WO2021192453A1

WO2021192453A1 - Negative electrode material for lithium ion secondary batteries

Info

Publication number: WO2021192453A1
Application number: PCT/JP2020/046857
Authority: WO
Inventors: 杉政　昌俊; 栄二關; 耕平本蔵; 渉太伊藤
Original assignee: 株式会社日立製作所
Priority date: 2020-03-24
Filing date: 2020-12-16
Publication date: 2021-09-30
Also published as: JPWO2021192453A1

Abstract

The purpose of this disclosure is to provide a negative electrode material that achieves a high capacity retention rate and can suppress resistance increases.　The present embodiment pertains to a negative electrode material for lithium ion secondary batteries that comprises an active substance, a binder, and an SEI film formed on the periphery of the active substance and binder, the negative electrode material being characterized in that the SEI film comprises lithium oxide (Li2O) and lithium fluoride (LiF); the XPS spectrum for oxygen 1s measured from the surface of the SEI film to a depth of 100 nm has a peak representing the double bond of C=O in the vicinity of 533 eV, a peak representing lithium oxide (Li2O) in the vicinity of 530 eV, a peak representing lithium fluoride (LiF) in the vicinity of 57 eV, and a peak representing the double bond of C=O in the vicinity of 56 eV; and the C-K edge spectrum of the SEI film by STEM-EELS has a broad peak over 285-310 eV.

Description

Negative electrode material for lithium-ion secondary batteries

The present disclosure relates to a negative electrode material for a lithium ion secondary battery.

Lithium-ion secondary batteries are one of the non-aqueous electrolyte secondary batteries, and because of their high energy density, they are also used as batteries for portable devices and electric vehicles. In a lithium ion secondary battery, a carbon material such as lithium metal oxide is generally used as the positive electrode active material and graphite is generally used as the negative electrode active material. The positive electrode and the negative electrode of a lithium ion secondary battery are formed, for example, by adding a binder, a conductive agent, or the like to minute active material particles to form a slurry, and then applying the mixture to a metal foil.

In a lithium ion secondary battery, the lithium ions released from the positive electrode active material are stored in the negative electrode active material during charging, and the lithium ions stored in the negative electrode active material are released and stored in the positive electrode active material during discharging. As the lithium ions move between the electrodes in this way, a current flows between the electrodes.

In such a lithium ion secondary battery, the capacity is reduced by the electrical isolation of the positive electrode active material, the electrical isolation of the negative electrode active material, and the immobilization of lithium ions passing between the electrodes. Among these factors, it is considered that one of the factors regarding the capacity reduction due to the immobilization of lithium ions passing between the electrodes is that lithium ions are taken into the SEI film as the SEI (Solid Electrolyte Interphase) film grows. Has been done. On the other hand, the SEI film is indispensable for stable charging and discharging of the lithium ion secondary battery.

As a method for forming an SEI film, for example, Patent Document 1 discloses a method for adding an additive for forming an SEI to an electrolytic solution. Further, Patent Document 2 discloses a method in which at least one storage place for a stabilizing additive capable of stabilizing the SEI layer is provided inside the battery, and when deterioration of SEI is detected, the stabilizing additive is released. There is.

Japanese Unexamined Patent Publication No. 2015-122236 Japanese Patent Publication No. 2014-517996

In Patent Document 1, an organic additive is added to an electrolytic solution and charging / discharging is repeated to form an SEI film, but the growth of the SEI film itself due to charging / discharging cannot be suppressed, so lithium ions are immobilized. In some cases, capacity deterioration due to the discharge could not be suppressed. The same can be said for Patent Document 2. Generally, in the conventional process, the capacity deterioration due to the immobilization of lithium ions cannot be suppressed, and the resistance tends to increase due to the thickening of the SEI film, so that there is a problem that the characteristics of the battery deteriorate. There is.

