Negative Electrode for Lithium Secondary Battery and Lithium Secondary Battery Including the Same

Abstract

Provided are a negative electrode for a lithium secondary negative electrode battery including: a current collector; a first negative electrode active material layer disposed on the current collector and including a silicon-based active material, a first graphite-based active material, and a linear conductive material; and a second negative electrode active material layer disposed on the first negative electrode active material layer and including a second graphite-based active material. The first graphite-based active material has a carbon coating layer on at least a part of a surface. Also provided is a lithium secondary battery including the negative electrode.

Claims

1. A negative electrode for a lithium secondary battery comprising: a current collector; a first negative electrode active material layer disposed on the current collector and including a silicon-based active material, a first graphite-based active material, and a linear conductive material; and a second negative electrode active material layer disposed on the first negative electrode active material layer and including a second graphite-based active material, wherein the first graphite-based active material has a carbon coating layer on at least a part of a surface.

2. The negative electrode for a lithium secondary battery of claim 1, wherein the first graphite-based active material is artificial graphite or a mixture of artificial graphite and natural graphite.

3. The negative electrode for a lithium secondary battery of claim 1, wherein the silicon-based active material and the first graphite-based active material are included at a weight ratio of 1:9 to 4:6.

4. The negative electrode for a lithium secondary battery of claim 1, wherein the carbon coating layer included on the first graphite-based active material is formed from hard carbon, soft carbon, heavy oil, or pitch.

5. The negative electrode for a lithium secondary battery of claim 1, wherein the linear conductive material is carbon nanotubes (CNT) and is included at 0.1 to 1 wt % with respect to a total weight of the first negative electrode active material layer.

6. The negative electrode for a lithium secondary battery of claim 1, wherein the first negative electrode active material layer satisfies the following Relation 1:
0.2<A.sub.1/A.sub.2(%)<1.7   [Relation 1] wherein A.sub.1 is a content (part by weight) of the linear conductive material, A.sub.2 is a content (part by weight) of the silicon-based active material, and A.sub.1/A.sub.2 is a percentage (%) of the content of the linear conductive material to the content of the silicon-based active material.

7. The negative electrode for a lithium secondary battery of claim 1, wherein the second negative electrode active material layer does not include a silicon-based active material, and the second graphite-based active material is the artificial graphite and does not have a carbon coating layer.

8. The negative electrode for a lithium secondary battery of claim 1, wherein the second negative electrode active material does not include the conductive material.

9. A lithium secondary battery comprising: the negative electrode of claim 1; a positive electrode; a separator; and an electrolyte.

Description

EXAMPLES

Example 1

[0060] <Production of Carbon-Coated First Graphite-Based Active Material>

[0061] 100 parts by mass of the artificial graphite (D50:20 μm) and 9.4 parts by mass of pitch derived from coal were added to a kneader having sigma or z-type stirring blades, preheated to 128° C. and were mixed for 20 minutes. The thus-obtained slurry type mixture was heated in a batch heating furnace at 350° C. for 1 hour under a nitrogen/oxygen mixed atmosphere, and was heat-treated for 1 hour while maintaining the temperature at 900° C. After radiational cooling under an inert atmosphere, the obtained powder was pulverized to obtain artificial graphite particles coated with non-crystalline carbon (first graphite-based active material).

[0062] <Production of Negative Electrode>

[0063] A negative electrode active material in which the produced non-crystalline carbon-coated first graphite-based active material and silicon oxide (SiO.sub.x, 0<x<2, D50:5 μm) were mixed at a weight ratio of 66.5:33.5, a CNT conductive material, and a binder (weight ratio of CMC/SBR=1.2/1.5) were mixed at a weight ratio of 97.1:0.2:2.7 and water was added to produce a first negative electrode slurry.

[0064] Artificial graphite having a bimodal particle diameter distribution (D50:20 μm) and the binder (weight ratio of CMC/SBR=1.2/1.5) were mixed at a weight ratio of 97.3:2.7 and water was added to produce a second negative electrode slurry.

[0065] On one surface of a copper current collector (copper foil having a thickness of 8 μm), the first negative electrode slurry and the second negative electrode slurry produced above were coated and dried to form a first negative electrode active material layer and a second negative electrode active material layer. Here, each loading was 4 mg/cm.sup.2 and 6.2 mg/cm.sup.2. This was rolled to have an electrode density of 1.77 g/cc to produce a negative electrode.

