Electrochemical Reactivity of KSi and NaSi Zintl Phases with Lithium

Abstract

The electrochemical reactions of NaSi and KSi Zintl phases with lithium (0-2.5 V) proceed very differently. Na/K is partially removed on first oxidation with a complete amorphization for NaSi or growth of a new crystallized K x Si phase for KSi. Upon discharge, metallic K is partially extruded and a Li-K-Si (Li 15 Si 4 -type) phase is formed. For NaSi, metallic Na is also chased on discharge but crystallization of Li-Na-Si (Li 15 Si 4 -type) phase is delayed at the second discharge. The amorphization of Li 15 K x Si 4 proceeds through a ∼0.40 Volts charge plateau but crystallized Li 15 Na x Si 4 does not show this feature and electrochemically behaves similarly to amorphous Li x Si.The design of high energy density electrochemical storage devices requires either or both high voltage difference ( E) and large amounts of exchanged species (i.e. large capacities) between the two electrodes. High E can be achieved through the use of highly electropositive metals (alkali, alkaline earth) whose corresponding monovalent or bivalent ions are either reversibly hosted in intercalation compounds (e.g. Li-ion technology) or electroplated (e.g. Li-polymer technology) during the charging step. To date, graphitic powders are the state-of-the-art intercalating material but alloys and alloying reactions have also been intensively investigated for decades with great performance improvements and some enthusiastic industrial announcements, but very slim commercialization success so far (MatsushitaWood's metal-80's, Fuji-Stalion-1995, Sony-Nexelion-2005.For Li-based systems, silicon is the most appealing candidate for Li-alloying element because it scores the highest discharge gravimetric (3579 mAh/g of Si) and volumetric (8330 mAh/cm 3 of Si) capacities, both ranking around ten times higher than those of carbon (Li 15 Si 4 1 vs. LiC 6 2 ) with a reaction taking place at very low potential (<200 mV vs. Li + /Li • ). This alloying process comes with very large volume changes 3 (≈275% for Si → Li 15 Si 4 ) leading to particle displacement, loss of contact/cohesion in composite electrodes, and formation of reactive surfaces, resulting in a poor cycle life. These major issues triggered many tackling approaches such as the use of small particles, 4,5 the confinement in active or inactive buffering matrices, 6-9 the use of suited polymeric binders, 10-15 thin films, 16-18 the application of specific cycling conditions, 19 and tuning the electrode porosity/formulation. Crystallized silicon undergoes a progressive amorphization during its first lithiation (a-Li x Si), but a crystallized phase can be spotted at very low voltage (c-Li 15 Si 4 ) and then back converted to amorphous a-Li x Si during charge.

2D Silicon–Germanium-Layered Materials as Anodes for Li-Ion Batteries

“…Apart from this, a comparison test of siloxene with bulk Si (Figure 8f) reveals some differences. Indeed, for Si, the CV presents two reduction peaks at 0.25 and 0.15 V and two oxidation ones at 0.32 and 0.50 V, 24,25 ascribed to the amorphization of bulk Si with lithiation and the final crystallization of Li 15 Si 4 in discharge followed by its delithiation into a-Si. Figure 8h shows bulk Si and siloxene in the sixth cycle, and it can be observed that the shoulder peak at 0.50 V in siloxene coincides with the oxidation peak of bulk Si, indicating the presence of an alloying reaction from the bulk Si impurities with Li.…”