KR102174482B1 - Cathode materials for lithium ion battery coated dimethoxydimethylsilane and preparing method thereof - Google Patents

Abstract

The present invention is a positive electrode active material; And dimethoxydimethylsilane (DODSi) coated on the surface of the positive electrode active material, and to a positive electrode active material for a lithium secondary battery. The NCM811 anode material functionalized with DODSi was successfully synthesized through a simple and convenient one-step coating process, and Si-O functional groups were formed on the surface of the NCM811 anode material within a few nanometers in thickness. According to the systematic analysis results, the Si-O functional groups impregnated in the artificial CEI layer played an important role in reducing the surface resistance of the cell by inhibiting electrolyte decomposition at the electrode/electrolyte interface. In addition, the Si-O functionalized artificial CEI layer reduced the F- concentration in the cell because the Si-O functional group chemically reacted with the F ^- species through a scavenging reaction to reduce the Ni dissolution action. As a result, since the surface stability of the NCM811 anode was improved by the formation of the Si-O functionalized artificial CEI layer, the cell cycled with the DODSi-functionalized NCM811 anode showed significantly improved cycling retention than the cell cycled with the untreated NCM811 at 55℃. .

Description

A cathode active material for lithium ion batteries coated with dimethoxydimethylsilane and its manufacturing method {CATHODE MATERIALS FOR LITHIUM ION BATTERY COATED DIMETHOXYDIMETHYLSILANE AND PREPARING METHOD THEREOF}

The present invention is a positive electrode active material; And dimethoxydimethylsilane (DODSi) coated on the surface of the positive electrode active material, and to a positive electrode active material for a lithium secondary battery.

Lithium-ion batteries (LIBs) are a promising power source driven primarily by energy conversion and storage reactions. In recent years, these applications have expanded exponentially from small devices to large-scale devices such as electric vehicles (EV) and energy storage systems (ESS). In this regard, the performance of the LIB is important because it affects the specifications of the target device, such as the driving distance of the EV or the stable time of ESS operation. Since these specifications are primarily influenced by the energy density of the LIB, many types of advanced electrode materials that allow high specific capacity and operating potential have been intensively studied.

Lithium nickel-cobalt-manganese oxide (Li[Ni _x Co _y Mn _z ]O ₂ , NCM) having a layered structure has received considerable attention as an alternative cathode material for LIBs. In particular, the presence of Ni is an important factor in increasing the specific capacity of the NCM anode material because the oxidation potential of Ni ²⁺ is lower than that of Co ³⁺ . That is, increasing the relative Ni composition than Co in the stacked structure of the NCM positive electrode material can effectively improve the energy density of the battery. This is due to the high specific capacity of the NCM anode at the same charging potential due to the high Ni composition. Therefore, a Ni-rich NCM positive electrode active material with an Ni content exceeding 60% is attracting attention. This is because it provides a higher specific capacity (>180mA h ^-1 ) than LiCoO ₂ (~145mA h ^-1 ), a lithium cobalt oxide.

However, the Ni-rich NCM anode material has poor cycle performance due to poor surface stability. In the charged state, the presence of chemically unstable Ni ⁴⁺ causes electrolyte decomposition at the surface of the Ni-rich NCM anode. This is because the unstable Ni ⁴⁺ species are liable to change to a more stable chemical state (Ni ³⁺ or Ni ²⁺ ) by reduction, so the electrolyte can be decomposed immediately at the electrolyte/electrode interface, giving electrons to Ni ⁴⁺ . In particular, when decomposed adducts accumulate continuously on the surface of the Ni-rich NCM anode, this will significantly hinder the Li ⁺ intercalation/de-intercalation reaction by increasing the surface resistance. And thus the dose reduction occurs rapidly. Moreover, electrolytic decomposition leads to the formation of nucleophilic fluorine (F ⁻ ) species within the cell, which will significantly corrode the transition metal components of the Ni-rich NCM anode. Therefore, ensuring the high surface stability of the Ni-rich NCM anode is an indispensable factor in achieving long cycle life, high cell performance and high specific capacity of LIB.

