Glassy Li metal anode for high-performance rechargeable Li batteries.

Lithium metal has been considered an ideal anode for high-energy rechargeable Li batteries, although its nucleation and growth process remains mysterious, especially at the nanoscale. Here, cryogenic transmission electron microscopy was used to reveal the evolving nanostructure of Li metal deposits at various transient states in the nucleation and growth process, in which a disorder-order phase transition was observed as a function of current density and deposition time. The atomic interaction over wide spatial and temporal scales was depicted by reactive molecular dynamics simulations to assist in understanding the kinetics. Compared to crystalline Li, glassy Li outperforms in electrochemical reversibility, and it has a desired structure for high-energy rechargeable Li batteries. Our findings correlate the crystallinity of the nuclei with the subsequent growth of the nanostructure and morphology, and provide strategies to control and shape the mesostructure of Li metal to achieve high performance in rechargeable Li batteries.

Lithium-electrolyte solvation and reaction in the electrolyte of a lithium ion battery: A ReaxFF reactive force field study.

In the electrode/electrolyte interface of a typical lithium-ion battery, a solid electrolyte interphase layer is formed as a result of electrolyte decomposition during the initial charge/discharge cycles. Electron leakage from the anode to the electrolyte reduces the Li+-ion and makes it more reactive, resulting in decomposition of the organic electrolyte. To study the Li-electrolyte solvation, solvent exchange, and subsequent solvent decomposition reactions at the anode/electrolyte interface, we have extended the existing ReaxFF reactive force field parameter sets to organic electrolyte species, such as ethylene carbonate, ethyl methyl carbonate, vinylene carbonate, and LiPF6 salt. Density Functional Theory (DFT) data describing Li-associated initiation reactions for the organic electrolytes and binding energies of Li-electrolyte solvation structures were generated and added to the existing ReaxFF training data, and subsequently, we trained the ReaxFF parameters with the aim of finding the optimal reproduction of the DFT data. In order to discern the characteristics of the Li neutral and cation, we have introduced a second Li parameter set to describe the Li+-ion. ReaxFF is trained for Li-neutral and Li+-cation to have similar solvation energies, but unlike the neutral Li, Li+ will not induce reactivity in the organic electrolyte. Solvent decomposition reactions are presumed to happen once Li+-ions are reduced to Li-atoms, which can be simulated using a Monte Carlo type atom modification within ReaxFF. This newly developed force field is capable of distinguishing between a Li-atom and a Li+-ion properly. Moreover, it is found that the solvent decomposition reaction barrier is a function of the number of ethylene carbonate molecules solvating the Li-atom.