JP2003007354A - Method and equipment for evaluating charasteristics of secondary lithium battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the method and equipment for quickly and conveniently evaluating the performance of the cathode active material applied to a secondary lithium battery. SOLUTION: The method for evaluating the property of the secondary lithium battery of this invention includes a step for measuring the electrical conductivity of the cathode active material used in the battery by changing the measuring temperature in a fixed temperature range, a step for approximating the relation between the reverse number (x) of the measured temperature and the logarithmic value (y) of the electric conductivity of the cathode active material by two lines on a xy plane, and a step for judging the character of the lithium secondary battery from the difference of the inclination of the two lines.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a characteristic evaluation method and a characteristic evaluation device for a lithium secondary battery.

[0002]

2. Description of the Related Art With the miniaturization of personal computers, video cameras, mobile phones, etc., in the field of information-related equipment and communication equipment, lithium secondary batteries are used as a power source for these equipment because of their high energy density. Has been put into practical use and has become widespread. On the other hand, also in the field of automobiles, the development of electric vehicles has been rushed due to environmental problems and resource problems, and lithium secondary batteries are also being considered as a power source for these electric vehicles.

Lithium secondary batteries have a layered rock salt structure of LiCoO ₂ , which can provide an operating voltage of 4V.
Positive electrode active materials such as LiNiO ₂ , LiMn ₂ O _{4 having} a spinel structure, and lithium transition metal composite oxides in which some of them are substituted with other elements are well known. Among these,
Batteries using LiCoO ₂ -based lithium-transition metal composite oxides as the positive electrode active material are now predominant because they are easy to synthesize and obtain the highest operating voltage.

By the way, these positive electrode active materials such as lithium-transition metal composite oxides have slight variations in synthesis conditions, such as composition of raw materials, heat treatment temperature, atmosphere (oxygen temperature, dew point and CO ₂ content), Its crystal structure changes with time. Even if the composition ratio of the positive electrode active material having a changed crystal structure is similar, the manner of occlusion / desorption of lithium is greatly different, and the performance when applied to a battery is greatly different.

Therefore, before constructing a battery using the produced positive electrode active material such as lithium transition metal composite oxide,
It is necessary to determine the crystal structure of each production lot and evaluate whether or not it has appropriate characteristics when used in a battery.

As a method for evaluating a positive electrode active material, conventionally, composition analysis by powder X-ray diffraction, ICP emission spectroscopic analysis, etc., and measurement of magnetic properties (Japanese Patent Laid-Open No. 9-180722).
No.), an electrode performance test using a small battery and the like are used, and the characteristics of the positive electrode active material are evaluated by using these alone or in combination.

[0007]

However, in ordinary powder X-ray diffraction, since the X-ray scattering cross-section of Li and oxygen is small and the X-ray reflection / diffraction intensity is weak, long-time measurement and precise structure are possible. Analysis is needed. These drawbacks of powder X-ray diffraction are solved to some extent by performing neutron diffraction. However, since a neutron source requires a nuclear reactor, it is extremely difficult to use neutron diffraction in general, and it is not suitable for applications such as lot inspection. Further, when the elements are non-uniformly distributed in the sample, a large error is involved in the estimation of the internal structure. In addition, although the compositional analysis can determine the elemental composition in the sample, the details of the compositional elements in the crystal structure cannot be determined, and lithium that is related to battery characteristics and lithium that is not related to the battery characteristics cannot be distinguished. Although the magnetic properties can be evaluated to some extent easily, the performance of the positive electrode active material alone cannot be evaluated, but only the performance of the positive electrode active material can be evaluated. This also applies to and. Therefore, the electrode performance test is the most reliable method for evaluating the performance of the positive electrode when applied to a battery. However, the electrode performance test is for powder X
Although it is a more reliable inspection method than line diffraction and composition analysis,
Since a battery is manufactured even though it is small in size, it is considerably complicated, and it is necessary to perform a long-term test in order to evaluate various characteristics. In other words, application to applications such as lot inspection is difficult.

Therefore, it is an object of the present invention to provide a method for quickly and simply evaluating the performance of a positive electrode active material when applied to a lithium secondary battery. Another object of the present invention is to provide a device for quickly and simply evaluating the performance of a positive electrode active material when applied to a lithium secondary battery.

[0009]

Means for Solving the Problems In order to solve the above problems, the present inventor has earnestly studied, and as a result, obtained the following findings. That is, the present inventor has found that the conductivity of the positive electrode active material is such that the transition metal oxide and the valence electrons in the transition metal composite oxide are determined by including lithium ions in the transition metal oxide such as nickel acid and cobalt acid. The positive electrode active material was considered to be an impurity semiconductor in which lithium ions act as impurities because it is exhibited by forming a conduction band derived from lithium ions in the forbidden band between the positive electrode active material and the band. That is, taking nickelic acid as an example, pure nickelic acid is essentially an insulator with a wide energy gap of about 4 eV in the forbidden band between its valence band and conduction band. Lithium ions are intercalated (become Li _X NiO ₂ ) in the crystal lattice of nickel acid, so that an impurity level derived from lithium ions is formed in the forbidden band. Since the impurity level is separated from the valence band by only a few meV, electrons can be excited by thermal entropy energy and free holes are formed in the valence band derived from nickel acid, resulting in conductivity. ing.

