Lithium secondary battery

Abstract

This lithium secondary battery is provided with: a positive electrode; a negative electrode that opposes the positive electrode; a separator disposed between the positive electrode and the negative electrode; a non-aqueous electrolyte having lithium ion conductivity; and, a spacer disposed between the negative electrode and the separator. Lithium metal is deposited on the negative electrode during charging, and lithium metal dissolves from the negative electrode during discharging. The spacer has a plurality of spot-shaped protrusions.

Description

Lithium secondary battery

[0001] The present disclosure relates to a lithium secondary battery having a lithium ion conducting electrolyte.

[0002] Non-aqueous electrolyte secondary batteries are used in applications such as ICT (Information Communication Technology) applications such as personal computers and smartphones, in-vehicle applications, and power storage. In these applications, non-aqueous electrolyte secondary batteries are required to have even higher capacities. Lithium-ion batteries are known as high-capacity non-aqueous electrolyte secondary batteries. Higher capacities in lithium-ion batteries can be achieved by using, for example, graphite and an alloy active material such as a silicon compound as the negative electrode active material. However, the capacity of lithium-ion batteries is reaching its limit.

[0003] Lithium secondary batteries (lithium metal secondary batteries) are promising non-aqueous electrolyte secondary batteries with a higher capacity than lithium ion batteries. In lithium secondary batteries, lithium metal precipitates on the negative electrode during charging and dissolves in the non-aqueous electrolyte during discharge.

[0004] Patent Document 1 proposes providing a spacer between the positive electrode and the separator to provide a space for accommodating lithium metal between the positive electrode and the negative electrode in order to prevent a decrease in charge / discharge efficiency due to lithium metal deposited in an isolated state and to prevent damage to the negative electrode current collector. By providing the spacer between the positive electrode and the separator, it is easier to maintain a space between the positive electrode and the separator during discharge, while it is difficult to maintain a space between the negative electrode and the separator. As a result, the lithium metal is deposited along the surface of the negative electrode due to the pressing force from the separator, making it difficult for it to grow into a dendrite shape.

[0005] International Publication No. 2010 / 066254

[0006] By controlling the deposition morphology of lithium metal, the cycle characteristics of lithium secondary batteries can be improved.

[0007] One aspect of the present disclosure relates to a lithium secondary battery including a positive electrode, a negative electrode facing the positive electrode, a separator disposed between the positive electrode and the negative electrode, a non-aqueous electrolyte having lithium ion conductivity, and a spacer disposed between the negative electrode and the separator, wherein lithium metal precipitates on the negative electrode during charging and dissolves from the negative electrode during discharging, and the spacer has a plurality of spot-shaped protrusions.

[0008] According to the present disclosure, it is possible to suppress the deterioration of the cycle characteristics of a lithium secondary battery.

[0009] 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.

[0010] Fig. 2 is a longitudinal sectional view schematically showing an example of a lithium secondary battery according to an embodiment of the present disclosure. Fig. 3 is a sectional view schematically showing the configuration of the positive electrode of Fig. 1. Fig. 4 is a sectional view schematically showing the configuration of the negative electrode unit of Fig. 1. Fig. 5 is a top view schematically showing an example of a negative electrode unit included in the lithium secondary battery of Fig. 1. Fig. 6 is an enlarged top view of a portion of the negative electrode unit, showing another example of the negative electrode unit.

[0011] Hereinafter, embodiments of the present disclosure will be described using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values, materials, etc. may be exemplified, but other numerical values, materials, etc. may be applied as long as the effects of the present disclosure are obtained. In this specification, the expression "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or more and numerical value B or less." In the following description, when lower and upper limits of numerical values ​​related to specific physical properties or conditions 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 the materials may be selected and used alone, or two or more of the materials may be used in combination.

[0012] The present disclosure also encompasses combinations of two or more features arbitrarily selected from the appended claims, i.e., two or more features arbitrarily selected from the appended claims can be combined unless a technical contradiction arises.

[0013] A lithium secondary battery according to one embodiment of the present disclosure includes a positive electrode, a negative electrode facing the positive electrode, a separator disposed between the positive electrode and the negative electrode, a non-aqueous electrolyte having lithium ion conductivity, and a spacer disposed between the negative electrode and the separator. The spacer has a plurality of spot-shaped protrusions. The lithium secondary battery is a type of secondary battery in which lithium metal precipitates on the negative electrode during charging and dissolves from the negative electrode during discharging. Hereinafter, the positive electrode and negative electrode may be collectively referred to as electrodes.

[0014] The spacer ensures a space for lithium metal to deposit on the surface of the negative electrode and reduces the volume change of the negative electrode that accompanies the deposition of lithium metal. When the spacer is placed between the negative electrode and the separator, lithium ions are released from the positive electrode more smoothly during the initial charge, which is advantageous in terms of improving the initial capacity, compared to when the spacer is placed between the positive electrode and the separator.

[0015] However, when a spacer is placed between the negative electrode and the separator, the surface pressure applied to the negative electrode during charging tends to vary, which can lead to uneven deposition of lithium metal. As a result, in areas with low surface pressure, lithium metal may deposit in structures such as dendrites that do not contribute to charge and discharge, resulting in a decrease in cycle performance. On the other hand, in areas with high surface pressure, the pressure applied to the electrode may increase locally, leading to electrode fracture.

[0016] For example, when multiple linear protrusions (straight-line protrusions) are arranged parallel to the longitudinal direction of the negative electrode as spacers, lithium metal tends to deposit along the longitudinal direction of the negative electrode. As a result, lithium metal deposition may become uneven between the longitudinal and width directions of the negative electrode. In addition, when multiple linear protrusions are provided, the area of ​​the negative electrode covered by the linear protrusions tends to increase, which may result in a decrease in capacity or a decrease in cycle characteristics.

