KR20090020288A - Mixed electrode active material and secondary battery comprising the same - Google Patents

Abstract

An electrode active material is provided to measure the charge and discharge state easily of the last stage of charging/discharging by applying 2 kinds of electrode active materials with different profiles to a secondary battery. An electrode active material comprises (a) the first electrode active material having potential plateau on the charging/discharging profile and (b) the second electrode active material which has potential plateau of the first electrode active material within working potential range, and having a negative or positive slope of the charging/discharging profile in the whole working potential. The charging/discharging profile over the potential plateau of the first electrode active material is compensated with the profile of the second electrode active material and is controlled so that the absolute value of the average slope of the profile over the potential plateau is 0.05 V.g/mAh or less.

Description

Mixed electrode active material, and a secondary battery comprising the same {MIXED ELECTRODE ACTIVE MATERIAL AND SECONDARY BATTERY COMPRISING THE SAME}

The present invention relates to an electrode active material in which two kinds of electrode active materials having different charge and discharge profiles are mixed.

Recently, with the development of portable devices such as mobile phones, notebook computers, camcorders, and the like, demand for small secondary batteries such as Ni-MH (Ni-MH) secondary batteries and lithium secondary batteries is increasing. In particular, lithium secondary batteries using lithium and nonaqueous electrolytes have been actively developed due to the high possibility of realizing small, lightweight and high energy density batteries. Generally, transition metal oxides such as LiCoO ₂ , LiNiO ₂ , and LiMn ₂ O ₄ are used as cathode materials of lithium secondary batteries, and lithium metal or carbon as anode materials. And the like, and a lithium secondary battery is constructed by using an organic solvent containing lithium ions as an electrolyte between two electrodes.

LiCoO ₂ has been reported to be useful as a cathode active material for lithium secondary batteries in 1980, and many studies have been conducted to date, and have been employed as a cathode active material for commercialized lithium secondary batteries. However, cobalt (Co) element is one of the scarce resources on the planet, so the development of a new cathode active material to replace it. In particular, LiFePO ₄ has a large volume density of 3.6 g / cm ² , generates a high potential of 3.4 V, and a theoretical capacity of 170 mAh / g. In addition, Fe is abundant in resources and can be manufactured at low cost and LiFePO ₄ is a new lithium secondary battery that replaces LiCoO ₂ since LiFePO ₄ contains one electrochemically detachable Li per Fe atom. Is expected to be a positive electrode active material. In addition, LiMPO ₄ which is substituted with a transition metal instead of Fe of LiFePO ₄ is also known. It is interesting that the potential of Li can be varied in various ways depending on the type of M.

General formula Li _x MPO ₄ (M is at least one of Fe, Mn, Co, Ni, Cu, Zn, Mg, Cr, V, Mo, Ti, Al, Nb, B, Ga, 0.05≤x≤1.2) The compound to be displayed has an olivine-type crystal structure and excellent cycle characteristics because the change in crystal structure accompanying charge and discharge is small. In addition, since oxygen atoms in the crystal are stably present by covalent bonds with phosphorus, there is an advantage that the possibility of oxygen emission is small and the safety is excellent even when the battery is exposed to a high temperature environment.

However, when fabricating a secondary battery using Li _x MPO ₄ as a positive electrode active material, the potential profile due to charge and discharge has a very flat characteristic, which makes it difficult to measure state of charge / discharge. There is a problem. In addition, the problem caused by the flat potential is not limited to the olivine-type positive electrode active material, and the voltage band at which the flat potential appears, but also appears in the spinel type active material of LiMn ₂ O ₄ series or LiTi ₂ O ₄ series, You need to solve the problem.

In the present invention, when the first electrode active material having the flat potential and the second electrode active material having the flat potential are properly mixed, the charge and discharge state can be easily measured while taking advantage of the first electrode active material having the flat potential. It turns out that it is.

Accordingly, the present invention provides an electrode active material in which a first electrode active material having a flat potential and a second electrode active material having a flat potential are appropriately mixed, a method of manufacturing the same, an indicator of a charge / discharge state corresponding to the second electrode active material, and the electrode active material. An object of the present invention is to provide an electrode and a secondary battery.

The present invention a) a first electrode active material having a flat potential section on the charge and discharge profile; And b) a second electrode active material including a flat potential section of the first electrode active material in an operating potential region, wherein the charge / discharge profile has a negative or positive slope in the entire region of the operating potential.

