Synthesis of Single-Phase LiSi by Ball-Milling: Electrochemical Behavior and Hydrogenation Properties

Abstract

In addition, Zintl phase LiSi, usually prepared under high temperature-pressure (873 K-4 GPa), has recently been added into the diagram as LiSi is also indeed stable under ambient conditions. A novel synthesis method for single-phase LiSi by highly energetic ball-milling is detailed herein. LiSi is investigated for two different applications: a negative electrode for lithium-ion batteries and a hydrogen storage material. Up to now, the electrochemistry of LiSi has not yet been explored. In order to obtain metastable Li 15 Si 4 at deep discharge, it is crucial to first form an amorphous Li-poor silicide by oxidation. Subsequent electrochemical behavior after the first cycle follows closely to that of classical silicon with fast capacity fading. On the contrary, when LiSi is first oxidized and x is kept below 1 in Li x Si, a unique electrochemical signature is observed with an initial capacity of 870 mAh.g −1 and good retention for the first 10 cycles. Concerning the hydrogenation properties, 2.8 wt% H 2 can be absorbed by LiSi at 573 K to form [LiH + Si], but the reaction is irreversible because of the slow LiSi formation kinetics.Zintl phases are an important class of materials, chemically located in between the intermetallic and ionic compounds, with extended polyanions that match the Zintl-Klemm concept, i.e. the polyanion structure is similar to that of a neighboring isoelectronic element. They possess a variety of interesting crystal chemistry toward a wide range of applications. 1-3 Various silicon based Zintl phases can be found in the literature and some examples given here are not exhaustive: 1) rare-earth metal silicides e.g. Eu 5 Si 3 with the formula 5[Eu 2+ ][Si] 4− [Si 2 ] 6− ; 4 2) ternary silicides, e.g. LiAlSi with the [AlSi] − zinc-blend network and Li + in octahedral sites next to the more electronegative silicon species, 5,6 and CaAl 2 Si 2 with twodimensional rafts of Ca 2+ cations tightly bound by [Al 2 Si 2 ] 2− layers (each Si is surrounded by 4 Al in a flipped tetrahedron or umbrella shaped); 6,7 3) alkaline-earth silicides e.g. CaSi 2 have puckered [Si − ] n polyanion corrugated layers (each silicon has three equidistant neighbors) separated from each other by planar mono-layers of Ca 2+ ; 8 4) alkali silicides, which are commonly found with [Si 4 ] 4− or/and [Si 9 ] 4− Zintl anions, e.g. Rb 12 Si 17 (in a 2:1 ratio). 9 Our interest lies in the alkali monosilicides, i.e. MSi (M = Li -Cs). For the heavier alkali monosilicides (MSi, M = Na, K, Rb, Cs), the silicon atoms form isolated [Si 4 ] 4− tetrahedra (which are isoelectronic with white phosphorus P 4 molecules) within the unit cell surrounded by four M + cations, with NaSi crystallizing in a monoclinic unit cell 10 and MSi (M = K, Rb, Cs) in cubic ones. 11 Unlike CaSi 2 which has very good stability toward moist air, 12 these alkali silicides are extremely air and moisture sensitive. 10,11 However, the missing piece in the alkali metal series is still the LiSi analog. Unlike the heavier alkali metal monosilicides, LiSi does not possess...

Thermodynamically stable lithium silicides and germanides from density functional theory calculations

“…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. 49 DFT predicts a novel P4/mmm phase with a formation energy of 0.07 eV p.f.u.…”

“…1, in increasing lithium content order; diamond-structure Fd3m c-Si; the I4 1 /a Li 1 Si 1 phase 48,49 which has recently been synthesized at ambient pressure 50 and is discussed in Sec. IV A; the Pnma Li 12 Si 7 phase 51 which contains silicon 5-membered rings and 4-membered stars, which have been studied using NMR; 14 17 and contrary to the reports that Li is anionic.…”

Electrochemical Reactivity of KSi and NaSi Zintl Phases with Lithium

“…1 Li / 100 h). This drastically contrasts with reports by Tang et al 39 showing that ca. 0.9 Li can be electrochemically removed (1 Li/30 h) from LiSi, another Zintl phase but having a very different crystal structure than NaSi/KSi.…”

The heat capacity and entropy of the lithium silicides Li17Si4 and Li16.42Si4 in the temperature range from (2 to 873) K

“…Lithium rich intermetallic compounds could act as an alternative of which lithium silicides appear attractive due to the highest theoretical specific capacity of ~4200 mAh·g -1 (Li 22 Si 5 ) [8,9] and low discharge potentials [10,11]. Furthermore, lithium silicide phases possess rather high reversible hydrogen storage capability and may be used as safe hydrogen storage materials [12,13].…”

“…Motivated by this view, we recently provided experimental heat capacities and entropies of the four lithium silicides Li 12 Si 7 , Li 7 Si 3 , Li 13 Si 4 , and Li 21 Si 5 /Li 22 Si 5 [34]. Wang et al used our data to recalculate the Li-Si phase diagram [35], still exhibiting weaknesses in the lithium rich part.…”