Negative electrode and battery

Abstract

This negative electrode 10 comprises a long negative electrode current collector 11 and a long negative electrode mixture layer 12. At least part of a negative electrode mixture layer (12) satisfies the relational expression (1) if: each region obtained by dividing said negative electrode mixture layer (12) into a center region (121) including the center in the width direction and end regions (122) including both ends in the width direction, and further dividing said layer into a surface-side region and a collector-side region by equally dividing the negative electrode mixture layer (12) into two in the thickness direction is defined as either a first region, which is a region of the negative electrode mixture layer which is an end region and a surface-side region, a second region, which is a region of the negative electrode mixture layer which is an end region and a collector-side region, a third region, which is a region of the negative electrode mixture layer which is a center region and a surface-side region, or a fourth region, which is a region of the negative electrode mixture layer which is a center region and a collector-side region; and the porosity of the first region is defined as A, the porosity of the second region is defined as B, and the porosity of the third region is defined as C.　(1): C>A>B

Description

Negative electrode and battery

[0001] This disclosure relates to a negative electrode and a battery.

[0002] In recent years, secondary batteries such as lithium-ion batteries have been widely used in applications requiring high capacity, such as automotive and energy storage. The electrodes that make up such batteries have a significant impact on their performance. For this reason, various studies have been conducted on electrodes.

[0003] Patent Document 1 proposes a negative electrode that can improve the input / output characteristics of a lithium-ion secondary battery by improving the permeability of the electrolyte. In the negative electrode described in Patent Document 1, the negative electrode mixture layer formed on the negative electrode current collector has a high-density negative electrode mixture layer and a low-density negative electrode mixture layer formed on the high-density negative electrode mixture layer, and the upper and side surfaces of the high-density mixture layer are covered with the low-density negative electrode mixture layer.

[0004] Japanese Patent Publication No. 2013-251213

[0005] Conventional negative electrodes, such as those described in Patent Document 1, which have a structure that changes density within the negative electrode mixture layer, can improve the permeability of the electrolyte and thus improve the input / output characteristics of the battery. However, such conventional negative electrodes still have room for improvement in terms of improving the battery's cycle characteristics.

[0006] Therefore, this disclosure provides a negative electrode that can improve the cycle characteristics of a battery.

[0007] The negative electrode of the present disclosure is a negative electrode including a long negative electrode current collector and a long negative electrode active material layer disposed on the negative electrode current collector. The negative electrode active material layer is divided in the width direction of the negative electrode active material layer into a central region including the center in the width direction and end regions including both ends in the width direction, and in the thickness direction of the negative electrode active material layer, when the negative electrode active material layer is equally divided into two in the thickness direction, each region obtained by dividing the negative electrode active material layer into a surface side region located on the surface side and a current collector side region located on the negative electrode current collector side is defined as follows: First region: The region of the negative electrode active material layer that is the end region and the surface side region. Second region: The region of the negative electrode active material layer that is the end region and the current collector side region. Third region: The region of the negative electrode active material layer that is the central region and the surface side region. Fourth region: The region of the negative electrode active material layer that is the central region and the current collector side region. When the porosity of the first region is A, the porosity of the second region is B, and the porosity of the third region is C, at least a part of the negative electrode active material layer satisfies the following relational expression (1). C > A > B... (1)

[0008] According to the negative electrode of the present disclosure, the cycle characteristics of the battery can be improved.

[0009] FIG. 1 is a plan view of a negative electrode according to Embodiment 1. FIG. 2 is a cross-sectional view taken along line II-II of the negative electrode shown in FIG. 1. FIG. 3 is a longitudinal cross-sectional view schematically showing an example of a battery according to Embodiment 2.

[0010] [Findings underlying the present disclosure] The inventors of the present invention noticed that when the density is changed as described in Patent Document 1 within the negative electrode active material layer, although the permeability of the electrolytic solution in the negative electrode is improved and good input / output characteristics of the battery can be obtained, the cycle characteristics are not sufficient. Therefore, for example, when the inventors intensively studied a long negative electrode configured to form a wound electrode group, they found that it is necessary to suppress the flow of the electrolytic solution flowing out of the electrode group during charging in order to improve the cycle characteristics of the battery. As a result, the inventors further intensively studied and arrived at the negative electrode of the present disclosure described below.

[0011] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. The present disclosure is not limited to the following embodiments.

[0012] [Embodiments of the Present Disclosure] (Embodiment 1) FIG. 1 is a plan view of a negative electrode according to Embodiment 1. FIG. 2 is a cross-sectional view taken along line II-II of the negative electrode shown in FIG. 1. The negative electrode 10 according to Embodiment 1 includes a long negative electrode current collector 11 and a long negative electrode active material layer 12 disposed on the negative electrode current collector 11.

[0013] The negative electrode active material layer 12 is divided in the width direction of the negative electrode active material layer 12 into a central region 121 including the center 12a in the width direction and an end region 122 including both end portions 12b in the width direction, and in the thickness direction of the negative electrode active material layer 12, when the negative electrode active material layer 12 is equally divided into two in the thickness direction, each region obtained by dividing it into a surface side region 123 located on the surface side of the negative electrode active material layer 12 and a current collector side region 124 located on the negative electrode current collector 11 side is defined as follows. First Region: The region of the negative electrode active material layer 12 that is the end region 122 and the surface side region 123. Second Region: The region of the negative electrode active material layer 12 that is the end region 122 and the current collector side region 124. Third Region: The region of the negative electrode active material layer 12 that is the central region 121 and the surface side region 123. Fourth Region: The region of the negative electrode active material layer 12 that is the central region 121 and the current collector side region 124.

[0014] Here, when the porosity of the first region is A, the porosity of the second region is B, and the porosity of the third region is C, at least a part of the negative electrode active material layer 12 satisfies the following relational expression (1). C > A > B... (1)

[0015] The porosity of the fourth region is defined as D.

[0016] In the present disclosure, the surface of the negative electrode active material layer 12 means the surface that contacts the electrolyte when the negative electrode 10 constitutes a battery, and is located on the opposite side to the surface facing the negative electrode current collector 11.

[0017] Furthermore, in this disclosure, the end region 122 including both ends 12b in the width direction refers to both the first end region including one end (first end) in the width direction and the second end region including the other end (second end). However, with respect to the void ratio A of the first region, the void ratio of the first end region and the void ratio of the second end region may be different from each other, as long as their respective void ratios satisfy relation (1). Similarly, with respect to the void ratio B of the second region, the void ratio of the first end region and the void ratio of the second end region may be different from each other, as long as their respective void ratios satisfy relation (1).

[0018] In Figure 2, for the sake of clarity, symbols representing the void ratio are added to each region: "A" for the first region, "B" for the second region, "C" for the third region, and "D" for the fourth region.

[0019] By satisfying the above relational expression (1) in the negative electrode mixture layer 12, that is, by having a porosity C in the third region located in the center of the surface side of the negative electrode mixture layer 12 greater than the porosity A in the first region located at the surface edge, the third region is secured as a space for holding the electrolyte, and the first region prevents the electrolyte from flowing out of the electrode group during charging. As a result, the negative electrode 10 according to Embodiment 1 can improve the cycle characteristics of the battery. Furthermore, since the second region, which has a porosity B smaller than porosity A and porosity C, is located at the current collector side edge, the electrolyte contained in the third region can be prevented from flowing into the second region, thereby improving the cycle characteristics.

