KR101233517B1 - Electrolyte for lithium ion rechargeable battery and lithium ion rechargeable battery comprising the same - Google Patents

Abstract

The electrolyte solution for lithium secondary batteries of this invention is a non-aqueous organic solvent; Lithium salts; Phosphoric acid compounds having a biphenyl group bonded to at least one of the oxygen atoms constituting the phosphoric acid group (PO ₄ ^-3 ); And halogenated ethylene carbonate, wherein the phosphoric acid compound is added in an amount of 0.1 to 10 wt% based on the non-aqueous organic solvent.

Description

ELECTROLYTE FOR LITHIUM ION RECHARGEABLE BATTERY AND LITHIUM ION RECHARGEABLE BATTERY COMPRISING THE SAME

1 is a cross-sectional view showing an upper portion of a lithium secondary battery in one embodiment of the present invention;

Figure 2 is a graph showing the results of the overcharge experiment of one embodiment of the present invention,

Figure 3 is a graph showing the results of the overcharge experiment of one comparative example of the present invention.

The present invention relates to a lithium ion secondary battery electrolyte and a lithium ion secondary battery comprising the same, and more particularly to a lithium ion secondary battery electrolyte and excellent lithium ion secondary battery having excellent life characteristics and low-temperature discharge characteristics.

With the recent rapid development of the electronics, telecommunications, and computer industries, the use of portable electronic products such as camcorders, mobile phones, notebook PCs, etc., as well as the compactness, light weight, and high functionality of the devices are becoming common. The demand for this is increasing. In particular, the rechargeable lithium secondary battery has a high energy density per unit weight of about 3 times higher than conventional lead acid batteries, nickel-cadmium batteries, nickel-hydrogen batteries, and nickel-zinc batteries, and can be rapidly charged. Development is underway.

Usually, lithium metal oxides capable of intercalation-deintercalation of lithium ions are used as a positive electrode active material of a lithium secondary battery, and various types of crystalline or amorphous carbons and carbons that can also occlude and desorb lithium ions are used as negative electrode active materials. Complexes are being used. The lithium secondary battery may be classified into a lithium ion battery, a lithium ion polymer battery, and a lithium polymer battery according to the type of separator and electrolyte used, and may be classified into a cylindrical shape, a square shape, or a pouch type according to a shape.

The average terminal voltage at the time of discharge of lithium secondary battery is 3.6 ~ 3.7V, which is higher than other secondary batteries such as alkali batteries, nickel metal hydride (Ni-MH) batteries, and Ni-Cd batteries. . In order to have such a high discharge voltage, an electrochemically stable electrolyte is required in the charge and discharge voltage range of 0 to 4.2V.

By the way, since the decomposition voltage of water is about 1.3 volts, an aqueous electrolyte solution cannot be used for a lithium secondary battery, and for this reason, the non-aqueous electrolyte solution which melt | dissolved lithium salt in the non-aqueous organic solvent is used as electrolyte solution for lithium secondary batteries. In this case, it is preferable to use an organic solvent having high ionic conductivity and low viscosity and low viscosity. However, since a single non-aqueous organic solvent that satisfies all of these conditions does not exist in reality, a mixed solvent of a high dielectric constant organic solvent and a low viscosity, high ion conductivity organic solvent is usually used as a lithium secondary battery electrolyte.

As examples of mixed solvents, U.S. Patent Nos. 6,114,070 and 6,048,637 disclose a mixture of chain carbonate and cyclic carbonate as dimethyl carbonate or diethyl carbonate and ethylene carbonate or propylene carbonate. A method of improving the ion conductivity of an organic solvent is disclosed.

U.S. Patent Nos. 5,352,548, 5,712,059 and 5,714,281 also disclose electrolyte solutions comprising an organic solvent having a vinylene carbonate (VC) content of at least 20%.

