JP2006172900A - Non-aqueous electrolytic liquid secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a non-aqueous electrolytic liquid secondary battery in which lithium titanate is used as a positive electrode active material and voltage drop at the initial stage of discharge is suppressed, while being able to obtain high capacity and high reliability. <P>SOLUTION: The non-aqueous electrolytic liquid secondary battery has a positive electrode, using lithium titanate as a positive electrode active material and a negative electrode, using a material capable of storing and releasing lithium ion as a negative electrode active material. The battery has lithium, capable of being used for charge and discharge at a quantity, corresponding to 105-120% of the negative electrode capacity. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a non-aqueous electrolyte secondary battery having lithium titanate as a positive electrode active material.

Currently, lithium-ion secondary batteries are mainly used as power sources for portable devices. The reason for this is that conventional secondary batteries represented by nickel-cadmium secondary batteries and metal hydride (MH) secondary batteries. Compared to the above, it is possible to reduce the weight and to achieve a higher voltage. In the current lithium ion battery, for example, a metal oxide such as LiCoO ₂ is used as a positive electrode, and graphite is used as a negative electrode. 4V.

  However, lowering the voltage of lithium ion secondary batteries is required for the following reasons. One reason is that the driving IC of portable devices is being lowered in voltage, and another is that solar cells are generally used for charging secondary batteries for portable devices. This is because, since the voltage of the solar cell is usually 1 V or less, charging of the high voltage lithium ion secondary battery has a problem that the number of stacked solar cells has to be increased.

  On the other hand, secondary batteries having a positive electrode using lithium titanate as an active material and a negative electrode using metal Li or carbon as an active material have been developed as secondary batteries capable of achieving low voltage (for example, patents). Literatures 1-2). In the secondary batteries disclosed in Patent Documents 1 and 2, since the voltage at the time of charging is 3 V or less and the average voltage at the time of discharging is 2 V or less, the above-described request for lowering the voltage can be met.

In a secondary battery using lithium titanate as disclosed in Patent Documents 1 and 2 as a positive electrode active material, as the negative electrode active material, in addition to the above metal lithium (Li) and carbon, Si, Sn, etc. that can be alloyed with Li Can be used. However, when metallic lithium is used, it is difficult to ensure satisfactory cycle performance. Further, unlike LiCoO ₂ or the like, lithium titanate does not allow mobility of Li as a constituent element during charge / discharge, and Li cannot be detached. Therefore, when carbon or the like is used as the negative electrode active material, Li is introduced from the outside because neither of the positive and negative electrodes has a Li source that can be used for charging and discharging.
Japanese Patent Laid-Open No. 10-64592 Japanese Patent Laid-Open No. 10-334917

  Even in a secondary battery using lithium titanate as a positive electrode active material as disclosed in Patent Documents 1 and 2, as in current lithium ion secondary batteries, high capacity, high reliability, and high load characteristics are achieved. There is also a demand. For example, as for load characteristics, since many recent portable devices using such secondary batteries have a communication function, it is required to be able to cope with an instantaneous large current, for example, in pulse discharge. In addition, there is a demand for suppression of polarization reduction when the portable device is used in a low temperature environment.

  In general, in secondary batteries, a voltage drop phenomenon occurs at the beginning of discharge, such as when the discharge starts, the voltage suddenly drops, then the voltage rises again to a certain level, and further when the discharge proceeds, the voltage gradually decreases. It is done. Therefore, if the timing of the pulse discharge matches the timing at which the above voltage drop occurs, if the degree of voltage drop is large, the portable device may stop because it falls below the cut voltage of the protection device of the portable device. Therefore, it is required to suppress such a voltage drop as much as possible. In addition, when the polarization increases in the battery in a low temperature environment, a voltage drop occurs and the same phenomenon as described above occurs.

  In order to suppress the voltage drop phenomenon at the initial stage of the discharge, a method of improving the load characteristics of the secondary battery by selecting a conductive additive used for the electrode can be considered. For example, artificial graphite, ketjen black, etc. can be used as a conductive aid, and by increasing the amount of these used, the load characteristics of the secondary battery can be improved. If it is increased, the amount of the active material must be reduced, resulting in a problem that the battery capacity decreases.

