Negative electrode for lithium secondary battery, lithium secondary battery comprising the same, and preparation method thereof

Abstract

A negative electrode for a lithium secondary battery, in which a LiF layer comprising amorphous LiF in an amount of 30 mol % or more is formed on a negative electrode active material layer comprising a carbon-based active material, a lithium secondary battery comprising the same, and a preparation method thereof.

Claims

1. A negative electrode for a lithium secondary battery, the negative electrode comprising: a negative electrode active material layer comprising a carbon-based active material; a LiF layer which is formed on the negative electrode active material layer and comprises amorphous LiF in an amount of 70 mol % to 95 mol % based on a total number of moles of LiF included in the LiF layer; and a metal oxide layer formed between the negative electrode active material layer and the LiF layer, and wherein the LiF layer has a thickness of 100 nm to 1,000 nm.

2. The negative electrode for a lithium secondary battery of claim 1, wherein the LiF layer further comprises at least one selected from the group consisting of Li.sub.2O, Li.sub.2CO.sub.3, and LiOH.

3. The negative electrode for a lithium secondary battery of claim 1, wherein the carbon-based active material comprises at least one selected from the group consisting of soft carbon, hard carbon, natural graphite, artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch based carbon fibers, meso-carbon microbeads, mesophase pitches, petroleum derived cokes, and coal tar pitch derived cokes.

4. The negative electrode for a lithium secondary battery of claim 1, wherein the metal oxide layer comprises at least one selected from the group consisting of titanium oxide, aluminum oxide, chromium trioxide, zinc oxide, copper oxide, magnesium oxide, zirconium dioxide, molybdenum trioxide, vanadium pentoxide, niobium pentoxide, iron oxide, manganese oxide, vanadium oxide, cobalt oxide, nickel oxide, and tantalum pentoxide.

5. The negative electrode for a lithium secondary battery of claim 1, further comprising a lithium compound layer formed on the LiF layer, wherein the lithium compound layer comprises at least one selected from the group consisting of Li.sub.2O, Li.sub.2CO.sub.3, and LiOH.

6. The negative electrode for a lithium secondary battery of claim 1, wherein the metal oxide layer has a thickness of 1 nm to 50 nm.

7. A preparation method of the negative electrode for a lithium secondary battery of claim 1, the method comprising steps of: (1) forming the negative electrode active material layer comprising the carbon-based active material on a negative electrode collector; (2) forming the metal oxide layer on the negative electrode active material layer; and (3) depositing the LiF layer on the metal oxide layer, and wherein the LiF layer has a thickness of 100 nm to 1,000 nm.

8. The method of claim 7, wherein the forming of the metal oxide layer is performed by drop coating, chemical vapor deposition, melting coating, electrodynamic coating, electrospraying, electrospinning, or dip coating.

9. A lithium secondary battery comprising the negative electrode for a lithium secondary battery of claim 1.

10. The negative electrode for a lithium secondary battery of claim 1, wherein the LiF layer is formed on the negative electrode active material layer by sputtering, E-Beam, physical vapor deposition (PVD) comprising evaporation or thermal evaporation, or chemical vapor deposition (CVD).

11. The negative electrode for a lithium secondary battery of claim 1, wherein the LiF layer is formed on the negative electrode active material layer by physical vapor deposition (PVD) in a temperature range of 700° C. to 900° C.

Description

EXAMPLES

(1) Hereinafter, the present invention will be described in detail, according to examples and experimental examples, but the present invention is not limited to these examples and experimental examples. The invention may be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these example embodiments are provided so that this description will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art.

Example 1

(2) A negative electrode active material slurry was prepared by adding 96 wt % of natural graphite, as a negative electrode active material, 1 wt % of Denka black (conductive agent), 2 wt % of SBR (binder), and 1 wt % of CMC (thickener) to water. After one surface of a copper current collector was coated with the prepared negative electrode active material slurry to a thickness of 65 μm, dried, and rolled, the coated copper current collector was punched into a predetermined size to prepare a negative electrode. LiF was deposited on the active material layer of the above-prepared negative electrode by physical vapor deposition (PVD). For the deposition, LiF powder was put in a thermal evaporator (Sunic System Co., Ltd.) and evaporated at 800° C. for 1 hour by thermal evaporation, and a final negative electrode was prepared by depositing a LiF layer to a thickness of 100 nm on a graphite electrode.

Example 2

(3) A negative electrode was prepared in the same manner as in Example 1 except that the deposition was performed at 800° C. for 6 minutes to form a LiF layer having a thickness of 10 nm.

