of the negative electrode to the carbon dioxide atmosphere. An exposure time of 12 hours or more, or less than 10 days, is acceptable.

[0071] The process of forming the first layer is preferably performed before forming the electrode body, but it is not excluded that it may be performed after forming the electrode body. That is, a positive electrode can be prepared, a negative electrode with a negative electrode mixture layer with attached lithium metal can be prepared, a spacer can be sandwiched between the positive electrode and the negative electrode to form the electrode body, the electrode body can be exposed to a carbon dioxide gas atmosphere, and the first layer can be formed on the surface of the negative electrode mixture layer.

[0072] It should be noted that the pre-doping process of lithium ions into the negative electrode flux layer is carried out, for example, by subsequently contacting the negative electrode flux layer with the electrolyte and then stopping it for a specified time. Such a process can be a process of forming a second layer in a manner that covers at least a portion of the first layer. For example, by performing at least one charge-discharge cycle on the electrochemical device, a second layer can be formed in the negative electrode flux layer, thus ending the pre-doping of lithium ions into the negative electrode. Alternatively, for example, the pre-doping of lithium ions into the negative electrode can also be ended by applying a specified charging voltage (e.g., 3.4–4.0V) between the terminals of the positive and negative electrodes for a specified time (e.g., 1–75 hours).

[0073] Figure 1 The configuration of an electrochemical device 200 according to one embodiment of the present invention is schematically shown. The electrochemical device 200 includes an electrode body 100, a non-aqueous electrolyte (not shown), a bottomed metal battery casing 210 housing the electrode body 100 and the non-aqueous electrolyte, and a sealing plate 220 sealing the opening of the battery casing 210. A sealing gasket 221 is disposed at the periphery of the sealing plate 220, and the interior of the battery casing 210 is sealed by riveting the sealing gasket 221 to the opening end of the battery casing 210. A positive electrode current collector 13 having a through hole 13h in the center is welded to the exposed portion 11x of the positive electrode current collector. One end of a tab lead 15 connected to the positive electrode current collector 13 is connected to the inner surface of the sealing plate 220. Thus, the sealing plate 220 functions as an external positive terminal. On the other hand, a negative electrode current collector 23 is welded to the exposed portion 21x of the negative electrode current collector. The negative electrode current collector 23 is directly welded to the welding component disposed on the inner bottom surface of the battery casing 210. Thus, the battery casing 210 functions as an external negative terminal.

[0074] The constituent elements of the electrochemical device according to embodiments of the present invention will be described in more detail below.

[0075] (negative electrode)

[0076] The negative electrode comprises a negative current collector and a negative electrode additive layer supported on the negative current collector. The negative electrode additive layer contains a negative electrode active material 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 additive layer on one side of the negative current collector is, for example, 10–300 μm.

[0077] The negative current collector uses a sheet-like metal material. This sheet-like metal material can be metal foil, porous metal, etched metal, etc. Suitable metal materials include copper, copper alloys, nickel, and stainless steel.

[0078] The negative current collector plate is a roughly disc-shaped metal plate. The material of the negative current collector plate can be, for example, copper, copper alloy, nickel, or stainless steel. The material of the negative current collector plate is the same as that of the negative current collector itself.

[0079] The interplanar spacing (i.e., the interplanar spacing between carbon layers) d002 of the (002) plane of non-graphitized carbon, as determined by X-ray diffraction, is... The theoretical capacity of the non-graphitized carbon is preferably 150 mAh / g or more. By using non-graphitized carbon, it is easy to obtain a negative electrode with low low-temperature DCR and small expansion and contraction during charging and discharging. The non-graphitized carbon preferably accounts for 50% or more, more than 80% or more, and more than 95% or more of the negative electrode active material. In addition, the non-graphitized carbon preferably accounts for 40% or more, more than 70% or more, and more than 90% or more of the negative electrode binder layer.

[0080] As a negative electrode active material, difficult-to-graphitize carbon can be used in combination with other materials besides difficult-to-graphitize carbon. Examples of materials other than difficult-to-graphitize carbon that can be used as a 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, tin alloy, etc.

