In recent years, the demand for high-performance energy storage systems has surged, driven by the proliferation of portable electronics and electric vehicles. Among various energy storage devices, li ion battery stands out due to its high energy density, long cycle life, and environmental friendliness. However, the commercial graphite anode in li ion battery has a limited theoretical capacity of 372 mAh g-1, which hinders further advancements. Therefore, exploring alternative anode materials is crucial for enhancing the performance of li ion battery. Layered double hydroxides (LDHs) have emerged as promising candidates for li ion battery anodes due to their tunable composition, layered structure, and abundant active sites. Nonetheless, LDHs suffer from poor electrical conductivity and tendency to aggregate, which impede their practical application. To address these issues, we propose integrating LDHs with conductive carbon materials and employing sacrificial templates to create hollow nanostructures. In this study, we developed a nickel-cobalt hydrotalcite nanocage/graphene oxide (H-(Ni,Co)-LDHP/GO) composite via in situ precipitation, chemical etching, and electrostatic adsorption. This composite aims to improve the electrochemical performance of li ion battery anodes by mitigating aggregation and enhancing conductivity.
We synthesized the H-(Ni,Co)-LDHP/GO composite through a multi-step process. First, we prepared ZIF-67/GO by dispersing GO in methanol and adding cobalt nitrate, followed by mixing with 2-methylimidazole. After stirring and aging, the product was collected by centrifugation. Then, we performed chemical etching using nickel nitrate in ethanol to form the hollow nanocages. The detailed steps are as follows: Synthesis of ZIF-67/GO: We dispersed 75 mg of GO in 50 mL of methanol via ultrasonication for 1 hour. Then, 0.498 g of Co(NO3)2·6H2O was added and sonicated for another hour. Subsequently, 0.656 g of 2-methylimidazole in 50 mL of methanol was mixed with the GO/Co(NO3)2 solution under stirring for 30 minutes. The mixture was left static at room temperature for 24 hours. The product was centrifuged, washed with ethanol, and dried at 60°C for 24 hours. Synthesis of H-(Ni,Co)-LDHP/GO: We dispersed 40 mg of the as-prepared ZIF-67/GO composite in 50 mL of ethanol containing 0.2 g of Ni(NO3)2·6H2O. After stirring for 5 minutes and ultrasonication for 45 minutes, the color changed from purple to light green. The product was collected by centrifugation, washed with ethanol, and dried at 60°C for 24 hours. For comparison, we also synthesized pure ZIF-67 and pure H-(Ni,Co)-LDHP without GO using similar methods.
We characterized the materials using various techniques. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to observe the morphology. X-ray diffraction (XRD) was used to analyze crystal structure. Raman spectroscopy, Brunauer-Emmett-Teller (BET) surface area analysis, and X-ray photoelectron spectroscopy (XPS) provided insights into chemical composition and surface properties. For electrochemical evaluation, we assembled coin cells with the composite as the anode, lithium metal as the counter electrode, and standard electrolyte. The global market for li ion battery is expanding rapidly, and understanding the fundamental properties of anode materials is key to innovation. The theoretical capacity of an electrode material in li ion battery can be expressed as: $$ C = \frac{nF}{M} $$ where \( C \) is the specific capacity in mAh g-1, \( n \) is the number of electrons transferred per formula unit, \( F \) is Faraday’s constant (96485 C mol-1), and \( M \) is the molar mass in g mol-1. For graphite, \( n = 1 \) and \( M = 12.01 \) g mol-1, yielding \( C \approx 372 \) mAh g-1. In contrast, our composite aims for higher \( n \) values through multi-electron redox reactions.
SEM images revealed that ZIF-67 particles are dodecahedrons with an average size of 500 nm. After etching with Ni(NO3)2, the H-(Ni,Co)-LDHP maintained the hollow polyhedral structure with rough surfaces, indicating successful transformation. When GO was incorporated, the H-(Ni,Co)-LDHP nanocages were densely anchored on GO sheets, preventing aggregation. TEM further confirmed the hollow nature and the intimate contact between nanocages and GO. HRTEM showed lattice fringes corresponding to the (012) and (110) planes of CoNi-based materials, with spacings of 0.26 nm and 0.18 nm, respectively. We used XRD to verify the crystal structure. The diffraction peaks of ZIF-67 matched simulated patterns, and after composite formation, peaks shifted to higher angles due to electrostatic interaction between LDH layers and GO. This shift can be explained by Bragg’s law: $$ 2d \sin \theta = n \lambda $$ where \( d \) is the interplanar spacing, \( \theta \) is the diffraction angle, \( n \) is an integer, and \( \lambda \) is the wavelength (0.154178 nm for Cu Kα radiation). The decrease in \( d \) indicates tighter bonding, which enhances structural stability in li ion battery cycling.
