Lithium-ion battery and control method thereof

Abstract

To provide a power storage device that has improved the abnormality detection accuracy when the device is overcharged.SOLUTION: A power storage device according to the present disclosure has a battery laminate, and a deformation detection sensor that detects the amount of deformation of the battery laminate. The average expansion coefficient of the battery laminate within a charge rate range of 80-100% is 2.8 times or more of the average expansion coefficient of the battery laminate within a charge rate range of 0-80%.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to a power storage device and a control method thereof. [Background technology]

[0002] Patent Documents 1 and 2 disclose a method for detecting deformation of a sealed secondary battery, which detects a change in an external field caused by deformation of a polymer matrix layer and detects the deformation of the sealed secondary battery based on the change.

[0003] Patent document 3 discloses a storage battery device that includes a measuring element that measures distortion caused by a force that changes the shape of the storage battery, detects the state of the storage battery based on the distortion measurement results obtained from the measuring element, and detects abnormalities in the storage battery from the detection results. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-99193 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-31360 [Patent Document 3] Japanese Patent Application Laid-Open No. 2012-234629 Summary of the Invention [Problem to be solved by the invention]

[0005] According to the conventional technology, there is a possibility that the accuracy of detecting the state of charge, particularly the accuracy of detecting an abnormality during overcharge, may be reduced.

[0006] The present disclosure has been made in consideration of these points, and provides a power storage device with improved accuracy in detecting the state of charge, particularly in detecting an abnormality during overcharge. [Means for solving the problem]

[0007] <Aspect 1> a battery stack and a deformation detection sensor that detects the amount of deformation of the battery stack; The average expansion rate of the battery stack in the range of a charge rate of 80 to 100% is 2.8 times or more the average expansion rate of the battery stack in the range of a charge rate of 0 to 80%. Energy storage device. <Aspect 2> the battery stack has, in this order, a positive electrode current collector layer, a positive electrode active material layer, a separator layer or a solid electrolyte layer, a negative electrode active material layer, and a negative electrode current collector layer; The negative electrode active material layer contains at least artificial graphite. The power storage device according to embodiment 1. <Aspect 3> 3. The method for controlling a power storage device according to aspect 1 or 2, wherein charging of the power storage device is stopped when the amount of deformation detected by the deformation detection sensor during charging exceeds a predetermined value. [Effects of the Invention]

[0008] According to the present disclosure, it is possible to obtain a power storage device with improved accuracy in detecting the state of charge, particularly in detecting an abnormality during overcharge. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a flowchart of a method for controlling a storage battery according to an embodiment. [Figure 2] FIG. 1 is a diagram showing the relationship between the charging rate and the battery cell expansion rate of a lithium ion battery fabricated in an example. DETAILED DESCRIPTION OF THE INVENTION

[0010] The power storage device according to the embodiment of the present disclosure may be a lithium ion battery, particularly a lithium ion battery used as an on-board battery for hybrid vehicles, electric vehicles, etc. An example of a power storage device according to the embodiment of the present disclosure will be specifically described below. The power storage device according to the present disclosure is not limited to this embodiment.

[0011] The power storage device according to the present disclosure includes a battery stack and a deformation detection sensor that detects the amount of deformation of the battery stack.

[0012] (battery stack) The average expansion rate of the battery stack varies depending on the charge rate. The average expansion rate of the battery stack at a charge rate of 80 to 100% is at least 2.8 times the average expansion rate of the battery stack at a charge rate of 0 to 80%. For example, the average expansion rate of the battery stack at a charge rate of 80 to 100% may be at least 3.0 times, at least 4.0 times, at least 5.0 times, at least 6.0 times, at least 7.0 times, at least 8.0 times, or at least 9.0 times the average expansion rate of the battery stack at a charge rate of 0 to 80%, or may be at most 20.0 times, at most 19.0 times, at most 18.0 times, at most 17.0 times, at most 16.0 times, at most 15.0 times, at most 14.0 times, at most 13.0 times, at most 12.0 times, at most 11.0 times, or at most 10.0 times.

