ZnMn₂O₄/V₂CT_x Composites Prepared as an Anode Material via High-Temperature Calcination Method for Optimized Li-Ion Batteries

Abstract

The ZnMn₂O₄/V₂CT_x composites with a lamellar rod-like bond structure were successfully synthesized through high-temperature calcination at 300 °C, aiming to enhance the Li storage properties of spinel-type ZnMn₂O₄ anode materials for lithium-ion batteries. Moreover, even though the electrode of the composites obtained at 300 °C had a nominal specific capacity of 100 mAh g⁻¹, it exhibited an impressive specific discharge capacity of 163 mAh g⁻¹ after undergoing 100 cycles. This represents an approximate increase of 64% compared to that observed in the pure ZnMn₂O₄ electrode (99.5 mAh g⁻¹). The remarkable performance of the composite can be credited to the collaborative impact between ZnMn₂O₄ and V₂CT_x, leading to a substantial improvement in its lithium ion storage capacity. Therefore, this study offers valuable insights into developing cost-effective, safe, and easily prepared anode materials.

1. Introduction

Recently, human society has witnessed a remarkable surge in progress, which has been accompanied by two significant challenges in the energy and environmental sectors that present major obstacles to the formulation of a national economic strategy [1,2]. Lithium-ion batteries (LIBs) have become widely used in various industries due to their exceptional characteristics, including a high capacity for storing energy, a minimal loss of charge over time, a lightweight construction, and an environmentally friendly nature. They play an indispensable role as vital energy storage devices that drive technological advancements and foster sustainable development [3,4,5]. Although LIBs exhibit exceptional performance in various aspects, they also possess certain drawbacks such as safety concerns, limited cycle life, temperature sensitivity, and capacity decay. Nevertheless, through continuous advancements in science and technology, along with technological enhancements, diligent efforts are being made to overcome these challenges. The ultimate goal is to further enhance the performance and safety of LIBs while promoting their extensive utilization across diverse fields [6,7,8].

Mixed transition metal oxides (MTMOs), exhibiting a spinel structure AB₂O₄, where A denotes divalent metal ions, such as Cu²⁺, Zn²⁺, Mn²⁺, and Mg²⁺, and B represents trivalent metal ions, including Fe³⁺, Al³⁺, Co³⁺, etc., have garnered significant attention owing to their remarkable electrochemical properties [9], being conversion anode materials exhibiting a theoretical capacity ranging from 700 to 1000 mAh g⁻¹, involving the transfer of at least eight moles of electrons during the initial discharge process. Among the various MTMOs, ZnMn₂O₄ exhibits two distinct lithium intercalation mechanisms (conversion and alloying), allowing it to achieve an initial discharge treatment with a theoretical capacity of 1008 mAh g⁻¹, surpassing other MTMOs without zinc [10,11]. In comparison to most Co- or Fe-based oxides, ZnMn₂O₄ operates within a voltage range of only 1.2–1.5 V (vs. Li⁺/Li), thereby expanding its potential window to battery applications [12]. Moreover, its abundant raw material sources, cost-effectiveness, and near-non-toxicity contribute to its suitability for large-scale utilization [13]. However, the inevitable volume changes during charge and discharge pose challenges in practical applications by causing structural collapse and comminution of the anode material, as well as reducing its capacity [14]. Additionally, similar to other MTMOs, ZnMn₂O₄ exhibits low electronic conductivity, which further hampers its performance. To address these limitations effectively, various strategies involve combining ZnMn₂O₄ with auxiliary materials, such as carbon nanotubes (CNTs) or graphene, that facilitate electron and ion transport while mitigating adverse effects associated with volumetric changes occurring throughout battery cycling [15,16].

