Secondary batteries, battery packs, and vehicles

Abstract

The present invention provides a secondary battery with excellent charge-discharge cycle life, a battery pack containing the secondary battery, and a vehicle containing the battery pack. [Solution] According to the embodiment, a secondary battery is provided comprising a positive electrode, a negative electrode, and an electrolyte. The negative electrode contains a niobium-containing oxide. The ratio B / A of the area A in the range of 200 eV to 215 eV and the area B in the range of 680 eV to 695 eV, as measured by hard X-ray photoelectron spectroscopy on the surface of the negative electrode, is 1.1 to 4.3. The electrolyte contains a primary carboxylic acid ester. The content of the primary carboxylic acid ester is 70% to 95% by mass relative to 100% by mass of the electrolyte excluding the lithium salt.

Description

[Technical Field]

[0001] Embodiments of the present invention relate to a secondary battery, a battery pack, and a vehicle. [Background technology]

[0002] Generally, it is preferable to have a high dielectric constant for the electrolyte. By using an electrolyte with a high dielectric constant, the input / output performance and low-temperature performance of the battery can be improved.

[0003] Carboxylic acid esters have a higher dielectric constant than carbonate esters, which are known as solvents for electrolytes. Therefore, the use of electrolytes containing carboxylic acid esters is being considered. However, carboxylic acid esters have a drawback: they tend to reductively decompose and produce gas at low potentials. For this reason, electrolytes containing carboxylic acid esters are primarily being considered for use in combination with electrodes containing lithium titanium oxide, which tends to have a higher operating potential. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] International Publication No. 2021 / 199485 [Patent Document 2] International Publication No. 2021 / 220745 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] The embodiment aims to provide a secondary battery with excellent charge-discharge cycle life, a battery pack containing the secondary battery, and a vehicle containing the battery pack. [Means for solving the problem]

[0006] According to the embodiment, a secondary battery is provided. The secondary battery comprises a positive electrode, a negative electrode, and an electrolyte. The negative electrode contains a niobium-containing oxide. The ratio B / A of the area A in the range of 200 eV to 215 eV and the area B in the range of 680 eV to 695 eV, as measured by hard X-ray photoelectron spectroscopy on the surface of the negative electrode, is 1.1 to 4.3. The electrolyte contains a lithium salt and a carboxylic acid ester represented by formula A. (Formula A)

[0007] [ka]

[0008] Each of R and R' in formula A is a hydrocarbon group. Let r be the number of carbon atoms in R and r' be the number of carbon atoms in R'. Then r and r' satisfy 5 ≤ (r+1) + r' ≤ 8, and the value of |(r+1) - r'| is 0 or 1. The carboxylic acid ester content is 70% to 95% by mass relative to 100% by mass of the electrolyte excluding the lithium salt. [Brief explanation of the drawing]

[0009] [Figure 1] A schematic cross-sectional view showing an example of a secondary battery according to this embodiment. [Figure 2] An enlarged cross-sectional view of section A of the secondary battery shown in Figure 1. [Figure 3] A schematic partial cutaway perspective view showing another example of a secondary battery according to the embodiment. [Figure 4] Figure 3 shows an enlarged cross-sectional view of section B of the secondary battery. [Figure 5] A schematic perspective view showing an example of a battery pack according to this embodiment. [Figure 6] An exploded perspective view schematically showing an example of a battery pack according to this embodiment. [Figure 7] A block diagram showing an example of the electrical circuit of the battery pack shown in Figure 6. [Figure 8] A partially transparent view schematically showing an example of a vehicle according to the embodiment. [Figure 9]A schematic diagram showing an example of a control system for the electrical system in a vehicle according to this embodiment. [Figure 10] A figure showing an example of a hard X-ray photoelectron spectroscopy spectrum of the negative electrode of a secondary battery according to the embodiment and other examples. [Figure 11] A figure showing another example of the hard X-ray photoelectron spectroscopy spectrum of the negative electrode of a secondary battery according to the embodiment and other examples. [Modes for carrying out the invention]

[0010] In secondary batteries using electrodes containing lithium titanium oxide as the active material, an electrolyte containing carboxylic acid esters can be used. By including a large amount of carboxylic acid esters in the electrolyte, the proportion of cyclic carbonates in the electrolyte can be reduced. Furthermore, secondary batteries containing electrolytes with carboxylic acid esters can maintain high output even after charge-discharge cycles. However, carboxylic acid esters are easily reductively decomposed at low potentials. When reductive decomposition of carboxylic acid esters occurs, there is a problem of gas generation.

[0011] Electrodes containing niobium-containing oxides tend to have a lower operating potential relative to the redox potential of lithium. Therefore, when electrodes containing niobium-containing oxides are combined with electrolytes containing carboxylic acid esters, gas generation due to the reductive decomposition of the carboxylic acid esters is likely to occur. Consequently, in secondary batteries using electrodes containing niobium-containing oxides, using electrolytes containing carboxylic acid esters to improve durability presents a challenge: the increased gas generation can actually degrade the charge-discharge cycle life performance.

[0012] (First embodiment) According to the first embodiment, a secondary battery is provided. The secondary battery comprises a positive electrode, a negative electrode, and an electrolyte. The negative electrode contains a niobium-containing oxide. The ratio B / A of the area A in the range of 200 eV to 215 eV and the area B in the range of 680 eV to 695 eV, as measured by hard X-ray photoelectron spectroscopy on the surface of the negative electrode, is 1.1 to 4.3. The electrolyte contains a lithium salt and a carboxylic acid ester represented by formula A.

[0013] (Formula A)

[0014] [ka]

[0015] Each of R and R' in formula A is a hydrocarbon group. Let r be the number of carbon atoms in R and r' be the number of carbon atoms in R'. Then r and r' satisfy 5 ≤ (r+1) + r' ≤ 8, and the value of |(r+1) - r'| is 0 or 1. The carboxylic acid ester content is 70% by mass or more and 95% by mass or less based on 100% by mass of the electrolyte excluding the lithium salt.

[0016] In this specification, a carboxylic acid ester represented by formula A, where r is the number of carbon atoms in the hydrocarbon group R and r' is the number of carbon atoms in the hydrocarbon group R', such that r and r' satisfy 5 ≤ (r+1) + r' ≤ 8, and the value of |(r+1) - r'| is 0 or 1, may be referred to as a first carboxylic acid ester.

[0017] The electrolyte contains a primary carboxylic acid ester.

[0018] The number of carbon atoms in the hydrocarbon groups R and R' contained in formula A, r and r', satisfy 5 ≤ (r+1) + r' ≤ 8. The smaller the number of (r+1) + r', that is, the fewer carbon atoms the primary carboxylic acid ester contains, the lower the viscosity of the primary carboxylic acid ester tends to be. As a result, the viscosity of the electrolyte can be lowered, which improves ionic conductivity and is therefore preferable. Since the primary carboxylic acid ester has (r+1) + r' ≤ 8, it contributes to improving the ionic conductivity of the electrolyte. Furthermore, the larger the number of (r+1) + r', that is, the larger the number of carbon atoms the primary carboxylic acid ester contains, the higher the stability at high temperatures tends to be. Since the primary carboxylic acid ester has (r+1) + r' ≥ 5, its stability at high temperatures can be improved.

[0019] A value of 0 or 1 for |(r+1)-r'| means that the structure of the carboxylic acid ester is symmetrical or nearly symmetrical. The 1 in |(r+1)-r'| represents the number of carbon atoms that are single-bonded to R and double-bonded to the oxygen atom in formula A. In other words, in |(r+1)-r'|, (r+1) is the value obtained by adding 1, which is the number of carbon atoms that are single-bonded to R and double-bonded to the oxygen atom, to r, the number of carbon atoms that are single-bonded to R and double-bonded to the oxygen atom. Note that "|(r+1)-r'|" means the absolute value of "(r+1)-r'".

[0020] Symmetry means that the value of (r+1) is equal to the number of carbon atoms r' contained in the hydrocarbon group R'. In other words, when the structure of a carboxylic acid ester is symmetric, the value of |(r+1)-r'| is 0.

[0021] The larger the value of |(r+1)-r'|, the further the carboxylic acid ester structure is from symmetry. When |(r+1)-r'| is 1, the carboxylic acid ester structure is said to be close to symmetry. The closer the structure is to symmetry, that is, the smaller the value of |(r+1)-r'|, the less likely charge imbalance is to occur within the carboxylic acid ester molecule. Therefore, first carboxylic acid esters with a |(r+1)-r'| value of 0 or 1 tend to have a low dielectric constant because they are less likely to polarize.

[0022] Carboxylic acid esters are generally substances with relatively high dielectric constants. However, for the reasons mentioned above, primary carboxylic acid esters have relatively low dielectric constants compared to other carboxylic acid esters.

[0023] In an electrolyte containing a lithium salt and a carboxylic acid ester, the degree of dissociation of the lithium salt tends to decrease as the dielectric constant of the carboxylic acid ester decreases. The electrolyte contained in the secondary battery according to this embodiment contains a first carboxylic acid ester with a low dielectric constant, so the degree of dissociation of the lithium salt in the electrolyte can be reduced.

[0024] The lower the degree of dissociation of lithium salts in the electrolyte, the lower their reduction resistance tends to be. Therefore, lithium salts are more susceptible to reductive decomposition, for example, by reacting with the negative electrode. When reductive decomposition of lithium salts occurs, a film may be formed on at least a portion of the surface of the negative electrode.

[0025] In hard X-ray photoelectron spectroscopy (HAXPES) on the surface of the negative electrode, the ratio B / A of the area A in the range of 200 eV to 215 eV and the area B in the range of 680 eV to 695 eV may be an indicator of the degree of exposure of niobium-containing oxide on the surface of the negative electrode.

[0026] For example, a small B / A ratio may mean that a large proportion of niobium-containing oxide occupies the surface of the negative electrode. In other words, it may mean that the degree of exposure of niobium-containing oxide on the surface of the negative electrode is large.

[0027] Conversely, a large B / A ratio may mean that the proportion of niobium-containing oxide on the negative electrode surface is small. For example, it may mean that the degree of exposure of niobium-containing oxide is small because a coating is formed on at least a portion of the negative electrode surface. Therefore, contact and reaction between the carboxylic acid ester contained in the electrolyte and the niobium-containing oxide can be suppressed. As a result, the decomposition of the carboxylic acid ester can be suppressed, and gas generation can be suppressed.

[0028] In the secondary battery according to this embodiment, the B / A ratio is 1.1 or higher. Therefore, the exposure of niobium-containing oxides on the surface of the negative electrode is suppressed. As a result, the decomposition of carboxylic acid esters can be suppressed. Also, since the B / A ratio is 4.3 or lower, the amount of film formed on the negative electrode surface is not excessive. Therefore, the resistance can be kept low even after charging and discharging. As a result, the charge-discharge cycle life can be improved.

[0029] The content of the first carboxylic acid ester is 70% to 95% by mass relative to 100% by mass of the electrolyte excluding lithium salt. A higher content of the first carboxylic acid ester relative to 100% by mass of the electrolyte excluding lithium salt tends to lower the viscosity of the electrolyte, thereby improving low-temperature performance and input / output performance. Since the content of the first carboxylic acid ester relative to 100% by mass of the electrolyte excluding lithium salt is 70% by mass or more, low-temperature performance and input / output performance can be improved. A lower content of the first carboxylic acid ester relative to the electrolyte excluding lithium salt tends to improve the degree of dissociation of lithium salt in the electrolyte. Since the content of the first carboxylic acid ester relative to 100% by mass of the electrolyte excluding lithium salt is 95% by mass or less, the reduction in cycle life caused by a decrease in the degree of dissociation of lithium salt can be suppressed. Because the content of the first carboxylic acid ester is 70% to 95% by mass relative to 100% by mass of the electrolyte excluding lithium salt, both high cycle life performance and high input / output performance can be achieved.

