Electrochemical device

Abstract

This electrochemical device includes a positive electrode, a negative electrode, and a lithium ion-conductive electrolyte, the positive electrode being provided with a positive electrode current collector, and a positive electrode material layer that is supported by the positive electrode current collector. The positive electrode material layer contains lithium transition metal oxide particles, each of which has a carbon layer formed on the surface thereof, and a polymer acid which has a carboxylic acid group or a salt of the polymer acid. The negative electrode is provided with a negative electrode current collector and a negative electrode material layer that is supported by the negative electrode current collector. The negative electrode material layer contains a carbon material which is reversibly doped with lithium ions, and the surface layer part of the negative electrode material layer has a first layer that contains lithium carbonate.

Description

Electrochemical Devices

[0001] The present invention relates to an electrochemical device.

[0002] Electrochemical devices using a carbon material doped with lithium ions in a negative electrode layer are known (see Patent Document 1). The electrochemical device includes a positive electrode, a negative electrode, and an electrolyte. The positive electrode layer is known to include a porous carbon material reversibly doped with anions as a positive electrode active material.

[0003] Patent Documents 2 and 3 disclose lithium ion secondary batteries in which the surface of the positive electrode active material is a lithium transition metal oxide coated with a carbon coating layer.

[0004] Patent Document 4 discloses that lithium iron phosphate particles are used as the active material of the positive electrode, and the lithium iron phosphate positive electrode sheet contains polyacrylic acid as a binder.

[0005] International Publication No. 2022 / 181608 JP 2014-143041 A JP 2013-149609 A JP 2023-519287 A

[0006] In electrochemical devices that utilize lithium ions, activated carbon with a large specific surface area, primarily composed of carbon, is typically used for the positive electrode, while carbon-based materials that can be doped with lithium ions are typically used for the negative electrode. In these electrochemical devices, a solid electrolyte interfacial film (i.e., SEI film) is formed on the negative electrode layer during charging and discharging. While the SEI film plays an important role in charge and discharge reactions, excessively thick SEI film increases the internal resistance of the electrochemical device.

[0007] Therefore, in electrochemical devices that use lithium ions, lithium ions are pre-doped into the negative electrode before charging and discharging. Pre-doping is performed, for example, by forming a film of metallic lithium on the surface of the negative electrode material layer using gas-phase methods such as chemical vapor deposition, physical vapor deposition, and sputtering, or by transfer. In this case, the SEI coating is homogeneous and of a moderate thickness. Since no additional reactions occur compared to the SEI coating formed by decomposing the electrolyte or additives, it can be made thinner, thereby reducing the internal resistance of the electrochemical device. However, lithium that cannot migrate between the lattices of the negative electrode remains on the surface, and the reaction between the electrolyte and lithium produces electrolyte decomposition products such as carboxylates.

[0008] The decomposition products of the electrolyte move to the positive electrode and react with the active sites on the surface of the positive electrode active material, which increases the resistance of the positive electrode material layer and tends to increase the internal resistance of the electrochemical device.

[0009] In particular, electrochemical devices that use a positive electrode material layer containing a lithium transition metal oxide such as lithium iron phosphate as a positive electrode active material in order to improve discharge capacity at high current densities have had the problem of increased reactivity between the positive electrode active material and the above-mentioned electrolyte decomposition products, resulting in increased internal resistance.

[0010] One aspect of the present invention relates to an electrochemical device including a positive electrode, a negative electrode, and a lithium ion conductive electrolyte, wherein the positive electrode includes a positive electrode current collector and a positive electrode material layer supported on the positive electrode current collector, and the positive electrode material layer includes lithium transition metal oxide particles having a carbon layer formed on the surface thereof, and a polymeric acid having a carboxylic acid group or a salt thereof, and the negative electrode includes a negative electrode current collector and a negative electrode material layer supported on the negative electrode current collector, and the negative electrode material layer includes a carbon material that is reversibly doped with lithium ions, and a surface layer portion of the negative electrode material layer includes a first layer containing lithium carbonate.

[0011] According to the present invention, an electrochemical device having good floating characteristics can be obtained.

[0012] The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0013] 1 is a longitudinal sectional view showing the configuration of an electrochemical device according to one embodiment of the present invention.

[0014] Hereinafter, embodiments of the present disclosure will be described using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values, materials, etc. may be exemplified, but other numerical values, materials, etc. may be applied as long as the effects of the present disclosure are obtained. Note that known components may be applied to components other than those characteristic of the present disclosure. In this specification, when a "range from numerical value A to numerical value B" is mentioned, the range includes numerical value A and numerical value B. In the following description, when a lower limit and an upper limit are exemplified for numerical values ​​of specific physical properties or conditions, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not equal to or greater than the upper limit. When multiple materials are exemplified, one of the materials may be selected and used alone, or two or more of the materials may be used in combination.

[0015] The present disclosure also encompasses combinations of two or more features arbitrarily selected from the appended claims, i.e., two or more features arbitrarily selected from the appended claims can be combined unless a technical contradiction arises.

[0016] An electrochemical device according to an embodiment of the present disclosure includes a positive electrode, a negative electrode, and a lithium ion conductive electrolyte. Generally, the positive electrode and the negative electrode, together with a separator interposed therebetween, constitute an electrode assembly. The electrode assembly may be formed, for example, as a columnar wound assembly by winding a strip-shaped positive electrode and a strip-shaped negative electrode with a separator interposed therebetween. Alternatively, the electrode assembly may be formed as a laminate by stacking a plate-shaped positive electrode and a strip-shaped negative electrode with a separator interposed therebetween.

[0017] The positive electrode comprises a positive electrode current collector and a positive electrode material layer supported on the positive electrode current collector. The positive electrode material layer contains a positive electrode active material that reversibly dopes and dedopes anions of a lithium salt. The greater the amount of lithium salt anions that can be doped and dedoped, the greater the capacity of the electrochemical device.

[0018] The positive electrode active material includes lithium transition metal oxide particles having a carbon layer formed on the surface. The lithium transition metal oxide has a large amount of lithium salt anions to be doped and dedoped, which can increase the capacity of the electrochemical device. Furthermore, by forming a carbon layer on the surface of the lithium transition metal oxide particles, electronic conductivity is improved, thereby increasing the discharge capacity at high current densities. Furthermore, an increase in resistance due to a reaction between the active surface of the lithium transition metal oxide particles and the electrolyte can be suppressed, thereby improving float characteristics.

[0019] The thickness of the carbon layer formed on the surface of the lithium transition metal oxide particles is preferably 1.0 to 10 nm. By having the thickness of the carbon layer formed on the surface of the lithium transition metal oxide particles within this range, the conductivity of the surface of the lithium transition metal oxide particles can be increased. Furthermore, if the lithium transition metal oxide crystals are in contact with each other via the carbon layer formed on the surface, the lithium transition metal oxide crystals will be electrically connected to each other, and the conductivity of the positive electrode material layer can be increased.

[0020] The lithium transition metal oxide may include at least one selected from the group consisting of olivine-type phosphate compounds, layered rock salt-type lithium transition metal composite oxides, and spinel-type manganese oxides. Among these, the lithium transition metal oxide preferably includes at least one metal oxide selected from the group consisting of lithium iron phosphate, lithium manganese iron phosphate, lithium cobalt phosphate, lithium cobalt oxide, manganese spinel, lithium nickel oxide, and ternary lithium nickel manganese cobalt oxide, and more preferably includes at least one metal oxide selected from the group consisting of lithium iron phosphate and lithium manganese iron phosphate. Lithium iron phosphate and lithium manganese iron phosphate, which have low electronic conductivity, can be improved by forming a carbon layer on their surfaces, thereby achieving a greater effect of improving discharge capacity at high current densities.

