The relentless depletion of fossil fuel reserves and the rapid advancement of the electric vehicle industry have created an urgent, global demand for efficient and high-capacity energy storage systems. Among the various technologies, the li ion battery stands out due to its superior energy density, long cycle life, and established manufacturing infrastructure, making it the dominant power source for portable electronics and electric mobility. However, the performance ceiling of conventional graphite anodes (theoretical capacity ~374 mAh g⁻¹) is increasingly becoming a bottleneck for next-generation applications requiring higher energy and power densities.
Transition metal oxides (TMOs) have emerged as compelling alternatives to graphite for li ion battery anodes. They offer significantly higher theoretical specific capacities (often approaching or exceeding 1000 mAh g⁻¹) through conversion and/or alloying reaction mechanisms, as opposed to the intercalation mechanism of graphite. Zinc oxide (ZnO), in particular, is an attractive candidate with a high theoretical capacity of 978 mAh g⁻¹, environmental benignity, and natural abundance. Its lithium storage involves a two-step process: a conversion reaction (ZnO + 2Li⁺ + 2e⁻ → Zn + Li₂O) followed by an alloying reaction (Zn + Li⁺ + e⁻ ↔ LiZn). This dual mechanism contributes to its high capacity potential. However, like most TMOs, ZnO suffers from severe intrinsic drawbacks that hinder its practical application in li ion batteries. These include drastic volume expansion/shrinkage during lithiation/delithiation, which pulverizes the electrode structure and leads to rapid capacity fading, as well as poor intrinsic electronic conductivity, which limits rate capability.
To overcome these challenges, material engineering strategies focusing on nanostructuring, composite formation, and conductive coating are essential. Constructing binary metal oxide composites, such as combining ZnO with another TMO like CuO, can create synergistic effects. The different metal cations can provide richer redox chemistry, potentially enhance electrical conductivity through the formation of more conductive phases (e.g., metallic Cu), and buffer volume changes more effectively than a single component. Furthermore, encapsulating active materials within a conductive carbon matrix is a highly effective strategy. The carbon coating not only improves the overall electronic conductivity of the electrode but also acts as a flexible mechanical buffer to accommodate volume strain, preserving the structural integrity of the active material over many cycles. Introducing heteroatoms like nitrogen into the carbon coating can further enhance its properties. N-doping creates defects and active sites (such as pyridinic and pyrrolic N) on the carbon surface, which can improve wettability with the electrolyte, facilitate faster Li⁺ ion adsorption/desorption, and boost charge transfer kinetics, all critical for high-performance li ion battery operation.
In this work, we report a rational and integrated synthesis strategy to fabricate N-doped carbon-coated ZnO/CuO microspheres (denoted as ZnO/CuO/N-C) as an advanced anode material for li ion batteries. Our approach employs Cu-doped ZnSe microspheres as a self-sacrificing template. A dopamine hydrochloride coating is subsequently polymerized on the template surface, followed by a one-step pyrolysis treatment under an inert atmosphere. This ingenious process simultaneously accomplishes three key objectives: (1) In-situ conversion of the active material: The ZnSe template is thermally oxidized and transformed into a porous ZnO/CuO composite. The evaporation of Se species during calcination creates intrinsic pores and channels within the microspheres. (2) Construction of an internal porous network: The generated pores enhance electrolyte infiltration and provide short diffusion paths for Li⁺ ions. (3) Formation of a conformal, N-doped carbon shell: The pyrolyzed dopamine forms a uniform, nitrogen-rich amorphous carbon layer coating the microspheres, ensuring excellent electrical contact and mechanical stability. The synergistic interplay between the porous ZnO/CuO core and the conductive, functional N-doped carbon shell endows the composite with exceptional electrochemical properties when evaluated as a li ion battery anode.