  The present invention has been made in view of the above circumstances, and uses lithium titanate as a positive electrode active material, can suppress a voltage drop at the initial stage of discharge, and can ensure high capacity and high reliability. It is an object to provide a liquid secondary battery.

  The present invention relates to lithium that can be used for charging and discharging in a nonaqueous electrolyte secondary battery having a positive electrode using lithium titanate as a positive electrode active material and a negative electrode using a material capable of occluding and releasing lithium ions as a negative electrode active material. Is solved in an amount corresponding to 105 to 120% of the negative electrode capacity.

  In the non-aqueous electrolyte secondary battery using lithium titanate as a positive electrode active material, the present invention can ensure high capacity and high reliability while suppressing a voltage drop at the initial stage of discharge in the non-aqueous electrolyte secondary battery using the positive electrode active material. did it.

  As described above, in a non-aqueous electrolyte secondary battery having a positive electrode using lithium titanate as a positive electrode active material, if a negative electrode such as metal Li that can become a mobility during charge and discharge is not used, it is separately used for charge and discharge. However, the present inventor has optimized the amount of Li that can be used for such charge and discharge to optimize the conventional secondary battery having a positive electrode using lithium titanate as a positive electrode active material. Have found that the above problems (suppression of voltage drop at the initial stage of discharge, increase in capacity, and ensuring high reliability) can be solved, and the present invention has been completed. Hereinafter, the present invention will be described in detail.

In the present invention, lithium titanate (Li ₄ Ti ₅ O ₁₂ ) is used as the positive electrode active material. Lithium titanate is obtained, for example, by heat-treating titanium oxide and a lithium compound at 760 to 1100 ° C. As the titanium oxide, either anatase type or rutile type can be used, and examples of the lithium compound include lithium hydroxide, lithium carbonate, and lithium oxide.

  In producing the positive electrode, it is preferable to pressure mold a positive electrode mixture prepared by mixing lithium titanate, a conductive additive, and a binder. The shape of the positive electrode thus obtained is, for example, a pellet shape. Can be mentioned. Examples of the conductive aid include flaky graphite, acetylene black, ketjen black, and carbon black. As the binder, a fluororesin is preferably used, and specific examples thereof include polytetrafluoroethylene and polyvinylidene fluoride. However, the method for producing the positive electrode is not limited to the above-described examples, and other methods may be used.

  What is necessary is just to comprise a negative electrode with the molded object (for example, pellet-shaped molded object) of the negative mix containing a negative electrode active material, for example. The negative electrode active material is not particularly limited as long as it can occlude and release Li ions. For example, carbon materials such as graphite, carbon nanotubes, vapor-grown carbon fibers, and low crystalline carbon, and metals such as Si and Sn can be used. An oxide or the like is preferably used.

  Moreover, when comprising a negative electrode with the pellet-shaped molded object of negative electrode mixture, in preparation, the binder is required other than the said negative electrode active material. The binder is not particularly limited, and for example, polyvinylidene fluoride, a mixture of styrene butadiene rubber and carboxymethyl cellulose, polyamideimide, and the like are preferable. Moreover, in preparing the negative electrode mixture, a conductive additive as exemplified in the case of the positive electrode can be used as necessary.

  As the electrolytic solution, an organic electrolytic solution prepared by dissolving a lithium salt as an electrolyte in an organic solvent is used.

Examples of the solvent for the electrolytic solution include cyclic carbonates such as ethylene carbonate, propylene carbonate, and butylene carbonate, acyclic carbonates such as dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate, 1,2-dimethoxyethane, and 1,2-diethyl. Ethers such as ethoxyethane, dimethoxypropane, 1,3-dioxolane, tetrahydrofuran and 2-methyltetrahydrofuran can be used, but two or more kinds of mixed solvents including cyclic carbonate and acyclic carbonate are particularly preferable. As the electrolyte, for _{example, LiC n F 2n + 1 SO} 3 (n> 1) , such as _{LiPF 6, LiCF 3 SO 3,} LiClO 4, LiBF 4, LiAsF 6, (C n F 2n + 1 SO 2) (C m F _{2m + 1} SO ₂ ) NLi (m, n ≧ 1), (RfOSO ₂ ) ₂ NLi (Rf is an alkyl halide having 2 or more carbon atoms), or the like can be used.

