dc.description.abstract | In recent years, the high energy density of Li-ion battery makes it become a popular battery type. Among various cathode materials, spinel LiNi0.5Mn1.5O4 (LNMO) has been investigated because of its high operating voltage (4.7 V vs. Li+/Li). However, LNMO is limited by the decomposition of commercial electrolyte at high voltage (> 4.5 V). Commercial electrolytes commonly use lithium hexafluorophosphate (LiPF6) dissolved in organic carbonates. However, decomposition of LiPF6 produces highly reactive PF5, which is thermally unstable and sensitive to moisture because of the unstable P-F bond. The instability of PF5 results in side reactions with water and organic carbonates, and eventually lead to capacity decay.
To improve the poor electronic conductivity of active material and binder, conductive additives are added into electrode. Carbon black (CB) is one of the common conductive additives considered to be inactive component. However, various functional groups were found on CB surface. Oxygen-containing functional groups could increase the absorption of water and lead to electrolyte decomposition. Nitrogen-doped carbon materials have been applied on anode materials of Li-ion batteries to improve Li-ion storage capacity, ion diffusion and electrical conductivity.
To study the influences of oxygen-functional groups and nitrogen-functional groups of CB on electrochemical performance of high-voltage cathode. In this work, oxygen-functional groups on CB were modified by thermal treatments and nitric acid treatment. In addition, hydrothermal treatment was applied on CB and oxidized CB for N-doping. Surface functional groups, elemental composition, specific surface area and defects degree of CB were analyzed by XPS, gas sorption analyzer, raman spectroscopy. These CB were used as conductive additives in high voltage cathode and assembled into LNMO/Li half-cell for electrochemical testing.
Among all of the samples, N-doped CB shows high discharge capacity (119.2 mAh g-1), excellent rate capability (Discharge capacity at 5 C retains 69.4% of capacity at 0.2 C) and high cycling stability (94.4% capacity retention after 100 cycles at 1 C). In the results of SEM and TEM, this sample shows less deposition on electrode. After electrochemical aging, the electrolyte decomposition products were analyzed by LC-MS/MS. The results suggested NCB with nitrogen functional groups would reduce amount of decomposition products and the consumption of main compositions of electrolyte. According to this study, N-doped CB can effectively improve discharge capacity and rate performance, even cycling stability of high-voltage cathode. | en_US |