By the way, charge and discharge of the lithium secondary battery is
It progresses by occluding and desorbing lithium ions in the crystal lattice that constitutes the positive electrode active material. That is, the impurity level of the positive electrode active material is filled with the absorption of lithium ions. Therefore, the property of the positive electrode active material absorbing and desorbing lithium ions, in other words, the property of the lithium secondary battery is determined by measuring the state of the impurity level (bandwidth) formed in the above-mentioned forbidden band. It is possible.

As a method for measuring the band width of the impurity level of the positive electrode active material, the present inventor has focused on the electric conductivity at low temperature. That is, by measuring at low temperature, the number of electrons thermally excited from the valence band to the impurity level gradually decreases, and the electric conductivity of the positive electrode active material also decreases accordingly. Since the relationship between the conductivity (σ) and the measured temperature (T) follows the Arrhenius law, it is activated from the Arrhenius equation (ln σ = −Ea / RT + C, Ea; activation energy, R; gas constant, C; constant). The excitation energy from the valence band to the impurity level is required as the energy for conversion.

The graph (Arrhenius plot) based on this Arrhenius equation includes a fine structure derived from the transition between impurity levels and the intermediate portion of the impurity levels, but it is roughly classified into high impurity levels on the high temperature side. It can be approximated by a portion derived from the energy level on the energy side and a portion derived from the energy level on the low energy side of the impurity level on the low temperature side.
In other words, the excitation energy that can be determined by measuring the electric conductivity also fills the high energy side of the impurity level at a relatively high temperature, and as a result, the energy level of the high energy side of the impurity conduction band affects the electric conductivity and This is because as the number of sides increases from several K to several tens of K), it becomes difficult to fill the high energy side, and finally the electrical conductivity largely depends on the energy level on the low energy side of the impurity level.

Therefore, it has been found that it is possible to obtain the band width of the impurity level by obtaining the difference between the inclinations of these two line segments. The characteristics of the lithium secondary battery can be evaluated by obtaining the band width of this impurity level. For example, the amount of lithium ions that the positive electrode active material can occlude / desorb, that is, the charge / discharge capacity of a lithium secondary battery formed from a positive electrode using the positive electrode active material can be evaluated. In addition, we found that the larger the impurity level bandwidth, the faster the absorption and desorption of lithium ions from the positive electrode active material, and the internal resistance and low-temperature output can also be determined from the bandwidth. I found that.

The present inventor, based on the above findings, a measurement step of changing the measurement temperature in a predetermined temperature range to measure the electric conductivity of the positive electrode active material used in the lithium secondary battery,
An approximation step of approximating the relationship between the reciprocal (x) of the measured temperature in a predetermined temperature range and the logarithmic value (y) of the electric conductivity of the positive electrode active material by two line segments on the xy plane; And a determination step of determining the characteristics of the lithium secondary battery from the difference in the slope of the line segment.

In this determination step, the charge / discharge capacity, internal resistance and low temperature output can be suitably determined, so that the characteristic determined in the determination step can be at least one of these.

By the way, it is important to reduce the internal resistance in order to improve the output of the lithium secondary battery. The internal resistance can be evaluated by the above-mentioned characteristic evaluation method, but if the method can be simply evaluated, it is better to be able to evaluate it in more detail. Further, in the evaluation method described above, it is possible to determine the internal resistance based on the positive electrode active material, but a part of the internal resistance is a resistance such as a conductive material and a conductive material that electrically connects the positive electrode active material and the current collector. It is also affected by the contact resistance of (the resistance of elements other than the positive electrode active material in the positive electrode). Therefore, measuring the internal resistance (electrical conductivity) due to elements other than the positive electrode active material in the positive electrode is important in evaluating the battery performance.

Here, the electric conductivity (σ) of the positive electrode is Max.
It is expressed by the following equation showing the two-layer electric conductivity of the well. σ =
σm (σc + Kc · σm + Kc (σc−σm)) / (σ
c + Kc.sigma.m-.phi. (. sigma.c-.sigma.m)), where .sigma.m is the electrical conductivity derived from materials other than the positive electrode active material, .sigma.c is the electrical conductivity derived from the positive electrode active material, and Kc is the shape factor of the positive electrode active material.
φ is the volume fraction of the positive electrode active material.

