JP5585470B2 - Lithium ion secondary battery - Google Patents

Description

  The present invention relates to a lithium ion secondary battery, and more particularly to 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 composed of 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, and a positive electrode active material. 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 if it is a homogeneous solid silicon monoxide SiO in which the ratio of Si to O is approximately 1: 1, it is separated into two phases of Si phase and SiO ₂ phase by solid internal reaction. To do. 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.

  By the way, in order to improve the performance of the lithium ion secondary battery, various treatments have been proposed. For example, as disclosed in Patent Documents 1, 2, and 3, after the positive electrode, the negative electrode, the separator, and the electrolytic solution are accommodated in the battery container, the battery is sealed after being precharged at a predetermined temperature. As disclosed in Patent Documents 4 and 5, after the secondary battery is assembled, a charge / discharge treatment is performed at a predetermined temperature. As disclosed in Patent Documents 6 and 7, after assembling the secondary battery, the battery is stored for a predetermined period while a charging voltage is applied at a predetermined temperature.

JP 2000-58130 A JP 2003-308880 A JP 2010-80116 A JP 2000-149996 A JP 2004-296179 A JP 2010-118161 A JP-A-10-289733

  This inventor performed the process disclosed by said patent documents 1-7 using silicon oxide as a negative electrode active material. As a result, it was found that the cycle characteristics of the battery were improved. Moreover, the film was formed in the negative electrode active material surface. From this, it is considered that the improvement of the cycle characteristics of the battery is due to the negative electrode active material being covered with the coating to suppress the electrolytic solution decomposition. The inventor of the present application has further eagerly searched for the film formed by the above-described treatment, and further improved the cycle characteristics of the battery.

  The present invention has been made in view of such circumstances, and by controlling the thickness of the coating formed on the surface of the negative electrode active material, cracks and defects on the surface of the negative electrode active material particles are suppressed, and the battery has excellent cycle characteristics. Another object is to provide a lithium ion secondary battery.

(1) A lithium ion secondary battery according to the present invention includes a positive electrode, a negative electrode, an electrolytic solution, and a separator.
The positive electrode has a positive electrode active material capable of inserting and extracting lithium ions,
The negative electrode is capable of inserting and extracting lithium ions, covers a core part made of silicon or / and a silicon compound, covers the surface of the core part, and allows lithium ions to pass therethrough and contains lithium fluoride. Having negative electrode active material particles comprising a coating,
The electrolytic solution includes a fluoride salt,
In the spectrum measured by X-ray photoelectron spectroscopy (XPS) using AlKα (monochromatic) radiation emitted under the conditions of a voltage of 15 kV and a current of 10 mA, this is due to the 2p orbit of silicon (Si) on the surface of the negative electrode active material particles The integrated intensity ratio of the peak due to the 1s orbit of fluorine (F) with respect to the peak is 70 or less.

  According to the said structure, the negative electrode active material particle which comprises a negative electrode consists of a core part and the film which coat | covers the surface. The core portion is made of silicon or / and a silicon compound, and the coating contains lithium fluoride. Lithium fluoride contained in the coating is produced by the decomposition reaction of the fluoride salt contained in the electrolytic solution by contacting silicon constituting the core part of the negative electrode active material particles. The negative electrode active material particles are analyzed by X-ray photoelectron spectroscopy (hereinafter referred to as XPS). XPS is a method of irradiating negative electrode active material particles with X-rays and measuring the energy intensity of photoelectrons emitted from the negative electrode active material particles. By measuring the energy intensity of photoelectrons, an element located on the negative electrode active material surface can be identified.

  Further, when the surface of the negative electrode active material particles is irradiated with X-rays, the photoelectrons emitted from the surface of the negative electrode active material particles are limited to those emitted from an element from the surface of the negative electrode active material particles to a predetermined depth H. It is done. For this reason, it is possible to analyze elements existing from the surface of the negative electrode active material particles to a predetermined depth H.

