Diffusion of Lithium in Bulk Amorphous Silicon: A Theoretical Study

Abstract

The rate performance of lithium-ion secondary batteries depends critically on the kinetic transport of Li within the anode material. Here we use first-principles theoretical calculations to study the diffusion of Li in the low-concentration limit, using model electrodes of crystalline and four-fold coordinated bulk amorphous silicon. We identify Li diffusion pathways that have relatively low energy barriers (<0.50 eV) in amorphous silicon and discuss how diffusion at short (∼2.5 Å), intermediate (∼10 Å), and long (>1 nm) distances depends on the atomic-scale features of the silicon host. We find that both the energy barriers for diffusion and the topology of the atomic structure control the diffusion. We estimate the diffusion rate in amorphous Si anode to be comparable to the rate in crystalline Si anodes. These findings shed light on the wide range of reported experimental results for Li diffusion in Si anodes.

Atomistic Study of Lithium Ion Dynamics in Li12Si7

“…The diffusivity value of Li in Si-based electrode materials has been investigated using experimental methods and computer simulations. [26][27][28][29][30][31][32][33][34][35] The local environments and dynamics of lithium ions in the binary lithium silicide were studied using NMR technique 36,37 . Kuhn et al 37 found that nine of thirteen crystallographically independent Li sites in the Li 12 Si 7 take part in an extreme fast long-range diffusion process characterized by an activation energy of only 0.18 eV in the Li 12 Si 7 .…”

Cs diffusion in SiC high-energy grain boundaries

“…The values ranged from -2.68 to 8.15 eV, with a standard deviation of 3.40 eV. The Cs, which is an impurity with a relatively larger size than those studied in the similar amorphous systems 46,47,51,[60][61][62][63] , turned out to be highly stable as interstitials in the a-SiC. The high stability of Cs defects vs. bulk is possibly due to the large relaxations available to the a-SiC system, analogous to the observation of Ag defects in the HEGB 24 , although the detailed mechanism of stabilizing Cs interstitials in a-SiC was not explicitly investigated here.…”

Insertion of Mono- vs. Bi- vs. Trivalent Atoms in Prospective Active Electrode Materials for Electrochemical Batteries: An ab Initio Perspective

“…This can be seen by comparing the insertion energies of Refs. [20,67], where default basis sets of the Spanish Initiative for Electronic Simulations with Thousands of Atoms (SIESTA) program were used, with those of Ref. [14], where bases tuned in this way were used, as well as with available plane wave results [12,59,60].…”

“…The rationale behind using an amorphous structure is two-fold: firstly, an amorphous structure may have larger-volume insertion sites, which would lead to lower strain energy; secondly, a metastable amorphous structure is expected to be more reactive towards Li, leading to a lower (stronger) E b . We used a previously published [67] amorphous Si (a-Si) structure. An amorphous structure has multiple insertion sites with different E b , and we found that many of those sites had E b much lower than that of c-Si; this is shown in Figure 1.…”