Patent Application
20070243468
The present invention relates to a lithium secondary battery having improved life characteristics. More specifically, the present invention relates to a lithium secondary battery having improved life characteristics, wherein metal ion impurities incorporated during an assembly process of the battery are precipitated and removed to thereby prevent electrodeposition of the metal ions on an anode, thus improving life characteristics of the battery, by the addition of a phosphate represented by Formula I which will be illustrated hereinafter to an electrode active material, an electrolyte or the surface of a separator.
Rapid expansion in use of portable electronic equipment such as mobile phones, notebook computers, camcorders, digital cameras and the like has led to increased demands for secondary batteries having a high-energy density as a power source for such equipment. In recent years, applicability of secondary batteries has been realized as power sources for electric vehicles (EVs) and hybrid electric vehicles (HEVs).
As examples of such secondary batteries, lithium secondary batteries comprising an anode of a carbonaceous material, a cathode of a lithium metal oxide, a separator of a polyolefin material and a non-aqueous lithium salt electrolyte are widely used. For optimal use in the electronic equipment of interest or vehicles, the lithium secondary batteries require excellent life characteristics. As such, efforts and attempts to improve a battery life are continuously undertaken, because the battery must undergo little decrease of the capacity even after repeated charge/discharge cycles.
Batteries undergo deterioration of life characteristics due to degradation of individual components caused by various factors. One of the main causes for the deterioration of the battery life characteristics is incorporation of impurities into the battery. For example, as the incorporation of water into the battery accelerates the degradation of the battery performance, Korean Patent Registration No. 414588 discloses a technique of inhibiting adverse side reactions and gas evolution by adsorption of water and water-borne by-products via the addition of zeolite to an electrolyte. In addition, Japanese Patent Application No. 2003-323916 A1 discloses a technique of suppressing battery degradation by adsorption and removal of water, hydrofluoric acid, a by-product from the reaction of water with lithium salts, and the like, via the addition of zeolite to an electrode active material or the like.
However, according to the experiments conducted by the inventors of the present invention, it was confirmed that internal short-circuiting occurs to thereby sharply decrease the battery capacity when metal impurities are incorporated into the battery, even after complete removal of water inside the battery or the by-products produced from the reaction of water with the lithium salts. Further, incorporation of large quantities of the metal impurities results in a failure to sufficiently fulfill functions of the battery. Therefore, maximum care should be taken to ensure that incorporation of the impurities does not occur upon fabrication of the lithium secondary battery. However, since it is in fact impossible to completely block the incorporation of the metal impurities into the battery, there is a need for the development of a technique to ensure that the internal short-circuiting of the battery does not take place even upon incorporation of the impurities.
Therefore, the present invention has been made to solve the above problems and other technical problems that have yet to be resolved.
As a result of a variety of extensive and intensive studies and experiments to solve the problems as described above, the inventors of the present invention have discovered that, upon the fabrication of a lithium secondary battery by inclusion of a phosphate of Formula I, which will be illustrated hereinafter, inside the battery, it is possible to easily remove metal impurities seriously harmful to the life characteristics of the battery by precipitation of the metal cations through binding of the phosphate with the metal cations of the impurities. The present invention has been completed based on these findings.
Therefore, a lithium secondary battery according to the present invention is characterized in that metal ion impurities incorporated during a fabrication process of the battery are precipitated to prevent electrodeposition of the metal ions on an anode, through the addition of one or more phosphates of Formula I below to an electrode active material, an electrolyte or the surface of a separator.
AxH(3−x)PO4 (I)
wherein, A is Li, Na or NH4; and O<x≦3.
That is, the secondary battery according to the present invention improves life characteristics of the battery by replacing metal ions of metal impurities with lithium ions, sodium ions and/or ammonium ions which are not detrimental to the operation of the battery, thereby precipitating and removing the impurities from the inside of the battery, through the incorporation of the above-mentioned phosphates of Formula I into the battery.
For example, where the metal impurities such as iron (Fe), copper (Cu), nickel (Ni) and cobalt (Co) are incorporated into a cathode, the impurities are eluted toward an electrolyte at an operation potential of the cathode, and once dissolved as a form of metal ions in the electrolyte, they are reduced at an anode and precipitated as metals. The thus-precipitated metals cause the occurrence of internal short-circuiting. Further, when the metal cations are also present in the electrolyte during the fabrication process of the battery, electrodeposition of the metal ions on the anode takes place, thus causing the internal short-circuiting of the battery. As a result, the metal ions eluted from the cathode or the metal ions in the impurities present in the electrolyte during the fabrication process of the battery undergo electrodeposition thereof on the anode during the battery operation, consequently resulting in the internal short-circuiting of the battery.