The MXene material is a two-dimensional (2D) layered material characterized by its highly conductive core and exceptional dispersibility in aqueous solutions owing to its hydrophilic surface [17]. Its unique nature makes fleck-like 2D MXene commonly used for constructing high-level structures for LiBs [18,19]. In this study, V₂CT_x MXene was utilized to enhance the low conductivity, slow ion diffusion, and volume expansion of ZnMn₂O₄. An effective method for synthesizing a ZnMn₂O₄/V₂CT_x composite electrode is reported, aiming to (1) develop a simple and applicable approach for preparing composite materials and (2) characterize the correlation between the morphology, structure, and electrochemical characteristics of ZnMn₂O₄/V₂CT_x. Additionally, rational energy storage mechanisms have been proposed based on cyclic voltammetry tests. This work provides valuable data supporting the enhancement of the electrochemical properties of ZnMn₂O₄.

2. Experimental Methods

2.1. Materials

The V₂AlC powder was supplied by 11 technology Co., Ltd. (China). Zinc sulfate heptahydrate (ZnSO₄·7H₂O), anhydrous oxalic acid (C₂H₂O₄), and hydrochloric acid (HCl) were supplied by Shanghai Aladdin Biochemical Technology Co., Ltd. (China). Ammonium fluoride (NH₄F) and polyethylene glycol 400 (H(OCH₂CH₂)_n OH) were supplied by Sinopharm Chemical Reagent Co., Ltd. (China). Manganese chloride (MnCl₂·4H₂O) was supplied by Tianjin Damao Chemical Reagent Factory. All chemicals and reagents utilized in the research are analyzed-grade or higher purity, obviating the need for addition purification.

2.2. Preparation Methods

2.2.1. Preparation of ZnMn₂O₄ Micron Rods

The mixture of 20 mL anhydrous ethanol, 10 mL polyethylene glycol 400, and 10 mL deionized water was supplemented with 0.18 g of anhydrous oxalic acid (C₂H₂O₄). Subsequently, the solution was further enriched with 0.144 g of zinc sulfate heptahydrate (ZnSO₄·7H₂O) and 0.198 g of manganese chloride (MnCl₂·4H₂O), followed by stirring at a temperature of 300 K for a period of 6 h. After being subjected to centrifugation at a speed of 5000 rpm for 5 min, the resulting precipitates underwent three washes with pure water and two washes with ethanol. Subsequently, they were dried in a preheated oven maintained at a temperature of 60 °C for up to 12 h to obtain precursor materials in the form of rods. The obtained precursor underwent calcination in a muffle furnace starting from room temperature until reaching a final temperature of 400 °C at an increment rate of 5 °C/min, followed by holding it steady for two hours prior to a natural cooling process, resulting in the formation of ZnMn₂O₄ micron rods labeled as the ZMO sample.

2.2.2. Preparation of V₂CT_x MXene by Hydrochloric Acid and Ammonium Fluoride Etching Method

The plastic beaker with a capacity of 50 mL was filled by combining 20 mL of hydrochloric acid and an equal volume of deionized water, and NH₄F powder particles weighing 2.0 g were introduced into the mixture. The solution underwent magnetic stirring for a duration of 0.5 h at a temperature of 300 K. Subsequently, 2.0 g of V₂AlC powder was gently added to the beaker and stirred for an additional 5 min. The mixture obtained was subsequently transferred into a 100 mL autoclave lined with Teflon and underwent hydrothermal treatment at a temperature of 120 °C for a period of 24 h. Afterwards, the remaining black sediment underwent several rinses with deionized water until the liquid above it had a pH value of ≥5. This was followed by an extra wash using alcohol. The damp black sediment produced from this procedure underwent filtration at a speed of 4500 rpm for 5 min, followed by dehydration in a vacuum oven set to 60 °C for a period of 12 h, resulting in the acquisition of the V₂CT_x MXene specimen.

2.2.3. Preparation of ZnMn₂O₄/V₂CT_x Composite by High Temperature Calcination Method

In this method, 1.0 g ZnMn₂O₄ and 1.0 g V₂CT_x MXene were ground together in a grinder until achieving homogeneity. The resulting mixture was divided into thirds: one portion remained uncalcined and labeled as the ZMO/V₂CT_x sample; while the other two portions were calcined by air in the muffle furnace at heating rates of 5 °C/min, and the temperature reached 300 °C and 400 °C, respectively, and was kept for 2 h before natural cooling. The corresponding samples were designated as ZMO/V₂CT_x-300 and ZMO/V₂CT_x-400.

