Electrochemical device

Abstract

This electrochemical device includes a positive electrode, a negative electrode, and a lithium-ion conductive electrolyte. The electrolyte includes an imide-based electrolyte. A surface layer portion of a negative electrode material layer of the negative electrode has a coating region. The ratio X (= ICO3 / (IF+ISO4)) of a peak intensity ICO3 to the sum of a peak intensity IF and a peak intensity ISO4 is 1.0 or more in a region spanning from the surface layer of the coating region to a depth of 50 nm inward, where the peak intensity ICO3 is of a first peak that appears in the binding energy range of 530–534 eV in an O1s spectrum that is obtained by measuring the coating region by X-ray photoelectron spectroscopy, the peak intensity IF is of a second peak that appears in the binding energy range of 684.8–685.3 eV in an F1s spectrum, and the peak intensity ISO4 is of a third peak that appears in the binding energy range of 167–171 eV in an S2p spectrum.

Description

Electrochemical Devices

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

[0002] Electrochemical devices using a carbon material in which lithium ions are inserted into the negative electrode layer are known (see Patent Documents 1 to 3). The electrochemical device includes a positive electrode, a negative electrode, and an electrolyte. LiPF is used as a lithium ion conductive electrolyte. ６ An electrolyte solution in which a lithium salt such as the above is dissolved in a non-aqueous solvent is known.

[0003] Patent Document 4 describes lithium bis(fluorosulfonyl)imide (LiFSI) and LiBF ４ an electrolyte solution containing a mixture of the above, a solvent containing at least one of a cyclic or chain carbonate compound, and a film-forming agent, and LiBF ４ A lithium ion capacitor has been proposed in which the molar ratio of LiFSI to LiF is 90 / 10 to 30 / 70, and the concentration of the mixture in the electrolyte is 1.2 to 1.8 mol / L.

[0004] JP 2014-123641 A International Publication No. 2007 / 88604 International Publication No. 2012 / 036249 Japanese Patent Application Laid-Open No. 2017-216310

[0005] One aspect of the present invention relates to an electrochemical device. The electrochemical device includes a positive electrode, a negative electrode, and a lithium ion conductive electrolyte. 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 to be reversibly doped with lithium ions, the negative electrode active material including a carbon material. The electrolyte includes an imide-based electrolyte salt. A surface portion of the negative electrode material layer has a coating region. An O1s spectrum of the coating region measured by X-ray photoelectron spectroscopy has a first peak in a binding energy range of 530 to 534 eV. An F1s spectrum of the coating region measured by X-ray photoelectron spectroscopy has a second peak in a binding energy range of 684.8 to 685.3 eV. An S2p spectrum of the coating region measured by X-ray photoelectron spectroscopy has a third peak in a binding energy range of 167 to 171 eV. The peak intensity I at the apex of the first peak in the O1s spectrum ＣＯ３and the peak intensity I at the apex of the second peak in the F1s spectrum is Ｆ and the peak intensity I at the apex of the third peak in the S2p spectrum is ＳＯ４ When Ｆ and peak intensity I ＳＯ４ Peak intensity I for the sum of ＣＯ３ The ratio X (= I ＣＯ３ / (I Ｆ +I ＳＯ４ ) is 1.0 or more in a region from the surface layer of the coating region to a depth of 50 nm inside.

[0006] According to the present invention, an increase in the internal resistance of an electrochemical device is suppressed.

[0007] 1 is a longitudinal cross-sectional view showing the configuration of an electrochemical device according to one embodiment of the present invention. Ｆ (peak intensity at the apex of the peak attributed to the lithium fluoride bond in the F1s spectrum) and peak intensity I ＳＯ４ (peak intensity at the apex of the peak attributed to lithium sulfate bonds in the S2p spectrum) ＣＯ３ (the peak intensity at the apex of the peak assigned to the lithium carbonate bond in the O1s spectrum) ＣＯ３ / (I Ｆ +I ＳＯ４ 1 is a graph showing the change in peak intensity I ) in the depth direction in the electrochemical devices of Example 1, Comparative Example 1, and Comparative Example 2. ＣＣ (peak intensity at the apex of the peak assigned to the C-C bond of the carbon material in the C1s spectrum at a depth of 100 nm from the surface layer of the coating region) Ｆ and I ＳＯ４ The ratio of the sum of Y (= (I Ｆ +I ＳＯ４ ) / I ＣＣ ) in the depth direction.

[0008] In electrochemical devices that utilize lithium ions, a solid electrolyte interfacial film (SEI film) is formed on the negative electrode material layer during charge and discharge. The SEI film plays an important role in charge and discharge reactions, but if the SEI film is formed too thick, the internal resistance of the electrochemical device increases.

[0009] In electrochemical devices using lithium ions, lithium ions are pre-doped into the negative electrode before charging and discharging. Pre-doping is performed, for example, by immersing the negative electrode in an electrolyte solution containing lithium ions and applying a voltage to the negative electrode. In this case, the SEI coating contains a large amount of LiF. An SEI coating containing a large amount of LiF is stable to the electrolyte solution but has high resistance.

[0010] In addition, LiPF ６ When using an electrolyte in which a fluorine-containing phosphate such as LiPF is dissolved, ６ is highly reactive with water and easily decomposes. HF is generated by the decomposition. The generated HF damages the SEI film. This makes it difficult to form a good quality SEI film, and the internal resistance of the device is likely to increase.

[0011] In contrast, when an electrolyte containing dissolved LiFSI is used, LiFSI is less likely to react with water and HF is less likely to be produced, but an SEI film containing a large amount of LiF is more likely to form, increasing the internal resistance of the device.

[0012] An electrochemical device according to one embodiment of the present invention 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.

[0013] 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.

[0014] The carbon material exhibits capacity through a Faraday reaction in which lithium ions are reversibly inserted and extracted. The doping of lithium ions into the negative electrode active material includes at least the insertion 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.

[0015] The electrolyte includes an imide-based electrolyte salt. The imide-based electrolyte salt includes an imide-based anion as an anion of a lithium salt. The electrolyte may be, for example, an electrolytic solution in which a lithium salt of the imide-based anion is dissolved in a solvent.

[0016] 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 a main component can reduce the internal resistance of an electrochemical device. On the other hand, an SEI coating containing lithium carbonate as a main component easily reacts with HF generated by the decomposition reaction of the electrolyte, and the SEI coating is easily damaged.

