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

Abstract

Figure S1. The electrocatalytic activity studied under Linear sweep voltammetry (LSV). (a) Koutecky-Levich plots (J −1 versus ω −1/2 , J is the current density in mA cm -2 and ω is the rotating speed in rpm -1/2 ). (b) LSV for NaCoFe 2 (PO 4 ) 3 at different rotating speed in O 2 saturated 0.1 M KOH electrolyte. (c) ORR stability performance of NaCoFe 2 (PO 4 ) 3 shown over 10 hours. (d) Number of electron transfer as calculated from K-L plot during the ORR process in potential window of 0.25 -0.50 V.

Potassium Cobalt Pyrophosphate as a Nonprecious Bifunctional Electrocatalyst for Zinc–Air Batteries

“…While noble metal-based electrocatalysts (e.g., RuO 2 , IrO 2 , Pt/C) offer efficient activity, their scarcity and high cost have triggered a search for economic catalysts acting as crucial components in energy conversion of electricity into fuels and chemicals . In this pursuit, a gamut of metal oxides, borides, nitrides, phosphides, (oxy)­hydroxide, and spinel/perovskite frameworks have been explored. − Off late, various (ortho/meta/pyro/mixed) phosphate polyanionic battery insertion materials have been exploited as electrocatalysts at times surpassing the activity of Pt/C or RuO 2 . − These phosphates offer structural/chemical stability owing to strong covalent bonds apart from catalytic activity stemming from transition-metal redox centers. Further, they exhibit structural diversity with regular/distorted octahedral, tetrahedral, and trigonal planar geometries, which can tune their electrocatalytic activity.…”

Unlocking New Redox Activity in Alluaudite Cathodes through Compositional Design

“…Barring just a few compositions, 38,41,42 only Fe 2+ /Fe 3+ redox has been harnessed, even in compounds containing multiple M species. 35,36,[43][44][45][46][47][48] For instance, Na2Fe3-xMnx(PO4)3 alluaudites can reach up to 140 mAh/g of reversible capacity for the x = 0 end-member and a steady decrease in capacity is observed as x increases. 36,37,49,50 These cathodes operate at 2.7 V vs. Na/Na + and no capacity can be attributed to the higher potential Mn 2+ /Mn 3+ couple, leading to rather low energy densities.…”

“…The development of Na-ion alluaudite cathodes has been hindered by the limited number of electrochemically active M species in this structure type. Barring just a few compositions, ,, only Fe 2+ /Fe 3+ redox has been harnessed, even in compounds containing multiple M species. ,,− For instance, Na 2 Fe 3– x Mn x (PO 4 ) 3 alluaudites can reach up to 140 mAh/g of reversible capacity for the x = 0 end-member, and a steady decrease in capacity is observed as x increases. ,,, These cathodes operate at 2.7 V vs Na/Na + , and no capacity can be attributed to the higher potential Mn 2+ /Mn 3+ couple, leading to rather low energy densities. In contrast, sulfate alluaudites exhibit unprecedently high operating voltages, so much so that the potentials of Ni- and Co-based systems fall outside of the electrochemical stability window of typical electrolytes. , The combination of a highly unstable structure on Na extraction (due to strong repulsion between edge-sharing MO 6 octahedra in the oxidized state) and a less negative “–2” charge on the sulfate group compared to the phosphate group results in low capacities for sulfate alluaudites. ,, Overall, a better understanding of the origin of the electrochemical inertness of Mn, with a redox potential well below the decomposition potential of organic electrolytes even in a sulfate alluaudite, is warranted, and the design of “beyond Fe” alluaudite cathodes is desirable.…”

Synthesis and characterization of a novel alkali mixed magnesium-aluminum phosphate with a layered structure–KMgAl(PO4)2

“…Aluminum phosphates find potential applications as ionic conductors [4,5,6], optical materials [7] and catalyzers [8,9,10]. Lithium aluminum titanium phosphate (Li1.3Al0.3Ti1.7(PO4)3, LATP) which can be synthesized by different methods has been widely studied due to its high density and an ionic conductivity of 1.88×10 −4 S⋅cm −1 at room temperature [11].…”