JP7079416B2 - Film formation method - Google Patents

Description

The present invention relates to a film forming method for forming a film on the negative electrode of a lithium ion secondary battery.

The secondary battery is widely used as a portable power source for personal computers and mobile terminals, or as a vehicle drive power source for EVs (electric vehicles), HVs (hybrid vehicles), PHVs (plug-in hybrid vehicles), and the like. Various techniques have been proposed for the purpose of maintaining the capacity of a lithium ion secondary battery, which is a kind of secondary battery. For example, in the capacity recovery method described in Patent Document 1, when the lithium ion secondary battery is discharged to the reference SOC, the change amount is smaller than the change amount of the discharge voltage dropped from 100% SOC to the reference SOC. In addition, the discharge voltage is controlled. As a result, the lithium ions accumulated in the non-opposing portion of the negative electrode active material layer that does not face the positive electrode active material layer are returned to the positive electrode active material layer.

JP-A-2015-187938

In a lithium ion secondary battery, which is a type of secondary battery, the components of the electrolytic solution are decomposed when charging is performed, so that the SEI (Solid Electrolyte Interphase) film made of LiF (lithium fluoride) or the like is formed as a negative electrode. It is formed on the surface of the active material layer. The SEI film suppresses the decomposition of the electrolytic solution caused by the direct contact between the negative electrode active material layer and the electrolytic solution, and facilitates the insertion of lithium ions into the negative electrode and the detachment from the negative electrode. On the other hand, since electric charges are consumed when the SEI film is formed, the battery capacity decreases when the SEI film is unnecessarily generated.

In the process of using the lithium ion secondary battery, changes such as expansion and contraction occur in the negative electrode (for example, the negative electrode active material layer). The SEI film formed on the surface of the negative electrode active material layer may be damaged (for example, cracks and peeling) due to expansion and contraction of the negative electrode. When the SEI film is damaged, the negative electrode active material layer and the electrolytic solution come into direct contact with each other to reshape the SEI film, which may reduce the battery capacity. Therefore, it is desirable to reduce the influence of damage to the SEI coating formed on the negative electrode.

A typical object of the present invention is to provide a film forming method capable of appropriately reducing the influence of damage to the SEI film on the negative electrode of a lithium ion secondary battery.

In order to realize such an object, one aspect of the film forming method disclosed herein is a film forming method for forming a film on the negative electrode of a lithium ion secondary battery including a positive electrode, a negative electrode, and an electrolytic solution. The temperature of the SOC adjustment step in which the SOC of the lithium ion secondary battery is set to 0% at room temperature and the temperature of the lithium ion secondary battery whose SOC is adjusted in the SOC adjustment step are in the range of -30 ° C or higher and -20 ° C or lower. Float charging to the lithium ion secondary battery in a state where the voltage of the temperature adjustment step and the lithium ion secondary battery whose temperature is adjusted in the temperature adjustment step is lowered to the set target voltage. In a battery comprising a float charging step to be performed, the target voltage having at least the same composition of the negative electrode and the electrolytic solution as the lithium ion secondary battery, the negative electrode at the working electrode and the lithium metal at the counter electrode. It is characterized in that the reaction current measured while sweeping the potential from the OCV toward the low potential is the difference between the negative electrode potential and the positive electrode potential corresponding to the negative electrode potential when forming a peak. ..

According to the film forming method according to the present disclosure, a film (hereinafter referred to as “solvent-derived film”) is further formed on the surface of the negative electrode on which the SEI film is formed by the substance in the solvent contained in the electrolytic solution. When the solvent-derived film is formed on the negative electrode, even if the SEI film is damaged, the possibility that the negative electrode active material layer and the electrolytic solution come into direct contact with each other and the SEI film is formed is reduced. Therefore, it is appropriately suppressed that the SEI film is damaged and the battery capacity is reduced.