[0030] The secondary battery according to this embodiment will be described further.

[0031] The secondary battery according to this embodiment may be, for example, a lithium secondary battery. Furthermore, the secondary battery may include a non-aqueous electrolyte secondary battery containing a non-aqueous electrolyte.

[0032] The amount of substance Ml (mol) of the lithium salt in the electrolyte and the amount of substance Mc (mol) of the primary carboxylic acid ester in the electrolyte satisfy the condition 0.1 ≤ Ml / Mc ≤ 0.4. It is preferable that Ml / Mc is within this range, as it allows for a low degree of dissociation of the lithium salt.

[0033] In hard X-ray photoelectron spectroscopy on the surface of the negative electrode, the ratio C / B of the area C within the range of 290 eV to 295 eV to the area B may be 0.2 or less. This will be explained below.

[0034] The negative electrode may contain a binder, as described later. The binder may also contain fluorine atoms. Examples of binders containing fluorine atoms include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and fluorinated rubber. Fluorine-containing binders may contain CF bonds.

[0035] In HAXPES measurements of the negative electrode surface, a C1s peak, attributed to carbon atoms in CF bonds, can be detected within the range of 290 eV to 295 eV.

[0036] In other words, if the negative electrode contains a binder containing fluorine atoms, the area C in the range of 290 eV to 295 eV may become relatively larger. The relative size of area C can be expressed as the ratio C / B of area C to area B in the range of 680 eV to 695 eV.

[0037] Therefore, in HAXPES for the surface of the negative electrode, a ratio C / B higher than 0.2 may indicate that the fluorine atoms present on the negative electrode surface largely originate from the binder. Conversely, if the negative electrode does not contain a binder containing fluorine atoms, the ratio C / B may be close to 0. For example, the ratio C / B may be 0.

[0038] A C / B ratio of 0.2 or less may indicate that the fluorine atoms present on the negative electrode surface largely originate from the coating formed on the negative electrode surface. For example, it may mean that they largely originate from lithium fluoride (LiF) that the coating may contain. LiF can be produced, for example, by the reductive decomposition of lithium salts and may be present on the negative electrode surface.

[0039] In the HAXPES of the negative electrode surface of the secondary battery according to this embodiment, the C / B ratio may be 0.2 or less. Therefore, contact between the carboxylic acid ester contained in the electrolyte and the niobium-containing oxide can be suppressed by the coating. As a result, gas generation can be suppressed.

[0040] In hard X-ray photoelectron spectroscopy of the negative electrode surface, an Nb3d peak, attributed to niobium atoms, may be detected in the range of 200 eV to 215 eV. In the range of 680 eV to 695 eV, an F1s peak, attributed to fluorine atoms, may be detected. In other words, the ratio B / A of the area A in the range of 200 eV to 215 eV to the area B in the range of 680 eV to 695 eV can be an indicator of the degree of fluorine atom presence relative to niobium atoms on the negative electrode surface. On the negative electrode surface, fluorine atoms may be contained in, for example, a coating. On the negative electrode surface, niobium atoms may be contained in, for example, a niobium-containing oxide.

[0041] A high ratio B / A may indicate a greater proportion of fluorine atoms relative to niobium atoms. In other words, it may mean a larger amount of coating formed on the surface of the negative electrode.

[0042] Conversely, a small B / A ratio may mean that the amount of coating formed on the negative electrode surface is small. In this case, resistance can be reduced, which contributes to improved low-temperature performance.

[0043] Next, the secondary battery according to the embodiment will be described in more detail with reference to the drawings.

[0044] Figure 1 is a schematic cross-sectional view showing an example of a secondary battery. Figure 2 is an enlarged cross-sectional view of part A of the secondary battery shown in Figure 1.

[0045] The secondary battery 100 shown in Figures 1 and 2 comprises a bag-shaped outer casing member 2 shown in Figure 1, an electrode group 1 shown in Figures 1 and 2, and an electrolyte (not shown). The electrode group 1 and the electrolyte are housed within the bag-shaped outer casing member 2. The electrolyte (not shown) is held by the electrode group 1.

[0046] The bag-shaped outer packaging member 2 consists of a laminate film comprising two resin layers and a metal layer interposed between them.

[0047] As shown in Figure 1, electrode group 1 is a flat, wound electrode group. As shown in Figure 2, the flat, wound electrode group 1 includes a negative electrode 3, a separator 4, and a positive electrode 5. The separator 4 is interposed between the negative electrode 3 and the positive electrode 5.

[0048] The negative electrode 3 includes a negative electrode current collector 3a and a negative electrode active material containing layer 3b. In the portion of the negative electrode 3 located in the outermost shell of the wound electrode group 1, the negative electrode active material containing layer 3b is formed only on the inner surface side of the negative electrode current collector 3a, as shown in Figure 2. In the other portions of the negative electrode 3, the negative electrode active material containing layer 3b is formed on both sides of the negative electrode current collector 3a.

[0049] The positive electrode 5 includes a positive electrode current collector 5a and positive electrode active material-containing layers 5b formed on both sides thereof.

[0050] As shown in Figure 1, the negative electrode terminal 6 and the positive electrode terminal 7 are located near the outer edge of the wound electrode group 1. The negative electrode terminal 6 is connected to the outermost part of the negative electrode current collector 3a. The positive electrode terminal 7 is connected to the outermost part of the positive electrode current collector 5a. These negative electrode terminals 6 and positive electrode terminals 7 extend outward from the opening of the bag-shaped outer casing member 2. A thermoplastic resin layer is installed on the inner surface of the bag-shaped outer casing member 2, and the opening is closed by heat fusion of this layer.

[0051] The secondary battery according to this embodiment is not limited to the secondary battery with the configuration shown in Figures 1 and 2, but may also be a battery with the configuration shown in Figures 3 and 4, for example.

[0052] Figure 3 is a schematic partially cutaway perspective view showing another example of a secondary battery. Figure 4 is an enlarged cross-sectional view of section B of the secondary battery shown in Figure 3.

[0053] The secondary battery 100 shown in Figures 3 and 4 comprises an electrode group 1 shown in Figures 3 and 4, an outer casing member 2 shown in Figure 3, and an electrolyte (not shown). The electrode group 1 and the electrolyte are housed within the outer casing member 2. The electrolyte is held within the electrode group 1.

[0054] The exterior component 2 consists of a laminate film comprising two resin layers and a metal layer interposed between them.

[0055] As shown in Figure 4, electrode group 1 is a stacked electrode group. The stacked electrode group 1 has a structure in which negative electrodes 3 and positive electrodes 5 are alternately stacked with separators 4 interposed between them.

[0056] The electrode group 1 includes a plurality of negative electrodes 3. Each of the plurality of negative electrodes 3 comprises a negative electrode current collector 3a and a negative electrode active material-containing layer 3b supported on both sides of the negative electrode current collector 3a. The electrode group 1 also includes a plurality of positive electrodes 5. Each of the plurality of positive electrodes 5 comprises a positive electrode current collector 5a and a positive electrode active material-containing layer 5b supported on both sides of the positive electrode current collector 5a.

[0057] Each negative electrode 3's negative electrode current collector 3a includes a portion on one side where the negative electrode active material-containing layer 3b is not supported on any surface. This portion functions as a negative electrode current collector tab 3c. As shown in Figure 4, the negative electrode current collector tab 3c does not overlap with the positive electrode 5. Furthermore, multiple negative electrode current collector tabs 3c are electrically connected to a strip-shaped negative electrode terminal 6. The tip of the strip-shaped negative electrode terminal 6 is extended to the outside of the outer casing member 2.

[0058] Although not shown in the diagram, the positive electrode current collector 5a of each positive electrode 5 includes a portion on one side where the positive electrode active material-containing layer 5b is not supported on any surface. This portion functions as a positive electrode current collector tab. The positive electrode current collector tab, like the negative electrode current collector tab 3c, does not overlap with the negative electrode 3. Furthermore, the positive electrode current collector tab is located on the opposite side of the electrode group 1 from the negative electrode current collector tab 3c. The positive electrode current collector tab is electrically connected to a strip-shaped positive electrode terminal 7. The tip of the strip-shaped positive electrode terminal 7 is located on the opposite side from the negative electrode terminal 6 and is extended to the outside of the outer casing member 2.

[0059] (Manufacturing method) An example of a method for manufacturing a secondary battery according to this embodiment includes preparing a positive electrode and a negative electrode, housing the positive electrode and negative electrode in an outer casing, injecting an electrolyte into the outer casing, sealing the outer casing to obtain a battery precursor, and obtaining a secondary battery by performing aging after the initial charging of the battery precursor. After aging, gases inside the battery may be removed.

[0060] The positive electrode can be manufactured, for example, by the following method. First, a slurry is prepared by suspending the positive electrode active material, conductive agent, and binder in a solvent. This slurry is applied to one or both sides of the positive electrode current collector. Next, the applied slurry is dried to obtain a laminate of the positive electrode active material-containing layer and the positive electrode current collector. After that, this laminate is pressed. In this way, the positive electrode is manufactured.

[0061] Alternatively, the positive electrode may be manufactured by the following method: First, a positive electrode active material, a conductive agent, and a binder are mixed to obtain a mixture. Next, this mixture is formed into pellets. Then, these pellets are placed on a positive electrode current collector to obtain the positive electrode.

[0062] The negative electrode can be manufactured, for example, by the following method. First, a slurry is prepared by suspending the negative electrode active material, conductive agent, and binder in a solvent. This slurry is applied to one or both sides of the negative electrode current collector. Next, the applied slurry is dried to obtain a laminate of the negative electrode active material-containing layer and the negative electrode current collector. After that, this laminate is pressed. In this way, the negative electrode is manufactured.

[0063] Alternatively, the negative electrode may be manufactured by the following method: First, a negative electrode active material, a conductive agent, and a binder are mixed to obtain a mixture. Next, this mixture is formed into pellets. Then, these pellets are placed on a negative electrode current collector to obtain the negative electrode.

[0064] An electrode group is created by placing a separator between the positive electrode and the negative electrode prepared as described above. Of the electrode group, the positive electrode terminal is electrically connected to the positive electrode, and the negative electrode terminal is electrically connected to the negative electrode.

[0065] An electrode group having positive and negative terminals is housed in a bag-shaped outer casing made of laminate film. The remaining portion is sealed by heat sealing, leaving an opening for electrolyte injection. Next, the electrolyte is injected into the bag-shaped outer casing through the opening, and the opening is sealed by heat sealing. Heat sealing may be performed under reduced pressure. This yields a battery precursor.

[0066] Next, the battery precursor is subjected to an initial charge at room temperature (e.g., 25°C), and then aging is performed at a temperature above room temperature. Alternatively, discharging may be performed after the initial charge before aging.

[0067] After the initial charge and aging process, decomposition of lithium salt occurs, resulting in the formation of a film on at least a portion of the negative electrode surface. Specifically, a film may form on at least a portion of the niobium-containing oxide surface. As a result, the proportion of the niobium-containing oxide on the negative electrode surface may decrease. Therefore, in HAXPES, the ratio B / A of the area A in the range of 200 eV to 215 eV and the area B in the range of 680 eV to 695 eV can be set to a range of 1.1 to 4.3.

