Electrochemical device

Abstract

This electrochemical device comprises a positive electrode, a negative electrode, and a lithium ion-conductive electrolyte, said negative electrode being provided with 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 that undergoes reversible doping with lithium ions. The negative electrode active material comprises a carbon material. The negative electrode material layer has a coated region. The surface of the negative electrode material layer is divided into: a first region, which contains a region in proximity to a connection region with a negative electrode lead terminal; and a second region outside the first region. A composition ratio X1 of lithium fluoride to lithium carbonate contained in the coated region in the first region of the negative electrode material layer is higher than a composition ratio X2 of lithium fluoride to lithium carbonate contained in the coated region in the second region.

Description

Electrochemical Devices

[0001] The present disclosure relates to electrochemical devices.

[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 Pamphlet International Publication No. 2012 / 036249 Pamphlet JP 2017-216310 A

[0005] 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.

[0006] 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 against the electrolyte solution but has high resistance. When an SEI coating containing a large amount of LiF is formed, the internal resistance of the device increases.

[0007] Because LiF is insulating, when an SEI coating containing LiF is formed, the current is less likely to diffuse throughout the electrode, and the current may concentrate at the connection portion with the lead terminal. As a result, heat may concentrate at the connection portion with the lead terminal, which may be one of the causes of reduced reliability of the electrochemical device.

[0008] One aspect of the present disclosure provides a battery including 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 carried on the negative electrode current collector, the negative electrode material layer comprising a negative electrode active material to which lithium ions are reversibly doped, the negative electrode active material comprising a carbon material, the negative electrode material layer having a coating region and further comprising a negative electrode lead terminal electrically connected to the negative electrode, the negative electrode having a first end and a second end intersecting the first end, and the negative electrode lead terminal having a first end and a second end intersecting the ... the negative electrode material layer protrudes from the negative electrode lead terminal, the surface of the negative electrode material layer is divided into a first region including a region close to a connection region with the negative electrode lead terminal, and a second region other than the first region, the coating region contains lithium carbonate and lithium fluoride, and a composition ratio X1 of the lithium fluoride to the lithium carbonate contained in the coating region in the first region of the negative electrode material layer is higher than a composition ratio X2 of the lithium fluoride to the lithium carbonate contained in the coating region in the second region.

[0009] According to the present disclosure, the reliability of electrochemical devices can be improved.

[0010] 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.

[0011] Fig. 1 is a longitudinal sectional view showing the configuration of an electrochemical device according to an embodiment of the present disclosure; Fig. 2 is a top view showing a first region and a second region of a negative electrode used in an electrochemical device according to an embodiment of the present disclosure; Fig. 3 is a top view showing another example of the first region and the second region of a negative electrode used in an electrochemical device according to an embodiment of the present disclosure; Fig. 4 is a graph showing the change in the depth direction of the ratio A / B of the peak intensity A assigned to lithium carbonate bonds in the O1s spectrum to the peak intensity B assigned to lithium fluoride bonds in the F1s spectrum, for each of the electrochemical devices of Example 1 and Comparative Example 1.

[0012] Embodiments of the electrochemical device according to the present disclosure are described below using examples. However, the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be used as examples, but other numerical values ​​and materials may be used as long as the effects of the present disclosure are obtained. Note that known components may be used as components characteristic of the present disclosure. In this specification, when a "range between numerical value A and numerical value B" is mentioned, the range includes numerical value A and numerical value B.

[0013] In the following description, when lower and upper limits of numerical values ​​relating to specific physical properties, conditions, etc. are exemplified, 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 them can be selected and used alone, or two or more can be used in combination, unless otherwise specified.

[0014] The present disclosure encompasses any combination of two or more features arbitrarily selected from the appended claims, i.e., any combination of 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 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.

[0017] 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.

[0018] The negative electrode layer has a coating region on its surface. The coating region is a region where an SEI coating is formed. The SEI coating is formed of 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.

[0019] 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.

[0020] The negative electrode has a first end and a second end that intersects the first end.

[0021] The first end and the second end are ends in the planar direction of the negative electrode formed in a plate or foil shape. Typically, the negative electrode is rectangular in shape, plate or foil, and in this case, the first end and the second end are ends along one or the other of two intersecting sides of the rectangle. The negative electrode may also have a third end facing the first end in a direction along the second end, and a fourth end that intersects with the first end and faces the second end in a direction along the first end. In a rectangular negative electrode, the third end is approximately parallel to the first end, and the fourth end is approximately parallel to the second end.

[0022] A negative electrode lead terminal is attached to the negative electrode so as to protrude from a first end of the negative electrode. The negative electrode lead terminal is electrically connected to the negative electrode, and the protruding portion thereof is electrically connected to an external terminal of the electrochemical device, thereby forming the device. The first end may be an end along the longitudinal direction of the strip-shaped negative electrode.

[0023] Here, the surface of the negative electrode material layer is divided into a first region and a second region other than the first region. The first region includes a region adjacent to the connection region with the negative electrode lead terminal. In the first region of the negative electrode material layer, a composition ratio (or molar ratio) X1 of lithium fluoride to lithium carbonate contained in the coating region is higher than a composition ratio (or molar ratio) X2 of lithium fluoride to lithium carbonate contained in the coating region.

