JP6268534B2 - Method for producing non-aqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a method for manufacturing a non-aqueous electrolyte secondary battery including a non-aqueous electrolyte.

Non-aqueous electrolyte secondary batteries such as lithium ion batteries are preferably used for high-output power sources mounted on vehicles because they are lighter and have higher energy density than existing batteries.
In the manufacture of this type of battery, typically, a battery assembly is constructed using a positive electrode, a negative electrode, and a non-aqueous electrolyte, and initial charging (conditioning) or the like is performed on the battery assembly. Or a SEI (Solid Electrolyte Interface) film is formed on the surface of the negative electrode (see, for example, Patent Documents 1 to 4).

  For example, in Patent Document 1, in the initial charging of a non-aqueous electrolyte secondary battery using a silicon-silicon oxide composite as a negative electrode active material, after constant voltage charging, for example, charging is suspended for 2 hours or more. It is disclosed that resuming charging is repeated about 1 to 3 times. Thereby, the effect that the cycling characteristics of this secondary battery can be improved reliably is acquired. Japanese Patent Application Laid-Open No. H10-228688 discloses that the charging potential of constant voltage charging is performed in multiple steps by a step voltage value (ΔV0) depending on the situation in forming the SEI film. Thereby, it is supposed that a stable SEI film can be formed regardless of the state of the battery.

JP 2013-045590 A JP 2011-108550 A JP 2005-071697 A JP 2012-227035 A

By the way, in the above-described initial charging process, charging with a relatively large current (for example, 1.4 C or more) can reduce the time required for the initial charging, but there is a tradeoff that the battery resistance increases. It was. For this reason, it is known that the charging current (charging rate) in the initial charging process is preferably suppressed to about 1 C or less because a better SEI film can be formed.
However, regarding the processing after the assembly of the non-aqueous electrolyte secondary battery, in addition to the above-mentioned initial charging, holding (aging) in a high temperature environment for the purpose of improving battery characteristics and quality inspection (for example, IV resistance) And self-discharge characteristics). A longer time is required to perform all of these charge / discharge processes. Therefore, for the initial charging, it is required to appropriately perform an activation process in a shorter time and to form a high-quality SEI film.
The present invention has been created in view of such a situation, and an object thereof is to provide a method for manufacturing a nonaqueous electrolyte secondary battery capable of reducing both initial charge time and forming a high-quality SEI film. That is.

  In order to solve the above problems, the present invention provides a method for producing a nonaqueous electrolyte secondary battery. In this manufacturing method, a battery assembly in which a positive electrode, a negative electrode, and a nonaqueous electrolyte are accommodated in a battery case is prepared, and initial charging is performed on the battery assembly at a charge rate of 1.4 C or more and 6 C or less. , Is included. The initial charge is characterized by including at least one minute pause period in which charging is not performed for 30 seconds or more and 1 minute and 30 seconds or less in a voltage range in which an SEI film is formed on the surface of the negative electrode. ing.

  As described above, in the technique disclosed herein, initial charging is performed at a relatively large current (high rate) of 1.4 C or more and 6 C or less. Thereby, the time of initial charge can be shortened. Further, in the initial charging, the above-described minute rest period is provided while the coating is formed on the surface of the negative electrode. Thereby, even if it is charge by a large current, the formation form of a film can be arranged favorably and a good-quality SEI film can be formed. As a result, a nonaqueous electrolyte secondary battery can be manufactured without increasing battery resistance. According to such SEI coating, for example, the high-temperature storage characteristics and cycle characteristics of the battery can be improved, and the characteristics of the secondary battery can be improved such that unintended decomposition of the electrolytic solution is suppressed.

It is a graph which illustrates the relationship between the charge rate and battery resistance in the initial charge which does not include the conventional minute pause period. It is a graph which illustrates the relationship between a charge rate and battery resistance when providing a micro rest period at 2.8V in initial charge. It is a graph which illustrates the relationship between a charge rate and battery resistance when providing a micro rest period at 3.0V in initial charge. It is a graph which illustrates the relationship between a charge rate and battery resistance when providing a micro rest period at 3.8V in initial charge. It is a graph which illustrates the relationship between a charge rate and battery resistance when providing a micro rest period at 4.0V in initial charge.

  Hereinafter, preferred embodiments of the present invention will be described. Note that matters other than the matters specifically mentioned in the present specification and necessary for implementation (for example, specific constituent materials and assembly methods of the secondary battery) are those skilled in the art based on the prior art in this field. It can be grasped as a design item. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field.

