LITHIUM SECONDARY BATTERY CONTAINING ADDITIVE FOR IMPROVED HIGH-TEMPERATURE CHARACTERISTICS
FIELD OF THE INVENTION
The present invention relates to a secondary battery containing an additive for improving high-temperature characteristics and, more particularly, to a lithium secondary battery with excellent high-temperature performance and lifespan, including an ionic salt based electrolyte as well as a particular additive.
BACKGROUND OF THE INVENTION
With increased development of technologies for mobile devices and requirements for the same, demand for a battery as an energy source of the mobile device has rapidly increased. Under such circumstances, a great deal of study and investigation into novel batteries satisfying various consumer needs has been conducted. Especially, a lithium secondary battery such as a lithium ion battery, a lithium ion polymer battery, etc. having excellent energy density, discharge voltage, output stability and the like are in great demand.
In general, a lithium secondary battery includes a cathode active material based on a metal oxide such as LiCoO2, an anode active material based on a carbon material, a porous polymer membrane interposed between a cathode and an anode, and a nonaqueous electrolyte containing a lithium salt such as LiPF6 provided therein. During
charge, lithium ions contained in the cathode active material are released and enter into a carbon layer in the anode, while lithium ions are released from the carbon layer and enter into the cathode active material during discharge. The non-aqueous electrolyte is a medium through which the lithium ions move between the anode and the cathode. Such a lithium secondary battery must be stable in operating voltage ranges of the battery and have a high rate of ion delivery.
If the non-aqueous electrolyte includes only cyclic carbonates with high polarity sufficient to dissociate lithium ions, a viscosity of the electrolyte may be increased, leading to a decrease in ion conductivity thereof.
US Patent Nos. 5,521,027 and 5,525,443 describe an electrolyte mixture including a linear carbonate having reduced viscosity and polarity as well as a cyclic carbonate in order to decrease a viscosity of the electrolyte. Representative examples of such a linear carbonate include dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC), etc. and, among these compounds, EMC having the lowest freezing point of -55 °C is preferably used to exhibit superior low-temperature performance and lifespan. Representative examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), etc. Among these compounds, although PC having a relatively low freezing point of -49 °C is preferably used to exhibit favorable low-temperature performance, this compound may vigorously react with an anode of a battery during charge when a graphitized carbon with a high capacity is used as the anode. Therefore, instead of PC, which is difficult to use in large amounts, EC capable of forming a stable protection film at the anode is generally used. However, since EC is not entirely non-active with the cathode (or may also be slightly active with the cathode), decomposition of the electrolyte occurring at the anode and the cathode during charge/discharge of a battery
may be a cause for deterioration in lifespan of the battery and EC has increased activity at a high temperature, thus being undesirable.
Accordingly, in order to solve the above problems and enhance lifespan of a battery at room temperature and high temperatures, Japanese Laid-Open Application No. 2000-123867 discloses a battery fabricated by adding a small amount of an ester compound, which has a cyclic molecular structure and C=C unsaturated bonds in a ring (such as vinylene carbonate), to an electrolyte. It is considered that such ester compound as an additive is decomposed at either an anode or a cathode and forms a film on a surface of the electrode so as to inhibit decomposition of the electrolyte. However, the above additive cannot completely prevent decomposition of the electrolyte.
Further, Japanese Laid-Open Application No. 2002-25611 discloses a battery fabricated by adding ethylene sulfite and vinylene carbonate to an electrolyte, while Japanese Laid-Open Application No. 2002-270230 proposes another battery fabricated by adding at least one of various ethylene sulfite compounds to an electrolyte. However, it was found that no additive described in these prior arts can exhibit desirable effects.
Accordingly, in consideration of performance at high temperatures, there is still a strong need for development of effective additives in secondary battery applications.
SUMMARY OF THE INVENTION
Therefore, the present invention has been made to solve the above problems and other technical problems that have yet to be resolved.
The present inventors have undertaken extensive research and studies and found that a novel lithium secondary battery with noticeably improved high-temperature performance and lifespan characteristics can be fabricated by adding polyalkyleneglycol diglycidylether to an electrolyte of the battery. As a result, the present invention has been accomplished.
On the basis of this finding, it is an object of the present invention to provide a secondary battery including an ionic salt based electrolyte, wherein a substance represented by the following formula (I) is added in an amount of 0.1 to 10% by weight ("wt.%") to the total weight of the electrolyte so as to enhance high-temperature characteristics of the battery:
(Formula Removed)
wherein n is an integer ranging from 0 to 10.
According to the present invention, when a battery is stored or used at a high temperature, a decomposition reaction between an electrode and an electrolyte of the battery may be decreased which in turn considerably reduces gas generated thereby, so that deterioration of a cycle at elevated temperature and/or capacity reduction caused by high temperature storage of the battery may be significantly decreased and, in addition, thickness swelling of the battery may be inhibited. Moreover, the battery containing the electrolyte prepared as described above does not suffer from performance deterioration, thus exhibiting enhanced safety.