On the other hand, in order to improve the specific properties of the battery with the mixed solvent, a lot of preparation of the functional electrolyte using a mixture of additives with a lithium salt in the solvent. For example, during the initial charging of the battery, the solvent forming material and the lithium ions on the negative electrode react to form a stable solid electrolyte interface (SEI) in which part of the electrolyte is decomposed and loss of capacity occurs. do. In order to reduce electrolyte decomposition, gas generation, and capacity loss during such initial charging, as a means of suppressing decomposition of the solvent on the cathode, a method of adding a compound that forms a solid electrolyte coating on the cathode to the electrolyte is disclosed in Japanese Patent Laid-Open. 2001-6729, etc. are proposed.

On the other hand, the secondary battery of the nonaqueous electrolyte may cause abnormal heat generation in the battery when the power circuit or the charging device of the electronic device is broken or overcharged, and in extreme cases, the battery may be damaged or ignited. Development of a method is required.

 As a countermeasure to prevent the battery from rupturing or ignition due to overcharging, a method of controlling the charging voltage of the charger is the mainstream, and there are other methods of additionally installing a protection circuit or a protection element. There is a burden on downsizing and cost reduction of the battery pack, and there is a possibility of failure. In order to solve this problem, there have been attempts to prevent the overcharging by adding an additive to the electrolyte of the lithium battery (see Japanese Patent Application Laid-Open No. 7-302614, Hei 9-50822, Hei-106835).

For example, Japanese Patent Application Laid-Open Nos. 7-302614 and 9-50822 are additives in an electrolyte, having an molecular weight of 500 or less, pi electron orbits, and having a reversible redox potential of more than the positive electrode potential during full charge of a secondary battery. It is proposed to add a compound, in which case, the additive acts as a redox shuttle to establish a protective mechanism that consumes the overcharge current during overcharge.

On the other hand, Japanese Patent Laid-Open No. 9-106835 discloses a technique in which an additive protects a battery during overcharge by starting a polymerization reaction at a voltage in an overcharge state and acting as a resistor.

An object of the present invention is to provide an electrolyte solution for a lithium ion secondary battery and a lithium ion secondary battery suitable for securing overcharge stability of a lithium ion secondary battery.

It is another object of the present invention to provide an electrolyte solution for a lithium ion secondary battery and a lithium ion secondary battery that can increase cycle life characteristics in addition to securing overcharge stability.

The lithium secondary battery electrolyte of the present invention for achieving the above object, a non-aqueous organic solvent; Lithium salts; Phosphoric acid compounds having a biphenyl group bonded to at least one of the oxygen atoms constituting the phosphoric acid group (PO ₄ ^-3 ); And halogenated ethylene carbonate, wherein the phosphate compound is added in an amount of 0.1 to 10 wt% based on the non-aqueous organic solvent.

In the present invention, the phosphoric acid compound may be composed of each of phosphoric acid compounds having a biphenyl group bonded to one to three oxygen atoms of the oxygen atoms forming a phosphoric acid group, as shown in the following formulas (1), (2) and (3), or a combination thereof. Tris biphenyl phosphate, in which a total of three biphenyl groups are bonded to each of three oxygen atoms of phosphoric acid, tends to have higher overcharge stability than phosphate compounds such as bis biphenyl phosphate and biphenyl phosphate having a small number of biphenyl groups.

[Formula 1]

(2)

[Formula 3]

It is preferable that the addition amount of the phosphoric acid compound of this invention shall be 3 to 5 weight% with respect to an organic solvent.

In the present invention, the halogenated ethylene carbonate may be included in an amount of 0.1 to 10% by weight relative to the non-aqueous organic solvent, and substantially 3 to 5% by weight is preferably added to increase the cycle life characteristics. While the effect on the improvement is in the form of saturation, it tends to lower the low-temperature discharge characteristics.

Considering the addition amount of the phosphoric acid compound and the halogenated ethylene carbonate, the relative addition ratio of these substances can be from 1: 100 to 100: 1, but the addition ratio of the phosphoric acid compound and the halogenated ethylene carbonate is 1: 1 to 2: 1. desirable. The more the number of biphenyl groups in the phosphoric acid compound, the lower the cycle life characteristics of the electrolyte solution. It is preferable to maintain the addition ratio of halogenated ethylene carbonate to 1: 1 to improve the cycle life characteristics.