Therefore, an object of the present disclosure is to provide a negative electrode material capable of achieving a high capacity retention rate and suppressing an increase in resistance.

An example of the embodiment of this embodiment is as follows, for example.

(1) A negative electrode material for a lithium ion secondary battery, which comprises an active material, a binder, and an SEI film formed around them.
The SEI film _{contains lithium oxide (Li 2} O) and lithium fluoride (Li F).
The XPS spectrum of oxygen 1s measured from the surface of the SEI film to a depth of 100 nm shows a peak showing a C = O double bond near 533 eV and lithium oxide (Li ₂ O) near 530 eV. The peak has a peak showing lithium fluoride (LiF) near 57 eV and a peak showing a C = O double bond near 56 eV.
A negative electrode material characterized in that the spectrum at the CK end of the SEI film by STEM-EELS has a broad peak from 285 to 310 eV.
(2) The negative electrode material according to (1), which is used as a negative electrode mixture layer.
(3) The negative electrode material according to (1), which is used as a negative electrode active material.
(4) A negative electrode comprising the negative electrode material according to any one of (1) to (3) and a negative electrode current collector as the negative electrode mixture layer.
This specification includes the disclosure of Japanese Patent Application No. 2020-052858, which is the basis of the priority of the present application.

According to the present disclosure, it is possible to provide a negative electrode material capable of achieving a high capacity retention rate and suppressing an increase in resistance.

It is a schematic cross-sectional view for demonstrating the cell structure of a lithium ion secondary battery. It is a schematic cross-sectional view for demonstrating the cell structure for manufacturing the negative electrode material which concerns on this embodiment. It is an XPS spectrum in the depth direction of the SEI film of the negative electrode obtained in Example 1. It is an XPS spectrum in the depth direction of the SEI film formed by charging and discharging the positive electrode and the negative electrode (the second positive electrode is not used) as a conventional product. It is an XPS spectrum of oxygen 1s of the SEI film of the negative electrode obtained in Example 1. It is an XPS spectrum of lithium 1s of the SEI film of the negative electrode obtained in Example 1. It is a spectrum of the CK end by STEM-EELS of the SEI film of the negative electrode obtained in Example 1. It is a spectrum of the CK end by STEM-EELS of the SEI film formed by charging and discharging the positive electrode and the negative electrode (the second positive electrode is not used) as a conventional product.

Hereinafter, the present embodiment will be described with reference to the drawings.

[Structure of lithium-ion secondary battery]
FIG. 1 is a schematic cross-sectional view showing the internal structure of the lithium ion secondary battery according to the present embodiment, but the lithium ion secondary battery is not limited to this structure, and is, for example, a laminated type. You may.

As shown in FIG. 1, the lithium ion secondary battery 1 has a positive electrode 10 and a negative electrode 12 that directly contribute to an electrochemical reaction, and in the lithium ion secondary battery 1, the positive electrode 10 and the negative electrode 12 face each other. It is arranged to do. A separator 11 is provided between the positive electrode 10 and the negative electrode 12. Further, the lithium ion secondary battery 1 contains a non-aqueous electrolytic solution for enabling the movement of Li between the positive electrode 10 and the negative electrode 12. The non-aqueous electrolyte solution contains at least a supporting electrolyte, a non-aqueous solvent, and a boroxine compound of the formula (1), which will be described in detail later. In the lithium ion secondary battery 1 of FIG. 1, the positive electrode 10, the negative electrode 12, and the separator 11 are impregnated with a non-aqueous electrolytic solution.

Regarding the specific structure, the lithium ion secondary battery 1 shown in FIG. 1 has a positive electrode 10, a separator 11, a negative electrode 12, a battery container 13, a positive electrode current collecting tab 14, a negative electrode current collecting tab 15, an inner lid 16, and an internal pressure release valve. It is composed of 17, a gasket 18, a positive temperature coefficient (PTC) resistance element 19, a battery lid 20, and a shaft center 21. The battery lid 20 is a component composed of an inner lid 16, an internal pressure release valve 17, a gasket 18, and a positive temperature coefficient resistance element 19. Further, a positive electrode 10, a separator 11 and a negative electrode 12 are wound around the axis 21.