[0017] In contrast, in the lithium secondary battery according to the embodiment of the present disclosure, the spacers are arranged in a spot pattern, which prevents the occurrence of areas where the surface pressure is locally low, thereby suppressing a decrease in cycle characteristics. In addition, the area of ​​the negative electrode covered by the spacers can be minimized, allowing the capacity and cycle characteristics to be maintained at a high level.

[0018] Furthermore, compared with linear protrusions, spot-shaped protrusions do not hinder the circulation of the electrolyte, thereby maintaining high cycle characteristics. In particular, in lithium secondary batteries in which lithium metal is deposited on the negative electrode, it is important to provide a sufficient supply of electrolyte near the surface of the negative electrode. The spot-shaped protrusions provided between the negative electrode and the separator not only ensure a space for lithium metal deposition, but also a space for convection of the electrolyte on the surface of the negative electrode. In this regard, it is more effective to provide the spot-shaped protrusions between the negative electrode and the separator than between the positive electrode and the separator.

[0019] The spacer may be provided on the surface of the negative electrode or on the surface of the separator facing the negative electrode. Hereinafter, a negative electrode having a spacer provided on its surface is also referred to as a negative electrode unit. Also, a separator having a spacer provided on its surface is also referred to as a composite separator.

[0020] The spacer may include multiple convex portion groups in which multiple spot-shaped convex portions are arranged at equal intervals in the lateral direction of the strip-shaped negative electrode. These convex portion groups may be arranged at equal intervals in the longitudinal direction of the negative electrode. In this case, the spot-shaped convex portions of adjacent convex portion groups in the longitudinal direction may be arranged with a shift in their positions in the lateral direction. "The spot-shaped convex portions are arranged with a shift in their positions in the lateral direction" means that, when comparing the lateral positions of multiple spot-shaped convex portions belonging to one adjacent convex portion group with the lateral positions of spot-shaped convex portions belonging to the other adjacent convex portion group, the positions of the spot-shaped convex portions belonging to one convex portion group are at a certain distance from the positions of the corresponding spot-shaped convex portions belonging to the other adjacent convex portion group. Note that, when evaluating the distance between spot-shaped convex portions, the position of the spot-shaped convex portions refers to the center position of the spot of the spot-shaped convex portion. The center position refers to the position of the center of gravity calculated based on the contour shape of the spot.

[0021] Lithium metal is likely to deposit around the spot-shaped protrusions, and pressure (stress) applied to the electrode during charging is likely to concentrate there. Therefore, when a group of protrusions is provided, stress-concentrated areas around the spot-shaped protrusions are connected to each other in the short direction, making the electrode (current collector) more likely to break. To avoid breakage of the electrode (current collector), the distance between the spot-shaped protrusions can be increased so that the stress-concentrated areas do not connect to each other. However, this makes it difficult to ensure space for lithium metal to deposit, and may actually result in a deterioration in cycle performance.

[0022] However, by staggering the positions of the spot-shaped protrusions in the lateral direction in adjacent groups of protrusions in the longitudinal direction, the density of the spot-shaped protrusions per negative electrode area can be maintained at a certain level or higher, and the distance between the spot-shaped protrusions in the lateral direction can be increased while ensuring space for lithium metal deposition. As a result, the deterioration of cycle characteristics is suppressed, and the stress concentration portions are prevented from connecting with each other in the lateral direction, thereby suppressing breakage of the electrode.

[0023] The amount of deviation between spot-shaped protrusions in adjacent protrusion groups is preferably half the arrangement interval between the spot-shaped protrusions in the protrusion groups, which makes it possible to highly effectively suppress both deterioration of cycle characteristics and breakage of the electrode.

[0024] From the viewpoint of ensuring the minimum necessary space between the negative electrode and the separator, the height of the spacer (i.e., the height of the spot-like protrusions) may be 0.02 mm or more and 0.09 mm or less, or 0.015 mm or more and 0.1 mm or less, depending on the battery size. The height h of the spacer is the average height of the spot-like protrusions, and is determined by arbitrarily selecting 10 spot-like protrusions and averaging the measured values ​​of the maximum heights at the positions farthest from the negative electrode current collector.

[0025] The contour shape of the spot-like protrusions when viewed from the normal direction of the surface of the negative electrode may be circular or polygonal, and is not particularly limited. ２ More than 20 mm ２ The outer diameter of the contour shape is, for example, 0.5 mm or more and 10 mm or less. The inner diameter of the contour shape is, for example, 0.5 mm or more and 8.8 mm or less. The area of ​​the contour shape means the projected area when the spot-like protrusions are projected onto the surface of the negative electrode. The outer diameter of the contour shape means the diameter of a circumscribing circle that circumscribes the contour shape. The inner diameter of the contour shape means the diameter of an inscribing circle that inscribes the contour shape.

[0026] From the viewpoint of improving the liquid circulation of the nonaqueous electrolyte on the negative electrode surface, the height of some of the spot-shaped protrusions may be different from the height of the remaining spot-shaped protrusions. For example, the heights of adjacent spot-shaped protrusions may be different. The plurality of spot-shaped protrusions may include a spot-shaped protrusion with a height h1 and a spot-shaped protrusion with a height h2 smaller than the height h1. In this case, the ratio of the height h2 to the height h1 (h2 / h1) may be, for example, 0.8 or more and less than 1.0, or 0.8 or more and 0.95 or less.

[0027] From the viewpoint of suppressing deposition of lithium metal on the surfaces of the spot-shaped protrusions, the plurality of line-shaped protrusions may be made of a material having lower conductivity than the negative electrode, or may be made of a resin material.

[0028] For a plurality of spot-shaped protrusions, when straight lines are drawn that pass through the spot-shaped protrusions and are parallel to the short-side direction of the negative electrode, the distance between adjacent lines is preferably 1.5 mm or more and 4.0 mm or less. This means that when the spot-shaped protrusions have a group of protrusions that are arranged at equal intervals in the short-side direction of the negative electrode, the distance between straight lines that are parallel to the short-side direction and connect the plurality of spot-shaped protrusions in the group of protrusions is 1.5 mm or more and 4.0 mm or less. In this case, deterioration of cycle characteristics can be significantly suppressed.

