Improving amorphous carbon anodes for Na ion batteries by surface treatment of a presodiated electrode with Al₂O₃

Disordered carbons are often investigated as potential materials for anodes in rechargeable sodium-ion batteries. In these materials, electrochemical activity takes place in sites such as surfaces edges, defects, porous (via insertion), and local nano-graphitic clusters (via intercalation). In fact, such carbons can insert reversibly Na ions at capacities up to 300 mAh/g.

One major drawback of these materials is the low Coulombic efficiency in the first charge/discharge cycles. Because of the relatively large surface area and the volumetric change between the sodiated and de-sodiated states, the surface films formed on the active mass at low potentials, which are supposed to passivate and protect the active mass, are not stable enough to prevent parasitic surface side reactions.

In order to prevent formation of thick surface films and excessive consumption of active Na- ions in side reactions, it is possible to pre-treat the carbon surface layers comprising nano-particles of metal oxides like alumina. Such coatings can be used as artificial SEI. However, disordered carbons have complex structures, which include several types of ion insertion sites. Upon coating electro-active insertion sites can be block, thus leading to low specific capacity. Here, we present a new method for implementing construction of effective passivation films for carbon anode materials. By this method, we succeeded to improve the Coulombic efficiency of carbon anodes for sodium ion batteries and hance, to increase their specific capacity.