Method of preparing irreversible additive included in cathode material for lithium secondary battery, cathode material including irreversible additive prepared by the same, and lithium secondary battery including cathode material

Abstract

There are provided a method of preparing an irreversible additive in which a content of a Li-based by-product such as unreacted lithium oxide generated in a process of preparing lithium nickel-based oxide is decreased, which may significantly reduce gelation of a composition including the irreversible additive, a cathode material including the irreversible additive prepared by the same, and a lithium secondary battery including the cathode material.

Claims

1. A method of preparing an irreversible additive included in a cathode material for a lithium secondary battery, the method comprising: mixing a lithium precursor with NiO and heat-treating the mixture to prepare a lithium-excess transition metal oxide, wherein the lithium precursor comprises Li.sub.2+aNi.sub.1−bM.sub.bO.sub.2+c, NiO, and Li.sub.2O physically bonded to each other at a mole ratio of x:y:z; wherein the lithium-excess transition metal oxide comprises Li.sub.2+aNi.sub.1−bM.sub.bO.sub.2+c, NiO, and Li.sub.2O physically bonded to each other at a mole ratio of x′:y′:z′, wherein, −0.2≤a≤0.2, 0≤b≤1, and 0≤c≤0.2, and x≥0.8, x′≥0.93, 0<z≤0.21, and 0≤z′≤0.09, all of which based on x<x′, y>y′, and z>z′, and x+y=1 and x′+y′=1, and M is at least one element selected from the group consisting of Cu, Mg, Pt, Al, Co, P, and B.

2. The method of claim 1, wherein: 0.8≤x≤0.9 and 0.93≤x′≤1.0.

3. The method of claim 1, wherein: 0≤z′≤0.055.

4. The method of claim 1, wherein: the NiO is mixed in an amount of 0.1 times to 1 times the mole ratio(z) of Li.sub.2O present in the lithium precursor per mole of the lithium precursor.

5. The method of claim 1, wherein: the heat treatment is performed in an inert atmosphere at 500 to 800° C. for 12 hours to 24 hours.

6. The method of claim 5, wherein: the inert atmosphere is a N.sub.2 atmosphere or an Ar atmosphere.

7. The method of claim 5, wherein: the heat treatment is performed at 650 to 700° C. for 12 hours to 24 hours.

8. A cathode material, comprising: the irreversible additive prepared by the method of claim 1; and a cathode active material, wherein the irreversible additive includes the lithium-excess transition metal oxide in which Li.sub.2+aNi.sub.1−bM.sub.bO.sub.2+c, NiO, and Li.sub.2O are physically bonded to each other at a mole ratio of x′:y′:z′, −0.2≤a≤0.2, 0≤b<1, and 0≤c≤0.2, x′ is 0.93 or more, and z′ is 0 to 0.09 based on x′+y′=1, and M is at least one element selected from the group consisting of Cu, Mg, Pt, Al, Co, P, and B.

9. A lithium secondary battery having a structure in which an electrode assembly is embedded in a battery case together with an electrolyte, wherein the electrode assembly includes: a cathode having a cathode current collector on which the cathode material of claim 8 is coated; an anode having an anode current collector on which an anode material including an anode active material is coated; and a separator interposed between the cathode and the anode.

10. The lithium secondary battery of claim 9, wherein: the anode active material includes a Si-based material represented by Formula (1),
SiO.sub.x   (1) here, 0<x<2.

11. The method of claim 1, wherein: the mixing step includes a heat treatment of the excess lithium precursor performed in an inert atmosphere at 650 to 700° C. for 12 hours to 24 hours.

Description

DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a graph showing results of viscosity measurement according to Experimental Example 3.

MODE FOR INVENTION

(2) Hereinafter, examples of the present invention will be described in detail in such a manner that it may easily be implemented by those skilled in the art to which the present invention pertains. However, the present invention may be implemented in various different forms and is not limited to the embodiment described herein.

Comparative Example 1

(3) 50 g of Li.sub.2O, 136 g of NiO, and 5.5 g of (NH.sub.4).sub.2HPO.sub.4 as the raw material of the element M were mixed, heated in the N.sub.2 atmosphere at 685° C. for 18 hours, and then the resultant reactant was cooled, and the irreversible additive particles were obtained.