[0029] The ratio of the area of ​​the negative electrode surface covered by the spacers to the area of ​​the negative electrode surface (hereinafter also referred to as the coverage rate of the negative electrode surface by the spacers) may be 5% or more and 20% or less, or 5% or more and 15% or less. When the coverage rate of the negative electrode surface by the spacers is 20% or less, the impact on battery performance caused by the negative electrode surface being covered by the spacers can be minimized. When the coverage rate of the negative electrode surface by the spacers is 5% or more, the spacers can sufficiently secure space for lithium metal deposition, and the effect of suppressing deterioration of cycle characteristics can be sufficiently obtained.

[0030] The spacers may be disposed on both sides of the negative electrode. That is, the negative electrode has a first surface and a second surface opposite to the first surface, and spacers having spot-shaped protrusions may be disposed between the first surface of the negative electrode and the separator disposed on the first surface side, and between the second surface of the negative electrode and the separator disposed on the second surface side. Hereinafter, the spacers (spot-shaped protrusions) disposed on the first surface side of the negative electrode will be referred to as first spacers (first spot-shaped protrusions), and the spacers (spot-shaped protrusions) disposed on the second surface side of the negative electrode will be referred to as second spacers (second spot-shaped protrusions).

[0031] The second spot-shaped protrusions may be arranged at positions on the second surface opposite to the first spot-shaped protrusions, or may be arranged at positions on the second surface opposite to an area on the first surface where the first spot-shaped protrusions are not arranged. In other words, when viewed from the normal direction of the first or second surface of the negative electrode, the second spot-shaped protrusions may be arranged at positions overlapping with the first spot-shaped protrusions, or may be arranged at positions not overlapping with the first spot-shaped protrusions.

[0032] When a wound electrode group is formed, the first spot-shaped protrusions (or the second spot-shaped protrusions) press the second surface (or the first surface) of the negative electrode located closer to the outer periphery or the inner periphery via the separator and the positive electrode, thereby increasing the surface pressure during charging, thereby suppressing the precipitation of lithium metal dendrites and the like, and suppressing a deterioration in cycle characteristics.

[0033] In addition, when the second spot-shaped protrusions are arranged at positions that do not overlap with the first spot-shaped protrusions, the second spot-shaped protrusions press the negative electrode located on the outer or inner periphery via the separator and the positive electrode, and press the spaces formed between the first spot-shaped protrusions from the second surface side toward the first surface side, thereby further increasing the surface pressure during charging and further improving the effect of suppressing the deterioration of cycle characteristics.

[0034] Furthermore, by arranging the first spot-shaped convex portion and the second spot-shaped convex portion so that they do not overlap, localized increase in the thickness of the negative electrode near the convex portion due to precipitation of lithium metal is suppressed, improving cycle characteristics.

[0035] Therefore, in terms of suppressing deterioration of cycle characteristics, it is preferable that the second spot-shaped protrusions be arranged on the second surface opposite to the region on the first surface where the first spot-shaped protrusions are not arranged (i.e., arranged at a position that does not overlap with the first spot-shaped protrusions). However, the second spot-shaped protrusions may be arranged at a position that overlaps with the first spot-shaped protrusions. In this case, because the second spot-shaped protrusions are arranged at a position that overlaps with the first spot-shaped protrusions, the second spot-shaped protrusions do not press against the spaces formed between the first spot-shaped protrusions. Therefore, the negative electrode current collector is less likely to be deformed in a way that protrudes toward the first surface at the position of the second spot-shaped protrusions, and breakage of the negative electrode current collector is more likely to be suppressed.

[0036] In the present disclosure, when the electrode group is a wound type, "when viewed from the normal direction of the surface" means when the surface is stretched out flat and viewed from the normal direction. Therefore, "the convex portions overlap" does not include the case where the convex portions overlap due to winding.

[0037] The lithium secondary battery may include a stacked electrode group configured by stacking a positive electrode and a negative electrode with a separator interposed therebetween, or may include a wound electrode group configured by spirally winding a positive electrode and a negative electrode with a separator interposed therebetween.

[0038] Each component of the lithium secondary battery will be described in more detail below. [Negative Electrode] The negative electrode includes a negative electrode current collector. In a lithium secondary battery, lithium metal is deposited on the surface of the negative electrode current collector upon charging. More specifically, lithium ions contained in the non-aqueous electrolyte receive electrons on the negative electrode current collector upon charging to become lithium metal, which is then deposited on the surface of the negative electrode current collector. The lithium metal deposited on the surface of the negative electrode current collector dissolves as lithium ions in the non-aqueous electrolyte upon discharging. The lithium ions contained in the non-aqueous electrolyte may be derived from a lithium salt added to the non-aqueous electrolyte, may be supplied from the positive electrode active material upon charging, or may be both.

[0039] The negative electrode current collector may be a strip-shaped conductive sheet, such as a foil or film.

[0040] The surface of the conductive sheet may be smooth. This makes it easier for lithium metal from the positive electrode to deposit evenly on the conductive sheet during charging. "Smooth" means that the maximum height roughness Rz of the conductive sheet is 20 μm or less. The maximum height roughness Rz of the conductive sheet may be 10 μm or less. The maximum height roughness Rz is measured in accordance with JIS B 0601:2013.

[0041] The material of the negative electrode current collector (conductive sheet) may be any conductive material other than lithium metal and lithium alloys. The conductive material may be a metallic material such as a metal or alloy. The conductive material is preferably a material that does not react with lithium. More specifically, a material that does not form an alloy or an intermetallic compound with lithium is preferred. Examples of such conductive materials include copper (Cu), nickel (Ni), iron (Fe), and alloys containing these metal elements, as well as graphite with a preferentially exposed basal plane. Examples of alloys include copper alloys and stainless steel (SUS). Among these, copper and / or copper alloys with high conductivity are preferred.