It is a schematic diagram which shows an example of a film forming system 100. It is a flowchart of the film formation method in this embodiment. It is a graph which shows an example of the relationship between the cycle endurance period and the measured value of the capacity retention rate of a lithium ion secondary battery 1. It is an example of a voltammogram regarding a negative electrode and an electrolytic solution of a lithium ion secondary battery 1. It is a graph which shows the relationship of a positive electrode potential, a negative electrode potential, and SOC. It is a table which shows the result of the comparative test for confirming the effect by a film formation treatment.

Hereinafter, one of the typical embodiments in the present disclosure will be described in detail with reference to the drawings. Matters other than those specifically mentioned in the present specification and necessary for implementation can be grasped as design matters of those skilled in the art based on the prior art in the art. The present invention can be carried out based on the contents disclosed in the present specification and the common general technical knowledge in the art. Moreover, the dimensional relationship in each drawing does not reflect the actual dimensional relationship.

As used herein, the term "battery" refers to a general storage device capable of extracting electrical energy, and is a concept including a primary battery and a secondary battery. "Secondary battery" refers to a general storage device that can be charged and discharged repeatedly, and includes so-called storage batteries (that is, chemical batteries) such as lithium ion secondary batteries, nickel hydrogen batteries, and nickel cadmium batteries, as well as electric double layer capacitors and the like. Includes capacitors (ie, physical batteries). That is, the lithium ion secondary battery is one of various secondary batteries. Hereinafter, the film forming method according to the present disclosure will be described in detail by exemplifying a case where a film is formed on the negative electrode of a lithium ion secondary battery used in a vehicle. However, it is not intended to limit the film forming method according to the present disclosure to those described in the following embodiments. For example, it is possible to apply at least a part of the techniques exemplified in the present disclosure to a lithium ion secondary battery used in a device other than a vehicle.

With reference to FIG. 1, an example of a film forming system 100 used for forming a film on the negative electrode of a lithium ion secondary battery (hereinafter, may be simply referred to as “secondary battery”) 1 is schematically described. explain. The film forming system 100 of the present embodiment includes a charge / discharge control device 10, a voltmeter 20, an ammeter 30, and a temperature adjusting device 40.

The charge / discharge control device 10 executes various controls such as charge / discharge control of the secondary battery 1. The charge / discharge control device 10 includes a controller that controls various controls and a storage device that stores programs and various data. As an example, in the present embodiment, the electronic control system (ECU) mounted on the vehicle is used as the charge / discharge control device 10. However, a device other than the ECU (for example, a charging device or a personal computer) may be used as the charge / discharge control device.

Although not shown, a power supply device (for example, a generator) and an output device (for example, an external device for output destination) are connected in parallel to the positive electrode terminal and the negative electrode terminal of the secondary battery 1, respectively. As an example, the power supply device of the present embodiment converts the kinetic energy generated when the vehicle is braked into electric power, and supplies the secondary battery 1 with charging electric power. Further, the output device of the present embodiment drives the vehicle by the discharge power supplied from the secondary battery 1.

A voltmeter 20 is connected in parallel to the secondary battery 1. The voltmeter 20 measures the voltage between the positive and negative electrodes of the secondary battery 1. Further, an ammeter 30 is connected in series to the secondary battery 1. The ammeter 30 measures the current flowing through the secondary battery 1. The charge / discharge control device 10 controls the charge / discharge of the secondary battery 1 based on the voltage measured by the voltmeter 20 and the current measured by the ammeter 30.

The temperature adjusting device 40 adjusts the temperature of the secondary battery 1. Various devices can also be used for the temperature control device 40. For example, a constant temperature bath or the like capable of adjusting the temperature may be used as the temperature adjusting device 40.

An example of the lithium ion secondary battery 1 to be formed into a film will be described. The secondary battery 1 includes a battery case, an electrode body, and an electrolytic solution. The battery case houses the electrode body and the electrolytic solution in a sealed state. For the battery case, for example, at least one of a flexible laminate or a lightweight metal material having good thermal conductivity such as aluminum can be used.

