Electrode, secondary battery, and battery pack

Abstract

One embodiment of the present invention provides an electrode comprising an active material–containing layer. The active material-containing layer includes a plurality of active material particles, and a coating that covers at least a part of the surfaces of the plurality of active material particles and contains boron or contains boron and at least one of Si and Al. The ratio of the mass of boron, Si and Al to the mass of the active material particles is in the range of 0.03 mass%-0.50 mass% inclusive.

Description

Electrode, secondary battery and battery pack

[0001] FIELD Embodiments of the present invention relate to an electrode, a secondary battery, and a battery pack.

[0002] Rapid advances in electronics technology in recent years have led to the miniaturization and weight reduction of electronic devices. There is a strong demand for secondary batteries, which are used as power sources for driving or backing up such electronic devices, that are small, lightweight, and have a high energy density. Furthermore, the need to reduce CO2 emissions has recently led to an urgent need for the development of larger-capacity energy storage systems for electric vehicles and for nighttime home power storage. High-volume density lithium secondary batteries have been attracting attention as a new type of secondary battery that can meet these demands.

[0003] Japanese Patent Publication No. 2013-62026 Japanese Patent Publication No. 2015-99746 International Publication No. 2016 / 147404 International Publication No. 2017 / 042956 International Publication No. 2018 / 110708 Japanese Patent Publication No. 2018-160416 Japanese Patent Publication No. 2019-169276 Japanese Patent Publication No. 2021-150160

[0004] An object of embodiments of the present invention is to provide an electrode capable of realizing a secondary battery that exhibits excellent cycle life performance and low-temperature performance, a secondary battery including this electrode, and a battery pack including this secondary battery.

[0005] According to an embodiment, an electrode is provided that includes an active material-containing layer. The active material-containing layer includes a plurality of active material particles and a coating that covers at least a portion of the surfaces of the plurality of active material particles and contains boron or boron and at least one of Si and Al. The ratio of the total mass of boron, Si, and Al to the mass of the active material particles is within a range of 0.03% by mass to 0.50% by mass.

[0006] According to another embodiment, a secondary battery is provided. The secondary battery includes a negative electrode, a positive electrode, and an electrolyte. The negative electrode includes an electrode according to an embodiment.

[0007] According to another embodiment, a battery pack is provided, which includes a secondary battery according to the embodiment.

[0008] FIG. 1 is a cross-sectional view schematically showing an example of a secondary battery according to an embodiment. FIG. 2 is a cross-sectional view enlarging part A of the secondary battery shown in FIG. 1. FIG. 3 is a partially cutaway perspective view schematically showing another example of a secondary battery according to an embodiment. FIG. 4 is a cross-sectional view enlarging part B of the secondary battery shown in FIG. 3. FIG. 5 is a perspective view schematically showing an example of a battery pack according to an embodiment. FIG. 6 is an exploded perspective view schematically showing an example of a battery pack according to an embodiment. FIG. 7 is a block diagram showing an example of an electrical circuit of the battery pack shown in FIG. 6. Embodiment

[0009] On the surface of the negative electrode, a film is formed due to self-discharge caused by a side reaction between the active material particles and the electrolyte. Repeated charge-discharge cycles tend to increase the resistance in low-temperature environments more easily than the room-temperature resistance due to this film formation. Furthermore, if the surface of the negative electrode active material is exposed, film formation due to self-discharge is thought to occur continuously. Therefore, repeated charge-discharge cycles of the secondary battery and / or deterioration over time cause film formation, resulting in a continuous increase in the positive electrode potential. As a result, there are problems with positive electrode deterioration due to a shift in the SOC (shift of operation window) between the positive and negative electrodes and gas generation due to oxidation of the electrolyte.

[0010] When the electrode according to the embodiment is included in a secondary battery as a negative electrode, self-discharge on the surface of the negative electrode active material can be suppressed, thereby suppressing an increase in the positive electrode potential in the secondary battery. As a result, a secondary battery exhibiting excellent cycle life performance and low-temperature performance can be realized.

[0011] The following describes embodiments with reference to the drawings. Common components throughout the embodiments are designated by the same reference numerals, and redundant explanations will be omitted. Each figure is a schematic diagram for explaining and facilitating understanding of the embodiments. While the shapes, dimensions, and ratios may differ from those of actual devices, these may be appropriately modified in design, taking into consideration the following explanation and known techniques.

[0012] (First embodiment) According to the first embodiment, an electrode is provided. The electrode includes an active material-containing layer. The active material-containing layer contains a plurality of active material particles. At least a portion of the surface of the active material particles is coated with a coating that contains at least boron (B) and may further contain at least one of Si and Al. The ratio of the mass of B or the total mass of B and Si and / or Al to the mass of the active material particles is within a range of 0.03 mass% to 0.50 mass% (mass ratio 3×10 -4 Over 5.0 x 10 -3 (See below).

[0013] The electrode is, for example, a battery electrode, a secondary battery electrode, or a lithium ion secondary battery electrode. More specifically, the electrode may be a battery negative electrode, a secondary battery negative electrode, or a lithium ion secondary battery negative electrode. Examples of secondary batteries include non-aqueous electrolyte secondary batteries containing a non-aqueous electrolyte.

[0014] The electrode according to the embodiment includes an active material-containing layer composed of a plurality of active material particles. The surfaces of the active material particles are at least partially coated with a material containing B or a material containing Si and / or Al in addition to B. In other words, a coating layer (film) containing B or a coating layer (film) containing silicon (Si) and / or aluminum (Al) in addition to B is formed on the surfaces of the active material particles. By forming a coating layer containing B, B + Si, B + Al, or B + Si + Al on the surfaces of the active material particles, contact between the surfaces of the active material particles and the electrolyte can be suppressed. This suppresses the formation of a coating on the surfaces of the active material particles due to self-discharge, thereby suppressing an increase in low-temperature resistance.

[0015] Furthermore, when such an electrode is used as a negative electrode, the increase in the positive electrode potential can be suppressed by suppressing the film formation reaction that accompanies self-discharge of the negative electrode, thereby suppressing the deterioration of the positive electrode due to the SOC deviation and the generation of gas due to the oxidation of the electrolyte components in the positive electrode. Therefore, the electrode according to the embodiment can achieve excellent cycle life performance and low-temperature performance.

[0016] A coating containing at least B thinly and evenly covers the surface of the active material particles. Some electrode active materials are known to undergo volume changes associated with the insertion and desorption of charge carrier ions, such as lithium ions, during battery charging and discharging. A coating containing B and possibly Si or Al does not contain localized concentrations at specific locations on the particle surface, making it easy to follow volume changes in the active material particles. Specifically, it is desirable for the B-containing coating that covers at least a portion of the active material particles to be a uniform film. Coatings containing Si or Al but not B can be formed to a thickness of several nanometers using, for example, metal alkoxides, but tend to be concentrated directly on the active material. If such a coating is formed on an active material that exhibits volume changes, the coating covering the active material surface may be damaged by the volume changes associated with charging and discharging. In this case, contact between the active material and the electrolyte may be insufficiently suppressed. The coating containing at least B can follow the volume change of the active material, so it can be maintained even after repeated charge and discharge, and can suppress the coating-forming reaction between the active material and the electrolyte that causes an increase in resistance over a long period of time.

[0017] The following advantages can be obtained when the coating layer formed on the active material particles contains Al. When using a battery in a high-temperature environment or in a high SOC (state of charge) range, the coating layer containing Al as well as B, Si, and Al, is more advantageous in suppressing resistance increase than a coating layer containing only B or B+Si, which does not contain Al. Furthermore, the various coating layers containing Al are particularly advantageous in suppressing resistance increase in low-temperature environments. However, as the Al content increases, the initial resistance tends to increase more easily than an electrode having a coating layer containing B or B+Si. Therefore, it is desirable to appropriately adjust the ratio of B and Si to Al, taking into account the usage environment.

[0018] The chemical structures of the B, Si, and Al components in the coating are unclear. However, they may exist in the form of a coating containing Al formed on the surface of the negative electrode active material and Li, C, P, F, and O derived from the electrolyte components. In the case of a coating containing only B among B, Si, and Al, after the first charge / discharge cycle is completed, B is present in a state similar to lithium borate, for example, within the coating containing Li, C, P, F, and O, which are electrolyte components. As described below, inorganic compounds such as boric acid are used as starting materials to form B-containing coatings, condensed silicates are used to form Si-containing coatings, and organoaluminum compounds are used to form Al-containing coatings. The aluminum atoms in the coating may be bonded or coordinated with organic substituents contained in the organoaluminum compound. Furthermore, the aluminum atoms may be bonded with, for example, methoxy, ethoxy, propyl, or butoxy groups, or may be coordinated with acetyl groups.

[0019] The bonding state of each element constituting the coating on the electrode surface can be investigated by time-of-flight secondary ion mass spectrometry (ToF-SIMS) described later. In the above-mentioned coating containing at least B contained in the electrode according to the embodiment, B reacts with positive secondary ions ( 57 Li2BO2 + , 87 Li4BO3 + ) and negative secondary ions ( 93 LiB2O4 - , 143 Li2B3O6 - ) In other words, B in the coating has a lithium borate bond. The lithium borate bond mentioned here is not limited to lithium borate, but also includes bonding states in structures similar to lithium borate.

[0020] It is known to use LiBF4 as a supporting salt for electrolytes containing B, but in such cases, the resistance increase suppression effect obtained by the coating containing B as described above does not appear. When electrodes removed from a battery using LiBF4 as an electrolyte are measured with ToF-SIMS, LiBOF3 is detected as a negative secondary ion. - , LiB2O3F2 -, LiB2OF6 - The positive and negative secondary ions ( 57 Li2BO2 + , 87 Li4BO3 + , 93 LiB2O4 - , 143 Li2B3O6 - ) was not detected. This suggests that even when a coating containing boron is formed on the surface of active material particles, the bonding state of boron in the coating varies depending on the added substance and addition method, and that the chemical bonding state of B in the coating formed on the active material surface has a significant impact on the resistance increase suppression effect.

[0021] Furthermore, LIBOB (lithium bis(oxalato)borate; LiB2(C2O4)2) is known as a common electrolyte additive containing B. However, batteries using LIBOB as an electrolyte additive also do not exhibit the resistance increase suppression effect, unlike batteries using active materials containing the above-mentioned B-containing coating. Generally, when components of an electrolyte additive are incorporated into a coating formed on the surface of an active material, the additive is incorporated into the coating on the active material while maintaining its chemical structure. For example, when LIBOB is used as an electrolyte additive, it is generally known that a substance having an oxalate structure is present in the resulting coating. When the chemical bonding state of B contained in such a substance is confirmed by ToF-SIMS analysis, it is found that the chemical bonding state of B in the coating formed on the active material surface differs from the bonding state of B in the coating of the electrode according to this embodiment.

[0022] Although other examples of coatings containing B have been shown, unlike those, as in the electrode according to the embodiment, B exists in a coating on the active material in a form having a structure similar to that of lithium borate, making it possible to suppress the coating formation reaction that accompanies self-discharge between the electrolyte and the active material during charging / discharging or storage of a battery using the electrode. As a result, an increase in battery resistance can be suppressed. On the other hand, when a compound containing B with a structure different from that of B in the electrode according to the embodiment (BF4 when LiBF4 is used as the electrolyte supporting salt) is used, -When a substance having an oxalate structure (such as a substance having an oxalate structure when LIBOB is used as an electrolyte additive) is present as a coating component on the surface of the active material, the resistance increase suppression effect obtained by the electrode according to this embodiment is not achieved. Therefore, a coating formation reaction accompanied by self-discharge proceeds between the active material and the electrolyte, which can lead to a large increase in resistance.

[0023] In the active material-containing layer, the ratio of the mass of B, Si, and Al to the mass of the active material particles is within the range of 0.03 mass% or more and 0.50 mass% or less ([mass of B] / [mass of active material particles], [mass of B + mass of Si] / [mass of active material particles], [mass of B + mass of Al] / [mass of active material particles], or [mass of B + mass of Si + mass of Al] / [mass of active material particles] is 3×10 -4 Over 5.0 x 10 -3 (Within the range below.) On the other hand, if the proportion is 0.03 mass% or more, the effect of suppressing the formation of a coating due to self-discharge is exerted. Therefore, the increase in resistance due to repeated charge-discharge cycles can be suppressed. On the other hand, if the proportion is 0.50 mass% or less, the coating covering the surface of the active material particles is not too thick, and the conductivity of Li ions within the coating is not inhibited during charge-discharge. As a result, not only can the resistance at the beginning of the charge-discharge cycle be kept low, but the rate of increase in resistance due to repeated charge-discharge cycles can also be reduced.