[0021] The positive electrode material layer includes a polymeric acid having a carboxylic acid group or a salt thereof. Specific examples of the polymeric acid having a carboxylic acid group include polycarboxylic acids such as polyacrylic acid, polymethacrylic acid, and polymaleic acid, and copolymers having these structural units. Specific examples of the salt of the polymeric acid having a carboxylic acid group include lithium salt, sodium salt, potassium salt, and ammonium salt. The polymeric acid having a carboxylic acid group or a salt thereof may be used alone or in combination of two or more.

[0022] The polymeric acid having carboxylic acid groups adsorbs to the lithium transition metal oxide particles contained in the positive electrode material layer, uniformly covering the particle surfaces and thereby binding the lithium transition metal oxide particles to each other or to the positive electrode current collector with strong binding strength. Carboxymethyl cellulose can also be used as a binder for the positive electrode material layer, but its oxidation resistance is poor and it decomposes due to anodic oxidation during the charging and discharging of the electrochemical device, which reduces the binding strength of the positive electrode material layer. Furthermore, carboxymethyl cellulose itself has hard and brittle physical properties, making it prone to peeling due to the expansion and contraction of the positive electrode associated with the charging and discharging of the electrochemical device. Therefore, combining styrene-butadiene rubber, which has a flexible polymer structure, with carboxymethyl cellulose can maintain high binding strength. However, due to the high insulating properties of styrene-butadiene rubber, its use in the positive electrode material layer increases the resistance of the positive electrode material layer, which can lead to an increase in the DCR of the electrochemical device before the float test.

[0023] The polymeric acid having a carboxylic acid group preferably includes one or more selected from the group consisting of polyacrylic acid, polymethacrylic acid, and copolymers containing acrylic acid and methacrylic acid. In the manufacturing process of a positive electrode, the polymeric acid having a carboxylic acid group typically functions not only as a binder but also as a thickener for a positive electrode mixture slurry in which a positive electrode mixture is dispersed in a liquid dispersion medium. Among these, polyacrylic acid and sodium polyacrylate are easily added to a positive electrode mixture slurry to achieve the desired viscosity with a small amount, and the positive electrode mixture slurry also has good applicability when applied to a positive electrode current collector. Furthermore, a positive electrode material layer using the above-mentioned positive electrode mixture slurry also has high binding strength with the etched aluminum foil used as a positive electrode current collector.

[0024] In the past, in positive electrode mixture slurries containing lithium transition metal oxides, the pH of the slurry tends to increase as lithium elutes from the lithium transition metal oxide. As a result, when polyacrylic acid is used in the positive electrode mixture slurry, the slurry viscosity increases over time, making it difficult to achieve the desired viscosity and resulting in poor coatability. On the other hand, in positive electrode mixture slurries using lithium transition metal oxides with a carbon layer formed on their surfaces, less lithium elutes from the lithium transition metal oxide and the pH of the positive electrode mixture slurry increases only slightly. As a result, positive electrode mixture slurries containing lithium transition metal oxides with a carbon layer formed on their surfaces can achieve stable slurry viscosity even when polyacrylic acid is used in combination, resulting in good coatability when applied to a positive electrode current collector.

[0025] The weight-average molecular weight of the polymeric acid or its salt having a carboxylic acid group is preferably in the range of 5,000 to 500,000. A positive electrode mixture slurry using a polymeric acid or its salt having a weight-average molecular weight of 5,000 or less has too low a viscosity, resulting in poor coating properties and making it impossible to uniformly coat the current collector with the desired thickness. Furthermore, a positive electrode material layer using the above-mentioned positive electrode mixture slurry also has insufficient binding strength with the positive electrode current collector. On the other hand, a positive electrode mixture slurry using a polymeric acid or its salt having a weight-average molecular weight of 500,000 or more has an excessively high viscosity, making it difficult to uniformly disperse the positive electrode active material, conductive additive, etc. As a result, the composition distribution within the positive electrode material layer becomes nonuniform, and electrochemical devices using the above-mentioned positive electrodes experience increased DCR after float tests.

[0026] The content of the polymeric acid having a carboxylic acid group or its salt in the positive electrode layer may be 1% by mass or more and 9% by mass or less, or may be 1% by mass or more and 5% by mass or less, relative to the total mass of the positive electrode layer. When the content of the polymeric acid having a carboxylic acid group or its salt is in the range of 1% by mass or more and 9% by mass or less, the effect of suppressing deterioration of float characteristics is sufficiently exhibited. When the content of the polymeric acid having a carboxylic acid group or its salt in the positive electrode layer is less than 1% by mass, the viscosity of the positive electrode mixture slurry becomes too low, making it difficult to uniformly apply it to the current collector. When the content of the polymeric acid having a carboxylic acid group or its salt in the positive electrode layer is more than 9% by mass, the resistance of the positive electrode layer increases, and the DCR of the electrochemical device before the float test increases.

[0027] The negative electrode includes a negative electrode current collector and a negative electrode material layer supported on the negative electrode current collector. The negative electrode material layer includes a carbon material that is reversibly doped with lithium ions.

[0028] The carbon material exhibits capacity through a Faraday reaction in which lithium ions are reversibly doped and dedoped. The doping of lithium ions into the negative electrode active material includes at least the doping phenomenon of lithium ions into the negative electrode active material, and may also include the adsorption of lithium ions into the negative electrode active material and chemical interactions between the negative electrode active material and lithium ions.

[0029] The surface portion of the negative electrode layer has a coating region. The coating region is a region where an SEI coating is formed. The SEI coating is formed by depositing lithium carbonate (Li ２ CO ３ An SEI coating containing lithium carbonate has low resistance to the movement of lithium ions, and forming an SEI coating containing lithium carbonate as its main component can reduce the internal resistance of an electrochemical device. On the other hand, an SEI coating containing lithium carbonate as its main component easily reacts with HF generated by the decomposition reaction of the electrolyte, and the SEI coating is easily destroyed.

[0030] The negative electrode mixture layer is pre-doped with lithium ions in advance. By pre-doping the negative electrode mixture layer with lithium ions in advance, the potential of the negative electrode decreases, the potential difference (i.e., voltage) between the positive electrode and the negative electrode increases, and the energy density of the electrochemical device improves.

[0031] The negative electrode potential is, for example, 0.2 V or less based on lithium (vs. Li / Li+). This negative electrode potential is the negative electrode potential (25°C) at the time of completion of pre-doping (or during charging). The amount of lithium to be pre-doped is set according to the negative electrode potential in the electrolyte after completion of pre-doping.

[0032] The surface layer portion of the negative electrode composite layer may have a first layer containing lithium carbonate. The first layer is formed mainly on the surface of the negative electrode active material. Normally, the larger the specific surface area of ​​the negative electrode composite layer, the more likely the float characteristics are to deteriorate. However, by forming the first layer, the deterioration of the resistance of the negative electrode is significantly suppressed, and the deterioration of the float characteristics is suppressed.

[0033] The surface layer portion of the negative electrode mixture layer may have a second layer containing a solid electrolyte. At least a portion of the second layer may cover at least a portion of the surface of the negative electrode mixture layer via the first layer. The second layer has a different composition from the first layer, and the second layer is distinguishable from the first layer. In an electrochemical device that uses lithium ions, a solid electrolyte interfacial film (i.e., an SEI film) is formed on the negative electrode mixture layer during charge and discharge. The second layer may be formed as an SEI film.