Whereas, according to the present invention, it is possible to previously prevent electrodeposition of the metal ions on the anode, due to replacement and precipitation of such metal ions with the lithium ions, sodium ions and/or ammonium ions in the phosphate of Formula I.
Further, a sodium phosphate among the above-mentioned phosphates provides precipitation of the metal ions as well as flame retardancy, thereby improving the safety of the battery.
There are several known methods of replacing conventional components of the lithium secondary battery with phosphates or of adding the phosphates to the battery components. For example, U.S. Pat. No. 6,720,110 discloses a lithium secondary battery using a lithium phosphate having a certain structure as an electrode active material, instead of conventional electrode active materials. Further, Japanese Patent Application No. 2005-5117 A1 discloses a technique of suppressing decomposition of electrolytes by inducing formation of stable coatings at an anode, via the use of fluoro-substituted [oxalato-O,O′] lithium phosphates as electrolyte salts. In addition, Korean Patent Application No. 2004-99606 A1 discloses a technique of adding a phosphate-based compound to crosslink an ion-conductive polymer, upon preparing a gelled electrolyte composition. However, to the best of our knowledge, no case has been found in the conventional prior arts wherein precipitation and removal of metal impurities are effected by addition of a certain phosphate as proposed in the present invention.
As preferred examples of the phosphates of Formula I, mention may be made of ammonium hydrogen phosphate, (NH4)2HPO4, ammonium dihydrogen phosphate, NH4H2PO4, lithium phosphate, Li3PO4, lithium dihydrogen phosphate, LiH2PO4, sodium phosphate, Na3PO4, sodium hydrogen phosphate, Na2HPO4 and sodium dihydrogen phosphate, NaH2PO4. These materials may be used alone or in any combination thereof. Among them, particularly preferred is lithium dihydrogen phosphate (LiH2PO4) which exhibits a high-precipitation rate per unit weight for metal cations of impurities and provides lithium ions directly acting on electrolytes of the lithium secondary batteries.
Therefore, a target part to which the phosphate may be added includes electrode active materials, electrolytes, and separator surface, as discussed above. Particularly preferably, the phosphate is added to a cathode upon fabrication thereof, or is added as a coating on the surface of a separator. In this connection, if the phosphate has a large particle size, it is difficult to coat the phosphate on the electrode or separator. Therefore, the particle size of the phosphate is preferably less than 50 μm.
The amount of the phosphate material added to the electrode active material or electrolyte is in a range of 0.005 to 5% by weight, based on the weight of the electrode active material or electrolyte. If the content of the phosphate added is excessively low, it may be difficult to substantially remove the metal impurities. If the content of the phosphate added is excessively high, this may undesirably lead to a decrease in an energy density of the battery or an increase in an internal resistance of the battery, thus causing deterioration of the battery performance.
When it is desired to coat the phosphate material on the surface of the separator, the phosphate, in conjunction with a fluorine-based material such as PVdF as a base material, is dispersed in a suitable solvent and then may be partially or completely coated on the surface of the separator by various coating methods known in the art. Preferably, the phosphate material is coated in a range of 0.005 to 50 g/m2 to the separator.
In one preferred embodiment, a cation exchange material, containing cations selected from the group consisting of lithium, sodium, ammonium and any combination thereof, may also be used, in conjunction with the phosphate of Formula I. The cation exchange material serves to remove metal ion impurities incorporated during an assembly process of the battery, via a cation exchange process.
The cation exchange material is a material containing lithium ions and the like while not exhibiting adverse side effects on the battery operation. Preferably, examples of the cation exchange material may include alumino-silicate, alumino-phosphate and the like. These materials may be used alone or in any combination thereof.
An amount of the cation exchange material to be added may be determined within the range where the total amount of the cation exchange material and the phosphate of Formula I does not exceed the above-specified content range.
Hereinafter, the other remaining components necessary for the lithium secondary battery according to the present invention will be described in more detail.
The lithium secondary battery of the present invention is comprised of a cathode, an anode, a separator and a lithium salt-containing, non-aqueous electrolyte, with inclusion of the cation exchange material as mentioned above.
The cathode is, for example, fabricated by applying a mixture of a cathode active material, a conductive material and a binder to a cathode current collector, followed by drying. If necessary, a filler may be further added to the above mixture.
