Electrochemical Catalytic Conversion Type Cobalt-based Anode Materials For Lithium Storage Performances

With the increase of the demand for high-performance lithium ion batteries(LIBs)in mobile electronics,electric vehicles,and other fields,it is of great significance to develop high-capacity and low-cost electrode materials for enhancing the energy and power density of LIBs.Transition metal oxysalts have become one of the research focuses of new lithium storage anode materials in recent years,due to their considerable capacities,simple synthesis,and low cost.Under the electrochemical catalysis of transition metals,the reversible conversion of inorganic components supplies ultrahigh extra capacities.But there are outstanding problems such as structural failures and side reactions in transition metal electrocatalytic conversion type materials,with an obviously fluctuant cycling performance during long charge and discharge processes.This dissertation aims at the unstability of lithium storage performances.On the one hand,the stepwise conversion of inorganic lithiated components(Li2CO3 and LiOH)is restricted by the rational regulation of the electrocatalytic behavior of transition metals,thus enhancing the structure and performance stability.On the other hand,on the basis of the conversion reaction of LiOH components under the electrocatalysis of transition metals,a capacity counterbalance approach in one compound is designed to achieve high and stable lithium storage performances.Lastly,an electrocatalytic conversion type anode material with core-shell structure is constructed,utilizing the partitioned versatility of transition metals to optimize electrochemical performances.1.Lithium storage performances of Co2(OH)2CO3/RGO anodes improved by a confined stepwise conversion process under the electrochemical catalysis of cobalt:Electrocatalytic conversion type anode materials containing single inorganic component(transition metal carbonates/hydroxides)often show the cycling unstability with obvious fluctuation in capacities.Therefore,one-dimensional Co2(OH)2CO3(CHC)nanowires are in-situ anchored on the surface of reduced graphene oxide(RGO)via a one-pot hydrothermal approach,forming surface independent confined domains to further restrict the electrochemical catalytic conversion process.Ex-situ XPS and TEM characterization results on cycled electrodes confirm that the added capacity of CHC arises from the step-by-step reversible conversion reactions of Li2CO3 and LiOH under the electrocatalysis of Co metal in-situ generated after the conversion of CHC.In the discharge process,the conversion of Li2CO3 occurs at 0.95 V followed by the conversion of LiOH at 0.01 V,while the order is reversed in the charge process.Such a stepwise electrochemical catalytic conversion procedure could restrict the two conversion degrees for each other,relieve the volume change,and avoid serious side reactions.The confined effect is further enhanced by precisely limiting the width and length of the CHC on the surface of RGO.The optimized CHC/RGO hybrid maintains a high reversible capacity of 1110 mA h g-1 after 100 cycles at the current density of 0.1 A g-1,which is much higher than the theoretical value of CHC(506 mA h g-1)based on the recognized conversion reaction.Furthermore,it keeps high reversible capacities of 755 and 506 mA h g-1 at current densities of 1 and 2 A g-1 over 200 cycles,respectively,exhibiting a high-rate cyclability with the lowest coefficient of variance of 9.4%among the reported ones.2.Lithium storage performances of CoGeO2(OH)2/RGO anodes enhanced by a compensation effect of the electrochemical catalysis of cobalt:To enhance lithium storage performances of germanates anodes,a novel capacity counterbalance approach in one compound is designed by introducing an electrocatalytic conversion type hydroxyl component,thereby effectively compensating the capacity loss of GeO2 components.CoGeO2(OH)2(CGH)nanoplates are chemically bonded with RGO sheets in situ with a mild one-pot hydrothermal synthesis,constructing maximal face-to-face contact interfaces with interfacial chemical bonds.The hydroxyl group(Co-OH)contents of CGH are regulated by thermal annealing,thus controlling the capacity contribution resulted from the electrocatalytic conversion reaction of LiOH to exactly offset the capacity fading of GeO2.The results of characterization on cycled electrodes at different potentials confirm the stepwise electrochemical reactions of Co,GeO2,and LiOH.The capacity equilibrium of these electrochemical reactions ensures an excellent cycling stability.Consequently,the CGH/RGO hybrid keeps high charge capacity of 1136 mA h g-1 at the current density of 0.1 A g-1 after 100 cycles.It also shows a high stable cyclability with a retained charge capacity of 560 mA h g-1 at the current density of 1 A g-1 over 1000 cycles.3.Lithium storage performances of CosGe3@Co/RGO anodes optimized by a core-shell partitioned versatility of cobalt:The introduction of an electrocatalytic conversion type component is an effective method for improving lithium storage performances of Ge anode materials.In order to further reduce the lithium loss of electrochemically inactive components,an electrocatalytic material(Co5Ge3@Co)with core-shell spacial arranged structure is designed.Co5Ge3@Co core-shell nanoparticles are successfully hybridized with RGO matrixes via a hydrothermal and sequential annealing procedure.The thickness of Co shell of Co5Ge3@Co nanoparticles could be regulated by utilizing the interfacial bonding enhanced Kirkendall effect during the annealing process.Tunable spacial arranged Co components exhibit diverse functions to enhance electrochemical performances of Ge components.Both the Co core and Co shell restrict the volume expansion of inner Ge components as well as improve the electronic conductivity.What’s more,the Co component takes part in the electrochemical reactions for the additional capacity contribution.The Co core as the electrocatalyst makes for the reversible conversions from GeO2 to Ge to Li4.4Ge;the Co shell and partial Co core are involved in the redox reactions from Co to Co2+ to Co3+.Above electrochemical reaction process is confirmed by the characterization on discharge/charge products.Based on the spacial arranged structures and diverse functions of Co components,Co5Ge3@Co/RGO hybrid delivers a reversible capacity as high as 1106 mA h g-1 at the current density of 0.1 A g-1 over 100 cycles.Additionally,it keeps reversible capacities of 864 and 576 mA h g-1 at current densities of 0.5 and 1 A g-1 over 500 cycles,respectively.