Biomass is rich, renewable, sustainable, and green resources, thereby excellent raw material for the fabrication of carbon materials. The diversity in structure and morphology of biomass are relevant in obtaining carbon materials with different structures and performances. The inherent ordered porous structure of biomass also benefits the activation process to yield porous carbons with ultrahigh specific surface area and pore volume. Besides, obtained biomass‐derived carbons (BDCs) are hard carbon with porous morphology, stable structure, superior hardness/strength, and good cycling performances when used in electrochemical capacitors (ECs). The inherent N, S, P, and O elements in biomass yield naturally self‐doped N, S, P, and O BDCs with unique intrinsic structures. In this paper, the synthesis approaches and applications of BDCs in ECs are reviewed. It shows that BDCs electrochemical performances are highly determined by their pore structures, specific surface areas, heteroatoms doping, graphitization degree, defects, and morphologies. The electrochemical performances of BDCs can further be improved by compositing with other materials, such as graphene, carbon nanofibers/nanotubes, transition metal oxides or hydroxides, and conducting polymers. The future challenges and outlooks of BDCs are also provided.
Atomically thin plasmonic MoO2 nanosheets were successfully obtained via a solid–solid reaction, which exhibits tremendous potential for the general fabrication of other two-dimensional nonlayered materials.
Graphene
sheets have a vast number of potential applications due
to their excellent properties. However, poor quality and harsh preparation
conditions restrict their application. Here, few-layer graphene (FLG)
sheet powder with high quality has been synthesized from waste expanded
polystyrene (EPS) at low temperature by dense Fe cluster catalysis.
The micron-sized FLG sheets comprising about three layers show high
crystallinity and good electrical conductivity that are comparable
to those of the shear-exfoliated graphene nanoplatelets. More than
70% carbon yield of FLG sheets from cheap EPS and their safe, controllable
synthesis conditions make it easy to expand production. The catalytic
formation mechanism of FLG sheets is studied.
Increasing the nickel content and broadening the voltage window are important means for LiNi x Co y Mn 1−x−y O 2 layered cathodes with low cost and high energy density, but these nickel-rich cathodes often suffer from structural instability and unsatisfactory cyclic performance. The systematic and detailed degradation mechanism especially under a high voltage is still unclear, which hinders the further development of nickel-rich cathodes. Our results show that due to the migration of high valence nickel ions to lithium sites, especially upon the deep removal of Li + ions, the nickel-rich cathode undergoes an irreversible phase transformation from a layered structure to a spinel or even rocksalt phase. Such irreversible phase transitions within a wide voltage window would cause insufficient lithium utilization and voltage decay, finally deteriorating the electrochemical performance of nickel-rich cathodes. In a narrow voltage range of 3.0−4.3 V, the capacity retention of the Ni-rich cathode is 93.4%, and the voltage fading is only 40 mV after 250 cycles. However, the cathode only exhibits a capacity retention of 77.4% with a significant voltage decay over 180 mV, as the voltage range further extends to 3.0−4.6 V. Furthermore, various characterizations and electrochemical performances demonstrate that the strengthened metal−oxygen bonds in the transition layer can produce stable structures and suppress phase transitions, thereby displaying superior electrochemical performance in the widened voltage window. As a result, the cycling retention of a Zr-doped cathode reaches 84.5%, and the voltage decay is only 50 mV after 250 cycles at 3.0−4.6 V, which exhibits excellent long-term cycle performance. These insights provide guidance for understanding the electrochemical mechanism and the design of high-voltage cathode materials.
Electrochemical capacitors are under
the spotlight due to their
high power density, but they have a low energy density. Redox electrolytes
have emerged as a promising approach to design high-energy electrochemical
energy storage devices. Herein, a chlorine-based redox electrochemical
capacitor is reported in an ionic liquid electrolyte. The commercial
activated carbon is employed as the working electrode to render the
reversible redox of chloride ions in an ionic liquid, by the restriction
of micropores on neutral chlorine. The carbon material can simultaneously
provide electrical double-layer capacitance. The effective integration
of a chlorine redox reaction and electrical double layer allows for
high-energy electrochemical capacitors. By this means, a rechargeable
chlorine-based redox electrochemical capacitor with reversible capacity
and good rate capability and cycling stability is obtained. This work
offers a solution for a new type of high-energy electrochemical capacitors.
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