JP2023092762A - Negative electrode active material for non-aqueous secondary batteries - Google Patents

Abstract

To inhibit the reaction between a non-aqueous electrolyte and a graphite surface while promoting the reaction between lithium ions and the graphite surface in the use of a negative electrode active material composed of graphite.SOLUTION: A negative electrode active material for a non-aqueous secondary battery comprises graphite particles. Edge surfaces of graphite crystals exposed on the surface of the particles are doped with a hetero element. When a change in electric potential V of a graphite with respect to a discharge electric quantity Q of the graphite in a non-aqueous electrolyte is measured for the graphite before doping the hetero element to the edge surface, a curve of [rate of change in discharge quantity Q with respect to potential V (dQ/dV)] vs [potential V] has a peak in a section from 0.5 V to 0.7 V of the potential V.SELECTED DRAWING: Figure 1

Description

The present invention relates to a negative electrode active material for non-aqueous secondary batteries, and more particularly to a material made of graphite.

Patent Document 1 discloses a method for measuring properties of amorphous carbon. A change in the potential V of amorphous carbon with respect to the amount of discharged electricity Q of amorphous carbon in the non-aqueous electrolyte is measured. The rate of change (dQ/dV) of the amount of discharged electricity (Q) of amorphous carbon with respect to the potential (V) represents the characteristics of amorphous carbon. Amorphous carbon having a rate of change (dQ/dV) within a predetermined range is useful for providing a power storage element that has a relatively high initial capacity while suppressing an increase in hysteresis during charging and discharging.

Patent Document 2 discloses the use of chemically modified graphite as an active material for non-aqueous secondary batteries. In this method, a boron source, a carbon source, and a nitrogen source are reacted on the surface of graphite to form a compound represented by the general formula BC _x N (wherein x satisfies 2≤x≤4) on the surface of graphite. to cover. A boron halide is used as the boron source, and acetonitrile is used as the carbon and nitrogen sources.

Non-Patent Document 1 discloses nitrogen-doped graphite. The edge faces of the graphite have pyridine-type nitrogen.

WO2019/103019 JP 2014-216283 A

Riku Shibuya, "Local Reactivity and Catalytic Activity of Graphitic Carbon Surface", [online], 2018, University of Tsukuba, Doctoral Dissertation, [Searched October 11, 2021], Internet<URL: https: //doi.org/10.15068/00152351> Toray Research Center, Inc., "Kinetic Analysis of Thermal Decomposition Reaction by TG-MS", [online], Case Studies, 2021-03-04, No.0466, [Searched on October 15, 2021], Internet <URL: https://cs2.toray.co.jp/news/trc/news_rd01.nsf/0/506210B45A92A06C4925867F00049223?open>

The present invention provides a negative electrode active material for a non-aqueous secondary battery comprising graphite. An object of the present invention is to suppress the reaction between the non-aqueous electrolyte and the graphite surface while promoting the reaction between lithium ions and the graphite surface.

[1] A negative electrode active material made of graphite particles,
Edge surfaces of graphite crystals exposed on the surface of the particles are doped with a hetero element,
Regarding the graphite before doping the hetero element on the edge surface, when the change in the electric potential V of the graphite with respect to the discharged electric quantity Q of the graphite in the non-aqueous electrolyte solution was measured, [change in the discharged electric quantity Q with respect to the electric potential V the curve of ratio (dQ/dV)] vs. [potential V] has a peak in the interval of potential V from 0.5 V to 0.7 V;
Negative electrode active material for non-aqueous secondary batteries.
[2] The area of the peak in the section is 3 mAh/g or more, provided that the baseline is the value of dQ/dV at potential V = 0.5 V, and at potential V = 0.7 V A straight line passing through (V, dQ/dV) = (0.5, a) and (V, dQ/dV) = (0.7, b), where b is the value of dQ/dV of
The negative electrode active material according to [1].
[3] the hetero element is at least one of boron, nitrogen, phosphorus and oxygen;
The negative electrode active material according to [1] or [2].
[4] the heteroatom is nitrogen,
The negative electrode active material according to [1] or [2].
[5] Used so that the edge surface doped with the hetero element is in direct contact with the non-aqueous electrolytic solution,
The negative electrode active material according to any one of [1] to [4].
[6] A negative electrode plate having the negative electrode active material according to any one of [1] to [5], a positive electrode plate, and a non-aqueous electrolytic solution,
The edge surface doped with the hetero element is exposed in a non-aqueous electrolytic solution,
Lithium-ion secondary battery.
[7] A method for producing a negative electrode active material made of graphite particles, comprising:
A step of heating the graphite mixed with the organic compound containing the hetero element in an inert atmosphere,
Regarding the graphite, when the change in the electric potential V of the graphite with respect to the discharged electric quantity Q of the graphite in the non-aqueous electrolytic solution is measured, [change rate of the discharged electric quantity Q with respect to the electric potential V (dQ/dV)] vs. [potential V] has a peak in the interval of 0.5 V to 0.7 V of potential V,
Method.