By the way, since the positive electrode active material is a semiconductor as described above, its electric conductivity is lowered as the temperature is lowered. On the other hand, other than the positive electrode active material, the conductive material (generally a carbon material such as graphite is used) and the current collector, which are elements that make a large contribution to the electric conductivity of the positive electrode, are good conductors and have a high temperature. The electrical conductivity increases as is decreased. Therefore, at an extremely low temperature, σm >> σc holds, and the elements other than the positive electrode active material dominate the electric conductivity (σ) of the positive electrode as in the following approximate expression. σ≈σm (Kc (1−φ)) / (Kc + φ) Therefore, by measuring the electric conductivity of the positive electrode at a low temperature, the electric conductivity of the positive electrode derived from sources other than the positive electrode active material is measured. I found that I can.

Therefore, the present inventor, based on the above findings,
A measurement step of measuring the electric conductivity of a positive electrode containing a positive electrode active material used in a lithium secondary battery by changing the measurement temperature within a predetermined temperature range until the electric conductivity becomes substantially constant, and the electric conductivity of the positive electrode. And a determination step of determining the electrical conductivity of the positive electrode derived from the positive electrode active material and the electrical conductivity derived from other than the lowest value of the positive electrode, the characteristic evaluation method of the lithium secondary battery, Invented

In the method for evaluating the characteristics of these lithium secondary batteries, the performance of a lithium secondary battery using the positive electrode can be evaluated quickly and simply by simply measuring the electric conductivity of the positive electrode for a short time.

Further, as an apparatus for carrying out the above-mentioned method for evaluating the characteristics of the lithium secondary battery, the present inventor has
An electric conductivity measuring means for measuring the electric conductivity of a positive electrode or a positive electrode active material used in a lithium secondary battery, a cooling means for cooling the positive electrode or the positive electrode active material to a temperature range near a cryogenic temperature, and a predetermined temperature. The relationship between the reciprocal (x) of the measured temperature in the range and the logarithmic value (y) of the electric conductivity of the positive electrode or the positive electrode active material measured by the electric conductivity measuring means is a combination of two line segments on the xy plane. The characteristic of the lithium secondary battery, comprising: an approximating means that approximates with the above; and a computing means that has a determining means that determines the characteristic of the lithium secondary battery from the difference between the inclinations of the two line segments. The evaluation device was invented.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of a method for evaluating characteristics of a lithium secondary battery and an embodiment of an apparatus for evaluating the characteristics of the present invention will be described below.

<Characteristic Evaluation Method of Lithium Secondary Battery> The target to which the characteristic evaluation method of the lithium secondary battery of the present invention can be applied is a positive electrode active material having semiconductor-like properties. Specifically, a positive electrode active material (layered rock salt structure LiCoO ₂ , L containing a complex oxide of a transition metal such as cobalt, nickel, and manganese and a lithium, which is generally used in a lithium secondary battery.
iNiO ₂ , spinel structure LiMn ₂ O _{4 and the} like) have almost all semiconducting properties and are objects of the method of the present invention.
Further, the positive electrode active material is not limited to a single composition and may be a mixture of positive electrode active materials having a plurality of compositions. Further, it can be applied to a positive electrode using the positive electrode active material. By applying the method of the present invention to a positive electrode in a form used in a lithium secondary battery, not only the characteristics of the positive electrode active material but also the characteristics of the positive electrode can be evaluated. When the method of the present invention is applied to the positive electrode, it can be evaluated as it is. For example, in the case of a sheet-shaped positive electrode, the electrical conductivity can be measured by bringing the electrodes into contact with both surfaces in the thickness direction of the positive electrode. When the method of the present invention is applied to the positive electrode active material, the positive electrode active material is often a powder,
It is preferable to evaluate by a method such that a certain shape can be maintained while maintaining conductivity. For example, the electric conductivity can be measured after the positive electrode active material is filled in the measuring cell or after being pressed.

As the positive electrode to be evaluated, one manufactured by a general method can be used as it is. To give an example of the positive electrode manufacturing method, a positive electrode active material to be evaluated is mixed with a conductive material and a binder, and a suitable solvent is added to form a paste-like positive electrode mixture. What was formed by apply | coating on the collector surface of this, drying, and compressing in order to raise positive electrode density can be used. The conductive material at this time is for ensuring the electrical conductivity of the positive electrode, and it is preferable to use one kind or a mixture of two or more kinds of carbon substance powders such as carbon black, acetylene black, and graphite. it can. The binder plays a role of binding the active material particles and the conductive material particles together, and it is preferable to use a fluorine-containing resin such as polytetrafluoroethylene, polyvinylidene fluoride, or fluororubber, or a thermoplastic resin such as polypropylene or polyethylene. it can. An organic solvent such as N-methyl-2-pyrrolidone can be used as a solvent for dispersing the active material, the conductive material, and the binder.

