| Summary: | 博士 === 國立清華大學 === 材料科學工程學系 === 107 === Li-ion batteries (LIBs) are the most promising energy storage system because of their long cycle life and high energy density. However, it is vital for anode and cathode materials which store Li-ions in their host structure to pursue higher energy density, power density, and safety. Increasing the stability of batteries prevents the damage as well as the explosion of batteries. Therefore, it is aimed to develop high-capacity and high-rate LIBs on high-safety anode/ cathode materials, anatase TiO2 and olivine LiMnPO4, by using the strategies of nano technology, morphology control, surface coating, and cation doping/substitution.
For anode materials, anatase TiO2 is a high-safety material as the operating voltage is above 1 V (v.s. Li/Li+) higher than the voltage range to form severe SEI layer. Anatase TiO2 has the high theoretical capacity (334 mAhg-1) among other high-safety materials, yet using the strategies as mentioned above is essential since it has a poor electrical conductivity and Li-ion diffusivity. The first part in this study aims to enhance the cycling life and rate capability of anatase TiO2 by combining the size reduction, morphology control, and cation substitution into one step. Nb-substituted TiO2 nanoplates (Nb-TiO2) were synthesized from hollow TiO2 in a hydrothermal process. Substituting large amounts of Nb5+ into anatase TiO2 promotes the morphology transformation from hollow to plate. Nb-TiO2 nanoplates with (001) preferred orientation show superior rate capability of 127 mAhg-1 at 10 C and cycling stability of 10,000 cycles at 20 C. By applying the cyclic voltammetry in wide scan rates, the mechanism of high rate is due to the enhanced pseudocapacitance which promotes fast Li-ion insertion/ extraction behaviors near the surface of Nb-TiO2.
In the second part, the study aims to improve the utility potential of anatase TiO2 in LIBs and SIBs (Na-ion batteries). Therefore, a novel preparation method is invented. The anatase TiO2 nanoplates with 10 nm particle sizes and uniform pores are produced by pyrolysing titanium-terephthalate hybrid materials. In SIBs, high-crystallinity anatase exhibits good rate capability, delivering 53 mAhg-1 at 30 C. Ex-situ XRD and XPS analysis show that anatase TiO2 forms metallic Ti and amorphous sodium titanate which is reversible with Ti4+/Ti 3+ redox reaction. Pseudocapacitance is found to comprise most capacity in the first cycle, and then the insertion capacity will enhance after activation, which proves that anatase TiO2 is a suitable host for accommodating Na-ion.
For cathode materials, LiMnPO4 is one of the olivine materials which show the features of high thermal stability, cost efficiency, and cycle life. LiMnPO4 with higher operating voltage (4.1 V v.s. Li/Li+) and energy density (701 Whkg-1) is expected to replace the commercialized LiFePO4. However, LiMnPO4 suffers from poor electric conductivity (< 10-9 Scm-1) and Li-ion diffusivity (< 10-14 cm2s-1) which limit its potential. Therefore, coating carbon, reducing particle size, and doping cation into LiMnPO4 is essential to improve the electrochemical performance. So far, LiMnPO4 is generally synthesized from the hydrothermal process since the produced particles are highly crystalline and small, but the yields are meager.
The study of LiMnPO4 in the first part reveals a novel preparation method, called diamine-assisted polymerization method to synthesize nano LiMnPO4 coated with homogenous N-doped carbon derived from polyamides. The p-phenylenediamine (PPD) is added into the synthesis process to suppress the particle growth. Moreover, PPD maintains the reaction pH, preventing the impurity formation. When coated carbon is prepared with sucrose, the LiMnPO4/C prepared with large amounts of PPD exhibits 134 mAhg-1 at 0.1 C. To cover a more homogenous and conductive carbon on LiMnPO4, PPD and acyl chlorides are in-situ polymerized into aromatic and semi-aliphatic polyamide. N-doped carbon pyrolyzed from the polyamide allows a fast Li-ion migration into the LiMnPO4. It is demonstrated that N is bonding with P and Mn on the LiMnPO4 surface, decreasing the contact resistance of carbon. Thus, LiMnPO4/N-doped C exhibits superior cycling performance.
In the final part, V4+ is substituted into LiMnPO4 to improve rate capability and relieve the strain during phase transition. V4+ acts to suppress the grain growth and promote (020) preferred orientation. The accommodation of V4+ on Mn site is accompanied with Mn vacancy. The carbon-coated LiMn1-2xVxPO4 exhibits a superior rate capability of 157 mAhg-1 at 0.1 C and 106 mAhg-1 at 20 C. In-situ XANES reveals that a continuous V3+/4+ and V4+/5+ redox reactions occur in the range of 2.0-3.5 V and 3.5-4.3 V. The V3+/4+ redox reaction promotes the solid-solution reaction and additional capacity below 3.5 V. Furthermore, in-situ XRD shows that the LiMn1-2xVxPO4 undergoes a fast crystalline-to-amorphous reaction. The formation of a metastable amorphous phase with wide Li contents will relieve the interfacial strain, which explains why the olivine with sluggish phase transition can exhibit fast lithiation/ delithiation after substituting with V4+.
Therefore, both anatase TiO2 and olivine LiMnPO4 exhibit outstanding performance, especially on rate capability by using novel preparation methods and several improving strategies. The electrochemical mechanisms are completely revealed which becomes the foundation to develop high-safety materials with superior performance.
|
|---|