[0020] In this disclosure, the porosity in each of the first to fourth regions of the negative electrode mixture layer 12 is a two-dimensional value obtained from the ratio of the area of ​​voids to the cross-sectional area of ​​each region in the cross-section of the negative electrode mixture layer 12.

[0021] The porosity in each region of the negative electrode mixture layer 12 can be measured by the following method: (1) Disassemble the battery to be evaluated and cut out the negative electrode to expose the cross-section of the negative electrode mixture layer 12. One method for exposing the cross-section is to cut out a part of the negative electrode 10 and process it with an ion milling device (for example, Hitachi High-Tech Corporation's IM4000PLUS) to expose the cross-section of the negative electrode mixture layer 12. (2) Use a scanning electron microscope (SEM) to take backscattered electron images of the exposed cross-section of the negative electrode mixture layer 12 for each region of the negative electrode mixture layer 12. The magnification for taking the backscattered electron images is, for example, 800x. Perform the following processes (3) to (4) for each region of the negative electrode mixture layer 12 and calculate the porosity of each. (3) The cross-sectional image obtained above is imported into a computer and binarized using image analysis software (for example, ImageJ from the National Institutes of Health, USA) to obtain a binarized image in which the particle cross-sections in the cross-sectional image are converted to black and the voids present in the particle cross-sections are converted to white. (4) In the binarized image obtained above, the area of ​​the voids is calculated by excluding the voids inside the particles (pores not connected to the particle surface) and pores with a width of 3 μm or less connected to the particle surface from the voids converted to white. The void ratio is calculated based on the following formula: Void ratio (%) = Area of ​​voids / Area of ​​each region in the cross-section of the negative electrode mixture layer × 100 (5) For each region of the negative electrode mixture layer 12, the void ratio is calculated three times using (3) and (4) above, and the average value is taken as the void ratio for each region of the negative electrode mixture layer 12.

[0022] The porosity in each region of the negative electrode mixture layer 12 can be controlled, for example, by the type and proportion of carbon material included, as well as the rolling conditions when manufacturing the negative electrode mixture layer 12.

[0023] As described above, in the first embodiment 1, the negative electrode 10 only needs to satisfy the above relational expression (1) if at least a part of the negative electrode mixture layer 12. Therefore, the elongated negative electrode mixture layer 12 may only partially satisfy the above relational expression (1). For example, in the elongated negative electrode mixture layer 12, it is preferable that 90% or more of the length in the longitudinal direction satisfies the above relational expression (1). The entire elongated negative electrode mixture layer 12 may also satisfy the above relational expression (1).

[0024] The negative electrode mixture layer 12 may also satisfy the following relation (2) in the portion that satisfies relation (1): C ≥ D > A ... (2)

[0025] By satisfying the above relational equation (2) in the negative electrode mixture layer 12, that is, by having a porosity D in the fourth region located in the center of the negative electrode mixture layer 12 on the current collector side, greater than the porosity A in the first region and the porosity B in the second region located in the end region 122, the fourth region, in addition to the third region, can be secured as a space for holding electrolyte. Furthermore, the electrolyte held in the fourth region is prevented from flowing out of the electrode group during charging by the first and second regions of the end region 122. As a result, the negative electrode 10 according to Embodiment 1 can further improve the cycle characteristics of the battery. Moreover, because the fourth region, which has a porosity D of less than or equal to porosity C, is located in the center on the current collector side, the liquid permeability from the electrode plate surface of the third region is not impaired, and the cycle characteristics can be further improved.

[0026] The negative electrode mixture layer 12 may also satisfy the following relation (3) in the portion that satisfies relation (1) above: 1.0 < C / A < 2.0 ... (3)

[0027] By satisfying the above relational equation (3), the electrolyte in the third region located in the center of the surface side of the negative electrode mixture layer 12 can be prevented from flowing into the first region located at the surface edge. This makes it possible to further improve the battery's cycle characteristics.

[0028] The negative electrode mixture layer 12 may also satisfy the following relation (4) in the portion that satisfies relation (1) above: 1.2 < C / A < 1.7 ... (4)

[0029] By satisfying the above relational equation (4) in the negative electrode mixture layer 12, it is possible to suppress the flow of the electrolyte from the third region located in the central part of the surface side of the negative electrode mixture layer 12 into the first region located at the edge of the surface side.

[0030] The porosity A in the first region may be, for example, 10.0% or more and 25.0% or less, or 12.0% or more and 20.0% or less.

[0031] The porosity B in the second region may be, for example, 5.0% or more and 20.0% or less, or 10.0% or more and 15.0% or less.

[0032] The porosity C in the third region may be, for example, 15.0% or more and 40.0% or less, or 15.0% or more and 30.0% or less.

[0033] The porosity D in the fourth region may be, for example, 13.0% or more and 33.0% or less, or 14.0% or more and 28.0% or less.

[0034] The curvature ratio in the thickness direction at the end region 122 of the negative electrode mixture layer 12 is T. AB The curvature ratio in the thickness direction in the central region 121 of the negative electrode mixture layer 12 is set to T. CD In this case, at least a portion of the negative electrode mixture layer 12 may satisfy the following relational expression (5). AB >T CD ... (5)

[0035] The elongated negative electrode mixture layer 12 may partially satisfy the above relational expression (5), or the entire elongated negative electrode mixture layer 12 may satisfy the above relational expression (5).

[0036] In a negative electrode mixture layer, the curvature ratio is an indicator of the degree of curvature of the voids (pores) through which the electrolyte forms in the negative electrode mixture layer. A smaller curvature ratio means that the path through the voids is less curved. The curvature ratio of the negative electrode mixture layer is the value obtained by dividing the path length from the start point to the end point of the voids in the negative electrode mixture layer by the straight-line distance from the start point to the end point of the voids in the negative electrode mixture layer. If the path length is the same as the straight-line distance from the start point to the end point of the voids in the electrode mixture layer, the curvature ratio is 1.

[0037] The tortuosity T in the end region 122 of the negative electrode mixture layer 12 AB is a value obtained by dividing the path (path length) from the start point to the end point of the voids in the end region 122 of the negative electrode mixture layer 12 by the straight-line distance from the start point to the end point of the voids in the end region 122 of the negative electrode mixture layer 12. The tortuosity T in the central region 121 of the negative electrode mixture layer 12 CD is a value obtained by dividing the path (path length) from the start point to the end point of the voids in the central region 121 of the negative electrode mixture layer 12 by the straight-line distance from the start point to the end point of the voids in the central region 121 of the negative electrode mixture layer 12. That the negative electrode mixture layer 12 satisfies the above relational expression (5) means that, namely, by increasing the tortuosity T of the end region 122 of the negative electrode mixture layer 12 to increase the packing density of the negative active material particles and decreasing the tortuosity T of the central region 121 to lower the packing density of the negative active material particles, the permeability of the electrolytic solution can be improved in the central region 121 while sufficiently retaining the electrolytic solution, and the outflow of the electrolytic solution from the end region 122 can be further suppressed. Therefore, when the above relational expression (5) is satisfied, the negative electrode 10 according to Embodiment 1 can further improve the cycle characteristics of the battery.

