Electrochemical device

Abstract

This electrochemical device comprises a positive electrode, a negative electrode, and a lithium-ion conductive electrolyte, wherein the positive electrode includes a positive electrode active material, and the positive electrode active material includes lithium transition metal oxide particles having carbon layers formed on the surfaces thereof. The negative electrode includes a negative electrode current collector and a negative electrode material layer supported by the negative electrode current collector. The negative electrode material layer comprises a negative electrode active material that reversibly dopes lithium ions. The negative electrode active material comprises a carbon material. A surface layer portion of the negative electrode material layer has a coated region, and when the coated region is measured using X-ray photoelectron spectroscopy, a peak of a binding energy is observed in a range of 530-534 eV in an O1s spectrum. The peak intensity in the O1s spectrum increases from the surface layer toward the inside of the coated region.

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] International Publication No. 2022 / 181608 Japanese Patent Application Laid-Open No. 2014-143041 Japanese Patent Application Laid-Open No. 2013-149609

[0005] 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.

[0006] Therefore, in electrochemical devices that utilize 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 a gas-phase method such as chemical vapor deposition, physical vapor deposition, or 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 electrolyte reacts with lithium to produce electrolyte decomposition products such as carboxylates.

[0007] 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.

[0008] 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.

[0009] One aspect of the present disclosure relates to 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 including a positive electrode active material, the positive electrode active material including lithium transition metal oxide particles having a carbon layer formed on a surface 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 including a negative electrode active material to be reversibly doped with lithium ions, the negative electrode active material including a carbon material, a surface portion of the negative electrode material layer has a coating region, and when the coating region is measured by X-ray photoelectron spectroscopy, a peak is observed in an O1s spectrum in a binding energy range of 530 to 534 eV, and the intensity of the peak in the O1s spectrum increases from the surface of the coating region toward the inside.

[0010] According to the present disclosure, an electrochemical device with good float characteristics can be obtained.

[0011] 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.

[0012] 1 is a longitudinal cross-sectional view showing the configuration of an electrochemical device according to an embodiment of the present disclosure; 2 is a graph showing changes in the depth direction of the peak intensity A at the apex of the peak attributable to lithium carbonate bonds in the O1s spectrum, the peak intensity B at the apex of the peak in the F1s spectrum, and the peak intensity ratio A / B for the electrochemical device A1 of Example 1; 3 is a graph showing changes in the depth direction of the peak intensity A at the apex of the peak attributable to lithium carbonate bonds in the O1s spectrum, the peak intensity B at the apex of the peak in the F1s spectrum, and the peak intensity ratio A / B for the electrochemical device A5 of Example 5; and 4 is a graph showing changes in the depth direction of the peak intensity A at the apex of the peak attributable to lithium carbonate bonds in the O1s spectrum, the peak intensity B at the apex of the peak in the F1s spectrum, and the peak intensity ratio A / B for the electrochemical device A9 of Example 9. FIG. 10 is a graph showing changes in peak intensity A at the apex of the peak attributed to lithium carbonate bonds in the O1s spectrum, peak intensity B at the apex of the peak in the F1s spectrum, and peak intensity ratio A / B in the depth direction for electrochemical device B4 of Comparative Example 4.

[0013] 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.

[0014] 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.

[0015] 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.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] 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 negative electrode active material that is reversibly doped with lithium ions. The negative electrode active material includes a carbon material.

[0021] 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.

[0022] 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 (Li２ 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.

[0023] The SEI coating may also contain lithium fluoride (LiF). An SEI coating containing lithium fluoride is stable in the electrolyte and has low reactivity with HF. However, an SEI coating containing lithium fluoride as the main component has high resistance to lithium ion migration and is likely to increase the internal resistance of the electrochemical device.

[0024] When the coating region containing lithium carbonate is measured by X-ray photoelectron spectroscopy (XPS), a peak attributed to lithium carbonate bonds is observed in the O1s spectrum. The peak attributed to lithium carbonate bonds is a peak attributed to the C═O bond (or C—O bond) of lithium carbonate, and can appear in the binding energy range of 530 to 534 eV.

[0025] On the other hand, when the coating region containing lithium fluoride is measured by X-ray photoelectron spectroscopy (XPS), a peak attributed to the lithium fluoride bond is observed in the F1s spectrum. The peak attributed to the lithium fluoride bond is a peak attributed to the Li—F bond, and can appear in the binding energy range of 684.8 to 685.3 eV.

[0026] In an electrochemical device according to an embodiment of the present disclosure, the intensity of the peak observed in the O1s spectrum in the binding energy range of 530 to 534 eV increases from the surface layer of the coating region toward the interior. This means that within the coating region, lithium carbonate is unevenly distributed in the depth direction, with greater amounts present in the central and deep portions (negative electrode active material side) of the coating region, and the amount of lithium carbonate present in the surface layer of the coating region is reduced. This prevents the SEI coating, which contains a large amount of lithium carbonate, from coming into contact with the electrolyte, thereby suppressing the collapse of the SEI coating while maintaining low resistance to lithium ion migration.

