JP5434274B2 - Lithium ion battery charging method, battery system, charge control device, lithium ion battery, assembled battery, vehicle - Google Patents

Description

  The present invention relates to a method for charging a lithium ion battery, a battery system, a charge control device, a lithium ion battery, an assembled battery, and a vehicle.

Some lithium ion batteries use an olivine type compound made of lithium iron phosphate (LiFePO ₄ ) containing iron and phosphoric acid in addition to lithium as a positive electrode active material. A lithium ion battery using lithium iron phosphate as a positive electrode active material is considered to be excellent in thermal stability. Conventionally, a battery using such a positive electrode active material is charged by a constant current / constant voltage charging method. At room temperature (25 ° C.), the charging voltage in the constant voltage charging region is 3.3 to 3.8V. (Patent Document 1).

JP 2004-87319 A

  However, when the lithium ion battery is charged, the battery itself may be at a temperature higher than room temperature due to the heat generated by the battery itself or the environmental temperature where the battery is placed. In such a case, for example, at a high temperature (55 ° C.), the lithium salt (F system) in the electrolyte is hydrolyzed by a small amount of water remaining in the electrolyte to generate HF. Therefore, Fe is eluted from the positive electrode by the action of HF (oxidation action), and the capacity of the positive electrode is reduced. For this reason, sufficient battery capacity cannot be obtained by repeating charging and discharging. That is, the conventional charging method has a problem that the cycle characteristics are deteriorated.

  Accordingly, an object of the present invention is to provide a charging method capable of improving cycle characteristics in a lithium ion battery using an olivine type compound containing iron and phosphoric acid in addition to lithium, and a lithium ion battery with improved cycle characteristics thereby. Is to provide. Another object of the present invention is to provide a battery system capable of improving charge / discharge cycle characteristics of a lithium ion battery using an olivine type compound containing iron and phosphoric acid in addition to lithium. Another object of the present invention is to provide a charge control device for improving charge / discharge cycle characteristics of a lithium ion battery using an olivine type compound containing iron and phosphoric acid in addition to lithium as a positive electrode active material. . Furthermore, another object of the present invention is to provide a vehicle having an improved battery life mounted using this battery system.

To achieve the above object, the charging method according to the present invention has a charging voltage of 3.8 V or less when the temperature during charging of a lithium ion battery using a positive electrode active material containing lithium, iron, and phosphoric acid is less than 45 ° C. The battery is charged and charged at a voltage of 45 to 55 ° C. with a charging voltage higher than 3.8 V and 4.2 V or lower.

The charge control device according to the present invention for achieving the above object is a device for charging a lithium ion battery using a positive electrode active material containing lithium, iron and phosphoric acid. When the temperature during charging of the lithium ion battery is less than 45 ° C., this charge control device charges with a charging voltage of 3.8 V or less, and when it is 45 to 55 ° C., the charging voltage is higher than 3.8 V and lower than 4.2 V. It is characterized by doing.

  According to the method for charging a lithium ion battery according to the present invention, a small amount of water inside the battery can be decomposed by charging at a charging voltage higher than 3.8 V and 4.2 V or lower. For this reason, elution of Fe from the positive electrode due to moisture and precipitation of the extracted Fe at the negative electrode can be prevented or suppressed, capacity reduction due to repeated charge and discharge can be reduced, and cycle characteristics can be improved. .

  According to the charge control device of the present invention, a small amount of water inside the battery is decomposed by charging with a charging voltage higher than 3.8 V and lower than 4.2 V. Therefore, elution of Fe from the positive electrode due to moisture and precipitation of the extracted Fe at the negative electrode can be prevented or suppressed, and the cycle characteristics can be improved with less capacity reduction due to repeated charge and discharge.

It is the cross-sectional schematic which represented typically the whole structure of the non-bipolar laminated lithium ion battery. 1 is a schematic cross-sectional view schematically showing the entire structure of a bipolar stacked lithium ion battery. It is a perspective view which shows the external appearance of a lithium ion battery. It is a block diagram for demonstrating a battery system. It is a flowchart which shows the operation | movement procedure of a battery system. It is drawing for demonstrating the assembled battery using a lithium ion battery. It is drawing for demonstrating the vehicle using an assembled battery. It is a graph which shows the cycle characteristic of the Example in a charging temperature condition of 55 degreeC, and a comparative example.

  Embodiments of the present invention will be described below with reference to the accompanying drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted. In addition, the dimensional ratios in the drawings are exaggerated for convenience of explanation, and may be different from the actual ratios.

  First, a lithium ion battery (also referred to as a lithium ion secondary battery) according to this embodiment will be described. Lithium ion batteries have various forms and structures, such as stacked (flat) batteries and wound (cylindrical) batteries, for example, when distinguished by form and structure. Although these embodiments can also be applied to the present embodiment, a case where a stacked (flat) battery structure is employed will be described here. Of course, the present invention itself can be implemented even with a structure other than a stacked (flat) battery structure such as a wound (cylindrical) battery.

  Further, when viewed in terms of electrical connection form (electrode structure) in a lithium ion battery, there are non-bipolar (internal parallel connection type) batteries and bipolar (internal series connection type) batteries. Of these, bipolar batteries are preferred because they have the advantage of having higher power density and higher voltage than non-bipolar batteries. This is because a non-bipolar battery takes a lead wire from each of the positive electrode and the negative electrode, and is connected to an adjacent battery via the lead wire. For this reason, the electron conduction path becomes longer corresponding to the length of the lead wire, and the output of the battery is lowered accordingly. On the other hand, in the bipolar battery, current flows in the vertical direction (electrode stacking direction) through the current collector, so that the electron conduction path can be shortened and the output is high. Below, both of these two forms will be described.

