Negative active core-shell material, method of preparing the same, and rechargeable lithium battery including the same

0001The present invention relates to a negative active material for a rechargeable lithium battery, a method of preparing the same, and a rechargeable lithium battery including the same. More particularly, the present invention relates to a negative active material for a rechargeable lithium battery that can improve stability at the interface between a negative electrode and an electrolyte, charge and discharge efficiency, and cycle-life, and can be applied along with many kinds of binders, a method of preparing the same, and a rechargeable lithium battery including the same.

0002A lithium rechargeable battery has recently drawn attention as a power source for a small portable electronic device. It uses an organic electrolyte solution and thereby has a discharge voltage that is twice as high as a conventional battery that uses an alkali aqueous solution, and accordingly has a high energy density.

0003A negative active material for a lithium battery can reversibly receive and supply lithium ions while maintaining its structural and electrical properties. For the negative active material, lithium metal, a lithium-containing alloy, or a carbon-based material, which has a chemical potential that is a similar level to lithium metal when lithium ions are intercalated/deintercalated, is usually used. A battery using lithium metal or a lithium alloy as a negative active material is called a lithium metal battery, and a battery using a carbon material is called a lithium ion battery.

0004A lithium metal battery using lithium metal or a lithium alloy for its negative electrode has a risk of explosion due to a battery short that may be caused by formation of dendrites.

0005Hence, the lithium metal battery is being replaced by a lithium ion battery that uses a carbon material as its negative active material and does not have such a risk. The lithium ion battery only transfers lithium ions during charge/ discharge, and it maintains the electrode active material as it is. Thus, the lithium ion battery has improved stability and cycle-life.

0006In the lithium ion battery, a non-aqueous system in which polyvinylidene fluoride (PVDF) is dissolved in N-methyl-2-pyrrolidone (NMP) or an organic acetone solvent is usually used. The non-aqueous system is widely known as a binder for a conventional negative electrode plate. However, when the PVDF/NMP non-aqueous system is used as a binder, the organic solvent such as NMP and acetone may contaminate the natural environment. Also, since the organic solvent is expensive, the manufacturing cost for a lithium battery is increased. In addition, since most organic solvents are highly volatile, there is a risk of explosion when they are used in an enclosed space. To prevent explosions, an anti-explosion unit is additionally required.

0007To overcome the problems, researchers have actively studied to develop a method of using an aqueous system in which styrene-butadiene rubber (SBR) is dispersed in water along with a thickener, e.g., carboxylmethylcellulose (CMC), when a negative electrode plate is manufactured. Since the SBR binder can be dispersed in water in the form of emulsion, it does not have to use an organic solvent, and the SBR binder has strong adherence. With the SBR binder, the amount of a binder is reduced and the amount of a negative active material is increased, which is advantageous in manufacturing a high-capacity lithium battery.

0008When an aqueous solvent is used with an aqueous binder, the high surface tension of the aqueous solvent degrades the coating property of negative active material slurry to thereby reduce the planarity of a negative electrode. The low planarity of the negative electrode makes the surface of the negative electrode non-uniform to thereby reduce a reaction surface area and deteriorate the cycle-life characteristic.

0009The present invention provides a negative active material for a rechargeable lithium battery that can improve stability at the interface between a negative electrode and an electrolyte, charge and discharge efficiency, and cycle-life, and can be applied along with all kinds of aqueous and non-aqueous binders, and a method of preparing the negative active material.

0010The present invention provides a rechargeable lithium battery including the negative active material and having excellent charge and discharge efficiency and cycle-life.

0011According to a first aspect of the present invention, there is provided a negative active material for a rechargeable lithium battery that includes a lithium-vanadium oxide core being capable of performing reversible electrochemical oxidation and reduction, and an inorganic oxide coating layer disposed on the surface of the core material.

0012The lithium-vanadium oxide may be represented by Chemical Formula I.          Chemical Formula I Li_xM_yV_zO_2+d wherein 0.1 ≤ x ≤ 2.5, 0 ≤ y ≤ 0.5, 0.5 ≤ z ≤ 1.5, 0 ≤ d ≤ 0.5, and M is a metal selected from the group consisting of Al, Cr, Mo, Ti, W, Zr, and combinations thereof.

0013The inorganic oxide may be selected from the group consisting of silica, alumina, zirconia, titania, and mixtures thereof.

