Material for lithium secondary battery of high performance

Abstract

Provided is a cathode active material containing a Ni-based lithium mixed transition metal oxide. More specifically, the cathode active material comprises the lithium mixed transition metal oxide having a composition represented by Formula I of LixMyO2 wherein M, x and y are as defined in the specification, which is prepared by a solid-state reaction of Li2CO3 with a mixed transition metal precursor under an oxygen-deficient atmosphere, and has a Li2CO3 content of less than 0.07% by weight of the cathode active material as determined by pH titration. The cathode active material in accordance with the present invention and substantially free of water-soluble bases such as lithium carbonates and lithium sulfates and therefore has excellent high-temperature and storage stabilities and a stable crystal structure. A secondary battery comprising such a cathode active material exhibits a high capacity and excellent characteristics, and can be produced by an environmentally friendly method with low production costs and high production efficiency.

Description

FIELD OF THE INVENTION [0001] The present invention relates to a cathode active material containing a Ni-based lithium mixed transition metal oxide. More specifically, the present invention relates to a cathode active material which comprises a lithium mixed transition metal oxide having a given composition, in which the lithium mixed transition metal oxide is prepared by a solid-state reaction of Li2CO3 with a mixed transition metal precursor under an oxygen-deficient atmosphere, and has a Li2CO3 content of less than 0.07% by weight of the cathode active material as determined by pH titration. BACKGROUND OF THE INVENTION [0002] Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as an energy source. Among other things, lithium secondary batteries having a high-energy density and voltage, a long cycle lifespan and a low self-discharge rate are commercially available and widely used. [0003] As cathode ...

AI Technical Summary

Benefits of technology

[0023] As a result of a variety of extensive and intensive studies and experiments and in view of the problems as described above, the inventors of the present invention provide herewith a cathode active material, containing a lithium mixed transition metal oxide having a given composition, prepared by a solid-state reaction of Li2CO3 with a mixed transition metal precursor under an oxygen-deficient atmosphere, and being substantially free of Li2CO3, exhibits a high capacity, excellent cycle characteristics, significantly improved storage and high-temperature stability, and can be produced with low production costs and improved production efficiency. The present invention has been completed based on these findings.

Problems solved by technology

Of the aforementioned cathode active materials, LiCoO2 is currently widely used due to superior general properties including excellent cycle characteristics, but suffers from low safety, expensiveness due to finite resources of cobalt as a raw material, and limitations in practical and mass application thereof as a power source for electric vehicles (EVs) and the like.

However, these lithium manganese oxides suffer from shortcomings such as low capacity and poor cycle characteristics.

However, the LiNiO2-based cathode active materials suffer from some limitations in practical application thereof, due to the following problems.

First, LiNiO2-based oxides undergo sharp phase transition of the crystal structure with volumetric changes accompanied by repeated charge / discharge cycling, and thereby may suffer from cracking of particles or formation of voids in grain boundaries.

Consequently, intercalation / deintercalation of lithium ions may be hindered to increase the polarization resistance, thereby resulting in deterioration of the charge / discharge performance.

However, the thus-prepared cathode active material, under the charged state, undergoes structural swelling and destabilization due to the repulsive force between oxygen atoms, and suffers from problems of severe deterioration in cycle characteristics due to repeated charge / discharge cycles.

Further, LiNiO2 particles have an agglomerate secondary particle structure in which primary particles are agglomerated to form secondary particles and consequently a contact area with the electrolyte further increases to result in severe evolution of CO2 gas, which in turn unfortunately leads to the occurrence of battery swelling and deterioration of desirable high-temperature safety.

Third, LiNiO2 suffers from a sharp decrease in the chemical resistance of a surface thereof upon exposure to air and moisture, and the gelation of slurries by polymerization of an N-methyl pyrrolidone / poly(vinylidene fluoride) (NMP-PVDF) slurry due to a high pH value.

These properties of LiNiO2 cause severe processing problems during battery production.

Fourth, high-quality LiNiO2 cannot be produced by a simple solid-state reaction as is used in the production of LiCoO2, and LiNiMO2 cathode active materials containing an essential dopant cobalt and further dopants manganese and aluminum are produced by reacting a lithium source such as LiOH.H2O with a mixed transition metal hydroxide under an oxygen or syngas atmosphere (i.e., a CO2-deficient atmosphere), which consequently increases production costs.

Further, when an additional step, such as intermediary washing or coating, is included to remove impurities in the production of LiNiO2, this leads to a further increase in production costs.

However, various problems, such as high production costs, swelling due to gas evolution in the fabricated batteries, poor chemical stability, high pH and the like, have not been sufficiently solved.

An excess of lithium “evaporates”; however, “evaporation” is a lab-scale effect and not an option for large-scale preparation.

That is, when applied to a large-scale production process, it becomes difficult to evaporate excess lithium, thereby resulting in problems associated with the formation of lithium hydroxides and lithium carbonates.

At higher temperatures, samples deteriorate dramatically.

In prior arts including the above, LiNiO2-based cathode active materials are generally prepared by high cost processes, in a specific reaction atmosphere, especially in a flow of synthetic gas such as oxygen or synthetic air, free of CO2, and using LiOH.H2O, Li nitrate, Li acetate, etc., but not the inexpensive, easily manageable Li2CO3.

Furthermore, the final cathode active materials have a high content of soluble bases, originating from carbonate impurities present in the precursors, which remain in the final cathode because of the thermodynamic limitation.

Further, the crystal structure of the final cathode active materials per se is basically unstable even when the final cathode active materials are substantially free of soluble bases.

Further, it was found through various experiments conducted by the inventors of the present invention that the aforesaid prior art composite oxide suffers from significant problems associated with a high-temperature safety, due to production of large amounts of impurities such as Li2CO3.

), but the resulting electrochemical properties are very poor.

Even where a complete coverage of the particle is accomplished, a significant improvement of air-stability could not be made and electrochemical properties were poor.

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