Co₃O₄/SnO₂ Hybrid Nanorods as High-Capacity Anodes for Lithium-Ion Batteries

Abstract

With the surging demand for high-performance energy storage devices, enhancing the energy density and charge-discharge efficiency of lithium-ion batteries has become an urgent need. Co₃O₄, with a high theoretical specific capacity of 890 mAh g⁻¹, is regarded as a promising anode candidate. In this work, rod-like hybrid Co₃O₄/SnO₂ composites were successfully prepared via the pyrolysis of cobalt-tin ethylene glycolate precursor. Notably, when the Co/Sn molar ratio is tuned to 3.8:1, the product evolves into nanorods. Lithium-ion batteries using Co_3.8Sn₁ as the anode deliver an initial specific capacity of 1588.9 mAh g⁻¹, and retain a reversible capacity of 427.9 mAh g⁻¹ after 500 cycles at 2 A g⁻¹, demonstrating that Sn-doping-induced optimization of morphology and conductivity effectively enhances electrochemical performance.

1. Introduction

With the rapid development of the pure electric vehicle market, there is a growing demand for lithium-ion batteries (LIBs) with higher energy density and improved charge-discharge efficiency [1]. However, graphite, the dominant commercial electrode material in commercial LIBs, has inherent limitations due to its relatively low theoretical specific capacity (372 mAh g⁻¹). Furthermore, it exhibits significant polarization at high current rates, making it increasingly difficult to meet the requirements of next-generation high-performance LIBs [2]. Previous studies have demonstrated that materials with high theoretical capacities, such as transition metal oxides, show promise as novel anode materials to boost LIB performance, thus holding significant application potential.

Among transition metal oxides, Co₃O₄ is considered a promising anode material due to its high theoretical specific capacity (890 mAh g⁻¹) [3]. The lithium storage mechanism of Co₃O₄ involves a conversion reaction, where redox reactions occur during charge and discharge processes. However, this process is accompanied by significant volume expansion [4]. Additionally, Co₃O₄ as a p-type semiconductor exhibits extremely low electronic conductivity at room temperature [5]. This results in rapid capacity decay and poor cycling stability in LIBs, severely hindering the commercial application of Co₃O₄.To address these issues, various strategies have been employed to modulate the structure and morphology of Co₃O₄. For instance, rod-like Co₃O₄ derived from metal-organic frameworks [6], capsule-shaped porous Co₃O₄ nanomaterials [7], and polyacrylonitrile microfibers embedded with Co₃O₄ nanoparticles prepared via centrifugal spinning [8] have been developed. After these structural and morphological modifications, the electrochemical performance of Co₃O₄ as an anode material for LIBs has been greatly improved.

Another effective approach is to fabricate nanocomposites by combining Co₃O₄ with metal oxides that exhibit different lithium storage mechanisms. SnO₂ offers a high theoretical specific capacity (approximately 780 mAh g⁻¹) and relatively good electrical conductivity. Unlike the lithium storage mechanism of Co₃O₄, SnO₂ follows a two-step reaction mechanism during lithiation. The first step involves reduction into Li₂O and metallic Sn, followed by the alloying reaction between Li and Sn in the second step [9]. By compositing the two, a mutual buffering effect can be achieved during the charge and discharge processes of LIBs, resulting in a synergistic enhancement of lithium storage performance [10]. For instance, Kim et al. synthesized SnO₂@Co₃O₄ hollow nanospheres with double shells via a template-based sol-gel coating method for application as anodes in LIBs. The unique hollow structure effectively alleviates volume variation while simultaneously accelerating lithium-ion transport and enhancing cycling stability. A high reversible discharge capacity of 962 mAh g⁻¹ was maintained after 100 cycles at a current density of 100 mA g⁻¹. SnO₂@Co₃O₄ hollow nanospheres demonstrates a superior performance over the SnO₂ electrode, which delivers a reversible discharge capacity of only 601 mAh g⁻¹ [11]. Xing et al. prepared SnO₂-Co₃O₄ core-shell nanoneedle arrays on copper foil. Leveraging the synergistic effect between SnO₂ and Co₃O₄, along with the nanoneedle array structure, the electrode demonstrated improved capacity and mitigated volume expansion [12]. In another study, Chen et al. loaded SnO₂ nanoparticles onto Co₃O₄ micro-flowers composed of porous nanosheets, obtaining a material labeled Co@Sn-MFs. Compared to pure Co₃O₄ microflowers, this composite showed enhanced specific capacity and improved electrical conductivity due to the synergistic interaction between its components and its unique spatial structure. The Co@Sn-MFs electrode delivered a discharge capacity of 767.5 mAh g⁻¹ after 100 cycles at a current density of 200 mA g⁻¹ [13].