The method of the present invention is a method for evaluating the characteristics of a lithium secondary battery from the impurity level and has a charge / discharge capacity, an internal resistance and a low temperature output (hereinafter referred to as a charge / discharge capacity) of a lithium secondary battery having a measuring step, an approximating step and a judging step. Abbreviated as "charge / discharge capacity") and a method of evaluating the characteristics of the lithium secondary battery from the value of electric conductivity at low temperature, which has a measurement step and a determination step. There is a method of evaluating the internal resistance.

In both measuring methods, the conductivity of a positive electrode active material or a positive electrode using the positive electrode active material (hereinafter abbreviated as "positive electrode or the like") is measured in a predetermined temperature range as a measuring step. The predetermined temperature range for evaluating charge / discharge capacity is
When the measured electrical conductivity is Arrhenius plotted, it shall be the range including at least the inflection point. As the predetermined temperature range, it is preferable to set the liquid He temperature (4.15K) to 140K so that the high energy side and the low energy side of the impurity level can be measured. This is because the lower limit is a low temperature at which the liquid He temperature can be inexpensively achieved, and the liquid He temperature is sufficient. And the upper limit of 140K
Is for the purpose of eliminating the influence by having a maximum point in the vicinity of 200 to 250 K when an Arrhenius plot is made from an experiment of the temperature dependence of the electric conductivity of the positive electrode.

The predetermined temperature range for evaluating the internal resistance is as low as 20 K or less. Preferably, the predetermined temperature range is up to a temperature at which the value of electric conductivity is substantially constant (plateau region).
It is preferably around the liquid He temperature because it is relatively easy to reach. However, both methods may measure the electrical conductivity in a temperature range other than these predetermined temperature ranges for any reason.

The measuring steps of both methods can be performed simultaneously. In other words, from about 140 K to room temperature, liquid He
By measuring the electrical conductivity in the temperature range up to near temperature, it is possible to evaluate charge and discharge capacity and internal resistance at the same time,
preferable.

The electric conductivity can be measured by directly measuring the electric conductivity, measuring the resistance, and obtaining the reciprocal thereof.
Needless to say, this can be achieved by measuring the relationship between voltage and current and other related physical quantities. The method of lowering the temperature of the positive electrode and the like is not particularly limited, and any method may be used as long as it can cool the positive electrode and the like to a required temperature. For example, a cryostat, a thermoelectric cooler, etc. can be used.

When measuring the electric conductivity, it is general to make a measurement by contacting a positive electrode with a measuring electrode. At that time, the contact resistance between the positive electrode and the measuring electrode is lowered. For this purpose, it is preferable to have a step of forming a gold coating on the surface of the positive electrode or the like by vapor deposition or the like, and a step of contacting an electrode made of nickel felt on the gold coating and then impregnating with a silver paste and pressure bonding. In that case, the contact resistance between the positive electrode or the like and the electrode can be obtained by changing the area where the measurement electrode contacts the positive electrode or the like to measure the electric conductivity, and the electric conductivity of the positive electrode or the like can be corrected. preferable.

When the charge and discharge capacity of a lithium secondary battery is evaluated, an approximation step is performed to estimate the impurity level derived from lithium. In the approximation step, the relationship between the reciprocal of the measured temperature (x) and the logarithmic value (y) of the electric conductivity of the positive electrode active material in a predetermined temperature range is plotted on the xy plane (Arrhenius plot), and the two values are plotted. This is a step of approximating with a line segment. Specifically, in order to approximate the Arrhenius plot with two line segments, it is preferable to perform the approximation before and after the inflection point of the Arrhenius plot, and further, the liquid He temperature to the temperature range of 20K and the temperature of 50 to 140K. It is preferable that they are approximated by the range and respectively. Since the inflection point of the Arrhenius plot often exists at 20 to 50K from the experiment, the range is excluded. This experiment will be described in detail in Examples described later. Here, the approximation method of the line segment is not particularly limited, and for example, the least square method or the like can be adopted.

The determination step in the method of evaluating the charge / discharge capacity and the like is a step of determining the charge / discharge capacity and the like of the lithium secondary battery from the difference in the slopes of the two line segments obtained in the approximation step. When the composition of the positive electrode active material is approximately the same, the larger the difference in this gradient, the larger the charge / discharge capacity and the low-temperature output, and the smaller the internal resistance. That is, the larger the difference in the inclination, the better the characteristics of the lithium secondary battery.

The determining step in the method for evaluating the internal resistance is based on the electric conductivity of the plateau portion (minimum electric conductivity) obtained in the measuring step, and the electric conductivity derived from other than the positive electrode active material of the positive electrode. Then, the electric conductivity derived from the positive electrode active material is derived from the value based on the following equation.

Σ≈σm (Kc (1-φ)) / (Kc +
φ), σm is the electrical conductivity derived from other than the positive electrode active material, σc is the electrical conductivity derived from the positive electrode active material, Kc is the shape factor of the positive electrode active material, and φ is the volume fraction of the positive electrode active material. .

