Material for lithium secondary battery of high performance

Abstract

Provided is a lithium mixed transition metal oxide having a composition represented by Formula I of LixMyO2 (M, x and y are as defined in the specification) having mixed transition metal oxide layers (“MO layers”) comprising Ni ions and lithium ions, wherein lithium ions intercalate into and deintercalate from the MO layers and a portion of MO layer-derived Ni ions are inserted into intercalation / deintercalation layers of lithium ions (“reversible lithium layers”) thereby resulting in the interconnection between the MO layers. The lithium mixed transition metal oxide of the present invention has a stable layered structure and therefore exhibits improved stability of the crystal structure upon charge / discharge. In addition, a battery comprising such a cathode active material can exhibit a high capacity and a high cycle stability. Further, such a lithium mixed transition metal oxide is substantially free of water-soluble bases, and thereby can provide excellent storage stability, decreased gas evolution and consequently superior high-temperature stability with the feasibility of low-cost mass production.

Description

FIELD OF THE INVENTION [0001] The present invention relates to a Ni-based lithium mixed transition metal oxide and a cathode active material for a secondary battery comprising the same. More specifically, the Ni-based lithium mixed transition metal oxide according to the present invention has a given composition and exhibits intercalation / deintercalation of lithium ions into / from mixed transition metal oxide layers (“MO layers”) and interconnection of MO layers via the insertion of a portion of MO layer-derived Ni ions into intercalation / deintercalation layers (reversible lithium layers) of lithium ions, thereby improving the structural stability of the crystal structure upon charge / discharge to provide an excellent sintering stability. In addition, a battery comprising such a cathode active material can exert a high capacity and a high cycle stability. Further, with substantially no water-soluble bases present, such a lithium mixed transition metal oxide exhibits excellent storage ...

AI Technical Summary

Benefits of technology

[0014] 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 lithium mixed transition metal oxide, as will be illustrated hereinafter, having a given composition and a specific atomic-level structure, with which it is possible to realize superior thermal stability, and high cycle stability in conjunction with a high capacity, due to improvements in the stability of the crystal structure upon charge / discharge. Further, such a lithium mixed transition metal oxide can be prepared in a substantially water-soluble base-free form and therefore exhibits excellent storage stability, reduced gas evolution and thereby excellent high-temperature safety in conjunction with the feasibility of industrial-scale production at low production costs. 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 gelation of slurries by polymerization of an N-methylpyrrolidone / 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 LiNiMeO2 (Me=Co, Mn or Al) 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, it was found through various experiments conducted by the inventors of the present invention that the aforementioned oxides include large amounts of impurities such as lithium carbonates, and suffer from significant problems associated with severe gas evolution at high temperatures and structural instability.

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