JP7375424B2 - Negative electrode active material and lithium ion secondary battery - Google Patents

Description

The present invention relates to a negative electrode active material and a lithium ion secondary battery, and more specifically to a novel negative electrode active material made of an oxide that has the ability to absorb and release Li ions, and a lithium ion secondary battery using the same as a negative electrode. Regarding batteries.

A lithium ion secondary battery is a secondary battery that charges and discharges by moving lithium ions between a positive electrode and a negative electrode. Most of the lithium ion secondary batteries currently in practical use use carbon (graphite) as the negative electrode material.
However, the specific capacitance per weight and volume of graphite is low at 372 mAh/g and 855 mAh/cm ³ , respectively, and the charge/discharge potential is also low at 0.07 to 0.23 V vs. Li ⁺ /Li (non- (See Patent Documents 1 to 3). Furthermore, in a battery system using a graphite negative electrode, since the charging and discharging potential of graphite is too low, there is a risk of lithium precipitation.

Recently, alloy negative electrodes have been attracting attention with the aim of further improving energy density. For example, silicon (Si) forms an intermetallic compound with Li and eventually occludes Li to an alloy composition of Li _4.4 Si. Most alloy negative electrodes are charged and discharged at a potential of 1V vs. Li ⁺ /Li or less (see Non-Patent Documents 4 and 5).
However, an alloy negative electrode made of silicon or the like undergoes a very large volume change of 100 to 350% during charging and discharging, and may become pulverized or crack or peel due to structural changes during charging and discharging cycles. Therefore, the problem is that the cycle life is extremely short.

On the other hand, an oxide negative electrode typified by lithium titanate (Li ₄ Ti ₅ O ₁₂ ) is a type of negative electrode that performs charging and discharging by intercalating and deintercalating Li ions while maintaining its crystal structure. Oxide negative electrodes generally exhibit small volume changes during charging and discharging.
However, conventional oxide negative electrodes have a charge/discharge potential of around 1.5 to 2.0 V, so the voltage as a battery cell is low. Further, in battery systems using conventional oxide negative electrodes, energy density is low, and increasing capacity is a problem (see Non-Patent Documents 1, 6, and 7).

Zenpachi Kokumi, Lithium secondary battery (Ohmsha, 2008) R. Chen, R. Luo, Y. Huang, F. Wu, and L. Li, Advanced Science 3, 1600051(2016) M. Obrovac and V. Chevrier, Chemical reviews 114, 11444(2014) R. A. Huggins, Journal of Power Sources 81-82, 13(1999) N. Nitta and G. Yushin, Particle & Particle Systems Characterization 31, 317(2014) X. W. Xuefeng Song, Zhuang Sun, Peng Zhang, and Lian Gao, Material Matters 8, 111(2014) M. Reddy, G. Subba Rao, and B. Chowdari, Chemical reviews 113, 5364(2013)

The problem to be solved by the present invention is to provide a novel negative electrode active material that has an appropriate Li ion charge/discharge potential, a relatively high capacity, and a relatively long cycle life.
Another problem to be solved by the present invention is to provide a lithium ion secondary battery using such a negative electrode active material as a negative electrode.

In order to solve the above problems, the negative electrode active material of the present invention is made of an oxide containing Fe and a metal element other than Fe, has the ability to absorb and release Li ions, and has a charge/discharge potential of Li ions. The gist is that it is more than 0V and less than 1.5V.

The oxide preferably has a composition represented by the following formula (1).
(Ca _1-x Sr _x )Ba(Fe _4-y M _y )O ₈ …(1)
however,
M is any one or more elements selected from the group consisting of Ti, Co, Ni, Cu, Zn, and Mn;
0≦x≦1, 0≦y≦0.6.

Further, the oxide preferably has a composition represented by the following formula (2).
Y(Fe1 _{-zM'z ) O3} ...(2)
however,
M' is Mn and/or Co,
1≦z≦0.2.

Furthermore, the gist of the lithium ion secondary battery according to the present invention is that the negative electrode active material according to the present invention is used for the negative electrode.