In the electrode group in which the separator 11 is inserted between the positive electrode 10 and the negative electrode 12 and wound around the axial center 21, the axial center 21 is particularly limited as long as it can support the positive electrode 10, the separator 11 and the negative electrode 12. Instead, for example, any known axis can be used. In this embodiment, the electrode group is formed in a cylindrical shape. The shape of the battery container 13 is formed in a cylindrical shape according to the shape of the electrode group.

The material of the battery container 13 is not particularly limited, and a material having corrosion resistance to the electrolytic solution can be preferably used. Examples of the material having corrosion resistance to the electrolytic solution include aluminum, stainless steel, nickel-plated steel and the like.

The electrode group is housed in the battery container 13, the negative electrode current collecting tab 15 is connected to the inner wall of the battery container 13, and the positive electrode current collecting tab 14 is connected to the bottom surface of the battery lid 20. The electrolytic solution is injected into the battery container 13 before sealing the battery. Examples of the method for injecting the electrolytic solution include a method of directly adding the electrolytic solution to the electrode group with the battery cover 20 open, a method of adding the electrolytic solution from the injection port installed in the battery cover 20, and the like.

After that, the battery lid 20 is brought into close contact with the battery container 13 to seal the entire battery. If there is an electrolyte inlet, seal it as well. The battery can be sealed by using a known technique such as welding or caulking.

A solid electrolyte may be used instead of the non-aqueous electrolyte and the separator. The solid electrolyte is not particularly limited, and examples thereof include ionic conductive polymers such as polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride, polymethyl methacrylate, polyhexafluoropropylene, and polyethylene oxide. When these solid polymer electrolytes are used, the non-aqueous electrolyte solution and the separator can be omitted.

[Negative electrode material]
The negative electrode material according to the present embodiment relates to a material used for the negative electrode 12 of a lithium ion secondary battery. The negative electrode is generally configured to include a negative electrode mixture layer containing a negative electrode active material and a binder, and a negative electrode current collector.

Here, the form of the negative electrode material according to the present embodiment is not particularly limited as long as it is a material used for the negative electrode, and may be, for example, a negative electrode mixture layer or a negative electrode active material. good.

When the form of the negative electrode material according to the present embodiment is a negative electrode mixture layer, the negative electrode according to the present embodiment includes the negative electrode material according to the present embodiment as the negative electrode mixture layer and the negative electrode current collector.

When the form of the negative electrode material according to the present embodiment is a negative electrode active material, the negative electrode according to the present embodiment is a binder contained in the negative electrode material according to the present embodiment as the negative electrode active material and the negative electrode material according to the present embodiment. It includes a negative electrode mixture layer containing another binder and a negative electrode current collector. That is, the negative electrode material according to the present embodiment is contained in the negative electrode mixture layer as a substance capable of reversibly occluding and releasing lithium ions (that is, a negative electrode active material). The binder contained in the negative electrode material and the additional binder contained in the negative electrode mixture layer may be of the same type or different types.

The negative electrode material according to the present embodiment includes a negative electrode active material, a binder, and an SEI film formed around them. Further, in the negative electrode material according to the present embodiment, the SEI film contains lithium oxide (Li ₂ O) and lithium fluoride (Li F). In the negative electrode material according to the present embodiment, the XPS spectrum of oxygen 1s measured from the surface of the SEI film to a depth of 100 nm oxidizes a peak showing a C = O double bond near 533 eV around 530 eV. It has a peak showing lithium (Li ₂ O), a peak showing lithium fluoride (LiF) near 57 eV, and a peak showing a double bond of C = O near 56 eV. Further, in the negative electrode material according to the present embodiment, the spectrum of the CK end of the SEI film by STEM-EELS has a broad peak from 285 to 310 eV. As for the negative electrode material having such characteristics, the negative electrode material according to the present embodiment has a novel structure and is not known in the past. Further, it has been confirmed in this embodiment that a lithium ion secondary battery capable of achieving a high capacity retention rate and suppressing an increase in resistance can be obtained by using the negative electrode material according to the present embodiment.

