JP5776932B2 - Lithium ion secondary battery operating method and battery device - Google Patents

Description

  The present invention relates to a method of operating a lithium ion secondary battery and a battery device.

  Lithium ion secondary batteries are small and have a large capacity, and are therefore used in a wide range of fields such as mobile phones and notebook computers.

  When the lithium ion secondary battery is charged and discharged, lithium ions are repeatedly inserted and desorbed between the positive electrode (negative electrode) active material and the electrolytic solution, and at that time, the electrolyte is partially reduced and decomposed. The reductive decomposition product of the electrolyte coats the surface of the negative electrode active material, and forms a film that allows lithium ions to pass but does not allow electrons to pass. This is called a solid electrolyte interphase (SEI).

  The SEI coating covers the surface of the negative electrode active material, thereby preventing direct contact between the electrolyte and the negative electrode active material, thereby preventing degradation of the electrolyte.

  By the way, in recent years, the use of particulate silicon oxide (SiOx: about 0.5 ≦ x ≦ 1.5) has been studied as a negative electrode active material capable of inserting and extracting lithium ions. Silicon oxide SiOx expands or contracts due to expansion and contraction of Li ions. For this reason, cracks are likely to occur in the SEI film formed on the surface of the negative electrode active material made of silicon oxide. If a crack occurs in the SEI film, the electrolyte and the negative electrode active material are in direct contact with each other, so that the electrolyte is decomposed and deteriorated, which may deteriorate the cycle characteristics of the battery.

  Conventionally, Patent Documents 1 to 7 propose that the battery characteristics are improved by controlling the temperature during charging and discharging to a predetermined temperature. These patent documents do not describe or suggest the relationship between the battery temperature and the SEI coating.

  As a result of earnest search, the inventors have clarified the relationship between the temperature of the battery and the SEI film on the surface of the negative electrode active material, and have improved the cycle characteristics of the battery.

JP-A-2005-65476 JP 2009-87814 A JP 2010-108873 A JP 2010-198759 A JP 2010-262879 A JP 2010-49968 A Japanese National Patent Publication No. 11-506867

  This invention is made | formed in view of this situation, and makes it a subject to provide the operating method and battery apparatus of a lithium ion secondary battery excellent in the cycling characteristics of a battery.

  (1) The operating method of the lithium ion secondary battery of the present invention comprises a positive electrode having a positive electrode active material capable of occluding and releasing lithium ions, and a negative electrode active comprising silicon and / or silicon compounds capable of occluding and releasing lithium ions. A lithium ion secondary battery including a negative electrode having a substance and an electrolytic solution is adjusted to a temperature of 40 ° C. or higher and 60 ° C. or lower at least during charging.

  In a lithium ion secondary battery, lithium ions are occluded in the negative electrode active material during charging, and an SEI coating is mainly formed on the surface of the negative electrode active material during this charging. The SEI coating refers to a coating generated from a decomposition product of an electrolytic solution. By adjusting the lithium ion secondary battery at least at 40 ° C. or more and 60 ° C. or less during charging, a stable SEI film is formed as described later. The stable SEI film is thin and flexibly follows the expansion and contraction of the negative electrode active material. For this reason, it is hard to produce a crack in a SEI film.

  In particular, the negative electrode active material is made of silicon or / and a silicon compound capable of inserting and extracting lithium ions. A negative electrode active material made of silicon or / and a silicon compound has a large volume expansion / contraction during charge / discharge. For this reason, the SEI coating can flexibly follow the volume change of the negative electrode active material, so that damage to the SEI coating can be effectively suppressed.

  Thereby, it can suppress that electrolyte solution contacts a negative electrode active material directly, and can prevent deterioration of electrolyte solution. Therefore, according to the operating method of the lithium ion secondary battery of this invention, the outstanding battery cycle characteristic can be exhibited.

  When the temperature of the lithium ion secondary battery is lower than 40 ° C., the thickness of the SEI film increases, and the negative electrode active material may expand or contract, and the SEI film may be cracked or damaged. There is a possibility that the electrolytic solution penetrates into the negative electrode active material from cracks or missing portions on the outermost surface of the coating, reacts with the negative electrode active material, deteriorates the electrolytic solution, and battery cycle characteristics are lowered. When the temperature of the lithium ion secondary battery exceeds 60 ° C., the components of the electrolytic solution, particularly the solvent, may be altered and the battery characteristics may be deteriorated.

