JP2012230779A - Method for manufacturing lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a lithium secondary battery excellent in cycle characteristics.SOLUTION: Initial charging and discharging of a battery body comprising a positive electrode comprising a positive electrode active material capable of absorbing and desorbing lithium ions, a negative electrode comprising a negative electrode active material capable of absorbing and desorbing lithium ions and comprising silicon or/and a silicon compound, a separator, and an electrolytic solution, are performed at temperatures of 35-80°C.

Description

  The present invention relates to a method for manufacturing a lithium ion secondary battery, and more particularly to a method for manufacturing a lithium ion secondary battery using silicon as a negative electrode active material.

  Secondary batteries such as 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.

  A lithium ion secondary battery is composed of a positive electrode, a negative electrode, an electrolytic solution, and a separator. The positive electrode is coated with a positive electrode active material made of a metal composite oxide of lithium and a transition metal, such as lithium / manganese composite oxide, lithium / cobalt composite oxide, lithium / nickel composite oxide, and the like. Current collector.

The negative electrode is formed by covering a current collector with a negative electrode active material capable of inserting and extracting lithium ions. In recent years, the use of 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. It is known that silicon oxide SiOx decomposes into Si and SiO ₂ when heat-treated. This is called a disproportionation reaction and is a reaction in which homogeneous solid silicon monoxide SiO having a ratio of Si to O of approximately 1: 1 is separated into two phases of Si phase and SiO ₂ phase by solid internal reaction. It is. The Si phase obtained by separation is very fine and is covered with the SiO ₂ phase. The Si phase contains silicon alone capable of inserting and extracting Li ions, and the volume expands and contracts due to the expansion and contraction of Li ions. The SiO ₂ phase absorbs the expansion and contraction of the Si phase, and prevents the electrolytic solution from contacting the Si phase, thereby suppressing the decomposition reaction of the electrolytic solution and improving the cycle characteristics of the battery.

  In a lithium secondary battery using silicon oxide as a negative electrode active material, various treatments have been proposed in order to improve battery performance. For example, in Patent Document 1, a negative electrode is formed by laminating a lithium metal foil on the surface of a negative electrode active material layer made of a negative electrode active material, and the negative electrode and the positive electrode are laminated so as to face each other with a separator interposed therebetween. It has been proposed to stand (aging) at a temperature of 10 to 70 ° C. In Patent Document 1, since the lithium metal foil is laminated on the surface of the negative electrode active material layer, the lithium ion is doped from the lithium metal foil into the negative electrode active material layer by leaving it at a predetermined temperature, and the battery Irreversible capacity is compensated.

  Patent Documents 2, 3, and 4 disclose that a negative electrode active material is not silicon monoxide but a lithium ion secondary battery that is a carbon-based material is charged and discharged (conditioning) at a predetermined temperature. ing.

JP 2010-160983 A JP 2004-296179 A JP 2010-282874 A JP 2006-351332 A

  However, Patent Document 1 aims to compensate for the irreversible capacity of the battery, and does not mention improvement of the cycle characteristics of the battery.

  In Patent Documents 2 to 4, since a carbon-based material is used as the negative electrode, it is unclear whether it can be applied when silicon oxide is used as the negative electrode.

  This inventor earnestly searched for the condition of the conditioning process of a lithium ion secondary battery in order to improve the cycling characteristics of a battery about the case where silicon oxide is used as a negative electrode.

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

  (1) A method of manufacturing a lithium ion secondary battery according to the present invention includes a positive electrode having a positive electrode active material capable of occluding and releasing lithium ions, and lithium and / or silicon compounds capable of occluding and releasing lithium ions. A battery body comprising a negative electrode having a negative electrode active material, a separator, and an electrolytic solution is subjected to initial charge / discharge under a temperature condition of 35 to 80 ° C.