Based on the strategy of using ethylene glycol as a ligand to synthesize spherical cobalt-glycolate precursors, which transform into yolk-shell Co₃O₄ microspheres after thermal decomposition [14,15], this study introduces 0.4 mmol of Sn²⁺ to modulate the precursor’s morphology. Specifically, the original spherical cobalt-glycolate evolves into rod-like hybrid Co₃O₄/SnO₂ composites (denoted as Co_3.8Sn₁). The resulting Co_3.8Sn₁ nanorods feature a unique structure with abundant voids (to accommodate volume expansion during lithiation) and enhanced electrical conductivity (attributed to the in-situ formed SnO₂ component). When employed as an anode material for LIBs, these nanorods demonstrate high specific capacity and excellent cycling/stability performance. This work aims to provide a feasible morphology-regulation strategy for developing high-performance transition metal oxide-based anodes, addressing the key bottlenecks of traditional anode materials in next-generation LIBs.

2. Materials and Methods

2.1. Preparation of Co₃O₄ Yolk-Shell Microspheres

4 mmol of Co(CH₃COO)₂·4H₂O were dissolved in 100 mL of ethylene glycol (EG) and stirred at 50 °C for 30 min. The mixture was then refluxed at 170 °C for 2 h. After cooling to room temperature, the light purple precipitate was collected, washed with acetone, and dried under vacuum. Subsequently, the product was transferred to a muffle furnace (Hefei Kejing Materials Technology Co., Ltd., Hefei, China) and heated to 350 °C at a heating rate of 1 °C min⁻¹ in air, followed by calcination for 2 h. The final product consisted of Co₃O₄ yolk-shell microspheres.

2.2. Preparation of Co₃O₄/SnO₂ Hybrid Nanorods

A mixture of 4 mmol of Co(CH₃COO)₂·4H₂O and 0.4 mmol of Sn(CH₃COO)₂, corresponding to a Co²⁺/Sn²⁺ molar ratio of 10:1, was dissolved in 100 mL of EG. The same synthetic procedure as in Section 2.1 was employed. The resulting mixture was stirred in an oil bath at 50 °C for 30 min, forming a light purple solution. Subsequently, the solution was heated to 170 °C and refluxed for 2 h, during which the color deepened to purple. After cooling to room temperature, the purple precipitate was collected by centrifugation, thoroughly washed with acetone, and dried under vacuum. The dried precursor was then transferred to a muffle furnace and calcined at 350 °C for 2 h in air. The final product was designated as Co_3.8Sn₁.

The same procedure was repeated using equimolar amounts of Co²⁺ and Sn²⁺ (i.e., a 1:1 molar ratio) to prepare the corresponding sample, which was denoted as Co₃O₄-SnO₂.

2.3. Material Characterization Methods

The crystal structure and phase composition of the as-synthesized materials were determined by X-ray diffraction (XRD) on a D8 ADVANCE diffractometer (Beijing Bruker Technology Co., Ltd., Beijing, China) using Cu-Kα radiation (λ = 0.15418 nm). The microscopic morphology and structural features were examined using a JSM7600F (NEC Electronics Corporation, Tokyo, Japan) scanning electron microscope (SEM) and a Talos F200X G2 (Thermo Fisher Scientific Inc., Waltham, MA, USA) transmission electron microscope (TEM). The elemental composition and chemical states were analyzed by X-ray photoelectron spectroscopy (XPS) on an ESCALab250Xi spectrometer with an Al-Kα excitation source. The binding energy scale of all XPS spectra was calibrated by referencing the C 1s peak of adventitious carbon to 284.8 eV. The molar ratios of the elements in the samples were determined using an Agilent 5110 (Agilent Technology Co., Ltd, Santa Clara, CA, USA) inductively coupled plasma optical emission spectrometer (ICP-OES).