Examples of the cathode active materials that can be used in the present invention may include, but are not limited to, layered compounds such as lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2), or compounds substituted with one or more transition metals; lithium manganese oxides such as compounds of Formula Li1+xMn2−xO4 (0≦x≦0.33), LiMnO3, LiMn2O3 and LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, V2O5 and Cu2V2O7; Ni-site type lithium nickel oxides of Formula LiNi1−xMxO2 (M=Co, Mn, Al, Cu, Fe, Mg, B or Ga, and 0.01≦x≦0.3); lithium manganese composite oxides of Formula LiMn2−xMxO2 (M=Co, Ni, Fe, Cr, Zn or Ta, and 0.01≦x≦0.1), or Formula Li2Mn3MO8 (M=Fe, Co, Ni, Cu or Zn); LiMn2O4 wherein a portion of Li is substituted with alkaline earth metal ions; disulfide compounds; Fe2(MoO4)3, LiFe3O4 and the like.
The cathode current collector is generally fabricated to have a thickness of 3 to 500 μm. There is no particular limit to materials for the cathode current collector, so long as they have high conductivity without causing chemical changes in the fabricated battery. As examples of materials for the cathode current collector, mention may be made of stainless steel, aluminum, nickel, titanium, sintered carbon, and aluminum or stainless steel which was surface-treated with carbon, nickel, titanium or silver. The cathode current collector may be fabricated to have fine irregularities on the surface thereof so as to enhance adhesive strength to the cathode active material. In addition, the cathode current collector may take various forms including films, sheets, foils, nets, porous structures, foams and non-woven fabrics.
The conductive material is typically added in an amount of 1 to 50% by weight, based on the total weight of the mixture including the cathode active material. There is no particular limit to the conductive material, so long as it has suitable conductivity without causing chemical changes in the fabricated battery. As examples of conductive materials, mention may be made of conductive materials, including graphite such as natural or artificial graphite; carbon blacks such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black and thermal black; conductive fibers such as carbon fibers and metallic fibers; metallic powders such as carbon fluoride powder, aluminum powder and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and polyphenylene derivatives.
The binder is a component assisting in binding between the active material and conductive material, and in binding with the current collector. The binder is typically added in an amount of 1 to 50% by weight, based on the total weight of the mixture including the cathode active material. As examples of the binder, mention may be made of polyvinylidene fluoride, polyvinyl alcohols, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinyl pyrollidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butadiene rubber, fluoro rubber and various copolymers.
The filler is an optional ingredient used to inhibit cathode expansion. There is no particular limit to the filler, so long as it does not cause chemical changes in the fabricated battery and is a fibrous material. As examples of the filler, there may be used olefin polymers such as polyethylene and polypropylene; and fibrous materials such as glass fiber and carbon fiber.
The anode is fabricated by applying anode materials to the anode current collector, followed by drying. If necessary, other components as described above may be further included.
The anode current collector is generally fabricated to have a thickness of 3 to 500 μm. There is no particular limit to materials for the anode current collector, so long as they have suitable conductivity without causing chemical changes in the fabricated battery. As examples of materials for the anode current collector, mention may be made of copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel having a surface treated with carbon, nickel, titanium or silver, and aluminum-cadmium alloys. Similar to the cathode current collector, the anode current collector may also be processed to form fine irregularities on the surfaces thereof so as to enhance adhesive strength to the anode active material. In addition, the anode current collector may be used in various forms including films, sheets, foils, nets, porous structures, foams and non-woven fabrics.
As examples of the anode active materials utilizable in the present invention, mention may be made of carbon such as non-graphitizing carbon and graphite-based carbon; metal composite oxides such as LixFe2O3 (0≦x≦1), LixWO2 (0≦x≦1) and SnxMe1−xMe′yOz (Me: Mn, Fe, Pb or Ge; Me′: Al, B, P, Si, Group I, Group II and Group III elements of the Periodic Table of the Elements, or halogens; 0<x≦1; 1≦y≦3; and 1≦z≦8); lithium metals; lithium alloys; silicon-based alloys; tin-based alloys; metal oxides such as SnO, SnO2, PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4, and Bi2O5; conductive polymers such as polyacetylene; and Li—Co—Ni based materials.
The separator is interposed between the cathode and anode. As the separator, an insulating thin film having high ion permeability and mechanical strength is used. The separator typically has a pore diameter of 0.01 to 10 μm and a thickness of 5 to 300 μm. As the separator, sheets or non-woven fabrics made of an olefin polymer such as polypropylene and/or glass fibers or polyethylene, which have chemical resistance and hydrophobicity, are used. When a solid electrolyte such as a polymer is employed as the electrolyte, the solid electrolyte may also serve as both the separator and electrolyte.
The lithium salt-containing, non-aqueous electrolyte is composed of a non-aqueous electrolyte and lithium. As the non-aqueous electrolyte, a non-aqueous electrolytic solution, solid electrolyte and inorganic solid electrolyte may be utilized.
