GeP3 with soft and tunable bonding nature enabling highly reversible alloying with Na ions

Abstract

A GeP3 compound is introduced here for the first time as a promising anode for sodium-ion batteries (SIBs). The compound shows a high capacity and good cyclic stability, which well agree with the first-principle calculation results regarding its soft bonding nature and related innovative mechanical endurance. The binding energies of Ge–P and P–P verify that Ge–P has softer bonding feature with smaller energy variations compared to P–P when the bonding length is changed. In order to confirm the bonding natures and their effect on the mechanical and electrochemical properties, two layered GePn compounds with different Ge–P content (i.e., GeP3 and GeP5) have been synthesized using a high energy mechanical ball-milling (HEMM) method. GeP3 maintains high discharge and charge capacities of 1.274 and 1.269 Ah g−1 even after 150 cycles, respectively, which correspond to capacity retentions of 87.6% and 88.0% from the 5th cycle, respectively. A comparative study on the elastic moduli of GeP3 and GeP5 demonstrates how the superior electrochemical performance of GeP3 is correlated with its more softened bonding feature compared to GeP5 both in the pristine state and during charge/discharge. The elastic modulus of GeP3 shows significantly softer features than that of GeP5, implying that mechanical stress or strain can be more easily alleviated in GeP3 than in GeP5 during charge/discharge. A morphological comparison between GeP3 and GeP5 electrodes reveals that GeP5 electrode undergoes very serious morphology and volume changes, whereas GeP3 electrode does not show any significant change in good accordance with the elastic modulus comparison from the first-principle calculations.