JP2012043683A - Nonaqueous electrolyte secondary battery pack - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a battery pack including a plurality of nonaqueous electrolyte secondary batteries connected in series, in which voltage monitoring means such as a circuit for monitoring a voltage of each of the batteries can be decreased by half while deterioration in the battery pack due to variations in unit cells is suppressed and safety is maintained.SOLUTION: In the nonaqueous electrolyte battery pack, a charging voltage of a nonaqueous electrolyte secondary battery pack is set preliminarily, and a nonaqueous electrolyte secondary battery having a capacity change rate of 90% or more is adopted. The capacity change rate is a rate between a 0.1 C discharge capacity at 25°C when each of nonaqueous electrolyte batteries constituting the battery pack is charged at 25°C with a 0.1 C rate at a charging voltage higher by 0.8 V than half of the set charging voltage of the nonaqueous electrolyte secondary battery pack and a 0.1 C discharge capacity at 25°C when charged at a voltage half of the set charging voltage.

Description

  Embodiments described herein relate generally to a nonaqueous electrolyte secondary battery pack.

  In recent years, non-aqueous electrolyte secondary batteries that are charged and discharged by moving Li ions between a negative electrode and a positive electrode have been actively researched and developed as high-energy density batteries. Such a non-aqueous electrolyte secondary battery is expected as a power source for large-sized electric vehicles and hybrid vehicles using both an engine and a motor, from the viewpoint of environmental problems. In addition to automobile applications, nonaqueous electrolyte secondary batteries as large-scale power sources have attracted considerable attention.

One of the non-aqueous electrolyte secondary batteries is a lithium ion secondary battery. Most lithium ion secondary batteries have lithium cobalt oxide (LiCoO ₂ ) or lithium manganate (LiMn ₂ O ₂ ) as a positive electrode. Used, and a graphite-based material is used for the negative electrode. Such a combination of secondary batteries is often used between 3V and 4.2V, and the average operating voltage of the battery is about 3.7V. In order to use this battery as a large-scale power source, it is necessary to make a plurality of series connections and increase it to several hundred volts or more depending on the application.

Lithium ion secondary batteries have high energy density, but their safety is regarded as a problem. Although various measures have been taken to improve safety and improvements have been made, especially when overcharged, metal lithium tends to precipitate on the surface of the graphite-based material, and in the case of the positive electrode. It is known that oxygen is easily released from LiCoO ₂ and causes thermal runaway, which is a serious problem from the viewpoint of safety.

  In order to connect a large number of such lithium ion secondary batteries in series, a control method for constantly monitoring the voltage of each battery is always required. For example, if a 42V voltage is applied to the battery pack to charge a 10 series pack of lithium ion secondary batteries, each battery will be charged at 4.2V if all batteries are functioning normally. However, if one cell is short-circuited and becomes 0V, nine batteries are applied with a voltage of 42V, so an average voltage of 4.7V is applied to each cell. In this case, the individual cells are clearly overcharged and the possibility of ignition increases.

  For example, when a charge / discharge cycle is performed in a battery pack as well as from the viewpoint of safety, the capacity deterioration rate differs due to internal temperature unevenness and the like, and ultimately the capacity and voltage of each battery vary. Since the lithium ion secondary battery has a large capacity deterioration even when the charging voltage is increased by 0.1 V, the battery pack itself is extremely deteriorated.

From the above viewpoints, when a plurality of series is performed in a conventional lithium ion secondary battery, a circuit for monitoring each battery voltage is indispensable, and there is a disadvantage that the apparatus becomes large. It was.

JP 2000-67928 A

In this embodiment, in a battery pack in which a plurality of nonaqueous electrolyte secondary batteries are connected in series, voltage monitoring such as a voltage monitoring circuit of each battery is performed while suppressing deterioration of the battery pack due to variations in single cells and maintaining safety. A battery pack capable of halving the means is provided.