Here, the value of Kc can be approximated by approximately 2. The value of φ can be measured by observing the cross-sectional structure with a focused beam-scanning ion microscope.

The actual evaluation may be carried out for each manufacturing lot of the manufactured positive electrode or the like, and in the case where the manufacturing lot is evaluated for each manufacturing lot, a slight change in the structure of the positive electrode active material and the positive electrode The manufacturing state can be detected quickly and easily, and conversely, by manufacturing, shipping, and using only lots within the specified limit values, variations in the charge / discharge capacity, internal resistance, etc. of the final lithium secondary battery can be reduced. It can be efficiently suppressed in the stage before the battery configuration.

<Characteristic Evaluation Device for Lithium Secondary Battery> The device of the present invention is a device suitable for performing the above-mentioned characteristic evaluation method for lithium secondary batteries. The device of the present invention can perform any of the two methods of evaluating the characteristics of the lithium secondary battery described above. The object to be evaluated that can be evaluated by the device of the present invention is the same as the object of the method for evaluating the characteristics of the lithium secondary battery described above, and therefore the description thereof is omitted here.

The device of the present invention has an electric conductivity measuring means, a cooling means and a computing means. The measuring step of the above-mentioned characteristic evaluation method can be performed by the electric conductivity measuring means and the cooling means. That is, the electric conductivity of the positive electrode or the positive electrode active material cooled to near the cryogenic temperature by the cooling means is measured by the electric conductivity measuring means. The measured value of the electric conductivity is held by some method and provided to the calculation unit described later. The electrical conductivity measuring means and the cooling means are not particularly limited, and general electrical conductivity measuring means and cooling means can be adopted. Specifically, means that can realize the method described in the characteristic evaluation method of the lithium secondary battery described above is applicable.

The electric conductivity measuring means is generally measured by bringing a measuring electrode into contact with a positive electrode or the like, but at that time, in order to reduce the contact resistance between the positive electrode or the like and the measuring electrode. To produce a gold coating on the surface of the positive electrode, etc.
It is preferable that an electrode made of a felt made of nickel is brought into contact with the gold coating, followed by impregnation with a silver paste and pressure bonding.

The calculating means is a means that can realize the approximating step and the determining step in the above-mentioned characteristic evaluation method of the lithium secondary battery. For example, the arithmetic means can be realized by logic on a computer.

It should be noted that the respective embodiments of the method and apparatus for evaluating characteristics of a lithium secondary battery of the present invention described above are merely examples, and these embodiments are not limited to the above-described embodiments, and those skilled in the art will understand. It is possible to adopt a form in which various changes and improvements are made based on the knowledge of.

[0042]

EXAMPLES In order to prove the superiority of the method for evaluating the characteristics of the lithium secondary battery of the present invention, positive electrodes using positive electrode active materials of various lithium transition metal composite oxides were prepared, and the electric conductivity of each positive electrode was increased. Was measured to evaluate the characteristics of the positive electrode active material and the positive electrode, and a lithium secondary battery using these positive electrodes was manufactured and the characteristics were actually confirmed. These will be described below as examples.

<Characteristic Evaluation Device for Lithium Secondary Battery> The characteristic evaluation of this embodiment was carried out by the following device. That is, as shown in FIG. 1, the present apparatus comprises a current generator 1, a multimeter 2, a set of copper electrodes 3 and a gold-iron chromel thermocouple 4 as electric conductivity measuring means, and a liquid He as cooling means. It comprises a dual 5 (having a cavity C having a temperature gradient from room temperature to the liquid He temperature from the entrance to the deepest part), and a computer 6 equipped with a logic as a calculation means. The sample held between the copper electrodes 3 is gradually inserted into the cavity C of the liquid He deur 4 together with the gold-iron chromel thermocouple 4 attached to the copper electrodes 3 and cooled.

A constant current (A) generated by the current generator 1 is applied to the sample via the copper electrode 3. At that time, the voltage (V) between the copper electrodes 3 is measured to measure the resistivity, and the sample temperature is measured from the electromotive force from the gold-iron chromel thermocouple 4 measured at the same time. The resistivity is obtained by multiplying the value obtained by dividing the surface area of the electrode by the sample thickness and the value obtained by dividing the voltage (V) by the current (A). The electrical conductivity is calculated as the reciprocal of the obtained resistivity. The sample temperature is calculated from the temperature corresponding to the electromotive force of the gold-iron chromel thermocouple 4, and a computer 6 is used to generate an Arrhenius plot from the calculated values of electrical conductivity at each measured temperature to calculate activation energy.

