Yingnan Dong

Development of novel lithium borate additives for designed surface modification of high voltage LiNi0.5Mn1.5O4 cathodes

A novel series of lithium alkyl trimethyl borates and lithium aryl trimethyl borates have been prepared and investigated as cathode film forming additives. The borates are prepared via the reaction of lithium alkoxides or lithium phenoxides with trimethyl borate. Incorporation of 0.5–2.0% (wt) of the lithium borates to a baseline electrolyte (1.0 M LiPF6 in 3 : 7 (EC/EMC)) results in improved capacity retention and efficiency of high voltage graphite/LiNi0.5Mn1.5O4 cells especially upon cycling at elevated temperature (55 °C). The improved performance results from the sacrificial oxidation of the lithium borate on the cathode surface to generate a cathode passivation film. The lithium borates can be readily structurally modified to act as a functional group delivery agent to modify the cathode surface. Ex situ surface analysis of the electrodes after cycling confirms that the lithium borates modify the cathode surface and generate a borate rich surface film which inhibits electrolyte oxidation and Mn dissolution.

Effect of Lithium Borate Additives on Cathode Film Formation in LiNi0.5Mn1.5O4/Li Cells

A direct comparison of the cathode-electrolyte interface (CEI) generated on high-voltage LiNi0.5Mn1.5O4 cathodes with three different lithium borate electrolyte additives, lithium bis(oxalato)borate (LiBOB), lithium 4-pyridyl trimethyl borate (LPTB), and lithium catechol dimethyl borate (LiCDMB), has been conducted. The lithium borate electrolyte additives have been previously reported to improve the capacity retention and efficiency of graphite/LiNi0.5Mn1.5O4 cells due to the formation of passivating CEI. Linear sweep voltammetry (LSV) suggests that incorporation of the lithium borates into 1.2 M LiPF6 in EC/EMC (3/7) electrolyte results in borate oxidation on the cathode surface at high potential. The reaction of the borates on the cathode surface leads to an increase in impedance as determined by electrochemical impedance spectroscopy (EIS), consistent with the formation of a cathode surface film. Ex-situ surface analysis of the electrode via a combination of SEM, TEM, IR-ATR, XPS, and high energy XPS (HAXPES) suggests that oxidation of all borate additives results in deposition of a passivation layer on the surface of LiNi0.5Mn1.5O4 which inhibits transition metal ion dissolution from the cathode. The passivation layer thickness increases as a function of additive structure LiCDMB > LPTB > LiBOB. The results suggest that the CEI thickness can be controlled by the structure and reactivity of the electrolyte additive.

Influence of the Oil on the Structure and Electrochemical Performance of Emulsion-Templated Tin/Carbon Anodes for Lithium Ion Batteries

Tin (Sn) is a useful anode material for lithium ion batteries (LIBs) because of its high theoretical capacity. We fabricated oil-in-water emulsion-templated tin nanoparticle/carbon black (SnNP/CB) anodes with octane, hexadecane, 1-chlorohexadecane, and 1-bromohexadecane as the oil phases. Emulsion creaming, the oil vapor pressure, and the emulsion droplet size distribution all affect drying and thus the morphology of the dried emulsion. This morphology has a direct impact on the electrochemical performance of the anode. SnNP/CB anodes prepared with hexadecane showed very few cracks and had the highest capacities and capacity retention. The combination of low vapor pressure, creaming, which forced the emulsion droplets into a close-packed arrangement on the surface of the continuous water phase, and the small droplets allowed for gentle evaporation of the liquids during drying. This led to lower differential stresses on the sample and reduced cracking. For octane, the vapor pressure was high, the droplet sizes were large for 1-cholorohexadecane, and there was no creaming for 1-bromohexadecane. All of these factors contributed to cracking of the anode surface during drying and reduced the electrochemical performance. Choosing an oil with balanced properties is important for obtaining the best cell performance for emulsion-templated anodes for LIBs.