[0027] In contrast, in the F1s spectrum, the intensity of the peak observed in the binding energy range of 684.8 to 685.3 eV may decrease from the surface layer of the coating region toward the inside, in a depth region where the intensity of the peak attributable to the lithium carbonate bond in the O1s spectrum increases from the surface layer of the coating region toward the inside. That is, an SEI coating rich in lithium fluoride may be formed on the surface layer of the coating region that comes into contact with the electrolyte. An SEI coating rich in lithium fluoride is stable against the electrolyte and is less likely to collapse. Furthermore, because the SEI coating rich in lithium carbonate is formed on the inside, the thickness of the SEI coating rich in lithium fluoride may be thin, and an increase in the migration resistance of lithium ions is suppressed.

[0028] In other words, the coating region has a first layer containing a large amount of lithium fluoride formed on the surface layer in contact with the electrolyte solution, and a second layer containing a large amount of lithium carbonate formed more internally than the first layer (on the side in contact with the negative electrode active material). The concentration of lithium fluoride (content per unit volume) in the first layer is greater than the concentration of lithium fluoride in the second layer. Meanwhile, the concentration of lithium carbonate (content per unit volume) in the second layer is greater than the concentration of lithium carbonate in the first layer. However, it is not necessary for there to be a clear difference in the concentrations of lithium carbonate or lithium fluoride across the boundary between the first and second layers. The concentration of lithium fluoride may gradually decrease from the first layer to the second layer and / or the concentration of lithium carbonate may gradually increase from the first layer to the second layer.

[0029] At any depth, the peak intensity at the apex of the peak in the O1s spectrum is designated as A, and the peak intensity at the apex of the peak in the F1s spectrum is designated as B. The distribution of peak intensity B in the depth direction (thickness direction of the surface layer portion) may have a maximum apex in the surface layer (first layer) of the coating region. In contrast, the distribution of peak intensity A in the depth direction (thickness direction of the surface layer portion) may have a peak intensity that increases toward the negative electrode active material in the surface layer of the coating region and has a maximum apex in the second layer (negative electrode active material side) more inward than the surface layer. In this case, the peak intensity ratio A / B increases and then decreases as it moves from the surface layer to the inside of the coating region. The peak intensity ratio A / B preferably has a maximum value within the coating region. In this case, when the surface layer portion of the negative electrode material layer is measured by X-ray photoelectron spectroscopy, peaks attributable to the carbon material are substantially not observed in the C1s spectrum at the depth position where the peak intensity ratio A / B is maximum.

[0030] The peaks attributable to carbon materials in the C1s spectrum are peaks attributable to C-C bonds in the plane of graphite, and may appear in the binding energy range of 281 to 283 eV. "Substantially no peaks attributable to carbon materials are observed" means that even if the peaks are slightly observed, the peak intensity at the apex is 0.2 times or less the peak intensity A.

[0031] The maximum value of the peak intensity ratio A / B in the depth direction is, for example, 0.5 to 2, and may be 1 to 1.8. The peak intensities A and B are determined from the height of the peak from the baseline.

[0032] The lithium carbonate-rich layer (second layer) may be formed on the surface of the negative electrode material layer before assembling the electrochemical device. In an electrochemical device assembled using the negative electrode, subsequent charging and discharging may result in the formation of a uniform, moderately thick lithium fluoride-rich first layer on the surface of the negative electrode active material, with the second layer as a base layer. The first layer is formed, for example, by a reaction between a fluorine-containing electrolyte and the negative electrode in the electrochemical device. The lithium carbonate-containing second layer promotes the formation of the first layer, which is a good SEI coating, and maintains the SEI coating in good condition even when repeatedly charged and discharged.

[0033] The thickness of the first layer containing a large amount of lithium fluoride may be, for example, 1 nm or more, 3 nm or more, or 5 nm or more. However, if the thickness of the first layer exceeds 20 nm, the first layer itself may become a resistance component. Therefore, the thickness of the first layer may be 20 nm or less, or may be 10 nm or less.

[0034] In contrast, the thickness of the second layer containing a large amount of lithium carbonate may be, for example, 1 nm or more, and may be 5 nm or more if a longer-term effect is expected, or 10 nm or more if a more reliable effect is expected. However, if the thickness of the second layer exceeds 50 nm, the second layer itself may become a resistance component. Therefore, the thickness of the second layer may be 50 nm or less, or may be 30 nm or less. For example, it is 1 nm to 50 nm.

[0035] The thickness of the coating region (the thickness of the first layer and the second layer) is measured by analyzing the surface portion of the negative electrode material layer at multiple locations (at least five locations). The average of the thicknesses of the first layer or the second layer measured at the multiple locations may be used as the thickness of the first layer or the second layer. The negative electrode material 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 carbon material that constituted the vicinity of the surface portion of the negative electrode material layer may be analyzed. In this case, the carbon material covered with the coating may be collected from the region of the negative electrode material layer that was disposed on the side opposite to the surface that was bonded to the negative electrode current collector and used for analysis.