[Manufacturing method of negative electrode material]
The negative electrode material according to this embodiment can be produced by a novel method described below. In the following description, the negative electrode material in the form of the negative electrode mixture layer will be given as a main example, but the present embodiment is not limited to this form.

FIG. 2 is a schematic cross-sectional view showing an electrode configuration of a cell that can be used in the production of the negative electrode material according to the present embodiment. By using the cell shown in FIG. 2, the SEI film according to the present embodiment can be formed.

In FIG. 2, the first positive electrode 111a and the negative electrode 112 are arranged with the first separator 113a interposed therebetween, and the second positive electrode 111b and the negative electrode 112 are arranged with the second separator 113b interposed therebetween. There is. A non-aqueous electrolyte solution is injected into this cell. The negative electrode material according to the present embodiment uses such a cell to inject lithium ions (first charge) from the first positive electrode 111a to the negative electrode 112, and then from the second positive electrode 111b to the negative electrode. It can be produced by injecting lithium ions into 112 (second charging).

The first charge is preferably fully charged. The injection amount of lithium ions in the second charge varies depending on the design of the amount of the negative electrode active material to be produced, but is, for example, 10 to 250 mAh for the negative electrode active material having a capacity of 460 mAh, that is, the entire design value of the active material capacity. It is preferably 2 to 55% of the above. The amount of lithium ions injected in the second charge is preferably 46 mAh or more, that is, 10% or more of the total design value, from the viewpoint of uniformly forming the SEI film. The amount of lithium ions injected in the second charge is preferably 200 mAh or less, that is, 43% or less, from the viewpoint of preventing lithium precipitation due to overcharging. The rate in the second charge is preferably 1/10 C to 1 / 200C from the viewpoint of uniformly forming the SEI film and / or preventing the precipitation of lithium. The first charge and the second charge are each performed at least once, and may be performed a plurality of times.

The negative electrode 112 is configured to include a negative electrode mixture layer containing a negative electrode active material capable of reversibly occluding and releasing lithium ions and a binder for binding the negative electrode active material, and a negative electrode current collector.

The negative electrode active material is not particularly limited, but is, for example, natural graphite, composite carbonaceous material, artificial graphite, silicon (Si), natural or artificial graphite mixed with silicon, graphitized carbon material, or titanium acid. Examples thereof include lithium Li ₄ Ti ₅ O _12. Examples of the composite carbonaceous material include those in which a film is formed on natural graphite by a dry CVD method or a wet spray method. Examples of artificial graphite include those produced by firing using a resin material such as epoxy or phenol or a pitch-based material obtained from petroleum or coal as a raw material. As the negative electrode active material, one type may be used alone, or two or more types may be used in combination.

The binder is not particularly limited, and as the binder, for example, either an aqueous binder that dissolves, swells, or disperses in water, or an organic binder that does not dissolve, swell, or disperse in water can be used. .. Specific examples of the aqueous binder include styrene-butadiene rubber, acrylic polymers, polymers having a cyano group, and copolymers thereof. Specific examples of the organic binder include polyvinylidene fluoride (PVDF), polytetrafluoroethylene, and copolymers thereof. One type of binder may be used alone, or two or more types may be used in combination. Further, a thickening binder such as carboxymethyl cellulose may be used in combination.

As the negative electrode current collector, for example, a material such as a metal foil made of copper or a copper alloy containing copper as a main component, a metal plate, a foamed metal plate, an expanded metal, or a punching metal can be appropriately used.