  As described above, in the non-aqueous electrolyte secondary battery of the present invention, since the positive and negative electrodes do not have Li for mobility, Li that can be used for charging and discharging is separately introduced. This Li can be introduced, for example, by attaching a metal Li foil to the positive electrode or the negative electrode. In the non-aqueous electrolyte secondary battery of the present invention, the amount of Li that can be used for charging and discharging is an amount corresponding to 105% or more and 120% or less of the negative electrode capacity.

  In a non-aqueous electrolyte secondary battery using lithium titanate as a positive electrode active material, when the positive electrode capacity is larger than the negative electrode capacity, when Li (Li ion) is completely transferred to one of the positive and negative electrodes, Li is not present in the negative electrode. Usually, the capacity of the negative electrode is increased because of easy precipitation. At this time, if the amount of Li introduced is less than the amount consumed as the irreversible capacity at the positive and negative electrodes during the first charge / discharge (details will be described later), the battery capacity will be small if it is small, and the battery capacity will be large if it is large. However, this battery capacity is regulated by the positive electrode capacity. On the other hand, when comparing the negative electrode capacity and the amount of Li introduced, if the amount of Li is small, Li dendrite does not precipitate but the battery capacity is small. On the other hand, if the amount of Li is large, the battery capacity is large. There is a possibility that the reliability of the battery may be impaired, for example, the dendrite is deposited and the battery is ignited due to a short circuit.

  However, the present inventor has found that if the amount of Li introduced into the non-aqueous electrolyte secondary battery is an amount corresponding to 105% or more and 120% or less of the negative electrode capacity, Li dendrite does not precipitate. Therefore, by making the amount of Li within the above range, it is possible to achieve a high capacity of the battery, to prevent a short circuit due to precipitation of Li dendrite, and to improve the reliability of the battery.

  Furthermore, the voltage drop at the initial stage of discharge can be suppressed by setting the amount of Li introduced into the non-aqueous electrolyte secondary battery to an amount corresponding to 105% to 120% of the negative electrode capacity. In the early stage of discharge, Li ions are occluded in the negative electrode, and normally there are no Li ions in the positive electrode. When the discharge is performed in the absence of Li ions on the positive electrode, the discharge voltage is lowered because the diffusion rate of Li ions is extremely small. However, when Li ions are occluded in the positive electrode to some extent, the diffusion rate of Li ions rapidly increases, so that the discharge voltage rises again. In the present invention, the amount of Li to be introduced is in the above range, so that when the charging is completed (that is, the state where Li ions are occluded in the negative electrode), the diffusion of Li ions is suppressed in order to suppress the voltage drop at the initial stage of discharge. A part of Li ions (that is, an amount corresponding to 5% or more and 20% or less of the negative electrode capacity) remains on the positive electrode to such an extent that the speed can be maintained. The lower limit of the amount of Li introduced into the battery is more preferably 110% with respect to the negative electrode capacity, and the upper limit is more preferably 115% with respect to the negative electrode capacity.

  In the non-aqueous electrolyte secondary battery using lithium titanate as the positive electrode active material, a part of the introduced Li forms a reaction product (compound) on the electrode surface during the first charge / discharge, and the next and subsequent times. It is impossible to participate in the charge / discharge. The “amount of Li that can be used for charging / discharging” as used in the present invention is an amount that becomes a reactant during the first charge / discharge from the amount of Li introduced into the battery and can no longer be involved in charge / discharge after the next time (irreversible capacity). This means the amount excluding the component amount (hereinafter, “Li that can be used for charging and discharging” may be referred to as “reversible Li”). Therefore, the amount of Li introduced at the time of manufacturing the battery is an amount obtained by adding the amount of Li that becomes irreversible capacity to the amount of reversible Li.