[0036] In the case of a negative electrode removed from a completed electrochemical device after a predetermined aging or at least one charge / discharge, the coating formed on the surface of the negative electrode material layer or the surface of the carbon material has an SEI coating (i.e., the first layer) formed within the electrochemical device. In this case, a peak attributed to lithium carbonate can also be observed in the first layer in the O1s spectrum. However, since the first layer formed within the electrochemical device has a different composition from the second layer formed in advance before assembling the electrochemical device, the two can be distinguished. For example, in XPS analysis of the first layer, an F1s peak attributed to LiF bonds is observed, but a substantial F1s peak attributed to LiF bonds is not observed in the second layer. Furthermore, the amount of lithium carbonate contained in the first layer is small. Note that, as a peak in the Li1s spectrum, for example, ROCO ２ Peaks derived from compounds such as Li and ROLi can be detected.

[0037] When the coating region containing lithium carbonate is analyzed by XPS, a peak (first peak) attributable to a C=O bond and a peak (second peak) attributable to a Li-O bond may be observed in the O1s spectrum. The coating region present near the surface of the carbon material contains a small amount of LiOH or Li ２ It is believed to contain O.

[0038] Specifically, when the coating region is analyzed in the depth direction, in the O1s spectrum, a first region in which a first peak (attributed to a C═O bond) and a second peak (attributed to a Li—O bond) are observed, in order of decreasing distance from the outermost surface of the surface layer portion, 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. Furthermore, a third region may also be present, which is closer to the outermost surface of the surface layer portion than the first region, and in which the first peak is observed but the second peak is not observed. The third region is more likely to be observed when the thickness of the second layer is large.

[0039] At the center of the second layer in the thickness direction, a peak attributed to a C—C bond is generally not observed in the C1s spectrum, or even if observed, its intensity is half or less of the peak intensity attributed to a C═O bond.

[0040] XPS analysis of the surface portion of the negative electrode material layer is performed, for example, by irradiating an argon beam onto the surface portion or a coating formed on the surface of the carbon material in the chamber of an X-ray photoelectron spectrometer, and observing and recording changes in the spectra attributable to C1s, O1s, or F1s electrons 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 a peak attributable to lithium fluoride is stably observed corresponds to the thickness of the first layer. The thickness of the region where a peak attributable to lithium carbonate is stably observed corresponds to the thickness of the second layer.

[0041] Next, a method for forming a coating region on the surface of the negative electrode layer will be described. First, a second layer containing lithium carbonate is formed on the surface of the negative electrode layer. The step of forming the second layer can be performed by, for example, a vapor phase method, a coating method, or a transfer method.

[0042] 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 material 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 material layer may be -20 to 80°C.

[0043] 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 second 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).

[0044] Furthermore, a step of forming a first layer containing lithium fluoride so as to cover at least a portion of the second layer is carried out to obtain a negative electrode. The surface layer portion of the obtained negative electrode material layer has a first layer and a second layer. The first 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 via the second layer (i.e., the second layer serves as a base layer).

[0045] The step of forming the first layer is carried out in a state where the negative electrode material layer is in contact with the electrolyte, and therefore may also serve as at least a part of a step of pre-doping lithium ions into the negative electrode material layer. For example, metallic lithium may be used as a source of the pre-doped lithium ions.

[0046] Metallic lithium may be attached to the surface of the negative electrode material layer (solid-phase doping). Note that a second 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 material layer to which metallic lithium has been attached to a carbon dioxide gas atmosphere.

[0047] The step of attaching metallic lithium to the surface of the negative electrode material layer can be carried out by, for example, a vapor phase method, transfer, etc. 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 material 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.

[0048] 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.

[0049] To form a thicker second layer, the partial pressure of carbon dioxide is set to, for example, 0.5 atmospheres (5.05×10 ４It is efficient if the pressure is greater than 1 atmosphere (1.01 x 10 ５ Pa) or more.

[0050] 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 second layer becomes.

[0051] The thickness of the second 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.

[0052] The step of forming the second 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 material 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 second layer on the surface portion of the negative electrode material layer.

[0053] The pre-doping process of lithium ions into the negative electrode material layer can be further carried out, for example, by subsequently contacting the negative electrode material layer with an electrolyte and then leaving the negative electrode material layer for a predetermined period of time to complete the process. This process can be a process of forming a first layer to cover at least a portion of the second layer. For example, by charging and discharging the electrochemical device at least once, the first layer can be formed on the negative electrode material layer and the pre-doping of lithium ions into the negative electrode can be completed. Furthermore, the 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).

[0054] 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 (second layer) primarily composed of lithium carbonate is formed on the surface of the negative electrode material. The SEI coating containing lithium carbonate has low resistance to lithium ion migration, and therefore can reduce the resistance of the negative electrode material layer.

[0055] Furthermore, in an electrochemical device assembled using a negative electrode having the second layer, a stable coating (first 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 second layer as a base layer. The first layer formed inside the second 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.