  The manufacturing method disclosed herein includes (a) preparing a battery assembly in which a positive electrode, a negative electrode, and a nonaqueous electrolyte are housed in a battery case; and (b) 1.4 C or more with respect to this battery assembly. Performing initial charging at a charging rate of 6C or less. In this initial charging, a minute pause period in which charging is not performed for 30 seconds or more and 1 minute and 30 seconds or less is included in the voltage range in which the SEI film is formed on the surface of the negative electrode. Hereinafter, each process is demonstrated in order.

(A) Battery Assembly Construction Step Here, a battery assembly in which the positive electrode, the negative electrode, and the nonaqueous electrolyte are accommodated in the battery case is constructed. More specifically, an electrode body having a positive electrode and a negative electrode facing each other (typically via a resin separator) and a nonaqueous electrolyte are accommodated in a predetermined battery case. Although it does not restrict | limit especially as a battery case, For example, what consists of lightweight metal materials, such as aluminum, can be used suitably.

As the positive electrode, for example, a material in which a positive electrode active material layer is formed on the surface of the positive electrode current collector by binding the positive electrode active material on the positive electrode current collector together with a conductive material, a binder, or the like can be used. As the positive electrode current collector, a conductive member made of a metal having good conductivity (for example, aluminum or an alloy thereof) can be suitably used. Examples of the positive electrode active material include layered rock salt type and spinel type lithium composite metal oxides (for example, LiNiO ₂ , LiCoO ₂ , LiFeO ₂ , LiMn ₂ O ₄ , LiNi _1/3 Co _1/3 Mn _1/3 O _2. LiNi _0.5 Mn _1.5 O ₄ , LiCrMnO _4, etc.) can be preferably used. As the conductive material, a carbon material such as carbon black (for example, acetylene black) or graphite can be suitably used. As the binder, polyvinylidene fluoride (PVdF), polyethylene oxide (PEO), or the like can be suitably used.

  It is appropriate to mix these materials so that the ratio of the positive electrode active material to the entire positive electrode active material layer is approximately 60% by mass or more (typically 60% by mass to 99% by mass). It is preferable that it is 70 mass%-95 mass%. The proportion of the conductive material in the entire positive electrode active material layer can be, for example, about 1% by mass to 20% by mass, and preferably about 2% by mass to 10% by mass. Moreover, when using a binder, the ratio of the binder to the whole positive electrode active material layer can be made into about 0.5 mass%-10 mass%, for example, and should be about 1 mass%-5 mass% normally. Is preferred.

  As the negative electrode, for example, a negative electrode active material layer formed on the surface of the negative electrode current collector by binding the negative electrode active material together with a binder or the like onto the negative electrode current collector can be used. As the negative electrode current collector, a conductive material made of a metal having good conductivity (for example, copper or an alloy thereof) can be suitably used. The negative electrode active material is not particularly limited, but graphite (graphite) -based carbon materials can be suitably used. In particular, amorphous-coated graphite having a form in which amorphous carbon is coated on the surface of graphite particles is used. Is preferred. As the binder, styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), polytetrafluoroethylene (PTFE), or the like can be suitably used.

  The proportion of the negative electrode active material in the entire negative electrode active material layer is suitably about 50% by mass or more, and usually 90% to 99% by mass (for example, 95% to 99% by mass). preferable. Thereby, a high energy density is realizable. When using a binder, the ratio of the binder to the whole negative electrode active material layer can be, for example, approximately 0.5% by mass to 10% by mass, and usually approximately 0.5% by mass to 5% by mass. Is preferred. Thereby, the mechanical strength (shape retention) of a negative electrode active material layer can be ensured suitably, and favorable durability can be implement | achieved. When using a thickener, the ratio of the thickener to the whole negative electrode active material layer can be made into about 0.5 mass%-10 mass%, for example, and usually about 0.5 mass%-5 mass. % Is preferable.

  As the separator, it is possible to use a separator that insulates the positive electrode and the negative electrode and enables movement (passage) and retention of charge carriers. Furthermore, a material that has a shutdown function that softens and melts at a predetermined temperature and stops the movement of charge carriers can be preferably used. As a suitable example, a porous resin sheet (film) made of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, polyamide or the like can be mentioned.

A non-aqueous electrolyte (typically a non-aqueous electrolyte) can be prepared, for example, by including a supporting salt in a non-aqueous solvent. As the supporting salt, lithium salt, sodium salt, magnesium salt and the like can be used, and lithium salts such as LiPF ₆ and LiBF ₄ are particularly preferable. As the non-aqueous solvent, aprotic solvents such as carbonates, esters, ethers, nitriles, sulfones, and lactones can be considered. Among these, carbonates such as ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) can be preferably used.
As used herein, the term “battery assembly” refers to any battery assembly that has been assembled up to the stage prior to the initial charging process using an electrode body and a non-aqueous electrolyte. It is not limited. In the battery assembly, for example, the battery case may be in a state before sealing or in a state after sealing.

