KR102547944B1 - Cathode materials for lithium ion battery containing calsium sulfate, manufacturing method thereof and lithium ion battery including the same - Google Patents

Abstract

In the present invention, it is proposed to use calcium sulfate (CaSO ₄ , CSO), a dual functionalized surface modifier, in an efficient one-step method to improve the cycling performance of Ni-rich NCM cathode materials. Heat treatment of the NCM811 cathode material using the CSO precursor can artificially form a Ca- and SOx-functionalized cathode electrolyte mesophase (CEI) layer on the surface of the Ni-rich NCM cathode material. Then, the CEI layer inhibits the electrolyte decomposition at the interface between the Ni-rich NCM anode and the electrolyte. The successful formation of the CSO-modified CEI layer is confirmed by SEM and FT-IR analysis, and this process does not affect the bulk structure of the Ni-rich NCM anode material. During cycling, the CSO-modified CEI layer significantly reduces electrolyte decomposition during cycling at both room temperature and 45 °C, resulting in a significant increase in the cycling retention rate of the cell.

Description

Cathode material for lithium secondary battery containing calcium sulfate, method for manufacturing the same, and lithium secondary battery including the same

The present invention relates to a positive electrode material for a lithium secondary battery containing calcium sulfate, a method for manufacturing the same, and a lithium secondary battery including the same, and specifically, a NCM-based positive electrode active material and a calcium sulfate (CaSO4, CSO) additive. It is an invention related to a cathode material for a lithium secondary battery, a manufacturing method thereof, and a lithium secondary battery including the same.

Lithium-ion batteries (LIBs) have been successfully commercialized in 1991 and have been applied to various electronic applications from portable devices to electric vehicles. Electrode materials are generally the main components of LIBs as they determine the specifications of their applications, so many types of electrode materials have been tested for their ability to meet the requirements of LIB applications. Among many electrode material candidates, lithium nickel-cobalt-manganese oxide (NCM) has received considerable attention as a major cathode material in the mid-2000s due to its high specific capacity that increases the energy density of cells. This increase is due to the unique electrochemical properties of nickel in the interlayer structure. The electrochemical oxidation potential of nickel is lower than that of the Co species, which leads to the same cut-off potential in the cell but with an increased specific capacity.

Therefore, LiNi0.33Co _0.33 Mn _0.33 O ₂ (NCM333), LiNi _0.6 Co _0.2 Mn _0.2 O ₂ (NCM622), LiNi _0.7 Co _0.2 Mn _0.1 O ₂ (NCM721), LiNi _0.8 Co _0.1 Mn _0.1 O ₂ (NCM811) A number of NCM cathode material candidates have been studied. However, it is necessary to improve the cycling performance of the NCM cathode material in order to use it with lithium cobalt oxide, which is a cathode material widely used in LIBs.

When the NCM cathode material is charged, unstable Ni ⁴⁺ is formed within the cell, which tends to severely decompose the electrolyte. Once the electrolyte is decomposed by interaction with Ni ⁴⁺ , the decomposed adducts accumulate on the surface of the NCM cathode material and hinder the movement of Li ⁺ ions. The decomposed adduct seriously hinders the intercalation/deintercalation of Li ⁺ in the cell, resulting in rapid degradation of cycling performance. Since these unwanted reactions are further accelerated in NCM cathode materials with high Ni content, improving the surface stability of NCM cathode materials is a major challenge to satisfy both the stable cycling performance of cells and the high energy density of LIBs.

Republic of Korea Patent Publication No. 10-2017-0048999

In the present invention, we propose a new approach for modifying the surface properties of NCM cathode materials using calcium sulfate (CaSO4, hereinafter abbreviated as CSO), which is a dual-functionalized coating precursor.

In order to solve the above technical problem, the present invention provides a cathode material for a lithium secondary battery, characterized in that it comprises an NCM-based cathode active material and a calcium sulfate (CaSO4, CSO) additive.