[0056] 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, electrochemical devices using a positive electrode layer containing a lithium transition metal oxide as the positive electrode active material experience increased reactivity between the positive electrode active material and the electrolyte decomposition products, resulting in increased internal resistance.

[0057] However, by forming a carbon layer on the surface of lithium transition metal oxide particles, the reaction between the lithium transition metal oxide and the above-mentioned electrolyte decomposition products is suppressed, and therefore, an electrochemical device using a doped negative electrode prepared by the solid-phase doping method and a positive electrode made of lithium transition metal oxide particles with a carbon layer formed on the particle surface can suppress an increase in internal resistance and exhibit good float characteristics.

[0058] 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 second layer, with an electrolyte containing lithium ions. In this case, the decomposition reaction of the electrolyte and the electrolyte, which occurs concurrently 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 a first layer composed mainly 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.

[0059] FIG. 1 schematically illustrates the configuration of an electrochemical device 200 according to an embodiment of the present disclosure. 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 a 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 a 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.

[0060] Each component of the electrochemical device according to the embodiment of the present disclosure will be described in more detail below.

[0061] (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.

[0062] 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.

[0063] 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.

[0064] (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.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] In this specification, the average particle size means the volume-based median diameter (D50) in the particle size distribution obtained by laser diffraction particle size distribution measurement.

[0069] The negative electrode layer contains a negative electrode active material as an essential component, and optionally contains a conductive material, a binder, etc. Examples of the conductive material include carbon black and carbon fiber. Examples of the binder include fluororesin, acrylic resin, rubber material, and cellulose derivative.

[0070] The negative electrode material layer is formed, for example, by mixing a negative electrode active material, a conductive agent, a binder, and the like together with a dispersion medium to prepare a negative electrode mixture slurry, applying the negative electrode mixture slurry to a negative electrode current collector, and then drying the negative electrode mixture slurry. The thickness of the negative electrode material layer is, for example, 10 to 300 μm per side.

[0071] 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.

[0072] (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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] The thickness of the carbon layer can be measured by TEM-EELS (JEM-2010FEF manufactured by JEOL Ltd.).

[0080] 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.

[0081] The positive electrode layer contains a positive electrode active material as an essential component, and optionally contains a conductive material, a binder, etc. Examples of the conductive material include carbon black and carbon fiber. Examples of the binder include fluororesin, acrylic resin, rubber material, and cellulose derivative.

[0082] The positive electrode material layer is formed, for example, by mixing a positive electrode active material, a conductive agent, a binder, etc., together with a dispersion medium to prepare a positive electrode mixture slurry, applying the positive electrode mixture slurry to a positive electrode current collector, and then drying 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.

[0083] (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.

[0084] (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.

[0085] 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.

[0086] 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.

[0087] 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, so it is unlikely to produce by-products and is believed to contribute to smooth charging and discharging without damaging the surface of the active material. Furthermore, the SEI coating formed on the surface of the negative electrode material layer by LiFSI contains a large amount of lithium fluoride and a small amount of lithium carbonate. This allows a stable coating (first layer) primarily composed of lithium fluoride to be formed over the second layer primarily composed of lithium carbonate.

[0088] 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.

[0089] 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.

[0090] 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.

[0091] (Additional Notes) The above description of the embodiments discloses the following techniques. (Technology 1) An electrochemical device comprising: a positive electrode, a negative electrode, and a lithium ion conductive electrolyte; 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 a positive electrode active material; the positive electrode active material comprises lithium transition metal oxide particles having a carbon layer formed on a surface 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 negative electrode active material to be reversibly doped with lithium ions; the negative electrode active material comprises a carbon material; a surface portion of the negative electrode material layer has a coating region; when the coating region is measured by X-ray photoelectron spectroscopy, a peak is observed in an O1s spectrum in a binding energy range of 530 to 534 eV; and the intensity of the peak in the O1s spectrum increases from the surface of the coating region toward the inside. (Technology 2) The electrochemical device according to Technology 1, wherein the lithium transition metal oxide comprises 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 carbon layer formed on the surface of the lithium transition metal oxide particles has a thickness of 1.0 nm or more and 10 nm or less. (Technology 4) The electrochemical device according to any one of Technology 1 to 3, wherein, when the coating region is measured by X-ray photoelectron spectroscopy, a peak is observed in the F1s spectrum in a binding energy range of 684.8 to 685.3 eV, and the intensity of the peak in the F1s spectrum decreases from the surface of the coating region toward the inside. (Technology 5) The electrochemical device according to Technology 4, wherein a ratio A / B of a peak intensity A at an apex of the peak in the O1s spectrum to a peak intensity B at an apex of the peak in the F1s spectrum increases and then decreases from a surface layer of the coating region toward the inside, and when a surface portion of the negative electrode material layer is measured by X-ray photoelectron spectroscopy, a peak attributed to a bond of the carbon material is not substantially observed in the C1s spectrum at a depth from the surface layer of the coating region where the ratio A / B is maximum.(Technology 6) The electrochemical device according to any one of Techniques 1 to 5, wherein the electrolyte includes an imide-based electrolyte. (Technology 7) The electrochemical device according to Technique 6, wherein the imide-based electrolyte includes an anion containing fluorine and sulfur.