The negative electrode active material according to the present invention suppresses the reaction between the non-aqueous electrolyte and the graphite surface while promoting the reaction between lithium ions and the graphite surface.

Cross section of a particle of graphite Change in [potential V] with respect to [discharge quantity Q] Change in [rate of change in discharge quantity Q with respect to potential V (dQ/dV)] with respect to [potential V] Explanatory diagram of peak area Hetero-element doping model formula Hypothetical graph showing change in lifetime with and without doping Flow chart of doping process for graphite Graph showing the difference in reaction resistance with and without doping Graph showing difference in battery life with and without doping

<Overview>

FIG. 1 shows a cross section of graphite particles used as a negative electrode active material. The graphite particles consist of graphite crystals. The crystal has edge faces and basal faces. In one embodiment shown in the figure, the graphite particles are aggregates of graphite crystals. In one aspect, the surface of the graphite particles has a mosaic of portions where edge surfaces are exposed and portions where basal surfaces are exposed. Edge surfaces of the graphite are preferably not coated with amorphous carbon or other carbon.

In this embodiment, the reaction between the lithium ions and the graphite particles is promoted by increasing the exposed edge surfaces per bulk volume of the graphite particles. Further, by doping the exposed edge surface with a hetero element, the reaction between the non-aqueous electrolyte and the graphite surface is suppressed.

<Graphite with exposed edges>

Graphite with many exposed edge faces will be described. As shown in FIG. 1, the edges of the graphite crystals exposed on the surface of the graphite particles promote the reaction between the lithium ions and the graphite particles, while also reacting well with the non-aqueous electrolyte. do. The characteristics of graphite with such exposed edge surfaces are evaluated based on the curve of the change in potential V with respect to the amount of discharge electricity Q. FIG.

First, an evaluation cell including a negative electrode plate having graphite as a negative electrode active material, a positive electrode plate, and a non-aqueous electrolytic solution is produced. Graphite is before doping the hetero element on the edge surface. A hetero element is an element other than carbon. Next, the potential V [V vs. Li/Li ⁺ ] of the negative electrode active material and the discharge quantity of electricity Q [mAh g ⁻¹ ] are measured in the evaluation cell. The amount of discharged electricity Q is the change in the electric capacity of graphite per gram of graphite.

FIG. 2 is a charge/discharge curve showing changes in the potential V with respect to the quantity Q of discharged electricity. A plateau appears in the section where the potential V is from 0.5V to 0.7V as indicated by the dashed line. This plateau is characteristic of graphite with many exposed edges. Graphite with exposed edge surfaces reacts well with both lithium and non-aqueous electrolyte as described above. Next, from this curve, the change rate dQ/dV [mAh g ^-1 V ^-1 ] of the discharged quantity of electricity Q with respect to the potential V is obtained.

FIG. 3 is a curve showing changes in the rate of change dQ/dV of the discharged quantity of electricity Q with respect to the potential V. In FIG. Note that the sign of dQ/dV is reversed in the graph. A peak appears in the section where the potential V is from 0.5V to 0.7V. This peak corresponds to the plateau shown in FIG. This peak is characteristic of graphite with many exposed edges. Graphite with exposed edge surfaces reacts well with both lithium and non-aqueous electrolyte as described above. Area (A) and area (B) will be described with reference to FIG.

