A Bifunctional-Modulated Conformal Li/Mn-Rich Layered Cathode for Fast-Charging, High Volumetric Density and Durable Li-Ion Full Cells.

Lithium- and manganese-rich (LMR) layered cathode materials hold the great promise in designing the next-generation high energy density lithium ion batteries. However, due to the severe surface phase transformation and structure collapse, stabilizing LMR to suppress capacity fade has been a critical challenge. Here, a bifunctional strategy that integrates the advantages of surface modification and structural design is proposed to address the above issues. A model compound Li1.2Mn0.54Ni0.13Co0.13O2 (MNC) with semi-hollow microsphere structure is synthesized, of which the surface is modified by surface-treated layer and graphene/carbon nanotube dual layers. The unique structure design enabled high tap density (2.1 g cm-3) and bidirectional ion diffusion pathways. The dual surface coatings covalent bonded with MNC via C-O-M linkage greatly improves charge transfer efficiency and mitigates electrode degradation. Owing to the synergistic effect, the obtained MNC cathode is highly conformal with durable structure integrity, exhibiting high volumetric energy density (2234 Wh L-1) and predominant capacitive behavior. The assembled full cell, with nanographite as the anode, reveals an energy density of 526.5 Wh kg-1, good rate performance (70.3% retention at 20 C) and long cycle life (1000 cycles). The strategy presented in this work may shed light on designing other high-performance energy devices.

ZnFe2O4@Carbon Core-Shell Nanoparticles Encapsulated in Reduced Graphene Oxide for High-Performance Li-Ion Hybrid Supercapacitors.

Li-ion hybrid supercapacitors (Li-HSCs) are attracting extensive attention because of their high energy/power densities. However, the performance of most Li-HSCs suffers from the limitation of the sluggish kinetics of battery-type anodes. Herein, we demonstrate that with dual protection of carbon and graphene, a three-dimensional, strongly coupled ZnFe2O4@C/reduced graphene oxide (RGO) composite anode provides an effective solution to this issue. The covalent C-O-M linkage between ZnFe2O4 nanoparticles and C/RGO promotes charge transfer and enhances structural stability. Two kinds of carbon-based buffering layers are able to well accommodate the volume change during charging/discharging, endowing the composite anode with high rate performance (692 mA h g-1 at 5 A g-1) and outstanding cycle life (98.3% of capacity retention after 700 cycles at 1 A g-1). The resulting ZnFe2O4@C/RGO//activated carbon Li-HSC shows an ultrahigh energy density of 174 W h kg-1, excellent power density of 51.4 kW kg-1 (at 109 W h kg-1), and superior cycle life (80.5% of retention capacity after 10 000 cycles at 5 A g-1).

Sub-nanometer, Ultrafine α-Fe2 O3 Sheets Realized by Controlled Crystallization Kinetics for Stable, High-Performance Energy Storage.

The development of energy devices based on iron oxides/hydroxides is largely hindered by their poor conductivity and large volume changes, especially with regard to specific capacitance and cycle stability. Herein, superior capacitance (1575 F g-1 at 1.25 A g-1 ) and high rate performance (955 F g-1 at 25 A g-1 ) were realized by synthesizing sub-nanometer, ultrafine α-Fe2 O3 sheets loaded on graphene (SU-Fe2 O3 -rGO). An assembled asymmetric supercapacitor showed outstanding cycle stability (106 % retention after 30 000 cycles). This excellent performance arises from the unique structural characteristics of the α-Fe2 O3 sheets, which not only enrich electrochemically reactive sites, but also largely eliminate the volume changes after long-term charge/discharge cycling. The synthesis of SU-Fe2 O3 -rGO critically depends on control of the crystallization kinetics during growth. A controlled heterogeneous nucleation mechanism results in the formation of atomically thin α-Fe2 O3 sheets on graphene rather than large particles in solvent, as clarified by theoretical calculations. This strategy paves a new way to synthesizing atomically thin transition metal oxide sheets and low-cost, eco-friendly iron-based energy storage.

Asymmetric All-Metal-Oxide Supercapacitor with Superb Cycle Performance.

Metal oxides have great potential for developing high-performance supercapacitors due to their high specific capacitances. However, achieving high energy densities while maintaining good rate capability and long cycle life has proved to be challenging. We propose herein a strategy for constructing all-metal-oxide asymmetric supercapacitors (ASCs), in which both the cathode and anode are based on metal oxides, and demonstrate their outstanding electrochemical performance. We anchored SnO2 nanoparticles on the surface of reduced graphene oxide (RGO) through Sn-O-C bonds (as the cathode of ACSs), and employed low-crystalline RGO/MoO3 nanosheets as the anode, based on the large work function difference between SnO2 and MoO3 . The resulting ASC can operate stably at 1.8 V in neutral aqueous electrolyte and deliver an energy density of up to 33 W h kg-1 , which remains at 13.8 W h kg-1 even at 37.5 kW kg-1 . Moreover, the ASC exhibits a good cycling stability of 92.5 % capacitance retention after 20 000 cycles.