0014The inorganic oxide may have a nano-sized particle having a diameter of 50 nm or less. According to one embodiment, the inorganic oxide has a nano-sized particle having a diameter of 30 nm or less. According to another embodiment, the inorganic oxide has a nano-sized particle having a diameter of 5 nm to 20 nm.

0015The inorganic oxide coating layer may have a thickness of 50 nm or less. According to one embodiment, the inorganic oxide coating layer has a thickness of 30 nm or less. According to another embodiment, the inorganic oxide coating layer has a thickness of 5 nm to 20 nm.

0016The inorganic oxide may be present in an amount of 1 to 30 parts by weight based on 100 parts by weight of the core material. According to one embodiment, the inorganic oxide is present in an amount of 1 to 10 parts by weight based on 100 parts by weight of the core material. The inorganic oxide may be coated in an amount of 5 parts by weight or 7 parts by weight based on 100 parts by weight of the core material.

0017According to a second aspect of the present invention, there is provided a method of a negative active material for a rechargeable lithium battery that includes steps of preparing a dispersion solution by dispersing lithium-vanadium oxide in a coating liquid including inorganic oxide and a solvent, volatilizing the solvent from the dispersion solution, and heat-treating the solvent-volatilized dispersion solution. The lithium-vanadium oxide is formed into a core.

0018According to a third aspect of the present invention, there is provided a rechargeable lithium battery that includes a negative electrode including the negative active material, a positive electrode including a positive active material, and an electrolyte.

0019According to a fourth aspect of the present invention, there is provided the use of a negative active material according to the present invention in its first aspect for a rechargeable lithium battery.

0020A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent by reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein: <ul id="ul0001" list-style="none" compact="compact"><li>FIGURE 1 schematically shows a negative active material for a rechargeable lithium battery constructed as one embodiment of the present invention;</li><li>FIGURE 2 schematically shows a rechargeable lithium battery using the negative active material, which is constructed as one embodiment of the present invention; and</li><li>FIGURE 3 is a scanning electron microscope photograph of a negative active material prepared according to the principles of the present invention.</li></ul>

0021The present invention relates to a negative active material for a rechargeable lithium battery that has a core-shell structure.

0022The core is made of lithium-vanadium oxide that is capable of performing reversible electrochemical oxidation and reduction, and may be a compound represented by the following Chemical Formula 1.          Chemical Formula 1 Li_xM_yV_zO_2+d wherein, in the above Chemical Formula 1, 0.1 ≤ x ≤ 2.5, 0 ≤ y ≤ 0.5, 0.5 ≤ z ≤ 1.5, 0 ≤ d ≤ 0.5, and M is a metal selected from the group consisting of Al, Cr, Mo, Ti, W, Zr, and combinations thereof.

0023The shell (coating layer) on the surface of the core may include an oxide including an element selected from the group consisting of Si, Ti, Zr, Al, and combinations thereof, or mixtures thereof.

0024The oxide includes an inorganic oxide that may be selected from the group consisting of silica, alumina, zirconia, titania, and mixtures thereof.

0025The inorganic oxide may be present in an amount of 1 to 30 parts by weight based on 100 parts by weight of the core material. According to one embodiment, the inorganic oxide is present in an amount of 1 to 10 parts by weight based on 100 parts by weight of the core material. The inorganic oxide may be coated in an amount of 5 parts by weight or 7 parts by weight based on 100 parts by weight of the core material. If the amount of the inorganic oxide is less than 1 part by weight, the coating effect is negligible. If the amount of the inorganic oxide is more than 30 parts by weight, a thick coating layer may be formed on the surface of the core resulting in an increase of electrode resistance.

0026The inorganic oxide may be a nano-sized particle having a diameter of 50 nm or less. According to one embodiment, the inorganic oxide has a nano-sized particle with a diameter of 30 nm or less. According to another embodiment, the inorganic oxide has a nano-sized particle with a diameter of 5 nm to 20 nm. If the particle size of the inorganic oxide exceeds 50 nm, coating property on the core deteriorates. Conversely, if the particle size is smaller than 5 nm, the inorganic oxide may coagulate with each other, or dispersion property is poor when coating is carried out. Thus, it is hard to form a uniform coating layer.