  Furthermore, when distinguished by the type of electrolyte in the lithium ion battery, a conventionally known battery such as a solution electrolyte type battery using a solution electrolyte such as a non-aqueous electrolyte as the electrolyte, a polymer battery using a polymer electrolyte as the electrolyte, etc. It can be applied to any electrolyte type. It is preferably applied to a polymer battery using a polymer electrolyte as an electrolyte. The polymer battery is further divided into a gel electrolyte type battery using a polymer gel electrolyte (also simply referred to as gel electrolyte) and a solid polymer (all solid) type battery using a polymer solid electrolyte (also simply referred to as polymer electrolyte). It is done. Among them, polymer batteries, especially solid polymer (all solid) batteries, do not cause liquid leakage, so there is no problem of liquid junctions, high reliability, and a simple configuration with excellent output characteristics. This is advantageous in that it can be done.

  FIG. 1 is a schematic cross-sectional view schematically showing the overall structure of a non-bipolar stacked lithium ion battery.

  As shown in FIG. 1, in the lithium ion battery 10 of this embodiment, a substantially rectangular power generation element (battery element) 21 in which a charge / discharge reaction actually proceeds is sealed inside a laminate sheet 29 that is an exterior. It has a structure. Specifically, the power generation element 21 is housed and sealed by using a polymer-metal composite laminate sheet as the battery exterior and joining all of its peripheral parts by thermal fusion.

The power generation element 21 includes a positive electrode in which the positive electrode active material layer 13 is disposed on both surfaces of the positive electrode current collector 11, an electrolyte layer 17, and a negative electrode in which the negative electrode active material layer 15 is disposed on both surfaces of the negative electrode current collector 12. It has a stacked configuration. In this embodiment, LiFePO ₄ is used as the positive electrode active material. On the other hand, a carbon-based material such as graphite is used for the negative electrode.

  The electrolyte layer 17 is formed by impregnating a separator with an electrolyte, a polymer electrolyte, or the like. The electrolyte of the electrolyte layer 17 contains an electrolyte salt (lithium salt) containing fluorine (F) (details will be described later).

  The power generation element 21 is configured such that one positive electrode active material layer 13 and a negative electrode active material layer 15 adjacent to the positive electrode active material layer 13 face each other with the electrolyte layer 17 therebetween, so that the positive electrode active material layer 13, the electrolyte layer 17, and the negative electrode active material layer The material layers 15 are laminated in this order to form one single cell layer 19. Therefore, the lithium ion battery 10 of the present embodiment has a configuration in which a plurality of single battery layers 19 are stacked so that they are electrically connected in parallel.

  Although the positive electrode active material layer 12 is disposed on only one side of the outermost layer positive electrode current collector located on both outermost layers of the power generation element 21, active material layers may be provided on both sides. Note that the arrangement of the positive electrode and the negative electrode is reversed from that in FIG. 3 described later, so that the outermost layer negative electrode current collector is positioned in both outermost layers of the power generation element 21, and only one side of the outermost layer negative electrode current collector is provided. A negative electrode active material layer may be disposed on the substrate.

  A positive electrode tab 25 and a negative electrode tab 27 are respectively attached to the positive electrode current collector 11 and the negative electrode current collector 12 (see FIG. 3 described later). The positive electrode current collector 11 and the negative electrode current collector 12 are attached to the positive electrode tab 25 and the negative electrode tab 27 by, for example, ultrasonic welding or resistance welding. The positive electrode tab 25 and the negative electrode tab 27 may be attached to the positive electrode current collector 11 and the negative electrode current collector 12 of each electrode through a positive electrode lead and a negative electrode lead (not shown), respectively, as necessary. .

  FIG. 2 is a schematic cross-sectional view schematically showing the entire structure of a bipolar stacked lithium ion battery.

In the bipolar battery 10 ′ of this embodiment shown in FIG. 2, a substantially rectangular power generation element (battery element; laminate) 21 in which a charge / discharge reaction actually proceeds is sealed inside a laminate film 29 that is a battery exterior material. It has a stopped structure. Even in this type of battery, LiFePO ₄ is used as the positive electrode active material in this embodiment.

  As shown in FIG. 2, in the power generation element 21 of the bipolar battery 10 ′ of the present embodiment, a positive electrode active material layer 13 that is electrically coupled to one surface of the current collector 11 is formed. And a plurality of bipolar electrodes having a negative electrode active material layer 15 electrically coupled to the opposite surface. Each bipolar electrode is stacked via the electrolyte layer 17 to form the power generation element 21.

  At this time, each bipolar type is formed such that the positive electrode active material layer 13 of one bipolar electrode and the negative electrode active material layer 15 of another bipolar electrode adjacent to the one bipolar electrode face each other through the electrolyte layer 17. Electrodes and electrolyte layers 17 are alternately stacked. That is, the electrolyte layer 17 is disposed between the positive electrode active material layer 13 of one bipolar electrode and the negative electrode active material layer 15 of another bipolar electrode adjacent to the one bipolar electrode. The positive electrode active material layer 13, the electrolyte layer 17, and the negative electrode active material layer 15 constitute one unit cell layer 19. Therefore, it can be said that the bipolar battery 10 ′ has a configuration in which the single battery layers 19 are stacked.