FIG. 4 is an explanatory diagram of the peak area in the curve showing the change in the rate of change dQ/dV of the discharged quantity of electricity Q with respect to the potential V. FIG. The sum of area (A) and area (B) represents the amount of discharged electricity Q in the section where the potential V is from 0.5V to 0.7V. Area (A) in the figure represents the discharge quantity of electricity in a range in which dQ/dV is larger than the baseline BL in the discharge quantity of electricity Q. FIG. That is, area (A) represents the "peak area" in the section where the potential V is from 0.5V to 0.7V. Area (B) in the figure represents the amount of discharged electricity in a range where dQ/dV is smaller than the baseline BL.

The baseline BL shown in FIG. 4 is represented by a straight line passing through (V, dQ/dV)=(0.5, a) and (V, dQ/dV)=(0.7, b). where a is the value of dQ/dV when potential V=0.5V. b is the value of dQ/dV at potential V=0.7V.

In FIG. 4, graphite having an area (A) size of 1.0 [mAh/g] or more, preferably 3.0 [mAh/g] or more is defined as "graphite with a large exposed edge surface".

In another embodiment of graphite with a large exposed edge surface, in which a peak appears in a section where the potential V is 0.5 V to 0.7 V, the graphite with a large exposed edge surface has a rate of change in the amount of discharged electricity Q with respect to the potential V It is defined as follows based on the curve showing the change in dQ/dV. That is, in graphite with many exposed edge surfaces, the sum of area (A) and area (B) in the section where the potential V is 0.5 V to 0.7 V is 6.5 [mAh/g] or more, preferably 8.5 [mAh/g]. That's it.

In the example shown in FIG. 3, the size of area (A) is 3.0 [mAh/g]. The size of area (B) is 5.5 [mAh/g]. The sum of these is 8.5 [mAh/g].

Graphite with many exposed edge surfaces can be selected based on the measurement results of the graphite particles by Raman spectroscopy and the specific surface area of the graphite particles. The method for sorting out graphite with many exposed edge surfaces is not limited to these. Graphite with exposed edge surfaces can be produced by spheroidizing flake graphite and increasing the number of rotations of a mill during granulation. Also, by increasing the oxygen partial pressure when firing graphite, the exposure of the edge plane of the graphite surface can be increased. Also, by irradiating the graphite with microwaves in an air atmosphere, the exposure of the edge surface of the graphite surface can be increased. The method for producing graphite with many exposed edge surfaces is not limited to these.

<Dope to exposed edge surface>

FIG. 5 shows a model formula for hetero element doping on the edge surface shown in FIG. With reference to this figure, the reason why the edges of graphite crystals exposed on the surfaces of graphite particles are likely to decompose the non-aqueous electrolytic solution and form the SEI film will be explained. Furthermore, it will be explained that these effects can be reduced by doping the edge surface with a hetero element.

Carbon atoms C with high reactivity as shown in FIG. 5 are exposed on the edge surface of the graphite crystal. Hereinafter, the terminal carbon atom C exposed on the edge surface may be referred to as a highly reactive site.

During charging of a secondary battery having graphite as a negative electrode active material, the carbon atoms C are likely to react with the non-aqueous electrolyte. The carbon atoms C decompose the non-aqueous electrolyte. A product generated by decomposition of the non-aqueous electrolyte becomes an SEI (Solid Electrolyte Interphase) film covering the edge surface. A significant increase in the SEI film impairs the reaction between graphite and lithium ions, thus shortening the life of the lithium ion secondary battery.

A heteroelement source is introduced by targeting the carbon atom C shown in FIG. This protects the highly reactive site with a heteroatom. In other words, the highly reactive sites are capped and inactivated. In one embodiment, heteroatom X is bonded to carbon atom C. In another embodiment the heteroatom X replaces the carbon atom C. In one aspect, the heteroatom is at least one of boron, nitrogen, phosphorus and oxygen. As an example, when a nitrogen atom N is selected as the heteroatom X and the carbon atom C is replaced with the nitrogen atom N, a pyridine-type (-N=) nitrogen is produced. The doping form of the hetero element X is not limited to this. Doping with the hetero element X may be performed by CVD (chemical vapor deposition).