0027The inorganic oxide coating layer may have a nano-sized thickness that can conducts lithium ions, and does not incur resistance. According to one embodiment, the inorganic oxide coating layer has a thickness of 50 nm or less. According to another embodiment, the inorganic oxide coating layer has a thickness of 30 nm or less. According to yet another embodiment, the inorganic oxide coating layer has a thickness of 5 nm to 20 nm. If the coating layer is thicker than 50 nm, the coating layer functions as a barrier that reduces an electron transfer rate into the surface of an active material that forms the core and reduces a lithium ion transfer rate into the inside of the active material. Eventually, the coating layer considerably degrades the electro-chemical characteristics of the active material, such as the high-efficiency characteristic and cycle-life characteristic. Thus, it is not desirable. Conversely, if the coating layer is thinner than 5 nm, the aforementioned effect cannot be acquired.

0028The structure of the active material is shown in FIGURE 1. As shown in FIGURE 1, active material 10 includes a core-shell structure including core 20 and coating layer (shell) 30 disposed on core 20.

0029An SEI (solid electrolyte interface) film is formed by electrochemical decomposition of an electrolyte at the interface between an electrolyte and a negative active material in a conventional battery system. According to one embodiment of the present invention, the inorganic oxide coating layer, which is disposed on the surface of the core, minimizes the formation of the SEI film during charging and discharging processes, and thereby improves reversible efficiency and cycle-life.

0030The negative active material having the above structure may be used in a mixture with a lithium metal, a lithium-containing alloy, a material that is capable of reversibly intercalating and deintercalating lithium ions, or a material that is capable of reversibly forming a lithium-containing compound by reacting with lithium ions.

0031The material capable of reversibly intercalating and deintercalating lithium ions may be a carbon-based negative active material such as crystalline carbon, amorphous carbon, or combinations thereof.

0032The material capable of reversibly forming a lithium-containing compound by reacting with lithium ions may include Si, Si oxide, Sn, Sn oxide, a tin alloy composite, a transition element oxide, a lithium metal nitride, or lithium-vanadium oxide that is not coated with an inorganic oxide.

0033The lithium-containing alloy may be an alloy of lithium and a metal selected from the group consisting of Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Al, Fe, and Sn.

0034The negative active material developed as one embodiment of the present invention may be prepared as follows.

0035First, a dispersion solution is prepared by dispersing lithium-vanadium oxide in a coating liquid in which nano-sized inorganic oxide particles are dispersed. Since the nano-sized inorganic oxide particles have a high surface energy, they are adsorbed onto the surface of a core material that exists in a form of large particle to maintain stability.

0036The inorganic oxide may be present in an amount of 1 to 30 parts by weight based on 100 parts by weight of the core material. According to one embodiment, the inorganic oxide is present in an amount of 1 to 10 parts by weight based on 100 parts by weight of the core material. According to another embodiment, the inorganic oxide is present in an amount of 5 parts by weight or 7 parts by weight based on 100 parts by weight of the core material. When the amount of the inorganic oxide is less than 1 part by weight, coating effect is negligible, whereas when it is more than 30 parts by weight, a thick coating layer may be formed on the surface of the core resulting in increase of electrode resistance.

0037The core material may be used in an amount of 1 to 10 g per 1000 ml of a solvent.

0038Herein, the solvent of the dispersion solution may include a material selected from the group consisting of water, methanol, ethanol, propanol, and mixtures thereof.

0039Subsequently, the solvent may be volatilized from the prepared dispersion solution, and then heat treatment is performed to obtain lithium-vanadium oxide coated with an inorganic oxide.

0040The process of volatilizing the solvent may be performed by heating the dispersion solution at temperature ranging from 50 °C to 100 °C, and more specifically at around 60 °C. The volatilization temperature may vary according to the kind of alcohol used for a solvent of the dispersion solution. If the volatilization is performed at temperature lower than 50 °C, the volatilization time becomes longer. If it is performed at temperature higher than 100 °C, there is no big difference in the effect. Thus, it is uneconomical.

0041The heat treatment may be performed at temperature ranging from 300 °C to 600 °C. When the heat treatment is performed at lower than 300 °C, impurities exist in the core material. When it is performed at higher than 600 °C, the process is uneconomical.

