Positive Electrode Additive, Manufacturing Method Thereof, and Positive Electrode and Lithium Rechargeable Battery Including the Same

Abstract

The present disclosure relates to a positive electrode additive, a manufacturing method thereof, and a positive electrode and a lithium rechargeable battery including the same. Specifically, one embodiment of the present disclosure provides a positive electrode additive for a lithium rechargeable battery comprising: a compound represented by the following Chemical Formula 1; a compound represented by the following Chemical Formula 2; and lithium phosphate (Li.sub.3PO.sub.4):

Li.sub.2+aNi.sub.bM.sub.1−bO.sub.2+c   [Chemical Formula 1] wherein, M is a metal element forming a divalent cation, −0.2≤a≤0.2, 0.5≤b≤1.0, and −0.2≤c≤0.2,

Ni.sub.2−eM.sub.1−eP.sub.4O.sub.12   [Chemical Formula 2] wherein, 0.5≤e≤1.0, and M is the same as defined in Chemical Formula 1.

Claims

1. A positive electrode additive for a lithium rechargeable battery comprising: a compound represented by the following Chemical Formula 1; a compound represented by the following Chemical Formula 2; and lithium phosphate (Li.sub.3PO.sub.4)
Li.sub.2+aNi.sub.bM.sub.1−bO.sub.2+c   [Chemical Formula 1] wherein, M is a metal element forming a divalent cation, −0.2≤a≤0.2, 0.5≤b≤1.0, and −0.2≤c≤0.2,
Ni.sub.2−eM.sub.1−eP.sub.4O.sub.12   [Chemical Formula 2] wherein, 0.5≤e≤1.0, and M is the same as defined in Chemical Formula 1.

2. The positive electrode additive for a lithium rechargeable battery according to claim 1, wherein the compound represented by Chemical Formula 2 is dissolved in the electrolyte containing a lithium salt and an organic solvent.

3. The positive electrode additive for a lithium rechargeable battery according to claim 1, wherein in the positive electrode additive, the compound represented by Chemical Formula 1 forms a secondary particle, and the lithium phosphate particle is attached to the surface of the secondary particle.

4. The positive electrode additive for a lithium rechargeable battery according to claim 1, wherein the positive electrode additive further comprises Li.sub.2O and NiO.

5. A method for manufacturing a positive electrode additive for a lithium rechargeable battery according to claim 1 comprising the steps of: preparing a raw material mixture containing a lithium raw material, a nickel raw material, and a phosphorus raw material; and heat-treating the raw material mixture at a temperature in the range of 600 to 900° C. in a reactor to an inert gas is supplied at a flow rate of 1.5 to 2.5 L/min,

6. The method for manufacturing a positive electrode additive for a lithium rechargeable battery according to claim 5, wherein the inert gas includes nitrogen (N.sub.2) gas.

7. The method for manufacturing a positive electrode additive for a lithium rechargeable battery according to claim 5, wherein in the raw material mixture, a molar ratio of lithium (Li):nickel (Ni) constituting the lithium raw material and the nickel raw is 3:1 to 3:2.

8. The method for manufacturing a positive electrode additive for a lithium rechargeable battery according to claim 5, wherein the phosphorus raw material is contained in an amount of 1 to 10% by weight based on the total amount (100% by weight) of the raw material mixture.

9. The method for manufacturing a positive electrode additive for a lithium rechargeable battery according to claim 5, wherein the phosphorus raw material includes secondary ammonium phosphate ((NH.sub.4).sub.2HPO.sub.4), primary ammonium phosphate (NH.sub.4H.sub.2PO.sub.4), or a mixture thereof.