<Preparation of Sample of Lithium Transition Metal Complex Oxide> Particle powders of lithium hydroxide monohydrate (LiOH.H ₂ O) and nickel hydroxide (Ni (OH) ₂ ) in molar ratio. They were mixed well so that the ratio of Li: Ni was 1: 1. This mixed particle powder was compression-molded into a tablet and then heat-treated at 650 ° C. for 24 hours in an oxygen stream. The specific surface area of the obtained tablets was changed by a crusher (impact mill) to prepare Sample 1 (specific surface area 0.31 m
² / g), sample 2 (specific surface area 0.43 m ² / g), sample 3
(Specific surface area 0.63 m ² / g).

Lithium hydroxide monohydrate (LiOH.H ₂
O) and manganese dioxide (MnO ₂ ) are mixed well in a molar ratio of Li: Mn = 1: 2, and the same treatment as in Samples 1 to 3 is performed to obtain lithium manganese composite oxide. Powder. This powder and a lithium-nickel composite oxide were mixed at a mass ratio of 75:25 to obtain Sample 4.

<Determination of Crystal Structure by X-Ray Diffraction Analysis> The crystal structure of each of the above-prepared samples was determined by Rietveld analysis of the powder X-ray diffraction pattern. The diffraction pattern was measured by a step scan of 2θ = 0.004 ° under the condition that the strongest diffraction line intensity was 80000 counts or more. The measurement time per sample was about 2 hours. Samples 1 to 3 were all single layers with a layered rock salt structure. Sample 4 had a two-phase mixed state of a layered rock salt structure and a spinel structure.

<Determination of Elemental Composition by ICP Optical Emission Spectroscopy> The elemental composition of each of the prepared samples was measured by inductively coupled plasma (ICP) optical emission spectroscopy. The results are shown in Table 1. The balance of the composition is oxygen.

[0049]

[Table 1]

<Preparation of Positive Electrode> To 85 parts by mass of each positive electrode active material of Samples 1 to 4, 10 parts by mass of graphite as a conductive material and 5 parts by mass of polyvinylidene fluoride as a binder were mixed, and then N-methyl was added. 2-Pyrrolidone was added as a solvent to prepare a positive electrode mixture paste. This paste was applied to both sides of a current collector made of aluminum foil, dried, and rolled to form sheet-shaped positive electrodes, which were used as positive electrodes of Samples 1 to 4.

<Measurement of Electric Conductivity and Calculation of Activation Energy etc.> The electric conductivity of each sample was measured (measuring step). As a pretreatment for measurement, each sample is 1.5 × 1.5c
After cutting to m, gold was vapor-deposited on both sides with a thickness of 150Å. After that, apply silver paste on both sides,
A 1.2 × 1.2 cm Ni felt was attached. Then, the copper electrodes 3 were attached to both surfaces of the sample, and the electrical conductivity at low temperature was measured. The electrical conductivity was derived by measuring the resistivity of each sample. The temperature dependence of the electrical conductivity was measured in the same manner for the aluminum foil as a blank. The measurement temperature range was from liquid He temperature to room temperature. As an example, the measurement results of the resistivity of Sample 1 are shown in FIG.

The following processing was performed by the logic installed in the computer 6. First, the relationship between the reciprocal of the measured temperature and the logarithm of the measured electric conductivity was plotted (Arrhenius plot). As an example, an Arrhenius plot for Sample 1 is shown in FIG. Here, the Arrhenius plots of the other samples are not shown, but as in the case of sample 1, the sample had an inflection point between 20 and 50K. this is,
The same applies to positive electrodes and the like other than the samples shown in this example.

Then, as the predetermined temperature range of the Arrhenius plot, the measured value of liquid He temperature to 140 K was taken out, and the range was approximated by two line segments. Specifically, the slopes of the line segment approximated from the temperature range of the liquid He to 20 K and the line segment approximated from the temperature range of 50 to 140 K were obtained. The activation energy (energy level) was determined by applying the slopes to the Arrhenius equation (ln σ = −Ea / RT + C, Ea; activation energy, R; gas constant, C; constant). Liquid H
The energy level at the lower end of the impurity level can be derived from the line segment approximated from the temperature range of e to 20 K, and the energy level at the upper end can be derived from the line segment approximated from the temperature range of 50 to 140 K. Then, the difference in energy level between the upper end and the lower end of each sample was defined as the band width of the impurity level.

Next, the electric conductivity (σ) when each sample is at the liquid He temperature is expressed by the following equation: σ≈σm (Kc (1-
φ)) / (Kc + φ), where σm is the electric conductivity derived from other than the positive electrode active material, Kc is the shape factor of the positive electrode active material, and φ
Is approximated by the volume fraction of the positive electrode active material, and elements other than the positive electrode active material are dominant. Therefore, the electric conductivity was obtained from the resistivity of each sample at the liquid He temperature, and the electric conductivity (σm) derived from other than the positive electrode active material was derived by applying the electric conductivity to the above equation. Here, Kc was set to 2, and φ was determined by observing a sectional structure with a focused ion beam-scanning ion microscope.

