Investigating Ternary Li–Mg–Si Zintl Phase Formation and Evolution for Si Anodes in Li-Ion Batteries with Mg(TFSI)₂ Electrolyte Additive

Abstract

Improved electrochemical performance of Si was recently reported by adding multivalent cation salts (such as Mg2+, Al3+, Ca2+, etc.) in the electrolyte. This is achieved via the formation in an in situ manner of relatively more stable Li–M–Si ternary phases with less chemical reactivity. These phases stabilize Si anions and thus reduce side reactions with electrolytes at the surface and eventually benefit the overall electrochemistry. To understand the mechanism of ternary Zintl phase formation and its dynamics upon lithiation/delithiation, high-resolution solid-state 7Li and 29Si nuclear magnetic resonance (NMR) are utilized to directly probe the local Li and Si environments on Si electrodes harvested from coin and pouch cells at various states of (de)­lithiation. The NMR spectra along with the electrochemical characterization reveal that lithiation of Si starts from the surface Si–O layer further confirmed by 7Li–29Si cross-polarization NMR. Lithiation progresses with heterogeneous silicon clustering with Si–4 anions at high states of lithiation. At a fully lithiated state, the formation of overlithiated Si species is detected. At a low-voltage region (below 100 mV), direct evidence for Mg-ion insertion is found, postulated by two possible mechanisms: ion exchange with fully or overlithiated binary domains (Li3.75+x Si) and/or a coinsertion with slightly underlithiated domains (∼Li3.55Si). Upon delithiation, Li extraction starts from overlithiated Si domains. No evidence is found for electrochemical Mg removal. Evidence for a lithium-deficient Li y Mg0.1Si phase is found as a result of Li removal during charging. This investigation sheds light on the possible mechanisms of a new Si anode chemistry, which could enable the development of stable Si-based anodes for lithium-ion batteries.

Utilization of 29Si MAS-NMR to Understand Solid State Diffusion in Energy Storage Materials

“…Intermediate phases, identified by MAS-NMR, included various silicon oxide, hydroxide, and hydride species that effected the cycling performance due to increased surface reactivity with the electrolyte as a function of charge. Building on the ability to study amorphous and nanoscale surface coatings, it was used to evaluate the products of an in-situ synthesis method that selectively coats the surface of an active silicon particle (BH Han et al, 2019a;BH Han et al, 2021;Li et al, 2021).…”

“…To form a coating of these phases on the silicon in an electrochemical cell, small amounts of the salt Mg (TFSI) 2 were added to the standard Gen2 electrolyte (1.2M LiPF 6 EMC/EC 3:7). After cycling, TEM, EDX, MAS-NMR, and XPS studies showed surface Mg +2 at the approximate stoichiometric ratio of Li 14.5 Mg 0.5 Si 4 , a point in the solid solution between Li 14 MgSi 4 and Li 15 Si 4 that may represent a limit based on magnesium diffusion under the conditions used for the study (BH Han et al, 2019a;BH Han et al, 2021;Li et al, 2021). Spectroscopic evidence for interfacial phase formation is shown in Figure 2.…”

“…With in-situ ternary salt formation at the surface, baseline Si electrode samples have extended cycle and calendar life compared to electrolytes without the additive. The mechanism of ternary Zintl phase formation and its dynamics upon lithiation/delithiation were also studied with 29 Si MAS NMR on Si electrodes harvested from cycled coin and pouch cells at various states of (de)lithiation, see Figure 3 (Li et al, 2021). The NMR data, along with other complimentary characterization techniques, reveal that lithiation of Si starts from the Si−O surface layer and progresses with heterogeneous silicon clustering with Si −4 anions at high states of lithiation.…”

Atomistic Diffusion Pathways of Lithium Ions in Crystalline Lithium Silicides fromab InitioMolecular Dynamics Simulations

“…Many composite materials act as buffering matrices for the volume expansion of silicon while simultaneously ensuring electronic contact of the active material and mechanical stability of the electrode . To avoid unwanted side reactions of the Si anode and electrolyte, suitable electrolytes matching the Si surface chemistry have to be chosen. − Moreover, compounds involving Si can achieve good electrochemical performance as active anode materials: for example, silicanes and ternary Li–Mg–Si phases …”

“…7−9 Moreover, compounds involving Si can achieve good electrochemical performance as active anode materials: for example, silicanes 10 and ternary Li−Mg−Si phases. 11 Experimental lithiation of crystalline Si in a LiCl-KCl melt at a temperature of 415 °C leads to various crystalline Li x Si y phases, 12 which can be understood as Zintl-like phases. 13,14 Lithium silicide crystal structures of the compositions Li 1 Si 1 , Li 12 Si 7 , Li 7 Si 3 , Li 13 Si 4 , Li 15 Si 4 , Li 4.11 Si 1 , Li 21 Si 5 , Li 17 Si 4 , and Li 22 Si 5 (refs 15−23) have been described in the literature so far.…”