A lithium secondary battery of the present invention for achieving the above object, a positive electrode comprising a positive electrode active material that can occlude and detach the lithium ion; A negative electrode including a negative electrode active material capable of occluding and desorbing lithium ions; Non-aqueous organic solvents, lithium salts, halogenated ethylene carbonates; And a phosphoric acid compound having a biphenyl group bonded to at least one of oxygen atoms constituting the phosphoric acid group (PO ₄ ^-3 ), which is added in an amount of 0.1 to 10 wt% based on the non-aqueous organic solvent.

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

The electrolyte solution of Examples and Comparative Examples of the present invention contains a non-aqueous organic solvent and a lithium salt. Lithium salt acts as a source of lithium ions in the battery to enable the operation of the basic lithium battery, the non-aqueous organic solvent acts as a passage through which the ions involved in the electrochemical reaction of the battery can move.

In the embodiment of the present invention as the halogenated ethylene carbonate may be used a compound of the formula (4), preferably fluoro ethylene carbonate using fluorine as the halogen element may be used.

[Formula 4]

(Wherein X is a halogen atom, Y is H or a halogen atom, and n and m are 1 or 2).

The content of the phosphoric acid compound in the present invention may be 0.1 to 10% by weight based on the non-aqueous organic solvent, the content of halogenated ethylene carbonate may be 0.1 to 10% by weight relative to the non-aqueous organic solvent.

The phosphoric acid compound and halogenated ethylene carbonate of the present invention having a biphenyl group should be mixed and used within the above ranges, respectively, so that the overcharge stability and cycle life characteristics of the battery are maintained in good condition. The use of only halogenated ethylene carbonate in the electrolyte alone is susceptible to overcharge and can not pass the overcharge test under conditions of discharge rate 1C, voltage 12V, and constant current.

On the other hand, when only the phosphoric acid compound having a biphenyl group is used, the overcharge test can be passed by adding 3 to 5% by weight of the organic solvent, but the cycle life characteristics are all 80% in view of the discharge capacity after 300 times of charge and discharge use. It appears to be low below and it turns out that utility is inferior. In the present invention in which the phosphoric acid compound and the halogenated ethylene carbonate are added together by 3% by weight or more, the overcharge stability enough to pass the overcharge test is secured, and the discharge capacity after 300 times of charge and discharge is almost 85% or more. It can be seen that it is maintained.

In the present invention, the lithium salt may be LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiC (SO ₂ CF ₃ ) ₃ , LiN (SO ₃ CF ₃ ) ₂ , LiC ₄ F ₉ SO ₃ , LiAlO ₄ , LiAlCl ₄ , one or two or more selected from the group consisting of LiI can be used in combination. The concentration of the lithium salt is preferably used in the range of 0.6 to 2.0M, more preferably in the range of 0.7 to 1.6M. If the concentration of the lithium salt is less than 0.6M, the conductivity of the electrolyte is lowered, the performance of the electrolyte is lowered, and if it exceeds 2.0M, the viscosity of the electrolyte is increased, there is a problem that the mobility of lithium ions decreases.

The non-aqueous organic solvent may include one or a mixture of carbonates, esters, ethers or ketones. In the case of a carbonate-based solvent in the non-aqueous organic solvent, it is preferable to use a mixture of cyclic carbonate and chain carbonate. In this case, the cyclic carbonate and the chain carbonate are preferably used by mixing in a volume ratio of 1: 1 to 1: 9, and more preferably used by mixing in a volume ratio of 1: 1.5 to 1: 4. The performance of the electrolyte is superior to other volume ratios by mixing at the volume ratio.

As the cyclic carbonate, ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, etc. may be used. Can be. Among the high dielectric constant ethylene carbonate and propylene carbonate, ethylene carbonate has a high melting point and is mixed with other solvents. When graphite is used as a negative electrode active material, propylene carbonate having a low decomposition voltage is not used or the content is lowered.