[0034] The SEI coating plays an important role in charge-discharge reactions, but if the SEI coating is formed too thick, the resistance of the negative electrode increases. In contrast, the first layer containing lithium carbonate promotes the formation of a good SEI coating and maintains the SEI coating in a good state even when repeated charge-discharge cycles are performed. Therefore, by forming the first layer on the surface layer of the negative electrode composite layer, it is possible to significantly suppress deterioration of the negative electrode, even when the specific surface area of ​​the negative electrode composite layer is increased to reduce the resistance of the negative electrode.

[0035] When the negative electrode composite layer 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. By using the first layer as an underlayer, the second layer is formed as a good-quality SEI coating.

[0036] 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. By using the first layer containing a large amount of lithium carbonate as the base layer, the second layer is formed as a better SEI coating.

[0037] The first layer can be formed on the surface of the negative electrode composite layer before assembling the electrochemical device. In an electrochemical device assembled using the negative electrode, a homogeneous, appropriately thick second layer (SEI coating) is formed on the surface of the negative electrode active material through subsequent charging and discharging. The SEI coating is formed, for example, by a reaction between the electrolyte and the negative electrode in the 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 coating. However, for convenience, the second layer will be referred to as the SEI coating in this specification to distinguish it from the first layer.

[0038] 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.

[0039] The thickness of the first layer may be, for example, 1 nm or more, or 5 nm or more if a longer-term effect from the SEI coating is expected, or 10 nm or more if a more reliable effect is expected. 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. The thickness of the first layer is, for example, 1 nm to 50 nm.

[0040] The thickness of the second layer may be, for example, 1 nm or more, or may be 3 nm or more, or 5 nm or more is sufficient. 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.

[0041] The thicknesses of the first and second layers are measured by analyzing the surface portion of the negative electrode composite layer at multiple locations (at least five locations). The average thickness of the first or second layer measured at multiple locations may be used as the thickness of the first or second layer. The negative electrode composite 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 composite layer may be analyzed. Specifically, the coating-covered negative electrode active material may be collected from the region of the negative electrode composite layer disposed on the side opposite to the surface bonded to the negative electrode current collector and used for analysis.

[0042] XPS analysis of the surface portion of the negative electrode composite 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 changes in the spectra attributable to C1s, O1s electrons, etc. with respect to irradiation time. In this case, the spectrum of the outermost surface of the surface portion may be ignored to avoid analytical errors. The thickness of the region where peaks attributable to lithium carbonate are stably observed corresponds to the thickness of the first layer.

[0043] In the case of a negative electrode removed from an electrochemical device after assembly and subsequent aging or at least one charge / discharge, the surface portion of the negative electrode composite layer has an SEI coating (i.e., second layer) containing a solid electrolyte. The thickness of the region where peaks attributable to bonds possessed by 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).

[0044] 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 substantially not contained in the first layer. LiF, for example, may be selected as the compound that contains the element that can be a label for the second layer.

[0045] 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.

[0046] 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 attributed to the LiF bond is not observed. Therefore, the thickness of the region where a peak attributed to the LiF bond is not stably observed may be defined as the thickness of the first layer.

[0047] 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 first layer formed in advance, so the two can be distinguished. For example, in an 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 small. The Li1s peak can be observed, for example, in ROCO ２ Peaks derived from compounds such as Li and ROLi can be detected.

[0048] When the first layer is analyzed by XPS, in addition to the first peak of O1s attributed to the C=O bond, a second peak of O1s attributed to the Li-O bond may be observed. The region of the coating present near the surface of the negative electrode active material contains a small amount of LiOH or Li ２ It may contain O.

[0049] Specifically, when the first layer constituting the surface layer portion of the negative electrode composite layer is analyzed in the depth direction, a first region in which a first peak (O1s attributable to a C═O bond) and a second peak (O1s 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 may be observed, in order of increasing distance from the outermost surface of the surface layer portion. 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 likely to be observed when the thickness of the lithium carbonate-containing region is large.

[0050] The magnitude of the peak intensity can be determined by the height of the peak from the baseline.

[0051] At the center in the thickness direction of the first layer, the C1s peak attributed to a C—C bond is generally not observed, or even if observed, its intensity is half or less of the peak intensity attributed to a C═O bond.

[0052] 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 performed by, for example, a vapor phase method, a coating method, or a transfer method.

[0053] Examples of the gas phase method include chemical vapor deposition, physical vapor deposition, and sputtering. For example, lithium carbonate may be deposited on the surface of the negative electrode composite layer using a vacuum deposition device. The pressure in the device chamber during deposition is, for example, 10 －２ ~10 －５ 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.

[0054] As a coating method, a solution or dispersion containing lithium carbonate can be applied to the surface of the negative electrode using, for example, a microgravure coater, followed by drying to form the first layer. 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 equal to or lower than the solubility (for example, about 0.9 to 1.3 mass % in the case of an aqueous solution at room temperature).

[0055] Metallic lithium may be attached to the surface of the negative electrode mixture layer (solid-phase doping). Note that a first layer containing lithium carbonate having a thickness of, for example, 1 nm or more and 50 nm or less can also be formed by exposing a negative electrode having a negative electrode mixture layer to which metallic lithium has been attached to a carbon dioxide gas atmosphere.

[0056] The step of attaching metallic lithium to the surface of the negative electrode mixture layer can be carried out by, for example, a vapor phase method, transfer, or the like. Examples of the vapor phase method include chemical vapor deposition, physical vapor deposition, and sputtering. For example, metallic lithium may be formed in the form of a film 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 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.

[0057] The carbon dioxide gas atmosphere is preferably a dry atmosphere containing no moisture, and may have a dew point of, for example, -40°C or lower or -50°C or lower. The carbon dioxide gas atmosphere may contain gases other than carbon dioxide, but the molar fraction of carbon dioxide is preferably 80% or higher, and more preferably 95% or higher. It is preferable that the atmosphere does not contain any oxidizing gases, and the molar fraction of oxygen should be 0.1% or lower.

[0058] To form a thicker first layer, the partial pressure of carbon dioxide is set to, for example, 0.5 atmospheres (5.05×10 ４ It is efficient if the pressure is greater than 1 atmosphere (1.01 x 10 ５ Pa) or more.

[0059] The temperature of the negative electrode exposed to the carbon dioxide gas atmosphere may be, for example, in the range of 15° C. to 120° C. The higher the temperature, the thicker the first layer becomes.

[0060] The thickness of the first layer can be easily controlled by changing the time for which the negative electrode is exposed to the carbon dioxide gas atmosphere. The exposure time may be, for example, 12 hours or more and less than 10 days.

[0061] The step of forming the first layer is preferably carried out before constructing the electrode body, but may be carried out after constructing the electrode body. That is, a positive electrode may be prepared, a negative electrode having a negative electrode mixture layer to which metallic lithium is attached may be prepared, a separator may be interposed between the positive electrode and the negative electrode to form an electrode body, and the electrode body may be exposed to a carbon dioxide gas atmosphere to form the first layer on the surface portion of the negative electrode mixture layer.

[0062] 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 composite 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).

[0063] The step of forming the second layer proceeds further after the step of forming the first layer by contacting the negative electrode mixture layer with an electrolyte and completing the process by leaving the mixture for a predetermined period of time. Such a step may be a step of forming a second layer so as to cover at least a portion of the first layer. For example, by performing at least one charge / discharge cycle on the electrochemical device, the second layer can be formed on the negative electrode mixture layer and pre-doping of lithium ions into the negative electrode can be completed. Furthermore, for example, pre-doping of lithium ions into the negative electrode can also be completed by applying a predetermined charging voltage (e.g., 3.4 to 4.0 V) between the terminals of the positive electrode and the negative electrode for a predetermined period of time (e.g., 1 to 75 hours).