(B) Initial charging Next, an initial charging process is performed on the battery assembly constructed as described above at a charging rate of 1.4 C or more and 6 C or less. Here, a minute pause period in which charging is not performed for 30 seconds or more and 1 minute and 30 seconds or less in the voltage range in which the SEI film is formed on the surface of the negative electrode is included at least once. Such initial charging is typically performed by connecting an external power source between the positive electrode and the negative electrode of the battery assembly, and at a predetermined charging rate, for example, a secondary battery driving voltage range (for example, 2.7 to 4.1 V). ) Can be carried out by charging within a range including. Although not particularly limited, this initial charging is performed until the SOC (State of Charge) of the battery assembly reaches 65% or more (typically 80% or more, for example, 80 to 105%). Can be implemented.

In the method disclosed herein, the charging rate is set to a relatively high rate of about 1.4C to 6C, preferably 1.5C to 5C. Thereby, initial charge can be implemented more rapidly and the time required for initial charge can be shortened. If the charge rate is less than 1.4C, the effect of shortening the initial charge cannot be greatly obtained, and the form of the film to be formed is not deteriorated so much that the features of the present technology are not effectively expressed. Is not preferable. If the charging rate exceeds 6C, the film formation rate is too rapid, and the effect of the minute rest period described later cannot be obtained.
Here, “1C” means a current value necessary for discharging the rated capacity (nominal capacity) (Ah) in one hour.

  The charging method is not particularly limited as long as the above charging rate can be realized. For example, the charging method may be a method of discharging at a constant current (CC discharge) or a method of discharging at a constant voltage (CV discharging), or You may carry out by the system (CCCV discharge) which discharges with a constant voltage after discharging with a constant current until it reaches a predetermined voltage.

  Note that if the initial charging is performed with a large current of 1.4 C or more, generally, for example, as shown in FIG. 1, the resistance of the obtained secondary battery increases. This is because, in large current charging, current unevenness is likely to occur between where the current on the negative electrode surface tends to flow and where it is difficult to flow, and furthermore, since the film is formed relatively quickly, the film form tends to be disturbed. It is thought that it becomes. On the other hand, in the technique disclosed herein, the initial charging includes at least one minute rest period in which charging is not performed in the voltage range in which the SEI film is formed on the surface of the negative electrode. In other words, by stopping charging in the middle of the film formation and providing time for slowly forming the film, a uniform film is formed, and the deterioration of the film state (and hence the increase in battery resistance) is alleviated. Yes. In this minute rest period, the flow of electrons between the positive and negative electrodes stops, but it is considered that the movement of the charge carriers and the formation of the film are carried out slowly because of the rapid charging with a large current.

  The minute rest period can be 30 seconds or more and 1 minute 30 seconds or less. If the minute pause period is shorter than 30 seconds, the effect of the charge pause cannot be clearly obtained, which is not preferable. In order to effectively realize the unevenness suppression of the SEI film, the minute rest period is preferably 30 seconds or more, more preferably 35 seconds or more. Further, during charging, Li is inserted into the negative electrode (negative electrode active material) and the negative electrode active material layer expands, so that the pressure between the positive and negative electrodes increases, and the gas generated in the initial charge is easily discharged from the electrode body. However, the negative electrode does not expand during the minute rest period, and if the minute rest period is too long, the pressure between the positive and negative electrodes is released due to creep, and gas biting may occur. Therefore, the minute rest period is preferably between 1 minute and 30 seconds or less, more preferably 1 minute and 20 seconds or less.

  In this way, the minute rest period has an effect of adjusting the film formation state. Therefore, as long as the film is formed on the negative electrode, a minute rest period may be provided two times or more (for example, two times, three times, four times, etc.). In this case, although not necessarily strictly defined, it is preferable that the sum of all the minute rest periods is between 30 seconds and 1 minute 30 seconds.

  Since charging is not performed in the minute rest period, it can be considered that the current is essentially 0 (zero). However, since the technology disclosed herein is based on charging with a large current of 1.4 C or more as described above, in this minute pause period, a predetermined charging current of 1.4 C or more and 6 C or less is obtained. On the other hand, a weak current that can be regarded as substantially zero is allowed to flow. As such a weak current, for example, about 1/300 or less (for example, 0.0001C to 0.003C) of the predetermined charging current can be used as a guide.