Comparative Example 2

(4) 50 g of Li.sub.2O, 136 g of NiO, and 5.5 g of (NH.sub.4).sub.2HPO.sub.4 as the raw material of the element M were mixed, heated in the N.sub.2 atmosphere at 685° C. for 18 hours, heated at 685° C. for 18 hours once again, and then the resultant reactant was cooled, and the irreversible additive particles were obtained.

Example 1

(5) 50 g of Li.sub.2O, 136 g of NiO, and 5.5 g of (NH.sub.4).sub.2HPO.sub.4 as the raw material of the element M were mixed and heated in the N.sub.2 atmosphere at 685° C. for 18 hours. Then, 7.6 g of NiO (0.5 times the mole ratio of Li.sub.2O present in the obtained reactant) was additionally mixed to the obtained reactant and heated at 685° C. for 18 hours, and the resultant was cooled, and the irreversible additive particles were obtained.

Example 2

(6) 50 g of Li.sub.2O, 136 g of NiO, and 5.5 g of (NH.sub.4).sub.2HPO.sub.4 as the raw material of the element M were mixed and heated in the N.sub.2 atmosphere at 685° C. for 18 hours. Then, 2.6 g of NiO (0.17 times the mole ratio of Li.sub.2O present in the obtained reactant) was additionally mixed to the obtained reactant and heated at 685° C. for 18 hours, and the resultant was cooled, and the irreversible additive particles were obtained.

Experimental Example 1

(7) Measurement of Content of Lithium By-Product by pH Titration

(8) A content of the lithium by-product was measured by performing pH titration on the irreversible additive particles prepared in Examples 1 and 2 and Comparative Examples 1 and 2.

(9) In detail, 10 g of each irreversible additive was added to 100 mL of distilled water, the mixtures were stirred for 5 minutes to melt the lithium by-product, and then the mixtures were titrated by acid-base titration using 0.1 M of HCl.

(10) A value was calculated by converting the melted lithium by-product into wt % based on the total weight of the obtained irreversible additive.

(11) The calculation is performed as below. The titration curve may be obtained by performing acid-base titration on the irreversible additives of the examples and the comparative examples. Such a curve has an inflection point. A content of Li.sub.2CO.sub.3 is calculated based on a value corresponding to two times (which is 2Y, this is because Li.sub.2CO.sub.3 needs a doubled amount of acid) the amount Y of acid corresponding to the right side of such an inflection and a content of LiOH is calculated based on a value corresponding to an amount of acid (X—Y) obtained by subtracting the amount Y of acid corresponding to the right side of the inflection from an amount X of acid corresponding to the left side of the inflection, and then the content of Li.sub.2CO.sub.3 and the content of LiOH are converted into wt % based on the total weight of the irreversible additive, respectively.

(12) As a result of the analysis, it was confirmed that the content of LiOH was measured as 2.5 wt % or less and the content of Li.sub.2CO.sub.3 was also measured very low as 0.35 wt % or less, based on the total weight of the irreversible additive in the irreversible additives of Examples 1 and 2.

(13) Meanwhile, in the case of Comparative Example 1 in which no secondary treatment with NiO was conducted, it was confirmed that the content of LiOH was 4 wt % or more and the content of Li.sub.2CO.sub.3 was 0.4 wt % or more.

(14) The lithium by-product was also decreased slightly in Comparative Example 2, likely due to the secondary heat treatment, but even with this second heat treatment, the absence of any additional NiO added for this second heat treatment resulted in an insignificant decrease, specifically, the content of LiOH is 3 wt % or more, and the content of Li.sub.2CO.sub.3 is 0.3 wt % or more. Therefore, a desired decrease effect is still not obtained.

Experimental Example 2

(15) XRD Analysis

(16) A series of cathode slurries were prepared by mixing each of the irreversible additive prepared in Examples 1 and 2 and Comparative Examples 1 and 2 with a carbon black conductive material and a PVdF binder in a weight ratio of 92.5:3.5:4 in an N-methylpyrrolidone solvent. Each prepared cathode slurry was then coated on an aluminum current collector, and then the current collector was dried and rolled to prepare a cathode.