[0092] EXAMPLES The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples.

[0093] 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, 3 parts by mass of acrylic rubber as a binder, 2 parts by mass of carboxymethyl cellulose as a thickener, and 5 parts by mass of Ketjen Black as 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 electrode current collector exposed portion having a width of 10 mm was formed at the longitudinal end of the positive electrode current collector.

[0094] (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 carboxycellulose, and 3 parts by mass of styrene-butadiene rubber were dispersed in water to prepare a negative electrode mixture slurry. The resulting 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.

[0095] Thereafter, 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.

[0096] 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.

[0097] (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.

[0098] (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.

[0099] (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.

[0100] 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 first layer containing a large amount of lithium fluoride can be formed on the surface of the negative electrode active material with the second layer as an underlying layer.

[0101] (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.

[0102] 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):

[0103] Internal resistance R1=ΔV / Id (A)

[0104] It can be said that the lower the DCR after the float test, the more the increase in internal resistance is suppressed and the better the float characteristics of the electrochemical device.

[0105] [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.

[0106] [Evaluation 3] (Measurement of capacity after float test of electrochemical device) Next, a float test was performed by applying a constant voltage of 3.6 V to the electrochemical device in an environment of 70° C. for 1000 hours. After that, 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, and then charged again to 3.6 V at a current value of 1 A. The discharge capacity when the device was then discharged to 2.2 V at a current value of 1 A was taken as the discharge capacity C2 after the float test.

[0107] Using the obtained C1 and C2, the capacity retention rate (%) was calculated according to the following formula (B).

[0108] Capacity maintenance rate = (C2 / C1) x 100 (B)

[0109] It can be said that the larger the capacity retention rate, the less the electrochemical device deteriorates in the float test.

[0110] (Examples 2 to 4, Comparative Examples 1 to 3) Electrochemical devices A2 to A4 and B1 to B3 were fabricated by the same method as in Example 1, except for the thickness of the carbon layer formed on the particle surface of the LFP, which was the positive electrode active material. Positive electrode active materials with different carbon layer thicknesses were prepared by changing the treatment conditions (gas flow rate or treatment time) in the step of forming a carbon layer on the particle surface of the positive electrode active material. For B1, the electrochemical device was fabricated using LFP powder without a carbon layer.

[0111] The thickness of the carbon layer was measured using TEM-EELS, and it was confirmed that each carbon layer had a uniform thickness.

[0112] (2) Electrochemical devices A2 to A4 and B1 to B3 were fabricated and evaluated in the same manner as in Example 1, except for the step of forming a carbon layer on the particle surface of the positive electrode active material.

[0113] Table 1 shows the carbon layer thickness, DCR after the float test, and capacity retention rate for electrochemical devices A1 to A4 and B1 to B3. In Table 1, the DCR is shown as a relative value, with the DCR after the float test for electrochemical device B1 set at 100. Electrochemical devices A1 to A4 are examples, and electrochemical devices B1 to B3 are comparative examples.

[0114]

[0115] Samples A1 to A4, in which the carbon layer on the surface of the positive electrode active material is 1 to 10 nm thick, have low DCR after the float test and high capacity retention. In particular, sample A1, in which the carbon layer is 3 nm thick, has the best float characteristics and the least capacity degradation among samples A1 to A4 and B1 to B3. On the other hand, samples B1 and B2, in which the carbon layer is less than 1.0 nm thick, have higher DCR after the float test and lower capacity retention compared to samples A1 to A4. However, sample B3, in which the carbon layer is too thick, had an increased DCR after the float test.

[0116] This is presumably because the thickness of the carbon layer on the surface of the positive electrode active material of B1 and B2 was insufficient, which prevented sufficient electrical conduction paths between the active materials. Furthermore, it is presumed that the positive electrode active material deteriorated due to a reaction between the surface of the active positive electrode active material and the electrolyte composition, resulting in an increase in DCR after the float test.

[0117] It is presumed that in A1 to A4, in which a carbon layer of sufficient thickness was formed on the surface of the positive electrode active material, the reaction between the surface of the positive electrode active material and the electrolyte composition was suppressed, and therefore the DCR after the float test was reduced.

[0118] Regarding the capacity retention rate after the float test, when a carbon layer of sufficient thickness is formed on the surface of the positive electrode active material, the electrolyte composition does not come into contact with the active surface of the positive electrode active material, and therefore, a decrease in the capacity retention rate due to deterioration of the positive electrode active material is suppressed.

[0119] On the other hand, if the thickness of the carbon layer on the surface of the positive electrode active material is insufficient, the positive electrode active material is degraded due to a reaction between the surface of the active positive electrode active material and the electrolyte composition, which is presumed to result in a decrease in capacity retention rate.

[0120] (Comparative Examples 4 to 6) 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 a battery container 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 the 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).

[0121] 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.

[0122] The 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 having a capacity of 1 A.