[0017] 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 tends to increase the internal resistance of the electrochemical device. In particular, the internal resistance (DCR) at low temperatures tends to increase.

[0018] The SEI coating also contains lithium sulfate (LiSO ４ Lithium sulfate (LiSO ４) is derived from imide-based anions and can be generated by the reaction of imide-based anions on the negative electrode surface. Lithium sulfate, like lithium fluoride, has high insulating properties, and an SEI coating containing lithium sulfate has high resistance to lithium ion migration, which tends to increase the internal resistance of the electrochemical device. Therefore, in electrochemical devices using imide-based electrolyte salts, it is important to reduce the amount of lithium sulfate that can be contained in the SEI coating in order to suppress an increase in the internal resistance of the device.

[0019] 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.

[0020] 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.

[0021] Furthermore, when the coating region containing lithium sulfate is measured by X-ray photoelectron spectroscopy (XPS), a peak attributed to lithium sulfate bonds is observed in the S2p spectrum. The peak attributed to lithium sulfate bonds is a peak attributed to S═O bonds, and can appear in the binding energy range of 167 to 171 eV.

[0022] In the O1s spectrum, the intensity of the peak observed in the binding energy range of 530 to 534 eV is referred to as peak intensity I ＣＯ３ The peak intensity I ＣＯ３ is the intensity at the apex of the peak in the O1s spectrum. ＣＯ３ is an index that reflects the content of lithium carbonate contained in the vicinity of the measurement point in the coating region.

[0023] Similarly, in the F1s spectrum, the intensity of the peak observed in the binding energy range of 684.8 to 685.3 eV is referred to as the peak intensity I Ｆ The peak intensity I Ｆ is the intensity at the apex of the peak in the F1s spectrum. Ｆ is an index that reflects the content of lithium fluoride contained in the vicinity of the measurement point in the coating region.

[0024] Similarly, in the S2p spectrum, the intensity of the peak observed in the binding energy range of 167 to 171 eV is referred to as the peak intensity I ＳＯ４ The peak intensity I ＳＯ４ is the intensity at the apex of the peak in the S2p spectrum. ＳＯ４ is an index that reflects the content of lithium sulfate contained in the vicinity of the measurement point in the coating region.

[0025] In the electrochemical device according to one embodiment of the present invention, the peak intensity I Ｆ and peak intensity I ＳＯ４ Peak intensity I relative to the sum of ＣＯ３ The ratio X (= I ＣＯ３ / (I Ｆ +I ＳＯ４ ) is 1.0 or greater throughout the range from the surface layer of the coating region to a depth of 50 nm inward. This means that in the region from the surface layer of the coating region to a depth of 50 nm inward, the SEI coating contains a large amount of lithium carbonate, but the contents of lithium fluoride and lithium sulfate are reduced, thereby maintaining low lithium ion migration resistance. As a result, the internal resistance (DCR) of the electrochemical device can be maintained low even at low temperatures. In order to obtain a significantly low internal resistance (DCR) even at low temperatures, the ratio X is preferably 2.0 or greater, and more preferably 3.0 or greater, throughout the range from the surface layer of the coating region to a depth of 50 nm inward.

[0026] In addition, the formation of a dense lithium carbonate coating helps to suppress side reactions with the electrolyte, thereby suppressing the generation of lithium fluoride and damage to the SEI coating that occurs during charge and discharge. Furthermore, because an SEI coating with low resistance to lithium ion migration is formed, even if the SEI coating is formed thick, as long as it is of an appropriate thickness, it does not hinder the migration of lithium ions, and forming a thick SEI coating makes it less susceptible to damage.

[0027] The ratio X may be 2.0 or greater over the range from the surface layer of the coating region to a depth of 10 nm inward. In this case, the lithium fluoride and lithium sulfate contents are further reduced in the outermost layer, resulting in a denser lithium carbonate coating, which makes it possible to form a coating that is more stable against the electrolyte solution and further suppress damage to the SEI coating. In terms of significantly suppressing damage to the SEI coating, the ratio X is preferably 3.0 or greater, and more preferably 4.0 or greater, over the range from the surface layer of the coating region to a depth of 10 nm inward.

[0028] When the coating region is measured by X-ray photoelectron spectroscopy (XPS), a peak can be observed in the C1s spectrum in a binding energy range of 281 to 283 eV. The peak appearing in the binding energy range of 281 to 283 eV is a peak attributed to a C—C bond within the carbon plane, and is derived from the carbon material contained in the negative electrode active material.

[0029] Here, in the C1s spectrum at a depth of 100 nm from the surface layer of the coating region, the intensity of the peak observed in the binding energy range of 281 to 283 eV is referred to as peak intensity I ＣＣ The peak intensity I ＣＣ is the intensity at the apex of the peak in the C1s spectrum. A depth of 100 nm from the surface layer of the coating region corresponds to a position within the coating region that is sufficiently far from the surface layer, or a position where the carbon material is exposed through the SEI coating. Therefore, the peak intensity I ＣＣ reflects the peak intensity originating from the carbon material in the region where the absolute amounts of lithium fluoride and lithium sulfate are small.

[0030] Peak intensity I ＣＣPeak intensity I Ｆ and I ＳＯ４ The ratio of the sum of Y (= (I Ｆ +I ＳＯ４ ) / I ＣＣ ) may be 2.0 or less over the range from the surface layer of the coating region to a depth of 50 nm inward. In this case, the amount of lithium fluoride and lithium sulfate produced is extremely small, and it can be said that an SEI coating essentially consisting of lithium carbonate is formed, and a dense coating that is stable against the electrolyte can be formed. As a result, it is believed that the production of lithium fluoride and damage to the SEI coating associated with charge and discharge can be significantly suppressed. The ratio Y may be 1.3 or less, or 1.0 or less over the range from the surface layer of the coating region to a depth of 50 nm inward.

[0031] In the XPS measurement, the peak intensity I ＣＯ３ , I Ｆ , and I ＳＯ４ Calculate the ratio X (= I ＣＯ３ / (I Ｆ +I ＳＯ４ )) is obtained. While etching the negative electrode material layer having a coating region, XPS measurement is performed to observe the changes in the O1s spectrum, F1s spectrum, and S2p spectrum in the depth direction. From the XPS spectra measured at multiple depths, the dependence of the ratio X in the depth direction can be obtained. Similarly, while etching the negative electrode material layer having a coating region, XPS measurement is performed to observe the changes in the F1s spectrum and S2p spectrum in the depth direction, and by comparing this with the C1s spectrum at a depth of 100 nm inside from the surface layer of the coating region, the ratio Y (=(I Ｆ +I ＳＯ４ ) / I ＣＣ ) can be obtained as a function of depth.