  (2) It is preferable to adjust the lithium ion secondary battery to a temperature of 45 ° C. or more and 60 ° C. or less at least during charging.

  In this case, the thickness of the SEI film can be further reduced while suppressing the alteration of the electrolytic solution. For this reason, the SEI film can flexibly follow due to the expansion / contraction of the negative electrode active material, and the crack of the SEI film can be suppressed. Therefore, deterioration of the electrolytic solution can be suppressed, and the cycle characteristics of the battery can be further improved.

  (3) The lithium ion secondary battery is preferably adjusted to a temperature of 35 ° C. or higher and 80 ° C. or lower during initial charge / discharge.

  The initial charge / discharge is charge / discharge that is initially performed on the lithium ion secondary battery, and is also referred to as a conditioning process. By performing initial charge / discharge on the lithium ion secondary battery, a thin and stable SEI film is formed on the surface of the negative electrode active material. When the lithium ion secondary battery is adjusted to a temperature of 35 ° C. or higher and 80 ° C. or lower at least during charging during subsequent battery operation, a thin and stable SEI coating can be maintained. Therefore, cracking of the SEI film can be prevented, and further, the deterioration of the electrolyte can be suppressed and the cycle characteristics of the battery can be improved.

  When the temperature of the lithium ion secondary battery for initial charge / discharge is less than 35 ° C., the thickness of the SEI film increases during initial charge / discharge, and cracks and defects occur in the outermost surface portion of the SEI film due to expansion / contraction of the negative electrode active material. May occur. When the temperature of the initial charge / discharge lithium ion secondary battery exceeds 80 ° C., the components of the electrolytic solution, particularly the solvent, may be altered and the battery characteristics may be deteriorated.

  (4) The battery device of the present invention includes a positive electrode having a positive electrode active material capable of occluding and releasing lithium ions, a negative electrode having a negative electrode active material composed of silicon and / or silicon compounds capable of occluding and releasing lithium ions, And a temperature adjusting means for adjusting the lithium ion secondary battery to a temperature of 40 ° C. or more and 60 ° C. or less at least during charging.

  A stable SEI film can be formed on the surface of the negative electrode active material by adjusting the lithium ion secondary battery to at least 40 ° C. and not more than 60 ° C. with a temperature adjusting means at least during discharge. For this reason, even when the negative electrode active material expands / shrinks, it is possible to suppress the SEI coating from flexibly following the deformation of the negative electrode active material and causing cracks and defects. Therefore, the electrolytic solution is not in direct contact with the negative electrode active material, and deterioration of the electrolytic solution can be suppressed. Therefore, battery cycle characteristics can be improved.

  According to the present invention, since the lithium ion secondary battery is adjusted to 40 ° C. or more and 60 ° C. or less at least during charging, the battery cycle characteristics are excellent.

It is a diagram which shows the relationship between the charging / discharging cycle number at the time of the operation test of the apparatuses 1-3 in experiment 1, and a battery capacity maintenance factor. It is a diagram which shows the relationship between the charging / discharging cycle number at the time of the operation test of the apparatuses 4-6 in experiment 2, and a battery capacity maintenance factor. It is a figure which shows the mass change rate of the active material in the apparatuses 7-9 provided with the battery which performed the conditioning process in Experiment 3. FIG. It is a figure which shows the thickness change rate of the active material in the apparatuses 7-9 provided with the battery which performed the conditioning process in Experiment 3. FIG. It is a figure which shows the mass change rate of the active material in the apparatuses 10-12 after the operation test in Experiment 4. FIG. It is a figure which shows the thickness change rate of the active material in the apparatuses 10-12 after the operation test in Experiment 4. FIG. It is a SEM photograph of the negative electrode of the apparatus 10 after the operation test in Experiment 4. It is a SEM photograph of the negative electrode of the apparatus 12 after the operation test in Experiment 4.

  In the present invention, the lithium ion secondary battery is adjusted to a temperature of 40 to 60 ° C. at least during charging during battery operation. Hereinafter, the lithium ion secondary battery, the operation method thereof, and the battery device will be described in detail.