[0042] The thickness of the negative electrode current collector is not particularly limited and is, for example, 5 μm or more and 300 μm or less.

[0043] A negative electrode mixture layer (not shown) may be formed on the surface of the negative electrode current collector. The negative electrode mixture layer is formed, for example, by applying a paste containing a negative electrode active material such as graphite to at least a portion of the surface of the negative electrode current collector. However, from the viewpoint of achieving a high-capacity lithium secondary battery exceeding that of a lithium ion battery, the thickness of the negative electrode mixture layer is set to be sufficiently thin so that lithium metal can be precipitated on the negative electrode.

[0044] [Spacer] The material constituting the spacer is not particularly limited. The spacer may be composed of a conductive material and / or an insulating material. The spacer may be provided on the surface of the negative electrode or on the surface of the separator (the surface facing the negative electrode).

[0045] The conductive material can be appropriately selected from the materials exemplified for the negative electrode current collector. Such spacers may be provided by forming protrusions on the negative electrode current collector by press processing or the like. Alternatively, a conductive paint may be applied to the surface of the negative electrode, or a conductive tape may be attached to the surface of the negative electrode.

[0046] Examples of insulating materials include resin materials. Examples of resin materials include polyolefin resins, acrylic resins, polyamide resins, polyimide resins, silicone resins, and fluorine-based resins. A cured product of a curable resin such as an epoxy resin may also be used. Furthermore, these resin materials may be mixed with an inorganic filler or the like. The spacer may be formed, for example, by attaching a resin adhesive tape to the surface of the negative electrode. Alternatively, the spacer may be formed by applying a solution or dispersion containing a resin material to the surface of the negative electrode or the surface of the separator facing the negative electrode and then drying the solution or dispersion. The spacer may also be formed by applying a curable resin to the surface of the negative electrode or the surface of the separator facing the negative electrode in a desired shape and then curing the resin.

[0047] [Positive Electrode] The positive electrode includes, for example, a positive electrode current collector and a positive electrode composite layer supported on the positive electrode current collector. The positive electrode composite layer includes, for example, a positive electrode active material, a conductive material, and a binder. The positive electrode composite layer may be formed on only one side of the positive electrode current collector, or may be formed on both sides. The positive electrode is obtained, for example, by applying a positive electrode composite slurry including the positive electrode active material, the conductive material, and the binder to both sides of the positive electrode current collector, drying the coating, and then rolling.

[0048] The positive electrode active material is a material that absorbs and releases lithium ions. Examples of the positive electrode active material include lithium-containing transition metal oxides, transition metal fluorides, polyanions, fluorinated polyanions, and transition metal sulfides. Among these, lithium-containing transition metal oxides are preferred because of their low production cost and high average discharge voltage.

[0049] Examples of transition metal elements contained in the lithium-containing transition metal oxide include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, and W. The lithium-containing transition metal oxide may contain one or more transition metal elements. The transition metal element may be Co, Ni, and / or Mn. The lithium-containing transition metal oxide may contain one or more typical elements as needed. Examples of typical elements include Mg, Al, Ca, Zn, Ga, Ge, Sn, Sb, Pb, and Bi. The typical element may be Al, etc.

[0050] Among lithium-containing transition metal oxides, composite oxides containing Co, Ni, and / or Mn as transition metal elements, and optionally containing Al, and having a layered rock-salt crystal structure are preferred in terms of obtaining high capacity. In this case, in the lithium secondary battery, the molar ratio mLi / mM of the total amount of lithium contained in the positive electrode and negative electrode to the amount mM of metal M other than lithium contained in the positive electrode is set to, for example, 1.1 or less.

[0051] The conductive material is, for example, a carbon material, such as carbon black, acetylene black, ketjen black, carbon nanotubes, and graphite.

[0052] Examples of the binder include fluororesin, polyacrylonitrile, polyimide resin, acrylic resin, polyolefin resin, rubber polymer, etc. Examples of the fluororesin include polytetrafluoroethylene, polyvinylidene fluoride, etc.

[0053] The positive electrode current collector may be a conductive sheet. Examples of the conductive sheet include foil and film. The surface of the positive electrode current collector may be coated with a carbon material.

[0054] Examples of the material of the positive electrode current collector (conductive sheet) include metal materials containing Al, Ti, Fe, etc. The metal material may be Al, an Al alloy, Ti, a Ti alloy, an Fe alloy, etc. The Fe alloy may be stainless steel (SUS).

[0055] The thickness of the positive electrode current collector is not particularly limited and is, for example, 5 μm or more and 300 μm or less.

[0056] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator. Examples of porous sheets include thin films, woven fabrics, and nonwoven fabrics having micropores. The material of the separator is not particularly limited, but may be a polymer material. Examples of polymer materials include olefin resins, polyamide resins, and cellulose. Examples of olefin resins include polyethylene, polypropylene, and copolymers of ethylene and propylene. The separator may contain additives as needed. Examples of additives include inorganic fillers.

[0057] [Non-aqueous electrolyte] The non-aqueous electrolyte having lithium ion conductivity contains, for example, a non-aqueous solvent and lithium ions and anions dissolved in the non-aqueous solvent. The non-aqueous electrolyte may be in a liquid state or a gel state.

[0058] The liquid non-aqueous electrolyte is prepared by dissolving a lithium salt in a non-aqueous solvent, which generates lithium ions and anions.

[0059] The gel-like non-aqueous electrolyte contains a lithium salt and a matrix polymer, or a lithium salt, a non-aqueous solvent, and a matrix polymer. The matrix polymer is, for example, a polymer material that absorbs the non-aqueous solvent and gels. Examples of the polymer material include fluororesin, acrylic resin, and polyether resin.