[0068] Details of the manufacturing conditions are described below.

[0069] Aging can be performed, for example, on a battery precursor whose battery voltage has been brought to a range of 2.8V to 3.2V by initial charging or discharge after initial charging, in an environment with a temperature of 60°C to 80°C for a period of 1 hour to 48 hours. The battery voltage during aging may also be within the range of 2.8V to 3.0V.

[0070] Performing an initial charge can lower the potential of the negative electrode. Lowering the potential of the negative electrode is preferable because it facilitates the reductive decomposition of lithium salts on the negative electrode surface.

[0071] The higher the battery voltage during aging, the higher the temperature during aging, or the longer the aging time, the more likely lithium salt decomposition will progress.

[0072] The negative electrode, positive electrode, and non-aqueous electrolyte will be described below. Separators, casing members, positive electrode terminals, and negative electrode terminals that the secondary battery of this embodiment may include in addition to these components will also be described below.

[0073] (Negative electrode) The negative electrode may include a negative electrode current collector and a negative electrode active material-containing layer. The negative electrode active material-containing layer may be formed on one or both sides of the negative electrode current collector. The negative electrode active material-containing layer may include the negative electrode active material and optionally a conductive agent and a binder.

[0074] A coating may be formed on at least a portion of the surface of the negative electrode. The coating may contain, for example, fluorine atoms. The fluorine atoms may be included in the coating as, for example, lithium fluoride (LiF). The coating may be present on at least a portion of the surface of the negative electrode active material-containing layer, or it may cover at least a portion of the surface of the negative electrode active material particles. The coating may cover at least a portion of the surface of the niobium-containing oxide. The coating may be in the form of a film or in the form of layers.

[0075] The coating preferably contains lithium fluoride. The coating containing lithium fluoride can effectively suppress the reduction decomposition of carboxylic acid ester on the surface of the negative electrode.

[0076] The negative electrode included in the secondary battery according to the embodiment may be changed from the negative electrode included in the battery precursor at the stage before being subjected to the first charging and aging. This is due to, for example, the reaction between the negative electrode and the electrolyte during the first charging and aging after assembling the battery precursor as described above. In this reaction, a coating may be formed on at least a part of the surface of the negative electrode.

[0077] The niobium-containing oxide may be included, for example, in the negative electrode active material-containing layer as a negative electrode active material. The negative electrode active material may further include other negative electrode active materials other than the niobium-containing oxide.

[0078] The negative electrode containing the niobium-containing oxide can lower the lower limit of the operating potential to 1 V (vs. Li / Li + ) in terms of lithium standard. The upper limit of the operating potential may be, for example, 3 V. In such a negative electrode where the potential is low, the reduction decomposition of the electrolyte tends to occur. However, in a negative electrode where the negative electrode active material is made of a carbon material and the operating potential is too low, the reduction decomposition rate of the carboxylic acid ester is excessively large, so gas is likely to be generated.

[0079] Examples of the niobium-containing oxide include niobium pentoxide (Nb2O5) and monoclinic niobium titanate. The type of the niobium-containing oxide can be one kind or two or more kinds.

[0080] As an example of the monoclinic niobium titanate, a compound represented by the general formula Li x Ti 1-y M1 y Nb 2-z M2 z O 7+δ , and a compound represented by the general formula Li x Ti 1-y M3 y+z Nb 2-z O 7-δCompounds represented by the formula are shown below. Here, M1 is at least one element selected from the group consisting of Zr, Si, and Sn. M2 is at least one element selected from the group consisting of V, Ta, Bi, K, Ca, B, Co, Fe, Mn, Ni, Si, P, and Mo. M3 is at least one element selected from the group consisting of Mg, Fe, Ni, Co, W, Ta, and Mo. The subscripts in the empirical formula represent 0≦x≦5, 0≦y≦1, 0≦z<2, and -0.3≦δ≦0.3. General formula Li x Ti 1-y M1 y Nb 2-z M2 z O 7+δ A specific example of monoclinic niobium titanium oxide represented by Li x Nb2TiO7 (0≦x≦5) is one example.

[0081] Other examples of negative electrode active materials besides niobium-containing oxides include lithium titanate (e.g., Li) having a ramsdelite structure. 2+y Li3O7 (0≦y≦3), lithium titanate having a spinel structure (e.g., Li 4+x Ti5O 12 Examples include titanium dioxide (TiO2), anatase-type titanium dioxide, rutile-type titanium dioxide, hollandite-type titanium composite oxide, and orthorhombic titanium composite oxide. The other types of negative electrode active materials besides niobium-containing oxides can be one or more.

[0082] As an example of the above orthorhombic titanium-containing composite oxide, Li 2+a M I 2-b Ti 6-c M II d O 14+σ Examples of compounds represented by are given. Here, M I It is at least one selected from the group consisting of Sr, Ba, Ca, Mg, Na, Cs, Rb, and K. IIis at least one selected from the group consisting of Zr, Sn, V, Nb, Ta, Mo, W, Y, Fe, Co, Cr, Mn, Ni, and Al. In the composition formula, each subscript has the following properties: 0≦a≦6, 0≦b<2, 0≦c<6, 0≦d<6, -0.5≦σ≦0.5. As a specific example of an orthorhombic titanium-containing composite oxide, Li 2+a Na2Li6O 14 (0 ≤ a ≤ 6) is one example.

[0083] As an example of the above orthorhombic titanium-containing composite oxide, Li 2+a M I 2-b Ti 6-c M II d O 14+σ Examples of compounds represented by are given. Here, M I It is at least one selected from the group consisting of Sr, Ba, Ca, Mg, Na, Cs, Rb, and K. II is at least one selected from the group consisting of Zr, Sn, V, Nb, Ta, Mo, W, Y, Fe, Co, Cr, Mn, Ni, and Al. In the composition formula, each subscript has the following properties: 0≦a≦6, 0≦b<2, 0≦c<6, 0≦d<6, -0.5≦σ≦0.5. As a specific example of an orthorhombic titanium-containing composite oxide, Li 2+a Na2Li6O 14 (0 ≤ a ≤ 6) is one example.

[0084] Conductive agents are added to enhance current collection performance and reduce contact resistance between the negative electrode active material and the negative electrode current collector. Examples of conductive agents include carbonaceous materials such as vapor-grown carbon fiber (VGCF), carbon black such as acetylene black, graphite, carbon nanotubes, and carbon nanofibers. One of these may be used as a conductive agent, or two or more may be used in combination. Alternatively, instead of using a conductive agent, the surface of the negative electrode active material particles may be coated with a carbon coating or an electronically conductive inorganic material coating.

[0085] Binding agents are added to fill the gaps between dispersed negative electrode active materials and to bind the negative electrode active materials to the negative electrode current collector. Examples of binding agents include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), fluororubber, styrene-butadiene rubber, polyacrylic acid compounds, imide compounds, carboxymethyl cellulose (CMC), and salts of CMC. One of these may be used as a binding agent, or two or more may be used in combination.

[0086] The binder may contain fluorine atoms, as explained earlier. However, it is preferable that the ratio C / B in the HAXPES on the surface of the negative electrode is 0.2 or less.

[0087] In the negative electrode active material-containing layer, it is preferable that the negative electrode active material, conductive agent, and binder are blended in proportions of 68% to 96% by mass, 2% to 30% by mass, and 2% to 30% by mass, respectively. By setting the amount of conductive agent to 2% by mass or more, the current collection performance of the negative electrode active material-containing layer can be improved. Furthermore, by setting the amount of binder to 2% by mass or more, sufficient bonding between the negative electrode active material-containing layer and the negative electrode current collector can be achieved, and excellent cycle performance can be expected. On the other hand, it is preferable to set the amount of conductive agent and binder to 30% by mass or less, respectively, in order to achieve high capacity. In addition, since the binder is an insulating component, by reducing its blending ratio in the negative electrode active material-containing layer to 30% by mass or less, the resistance of the negative electrode can be reduced. Therefore, this contributes to improving output performance.

[0088] The density of the negative electrode active material layer (excluding the current collector) is 2.0 g / cm³. 3 More than 3.2g / cm 3 The following is preferable. A negative electrode with a density of the negative electrode active material-containing layer within this range exhibits excellent energy density and electrolyte retention. The density of the negative electrode active material-containing layer is 2.2 g / cm³. 3 More than 3.0g / cm 3It is more preferable that it is as follows.

[0089] For the negative electrode current collector, a material that is electrochemically stable at the potential at which lithium (Li) is inserted into and extracted from the negative electrode active material is used. For example, the negative electrode current collector is preferably made of copper, nickel, stainless steel or aluminum, or an aluminum alloy containing one or more elements selected from Mg, Ti, Zn, Mn, Fe, Cu, and Si. The thickness of the negative electrode current collector is preferably 5 μm or more and 20 μm or less. The negative electrode current collector having such a thickness can balance the strength and weight reduction of the negative electrode.

[0090] Further, the negative electrode current collector can include a portion where the negative electrode active material-containing layer is not formed on its surface. This portion can function as a negative electrode current collecting tab.

[0091] (Positive electrode) The positive electrode can include a positive electrode current collector and a positive electrode active material-containing layer. The positive electrode active material-containing layer can be formed on one or both sides of the positive electrode current collector. The positive electrode active material-containing layer can include a positive electrode active material, and optionally a conductive agent and a binder.

[0092] As the positive electrode active material, for example, an oxide or a sulfide can be used. The positive electrode may contain one kind of compound alone as the positive electrode active material, or may contain a combination of two or more kinds of compounds. Examples of the oxide and the sulfide include compounds into which Li or Li ions can be inserted and extracted.

[0093] Examples of such compounds include manganese dioxide (MnO2), iron oxide, copper oxide, nickel oxide, lithium manganese composite oxide (for example, Li x Mn2O4 or Li x MnO2; 0 <x ≦ 1), lithium nickel composite oxide (for example, Li x NiO2; 0 <x ≦ 1), lithium cobalt composite oxide (for example, Li x CoO2; 0 <x ≦ 1), lithium nickel cobalt composite oxide (for example, Li xNi 1-y Co y O2; 0 < x ≤ 1, 0 < y < 1), lithium manganese cobalt composite oxide (e.g., Li x Mn y Co 1-y O2; 0 < x ≤ 1, 0 < y < 1), lithium manganese nickel composite oxide with spinel structure (e.g., Li x Mn 2-y Ni y O4; 0 < x ≤ 1, 0 < y < 2), lithium phosphate with olivine structure (e.g., Li x FePO4; 0 < x ≤ 1, Li x Fe 1-y Mn y PO4; 0 < x ≤ 1, 0 < y ≤ 1, Li x CoPO4; 0 < x ≤ 1), iron sulfate (Fe2(SO4)3), vanadium oxide (e.g., V2O5), and lithium nickel cobalt manganese composite oxide (Li x Ni 1-y-z Co y Mn z O2; 0 < x ≤ 1, 0 < y < 1, 0 < z < 1, y + z < 1) is included.