10. The method for manufacturing a positive electrode additive for a lithium rechargeable battery according to claim 5, wherein the step of heat-treating the raw material mixture at a temperature in the range of 600 to 900° C. in a reactor to which an inert gas is supplied, comprises the following steps: a) reacting the lithium raw material and the nickel raw material to produce a compound represented by the following Chemical Formula 1; b) reacting the nickel raw material that has not reacted in step a) with the phosphorous raw material to produce a compound represented by the following Chemical Formula 2; c) reacting the lithium raw material that has not reacted in step a) with the phosphorus raw material that has not reacted in step b) to produce lithium phosphate (Li.sub.3PO.sub.4); and d) obtaining a positive electrode additive including the compound represented by Chemical Formula 1 produced in step a), the compound represented by Chemical Formula 2 produced in step b), and the lithium phosphate (Li.sub.3PO.sub.4) produced in step c).
Li.sub.2+aNi.sub.bM.sub.1−bO.sub.2+c   [Chemical Formula 1] wherein, M is a metal element forming a divalent cation, −0.2≤a≤0.2, 0.5≤b≤1.0 and −0.2≤c≤0.2,
Ni.sub.2−eM.sub.1−eP.sub.4O.sub.12   [Chemical Formula 2] wherein, 0.5≤e≤1.0, and M is the same as defined in Chemical Formula 1.

11. The method for manufacturing a positive electrode additive for a lithium rechargeable battery according to claim 10, wherein the positive electrode additive obtained in step d) also includes a lithium raw material, a nickel raw material, or a mixture thereof which are not reacted in a) to c).

12. A lithium rechargeable battery comprising: a positive electrode containing the positive electrode additive of claim 1; an electrolyte containing a lithium salt and an organic solvent; and a negative electrode.

13. The lithium rechargeable battery according to claim 12, wherein during charging formation of the lithium rechargeable battery, the compound represented by Chemical Formula 2 is eluted from the positive electrode additive in the positive elective mixture, and then reduced to Ni metal on the surface of the negative electrode.

14. The lithium rechargeable battery according to claim 12, wherein 200 to 4000 ppm of Ni metal is detected from the surface of the negative electrode separated after the lithium rechargeable battery is formed and charged up to 4.2 V.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0110] FIG. 1 is an XRD of Ni.sub.2P.sub.4O.sub.12 separately synthesized in Experimental Example 1.

[0111] FIG. 2a is an FE-SEM image of the positive electrode active material of Example 1.

[0112] FIG. 2b is an EDS image of a specific portion in FIG. 2a.

[0113] FIG. 3a is a SEM image of a negative electrode separated after the lithium rechargeable battery of Example 1 was charged once.

[0114] FIGS. 3b and 3c are EDS images of specific parts in FIG. 3a.

[0115] FIG. 4 shows the results of evaluation of the capacity (life) maintenance rate according to charge and discharge of each lithium rechargeable battery of Examples 1 and 3 and Comparative Examples 1 and 3.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0116] Hereinafter, the present disclosure will be described in more detail with reference to examples, but these examples are provided for illustrative purposes only, and should not be construed as limiting the scope and spirit of the present disclosure.\

Example 1

[0117] (1) Manufacture of Positive Electrode Additive

[0118] Li.sub.2O, NiO, and (NH.sub.4).sub.2HPO.sub.4 were mixed to prepare a raw material mixture.

[0119] However, the Li.sub.2O and the NiO were mixed at a molar ratio of 1:1 such that the molar ratio of Li:Ni in the raw material mixture was 2:1, and the (NH.sub.4).sub.2HPO.sub.4 was set to 5 wt % based on the total amount (100 wt %) of the raw material mixture.

[0120] The raw material mixture was heat-treated at 680° C. for 10 hours under the conditions where nitrogen gas, which is a kind of inert gas, was introduced at 2 L/min, to obtain a positive electrode additive of Example 1.

[0121] (2) Manufacture of Positive Electrode and Lithium Rechargeable Battery (Coin Full-Cell)

[0122] The positive electrode additive of Example 1, a conductive material (Super-P, Denka Black) and a binder (PVdF) were mixed at a weight ratio of 97:2:1 (positive electrode additive: conductive material: binder) in an organic solvent to manufacture a positive electrode mixture in a slurry form. Then, the positive electrode mixture was coated onto an aluminum current collector and dried in a vacuum oven at 120° C. for 30 minutes to manufacture a positive electrode.