  Further, for the purpose of preventing liquid junction due to leakage of the electrolytic solution from the electrolyte layer 17, a seal portion 31 is disposed on the outer peripheral portion of the unit cell layer 19. By providing the seal portion 31, it is possible to insulate between the adjacent current collectors 11 and prevent a short circuit due to contact between adjacent electrodes. By installing this seal portion 31, long-term reliability and safety are ensured, and a high-quality bipolar battery 10 'can be provided.

  A positive electrode active material layer 13 is formed only on one side of the positive electrode outermost layer current collector 11 a located in the outermost layer of the power generation element 21. The negative electrode active material layer 15 is formed only on one surface of the outermost current collector 11b on the negative electrode side located in the outermost layer of the power generation element 21. However, the positive electrode active material layer 13 may be formed on both surfaces of the outermost layer current collector 11a on the positive electrode side. Similarly, the negative electrode active material layer 13 may be formed on both surfaces of the outermost layer current collector 11b on the negative electrode side.

  The electrolyte layer 17 is formed by impregnating a separator with an electrolyte, a polymer electrolyte, or the like. The electrolyte of the electrolyte layer 17 contains an electrolyte salt (lithium salt) containing fluorine (F) (details will be described later).

  Further, in the bipolar secondary battery 10 ′ shown in FIG. 2, the positive electrode tab 25 is disposed adjacent to the positive electrode side outermost layer current collector 11a, and this is extended and led out from the laminate sheet 29 which is a battery exterior material. ing. On the other hand, the negative electrode tab 27 is disposed so as to be adjacent to the negative electrode side outermost layer current collector 11b, and is similarly extended and led out from the laminate sheet 29 which is an exterior of the battery.

  Note that the number of stacks of the unit cell layers 19 is adjusted according to a desired voltage. In the bipolar battery 10 ′, the number of times the single battery layer 19 is stacked may be reduced if a sufficient output can be secured even if the battery is made as thin as possible. Even in the bipolar battery 10 ′, the power generation element 21 is sealed in a laminate sheet 29, which is a battery exterior material, and the positive electrode tab 25 and the negative electrode tab 27 are laminated in order to prevent external impact and environmental degradation during use. It is preferable that the structure be taken out of the sheet 29.

  The constituent elements and the manufacturing method of the lithium ion battery 10 and the bipolar battery 10 ′ are basically the same except that the electrical connection form (electrode structure) in both batteries is different. Since existing manufacturing methods may be used, description thereof is omitted.

  FIG. 3 is a perspective view showing an appearance of the above-described stacked flat non-bipolar or bipolar lithium ion secondary battery.

  As shown in FIG. 3, in a stacked flat lithium ion battery 10 (or 10 ′), a power generation element (battery element) 21 is wrapped by a laminate sheet 29 that is a battery exterior material. The periphery of the laminate sheet 29 is heat-sealed. From the power generation element 21, the positive electrode tab 25 and the negative electrode tab 27 are sealed in a state of being drawn out. Here, the power generation element (battery element) 21 enclosed by the battery outer packaging material 52 is the non-bipolar or bipolar lithium-ion secondary battery 10 (or 10 ′) shown in FIG. 1 or FIG. This is a power generation element (battery element) 21.

  The laminate sheet as the exterior material is, for example, a laminate film having a laminated structure of aluminum and a resin material. Weight reduction can be achieved by such a laminate sealing form using a laminate sheet.

  In addition, the method of taking out the positive electrode tab 25 and the negative electrode tab 27 is not particularly limited. For example, the positive electrode tab 58 and the negative electrode tab 59 may be pulled out from the same side in addition to the form shown in FIG. However, the positive electrode tab 25 and the negative electrode tab 27 may be divided into a plurality of parts and taken out from each side.

  In addition, in place of laminate sealing, in a wound type lithium ion battery, instead of a tab, for example, a terminal may be formed using a cylindrical can (metal can).

  In this embodiment, the temperature sensor 101 is provided in close contact with the laminate-sealed battery outer package. The temperature sensor 101 is used as part of a battery system described later. A lead wire 111 for transmitting the signal is output from the temperature sensor 101 and is connected to a control device 105 described later.

  FIG. 4 is a block diagram for explaining the battery system in the present embodiment. In the figure, solid lines indicate electrical wiring, and broken lines indicate control signal lines from the control device 105.

  The battery system 100 includes the above-described lithium ion battery 10 (or 10 ′), a charge control device 110 (charge control means) that charges the lithium ion battery 10 (or 10 ′), and the lithium ion battery 10 (or 10 ′). ).

  The charging control device 110 includes a temperature sensor 101, a switch 102, a voltmeter 103, an ammeter 104, and a control device 105. The temperature sensor 101 measures the temperature of the lithium ion battery 10 (or 10 '). The voltmeter 103 is connected between the positive electrode tab 25 and the negative electrode tab 27 and measures the voltage therebetween. That is, the received voltage between the positive electrode and the negative electrode of the lithium ion battery 10 (or 10 ') is measured. The ammeter 104 is provided between the negative electrode tab 27 and the DC power source 120 and measures the current flowing in the battery. The cooling device 130 is an air cooling device, for example, and cools the lithium ion battery 10 (or 10 ') by sending air to the lithium ion battery 10 (or 10').