The negative electrode 112 is manufactured, for example, by mixing a negative electrode active material and a binder together with an appropriate solvent to obtain a negative electrode mixture, applying this negative electrode mixture to a negative electrode current collector, and then drying and compression molding. Can be done. As a method for applying the negative electrode mixture, for example, a doctor blade method, a dipping method, a spray method, or the like can be used.

Separators (first separator and second separator) are provided to prevent a short circuit from occurring due to direct contact between the positive electrode (including the first positive electrode and the second positive electrode) and the negative electrode 112. The separator is not particularly limited, but for example, a microporous film such as polyethylene, polypropylene, or aramid resin, or a film in which the surface of such a microporous film is coated with a heat-resistant substance such as alumina particles is used. Can be used.

The non-aqueous electrolytic solution to be injected into the cell is not particularly limited, and for example, a general-purpose product used in a commercially available lithium ion secondary battery can be used. The non-aqueous electrolyte solution includes, for example, a non-aqueous solvent and a supporting electrolyte. Examples of the non-aqueous solvent include an aprotic organic solvent. Examples of the aproton organic solvent include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and methylpropyl. Examples include carbonate (MPC), ethylpropyl carbonate (EPC), or mixtures thereof. As the aprotic organic solvent, one type may be used alone, or two or more types may be used in combination. Examples of the supporting electrolyte include lithium salts. Examples of the lithium salt include lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium iodide, lithium chloride, lithium bromide, LiB [OCOCF ₃ ] ₄ , LiB [OCOCF ₂ CF _3]. ] ₄ , LiPF ₄ (CF ₃ ) ₂ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ CF ₂ CF ₃ ) ₂ , or a mixture thereof. As the supporting electrolyte, one type may be used alone, or two or more types may be used in combination. The lithium salt preferably contains a fluorine atom.

The positive electrode 10 includes, for example, a positive electrode mixture layer containing a positive electrode active material, a conductive material, and a binder, and a positive electrode current collector in which the positive electrode mixture layer is coated on one or both sides. The

positive electrodes

111a and 111b are not particularly limited as long as lithium ions can be injected into the negative electrode 112. The type of positive electrode active material is not particularly limited. From the viewpoint of cost reduction and simplification of the manufacturing process, it is preferable that the structures of the first positive electrode 111a and the second positive electrode 111b are the same, and the positive electrode active material and the binder used are also of the same type. Is preferable.

As the positive electrode active material, for example, lithium cobalt oxide, lithium manganese substituted cobaltate, lithium manganate, lithium nickel acid, transition metal phosphate lithium such as olivine-type lithium iron _{_{_{_{phosphate, Li w Ni x Co y Mn}}}} z O 2 (Here, w, x, y, z are 0 or positive values). As the positive electrode active material, one type may be used alone, or two or more types may be used in combination.

As the conductive agent, for example, carbon particles such as graphite, carbon black, acetylene black, ketjen black, and channel black, carbon fibers, and the like can be used. These conductive agents may be used alone or in combination of two or more.

The binder is not particularly limited, but is, for example, polyvinylidene fluoride, styrene / butadiene rubber, carboxyl / methylcellulose, cellulose acetate, ethylcellulose, fluororubber, ethylene / propylene rubber, polyacrylic acid, polyimide, polyamide, or A mixture of these can be mentioned. One type of binder may be used alone, or two or more types may be used in combination. Further, a thickening binder such as carboxymethyl cellulose may be used in combination.

As the positive electrode current collector, for example, an appropriate material such as a metal foil made of aluminum, stainless steel, titanium or the like, a metal plate, a foamed metal plate, an expanded metal, a punching metal or the like can be used.