2.4. Electrochemical Measurements

The electrochemical performance of the Co₃O₄/SnO₂ hybrid nanorods was evaluated by assembling CR2025-type coin cells in an argon-filled glove box (H₂O and O₂ levels < 0.1 ppm; MBraun, Stratham, NH, USA). The working electrode was prepared by thoroughly mixing 70 wt% active material, 20 wt% Ketjen black, and 10 wt% carboxymethyl cellulose (CMC) binder in deionized water to form a homogeneous slurry via magnetic stirring. This slurry was then uniformly coated onto a copper foil current collector and dried at 80 °C under vacuum for 12 h. The resulting electrode had an active material loading mass of approximately 1.0–1.2 mg cm⁻². The electrolyte was 1.0 M LiPF₆ dissolved in a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) (1:1:1 by volume). A lithium metal foil was used as the counter/reference electrode, and a Celgard 2400 membrane (Suzhou Sinero Technology Co., Ltd., Suzhou, China) was employed as the separator. Galvanostatic charge-discharge tests were performed on a Land CT2001A battery test system within a voltage window of 0.001–3.0 V (vs. Li/Li⁺) at various current densities to assess the specific capacity, cycling stability, and rate capability. Cyclic voltammetry (CV) was conducted on a VSP electrochemical workstation at a scan rate of 0.1 mV s⁻¹. Electrochemical impedance spectroscopy (EIS) measurements were carried out over a frequency range from 100 kHz to 100 mHz to evaluate the electrode’s interfacial and charge-transfer resistance.

3. Results

3.1. Structural Characterization and Phase Analysis of Materials

The synthetic route for Co_3.8Sn₁ is illustrated in Scheme 1. A cobalt-tin glycolate rod-like precursor (CoSn-gly) was first synthesized via a solvothermal reaction in EG from Co(CH₃COO)₂·4H₂O and Sn(CH₃COO)₂. Subsequent calcination of this precursor at 350 °C in air yielded the final material. ICP-OES measurements confirmed a Co:Sn molar ratio of 3.8:1 in the final product, consequently, the product was designated as Co_3.8Sn₁. The reduced molar amount of Co in the product is attributed to the loss of a portion of Co²⁺ ions that did not participate in the reaction.

Figure S1 presents the SEM images of the precursors for Co₃O₄ and Co_3.8Sn₁. As shown in Figure S1a,b, the Co-gly precursor consists of microspheres with a uniform diameter of approximately 300 nm. In contrast, the introduction of Sn²⁺ significantly alters the morphology of the precursor. Figure S1c,d reveal that the presence of Sn promotes the formation of a composite structure comprising both nanorods and nanospheres, indicating a tendency for rod-like growth. These precursors were subsequently calcined at 350 °C for 2 h. The resulting products are shown in Figure S2. After calcination, the Co-gly microspheres transform into yolk-shell Co₃O₄ microspheres. Meanwhile, the Co_3.8Sn₁ material maintains its rod-like morphology. Despite the persistence of numerous nanospheres, the rod-like morphology of the final Co_3.8Sn₁ material was largely retained.

The microstructure of the samples was further characterized by TEM. Figure S3 presents TEM images of the Co₃O₄ sample, revealing that the precursor transforms into yolk-shell microspheres after calcination (Figure S3b). The HRTEM image in Figure S3c shows lattice fringes with an interplanar spacing of 0.243 nm, corresponding to the (311) plane of Co₃O₄. Elemental mapping images (Figure S3d–f) confirm the uniform distribution of Co and O throughout the microspheres. As shown in Figure 1a,b, the Co_3.8Sn₁ composite consists of numerous nanoparticles assembled into rod-like architectures with substantial internal voids, a structure conducive to rapid ion transport. The HRTEM image in Figure 1c displays two distinct sets of lattice fringes: spacings of 0.237 nm and 0.341 nm, which are assigned to the (311) plane of Co₃O₄ and the (110) plane of SnO₂, respectively. This, combined with the elemental mapping results in Figure 1d–f which show a homogeneous distribution of Co, Sn, and O, indicates that the Co_3.8Sn₁ material is composed of Co₃O₄ nanoparticles encapsulated by a SnO₂ outer layer.

