Additive for cathode, method for preparing the same, cathode including the same, and lithium secondary battery including the same

Abstract

The present disclosure relates to a cathode additive of a lithium secondary battery, and a method for preparing the same. The cathode additive exhibits high irreversible capacity, and may be effectively applied to a battery using an anode material having high energy density. In one embodiment, the cathode additive includes a compound represented by the following Chemical Formula 1:
y(Li.sub.2Ni.sub.1-xM.sub.xO.sub.2)-z(Li.sub.6Co.sub.1-xM.sub.xO.sub.4)   [Chemical Formula 1]

Claims

1. A cathode additive for a lithium secondary battery, comprising: a compound represented by the following Chemical Formula 1:
y(Li.sub.2Ni.sub.1-xM.sub.xO.sub.2)-z(Li.sub.6Co.sub.1-xM.sub.xO.sub.4)   [Chemical Formula 1] in Chemical Formula 1, M is one or more elements selected from the group consisting of P, B, F, W, Ti and Zr, 0≤x<1.0, y and z are molar ratios of Li.sub.2Ni.sub.1-xM.sub.xO.sub.2 and Li.sub.6Co.sub.1-xM.sub.xO.sub.4 contained in the compound of Chemical Formula 1, and y:z is 2:1 to 30:1.

2. The cathode additive for a lithium secondary battery of claim 1, wherein the compound comprises primary particles of Li.sub.2Ni.sub.1-xM.sub.xO.sub.2 and primary particles of Li.sub.6Co.sub.1-xM.sub.xO.sub.4 physically mixed and connected to form a single particulate or a complex.

3. The cathode additive for a lithium secondary battery of claim 2, wherein the compound of Chemical Formula 1 in the form of a single particulate or a complex has an additional peak at 2θ of 23.5°±0.2° or 36.3°±0.2° in XRD (X-ray diffraction) analysis by Cu Kα X-ray (X-rα), and an intensity of the additional peak is 10% or less based on an intensity of the peak at 2θ of 25. 67°±0.2°.

4. The cathode additive for a lithium secondary battery of claim 1, wherein the compound of Chemical Formula 1 further comprises residual Li.sub.2O in an amount of about 1.5 wt % or less.

5. The cathode additive for a lithium secondary battery of claim 1, wherein the (Li.sub.2Ni.sub.1-xM.sub.xO.sub.2) is Li.sub.2NiO.sub.2 and the (Li.sub.6Co.sub.1-xM.sub.xO.sub.4) is Li.sub.6CoO.sub.4.

6. The cathode additive for a lithium secondary battery of claim 1, wherein y:z is 2.5:1 to 20:1.

7. The cathode additive for a lithium secondary battery of claim 1, wherein y:z is 3:1 to 10:1.

8. A method for preparing the cathode additive of a lithium secondary battery of claim 1, comprising: a first calcination step of calcining a mixture comprising a nickel precursor, an M-containing precursor, and a lithium precursor comprising Li.sub.2O to form a complex comprising Li.sub.2Ni.sub.1-xM.sub.xO.sub.2 and Li.sub.2O; and a second calcination step of further reacting the lithium precursor contained in the complex with a cobalt (Co) precursor to form a compound of Chemical Formula 1.

9. The method for preparing the cathode additive of claim 8, wherein the first calcination step is carried out under an inert atmosphere at a temperature of 500° C. to 800° C., and the second calcination step is carried out under an inert atmosphere at a temperature of 400° C. to 800° C.

10. The method for preparing the cathode additive of claim 8, wherein the nickel precursor comprises nickel oxide or nickel hydroxide, and the M-containing precursor, which includes the element M, comprises one or more selected from the group consisting of its oxide, hydroxide, oxyhydroxide, sulfate, nitrate, acetate, carbonate, oxalate, citrate, halide, phosphate and hydrate thereof.

11. A cathode mix comprising the cathode additive of claim 1; and a cathode active material.

12. The cathode mix of claim 11, wherein a weight ratio of the cathode additive: the cathode active material is 1:99 to 35:65.

13. The cathode mix of claim 11, wherein the cathode active material comprises one or more composite oxides of a metal selected from the group consisting of cobalt, manganese, nickel, and a combination thereof; and lithium.

14. A lithium secondary battery comprising: a cathode comprising the cathode mix of claim 11; an electrolyte; and an anode.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a diagram schematically showing a mechanism by which oxygen gas is generated from unreacted lithium oxide in a conventional irreversible cathode additive.