The positive electrode is formed by, for example, mixing a positive electrode active material, a conductive agent, and a binder together with an appropriate solvent to form a positive electrode mixture, applying the positive electrode mixture to a positive electrode current collector, and then drying and compression molding. Can be made. As a method for applying the positive electrode mixture, for example, a doctor blade method, a dipping method, a spray method, or the like can be used. Further, as a method for compression molding the positive electrode mixture, for example, a roll press or the like can be used. The solvent may be any solvent as long as it can dissolve the binder, and for example, 1-methyl-2-pyrrolidone, water, or the like can be used, but the solvent is not limited thereto.

The step of forming the SEI film on the negative electrode 112 using the cell configuration of FIG. 2 will be specifically described below. The first positive electrode 111a and the second positive electrode 111b are manufactured so that their capacities are, for example, 300 mAh. The negative electrode 112 is manufactured so that its capacity is, for example, 460 mAh. The first positive electrode 111a and the second positive electrode 111b have the same configuration. Next, first, the first positive electrode 111a and the negative electrode 112 are connected to a power source, and lithium ions are injected from the first positive electrode 111a into the negative electrode 112. After the injection of lithium ions from the first positive electrode 111a is completed, the first positive electrode 111a is disconnected from the power source, and instead, the second positive electrode 111b is connected to the power source. Then, lithium ions are injected from the second positive electrode 111b into the negative electrode 112. In this example, the injection amount of lithium ions is preferably 10 to 250 mAh. Further, from the viewpoint of uniformly forming the SEI film, the injection amount of lithium ions is preferably 40 mAh or more. Further, from the viewpoint of suppressing the precipitation of lithium due to overcharging, the injection amount of lithium ions is preferably 200 mAh or less. Further, the injection of lithium ions from the second positive electrode 111b is preferably performed at a low rate of about 1 / 100C. After the injection of lithium ions is completed, discharge treatment and cleaning treatment can be performed as appropriate. Further, a SEI film showing the characteristics of the present embodiment is formed on the negative electrode mixture layer of the negative electrode obtained in the above steps, and by using this negative electrode, a high capacity retention rate is achieved and the resistance is increased. It is possible to manufacture a lithium ion secondary battery capable of suppressing the above. Alternatively, the negative electrode mixture layer may be peeled off from the negative electrode, crushed, and used as the negative electrode active material. The crushing method is not particularly limited, and for example, a conventionally known crushing means can be used.

Hereinafter, the present embodiment will be described with reference to examples, but the present embodiment is not limited to the following examples.

(Example 1)
[Method for manufacturing negative electrode]
Using the cell configuration shown in FIG. 2, a negative electrode was produced by the following steps. The first positive electrode 111a and the second positive electrode 111b were manufactured so that their capacities were 300 mAh, respectively. The first positive electrode 111a and the second positive electrode 111b have the same configuration. LiNi _0.5 Mn _0.2 Co _0.3 was used as the positive electrode active material, a carbon material was used as the positive electrode conductive agent, and PVDF was used as the positive electrode binder. The negative electrode 112 was manufactured so that its capacity was 460 mAh. Graphite was used as the negative electrode active material, carbon material was used as the negative electrode conductive agent, and SBR and CMC were used as the negative electrode binder.

First, the first positive electrode 111a and the negative electrode 112 were connected to a power source, and lithium ions were injected from the first positive electrode 111a into the negative electrode 112 (fully charged, rate: 0.5C). After the injection of lithium ions from the first positive electrode 111a was completed, the first positive electrode 111a was disconnected from the power source, and instead, the second positive electrode 111b was connected to the power source. Then, lithium ions were injected from the second positive electrode 111b into the negative electrode 112. In the second charging, the injection amount of lithium ions from the second positive electrode 111b was set to 200 mAh. In addition, the injection of lithium ions from the second positive electrode 111b was performed at a low rate of 1 / 100C. After the injection of lithium ions was completed, a discharge treatment was appropriately performed.

Through the above steps, a negative electrode having a negative electrode mixture layer on which an SEI film was formed was produced.