  According to the said structure, the initial stage charge / discharge is performed on the battery main body which consists of a positive electrode, a negative electrode, a separator, and electrolyte solution on 35-80 degreeC temperature conditions. For this reason, the cycle characteristics of the battery are improved. The reason is considered as follows. When the battery body is initially charged and discharged under a temperature condition of 35 to 80 ° C., a relatively thin and stable film is formed on the surface of the negative electrode active material. The negative electrode active material is made of silicon oxide, and expands and contracts by inserting and extracting Li ions. When the negative electrode active material expands / shrinks, the coating on the surface of the negative electrode active material is relatively thin, so that the stress applied to the outer surface of the coating is reduced, and the occurrence of cracks and defects on the outer surface of the coating can be suppressed. . Therefore, it is difficult for the negative electrode active material to come into contact with the electrolytic solution, and the decomposition reaction of the electrolytic solution can be suppressed. Therefore, the cycle characteristics of the battery can be improved.

  On the other hand, when the initial charge / discharge temperature is less than 35 ° C., the cycle characteristics of the battery may be deteriorated. When the initial charge / discharge temperature exceeds 80 ° C., the components of the electrolytic solution, particularly the solvent, may be altered to deteriorate the battery characteristics.

  (2) It is preferable to perform initial charge / discharge on the battery body under a temperature condition of 40 to 60 ° C. In this case, the cycle characteristics of the battery can be further improved while suppressing the deterioration of the electrolytic solution.

  (3) The electrolyte solution preferably contains a fluoride salt. In this case, during the initial charge / discharge, the negative electrode active material reacts with the fluoride salt of the electrolytic solution to form a stable film on the surface of the negative electrode active material. For this reason, it is possible to further exhibit excellent cycle characteristics.

  (4) The negative electrode is preferably formed by binding the negative electrode active material with polyamideimide. Polyamideimide has a lower irreversible capacity for lithium ions than polyimide, and increases the initial efficiency of the battery. For this reason, it is possible to form a negative electrode without defects while keeping the irreversible capacity of the negative electrode low.

  According to the method for producing a lithium ion secondary battery of the present invention, the battery body is initially charged and discharged under a temperature condition of 35 to 80 ° C. For this reason, the lithium ion secondary battery excellent in cycling characteristics can be produced.

It is sectional explanatory drawing of the negative electrode active material particle when the film thickness of a film is thin (case A). It is sectional explanatory drawing of the negative electrode active material particle when the film thickness of a film is thick (case B). It is a diagram which shows the discharge capacity maintenance factor for every cycle of the secondary battery of Example 1 and Comparative Example 1. 4 is a diagram showing internal resistance values for each cycle of the secondary batteries of Example 1 and Comparative Example 1. FIG. 2 is a diagram showing a spectrum of binding energy of a negative electrode active material particle of Example 1 in a range of 0 to 1000 eV. FIG. 4 is a diagram showing a spectrum of binding energy in a range of 680 to 692 eV of negative electrode active material particles of Example 1. FIG. 4 is a diagram showing a spectrum of binding energy in a range of 0 to 1000 eV of negative electrode active material particles of Comparative Example 1. FIG.

  In the method for producing a lithium secondary battery of the present invention, the battery body is charged and discharged under a temperature condition of 35 to 80 ° C. Hereinafter, the battery main body and charging / discharging to the battery main body will be described in detail.

(Battery body)
The battery body includes a positive electrode, a negative electrode, a separator, and an electrolytic solution.

  The negative electrode has a negative electrode active material that can occlude and release lithium ions and is made of silicon or / and a silicon compound. 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 have 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 covered by 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. The nonaqueous electrolytic solution is obtained by dissolving a fluoride salt as an electrolyte in an organic solvent. 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. It is preferable to connect the positive electrode current collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal that communicate with the outside using a current collecting lead or the like, and then impregnate the electrode body with a non-aqueous electrolyte to form a battery body. .

  The shape of the non-aqueous 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 adopted.