[0123] As a counter electrode, graphite, a conductive material (super P) and a binder (CMC) were mixed at a weight ratio of 95:1:4, and water was used as a solvent to manufacture a negative electrode mixture slurry. The negative electrode mixture slurry was coated onto a copper current collector and dried in a vacuum oven at 50° C. for 20 minutes to manufacture a negative electrode.

[0124] Using the respective components, a 2032 coin full-cell was manufactured according to a conventional manufacturing method.

Example 2

[0125] (1) Manufacture of Positive Electrode Additive

[0126] The positive electrode additive of Example 2 was obtained in the same manner as in Example 1, except that the heat treatment temperature was changed from 680° C. to 730° C.

[0127] (2) Manufacture of Positive Electrode and Lithium Rechargeable Battery (Coin Full-Cell)

[0128] The positive electrode and the lithium rechargeable battery of Example 2 were prepared in the same manner as in Example 1, except that the positive electrode additive of Example 2 was used instead of the positive electrode additive of Example 1.

Example 3

[0129] (1) Manufacture of Positive Electrode Additive

[0130] The positive electrode additive of Example 3 was obtained in the same manner as in Example 1, except that the heat treatment temperature was changed from 680° C. to 850° C.

[0131] (2) Manufacture of Positive Electrode and Lithium Rechargeable Battery (Coin Full-Cell)

[0132] The positive electrode and the lithium rechargeable battery of Example 3 were manufactured in the same manner as in Example 1, except that the positive electrode additive of Example 3 was used instead of the positive electrode additive of Example 1.

Example 4

[0133] (1) Manufacture of Positive Electrode Additive

[0134] The positive electrode additive was manufactured in the same manner as in Example 1

[0135] (2) Manufacture of Positive Electrode and Lithium Rechargeable Battery (Coin Full-Cell)

[0136] A positive electrode active material (LiNi.sub.0.8Co.sub.0.1Mn.sub.0.1O.sub.2), the positive electrode additive, a conductive material (Super-P, Denka Black) and a binder (PVdF) were mixed at a weight ratio of 88:9:2:1 (positive electrode active material: positive electrode additive: conductive material: binder) in an organic solvent (NMP) to manufacture a positive electrode mixture in a slurry form. Then, the positive electrode mixture was coated onto an aluminum current collector and dried in a vacuum oven at 120° C. for 30 minutes to manufacture a positive electrode.

[0137] As a counter electrode, graphite, a conductive material (super P) and a binder (CMC) were mixed at a weight ratio of 95:1:4, and water was used as a solvent to manufacture a negative electrode mixture slurry. The negative electrode mixture slurry was coated onto a copper current collector and dried in a vacuum oven at 50° C. for 20 minutes to manufacture a negative electrode.

Comparative Example 1

[0138] (1) Manufacture of Positive Electrode Additive

[0139] The positive electrode additive of Comparative Example 1 was manufactured in the same manner as in Example 1, except that the N.sub.2 flow was changed from 2 L/min to 0.5 L/min.

[0140] (2) Manufacture of Positive Electrode and Lithium Rechargeable Battery (Coin Full-Cell)

[0141] The positive electrode and the lithium rechargeable battery of Comparative Example 1 were manufactured in the same manner as in Example 1, except that the positive electrode additive of Comparative Example 1 was used instead of the positive electrode additive of Example 1.

Comparative Example 2

[0142] (1) Manufacture of Positive Electrode Additive

[0143] The positive electrode additive of Comparative Example 2 was obtained in the same manner as in Example 1, except that the heat treatment temperature was changed from 680° C. to 920° C.

[0144] (2) Manufacture of Positive Electrode and Lithium Rechargeable Battery (Coin Full-Cell)

[0145] The positive electrode and the lithium rechargeable battery of Comparative Example 2 were manufactured in the same manner as in Example 1, except that the positive electrode additive of Comparative Example 2 was used instead of the positive electrode additive of Example 1.