  When the thickness of the coating formed on the surface of the negative electrode active material particles changes, the elements detected by XPS also change. 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 negative electrode active material particles 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 present invention defines the thickness of the film from the area ratio between the intensity caused by the photoelectrons generated from the substance in the film detected by XPS and the intensity caused by the photoelectrons generated from the substance in the core. I am going to do that.

  That is, in the present invention, 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 negative electrode active material particle is measured by XPS. The peak due to the 2P orbit of silicon appears in the bond energy region of 98 to 105 eV. A peak due to the 1s orbital of fluorine appears in a 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 fluorine to the peak caused by the 2p orbit of silicon on the surface of the negative electrode active material particles is 70 or less. In this case, silicon contained in the core portion exhibits a relatively strong spectral intensity. For this reason, the thickness of the coating is thin, and accordingly, the core part existing inside the coating is located within the predetermined depth H from the surface of the negative electrode active material particles or to the vicinity thereof. I understand.

  The core part expands and contracts by inserting and extracting Li ions. Since the coating is relatively thin when the core portion expands and contracts, the stress applied to the outer surface of the coating is reduced, and cracks and defects on the outer surface of the coating can be suppressed. Therefore, it is difficult for the core part to come into contact with the electrolytic solution, it is possible to suppress the elution of Li ions present in the core part, and to suppress the decomposition reaction of the electrolytic solution. Therefore, the cycle characteristics of the battery can be improved.

  On the other hand, when the integral intensity ratio is larger than 70, the coating surface expands and contracts, and the coating tends to crack or chip. The electrolyte enters the coating from the crack, reaches the core, contacts the silicon in the core, the electrolyte deteriorates, and Li ions occluded in the core are eluted, resulting in a battery cycle. There is a risk that the characteristics will deteriorate.

(2) the core part of the negative electrode active material particles are preferably composed of a Si phase and SiO ₂ phase.

In the above configuration, the Si phase is a phase made of silicon alone capable of inserting and extracting Li ions, and causes expansion and contraction as the Li ions are stored and released. Even if the Si phase expands and contracts, the volume change is absorbed by the SiO ₂ phase. Therefore, the expansion / contraction of the whole negative electrode active material particles can be suppressed, and the cycle characteristics of the battery can be further enhanced.

(3) The integrated intensity ratio is preferably 50 or less . In this case, the thickness of the coating covering the core portion is further reduced, and it is possible to more effectively suppress the occurrence of cracks and defects on the outer surface of the coating. Therefore, it is difficult for the core portion to come into contact with the electrolytic solution, the decomposition reaction of the electrolytic solution can be effectively suppressed, and the cycle characteristics of the battery can be further enhanced.

  According to the lithium ion secondary battery of the present invention, a coating containing lithium fluoride is formed on the surface of the negative electrode active material particles of the negative electrode, and is detected when the surface of the negative electrode active material particles is irradiated with X-rays by XPS. In the spectrum obtained, the integrated intensity ratio of the peak due to the 1s orbit of fluorine to the peak due to the 2p orbit of silicon on the surface of the negative electrode active material particles is 50 or less. For this reason, by forming the thickness of the coating appropriately, cracks and defects on the surface of the negative electrode active material particles can be suppressed, and the cycle characteristics of the battery can be improved.

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). 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. 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.

  The lithium ion secondary battery of this invention has a negative electrode, a positive electrode, electrolyte solution, and a separator.

(Negative electrode)
The negative electrode has negative electrode active material particles. 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 particles 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.

As shown in FIG. 1, the negative electrode active material particles 3 include a core portion 1 made of silicon or / and a silicon compound, and a coating 2 covering the core portion 1. The core unit 1 includes a Si phase composed of 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 core part is preferably 1 to 3. When the mass ratio is less than 1, the core portion is greatly expanded and contracted, and there is a possibility that a crack may occur in the negative electrode active material layer. On the other hand, when the mass ratio exceeds 3, the amount of insertion / extraction of Li in the core portion is small, and the electric capacity may be lowered.