  The control device 105 controls the voltage of the electric power supplied from the DC power source 120 to the lithium ion battery 10 (or 10 ') according to the temperature of the temperature sensor 10 according to the procedure described later. At this time, the control device 105 monitors the voltmeter 103 and the ammeter 104 to monitor whether the supplied voltage is a control voltage and whether a current exceeding a specified current is flowing. . In addition, the control device 105 operates the cooling device 130 to cool the lithium ion battery 10 (or 10 ') when the lithium ion battery 10 (or 10') reaches a predetermined temperature (described later) or higher.

  The cooling device 130 is not limited to an air cooling device, and well-known cooling means such as a water cooling device can be applied. Further, a known heat sink or the like may be attached to the lithium ion battery 10 (or 10 ') to increase the cooling efficiency. When the cooling device 130 is not provided, natural cooling of the lithium ion battery 10 (or 10 ') can be aided by attaching a heat sink.

  Further, the cooling device 130 is not dedicated to the present battery system, and a cooling means attached to a device (for example, a vehicle described later) using the lithium ion battery 10 (or 10 ') may be used.

  The DC power source 120 is an external power source. The switch 102 is for switching charging on and off, and is controlled by the control device 105. The control device 105 is a small electronic computer having, for example, a microprocessor, a clock, a memory, and the like. The control device 105 stores a program having a procedure described later, and controls charging by executing the program.

  The charge control device 110 may be provided as a battery system 100 integrated with the lithium ion battery 10 (or 10 ′), or may be detachable from the lithium ion battery 10 (or 10 ′) to be a single charger. May be provided as In the case of a charger, the charging control device 110 may have other functions used in a normal charger. In the case of a charger, a temperature sensor for measuring the temperature of the lithium ion battery 10 (or 10 ') is attached to the lithium ion battery 10 (or 10') for charging.

  FIG. 5 is a flowchart showing the charging operation procedure of this battery system.

  The basis of this charging operation procedure is constant current constant voltage charging. The charging voltage in the constant voltage charging region is set to a voltage higher than 3.8V and lower than 4.2V when a predetermined battery temperature is reached.

  Hereinafter, a description will be given with reference to FIG. In the following description, the lithium ion battery 10 (or 10 ') is simply referred to as a battery.

  First, the control device 105 turns on the switch 102 and starts constant current charging (S1). At this time, the control device 105 monitors the temperature sensor 101, the voltmeter 103, and the ammeter 104 until charging is completed simultaneously with the start of charging. Charging at this time is constant current charging, and the output of the DC power source 120 is controlled so that a constant current flows from the value of the ammeter 104 to the battery.

  Thereafter, the control device 105 determines whether or not the current being charged cannot be a constant current (S2), and controls the power supply 120 so that the constant voltage charging is performed when the constant current charging cannot be performed. (S3). The constant voltage charging here may be, for example, a voltage of 3.8 V or less.

Then, the control device 105 determines whether or not the battery temperature is within a predetermined range (S4). Here, the temperature in the predetermined range is 45 to 55 ° C. Here, when the temperature of the battery is outside the predetermined range (S4: No), the processing of steps after S10 described later is performed.

  When the battery temperature is in a predetermined range (S4: Yes), the control device 105 has a charging voltage (that is, a voltage between the positive electrode tab 25 and the negative electrode tab 27) that is constant between 3.8V and 4.2V or less. The voltage is controlled to be a voltage (S5). Here, the constant voltage may vary somewhat as long as it is a voltage within a range higher than 3.8V and lower than 4.2V.

  On the other hand, when the battery temperature is outside the predetermined range in S4 (S4: No), the control device 105 determines whether or not the battery temperature exceeds 55 ° C., which is the upper limit of the predetermined temperature range (S10). Here, when the battery temperature exceeds the upper limit of the predetermined temperature range, the cooling device is operated so as to be within the predetermined temperature range (S11). When the cooling device 130 is not attached, the charging operation may be temporarily stopped to wait until the battery temperature is within a predetermined temperature range. This temporarily stops charging, suppresses heat generation during charging, and waits for the battery to cool naturally.

  After S11 and when the battery temperature does not exceed the upper limit of the predetermined range of temperature in S10 (that is, the temperature is lower than the lower limit of the predetermined range), the process proceeds to S6.

  In S6, the control device 105 determines whether or not the current value is equal to or smaller than the charging end current value (S6). If the current value is equal to or smaller than the end current value, the switch 102 is turned off and the charging operation is terminated. On the other hand, if the end current value is not reached, the process returns to S4 to determine the temperature. Then, the subsequent processing is continued. That is, when the temperature is less than the lower limit of the predetermined range, the first constant voltage charging is continued, and when the temperature is within the predetermined range, the constant voltage charging is continued at a voltage higher than 3.8V and lower than 4.2V. And if temperature exceeds the upper limit of a predetermined range, it will become processing after S10.

  The operation of the first embodiment will be described.

  The lithium ion battery 10 (or 10 ') has a lithium salt containing fluorine as an electrolyte. During charging, the lithium salt in the electrolyte causes a hydrolysis reaction with a small amount of water inside the battery, resulting in the generation of HF (see the following formula (1)).

