The escalating demand for high-performance energy storage systems, driven by the depletion of fossil fuels and the rapid growth of electric vehicles, has catalyzed significant advancements in battery technology. Among various energy storage devices, the lithium-ion battery stands out due to its superior cycle life, high energy density, and stability, making it the dominant power source for portable electronics and electric mobility. However, the commercial graphite anode, with a modest theoretical capacity of 372 mAh g-1, is increasingly becoming a bottleneck for achieving higher energy densities required for next-generation applications.
Transition metal oxides (TMOs) have emerged as promising alternative anode materials for lithium-ion batteries, offering substantially higher theoretical capacities (approximately 1000 mAh g-1), abundant valence states, and low cost. Zinc oxide (ZnO), in particular, is an attractive candidate with a high theoretical capacity of 978 mAh g-1, which originates from a combined conversion and alloying reaction mechanism:
Conversion Reaction: $$ \text{ZnO} + 2\text{Li}^+ + 2e^- \leftrightarrow \text{Zn} + \text{Li}_2\text{O} $$
Alloying Reaction: $$ \text{Zn} + \text{Li}^+ + e^- \leftrightarrow \text{LiZn} $$
This dual mechanism provides more lithium storage sites compared to the intercalation mechanism of graphite. Furthermore, ZnO offers a higher Li+ ion diffusion coefficient and is environmentally benign. Despite these advantages, the practical application of ZnO anodes is hampered by severe volume expansion during lithiation/delithiation cycles, leading to rapid capacity fading and poor structural integrity.
To address these intrinsic challenges, material engineering strategies such as nanostructuring, compositing, and carbon coating are widely employed. Constructing binary transition metal oxides can leverage synergistic effects between different cations to enhance electrical conductivity and create more electroactive sites. Introducing a conductive carbon matrix, especially one doped with heteroatoms like nitrogen, is a highly effective strategy. The carbon coating acts as a mechanical buffer to accommodate volume changes, while nitrogen doping (creating pyridinic N, pyrrolic N, and graphitic N) can significantly increase surface active sites, improve wettability with the electrolyte, and enhance electronic conductivity, thereby accelerating charge transfer kinetics.
In this work, we present a facile and integrated synthesis strategy to fabricate nitrogen-doped carbon-coated ZnO/CuO microspheres (ZnO/CuO/N-C) as a high-performance anode for lithium-ion batteries. This approach simultaneously achieves three critical goals: (1) in-situ conversion of the active material, (2) creation of an internal porous structure, and (3) formation of a conductive, N-doped carbon protective layer.
Synthesis Strategy and Material Characterization
The synthesis involves a templating method using Cu-doped ZnSe microspheres, followed by polydopamine coating and a one-step calcination process. The detailed procedure is summarized below.
| Step | Process | Purpose |
|---|---|---|
| 1 | Hydrothermal synthesis of Cu-ZnSe microspheres. | To form a spherical template with a homogeneous distribution of Cu and Zn. |
| 2 | Polydopamine (PDA) coating on Cu-ZnSe. | To provide a carbon and nitrogen source for the subsequent coating layer. |
| 3 | Calcination in N2 atmosphere at 400°C. | To (i) convert Cu-ZnSe to ZnO/CuO, (ii) remove Se to create internal pores, and (iii) carbonize PDA to form an N-doped carbon shell. |
For comparison, bare ZnO/CuO was obtained by calcining Cu-ZnSe in air at 700°C, and the pristine Cu-ZnSe was also tested.
X-ray diffraction (XRD) analysis confirmed the complete phase transformation. The diffraction peaks for the final product were indexed to hexagonal ZnO (PDF#36-1451) and monoclinic CuO (PDF#44-0706), with no residual ZnSe phases detected. The crystallite size, calculated using the Scherrer equation, decreased significantly for the carbon-coated sample, indicating that the N-C coating process refined the crystal grains.
$$ D = \frac{K \lambda}{\beta \cos\theta} $$
where \(D\) is the crystallite size, \(K\) is the shape factor (0.9), \(\lambda\) is the X-ray wavelength, \(\beta\) is the full width at half maximum, and \(\theta\) is the Bragg angle. Smaller crystallites create more grain boundaries, which can facilitate faster Li+ ion transport.