2.3. Characterization

The XRD patterns were acquired utilizing a Rigaku D/Max-2500 X-ray diffractometer (Japan) within the range of 5–65°. Raman spectroscopic investigations were conducted employing a Renishaw in Via RFS100/S micro-Raman spectrometer (Britain) with an excitation wavelength of 514 nm. Fourier transform infrared (FTIR) spectroscopy analysis was performed using a Thermo Scientific Nicolet iS50 spectrophotometer (USA). The surface morphology and composition analysis were carried out utilizing a JEOL JSM-7800F scanning electron microscope (SEM) (Japan). The surface composition was determined through X-ray photoelectron spectroscopy (XPS) using an Escalab 250Xi spectrometer (USA).

2.4. Electrochemical Tests

The electrochemical performance of the prepared samples was investigated employing the 2032 button battery technique, wherein a copper sheet was utilized as the substrate for coating the sample to create an anode, while lithium ions were employed as the reference electrode.

The active ingredient (sample obtained), conductive material (acetylene black), and binder (PVDF) were combined in a ratio of 8:1:1 by weight and dissolved in N-methyl pyrrolidone (NMP) solvent to create a slurry. This slurry was then applied onto circular copper sheets with a radius of 6 mm, followed by vacuum drying at 60 °C for 12 h. The electrochemical performance of the prepared electrodes was examined using cyclic voltammetry (CV) and constant current charge–discharge (GCD) measurements, conducted on electrochemical workstations (Chenhua CHI660E and Metrohm Autolab PGSTAT302N) (Switzerland), as well as a battery test system NEWARE CT-4008T (China). All electrochemical experiments utilized approximately 1 mg of active ingredients, and the mass loading of the active materials in the electrode was 1 mg/cm².

3. Results and Discussion

3.1. Structural Characterization

In Figure 1a, an anode composed of ZnMn₂O₄/V₂CT_x and a reference electrode consisting of a lithium sheet were utilized to fabricate LIBs. The two parallel electrodes were separated by a filter paper.

Figure 1b clearly illustrates the diffraction peaks of ZMO, which match well with standard PDF#71-2499. After etching V₂AlC (PDF#29-0101) with NH₄F+HCl, the characteristic diffraction peak at approximately 9.16° can be attributed to the (002) plane of V₂CTx MXene. The results demonstrate successful fabrication of pure ZnMn₂O4 and V₂CT_x MXene materials. For the ZMO/V₂CT_x sample, observed peaks correspond to crystal planes for both ZnMn₂O4 and V₂CT_x MXene without any significant impurity peak detected. It is noteworthy that compared to V₂CT_x alone, in the ZMO/V₂CT_x composite material, there is a slight shift in position from 9.16° to 9.24° for peak (002), indicating a more compact and stacked structure resulting from the grinding process on V₂CT_x MXene. Upon calcination at 300 °C, the XRD pattern shows no apparent changes except for a gradual decrease in diffraction peak intensity of V₂CT_x MXene, suggesting structural breakdown at high temperatures. However, when the temperature reaches 400 °C, the characteristic peaks strength of ZnMn₂O₄ gradually increases, along with the appearance of MnV₇O₁₄ phase (PDF#89-0484). As shown in Figure 1c, the Raman spectroscopy tests demonstrate that the 321 cm⁻¹ Raman peak observed in the ZMO electrode is a result of the vibration of the Zn-O bond within the tetrahedral structure of ZnFe₂O₄. Additionally, it can be inferred from the presence of a 635 cm⁻¹ Raman peak that there are symmetric stretching vibrations occurring in MnO₆ groups, indicating the existence of a spinel phase. The peak at around 138 cm⁻¹ represents a characteristic feature of V₂CT_x MXene, confirming successful preparation of the ZnMn₂O₄/V₂CT_x composite sample. Additionally, a characteristic peak near 844 cm⁻¹ confirms the synthesis of Mn₂V₂O₇ for ZMO/V₂CT_x-400 and is consistent with the XRD results. The FTIR spectra for synthesized ZMO and ZnMn₂O₄/V₂CT_x are plotted over a range spanning from 400 to 4000 cm⁻¹ and are shown in Figure 1d. In terms of pure ZMO (as seen in figure), an observed peak appears between regions ranging from 500 to 700 cm⁻¹, which may be due to M-O-M (where M = Zn, Mn) bonding; weak bands at approximately 1001.84 cm⁻¹ correspond to -OH vibrations; strong absorption around 1613 cm⁻¹ arises from C-O stretching vibrations; and finally, the peak located at 3436 cm⁻¹ corresponds to -OH stretching vibrations. The FTIR spectra for ZMO/V₂CT_x, ZMO/V₂CT_x-300, and ZMO/V₂CT_x-400 are generally consistent with those obtained for pure ZMO, except for the overall weaker intensity.