[0064] As described above, solid-phase doping refers to a method in which lithium ions are transferred into the negative electrode active material using a negative electrode material layer to which metallic lithium has been attached in a pre-doping step of the negative electrode material layer. At the same time, an SEI coating (first layer) primarily composed of lithium carbonate is formed on the surface layer of the negative electrode material. The SEI coating (first layer) containing lithium carbonate has low resistance to lithium ion migration, and therefore can reduce the resistance of the negative electrode material layer.

[0065] On the other hand, liquid-phase doping refers to a method in which lithium ions are transferred into the negative electrode active material in a pre-doping step of the negative electrode material layer by directly contacting the negative electrode material layer, which does not have the first layer, with an electrolyte containing lithium ions. In this case, the decomposition reaction of the electrolyte and the electrolyte, which occurs in parallel with the pre-doping step, cannot be controlled, resulting in the formation of an SEI coating containing a mixture of lithium fluoride and lithium carbonate. Furthermore, the SEI coating formed by liquid-phase doping is a relatively thick SEI coating with a thickness of 70 nm or more. Therefore, in the liquid-phase doping method, only the first layer, primarily composed of lithium fluoride, can be formed on the surface of the negative electrode material layer. The SEI coating tends to increase the internal resistance of the electrochemical device due to the influence of lithium fluoride, which has a high resistance to lithium ion migration.

[0066] Furthermore, in an electrochemical device assembled using a negative electrode having the second layer, a stable coating (second layer) containing a large amount of lithium fluoride can be formed on the surface of the negative electrode active material through subsequent charge and discharge cycles, with the first layer as a base layer. The second layer formed inside the first layer can be thin, which suppresses an increase in the resistance to lithium ion migration and can suppress an increase in the resistance of the negative electrode material layer.

[0067] While the solid-phase doping method reduces the resistance of the negative electrode layer, the electrolyte reacts with the remaining lithium that cannot migrate into the negative electrode active material, generating electrolyte decomposition products such as carboxylates. These electrolyte decomposition products migrate to the positive electrode and react with active sites on the surface of the positive electrode active material, which tends to increase the resistance of the positive electrode layer. In particular, in electrochemical devices using a positive electrode layer containing a lithium transition metal oxide as the positive electrode active material, the reaction between the positive electrode active material and the electrolyte decomposition products is not sufficiently suppressed even when a carbon layer is formed on the surface of the lithium transition metal oxide, and this can result in an increase in internal resistance.

[0068] However, when the positive electrode material layer contains a polymeric acid having a carboxylic acid group or its salt, the surface of the positive electrode active material, such as a lithium transition metal oxide, is adequately covered with the polymeric acid, thereby effectively suppressing the reaction between the positive electrode active material and the decomposition products of the electrolyte. As a result, an electrochemical device using a doped negative electrode prepared by the solid-phase doping method, lithium transition metal oxide particles having a carbon layer formed on their surfaces, and a positive electrode containing a polymeric acid having a carboxylic acid group or its salt, exhibits suppressed increases in internal resistance and exhibits good float characteristics.

[0069] 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 electrode current collector 13 having a central through-hole 13h is welded to the positive electrode core material exposed portion 11x. One end of a tab lead 15 is connected to the positive electrode 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 electrode terminal. Meanwhile, a negative electrode current collector 23 is welded to the negative electrode core material 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.

[0070] Each component of the electrochemical device according to the embodiment of the present disclosure will be described in more detail below.

[0071] (Negative Electrode) The negative electrode includes a negative electrode current collector and a negative electrode material layer (negative electrode mixture layer) supported on the negative electrode current collector.

[0072] The negative electrode current collector is made of a sheet-like metal material, such as a metal foil, a porous metal, or an etched metal. Examples of the metal material include copper, a copper alloy, nickel, and stainless steel.

[0073] 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.

[0074] (Negative electrode material layer) The negative electrode material layer includes a carbon material that electrochemically dopes and dedopes lithium ions as a negative electrode active material. As the carbon material, graphite, non-graphitizable carbon (hard carbon), and easily graphitizable carbon (soft carbon) are preferred, and graphite and hard carbon are particularly preferred. The carbon material may be used in combination with other materials.

[0075] Non-graphitizable carbon has a (002) plane spacing (i.e., the spacing between carbon layers) d ００２ The non-graphitizable carbon may have a theoretical capacity of 3.8 Å or more. The theoretical capacity of the non-graphitizable carbon is preferably, for example, 150 mAh / g or more. By using non-graphitizable carbon, it becomes easier to obtain 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. Furthermore, 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.

[0076] 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.

[0077] From the viewpoint of achieving high packing properties 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.

[0078] In this specification, the average particle size refers to the volume-based median diameter (D ５０ ) means

[0079] The negative electrode material layer contains a negative electrode active material as an essential component, and optionally contains a conductive material and a binder for the negative electrode material layer (hereinafter referred to as a negative electrode binder). Examples of conductive materials include carbon black and carbon fiber. Examples of negative electrode binders include fluororesins, acrylic resins, rubber materials, and cellulose derivatives. Examples of fluororesins include polyvinylidene fluoride, polytetrafluoroethylene, and tetrafluoroethylene-hexafluoropropylene copolymers. Examples of acrylic resins include polyacrylic acid and acrylic acid-methacrylic acid copolymers. Examples of rubber materials include styrene butadiene rubber (hereinafter also referred to as "SBR"). Examples of cellulose derivatives include at least one selected from the group consisting of carboxymethyl cellulose and carboxymethyl cellulose salts (hereinafter at least one selected from the group consisting of carboxymethyl cellulose and carboxymethyl cellulose salts is also referred to as "CMC").

[0080] CMC has a smaller effect on the resistance of the negative electrode mixture layer than binders such as rubber materials. When a carboxymethyl cellulose salt is used as the CMC, a sodium salt, a lithium salt, a potassium salt, an ammonium salt, or the like can be used. Among these, it is preferable that the carboxymethyl cellulose salt contains an ammonium salt. Among CMCs, the ammonium salt of carboxymethyl cellulose is particularly unlikely to increase the resistance of the negative electrode, and is therefore preferably used as a negative electrode binder.

[0081] On the other hand, when the electrochemical device is used at high temperatures for a long period of time, CMC alone does not provide sufficient durability to maintain the binding of the negative electrode mixture layer. Therefore, in order to achieve durability, the negative electrode mixture layer preferably contains a binder such as a rubber material in addition to CMC. The binder, such as a rubber material, has a flexible polymer structure that allows it to follow the expansion and contraction of the negative electrode mixture layer under the temperature environment in which the electrochemical device is used, thereby maintaining the binding of the negative electrode mixture layer.

[0082] CMC not only acts as a binder, but also stabilizes the dispersion of the negative electrode mixture slurry in the negative electrode manufacturing process, and can homogenize the negative electrode active material and the conductive agent in the negative electrode mixture layer when the negative electrode mixture layer is formed.

[0083] The negative electrode material layer is formed, for example, by mixing a negative electrode active material, a conductive agent, a negative electrode 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 negative electrode mixture slurry. The thickness of the negative electrode material layer is, for example, 10 to 300 μm per side.

[0084] The negative electrode material 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 doped into the negative electrode material layer.

[0085] (Positive Electrode) The positive electrode includes a positive electrode current collector and a positive electrode material layer (positive electrode mixture layer) supported on the positive electrode current collector.

[0086] The positive electrode current collector is made of a sheet-like metal material, such as a metal foil, a porous metal, or an etched metal. Examples of the metal material include aluminum, an aluminum alloy, nickel, and titanium.

[0087] 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.

