| The surging demand for efficient yet low-cost renewable energy technologies boosts great interests in seeking for high-performance power source.Li-ion batteries（LIBs）are very appealing candidate towards this goal due to high energy density,long calendar life and environmental benignity.However,the scarcity of Li resources（only 20 ppm in Earth’s crust）offsets the potential of LIBs in large-scale energy applications such as power plants and electric grids with MWh-level or above.Compared to Li,Na is more abundant in reserve（2.83 % in Earth’s crust and 30.6 % in the ocean）for the design of rechargeable Na-ion batteries（NIBs）with similar electrochemical properties but greatly reduced cost.This feature is particularly attractive for energy storage applications,where the requirement on specific volumetric and gravimetric energy density are not a critical issue,for example,the electrical grid storage of solar or wind energy.Although the chemistry of Na intercalation is very similar to that of Li in terms of the voltage,stability and diffusion barrier,many electrode materials（e.g.,graphite,non-layered metal oxides）normally working in LIBs lose the activity towards Na storage due to the limited intercalation of Na with higher ionization potential and larger ionic diameter（1.02 ? vs.0.76 ? for Li）.Hard carbon materials,metals（e.g.,Sn,Sb）or elementary substances（e.g.,P）,tunnel-structured transitional metal oxides,fluoride-based materials and metal sulfides/oxides have been investigated as the anode materials in NIBs,delivering greatly enhanced performance in terms of insertion or conversion-type reactions.A major problem of these materials,however,is much drastic volume change than the case in Li-intercalated electrodes upon repeated electrochemical cycling,which leads to severe electrode pulverization and short cycle life for Na storage.The development of truly durable anode materials is still far from a satisfactory breakthrough for the construction of high-performance NIBs.In this work,we report a new type of nanohybrids composed of amorphous binary metal sulfide hollow nanostructures with N-doped carbon sheath for highly reversible and ultralong-life Na storage.This unique design well integrates three main design principles to achieve excellent performance for Na storage.Firstly,the atomically mixed system of binary metal sulfides with distinct electrochemical behavior may act as the buffer zone to mitigate the internal strain between each other,which efficiently relieves the electrode from the isotropic volume change upon cycling.The loose amorphous structure particularly facilitates the diffusion of Na ions with large radius by enhancing the atomic/ionic mobility within the materials.In this sense,the atomically mixed matrix is advantageous over the mechanically mixed heterogeneous powders.Second,the hollow nanostructure could sufficiently tolerate the volume variation upon Na insertion/extraction by the presence of interior structure,which simultaneously provides a large interfacial active area for fast Na diffusion across the shells with nanoscale thickness.The hollow nanocage of amorphous CoSnSx with high capacity for Na storage is chosen to demonstrate our concept,which has been rarely reported.On this base,the introduction of a robust N-doped conductive carbon sheath around the amorphous hollow nanostructures further reinforce them against particle aggregation and pulverization while enhancing the electrode conductivity.As a result,for sodium storage,the N-doped carbon coated nanoboxes of amorphous cobalt-tin binary sulfides（denoted as CoSnSx@NC）exhibit exceptionally CoSnSx@NC nanobox electrodes exhibit ultra-long lifespan of over 4000 cycles at high current density of 1.0 A g-1 with stable capacity retention of over 70 % since the 10 th cycle.It corresponds to a very slow capacity loss of 0.0075 % per cycle,rendering the exceptional durability for practical use.After deep cycling,a comparable capacity of 180 mAh g-1 is retained with nearly 100% Coulombic efficiency,which is still nearly two-fold higher than that of the state-of-the-art cathode materials and is sufficient for the construction of high-energy NIBs.This electrodes can also be applied for lithium storage,which also delivered excellent electrochemical capacity.The asymmetric Na-ion full cells assembled from them and Na3V2（PO4）3/C cathode exhibit high energy density of 86.6 Wh kg-1 and good capacity retention for 500 cycles at high current rate. |