The morphology of ZMO and ZnMn₂O₄/V₂CT_x composites is depicted in Figure 2, revealing the binding between the micrometer rods of ZMO and the lamellae of V₂CT_x MXene. As the temperature increases, the bonding becomes more pronounced, leading to a spherical structure at 400 °C, with severe aggregation phenomena observed. The presence of Zn, Mn, O, V, and C elements on the sample surface is confirmed through elemental mapping (Figure 3).

The change in the valence state of ZMO/V₂CT_x-300 was investigated using XPS. The obtained spectra from the investigation (Figure 4a) indicate the presence of elements, including zinc (Zn), manganese (Mn), oxygen (O), vanadium (V), and carbon (C).

In Figure 4b, the observed peaks corresponding to energy levels of 1022.1 and 1045.05 eV can be assigned to the Zn 2p 3/2 and Zn 2p 1/2 orbitals, respectively. Similarly, in Figure 4c, the Mn 2p spectra exhibit two separate peaks at energies around 641.65 and 653.95 eV, suggesting the presence of distinct states corresponding to Mn 2p 3/2 and Mn 2p 1/2. In particular, it is worth noting that the peak corresponding to Mn³⁺ can be further resolved into four distinct peaks at approximate binding energies of around 641.6 eV (653.4 eV) and around binding energies of approximately 642.95 eV (654.8 5eV), which are attributed to different oxidation states: Mn³⁺ and Mn⁴⁺. The analysis of the O1s spectrum (Figure 4d) exhibited three separate peaks at binding energies of 530.15, 530.65, and 531.95 eV, respectively, suggesting the existence of diverse oxygen components on the surface. These peaks correspond to Mn-bonded lattice oxygen (OL), oxygen vacancy sites (Ov), and surface-absorbed hydroxyl groups (-OH). The oxygen vacancies act as adsorption sites for various oxygen species like O⁻, O²⁻, and Mn-OH. The presence of adsorbed surface oxygen components is responsible for the observed peak at 530.65 eV, which affects the oxygen vacancy state of the active substance. The V 2p spectrum depicted in Figure 4e displays two distinct peaks at approximately 517.45 and 525.25 eV, representing the binding energies of V 2p 3/2 and V 2p 1/2, respectively. Specifically, the V 2p spectrum can be accurately modeled with four peaks: V⁴⁺ at around 525.4 and 517.85 eV, as well as the coexisting V⁵⁺ at approximately 524.5 and 517.25 eV. Furthermore, the C1s spectra (Figure 4f) were fitted using a three-peak model with assignments made for C=O, C-O, and C-C at energies of about 286.55, 285, and 284.6 eV, respectively. The XPS results confirm the successful synthesis of the ZMO/V₂CT_x-300 composite.