[0088] The positive electrode layer contains a positive electrode active material that reversibly dopes and dedopes anions of a lithium salt. The positive electrode active material contains lithium transition metal oxide particles having a carbon layer formed on the surface thereof.

[0089] The method for preparing lithium transition metal oxide particles having a carbon layer formed on their surfaces is described below. Specifically, powdered lithium transition metal oxide is placed in an electric furnace and heated for 10 minutes to 8 hours at 300 to 800°C in a reducing atmosphere, such as a tubular furnace or muffle furnace, while flowing carbon dioxide gas or a hydrocarbon gas (e.g., acetylene gas or methane gas). It is preferable to rotate the electric furnace and stir the metal oxide powder during the treatment so that the carbon layer is formed on the entire surface of the metal oxide particles. To form a uniform and dense carbon layer, it is preferable to carry out the reaction at a relatively low temperature of 300 to 500°C for a long period of 1 to 5 hours, or to moderate the reaction by mixing a small amount of nitrogen gas into the raw material gas. Electrochemical devices using lithium transition metal oxide particles having a uniform and dense carbon layer exhibit excellent float characteristics.

[0090] The lithium transition metal oxide used as the base is not particularly limited, and may be, for example, at least one metal oxide selected from the group consisting of olivine-type phosphate compounds, layered rock salt-type lithium transition metal composite oxides, and spinel-type manganese oxides. More specifically, powdered lithium iron phosphate, lithium manganese iron phosphate, lithium cobalt phosphate, lithium cobalt oxide, manganese spinel, lithium nickel oxide, and ternary lithium nickel manganese cobalt oxide, or mixtures thereof, may be used.

[0091] The thickness of the carbon layer can be adjusted by adjusting the gas flow rate, treatment time, and reaction temperature of the carbon dioxide gas or other gases used during the surface treatment. The reason for limiting the thickness of the carbon layer to a range of 1 to 10 nm is that a carbon layer thickness of less than 1 nm would result in insufficient conductivity between the positive electrode active material and the carbon layer. Furthermore, decomposition products of the electrolyte generated by the reaction between the active surface of the positive electrode active material and the electrolyte form a film on the positive electrode active material, increasing resistance. This prevents electrochemical devices using the above-described positive electrode from achieving a sufficiently high discharge capacity. On the other hand, a carbon layer thickness exceeding 10 nm increases the volume ratio of the carbon layer to the positive electrode material layer, reducing the density of the active material and impeding the movement of lithium ions, resulting in a decrease in discharge capacity at high current densities.

[0092] The thickness of the carbon layer can be measured by TEM-EELS (JEM-2010FEF manufactured by JEOL Ltd.).

[0093] The lithium transition metal oxide 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 material layer.

[0094] The positive electrode material layer contains, as essential components, a positive electrode active material and a polymeric acid having a carboxylic acid group or its salt, which is a binder for the positive electrode material layer (hereinafter referred to as a positive electrode binder), and optionally contains a conductive material, other binders, etc. Examples of conductive materials include carbon black and carbon fiber. Examples of other binders include a negative electrode binder.

[0095] The term "polymeric acid having a carboxylic acid group or its salt" is a broad concept that includes not only unneutralized polymeric acids but also their neutralized salts. The polymeric acid or its salt includes one or more selected from the group consisting of polyacrylic acid, polymethacrylic acid, and copolymers containing acrylic acid and methacrylic acid, and may be used singly or in combination of two or more.

[0096] The positive electrode material layer is formed, for example, by mixing a positive electrode active material, a conductive agent, a positive electrode 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. The thickness of the positive electrode material layer is, for example, 10 to 300 μm per side of the positive electrode current collector.

[0097] (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.

[0098] (Electrolyte) The electrolyte has lithium ion conductivity and contains a lithium salt and a solvent for dissolving the lithium salt. The anions of the lithium salt are reversibly doped and dedoped to the positive electrode. The lithium ions derived from the lithium salt are reversibly doped and dedoped to the negative electrode.

[0099] Examples of lithium salts include LiClO ４ , LiBF ４ , LiPF ６ , LiAlCl ４ , LiSbF ６ , LiSCN, LiCF ３ SO ３ , LiFSO ３ , LiCF ３ CO ２ , LiAsF ６ , LiB １０ Cl １０ , LiCl, LiBr, LiI, LiBCl ４ , LiN(SO ２ F) ２ , LiN(SO ２ CF ３ ) ２ These may be used alone or in combination of two or more. The lithium salt is preferably a salt having a fluorine-containing anion, since it has a high degree of dissociation, can provide an electrolyte solution with low viscosity, and can improve the withstand voltage characteristics of an electrochemical device.

[0100] The electrolyte preferably includes an imide-based electrolyte. The imide-based electrolyte includes an imide-based anion as the anion of the lithium salt. The imide-based anion may be an anion containing fluorine and sulfur, and in particular, lithium bis(fluorosulfonyl)imide, i.e., LiN(SO ２ F) ２ It is preferable to use LiFSI. For example, 80 mass % or more of the lithium salt may be LiFSI.

[0101] LiFSI is believed to have the effect of reducing the degradation of the positive electrode active material and the negative electrode active material. Among salts containing fluorine-containing anions, the FSI anion is highly stable, and therefore is less likely to produce by-products, contributing to smooth charge and discharge without damaging the surface of the active material. Furthermore, the SEI coating formed on the surface of the negative electrode layer by LiFSI contains a large amount of lithium fluoride and a small amount of lithium carbonate. This allows a stable coating (second layer) primarily composed of lithium fluoride to be formed over the first layer primarily composed of lithium carbonate.

[0102] The concentration of the lithium salt in the non-aqueous electrolyte in a charged state (state of charge (SOC) 90 to 100%) is, for example, 0.2 to 5 mol / L.

[0103] 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.

[0104] 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.

[0105] (Additional Notes) The above embodiments disclose the following technologies. (Technology 1) An electrochemical device including a positive electrode, a negative electrode, and a lithium ion conductive electrolyte, wherein the positive electrode comprises a positive electrode current collector and a positive electrode material layer supported on the positive electrode current collector, the positive electrode material layer includes lithium transition metal oxide particles having a carbon layer formed on their surfaces, and a polymeric acid having a carboxylic acid group or a salt thereof, the negative electrode comprises a negative electrode current collector and a negative electrode material layer supported on the negative electrode current collector, the negative electrode material layer includes a carbon material reversibly doped with lithium ions, and a surface layer portion of the negative electrode material layer has a first layer containing lithium carbonate. (Technology 2) The electrochemical device according to Technology 1, wherein the lithium transition metal oxide includes at least one metal oxide selected from the group consisting of an olivine-type phosphate compound, a layered rock salt-type lithium transition metal composite oxide, and a spinel-type manganese oxide. (Technology 3) The electrochemical device according to Technology 1 or 2, wherein the polymeric acid comprises at least one selected from the group consisting of polyacrylic acid, polymethacrylic acid, and a copolymer containing acrylic acid and methacrylic acid. (Technology 4) The electrochemical device according to any one of Technology 1 to 3, wherein the polymeric acid has a weight-average molecular weight of 5,000 or more and 500,000 or less. (Technology 5) The electrochemical device according to any one of Technology 1 to 4, wherein the content of the polymeric acid is 1% by mass or more and 9% by mass or less with respect to the total mass of the positive electrode material layer. (Technology 6) The electrochemical device according to any one of Technology 1 to 5, wherein the negative electrode material layer comprises at least one selected from the group consisting of carboxymethyl cellulose, carboxymethyl cellulose salt, and styrene butadiene rubber. (Technology 7) The electrochemical device according to any one of Technology 1 to 6, wherein a surface layer portion of the negative electrode material layer has a second layer containing a solid electrolyte, and at least a portion of the second layer covers at least a portion of the surface of the negative electrode material layer via the first layer. (Technology 8) The electrochemical device according to Technology 7, wherein the second layer contains lithium carbonate, and the content of the lithium carbonate contained in the second layer is less than the content of the lithium carbonate contained in the first layer.(Technology 9) The electrochemical device according to any one of Techniques 1 to 8, wherein the electrolyte includes an imide-based electrolyte. (Technology 10) The electrochemical device according to Technique 9, wherein the imide-based electrolyte includes an anion containing fluorine and sulfur.