The electrode body includes a positive electrode, a negative electrode, and a separator. As an example, a long positive electrode (positive electrode sheet), a long negative electrode (negative electrode sheet), and two long separators (separator sheets) are superposed on the electrode body of the present embodiment. A wound electrode body wound in the longitudinal direction is adopted. In the positive electrode, a positive electrode active material layer is formed along the longitudinal direction on one side or both sides (both sides in this embodiment) of a long positive electrode current collector. In the negative electrode, a negative electrode active material layer is formed along the longitudinal direction on one side or both sides (both sides in this embodiment) of a long negative electrode current collector. A non-formed portion of the positive electrode active material layer (that is, a positive electrode active material layer) formed so as to protrude outward from both sides in the winding axis direction (the sheet width direction orthogonal to the longitudinal direction) of the electrode body is formed. The positive electrode current collector plate is located in the portion where the positive electrode current collector is exposed and the negative electrode active material layer is not formed (that is, the portion where the negative electrode active material layer is not formed and the negative electrode current collector is exposed). And the negative electrode current collector plate is joined. The positive electrode terminal is electrically connected to the positive electrode current collector plate, and the negative electrode terminal is electrically connected to the negative electrode current collector plate. It is also possible to change the configuration of the electrode body. For example, a laminated electrode body may be used instead of the wound electrode body.

As the positive electrode current collector, those used as the positive electrode current collector of this type of lithium ion secondary battery can be used without particular limitation. Typically, a metal positive electrode current collector having good conductivity is preferred. For example, a metal material such as aluminum, nickel, titanium, or stainless steel can be used as the positive electrode current collector. In particular, aluminum (for example, aluminum foil) is preferable. Examples of the positive electrode active material of the positive electrode active material layer include lithium composite metal oxides such as a layered structure and a spinel structure (for example, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiNiO ₂ , LiCoO ₂ , LiFeO ₂ ). , LiMn ₂ O ₄ , LiNi _0.5 Mn _1.5 O ₄ , LiCrMnO ₄ , LiFePO ₄ , etc.). In the positive electrode active material layer, the positive electrode active material and a material (conductive material, binder, etc.) used as needed are dispersed in an appropriate solvent (for example, N-methyl-2-pyrrolidone: NMP), and a paste (or slurry) is formed. It can be formed by preparing a composition of (shape), applying an appropriate amount of the composition to the surface of the positive electrode current collector, and drying the composition.

As the negative electrode current collector, those used as the negative electrode current collector of this type of lithium ion secondary battery can be used without particular limitation. Typically, a metal negative electrode current collector having good conductivity is preferable, and for example, copper (for example, copper foil) or an alloy mainly composed of copper can be used. Examples of the negative electrode active material of the negative electrode active material layer include a particle-like (or spherical or scaly) carbon material containing a graphite structure (layered structure) at least in part, and a lithium transition metal composite oxide (for example, Li ₄ Ti). ₅ O ₁₂ and the like lithium-titanium composite oxide), lithium transition metal composite nitride and the like. In the negative electrode active material layer, the negative electrode active material and a material (binder or the like) used as needed are dispersed in an appropriate solvent (for example, ion-exchanged water) to prepare a paste-like (or slurry-like) composition. It can be formed by applying an appropriate amount of the composition to the surface of the negative electrode current collector and drying it.

As the separator, a separator made of a conventionally known porous sheet can be used without particular limitation. For example, a porous sheet (film, non-woven fabric, etc.) made of a polyolefin resin such as polyethylene (PE) and polypropylene (PP) can be mentioned. The porous sheet may have a single-layer structure or a plurality of structures having two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). Further, the porous sheet may be configured to have a porous heat-resistant layer on one side or both sides. This heat-resistant layer may be, for example, a layer containing an inorganic filler and a binder (also referred to as a filler layer). As the inorganic filler, for example, alumina, boehmite, silica and the like can be preferably adopted.

The electrolytic solution housed in the battery case together with the electrode body contains a supporting salt in a suitable non-aqueous solvent, and a conventionally known electrolytic solution can be used without particular limitation. For example, as the non-aqueous solvent, ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and the like can be used. Further, as the supporting salt, for example, a lithium salt can be preferably used, and LiPF ₆ is adopted in this embodiment.

