Researches Of Carbon Gel And Their Composites As Anode Materials For Lithium-Ion Batteries

Lithium-ion batteries are considered as the most promising power sources in the 21st century for the wide application range from microbatteries for small-size electronic devices to power sources for electrical vehicles. Although, the capacity of the material conventionally used for the anodes, graphite, is increasingly approaching its theoretical limit of 372 mAh·g-1, it is still insufficient for meeting the needs of future electronic equipment. Therefore, a lot of efforts have been made to develop new material in recent years.As a kind of novel porous carbonaceous materials with merits of low mass densities, continuous porosities, high surface areas, and high electrical conductivity, carbon gel (aerogel, xerogel or cryogel, CG) has been paid worldwide attentions as promising material for anode electrode of supercapacitors and rechargeable lithium-ion batteries, advanced catalyst supports, adsorbents, chromatographic packing, thermal insulators, and a variety of other applications.In present work, the polymerization of resorcinol (R) with formaldehyde (F) catalyzed by sodium carbonate results in the formation of dark red, transparent RF organic gels via a sol-gel process, acid aging process, and drying (supercritical drying, ambient pressure drying or freeze-drying). Upon pyrolysis of RF organic precursors at high temperature, the monolithic RF carbon gels can be obtained. The different CG samples obtained by different drying condition were denoted by carbon aerogel (CA), carbon xerogel (CX) and carbon cryogel (CC) in this article, respectively.The structures and properties of CG were investigated by using techniques such as SEM, TEM, XRD and BET, respectively. Effectes of synthetic condition on the structures and properties of CG were researched in details. The results showed that the carbon particles of CG are interconnected into the aggregates like bunches of grapes, which are connected into a bulk network. Due to the supercritical drying can keep the formed skeleton structure of wet gels, the particle size of CA is the smallest and most uniform than that of the other samples. CA has a BET surface area higher than 556 m2·g-1 and a fairly uniform pore size distribution in the range 10-20nm. The prepared CA possesses several small and broad peaks related to active carbon and graphite, indicating that the obtained CA is composed of active carbon and graphite. Compared with CA, CX and CC are more crystalline. The density of the reactants in the initial solution,catalyst ratio (initial solution pH) and drying condition have many considerable effectes on the final structure of CG.In this dissertation, the electrochemical performances of CG as anode materials for lithium-ion batteries were investigated by using electrochemical techniques such as charge-discharge test, cyclic voltammetry and impedance spectroscopy (EIS) analysis, respectively. The results show that CA exhibits the highest reversible capacity (500.7 mAh·g-1) and preferable cycling performance than the other samples. Moreover, the reversible capacity maintains 317.7 mAh·g-1 after the 50th cycle with the capacity retention of 63.3% and the coulomb efficiency of over 99%.The CG-SiO (or CG-Si) composite anode material was synthesized by high energy mechanical ball-milling of CG and SiO (or Si) at room temperature and atmosphere. The influences of doped chemical, doped ratio and ball-milling condition on crystal structure and eletrochmical performance of CG-SiO (or CG-Si) composite were investigated. The results showed that, CG-SiO (or CG-Si) composite is composed of active carbon, graphite, SiO (or Si) composite and dispersed Si crystal while CG consists of active carbon and graphite, and CG-SiO has smaller and much more uniform particles than CG. Electrochemical tests showed that the electrochemical performance of CG-SiO (1:1 wt%) (or CG-Si (3:1 wt%)) is the best, and the electrochemical performance of CG-SiO is batter than that of CG-Si. SiO can greatly improve discharge capacity of CG with an acceptable sacrifice of cycling stability, and the charge-discharge capacity of CG-SiO comes mainly from lithium insertion-extraction in Si-SiO in the sample. Results show that the CG-SiO (or CG-Si) composite, which is obtained by ball-milling 10 hours and 400 rotating per minute (rpm), exhibits higher reversible capacity and preferable cycling performance.CA-SiO and CA-Si have excellent high-rate dischargeability and are promising anode materials of lithium-ion batteries for use of high power density purpose. The CA-SiO composite, which was synthesized by ball-milling, reveals an initial reversible capacity of 1942 mAh·g-1 and reversible capacity of 984.6 mAh·g-1. After the 50th cycle, the reversible capacity maintains 635.3 mAh·g-1 with the capacity retention of 66.4% the coulomb efficiency of over 98%.