[0106] EXAMPLES The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples.

[0107] Example 1 (1) Preparation of Positive Electrode Active Material Commercially available lithium iron phosphate (LFP) powder manufactured by Merck was used as the positive electrode active material in Example 1. (2) Step of Forming a Carbon Layer on the Particle Surface of the Positive Electrode Active Material A carbon layer was formed on the surface of LFP particles by the following method. The LFP powder was placed in an electric furnace and heated at 450°C for 3 hours in a reducing atmosphere while flowing acetylene gas, thereby obtaining LFP powder with a 3 nm-thick carbon layer. (3) Fabrication of Positive Electrode 90 parts by mass of the positive electrode active material obtained in step (2) above, 5 parts by mass of polyacrylic acid (weight average molecular weight: 25,000), and 5 parts by mass of Ketjen Black, a conductive material, were dispersed in water to prepare a positive electrode mixture slurry. The resulting positive electrode mixture slurry was applied to both sides of an etched aluminum foil (20 μm thick) serving as a positive electrode current collector, and the coating was dried and rolled to form a positive electrode material layer, thereby obtaining a positive electrode. A positive current collector exposed portion having a width of 10 mm was formed at the longitudinal end of the positive electrode current collector.

[0108] (4) Preparation of Negative Electrode 90 parts by mass of non-graphitizable carbon, 5 parts by mass of Ketjen Black, 2 parts by mass of CMC, and 3 parts by mass of SBR were dispersed in water to prepare a negative electrode mixture slurry. The obtained negative electrode mixture slurry was applied to both sides of a copper foil (thickness: 8 μm) serving as a negative electrode current collector, and the coating was dried and rolled to form a negative electrode material layer, thereby obtaining a negative electrode.

[0109] A certain amount of metallic lithium for pre-doping was attached to the entire surface of the negative electrode mixture layer. 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.

[0110] Thereafter, the chamber of the device was purged with carbon dioxide to create a carbon dioxide gas atmosphere, thereby forming a coating containing lithium carbonate on the surface of the negative electrode mixture layer. The dew point of the carbon dioxide gas atmosphere was −40° C., the molar fraction of carbon dioxide was 100%, and the pressure in the chamber was 1 atmosphere (1.01×10 ５ The temperature of the negative electrode exposed to the carbon dioxide gas atmosphere at 1 atmosphere was 25° C. The time for exposing the negative electrode to the carbon dioxide gas atmosphere was 22 hours.

[0111] (5) Preparation of Electrode Assembly The positive electrode and negative electrode were wound into a cylindrical shape with a polypropylene separator (20 μm thick) interposed therebetween to form an electrode assembly with a discharge capacity of 1 A. The exposed positive electrode core material protruded from one end face of the wound assembly, and the exposed negative electrode core material protruded from the other end face of the electrode assembly. A disk-shaped positive electrode current collector and a disk-shaped negative electrode current collector were welded to the exposed positive electrode core material and the exposed negative electrode core material, respectively. The prepared electrode assembly was then left to stand in a carbon dioxide gas atmosphere for 20 hours until lithium ions were doped between the lattices of the negative electrode.

[0112] (6) Preparation of Non-Aqueous Electrolyte An electrolyte was prepared by dissolving a lithium salt in a solvent. A mixture of a cyclic carbonate and a chain carbonate was used as the solvent. Propylene carbonate and ethyl methyl carbonate were used as the cyclic carbonate and the chain carbonate, respectively. Specifically, propylene carbonate and dimethyl carbonate were mixed in a volume ratio of 30:70 to prepare the solvent. LiFSI was dissolved as a lithium salt in the resulting solvent at a concentration of 1.0 mol / L to prepare a non-aqueous electrolyte.

[0113] (7) Assembly of the electrochemical device The electrode assembly was housed in a cell case with an opening and a bottom, 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 the nonaqueous electrolyte was poured into the cell case, the opening of the cell case was sealed with a sealing plate to assemble the electrochemical device A1 as shown in FIG.

[0114] Thereafter, aging was performed by applying a current of 1 A and charging / discharging for 10 cycles in a voltage range of 2.2 to 3.6 V in an atmosphere at 80° C. During this process, a second layer containing a large amount of lithium fluoride can be formed on the surface of the negative electrode active material with the first layer as a base layer.

[0115] (8) Evaluation [Evaluation 1] (Measurement of internal resistance of electrochemical device) After aging, the electrochemical device was charged at a constant current of 1 A in an environment of −30° C. until the voltage reached 3.6 V, and then the state in which a voltage of 3.6 V was applied was maintained for 10 minutes. Thereafter, the device was discharged at a constant current of 1 A in an environment of −30° C. until the voltage reached 2.2 V.

[0116] Using the discharge curve obtained from the above discharge (vertical axis: discharge voltage, horizontal axis: discharge time), a linear approximation line was calculated for the range from 0.5 seconds to 2 seconds after the start of discharge of the discharge curve, and the voltage VS at the intercept of the approximation line was determined. ΔV was calculated as the value (V0-VS) obtained by subtracting the voltage VS from the voltage VO at the start of discharge (0 seconds after the start of discharge). The internal resistance (DCR) R1 (Ω) of the electrochemical device was calculated using ΔV (V) and the current value Id during discharge according to the following formula (A):

[0117] Internal resistance R1=ΔV / Id (A)

[0118] [Evaluation 2] (Measurement of initial capacity of electrochemical device) After pre-doping, the electrochemical device was discharged to 2.2 V at a current value of 1 A in an atmosphere of 25° C., and then charged again to 3.6 V at a current value of 1 A, and then discharged to 2.2 V at a current value of 1 A. The discharge capacity C1 before floating was determined.

[0119] [Evaluation 3] (Measurement of Capacity of Electrochemical Device After Float Test) Next, a float test was performed by applying a constant voltage of 3.6 V to the electrochemical device for 1000 hours in an environment of 70° C. Thereafter, the device was left in an atmosphere of 25° C. for 2 hours, discharged to 2.2 V at a current value of 1 A, charged again to 3.6 V at a current value of 1 A, and then discharged to 2.2 V at a current value of 1 A. The discharge capacity in this case was taken as the discharge capacity C2 after the float test.

[0120] Using the obtained C1 and C2, the capacity retention rate (%) was calculated according to the following formula (B).

[0121] Capacity maintenance rate = (C2 / C1) x 100 (B)

[0122] It can be said that the larger the capacity retention rate, the less the electrochemical device deteriorates in the float test.

[0123] Comparative Example 1 Electrochemical device B1 was produced in the same manner as in Example 1, except that 2 parts by mass of CMC and 3 parts by mass of SBR were used instead of 5 parts by mass of polyacrylic acid (weight average molecular weight: 25,000) used in preparing the positive electrode mixture slurry. Electrochemical device B1 was evaluated in the same manner as in Example 1.

