Abstract
In the last two decades, Li-ion batteries (LIBs) has played a critically important role in the market as a primary power supplier with a higher gravimetric and volumetric capacity than other rechargeable-battery systems. However, current LIB technologies cannot satisfy the energy and power requirements of a wide range of applications, from portable electronic devices to all-electric vehicles and smart grids. In this regard, Asia Carbons and Technology INC company which locates at Taoyuan country, Taiwan (R.O.C) and Prof. Hsing-Yu Tuan Labortary have cooperated to dedicate to the progressive LIBs. Hence, the study was a cooperative work between Prof. Hsing-Yu Tuan Lab and the company. The aim was to find out the optimal conditions for assembling modified-graphite based on LIBs with high capacity, high coulombic efficiency, cycling stable. These modified samples were originally graphite materials, which has been treated under the high pressure to transform their morphologies. The sample treatment process was carried out by the company. The number of samples were 32 samples, namely G201, G202, G203 to G232 in order. In order to achieve the purpose, those modified graphite samples were initially assembled anode electrode in half-cells under the same process (described lately in chapter 2) to find out the good samples in terms of high capacity, good coulombic efficiency and cycling stable. All the produced slurries included 83,50 wt% active materials, 8.0 wt% super P and 8.5 wt% PVDF stirred in NMP solution for 1.5 hours. Then, their cycling performances were carefully compared to each other, based on the predefined goals (KPI), which resulted in obtaining four best samples. These samples were investigated further under different conditions such as (i) various coating thickness and (ii) different kinds of electrolytes to find out the best sample as well as to gain the recipe for assembling modified-graphite based on lithium-ion batteries. Besides that, those samples were analyzed via SEM images, IV test to obtain more understanding about their characteristics. It is found that the modified sample named G227 stood out as a potential candidate for anode electrode in lithium battery, since it performed stably over 50 cycles with high capacity above 320 mA h/g. Another finding was that electrolyte system which includes ethylene carbonate (EC): dimethyl carbonate (DMC): fluoroethylene carbonate (FEC) 4.5:4.5:1 (v:v) in LPF6 has enhanced the good cycling performance rather than fluoroethylene carbonate (FEC): diethyl carbonate (DEC) 3:7 (v.v) and ethylene carbonate (EC): dimethyl carbonate (DMC) 1:1 (v/v) in LPF6 system. However, this work currently facing an issue that the average capacity is quite low, which needs a lot effort to solve. In order to surpass the weakness of low capacity of modified graphite based on negative electrodes, the author continuously carried out the work named “silicon/graphite composites as anode materials” to overcome the problem. The study indicated that silicon/graphite composite has a significant improvement on specific capacity and the initial efficiency. The electrodes sustain a higher number of charge/discharge cycles with a more stable discharge capacity compared to the graphite powders only. Two full-cell coins show outstanding stability for the first 50 cycles, leading to a capacity retention of 1st sample and 2nd one are 94.68% and 92.56%, respectively. The average initial efficiency is over 86.00%. This result has exceeded the standard KPI (>85%). Besides that, the cycle performance of Si/graphite composites based-on pouch cell is also investigated. Its areal capacity and specific capacity are 3.92 mA h/cm2 and 604 mA h/g, respectively, which is much better than expected.