The core part 1 of the negative electrode active material particles 3 may be composed of only the Si phase and the SiO ₂ phase. The core portion 1, while the main component and Si phase and SiO ₂ phase, and other, as a component of the core portion 1 of the anode active material particles 3, may contain known active materials. Specifically, at least one of Me _x Si _y O _z (Me is Li, Ca, etc.) may be mixed.

  The coating 2 of the negative electrode active material particles 3 is an insulating film through which Li ions can pass and contains lithium fluoride. Lithium fluoride is formed by the decomposition reaction of the fluorinated salt contained in the electrolytic solution in contact with the core portion. The coating 2 may contain, in addition to lithium fluoride, silicon or / and a silicon compound that is a component of the core portion 1, 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 of the negative electrode active material particles 3. This is because the core part 1 is brought into contact with the electrolytic solution to suppress decomposition of the electrolyte in the electrolytic solution, and the elution of Li ions occluded in the core part 1 is suppressed.

  The thickness of the coating 2 is adjusted, for example, according to the condition of the conditioning process as described later. The thickness of the coating 2 is measured by X-ray photoelectron spectroscopy (XPS). In XPS, negative electrode active material particles are 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 detectable depth of the negative electrode active material surface also changes. Therefore, in the present invention, a current of 15 kV and 10 mA is applied. The generation position of photoelectrons emitted from the surface of the negative electrode active material when a current of 15 kV and 10 mA is applied to the X-ray tube is said to have a depth H of about 5 to 50 nm from the outermost surface of the negative electrode active material.

  The integrated intensity ratio of the peak due to the 1s orbit of fluorine to the peak due to the 2p orbit of silicon on the surface of the negative electrode active material particles is 70 or less, 60 or less, preferably 50 or less. 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 mountain-shaped peak portion between the ends on both sides of the energy region of each peak corresponds to the superposition of the peak due to the silicon 2p orbit and the peak due to the fluorine 1s orbit. The area ratio of the peak portion is the integrated intensity ratio. When 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 negative electrode active material particle is large from the surface of the negative electrode active material, for example, when it exceeds 70, 60 or even 50 The thickness of the coating film containing silicon is too large, and the outermost surface of the coating film cannot follow the expansion / contraction of the core portion, so that stress concentrates and cracks and defects may occur. That is, the electrolytic solution may permeate into the core portion through cracks or defects on the outermost surface of the coating, or Li ions occluded in the core portion 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 negative electrode active material particles 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 portion 1, it is possible to suppress the silicon contained in the core portion from coming into contact with the electrolytic solution, to suppress the deterioration of the electrolytic solution, and to increase the battery. Cycle characteristics can be exhibited. Moreover, elution of Li ions occluded in the core portion can be suppressed, and a decrease in battery capacity can be suppressed.

  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 a conductive support material, a material generally used for an electrode of a lithium ion 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.

(Positive electrode)
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, and preferably further includes a binder and / or a conductive additive. 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 ₂ , 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.

(Separator)
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.

(Electrolyte)
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.

  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. Regardless of the shape, a separator is sandwiched between the positive electrode and the negative electrode to form an electrode body, and the space between the positive electrode current collector and the negative electrode current collector to the positive electrode terminal and the negative electrode terminal is used for current collection. After connecting using a lead or the like, this electrode body is sealed in a battery container together with a non-aqueous electrolyte to form a battery.