The charge and discharge profile after the flat potential section of the first electrode active material is compensated by the profile of the second electrode active material so that the absolute value of the average slope of the profile after the flat potential section is adjusted to 0.05 V · g / mAh or less. It provides a mixed electrode active material.

In addition, the present invention a) a first electrode active material having a flat potential section on the charge and discharge profile; And b) mixing a second electrode active material including a flat potential section of the first electrode active material in an operating potential region, and having a negative or positive slope of a charge / discharge profile in an entire region of the operating potential. As a method for producing an electrode active material,

The charging and discharging profile after the flat potential section of the first electrode active material is compensated by the profile of the second electrode active material so that the absolute value of the average slope after the flat potential section is adjusted to be 0.05 V · g / mAh or less. It provides a method for producing an electrode active material.

The present invention may be mixed with a first electrode active material having a flat potential section on a charge / discharge profile to act as a second electrode active material, and include a flat potential section of the first electrode active material within an operating potential region. An indicator material for a charge / discharge state, in which the charge / discharge profile in all areas has a negative or positive slope,

Compensating the charge-discharge profile after the flat potential section of the first electrode active material, thereby providing an indicator material of the charge-discharge state that can be adjusted so that the absolute value of the average slope after the flat potential section is 0.035 V · g / mAh or less.

The present invention also provides an electrode and a secondary battery comprising the electrode active material described above.

The present invention includes a) a first electrode active material having a flat potential on a charge and discharge profile, and b) a flat potential section of the first electrode active material in an operating potential region, wherein the charge and discharge profile is negative or positive in all regions of the operating potential. A secondary battery capable of easily measuring the charge / discharge state at the end of charge / discharge, while utilizing the advantages of the electrode active material having a flat potential, by using the electrode active material in which the second electrode active material having the slope of Can be provided.

In the present invention, the state of charge / discharge refers to a degree of remaining capacity that can be charged or discharged based on the fully charged state of the electrode active material in the secondary battery.

In addition, in the present invention, the flat potential means a region in which the potential of the electrode active material is kept constant in the reversible capacitance region except for the initial and / or end of charge / discharge on the charge / discharge profile. It means a section in which the absolute value of the slope is close to zero.

The measuring method of the charge / discharge state includes a method of recognizing the charge / discharge degree by measuring the voltage change according to the charge / discharge state, and a method of recognizing the charge / discharge degree by measuring the voltage and current values, but only measuring the charge / discharge voltage. The easiest way is to do it. In order to measure the charge / discharge state by using the charge / discharge voltage value, the charge / discharge capacity and the charge / discharge voltage should generally be shown to be proportional to each other. However, in the case of the first electrode active material having a flat potential, even if the charge / discharge proceeds, It has a flat section where the potential does not change, and the charge / discharge potential rapidly changes after the flat section, and thus it is very difficult to measure the degree of charge / discharge during charging or use of the battery.

Therefore, in the present invention, a) a first electrode active material having a flat potential section on the charge-discharge profile; And b) a flat potential section of the first electrode active material in the operating potential region, and using the electrode active material in which the charge and discharge profile is mixed with the second electrode active material having a negative or positive slope in the entire region of the operating potential, Even when the first electrode active material having the flat potential is used as the electrode active material of the secondary battery, the charging and discharging state can be easily measured.

That is, when only a material having a flat potential is used as an electrode active material, even when charging and discharging proceeds, the potential does not change in the flat potential region, and a sudden change in voltage occurs when charging and discharging is almost completed. The measurement of the discharge state using the discharge voltage value was difficult. However, in the case of using the electrode active material mixed with the second electrode active material of the present invention, since the second electrode active material participates in charging and discharging even after the flat potential section of the first electrode active material, the potential change is observed. The abrupt change in potential can be avoided.

For example, when the first electrode active material exhibits a flat potential at 3.5V, when the second electrode active material having a constant slope is mixed therein, the action principle of discharge may be as follows.

1) 4V to 3.5V: The second electrode active material participates in the discharge.

2) 3.5V: The first electrode active material participates in the discharge until the end of the flat potential.

3) 3.5V ~ 3V: The second electrode active material participates in the discharge after the flat potential section of the first electrode active material.