An example of a film forming method for forming a solvent-derived film on the negative electrode of the secondary battery 1 will be described with reference to FIGS. 2 to 5. First, the film forming treatment (S1 to S4) for the manufactured unused secondary battery 1 is executed. The film forming process on the unused secondary battery 1 may be performed by the vehicle film forming system 100 described above, or may be activated before the secondary battery 1 is mounted on the vehicle (for example, activation on the secondary battery 1). During the process), it may be performed by a system other than the vehicle.

The film forming treatment (S1 to S4) will be described. First, the SOC of the secondary battery 1 is adjusted to 0% while the temperature of the secondary battery 1 is at room temperature (S1). Next, the temperature of the secondary battery 1 is adjusted to a range of −30 ° C. or higher and −20 ° C. or lower (S2). In the film forming treatment, the film formed by the substance in the solvent contained in the electrolytic solution (solvent-derived film) is preferentially formed on the negative electrode while suppressing the formation of the salt-derived film (salt-derived film) in the electrolytic solution. It is desirable to do. Details will be described later with reference to FIG. 6, but by adjusting the temperature of the secondary battery to a low temperature in S2, the diffusion of salt, water, and acid in the electrolytic solution is suppressed, and a salt-derived film is formed. Is presumed to be suppressed.

Next, the voltage of the secondary battery 1 is lowered to the target voltage at an appropriate drop speed (for example, 1 mV / s in this embodiment). Float charging is executed for the secondary battery 1 in a state where the voltage of the secondary battery 1 is lowered to the target voltage (S3). Although the details will be described later, the target voltage is set to a voltage at which the solvent-derived film is preferentially formed on the surface of the negative electrode of the secondary battery 1. Further, float charging refers to a charging method in which the voltage of the secondary battery 1 is maintained without substantially passing the charging current through the secondary battery 1. In S3, for example, a weak current may be supplied to the secondary battery 1 so that the voltage of the secondary battery 1 is maintained. Further, when the charge amount of the secondary battery decreases due to self-discharge or the like, the voltage of the secondary battery 1 may be maintained by supplying a current to the secondary battery 1. While the voltage of the secondary battery 1 is maintained at the target voltage, the formation of the solvent film on the surface of the negative electrode proceeds. The time for continuing the float charge may be appropriately set according to the type of the electrolytic solution in the secondary battery 1 and the like. As an example, in this embodiment, float charging is performed for 1 hour. After that, the SOC of the secondary battery 1 is returned to 0% (S4), and the film forming process is completed.

The secondary battery 1 that has been subjected to the film forming treatment is used for traveling the vehicle (S5). Then, it is determined whether or not the film needs to be reformed (S6). Criteria for determining whether or not the film needs to be reformed can be set as appropriate. For example, when the period in which the secondary battery 1 has been used for running the vehicle reaches a predetermined period (for example, 2 years) after the film forming treatment (S1 to S4) for the secondary battery 1 is executed, the film is formed. It may be determined that reformation is necessary.

Further, in S6, it may be determined whether or not the film needs to be reformed based on the capacity retention rate of the secondary battery 1. With reference to FIG. 3, an example of a method for determining the necessity of remodeling the coating film based on the capacity retention rate will be described. FIG. 3 shows an example of the relationship between the cycle durability period and the measured value of the capacity retention rate of the secondary battery 1 in which the film forming treatment (S1 to S4) is performed and the secondary battery 1 in which the film forming treatment is not performed. It is a graph which shows. In the graph of FIG. 3, the horizontal axis shows the square root of the cycle endurance period, and the vertical axis shows the capacity retention rate of the secondary battery 1. As shown in FIG. 3, the capacity retention rate of the secondary battery 1 subjected to the film forming treatment is unlikely to decrease as compared with the capacity retention rate of the secondary battery 1 not subjected to the film forming treatment. However, in the secondary battery 1 subjected to the film forming treatment, the degree to which the capacity retention rate decreases (that is, the slope of the graph in FIG. 3) becomes large after a certain cycle endurance period elapses. Therefore, in the present embodiment, the relationship between the cycle durability period and the capacity retention rate is obtained in advance by experiments for the batteries having the same configuration as the secondary battery 1 and having the film forming treatment (S1 to S4) executed. To. Next, the inflection point of the transition of the capacity retention rate is specified, and the value of the capacity retention rate at the inflection point is set as the threshold value of the capacity retention rate of the secondary battery 1. In S6, when the capacity retention rate of the secondary battery 1 becomes less than the threshold value, it is determined that the film needs to be reformed.

