Oxide cathodes for sodium-ion batteries:Designs,challenges,and perspectives

Sodium-ion batteries(SIBs),which are an alternative to lithium-ion batteries(LIBs),have attracted increasing attention due to their low cost of Na resources and similar Na storage mechanism to LIBs.Compared with anode materials and electrolytes,the development of cathode materials lags behind.Therefore,the key to improving the specific energy and promoting the application of SIBs is to develop high-performance sodium intercalation cathode materials.Transition-metal oxides are one of the most promising cathode materials for SIBs owing to their excellent energy density,high specific discharge capacity,and environmentally friendly nature.In the present work,the latest progress in the research of transition-metal oxides is summarized.Moreover,the existing challenges are discussed,and a series of strategies are proposed to overcome these drawbacks.This review aims at providing guidance for the development of metal oxides in the next stage.

Determination of nighttime VTEC average in the Klobuchar ionospheric delay model

The Klobuchar model has been widely used to correct the ionospheric delay in applications. However, the NVTEC(Nighttime Vertical Total Electron Content) of the Klobuchar model employs an empirical constant of 9 TECU(Total Electron Content Unit) at L1 frequency. In this paper, the rationality and reliability of the nighttime constant setting are investigated using the GIM(Global Ionosphere Map) product of the IGS(International GNSS Service) from 1998 to 2015. Our study indicates that the suitable time span of NVTEC average in nighttime should be between 20:00 and 06:00 LT(local time). The NVTEC is highly correlated with seasons, having positive extremes in spring and autumn and negative extremes in summer through the mean values in all latitudes. In addition to seasonal dependence, solar activity in the solar cycle 23 strongly influences NVTEC as well and leads to its variation within a range between 25 and30 TECU in spring and autumn at solar maximum, which is about 1.5 times greater than that in summer and winter. The NVTEC also has a dependence on the latitude at solar maximum, with the mean value from 30 TECU in low latitudinal regions to 15 TECU in high latitudinal regions. Therefore, these results demonstrate that the nighttime VTEC has much greater deviations from the imperial constant in the Klobuchar model, and the newly estimated constant is expected to bring improvement to the predictability of the Klobuchar ionospheric delay model in nighttime.

Autotrophic denitrification for nitrate and nitrite removal using sulfur-limestone

Sulfur-limestone was used in the autotrophic denitrification process to remove the nitrate and nitrite in a lab scale upflow biofilter.Synthetic water with four levels of nitrate and nitrite concentrations of 10,40,70 and 100 mg N/L was tested.When treating the low concentration of nitrate-or nitrite-contaminated water(10,40 mg N/L),a high removal rate of about 90% was achieved at the hydraulic retention time(HRT) of 3 hr and temperature of 20-25°C.At the same HRT,50% of the nitrate or nitrite could be removed even at the low temperature of 5-10°C.For the higher concentration nitrate and nitrite(70,100 mg N/L),longer HRT was required.The batch test indicated that influent concentration,HRT and temperature are important factors afiecting the denitrification eficiency.Molecular analysis implied that nitrate and nitrite were denitrified into nitrogen by the same microorganisms.The sequential two-step-reactions from nitrate to nitrite and from nitrite to the next-step product might have taken place in the same cell during the autotrophic denitrification process.

Sulfur-based autotrophic denitrification from the micro-polluted water

Eutrophication caused by high concentrations of nutrients is a huge problem for many natural lakes and reservoirs. Removing the nitrogen contamination from the low C/N water body has become an urgent need. Autotrophic denitrification with the sulfur compound as electron donor was investigated in the biofilter reactors. Through the lab-scale experiment,it was found that different sulfur compounds and different carriers caused very different treatment performances. Thiosulfate was selected to be the best electron donor and ceramsite was chosen as the suitable carrier due to the good denitrification efficiency, low cost and the good resistibility against the high hydraulic loads. Later the optimum running parameters of the process were determined. Then the pilot-scale experiment was carried out with the real micro-polluted water from the West Lake, China. The results indicated that the autotrophic denitrification with thiosulfate as electron donor was feasible and applicable for the micro-polluted lake water.

Removal of anaerobic soluble microbial products in a biological activated carbon reactor

The soluble microbial products (SMP) in the biological treatment effluent are generally of great amount and are poorly biodegradable. Focusing on the biodegradation of anaerobic SMP, the biological activated carbon (BAC) was introduced into the anaerobic system. The experiments were conducted in two identical lab-scale up-flow anaerobic sludge blanket (UASB) reactors. The high strength organics were degraded in the first UASB reactor (UASB1) and the second UASB (UASB2, i.e., BAC) functioned as a polishing step to remove SMP produced in UASB1. The results showed that 90% of the SMP could be removed before granular activated carbon was saturated. After the saturation, the SMP removal decreased to 60% on the average. Analysis of granular activated carbon adsorption revealed that the main role of SMP removal in BAC reactor was biodegradation. A strain of SMP-degrading bacteria, which was found highly similar to Klebsiella sp., was isolated, enriched and inoculated back to the BAC reactor. When the influent chemical oxygen demand (COD) was 10,000 mg/L and the organic loading rate achieved 10 kg COD/(m 3 ·day), the effluent from the BAC reactor could meet the discharge standard without further treatment. Anaerobic BAC reactor inoculated with the isolated Klebsiella was proved to be an effective, cheap and easy technical treatment approach for the removal of SMP in the treatment of easily-degradable wastewater with COD lower than 10,000 mg/L.

Mesoporous iron oxide-silica supported gold catalysts for low-temperature CO oxidation

Mesoporous iron oxide-silica composite with a high silica content was synthesized by hydrothermal method,and another composite material with a high iron content was obtained by etching part of silica in alkaline solution.Gold catalysts were loaded onto both composites by a deposition-precipitation method,and used for CO oxidation.The samples were characterized by BrumauerEmmet-Teller(BET),X-ray diffraction(XRD),X-ray photoelectron spectra(XPS),transmission electron microscope(TEM)and scanning electron microscope(SEM)techniques.Both composites had high specific surface areas and were amorphous.The Au nanoparticles dispersed on the surface of the composites existed in metallic state.Composite with high silica content was not suitable for Au loading,and its supported gold catalyst showed poor performance in catalytic reaction.In contrast,composite with high iron content allowed efficient Au loading,and CO could be oxidized completely at low temperature on its supported gold catalyst.The effects of deposition-precipitation pH values on Au loading and activity of the catalyst were investigated,and the results indicated that Au loading was the highest and the catalyst was the most active for CO oxidation when the synthesis pH was adjusted to 8.