[0038] The negative electrode mixture layer 12 may further satisfy the following relational expression (6) in a portion satisfying the above relational expression (5). 1.0 < T AB / T CD < 1.5... (6)

[0039] When the negative electrode mixture layer 12 satisfies the above relational expression (6), that is, when the ratio (T CD of the tortuosity T of the end region 122 to the tortuosity T of the central region 121 AB is more than 1.0 and less than 1.5, a battery with further improved cycle characteristics can be realized. Hereinafter, the ratio of the tortuosity T of the end region 122 to the tortuosity T of the central region 121 AB / T CD will be referred to as the tortuosity ratio T CD / T AB . AB / T CD and described as such.

[0040] The tortuosity T in the end region 122 AB ​​​​For example, it may be 3.0 or more and 6.5 or less, or 3.5 or more and 5.5 or less.

[0041] Curvature ratio T in the central region 121 CD For example, it may be 2.0 or more and 5.5 or less, or 3.0 or more and 4.5 or less.

[0042] In this specification, the curvature ratio T in the end region 122 AB and the curvature ratio T in the central region 121 CD These are calculated by the following equations (I) and (II), respectively. In equation (I), f AB This is the path length (abbreviated as path length) of the medial axis that penetrates opposing surfaces in the thickness direction in the end region 122, s AB is f AB This is the length of the straight line connecting the starting point and the ending point of the path (abbreviated as the straight-line distance between the starting and ending points). In equation (II), f CD This is the path length (abbreviated as path length) of the medial axis that penetrates opposing surfaces in the thickness direction in the central region 121, s CD is f CD This is the length of the straight line connecting the start and end points of the path (abbreviated as the straight-line distance between the start and end points). Furthermore, the sample used to evaluate the curvature ratio is one that is in a fully discharged state. AB = f AB / s AB ... (I) T CD = f CD / s CD ... (II)

[0043] The above path length and straight-line distance are determined by cross-sectional observation and image analysis of the negative electrode mixture layer 12 using a 3D scanning electron microscope (3DSEM, for example, Ethos NX-5000 manufactured by Hitachi High-Tech Corporation).

[0044] The specific method for calculating the curvature ratio is as follows:

[0045] (1) Construction of a three-dimensional structure The negative electrode mixture layer 12, on which the three-dimensional structure is constructed using a 3DSEM, is placed on the sample stage of the 3DSEM, and continuous cross-sectional slicing and cross-sectional observation are performed alternately. The observation is performed with an acceleration voltage of 5 kV. The obtained two-dimensional continuous images are binarized using three-dimensional image analysis software (for example, EX FACT VR manufactured by Visual Science Japan Co., Ltd.), and the images are stitched together to construct a three-dimensional structure. The three-dimensional structure is preferably 100 μm × 100 μm × 100 μm or larger.

[0046] (2) f in the end region 122 and the central region 121 AB , f CD Determination In the three-dimensional structure image obtained in (1) above, the voids extracted by binarization are thinned, and the axis (Medial Axis) passing through the center of the void is determined. Among the Medial Axis present in the cube, those that penetrate perpendicularly to the surface of the negative electrode current collector 11 are extracted, and for those with branches in a single path, the shortest path is determined by the path length (f) of the end region 122 and the central region 121, respectively. AB and f CD ) was decided.

[0047] (3) Calculation of the curvature ratio in the end region 122 and the central region 121 The path length (f) of the end region 122 and the central region 121 obtained in (2) above AB and f CD ) the average value, the straight-line distance (s) connecting the paths AB and s CD Using the average value of ), the curvature ratio of the end region 122 and the central region 121 is calculated using the above formulas (I) and (II).

[0048] Curvature ratio T in the end region 122 AB and the curvature ratio T in the central region 121 CDTo produce the negative electrode mixture layer 12 such that the above relational equations (5) and (6) are satisfied, for example, multiple types of negative electrode active materials may be used. For example, two active materials A and B made of different materials may be prepared as negative electrode active materials, and multiple negative electrode mixture slurries with different mixing ratios of active material A and active material B may be prepared. For example, by changing the mixing ratio of active material A and active material B in the negative electrode mixture slurry for producing the region corresponding to the end region 122 and the negative electrode mixture slurry for producing the region corresponding to the central region 121, the curvature ratio T in the end region 122 can be changed. AB and the curvature ratio T in the central region 121 CD The system can be controlled to satisfy the above relations (5) and (6).

[0049] The ratio of the length of the central region 121 in the width direction to the total width of the negative electrode mixture layer 12 is, for example, 10% or more and 50% or less. By setting the length of the central region 121 in the width direction within this range, the effects obtained by satisfying the above relational equations (1) to (6) become easier to obtain. Therefore, the cycle characteristics of the battery can be further improved.

[0050] The thickness of the negative electrode mixture layer 12 in the central region 121 may be greater than the thickness of the negative electrode mixture layer 12 in the edge region 122. This allows a sufficient amount of electrolyte to be held in the third region, which is the central region 121, thereby further improving the cycle characteristics.

[0051] The configurations of the negative electrode 10 in Embodiment 1 will be described in detail below.

[0052] [Negative Electrode Current Collector] A sheet or film made of a metallic material such as stainless steel, nickel, copper, or alloys thereof may be used as the negative electrode current collector 11. The sheet or film may be porous or non-porous. Metal foil, metal mesh, etc., may be used as the sheet or film. A carbon material such as carbon may be coated on the surface of the negative electrode current collector 11 as a conductive auxiliary material.

[0053] The thickness of the negative electrode current collector 11 is not particularly limited, but from the viewpoint of balancing the strength and weight reduction of the negative electrode 10, it may be, for example, 1 μm or more and 50 μm or less, or 5 μm or more and 20 μm or less.

[0054] [Negative electrode mixture layer] The negative electrode mixture layer 12 contains a negative electrode active material. The negative electrode active material may be a material having the ability to intercept and release metal ions (e.g., lithium ions). The negative electrode active material includes, for example, at least one selected from the group consisting of carbon materials and materials capable of forming alloys with lithium.

[0055] The carbon material that functions as the negative electrode active material is, for example, at least one selected from the group consisting of graphite, soft carbon, and hard carbon. The carbon material is preferably graphite. The graphite may be natural graphite or artificial graphite.

[0056] The negative electrode mixture layer 12 may contain artificial graphite and natural graphite as negative electrode active material. Artificial graphite and natural graphite have different ranges of internal porosity. Therefore, for example, by adjusting the mass ratio of artificial graphite and natural graphite contained in the negative electrode mixture layer 12, it is possible to control the porosity and curvature of the negative electrode mixture layer 12 to a desired range. For example, the mass ratio of artificial graphite and natural graphite in the end region 122 of the negative electrode mixture layer 12 may be different from the mass ratio of artificial graphite and natural graphite in the central region 121 of the negative electrode mixture layer 12. This makes it easy to control the porosity and curvature of the negative electrode mixture layer 12 between the end region 122 and the central region 121. For example, the mass ratio of artificial graphite to the total amount of artificial graphite and natural graphite in the third region of the negative electrode mixture layer 12 may be greater than the mass ratio of artificial graphite to the total amount of artificial graphite and natural graphite in the second region of the negative electrode mixture layer 12.

