Dimensionality-Reduction Regulation of C@M-Zn₂SnO₄(H⁺) for High-Capacity and Durable Lithium-Ion Battery Anodes

1. Introduction

The rapid expansion of electric transportation and renewable-energy systems has spurred the damand for lithium-ion batteries (LIBs) with higher energy density and extended lifespans [1,2,3,4,5,6]. However, the practical energy density of current LIBs is still constrained by the limited capacity of commercial graphite anodes, whose theoretical specific capacity is only 372 mAh g⁻¹ [7,8]. Developing alternative anode materials with higher capacity while maintaining structural stability during long-term cycling is therefore of great importance [9,11,12].

Among various candidates, Zn₂SnO₄, has attracted considerable attention as a Sn-Zn-based composite oxide because of its high theoretical capacity (1231 mAh g⁻¹), [12,13] low cost, and abundant elemental resource. Nevertheless, its practical application remains hindered by sluggish reaction kinetics, large volume variation, and unstable electrode/electrolyte interfaces [9,14,15,16]. During repeated lithiation/delithiation, Zn₂SnO₄ undergoes complex conversion and alloying reactions, which generate severe internal stress, particle pulverization, electrical disconnection, and continuous interfacial side reactions [14,15]. These issues usually lead to low initial Coulombic efficiency, aggravated polarization, rapid capacity decay and poor cycling durability [16]. Thus, for Zn₂SnO₄ anodes, the central challenge lies not only in utilizing their high capacity, but in reconciling high-capacity lithium storage with structural and interfacial stability [9,12,16].

To address these issue, extensive efforts have focused on nanostructure design and carbon modification [16,17]. Constructing nanosheets, porous frameworks, or hierarchical structures can shorten Li⁺ diffusion pathways and alleviate lithiation-induced strain, [18,19] whereas carbon incorporation can enhance electronic conductivity and suppress direct electrolyte attack [20,21]. Despite these advances, most reported strategies improve only part of the problem, because the electrochemical instability of Zn₂SnO₄ originates from the coupling of bulk transport limitation, internal stress accumulation, and interface deterioration [22,23]. In particular, increasing structural openness may accelerate ion transport but also expose more reactive surface to the electrolyte, while simple carbon coating may stabilize the surface without fundamentally relieving diffusion limitation or stress concentration in compact particles. Therefore, an integrated regulation strategy that simultaneously coordinates transport kinetics, strain accommodation, and interfacial stability is still highly needed [25].

With these considerations in mind, we focused on dimensionality reduction as a promising route for the rational design of Zn₂SnO₄-based anodes [13,18]. More importantly, dimensionality reduction here is not merely a geometric size decrease, but a controlled transformation from compact 3D particles to low-dimensional assembled architectures with reconfigured ion/electron transport pathways, stress-release modes, and surface-exposure characteristics. When further coupled with interfacial carbon confinement, such a structure is expected to promote charge transfer, shorten Li⁺ diffusion length, buffer volume fluctuation, and stabilize the solid-electrolyte interphase (SEI), thereby enabling a better balance between high capacity and long-term durability [21,24]. Realizing this design, however, requires precise chemical synthesis capable of continuously regulating crystal growth, microstructural reconstruction, and surface stabilization [25,26].

Herein, we report a dimensionality-reduction-assisted structural regulation strategy for Zn₂SnO₄ anodes. Through surfactant-directed crystal growth, acid-etching reconstruction, and hydrothermal carbon coating, compact Zn₂SnO₄ octahedra are progressively transformed into sheet-assembled architectures and finally into a carbon-confined core-shell composite, denoted as C@M-Zn₂SnO₄(H⁺). This structural evolution is designed to simultaneously enhance transport kinetics, relieve mechanical stress, and stabilize the electrode/electrolyte interface, while preserving the intrinsic lithium-storage mechanism of Zn₂SnO₄. The optimized C@M-Zn₂SnO₄(H⁺) electrode exhibits favorable electrochemical performance, delivering 620 mAh g⁻¹ after 600 cycles at 200 mA g⁻¹, with Coulombic efficiency close to 100%.