(2) FIG. 2 is a graph showing the XRD analysis result of the cathode. The cathode is separated from a battery obtained using the cathode additive prepared in Comparative Example 1 after charging the battery to a predetermined voltage.

(3) FIG. 3 is a graph showing the XRD analysis result of the cathode. The cathode is separated from a battery obtained using the cathode additive prepared in Example 2 after charging the battery to a predetermined voltage.

(4) FIG. 4 is a graph showing the XRD analysis result of the cathode additives obtained in Comparative Example 1 and Example 2.

(5) FIG. 5 is a graph showing the charge-discharge profile of the cathode additives obtained in Example 2 and Comparative Example 1.

(6) FIG. 6 is a graph showing the evaluation results of an amount of oxygen gas generated during storage after charging in Experimental Example 2 for the battery prepared by using the cathode additive obtained in Example 2 and Comparative Example 1 together with the cathode active material.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(7) Hereinafter, the function and effect of the present invention will be described in more detail through specific examples of the present invention. However, these examples are for illustrative purposes only, and the invention is not intended to be limited by these examples.

EXAMPLE 1

(8) 60 g of Li.sub.2O as a raw material for lithium, 150 g of NiO as a raw material of nickel and 6 g of ammonium phosphate as a raw material for element M were mixed, and then heat-treated and calcined at 685° C. for 18 hours under a nitrogen atmosphere.

(9) Then, 5.13 g of CoO was further added to the product, followed by heat-treating and calcining at 550° C. for 18 hours under a nitrogen atmosphere. The product was cooled down to obtain cathode additive particles.

EXAMPLE 2

(10) 60 g of Li.sub.2O as a raw material for lithium, 150 g of NiO as a raw material of nickel and 6 g of ammonium phosphate as a raw material for element M were mixed, and then heat-treated and calcined at 685° C. for 18 hours under a nitrogen atmosphere.

(11) Then, 15.4 g of CoO was further added to the product, followed by heat-treating and calcining at 550° C. for 18 hours under a nitrogen atmosphere. The product was cooled down to obtain cathode additive partic

EXAMPLE 3

(12) 60 g of Li.sub.2O as a raw material for lithium, 150 g of NiO as a raw material of nickel and 6 g of ammonium phosphate as a raw material for element M were mixed, and then heat-treated and calcined at 685° C. for 18 hours under a nitrogen atmosphere.

(13) Then, 45 g of CoO was further added to the product, followed by heat-treating and calcining at 550° C. for 18 hours under a nitrogen atmosphere. The product was cooled down to obtain cathode additive particles.

EXAMPLE 4

(14) 60 g of Li.sub.2O as a raw material for lithium, 150 g of NiO as a raw material of nickel and 0.5 g of boric acid as a raw material for element M were mixed, and then heat-treated and calcined at 685° C. for 18 hours under a nitrogen atmosphere.

(15) Then, 15.4 g of CoO was further added to the product, followed by heat-treating and calcining at 550° C. for 18 hours under a nitrogen atmosphere. The product was cooled down to obtain cathode additive particles.

COMPARATIVE EXAMPLE 1

(16) 60 g of Li.sub.2O as a raw material for lithium, 150 g of NiO as a raw material of nickel and 6 g of ammonium phosphate as a raw material for element M were mixed, and then heat-treated and calcined at 685° C. for 18 hours under a nitrogen atmosphere. The product was cooled down to obtain cathode additive particles.

EXPERIMENTAL EXAMPLE 1

Analysis of the Cathode Additive

(17) X-ray diffraction (XRD) analysis using Cu Kα X-ray (X-rα) was performed on the cathode additive particles prepared in Example 2 and Comparative Example 1, and the result is shown in FIG. 4. For this XRD analysis, a XRD analysis equipment from Bruker (product name: D4 Endeavor) was used.

(18) Referring to the upper graph of FIG. 4, it was confirmed that Example 2 had an additional peak at 2θ of 23.5°±0.2° or 36.3°±0.2°, and this additional peak had an intensity of 5%, based on an intensity of the peak at 2θ of 25.67°±0.2°. This result indicated that Li.sub.6Co.sub.1-xM.sub.xO.sub.4 was included in the cathode additive of Example 2, and the entire cathode additive had a single complex form.