It is well known that the cathode-electrolyte interface (CEI) layer plays an important role in improving the surface stability of the anode material. This is because the electron transfer reaction between the electrode and the electrolyte does not occur, the CEI layer can effectively suppress the electrolyte decomposition at the electrolyte/electrode interface. In particular, since dimethoxydimethylsilane (DODSi) is mainly composed of organic functional groups, the thermal decomposition temperature is low, which means that CEI can be formed at relatively low temperatures in an alternative round of commonly tried inorganic-based coating approaches. do. In addition, the silyl ether (Si-O) residues of DODSi are expected to effectively scavenging F ^- species, which severely promotes the irreversible dissolution of the transition metal component from the Ni-rich NCM anode.

Korean Patent Registration No. 10-1775097

The present inventors developed an artificial CEI layer on the surface of LiNi _0.8 Co _0.1 Mn _0.1 O ₂ (NCM811) to improve the surface stability of the Ni-rich NCM anode, which was transformed into a silyl ether (Si-O) functional group (Fig. One). The present inventors performed a heat treatment process using DODSi as a CEI forming precursor after efficiently introducing an artificial CEI layer onto the surface of NCM811 by a wet coating method. Based on these considerations, surface-functionalized Ni-rich NCM was synthesized through a one-step process, and its physicochemical and electrochemical properties were investigated.

In order to solve the above technical problem, the present invention is a positive electrode active material; And it provides a positive electrode active material for a lithium secondary battery including dimethoxydimethylsilane (DODSi) coated on the surface of the positive electrode active material.

In the present invention, the positive electrode active material is a positive electrode active material for a lithium secondary battery, characterized in that represented by the following [Chemical Formula 1];

[Formula 1]

LiNi _a Co _b Mn _c O ₂ (0.6≤a≤0.9, a+b+c=1)

The positive electrode active material is preferably LiNi _0.8 Co _0.1 Mn _0.1 O ₂ .

The dimethoxydimethylsilane and the positive electrode active material have a weight ratio of 1:1 to 1:100 and more preferably are mixed in a weight ratio of 1:1 to 1:10.

In order to solve the above other problems, the present invention provides a positive electrode including the positive electrode active material; It provides a lithium secondary battery including an electrolyte; and a negative electrode, wherein the electrolyte includes 1.0 M LiPF ₆ in EC (ethylene carbonate): EMC (ethyl methyl carbonate) = 1:2.

In order to solve the another problem, the present invention includes mixing dimethoxydimethylsilane and a positive electrode active material to coat dimethoxydimethylsilane on the surface of a positive electrode active material; And heat treating the coated positive electrode active material It provides a method of manufacturing a cathode active material for a lithium secondary battery.

The heat treatment is performed at 400 to 800°C, preferably at 600°C. When the heat treatment is performed at less than 400° C., dimethoxydimethylsilane is not properly coated, and when it exceeds 800° C., the dimethoxydimethylsilane coating layer may be decomposed by a high heat temperature.

The NCM811 anode material functionalized with DODSi was successfully synthesized through a simple and convenient one-step coating process, and Si-O functional groups were formed on the surface of the NCM811 anode material within a few nanometers in thickness. According to the systematic analysis results, the Si-O functional groups impregnated in the artificial CEI layer played an important role in reducing the surface resistance of the cell by inhibiting electrolyte decomposition at the electrode/electrolyte interface. In addition, the Si-O functionalized artificial CEI layer reduced the F- concentration in the cell because the Si-O functional group chemically reacted with the F ^- species through a scavenging reaction to reduce the Ni dissolution action. As a result, since the surface stability of the NCM811 anode was improved by the formation of the Si-O functionalized artificial CEI layer, the cell cycled with the DODSi-functionalized NCM811 anode showed significantly improved cycling retention than the cell cycled with the untreated NCM811 at 55℃. .

