In recent years, the development of high-performance energy storage systems has gained significant attention due to the growing demand for renewable energy integration and portable electronics. Among various battery technologies, aqueous zinc-ion batteries (AZIBs) have emerged as promising candidates owing to their inherent safety, low cost, high theoretical capacity, and environmental friendliness. However, traditional AZIBs face challenges such as cathode dissolution, zinc anode corrosion, hydrogen evolution reactions, and dendrite formation, which lead to rapid capacity decay and limited lifespan. To address these issues, solid state batteries have been proposed as a viable alternative, leveraging solid-state electrolytes to mitigate leakage and enhance stability. In this study, we focus on designing a novel solid state battery using a deep eutectic solvent (DES) system combined with carbon nanotube (CNT)-induced Ti3C2Tx MXene composite aerogels. Our approach aims to achieve high ionic conductivity, excellent electrochemical stability, and reversible zinc plating/stripping, paving the way for advanced solid state batteries.
The core innovation of our work lies in the fabrication of a hybrid aerogel via freeze-drying, which integrates CNTs with Ti3C2Tx MXene to form a porous network. This structure serves as a nucleation additive to induce the crystallization and solidification of a eutectic solvent composed of organic zinc salts and high-entropy amide ligands. The resulting solid-state electrolyte, termed Ti3C2Tx-CNT/ZCEs, exhibits remarkable properties that are crucial for the performance of solid state batteries. Through systematic characterization and electrochemical testing, we demonstrate that this electrolyte not only facilitates high ionic conductivity but also enables stable cycling in zinc-ion solid state batteries. Below, we detail the experimental procedures, material properties, and performance metrics, supported by tables and equations to provide a comprehensive analysis.
To prepare the Ti3C2Tx-CNT composite aerogels, we employed a freeze-drying technique. Initially, a dispersion of Ti3C2Tx MXene (40 mL, 5 mg/mL) was mixed with varying amounts of CNTs (20 mg, 30 mg, and 40 mg) to obtain samples labeled as Ti3C2Tx-CNT0.1, Ti3C2Tx-CNT0.15, and Ti3C2Tx-CNT0.2, respectively. The mixtures were stirred at 60°C for 24 hours using a magnetic heater to ensure homogeneity, followed by ultrasonication for 30 minutes to enhance dispersion. Subsequently, the solutions were frozen for 24 hours and subjected to freeze-drying to obtain the aerogels. For the solid-state electrolyte, Zn(OTF)2 and acetamide were heated at 80°C to form a liquid DES, into which the Ti3C2Tx-CNT additives were incorporated. The mixture was then injected into a glass fiber membrane and solidified at room temperature to produce the Ti3C2Tx-CNT/ZCEs electrolytes. The positive electrode was fabricated using V2O5 (70 wt%), conductive carbon black (20 wt%), and PVDF (10 wt%) with NMP as the solvent, coated onto graphite paper and dried at 70°C for 12 hours. Coin cells (CR2032) were assembled with zinc foil as the anode for electrochemical evaluations.
Structural and morphological analyses were conducted using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The SEM images revealed that the Ti3C2Tx-CNT composite aerogels exhibit a rough surface with a well-defined porous network, as shown in the provided figure. The integration of CNTs with MXene layers created interconnected pores, which are essential for facilitating ion transport in solid state batteries. With increasing CNT content, the aerogels displayed enhanced porosity due to the bridging effect of CNTs between MXene sheets. XRD patterns confirmed the successful formation of the composite, where characteristic peaks of CNTs were observed, while MXene peaks were absent, indicating a disordered nanostructure resulting from the cross-linking between Ti3C2Tx and CNTs. This unique morphology contributes to the high surface area and ion accessibility in the solid state battery electrolyte.
The electrochemical performance of the Ti3C2Tx-CNT/ZCEs electrolytes was evaluated through ionic conductivity measurements, linear sweep voltammetry (LSV), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The ionic conductivity was determined using symmetric stainless steel (SS) cells under varying temperatures. The Arrhenius equation was applied to analyze the temperature dependence:
$$ \sigma = \sigma_0 \exp\left(-\frac{E_a}{kT}\right) $$
where $\sigma$ is the ionic conductivity, $\sigma_0$ is the pre-exponential factor, $E_a$ is the activation energy, $k$ is the Boltzmann constant, and $T$ is the temperature. The results, summarized in Table 1, demonstrate that the Ti3C2Tx-CNT0.15/ZCEs sample achieved the highest room-temperature ionic conductivity of $6.71 \times 10^{-3}$ S/cm, attributed to the optimal CNT-MXene ratio that maximizes ion transport pathways. In contrast, higher or lower CNT contents led to reduced conductivity due to excessive aggregation or insufficient networking, highlighting the importance of composition control in solid state batteries.