[0057] When preparing the first to fourth regions of the negative electrode mixture layer 12, the mass ratio of artificial graphite to natural graphite in each region may be adjusted to satisfy the above relation (1). Furthermore, the mass ratio of artificial graphite to natural graphite in each region may be adjusted to satisfy a desired relation from the above relations (2) to (6).

[0058] The negative electrode mixture layer 12 contains, for example, 0.5 m of negative electrode active material. 2 / g or more and 3.0m 2 Artificial graphite having a BET specific surface area of ​​less than 3.0 m² 2 / g or more and 10.0m 2 It may also contain natural graphite having a BET specific surface area of ​​less than or equal to / g.

[0059] Whether the graphite contained in the negative electrode mixture layer 12 is artificial graphite or natural graphite can be determined, for example, by examining the voids present within the graphite particles using SEM images of the cross-section of the graphite particles. Generally, natural graphite contains a relatively large number of voids within its particles, while artificial graphite contains very few voids. Therefore, it is possible to determine whether the graphite contained in the negative electrode mixture layer 12 is artificial graphite or natural graphite based on its state.

[0060] Furthermore, to increase the capacity of the secondary battery, the negative electrode active material may include a Si-based material. Here, a Si-based material means a material containing Si. Examples of Si-based materials include Si, Si alloys, and Si compounds. The Si-based material may also be a composite particle containing, for example, an ion-conducting phase and a silicon phase (silicon particles in one respect) dispersed within the ion-conducting phase. The ion-conducting phase is a phase that conducts ions and includes, for example, at least one selected from the group consisting of silicate phase, aluminate phase, carbon phase, and silicon oxide phase. One of these negative electrode active materials may be used, or two or more may be used in combination.

[0061] The negative electrode mixture layer 12 may contain at least one selected from the group consisting of graphite and Si-based materials as the negative electrode active material. Graphite is recommended because it does not degrade easily even when repeatedly charged and discharged at great depths. Carbon materials other than graphite may also be used as the negative electrode active material. Si-based materials exhibit a larger capacity than graphite, which is advantageous for increasing battery capacity. In order to achieve high capacity while maintaining good cycle characteristics, graphite and Si-based materials may be used in combination as the negative electrode active material.

[0062] When the Si-based material is the above-mentioned composite particle and the ion-conducting phase is the carbon phase, the carbon phase may be composed of, for example, amorphous carbon. Examples of amorphous carbon constituting the carbon phase include hard carbon, soft carbon, and other amorphous carbons. Amorphous carbon is a carbon material in which the average interplanar spacing d002 of the (002) planes, as measured by X-ray diffraction, exceeds 0.34 nm.

[0063] When the Si-based material is the above-mentioned composite particle and the ion-conducting phase is a silicon oxide phase, the main component of the silicon oxide phase may be silicon dioxide. Here, the main component of the silicon oxide phase is, for example, a component that accounts for 95% or more and 100% or less by mass of the silicon oxide phase. The composition of the composite particle containing the silicon oxide phase and the silicon phase dispersed therein is, as a whole, SiO x It can be expressed as SiO x For example, it has a structure in which silicon fine particles are dispersed in amorphous SiO2. The oxygen content ratio x to silicon is preferably 0.5 ≤ x < 2.0, and more preferably 0.8 ≤ x ≤ 1.5.

[0064] If the Si-based material is the above-mentioned composite particle and the ion-conducting phase is a silicate phase, the silicate phase may satisfy the following conditions (A) and / or (B): (A) The silicate phase contains at least one element selected from the group consisting of alkali metal elements and Group 2 elements (Group 2 elements of the long-period periodic table). (B) The silicate phase contains element L. Element L is at least one element selected from the group consisting of B, Al, Zr, Nb, Ta, V, lanthanides, Y, Ti, P, Bi, Zn, Sn, Pb, Sb, Co, Er, F, and W. Lanthanides are a collective term for 15 elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71.

[0065] Regarding the above condition (A), examples of alkali metal elements include lithium (Li), potassium (K), and sodium (Na). Examples of group 2 elements include magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). The inclusion of alkali metal elements and / or group 2 elements may reduce the irreversible capacity of the silicate phase. A lithium-containing silicate phase (hereinafter sometimes referred to as the "lithium silicate phase") is preferred, for example, for its low irreversible capacity and high initial charge-discharge efficiency.

[0066] The lithium silicate phase may be any oxide phase containing Li, Si, and O, and may also contain other elements. The atomic ratio of O to Si in the lithium silicate phase, O / Si, is, for example, greater than 2 and less than 4. Preferably, O / Si is greater than 2 and less than 3. The atomic ratio of Li to Si in the lithium silicate phase, Li / Si, is, for example, greater than 0 and less than 4.

[0067] The lithium silicate phase is given by formula: Li 2z SiO (2+z) The material may contain a lithium silicate phase represented by (0 < z < 2), or may be composed of such lithium silicate phase. Preferably, z satisfies the relationship 0 < z < 1, and more preferably z = 1 / 2 (i.e., Li2Si2O5).

[0068] Furthermore, the Si-based material may also include composite particles comprising an ionic conductive phase and a silicon phase dispersed within the ionic conductive phase, and a coating layer covering at least a portion of the surface of the composite particles.

[0069] The coating layer present on the surface of the composite particles may include, for example, a conductive layer. By forming a conductive layer on the surface of the composite particles, the conductivity of the Si-based material can sometimes be enhanced. As the conductive material constituting the conductive layer, a conductive material containing carbon is preferred. Examples of carbon-containing conductive materials include conductive carbon materials. Examples of conductive carbon materials include carbon black, graphite, and amorphous carbon with low crystallinity. Amorphous carbon is preferred because it provides a large buffering effect against the silicon phase, which undergoes volume changes during charging and discharging. Amorphous carbon may be either easily graphitized carbon (soft carbon) or difficult-to-graphitize carbon (hard carbon). Examples of carbon black include acetylene black and Ketjen black. The thickness of the conductive layer may be, for example, in the range of 1 nm or more and 200 nm or less. The thickness of the conductive layer can be measured by cross-sectional observation of the Si-containing material using a scanning electron microscope (SEM) or transmission electron microscope (TEM).

[0070] Regarding the Si-based material content, in terms of increasing the capacity of the secondary battery, it is preferably 3% by mass or more, more preferably 5% by mass or more, and even more preferably 8% by mass or more, relative to the total amount of negative electrode active material. Furthermore, in terms of increasing the capacity of the secondary battery and suppressing the swelling of the negative electrode, the Si-based material content is preferably 3% by mass or more and 30% by mass or less, more preferably 5% by mass or more and 25% by mass or less, and even more preferably 8% by mass or more and 20% by mass or less, relative to the total amount of negative electrode active material.

[0071] The negative electrode mixture layer 12 may further contain a binder. Examples of binders include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resin, polyolefin, and styrene-butadiene rubber (SBR). In addition, these resins may be used in combination with carboxymethylcellulose (CMC) or its salts, polyacrylic acid (PAA) or its salts, polyvinyl alcohol (PVA), etc.