[0066] <Production of Positive Electrode>

[0067] Li[Ni.sub.0.88Co.sub.0.1Mn.sub.0.02]O.sub.2 as a positive electrode active material, carbon black as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were mixed at a weight ratio of 96.5:2:1.5 to produce a slurry. The slurry was uniformly coated on an aluminum foil having a thickness of 12 μm and dried under vacuum to produce a positive electrode for a secondary battery.

[0068] <Production of Half Battery>

[0069] The produced negative electrode and a lithium metal were used as a counter electrode, a PE separator was interposed between the negative electrode and the counter electrode, an electrolyte was injected, and a coin cell (CR2032) was assembled. The assembled coin cell was paused at room temperature for 3-24 hours to produce a half battery. Here, the electrolyte was obtained by mixing a lithium salt 1 M LiPF.sub.6 with an organic solvent (volume ratio of EC:EMC=3:7) and mixing with an electrolyte additive FEC 3 vol %.

[0070] <Production of Secondary Battery>

[0071] The positive electrode and the negative electrode were notched at a predetermined size, respectively and laminated, a separator (polyethylene, thickness 13 μm) was interposed between the positive electrode and the negative electrode to form an electrode cell, and then each tab part of the positive electrode and the negative electrode was welded. The welded assembly of positive electrode/separator/negative electrode was placed in a pouch, and three sides except an electrolyte injection part side were sealed. Here, a portion where there was an electrode tab was included in a sealing part.

[0072] The electrolyte was injected through the other side except the sealing part, the other side was sealed, and the battery was immersed for 12 hours or more.

[0073] The electrolyte was obtained by dissolving 1 M LiPF.sub.6 in a mixed solvent of EC/EMC/DEC (volume ratio of 25/45/30) and adding 1 wt % of vinylene carbonate (VC), 0.5 wt % of 1,3-propene sultone (PRS), and 0.5 wt % of lithium bis(oxalato)borate (LiBOB).

[0074] Thereafter, pre-charge was performed with a current corresponding to 0.25 C for 36 minutes. Degassing was performed after 1 hour, aging was performed for 24 hours or more, and then formation charge and discharge were performed (charge condition: CC-CV 0.2 C 4.2 V 0.05 C CUT-OFF, discharge condition: CC 0.2 C 2.5 V CUT-OFF). Thereafter, standard charge and discharge were performed (charge condition: CC-CV 0.5 C, 4.2 V, 0.05 C CUT-OFF, discharge condition: CC 0.5 C, 2.5 V CUT-OFF).

[0075] Evaluation Example

[0076] [Evaluation Example 1]: Evaluation of Adhesive Strength Characteristic by Formation of Multilayer Negative Electrode Active Material Layer and Application of Carbon-Coated Graphite-Based Active Material

Example 2

[0077] A negative electrode, a coin cell, and a secondary battery were produced in the same manner as in Example 1, except that the non-crystalline carbon-coated first graphite-based active material produced was used in place of the second graphite-based active material.

Comparative Example 1

[0078] A negative electrode and a coil cell were produced in the same manner as in Example 1, except that the second graphite-based active material without carbon coating was used in the first negative electrode slurry in place of the carbon-coated first graphite-based active material, and the carbon-coated first graphite-based active material was used in the second negative electrode slurry in place of the second graphite-based active material without carbon coating.

Comparative Example 2

[0079] A negative electrode and a coil cell were produced in the same manner as in Example 1, except that the first negative electrode active material layer was loaded at 10.2 mg/cm.sup.2 on a current collector, without forming the second negative electrode active material layer.

[0080] (Evaluation Method)

[0081] Evaluation of Interfacial Adhesive Strength Between Negative Electrode Active Material Layer and Current Collector

[0082] The produced negative electrode was cut into a size of 18 mm wide and 150 mm long, a tape having a width of 18 mm was attached to a foil layer of the negative electrode, and sufficient adhesion was made with a roller having a load of 2 kg. The active material layer of the negative electrode was attached to one side of a tensile tester using a double-sided tape. A tape attached to a foil was engaged to the other side of the tensile tester, and adhesive strength was measured. Measurement results are shown in the following Table 1.

[0083] Evaluation of Interfacial Resistance (EIS, Electrochemical Impedance Spectroscopy)

[0084] The produced coil cell was set at SOC 50%, and the interfacial resistance value of the negative electrode was measured in a range of 10 kHz to 100 mHz using EIS. Measurement results are shown in the following Table 1.