[0032] The maximum value of the ratio X in the depth direction is, for example, 3.0 or more, and may be 4.0 or more, or 6.0 or more. ＣＯ３ , I Ｆ , I ＳＯ４ , and I ＣＣ are each determined from the height of the peak from the baseline.

[0033] To form a coating region, a layer containing lithium carbonate 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, a coating region can be formed on the surface of the negative electrode active material by subsequent charging and discharging. For example, a reaction between the electrolyte and the negative electrode in the electrochemical device generates lithium fluoride, forming a coating region containing lithium fluoride. The coating region can be formed, for example, by ROCO ２ These compounds may include compounds such as Li, ROLi, where R is a hydrocarbon group. These compounds can be identified by analyzing the peak of the Li1s spectrum in XPS measurement.

[0034] By controlling the conditions for forming the layer containing lithium carbonate on the surface layer portion of the negative electrode material layer and / or the conditions for bringing the negative electrode material layer into contact with an electrolyte in an electrochemical device, it is possible to control the content ratios of lithium fluoride and lithium sulfate contained in the coating region and the density of the layer containing lithium carbonate.

[0035] When the coating region containing lithium carbonate is analyzed by XPS, a peak (second peak) attributed to a Li-O bond may be observed in the O1s spectrum in addition to a peak (first peak) attributed to a C=O bond. The second peak is due to a trace of LiOH or Li that may be present near the surface of the carbon material. ２ It is thought to be derived from O.

[0036] For example, XPS analysis of the surface portion of the negative electrode material layer involves irradiating an argon beam onto the surface portion or a coating formed on the surface of a carbon material in the chamber of an X-ray photoelectron spectrometer, and while etching the surface portion, observing and recording changes in the spectra attributable to C1s, O1s, or F1s electrons versus irradiation time. At this time, the position where a peak attributable to a C=O bond begins to be observed in the O1s spectrum, or where a peak attributable to a Li-F bond begins to be observed in the F1s spectrum, is used as the reference for the depth of the coating region (the position where the depth is 0), and the depth of the coating region is determined. The depth of the coating region is determined by measuring the depth of SiO under the same etching conditions. ２ When etching SiO ２ The depth to which SiO２ This means the equivalent depth.

[0037] 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 constituting 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 disposed on the side opposite to the surface bonded to the negative electrode current collector and used for analysis.

[0038] Next, a method for forming a coating region on the surface layer portion of the negative electrode material layer will be described. First, a layer containing lithium carbonate is formed on the surface layer portion of the negative electrode material layer. The step of forming the lithium carbonate layer can be performed by, for example, a gas phase method, a coating method, a transfer method, or the like. Examples of the gas phase method include chemical vapor deposition, physical vapor deposition, and sputtering. Examples of the coating method include a method in which a solution or dispersion containing lithium carbonate is applied to the surface of the negative electrode using, for example, a microgravure coater, followed by drying.

[0039] A layer containing lithium carbonate may 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. The step of attaching metallic lithium to the surface of the negative electrode material layer can be carried out by, for example, a gas phase method, transfer, or the like. Examples of the gas 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 may be, 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.

[0040] The carbon dioxide gas atmosphere is preferably a dry atmosphere that does not contain 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 0.4% or lower.

[0041] In a carbon dioxide gas atmosphere, the partial pressure of carbon dioxide is, 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.

[0042] The temperature of the negative electrode exposed to the carbon dioxide gas atmosphere may be, for example, in the range of 25° C. to 200° C. or 40° C. to 180° C. The higher the temperature, the thicker the lithium carbonate layer becomes.

[0043] The time for exposing the negative electrode to the carbon dioxide gas atmosphere may be at least 1 minute, and may be at most 12 hours, at most 5 hours, or at most 1 hour. The longer the exposure time, the thicker the lithium carbonate layer becomes.

[0044] The negative electrode layer is then brought into contact with an electrolyte to form a coating region. The step of bringing the negative electrode layer into contact with the electrolyte may also serve as at least a part of a step of pre-doping lithium ions into the negative electrode layer. For example, metallic lithium may be used as a source of the pre-doped lithium ions.

[0045] The pre-doping process of lithium ions into the negative electrode material layer proceeds, for example, by subsequently contacting the negative electrode material layer with an electrolyte and completing the process by leaving the negative electrode material layer for a predetermined period of time. Such a process can be a process of forming a coating region on the surface of the negative electrode material layer. For example, by charging and discharging the electrochemical device at least once, a coating region can be formed on the surface of 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).

[0046] However, the lithium carbonate layer formed by exposure to a carbon dioxide gas atmosphere may have defects such as pinholes. When the negative electrode material layer is brought into contact with the electrolyte, lithium is doped into the carbon material, and as the pre-doping process progresses, the potential of the carbon material decreases. If defects are present in the lithium carbonate layer, an interface between the electrolyte and the lithium-doped carbon material, which has a low potential, may form at the defects, allowing the reductive decomposition reaction of imide anions to proceed. As a result, an SEI coating containing lithium fluoride and lithium sulfate may be formed to seal the defects. As a result, a coating region containing lithium fluoride and lithium sulfate is formed on the surface of the negative electrode material layer. As described above, lithium fluoride and lithium sulfate are insulating. Therefore, from the perspective of reducing the internal resistance of the electrochemical device, it is important to control the conditions for forming the lithium carbonate layer and the conditions during pre-doping to minimize the formation of an SEI coating caused by a chemical reaction containing lithium fluoride and lithium sulfate.

[0047] One possible method for reducing the content of lithium fluoride and lithium sulfate in the coating region is to contact the negative electrode material layer with the electrolyte and then immediately apply a charging voltage between the positive and negative electrode terminals. In this case, applying the charging voltage causes the negative electrode material layer, whose potential has been sufficiently reduced, to come into contact with the electrolyte. This electrochemically forms a stable SEI coating through the decomposition of components (such as additives) contained in the electrolyte other than the imide-based electrolyte salt, thereby sealing defects. This reduces the content of lithium fluoride and lithium sulfate in the coating region.

