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Electrochemical and Microstructural Analysis of Aging Mechanism of 18650 LiFePO4/Graphite Li-ion Batteries under Different C-Rate and Temperature Conditions

Sharifi, Hossein | 2017

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 50952 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Asgari, Sirous
  7. Abstract:
  8. In this study, the aging of the LiFePO4/graphite cell was investigated in two different types of 18650 Li-ion batteries during cycling at various C-rates (0.5, 1, 2, 3, 4C) and high temperature under long-term cycling. An amount of 20% Capacity loss was considered as the end of the cycling. Batteries with a capacity of 1500 mAh after this capacity drop, experience 60, 120, 1502, and 2155 cycles, at the rates of 4, 3, 2, and 1C , and batteries with capacity of 1400 mAh was also 60, 360, 1100, 1000, and 805 cycles at a rate of 0.5C. Capacity decrease of the cell is in linear relationship with cycle number and the slope of the capacity-fading line increases with elevating current rate. Aging mechanisms were investigated using a combination of electrochemical and microstructural analysis. A half-cell study was performed after full cell disassembly by using both LiFePO4 and graphite electrodes. The LiFePO4 half-cell study showed that the amount of capacity loss in the first charge cycle compared to the fresh electrode for cycled samples in 1, 2, 3, and 4 C was 25.03, 27.61, 29.76 and 33.51%, which is directly related to the loss of active lithium ions in full cell. Also, the results show that the initial capacity of the LiFePO4 half-cell is largely recovered, and the discharge capacity loss, which is related to irreversible capacity, is 1.18, 1.56, 7.32, 11.3%. But the capacity loss was higher in graphite as well as higher rates, indicating a higher capacity fading in this electrode. At low C-rates, loss of the active lithium source from the system is proposed as a primary source of capacity fading for the full cells, and at high temperatures and high C-rates, this factor deteriorates by the instability of the electrode/electrolyte interface and unstable SEI layer. These are expected to form on the newly exposed graphite anode surface, which cause sustainable consumption of active lithium and further leads to the performance degradation of active materials. Significant increase of resistance interface between these electrodes and electrolytes in EIS tests is also due to this. The increase in the elements of the SEI compounds by EDS and the results of Raman spectroscopy also confirms this phenomenon. The electrochemical impedance spectroscopy of the half-cell represents an increase in the resistance of the positive electrode which is the main reason for power fading in high-rate discharge cycling
  9. Keywords:
  10. Graphite Electrode ; Aging ; Flow Rate ; Solid Electrolyte ; Lithium Ferrite Phosphate Cathode ; High-Rate Discharge ; Active Lithium Loss

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