JP6171821B2 - Power storage device having life determination function, and battery life determination method - Google Patents

Description

  The present invention relates to a power storage device having a life determination function and a life determination method for an assembled battery.

  In recent years, it is desirable to develop an energy-saving society by using electric vehicles and smart grids that make use of natural energy. Among them, the secondary battery has a large role as a power storage device. In particular, lithium ion secondary batteries have excellent capacity and output, and can contribute to downsizing of the system. At that time, since one lithium ion secondary battery has a small amount of electric power, it is often used as an assembled battery of lithium ion secondary batteries combined in series and in parallel. In this case, it is an indispensable technique to sequentially grasp the deterioration state of the entire assembled battery that changes depending on various use conditions. For example, when a lithium ion secondary battery deteriorates and the battery capacity that can be stored decreases, sufficient power cannot be supplied, and problems such as failure of a device that receives power supply to operate properly occur.

  Conventionally, in order to evaluate the deterioration state of a single battery or an assembled battery, a technique is known in which the impedance of a lithium ion secondary battery is measured and compared with a predetermined standard. In patent document 1, when the impedance of the whole assembled battery is evaluated and it shows more than a specified impedance, the assembled battery is determined to have a lifetime.

  However, in the technique using impedance as in Patent Document 1, the determination accuracy may be reduced depending on the external environment. The lithium ion secondary battery is sensitive to an external temperature. For example, when the temperature is high, the impedance is a low value, and when the temperature is low, the impedance is a high value. Under conditions where the temperature is high, even if the impedance is supposed to be considered as a lifetime, it is estimated to be low and can be used. Conversely, under conditions where the temperature is low, it should be normal. In some cases, it is estimated that the life is long estimated even though the impedance is. In addition, although it is possible to perform temperature correction by a logic circuit, it is necessary to accurately grasp the impedance characteristics of each temperature after deterioration as well as the initial stage, which is not realistic.

JP-A-9-114588

  An object of the present invention is to provide a power storage device capable of accurately determining the life of an assembled battery regardless of the external temperature environment, and a method for determining the life of the assembled battery.

  The power storage device according to the present invention includes a plurality of unit cells connected in series, and at least two of the plurality of unit cells include an assembled battery including a first unit cell and a second unit cell, The first voltage measuring device that measures the voltage of the first cell, the second voltage measuring device that measures the voltage of the second cell, and the voltages of the first cell and the second cell are compared. And a charge / discharge characteristic of the first unit cell and the second unit cell has an inflection region in a charge / discharge curve, and the first unit cell is more than the second unit cell. The normal state of reaching the predetermined voltage in the inflection region first has a normal state, and the comparison unit, when charging and discharging, the second unit cell reaches the predetermined voltage in the inflection region before the first unit cell This is a power storage device having a life determination function characterized by determining a life.

  According to this configuration, in the charging / discharging process, each voltage of the first unit cell and the second unit cell is measured, and from the normal state where the first unit cell reaches the inflection region voltage earlier than the second unit cell, The state in which the second cell reaches the inflection region voltage before the single cell is determined as the life. In other words, when the battery that first becomes the inflection region voltage at the time of discharging is replaced from the first unit cell to the second unit cell, it can be determined that the life of the storage battery is reached. The inflection region is a region where the amount of change in voltage with respect to the discharge capacitance is large, and by measuring the voltage, it can be determined that the state of the inflection region is accurate. In this way, by relatively evaluating the two types of single cells installed in the power storage device, it is possible to accurately determine the life of the assembled battery regardless of the environmental temperature.

  Moreover, it is desirable that the first unit cell and the second unit cell are power storage devices having a life determination function that are configured adjacent to each other in the assembled battery.

  According to this configuration, since the temperature difference between the first unit cell and the second unit cell can be further reduced, the lifetime of the power storage device can be determined with high accuracy.

  Moreover, the life determination method according to the present invention is a method of determining the life of a battery pack in which a plurality of single cells are connected in series, and at least two of the plurality of single cells are charge / discharge exhibiting charge / discharge characteristics. A first cell and a second cell having an inflection region in a curve, and a predetermined voltage in the inflection region of the first cell and a predetermined voltage in the inflection region of the second cell. When the first cell reaches the inflection region voltage at the time of discharge, it is determined as a normal state, and the second cell is first in the inflection region by repeated charge and discharge. This is a method for determining the life of an assembled battery in which the life of the power storage device is determined when a predetermined voltage is reached.

  According to this method, the life of the assembled battery can be accurately determined regardless of the environmental temperature by relatively evaluating the voltages of the two types of unit cells installed in the assembled battery.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide the electrical storage apparatus which can determine the lifetime of an assembled battery accurately irrespective of the temperature environment of an assembled battery, and also to provide the lifetime determination method of an assembled battery.

Power storage device of first embodiment A diagram schematically showing a single cell A charge / discharge curve showing the charge / discharge characteristics of the first cell and the second cell of the first embodiment Cycle characteristics in the high voltage region of the first cell and the second cell of the first embodiment Power storage device of second embodiment Charging / discharging curve showing the charging / discharging characteristics of the first cell, the second cell, and the third cell of the second embodiment Cycle characteristics in the high voltage region of the first cell, the second cell, and the third cell of the second embodiment Cycle characteristics in the high voltage region of the first unit cell and the second unit cell of Example 1 Cycle characteristics when designing the cycle deterioration of Example 2 Cycle characteristics when designing the cycle deterioration of Example 3 Relationship between cycle number and impedance at each temperature in the comparative example

  DESCRIPTION OF EMBODIMENTS Embodiments (embodiments) for carrying out the present invention will be described in detail with reference to the drawings. It should be noted that matters other than the matters specifically mentioned in the present specification and necessary for the implementation of the present invention (for example, the construction means of the assembled battery, the construction of the electrode body unit or the electrolyte constituting the unit cell, the construction of the battery Can be modified based on common technical knowledge in the field.