Synthesis and Structural Characterization
The synthesis pathway for the ZnO/CuO/N-C microspheres is designed for efficiency and control. It begins with the hydrothermal synthesis of uniform Cu-doped ZnSe (Cu-ZnSe) microspheres, which serve as the foundational template. These microspheres are then subjected to an in-situ polymerization process where dopamine hydrochloride, in a tris-buffer solution at a pH of 8.5-9.5, forms a polydopamine (PDA) layer uniformly coating the surface. The final and crucial step involves pyrolysis under a nitrogen atmosphere. During this calcination, multiple transformations occur concurrently: the PDA coating carbonizes into a N-doped carbon layer, while the core Cu-ZnSe is converted into ZnO/CuO, with Se being removed as gaseous species, thereby creating a porous interior structure. For comparison, bare ZnO/CuO microspheres were prepared by direct calcination of the Cu-ZnSe template in air.
X-ray diffraction (XRD) analysis confirms the successful phase transformation. The diffraction patterns of the precursor Cu-ZnSe match perfectly with the cubic phase of ZnSe. After the carbon-coating and pyrolysis process, all characteristic peaks of ZnSe disappear, and new peaks emerge that are indexed to the hexagonal wurtzite structure of ZnO and the monoclinic structure of CuO, with no detectable impurities. The crystalline size, estimated using the Scherrer equation, reveals a significant refinement for the carbon-coated sample. The calculated crystallite size for ZnO/CuO/N-C is approximately 8 nm, which is notably smaller than that of the bare ZnO/CuO (28 nm) or the precursor (29 nm). This nanoscale crystallinity is highly beneficial for li ion battery electrodes, as it drastically shortens the diffusion length for both Li⁺ ions and electrons, while the abundant grain boundaries formed between tiny crystallites can create efficient percolation networks for rapid charge transport.
The chemical states and surface composition of the ZnO/CuO/N-C composite were probed by X-ray photoelectron spectroscopy (XPS). The survey spectrum confirms the presence of Zn, Cu, O, N, and C, with no trace of Se, indicating its complete removal. The high-resolution N 1s spectrum can be deconvoluted into three characteristic peaks corresponding to pyridinic N (398.4 eV), pyrrolic N (400.0 eV), and graphitic N (401.0 eV). The combined contribution of pyridinic and pyrrolic N accounts for about 94% of the total nitrogen content. These nitrogen species are crucial: pyridinic and pyrrolic N create Lewis basic sites that enhance the adsorption and interaction with Li⁺ ions, effectively providing additional pseudo-capacitive active sites, while graphitic N improves the electronic conductivity of the carbon matrix. The high-resolution C 1s spectrum further corroborates the successful incorporation of N into the carbon lattice, showing signatures of C-N/C-O bonds. Raman spectroscopy of the composite shows two prominent bands at approximately 1360 cm⁻¹ (D band, disorder) and 1585 cm⁻¹ (G band, graphitic). The intensity ratio ID/IG is calculated to be around 2.12, indicating a highly disordered carbon structure with abundant defects. These defects are not detrimental; rather, they are advantageous for a li ion battery anode as they can serve as additional active sites for electrochemical reactions and facilitate ion storage via surface-driven processes.
The morphological evolution was investigated by scanning and transmission electron microscopy (SEM/TEM). The precursor Cu-ZnSe microspheres exhibit a well-defined spherical morphology with a diameter of about 3 μm, composed of aggregated nanoparticles. Remarkably, this spherical secondary structure is perfectly preserved after the dopamine coating and pyrolysis process to form ZnO/CuO/N-C. The surface of the microspheres appears smoother due to the conformal carbon coating, and the primary particle size is reduced to around 100 nm. TEM images clearly reveal a core-shell structure, where a dark contrasted core is encapsulated by a lighter gray shell with a thickness of approximately 50 nm, corresponding to the N-doped carbon layer. High-resolution TEM (HRTEM) shows distinct lattice fringes with spacings of 0.28 nm and 0.25 nm, assignable to the (100) plane of ZnO and the (002) plane of CuO, respectively, confirming the coexistence of both oxides. The porous nature of the material, resulting from Se removal, is supported by nitrogen adsorption-desorption analysis. The bare ZnO/CuO shows a higher specific surface area, while the carbon-coated sample has a slightly lower value, as some pores are filled or coated by the carbon layer, but it retains a significant mesoporous structure beneficial for electrolyte access.
