Structure of Surface Entrance Sites for Li Intercalation into TiO₂ Nanoparticles, Nanosheets, and Mesoporous Architectures with Application for Li-Ion Batteries

Abstract

Power output is central to the viability of a Liion battery and is, in part, dependent upon the activation energy barrier associated with Li intercalation/deintercalation into the host lattice (electrode). The lower the energy barrier, the faster the intercalation reaction rate and greater the power. The activation energy is governed by the atomistic structure(s) of the entrance sites for Li intercalation. Accordingly, a first step in optimizing battery power via structural manipulation of entrance sites is to understand the structure of these entrance sites. However, HRTEM is (presently) unable to characterize the structures of entrance sites with atomistic resolution. Accordingly, we generate models of the entrance sites using molecular dynamics. In particular, we simulate the synthetic protocol used to fabricate nanostructured TiO 2 experimentally. The resulting atomistic models reveal a highly complex and diverse structural distribution of entrance sites, which emanate from the surface curvature of the nanostructured material. In particular, we show how nanostructuring can be used to change profoundly the nature and concentration of such entrance sites. ■ INTRODUCTIONNanostructured materials have shown great promise recently as potential electrodes for Li-ion batteries. 1,2 In particular, Ren et al. showed that mesoporous TiO 2 , with a 3D pore structure, can be used as an anode-replacing graphite. 3 Similarly, mesoporous MnO 2 can act as a cathode. 4 Nanostructuring of the electrodes was shown, in both cases, to confer electrochemical activity upon the materials; 5 the parent bulk materials are electrochemically inactive. A recent simulation study revealed that mesoporous materials are able to expand and contract elastically (during charge cycling) as pseudo "breathingcrystals"enabling retention of the structure and crucially the 1 × 1 tunnels in which the Li ions enter and reside. 6 Conversely, the bulk parent material deforms plastically during intercalation, pulverizing the tunnels. 4,6 Central to the power output and charge time of batteries is the activation energy barriers associated with Li intercalation/ deintercalation from the host electrodes. The lower the energy barriers, the faster the reactions facilitating higher power and lower charge times. The activation energy is governed by the atomistic structure(s) of the entrance sites for Li intercalation. Accordingly, a first step in tuning battery power, via structural manipulation of entrance sites, is to understand their structure.Inspection of HRTEM images of mesoporous TiO 2 ( Figure 1) reveals that the entrance sites are not structurally uniform; 3 rather the figure reveals a diverse structural complexity, which emanates from the three-dimensional curvature of the pore. Accordingly, if one were to use a model of the perfect surface to calculate energy barriers associated with Li intercalation, the results would prove erroneous because the model would not capture the structural perturbations emanating from the curvature of the surface. Indeed,...

Atomistic Simulation and Characterization of Spinel Li1+xMn2O4 (0 ≤ x ≤ 1) Nanoparticles

“…Related composites or heterostructures were previously synthesized by simulations for binary MnO 2 , and TiO 2 , nanoparticles, together with experimental syntheses of lithium manganese oxides. , In contrast to previously simulated binaries, the complexity of the current Li–Mn–O heterostructured nanoparticle emanates from it being a ternary compound, compounded by the possibility of adopting several polymorphic crystal phases when synthesized at high temperatures. Here, we have captured such complexity by evolving the structures, starting from amorphous precursors, rather than generating models using crystallographic symmetry operators.…”

“…Microstructures are consistent with those observed from the high resolution transmission microscope. Indepth understanding of such microstructural features and active sites of ions can aid in the design and modification of high energy density electrode materials [34,35],…”

“…27,28 Simulated amorphization and recrystallization (A+R), based on classical molecular dynamical methods, have been successfully employed to generate various (model) nanoarchitectures (nanospheres, sheets, and porous) of binary CeO 2 29 and Li-ion battery electrodes such as MnO 2 30,31 and TiO 2 . 32 In addition, the method yielded heterostructured or composite compounds with ramsdellite-pyrolusite and rutilebrookite polymorphs for MnO 2 and TiO 2 , respectively. A model ternary Li 2 MnO 3 nanosphere has also been synthesized using this approach.…”

Materials modelling in the University of Limpopo

“…Their enhanced performance is now achieved with nano-architectured electrodes, including primary nanoparticles. The MMC has applied some of the world-leading simulation approaches to produce such nanostructures in Li-ion batteries [39,40] and first principles methods [41], which shed valuable insights on precursor reactor processes occurring in laboratories, industrial pilot and production plants. It has also applied machine learning cluster expansion methods to determine the stability of doped NMC cathodes [42] and has carried out surface calculations for cathodes [43] and metal air batteries [44,45].…”

The effects of structural properties on the lithium storage behavior of mesoporous TiO2

“…A popular and effective way to address these issues is to utilize mesoporous TiO 2 with micron or submicron particle sizes instead of monodispersed nanoparticles [13,[19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36]. For instance, a superior reversible capacity of 220 mA h g −1 at 1C has been reported by Saravanan et al for the micronsized mesoporous TiO 2 with high packing density [19].…”