As described with reference to FIGS. 1 to 5, by doping the edge surface with the hetero element, the reaction between the highly reactive site and the non-aqueous electrolytic solution, the so-called side reaction, is eliminated or significantly suppressed. Therefore, the decomposition of the non-aqueous electrolytic solution by the highly reactive sites is eliminated or significantly suppressed. In addition, the SEI film formed by the decomposition of the non-aqueous electrolytic solution does not shorten the life of the lithium ion secondary battery, or is significantly suppressed. On the other hand, the doped edge face reacts with lithium ions in the same way as the edge face before doping. In other words, only highly reactive points that cause side reactions on the edge surface are selectively protected. Therefore, heteroelements do not affect other battery elements. Therefore, a battery made of the graphite has a high output and, in return, does not lose its life.

In one embodiment, the edges of the doped graphite are not coated with amorphous carbon or other carbon. When producing a secondary battery, graphite is used so that the edge surface of the doped graphite is in direct contact with the non-aqueous electrolyte.

<Change in Life of Lithium-Ion Secondary Battery>

FIG. 6 shows a hypothetical model of change in life of a lithium ion secondary battery with or without doping. In the absence of edge-face doping, batteries made of graphite with more highly reactive sites on the edge face have a shorter lifetime than batteries made of graphite with less. This is because, as described above, the highly reactive sites on the edge surface decompose a large amount of the non-aqueous electrolytic solution.

As shown in FIG. 6, when the edge surface is doped, the life of a battery made of graphite with many highly reactive sites on the edge surface is not shorter than that of a battery made of graphite with a small number of highly reactive sites. This is because, as described above, the highly reactive sites on the edge surface are protected by the doped heteroelement. Even if graphite, which has few highly reactive sites on the edge surface, is doped with a hetero element, no further improvement in life can be expected. In other words, the effect of hetero element protection on graphite with many highly reactive sites is greater than the effect of hetero element protection on graphite with few highly reactive sites.

<Method of doping graphite>

FIG. 7 shows the flow of processes for doping graphite with a hetero element. The step of doping graphite with a hetero element will be described with reference to this figure. Graphite is spheroidized in step S01. The spheronization treatment increases the ratio of the edge surface to the entire surface of the graphite particles. As described above, for graphite, when the change in the potential V of graphite with respect to the amount of discharge electricity Q of graphite in a non-aqueous electrolyte is measured, [rate of change in the amount of discharge electricity Q with respect to potential V (dQ/dV)] vs. [potential V] has a peak in the interval of potential V from 0.5V to 0.7V.

At step S02 shown in FIG. 7, graphite is mixed with a compound containing a hetero element (for example, B, N or P). In one aspect the compound is an organic compound. In one aspect the organic compound is a nitrogen-containing compound. Moreover, a polyacrylonitrile is mentioned as a nitrogen-containing compound. The amount of compound should be sufficient to coat the entire surface area of the graphite particles.

Nitrogen-containing compounds other than polyacrylonitrile include, for example, nitrogen-containing cyclic compounds such as N-methylpyrrolidone, pyridine, melamine, and pyrrole; compounds having amino groups such as amino acids, urea, and aniline; and amides such as formamide, acetamide, and acetanilide. A compound having a bond and a compound having a cyano group such as acetonitrile can be mentioned.

In one example, the amount (g) sufficient to coat the entire surface area of the graphite particles is determined by the following formula.

W×{A/( ^πr2 ×6.02× ¹⁰²³ /M)}

π = [pi]
r = [molecular radius of compound containing heteroatom (m)]
M = [molecular weight of hetero element-containing compound (g/mol)]
A = [specific surface area of graphite (m ² /g)]
W = [input graphite amount (g)]

For example, when polyacrylonitrile is selected as a compound to be mixed with graphite, the amount of polyacrylonitrile added may be 10 parts by weight with respect to 100 parts by weight of graphite.

In step S03 shown in FIG. 7, the hetero-element-containing compound and graphite are heated in an inert atmosphere. Add about 50 to 100°C to the thermal decomposition temperature of the compound. It also allows sufficient time for the compound to thermally decompose. A sufficient time for pyrolysis may be estimated by kinetic analysis of pyrolysis reaction by TG-MS (Non-Patent Document 2).