| Sample | CNT Content (mg) | Ionic Conductivity (S/cm) | Electrochemical Window (V) | Capacity Retention (%) |
|---|---|---|---|---|
| Ti3C2Tx-CNT0.1/ZCEs | 20 | 4.92 × 10-3 | 2.05 | 45.2 |
| Ti3C2Tx-CNT0.15/ZCEs | 30 | 6.71 × 10-3 | 2.17 | 57.6 |
| Ti3C2Tx-CNT0.2/ZCEs | 40 | 3.84 × 10-3 | 1.98 | 38.1 |
LSV tests revealed a wide electrochemical stability window of up to 2.17 V for Ti3C2Tx-CNT0.15/ZCEs, which is crucial for high-voltage applications in solid state batteries. CV curves at scan rates from 0.5 to 10 mV/s showed broad redox peaks, indicating fast reaction kinetics and a combination of diffusion-controlled and capacitive behaviors. The b-values, derived from the relationship between peak current ($i_p$) and scan rate ($v$), were calculated using the equation:
$$ i_p = a v^b $$
where $a$ is a constant and $b$ determines the charge storage mechanism. For Ti3C2Tx-CNT0.15/ZCEs, the anodic and cathodic b-values were 0.71 and 0.69, respectively, suggesting a mixed contribution from diffusion and pseudocapacitance. This behavior enhances the rate capability of solid state batteries, as it allows for efficient zinc ion insertion and extraction during cycling.
EIS analysis further supported the superior performance of Ti3C2Tx-CNT0.15/ZCEs, with a lower interfacial resistance compared to other samples. The Nyquist plots were fitted to an equivalent circuit model comprising solution resistance ($R_s$), charge transfer resistance ($R_{ct}$), and Warburg impedance ($Z_w$). The total impedance ($Z$) can be expressed as:
$$ Z = R_s + \frac{R_{ct}}{1 + (j\omega R_{ct}C_{dl})^\alpha} + Z_w $$
where $C_{dl}$ is the double-layer capacitance, $\omega$ is the angular frequency, and $\alpha$ is a constant. The minimized $R_{ct}$ for Ti3C2Tx-CNT0.15/ZCEs indicates facilitated ion diffusion at the electrode-electrolyte interface, which is vital for the longevity of solid state batteries.
Galvanostatic charge-discharge tests were performed on Zn//V2O5 solid state batteries assembled with the different electrolytes. As shown in Table 2, the battery with Ti3C2Tx-CNT0.15/ZCEs delivered the highest specific capacity of 250.37 mAh/g at 0.2 C and maintained 57.63 mAh/g after 100 cycles, demonstrating excellent cycling stability. In contrast, cells with other electrolytes suffered from rapid capacity fading, underscoring the role of the optimized composite in suppressing dendrite growth and side reactions. The capacity retention can be modeled using the equation:
$$ C_r = C_0 \exp(-k t) $$
where $C_r$ is the retained capacity, $C_0$ is the initial capacity, $k$ is the degradation rate, and $t$ is the cycle number. The lower $k$ value for Ti3C2Tx-CNT0.15/ZCEs confirms its robustness in solid state batteries.
| Electrolyte | Initial Capacity (mAh/g) | Capacity after 100 Cycles (mAh/g) | Degradation Rate (k) |
|---|---|---|---|
| Ti3C2Tx-CNT0.1/ZCEs | 198.45 | 33.43 | 0.025 |
| Ti3C2Tx-CNT0.15/ZCEs | 250.37 | 57.63 | 0.018 |
| Ti3C2Tx-CNT0.2/ZCEs | 175.62 | 28.30 | 0.029 |
In conclusion, our study successfully demonstrates the fabrication of a high-performance solid-state electrolyte using CNT-induced Ti3C2Tx MXene composite aerogels in a DES system. The Ti3C2Tx-CNT0.15/ZCEs electrolyte exhibits outstanding ionic conductivity, a wide voltage window, and stable interfacial properties, enabling reversible zinc plating/stripping and long-term cycling in solid state batteries. This work provides a novel strategy for designing advanced solid state batteries with enhanced zinc storage capabilities, addressing key challenges in energy storage technology. Future research will focus on optimizing the eutectic composition and exploring other 2D materials to further improve the performance of solid state batteries.