[0060] As the lithium salt or anion, any known material used in non-aqueous electrolytes for lithium secondary batteries can be used. Specifically, BF ４ － , ClO ４ － , P.F. ６ － , C.F. ３ SO ３ － , C.F. ３ CO ２ － , anions of imides, anions of oxalate complexes, etc. Examples of the anions of imides include N(SO ２ CF ３ )２ － , N(C ｍ F ２ｍ＋１ SO ２ ) ｘ (C ｎ F ２ｎ＋１ SO ２ ) ｙ － (m and n are each independently an integer of 0 or 1 or more, and x and y are each independently 0, 1, or 2, satisfying the relationship x+y=2.) The anion of the oxalate complex may contain boron and / or phosphorus. Examples of the anion of the oxalate complex include bisoxalate borate anion, BF ２ (C ２ O ４ ) － , P.F. ４ (C ２ O ４ ) － , P.F. ２ (C ２ O ４ ) ２ － The non-aqueous electrolyte may contain one of these anions alone or two or more of them.

[0061] From the viewpoint of suppressing the deposition of lithium metal in a dendritic form, the nonaqueous electrolyte preferably contains at least an anion of an oxalate complex. The interaction between the anion of the oxalate complex and lithium facilitates the uniform deposition of lithium metal in the form of fine particles. This facilitates the suppression of localized deposition of lithium metal. The anion of the oxalate complex may be combined with another anion. The other anion may be PF ６ － and / or an anion of an imide.

[0062] Examples of non-aqueous solvents include esters, ethers, nitriles, amides, and halogen-substituted derivatives thereof. The non-aqueous electrolyte may contain one or more of these non-aqueous solvents. Examples of halogen-substituted derivatives include fluorides.

[0063] Examples of esters include carbonate esters and carboxylic acid esters. Examples of cyclic carbonate esters include ethylene carbonate, propylene carbonate, and fluoroethylene carbonate (FEC). Examples of chain carbonate esters include dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate. Examples of cyclic carboxylic acid esters include γ-butyrolactone and γ-valerolactone. Examples of chain carboxylic acid esters include ethyl acetate, methyl propionate, and methyl fluoropropionate.

[0064] Examples of the ether include cyclic ethers and chain ethers. Examples of the cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, and 2-methyltetrahydrofuran. Examples of the chain ethers include 1,2-dimethoxyethane, diethyl ether, ethyl vinyl ether, methyl phenyl ether, benzyl ethyl ether, diphenyl ether, dibenzyl ether, 1,2-diethoxyethane, and diethylene glycol dimethyl ether.

[0065] The concentration of the lithium salt in the non-aqueous electrolyte is, for example, 0.5 mol / L or more and 3.5 mol / L or less. The concentration of the anion in the non-aqueous electrolyte may be 0.5 mol / L or more and 3.5 mol / L or less. Furthermore, the concentration of the anion of the oxalate complex in the non-aqueous electrolyte may be 0.05 mol / L or more and 1 mol / L or less.

[0066] The non-aqueous electrolyte may contain an additive. The additive may form a coating on the negative electrode. The formation of a coating derived from the additive on the negative electrode makes it easier to suppress the formation of dendrites. Examples of such additives include vinylene carbonate, FEC, and vinyl ethyl carbonate (VEC).

[0067] The configuration of a lithium secondary battery according to the present disclosure will be described below with reference to the drawings, taking as an example a cylindrical battery including a wound electrode group, although the present disclosure is not limited to the following configuration.

[0068] Fig. 1 is a longitudinal sectional view schematically showing an example of a lithium secondary battery according to an embodiment of the present disclosure. Fig. 2 is a sectional view schematically showing the configuration of the positive electrode of Fig. 1, and is an enlarged view of a portion surrounded by region II in Fig. 1. Fig. 3 is a sectional view schematically showing the configuration of the negative electrode unit of Fig. 1, and is an enlarged view of a portion surrounded by region III in Fig. 1. Fig. 4 is a top view schematically showing an example of a negative electrode unit included in the lithium secondary battery of Fig. 1.

[0069] The lithium secondary battery 10 is a cylindrical battery including a cylindrical battery case, a wound electrode group 14 housed in the battery case, and a non-aqueous electrolyte (not shown). The battery case is composed of a case body 15, which is a cylindrical metal container with a bottom, and a sealing body 16 that seals the opening of the case body 15. A gasket 27 is disposed between the case body 15 and the sealing body 16, thereby ensuring the airtightness of the battery case. Within the case body 15, insulating plates 17 and 18 are disposed at both ends of the electrode group 14 in the winding axis direction.

[0070] Case body 15 has a step 21 formed, for example, by pressing a portion of the side wall of case body 15 from the outside. Step 21 may be formed in an annular shape along the circumferential direction of case body 15 on the side wall of case body 15. In this case, sealing body 16 is supported by the surface of step 21 on the opening side.

[0071] The sealing body 16 includes a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26. These components are stacked in this order in the sealing body 16. The sealing body 16 is attached to the opening of the case body 15 so that the cap 26 is located outside the case body 15 and the filter 22 is located inside the case body 15. The above-mentioned components constituting the sealing body 16 are, for example, disk-shaped or ring-shaped. The lower valve body 23 and the upper valve body 25 are connected to each other at their respective centers, and an insulating member 24 is interposed between their respective peripheral edges. The filter 22 and the lower valve body 23 are connected to each other at their respective centers. The upper valve body 25 and the cap 26 are connected to each other at their respective centers. In other words, all components except the insulating member 24 are electrically connected to each other.

[0072] A vent hole (not shown) is formed in the lower valve body 23. Therefore, if the internal pressure of the battery case increases due to abnormal heat generation or the like, the upper valve body 25 bulges toward the cap 26 and separates from the lower valve body 23. This cuts off the electrical connection between the lower valve body 23 and the upper valve body 25. If the internal pressure increases further, the upper valve body 25 breaks, and gas is discharged from an opening (not shown) formed in the cap 26.

