Facile synthesis of hierarchical Na2Fe(SO4)2@rGO/C as high-voltage cathode for energy density-enhanced sodium-ion batteries

Fe-based sulfates are ideal cathode candidates for sodium-ion batteries(SIBs) owing to their high operating voltage and low cost but suffer from the nature of poor power performance. Herein, a hierarchical porous Na2Fe(SO4)2@reduced graphene oxide/carbon dot(Na2Fe(SO4)2@rGO/C) with low carbon content(4.12 wt%) was synthesized via a facile homogeneous strategy benefiting for engineering application,which delivers excellent sodium storage performance(high voltage plateau of 3.75 V, 85 m Ah g-1 and330 Wh kg-1 at 0.05 C;5805 W kg-1 at 10 C) and high Na+diffusion coefficient(1.19 × 10-12 cm2 s-1).Moreover, the midpoint voltage of assembled full cell could reach 3.0 V. The electron transfer and reaction kinetics are effectively boosted since the nanoscale Na2Fe(SO4)2 is supported by a robust crosslinked carbon matrix with rGO sheets and carbon dots. The slight rGO sheets sufficiently enhance the electron transfer like a current collecter and restrain the aggregation, as well as ensure smooth ion channels. Meanwhile, the carbon dots in the whole space connect with Na2Fe(SO4)2 and help rGO to promote the conductivity of the electrode. Ex-situ X-ray powder diffraction and X-ray photoelectron spectrometry analysis confirm the high reversibility of this sodiation/desodiation process.

Layered K_(0.54)Mn_(0.78)Mg_(0.22)O_(2)as a high-performance cathode material for potassium-ion batteries

Layered Mn-based oxides are one of the promising cathode materials for potassium-ion batteries(KIBs)owing to their high theoretical capacities,abundant material supply,and simple synthesis method.However,the structural deterioration resulting from the Jahn-Teller effect of Mn ions hinders their further development in KIBs.Herein,a novel Mn-based layered oxide,K_(0.54)Mn_(0.78)Mg_(0.22)O_(2),is successfully designed and fabricated as KIBs cathode for the first time.It delivers smooth charging/discharging curves with high specific capacity of 132.4 mAh·g^(‒1)at 20 mA·g^(‒1)and good high-rate cycling stability with a capacity retention of 84%over 100 cycles at 200 mA·g^(‒1).Combining in-situ X-ray diffraction(XRD)and ex-situ X-ray photoelectron spectroscopy(XPS)analysis,the storage of K-ions by K_(0.54)Mn_(0.78)Mg_(0.22)O_(2)is revealed to be a solid-solution processes with reversible slip of the crystal lattice.The studies suggest that the rational doping of inactive Mg2+can effectively suppress the Jahn-Teller effect and provide outstanding structure stability.This work deepens the understanding of the structural evolution of Mn-based layered materials doped with inactive materials during de/potassiation processes.