[0072] The negative electrode mixture layer 12 may further contain a conductive additive. The conductive additive is used to reduce the resistance of the negative electrode. Examples of conductive additives include carbon materials and conductive polymer compounds. Examples of carbon materials include carbon black, graphite, acetylene black, carbon nanotubes, carbon nanofibers, graphene, fullerene, and graphite oxide. Examples of conductive polymer compounds include polyaniline, polypyrrole, and polythiophene.

[0073] (Embodiment 2) The battery in Embodiment 2 comprises a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the same as the negative electrode in Embodiment 1. With this configuration, the battery in Embodiment 2 can improve its cycle characteristics.

[0074] Figure 3 is a schematic longitudinal cross-sectional view showing an example of a battery according to Embodiment 2. The battery 100 is a cylindrical battery comprising a cylindrical battery case, a wound electrode group 24, and an electrolyte (not shown). The electrode group 24 is housed within the battery case and is in contact with the electrolyte.

[0075] The battery case consists of a case body 25, which is a bottomed cylindrical metal container, and a sealing body 26 that seals the opening of the case body 25. A gasket 37 is placed between the case body 25 and the sealing body 26. The gasket 37 ensures that the battery case is airtight. Inside the case body 25, insulating plates 27 and 28 are placed at both ends of the electrode group 24 in the winding axis direction.

[0076] The case body 25 has, for example, a stepped portion 31. The stepped portion 31 can be formed by partially pressing the side wall of the case body 25 from the outside. The stepped portion 31 may be formed in an annular shape on the side wall of the case body 25 along the circumferential direction of a virtual circle defined by the case body 25. In this case, the sealing body 26 is supported, for example, by the opening side surface of the stepped portion 31.

[0077] The sealing body 26 comprises a filter 32, a lower valve body 33, an insulating member 34, an upper valve body 35, and a cap 36. In the sealing body 26, these members are stacked in this order. The sealing body 26 is installed in the opening of the case body 25 such that the cap 36 is located on the outside of the case body 25 and the filter 32 is located on the inside of the case body 25.

[0078] Each of the above-mentioned components constituting the sealing body 26 is, for example, disc-shaped or ring-shaped. Except for the insulating member 34, each of the above-mentioned components is electrically connected to one another.

[0079] The electrode group 24 includes a positive electrode 21, a separator 22, and a negative electrode 23. The positive electrode 21, the separator 22, and the negative electrode 23 are all strip-shaped. The width direction of the strip-shaped positive electrode 21 and negative electrode 23 is, for example, parallel to the winding axis of the electrode group 24. The separator 22 is positioned between the positive electrode 21 and the negative electrode 23. The positive electrode 21 and the negative electrode 23 are wound in a spiral shape with the separator 22 interposed between them.

[0080] When observing a cross-section of the battery 100 in a direction perpendicular to the winding axis of the electrode group 24, the positive electrode 21 and the negative electrode 23 are alternately stacked in the radial direction of a virtual circle defined by the case body 25, with a separator 22 interposed between them.

[0081] The positive electrode 21 is electrically connected to the cap 36, which also serves as the positive electrode terminal, via a positive electrode lead 29. One end of the positive electrode lead 29 is connected, for example, near the center of the positive electrode 21 in the longitudinal direction. The positive electrode lead 29 extends from the positive electrode 21 to the filter 32 through a through hole formed in the insulating plate 27. The other end of the positive electrode lead 29 is welded, for example, to the electrode group 24 side of the filter 32.

[0082] The negative electrode 23 is electrically connected to the case body 25, which also serves as the negative electrode terminal, via a negative electrode lead 30. One end of the negative electrode lead 30 is connected, for example, to the end of the negative electrode 23 in the longitudinal direction of the negative electrode 23. The other end of the negative electrode lead 30 is welded, for example, to the inner bottom surface of the case body 25.

[0083] The components of battery 100 will be described in detail below.

[0084] The positive electrode 21 includes a material having the property of intercalating and releasing metal ions (e.g., lithium ions). The positive electrode 21 includes, for example, a positive electrode active material. The positive electrode 21 comprises, for example, a positive electrode current collector and a positive electrode mixture layer supported on the surface of the positive electrode current collector.

[0085] As the positive electrode current collector, a sheet or film made of a metallic material such as aluminum, stainless steel, titanium, or their alloys can be used. Aluminum and its alloys are suitable as materials for positive electrode current collectors because they are inexpensive and easy to make into thin films. The sheet or film may be porous or non-porous. Metal foil, metal mesh, etc., can be used as the sheet or film. A carbon material such as carbon may be coated on the surface of the positive electrode current collector as a conductive auxiliary material.

[0086] The positive electrode mixture layer contains a positive electrode active material. The positive electrode active material may be a material that has the ability to intercept and release metal ions (e.g., lithium ions). As the positive electrode active material, lithium-containing transition metal oxides, lithium-containing transition metal phosphates, transition metal fluorides, polyanionic materials, fluorinated polyanionic materials, transition metal sulfides, transition metal oxysulfides, transition metal oxynitrides, etc., can be used. In particular, when lithium-containing transition metal oxides or lithium-containing transition metal phosphates are used as the positive electrode active material, the manufacturing cost of the battery can be reduced and the average discharge voltage can be increased. Examples of lithium-containing transition metal oxides include lithium cobalt oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese oxide, and lithium nickel manganese oxide. Examples of lithium-containing transition metal phosphates include lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, and lithium nickel phosphate.

[0087] The positive electrode mixture layer may further contain a binder. As the binder, the material described in Embodiment 1 as a binder usable in the negative electrode mixture layer can also be used in the positive electrode mixture layer.

[0088] The positive electrode mixture layer may further contain a conductive agent. As the conductive agent, the material described in Embodiment 1 as a conductive agent usable in the negative electrode mixture layer can also be used in the positive electrode mixture layer.

[0089] The negative electrode 23 includes a material having the property of intercalating and releasing metal ions (e.g., lithium ions). The negative electrode 23 is the negative electrode 10 according to Embodiment 1.

[0090] The electrolyte solution used as the electrolyte may contain a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. The concentration of the lithium salt in the electrolyte solution may be, for example, 0.5 mol / liter or more and 2 mol / liter or less. By controlling the lithium salt concentration within the above range, an electrolyte solution with excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0091] As non-aqueous solvents, cyclic carbonate esters, linear carbonate esters, cyclic ethers, linear ethers, nitriles, amides, etc., may be used. One of these solvents may be used, or two or more may be used in combination.

[0092] Examples of lithium salts that can be used include lithium hexafluoride phosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bisperfluoroethylsulfonylimide (LiN(SO2C2F5)2), LiAsF6, LiCF3SO3, and lithium difluoro(oxalato)borate. One of these electrolyte salts may be used, or two or more may be used in combination.

[0093] Typically, it is desirable to interpose a separator between the positive and negative electrodes. The separator 22 has high ion permeability and appropriate mechanical strength and insulating properties. As the separator 22, a microporous thin film, woven fabric, and nonwoven fabric can be used. As the material of the separator 22, for example, a polymer can be used. The polymer may be polyolefin such as polypropylene and polyethylene.

[0094] In the battery according to Embodiment 2, the electrolyte may be impregnated into a polymer provided as a separator, for example. That is, the battery according to Embodiment 2 may have a structure in which both the electrolyte and the polymer are used in combination.