1 is a diagram schematically showing the role of both the surface-modified NCM811 materials functionalized by DODSi.
2 shows SEM analysis of (a) untreated NCM811, (b) NCM811 modified with 1% DODSi, (c) NCM811 modified with 2% DODSi and (d) NCM811 modified with 10% DODSi, and (e) no It is an image showing the TEM analysis results of treated NCM811 and (f) NCM811 modified with 1% DODSi.
3 is a graph showing the results of FT-IR analysis of the NCM811 electrode modified with DODSi (black: NCM811 without treatment, red: NCM811 modified with 1% DODSi, blue: NCM811 modified with 2% DODSi, green: 10 % DODSi modified NCM811).
4 is a graph showing the XRD spectra of untreated NCM811 (black) and NCM811 (red) modified with 10% DODSi.
Fig. 5 shows (a) a potential profile of a cell in an initial state (line) and after 100 cycles (point); (b) cycling behavior of the cell at room temperature; (c) the potential profile of the cell at the initial state (line) and after 100 cycles (point); (d) a graph showing the cycling behavior of the cell at high temperature (black: NCM811 untreated, red: NCM811 modified with 1% DODSi, blue: NCM811 modified with 2% DODSi, green: NCM811 modified with 10% DODSi) to be.
6 is a CV (Cyclic voltammetry) graph of NCM811 untreated and NCM811 modified with 1% DODSi; (a) the first cycle of NCM811 modified with 1% DODSi and untreated NCM811 and (b) the second cycle of NCM811 modified with 1% DODSi and untreated NCM811.
7 is a graph showing EIS results in (a) 1 cycle and (b) 25 cycles (black: NCM811 and red: DODSi-NCM811) and analyzed by SEM (c) cycled NCM811 and (d) cycled This is an image of the surface form of DODSi-NCM811.
8 is a graph showing the XPS analysis results of cycled NCM811 (top) and NCM811 (bottom) modified with DODSi; (a) C1s, (b) F1s, (c) P2p and (d) Si2p.
9 is a graph showing the ICP-MS analysis results of lithium metal cycled with (a) untreated NCM811 and (b) NCM811 modified with 1% DODSi.

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

(1) Preparation of DODSi-functionalized NCM811 anode

The DODSi-functionalized NCM811 cathode material was prepared as follows. After dissolving DODSi (Sigma-Aldrich) in 50 mL of N-methyl-2-pyrrolidone (NMP, Sigma-Aldrich) at ambient temperature, 5 g of NCM811 was added to the solution. DODSi used 1, 2 and 10 wt% compared to NCM811. The resulting mixture was stirred for 1 hour and then filtered to recover the wet coated NCM811 material. The recovered NCM811 solid was heated from room temperature to 600° C. at a rate of 10° C./min and maintained under an air atmosphere for 3 hours.

The surface morphology of the NCM811 anode material functionalized with DODSi was measured using a field-emission scanning electron microscope (FESEM, JSM-7001F, JEOL) and a transmission electron microscope (TEM, TALOS F200X, FEI). The crystal structure of the prepared cathode material was investigated using an X-ray diffractometer (XRD, SmartLab, Rigaku) equipped with a single color Cu Kα ray (λ=1.54056Å). In addition, the chemical composition of the surface area of the NCM811 anode material functionalized with DODSi was analyzed by Fourier transform infrared spectroscopy (FT-IR, VERTEX 70, Bruker) in attenuated total reflection (ATR) mode.

(2) Manufacture of secondary battery including DODSi-functionalized NCM811 positive electrode and evaluation of electrochemical properties

For evaluation of the electrochemical performance, the NCM811 anode was prepared as follows. NCM811 positive electrode, poly(vinylidene fluoride) (PVDF, KF3000, Kureha) and carbon black (Super P) were mixed in the NMP in a ratio of 90:5:5 (wt%). The resulting mixture was coated on an Al current collector and dried in a vacuum oven at 120°C. The loading density of the positive electrode was about 12.5 mg cm ^-2 . Cycling performance is a mixture of ethylene carbonate (EC) and ethylmethyl carbonate (EMC) in each anode, Li metal anode, poly(ethylene) (PE) separator (Celgard), and electrolyte (1:2 (v/v)) PanaxEtec) was investigated using a 2032 coin cell consisting of 1M LiPF ₆ ). Cells were charged/discharged at a potential range of 3.0-4.3V (vs.Li/Li ⁺ ) at 0.1 C-rate for 2 cycles (formation phase) and they were 25°C or 55°C by charging and discharging units (WBCS3000, Wonatech). At 1.0 C-rate (180 mA g ^-1 ) (cycle step).