The crystal structures and phase compositions of the synthesized materials were characterized by XRD, as shown in Figure 2a. For Co₃O₄, the distinct diffraction peaks at 31.2°, 36.8°, 44.8°, 59.3°, and 65.2° are indexed to the (220), (311), (400), (511), and (440) planes, respectively, of a face-centered cubic spinel structure (JCPDS No. 42-1467). The XRD pattern of the Co₃O₄-SnO₂ sample shows three prominent peaks at 26.6°, 33.8°, and 51.7°, which correspond to the (110), (101), and (211) planes of tetragonal SnO₂ (JCPDS No. 41-1445), confirming its successful formation. In the case of the Co_3.8Sn₁ composite, diffraction peaks from both Co₃O₄ and SnO₂ are present. However, the characteristic peaks of SnO₂ are of relatively low intensity due to its lower concentration in the composite. The XRD pattern further confirms that the Co_3.8Sn₁ is a composite material consisting of SnO₂ and Co₃O₄.

The elemental composition and chemical states of the synthesized samples were analyzed by XPS. As shown in Figure 2b, the high-resolution O 1s spectrum was deconvoluted into three peaks at binding energies of 529.5 eV, 531.2 eV, and 533.4 eV, which are assigned to lattice oxygen (O_lat), adsorbed oxygen species (O_ads), and oxygen from surface-adsorbed water or hydroxyl groups (O_w), respectively [16]. The introduction of Sn²⁺ caused a positive shift in the O 1s peak, indicating a decrease in the electron density around oxygen atoms due to a strong electronic interaction with SnO₂. Figure 2c displays the Co 2p spectrum, characterized by two spin-orbit doublets. The main peaks at 779.4 eV and 794.5 eV are attributed to Co2p_3/2 and Co 2p_1/2, respectively. Each doublet could be further fitted with two components: the peaks at 779.4 eV and 794.5 eV correspond to Co²⁺, while those at 781.2 eV and 796.3 eV are indicative of Co³⁺, and the spectrum is also accompanied by satellite features confirms [17]. These results demonstrate that cobalt in Co_3.8Sn₁ exists in a mixed-valence state, with the coexistence of Co²⁺ and Co³⁺ [18]. Notably, the Co 2p peaks in the SnO₂-containing composite Co_3.8Sn₁ shifts to higher binding energies compared to pure Co₃O₄, suggesting the formation of a p-n heterojunction at the interface between Co₃O₄ and SnO₂, which modifies the local electron density [19]. The Sn 3d spectrum of Co_3.8Sn₁ is presented in Figure 2d. The peaks located at 486.3 eV and 494.7 eV are assigned to Sn 3d_5/2 and Sn 3d_3/2, respectively. The spin-orbit splitting of 8.4 eV is consistent with the standard value for Sn⁴⁺ in SnO₂, confirming the successful incorporation of tin oxide [20].

3.2. Electrochemical Properties

The electrochemical reactions of Co₃O₄ and Co_3.8Sn₁ were investigated by CV, as shown in Figure 3a,b. A distinct difference is observed between the first-cycle CV curves and the subsequent cycles for both materials, which is attributed to irreversible reactions and structural changes during the initial cycle, leading to the formation of a solid electrolyte interphase (SEI) layer [21]. For Co₃O₄ (Figure 3a), the first cathodic scan exhibited a reduction peak at approximately 0.82 V, attributed to the reduction of Co₃O₄ to metallic Co and the formation of Li₂O [22]. The subsequent anodic scan showed an oxidation peak at around 2.10 V, corresponding to the reversible oxidation of Co to Co₃O₄. For Co_3.8Sn₁ (Figure 3b), in its first cathodic scan, the peak at 0.03 V is assigned to the alloying reaction of Li⁺ with Sn to form Li_xSn alloys [23], while the prominent peak at 0.69 V corresponds to the further reduction of Co₃O₄ and SnO₂ to metallic Co and Sn. Compared to the reduction peak of pure Co₃O₄ at 0.82 V, this peak is attributed to the formation of the Co₃O₄/SnO₂ heterojunction in the Co_3.8Sn₁ composite, which modifies the electronic structure and consequently enhances the electrochemical performance of the material. In the anodic scan, the peaks at 0.56 V, 1.07 V, and 2.11 V are related to the de-alloying of Li_xSn (x represents the stoichiometric coefficient of Li in the Li_xSn alloy), the oxidation of Sn to SnO₂, and the oxidation of Co to Co₃O₄, respectively [24]. The curves for the second and third cycles overlapped well, demonstrating excellent cycling stability. The diverse lithium storage mechanisms and highly reversible CV curves suggest that the Co_3.8Sn₁ electrode possesses high structural stability, low polarization, and the potential for high specific capacity. The relevant reaction equations are as follows:

