Ionic and Electronic Transport in Alluaudite Na_2+2xFe_2−x(SO₄)₃

Abstract

Alluaudite-type Na 2 + 2x Fe 2Àx (SO 4 ) 3 has recently been discovered as a3 .8 V( higheste ver Fe 3 + /Fe 2 + redox potential) cathode materialf or sodium-ion batteries. Contrary to the sluggishk inetics caused by the large Na + radius in most cathode materials for sodium-ion batteries, this alluaudite-type compound shows superior high-rate capability during electrochemical cycling. Here, to explore the underlying factors for the fast kinetics, we experimentallyi nvestigate the inherent ionic and electronic transport of this alluaudite phase by using impedance spectroscopy and potentiostatic polarization.C ompared with the commercialc athode material, LiFePO 4 ,w ed emonstrate four and two orders of magnitude higher intrinsic ionic and electronic conductivity,r espectively,w hichu nderpin its superior high-rate capability in terms of electrode performance.Rechargeable lithium-ion batteries, as the key components of consumer electronic devices,e lectric vehicles, and electric energy-storage systems, are now penetrating into our modern life. Modern lithium-ion batteries are ten times cheaper than the first commercial ones introduced by Sony in 1991, but further reducing their costs seems to be extremely hard, owing to the limitation and uneven distribution of lithium reserves. In the faceo fp redictable increasing energy-storage demands in the future, ac heaper and more easily available alternative should replace or at least supplementp resent lithium-ion batteries.Over the past few years, sodium-ion batteriesh ave received ar esurgenceo fi nterest, owing to the low cost and earth abundance of elemental sodium.[1] In the field of large-scale stationery energy storage, sodium-ion batteries are considered to be very promising candidates. To further lower the costs and environmental hazards, expensive and toxic transitionmetal elements (Co and Ni)i nc athode materials should be avoided.Enlightened by the rich experience of compound structures in lithium-ion batteries, cathode materials based on layered oxides [1b, 2] (e.g. P2-Na x Mn 0.5 Fe 0.5 O 2 ,O 3-type NaFeO 2 )a nd polyanions [1c, e, 3] (e.g. Na 2 FePO 4 F, NaFePO 4 and Na 2 FeP 2 O 7 )h ave been continuouslyf ound. However,a lmosta ll of them suffer from al ower redox potential in Na systems compared with their Li analogues,r esulting in al imited energy density.T he inductive effect of the SO 4 2À polyanion is considered to be ar ational strategy to solvet his problem.[ (3.45 Vv s. Li + /Li). Therefore, SO 4 2À -based polyanion compounds consistingo fl ow-cost elements (chargec arrierN aa nd redox-active transition-metal Fe) can act as economic and environmentally friendly cathode materials without scarifying the energy density.Recently,w er eported an ew alluaudite-type sulfate polyanion compound,N a 2 + 2x Fe 2Àx (SO 4 ) 3 ,a sap romising cathode material for sodium-ion batteries.[11] Thisc ompound can easily be prepared through al ow-temperature (350 8C) solid-state reaction by using economic and readily accessible precursors Na 2 SO 4 and anhydrous Fe...

Investigation into the dehydration of selenate doped Na2M(SO4)2·2H2O (M = Mn, Fe, Co and Ni): Stabilisation of the high Na content alluaudite phases Na3M1.5(SO4)3-1.5x(SeO4)1.5x (M = Mn, Co and Ni) through selenate incorporation

“…Reports regarding the alluaudite system often refer to off-stoichiometric behaviour, with compositions such as Na 2+2y Fe 2-y (SO 4 ) 3 (y ≤ 0.3) common for this material. [19,20] Similar formation of an alluadite-type phase has been observed on dehydration of the Mn based system [2], while for the M = Ni, Co systems dehydration leads to the formation of the simple bimetallic sulfate Na 2 M(SO 4 ) 2 [6]. We 0≤x≤1.0) materials, illustrating the significant effect of the incorporation of selenate on the products obtained.…”

High‐Abundance and Low‐Cost Metal‐Based Cathode Materials for Sodium‐Ion Batteries: Problems, Progress, and Key Technologies

“…A very small volume change was observed (Δ V ≈ 3.5%) during the subsequent cycles, which indicates satisfactory cycling stability. The single‐phase reaction was further verified by studies of off‐stoichiometry Na 2+2 x Fe 2− x (SO 4 ) 3 materials . Since then, Barpanda's group developed an ion‐thermal synthetic method with low temperature of only 300 °C .…”

Sulfate‐Based Cathode Materials for Li‐ and Na‐Ion Batteries

“…DFT calculations suggest that a full desodiation might be correlated to a volume shrinkage of 8 % and a possibly irreversible phase transition rendering a complete oxidation unfavorable ,. Na 2+2x Fe 2‐x (SO 4 ) 3 displays room temperature ionic and electronic conductivities of 1×10 −7 S cm −1 and 2×10 −9 S cm −1 as shown by impedance measurements (Figure c), which is comparable to materials such as NASICON Na 3 V 2 (PO 4 ) 3 . The SO 4 ‐based alluaudite can be prepared via various synthesis approaches:…”