[0024] In order to further suppress an increase in the room temperature resistance and low temperature resistance during battery use, it is desirable that the ratio of the mass of B, Si, and Al to the mass of the active material be within the range of 0.05 mass% or more and 0.50 mass% or less ([mass of B] / [mass of active material particles], [mass of B + mass of Si] / [mass of active material particles], [mass of B + mass of Al] / [mass of active material particles], or [mass of B + mass of Si + mass of Al] / [mass of active material particles] is 5×10 -4 Over 5.0 x 10 -3In order to suppress an increase in the initial resistance due to coating on the active material surface and to suppress an increase in resistance, the ratio of the mass of B, Si, and Al to the mass of the active material is preferably within a range of 0.05% by mass or more and 0.30% by mass or less ([mass of B] / [mass of active material particles], [mass of B + mass of Si] / [mass of active material particles], [mass of B + mass of Al] / [mass of active material particles], or [mass of B + mass of Si + mass of Al] / [mass of active material particles] is 5×10 -4 Over 3.0 x 10 -3 within the following range).

[0025] The electrodes according to the embodiment will be described in detail below.

[0026] Such an electrode may further include a current collector. For example, an active material-containing layer may be provided on at least a portion of the current collector.

[0027] The active material-containing layer may be, for example, a sheet-shaped layer formed on one side or both sides of the current collector. The active material-containing layer may further contain a conductive agent and / or a binder in addition to the active material particles.

[0028] The thickness of the active material-containing layer is not particularly limited. For example, the thickness of the sheet-shaped active material-containing layer may be in the range of 10 μm to 50 μm.

[0029] The active material particles contain, for example, a titanium-containing oxide. The multiple active material particles may further contain another active material different from the titanium-containing oxide. The mass ratio of the titanium-containing oxide contained in the active material particles may be 80 mass% or more. The mass ratio of the titanium-containing oxide to the active material particles may be 90 mass% or more, or even 100 mass%. That is, each of the multiple active material particles contained in the active material-containing layer may be composed solely of a titanium-containing oxide. Furthermore, among the multiple active material particles, for example, the titanium-containing oxide particles may account for 80 mass% or more, and the other active material particles may account for 20 mass% or less. The type of titanium-containing oxide contained in the multiple active material particles may be one type or two or more types.

[0030] A preferred example of the titanium-containing oxide is lithium titanate having a spinel structure. The lithium titanate having a spinel structure is, for example, represented by the general formula Li 4+a TiO 12 The subscript a in the general formula is a variable that can vary within the above range depending on the state of charge of the lithium titanate. The subscript a is, for example, within the range of -1≦a≦3. Compared to carbon materials typically used as negative electrode active materials, lithium titanate with a spinel structure exhibits less volume change during charge and discharge when used as an electrode active material. Therefore, in a battery using an electrode containing lithium titanate with a spinel structure, the decrease in capacity and electrode deterioration associated with lithium insertion and extraction are less likely to progress.

[0031] Examples of titanium-containing oxides other than lithium titanate having a spinel structure include lithium titanate having a ramsdellite structure (e.g., Li 2+y Ti3O7, 0≦y≦3), monoclinic titanium dioxide (TiO2), anatase titanium dioxide, rutile titanium dioxide, hollandite titanium composite oxide, monoclinic niobium titanium oxide (e.g., Li x TiNbO 7+δ , 0≦x≦5 and −0.3≦δ≦0.3) and orthorhombic titanium composite oxides. These titanium-containing oxides undergo larger volume changes during charge and discharge than lithium titanate with a spinel structure. As mentioned above, the coating containing B can follow the volume changes of the active material particles, so even when these titanium-containing oxides, which exhibit large volume changes, are used as coatings, the above-mentioned effects can be achieved and maintained.

[0032] The active material particles may exist in the form of primary particles or in the form of secondary particles formed by aggregation of primary particles. Whether the active material particles are secondary particles or primary particles can be determined by observation with a scanning electron microscope (SEM).

[0033] The average primary particle diameter of the active material particles (for example, D ５０(i.e., the particle size at which the cumulative frequency from the small particle size side in the particle size distribution is 50%) may be, for example, in the range of 0.1 μm to 30 μm, preferably in the range of 1 μm to 10 μm.

[0034] The binder is blended to fill the gaps between the dispersed active material particles and to bind the active material and the current collector. Examples of binders include polyvinylidene fluoride (PVdF), polyimide, polyamide, polyvinylpyrrolidone (PVP), carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA), polyethylene glycol (PEG), polyvinyl chloride (PVC), polyvinyl acetate (PVAC), and polyacrylic acid. One of these may be used as the binder, or two or more may be combined and used as the binder. The ratio of the mass of the binder to the total mass of the active material particles, conductive agent, and binder in the active material-containing layer may be, for example, in the range of 1.5% by mass to 30.0% by mass.

[0035] From the viewpoint of minimizing the increase in battery resistance due to an increase in the binder content, it is preferable that the binder contains PVdF. Furthermore, it is desirable that the mass ratio of the binder is in the range of 1.5 mass % to 5.0 mass %, since this can suppress the increase in initial cell resistance due to the binder content.

[0036] The conductive agent is blended to improve current collection performance and reduce contact resistance between the active material and the current collector. Examples of conductive agents include vapor-grown carbon fiber (VGCF), carbon black such as acetylene black, fibrous carbon such as carbon nanotubes, and carbonaceous materials such as graphite. One of these may be used as the conductive agent, or two or more may be used in combination as the conductive agent.

[0037] The blending ratios of the active material particles, the conductive agent, and the binder in the active material-containing layer can be appropriately changed depending on the application of the electrode. For example, the active material particles, the conductive agent, and the binder are preferably blended in ratios of 68% by mass to 96% by mass, 2% by mass to 30% by mass, and 1.5% by mass to 30% by mass, respectively.

[0038] The current collector can be made of a material that is electrochemically stable at the potential at which lithium (Li) is inserted into and extracted from the active material. For example, when the electrode is used as a negative electrode, the current collector is preferably made of copper, nickel, stainless steel, aluminum, or an aluminum alloy containing one or more elements selected from Mg, Ti, Zn, Mn, Fe, Cu, and Si. The thickness of the current collector is preferably 5 μm or more and 20 μm or less. A current collector having such a thickness can balance the strength and weight of the electrode.

[0039] The current collector may also include a portion on the surface of which no active material-containing layer is formed, and this portion can function as an electrode current collecting tab.

[0040] <Measurement Methods> Various measurements of the electrode are described below. Specifically, the presence or absence of a coating containing at least B can be confirmed by scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDX), scanning transmission electron microscope-energy dispersive X-ray spectroscopy (STEM-EDX), or glow discharge optical emission spectrometry (GD-OES), the bonding state of the constituent elements of the coating can be confirmed by time-of-flight secondary ion mass spectrometry (ToF-SIMS), the crystalline structure of the active material can be confirmed by powder X-ray diffraction (XRD), and the elemental composition of the active material and coating can be confirmed by inductively coupled plasma (ICP) emission spectroscopy.

[0041] When measuring an electrode incorporated in a secondary battery, the electrode is removed and used as a measurement sample according to the following procedure.

[0042] For example, when measuring an electrode used as a negative electrode, the battery is placed in a fully discharged state. For example, to understand the crystalline state of the active material, the battery is placed in a state in which lithium ions are completely released from the active material. To place the battery in a fully discharged state, for example, the battery is discharged multiple times at a current of 0.1 C in a 25°C environment until the rated end voltage or the battery voltage reaches 1.0 V, so that the current value during discharge is 1 / 100 or less of the rated capacity. Even in a discharged state, residual lithium ions may remain.

[0043] A secondary battery in a fully discharged state (State of Charge: 0%) is disassembled in a glove box filled with argon. The electrode to be measured is removed from the disassembled secondary battery. The electrode is washed with an appropriate solvent and then dried. The solvent used for washing is a solvent that does not contain electrolyte salt, such as ethyl methyl carbonate. If the washing is insufficient, it may be difficult to observe particles of the electrode material, such as the active material, due to the influence of lithium carbonate or lithium fluoride remaining in the electrode.

[0044] <Scanning Electron Microscope-Energy Dispersive X-ray Spectroscopy (SEM-EDX)> This section explains a method for performing SEM-EDX on a cross section of an electrode to obtain elemental mapping. By performing SEM-EDX, it is possible to confirm whether the surface of the electrode active material particles is coated with a substance containing at least B, or a substance containing Si and / or Al in addition to B.

[0045] The electrode to be measured is cut using an ion milling device. When cutting the electrode, the electrode is cut along the thickness direction. The cross section of the cut electrode is attached to an SEM sample stage. At this time, the electrode is treated with conductive tape or the like to prevent peeling or lifting from the sample stage. The electrode (active material-containing layer) attached to the SEM or STEM sample stage is observed with an SEM to obtain an SEM image. During SEM measurement, observation is performed at a magnification of 10,000 times. It is also preferable to maintain an inert atmosphere when introducing the electrode into the sample chamber.

[0046] Furthermore, an elemental mapping image corresponding to the SEM image is obtained by EDX. By considering the SEM image and the elemental mapping image in combination, it is possible to confirm whether or not at least a portion of the surface of the active material particle is covered with a coating containing B or a coating containing B and at least one of Si and Al.

[0047] Although the measurement method using SEM-EDX was described above, the elements coated on the surface of the active material particles can also be detected by element mapping using STEM-EDX. In particular, mapping the cross section of the active material particles using EDX allows confirmation of a layer on the surface of the active material particles in which the electrolyte components C, O, F, and P coexist with B or B and Si or Al.

[0048] In SEM and STEM images, the boundary between the surface of the active material particles and the coating can be distinguished, and therefore the state of coating of the active material with the coating and the thickness of the coating can be determined from the SEM / STEM images.

[0049] Because B is a light element, for example, when the B content is low, it may be difficult to detect the presence of B in an electrode containing active material particles on which a coating containing B or a coating containing B and at least one of Si and Al is formed by SEM-EDX and STEM-EDX analysis due to the influence of pretreatment conditions used to prepare the measurement sample. In such cases, the presence of B and its chemical bonding state can be confirmed by glow discharge optical emission spectrometry and time-of-flight secondary ion mass spectrometry, which will be described below.

[0050] <Glow Discharge Optical Emission Spectroscopy (GD-OES)> Electrode surface analysis by glow discharge optical emission spectroscopy (GD-OES) can confirm whether the coating on the active material particles contains B. Surface analysis by GD-OES can be performed, for example, as follows.

[0051] As described above, the battery is discharged, removed, washed, and dried to prepare a measurement sample. The dried electrode surface is then placed in a glow discharge light-emitting device in an environment not exposed to the atmosphere, and measurements are performed.

[0052] When a coating containing B is provided, the same measurement can be performed to confirm that B is uniformly distributed in the region about 20 μm deep from the electrode surface. If the increase or decrease in the B emission intensity in the region 5 μm to 20 μm deep from the surface remains within ±10%, it can be determined that B is distributed uniformly. In this case, in addition to B, the coating and underlying active material components containing Li, C, O, F, and P that can be formed by reaction with the electrolyte (Li4Ti5O 12 In the case of using a material that serves as a Si source and an Al source, Si and Al are observed simultaneously.

[0053] <Time-of-flight secondary ion mass spectrometry (ToF-SIMS)> If the dried electrode is subjected to electrode surface analysis by time-of-flight secondary ion mass spectrometry (ToF-SIMS), the bonding state of each element constituting the coating on the electrode surface can be investigated. As described above, the battery is removed from the discharged battery, washed, and dried to prepare a measurement sample. The dried electrode is introduced into a time-of-flight secondary ion mass spectrometry (ToF-SIMS) device and subjected to electrode surface analysis. In an electrode that includes a coating containing at least B, such as the electrode according to the embodiment, positive secondary ions ( 57 Li2BO2 + , 87 Li4BO3 + ) and negative secondary ions ( 93 LiB2O4 - , 143 Li2B3O6 - ) B is detected.

[0054] <X-ray powder diffraction (XRD) measurement of active material> X-ray powder diffraction measurement of active material can be carried out, for example, as follows.

[0055] The electrode to be measured can be cut to an area equivalent to the holder area of ​​the wide-angle X-ray diffraction instrument and directly attached to a glass holder for measurement. In this case, XRD measurements are performed in advance depending on the type of metal foil of the electrode current collector to determine the position of the peaks derived from the current collector. The presence or absence of peaks from composite materials such as conductive agents and binders should also be determined in advance. If the peaks of the current collector and the active material overlap, it is desirable to peel the active material-containing layer from the current collector and measure it. This is to separate the overlapping peaks when quantitatively measuring the peak intensity. Of course, if these are known in advance, this procedure can be omitted. The active material-containing layer can also be physically peeled off. The active material-containing layer is easily peeled off by applying ultrasonic waves in a solvent. Specifically, the active material-containing layer containing the electrode active material can be peeled off from the electrode current collector by placing the electrode in ethyl methyl carbonate in a glass beaker and vibrating it in an ultrasonic cleaner.

[0056] When ultrasonic treatment is performed to peel off the active material-containing layer from the current collector, the electrode composite powder (including the active material, conductive agent, and binder) can be recovered by volatilizing the solvent. The recovered composite powder can be filled into, for example, a Lindemann glass capillary and measured, allowing powder X-ray diffraction measurement of the active material to be performed.

[0057] Furthermore, as mentioned above, if the electrodes are not sufficiently cleaned after removal from the battery, the lithium ions remaining in the electrodes may cause impurity phases such as lithium carbonate and lithium fluoride to be mixed in. If there is a concern about this, it is advisable to use an airtight container in which the measurement atmosphere can be kept in an inert gas.

