Boosting cycling stability through Al(PO3)3 loading in a Na4MnV(PO4)3/C cathode for high-performance sodium-ion batteries.

Mn-based NASICON-type Na4MnV(PO4)3 (NMVP) has been widely investigated as one of the most promising alternatives to Na3V2(PO4)3 cathodes for sodium-ion batteries (SIBs) due to its higher energy density, higher abundance, and lower cost and toxicity compared to V. However, electrochemical performance for large-scale applications is limited by NMVP's inferior conductivity and structural degradation during cycling. Herein, a facile strategy to modify the surface/interphase properties of NMVP/C was reported using the thermally stable Al(PO3)3 precursor with a wet process followed by heat treatment to enhance the interface stability of electrodes. The nanomodified layer has the benefits of an ionic conductor (slight NaPO3) and robust composite (Al(PO3)3), which can facilitate the stability of Mn-based cathode materials and ionic conductivity. These merits endow 1 wt% Al(PO3)3-loaded NMVP/C cathodes with a high rate performance (102/61 mAh g-1 at 0.2/50 C) and impressive cyclability (88.5%/89.7% at 5 C/10 C after 3000/4000 cycles) in Na-ion batteries at 2.5-3.8 V. Moreover, when the cutoff voltage is raised to 4 V, improved electrochemical properties (111.6/50.8 mAh g-1 at 0.2/10 C and 71.4% after 1000 cycles at 5 C) are also realized. Such an enhancement indicates that facial surface modification engineering limits organic electrolyte erosion, inhibits transition metal dissolution and suppresses surface lattice degradation, which is confirmed by ex situ X-ray diffractometry and transmission electron microscopy. Therefore, the Al(PO3)3 surface modification strategy combined with mechanism analysis can provide a possible reference for advanced electrochemical properties in energy storage devices.

Na Storage Activity or Inertness of P-Configurations in N, P Dual-Doped Carbon Nanofibers: Bulk vs Surface.

Heteroatom doping is an effective method to improve the electrochemical properties of hard carbon anodes for sodium-ion batteries. However, the different roles of surface and bulk heteroatoms in Na storage have not been explored much. Herein, N, P dual-doped carbon nanofibers (NP-CNFs) with high doping contents and low surface area are designed to clarify the above issue. It is confirmed that P plays a more crucial role in Na storage compared with N. In addition, surface and bulk P not only possess different configurations but also show distinct Na storage activity. There are only oxidized POx groups on the surface, which are inactive for Na storage but promote the stability of electrochemistry interphase, while in the bulk phase, unoxidized P-C bonds also emerge except POx, which shows preeminently reversible Na storage activity, and the POx groups are activated simultaneously. Furthermore, P-doping changes the reactivity of N-configurations with Na both on the surface and in the bulk phase, exhibiting interesting synergism. As expected, the surface stability, bulk activity, and synergism enable NP-CNFs to achieve superior performance. It could deliver a prominent capacity of 105.6 mAh g-1 at 10 A g-1 after 3000 cycles in half cells and 164.3 mAh g-1 at 1 A g-1 after 200 cycles in full cells.

Three-Dimensional Self-assembled Hairball-Like VS4 as High-Capacity Anodes for Sodium-Ion Batteries.

Sodium-ion batteries (SIBs) are considered to be attractive candidates for large-scale energy storage systems because of their rich earth abundance and consistent performance. However, there are still challenges in developing desirable anode materials that can accommodate rapid and stable insertion/extraction of Na+ and can exhibit excellent electrochemical performance. Herein, the self-assembled hairball-like VS4 as anodes of SIBs exhibits high discharge capacity (660 and 589 mAh g-1 at 1 and 3 A g-1, respectively) and excellent rate property (about 100% retention at 10 and 20 A g-1 after 1000 cycles) at room temperature. Moreover, the VS4 can also exhibit 591 mAh g-1 at 1 A g-1 after 600 cycles at 0 °C. An unlike traditional mechanism of VS4 for Na+ storage was proposed according to the dates of ex situ characterization, cyclic voltammetry, and electrochemical kinetic analysis. The capacities of the final stabilization stage are provided by the reactions of reversible transformation between Na2S and S, which were considered the reaction mechanisms of Na-S batteries. This work can provide a basis for the synthesis and application of sulfur-rich compounds in fields of batteries, semiconductor devices, and catalysts.