TABLE-US-00001 TABLE 1 Presence or Composition of Inter- absence of active material Ad- facial carbon coating on of negative hesive resist- graphite particle electrode active strength ance surface material layer (N) (Ohm) First Second First Second graphite- graphite- layer layer based based active active material material Example Carbon Carbon Graphite- Graphite- 0.50 3.50 1 coating ○ coating x based + based silicon- alone based Example Carbon Carbon Graphite- Graphite- 0.50 3.41 2 coating ○ coating ○ based + based silicon- alone based Compar- Carbon Carbon Graphite- Graphite- 0.32 5.42 ative coating x coating ○ based + based Example silicon- alone 1 based Compar- Carbon coating ○ Blending, 0.34 4.99 ative single layer Example (graphite-based + 2 silicon-based)

[0085] Referring to Table 1, it was confirmed that the electrodes produced according to Examples 1 and 2 of the present invention included the non-crystalline carbon-coated first graphite-based active material (first layer) to show excellent interfacial adhesive strength between the negative electrode active material layer and the current collector, as compared with Comparative Example 1.

[0086] In Comparative Example 1, it was confirmed that the graphite-based active material without a carbon coating layer was disposed in the first layer to decrease adhesive strength, whereby coil cell interfacial resistance (R.sub.high-frequency) was increased.

[0087] In Comparative Example 2, it was confirmed that the silicon-based material was included in the upper layer portion of the negative electrode having a large contact area with the electrolyte to increase the side reaction with the electrolyte, whereby the coin cell interfacial resistance was increased.

[0088] Meanwhile, in Example 2, it was confirmed that the second layer graphite-based active material as well as the first layer graphite-based active material was coated with carbon so that the interfacial resistance value was somewhat decreased, but this may not be preferred in terms of output and quick charge.

[0089] [Evaluation Example 2]: Evaluation of Battery Characteristic Depending on Application of CNT Conductive Material

Examples 3 and Comparative Examples 3 to 6

[0090] A negative electrode, a coin cell, and a secondary battery were produced in the same manner as in Example 1, except that the conductive material was used in the first and second negative electrode slurries as shown in the following Table 2.

[0091] Here, the content of the active material was changed depending on the increase or decrease in the content of the conductive material in each slurry, and the content of the binder (2.7 wt %) was the same.

[0092] (Evaluation Method)

[0093] Evaluation of (Normal) Charge Life Characteristic

[0094] The secondary batteries produced in Examples 1 and 3 and Comparative Examples 3 to 5 were subjected to normal charge life characteristic evaluation in a range of DOD94 (SOC2-96) in a chamber in which 25° C. was maintained. The battery was charged to the voltage corresponding to SOC96 at 0.3 C under a constant current/constant voltage (CC/CV) condition, cut-off at 0.05 C, and then discharged to the voltage corresponding to SOC2 under a constant current (CC) condition, and the discharge capacity was measured. This was repeated in 100 cycles, the discharge capacity retention rate of the evaluation of (normal) charge life characteristic was measured, and the results are summarized in the following Table 2.

TABLE-US-00002 TABLE 2 Use of conductive material First Second Life characteristic layer layer Type (@ 100 (wt %) (wt %) (shape) cycle, %) Example 1 Used (0.2) — CNT 95.7 (linear) Example 3 Used (0.1) Used (0.1) CNT 94.2 (linear) Comparative — Used (0.2) CNT 67.8 Example 3 (linear) Comparative Used (1) Used (1) CB (point- 89.6 Example 4 shaped) Comparative Used (2) — CB (point- 91.2 Example 5 shaped) Comparative Used (5) — Artificial 88.3 Example 6 graphite (plate- shaped)

[0095] Referring to Table 2, it was seen that the secondary batteries produced according to Examples 1 to 3 of the present invention included the linear conductive material CNT in the first layer, whereby the life characteristic was improved as compared with the Comparative Example.

[0096] In Comparative Example 3 including the conductive material in the second layer, it is analyzed that an active material isolation phenomenon due to expansion of the silicon-based active material in the first layer was not suppressed, whereby the life characteristic was deteriorated. It was seen that since the point-shaped or plate-shaped conductive material of Comparative Examples 4 to 6 had a significantly smaller specific surface area than the linear conductive material, a life characteristic was deteriorated.

[0097] Meanwhile, in Example 3 using the same content of CNT distributed in the first layer and the second layer, battery performance was somewhat deteriorated as compared with Example 1. The results as such suggest that since the artificial graphite active material in the second layer did not show volume expansion substantially during battery charge and discharge as compared with the silicon-based material included in the first layer, there was no particular improvement of the effect even with the use of the conductive material in the second layer in the present invention.

[0098] In order to improve the life characteristic while maintaining an energy density identically therefrom, it was seen that it is advantageous to use an optimal content of the linear conductive material only in the first layer.