(Single cell)
FIG. 3 shows an example of a discharge curve showing the charge / discharge characteristics of the first unit cell 14 and the second unit cell 16. In FIG. 3, the horizontal axis represents the normal cell 1 (first embodiment).
(Power storage device)
First, the first embodiment will be described. FIG. 1 schematically shows a power storage device 40 of the present embodiment. The power storage device 40 of the present embodiment includes a first cell 14, a voltage measurement device 30 that measures the voltage of the first cell 14, a second cell 16, and a voltage that measures the voltage of the second cell 16. A measurement device 35 and a comparison unit 38 are included. Further, the first unit cell 14 and the second unit cell 16 are connected in series, and are further connected in series with the plurality of unit cells 12 to constitute the assembled battery 20.

  The battery pack 20 is connected to the first unit cell 14 and the second unit cell 16 installed therein, and the voltage measurement device 30 and the voltage measurement device 35 are connected to the first cell 14 and the second cell 16, respectively. The unit cell 14 and the second unit cell 16 are respectively connected in parallel. The comparison unit 38 compares the voltages of the voltage measuring device 30 and the voltage measuring device 35, and determines the life of the power storage device 40 by a life determination method described later.

  FIG. 3 shows an example of a discharge curve showing the charge / discharge characteristics of the first unit cell 14 and the second unit cell 16. In FIG. 3, the horizontal axis indicates the depth of discharge (electric capacity of discharge) where the full charge state in the normal state of the single battery 1 is 0, and the discharge end time is 100, and the vertical axis indicates the cell voltage of the single battery. Indicates the state of charge. In FIG. 3, the discharge curve drawn with a solid line shows the charge / discharge characteristics of the first unit cell, and the discharge curve drawn with a dotted line shows the charge / discharge characteristic of the second unit cell. As is apparent from the discharge curve, the charge / discharge characteristics of the first cell 14 and the second cell 16 have an inflection region 100 in the charge / discharge curve, and each charge / discharge curve has an inflection region. It has a high voltage region 101 having a voltage larger than 100 and a low voltage region 102 having a voltage smaller than the inflection region.

(Relationship between first cell and second cell)
Further, the electric capacity in the high voltage region 101 of the first unit cell 14 is set to be smaller than the electric capacity in the high voltage region 101 of the second unit cell 16 by a predetermined amount. When discharged, the first unit cell has a normal state that reaches the predetermined voltage in the inflection region 100 earlier than the second unit cell.

  Further, the cycle deterioration rate of the battery capacity of the first unit cell 14 is slower than the cycle deterioration rate of the battery capacity of the second unit cell 16. In particular, the cycle deterioration rate of the electric capacity in the high voltage region 101 is important, a capacity difference in the high voltage region 101 between the first unit cell 14 and the second unit cell 16 is provided, and a predetermined voltage in the inflection region 100 is set. By using it, the lifetime of the power storage device 40 with higher accuracy can be determined.

  Normally, when forming a unit cell into an assembled battery, an equivalent unit cell is selected by grouping according to characteristics, and the assembled battery 20 is manufactured. The difference in electric capacity in the high voltage region between the first unit cell 14 and the second unit cell 16 is not particularly limited as long as the effect of the present invention is exhibited, but due to variation in the battery capacity at the time of grouping. About 1% or more is sufficient. For example, the difference in electric capacity between the first unit cell 14 and the second unit cell 16 in the high voltage region 101 is preferably equal to or greater than the difference in electric capacity when the unit cell is manufactured (for example, 3% or more). . Such a design facilitates discrimination without using a highly sensitive electric circuit.

  Moreover, since the 1st cell 14 and the 2nd cell 16 may be equivalent battery capacity or different battery capacity, the capacity | capacitance in a low voltage area | region is not specifically limited. That is, FIG. 3 shows an example in which the first unit cell 14 has a battery capacity of 100, and the second unit cell also uses 100, but the second one has a very large electric capacity in the low voltage region. It may be more than 100 using the single battery 16.

(Battery)
Moreover, these unit cells can be used as the assembled battery 20 capable of outputting a high voltage by connecting a plurality of unit cells 12 in series as necessary. Thereby, the voltage of the whole electrical storage apparatus 40 can be adjusted. Moreover, although omitted in the drawings, each unit cell connected in series may be individually connected to the same unit cell in parallel, and each unit cell 12 that occupies most of the assembled battery is also each. Individually, a voltage measuring device may be provided to prevent overdischarge and overcharge, and an electric circuit may be provided.

(Life judgment method)
The power storage device 40 having the first unit cell 14 and the second unit cell 16 described above determines the life by the following procedure. First, in the normal state before the lifetime in which charging and discharging are repeatedly performed from the initial state, the first voltage measured by the voltage measuring device 30 is more measured than the second unit cell 16 measured by the voltage measuring device 35 when the power storage device 40 is discharged. The unit cell 14 first reaches a predetermined voltage in a predetermined inflection region. In this case, all the cells in the assembled battery are discharged evenly and discharged so as to have the same depth of discharge. Accordingly, both the first unit cell 14 and the second unit cell 16 are always discharged at the same discharge depth from the fully charged state to the end of discharge. In the normal state, even if the first single cell 14 reaches a predetermined voltage in the inflection region 100 shown in FIG. 3, the second single cell still shows a high voltage in the high voltage region 101. The determination as to whether or not it is in the normal state is that the voltage of the first unit cell 14 and the second unit cell 16 is measured by the voltage measuring device (30, 35), and the comparison unit 38 compares the voltages of the two unit cells. Determine by. On the other hand, in the state where the lifetime has been reached, the second unit cell 16 reaches the predetermined voltage in the inflection region earlier than the first unit cell 14 when the power storage device 40 is discharged. The state that has reached the end of its life is also determined by comparing the voltages of the first unit cell 14 and the second unit cell 16 in the same manner as in determining the normal state.

