In recent years, the development of high-performance energy storage systems has become crucial for meeting global energy demands. Among various technologies, solid-state batteries have garnered significant attention due to their enhanced safety, stability, and potential for high energy density. Specifically, solid-state zinc-ion batteries represent a promising alternative to conventional lithium-based systems, offering advantages such as low cost, environmental friendliness, and high theoretical capacity. However, challenges like zinc dendrite formation, cathode dissolution, and low ionic conductivity at room temperature hinder their practical application. To address these issues, we explore the use of composite aerogels based on carbon nanotubes (CNT) and Ti3C2Tx MXene as additives in deep eutectic solvent (DES)-based solid electrolytes. This approach aims to improve ionic transport, mechanical strength, and overall performance of solid-state batteries.
Our work focuses on synthesizing Ti3C2Tx-CNT composite aerogels via a freeze-drying method and incorporating them into a zinc-based deep eutectic solvent to form solid-state electrolytes. The DES consists of zinc triflate (Zn(OTF)2) and acetamide ligands, which facilitate high ionic conductivity upon crystallization. By varying the CNT content in the aerogel, we optimize the electrolyte’s properties, including ion transport and interfacial stability. The resulting solid-state batteries exhibit remarkable cycling performance and dendrite-free zinc plating/stripping, making them suitable for large-scale energy storage applications. Throughout this study, we emphasize the role of solid-state battery components in enhancing electrochemical behavior, and we present data through tables and mathematical models to elucidate key mechanisms.
The preparation of Ti3C2Tx-CNT composite aerogels involves dispersing Ti3C2Tx MXene in aqueous solution and adding specific amounts of CNT (e.g., 20 mg, 30 mg, and 40 mg) to achieve different mass ratios. After stirring and ultrasonication, the mixtures are freeze-dried to form porous aerogels, labeled as Ti3C2Tx-CNT0.1, Ti3C2Tx-CNT0.15, and Ti3C2Tx-CNT0.2, respectively. These aerogels serve as nucleation agents to induce the crystallization of the DES, which is prepared by heating Zn(OTF)2 and acetamide at 80°C until a homogeneous liquid forms. The composite additives are then mixed into the DES, and the resulting blend is cast onto a glass fiber membrane and solidified at room temperature to produce the solid-state electrolytes, termed Ti3C2Tx-CNT0.1/ZCEs, Ti3C2Tx-CNT0.15/ZCEs, and Ti3C2Tx-CNT0.2/ZCEs.
Structural characterization reveals that the Ti3C2Tx-CNT composite aerogels exhibit a highly porous network, with CNT acting as a bridging agent between MXene layers. This morphology enhances the surface area and pore volume, facilitating ion diffusion in the solid-state battery electrolyte. X-ray diffraction (XRD) analysis confirms the presence of CNT characteristic peaks, while MXene peaks are absent, indicating a disordered nanostructure due to strong interactions between the components. Scanning electron microscopy (SEM) images show rough surfaces and interconnected pores, which become more pronounced with increasing CNT content. This hierarchical structure is critical for providing continuous pathways for zinc ion transport in solid-state batteries.
To evaluate the electrochemical performance of the solid-state electrolytes, we measure ionic conductivity using electrochemical impedance spectroscopy (EIS) on symmetric stainless steel cells. The ionic conductivity (σ) is calculated using the formula:
$$ \sigma = \frac{L}{R_b \times A} $$
where L is the thickness of the electrolyte, Rb is the bulk resistance obtained from the EIS Nyquist plot, and A is the contact area. The results for different electrolytes are summarized in Table 1, demonstrating that Ti3C2Tx-CNT0.15/ZCEs achieves the highest ionic conductivity of 6.71 × 10−3 S/cm at room temperature. This enhancement is attributed to the optimal CNT content, which balances pore structure and ion transport channels. In contrast, lower or higher CNT amounts lead to reduced conductivity due to excessive aggregation or blocked pathways.
