Designs And Property Investigations On Interfacial Films Between Electrode/Electrolyte In Lithium Ion Battery

Due to its high energy density and long cycling life compared to other secondary batteries, lithium-ion battery has been successfully used in electronic devices and is regarded as a promising power source for electric vehicles. In order to meet the demands of advanced portable devices and achieve energy sustainability, considerable improvements and advancements of lithium ion battery（e. g., high performance at high voltage, elevated and low temperature, etc.） are still required. To meet these requirements, it is necessary to modify the interfacial properties of electrode/electrolyte. In this dissertation, two novel electrolyte additives were developed to improve the interfacial property improvements of several electrode/electrolyte systems, including graphite/propylene carbonate（PC） based electrolyte, LiMn₂O₄ and graphite at elevated temperature, LiNi_0.5Mn_1.5O₄, and LiCoO₂/graphite at high operation voltage, related mechanisms were understood. The obtained results are as follows:（1） To prevent the co-intercalation of PC, which has low melting point（about-49 oC）, 4-fluorophenyl acetate（FPA） was selected as an solid electrolyte interphase（SEI） formation additive for the graphite anode, based on density functional theory（DFT） calculations. Charge/discharge tests in graphite/Li and Li FePO₄/graphite cells show that the cells using 4-FPA exhibit better performance than that using phenyl acetate（PA） as the SEI formation additive. The results from scanning electron microscopy（SEM）, energy dispersive X-ray spectroscopy（EDS） and Fourier transform infrared spectrum（FTIR）, show that fluorine not only makes 4-FPA more reducible but also improves the stability of SEI on graphite, which inhibits the decomposition and co-intercalation of PC more effective than that of PA.（2） To improve the compatibility of PC and graphite anode, prop-1-ene-1, 3-sultone（PES） was selected as the anode SEI formation additive. The charge/discharge test in LiCoO₂/graphite cell shows that the cell with PES exhibits better performance than that with propane sultone（PS） as the SEI formation additive. The results from SEM, EDS, FTIR and X-ray photoelectron spectroscopy（XPS）, show that the C=C structure in PES increases the electro-withdrawing ability of the additive, resulting in PES has higher reduction activity and favors to build a more protective SEI than PS.（3） PES is applied as an electrolyte additive to improve the cycling stability of LiMn₂O₄/graphite cells at elevated temperatures, with a comparison of vinylene carbonate（VC）. Charge/discharge tests show that the cell using PES exhibits better performance than that using VC as the additive. The results from SEM, XPS, X-ray diffraction（XRD）, Thermogravimetric analysis（TGA） and DFT calculations, show that PES simultaneously forms protective interfacial films on the anode and cathode of the LiMn₂O₄/graphite battery. The cathodic interfacial film prevents the manganese dissolution from Li Mn₂O₄, while the SEI on the anode protects the graphite from the co-insertion of solvent, prevents the electrolyte decomposition and inhibits the deposition of the dissolved manganese from cathode.（4） To improve the cycling stability of LiNi_0.5Mn_1.5O₄ cathode, the cathode/electrolyte interface was modified by using PES as an electrolyte additive. Constant current charge/discharge test shows that the application of PES improves significantly the cycling stability of LiNi_0.5Mn_1.5O₄. The SEM, transmission electron microscopy（TEM）, XRD, and XPS analysis results show that PES decomposed preferentially to the electrolyte and the sulfur-containing compounds on the LiNi_0.5Mn_1.5O₄ cathode may poison the catalyst of the nickel manganese oxide. Thus suppressed the electrolyte decomposition at high voltage（>4.7 V vs. Li+/Li） and protects the cathode material, leading to the improved cycling performance of the LiNi_0.5Mn_1.5O₄/Li cell.（5） Vinyl ethylene carbonate（VEC） was used as an electrolyte additive to improve the cycling stability of Li CoO₂/graphite battery at high operation voltage（3.0-4.5 V）. Charge/discharge tests demonstrate that the battery with VEC exhibits significantly improved cycling stability. The capacity retention is increased from 38% to 87% after the using of VEC. The results from SEM, TEM, and XPS show that VEC simultaneously forms stable interfacial films on anode and cathode of the LiCoO₂/graphite battery, which protects the electrode particles from destruction, reduces the Co dissolution from cathode and deposition on anode, and suppresses the electrolyte decomposition on the electrodes.