Enhancing Electrochemical Performance and Thermal Stability of Li[(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2]O2 through Core-Shell Structure Modification

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1. Original link

DOI Link:

https://pubs.acs.org/doi/full/10.1021/ja053675g

2.Corresponding Author:

• Google Scholar:

https://scholar.google.com/citations?user=aHUXXmwAAAAJ&hl=en&oi=ao

• Science Direct:

https://www.scopus.com/authid/detail.uri?authorId=59211172700

• University Web:

https://www.anl.gov/profile/khalil-amine

3. Inclusion Date

Received:

Accepted: 20 June 2005

Published: 31 August 2005

4. Research Content

1. Scientific Issues

• Improving the electrochemical performance and thermal stability of Li[(Ni₀.₈Co₀.₁Mn₀.₁)₀.₈(Ni₀.₅Mn₀.₅)₀.₂]O₂ through core-shell structure modification

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2. Experimental and Modeling Methods

2.1 Experimental Methods

1. Synthesis and chemical analysis of metal precursors

(1) Method: Co-precipitation method.

(2) Materials:

• Raw materials:NiSO₄·6H₂O, CoSO₄·7H₂O, MnSO₄·H₂O (ionic ratio Ni:Co:Mn = 8:1:1).

• Additives:2.0 mol/dm³ NaOH aqueous solution and NH₄OH aqueous solution (chelator).

(3) Reaction conditions:

• Under nitrogen protection, using a 4 L continuous stirring reactor (CSTR), control the solution concentration, pH value, temperature, and stirring speed.

(4) Core-shell structure synthesis:

• By adding NiSO₄·6H₂O and MnSO₄·H₂O (ionic ratio Ni:Mn = 1:1) to spherical Ni₀.₈Co₀.₁Mn₀.₁₂, prepare (Ni₀.₈Co₀.₁Mn₀.₁)₁₋ₓ(Ni₀.₅Mn₀.₅)ₓ₂.

(5) Subsequent treatment:

• After mixing with LiOH·H₂O, heat in air at 750 °C for 12 hours or 770 °C for 20 hours respectively.

• Analysis:Use atomic absorption spectroscopy (AAS) to analyze the chemical composition of the final compound.

2. Structural and morphological characterization

(1) XRD:

• Instrument:Rint-2000, Cu Kα radiation.

• Measurement range:2θ = 10−80°, step size 0.03°, count time 5 s.

• Data processing:Calculate lattice parameters using the least squares method.

(2) SEM:

• Instrument:JSM 6400, equipped withenergy dispersive spectrometer (EDS).

• Function:Observe powder morphology and element concentration.

(3) XPS:

• Instrument:PHI 5600.

• Function:Surface analysis and depth profiling (using Ar ion etching, rate 1.5 nm/min).

3. Electrochemical performance testing

(1) Electrode preparation:

• Material ratio:Active material: Carbon black: Polyvinylidene fluoride (PVDF) = 80:10:10.

• Process:Coat the slurry on aluminum foil, roll press at 120 °C and vacuum dry.

(2) Battery type:

• Preliminary test:2032 coin cell (negative electrode is lithium metal).

• Long-life test:Stacked full battery packaged in aluminum foil bag (capacity 120 mAh, negative electrode is intermediate phase carbon microspheres).

(3) Testing conditions:

• Electrolyte:1 M LiPF₆ (dissolved in a 1:1 volume ratio of ethylene carbonate and diethyl carbonate).

• Battery formation:Cycle three times at 0.1, 0.2, 0.5 C rates between 3.0−4.3 V, then cycle test at 1 C rate.

4. Differential scanning calorimetry (DSC)

(1) Sample preparation:

• Open the battery in an argon dry box after charging to 4.3 V.

• Remove the electrolyte and recover the positive electrode material.

(2) Experimental conditions:

• Instrument:Pyris 1 DSC.

• Sample amount:3–5 mg.

• Sealing:Stainless steel sealed pot, copper sealing ring plated with gold.

• Heating rate:1 °C/min.

Scheme. The substitute ISC fault experimental platform

3. Research Results

3.1 Core-Shell Structure Li[(Ni₀.₈Co₀.₁Mn₀.₁)₁₋ₓ(Ni₀.₅Mn₀.₅)ₓ]O₂ Synthesis and Characterization

1. Research Background and Issues

• Although Li[Ni₀.₅Mn₀.₅]O₂ has good structure and thermal stability, its poor rate performance may affect the electrochemical performance of Li[(Ni₀.₈Co₀.₁Mn₀.₁)₁₋ₓ(Ni₀.₅Mn₀.₅)ₓ]O₂ spherical core-shell powder.