[0081] From the viewpoint that the negative electrode active material has high filling capacity and can easily suppress side reactions with the electrolyte, the average particle size of the negative electrode active material (especially the non-graphitized carbon) is preferably 1 μm to 20 μm, and more preferably 2 μm to 15 μm.

[0082] It should be noted that, in this specification, the average particle size refers to the median particle size (D0) of the volume reference in the particle size distribution obtained by laser diffraction particle size distribution measurement. 50 ).

[0083] The negative electrode mixture layer contains a negative electrode active material as a necessary component, and conductive materials, binders, etc. as optional components. Examples of conductive agents include carbon black and carbon fiber. Examples of binders include fluoropolymers, acrylic resins, rubber materials, and cellulose derivatives.

[0084] The negative electrode binder layer can be formed, for example, as described below: a negative electrode active material, a conductive agent, and a binder are mixed together with a dispersion medium to prepare a negative electrode binder slurry, and the negative electrode binder slurry is coated onto a negative electrode current collector and then dried to form the negative electrode binder layer.

[0085] Lithium ions are pre-doped into the negative electrode flux layer. This lowers the negative electrode potential, thus 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 pre-doped lithium can be set to, for example, approximately 50% to 95% of the maximum amount that can be absorbed in the negative electrode flux layer.

[0086] The capacitance per unit mass of the negative electrode active material should be, for example, 1000 F / g or higher. However, from the viewpoint of improving the capacity density of the electrochemical device, the capacitance per unit mass of the negative electrode active material should be, for example, 30000 F / g or lower. The capacitance per unit mass of the negative electrode active material is typically greater than that of the positive electrode active material, for example, 20 to 800 times greater. It should be noted that the capacitance per unit mass of the negative electrode active material can be determined using the following methods.

[0087] First, an evaluation negative electrode is prepared, cut to a size of 31mm × 41mm. As the counter electrode, a 100μm thick lithium metal foil, cut to a size of 40mm × 50mm, is prepared. The negative electrode mixture layer and the lithium metal foil are placed face-to-face with a 25μm thick cellulose paper (e.g., product number TF4425) manufactured by Japan Kotaka Paper Industry Co., Ltd. as a spacer to form an electrode body. The electrode body is then immersed in the electrolyte of Example 1, described later, to assemble a battery.

[0088] The battery was charged with a constant current (CC) of 0.5mA until the battery voltage reached 0.01V, then charged with a constant voltage (CV) for 1 hour, and finally discharged with 0.5mA until the battery voltage reached 1.5V. The capacitance per unit mass of the negative electrode active material was calculated based on the discharge time during which the potential of the negative electrode changed by 0.1V starting 1 minute after the start of discharge.

[0089] (positive electrode)

[0090] The positive electrode comprises a positive current collector and a positive electrode flux layer supported on the positive current collector. The positive electrode flux layer contains a positive electrode active material reversibly doped with anions, such as a carbon material or a conductive polymer. The thickness of the positive electrode flux layer on one side of the positive current collector is, for example, 10–300 μm.

[0091] The positive current collector uses a sheet-like metal material. This sheet-like metal material can be metal foil, porous metal, etched metal, etc. Suitable metal materials include aluminum, aluminum alloys, nickel, and titanium.

[0092] The positive current collector is a generally disc-shaped metal plate. Preferably, a through hole is formed in the center of the positive current collector to provide a passage for the non-aqueous electrolyte. The material of the positive current collector is, for example, aluminum, aluminum alloy, titanium, stainless steel, etc. The material of the positive current collector can be the same as that of the positive current collector.

[0093] The carbon material used as the positive electrode active material is preferably a porous carbon material, such as activated carbon, or a carbon material exemplified as the negative electrode active material (e.g., non-graphitizable carbon). Examples of raw materials for activated carbon include wood, coconut shells, coal, asphalt, and phenolic resins. The activated carbon is preferably activated.

[0094] The average particle size of activated carbon is not particularly limited, but it is preferably less than 20 μm, and more preferably 3 μm to 15 μm.