[0048] In general electrochemical device manufacturing, after injecting the electrolyte, the device is left standing for a predetermined period (usually 18 to 24 hours) without applying a voltage to allow the electrolyte to penetrate into the pores of the negative electrode material layer, and then an aging treatment is performed in which a charging voltage is applied to the device after leaving it standing. In order to reduce the content of lithium fluoride and lithium sulfate in the coating region, the period for leaving the device standing after injecting the electrolyte is preferably short, and preferably so short that it can be said to be essentially zero in order to suppress the progress of chemical reactions.

[0049] The period from when the electrolyte is poured into the device until the aging treatment is started (the period from when the negative electrode material layer having the lithium carbonate layer formed on the surface thereof is brought into contact with the electrolyte until the charging voltage is applied) may be 12 hours or less, 6 hours or less, and preferably 1 hour or less. The period may be substantially zero, and although there is no lower limit for the period, it may be 10 minutes or more in consideration of the device manufacturing process.

[0050] In order to suppress the generation of lithium fluoride and lithium sulfate, the device may be cooled to reduce the chemical reaction rate (for example, at −30° C.) before the electrolyte is injected and a charging voltage is applied to start the aging treatment.

[0051] The step of forming a layer containing lithium carbonate on the surface of the negative electrode material layer is preferably carried out before constructing the electrode body, but this does not exclude the possibility of performing the step 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 a layer containing lithium carbonate on the surface of the negative electrode material layer.

[0052] The electrochemical device according to the present invention includes electrochemical devices such as lithium ion secondary batteries, lithium ion capacitors, and electric double layer capacitors. For example, a polarizable electrode layer may be formed as the positive electrode of the electrochemical device using a positive electrode material layer containing a carbon material as the positive electrode active material. In this case, an electric double layer is formed by the adsorption of ions to the positive electrode active material, and capacitance is generated on the positive electrode side. The carbon material is, for example, activated carbon. The carbon material (e.g., activated carbon) has a specific surface area of ​​1500 m ２ / g or more 2500m ２ / g or less, average particle size is 10 μm or less, and total pore volume is 0.5 cm ３ / g or more 1.5cm ３ / g or less and an average pore diameter of 1 nm or more and 3 nm or less can be preferably used.

[0053] The positive electrode of the electrochemical device may be, for example, a non-polarizable electrode using a lithium transition metal oxide as the positive electrode active material, or a polarizable electrode layer may be formed using a conductive polymer or a carbon material capable of reversibly inserting and desorbing lithium salt anions as the positive electrode active material. Examples of lithium transition metal oxides include known materials used in lithium ion secondary batteries, such as lithium cobalt oxide, manganese spinel, lithium nickel oxide, lithium iron phosphate, lithium manganese iron phosphate, lithium cobalt phosphate, and lithium nickel manganese cobalt oxide. Examples of carbon materials capable of reversibly inserting and desorbing lithium salt anions include known materials such as graphite, graphene, non-graphitizable carbon (hard carbon), and easily graphitizable carbon (soft carbon).

[0054] 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.

[0055] Hereinafter, each component of the electrochemical device according to the embodiment of the present invention will be described in more detail, taking as an example a case where the positive electrode is a polarizable electrode and the negative electrode is a non-polarizable electrode.

[0056] (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.

[0057] 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.

[0058] 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.

[0059] (Negative electrode material layer) The negative electrode material layer includes a carbon material that electrochemically inserts and extracts 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, with graphite and hard carbon being particularly preferred. The carbon material may be used in combination with other materials.

[0060] 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.

[0061] 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.

[0062] From the viewpoint of high packing property of the negative electrode active material in the negative electrode and easy suppression of side reactions with the electrolyte, the average particle size of the negative electrode active material (particularly non-graphitizable carbon) is preferably 1 μm or more and 20 μm or less, and more preferably 2 μm or more and 15 μm or less. More preferably, the negative electrode active material has an average particle size of 10 μm or less and a specific surface area of ​​30 m ２ / g or more 60m ２ / g or less of a carbon material.

[0063] The specific surface area of ​​the negative electrode material layer is 10 m ２ / g or more 70m ２ / g or less. ２ When the specific surface area of ​​the negative electrode material layer is 70 m / g or more, the resistance of the negative electrode is significantly reduced, and thereby the internal resistance of the electrochemical device is significantly reduced. ２ When the specific surface area of ​​the negative electrode material layer is 20 m / g or less, the reactivity of the negative electrode does not become too high, and the negative electrode deterioration is easily suppressed. Therefore, excellent float characteristics can be obtained, and reliability can be improved. More preferably, the specific surface area of ​​the negative electrode material layer is 20 m / g or less. ２ / g or more, 60m ２ / g or less. The float characteristic is an index of the degree of deterioration of an electrochemical device when float charging is performed using an external DC power source to maintain a constant voltage. The smaller the capacity decrease and the smaller the increase in internal resistance during float charging, the better the float characteristic can be said to be.

[0064] The specific surface area of ​​the negative electrode material layer roughly reflects the specific surface area of ​​the negative electrode active material. ２ / g or more 60m ２ / g or less as the negative electrode active material, the specific surface area of ​​the negative electrode material layer is ２ / g or more 70m ２ The specific surface area of ​​the negative electrode layer can be easily controlled to a range of 800 m / g or less. The specific surface area of ​​the negative electrode layer can also be adjusted by the specific surface area of ​​the conductive agent added to the negative electrode layer. The specific surface area of ​​the conductive agent is, for example, 800 m ２ / g or more is desirable, and 1000m ２ / g or more. Examples of such conductive agents include carbon black (e.g., ketjen black). The specific surface area of ​​the negative electrode layer can be determined by the same method as that for measuring the specific surface area of ​​the positive electrode layer, which will be described later.

[0065] In this specification, the average particle size refers to the volume-based median diameter (D ５０ ) means

[0066] The negative electrode layer contains a negative electrode active material as an essential component, and optionally contains a conductive agent, a binder, etc. Examples of the conductive agent include carbon black and carbon fiber. Examples of the binder include fluororesin, acrylic resin, rubber material, and cellulose derivative.

[0067] 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.

[0068] 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 lithium pre-doped may be, for example, approximately 50% to 95% of the maximum amount that can be inserted into the negative electrode material layer.

[0069] (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.

[0070] 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.

[0071] 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.

[0072] (Positive Electrode Material Layer) The positive electrode material layer contains a material that reversibly dopes anions as a positive electrode active material. The positive electrode active material is, for example, a carbon material or a conductive polymer.