0042In addition, additives such as a dispersing agent and a surfactant may be added to the dispersion solution to help dispersion of the lithium-vanadium oxide. The additives stabilize the surface of the nano-sized inorganic oxide. Since the inorganic oxide with a stabilized surface cannot be adsorbed onto the lithium-vanadium oxide, the additives should be used by an amount that the additives do not inhibit the adsorption. The kinds and amount of the additives are widely known to those skilled in the art to which the present invention pertains.

0043The rechargeable lithium battery includes a negative electrode including the negative active material described above, a positive electrode, and an electrolyte.

0044The negative electrode includes a current collector and a negative active material layer disposed on the current collector. The negative active material layer is made of a material that includes the negative active material described above.

0045The negative electrode may be fabricated as follows. A composition, including the negative active material of the present invention, a binder, and optionally a conductive agent mixed in a solvent, is applied on a negative current collector made of a material such as copper.

0046The negative active material of the present invention is the same as above-described. According to one embodiment of the present invention, the negative active material is included in an amount of 50 wt% to 99 wt% based on the total weight of the negative active material layer. According to another embodiment, the negative active material is included in an amount of 80 wt% to 98 wt% based on the total weight of the negative active material layer. When the amount of the negative active material is less than 50 wt%, battery capacity may decreases, whereas when it is more than 99 wt%, a relative amount of a binder is reduced and thus binding force between the negative active material layer and a current collector may decreases.

0047The amount of the binder ranges from 0.8 wt% to 5 wt% based on the total weight of the negative active material layer. According to one embodiment, the amount of the binder ranges from 1 wt% to 5 wt%. According to another embodiment, the amount of the binder ranges from 1 wt% to 2 wt%. When the amount of the binder is less than 0.8 wt%, binder content is significantly low and adherence between a negative active material and a current collector is not sufficient. On the contrary, when it is more than 5 wt%, the amount of the negative active material is low and thus a high-capacity battery cannot be realized.

0048Herein, aqueous or non-aqueous binders may be used.

0049For the aqueous binder, at least one selected from the group consisting of a styrene-butadiene rubber (SBR), polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, and mixtures thereof may be used. Since the styrene-butadiene rubber binder can be dispersed in water in the form of an emulsion, it does not have to use an organic solvent. Also, since it is highly adhesive, the amount of the binder to be used can be reduced and the amount of the negative active material can be increased. Therefore, it is advantageous in the manufacture of a high-capacity lithium battery. The aqueous binder is used together with water or an aqueous solvent of water and an alcohol solvent.

0050When an aqueous binder is used, a thickener may be added thereto to adjust the thickness. The thickener may be at least one selected from the group consisting of carboxyl methyl cellulose, hydroxymethyl cellulose, hydroxy ethyl cellulose, hydroxy propyl cellulose, and combinations thereof. The amount of the thickener may range from 0.8 wt% to 5 wt% based on the entire negative active material layer, specifically from 1 wt% to 5 wt%, and more specifically from 1 wt% to 2 wt%.

0051If the amount of the thickener is less than 0.8 wt%, there may be a problem that the negative active material may be too fluid and flows down during a coating process of the negative active material. If the amount of the thickener is more than 5 wt%, the negative active material becomes too thick for coating and functions as a resistor. Also, since the amount of the negative active material is reduced as much as an excessive amount of the thickener, there is a problem that the produced battery does not have high capacity.

0052Examples of the non-aqueous binder include at least one selected from the group consisting of polyvinylchloride, polyvinylpyrrolidone, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, and mixtures thereof. These binders are used along with non-aqueous solvents such as N-methyl-2-pyrrolidone (NMP), dimethylformamide, tetrahydrofuran, and mixtures thereof.

0053Any electrically conductive material may be used as a conductive agent unless it causes a chemical reaction. Examples of the conductive agent include natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, a metal powder or a metal fiber including copper, nickel, aluminum, silver, and so on, a polyphenylene derivative, or combinations thereof.

0054The current collector may be selected from the group consisting of a copper foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foam, a copper foam, a polymer substrate coated with a conductive metal, and combinations thereof.

