Longjun Li

Advanced hybrid Li–air batteries with high-performance mesoporous nanocatalysts

Hybrid Li–air batteries fabricated with mesoporous NiCo2O4 nanoflakes directly grown onto nickel foam and N-doped mesoporous carbon loaded onto a hydrophobic carbon paper, respectively, as the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts are found to exhibit the best reported cycle life.

Delineating the roles of Co3O4 and N-doped carbon nanoweb (CNW) in bifunctional Co3O4/CNW catalysts for oxygen reduction and oxygen evolution reactions

While many synthesis methods have been reported for Co3O4–carbon nanocomposites as a bifunctional electrocatalyst for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) in alkaline solutions, the individual contributions of Co3O4 and CNW towards ORR and OER are still not clear. Here, we report the individual functionality of Co3O4 and the N-doped carbon nanoweb (CNW) in ORR and OER. The Co3O4/CNW bifunctional catalysts were synthesized by an in situ growth of Co precursors onto CNW followed by a controlled heat treatment. The specific functions of Co3O4 and CNW were delineated by rotating disk electrode measurements. It was found that Co3O4 alone exhibited poor ORR catalytic activity. However, in the presence of CNW, Co3O4 assisted the selective four-electron oxygen reduction over the two-electron pathway. Analyses of the Tafel slope and reaction order suggested that Co3O4 acted as the primary catalytic site for OER. CNW improved the electronic conduction between Co3O4 and the current collector. CNW underwent serious degradation at the high potential of the OER but its stability improved greatly upon the deposition of Co3O4. Two possible mechanisms for the improved catalytic stability are discussed. The findings demonstrate the specific functions of Co3O4 and CNW in catalyzing the OER and ORR and further establish an understanding of the synergy of the composite in electrocatalysis.

Hierarchical pore-in-pore and wire-in-wire catalysts for rechargeable Zn– and Li–air batteries with ultra-long cycle life and high cell efficiency

We report here the design of low-cost hierarchical oxygen reduction/evolution reaction (ORR/OER) catalysts that could achieve higher cell efficiency and several times longer cycle life than conventional Pt/C + IrO2 bifunctional catalysts in metal–air batteries (Zn–air and Li–air batteries).

Enhanced cycling stability of hybrid Li-air batteries enabled by ordered Pd3Fe intermetallic electrocatalyst.

We report an ordered Pd3Fe intermetallic catalyst that exhibits significantly enhanced activity and durability for the oxygen reduction reaction under alkaline conditions. Ordered Pd3Fe enables a hybrid Li-air battery to exhibit the best reported full-cell cycling performance (220 cycles, 880 h).

VO2/rGO nanorods as a potential anode for sodium- and lithium-ion batteries

VO2 (B) is an interesting cathode candidate in lithium-ion batteries with a high theoretical capacity and fast charge/mass transfer rate. Most of the studies on this material have focused on the lithium insertion reaction in the range of 4.0–1.0 V. In this paper, VO2/rGO nanocomposite was prepared by a microwave-assisted solvothermal method and investigated in both lithium-ion and sodium-ion batteries as a potential anode. The VO2 (B) nanorods have an average diameter of ∼200 nm, and is encapsulated by reduced graphene oxide (rGO) nanosheets. Electrochemical results reveal that the VO2/rGO electrodes exhibit stable cycling and good rate performance in both Li and Na cells. Reversible capacities of 400 mA h g−1 over 400 cycles (vs. Li/Li+) and 200 mA h g−1 over 200 cycles (vs. Na/Na+) have been achieved, indicating it is potential as an anode candidate either in lithium- or sodium-ion batteries. Nevertheless, the reaction mechanisms seem to be different in Li and Na cells. The crystal structure of VO2 (B) is maintained at a low discharge potential of 0.05 V in lithium-ion cells, while phase amorphization occurs in sodium-ion cells below 0.5 V. This result is consistent with the previous study on TiO2, further confirming that some stable metal oxides may show rather different behaviors in Na cells than expected.

Dual-electrolyte lithium–air batteries: influence of catalyst, temperature, and solid-electrolyte conductivity on the efficiency and power density

Two major issues limiting the conversion efficiency and power density of dual-electrolyte Li–air cells are the lack of efficient oxygen evolution catalysts and high internal resistance associated with the solid electrolyte. In this context, the charge voltage is lowered by 0.11 V at a charge current density of 2 mA cm−2 by employing nanocrystalline IrO2 synthesized by a modified Adams fusion method. Similarly, the overall internal resistance of the cell is reduced substantially by increasing the operating temperature of the cell from 20 to 40 °C, resulting in a nearly three-fold increase in the maximum power density. Overall, the conversion efficiency at 2 mA cm−2 is improved from 61% to 74% at 40 °C with the nanocrystalline IrO2. The internal resistance is further reduced by employing a more conductive solid electrolyte at 40 °C, resulting in a maximum power density and conversion efficiency at 2 mA cm−2 of, respectively, 40 mW cm−2 and 80%.

Understanding the Redox Obstacles in High Sulfur-Loading Li–S Batteries and Design of an Advanced Gel Cathode

Lithium-sulfur batteries with a high energy density are being considered a promising candidate for next-generation energy storage. However, realization of Li-S batteries is plagued by poor sulfur utilization due to the shuttle of intermediate lithiation products between electrodes and its dynamic redistribution. To optimize the sulfur utilization, an understanding of its redox behavior is essential. Herein, we report a gel cathode consisting of a polysulfide-impregnated O- and N-doped porous carbon and an independent, continuous, and highly conducting 3-dimensional graphite film as the charge-transfer network. This design decouples the function of electron conduction and polysulfide absorption, which is beneficial for understanding the sulfur redox behavior and identifying the dominant factors leading to cell failure when the cells have high sulfur content and insufficient electrolyte. This design also opens up new prospects of tuning the properties of Li-S batteries via separately designing the material functions of electron conduction and polysulfide absorption.