As the chain carbonate, dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylmethyl carbonate (EMC), ethylpropyl carbonate (EPC), etc. may be used. Among these, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate having low viscosity are mainly used.

As ester, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, γ-valerolactone, γ-caprolactone, δ-valerolactone, ε-caprolactone, etc. Tetrahydrofuran, 2-methyltetrahydrofuran, dibutyl ether and the like may be used as the ether. As the ketone, polymethylvinyl ketone may be used.

The electrolyte solution of the present invention may further include an aromatic hydrocarbon organic solvent in the carbonate solvent. As the aromatic hydrocarbon organic solvent, an aromatic hydrocarbon compound of Formula 5 may be used.

[Chemical Formula 5]

In the above formula, R is halogen or an alkyl group having 1 to 10 carbon atoms and q is an integer of 0 to 6. Specific examples of the aromatic hydrocarbon organic solvent may include benzene, fluorobenzene, bromobenzene, chlorobenzene, toluene, xylene, mesitylene, and the like, which may be used alone or in combination. When the volume ratio of carbonate solvent / aromatic hydrocarbon organic solvent is 1: 1 to 30: 1 in an electrolyte solution containing an aromatic hydrocarbon organic solvent, the properties, such as stability, safety, and ion conductivity, which are generally required, are superior to other ratio compositions. Do.

The lithium secondary battery of the present invention includes a positive electrode and a negative electrode.

The positive electrode includes a positive electrode active material capable of occluding and desorbing lithium ions, and the positive electrode active material is preferably at least one selected from cobalt, manganese, nickel, and a composite metal oxide with lithium. In addition to these metals, the employment rate of the metals may be varied. In addition to these metals, Mg, Al, Co, K, Na, Ca, Si, Ti, Sn, V, Ge, Ga, B, As, Zr, Mn, Sr, V, and a rare earth element.

The negative electrode includes a negative electrode active material capable of occluding and desorbing lithium ions, and examples of the negative electrode active material include carbon materials such as crystalline carbon, amorphous carbon, carbon composite, carbon fiber, lithium metal, alloys of lithium and other elements, and the like. Can be. Examples of the amorphous carbon include hard carbon, coke, mesocarbon microbead (MCMB) calcined at 1500 ° C. or lower, and mesophase pitch-based carbon fiber (MPCF). The crystalline carbon is a graphite-based material, specifically natural graphite, graphitized coke, graphitized MCMB, and graphitized MPCF. The carbonaceous material is preferably a material having an d002 interplanar distance of 3.35 to 3.38 Å and an Lc (crystallite size) of at least 20 nm by X-ray diffraction. Other elements constituting the alloy with lithium may be aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium or indium.

The positive electrode or the negative electrode may be prepared by dispersing an electrode active material, a binder and a conductive material, if necessary, a thickener in a solvent to prepare an electrode slurry composition, and applying the slurry composition to an electrode current collector. As the positive electrode current collector, aluminum or an aluminum alloy may be commonly used, and copper or a copper alloy may be used as the negative electrode current collector. The anode current collector and the anode current collector may be in the form of a foil or a mesh.

The binder is a material that plays a role of pasting the active material, mutual adhesion of the active material, adhesion with the current collector, buffering effect on the expansion and contraction of the active material, and the like, for example, polyvinylidene fluoride (PVdF), polyhexafluoro Copolymer of propylene-polyvinylidene fluoride (P (VdF / HFP)), poly (vinylacetate), polyvinyl alcohol, polyethylene oxide, polyvinylpyrrolidone, alkylated polyethylene oxide, polyvinyl ether, poly ( Methyl methacrylate), poly (ethylacrylate), polytetrafluoroethylene, polyvinylchloride, polyacrylonitrile, polyvinylpyridine, styrene-butadiene rubber, acrylonitrile-butadiene rubber and the like. The content of the binder is 0.1 to 30% by weight, preferably 1 to 10% by weight, based on the electrode active material. When the content of the binder is too small, the adhesion between the electrode active material and the current collector is insufficient, and when the content of the binder is too high, the adhesion is improved, but the content of the electrode active material decreases by that amount, which is disadvantageous in increasing battery capacity.