As the non-aqueous electrolytic solution that can be used in the present invention, for example, mention may be made of non-protic organic solvents such as N-methyl-2-pyrollidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyro lactone, 1,2-dimethoxy ethane, tetrahydroxy Franc, 2-methyl tetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxy methane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ether, methyl propionate and ethyl propionate.
As examples of the organic solid electrolyte utilized in the present invention, mention may be made of polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphoric acid ester polymers, poly agitation lysine, polyester sulfide, polyvinyl alcohols, polyvinylidene fluoride, and polymers containing ionic dissociation groups.
As examples of the inorganic solid electrolyte utilized in the present invention, mention may be made of nitrides, halides and sulfates of lithium such as Li3N, LiI, Li5NI2, Li3N—LiI—LiOH, LiSiO4, LiSiO4—LiI—LiOH, Li2SiS3, Li4SiO4, Li4SiO4Li I—LiOH and Li3PO4—Li2S—SiS2.
The lithium salt is a material that is readily soluble in the above-mentioned non-aqueous electrolyte and may include, for example, LiCl, LiBr, LiI, LiClO4, LiBF4, LiB10Cl10, LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, (CF3SO2)2NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium tetraphenyl borate and imide.
Additionally, in order to improve charge/discharge characteristics and flame retardancy, for example, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxy ethanol, aluminum trichloride or the like may be added to the non-aqueous electrolyte. If necessary, in order to impart incombustibility, the non-aqueous electrolyte may further include halogen-containing solvents such as carbon tetrachloride and ethylene trifluoride. Further, in order to improve high-temperature storage characteristics, the non-aqueous electrolyte may additionally include carbon dioxide gas.
As discussed hereinbefore, sodium phosphates, which correspond to phosphates of Formula I wherein A is Na, can perform precipitation of metal ions as the impurities, simultaneously with provision of flame retardancy.
In general, one of the most significant problems, suffered by lithium secondary batteries, is the low safety of the battery. The lithium secondary batteries are susceptible to the high-risk of ignition under various circumstances such as overcharge, heating from external sources, physical deformation and the like. A variety of methods have been proposed for prevention of overcharge as a cause for such a risk of ignition and for prevention of internal short circuiting resulting from physical deformation. However, in spite of such various preventive measures, there was needed means that can prevent ignition, or can at least inhibit a further progress of ignition when ignition is initiated. For this purpose, several techniques of preventing and suppressing ignition of the battery via the addition of a flame retardant are known in the related art. However, these techniques suffer from inevitable problems associated with deterioration of battery performance caused by direct action of the thus-added flame retardant on main functional elements of the secondary battery to thereby lower an ionic conductivity, consequently resulting in an increased internal resistance of the battery and therefore a decreased discharge capacity.
On the other hand, the sodium phosphate according to the present invention can improve the battery performance by preventing electrodeposition of the metal ions on an anode, via precipitation of the metal ions incorporated during fabrication of the battery, and can also significantly improve the safety of the battery by exerting excellent flame retardancy.
Now, the present invention will be described in more detail with reference to the following examples. These examples are provided only for illustrating the present invention and should not be construed as limiting the scope and spirit of the present invention.
Iron (II) perchlorate hydrate (Fe(ClO4)2.xH2O) was dissolved in a solution of ethylene carbonate (EC) and ethylmethyl carbonate (EMC) (1:2, v/v) containing 1M LiPF6 salt dissolved therein, which is an electrolyte for a lithium secondary battery, thereby preparing a solution containing 500 ppm of Fe. 2% by weight of ammonium hydrogen phosphate ((NH4)2HPO4) was added to the thus-prepared electrolyte which was then left at room temperature for 24 hours, and the concentration of Fe was determined using inductively coupled plasma-atomic emission spectrophotometer (ICP-AES). The results thus obtained are given in Table 1 below.
An experiment was carried out in the same manner as in Example 1, except that ammonium dihydrogen phosphate (NH4H2PO4) was used instead of ammonium hydrogen phosphate ((NH4)2HPO4). The experimental results thus obtained are given in Table 1 below.
An experiment was carried out in the same manner as in Example 1, except that lithium phosphate (Li3PO4) was used instead of ammonium hydrogen phosphate ((NH4)2HPO4). The experimental results thus obtained are given in Table 1 below.
An experiment was carried out in the same manner as in Example 1, except that lithium dihydrogen phosphate (LiH2PO4) was used instead of ammonium hydrogen phosphate ((NH4)2HPO4). The experimental results thus obtained are given in Table 1 below.