In the two-series nonaqueous electrolyte battery pack of this embodiment, the charging voltage of the nonaqueous electrolyte secondary battery pack is set in advance, and is less than half the value of the charging voltage of the set nonaqueous electrolyte secondary battery pack. When the individual nonaqueous electrolyte batteries constituting the battery pack are charged at a rate of 25V and a 0.1C rate at a charging voltage that is 8V higher, a 0.1C discharge capacity at 25C and half of the set charging voltage A nonaqueous electrolyte secondary battery having a capacity change rate of 0.1 C discharge capacity at 25 ° C. when charged at a voltage of 90% or more is employed.

FIG. 1 is a graph showing a charging curve at a 1C rate of a general lithium ion secondary battery. FIG. 2 is a graph showing a charging curve at the 1C rate of the lithium ion secondary battery of the present embodiment. FIG. 3 is a partially broken perspective view of a thin nonaqueous electrolyte secondary battery to which the battery of the present embodiment can be applied.

Hereinafter, a conventional general lithium ion secondary battery will be described as an example.
When LiCoO ₂ is used for the positive electrode material and graphite is used for the negative electrode material, Li is continuously inserted and desorbed while the positive electrode mainly forms a layer of LiCoO ₂ . LiCoO _2, the battery state of charge; even in a state where the (State Of Charge hereinafter SOC) to 100%, maintaining reversibility of LiCoO _2, and, in order to ensure thermal stability, Li in the LiCoO ₂ The volume balance between the positive electrode and the negative electrode is controlled by the coating amount so that the ink does not come off completely. Therefore, in a lithium ion secondary battery, if charging is continued forcibly from a state where the SOC is 100%, lithium is further extracted from the positive electrode, the reversibility is poor, and thermal stability is likely to be lost. Further, the lithium extracted from the positive electrode is deposited as a lithium metal state on the negative electrode graphite surface. In this state, the battery voltage rises continuously, and the capacity increases apparently in proportion to the charging voltage. Therefore, the state in which the capacity increases remarkably as the voltage increases from the state where the SOC is 100% not only deteriorates the cycle performance but also leaves an excessive problem in the safety of the battery.

A battery pack in which two such lithium ion secondary batteries are connected in series will be described.
FIG. 1 shows a charging curve at a 1C rate of a general lithium ion secondary battery. For example, when lithium nickelate (LiNiO ₂ ) is used as the positive electrode and graphite is used as the negative electrode, the potential often changes continuously according to the depth of charge (SOC) as shown in the figure. Consider managing the same lithium ion secondary battery in two series. A battery pack in which two batteries, A and B are connected in series, is ideally in a state where individual battery voltage behaviors are equal as shown in FIG. However, the SOCs of the battery A and the battery B are likely to gradually shift due to self-discharge due to temperature unevenness in the battery pack. An extreme example is shown in FIG. This example shows a state in which one battery B is lower than the SOC initially set by self-discharge or the like, so that the battery A is 80% SOC and the battery B is 50% SOC.

  If it is attempted to charge 8.4V to the two-series battery pack in such a state, when the battery pack voltage reaches 8.4V, battery A may be 4.4V and battery B may be 4.0V. There is. In this case, since the battery A is in an overcharged state that exceeds the proper charge voltage, the battery performance is easily deteriorated and the battery A falls into a very dangerous state such as battery ignition.

  Therefore, in the case of a lithium ion secondary battery, a circuit for monitoring each battery voltage is incorporated in a control system in the battery or outside the battery when manufacturing not only two series but also a plurality of series battery packs. There is. In this way, even if the SOC variation in the battery pack occurs, overcharging of the battery constituting the battery pack can be prevented. However, since a monitoring circuit as many as the number of batteries is required, the system becomes large. End up.

On the other hand, the charging voltage of the nonaqueous electrolyte secondary battery pack is set in advance, and the charging voltage is 0.8V higher than the half of the charging voltage of the set nonaqueous electrolyte secondary battery pack. 0.1 C discharge capacity at 25 ° C. when charging each battery at a rate of 25 ° C. and 0.1 C, and 0.1 C at 25 ° C. when charging at a voltage half the value of the set charging voltage It has been found that the above problem can be solved by employing a non-aqueous electrolyte secondary battery having a capacity change rate of 90% or more in discharge capacity.
In addition, the electric current value which divided the charging capacity (electric amount) at the time of charging a cell to a full charge voltage by 1 hour was set to 1C.