Referring to FIG. 1 , according to an embodiment of the present invention, the NCM cathode material is mixed with CaSO ₄ and then the surface of the NCM cathode material is modified in a one-step process through heat treatment. Heat treatment of CaSO ₄ can separate both Ca ²⁺ and SO ₄ ^2- species. Thus, 1) Ca ²⁺ can combine with elemental oxygen due to its strong binding affinity, and 2) SO ₄ ^2- can also form a residue such as Li ⁺ remaining on the NCM anode surface from unreacted residual lithium species (eg lithium hydroxide or lithium carbonate). Can bind cationic species.

In previous studies, since direct contact between the electrode and the electrolyte is prevented during the electrochemical charge/discharge process to suppress electrolyte decomposition, the formation of a SO _x- based Cathode-Electrolyte Interface (CEI) layer is effective in improving the cycle performance of the cell. There was a report. Therefore, Ca ²⁺ can increase the mechanical properties of NCM cathode materials by increasing particle hardness based on the strong bonding affinity between Ca and O elements, and SO ₄ ^2- can suppress electrolyte decomposition, which is useful for improving the interfacial stability of NCM cathode materials. can contribute In the present invention, based on this material strategy, surface modification of Ni-rich NCM cathode materials is attempted through a simple and convenient one-step process using CaSO ₄ , and changes in material properties are characterized in detail. The electrochemical performance is evaluated to review the compatibility of the CaSO ₄ modified NCM cathode material, and the role of the CaSO ₄ -based CEI layer is verified by systematic spectroscopic analysis.

The calcium sulfate (CaSO ₄ , CSO) additive is characterized in that it is separated into Ca ²⁺ and SO ₄ ^2- ions in the cathode material.

It is characterized in that the Ca ²⁺ ion is combined with an oxygen (O) element, and the SO ₄ ^2- ion is combined with a cationic species (Group) including Li ⁺ .

The calcium sulfate (CaSO ₄ , CSO) additive is characterized in that it contains 0.1 to 0.5 wt% based on the weight of the cathode material.

The NCM-based cathode active material is LiNi _0.8 Co _0.1 Mn _0.1 O ₂ (NCM811).

According to another embodiment of the present invention, mixing the NCM-based positive electrode active material and calcium sulfate (CaSO ₄ , CSO) additive; and heat-treating a mixture of the NCM-based cathode active material and calcium sulfate (CaSO ₄ , CSO) additive.

In the heat treatment step, the calcium sulfate (CaSO ₄ , CSO) additive is characterized in that it is separated into Ca ²⁺ and SO ₄ ^2- ions in the cathode material.

It is characterized in that the Ca ²⁺ ion is combined with an oxygen (O) element, and the SO ₄ ^2- ion is combined with a cationic species (Group) including Li ⁺ .

According to another embodiment of the present invention, the anode; cathode; and an electrolyte layer disposed between the anode and the cathode, wherein the electrolyte layer includes the above-described electrolyte solution.

The positive electrode is characterized by including LiNi _0.8 Co _0.1 Mn _0.1 O ₂ (NCM811).

It is characterized in that a Ca- and SO _x -functionalized cathode-electrolyte interphase (CEI) layer is formed on the surface of the anode.

According to one embodiment of the present invention, calcium sulfate (CaSO4, CSO), a dual functionalized surface modifier, was used in an efficient one-step method to improve the cycling performance of the Ni-rich NCM cathode material. Heat treatment of the NCM 811 cathode material using the CSO precursor can artificially form a Ca- and SO _x -functionalized cathode electrolyte mesophase (CEI) layer on the surface of the Ni-rich NCM cathode material. Then, the CEI layer inhibits the electrolyte decomposition at the interface between the Ni-rich NCM anode and the electrolyte. The successful formation of the CSO-modified CEI layer is confirmed by SEM and FT-IR analysis, and this process does not affect the bulk structure of the Ni-rich NCM anode material.