The substance of formula I according to the present invention is contained in the electrolyte as an additive and, as defined above, a content of the substance may range from 0.1 to 10wt.% and, preferably, 1 to 7wt.% to the total weight of the
electrolyte. This substance is duly distinguished from other substances as a cross-linking agent added to a gel polymer electrolyte.
If the amount of the additive is too small, the additive may be little effective. Conversely, when the amount is too large, a viscosity of the electrolyte may be increased while a battery manufactured using the electrolyte may exhibit high resistance, causing undesirable deterioration in performance of the battery.
In formula I, n may be an integer ranging from 0 to 10 as defined above and, preferably, 1 to 6.
The compound of formula I according to the present invention may be easily prepared by those skilled in the art based on a chemical formula thereof or may be commercially available and, therefore, further detailed description thereof will be omitted hereinafter for brevity and to prevent the present invention from being unclear.
The ionic salt used herein may include a lithium salt and the lithium salt is a material readily soluble in the electrolyte of the present invention and may include, for example, LiCl, LiBr, LiI, LiC1O4, LiBF4, LiB10Cl10, LiPF6, LiCF3SO3, LiCF3CO2) LiAsF6, LiSbF6, LiAlCU, CH3SO3Li, CF3SO3Li, (CF3SO2)2NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium 4-phenyl borate, imides, etc. Among these, LiC1O4, LiCF3SO3, LiC4F9SO3, LiBF4, LiAsF6) LiN(C2F5SO2)2 and LiN(CF3SO2)2 are especially preferred.
The electrolyte may include a non-aqueous electrolyte and such non-aqueous electrolyte may include an aprotic organic solvent such as N-methyl-2-pyrrolidinone, propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC),
dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC), γ -butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyl tetrahydrofuran, dimethylsulfoxide, 1,3-dioxolan, formamide, dimethylforraamide, dioxolan, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxy methane, dioxolan derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ether, methyl propionate, ethyl propionate, etc. These materials may be used alone or in combination with two or more thereof.
Among these materials, a mixture of at least one linear carbonate compound and at least one cyclic carbonate compound is more preferred.
Occasionally, the non-aqueous electrolyte of the present invention may include, for example, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidinine, ethyleneglycol dialkylether, ammonium salts, pyrrol, 2-methoxy ethanol, tri-chloro aluminum, vinylene carbonate, propylene sulfone, etc., which is added to the electrolyte to enhance charge/discharge characteristics and/or flame resistance. Moreover, in order to make the electrolyte flame retardant, a halogen based solvent such as carbon tetrachloride, ethylene trifluoride, etc. may further be added to the electrolyte.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
FIG. 1 contains graphs illustrating variation in capacity of a battery cell according to the present invention fabricated in Example 1, compared to a conventional battery cell fabricated in Comparative Example 1, according to Experimental Example 1 of the present invention;
FIG. 2 contains graphs illustrating thickness change of the battery cell according to the present invention fabricated in Example 1, compared to the conventional battery cell fabricated in Comparative Example 1, according to Experimental Example 1 of the present invention; and
FIG. 3 contains graphs illustrating thickness change of the battery cell according to the present invention fabricated in Example 1, compared to the battery cell fabricated in Comparative Example 1, according to Experimental Example 2 of the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The secondary battery of the present invention may be a lithium secondary battery and various components constituting the lithium secondary battery will be described in detail below.
In general, a lithium secondary battery includes an electrode assembly in which a cathode, an anode and a separation membrane interposed between the cathode and the anode, as well as an electrolyte are provided.
The cathode may be formed by applying a mixture of a cathode active material, a conductive material and a binder to a cathode (current) collector, drying the coated collector and pressing the treated collector and, if necessary, the mixture may additionally include filler.
The cathode collector may have a thickness ranging from 3 to 500jwn. Such cathode collector is not particularly restricted so long as it does not induce chemical modification of a battery while having excellent conductive properties. For example, the cathode collector may include stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., and so forth. The cathode collector may have microfine roughness on a surface thereof so as to reinforce adhesion of the cathode active material and, in addition, may be fabricated in various forms such as a film, a sheet, a foil, a net, a porous material, a foamed material, a non-woven fabric material, and the like.
The cathode active material may include, but is not limited to, for example: a lamellar compound such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2), or a compound substituted by one or more transition metal elements; a lithium manganese oxide represented by Li1+xMn2-xO4 (wherein x ranges from 0 to 0.33), LiMnO3, LiMn2O3, LiMnO2, etc.; a lithium copper oxide such as Li2CuO2; a vanadium oxide such as L1V3O8, LiFe3O4, V2O5, CU2V2O7, etc.; a Ni site type lithium nickel oxide represented by LiNi1-xMxO2 (wherein M is Co, Mn, Al, Cu, Fe, Mg, B or Ga and x ranges from 0.01 to 0.3); a lithium manganese composite oxide represented by LiMn2-xMx02 (wherein M is Co, Ni, Fe, Cr, Zn or Ta and x ranges from 0.01 to 0.1) or Li2Mn3MO8 (wherein M is Fe, Co, Ni, Cu or Zn); LiMn2O4 containing Li partially substituted by alkali-earth metal ions; a disulfide compound; Fe2(MoO4)3, and the like.