After cycling, the cell was disassembled and each cycled NCM811 anode was recovered from an argon filled glove box. The surface morphology of the cycled NCM811 anode was observed by SEM, and the chemical composition of the surface area was analyzed by X-ray photoelectron spectroscopy (XPS, K alpha, PHI 5000 versa Probe II) under N ₂ atmosphere. In addition, the transition metal component deposited on the Li metal cathode was quantified by inductively coupled plasma-mass spectrometry (ICP-MS, Bruker). Electrochemical impedance spectroscopy (EIS) analysis was performed with an AC signal at an amplitude of 10 mV in a frequency range of 1 MHz to 10 mHz using an electrochemical analyzer (Zive MP1, Wonatech).

Experimental example

(1) SEM analysis result of the surface morphology of DODSi-functionalized NCM811 anode

The surface morphology of various amounts of DODSi-functionalized NCM811 anode was analyzed by SEM (FIG. 2). The present inventors confirmed that the surface of the NCM811 anode treated with DODSi after surface modification was well coated with a new DODSi-based CEI layer. According to the TEM image (Figs. 2e-2f), a CEI layer based on about 5 nm was found on the surface of the NCM811 anode material. This indicates that the surface of the NCM811 anode material was altered after one-step functionalization using DODSi. In order to clarify the origin of this chemical change, the prepared NCM811 anode was analyzed by FT-IR spectroscopy (Fig. 3). Si-O vibration peaks observed at 1410 and 1470 cm ^-1 indicated that a silyl ether (Si-O) functional group was formed on the DODSi functionalized NCM811 anode surface. In particular, the bulk structure of the NCM811 cathode material was not affected by the heat treatment using DODSi. This was revealed by a comparison of the XRD patterns of NCM811 and DODSi-functionalized NCM811 materials showing a layered α-NaFeO ₂ structure with R-3m symmetry (Table 1).

[Table 1]

The lattice parameters (a, c) of the two materials and the intensity ratio of the (104) peak in (003) were almost the same, so it was confirmed that DODSi did not impair the structural stability of the NCM811 cathode material (FIG.

Therefore, only heat treatment using DODSi changed the surface characteristics of the NCM811 anode, not the bulk structure itself.

(2) Evaluation of electrochemical properties of DODSi-functionalized NCM811 anode

The electrochemical performance of the NCM811 positive electrode functionalized with DODSi was evaluated at 25° C. (FIGS. 5A and 5B ). In the first cycle, the DODSi functionalized NCM811 anode showed greater polarization than the NCM811 anode. This may be because a new CEI layer is formed on the NCM811 anode, which increases the surface resistance. Thus, the initial discharge-specific capacity (after the formation step) of the cell cycled with DODSi-functionalized NCM811 was slightly lower than that of the NCM811 anode (179.7 mA h-1) (1% DODSi: 181.1 mA hg ^-1 ; 2% DODSi: 176.4 mA hg ^-1 ; 10% DODSi: 172.4 mA hg ^-1 ). These results are in good agreement with the results of measuring the cyclic voltage current of the cell (Fig. 6). In particular, cells with DODSi-functionalized NCM811 exhibited a higher oxidation peak with respect to the first delithium reaction (3.87V) than those with NCM811 (3.80V). This is believed to be due to the increasing surface resistance in the first cycle. Interestingly, the electrochemical potential of the second delithiation reaction of DODSi-functionalized NCM811 decreased to 3.70V, which is similar to the results obtained for NCM811 (3.66V). These results indicate that the kinetic behavior on the NCM811 surface stabilized after the first activation step. As a result, all cells showed a similar cycle holding power of about 85% after 100 cycles, similar to one of the cells cycled with NCM811 at 25°C.