X-ray photoelectron spectroscopy (XPS) was employed to investigate the surface chemical composition and states. The survey spectrum confirmed the presence of Zn, Cu, O, N, and C, with no detectable Se signal, verifying its complete removal. The high-resolution N 1s spectrum could be deconvoluted into three components, as summarized below.
| Binding Energy (eV) | Assignment | Function | Atomic % |
|---|---|---|---|
| 398.4 | Pyridinic N | Provides active sites for Li+ adsorption. | ~45% |
| 400.0 | Pyrrolic N | Enhances surface wettability and offers redox-active sites. | ~49% |
| 401.0 | Graphitic N | Improves overall electronic conductivity. | ~6% |
The dominance of pyridinic and pyrrolic N (totaling ~94%) is particularly beneficial for providing numerous electrochemically active sites and promoting rapid charge transfer, which is crucial for the performance of the lithium-ion battery anode.
Scanning electron microscopy (SEM) images revealed that the spherical morphology of the initial Cu-ZnSe template (≈3 μm in diameter) was well preserved after the N-C coating and conversion process. The primary particles on the surface became smaller (≈100 nm) due to the polydopamine coating. In contrast, the bare ZnO/CuO sample also showed a spherical structure but with visible surface pores formed by Se evaporation. Transmission electron microscopy (TEM) confirmed the core-shell structure, with a uniform N-doped carbon layer of about 50 nm thickness encapsulating the ZnO/CuO core. High-resolution TEM (HRTEM) showed lattice fringes corresponding to the (103) plane of ZnO and the (002) plane of CuO.
Raman spectroscopy further confirmed the nature of the carbon layer. The intensity ratio of the D band (disordered carbon, ~1366 cm-1) to the G band (graphitic carbon, ~1589 cm-1), ID/IG, was approximately 2.12. This high value indicates a highly disordered carbon structure with abundant defects, which can serve as additional active sites for lithium storage and contribute to pseudocapacitive behavior, enhancing the rate capability of the li ion battery anode.
Nitrogen adsorption-desorption measurements showed that the specific surface area of ZnO/CuO/N-C (7.01 m2 g-1) was lower than that of bare ZnO/CuO (7.57 m2 g-1) but significantly higher than the template Cu-ZnSe (1.93 m2 g-1). The reduced surface area for the coated sample is attributed to some pore-filling by the N-doped carbon, but the retained mesoporosity is still beneficial for electrolyte infiltration.
Electrochemical Performance in Lithium-ion Battery
The electrochemical properties of the ZnO/CuO/N-C microspheres as an anode for lithium-ion batteries were systematically evaluated using coin-type half-cells with metallic lithium as the counter electrode.
Cyclic voltammetry (CV) curves at a scan rate of 1 mV s-1 revealed the complex redox processes involved. The initial cathodic scan showed an irreversible peak around 0.3 V, corresponding to the formation of a solid electrolyte interphase (SEI) layer. In subsequent cycles, highly overlapping CV curves indicated excellent electrochemical reversibility. The redox peaks were assigned to the stepwise conversion and alloying reactions of CuO and ZnO, as described in the introduction.
Galvanostatic charge-discharge profiles at 0.1 A g-1 showed an initial discharge capacity of 692.7 mAh g-1 and a charge capacity of 356.6 mAh g-1, yielding a first-cycle Coulombic efficiency (CE) of 51.4%. The low initial CE is common for conversion-type anodes due to SEI formation and irreversible phase changes. Remarkably, the CE quickly increased and stabilized above 98.6% after a few cycles. The discharge plateaus gradually diminished with cycling, which is associated with the activation process where more active sites are exposed, leading to a continuous increase in capacity.
The long-term cycling stability is a critical metric for any practical lithium-ion battery anode. As shown in the comparative analysis below, the ZnO/CuO/N-C electrode exhibited superior performance.
| Material | Current Density (A g-1) | Cycle Number | Discharge Capacity (mAh g-1) | Capacity Retention / Trend |
|---|---|---|---|---|
| ZnO/CuO/N-C | 0.1 | 200 | 1010.4 | Increases to 146% of initial capacity |
| ZnO/CuO (Bare) | 0.1 | 200 | ~300 (est.) | Continuous decay |
| Cu-ZnSe | 0.1 | 200 | ~200 (est.) | Rapid decay |
| ZnO/CuO/N-C | 1.0 | 500 | 385.0 | Stable |
| ZnO/CuO/N-C | 1.0 | 1000 | 447.1 | Stable, even increases |
The exceptional performance of the ZnO/CuO/N-C anode, especially its capacity increase during cycling, can be attributed to the progressive activation of the material and the excellent structural stability imparted by the N-doped carbon coating. The coating effectively buffers the volume changes of the ZnO/CuO core, preventing pulverization and maintaining electrical contact throughout extended cycling in the lithium-ion battery.