Table 2 shows the energy level, the band width, the resistivity at the liquid He temperature, and σm for each sample.

[0056]

[Table 2]

<Production of Lithium Secondary Battery> A lithium secondary battery was produced using each of the above samples as a positive electrode.
The structure of the manufactured lithium secondary battery will be described with reference to FIG. The lithium secondary battery 10 has an electrode body 14 in which a positive electrode and a negative electrode are spirally wound via a separator in a battery can 12. The positive electrode current collector lead 16 drawn from the positive electrode is connected to the cap 17 attached to the battery can 12, and the negative electrode current collector lead 18 drawn from the negative electrode is connected to the battery can 12.
An insulating plate 20 is attached to each of the inner bottom surface of the battery can 12 and the upper portion of the electrode assembly 14.

Next, a method of making this battery will be described.

Natural graphite was used as the negative electrode active material. After mixing 92.5 parts by mass of this negative electrode active material with 7.5 parts by mass of polyvinylidene fluoride as a binder, N-methyl-2-
Pyrrolidone was added as a solvent to prepare a negative electrode mixture paste. This paste was applied on both sides of a current collector made of copper foil, dried and rolled to obtain a sheet-shaped negative electrode.

As the separator, a polypropylene porous film cut into a wider area than the positive electrode plate and the negative electrode plate was used. A separator was interposed between the positive electrode plate and the negative electrode plate, and the whole was wound into a roll to form the electrode body 14. An insulating plate 20 is attached under the electrode body 14, and the battery can 12
Then, the negative electrode lead wire 18 was spot-welded to the battery can 12. Also, the insulating plate 20 was attached to the upper side of the electrode body, and then the non-aqueous electrolyte solution was injected.

In the non-aqueous electrolyte solution, ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed in a volume ratio of 3: 4, and this was mixed with lithium hexafluorophosphate (L).
iPF ₆ ) was dissolved to a concentration of 1 mol / L and used. After spot welding the cap 17 incorporating the gasket and the positive electrode lead wire 16, the cap 17 is mounted on the battery can 12, and the upper part of the battery can 12 is caulked to hermetically seal the inside of the battery to form a cylindrical lithium secondary battery. Was completed.

<Measurement of Specific Capacity> For each of the above lithium secondary batteries using Sample 1 to Sample 4 as the positive electrode,
First, I did conditioning. Conditioning condition is 25 ℃ ambient temperature, current density 0.25mA
Constant-current constant-voltage charge of 6 hours and constant-current constant-charge with a total charge time of 6 hours at a constant current of / cm ² until the end-of-charge voltage of 4.1V is reached and further charging at a constant voltage of 4.1V.
One charge / discharge cycle consisting of constant current discharge until reaching a discharge end voltage of 3 V at a constant current of mA / cm ² was performed, and then a charge end voltage of 4.1 V was obtained at a constant current of current density of 1 mA / cm ^2. 3. Charge the battery until it reaches the destination.
A charging / discharging cycle consisting of a constant current constant voltage charging with a total charging time of 2 hours for charging with a constant voltage of 1 V and a constant current discharging until a discharge end voltage of 3 V is reached with a constant current of a current density of 1 mA / cm ^2. Five cycles were performed. The discharge capacity in the final cycle was measured, and the value obtained by dividing the value by the mass of the positive electrode active material was taken as the specific capacity.

<Measurement of Internal Resistance> The internal resistance of each lithium secondary battery at 25 ° C. was measured. The internal resistance was measured by measuring the value of the terminal voltage of the battery when the value of the current applied to the battery was changed and deriving from the slope of the line connecting each current value and the voltage value.

<Measurement of Low Temperature Output> The output at −30 ° C. was measured for each lithium secondary battery. The output value is a current value I _2.5 (−30 ° C.) at which the battery voltage becomes 2.5 V from the current dependency of the voltage drop amount in 10 seconds after the fully charged battery of each test example was cooled to −30 ° C. ) Was obtained and 2.5 × I _2.5 (W) was taken as the 2.5V cut 10 second output value (−30 ° C.).

<List of Experimental Results and Characteristic Evaluation of Lithium Secondary Battery> The results of the above series of experiments are shown in Table 3 below.
The data of each sample are shown in the form of a list.

[0066]

[Table 3]

Further, the band width shown in Table 2 and the specific capacity, internal resistance and low temperature output shown in Table 3 are linearly approximated by the least square method. In specific capacity, (specific capacity (mAh / g)) = 32.1 × (impurity level (me
V)) + 139.07, R ² = 0.999, for internal resistance (internal resistance (mΩ)) = − 0.5936 × (impurity level (meV)) + 1.003, R ² = 0.989, low temperature In the output, (low temperature output (W)) = 2.6347 × (impurity level (meV)) + 1.3075, R ² = 0.9995
Became. It can be seen that in all cases, the correlation coefficient (R ² ) is extremely close to 1, and there is a high positive correlation.