The conductive material may be at least one selected from the group consisting of a graphite-based conductive agent, a carbon black-based conductive agent, a metal or a metal compound-based conductive agent as a material for improving electronic conductivity. Examples of the graphite conductive agent include artificial graphite and natural graphite, and examples of the carbon black conductive agent include acetylene black, ketjen black, denka black, thermal black, and channel. Channel black and the like, and examples of the metal or metal compound conductive agent include perovskite such as tin, tin oxide, tin phosphate (SnPO ₄ ), titanium oxide, potassium titanate, LaSrCoO ₃ , and LaSrMnO _3. ) There is a substance. However, it is not limited to the conductive agents listed above.

The content of the conductive agent is preferably 0.1 to 10% by weight based on the electrode active material. When the content of the conductive agent is less than 0.1% by weight, the electrochemical properties are lowered, and when the content of the conductive agent is greater than 10% by weight, the energy density per weight decreases.

The thickener is not particularly limited as long as it can play a role of controlling the viscosity of the active material slurry. For example, carboxymethyl cellulose, hydroxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, and the like may be used.

As the solvent in which the electrode active material, the binder, the conductive material and the like are dispersed, a non-aqueous solvent or an aqueous solvent is used. Examples of the non-aqueous solvent include N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, N, N-dimethylaminopropylamine, ethylene oxide and tetrahydrofuran.

The lithium secondary battery may include a separator that prevents a short circuit between the positive electrode and the negative electrode and provides a passage for lithium ions, and such separators include polypropylene, polyethylene, polyethylene / polypropylene, polyethylene / polypropylene / polyethylene, poly Polyolefin polymer membranes, such as propylene / polyethylene / polypropylene, or a multilayer of these, a microporous film, a woven fabric, and a nonwoven fabric can be used. Further, a film coated with a resin having excellent stability may be used for the porous polyolefin film.

1 is a cross-sectional view of a rectangular lithium secondary battery shown as one embodiment of the present invention.

Referring to FIG. 1, in a lithium secondary battery, an electrode assembly 12 including an anode 13, a cathode 15, and a separator 14 is accommodated in a can 10 together with an electrolyte solution. It is formed by sealing the upper end with the cap assembly 20. The cap assembly 20 includes a cap plate 40, an insulating plate 50, a terminal plate 60, and an electrode terminal 30. The cap assembly 20 is combined with the insulating case 70 to seal the can 10.

The electrode terminal 30 is inserted into the terminal through-hole 41 formed in the center of the cap plate 40. When the electrode terminal 30 is inserted into the terminal through-hole 41, the tubular gasket 46 is coupled to the outer surface of the electrode terminal 30 and inserted together to insulate the electrode terminal 30 and the cap plate 40. . After the cap assembly 20 is assembled to the upper end of the can 10, the electrolyte is injected through the electrolyte injection hole 42, and the electrolyte injection hole 42 is closed by a stopper 43. The electrode terminal 30 is connected to the negative electrode tab 17 of the negative electrode 15 or the positive electrode tab 16 of the positive electrode 13 to act as a negative electrode terminal or a positive electrode terminal.

The lithium secondary battery of the present invention may be formed in other shapes, such as cylindrical, pouch type, in addition to the illustrated square.

Hereinafter, examples and comparative examples of the present invention will be described. However, the following examples are only a preferred embodiment of the present invention, and the present invention is not limited to the following examples. Meanwhile, in Examples and Comparative Examples, ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate were mixed in a volume ratio of 1: 1: 1 as the basic solvent, and the lithium salt was added so that the lithium ion concentration was 1.15 mol in the basic solvent. It is assumed that the dissolved base electrolyte is used. It can be seen that all of the lithium salts dissociate to have a lithium ion concentration of 1.15 mol, so that a corresponding amount of the lithium salt such as LiPF ₆ can be dissolved in a basic solvent to form a base electrolyte.