[0024] The first region contains an insulating coating rich in lithium fluoride, resulting in high internal resistance. Meanwhile, the second region contains a coating rich in lithium carbonate, resulting in low internal resistance. By providing the first and second regions with different resistances within the negative electrode, current is dispersed throughout the electrode, making it easier to flow, and heat is prevented from concentrating at the connection point with the lead terminal. This improves the reliability of the electrochemical device.

[0025] The first region having high resistance is disposed in a region close to the connection region with the negative electrode lead terminal. In contrast, the second region having low resistance can be disposed in a region far away from the connection region with the negative electrode lead terminal. This facilitates current flow from the connection region with the negative electrode lead terminal through the first region to the second region, and the current is easily diffused and flows throughout the electrode.

[0026] Here, dividing the surface of the negative electrode material layer into a first region and a second region does not necessarily mean that there is a clear difference in the composition ratio of lithium fluoride to lithium carbonate contained in the coating region across the boundary between the first and second regions. The composition ratio may be distributed so that it gradually decreases from the first region across the boundary between the first and second regions toward the second region. The first region and / or the second region may include multiple regions separated from each other. The method of division is not limited, but may be, for example, divided into two so that the areas of the first region and the second region are the same, or so that the area of ​​one region is 50% to 200% of the area of ​​the other. Alternatively, the region may be divided into two so that at least one of the areas of the first region and the second region is rectangular.

[0027] The first region may include a region along the first end and close to the first end. The first region may be disposed along the first end from which the negative electrode lead terminal protrudes. The first region may or may not include the first end. There may be a case where an exposed portion where the negative electrode material layer is not formed and the current collector is exposed is provided along the first end. In this case, the first region may include the boundary between the exposed portion and the negative electrode material layer on the first end side, and may include a region along the first end close to the first end.

[0028] The first region and the second region may be divided into two in a direction along the second end (i.e., a direction from the first end to the third end). In this case, the first region may include a region located closer to the first end than the second region. The second region may include a region located closer to the third end than the first region (the opposite side from the first end). "The first region and the second region being divided into two in a direction along the second end" means that, at least at a certain position in the direction along the first end in the negative electrode material layer, when a line is drawn through the position and in a direction along the second end, the boundary between the first region and the second region intersects with the line.

[0029] The first region may also include a region along the second end and close to the second end. The negative electrode lead terminal may be disposed, for example, at a position offset from the center of the negative electrode in the direction along the first end (or from a position where the distance from the second end is equal to the distance from the fourth end). In this case, the first region may be disposed along the second end, which is closer to the negative electrode lead terminal than the fourth end. The first region may or may not include the second end. Along the second end, no negative electrode material layer may be formed, leaving an exposed portion where the current collector is exposed. In this case, the first region may include the boundary between the exposed portion and the negative electrode material layer on the second end side, and may include a region along the second end close to the second end.

[0030] The first region and the second region may be divided into two in a direction along the first end (i.e., a direction from the second end to the fourth end). In this case, the first region may include a region located closer to the second end than the second region. The second region may include a region located closer to the fourth end than the first region (opposite the second end). "The first region and the second region being divided into two in a direction along the first end" means that, at least at a certain position in the direction along the second end in the negative electrode material layer, when a line is drawn through the position and in a direction along the first end, the boundary between the first region and the second region intersects with the line.

[0031] The first region may include at least a region adjacent to the first end and a region adjacent to the second end, while the second region may include at least a region adjacent to a third end opposite the first end and a region adjacent to a fourth end opposite the second end.

[0032] In each of the first and second regions, the composition ratio of lithium fluoride to lithium carbonate contained in the coating region can be evaluated by X-ray photoelectron spectroscopy of the coating region.

[0033] 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. The intensity of this peak attributed to lithium carbonate bonds is thought to be proportional to the abundance ratio (or molar fraction) of lithium carbonate contained in the coating.

[0034] On the other hand, when the coating region containing lithium fluoride is measured by X-ray photoelectron spectroscopy (XPS), a peak attributed to lithium fluoride bonds is observed in the F1s spectrum. The peak attributed to lithium fluoride bonds is a peak attributed to Li—F bonds, and can appear in the binding energy range of 684.8 to 685.3 eV. The intensity of this peak attributed to lithium fluoride bonds is thought to be proportional to the abundance ratio (or molar fraction) of lithium fluoride contained in the coating.

[0035] In the O1s spectrum, the intensity of a peak observed in a binding energy range of 530 to 534 eV is defined as peak intensity A. Peak intensity A is the intensity at the apex of the peak in the O1s spectrum. Similarly, in the F1s spectrum, the intensity of a peak observed in a binding energy range of 684.8 to 685.3 eV is defined as peak intensity B. Peak intensity B is the intensity at the apex of the peak in the F1s spectrum. The ratio B / A of peak intensity B to peak intensity A can be said to reflect the composition ratio of lithium fluoride to lithium carbonate at the measurement position in the coating region.