• The research balances the stability and rate performance of the material by controlling the shell thickness at 1−1.5 μm.

2. Synthesis Method

(1) Core-shell structure synthesis (Fig. 1):

• First, synthesize Ni₀.₈Co₀.₁Mn₀.₁₂ through co-precipitation method, presenting a typical M(OH)₂ layered structure (Fig. 1a).

• Continuously add Ni and Mn solution on the surface of Ni₀.₈Co₀.₁Mn₀.₁₂ to synthesize Ni₀.₅Mn₀.₅₂ and form a shell.

• Comparison experiment:Independently prepare Ni₀.₅Mn₀.₅₂ to evaluate the performance differences of different structures.

3. Material Characterization

(1) Morphology and particle size (Fig. 2):

• SEM images show that Ni₀.₈Co₀.₁Mn₀.₁₂ has a spherical structure with an average particle size of about 12−13 μm (Fig. 2a, b).

(2) Crystal structure (Fig. 1b):

• XRD patterns show that (Ni₀.₈Co₀.₁Mn₀.₁)₁₋ₓ(Ni₀.₅Mn₀.₅)ₓ₂ has two diffraction peaks corresponding to Ni₀.₈Co₀.₁Mn₀.₁₂ and Ni₀.₅Mn₀.₅₂ phases.

4. Results and Significance

• Successfully prepared core-shell structured Li[(Ni₀.₈Co₀.₁Mn₀.₁)₁₋ₓ(Ni₀.₅Mn₀.₅)ₓ]O₂, providing an effective strategy to balance electrochemical performance and structural stability.

• XRD and SEM data of core-shell structure materials validate the accuracy of their design and synthesis.

Fig. 1 XRD patterns of as-prepared (a) pristine [Ni0.8Co0.1Mn0.1](OH)2, (b) [(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2](OH)2, and (c) [Ni0.5Mn0.5](OH)2. The synthesis of the hydroxide was carried out using coprecipitation method.

Fig. 2 SEM images: (a) low- and (b) high-magnification of pristine [Ni0.8Co0.1Mn0.1](OH)2; (c) low- and (d) high-magnification of core−shell [(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2](OH)2; cross-section images of (e) [Ni0.8Co0.1Mn0.1](OH)2 and (f) [(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2](OH)2.

3.2 Structure and Performance Characterization of Core-Shell Structure Li[(Ni₀.₈Co₀.₁Mn₀.₁)₀.₈(Ni₀.₅Mn₀.₅)₀.₂]O₂

1. Morphology and Composition of Core-Shell Structure (Fig. 2)

(1) Particle size and morphology:

• (Ni₀.₈Co₀.₁Mn₀.₁)₁₋ₓ(Ni₀.₅Mn₀.₅)ₓ₂ particles are spherical, with an average particle size of about 15 μm (Fig. 2c, d).

• The secondary particles of the core-shell structure Ni₀.₈Co₀.₁Mn₀.₁₂ are wrapped by Ni₀.₅Mn₀.₅₂, leading to an increase in particle size (Fig. 2f).

(2) Verification of core-shell structure:

• SEM images of partially fractured particles show that (Ni₀.₈Co₀.₁Mn₀.₁)₀.₈(Ni₀.₅Mn₀.₅)₀.₂₂ displays a clear core-shell interface, with a shell thickness of about 1 μm (Fig. 2f).

• Chemical analysis indicates that the composition of the core-shell material is Ni₀.₇₄Co₀.₀₈Mn₀.₁₈₂, which can be represented as (Ni₀.₈Co₀.₁Mn₀.₁)₀.₈(Ni₀.₅Mn₀.₅)₀.₂₂.

2. Crystal structure after high-temperature calcination (Fig. 3)

(1) Calcination conditions:

• The core-shell structure (Ni₀.₈Co₀.₁Mn₀.₁)₀.₈(Ni₀.₅Mn₀.₅)₀.₂₂ is calcined with lithium salt at 770 °C for 20 hours to obtain Li[(Ni₀.₈Co₀.₁Mn₀.₁)₀.₈(Ni₀.₅Mn₀.₅)₀.₂]O₂.