An experiment was carried out in the same manner as in Example 1, except that sodium hydrogen phosphate (Na2HPO4) was used instead of ammonium hydrogen phosphate ((NH4)2HPO4). The experimental results thus obtained are given in Table 1 below.
An experiment was carried out in the same manner as in Example 1, except that ammonium hydrogen phosphate ((NH4)2HPO4) was not added. The experimental results thus obtained are given in Table 1 below.
An experiment was carried out in the same manner as in Example 1, except that ammonium benzoate (C6H5COONH4) was used instead of ammonium hydrogen phosphate ((NH4)2HPO4). The experimental results thus obtained are given in Table 1 below.
As can be seen from Table 1, electrolytes of Examples 1 through 5 according to the present invention exhibited a significant decrease in the concentration of Fe ions. In particular, it can be confirmed that an electrolyte of Example 4 using lithium dihydrogen phosphate (LiH2PO4) shows a significant decrease in the concentration of Fe ions. Whereas, electrolytes of Comparative Examples 1 and 2 showed substantially no change in the concentration of Fe ions.
Upon fabrication of a cathode, 0.5% by weight of ammonium hydrogen phosphate ((NH4)2HPO4) was added to fabricate a cathode. The thus-fabricated cathode and an anode made of graphite were used to fabricate a battery. In addition, iron (II) perchlorate hydrate (Fe(ClO4)2.xH2O) was dissolved in a solution of ethylene carbonate (EC) and ethylmethyl carbonate (EMC) (1:2, v/v) containing 1M LiPF6 salt dissolved therein, thereby preparing a solution containing 500 ppm of Fe which was then used as an electrolyte. 10 batteries thus fabricated were left in the fully-charged state for one week. As compared to a potential obtained upon completion of battery charge, the number of batteries showing a voltage drop of more than 100 mV is given in Table 2 below.
An experiment was carried out in the same manner as in Example 6, except that ammonium dihydrogen phosphate (NH4H2PO4) was used instead of ammonium hydrogen phosphate ((NH4)2HPO4). The experimental results thus obtained are given in Table 2 below.
An experiment was carried out in the same manner as in Example 6, except that lithium phosphate (Li3PO4) was used instead of ammonium hydrogen phosphate ((NH4)2HPO4). The experimental results thus obtained are given in Table 2 below.
An experiment was carried out in the same manner as in Example 6, except that lithium dihydrogen phosphate (LiH2PO4) was used instead of ammonium hydrogen phosphate ((NH4)2HPO4). The experimental results thus obtained are given in Table 2 below.
An experiment was carried out in the same manner as in Example 6, except that sodium hydrogen phosphate (Na2HPO4) was used instead of ammonium hydrogen phosphate ((NH4)2HPO4). The experimental results thus obtained are given in Table 2 below.
An experiment was carried out in the same manner as in Example 6, except that, upon fabrication of a cathode, 0.5% by weight of ammonium hydrogen phosphate ((NH4)2HPO4) and 0.5% by weight of alumino-silicate containing ammonium ions (available from Aldrich) were simultaneously added to fabricate a cathode. The experimental results thus obtained are given in Table 2 below.
An experiment was carried out in the same manner as in Example 6, except that ammonium hydrogen phosphate ((NH4)2HPO4) was not used. The experimental results thus obtained are given in Table 2 below.
As can be seen from Table 2, electrolytes of Examples 6 to 10 according to the present invention exhibited a significant decrease in the number of batteries undergoing a voltage drop. In particular, the electrolyte of Example 9 using lithium dihydrogen phosphate (LiH2PO4) was found to exert excellent performance. In addition, no occurrence of a voltage drop was observed in the electrolyte of Example 11 involving simultaneous addition of 0.5% by weight of ammonium hydrogen phosphate ((NH4)2HPO4) and 0.5% by weight of alumino-silicate containing ammonium ions (available from Aldrich). Therefore, it can be seen that combined use of the above two components exerts higher effects. Whereas, the electrolyte of Comparative Example 3 exhibited the occurrence of a voltage drop in 8 out of 10 batteries, due to internal short circuiting of the batteries.
As apparent from the above description, a lithium secondary battery according to the present invention improves life characteristics of the battery by replacing metal cations of metal impurities with the lithium ions, sodium ions and/or ammonium ions which are not detrimental to the operation of the battery, thereby removing the metal impurities and consequently preventing electrodeposition of the metal ions on an anode, through the addition of a certain phosphate compound. In particular, a sodium phosphate provides effects of improving life characteristics while exerting excellent flame retardancy.
Although the preferred embodiments 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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2005-0101016 | Oct 2005 | KR | national |
