Study On Cost-effectivesynthetic Process Andperformances Of Lithium Ironphosphate

Lithium iron phosphate?LiFePO₄?battery,which possesses the characteristics of good safety and long cycle life,has been extensively adopted for increasingly larger scale applications in electric buses,stationary electric power storage and other fields.Whereas,there is a need for a manufacturing process that produces electrochemically active LiFePO₄/C at low cost.Current preparation technology of LiFePO₄/C is mainly based on iron phosphate?FePO₄?as precursor,whose cost and performance play a decisive role in those of LiFePO₄/C.Therefore,the present work focuses on the following three aspects:?a?Dehydration process of iron phosphate dihydrate?FePO₄·2H₂O?,aimed at obtaining the most economical dehydration temperature and optimal performance parameters;?b?Purification technique of industrial grade copperas,aimed at not only reducing the cost of raw-material FeSO₄·7H₂O,but also improving the performance of LiFePO₄/C;?c?Doping a high amount of titanium dioxide?TiO₂?to LiFePO₄/C,in order to improve the electrochemical properties.The samples were characterized by chemical analysis,XRD,SEM,TG-DTA and charge-discharge test in terms of element content,crystalline structure,morphology,thermogravimetric change and eletrochemical performances.?1?During the dehydration process of FePO₄·2H₂O,the crystalline structure,particle size and surface state of FePO₄ sample vary greatly with the dehydration temperature.Although the TG-DTA curves show that the crystal water of FePO₄·2H₂O can be completely removed at the temperature of 202.4°C and there is neither weight loss nor thermal behavior between 202.4⁶71.8°C,the XRD results reveal that there exists the phase transition at this temperature range.Monoclinic FePO₄·2H₂O belonging to the space group P2₁/n is transformed into hexagonal FePO₄ belonging to the space group P6₃mc at250⁴50°C and hexagonal FePO₄ belonging to the space group P3₁21 at 550⁶50°C,respectively.Partial glass phase appears in the hexagonal FePO₄ sample if the temperature exceeds 671.8°C.Additionally,the particle size of the dehydrated FePO₄ increases gradually with dehydration temperature increasing.The LiFePO₄/C sample prepared by FePO₄ dehydrated at 550°C shows the optimal electrochemical performance,whose 0.2 C specific discharge capacity is 157.4 and 116.5 mAh·g^-11 at 25 and-20°C,respectively,and compaction density is 2.45 g·cm^-3.?2?The purified copperas solution is an alternative of high grade FeSO₄·7H₂O for FePO₄preparation.The chemical analysis results confirm that the purified copperas solution with titanium content less than 10 ppm?wt.,vs.FeSO₄?can be obtained by using Fe powder and Na₃PO₄ as clarifying and sedimentation agent.The specific capacity of LiFePO₄/C sample synthesized with the purified solution is 162.5 mAh·g^-11 at 0.2 C,while that of LiFePO₄/C prepared by commercial FeSO₄·7H₂O?A.R.?with titanium content of 49.1 ppm?wt.,vs.FeSO₄?is only 157.4 mAh·g^-1.If the purified solution is applied in the production of LiFePO₄/C instead of FeSO₄·7H₂O?A.R.?,the cost of raw material FeSO₄·7H₂O will be reduced by 78.3%.?3?In order to study the effect of a high doping amount of TiO₂ on the properties of LiFePO₄/C,8mol%TiO₂ was added to the mixture of FePO₄,Li₂CO₃ and glucose to synthesize LiFePO₄/C.The doped LiFePO₄/C sample shows smaller particle size,higher specific capacity and better C-rate capability than the undoped one,nevertheless,the low-temperature performance is deteriorated.