[0123] Using the above wound body, electrochemical devices B4 to B6 were fabricated in the same manner as in Example 1 and evaluated.

[0124] Table 2 shows the carbon layer thickness, DCR after the float test, and capacity retention rate for electrochemical devices A1, B1, and B4 to B6. In Table 2, the DCR is shown as a relative value, with the DCR after the float test for electrochemical device B1 set at 100. Electrochemical device A1 is an example, and devices B1 and B4 to B6 are comparative examples.

[0125]

[0126] In electrochemical devices B4 to B6, the DCR increased after the float test regardless of whether or not a carbon layer was present on the surface of the positive electrode active material. 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.

[0127] Furthermore, in electrochemical devices B4 to B6, the capacity retention rate after the float test decreased regardless of whether a carbon layer was present on the surface of the positive electrode active material. This is presumably because the SEI coating formed on the surface of the doped negative electrode prepared by the liquid phase doping method was non-uniform, and the surface of the active negative electrode active material on which the SEI coating was not formed reacted with the electrolyte, resulting in the deactivation of part of the negative electrode active material.

[0128] Furthermore, in B5, a carbon layer was not formed on the surface of the positive electrode active material and a doped negative electrode prepared by the liquid phase doping method was used, resulting in a larger DCR after the float test and a lower capacity retention rate than in B4.

[0129] (Example 5) (1) Preparation of Positive Electrode Active Material Lithium manganese iron phosphate (LMFP) was synthesized as the positive electrode active material in Example 5 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.

[0130] For the ball mill treatment (first time), Easy Nano RMBII type 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.

[0131] The ball milled mixture was dried and then calcined in a nitrogen atmosphere at 400° C. for 10 hours.

[0132] 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.

[0133] 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).

[0134] (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 4-nm-thick carbon layer formed thereon.

[0135] Electrochemical device A5 was fabricated and evaluated 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.

[0136] (Examples 6 to 8, Comparative Examples 7 to 9) Electrochemical devices A6 to A8 and B7 to B9 were fabricated using the same method as in Example 5, except for the thickness of the carbon layer formed on the surface of the LMFP particles, which were the positive electrode active material. Positive electrode active materials with different carbon layer thicknesses were prepared by changing the treatment conditions (gas flow rate or treatment time) in the step of forming the carbon layer on the surface of the LMFP. For B7, an electrochemical device was fabricated using LMFP without a carbon layer.

[0137] The thickness of the carbon layer was measured using TEM-EELS, and it was confirmed that each carbon layer had a uniform thickness.

[0138] (2) Electrochemical devices A5 to A8 and B7 to B9 were fabricated in the same manner as in Example 1, except for the step of forming a carbon layer on the particle surfaces of the positive electrode active material. The electrochemical devices A5 to A8 and B7 to B9, which used LMFP as the positive electrode active material, were aged by 10 charge / discharge cycles at a voltage ranging from 2.2 to 4.2 V. The applied voltage for evaluation of the internal resistance and capacity of the electrochemical devices was also set to 4.2 V.

[0139] Table 3 shows the carbon layer thickness, DCR after the float test, and capacity retention rate for electrochemical devices A5 to A8 and B7 to B9. In Table 3, the DCR (%) after the float test is shown as a relative value, with the DCR after the float test for electrochemical device B7 set at 100. Electrochemical devices A5 to A8 are examples, and B7 to B9 are comparative examples.

[0140]

[0141] A5 to A8, which have carbon layer thicknesses of 1 to 10 nm, have low DCR after the float test and high capacity retention. In particular, A5, which has a carbon layer thickness of 4 nm, has the best float characteristics and the least capacity degradation among A5 to A8 and B7 to B9. On the other hand, B7 and B8, which have carbon layer thicknesses of less than 1.0 nm, have higher DCR after the float test and lower capacity retention than A5 to A8.

[0142] The results of the DCR and capacity retention rate after the float test for electrochemical devices A5 to A8 and B7 to B9 showed similar trends in the relative magnitude relationships between the samples as those for electrochemical devices A1 to A3 and B1 to B3, although the absolute values ​​differed. This is presumably due to an increase in internal resistance and deterioration of the active material resulting from differences in the positive electrode active materials used.

[0143] Example 9 (1) Preparation of Positive Electrode Active Material Commercially available ternary nickel manganese cobalt oxide (NMC) manufactured by Merck was used as the positive electrode active material in Example 9. (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 the NMC by the following method. The NMC was placed in an electric furnace and heated at 450°C for 1 hour in a reducing atmosphere while flowing acetylene gas, thereby obtaining NMC powder with a 2 nm carbon layer formed thereon.

[0144] (2) Electrochemical device A9 was produced and evaluated in the same manner as in Example 1, except for the step of forming a carbon layer on the particle surface of the positive electrode active material.