The key characteristics of the synthesized materials are summarized in the table below:
| Material | Crystallite Size (nm) | Specific Surface Area (m² g⁻¹) | Dominant Feature |
|---|---|---|---|
| Cu-ZnSe (Precursor) | 29 | 1.93 | Dense microspheres |
| ZnO/CuO | 28 | 7.57 | Porous microspheres |
| ZnO/CuO/N-C | 8 | 7.01 | Porous core, N-doped Carbon Shell |
Electrochemical Performance in Li-Ion Batteries
The electrochemical performance of the ZnO/CuO/N-C microspheres as an anode was systematically evaluated in half-cell configurations (vs. Li/Li⁺). Cyclic voltammetry (CV) was first conducted to understand the redox behavior. The initial cathodic scan shows a large, irreversible reduction peak near 0.3 V, associated with the electrolyte decomposition and the formation of a solid electrolyte interphase (SEI) layer, which is common for conversion-type anodes. In subsequent cycles, the CV curves become highly superimposable, indicating excellent electrochemical reversibility after the initial activation. The redox peaks can be ascribed to the multi-step reactions of both ZnO and CuO. For ZnO, the reduction involves its conversion to Zn metal followed by alloying with Li to form LixZn, with corresponding oxidation peaks for de-alloying and re-oxidation. For CuO, stepwise reduction to Cu2O and finally to metallic Cu occurs, with oxidation peaks corresponding to the reverse process. The presence of multiple peaks confirms the rich redox activity provided by the binary oxide system, contributing to the high capacity of this li ion battery anode.
Galvanostatic charge-discharge profiling at 0.1 A g⁻¹ provides direct evidence of the high capacity. The initial discharge and charge capacities of the ZnO/CuO/N-C electrode are 692.7 and 356.6 mAh g⁻¹, respectively, yielding a first-cycle Coulombic efficiency (CE) of 51.4%. The irreversible capacity loss is primarily attributed to the inevitable SEI formation and some irreversible side reactions. Encouragingly, the CE quickly rises and stabilizes above 98% after a few cycles. More importantly, unlike typical electrodes that undergo continuous capacity decay, the ZnO/CuO/N-C anode exhibits a unique capacity rise upon cycling. After 200 cycles, it delivers a remarkably high reversible discharge capacity of 1010.4 mAh g⁻¹, exceeding its theoretical value based on ZnO alone. This phenomenon, often observed in nanostructured metal oxides, can be attributed to the progressive activation of the electrode material: the conductive carbon coating and the porous structure facilitate gradual electrolyte penetration and access to more active sites; the metal nanoparticles formed during conversion (Zn, Cu) may act as catalysts for the reversible decomposition of the polymeric/gel-like SEI layer, contributing extra capacity; and the N-doped carbon itself can provide additional Li⁺ storage sites.
The long-term cycling stability and rate capability, critical metrics for any practical li ion battery, were rigorously tested. At a higher current density of 1 A g⁻¹, the superior stability of the carbon-coated composite becomes starkly evident. The bare ZnO/CuO electrode suffers from rapid capacity fading, dropping to a negligible capacity after several hundred cycles due to structural degradation and pulverization. In striking contrast, the ZnO/CuO/N-C anode demonstrates outstanding cycling resilience. It maintains a capacity of 385.0 mAh g⁻¹ after 500 cycles. Even after 1000 cycles at this demanding rate, it retains a substantial capacity of 447.1 mAh g⁻¹, showcasing exceptional longevity. This performance underscores the vital role of the N-doped carbon shell in mechanically confining the active material, buffering volume changes, and maintaining electrical connectivity throughout extended cycling.
