Studies On The Micro/Nanostructural Constructions And High Lithium Storage Capabilities Of Transition Metal Carbonates

Non-renewable fossil fuels are the main energy sources nowadays,but bring serious environmental pollution when used.Developing renewable energy sources has been recognized as an effective solution for solving the energy and environmental crises,which has been one of hot research topics in recent decades.The storage or conversion of green energy sources,such as wind and solar powers,in secondary batteries for later use can improve the utilization of green energy sources and can overcome their restrictions in time and geographical factors.In addition,the high-energy and high-power demands for advanced secondary batteries are significantly improved along with the developments of smart power grids,hybrid electric vehicles,electric vehicles,information technologies and so on.Lithium ion batteries（LIBs）have been widely used in mobile electronic devices due to the advantages of high energy density,high output voltage,long lifespan,no memory effect and low self-discharge rate.To realize the future applications of LIB technology in the high-energy and/or high-power density devices,it is crucial to develop new LIB electrode materials with higher capacity,lower cost and longer lifespan.Recently,transition metal carbonates（MCO₃,M = Fe,Co,Mn and Ni）have attracted considerable attentions as a new kind of promising anode materials for LIBs,owing to their low cost,easy preparation and high capacity features.In tradition,theoretical capacity of MCO₃ is calculated to be～460 mAh g-1 based on the reversible reaction of "MCO₃ + 2Li（?）Li2CO₃ + M".However,according to the literature reports and our experimental results,as LIB anodes several MCO₃ materials could exhibit a reversible capacity as high as 1000 mAh g-1,or higher,even after a long-term charge-discharge cycle.For instances,herein as-synthesized FeCO₃ rhombohedra remain a discharge capacity of 1018 mA g"1 over 120 galvanostatic cycles at a current density of 200 mA g-1;CoCO₃ dumbbells keep 1346 mAh g-1 after 600 cycles at 100 mA g-1;The capacity of rod-like Co0.8Fe0.22CO₃ structures remain 1263 mAh g-1 in the 250th discharge-charge cycle at 100 mA g-1.Also,MnCO₃ spindle-GO composites deliver a reversible capacity of 1560 mAh g-1 in the 500th cycle at 100 mA g-1,and interconnected Ni（HCO₃）2 hollow spheres present the 80th reversible capacity of 1442 mAh g-1 at a current rate of 100 mA g-1.As active substances of LIB anodes,actual capacities of the above mentioned MCO₃ are much higher than their traditional theoretical capacities.This not only proves the potential applications of serial MCO₃ in LIB fields but also suggest their novel energy storage mechanisms towards metallic lithium.As an example,the further reversible reactions between Li2CO₃ and lower valence carbon-containing materials catalyzed by the in situ generated M0 nanocrystals could be used to account for the additional capacity of MCO₃:Li2CO₃+（4 + 0.5x）Li（？）0.5 LixC2 + 3 Li2O（x=0,1 or 2）.That is,the "new" theoretical capacity of MCO₃ should be～1600 mAh g-1 which is 4.3 times of the theoretical capacity of commercial anode graphite（372 mAh g-1）.Nevertheless,the generally unsatisfactory low capacities and bad cycling stabilities of crystalline MCO₃ should be mentioned,mainly owing to their low electronic conductivity and irreversible electrochemical conversion reaction towards metallic lithium.This thesis focuses on the morphological-controlling constructions of micro-nanostructural MCO₃ crystals as advanced LIB anodes,the possible relationships between as-synthesized hierarchicial structures and electrochemical properties,the reversible charge-discharge reactions of micro-nanostructural MCO₃ cystallites towards metallic Li and the performance improvement mechanisms,shown as below.1.Hydrothermal Construction of Micro/Nanostructural MCO₃ Under the assistance of additives（i.e.,ascorbic acid abbreviated as AA or criticacid abbreviated as CA）,facile hydrothermal routes are used to synthesize FeCO₃ rhombohedra,CoCO₃ dumbbell-shaped