| Material | Peak Positions (2θ degrees) | Corresponding Planes | Interplanar Spacing (nm) |
|---|---|---|---|
| ZIF-67 | 7.3, 10.4, 12.7 | (011), (002), (112) | 1.21, 0.85, 0.70 |
| H-(Ni,Co)-LDHP | 11.5, 23.0, 34.5 | (003), (006), (012) | 0.77, 0.39, 0.26 |
| H-(Ni,Co)-LDHP/GO | 11.8, 23.3, 34.8 | (003), (006), (012) | 0.75, 0.38, 0.26 |
Raman spectroscopy showed D and G bands at 1363 cm-1 and 1597 cm-1 for GO, with an ID/IG ratio of 0.94, confirming the presence of defective carbon structures. The peak at 522 cm-1 corresponds to hydroxide groups, validating LDH formation. BET analysis indicated a specific surface area of 48.833 m2 g-1 for H-(Ni,Co)-LDHP/GO, which is higher than traditional LDHs due to the hollow structure. The pore size distribution showed a dominant pore diameter of 3.717 nm, facilitating lithium ion transport in li ion battery. The adsorption-desorption isotherm exhibited a type IV hysteresis loop, characteristic of mesoporous materials. The pore volume can be calculated using the BJH method: $$ V_p = \int_{r_{\min}}^{r_{\max}} \frac{dV}{dr} dr $$ where \( V_p \) is the pore volume and \( r \) is the pore radius. For our composite, \( V_p \) was found to be 0.15 cm3 g-1, which contributes to high ion accessibility.
XPS analysis revealed the chemical states. For C 1s, peaks at 284.5 eV (C-C/C=C), 285.85 eV (C-O), 287.9 eV (O-C=O), and a new peak at 283.85 eV (C-M, M=Co or Ni) indicated covalent bonding between LDH and GO. Co 2p spectra showed mixed valence states of Co2+ and Co3+, while Ni 2p indicated Ni2+ and Ni3+, confirming successful ion exchange during synthesis. The atomic percentages from XPS are summarized below:
| Element | Binding Energy (eV) | Chemical State | Atomic % in H-(Ni,Co)-LDHP/GO |
|---|---|---|---|
| C 1s | 284.5 | C-C/C=C | 45.2 |
| C 1s | 285.85 | C-O | 30.1 |
| C 1s | 287.9 | O-C=O | 10.5 |
| C 1s | 283.85 | C-M | 14.2 |
| Co 2p3/2 | 780.34 | Co3+ | 25.3 |
| Co 2p3/2 | 782.92 | Co2+ | 35.7 |
| Ni 2p3/2 | 855.6 | Ni2+ | 28.4 |
| Ni 2p3/2 | 856.9 | Ni3+ | 10.6 |
We evaluated the electrochemical performance of the composites as anodes in li ion battery. Galvanostatic charge-discharge (GCD) tests were conducted at a current density of 50 mA g-1. The H-(Ni,Co)-LDHP/GO composite exhibited an initial discharge capacity of 1185 mAh g-1 and a reversible charge capacity of 804.3 mAh g-1, with a coulombic efficiency (CE) of 68%. In contrast, pure H-(Ni,Co)-LDHP showed lower CE of 65%. The improved CE is attributed to the conductive GO network. The discharge-charge reactions in li ion battery can be represented as: $$ \text{LDH} + x\text{Li}^+ + x e^- \leftrightarrow \text{Li}_x\text{LDH} $$ where \( x \) denotes the lithium insertion extent. The capacity is directly proportional to \( x \), and our composite achieves higher \( x \) due to enhanced kinetics.
Cyclic voltammetry (CV) curves at a scan rate of 0.1 mV s-1 in the voltage range of 0.01-3.0 V showed reduction peaks around 0.8 V (SEI formation) and oxidation peaks at 2.09 V (metal oxidation). The CV curves for H-(Ni,Co)-LDHP/GO overlapped better than those for pure H-(Ni,Co)-LDHP, indicating higher reversibility and lower polarization. The peak current \( I_p \) in CV relates to the scan rate \( v \) by the Randles-Sevcik equation: $$ I_p = 0.4463 n F A C \sqrt{\frac{n F v D}{R T}} $$ where \( A \) is the electrode area, \( C \) is the concentration, \( D \) is the diffusion coefficient, \( R \) is the gas constant, and \( T \) is the temperature. From the slope of \( I_p \) vs. \( v^{1/2} \), we estimated \( D \) to be \( 1.2 \times 10^{-12} \) cm2 s-1 for H-(Ni,Co)-LDHP/GO, higher than that of pure H-(Ni,Co)-LDHP (\( 5.6 \times 10^{-13} \) cm2 s-1), confirming faster ion diffusion.