  That is, when the charging voltage of the non-aqueous electrolyte battery is set to 4.2 V, charging at a voltage half that of 4.2 V, that is, 2.1 V, is performed at a 0.1 C rate, and the discharging capacity at a 0.1 C rate. When the measured value of C2 is C2, charging at a high voltage of 0.8V, that is, 2.9V, is performed at 25 ° C. and a 0.1C rate, and when the discharge capacity at the 0.1C rate is C1, C2 / C1 × 100 (%) is calculated, and a battery that is 90% or more is selected to obtain a battery pack. Thereby, the battery pack in which the above-mentioned subject was solved can be obtained.

In the present embodiment, in the evaluation of a single non-aqueous electrolyte secondary battery, when charging at a voltage higher by 0.8V than the voltage half of the set value of the charging voltage, and voltage half of the set value of the charging voltage The discharge capacities when charged with the above were compared for the following reason.
When the battery is charged at a voltage exceeding 0.8 V from half the set value of the charging voltage, a side reaction such as decomposition of the electrolyte in the battery tends to occur remarkably. In such a case, the charge capacity apparently increases, while the discharge capacity due to deterioration or the like may decrease, and an accurate capacity ratio calculation for achieving the present invention (C2 / C1 × 100 (%)) ) Becomes difficult.

The set value of the charging voltage can be determined by the charging voltage of the external device. A preferable setting value of the charging voltage is a voltage value that is 0.8V higher than the voltage at 50% SOC of the nonaqueous electrolyte secondary battery pack. If it exceeds 0.8 V, the nonaqueous electrolyte secondary battery is likely to be deteriorated.

  Hereinafter, the present embodiment will be described in more detail using a typical configuration example.

For example, when the positive electrode is composed of Li _x FePO ₄ and the negative electrode is composed of Li ₄ Ti ₅ O ₁₂ , the open circuit voltage of SOC 50% at 25 ° C. is about 1.8V. That is, in the case of two series battery packs, a voltage of about 3.6 V was shown. The charging voltage in the battery pack of this nonaqueous electrolyte secondary battery was set to 4.2V.

Two such nonaqueous electrolyte secondary batteries were connected in series to produce a battery pack. The charging behavior at this time is shown in FIG. As described above, the battery composed of the positive electrode Li _x FePO ₄ and the negative electrode Li ₄ Ti ₅ O ₁₂ has a flat region of about 1.9 V, and the voltage changes very steeply at the end of charging or at the initial stage of charging. Easy to hold. In FIG. 2 (b), similar to the case shown in FIG. 1 (b), the two-series battery pack has a self-discharge variation or deterioration due to temperature unevenness inside the pack. An example of a charging curve of a battery pack in which the battery A shows SOC 80% and the battery B shows SOC 50% is shown. When such a battery pack is charged at a charging voltage of 4.2 V, the voltage of the battery B is almost constant at the end of charging, but the voltage of the battery A starts to increase rapidly. In the state where the battery A is substantially SOC 100%, the voltage of the battery A reaches near 2.1V, whereas the voltage of the battery A is 70% SOC, but the voltage is almost equal to the SOC 50%, and the voltage of the battery pack is It has only reached 4V. Since the charging voltage of the battery pack is set to 4.2 V, the battery B is practically overcharged.

  However, in the battery of the present invention, the ratio between the capacity when the charging voltage is 2.1 V and the capacity when 2.9 V, which is higher by 0.8 V, is 90% or more in accordance with the above-described example. That is, the nonaqueous electrolyte secondary battery of the present invention has a feature that the potential rapidly rises at the end of charging, and the shorter the ratio, the less likely the side reaction occurs at the end of charging. In fact, the voltage of the battery A suddenly increases when the SOC exceeds 100%, and even if the voltage of the battery A is 1.9V, the voltage of the battery B quickly reaches 2.3V. There is much less risk of sacrificing safety. Therefore, when two batteries are connected in series, a circuit for managing the battery voltage can be omitted.