[0013] The phenomenon of the battery stack swelling specifically is undesirable from the viewpoint of restraining the batteries, but it can improve the accuracy of detecting the state of charge, particularly the accuracy of detecting abnormalities during overcharge. In order to specifically swell the battery stack as in the present disclosure, that is, the expansion rate of the battery stack in the normal range of use must be low and the expansion rate must be high. 80~100% In order to increase the expansion rate of the battery stack in this range, it is possible to use a specific negative electrode active material, adjust the capacity ratio of the positive electrode to the negative electrode, adjust the compounding ratio of the active materials in the active material layer, or adjust the density of the active material layer.

[0014] The battery stack may have a positive electrode current collector layer, a positive electrode active material layer, a separator layer or a solid electrolyte layer, a negative electrode active material layer, and a negative electrode current collector layer in this order. The power storage device may have a bipolar structure in which a plurality of battery stacks are electrically connected.

[0015] (Positive electrode current collector layer) The positive electrode current collector layer may include a metal foil. The metal foil may be an aluminum foil, a metal foil such as SUS, Cu, Ni, Fe, Ti, Co, or Zn, a foil using a clad material of a metal such as SUS, Cu, Ni, Fe, Ti, Co, or Zn with Al, or a foil in which a metal surface is coated with Al. In particular, the metal foil may be an aluminum foil.

[0016] The metal foil may have a carbon material and a binder binding the carbon material on its surface. The carbon material may be acetylene black, furnace black, channel black, thermal black, activated carbon, carbon, carbon fiber, graphite, etc. The binder may be PESE (polyethylene sebacate), PPSU (polyphenylsulfone), PBI (polybenzimidazole), PESU (polyethersulfone), etc. The binder may be PVdF (polyvinylidene fluoride), BR (butylene rubber), SBR (styrene butadiene rubber), etc.

[0017] (Cathode active material layer) The positive electrode active material may be, for example, an oxide active material. A ternary active material or a lithium iron phosphate active material is preferred. The positive electrode active material may be, for example, in the form of particles. The content of the positive electrode active material in the positive electrode active material layer is not limited. The content may be, for example, 40% by mass or more and 99% by mass or less, and a content of 90% by mass or more is preferred.

[0018] (separator layer) The separator layer may be an inorganic porous layer impregnated with an electrolytic solution. For example, the inorganic porous layer may contain inorganic fine particles such as aluminum oxide / hydroxide, boehmite, titania, magnesia, and zirconia. The average particle size of the inorganic fine particles may be in the range of 10 nm to 50 μm. Alternatively, the separator layer may be a porous polymer layer impregnated with an electrolytic solution.

[0019] (Solid electrolyte layer) The solid electrolyte layer contains at least a solid electrolyte. It may further contain a binder. The solid electrolyte may be, for example, an inorganic solid electrolyte such as a sulfide solid electrolyte. The solid electrolyte may be, for example, in the form of particles. The average particle size of the particulate solid electrolyte may be, for example, 0.1 μm or more and 50 μm or less. The binder may be PVdF (polyvinylidene fluoride), BR (butylene rubber), SBR (styrene butadiene rubber), etc.

[0020] (Negative electrode active material layer) The negative electrode active material is preferably one that can absorb and release metal ions and that exhibits a high expansion rate at the end of charging. Artificial graphite may be used as such a material. Artificial graphite is artificially produced graphite and is sometimes referred to as synthetic graphite. Artificial graphite is generally obtained by heat-treating coal coke as a raw material to graphitize it.

[0021] The particles of the negative electrode active material may be formed by granulating single particles to form secondary particles. The negative electrode active material layer may further contain a binder. The binder may be PVdF (polyvinylidene fluoride), BR (butylene rubber), SBR (styrene butadiene rubber), or the like. The content of the negative electrode active material in the negative electrode active material layer is not limited. The content may be, for example, 40% by mass or more and 100% by mass or less, and a content of 90% by mass or more is preferred.

[0022] (negative electrode current collector layer) The negative electrode current collector layer may be the same as the positive electrode current collector layer, but it is preferable to use copper foil as the metal foil.