[0099] [Evaluation Example 3] Evaluation of Charge Characteristic by Rate and Evaluation of Quick Charge Life Characteristic

Examples 4 and 5

[0100] A negative electrode, a coin cell, and a secondary battery were produced in the same manner as in Example 1, except that the first and second graphite-based active materials were used as shown in the following Table 3.

[0101] In Example 5, as the first graphite-based active material, a mixture of surface-coated natural graphite and surface-coated artificial graphite at a weight ratio of 1:9 was used.

[0102] (Evaluation Method)

[0103] Evaluation of High-Rate Charge Characteristic

[0104] The negative electrodes produced according to Examples 1, 2, 4, and 5 and the same positive electrode were used to manufacture a pouch type secondary battery (cell) having a large capacity of 20 Ah or more, and the battery was charged with a constant current at a current of 0.2 C rate until the voltage reached 4.2 V and charged with a constant voltage by cutting-off at a current of 0.05 C rate while maintaining 4.2 V in a constant voltage mode. Thereafter, the battery was discharged to 2.5 V at 0.2 C, charged at a constant current of 0.2 C rate until the voltage reached 4.2 V, and cut-off at a current of 0.05 C rate while maintaining 4.2 V in a constant voltage mode, thereby performing constant voltage charge evaluation. The evaluation of high-rate charge characteristic was performed at 2.0 C in a chamber in which a constant temperature (25° C.) was maintained. A 2 C constant current charge capacity (%) for the initial 0.2 C rate constant current charge capacity was measured, and the results are summarized in the following Table 3.

[0105] Evaluation of (Normal) Charge Life Characteristic

[0106] The negative electrodes produced according to Examples 1, 2, 4, and 5, and the same positive electrode were used to manufacture a pouch type secondary battery (cell) having a large capacity of 20 Ah, and a normal charge life characteristic evaluation in a range of DOD94 (SOC2-96) was performed in a chamber in which 25° C. was maintained. The battery was charged to the voltage corresponding to SOC96 at 0.3 C under a constant current/constant voltage (CC/CV) condition, cut-off at 0.05 C, and then discharged to the voltage corresponding to SOC2 under 0.5 C of a constant current (CC) condition, and the discharge capacity was measured. This was repeated in 100 cycles, the discharge capacity retention rate of the evaluation of (normal) charge life characteristic was measured, and the results are summarized in the following Table 3.

[0107] Quick Charge Life Characteristic Evaluation

[0108] The negative electrodes produced according to Examples 1, 2, 4, and 5, and the same positive electrode were used to manufacture a pouch type secondary battery (cell) having a large capacity of 20 Ah, and quick charge evaluation was performed with step charge at 1.25 C/1.0 C/0.75 C/0.5 C C-rate and at ⅓ C discharge C-rate in a chamber in which a constant temperature (25° C.) was maintained, set in a range of DOD72 (SOC8-80). There was a rest time of 10 minutes between charge and discharge cycles and after repeating 100/200/300 cycles, a quick charge capacity retention rate was measured, and the results are summarized in the following Table 4.

TABLE-US-00003 TABLE 3 Type and presence or absence of (carbon 2 C charge Normal capacity coating) of graphite- capacity retention rate based active material Adhesive retention (0.3 C charge- First Second strength rate 0.5 C discharge) layer layer (N) (%) (100 cycle, %) Example Artificial Artificial 0.5 72.4 94.9 1 graphite graphite (∘) (x) Example Artificial Artificial 0.5 72.9 93.0 2 graphite graphite (∘) (∘) Example Artificial Natural 0.5 65.1 83.4 4 graphite graphite (∘) (∘) Example Natural Artificial 0.52 72.6 95.0 5 (∘) + graphite artificial (x) graphite (∘)

TABLE-US-00004 TABLE 4 Discharge capacity retention rate (%) by quick charge cycle (35 minutes protocol) After 100 cycles After 200 cycles After 300 cycles Example 1 92.7 90.7 88.4 Example 2 92.8 90.0 87.6 Example 4 82.6 — — Example 5 92.6 90.5 88.2

[0109] Referring to Tables 3 and 4, it was confirmed that i) as seen from Example 2, when surface-coated artificial graphite was used in the second layer, a C-rate characteristic was somewhat improved with a decreased interfacial resistance by a surface coating layer, but as the normal and quick charge life evaluation (long-term evaluation) proceeded, life deterioration was accelerated due to the side reaction of the electrolyte and the surface coating layer. ii) in Example 4, it is analyzed that the high-rate characteristic was not good due to the intrinsic characteristics of the natural graphite in the upper layer, and thus, the charge characteristic by rate and the quick charge life characteristic were deteriorated.