[0095] The battery according to Embodiment 2 may further contain a solid electrolyte as the electrolyte. That is, the battery of this disclosure may have a hybrid structure in which an electrolyte and a solid electrolyte are used in combination. Examples of solid electrolyte materials are halide solid electrolytes, sulfide solid electrolytes, oxide solid electrolytes, or organic polymer solid electrolytes. In this disclosure, "halide solid electrolyte" means a solid electrolyte in which a halogen element is the main component of the anions. "Sulfide solid electrolyte" means a solid electrolyte in which sulfur is the main component of the anions. "Oxide solid electrolyte" means a solid electrolyte in which oxygen is the main component of the anions. The main component of the anions means the anion with the largest amount of substance among all the anions that make up the solid electrolyte.

[0096] The battery according to Embodiment 2 is not limited to the cylindrical non-aqueous electrolyte secondary battery shown in Figure 3, and may have other configurations as long as it includes a wound electrode group in which a long negative electrode and a long positive electrode according to Embodiment 1 are wound around each other with a separator in between.

[0097] (Other Embodiments) (Note) The above description of embodiments discloses the following technologies.

[0098] (Technical 1) A negative electrode comprising: an elongated negative electrode current collector; and an elongated negative electrode mixture layer disposed on the negative electrode current collector, wherein the negative electrode mixture layer is divided in the width direction into a central region including the center in the width direction and end regions including both ends in the width direction, and the negative electrode mixture layer is divided in the thickness direction into two equal parts in the thickness direction, thereby dividing the negative electrode mixture layer into a surface-side region located on the surface side and a current collector-side region located on the negative electrode current collector side, and each of the resulting regions is, The first region is defined as the end region and the surface-side region of the negative electrode mixture layer. The second region is defined as the end region and the current collector-side region of the negative electrode mixture layer. The third region is defined as the central region and the surface-side region of the negative electrode mixture layer. The fourth region is defined as the central region and the current collector-side region of the negative electrode mixture layer. If the porosity of the first region is A, the porosity of the second region is B, and the porosity of the third region is C, then at least a part of the negative electrode mixture layer satisfies the following relationship (1): C > A > B ... (1)

[0099] This configuration allows the negative electrode according to Technology 1 to improve the battery's cycle characteristics.

[0100] (Technology 2) When the porosity of the fourth region is D, the negative electrode mixture layer further satisfies the following relation (2) in the portion that satisfies relation (1), as described in Technology 1: C ≥ D > A ... (2)

[0101] This configuration allows the negative electrode related to Technology 2 to further improve the battery's cycle characteristics.

[0102] (Technical 3) The negative electrode mixture layer, in the portion that satisfies relation (1), further satisfies the following relation (3), as described in Technical 1 or 2: 1.0 < C / A < 2.0 ... (3)

[0103] This configuration allows the negative electrode according to technology 3 to suppress the flow of the electrolyte from the third region located in the center of the surface side of the negative electrode mixture layer into the first region located at the surface edge, thereby further improving the battery's cycle characteristics.

[0104] (Technical 4) The negative electrode mixture layer, in the portion that satisfies relation (1), further satisfies the following relation (4), as described in Technical 3: 1.2 < C / A < 1.7 ... (4)

[0105] This configuration allows the negative electrode according to technology 4 to suppress the flow of the electrolyte from the third region located in the center of the surface side of the negative electrode mixture layer into the first region located at the surface edge, thereby further improving the battery's cycle characteristics.

[0106] (Technical 5) The curvature ratio in the thickness direction at the end region of the negative electrode mixture layer is T AB The curvature ratio in the thickness direction in the central region of the negative electrode mixture layer is set to T. CD In this case, at least a portion of the negative electrode mixture layer satisfies the following relational expression (5), and is a negative electrode according to any one of the technical items 1 to 4. AB >T CD ... (5)

[0107] This configuration allows the negative electrode related to technology 5 to further improve the battery's cycle characteristics.

[0108] (Technical 6) The negative electrode mixture layer, in the portion that satisfies relation (5), further satisfies the following relation (6), as described in Technical 5. 1.0 < T AB / T CD <1.5 ... (6)

[0109] This configuration allows the negative electrode related to technology 6 to further improve the battery's cycle characteristics.

[0110] (Technical 7) The negative electrode according to any one of Technical 1 to 6, wherein the length of the central region in the width direction is the ratio of the total width of the negative electrode mixture layer to 10% or more and 50% or less.

[0111] This configuration allows the negative electrode related to technology 7 to further improve the battery's cycle characteristics.

[0112] (Technical 8) The negative electrode according to any one of Technical 1 to 7, wherein the thickness of the negative electrode mixture layer in the central region is greater than the thickness of the negative electrode mixture layer in the end region.

[0113] This configuration allows the negative electrode related to technology 8 to further improve the battery's cycle characteristics.

[0114] (Technical 9) The negative electrode according to any one of Technical 1 to 8, wherein the negative electrode mixture layer comprises artificial graphite and natural graphite, and the mass ratio of the artificial graphite to the sum of the artificial graphite and natural graphite in the third region of the negative electrode mixture layer is greater than the mass ratio of the artificial graphite to the sum of the artificial graphite and natural graphite in the second region of the negative electrode mixture layer.

[0115] With this configuration, the negative electrode according to technology 9 can easily control the void ratio and curvature ratio between the edge region and the central region of the negative electrode mixture layer.

[0116] (Technical 10) A battery comprising a negative electrode, a positive electrode, and an electrolyte, as described in any one of Technical 1 to 9.

[0117] This configuration allows the battery according to technology 10 to have improved cycle characteristics.

[0118] The present disclosure will be described in more detail below with reference to examples. The following examples are merely illustrative and not limited to any one aspect.

[0119] [Example 1] (Preparation of negative electrode mixture slurry) A first active material and a second active material were prepared as negative electrode active materials. The first active material was graphite, and the graphite had a BET specific surface area of ​​1.5 m². 2 It was synthetic graphite at a concentration of / g. The second active material was graphite, and this graphite had a BET specific surface area of ​​4.2 m². 2 The material was natural graphite at a concentration of / g. Carboxymethylcellulose, polyacrylic acid, and styrene-butadiene copolymer rubber were prepared as binders, and carbon nanotubes were prepared as conductive additives.

[0120] In Example 1, four negative electrode mixture slurries were prepared: a first slurry for preparing the first region, a second slurry for preparing the second region, a third slurry for preparing the third region, and a fourth slurry for preparing the fourth region.

[0121] In the first slurry, a mixed active material was used, in which the first active material and the second active material were mixed in a mass ratio of first active material:second active material = 80:20. The first slurry was prepared by mixing the mixed active material, a binder, and a conductive additive in a mass ratio of mixed active material:binder:conductive additive = 100:3:1 to a negative electrode mixture, to which water was added as a dispersion medium, and then stirred using a mixer.

[0122] In the second slurry, only the second active material was used as the negative electrode active material. The second slurry was prepared by mixing the second active material, binder, and conductive additive in a mass ratio of second active material:binder:conductive additive = 100:3:1, adding water as a dispersion medium, and then stirring with a mixer.

[0123] The third slurry was prepared in the same manner as the first slurry.

[0124] The fourth slurry was prepared in the same manner as the first slurry.