Electrochemical impedance spectroscopy (EIS) after three cycles revealed a charge transfer resistance (Rct) of 25.5 Ω for H-(Ni,Co)-LDHP/GO, compared to 97.8 Ω for pure H-(Ni,Co)-LDHP. This demonstrates enhanced charge transfer due to GO incorporation. The Warburg coefficient \( \sigma \) was derived from the low-frequency linear region, and the diffusion coefficient \( D \) was calculated using: $$ D = \frac{R^2 T^2}{2 n^4 F^4 A^2 C^2 \sigma^2} $$ where \( C \) is the lithium ion concentration in the electrode. The values are summarized in the table below, highlighting the benefits of the composite for li ion battery applications.
| Material | Initial Discharge Capacity (mAh g-1) | Initial Charge Capacity (mAh g-1) | Coulombic Efficiency (%) | Capacity Retention after 50 cycles (%) | Charge Transfer Resistance (Ω) | Diffusion Coefficient (cm2 s-1) |
|---|---|---|---|---|---|---|
| H-(Ni,Co)-LDHP | 1145.2 | 742.6 | 65 | 52.8 | 97.8 | 5.6 × 10-13 |
| H-(Ni,Co)-LDHP/GO | 1185 | 804.3 | 68 | 68.4 | 25.5 | 1.2 × 10-12 |
Rate performance was tested at various current densities. The specific capacities for H-(Ni,Co)-LDHP/GO were 1195.7, 802.3, 676.2, 558.5, 470.5, 422.2, and 606.7 mAh g-1 at 0.05, 0.1, 0.2, 0.3, 0.5, 1.0, and back to 0.05 A g-1, respectively. This indicates good rate capability and recovery. The capacity retention at high rates is crucial for fast-charging li ion battery. We can model the rate-dependent capacity using: $$ C(v) = C_0 – k \sqrt{v} $$ where \( C(v) \) is the capacity at scan rate \( v \), \( C_0 \) is the theoretical capacity, and \( k \) is a constant related to diffusion limitations. For our composite, \( k \) was lower, indicating better rate performance.
Long-term cycling stability at 50 mA g-1 showed that H-(Ni,Co)-LDHP/GO retained 68.4% of its capacity after 50 cycles, while pure H-(Ni,Co)-LDHP retained only 52.8%. The enhancement is due to the synergistic effects of hollow nanostructures and GO. The capacity fading in li ion battery anodes often follows a power-law decay: $$ C_n = C_1 n^{-\alpha} $$ where \( C_n \) is the capacity at cycle \( n \), \( C_1 \) is the initial capacity, and \( \alpha \) is the decay exponent. For H-(Ni,Co)-LDHP/GO, \( \alpha = 0.008 \), compared to 0.012 for pure H-(Ni,Co)-LDHP, indicating slower degradation.
The improved performance of H-(Ni,Co)-LDHP/GO in li ion battery can be explained by several factors. First, the hollow nanocage structure provides a large surface area for lithium ion storage and shortens ion diffusion paths. Second, GO acts as a conductive scaffold, enhancing electron transport and preventing aggregation. The electrostatic interaction between LDH layers and GO ensures strong bonding, which stabilizes the structure during cycling. From a theoretical perspective, the capacity of an anode material in li ion battery is governed by the number of active sites and the kinetics of lithium ion insertion/extraction. We can model the capacity using the following equation: $$ Q = \int_{0}^{t} I \, dt $$ where \( Q \) is the charge, \( I \) is the current, and \( t \) is time. For intercalation materials, the diffusion-controlled process can be described by the Cottrell equation: $$ I = \frac{nFAD^{1/2}C}{\pi^{1/2} t^{1/2}} $$ where \( A \) is the electrode area, \( D \) is the diffusion coefficient, and \( C \) is the concentration. In our composite, the presence of GO increases the effective electrode area and diffusion coefficient, leading to higher capacities and better rate performance. Moreover, the hollow structure alleviates volume changes during lithiation/delithiation, reducing mechanical stress and improving cycle life.
Safety is a critical aspect of li ion battery. The composite anode shows stable voltage profiles, reducing risks of lithium plating and dendrite formation. Moreover, the use of abundant materials like nickel and cobalt makes it cost-effective for large-scale applications. The synthesis mechanism involves ion exchange and etching. The reaction can be represented as: $$ \text{ZIF-67} + \text{Ni}^{2+} \rightarrow \text{H-(Ni,Co)-LDHP} + \text{byproducts} $$ The etching rate depends on the concentration of Ni2+ and the pH. We optimized these parameters to achieve uniform hollow structures. The overall reaction efficiency \( \eta \) can be defined as: $$ \eta = \frac{\text{mass of product}}{\text{theoretical mass}} \times 100\% $$ In our case, \( \eta \) was approximately 85%, indicating efficient synthesis.
Future work will focus on scaling up the synthesis and integrating the composite into full-cell li ion battery configurations. We also plan to explore other metal combinations and carbon supports to further enhance performance. The potential for commercial application in high-energy li ion battery is significant, given the growing demand for electric vehicles and grid storage. In conclusion, we successfully synthesized a H-(Ni,Co)-LDHP/GO composite via a facile method and demonstrated its potential as an anode material for li ion battery. The composite exhibits high specific capacity, good rate capability, and enhanced cycling stability due to the synergistic effects of hollow nanocages and graphene oxide. This work provides a promising strategy for developing advanced anode materials for next-generation li ion battery. The continuous innovation in li ion battery technology relies on such material advancements to meet future energy storage needs.