( Example  One)

LiCoO ₂ as a positive electrode active material, polyvinylidene fluoride (PVDF) as a binder and carbon as a conductive agent were mixed in a weight ratio of 92: 4: 4, and then dispersed in N-methyl-2-pyrrolidone to prepare a positive electrode slurry. It was. The slurry was coated on an aluminum foil having a thickness of 15 μm, dried, and rolled to prepare a positive electrode. Synthetic graphite as a negative electrode active material, styrene-butadiene rubber as a binder, and carboxymethyl cellulose as a thickener were mixed in a weight ratio of 96: 2: 2, and then dispersed in water to prepare a negative electrode active material slurry. The slurry was coated on a copper foil having a thickness of 10 μm, dried, and rolled to prepare a negative electrode.

A film separator made of a polyethylene (PE) material having a thickness of 20 μm was inserted between the prepared electrodes and then wound and compressed to insert into a rectangular can. An electrolyte solution was injected into a square can to prepare a lithium secondary battery. Electrolyte solution is ethylene carbonate / ethyl methyl carbonate / diethyl carbonate mixed solvent of one or a simple covalent bond to the (1: 1 volume ratio: 1) was dissolved to a 1.15M solution of LiPF _6, then phosphoric acid (H ₃ PO ₄₎ in 5% by weight and 3% by weight of the non-aqueous organic solvent were added to biphenyl phosphate and fluoro ethylene carbonate in which hydrogen bonded to oxygen was substituted with a biphenyl group.

( Example 2)

In the same conditions as in Example 1, biphenyl phosphate and fluoroethylene carbonate, each of which each hydrogen bonded to two oxygens which are simply covalently bonded to phosphorus of phosphoric acid (H ₃ PO ₄ ), were substituted with a biphenyl group, were respectively 5% and 3% by weight of the nonaqueous organic solvent was added.

( Example 3)

Under the same conditions as in Example 1, biphenyl phosphate and fluoroethylene carbonate in which each hydrogen bonded to three oxygen simply covalently bonded to phosphorus of phosphoric acid (H ₃ PO ₄ ) were substituted with a biphenyl group, respectively 3% and 3% by weight of the nonaqueous organic solvent was added.

( Example 4)

Under the same conditions as in Example 1, biphenyl phosphate and fluoroethylene carbonate each substituted with biphenyl groups for each hydrogen bonded to three oxygen simply covalently bonded to phosphorus of phosphoric acid (H ₃ PO ₄ ), respectively 5% and 3% by weight of the nonaqueous organic solvent was added.

(Comparative Example 1)

Under the same conditions as in Example 1, biphenyl phosphate in which hydrogen bonded to one oxygen that is simply covalently bonded to phosphorus of phosphoric acid (H ₃ PO ₄ ) was substituted with a biphenyl group, without adding fluoroethylene carbonate. 5 weight% of the non-aqueous organic solvent was added in the state.

Comparative Example 2

Addition of fluoro ethylene carbonate to biphenyl phosphate in which each hydrogen bonded to two oxygens which are simply covalently bonded to phosphorus of phosphoric acid (H ₃ PO ₄ ) is substituted with a biphenyl group under different conditions as in Example 1 5% by weight of the non-aqueous organic solvent was added.

Comparative Example 3

The addition of fluoroethylene carbonate to biphenyl phosphate in which each hydrogen bonded to three oxygens simply covalently bonded to phosphorus of phosphoric acid (H ₃ PO ₄ ) was substituted with a biphenyl group under different conditions as in Example 1 3% by weight of the non-aqueous organic solvent was added.

Comparative Example 4

In the same conditions as in Example 1, 3% by weight of the non-aqueous organic solvent was added to fluoro ethylene carbonate without addition of biphenyl phosphate.