[0125] (Fabrication of the negative electrode) A long copper foil with a width of 65.0 mm was used as the negative electrode current collector. On both sides of this long negative electrode current collector, the fourth slurry was applied to the central region including the center in the width direction, so that its width was 20% of the total width of the copper foil. Furthermore, the second slurry was applied to the areas on both sides of the negative electrode current collector where the fourth slurry was not applied. These coatings of the fourth slurry and the second slurry were dried to form a film with a thickness of 70 μm on both sides of the copper foil. The third slurry was applied to the surface of the film formed using the fourth slurry, and the first slurry was applied to the surface of the film made using the second slurry. These coatings of the third slurry and the first slurry were dried to form a film with a thickness of 70 μm, and then rolled to form the negative electrode mixture layer. An exposed portion was provided on a part of the negative electrode where the surface of the negative electrode current collector was exposed.

[0126] (Preparation of positive electrode active material) [Ni obtained by coprecipitation method 0.90 Al 0.05 Mn 0.05 The composite hydroxide represented by ](OH)2 is calcined at 500°C for 8 hours to obtain the composite oxide (Ni 0.90 Al 0.05 Mn 0.05 O2) was obtained. Next, LiOH and the composite oxide were mixed so that the molar ratio of Li to the total amount of Ni, Al, and Mn was 1.03:1 to obtain a mixture. This mixture was subjected to an oxygen stream (10 cm) with an oxygen concentration of 95%. 3 A lithium-containing composite oxide was obtained by calcining from room temperature to 650°C at a heating rate of 2.0°C / min (at a flow rate of 2 mL / min per 1 kg of mixture) and then from 650°C to 780°C at a heating rate of 0.5°C / min.

[0127] (Preparation of the positive electrode) The above positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed in a mass ratio of 98:1:1, and N-methyl-2-pyrrolidone (NMP) was used as a dispersion medium to prepare a positive electrode mixture slurry. Next, the positive electrode mixture slurry was applied to a positive electrode current collector made of aluminum foil, and after drying and compressing the coating film, the positive electrode current collector was cut to a predetermined electrode size to obtain a positive electrode in which positive electrode mixture layers were arranged on both sides of the positive electrode core. In addition, an exposed portion was provided on a part of the positive electrode in which the surface of the positive electrode current collector was exposed.

[0128] (Preparation of non-aqueous electrolyte) A non-aqueous electrolyte was prepared by dissolving LiPF6 at a concentration of 1.2 mol / liter in a mixed solvent prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 3:3:4 (at 25°C).

[0129] (Preparation of test cell (secondary battery)) An aluminum lead was attached to the exposed part of the positive electrode and a nickel lead was attached to the exposed part of the negative electrode. The positive and negative electrodes were wound in a spiral shape via a polyolefin separator to create a wound electrode body. Insulating plates were placed above and below the electrode body, and the electrode body was housed in an outer casing. The negative electrode lead was welded to the bottom of the bottomed cylindrical outer casing, and the positive electrode lead was welded to the sealing body. Electrolyte was injected into the outer casing, and the opening of the outer casing was sealed with the sealing body via a gasket to create a secondary battery as a test cell.

[0130] [Example 2] (Preparation of negative electrode mixture slurry) The first slurry, second slurry, third slurry, and fourth slurry were prepared in the same manner as in Example 1.

[0131] (Preparation of the negative electrode) The negative electrode mixture layer was formed in the same manner as in Example 1, except that the width to which the fourth slurry was applied was set to 40% of the total width of the copper foil.

[0132] (Preparation of positive electrode active material and positive electrode) The positive electrode active material and positive electrode were prepared in the same manner as in Example 1.

[0133] (Preparation of non-aqueous electrolyte) The non-aqueous electrolyte was prepared in the same manner as in Example 1.

[0134] (Preparation of test cell (secondary battery)) A secondary battery to be used as a test cell was prepared in the same manner as in Example 1, except that the negative electrode prepared in Example 2 was used.

[0135] [Comparative Example 1] (Preparation of negative electrode mixture slurry) The first slurry and the second slurry were prepared in the same manner as in Example 1. In Comparative Example 1, only these two types of slurries were prepared.

[0136] (Fabrication of the negative electrode) The same elongated copper foil as in Example 1 was used as the negative electrode current collector. The second slurry was applied to both sides of this elongated negative electrode current collector, and the coating was dried to form a film with a thickness of 70 μm on each side. The first slurry was applied to the surface of each of the fabricated films, and the coating was dried to form a film with a thickness of 70 μm, which was then rolled to form the negative electrode mixture layer. That is, the negative electrode mixture layer of Comparative Example 1 was a two-layer negative electrode mixture layer consisting of a surface layer and a current collector layer, and was not divided into a central region and an end region in the width direction. As in Example 1, an exposed portion was provided on a part of the negative electrode where the surface of the negative electrode current collector was exposed.

[0137] (Preparation of positive electrode active material and positive electrode) The positive electrode active material and positive electrode were prepared in the same manner as in Example 1.

[0138] (Preparation of non-aqueous electrolyte) The non-aqueous electrolyte was prepared in the same manner as in Example 1.

[0139] (Preparation of test cell (secondary battery)) A secondary battery to be used as a test cell was prepared in the same manner as in Example 1, except that the negative electrode prepared in Comparative Example 1 was used.

[0140] [Comparative Example 2] (Preparation of negative electrode mixture slurry) The first slurry and the second slurry were prepared in the same manner as in Example 1. In Comparative Example 2, only these two types of slurries were prepared.

[0141] (Fabrication of the negative electrode) The same elongated copper foil as in Example 1 was used as the negative electrode current collector. The second slurry was applied to both sides of this elongated negative electrode current collector, and the coating was dried to form a film with a thickness of 50 μm on each side. The first slurry was applied to the surface of each of the fabricated films, and the coating was dried to form a film with a thickness of 90 μm, which was then rolled to form the negative electrode mixture layer. In other words, the negative electrode mixture layer of Comparative Example 3 was a two-layer negative electrode mixture layer consisting of a surface layer and a current collector layer, and was not divided into a central region and an end region in the width direction.

[0142] (Preparation of positive electrode active material and positive electrode) The positive electrode active material and positive electrode were prepared in the same manner as in Example 1.

[0143] (Preparation of non-aqueous electrolyte) The non-aqueous electrolyte was prepared in the same manner as in Example 1.

[0144] (Preparation of test cell (secondary battery)) A secondary battery to be used as a test cell was prepared in the same manner as in Example 1, except that the negative electrode prepared in Comparative Example 2 was used.

[0145] [Comparative Example 3] (Preparation of negative electrode mixture slurry) The first slurry was prepared in the same manner as in Example 1.

[0146] In the second slurry, a mixed active material was used, in which the first active material and the second active material were mixed in a mass ratio of first active material:second active material = 40:60. The second slurry was prepared by mixing the mixed active material, a binder, and a conductive additive in a mass ratio of mixed active material:binder:conductive additive = 100:3:1 to a negative electrode mixture, adding water as a dispersion medium, and then stirring with a mixer.

[0147] In Comparative Example 3, only the two types of slurries described above were prepared.