• Comparison sample Li[Ni₀.₈Co₀.₁Mn₀.₁]O₂ is prepared by calcining at 750 °C for 12 hours.

(2) XRD analysis:

• Both samples exhibit good layered structure with R3̄m space group (Fig. 3).

• The XRD peaks of core-shell structure Li[(Ni₀.₈Co₀.₁Mn₀.₁)₀.₈(Ni₀.₅Mn₀.₅)₀.₂]O₂ overlap with peaks of Li[Ni₀.₈Co₀.₁Mn₀.₁]O₂ and Li[Ni₀.₅Mn₀.₅]O₂, showing no secondary phase.

3. Lattice parameters and microstructure (Table 1)

• The lattice parameters of the core-shell structure are slightly larger than those of Li[Ni₀.₈Co₀.₁Mn₀.₁]O₂, reflecting the influence of the [Ni₀.₅Mn₀.₅]O₂ shell on the core.

• The (210) and (116) diffraction peaks of the core-shell material are less pronounced than those of Li[Ni₀.₈Co₀.₁Mn₀.₁]O₂, indicating minor structural differences exist in the core-shell structure.

Fig. 3 XRD patterns of (a) core−shell Li[(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2]O2 and (b) Li[Ni0.8Co0.1Mn0.1]O2 powders.

3.3 Stability and Element Diffusion Analysis of Core-Shell Structure after High-Temperature Calcination

1. Integrity of Core-Shell Structure (Fig. 4)

(1) Integrity of core-shell structure after calcination:

• The core-shell particles Li[(Ni₀.₈Co₀.₁Mn₀.₁)₀.₈(Ni₀.₅Mn₀.₅)₀.₂]O₂ remain intact after high-temperature calcination (Fig. 4b).

• The shell thickness is 1−1.5 μm, indicating that the core-shell structure remains intact after high-temperature calcination.

2. Element diffusion and concentration gradient analysis (Fig. 5)

(1) EDS result analysis:

• In the core region (5−15 μm), the abundance of Ni and Co elements is higher, while the Mn content is lower (Fig. 5b), consistent with the assumption that the core mainly consists of Li[Ni₀.₈Co₀.₁Mn₀.₁]O₂.

• The presence of a concentration gradient for Co indicates that Co diffuses from the core to the shell until equilibrium is reached at high temperatures.

3. Surface element distribution and diffusion mechanism (Fig. 6)

(1) XPS surface analysis:

• After calcination, the shell layer has a very low Co content, while Ni and Mn are significantly present (Fig. 6).

• As the Ar⁺ ion etching time increases (depth of about 150 nm), the signal intensity of Co and Mn increases, indicating that the diffusion of Co from the core to the shell is caused by high-temperature calcination.

(3) Changes in element valence states:

1. The oxidation states of transition metal elements in the core Li[Ni₀.₈Co₀.₁Mn₀.₁]O₂ are Ni³⁺, Co³⁺, and Mn³⁺.

2. In the shell Li[Ni₀.₅Mn₀.₅]O₂, Ni²⁺ and Mn⁴⁺ dominate, and calcination did not significantly cause Ni diffusion from the core to the shell or from the shell to the core.

3. Changes in Mn valence state:

• After 1 hour of etching (150 nm depth), the binding energy difference between Mn 2p₃/₂ and 2p₁/₂ decreases from 11.7 eV to about 9.4 eV.

• This indicates that the average oxidation state of Mn partially changes from +4 to +3, mainly due to the diffusion of trivalent Co from the core Li[Ni₀.₈Co₀.₁Mn₀.₁]O₂ to the shell.

• Therefore, a small amount of Co-doped Li[Ni₀.₅Mn₀.₅]O₂ is formed in the shell.

4. Summary of Chemical Composition of Core-Shell Structure

• Due to analysis difficulties, the detailed chemical composition of the core and shell of Li[(Ni₀.₈Co₀.₁Mn₀.₁)₀.₈(Ni₀.₅Mn₀.₅)₀.₂]O₂ cannot be clearly defined at present.

• The results indicate that the core-shell structure Li[(Ni₀.₈Co₀.₁Mn₀.₁)₀.₈(Ni₀.₅Mn₀.₅)₀.₂]O₂ consists of a Li[Ni₀.₅Mn₀.₅]O₂ shell with a Co concentration gradient and a Li[Ni₀.₈Co₀.₁Mn₀.₁]O₂ core.