[0073] The carbon material used as the positive electrode active material is preferably a porous carbon material, such as activated carbon or the carbon materials exemplified as the negative electrode active material (e.g., non-graphitizable carbon). Examples of raw materials for activated carbon include wood, coconut shells, coal, pitch, and phenolic resin. The activated carbon is preferably activated.

[0074] The average particle size (volume-based median diameter D50) of the carbon material is not particularly limited, but is preferably 20 μm or less, more preferably 10 μm or less, and may be 3 μm to 10 μm.

[0075] The specific surface area of ​​the positive electrode material layer generally reflects the specific surface area of ​​the positive electrode active material. ２ / g or more, 4000m ２ / g or less is sufficient, and ２ / g or more, 3000m ２ More preferably, the specific surface area of ​​the positive electrode material layer is 1500 m ２ / g or more, 2500m ２ / g or less.

[0076] The specific surface area of ​​the positive electrode mixture layer is the BET specific surface area determined using a measuring device conforming to JIS Z8830 (e.g., a Tristar II 3020 manufactured by Shimadzu Corporation). Specifically, the electrochemical device is disassembled, and the positive electrode is removed. Next, the positive electrode is washed with dimethyl carbonate (DMC) and dried. Thereafter, the positive electrode mixture layer is peeled from the positive electrode current collector, and approximately 0.5 g of a sample of the positive electrode mixture layer is collected.

[0077] Next, the collected sample is heated at 150°C for 12 hours under a reduced pressure of 95 kPa or less, and then nitrogen gas is adsorbed onto a sample of known mass to obtain an adsorption isotherm over a relative pressure range of 0 to 1. The surface area of ​​the sample is then calculated from the monolayer adsorption amount of gas obtained from the adsorption isotherm. Here, the specific surface area is calculated using the BET single-point method (relative pressure 0.3) using the following BET equations (Equations (1) and (2)).

[0078] P / V × (P0 - P) = (1 / Vm × C) + {(C - 1) / Vm × C} (P / P0) (1) S = k × Vm (2) P0: saturated vapor pressure P: adsorption equilibrium pressure V: adsorption amount at adsorption equilibrium pressure P Vm: monolayer adsorption amount C: parameters related to heat of adsorption, etc. S: specific surface area k: nitrogen monomolecular occupation area 0.162 nm ２ The activated carbon preferably accounts for 50% by mass or more, more preferably 80% by mass or more, and even 95% by mass or more of the positive electrode active material, and more preferably 40% by mass or more, more preferably 70% by mass or more, and even more preferably 90% by mass or more of the positive electrode material layer.

[0079] The positive electrode layer contains a positive electrode active material as an essential component, and optionally contains a conductive agent, a binder, etc. Examples of the conductive agent include carbon black and carbon fiber. Examples of the binder include fluororesin, acrylic resin, rubber material, and cellulose derivative.

[0080] 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 applied positive electrode mixture slurry. The thickness of the positive electrode material layer is, for example, 10 μm to 300 μm per side of the positive electrode current collector.

[0081] The conductive polymer used as the positive electrode active material is preferably a π-conjugated polymer. Examples of π-conjugated polymers that can be used include polypyrrole, polythiophene, polyfuran, polyaniline, polythiophene vinylene, polypyridine, and derivatives thereof. These may be used alone or in combination of two or more. The weight-average molecular weight of the conductive polymer is, for example, 1,000 to 100,000. Note that a derivative of a π-conjugated polymer refers to a polymer having a π-conjugated polymer skeleton such as polypyrrole, polythiophene, polyfuran, polyaniline, polythiophene vinylene, or polypyridine. For example, polythiophene derivatives include poly(3,4-ethylenedioxythiophene) (PEDOT).

[0082] Conductive polymers are formed, for example, by immersing a positive electrode current collector with a carbon layer in a reaction solution containing raw material monomers of the conductive polymer and electrolytically polymerizing the raw material monomers in the presence of the positive electrode current collector. In electrolytic polymerization, a positive electrode current collector and a counter electrode are immersed in a reaction solution containing the raw material monomers, and a current is passed between them using the positive electrode current collector as the anode. Conductive polymers may also be formed by methods other than electrolytic polymerization. For example, a conductive polymer may be formed by chemical polymerization of raw material monomers. In chemical polymerization, raw material monomers are polymerized using an oxidizing agent or the like in the presence of the positive electrode current collector.

[0083] The raw material monomer used in electropolymerization or chemical polymerization may be any polymerizable compound capable of producing a conductive polymer by polymerization. The raw material monomer may include an oligomer. Examples of the raw material monomer include aniline, pyrrole, thiophene, furan, thiophene vinylene, pyridine, or derivatives thereof. These may be used alone or in combination of two or more. Among these, aniline is particularly easy to grow on the surface of the carbon layer by electropolymerization.

[0084] Electropolymerization or chemical polymerization can be carried out using a reaction solution containing an anion (dopant). By doping a π-electron conjugated polymer with a dopant, excellent conductivity can be achieved. Examples of dopants include sulfate ions, nitrate ions, phosphate ions, borate ions, benzenesulfonate ions, naphthalenesulfonate ions, toluenesulfonate ions, methanesulfonate ions, perchlorate ions, tetrafluoroborate ions, hexafluorophosphate ions, and fluorosulfate ions. The dopant may also be a polymeric ion. Examples of polymeric ions include ions of polyvinylsulfonic acid, polystyrenesulfonic acid, polyallylsulfonic acid, polyacrylicsulfonic acid, polymethacrylic acid, poly(2-acrylamido-2-methylpropanesulfonic acid), polyisoprenesulfonic acid, and polyacrylic acid.

[0085] (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, from 8 μm to 300 μm, and preferably from 8 μm to 40 μm.

[0086] (Electrolyte) The electrolyte has lithium ion conductivity and includes a lithium salt and a solvent that dissolves the lithium salt. The anion of the lithium salt is repeatedly adsorbed to and desorbed from the positive electrode in a reversible manner. The lithium ion derived from the lithium salt is reversibly inserted into and desorbed from the negative electrode. The electrolyte includes an imide-based electrolyte salt. The imide-based electrolyte salt includes an imide-based anion as the anion of the lithium salt.