(Initial charge / discharge to the battery body)
The battery body is initially charged and discharged under a temperature condition of 35 ° C. or higher and 80 ° C. or lower. This initial charge / discharge is an initial charge / discharge performed on the battery body, and is also referred to as a conditioning process. By performing initial charge / discharge on the battery body, a film is formed on the surface of the negative electrode active material of the negative electrode constituting the battery body, and the coated particles are composed of a core part made of the negative electrode active material and a film covering the core part Is formed.

  The temperature at which the battery body is initially charged / discharged is 35 ° C. or higher and 80 ° C. or lower. Further, the lower limit is preferably 40 ° C., and the upper limit is preferably 60 ° C., more 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 missing portions 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 initial charge / discharge treatment is too high, the components of the electrolytic solution, particularly the solvent, may change, and the battery characteristics may deteriorate.

  A battery body composed of a negative electrode, a positive electrode, a separator, and an electrolytic solution is housed in a battery container and sealed, thereby producing a lithium ion secondary battery. There is no particular limitation as to which stage the initial charging / discharging of the battery body 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 as a battery container. After being accommodated in the container and injecting the electrolyte, initial charge / discharge is performed and sealed. Second, the electrode body and the electrolytic solution are accommodated in a battery container and sealed to perform initial charge / discharge after the secondary battery is assembled. Among these, from the viewpoint of workability of charge / discharge, it is preferable to perform initial charge / discharge after assembling the secondary battery.

  By performing initial charge and discharge on the battery body under a temperature condition of 35 ° C. or higher and 80 ° C. or lower, a relatively thin and stable coating is formed on the surface of the negative electrode active material particles. That is, as shown in FIG. 1, coated particles 3 including a core portion 1 made of a negative electrode active material and a coating 2 that covers the surface of the core portion 1 are formed. The coating 2 is an insulating film through which Li ions can pass. The coating 2 is generated, for example, when a component in the electrolytic solution is decomposed by coming into contact with silicon constituting the core portion 1 and the decomposition product adheres to the surface of the core portion 1.

  When the electrolytic solution contains a fluorinated salt, initial charge / discharge is performed on the battery body to form a coating 2 containing lithium fluoride on the surface of the core portion 1 made of the negative electrode active material. Lithium fluoride (LiF) is formed by decomposing as shown in the following formula (1) when the fluoride salt in the fluorinated salt of the electrolytic solution comes into contact with silicon constituting the core portion 1. It is a thing.

LiPF ₆ → LiF + PF ₅ (1)
The coating 2 may contain, in addition to lithium fluoride, silicon or / and a silicon compound that is 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 2 may be uniform in the thickness direction of the coating 2 or may have a gradient in the thickness direction of the coating 2. In the latter case, it is considered that the interface between the core portion 1 and the coating 2 has the largest lithium fluoride content, and the lithium fluoride content gradually decreases toward the outside of the coating 2 in the thickness direction. It is done.

  The coating 2 may cover the entire surface of the core portion 1. In order to prevent the negative electrode active material constituting the core part 1 from coming into contact with the electrolytic solution and decomposing the electrolyte in the electrolytic solution, and to suppress the elution of Li ions occluded in the negative electrode active material It is.

  The thickness of the film 2 varies depending on, for example, the temperature condition of the conditioning process described above. The higher the temperature of the conditioning process, the thinner and more stable the coating 2 is formed. The thickness of the coating 2 is measured by X-ray photoelectron spectroscopy (XPS). In XPS, X-rays are applied to the coated particle 3 composed of the core portion 1 with the coating 2 and the energy intensity of photoelectrons emitted from the coated particle 3 is measured. By measuring the energy intensity of the photoelectrons, the element located on the surface of the coated particle 3 can be identified. Further, when the surface of the coated particle 3 is irradiated with X-rays, the photoelectrons emitted from the surface of the coated particle 3 are limited to those emitted from an element from the surface of the coated particle 3 to a predetermined depth H. . For this reason, elements existing from the surface of the coated particle 3 to a predetermined depth H can be analyzed.