[0094] Among the above, examples of more preferable compounds as the positive electrode active material include lithium manganese composite oxide with spinel structure (e.g., Li x Mn2O4; 0 < x ≤ 1), lithium nickel composite oxide (e.g., Li x NiO2; 0 < x ≤ 1), lithium cobalt composite oxide (e.g., Li x CoO2; 0 < x ≤ 1), lithium nickel cobalt composite oxide (e.g., Li x Ni 1-y Co y O2; 0 < x ≤ 1, 0 < y < 1), lithium manganese nickel composite oxide with spinel structure (e.g., Li x Mn 2-y Ni y O4; 0 < x ≤ 1, 0 < y < 2), lithium manganese cobalt composite oxide (e.g., Li x Mn y Co 1-y O2; 0 < x ≤ 1, 0 < y < 1), lithium iron phosphate (e.g., Li xFePO4; 0 < x ≤ 1), and lithium nickel cobalt manganese composite oxide (Li x Ni 1-y-z Co y Mn z O2; 0 < x ≤ 1, 0 < y < 1, 0 < z < 1, y + z < 1) are included. When these compounds are used as the positive electrode active material, the positive electrode potential can be increased.

[0095] When a room-temperature molten salt is used as the electrolyte of the battery, it is preferable to use a positive electrode active material containing lithium iron phosphate, Li x VPO4F (0 ≤ x ≤ 1), lithium manganese composite oxide, lithium nickel composite oxide, lithium nickel cobalt composite oxide, or a mixture thereof. Since these compounds have low reactivity with the room-temperature molten salt, the cycle life can be improved. Details of the room-temperature molten salt will be described later.

[0096] The primary particle size of the positive electrode active material is preferably 100 nm or more and 1 μm or less. A positive electrode active material with a primary particle size of 100 nm or more is easy to handle in industrial production. A positive electrode active material with a primary particle size of 1 μm or less can allow the solid-state diffusion of lithium ions to proceed smoothly.

[0097] The specific surface area of the positive electrode active material is preferably 0.1 m 2 / g or more and 10 m 2 / g or less. A positive electrode active material with a specific surface area of 0.1 m 2 / g or more can sufficiently secure the sites for the insertion and extraction of Li ions. A positive electrode active material with a specific surface area of 10 m 2 / g or less is easy to handle in industrial production and can ensure good charge-discharge cycle performance.

[0098] A binder is added to fill the gaps between dispersed positive electrode active materials and to bond the positive electrode active materials to the positive electrode current collector. Examples of binders include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), fluororubber, polyacrylic acid compounds, imide compounds, carboxymethyl cellulose (CMC), and salts of CMC. One of these may be used as a binder, or two or more may be used in combination.

[0099] Conductive agents are added to enhance current collection performance and reduce contact resistance between the positive electrode active material and the positive electrode current collector. Examples of conductive agents include vapor-grown carbon fiber (VGCF), carbon black such as acetylene black, and carbonaceous materials such as graphite. One of these may be used as a conductive agent, or two or more may be used in combination. Conductive agents may also be omitted.

[0100] In the positive electrode active material-containing layer, it is preferable that the positive electrode active material and the binder are blended in proportions of 80% to 98% by mass and 2% to 20% by mass, respectively.

[0101] Sufficient electrode strength can be obtained by using a binder amount of 2% by mass or more. Furthermore, the binder can function as an insulator. Therefore, reducing the binder amount to 20% by mass or less reduces the amount of insulator contained in the electrode, thereby reducing internal resistance.

[0102] When a conductive agent is added, it is preferable that the positive electrode active material, binder, and conductive agent are blended in proportions of 77% to 95% by mass, 2% to 20% by mass, and 3% to 15% by mass, respectively.

[0103] The above-mentioned effects can be achieved by increasing the amount of conductive agent to 3% by mass or more. Furthermore, by reducing the amount of conductive agent to 15% by mass or less, the proportion of conductive agent in contact with the electrolyte can be reduced. This lower proportion reduces the decomposition of the electrolyte under high-temperature storage conditions.

[0104] The positive electrode current collector is preferably an aluminum foil, or an aluminum alloy foil containing one or more elements selected from Mg, Ti, Zn, Ni, Cr, Mn, Fe, Cu, and Si.

[0105] The thickness of the aluminum foil or aluminum alloy foil is preferably 5 μm or more and 20 μm or less, and more preferably 15 μm or less. The purity of the aluminum foil is preferably 99% by mass or more. The content of transition metals such as iron, copper, nickel, and chromium in the aluminum foil or aluminum alloy foil is preferably 1% by mass or less.

[0106] Furthermore, the positive electrode current collector may include portions on its surface where the positive electrode active material-containing layer is not formed. These portions can function as positive electrode current collector tabs.

[0107] The positive electrode can be fabricated, for example, using a positive electrode active material in the same manner as the negative electrode.

[0108] (electrolyte) As the electrolyte, for example, a liquid non-aqueous electrolyte or a gel-type non-aqueous electrolyte can be used. A liquid non-aqueous electrolyte is prepared by dissolving an electrolyte salt as a solute in a solvent. The concentration of the electrolyte salt is preferably 0.5 mol / kg or more and 2.5 mol / kg or less.

[0109] Lithium salts may be included in the electrolyte as electrolyte salts. Primary carboxylic acid esters may be included in the electrolyte, for example, as solvents. The electrolyte may further contain other electrolyte salts of lithium salts, and may further contain other solvents of primary carboxylic acid esters.

[0110] The composition of the electrolyte contained in the secondary battery of the embodiment, particularly the concentration of lithium salt, may have changed from the electrolyte composition immediately after preparation (initial composition). This is because, for example, after assembling the battery precursor using the electrolyte immediately after preparation, the lithium salt may decompose during the initial charging, aging, etc.

[0111] Examples of lithium salts include lithium hexafluoride phosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(pentafluoroethylsulfonyl)imide (LiBETI), lithium tetrafluoroborate (LiBF4), lithium difluoro(oxalato)borate (LiDFOB), lithium difluorophosphate (LiDFP), lithium arsenide hexafluoride (LiAsF6), lithium trifluoromethanesulfonate (LiCF3SO3), and lithium bistrifluoromethylsulfonylimide (LiN(CF3SO2)2) and lithium perchlorate (LiClO4). The number of lithium salts can be one or more. The lithium salt is preferably one that is difficult to oxidize even at high potentials, and LiPF6 is the most preferred.

[0112] The lithium salt preferably contains fluorine atoms. A lithium salt containing fluorine atoms can be reduced and decomposed, for example, during the initial charge and / or aging. Therefore, when the electrolyte contains a lithium salt containing fluorine atoms, a film containing lithium fluoride (LiF) can be formed on the negative electrode surface.

[0113] Preferred examples of lithium salts containing a fluorine atom include lithium hexafluoride phosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(pentafluoroethylsulfonyl)imide (LiBETI), lithium tetrafluoroborate (LiBF4), lithium difluoro(oxalato)borate (LiDFOB), and lithium difluorophosphate (LiDFP).

[0114] Examples of primary carboxylic acid esters that the organic solvent may contain include, for example, propyl propionate (PP), butyl butanoate, and ethyl propionate. The type of primary carboxylic acid ester can be one or more.

[0115] Other examples of solvents for primary carboxylic acid esters include cyclic carbonates such as propylene carbonate (PC), ethylene carbonate (EC), and vinylene carbonate (VC); linear carbonates such as diethyl carbonate (DEC), dimethyl carbonate (DMC), and methyl ethyl carbonate (MEC); cyclic ethers such as tetrahydrofuran (THF), 2-methyl tetrahydrofuran (2MeTHF), and dioxolane (DOX); linear ethers such as dimethoxyethane (DME) and diethoxyethane (DEE); and γ-butyrolactone (GBL), acetonitrile (AN), and sulfolane (SL). The other solvent types can be one or more.

[0116] Gel-like non-aqueous electrolytes are prepared by compounding a liquid non-aqueous electrolyte with a polymer material. Examples of polymer materials include polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyethylene oxide (PEO), or mixtures thereof.

[0117] In addition to liquid nonaqueous electrolytes and gel-type nonaqueous electrolytes, the nonaqueous electrolyte may further include lithium-ion-containing room-temperature molten salts (ionic melts), polymer solid electrolytes, and inorganic solid electrolytes.

[0118] Room temperature molten salts (ionic melts) refer to organic salts consisting of a combination of organic cations and anions that can exist as liquids at room temperature (15°C to 25°C). Room temperature molten salts include room temperature molten salts that exist as liquids on their own, room temperature molten salts that become liquid when mixed with an electrolyte salt, room temperature molten salts that become liquid when dissolved in an organic solvent, or mixtures thereof. Generally, the melting point of room temperature molten salts used in secondary batteries is 25°C or lower. Also, organic cations generally have a quaternary ammonium skeleton.

[0119] Polymeric solid electrolytes are prepared by dissolving an electrolyte salt in a polymer material and then solidifying it.

[0120] Inorganic solid electrolytes are solid materials that have lithium ion conductivity. Here, "having lithium ion conductivity" means that at 25°C, they have a conductivity of 1 × 10⁻⁶. -6 This refers to exhibiting a lithium ion conductivity of S / cm or higher. Examples of inorganic solid electrolytes include oxide-based solid electrolytes and sulfide-based solid electrolytes. Specific examples of inorganic solid electrolytes are as follows.

[0121] As an oxide-based solid electrolyte, it has a NASICON (Sodium (Na) Super Ionic Conductor) type structure, and its general formula is Li 1+x It is preferable to use a lithium phosphate solid electrolyte represented by Mα2(PO4)3. In the above general formula, Mα is one or more selected from the group consisting of, for example, titanium (Ti), germanium (Ge), strontium (Sr), zirconium (Zr), tin (Sn), aluminum (Al), and calcium (Ca). The subscript x is in the range of 0 ≤ x ≤ 2.

[0122] A specific example of a lithium phosphate solid electrolyte having a NASICON-type structure is Li1+x Al x Ti 2-x (PO4)3 - represented LATP compound where 0.1 ≦ x ≦ 0.5; Li 1+x Al y Mβ 2-y (PO4)3 - represented compound where Mβ is one or more selected from the group consisting of Ti, Ge, Sr, Zr, Sn, and Ca and 0 ≦ x ≦ 1 and 0 ≦ y ≦ 1; Li 1+x Al x Ge 2-x (PO4)3 - represented compound where 0 ≦ x ≦ 2; and, Li 1+x Al x Zr 2-x (PO4)3 - represented compound where 0 ≦ x ≦ 2; Li 1+x+y Al x Mγ 2-x Si y P 3-y O 12 - represented compound where Mγ is one or more selected from the group consisting of Ti and Ge and 0 < x ≦ 2, 0 ≦ y < 3; Li 1+2x Zr 1-x Ca x (PO4)3 - represented compounds where 0 ≦ x < 1 can be mentioned.

[0123] Also, as the oxide - based solid electrolyte, in addition to the above - mentioned lithium phosphate solid electrolyte, Li x PO y N z - represented amorphous LIPON compound where 2.6 ≦ x ≦ 3.5, 1.9 ≦ y ≦ 3.8, and 0.1 ≦ z ≦ 1.3 (for example, Li 2.9 PO 3.3 N 0.46 ); Garnet - type structure La 5+x A x La 3-x Mδ2O 12 - represented compound where A is one or more selected from the group consisting of Ca, Sr, and Ba and Mδ is one or more selected from the group consisting of Nb and Ta and 0 ≦ x ≦ 0.5; Li3Mδ 2-x L2O 12 - represented compound where Mδ is one or more selected from the group consisting of Nb and Ta and L may contain Zr and 0 ≦ x ≦ 0.5; Li7-3x Al x La3Zr3O 12 Compounds represented by 0 ≤ x ≤ 0.5; Li 5+x La3MCSR 2-x Zr x O 12 Represented by , where Mδ is 1 or more selected from the group consisting of Nb and Ta, and 0 ≤ x ≤ 2, it is an LLZ compound (e.g., Li7La3Zr2O 12 ); and having a perovskite-type structure La 2 / 3-x Li x Examples include compounds represented as TiO3 where 0.3 ≤ x ≤ 0.7.

