Preparation And Modification Of LiFePO₄ By Hydrothermal Process As Cathode Materials For Lithium Ion Batteries

LiFePO₄ as cathode material was probed by Goodenough et al. in 1997. LiFePO₄ is reversible in storing up Li+. Its charge-discharge potential plateaus are 3.5V and 3.4V vs. Li/Li+, respectively. Its theoretical specific capacity is nearly 170 mAh g^-1 with good electrochemical reversibility. Afterwards, the phospho cathode materials, especially LiFePO₄, became the researching hotspot for its many merits, such as heat stability, safety, abundant reserves, low-cost and environmental friendship.LiFePO₄ has many merits as lithium ion battery cathode materials. However, there are also some intrinsic drawbacks, for example, 1. low electronic conductivity; 2. poor dynamic properties; 3. quick fading capacity.Various methods were used to synthesize LiFePO₄, such as solid state, hydrothermal, sol-gel and coprecipitation processes, etc. Each method has its merits and drawbacks. The hydrothermal synthesis method and improvement of LiFePO₄ are investigated in this paper. Hydrothermal process is prevailed for its many merits, such as reduced particle size, low energy consumption, facility to produce, etc. Pristine LiFePO₄ was synthesized at first, then, its electrochemical properties were tested. C-coating is a usual process to improve the property of LiFePO₄. Pristine LiFePO₄ was coated with carbon by adding sucrose into the reaction kettle as carbon resource. Super-cation doping is another process to improve the property of LiFePO₄, so Co(CH₃CO₂)2·4H₂O was added into the reaction materials when LiFePO₄ was synthesized by hydrothermal method to substitute part Fe²⁺ with Co²⁺ in the Fe position.After Co-doping, the particle size of LiFePO₄ is markedly reduced, as demonstrated in SEM images, which can be partly explained by the less radius of Co²⁺ ion (0.74 (?)) than Fe²⁺ ion (0.76 (?)). The particle size is reduced as Fe²⁺ ions were partly substituted by Co²⁺ ions in the crystal lattice of LiFePO₄. The initial discharge specific capacity of LiFePO₄ increased from 88 mAh g^-1 to 116 mAh g^-1 by galvanostatic cycling before annealing. After annealing, the initial specific capacity enhanced from 115 mAh g^-1 to 132 mAh g^-1. The polarization of Co-doped LiFePO₄ is greatly reduced as can be seen from the cyclic voltammetry (CV) plot, as indicates that the rate performance of LiFePO₄ is enhanced obviously.In order to investigate the effect of different annealing temperature to the electrochemical property of LiFePO₄, the hydrothermally synthesized samples of LiFePO₄ were annealed at various temperatures without C-coating before post annealing. The electrochemical property of LiFePO₄ after annealing is not the best without C-coating secondly, but it is convenient to investigate the relationship between the different anneling temperature and the electrochemical property of LiFePO₄, especially the particle sizes and the electrochemical property of LiFePO₄.To the Co-doped LiFePO₄, the best annealing temperature is 400 oC, the electrochemical property of LiFePO₄ is enhanced after annealing, nevertheless it will decend when the annealing temperature rised. The particles will agglomerate at higher temperature, which would block extraction-insertion of lithium ions, so the electrochemical property of LiFePO₄ becomes poor. To the pristine and C-coated LiFePO₄, the best annealing temperature is 500 oC. The different particle sizes correspond to different optimal annealing temperatures.The effects of different mixing sequences of the solutions by which LiFePO₄ were hydrothermally synthesized were compared. The solutions of H₃PO₄ and LiOH·H₂O were mixed to synthesize Li3PO₄ at first, and the solutions of FeSO₄·7H₂O, Co(CH₃CO₂)2·4H₂O and sucrose were added. This sequence is more appropriate to the hydrothermal synthetization of LiFePO₄ based on the investigation of electrochemical properties.Generally speaking, the various electrochemical properties of Co-doped LiFePO₄ are enhanced greatly. Further efforts are underway to determine the optimal preparation condition including the optimal carbon and Co contents, carbon and Co sources, synthesis and annealing conditions are underway. In order to break the bottle neck limitation of electric vehicle applying, the electrochemical properties of LiFePO₄ would have to be improved greatly.