KR20070033839A - Lithium secondary battery which can prevent elution of copper current collector - Google Patents

Abstract

A lithium secondary battery according to the present invention is a lithium secondary battery having an electrode assembly comprising two electrodes having different polarities and a separator interposed between the two electrodes, a case containing the electrode assembly and the electrolyte, wherein The positive electrode is characterized in that at least a portion of the metal of the lithium metal oxide active material is nickel, and the electrolyte solution includes a certain component or more of nitrile succinate (Succinonitril: SN).

According to the present invention, even when the overdischarge of the lithium secondary battery is made, even when the potential is 0 volts, the copper is not eluted into the electrolyte and the basic electrode structure of the negative electrode is stably maintained, so that subsequent charging can be smoothly performed. Aging can be prevented from seriously occurring at the dose of.

Description

Lithium rechargeable battery free from gushing out of copper

1 is a graph of the total potential and discharge characteristics of each electrode of a conventional lithium secondary battery.

Figure 2 is a graph of the discharge potential of each electrode and the total potential of the lithium secondary battery according to the present invention.

The present invention relates to a secondary battery, and more particularly, to a lithium secondary battery capable of preventing battery aging and battery capacity reduction by maintaining an electrode structure even when battery voltage is over-discharged than a normal level.

In general, a lithium secondary battery such as a lithium ion battery or a lithium polymer battery includes an electrode assembly in which two electrodes and a separator for preventing a short circuit between the electrodes are stacked or stacked and an electrolyte and a case in which the electrode assembly and the electrolyte are stored. It is made.

Here, the electrode assembly includes a positive electrode coated with a positive electrode active material such as lithium compound (for example, lithium cobalt, lithium manganate, lithium nickelate) on a positive electrode current collector such as aluminum, and a graphite and a negative electrode current collector such as copper. A negative electrode coated with the same negative electrode active material, and a separator interposed between the positive electrode and the negative electrode.

In addition, the electrolyte consists of lithium salts and high-purity organic solvents, and serves as a transport medium for lithium ions generated by electrochemical reactions at the anode and cathode.

The case is formed of a cylindrical can, a rectangular can, or a pouch, and the like, and a control circuit and various other safety devices may be installed inside and outside the case to prevent overcharge or overdischarge.

The charging and discharging characteristics of the secondary battery are as follows.

First, during charging, in which electrons are provided to the negative electrode from the charger, lithium ions move from the positive electrode active material to the negative electrode active material. That is, lithium ions from the lithium compound are intercalated to the graphite having a layered structure through the electrolyte and the separator. By the way, if the charge is continued without limiting the charging voltage during the charging has the characteristic that the voltage continues to rise. Therefore, various safety devices, control circuits, and the like are attached to the charger and the battery itself to prevent such overcharge.

In addition, lithium ions move from the negative electrode active material to the positive electrode active material at the time of discharge in which electrons are discharged from the negative electrode as described above. That is, lithium ions pass through the electrolyte and the separator from the graphite and return to the lithium compound (deintercalation).

Referring to FIG. 1, a discharge characteristic diagram of a conventional secondary battery is shown as a graph. As shown, the total voltage of the battery becomes the potential difference between the positive electrode (aluminum current collector and positive electrode active material) and the negative electrode (copper current collector and negative electrode active material). The potential of each electrode can be seen as the potential relative to the lithium metal standard reduction potential. If the discharge continues to pass through the period where the battery potential is maintained almost constant, the positive voltage of the positive electrode using lithium cobalt as an active material is still constant, but the self voltage of the lithium intercalated carbon (LIC) negative electrode gradually increases, There is almost no potential difference between the negative electrode and the negative electrode, and accordingly, the total voltage of the battery decreases.

That is, in the current lithium secondary battery system, since the irreversible capacity of the negative electrode is greater than that of the positive electrode, when overdischarge occurs in which the battery is continuously discharged to 0 V with low current or constant resistance, the voltage of the negative electrode having the large irreversible capacity is increased. You will rise first.

However, as shown in the drawing, a negative electrode made of a conventional copper current collector and a negative electrode active material has a problem in that the current collector is eluted while the total voltage of the battery approaches approximately 0V. That is, when the total voltage of the battery is about 0.5 V or less or the voltage of the copper current collector itself is about 3.5 V or more, copper starts to elute from the copper current collector. Elution of this copper current collector is indicated by section A in the graph.

