Synthesis And Electrochemical Performance Of Oxides And Carbonates Of Iron And Cobalt As Anode Materials For Lithium Ion Batteries

Due to its low capacity and density, graphite as the anode material can hardly meet the requirement of high energy density and portability for the new generation of Li ion batteries which calls out for the materials with higher energy and volume density. Oxides of iron and cobalt, being highly electrochemical reactive and inexpensive, which are of great scientific and application interest, are considered potential candidates for the present anode material. In addition, as newly emerged conversion anodes, iron and cobalt carbonates have much improvement could be made on the lithium storage properties, the lithium storage mechanism need be further proven by research as well.The aim of this thesis is listed as follows:prepare CO3O4, Fe3O4, FeCO3, COCO3and the composite of one of first three and grapheme by simple one-step hydro-thermo or solu-thermo route; study the electrochemical performance of oxides and carbonates decorated with grapheme of different kinds; coat CO3O4and COCO3prepared by hydrothermo route with poly-pyrrole via chemical oxidation method and study the influence of poly-pyrrole coating on lithium storage and electrochemical properties; prepare CO3O4/C, FesCVC and CoFe2O4/nitrogen doped carbon composites by simple solid state reaction and study the influence of carbon and N doped carbon coating on the lithium storage mechanism and electrochemical performance. Detailed works carried out are presented as follows:1. CO3O4were synthesized by low temperature rapid one-step solu-thermo route with diameter of5,13,12nm, namely CO1, CO2, CO3, CO2having the best dispersity. Capacity of CO1and CO2decreased faster than CO3as cycling, indicating it not being an advantage to achieve mono-dispersed and fine particles in morphology and buffering stress in cycling being the key factor. As with CO2coated with poly-pyrrole, electrochemical performance is evidently hindered because of the PPy coating being too thick. However, an increase in capacity is observed, indicating PPy decorating plays a positive role on the cycling stability of CO3O4. Based on the same solu-thermo system, introducing GO and rGO in preparing process resulted two grapheme decorated composite namely, CO/rGO1and CO/rGO2. Owing to the poor composite status, both sample presented capacity decay at cycling and rate tests, achieving a capacity less than500mAh g’1after cycling62times at0.1to0.2C, with last charging capacity of～280mAh g-1at2C. Nevertheless, both sample showed an increased reversibility than bare CO3O4. Flake structured graphitized carbon encapsulated CO3O4/COO/CO composite (GCCC) with carbon content of25.1wt.%was prepared by two-step thermo treatment using glucose as carbon source, Co(NO3)2·6H2O as cobalt source and NaCl as template. GCCC exhibited superior cycling stability and rate lithium storage ability, cycling capacity at0.2C having surpassed that of0.1C as the cycle proceeds,983.2mAh g-1achieved at low rate after62cycles, without decay at0.2to2C cycling for70cycles. The last charge capacities at1,2,3,4and5C were690.2,583.1,512.7,456.3and412.4mAh g-1. Capacity recovered1123.5mAh g-1when rate decreased to0.1C.2. Fe3O4nano particles were synthesized by low temperature rapid one-step hydrothermo route, introducing GO and rGO in preparing process resulted two grapheme decorated composite namely, FO/rGO1and FO/rGO2. Both composite samples exhibited increased cycling stability than bare Fe3O4. FO/rGO2had a high phase purity and good composite status; owing to the excessive ascorbic acid used, FO/rGO1had two phases Fe3O4and FeCO3with poor composite status and dispersity of rGO. Correspondingly, FO/rGO2exhibited higher electrochemical reactivity, the last capacities at1,2,3,4,5C being759.9,612.3,472.5,364.5and361.1mAh g-1. Cycled at0.1-5C for182cycles, capacity recovered1406.6and1363.9mAh g-1at0.1and0.2C, being133.7%and138.2%of initial capacity at same rate. graphene/Fe3O4/Fe/graphitized carbon composite with carbon content of31.3wt.