[0124] (Comparative Examples 2 to 3) For Comparative Examples 2 to 3, positive electrode mixture slurries were prepared by the same method as in Example 1, except that the weight-average molecular weight and the amount of polyacrylic acid added were changed. However, these positive electrode mixture slurries had low slurry viscosity, which deteriorated the coating properties, and therefore could not be uniformly coated on the current collector to the desired thickness. Because electrochemical devices B2 to B3 could not be fabricated due to the above, evaluation of electrochemical devices B2 to B3 was discontinued.

[0125] Table 1 shows the type of positive electrode active material, the type and amount of binder, the weight-average molecular weight of the polyacrylic acid used as the binder, the DCR before the float test, and the capacity retention rate after the float test for electrochemical devices A1 and B1 to B3. In Table 1, the DCR before the float test and the capacity retention rate after the float test are expressed as relative values, with the DCR before the float test and the capacity retention rate after the float test for electrochemical device B1 set to 100. Electrochemical device A1 is an example, and electrochemical device B1 is a comparative example.

[0126]

[0127] The electrochemical device A1 had a lower DCR before the float test than the electrochemical device B1. This is presumably because the positive electrode material layer of the electrochemical device A1 contains polyacrylic acid, which improves the binding strength of the positive electrode material layer of the electrochemical device A1 and improves the conductivity of the positive electrode material layer. On the other hand, the electrochemical device B1 differs from the electrochemical device A1 in that the positive electrode material layer contains CMC and SBR. The CMC and SBR contained in the positive electrode material layer of the electrochemical device B1 have weaker binding strength than the acrylic acid contained in the positive electrode material layer of the electrochemical device A1, which is presumably why the DCR before the float test increased. Furthermore, the capacity retention rate of the electrochemical device A1 after the float test was improved compared to the electrochemical device B1. This is presumably because the polyacrylic acid contained in the positive electrode material layer of the electrochemical device A1 moderately coats the surface of the positive electrode active material, thereby suppressing the reaction between the positive electrode active material and decomposition products of the electrolyte, thereby suppressing the capacity loss of the electrochemical device A1. On the other hand, it is presumed that the CMC and SBR contained in the positive electrode material layer of electrochemical device B1 were unable to sufficiently suppress the reaction between the positive electrode active material and the electrolyte decomposition products, resulting in a decrease in the capacity of electrochemical device A1.

[0128] (Comparative Example 4) In the preparation of the negative electrode, instead of the method of attaching metallic lithium for pre-doping (hereinafter referred to as solid-phase doping method), the negative electrode was pre-doped by contacting it with an electrolyte solution having lithium ion conductivity. Specifically, a working electrode (SUS metal plate) to which a lithium foil serving as a negative electrode and a lithium ion source was attached was placed in an electrolytic cell filled with an electrolyte solution having lithium ion conductivity, with a separator interposed between the negative electrode and the working electrode. Next, the working electrode was used as a positive electrode, and a voltage was applied between the positive electrode and the negative electrode to dope lithium ions into the carbon layer (hereinafter referred to as liquid-phase doping method).

[0129] The lithium foil and the negative electrode were wound in a columnar shape with a polypropylene resin separator (thickness: 20 μm) interposed therebetween, and the electrolyte solution prepared in Example 1 was added. Then, a current was applied so that the negative electrode potential was 0.2 V or less, thereby producing a negative electrode doped with lithium.

[0130] The above-mentioned wound body was disassembled, and the doped negative electrode (hereinafter referred to as the doped negative electrode) was taken out. The positive electrode and the doped negative electrode used in Example 1 were wound in a columnar shape with a polypropylene resin separator (thickness: 20 μm) interposed therebetween to prepare a wound body.

[0131] Using the above wound body, an electrochemical device B4 was produced in the same manner as in Example 1 and evaluated.

[0132] Comparative Example 5 A wound body was produced using a positive electrode in which the CMC and SBR contained in the positive electrode material layer of Comparative Example 1 were replaced with only CMC, and an electrochemical device B5 was produced and evaluated in the same manner as electrochemical device A1.

[0133] Comparative Example 6: An attempt was made to form a positive electrode material layer by replacing the CMC and SBR contained in the positive electrode mixture slurry of Comparative Example 1 with only SBR. However, these positive electrode mixture slurries had low slurry viscosity and poor coating properties, making it impossible to uniformly coat the current collector with the desired thickness. Because of the above, it was not possible to fabricate electrochemical device B6, and evaluation of electrochemical device B6 was therefore discontinued.

[0134] Table 2 shows the type of positive electrode active material, the type and amount of binder, the weight-average molecular weight of the polyacrylic acid used as the binder, the negative electrode pre-doping method, the DCR before the float test, and the capacity retention rate after the float test for electrochemical devices A1, B1, and B4 to B6. In Table 2, the DCR before the float test and the capacity retention rate after the float test are expressed as relative values, with the DCR before the float test and the capacity retention rate after the float test for electrochemical device B1 set to 100.

[0135]

[0136] In electrochemical device B4, the DCR before the float test was increased and the capacity retention rate after the float test was decreased compared to electrochemical device B1. This is presumably because the use of a doped negative electrode prepared by the liquid phase doping method increased the internal resistance of the negative electrode.

[0137] The DCR of electrochemical device B5 before the float test was higher than the DCR of electrochemical devices A1 and B1 before the float test. This is thought to be due to the low oxidation resistance of the CMC contained in the positive electrode material layer of electrochemical device B5, which led to decomposition of the CMC during aging, reducing the binding strength of the positive electrode material layer and resulting in a decrease in conductivity. Furthermore, the capacity retention rate of electrochemical device B5 decreased after the float test. This is thought to be due to further oxidative degradation of the CMC during the float test, which led to peeling of the positive electrode material layer and a significant decrease in capacity.

[0138] (Example 2) (1) Preparation of Positive Electrode Active Material Lithium manganese iron phosphate (LMFP) was synthesized as the positive electrode active material in Example 2 by the following method. Lithium carbonate (Li ２ CO ３ ), iron(II) oxalate dihydrate (FeC ２ O ４ ・2H ２ O), manganese(II) oxalate dihydrate (MnC ２ O ４ ・2H ２ O) and ammonium dihydrogen phosphate (NH ４ H ２ P.O. ４ ) were weighed out so as to have a molar ratio of Li:Mn:Fe:P=1:0.5:0.5:1, and mixed using a ball mill.

[0139] For the ball mill treatment (first time), an Easy Nano RMBII model manufactured by Imex Co., Ltd. was used, and treatment was carried out under the conditions of a rotation speed of 1000 rpm and a rotation time of 1 hour, with zirconia balls of 1 mm in diameter and acetone as a solvent.

[0140] The ball milled mixture was dried and then calcined in a nitrogen atmosphere at 400° C. for 10 hours.

[0141] After firing, the fired product was pulverized in a mortar. Acetone was added to the pulverized fired product again, and the mixture was pulverized using the Easy Nano RMB II at 3,000 rpm for 1 hour, in the same manner as in the first ball mill treatment.

[0142] After the second ball mill treatment, the fired product was again pulverized in a mortar and heat-treated at 550° C. for 1 hour in a nitrogen atmosphere to obtain LMFP (Mn:Fe=1:1).

[0143] (2) Step of forming a carbon layer on the particle surface of the positive electrode active material: A positive electrode active material was produced by forming a carbon layer on LMFP by the following method: The LMFP synthesized by the above method was placed in an electric furnace and heated at 400°C for 1 hour in a reducing atmosphere while flowing acetylene gas, thereby obtaining LMFP powder with a 3-nm-thick carbon layer formed thereon.

[0144] Electrochemical device A2 was fabricated in the same manner as in Example 1, except for the steps of (1) preparing the positive electrode active material and (2) forming a carbon layer on the particle surfaces of the positive electrode active material. Electrochemical device A2 was then evaluated in the same manner as in Example 1.