During cycling, the CSO-modified CEI layer significantly reduces electrolyte decomposition during cycling at both room temperature and 45 °C, resulting in a significant increase in the cycling retention rate of the cell. Cells cycled with NCM811 anodes modified with 0.1 wt% content of CSO show a specific capacity retention of 90.0%, whereas cells cycled with NCM811 anodes continue to fade after cycling retention (74.0%) after 100 cycles. suffer from symptoms

SEM, EIS, XPS, and ICP-MS analyzes of the recovered electrodes showed that undesirable surface reactions such as electrolyte decomposition and metal decomposition were well controlled within the cell due to the artificial CSO-modified CEI layer present on the surface of the Ni-rich NCM811 anode. show

1 is a schematic diagram showing the role of the surface-modified NCM811 cathode material functionalized by CSO.
2 shows EDS analysis of (a) NCM811, (b) NCM811 modified with 0.1 wt% CSO, (c) NCM811 modified with 0.5 wt% CSO, and (d) NCM811 modified with 0.5 wt% CSO. am.
Figure 3 shows (a) initial cycle potential profile and (b) room temperature cycling performance and (c) initial cycle potential profile and (d) high temperature cycling performance (black: NCM811, red: modified with 0.1 wt% CSO). NCM811 modified, blue: NCM811 modified with 0.5 wt% CSO).
4 shows the surface morphology of (a) cycled NCM811 and (b) cycled 0.1 wt% CSO-modified NCM811 and EIS results of cycled NCM811 anode at 70 cycles (c) cycled NCM811 and (d) cycled 0.1 wt% NCM811. Plot showing % CSO modified NCM811 (black: NCM811 and red: NCM811 modified with 0.1 wt% CSO).
Figure 5 shows XPS analysis ((a) C1s, (b) F1s, (c) P2p) for a cycled NCM811 anode (top) and a cycled 0.1 wt% CSO modified NCM811 anode (bottom).
6 is a graph showing ICP-MS results of lithium metal (black: NCM811 and red: 0.1 wt% CSO-modified NCM811) in cycled NCM811 and cycled 0.1 wt% CSO-modified NCM811.
7 is a diagram showing FT-IR analysis of NCM811 electrodes using CSO (black: NCM811 and pink: NCM811 modified with 10.0 wt% CSO).
8 is a diagram of XRD spectra (red: 0.1 wt%, blue: 0.5 wt%) for unmodified NCM811 (black) and NCM811 modified with CSO.

Hereinafter, the present invention will be described in more detail through examples. However, these examples are only for helping the understanding of the present invention, and the scope of the present invention is not limited to these examples in any sense.

Example

CSO-modified NCM811 cathode material is NCM811 (LiNi _0.8 Co _0.1 Mn _0.1 O ₂ ) 10.0 g, CSO (Aldrich) 0.01 g (abbreviated as 0.1 CSO-modified NCM811) and CSO (Aldrich) 0.05 g (abbreviated as 0.5 CSO-modified NCM811) ) CSO (Aldrich) was prepared by elaborate mixing for 20 minutes. Each mixture was then placed in a furnace and heated to 400 °C for 3 hours. After cooling to room temperature, the CSO-modified NCM811 cathode material was collected and the surface morphology was analyzed by scanning electron microscopy (FE-SEM/EDS-7800F, JEOL). Their chemical composition was also characterized by Fourier Transform Infrared (FT-IR) spectroscopy (Bruker, SPEXX 80V), and their bulk structure was measured by X-ray diffraction (XRD, SmartLab, Rigaku).

Bare NCM811 cathode material and CSO-modified NCM811 cathode material were prepared as follows: 1.8 g of NCM811 cathode material in 1.5 mL of N -methylpyrrolidone (NMP, Aldrich), poly(vinylidene difluoride) (PVDF, Kureha ) 0.1 g and 0.1 g of carbon conductive agent (Super P) were mixed with a homogenizer for 1 hour. Then, the mixed NCM811 slurry was coated on an aluminum (Al) current collector and dried at 120 °C for 3 h and then overnight at 120 °C in a vacuum oven. The load density of the NCM811 anode was 9.4 ± 0.6 mg cm ^-2 . The anode was assembled as a 2032-coin cell with a poly(ethylene) separator and electrolyte (ethylene carbonate: ethylmethyl carbonate = 1:2 (vol%) + 1 M LiPF ₆ ; assimilated electrolyte). The cell was charged to 4.3 V (vs. Li/Li+) and discharged to 3.0 V (vs. Li/Li+) with a current density of 0.1 C for the first two cycles (formation phase) and 0.5 C for 100 cycles. . Cycling performance was evaluated at room temperature or 45 °C.