Co₃O₄ + 8Li⁺ + 8e⁻ ↔ 3Co + 4Li₂O,

(1)

SnO₂ + 4Li⁺ + 4e⁻ ↔ Sn + 2Li₂O,

(2)

Sn + xLi⁺ + xe⁻ ↔ Li_xSn (0 ≤ x ≤ 4.4).

(3)

The EIS results for Co₃O₄ and Co_3.8Sn₁ are presented in Figure 3c. The Nyquist plots for both materials consist of a depressed semicircle in the high-to-medium frequency region and an inclined line in the low-frequency region. The intercept on the Z’-axis at high frequency corresponds to the ohmic resistance (R_s) of the electrolyte. The diameter of the semicircle reflects the charge-transfer resistance (R_ct) at the electrode/electrolyte interface, while the sloping line is associated with the diffusion resistance Z_w of lithium ions in the electrolyte. It is evident that the Co_3.8Sn₁ electrode exhibits a significantly smaller semicircular diameter compared to the Co₃O₄ electrode, indicating a much lower R_ct. This reduction can be attributed to the altered microstructure induced by Sn²⁺ incorporation, which increases the electrode/electrolyte contact area and facilitates ion diffusion. The synergistic effect between Co and Sn species in the Co_3.8Sn₁ composite accelerates charge transfer kinetics, thereby leading to improved electrochemical performance.

The rate capability of the Co_3.8Sn₁ anode was evaluated at various current densities, as shown in Figure 4a. The electrode delivered high average specific capacities of 2084.84, 1996.96, 1623.31, 1235.13, and 888.46 mAh g⁻¹ at current densities of 0.1, 0.2, 0.5, 1 and 2 A g⁻¹, respectively. Notably, even at a high current density of 2 A g⁻¹, the capacity of Co_3.8Sn₁ significantly surpassed that of the pure Co₃O₄ anode. Furthermore, when the current density was returned to 0.2 A g⁻¹, the capacity recovered to 1365.22 mAh g⁻¹, demonstrating excellent reversibility and robust lithium storage performance.

The cycling performance of the Co_3.8Sn₁ and Co₃O₄ electrodes was evaluated at a current density of 0.2 A g⁻¹, as shown in Figure 4b. The Co_3.8Sn₁ anode demonstrated a high initial specific capacity and exceptional stability, maintaining a maximum specific capacity of 1402.7 mAh g⁻¹ after 150 cycles with a Coulombic efficiency of approximately 98%. In contrast, the Co₃O₄ anode not only started with a lower initial capacity but also suffered from a rapid capacity decay over 100 cycles. The incorporation of SnO₂ effectively modifies the structure of Co₃O₄, mitigating the agglomeration of nanoparticle and enhancing electrical conductivity. These synergistic effects collectively contribute to the significantly improved cycling stability and enhanced capacity of the Co_3.8Sn₁ composite. In the long-term cycling tests conducted at various current densities, the specific capacity of the Co_3.8Sn₁ electrode consistently demonstrates a trend of initial decrease followed by gradual increase. This phenomenon can be attributed to the presence of transition metal Co, which may catalyze electrolyte decomposition and the formation of a gel-like film, thereby contributing additional pseudocapacitive lithium storage [9]. The galvanostatic charge-discharge profiles of the Co_3.8Sn₁ and Co₃O₄ electrodes for the 1st, 2nd, 5th, and 10th cycles are presented in Figure 4c. The Co_3.8Sn₁ electrode exhibited an initial discharge capacity of 2645.8 mAh g⁻¹ and a charge capacity of 1820.4 mAh g⁻¹, yielding a first-cycle Coulombic efficiency of 68.8%. The initial irreversible capacity loss is primarily associated with SEI formation. The highly overlapping voltage profiles from the 2nd to the 10th cycle indicate excellent cycling reversibility thereafter. In the initial discharge curve, the voltage plateau observed between 0.4–1.0 V can be attributed to the conversion of SnO₂ to Sn, the reduction of Co₃O₄ to Co. The subsequent plateau in the range of 0.02–0.20 V corresponds to the alloying reaction between Sn and Li⁺ to form Li_xSn alloys. During the charging process, three distinct voltage plateaus emerge: the first plateau (0.4–0.7 V) is associated with the de-alloying of Li_xSn, while the subsequent two plateaus (0.8–1.2 V and 1.8–2.2 V) correspond to the stepwise oxidation of Sn to SnO₂ and Co to Co₃O₄, respectively [25].