[0124] One or more of the above compounds can be used as a solid electrolyte. Two or more of the above solid electrolytes may also be used.

[0125] (Separator) The separator is formed from a porous film containing, for example, polyethylene (PE), polypropylene (PP), cellulose, or polyvinylidene fluoride (PVdF), or from a synthetic resin nonwoven fabric. From a safety standpoint, it is preferable to use a porous film made of polyethylene or polypropylene. This is because these porous films can melt at a certain temperature and interrupt the electric current.

[0126] (Exterior components) For example, the outer packaging material can be a container made of laminate film or a metal container.

[0127] The thickness of the laminating film is, for example, 0.5 mm or less, and preferably 0.2 mm or less.

[0128] As the laminate film, a multilayer film is used that includes multiple resin layers and a metal layer interposed between these resin layers. The resin layers include polymer materials such as polypropylene (PP), polyethylene (PE), nylon, and polyethylene terephthalate (PET). The metal layer is preferably made of aluminum foil or aluminum alloy foil for weight reduction. The laminate film can be molded into the shape of an exterior component by sealing it by heat fusion.

[0129] The thickness of the metal container wall is, for example, 1 mm or less, more preferably 0.5 mm or less, and even more preferably 0.2 mm or less.

[0130] Metal containers are made from, for example, aluminum or aluminum alloys. Aluminum alloys preferably contain elements such as magnesium, zinc, and silicon. If aluminum alloys contain transition metals such as iron, copper, nickel, and chromium, their content is preferably 100 ppm by mass or less.

[0131] The shape of the exterior components is not particularly limited. For example, the exterior components may be flat (thin), rectangular, cylindrical, coin-shaped, or button-shaped. The exterior components can be appropriately selected according to the battery dimensions and intended use.

[0132] (Negative terminal) The negative terminal has a potential range of 1V to 3V relative to the oxidation-reduction potential of lithium (vs.Li / Li +The negative electrode terminal can be formed from an electrically stable and conductive material. Specifically, examples of materials for the negative electrode terminal include copper, nickel, stainless steel, or aluminum, or an aluminum alloy containing at least one element selected from the group consisting of Mg, Ti, Zn, Mn, Fe, Cu, and Si. It is preferable to use aluminum or an aluminum alloy as the material for the negative electrode terminal. It is preferable that the negative electrode terminal be made of the same material as the negative electrode current collector in order to reduce contact resistance with the negative electrode current collector.

[0133] (Positive terminal) The positive terminal has a potential range of 3V to 4.5V relative to the oxidation-reduction potential of lithium (vs.Li / Li + The positive electrode terminal can be formed from an electrically stable and conductive material. Examples of positive electrode terminal materials include aluminum, or an aluminum alloy containing at least one element selected from the group consisting of Mg, Ti, Zn, Mn, Fe, Cu, and Si. It is preferable that the positive electrode terminal be formed from the same material as the positive electrode current collector in order to reduce contact resistance with the positive electrode current collector.

[0134] (Measurement method) The measurement method for a secondary battery according to this embodiment will be described below.

[0135] When measuring electrodes, if the electrode to be measured (e.g., the negative electrode) is incorporated into a battery, the electrode to be measured is removed from the battery as follows: First, the battery containing the electrode is disassembled in a glove box filled with argon. The electrode to be measured is removed from the disassembled battery. This electrode is washed with a suitable solvent. For example, methyl ethyl carbonate can be used as the solvent for washing. The washed electrode is then vacuum-dried. Thus, a measuring electrode is obtained.

[0136] To measure the electrolyte, disassemble the battery in a glove box filled with argon, as described above. Then, collect the electrolyte using a pipette or similar tool. If it is difficult to collect the electrolyte with a pipette, the electrolyte may be extracted with a solvent such as super-dehydrated acetonitrile. Thus, obtain the electrolyte for measurement.

[0137] (Electrolyte composition analysis) The types and amounts of substances contained in an electrolyte can be analyzed by ion chromatography. By identifying the types of substances, their structures can also be determined, allowing for the identification of the r and r' of the carboxylic acid esters contained in the electrolyte. Gas chromatography can be used as the ion chromatography method.

[0138] The electrolyte obtained as described above is diluted 20 times in volume with acetonitrile to prepare the measurement sample. Gas chromatography-mass spectrometry (GC-MS) is performed on the measurement sample under the conditions shown in Table 1.

[0139] [Table 1]

[0140] The content (mass %) of carboxylic acid esters in an electrolyte excluding lithium salts can be measured as follows. First, the content of lithium ions, the anions corresponding to lithium ions, and the carboxylic acid esters in the electrolyte is measured using the method described above. The content of lithium ions and the anions corresponding to lithium ions corresponds to the content of lithium salts contained in the electrolyte. Therefore, by subtracting the content of lithium salts from the amount of electrolyte, the amount of electrolyte excluding lithium salts can be calculated. The mass percentage concentration of carboxylic acid esters when this calculated "amount of electrolyte excluding lithium salts" is taken as 100 mass % can be calculated as the "content of carboxylic acid esters in 100 mass % of electrolyte excluding lithium salts." In other words, "excluding lithium salts" actually means subtracting the amount of lithium salts from the amount of electrolyte in order to use it as a reference value in the formulation calculation for electrolytes that actually contain lithium salts.

[0141] (HAXPES analysis) Analysis using HAXPES can be performed as follows:

[0142] The measurement electrode (e.g., negative electrode) extracted using the method described above is attached to the HAXPES sample folder with carbon tape in a glove box filled with argon. The sample folder is placed in a transfer vessel, sealed, and the vessel is removed from the glove box and attached to the sample inlet of the HAXPES instrument. By transferring the sample folder within the instrument, the measurement electrode is introduced into the instrument without exposure to air. The HAXPES analysis is performed as follows: The energy is calibrated with Au4f7 / 2 (5870eV) before measurement. The excitation energy is 5956.01eV. The beam size is 20μm x 30μm. The electron analyzer (Scienta Omicron R4000 L1) is set to 10keV. The photoelectron detection angle is set to an oblique incidence configuration of approximately 88.5°. Charge neutralization is not used.

[0143] The background is subtracted from the results obtained from the above analysis.

[0144] Background subtraction and area calculation can be performed using the Shirley method. For example, when measuring area A in the range of 200 eV to 215 eV, background subtraction is performed using the Shirley method for the binding energy in the range of 200 eV to 215 eV from the HAXPES analysis results. After background subtraction, a chart is obtained with normalized intensity on the vertical axis and binding energy (in eV) on the horizontal axis. This chart is taken as the HAXPES spectrum. The HAXPES spectrum is integrated with the binding energy in the range of 200 eV to 215 eV as the integration range. In this way, area A is measured.

[0145] Areas B and C can also be measured from the HAXPES spectra obtained in the same manner as described above. However, the range of binding energies used for background subtraction and integration in area measurement is changed to 680 eV to 695 eV for area B and 290 eV to 295 eV for area C.

[0146] Figure 10 shows an example of a HAXPES spectrum obtained using the method described above. Figure 10 illustrates the range of binding energies from 200 eV to 216 eV. In Figure 10, a1 is the HAXPES spectrum obtained for the negative electrode of Example 1, and a2 is the HAXPES spectrum obtained for the negative electrode of Comparative Example 1, both obtained by performing the HAXPES and background subtraction described above.

[0147] Furthermore, Figure 11 shows other examples of HAXPES spectra obtained using the method described above. Figure 11 illustrates the range of binding energies from 680 eV to 695 eV. In Figure 11, b1 represents the HAXPES spectrum obtained for the negative electrode of Example 1, and b2 represents the HAXPES spectrum obtained for the negative electrode of Comparative Example 1, using the HAXPES and background subtraction methods described above.

[0148] In other words, the area within the range of 200 eV to 215 eV for spectrum a1 in Figure 10 corresponds to area A for Example 1. The area within the range of 680 eV to 695 eV for spectrum b1 in Figure 11 corresponds to area B for Example 1. From this, the ratio B / A for Example 1 can be calculated. Similarly, from the area within the range of 200 eV to 215 eV for spectrum a2 in Figure 10, and the area within the range of 680 eV to 695 eV for spectrum b2 in Figure 11, the areas A, B, and ratio B / A for Comparative Example 1 can be calculated.

[0149] As is clear from Figures 10 and 11, in the range of 200 eV to 215 eV, the difference in area between spectrum a1 and spectrum a2 is relatively small. In contrast, in the range of 680 eV to 695 eV, the area of ​​spectrum b2 is significantly smaller than that of spectrum b1. In other words, from Figures 10 and 11, it is clear that the ratio B / A in Example 1 is larger than the ratio B / A in Comparative Example 1.

[0150] According to the first embodiment, a secondary battery is provided. The secondary battery comprises a positive electrode, a negative electrode, and an electrolyte. The negative electrode contains a niobium-containing oxide. The ratio B / A of the area A in the range of 200 eV to 215 eV and the area B in the range of 680 eV to 695 eV, as measured by hard X-ray photoelectron spectroscopy on the surface of the negative electrode, is 1.1 to 4.3. The electrolyte contains a lithium salt and a carboxylic acid ester represented by formula A. (Formula A)

[0151] [ka]

[0152] In formula A, R and R' are both hydrocarbon groups. Let r be the number of carbon atoms in R and r' be the number of carbon atoms in R'. Then r and r' satisfy 5 ≤ (r+1) + r' ≤ 8, and the value of |(r+1) - r'| is 0 or 1. The carboxylic acid ester content is 70% to 95% by mass relative to 100% by mass of the electrolyte excluding the lithium salt. Therefore, a secondary battery with excellent charge-discharge cycle life can be realized.

[0153] (Second embodiment) According to the second embodiment, a battery pack is provided, which comprises a plurality of secondary batteries according to the first embodiment.

[0154] In such a battery pack, each individual cell may be arranged in series or parallel connections, or a combination of series and parallel connections may be used.

[0155] Next, an example of a battery pack according to the embodiment will be described with reference to the drawings.

[0156] Figure 5 is a schematic perspective view showing an example of a battery pack. The battery pack 200 shown in Figure 5 comprises five single cells 100a to 100e, four busbars 21, a positive electrode lead 22, and a negative electrode lead 23. Each of the five single cells 100a to 100e is a secondary battery according to the first embodiment.

[0157] The busbar 21 connects, for example, the negative terminal 6 of one cell 100a to the positive terminal 7 of the adjacent cell 100b. In this way, the five cells 100 are connected in series by four busbars 21. That is, the battery pack 200 in Figure 5 is a battery pack with five cells in series. Although not illustrated, in a battery pack containing multiple cells that are electrically connected in parallel, the multiple cells can be electrically connected, for example, by connecting multiple negative terminals to each other and multiple positive terminals to each other by busbars.

[0158] The positive terminal 7 of at least one of the five single cells 100a to 100e is electrically connected to the positive lead 22 for external connection. In addition, the negative terminal 6 of at least one of the five single cells 100a to 100e is electrically connected to the negative lead 23 for external connection.

[0159] The battery pack according to the second embodiment comprises the secondary battery according to the first embodiment. Therefore, the charge-discharge cycle life performance can be improved.

[0160] (Third embodiment) According to a third embodiment, a battery pack is provided. This battery pack comprises a battery pack according to the second embodiment. This battery pack may also comprise a single secondary battery according to the first embodiment instead of the battery pack according to the second embodiment.