After completing the cycling performance test, the cell was disassembled in an Ar-filled glove box and each anode was recovered. The cycled NCM811 anode was analyzed by SEM to determine surface morphology and changes in chemical composition were detected by X-ray photoelectron spectroscopy (XPS, Thermo-Scientific). Electrochemical impedance spectroscopy (EIS) was performed on the charged cells scanned from 1 M to 10 mHz with an electrochemical tester (Wonatech, ZiveMP1), and periodicity was determined by inductively coupled plasma mass spectrometry (ICP-MS, Thermo, FluoTime300/MicroTime100). Metal degradation at the anode was analyzed. All assays were performed at room temperature.

Experiment result

The surface morphology of the CSO-modified NCM811 cathode material was analyzed by SEM and EDS (FIG. 2). The unmodified NCM811 cathode material maintains a relatively clean topological surface state. In contrast, the surface of the CSO-modified NCM811 cathode material appears to be covered with a thin layer: the new surface layer is sparsely present on the surface of the primary particles of the NCM811 cathode material, indicating that the surface modification by the CSO precursor may have caused the surface modification of the NCM811 cathode material to occur. to create a new artificial CEI layer.

As a result of additional EDS analysis, elements Ca and S (originating from the CSO precursor) were found on the surface of the CSO-modified NCM811 cathode material, confirming the formation of a CSO-modified CEI layer on the surface of the cathode.

The FT-IR spectrum supports this explanation as the S=O stretching is clearly observed at 1,150 cm ^-1 whereas the unmodified NCM811 anode material does not show a similar signal (Fig. 7).

Although the chemical state of the CSO-modified NCM811 cathode material is changed, the layered structure is well preserved, as confirmed by XRD analysis (Fig. 8). This demonstrates that the CSO precursor forms an artificial CEI layer functionalized with Ca and S-O only on the surface, but does not alter the bulk structure of the NCM811 anode material.

Electrochemical performance evaluation of the CSO-modified NCM811 cathode material is shown in FIG. 3. A cell cycled with NCM811 cathode material at room temperature shows a specific capacity of 208.9 mA hg ^-1 in the initial cycle. In contrast, cells cycled with the CSO-modified NCM811 anode material exhibited relatively low specific capacities (200.2 mA hg ^-1 for CSO-modified NCM811 with 0.1 wt% content and 200.3 mA hg ^-1 for CSO-modified NCM811 with 0.5 wt% content). show This reaction can be explained by the increase in the distance of the Li ⁺ diffusion path at the interface between the NCM811 anode and the electrolyte as the artificial CEI layer is placed on the anode surface. Nevertheless, cells cycled with unmodified NCM811 anodes show only 74.0% retention at 100 cycles, whereas NCM811 anodes modified with 0.1 wt% and 0.5 wt% CSO have retention rates of 90.0% and 85.6%, thus CSO modification. The cycling performance of the NCM811 cathode material is clearly improved compared to the bare NCM811 cathode material, which can be considered as the more stable cycling behavior of the CSO-modified NCM811 cathode material.

In particular, when cycling a cell assembled with Ni-rich NCM anode material, electrolyte decomposition is significantly accelerated, indicating that the CSO-based artificial CEI layer can effectively suppress the electrolyte decomposition in the cell. In fact, the electrochemical performance of the CSO-modified NCM811 anode material was significantly improved when cycled at 45 °C. The initial potential profile of the CSO-modified NCM811 anode material shows a lower initial specific discharge capacity (215.3 mA hg ^-1 for 0.1 CSO-modified NCM811 and 217.5 mA hg ^-1 for 0.5 CSO-modified NCM811) than that of unmodified NCM811 (224.3 mA hg-1 ^{). 1} ). However, this difference is negligible because the kinetic behavior of Li+ migration can accelerate at high temperatures.