Comparative Example 3

[0146] (1) Manufacture of Positive Electrode Additive

[0147] Li.sub.2O and NiO were mixed at a molar ratio of 1:1, and then the mixture was heat-treated at 680° C. for 10 hours under the conditions where nitrogen gas, which is a kind of inert gas, was introduced at 2 L/min, thereby producing lithium nickel oxide.

[0148] After recovering the lithium nickel oxide, it was mixed with NH.sub.4H.sub.2PO.sub.4. Here, NH.sub.4H.sub.2PO.sub.4 was set to 5 wt % based on the total weight (100 wt %) of the mixture of the lithium nickel oxide and NH.sub.4H.sub.2PO.sub.4. In addition, the mixing conditions used at this time were the same as those used when mixing the Li.sub.2O and NiO.

[0149] The mixture of the lithium nickel oxide and NH.sub.4H.sub.2PO.sub.4 was heat-treated at 730° C. for 10 hours under the condition where a nitrogen gas as an inert gas were introduced at 2 L/min. Thereby, the positive electrode additive of Comparative Example 3 was obtained.

[0150] (2) Preparation of Positive Electrode and Lithium Rechargeable Battery (Coin Full-Cell)

[0151] The positive electrode and the lithium rechargeable battery of Comparative Example 3 were manufactured in the same manner as in Example 1, except that the positive electrode additive of Comparative Example 3 was used instead of the positive electrode additive of Example 1.

Experimental Example 1: Qualitative and Quantitative Analysis of the Positive Electrode Additive by XRD (X-Ray Diffraction)

[0152] XRD analysis of each of the positive electrode additives of Examples 1 to 3 and Comparative Examples 1 to 3 was performed by an X-ray diffraction analyzer (Bruker AXS D4-Endeavor XRD) using Cu Kα X-rays (X-rα).

[0153] The results of X-ray diffraction analysis of each of the positive electrode additives were qualitatively analyzed using Bruker's Evaluation program to confirm the corresponding peaks for Li.sub.2NiO.sub.2, Li.sub.3PO.sub.4, Li.sub.2O and NiO. For these corresponding peaks, quantitative analysis between two phases was performed by Rietveld refinement using TOPAS program (Bruker-AXS, TOPAS4, Karlsruhe, Germany). Rietveld refinement is a feedback process that repetitively adjusts the variables capable of using until the pattern calculated from each structural model of Li.sub.2NiO.sub.2, Li.sub.3PO.sub.4, Li.sub.2O and NiO matches best. In this process, not only the position of the diffraction peak, but also the intensity and intensity ratio of the peak were analyzed and the contents of the two phases were quantitatively analyzed (see, Rietveld, HML “Line Profiles of Neutron Powder-diffraction Peaks for Structure Refinement” Axta. Cryst., 22, 151-2, 1967 and Bish D L & Howard C J, “Quantitative phase analysis using the Rietveld method” J. Appl. Cryst., 21, 86-81, 1988).

[0154] The results of such XRD analysis were shown in Table 1 below. From this, it could be confirmed that Examples 1 to 3 and Comparative Examples 1 to 3 contained Li.sub.2NiO.sub.2, Li.sub.3PO.sub.4, Li.sub.2O and NiO.

[0155] However, referring to Table 1 below, it can be seen that collective mixing of the raw materials (the number of heat treatments), the heat treatment temperature, the supply state of the inert gas during the heat treatment, and the supply flow rate thereof have an influence on the mixing ratio of each component.

TABLE-US-00001 TABLE 1 Content of respective components based on the total content (100 wt %) of Li.sub.2NiO.sub.2, NiO, Li.sub.3PO.sub.4, and Li.sub.2O (unit: wt %) Li.sub.2NiO.sub.2, NiO Li.sub.3PO.sub.4 Li.sub.2O Example 1 81.8 13 3.1 2.1 Example 2 85.2 10.7 2.9 1.2 Example 3 84.9 10.1 2.8 2.2 Comparative 86.4 10.8 2.8 0 Example 1 Comparative 64.5 21.5 2.5 2.5 Example 2 Comparative 76.7 13.4 3.1 6.9 Example 3

[0156] According to Table 1, when the flow rate of the nitrogen gas supplied in the heat treatment process was less than 1.5 L/min (Comparative Example 1), Li.sub.2O was not detected at all.