As a negative electrode active material for a lithium ion secondary battery, an oxide containing Fe and a metal element other than Fe, which has the ability to absorb and release Li ions, and whose charge/discharge potential of Li ions exceeds 0 V and 1 When using an oxide with a voltage of less than .5V (particularly an oxide having a composition represented by formula (1) or formula (2)), lithium ions with high cell voltage, high capacity, and excellent cycle life can be produced. A secondary battery is obtained.

It is _a charge-discharge curve of _CaBaFe4O8 .

An embodiment of the present invention will be described in detail below.
[1. Negative electrode active material]
[1.1. overview]
The negative electrode active material according to the present invention is made of an oxide containing Fe and a metal element other than Fe, has the ability to absorb and release Li ions, and has a charge/discharge potential of more than 0V and less than 1.5V. It is.
In order to obtain high-performance lithium-ion secondary batteries, the oxides used for the negative electrode must be
(a) has a site that allows Li ions to be inserted and removed reversibly within the crystal structure;
(b) When Li ions are inserted into the site, it becomes energetically stable,
(c) has an appropriate Li ion charge/discharge potential (more than 0 V and less than 1.5 V, preferably 0.5 V or more and less than 1.5 V), and
(d) It must have a relatively high capacity.
Specific examples of oxides that satisfy these conditions include the following.

[1.2. Specific example 1 (CaBaFe ₄ O ₈ based oxide)]
A first specific example of the negative electrode active material is made of an oxide having a composition represented by the following formula (1).
(Ca _1-x Sr _x )Ba(Fe _4-y M _y )O ₈ …(1)
however,
M is any one or more elements selected from the group consisting of Ti, Co, Ni, Cu, Zn, and Mn;
0≦x≦1, 0≦y≦0.6.

[1.2.1. Crystal structure]
The oxide represented by formula (1) is composed of a CaBaFe ₄ O ₈ based oxide. "CaBaFe ₄ O ₈ -based oxide" refers to CaBaFe ₄ O ₈ and oxides in which the Ca site and/or Fe site of CaBaFe ₄ O ₈ is replaced with another metal element.
CaBaFe ₄ O ₈ -based oxides have a crystal structure belonging to space group P-31m (162) before insertion of Li ions, and within that crystal structure there are many crystal structures into which Li ions can be inserted. There is a site. CaBaFe ₄ O ₈ -based oxides can insert up to 12 Li ions per formula weight, as predicted by first-principles calculations, but some of these Li ions become reversible during charging and discharging. It is thought that they are inserted and removed in a controlled manner.

[1.2.2. composition]
[A. x]
Sr can replace Ca sites at any ratio. That is, x can take any value in the range of 0 or more and 1 or less.
CaBaFe ₄ O ₈ and SrBaFe ₄ O ₈ have the same crystal structure and have similar characteristics regarding Li ion insertion/extraction potential and energy density. (Ca, Sr)BaFe ₄ O ₈ in which a part of the Ca site is replaced with Sr has the effect of increasing the current capacity per weight and volume compared to the end component.

[B. y]
The valence of Fe changes as Li ions are inserted and removed. Element M represents a metal element that replaces the Fe site. The element M is also preferably one whose valence changes as Li ions are intercalated and deintercalated.
In particular, element M is preferably any one or more elements selected from the group consisting of Ti, Co, Ni, Cu, Zn, and Mn. This is because these elements are easily substituted into Fe sites and are likely to cause electronic conductivity.

Element M is not necessarily required. That is, y may be zero. However, when some of the Fe sites are replaced with element M, the electronic conductivity improves and the polarization during Li insertion and desorption becomes smaller. In order to obtain such an effect, y is preferably 0.1 or more. y is preferably 0.2 or more, more preferably 0.3 or more.
On the other hand, if y becomes too large, another phase (impurity) will be generated and the battery characteristics will deteriorate. Therefore, y is preferably 0.6 or less. y is preferably 0.5 or less, more preferably 0.4 or less.