  The comparison unit is not particularly limited as long as it is a general circuit that can detect and compare voltages, but it is preferable to use a circuit that can detect a difference in voltage and output a signal.

  In the embodiment of FIG. 1, the first unit cell 14 and the second unit cell 16 are in an equivalent environment within the assembled battery 20. For this reason, even if there is a change in the environmental temperature, the characteristics of the first single cell 14 and the second single cell 16 to be compared change in the same way, and more accurately indicate that the charge / discharge state is established regardless of the environmental temperature. I can judge. At this time, the temperature characteristics of the first unit cell 14 and the second unit cell 16 do not necessarily have to be the same, but it is desirable that the temperature properties be approximately the same for determining the life with higher accuracy.

  In the present specification, the “inflection region 100” refers to the unit change amount of the electric capacity in the charge / discharge curve of the charge / discharge characteristics defined by the electric capacity and voltage stored in each unit cell unless otherwise specified. On the other hand, it refers to a region where the amount of change in voltage is large (locally). In the inflection region 100, since the amount of change in voltage is larger than the amount of change in electric capacity stored in the unit cell, when determining the electric capacity stored in the unit cell by voltage, the lithium ion secondary battery is accurately used. The stored electrical capacity can be evaluated. When the voltage value error in the voltage measuring device is 0.05V, if the voltage change amount in the inflection region is 0.05V or more when the electric capacity changes by 5%, the determination is made with an error of about 5%. be able to.

  In the present specification, the “inflection region voltage” is a central voltage in an inflection region set for each unit cell unless otherwise specified, in other words, the center of the voltage width indicating the inflection region. Is the voltage. Usually, it is shown using the unit of V. The set voltage value can be appropriately adjusted depending on the combination of the positive electrode active material and the negative electrode active material used.

  In this specification, the normal state refers to a state immediately before reaching the life of the lithium ion secondary battery from the beginning, and is a state that can be used safely in the market.

  Note that cycle deterioration is not only deterioration due to pure charge / discharge cycle load, but also charging by taking into account various factors that can be affected by various user environments such as use at high temperatures, long-term storage, temperature shock, etc. In addition, deterioration due to normal use due to repeated discharge can be taken into account.

  The present embodiment is a power storage device capable of measuring a lifetime regardless of various environmental changes that change from moment to moment.

  The components of this embodiment will be described in further detail. In FIG. 2, as the first unit cell 14 and the second unit cell 16, a lithium ion secondary battery is taken as an example, and the internal structure thereof is shown as a battery element 50. The battery elements 50 of the first cell 14 and the second cell 16 both have a positive electrode 60 in which a positive electrode mixture layer 61 is formed on both surfaces of a positive electrode current collector 62 and a negative electrode composite on both surfaces of a negative electrode current collector 72. A negative electrode 70 on which the agent layer 71 is formed and a separator 80 containing an electrolyte having lithium ion conductivity are included, and the separator 80 is sandwiched between the positive electrode 60 and the negative electrode 70. The positive electrode mixture layer 61 includes a positive electrode active material, a conductive auxiliary agent, and a binder, and the negative electrode mixture layer 71 includes a negative electrode active material, a conductive auxiliary agent, and a binder.

In order to give an inflection region to the charge / discharge curve showing the charge / discharge characteristics as described above in the configuration of the unit cell, the following three configuration examples are given.
(1) A configuration in which a plurality of types of positive electrode active materials in the positive electrode mixture layer 61 are mixed and used.
(2) A configuration in which a plurality of types of negative electrode active materials in the negative electrode mixture layer 71 are mixed and used.
(3) The structure of said (1) and (2) in which the positive electrode 60 and the negative electrode 70 are both.

Each of the positive electrode active material and the negative electrode active material has a unique potential. The voltage of the unit cell is a potential difference caused by the intrinsic potential between the positive electrode active material and the negative electrode active material. By mixing a plurality of active material materials in each electrode, a steep potential difference caused by a potential break between the active materials is expressed in the charge / discharge curve, and the inflection region 100 can be obtained. Of course, the method of expressing the inflection region is not limited to that described above, and an active material having two intrinsic potentials can be selected with one active material, such as LiMn _1-k Fe _k PO ₄ (0 <k <1). For example, an inflection region 100 can be obtained by expressing a steep potential difference in the charge / discharge curve with one type of active material.

In the case of (1) or (3) in which the inflection region 100 originates from the positive electrode 60, it is desirable that the potentials of the positive electrode active materials in the positive electrode mixture layer 61 are sufficiently different. That is, it is desirable to use together positive electrode active materials having different potentials. The voltage range of the positive electrode active material for lithium ion secondary batteries currently widely used is about 3.0 to about 4.0 V with respect to metallic lithium. In this embodiment, it can classify | categorize into a low voltage positive electrode active material and a high voltage positive electrode active material on the basis of the metallic lithium. The low voltage positive electrode active material is an active material whose voltage in the vicinity of full charge (charge depth 95%) is 3.5 V or less with respect to lithium metal, and specific examples include LiTiO ₂ and LiFePO ₄ . The high-voltage positive electrode active material is an active material having a voltage of 3.5 V or more at the initial stage of charging (charging depth 5%), specifically, Li (Ni _1-xy Co _x Mn _y ) O ₂ (hereinafter, , “NCM”, 0.1 ≦ x ≦ 0.5, 0.1 ≦ y ≦ 0.5), Lia (Ni _1- bc Co _b Al _c ) O ₂ (0.9 ≦ a ≦ 1) .3, 0 <b ≦ 0.5, 0 <c ≦ 0.7), LiMnO ₂ , LiVPO ₄ , LiVOPO ₄ , LiCoO ₂ , LiMnPO ₄ , LiCoPO ₄ , LiNiPO _{4 and the} like.