[0036] Therefore, the composition ratio X1 of lithium fluoride to lithium carbonate contained in the coating region in the first region can be evaluated from the peak intensity ratio B / A measured in the first region. Similarly, the composition ratio X2 of lithium fluoride to lithium carbonate contained in the coating region in the second region can be evaluated from the peak intensity ratio B / A measured in the second region. Therefore, ratio X1 being higher than ratio X2 means that the peak intensity ratio B / A measured in the first region is higher than the peak intensity ratio B / A measured in the second region, and that the peak intensity ratio A / B measured in the first region is lower than the peak intensity ratio A / B measured in the second region.

[0037] In an electrochemical device according to an embodiment, the ratio A / B of peak intensity A to peak intensity B may be less than 2.5 in the first region over a range from the surface of the coating region to a depth of 50 nm or less, whereas in the second region, the ratio A / B may be 2.5 or greater over a range from the surface of the coating region to a depth of 50 nm or less.

[0038] In the second region, the ratio A / B may be 4.0 or greater throughout a range extending from the surface of the coating region to a depth of 50 nm or less. A ratio A / B of 4.0 or greater means that the SEI coating contains a large amount of lithium carbonate, but the lithium fluoride content is significantly reduced. By having a ratio A / B of 4.0 or greater throughout a range extending from the surface of the coating region to a depth of 50 nm or less, the lithium ion migration resistance can be maintained low, thereby maintaining a low internal resistance (DCR) of the electrochemical device even at low temperatures. In addition, the formation of a dense lithium carbonate coating facilitates the suppression of side reactions with the electrolyte, thereby suppressing the generation of lithium fluoride and damage to the SEI coating associated with charge and discharge. Furthermore, since an SEI coating with low lithium ion migration resistance is formed, even if the SEI coating is formed thick, as long as the thickness is appropriate, it does not hinder the migration of lithium ions, and forming a thick SEI coating makes it less susceptible to damage. In the second region, the ratio A / B may be 6.0 or more over a range from the surface layer of the coating region to a depth of 10 nm or less inside.

[0039] Furthermore, 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.

[0040] In the C1s spectrum at a depth of 100 nm from the surface layer of the coating region, the intensity of a peak observed in the binding energy range of 281 to 283 eV is defined as peak intensity C. Peak intensity C 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. Thus, peak intensity C reflects the peak intensity derived from the carbon material in a region where the absolute amount of lithium fluoride is small.

[0041] The ratio B / C of peak intensity B to peak intensity C may be 1.0 or greater in the first region over a range from the surface of the coating region to a depth of 50 nm or less. In contrast, the ratio B / C may be less than 1.0 in the second region over a range from the surface of the coating region to a depth of 50 nm or less. When the ratio B / C is less than 1.0, the amount of lithium fluoride produced is extremely small, and an SEI coating essentially consisting of lithium carbonate is formed, resulting in the formation of a dense coating that is stable against the electrolyte. It is believed that a ratio B / C of less than 1.0 over a range from the surface of the coating region to a depth of 50 nm or less can significantly suppress the production of lithium fluoride and damage to the SEI coating during charge and discharge. The ratio B / C may be 0.6 or less, or 0.5 or less, in the second region over a range from the surface of the coating region to a depth of 50 nm or less.

[0042] In the XPS measurement, peak intensities A and B are determined at any depth, and the ratio A / B is calculated. While etching the negative electrode material layer having a coating region, XPS measurement is performed to observe the changes in the O1s spectrum and the F1s spectrum in the depth direction. The depth dependency of the ratio A / B can be determined from the XPS spectra measured at multiple depths. Similarly, while etching the negative electrode material layer having a coating region, XPS measurement is performed to observe the changes in the F1s 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 depth dependency of the ratio B / C can be determined.

[0043] The maximum value of the peak intensity ratio A / B in the depth direction in the second region is, for example, 8 or more, and may be 10 or more, or 11 or more. Note that the peak intensity A, peak intensity B, and peak intensity C are each determined from the height of the peak from the baseline.

[0044] 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, subsequent charging and discharging can form a coating region on the surface of the negative electrode active material. For example, lithium fluoride is produced by reaction between the electrolyte and the negative electrode in the electrochemical device, forming a coating region containing lithium fluoride. The coating region can include compounds such as ROCOLi and ROLi, where R is a hydrocarbon group. These compounds can be identified by analyzing the peak of the Li1s spectrum in XPS measurements.

[0045] By controlling the conditions for forming the lithium carbonate-containing layer on the surface of the negative electrode layer and / or the conditions for contacting the negative electrode layer with an electrolyte in an electrochemical device, it is possible to control the lithium fluoride content in the coating region and the density of the lithium carbonate-containing layer. By varying these conditions within the negative electrode layer, it is possible to form a first region and a second region having different lithium fluoride to lithium carbonate composition ratios in the coating region.

[0046] 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.

