Achieving High Pseudocapacitance Anode SnP2O7@C By an in Situ Self-Nanocrystallization Method for Ultrastable Sodium Storage
Authors: Mu Yuhe , He Wei , Zhou Yue 
Published: September 2023

Title: High Pseudocapacitance Anode SnP2O7@C by In Situ Self-Nanocrystallization Method for Ultrastable Sodium Storage
The rich resource and cheap price of Na have attracted attention in energy storage research and the market, becoming an optional choice for Li-ion batteries. The large radius of sodium-ion leads to a high diffusion barrier that restricted the development of sodium-ion batteries. Therefore, anodes with high capacity and stable cyclability are urgently addressed before further utilization.
The mechanism of sodium-ion batteries can be classified into three categories: intercalation, alloying and conversion. Anode based on alloying and conversion mechanism became a research hotspot due to superior theoretical capacity. In pursuit of surpassing capacity, recent studies concentrated on anode with mixed mechanism, which had the potential to marriage the benefit of alloying and conversion mechanisms. However, the large volume expansion and short lifetime hindered the application in batteries.
Here, we demonstrated an ultrastable sodium battery based on a novel anode, pomegranate-like SnP2O7 nanoparticles with a mixed mechanism homogeneously dispersed in the N-doped carbon matrix. Without the constraint of the grinding method, in situ self-nanocrystallization method has been first used to generate ultra-fine SnP2O7 particles.
The results of the Ex-situ transmission electron microscope (TEM) characterization exhibited that the size range of SnP2O7 particles was efficiently decreased by the new fabrication method, from 66 to 20nm. During cycling tests, the nanostructure SnP2O7 particles remained well distributed in the N-doped carbon matrix rather than aggregation under long cycling tests. The self-nanocrystallization SnP2O7 nanoparticles ameliorated the diffusion of sodium-ion, which attained the acceleration of reaction kinetics. The N-doped carbon matrix contributed to the conductivity of composite material and meanwhile form strong adhesion with the carbon matrix to maintain the nanostructure SnP2O7 particles active during charge and discharge cycling. The pomegranate-like SnP2O7@C nanomaterial enhanced the pseudocapacitance to attain higher capacity and fast-charging storage. Under the capacitive-controlled behaviour test at 0.4mVs-1, the calculated capacitive showed a dominant contribution of total capacity, made up 75.7% of all. The participation of capacitive promoted avert the slow diffusion of sodium ion and obviate the destruction of the structure. The SnP2O7 carbon matrix anode exhibited superior specific capacity 403 mAh g-1 at 200 mA g-1 and cycling stability (185 mAh g-1 at 1000 mA g-1). The cycling stability of SnP2O7 nanomaterial has been ameliorated by the protection of N-doped carbon. The coulombic efficiency of SnP2O7@C showed no obvious changes and maintained around 100%, while the CE of SnP2O7 had appreciable variation. A new fabrication route of conversion and alloying anode nanomaterial was introduced in this work and has the potential to be utilized in large-scale manufacture.

Figure 1