0055The positive electrode includes a positive active material that is capable of performing a reversible electrochemical oxidation and reduction reaction, and is a lithiated intercalation compound that is used in a rechargeable lithium battery. The lithiated intercalation compound includes compounds of the following Chemical Formulas 2 to 25.          Chemical Formula 2 Li_aA_1-bB_bD₂ wherein, in the Formula 2, 0.95 ≤ a ≤ 1.1, and 0 ≤ b ≤ 0.5.          Chemical Formula 3 Li_aE_1-bB_bO_2-cF_c wherein, in the above Formula 3, 0.95 ≤ a ≤ 1.1, 0 ≤ b ≤ 0.5, and 0 ≤ c ≤ 0.05.          Chemical Formula 4 LiE_2-bB_bO_4-cF_c wherein, in the above Formula 4, 0 ≤ b ≤ 0.5, and 0 ≤ c ≤ 0.05.          Chemical Formula 5 Li_aNi_1-b-cCo_bBcD_α wherein, in the above Formula 5, 0.95 ≤ a ≤1.1, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, and 0 &lt; α ≤ 2.          Chemical Formula 6 Li_aNi_1-b-cCo_bB_cO_2-αF_α wherein, in the above Formula 6, 0.95 ≤ a ≤ 1.1, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, and 0 &lt; α &lt; 2.          Chemical Formula 7 Li_aNi_1-b-cCo_bB_cO_2-αF₂ wherein, in the above Formula 7, 0.95 ≤ a ≤ 1.1, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, and 0 &lt; α &lt; 2.          Chemical Formula 8 Li_aNi_1-b-cMn_bB_cD_α wherein, in the above Formula 8, 0.95 ≤ a ≤ 1.1, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, and 0 &lt; α ≤ 2.          Chemical Formula 9 Li_aNi_1-b-cMn_bB_cO_2-αF_α wherein, in the above Formula 9, 0.95 ≤ a ≤ 1.1, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, and 0 &lt; α &lt; 2.          Chemical Formula 10 Li_aNi_1-b-cMn_bB_cO_2-αF₂ wherein, in the above Formula 10, 0.95 ≤ a ≤ 1.1, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.05, and 0 &lt; α &lt; 2.          Chemical Formula 11 Li_aNi_bE_cG_dO₂ wherein, in the above Formula 11, 0.90 ≤ a ≤ 1.1, 0 ≤ b ≤ 0.9, 0 ≤ c ≤ 0.5, and 0.001 ≤ d ≤ 0.1.          Chemical Formula 12 Li_aNi_bCo_cMn_dGeO₂ wherein, in the above Formula 12, 0.90 ≤ a ≤ 1.1, 0 ≤ b ≤ 0.9, 0 ≤ c ≤ 0.5, 0 ≤d≤0.5, and 0.001 ≤e≤0.1.          Chemical Formula 13 Li_aNiG_bO₂ wherein, in the above Formula 13, 0.90 ≤ a ≤ 1.1, and 0.001 ≤ b ≤ 0.1.          Chemical Formula 14 Li_aCoG_bO₂ wherein, in the above Formula 14, 0.90 ≤ a ≤ 1.1, and 0.001 ≤ b ≤ 0.1.          Chemical Formula 15 Li_aMnG_bO₂ wherein, in the above Formula 15, 0.90 ≤ a ≤ 1.1, and 0.001 ≤ b ≤ 0.1.          Chemical Formula 16 Li_aMn₂G_bO₄ wherein, in the above Formula 16, 0.90 ≤ a ≤ 1.1, and 0.001 ≤ b ≤ 0.1.          Chemical Formula 17          Chemical Formula 18 QS₂          Chemical Formula 19 LiQS₂          Chemical Formula 20 V₂O₅          Chemical Formula 21 LiV₂O₅          Chemical Formula 22 LiIO₂          Chemical Formula 23 LiNiVO₄          Chemical Formula 24 Li_3-fJ₂(PO₄)₃ (0 ≤ f ≤ 3)          Chemical Formula 25 Li_3-fFe₂(PO₄)₃ (0 ≤ f ≤ 2)

0056In the above Formulas 2 to 25, "A" is selected from the group consisting of Ni, Co, Mn, and combinations thereof. "B" is selected from the group consisting of Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, a rare earth element, and combinations thereof. "D" is selected from the group consisting of O, F, S, P, and combinations thereof. "E" is selected from the group consisting of Co, Mn, and combinations thereof. "F" is selected from the group consisting of F, S, P, and combinations thereof. "G" is selected from the group consisting of Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V, a lanthanide element, and combinations thereof. "Q" is selected from the group consisting of Ti, Mo, Mn, and combinations thereof. "I" is selected from the group consisting of Cr, V, Fe, Sc, Y, and combinations thereof. "J" is selected from the group consisting of V, Cr, Mn, Co, Ni, Cu, and combinations thereof.