[0047] 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 SiO ２ This means the equivalent depth.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] The higher the temperature of the negative electrode exposed to the carbon dioxide gas atmosphere, the more the lithium fluoride content in the coating region is reduced when an electrochemical device is constructed, allowing for the formation of a dense lithium carbonate coating. On the other hand, the higher the temperature of the negative electrode exposed to the carbon dioxide gas atmosphere, the more likely it is that a thick lithium carbonate layer will be formed. By shortening the exposure time to the carbon dioxide gas atmosphere, it is possible to suppress an increase in lithium ion migration resistance due to the formation of an excessively thick lithium carbonate coating, and to reduce the lithium fluoride content in the coating region. For example, by setting the exposure time to 12 hours or less, the lithium fluoride content in the coating region is reduced, allowing for the formation of a dense lithium carbonate coating of appropriate thickness.

[0056] 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.

[0057] When exposing the negative electrode to a carbon dioxide gas atmosphere, the temperature conditions of the negative electrode may be made different between the first region and the second region of the negative electrode, thereby obtaining a negative electrode in which the composition ratio X1 of lithium fluoride to lithium carbonate contained in the coating region in the first region is higher than the composition ratio X2 of lithium fluoride to lithium carbonate contained in the coating region in the second region.

[0058] For example, when exposing the negative electrode to a carbon dioxide gas atmosphere, a heater is positioned so as to overlap a portion of the negative electrode that does not include the connection region with the negative electrode lead terminal, as viewed from the normal direction of the main surface of the negative electrode, and the portion of the negative electrode is heated. As described above, in the portion of the negative electrode heated by the heater, when an electrochemical device is constructed, the lithium fluoride content in the coating region is reduced, resulting in a lower lithium fluoride composition ratio. In contrast, the remaining portion of the negative electrode is not actively heated by the heater, and therefore the lithium fluoride content in the coating region is higher when an electrochemical device is constructed. In other words, the portion of the negative electrode heated by the heater becomes the second region, and the remaining region that is not actively heated becomes the first region.

[0059] The temperature in the partial region (second region) of the negative electrode heated by the heater is, for example, 25° C. to 200° C. The temperature in the remaining region (first region) of the negative electrode is, for example, 0° C. to 25° C. The time for which the negative electrode is exposed to the carbon dioxide gas atmosphere is, for example, in the range of 1 minute to 12 hours.

[0060] 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).

[0061] The electrochemical device according to the present disclosure encompasses 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 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.

[0062] 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).

[0063] 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 formed into a columnar shape by winding a positive electrode 10 and a negative electrode 20 with a separator 30 interposed therebetween, a non-aqueous electrolyte (not shown), a metal cell case 210 with a bottom that accommodates the electrode assembly 100 and the non-aqueous 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 end of the cell case 210 to the gasket 221.

[0064] The positive electrode 10 is in the form of a long sheet and includes a positive electrode current collector and a positive electrode material layer supported thereon. The positive electrode material layer is formed on both sides of the positive electrode current collector. However, one longitudinal end of the positive electrode current collector is formed with a positive electrode current collector exposed portion 11x that does not have a positive electrode material layer. A positive electrode current collector 13 having a through hole in the center is welded to the positive electrode current collector 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 a sealing plate 220. Thus, the sealing plate 220 functions as an external positive electrode terminal.

[0065] The negative electrode 20 is in the form of a long sheet and includes a negative electrode current collector and a negative electrode material layer supported on the negative electrode current collector. The negative electrode current collector has a negative electrode current collector exposed portion 21x that does not have a negative electrode material layer, and a tab lead (negative electrode lead terminal) 24 is connected to the negative electrode current collector exposed portion 21x. The tab lead 24 extends from one end face of the electrode body 100 and is directly welded to the inner bottom surface of the cell case 210. Thus, the cell case 210 functions as an external negative electrode terminal.

[0066] Fig. 2 is a top view showing the configuration of the negative electrode 20 before winding, which is used in the electrochemical device 200. Note that, although one tab lead 24 is connected to the negative electrode in Fig. 2, a plurality of tab leads 24 may be connected.

[0067] The negative electrode 20 has a negative electrode current collector 21 and a negative electrode material layer 22 supported on the negative electrode current collector 21. The negative electrode 20 has four ends, a first end E1, a second end E2, a third end E3, and a fourth end E4, which correspond to the four sides of the rectangular outline of the negative electrode 20. A tab lead (negative electrode lead terminal) 24 is connected to the negative electrode current collector exposed portion 21x, and a portion of the tab lead 24 protrudes from the negative electrode. The tab lead 24 protrudes from the negative electrode on the side of the first end E1 along the longitudinal direction of the negative electrode 20.

[0068] The surface of the negative electrode material layer 22 is divided into a first region 20A and a second region 20B by the boundary indicated by dashed line A1 in FIG. 2 . The first region 20A includes a region close to the connection region with the tab lead 24. The first region 20A includes a region along the first end E1 and close to the first end E1. The first region 20A and the second region 20B are divided in half by a straight line (dashed line A1) along the first end E1 in the direction along the second end E2 (i.e., in the direction from the first end E1 toward the third end E3).