[0087] The imide-based anion may be a fluorine- and sulfur-containing anion, 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. The imide-based anion is LiN(SO ２ CF ３ ) ２ , LiN(SO ２ CF ２ CF ３ )２ may include:

[0088] A lithium salt other than the imide-based electrolyte salt may be added together with the imide-based electrolyte salt to form the electrolyte. Examples of the other lithium salt 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 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 a low viscosity, and can improve the withstand voltage characteristics of an electrochemical device.

[0089] Imide-based electrolyte salts, such as LiFSI, have a high degree of lithium ion dissociation and are therefore likely to increase the lithium ion conductivity of the electrolyte. On the other hand, the strong covalent bond of F within the imide-based anion makes it difficult for the reaction in which F dissociates and generates LiF to occur in the surface portion of the negative electrode material layer, making it easy to reduce the amount of lithium fluoride that may be contained in the coating region of the surface portion of the negative electrode material layer. As a result, it is easy to form an SEI coating with low lithium ion migration resistance in the coating region, making it easy to reduce the internal resistance of the electrochemical device.

[0090] From the viewpoint of reducing the internal resistance of the electrochemical device, the concentration of the imide-based electrolyte (LiFSI) in the electrolyte solution is preferably 1.0 mol / L or less. On the other hand, from the viewpoint of obtaining sufficiently high lithium ion conductivity, the concentration of the imide-based electrolyte (LiFSI) in the electrolyte solution may be 0.5 mol / L or more, and preferably 0.7 mol / L or more.

[0091] LiFSI is believed to have the effect of reducing the deterioration of the positive and negative electrode active materials. Among salts having fluorine-containing anions, the FSI anion is highly stable, so it is thought to be less likely to produce by-products, not damage the surface of the active materials, and contribute to smooth charge and discharge.

[0092] The concentration of the lithium salt in the nonaqueous electrolyte in a charged state (state of charge (SOC) 90 to 100%) is, for example, 0.2 mol / L or more and 5 mol / L or less, preferably 0.2 mol / L or more and 1 mol / L or less, and more preferably 0.7 mol / L or less and 1 mol / L or less.

[0093] 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.

[0094] 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.

[0095] (Additional Notes) The above description of the embodiments discloses the following techniques.

[0096] (Technology 1) A lithium-ion conductive electrolyte is provided, the lithium-ion conductive electrolyte comprising: a positive electrode, a negative electrode, and a lithium-ion conductive electrolyte; 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; the electrolyte comprises an imide-based electrolyte salt; a surface layer portion of the negative electrode material layer has a coating region; an O1s spectrum of the coating region measured by X-ray photoelectron spectroscopy has a first peak in a binding energy range of 530 to 534 eV; an F1s spectrum of the coating region measured by X-ray photoelectron spectroscopy has a second peak in a binding energy range of 684.8 to 685.3 eV; and an S2p spectrum of the coating region measured by X-ray photoelectron spectroscopy has a third peak in a binding energy range of 167 to 171 eV; and a peak intensity I at an apex of the first peak in the O1s spectrum is ＣＯ３ and the peak intensity I at the apex of the second peak in the F1s spectrum is Ｆ and the peak intensity I at the apex of the third peak in the S2p spectrum is ＳＯ４ When Ｆ and peak intensity I ＳＯ４ Peak intensity I for the sum of ＣＯ３ The ratio X (= I ＣＯ３ / (I Ｆ +I ＳＯ４ ) is 1.0 or more in a region from the surface layer of the coating region to a depth of 50 nm inside.

[0097] (Technology 2) The electrochemical device according to Technology 1, wherein the ratio X is 2.0 or more in a region extending from the surface layer of the coating region to a depth of 10 nm inside.

[0098] (Technology 3) A C1s spectrum of the coating region measured by X-ray photoelectron spectroscopy has a fourth peak in a binding energy range of 281 to 283 eV, and a peak intensity I at the apex of the fourth peak in the C1s spectrum at a depth of 100 nm from the surface layer of the coating region to the inside is ＣＣ When ＣＣ The peak intensity IＦ and the peak intensity I ＳＯ４ The ratio of the sum of Y (= (I Ｆ +I ＳＯ４ ) / I ＣＣ 3. The electrochemical device according to claim 1, wherein the ρ (ρ) of the coating region is 2.0 or less in the region from the surface layer of the coating region to a depth of 50 nm inside.

[0099] (Technology 4) The electrochemical device according to any one of Technologies 1 to 3, wherein the electrolyte is an electrolytic solution containing a solvent and the imide-based electrolyte salt, and the concentration of the imide-based electrolyte salt in the electrolytic solution is 1.0 mol / L or less.

[0100] (Technology 5) The electrochemical device according to Technology 4, wherein the imide-based electrolyte salt contains an anion containing fluorine and sulfur.

[0101] (Technology 6) The electrochemical device according to any one of Technologies 1 to 5, wherein the positive electrode comprises a positive electrode current collector and a positive electrode material layer supported on the positive electrode current collector, and the positive electrode material layer is a polarizable electrode layer containing a carbon material as a positive electrode active material.

[0102] (Technology 7) The electrochemical device according to any one of Technologies 1 to 5, wherein the positive electrode comprises a positive electrode current collector and a positive electrode material layer supported on the positive electrode current collector and containing a positive electrode active material, and the positive electrode active material contains at least one material selected from the group consisting of a conductive polymer, a lithium transition metal oxide, and a carbon material capable of reversibly inserting and desorbing anions of a lithium salt.

[0103] (Technology 8) The carbon material contained as the negative electrode active material has an average particle size of 10 μm or less and a particle size of 30 μm or less. ２ / g or more 60m ２ 8. The electrochemical device according to any one of claims 1 to 7, having a specific surface area of ​​0.1 wt. / g or less.

[0104] EXAMPLES The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples.

[0105] (Example 1) (1) Preparation of Positive Electrode A 30 μm thick aluminum foil (positive electrode current collector) was prepared. Separately, 88 parts by mass of activated carbon (average particle size 5.5 μm) as a positive electrode active material, 6 parts by mass of polytetrafluoroethylene as a binder, and 6 parts by mass of acetylene black as a conductive agent were dispersed in water to prepare a positive electrode mixture slurry. The obtained positive electrode mixture slurry was applied to both sides of aluminum foil, and the coating was dried and rolled to form a positive electrode material layer, thereby obtaining a positive electrode. A 10 mm wide positive electrode current collector exposed portion was formed at the longitudinal end of the positive electrode current collector.