Example 3

(4) A negative electrode active material slurry was prepared by adding 96 wt % of natural graphite, as a negative electrode active material, 1 wt % of Denka black (conductive agent), 2 wt % of SBR (binder), and 1 wt % of CMC (thickener) to water. After one surface of a copper current collector was coated with the prepared negative electrode active material slurry to a thickness of 65 μm, dried, and rolled, the coated copper current collector was punched into a predetermined size to prepare a negative electrode.

(5) An Al.sub.2O.sub.3 layer was formed to a thickness of 2 nm on the active material layer of the above-prepared negative electrode by applying a power of 100 W for 10 seconds using a sputter.

(6) LiF was deposited on the Al.sub.2O.sub.3 layer by physical vapor deposition (PVD). For the deposition, LiF powder was put in a thermal evaporator (Sunic System Co., Ltd.) and a final negative electrode was prepared by depositing a LiF layer to a thickness of 100 nm on a graphite electrode through thermal evaporation.

Example 4

(7) A negative electrode was prepared in the same manner as in Example 3 except that an Al.sub.2O.sub.3 layer was formed to a thickness of 5 nm by applying a power of 100 W for 30 seconds.

Example 5

(8) A negative electrode was prepared in the same manner as in Example 3 except that a LiF layer was deposited to a thickness of 10 nm.

Example 6

(9) A negative electrode active material slurry was prepared by adding 96 wt % of natural graphite, as a negative electrode active material, 1 wt % of Denka black (conductive agent), 2 wt % of SBR (binder), and 1 wt % of CMC (thickener) to water. After one surface of a copper current collector was coated with the prepared negative electrode active material slurry to a thickness of 65 μm, dried, and rolled, the coated copper current collector was punched into a predetermined size to prepare a negative electrode.

(10) Both LiF and Li.sub.2CO.sub.3 were deposited on the active material layer of the above-prepared negative electrode by physical vapor deposition (PVD). For the deposition, LiF powder and Li.sub.2CO.sub.3 powder, as raw materials, were put in a thermal evaporator (Sunic System Co., Ltd.) and evaporated at 800° C. for 1 hour and 10 minutes by thermal evaporation, and a final negative electrode was prepared by depositing a mixed layer of LiF and Li.sub.2CO.sub.3 to a thickness of 110 nm on a graphite electrode.

Example 7

(11) A final negative electrode was prepared by further forming a 10 nm thick Li.sub.2CO.sub.3 layer by depositing Li.sub.2CO.sub.3 on the LiF layer of the negative electrode, on which the LiF layer having a thickness of 100 nm and prepared in Example 1 was deposited, for 6 minutes by physical vapor deposition (PVD).

Comparative Example 1

(12) A negative electrode active material slurry was prepared by adding 96 wt % of natural graphite, as a negative electrode active material, 1 wt % of Denka black (conductive agent), 2 wt % of SBR (binder), and 1 wt % of CMC (thickener) to water. After one surface of a copper current collector was coated with the prepared negative electrode active material slurry to a thickness of 65 μm, dried, and rolled, the coated copper current collector was punched into a predetermined size to prepare a negative electrode.

Comparative Example 2

(13) A negative electrode was prepared in the same manner as in Example 2 except that a highly crystalline LiF layer having a thickness of 10 nm was formed by changing the deposition temperature of Example 2 to 1,000° C.

Examples 1-1 to 7-1 and Comparative Examples 1-1 and 2-1: Preparation of Lithium Secondary Battery

(14) A Li metal foil (150 μm) was used as a counter electrode, a polyolefin separator was disposed between each of the negative electrodes prepared in Examples 1 to 7 and Comparative Examples 1 and 2 and the Li metal, and a coin-type half cell was then prepared by injecting an electrolyte in which 1 M LiPF.sub.6 was dissolved in a solvent that was prepared by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 50:50.

Experimental Example 1—Measurement of Amount of Amorphous LiF

(15) An amount of amorphous LiF included in the LiF layer of the negative electrodes respectively prepared in Examples to 7 and Comparative Example 2 was measured by a transmission electron microscope (TEM), and the results thereof are presented in Table 1 below.

Experimental Example 2—Rapid Charging Test

(16) Rapid charging tests were performed on the batteries respectively prepared in Examples 1-1 to 7-1 and Comparative Examples 1-1 and 2-1 using an electrochemical charger/discharger. Before the rapid charging tests, an activation process was performed, and, specifically, the activation process was performed by performing 3 cycles of charging and discharging of each battery at a current density of 0.1 C-rate in a voltage range of 1.5 V to 0.005 V.