(17) X-ray diffraction analysis (XRD) was performed on each prepared cathode using Cu—Kα X-ray, and an X-ray diffractometer (D4 Endeavor, manufactured by Bruker Corporation) was used for the XRD analysis. The analysis results are shown in Table 1.

(18) As described above, the mole ratios of Li.sub.2+aNi.sub.1−bM.sub.bO.sub.2+c and NiO are calculated from the cathode before the charging.

(19) The mole ratio is calculated as below. The conductive material and the binder are not under consideration because observed peaks thereof are too small when the XRD analysis of the cathode is performed. The mole ratios of the Li.sub.2+aNi.sub.1−bM.sub.bO.sub.2+c and the NiO are calculated by using values of intensity (the maximum point of a peak) and width (width of the middle of the peak) of the Li.sub.2+aNi.sub.1−bM.sub.bO.sub.2+c and the NiO, respectively, based on the Rietveld method.

(20) As a result of the analysis, it was confirmed that the content of the Li.sub.2+aNi.sub.1−bM.sub.bO.sub.2+c was increased up to 93% or more based on the total of the Li.sub.2+aNi.sub.1−bM.sub.bO.sub.2+c and the NiO in the irreversible additive prepared by the method of the present invention (Examples 1 and 2). Meanwhile, in the case of Comparative Example 1, the content of the Li.sub.2+aNi.sub.1−bM.sub.bO.sub.2+c was below 90% based on the total of the Li.sub.2+aNi.sub.1−bM.sub.bO.sub.2+c and the NiO, and in the case of Comparative Example 2, while the content of the Li.sub.2+aNi.sub.1−bM.sub.bO.sub.2+c was increased slightly likely due to the secondary heat treatment, once again, the absence of a second NiO addition meant that the content did not reach 93% based on the total of the Li.sub.2+aNi.sub.1−bM.sub.bO.sub.2+c and the NiO.

(21) Analysis of Content of Li.sub.2O

(22) The prepared cathodes as described above were prepared in the form of coin-cell and discharged up to 4.25 V with 0.1 C in a constant current (CC)/constant voltage (CV) mode, and then a peak density thereof was analyzed by the XRD analysis of the charged cathode to calculate the mole ratio of the Li.sub.2O. The calculation results are shown in Table 1.

(23) Here, the samples used in the XRD analysis are the cathode obtained by decomposing the cells after the charging, and were analyzed as below.

(24) Since a structure of Li.sub.2+aNi.sub.1−bM.sub.bO.sub.2+c and a position of the peak of Li.sub.2+aNi.sub.1−bM.sub.bO.sub.2+c are changed due to the charge, the number of moles of Li.sub.2O is estimated based on the content of the NiO calculated by the Rietveld method by comparing the intensity of the peak of the NiO before the charging to the intensity of the peak of the Li.sub.2O after the charging.

(25) The calculated mole number of the Li.sub.2O was converted based on 1 mole of the Li.sub.2+aNi.sub.1−bM.sub.bO.sub.2+c+NiO calculated above.

(26) For example, if the intensity of the peak of the NiO before the charging is 100 and a ratio of the NiO is 5 mol %, when the peak of the Li.sub.2O is 50, a ratio of the Li.sub.2O is estimated as 2.5 mol %.

(27) It could be confirmed that the content of the Li.sub.2O according to the examples of the present invention is 0.09 mole or less, but the content of the Li.sub.2O according to the comparative examples exceeds 0.14 by comparing the calculated values.

Experimental Example 3

(28) Viscosity Measurement

(29) The irreversible additive prepared in Examples 1 and 2 and Comparative Examples 1 and 2, LiNi.sub.0.8Mn.sub.0.1Co.sub.0.1O.sub.2 as a cathode active material, a carbon black conductive material, and a PVdF binder were mixed in a weight ratio of 4.6:87.9:3.5:4 in an N-methylpyrrolidone solvent, and then a cathode slurry for each additive was prepared.