Cells cycled with 0.1 wt% and 0.5 wt% CSO-modified NCM811 anodes showed increased cycling retention (0.1 wt% CSO-modified NCM811: 78.0%, 0.5 wt% CSO-modified NCM811: 73.0%), whereas with unmodified NCM811 anodes. Cycled cells showed a sustained decrease in cycling retention.

Thus, unwanted reactions occurring at the interface between NCM811 and electrolyte appear to be well managed in cells cycling with CSO-modified NCM811 anodes as the artificially introduced CSO-based CEI layer inhibits electrolyte decomposition during the electrochemical process.

To confirm the above, the cycled NCM811 anode was recovered and analyzed by SEM (Figs. 4a and 4b).

The recovered untreated NCM811 anode surface contains many decomposed adducts most likely from electrolyte decomposition during electrochemical cycling.

In contrast, the recovered 0.1 wt% CSO-modified NCM811 anode has a clean surface state.

These notable morphological differences clearly indicate that much less undesirable side reactions occur in cells cycled with NCM811 anodes modified with 0.1 wt% CSO upon formation of the CSO-based CEI layer.

Additional EIS results also support this explanation (Figs. 4c and 4d). The internal resistance of the cell cycled with the unmodified NCM811 anode increases significantly after 70 cycles (R _CEI : 27.7 Ω, R _CT : 119.4 Ω), but the cell cycled with the NCM811 anode modified with 0.1 wt% CSO The increase in is much smaller (R _CEI : 23.4 Ω and R _CT : 70.9 Ω).

These results strongly suggest that severe electrolyte decomposition occurs in cells cycled with unmodified NCM811 anodes, resulting in a significant increase in overall resistance within the cell.

The XPS results indicate that unwanted surface reactions related to electrolyte degradation were well suppressed in the cell cycled with the CSO-modified NCM811 anode (Fig. 5). The C 1s spectrum shows relatively higher intensities of C-C (285.0 eV from carbon conductor) and C-F (291.1 eV from PVDF binder) in NCM811 anodes modified with recovered CSO than in unmodified NCM811 anodes recovered.

Instead, much higher C 1s peaks such as C=O (289.1 eV), R-C(=O)-R (287.7 eV), and C-O (286.4 eV) are observed in the recovered unmodified NCM811 anode, which is due to electrolyte degradation in the cell. Provide evidence to support its occurrence.

On the other hand, similar spectroscopic behavior is observed in the F 1s and P 2p assays. The cycled unmodified NCM811 anode was considered decomposed: Li _x PO _y F _z (687.3 eV in F 1s, 135.3 eV in P 2p), LiF (685.5 eV in F 1s) and Li _x PF _y (687.3 eV in P 2p). 137.2 eV).

Therefore, less electrolyte decomposition occurs in the cell cycled with the CSO-modified NCM811 anode, and thus the interfacial stability of the CSO-modified NCM811 anode can be maintained even after 100 cycles.

Quantification of dissolved transition metal components by ICP-MS analysis of the cycled cathode can explain these results (FIG. 6). A significant amount of dissolved transition metal components (Ni: 27,804 ppb, Co: 4,143 ppb, Mn: 4,002 ppb) exist in the cycled anode, which means that severe electrolyte decomposition in the cell rapidly accelerates the dissolution of transition metal components by chemical reactions. This implies that the F ^- species may be present in high concentrations in the cell. Conversely, the negative electrode combined with the CSO-modified NCM811 positive electrode contained only a small amount of dissolved transition metal components (Ni: 19,067 ppb, Co: 2,095 ppb, Mn: 2,611 ppb), indicating that electrolyte decomposition was not serious in the cell. . These results suggest that the surface modification of the NCM811 anode with the CSO precursor is obviously effective in improving the interfacial stability of the Ni-rich anode material, thus significantly improving the electrochemical performance of the cell.

conclusion

In order to enhance the cycle retention rate of the NCM811 cathode material, the present invention proposes to use an efficient one-step surface modification method using CSO, a dual functionalized coating precursor.