0057The electrolyte includes a non-aqueous organic solvent and a lithium salt.

0058The lithium salt is dissolved in the non-aqueous organic solvent to supply lithium ions in the battery. It may operate a basic operation of a rechargeable lithium battery, and it facilitates transmission of lithium ions between positive and negative electrodes. Non-limiting examples of the lithium salt include at least one supporting electrolyte salt selected from the group consisting of LiPF₆, LiBF₄, LiSbF₆, LiAsF₆, LiCF₃SO₃, LiN(SO₂C₂F₅)₂, LiN(CF₃SO₂)₂, LiC₄F₉SO₃, LiClO₄, LiAlO₂, LiAlCl₄, LiN(C_xF_2x+1SO₂)(C_yF_2y+1SO₂) (where x and y are natural numbers), LiCl, LiI, and lithium bisoxalate borate. The lithium salt may be used at a 0.1M to 2.0M concentration. When the lithium salt concentration is less than 0.1M, electrolyte performance may deteriorate due to low electrolyte conductivity, whereas when it is more than 2.0M, lithium ion mobility may be reduced due to an increase of electrolyte viscosity.

0059The non-aqueous organic solvent acts as a medium for transmitting ions taking part in the electrochemical reaction of the battery. The non-aqueous organic solvent may include, but is not limited to, benzene, toluene, fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene, 1,4-difluorobenzene, 1,2,3-trifluorobenzene, 1,2,4-trifluorobenzene, chlorobenzene, 1,2-dichlorobenzene, 1,3-dichlorobenzene, 1,4-dichlorobenzene, 1,2,3-trichlorobenzene, 1,2,4-trichlorobenzene, iodobenzene, 1,2-diiodobenzene, 1,3-diiodobenzene, 1,4-diiodobenzene, 1,2,3-triiodobenzene, 1,2,4-triiodobenzene, fluorotoluene, 1,2-difluorotoluene, 1,3-difluorotoluene, 1,4-difluorotoluene, 1,2,3-trifluorotoluene, 1,2,4-trifluorotoluene, chlorotoluene, 1,2-dichlorotoluene, 1,3-dichlorotoluene, 1,4-dichlorotoluene, 1,2,3-trichlorotoluene, 1,2,4-trichlorotoluene, iodotoluene, 1,2-diiodotoluene, 1,3-diiodotoluene, 1,4-diiodotoluene, 1,2,3-triiodotoluene, 1,2,4-triiodotoluene, R-CN (wherein R is a C2 to C50 linear, branched, or cyclic hydrocarbon, a double bond, an aromatic ring, or an ether bond), dimethylformamide, dimethylacetate, xylene, cyclohexane, tetrahydrofuran, 2-methyltetrahydrofuran, cyclohexanone, ethanol, isopropyl alcohol, dimethyl carbonate, ethylmethyl carbonate, diethyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, methyl propionate, ethyl propionate, methyl acetate, ethyl acetate, propyl acetate, dimethoxyethane, 1,3-dioxolan, diglyme, tetraglyme, ethylene carbonate, propylene carbonate, γ-butyrolactone, sulfolane, valerolactone, decanolide, and mevalolactone. The non-aqueous organic solvent may be used singularly or in a mixture. When the organic solvent is used in a mixture, a mixing ratio can be controlled in accordance with a desirable cell performance.

0060FIGURE 2 shows a rechargeable lithium battery having the above-mentioned structure constructed as an embodiment of the present invention.

0061Referring to FIGURE 2, rechargeable lithium battery 3 is composed of electrode assembly 4 including positive electrode 5, negative electrode 6, and separator 7 interposed between positive electrode 5 and negative electrode 6, all of which are positioned in battery case 8. The upper part of battery case 8 is sealed with cap plate 11 and gasket 12. The rechargeable lithium battery of the present embodiment is not limited to the above-mentioned shape, and may have a prismatic shape, a pouch, or so on.

0062The rechargeable lithium battery generally includes a positive electrode, a negative electrode, and an electrolyte. The battery may further include a separator as needed. The separator may include any material used in conventional lithium secondary batteries. Non-limiting examples of suitable separator materials include polyethylene, polypropylene, polyvinylidene fluoride, and multi-layers thereof such as a polyethylene/polypropylene double-layered separator, a polyethylene/polypropylene/polyethylene three-layered separator, and a polypropylene/polyethylene/polypropylene three-layered separator.

