The Synthesis And Modification Of LiFeBO₃/C As Cathode Material For Lithium-Ion Batteries

LiFeBO3 is one of the promising cathode materials for lithium ion batteries as it offers high voltage window, low cost, high theoretical capacity and environmental benignity. Reduce the particle size to nanometer range can shorten the diffusion distance of the lithium ions within the active materials and hence increase high rate charging performance. But with the increased specific surface of the materials, the risk of unwanted side reaction will increase as well. To contain this issue, various coating are used. In this thesis, we used citric acid method to prepare active materials, along with carbon coating. In order to enhance the electrochemical performance of LiFeBO3, a series of problems such as sensitive to ambient humidity as well as oxygen, low electronic conductivity, and poor performance in large current charging density are investigated. The main contents are summarized as followings:1. Synthesis of LiFeBO3/C Composite Materials.(1) The LiFeBO3/C composite materials were prepared by citric acid method. The pH values, ratio of citric acid n (n:the molar ratio of citric acid and other raw materials) were studied for the influence of the phase, morphology and electrochemical properties of the materials. LiFeBO3/C composite materials are achieved at 600℃ for 10h under the different molar ratio of citric acid. The results show that high performance LiFeBO3/C materials are achieved under the molar ratio of citric acid n=1. The particles of the materials have a uniform fine microstructure and medium carbon content (12.6wt.%). The galvanostatic charge-discharge test shows the particles can deliver a high discharge capacity of 161.7mAh·g-1 at the rate of 0.05C (1C=220mAh·g-1). The discharge capacity is 83.3mAh·g-1 under the rate of 1C.(2) The sintering temperature parameters were optimized to obtain finer performance LiFeBO3/C particles. Different morphology of LiFeBO3/C composite materials were prepared at 500℃～700℃ under the molar ratio of citric acid n=1. The results show that the products with a monoclinic structure can successfully achieved at 500℃～650℃. In 500℃～600℃ range, the rising sintering temperature can improve the crystal structure and electrochemical performance. At 650℃, the particle size will increase, causing the electrochemical properties degrade. When the temperature further increase to 700℃, Fe2+ can be partially reduced to Fe.(3) Based on the parameters explored in Step 1 and 2. The sintering time were optimized to obtain smaller and better performance LiFeBO3/C particles. Particles with different sintering time ranging from 2 hours to 14 hours were synthesized at 600℃ under the molar ratio of citric acid n=1. The results show that citrate may not be decomposed completely and the particles can’t form the best pyrolytic carbon layers under the short sintering time (2hours).When the time is greater than 6 hours, the pyrolytic carbon layers may be destroyed along with the crystal growth. The materials which were synthesized for 6 hours show the best performance. The initial discharge capacity of the best sample is 198.4mAh·g-1 under the rate of 0.05C. The discharge capacity of the best sample is 83.7mAh·g-1 under the rate of 1C.2. Modification of LiFeBO3/C Composite Materials.(1) Styrene-acrylonitrile (PSAN) copolymer was used as a carbon coating source to enhance the conductivity of LiFeBO3/C composite materials and strengthen its resistance to moist in the air. The results showed that the conductive carbon network formed from pyrolytic PSAN can increase the conductivity of the cathode materials. The coated materials can defend moist in the air to some extent. The 1st cycle discharge capacity of the sample which was exposed in the air for 24H is about 160mAh·g-1 under the rate of 0.05C. However, the as-prepared sample can only deliver 140mAh·g-1. Under high discharge rate of 1C, the coated materials can reach 80mAh·g-1, while the pristine ones can only deliver 60mAh·g-1.(2) Single Wall Carbon Nanotubes (SWNT) were used to improve the rate performance by forming the LiFeBO3/C/SWNT composite materials. The results demonstrated that the LiFeBO3/C/SWNT composite electrode showed excellent rate performance under the high rate. At the rate of 5C, LiFeBO3/C/SWNT sample discharge capacity maintains at 41.6mAh·g-1 than 25.5mAh·g-1 of LiFeBO3/C sample. At the 10C rate, LiFeBO3/C/SWNT sample discharge capacity maintains at 34.4mAh·g-1 but the LiFeBO3/C sample can just deliever 20.3mAh·g-1. After 500 cycles under the rate of IOC, the discharge capacity of LiFeBO3/C/SWNT composite can be still kept up to 90% of its initial capacitance.