[0095] The specific surface area of ​​the positive electrode flux layer roughly reflects the specific surface area of ​​the positive electrode active material. For example, the specific surface area of ​​the positive electrode flux layer might be 600 m². 2 / g or more and 4000m 2 / g or less is acceptable, preferably 800m 2 / g or more and 3000m 2 / g or less. The specific surface area of ​​the positive electrode mixture layer is the BET specific surface area determined using a measuring apparatus according to JIS Z8830 (e.g., the TriStarII3020 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. Then, the positive electrode mixture layer is peeled off from the positive electrode current collector, and approximately 0.5g of the positive electrode mixture layer sample is collected. Next, the specific surface area of ​​the collected sample is determined according to the method for determining the specific surface area of ​​the negative electrode mixture layer described above.

[0096] Activated carbon preferably constitutes 50% or more, 80% or more, or 95% or more of the positive electrode active material. Furthermore, activated carbon preferably constitutes 40% or more, 70% or more, or 90% or more of the positive electrode binder layer.

[0097] The positive electrode binder layer contains a positive electrode active material as a necessary component, and conductive materials, binders, etc., as optional components. Examples of conductive agents include carbon black and carbon fiber. Examples of binders include fluoropolymers, acrylic resins, rubber materials, and cellulose derivatives.

[0098] The positive electrode additive layer is formed, for example, as described below: a positive electrode active material, a conductive agent, and a binder are mixed together with a dispersion medium to prepare a positive electrode additive slurry, and the positive electrode additive slurry is coated onto a positive electrode current collector and then dried to form the layer.

[0099] As the conductive polymer used as the positive electrode active material, a π-conjugated polymer is preferred. Examples of π-conjugated polymers include polypyrrole, polythiophene, polyfuran, polyaniline, polythiophene ethylene, polypyridine, or derivatives thereof. These conductive polymers can be used alone or in combination of two or more. The weight-average molecular weight of the conductive polymer is, for example, 1000 to 100000. It should be noted that derivatives of π-conjugated polymers refer to polymers such as polypyrrole, polythiophene, polyfuran, polyaniline, polythiophene ethylene, and polypyridine, which use a π-conjugated polymer as their basic backbone. For example, polythiophene derivatives include poly(3,4-ethylenedioxythiophene) (PEDOT).

[0100] 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 of the conductive polymer, and then electrolytically polymerizing the raw material monomers in the presence of the positive electrode current collector. In the electrolytic polymerization, the positive electrode current collector and a counter electrode are immersed in the reaction solution containing the raw material monomers, with the positive electrode current collector acting as the anode, and current is allowed to flow between them. Conductive polymers can also be formed by methods other than electrolytic polymerization. For example, conductive polymers can be formed by the chemical polymerization of raw material monomers. In chemical polymerization, the raw material monomers are polymerized in the presence of the positive electrode current collector using an oxidizing agent or the like.

[0101] In electrolytic polymerization or chemical polymerization, any polymeric compound capable of forming a conductive polymer can be used as a starting monomer. The starting monomer may include oligomers. Examples of starting monomers include aniline, pyrrole, thiophene, furan, thiophene vinylene, pyridine, or their derivatives. These starting monomers can be used alone or in combination of two or more. Aniline, in particular, readily grows on the surface of a carbon layer via electrolytic polymerization.

[0102] Electrolytic polymerization or chemical polymerization can be carried out using reaction solutions containing anions (dopants). Excellent conductivity is 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 can also be polymeric ions. Examples of polymeric ions include polyvinylsulfonic acid, polystyrene sulfonic acid, polyallyl sulfonic acid, polyacrylic acid sulfonic acid, polymethacrylic acid sulfonic acid, poly(2-acrylamido-2-methylpropanesulfonic acid), polyisoprene sulfonic acid, and polyacrylic acid ions.

[0103] (spacer)

[0104] As spacers, nonwoven fabrics made of cellulose fibers, nonwoven fabrics made of glass fibers, microporous membranes made of polyolefins, woven fabrics, or nonwoven fabrics can be used. The thickness of the spacer is, for example, 8 to 300 μm, preferably 8 to 40 μm.

[0105] (electrolytes)

[0106] Electrolytes exhibit lithium-ion conductivity and may contain lithium salts and solvents that dissolve them. The lithium salt anions can reversibly and repeatedly undergo doping and dedoping at the positive electrode. Lithium ions from the lithium salt are reversibly absorbed and released at the negative electrode.

