Properties Of Electrode/Electrolyte Interfaces For Intercalation Electrodes And Electrochemical Properties Of Manganese Oxide Electrodes For Lithium-ion Batteries

Rechargeable lithium-ion batteries are regarded as promising energy storagedevices for portable electronic devices as well as electric vehicles (EVs) and hybridelectric vehicles (HEVs), due to their high energy density, high power density, lowcost, superior safety, and stable cycling life. This dissertation is focused on threeaspects related to the development and research of lithium-ion batteries. They are thepreparation of new type anode and cathode materials for Li-ion batteries, theoptimization of the electrolyte and the properties of electrode/electrolyte interfaces forLi-ion batteries, respectively.The main research contents and conclusions are as follows:(1) Single intercalation particles, mixed particles, homogeneous porouselectrodes and nonhomogeneous, multilayered porous electrode models are proposed.Meanwhile, nonhomogeneous, multilayered porous graphite electrodes are prepared,and the first lithium-ion insertion and extraction processes of nonhomogeneous,multilayered porous graphite electrode at different potentials are studied byelectrochemical impedance spectroscopy (EIS). The results reveal that a newsemicircle is observed in the middle frequency region. There are three semicircles andone line appeared in the whole frequency region. This new phenomenon has beeninvestigated through the detailed analysis of the change of kinetic parameters obtainedfrom simulating the experimental EIS data as a function of potential. It has found thatthe two semicircles in the intermediate frequency region were strongly potentialdependent, and they were both attributed to the charge transfer process. A detailedanalysis reveal that a different particle size distribution could lead to the appearanceof a new arc and a different layer distribution (a thicker layer and a thinner layer),which could lead to a well-developed semicircle.(2) The effects of vinylethylene carbonate (VEC) and fluoroethylene carbonate(FEC) as electrolyte additives, and the content of VEC or FEC in ethylene carbonate(EC)-based electrolyte on the formation mechanisms of solid electrolyte interface(SEI) film and the electrochemical properties of the graphite electrodes in lithium-ionbatteries are investigated by cyclic voltammetry (CV) measurement andcharge-discharge test. The results show that in the case of electrolyte containing lowcontent of VEC(<5vol%), the formation of ROCO2Li due to the double electronsreduction process of EC can be suppressed, thus improvs the electrochemical performance of graphite electrodes. In the case of electrolyte containing high contentof VEC, the formation of Li2CO3due to the single electron reduction process of ECcan be also suppressed, that may take an adverse effect on the cycle performance ofgraphite electrodes. Scanning electron microscopy (SEM) and Fourier transforminfrared (FTIR) spectroscopy are used to investigate the morphology and the surfacechemistry of graphite electrodes cycled in VEC-free and VEC-containing electrolytes.Finally, EIS is used in order to better understand the formation mechanisms of SEIfilm in VEC-containing electrolyte. The results reveal that the main reductionproducts of the SEI film formed in VEC-containing electrolyte are VEC polymerizes,Li2CO3and ROCO2Li. The SEI film covering graphite electrodes in VEC-containingelectrolyte can be more stable during lithium ions insertion, and be flexible toaccommodate the volume changes of graphite material, resulting in a betterreversibility of lithium ions insertion and extraction.For the electrolyte contains FEC, the double electrons reduction process of ECcan also be suppressed. When the content of FEC is1vol%, the graphite electrode hashigher reversible capacity and better cycle performance. The graphite electrode cycledin FEC-containing electrolyte is uniformly covered by small particles with the particlesize of100nm according to SEM results. The surface chemistry of graphite electrodecycled in FEC-containing electrolyte is investigated by X-ray photoelectronspectroscopy (XPS). The results demonstrate the compositions of SEI film are LiF,Li2CO3, ROCO2Li and Li2O. It is believed that inorganic Li2CO3and LiF have ahigher conductivity than organic ROCO2Li; hence the discharge capacity and theconductivity of lithium ions are increased. EIS results reveal that adding FEC to theelectrolyte can cause the formation of a stable SEI film with low resistance on thegraphite electrode, which effectively prevents carbon exfoliation and the volumechange during the lithium ions insertion and extraction processes. These resultsindicate that the improvement of electrochemical performance of the graphiteelectrode can be attributed to the formation of a uniform, thin, compact and stable SEIfilm with low resistance.(3) Carbon micro-coils (CMCs) are synthesized by chemical vapor deposition(CVD) method. CV, charge-discharge test and EIS are introduced to characterize theelectrochemical properties of CMCs electrode and discuss the properties ofelectrode/electrolyte interfaces during the fist lithium ions insertion process.Charge-discharge results show that the initial irreversible capacities of CMCs electrodes are both large at30and100mA·g-1. But for electrode at the current densityof100mA·g-1, it has a better cycle performance with the reversible capacity of226.9mAh·g-1after250cycles. EIS results demonstrate the SEI film is mainly formed in thefirst discharge process, and there are three main stages in the SEI film formationprocess. First, the rate of SEI film formation is slow, and the thickness of SEI filmincreases slightly in the potential range of1.5to1.0V; second, when the potential isdecreased from1.0to0.6V, SEI film develops and the thickness increase rapidly; thethird stage is the stable stage when the potential is further decreased, SEI film growthtends to be stable, the thickness and the resistance of SEI film are nearly invariable. Astable and compact SEI film can pronounce a positive effect on the cycling behaviorof the CMCs electrode.(4) Mn3O4nanoparticles and Mn3O4/carbon nanotubes (CNTs) composites areprepared via a hydrothermal synthesis method. MnO and MnO/CNTs composites areobtained by heating Mn3O4and Mn3O4/CNTs at500℃for3h in flowing Ar/H2. Thephase structure, composition and morphology of the composites are characterized byXRD and field emission scanning electron microscopy (FESEM). Theelectrochemical properties of the composite electrodes are studied by performing CV,galvanostatic charge and discharge tests. The results reveal that the Mn3O4/CNTs andMnO/CNTs electrodes exhibit higher specific capacity at the current density of100mAh·g-1and better cycle performance than pure Mn3O4and MnO electrodes. Theexcellent electrochemical properties of Mn3O4/CNTs and MnO/CNTs electrodes canbe attributed to the presence of CNTs in the composites offering an electronicconducting network and suppressing the volume expansion of Mn3O4and MnOparticles efficiently during the charge and discharge processes.(5) Li2MnSiO4and Li2MnSiO4/C with glucose, adipic acid and sugar as carbonsources, are synthesized by sol-gel method. The crystalline structure and morphologyare determined by XRD and SEM. Charge-discharge data reveal that, carbon coatingcan increase the discharge capacity of Li2MnSiO4, the initial discharge capacities ofLi2MnSiO4/C with glucose, adipic acid and sugar as carbon sources are23.3,179.7,184.4and81.4mAh·g-1, respectively. EIS results demonstrate that carbon coating candecrease the charge-transfer resistance, and improve the velocity of lithium iondiffusions in the bulk of the electrode. According to the above results, it can beconcluded that the addition of C can noticeably increase the discharge capacity ofLi2MnSiO4and improve the electrochemical activity of material. Meanwhile, Li2MnSiO4/C with glucose as carbon source can achieve the best electrochemicalproperties.In this paper, there are84figures,12tables and263reference articles.