LiPF ₆ + H ₂ O → HF + LiOH + PF ₅ (1)
The HF in the electrolytic solution generated at this time acts on the positive electrode, and Fe is eluted as Fe ²⁺ and Fe ³⁺ from the positive electrode active material LiFePO ₄ . In the elution of Fe, not only the capacity of the positive electrode is reduced, but the dissolved Fe ²⁺ or Fe ³⁺ ions are reduced on the carbon negative electrode having a base potential and deposited as an Fe compound. Since the carbon of the negative electrode is involved in the charge / discharge reaction of Li on the edge surface, if Fe deposits there, the reaction of Li (that is, the path through which Li enters and exits) is greatly hindered, leading to a further decrease in capacity. This phenomenon occurs remarkably when the charging voltage is about 3.3 to 3.8 V as in the conventional case at 45 ° C. or higher. Since Fe once deposited on the negative electrode does not return to its original state, it is considered that such a phenomenon occurs every time the battery is charged, resulting in a decrease in battery capacity and deterioration in cycle characteristics (described later). Examples and Comparative Examples).

  Therefore, in this embodiment, the charging voltage is increased when the battery temperature during charging becomes 45 ° C. or higher. By increasing the charging voltage, a small amount of water present in the battery can be removed by oxidative decomposition. Therefore, the reaction (1) can be prevented or suppressed, the hydrolysis of Fe can be prevented or suppressed, and the elution of Fe in the positive electrode active material can be suppressed. Therefore, even if the upper discharge is repeated, the capacity does not decrease, and as a result, the cycle characteristics can be improved (see Examples and Comparative Examples described later).

  In addition, when the battery temperature during charge was less than 45 degreeC, it was confirmed from the comparative example mentioned later that a charge voltage does not affect cycling characteristics. For this reason, it is not necessary to increase the charging voltage when the battery temperature is lower than 45 ° C. On the other hand, when the battery temperature during charging is 55 ° C. or higher, it is not preferable as the temperature during charging. This is because if the battery voltage is higher than 4.0 V at a temperature of 55 ° C. or higher, the electrolytic solution itself undergoes oxidative decomposition, and the battery performance may be degraded. For this reason, in this embodiment, the battery temperature is prevented from reaching 55 ° C. or higher.

  Next, each member constituting this embodiment will be described.

(Positive electrode and negative electrode)
The positive electrode and the negative electrode each include a positive electrode active material and a negative electrode active material.

As the positive electrode active material, the olivine type lithium iron phosphate represented by the chemical formula LiFePO ₄ already described is used.

On the other hand, as a negative electrode active material, Preferably, carbon materials, such as graphite, soft carbon, and hard carbon, are mentioned, for example. The negative electrode active material is not particularly limited as long as it can reversibly occlude and release lithium even if it is an active material other than carbon. Examples of non-carbon-based negative electrode active materials include metals such as Si and Sn, or metal oxides such as TiO, Ti ₂ O ₃ , TiO ₂ , SiO ₂ , SiO, and SnO ₂ , Li _4/3 Ti _{5 / 3} O ₄ or Li ₇ MnN, such as a composite oxide of lithium and a transition metal, a Li—Pb alloy, a Li—Al alloy, or the like.

  Further, these positive electrode and negative electrode may contain, for example, a binder, a conductive additive, an electrolyte salt (lithium salt), an ion conductive polymer, and the like as additives.

  Among these, examples of the conductive assistant include carbon materials such as acetylene black, carbon black, graphite, and vapor grown carbon fiber. These effectively form an electronic network inside the active material layer and contribute to improving the output characteristics of the battery.

As the electrolyte salt (lithium salt), Li (C ₂ F ₅ SO ₂ ) ₂ N, Li (CF ₃ SO ₂ ) ₂ N, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiPF _{3 (CF 2) 3, LiH 2} BC 6 O 8 and the like.

  Examples of the ion conductive polymer include polyethylene oxide (PEO) -based and polypropylene oxide (PPO) -based polymers.

  The binder is an additive that plays a role in binding the active material and the conductive additive. The binder is not particularly limited as long as it can bind the active material and the conductive additive. For example, a thermoplastic polymer, a fluororesin, a fluororubber, a polyacrylic resin, a polyimide resin, a mixed system of styrene butadiene rubber and carboxymethylcellulose, or the like is used.

  Since the mixing ratio of these positive electrode active material and negative electrode active material and the components of the additives contained in them can be implemented by appropriately referring to known knowledge as a non-aqueous solvent secondary battery, a detailed explanation is provided. Omitted. Moreover, there is no restriction | limiting in particular also about the thickness (thickness of each active material layer) of a positive electrode and a negative electrode, The conventionally well-known knowledge about a battery can be referred suitably.

(Electrolyte layer)
In the electrolyte layer, the separator is impregnated with the electrolyte. Examples of the separator include polyolefin such as polyethylene and polypropylene, and a microporous film made of aramid fiber.

  The electrolyte can be, for example, a liquid electrolyte. The liquid electrolyte has a form in which an electrolyte salt (lithium salt) as a supporting salt is dissolved in an organic solvent as a plasticizer. Examples of the organic solvent that can be used as the plasticizer include carbonates such as ethylene carbonate (EC) and propylene carbonate (PC), and are not particularly limited. They may be used alone or in the form of a mixture of two or more.