[0161] The battery pack may further include a protection circuit. The protection circuit has the function of controlling the charging and discharging of the secondary battery. Alternatively, a circuit included in a device that uses the battery pack as a power source (e.g., electronic equipment, automobile, etc.) may be used as the protection circuit for the battery pack.

[0162] Furthermore, such a battery pack may also be equipped with external terminals for power supply. These external terminals are for outputting current from the secondary battery to the outside and / or for inputting current from the outside to the secondary battery. In other words, when the battery pack is used as a power source, current is supplied to the outside through the external terminals. Also, when charging the battery pack, the charging current (including regenerative energy from the power of an automobile, etc.) is supplied to the battery pack through the external terminals.

[0163] Next, an example of a battery pack according to the embodiment will be described with reference to the drawings.

[0164] Figure 6 is an exploded perspective view schematically showing an example of a battery pack. Figure 7 is a block diagram showing an example of the electrical circuit of the battery pack shown in Figure 6.

[0165] The battery pack 300 shown in Figures 6 and 7 comprises a housing container 31, a lid 32, a protective sheet 33, a battery pack 200, a printed circuit board 34, wiring 35, and an insulating plate (not shown).

[0166] The container 31 shown in Figure 6 is a bottomed rectangular container with a rectangular base. The container 31 is configured to accommodate a protective sheet 33, a battery pack 200, a printed circuit board 34, and wiring 35. The lid 32 has a rectangular shape. The lid 32 covers the container 31, thereby housing the battery pack 200 and the other components. The container 31 and the lid 32 are provided with openings or connection terminals for connecting to external devices, etc., although these are not shown in the figures.

[0167] The battery pack 200 comprises multiple individual cells 100, a positive electrode lead 22, a negative electrode lead 23, and an adhesive tape 24.

[0168] At least one of the multiple single cells 100 is a secondary battery according to the first embodiment. Each of the multiple single cells 100 is electrically connected in series, as shown in Figure 7. The multiple single cells 100 may also be electrically connected in parallel, or they may be connected in a combination of series and parallel connections. When the multiple single cells 100 are connected in parallel, the battery capacity increases compared to when they are connected in series.

[0169] The adhesive tape 24 fastens multiple single cells 100 together. Alternatively, heat-shrinkable tape may be used to secure the multiple single cells 100 instead of the adhesive tape 24. In this case, protective sheets 33 are placed on both sides of the battery pack 200, the heat-shrinkable tape is wrapped around it, and then the heat-shrinkable tape is heat-shrinked to bundle the multiple single cells 100 together.

[0170] One end of the positive lead 22 is connected to the battery pack 200. One end of the positive lead 22 is electrically connected to the positive terminal of one or more single cells 100. One end of the negative lead 23 is connected to the battery pack 200. One end of the negative lead 23 is electrically connected to the negative terminal of one or more single cells 100.

[0171] The printed circuit board 34 is installed along one of the shorter sides of the inner surface of the housing container 31. The printed circuit board 34 includes a positive terminal connector 342, a negative terminal connector 343, a thermistor 345, a protection circuit 346, wiring 342a and 343a, an external terminal 350 for energization, a positive side wiring (positive wiring) 348a, and a negative side wiring (negative wiring) 348b. One main surface of the printed circuit board 34 faces one side of the battery pack 200. An insulating plate (not shown) is interposed between the printed circuit board 34 and the battery pack 200.

[0172] The other end 22a of the positive lead 22 is electrically connected to the positive connector 342. The other end 23a of the negative lead 23 is electrically connected to the negative connector 343.

[0173] The thermistor 345 is fixed to one main surface of the printed circuit board 34. The thermistor 345 detects the temperature of each of the single cells 100 and transmits the detection signal to the protection circuit 346.

[0174] The external power supply terminal 350 is fixed to the other main surface of the printed circuit board 34. The external power supply terminal 350 is electrically connected to equipment located outside the battery pack 300. The external power supply terminal 350 includes a positive terminal 352 and a negative terminal 353.

[0175] The protection circuit 346 is fixed to the other main surface of the printed circuit board 34. The protection circuit 346 is connected to the positive terminal 352 via the positive side wiring 348a. The protection circuit 346 is connected to the negative terminal 353 via the negative side wiring 348b. The protection circuit 346 is also electrically connected to the positive side connector 342 via wiring 342a. The protection circuit 346 is also electrically connected to the negative side connector 343 via wiring 343a. Furthermore, the protection circuit 346 is electrically connected to each of the multiple single cells 100 via wiring 35.

[0176] The protective sheet 33 is positioned on both inner surfaces in the long-side direction of the housing container 31 and on the inner surface in the short-side direction facing the printed circuit board 34 via the battery pack 200. The protective sheet 33 is made of, for example, resin or rubber.

[0177] The protection circuit 346 controls the charging and discharging of multiple single cells 100. The protection circuit 346 also disconnects the electrical connection between the protection circuit 346 and the external terminals 350 (positive terminal 352, negative terminal 353) for supplying power to external devices, based on a detection signal transmitted from the thermistor 345 or a detection signal transmitted from an individual single cell 100 or a battery pack 200.

[0178] An example of a detection signal transmitted from the thermistor 345 is a signal indicating that the temperature of a single cell 100 is above a predetermined temperature. An example of a detection signal transmitted from an individual single cell 100 or a battery pack 200 is a signal indicating that overcharging, over-discharging, or overcurrent has been detected in a single cell 100. When detecting overcharging, etc., in an individual single cell 100, the battery voltage may be detected, or the positive electrode potential or negative electrode potential may be detected. In the latter case, a lithium electrode to be used as a reference electrode is inserted into each individual single cell 100.

[0179] Furthermore, the protection circuit 346 may be a circuit included in a device that uses the battery pack 300 as a power source (for example, an electronic device, an automobile, etc.).

[0180] Furthermore, as described above, the battery pack 300 is equipped with an external terminal 350 for power supply. Therefore, the battery pack 300 can output current from the battery pack 200 to an external device and input current from an external device to the battery pack 200 via the external terminal 350. In other words, when the battery pack 300 is used as a power source, current from the battery pack 200 is supplied to the external device through the external terminal 350. Also, when charging the battery pack 300, charging current from an external device is supplied to the battery pack 300 through the external terminal 350. When this battery pack 300 is used as an on-board battery, the regenerative energy of the vehicle's power can be used as the charging current from the external device.

[0181] The battery pack 300 may have multiple battery packs 200. In this case, the multiple battery packs 200 may be connected in series, in parallel, or in a combination of series and parallel connections. The printed circuit board 34 and wiring 35 may also be omitted. In this case, the positive lead 22 and the negative lead 23 may be used as the positive terminal 352 and negative terminal 353 of the external terminal 350 for energization, respectively.

[0182] Such battery packs are used in applications where excellent cycle performance is required, for example, when drawing high currents. Specifically, these battery packs are used as power supplies for electronic devices, stationary batteries, and on-board batteries for various vehicles. Examples of electronic devices include digital cameras. These battery packs are particularly suitable for use as on-board batteries.

[0183] The battery pack according to the third embodiment includes a secondary battery according to the first embodiment or a battery pack according to the second embodiment. Therefore, the charge-discharge cycle life performance can be improved.

[0184] (Fourth embodiment) According to the fourth embodiment, a vehicle is provided, which is equipped with the battery pack according to the third embodiment.

[0185] In such a vehicle, the battery pack, for example, recovers regenerative energy from the vehicle's power. The vehicle may also include a mechanism (regenerator) that converts the vehicle's kinetic energy into regenerative energy.

[0186] Examples of vehicles include, for example, two-wheeled or four-wheeled hybrid electric vehicles, two-wheeled or four-wheeled electric vehicles, electric assist bicycles, and railway vehicles.

[0187] The mounting location of the battery pack in a vehicle is not particularly limited. For example, when a battery pack is installed in an automobile, it can be mounted in the engine compartment, at the rear of the vehicle, or under the seats.

[0188] A vehicle may be equipped with multiple battery packs. In this case, the batteries contained in each battery pack may be electrically connected in series, electrically connected in parallel, or electrically connected in a combination of series and parallel connections. For example, if each battery pack contains a battery pack, the battery packs may be electrically connected in series, electrically connected in parallel, or electrically connected in a combination of series and parallel connections. Alternatively, if each battery pack contains a single battery, the batteries may be electrically connected in series, electrically connected in parallel, or electrically connected in a combination of series and parallel connections.

[0189] Next, an example of a vehicle according to the embodiment will be described with reference to the drawings.

[0190] Figure 8 is a schematic partial transparency drawing showing an example of a vehicle.

[0191] The vehicle 400 shown in Figure 8 includes a vehicle body 40 and a battery pack 300 according to the third embodiment. In the example shown in Figure 8, the vehicle 400 is a four-wheeled automobile.

[0192] This vehicle 400 may be equipped with multiple battery packs 300. In this case, the batteries contained in the battery pack 300 (for example, single cells or battery packs) may be connected in series, in parallel, or in a combination of series and parallel connections.

[0193] Figure 8 illustrates an example in which the battery pack 300 is mounted in the engine compartment located in front of the vehicle body 40. As described above, the battery pack 300 may also be mounted, for example, in the rear of the vehicle body 40 or under the seats. This battery pack 300 can be used as a power source for the vehicle 400. In addition, this battery pack 300 can recover regenerative energy from the vehicle 400's power.

[0194] Next, an embodiment of the vehicle according to the embodiment will be described with reference to Figure 9.

[0195] Figure 9 is a schematic diagram illustrating an example of a control system for the electrical system in a vehicle. The vehicle 400 shown in Figure 9 is an electric vehicle.

[0196] The vehicle 400 shown in Figure 9 comprises a vehicle body 40, a vehicle power supply 41, a vehicle ECU (ECU: Electric Control Unit) 42 which is a higher-level control device for the vehicle power supply 41, an external terminal (terminal for connecting to an external power supply) 43, an inverter 44, and a drive motor 45.

[0197] Vehicle 400 has its vehicle power supply 41 mounted, for example, in the engine compartment, at the rear of the vehicle body, or under the seats. Note that in the vehicle 400 shown in Figure 9, the mounting location of the vehicle power supply 41 is shown in a schematic manner.

[0198] The vehicle power supply 41 comprises a plurality (for example, three) of battery packs 300a, 300b, and 300c, a battery management unit (BMU) 411, and a communication bus 412.

[0199] Battery pack 300a comprises a battery pack 200a and a battery pack monitoring device 301a (e.g., VTM: Voltage Temperature Monitoring). Battery pack 300b comprises a battery pack 200b and a battery pack monitoring device 301b. Battery pack 300c comprises a battery pack 200c and a battery pack monitoring device 301c. Battery packs 300a to 300c are similar to the aforementioned battery pack 300, and battery packs 200a to 200c are similar to the aforementioned battery pack 200. Battery packs 200a to 200c are electrically connected in series. Battery packs 300a, 300b, and 300c can each be independently removed and replaced with another battery pack 300.

[0200] Each of the battery packs 200a to 200c comprises multiple single cells connected in series. At least one of the multiple single cells is a secondary battery according to the first embodiment. Each of the battery packs 200a to 200c is charged and discharged through a positive terminal 413 and a negative terminal 414.

[0201] The battery management device 411 communicates with the battery pack monitoring devices 301a to 301c and collects information such as voltage and temperature for each of the single cells 100 included in the battery packs 200a to 200c included in the vehicle power supply 41. In this way, the battery management device 411 collects information related to the maintenance of the vehicle power supply 41.

