Comparative computational study of the energetics of Li, Na, and Mg storage in amorphous and crystalline silicon

Abstract

To assess the potential of amorphous Si (a-Si) as an anode for Li, Na, and Mg-ion batteries, the energetics of Li, Na, and Mg atoms in a-Si are computed from first-principles and compared to those in crystalline Si (c-Si). It is shown that Si preamorphization increases the average anode voltage and reduces the volume expansion of the anode during the insertion of the metal atoms.Analysis of computed formation energies of Li, Na, and Mg defects in a-Si and c-Si suggests that the energetics of the single atoms into a-Si are thermodynamically more favorable. For instance, defect formation energies of Li, Na, and Mg defects in a-Si are respectively 0.71, 1.72, and 1.82 eV lower compared to those in c-Si. Moreover, the defect formation energies of Li, Na, and Mg defects (vs. vacuum reference states) in a-Si are comparable with the metal cohesive energies and consequently the insertion of the metal atoms might be possible with appropriate control of charging process. This is in contrast to c-Si, where the storage of Na and Mg atoms is limited due to high energy cost of Na and Mg insertion into c-Si.

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

“…Some of these studies considered for the first time phononic effects on lithiation, sodiation, and magnesiation, and effects on voltage of doping. While most of these studies helped explain the mechanism and parameters of performance which had already been observed experimentally, some of them predicted electrochemical activity before it was confirmed experimentally, for example, the recently confirmed sodiation of amorphous silicon [20,38] or voltages achieved with magnesiated TiO 2 [22,39]. Some of the calculations we performed and review below still await experimental confirmation such as our prediction of significantly enhanced insertion energetics by p-doping.…”

“…Here, we only introduce key computed quantities which are important for a material's performance as an active electrode material and which are discussed in Sections 2 and 3. To analyze the thermodynamics of interaction of a potential electrode material with Li, Na, K, Mg, or Al, we use the binding energy E b (n) of n of these atoms to the active material, which is effectively the defect formation energy (E f , which is the term used in many studies [11][12][13]15,16,[19][20][21][22][23][24][25]). It is computed as:…”

“…We produced several such studies and they are surveyed next. In references [11,12,[14][15][16]20,24], we produced comparative studies of Li, Na, and Mg insertion in crystalline and amorphous silicon. While lithiation of Si is an alloying reaction, we will further, for the sake of consistency, use the word "insertion", since for small concentrations and only interstitial sites occupied, the two mechanisms do not differ.…”

“…While lithiation of Si is an alloying reaction, we will further, for the sake of consistency, use the word "insertion", since for small concentrations and only interstitial sites occupied, the two mechanisms do not differ. Some of these studies used plane wave bases [12] and some used localized basis sets [14,20]. Specifically, when using the localized basis sets, which are more computationally efficient for many kinds of calculations, the issue of basis size and shape selection arises.…”

“…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].…”