Highly Reversible and Rapid Sodium Storage in GeP3 with Synergistic Effect from Outside-In Optimization.

The composite GeP3/C@rGO as a sodium ion battery anode material was fabricated by introducing carbon matrix into GeP3 through high energy ball milling, followed by encapsulating the resultant composite with graphene via a solution-based ultrasonic method. To delineate the individual role of carbon matrix and graphene, material characterization and electrochemical analyses were performed for GeP3/C@rGO and three other samples: bare GeP3, GeP3 with graphene coating (GeP3@rGO) and GeP3 with carbon matrix (GeP3/C). GeP3/C@rGO exhibits the highest electric conductivity (5.89×10-1 S•cm-1) and the largest surface area (167.85 m2 g-1) among the four samples. The as-prepared GeP3/C@rGO delivered a reversibly high capability of 1084 mA h g-1 at 50 mA g-1, excellent rate capability (435.4 mA h g-1 at a high rate of 5 A g-1) and long-term cycling stability (400 cycles with a reversible capacity of 823.3 mA h g-1 at 0.2 A g-1), all of which out-performance the other three samples. The kinetics investigation reveals a "pseudocapacitive behaviour" in GeP3/C and GeP3/C@rGO, where solely faradic reactions took place in bare GeP3 and GeP3@rGO with a typical "battery behaviour". Based on ex-situ X-ray photoelectron spectroscopy (XPS) and ex-situ Electrochemical impedance spectroscopy (EIS), the carbon matrix serves to activate and stabilize the interior of composite, while the graphene protects and restrains the exterior surface. Benefitting from the synergistic combination of these two components, GeP3/C@rGO achieved extremely stable cycling stability as well as outstanding rate performance.