(Lithium ion secondary battery)
A lithium ion secondary battery includes a positive electrode, a negative electrode, a separator, and an electrolyte.

  The negative electrode has a negative electrode active material capable of inserting and extracting lithium ions. The negative electrode active material is generally pressure-bonded to a current collector as a negative electrode active material layer. As the current collector, for example, a metal mesh or metal foil such as copper or copper alloy may be used.

  The negative electrode active material constitutes negative electrode active material particles that are in the form of particles or powder. The average particle diameter of the negative electrode active material particles is preferably 0.01 to 10 μm, and more preferably 0.01 to 5 μm.

The negative electrode active material particles are made of silicon or / and a silicon compound capable of inserting and extracting lithium ions, and have, for example, a Si phase and a SiO ₂ phase. The Si phase is composed of simple silicon, and is a phase that can occlude and release Li ions, and expands and contracts as Li ions are occluded and released. The SiO ₂ phase is made of SiO ₂ and absorbs expansion and contraction of the Si phase. By Si phase is coated with SiO ₂ phase, it may form a negative electrode active material particles composed of the Si phase and SiO ₂ phase. Furthermore, it is preferable that a plurality of refined Si phases are covered with a SiO ₂ phase and integrated to form one particle, that is, a negative electrode active material particle. In this case, the volume change of the whole negative electrode active material particle can be suppressed effectively.

The mass ratio of the SiO ₂ phase to the Si phase in the negative electrode active material particles is preferably 1 to 3. When the mass ratio is less than 1, the negative electrode active material particles are greatly expanded and contracted, and there is a possibility that cracks may occur in the negative electrode active material layer composed of the negative electrode active material particles. On the other hand, when the mass ratio exceeds 3, the amount of insertion and extraction of Li in the negative electrode active material particles is small, and the electric capacity may be lowered.

The negative electrode active material particles may be composed only of the Si phase and the SiO ₂ phase. Further, the negative electrode active material particles are mainly composed of a Si phase and a SiO ₂ phase, but in addition, a known active material may be included as a component of the negative electrode active material particles, specifically, Me. _At least one of _x Si _y O _z (Me is Li, Ca, etc.) may be mixed.

As a raw material for the negative electrode active material particles, a raw material powder containing silicon monoxide may be used. In this case, silicon monoxide in the raw material powder is disproportionated into two phases of SiO ₂ phase and Si phase. In disproportionation of silicon monoxide, silicon monoxide (SiOn: n is 0.5 ≦ n ≦ 1.5), which is a homogeneous solid having an atomic ratio of Si to O of approximately 1: 1, is a reaction inside the solid. To separate into two phases of SiO ₂ phase and Si phase. The silicon oxide powder obtained by disproportionation includes a SiO ₂ phase and a Si phase.

  The disproportionation of silicon monoxide in the raw material powder proceeds by applying energy to the raw material powder. As an example, a method of heating or milling the raw material powder can be mentioned.

When the raw material powder is heated, it is generally said that almost all silicon monoxide is disproportionated and separated into two phases at 800 ° C. or higher if oxygen is removed. Specifically, the raw material powder containing the amorphous silicon monoxide powder is subjected to heat treatment at 800 to 1200 ° C. for 1 to 5 hours in an inert atmosphere such as vacuum or in an inert gas. A silicon oxide powder containing two phases of an amorphous SiO ₂ phase and a crystalline Si phase is obtained.

  When milling the raw material powder, part of the mechanical energy of the milling contributes to chemical atomic diffusion at the solid phase interface of the raw material powder, and generates an oxide phase, a silicon phase, and the like. In milling, the raw material powder may be mixed using a V-type mixer, a ball mill, an attritor, a jet mill, a vibration mill, a high energy ball mill or the like in an inert gas atmosphere such as vacuum or argon gas. Further heat treatment may be performed after milling to further promote disproportionation of silicon monoxide.

  In addition, after making said negative electrode active material particle into the main negative electrode active material, you may add and use other well-known negative electrode active materials (for example, graphite, Sn, Si, etc.).