[0058] <Inductively Coupled Plasma (ICP) Emission Spectroscopy> The composition of the active material can be analyzed using inductively coupled plasma (ICP) emission spectroscopy. In this case, the abundance ratio (molar ratio) of each element depends on the sensitivity of the analytical device used. Therefore, the measured molar ratio may deviate from the actual molar ratio by the error of the measuring device. For example, if the molar ratio obtained by measuring spinel-type lithium titanate is 4+a TiO 12 The numerical values ​​may deviate from the above due to the error range of the analytical device, but even with such measurement results, the performance of the electrode according to the embodiment can be fully demonstrated.

[0059] A portion containing an electrode active material, such as an active material-containing layer, is peeled off from the electrode to be measured. For example, the active material-containing layer can be peeled off by irradiating it with ultrasonic waves. As a specific example, the electrode can be placed in ethyl methyl carbonate in a glass beaker and vibrated in an ultrasonic cleaner, thereby peeling off the active material-containing layer from the electrode current collector.

[0060] Next, the peeled portion is heated in air for a short period of time (e.g., at 500°C for approximately one hour) to burn off unnecessary components such as binder and carbon. This residue is dissolved in acid or alkali to produce a liquid sample containing the active material and the coating on the active material particles. Acids that can be used include hydrochloric acid, nitric acid, sulfuric acid, and hydrogen fluoride. Carbonates such as sodium carbonate are used as alkalis to melt components that are less soluble in acid (e.g., silicon and aluminum oxide), which are then dissolved in acid again and filtered to produce a solution. This liquid sample can be subjected to ICP analysis to determine the composition of the active material coated with the coating.

[0061] The crystalline phase and composition of the active material can be determined based on the results of quantitative analysis using ICP analysis and the crystalline structure of the active material determined by X-ray diffraction measurement. The total amount of active material can be calculated from the determined crystalline phase and active material composition and the amounts of various elements obtained by ICP analysis. Based on the determined total amount of active material, the ratio of the amounts of B, Si, and Al measured at the same time to the amount of active material can be determined.

[0062] <Method for Manufacturing Electrode> The electrode according to the embodiment can be manufactured by, for example, the following method.

[0063] First, active material particles, a conductive agent, and a binder are suspended in an N-methyl-2-pyrrolidone (NMP) solvent to prepare a slurry. At this time, a B source, or optionally at least one of an Si source and an Al source together with the B source, is added to the slurry. The B source and optional Si and Al sources are added, for example, after dilution using a solvent such as NMP used in preparing the slurry. The solid content concentration of the resulting slurry is, for example, in the range of 40% by mass to 65% by mass. The active material particles, the conductive agent, and the binder are blended in the proportions described above. When only the B source is added, the amount of B element added is adjusted so that the content ratio of B element is the aforementioned mass ratio of Si and Al to the active material particles. When an Si source and / or an Al source is added in addition to the B source, the amounts of the B source, Si source, and Al source added are adjusted so that the overall content ratio of B element, Si element, and Al element is the aforementioned total mass ratio of B, Si, and Al to the active material particles.

[0064] When a Si source or an Al source is added, the order of adding the B source, Si source, and Al source to the slurry is not particularly limited. As described later, metal alkoxides can be used as the Si source or Al source, but from the viewpoint of slowly proceeding the hydrolysis reaction of the Si material or Al material using the metal alkoxide as a raw material, it is preferable to add the B source after the Si source or Al source.

[0065] The prepared slurry is applied to a current collector such as aluminum foil at a desired basis weight using a coating machine, and the coating is dried. When a Si source or an Al source is added, it is preferable to perform additional drying on the electrode after coating. Further drying of the electrode is preferable because it can remove residual components (organic substances) from the materials used in the Si source or Al source. For example, vacuum drying at 120°C for 48 hours is desirable as additional drying conditions. In this way, a member comprising a current collector and an active material-containing layer formed on the current collector is produced. The resulting member has, for example, a strip shape. The member is then pressed to achieve a predetermined electrode density. In this way, the electrode according to the embodiment can be produced.

[0066] Such an electrode may be fabricated in the following manner.

[0067] First, active material particles, a conductive agent, and a binder are suspended in a suitable solvent to prepare a slurry. This slurry is then applied, dried, and pressed in the same manner as in the above-described manufacturing method to obtain an electrode structure whose active material-containing layer does not contain B, Si, or Al components. The electrode structure is then immersed in an ethanol solution containing a B source. The immersion treatment is preferably performed by immersing the electrode structure at room temperature for 10 to 30 seconds. The electrode structure is then removed from the B-containing solution, and any excess solution adhering to the member is wiped off. The electrode structure is then dried.

[0068] When a coating layer (film) containing Si and / or Al components in addition to B is formed on the active material surface, an electrode structure whose active material-containing layer does not contain B, Si, or Al components is first immersed in an ethanol solution containing a Si source, a Si source and an Al source, or an Al source, and then removed from the Si / Al-containing solution, and excess solution adhering to the member is wiped off. The electrode structure is then dried at room temperature. This forms a coating containing at least one of Si and Al near the active material surface. The electrode structure is then immersed in an ethanol solution containing a B source. The electrode structure is then dried.

[0069] The electrode structure is sufficiently dried, for example, at room temperature (25°C) until excess liquid components are no longer visible. After drying in this manner, it is desirable to further dry the electrode structure on a hot plate set at 120°C, and then dry it in a vacuum dryer set at 120°C for 48 hours or more. As described above, when a Si source or an Al source is used, such additional drying is preferable because it can remove organic components derived from the Si source or the Al source. In this way, at least a portion of the surfaces of the plurality of active material particles can be coated with a component containing B, B and Si, B and Al, or B, Si, and Al.

[0070] Alternatively, a coating may be formed on the surface of the active material particles prior to preparing the slurry. Specifically, first, the active material particles and the B source, or the active material particles, the B source, and the Si source and / or the Al source are mixed. During mixing, dilution is performed using, for example, ethanol as a solvent. When only the B source is used among the B source, Si source, and Al source, a coating layer containing B is formed on the active material surface by drying after mixing. When the Si source or the Al source is used in addition to the B source, a heat treatment is performed in the range of 200°C to 500°C after mixing to remove organic substances contained in the Si source or the Al source. In this manner, a coating layer containing B and Si and / or Al is formed on the active material surface.

[0071] An inorganic compound containing B is used as the B source for forming a coating layer (film) containing B on the surface of active material particles. Specific examples of inorganic compounds containing B include boric acid (H3BO3), diboron trioxide (BO3), lithium tetraborate (Li2BO7), and other forms of lithium borate compounds. Considering the cost of raw materials, ease of handling, solubility in solvents, and the formation of a coating in the form of lithium borate on the surface of the active material, it is desirable to use boric acid as the B source.

[0072] ToF-SIMS analysis results indicate that the B in the coating on the active material, which inhibits the reaction between the active material and electrolyte in the battery, exists in a form similar to lithium borate. Although the details of the formation of this lithium borate-form B component cannot be determined with certainty, it is possible that it was formed by the reaction of boric acid with Li eluted from the lithium-containing active material during the slurry stage of electrode fabrication. Furthermore, when a battery is constructed using a lithium-containing electrolyte, the Li in the electrolyte may react with boric acid to form the lithium borate-form B component. BO3 has low reactivity with Li. Therefore, considering its reactivity with Li, boric acid is a more desirable source of B than BO3, even among compounds containing the same B.

[0073] Among the above-mentioned active materials, lithium titanate having a spinel structure (Li 4+a TiO 12 ; -1≦a≦3) contains Li. Also, lithium titanate having a ramsdellite structure (e.g., Li 2+y Ti3O7, 0≦y≦3) and monoclinic niobium titanium oxide (e.g., Li x TiNbO 7+δ , 0≦x≦5 and −0.3≦δ≦0.3) are active materials that can contain Li. Because Li can be eluted from these active materials, if boric acid is used as a B source, a B-containing coating in the form of lithium borate can be efficiently formed.

[0074] In contrast, when a Li-free active material such as monoclinic titanium dioxide (TiO2) is used, for example, a boron-containing material is fixed onto the Li-free active material by heat treatment or the like, and when a coating is formed on the active material by charging and discharging, the electrolyte components Li and boron are incorporated into the coating, forming a coating such as lithium borate. To more efficiently form a B-containing coating in the form of lithium borate on the active material surface, it is preferable to use lithium tetraborate (Li2B4O7) or lithium borate, or to add a Li source such as lithium hydroxide or lithium carbonate simultaneously with boric acid. In the latter case, simultaneous addition of lithium hydroxide and boric acid is preferred from the viewpoint of preventing gas generation.

[0075] When using boric acid as a B source to directly form a coating layer (film) containing only B among B, Si, and Al on the surface of active material particles, it is desirable to mix the active material with a diluted solution of boric acid and then dry the mixture at a temperature of 25° C. to 200° C. in order to uniformly form a coating containing lithium borate on the active material. In this case, the raw material solution (diluted boric acid solution) does not contain any organic components that need to be removed at high temperatures, and boric acid has a melting point of 170° C. and a boiling point of 300° C. In order to fix the B-containing compound on the outer surface of the active material particles and prevent B from diffusing into the active material to reduce the B concentration on the outer surface of the active material particles, it is desirable to limit the treatment after mixing to drying at the above temperature.

[0076] As mentioned above, it is known that LiBF4 is used as an electrolyte supporting salt in lithium-ion secondary batteries. Furthermore, boron-containing compounds such as TMSB (tris(trimethylsilyl)borate) are sometimes used as electrolyte additives. Although these battery constituent materials also contain boron, using them as a boron source to form a coating layer does not effectively suppress the formation of a coating on the active material surface that is associated with self-discharge. While the exact reason is unclear, it is thought that when the boron component derived from these compounds is incorporated into the coating on the active material surface, a coating (such as one containing lithium borate) that deactivates the reactive sites on the active material surface cannot be formed on the active material surface.

[0077] Metal alkoxides are used as the silicon source when forming a silicon-containing coating layer (film) on the surface of active material particles. The reaction between the silicon-containing metal alkoxide and water adsorbed on the active material surface causes a hydrolysis polymerization reaction, forming a silicon-containing coating on the active material surface. Silicon-containing metal alkoxides are available in monomers (silicate monomers) and polymers (silicate oligomers). Examples of monomers include ethyl orthosilicate (also known as tetraethoxysilane or tetraethyl orthosilicate; TEOS), methyl orthosilicate (also known as tetramethyl orthosilicate; TMOS), propyl silicate, and butyl silicate. Polymers often contain solid components in which the monomers are pre-condensed. Examples of polymers include chemically stable condensed silicates (e.g., silicate 40 (manufactured by Tama Chemicals Co., Ltd.) containing ethyl polysilicate, ethyl silicate 40 (manufactured by Colcoat Co., Ltd.), ethyl silicate 48 (manufactured by Colcoat Co., Ltd.), methyl silicate 51 (manufactured by Colcoat Co., Ltd.), and methyl silicate 53 (manufactured by Colcoat Co., Ltd.)). When forming a Si-containing coating, it is desirable to use a polymer as a raw material. Polymers tend to form a better coating on the active material than monomeric Si metal alkoxides alone.

[0078] An organoaluminum compound is used as the Al source when forming an Al-containing coating layer (film) on the surface of active material particles. Similar to the formation of the Si coating layer, this organoaluminum reacts with water adsorbed on the active material surface to undergo a hydrolysis polymerization reaction, forming a coating on the active material surface. Examples of organoaluminum compounds include at least one selected from the group consisting of aluminum alkoxides and aluminum chelates (complexes). Examples of aluminum alkoxides that can be used include di-2-butoxyaluminum ethyl acetoacetate (Al(C4H9O)2(C6H9O3)), aluminum tri-2-butoxide (Al(OC4H9)3), di-2-butoxyaluminum acetylacetate (Al(C4H9O)2(C5H7O2)), aluminum sec-butoxide (Al(O-sec-C4H9)3), aluminum isopropylate (Al(O-iC3H7)3), and aluminum ethoxide (Al(O-C2H5)3). Examples of aluminum chelates that can be used include aluminum trisacetylacetate (Al(C5H7O2)3), aluminum bisethylacetoacetate monoacetylacetate (Al(C5H7O2)(C6H9O3)2), aluminum trisethylacetoacetate (Al(C6H9O3)3), etc. These may be used alone or as a mixture of two or more.

[0079] Furthermore, although ethanol was exemplified as a solvent used to dilute boron-containing compounds such as boric acid, silicon-containing metal alkoxides, and organoaluminum compounds, the diluent is not limited to ethanol. The diluent is preferably a solvent with a relatively low boiling point that does not remain in the electrode after drying and that is unlikely to react with boric acid, metal alkoxides, or electrode components. Using a diluent solvent that is unlikely to remain in the electrode can prevent a decrease in efficiency due to residual solvent during battery charging and discharging. A solvent with a low water content is desirable to prevent reaction with metal alkoxides and electrode components. Considering safety, cost, and ease of handling, ethanol is preferably used. Another example is methanol, but caution is required when handling it due to its toxicity.