[0157] In addition, when the heat treatment temperature exceeded 900° C. (Comparative Example 2), and when the mixture of Li.sub.2O and NiO was heat-treated to produce Li.sub.2NiO.sub.2, which was then separately mixed with (NH.sub.4).sub.2HPO.sub.4 and heat-treated (Comparative Example 3), the detection amount of Li.sub.2NiO.sub.2 was significantly reduced as compared with the Examples.

[0158] When the heat treatment temperature exceeds 900° C. in this way, or when the heat treatment is performed twice, Li.sub.2NiO.sub.2 may be decomposed and then re-synthesized, and the rate of return to Li.sub.2O without being disappeared or re-synthesized can be increased. Meanwhile, Ni.sub.2P.sub.4O.sub.12 synthesized in Examples 1 to 3 is a trace amount, and thus, it was confirmed whether Ni.sub.2P.sub.4O.sub.12 was synthesized by separately mixing only NiO and (NH.sub.4).sub.2HPO.sub.4 at a molar ratio of Ni:P=1:2 and heat-treating the mixture, rather than confirming the content in the final material.

[0159] The resulting material was subjected to XRD analysis by the same method as before, and the results are shown in FIG. 1.

[0160] According to FIG. 1, XRD peaks of the material made of the NiO and (NH.sub.4).sub.2HPO.sub.4 raw materials are observed at the same position as Ni.sub.2P.sub.4O.sub.12. Thereby, even in the process of heat-treating the raw material mixture containing Li.sub.2O, NiO, and (NH.sub.4).sub.2HPO.sub.4, (unreacted) NiO not participating in the production of Li.sub.2NiO.sub.2 can react with (NH.sub.4).sub.2HPO.sub.4. As a result, it is inferred that Ni.sub.2P.sub.4O.sub.12 can be produced.

[0161] Further, since Ni.sub.2P.sub.4O.sub.12 is a highly reactive compound due to an unstable structure, it was expected that the entire amount could be dissolved in the electrolyte and reduced to Ni on the surface of the negative electrode.

Experimental Example 2: Confirmation of the Presence of Li.SUB.3.PO.SUB.4 .in the Positive Electrode Additive by SEM (Scanning Electron Microscope) and EDS (Energy Dispersive X-ray Spectroscopy)

[0162] The surface of the positive electrode additive of Example 1 was photographed using a scanning electron microscope (FE-SEM) (JSM-7610F Schottky Field Emission Scanning Election Microscope), and the photographed image is shown in FIG. 2a.

[0163] In FIG. 2, the first phase secondary particle and the second phase particle (the part indicated by a circle in FIG. 2) attached to the surface were observed.

[0164] Among the components of the positive electrode additive analyzed in Table 1, the component present in the largest amount is Li.sub.2NiO.sub.2 and thus, it is determined that the first phase secondary particle is Li.sub.2NiO.sub.2.

[0165] In order grasp the particle composition of the second phase, energy dispersive spectroscopy (EDS) (JSM-7610F Schottky Field Emission Scanning Election Microscope) was performed. As a result, element P was recognized at a position corresponding to the part shown in FIG. 2a. From this, it can be seen that the coating layer contains P.

[0166] it can be seen from Table 1 that among the components of the positive electrode additive analyzed, Li.sub.3PO.sub.4 is the only compound containing P and thus, the second phase particle is Li.sub.3PO.sub.4.

[0167] Of course, in the Examples, Ni.sub.2P.sub.4O.sub.12 can also be synthesized, but it is regarded as Ni.sub.2P.sub.4O.sub.12 since the amount is very small

Experimental Example 3: Evaluation of Negative Electrode Surface Characteristics after Charge/Discharge of Lithium Rechargeable Battery

[0168] The lithium rechargeable battery of Example 1 was charged once at 25° C. under the following conditions, and then the negative electrode was separated. SEM (Scanning Electron Microscope) and EDS (Energy Dispersive X-ray Spectroscopy) analysis of the negative electrode surface was performed.

