Synthesis and improvement of Fe, Mn based phosphate cathode materials for Lithium ion batteries

Abstract

After 30 years of vigorous development, rechargeable Li-ion batteries have become the most popular batteries which are widely used in various of electronic devices such as mobile phone, video cameras and computers, owning to their good cyclic life, high energy density and high capacity to other secondary cells. With the exhaustion of fossil energy reserve and the increase of air pollution, further research and development of batteries with higher power, improved safety, longer life, lower cost and larger battery systems for portable electronics, electric vehicles (EVs), plug-in hybrid EVs and energy storage is desperately needed for sustainable development. Among the family of the cathode materials, olivine-structured orthophosphates such as LiFePO4 (LFP) and LiMnPO4 (LMP) are considered to be two of the most promising cathode materials for high-power batteries. Comparing with the cobalt oxide- and manganese oxide-based materials, LFP has the virtues of low cost, environmentally benign, safety and thermal stability plus the characteristics of high reversible theoretical capacity (170 mAh g-1) and perfect flat voltage profile at 3.45 V vs Li/Li+. On the other hand, LMP has higher theoretical energy density (701 Wh kg-1 = 171 mAh g-1 × 4.1 V) than that of LFP (586 Wh kg-1 = 170 mAh g-1 × 3.45 V) and a mild voltage (4.1 V vs Li/Li+) within the stable window of commercially used carbonate ester-based electrolytes, which makes it a practically applicable material compared with LiCoPO4 and LiNiPO4 that with higher potential of 4.8 V and 5.1 V vs Li/Li+ respectively. However, both of the LFP and LMP have poor lithium ion and electronic conductivities of ~10-9 S cm-1 and < 10-10 S cm-1 respectively, leading to poor rate and cycling performances. In order to overcome the inherent kinetic limitations and poor electrochemical performances of these materials, tremendous efforts are focused on preparing nanosized or microsized hierarchical structured materials accompanied by conductive coating and internal cationic doping. The objective of this dissertation is to improve the rate and cycling performances of these Fe and Mn based phosphate cathode materials using Li3PO4 as the precursor and discuss the reaction mechanisms. To achieve this goal, a research program comprising three major parts was performed: Firstly, A simple one-pot solvothermal approach was employed to synthesis LMP nanomaterial using Li3PO4 nanorod and MnSO4H2O as the precursors. Various experimental parameters such as volume ratio of polyethylene glycol 600 (PEG 600) to water, reactant feeding order, reaction time, pH value, were studied. A reaction mechanism was proposed with evidence from the XRD and TEM results. The charge-discharge properties show that the LMP nanomaterial synthesized at 180 ºC for 4 h under pH value of 6.46 followed by sintering with glucose at 600 ºC for 3 h under argon circumstance present the highest discharge capacity of 147 mAh g-1 at 0.05 C rate (i.e. 8.55 mA g-1 of current rate) under a galvanostatic charging-discharging mode. It can retain 93% of the initial capacity of 46.6 mAh g-1 after cycling 200 times at 1 C rate. The pH value of the system was found to have a strong influence on the unit cell parameters of the final products. The as-prepared LMP products with smaller particle size, larger crystal cell volume and larger lattice parameters along b-axis had higher lithium ion diffusion coefficient and better electrochemical performance. Secondly, monodisperse LFP micro hollow spheres were synthesized by ion exchange reaction between spherical Li3PO4 templates and Fe2+ ions in ethylene glycol (EG) medium in solvothermal condition. The direct phase transformation from Li3PO4 to pure LFP could be clearly observed from the XRD patterns of the samples collected after different reaction time at 180 oC. SEM, TEM images and electron diffraction pattern show that the LFP micro hollow spheres exhibit a quite uniform size of 1 m and a polycrystalline structure composing of aggregated nanoparticles. The influences of the ferrous salt and water on the phase and morphology of the final LFP products had been systematically studied. The polycrystal LFP micro hollow spheres were carbon coated by sintering with glucose, resulting in a high rate discharge capability (72.5 mAh g-1) at 50 C and an excellent cycling stability at 20 C over 2000 cycles (maintaining 80% of its initial discharge capacity of 101 mAh g-1). This approach may provide an effective strategy to prepare other LiMPO4 (M= Mn, Co, or Ni) micro hollow spheres. Thirdly, a series of Li3PO4 micro hollow sphere with different sizes were prepared. The size effect of the electrochemical properties of the LFP microspheres was studied. It was found that the LFP prepared form Li3PO4 micro hollow sphere with size around 1 m had the highest capacity and best rate performances. The reactions between Li3PO4 micro hollow sphere and Mn2+, Co2+, and Ni2+ were also investigated. The preliminary results showed that the LMP and Fe doped LMP could also be obtained by slovothermally treating the Li3PO4 micro hollow sphere with MnCl24H2O or FeCl24H2O/MnCl24H2O mixture in EG (ethylene glycol) solution. However, the electrochemical performances of the carbon coated LMP and Fe doped LMP materials were poor, and further modification was needed. The reaction between Li3PO4 micro hollow sphere and Co2+ was not clear yet, because an unknown intermediate product formed after the solvothermal treatment. This intermediate product could also transform into LiCoPO4 and other impurity after calcination at high temperature. The Ni2+ could not react easily with the Li3PO4 micro hollow sphere under solvothermal condition.

Date of Award	2 Oct 2013
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chi Yuen CHUNG (Supervisor)