[0069] When forming the coating region, the SEI coating is formed so that the composition ratio X1 of lithium fluoride to lithium carbonate contained in the coating region in the first region 20A is higher than the composition ratio X2 of lithium fluoride to lithium carbonate contained in the coating region in the second region 20B. A coating having different composition ratios of lithium fluoride to lithium carbonate in the first region 20A and the second region 20B can be formed, for example, by exposing a negative electrode having a negative electrode material layer to which metallic lithium has been attached to a carbon dioxide atmosphere and forming a layer containing lithium carbonate in the surface layer portion of the negative electrode material layer, while the exposure to the carbon dioxide atmosphere is performed while heating the second region 20B and while suppressing heating of the first region 20A.

[0070] For example, when exposing the negative electrode to a carbon dioxide gas atmosphere, the negative electrode 20 can be positioned slightly offset from a heater provided in the device, so that when viewed from the normal direction of the main surface of the negative electrode 20, only the area below the dashed line A1 (the third end E3 side) overlaps with the heater, and the area above the dashed line A1 (the first end E1 side) does not overlap with the heater.

[0071] 3A and 3B show examples in which the boundary between the first region 20A and the second region 20B in FIG. 2 is different. In FIG. 3A , the surface of the negative electrode material layer 22 is divided into the first region 20A and the second region 20B by the boundary indicated by the dashed line A2. The first region 20A includes a region along the second end E2 and close to the second end E2. The first region 20A and the second region 20B are divided in half by the straight line (dashed line A2) along the second end E2 in the direction along the first end E1 (i.e., in the direction from the second end E2 toward the fourth end E4). In this case, when exposing the negative electrode to a carbon dioxide gas atmosphere, the negative electrode is positioned slightly offset from the heater provided in the device, and when viewed from the normal direction of the main surface of the negative electrode 20, only the area to the left of the dashed line A2 (the fourth end E4 side) overlaps with the heater, and the area to the right of the dashed line A2 (the second end E22 side) does not overlap with the heater.

[0072] 3B , the surface of the negative electrode 20 on which the negative electrode material layer 22 is disposed is divided into a first region 20A and a second region 20B by the boundary indicated by dashed line A3. The first region 20A and the second region 20B are divided in half in the direction along the first end E1 and also in half in the direction along the second end E2. In this case, when exposing the negative electrode to a carbon dioxide gas atmosphere, the negative electrode is positioned slightly offset from a heater provided in the device so that only the region below and to the left of dashed line A3 (the third end E3 side and the fourth end E4 side) overlaps with the heater when viewed from the normal direction of the main surface of the negative electrode 20.

[0073] 2, 3A, and 3B, the first region 20A includes at least a region adjacent to the first end E1 and a region adjacent to the second end E2, while the second region 20B includes at least a region adjacent to the third end E3 and a region adjacent to the fourth end E4.

[0074] Hereinafter, each component of the electrochemical device according to the embodiment of the present disclosure 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.

[0075] (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.

[0076] 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.

[0077] 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.

[0078] (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.

[0079] 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.

[0080] 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.

[0081] 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 to 20 μm, and more preferably 2 μm to 15 μm. 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.

[0082] The specific surface area of ​​the negative electrode material layer is 10 m ２ / g~70m ２ / g. The specific surface area of ​​the negative electrode material layer may be in the range of 10 m 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.

[0083] 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~70m ２ The specific surface area of ​​the negative electrode layer can be easily controlled within the range of 800 m / g. 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. 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.

[0084] In this specification, the average particle size refers to the volume-based median diameter (D ５０ ) means

[0085] 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.

[0086] 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.

[0087] 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.

[0088] (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.

[0089] 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.

[0090] 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.

[0091] (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.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] 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.

[0096] Next, the collected sample is heated at 150°C for 12 hours under a reduced pressure of 95 kPa or less, and then nitrogen gas is adsorbed onto a sample of known mass to obtain an adsorption isotherm over a relative pressure range of 0 to 1. The surface area of ​​the sample is then calculated from the monolayer adsorption amount of gas obtained from the adsorption isotherm. Here, the specific surface area is calculated using the BET single-point method (relative pressure 0.3) using the BET formula below.

[0097] P / V (P0-P) = (1 / VmC) + {(C-1) / VmC} (P / P0)... (1)

[0098] S=kVm...(2)

[0099] 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

[0100] 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.

[0101] 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.

[0102] 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.

[0103] 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. The derivative of a π-conjugated polymer refers to a polymer having a π-conjugated polymer skeleton such as polypyrrole, polythiophene, polyfuran, polyaniline, polythiophene vinylene, or polypyridine. For example, polythiophene derivatives include poly(3,4-ethylenedioxythiophene) (PEDOT).

[0104] 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.

[0105] 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.

[0106] 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.

[0107] (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.

[0108] (Electrolyte) The electrolyte has lithium ion conductivity and contains a lithium salt and a solvent that dissolves the lithium salt. The anions of the lithium salt are reversibly adsorbed to and desorbed from the positive electrode. The lithium ions derived from the lithium salt are reversibly inserted into and desorbed from the negative electrode.

[0109] Examples of lithium salts include LiClO4, LiBF4, LiPF6, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiFSO3, LiCF3CO2, LiAsF6, and LiB10 Cl 10 , LiCl, LiBr, LiI, LiBCl, LiN(SOF), LiN(SOCF), LiN(SOCFCF), etc. 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 the electrochemical device.

