Study The Influence Of Fluorinated Solvents On The Electrochemical Performance Of High Voltage Cobalt-based Cathode

With the increase demand of high energy density in electric vehicles?EVs?and hybrid electric vehicles?HEVs?market,huge challenge falls on lithium-ion batteries.So the development and research of high energy density battery is the market demand and also the goal that researchers continue to pursue.Usually the energy density of Li-ion batteries can be improved from two aspects,to improve the battery discharge capacity or increase the operating voltage plateau.Although very high specific capacity has been achieved in Li-S and Li-air batteries,bottleneck still exists in practical application.Recently,Li₂CoPO₄F and LiCoPO₄ as the cathode materials for Li-ion batteries have attracted much attention due to their high operating voltage plateau.However,their high working voltage of 4.8 V and above is beyond the electrochemical stability window of conventional organic carbonate-based electrolytes.Therefore,the study of new high voltage electrolytes is very essential and urgent.In this paper,with the aim to enhance the electrochemical performance of these cathode materials,we use FEC and F-EPE as co-solvent for high voltage electrolyte,and study the effects on the cathode material of Li₂CoPO₄F and LiCoPO₄.At the same time,stability of the electrolyte at high voltage,electrochemical performance of cathode materials in these electrolytes,the stability of electrode/electrolyte interface and its effects on electrochemical performance have been investigated systematically.High-voltage fluorinated solvents FEC and F-EPE show excellent compatibility with Li₂CoPO₄F cathode material,and can significantly improve the electrochemical performance of Li₂CoPO₄F.The LSV results show that FEC and F-EPE as co-solvents can significantly improve the high voltage stability of electrolytes compared with the traditional carbonate electrolyte.Electrochemical studies show that FEC/DMC =1:1 and F-EPE/DMC = 1:2 systems exhibit better performance.When cycled in 1 M LiPF6 FEC/DMC = 1:2?wt/wt?,F-EPE/DMC = 1:2?wt/wt?and EC/DMC = 1:1?wt/wt?between 3-5.4 V,at 0.2 C(1C = 143 mA g^-1)rate,Li₂CoPO₄F shows an initial discharge capacity of 121.7,111.3 mAh g^-1 and 95.9 mAh g^-1 and a capacity retention of 35.9%,28.2%and 0.029%after 100 cycles,respectively.The possible functional mechanisms of fluorinated solvents FEC and F-EPE are inverstigated by LSV,CV,EIS,SEM and XPS measurements.The results show that the products of electrolytes oxide decomposition?including ROLi,ROCO2Li,LixPFyO2,LixPFy,et al?can be observed on the surface of Li₂CoPO₄F electrode after cycling in all the three electrolytes,the thickness of interface films formed on FEC and F-EPE based electrolyte is lower than EC/DMC based electrolyte,limited to few nm.Compared with the traditional carbonate electrolyte,the SEI layer formed on FEC and F-EPE based electrolyte is more stable,which suppresses the side reactions at electrode/electrolyte interface when charge to high voltage,and improves the structure stability of Li₂CoPO₄F during cycling,thus singnificaltly improves the electrochemical performance of Li₂CoPO₄F.Fe³⁺ doped Li_1-xCo_1-xFe_xPO₄?x = 0,0.05?materials are synthesized by sol-gel method,the effects of Fe³⁺ doping on the electrochemical performance of LiCoPO₄ are studied.And the above high-voltage electrolyte 1 M LiPF6 FEC/DMC = 1:1 is applied to further improve the electrochemical performance of Li_0.95Co_0.95Fe_0.05PO₄.The electrochemical results show that Fe³⁺ doping can improve the electrical conductivity of Li_0.95Co_0.95Fe_0.05PO₄ material,decrease the polarization of Li0.95Co0.94Fe0.05PO₄ electrode,and reduce the impedance of battery,thus improve the electrochemical performance of Li_0.95Co_0.95Fe_0.05PO₄ material,especially at high current rate.Compared with the traditional carbonate electrolyte,1 M LiPF6 FEC/DMC = 1:1 electrolyte has higher stability at high voltage,can further improve the electrochemical performance of Li_0.95Co_0.95Fe_0.05PO₄ material.