Tin-Based Composite Oxide Confined by Reduced Graphene Oxide as a High-Rate Anode for Sodium-Ion Capacitors

Abstract

Conversion-alloy type anode, Sn2P2O7, is a potential candidate for sodium-ion capacitors (SICs) due to the 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 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 cycles. 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 Wh kg-1/ 2523.3 W kg-1. This work provides a basis for the structural design of high-rate and long-life tin-based composite oxide anodes.