The rate performance was evaluated by subjecting the electrode to progressively higher current densities from 0.1 to 1 A g⁻¹ and back to 0.1 A g⁻¹. The ZnO/CuO/N-C electrode delivers average discharge capacities of 409.9, 334.2, 277.0, and 190.9 mAh g⁻¹ at 0.1, 0.2, 0.5, and 1 A g⁻¹, respectively. When the current density is switched back to 0.1 A g⁻¹, the capacity recovers to 401.2 mAh g⁻¹, demonstrating excellent structural stability and reversibility. The bare ZnO/CuO electrode shows much poorer rate capability and irreversible capacity loss. The superior rate performance of the composite anode is a direct benefit of its engineered architecture: the conductive N-doped carbon network ensures fast electron transport, the nanoscale crystallites and porous structure shorten Li⁺ diffusion paths, and the surface N-functional groups enhance charge transfer kinetics at the electrode-electrolyte interface.
A comparative summary of the cycling performance against other reported ZnO-based anodes highlights the competitiveness of our material:
| Anode Material | Current Density (A g⁻¹) | Cycle Number | Capacity (mAh g⁻¹) |
|---|---|---|---|
| ZnO/CuO/N-C (This work) | 0.1 | 200 | 1010.4 |
| ZnO/CuO/N-C (This work) | 1.0 | 1000 | 447.1 |
| ZnO/MnO@Porous Carbon | 1.0 | 1000 | ~315 |
| ZnO@Carbon | 0.1 | 100 | ~500 |
| ZnO/CuO/Graphene | 0.3 | 50 | ~186 |
Kinetic Analysis and Storage Mechanism
To gain deeper insight into the reasons behind the excellent rate performance, we analyzed the electrochemical kinetics of the ZnO/CuO/N-C anode. CV measurements were performed at various scan rates (ν). The relationship between the peak current (i) and the scan rate can be expressed by the power-law equation:
$$ i = a\nu^{b} $$
where a and b are adjustable parameters. The value of the exponent b provides information on the charge storage mechanism. A b-value of 0.5 indicates a diffusion-controlled process (battery-type behavior, limited by semi-infinite linear diffusion of Li⁺ ions), while a b-value of 1.0 signifies a surface-controlled capacitive process (capacitor-type behavior). By plotting log(i) versus log(ν), the b-value can be determined from the slope of the linear fit. For the ZnO/CuO/N-C electrode, the calculated b-values for the main anodic and cathodic peaks are approximately 0.77 and 0.91, respectively. These values, being much closer to 1 than to 0.5, reveal that the charge storage is predominantly governed by surface-controlled capacitive processes, including electric double-layer capacitance and surface faradaic pseudocapacitance. This is a highly desirable characteristic for a high-power li ion battery anode, as capacitive processes are inherently faster and less diffusion-limited than bulk intercalation or conversion reactions.
The quantitative contribution of the capacitive effect can be further separated at a fixed potential using the equation:
$$ i(V) = k_{1}\nu + k_{2}\nu^{1/2} $$
where i(V) is the total current, $k_{1}\nu$ represents the current from the capacitive effects, and $k_{2}\nu^{1/2}$ represents the current from the diffusion-controlled processes. The capacitive contribution increases with scan rate, and at a typical scan rate of 1 mV s⁻¹, it accounts for a remarkably high proportion of approximately 90% of the total charge storage. This dominant pseudocapacitive behavior is attributed to several factors intrinsic to the ZnO/CuO/N-C design: (1) The ultra-small nanocrystallites provide an enormous electrochemically active surface area. (2) The N-doped carbon coating, with its rich defect sites (pyridinic/pyrrolic N), enables fast surface Li⁺ ion adsorption/desorption. (3) The porous structure allows the electrolyte to access almost the entire surface area, making the surface-driven reactions highly efficient.