[C. Preferred composition]
The CaBaFe ₄ O ₈ -based oxide is particularly preferably CaBaFe ₄ O ₈ , SrBaFe ₄ O ₈ , Ca _0.5 Sr _0.5 BaFe ₄ O ₈ , or CaBaFe _3.6 M _0.4 O ₈ . This is because these materials have a lower average potential and a higher current capacity per weight and volume than Li ₄ Ti ₅ O ₁₂ .
The negative electrode active material may be made of any one of these oxides, or may be made of two or more of these oxides.

[1.2.3. Average particle size]
"Average particle size" refers to the median diameter (D ₅₀ ) obtained by laser diffraction scattering method.
If the average particle size of the CaBaFe ₄ O ₈ -based oxide becomes too large, it takes time for Li to be inserted and removed from the surface into the interior, resulting in deterioration of battery characteristics. Therefore, the average particle size of the CaBaFe ₄ O ₈ based oxide is preferably 10 μm or less. The average particle size is preferably 7 μm or less, more preferably 5 μm or less.

[1.3. Specific example 2 (YFeO _3- based oxide)]
A second specific example of the negative electrode active material is made of an oxide having a composition represented by the following formula (2).
Y(Fe1 _{-zM'z ) O3} ...(2)
however,
M' is Mn and/or Co,
0≦z≦0.2.

[1.3.1. Crystal structure]
The oxide represented by formula (2) is composed of YFeO ₃ -based oxide. "YFeO ₃ -based oxide" refers to YFeO ₃ and oxides in which the Fe site of YFeO ₃ is substituted with another metal element.
The YFeO ₃ -based oxide has a crystal structure belonging to the space group Pbnm (62) before insertion of Li ions, and within the crystal structure there are many sites into which Li ions can be inserted. As predicted by first-principles calculations, YFeO ₃ -based oxides can insert up to two Li ions per formula weight, but some of these Li ions are reversibly inserted during charging and discharging. It is thought that insertion and removal occur.

[1.3.2. composition]
[A. z]
The valence of Fe changes as Li ions are inserted and removed. Element M' represents a metal element that replaces the Fe site. The element M' is also preferably one whose valence changes as Li ions are intercalated and deintercalated.
The element M' is particularly preferably Mn and/or Co. This is because these elements are easily substituted into Fe sites, improving electronic conductivity.

Element M' is not necessarily required. That is, z may be zero. However, when some of the Fe sites are replaced with element M', the electronic conductivity improves and the polarization during Li insertion and desorption becomes smaller. In order to obtain such an effect, z is preferably 0.03 or more. z is preferably 0.05 or more, more preferably 0.07 or more.
On the other hand, if z becomes too large, another phase (impurity phase) will be generated and the battery characteristics will deteriorate. Therefore, z is preferably 0.2 or less. z is preferably 0.15 or less, more preferably 0.1 or less.

[B. Preferred composition]
The YFeO ₃ -based oxide is particularly preferably YFeO ₃ or YFe _0.9 Mn _0.1 O ₃ . This is because these materials have a lower Li insertion/extraction potential and a larger current capacity per volume than Li ₄ Ti ₅ O ₁₂ .
The negative electrode active material may be made of any one of these oxides, or may be made of two or more of these oxides.

[1.3.3. Average particle size]
"Average particle size" refers to the median diameter (D ₅₀ ) obtained by laser diffraction scattering method.
If the average particle size of the YFeO ₃ -based oxide becomes too large, it takes time for Li to be inserted and removed from the surface into the interior, resulting in deterioration of battery characteristics. Therefore, the average particle size of the YFeO ₃ -based oxide is preferably 10 μm or less. The average particle size is preferably 7 μm or less, more preferably 5 μm or less.

[2. Manufacturing method of negative electrode active material]
The negative electrode active material according to the present invention is
(a) Mix raw materials to have a predetermined composition,
(b) calcining the raw material mixture under predetermined conditions;
(c) If necessary, it can be produced by appropriately pulverizing calcined powder.
The manufacturing conditions are not particularly limited, and it is preferable to select optimal conditions depending on the intended composition.