[0080] The organoaluminum used as the Al source, as mentioned above, is more susceptible to polymerization due to hydrolysis than the Si-containing metal alkoxide used as the Si source, so caution is advised. To coat the surface of an active material with a coating containing Si or Al, two methods are used: 1. immersing the electrode in a diluted solution, and 2. diluting the solution with NMP or similar and adding it to the electrode slurry. In the former method (1), it is desirable to avoid the formation of cloudy or other precipitates during the preparation of the diluted solution, and in the latter method (2), it is desirable to avoid the formation of such precipitates during the dilution with NMP or similar solvent. The formation of such precipitates makes it difficult to form a uniform coating on the active material surface, which is necessary to suppress an increase in low-temperature resistance, and is therefore desirable.

[0081] 3. Even when a coating is formed on the surface of the active material in advance, it is desirable that no precipitate such as cloudiness is formed when the active material is diluted with a solvent.

[0082] Furthermore, the organoaluminum used as the Al source and the metal alkoxide used as the Si source mentioned above can be divided into the metal portion of Al or Si and the organic component. If a compound with a sufficiently high organic component compared to the metal component is used, a large amount of organic component will remain in the completed electrode, which can cause gas generation. Furthermore, if a compound with a low organic component compared to the Al component is used as the Al source, the Al component will also be dispersed along with the organic component during the drying process during electrode fabrication, making it difficult to fabricate an electrode containing a consistent amount of Al. To avoid the above problems, the raw materials used for the Al and Si sources must be selected taking into account the electrode fabrication conditions, particularly the drying conditions and the thermal properties of the raw materials.

[0083] Each step in forming the coating layer containing Si and / or Al is carried out in a low dew point environment. For example, the coating layer is formed in an environment with a dew point of -50°C or higher and -10°C or lower, preferably -20°C. In this coating formation reaction, water adsorbed on the active material surface reacts with a metal alkoxide or aluminum chelate, forming a coating containing Si and / or Al on the active material surface. By forming a coating of a metal alkoxide or aluminum chelate, which serves as a Si source or an Al source, without supplying water from an external source or adding an acid or basic catalyst to facilitate the reaction, a uniform coating can be formed on the active material surface.

[0084] By coating the surface of the active material with a film containing B or a film containing at least one of Si and Al in addition to B, the film-forming reaction between the electrolyte and the surface of the active material that occurs during charging and discharging of the battery can be suppressed, thereby suppressing an increase in low-temperature resistance.

[0085] According to the first embodiment described above, an electrode including an active material-containing layer is provided. The active material-containing layer includes a plurality of active material particles and a coating covering at least a portion of the surface of the particles. The coating contains at least B and may further contain at least one of Si and Al. The ratios of the mass of B, Si, and Al to the mass of the active material are in the range of 0.03% by mass to 0.50% by mass.

[0086] Since at least a portion of the surface of the active material particles is coated with a substance containing at least B, it is possible to suppress the formation of a coating on the surface of the active material particles due to self-discharge. Therefore, in a battery using such an electrode as a negative electrode, it is possible to suppress the increase in low-temperature resistance due to charge-discharge cycles, the deterioration of the positive electrode due to the increase in positive electrode potential, and the generation of oxygen gas at the positive electrode. Therefore, the electrode according to the embodiment can achieve excellent cycle life performance and low-temperature performance.

[0087] Second Embodiment According to a second embodiment, a secondary battery is provided that includes a negative electrode, a positive electrode, and an electrolyte. The negative electrode includes the electrode according to the first embodiment. The secondary battery may be, for example, a lithium ion secondary battery. The secondary battery may also be a non-aqueous electrolyte secondary battery that includes a non-aqueous electrolyte.

[0088] The secondary battery may further include a separator disposed between the positive electrode and the negative electrode. The negative electrode, the positive electrode, and the separator may constitute an electrode assembly. The electrolyte may be held in the electrode assembly.

[0089] The secondary battery may further include an exterior member that houses the electrode group and the electrolyte.

[0090] In addition, the secondary battery may further include a negative electrode terminal electrically connected to the negative electrode and a positive electrode terminal electrically connected to the positive electrode.

[0091] The negative electrode, positive electrode, electrolyte, separator, exterior member, negative electrode terminal, and positive electrode terminal will be described in detail below.

[0092] (1) Negative Electrode The electrode according to the first embodiment can be used as the negative electrode included in the secondary battery according to the second embodiment.

[0093] The density of the negative electrode active material-containing layer (excluding the current collector) is 1.8 g / cm 3 2.8g / cm or more 3 A negative electrode having a negative electrode active material-containing layer with a density within this range is excellent in energy density and electrolyte retention. The density of the negative electrode active material-containing layer is preferably 2.1 g / cm or less. 3 2.6g / cm or more 3 More preferably, it is:

[0094] (2) Positive Electrode The positive electrode may include a positive electrode current collector and a positive electrode active material-containing layer. The positive electrode active material-containing layer may be formed on one or both sides of the positive electrode current collector. The positive electrode active material-containing layer may include a positive electrode active material and, optionally, a conductive agent and a binder.

[0095] The positive electrode active material may be, for example, an oxide or a sulfide. The positive electrode may contain one type of compound alone or two or more types of compounds in combination as the positive electrode active material. Examples of oxides and sulfides include compounds capable of inserting and extracting Li or Li ions.

[0096] Examples of such compounds include manganese dioxide (MnO2), iron oxide, copper oxide, nickel oxide, and lithium manganese composite oxide (e.g., Li x Mn2O4 or Li x MnO2, where 0<x≦1), lithium nickel composite oxide (e.g., Li x NiO2, where 0<x≦1), lithium cobalt composite oxide (e.g., Li x CoO2, where 0<x≦1), lithium nickel cobalt composite oxide (e.g., Li x Ni 1-y Co yO2, where 0<x≦1 and 0<y<1), lithium manganese cobalt composite oxide (e.g., Li x Mn y Co 1-y O2, where 0<x≦1 and 0<y<1), lithium manganese nickel composite oxides having a spinel structure (e.g., Li x Mn 2-y Ni y O4, where 0<x≦1 and 0<y<2), lithium phosphate oxides having an olivine structure (e.g., Li x Represented by FePO4, where 0<x≦1; Li x Fe 1-y Mn y PO4, where 0<x≦1 and 0<y<1; Li x CoPO4, where 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, where 0<x≦1, 0<y<1, 0<z<1, and y+z<1).

[0097] Among the above, examples of compounds more preferred as the positive electrode active material include lithium manganese composite oxides having a spinel structure (e.g., Li x Mn2O4, where 0<x≦1), lithium nickel composite oxide (e.g., Li x NiO2, where 0<x≦1), lithium cobalt composite oxide (e.g., Li x CoO2, where 0<x≦1), lithium nickel cobalt composite oxide (e.g., Li x Ni 1-y Co y O2, where 0<x≦1 and 0<y<1), lithium manganese nickel composite oxides having a spinel structure (e.g., Li x Mn 2-y Ni y O4, where 0<x≦1 and 0<y<2), lithium manganese cobalt composite oxide (e.g., Li x Mn y Co1-y O2, where 0<x≦1 and 0<y<1), lithium iron phosphate (e.g., Li x FePO4, where 0<x≦1), and lithium nickel cobalt manganese composite oxide (Li x Ni 1-y-z Co y Mn z O2, where 0<x≦1, 0<y<1, 0<z<1, and y+z<1. When these compounds are used as positive electrode active materials, the positive electrode potential can be increased.

[0098] When room temperature molten salt is used as the electrolyte of the battery, lithium iron phosphate, Li x It is preferable to use a positive electrode active material containing VPO4F (0≦x≦1), lithium manganese composite oxide, lithium nickel composite oxide, lithium nickel cobalt composite oxide, or a mixture thereof. These compounds have low reactivity with room-temperature molten salts, which can improve cycle life. Details of room-temperature molten salts will be described later.

[0099] The primary particle diameter 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 diameter of 100 nm or more is easy to handle in industrial production. A positive electrode active material with a primary particle diameter of 1 μm or less can smoothly diffuse lithium ions within the solid.

[0100] The specific surface area of ​​the positive electrode active material is 0.1 m 2 / g or more 10m 2 / g or less. 2 A positive electrode active material having a specific surface area of ​​10 m / g or more can secure sufficient sites for absorbing and releasing Li ions. 2 A positive electrode active material having a specific surface area of ​​0.1 / g or less is easy to handle in industrial production and can ensure good charge-discharge cycle performance.

[0101] The binder is blended to fill gaps between the dispersed positive electrode active material and to bind the positive electrode active material and the positive electrode current collector. Examples of binders include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), fluorine-based rubber, polyacrylic acid compounds, imide compounds, carboxymethyl cellulose (CMC), and CMC salts. One of these may be used as the binder, or two or more may be used in combination as the binder.

[0102] The conductive agent is blended to improve current collection performance and reduce contact resistance between the positive electrode active material and the positive 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 nanofibers, and carbon nanotubes. One of these may be used as the conductive agent, or two or more may be used in combination as the conductive agent. The conductive agent may also be omitted.

[0103] In the positive electrode active material-containing layer, the positive electrode active material and the binder are preferably mixed in proportions of 80% by mass to 98% by mass and 2% by mass to 20% by mass, respectively.

[0104] By setting the amount of binder to 2% by mass or more, sufficient electrode strength can be obtained. Furthermore, the binder can function as an insulator. Therefore, by setting the amount of binder to 20% by mass or less, the amount of insulator contained in the electrode is reduced, thereby reducing the internal resistance.

[0105] When a conductive agent is added, the positive electrode active material, the binder, and the conductive agent are preferably blended in proportions of 77% by mass or more and 95% by mass or less, 2% by mass or more and 20% by mass or less, and 3% by mass or more and 15% by mass or less, respectively.

[0106] By setting the amount of the conductive agent to 3% by mass or more, the above-mentioned effects can be achieved. Furthermore, by setting the amount of the conductive agent to 15% by mass or less, the proportion of the conductive agent in contact with the electrolyte can be reduced. This low proportion can reduce decomposition of the electrolyte during high-temperature storage.

[0107] 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.

[0108] The thickness of the aluminum foil or aluminum alloy foil is preferably 5 μm to 20 μm, 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 contained in the aluminum foil or aluminum alloy foil is preferably 1% by mass or less.

[0109] The positive electrode current collector may also include a portion on the surface of which the positive electrode active material-containing layer is not formed, and this portion can function as a positive electrode current collecting tab.

[0110] The positive electrode can be produced, for example, by the following method. First, an active material, a conductive agent, and a binder are suspended in a solvent to prepare a slurry. This slurry is applied to one or both sides of a current collector. Next, the applied slurry is dried to obtain a laminate of an active material-containing layer and a current collector. After that, this laminate is pressed. In this manner, a positive electrode is produced.

[0111] Alternatively, the positive electrode may be prepared by the following method: First, an active material, a conductive agent, and a binder are mixed to obtain a mixture, and then the mixture is formed into pellets. The pellets are then placed on a current collector to obtain a positive electrode.

[0112] (3) Electrolyte The electrolyte may be, for example, a liquid nonaqueous electrolyte or a gel nonaqueous electrolyte. The liquid nonaqueous electrolyte is prepared by dissolving an electrolyte salt as a solute in an organic solvent. The concentration of the electrolyte salt is preferably 0.5 mol / L or more and 2.5 mol / L or less.

[0113] Examples of electrolyte salts include lithium salts such as lithium perchlorate (LiClO), lithium hexafluorophosphate (LiPF), lithium tetrafluoroborate (LiBF), lithium hexafluoride (LiAsF), lithium trifluoromethanesulfonate (LiCFSO), and lithium bistrifluoromethylsulfonylimide (LiN(CFSO)), and mixtures thereof. The electrolyte salt is preferably one that is difficult to oxidize even at high potentials, and LiPF is most preferred.

[0114] Examples of organic solvents include cyclic carbonates such as propylene carbonate (PC), ethylene carbonate (EC), and vinylene carbonate (VC); chain 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); chain ethers such as dimethoxyethane (DME) and diethoxyethane (DEE); γ-butyrolactone (GBL), acetonitrile (AN), and sulfolane (SL). These organic solvents can be used alone or as a mixed solvent.

[0115] The gelled non-aqueous electrolyte is prepared by combining a liquid non-aqueous electrolyte with a polymeric material. The polymeric material may be, for example, at least one selected from the group consisting of polyvinylidene fluoride, polyacrylonitrile, polyethylene oxide, acrylic polymer, polyamide, and polyvinyl alcohol. The use of a gelled electrolyte as the electrolyte can suppress metal elution from the positive electrode and the generation of gases such as hydrogen. As a result, excellent cycle life performance can be achieved.

[0116] Alternatively, in addition to liquid nonaqueous electrolytes and gel nonaqueous electrolytes, room temperature molten salts containing lithium ions (ionic melts), polymer solid electrolytes, inorganic solid electrolytes, and the like may be used as the nonaqueous electrolyte.

