Alluaudite NaCoFe₂(PO₄)₃ as a 2.9 V Cathode for Sodium-Ion Batteries Exhibiting Bifunctional Electrocatalytic Activity

Developing novel earth-abundant and high energy density cathode materials is pivotal to realize the enduring energy storage revolution. Alluaudites Na_xM_y(XO₄)₃ (M = Mn, Fe, Co, Ni; X = Mo, W, P, As, S), as a competent series of sodium insertion cathode contenders, have attracted wide scientific attention in recent years due to their unique open framework geometry, structural flexibility, scalable synthesis, and desirable electrochemical performance. Exploring the alluaudite family of sodium insertion systems, we herein present a hitherto unknown NaCoFe₂(PO₄)₃ alluaudite prepared by an economic solution combustion technique. Rietveld analysis of powder X-ray diffraction pattern identified the formation of alluaudite-type monoclinic C2/c phase with a = 11.750(3) Å, b = 12.459(1) Å, c = 6.383(3) Å, and unique angle β = 113.711(7)°. As confirmed by bond valence site energy calculations, the structure renders two distinct tunnels: Na(1) and Na(2), for the one-dimensional migration of Na⁺ ions along the c-direction. Computational modeling revealed a migration barrier of E_a ∼ 0.31 eV for Na(2), which is one of the lowest values for Na⁺-conducting materials. Preliminary electrochemical study on the as-synthesized NaCoFe₂(PO₄)₃ alluaudite exhibited reversible sodium intercalation involving a 2.9 V Fe³⁺/Fe²⁺ redox activity delivering capacity ∼70 mAh/g with good cyclability over 100 cycles. Taking advantage of transition metal active centers and PO₄ linkage, NaCoFe₂(PO₄)₃ further showed efficient bifunctional electrocatalytic activity with near four electron transfer reaction. With favorable diffusional and electrochemical performance, the discovery of alluaudite NaCoFe₂(PO₄)₃ introduces a novel 3 V class of cathode for sodium-ion batteries. It not only enriches the materials database of sodium insertion compounds, but also enables its possible application in metal–air batteries and water splitting.