Exceptionally highly performing Na-ion battery anode using crystalline SnO2 nanoparticles confined in mesoporous carbon

Abstract

Confined and unconfined SnO₂ nanoparticles in the pores of mesoporous carbon were prepared and tested as anode materials vs. Na. Both composites present small crystalline SnO₂ particles (∼3 nm) but different location and dispersion in the carbon matrix. When the particles are homogeneously distributed and confined in the carbon pores, an initial reversible capacity of 780 mA h g⁻¹ is achieved with unprecedented capacity retention of 80 and 54% after 100 and 4000 cycles, respectively, at a high current rate (50 C, 1800 mA g⁻¹). Unexpectedly, over two current rate variation cycles from 1 C to 500 C, the composite recovers 81% and 97%, respectively after returning from the 500 C to the 1 C rate. To our knowledge, no other material with such a long cycling life and superior performance in terms of capacity and rate capability has been reported so far for sodium ion batteries. HRTEM, XRD, N₂ adsorption, XPS and galvanostatic cycling results suggest that confined SnO₂ particles undergo an enhanced sodium alloying/dealloying process due to their special confinement inside the pores, which increases their conductivity, facilitates the diffusion of Na⁺ ions and buffers the large volumetric changes during charge/discharge. These high performances cannot be delivered when SnO₂ is not confined and not well dispersed in the carbon pores. This work demonstrates that nano-confinement of anode species in carbon is a valuable concept affording the modification of the fundamental properties of guest species along with their electrochemical performances leading to highly stable and performing materials with a long life for Na-ion batteries.