The rate capability was evaluated by testing the electrode at progressively higher current densities. The ZnO/CuO/N-C anode delivered discharge capacities of 409.9, 334.2, 277.0, and 190.9 mAh g-1 at 0.1, 0.2, 0.5, and 1.0 A g-1, respectively. When the current density was returned to 0.1 A g-1, the capacity recovered to 401.2 mAh g-1, demonstrating outstanding reversibility and structural robustness. In contrast, the bare ZnO/CuO and Cu-ZnSe electrodes showed much poorer rate performance and irreversible capacity loss.
Kinetic Analysis and Storage Mechanism
To understand the superior rate performance, we analyzed the electrochemical kinetics through CV measurements at various scan rates (\(v\)). The current (\(i\)) response at a specific potential obeys a power-law relationship:
$$ i = a v^b $$
where \(a\) and \(b\) are adjustable parameters. The \(b\)-value, determined from the slope of \(\log(i)\) vs. \(\log(v)\), indicates the charge storage mechanism. A \(b\)-value of 0.5 suggests a diffusion-controlled process (battery-type behavior), while a value of 1.0 signifies a surface-controlled capacitive process. For the ZnO/CuO/N-C anode, the \(b\)-values for the main redox peaks were calculated to be 0.91 and 0.77, indicating that the charge storage is dominated by capacitive processes. This pseudocapacitive contribution is highly beneficial for achieving fast charge/discharge rates in a li ion battery.
The total current at a fixed potential can be further partitioned into capacitive (\(k_1 v\)) and diffusion-controlled (\(k_2 v^{1/2}\)) contributions using the equation:
$$ i(v) = k_1 v + k_2 v^{1/2} $$
Quantitative analysis revealed that the capacitive contribution increased with scan rate, reaching approximately 90% at 1 mV s-1. This high pseudocapacitive ratio is a direct consequence of the N-doped carbon coating and the nanoscale structure, which provide a large number of surface and near-surface active sites for rapid Li+ ion storage without deep solid-state diffusion limitations.
Electrochemical impedance spectroscopy (EIS) was performed to investigate the charge transfer resistance. The Nyquist plots consisted of a depressed semicircle in the high-medium frequency region (related to charge transfer resistance, \(R_{ct}\)) and a sloping line in the low-frequency region (related to Li+ ion diffusion). The fitted \(R_{ct}\) values for ZnO/CuO/N-C, bare ZnO/CuO, and Cu-ZnSe were 152 Ω, 217 Ω, and 375 Ω, respectively. The significantly lower \(R_{ct}\) for the N-C coated sample underscores the role of the conductive N-doped carbon network in facilitating faster charge transfer kinetics at the electrode/electrolyte interface.
The Li+ ion diffusion coefficient (\(D_{Li^+}\)) can be estimated from the low-frequency Warburg region using the formula:
$$ D_{Li^+} = \frac{R^2 T^2}{2 A^2 n^4 F^4 C^2 \sigma^2} $$
where \(R\) is the gas constant, \(T\) is the absolute temperature, \(A\) is the electrode area, \(n\) is the number of electrons per reaction, \(F\) is Faraday’s constant, \(C\) is the concentration of Li+ ions, and \(\sigma\) is the Warburg coefficient obtained from the slope of \(Z’\) vs. \(\omega^{-1/2}\). The calculated \(D_{Li^+}\) for ZnO/CuO/N-C was higher than those for the other two samples, confirming enhanced ion transport kinetics, which is vital for the high-rate performance of the lithium-ion battery anode.
Conclusion
In summary, we have successfully developed a novel and efficient strategy to synthesize nitrogen-doped carbon-coated ZnO/CuO microspheres as a high-performance anode material for lithium-ion batteries. This integrated approach, involving templating, polydopamine coating, and one-step calcination, concurrently achieves the in-situ generation of active ZnO/CuO, the creation of an internal porous architecture, and the formation of a protective, conductive N-doped carbon shell.
The synergistic effects of this unique structure deliver outstanding electrochemical properties: a high reversible capacity of 1010.4 mAh g-1 at 0.1 A g-1 after 200 cycles, exceptional long-term stability with a capacity of 447.1 mAh g-1 at 1 A g-1 after 1000 cycles, and remarkable rate capability. The performance enhancement is attributed to multiple factors: the N-doped carbon coating effectively accommodates volume strain and maintains electrical connectivity; the internal porosity facilitates electrolyte penetration; the dual metal oxides (ZnO and CuO) provide high capacity through conversion and alloying reactions; and the nitrogen doping introduces abundant active sites and improves conductivity, leading to a dominant pseudocapacitive charge storage mechanism.
This work not only presents a superior anode material but also provides a versatile and scalable design principle for developing other high-performance, conversion-type electrode materials for advanced energy storage systems, particularly for the next generation of high-energy-density lithium-ion batteries.