<Standard capacity>

Table 1 shows the relative capacities when the batteries prepared according to Examples 1 to 4 and Comparative Examples 1 to 4 were charged for 3 hours under 0.5C / 4.2V constant current-constant voltage conditions.

<300 doses (%)>

The batteries prepared according to Examples 1 to 4 and Comparative Examples 1 to 4 were subjected to 1C / 4.2V constant current / constant voltage, 0.05C cutoff charging at 25 ° C. at room temperature, and discharged at 1C 3.2V constant current. After repeating this process 300 times, the capacity (%) of the 300th time of normal temperature was measured, respectively, and the result is shown in Table 1.

<Overcharge test result>

The state of the battery was observed while charging the battery prepared according to Examples 1 to 4 and Comparative Examples 1 to 4 at a normal temperature of 25 ° C. for 2 hours and 30 minutes at 1 C / 12V constant current constant voltage. Test results were marked with NG (NOT GOOD) and OK. The overcharge stability of the lithium ion secondary battery is classified as shown in Table 2 below, and specifically, the progress of the overcharge test for Example 3 and Comparative Example 4 is shown in the graphs of FIGS. 2 and 3. 2 and 3, the thick line represents the battery surface temperature, the thinnest line represents the voltage, and the middle line represents the current.

Biphenyl phosphate
Of organic solvents
weight%) FEC (% by weight of organic solvent) Standard capacity
(%) Overcharge test result 300 capacity (%) Example 1 5 3 100 OK 85 Comparative Example 1 5 0 96 OK 80 Example 2 5 3 100 OK 85 Comparative Example 2 5 0 96 OK 80 Example 3 3 3 100 OK 85 Example 4 5 3 100 OK 83 Comparative Example 3 3 0 96 OK 75 Comparative Example 4 0 3 100 NG 90

FEC: fluoroethylene carbonate

level L0 L1 L2 L3 L4 L5 phenomenon No change Leak Smoke below 200 ℃ Smoke over 200 ℃ Fire Explosion, burst

As can be seen from Table 1 and Figure 3, when only fluoroethylene carbonate is added to the base electrolyte as in Comparative Example 4, the voltage of the battery in the overcharge step to maintain a steady voltage state of the lithium ion battery to some extent but internally The overcharge current is accumulated in the electrode and the electrode becomes unstable, and the unstable electrode may react with the electrolyte to decompose and polymerize, and heat generation may be intensified. If the separator shuts down due to the temperature rise and thermal runaway already occurs even though no current flows, ignition and battery rupture may occur.

In terms of voltage and current, the voltage rises almost vertically at any point in time so that the voltage of the rechargeable battery is applied to the two electrodes of the battery as it is. At this time, it can be seen that the flow of current is interrupted inside the battery. At this point in time, the battery surface temperature is 160 degrees Celsius, and ignition occurs, resulting in an L4 state.

When the disconnection state of the ion current continues for a certain period of time, and the separator melts as the inside of the battery is burned away, an electrical short circuit occurs between the positive electrode and the negative electrode to allow the conductive current to flow. At this time, the constant current flows from the charger, and the voltage between the two terminals is 0V. Therefore, when only fluoroethylene carbonate is added to the base electrolyte solution, there is no stability during overdischarge and it is difficult to adopt.

On the other hand, when 3 to 5% by weight of biphenyl phosphate relative to the organic solvent is added, thermal runaway due to the rise of the secondary battery temperature is suppressed as shown in FIG. At this time, the voltage rises slightly from the normal charging voltage and then returns to the normal state or remains slightly high.

This phenomenon occurs when the electrode becomes unstable due to overcharging, and as the voltage between the two electrodes exceeds the designed charging voltage, the biphenyl phosphate contained in the electrolyte decomposes before the non-aqueous organic solvent, so that the overcharge current is consumed in this reaction and heat. It can be considered to play a role in controlling generation and voltage rise.

Subsequently, as shown in the overcharge progress graph for Example 3, the battery maintains a temperature of 70 degrees C or more even during overcharging, but does not ignite or rupture due to an abnormal voltage or current or thermal runaway. Did.