(17) The rapid charging tests were performed on the batteries completing the activation process, and, specifically, rapid charging was performed at a current density of 2.9 C-rate for 12 minutes, and a state of charge (SOC) before the occurrence of lithium precipitation was measured based on a voltage graph thus obtained. SOCs when the lithium precipitation occurred are presented in Table 1 below.

Experimental Example 3—Calorimetry Experiment

(18) An activation process was performed on the batteries respectively prepared in Examples 1-1 to 7-1 and Comparative Examples 1-1 and 2-1 by performing 3 cycles of charging and discharging of each battery at a current density of 0.1 C-rate in a voltage range of 1.5 V to 0.005 V.

(19) After powder was obtained by scraping off the negative electrode layer in the battery charged to 0.005 V, 0.1 mL of an electrolyte solution (1 M LiPF.sub.6 was dissolved in a solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed in a volume ratio of 50:50) was added to 13 mg of the powder, and the mixture thus obtained was then loaded into a differential scanning Calorimeter (DSC, Mettler Toledo) and heated at a rate of 10° C./min to measure a heat flow. The results thereof are presented in Table 1.

(20) TABLE-US-00001 TABLE 1 Faction of Lithium amorphous precipitation Main Calorific LiF SOC Onset peak value (mol %) (%) (° C.) (° C.) (J/g) Example 1-1 61 45 132 293 1,900 Example 2-1 57 43 106 275 2,900 Example 3-1 81 54 147 296 1,800 Example 4-1 87 57 152 298 1,700 Example 5-1 79 53 122 278 2,700 Example 6-1 63 47 139 297 1,830 Example 7-1 60 44 135 295 1,850 Comparative — 34 97 260 4,300 Example 1-1 Comparative 27 37 101 267 3,700 Example 2-1

(21) Referring to Table 1, the lithium secondary batteries of Examples 1-1 to 7-1 respectively comprising the negative electrodes of Examples 1 to 7 had a SOC when lithium precipitation occurred of 43% or more, wherein it may be confirmed that the SOCs when lithium precipitation occurred were larger than 34%, a SOC of the lithium secondary battery of Comparative Example 1-1 comprising the negative electrode of Comparative Example 1, and 37%, a SOC of the lithium secondary battery of Comparative Example 2-1 comprising the negative electrode of Comparative Example 2. That is, it may be understood that the lithium secondary batteries of Examples 1-1 to 7-1 may stably have a larger amount of charge before lithium precipitated during rapid charging than the lithium secondary batteries of Comparative Examples 1-1 and 2-1.

(22) Also, when the lithium secondary batteries of Comparative Examples 1-1 and 2-1 were compared, it may be confirmed that lithium secondary battery of Comparative Example 2-1 had a larger state of charge when lithium precipitation occurred.

(23) From the above results, with respect to the negative electrode in which LiF was deposited on the negative electrode active material layer comprising natural graphite, since a stable and thin SEI is allowed to be formed by the LiF layer during the activation process, it is considered that the rapid charging was improved.

(24) However, when the lithium secondary batteries of Example 2-1 and Comparative Example 2-1 were compared, all of the negative electrodes included in these lithium secondary batteries had the 10 nm thick LiF layer formed on the negative electrode active material layer, but the state of charges when lithium precipitation occurred of the lithium secondary batteries of Example 2-1 and Comparative Example 2-1 were 43% and 37%, respectively. With respect to the negative electrodes included in the lithium secondary batteries of Example 2-1 and Comparative Example 2-1, since there was a difference only in the amount of the amorphous LiF included in the LiF layer, it may be confirmed that, in a case in which the LiF layer formed on the negative electrode active material layer included a predetermined amount or more of the amorphous LiF, excellent rapid charging characteristics may be obtained.

(25) Furthermore, referring to the Calorimetry experiment of Table 1, the negative electrodes included in the lithium secondary batteries of Examples 1-1 to 7-1 had higher onset temperatures and higher main peak temperatures than the negative electrodes included in the lithium secondary batteries of Comparative Examples 1-1 and 2-1. From these results, it may be considered that the lithium secondary batteries of Examples 1-1 to 7-1 may be more safely maintained at high temperature. Also, the negative electrodes included in the lithium secondary batteries of Examples 1-1 to 7-1 had lower calorific values than the negative electrodes included in the lithium secondary batteries of Comparative Examples 1-1 and 2-1, wherein, from these results, it may be predicted that the lithium secondary batteries of Examples 1-1 to 7-1 may be safer when exposed to high temperature than the lithium secondary batteries of Comparative Examples 1-1 and 2-1.