LiFePO ₄ (hereinafter, referred to as _{"LFP"), LiVPO 4, LiVOPO 4,} LiMnPO 4, LiCoPO 4, LiNiPO positive electrode active substance groups, such as _4, less voltage change in normal use zone, the initial charging stage, rapidly charging end It is a positive electrode active material whose voltage changes. Therefore, when the positive electrode active materials having such a structure are used in combination, the change in charge depth with respect to the change in voltage in the inflection region can be reduced, and the detection accuracy can be further improved. For example, the LiFePO ₄ as a low-voltage positive electrode active _{material, LiVPO 4, LiVOPO 4, LiMnPO} 4, LiCoPO 4, and olivine skeletal positive electrode active material or the like selected from LiNiPO _4, it is preferable to combine as a high voltage positive electrode active material.

Similarly, in the case of (2) where the inflection region 100 originates from the negative electrode 70, the inflection region can be obtained by selecting a plurality of types of negative electrode active materials in the negative electrode mixture layer 71. Also, a combination with a large potential difference of the negative electrode active material is desirable. In the case of the negative electrode, it can be distinguished as a low-voltage negative electrode active material and a high-voltage negative electrode active material with a boundary of about 0.5 V. For example, graphite is exemplified as the low voltage negative electrode active material, and hard carbon, Li ₄ Ti ₅ O ₁₂ , SiO _w (w is 0.5 or more and 2.5 or less), Al ₂ as the high voltage negative electrode active material. Examples include O ₃ .

  In the present embodiment, the cycle deterioration rate of the electric capacity in the high voltage region 101 of the first unit cell 14 is slower than the cycle deterioration rate of the electric capacity in the high voltage region 101 of the second unit cell 16. Yes. As a method for controlling the cycle deterioration rate, for example, the selection of the active material, the amount of the conductive assistant, the molecular weight of the binder, the additive capable of controlling the cycle life, the coating per unit area of the positive electrode 60 and the negative electrode 70 facing each other. It is possible to design according to the work ratio, etc., which is preferable.

  Specifically, the lithium ion secondary battery has various characteristics depending on each material used in the battery. In particular, the positive electrode active material used for the positive electrode 60, the negative electrode active material used for the negative electrode 70, and the electrolyte determine the main characteristics of the lithium ion secondary battery. For example, when the characteristics of the positive electrode active material A and the positive electrode active material B are compared, and it is found that the deterioration rate of the positive electrode active material A is fast, the positive electrode active material A is used for the second unit cell 16, The positive electrode active material B may be used for the single cell 14.

  Preferably, the positive electrode active materials A and B are active materials of the same kind and different diameters, and if the fact that the cycle deterioration rate as a lithium ion secondary battery increases as the particle size decreases, a characteristic difference other than the deterioration characteristics is obtained. Can be reduced. For example, 10 μm NCM can be used as the positive electrode active material of the first unit cell, and 5 μm NCM can be used as the positive electrode active material of the second unit cell. Note that the same type of active material means that the types of materials are the same, and indicates materials represented by the same composition formula. Of course, variations in the composition formula to the extent that the various characteristics of the material do not change, such as oxygen vacancies, are within the same range. The particle size may be measured and managed with a laser diffraction particle size distribution measuring device.

  Further, the cycle deterioration characteristics of the unit cell can also be designed by the amount of binder used for the positive electrode mixture layer 61 and the negative electrode mixture layer 71 and the molecular weight of the binder. When the amount of the binder is large or when the molecular weight of the binder is large, the deterioration of the mixture layer can be suppressed. For example, assuming that the molecular weight of PVDF (polyvinylidene fluoride) in the positive electrode mixture layer of the first unit cell is 1,000,000 and the molecular weight of PVDF in the positive electrode mixture layer of the second unit cell is 500,000, The deterioration rate can be made slower than that of the second unit cell, and can be applied to the power storage device of the present embodiment. The molecular weight may be measured and managed by gel permeation chromatography (GPC).

  Moreover, the deterioration characteristic of a single cell can be designed by putting the additive which can design the cycle deterioration rate of a single cell in a positive mix layer and a negative mix layer. An additive capable of designing the cycle deterioration characteristics of the unit cell is NMP. When the amount of NMP added to the positive electrode mixture layer is small, the deterioration of the unit cell tends to be suppressed. For example, by adding 0.1% by weight to the positive electrode mixture layer of the first unit cell and 0.5% to the positive electrode mixture layer of the second unit cell, the deterioration rate of the first unit cell can be reduced to the second level. It can be slower than the unit cell, and can be applied to the power storage device of this embodiment.

  As described above, the configuration example for controlling the characteristics of the unit cell according to the present embodiment has been described, but other configurations configuring the unit cell will also be described.

(Conductive aid)
The positive electrode mixture layer 61 and the negative electrode mixture layer 71 may contain a conductive auxiliary agent, and the conductive auxiliary agent includes acetylene black, which is widely used for non-aqueous electrochemical devices, and needles containing carbon nanotubes. Carbon or the like can be used.