(3) Evaluation of electrochemical properties of DODSi-functionalized NCM811 anode at high temperature

In a high temperature environment of 55° C. (FIGS. 5C and 5D ), DODSi-functionalized NCM811 still showed slightly increased polarization in the first cycle. However, since the Li ⁺ migration of the cell is strongly promoted at the enhanced temperature, it is significantly reduced compared to cycling at room temperature. Also, the initial discharge specific capacity of the cell is similar to the tendency seen at room temperature. That is, it decreases with increasing DODSi content (NCM811: 209.6 mA h ^-1 ; 1% DODSi: 202.9 mA h ^-1 ; 2% DODSi: 201.6 mA h ^-1 ; 10% DODSi: 200.9 mA h ^-1 ). Considering the cycling performance, DODSi-functionalized NCM811 showed a significant improvement in cycling retention. More specifically, after 100 cycles at 55°C, the DODSi-functionalized NCM811 anode showed a retention rate of 71.8% (1% DODSi), 70.8% (2% DODSi) and 69.5% (10% DODSi), but the NCM811 was 47.1%. Was maintained only.

(4) EIS analysis results of DODSi-functionalized NCM811 anode after cycle

These results are consistent with those obtained in the EIS analysis (Figs. 7A and 7B). After the initial activation process (formation step), the surface resistance (RCEI: 3.0Ω and RCT: 37.3Ω) of the DODSi-functionalized NCM811 anode was slightly lower than that of NCM811 (RCEI: 5.1Ω and RCT: 44.3). After 25 cycles, the cell cycled with the DODSi-functionalized NCM811 anode showed a lower surface resistance with charge transfer resistance (RCEI: 4.6 Ω, RCT: 46.7 Ω), and the surface and charge transfer of the cell cycled with untreated NCM811. The resistance increased significantly (RCEI: 7.6 Ω, RCT: 84.1 Ω). In particular, as the number of cycles increases, decomposition products may continuously accumulate on the NCM811 surface, thereby increasing the internal resistance of the cell.

(5) SEM analysis results of DODSi-functionalized NCM811 anode after cycle

In general, the surface stability of the NCM811 cathode material tends to decrease significantly at elevated temperatures because the unstable Ni ⁴⁺ paper strongly accelerates the electrochemical decomposition of the electrolyte, and as a result, cell cycling performance rapidly deteriorates. The DODSi-functionalized NCM811 anode showed high cycle life, which means that these materials were effective in inhibiting electrolyte decomposition and thus exhibited better cycle holding power than at high temperatures. SEM analysis of the cycled NCM811 anode supported this hypothesis (Figs. 7c, d). The SEM image of the cycled NCM811 anode showed a distinct thick layer on the anode surface, due to the decomposed products of the electrolyte. This result indicates that the NCM811 material continuously decomposes the electrolyte during the electrochemical charging/discharging process, resulting in poor cycling performance at high temperatures. In contrast, the cycled DODSi-functionalized NCM811 anodes exhibited a relatively uniform and clean surface condition, indicating less electrolyte degradation in these cells.

(6) XPS analysis result of DODSi-functionalized NCM811 anode after cycle

XPS analysis of the cycled anode also demonstrated the effect of a DODSi-derived artificial layer on the NCM811 surface (Figure 8). In the C1s spectrum of Figure 8A, all cycled NCM811 anodes have similar chemical compositions, i.e. -CF (291.1eV), -C=O (289.1eV), RC=O)-R (287.7 eV), -CO- (286.4 eV). ), -CC- (285.0 eV). In particular, the formation of carbonyl (-C=O and RC(=O)-R) and ether (-CO-) functional groups is regarded as spectroscopic evidence for the electrolytic degradation that occurs, and this chemical part is due to electrochemical degradation. This may be due to the carbonate-based electrolyte component formed. In addition, the C1s spectrum indicated that the amount of carbon species derived from electrolyte decomposition was clearly higher in cycled NCM811 than in cycled DODSi-functionalized NCM811. This result is supported by additional XPS spectra (F1s and P2p). More specifically, in the F1s spectrum of FIG.8B, higher amounts of LiF (686.4 eV) and PVDF (688.3 eV), which can be considered as degraded adducts during the electrochemical cycle, were cycled in the cycled DODSi-functionalized NCM811. Found in NCM811. In addition, in the P2p spectrum, relatively high concentrations of LixPFy (138.1 eV) and LixPOyFz (135.7 eV) species were found in the cycled NCM811, which could be considered as additives from electrolytic degradation compared to the cycled DODSi-functionalized NCM811. These results indicate that the artificial CEI layer functionalized with DODSi greatly reduced the electrolyte decomposition on the NCM811 surface, resulting in improved interfacial stability and improved cell cycle performance.