[3. Lithium ion secondary battery]
The lithium ion secondary battery according to the present invention uses the negative electrode active material according to the present invention as a negative electrode. The material of the positive electrode active material, the material of the electrolyte, the structure of the lithium ion secondary battery, etc. are not particularly limited, and the most suitable one can be selected depending on the purpose.

[4. Effect】
As a negative electrode active material for a lithium ion secondary battery, an oxide containing Fe and a metal element other than Fe, which has the ability to absorb and release Li ions, and whose charge/discharge potential of Li ions exceeds 0 V and 1 When using an oxide with a voltage of less than .5V (particularly an oxide having a composition represented by formula (1) or formula (2)), lithium ions with high cell voltage, high capacity, and excellent cycle life can be produced. A secondary battery is obtained.

The reason why the negative electrode active material according to the present invention exhibits a high cell voltage is because the insertion and desorption potential of Li ions is in a range suitable for a negative electrode of a lithium ion secondary battery (more than 0 V and less than 1.5 V). Conceivable.
In addition, the negative electrode active material according to the present invention exhibits high capacity because it has a large number of sites into which Li ions can be inserted in the crystal structure, and because the valence of cations contained in the oxide changes. This is thought to be because more Li ions can be stored.
Furthermore, the reason why the negative electrode active material according to the present invention exhibits a high cycle life is considered to be because the change in volume accompanying the intercalation and desorption of Li ions is small.

(Example 1: Evaluation of charging/discharging potential and volume change)
[1. Method of calculation]
First-principles calculations were performed on the oxides before and after Li insertion, and the Li insertion potential and volume change were evaluated. Bond Valence Sum (BVS) is one of the indicators for evaluating the stability of atoms (ions) in a crystal structure (see Reference 1). BVS represents the valence (formal charge) of the ion of the polyhedral central atom in the crystal structure, and is used for stable position evaluation in crystal structure analysis.
[Reference 1] ID Brown, Chemical Reviews 109, 7858(2009)

Since Li in the oxide exists as an ion with a valence of +1, it is expected that the site where BVS=1 corresponds to a stable position of Li. Therefore, Li was inserted at a site where BVS was close to 1 and at a sufficient distance from other atoms in the crystal structure.
From the internal energy difference of the Li insertion reaction expressed by the following equation (3), the charging (discharging) potential (V) was estimated using the following equation (4) (Reference document 2).
[Reference 2] YS Meng and ME Arroyo-de Dompablo, Energy & Environmental Science 2, 589(2009)

MO _y +xLi → Li _x MO _y …(3)
V≒-[E(Li _x MO _y )-E(MO _y )-xE(Li)]/xF...(4)
However, F is Faraday constant.

Note that, according to Nernst's equation, the potential is estimated from the free energy difference ΔG, but since the reaction in a solid system near room temperature is considered, the effect of entropy is ignored. The volume change associated with Li insertion was directly calculated from the crystal volume after structure optimization.

[2. result]
Table 1 shows the charge/discharge potential and volume change of CaBaFe ₄ O ₈ -based oxide and YFeO ₃ -based oxide obtained from calculations.
The potential when 12 Li atoms are inserted into CaBaFe ₄ O ₈ is predicted to be 0.84 V vs. Li ⁺ /Li, which is preferable as a negative electrode active material. From the viewpoint of Li number density and potential, CaBaFe ₄ O ₈ is superior to lithium titanate (Li ₄ Ti ₅ O ₁₂ ).

The potential when one Li is inserted into SrBaFe ₄ O ₈ is as high as 1.63 V vs. Li ⁺ /Li. However, the charging and discharging potentials during the reaction in which 5 Li atoms are inserted into LiSrBaFe ₄ O ₁₂ and the reaction in which 6 Li atoms are inserted into Li ₆ SrBaFe ₄ O ₁₂ are respectively 1.06 V vs. Li ⁺ / Li and 0.65V vs. Li ⁺ /Li, which is preferable as a negative electrode active material. From the viewpoint of Li number density and potential, SrBaFe ₄ O ₈ is superior to lithium titanate (Li ₄ Ti ₅ O ₁₂ ).

