Lithium Insertion in Nanostructured Si_1-xTi_xAlloys

Abstract

Nanostructured Si 1-x Ti x alloys (0 ≤ x ≤ 0.3) prepared by ball milling were studied as negative electrode materials in Li cells. The alloys comprised a nanocrystalline and amorphous Si phase and a nanocrystalline C49 TiSi 2 phase. When x ≥ 0.15 the nanocrystalline Si phase was completely eliminated and such alloys consisted only of amorphous Si and the C49 TiSi 2 phase. The alloys with x ≥ 0.15 also completely suppressed the formation of Li 15 Si 4 during cycling, resulting in good capacity retention, and, unlike other Si transition metal alloys, without introducing noticeable cell polarization. The observed capacity of the Si 1-x Ti x alloys suggested that the TiSi 2 is an inactive phase toward Li. Silicon-based materials are of high interest in the pursuit of obtaining higher energy density lithium ion batteries. Pure silicon has an outstanding theoretical capacity of 2194 Ah/L, which has been estimated to provide an increase in energy density of up to 34% in Li-ion cells, compared to cells with conventional graphite-based electrodes. However, the cycle life of Si electrodes is difficult to maintain because of its drastic volume expansion during lithiation (up to 280%). Reducing Li uptake and providing an inactive phase framework is one possible way to alleviate the expansion problem. Some inactive M-Si alloys, such as Ni-Si, 2-4 Fe-Si, 5 Cu-Si, 6 B-Si, 7 have been extensively studied in the past two decades.Si-Ti alloys are of interest as electrode materials for Li-ion batteries because of their thermal stability and because they are composed of high abundance elements. They also have low density and TiSi 2 has high electrical conductivity compared to other silicides. 8 Because of these favorable properties, this system has been studied by other groups in the past. Lee et al. synthesized Si-Ti alloys via melt spinning, forming alloys comprised of well crystallized Si and TiSi 2 phases. 9The observed capacities of these samples in Li cells were far less than expected from the amount of Si present. All the Si in these samples might not have been fully accessible to lithium ions because of the morphology that resulted from rapid solidification processing. Zhou et al. synthesized Si coated TiSi 2 nanonets via chemical vapor deposition.10 When the nanonets were cycled between 0.03-3.0 V, high capacity fade during cycling resulted because of the formation of Li 15 Si 4 . When cycling was conducted above 0.09 V, capacity retention was improved because the formation of Li 15 Si 4 was avoided. This indicates the importance of avoiding Li 15 Si 4 formation to obtain good cycling performance. Park et al. designed a TiSi 2 -coated Si anode via a silicothermic reduction process.11 The conductive silicide coating resulted in improved rate performance. It is apparent from these studies that microstructure and Li 15 Si 4 avoidance is important in obtaining good cycling in Si-Ti alloys. However, to our knowledge a systematic study of the effect of composition in the Si-Ti alloy system has not previously been reported.Here a...

Adhesion and Surface Layers on Silicon Anodes Suppress Formation of c-Li3.75Si and Solid-Electrolyte Interphase

“…Since c -Li 3.75 Si only forms below 50 mV vs Li, ,,, sufficiently high stress can prevent this phase from forming. In one approach, Obrovac and co-workers prepared a series of silicon alloys with transition metals including Ti, Ni, and Cu, both as powders and thin films. − c -Li 3.75 Si was suppressed in both the powder and thin film electrodes which was ascribed to increased stress induced by crystalline intermetallic inclusions in the powders and the dissolved transition metal atoms in the thin films. − …”

Adhesion and Surface Layers on Silicon Anodes Suppress Formation of c-Li3.75Si and Solid Electrolyte Interphase

“…The Cu/Ni/Si85Ti15 electrode shows improved capacity retention of 82% after 200 cycles; Wang et al reported similar results for Ti-Si alloys and attributed the observation to the suppression of the c-Li3.75Si phase. 25 Compared to the Cu/Ni/Si85Ti15 electrode, addition of carbon (10 nm) or TiO2 (5 or 10 nm) capping layers exhibits further improvement in capacity retention as well as CE. The improvement in CE might result from less (or different) SEI formation, although this remains to be determined (vide infra).…”

“…This is also true for alloying with the hard non-active transition metal inclusions, titanium in our case, which generate internal stresses in the silicon film. [25][26][27][28] However, with each delithiation cycle, there is a non-zero amount of fracture that occurs. Pores and fractures between 'islands' result in additional free volume for expansion in the electrode material, as is visible in the SEM micrographs in Figure 2 and is often observed during lithiation-delithiation cycling of silicon thin films.…”

“…In one approach, Obrovac and co-workers prepared a series of alloys of silicon with transition metals including Ti, Ni, and Cu, both as powders and thin films. [25][26][27][28] c-Li3.75Si was suppressed in both the powder and thin film electrodes, and was ascribed to increased stress induced by crystalline intermetallic inclusions in the powders, 25 and the dissolved transition metal atoms in the thin films. [26][27][28] Another approach to inhibiting the formation of c-Li3.75Si is through induction of mechanical stress at the interface between a silicon film and the substrate.…”

Beyond Thin Films: Clarifying the Impact of c-Li15Si4 Formation in Thin Film, Nanoparticle, and Porous Si Electrodes

“…Despite the relatively small additional capacity and expansion associated with its formation, c -Li 15 Si 4 has been linked to an increased loss of capacity in both silicon thin films , and nanoparticles. , The observed links between delamination, capacity loss, and the c -Li 15 Si 4 phase are convoluted in thin films. The presence of c -Li 15 Si 4 is near-ubiquitous when delamination is observed in thin films, but this crystalline phase is also generally accepted to play an intrinsic role in capacity degradation beyond catastrophic delamination .…”