  Note that the voltage range in which the SEI film is formed on the surface of the negative electrode can vary depending on the battery configuration (typically, a combination of the positive and negative electrode active materials and the non-aqueous electrolyte). It can set suitably about a secondary battery. For example, in a secondary battery having a structure using a graphite diameter material as the negative electrode active material, typically, a voltage range in which a SEI film is formed may be in a range of 3.0 V to 3.8 V.

  Such initial charging can be performed in a normal temperature region (15 ° C. to 30 ° C., typically 25 ° C.). Thereby, an increase in resistance can be suppressed and initial charging can be performed in a short time with a large current. Although it cannot be generally stated because it depends on the battery configuration (for example, rated capacity) and the charging rate, the overall required time for this initial charging can be approximately 10 minutes to 5 hours. .

  In addition, following the above initial charging, for example, aging at normal temperature and high temperature, self-discharge inspection, and the like can be performed. Aging is not limited to this, but, for example, the battery assembly after initial charging is heated at a high temperature of 40 ° C. or higher (eg, 40 to 80 ° C., preferably 50 to 70 ° C., more preferably 55 to 65 ° C.). In this region, it can be carried out by holding (leaving) for several hours (for example, until the total time from the start of temperature rise is 1 to 100 hours, preferably 10 to 48 hours). In addition, the self-discharge test measures the amount of discharge that is spontaneously discharged when a battery assembly in a predetermined state of charge is left at a predetermined temperature. A non-defective product / defective product is discriminated by judging the presence or absence of a short circuit.

  Several examples relating to the present invention will be described below, but the present invention is not intended to be limited to the specific examples.

The inventors of the present invention constructed a battery assembly having the following configuration, and studied the detection sensitivity of a micro short circuit and the conditions affecting dV / dQ.
Positive electrode active material as a positive electrode active material: Li _1.14 Ni _0.335 Co _0.335 Mn _0.33 O _2-δ , acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder And a composition prepared by mixing so as to have a mass ratio of 90: 8: 2 was applied to an aluminum foil (positive electrode current collector) to form a positive electrode active material layer. The obtained positive electrode was dried and pressed to produce a sheet-like positive electrode (positive electrode sheet).

Also prepared by mixing amorphous coated graphite as a negative electrode active material, styrene butadiene rubber (SBR) as a binder and carboxymethyl cellulose (CMC) as a thickener so that the mass ratio is 98: 1: 1. The composition was applied to a copper foil (negative electrode current collector) to form a negative electrode active material layer. The obtained negative electrode was dried and pressed to prepare a sheet-like negative electrode (negative electrode sheet).
The prepared positive electrode sheet and negative electrode sheet are overlapped via a resin separator (here, a three-layer structure in which a polypropylene (PP) layer is laminated on both sides of a polyethylene (PE) layer) to form a flat shape. I turned around. This electrode body was accommodated in a battery case, and a nonaqueous electrolyte was injected. As the non-aqueous electrolyte, a mixed solvent containing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) at a volume ratio of 3: 4: 3, and LiPF ₆ as an electrolyte is about 1 mol. A solution dissolved at a concentration of / L was used. Then, a battery assembly (capacity ratio (CN / CP) is 1.85, rated capacity is 4 Ah) was constructed by attaching a lid to the opening of the battery case and welding and joining.

(No pause)
With respect to the battery assembly constructed as described above, the positive and negative terminal voltages are 4.4 with seven constant currents with charge rates of 0.5C, 1C, 1.2C, 1.4C, 3C, 6C, and 7C. Constant current charging was performed until 1V was reached. After the secondary battery that has been activated by the initial charging in this way is adjusted to 3.72 V (SOC 60%) and then discharged at 100 A for 10 seconds, the resistance value ( (IV resistance) was calculated, and the result is shown in FIG.

(2.8V pause)
In the above “no pause”, when the voltage between the positive and negative terminals reaches 2.8V, there is a minute pause period for stopping charging for 15 seconds, 30 seconds, 1 minute 30 seconds, 3 minutes, The initial charge was performed under the same conditions. Thus, IV resistance was measured with respect to the secondary battery by which the activation process by the initial charge was performed, and the result was shown in FIG.

(3.0V pause)
In the above “no pause”, when the voltage between the positive and negative terminals reaches 3.0 V, a minute pause period is provided in which charging is stopped for 15 seconds, 30 seconds, 1 minute 30 seconds, 3 minutes. The initial charge was performed under the same conditions. Thus, IV resistance was measured with respect to the secondary battery by which the activation process by the initial charge was performed, and the result was shown in FIG.

