Browsing by Author "Umesh, Bharath"

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Composition Modulation of Ionic Liquid Hybrid Electrolyte for 5 V Lithium-Ion Batteries
(2019-11-13)
Wu, Chia-Jung
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Rath, Puma Chandra
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Patra, Jagabandhu
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Bresser, Dominic
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Passerini, Stefano
Electrolyte is a key component in high-voltage lithium-ion batteries (LIBs). Bis(trifluoromethanesulfonyl)imide-based ionic liquid (IL)/organic carbonate hybrid electrolytes have been a research focus owing to their excellent balance of safety and ionic conductivity. Nevertheless, corrosion of Al current collectors at high potentials usually happens for this kind of electrolyte. In this study, this long-standing problem is solved via the modulation of the IL/carbonate ratio and LiPF6 concentration in the hybrid electrolyte. The proposed electrolyte suppresses Al dissolution and electrolyte oxidation at 5 V (vs Li+/Li) and thus allows for ideal lithiation/delithiation performance of a high-voltage LiNi0.5Mn1.5O4 (LNMO) cathode even at 55 degrees C. The underlying mechanism is examined in this work. Excellent cycling stability (97% capacity retention) for an LNMO cathode after 300 cycles is achieved. This electrolyte shows good wettability toward a polyethylene separator and low flammability. In addition, satisfactory compatibility with both graphite and Si-based anodes is confirmed. The proposed electrolyte design strategies have great potential for applications in high-voltage LIBs.
Facile synthesis of core-shell structured Si@graphene balls as a high-performance anode for lithium-ion batteries
(2020-05-07)
Jamaluddin, Anif
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Umesh, Bharath
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Chen, Fuming
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Chang, Jeng-Kuei
;
Su, Ching-Yuan
Encapsulating silicon (Si) nanoparticles with graphene nanosheets in a microspherical structure is proposed to increase electrical conductivity and solve stability issues when using Si as an anode material in lithium-ion batteries (LIBs). Currently the main strategies to produce high-quality Si-graphene (Si@Gra) electrodes are (1) chemical vapor deposition (CVD) of graphene grown in situ on Si by hydrocarbon precursors and (2) encapsulating Si with a graphene oxide followed by postannealing. However, both methods require a high-temperature and are costly and time-consuming procedures, which hinders their mass scalability and practical utilization. Herein, we report a Si@Gra composite with a ball-like structure that is assembled by a facile spray drying process without a postannealing treatment. The graphene sheets are synthesized by an electrochemical exfoliation method from natural graphite. The resulting Si@Gra composite exhibits a unique core-shell structure, from which the ball-like morphology and the number of graphene layers in the Si@Gra composites are found to affect both the electric conductivity and ionic conductivity. The Si@Gra composites are found to increase the capacity of the anode and provide excellent cycling stability, which is attributed to the high electrical conductivity and mechanical flexibility of the layered graphene; additionally, a void space in the core-shelled ball structure inside the Si@Gra compensates for the Si volume expansion. As a result, the Si@few-layer graphene ball anode exhibits a high initial discharge capacity of 2882.3 mA h g(-1) and a high initial coulombic efficiency of 86.9% at 0.2 A g(-1). The combination of few-layer graphene sheets and the spray drying process can effectively be applied for large-scale production of core-shell structured Si@Gra composites as promising anode materials for use in high-performance LIBs.
Prior vacuuming for supercritical fluid synthesis of SnO2/graphene nanocomposites with superior electrochemical Li+ storage performance
(2018-12-01)
Xie, Jian-De
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Li, Hui-Ying
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Umesh, Bharath
;
Lee, Tai-Chou
;
Chang, Jeng-Kuei
Supercritical CO2 (SCCO2) fluid, exhibiting gas-like diffusivity, extremely low viscosity, and near-zero surface tension, is used to synthesize uniformly dispersed and tightly anchored SO2 nanoparticles (a 3-nm diameter was achieved) on graphene nanosheets (GNSs). Usually, the conventional synthesis processes (in the absence of SCCO2) results in aggregated SnO2 clusters; whereas the technique described in this work eliminates this limitation. This study reveals the significance of two crucial factors (the SCCO2 pressure (i.e., fluid density) and the degassing step (i.e., vacuuming stage) in autoclave before injecting CO2) on the uniform distribution of the synthesized SnO2 nanoparticles on GNSs. Increasing the pressure leads to an increase in SCCO2 density (and viscosity), suppressing the transport of SnO2 precursors throughout the sample. On the other hand, vacuuming the autoclave before injecting CO2 improves the uniformity of SnO2 particle distributions. To assess the electrochemical performance of the synthesized nanoparticles, the specific capacity, rate capability, and cyclic stability were determined for various samples. A capacity of similar to 787 mAh g(-1) at 100 mA g(-1) was achieved for an optimal configuration of the SnO2/GNS electrodes. The capacity retention was 60% when the charge-discharge rate increased to 6000 mA g(-1). (C) 2018 Elsevier Ltd. All rights reserved.