The potential when inserting 0.25 pieces of Li into YFeO ₃ is as high as 1.99V vs. Li ⁺ /Li. However, the charge/discharge potential during the reaction of inserting 7/4 Li into Li _1/4 YFeO ₃ is 0.81 V vs. Li ⁺ /Li, which is preferable as a negative electrode active material. From the viewpoint of Li number density and potential, YFeO ₃ is superior to lithium titanate (Li ₄ Ti ₅ O ₁₂ ).
The volume change per mole of intercalated lithium of alloy-based materials such as Si is 9 cm ³ /mol. It was found that the volume change of the negative electrode active material according to the present invention was smaller than that, and the cycle life was excellent.

(Examples 2.1 to 2.11, Comparative Example 1)
[1. Preparation of sample]
[1.1. Examples 2.1 to 2.11]
The sample was synthesized using a conventional solid phase reaction method. As raw materials, commercially available oxides or carbonates of reagents were used. The charging composition is as follows.
(a) CaBaFe ₄ O ₈ (Example 2.1)
(b) SrBaFe ₄ O ₈ (Example 2.2)
(c) Ca _0.5 Sr _0.5 BaFe ₄ O ₈ (Example 2.3)
(d) CaBaFe _3.6 Co _0.4 O ₈ (Example 2.4)
(e) CaBaFe _3.6 Mn _0.4 O ₈ (Example 2.5)
(f) CaBaFe _3.6 Ti _0.4 O ₈ (Example 2.6)
(g) CaBaFe _3.6 Zn _0.4 O ₈ (Example 2.7)
(h) CaBaFe _3.6 Cu _0.4 O ₈ (Example 2.8)
(i) CaBaFe _3.6 Ni _0.4 O ₈ (Example 2.9)
(j) YFeO ₃ (Example 2.10)
(k) YFe _0.9 Mn _0.1 O ₃ (Example 2.11)

The raw materials were weighed to a total amount of 10 g using a stoichiometric composition, and placed in a 250 mL polypot. Further, 80 mL of zirconia balls with a diameter of 5 mm and 80 mL of ethanol as a solvent were added, and the mixture was mixed for 20 hours using a ball mill. After mixing, the slurry was collected, the ethanol was evaporated on a rotary evaporator, and the powder was collected. The obtained powder was calcined in a muffle furnace in the atmosphere under the conditions shown in Table 2. The calcined powder was pulverized in an alumina mortar until there was no coarse powder (until the particle size became several μm or less).

[1.2. Comparative example 1]
Regarding Li ₄ Ti ₅ O ₁₂ (Comparative Example 1), the sample was not synthesized, and literature values (Reference Document 3) were used for the charge/discharge characteristics.
[Reference 3] Nitta, Naoki, et al, "Li-ion battery materials: present and future," Materials today 18.5(2015):252-264

[2. Test method]
[2.1. X-ray diffraction]
The crystal phase of the obtained sample was identified by X-ray diffraction (XRD).

[2.2. Charge/discharge characteristics]
The above powder was used as the negative electrode active material (S). Carbon black (Toka Black 5500, manufactured by Tokai Carbon Co., Ltd.) was used as the conductive aid (C). As the binder (P), polyvinylidene fluoride (manufactured by Kureha Chemical Industry Co., Ltd., KF Polymer #1120) was used. NMP (N-methyl-2-pyrrolidone) was used as the organic solvent.

The negative electrode active material (S), the conductive aid (C), and the binder (P) were weighed to have the mass ratio shown in Table 2. The negative electrode active material (S) and the conductive additive (C) were mixed in a mortar for 10 minutes. A predetermined amount of binder (P) was added to this, and the mixture was kneaded while adding an organic solvent little by little to obtain a paste-like negative electrode composite material. The negative electrode composite material was applied onto the copper foil using a doctor blade (film thickness: 150 to 200 μm) and dried in the air at 140° C. for 20 minutes. Furthermore, it was dried in a vacuum dryer at 120° C. for about 15 hours. This was punched out into a disk shape (φ16.2 mm), and the surface was pressed at 5 kN using a press machine to produce a negative electrode.

