【Background】
Compared with lithium-ion batteries (LIBs), sodium-ion batteries (SIBs) have the advantages of low cost, abundant sodium resources, and high safety, so they show greater potential in the field of energy storage. However, due to the large radius of Na+ ions, sodium storage kinetics is slow, and the cycling and rate performance are poor. Metal thiophosphite (MTPs) is considered to be a promising new anode material for secondary batteries due to its unique two-dimensional layered structure, rich composition and high theoretical specific capacity. However, the low conductivity and fragility impair the rate and cycling performance of the MTP electrode. Compounding it with MXene conductive substrates has proven to be an effective strategy to improve the energy storage performance of various secondary materials. However, there are few reports on MTP/MXene composites. In addition, traditional mechanical or liquid-phase hybrid composite methods are time-consuming and labor-intensive. Stripping of MXene often requires the use of toxic hydrofluoric acid (HF), which poses a threat to the safety of the experiment. Therefore, it is necessary to develop an efficient, safe, and environmentally friendly strategy for the construction of MTP/MXene composites.
【Job Introduction】
Recently, the team of Professor Lei Shuijin of Nanchang University and Professor Xiong Shenglin of Shandong University proposed a new composite strategy, namely Lewis acid molten salt etching and synchronous phosphorus vulcanization strategy, and successfully prepared a series of MTP (FePS3, CoPS, NiPS3, CdPS3)/Ti3C2Tx MXene composites. This method greatly improves the preparation efficiency, avoids the use of HF acid, and improves the safety of the experiment. The introduction of Ti3C2Tx MXene enhances the conductivity of the composites and accelerates the transport of electrons and ions. The two-dimensional MTP is distributed in a vertical array on the surface of MXene, which effectively avoids the agglomeration between MTP nanosheets. Based on this, the prepared series of MTP/Ti3C2Tx MXene composite electrodes all exhibited excellent sodium storage performance. The article was published in the top international journal Advanced Functional Materials. Zhong Longsheng, a doctoral student at Nanchang University, is the first author of this paper.
【Core Contents】
MTP layered materials have a wide layer spacing (~0.65 nm), which provides ample space for the insertion and expulsion of Na+ ions. In addition, MTP contains both P and S elements, which can form different compounds with Na at different output voltages, thus providing a high theoretical specific capacity (~1300 mAh g−1). Therefore, MTP is a class of high-performance sodium electrode materials with potential. Due to its high electrical conductivity (typically 6000~8000 S cm−1), tunable surface electrochemistry and two-dimensional layered structure, MXene is often used as a substrate to improve the electrochemical properties of various secondary materials. The usual composite strategy involves a complex preparation process of MXene followed by mechanical or liquid-phase mixing, which greatly reduces experimental efficiency. In recent years, the Lewis acid melt salt method is an efficient MXene preparation strategy, which avoids the use of toxic HF acids and improves the safety of experiments. Importantly, during the etching process of MXene, an interesting intermediate can be generated, namely transition metal particles (e.g., Fe, Co, Ni, Cd, etc.)/MXene composites. These transition metal particles are evenly loaded on the MXene layer. Inspired by this, simultaneous phosphorus vulcanization of these intermediates is expected to yield a series of MTP/MXene complexes.
In this study, a series of MTP (FePS3, CoPS, NiPS3, CdPS3)/MXene composites were successfully prepared by using different molten salts. These in situ grown MTPs exhibit a distinct two-dimensional sheet-like structure that is uniformly distributed on the MXene surface, forming a three-dimensional cross-linked structure. This unique composite structure hinders the stacking of 2D MTP nanosheets, greatly enhancing the stability of the material and prolonging the cycle life of the electrode. In addition, the high conductivity MXene substrate promotes rapid electron/charge transport and increases the rate capability of the electrode.
Taking the FePS3/Ti3C2Tx MXene composite anode as an example, the material exhibits excellent sodium storage performance. At current densities of 0.1 A g-1 and 10 A g-1, they exhibit high specific capacities of 828 mAh g-1 and 598 mAh g-1, respectively. After 2000 cycles at a current density of 5 A g-1, it is still able to provide a discharge specific capacity of 596 mAh g-1. Due to the enhanced conductivity, the FePS3/Ti3C2Tx complex exhibits a smaller charge transfer resistance and a higher Na⁺ diffusion coefficient compared to the pure FePS3 electrode. Similarly, NiPS3/Ti3C2Tx, CdPS3/Ti3C2Tx, and CoPS/Ti3C2Tx anodes also exhibit excellent sodium storage performance.
In situ XRD and ex situ HRTEM characterization techniques have been applied to study sodium storage mechanisms. The results show that the weak P–S bond is irreversibly broken during the first charge and discharge, and Na3FeS3 and P products are formed. These products together contribute to a high reversible specific capacity through a conversion-alloy reaction with Na+ during the subsequent charge-discharge process. In addition, DFT theoretical calculations were used to study the effect of the introduction of MXene substrate on the improvement of electrochemical performance. The results show that the FePS3/Ti3C2Tx complex exhibits higher density of states, greater Na+ adsorption energy and lower diffusion barrier near the Fermi level than the pure FePS3 anode. As a result, the FePS3/Ti3C2Tx complex exhibits better rate performance.
【Graphic Guide】
Figure 1 Synthesis roadmap of MTP/MXene
Fig.2 Structural characterization of MTP/MXene complexes
Fig.3 Sodium storage performance of FePS3/Ti3C2Tx composite electrode
Fig.4 Sodium storage mechanism of FePS3/Ti3C2Tx composite electrode
【Conclusion】
This study has developed a simple, safe and environmentally friendly new method to efficiently prepare phosphorothioate/Ti3C2Tx composites for use as anode materials for sodium-ion batteries by Lewis molten salt etching and simultaneous phosphorus vulcanization process. The in-situ modified phosphosate nanosheets on the surface of Ti3C2Tx MXene form a three-dimensional cross-linked structure, which not only facilitates the penetration of the electrolyte, but also increases the active site and shortens the ion diffusion path. The introduction of Ti3C2Tx substrate improved the conductivity of phosphorothiothioate and mitigated the volume expansion during sodium/desodification. These positive effects make the MTP/Ti3C2Tx composites exhibit excellent sodium storage properties. In situ XRD and ex situ HRTEM characterization revealed the energy storage mechanism of the FePS3/Ti3C2Tx anode, involving the cleavage of P−S bonds to form P and Na3FeS3, which together provide high specific capacity. This study further expands the application scope of Lewis molten salt etching and provides a new method for the preparation of high-performance rechargeable battery anodes by Lewis molten salt etching and simultaneous phosphorus vulcanization.
Longsheng Zhong, Ming Yue, Yazhan Liang, Baojuan Xi, Xuguang An, Yanhe Xiao, Baochang Cheng, Shuijin Lei*, and Shenglin Xiong*, In–Situ universal construction of thiophosphite/MXene hybrids via Lewis acidic etching for superior sodium storage. Advanced Functional Materials, 2024. DOI:0.1002/adfm.202407740
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