(binder)
As the binder used for the positive electrode mixture layer 61 and the negative electrode mixture layer 71, styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), or the like can be used in addition to the above-described poly (vinylidene fluoride) (PVDF). In addition, when apply | coating and forming the positive mix layer 61 and the negative mix layer 71, the solvent which melt | dissolves these binders, for example, N-methylpyrrolidone (NMP), a pure water etc. should just be used.

(Current collector)
As the current collector used for the positive electrode current collector 62 and the negative electrode current collector 72, various known materials generally used for lithium ion secondary batteries can be used. Specifically, as the negative electrode current collector 72, A Cu foil is an Al foil as the positive electrode current collector 62.

(Separator)
There is no restriction | limiting in particular in a separator, A widely well-known material can be used. For example, a microporous film of a polyolefin resin such as polyethylene or polypropylene can be used.

(Electrolytes)
As the electrolyte, a nonaqueous electrolytic solution, a gel electrolyte, an inorganic or organic solid electrolyte can be widely used. For example, the non-aqueous electrolyte can use a substance containing a solvent and a salt, and this may contain additives as appropriate.

As the solvent for the non-aqueous electrolyte, a solvent having lithium ion conductivity is desirable. For example, emotion carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) can be used alone or in appropriate combination. Dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), diethyl carbonate (DEC), difluoro carbonate (FEC), etc. may be used in combination in order to increase the electrical conductivity and obtain an electrolyte having an appropriate viscosity. . LiPF ₆ , LiBF ₄ , LiClO ₄ or the like can be used as the salt in the non-electrolytic solution.

(Exterior body)
Although not shown in FIG. 2, there is no particular limitation on the outer package that encloses the battery element in which the positive electrode, the negative electrode, and the separator are laminated, and an iron, aluminum or stainless steel can, or an aluminum laminated outer bag is appropriately selected. be able to.

(Single cell manufacturing method)
Below, an example of the manufacturing method of a cell is demonstrated. First, the positive electrode 60 is prepared by mixing a positive electrode active material to be used together, a predetermined conductive auxiliary agent, and a predetermined binder together with a solvent to prepare a paint, and then applying the mixture onto an aluminum foil as the positive electrode current collector 62. Drying to obtain the positive electrode 60.

  Next, the negative electrode 70 is the negative electrode current collector 72 after the negative electrode active material to be used in combination, a predetermined conductive auxiliary agent, and a predetermined binder are mixed together with a solvent to prepare a paint, similarly to the positive electrode 60. The negative electrode 70 is obtained by applying and drying on the copper foil.

  The produced positive electrode 60 and negative electrode 70 are laminated or wound via a separator and inserted into the outer package as a battery element.

  After the battery element is inserted into the outer package, an electrolyte is added. If the outer package is vacuum-sealed, a lithium ion secondary battery is completed as a single cell.

(Method of manufacturing assembled battery)
Next, a method for manufacturing the assembled battery will be described. A unit cell obtained by the above-described configuration and manufacturing method is prepared in advance, and the first unit cell 14 and the second unit cell having a higher capacity of the high voltage region 101 and a faster cycle deterioration rate than the first unit cell 14. A necessary number of batteries 16 and other lithium-on secondary batteries 12 are prepared. Each unit cell is connected in series, and the assembled battery 20 is obtained.

  When connecting the cells in series, it is desirable to connect the cells in the same state of charge in advance. Here, the state of charge refers to the ratio of the electric capacity actually accumulated to the battery capacity. More preferably, it is desirable to connect in a state of charge near full charge. When connected in a charged state near full charge, voltage variation near full charge is minimized, and the possibility of being overcharged is reduced, and a safe assembled battery can be obtained.

(Method for manufacturing storage battery)
Next, a method for manufacturing the power storage device 40 will be described. First, the voltage measuring device 30 and the voltage measuring device 35 are connected in parallel to the first unit cell 14 and the second unit cell 16 of the assembled battery 20 obtained by the above manufacturing method, respectively. Further, the power storage device 40 is obtained by connecting a signal line from the voltage measurement device 30 and the voltage measurement device 35 to the comparison unit 38 that determines the life of the power storage device.

(Second Embodiment)
Next, the power storage device 240 of the second embodiment is schematically shown in FIG. 5, and only the parts different from the first embodiment will be described regarding the structure and the manufacturing method.

  The power storage device 240 of the present embodiment includes a first cell 214, a voltage measurement device 230 that measures the voltage of the first cell 214, a second cell 216, and a voltage that measures the voltage of the second cell 216. It has a measuring device 35, a third unit cell 217, a voltage measuring unit 237 that measures the voltage of the third unit cell, and a comparison unit 238. It differs from the first embodiment in that it has a third unit cell 217.

  The first unit cell 214, the second unit cell 216, and the third unit cell 217 are connected in series, and in order to measure the voltage of each unit cell, the voltage measurement device 230, the voltage measurement device 235, and the voltage measurement device 237 are measured. Are connected in parallel to the first cell 214, the second cell 216, and the third cell 217, respectively. Further, the comparison unit 237 determines the deterioration state and the life of the power storage device 240 based on the voltages of the voltage measurement device 230, the voltage measurement device 235, and the voltage measurement device 237.

  FIG. 6 shows an example of a discharge curve showing the charge / discharge characteristics of the first cell 214, the second cell 216, and the third cell 217. The horizontal axis of FIG. 6 indicates the battery capacity of the first unit cell 214 at the initial stage as 100.

(Relationship between the first cell, the second cell, and the third cell)
The electric capacity in the high voltage region 201 is larger in the order of the first unit cell 214, the third unit cell 217, and the second unit cell 216. In addition, the cycle deterioration rate of the electric capacity in the high voltage region 201 is higher in the order of the second unit cell 214, the third unit cell 217, and the first unit cell 216.