  When the thickness of the coating 2 formed on the surface of the coated particle 3 changes, the element detected by XPS also changes. This will be conceptually described with reference to FIGS. FIG. 1 shows a case A in which the thickness of the coating 2 is smaller than the thickness H from the surface of the coated particle 3 that can be detected by XPS. In case A, XPS detects not only the elements of the film 2 but also the elements of the core part 1. The coating 2 contains lithium fluoride, and the core 1 is made of silicon or / and a silicon compound. For this reason, XPS detects not only fluorine contained in the coating 2 but also silicon and / or silicon compounds contained in the core portion 1.

  FIG. 2 shows a case B in which the thickness of the coating 2 is larger than the thickness H from the surface of the negative electrode active material particles that can be detected by XPS. In case B, the elements detected by XPS are generally limited to those in the coating 2, and almost no elements in the core portion 1 are detected. For this reason, in case B, the detected amount of fluorine is larger than in case A, and the detected amount of silicon is smaller.

  Thus, when the thickness of the coating 2 changes, the detection amount of each element detected by XPS also changes. Therefore, the thickness of the coating 2 is defined from the area ratio between the intensity caused by the photoelectrons generated from the substance in the coating 2 detected by XPS and the intensity caused by the photoelectrons generated from the substance in the core 1. Can do.

  That is, the integrated intensity ratio of the peak caused by the 1s orbit of F (fluorine) to the peak caused by the 2p orbit of silicon on the surface of the negative electrode active material particles is preferably measured by XPS. The peak due to the 2p orbit of silicon appears in the bond energy region of 98 to 105 eV. The peak due to the 1s orbit of F (fluorine) appears in the binding energy region of 680 to 692 eV. The integrated intensity ratio when the integrated intensity ratio of the peak caused by the 1s orbit of F (fluorine) to the peak caused by the 2p orbit of silicon on the surface of the negative electrode active material particles is preferably 70 or less. In this case, silicon contained in the core portion 1 exhibits a relatively strong spectral intensity. For this reason, the thickness of the coating 2 is thin, and accordingly, the core portion 1 existing on the inner side of the coating 2 is located within a predetermined depth H from the surface of the negative electrode active material particles or to the vicinity thereof. You can see that

  In XPS, the coated particles 3 may be irradiated with AlKα rays (monochrome) as excitation X-rays. Excited X-rays are generated by applying a current of 15 kV and 10 mA to the AlKα pole of an X-ray tube that is an X-ray source. When the voltage or current value of the applied current changes, the depth from the surface of the coated particle 3 that can be detected also changes, so that a current of 15 kV and 10 mA is applied. The generation position of the photoelectrons emitted from the surface of the coated particle 3 when a current of 15 kV and 10 mA is applied to the X-ray tube is assumed to have a depth H of about 5 to 50 nm from the outermost surface of the coated particle 3. Yes.

  Here, the peak caused by the 2p orbit of silicon and the peak caused by the 1s orbit of fluorine each protrude in a mountain shape. The peak-shaped peak portion between the end portions on both sides of the energy region of each peak includes a peak-shaped peak portion corresponding to the superposition of the peak caused by the silicon 2p orbit and the peak caused by the fluorine 1s orbit. Is formed. The area ratio of the peak portion is the integrated intensity ratio. When the integrated intensity ratio of the peak due to the 1s orbit of fluorine with respect to the peak due to the 2p orbit of silicon on the surface of the coated particle 3 is large, for example, when it exceeds 70, 60, or 50, the coating 2 containing silicon Since the thickness is too large, the outermost surface of the coating 2 cannot follow the expansion / contraction of the core portion 1, and stress may concentrate, causing cracks and defects. That is, the electrolytic solution may permeate into the core portion through the cracks and defects on the outermost surface of the coating 2 or Li ions occluded in the core portion 1 may be eluted, thereby reducing the cycle characteristics of the battery. .