%(GN-FFG) was prepared by two-step thermo treating Fe(NO3)3·9H2O, glucose and NaCl mixture, whose graphitized carbon cages were dispersed in grapheme substrates, Fe3O4encapsulated in carbon cages or directly embedded in substrates. Combination effects of Fe3O4and carbon materials made GN-FFG present high cycle reversibility, superior rate lithium storage ability and ever increasing electrochemical reactivity. The cycling capacity at3,4and5C for30cycles were569.2,523.3and480.6mAh g-1, respectively. After cycling at0.1-5C for249cycles, capacity retrieved1179.9mAh g-1, which was132.9%of initial capacity at0.1C.3. Nitrogen doped carbon coating of CoFe2O4was realized by simple pressure assisted pyrolysis route using pyrrole as carbon source. The as prepared N doped C/CoFe2O4composite delivered a capacity of646.2mAh g-1after80cycles at0.1C and can cycle stably at0.2-1.6C, retrieving662.8mAh g-1when returned to0.1C which shows far better lithium storage ability than bare CoFe2O4. Combining CV and galvano discharge/charge curves, the discharge mechanism change in first discharge of nitrogen doped carbon coated and without coating samples was analyzed in detail, encompassing large amount of lithium intercalation before conversion reaction of CoFe2O4, different SEI building and formation potential, possible diverse capacitive surface lithium storage, capacity brought by nitrogen doped carbon and the resulting enlarged irreversible capacity in first cycle. SEM, DC conductivity and EIS confirmed the positive effects of nitrogen doped carbon coating on the cycling stability and lithium storage kinetics.4. Urchin micro sphere COCO3(CC) was prepared by simple hydrothermo route, and further decorated by poly-pyrrole. Compared with CC, the as prepared CoCO3-PPy (CC-PPy) exhibited much enhanced cycling stability, superior rate performance and capacity retrieving ability. The capacities after100cycles at0.1,1,2,3,4and5C were1070.7,811.2,737.6,518.7,504.5and559mAh g-1, respectively, retrieving1787mAh g-1after500cycles at1～5C. Based on the studies reported previously, a more comprehensive lithium storage mechanism was proposed with respect to the experimental data above which comprised two step conversion reaction,7mol lithium being stored per mol COCO3. First step conversion was consistent with the reduction of COCO3to Co and the formation of Li2CO3. Second step conversion was consistent with the reduction of Li2CO3to LixC2(x=0,1,2) and the formation of Li2O. Based on EIS Nyquist plot, kinetic origin of the enhancement of CC-PPy was analyzed. The ex-situ FTIR spectroscopy at different state of charge/discharge gave sufficient evidence of reversible COCO3â†'Li2CO3â†'Li2O reaction.5. Micro-sized nano flower sphere FeCO3was synthesized by low temperature rapid one-step hydrothermo route, introducing GO and rGO in preparing process resulted two grapheme decorated composite namely, FC/rGO1and FC/rGO2, in which FeCO3was disperse in form of nano wires and nano particles in rGO substrates. Resemble with CoCO3, the conversion reaction of FeCO3anode was not restricted to FeCO3â†'Li2CO3as indicated by galvano profiles and CV curves, but a secondary reaction took place Li2CO3â†'LixC2(x=0,1,2), as well. Unlike Co3O4and Fe3O4nano particles, micro-sized nano flower sphere FeCO3exhibited fairly good cycling and rate performance, capacity being609.4mAh g-1after62cycles at0.1～0.2C,452.2and320.6mAh g-1for1C and2C, respectively. It was because of the spacious buffering space for the volume change in cycling process. FC/rGO1and FC/rGO2 presented better electrochemical reactivity and cycling stability than micro-sized nano flower sphere FeCO3. However, FC/rGO2lost advantage over FeCO3at fairly high rates. FC/rGO1kept high reactivity at all rates, delivering a capacity of842mAh g-1after60cycles at0.1～0.2C,540.9,423.4,328.9and213.9mAh g-1at1.5,3,4and5C, respectively. After cycling for253cycles at0.1-5C, last charge capacity at0.2C was1166mAh g-1, exhibiting good retrieving ability, cycling and rate performance.