2. Materials and Methods

2.1. Materials

SnCl₄·5H₂O, glucose (C₆H₁₂O₆), N-methyl-2-pyrrolidone (NMP), polyvinylidene fluoride (PVDF), copper foil, NaOH, lithium battery electrolyte (LiPF₆), Celgard separator, ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), HCl (38 wt%), HNO₃ (69 wt%), coin cell components (CR2032) were purchased from China. Ketjen black (C) was purchased from Japan. Pluronic P123 (PEO-PPO-PEO) was purchased from the USA.

2.2. Synthesis of M-Zn₂SnO₄(H⁺) and C@M-Zn₂SnO₄(H⁺)

In a typical synthesis, 0.4 g Pluronic P123 was dissolved in 10 mL deionized water (DIW, 18.23 MΩ) at 65 °C under ultrasonication, followed by magnetic stirring until a transparent pale-white solution was obtained. Separately, 2 mmol ZnSO₄·7H₂O and 2 mmol SnCl₄·5H₂O were dissolved in 10 mL DIW. To prevent premature coprecipitation, 2 mL concentrated HCl was added to the SnCl₄ solution. After 10 minutes of equilibration, the ZnSO₄ solution was slowly introduced into the SnCl₄ solution under stirring, and the mixture was stirred for an additional 20 minutes. A 2 M NaOH solution (26.75 mL) was then added using a micropump at a rate of 0.5 mL min^-1. The final volume was adjusted to 70 mL with DIW, and the mixture was transferred to a Teflon-lined stainless-steel autoclave, where it was heated at 200 °C for 20 h. After cooling to room temperature, the precipitate was collected by centrifugation, washed with ethanol and DIW, and dried to obtain the precursor product.

To reconstruct the microstructure and eliminate secondary phases, the as-synthesized product was treated with 0.2 M HNO₃ for 2 h, followed by washing and drying. The resulting sample was denoted as M-Zn₂SnO₄(H⁺), with the untreated sample referred to as M-Zn₂SnO₄ for comparison.

For carbon-shell construction, 0.4 g of M-Zn₂SnO₄(H⁺) was dispersed in DIW, followed by the addition of 0.4 g of Pluronic P123 and 1 g of glucose. After hydrothermal treatment, the collected precipitate was dried and annealed in a tube furnace under flowing high-purity N₂. The sample was heated to 600 °C at a rate of 2 °C min^-1 and held for 4 h, yielding the final gray-brown product, denoted as C@M-Zn₂SnO₄(H⁺).

2.3. Electrochemical Test

Electrochemical performance was evaluated using CR2032 coin cells assembled with Zn₂SnO₄-based electrodes as the working electrodes and lithium foil as both the counter and reference electrode. The active materials were mixed with Ketjen black and PVDF in NMP at a mass ratio of 8:1:1 to form homogeneous slurries. The slurries were uniformly coated onto copper foil, dried under vacuum at 100 °C for 6 h, and then punched into circular disks with a diameter of 14 mm.

Cell assembly was conducted in an argon-filled glove box with H₂O and O₂ contents below 0.1 ppm and 0.5 ppm, respectively. The electrolyte was 1.0 M LiPF₆ dissolved in EC, DMC, and DEC with a volume ratio of 1:1:1. Celgard 2500 membranes were used as separators. After assembly, the cells were rested for 2 h before testing.

Galvanostatic charge/discharge tests and rate-performance measurements were performed on a LAND battery testing system (model BT-2018AS, Hubei Land New Energy Equipment Co., Ltd.) at room temperature within a voltage window of 0.2-3.0 V. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were carried out on a CHI 760e electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd.). CV measurements were conducted at a scan rate of 0.2 mV s^-1 between 0.2 and 3.0 V, starting from the open-circuit voltage. EIS measurements were performed at room temperature to evaluate interfacial charge-transfer behavior and Li⁺ diffusion characteristics.