0063The following examples illustrate the present invention in more detail. However, it is understood that the present invention is not limited by these examples.

Example 1

0064A coating liquid was prepared by dispersing silica with an average particle diameter of 12 nm in ethanol and performing ultrasonic wave treatment thereon for one hour.

0065A dispersion solution was prepared by dispersing Li_1.1Ti_0.1V_0.9O₂ in the coating liquid. Herein, one part by weight of silica was used based on 100 parts by weight of Li_1.1Ti_0.1V_0.9O₂. The dispersion solution was heated at 60 °C to volatilize ethanol. Then, a negative active material with a silica coating layer formed on the surface of Li_1.1Ti_0.1V_0.9O₂ in a thickness of 20 nm was prepared by performing heat treatment at 450 °C.

Example 2

0066A negative active material was prepared according to the same method as described in Example 1, except that a silica coating layer was formed in a thickness of 22 nm, 3 parts by weight of silica was used based on 100 parts by weight of Li_1.1Ti_0.1V_0.9O₂, and an average particle diameter of the silica was 10 nm.

Example 3

0067A coating liquid was prepared by dispersing alumina, which has an average particle diameter of 12 nm, in ethanol and performing ultrasonic wave treatment thereon for one hour.

0068A dispersion solution was prepared by dispersing Li_1.1Ti_0.1V_0.9O₂ in the coating liquid. Herein, one part by weight of alumina was used based on 100 parts by weight of Li_1.1Ti_0.1V_0.9O₂. The dispersion solution was heated at 60 °C to volatilize ethanol. Then, a negative active material with an alumina coating layer formed on the surface of Li_1.1Ti_0.1V_0.9O₂ in a thickness of 24 nm was prepared by performing heat treatment at 450 °C.

Example 4

0069A negative active material was prepared according to the same method as described in Example 3, except that an alumina coating layer was formed in a thickness of 35 nm by using 7 parts by weight of alumina, which has an average particle diameter of 15 nm, based on 100 parts by weight of Li_1.1Ti_0.1V_0.9O₂.

Example 5

0070A coating liquid was prepared by dispersing zirconia with an average particle diameter of 10 nm in propanol and performing ultrasonic wave treatment thereon for one hour.

0071A dispersion solution was prepared by dispersing Li_1.1Ti_0.1V_0.9O₂ in the coating liquid. Herein, five parts by weight of zirconia was used based on 100 parts by weight of Li_1.1Ti_0.1V_0.9O₂. The dispersion solution was heated at 80 °C to volatilize propanol. Then, a negative active material with a zirconia coating layer formed on the surface of Li_1.1Ti_0.1V_0.9O₂ in a thickness of 25 nm was prepared by performing heat treatment at 450 °C.

Example 6

0072A negative active material was prepared according to the same method as described in Example 5, except that a zirconia coating layer was formed in a thickness of 24 nm by using 3 parts by weight of zirconia, which has an average particle diameter of 15 nm, based on 100 parts by weight of Li_1.1Ti_0.1V_0.9O₂.

Example 7

0073A coating liquid was prepared by dispersing titania with an average particle diameter of 7 nm in ethanol and performing ultrasonic wave treatment thereon for one hour.

0074A dispersion solution was prepared by dispersing Li_1.1Ti_0.1V_0.9O₂ in the coating liquid. Herein, 15 parts by weight of titania was used based on 100 parts by weight of Li_1.1Ti_0.1V_0.9O₂. The dispersion solution was heated at 80 °C to volatilize ethanol. Then, a negative active material with a titania coating layer formed on the surface of Li_1.1Ti_0.1V_0.9O₂ in a thickness of 34 nm was prepared by performing heat treatment at 450 °C.

Example 8

0075A negative active material was prepared according to the same method as described in Example 7, except that a titania coating layer was formed in a thickness of 45 nm by using 15 parts by weight of titania, which has an average particle diameter of 15 nm, based on 100 parts by weight of Li_1.1Ti_0.1V_0.9O₂.