[0117] Room-temperature molten salts (ionic melts) refer to organic salts formed from a combination of organic cations and anions that can exist as a liquid at room temperature (15°C or higher and 25°C or lower). Room-temperature molten salts include room-temperature molten salts that exist as a liquid by themselves, 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, and mixtures thereof. Generally, the melting point of room-temperature molten salts used in secondary batteries is 25°C or lower. In addition, the organic cation generally has a quaternary ammonium skeleton.

[0118] The solid polymer electrolyte is prepared by dissolving an electrolyte salt in a polymer material and solidifying it.

[0119] The inorganic solid electrolyte is a solid substance that has Li ion conductivity.

[0120] (4) Separator The separator is formed from, for example, a porous film containing polyethylene (PE), polypropylene (PP), cellulose, or polyvinylidene fluoride (PVdF), or a synthetic resin nonwoven fabric. From the viewpoint of safety, it is preferable to use a porous film formed from polyethylene or polypropylene. This is because these porous films melt at a certain temperature and are capable of interrupting current.

[0121] A solid electrolyte layer containing solid electrolyte particles can also be used as the separator. The solid electrolyte layer may contain one type of solid electrolyte particles or multiple types of solid electrolyte particles. The solid electrolyte layer may be a solid electrolyte composite membrane containing solid electrolyte particles. The solid electrolyte composite membrane is, for example, a membrane formed by molding solid electrolyte particles using a polymer material. The solid electrolyte layer may contain at least one selected from the group consisting of a plasticizer and an electrolyte salt. When the solid electrolyte layer contains an electrolyte salt, for example, the alkali metal ion conductivity of the solid electrolyte layer can be further increased.

[0122] Examples of polymeric materials include polyethers, polyesters, polyamines, polyethylenes, silicones, and polysulfides.

[0123] The lithium ion conductivity of the solid electrolyte is 1×10 at 25°C. －１０ It is preferable that the lithium ion conductivity of the solid electrolyte at 25° C. is 1×10 S / cm or more. －１０ When the lithium ion conductivity of the solid electrolyte is 1×10 S / cm or more, the lithium ion concentration near the surface of the solid electrolyte is likely to be high, thereby improving the rate performance and life performance. －６ The upper limit of the lithium ion conductivity of the solid electrolyte is, for example, 2×10 －２ S / cm.

[0124] Examples of the solid electrolyte include sulfide solid electrolytes, oxide solid electrolytes, nitride solid electrolytes, and polymer solid electrolytes. Specific examples of the solid electrolyte include sulfide-based Li ２ SeP ２ S ５ and lithium lanthanum titanium composite oxides (e.g., Li ０．５ La ０．５ TiO ３ ), inorganic compounds having a LISICON structure (e.g., Li ３．６ Si ０．６ P０．４ O ４ ), inorganic compounds having a NASICON skeleton, amorphous LIPON (Li ２．９ P.O. ３．３ N ０．４６ ), lithium calcium zirconium oxide (Li １．２ Zr １．９ Ca ０．１ (PO ４ ) ３ The coating layer may contain one type of solid electrolyte or two or more types of solid electrolytes.

[0125] The inorganic compound having a NASICON skeleton is preferably an inorganic compound represented by the general formula LiM2(PO4)3 (M is one or more selected from Ti, Ge, Sr, Zr, Sn, and Al). １＋ｘ Al ｘ Ge ２－ｘ (PO ４ ) ３ , Li １＋ｘ Al ｘ Zr ２－ｘ (PO ４ ) ３ , Li １＋ｘ Al ｘ Ti ２－ｘ (PO ４ ) ３ (LATP) is preferred because it has high ionic conductivity and high electrochemical stability against water. In the above, x preferably satisfies 0≦x≦0.5.

[0126] Examples of inorganic compounds having a garnet structure include Li 5+x A y La 3-y M2O 12 (A is at least one selected from the group consisting of Ca, Sr, and Ba, and M is at least one selected from the group consisting of Nb and Ta), Li3M 2-x ZrO 12 (M is at least one selected from the group consisting of Ta and Nb), Li 7-3x Al x La3Zr3O12 , and LiLaZrO 12 In the above, x is, for example, 0≦x<0.8, preferably 0≦x≦0.5. y is, for example, 0≦y<2. The oxide having a garnet structure may be composed of one of these compounds, or may contain a mixture of two or more of these compounds. Among these, Li 6.25 Al 0.25 La3Zr3O 12 and Li7La3Zr2O 12 has high ionic conductivity and is electrochemically stable, resulting in excellent discharge performance and cycle life.

[0127] If the solid electrolyte contains elemental sulfur, the sulfur component will dissolve in the organic electrolyte, which will be described later, and therefore it is preferable that the solid electrolyte does not contain elemental sulfur.

[0128] A preferred solid electrolyte is LATP (Li １＋ｘ Al ｘ Ti ２－ｘ (PO ４ ) ３ ), amorphous LIPON, garnet-type lithium lanthanum zirconium-containing oxides (e.g., Li ７ La ３ Zr ２ O １２ :LLZ).

[0129] The lithium ion conductivity of an inorganic compound having a NASICON skeleton and represented by LiM2(PO4)3 at 25°C is, for example, 1 × 10 -3 S / cm to 1×10 -5 S / cm. 2.9 P.O. 3.3 N 0.46 ) at 25 ° C. is 3 × 10 -6 S / cm. Garnet-type LLZ (LiLaZrO 12 ) at 25 ° C. is 3 × 10 -4 S / cm.

[0130] Among these, the solid electrolyte is preferably LATP having a NASICON-type skeleton.

[0131] (5) Exterior Member As the exterior member, for example, a container made of a laminate film or a metal container can be used.

[0132] The thickness of the laminate film is, for example, 0.5 mm or less, preferably 0.2 mm or less.

[0133] The laminate film is a multilayer film containing multiple resin layers and metal layers interposed between the resin layers. The resin layers include polymeric materials such as polypropylene (PP), polyethylene (PE), nylon, and polyethylene terephthalate (PET). The metal layers are preferably made of aluminum foil or aluminum alloy foil to reduce weight. The laminate film can be molded into the shape of the exterior component by sealing it by heat fusion.

[0134] The thickness of the wall of the metal container is, for example, 1 mm or less, more preferably 0.5 mm or less, and even more preferably 0.2 mm or less.

[0135] The metal container is made of, for example, aluminum or an aluminum alloy. The aluminum alloy preferably contains elements such as magnesium, zinc, and silicon. When the aluminum alloy contains transition metals such as iron, copper, nickel, and chromium, the content of these metals is preferably 100 mass ppm or less.

[0136] The shape of the exterior member is not particularly limited. The shape of the exterior member may be, for example, flat (thin), rectangular, cylindrical, coin-shaped, or button-shaped. The exterior member can be appropriately selected depending on the battery dimensions and the intended use of the battery.

[0137] (6) Negative Electrode Terminal The negative electrode terminal can be formed from a material that is electrochemically stable at the Li absorption / desorption potential of lithium titanate having a spinel structure and is electrically conductive. Specifically, the material for the negative electrode terminal can be copper, nickel, stainless steel, aluminum, or an aluminum alloy containing at least one element selected from the group consisting of Mg, Ti, Zn, Mn, Fe, Cu, and Si. Aluminum or an aluminum alloy is preferably used as the material for the negative electrode terminal. The negative electrode terminal is preferably made of the same material as the negative electrode current collector in order to reduce contact resistance with the negative electrode current collector.

[0138] (7) Positive electrode terminal The positive electrode terminal has a potential range of 3 V to 5 V with respect to the oxidation-reduction potential of lithium (vs. Li / Li ＋ The positive electrode terminal can be formed from a material that is electrically stable and conductive in the above-mentioned conditions. Examples of the material for the positive electrode terminal include aluminum and aluminum alloys containing at least one element selected from the group consisting of Mg, Ti, Zn, Mn, Fe, Cu, and Si. The positive electrode terminal is preferably formed from the same material as the positive electrode current collector in order to reduce contact resistance with the positive electrode current collector.

[0139] Next, the secondary battery according to the second embodiment will be described in more detail with reference to the drawings.

[0140] Fig. 1 is a cross-sectional view schematically showing an example of a secondary battery according to a second embodiment. Fig. 2 is an enlarged cross-sectional view of part A of the secondary battery shown in Fig. 1.

[0141] The secondary battery 100 shown in Figures 1 and 2 includes a bag-shaped exterior member 2 shown in Figures 1 and 2, an electrode group 1 shown in Figure 1, and an electrolyte (not shown). The electrode group 1 and the electrolyte are housed in the bag-shaped exterior member 2. The electrolyte (not shown) is held in the electrode group 1.

[0142] The bag-shaped exterior member 2 is made of a laminate film including two resin layers and a metal layer interposed between them.

[0143] As shown in Fig. 1 , the electrode group 1 is a flat wound electrode group. As shown in Fig. 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.

[0144] The negative electrode 3 includes a negative electrode current collector 3 a and a negative electrode active material-containing layer 3 b. In the portion of the negative electrode 3 located at the outermost shell of the wound-type electrode group 1, the negative electrode active material-containing layer 3 b is formed only on the inner surface side of the negative electrode current collector 3 a, as shown in Fig. 2. In the other portions of the negative electrode 3, the negative electrode active material-containing layer 3 b is formed on both sides of the negative electrode current collector 3 a.

[0145] The positive electrode 5 includes a positive electrode current collector 5a and positive electrode active material-containing layers 5b formed on both sides of the positive electrode current collector 5a.

[0146] As shown in Fig. 1, the negative electrode terminal 6 and the positive electrode terminal 7 are located near the outer peripheral edge of the wound electrode group 1. The negative electrode terminal 6 is connected to a portion located at the outermost shell of the negative electrode current collector 3a. The positive electrode terminal 7 is connected to a portion located at the outermost shell of the positive electrode current collector 5a. The negative electrode terminal 6 and the positive electrode terminal 7 extend to the outside from an opening of the bag-shaped exterior member 2. A thermoplastic resin layer is provided on the inner surface of the bag-shaped exterior member 2, and the opening is closed by heat sealing the thermoplastic resin layer.

[0147] The secondary battery according to the second embodiment is not limited to the secondary battery having the configuration shown in FIGS. 1 and 2, but may be a battery having the configuration shown in FIGS. 3 and 4, for example.

[0148] Fig. 3 is a partially cutaway perspective view schematically showing another example of the secondary battery according to the second embodiment. Fig. 4 is an enlarged cross-sectional view of part B of the secondary battery shown in Fig. 3.

[0149] The secondary battery 100 shown in Figures 3 and 4 includes an electrode group 1 shown in Figures 3 and 4, an exterior member 2 shown in Figure 3, and an electrolyte (not shown). The electrode group 1 and the electrolyte are housed in the exterior member 2. The electrolyte is held in the electrode group 1.

[0150] The exterior member 2 is made of a laminate film including two resin layers and a metal layer interposed between them.

[0151] The electrode group 1 is a laminated electrode group as shown in Fig. 4. The laminated electrode group 1 has a structure in which negative electrodes 3 and positive electrodes 5 are alternately laminated with separators 4 interposed therebetween.

[0152] The electrode group 1 includes a plurality of negative electrodes 3. Each of the plurality of negative electrodes 3 includes 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 includes 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.

[0153] The negative electrode current collector 3a of each negative electrode 3 includes a portion on one side where the negative electrode active material-containing layer 3b is not supported on any surface. This portion serves as a negative electrode current collector tab 3c. As shown in FIG. 4 , the negative electrode current collector tab 3c does not overlap with the positive electrode 5. The 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 pulled out to the outside of the exterior member 2.

[0154] Although not shown, 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 collecting tab. Like the negative electrode current collecting tab 3c, the positive electrode current collecting tab does not overlap with the negative electrode 3. The positive electrode current collecting tab is located on the opposite side of the electrode group 1 from the negative electrode current collecting tab 3c. The positive electrode current collecting 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 drawn out to the exterior of the exterior member 2.

[0155] The secondary battery according to the second embodiment includes the electrode according to the first embodiment as the negative electrode, and therefore, this secondary battery can achieve excellent cycle life performance and low-temperature performance.

[0156] Third Embodiment According to a third embodiment, a battery pack is provided. The battery pack according to the third embodiment includes a plurality of secondary batteries according to the second embodiment.

[0157] In such a battery pack, the individual cells may be electrically connected in series or in parallel, or may be connected in a combination of series and parallel.

[0158] Next, an example of a battery pack will be described with reference to the drawings.

[0159] Fig. 5 is a perspective view schematically illustrating an example of a battery pack according to the third embodiment. The battery pack 200 shown in Fig. 5 includes five cells 100a to 100e, four bus bars 21, a positive electrode lead 22, and a negative electrode lead 23. Each of the five cells 100a to 100e is a secondary battery according to the second embodiment.

[0160] The bus bar 21 connects, for example, the negative electrode terminal 6 of one cell 100a to the positive electrode terminal 7 of the adjacent cell 100b. In this way, the five cells 100 are connected in series by four bus bars 21. That is, the battery pack 200 in FIG. 5 is a five-series battery pack. Although an example is not shown, in a battery pack including a plurality of cells electrically connected in parallel, the plurality of cells can be electrically connected by, for example, connecting the plurality of negative electrode terminals to each other by a bus bar and connecting the plurality of positive electrode terminals to each other by a bus bar.

