Researches On Modification And Electrochemical Performances Of Hard Carbon As The Anode Materials For Lithium-ion Batteries

Hard carbon possesses the advantages of high specific capacity, excellent rate performance, stable structure and good compatibility of electrolyte, which make it a promising anode material for lithium ion batteries. However, its high potential platform, low initial efficiency restrict its application in lithium ion batteries industry. In this paper, hard carbon has been modified, doping boric acid, activator (polyethylene glycol, KOH) generating pore and coating with polyvinylidene fluoride. Its aims at finding a way which not only benefit to the mass production, but also ensure it possesses excellent electrochemical performances. The main results were as follows:1. The modified hard carbon anode materials are prepared by low temperature pyrolyse, in which phenolic epoxy resin is used as precursor and boric acid is used as dopant. The results show that boric acid increases the order degree of the structure of hard carbon, decreases the interlayer space, minize the interphase film resistance of solid electrolyte, improves power performance, while the cycle performance drops. When the mass ratio of boric acid to hard carbon is 10%, the reversible specific capacity reaches the maximum value of 461.1 mAh/g, and the interphase film resistance of solid electrolyte comes only 24.53?.2. The activator is used to prepared modify hard carbon materials with pore. Studies shows that phenolic epoxy resin and thermally unstable polymer polyethylene glycol is incompatible and The micro phase separation structure of their blending is the main reason for the formation of pore structure. The pore size of hard carbon concentrate between 2 and 10 nm and the porosity of up to 80%. When ratio of polyethylene glycol to resin comes to 0.5, the reversible capacity reaches the maximum 432.6 mAh/g. With the increasing of polyethylene glycol, the initial coulombic efficiency and ratio performances at 2 C and 5 C of hard carbon are improved, while the capacity of 1C and cyclic performance becomes poor. The main pore size is less than 2 nm when KOH is used as activator, when the dosage of activator KOH increases, the total pore volume and specific surface area of hard carbon increase. Its initial charge-discharge specific capacities increase, and the cycle performances is enhanced, At the same time, the average pore diameter, yield of hard carbon and tap density decrease.3. Hard carbon is coating with carbon source. With the increasing of PVDF content, the rate performance and cycle performance of hard carbon improve, and the irreversible capacity also increases. When ratio of the PVDF to hard carbon reaches 1, the reversible specific capacity and cyclic rentential of hard carbon come to the highest pyrolysed at 850 ?,438.7 mAh/g and 90.83%, respectively. The coating of PVDF on hard carbon also reduce the impedance of charge transfer and SEI film.