2.4. Material Characterization

X-ray diffraction (XRD) patterns were recorded on a Rigaku D/max-rB diffractometer using Cu Kα radiation over a 2θ range of 20–80° at a scanning rate of 2° min^-1. Raman spectra were collected on a LRS-5G-FG Raman spectrometer with a 532 nm excitation laser in the range of 200–2200 cm^-1. Thermogravimetric analysis (TGA) was carried out on a Mettler-Toledo simultaneous thermal analyzer at a heating rate of 5 °C min^-1 from room temperature to 1200 °C. Morphological and microstructural features were investigated using a Nova Nano SEM450 field-emission scanning electron microscope and a Talos F200S G2 transmission electron microscope. Elemental distributions were analyzed by energy-dispersive X-ray spectroscopy (EDS), and high-angle annular dark-field (HAADF) imaging was used to further probe the microstructural characteristics of the composite samples.

3. Results

3.1. Dimensionality-Reduction-Induced Structural Evolution

By regulating the structure via dimensionality reduction, we progressively transform Zn₂SnO₄ octahedra into low-dimensional assemblies encapsulated in a carbon shell. This structural transformation shortens Li⁺ transport paths, alleviates structural stress, and stabilize the electrode/electrolyte interface. As shown in Figure 1, C@M-Zn₂SnO₄(H⁺) was fabricated through surfactant-directed crystal growth, acid-etching induced reconstruction, hydrothermal carbon coating, and subsequent annealing, which enables precise modulation of phase composition, microstructure, and interfacial properties.

As shown in Figure 2a, pristine Zn₂SnO₄ dispalys a well-defined regular octahedral morphology, characteristic of a compact and highly ordered three-dimensional crystal structure. Through surfactant-mediated dimensionality reduction structural modulation, the original well-defined octahedral geometry of M-Zn₂SnO₄ is disrupted, evolving instead into irregular architecture assembled from sheet-like subunits with distinct local stacking features (Figure 2b). This result indicates that the surfactant effectively modulates the nucleation pathway and preferential growth kinetics of the resultant crystals. To correlate the observed morphological transformation to its underlying phase and structural evolution, we collected X-ray diffraction (XRD) patterns (Figure 2c). All diffraction peaks of pristine Zn₂SnO₄ are indexed to the cubic inverse spinel phase (JCPDS No. 24-1470), while M-Zn₂SnO₄ retains the signature diffraction peaks of Zn₂SnO₄ phase, alongside additional peaks assignable to ZnSnO₃ and ZnO. This finding confirms that the surfactant both mediates the observed morphological evolution and tunes the local reaction environment and phase formation pathway during crystallization. Further acid treatment generated abundant surface pores and perforations in M-Zn₂SnO₄(H⁺), while completely eliminating the ZnSnO₃ and ZnO impurity phases (Figure 2c,d). Together, these results show that surfactant-mediated assembly followed by acid etching drives the evolution of pristine Zn₂SnO₄ from a densely packed octahedral structure into a low-dimensional porous assembly structure, thereby establishing the structural basis for high-performance LIBs.

The internal microstructure of Zn₂SnO₄-based materials was comprehensively examined using transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). Compared with pristine Zn₂SnO₄, M-Zn₂SnO₄ exhibits distinct mass-thickness contrast variations in bright-field TEM images, indicating a structural evolution towards a thinner sheet-like morphology, in agreement with the scanning electron microscopy (SEM) observations (Figure S4a,b). HRTEM images of the M-Zn₂SnO₄ (Figure S4c,d) show lattice spacings of 0.24 and 0.26 nm, which we index to the (222) and (311) planes of the Zn₂SnO₄ cubic inverse spinel phase, respectively. The selected area electron diffraction (SAED) pattern reveals clear, regular diffraction spots, indicating high crystallinity and distinct crystal orientation. In addition, the pattern is consistent with the cubic inverse spinel Zn₂SnO₄, matching well with XRD patterns.

