Design of new anode materials based on hierarchical, three dimensional ordered macro-mesoporous TiO₂ for high performance lithium ion batteries

As anode materials for lithium ion batteries, two three dimensionally ordered macroporous TiO₂, one with disordered inter-particle mesopores formed by the aggregation of nanoparticles (3DOM) and another with inner-particle mesopores generated by a surfactant templating strategy (3DOMM), have been synthesized using poly(styrene-methyl methacrylate-3-sulfopropyl methacrylate potassium) (P(St-MMA-SPMAP)) spheres as a hard template and their electrochemical properties are compared. SEM and TEM observations reveal that both 3DOM TiO₂ and 3DOMM TiO₂ have well-ordered macropores and interconnected macropore walls with a regular periodicity. 3DOMM TiO ₂ demonstrates a specific surface area of 139 m² g ^-1, which is higher than that of 3DOM TiO₂ (99 m ² g^-1) due to the smaller crystallite size and inner-particle mesopores. The electrolyte adsorption results show that both 3DOM TiO₂ and 3DOMM TiO₂ have similar adsorption capacities despite a difference in the surface area. Electrochemical impendence spectroscopy analysis shows that 3DOMM TiO₂ has a lower charge transfer resistance and faster Li⁺ diffusion coefficient than 3DOM TiO₂. Moreover, both 3DOM TiO₂ and 3DOMM TiO₂ possess excellent initial capacity of 248 mA h g^-1 and 235 mA h g^-1 at 0.2 C and 208 mA h g^-1 and 202 mA h g^-1 at 1 C, respectively. The reversibility study demonstrates that the 3DOMM TiO₂ displays higher cycling capacity, superior rate behavior and higher Coulombic efficiency because the higher surface area provides more active sites and the presence of the inner-particle mesopores in the walls of macropores serve as a bicontinuous transport path and affords a shorter path length for diffusion of Li ions compared with the 3DOM TiO₂ and its crystallite aggregated mesopores. The reversible capacity of 106 mA h g ^-1 observed for the 3DOMM TiO₂ can be retained after 200 charge-discharge cycles at a relatively high current rate of 4 C. This cycle stability performance can be equally attributed to the crystallite size and inner-particle mesopores in the 3DOMM TiO₂. Moreover, the existence of a bicontinuous porous structure in the 3DOMM TiO₂ can further enhance the lithium insertion/extraction capacity at high rates. We believe that this study can shed light on the 3DOMM structure as a promising material for highly enhanced performance in lithium ion batteries. This journal is