Jun Liu

Symmetric cell approach and impedance spectroscopy of high power lithium-ion batteries

High power lithium-ion cells are a very promising energy source for practical hybrid vehicles. It is found that the impedance of the 18650 high-power cells using LiNi{sub 0.8}Co{sub 0.2}O{sub 2} chemistry increases with time during the beginning period of storage. A symmetric cell approach is developed to distinguish the anode and cathode effects on the impedance rise. Cathode impedance, especially charge-transfer resistance, is identified as the main component of the cell impedance and is most responsible for the rise of the cell impedance during storage at room temperature. With analysis of impedance spectra from a variety of cells, the charge-transfer process is thought to take place at the interface between the electrolyte solution and the surface of surface layers on the electrode. We also propose that the surface layers might be mixed conductors of electrons and lithium ions, instead of pure lithium-ion conductors. The nature of the surface layers on the cathode is likely different from that of the surface layers on the anode.

Factors responsible for impedance rise in high power lithium ion batteries

Abstract High-power, 18,650 lithium-ion cells have been designed and fabricated in order to understand the factors limiting the calendar life of the lithium-ion system. Each cell consisted of a LiNi0.8Co0.2O2 positive electrode, a blend of MCMB-6 and SFG-6 carbon negative electrode, and a LiPF6 in EC:DEC (1:1) electrolyte. These cells, which initially meet the power requirement set by the partnership for a new generation of vehicles (PNGV), were subjected to accelerated calendar life and cycle life testing. After testing at elevated temperatures, the cells experienced a significant impedance rise and loss of power. The fade rate of power in these cells was dependent of the state of charge and the temperature of testing. Micro-reference electrode and ac-impedance studies on symmetrical cells have confirmed that the interfacial resistance at the positive electrode was the main reason behind the impedance rise in the high power cell.

Mechanism of capacity fade of MCMB/Li1.1[Ni1/3Mn1/3Co1/3]0.9O2cell at elevated temperature and additives to improve its cycle life

The performance degradation of graphite/Li1.1[Ni1/3Mn1/3Co1/3]0.9O2lithium-ion cells at elevated temperature was investigated. The electrochemical data suggest that the migration of dissolved transition metals from the cathode to the anode is the key contributor to the performance degradation. With the help of density function theory calculations, lithium difluoro[oxalato] borate was tested to be an effective electrolyte additive to mitigate the performance degradation of lithium-ion cells. The application of this novel electrolyte additive was found to significantly improve both the life and safety characteristics of graphite/Li1.1[Ni1/3Mn1/3Co1/3]0.9O2lithium-ion cells.

Lithium Tetrafluoro Oxalato Phosphate as Electrolyte Additive for Lithium-Ion Cells

Lithium tetrafluoro oxalato phosphate LTFOP was investigated as an electrolyte additive to improve the life of mesocarbon microbead MCMB/Li1.1Ni1/3Co1/3Mn1/30.9O2 NCM cells for high power applications. With the addition of 1‐3 wt % LTFOP to MCMB/NCM cells, the capacity retention after 200 cycles at 55°C significantly improved. Electrochemical impedance spectroscopy showed that the LTFOP addition in the electrolyte increased the initial impedance but lowered the impedance growth rate during cycling. Aging tests at 55°C indicated that the capacity retention of the negative electrode significantly benefited as a result of the LTFOP addition. Differential scanning calorimetry showed that the safety of the lithiated MCMB is significantly improved with the LTFOP addition.