In addition, it can be seen that in Example 1 to 4, the cycle life characteristics are improved by adding fluoroethylene carbonate to the base electrolyte as compared with the case where the cycle life characteristics are lowered as in Comparative Examples 1 to 3 while biphenyl phosphate is added. . In particular, in the case of biphenyl phosphate in which all of the hydrogen of phosphoric acid is substituted with a biphenyl group, the overcharge stability is higher than that of other biphenyl phosphate compounds, so that the overcharge safety can be obtained even when a small amount compared to the number of moles is used.

As described above, the present invention provides a lithium ion secondary battery electrolyte and a lithium ion secondary battery excellent in cycle life characteristics and overcharge characteristics.

Although the present invention has been described with reference to the above embodiments, it is merely illustrative, and those skilled in the art will understand that various modifications and equivalent other embodiments are possible therefrom. .

Claims (13)

Non-aqueous organic solvents; Lithium salts; Halogenated ethylene carbonate; And Lithium ion secondary characterized in that it comprises a phosphoric acid compound having a biphenyl group bonded to at least one of the oxygen atoms constituting the phosphoric acid group (PO ₄ ^-3 ), the phosphoric acid compound is added to 0.1 to 10% by weight based on the non-aqueous organic solvent Battery electrolyte. The method of claim 1, The halogenated ethylene carbonate is an electrolyte solution for a lithium ion secondary battery, characterized in that contained 0.1 to 10% by weight relative to the non-aqueous organic solvent. The method of claim 2, The halogenated ethylene carbonate is an electrolyte solution for a lithium ion secondary battery, characterized in that 3 to 5% by weight based on the non-aqueous organic solvent. The method of claim 1, The amount of the phosphoric acid compound added is 3% by weight to 5% by weight with respect to the non-aqueous organic solvent. The method of claim 1, The halogenated ethylene carbonate is fluoro ethylene carbonate, the electrolyte solution for lithium ion secondary batteries. The method of claim 1, The phosphoric acid compound is a lithium ion secondary battery electrolyte, characterized in that each one or a combination of phosphoric acid compounds having a biphenyl group bonded to one to three oxygen atoms of the oxygen atoms forming a phosphoric acid group. The method of claim 6, The phosphoric acid compound is a tris biphenyl phosphate in which a total of three biphenyl groups are bonded to each of three oxygen atoms constituting the phosphoric acid group, an electrolyte for a lithium ion secondary battery. The method of claim 1, The amount of the phosphate compound added is from 3% by weight to 5% by weight based on the non-aqueous organic solvent. The method of claim 1, The addition ratio of the phosphate compound and the halogenated ethylene carbonate is 1: 1 to 2: 1 electrolyte solution for lithium ion secondary batteries. The method of claim 1, The non-aqueous organic solvent is a mixed solvent of a cyclic carbonate and a chain carbonate, characterized in that the electrolyte solution for a lithium ion secondary battery. The method of claim 1, The non-aqueous organic solvent is a mixed solvent of a carbonate solvent and an aromatic hydrocarbon organic solvent, wherein the aromatic hydrocarbon organic solvent is an aromatic compound of the formula (5). [Chemical Formula 5]

(Wherein R is a halogen or an alkyl group having 1 to 10 carbon atoms and q is an integer of 0 to 6). The method of claim 1, The lithium salt is LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiC (SO ₂ CF ₃ ) ₃ , LiN (SO ₃ CF ₃ ) ₂ , LiC ₄ F ₉ SO ₃ , LiAlO ₄ , LiAlCl ₄ , LiCl and LiI electrolyte solution for a lithium secondary battery, characterized in that at least two or more selected from the group consisting of. The electrolyte of any one of claims 1 to 12; A positive electrode including a positive electrode active material capable of occluding and detaching lithium ions; A negative electrode including a negative electrode active material capable of occluding and desorbing lithium ions; And And a case sealed by incorporating the electrolyte, the positive electrode, and the negative electrode.

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