  In addition to the negative electrode active material, the negative electrode active material layer may contain a binder, a conductive additive, and the like.

  The binder is not particularly limited, and a known one may be used. For example, a resin that does not decompose even at a high potential, such as a fluorine-containing resin such as polytetrafluoroethylene or polyvinylidene fluoride, can be used. The blending ratio of the binder is preferably a mass ratio of negative electrode active material: binder = 1: 0.05 to 1: 0.5. This is because when the amount of the binder is too small, the moldability of the electrode is lowered, and when the amount of the binder is too large, the energy density of the electrode is lowered.

  As the conductive aid, a material generally used for an electrode of a lithium secondary battery may be used. For example, it is preferable to use conductive carbon materials such as carbon black (carbonaceous fine particles) such as acetylene black and ketjen black, and carbon fibers. Besides conductive carbon materials, known conductive materials such as conductive organic compounds are also used. An auxiliary agent may be used. One of these may be used alone or in combination of two or more. The blending ratio of the conductive additive is preferably a mass ratio of negative electrode active material: conductive additive = 1: 0.01 to 1: 0.5. This is because if the amount of the conductive aid is too small, an efficient conductive path cannot be formed, and if the amount of the conductive aid is too large, the moldability of the electrode is deteriorated and the energy density of the electrode is lowered.

The positive electrode includes a current collector and a positive electrode active material layer that covers the surface of the current collector. The positive electrode active material layer includes a positive electrode active material capable of inserting and extracting lithium ions, and preferably further includes a binder and / or a conductive aid. There are no particular limitations on the conductive additive and the binder, and any conductive auxiliary material and binder can be used as long as they can be used in non-aqueous secondary batteries. As the positive electrode active material, for example, a metal composite oxide of lithium and a transition metal such as a lithium / manganese composite oxide, a lithium / cobalt composite oxide, or a lithium / nickel composite oxide is used. Specific examples include LiCoO ₂ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , Li ₂ MnO ₃ , and S. The current collector may be any material that is generally used for the positive electrode of a non-aqueous secondary battery, such as aluminum, nickel, and stainless steel.

  The separator separates the positive electrode and the negative electrode and holds the non-aqueous electrolyte, and a thin microporous film such as polyethylene or polypropylene can be used.

The electrolytic solution may be a nonaqueous electrolytic solution in which an electrolyte is dissolved in an organic solvent. The electrolyte may be a fluoride salt. The electrolyte fluoride salt is preferably an alkali metal fluoride salt soluble in an organic solvent. The alkali metal fluoride salt, _{e.g., LiPF 6, LiBF 4, LiAsF} 6, NaPF 6, NaBF 4, and may be used at least one selected from the group of NaAsF _6. The organic solvent of the non-aqueous electrolyte is preferably an aprotic organic solvent, such as propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate ( One or more selected from EMC) and the like can be used.

  A separator is sandwiched between the positive electrode and the negative electrode to form an electrode body. Lithium ion secondary battery in which a non-aqueous electrolyte is impregnated in the electrode body after connecting between the positive electrode current collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal leading to the outside using a current collecting lead or the like It is good to do.

  The shape of the lithium ion secondary battery is not particularly limited, and various shapes such as a cylindrical shape, a stacked shape, a coin shape, and a laminated shape can be employed.

(Operation method of lithium ion secondary battery)
A lithium ion secondary battery using a Si-based negative electrode is adjusted to a temperature of 40 ° C. or more and 60 ° C. or less at least during charging. The lithium ion secondary battery using the Si-based negative electrode is preferably adjusted to a temperature of 45 ° C. or more and 60 ° C. or less, and further 50 ° C. or more and 60 ° C. or less. A lithium ion secondary battery using a Si-based negative electrode is charged at least under a temperature condition of 40 ° C. or higher and 60 ° C. or lower, whereby a relatively thin and stable SEI coating (hereinafter referred to as coating) is formed on the surface of the negative electrode active material particles. ) Is formed. That is, coated particles comprising a core part composed of a negative electrode active material and a film covering the surface of the core part are formed. The coating is an insulating film through which Li ions can pass. The coating is generated, for example, when a component in the electrolytic solution is decomposed when it comes into contact with silicon constituting the core portion, and the decomposition product adheres to the surface of the core portion.

