Aluminum doping improves the energetics of lithium, sodium, and magnesium storage in silicon: A first-principles study

Abstract

While Si is an effective insertion type anode for Li-ion batteries, crystalline Si has been shown to be unsuitable for Na and Mg storage due, in particular, to insufficient binding strength. It has recently been reported that Si nanowires could be synthesized with high-concentration (several atomic %) and dispersed Al doping. Here we show based on density functional theory calculations that Al doping significantly improves the energetics for Na and Mg insertion, specifically, making it thermodynamically favored versus vacuum reference states. For high Al concentrations, the energy of Mg in Al-doped Si approaches the cohesive energy of Mg. However, the migration barriers for the diffusion of Li (0.57-0.70 eV), Na (1.07-1.19 eV) and Mg (0.97-1.18 eV) in Al-doped Si are found to remain about as high as in pure Si, likely preventing effective electrochemical sodiation and magnesiation.

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

“…Naturally, one could use explicit p-doping to achieve the same effect of stabilization of E b . This was confirmed by our studies on p-doped Si (doped with Al) and Ge (doped with Ga) [24,37]. Figure 8 shows that p-doping leads to a substantial strengthening of binding of all Li, Na or Mg. At concentrations of p-dopants which have been shown to be feasible [99], it is possible to make Na and Mg insertion favorable (E b becomes negative).…”

“…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.…”

“…[17] and a passivated nanosheet model in [11]. The electronic mechanism of Li, Na, and Mg accommodation is donation of Li, Na, or Mg valence electrons to the conduction band of Si, as evidenced by the shift of the Fermi level to the conduction band [24,60]. Li and Na donate their only valence electron, as reflected in their Bader charges of about 0.9 |e|.…”

Feasibility of Reversible Electrochemical Na-Storage and Cyclic Stability of Amorphous Silicon and Silicon-Graphene Film Electrodes

“…11 Such predicted Na-capacity, even though significantly lesser compared to the Li-capacity, would still be considerably superior to those of most other potential anode materials. [3][4][5][6]15,16 Furthermore, lower Na-intake in Si would result in lower volume expansion compared to the case of Li-intake (viz., ∼114% for Na vs. ∼400% for Li), 11 which might just lead to lesser problems related to stress induced degradation and capacity fade in the case of Na.Due to the earlier belief concerning electrochemical Na-insertion being difficult in Si, [6][7][8][9][10]13 only very recently few experimental works 12,17,18 have explored the possibilities of electrochemical sodiation/de-sodiation in crystalline (c-Si) 6,17 and amorphous Si (a-Si) 12,18 in the form of nanoparticles. Indeed, both Ellis et al 7 and Komaba et al, 6 in their studies with Si particle sizes of 325 mesh * Electrochemical Society Student Member.…”