[0161] The positive electrode terminal 7 of at least one of the five cells 100a to 100e is electrically connected to a positive electrode lead 22 for external connection. The negative electrode terminal 6 of at least one of the five cells 100a to 100e is electrically connected to a negative electrode lead 23 for external connection.

[0162] The battery pack according to the third embodiment includes the secondary battery according to the second embodiment, and therefore this battery pack can achieve excellent cycle life performance and low-temperature performance.

[0163] Fourth Embodiment According to a fourth embodiment, a battery pack is provided. This battery pack includes the battery assembly according to the third embodiment. The battery pack may include a single secondary battery according to the second embodiment instead of the battery assembly according to the third embodiment.

[0164] Such a battery pack may further include a protection circuit. The protection circuit has a 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.

[0165] The battery pack may further include external terminals for current supply. The external terminals for current supply are for outputting current from the secondary battery to the outside and / or 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 for current supply. When the battery pack is charged, charging current (including regenerative energy from the power of a vehicle or the like) is supplied to the battery pack through the external terminals for current supply.

[0166] Next, an example of a battery pack will be described with reference to the drawings.

[0167] Fig. 6 is an exploded perspective view schematically showing an example of a battery pack according to a fourth embodiment. Fig. 7 is a block diagram showing an example of an electric circuit of the battery pack shown in Fig. 6.

[0168] The battery pack 300 shown in FIGS. 6 and 7 includes a container 31, a lid 32, a protective sheet 33, a battery pack 200, a printed wiring board 34, wiring 35, and an insulating plate (not shown).

[0169] The storage container 31 shown in Fig. 6 is a bottomed, prismatic container having a rectangular bottom. The storage container 31 is configured to be able to accommodate a protective sheet 33, a battery pack 200, a printed wiring board 34, and wiring 35. The lid 32 has a rectangular shape. The lid 32 covers the storage container 31 to accommodate the battery pack 200 and other components. Although not shown, the storage container 31 and the lid 32 are provided with openings or connection terminals for connection to external devices and the like.

[0170] The battery pack 200 includes a plurality of cells 100 , a positive electrode lead 22 , a negative electrode lead 23 , and an adhesive tape 24 .

[0171] At least one of the plurality of cells 100 is a secondary battery according to the second embodiment. The plurality of cells 100 are electrically connected in series as shown in FIG. 7 . The plurality of cells 100 may be electrically connected in parallel, or may be connected in a combination of series and parallel connections. When the plurality of cells 100 are connected in parallel, the battery capacity increases compared to when they are connected in series.

[0172] The adhesive tape 24 fastens the plurality of unit cells 100. Heat-shrinkable tape may be used to secure the plurality of unit cells 100 instead of the adhesive tape 24. In this case, protective sheets 33 are placed on both side surfaces of the battery pack 200, and the heat-shrinkable tape is wrapped around the battery pack 200, and the heat-shrinkable tape is then thermally shrunk to bind the plurality of unit cells 100 together.

[0173] One end of the positive electrode lead 22 is connected to the battery pack 200. One end of the positive electrode lead 22 is electrically connected to the positive electrode of one or more single cells 100. One end of the negative electrode lead 23 is connected to the battery pack 200. One end of the negative electrode lead 23 is electrically connected to the negative electrode of one or more single cells 100.

[0174] The printed wiring board 34 is installed along one of the shorter sides of the inner surface of the container 31. The printed wiring board 34 includes a positive connector 342, a negative connector 343, a thermistor 345, a protection circuit 346, wires 342a and 343a, an external terminal 350 for supplying current, a positive wire 348a, and a negative wire 348b. One main surface of the printed wiring board 34 faces one side of the battery pack 200. An insulating plate (not shown) is interposed between the printed wiring board 34 and the battery pack 200.

[0175] The other end 22a of the positive electrode lead 22 is electrically connected to the positive electrode connector 342. The other end 23a of the negative electrode lead 23 is electrically connected to the negative electrode connector 343.

[0176] The thermistor 345 is fixed to one main surface of the printed wiring board 34. The thermistor 345 detects the temperature of each of the cells 100 and transmits the detection signal to the protection circuit 346.

[0177] The external terminals 350 for applying current are fixed to the other main surface of the printed wiring board 34. The external terminals 350 for applying current are electrically connected to devices located outside the battery pack 300. The external terminals 350 for applying current include a positive terminal 352 and a negative terminal 353.

[0178] The protection circuit 346 is fixed to the other main surface of the printed wiring board 34. The protection circuit 346 is connected to the positive terminal 352 via a positive wiring 348a. The protection circuit 346 is connected to the negative terminal 353 via a negative wiring 348b. The protection circuit 346 is also electrically connected to the positive connector 342 via a wiring 342a. The protection circuit 346 is electrically connected to the negative connector 343 via a wiring 343a. The protection circuit 346 is also electrically connected to each of the plurality of single cells 100 via wiring 35.

[0179] The protective sheet 33 is disposed on both inner surfaces of the long sides of the container 31 and on the inner surface of the short side that faces the printed wiring board 34 across the battery pack 200. The protective sheet 33 is made of, for example, resin or rubber.

[0180] The protection circuit 346 controls the charging and discharging of the plurality of cells 100. Furthermore, the protection circuit 346 cuts off the electrical connection between the protection circuit 346 and external terminals 350 (positive terminal 352, negative terminal 353) for supplying electricity to external devices, based on a detection signal transmitted from the thermistor 345 or a detection signal transmitted from each cell 100 or the battery pack 200.

[0181] An example of the detection signal transmitted from the thermistor 345 is a signal indicating that the temperature of the cell 100 is equal to or higher than a predetermined temperature. An example of the detection signal transmitted from each cell 100 or the battery pack 200 is a signal indicating that overcharging, overdischarging, or overcurrent of the cell 100 is detected. When detecting overcharging or the like for each cell 100, the battery voltage may be detected, or the positive electrode potential or the negative electrode potential may be detected. In the latter case, a lithium electrode used as a reference electrode is inserted into each cell 100.

[0182] The protection circuit 346 may be a circuit included in a device (such as an electronic device or an automobile) that uses the battery pack 300 as a power source.

[0183] As described above, the battery pack 300 is also provided with the external terminals 350 for current application. Therefore, the battery pack 300 can output current from the battery assembly 200 to an external device and input current from the external device to the battery assembly 200 via the external terminals 350 for current application. In other words, when the battery pack 300 is used as a power source, the current from the battery assembly 200 is supplied to the external device via the external terminals 350 for current application. When the battery pack 300 is charged, a charging current from the external device is supplied to the battery pack 300 via the external terminals 350 for current application. When the battery pack 300 is used as an in-vehicle battery, regenerative energy from the vehicle's power can be used as the charging current from the external device.

[0184] The battery pack 300 may include a plurality of assembled batteries 200. In this case, the assembled batteries 200 may be connected in series, in parallel, or in a combination of series and parallel connections. The printed wiring board 34 and the wiring 35 may be omitted. In this case, the positive electrode lead 22 and the negative electrode lead 23 may be used as the positive and negative terminals of the external terminals for supplying current, respectively.

[0185] Such a battery pack is used in applications requiring excellent cycle performance when drawing a large current, for example. Specifically, this battery pack is used, for example, as a power source for electronic devices, a stationary battery, or an on-board battery for various vehicles. Examples of electronic devices include digital cameras. This battery pack is particularly suitable for use as an on-board battery.

[0186] The battery pack according to the fourth embodiment includes the secondary battery according to the second embodiment or the battery pack according to the third embodiment, and therefore this battery pack can achieve excellent cycle life performance and low-temperature performance.

[0187] Examples will be described below, but the embodiments are not limited to the following examples.

[0188] In all of these examples, the operation of forming the coating layer on the negative electrode was carried out in a low dew point environment (dew point: -20°C or lower).

[0189] Example 1-1 A laminate cell was fabricated as a secondary battery as described below.

[0190] <Preparation of Negative Electrode> As a negative electrode active material, ５０ ) is 0.8 μm of lithium titanate (Li4Ti5O 12 (TLO) powder, acetylene black as a conductive agent, polyvinylidene fluoride (PVdF; Kureha Corporation, Kureha KF Polymer Grade No. W#1700) as a binder, and boric acid as a B source for forming a B-containing coating on the lithium titanate surface were prepared. These materials were mixed so that the mass ratio of the resulting negative electrode active material-containing layer was active material (TLO): conductive agent: binder: B = 100:3:3:0.2. This mixture was dispersed in N-methyl-2-pyrrolidone (NMP) to prepare a negative electrode slurry. The solids concentration in the negative electrode slurry was 58 mass%.

[0191] The prepared negative electrode slurry was applied to a negative electrode having a capacity per unit area of ​​1.05 mAh / cm on one side. 2 After that, the coating was applied to both sides of a strip-shaped aluminum foil so that the coating film was dried to obtain a laminate. Thereafter, the electrode density (the portion of the active material-containing layer excluding the aluminum foil) was 2.3 g / cm 3 The laminate was then press-molded so that the laminate had a thickness of 130° C. Thereafter, the laminate was dried in a vacuum at a temperature of 130° C. for 12 hours to prepare a negative electrode.

[0192] <Preparation of Positive Electrode> 100 parts by mass of commercially available spinel-type lithium manganese oxide (LiMn2O4) was mixed with 5 parts by mass of acetylene black as a conductive agent to obtain a mixture. Next, this mixture was dispersed in NMP to obtain a dispersion. PVdF (Kureha KF Polymer Grade No. W#1700) as a binder was mixed with this dispersion in a ratio of 5 parts by mass relative to the lithium manganese oxide to prepare a positive electrode slurry. This slurry was applied to both sides of a strip-shaped current collector made of aluminum foil using a blade. This was dried under vacuum at 130°C for 12 hours. Thereafter, the obtained laminate was coated with a coating material having a density of 2.1 g / cm3 of the positive electrode active material-containing layer (excluding the current collector). 3 The thickness of each active material-containing layer of the positive and negative electrodes was adjusted so that the negative electrode capacity / positive electrode capacity ratio became 1.1.

[0193] <Preparation of Electrode Assembly> A strip-shaped separator, 19 μm thick and 76 mm wide, composed of 75% cellulose fiber by mass and 25% polyester fiber (melting point 260°C) by mass, was prepared. A rectangular negative electrode measuring 76 mm x 56 mm was cut from the previously prepared strip-shaped negative electrode. A rectangular positive electrode measuring 75 mm x 55 mm was cut from one side of the previously prepared strip-shaped positive electrode. The separator was folded every 57 mm, and positive and negative electrodes were alternately inserted to achieve a design capacity of 2.5 Ah, to prepare an electrode assembly with a zigzag-folded separator. A positive electrode terminal and a negative electrode terminal were electrically connected to this electrode assembly.

[0194] <Preparation of Liquid Non-Aqueous Electrolyte> A mixed solvent of propylene carbonate and diethyl carbonate (volume ratio 1:2) was prepared. Lithium hexafluorophosphate (LiPF) was dissolved in this solvent to a concentration of 1 M. Thus, a liquid non-aqueous electrolyte was prepared.

[0195] <Fabrication of Laminated Cell> An exterior member made of a multilayer film including multiple resin layers and metal layers interposed between these resin layers was prepared. The fabricated electrode group was incorporated into this exterior member, and vacuum dried for 8 hours at 120°C. The previously prepared electrolyte solution was then poured into the exterior member, and the exterior member was heat-sealed to fabricate a laminated cell-type lithium-ion secondary battery with a design capacity of 2.5 Ah.

[0196] (Examples 1-2 to 1-7) Laminate cells were produced in the same manner as in Example 1-1, except that the amount of the B source mixed was changed so that the B mass ratio in the resulting negative electrode active material-containing layer would be as shown in the following Table 1. The B content means the ratio of the mass of B to the total mass of the active material particles.

[0197] (Examples 1-8 and 1-9) Unlike Example 1-1, negative electrodes were fabricated without adding boric acid as a B source during fabrication. Specifically, negative electrodes were fabricated in the same manner as in Example 1-1, except that boric acid was omitted when preparing the negative electrode slurry. The fabricated negative electrode was immersed in a solution of ethanol and boric acid in a mass mixture ratio (boric acid / ethanol) of 0.1 to 0.6. The negative electrode was then removed from the solution and dried to fabricate a negative electrode in which B was coated on the surface of the lithium titanate active material. Quantitative analysis of the B content of the negative electrode active material-containing layer was performed using ICP emission spectroscopy, and it was confirmed that the B content relative to the active material was as shown in Table 1. A laminate cell was fabricated in the same manner as in Example 1-1, except for the procedure for coating B on the negative electrode.

