Critical design factors for kinetically favorable P-based compounds toward alloying with Na ions for high-power sodium-ion batteries

Abstract

First-principles calculations with experimental validation enable us to radically predict and compare the electron transport, elastic softness, and thermodynamic phase stabilities of black phosphorus (BP) and GeP₃ to figure out the crucial factors for designing the most kinetically favorable P-based compounds toward alloying with Na ions. From the viewpoint of electron transport, Ge is able to provide P with electrons due to the difference of electronegativity, causing GeP₃ to have a significantly reduced band gap and a lower electron-hopping barrier compared to BP. Because the structure of GeP₃ looks more elastically softened than that of BP, its Na⁺ migration barriers in the sodiated states must be lower than those of the sodiated BP structure as proven by the higher Na⁺ diffusion coefficients of GeP₃. The thermodynamic phase stabilities of BP and GeP₃ could be predicted by calculating their mixing enthalpy values and thereby homogeneous bulk free energies were obtained for Na_1−x(GeP₃) (0.0 ≤ x ≤ 0.5) indicating that no phase separation occurs in the sodiated GeP₃ because of an additional stable single-phase at x = 0.25. By contrast, the sodiated P easily gets separated into NaP and Na_0.5P with the same sodiation content. The suppressed phase separation in GeP₃ makes the Na⁺ distribution homogeneous as confirmed by TEM-EDS color mapping and thus contributes to increasing Na⁺ diffusion coefficients because Na⁺ migration can be primarily interrupted by the interfaces or grain boundaries between the separated phases. Taking into account three enhanced physicochemical properties, the GeP₃ electrodes may well present a superior rate capability to the BP electrode, underscoring that electron transport, elastic softness, and thermodynamic phase stabilities must be the most important design factors for realizing kinetically favorable P-based compounds toward alloying with Na⁺. In detail, even at an ultrahigh current density of 20 A g⁻¹, the capacity of the GeP₃ electrode still comes to 0.197 A h g⁻¹. At 1 A g⁻¹, GeP₃ could maintain a discharge capacity of 0.499 A h g⁻¹ even after 1000 cycles. This study may provide promising guidelines for the design of alloy-based electrode materials for high power and energy SIBs.