[Analysis by XPS]
FIG. 3A shows the XPS spectrum of the obtained negative electrode in the depth direction of the SEI film. The measured elements were fluorine, oxygen, carbon and lithium. From the XPS spectrum, it can be seen that the ratio of oxygen increases from the surface to a depth of about 25 nm to form a peak. Further, from the XPS spectrum, it can be seen that the ratio of lithium increases from the surface to a depth of about 50 nm to form a peak. FIG. 3B shows the measurement spectrum of the SEI film formed by charging and discharging the positive electrode and the negative electrode (the second positive electrode is not used) as a conventional product by XPS. It can be seen that oxygen and lithium are present near the surface of the SEI film. From this analysis result, it can be confirmed that the SEI film according to this example contains lithium and oxygen in the vicinity of the surface.

FIG. 4 is an XPS spectrum of oxygen 1s of the obtained negative electrode SEI film. The XPS spectrum was measured at a depth of 0, 24, 48 or 90 nm from the surface. In the XPS spectrum, it can be seen that the peak showing the double bond of C = O exists near 533 eV, and the peak showing _{Li 2 O exists near 530 eV.} The peak showing the double bond of C = O decreases sharply from the depth of 48 nm. On the other hand, the _{peak showing Li 2} O is also present on the surface, but increases at 24-48 nm. From this, it is considered that a large amount _{of Li 2} O is present in the vicinity of 24-48 nm. This is also consistent with the results of the oxygen and lithium XPS spectra of FIG. 3A.

FIG. 5 is an XPS spectrum of lithium 1s of the obtained negative electrode SEI film. The XPS spectrum was measured at a depth of 0, 24, 48 or 90 nm from the surface. In the XPS spectrum, it can be seen that the peak showing LiF exists near 57 eV and the peak showing the double bond of C = O exists near 56 eV. The peak showing LiF decreases sharply from 48 nm. On the other hand, a peak showing a C = O double bond is also present on the surface, but increases at 24-90 nm. From this, it is considered that there are many C = O double bonds in the vicinity of 24 to 90 nm.

[Analysis by STEM-EELS]
FIG. 6A is a spectrum of the CK end of the obtained negative electrode SEI film by STEM-EELS. There is one broad peak from 285 to 310 eV. On the other hand, as shown in FIG. 6B, there is a sharp edge around 287 to 290 eV, which is considered to exhibit _{Li 2} CO _3, which is characteristic of the SEI film formed by continuous charging and discharging of a general lithium ion secondary battery. There are no peaks.

From the measurement spectra of FIGS. 4 to 6, it can be seen that the SEI film of the obtained negative electrode mixture layer contains lithium oxide (Li _{2 O) and lithium fluoride (Li F).} Further, from the XPS spectra of FIGS. 4 and 5, in the XPS spectrum of oxygen 1s measured from the surface of the SEI film to a depth of 100 nm, there is a peak showing a double bond of C = O near 533 eV. There is a peak showing lithium oxide (Li ₂ O) near 530 eV, a peak showing lithium fluoride (LiF) near 57 eV in the XPS spectrum of lithium 1s, and a double C = O near 56 eV. It can be seen that there is a peak indicating binding. Further, it can be seen that a broad peak exists from 285 to 310 eV in the spectrum at the CK end by STEM-EELS in FIG. 6A.

(Example 2)
Hereinafter, a lithium ion secondary battery was prototyped using the negative electrode obtained in Example 1 and its performance was evaluated.

The positive electrode was prepared using LiNi _0.5 Mn _0.2 Co _0.3 as the positive electrode active material, a carbon material as the positive electrode conductive agent, and PVDF as the positive electrode binder. The capacity of the positive electrode was set to 300 mAh.

The produced lithium ion secondary battery was subjected to continuous charging / discharging of 4.2 to 3.0 V and 1 C for 50 cycles in an environment of 50 ° C., and the capacity retention rate and the DC resistance at the time of full charge were evaluated.

