Studies On Lithium Storage Materials Based On Multi-Electron Redox Reaction

Chemical power sources have been recognized to be the key link in thedevelopment of current sustainable new energy technologies. Being regarded as therepresentative of advanced secondary batteries, lithium-ion battery has the highestenergy density among present electrochemical energy storage systems, and isconsidered as the ideal choice for future electric vehicles (EVs) and energy-storagepower stations. However, it is very difficult to enhance the energy density of Li-ionbattery due to the limitation of theoretical capacity and structural stability of thetraditional intercalation materials. Therefore, it is assuredly a most important subjectfor searching new structural material and exploring new lithium-storage mechanism inresearch of Li-ion batteries. Alloying reaction and conversion reaction based onvalance changes of active components or reversible structure conversion in the hostseem to provide approach to enable muti-electron transfer reactions and realize thesignificant breakthrough in the capacity of the electrode materials for Li-ion batteries.In this thesis, we tried to explore both cathode and anode materials based on suchmuti-electron transfer reactions through building novel composite structure to benefiteto the reversibility of alloying reaction, conversion reaction and special intercalationreaction and to realize high specific capacity of electrode materials for thedevelopment of high-energy Li-ion batteries. The main results and new findings inthis work are summarized as follows:1. A new approach was proposed to prepare composite anodes with muti-layer core-shell structure, in which the pulverization and agglomeration of anode materials areimmensely avoided and the electric contact is always maintained during the wholeelectrochemical process. Therefore, we synthesized sandwiched nanocomposites anode with inert matrix (SiC, WC, TiN) as the nanocore, active anode materials (Sn,Si, Al, Sb) as the interlayer, and graphite as the shell by simple ball-milling process,and investigated their electrochemical performance in this work. The experimentalresults showed that the synergy of inner inert matrix and outer graphite can effectivelyaccommodate the volumetric expansion of active anode materials during lithiuminsertion process, ensure good electric contact between active particles, and maintainthe structural stability of the composite. Thus, such core-shell nanocomposites greatlyimprove the electrochemical performance of active anodes. Among them, SiC-Sn-Ccomposite anode exhibits superior electrochemical performance with a high capacityof671mAh g-1and a capacity attenuation rate of only0.066%per cycle. On the basisof these results, we also investigated the formation mechanism of this sandwichedstructure and the relationship between the electrochemical performance and thestructure of the composite electrodes.2. A controllable, cheap, and scaleable one-step ball milling is employed to prepareinactive/active/carbon ternary compounds including Co-Sn-C and Fe-Al-C, and theirelectrochemical performances are also investigated in this work. The experimentalresults show that alloying with inert metal can effectively alleviate the volume changeof Sn and Al anode during charge/discharge process, thus improve theirelectrochemical performance. Especially Fe-Al-C compound evidently enhances thelithium-storage utilization of active Al when compared with elemental Al anode.3. For the reversibility of the electrochemical conversion reaction, a located domainmodel in nano-sized range is proposed to improve the kinetics of the conversionreaction. We synthesized two nanocomposites, SnO2-SiC/C and MnO-Li2CO3/C,through respective ball milling and liquid impregnation methods, and investigatedtheir electrochemical performance when used as anode and cathode materialsrespectively for Li-ion batteries. The experimental results show that the conversion reaction can proceed reversibly as long as all the phases are well dispersed innanoscale and closely contacted to create electrochemically favorable nano-domainsin the electrode. The SnO2-SiC/C composite anode with core-shell structure canrealize completely reversible conversion and alloying reaction, involving8.4electrontransfer in total, and deliver a reversible capacity of1451mAh g-1(close to thetheoretical capacity of SnO2) with excellent cycling stability and rate capability. TheMnO-Li2CO3nanocomposite supported by mesoporous carbon (MC-1) can deliver ahigh capacity of493mAh g-1, corresponding to two-electron reversible transfer,which is three times of the traditional intercalation cathode materials in capacity, andshow a good cycling stability.4. Hierarchical macro/meso porous Li2FeSiO4with interconnected carbon (P-Li2FeSiO4@C) was synthesized via template-assisted approach in this work. We alsoinvestigated the formation mechanism of this porous structure and further tested theelectrochemical performance of the P-Li2FeSiO4@C cathode. The experimentalresults show that the hierarchical porous structure and carbon coating benefit largeelectrochemical reaction surface, downsizing particle and excellent electron and iondiffusion path, leading to superior high capacity as well as fast kinetics of Li ionstransportation and charge transfer within the interconnected frameworks. At normaltemperature (25°C), the P-Li2FeSiO4@C cathode can not only realize stable cyclingwith high capacity (254.3mAh g-1), but also show a strong power capability with aquite high rate output of95mAh g-1even at40C (6000mA g-1).