In S6, a plurality of determination criteria may be used. For example, when one of the condition that the elapsed period after the film formation treatment is executed reaches a predetermined period and the condition that the capacity retention rate is less than the threshold value is satisfied, it is determined that the film formation is necessary. It is also good.

If the remodeling of the coating is not necessary (S6: NO), the secondary battery 1 is continuously used for running the vehicle (S5). If it is necessary to reshape the film (S6: YES), the film forming process (S1 to S4) is executed again. The film forming process for the used secondary battery 1 may be executed by the film forming system 100 of the vehicle described above, or may be executed by a system other than the vehicle after the secondary battery 1 is removed from the vehicle. You may. Further, the film forming process may be performed only on one of the unused secondary battery 1 and the used secondary battery 1.

A method of setting the target voltage of the secondary battery 1 when the float charge (S3) is executed will be described with reference to FIGS. 4 and 5. The target voltage is set in advance by an experiment or the like. First, LSV (Linear Swep Voltammetry) is executed for a battery having at least the same composition of the negative electrode and the electrolytic solution as that of the secondary battery 1. LSV is a method of sweeping the potential of an electrode in a certain direction and measuring the reaction current flowing in response to the sweep of the potential. In the present embodiment, the negative electrode is used as a working electrode and the lithium (Li) metal is used as a counter electrode, and the reaction current is generated while linearly sweeping the potential from the OCV in the low potential direction at an appropriate speed (1 mV / s in this embodiment). Be measured. FIG. 4 is an example of a voltammogram relating to the negative electrode potential of the secondary battery 1 and the electrolytic solution, in which the horizontal axis represents the potential and the vertical axis represents the reaction current value. When the potential is swept in the direction of low potential, a peak appears in the reaction current. It is considered that the reaction current at the time of forming this peak is due to the decomposition of the solvent in the electrolytic solution and the formation of the salt-derived film. Further, the details will be described later with reference to FIG. 6, but when the potential is swept in the direction of a lower potential than the potential at the time of peak formation, an increase in the reaction current value due to the lithium insertion reaction into the negative electrode can be seen. .. Therefore, in the present embodiment, the negative electrode potential PVn when the reaction current forms a peak is specified. As an example, in the present embodiment, the negative electrode potential at the start of the peak reaction current when the negative electrode battery is swept in the direction of the low potential is specified. However, the specified negative electrode potential may be any negative electrode potential after the peak of the reaction current starts and before the peak ends. Therefore, for example, the negative electrode potential at the time of the maximum peak may be specified, or the negative electrode potential at the end of the peak may be specified.

Next, as shown in FIG. 5, the difference between the specified negative electrode potential (that is, the negative electrode potential when the reaction current forms a peak) PVn and the positive electrode potential PVp corresponding to the negative electrode potential PVn is the secondary battery 1. It is set as the target voltage of. That is, the value obtained by subtracting PVn from the positive electrode potential PVp in the battery when the negative electrode potential takes the value PVn is set as the target voltage. In other words, the positive electrode potential PVp corresponding to the negative electrode potential PVn is the potential of the positive electrode when the SOC of the battery is set to a value at which a peak is formed in the reaction current (see FIG. 4). The target voltage of the secondary battery 1 set by the above method is a voltage at which the solvent-derived film is likely to be preferentially formed on the surface of the negative electrode of the secondary battery 1.

