A bi-functional strategy involving surface coating and subsurface gradient co-doping for enhanced cycle stability of LiCoO_(2) at 4.6 V

Layered LiCoO_(2)(LCO)acts as a dominant cathode material for lithium-ion batteries(LIBs)in 3C products because of its high compacted density and volumetric energy density.Although improving the high cutoff voltage is an effective strategy to increase its capacity,such behavior would trigger rapid capacity decay due to the surface or/and structure degradation.Herein,we propose a bi-functional surface strategy involving constructing a robust spinel-like phase coating layer with great integrity and compatibility to LiCoO_(2) and modulating crystal lattice by anion and cation gradient co-doping at the subsurface.As a result,the modified LiCoO_(2)(AFM-LCO)shows a capacity retention of 80.9%after 500 cycles between 3.0and 4.6 V.The Al,F,Mg enriched spinel-like phase coating layer serves as a robust physical barrier to effectively inhibit the undesired side reactions between the electrolyte and the cathode.Meanwhile,the Al,F,Mg gradient co-doping significantly enhances the surficial structure stability,suppresses Co dissolution and oxygen release,providing a stable path for Li-ions mobility all through the long-term cycles.Thus,the surface bi-functional strategy is an effective method to synergistically improve the electrochemical performances of LCO at a high cut-off voltage of 4.6 V.

Synergistic effect of fluorinated solvent and Mg^(2+)enabling 4.6 V LiCoO_(2)performances

Increasing the charging cut-off potential of lithium cobalt oxide(LiCoO_(2),LCO)can effectively improve the energy density of the lithium-ion batteries,which are the mainstream energy storage devices used in 3C electronic products.However,the continuous decomposition of the electrolyte and dissolution of Co from the electrode will occur at high-potential operation,which deteriorate the performances of LCO.Here,a cathode-electrolyte interface(CEI)layer containing Mg F_(2)is constructed to enhance the electrochemical stability of LCO at 4.6 V(vs.Li^(+)/Li).The Mg^(2+)added to the cathode gradually releases into the electrolyte during cycling,which forms a stable Mg F_(2)-rich protective layer.In addition,1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropylether(TTE)is added to the electrolyte acting as a F source to increase the content of Mg F_(2)in the CEI layer.The Mg F_(2)-rich CEI layer effectively suppresses the decomposition of electrolyte components and the dissolution of Co of LCO,which makes the Li||LiCoO_(2)(Li||LCO)cell cycled stably at 3~4.6 V(vs.Li^(+)/Li)in 200 cycles with a retention of 83.9%.