[0169] Charge: 0.2 C, CC/CV, 4.2V, 0.05 C cut-off

[0170] According to the SEM image of FIG. 3a, the film was observed on the surface of the negative electrode separated from the lithium rechargeable battery of Example 1 after charging once. As a result of EDS analysis of the corresponding portion, it is confirmed that the main component of the film is Ni (FIG. 3c). This is inferred to be because Ni.sub.2P.sub.4O.sub.12 contained in the positive electrode additive of Example 1 is dissolved in the electrolyte, and then reduced on the surface of the negative electrode to form a Ni metal layer.

[0171] In addition, inductively coupled plasma (ICP) analysis was performed for Examples 1 to 4 and Comparative Examples 1 to 6, and the results are shown in Table 2 below.

TABLE-US-00002 TABLE 2 Result of ICP analysis: Detection amount of Ni (unit: ppm) Example 1 3378 Example 2 3780 Example 3 3570 Example 4 165 Comparative Example 1 15 Comparative Example 2 5 Comparative Example 3 10

[0172] As shown in Table 2, it is confirmed that the amount of Ni reduction in Examples 1 to 3 is significantly excessive as compared with Comparative Examples 1 to 3.

[0173] In the case of Examples 1 to 3, (unreacted) NiO not participating in the formation reaction of Li.sub.2NiO.sub.2 may react with the (NH.sub.4).sub.2HPO.sub.4, and as a result, it is inferred that Ni.sub.2P.sub.4O.sub.12 can be produced. Further, it is inferred that since Ni.sub.2P.sub.4O.sub.12 is a highly reactive compound due to its unstable structure, the whole amount may be dissolved in the electrolyte and reduced to Ni on the surface of the negative electrode.

[0174] On the other hand, the reaction temperature itself of Comparative Example 1 is included in the range of Examples 1 to 3, but as the supply flow rate of the inert gas was lowered to less than 1.5 L/min, Ni.sub.2P.sub.4O.sub.12 was not produced, and such an additive not containing Ni.sub.2P.sub.4O.sub.12 would not been able to form a Ni metal layer on the surface of the negative electrode.

[0175] Further, when the heat treatment temperature exceeds 900° C. (Comparative Example 2), and when a mixture of Li.sub.2O and NiO was heat-treated to produce Li.sub.2NiO.sub.2 which was separately mixed with (NH.sub.4).sub.2HPO.sub.4 and heat-treated (Comparative Example 3), structurally unstable Li.sub.2NiO.sub.2 was decomposed under the heat treatment conditions and then re-synthesized, and Ni.sub.2P.sub.4O.sub.12 would not have been produced in that process.

[0176] However, Ni detected in Comparative Examples 1 to 3 is inferred from the result that a very small amount of NiO is eluted from the positive electrode additive.

Experimental Example 4: Evaluation of Capacity Maintenance Rate according to Charging and Discharging of Lithium Rechargeable Battery

[0177] For each lithium rechargeable batteries of Examples 1 and 3 and Comparative Examples 1 and 3, 50 cycle charge/discharge were performed at 25° C. under the following conditions. After the completion of the 50th cycle compared to the initial (1st cycle) discharge capacity of each battery, the discharge capacity was evaluated, and the results are shown in FIG. 4.

[0178] Charge: 0.2 C, CC/CV, 4.25V, 0.05 C cut-off

[0179] Discharge: 0.2 C , CC, 2.5 V, cut-off

[0180] From FIG. 4, it can be seen that the Ni metal layers formed on the surfaces of the negative electrodes of Examples 1 and 3 do not inhibit the electrochemical properties of the battery, but allow the capacity of Comparative Examples 1 and 3 to maintain at the same cycle.

INDUSTRIAL APPLICABILITY

[0181] The lithium ion battery to which the positive electrode additive of the one embodiment is applied suppresses gas generation and can be stably driven while effective offsetting initial irreversible capacities of the positive electrode and the negative electrode.