[0148] (Fabrication of the negative electrode) The same elongated copper foil as in Example 1 was used as the negative electrode current collector. The second slurry was applied to both sides of this elongated negative electrode current collector, and the coating was dried to form a film with a thickness of 70 μm on each side. The first slurry was applied to the surface of each of the fabricated films, and the coating was dried to form a film with a thickness of 70 μm, which was then rolled to form the negative electrode mixture layer. In other words, the negative electrode mixture layer of Comparative Example 3 was a two-layer negative electrode mixture layer consisting of a surface layer and a current collector layer, and was not divided into a central region and an end region in the width direction.

[0149] (Preparation of positive electrode active material and positive electrode) The positive electrode active material and positive electrode were prepared in the same manner as in Example 1.

[0150] (Preparation of non-aqueous electrolyte) The non-aqueous electrolyte was prepared in the same manner as in Example 1.

[0151] (Preparation of test cell (secondary battery)) A secondary battery to be used as a test cell was prepared in the same manner as in Example 1, except that the negative electrode prepared in Comparative Example 3 was used.

[0152] [Measurement of porosity in each region of the negative electrode mixture layer] For the negative electrodes of each example and comparative example, the porosity of each region in the negative electrode mixture layer was determined using the method described in Embodiment 1. In the negative electrode mixture layers of Comparative Examples 1 to 3, since the regions were not divided in the width direction, the porosity was determined for the current collector side layer made using the second slurry and the surface side layer made using the first slurry. However, instead of disassembling the battery and cutting out the negative electrode, the porosity of the negative electrode mixture layer was determined using the prepared negative electrode. The results are shown in Table 2.

[0153] Here, for example, in Example 1, even though the same slurry is used for the first slurry, the third slurry, and the fourth slurry, the reason why the void ratio A of the first region, the void ratio C of the third region, and the void ratio D of the fourth region are different from each other is thought to be as follows. For example, the difference in void ratio between the first region and the third region is caused by rolling during the production of the negative electrode. When the end region, which includes the first and second regions, and the central region, which includes the third and fourth regions, are compressed simultaneously, the end region has a lower proportion of the first active material (i.e., artificial graphite) than the central region, resulting in higher compressibility and a smaller void ratio. Conversely, the central region has a higher proportion of the first active material (i.e., artificial graphite) than the end region, resulting in lower compressibility and a larger void ratio. It is thought that these differences in compressibility cause the void ratios of each region to differ even when the same slurry is used. Furthermore, the difference in void ratio between the third region and the fourth region is thought to be because, during rolling in the production of the negative electrode, the fourth region, which is located on the lower layer side, was compressed more than the third region, which is located on the upper layer side. The same applies to Example 2.

[0154] [Measurement of curvature ratio in each region of the negative electrode mixture layer] For the negative electrode of each embodiment, the curvature ratio T in the thickness direction in the edge region of the negative electrode mixture layer was measured using the method described in Embodiment 1. AB And the curvature ratio T in the thickness direction in the central region of the negative electrode mixture layer. CD The following was determined. In the negative electrode mixture layers of Comparative Examples 1 to 3, the region was not divided in the width direction, so the curvature in the thickness direction was determined for the entire negative electrode mixture layer. The results are shown in Table 2.

[0155] [Evaluation of Charge / Discharge Cycle Characteristics] The test cells of each example and comparative example were charged at a constant current of 0.3C at a temperature of 25°C until the battery voltage reached 4.2V, and then charged at a constant voltage of 4.2V until the current value was 0.02C. After that, the batteries were discharged at a constant current of 0.5C until the battery voltage reached 2.5V, and the discharge capacity at this time was defined as the initial discharge capacity. This charge / discharge cycle was considered one cycle, and 100 cycles were performed. The initial discharge capacity and the discharge capacity at the 100th cycle were determined, and the capacity retention rate was calculated using the following formula: Capacity retention rate (%) = Discharge capacity at the 100th cycle / Initial discharge capacity × 100

[0156] The obtained capacity retention rate was defined as the cycle retention rate. The results are shown in Table 2. Table 2 also shows the relative values ​​when the cycle retention rate of Comparative Example 1 is set as the baseline (100%).

[0157]

[0158]

[0159] (Discussion) As shown in Table 2, the cycle characteristics of all the test cells in the examples were improved compared to the test cells of Comparative Examples 1 to 3. In other words, when the negative electrode mixture layer is divided into the first, second, third, and fourth regions described above, a negative electrode equipped with a negative electrode mixture layer in which the porosity of each region satisfies the above relational equation (1) was able to improve the cycle characteristics of the battery.

[0160] The technology disclosed herein is useful for batteries such as lithium-ion secondary batteries that require excellent cycle characteristics.

Claims

1. A negative electrode comprising: an elongated negative electrode current collector; and an elongated negative electrode mixture layer disposed on the negative electrode current collector, wherein the negative electrode mixture layer is divided in the width direction into a central region including the center in the width direction and end regions including both ends in the width direction, and the negative electrode mixture layer is divided in the thickness direction into two equal parts, thereby dividing the negative electrode mixture layer into a surface-side region located on the surface side and a current collector-side region located on the negative electrode current collector side, and each of the resulting regions is, The first region is defined as the end region and the surface-side region of the negative electrode mixture layer. The second region is defined as the end region and the current collector-side region of the negative electrode mixture layer. The third region is defined as the central region and the surface-side region of the negative electrode mixture layer. The fourth region is defined as the central region and the current collector-side region of the negative electrode mixture layer. If the porosity of the first region is A, the porosity of the second region is B, and the porosity of the third region is C, then at least a part of the negative electrode mixture layer satisfies the following relationship (1): C > A > B ... (1) 2. When the porosity of the fourth region is D, the negative electrode mixture layer further satisfies the following relation (2) in the portion that satisfies relation (1), as described in claim 1: C ≥ D > A ... (2) 3. The negative electrode according to claim 1, wherein the negative electrode mixture layer further satisfies the following relation (3) in the portion that satisfies relation (1): 1.0 < C / A < 2.0 ... (3) 4. The negative electrode according to claim 3, wherein the negative electrode mixture layer further satisfies the following relation (4) in the portion that satisfies relation (1): 1.2 < C / A < 1.7 ... (4) 5. The curvature ratio in the thickness direction at the end region of the negative electrode mixture layer is T AB The curvature ratio in the thickness direction in the central region of the negative electrode mixture layer is set to T. CD In this case, at least a portion of the negative electrode mixture layer satisfies the following relational expression (5), the negative electrode according to claim 1. AB >T CD ... (5) 6. The negative electrode according to claim 5, wherein the negative electrode mixture layer further satisfies the following relation (6) in the portion that satisfies relation (5). 1.0 < T AB / T CD <1.5 ... (6) 7. The negative electrode according to claim 1, wherein the length of the central region in the width direction is 10% or more and 50% or less of the total width of the negative electrode mixture layer.

8. The anode according to claim 1, wherein the thickness of the anode mixture layer in the central region is greater than the thickness of the anode mixture layer in the end region.

9. The negative electrode according to claim 1, wherein the negative electrode mixture layer comprises artificial graphite and natural graphite, and the mass ratio of the artificial graphite to the total of the artificial graphite and natural graphite in the third region of the negative electrode mixture layer is greater than the mass ratio of the artificial graphite to the total of the artificial graphite and natural graphite in the second region of the negative electrode mixture layer.

10. A battery comprising a negative electrode, a positive electrode, and an electrolyte, as described in any one of claims 1 to 9.

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