[0106] (2) Preparation of Negative Electrode: A 10 μm thick copper foil (negative electrode current collector) was prepared. 84 parts by mass of non-graphitizable carbon (average particle size 5 μm), 5 parts by mass of carboxycellulose, 1 part by mass of styrene-butadiene rubber, and 10 parts by mass of Ketjen Black (a conductive agent) were dispersed in water to prepare a negative electrode mixture slurry. The resulting negative electrode mixture slurry was applied to both sides of copper foil, and the coating was dried and rolled to form a negative electrode material layer, resulting in a negative electrode.

[0107] Thereafter, a thin film of metallic lithium for pre-doping was formed on the entire surface of the negative electrode mixture layer by vacuum deposition. The amount of lithium to be pre-doped was set so that the negative electrode potential in the non-aqueous electrolyte after pre-doping was 0.2 V or less relative to metallic lithium.

[0108] 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 material 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 100° C. The time for exposing the negative electrode to the carbon dioxide gas atmosphere was 30 minutes.

[0109] (3) Preparation of Electrode Assembly: The positive electrode and negative electrode were wound into a cylindrical shape with a polyolefin microporous membrane separator (thickness: 16 μm) interposed therebetween. The exposed portion of the positive electrode core material protruded from one end face of the wound assembly, and the exposed portion of the 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 portion of the positive electrode core material and the exposed portion of the negative electrode core material, respectively.

[0110] (4) Preparation of Non-Aqueous Electrolyte: A solvent was prepared by adding 0.1% by mass of vinylene carbonate (VC) to a mixture of propylene carbonate (PC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7. LiFSI (lithium salt) was dissolved in the resulting solvent at a concentration of 0.9 mol / L to prepare a non-aqueous electrolyte.

[0111] (5) Assembly of an electrochemical device: The electrode assembly was housed in a bottomed cell case having an opening, 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 a nonaqueous electrolyte was placed in the cell case, the opening of the cell case was sealed with a sealing plate, and an electrochemical device as shown in FIG. 1 was assembled.

[0112] Thereafter, aging was performed at 60° C. while a charging voltage of 3.8 V was applied between the terminals of the positive electrode and the negative electrode to complete pre-doping of lithium ions into the negative electrode.

[0113] Between the completion of assembly of electrochemical device A1 and the application of the charging voltage, no resting period was provided except for the minimum period required for the manufacturing process, and aging was performed with essentially no resting period (25°C, 30 minutes or less), thereby obtaining electrochemical device A1.

[0114] (6) Evaluation [Evaluation 1] (XPS analysis of coating region) The negative electrode was removed from the electrochemical device, and the surface layer of the negative electrode material layer was analyzed by XPS for C1s spectrum, O1s spectrum, F1s spectrum, and S2p spectrum. For the analysis, an X-ray photoelectron spectrometer (trade name: PHI Quantera SXM, manufactured by ULVAC-PHI, Inc.) was used. The measurement conditions are shown below.

[0115] 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[Evaluation 2] (Measurement of internal resistance of electrochemical device) Immediately after aging, the electrochemical device was subjected to a current of 2 mA / cm per positive electrode area in an environment of −30° C. until the voltage reached 3.8 V. ２ After that, the battery was charged at a constant current density of 2 mA / cm per positive electrode area in an environment of −30° C. until the voltage reached 2.2 V. ２ A constant current discharge was carried out at a current density of .

[0116] Using the discharge curve (vertical axis: discharge voltage, horizontal axis: discharge time) obtained in the above discharge, a linear approximation line was calculated for the range of 0.5 to 2 seconds after the start of discharge of the discharge curve, and the voltage VS at the intercept of the approximation line was calculated. 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) was calculated as ΔV. ΔV (V) and the current value during discharge (current density per positive electrode area 2 mA / cm ２ × positive electrode area) Id was used to calculate the internal resistance (DCR) R1 (Ω) of the electrochemical device according to the following formula (A).

[0117] Internal resistance R1 = ΔV / Id (A) (Comparative Example 1) An electrochemical device was assembled in the same manner as in Example 1. After the assembly was completed, the electrochemical device was left to stand at 25°C for 18 hours without applying voltage, and then aged at 60°C while applying a charging voltage of 3.8 V between the terminals of the positive electrode and the negative electrode, completing the pre-doping of lithium ions into the negative electrode, thereby obtaining electrochemical device B1. Electrochemical device B1 was evaluated in the same manner as in Example 1.

[0118] Comparative Example 2 An electrochemical device was assembled in the same manner as in Example 1. The assembled electrochemical device was left standing at 25°C for 168 hours without applying voltage, and then aged at 60°C while applying a charging voltage of 3.8 V between the positive and negative electrode terminals to complete pre-doping of lithium ions into the negative electrode, thereby obtaining electrochemical device B2. Electrochemical device B2 was evaluated in the same manner as in Example 1.

[0119] In the XPS analysis of the coating region, the O1s spectrum showed a peak intensity I at the apex of a peak appearing in the binding energy range of 530 to 534 eV. ＣＯ３ was calculated as the intensity of the peak attributed to the lithium carbonate bond. Also, from the F1s spectrum, the peak intensity I Ｆ was calculated as the intensity of the peak attributed to the lithium fluoride bond. Also, from the S2p spectrum, the peak intensity I ＳＯ４ was calculated as the intensity of the peak attributed to the lithium sulfate bond. In addition, the peak intensity I at the apex of the peak appearing in the binding energy range of 281 to 283 eV from the C1s spectrum at a depth of 100 nm from the surface layer of the coating region was calculated. ＣＣ was determined as the intensity of the peak attributed to the C—C bond of the carbon material contained in the negative electrode active material.

[0120] While etching the surface portion of the negative electrode material layer, the peak intensity I ＣＯ３ , I Ｆ , and I ＳＯ４ The change in the depth direction (thickness direction of the surface layer) was measured, and the peak intensity ratio X (= I ＣＯ３ / (I Ｆ +I ＳＯ４ The change in the depth direction (thickness direction of the surface layer) of the peak intensity I Ｆ +I ＳＯ４ The change in the depth direction (thickness direction of the surface layer) is expressed as the peak intensity I ＣＣ Compared with the peak intensity ratio Y (= (I Ｆ +I ＳＯ４ ) / I ＣＣ ) in the depth direction (thickness direction of the surface layer) was determined.