  The integrated intensity ratio of the peak caused by the 1s orbit of fluorine to the peak caused by the 2p orbit of silicon on the surface of the coated particle 3 is preferably 70 or less, 60 or less, and further 50 or less. The lower limit of the integrated intensity ratio is preferably 5, and more preferably 10. In this case, since the coating 2 covers the entire surface of the core part 1, it is possible to suppress the silicon contained in the core part 1 from coming into contact with the electrolytic solution, to suppress the deterioration of the electrolytic solution, High cycle characteristics can be exhibited. Moreover, elution of Li ions occluded in the core part 1 can be suppressed, and a decrease in battery capacity can be suppressed.

  On the other hand, when the integral intensity ratio is greater than 70, the surface of the coating 2 is greatly expanded and contracted, and cracks and defects are likely to occur in the coating. The electrolyte enters the coating from the crack, reaches the core 1 and contacts the silicon in the core 1, and the electrolyte is deteriorated or Li ions occluded in the core 1 are eluted. Thus, the cycle characteristics of the battery may be deteriorated.

Example 1
The lithium secondary battery of this example was manufactured as follows, and the cycle evaluation test of the battery was performed.

  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 is a percentage, and the negative electrode active material particles / graphite powder / Ketjen black / polyamideimide = 48 / 39.4 / 2.6 / 10. there were.

  Next, the slurry mixture was formed into a film on one side of a copper foil as a current collector using a doctor blade, pressed at a predetermined pressure, heated at 200 ° C. for 2 hours, and allowed to cool. 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 and polyvinylidene fluoride (PVDF) as a binder are mixed to form a slurry. Was applied to one side of an aluminum foil 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 and diethyl carbonate in a mixing ratio of 3 parts by mass and 7 parts by mass. 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 laminate battery was subjected to a conditioning treatment (initial charge / discharge). The conditioning treatment was repeated three times at 45 ° C. The first time, the charging condition was set to CC (constant current) charging at 0.2C and 4.1V, and the discharging condition was set to CC discharging at 0.2C, 3V and cut-off. For the second time, the charging condition was set to CC-CV charging of 0.2C and 4.1V, and the discharging condition was set to CC discharge of 0.1C, 3V and cut-off. In the third time, the charging condition was 1C, 4.2V CC-CV charging, and the discharging condition was 1C, 3V, cut-off CC discharge. After the conditioning treatment, the lithium ion secondary battery was returned to room temperature (25 ° C.).

  By the conditioning treatment, a film was formed on the surface of the negative electrode active material particles, and coated particles composed of a core portion made of the negative electrode active material and a film were obtained.

(Comparative Example 1)
In this comparative example, a lithium ion secondary battery was produced in the same manner as in Example 1 except that the temperature of the conditioning treatment was 25 ° C.

<Battery cycle experiment>
The cycle test of the lithium ion secondary battery of Example 1 and Comparative Example 1 was performed. The cycle test was performed at 25 ° C., the charge condition was 1 C, 4.2 V CC-CV (constant current constant voltage) charge, and the discharge condition was 1 C, 2.5 V, cut-off CC discharge. The first charge / discharge test after the conditioning treatment was taken as the first cycle, and the same charge / discharge was repeated until the 300th cycle. The discharge capacity 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. 3, the discharge capacity maintenance factor for every cycle about each lithium ion secondary battery of Example 1 and Comparative Example 1 was shown. In FIG. 4, the internal resistance value of each lithium ion secondary battery of Example 1 and Comparative Example 1 measured for each cycle is shown.

  As shown in FIG. 3, the lithium ion secondary battery of Example 1 had a higher discharge capacity retention rate than that of Comparative Example 1. As shown in FIG. 4, the lithium ion secondary battery of Example 1 had substantially the same internal resistance value at the initial number of cycles as compared with that of Comparative Example 1. However, as the number of cycles increased, the internal resistance value in Comparative Example 1 gradually increased from that in Example 1. From this, it can be seen that Example 1 in which the initial charge / discharge was performed at 45 ° C. is superior in the cycle characteristics of the battery.