<Comparative test>
With reference to FIG. 6, the results of a comparative test for confirming the effect of the film forming treatment (see S1 to S4 in FIG. 2) will be described. In the comparative test shown in FIG. 6, first, 11 lithium ion secondary batteries 1 described above were prepared, and the capacity A of each of the 11 secondary batteries 1 was measured. Of the 11 secondary batteries 1, No. The secondary battery 1 of No. 1 was not subjected to the film forming treatment. In addition, No. 1 in which the capacity A was measured. 2-No. For each of the 11 secondary batteries 1, the film forming process was executed in the same procedure as in S1 to S4 (see FIG. 2) described above while changing the temperature and voltage of the secondary battery 1 during float charging. The conditions of the film forming treatment executed for each secondary battery 1 are as shown in FIG. Here, the “target voltage Vt” shown in FIG. 6 is a target voltage set by the method described with reference to FIGS. 4 and 5. Further, “Vl” shown in FIG. 6 is a voltage higher than the target voltage Vt. Then, a cycle test was performed on each secondary battery 1. In the cycle test, the temperature of the secondary battery 1 was set to a high temperature, and a rectangular wave charge / discharge was executed in a predetermined cycle at a charge / discharge rate of 2C in a SOC range of 0% to 100%. The rest period between charging and discharging was 10 minutes. After that, No. 1 to No. The capacity B of each of the secondary batteries 1 of 11 was measured, and the capacity retention rate (B / A (%)) of each battery was calculated.

As shown in FIG. 6, No. The capacity retention rates of the secondary batteries 1 of 8 to 10 are No. 2 which is a comparative example. It was higher than the capacity retention rate of the secondary battery 1 of 1. In particular, No. 9 and No. The capacity retention rate of the secondary battery 1 of 10 is No. It was significantly higher than the capacity retention rate of the secondary battery 1 of 1. As a result, it can be confirmed that the above-mentioned film forming treatment (see S1 to S4 in FIG. 2) suppresses the decrease in battery capacity. This is because a soft solvent-derived film is further formed on the surface of the negative electrode on which the SEI film is formed, so that even if the SEI film is damaged due to expansion and contraction of the negative electrode, the negative electrode active material layer and electrolysis are performed. It is considered that this is because the direct contact of the liquid is suppressed.

Further, when the voltage during float charging was Vl (> Vt), the capacity retention rate did not improve regardless of the temperature during float charging. It is speculated that this is because if the negative electrode potential during float charging is set too low (that is, if the battery voltage is set too high), a salt-derived film is preferentially formed regardless of the temperature during float charging. Will be done. From the above, it can be seen that it is desirable to set the voltage at the time of float charging by the method described with reference to FIGS. 4 and 5.

In addition, No. 7-No. Looking at the results of No. 11, it can be seen that the capacity retention rate is improved by setting the temperature during float charging to −10 ° C. to −30 ° C., more preferably −20 ° C. to −30 ° C. It is considered that this is because by lowering the temperature during float charging, the diffusion of salt, water, and acid in the electrolytic solution is suppressed, and it becomes difficult to form a salt-derived film.

Although the detailed description has been given with reference to specific embodiments, these are merely examples and do not limit the scope of the claims. The techniques described in the claims include various modifications and modifications of the above-described embodiments.

1 Lithium-ion secondary battery 10 Charge / discharge control device 40 Temperature control device

Claims (1)

A film forming method for forming a film on the negative electrode of a lithium ion secondary battery including a positive electrode, a negative electrode, and an electrolytic solution.
The SOC adjustment step of setting the SOC of the lithium ion secondary battery to 0% at room temperature,
A temperature adjustment step for adjusting the temperature of the lithium ion secondary battery whose SOC has been adjusted in the SOC adjustment step to a range of −30 ° C. or higher and −20 ° C. or lower,
A float charging step of executing float charging of the lithium ion secondary battery in a state where the voltage of the lithium ion secondary battery whose temperature has been adjusted in the temperature adjusting step is lowered to a set target voltage, and a float charging step.
Including
The target voltage is
In a battery having at least the same composition of the negative electrode and the electrolytic solution as the lithium ion secondary battery, the negative electrode as the working electrode, and the lithium metal as the counter electrode, while the potential is swept from the OCV toward the low potential. A film forming method, characterized in that the measured reaction current is the difference between the negative electrode potential at the time of forming a peak and the positive electrode potential corresponding to the negative electrode potential.

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