Coupling carbon coating with dimensionality reduction provides a synergistic route to improved electronic/ionic transport, structural robustness and electrode/electrolyte interfaces, resulting in superior rate capability and cycling stability. We therefore hydrothermally coated M-Zn₂SnO₄(H⁺) with a controlled carbon layer to form a core-shell C@M-Zn₂SnO₄(H⁺) composite. As shown in Figure 3a–d, C@M-Zn₂SnO₄(H⁺) largely retains the particle morphology of the etched precursor, while a continuous shell approximately 35 nm thick is clearly observed on the surface. The absence of discernible lattice fringes in this shell indicates its amorphous carbon layer. The weak and broadened XRD diffraction peaks observed for C@M-Zn₂SnO₄(H⁺) can be ascribed to the shielding effect of the amorphous carbon shell surrounding the particles (Figure 2c). Elemental mapping images of C@M-Zn₂SnO₄(H⁺) revealed that Zn, Sn, and O are concentrated primarily in the inner region, exhibiting significant overlap, indicating that the Zn-Sn-O framework remains the structural core. In contrast, C is predominantly located on the outer surface, rather than being randomly dispersed within the bulk, confirming the core-shell structure of the C@M-Zn₂SnO₄(H⁺) composite (Figure 3f–j). Similar results can also be found in the SEM characterizations presented in Figure S2.

Raman spectroscopy further revealed local structural variations induced by surfactant-mediated dimensionality reduction. The characteristic Raman bands of Zn₂SnO₄ became weaker and broader after dimensionality reduction, indicating increased lattice distortion and reduced local order, consistent with the XRD results (Figure S3a). Thermogravimetric analysis further indicated a carbon content of ~13.14 wt% in C@M-Zn₂SnO₄(H⁺) (Figure S3b). Together, these results demonstrate the formation of a dimensionally reduced, structurally reconstructed Zn₂SnO₄-based composite with a porous oxide core and a continuous carbon shell, a configuration conducive to fast ion transport and structural stability.

3.2. Kinetic and Interfacial Effects of Structural Regulation

Electrochemical impedance spectroscopy (EIS) shows that all Zn₂SnO₄-based samples exhibit similar Nyquist plots, consisting of a semicircle in the high-frequency region and an inclined line in the low-frequency region, indicating that the electrode kinetics are jointly governed by interfacial charge transfer and Li⁺ diffusion (Figure 4a). Relative to pristine Zn₂SnO₄, M-Zn₂SnO₄ displays a markedly smaller semicircle, confirming that dimensionality reduction effectively lowers the charge-transfer resistance. Following acid etching, M-Zn₂SnO₄(H⁺) shows a further decrease in semicircle radius together with an increased slope in the low-frequency region, suggesting improved interfacial contact and facilitated Li⁺ diffusion. Notably, C@M-Zn₂SnO₄(H⁺) exhibits the smallest semicircle and the smallest low-frequency Warburg tail, indicative of the lowest charge-transfer resistance and optimal Li⁺ diffusion kinetics among all samples. These results highlight a pronounced synergy between carbon coating and the the reconstructed framework induced by dimensionality reduction: the continuous carbon network promotes electron transport while stabilizing the electrode interface, thereby effectively mitigating polarization.

As shown in Figure S4, all Zn₂SnO₄-based electrodes exhibit similar redox features within 0.2-3.0 V, indicating that these electrodes follow the same intrinsic lithium-storage chemistry, namely the typical conversion reaction coupled with alloying/dealloying processes of Zn₂SnO₄-based anodes. The initial cathodic peaks in the low-potential region are associated with solid electrolyte interphase (SEI) formation, reduction of Zn₂SnO₄ to Zn/Sn and Li₂O, and the formation of Li_yZn and Li_xSn alloys, whereas the anodic peaks arise from dealloying and partial reversible conversion reactions. The discrepancy between the first and subsequent cycles reflects irreversible interfacial reactions and electrode activation.