<XPS>
The spectral intensity of the binding energy of the negative electrode active material particles of the lithium ion secondary batteries of Example 1 and Comparative Example 1 was measured by XPS. In XPS, the surface of the negative electrode active material particles was irradiated with AlKα rays emitted at a temperature of 25 ° C. under conditions of 15 kV and 10 mA. As shown in FIG. 1, the irradiation angle θ of the AlKα ray was set to 35 ° with respect to the tangent to the surface of the negative electrode active material particles. FIG. 5 shows a spectrum of binding energy of the negative electrode active material particles of Example 1 in the range of 0 to 1000 eV, and FIG. 6 shows a spectrum of binding energy of the negative electrode active material particles of Example 1 in the range of 680 to 692 eV. FIG. 7 shows the spectrum of the binding energy of the negative electrode active material particles of Comparative Example 1 in the range of 0 to 1000 eV.

As shown in FIGS. 5 and 6, in the X-ray spectrum emitted from the negative electrode active material particles of Example 1, a peak due to the 1s orbital of fluorine appears in the range where the binding energy is 680 to 692 eV, A peak due to the 2P orbital of silicon appeared in the range where the binding energy was 98 to 105 eV. Since the 1s orbital of fluorine is an orbital bonded to Li, the appearance of a peak of 1s orbital of fluorine indicates that a compound containing fluorine is generated on the surface of the negative electrode active material particles. . In FIG. 6, the peak appearing in the vicinity of 684 eV among the 1 s peak of fluorine appears due to lithium fluoride. That is, the appearance of a peak near 684 eV indicates that lithium fluoride is generated on the surface of the negative electrode active material particles. The peak of 1 s of fluorine appears even if there is a compound containing fluorine even without lithium fluoride. The peak that appears due to lithium fluoride is the peak that appears in the vicinity of 684 eV among the 1 s peak of fluorine (FIG. 6). It is estimated that the Li source and the fluorine source of LiF are the electrolyte LiPF ₆ of the electrolytic solution. That is, the electrolyte LiPF ₆ is decomposed as shown in the following formula (1) to generate LiF.

LiPF ₆ → LiF + PF ₅ (1)
As shown in FIGS. 5 and 7, the integrated intensity ratio of the peak caused by the 1s orbit of fluorine to the peak caused by the 2p orbit of silicon on the negative electrode active material particle surfaces of Example 1 and Comparative Example 1 was determined. As shown in Table 1, the integrated intensity ratio of Example 1 was 44, and the integrated intensity ratio of Comparative Example 1 was 77. The negative electrode active material particles of Example 1 were found to have a thinner fluorine-containing film than Comparative Example 1 because the relative strength of the 1s orbit of fluorine was lower than that of Comparative Example 1.

  As described above, the coated particles of the negative electrode of Example 1 had better battery cycle identification than Comparative Example 1. The reason for this is that the temperature at the conditioning process (during initial charge / discharge) is as high as 45 ° C., so a thin and stable film is formed on the surface of the core, and the stress on the outermost surface of the film due to expansion / contraction of the core This is thought to be because the cracks and defects were hardly generated on the outermost surface of the coating.

1: Core part, 2: Coating, 3: Covered particle.

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 capable of occluding and releasing lithium ions and made of silicon or / and a silicon compound, a separator, and an electrolytic solution. A method for producing a lithium ion secondary battery, wherein the battery body is initially charged and discharged under a temperature condition of 35 to 80 ° C.   The method for producing a lithium ion secondary battery according to claim 1, wherein the battery body is initially charged and discharged under a temperature condition of 40 to 60 ° C.   The method for manufacturing a lithium ion secondary battery according to claim 1, wherein the electrolytic solution contains a fluoride salt.   The method for producing a lithium ion secondary battery according to claim 1, wherein the negative electrode is formed by binding the negative electrode active material with polyamideimide.

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