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

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 GeP_n 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.