【作者】 丁昭郡；

【导师】 张建新；

【作者基本信息】 山东大学 ， 材料学， 2014， 博士

【摘要】 石墨负极由于容量和密度偏低已难以满足新一代锂电池对高能量密度和便携性的要求,亟需有能量和体积密度更高的材料出现。铁、钴金属氧化物(C0304,Fe3O4, CoFe2O4)由于其高电化学活性和低成本显示出极大的研究和应用价值,是非常有潜力的锂电负极替代材料。此外,作为新型转换型负极,铁、钴碳酸盐(COCO3、FeCO3)的储锂性能存在很大的发展空间,储锂机制也需要进一步研究和证实。本论文旨在通过一步简单水热或溶剂热法制备Co3O4, Fe3O4, FeCO3, CoCO3及前三者的rGO复合物,研究不同类型的rGO复合对氧化物和碳酸盐电化学性能的影响；通过化学氧化法对水热所得C0304和COCO3进行后续PPy包覆,研究PPy包覆对其储锂机制和电化学性能的影响。通过简单固相法制备新型C0304、Fe304基碳复合材料及CoFe2O4基N掺杂碳复合材料,研究碳或氮掺杂碳包覆对其储锂机制和电化学性能的影响,开展的主要工作如下：1、一步低温快速溶剂热制得平均粒径为5,13,12nm的Co3O4颗粒CO1, CO2和C03,其中C02的分散性最好。CO1和C02比C03更迅速的循环衰减表明：对纯转换型负极而言,单分散性颗粒和小粒径不是优势形貌,应力缓冲能力才是关键。对C02进行后续PPy包覆,由于PPy层过厚,复合物的电化学活性明显下降,但容量呈现上升趋势,表明PPy复合对改善Co304颗粒的循环稳定性仍有积极意义；基于相同溶剂热体系,分别引入GO和rGO制得Co3O4/rGO复合物CO/rGO1和CO/rG02。由于C0304与rGO的复合状态不理想,两复合样品的循环和倍率容量均表现出明显的衰减,62次0.1～0.2C循环后容量不足500mAhg-1,末次2C充电约280mAhg-1。尽管如此,两者的循环可逆性仍远优于裸露的C0304颗粒；以葡萄糖为碳源,NaC1为模板,Co(NO3)2-6H2O为钴源,通过两步热处理方法制得片层结构的石墨化碳笼包裹的CO3O4/COO/CO复合负极(GCCC),其中碳含量为25.1wt%。 GCCC表现出优异的循环稳定性和倍率储锂能力：其在0.2C的循环容量逐渐超过0.1C,62周低倍率循环后容量为983.2mAhg-1。；70周0.2-1C循环没有明显衰减,末次1,2,3,4,5C分别充电690.2,583.1,512.7,456.3,412.41mAhg-1;倍率降至0.1C后,容量可恢复至1123.5mAh g-1。2、一步低温快速水热制得Fe304纳米颗粒,并分别引入GO和rGO制得复合物FO/rGO1和FO/rGO2。两复合样品的循环稳定性明显优于纯Fe304,其中FO/rGO2的相纯度高,复合形态好,FO/rGO1由于抗坏血酸加入过量含有Fe3O4和FeCO3两种活性物质,复合形态和rGO片的分散度较差。相应的,FO/rGO2表现出更高的电化学活性,其末次1,2,3,4,5C分别容量为759.9,612.3,472.5,364.5和361.1mAhg-1,经历182次0.1-5C多倍率循环后,0.1C和0.2C容量分别可恢复至1406.6和1363.9mAhg-1,为原同倍率容量的133.7%和138.2%；通过将Fe(NO3)3·9H2O,葡萄糖,NaCl的均匀混合物两步热处理的方法制得石墨烯/Fe3O4/Fe/石墨化碳复合物(GN-FFG),其中石墨化碳笼分布在石墨烯基体中,Fe304包裹在碳笼中或直接分布在基体中,碳含量为31.3wt.%。在Fe304和碳基材料的协同作用下,GN-FFG表现出高度循环可逆性,优异的倍率储锂能力,以及不断增强的电化学活性：其在3,4和5C循环30周后容量分别569.2,523.3和480.6mAhg-1,经历249周0.1-5C循环后,0.2C容量可恢复至1179.9mAhg1,为首次0.1C的132.9%。