[0110] 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 lithium bis(fluorosulfonyl)imide, i.e., LiN(SOF) (LiFSI), is particularly preferred. For example, 80% by mass or more of the lithium salt may be LiFSI.

[0111] Imide-based electrolytes, 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.

[0112] 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.

[0113] 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.

[0114] 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, preferably 0.2 to 1 mol / L, and more preferably 0.7 to 1 mol / L.

[0115] 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.

[0116] 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.

[0117] (Additional Notes) The above embodiments disclose the following techniques: (Technology 1) A battery includes 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 carried 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, wherein the negative electrode material layer has a coating region, and further comprises a negative electrode lead terminal electrically connected to the negative electrode, wherein the negative electrode has a first end and a second end intersecting the first end, and the negative electrode lead terminal protrudes from the negative electrode at the first end, and the surface of the negative electrode material layer is divided into a first region including a region adjacent to a connection region with the negative electrode lead terminal, and a second region other than the first region, and the coating region comprises lithium carbonate and lithium fluoride, An electrochemical device, wherein a composition ratio X1 of the lithium fluoride to the lithium carbonate contained in the coating region in the first region of the negative electrode material layer is higher than a composition ratio X2 of the lithium fluoride to the lithium carbonate contained in the coating region in the second region. (Technology 2) The electrochemical device according to Technology 1, wherein the first region includes a region along the first end and close to the first end. (Technology 3) The electrochemical device according to Technology 2, wherein the first region and the second region are divided into two in a direction along the second end. (Technology 4) The electrochemical device according to any one of Technology 1 to 3, wherein the first region includes a region along the second end and close to the second end. (Technology 5) The electrochemical device according to Technology 4, wherein the first region and the second region are divided into two in a direction along the first end. (Technology 6) The electrochemical device according to any one of Technologies 1 to 5, wherein the negative electrode has a third end opposite the first end and a fourth end intersecting the first end and opposite the second end, the first region includes at least a region proximate to the first end and the second end, and the second region includes at least a region proximate to the third end and the fourth end. (Technology 7) The electrochemical device according to any one of Technologies 1 to 6, wherein the first end extends along the longitudinal direction of the negative electrode.

[0118] (Technology 8) An electrochemical device according to any one of Techniques 1 to 7, wherein an O1s spectrum of the coating region measured by X-ray photoelectron spectroscopy has a peak in a binding energy range of 530 to 534 eV, and an F1s spectrum of the coating region measured by X-ray photoelectron spectroscopy has a peak in a binding energy range of 684.8 to 685.3 eV, and in the first region, 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 is less than 2.5 throughout a range from the surface of the coating region to a depth of 50 nm or less, and in the second region, the ratio A / B is 2.5 or greater throughout a range from the surface of the coating region to a depth of 50 nm or less. (Technology 9) An electrochemical device according to Technique 8, wherein the ratio A / B in the second region is 4.0 or greater throughout a range from the surface of the coating region to a depth of 50 nm or less. (Technology 10) An electrochemical device according to Technology 8 or 9, wherein a C1s spectrum of the coating region measured by X-ray photoelectron spectroscopy has a peak in a binding energy range of 281 to 283 eV, and in the first region, a ratio B / C of the peak intensity B in the F1s spectrum to a peak intensity C at an apex of the peak in the C1s spectrum at a depth of 100 nm from the surface of the coating region is 1.0 or more throughout a range from the surface of the coating region to a depth of 50 nm or less, and the ratio B / C in the second region is less than 1.0 throughout a range from the surface of the coating region to a depth of 50 nm or less. (Technology 11) An electrochemical device according to any one of Technology 1 to 10, wherein the electrolyte is an electrolytic solution containing a solvent and an imide-based electrolyte, and a concentration of the imide-based electrolyte in the electrolytic solution is 1.0 mol / L or less. (Technology 12) The electrochemical device according to Technology 11, wherein the imide-based electrolyte contains an anion containing fluorine and sulfur. (Technology 13) The electrochemical device according to any one of Technology 1 to 12, 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.(Technology 14) The electrochemical device according to any one of Technologies 1 to 13, 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. (Technology 15) The carbon material contained as the negative electrode active material has an average particle size of 10 μm or less, and the specific surface area of ​​the negative electrode material layer is 10 m. ２ / g or more 70m ２ 15. The electrochemical device according to any one of claims 1 to 14, wherein the electrochemical device has a surface area of ​​1000 Å or less.

[0119] EXAMPLES The present disclosure will be described in more detail below based on examples, but the present disclosure is not limited to these examples.

[0120] (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.

[0121] (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. A copper negative electrode tab lead (negative electrode lead terminal) was attached to the exposed portion of the negative electrode core material of the negative electrode.

[0122] 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.