(30) The DV2TLV viscometer (manufactured by Brookfield Engineering) was used for measuring viscosity of the cathode slurries, 300 ml of the slurries were prepared, and the viscosity of the slurries was measured by rotating blades of the viscometer at 1,200 rpm, and the measurement results are illustrated in FIG. 1. For reference, as a reference example, a slurry including no irreversible additive and including only 92.5 wt % of LiNi.sub.0.8Mn.sub.0.1Co.sub.0.1O.sub.2 used as the cathode active material was also measured.

(31) As a result of the analysis, it could be confirmed that the cathode slurry including the irreversible additive of Examples 1 and 2 has a viscosity characteristic similar with the cathode slurry including only LiNi.sub.0.8Mn.sub.0.1Co.sub.0.1O.sub.2, meanwhile, the cathode slurry including the irreversible additive of Comparative Examples 1 and 2 has a relatively high viscosity. Therefore, it can be expected that in the case where a content of the lithium by-product is still high as 4 wt % or more as in Comparative Example 1 and 2, improvement effect of gelation of the composition is small.

Experimental Example 4

(32) Manufacture of Lithium Secondary Battery

(33) The lithium secondary battery was manufactured by a method described below using each of the irreversible additive prepared in Examples 1 and 2 and Comparative Examples 1 and 2.

(34) In detail, the cathode slurries prepared in Experimental Example 3 were used.

(35) In addition, a composition for forming an anode was prepared by mixing mesocarbon microbead (MCMB) having 10 wt % of SiO as an anode active material, a carbon black conductive material, and a PVdF binder in a weight ratio of 90:5:5 in an N-methylpyrrolidone solvent, and the composition was applied on a copper current collector to prepare a cathode.

(36) An electrode assembly was prepared by placing a porous polyethylene separator between the cathode and the anode prepared as described above, the electrode assembly was placed inside a case, and then an electrolyte was injected into the case to manufacture a lithium secondary battery. Here, the electrolyte was prepared by dissolving 1.15 M concentration of lithium hexafluorophosphate (LiPF.sub.6) in an organic solvent containing ethylene carbonate (EC)/dimethyl carbonate (DMC)/ethylmethyl carbonate (EMC) (mixing volume ratio of EC/DMC/EMC=3/4/3).

(37) Measurement of Charge Capacity

(38) The manufactured secondary battery was charged up to 4.3 V with 0.1 C in a CC/CV mode (until 0.005 C cut-off) and charged up to 2.5 V with 0.1 C in a CC mode to measure charge capacity, and the measurement results are shown in Table 1.

(39) In the analyzing result, it could be confirmed that in the case where the cathode including the irreversible additive of Examples 1 and 2 is used, the charge capacity of the battery increases. That is because, in the case where the irreversible additive according to the present invention is used, a content of Li.sub.2+aNi.sub.1−bM.sub.bM.sub.bO.sub.2+c is relatively increased so that Li is more sufficiently supplied to the SiO-based active material which has a low initial efficiency and exhibits a large amount of initial irreversible capacity loss during initial charge and discharge, and gelation is prevented due to a low amount of lithium by-product, which may exhibit a high capacity of the cathode active material.

(40) TABLE-US-00001 TABLE 1 Mole of Li.sub.2O (based on 1 One-time pH titration mole of charge XRD analysis LiOH Li.sub.2CO.sub.3 Li.sub.2+aNi.sub.1−bM.sub.bO.sub.2+c + capacity Li.sub.2+aNi.sub.1−bM.sub.bO.sub.2+c NiO (wt %) (wt %) NiO) (mAh/g) Example 1 96.1%  3.9% 1.25 0.30 0.054 406 Example 2 94.2%  5.8% 2.2 0.33 0.087 402 Comparative 88.7% 11.3% 4.177 0.458 0.157 385 Example 1 Comparative 92.7%  7.3% 3.803 0.351 0.142 394.2 Example 2

INDUSTRIAL APPLICABILITY

(41) In the method of preparing an irreversible additive according to the present invention, NiO is additionally added and heat-treated, such that a fraction of the lithium nickel-based oxide of the irreversible additive is increased and a content of a Li-based by-product such as unreacted lithium oxide is decreased, thereby reducing viscosity increase or gelation of the composition. Accordingly, the lithium secondary battery prepared using the cathode material including the irreversible additive exhibiting excellent electrochemical characteristics.