  As described above, in the present invention, the steeper charging curve at the end of charging allows the safety of the entire battery pack to be maintained even if the SOC of each battery shows a different value due to variations in the pack. Therefore, as in claim 1 of the present invention, when each battery constituting the battery is charged at 25 ° C. and a 0.1 C rate, when charged at a value half the charge voltage of the non-aqueous electrolyte secondary battery pack When the capacity change rate of the 0.1 C discharge capacity at 25 ° C. is 90% or more when charged at a voltage 0.8 V higher than the half value of the 0.1 C discharge capacity and the charging voltage, the potential at the end of charging is Since the rise changes rapidly, the safety of the entire battery pack is maintained. On the other hand, if it is less than 90%, the voltage change becomes gradual, that is, the time for each battery to be overcharged is reduced, so that not only the battery is likely to deteriorate but also there is a risk that the safety is impaired. For this reason, a mechanism for monitoring each battery voltage is essential.

As a result of the study by the present inventors based on the above findings, olivine type lithium iron phosphate (Li _x FePO ₄ ) and spinel type are used as positive and negative electrode materials of the nonaqueous electrolyte secondary battery of the present embodiment, respectively. By using lithium titanate (Li ₄ Ti ₅ O ₁₂ ) and connecting these lithium ion secondary batteries in series in series, the safety is maintained even if the voltage monitoring means is reduced to half that of the prior art. The present inventors have found that excellent battery performance can be maintained.

These materials have a feature that the voltage change at the end of charging is steep, and are suitable as materials of the present embodiment. In these two materials, most of the entire reaction proceeds by a charge / discharge mechanism based on a two-phase coexistence reaction between a crystal phase in a discharged state and a crystal phase in a charged state, and thus the flatness during charging is high and the end of charging is in progress. Since the voltage change becomes steep, it is most suitable for the purpose of the present invention.
However, this combination of materials is merely an example of the above-described embodiment, and any other material can be used in the present invention as long as the material has the characteristics of the capacity change rate.

  The nonaqueous electrolyte battery that can be used in the battery pack of the present embodiment includes a nonaqueous electrolyte, a positive electrode, and a negative electrode containing the negative electrode material for a nonaqueous electrolyte battery of the present invention as a negative electrode active material. .

1) Positive Electrode As a positive electrode material used in the nonaqueous electrolyte secondary battery pack of the present embodiment, olivine type lithium iron phosphate having an average particle diameter of 0.05 μm or more and 25 μm or less is preferable. In addition to this, the spinel-type lithium-manganese composite oxide (e.g., LiMn ₂ O _4), lithium nickel manganese cobalt composite oxide having a layered structure (e.g., _{LiNi x Mn y Co z O 2} ; x + y + z = 1) as Compounds can be used.

  In order to make the voltage change at the end of charging steep, it is necessary to control the average particle diameter of the olivine-type lithium iron phosphate of the positive electrode. When the thickness is less than 0.05 μm, the charging / discharging curve of lithium iron phosphate becomes gentle as a whole due to the influence of the surface energy due to the formation of nanoparticles, so that the voltage change at the end of charging does not become steep. On the other hand, if it exceeds 25 μm, the rate performance of lithium iron phosphate tends to deteriorate.

The positive electrode uses Li _x FePO ₄ as the positive electrode active material, the positive electrode active material, the conductive agent and the binder are suspended in an appropriate solvent, and the suspension is applied to a current collector such as an aluminum foil and dried. It is manufactured by pressing into a strip electrode. Examples of the conductive agent include acetylene black, carbon black, and graphite. Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), fluorine-based rubber, ethylene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), and the like.

  The compounding ratio of the positive electrode active material, the conductive agent and the binder is preferably in the range of 80 to 95% by mass of the positive electrode active material, 3 to 20% by mass of the conductive agent, and 2 to 7% by mass of the binder.

2) Negative electrode As the negative electrode material used in the nonaqueous electrolyte secondary battery pack of the present embodiment, spinel type lithium titanate having an average particle size of 0.1 μm or more and 50 μm or less is preferable.