[0073] The electrode group 14 has a positive electrode 11, a negative electrode unit 12, and a separator 13. The positive electrode 11, the negative electrode unit 12, and the separator 13 are all strip-shaped. The positive electrode 11 and the negative electrode unit 12 are spirally wound with the separator 13 interposed therebetween so that the width directions of the strip-shaped positive electrode 11 and the negative electrode unit 12 are parallel to the winding axis. In a cross section perpendicular to the winding axis of the electrode group 14, the positive electrode 11 and the negative electrode unit 12 are alternately stacked in the radial direction of the electrode group 14 with the separator 13 interposed therebetween. In other words, the longitudinal direction of each electrode is the winding direction, and the width direction of each electrode is the axial direction.

[0074] The positive electrode 11 is electrically connected to a cap 26, which also serves as a positive electrode terminal, via a positive electrode lead 19. One end of the positive electrode lead 19 is connected, for example, near the center of the positive electrode 11 in the longitudinal direction. The positive electrode lead 19 extends from the positive electrode 11 to a filter 22 through a through-hole (not shown) formed in the insulating plate 17. The other end of the positive electrode lead 19 is welded to the surface of the filter 22 on the electrode group 14 side.

[0075] The positive electrode 11 includes a positive electrode current collector 30 and a positive electrode composite layer 31 (see FIG. 2 ), and is electrically connected to a cap 26, which functions as a positive electrode terminal, via a positive electrode lead 19. One end of the positive electrode lead 19 is connected, for example, near the center of the positive electrode 11 in the longitudinal direction. The positive electrode lead 19 extends from the positive electrode 11 through a through-hole (not shown) formed in the insulating plate 17 and reaches the filter 22. The other end of the positive electrode lead 19 is welded to the surface of the filter 22 on the electrode group 14 side.

[0076] As shown in Figures 3 and 4, the negative electrode unit 12 includes a strip-shaped negative electrode 40. The negative electrode 40 has a first surface S1 and a second surface S2 opposite the first surface S1. The negative electrode 40 includes at least a strip-shaped negative electrode current collector, and may include a strip-shaped negative electrode current collector and a negative electrode composite layer formed on both sides of the negative electrode current collector. The negative electrode current collector of the negative electrode 40 is electrically connected to the case body 15, which functions as a negative electrode terminal, via a negative electrode lead 20. One end of the negative electrode lead 20 is connected to, for example, a longitudinal end of the negative electrode current collector of the negative electrode 40, and the other end is welded to the inner bottom surface of the case body 15.

[0077] 3 and 4, the negative electrode unit 12 includes spacers 50 provided on the first surface S1 and the second surface S2 of the negative electrode 40. As shown in Fig. 4, the spacers 50 have a plurality of spot-like protrusions 51 dispersedly arranged on the surface of the negative electrode 40. The presence of the spot-like protrusions 51 forms spaces 35 between the first surface S1 and the separator 13 and between the second surface S2 and the separator 13.

[0078] In lithium secondary battery 10, lithium metal is deposited in space 35 above negative electrode 40 upon charging, and the deposited lithium metal dissolves in the non-aqueous electrolyte upon discharging. By accommodating the lithium metal deposited on the surface of the negative electrode within space 35, the volume change of the negative electrode due to the deposition of lithium metal is reduced, improving cycle characteristics.

[0079] As shown in Fig. 4, the spacer 50 includes a plurality of convex portion groups 52 (52A, 52B) in which a plurality of spot-shaped convex portions 51 are arranged at equal intervals in the lateral direction (band width direction) of the negative electrode 40. The plurality of convex portion groups 52 are arranged at equal intervals in the longitudinal direction of the negative electrode 40. Among the plurality of convex portion groups 52, convex portion groups 52A and 52B adjacent in the longitudinal direction are arranged such that the positions of the spot-shaped convex portions 51 in the convex portion group in the lateral direction are shifted and the convex portions are arranged alternately. The shift in the positions of the spot-shaped convex portions 51 in the lateral direction between adjacent convex portion groups 52A and 52B is approximately half the separation distance between the spot-shaped convex portions 51 in the lateral direction.

[0080] 3 and 4 , when viewed from the normal direction of the first surface S1, the spot-shaped protrusions 51 provided on the second surface S2 of the negative electrode 40 are arranged at positions overlapping the spot-shaped protrusions 51 provided on the first surface S1 of the negative electrode 40. However, the spot-shaped protrusions 51 provided on both surfaces of the negative electrode may be at the same positions on the front and back of the negative electrode or at different positions on the front and back of the negative electrode when viewed from the normal direction of one surface.

[0081] FIG. 5 is an enlarged view of a portion of a negative electrode unit showing another example of the negative electrode unit. In FIG. 5, in addition to the spacers provided on the front surface (first surface) of the negative electrode, the positions of the spacers provided on the back surface (second surface) of the negative electrode are indicated by dotted lines. In the example of FIG. 5, when viewed from the normal direction of the first surface S1, the spot-shaped protrusions 51 (spot-shaped protrusions 51 shown by dashed lines in FIG. 5) provided on the second surface S2 of the negative electrode 40 are positioned so as not to overlap with the spot-shaped protrusions 51 (spot-shaped protrusions 51 shown by solid lines in FIG. 5) provided on the first surface S1 of the negative electrode 40. Like the spacers 50 provided on the first surface S1, the spacers 50 provided on the second surface S2 also include a plurality of protrusion groups 52 (52C, 52D) in which a plurality of spot-shaped protrusions 51 are arranged at equal intervals in the short direction (width direction) of the negative electrode 40. The group of protrusions 52C provided on the second surface corresponds to the group of protrusions 52A provided on the first surface, and the group of protrusions 52D provided on the second surface corresponds to the group of protrusions 52B provided on the first surface.

[0082] When viewed from the normal direction of the first surface S1, the positions of the spot-shaped protrusions 51 in the longitudinal direction are the same in the protrusion groups 52A and 52C, but the positions in the lateral direction within the protrusion groups are shifted. Similarly, when viewed from the normal direction of the first surface S1, the positions of the spot-shaped protrusions 51 in the longitudinal direction are the same in the protrusion groups 52B and 52D, but the positions in the lateral direction within the protrusion groups are shifted.