[0107] Examples of lithium salts include LiClO4, LiBF4, LiPF6, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiFSO3, LiCF3CO2, LiAsF6, and LiB. 10 Cl 10 Lithium salts such as LiCl, LiBr, LiI, LiBCl4, LiN(FSO2)2, and LiN(CF3SO2)2 are used. These lithium salts can be used alone or in combination of two or more. Salts containing fluoride anions are preferred, and lithium bis(fluorosulfonyl)imide, i.e., LiN(SO2F)2, is particularly preferred. The concentration of the lithium salt in the electrolyte at the state of charge (SOC) of 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 can be LiFSI.

[0108] By using LiFSI, the increase rate of low-temperature DCR tends to be significantly reduced. It is believed that LiFSI has the effect of reducing the degradation of both positive and negative electrode active materials. It is thought that in salts containing fluoride anions, the stability of FSI anions is excellent, therefore, byproducts are less likely to be generated, and the surface of the active materials is not damaged, thus facilitating smooth charge and discharge. Especially when increasing the capacity of the positive electrode and the specific surface area of ​​the negative electrode binder layer, the degradation inhibition effect (inhibition of low-temperature DCR increase) brought about by using LiFSI, which significantly reduces the impact of byproducts on each active material, becomes significant.

[0109] As solvents, cyclic carbonates such as ethylene carbonate, propylene carbonate, and butenyl 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; ethylene glycol diethyl ether; trimethoxymethane; sulfolane; methylsulfolane; and 1,3-propanesulfonic acid lactone can be used. These solvents can be used alone or in combination of two or more.

[0110] Depending on the requirements, the electrolyte can contain various additives. For example, as an additive to form a lithium-ion conductive film on the negative electrode surface, unsaturated carbonates such as vinylene carbonate, vinyl ethylene carbonate, and divinyl ethylene carbonate can be added.

[0111] Example

[0112] The present invention will now be described in more detail based on embodiments, but the invention is not limited to these embodiments. It should be noted that a summary of the configuration of each of the following fabricated devices is shown in Table 1.

[0113] (Device A1)

[0114] (1) Production of the positive electrode

[0115] A 30 μm thick aluminum foil (positive electrode current collector) was prepared. Meanwhile, 88 parts by mass of activated carbon (average particle size 5.5 μm), 6 parts by mass of polytetrafluoroethylene (PTFE), and 6 parts by mass of acetylene black (acetylene black), were dispersed in water to prepare a positive electrode slurry. The obtained positive electrode slurry was coated onto both sides of the aluminum foil, the coating was dried and calendered to form a positive electrode slurry layer, thus obtaining the positive electrode. A 10 mm wide exposed portion of the positive electrode current collector was formed at the end along the length direction of the positive electrode current collector.

[0116] The mass Mp of the positive electrode active material supported per unit area is 3.7 mg / cm². 2 The electrostatic capacitance of the positive electrode mixture layer is 90 F / g, and the BET specific surface area of ​​the positive electrode mixture layer is 1700 m². 2 / g.

[0117] (2) Fabrication of the negative electrode

[0118] A copper foil with a thickness of 10 μm (negative electrode current collector) was prepared. Meanwhile, a negative electrode slurry was prepared by dispersing 97 parts by mass of non-graphitized carbon (average particle size 5 μm), 1 part by mass of carboxycellulose, and 2 parts by mass of styrene-butadiene rubber in water. The obtained negative electrode slurry was coated onto both sides of the copper foil, the coating was dried, and then calendered to form a negative electrode slurry layer, thus obtaining the negative electrode.

[0119] The mass (Mn) of the negative electrode active material loaded per unit area is 3.2 mg / cm³. 2 (Therefore, the Mp / Mn ratio is 1.1), the electrostatic capacitance of the negative electrode mixture layer is 5000 F / g, and the BET specific surface area of ​​the negative electrode mixture layer is 10 m². 2 / g.