  If the temperature of the lithium ion secondary battery using the Si-based negative electrode is too low, the coating becomes thick, and there is a possibility that cracks and defects occur in the outermost surface of the coating due to expansion and contraction of the core. There is a possibility that the electrolytic solution permeates from cracks or defects on the outermost surface portion of the coating, reacts with silicon in the core portion, deteriorates the electrolytic solution, and lowers the cycle characteristics of the battery. If the temperature of the lithium ion secondary battery is too high, the components of the electrolytic solution, particularly the solvent, may be altered and the battery characteristics may be deteriorated.

  The reason why the temperature should be adjusted to the predetermined temperature at least during charging is mainly because a stable film is formed on the surface of the negative electrode active material during charging. Preferably, the predetermined temperature is adjusted during charging and discharging. In this case, the temperature change is small, and the operation of the lithium ion secondary battery using the Si-based negative electrode can be stabilized.

  The electrolytic solution may contain a fluorinated salt. When the electrolytic solution contains a fluorinated salt, a lithium ion secondary battery is charged to form a film containing lithium fluoride on the surface of the core portion made of the negative electrode active material. Lithium fluoride (LiF) was formed by decomposition as shown in the following formula (1) when the fluoride salt in the fluorinated salt of the electrolytic solution was in contact with silicon constituting the core portion. Is.

LiPF ₆ → LiF + PF ₅ (1)
In addition to lithium fluoride, the coating film may contain silicon or / and a silicon compound as a component of the negative electrode active material, a component of an electrolytic solution, and the like. In this case, the content of lithium fluoride in the coating may be uniform in the thickness direction of the coating or may have a gradient in the thickness direction of the coating. In the latter case, it is considered that the lithium fluoride content is highest at the interface between the core portion and the coating, and the lithium fluoride content gradually decreases toward the outside in the thickness direction of the coating.

  The coating may cover the entire surface of the core portion. This is because the negative electrode active material constituting the core part is prevented from contacting the electrolytic solution and decomposing the electrolyte in the electrolytic solution, and also suppressing the elution of Li ions occluded in the negative electrode active material. is there.

  The thickness of the coating varies depending on, for example, the temperature conditions during charging / discharging of the lithium ion secondary battery described above. The higher the temperature of the lithium ion secondary battery using the Si-based negative electrode, the thinner and more stable the film is formed.

  Furthermore, the lithium ion secondary battery using the Si-based negative electrode may be initially charged and discharged under a temperature condition of 35 ° C. or higher and 80 ° C. or lower. By performing initial charge / discharge on the lithium ion secondary battery, a film is formed on the surface of the negative electrode active material.

  The number of times of charge / discharge as the initial charge / discharge may be one or more, and more preferably two or more and five or less. Charging and discharging at the initial charging / discharging is preferably performed under predetermined conditions, for example, at a constant current. Moreover, initial charge / discharge may be performed at a predetermined temperature.

  The temperature at which initial charge / discharge is performed on a lithium ion secondary battery using a Si-based negative electrode is 35 ° C. or more and 80 ° C. or less, and the lower limit is preferably 40 ° C., and the upper limit is 60 ° C. Is preferably 55 ° C. If the temperature at the time of initial charge / discharge is too low, the coating becomes thick, and there is a possibility that cracks and defects may occur on the outermost surface of the coating due to expansion / contraction of the core. There is a possibility that the electrolytic solution permeates from cracks or defects on the outermost surface portion of the coating, reacts with silicon in the core portion, deteriorates the electrolytic solution, and lowers the cycle characteristics of the battery. If the initial charging / discharging temperature is too high, the components of the electrolytic solution, particularly the solvent, may change, and the battery characteristics may be deteriorated.

  A negative electrode, a positive electrode, a separator, and an electrolytic solution are housed in a battery container and sealed, thereby producing a lithium ion secondary battery. There is no particular limitation on the stage at which the initial charge / discharge of the lithium ion secondary battery is performed in the production of the lithium ion secondary battery. For example, first, an electrode body composed of a negative electrode, a positive electrode, and a separator is used. When initial charging / discharging is performed after sealing and injecting the electrolytic solution in the battery container, secondly, the initial charging is performed after the secondary battery is assembled by storing and sealing the electrode body and the electrolytic solution in the battery container. Discharging may occur. Among these, from the viewpoint of workability of charge / discharge, it is preferable to perform initial charge / discharge after assembling the secondary battery.