More specifically, the positive electrode active material has a higher standard reduction potential than lithium intercalated carbon or a copper current collector of a negative electrode. Therefore, a copper current collector can also elute by battery reaction. However, when the potential difference between the positive electrode and the negative electrode is large, it is difficult for the current collector of copper to elute with copper ions. That is, even though the copper current collector melts into copper ions while contacting the electrolyte solution, lithium is present in abundance and lithium ions of lithium intercalated carbon, a negative electrode active material, are introduced into the electrolyte, and electrons in the negative electrode active material flow into the copper current collector. do. Therefore, the copper ions immediately obtain electrons from a relatively low-potential, electron-rich copper negative electrode current collector, become copper atoms, and continue to form a negative electrode current collector. However, as the discharge time continues, as the potential difference between the positive electrode and the negative electrode gradually disappears, and eventually the potential difference between the positive electrode and the negative electrode becomes similar (approximately 0.5 V or less with the total battery voltage), the standard reduction potential difference between the copper active material and the positive electrode active material is negative. When compared with the standard reduction potential difference of the active material becomes equal or larger. The surrounding lithium is rarely present, and the inflow of electrons from the active material to the current collector is also reduced, and copper in the copper current collector is actively eluted with ions.

When the copper current collector, which is a negative electrode, is eluted, internal resistance due to the absence of the current collector also increases, and copper, which is dissolved and exists in the form of copper ions, precipitates again as a metal on the surface of the negative electrode during charging. The precipitated copper may cause a dendrite phenomenon, which causes a fine short circuit between the anode and the cathode as it grows, and may interfere with the intercalation of lithium ions on the surface of the cathode, thereby lowering the charge / discharge capacity. . In addition, when a dendrites occur, various side effects such as swelling in which a gas is generated and a battery swells due to gas generation inside the battery may occur.

That is, dissolution of copper due to overdischarge may cause damage to a cell, prevent charging and discharging properly after overdischarge occurs, and cause an aging phenomenon in which battery capacity is sharply lowered. Therefore, after a considerable amount of copper has eluted, even if the secondary battery is charged, it becomes difficult to use it as a battery.

Of course, in order to prevent such over-discharge, the over-discharge control circuit is attached to the outside of the case in the form of a protective circuit board. However, it is also conceivable that the overdischarge control circuit does not work poorly or is damaged. In addition, even if the over-discharge control circuit terminates discharging by disconnecting an external circuit, if the charging is not performed at a proper time by continuously consuming current in any form in the control circuit itself, the battery is completely discharged and used again after a certain period of time. Unexpected problems may arise.

An object of the present invention is to provide a lithium secondary battery that can eliminate the problems caused by over-discharge of the battery by preventing the copper used as a negative electrode current collector to elute with copper ions through the composition of the negative electrode, the positive electrode and the electrolyte solution itself.

In order to achieve the above object, a lithium secondary battery according to the present invention has an electrode assembly including two electrodes having different polarities and a separator interposed between the two electrodes, a lithium secondary battery having a case containing the electrode assembly and the electrolyte. In claim 2, wherein the positive electrode of the two electrodes is at least a portion of the metal of the lithium metal oxide active material is nickel, the electrolyte solution is characterized in that it comprises at least a certain component (Succinonitril: SN).

Nickel, manganese, cobalt may be used as the metal in the present invention, and may include at least nickel, but may include a nickel mixed system in which the proportion of nickel among the three metals may be 1% or more and 50% or less with respect to the weight of the positive electrode active material. The active material may be, for example, a substance such as Li (Ni _X Co _Y Mn _Z O ₂ ), Li (Ni _X Mn _Z O ₂ ), Li (Ni _X Co _Y O ₂ ), and lithium cobalt acid when x is an arbitrary minority. It can be formed by mixing (CoO _2).