Fig. 4 SEM images of (a) core−shell Li[(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2]O2 and (b) Li[Ni0.8Co0.1Mn0.1]O2 powders.

Fig. 5 The energy dispersive spectroscopic (EDS) image of the Li[(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2]O2 particle.

Fig. 6 X-ray photoelectron spectroscopy (XPS) data of the core−shell structured Li[(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2]O2 particles

3.4 Comparative Analysis of Electrochemical Performance of Core-Shell Materials

1. Initial Electrochemical Performance of Core-Shell Structure Materials and Comparison Materials (Fig. 7)

(1) Battery testing setup:

• Using Li as the negative electrode, Li[Ni₀.₈Co₀.₁Mn₀.₁]O₂ and Li[(Ni₀.₈Co₀.₁Mn₀.₁)₀.₈(Ni₀.₅Mn₀.₅)₀.₂]O₂ as positive electrodes for comparative electrochemical performance.

• Charge and discharge tests at a constant current density of 40 mA/g within a voltage range of 3.0−4.3 V.

(2) Initial capacity and efficiency:

• Li/Li[Ni₀.₈Co₀.₁Mn₀.₁]O₂ has an initial discharge specific capacity of 200 mAh/g, consistent with literature values.

• Li/Li[(Ni₀.₈Co₀.₁Mn₀.₁)₀.₈(Ni₀.₅Mn₀.₅)₀.₂]O₂ has a slightly lower initial specific capacity of 188 mAh/g.

• Both batteries have similar initial charge-discharge efficiencies of about 87−89%.

(3) Comparative analysis of core-shell structure:

• The presence of Li[Ni₀.₅Mn₀.₅]O₂ leads to a reduction in the total specific capacity of the core-shell material, as its specific capacity is typically around 150 mAh/g.

• The working voltage of the core-shell material during the charging process is 0.1 V higher than that of Li[Ni₀.₈Co₀.₁Mn₀.₁]O₂, and polarization in the low voltage region (<3.8 V) is larger.

2. Li⁺ Insertion/Extraction Mechanism of Core-Shell Structure

• Li⁺ extraction from the core-shell particles starts from the shell Li[Ni₀.₅Mn₀.₅]O₂, then extends to the core Li[Ni₀.₈Co₀.₁Mn₀.₁]O₂; the insertion process shows the opposite behavior.

• The larger polarization at low voltage is attributed to the high impedance of the shell material Li[Ni₀.₅Mn₀.₅]O₂, while the lower polarization at high voltage is related to the properties of the core material Li[Ni₀.₈Co₀.₁Mn₀.₁]O₂.

3. Long-Cycle Performance of Core-Shell Structure (Fig. 8)

(1) Testing method:

• Using a lithium-ion battery packaged in an aluminum bag with a capacity of about 120 mAh, and cycling at a rate of 1 C within a voltage range of 3.0−4.3 V for 500 times.

(2) Comparison of cycling performance:

a. C/Li[Ni₀.₈Co₀.₁Mn₀.₁]O₂ battery maintains a capacity retention of 81% after 500 cycles.

• During cycling, the initial discharge voltage gradually decreases, and the working voltage drops by about 0.2 V after 500 cycles.

• This performance decline is attributed to the structural instability of Li[Ni₀.₈Co₀.₁Mn₀.₁]O₂ and the dissolution of Co under HF corrosion.

b. C/Li[(Ni₀.₈Co₀.₁Mn₀.₁)₀.₈(Ni₀.₅Mn₀.₅)₀.₂]O₂ battery shows significantly improved cycling performance, with a capacity retention of up to 98% after 500 cycles.。

• The voltage difference during cycling is smaller, indicating more stable electrochemical performance.

(3) Role of the shell:

• The chemical stability of the shell Li[Ni₀.₅Mn₀.₅]O₂ enhances the cycling performance of the battery.

• The shell Li[Ni₀.₅Mn₀.₅]O₂ completely wraps the core Li[Ni₀.₈Co₀.₁Mn₀.₁]O₂, preventing the corrosion of the core material by HF in the electrolyte and the dissolution of Co.

• If there are portions of Li[Ni₀.₈Co₀.₁Mn₀.₁]O₂ that are not fully covered by the shell, the cycling stability will be significantly affected.