In step S03 shown in FIG. 7, the liquefied or vaporized hetero-element-containing compound is brought into contact with the highly reactive sites of graphite. This replaces the carbon atoms in the highly reactive sites with heteroatoms. Or a hetero element is added to the carbon atom. Furthermore, the hetero-element-containing compound is removed from the mixture of graphite and the hetero-element-containing compound by thermally decomposing the remaining hetero-element-containing compound. As described above, graphite in which the highly reactive sites are protected by the hetero element is obtained.

In one example, a mixture of graphite and polyacrylonitrile is heated to 800° C. for 1 hour in an inert atmosphere. Polyacrylonitrile, which is liquefied by heating, contacts the highly reactive sites of graphite. Thus, the carbon atoms of the highly reactive sites are replaced with nitrogen atoms from the polyacrylonitrile, or nitrogen atoms are added to the carbon atoms, as shown in FIG. Further heating causes the remaining polyacrylonitrile to thermally decompose. As described above, graphite in which highly reactive sites are protected by nitrogen is obtained. Therefore, as explained with reference to FIG. 6, it is expected that the deterioration of life is suppressed by about 10% compared to the case where the highly reactive site is not protected by nitrogen.

<Production of secondary battery>

Using graphite doped with a hetero element as described above, a lithium ion secondary battery is produced according to a method known to those skilled in the art. A lithium-ion secondary battery is produced from the negative electrode plate having the negative electrode active material made of graphite, the positive electrode plate, and the non-aqueous electrolyte. Add separators and other materials as needed. With the secondary battery assembled, the edge surface doped with the hetero element is exposed to the non-aqueous electrolyte.

(1) Production of graphite particles with less exposed edge surfaces

Particles of natural graphite were prepared by a general process of spheronization. The average particle size was 10 µm. This graphite was used as graphite particles with "small edge surface exposure".

(2) Fabrication of graphite particles with exposed edge surfaces

50 g of raw material natural graphite particles were placed in a crucible. The average particle size of the natural graphite particles was 10 µm. The crucible was placed in a muffle furnace maintained at 800° C. in an air atmosphere and maintained for 30 minutes. After allowing the graphite particles to cool naturally, they were collected. This graphite was used as graphite particles with "extremely exposed edge surfaces".

(3) Nitrogen atom doping to edge planes 10 parts by weight of polyacrylonitrile (manufactured by sigma aldrich ) were added and mixed. The mixture placed in the crucible was placed in a batch-type firing furnace, heated to 750° C. in a nitrogen atmosphere, and held for 60 minutes to fire the graphite particles. The fired graphite particles were allowed to cool naturally until they reached room temperature. The graphite particles thus obtained were treated as "doped" graphite particles.

(4) Production of Lithium Ion Secondary Battery To 98 parts by weight of each graphite particle, 1.0 part by weight of CMC (carboxymethyl cellulose) powder was added as a thickening agent. They were mixed by stirring for 1 minute while they were in powder form. A paste was obtained by adding purified water to the graphite particles and kneading them for 2 minutes. At this time, the amount of purified water to be added was selected so that the ratio of the total of graphite particles and CMC as a solid content to the entire paste, hereinafter referred to as total solid content concentration, was 57 to 63% by weight. Purified water was further added to the hard-kneaded graphite particles, and the paste was further kneaded for 1 minute. The amount of purified water added was selected to reduce the total solids concentration to 55-50% by weight. An aqueous dispersion of SBR (styrene-butadiene rubber) as a binder was added to the paste and the paste was stirred for 1 minute. In this dispersion, SBR, which becomes a solid content, occupied 45% by weight. The amount of the dispersion liquid added was selected so that SBR, which becomes the solid content, occupied 1.0 part by weight with respect to 98 parts by weight of the graphite particles. The obtained paste was passed through a mesh with a diameter of 75 μm to remove coarse particles. As described above, a negative electrode mixture paste was obtained.

A negative electrode layer was formed on the copper foil by applying the negative electrode mixture paste to the copper foil. The amount of the negative electrode mixture paste applied was selected so that the amount of the mixture per unit area of the negative electrode layer was 3.7 to 4.0 mg/cm ² . A negative electrode layer with a density of 1.10 to 1.20 g/cm ³ was obtained by roll-pressing a copper foil. A negative electrode plate was obtained by cutting the copper foil into a size of 40 mm×50 mm.

