Positive active material for rechargeable lithium battery and method of preparing same

(a) Field of the Invention

The present invention relates to a positive active material for rechargeable lithium batteries and a method of preparing the same and, more particularly, to a manganese-based positive active material for rechargeable lithium batteries which has a good cycle life characteristic.

(b) Description of the Related Art

Generally, manganese-based compounds such as LiMn 2 O 4 and LiMnO 2 are the positive active material of choice for rechargeable lithium batteries because of their low cost, abundance and environmentally friendly characteristics. Among such manganese-based compounds, LiMn 2 O 4 is particularly stable for the battery use and thus attractive for the electric vehicle application.

However, as compared to other lithiated transition metal oxides such as LiCoO 2 and LiNiO 2 , LiMn 2 O 4 has a relatively low discharge capacity. Furthermore, when high rate of charge and discharge operations are cycled, the discharge capacity is excessively reduced. In particular, when the charge and discharge operations are continuously performed at high temperatures, manganese distributed in the surface of LiMn 2 O 4 readily elutes to the electrolyte, causing a disproportionation reaction. This reaction seriously deteriorates the cycle life characteristic of the battery.

It is an object of the present invention to provide a manganese-based positive active material for rechargeable lithium batteries which exhibits a good cycle life characteristic at high temperatures.

This and other objects may be achieved by a positive active material for rechargeable lithium batteries including an active material component processed from a manganese-based compound. The manganese-based compound is selected from Li x MnO 2 , Li x MnF 2 , Li x MnS 2 , Li x MnO 2-z F z , Li x MnO 2-z S z , Li x Mn 1-y M y O 2 , Li x Mn 1-y M y F 2 , Li x Mn 1-y M y S 2 , Li x Mn 1-y M y O 2-z F z , Li x Mn 1-y M y O 2-z S z , Li x Mn 2 O 4 , Li x Mn 2 F 4 , Li x Mn 2 S 4 , Li x Mn 2 O 4-z F z , Li x Mn 2 O 4-z S z , Li x Mn 2-y M y O 4 , Li x Mn 2-y M y F 4 , Li x Mn 2-y M y S 4 , Li x Mn 2-y M y O 4-z F z , or Li x Mn 2-y M y O 4-z S z where 0<x≦1.5, 0.05≦y≦0.3, z≦1.0 and M is selected from Al, Co, Cr, Mg, Fe or La. A metallic oxide is coated on the active material component.

A method of preparing the positive active material is performed by obtaining a powder from a source material. The source material is selected from Li x MnO 2 , Li x MnF 2 , Li x MnS 2 , Li x MnO 2-z F z , Li x MnO 2-z S z , Li x Mn 1-y M y O 2 , Li x Mn 1-y M y F 2 , Li x Mn 1-y M y S 2 , Li x Mn 1-y M y O 2-z F z , Li x Mn 1-y M y O 2-z S z , Li x Mn 2 O 4 , Li x Mn 2 F 4 , Li x Mn 2 S 4 , Li x Mn 2 O 4-z F z , Li x Mn 2 O 4-z S z , Li x Mn 2-y M y O 4 , Li x Mn 2-y M y F 4 , Li x Mn 2-y M y S 4 , Li x Mn 2-y M y O 4-z F z , or Li x Mn 2-y M y O 4-z S z , where 0<x≦1.5, 0.05≦y≦0.3, z≦1.0 and M is selected from Al, Co, Cr, Mg, Fe or La. The powder is then coated with a metallic alkoxide solution to make an alkoxide-coated powder. Thereafter, the metallic alkoxide-coated powder is heat-treated such that it is changed into a metallic oxide-coated powder.

A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein:

FIG. 1 is a graph illustrating high-temperature charge and discharge characteristics of rechargeable lithium cells according to an example of the present invention and a comparative example;

FIG. 2 is a graph illustrating high-temperature cycle life characteristics of the rechargeable lithium cells of FIG. 1 ; and

FIG. 3 is a graph illustrating SIMS analysis results with respect to a rechargeable lithium cell according to another example of the present invention.

In a method of preparing a positive active material for rechargeable lithium batteries, a powder, being an active material precursor, is first processed from a manganese-based compound. The manganese-based compound is selected from Li x MnO 2 , Li x MnF 2 , Li x MnS 2 , Li x MnO 2-z F z , Li x MnO 2-z S z , Li x Mn 1-y M y O 2 , Li x Mn 1-y M y F 2 , Li x Mn 1-y M y S 2 , Li x Mn 1-y M y O 2-z F z , Li x Mn 1-y M y O 2-z S z , Li x Mn 2 O 4 , Li x Mn 2 F 4 , Li x Mn 2 S 4 , Li x Mn 2 O 4-z F z , Li x Mn 2 O 4-z S z , Li x Mn 2-y M y O 4 , Li x Mn 2-y M y F 4 , Li x Mn 2-y M y S 4 , Li x Mn 2-y M y O 4-z F z , or Li x Mn 2-y M y O 4-z S z , where 0<x≦1.5, 0.05≦y≦0.3, z≦1.0 and M is selected from Al, Co, Cr, Mg, Fe or La. The powder processing step can be performed with a technique known in the related art.

Thereafter, the powder is coated with an alkoxide solution. The alkoxide solution is formed by the reaction of an alcohol with a metal being 1 to 50 weight percent of the alcohol. The metal may be preferably selected from Mg, Al, Co, K, Na, Ca, Ti or Sr. More preferably, the metal is selected from Mg, Ti or Al. An alternative alkoxide is a silicon alkoxide formed by a reaction of an alcohol with silicon. The alcohol is preferably selected from methanol or ethanol. When the metal or silicon is less than 1 weight percent of the alcohol, the coating effect of the metallic alkoxide solution onto the powder is not induced. In contrast, when the metal or silicon is more than 50 weight percent of the alcohol, the coating layer of the metallic alkoxide solution becomes undesirably thick. A sputtering technique, a chemical vapor deposition (CVD) technique, a dip coating technique and other general-purpose coating techniques may be employed for the coating use. Among the techniques, the dip coating technique may be preferably used for coating the alkoxide solution onto the powder.

The alkoxide-coated powder is then dried at 120° C. for about 5 hours in an oven. The drying step is to uniformly distribute lithium salts in the powder. Thereafter, the dried powder is heat-treated at temperatures ranged from 200 to 1000° C. for 1 to 20 hours under an oxidation atmosphere where dry air or oxygen is blowing. When the heat-treating temperature is lower than 200° C., the metallic or silicon alkoxide solution coated on the powder is not crystallized so that it prohibits free movement of lithium ions in the active material. It is preferable that the heat-treating step is performed at temperatures ranged from 300 to 900° C. for 1 to 10 hours. This heat-treating operation makes the metallic or silicon alkoxide to be changed into an oxide. In this way, a metallic or silicon oxide-coated active material is prepared.

The metallic or silicon oxide formed on the surface of the power may be derived from the single metallic or silicon alkoxide source or the composite sources of manganese of lithiated transition metal compound and metallic alkoxide. The thickness of the metallic oxide layer reaches up to 1 to 100 nm and the quantity of metal content is ranged from 1.0 to 10 weight percent of the metallic oxide.

The following examples further illustrate the present invention.