[0120] Then, a thin film of pre-doped lithium metal is formed on the entire surface of the negative electrode mixture layer by vacuum evaporation. The amount of pre-doped lithium is set such that the negative electrode potential in the non-aqueous electrolyte after pre-doping is below 0.2V relative to the lithium metal.

[0121] Then, the chamber of the device is 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 is set to -40°C, the mole fraction of carbon dioxide is set to 100%, and the pressure inside the chamber is set to 1 atmosphere (1.01 × 10⁻⁶). 5 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. The first layer substantially contained no F (or LiF).

[0122] (3) Fabrication of the electrode body

[0123] An electrode body is formed by winding a nonwoven cellulose fabric spacer (25 μm thick) between the positive and negative electrodes into a cylindrical shape. The positive current collector protrudes from one end face of the winding body, and the negative current collector protrudes from the other end face. Disc-shaped positive and negative current collector plates are then welded to the exposed positive and negative current collector portions, respectively.

[0124] (4) Preparation of non-aqueous electrolyte

[0125] A solvent was prepared by adding 0.2% by mass of vinylene carbonate to a mixture of propylene carbonate and dimethyl carbonate in a volume ratio of 1:1. A non-aqueous electrolyte was prepared by dissolving LiFSI as a lithium salt in the obtained solvent at a concentration of 1.2 mol / L.

[0126] (5) Assembly of electrochemical devices

[0127] The electrode bodies are housed in a bottomed battery casing with an opening. The lead wires connecting to the positive current collector are connected to the inner surface of the sealing plate. Additionally, the negative current collector is soldered to the inner bottom surface of the battery casing. After placing the non-aqueous electrolyte inside the battery casing, the opening of the battery casing is sealed with the sealing plate. The assembly is then as follows: Figure 1 The electrochemical device shown.

[0128] Then, while applying a charging voltage of 3.8V between the terminals of the positive and negative electrodes, aging is carried out at 60°C to end the pre-doping of lithium ions into the negative electrode.

[0129] (6) Evaluation

[0130] [Rating 1]

[0131] <Level 1 XPS Analysis>

[0132] The C1s, O1s, and Li1s spectra of the surface layer of the negative electrode mixture after exposure to a carbon dioxide atmosphere were analyzed by XPS. An X-ray photoelectron spectroscopy apparatus (trade name: Model 5600, manufactured by ULVAC-PHI Co., Ltd.) was used in the analysis. The measurement conditions are shown below.

[0133] X-ray source: Al-mono (1486.6eV) 14kV / 200W

[0134] Measured diameter: 800μmφ

[0135] Photoelectron extraction angle: 45°

[0136] Etching conditions: accelerating voltage 3kV, etching rate approximately 3.1nm / min (SiO2 conversion), grating area 3.1mm × 3.4mm.

[0137] Analysis of C1s, O1s, and Li1s spectra confirmed that the thickness of the first layer was approximately 18 nm. Specifically, peaks inferred to be C=O bonds (such as those of impurity carbon) were observed at the outermost surface, but these peaks decreased sharply near a depth of 1–2 nm in the first layer. On the other hand, a first peak belonging to C=O bonds was observed from the outermost surface of the surface layer up to a depth of 18 nm. Peaks belonging to Li-O bonds were also observed near a depth of 18 nm. Furthermore, the presence of Li was consistently confirmed from the outermost surface of the surface layer down to a depth of 18 nm. No peaks belonging to LiF were observed.

[0138] [Rating 2]

[0139] Similarly, XPS analysis of the surface layer of the negative electrode mixture taken from the electrochemical device confirmed the formation of a 10 nm thick SEI film (second layer) with a different composition from the first layer. Additionally, a peak belonging to LiF was observed.

[0140] [Rating 3]

[0141] (Determination of the capacity of electrochemical devices)

[0142] For electrochemical devices that have just undergone aging, at an environment of -30°C, with a positive electrode area of ​​2 mA / cm², 2 A constant current was applied to charge the electrode until the voltage reached 3.8V, and then the applied voltage of 3.8V was maintained for 10 minutes. Then, at a current density of -30°C, the electrode was charged at 2mA / cm² per unit area of ​​the positive electrode. 2 A constant current discharge was performed until the voltage reached 2.2V. During the discharge, the time t (sec) required for the voltage to decrease from 3.3V to 3.0V was measured. Using the measured time t, the initial capacity C1 of the electrochemical device was calculated using the following equation (A).