[0145] (Examples 10 to 12, Comparative Examples 10 to 12) Electrochemical devices A10 to A12 and B10 to B12 were fabricated using the same method as in Example 9, except for the thickness of the carbon layer formed on the surface of the NMC particles, which were the positive electrode active material. Positive electrode active materials with different carbon layer thicknesses were prepared by changing the treatment conditions (gas flow rate or treatment time) in the step of forming the carbon layer on the surface of the NMC particles. For B10, an electrochemical device was fabricated using NMC particles without a carbon layer.

[0146] The thickness of the carbon layer was measured using TEM-EELS, and it was confirmed that each carbon layer had a uniform thickness.

[0147] (2) Electrochemical devices A9 to A12 and B10 to B12 were fabricated and evaluated in the same manner as in Example 1, except for the step of forming a carbon layer on the particle surface of the positive electrode active material. Electrochemical devices A5 to A8 and B7 to B9, which used LMFP as the positive electrode active material, were aged by 10 charge / discharge cycles at a voltage range of 2.2 to 4.3 V. The applied voltage for evaluation of the internal resistance and capacity of the electrochemical devices was also set to 4.3 V.

[0148] Table 4 shows the carbon layer thickness, DCR after the float test, and capacity retention rate for electrochemical devices A9 to A12 and B10 to B12. In Table 4, the DCR is shown as a relative value, with the DCR after the float test for electrochemical device B10 set to 100. Electrochemical devices A9 to A12 are Examples 9 to 12, and B10 to B12 are Comparative Examples 10 to 12.

[0149]

[0150] A9 to A12, in which the carbon layer formed on the surface of the positive electrode active material was 1 to 10 nm thick, showed low DCR after the float test and high capacity retention. However, compared to B10 and B11, in which the carbon layer was less than 1.0 nm thick, the DCR and capacity retention after the float test for A9 to A12 showed relatively small differences compared to when LFP and LMFP were used as the positive electrode active material. This is presumably due to the difference in conductivity of the positive electrode active material used.

[0151] That is, since NMC particles have lower conductivity than LFP and LMFP, it is estimated that the effect of forming a carbon layer on the surface of an electrochemical device using NMC particles as a positive electrode active material on the DCR and capacity retention rate after a float test is smaller than that of an electrochemical device using LFP or LMFP as a positive electrode active material. In other words, it can be said that in an electrochemical device using LFP or LMFP, which have low conductivity, as a positive electrode active material, forming a carbon layer on the surface of the positive electrode active material has a large effect on the DCR and capacity retention rate after a float test.

[0152] [Evaluation 4] (XPS analysis of coating region) After aging, the negative electrodes were removed from the electrochemical devices A1, A5, and A9, and the surface portion of the negative electrode material layer was analyzed by XPS for the C1s spectrum (a peak appearing in a binding energy range of 281 to 283 eV), the O1s spectrum (a peak appearing in a binding energy range of 530 to 534 eV), and the F1s spectrum (a peak appearing in a binding energy range of 684.8 to 685.3 eV). For the analysis, an X-ray photoelectron spectrometer (X-ray photoelectron spectrometer (trade name: PHI Quantera SXM, manufactured by ULVAC-PHI, Inc.) was used. The measurement conditions are shown below.

[0153] X-ray source: Al-mono (1486.6 eV) 15 kV / 25 W Measurement diameter: 100 μmφ Photoelectron take-off angle: 45° Etching conditions: Acceleration voltage 2 kV, etching rate approximately 7.05 nm / min (SiO ２ Conversion), raster area 2mm x 2mm

[0154] (Electrochemical Device) In the XPS analysis of the coating region, the peak intensity A at the apex of the peak appearing in the binding energy range of 530 to 534 eV in the O1s spectrum was determined as the intensity of the peak attributed to lithium carbonate bonds. Furthermore, the peak intensity B at the apex of the peak appearing in the binding energy range of 684.8 to 685.3 eV in the F1s spectrum was determined as the intensity of the peak attributed to lithium fluoride bonds. While etching the surface portion of the negative electrode material layer, the changes in peak intensity A, peak intensity B, and peak intensity ratio A / B in the depth direction (thickness direction of the surface portion) were measured. Figure 2 shows the changes in peak intensity A, peak intensity B, and peak intensity ratio A / B in the depth direction for electrochemical device A1. Figure 3 shows the changes in peak intensity A, peak intensity B, and peak intensity ratio A / B in the depth direction for electrochemical device A5. FIG. 4 shows the changes in peak intensity A, peak intensity B, and peak intensity ratio A / B in the depth direction in electrochemical device A9.

[0155] As can be seen from FIG. 2, in the electrochemical device A1 of Example 1, the peak intensity A (marked with a triangle in FIG. 2) attributed to the lithium carbonate bond in the O1s spectrum increases from the surface layer of the coating region toward the inside (negative electrode active material side), and ２ The peak intensity B (indicated by the square in Figure 2) attributable to lithium fluoride bonds in the F1s spectrum decreases from the surface layer of the coating region toward the inside (negative electrode active material side). This means that the SEI coating is formed so that the coating (first layer) containing a large amount of lithium fluoride covers the coating (second layer) containing a large amount of lithium carbonate. The peak intensity ratio A / B (indicated by the square in Figure 2) is the ratio of the SiO ２ This is converted to 1.60 at a depth of about 20 nm, which is the maximum value.