Therefore, when the electrode active material is discharged from 4V to 3V, even if most of the discharge section is seen at 3.5V, the drop in potential can be measured in the above 3) area, so the degree of discharge and the end point of discharge are measured. The charging / discharging capacity ratio between the 2) regions and the 1) and 3) regions will vary depending on the mixing ratio of the first electrode active material and the second electrode active material and / or the reversible capacity of each material.

On the other hand, in the region 3) after the flat potential section of the first electrode active material, although the first electrode active material also participates in the discharge, its potential drop is sharp, and thus it is difficult to contribute to the measurement of the charged / discharged state. However, since the second electrode active material participates in the discharge together in the region 3) to compensate for the sharp decrease in the charge / discharge profile, the absolute value of the average slope of the profile after the flat potential period may be 0.05 V · g / mAh or less. . The slope value may be inferred from, for example, the charge and discharge curves as shown in FIGS. 1 to 4, and corresponds to the average slope value of the section after the planar potential as shown in regions (C) of FIGS. 1 and 3. It is preferable that the charge and discharge profile of the first electrode active material is smaller than the absolute value of the average slope of the profile after the flat potential section.

In the present invention, the average inclination is a value in consideration of the fact that the inclination of the actual profile cannot be represented since the charge / discharge profile is not a straight line, and represents the change amount of potential / change in reversible capacity in the set charge / discharge interval.

In the present invention, the first electrode active material having the flat potential section has a discharge capacity vs. The discharge voltage may be an active material having a constant discharge voltage in the curve, non-limiting examples of Li _x MPO ₄ (M is at least one selected from the group consisting of Fe, Mn, Co, Ni, 0.05≤x≤ 1.2), LiMn _{2 -x} Ni _x O ₄ (0≤x≤0.5), LiTi ₂ O ₄ , Li ₄ Ti ₅ O _{12 And the like} , as well as the active material is used as a mixture of two or more The same may be the case.

The reason why the electrode active material has a flat potential is that when lithium is inserted / deleted during charging and discharging, there is almost no change in structure, or during charging or discharging, a two-phase reaction is performed instead of a one-phase reaction. In many cases, the present invention is not limited to the above examples, and any case of the electrode active material exhibiting a flat potential for the above reason may fall within the scope of the present invention.

The second electrode active material of the present invention can act as an indicator in the state of charge and discharge, and since the change in potential due to charge and discharge is continuously shown throughout the charge and discharge period, it is mixed with the first electrode active material having a flat voltage, thereby charging and discharging It serves to facilitate the measurement of the state, and has a reversible electrochemical capacity by insertion and desorption of lithium, and can be used without particular limitation as long as the potential change due to charging and discharging continuously appears throughout the discharge period. Non-limiting examples of the second electrode active material may be LiNi _x Mn _y Co _{1 -x- y} O ₂ (0≤x≤1, 0≤y≤1), Li ₂ Ti ₃ O ₇ , TiO _2, etc. The above materials may be used alone or in a mixture of two or more thereof.

On the other hand, the operating voltage range of the first electrode active material (that is, the active material having a flat potential), in particular, the flat potential section is preferably included in the operating potential region of the second electrode active material.

At this time, it is preferable that the upper limit and the lower limit of the operating potential region of the second electrode active material differ by about 0.5V from the flat voltage region of the first electrode active material. If the difference is too small, it may be difficult to distinguish the voltage difference between the two materials, it is good to show a difference of about 0.5V.

For example, when LiMnPO ₄ is used as the first electrode active material, since the operating voltage range of LiMnPO ₄ is around 4.1V, a preferred material for the second electrode active material is LiCoO ₂ or the like having an operating voltage range of 3.5V to 4.5V. This will be possible.

On the other hand, the second electrode active material may be mixed in the range of 0.1 parts by weight to 80 parts by weight relative to 100 parts by weight of the first electrode active material having a flat potential, preferably in the range of 1 part by weight to 50 parts by weight. When the amount of the second electrode active material is less than 0.1 part by weight, it is difficult to expect the effect of the present invention. When the amount of the second electrode active material is added more than 80 parts by weight, it is difficult to take advantage of the first electrode active material having the flat voltage as the main active material. . According to the present invention, even if the first electrode active material having the flat potential is used in the secondary battery, the charge / discharge profile may have an appropriate slope after the flat potential section of the first electrode active material by mixing with the second electrode active material. Therefore, by measuring the charging and discharging potential at that time, it is possible to measure the charging and discharging capacity corresponding thereto, and the correspondence relation between the charging and discharging potential and the charging and discharging capacity is determined by measuring the discharge capacity in advance for each charging and discharging potential. And then you can get it from there.