As the electrolyte salt, _{e.g., LiPF 6, LiBF 4, LiClO} 4, LiAsF 6, LiTaF 6, LiSbF 6, LiAlCl 4, Li 2 B 10 Cl 10, LiI, LiBr, LiCl, LiAlCl, halogenated inorganic such LiHF ₂ Acid anion salts are preferred.

  These halogen-based electrolyte salts promote dissociation of the electrolyte of the lithium battery by introducing a fluorine element having a high electronegativity as an anion molecule.

  Further, instead of the separator impregnated with the liquid electrolyte, the polymer electrolyte itself may be used as the separator. The polymer electrolyte is classified into, for example, a gel electrolyte containing an electrolytic solution and an intrinsic polymer electrolyte containing no electrolytic solution.

  The gel electrolyte has a configuration in which the above liquid electrolyte is injected into a matrix polymer made of an ion conductive polymer. Examples of the ion conductive polymer used as the matrix polymer include polyethylene oxide (PEO), polypropylene oxide (PPO), and copolymers thereof. In such a polyalkylene oxide polymer, the above electrolyte salt can be dissolved well.

  In these electrolytes, vinylene carbonate (VC), ethylene carbonate (EC), diethylene carbonate auto (DEC), lithium bisoxalate borate (LiBOB), ethylene sulfite (ES), chloroethylene carbonate (Cl) -EC) or other conventionally known additives may be included. As described above, this additive forms a film on the negative electrode and suppresses deterioration.

(Current collector)
The current collector is made of a conductive material on both the positive electrode side and the negative electrode side. The positive electrode side and the negative electrode side may be made of the same material or different materials. For example, a metal or a conductive polymer can be employed. Specifically, metal materials, such as aluminum, nickel, iron, stainless steel, titanium, copper, are mentioned, for example. In addition to these, a clad material of nickel and aluminum, a clad material of copper and aluminum, or a plating material of a combination of these metals can be preferably used. Moreover, the foil by which aluminum is coat | covered on the metal surface may be sufficient. Of these, aluminum and copper are preferable from the viewpoints of electron conductivity and battery operating potential.

  The size and thickness of the current collector are not particularly limited as long as the current collector has a size suitable for the battery capacity and can function as a current collector.

(Positive electrode tab and negative electrode tab)
The positive electrode tab and the negative electrode tab are electrically connected to each current collector for the purpose of taking out current outside the battery. The material may be the same as or different from the current collector. Furthermore, it is good also as a positive electrode tab and a negative electrode tab by extending each electrical power collector.

(Battery exterior material)
As the battery exterior material, a bag-like case using a laminate film containing aluminum can be used. As this laminate film, for example, a laminate film having a three-layer structure in which polypropylene, aluminum, and a polyamide-based synthetic resin are laminated in this order can be used. However, the laminate film is not limited thereto. Although not shown, a metal can case may be used in place of the battery outer packaging material in the form of a laminate.

  Next, a usage example of the lithium ion battery 10 (or 10 ') in the above-described embodiment will be described.

(Battery)
6A and 6B are explanatory diagrams for explaining the assembled battery according to the first embodiment. FIG. 6A is a plan view of the assembled battery, FIG. 6B is a front view of the assembled battery, and FIG. 6C is an assembled battery. It is a side view.

  The assembled battery 300 constitutes a small assembled battery 250 in which a plurality of the above-described lithium ion batteries 10 (or 10 ') are connected in series or in parallel and can be attached and detached. Further, a plurality of small assembled batteries 250 are further connected in series or in parallel to form an assembled battery 300 having high volume energy density and high volume output density. Such an assembled battery 300 can have a large capacity and a large output suitable for a vehicle driving power source and an auxiliary power source.

  The small assembled batteries 250 are connected to each other using an electrical connection means such as a bus bar, and are stacked in multiple stages using a connection jig 310. The number of battery packs 250 used may be determined according to the battery capacity and output required by the vehicle (electric vehicle).

  In the case of such a configuration of the assembled battery 300, when the battery system is configured using the assembled battery 300, there is only one charging control device 110 for the entire assembled battery. The control itself is the same as the procedure shown in FIG. In this case, at least one temperature sensor 101 is required in the assembled battery 300. In the example of FIG. 6, the battery pack 250 is provided at three locations, that is, the battery pack 250 on the outermost side of the battery pack 300 as a whole and the battery pack 250 in the middle. In the figure, the lead wire from the temperature sensor 101 is not shown. In charging control, the charging voltage may be set to a voltage higher than 3.8 V and lower than 4.2 V when at least one of the three temperature sensors falls within a predetermined range. However, when any one of the temperature sensors exceeds the upper limit of the temperature range, the cooling device is operated or the charging is temporarily stopped to lower the temperature.

  The temperature sensor 101 is not limited to this, and may be provided in each lithium ion battery 10 (or 10 ').

  The assembled battery 300 of this embodiment has good cycle characteristics of the individual lithium ion batteries 10 (or 10 ′) constituting the assembled battery 300, so that the assembled battery 300 has not only a large capacity, but also its usage time. Can be long. In addition, since the assembled battery 300 has a structure in which a plurality of detachable small assembled batteries 250 are provided, even if a defect or the like partially occurs, only the small assembled battery 250 inside needs to be replaced.

(vehicle)
Next, a vehicle using such an assembled battery will be described.

  FIG. 7 is a diagram showing an example in which the above-described assembled battery is mounted on an electric vehicle.