(Battery device)
The battery device of the present invention includes the above lithium ion secondary battery and temperature adjusting means for adjusting the lithium ion secondary battery to a temperature of 40 ° C. or higher and 60 ° C. or lower at least during charging.

  The temperature adjusting means is not particularly limited as long as it can adjust the lithium ion secondary battery to the above-mentioned predetermined temperature. For example, a heater capable of temperature control can be mentioned. In addition, for example, when a lithium ion secondary battery is mounted on a vehicle equipped with an internal combustion engine, the temperature adjusting means is an exhaust pipe that circulates high-temperature exhaust gas provided around the lithium ion secondary battery. Or you may.

(Device 1)
This device is a battery device including a lithium ion secondary battery and temperature adjusting means.

  The lithium ion secondary battery was produced as follows.

  First, commercially available SiO powder, graphite powder as a conductive aid, ketjen black, and polyamideimide as a binder were mixed, and a solvent was added to obtain a slurry mixture. The solvent was N-methyl-2-pyrrolidone (NMP). The mass ratio of the negative electrode active material particles, the graphite powder, the ketjen black, and the polyamideimide was, as a percentage, negative electrode active material particles / graphite powder / Ketjen black / polyamideimide = 42/40/3/15.

  Next, the slurry mixture is formed into a film on one side of a copper foil (thickness 20 μm) as a current collector using a doctor blade, pressed at a predetermined pressure, heated at 200 ° C. for 2 hours, and allowed to cool. did. Thereby, the negative electrode formed by fixing the negative electrode active material layer on the current collector surface was formed.

Next, a lithium / nickel composite oxide LiNi _1/3 Co _1/3 Mn _1/3 O ₂ as a positive electrode active material, acetylene black, and polyvinylidene fluoride (PVDF) as a binder are mixed to form a slurry. The slurry was applied to one side of an aluminum foil (thickness 19 μm) as a current collector, pressed and fired. This obtained the positive electrode formed by fixing a positive electrode active material layer on the surface of a collector. A polypropylene porous membrane as a separator was sandwiched between the positive electrode and the negative electrode. A plurality of electrode bodies composed of the positive electrode, the separator, and the negative electrode were stacked. The periphery of the two aluminum films was sealed by heat-welding except for a part to make a bag shape. The laminated electrode body was put in a bag-like aluminum film, and an electrolytic solution was further put. The electrolytic solution is obtained by dissolving LiPF ₆ as an electrolyte in an organic solvent. The organic solvent was prepared by mixing ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate at a blending ratio of 3/3/7 (vol%). The concentration of LiPF ₆ in the electrolytic solution was 1 mol / L.

  Then, the opening part of the aluminum film was completely airtightly sealed while evacuating. At this time, the tips of the positive electrode side and negative electrode side current collectors were protruded from the edge portions of the film to enable connection to external terminals, thereby obtaining a laminated battery.

  The temperature adjusting means is a heater. The heater is disposed around the lithium ion secondary battery. The heater incorporates a control circuit that can adjust the lithium ion secondary battery to a desired temperature.

  The battery device is operated while adjusting the lithium ion secondary battery to a predetermined temperature described later by the temperature adjusting means.

<Experiment 1>
About the said battery apparatus, after performing the conditioning process of a lithium ion secondary battery, the operation test which repeats charging / discharging was done.

  The conditioning process was repeated 3 times at 25 ° C. The first time, the battery was charged to 4.1 V with a constant current (CC) of 0.2 C, and discharged to 3.0 V with a constant current of 0.2 C. The second time was charged to 4.1 V with a constant current constant voltage (CC-CV) of 0.2 C, and discharged to 3.0 V with a constant current of 0.1 C. The third time was charged to 4.2V with a constant current constant voltage of 1C and discharged to 3.0V with a constant current of 1C. After the conditioning treatment, the lithium ion secondary battery was returned to room temperature (25 ° C.).