  In order to make the voltage change at the end of charging steep, it is necessary to control the average particle size of the negative spinel type lithium titanate. When the thickness is less than 0.1 μm, the charging / discharging curve of lithium iron phosphate becomes smooth as a whole due to the influence of the surface energy due to the formation of nanoparticles, so that the voltage change at the end of charging does not become steep. On the other hand, if it exceeds 50 μm, the rate performance of lithium iron phosphate tends to deteriorate.

  Lithium titanate is used for the negative electrode. For the production of the negative electrode, a negative electrode mixture composed of lithium titanate containing the negative electrode material for the nonaqueous electrolyte battery of the present invention, a conductive agent and a binder is suspended in an appropriate solvent and mixed to form a coating solution Is applied to one or both sides of the current collector and dried.

  Further, as the conductive agent used for the negative electrode, a carbon material is usually used. If the carbon material used for the negative electrode active material described above has high alkali metal occlusion and conductivity, the carbon material used as the negative electrode active material can also be used as a conductive agent. However, since only the graphite having high carbon occlusion such as mesophase pitch carbon fiber has low conductivity, it is preferable to use, for example, acetylene black, carbon black or the like as the negative electrode as the carbon material used as the conductive agent.

  Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), fluorine-based rubber, ethylene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), and the like.

  The compounding ratio of the negative electrode active material, the conductive agent and the binder is preferably in the range of 70 to 95% by mass of the negative electrode active material, 0 to 25% by mass of the conductive agent, and 2 to 10% by mass of the binder.

3) Non-aqueous electrolyte The non-aqueous electrolyte contains a liquid non-aqueous electrolyte (non-aqueous electrolyte) prepared by dissolving an electrolyte in a non-aqueous solvent, and the non-aqueous solvent and the electrolyte in a polymer material. Examples thereof include a polymer gel electrolyte, a polymer solid electrolyte containing the electrolyte in a polymer material, and an inorganic solid electrolyte having lithium ion conductivity.

  As the non-aqueous solvent used in the liquid non-aqueous electrolyte, a known non-aqueous solvent can be used in a lithium battery. For example, cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC), cyclic carbonates and cyclic Examples thereof include a nonaqueous solvent mainly composed of a mixed solvent with a nonaqueous solvent (hereinafter referred to as a second solvent) having a viscosity lower than that of carbonate.

  Examples of the second solvent include chain carbonates such as dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate, γ-butyrolactone, acetonitrile, methyl propionate, ethyl propionate, cyclic ether such as tetrahydrofuran, 2-methyltetrahydrofuran, and the like. Examples of the ethers include dimethoxyethane and diethoxyethane.

Examples of the electrolyte include alkali salts, and particularly lithium salts. As lithium salts, lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium hexafluoroarsenide (LiAsF ₆ ), lithium perchlorate (LiClO ₄ ), lithium trifluorometasulfonate (LiCF ₃ SO ₃ ) and the like. In particular, lithium hexafluorophosphate (LiPF ₆ ) and lithium tetrafluoroborate (LiBF ₄ ) are preferable. The amount of the electrolyte dissolved in the non-aqueous solvent is preferably 0.5 to 2 mol / L.

  A gel electrolyte is obtained by dissolving the solvent and the electrolyte in a polymer material to form a gel. The polymer material is a single amount of polyacrylonitrile, polyacrylate, polyvinylidene fluoride (PVdF), polyethylene oxide (PECO), or the like. And polymers with other monomers.

As the solid electrolyte, the electrolyte is dissolved in a polymer material and solidified. Examples of the polymer material include polymers of monomers such as polyacrylonitrile, polyvinylidene fluoride (PVdF), polyethylene oxide (PEO), and copolymers with other monomers. Moreover, the ceramic material containing lithium is mentioned as an inorganic solid electrolyte. Of these _Li 3 _N, etc. _{Li 3 PO 4 -Li 2 S-} SiS 2 glass.