The negative electrode capacity according to the non-aqueous electrolyte secondary battery of the present invention is such that the negative electrode is a working electrode, Li metal foil is used as a counter electrode and a reference electrode, and ethylene carbonate and methyl ethyl carbonate are used as the electrolyte in a ratio of 1: 1. A model cell is formed by using a solution obtained by dissolving LiPF ₆ at a concentration of 1 mol / L in a mixed solution mixed at (volume ratio), and using a microporous film made of polyethylene as a separator. The model cell is discharged at a current density of 0.05 mA / cm ² to 10 mV (Li potential, the same applies hereinafter) to obtain a discharge capacity, and then charged to 2 V at the same current density to obtain the charge capacity. The charge capacity is defined as the negative electrode capacity, and the capacity obtained by subtracting the charge capacity from the discharge capacity is defined as the irreversible capacity. The amount of Li introduced into the battery can be theoretically determined based on these negative electrode capacity and irreversible capacity. In addition, the positive electrode capacity | capacitance for comparing with a negative electrode capacity | capacitance shall be the charge capacity measured using the said model cell which changed the working electrode into the said positive electrode, and the voltage at the time of charge to 0.5-3V.

  In the present invention, the positive electrode can, the negative electrode can, the separator, the insulating gasket and the like are not particularly limited, and those having a conventional configuration can be used.

  Hereinafter, the present invention will be described in detail based on examples. However, the following examples are not intended to limit the present invention, and all modifications made without departing from the spirit of the preceding and following descriptions are included in the technical scope of the present invention.

Example 1
Li ₄ Ti ₅ O _{12 as} a positive electrode active material and Ketjen black as a conductive auxiliary agent are mixed so that the mass ratio is 85: 7, and this is preliminarily mixed with polyvinylidene fluoride (PVDF) as a binder. It mixed with the N-methyl-2-pyrrolidone (NMP) solution dissolved so that it might become 8 mass% in all the positive mixes, and stirred, and the positive mix mixture containing slurry was produced. This positive electrode mixture-containing slurry was once dried to remove NMP, and then pulverized in a mortar, followed by pressure molding to produce a pellet-like positive electrode having a diameter of 6 mm and a thickness of 0.95 mm.

  Next, as a negative electrode active material, natural graphite and MCMB (mesocarbon microbeads) obtained by graphitization at 3000 ° C. are mixed so as to have a mass ratio of 45:45. Was mixed with an NMP solution in which 10% by mass of PVDF was dissolved in the total negative electrode mixture, and stirred to prepare a negative electrode mixture-containing slurry. This negative electrode mixture-containing slurry was once dried to remove NMP, and then pulverized in a mortar, and this was pressure-molded to produce a pellet-shaped negative electrode having a diameter of 7 mm and a thickness of 0.63 mm. The negative electrode capacity measured by the model cell was 10.9 mAh, and the irreversible capacity was 1.1 mAh.

The positive electrode and the negative electrode thus obtained were fixed to the positive electrode can and the negative electrode can, respectively, and a 0.12 mm metal Li foil was pasted on the negative electrode, and then the volume of ethylene carbonate and dimethyl carbonate was used as the electrolyte. By injecting an organic electrolyte prepared by dissolving 1 mol / L of LiPF _{6 in} a mixed solvent of a ratio 1: 3, and sealing through a separator made of a microporous film made of polypropylene, the structure shown in FIG. A non-aqueous electrolyte secondary battery having a diameter of 10 mm and a thickness of 2.5 mm was produced. The amount of the introduced metal Li foil was 10.9 mAh in terms of reversible Li amount, 107% with respect to the negative electrode capacity.

  Here, the battery shown in FIG. 1 will be described. 1 is a positive electrode, 2 is a negative electrode, 3 is a separator, 4 is a positive electrode can, 5 is a negative electrode can, and 6 is an insulating packing. The separator 3 is disposed between the positive electrode 1 and the negative electrode 2. In the negative electrode 2, a metal Li foil is disposed on the side facing the separator 3 (not shown). The positive electrode 1, the negative electrode 2, the separator 3, and the electrolyte (not shown) are sealed in a space formed by the stainless steel positive electrode can 4, the stainless steel negative electrode can 5, and the polypropylene insulating packing 6. Has been.

