Sodium-Storage Mechanism Of Metal Selenides In Ether-Based Electrolytes

With the rapid development of energy-intensive systems,such as portable power tools and electric vehicles,sodium-ion batteries have developed rapidly due to their low cost,high energy density,and electrochemical properties similar to lithium-ion batteries.Among many anode electrode materials for sodium ion batteries,metal sulfur/selenide exhibits excellent electrochemical performance by virtue of good electrical conductivity,diverse stoichiometric compositions,unique crystal structures and abundant redox sites.However,with copper foil as the current collector and ether as the electrolyte solvent,the discharge/charge curves of metal sulfur/selenide continue to change until hundreds of cycles,although there are excellent cycle/rate performances.So far,the research on this phenomenon is still superficial and contradictory.Here,we take metal selenide as the model to re-examine sodium storage mechanism of metal selenide in ether-based electrolyte.The main contents are as follows:(1)Here,CoSe2 is used as a model of metal selenides to disclose the electro-chemical reactions in ether-based electrolytes,where Cu species from Cu foil gradually permeate into the electrode and replace Co to bind with Se upon cycling.The driving force of this transition in thermodynamics can be qualitatively illustrated by Hard-Soft-Acid-Base theory,as confirmed by a series of metal selenides.Because Cu species pass through a solid-electrolyte-interphase(SEI)film during this transition,electrolytes and voltages control the formation of the SEI film,become important in kinetics.To our knowledge,it is the first time to realize the crucial role of SEI in this transition.It is found that SEI in NaPF6/DGM is rich in NaF,with less organic matter and small thickness.Thus,compared with NaPF6/EC/DEC,the conversion resistance in NaPF6/DGM is smaller.This result is also supported by density functional theory(DFT)calculations.These results are important to understand the electrochemical properties of metal selenides in sodium ion batteries.(2)Based on the research on the sodium storage mechanism of metal selenides in ether-based electrolytes,some effective strategies have been proposed to inhibit and eradicate this transformation.First,by adjusting the voltage range,the formation of a thick SEI film in 0.01-3.0 V effectively inhibits this electrochemical transformation.Then,using titanium foil or aluminum foil instead of copper foil as the current collector,can eradicate the electrochemical conversion.Finally,the reversible capacity and cycle stability of CoSe2 on the titanium foil were improved by increasing the discharge cut-off voltage to 1 V.After 500 cycles of CoSe2,the reversible capacity was increased from 81.8 mAh g-1 to 133.8 mAh g-1.