Red Phosphorous‐Derived Protective Layers with High Ionic Conductivity and Mechanical Strength on Dendrite‐Free Sodium and Potassium Metal Anodes

Abstract

Sodium metal anodes are ideal candidates for advanced high energy density Na metal batteries. Nevertheless, the unstable solid electrolyte interphase (SEI), the uncontrollable dendrite growth, and low Coulombic efficiency during cycling have prevented their applications. Herein, a high‐performance Na anode is achieved by introduction of an ex situ artificial Na3P layer on the surface via a simple red phosphorus pretreatment method. The artificial SEI layer possesses high ionic conductivity and high Young's modulus, which regulates uniform deposition of ions and prevents the dendrite growth. Benefiting from these merits, the Na||Na cells with the protected layers demonstrate excellent electrochemical performance (780 h at 1.0 mA cm–2, 1.0 mAh cm–2). When assembled into a full battery with a Na3V2(PO4)3 cathode, the Na metal battery exhibits a long lifespan of 400 cycles at 15 C and a high rate capacity of ≈53.2 mAh g–1 at 30 C. In addition the red P pretreatment method can be applied to potassium metal anodes. Outstanding performance is also achieved in K||K cells with the formation of a KxPy protecting layer (550 h at 0.5 mA cm–2, 0.5 mAh cm–2). The artificial P‐derived protection approach can also be extended to solid‐state alkali metal batteries with high power density and energy density.

Artificial Heterogeneous Interphase Layer with Boosted Ion Affinity and Diffusion for Na/K‐Metal Batteries

“…The time-voltage curve of the Na 2 S/V/Na cell was stable, flat and smooth during the entire cycle process, with a low voltage polarization of 38 mV, indicating a stable artificial protective layer and rapid Na + transport. [28,39,40] In contrast, the bare Na and Na 2 S/Na symmetric cells exhibited short cycle lives of 24 and 292 h, respectively. A remarkably higher voltage polarization (100 mV) was observed for the bare Na symmetric cell at the beginning and end of each plating/stripping process, which indicates that the consumption of previously unused fresh Na below the traditional SEI layer.…”

“…The K 2 S/V/K cell showed the best cyclic stability in a carbonated-based electrolyte compared with previously reported results (Figure 6d). [15,28,[63][64][65][66] The rate performance is shown in Figure 6e. A low overpotential of 110 mV was obtained for the symmetric K 2 S/V/K cell at 0.5 mA cm −2 , which is considerably lower than that of the symmetric bare K cell (229 mV).…”

“…The ideal stable protective layer should have the following characteristics: i) high ionic conductivity, ii) excellent chemical stability over a wide operating voltage window, and iii) high mechanical strength. [26] Based on these requirements, various artificial protective layers have been constructed to improve the electrochemical performance of NMAs (PMAs), such as a sulfide-rich SEI layer, [27] Na 3 P, [28] Na-Bi, [29] KSb x F y , [30] and K 2 Te. [31] Among them, inorganic protective layers with a high Na + (K + ) conductivity and Young's modulus have improved the cyclic stability of Na-metal and K-metal batteries.…”

In Situ Constructed Ionic‐Electronic Dual‐Conducting Scaffold with Reinforced Interface for High‐Performance Sodium Metal Anodes

“…As the sputtering process is performed, the intensity of NaF in EC scaffold initially increases within 1 min and then gradually decreases after 2 min (Figure 6a), while the intensity of NaF in IEDC scaffold delivers continuous growing tendency with increased sputtering depth (Figure 6b), indicating there is more NaF content in the inner layer of SEI of IEDC scaffold, which is beneficial to uniformize the distribution of Na ions and suppress the growth of Na dendrites due to the fast Na-ion conducting nature and high shear modulus (31.4 GPa) of inorganic NaF. [14] Interestingly, after etching for 1 min to remove the surface chemical composition, the distinct peak at 129.0 eV representing Na 3 P emerges in the P 2p spectrum of IEDC scaffold (Figure 6d) and its intensity increases continuously along with the sputtering time, even after sputtering for 8 min (equals to a depth of 20 nm), [48] indicating that reaction products of RP during electrochemical process participates in the formation of SEI (compared with the P 2p spectrum of EC scaffold in Figure 6c). The sputtering data of a growing amount of Na 3 P co-existing with Na x PF y and Na x PO y F z confirm its function acting as one important component in the interior of SEI layer.…”

“…Moreover, the high resolution TEM (HRTEM) (Figure 3h,i) of IEDC scaffold shows distinct lattice fringes with spacings of 0.25, 0.24, and 0.20 nm, corresponding to the (110), (103), and (104) planes of crystallite Na 3 P, further verifying the successful synthesis of Na 3 P phase. [48] These substantial, ultrafine and well-dispersed Na 3 P nanoparticles would be able to serve as a floating skeleton to support plated Na, and are beneficial to forming a stable SEI film containing Na 3 P component with higher ion-conducting capability and robust mechanical strength.…”