(Degradation state of battery and life judgment method)
FIG. 7 schematically shows the cycle deterioration characteristics of the electric capacity of the high voltage region 201 of the first unit cell 214, the second unit cell 216, and the third unit cell 217. The power storage device 240 including the first unit cell 214, the second unit cell 216, and the third unit cell 217 described above determines the deterioration state and the life by the following procedure. First, in the initial stage, the first unit cell 214 measured by the voltage measurement device 230 is set in advance before the second unit cell 216 measured by the voltage measurement device 235 when the power storage device 240 is discharged. Reaches a predetermined voltage in the inflection region. At this time, the comparison unit 238 determines that the normal state. After that, when charging and discharging are repeated, the second unit cell 216 measured by the voltage measurement device 235 is ahead of the first unit cell 214 measured by the voltage measurement device 230 when the power storage device 240 is discharged. Reach the set inflection region voltage. At this time, the comparison unit 238 determines that the predetermined cycle deterioration state set in advance has been exceeded, and determines that this is the lifetime. The life determination method described above can be determined based on the same principle as in the first embodiment.

  In the second embodiment, for example, if the inflection region voltage of the third cell 217 is designed as a deterioration state of 50% of the life of the assembled battery 220, the progress of the deterioration state until the life of the assembled battery 220 can be grasped. it can.

  As described above, the cycle of the power storage device 240 is provided by using the fourth and fifth unit cells that have the inflection region 202 and the cycle deterioration of the electric capacity in the high voltage region is slower than that of the second unit cell 216. It is desirable that the deterioration state can be further subdivided and determined.

  By using the configuration as described above, the power storage device 240 can accurately determine the deterioration state and the lifetime regardless of the environmental temperature.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to a following example.

Example 1
(Preparation of positive electrode)
As a positive electrode active material, NCM (LiNi _1/3 Mn _1/3 Co _1/3 O ₂ ), LiFePO ₄ (hereinafter LFP), carbon black and graphite as a conductive aid, and PVDF (polyvinylidene fluoride) as a binder. A positive electrode was produced. For NCM, an average particle diameter (D50) of 10 μm was used. The mixing ratio was 25 g of NCM, 60 g of LFP, 5 g of carbon black, and 5 g of graphite. An N-methyl-2-pyrrolidinone (NMP) solution (50 g, 10 wt%) of PVDF (manufactured by Kureha Chemical Industry Co., Ltd., KF7305) was added thereto and mixed to prepare 145 g of paint. This paint was applied to an aluminum foil (thickness 20 μm) as a current collector by a doctor blade method, dried at 90 ° C., and rolled to form a positive electrode mixture layer.

(Preparation of negative electrode)
After dry-mixing 45 g of natural graphite as a negative electrode active material and 2.5 g of carbon black as a conductive additive, 22.5 g of PVDF solution was added as a binder to prepare a negative electrode paint. This paint was applied to a copper foil (thickness: 16 μm) as a current collector by a doctor blade method, dried (90 ° C.), and rolled to form a negative electrode mixture layer.

(Production of first cell)
The obtained positive electrode and negative electrode were cut into predetermined dimensions together with a separator (microporous membrane made of polyolefin). The positive electrode and the negative electrode were provided with portions where no paint (mixture layer) was applied or formed in order to weld the external lead terminals. A positive electrode, a separator, and a negative electrode were laminated in this order. At this time, the lithium ion secondary battery was laminated so that the capacity was 200 mAh. An aluminum foil (width 4 mm, length 40 mm, thickness 100 μm) and nickel foil (width 4 mm, length 40 mm, thickness 100 μm) were ultrasonically welded to the positive electrode and the negative electrode, respectively, as external lead terminals. Polypropylene (PP) was wrapped around this external lead terminal and thermally bonded.

A battery element in which a positive electrode, a negative electrode, and a separator were laminated was housed in an exterior body made of an aluminum laminate material. After putting a battery element in the outer package, LiPF ₆ is dissolved in 1M in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) (EC: DEC = 30: 70 vol%) as an electrolytic solution. Was added, and the outer package was vacuum-sealed to produce a lithium ion secondary battery. The lithium ion secondary battery was initially charged at 10 mA (0.05 C) after sealing. The initial average discharge capacity of the obtained lithium ion secondary battery was about 200 mAh. An inflection region appeared when discharging about 60 mAh from the time of full charge.

(Production of second cell)
At the time of producing the positive electrode, the average particle diameter (D50) of NCM was 5 μm, 42.5 g of NCM and 42.5 g of LFP were used. Others were produced in the same manner as the first unit cell 14. A lithium ion secondary battery having an initial average discharge capacity of about 200 mAh was obtained. From the time of full charge, an inflection region appeared when discharging about 100 mAh.

  The cycle deterioration load from the fully discharged state to the fully charged state was repeated a predetermined number of times for each of the first unit cell and the second unit cell. When the electric capacity in the high voltage region of the first unit cell 14 and the second unit cell 16 is evaluated every 200 cycles, and the battery capacity of the first unit cell in the initial stage is set to 100%, the cycle deterioration characteristics are evaluated. It became like this. After about 1000 cycles, it was confirmed that the electric capacity in the high voltage region became equivalent, and the capacity of the second unit cell decreased in the subsequent cycles.

(Production of assembled battery)
One assembled first battery and one second single battery were charged until they were fully charged and connected in series to obtain an assembled battery. In addition, the lifetime of the assembled battery in Example 1 was set to 1000 cycles.