[0156] In the electrochemical device A1, SiO ２ A peak derived from the carbon material, which is the negative electrode active material, was observed in the C1s spectrum from a depth of 50 nm in terms of SiO. ２ This was evaluated as a thickness of 50 nm.

[0157] As can be seen from FIG. 3, in the electrochemical device A5 of Example 5, the peak intensity A (marked with a triangle in FIG. 3) attributed to the lithium carbonate bond in the O1s spectrum increases from the surface layer of the coating region toward the inside (negative electrode active material side), and ２ The peak intensity B (indicated by the square in Figure 3) attributable to lithium fluoride bonds in the F1s spectrum decreases from the surface layer of the coating region toward the inside (negative electrode active material side). This means that the SEI coating is formed so that the coating (first layer) containing a large amount of lithium fluoride covers the coating (second layer) containing a large amount of lithium carbonate. The peak intensity ratio A / B (indicated by the square in Figure 3) is the ratio of the SiO ２ This is converted to 1.84 at a depth of about 20 nm, which is the maximum value.

[0158] In the electrochemical device A5, SiO ２A peak derived from the carbon material, which is the negative electrode active material, was observed in the C1s spectrum from a depth of 60 nm in terms of SiO. ２ This was evaluated as a thickness of 60 nm.

[0159] As can be seen from FIG. 4, in the electrochemical device A9 of Example 9, the peak intensity A (marked with a triangle in FIG. 4) attributed to the lithium carbonate bond in the O1s spectrum increases from the surface layer of the coating region toward the inside (negative electrode active material side), and ２ The peak intensity B (indicated by the square in Figure 4) attributable to lithium fluoride bonds in the F1s spectrum decreases from the surface layer of the coating region toward the inside (negative electrode active material side). This means that the SEI coating is formed so that the coating (first layer) containing a large amount of lithium fluoride covers the coating (second layer) containing a large amount of lithium carbonate. The peak intensity ratio A / B (indicated by the square in Figure 4) is the ratio of the SiO ２ This is converted to 1.07 at a depth of about 40 nm, which is the maximum value.

[0160] In the electrochemical device A9, SiO ２ A peak derived from the carbon material, which is the negative electrode active material, was observed in the C1s spectrum from a depth of 65 nm in terms of SiO. ２ This was evaluated as a thickness of 65 nm.

[0161] 5, for electrochemical device B4 of Comparative Example 4, the peak intensity A (indicated by triangles in FIG. 5) attributable to lithium carbonate bonds in the O1s spectrum decreased from the surface layer of the coating region toward the inside (negative electrode active material side). On the other hand, the peak intensity (indicated by squares in FIG. 5) attributable to lithium fluoride bonds in the F1s spectrum increased and then decreased from the surface layer of the coating region toward the inside (negative electrode active material side). The peak intensity ratio A / B (indicated by circles in FIG. 5) did not show a maximum value due to changes in the depth direction.

[0162] In the electrochemical device B4, SiO ２A peak derived from the carbon material, which is the negative electrode active material, was observed in the C1s spectrum from a depth of 80 nm in terms of SiO. ２ This was evaluated as a thickness of 80 nm.

[0163] The electrochemical device according to the present invention is suitable for use in a vehicle, for example.

[0164] 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.

[0165] 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 a positive electrode active material, and the positive electrode active material comprises lithium transition metal oxide particles having a carbon layer formed on a surface 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 negative electrode active material reversibly doped with lithium ions, and the negative electrode active material comprises a carbon material, a surface portion of the negative electrode material layer has a coating region, and when the coating region is measured by X-ray photoelectron spectroscopy, a peak is observed in an O1s spectrum with a binding energy in the range of 530 to 534 eV, and the intensity of the peak in the O1s spectrum increases from the surface of the coating region toward the inside.

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 carbon layer formed on the surface of the lithium transition metal oxide particles has a thickness of 1.0 nm or more and 10 nm or less.

4. The electrochemical device according to claim 1, wherein, when the coating region is measured by X-ray photoelectron spectroscopy, a peak is observed in the F1s spectrum in a binding energy range of 684.8 to 685.3 eV, and the intensity of the peak in the F1s spectrum decreases from the surface of the coating region toward the inside.

5. The electrochemical device according to claim 4, wherein a ratio A / B of a peak intensity A at an apex of the peak in the O1s spectrum to a peak intensity B at an apex of the peak in the F1s spectrum increases and then decreases from the surface of the coating region toward the inside, and when a surface portion of the negative electrode material layer is measured by X-ray photoelectron spectroscopy, a peak attributable to bonds in the carbon material is substantially not observed in the C1s spectrum at a depth from the surface of the coating region where the ratio A / B is maximum.

6. The electrochemical device according to any one of claims 1 to 5, wherein the electrolyte includes an imide-based electrolyte.

7. The electrochemical device of claim 6, wherein the imide-based electrolyte includes anions containing fluorine and sulfur.

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