  In the operation test after the conditioning treatment, charging and discharging of the lithium ion secondary battery were repeated. At the time of charge, the lithium ion secondary battery was charged to 4.2V with CC-CV (constant current constant voltage) of 1C. At the time of discharging, the lithium ion secondary battery was discharged to 2.5 V with a CC (constant current) of 1C. The first charge / discharge after the conditioning treatment was regarded as the first cycle, and the same charge / discharge was repeated until the 500th cycle. The battery temperature during the operation test was set to 25 ° C, 45 ° C, and 55 ° C. When the temperature of the battery at the time of the operation test was 25 ° C., the device 1 was designated as device 1, when 45 ° C. was designated as device 2 and when 55 ° C. was designated as device 3.

  The discharge capacity of the battery was measured for each cycle, and the discharge capacity retention rate in each cycle was calculated. The discharge capacity maintenance ratio is a value obtained by dividing the discharge capacity at the Nth cycle by the initial discharge capacity ((discharge capacity at the Nth cycle) / (discharge capacity at the first cycle) × 100). . N is an integer of 1-20.

  In FIG. 1, the discharge capacity maintenance rate for each battery cycle for the devices 1, 2, and 3 is shown. As shown in FIG. 1, the batteries of the devices 2 and 3 showed a higher capacity retention rate until after a large number of cycles had elapsed than those of the device 1. After 500 cycles, the device 3 showed a higher discharge capacity maintenance rate than the device 2. This indicates that excellent cycle characteristics are exhibited when the operating temperature is 40 ° C. or higher, further 45 ° C. or higher, and preferably 50 ° C. or higher.

<Experiment 2>
In this experiment, the effect of the temperature during the conditioning process and the operation test on the cycle characteristics was examined. When the temperature during the conditioning process and the temperature during the operation test of the battery of the battery device are both 25 ° C., the device 4 is used. The temperature during the conditioning process is 25 ° C. and the temperature during the cycle test is 45 ° C. The apparatus 5 was used, and the case where the temperature during the conditioning process was 45 ° C. and the temperature during the cycle test was 25 ° C. was used as the apparatus 6.

  The conditioning process and the operation test were performed under the same conditions as in Experiment 1 except for the temperature conditions. The discharge capacity retention rate of the battery during the 250 cycle charge / discharge is shown in FIG. As shown in FIG. 2, when the battery temperature during the operation test is 45 ° C. (device 5), the battery temperature is lower than when the battery temperature during the operation test is 25 ° C. (devices 4 and 6). Cycle characteristics were excellent. When the temperature during the operation test is 25 ° C. and the temperature during the conditioning process is 45 ° C. (apparatus 6), the cycle characteristics are superior to that when the temperature during the conditioning process is 25 ° C. (apparatus 4). It was. Therefore, by keeping the temperature of the battery during the conditioning process at 45 ° C., which is slightly higher than room temperature, the cycle characteristics are improved compared to the case of 25 ° C. (room temperature), and the temperature during the conditioning process is further improved. It can be seen that the cycle characteristics are improved when the temperature of the battery during the operation test is 45 ° C. (device 5) than when the temperature is 45 ° C. (device 6).

<Experiment 3>
In this experiment, the temperature during the conditioning treatment of the battery and the change in the mass and thickness of the active material before and after the conditioning treatment were measured. The battery device was conditioned at 25 ° C., 45 ° C., and 55 ° C., and the device 7, device 8, and device 9 were used in this order. The charging / discharging conditions of the conditioning treatment were the same as in Experiment 1 above. About each apparatus, the mass change rate and thickness change rate of the positive electrode and negative electrode before and after a conditioning process were measured.

  The mass after conditioning is taken out of the battery after removing the positive electrode and negative electrode, washed, dried and weighed to determine the ratio of the mass after conditioning to the mass before conditioning. The mass change rate was used. Then, the ratio of the thickness of the electrode excluding the current collector after the conditioning treatment to the thickness of the electrode excluding the current collector before the conditioning treatment was determined, and this was used as the thickness change rate of the positive electrode and the negative electrode. FIG. 3 shows mass change rates of the positive electrode and the negative electrode, and FIG. 4 shows thickness change rates of the positive electrode and the negative electrode.

