Preparation And Performance Of LiMnPO₄ As Cathode Material For Lithium-ion Battery

It is very important to develop new cathode materials with low cost, high safety and excellent properties for Lithium-ion batteries scale-up application. In view of this issue, olivine LiMnPO₄ with good commercialized electrolyte compatibility, has been considered as one of the most promising cathode materials. However, its poor conductivity restricts application. In order to improve the electrochemical properties of LiMnPO₄, the hydrothermal preparation and carbon coating process for nano-LiMn_1-xFe_xPO₄/C composite materials with high crystalloid and dispersion were investigated in this work, the structural and electrochemical properties of as-prepared materials were studied by using XRD, SEM, electrochemical impedance and charge-discharge tests etc. technologies. The obtained innovative results are described as the following:1. Structure and properties characterization of LiMn_1-xFe_xPO₄/C（x=0.1, 0.2, 0.3, 0.4, 0.5） prepared by hydrothermal method followed by carbon-coating, were studied firstly. The results show that as-prepared materials are pure olivine LiMn_1-xFe_xPO₄/C solid solution. With increase of Fe content, the obtained samples present improved crystallinity and particle dispersion, enhanced Li+ diffusion coefficient, reduced electric charge transfer resistance, leading to remarkable enhancement on capacity and rate capability. The sample with x=0.5 shows the best performance, and its discharge capacity reaches 161mAh/g, 146.9mAh/g and 120mAh/g at 0.2C, 1C and 5C, respectively. Compared with sample with x=0.5,the sample with x=0.3 exhibits a little lower energy density but higher average voltage, which implies that its energy density may be further improved by optimizing the synthesis conditions.2. Secondly, the effects of carbon-coating process on the structure and properties of nano-LiMn0.7Fe0.3PO4/C materials were investigated. The results reveal that in-situ carbon coating during hydrothermal process make LiMn0.7Fe0.3PO4/C obtain more uniform coated carbon, more controlled crystal particle growth and dispersion, more excellent electrochemical performance while compared with the hydrothermal method followed by carbon-coating process. It presents a discharge capacity of 108.8mAh/g at 5C, while the sample prepared by the hydrothermal method followed by carbon-coating process only shows a capacity of 69.9mAh/g.3. Next, the effects of CTAB content on the structure and properties of nano-LiMn0.7Fe0.3PO4/C materials prepared by in-situ carbon coating during hydrothermal process were investigated. The results display that CTAB content exhibits little effect on crystal lattice parameters, structure and coated-carbon amounts of as-prepared materials. However, it displays great impact on the crystal particle size, carbon coating uniformity and particle dispersion. With gradual increase of CTAB content to 2.5g（in 40 ml reaction liquid）,as-prepared samples show an improved carbon coating uniformity, elevated Li+ diffusion coefficient, reduced electric charge transfer resistance, and remarkable enhanced capacity, rate capability and energy density. However, too much CTAB can lead to deteriorated particle aggregation and properties. The sample prepared by using 2.5g CTAB shows the lowest electrochemical impedance（62.2?）, the biggest Li+ diffusion coefficient（2.143×10-15 cm2/s）. It presents a capacity of 143.5 mAh/g at 0.5C and 112.6 mAh/g at 5C, respectively. It is also noticeable that the sample exhibits an energy density of 499.3Wh/kg, 457.8Wh/kg and 349.7Wh/kg at 1C, 2C and 5C rate respectively. It shows almost same energy density but longer 4V voltage plateau while compared with LiMn0.5Fe0.5PO4/C prepared by the hydrothermal method followed by carbon-coating process.4. Finally, LiMn0.7Fe0.3PO4/C material was solvothermally synthesized in a mixed solvent of water and ethylene glycol（EG）, and the effects of the volume ratio of water and EG on the structure and properties of as-prepared materials were investigated. The results indicate that the sample prepared at 180 oC has typical olivine structure, but remains impure phase of Li3PO4 and Fe3（PO4）2. The fine crystal particles are agglomerated intensively, and it presents lower capacity（⁷0mAh/g）and obscure 3.5V voltage plateau mainly due to unsuccessful Fe-doping at 180 oC. In spite of this, this preparation method should be further studied in future with a view of the possibility of particle control and easy treatment of the waste produced during large scale production.