  In this electric vehicle 400, the assembled battery 300 is mounted under the seat in the center of the vehicle body and used as a motor power source for the electric vehicle 400. This is because if it is installed under the seat, the interior space and the trunk room can be widened. The place where the assembled battery 300 is mounted is not limited to the position under the seat, but may be a lower part of the rear trunk room or an engine room in front of the vehicle.

  As described above, in the electric vehicle 400 using the assembled battery 300, the charging control device 110 may be incorporated into a controller (ECU) in the vehicle as a control device for charging the assembled battery 300.

  Thus, in the electric vehicle 400 using the assembled battery 300, the lithium ion battery 10 (or 10 ') constituting the assembled battery 300 has a good cycle characteristic, and thus provides a sufficient output even when used for a long time. Yes.

  Such a vehicle can be used for, for example, a complete electric vehicle that does not use gasoline, a hybrid vehicle such as a series hybrid vehicle and a parallel hybrid vehicle, and a sub storage battery in a fuel cell vehicle. In addition, it can also be used as various power sources and secondary batteries for two-wheeled vehicles (motorcycles), tricycles, and moving bodies such as trains.

Production of Lithium Batteries as Examples and Comparative Examples (1) Production of Positive Electrode LiFePO ₄ (86 mass%) having a carbon content of 2 mass% and an average particle size of 1.0 μm was used as a positive electrode active material. To this positive electrode active material, acetylene black (6% by mass in the mixture) as a conductive additive and PVdF (8% by mass) as a binder were added and mixed. An appropriate amount of N-methyl-2-pyrrolidone (NMP) was added as a solvent to the positive electrode mixture thus prepared, and stirred sufficiently to prepare a slurry having a viscosity of 20000 cps.

The slurry thus obtained was applied onto an aluminum foil (thickness 20 μm) serving as a positive electrode current collector so as to be 6.0 mg / cm ² , dried and pressed to form a positive electrode. The size of the positive electrode is 6.8 cm × 6.8 cm.

(2) Production of negative electrode MCMB (90 mass%) having an average particle diameter of 10 μm was used as the negative electrode active material. PVdF (10% by mass) as a binder was added to the negative electrode active material and mixed. MCMB (mesocarbon microbeads) is artificial graphite. An appropriate amount of NMP was added as a solvent to this mixture and stirred sufficiently to prepare a slurry having a viscosity of 5000 cps. The slurry thus obtained was applied on a copper foil (thickness 10 μm) as a negative electrode current collector so as to be 1.3 mg / cm ² , dried and pressed, did. The size of the negative electrode is 7.0 cm × 7.0 cm.

(3) Separator A 25 μm-thick polyethylene (PE) microporous membrane was used as the separator. The size of the separator is 7.8 cm × 7.8 cm.

(4) Electrolyte A mixed solvent of ethylene carbonate and diethyl carbonate (mixing volume ratio 1: 1) containing LiPF ₆ at a ratio of 1 mol / L was used.

(5) Battery The separator was sufficiently impregnated with an electrolyte, and this was sandwiched between the positive electrode and the negative electrode to obtain a single-layer battery.

  The capacity confirmation test of the battery was carried out in a constant temperature bath, kept at 25 ° C., carried out with constant current and constant voltage charge up to 4.0 V at 35 mA for 3 hours, and at the capacity when discharged at constant current up to 2.0 V at 35 mA. Calculated.

(Experiment)
A cycle test was performed as an example and a comparative example using the single-layer battery produced above.

  In the cycle test, after constant current charging at 105 mA, when the current value became less than 105 mA, constant voltage charging with the charging voltage of each example and each comparative example was performed for 3 hours, and then 2. Discharge to 0V was performed. This charge / discharge cycle was performed at 55 ° C. and 45 ° C. for 300 cycles. The capacity retention rate indicates the ratio of the capacity after 300 cycles to the initial capacity. It should be noted that both charging and discharging were performed in a constant temperature bath to maintain the temperature condition.

  The capacity retention was shown as a percentage based on the capacity in the capacity confirmation test. That is, capacity retention ratio = (capacity after 300 cycles) / (capacity in capacity confirmation test).

  The results of the cycle test are shown in Tables 1 and 2. Table 1 shows the case where the charging temperature is 55 ° C. (charging temperature condition 55 ° C.), and Table 2 shows the case where the charging temperature is 45 ° C. (charging temperature condition 45 ° C.).

  From the results of Tables 1 and 2, the Examples 1 to 6 in which the charging voltage was 3.8 to 4.2 V were higher than those of Comparative Examples 1 and 2 at any of the charging temperature conditions 55 ° C and 45 ° C. Good cycle characteristics.

  FIG. 8 is a graph showing the cycle characteristics of the example and the comparative example under the charging temperature condition of 55 ° C. As can be seen from this figure, as the number of charge / discharge increases, the capacity retention rate in the first comparative example greatly decreases, but in Embodiments 1 to 3, the decrease is small.

  From these results, it is understood that the cycle characteristics are improved by setting the charging voltage to be higher than 3.8V and 4.2V or lower.

  Table 3 shows the cycle characteristics when charged at a charging temperature of 25 ° C. (charging temperature condition of 25 ° C.). Other conditions are the same as those in the above examples and comparative examples.

  As can be seen from Comparative Examples 3 to 6 shown in Table 3, when the charging temperature is 25 ° C., the cycle characteristics are not affected even if the charging voltage is changed.