[0198] (Example 1-10) Unlike Example 1-3, boric acid as a B source was not added during the preparation of the negative electrode, and instead, boron was directly coated on the active material before the electrode was prepared. The average particle diameter (D ５０A TLO powder with a particle size of 0.8 μm was wet-mixed with boric acid and pure water. The resulting mixture was dried at 200°C in an air atmosphere. The dried active material was quantitatively analyzed for B content using ICP optical emission spectroscopy, confirming that the B content relative to the active material was as shown in Table 1. Using this active material, a negative electrode slurry was prepared to have the same electrode composition as in Example 1-3, and a negative electrode was fabricated using the same procedure as in Example 1-3. The state of the negative electrode was examined using STEM and glow discharge optical emission spectroscopy, confirming that a uniform coating containing B was formed on the surface of the negative electrode active material in the resulting negative electrode. A laminate cell was fabricated using the same method as in Example 1-3, except for using this negative electrode.

[0199] (Example 1-11) A negative electrode was produced in the same manner as in Example 1-10, except that the mass ratio of active material to boron was changed from 100:0.15 to 100:0.22. The state of the negative electrode was examined by STEM and glow discharge optical emission spectroscopy, and it was confirmed that a uniform coating containing B was formed on the surface of the negative electrode active material in the obtained negative electrode. A laminate cell was produced in the same manner as in Example 1-10, except that the obtained negative electrode was used.

[0200] Example 1-12 A laminate cell was produced in the same manner as in Example 1-10, except that monoclinic titanium dioxide (TiO2(B)) was used as the negative electrode active material instead of TLO powder, and the mass ratio of the conductive agent to the binder relative to the negative electrode active material (TiO2(B)) (active material:conductive agent:binder) was changed from 100:3:3 to 100:10:10.

[0201] Example 1-13 A laminate cell was produced in the same manner as in Example 1-10, except that monoclinic niobium titanium oxide (TiNbO; TNO) was used as the negative electrode active material instead of TLO powder, and the mass ratio of the conductive agent to the binder relative to the negative electrode active material (TNO) (active material:conductive agent:binder) was changed from 100:3:3 to 100:10:10.

[0202] (Example 1-14) A negative electrode was produced in the same manner as in Example 1-13, except that when the active material and boric acid were mixed with pure water to directly coat the active material with a B-containing coating, lithium hydroxide was added until the pH of the pure water reached 7. A laminate cell was produced in the same manner as in Example 1-13, except that the obtained negative electrode was used.

[0203] Comparative Example 1-1 A laminate cell was produced in the same manner as in Example 1-1, except that the addition of boric acid was omitted when producing the negative electrode.

[0204] Comparative Examples 1-2 to 1-5 Laminate cells were produced in the same manner as in Example 1-1, except that the amount of B source mixed was changed so that the B mass ratio in the resulting negative electrode active material-containing layer would be as shown in Table 1 below.

[0205] (Comparative Example 1-6) As in Example 1-10, boric acid, the B source, was not added during negative electrode preparation, and direct coating was performed on the active material prior to electrode preparation. However, the temperature at which the mixture of TLO powder, boric acid, and pure water was dried in an air atmosphere was changed from 200°C to 600°C. Aside from the change in drying temperature, a negative electrode was prepared using the same procedure as in Example 1-10. STEM observation of the cross section of the negative electrode active material particles confirmed that B diffused into the active material particles upon heat treatment and coexisted with the negative electrode active material constituent elements Li, Ti, and O at a depth of several nanometers within the active material particles, and that the amount of B on the outer surface was reduced by heat treatment. Thus, it was confirmed that no coating containing B was formed in this example. A laminate cell was prepared using the same method as in Example 1-10, except for using the resulting negative electrode.

[0206] Comparative Example 1-7 A laminate cell was produced in the same manner as in Comparative Example 1-1, except that monoclinic titanium dioxide (TiO2(B)) was used instead of TLO powder as the negative electrode active material, and the mass ratio of the conductive agent to the binder relative to the negative electrode active material (TiO2(B)) (active material:conductive agent:binder) was changed from 100:3:3 to 100:10:10.

[0207] Comparative Example 1-8 A laminate cell was produced in the same manner as in Comparative Example 1-1, except that monoclinic niobium titanium oxide (TiNbO; TNO) was used as the negative electrode active material instead of TLO powder, and the mass ratio of the conductive agent to the binder relative to the negative electrode active material (TNO) (active material:conductive agent:binder) was changed from 100:3:3 to 100:10:10.

[0208] Comparative Example 1-9 A laminate cell was produced in the same manner as in Comparative Example 1-1, except that the electrolyte salt was changed from LiPF6 to lithium tetrafluoroborate (LiBF4).

[0209] Example 2-1 A laminate cell was produced in the same manner as in Example 1-1, except that the negative electrode slurry was prepared as follows.

[0210] The negative electrode active material has an average particle diameter (D ５０ ) is 0.8 μm in diameter and has a spinel structure. 12 ) powder, acetylene black as a conductive agent, polyvinylidene fluoride (Kureha KF Polymer Grade No. W#1700) as a binder, boric acid as a B source, and di-2-butoxyaluminum acetylacetate (manufactured by Matsumoto Fine Chemical Co., Ltd., product name A-1033R) as an Al source were prepared. These materials were mixed so that the mass ratio (active material: conductive agent: binder: B: Al) in the resulting negative electrode active material-containing layer was 100:3:3:0.15:0.1. This mixture was dispersed in NMP to prepare a negative electrode slurry. The solids concentration in the negative electrode slurry was 58% by mass.

[0211] Examples 2-2 to 2-6 Laminate cells were produced in the same manner as in Example 2-1, except that the amount of boric acid mixed as the B source and the amount of di-2-butoxyaluminum acetylacetate mixed as the Al source were changed so that the B mass ratio and Al mass ratio in the resulting negative electrode active material-containing layer would be as shown in Table 2 below.

[0212] (Example 2-7) Unlike Example 2-1, a negative electrode was prepared without adding a B source or an Al source during preparation. The prepared negative electrode was immersed in a solution containing ethanol and an Al source. The negative electrode was then removed from the solution and dried. This electrode was then impregnated in a solution containing ethanol and boric acid, which is a B source. The negative electrode was then removed from the solution and dried to prepare a negative electrode in which a coating containing B and Al was coated on the surface of the lithium titanate active material. The amounts of B and Al were as shown in Table 2. A laminate cell was prepared in the same manner as in Example 2-1, except for the procedure for coating the negative electrode with B and Al.

[0213] Example 2-8 A laminate cell was produced in the same manner as in Example 2-1, except that monoclinic titanium dioxide (TiO2(B)) was used as the negative electrode active material instead of the TLO powder, and the mixing amounts of various components were changed so that the mass ratio of negative electrode active material (TiO2(B)):conductive agent:binder:B:Al in the resulting negative electrode active material-containing layer was changed from 100:3:3:0.15:0.1 to 100:10:10:0.1:0.05.

[0214] Example 2-9 A laminate cell was produced in the same manner as in Example 2-1, except that monoclinic niobium titanium oxide (TiNbO; TNO) was used instead of TLO powder as the negative electrode active material, and the mixing amounts of various components were changed so that the mass ratio of negative electrode active material (TNO):conductive agent:binder:B:Al in the resulting negative electrode active material-containing layer was changed from 100:3:3:0.15:0.1 to 100:10:10:0.1:0.05.

[0215] Comparative Examples 2-1 and 2-2 Laminate cells were produced in the same manner as in Example 2-1, except that the amounts of boric acid as the B source and di-2-butoxyaluminum acetylacetate as the Al source were changed so that the B mass ratio and Al mass ratio in the resulting negative electrode active material-containing layer would be as shown in Table 2 below.

[0216] Example 3-1 A laminate cell was produced in the same manner as in Example 1-1, except that the negative electrode slurry was prepared as follows.

[0217] The negative electrode active material has an average particle diameter (D ５０ ) is 0.8 μm in diameter and has a spinel structure. 12 ) powder, acetylene black as a conductive agent, polyvinylidene fluoride (Kureha KF Polymer Grade No. W#1700) as a binder, boric acid as a B source, and ethyl silicate 40 (manufactured by Colcoat Co., Ltd.) as a Si source were prepared. These materials were mixed so that the mass ratio (active material: conductive agent: binder: B: Si) in the resulting negative electrode active material-containing layer was 100:3:3:0.1:0.05. This mixture was dispersed in NMP to prepare a negative electrode slurry. The solids concentration in the negative electrode slurry was 58% by mass.

[0218] Examples 3-2 to 3-6 Laminate cells were produced in the same manner as in Example 3-1, except that the amounts of boric acid as the B source and ethyl silicate 40 as the Si source were changed so that the B mass ratio and Si mass ratio in the resulting negative electrode active material-containing layer would be as shown in Table 3 below.

[0219] (Example 3-7) Unlike Example 3-1, a negative electrode was prepared without adding a B source or a Si source during preparation. The prepared negative electrode was immersed in a solution containing ethanol and a Si source. The negative electrode was then removed from the solution and dried to prepare a negative electrode in which a Si-containing coating was coated on the surface of the lithium titanate active material. This electrode was then impregnated in a solution containing ethanol and boric acid, which is a B source. The negative electrode was then removed from the solution and dried to prepare a negative electrode in which B and Si were coated on the surface of the lithium titanate active material. The B and Si amounts were as shown in Table 3. A laminate cell was prepared in the same manner as in Example 3-1, except for the procedure for coating the B+Si on the negative electrode.

[0220] Comparative Examples 3-1 to 3-4 Laminate cells were produced in the same manner as in Example 3-1, except that the amounts of boric acid as the B source and ethyl silicate 40 as the Si source were changed so that the B mass ratio and Si mass ratio in the resulting negative electrode active material-containing layer would be as shown in Table 3 below.

[0221] Example 4-1 A laminate cell was produced in the same manner as in Example 1-1, except that the negative electrode slurry was prepared as follows.

[0222] The negative electrode active material has an average particle diameter (D ５０ ) is 0.8 μm in diameter and has a spinel structure. 12 ) powder, acetylene black as a conductive agent, polyvinylidene fluoride (Kureha KF Polymer Grade No. W#1700) as a binder, boric acid as a B source, ethyl silicate 40 (Colcoat Co., Ltd.) as a Si source, and di-2-butoxyaluminum acetylacetate (manufactured by Matsumoto Fine Chemical Co., Ltd., product name A-1033R) as an Al source were prepared. These materials were mixed so that the mass ratio (active material: conductive agent: binder: B: Si: Al) in the resulting negative electrode active material-containing layer was 100:3:3:0.1:0.05:0.05. This mixture was dispersed in NMP to prepare a negative electrode slurry. The solids concentration in the negative electrode slurry was 58% by mass.

[0223] Examples 4-2 to 4-6 Laminate cells were produced in the same manner as in Example 4-1, except that the amounts of boric acid as the B source, ethyl silicate 40 as the Si source, and di-2-butoxyaluminum acetylacetate as the Al source were changed so that the B mass, Si mass, and Al mass ratios in the resulting negative electrode active material-containing layer would be as shown in Table 4 below.

[0224] Example 4-7 Unlike Example 4-1, a negative electrode was prepared without adding a B source, Si source, or Al source during preparation. The prepared negative electrode was immersed in a solution containing ethanol, an Al source, and a Si source. The negative electrode was then removed from the solution and dried to prepare a negative electrode in which a coating containing Si and Al was coated on the surface of the lithium titanate active material. This electrode was then impregnated in a solution containing ethanol and boric acid, the B source. The negative electrode was then removed from the solution and dried to prepare a negative electrode in which B, Si, and Al were coated on the surface of the lithium titanate active material. The amounts of B, Si, and Al were as shown in Table 3. A laminate cell was prepared in the same manner as in Example 3-1, except for the B+Si+Al coating procedure.

[0225] Comparative Examples 4-1 to 4-4 Laminate cells were produced in the same manner as in Example 4-1, except that the mixing amounts of boric acid as the B source, ethyl silicate 40 as the Si source, and di-2-butoxyaluminum acetylacetate as the aluminum source were changed so that the B mass ratio, Si mass ratio, and Al mass ratio in the resulting negative electrode active material-containing layer would be as shown in Table 3 below.

[0226] <Negative Electrode Measurement> The negative electrode active material-containing layer was removed from the negative electrode prepared in each example using the procedure described above, and the amounts of Ti, B, Si, and Al were quantitatively analyzed using an ICP optical emission spectrometer. The analysis results confirmed that the electrodes prepared by adding boric acid, adding an organoaluminum compound together with boric acid, adding a condensed silicate together with boric acid, or adding a condensed silicate and an organoaluminum compound together with boric acid contained the designed amounts of B, Si, and Al in the negative electrode. Furthermore, it was confirmed that a trace amount of B derived from LiBF was present in the negative electrode in Comparative Example 1-9, in which LiBF was used as the electrolyte supporting salt.

[0227] Furthermore, SEM-EDX and GD-OES confirmed that B, Si, and Al were present on the electrode surface and the active material surface. However, in Comparative Examples 1-1, 1-7, and 1-8, in which boric acid, Si-based metal alkoxide, and organoaluminum compound were not added, B, Si, and Al were not detected. ToF-SIMS confirmed that the B-containing compound on the active material surface in Examples 1-1 to 1-14 was in the form of lithium borate. Specifically, positive secondary ions ( 57 Li2BO2 + , 87 Li4BO3 + ) and negative secondary ions ( 93 LiB2O4 - , 143 Li2B3O6 - ) was detected.

