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^-3) 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 × 10^-12 cm² s^-1 at 52% SOC and 3.8 × 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. (Figure Presented).