[0202] The battery management device 411 and the battery pack monitoring devices 301a to 301c are connected via a communication bus 412. On the communication bus 412, one set of communication lines is shared by multiple nodes (the battery management device 411 and one or more battery pack monitoring devices 301a to 301c). The communication bus 412 is a communication bus configured, for example, based on the CAN (Control Area Network) standard.

[0203] The battery pack monitoring devices 301a to 301c measure the voltage and temperature of each individual cell constituting the battery packs 200a to 200c based on commands communicated from the battery management device 411. However, temperature can be measured at only a few locations per battery pack, and it is not necessary to measure the temperature of all individual cells.

[0204] The vehicle power supply 41 may also have an electromagnetic contactor (for example, a switch device 415 shown in Figure 9) that switches the presence or absence of an electrical connection between the positive terminal 413 and the negative terminal 414. The switch device 415 includes a pre-charge switch (not shown) that turns on when charging is performed on the battery packs 200a to 200c, and a main switch (not shown) that turns on when the output from the battery packs 200a to 200c is supplied to the load. Each of the pre-charge switch and the main switch includes a relay circuit (not shown) that is switched on or off by a signal supplied to a coil located near the switch element. Electromagnetic contactors such as the switch device 415 are controlled based on a control signal from the battery management device 411 or the vehicle ECU 42 that controls the operation of the entire vehicle 400.

[0205] The inverter 44 converts the input DC voltage into a high voltage of three-phase alternating current (AC) for motor drive. The three-phase output terminals of the inverter 44 are connected to the three-phase input terminals of the drive motor 45. The inverter 44 is controlled based on control signals from the battery management device 411 or the vehicle ECU 42 for controlling the operation of the entire vehicle. By controlling the inverter 44, the output voltage from the inverter 44 is adjusted.

[0206] The drive motor 45 rotates using power supplied from the inverter 44. The driving force generated by the rotation of the drive motor 45 is transmitted to the axle and drive wheels W, for example, via a differential gear unit.

[0207] Although not shown in the diagram, vehicle 400 is also equipped with a regenerative braking mechanism (regenerator). The regenerative braking mechanism rotates the drive motor 45 when vehicle 400 is braked, converting kinetic energy into regenerative energy as electrical energy. The regenerative energy recovered by the regenerative braking mechanism is input to the inverter 44 and converted into a DC current. The converted DC current is input to the vehicle power supply 41.

[0208] One terminal of connection line L1 is connected to the negative terminal 414 of the vehicle power supply 41. The other terminal of connection line L1 is connected to the negative input terminal 417 of the inverter 44. A current detection unit (current detection circuit) 416 within the battery management device 411 is provided on connection line L1 between the negative terminal 414 and the negative input terminal 417.

[0209] One terminal of connection line L2 is connected to the positive terminal 413 of the vehicle power supply 41. The other terminal of connection line L2 is connected to the positive input terminal 418 of the inverter 44. A switch device 415 is provided between the positive terminal 413 and the positive input terminal 418 of connection line L2.

[0210] External terminal 43 is connected to battery management device 411. External terminal 43 can be connected to an external power supply, for example.

[0211] The vehicle ECU 42, in response to operational inputs from the driver and others, coordinates control of the vehicle power supply 41, switch device 415, inverter 44, etc., together with other management and control devices, including the battery management device 411. Through the coordinated control of the vehicle ECU 42, etc., the output of power from the vehicle power supply 41 and the charging of the vehicle power supply 41 are controlled, and the entire vehicle 400 is managed. Data related to the maintenance of the vehicle power supply 41, such as the remaining capacity of the vehicle power supply 41, is transferred between the battery management device 411 and the vehicle ECU 42 via a communication line.

[0212] The vehicle according to the fourth embodiment is equipped with a battery pack according to the third embodiment. Because the battery pack has excellent charge-discharge cycle life, a highly reliable vehicle can be provided.

[0213] (Examples) Examples are described below, but the embodiments are not limited to those described below.

[0214] (Example 1) <Fabrication of the negative electrode> Nb2O5 particles and TiO2 particles were mixed in a 1:1 molar ratio. The resulting powder was placed in an alumina crucible and heated at 800°C for 10 hours. After that, it was crushed and mixed, and calcined again at 800°C for 10 hours to obtain precursor particles. Furthermore, the obtained precursor particles were fully calcined at 1100°C for 5 hours to obtain Nb2TiO7(TNO) powder, a niobium-containing oxide.

[0215] As the negative electrode active material, Nb2TiO7 (TNO) powder was prepared; as conductive materials, acetylene black (AB) and multi-walled carbon nanotubes (MWCNTs) were prepared; as binders, carboxymethylcellulose (CMC) sodium salt powder and styrene-butadiene rubber (SBR) dispersions were prepared. These materials were mixed in pure water, the solvent, in the ratio of 86 parts by mass of TNO powder, 5 parts by mass of AB, 5 parts by mass of MWCNTs, 2 parts by mass of CMC sodium salt powder, and 2 parts by mass of SBR. The mixing was carried out in the following order while stirring the pure water. After dissolving the CMC sodium salt in the pure water, the SBR dispersion was further mixed in to obtain a dispersion. AB and MWCNTs were dispersed in this dispersion, and finally the TNO powder was dispersed. Thus, a negative electrode slurry was obtained. This negative electrode slurry was coated on both sides of a negative electrode current collector made of 12 μm thick aluminum foil. After that, the negative electrode slurry was dried and pressed to obtain the negative electrode. When cutting the negative electrode into a rectangle, a portion was cut along one side of the rectangle so that no coating was formed on the negative electrode current collector. This portion was designated as the negative electrode current collector tab.

[0216] <Fabrication of the positive electrode> As the positive electrode active material, LiNi 0.8 Co 0.1 Mn 0.1 3 g of O2 (NCM) was prepared. Acetylene black (AB) was prepared as a conductive agent. A PVdF dispersion was also prepared. The PVdF dispersion consisted of PVdF dispersed in N-methyl-2-pyrrolidone (NMP) at a solid content of 8% as a binder.

[0217] The above materials were mixed in a ratio of 90 parts by mass of NCM, 5 parts by mass of AB, and 5 parts by mass of PVdF in NMP as a solvent to prepare a positive electrode slurry. This positive electrode slurry was applied to both sides of a positive electrode current collector made of 12 μm thick aluminum foil. The positive electrode slurry was then dried and pressed to obtain the positive electrode. When cutting the positive electrode into a rectangle, a portion was cut along one side of the rectangle so that no coating film was formed on the positive electrode current collector. This portion was designated as the positive electrode current collector tab.

[0218] <Fabrication of electrode groups> A cellulose separator with a thickness of 15 μm was prepared as the separator. The negative electrode, separator, positive electrode, and separator were stacked in this order to create a stacked electrode array.

[0219] <Electrolyte adjustment> A mixed solvent was obtained by mixing propylene carbonate (PC) as a cyclic carbonate and propyl propionate (PP) as a carboxylic acid ester in a mass ratio of PC:PP = 25:75. A liquid non-aqueous electrolyte was prepared by dissolving lithium hexafluoride phosphate (LiPF6) as a lithium salt in the mixed solvent to a concentration of 1.2 mol / kg.

[0220] <Assembly of battery precursor> The electrode group was housed in a pack made of a laminate film composed of an aluminum foil and polypropylene layers formed on both sides thereof. Thereafter, a liquid non-aqueous electrolyte was injected into the laminate film pack housing the electrode group. The laminate film pack was completely sealed by heat sealing to produce a battery precursor.

[0221] <First charge and aging> The battery precursor was charged at a battery voltage of 3.1 V and 0.2 C. Aging was performed by leaving the battery precursor with a battery voltage of 3.1 V in a thermostat at 70 °C for 24 hours. Thus, a secondary battery was produced.

[0222] (Example 2) A secondary battery was produced in the same manner as in Example 1, except that the type of niobium-containing oxide was changed to niobium pentoxide represented by Nb2O5.

[0223] (Examples 3, 4) A secondary battery was produced in the same manner as in Example 1, except that the type of carboxylic acid ester was changed as described in Table 2.

[0224] (Examples 5, 6) A secondary battery was produced in the same manner as in Example 1, except that the amount of the carboxylic acid ester was changed as described in Table 2.

[0225] (Example 7) In the preparation of the electrolyte, propylene carbonate (PC) and propyl propionate (PP) were mixed at a mass ratio of PC:PP = 25:75 to obtain a mixed solvent. LiPF6 was dissolved in this mixed solvent to a concentration of 0.86 mol / kg. A secondary battery was produced in the same manner as in Example 1, except for the above.

[0226] (Example 8) In preparing the electrolyte, propylene carbonate (PC) and propyl propionate (PP) were mixed in a mass ratio of PC:PP = 25:75 to obtain a mixed solvent. LiPF6 was dissolved in this mixed solvent to a concentration of 1.6 mol / kg. A secondary battery was prepared in the same manner as in Example 1, except for the above.

[0227] (Example 9) In preparing the electrolyte, propylene carbonate (PC) and propyl propionate (PP) were mixed in a mass ratio of PC:PP = 25:75 to obtain a mixed solvent. LiPF6 was dissolved in this mixed solvent to a concentration of 0.74 mol / kg. A secondary battery was prepared in the same manner as in Example 1, except for the above.

[0228] (Example 10) In preparing the electrolyte, propylene carbonate (PC) and propyl propionate (PP) were mixed in a mass ratio of PC:PP = 25:75 to obtain a mixed solvent. LiPF6 was dissolved in this mixed solvent to a concentration of 2.2 mol / kg. A secondary battery was prepared in the same manner as in Example 1, except for the above.

[0229] (Example 11) A secondary battery was fabricated in the same manner as in Example 1, except that the battery voltage during aging was changed to the value shown in Table 2.

[0230] (Examples 12 and 13) A secondary battery was fabricated in the same manner as in Example 1, except that the temperature during aging was changed to the value shown in Table 2.

[0231] (Comparative Examples 1-3) A secondary battery was prepared in the same manner as in Example 1, except that the type of carboxylic acid ester was changed as shown in Table 2.

[0232] (Comparative Examples 4, 5) A secondary battery was prepared in the same manner as in Example 1, except that the amount of carboxylic acid ester was changed as shown in Table 2.

[0233] (Comparative Example 6) A secondary battery was fabricated in the same manner as in Example 1, except that the battery voltage during aging was changed to the value shown in Table 2.

[0234] (Comparative Examples 7, 8) A secondary battery was fabricated in the same manner as in Example 1, except that the temperature during aging was changed to the value shown in Table 2.

[0235] (HAXPES analysis) The negative electrodes of the secondary batteries in the examples and comparative examples were subjected to HAXPES analysis using the method described above, and the ratios B / A and C / B were measured.

[0236] (60°C cycle maintenance rate test) In a 60°C constant temperature bath, secondary batteries were charged using CCCV (Constant Current Constant Voltage) at a 1C rate, followed by CC (Constant Current) discharge at a 1C rate. The discharge capacity was measured after the above charge and discharge cycles. The above CCCV charging and CC discharge cycles were repeated 200 times, with C1 being the discharge capacity after the first cycle and C200 being the discharge capacity after 200 cycles. The capacity retention rate after 200 cycles was defined as C200 / C1. Note that 1C refers to the current value at which the battery's State of Charge (SOC) drops to 0% in one hour when discharged from 100%.

[0237] The manufacturing conditions and measurement results for the secondary batteries of each example and comparative example are shown in Tables 2 to 4.