[0023] The amount of the negative electrode active material layer carried on the negative electrode current collector layer is, for example, 10 mg / cm 2 More than 15-50mg / cm 2 In addition, the capacity ratio between the positive electrode active material and the negative electrode active material can be adjusted so that the battery stack expands appropriately within the range of normal use and the expansion rate of the battery stack increases only when the battery stack falls into an overcharged state, and this capacity ratio may be set to, for example, 1.05 to 1.2.

[0024] (deformation detection sensor) The deformation detection sensor is not limited as long as it detects the amount of deformation of the battery stack. For example, the deformation detection sensor may be a linear potentiometer, whose output voltage changes with the linear movement of the contact. The deformation detection sensor may also be a sensor that uses a piezoelectric element to detect the displacement of the battery stack.

[0025] (Method for controlling an electricity storage device) As shown in FIG. 1, the energy storage device of this embodiment can prevent overcharging by stopping charging of the energy storage device when the amount of deformation of the battery stack detected by the deformation detection sensor during charging exceeds a predetermined value.

[0026] Generally, automotive batteries are frequently used when the charge rate is between 10 and 90%. If the battery falls into an overcharged state due to some kind of failure mode, the battery may become unstable. Therefore, it is necessary to detect this in advance and maintain the battery in a safe state. According to this embodiment, when the battery is charged to a level outside the normal use range, the expansion rate of the battery stack increases significantly, making it easy to detect abnormal expansion and maintaining the battery in a safe state.

[0027] The mechanism behind this expansion characteristic at the end of charging is unclear, but it is thought to be related to the crystal arrangement and crystallite size within the active material. Specifically, as Li is inserted between the graphite layers, strain accumulates between adjacent crystals, and this strain can no longer be absorbed by voids, triggering the outward expansion.

[0028] Furthermore, when the negative electrode active material is granulated into secondary particles, strain is thought to accumulate inside the secondary particles because the particles are compressed during granulation. When the individual particles expand in this state, the expansion strain is partially absorbed by the voids inside the secondary particles. However, once a certain expansion rate is reached, the voids can no longer absorb the strain, and the secondary particles expand outward. At the same time, it is thought that the strain generated during granulation compression is alleviated, resulting in a greater amount of outward expansion. [Example]

[0029] The present disclosure will be described in more detail below with reference to examples. The following examples are illustrative and do not limit the present disclosure. In the following examples, "Graphite A" used as the negative electrode active material is an artificial graphite that expands at a high rate toward the end of the charging reaction (6C+Li→C6Li), "Graphite B" is an artificial graphite that expands at a slower rate than Graphite A, and "Graphite C" is natural graphite.

[0030] Example 1 (Preparation of negative electrode) First, a negative electrode consisting of a negative electrode active material layer and a negative electrode current collector layer was fabricated. Graphite A, a binder (styrene butadiene rubber: SBR), and a thickener (carboxymethyl cellulose: CMC) were mixed in a weight ratio of 97:1.5:1.5 as the negative electrode active material, and distilled water was added to achieve a solid content of 55-60%. The mixture was then thoroughly kneaded using a kneader to obtain a negative electrode paste for the negative electrode active material layer. The resulting negative electrode paste was applied to a copper foil current collector layer using a doctor blade and dried at 100°C for 15 minutes to completely evaporate the water, forming a negative electrode layer. The loading of the negative electrode layer after drying was 12 mg / cm. 2 The produced negative electrode was compressed by a biaxial roll press to a density of 1.5 g / cc, to obtain a pressed negative electrode.

[0031] (Preparation of positive electrode) Next, a positive electrode was fabricated, consisting of a positive electrode active material layer and a positive electrode current collector layer. 0.33 Mn0.33 Co 0.33 Using O2, the positive electrode active material, conductive additive (acetylene black: AB), and binder (polyvinylidene fluoride: PVdF) were mixed in a weight ratio of 95:2.5:2.5, and N-methylpyrrolidone (NMP) was added as a solvent. The particles were thoroughly dispersed using a kneader or similar device to obtain a positive electrode paste. The positive electrode paste was applied to an aluminum foil current collector using a doctor blade or similar device, and dried at 80°C for at least 15 minutes to form a positive electrode layer. The loading of the positive electrode layer was 22 mg / cm. 2 This positive electrode layer was roll pressed to a density of 2.8 g / cc to obtain a positive electrode.