[0143] Capacity C1 = Id × t / V (A)

[0144] It should be noted that in equation (A), Id is the current value during discharge (current density per unit area of ​​the positive electrode, 2 mA / cm²). 2 × Positive electrode area), V is the value obtained by subtracting 3.0V from 3.3V (0.3V). The evaluation results are shown in Table 2.

[0145] (Measurement of the internal resistance of electrochemical devices)

[0146] Next, using the discharge curve obtained through the above discharge (vertical axis: discharge voltage, horizontal axis: discharge time), an approximate straight line is calculated within the range of 0.5 seconds to 2 seconds after the start of the discharge. The intercept voltage VS of this approximate straight line is then calculated. The value (V0-VS) obtained by subtracting the voltage VS from the voltage V0 at the start of the discharge (0 seconds after the start of the discharge) is then used as ΔV. ΔV (V) and the discharge current value (current density of 2 mA / cm² per unit area of ​​the positive electrode) are then used. 2 (×positive electrode area), the internal resistance (DCR) R1 (Ω) of the electrochemical device is calculated using the following formula (B). The evaluation results are shown in Table 2.

[0147] Internal resistance R1 = ΔV / Id (B)

[0148] (Float charge test of electrochemical devices)

[0149] Next, a float charge test was conducted on the electrochemical device at 85°C with a constant voltage of 3.8V for 1000 hours. Then, the low-temperature DCR was calculated, and the increase rate of low-temperature DCR was calculated based on the difference between the initial low-temperature DCR and the low-temperature DCR after repeated charge and discharge cycles (ΔDCR). The evaluation results are shown in Table 2.

[0150] (Components A2 to A7)

[0151] Mp and Mn were varied as described below, and Mp / Mn was varied as shown in Table 1. Otherwise, devices A2 to A7 were assembled in the same manner as device A1, and evaluated in the same way. The results are shown in Table 2.

[0152] (Device A2)

[0153] The mass Mp of the positive electrode active material loaded per unit area is set to 3.0 mg / cm². 2 The mass Mn of the negative electrode active material loaded per unit area was set to 4.2 mg / cm³. 2 (Therefore, the Mp / Mn ratio is 0.7).

[0154] (Device A3)

[0155] The mass Mp of the positive electrode active material supported per unit area was set to 3.9 mg / cm². 2 The mass Mn of the negative electrode active material loaded per unit area was set to 2.8 mg / cm³. 2 (Therefore, the Mp / Mn ratio is 1.4).

[0156] (Device A4)

[0157] The mass Mp of the positive electrode active material supported per unit area was set to 4.1 mg / cm². 2 The mass Mn of the negative electrode active material loaded per unit area was set to 2.6 mg / cm³. 2 (Therefore, the Mp / Mn ratio is 1.6).

[0158] (Component A5)

[0159] The mass Mp of the positive electrode active material loaded per unit area was set to 4.2 mg / cm². 2 The mass Mn of the negative electrode active material loaded per unit area was set to 2.3 mg / cm³. 2 (Therefore, the Mp / Mn ratio is 1.8).

[0160] (Device A6)

[0161] The mass Mp of the positive electrode active material loaded per unit area is set to 4.5 mg / cm².2 The mass Mn of the negative electrode active material loaded per unit area was set to 1.8 mg / cm³. 2 (Therefore, the Mp / Mn ratio is 2.5).

[0162] (Device A7)

[0163] The mass Mp of the positive electrode active material loaded per unit area was set to 4.8 mg / cm². 2 The mass Mn of the negative electrode active material supported per unit area was set to 1.3 mg / cm³. 2 (Therefore, the Mp / Mn ratio is 3.7).

[0164] (Components B1 to B7)

[0165] With the Mp / Mn ratio fixed at 1.6, the specific surface area of ​​the negative electrode flux layer was varied as shown in Table 1. Devices B1 to B7 were assembled in the same manner as device A1, and evaluated in the same way. The results are shown in Table 2. It should be noted that the specific surface area of ​​the negative electrode flux layer was varied by changing the specific surface area of ​​the non-graphitized carbon.