[0110] Therefore, the battery performance of Example 1 which was produced with the preferred graphite-based active material and the non-crystalline coating composition was somewhat improved as compared with that of Examples 2 and 4 which were not produced like Example 1.

[0111] Meanwhile, in Example 5, it was confirmed that the artificial graphite and the natural graphite were blended and used in the first layer, whereby the adhesive strength was improved and the 2 C charge capacity retention rate and the normal charge life characteristic were somewhat improved as compared with Example 1.

[0112] [Evaluation Example 4] Evaluation of Battery Characteristic by Content of Silicon-Based Material in First Layer

Examples 6 to 10

[0113] A negative electrode and a secondary battery were produced in the same manner as in Example 1, except that the content of the silicon-based material was as shown in the following Table 5, in the first negative electrode active material layer.

[0114] (Evaluation Method)

[0115] Evaluation of Cell Energy Density

[0116] The negative electrodes produced according to Examples 1 and 6 to 10 and the same positive electrode were used to manufacture a pouch type secondary battery (cell) having a large capacity of 20 Ah or more, and the battery was charged with a constant current at a current of 0.3 C rate until the voltage reached 4.2 V and charged with a constant voltage by cutting-off at a current of 0.05 C rate while maintaining 4.2 V in a constant voltage mode. Thereafter, the battery was discharged at a constant current of 0.3 C rate until the voltage reached 2.5 V to measure a discharge capacity (Ah) and energy (Wh), the volume of each battery in a 4.2 V charge state was measured to calculate a volume-energy density, and the results are summarized in the following Table 5.

[0117] Quick Charge Life Characteristic Evaluation

[0118] The secondary batteries produced in Examples 1 and 6 to 10 were subjected to the same evaluation of quick charge life characteristic of Evaluation Example 3, and the results are summarized in the following Table 5.

[0119] Evaluation of (Normal) Charge Life Characteristic

[0120] The secondary batteries produced in Examples 1 and 6 to 10 were subjected to the same evaluation of normal charge life characteristic of Evaluation Example 2, and the results are summarized in the following Table 5.

TABLE-US-00005 TABLE 5 First negative electrode active material layer Capacity Content Weight ratio retention rate (%) of (%) of After 100 After 100 silicon- CNT/silicon- Energy cycles of cycles of based based Density quick normal material material (Wh/L) charge charge Example 1 33.5 0.59 724 92.7 94.9 Example 6 20 1 720 93.5 96.0 Example 7 35 0.28 724 92.5 94.8 Example 8 45 0.5 727 87.1 89.4 Example 9 33.5 0.15 725 91.1 93.0 Example 10 33.5 1.79 721 90.4 92.1

[0121] (The content of the silicon-based material is wt % with respect to the total weight of the active material in the first negative electrode active material layer.)

[0122] Referring to Table 5, in Example 9, it was confirmed that the ratio of conductive material/silicon-based material was low, whereby the conductive path was non-uniform and the life characteristic was deteriorated. However, in Example 10, it is analyzed that since the ratio of the conductive material was increased, the porosity in the electrode was decreased, whereby the life characteristic was deteriorated. In addition, it was seen that when the content of silicon in the lower layer was excessive as in Example 8, desorption between the foil and the electrode layer occurred depending on volume change during charge and discharge, and the life characteristic was deteriorated.

[0123] By coating the surface of a graphite-based active material with carbon, adhesive strength of an interface between an electrode current collector and an active material layer may be improved, and by disposing a conductive material having excellent electrical conductivity on a negative electrode active material layer on an electrode current collector including a silicon-based active material, isolation of an electrode or electrical conductive path cutting off due to volume expansion of the silicon-based material may be improved.

[0124] In addition, by introducing a negative electrode active material layer having a multilayer structure and applying a binder and a conductive material differently to each layer, respectively, the total contents of the binder and the conductive material may be decreased, and by increasing the content of the silicon-based active material, a high energy density may be implemented.

[0125] In addition, by disposing the graphite-based active material having excellent output characteristics on the upper layer of the electrode to minimize the side reaction of the silicon-based material and an electrolyte, a high-capacity negative electrode having both decreased interfacial resistance and improved output characteristics may be manufactured.

[0126] Although the exemplary embodiments of the present invention have been described above, the present invention is not limited to the exemplary embodiments but may be made in various forms different from each other, and those skilled in the art will understand that the present invention may be implemented in other specific forms without departing from the spirit or essential feature of the present invention. Therefore, it should be understood that the exemplary embodiments described above are not restrictive, but illustrative in all aspects.