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 a 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 reversible high capacity of 1084 mA h g^-1 at 50 mA g^-1, excellent rate capacity (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 outperform the other three samples. The kinetics investigation reveals a "pseudocapacitive behavior" in GeP₃/C and GeP₃/C@rGO, where solely faradic reactions took place in bare GeP₃ and GeP₃@rGO with a typical "battery behavior". Based on ex-situ X-ray photoelectron spectroscopy and ex-situ electrochemical impedance spectroscopy, the carbon matrix serves to activate and stabilize the interior of the composite, while the graphene protects and restrains the exterior surface. Benefiting from the synergistic combination of these two components, GeP₃/C@rGO achieved extremely stable cycling stability as well as outstanding rate performance.