The long-term cycling performance of the Co_3.8Sn₁ electrode was further evaluated at a high current density of 1 A g⁻¹, as shown in Figure 4d. Following activation at 0.2 A g⁻¹, the cells were cycled at 1 A g⁻¹. The corresponding galvanostatic charge-discharge profiles for Co₃O₄ and Co_3.8Sn₁ are presented in Figure 4e,f, respectively. The Co_3.8Sn₁ electrode delivered a superior initial specific capacity and demonstrated exceptional stability, retaining a capacity of 496.8 mAh g⁻¹ after 500 cycles. In stark contrast, the Co₃O₄ electrode retained only 139.8 mAh g⁻¹ after the same number of cycles, underscoring the significant performance enhancement afforded by the Co_3.8Sn₁ composite.

As shown in Figure 4g, at a current density of 2 A g⁻¹, the Co_3.8Sn₁ electrode delivered a specific capacity of 427.9 mAh g⁻¹ after 500 cycles, significantly outperforming the mere 84.5 mAh g⁻¹ retained by the Co₃O₄ electrode. The Coulombic efficiency of Co_3.8Sn₁ remained at approximately 98.7%. Figure 4h,i present the corresponding galvanostatic charge-discharge profiles of Co₃O₄ and Co_3.8Sn₁ at 2 A g⁻¹, respectively. With increasing cycle numbers, Co_3.8Sn₁ exhibited a markedly slower capacity decay and more stable voltage profiles compared to Co₃O₄. These results indicate that Sn doping effectively enhances the cycling stability and reversibility of the material, enabling Co_3.8Sn₁ to maintain a high capacity retention rate and superior electrochemical performance during long-term cycling at high current densities.

4. Conclusions

In summary, Co₃O₄/SnO₂ hybrid nanorods were successfully synthesized via a solvothermal reaction coupled with thermal decomposition. The introduction of Sn²⁺ induced the transformation of spherical cobalt-glycolate precursors into rod-like architectures with abundant voids, which provided buffer space to accommodate volume expansion during lithiation/delithiation cycles. Meanwhile, the in-situ formed SnO₂ component significantly enhanced the electrical conductivity of the hybrid system, addressing the poor electron transport issue of pristine Co₃O₄. Furthermore, the synergistic interaction between Co and Sn species optimized the lithium storage behavior, strengthening the adsorption and diffusion of Li⁺ while promoting reversible redox reactions between multi-valent metal species. When evaluated as an anode for LIBs, the Co_3.8Sn₁ electrode exhibited remarkable electrochemical performance, which delivered high reversible specific capacities of 496.8 and 427.9 mAh g⁻¹ after 500 cycles at high current densities of 1 and 2 A g⁻¹, respectively. These results confirm that Sn-doping-mediated microstructure engineering is an effective strategy to overcome the inherent bottlenecks of volume expansion and low conductivity of transition metal oxide anodes. More importantly, the Co_3.8Sn₁ hybrid nanorods, with their balanced structural stability and electrochemical activity, emerge as a promising candidate material for next-generation high-performance LIBs.