In a glove box under an argon atmosphere, the above negative electrode and a porous polyethylene separator (φ26 mm) were placed in a Tomcell (manufactured by Nippon Tomcell), and 200 μdl of electrolyte was added. The electrolyte used was a mixture of ethylene carbonate/dimethyl carbonate/ethyl methyl carbonate = 3/4/3 (volume ratio) in which LiTFSI was dissolved as an electrolyte at a concentration of 1 mol/kg.
Thereafter, a Li metal electrode (φ18 mm) adhered to a current collector plate (SUS304) with a roller was placed on the cell as a counter electrode, and a stainless steel case was placed on the cell and caulked to prepare an evaluation cell.

The charge/discharge measurement was performed by setting the measurement cell in a low temperature constant temperature and humidity chamber (PL-3KTH, manufactured by Espec Co., Ltd.) and using a battery charging/discharging device (manufactured by Asuka Electronics Co., Ltd.). Measurements were performed at a temperature of 25° C., a current density of 0.05 mA/cm ² , a voltage range of 0.5 V to 2.0 V, and repeated constant current charging and discharging for 5 cycles.

[3. result]
[3.1. X-ray diffraction]
As a result of XRD analysis, a sample of CaBaFe ₄ O ₈ based oxide was obtained with space group P-31m (162) as reported in the paper. However, in some samples, an impurity phase that appeared to be BaFe ₂ O ₄ was also confirmed, but it was judged that it would not affect the evaluation of battery characteristics because it was in a very small amount.
For the YFeO ₃ -based oxide, a sample with the space group Pbnm (62) as reported in the paper was obtained.

[3.2. Charge/discharge characteristics]
Table 2 shows the charge/discharge characteristics of each sample. Table 2 also shows the material composition, the conditions for the synthesis process of the negative electrode material, and the composition of the negative electrode. Further, FIG. 1 shows a charge/discharge curve of CaBaFe ₄ O ₈ . From Table 2 and FIG. 1, the following can be seen.

(1) In the case of CaBaFe ₄ O ₈ , a capacity of about 218 mAh/g was developed on average in the second and subsequent charge/discharge cycles. Since the formula weight of CaBaFe ₄ O ₈ is 528.7882, when these capacities are converted into the number of Li per CaBaFe ₄ O ₈ , it corresponds to 4.3. Moreover, the average voltage during charging and discharging was 1.0V.
(2) In Examples 2.1 to 2.11, the current capacity was 100 mAh/g or more, and the average charge/discharge voltage was 1.5 V or less. In particular, the current capacities per volume of Examples 2.1 to 2.11 were all superior to lithium titanate (Li ₄ Ti ₅ O ₁₂ ).

Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the gist of the present invention.

The negative electrode active material according to the present invention can be used as a material for a negative electrode of a lithium ion secondary battery.

Claims (4)

It is made of an oxide containing Fe and a metal element other than Fe and has a composition expressed by the following formula (1) , has the ability to absorb and release Li ions, and has a charge/discharge potential of Li ions of 0 V. A negative electrode active material having a voltage of more than 1.5V.
(Ca _1-x Sr _x )Ba(Fe _4-y M _y )O ₈ …(1)
however,
M is any one or more elements selected from the group consisting of Ti, Co, Ni, Cu, Zn, and Mn;
0≦x≦1, 0≦y≦0.6. The negative _electrode active _material according to claim 1, _{wherein the oxide comprises CaBaFe4O8} , _{SrBaFe4O8 , Ca0.5Sr0.5BaFe4O8} , or _{CaBaFe3.6M0.4O8} . The negative electrode active material according to claim 1 or 2, comprising a powder having an average particle size of 10 μm or less. A lithium ion secondary battery using the negative electrode active material according to any one of claims 1 to 3 as a negative electrode.

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