(Comparative Example 1)
The negative electrode was produced using graphite as the negative electrode active material, a carbon material as the negative electrode conductive agent, and SBR and CMC as the negative electrode binder. The capacity of the negative electrode was 460 mAh. A lithium ion secondary battery was produced and evaluated in the same manner as in Example 2 except that this negative electrode was used.

[Results and discussion]
The results of Example 2 and Comparative Example 1 are shown in Table 1. The lithium ion secondary battery of Example 2 showed a high capacity retention rate as compared with the lithium ion secondary battery of Comparative Example 1. Regarding the DC resistance, the lithium ion secondary battery of Comparative Example 1 had an increased resistance after the cycle test, whereas the lithium ion secondary battery of Example 1 had almost no change in resistance even after the cycle test. rice field. In the lithium ion secondary battery of Comparative Example 1, it is presumed that the capacity capable of immobilizing lithium decreased due to the growth of the SEI film during the charge / discharge cycle, and the DC resistance also increased due to the thickening of the SEI film. Will be done. On the other hand, in the lithium ion secondary battery of Example 2, since a stable SEI film was already formed on the surface, the SEI film did not grow due to the charge / discharge cycle, and as a result, the capacity decreased and the DC resistance increased. It is presumed that it was suppressed.

The upper limit value and / or the lower limit value of the numerical range described in the present specification can be arbitrarily combined to specify a preferable range. For example, an upper limit value and a lower limit value of the numerical range can be arbitrarily combined to specify a preferable range, an upper limit value of the numerical range can be arbitrarily combined to specify a preferable range, and a lower limit of the numerical range can be specified. A preferable range can be defined by arbitrarily combining the values.

The scope of claims following this stated disclosure is expressly incorporated herein by this stated disclosure, and each claim is independent as a separate embodiment. The present disclosure includes all independent claims replaced by their dependent claims. In addition, additional embodiments derived from the independent claims and subsequent dependent claims are also expressly incorporated herein by this description.

Those skilled in the art can use the above description to make the best use of this disclosure. The claims and embodiments disclosed herein are merely explanatory and exemplary and should be construed as not limiting the scope of the present disclosure in any way. With the help of the present disclosure, changes can be made to the details of the above embodiments without departing from the basic principles of the present disclosure. In other words, the various modifications and improvements of the embodiments specifically disclosed herein are within the scope of this disclosure.

1 Lithium ion secondary battery 10 Positive electrode 11 Separator 12 Negative electrode 13 Battery container 14 Positive electrode current collection tab 15 Negative electrode current collection tab 16 Inner lid 17 Internal pressure release valve 18 Gasket 19 Positive temperature coefficient resistance element 20 Battery lid 21 Axis center 111a First Positive electrode 111b Second positive electrode 112 Negative electrode 113a First separator 113b Second separator All publications, patents and patent applications cited herein are incorporated herein by reference in their entirety.

Claims

A negative electrode material for a lithium ion secondary battery, which comprises an active material, a binder, and an SEI film formed around them.
The SEI film contains lithium oxide (Li 2 O) and lithium fluoride (Li F).
The XPS spectrum of oxygen 1s measured from the surface of the SEI film to a depth of 100 nm shows a peak showing a C = O double bond near 533 eV and lithium oxide (Li 2 O) near 530 eV. The peak has a peak showing lithium fluoride (LiF) near 57 eV and a peak showing a C = O double bond near 56 eV.
A negative electrode material characterized in that the spectrum at the CK end of the SEI film by STEM-EELS has a broad peak from 285 to 310 eV.
The negative electrode material according to claim 1, which is used as a negative electrode mixture layer.
The negative electrode material according to claim 1, which is used as a negative electrode active material.
A negative electrode comprising the negative electrode material according to any one of claims 1 to 3 as a negative electrode mixture layer and a negative electrode current collector.