A tin-based composite oxide confined by reduced graphene oxide as a high-rate anode for sodium-ion capacitors†

Abstract

A conversion-alloy type anode, Sn₂P₂O₇, is a promising candidate for sodium-ion capacitors (SICs) due to its high theoretical capacity, low cost, and nontoxic nature, but suffers from poor conductivity and large volume expansion. Herein, we propose a mild self-assembly strategy that achieves Sn₂P₂O₇ confined by reduced graphene oxide (rGO) as an anode for sodium storage, thereby yielding an impressive specific capacity of 433.3 mA h g⁻¹ at 0.1 A g⁻¹ and exceptional rate capability of 185.7 mA h g⁻¹ at a high current density of 10 A g⁻¹. Notably, ex situ TEM reveals the underlying evolution that Sn₂P₂O₇ particles are pulverized into nanodots with stable size as the cycle progresses and the rGO continuously sustains the electron conduction of Sn₂P₂O₇ nanodots after long-term cycling. Quantitative kinetic analysis quantitatively deciphers the dominant role of pseudocapacitance in the sodium storage process. Meanwhile, density functional theory calculations indicate that the interfacial binding between rGO and Sn₂P₂O₇ is dramatically conducive to accelerating electron transfer. The assembled Sn₂P₂O₇/rGO//AC SIC delivers a superior gravimetric energy/power density of 158.3 W h kg⁻¹/2523.3 W kg⁻¹. This work provides a foundation for the structural design of high-rate and long-life tin-based composite oxide anodes.