[0121] Figure 2 shows the change in the intensity ratio X in the depth direction for electrochemical devices A1, B1 and B2. Figure 3 shows the change in the intensity ratio Y in the depth direction for electrochemical devices A1, B1 and B2.

[0122] As can be seen from FIG. 2, in the electrochemical device A1 of Example 1, the intensity ratio X is２ The SiO2 content is 3.0 or more (1.0 or more) over a range of depth up to the inner 50 nm, and the SiO2 content is 3.0 or more (1.0 or more) over a range of depth up to the inner 50 nm. ２ In contrast, in electrochemical devices B1 and B2, the intensity ratio X is about 1.5 to 2.0 in the surface layer of the coating region, but decreases from the surface layer of the coating region toward the inside, and the intensity ratio X is 4.0 or more (2.0 or more) over a range of 10 nm deep in terms of SiO ２ In terms of conversion, the intensity ratio X remains below 1.0 at a depth of 20 nm or more.

[0123] As can be seen from FIG. 3, in the electrochemical device A1 of Example 1, the intensity ratio Y is ２ In contrast, in electrochemical devices B1 and B2, the intensity ratio Y increases rapidly from the surface layer of the coating region toward the inside, reaching a maximum of 5.3 in device B1 and a maximum of 4.0 in device B2 at a depth of 10 nm from the surface.

[0124] Table 1 shows the evaluation results of the internal resistance (DCR) of electrochemical devices A1, B1, and B2. In Table 1, the internal resistance values ​​are shown as relative values, with device A1 set to 100. Electrochemical device A1 achieved a significantly lower internal resistance than electrochemical devices B1 and B2.

[0125] As can be seen from Table 1, the internal resistance (DCR) increases as the resting period from the completion of assembly to the start of the aging treatment increases, but the internal resistance (DCR) reaches its maximum value after the resting period of about 18 hours, and then tends to decrease. This is due to the LiF coating and LiSO4 formed by the reaction of LiFSI during the resting period. ４ The coating is fragile and is likely to be decomposed by reaction with the electrolyte as the negative electrode potential decreases with the progress of pre-doping. ４It is believed that the coating is decomposed and replaced with a stable coating made of other components of the electrolyte (e.g., VC), and as a result, the internal resistance (DCR) of Device B2 is thought to be lower than that of Device B1.

[0126] In this regard, in order to suppress an increase in the internal resistance (DCR), it may be possible to lengthen the resting period from the completion of assembly to the start of the aging treatment. By setting the resting period from the completion of assembly to the start of the aging treatment to, for example, 200 hours or more, it may be possible to achieve a low internal resistance value comparable to that of Device A1. However, a longer resting period increases the cost (particularly the time cost) of manufacturing the electrochemical device. From the viewpoint of reducing manufacturing costs, a shorter resting period is better, and a period of 12 hours or less, 6 hours or less, or 1 hour or less is preferable.

[0127]

[0128] The electrochemical device according to the present invention is suitable for use in a vehicle, for example.

[0129] 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. A lithium ion conductive electrolyte comprising a positive electrode, a negative electrode, and a lithium ion conductive electrolyte, wherein the negative electrode comprises a negative electrode current collector and a negative electrode material layer supported on the negative electrode current collector, wherein the negative electrode material layer comprises a negative electrode active material to be reversibly doped with lithium ions, wherein the negative electrode active material comprises a carbon material, and wherein the electrolyte comprises an imide-based electrolyte salt, wherein a surface portion of the negative electrode material layer has a coating region, wherein an O1s spectrum of the coating region measured by X-ray photoelectron spectroscopy has a first peak in a binding energy range of 530 to 534 eV, an F1s spectrum of the coating region measured by X-ray photoelectron spectroscopy has a second peak in a binding energy range of 684.8 to 685.3 eV, and an S2p spectrum of the coating region measured by X-ray photoelectron spectroscopy has a third peak in a binding energy range of 167 to 171 eV, and wherein a peak intensity I at an apex of the first peak in the O1s spectrum is ＣＯ３ and the peak intensity I at the apex of the second peak in the F1s spectrum is Ｆ and the peak intensity I at the apex of the third peak in the S2p spectrum is ＳＯ４ When Ｆ and peak intensity I ＳＯ４ Peak intensity I for the sum of ＣＯ３ The ratio X (= I ＣＯ３ / (I Ｆ +I ＳＯ４ ) is 1.0 or more in a region from the surface layer of the coating region to a depth of 50 nm inside.

2. The electrochemical device according to claim 1, wherein the ratio X is 2.0 or greater in a region extending from the surface of the coating region to a depth of 10 nm inside.

3. The C1s spectrum of the coating region measured by X-ray photoelectron spectroscopy has a fourth peak in the binding energy range of 281 to 283 eV, and the peak intensity I at the apex of the fourth peak in the C1s spectrum at a depth of 100 nm from the surface layer of the coating region ＣＣ When ＣＣ The peak intensity I Ｆ and the peak intensity I ＳＯ４ The ratio of the sum of Y (= (I Ｆ +I ＳＯ４ ) / I ＣＣ 2. The electrochemical device according to claim 1, wherein the value of (x, y) is 2.0 or less in the region extending from the surface of the coating region to a depth of 50 nm inside.

4. The electrochemical device according to any one of claims 1 to 3, wherein the electrolyte is an electrolytic solution containing a solvent and the imide-based electrolyte salt, and the concentration of the imide-based electrolyte salt in the electrolytic solution is 1.0 mol / L or less.

5. The electrochemical device of claim 4, wherein the imide-based electrolyte salt includes anions containing fluorine and sulfur.

6. The electrochemical device according to any one of claims 1 to 3, wherein the positive electrode comprises a positive electrode current collector and a positive electrode material layer supported on the positive electrode current collector, and the positive electrode material layer is a polarizable electrode layer containing a carbon material as a positive electrode active material.

7. The electrochemical device according to any one of claims 1 to 3, wherein the positive electrode comprises a positive electrode current collector and a positive electrode material layer supported on the positive electrode current collector and containing a positive electrode active material, and the positive electrode active material contains at least one material selected from the group consisting of conductive polymers, lithium transition metal oxides, and carbon materials capable of reversibly inserting and desorbing anions of lithium salts.

8. The carbon material contained as the negative electrode active material has an average particle size of 10 μm or less and a particle size of 30 μm or less. ２ / g or more 60m ２ 4. The electrochemical device according to claim 1, having a specific surface area of ​​0.1 wt. / g or less.

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