Crystallite Size Dependence of Lithium Intercalation in Nanocrystalline Rutile

An electrochemical investigation has been undertaken into the dependence of particle size on the mechanism of lithium intercalation in rutile. A thoroughly characterized suite of rutile nanoparticles with diameters in the broad size range 4-50 nm was used in the study. Variation in the cyclic voltammetry of rutile over cycling is presented for the first time and numerous new electrode reactions identified. While a crystallite size effect was witnessed during fast voltammetry experiments, this effect disappeared as the rate was reduced and no variation in capacity was witnessed in galvanostatic cycling experiments. The intercalation potential was seen to increase with decreasing crystallite size, the opposite of what was predicted from quantum confinement, Enhanced lithium intercalation into the smaller nanoparticles has been ascribed to the slightly expanded structure of smaller nanocrystals relative to larger ones taking the dimensions of the tunnels into which Li inserts beyond a threshold value. This lattice expansion or volume dilation with decreasing crystallite size is due to the expected increase in surface tension with decreasing crystallite size. This would correspondingly increase the intercalation potential as the crystallite size decreased.