Fig. 7 The initial charge−discharge curves of Li/Li[Ni0.8Co0.1Mn0.1]O2 and Li/Li[(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2]O2 cell at the voltage range of 3.0−4.3 V

Fig. 8 (a) Continuous charge−discharge curves of C/Li[Ni0.8Co0.1Mn0.1]O2 cell; (b) continuous charge−discharge curves of C/Li[(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2]O2 cell; (C) corresponding discharge capacity versus cycling number.

3.5 Thermal Stability Analysis of Core-Shell Structure

1. Thermal stability testing (Fig. 9)

(1) Testing method:

• A differential scanning calorimetry (DSC) test was conducted on Li[(Ni₀.₈Co₀.₁Mn₀.₁)₀.₈(Ni₀.₅Mn₀.₅)₀.₂]O₂ electrode charged to 4.3 V.

(2) Starting temperature of thermal reaction:

• The exothermic peak starting temperature of Li[Ni₀.₈Co₀.₁Mn₀.₁]O₂ electrode is about 180 °C, with a sudden exothermic reaction occurring at 220 °C, releasing heat of 3285 J/g.

• The exothermic reaction starting temperature of Li[(Ni₀.₈Co₀.₁Mn₀.₁)₀.₈(Ni₀.₅Mn₀.₅)₀.₂]O₂ electrode is 250 °C, and the exothermic amount is 2261 J/g, which is reduced in comparison.

2. The effect of core-shell structure on thermal stability

(1) Thermal instability of Li[Ni₀.₈Co₀.₁Mn₀.₁]O₂:

• Li₁₋ₓNiO₂ material exhibits poor thermal stability due to the release of oxygen from the structure.

• As mentioned earlier, the exothermic temperature of Li[Ni₀.₅Mn₀.₅]O₂ is about 270−280 °C, and it has a lower thermal generation amount.

(2) Role of the shell:

• The outer layer Li[Ni₀.₅Mn₀.₅]O₂ shell has good thermal stability and can suppress the release of oxygen in Li₁₋ₓ[Ni₀.₈Co₀.₁Mn₀.₁]O₂, which is highly delithiated.

• This role significantly improves the thermal stability of the core-shell structure Li[(Ni₀.₈Co₀.₁Mn₀.₁)₀.₈(Ni₀.₅Mn₀.₅)₀.₂]O₂.

Fig. 9 Differential scanning calorimetry (DSC) traces of Li[(Ni0.8Co0.1Mn0.1)0.8(Ni0.5Mn0.5)0.2]O2 and Li[Ni0.8Co0.1Mn0.1]O2 at charged state to 4.3 V.

4. Important Conclusions

• Core-shell structure remains stable: After high-temperature calcination, the core-shell structure of Li[(Ni₀.₈Co₀.₁Mn₀.₁)₀.₈(Ni₀.₅Mn₀.₅)₀.₂]O₂ is maintained, although the shell layer experiences slight expansion, and Co diffuses from the core to the shell.

• Element diffusion and concentration gradient:Co diffuses from the core Li[Ni₀.₈Co₀.₁Mn₀.₁]O₂ to the shell during high-temperature calcination, resulting in a gradient in Co concentration on the surface and in the core.

• Electrochemical performance:Li[(Ni₀.₈Co₀.₁Mn₀.₁)₀.₈(Ni₀.₅Mn₀.₅)₀.₂]O₂ provides a capacity of about 188 mAh/g during the first charge-discharge cycle, slightly lower than Li[Ni₀.₈Co₀.₁Mn₀.₁]O₂, but exhibits higher working voltage and lower polarization, showing good cycling stability.

• Long cycling stability:Li[(Ni₀.₈Co₀.₁Mn₀.₁)₀.₈(Ni₀.₅Mn₀.₅)₀.₂]O₂ battery maintains a capacity retention of 98% after 500 charge-discharge cycles, compared to 81% for Li[Ni₀.₈Co₀.₁Mn₀.₁]O₂ battery.

• Improved thermal stability:The Li[Ni₀.₅Mn₀.₅]O₂ shell in the core-shell structure effectively suppresses oxygen release, enhancing the overall material’s thermal stability. Compared to Li[Ni₀.₈Co₀.₁Mn₀.₁]O₂ electrode, Li[(Ni₀.₈Co₀.₁Mn₀.₁)₀.₈(Ni₀.₅Mn₀.₅)₀.₂]O₂ electrode exhibits a higher starting temperature of thermal reaction and lower exothermic amount.

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