[0083] The positional deviation in the short-side direction of the spot-like protrusions 51 between the protrusion groups 52A and 52C is approximately half the separation distance between the spot-like protrusions 51 in the short-side direction. Similarly, the positional deviation in the short-side direction of the spot-like protrusions 51 between the protrusion groups 52B and 52D is approximately half the separation distance between the spot-like protrusions 51 in the short-side direction. In this case, the spot-like protrusions 51 provided on the second surface are located at the center of a parallelogram formed with the four spot-like protrusions 51 provided on the first surface as vertices. The spot-like protrusions 51 provided on the first surface are located at the center of a parallelogram formed with the four spot-like protrusions 51 provided on the second surface as vertices. As a result, the spot-like protrusions 51 provided on the second surface efficiently press the space 35 between the first surface S1 and the separator 13, and the spot-like protrusions 51 provided on the first surface efficiently press the space 35 between the second surface S2 and the separator 13. This makes it possible to maintain a high surface pressure during charging, and significantly suppress deterioration of cycle characteristics.

[0084] The arrangement of the spacers 50 (protrusion groups 52C, 52D) provided on the second surface S2 is not limited to the example in Fig. 5. For example, the spacers 50 (protrusion groups 52C, 52D) provided on the second surface S2 may be arranged at positions overlapping the spacers 50 (protrusion groups 52A, 52B) provided on the first surface S1. The spacers 50 (protrusion groups 52C, 52D) provided on the second surface S2 may be arranged at positions off-center within a parallelogram formed by the four spot-like protrusions 51 provided on the first surface S1.

[0085] In the illustrated example, a cylindrical lithium secondary battery having a wound electrode group has been described, but this embodiment is not limited to this case and can be applied to other cases. The shape of the lithium secondary battery can be appropriately selected from various shapes such as a cylindrical shape, a coin shape, a square shape, a sheet shape, and a flat shape depending on the application. The shape of the electrode group is not particularly limited, and may be a laminated type. In addition, known configurations other than the electrode group and non-aqueous electrolyte of the lithium secondary battery can be used without particular limitation.

[0086] [Examples] The lithium secondary battery according to the present disclosure will be specifically described below based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0087] Example 1 (1) Preparation of Positive Electrode A rock salt type lithium-containing transition metal oxide (NCA; positive electrode active material) containing Li, Ni, Co, and Al (the molar ratio of Li to the total of Ni, Co, and Al is 1.0) and having a layered structure, acetylene black (AB; conductive material), and polyvinylidene fluoride (PVdF; binder) were mixed in a mass ratio of NCA:AB:PVdF = 95:2.5:2.5, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added and stirred to prepare a positive electrode mixture slurry. Next, the obtained positive electrode mixture slurry was applied to both sides of an Al foil (positive electrode current collector), dried, and the coating of the positive electrode mixture was rolled using a roller. Finally, the resulting laminate of the positive electrode current collector and the positive electrode mixture was cut to a predetermined electrode size to prepare a positive electrode having a positive electrode mixture layer on both sides of the positive electrode current collector.

[0088] (2) Fabrication of Negative Electrode Unit: A rectangular electrolytic copper foil (thickness: 12 μm) was prepared as a negative electrode (negative electrode current collector). Polyimide ink was ejected onto one surface of the electrolytic copper foil using a dispenser, followed by vacuum drying to form a polyimide resin spacer in the shape of a spot, as shown in FIG.

[0089] Then, the same spacers as above were formed on the other surface of the electrolytic copper foil in the same manner as described above. The positions of the 0.8 mm diameter holes formed at the four corners of the electrolytic copper foil were confirmed with a CCD camera, and the positions of the spacers were adjusted so that, as viewed from one surface, the spacers formed on the other surface overlapped the spacers formed on the one surface. The electrolytic copper foil was then cut to a predetermined electrode size. In this way, a negative electrode unit was obtained, comprising a strip-shaped negative electrode and a spacer having a plurality of spot-shaped protrusions arranged on both sides of the negative electrode. The width of the electrolytic copper foil cut to the electrode size was 65 mm, and the longitudinal length was 1000 mm.

[0090] The contour shape of the multiple spot-like protrusions was a circle with a diameter of 1 mm. The arrangement interval of the spot-like protrusions in the short-side direction within the protrusion group (the center-to-center distance of the spot-like protrusions) was 3.536 mm. The arrangement interval of the multiple protrusion groups in the long-side direction (the distance between straight lines parallel to the short-side direction connecting the spot-like protrusions) was 1.768 mm. The ratio of the area of ​​the surface (one side) of the negative electrode covered by spacers to the area of ​​the surface (one side) of the negative electrode (the coverage rate of the negative electrode surface by spacers) was 12.6%.

[0091] (3) Preparation of non-aqueous electrolyte EC and DMC were mixed in a volume ratio of EC:DMC=30:70. LiPF ６ 1 mol / L and LiBF ２ (C ２ O ４ ) were dissolved in water to a concentration of 0.1 mol / L to prepare a liquid non-aqueous electrolyte.

[0092] (4) Battery Fabrication An Al tab was attached to the positive electrode obtained above. A Ni tab was attached to the negative electrode current collector of the negative electrode unit obtained above. The positive electrode and negative electrode were spirally wound with a separator in an inert gas atmosphere to produce a wound electrode assembly. Since all of the lithium contained in the electrode assembly originated from the positive electrode, the molar ratio mLi / mM of the total amount of lithium contained in the positive electrode and negative electrode to the amount mM of metal M (here, Ni, Co, and Al) contained in the positive electrode was 1.0. A polyethylene microporous membrane was used as the separator. The obtained electrode assembly was housed in a bag-shaped exterior formed of a laminate sheet with an Al layer. The nonaqueous electrolyte was injected into the exterior housing containing the electrode assembly, and the exterior housing was sealed to produce lithium secondary battery A1.