  From the results of the above Examples and Comparative Examples, it can be seen that the influence of moisture inside the battery occurs at a high temperature. Moreover, when the charging temperatures of 55 ° C. and 45 ° C. are compared, the higher the temperature, the worse the cycle exception. However, as shown in this example, at 45 ° C. and 55 ° C., it can be seen that by making the charging voltage higher than 3.8 V and lower than 4.2 V, the cycle characteristics can be improved and used for a long time.

  According to the embodiments and examples described above, the following effects are obtained.

  In the charging method of this embodiment, when the battery temperature during charging reaches 45 to 55 ° C., the charging voltage is set to be higher than 3.8 V and lower than 4.2 V. Charging at a voltage exceeding 3.8 V decomposes a small amount of water inside the battery and suppresses generation of HF due to fluorine (F) components in the electrolyte due to water. Thereby, the elution of Fe from the positive electrode can be prevented or suppressed, the capacity reduction due to repeated charge and discharge can be reduced, and the cycle characteristics of the battery can be improved. Moreover, the battery charged by this method and the assembled battery in which this battery is combined are less likely to have a reduced capacity due to multiple charging. In particular, when a carbon-based material is used as the negative electrode active material, it is possible to prevent or suppress precipitation of the extracted Fe at the negative electrode. A structure with more cycle characteristics can be expected.

  In addition, the battery system of this embodiment can reliably perform the above charging method while monitoring the current, voltage, and battery temperature during charging. Further, the charge control device can be provided as a single device separately from the lithium ion battery. Therefore, the charging method according to the present embodiment can be performed on an existing lithium ion battery using a positive electrode active material containing lithium, iron, and phosphoric acid to improve cycle characteristics.

  Furthermore, since the assembled battery according to the present embodiment is an assembled battery using a plurality of lithium ion batteries with improved cycle characteristics, the overall duration of the assembly is small because there is little decrease in capacity in the charge / discharge cycle. , Cycle characteristics are improved.

  In addition, the battery system, the lithium ion battery, and the assembled battery mounted on the battery system according to the present embodiment have good cycle characteristics of the secondary battery (lithium ion battery, assembled battery, or battery in the battery system) as a power source, and are long. Even if it is used for a period (that is, repeated charging and discharging many times), since the capacity is hardly reduced, the duration is long and it is suitable for long-distance running.

  Although the embodiments and examples of the present invention have been described above, the present invention is not limited to the above-described embodiments and examples.

  For example, the charging voltage that is higher than 3.8 V and lower than 4.2 V may not be performed every time charging is performed, and may be performed once every plural times of charging. Specifically, it may be performed periodically, for example, once every 10 times or once every 50 times, or irregularly within the number of times not exceeding these numbers. In the above-described embodiment, the charging voltage is set to be higher than 3.8 V and lower than 4.2 V in the constant voltage charging region when the battery temperature is in a predetermined range every time charging is performed. This charging voltage is for removing a very small amount of moisture inside the battery as already described. Therefore, if the voltage is raised once to remove the moisture in the interior, there will be no moisture in the battery for a while. However, since the water once decomposed is generated again over time, if the charging voltage is increased once every plural times, the water is decomposed again at that stage. Therefore, improvement in cycle characteristics can be expected only by increasing the charging voltage once in a plurality of times.

  The lithium ion battery 10, the assembled battery 300, or the battery system 100 or 200 of the present embodiment is used as a mounting power source (fixed power source) such as an uninterruptible power supply, in addition to being mounted on a vehicle. It is also possible.

  In addition, it goes without saying that various embodiments of the present invention are possible within the scope of the technical idea of the present invention.

10, 10 'lithium ion battery,
11 positive electrode current collector,
12 negative electrode current collector,
13 positive electrode active material layer,
15 negative electrode active material layer,
17 separator,
19 cell,
21 power generation elements,
25 positive electrode tab,
27 negative electrode tab,
29 Battery exterior materials,
100 battery system,
110 charge control device,
101 temperature sensor,
102 switch,
103 Voltmeter,
104 ammeter,
105 control device,
120 DC power supply,
130 Cooling device.

Claims (6)

A method for charging a lithium ion battery using a positive electrode active material containing lithium, iron, and phosphoric acid,
When the temperature during charging of the lithium ion battery is less than 45 ° C., the battery is charged at a charging voltage of 3.8 V or less,
A charging method for a lithium ion battery, wherein the charging voltage is set to be higher than 3.8 V and lower than 4.2 V when the temperature during charging of the lithium ion battery is 45 to 55 ° C. Multiple least one of the charging method of charging a lithium-ion battery of claim 1 Symbol mounting, characterized in that the charging at the charging voltage of the following higher 4.2V than the 3.8 V. The lithium-ion battery, the charging process of a lithium ion battery according to claim 1 or 2, characterized in that it comprises a carbon anode active material. When the temperature during charging of the lithium ion battery used for the positive electrode active material containing lithium, iron, and phosphoric acid is less than 45 ° C., the battery is charged with a charging voltage of 3.8 V or less, and when the temperature is 45 to 55 ° C., the charging voltage is 3 A charge control device comprising charge control means for making the voltage higher than .8V and lower than 4.2V. The charge control device according to claim 4 , wherein the charge control unit charges at least one of a plurality of times of charge with a charge voltage higher than 3.8 V and lower than 4.2 V. The charge control device according to claim 4 , wherein the lithium ion battery includes carbon in a negative electrode active material.