[0166] (Components C1 to C5)

[0167] The Mp / Mn ratio was fixed at 1.6, and the specific surface area of ​​the negative electrode mixture layer was fixed at 50 m². 2 / g, the thickness of the first layer was varied as shown in Table 1. Otherwise, devices C1 to C5 were assembled in the same manner as device A1, and evaluated in the same way. The results are shown in Table 2. It should be noted that the thickness of the first layer varied depending on the time the negative electrode was exposed to the carbon dioxide atmosphere. However, in device C1, the chamber after the lithium metal was deposited into the negative electrode mixture layer was not purged with carbon dioxide. Therefore, the first layer was not formed on the negative electrode of device C1.

[0168] (Device D1)

[0169] Graphite (average particle size 7 μm) was used instead of non-graphitized carbon as the negative electrode active material, and the Mp / Mn ratio was set to 1.6. Otherwise, device D1 was assembled in the same manner as device A1, and the same evaluation was performed. The results are shown in Table 2.

[0170] (Device D2)

[0171] Graphite (average particle size 7 μm) was used instead of non-graphitized carbon as the negative electrode active material, and the Mp / Mn ratio was set to 1.6. Consequently, the specific surface area of ​​the negative electrode additive layer was set to 50 m². 2 / g, except that device D2 was assembled in the same manner as device A1 and evaluated in the same way. The results are shown in Table 2.

[0172] (Device D3)

[0173] For the chamber after the lithium metal was deposited into the negative electrode mixture layer, no carbon dioxide purging was performed. Otherwise, device D3 was assembled in the same manner as device D2, and the same evaluation was conducted. As a result, no first layer was formed on the negative electrode of device D3. The results are shown in Table 2.

[0174] (Device E1)

[0175] The Mp / Mn ratio was set to 1.6, and the specific surface area of ​​the negative electrode mixture layer was set to 50 m². 2 / g, LiPF6 was used instead of LiFSI as the lithium salt for the electrolyte. Otherwise, device E1 was assembled in the same manner as device A1 and evaluated in the same way. The results are shown in Table 2.

[0176] (Device E2)

[0177] The Mp / Mn ratio was set to 0.7, and the specific surface area of ​​the negative electrode mixture layer was set to 50 m². 2 / g, LiPF6 was used instead of LiFSI as the lithium salt for the electrolyte. Otherwise, device E2 was assembled in the same manner as device A2 and evaluated in the same way. The results are shown in Table 2.

[0178] It should be noted that in Table 1, "HC" represents "difficult-to-graphitize carbon (hard carbon)". In Table 2, the evaluation results are expressed as an index with the evaluation result of device D1 set to 100. A higher low-temperature capacitance value is preferred, as are lower low-temperature DCR and DCR increase rate values.

[0179] [Table 1]

[0180]

[0181] [Table 2]

[0182]

[0183] Based on the comparison of devices A1 to A7, it can be understood that the larger the Mp / Mn ratio, the larger the low-temperature capacitance. However, considering the balance with low-temperature DCR, it is known that an Mp / Mn ratio in the range of 1.1 to 2.5, and further 1.4 to 1.8, is preferred.

[0184] Based on the comparison of devices B1 to B7, it can be seen that the larger the specific surface area of ​​the negative electrode flux layer, the smaller the low-temperature DCR, and on the other hand, the greater the DCR increase rate. Considering the balance between low-temperature DCR and DCR increase rate, the preferred specific surface area of ​​the negative electrode flux layer is 10–70 m². 2 / g, and more preferably 25-50m 2 / g.

[0185] A comparison of devices C1 to C5 shows that even with a relatively large specific surface area of ​​the negative electrode flux layer, the DCR increase rate is significantly reduced by adding the first layer. This is believed to be because forming the first layer stabilizes the second layer under repeated charge-discharge conditions, improving the reliability of the negative electrode. Furthermore, it is known that if the thickness of the first layer does not become extremely large, significant effects can be achieved even with a small second layer thickness.