Comparative Example 1

0076Li_1.1Ti_0.1V_0.9O₂ not coated with silica nano-particles was used in a negative active material instead of nano-particle coated Li_1.1Ti_0.1V_0.9O₂ as described in Example 1 to 8.

Experimental Example 1

0077The negative active material prepared according to Example 1 was observed with a scanning electron microscope (SEM), and a photograph thereof is presented in FIGURE 3.

0078FIGURE 3 shows a SEM photograph of the negative active material prepared according to Example 1. Referring to FIGURE 3, the surface of Li_1.1Ti_0.1V_0.9O₂ was coated with silica after heat treatment.

Manufacturing of Battery

0079A mixture was prepared by mixing a positive active material, e.g., lithium cobalt oxide (LiCoO₂) with a conductive agent, e.g., carbon black. A binder liquid was prepared by dissolving a polyvinylidenefluoride binder in an N-methyl-2-pyrrolidone solvent, and positive active material slurry was prepared by adding the mixture to the binder liquid.

0080The positive active material slurry was applied to aluminum foil having a thickness of 20 µm by using a doctor blade method. A positive electrode was prepared by drying the aluminum foil coated with the positive active material slurry at 120°C for 10 hours in a vacuum atmosphere to thereby volatilize N-methyl-2-pyrrolidone, and compressing it.

0081Negative active material slurries were prepared by mixing the negative active materials prepared according to Examples 1 to 3 and Comparative Example 1 with a carbon black conductive agent in an N-methylpyrrolidone solution where a polyvinylidenefluoride binder was dissolved.

0082The negative active material slurries were applied to copper foil having a thickness of 12 µm by using the doctor blade method. Then, negative electrodes were prepared by drying the copper foils coated with the negative active material slurries at 120 °C for 10 hours in the vacuum atmosphere to volatilize N-methyl-2-pyrrolidone, and compressing them.

0083Rechargeable lithium batteries were manufactured in a conventional method by using the positive electrodes, the negative electrodes, and polyolefin-based separators, e.g., polyethylene separators and polypropylene separators.

Experimental Example 2

0084Rechargeable lithium batteries including the negative active materials prepared according to Examples 1 to 3 and Comparative Example 1 were charged at 0.5 C up to 4.2 V and then discharged at 0.5 C down to 3.0 V. The charge/discharge was performed 50 times. Coulomb efficiencies were measured to estimate the initial reversible efficiency, and capacity retentions were measured after the charge/discharge was performed 50 times. The results were as shown in the following Table 1. The capacity retention, after performing charge/discharge 50 times, signifies a ratio of discharge capacity after 50-times charge/discharge to discharge capacity after one-time charge/discharge, which is referred to as a 50th cycle-life in Table 1. <tables id="tabl0001" num="0001"><table frame="all"><title>Table 1</title><tgroup cols="3"><colspec colnum="1" colname="col1" colwidth="39mm" /><colspec colnum="2" colname="col2" colwidth="33mm" /><colspec colnum="3" colname="col3" colwidth="25mm" /><thead><row><entry align="center" valign="top" /><entry align="center" valign="top">Coulomb efficiency</entry><entry align="center" valign="top">50th cycle-life</entry></row></thead><tbody><row><entry align="center">Example 1</entry><entry align="center">92.5%</entry><entry align="center">80.8%</entry></row><row><entry align="center">Example 2</entry><entry align="center">91.9%</entry><entry align="center">78.3%</entry></row><row><entry align="center">Example 3</entry><entry align="center">92.2%</entry><entry align="center">79.1 %</entry></row><row><entry align="center">Comparative Example 1</entry><entry align="center">91.7%</entry><entry align="center">76%</entry></row></tbody></tgroup></table></tables>

0085It is shown from Table 1 that the batteries of Examples 1 to 3 had a high Coulomb efficiency and excellent cycle-life, compared to the battery of Comparative Example 1.

0086These results show that reactivity at the interface between a negative electrode and an electrolyte is improved during cell operation due to an inorganic oxide coating layer on the surface of the negative active material. Thereby charge and discharge efficiency and cycle-life of a rechargeable lithium battery are also improved.

0087The negative active material for a rechargeable lithium battery can be used along with an aqueous binder, and can provide a rechargeable lithium battery having high capacity, charge and discharge efficiency, and improved cycle-life, particularly high capacity during charging and discharging at a high rate.

0088While this invention has been described in connection with the aforementioned exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.