[0032] (Making lithium-ion batteries) A lithium-ion battery was fabricated using the fabricated negative and positive electrodes. The positive and negative electrode layers were stacked facing each other, and a separator was placed between the electrodes to provide electrical insulation. A polyethylene-based film was used as the separator. The fabricated stack was wrapped in a laminate film, and after injecting the electrolyte, it was sealed under a vacuum of -80 kPa to form a lithium-ion battery. The electrolyte was a carbonate-based solvent mixed in a ratio of EC:DMC:EMC = 3:4:3, with 1.2 M of lithium hexafluorophosphate (LiPF6) dissolved as the lithium salt.

[0033] <<Examples 2 to 3 and Comparative Examples 1 to 6>> As shown in Table 1 below, lithium ion batteries of Examples 2 and 3 and Comparative Examples 1 to 6 were fabricated in the same manner as in Example 1, except that the negative electrode active material and the electrode density after pressing were changed.

[0034] <Evaluation method> The lithium ion batteries of the examples and comparative examples were evaluated by the following methods.

[0035] (Expansion rate) The fabricated lithium-ion battery was attached to a restraining jig equipped with a contact displacement meter, and connected to a charge / discharge device to perform charging and discharging. When measuring the expansion rate, the battery was charged from 2.5 V to 4.25 V at 0.1 C. The expansion rate (%) was calculated by dividing the displacement obtained at this time by the thickness of the battery at the time of fabrication.

[0036] (Expansion characteristics) For the above charge / discharge results, 2.5 V was set to a charging rate of 0% and 4.25 V to a charging rate of 100%, and the slope of the change in the expansion rate up to a charging rate of 80% was measured and defined as expansion rate A (expansion rate (%) / charging rate (%)). Similarly, the slope of the change in the expansion rate (%) with respect to the charging rate from a charging rate of 80% to a charging rate of 100% was measured and defined as expansion rate B (expansion rate (%) / charging rate (%)). The expansion rate X was then defined as expansion rate X = expansion rate B / expansion rate A.

[0037] The results are shown in Table 1. Figure 2 shows the relationship between the normalized charge rate (%) and the expansion rate (%). In the lithium-ion batteries of Examples 1 to 3, in which the battery expansion rate X was 2.8 or higher, abnormality detection during overcharging was possible. In the lithium-ion batteries of Comparative Examples 1 to 6, in which the expansion rate X was less than 2.8, abnormality detection during overcharging was sometimes not possible.

[0038] [Table 1]

Claims

1. a battery stack and a deformation detection sensor that detects the amount of deformation of the battery stack; When charging from 2.5V to 4.25V at 0.1C, 2.5V is a charging rate of 0% and 4.25V is a charging rate of 100%, and the value obtained by dividing the amount of displacement of the battery stack at a certain charging rate (%) by the thickness of the battery at the time of fabrication is taken as the expansion rate (%) at that charging rate. With respect to expansion rate A (expansion rate (%) / charging rate (%)), which is the slope of the change in expansion rate from a charging rate of 0% to 80%, and expansion rate B (expansion rate (%) / charging rate (%)), which is the slope of the change in expansion rate (%) with respect to the charging rate from a charging rate of 80% to a charging rate of 100%, the ratio of expansion rate B / expansion rate A is 2.8 or more, the battery stack has, in this order, a positive electrode current collector layer, a positive electrode active material layer, a separator layer or a solid electrolyte layer, a negative electrode active material layer, and a negative electrode current collector layer; The negative electrode active material layer contains at least artificial graphite. Lithium-ion battery.

2. 2. The method for controlling a lithium ion battery according to claim 1, wherein charging of the lithium ion battery is stopped when the amount of deformation detected by the deformation detection sensor during charging exceeds a predetermined value.

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