[0186] It should be noted that since graphite is used in the negative electrode active material of devices D1-D3, it is difficult to reduce the low-temperature DCR and the rate of increase in DCR. Furthermore, a comparison between devices C3 and E1 shows that lithium salt LiFSI is effective as an electrolyte. On the other hand, a comparison between devices A2 and E2 shows that when the Mp / Mn ratio is less than 1.1, LiFSI does not offer any advantages; however, increasing the Mp / Mn ratio brings out the advantages of LiFSI.

[0187] Industrial availability

[0188] The electrochemical device of the present invention is suitable for, for example, automotive applications.

[0189] Explanation of reference numerals in the attached figures

[0190] 100: Electrode body

[0191] 10: Positive electrode

[0192] 11x: Exposed portion of the positive current collector

[0193] 13: Positive current collector

[0194] 15: Electrode lead wire

[0195] 20: Negative electrode

[0196] 21x: Exposed portion of the negative current collector

[0197] 23: Negative current collector

[0198] 30: Spacer

[0199] 200: Electrochemical Devices

[0200] 210: Battery casing

[0201] 220: Sealing board

[0202] 221: Sealing gasket

Claims

1. An electrochemical device comprising a positive electrode, a negative electrode, and a lithium ion-conductive electrolyte, the positive electrode having 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 that reversibly dopes anions, the negative electrode having a negative electrode current collector, and a negative electrode mixture layer supported on the negative electrode current collector, the negative electrode mixture layer containing a negative electrode active material that reversibly dopes lithium ions, the negative electrode active material containing a non-graphitizable carbon, a ratio of a mass Mp of the positive electrode active material supported per unit area of the positive electrode to a mass Mn of the negative electrode active material supported per unit area of the negative electrode: Mp / Mn is 1.1 or greater and 2.5 or less, The specific surface area of the negative electrode mixture layer is 10 m 2 / g or more and 70 m 2 / g or less.

2. The electrochemical device according to claim 1, wherein the ratio: Mp / Mn is 1.4 or greater and 1.8 or less.

3. The electrochemical device according to claim 1, wherein The specific surface area of the negative electrode mixture layer is 25 m 2 / g or more and 70 m 2 / g or less.

4. The electrochemical device according to claim 1, wherein The specific surface area of the negative electrode mixture layer is 25 m 2 / g or more and 50 m 2 / g or less.

5. The electrochemical device according to claim 1, wherein a proportion of the non-graphitizable carbon in the negative electrode active material is 50 mass% or greater.

6. The electrochemical device according to claim 1, wherein a proportion of the non-graphitizable carbon in the negative electrode active material is 80 mass% or greater.

7. The electrochemical device according to claim 1, wherein a proportion of the non-graphitizable carbon in the negative electrode mixture layer is 40 mass% or greater.

8. The electrochemical device according to claim 1, wherein a proportion of the non-graphitizable carbon in the negative electrode mixture layer is 70 mass% or greater.

9. The electrochemical device according to any one of claims 1 to 8, wherein a surface layer portion of the negative electrode mixture layer has a first layer containing lithium carbonate.

10. The electrochemical device according to claim 9, wherein a surface layer portion of the negative electrode mixture layer has a second layer containing a solid electrolyte, at least a portion of the second layer covers at least a portion of a surface of the negative electrode mixture layer with the first layer interposed therebetween.

11. The electrochemical device according to claim 10, wherein the second layer contains lithium carbonate, a content of the lithium carbonate contained in the second layer is less than a content of the lithium carbonate contained in the first layer.

12. The electrochemical device according to claim 9, wherein a thickness of the first layer is 1 nm or greater and 50 nm or less.

13. The electrochemical device according to claim 9, wherein a thickness of the first layer is 1 nm or greater and 18 nm or less.

14. The electrochemical device according to claim 10, wherein when the first layer is measured by X-ray photoelectron spectroscopy, a peak of substantial F1s attributable to a LiF bond is not observed, when the second layer is measured by X-ray photoelectron spectroscopy, a peak of substantial F1s attributable to a LiF bond is observed.

15. The electrochemical device according to claim 1, wherein the lithium ion-conductive electrolyte contains lithium bisfluorosulfonylimide: LiN(SO2F)2.

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