  As shown in FIG. 3 and FIG. 4, before and after the conditioning treatment, all of the devices 7, 8, and 9 had almost no change in the mass and thickness of the positive electrode, but the mass and thickness of the negative electrode increased. The device 8 had a large increase in mass compared with the devices 7 and 9, but the thickness increase was small. The reason is presumed that a compound having a high density is produced as SEI.

<Experiment 4>
In this experiment, the temperature during battery operation test and the mass change rate and thickness change rate after 500 cycles of charge and discharge were measured in the operation test. About the said battery apparatus, after performing the conditioning process at the temperature of 25 degreeC, the operation test which repeats charging / discharging was done. The charging / discharging conditions of the conditioning treatment were the same as in Experiment 1 above. The charge / discharge conditions during the operation test after the conditioning treatment were the same as in Experiment 1 above, and the temperature of the battery was changed to 25 ° C., 45 ° C., and 55 ° C. Device 10, device 11, and device 12 were used in the order of the heat retention temperature of the battery: 25 ° C., 45 ° C., and 55 ° C. The mass change rate and thickness change rate of the positive electrode and the negative electrode were measured before and after the operation test for each device and after 500 times of charge and discharge were repeated in the operation test. The mass change rate and the thickness change rate were calculated by the same method as in Experiment 3 above. The results are shown in Table 1, the mass change rate of the positive electrode and the negative electrode is shown in FIG. 5, and the thickness change rate is shown in FIG.

  As shown in Table 1, FIG. 5, and FIG. 6, after the battery was charged and discharged 500 times, the mass and thickness of the negative electrode increased greatly. Especially, the mass change rate and thickness change rate of the negative electrode in the apparatus 10 were large, and the apparatus 12 was small. From this, it can be seen that the higher the temperature during the operation test, the smaller the increase in the thickness of the negative electrode.

  On the other hand, the positive electrode active material had less mass change and thickness change of the active material before and after the operation test than the negative electrode active material.

  The SEM (scanning electron microscope) image of the cross section of the negative electrode of the battery of the apparatuses 10 and 12 after 500 charge / discharge cycles is shown in FIGS. As shown in FIGS. 7 and 8, an active material layer composed of negative electrode active material particles was formed on the surface of the negative electrode current collector of the batteries of the devices 10 and 12. As shown in FIG. 7, polygonal particles in the active material layer are active material particles. In the apparatus 10, there were few gaps in the active material layer. On the other hand, as shown in FIG. 8, relatively large particles in the active material are active material particles. In the device 12 in which the battery temperature during the operation test was 55 ° C., there were more gaps in the active material layer than in the case of the device 10 at 25 ° C. The reason why there are many gaps in the active material layer of the device 12 is considered to be that the thickness of the SEI film on the surface of the active material is thin.

Claims (4)

A positive electrode having a positive electrode active material capable of occluding and releasing lithium ions, a negative electrode having a negative electrode active material composed of silicon and / or silicon compounds capable of occluding and releasing lithium ions, and an electrolyte containing a fluorinated salt A lithium ion secondary battery comprising a lithium ion secondary battery , wherein a film containing lithium fluoride is formed on a surface of the negative electrode active material by adjusting the lithium ion secondary battery to a temperature of 40 ° C. or more and 60 ° C. or less at least during charging. How to operate the battery.   The method for operating a lithium ion secondary battery according to claim 1, wherein the lithium ion secondary battery is adjusted to a temperature of 45 ° C. or more and 60 ° C. or less at least during charging.   The operation method of the lithium ion secondary battery according to claim 1 or 2, wherein the lithium ion secondary battery is adjusted to a temperature of 35 ° C or higher and 80 ° C or lower during initial charge / discharge. A positive electrode having a positive electrode active material capable of occluding and releasing lithium ions, a negative electrode having a negative electrode active material composed of silicon and / or silicon compounds capable of occluding and releasing lithium ions, and an electrolyte containing a fluorinated salt Lithium ion secondary battery equipped,
And a temperature adjusting means for forming a film containing lithium fluoride on the surface of the negative electrode active material by adjusting the lithium ion secondary battery to a temperature of 40 ° C. or more and 60 ° C. or less at least during charging. Battery device.