[0123] Thereafter, the chamber of the heater-equipped 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 negative electrode was positioned offset from the center of the heater so that only a portion of the negative electrode located below the dashed line A1 in FIG. 2 (the third end E3 side) overlapped with the heater, as viewed from the normal direction of the main surface of the negative electrode. Using the heater, the second region of the negative electrode was heated so that the temperature of the second region was maintained at 100°C, as shown in FIG. 2. The dew point of the carbon dioxide gas atmosphere was −40°C, the mole fraction of carbon dioxide was 100%, and the pressure in the chamber was 1 atmosphere (1.01 × 10 5 The temperature of the carbon dioxide gas atmosphere at 1 atmosphere was 20° C. The negative electrode was exposed to the carbon dioxide gas atmosphere for 30 minutes.

[0124] (3) Preparation of Electrode Assembly The positive electrode and negative electrode were wound into a cylindrical shape with a polyolefin microporous membrane separator (20 μm thick) interposed therebetween. The exposed portion of the positive electrode core material protruded from one end of the wound assembly, and the negative electrode tab lead protruded from the other end of the electrode assembly. A disk-shaped positive electrode current collector was welded to the exposed portion of the positive electrode core material.

[0125] (4) Preparation of Non-Aqueous Electrolyte Solution: A solvent was prepared by adding 0.1% by mass of vinylene carbonate to a mixture of propylene carbonate and ethyl methyl carbonate in a volume ratio of 3:7. LiFSI (a lithium salt) was dissolved in the resulting solvent at a concentration of 0.9 mol / L to prepare a non-aqueous electrolyte.

[0126] (5) Assembly of the electrochemical device The electrode assembly was housed in a cell case with an opening and a bottom, the tab lead connected to the positive electrode current collector plate was connected to the inner surface of the sealing plate, and the protruding portion of the negative electrode tab lead was welded to the inner bottom surface of the cell case. After the nonaqueous electrolyte was placed in 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.

[0127] 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.

[0128] Comparative Example 1 In the preparation of the negative electrode, when a coating containing lithium carbonate was formed on the surface layer of the negative electrode material layer, the negative electrode was positioned so that the entire surface of the negative electrode overlapped the heater when viewed from the normal direction of the main surface of the negative electrode. The heater was used to heat the negative electrode so that the entire negative electrode was maintained at 100° C. Except for this, an electrochemical device B1 was prepared in the same manner as in Example 1.

[0129] For each of electrochemical devices A1 and B1, an XPS analysis was performed on the coating region of the negative electrode as shown below.

[0130] (XPS analysis of coating region) The negative electrode was removed from the electrochemical device, and the surface of the negative electrode material layer was analyzed by XPS for C1s spectrum, O1s spectrum, and F1s spectrum. An X-ray photoelectron spectrometer (trade name: PHI QuanteraSXM, manufactured by ULVAC-PHI, Inc.) was used for the analysis. The measurement conditions are shown below.

[0131] 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

[0132] In the XPS analysis, the peak intensity A at the apex of a peak appearing in the binding energy range of 530 to 534 eV from 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 a peak appearing in the binding energy range of 684.8 to 685.3 eV from the F1s spectrum was determined as the intensity of the peak attributed to lithium fluoride bonds. Furthermore, the peak intensity C at the apex of a 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 of the coating region was determined as the intensity of the peak attributed to C—C bonds of the carbon material contained in the negative electrode active material.

[0133] While etching the surface layer portion of the negative electrode material layer, changes in peak intensity A and peak intensity B in the depth direction (thickness direction of the surface layer portion) were measured, and changes in the peak intensity ratio A / B in the depth direction (thickness direction of the surface layer portion) were determined. In addition, changes in peak intensity B in the depth direction (thickness direction of the surface layer portion) were compared with peak intensity C, and changes in the peak intensity ratio B / C in the depth direction (thickness direction of the surface layer portion) were determined.

[0134] 4 shows the change in the peak intensity ratio A / B in the depth direction for electrochemical devices A1 and B1. For electrochemical device A1, Fig. 4 shows the change in the peak intensity ratio A / B in the depth direction at positions corresponding to first region 20A and second region 20B in Fig. 2.

[0135] As can be seen from FIG. 4, in the electrochemical device A1 of Example 1, the peak intensity ratio A / B in the first region is ２ On the other hand, the peak intensity ratio A / B in the second region is less than 2.5 from the surface layer of the coating region to the SiO ２ The peak intensity ratio A / B in the first region is lower than the peak intensity ratio A / B in the second region. This means that the composition ratio X1 of lithium fluoride to lithium carbonate contained in the coating region in the first region is higher than the composition ratio X2 of lithium fluoride to lithium carbonate contained in the coating region in the second region. In this case, current easily flows from the connection region with the negative electrode lead terminal through the first region to the second region, and the current easily diffuses and flows throughout the electrode. As a result, heat concentration at the connection portion with the lead terminal is suppressed. This improves the reliability of the electrochemical device.

[0136] In the electrochemical device A1 of Example 1, the peak intensity ratio B / C in the first region is ２ On the other hand, the peak intensity ratio B / C in the second region was 1.0 or more from the surface layer of the coating region to the SiO ２This was converted to a value less than 1.0 over the range of the inner depth of 50 nm or less.

