Study On The Technology And Electrochemical Performance Of Lithium Iron Phosphate/Hard Carbon Lithium Ion Battery

Since lithium ion batteries have been successfully investigated and commercialized, they attract people's attention for their properties such as long cycling life, high voltage, security, no memory effort. However, the electrochemical performance of lithium ion battery is affinitive with its cathode and anode materials, conductive agent, binder, electrolyte, separator et al. Lithium iron phosphate (LiFePO4) has been considered as a promising lithium ion battery because of its rich raw materials, high capacity, stable structure, safety et al. As well, hard carbon (HC) with an inordinance structure which can be used for an anode material of lithium ion battery has been attracted people's interest for its high capacity, excellent cycling performance and low cost et al. In this thesis, we have developed a lithium ion battery-LiFePO4/HC using LiFePO4 as cathode and hard carbon as anode to study the conductive agent and binder influence of the electrochemical properties of LiFePO4/Li half cell and LiFePO4/HC full cell. In addition, we compared the electrochemical performance of LiFePO4/HC battery and LiFePO4/graphite (AGP-3). Through the experiments, we got the following conclusions:1. For LiFePO4/Li half cell, using Super P Li as conductive agent, the resistance of battery was smaller. At 0.2 C or 1 C rate, the discharge voltage plateau of the cell using Super P Li as the conductive agent was more stable than that of using acetylene black. After 150 cycles at 1 C rate, the capacity retention of the cell using Super P Li as conductive agent was higher. Cyclic voltammetry indicated that the LiFePO4 material has a good cyclic reversibility, which may be caused by the good conductivity results from the carbon fibers among LiFePO4 particles.2. For LiFePO4/HC full cell, we also got the conclusion that using Super P Li as conductive agent, the resistance of the cell was smaller and the capacity of it was higher. Rate performance test has shown that the cell using Super P Li as conductive had better rate performance, however, the discharge capacity of the cell was small at 10 C rate neither using Super P Li or acetylene black as conductive agent, which maybe due to the the unsatisfactory bond performance of the PVDF binder. Comparing with the LiFePO4/Li half cell, the resistance of the full cell was smaller and the capacity retention was higher after 150 cycles at 1 C rate.3. We have used a water binder (SBR) and an oiliness binder (PVDF) to make LiFePO4 cathode electrode, and assembled with lithium metal composing to LiFePO4Li lithium ion battery. Both of the water-based binder system and the oil-based binder system, the discharge voltage plateau (about 3.38 V) of the cell were stable and the discharge capacity were almostly the same at 0.2 C discharge rate, however, the water-based binder system was a little lower. While at the discharge rate of 1 C, in the water-based binder system, the first and second discharge capacity of the cell was higher than that of the oil-based binder system. From the results of the EIS and cycle life tests demonstrated that the cell with water-based binder system had a smaller resistance with Rct equates to 89.68Ωand had better capacity retention which was 65% after 150 cycles at 1 C discharge rate.4. We had used two binders to assemble LiFePO4/HC full batteries, the initial discharge capacity of the cell with oil-based binder system was higher than the water-based binder system in the charge-discharge process at 0.2 C rate. However, as cycles proceed, the discharge capacity of the cell with oil-based binder system was decreased, while, the discharge capacity of the cell with water-based binder system had a little increased. As the same as the results of LiFePO4/Li half cell tests, the discharge capacity of the cell with water-based binder system was higher than the cell with oil-based binder system, and its capacity retention was higher which was 97.9%. Rate performance test indicated that the cell with water-based binder system had a better rate performance. In addition, the water-based binder system had smaller resistance whose Rct was 5.08Ω, however, whatever the binder we used, the resistance of the LiFePO4/HC full cell was smaller than the LiFePO4/Li half cell.5 The rate performance of the cell using hard carbon as anode was better. When the cells charge-discharge cycling at 1 C rate, the initial discharge capacity of the LiFePO4/AGP-3 and LiFePO4/HC was 84.3% and 91.0% of the discharge capacity at 0.2 C rate. The discharge capacity of LiFePO4/AGP-3 cell was a little higher than LiFePO4/HC cell at 1 C rate or 2 C rate, however, on the contrary, the discharge capacity of LiFePO4/HC was higher when charge-discharged at 5 C or 10 C rate. 6. The resistance was almost the same when using hard carbon or graphite as anode, and the resistance of LiFePO4/HC was a little lower. The cycle life of LiFePO4/HC cell was longer than that of LiFePO4/HC cell, besides, the cycle life of the LiFePO4/HC full cell was longer than the LiFePO4/Li and HC/Li half cell, with its discharge capacity retention of 60% after 2450 cycles at 10 C rate.