Ionic-liquid-bifunctional wrapping of ultrafine SnO2 nanocrystals into N-doped graphene networks: high pseudocapacitive sodium storage and high-performance sodium-ion full cells

Conversion-Alloying Anode Materials for Sodium Ion Batteries

The past decade has witnessed a rapidly growing interest toward sodium ion battery (SIB) for large-scale energy storage in view of the abundance and easy accessibility of sodium resources. Key to addressing the remaining challenges and setbacks and to translate lab science into commercializable products is the development of high-performance anode materials. Anode materials featuring combined conversion and alloying mechanisms are one of the most attractive candidates, due to their high theoretical capacities and relatively low working voltages. The current understanding of sodium-storage mechanisms in conversion-alloying anode materials is presented here. The challenges faced by these materials in SIBs, and the corresponding improvement strategies, are comprehensively discussed in correlation with the resulting electrochemical behavior. Finally, with the guidance and perspectives, a roadmap toward the development of advanced conversion-alloying materials for commercializable SIBs is created.

Achieving High Pseudocapacitance Anode by An In Situ Nanocrystallization Strategy for Ultrastable Sodium-Ion Batteries

Conversion/alloying type anodes have shown great promise for sodium-ion batteries (SIBs) because of their high theoretical capacity. However, the poor structural stability derived from the large volume expansion and short lifetime impedes their further practical applications. Herein, we report a novel anode with a pomegranate-like nanostructure of SnP₂O₇ particles homogeneously dispersed in the robust N-doped carbon matrix. For the first time, we make use of in situ self-nanocrystallization to generate ultrafine SnP₂O₇ particles with a short pathway of ions and electrons to promote the reaction kinetics. Ex situ transmission electron microscope (TEM) shows that the average particle size of SnP₂O₇ decreases from 66 to 20 nm successfully based on this unique nanoscale-engineering method. Therefore, the nanoparticles together with the N-doped carbon contribute a high pseudocapacitance contribution. Moreover, the N-doped carbon matrix forms strong interaction with the self-nanocrystallization ultrafine SnP₂O₇ particles, leading to a stable nanostructure without any particle aggregation under a long-cycle operation. Benefiting from these synergistic merits, the SnP₂O₇@C anode shows a high specific capacity of 403 mAh g^-1 at 200 mA g^-1 and excellent cycling stability (185 mAh g^-1 after 4000 cycles at 1000 mA g^-1). This work presents a new route for the effective fabrication of advanced conversion/alloying anodes materials for SIBs.

Inside the failure mechanism of tin oxide as anode for sodium ion batteries

The conversion-alloying compounds have been identified as promising anode materials for sodium ion batteries (SIBs). One of them, SnO2, with an enormous theoretical capacity of 1558 mAh g−1 is an interesting candidate, also due to its low cost, environmental friendliness and wide availability of tin. However, many drawbacks limit its application in commercial batteries. In this paper, SnO2 has been synthesized from cheap reagents by using simple and easily scalable coprecipitation synthesis routes obtaining nanoparticles with sizes between 2 and 14 nm with almost spherical morphologies. The reasons of the failure of the alloying/de-alloying process were investigated by combining the results obtained from common electrochemical techniques, providing useful examples for the investigation of every material with analogous electrochemical features. Thanks to cyclic voltammetry, different reaction paths were detected for the two samples. The first cycle irreversibility was well characterized with electrochemical impedance spectroscopy, showing interesting trends in the values of the resistance. Galvanostatic cycling with potential limitations was employed to quantify the irreversibility, finding out that the most crystalline sample reached the terminal phase in the Sn-Na system (Na15Sn4), while the least crystalline sample could not achieve such a result (Na3Sn). The crystallinity of SnO2 was determined to be a key parameter, often neglected, for the realization of satisfactory anode compounds.

Ferrocene as a Novel Additive to Enhance the Lithium-Ion Storage Capability of SnO2/Graphene Composite

Improving the reversibility of conversion reaction is a promising way to enhance the lithium-ion storage capability of SnO₂-based anodes. Herein, we report ferrocene as a novel additive to improve the Li-ion storage performance of the SnO₂/graphene (SnO₂/G) composite. Through a simple mixing method, ferrocene can be uniformly dispersed into the SnO₂/G electrode. It is found that the ferrocene additive can effectively suppress the agglomeration of Sn/SnO₂ and retain the nanoscale Sn/Li₂O interface. Furthermore, metallic Fe is formed from ferrocene in the discharge process and acts as a catalyst to promote the reversible conversion between Sn/Li₂O and SnO₂. As a result, the SnO₂/G electrode with the addition of 10 wt % ferrocene (10%Fc-SnO₂/G) exhibits a superior Li-ion storage performance. It displays a reversible capacity of up to 1084.5 mAh g^-1 at 0.1 A g^-1 after 150 cycles with a good rate capability (752 mAh g^-1 at 1 A g^-1). In addition, the 10%Fc-SnO₂/G electrode can retain a capacity of 787.2 mAh g^-1 at 0.5 A g^-1 after 220 cycles. This work demonstrates the promising additive of ferrocene in enhancing the reversible capacity of SnO₂-based anodes for lithium-ion batteries.