Electrochemical impedance spectroscopy (EIS) was employed to analyze the charge transfer resistance and Li⁺ ion diffusion. The Nyquist plots consist of a semicircle in the high-to-medium frequency region, corresponding to the charge transfer resistance (Rct) at the electrode/electrolyte interface, and a sloping line in the low-frequency region, related to the Warburg impedance (Zw) associated with Li⁺ ion diffusion in the solid state. The fitted Rct value for the ZnO/CuO/N-C electrode (152 Ω) is significantly lower than that of the bare ZnO/CuO (217 Ω) and the precursor (375 Ω). This confirms that the N-doped carbon coating dramatically improves the electronic conductivity and facilitates faster charge transfer kinetics, which is crucial for achieving high rate capability in a li ion battery. The Li⁺ ion diffusion coefficient (DLi+) can be estimated from the low-frequency Warburg region. The calculated DLi+ for ZnO/CuO/N-C is higher than that of the other two samples, indicating enhanced ionic conductivity due to the shortened diffusion paths and favorable surface properties introduced by the composite structure.
The superior performance of the ZnO/CuO/N-C anode can be mechanistically summarized by the synergistic effects of its multi-scale design:
- Multi-Mechanism Capacity Contribution: The composite leverages both the conversion reaction of CuO and the conversion-alloying dual mechanism of ZnO, providing a high theoretical capacity base.
$$ \text{CuO} + 2\text{Li}^+ + 2e^- \rightleftharpoons \text{Cu} + \text{Li}_2\text{O} $$
$$ \text{ZnO} + 2\text{Li}^+ + 2e^- \rightleftharpoons \text{Zn} + \text{Li}_2\text{O} $$
$$ \text{Zn} + x\text{Li}^+ + xe^- \rightleftharpoons \text{Li}_x\text{Zn} $$ - Structural Stability via Carbon Nanoconfinement: The conformal N-doped carbon shell acts as a robust mechanical buffer, effectively containing the volume expansion of the active oxides and preventing particle aggregation and detachment from the current collector.
- Enhanced Kinetics from Architecture and Doping: The porous interior ensures full electrolyte wetting and shortens Li⁺ transport distances. The N-doped carbon network, rich in graphitic N, provides a “highway” for electron conduction. The pyridinic/pyrrolic N sites at the surface lower the energy barrier for Li⁺ charge transfer, promoting pseudocapacitive storage.
- Synergy Between Components: The in-situ formed metallic Cu nanoparticles during discharge can enhance the overall electronic conductivity of the electrode matrix. The porous carbon coating also allows for the reversible storage of Li⁺ ions itself, adding to the total capacity.
Conclusion
In summary, we have successfully developed a novel and efficient template-engaged strategy to synthesize N-doped carbon-coated ZnO/CuO porous microspheres. This integrated approach concurrently achieves the in-situ generation of active binary oxides, the creation of an internal porous network, and the encapsulation within a functional conductive carbon shell. When evaluated as an anode material for li ion batteries, this hierarchically structured composite exhibits a remarkable combination of high specific capacity, outstanding long-term cycling stability, and excellent rate capability. The electrode delivers a high reversible capacity of over 1000 mAh g⁻¹ at 0.1 A g⁻¹ after 200 cycles and maintains a stable capacity of 447 mAh g⁻¹ even after 1000 cycles at a high current density of 1 A g⁻¹. Detailed kinetic analysis reveals that the charge storage is predominantly surface-controlled pseudocapacitive, which accounts for its superior power performance. The exceptional electrochemical properties are attributed to the synergistic interplay between the porous ZnO/CuO core, which provides high capacity through multi-step redox reactions, and the N-doped carbon shell, which ensures structural integrity, enhances conductivity, and facilitates rapid surface kinetics. This work presents a rational and scalable material design paradigm that effectively addresses the key challenges of volume expansion and poor conductivity in transition metal oxide anodes, paving a promising way for the development of high-performance, durable li ion batteries for future energy storage demands.