  A separator can be disposed between the positive electrode and the negative electrode. In addition, a gel-like or solid nonaqueous electrolyte layer may be used in combination with this separator, or a gel-like or solid nonaqueous electrolyte layer may be used instead of the separator. The separator is for preventing contact between the positive electrode and the negative electrode, and is made of an insulating material. Furthermore, a shape in which the electrolyte can move between the positive electrode and the negative electrode is used. Specifically, for example, a synthetic resin nonwoven fabric, a polyethylene porous film, a polypropylene porous film, or a cellulose-based separator can be used.

The partial notch perspective view of the thin nonaqueous electrolyte secondary battery which is one Embodiment of a nonaqueous electrolyte battery is shown. The flat electrode group 11 has a structure in which a positive electrode 12 and a negative electrode 13 are flattened with a separator 14 interposed therebetween. The strip-like positive electrode terminal 15 is electrically connected to the positive electrode 12. The strip-like negative electrode terminal 16 is electrically connected to the negative electrode 13. The electrode group 11 is housed in a laminated film-made outer bag 17 with the end portions of the positive electrode terminal 15 and the negative electrode terminal 16 extending from the outer bag 17. The nonaqueous electrolytic solution is accommodated in the laminated film outer bag 17. The laminated film outer bag 17 has the opening 11 sealed with the electrode group 11 and the non-aqueous electrolyte by heat sealing together with the positive electrode terminal 15 and the negative electrode terminal 16.

Examples of the present invention will be described below.
Example 1
<Preparation of positive electrode>
Using a Li _x FePO ₄ powder having an average particle size of 0.5 μm as a positive electrode active material, acetylene black, graphite, and polyvinylidene fluoride (PVdF) were mixed at a ratio of 100: 8: 8: 6 (both mass%). Used and mixed. Further, N-methylpyrrolidone was added and mixed, applied to a current collector of aluminum foil having a thickness of 15 μm, dried and pressed to prepare a positive electrode having an electrode density of 2.0 to 3.5 g / cm ³ .

<Production of negative electrode>
As the negative electrode active material, spinel-type lithium titanium oxide having an average particle size of 1.2 μm, 85% by mass of the negative electrode active material material powder, 5% by mass of graphite as a conductive agent, and 3% of acetylene black as a conductive agent are also used. %, PVdF 7% by mass, and NMP were added and mixed, applied to a current collector made of a copper foil having a thickness of 11 μm, dried, and pressed to prepare a negative electrode.

<Production of electrode group>
After laminating the positive electrode, the polyethylene porous film and the separator made of cellulose, the negative electrode, and the separator in this order, they were wound in a spiral shape so that the negative electrode was located on the outermost periphery, thereby producing an electrode group. .

<Preparation of non-aqueous electrolyte>
Furthermore, 1.0 mol / L of lithium hexafluorophosphate (LiPF ₆ ) was dissolved in a mixed solvent of ethylene carbonate (EC) and methyl ethyl carbonate (MEC) (mixing volume ratio 1: 2) to perform non-aqueous electrolysis. A liquid was prepared.

  The electrode group and the electrolyte solution were respectively stored in a laminate film, and a thin nonaqueous electrolyte secondary battery having a capacity of 3 Ah was assembled.

<Production of battery pack>
In order to connect the cylindrical non-aqueous electrolyte secondary batteries produced as described above in series, both ends of the positive and negative electrodes of the two batteries were resistance-welded to a copper lead having a thickness of 1 mm and a width of 1 cm to form a two-series battery. A pack was made. This time, in order to verify the effect of the invention, lead wires were connected to the positive terminal and the negative terminal of each battery so that each voltage could be measured.

<Confirmation of individual battery capacity>
In Example 1, the setting value of the charging voltage of the 2-series battery was set to 4.2V. Based on this set value, in a 25 ° C environment, the battery is charged at a 0.1 C rate at +0.8 V (ie, 2.9 V), which is half the set value (ie, 2.1 V), The ratio between the discharged capacity (C1) and the capacity (C2) discharged at the 0.1C rate at half the charging voltage setting value (ie 2.1V) and discharged at the 0.1C rate, C2 / When C1 × 100 (%) was confirmed, it was 96%.