The internal resistance was not highly correlated with the value of the resistivity derived from other than the positive electrode active material derived from the electric conductivity at the liquid He temperature shown in Table 2, which was 25 ° C. when the internal resistance was measured. Therefore, it is considered that the positive electrode active material has a relatively high electric conductivity, and the influence of the resistance derived from other than the positive electrode active material is reduced. However, it goes without saying that it is preferable to lower the resistivity derived from other than the positive electrode active material. In that case, the resistivity derived from other than the positive electrode active material can be determined by this evaluation method.

[0069]

The method for evaluating the characteristics of the lithium secondary battery of the present invention is an evaluation method suitable for evaluating the characteristics of the positive electrode or the positive electrode active material, and the temperature dependence of the electrical conductivity of the positive electrode or the like is measured. The characteristics of the lithium secondary battery using the positive electrode, etc. (charge / discharge capacity, internal resistance, low temperature output, etc.)
Is determined. By such a method,
INDUSTRIAL APPLICABILITY The method for evaluating characteristics of a lithium secondary battery of the present invention is an evaluation method capable of evaluating characteristics of a lithium secondary battery with high reliability, quickly and easily.

[Brief description of drawings]

FIG. 1 is a schematic view showing a characteristic evaluation device used for measuring the electric conductivity of a positive electrode in an example.

FIG. 2 is a graph showing the temperature dependence of the electrical conductivity of the positive electrode of Sample 1 in the example.

FIG. 3 is an Arrhenius plot of the electric conductivity and the measurement temperature of the positive electrode of sample 1 in the example.

FIG. 4 is a diagram showing a configuration of a lithium secondary battery manufactured in an example.

[Explanation of symbols]

1: Current generator 2: Multimeter 3: Copper electrode 4: Gold-iron chromel thermocouple 5: Liquid Hedur C: Cavity 6: Computer 10: Lithium secondary battery 12: Battery can 14: Electrode body 16: Positive electrode current collecting lead 17: Cap 18: Negative electrode current collecting lead 20: Insulation plate

Claims (7)

[Claims] 1. A measurement step of measuring the electric conductivity of a positive electrode active material used in a lithium secondary battery by changing the measurement temperature within a predetermined temperature range, the reciprocal of the measured temperature (x) and the positive electrode active material An approximation step of approximating the relationship with the logarithmic value (y) of the electrical conductivity by two line segments on the xy plane, and determining the characteristics of the lithium secondary battery from the difference in the slopes of the two line segments A method for evaluating characteristics of a lithium secondary battery, comprising: a determining step. 2. The characteristic determined in the determination step is
The method for evaluating characteristics of a lithium secondary battery according to claim 1, wherein the method is at least one of charge / discharge capacity, internal resistance, and low-temperature output. 3. The measuring step is a step of measuring electric conductivity of a positive electrode containing the positive electrode active material.
Alternatively, the method for evaluating characteristics of the lithium secondary battery according to item 2. 4. The predetermined temperature range is a liquid He temperature of ˜1.
The secondary lithium according to any one of claims 1 to 3, wherein the inclination of the two line segments is approximated by a temperature range of liquid He temperature to 20K and a temperature range of 50 to 140K, respectively. Battery characteristics evaluation method. 5. A measurement step of measuring the electric conductivity of a positive electrode containing a positive electrode active material used in a lithium secondary battery by changing the measurement temperature in a predetermined temperature range until the electric conductivity becomes substantially constant, A lithium secondary comprising: a determination step of determining the electrical conductivity of the positive electrode derived from the positive electrode active material and the electrical conductivity of the other derived from the lowest value of the electrical conductivity of the positive electrode. Battery characteristics evaluation method. 6. A pretreatment step before the measuring step, which is a step of forming a gold coating on both surfaces of the positive electrode, and an electrode made of nickel felt is brought into contact with the gold coating and then impregnated with a silver paste. And a step of crimping.
5. The method for evaluating characteristics of a lithium secondary battery according to any one of 5 above. 7. An electric conductivity measuring means for measuring the electric conductivity of a positive electrode or a positive electrode active material used in a lithium secondary battery, and a cooling means for cooling the positive electrode or the positive electrode active material to a temperature range near a cryogenic temperature. And the reciprocal (x) of the measured temperature in a predetermined temperature range and the logarithmic value (y) of the electric conductivity of the positive electrode or the positive electrode active material measured by the electric conductivity measuring means, two on the xy plane. Lithium, which has an approximating means for approximating the combination of the two line segments and a determining means for determining the characteristics of the lithium secondary battery from the difference in inclination between the two line segments. Characteristic evaluation device for secondary batteries.