Compared with pristine Zn₂SnO₄, the samples with dimensionality reduction modulation exhibit more defined redox peaks, improved curve overlap and reduced peak shifts, indicating enhanced reversibility and faster interfacial kinetics without altering the intrinsic reaction pathway (Figure S4). Among them, C@M-Zn₂SnO₄(H⁺) shows the strongest current response, largest CV area, sharpest redox peaks and smallest peak separation (Figure 4b, c), confirming reduced polarization and superior reaction kinetics enabled by the synergistic effects of dimensional regulation, acid etching and carbon coating.

3.3. Enhanced Cycling Durability and Rate Capability

We further evaluated the cycling and rate performance of the prepared electrodes. As shown in Figure 5a, the C@M-Zn₂SnO₄(H⁺) electrode exhibits excellent long-term cycling stability at 200 mA g^-1. Its discharge capacity initially stabilizes at 650 mAh g^-1, followed by a slight recovery after ~350 cycles, reaching ~800 mAh g^-1, which may arise from gradual electrode activation and microstructural rearrangement during cycling. Although gradual capacity decay occurs over prolonged cycling, the electrode still retains ~620 mAh g^-1 after 600 cycles, with the Coulombic efficiency remaining close to 100% throughout, indicating reversible lithiation/delithiation and a stable electrode/electrolyte interface. The comparison of the first 50 cycles across all samples reveals the highest capacity retention for C@M-Zn₂SnO₄(H⁺), followed by M-Zn₂SnO₄(H⁺) and M-Zn₂SnO₄, whereas pristine Zn₂SnO₄ shows the lowest retention (Figure 5b).

Rate performance analysis shows that the capacities of all samples decrease progressively as the current density increases from 0.1 to 2 A g^-1 (Figure 5c). Notably, C@M-Zn₂SnO₄(H⁺) maintains higher capacities across the full current range and shows significant capacity recovery when the current returns to lower values, indicating superior rate capability and structural stability. Although M-Zn₂SnO₄(H⁺) and M-Zn₂SnO₄ outperform pristine Zn₂SnO₄, their capacities and recovery capability remain inferior to those of C@M-Zn₂SnO₄(H⁺). These findings further indicate that the synergistic effect between dimensionality reduction and carbon coating enhances both electronic conductivity and Li⁺ transport/diffusion kinetics, thereby improving cycling stability and rate performance.

3.4. Structural Evolution of Long-Term Stability

In addition, we characterized the morphology and structure of the Zn₂SnO₄-based electrodes after extended cycling measurements to systematically assess their stability. As shown in Figure 6, after 600 cycles, the C@M-Zn₂SnO₄(H⁺) electrode retains excellent surface continuity and structural integrity, with only minor micropores and slight surface deformations observed. The absence of large cracks indicates that the material retains exceptional structural stability over extended lithiation/delithiation cycles. In contrast, the pristine Zn₂SnO₄ electrode shows significant cracking after just 50 cycles, with cracks through the entire thickness dividing the electrode into irregular regions, accompanied by the precipitation of local bright impurities. This suggests severe structural degradation and interfacial instability induced by volume expansion during cycling. It is evident from these results that dimensionality reduction combined with carbon coating effectively mitigate structural damage during cycling, significantly enhancing the electrode’s morphology retention.

4. Conclusions

In summary, we have transformed Zn₂SnO₄ from regular octahedral particles into a sheet-assembled core-shell composite through dimensionality reduction and carbon coating strategies, significantly enhancing the material’s ion transport, electronic conductivity, and volume-buffering capabilities. The C@M-Zn₂SnO₄(H⁺) electrode exhibits a stable initial reversible capacity of 650 mAh g^-1 at a current density of 200 mA g^-1 and retains approximately 620 mAh g^-1 after 600 cycles, with Coulombic efficiency remaining close to 100% throughout the cycling process. Morphological analysis after cycling reveals that the electrode maintains excellent surface integrity, confirming that this integrated dimensionality reduction and conductive coating strategy offers a novel approach for optimizing material structural stability, interfacial dynamics, and long-term cycling performance. This work provides a viable pathway for the development of high-performance anode materials for LIBs.