3、以吡咯为碳源,通过简单压力辅助热解方法成功实现了CoFe204纳米晶表面的N掺C包覆。所得N掺杂C/CoFe2O4复合物在0.1C循环80周容量为646.2mAhg1,0.2～1.6C各倍率下循环稳定,倍率恢复到0.1C时容量为662.8mAhg-1,表现出远优于纯CoFe204的可逆储锂能力。结合CV和电压容量曲线详细分析了N掺杂碳包覆前后首次放电机制的变化,包括转换反应前CoFe204更大的锂插入容量,不同的SEI膜载体和形成电位,可能的更多样化的电容性界面储锂,以及N掺C本身的储锂和随之而来的更大的首次不可逆容量。SEM,直流电导率和EIS测试证实了N掺杂碳层对电极稳定性和储锂动力学的良性影响。4、通过简单水热法制得CoCO3海胆微球(CC),并进一步对其进行PPy修饰。与CC相比,所得CoCO3-PPy复合物(CC-PPy)表现出显著增强的循环稳定性,优异的倍率性能和超强容量恢复能力：在0.1,1,2,3,4和5C下循环100周后可逆容量分别为1070.7,811.2,737.6,518.7,504.5和559mAh g-1,500次1-5C循环后容量可恢复至1787mAhg-1。为支持所得实验数据,在前人工作基础上提出了更全面的COC03负极储锂机制,其中包含两步转换反应,每摩尔COC03的理论储锂量为7mol。一级反应对应着CoCO3还原为金属Co和Li2CO3的形成,二级反应对应着Li2CO3进一步还原为LixC2(x＝0,1,2)和Li20的生成。基于电压容量曲线和CV曲线比较了CC和CC-PPy的锂化和去锂化过程；基于Nyquist图分析了CC-PPy更好的电化学表现背后的动力学因素；CC-PPy电极在不同放电/充电态下的非原位红外光谱有力证实了CoCO3→Li2CO3→Li2O可逆转变过程。5、一步低温快速水热法制得FeCO3微纳米花球(FCMS),并在相同体系中分别基于GO和rGO分散液制得FeCO3/rGO复合物FC/rGO1和FC/rGO2,其中的FeCO3分别以纳米线和纳米颗粒的形式分布在rGO基体中。电压容量曲线和CV表明,与CoCO3一样,FeCO3基负极的转换反应不局限于FeCO3→Li2CO3转变,而是有Li2CO3→LixC2(x=0,1,2)二级反应发生。不同于C0304和Fe304纳米颗粒,FeCO3微纳米花球有不错的循环和倍率表现,0.1～43.2C循环62周容量达到609.4mAhg-1,1C和2C容量分别为452.2和320.6mAh g-1,这与其结构中存在可缓冲充放电过程中体积变化的空间有关。两个rGO复合样品在低倍率下均表现出比FeCO3花球更好的电化学活性和循环稳定性,但FC/rGO2在较高倍率下逐渐失去其容量优势,FC/rGO1则始终保持高活性,在0.1～0.2C循环60次后容量为842mAhg-1,1.5,3,4,5C容量分别为540.9,423.4,328.9,213.9mAhg-1;0.1-5C循环253周后,末次0.2C可充电1166mAhg-1,表现出良好的循环,倍率和容量恢复能力。更多还原

【Abstract】 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 COCO3→Li2CO3→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 FeCO3→Li2CO3as indicated by galvano profiles and CV curves, but a secondary reaction took place Li2CO3→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.更多还原