  After that, voltage measurement wiring is used for the first unit cell and the second unit cell, the voltage measurement devices are attached in parallel, and then wiring for transmitting the voltage value of the voltage measurement device to the comparison unit is performed. The device.

(Life test)
Table 1 shows the results of performing lifespan judgment in each temperature environment after repeating the cycle deterioration load from the fully discharged state to the fully charged state for the manufactured power storage device a predetermined number of times. After 1000 cycles, the comparison unit determined that the life was long, and if the power storage device in Example 1 was used, the life could be determined regardless of the environmental temperature and the discharge rate.

(Example 2)
In Example 2, the cycle deterioration characteristics of the lithium ion secondary battery were designed by using the molecular weight of the binder used for the high-voltage positive electrode mixture layer as a parameter.

(Production of battery electrode)
(Preparation of positive electrode)
When producing the positive electrode, a paint mainly composed of NCM and a paint mainly composed of LFP were prepared separately. When preparing the NCM paint, the paint was prepared so that the mixing ratio was 85 g of NCM, 5 g of carbon black, 5 g of graphite, and 5 g of PVDF. At this time, three types of paints were prepared using three types of PVDF molecular weights of about 250,000, about 500,000, and about 1 million, respectively. At the time of preparing the paint mainly composed of LFP, the paint was prepared so as to have a mixing ratio of 85 g of LFP, 5 g of carbon black, 5 g of graphite, and 5 g of PVDF. Then, using a doctor blade method on an aluminum foil, LFP paint and NCM paint were applied in this order. At this time, the amount of LFP and the amount of NCM per unit area on the aluminum foil were made comparable. Otherwise, three types of positive electrodes were produced in the same manner as in Example 1.

(Preparation of negative electrode)
A negative electrode equivalent to the negative electrode of Example 1 was used.

(Production of first cell)
The battery was made in the same manner as in Example 1, and three types of lithium ion secondary batteries with different molecular weights of the binder in the positive electrode mixture layer were obtained. Each of the obtained lithium ion secondary batteries had an initial average discharge capacity of about 200 mAh, and when the battery was discharged from a fully charged state, an inflection region was developed when the discharge capacity was about 100 mAh.

(Evaluation of cycle deterioration characteristics)
About the obtained three types of lithium ion secondary batteries, the test which repeats charging / discharging in the range of a complete discharge state to a full charge state was done, and the electric capacity of the high voltage area | region for every 200 cycles was evaluated. The results are shown in FIG. FIG. 9 confirmed that the cycle deterioration characteristics of the lithium ion secondary battery usable in the present invention can be designed by adjusting the molecular weight of the binder of the positive electrode mixture layer.

(Example 3)
In Example 3, the cycle deterioration characteristics of the lithium ion secondary battery were designed by using NMP as an additive capable of adjusting the cycle deterioration in the positive electrode mixture layer containing the high-voltage positive electrode active material.

(Production of battery electrode)
(Preparation of positive electrode)
First, paints mainly composed of NCM and LFP were prepared separately. When preparing the NCM paint, the paint was prepared so that the mixing ratio was 85 g of NCM, 5 g of carbon black, 5 g of graphite, and 5 g of PVDF. At the time of making the paint mainly composed of LFP, the paint was made so as to have a mixing ratio of 85 g of LFP, 5 g of carbon black, 5 g of graphite, 2 g of carboxymethylcellulose (CMC), and 3 g of styrene-butadiene rubber (SBR). . At this time, pure water was used as a solvent for the LFP paint. Thereafter, an NCM paint was applied onto the aluminum foil using a doctor blade method and dried. After that, NMP was added in a saturated atmosphere of NMP for 1 minute and 1 hour. The NCM (high voltage positive electrode active material) mixture layer to which NMP was added was measured for the concentration of NMP by using gas chromatography. Two types of NCM (high voltage positive electrode active material) mixture layers of about 0.8% were obtained. Thereafter, an LFP coating was applied on the NCM mixture layer, dried, and rolled to obtain two types of positive electrodes. At this time, the amount of LFP and the amount of NCM per unit area on the aluminum foil were made comparable.

(Preparation of negative electrode)
A negative electrode equivalent to the negative electrode of Example 1 was used.

(Production of battery)
The battery was made in the same manner as in Example 1, and two types of lithium ion secondary batteries with different amounts of NMP added to the positive electrode mixture layer containing the high-voltage positive electrode active material were obtained. Each of the obtained lithium ion secondary batteries had an initial average discharge capacity of about 200 mAh. When discharged from the fully charged state, an inflection region appeared when about 100 mAh was applied.

(Evaluation of cycle deterioration characteristics)
The obtained two types of lithium ion secondary batteries were subjected to a charge / discharge test in a range from a fully discharged state to a fully charged state, and the electric capacity in the high voltage region every 200 cycles was evaluated. The results are shown in FIG. FIG. 10 confirms that the cycle deterioration characteristics of the available lithium ion secondary battery can be designed by the amount of NMP added to the positive electrode mixture layer containing the high-voltage positive electrode active material.

  As described above, the lithium ion secondary battery that can be designed for cycle deterioration characteristics manufactured in Examples 2 and 3 is assembled into a battery as described in Example 1 to form a power storage device. Can be evaluated

(Comparative Example 1)
(Production of lithium ion secondary battery)
(Preparation of positive electrode)
A positive electrode was produced using NCM (LiNi _1/3 Mn _1/3 Co _1/3 O ₂ ) as the positive electrode active material, carbon black and graphite as the conductive assistant, and PVDF (polyvinylidene fluoride) as the binder. These were mixed at a mixing ratio of 85 g of NCM, 5 g of carbon black, and 5 g of graphite. To this, an N-methyl-2-pyrrolidinone (NMP) solution of PVDF (50 g, 10 wt%) was added and mixed. Produced. This paint was applied to an aluminum foil (thickness 20 μm) as a current collector by a doctor blade method, dried at 90 ° C., and rolled.