<Production of Electrode and Secondary Battery>

An electrode comprising the material described in the present invention as an electrode active material can be prepared by a method known to those skilled in the art. For example, in addition to using the material as an active material according to the present invention, the electrode may further use a conductive agent for providing electrical conductivity and a binder that enables adhesion between the material and the current collector.

The paste was prepared by adding and stirring a conductive agent to 1 to 30 wt% and a binder to 1 to 10 wt% with respect to the electrode active material prepared by the above method, and then adding the mixture to the dispersion solvent and stirring the mixture. It may be applied to, pressed and dried to prepare a laminate electrode.

The conducting agent generally uses carbon black. Products currently marketed as conductive agents include acetylene black series (Chevron Chemical Company or Gulf Oil Company), Ketjen Black EC series (Armak Company ), Vulcan XC-72 (manufactured by Cabot Company), and Super P (manufactured by MMM).

Representative examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF) or copolymers thereof, cellulose, and the like, and representative examples of the dispersant are isopropyl alcohol and N-methylpyrrolidone. (NMP), acetone and the like.

The current collector of the metal material is a highly conductive metal, and a metal to which the paste of the material can easily adhere can be used as long as it is not reactive in the voltage range of the battery. Representative examples include meshes, foils, and the like, such as aluminum or stainless steel.

The present invention also provides a secondary battery comprising the electrode of the present invention. The secondary battery of the present invention can be produced using a method known in the art, and is not particularly limited. For example, the separator may be placed between the positive electrode and the negative electrode to add a nonaqueous electrolyte. In addition, the electrode, the separator and the nonaqueous electrolyte and, if necessary, other additives, may be those known in the art.

In addition, a porous separator may be used as a separator in manufacturing a battery of the present invention, and for example, a polypropylene-based, polyethylene-based, or polyolefin-based porous separator may be used, but is not limited thereto.

The nonaqueous electrolyte of the secondary battery that can be used in the present invention may include a cyclic carbonate and / or a linear carbonate. Examples of the cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), gamma butyrolactone (GBL), and the like. Examples of the linear carbonates include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), methyl propyl carbonate (MPC), and the like. In addition, the nonaqueous electrolyte of the secondary battery of the present invention contains a lithium salt together with the carbonate compound. Specific examples of lithium salts include LiClO ₄ , LiCF ₃ SO ₃ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , and LiN (CF ₃ SO ₂ ) ₂ .

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples. However, the present invention is not limited thereto.

Example 1

LiMnPO ₄ and LiCoO ₂ were mixed at a weight ratio of 50:50 to prepare a cathode active material. The positive electrode active material was mixed so that the weight ratio of active material: conductive agent: binder was 95: 2.5: 2.5, a slurry was prepared using NMP as a solvent, and the slurry was coated on Al foil to prepare a positive electrode. Li metal was used for the negative electrode, and an electrolytic solution was prepared using a coin type _half- cell using EC / EMC (weight ratio 1: 2) in which 1 M LiPF ₆ was dissolved.

1 shows a charge and discharge curve of the coin-type half-cell of the first embodiment. Above 4.0 V, LiCoO ₂ starts to discharge (section A), and when near 4.0 V, LiMnPO ₄ discharges while exhibiting a flat potential (section B). Subsequently, when the discharge capacity at the flat voltage of LiMnPO ₄ is exhausted (see FIG. 2, about 75 mAh / g), LiCoO ₂ continues to discharge while exhibiting a constant potential gradient (section C). It can measure and avoid sudden full discharge.

Example 2

A coin-type half-cell was prepared in the same manner as in Example 1 except that a positive electrode active material in which Li ₄ Ti ₅ O ₁₂ and Li ₂ Ti ₃ O ₇ were mixed at a weight ratio of 50:50 was used.

2 shows the charge and discharge curves of the coin-type half-cell of Example 2. Above 2V, Li ₂ Ti ₃ O ₇ starts to discharge (section A), and when it reaches 1.5V, Li ₄ Ti ₅ O ₁₂ starts to discharge while showing flat potential (near 1.5V) (section B). Afterwards, when the discharge capacity at the flat voltage of Li ₄ Ti ₅ O ₁₂ is consumed (see FIG. 4, about 100 mAh / g), Li ₂ Ti ₃ O ₇ continues to discharge while exhibiting a constant potential gradient (section C). It is possible to measure the state of charge and discharge by the potential gradient and to avoid a sudden full discharge.