In the present invention, the nitrile succinate may be mixed with the whole electrolyte in a volume ratio of about 0.1 to 10%, and the material composition of the mixed electrolyte except for the succinate nitrile is a conventional lithium secondary battery electrolyte, such as propylene carbonate, ethylene carbonate, and die A mixture of lithium salt materials with chilled carbonate, ethylmethyl carbonate and the like may be used.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

2 is a graph showing a change in the potential of the positive electrode and the negative electrode and the potential of the entire battery over time in the lithium secondary battery according to an embodiment of the present invention. This graph shows a conventional electrode potential change and an electric potential change of the entire battery for comparison with the related art.

According to FIG. 2, in the nickel-based or nickel-mixed system of the present invention, the reduction potential decreases as a whole to about 4 volts from about 4.2 volts compared to lithium, compared to the conventional case indicated by dotted lines using lithium cobalt as a cathode active material. The dislocation curves show curves that are substantially parallel to the downward movement than the conventional case. This is because lithium nickelate has a relatively low reduction potential compared to lithium cobalt or lithium manganate (LiMn ₂ O ₄ ), and especially during discharge.

In order to have such a low reduction potential, the nickel mixed cathode active material may be 1% to 50% of the weight of the cathode active material of nickel compared to other mixed metals. The reduction potential of the positive electrode, for example nickel, is more than 0.2 volts, which may be a good condition for achieving the object of the present invention. When the molar ratio of nickel is close to 100%, the reduction potential of the positive electrode may be lower than the reduction potential of copper at the end of discharge, thereby preventing the elution of copper. But. In such a nickel content, it is preferable not to use more than 50% because there is a problem of the flat line of the use potential, the fall of the use potential, and the decrease of the energy content.

On the other hand, the battery reaction is basically made by the reduction potential difference between the negative electrode and the positive electrode active material may be affected by the properties of the electrolyte and the electrode material. Nitrile succinate nitrile of the present invention serves to increase the reduction potential of the negative electrode when the copper is difficult to be eluted when mixed with the carbonate-based electrolyte solution containing lithium salt in 0.1 to 10% by volume. Therefore, the potential rise graph of the conventional negative electrode shows a form in which the potential A is continuously increased until the same as the potential of the positive electrode.

Thus, elution of copper does not occur so that the total battery voltage becomes zero volts. Since elution of copper is not made, the basic structure applied to the negative electrode current collector together with the binder of lithium intercalated carbon as a negative electrode active material does not undergo any change, and thus, the negative electrode structure is not damaged even after charging, and thus can be made well. Of course, at this time, the lithium intercalated carbon is a state in which most of the lithium is released, but contains a lithium present in some of the carbon and the irreversible compound in the carbon state after the supercharge and discharge after a special sudden change during charging and discharging Will not be.

In the present invention, the higher the content of nitrile succinate, the more difficult the dissolution of copper in the negative electrode current collector, and the higher the content of nickel in the nickel-based positive electrode active material, the lower the reduction potential of the positive electrode. By reaching the zero potential of, it eventually serves to prevent the elution of copper. Therefore, if the content of nitrile succinate is sufficient, the composition of nickel in the nickel mixture can be relatively reduced. However, increasing the content of nitrile succinate lowers the overall cell voltage resulting from the use of nickel-based electrolytes, thereby reducing the possibility of decomposition of the electrolyte. Thus, this problem may be detrimental to properties such as lithium ion concentration and lithium ion mobility of the electrolyte, even if not considered. May be used and excessive use is undesirable.

On the other hand, if the content of nitrile succinate is too low to 0.1% or less by volume ratio, the reduction potential of copper may be relatively increased, and thus the effect of preventing the dissolution of copper may not be sufficiently obtained.

According to one embodiment of the present invention, in the positive electrode for a lithium secondary battery including a lithium transition metal oxide, 0.1 to 10 w% of an overdischarge prevention agent such as Li ₂ NiO ₂ may be contained in the entire positive electrode active material. In addition, the anti-discharge agent or Li ₂ Ni _{1- x} M _x O ₂ (0≤x <1, where M is a metal having an oxidation number of +2 selected from Mn and Co), which may serve as a positive electrode active material, may be used. Can be used.

The lithium nickel oxide controls the lithium irreversibility of the positive electrode and the negative electrode in the lithium secondary battery, thereby preventing the potential of increasing the potential of only the negative electrode first. Therefore, the electromotive force of the battery becomes zero volts before reaching the reduction potential of copper with respect to lithium as the potential of the positive electrode drops together during overdischarge.