| Electrolyte | CNT Content (mg) | Ionic Conductivity (S/cm) |
|---|---|---|
| Ti3C2Tx-CNT0.1/ZCEs | 20 | 5.23 × 10−3 |
| Ti3C2Tx-CNT0.15/ZCEs | 30 | 6.71 × 10−3 |
| Ti3C2Tx-CNT0.2/ZCEs | 40 | 4.89 × 10−3 |
Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) tests are conducted to assess the electrochemical stability window and reaction kinetics of the solid-state batteries. The LSV curves show that Ti3C2Tx-CNT0.15/ZCEs exhibits a wide stability window up to 2.17 V, which is sufficient for zinc-ion battery operations. CV measurements at scan rates from 0.5 to 10 mV/s reveal broad oxidation and reduction peaks, indicating fast ion transport and pseudocapacitive behavior. The peak currents (ip) are analyzed using the power-law relationship:
$$ i_p = a \times v^b $$
where v is the scan rate, and b is a parameter that distinguishes between diffusion-controlled (b = 0.5) and capacitive (b = 1) processes. For Ti3C2Tx-CNT0.15/ZCEs, the b values for anodic and cathodic peaks are 0.71 and 0.69, respectively, suggesting a mixed mechanism of diffusion and pseudocapacitance. This synergy enhances the rate capability of solid-state batteries, as confirmed by galvanostatic charge-discharge tests.
We assemble solid-state zinc-ion batteries using V2O5 as the cathode material and the composite electrolytes. The V2O5 electrodes are prepared by mixing 70 wt% V2O5, 20 wt% conductive carbon black, and 10 wt% polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (NMP) solvent, followed by coating on graphite paper and drying. The batteries are cycled at 0.2 C rate within a voltage range of 0.4 to 1.6 V. The cycling performance, illustrated in Table 2, shows that Ti3C2Tx-CNT0.15/ZCEs-based battery delivers a high reversible capacity of 250.37 mAh/g and retains 57.63 mAh/g after 100 cycles, outperforming the other electrolytes. This stability is attributed to the robust interface between the electrolyte and electrodes, which suppresses zinc dendrite growth and side reactions.
| Electrolyte | Initial Capacity (mAh/g) | Capacity after 100 cycles (mAh/g) | Capacity Retention (%) |
|---|---|---|---|
| Ti3C2Tx-CNT0.1/ZCEs | 180.45 | 33.43 | 18.5 |
| Ti3C2Tx-CNT0.15/ZCEs | 250.37 | 57.63 | 23.0 |
| Ti3C2Tx-CNT0.2/ZCEs | 160.89 | 28.30 | 17.6 |
Electrochemical impedance spectroscopy (EIS) is employed to investigate the interfacial resistance and ion diffusion in the solid-state batteries. The Nyquist plots consist of a semicircle at high frequencies, representing charge transfer resistance (Rct), and a linear region at low frequencies, associated with Warburg diffusion. The data is fitted using an equivalent circuit model, and the diffusion coefficient (D) of zinc ions is estimated from the Warburg impedance using the equation:
$$ D = \frac{R^2 T^2}{2 A^2 n^4 F^4 C^2 \sigma_W^2} $$
where R is the gas constant, T is temperature, A is electrode area, n is number of electrons, F is Faraday’s constant, C is ion concentration, and σW is the Warburg coefficient. For Ti3C2Tx-CNT0.15/ZCEs, the low Rct and high D values indicate facilitated ion transport and reduced polarization, contributing to the superior performance of solid-state batteries.
Furthermore, we analyze the mechanical properties of the composite electrolytes, as they play a vital role in preventing short circuits caused by zinc dendrites. The stress-strain behavior is modeled using the following relation for porous materials:
$$ \sigma = E \epsilon (1 – \phi) $$
where σ is stress, E is Young’s modulus, ε is strain, and φ is porosity. The Ti3C2Tx-CNT composite aerogels exhibit high elasticity and strength due to the cross-linking between MXene sheets and CNT, which enhances the durability of solid-state batteries during repeated cycling. This mechanical integrity, combined with high ionic conductivity, makes these electrolytes promising for long-lasting energy storage.
In conclusion, our study demonstrates that Ti3C2Tx-CNT composite aerogels effectively improve the properties of deep eutectic solvent-based solid electrolytes for zinc-ion batteries. The optimized Ti3C2Tx-CNT0.15/ZCEs electrolyte achieves high ionic conductivity, wide electrochemical window, and excellent cycling stability, enabling dendrite-free operation in solid-state batteries. These findings highlight the potential of MXene-CNT hybrids in advancing solid-state battery technology, particularly for applications requiring safety and high energy density. Future work will focus on scaling up the synthesis and integrating these electrolytes into flexible and wearable solid-state batteries to broaden their practical impact.
Overall, the development of such composite materials underscores the importance of interdisciplinary approaches in material science and electrochemistry. By leveraging the unique properties of MXene and CNT, we can overcome existing limitations in solid-state batteries and pave the way for next-generation energy storage systems. The continuous optimization of solid-state battery components will undoubtedly lead to more efficient and sustainable solutions for global energy challenges.