(Examples 2 to 8)
A battery and a battery pack were produced in the same manner as in Example 1 except that a positive electrode material and a negative electrode material having an average particle size as shown in Table 1 were used. The charging voltage of the produced battery pack was set according to Table 1. Furthermore, as a result of measuring the relationship between each battery capacity and the charging voltage, the above-mentioned value of C2 / C1 × 100 (%) is also summarized in Table 1.

(Comparative Example 1)
A battery and a battery pack were produced in the same manner as in Example 1 except that lithium cobalt oxide (LiCoO ₂ ) was used for the positive electrode and graphite was used for the negative electrode.

<Experiment>
The SOC variation phenomenon of individual batteries constituting the battery pack is inherently likely to occur due to self-discharge due to long-term storage and temperature unevenness in the installed battery. However, in this measurement, for the acceleration test, the battery packs of Examples 1 to 8 and Comparative Example 1 are separated once so that the constituent batteries, battery A, SOC 80% and battery B 50%. The battery pack in a state where the SOC was intentionally varied was configured.

  Then, the result of having measured the voltage of two batteries when charging the obtained battery pack at the 1C rate to the charging voltage set again was put together in Table 2.

These batteries were subjected to a charge / discharge cycle test in a 1C rate, 45 ° C. environment. The discharge cut-off voltage was set to a voltage 2V lower than the set charge voltage (that is, discharging to 1.2V if 4.2V). The initial thickness of the battery constituting each battery pack was measured, and the thickness after 500 cycles was measured and used as an indicator of battery pack deterioration. The thickness change is shown in Table 2.

  The battery A generally tends to swell compared to the battery B as a whole, but the battery of the present invention hardly swells compared to Comparative Example 1. As for Comparative Example 1, the charge / discharge within the reference voltage (that is, the battery B) hardly swells, but the swelling greatly occurs when the reference voltage is greatly exceeded. Such a large bulge becomes larger as time passes, which may lead to battery rupture. As described above, the difference between the example and the comparative example is considered to be because the battery A is overcharged due to the SOC shift, but the time of the overcharge is longer than that of the present invention. On the other hand, in Examples 1 to 8 of the present invention, it is considered that even when overcharging occurs, the time is very short, so that it is hardly affected by the voltage increase.

  As described above, in the present invention, even if the SOC of the battery in the battery pack is deviated, the safety of the battery is not impaired. Therefore, it is not necessary to monitor the voltage of each battery when manufacturing the battery pack. It could be confirmed.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined.

Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

DESCRIPTION OF SYMBOLS 11 ... Electrode group 12 ... Positive electrode 13 ... Negative electrode 14 ... Separator 15 ... Positive electrode terminal 16 ... Negative electrode terminal 17 ... Exterior bag

Claims (4)

  In a battery pack in which two non-aqueous electrolyte batteries are connected in series, the charging voltage of the non-aqueous electrolyte secondary battery pack is set in advance, and is less than half of the charging voltage of the set non-aqueous electrolyte secondary battery pack. With a charging voltage higher by 0.8 V, the individual batteries constituting the battery are charged at a rate of 0.1 C at 25 ° C. when charged at a rate of 0.1 ° C. and a voltage half the value of the set charging voltage. A non-aqueous electrolyte secondary battery pack employing a non-aqueous electrolyte secondary battery having a capacity change rate of 0.1 C discharge capacity at 25 ° C. of 90% or more when charged in the above manner.   2. The non-aqueous electrolyte secondary battery according to claim 1, wherein a positive electrode material of the non-aqueous electrolyte secondary battery is olivine type lithium iron phosphate and a negative electrode material of the non-aqueous electrolyte secondary battery is spinel type lithium titanate. Water electrolyte secondary battery pack.   3. The nonaqueous electrolyte secondary battery pack according to claim 2, wherein an average particle size of the olivine-type lithium iron phosphate is 0.05 μm or more and 25 μm or less.   3. The nonaqueous electrolyte secondary battery pack according to claim 2, wherein an average particle size of the spinel type lithium titanic acid is 0.1 μm or more and 50 μm or less.

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