(Preparation of negative electrode)
After dry-mixing 45 g of natural graphite as a negative electrode active material and 2.5 g of carbon black as a conductive additive, 22.5 g of PVDF solution was added as a binder to prepare a negative electrode paint. This paint was applied to a copper foil (thickness 16 μm) as a current collector by a doctor blade method, dried (90 ° C.) and rolled.

(Production of battery)
The obtained positive electrode and negative electrode were cut into predetermined dimensions together with a separator (microporous membrane made of polyolefin). The positive electrode and the negative electrode were provided with portions where no paint (mixture layer) was applied or formed in order to weld the external lead terminals. A positive electrode, a separator, and a negative electrode were laminated in this order. At this time, the lithium ion secondary battery was laminated so that the capacity was 200 mAh. An aluminum foil (width 4 mm, length 40 mm, thickness 100 μm) and nickel foil (width 4 mm, length 40 mm, thickness 100 μm) were ultrasonically welded to the positive electrode and the negative electrode, respectively, as external lead terminals. Polypropylene (PP) was wrapped around this external lead terminal and thermally bonded.

A battery element in which a positive electrode, a negative electrode, and a separator were laminated was housed in an exterior body made of an aluminum laminate material. After putting a battery element in the outer package, LiPF ₆ is dissolved in 1M in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) (EC: DEC = 30: 70 vol%) as an electrolytic solution. Was added, and the outer package was vacuum-sealed to produce a lithium ion secondary battery. The lithium ion secondary battery was initially charged at 10 mA (0.05 C) after sealing to obtain a lithium ion secondary battery. Then, when it charged and the initial average discharge capacity was confirmed, it was about 200 mAh.

(Impedance evaluation of lithium ion secondary battery)
In order to judge the assembled battery life from the impedance value, the impedance of the obtained lithium ion secondary battery was measured every 100 cycles. The results are shown in FIG. At this time, when the AC impedance at 1 kHz was about 160 mΩ, the life of the assembled battery was set (dotted line 300 in FIG. 11). Similarly, the results of evaluating the impedance after each environmental temperature is set every 100 cycles are also shown.

(Production of battery pack and life test)
Five lithium ion secondary batteries produced were fully charged and connected in series to obtain an assembled battery. Thereafter, a cycle deterioration test of the assembled battery was performed. At that time, the life determination was performed in the same manner as in Patent Document 1 and also at each environmental temperature. The results are shown in Table 1.

  The results are shown in Table 1. In an environment other than 25 ° C, an incorrect determination was made. In addition, items that have been determined to have a lifetime have not been evaluated since then.

In Table 1, in the measurement in the −10 ° C. environment after 500 cycles, Comparative Example 1 was judged as a life cycle earlier by 500 cycles, resulting in an erroneous determination. In the measurement in the 0 ° C. environment after 800 cycles, Comparative Example 1 was 200 cycles. It was judged that the life was early and erroneously judged, and in the measurement in a 45 ° C. environment after 1100 cycles, Comparative Example 1 judged that the life was late by 100 cycles and misjudged.

  The present invention provides a battery pack and a power storage device for a lithium ion secondary battery that can accurately evaluate the depth of charge without requiring a complicated determination circuit even in an environment where the environmental temperature fluctuates greatly. Since it contributes to the manufacture and sale of secondary battery assemblies and power storage devices, it has industrial applicability.

12, 212 cell 14, 214 first cell 16, 216 second cell 217 third cell 20, 220 battery pack 30, 230 voltage measurement device for the first cell
35, 235 Voltage measurement device for second cell 237 Voltage measurement device for third cell
38, 238 Comparison unit for judging the life of the power storage device 40, 240 Power storage device 50 Lithium ion secondary battery 60 Positive electrode 61 Positive electrode mixture layer 62 Positive electrode current collector 70 Negative electrode 71 Negative electrode mixture layer 72 Negative electrode current collector 80 Separator 100 , 202 Inflection region 101, 201 High voltage region 102, 203 Low voltage region 300 Impedance that becomes the life of the assembled battery of Comparative Example 1

Claims (3)

It has a plurality of unit cells connected in series, and at least two of the unit cells are a battery pack composed of a first unit cell and a second unit cell, and a voltage of the first unit cell. A first voltage measuring device for measuring, a second voltage measuring device for measuring the voltage of the second unit cell, and a comparison unit for comparing the voltages of the first unit cell and the second unit cell, The charge / discharge characteristics of the first unit cell and the second unit cell have an inflection region in a charge / discharge curve, and the first unit cell is in the inflection region prior to the second unit cell. The comparator has a normal state of reaching a predetermined voltage, and the comparison unit determines that the battery life is reached when the second unit cell reaches a predetermined voltage in the inflection region prior to the first unit cell during discharge. A power storage device having a life determination function.   The power storage device having a life determination function according to claim 1, wherein the first unit cell and the second unit cell are adjacent to each other in the assembled battery. A method for determining the life of an assembled battery comprising a plurality of cells connected in series,
At least two of the plurality of unit cells include a first unit cell and a second unit cell having an inflection region in a charge / discharge curve indicating charge / discharge characteristics,
The predetermined voltage in the inflection region of the first unit cell and the predetermined voltage in the inflection region of the second unit cell are measured, respectively, and the first unit cell is the first inflection region voltage during discharge. The battery life is determined to be the life of the power storage device when it is determined that the battery has reached the predetermined voltage in the inflection region due to repeated charge and discharge. Method.

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