(Method for producing lithium ion secondary battery)
A method for producing negative electrode active material particles used for a negative electrode of a lithium ion secondary battery will be described. A raw material powder containing silicon monoxide may be used as a raw material for the core of the negative electrode active material particles. 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 covering the outer surface of the core portion with the coating, for example, a conditioning process for charging and discharging under predetermined conditions may be performed during or after the secondary battery is assembled. As a specific example of the conditioning treatment, first, after an electrode body composed of a negative electrode, a positive electrode, and a separator is accommodated in a battery container and an electrolytic solution is injected, charging / discharging is performed at a predetermined temperature, and then sealing is performed. Second, the electrode body and the electrolytic solution are housed in a battery container and sealed to charge and discharge at a predetermined temperature after the secondary battery is assembled. Among these, from the viewpoint of workability of charge / discharge, it is preferable to charge / discharge at a predetermined temperature after assembling the secondary battery. The temperature at the time of charging / discharging is preferably 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 charging / discharging is too low, the coating becomes thick, and cracks and defects may occur in the outermost surface portion of the coating due to expansion and contraction of the core portion of the negative electrode active material particles. 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 charge / discharge treatment is too high, the components of the electrolytic solution, particularly the solvent, may be altered, and the battery characteristics may be deteriorated.

  In addition, the assembly method of the whole lithium ion secondary battery can be performed by a well-known method.

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

First, a commercially available SiO powder was heat-treated at 900 ° C. for 2 hours in an inert gas atmosphere. Thereby, SiO powder was disproportionated and the negative electrode active material particle was obtained. When the XRD measurement using CuKα was performed on the negative electrode active material particles, a unique peak derived from simple silicon and silicon dioxide was confirmed. From this, it was found that elemental silicon and silicon dioxide were generated in the negative electrode active material particles.

  Next, the prepared negative electrode active material particles, graphite powder as a conductive additive, 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 conductive additive, and the binder was 48/37/15 as a percentage.

  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 laminated battery was conditioned. The conditioning treatment was repeated three times at 45 ° C. The first time, the charging condition was set to 0.2C, 4.1V CC-CV (constant current constant voltage) charging, and the discharging condition was set to 0.2C, 3V, cut-off CC discharge. 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.).

(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 during the conditioning treatment was 25 ° C.

<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. 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. The measurement results are shown in FIGS. FIG. 3 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. 4 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. 5 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. 3 and 4, 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. 4, 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. 4). 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. 3 and 5, the integrated intensity ratio of the peak due to the 1s orbit of fluorine to the peak due to 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.

<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. The discharge capacity retention rate for each cycle is shown in FIG. FIG. 6 shows the results of a cycle test performed on each of the two lithium ion secondary batteries of Example 1 and Comparative Example 1 (n = 2 times).

  As shown in FIG. 6, the lithium ion secondary battery of Example 1 had a higher discharge capacity maintenance rate than that of Comparative Example 1.

  As described above, the negative electrode active material particles of the negative electrode of Example 1 had a thinner coating film containing lithium fluoride (LiF) than that of Comparative Example 1, and the cycle specification of the battery was good. The reason is that the negative electrode active material particle coating of the negative electrode of Example 1 was thin, the stress on the outermost surface of the coating due to the expansion / contraction of the core portion was kept low, and cracks and defects were less likely to occur on the outermost surface of the coating. it is conceivable that.

1: Core part, 2: Coating, 3: Negative electrode active material particle

Claims (2)

A lithium ion secondary battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator,
During or after battery assembly, the battery is charged and discharged at a temperature of 35 ° C or higher and 80 ° C or lower.
The positive electrode has a positive electrode active material capable of inserting and extracting lithium ions,
The negative electrode is capable of inserting and extracting lithium ions, covers a core part made of silicon or / and a silicon compound, covers the surface of the core part, and allows lithium ions to pass therethrough and contains lithium fluoride. Having negative electrode active material particles comprising a coating,
The electrolytic solution includes a fluoride salt,
In a spectrum measured by X-ray photoelectron spectroscopy (XPS) using AlKα (monochromatic) radiation radiated under conditions of a voltage of 15 kV and a current of 10 mA, fluorine with respect to a peak due to the 2p orbit of silicon on the surface of the negative electrode active material particles the integrated intensity ratio of the peak due to the 1s orbital Ri der 10 or more 70 or less,
The lithium ion secondary battery , wherein the core part of the negative electrode active material particles is composed of a Si phase and a SiO ₂ phase . The lithium ion secondary battery according to claim 1 , wherein the integrated intensity ratio is 50 or less.

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