JPH11339804A - Positive electrode material for nonaqueous electrolyte secondary battery and battery with the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode material excellent in cycle characteristic and high-load characteristic by using the spinel LiMn2 O4 , specified by the specific surface area, the ratio of the crystallization parameter of the X-ray diffraction pattern and the solid NMR spectrum using lithium nuclei, as the positive electrode active material. SOLUTION: The spinel LiMn2 O4 used as a positive electrode material has the specific surface area of 0.5-1.9 m<2> /g, the crystallization parameters α for diffractions (311), (222), (400), (440) has a values in the range of 1.1-2.3 when the ratio of the half width to the peak intensity of the diffraction peak (111) of the power X-ray diffraction pattern, has the area ratio of 30-50% of the spectrum B against the spectrum A within the spectrum constituted of the broad spectrum A having the line width exceeding 1000 ppm and the spinning side band spectrum B of at least below 1000 ppm in the solid NMR spectrum using lithium nuclei, and mainly belongs to the space group Fd3m (No.227).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to a positive electrode active material.

[0002]

2. Description of the Related Art Non-aqueous electrolyte secondary batteries are small, lightweight,
In addition, due to the high energy density, the expectation is increasing as portable and cordless devices are progressing.

Conventionally, lithium-containing metal oxides such as LiCoO ₂ , LiNiO ₂ , and LiMn ₂ O ₄ have been proposed as a positive electrode active material for a non-aqueous electrolyte secondary battery. On the other hand, as a negative electrode, metallic lithium, a lithium alloy, a graphite material capable of inserting and extracting lithium ions, and the like have been proposed and put to practical use. Among them, LiMn ₂ O ₄ is particularly attracting attention as an inexpensive cathode material.

However, this spinel-structured LiM
n ₂ O ₄ is extremely sensitive to synthesis conditions such as starting materials and processing temperature conditions, and has a great influence on electrochemical behavior. Electrochemical activity is high, and even if a high capacity is obtained in the initial stage, it is not always the case that sufficient electrochemical reversibility can be obtained. Properties were inferior. This has been caused by a crystal phase gradually changing due to a crystal phase change accompanying charge / discharge, or a part of manganese ions dissolving in the electrolytic solution.

For this reason, various additive elements are dissolved in a part of manganese to reduce the crystal phase change accompanying charge and discharge (J. Electrochem. Soc., 138 ('9)
1) 2859) has been proposed to improve reversibility. In addition, efforts to improve the reversibility by controlling the dissolution of manganese or stabilizing the crystal structure by coating the surface of LiMn ₂ O ₄ with another material phase as well as improving the Mn portion (J. Electrochem. Soc. , 145 ('
98) 194) have been proposed.

[0006]

Although the reversibility can be improved by making full use of the above-mentioned conventional elemental technology, other characteristics useful for battery characteristics, such as a decrease in specific capacity at an electrode or lithium. Dynamic factors such as ion migration are reduced, which adversely affects high load characteristics and the like.

The decrease in the electrode capacity is due to the fact that various manganese valences of LiMn ₂ O ₄ are dissolved in solid solution, so that the movable manganese valency is limited and a sufficient reversible capacity cannot be obtained. It is considered that the decrease in the electrode high-load characteristics is caused by coating the surface of the LiMn ₂ O ₄ particles with another material phase, which adversely affects electrochemical mass transport at the interface.

The present invention solves such a problem. When a spinel-type lithium-containing manganese oxide represented by the general formula LiMn ₂ O ₄ is used as a cathode material, other elements may be added to manganese. Without coating the surface,
It is intended to provide a material having high capacity and excellent high load characteristics.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In order to solve these problems,
The present invention mainly relates to a spinel-type lithium-containing manganese oxide belonging to the space group Fd3m (No. 227) and represented by the general formula LiMn ₂ O ₄ and having a specific surface area of 0.5 to
The crystallization parameter α using the ratio of the half width to the peak intensity of the diffraction peak (111) of the powder X-ray diffraction pattern was set to 1.9 m ² / g, and the crystallization parameter α was calculated for each diffraction peak (311), (22
2), (400), 1.1 to 2.440 with respect to α of (440).
3, and LiCl measured by a MAS / spin-echo method (combined use of a magic angle rotation method (rotation speed of 8 to 10 kHz) and a spin echo method), and based on 0 ppm LiCl. In the spectrum, among the broad spectrum (a) having a line width exceeding 1000 ppm and the spinning side band (b) having at least 100 ppm or less, the area ratio of the spectrum (b) to the spectrum (a) is 30% or more. A non-aqueous electrolyte secondary battery is provided by using a positive electrode material that is less than 50%, a positive electrode using this material, a negative electrode using a negative electrode material capable of inserting and extracting lithium, and a non-aqueous electrolyte. .