[0238] The manufacturing conditions include the type of negative electrode active material, the type of carboxylic acid ester, the (r+1)+r' and |(r+1)-r'| values ​​of the carboxylic acid ester, the amount of carboxylic acid ester (mass%), Ml / Mc, the battery voltage during aging (V), and the aging temperature (°C). The amount of carboxylic acid ester is expressed as the mass% of the carboxylic acid ester relative to the electrolyte excluding the lithium salt.

[0239] As measurement results, the ratio B / A and the ratio C / B obtained by HAXPES for the negative electrodes included in the secondary batteries of the examples and comparative examples, and the results of the 60°C cycle retention rate test are shown.

[0240] [Table 2]

[0241] [Table 3]

[0242] [Table 4]

[0243] In all of Examples 1 to 13, the 60°C cycle retention rate was superior to that of Comparative Examples 1 to 9.

[0244] It was revealed from Examples 1 and 2 that even if the type of niobium-containing oxide was changed, the charge-discharge cycle life could be improved.

[0245] It was revealed from Examples 1, 3, and 4 that even if the type of the first carboxylic acid ester was changed, the charge-discharge cycle life could be improved. However, among the examples in which the type of carboxylic acid ester was changed, Comparative Examples 1 to 3 had a charge-discharge cycle life inferior to that of the examples. Ethyl acetate included in the secondary battery of Comparative Example 1 and butyl pentanoate included in the secondary battery of Comparative Example 2 were outside the range where the value of (r + 1) + r' was 5 or more and 8 or less. Methyl propionate included in the secondary battery of Comparative Example 3 had a large |(r + 1) - r'| value of 2. Therefore, the carboxylic acid esters included in Comparative Examples 1 to 3 did not correspond to the first carboxylic acid ester.

[0246] Also, it was revealed from Examples 1, 5, and 6 and Comparative Examples 4 and 5 that when the content of the first first carboxylic acid ester was 70% by mass or more and 95% by mass or less with respect to 100% by mass of the electrolyte excluding the lithium salt, the charge-discharge cycle life could be improved.

[0247] Examples 1, 7-10 are examples in which the ratio Ml / Mc, the amount of lithium salt Ml (mol) to the amount of carboxylic acid ester Mc (mol) contained in the electrolyte, was varied. Comparing these examples, it became clear that Examples 1, 7, 8, and 10, which satisfy 0.1 ≤ Ml / Mc ≤ 0.4, showed a particularly large improvement in charge-discharge cycle life.

[0248] Examples 1 and 11, and Comparative Example 6 revealed that increasing the battery voltage during aging tends to increase the B / A ratio. Furthermore, Examples 1, 12, 13, and Comparative Examples 7 and 8 revealed that increasing the temperature during aging tends to increase the B / A ratio.

[0249] According to at least one of these embodiments or examples, a secondary battery is provided. The secondary battery comprises a positive electrode, a negative electrode, and an electrolyte. The negative electrode comprises a niobium-containing oxide. The ratio B / A of the area A in the range of 200 eV to 215 eV and the area B in the range of 680 eV to 695 eV, as measured by hard X-ray photoelectron spectroscopy on the surface of the negative electrode, is 1.1 to 4.3. The electrolyte comprises a lithium salt and a carboxylic acid ester represented by formula A.

[0250] (Formula A)

[0251] [ka]

[0252] Each of R and R' in formula A is a hydrocarbon group. When r is the number of carbon atoms in R and r' is the number of carbon atoms in R', r and r' satisfy 5 ≤ (r+1) + r' ≤ 8, and the value of |(r+1) - r'| is 0 or 1. The carboxylic acid ester content is 70% to 95% by mass relative to 100% by mass of the electrolyte excluding the lithium salt. Therefore, a secondary battery with excellent charge-discharge cycle life can be provided.

[0253] While several embodiments of the present invention have been described, these embodiments are presented as examples only and are not intended to limit the scope of the invention. These novel embodiments can be carried out in a variety of other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims of the invention and its equivalents.

[0254] The invention according to the embodiment is described below.

[0255] [1] comprising a positive electrode, a negative electrode, and an electrolyte, The aforementioned negative electrode contains a niobium-containing oxide, In hard X-ray photoelectron spectroscopy on the surface of the negative electrode, the ratio B / A of the area A in the range of 200 eV to 215 eV and the area B in the range of 680 eV to 695 eV is 1.1 to 4.3. The electrolyte comprises a lithium salt and a carboxylic acid ester represented by formula A. (Formula A)

[0256] [ka]

[0257] Each of R and R' in the above formula A is a hydrocarbon group. Let r be the number of carbon atoms contained in R, and r' be the number of carbon atoms contained in R'. Then r and r' satisfy 5 ≤ (r+1) + r' ≤ 8, and the value of |(r+1) - r'| is 0 or 1. A secondary battery in which the content of the carboxylic acid ester is 70% by mass or more and 95% by mass or less based on 100% by mass of the electrolyte excluding the lithium salt.

[0258] [2] The secondary battery according to [1], wherein the amount of substance Ml (mol) of the lithium salt contained in the electrolyte and the amount of substance Mc (mol) of the carboxylic acid ester contained in the electrolyte satisfy 0.1 ≤ Ml / Mc ≤ 0.4.

[0259] [3] The secondary battery according to [1] or [2], wherein the ratio C / B of the area C in the range of 290 eV to 295 eV to the negative electrode surface in the hard X-ray photoelectron spectroscopy is 0.2 or less.

[0260] [4] The niobium-containing oxide is of the general formula Li x Ti 1-y M1 y Nb 2-z M2 z O 7+δ and the general formula Li x Ti 1-y M3 y+z Nb 2-z O 7-δ It comprises at least one compound selected from the group consisting of, The aforementioned M1 is at least one element selected from the group consisting of Zr, Si, and Sn, the aforementioned M2 is at least one element selected from the group consisting of V, Ta, Bi, K, Ca, B, Co, Fe, Mn, Ni, Si, P, and Mo, and the aforementioned M3 is at least one element selected from the group consisting of Mg, Fe, Ni, Co, W, Ta, and Mo. A secondary battery according to any one of [1] to [3], wherein x satisfies 0 ≤ x ≤ 5, y satisfies 0 ≤ y ≤ 1, z satisfies 0 ≤ z ≤ 0.5, and δ satisfies -0.3 ≤ δ ≤ 0.3.

[0261] [5] A battery pack comprising a secondary battery as described in any one of items [1] to [4].

[0262] [6] External terminals for power supply, Protection circuit and The battery pack described in [5] further comprises the following:

[0263] [7] comprising a plurality of the aforementioned secondary batteries, The battery pack described in [5] or [6], wherein the secondary batteries are electrically connected in series, in parallel, or in a combination of series and parallel.

[0264] A vehicle equipped with a battery pack as described in any one of the following items: [8] [5] to [7].

[0265] [9] The vehicle according to [8], which includes a mechanism for converting the kinetic energy of the vehicle into regenerative energy. [Explanation of Symbols]

[0266] 1…Electrode group, 2…Outer casing, 3…Negative electrode, 3a…Negative electrode current collector, 3b…Negative electrode active material containing layer, 3c…Negative electrode current collector tab, 4…Separator, 5…Positive electrode, 5a…Positive electrode current collector, 5b…Positive electrode active material containing layer, 6…Negative electrode terminal, 7…Positive electrode terminal, 21…Bus bar, 22…Positive electrode side lead, 22a…Other end, 23…Negative electrode side lead, 23a…Other end, 24…Adhesive tape, 31…Housing container, 32…Lid, 33…Protective sheet, 34…Printed circuit board, 35…Wiring, 40…Vehicle body, 41…Vehicle power supply, 42…Electrical control device, 43…External terminals, 44…Inverter, 45…Drive motor, 100…Secondary battery, 200…Battery pack, 200a…Battery pack, 200b…Battery pack, 200c…Battery pack, 300…Battery pack, 30 0a...Battery pack, 300b...Battery pack, 300c...Battery pack, 301a...Battery pack monitoring device, 301b...Battery pack monitoring device, 301c...Battery pack monitoring device, 342...Positive side connector, 343...Negative side connector, 345...Thermistor, 346...Protection circuit, 342a...Wiring, 343a...Wiring, 350...External terminal for energization, 352...Positive side terminal, 353...Negative side terminal, 348a...Positive side wiring, 348b...Negative side wiring, 400...Vehicle, 411...Battery management device, 412...Communication bus, 413...Positive terminal, 414...Negative terminal, 415...Switching device, 416...Current detection unit, 417...Negative input terminal, 418...Positive input terminal, L1...Connection line, L2...Connection line, W...Drive wheel.

Claims

1. It includes a positive electrode, a negative electrode, and an electrolyte. The aforementioned negative electrode contains a niobium-containing oxide, In hard X-ray photoelectron spectroscopy on the surface of the negative electrode, the ratio B / A of the area A in the range of 200 eV to 215 eV and the area B in the range of 680 eV to 695 eV is 1.1 to 4.

3. The electrolyte comprises a lithium salt and a carboxylic acid ester represented by formula A. (Formula A) 【Chemistry 1】 Each of R and R' in the above formula A is a hydrocarbon group. Let r be the number of carbon atoms contained in R, and r' be the number of carbon atoms contained in R'. Then r and r' satisfy 5 ≤ (r + 1) + r' ≤ 8, and the value of |(r + 1) - r'| is 0 or 1. A secondary battery in which the content of the carboxylic acid ester is 70% by mass or more and 95% by mass or less based on 100% by mass of the electrolyte excluding the lithium salt.

2. The secondary battery according to claim 1, wherein the amount of substance Ml (mol) of the lithium salt contained in the electrolyte and the amount of substance Mc (mol) of the carboxylic acid ester contained in the electrolyte satisfy 0.1 ≤ Ml / Mc ≤ 0.

4.

3. The secondary battery according to claim 1, wherein the ratio C / B of the area C in the range of 290 eV to 295 eV to the negative electrode surface, as measured by hard X-ray photoelectron spectroscopy, is 0.2 or less.

4. The niobium-containing oxide has the general formula Li ｘ Ti １－ｙ M1 ｙ Nb ２－ｚ M2 ｚ O ７＋δ , and the general formula Li ｘ Ti １－ｙ M3 ｙ＋ｚ Nb ２－ｚ O ７－δ and includes at least one compound selected from the group consisting of The aforementioned M1 is at least one element selected from the group consisting of Zr, Si, and Sn; the aforementioned M2 is at least one element selected from the group consisting of V, Ta, Bi, K, Ca, B, Co, Fe, Mn, Ni, Si, P, and Mo; and the aforementioned M3 is at least one element selected from the group consisting of Mg, Fe, Ni, Co, W, Ta, and Mo. The secondary battery according to claim 1, wherein x satisfies 0 ≤ x ≤ 5, y satisfies 0 ≤ y ≤ 1, z satisfies 0 ≤ z ≤ 0.5, and δ satisfies -0.3 ≤ δ ≤ 0.

3.

5. A battery pack comprising a secondary battery according to any one of claims 1 to 4.

6. External terminals for power supply, Protection circuit and The battery pack according to claim 5, further comprising the above.

7. The device comprises multiple secondary batteries, The battery pack according to claim 5, wherein the secondary batteries are electrically connected in series, in parallel, or in a combination of series and parallel.

8. A vehicle equipped with the battery pack described in claim 5.

9. The vehicle according to claim 8, which includes a mechanism for converting the kinetic energy of the vehicle into regenerative energy.

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