[0228] <Charge-Discharge Cycle Test> A charge-discharge cycle test was carried out on the secondary batteries (laminated cells) fabricated in the above examples under the following conditions.

[0229] A secondary battery using lithium titanate (hereinafter referred to as TLO) having a spinel structure as the negative electrode active material was subjected to constant current constant voltage (CCCV) charging at a voltage of 2.7 V and a current of 2 C (5 A), and then constant current (CC) discharging at a voltage of 1.5 V and a current of 2 C (5 A).

[0230] Furthermore, for secondary batteries using monoclinic titanium dioxide (TiO2(B)) as the negative electrode active material, constant current constant voltage (CCCV) charging was performed at a voltage of 2.8 V and a current of 2 C (5 A), followed by constant current (CC) discharging at a voltage of 1.5 V and a current of 2 C (5 A).

[0231] The secondary battery using monoclinic niobium titanium oxide (hereinafter referred to as TNO) as the negative electrode active material was subjected to constant current constant voltage (CCCV) charging at a voltage of 2.9 V and a current of 2 C (5 A), and then constant current (CC) discharging at a voltage of 1.5 V and a current of 2 C (5 A).

[0232] Each secondary battery fabricated was subjected to a charge-discharge cycle test in a 45°C environment to examine the change in battery resistance at a 50% state of charge (SOC). Specifically, the change in resistance before and after 3,000 charge-discharge cycles under the aforementioned conditions was examined in both 25°C and −20°C environments. For each secondary battery, the SOC was adjusted to 50% by 1C charging in a 25°C environment before 3,000 charge-discharge cycles. The initial resistance was determined by measuring the 10-second discharge resistance at 1C (2.5 Ah) in a thermostatic chamber at 25°C and −20°C. To examine the resistance after 3,000 cycles, each secondary battery was discharged at 1.5V and 1C in a 25°C environment, then adjusted to 50% SOC by 1C charging. The 10-second discharge resistance at 1C (2.5 Ah) was then examined in a thermostatic chamber at 25°C and −20°C.

[0233] The results are shown in Tables 1 to 4 below. Tables 1 to 4 show the type of negative electrode active material, the B mass ratio, the Si mass ratio, and the Al mass ratio for each negative electrode example. Tables 1 to 4 also show the results of the charge-discharge cycle test for each example.

[0234] The B mass ratio, Si mass ratio, and Al mass ratio are the mass ratios of B, Si, and Al contained in the coating that covers the surface of the negative electrode active material particles, expressed as percentages, relative to the respective negative electrode active materials. For example, since the content ratio of the active material (TLO) and Si in the negative electrode active material-containing layer obtained in Example 1-1 was 100:0.2 in mass ratio, the mass ratio of Si to the active material was 0.2 / 100 = 2 × 10 -3 This figure is expressed as "0.2%".

[0235] The column "Initial resistivity at 25°C (%)" shows the initial resistance value of the secondary battery of each example in a 25°C environment as a ratio, with the corresponding value of Comparative Example 1-1 for the TLO cell, Comparative Example 1-7 for the TiO2(B) cell, and Comparative Example 1-8 for the TNO cell set to 100. The column "Resistance increase rate (%) at 25°C after 3000 cycles" shows the resistance increase rate of the secondary battery of each example in a 25°C environment (the ratio of the resistance value measured after 3000 cycles to the initial resistance value) as a ratio to the initial resistivity of the secondary battery at 25°C.

[0236] The column "-20°C initial resistivity (%)" shows the initial resistance value of the secondary battery of each example in a -20°C environment as a ratio, with the corresponding value of Comparative Example 1-1 for the TLO cell, Comparative Example 1-7 for the TiO2(B) cell, and Comparative Example 1-8 for the TNO cell set to 100. The column "After 3000 cycles, -20°C, resistance increase rate (%)" shows the resistance increase rate of the secondary battery of each example in a -20°C environment (the ratio of the resistance value measured after 3,000 cycles to the initial resistance value) as a ratio to the -20°C initial resistivity of that secondary battery.

[0237]

[0238]

[0239]

[0240]

[0241] As shown in Examples 1-1 to 1-11 (Table 1), in secondary batteries having a negative electrode in which the active material particles contained lithium titanate having a spinel structure and a predetermined amount of B, the increase in initial resistance at 25° C. due to the coating treatment was small, and the rate of increase in resistance was also kept low. That is, in Examples 1-1 to 1-11, excellent cycle life performance and low-temperature performance were achieved.

[0242] In contrast, in Comparative Example 1-1, in which the active material surface was not coated with B, and in Comparative Examples 1-2 and 1-3, in which the coating amount was small, the increase in resistance after cycling was large. As shown in Comparative Examples 1-4 and 1-5, when the B content exceeded 0.50 mass%, not only was the initial resistance high, but a large rate of increase in resistance was also observed.

[0243] When Examples 1-10 and 1-11, in which a boron-containing coating layer was formed on the active material particles by mixing active material particles with boric acid, were compared with Comparative Example 1-6, in which boron was similarly provided on the active material particles by mixing active material particles with boric acid, the following observations were made. In Examples 1-10 and 1-11, in which the drying temperature during the formation of the boron-containing coating layer on the active material particles was 200°C, the effect of suppressing the increase in resistance after the cycle test was confirmed. However, in Comparative Example 1-6, in which the drying temperature was 600°C, the effect of suppressing the increase in resistance after the cycle test was not as good as in Examples 1-10 and 1-11. As mentioned above, in the negative electrodes of Examples 1-10 and 1-11 before the cycle test, a uniform coating containing B was evenly formed on the surface of the negative electrode active material. In contrast, in the negative electrode of Comparative Example 1-6, although areas with a high B concentration were confirmed near the surface of the negative electrode active material, B was not present on the outer surface of the active material particles. B was present inside the particles near the surface, and no coating covering the surface of the active material was formed.

[0244] From the results of Examples 1-12 to 1-14 and Comparative Examples 1-7 and 1-8, it was found that even in cells using negative electrodes in which the active material was changed from lithium titanate to TiO2(B) or TNO, the increase in resistance during cycling could be suppressed by forming a coating layer containing boron on the surface of the active material.

[0245] As mentioned above, in the negative electrodes of Examples 1-1 to 1-14, ToF-SIMS before the cycle test showed positive secondary ions ( 57 Li2BO2 + , 87 Li4BO3 + ) and negative secondary ions ( 93 LiB2O4 - , 143 Li2B3O6 - ) was detected, it was confirmed that the compound containing B on the surface of the active material was in the form of lithium borate. In contrast, in Comparative Example 1-9, in which lithium tetrafluoroborate was used as the electrolyte supporting salt, a trace amount of boron was detected by ICP, but no peak suggesting the presence of lithium borate bonds was observed by ToF-SIMS, and it was confirmed that LiBOF3 - , LiB2O3F2 -, and LiB2OF6 - As shown in Table 1, in Comparative Example 1-9, the increase in resistance after the cycle test was large. This result shows that the effect of suppressing the increase in resistance during cycling can be achieved in a negative electrode in which lithium borate bonds are present in the coating layer on the surface of the active material.

[0246] As shown in Examples 2-1 to 2-7 (Table 2), secondary batteries having negative electrodes in which active material particles contained lithium titanate having a spinel structure and a coating containing predetermined amounts of both B and Al had low initial resistance in environments of 25° C. and −20° C., and also had low rates of resistance increase. That is, Examples 2-1 to 2-7 were able to achieve excellent cycle life performance and low-temperature performance.

[0247] Furthermore, from the results of Examples 2-8 and 2-9, it was found that even in cells using negative electrodes in which the active material was changed from lithium titanate to TiO2(B) or TNO, the increase in resistance after cycle testing could be suppressed by forming a coating layer containing boron and Al on the surface of the active material.

[0248] As shown in Comparative Example 2-1, when the total content of B+Al was less than 0.03 mass%, the initial resistance was small but the rate of increase in resistance after the cycle test was large. Also, as shown in Comparative Example 2-2, when the total content of B+Al exceeded 0.50 mass%, not only did the rate of increase in resistance increase, but the initial resistance also tended to increase.

[0249] As shown in Examples 3-1 to 3-7 (Table 3), it was confirmed that the increase in resistance of the secondary battery having a negative electrode with a coating containing both B and Si in predetermined amounts was suppressed in the cell after the cycle test. On the other hand, as shown in Comparative Examples 3-1 to 3-3, it was found that the increase in resistance was large when the B+Si coating amount deviated from the appropriate value.

[0250] As shown in Examples 4-1 to 4-7 (Table 4), it was confirmed that the increase in resistance of the secondary battery having a negative electrode with a coating containing all of B, Si, and Al in predetermined amounts was suppressed in the cell after the cycle test. On the other hand, as shown in Comparative Examples 4-1 to 4-4, it was found that the increase in resistance was large when the B+Si+Al coating amount deviated from the appropriate value.

[0251] According to at least one of the embodiments and examples described above, an electrode is provided. The electrode includes an active material-containing layer. The active material-containing layer includes a plurality of active material particles and a coating covering at least a portion of the surface of the particles, the coating containing at least boron (B) and may further contain Si, or Si+Al, or Al. The mass proportions of B, B+Si, B+Al, and B+Si+Al relative to the mass of the active material particles are each within the range of 0.03% by mass to 0.50% by mass.

[0252] By forming a coating containing B, B + Si, B + Al, or B + Si + Al on the surface of the active material particles, the coating formation reaction accompanying self-discharge between the electrolyte and the active material can be suppressed. Therefore, by using an electrode with the above configuration as a negative electrode, the increase in positive electrode potential due to charge / discharge cycles can be suppressed, and positive electrode deterioration due to SOC deviation and oxygen gas generation at the positive electrode can be suppressed. Furthermore, by suppressing the formation of a coating on the active material surface, the increase in low-temperature resistance can be suppressed. Therefore, an electrode with the above configuration can achieve excellent cycle life performance and low-temperature performance.

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

[0254] Several embodiments of the present invention are described below. [1] An electrode comprising an active material-containing layer, the active material-containing layer including a plurality of active material particles and a coating that covers at least a portion of the surfaces of the plurality of active material particles and contains boron or boron and at least one of Si and Al, and the ratio of the total mass of the boron, Si and Al to the mass of the active material particles is in the range of 0.03 mass % to 0.50 mass %. [2] The boron is an electrode that is characterized by a positive secondary ion in time-of-flight secondary ion mass spectrometry.57 Li2BO2 + or 87 Li4BO3 + Negative secondary ions include 93 LiB2O4 - or 143 Li2B3O6 - [3] The electrode according to [1] or [2], wherein the active material particles contain a titanium-containing oxide. [4] The active material particles have a spinel structure and are bonded to a lithium borate bond containing a compound represented by the general formula Li 4+a TiO 12 The electrode according to any one of [1] to [3], which contains lithium titanate represented by the formula: where the subscript a is within the range of -1≦a≦3. [5] A secondary battery comprising: a negative electrode; a positive electrode; and an electrolyte, wherein the negative electrode comprises the electrode according to any one of [1] to [4]. [6] A battery pack comprising the secondary battery according to [5]. [7] The battery pack according to [6], which comprises a plurality of the secondary batteries, the secondary batteries being electrically connected in series, in parallel, or in a combination of series and parallel.

[0255] DESCRIPTION OF SYMBOLS 1...electrode group, 2...exterior member, 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...container, 32...lid, 33...holding Protective sheet, 34...printed wiring board, 35...wiring, 100...secondary battery, 200...battery assembly, 300...battery pack, 342...positive electrode connector, 343...negative electrode connector, 345...thermistor, 346...protective circuit, 342a...wiring, 343a...wiring, 350...external terminal for supplying current, 352...positive terminal, 353...negative terminal, 348a...positive wiring, 348b...negative wiring.

Claims

1. An electrode comprising an active material-containing layer, the active material-containing layer comprising a plurality of active material particles and a coating covering at least a portion of the surface of the plurality of active material particles and containing boron or containing boron and at least one of Si and Al, wherein the ratio of the total mass of the boron, Si and Al to the mass of the active material particles is within the range of 0.03 mass % or more and 0.50 mass % or less.

2. The boron is a positive secondary ion in time-of-flight secondary ion mass spectrometry. 57 Li2BO2 + or 87 Li4BO3 + Negative secondary ions include 93 LiB2O4 - or 143 Li2B3O6 - 10. The electrode of claim 1 having a lithium borate bond comprising:

3. The electrode according to claim 1 or 2, wherein the active material particles comprise a titanium-containing oxide.

4. The active material particles have a spinel structure and have the general formula Li 4+a TiO 12 The electrode of claim 1 or 2, comprising lithium titanate represented by the formula:

5. A secondary battery comprising: a negative electrode; a positive electrode; and an electrolyte, wherein the negative electrode includes the electrode according to claim 1 or 2.

6. A battery pack comprising the secondary battery according to claim 5.

7. The battery pack according to claim 6, comprising a plurality of said secondary batteries, said secondary batteries being electrically connected in series, in parallel, or in a combination of series and parallel.

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