superstructures,full-molar-ratio CoxFe1-xCO₃ rod-like composites and interconnected Ni（HCO₃）2 hollow spheres.Also,in the absence of graphene oxide（GO）,MnCO₃ hierarchical flower-like architectures composed of secondary MnCO₃ nano-spindles could be obtained,while in the presence of nanosheet-shaped GO,uniform MnC03 spindle-GO nanocomposites generated therein.As for the one-pot hydrothermal preparation of each micro-nanostructural MCO₃,time-dependent experimental results are used to uncover nucleation,crystal growth and/or oriented attachment of nanocrystalline building blocks for the formation of the micro-nanostructures and to understand the complex actions of addtives.Especially,several aspects should be summarized and emphasized as the following:（a）the effects of AA or CA additives on the nucleation,crystal growth and/or oriented attachment of nanocrystalline building blocks could assist the formation of micro-nanostructures of MCO₃;（b）the complexation between M2+ ions and AA additives can uniformly induce the co-precipitation of Fe2+ and Co2+ ions to generate full-molar-ratio CoxFe1-xCO₃（0<x<1）rod-like crystallites;（c）the reducibility of AA may prevent the solution-based oxidation of Fe2+ and Ni2+ for the formation of crystallographically phase-pure FeCO₃ and Ni（HCO₃）2 structures;（d）the addition of nanosheet-shaped GO can impede the aggregation of MnCO₃ secondary nano-spindles for the formation of MnCO₃ spindle-GO nanocomposites;（e）an AA-assisted self-sacrificial templating formation mechanism explains the intriguing interconnected configuration of Ni（HCO₃）2 hollow nanospheres.2.Electrochemical Reaction Mechanisms of High-Capacity MCO₃ Aside from the common electrochemical conversion reaction of crystallineMCO₃ towards metallic lithium（MCO₃ + 2Li（?）Li2CO₃ + M）,the further reversible reactions between intermediate Li2CO₃ and metallic lithium[Li2CO₃+（4 + 0.5x）Li（?）0.5 LixC2 + 3 Li2O（x = 0,1 or 2）],catalyzed by the in situ generated M0 nanocrystals,has been proved,which accounts for the actually high-capacity feature of as-obtained MCO₃ micro-nanostructures herein.Furthermore,the possible oxidation of M2+ in micro-nanostructural MCO₃ into its high valences（e.g.M3+）in electrochemically cycled carbonates should not be omitted,which may give a hand to the high-capacity feature of MCO₃ to some extent.According to the structural characterizations and composition analysis of freshly prepared and electrochemically cycled MCO₃ micro-nanostructures,it is easy to visually determine the structural stabilities,and it is reproducible to detect both XPS C1s signals for the possible generation of Li2C2 originating from CO₃2-groups and XPS M 2P3/2 peaks for the further oxidation of M2+ ions.3.Performance Improvement Mechanisms of MCO₃Based on the systematical experimental results of freshly prepared and electrochemically cycled micro-nanostructural MCO₃ electrodes,several aspects should be mentioned to explain the involved performance-enhanced properties:（a）the doping of AA or CA additives grants high specific surface areas and porous features to the micro-nanostructures,which can facilitate their closer contact with electrolyte,effectively buffur the volume changes of MCO₃ occurring during the lithiation-delithiation processes,shorten the diffusion pathway of Li+ ions and even improve structural stability and electronic conductivity of the working electrodes;（b）the atomic-scale synergistic effectiveness of FeCO₃ and CoCO₃ components in full-molar-ratio CoxFe1-xCO₃ composites,by comparison with the electrochemical properties of the single-component FeCO₃ or CoCO₃ and the corresponding xFeCO₃+（1-x）CoCO₃ mixtures;（c）the nanosheet-shaped GO can not only improve the electron conductivity but also increase the specific surface area of active substances,which unexpectedly improves the rate performance and interfacial storage capability of MnCO₃ spindles;（d）the special structural characteristics of interconnected Ni（HCO₃）2 hollow spheres may speed up the diffusion rate of Li+ ions,and thus improve high-rate performance and cycling stability of the working electrodes.