[0093] Example 2 In the preparation of the negative electrode unit, the longitudinal arrangement interval (the distance between the straight lines parallel to the short side connecting the spot-like protrusions) of the plurality of protrusion groups was set to 3.536 mm. A lithium secondary battery A2 was prepared in the same manner as in Example 1, except for the above. The ratio of the area of ​​the negative electrode surface (one side) covered by spacers to the area of ​​the negative electrode surface (one side) was 6.3%.

[0094] Example 3 In preparing the negative electrode unit, multiple convex groups were arranged in the longitudinal direction so that the positions of the spot-shaped convex portions 51 in the lateral direction within each convex group coincided among the multiple convex groups. The arrangement interval in the lateral direction of the spot-shaped convex portions within each convex group (the distance between the centers of the spot-shaped convex portions) was 1.768 mm. The arrangement interval in the longitudinal direction of the multiple convex groups (the distance between the straight lines parallel to the lateral direction connecting the spot-shaped convex portions) was 3.536 mm. The ratio of the area of ​​the surface (one side) of the negative electrode covered by spacers to the area of ​​the surface (one side) of the negative electrode (the coverage rate of the spacers on the negative electrode surface) was 12.6%. A lithium secondary battery A3 was prepared in the same manner as in Example 1, except for the above.

[0095] Comparative Example 1 In the preparation of the negative electrode unit, spacers were formed so as to extend in lines along the longitudinal direction of the negative electrode. The width of the longitudinally extending spacers (linear protrusions) in the lateral direction was 1 mm. The arrangement interval of the linear protrusions in the lateral direction (the distance between the center lines of the linear protrusions) was 5 mm. The proportion of the area of ​​the surface (one side) of the negative electrode covered by the spacers (the coverage rate of the negative electrode surface by the spacers) was 20%. A lithium secondary battery B1 was prepared in the same manner as in Example 1, except for the above.

[0096] [Evaluation 1] A charge / discharge test was carried out on each of the obtained batteries. In the charge / discharge test, the batteries were charged in a thermostatic chamber at 25°C under the following conditions, then rested for 20 minutes, and then discharged under the following conditions.

[0097] (Charging) The battery was charged at a constant current of 2.15 mA per unit area (cm 2 ) of the electrode until the battery voltage reached 4.1 V, and then charged at a constant voltage of 4.1 V until the current value per unit area of ​​the electrode reached 0.54 mA.

[0098] (Discharge) The battery was discharged at a constant current of 2.15 mA per unit area (cm 2 ) of the electrode until the battery voltage reached 3.75 V.

[0099] The above charge and discharge cycle was counted as one cycle, and charge and discharge were repeated up to 100 cycles. The ratio (%) of the discharge capacity at the 100th cycle to the discharge capacity at the first cycle was calculated as the capacity retention rate.

[0100] The evaluation results are shown in Table 1.

[0101]

[0102] Batteries A1 to A3 exhibited higher capacity retention rates than Battery B1. In particular, Batteries A1 and A2, in which the positions of the spot-like protrusions in the lateral direction within the protrusion groups adjacent in the longitudinal direction were shifted and arranged alternately, exhibited significantly improved capacity retention rates.

[0103] The lithium secondary battery of the present disclosure can be used in electronic devices such as mobile phones, smartphones, and tablet terminals, electric vehicles including hybrids and plug-in hybrids, and home storage batteries combined with solar cells.

[0104] 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.

[0105] REFERENCE SIGNS LIST 10 Lithium secondary battery 11 Positive electrode 12 Negative electrode unit 13 Separator 14 Electrode group 15 Case body 16 Sealing body 17, 18 Insulating plate 19 Positive electrode lead 20 Negative electrode lead 21 Step portion 22 Filter 23 Lower valve body 24 Insulating member 25 Upper valve body 26 Cap 27 Gasket 30 Positive electrode current collector 31 Positive electrode mixture layer 40 Negative electrode 50 Spacer 51 Spot-shaped convex portion 52A to 52D Convex portion group

Claims

1. a positive electrode, a negative electrode facing the positive electrode, a separator disposed between the positive electrode and the negative electrode, a non-aqueous electrolyte having lithium ion conductivity, a spacer disposed between the negative electrode and the separator, comprising, lithium metal is deposited on the negative electrode during charging and dissolved from the negative electrode during discharging, the spacer has a plurality of spot-like convex portions, a lithium secondary battery.

2. the spacer includes a plurality of convex portion groups in which the plurality of spot-like convex portions are arranged at equal intervals in the short direction of the strip-shaped negative electrode, the plurality of convex portion groups are arranged at equal intervals in the longitudinal direction of the negative electrode, and in adjacent convex portion groups, the positions of the spot-like convex portions in the short direction are offset, the lithium secondary battery according to claim 1.

3. in adjacent convex portion groups, the positions of the spot-like convex portions in the short direction are offset by half of the interval between the arrangements of the plurality of spot-like convex portions in the convex portion group, the lithium secondary battery according to claim 2.

4. the height h of the spacer is 0.015 mm or more and 0.1 mm or less, the lithium secondary battery according to any one of claims 1 to 3.

5. the height of a part of the plurality of spot-like convex portions is different from the height of the remaining part of the plurality of spot-like convex portions, the lithium secondary battery according to any one of claims 1 to 3.

6. the plurality of convex portions are made of a resin material, the lithium secondary battery according to any one of claims 1 to 3.

7. when a straight line passing through the spot-like convex portions and parallel to the short direction of the negative electrode is drawn with respect to the plurality of spot-like convex portions, the distance between adjacent straight lines is 1.5 mm or more and 4.0 mm or less, the lithium secondary battery according to any one of claims 1 to 3.

8. the ratio of the area of the surface of the negative electrode covered by the spacer to the area of the surface of the negative electrode is 5% or more and 20% or less, the lithium secondary battery according to any one of claims 1 to 3.