3.2. Electrochemical Performance

The cyclic behavior of the ZMO and ZnMn₂O₄/V₂CT_x electrodes is depicted in Figure 5a. The ZMO/V₂CT_x-300 electrode demonstrates superior cycling performance compared to ZMO, exhibiting a remarkable reversible capacity of 163 mAh g⁻¹ after undergoing 100 cycles. In contrast, the capacity achieved by the ZMO electrode is lower at 99.5 mAh g⁻¹. It is important to mention that the Coulombic efficiency of the ZMO/V₂CT_x-300 electrode remains consistently close to 100% throughout continuous cycling, providing additional evidence supporting the electrochemical stability of this composite. The distinct advantages can be ascribed to the composite nature of the material and its calcination process, which enhance ion diffusion pathways and afford a greater abundance of active sites. Figure 5b illustrates the average discharge capacities at various current rates, where the values of 212.9, 169.4, 129.1, 72.4, and 29.7 mAh g⁻¹ correspond to current rates of 0.5, 1, 2, 5, and 10 C, respectively. Furthermore, after undergoing cycling at different current densities, electrodes demonstrated discharge capacities around approximately 167.99 and 216.3 mAh g⁻¹ when operated at rates of 1 and 0.5 C, respectively. The electrochemical charge–discharge curve shown in Figure 5c depicts the performance of ZMO/V₂CT_x-300 electrodes when discharged and charged at a rate of 1 C. In the second cycle, these electrodes exhibited a discharge capacity of 396.7 mAh g⁻¹, which remained consistent at a specific capacity of 163 mAh g⁻¹, even after undergoing 100 cycles. Additionally, it is worth noting that the charge–discharge curves have maintained their shape since the second cycle, indicating remarkable periodic electrochemical properties of the material.

The CV analysis was conducted on the ZMO and ZMO/V₂CT_x-300 cells to investigate the underlying reaction mechanism and chemical kinetics. Figure 6a,b illustrate the CV curves of the ZMO and ZMO/V₂CT_x-300 electrodes, respectively, using a scanning rate of 1 mV s⁻¹. It can be observed that the maximum point observed at around 1.2 V in the cyclotron voltammetry curve of ZMO corresponds to the conversion of Mn³⁺ into Mn²⁺. Similarly, the peak at approximately 0.3 V corresponds to the transformation of both Mn²⁺ and Zn²⁺ ions into elemental forms of Mn and Zn, respectively, resulting in the subsequent creation of the Li-Zn alloy at this specific potential. This can be represented by Equation [20]:

ZnMn₂O₄ + 8Li⁺ + 8e⁻ ↔ Zn + 2Mn + 4Li₂O,

(1)

Zn + xLi⁺ + xe⁻ ↔ Li_xZn (0 < x< 1).

(2)

The subsequent similarity of the curves demonstrates the robustness of the ZMO electrochemical properties. In comparison to Figure 6a,b exhibits certain alterations, such as a shift in its oxidation peak from 1.2 to 1.4 V, which aligns more closely with the oxidation peak position observed for V₂CT_x. The corresponding reaction equations for this transformation are provided below [21]:

V₂C/V₂CY₂ + xLi⁺ + xe⁻ ↔ V₂CLi_x/V₂CY₂Li_x (Y = F/OH).

(3)

The chemical kinetics of the cell were investigated using cyclovoltammetry at various scan rates (0.5, 1, 2, 5, and 10 mV s⁻¹), as depicted in Figure 6c,d. It was noted in the CV spectrum that a higher scanning rate led to a displacement of the oxidation peak towards higher potential, a movement of the reduction peak towards lower potential, and an enlargement of the current area due to Li⁺ polarization during continuous insertion and extraction processes.

4. Conclusions

In summary, ZnMn₂O₄/V₂CT_x composites were synthesized through high-temperature calcination. The results demonstrate that the structure and morphology of the ZnMn₂O₄/V₂CT_x material undergo alterations upon different temperature treatments. At 400 °C, the Mn₂V₂O₇ phase emerges, leading to a transformation in the morphology of the ZnMn₂O₄/V₂CT material from rod-like bonds to spherical structures and subsequently scaffold-like structures. Despite undergoing 100 cycles at a nominal specific capacity of 100 mAh g⁻¹, the ZMO/V₂CT_x-300 electrode exhibits an enduring specific discharge capacity of 163 mAh g⁻¹. This signifies an estimated surge of around 64% in contrast to the discharge capability (99.5 mAh g⁻¹) witnessed in the electrode composed solely of ZnMn₂O₄. These findings suggest that recombination and calcination processes effectively enhance both ionic transport capacity and conductivity within ZnMn₂O₄ materials. This research offers valuable perspectives on the effective utilization of AB₂O₄-based materials and MXene materials as electrodes with exceptional performance.