The conductive material may be added in an amount of 1 to 50wt.% of the total weight of a mixture containing the anode active material. Such conductive material is not particularly restricted so long as it does not cause chemical modification of a battery while having desired conductivity. For instance, the conductive material may include: graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, summer black, etc.; conductive fiber such as carbon fiber, metal fiber, etc.; metal powder such as carbon fluoride, aluminum, nickel powder, etc.; conductive whiskers such as zinc oxide, potassium titanate, etc.; conductive metal oxides such as titanium oxide; polyphenylene derivatives, and so forth.
The binder used herein may comprise a component for supporting combination of an active material with the conductive material and/or binding to the collector and, in general, may be added in an amount of 1 to 50wt.% of the total weight of a mixture comprising a cathode active material. Such binder may include, for example, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butyrene rubber, fluorine rubber, various copolymers, etc.
The filler used herein may optionally be used to inhibit expansion of a cathode and, is not particularly restricted so long as it is a fibrous material which does not cause chemical modification of a battery. For example, the filler may include an olefin based polymer such as polyethylene, polypropylene, etc.; or a fibrous material such as glass fiber, carbon fiber, etc.
An anode is fabricated by applying an anode active material to an anode collector and drying the coated collector and may optionally include other components described above.
The anode collector generally has a thickness of 3 to 500µm. Such anode collector is not particularly restricted so long as it does not induce chemical modification of a battery while having favorable conductive properties. For example, the anode collector may include copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel that was surface-treated with carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, and so forth. Similar to a cathode collector, the anode collector may have microfine roughness on a surface thereof so as to reinforce adhesion of the anode active material, and the anode collector may be fabricated in various forms such as a film, a sheet, a foil, a net, a porous material, a foamed material, a non-woven fabric material, and the like.
The anode active material may include, for example: carbon such as graphitization retardant carbon, graphite based carbon, etc.; metal composite oxides such as LixFe2O3 (0≤x≤l), LixWO2 (0≤x≤l), SnxMei.xMe'yOz (Me: Mn, Fe, Pb, Ge; and Me': Al, B, P, Si, Group 1, 2 or 3 elements; 0
(Table Removed )
As shown in FIG. 3, a thickness of the battery according to Example 1 was little increased after 2.5 hours with a maximum thickness of 0.6mm only which means the thickness change At compared to that of the battery before charge/discharge. Conversely, the thickness of the battery according to Comparative Example 1 was continuously increased even after 5 hours and the maximum thickness, that is, Δt was 0.82mm. Therefore, a difference in thickness changes between the batteries according to Example 1 and Comparative Example 1 was nearly 0.22mm. Accordingly, it can be seen that the battery according to Example 1 fabricated by adding polyethyleneglycol diglycidylether to the electrolyte may considerably inhibit thickness swelling during storage at a high temperature, thereby exhibiting improved performance thereof.
As listed in TABLE 1, an amount of a gas generated during reduction and exhausted outside, such as H2, CH4, C2H6, C2H4, etc. was approximately the same in both the inventive battery according to Example 1 and the conventional battery according to Comparative Example 1. Therefore, it was considered that the above both cases have little difference in reaction between the anode and the electrolyte. Meanwhile, as for amounts of CO and CO2 gases which are generated during oxidation
and/or reduction, the battery according to Example 1 showed a little higher CO generation than the battery according to Comparative Example 1 while considerably reduced amount of CO2, compared to the battery according to Comparative Example 1. In consideration of the above results, it is understood that a secondary battery according to the present invention exhibits a considerably lower decomposition reaction rate between a cathode and an electrolyte, thus achieving noticeably enhanced performance at high temperatures.
INDUSTRIAL APPLICABILITY
As described above, the secondary battery containing polyalkyleneglycol diglycidylether as an additive in accordance with the present invention may considerably inhibit thickness swelling so as to attain a great improvement in lifespan and safety of the battery.
Although the examples and comparative examples of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

WHAT IS CLAIMED IS:
1. A secondary battery including an ionic salt based electrolyte, wherein a
substance represented by the following formula (I) is added in an amount of 0.1 to
10wt.% to the total weight of the electrolyte so as to enhance high-temperature
characteristics of the battery:
(Formula Removed)
wherein n is an integer ranging from 0 to 10.
2. The secondary battery according to claim 1, wherein n ranges from 1 to 6.
3. The secondary battery according to claim 1, wherein the electrolyte is a mixture of at least one linear carbonate compound and at least one cyclic carbonate compound.
4. The secondary battery according to claim 1, wherein the ionic salt is a lithium salt.
5. The secondary battery according to claim 1, wherein the secondary battery is
a lithium secondary battery.
6. An electrolyte for a lithium secondary battery including a carbonate based
electrolyte, a lithium salt, and the substance represented by formula I as defined in
claim 1.