Lithiation of silicon via lithium Zintl-defect complexes from first principles

Abstract

An extensive search for low-energy lithium defects in crystalline silicon using density-functional-theory methods and the ab initio random structure searching (AIRSS) method shows that the four-lithium-atom substitutional point defect is exceptionally stable. This defect consists of four lithium atoms with strong ionic bonds to the four under-coordinated atoms of a silicon vacancy defect, similar to the bonding of metal ions in Zintl phases. This complex is stable over a range of silicon environments, indicating that it may aid amorphization of crystalline silicon and form upon delithiation of the silicon anode of a Li-ion rechargeable battery. }).For example, the lithium substitutional defect is denoted by {Li,V } since it comprises a lithium atom within a silicon vacancy. A lithium-vacancy complex was assigned to an electron paramagnetic resonance (EPR) center by Goldstein 2 and lithium drift rate experiments have also shown that lithium binds to vacancies, preventing further mobility.3 Our density-functional-theory (DFT) calculations show that the formation of a Frenkel-type impurity defect ({Li,V ,I }) from a T d interstitial Li atom requires an energy of 3.73 eV, which is in excellent agreement with the value of 3.75 eV calculated by Wan et al. 4 Wan et al. suggested that the substitutional lithium defect, {Li,V } will not form due to its large formation energy. However, we calculate its formation energy from bulk silicon and lithium metal to be 3.09 eV, and hence {Li,V } is more stable than a separated vacancy and lithium interstitial ({V } and {Li}). It is likely that silicon vacancies play a significant role in the interaction between lithium and silicon.Along with the wide range of technological uses of silicon, such as semiconductor devices and photovoltaics, it has recently been proposed as an anode for lithium-ion batteries. Lithium intercalated graphite is the standard lithium-ion battery (LIB) negative electrode material, due to its good rate capability and cyclability, but demand for even higher performance LIBs has motivated the investigation of other materials. Silicon is an attractive alternative since it has 10 times the gravimetric and volumetric capacity of graphite (calculated from the initial mass and volume of silicon) but, unlike graphite, silicon undergoes structural changes on lithiation. 5-7In the first stages of lithiation, in which silicon atoms greatly outnumber lithium, it has been proposed that lithiation occurs as interstitial lithium defects form near the silicon surface. 8 Previous theoretical studies have shown that a single lithium atom in bulk silicon resides at the T d site and that at higher concentrations lithium clusters can promote the breaking of silicon-silicon bonds. 4,9,10 There are equal numbers of T d sites and silicon atoms in c-Si and, since a-Li y Si forms at y ≈ 0.3 in micron-sized silicon clusters (a Li:Si ratio of ≈ 1 : 3 11,12 ), the presence of lithium must lead to the breaking of silicon-silicon bonds before all T d sites are filled with lithium. Chan e...

Thermodynamically stable lithium silicides and germanides from density functional theory calculations

“…34,35 More recently is has also been applied to the Li-P system 36 and defects in technologically relevant ceramics, 37,38 semiconductors 39,40 and LIBs. 41,42 Since in an AIRSS calculation each random starting configuration is independent from another, the search algorithm is trivially parallelisable, making high-throughput computation straightforward. AIRSS searches were performed for stoichiometries Li x Si y and Li x Ge y where x, y = 1 − 8.…”

“…The DFT ground state at 0 GPa is a I4 1 /a phase comprising a 3-fold coordinated silicon network hosting lithium tetrahedra similar to the {4Li,V} Zintl defect in silicon. 42 Recently the I4 1 /a phase has been synthesized via ball-milling and shown to be stable under ambient conditions. 50 Mulikan analysis yields a charge of 0.34|e| for each Li and −0.34|e| for each Si establishing Li as cationic contrary to a previous analysis.…”

Revealing lithium–silicide phase transformations in nano-structured silicon-based lithium ion batteries via in situ NMR spectroscopy

“…The Brillouin zone was integrated with a grid spacing finer than 2p  0.03 Å À 1 . Ab initio random structure searching (AIRSS) 47,48 has previously been applied to point defects in the Li-Si system 49,50 . Using AIRSS, B3,500 searches were performed where a Li or Si atom was randomly added or removed from the two-formula-unit c-Li 15 Si 4 (c-Li 3.75 Si) simulation cell and the resulting structure and lattice parameters were relaxed to a local energy minimum.…”

Stable Crystalline Forms of Na Polysulfides: Experiment versus Ab Initio Computational Prediction

“…CSP algorithms dealing with the latter problem have substantially improved in recent years and several approaches, including simulated annealing, minima hopping, metadynamics, and evolutionary approaches have shown promising results. Some of these approaches have already started to be employed in the investigations of battery materials …”