Enhanced Performance of Lithium–Sulfur Batteries with Co-Doped g-C3N4 Nanosheet-Based Separator

Vanadium-doped graphitic carbon nitride for high performance lithium-sulfur batteries

To promote polysulfide conversion in lithium sulfur batteries (LSB) and alleviate the shuttle effect, we designed and fabricated a novel catalyst of vanadium-doped graphite phase carbon nitride with nitrogen defects (V@gC3N4-ND) and high vanadium loading (3.46 at%) by defect engineering and two-step pyrolysis. Employing a V@gC3N4-ND modified separator, the LSB yielded capacities of 934 mA h g-1 at 1C and 404 mA h g-1 at 4C; the former was retained by 61% and 45% after 500 and 1000 cycles, respectively. In particular, the initial capacity of the battery reached 969 mA h g-1 at a sulfur loading of 10.0 mg cm-2. This work provides a facile route to the preparation of high-loading vanadium active site catalysts with nitrogen defects in the support, which are promising for high performance LSB applications.

Ultrafine Co-Species Interspersed g-C3N4 Nanosheets and Graphene as an Efficient Polysulfide Barrier to Enable High Performance Li-S Batteries

Lithium-sulfur (Li-S) batteries are regarded as one of the promising advanced energy storage systems due to their ultrahigh capacity and energy density. However, their practical applications are still hindered by the serious shuttle effect and sluggish reaction kinetics of soluble lithium polysulfides. Herein, g-C₃N₄ nanosheets and graphene decorated with an ultrafine Co-species nanodot heterostructure (Co@g-C₃N₄/G) as separator coatings were designed following a facile approach. Such an interlayer can not only enable effective polysulfide affinity through the physical barrier and chemical binding but also simultaneously have a catalytic effect on polysulfide conversion. Because of these superior merits, the Li-S cells assembled with Co@g-C₃N₄/G-PP separators matched with the S/KB composites (up to ~70 wt% sulfur in the final cathode) exhibit excellent rate capability and good cyclic stability. A high specific capacity of ~860 mAh g^-1 at 2.0 C as well as a capacity-fading rate of only ~0.035% per cycle over 350 cycles at 0.5 C can be achieved. This bifunctional separator can even endow a Li-S cell at a low current density to exhibit excellent cycling capability, with a capacity retention rate of ~88.4% at 0.2 C over 250 cycles. Furthermore, a Li-S cell with a Co@g-C₃N₄/G-PP separator possesses a stable specific capacity of 785 mAh g^-1 at 0.2 C after 150 cycles and a superior capacity retention rate of ~84.6% with a high sulfur loading of ~3.0 mg cm^-2. This effective polysulfide-confined separator holds good promise for promoting the further development of high-energy-density Li-S batteries.

Sodium doped flaky carbon nitride with nitrogen defects for enhanced photoreduction carbon dioxide activity

Sodium doped flaky carbon nitride (g-C₃N₄) with nitrogen defects (bmw-DCN-x) were synthesized via two steps method to enhance photocatalytic reduction of carbon dioxide (CO₂). After ball milling and calcination, dicyandiamide was evenly dispersed on the sodium chloride (NaCl) template to form a flaky structure. The NaCl not only provided part of sodium (Na) source for Na doped g-C₃N₄, but also introduced a large number of nitrogen (N) defects. Meanwhile, sodium hydroxide (NaOH) significantly enhanced Na doping. The bmw-DCN-30, a proportion of modified g-C₃N₄, showed heightened photo-reduction CO₂ performance, with satisfactory carbon monoxide (CO) and methane (CH₄) productivity at a rate of 30.6 μmol·g^-1·h^-1 and 5.4 μmol·g^-1·h^-1 respectively. This productivity was 15 and 11 times as much as that of bulky g-C₃N₄ (BCN). The related characterizations confirmed that N defects produced more reactive sites and enhanced the adsorption capacity of carbon nitride to CO₂. The accompanying Na doping and flaky structure characteristics improved the optical absorption ability and the effective separation of photogenerated carriers. Accordingly, this work provides further insights into constructing modified materials based on carbon nitride for CO₂ reduction.