[0145] Comparative Example 7 Electrochemical device B7 was produced in the same manner as in Example 2, except that 2 parts by mass of CMC and 3 parts by mass of SBR were used instead of 5 parts by mass of polyacrylic acid (weight average molecular weight: 25,000) used in preparing the positive electrode mixture slurry. Electrochemical device B7 was evaluated in the same manner as in Example 2.

[0146] Electrochemical devices A2 and B7, which used LMFP as the positive electrode active material, were aged by 10 cycles of charge and discharge in the voltage range of 2.2 to 4.2 V. The applied voltage set for evaluation of the internal resistance and capacity of the electrochemical devices was also 4.2 V.

[0147] Table 3 shows the type of positive electrode active material, the type and amount of binder, the weight-average molecular weight of the polyacrylic acid used as the binder, the DCR before the float test, and the capacity retention rate after the float test for electrochemical devices A2 and B7. In Table 3, the DCR before the float test and the capacity retention rate after the float test are expressed as relative values, with the DCR before the float test and the capacity retention rate after the float test for electrochemical device B7 set to 100. Electrochemical device A2 is an example, and electrochemical device B7 is a comparative example.

[0148]

[0149] The DCR of electrochemical device A2 before the float test was lower than that of electrochemical device B7. This is presumably because the positive electrode material layer of electrochemical device A2 contains polyacrylic acid, which improves the binding strength of the positive electrode material layer of electrochemical device A2 and improves the conductivity of the positive electrode material layer. On the other hand, electrochemical device B7 differs from electrochemical device A1 in that the positive electrode material layer contains CMC and SBR. The CMC and SBR contained in the positive electrode material layer of electrochemical device B7 have weaker binding strength than the acrylic acid contained in the positive electrode material layer of electrochemical device A2, which is presumably why the DCR before the float test increased. Furthermore, the capacity retention rate of electrochemical device A2 after the float test was improved compared to electrochemical device B7. This is presumably because the polyacrylic acid contained in the positive electrode material layer of electrochemical device A2 moderately coats the surface of the positive electrode active material, thereby suppressing the reaction between the positive electrode active material and electrolyte decomposition products, thereby suppressing the capacity loss of electrochemical device A2. On the other hand, it is presumed that the CMC and SBR contained in the positive electrode material layer of electrochemical device B7 were unable to sufficiently suppress the reaction between the positive electrode active material and the decomposition products of the electrolyte, which resulted in a decrease in the capacity of electrochemical device B7.

[0150] Electrochemical device A1 and electrochemical device A2 have low DCR before the float test and high capacity retention after the float test, regardless of the type of positive electrode active material. In other words, an electrochemical device using a doped negative electrode produced by a solid-phase doping method, a positive electrode containing lithium transition metal oxide particles having a carbon layer formed on the surface, and a polymeric acid having a carboxylic acid group or a salt thereof suppresses an increase in internal resistance and achieves good float characteristics.

[0151] [Evaluation 4: XPS analysis of the surface portion of the negative electrode composite layer] The surface portion of the negative electrode composite layer after exposure to a carbon dioxide gas atmosphere was analyzed by XPS for C1s spectrum, O1s spectrum, and Li1s spectrum. For the analysis, an X-ray photoelectron spectrometer (trade name: Model 5600, manufactured by ULVAC-PHI, Inc.) was used. The measurement conditions are shown below.

[0152] X-ray source: Al-mono (1486.6 eV) 14 kV / 200 W Measurement diameter: 800 μmφ Photoelectron take-off angle: 45° Etching conditions: Acceleration voltage 3 kV, etching rate approximately 3.1 nm / min (SiO ２ Conversion), raster area 3.1mm x 3.4mm

[0153] As an example, the results of XPS analysis of the negative electrode removed from electrochemical device A1 before the float test are shown below. Analysis of the C1s spectrum, O1s spectrum, and Li1s spectrum confirmed that the thickness of the first layer was approximately 18 nm. Specifically, peaks such as C-C bonds, which are presumed to be impurity carbon, were observed on the outermost surface, but these peaks rapidly decreased around 1 to 2 nm deep in the first layer. Meanwhile, a first peak attributed to C=O bonds was observed from the outermost surface to a depth of 18 nm in the surface layer. A peak attributed to Li-O bonds was also observed from a depth of around 18 nm. Furthermore, the steady presence of Li was confirmed from the outermost surface to a depth of 18 nm in the surface layer. No peaks attributed to LiF were observed.

[0154] The surface of the negative electrode composite layer of the negative electrode removed from the electrochemical device was subjected to XPS analysis in the same manner as above, and it was confirmed that a 10 nm thick SEI coating (second layer) was formed, which had a different composition from the first layer and was distinguishable from the first layer. In addition, a peak attributed to LiF was observed.

[0155] The electrochemical device according to the present invention is suitable for use in a vehicle, for example.

[0156] While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention.

[0157] 100: Electrode body 10: Positive electrode 11x: Positive electrode core material exposed portion 13: Positive electrode current collector plate 15: Tab lead 20: Negative electrode 21x: Negative electrode core material exposed portion 23: Negative electrode current collector plate 30: Separator 200: Electrochemical device 210: Cell case 220: Sealing plate 221: Gasket

Claims

1. An electrochemical device comprising a positive electrode, a negative electrode and a lithium ion conductive electrolyte, wherein the positive electrode comprises a positive electrode current collector and a positive electrode material layer supported on the positive electrode current collector, the positive electrode material layer comprises lithium transition metal oxide particles having a carbon layer formed on the surface thereof, and a polymeric acid having a carboxylic acid group or a salt thereof, the negative electrode comprises a negative electrode current collector and a negative electrode material layer supported on the negative electrode current collector, the negative electrode material layer comprises a carbon material reversibly doped with lithium ions, and a surface portion of the negative electrode material layer has a first layer containing lithium carbonate.

2. The electrochemical device according to claim 1, wherein the lithium transition metal oxide comprises at least one metal oxide selected from the group consisting of olivine-type phosphate compounds, layered rock-salt-type lithium transition metal composite oxides, and spinel-type manganese oxides.

3. The electrochemical device according to claim 1, wherein the polymeric acid comprises at least one selected from the group consisting of polyacrylic acid, polymethacrylic acid, and a copolymer containing acrylic acid and methacrylic acid.

4. The electrochemical device according to any one of claims 1 to 3, wherein the weight average molecular weight of the polymer acid is 5,000 or more and 500,000 or less.

5. The electrochemical device according to any one of claims 1 to 3, wherein the content of the polymer acid is 1 mass % or more and 9 mass % or less with respect to the total mass of the positive electrode material layer.

6. The electrochemical device according to any one of claims 1 to 3, wherein the negative electrode material layer contains at least one material selected from the group consisting of carboxymethyl cellulose, carboxymethyl cellulose salts, and styrene-butadiene rubber.

7. The electrochemical device according to any one of claims 1 to 3, wherein a surface portion of the negative electrode material layer has a second layer containing a solid electrolyte, and at least a portion of the second layer covers at least a portion of the surface of the negative electrode material layer via the first layer.

8. The electrochemical device according to claim 7, wherein the second layer contains lithium carbonate, and the content of the lithium carbonate contained in the second layer is less than the content of the lithium carbonate contained in the first layer.

9. The electrochemical device according to any one of claims 1 to 3, wherein the electrolyte includes an imide-based electrolyte.

10. The electrochemical device of claim 9, wherein the imide-based electrolyte includes anions containing fluorine and sulfur.

Images