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To limit global climate change and promote the transition to a less carbon-intensive and more sustainable energy system, the world is rapidly turning to renewable energy sources, and the development of efficient energy storage devices to integrate intermittent energy production is a key issue. Undoubtedly, rechargeable batteries have been regarded as a highly promising energy storage system, and lithium-ion batteries occupy a pivotal position in energy storage systems due to their advantages such as high energy density, operating voltage, specific capacity, and long cycle life. However, the non-universal distribution of lithium resources and their depletion have inevitably led to price increases and raised concerns about large-scale applications: for these reasons, alternative energy storage systems involving less critical elements need to be explored. Sodium-ion batteries (SIBs) appear to be a valuable option because sodium is an element that is ubiquitous, abundant, and low-cost. Notably, sodium and lithium have similar chemical and physical properties and similar intercalation chemistry. Despite these attractive features and recent progress, several shortcomings still need to be addressed to develop SIBs with high energy and power density, good life, and fast sodium ion diffusion and kinetics.
The main point of this paper
Research progress of cathode materials:
Layered/tunnel-shaped transition metal oxides: widely studied as SIB cathodes, but there are problems with volume change and phase instability.
Prussian blue analogs: promising due to their low cost, easy synthesis and fast Na+ migration characteristics.
Polyanion compounds: provide a strong three-dimensional polyanion framework and ion migration channels, and are suitable positive electrode materials.
NASICON structure cathode: by changing the transition metal ions to adjust the electrochemical properties, compounds such as Na3V2(PO4)3, Na3Cr2(PO4)3 and Na3Fe2(PO4)3 have been widely studied, and their electronic conductivity is improved by carbon coating or doping them.
Application of carbon nanofibers (CNFs):
CNFs are favorable fillers and matrices for anode materials or electrode materials in the energy field, with good electronic conductivity and high specific surface area.
CNFs are synthesized by electrospinning, and the resulting non-woven fabric sheets have ideal porosity, which is conducive to electrolyte penetration and allows volume changes during sodiumation/de-sodiumation.
CNFs are suitable for making stand-alone electrodes, avoiding the use of Al foil as active material support and current collector.
Preparation and characterization of self-standing cathode:
In this study, Na3MnZr(PO4)3/CNF self-standing cathode was prepared and characterized for the first time, and its electrochemical performance for application in SIB was tested.
The self-standing cathode was prepared on pre-electrospun CNFs by droplet method, and the pre-synthesized Na3MnZr(PO4)3 was dispersed in the polymer solution to be electrospun.
Different electrospinning settings (horizontal and vertical settings) were applied to optimize the preparation method.
Evaluation of electrochemical performance:
The electrochemical performance of the stand-alone electrode was evaluated and compared with the conventional tape-cast cathode, expecting to obtain better electrochemical performance, especially at high carbonization rate.
Determination of synthesis method:
Based on the physicochemical characteristics and electrochemical results, the most effective synthesis method for preparing binder-free electrodes was determined
Material advantages:
Na3MnZr(PO4)3 is a high-voltage cathode material suitable for sodium-ion batteries.
Preparation technology:
Na3MnZr(PO4)3 was loaded onto CNFs using an electrospinning method to form a self-standing cathode.
Active material loading strategy:
Na3MnZr(PO4)3 powder was mixed with polymer solution using different strategies and formed into composites by electrospinning.
Characterization confirmation:
The successful synthesis and structure of the material were verified using techniques such as XRPD, SEM, TEM, EDS, TGA and Raman spectroscopy.
Electrochemical performance:
The Na3MnZr(PO4)3/CNF self-standing cathode exhibited higher specific capacity than traditional tape-cast electrodes, especially at high C rates.
Structural advantages:
The porous CNF matrix improves the adaptability to electrolyte penetration and volume changes.
Optimal preparation conditions:
The vertically set electrospinning method achieves uniform distribution and quantitative loading of active materials
In this study, Na3MnZr(PO4)3/CNF standalone cathodes for SIBS were prepared by different synthesis methods based on electrospinning technology. Regardless of the method used, the active materials were successfully loaded into CNFs and maintained the nasicon-type crystal structure. In fact, the synthesis route has an impact on the amount and distribution of active materials loaded on the carbon nanofiber sheets. Compared with the drop-coating method, the dispersion of the pre-synthesized active materials method allows the Na3MnZr(PO4)3 particles to be evenly distributed along the film thickness, and the aggregates are both connected and covered on the CNF surface and embedded in the CNF interior. This close contact between the active material and the CNF is conducive to the improvement of the electrochemical performance. In terms of setup, the vertical setup seems to be more effective because the dumping of particles is avoided and the active materials are quantitatively loaded into CNFs. In fact, the v-30%MnZr/CNF sample showed the best electrochemical performance.
Independent of the synthesis method, the electrochemical performance of the standalone cathode in terms of specific discharge capacity at different C-rates is improved compared with the tape-cast cathode. It is noteworthy that this enhancement is particularly evident at high carbon rates because the porosity of the nonwoven nanofibers ensures the diffusion of the electrolyte and easy contact with the active material aggregates. The v-30%MnZr/CNF electrode exhibits the best electrochemical performance and a long cycle life compared to the tape-cast electrode. The results of this study indicate that the ex situ synthesis of Na3MnZr(PO4)3 and its addition (30 wt%) to the carbon precursor solution for electrospinning is a simple and feasible method to obtain a self-standing cathode with enhanced electrochemical performance compared to the tape-cast cathode (70 wt% of active material).