In the spinel-type lithium-containing manganese oxide, when repeatedly charged and discharged, various diffraction peaks indicated by a powder X-ray diffraction pattern gradually broaden, and the cycle characteristics are reduced accordingly. This is thought to be due to the fact that the crystal structure changes due to charge / discharge and causes structural destruction, and that the manganese in a part of the solid phase dissolves in the electrolyte, resulting in a low crystal structure. .

Originally, the principle of absorbing and releasing lithium ions in a space having a layered structure formed by an intermolecular force, for example, of a cobalt-based compound, whereas the spinel-based lithium-containing manganese oxide attracts by Coulomb force. Because of the principle of absorbing and releasing lithium ions in the space of the three-dimensional matrix, the manganese-based crystal structure after lithium is electrochemically extracted tends to be slightly unstable. Therefore, when the absorption and desorption of lithium are repeatedly and dynamically repeated by charging and discharging, the structure is changed, and it is easy to have a form different from the crystal state at the beginning of the cycle. In addition, because of the instability during the lithium extraction reaction process, a phenomenon occurs in which some manganese ions are discharged into the electrolyte, which further promotes the collapse of the crystal structure.

On the basis of such analysis facts, the present inventors have attempted to improve the crystallinity of the spinel-type lithium-containing manganese oxide without lowering the specific capacity density or high load characteristics and improving the cycle characteristics. In order to reduce the activity of dissolving manganese and manganese, it was decided to reduce the contact interface with the electrolytic solution, that is, to reduce the specific surface area.

Further, in order to provide a material in which not only the bulk crystal structure and specific surface area but also the chemical state of lithium contained therein are controlled, the solid state NMR of lithium is used to determine the state of lithium in the solid phase. Was detected.

That is, the characteristics are improved by controlling the occupation amount at an appropriate lithium accommodating site for stabilizing the three-dimensional structure of the spinel system existing in the spinel-type lithium-containing manganese oxide based on NMR analysis information. is there.

In solid-state NMR measurement, the MAS method is often used to observe the state of lithium in a lithium-containing transition metal oxide. However, it is very difficult to interpret the state of lithium in the transition metal oxide due to the paramagnetism of the transition metal only by the measurement using the MAS method. Therefore, further pulse-spi
n-echo method by measuring the electron state of the lithium by ⁷ Li-NMR in combination with, it is possible to consider the presence state of lithium in more detail in terms of relaxation times.

There are various methods for measuring the degree of crystallinity. Here, the diffraction intensity for the half width of each diffraction peak is used. This is not only a judgment based on the diffraction intensity factor, but also because the half width is a factor that knows the degree of growth of crystallites and so on. did.

(111) is a main diffraction peak which clearly shows the arrangement of manganese atoms, and the crystallinity of this diffraction peak does not show a significant change even if synthesis or surface modification is performed by any method. Therefore, when this peak is used as a reference, the change in crystallinity of the other diffraction peaks is determined by the parameter α.
It was digitized as The present inventors have conducted intensive studies on the improvement of cycle characteristics using various techniques. As a result, α of (111) is (311) (222) (4).
00) (440) 1.1 to 2.3 with respect to α of each peak.
Range.

The increase and sharpening of the diffraction intensity of the diffraction peaks other than (111) means that the improvement in crystallinity is apparent in the oxygen arrangement and the arrangement between oxygen and manganese, and this is the crystallization of the bulk solid as a whole. It is thought that it has greatly contributed to the improvement of the degree. During the measurement, when the crystallinity was increased to α of 2.3 or more, the initial specific capacity density decreased. Therefore, α is 2.
3 is considered the upper limit.

The leaching of manganese into the electrolyte was carried out in accordance with the above synthesis method.
It was found that elution was suppressed at 9 m ² / g. The measurement was performed by the BET method using nitrogen gas.
m ² / g or less could not be measured. Also,
At 1.9 m ² / g or more, the reason is not clear, but since the amount of manganese dissolved in the electrolytic solution starts to increase rapidly, the upper limit is set to 1.9 m ² / g.

Further, the following was derived from the analysis information of solid-state NMR. The spinel structure (space group Fd3m) has M
It is a known fact that there are two types of Li sites, 8a site and 16c site, of 4-coordinate and 6-coordinate having different distances from n. The present inventors consider the influence on these two Li sites that are different in distance from Mn, which has strong paramagnetic properties, and that the 16c site that is closer to Mn is more affected by stronger paramagnetism and has a lower magnetic field. Chemical shift to the side, 8
The a-site was considered not to be strongly affected by paramagnetism and was chemically shifted to a high magnetic field, and it was thought that it was possible to distinguish heterogeneous lithium sites in the lattice by NMR analysis. There is a clear correlation between the electrochemical activity and the signal reflecting the 8a site (spinning sideband spectrum b), and the peak area of this signal is determined by the area ratio of the broad signal (spectrum a having a line width of about 1000 ppm). If it is 30% or more, the specific capacity density of the monopole, which indicates the initial capacity of the cycle, becomes good, and how strongly the chemical environment where lithium in the crystal structure is placed is affected by the paramagnetic manganese. Since it can be understood by the chemical shift, the crystal periodicity of lithium and manganese, that is, the fine crystallinity in the crystal can be determined, and can be positioned as a condition showing good cycle characteristics.