[0137] In contrast, in the electrochemical device B1 of Comparative Example 1, as shown in FIG. 4, the peak intensity ratio A / B is higher in the range from the surface layer of the coating region to the SiO ２ In this case, the internal resistance of the coating is uniform throughout the negative electrode, making it difficult for current to diffuse throughout the electrode, and current may concentrate at the connection portion with the lead terminal. As a result, heat may concentrate at the connection portion with the lead terminal, potentially reducing the reliability of the electrochemical device.

[0138] In the electrochemical device A1, the peak intensity ratio A / B in the second region is ２ However, by changing the heating conditions of the negative electrode when forming a coating containing lithium carbonate on the surface layer portion of the negative electrode material layer, the peak intensity ratio A / B in the first region can be increased from the surface layer of the coating region to the SiO ２ The peak intensity ratio A / B in the second region is set to be less than 2.5 over a range of 50 nm or less in depth in terms of SiO2 equivalent, similarly to the electrochemical device B1. ２ It is also possible to make the conversion coefficient 4.0 or more over a range of a depth of 50 nm or less inside.

[0139] The electrochemical device according to the present disclosure is suitable for use in a vehicle, for example.

[0140] 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.

[0141] 100: Electrode body 10: Positive electrode 11x: Positive electrode current collector exposed portion 13: Positive electrode current collector plate 15: Tab lead 20: Negative electrode 20A: First region 20B: Second region 21x: Negative electrode current collector exposed portion 24: Tab lead (negative electrode lead terminal) 30: Separator 200: Electrochemical device 210: Cell case 220: Sealing plate 221: Gasket

Claims

1. A battery 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, wherein the negative electrode material layer has a coating region and further comprises a negative electrode lead terminal electrically connected to the negative electrode, wherein the negative electrode has a first end and a second end intersecting the first end, and the negative electrode lead terminal protrudes from the negative electrode at the first end, wherein the surface of the negative electrode material layer is divided into a first region including a region adjacent to the connection region with the negative electrode lead terminal, and a second region other than the first region, and wherein the coating region comprises lithium carbonate and lithium fluoride, an electrochemical device, wherein a composition ratio X1 of the lithium fluoride to the lithium carbonate contained in the coating region in the first region of the negative electrode material layer is higher than a composition ratio X2 of the lithium fluoride to the lithium carbonate contained in the coating region in the second region.

2. The electrochemical device of claim 1, wherein said first region includes a region along and adjacent to said first end.

3. The electrochemical device according to claim 2, wherein the first region and the second region are divided into two in a direction along the second end portion.

4. The electrochemical device of claim 1, wherein said first region includes a region along and adjacent said second end.

5. The electrochemical device according to claim 4, wherein the first region and the second region are divided into two in a direction along the first end.

6. The electrochemical device according to claim 1, wherein the negative electrode has a third end opposite the first end and a fourth end intersecting the first end and opposite the second end, the first region includes at least regions adjacent to the first end and the second end, and the second region includes at least regions adjacent to the third end and the fourth end.

7. The electrochemical device according to any one of claims 1 to 6, wherein the first end extends along the longitudinal direction of the negative electrode.

8. The electrochemical device according to claim 1, wherein an O1s spectrum of the coating region measured by X-ray photoelectron spectroscopy has a peak in a binding energy range of 530 to 534 eV, and an F1s spectrum of the coating region measured by X-ray photoelectron spectroscopy has a peak in a binding energy range of 684.8 to 685.3 eV, and in the first region, 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 is less than 2.5 throughout a range from the surface of the coating region to a depth of 50 nm or less, and in the second region, the ratio A / B is 2.5 or greater throughout a range from the surface of the coating region to a depth of 50 nm or less.

9. The electrochemical device according to claim 8, wherein the ratio A / B in the second region is 4.0 or greater over a range from the surface of the coating region to a depth of 50 nm or less inside.

10. The electrochemical device according to claim 8, wherein a C1s spectrum of the coating region measured by X-ray photoelectron spectroscopy has a peak in a binding energy range of 281 to 283 eV, wherein in the first region, a ratio B / C of the peak intensity B in the F1s spectrum to a peak intensity C at the apex of the peak in the C1s spectrum at a depth of 100 nm from the surface of the coating region is 1.0 or greater throughout a range from the surface of the coating region to a depth of 50 nm or less, and wherein the ratio B / C in the second region is less than 1.0 throughout a range from the surface of the coating region to a depth of 50 nm or less.

11. The electrochemical device according to any one of claims 1 to 6 and 8 to 10, wherein the electrolyte is an electrolytic solution containing a solvent and an imide-based electrolyte, and the concentration of the imide-based electrolyte in the electrolytic solution is 1.0 mol / L or less.

12. The electrochemical device of claim 11, wherein the imide-based electrolyte comprises anions containing fluorine and sulfur.

13. The electrochemical device according to any one of claims 1 to 6 and 8 to 10, 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.

14. The electrochemical device according to any one of claims 1 to 6 and 8 to 10, 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.

15. The carbon material contained as the negative electrode active material has an average particle size of 10 μm or less, and the specific surface area of ​​the negative electrode material layer is 10 m ２ / g or more 70m ２ The electrochemical device according to any one of claims 1 to 6 and 8 to 10, wherein the surface area of ​​the electrochemical device is 0.1 μm or less.

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