Heat treatment of NCM811 cathode material and CSO precursor creates an artificially induced functionalized CEI layer on the surface of NCM811 cathode material, preventing acceleration of electrolyte decomposition in Ni-rich NCM cathode material.

As a result of SEM and FT-IR analysis, it was confirmed that Ca and SO _x functional groups were formed on the surface of the NCM811 cathode material, but the layered structure of the NCM811 cathode material was well maintained.

The artificially induced CEI layer slightly increases the internal resistance in the initial cycle, but significantly reduces the electrolyte decomposition in the cell at both room temperature and 45 °C, thereby increasing the cycle retention rate of the cell.

Cells cycled with NCM811 anodes modified with 0.1 wt% CSO show 90.0% specific capacity retention, whereas cells cycled with unmodified NCM811 anodes suffer from poor cycle retention (74.0%).

Additional SEM, EIS, XPS and ICP-MS results on the recovered electrodes demonstrate the effectiveness of the CSO-based CEI layer in improving electrochemical cycling.

A relatively clean surface is observed for the recovered NCM811 anode modified with 0.1 wt% CSO, which clearly shows a decrease in internal resistance and a decrease in the dissolution of transition metal components.

This means that unwanted surface reactions are significantly suppressed by the presence of a CSO-based artificial CEI layer on the NCM811 anode surface.

Therefore, the one-step surface modification method using the CSO precursor is considered to be beneficial for improving the electrochemical performance of the Ni-rich NCM cathode material.

The present invention described above is not limited to the above-described embodiments, since various substitutions and changes can be made to those skilled in the art within the scope of the technical idea of the present invention. .

Claims (11)

Characterized in that it comprises a NCM-based positive electrode active material,
The calcium sulfate (CaSO4, CSO) additive is separated into Ca ²⁺ and SO ₄ ^2- ions in the cathode material, the Ca ²⁺ ions are combined with oxygen (O) elements, and the SO ₄ ^2- ions are Li A positive electrode material for a lithium secondary battery, characterized in that it binds to a cationic species (Group) containing ⁺ .
delete delete delete According to claim 1,
The NCM-based cathode active material is a cathode material for a lithium secondary battery, characterized in that LiNi _0.8 Co _0.1 Mn _0.1 O ₂ (NCM811).
Mixing an NCM-based cathode active material and calcium sulfate (CaSO ₄ , CSO) additive; and
Including; heat-treating a mixture of the NCM-based positive electrode active material and calcium sulfate (CaSO ₄ , CSO) additive,
In the heat treatment step, the calcium sulfate (CaSO ₄ , CSO) additive is separated into Ca ²⁺ and SO ₄ ^2- ions in the cathode material, the Ca ²⁺ ions combine with oxygen (O) elements, and the SO ₄ ^2- The ion combines with a cationic species (Group) including Li ⁺ ,
The calcium sulfate (CaSO ₄ , CSO) additive is characterized in that it contains 0.1 to 0.5% by weight relative to the weight of the cathode material
Manufacturing method of cathode material for lithium secondary battery.
delete delete anode; cathode; and
An electrolyte layer disposed between the anode and the cathode,
The positive electrode is a lithium secondary battery characterized in that it comprises the positive electrode material according to any one of claims 1 to 5.
According to claim 9,
The positive electrode is a lithium secondary battery, characterized in that it comprises LiNi _0.8 Co _0.1 Mn _0.1 O ₂ (NCM811).
According to claim 9,
A lithium secondary battery, characterized in that a Ca- and SO _x -functionalized cathode electrolyte interphase (CEI) layer is formed on the surface of the cathode.

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