When the rotational speed of the MAS method is further increased, the spectral area ratio is slightly different, and the resolution of the 16c site can be increased, and sharpening of the signal and appearance of the spinning side band can be expected. It is difficult to determine that the area ratio of (a) is 30% or more, but it is preferable that the area ratio is 30% or more as a condition for extracting high characteristics under the rotation condition of 8 to 10 kHz. Experimentally, the peak ratio did not exceed 50%, but this is closely related to the resolution of the previous equipment, and it is difficult to limit this value.

[0022]

Embodiments of the present invention will be described below with reference to the drawings.

The measurement conditions of the MAS / spin-echo method are described. Measurement frequency 116.75 MHz, observation width 125
kHz, pulse width 90 °, pulse (3 μsec), Pr
e-refocus (100 μsec), Post-r
efocus (98.4 μsec), repetition time (1
sec), sample rotation speed (10 kHz), integration frequency (51
2), measurement (room temperature), chemical shift standard (LiCl; 0)
ppm) The specific surface area was measured by the BET method of nitrogen gas adsorption.

The crystallinity was measured at a power of 40 kV and 40 m.
In A, a Cukα X-ray diffractometer was used.

Next, FIG. 1 shows a longitudinal sectional view of an evaluation battery for evaluating the positive electrode material of the present invention. In FIG. 1, reference numeral 1 denotes a battery case processed from a stainless steel plate having resistance to organic electrolyte, 2 denotes a sealing plate of the same material, 3 denotes an insulating packing, 4 denotes an electrode group, and 5 denotes a general formula LiMn ₂ O _4. Plate 6 using a carbon material capable of occluding and releasing lithium, 7 a microporous separator made of polypropylene resin, and 8 for improving the insulation of the upper and lower surfaces of the electrode plate group. Is an insulating ring arranged in the

The size of the battery for evaluation is 20 mm in diameter and the total height of the battery is 1.6 mm. As the electrolytic solution, a solution in which 1.5 mol / L of LiPF6 was dissolved in an EC-DEC equal volume mixed solvent was used.

The charge / discharge condition is a current density of 0.33 mA /
The reversible capacity and the open-circuit potential of the oxidation-reduction were measured in a potential range of ² V to 4.3 V (based on Li) with constant current charging and discharging of cm ² . The downtime was one hour.

In the synthesis of the spinel-type lithium-containing manganese oxide of the present invention, for example, manganese dioxide can be used as a manganese source, and lithium carbonate or lithium hydroxide can be used as a lithium source. These starting materials are LiM
It is obtained by mixing at a predetermined ratio so as to have an nO ₄ composition and calcining at a temperature of 750 to 900 ° C. in a reaction environment adjusted to an oxidizing atmosphere. It is necessary to adjust the particle size, the specific surface area, the temperature step in the reaction process and the reaction atmosphere.

Prototype coin-type batteries using various samples were prepared.
The specific capacity density at the time of initial discharge and the discharge capacity retention ratio with respect to the initial state after 100 cycles were examined in relation to the crystallinity parameter α. The result is shown in FIG. As can be seen from the figure, each of (311) and (222) for α of (111)
When the ratio of α of the (400) and (440) peaks exceeds 1.1, it supports that the capacity retention ratio is high and the cycle characteristics are good. The specific capacity density of the initial discharge does not depend and is approximately 125
Although the characteristics are around mAh / g, when the ratio of α exceeds 2.3, the capacity density rapidly decreases. Therefore, the ratio of α should be 1.1 to 2.3. Next, the relationship with the specific surface area was examined. The result is shown in FIG. As can be seen from the figure, up to a specific surface area of 1.9 m ² / g, it is supported that the capacity retention ratio is high and the cycle characteristics are good. The specific capacity density of the initial discharge shows a characteristic of about 125 mAh / g without depending on the ratio of α. Therefore, the specific surface area is 0.5 to 1.9 m ² / g.
Should be. Next, the relationship between the NMR spectrum (a) and the area ratio of (b) to (a) was examined. FIG. 4 shows the results. As can be seen from the figure, the spectral area ratio is 30.
%, The capacity retention rate is increased to support good cycle characteristics, but below this value, excellent characteristics are not exhibited. In addition, the specific capacity density of the initial discharge has the same tendency and tends to increase as the spectral area ratio increases. Therefore, the spectral area ratio should be 30 to 50%. If the value is 50% or more, reliable results cannot be obtained experimentally as described above.
Within the range of z, high characteristics can be expected in this range.