(19) A cathode was prepared using the cathode additive particles prepared in Example 2 and Comparative Example 1, and then X-ray diffraction (XRD) analysis was performed at various voltages. The results are shown in FIG. 2 and FIG. 3, respectively.

(20) Specifically, the cathode additive prepared in Example 2 or Comparative Example 1, a carbon black conductive material and a PVdF binder were mixed in a weight ratio of 85:10:5 in N-methylpyrrolidone solvent to prepare a composition for forming a cathode. This composition was applied to an aluminum current collector, followed by drying and rolling. Li-metal was used as an anode, and a coin-cell type battery was manufactured using an electrolyte containing 1.0 M of LiPF.sub.6 in a solvent having a mixing volume ratio of EC:DMC:DEC to be 1:2:1.

(21) The prepared battery was charged to a predetermined voltage shown in FIG. 2 and FIG. 3 at 0.1C and a temperature of 25° C., and then the cathode was separated and subjected to XRD analysis.

(22) Referring to FIG. 2 and FIG. 3, it was confirmed that Comparative Example 1 had a clear peak of unreacted lithium oxide (Li.sub.2O), so that a relatively large amount of by-products were contained (see FIG. 2; greater than 1.5 wt %). On the other hand, in Example 2, the peak of unreacted lithium oxide (Li.sub.2O) was substantially absent and the content of by-products was minimized to 1.5 wt % or less, more specifically, 0.6 wt % or less (see FIG. 3).

(23) The composition of the cathode additives of the Examples and Comparative Example was quantitatively calculated from the XRD results, and the results are shown in Table 1 below.

(24) TABLE-US-00001 TABLE 1 The molar ratio of Type of M and Li.sub.2Ni.sub.1−xM.sub.xO.sub.2:Li.sub.6Co.sub.1−xM.sub.xO.sub.4 (y:z) the content (x) Example 1 3:1 P(x = 0.07) Example 2 9:1 P(x = 0.07) Example 3 27:1  P(x = 0.07) Example 4 9:1 B(x = 0.07) Comparative 1:0 P(x = 0.07) Example 1

(25) Referring to Table 1 above, it was confirmed that the cathode additives of Examples 1 to 4 satisfy the composition of Chemical Formula 1.

(26) Meanwhile, the cathode additive prepared in Example 2 or Comparative Example 1, an acetylene black conductive material and a PVdF binder were mixed in a weight ratio of 85:10:5 in N-methylpyrrolidone solvent to prepare a composition for forming a cathode of each exemplary additive. This composition was applied to an aluminum current collector, followed by drying and rolling. A Li-metal anode and a PE separator were used to manufacture a coin half-cell type battery.

(27) The battery was charged to 4.25 V, and then discharged to 2.5V to obtain a charge-discharge profile. This is shown in FIG. 5.

(28) Referring to FIG. 5, it was confirmed that the cathode additive of Example 2 exhibited higher irreversible capacity (greater than 400 mAh/g) than Comparative Example 1 (about 375 mAh/g).

EXPERIMENTAL EXAMPLE 3

Evaluation of Oxygen Gas Generation

(29) The cathode additive prepared in Example 2 or Comparative Example 1, a cathode active material of Li(Ni.sub.0.8Co.sub.0.1Mn.sub.0.1)O.sub.2, an acetylene black conductive material and a PVdF binder were mixed in a weight ratio of 5:80:10:5 in N-methylpyrrolidone solvent to prepare a composition for forming a cathode from each additive. These compositions were each applied to an aluminum current collector, followed by drying and rolling to prepare a cathode of each additive. A Li-metal anode and a PE separator were used to manufacture a coin half-cell type battery of each cathode.

(30) The battery was charged to 4.25 V and stored at 60° C. for 6 weeks. In this experiment, the amount of generated oxygen gas when using the additive of Example 2 and Comparative Example 1 was evaluated using a volumetric method with the principle of Archimedes. The results are shown in FIG. 6.

(31) Referring to FIG. 6, it was confirmed that the amount of oxygen gas generated during storage after charging was reduced in the case of using the cathode additive of Example 2, compared with the case of using the cathode additive of Comparative Example 1 (Example 2: about 4.2 cm.sup.3 after 6 weeks of storage, versus Comparative Example 1: about 6.0 cm.sup.3 after 6 weeks of storage). This is presumably because the amount of by-products such as lithium oxide (Li.sub.2O) was reduced in the cathode additive of Example 2, thereby reducing the amount of generated oxygen gas derived therefrom.