Comparative Example 1

A coin-type half-cell was prepared in the same manner as in Example 1 except that only LiMnPO ₄ was used as the cathode active material.

2 shows the charge and discharge curves of the coin-type half-cell of Comparative Example 1. After showing the flat potential at 4.1V and showing a sudden potential drop at the end of the discharge, it is difficult to measure the state of charge and discharge and it is difficult to predict when the complete discharge occurs.

Comparative Example 2

A coin-type half-cell was prepared in the same manner as in Example 1 except that only Li ₄ Ti ₅ O ₁₂ was used as the cathode active material.

4 shows the charge and discharge curves of the coin-type half-cell of Comparative Example 2. The flat potential at 1.5V shows a sudden potential drop at the end of discharge, so it is difficult to measure the state of charge and discharge and it is difficult to predict when complete discharge occurs.

1 is a charge and discharge curve for an electrode prepared using a LiMnPO ₄ + LiCoO ₂ mixture as an active material. In this case, sections A and C represent charge and discharge regions of LiCoO ₂ , and section B represents charge and discharge regions of LiMnPO ₄ .

2 is a charge / discharge curve of an electrode prepared using LiMnPO ₄ as an active material.

3 is a charge / discharge curve of an electrode prepared by using a mixture of Li ₄ Ti ₅ O ₁₂ + Li ₂ Ti ₃ O ₇ as an active material. In this case, sections A and C represent charge and discharge regions of Li ₂ Ti ₃ O ₇ , and section B represents charge and discharge regions of Li ₄ Ti ₅ O ₁₂ .

4 is a charge / discharge curve of an electrode prepared using Li ₄ Ti ₅ O ₁₂ as an active material.

Claims (8)

a) a first electrode active material having a flat potential section on the charge / discharge profile; And b) a mixed electrode active material including a second electrode active material including a flat potential section of the first electrode active material in an operating potential region, and having a negative or positive slope in a charge / discharge profile in an entire region of the operating potential, The charge and discharge profile after the flat potential section of the first electrode active material is compensated by the profile of the second electrode active material so that the absolute value of the average slope of the profile after the flat potential section is adjusted to 0.05 V · g / mAh or less. Mixed electrode active material. The method of claim 1, wherein the first electrode active material is Li _x MPO ₄ (M is at least one selected from the group consisting of Fe, Mn, Co, Ni, 0.05≤x≤1.2), LiMn _{2 - x} Ni _x O ₄ (0 ≦ x ≦ 0.5), LiTi ₂ O ₄ , and Li ₄ Ti ₅ O ₁₂ . The method of claim 1, wherein the second electrode active material comprises LiNi _x Mn _y Co _{1 -x- y} O ₂ (0 ≦ x ≦ 1, 0 ≦ _y ≦ 1), Li ₂ Ti ₃ O ₇ , and TiO ₂ . Mixed electrode active material, characterized in that selected from the group. The mixed electrode active material of claim 1, wherein the second electrode active material is mixed in an amount of 0.1 part by weight to 80 parts by weight with respect to 100 parts by weight of the first electrode active material. a) a first electrode active material having a flat potential section on the charge / discharge profile; And b) a mixed electrode active material including a flat potential section of the first electrode active material in an operating potential region, and including a second electrode active material having a negative or positive slope having a charge / discharge profile in an entire region of the operating potential; As a manufacturing method of The charging and discharging profile after the flat potential section of the first electrode active material is compensated by the profile of the second electrode active material so that the absolute value of the average slope after the flat potential section is adjusted to be 0.05 V · g / mAh or less. Method for producing an electrode active material. It can be mixed with the first electrode active material having a flat potential section on the charge and discharge profile to act as a second electrode active material, and includes a flat potential section of the first electrode active material in the operating potential region, and charge and discharge in the entire region of the operating potential. An indicator material for a state of charge and discharge, in which the profile has a negative or positive slope, An indicator material in a charge / discharge state that can be adjusted such that the absolute value of the average slope after the flat potential section is 0.05 V · g / mAh or less by compensating the charge / discharge profile after the flat potential section of the first electrode active material. The electrode containing the electrode active material in any one of Claims 1-4. The secondary battery containing the electrode of Claim 7.

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