The three physical factors that determine the cycle characteristics described above are independent events, but in reality, these factors have a commonality, and the initial discharge specific capacity density is 125 m.
In order to obtain a discharge capacity retention rate of about Ah / g and a discharge capacity retention rate after 100 cycles of 90% or more, a condition setting in which these three factors are combined is adopted. Although the reason for this is not clear, the degree of crystallinity naturally increases during the process of improving the crystallinity, and it is equivalent to occupy a large amount of lithium contained in the appropriate site that has the effect of maximizing the degree of crystallinity. It is presumed to occur.

At the same time, the specific surface area of the particles does not decrease in the direction of improving the crystallinity of the surface by growing crystallites and improving the regularity of the crystal orientation plane so that the change in the specific surface area also supports this. It is thought.

When a spinel-type lithium-containing manganese oxide having these physical factors is used as a positive electrode active material, Mn does not contain other solid solution elements and the surface of the particles is not coated, so that high load is applied. Does not suffer from a large reaction overpotential during the electrochemical mass transfer at the interface at, and does not drop below 125 mAh / g at the expense of the specific capacity density of the initial discharge.

On the other hand, when a positive electrode active material in which another element is dissolved in part of Mn is used, the current density is 3.3 mA / c.
When charge / discharge is performed with a high load of m ² , the discharge capacity retention ratio is 80
%, But the specific capacity density of the initial discharge is 120 mA.
h / g.

In the case where the surface of the positive electrode active material particles was coated, the specific capacity density of the initial discharge did not decrease, but the discharge capacity was reduced by half in the high load test.

According to the present invention, the sacrifice of either the initial discharge or the specific capacity density was minimized even in the high-load charge / discharge described above.

Further, in the present invention, since the capacity retention rate is excellent, it is considered that the frequency of manganese elution from the solid phase into the electrolyte during charging / discharging is extremely low, and it is presumed that the cycle characteristics were improved. doing.

As described above, a material capable of inserting and extracting lithium, such as graphite, is used as the positive electrode active material with the spinel-type lithium-containing manganese oxide of the present invention having excellent initial discharge specific capacity density and excellent cycle characteristics. Carbon materials, alloys,
A secondary battery can be formed using an inorganic compound as a negative electrode active material and a nonaqueous electrolyte containing lithium.

[0038]

As described above, the present invention relates to the general formula LiMn
A spinel-type lithium-containing manganese oxide represented by ₂ O ₄ in which a specific surface area and a ratio of a predetermined crystallization parameter in an X-ray diffraction pattern, and a ratio of a predetermined spectrum in a solid-state NMR spectrum using lithium nuclei are specified. Is used for the positive electrode active material, the specific capacity density of the active material,
A nonaqueous electrolyte secondary battery having a high discharge capacity retention rate and excellent battery cycle characteristics and high load characteristics can be provided.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of a non-aqueous electrolyte secondary battery of the present invention.

FIG. 2 is a diagram showing the relationship between the ratio of the crystallinity parameter α of the positive electrode active material and the electrochemical characteristics.

FIG. 3 is a diagram showing a relationship between a specific surface area of a positive electrode active material and electrochemical characteristics.

FIG. 4 is a diagram showing a relationship between an NMR spectrum area ratio of a positive electrode active material and electrochemical characteristics.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Battery case 2 Sealing plate 3 Insulation packing 4 Electrode plate group 5 Positive electrode plate 6 Negative electrode plate 7 Separator 8 Insulation ring

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Akira Hashimoto 1006 Kazuma Kadoma, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.

Claims (2)

[Claims] 1. A space group Fd3m (No. 22)
7) A spinel-type lithium-containing manganese oxide represented by the general formula LiMn ₂ O ₄ , which has a specific surface area of 0.5 to 1.9 m ² / g and a diffraction peak of a powder X-ray diffraction pattern The crystallization parameter α using the ratio of the half width to the peak intensity of (111) is calculated for each diffraction peak (311),
1.1 for α of (222), (400), and (440)
To 2.3 and measured by the solid-state NMR spectrum using a lithium nucleus, particularly the MAS / spin-echo method (combination of magic angle rotation method and spin echo method).
A broad spectrum with a line width exceeding 1000 ppm in a spectrum based on Cl at 0 ppm (a)
And a cathode material for a non-aqueous electrolyte secondary battery, wherein the area ratio of the spectrum (b) to the spectrum (a) is at least 30% and less than 50% in the spectrum composed of the spinning side band (b) of at least 100 ppm or less. 2. A non-aqueous electrolyte secondary battery comprising the positive electrode material, a negative electrode material capable of inserting and extracting lithium, and a non-aqueous electrolyte.