Combining Fast Li-Ion Battery Cycling with Large Volumetric Energy Density: Grain Boundary Induced High Electronic and Ionic Conductivity in Li4Ti5O12 Spheres of Densely Packed Nanocrystallites

One of the key challenges toward high-power Li-ion batteries is to develop cheap, easy-to-prepare materials that combine high volumetric and gravimetric energy density with high power densities and a long cycle life. This requires electrode materials with large tap densities, which generally compromises the charge transport and hence the power density. Here densely packed Li4Ti5O12 (LTO) submicrospheres are prepared via a simple and easily up-scalable self-assembly process, resulting in very high tap densities (1.2 g.cm(-2)) and displaying exceptionally stable long-term high rate cyclic performance. The specific capacities at a (dis) charge rate of 10 and 20 C reach 148.6 and 130.1 mAh g(-1), respectively. Moreover, the capacity retention ratio is 97.3% after 500 cycles at 10 C in a half cell, and no obvious capacity reduction is found even after 8000 cycles at 30 C in a full LiFePO4/LTO battery. The excellent performance is explained by the abundant presence of grain boundaries between the nanocrystallites in the submicron spheres creating a 3D interconnected network, which allows very fast Li-ion and electron transport as indicated by the unusually large Li-ion diffusion coefficients and electronic conductivity at (6.2 x 10(-12) cm(2) s(-1) at 52% SOC and 3.8 X 10(-6) S cm(-1), respectively). This work demonstrates that, unlike in porous and nanosheet LTO structures with a high carbon content, exceptionally high rate charge transport can be combined with a large tap density and hence a large volumetric energy density, with the additional advantage of a much longer cycle life. More generally, the present results provide a promising strategy toward electrode materials combining high rate performances with high volumetric energy densities and long-term cyclic stability as required for the application in electric vehicles and tools.