Yuzi Zhang

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.

All-Aqueous Directed Assembly Strategy for Forming High-Capacity, Stable Silicon/Carbon Anodes for Lithium-Ion Batteries

Silicon (Si) particles have emerged as a promising active material for next-generation lithium-ion battery anodes. However, the large volume changes during lithiation/delithiation cycles result in fracture and pulverization of Si, leading to rapid fading of performance. Here, we report a simple, all-aqueous, directed assembly-based strategy to fabricate Si-based anodes that show capacity and capacity retention that are comparable or better than other more complex methods for forming anodes. We use a cationic surfactant, cetyltrimethylammonium bromide (CTAB), to stabilize Si nanoparticles (SiNPs) in water. This suspension is added to an aqueous suspension of para-amino benzoic acid-terminated carbon black (CB), pH 7. Charge interactions cause the well-dispersed SiNP to bind to the CB, allowing most of the SiNP to be available for lithiation and charge transfer. The CB forms a conducting network when the suspension pH is lowered. The dried SiNP/CTAB/CB anode exhibits a capacity of 1580 mAh g(-1) and efficiency of 97.3% after 50 cycles at a rate of 0.1C, and stable performance at cycling rates up to 5C. The directed spatial organization of the SiNP and CB using straightforward colloidal principles allows good contact between the well-dispersed active material and the electrically conducting network. The pore space in the CB network accommodates volume changes in the SiNPs. When CTAB is not used, the SiNPs form aggregates in the suspension, and do not contact the CB effectively. Therefore, the electrochemical performance of the SiNP/CB anode is inferior to that of the SiNP/CTAB/CB anode. This aqueous-based, room temperature, directed assembly technique is a new, but simple, low-cost scalable method to fabricate stable Si-based anodes for lithium-ion batteries with performance characteristics that match those made by other more sophisticated techniques.

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.