Such an anti-discharge agent may compensate for the irreversibility of the negative electrode by discharging more than a certain amount of lithium during the supercharge, and then responsible for intercalation deintercalation of less than lithium ions in the cycle, thereby making the positive electrode irreversible.

When the irreversibility of the positive electrode increases, the voltage on the positive side drops when overdischarge occurs. These materials added to conventional lithium cobalt anodizing oxides have an oxidation number of Ni or M from +2 to +4 during charge and discharge, and Li ions while the structure of Li ₂ Ni _{1 - x} M _x O ₂ performs charge and discharge. there is a de-insert Li _{2 -} may have a nominal composition (nominal composition) to _{z Ni 1 -x M x O 2} (0≤z <2).

The amount of the nickel-based anti-discharge inhibitor is preferably used in an amount of 0.1 to 10 w% with respect to the whole positive electrode active material. If the content of the overdischarge inhibitor is too small, the effect of the potential drop of the anode during overdischarge cannot be seen, and thus the dislocation of the overdischarge can be prevented. If excessive nickel-based lithium metal oxides such as Li ₂ NiO ₂ are used, the cycle characteristics of the battery deteriorate, and Li ₂ NiO ₂ is stable due to LiNiO _{2 which} is present through phase change after supercharge. It is not as good as it is.

As the active material of the negative electrode, carbon-based materials such as graphite, soft carbon, and hard carbon, which can be in charge of intercalation and deintercalation of lithium, can be used.

Separators that electrically separate the two electrodes in the electrode assembly may use a polypropylene-based, polyethylene-based, polyolefin-based porous separator.

As the electrolyte, cyclic carbonate and linear carbonate may be used as the substrate. Cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), gamma butyrolactone (GBL), and the like. Linear carbonates include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), methyl propyl carbonate (MPC), and the like. 0.1-10% of succinic acid nitrile is added to the base material which selected at least 1 among these by volume ratio.

The electrolyte includes a lithium salt, and the lithium salt may be specifically selected from the group consisting of LiClO ₄ , LiCF ₃ SO ₃ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , and LiN (CF ₃ SO ₂ ) ₂ .

According to the present invention, on the one hand, as the nitrile succinate is added to the electrolyte, the copper reduction potential is increased to make it more difficult to elute copper from the negative electrode current collector. The reduction potential is lowered to reach zero potential of the cell before copper is eluted.

Therefore, even when the battery is over-discharged and its potential is 0 volts, copper is not eluted with the electrolyte and the basic electrode structure of the negative electrode is stably maintained, so that subsequent charging can be smoothly performed. As a result, even in the event of a discharge control circuit problem or an unexpected long time, the battery performance can be maintained and disposal can be prevented.

In addition, even when overdischarge is performed to some extent, it is possible to prevent serious aging of the battery capacity.

Claims (5)

An electrode assembly having two electrodes of different polarities and a separator interposed between the two electrodes, In a lithium secondary battery comprising a case containing the electrode assembly and the electrolyte, Among the two electrodes, the positive electrode active material constituting the positive electrode includes at least nickel among the lithium metal oxides, and the negative electrode current collector constituting the negative electrode among the two electrodes is made of copper, and the electrolyte is made of succinonitril (SN). Lithium secondary battery comprising a. The method of claim 1, The metal may further include manganese or cobalt, wherein the proportion of nickel contained in the metal is 1% to 50% by weight of the positive electrode active material. The method of claim 1, The metal is mainly cobalt, a lithium secondary battery, characterized in that Li ₂ NiO ₂ is contained 0.1 to 10 w% of the positive electrode active material as a nickel-based over-discharge prevention agent. The method of claim 1, The metal is mainly cobalt, and Li ₂ Ni _{1- x} M _x O ₂ (0 ≦ x <1, M is a metal having an oxidation number of +2 selected from Mn and Co) as an overdischarge inhibitor is 0.1 to 10 relative to the active material. A lithium secondary battery, characterized in that w% is contained. The method according to any one of claims 1 to 4, The nitrile succinate is a lithium secondary battery, characterized in that the mixture is present in the whole electrolyte in a volume ratio of about 0.1 to 10%.

Images