Interestingly, the Si2p spectrum of the cycled DODSi-functionalized NCM811 anode provided an informative clue to assess the role of the DODSi-functionalized artificial CEI layer in FIG. 8D. In particular, the cycled DODSi-functionalized NCM811 showed a Si-O chemical composition (103.0 eV), which was considered the original chemical constituent of the CEI layer. In addition, a Si-F peak was observed at 103.9 eV, indicating that the initially induced Si-O functionalized artificial CEI layer not only inhibited electrolyte decomposition during the electrochemical cycle, but also selectively removed the scavenged F ^- species to remove Ni -rich promoted dissolution of the transition metal component from the cathode.

(7) ICP-MS analysis result of Li negative electrode recovered after cycle

Indeed, ICP-MS analysis of the recovered Li negative electrode supports this hypothesis (Fig. 9). Effective inhibition of electrolyte degradation at the NCM811/electrolyte interface by the DODSi-functionalized CEI layer reduces the formation of nucleophilic F ^- species that significantly promote dissolution of the Ni component from the NCM811 anode in the cell. Therefore, in the DODSi-functionalized NCM811 anode, the irreversible Ni dissolution reaction can be greatly suppressed. In particular, once Ni dissolution occurs, the component migrates to the cathode surface and is substantially reduced at the cathode surface. Therefore, quantifying the amount of Ni adsorbed on the Li cathode in the NCM811 and DODSi-functionalized NCM811 anodes after 100 cycles was an efficient way to compare the efficiency of DODSi-based surface modification. The inventors found that a significant amount of Ni was detected in the Li cathode cycled with NCM811 (129.7 ppm). This indicates that NCM811 was severely deformed by irreversibly dissolved Ni caused by electrolyte decomposition. In contrast, a significantly smaller amount of Ni was detected (95.4 ppm) in the Li cathode cycled with the DODSi-functionalized NCM811 anode. This indicated that the surface of NCM811 was protected by the DODSi-functionalized CEI layer during the electrochemical reaction, and as a result, the surface stability of the DODSi-functionalized NCM811 anode was improved, thereby improving the cell cycling performance.

Claims (11)

Positive electrode active material; And dimethoxydimethylsilane (DODSi) coated on the surface of the positive electrode active material,
The positive electrode active material is characterized in that it is represented by the following [Chemical Formula 1],
[Formula 1]
LiNi _a Co _b Mn _c O ₂ (0.6≤a≤0.9, a+b+c=1),
The dimethoxydimethylsilane is included in an amount of 1 wt% to 10 wt% relative to 100 wt% of the positive electrode active material,
The positive electrode active material coated on the surface of the dimethoxydimethylsilane (DODSi) is a positive electrode active material for a lithium secondary battery heat-treated at 400 ~ 800 ℃.
delete The method of claim 1,
The positive electrode active material is a positive active material for a lithium secondary battery, characterized in that LiNi _0.8 Co _0.1 Mn _0.1 O ₂ .
delete delete A positive electrode comprising the positive electrode active material of any one of claims 1 or 3;
Electrolyte; and
Lithium secondary battery comprising a negative electrode.
The method of claim 6,
The electrolyte is a lithium secondary battery, characterized in that 1.0 M LiPF ₆ is included in EC (ethylene carbonate): EMC (ethyl methyl carbonate) = 1:2.
Mixing dimethoxydimethylsilane and a positive electrode active material in an N-methyl-2-pyrrolidone (NMP) solvent to coat dimethoxydimethylsilane on the surface of the positive electrode active material; And
Including the step of heat-treating the coated positive active material,
The positive electrode active material is characterized in that it is represented by the following [Chemical Formula 1],
[Formula 1]
LiNi _a Co _b Mn _c O ₂ (0.6≤a≤0.9, a+b+c=1),
The dimethoxydimethylsilane is included in an amount of 1 wt% to 10 wt% relative to 100 wt% of the positive electrode active material,
The heat treatment method for manufacturing a positive electrode active material for a lithium secondary battery, characterized in that performed at 400 ~ 800 ℃.
delete The method of claim 8,
The heat treatment method for producing a positive electrode active material for a lithium secondary battery, characterized in that performed at 600 ℃.
delete

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