Example 2
A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the metal Li foil to be introduced was changed to one having a thickness of 0.14 mm. The amount of the introduced metal Li foil was a reversible Li amount and was 119% with respect to the negative electrode capacity.

Comparative Example 1
A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the metal Li foil to be introduced was changed to one having a thickness of 0.11 mm. The amount of the introduced metal Li foil was a reversible Li amount and was 90% with respect to the negative electrode capacity.

Comparative Example 2
A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the metal Li foil to be introduced was changed to one having a thickness of 0.16 mm. The amount of the introduced metal Li foil was a reversible Li amount and was 130% with respect to the negative electrode capacity.

The nonaqueous electrolyte secondary batteries of Examples 1 and 2 and Comparative Examples 1 and 2 were stored at 60 ° C. for 10 days for the purpose of Li doping. For these non-aqueous electrolyte secondary batteries, the charging time is 40 hours from the beginning of charging to the end of charging with a constant current of 0.5 mA / cm ² up to 2.6 V, and subsequently with a constant voltage of 2.6 V. The discharge capacity at the first cycle was determined by assuming that the operation of charging at 1 mA / cm ² and discharging the battery to a final voltage of 0.5 V was taken as one cycle. Further, for the battery that had been charged up to the fifth cycle, the discharge conditions were changed to determine the closed circuit voltage at the beginning of discharge. Specifically, the battery that had been charged up to the fifth cycle was discharged at 0.2 mA for 10 seconds, and the closed circuit voltage was measured using this as the initial stage of discharge. Furthermore, the battery after the end of the fifth cycle was overcharged for 20 hours until it reached 3.1 V at a constant current of 0.5 mA / cm ² , and then the battery was disassembled, and the state of the negative electrode surface (presence of Li deposition) ) Was confirmed as follows. The negative electrode after battery decomposition was used as a working electrode, Li metal was used as a counter electrode, and the cell was inserted into a cell containing an electrolytic solution to constitute a model cell. When a voltage is swept through the cell by cyclic voltammetry, if Li is deposited, a peak occurs in the current at 0V. The presence or absence of Li precipitation was confirmed by the presence or absence of this peak. These results are shown in Table 1.

  In Table 1, “negative electrode surface state” means the surface state of the negative electrode after being overcharged to 3.1 V after the end of the fifth cycle, and “closed voltage at the beginning of discharge” means the closed circuit voltage at the beginning of discharge in the fifth cycle. is doing.

  As can be seen from Table 1, in the non-aqueous electrolyte secondary batteries of Examples 1 and 2, the initial capacity was relatively large, and Li deposition was observed on the negative electrode surface even during overcharge after the end of 5 cycles. In addition, it has a high capacity and high reliability. Further, the closed circuit voltage at the initial stage of the fifth cycle discharge is high, and the voltage drop at the initial stage of the discharge is suppressed.

  On the other hand, in the nonaqueous electrolyte secondary battery of Comparative Example 1 in which the amount of reversible Li introduced was small, the initial capacity and the closed circuit voltage at the beginning of discharge in the fifth cycle were inferior. The voltage drop suppression is not achieved. In addition, in the nonaqueous electrolyte secondary battery of Comparative Example 2 in which the amount of reversible Li introduced was large, the initial capacity and the closed circuit voltage at the beginning of discharge of the fifth cycle were excellent, but at the time of overcharge after the end of the fifth cycle. In this case, Li is deposited on the negative electrode surface, which may cause a short circuit and impair reliability.

It is a partial cross section figure which shows an example of the nonaqueous electrolyte secondary battery of this invention.

Explanation of symbols

1 Positive electrode 2 Negative electrode 3 Separator 4 Positive electrode can 5 Negative electrode can 6 Insulation packing

Claims (1)

A non-aqueous electrolyte secondary battery having a positive electrode using lithium titanate as a positive electrode active material and a negative electrode using a material capable of occluding and releasing lithium ions as a negative electrode active material,
A nonaqueous electrolyte secondary battery comprising lithium that can be used for charging and discharging in an amount corresponding to 105 to 120% of a negative electrode capacity.

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