(3.8V pause)
In the above "no pause", when the voltage between the positive and negative terminals reaches 3.8V, a minute pause period is provided to stop charging for 15 seconds, 30 seconds, 1 minute 30 seconds, 3 minutes, The initial charge was performed under the same conditions. Thus, IV resistance was measured with respect to the secondary battery by which the activation process by the initial charge was performed, and the result was shown in FIG.

(4.0V pause)
In the above “no pause”, when the voltage between the positive and negative terminals reaches 4.0 V, a minute pause period is provided in which charging is stopped for 15 seconds, 30 seconds, 1 minute 30 seconds, 3 minutes. The initial charge was performed under the same conditions. The IV resistance was measured for the secondary battery subjected to the activation process by the initial charging in this way, and the result is shown in FIG.

In addition, in FIG. 2-5, the resistance measured value when not providing a micro rest period was also shown for reference.
As can be seen from FIG. 1, it was found that when the charging rate was set to 1.4 C or more in order to shorten the time required for the initial charging, the battery resistance of the secondary battery after adjustment increased rapidly.
FIG. 2 shows an example in which a minute pause period is provided at a potential between the positive and negative terminals of 2.8 V, which is lower than the potential at which the SEI film is formed. As can be seen from FIG. 2, the battery resistance could not be lowered even if a minute rest period was provided during the initial charging, but the resistance was increased.

  However, as shown in FIG. 3, when the voltage between the positive and negative terminals is 3.0 V and a minute pause period is provided at the potential at which the SEI film starts to be formed, the minute pause time is in the range of 30 seconds to 1 minute 30 seconds. It was confirmed that the resistance was hardly raised. In particular, it should be noted that the increase in battery resistance is significantly suppressed when charging with a large current of 3C to 6C. However, it was also confirmed that the battery resistance increases when the minute pause time is too short or too long. As shown in FIG. 4, the same tendency was also observed when the voltage between the positive and negative terminals was 3.8 V and a minute pause period was provided at a potential near the end of the formation of the SEI film.

  As can be seen from FIG. 5, when the voltage between the positive and negative terminals is 4.0 V, and the minute rest period is provided at the potential at which the formation of the SEI film is completely completed, the battery resistance is reduced as in the case of 2.8 V. It could not be lowered, but rather increased resistance.

  According to the inventor's examination based on the above measurement results, the SEI film is a non-aqueous electrolyte component decomposed by a battery reaction and deposited on the surface of the lower potential negative electrode. When this is performed, the formation speed of the SEI film formed (deposited) on the surface of the negative electrode is also accelerated, and it is considered that unevenness occurs in the “quality” based on the film thickness, the deposition mode, and the like. Such a decrease in the quality of the SEI film is thought to be associated with an increase in battery resistance.

  On the other hand, by temporarily stopping charging during the formation of the SEI film, the concentration unevenness of the film forming component moving toward the negative electrode in the non-aqueous electrolyte is alleviated, and the deposition rate is alleviated and the deposition mode is reduced. As a result, it is considered that a high-quality SEI film was formed. However, the length of the micro-pause period for stopping charging is too short at 30 seconds or less, and a sufficient effect cannot be obtained. Moreover, when 1 minute and 30 seconds were exceeded, it was thought that other problems, such as a gas bite, will generate | occur | produce this time and battery resistance will fall.

  The manufacturing method of the nonaqueous electrolyte secondary battery (for example, lithium ion battery) disclosed herein can reduce the manufacturing time without deteriorating the battery performance, and contributes to the cost reduction. Therefore, it can be suitably applied to the production of batteries for various uses. Among them, batteries for applications using a relatively wide SOC range, for example, a power source of a motor (electric motor) mounted on a vehicle such as a plug-in hybrid vehicle (PHV), a hybrid vehicle (HV), and an electric vehicle (EV). It can be suitably used as (driving power source).

  As mentioned above, although the specific example of this invention was demonstrated in detail, these are only illustrations and do not limit a claim. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.

Claims (2)

Preparing a battery assembly in which a positive electrode, a negative electrode, and a nonaqueous electrolyte are housed in a battery case;
Performing an initial charge on the battery assembly, the initial charge applied to the battery assembly ;
Including
The initial charge is
While charging at a charge rate of 1.4C or more and 6C or less,
To include at least one minute rest period does not charge during the following 1 minute 30 seconds 30 seconds Tei Ru voltage range SEI film is formed on the surface of the negative electrode,
A method for producing a non-aqueous electrolyte secondary battery. The positive electrode includes a lithium composite metal oxide, the negative electrode includes a carbon material,
The method for manufacturing a nonaqueous electrolyte secondary battery according to claim 1, wherein the minute pause period is provided in a voltage range of 3.0 V to 3.8 V.

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