A laminate type lithium ion secondary battery was produced by combining a separator, a positive electrode plate and an electrolytic solution with each of the produced negative electrode plates. A metal oxide LiNi _1/3 Co _1/3 Mn _1/3 O ₂ generally called NCM111 was commonly used as a positive electrode active material for the positive plate. As the electrolytic solution, 1.0 mol/L of LiPF ₆ was dissolved as an electrolyte in a mixed solvent consisting of ethylene carbonate, methyl ethyl carbonate and dimethyl carbonate, and this was commonly used.

FIG. 8 is a graph showing the difference in reaction resistance depending on whether the edge surface is exposed more or less and whether the edge surface is doped or not. The reaction resistance of the negative electrodes of the batteries fabricated using each of the four types of graphite particles was measured. Each measured value was converted to 100 as the measured value of undoped graphite particles with less exposed edge surfaces.

As shown in FIG. 8, in the absence of hetero-element doping, the surface of the graphite particles had more edge planes, so that the reaction resistance of the negative electrode made of the graphite particles was reduced. The reaction resistance of the negative electrode was also reduced when the hetero element was doped. Generally, a battery with a low reaction resistance has a high output. The test results also suggest that increasing the exposure of the edge surface promotes the reaction between the lithium ions and the graphite surface. In addition, the reaction resistance of the negative electrode was reduced by doping with the hetero element, regardless of whether the exposure of the edge surface was small or large.

FIG. 9 is a graph showing the difference in battery life with and without doping. We measured the capacity retention rate of batteries made using two types of graphite particles with many exposed edge surfaces. Each measured value was converted by setting the measured value of undoped graphite particles to 100. As shown in the graph, when the exposure of the edge surface was large, the doping of the hetero element increased the capacity retention rate of the battery. Generally, a battery with a high capacity retention rate has a long life. This test result also indicates that doping the edge surface with a hetero element suppresses the reaction between the non-aqueous electrolyte and the graphite surface.

S01-S03 Each step of doping method for graphite

Claims (7)

A negative electrode active material made of graphite particles,
Edge surfaces of graphite crystals exposed on the surface of the particles are doped with a hetero element,
Regarding the graphite before doping the hetero element on the edge surface, when the change in the electric potential V of the graphite with respect to the discharged electric quantity Q of the graphite in the non-aqueous electrolyte solution was measured, [change in the discharged electric quantity Q with respect to the electric potential V the curve of ratio (dQ/dV)] vs. [potential V] has a peak in the interval of potential V from 0.5 V to 0.7 V;
Negative electrode active material for non-aqueous secondary batteries. The area of the peak in the interval is 3 mAh/g or more, provided that the baseline is the value of dQ/dV at potential V = 0.5V and dQ/dV at potential V = 0.7V. When the value of dV is b, it is represented by a straight line passing through (V, dQ/dV) = (0.5, a) and (V, dQ/dV) = (0.7, b),
The negative electrode active material according to claim 1. the heteroatom is at least one of boron, nitrogen, phosphorus and oxygen;
The negative electrode active material according to claim 1 or 2. the heteroatom is nitrogen,
The negative electrode active material according to claim 1 or 2. The edge surface doped with the hetero element is used so as to be in direct contact with the non-aqueous electrolytic solution,
The negative electrode active material according to any one of claims 1 to 4. A negative electrode plate having the negative electrode active material according to any one of claims 1 to 5, a positive electrode plate, and a non-aqueous electrolytic solution,
The edge surface doped with the hetero element is exposed in a non-aqueous electrolytic solution,
Lithium-ion secondary battery. A method for producing a negative electrode active material made of graphite particles,
A step of heating graphite mixed with an organic compound containing a hetero element in an inert atmosphere,
Regarding the graphite, when the change in the electric potential V of the graphite with respect to the discharged electric quantity Q of the graphite in the non-aqueous electrolytic solution is measured, [change rate of the discharged electric quantity Q with respect to the electric potential V (dQ/dV)] vs. [potential V] has a peak in the interval of 0.5 V to 0.7 V of the potential V,
Method.

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