Enhancement of energy density and safety aspects of Li-ion cells necessitate the usage of 'alloying reaction' based anode materials in lieu of the presently used intercalation-based graphitic carbon. This becomes even more important for the upcoming Na-ion battery system since graphitic carbon does not intercalate sufficient Na-ions to qualify as an anode material. Among the potential 'alloying reaction' based anode materials for Li-ion batteries and beyond (viz., Na-ion, K-ion battery systems), Si and Sn have received the major focus; with the inherently ductile nature of Sn (as against the brittleness of Si) and the considerably better performance in the context of electrochemical Na-/K-storage, of late, tilting the balance somewhat in favor of Sn. Nevertheless, similar to Si and most other 'alloying reaction' based anode materials, Sn also undergoes volume expansion/contraction and phase transformations during alkali metal-ion insertion/removal. These cause stress-induced cracking, pulverization, delamination from current collector, accrued polarization and, thus, rapid capacity fade upon electrochemical cycling. Unlike Si, the aforementioned loss in mechanical integrity is believed to be primarily caused by some of the deleterious 1st order phase transformations and concomitant formation of brittle intermetallic phases during the alloying/de-alloying process. Against this backdrop, this review article focuses on aspects related to deformation, stress development and associated failure mechanisms of Sn-based electrodes for alkali-metal ion batteries; eventually establishing correlations between phase assemblage/transformation, stress development, mechanical integrity, electrode composition/architecture and electrochemical behavior.
Moving toward the next-generation alkali metal-ion battery systems, poor cyclic stability of "alloying-reaction"-based anode materials, such as Sn, is still an unresolved issue of immense significance. There is some interest/ promise with Cu−Sn intermetallic-based anodes (viz., primarily Cu 6 Sn 5 -based), but with the stability still lagging behind the desired level and lithiation/ delithiation mechanisms not being understood. Against this backdrop, starting with a less explored Cu−Sn intermetallic (viz., ε-Cu 3 Sn), operando synchrotron X-ray diffraction scans obtained during galvanostatic lithiation/delithiation cycles have indicated that changes in phase assemblage/evolution take place not only during an individual Li-alloying/dealloying cycle but also across different cycles. In this context, by the end of just one full lithiation/delithiation cycle, Cu 3 Sn gives way to a Sn-deficient Cu 41 Sn 11 phase and some (ejected) β-Sn. Cu 41 Sn 11 behaves fairly similar to Cu 3 Sn during lithiation, forming primarily Li 7 Sn 2 toward the end, but with itself getting reformed upon delithiation (unlike Cu 3 Sn). With continued cycling, Cu 41 Sn 11 gives way to ternary LiCu 2 Sn as the primary phase, which becomes dominant by ∼25 cycles. Upon lithiation, LiCu 2 Sn forms Li 3 CuSn (via Li 2 CuSn), which reverts back to LiCu 2 Sn upon delithiation. This process does not involve dissociation to first form Sn during lithiation; which, otherwise, is the norm with Cu−Sn binary intermetallics. Such changes in phase assemblage and associated lithiation/delithiation pathways resulted in reduction of potential hysteresis and progressive increase of Li-storage capacity upon cycling beyond 25 cycles. Accordingly, a reversible capacity of ∼400 mA h/g could be obtained after 100 cycles, with the increasing trend promising further increment upon continued cycling. On a practical front, such excellent cyclic stability and "safe" operating potential of Cu 3 Sn-based electrodes are very encouraging and ultimate proofs toward their "activity", with the insights into various changes in phase assemblage/evolution and associated lithiation/delithiation mechanisms being expected to lead to the development of stable-cum-"safe" intermetallic anodes for Li-ion batteries and beyond.
Silicon is a high-capacity and safer next-generation anode material for Li-ion batteries, with challenges from rapid capacity fade due to colossal volume changes during Li alloying/de-alloying. Nanostructured Si is deemed to address the above issue, with the possible usage of Si nanowires (SiNWs) on copper substrates (sans any binder or conducting additive) offering the highest performance in terms of anode capacity. However, the direct growth of SiNW on copper current collector foils is challenging and not reported earlier. Against this backdrop, we demonstrate here, for the first time, the successful growth of SiNW, with controllable features, on battery-grade copper substrates via a hot-wire-assisted vapor−liquid−solid (VLS) route. The usage of Sn as a nanotemplate has allowed bringing down the growth temperature to 400 °C, with the SiH 4 pressure and growth duration being other crucial parameters controlling various features of SiNWs, such as length, diameter, aspect ratio, effective crystalline core-to-amorphous shell ratio, morphology of the shell, and orientation with respect to the substrate. The emphasis here is on the variations of different morphological features of these nanowires with changes in process conditions as these are bound to have important implications for various electronic applications. One such application that we explore is their usage as an anode in Li-ion batteries. In the Li "half" cell, the free-standing SiNWs on copper foil exhibit reversible Li-storage capacities of ∼3556 mAh/g @ C/5 and ∼2462 mAh/g @ 1C while retaining ∼89% of the capacity after 100 cycles @ 1C. In the Li-ion "full" cell (with a home-made LiFePO 4 -based cathode), ∼97% capacity retention has been obtained after 100 cycles @ 341 μA/cm 2 . The superior electrochemical performance as an anode, the scalability of the growth technique, and the ability to tune the SiNW characteristics open up the possibility of industrial-scale application of the as-grown SiNWs on copper foil.
The attainment of "true reinforcement" in a composite and harnessing of the associated beneficial effects have been demonstrated here through the development of faceted crystalline Sb particles having the interiors reinforced with reduced graphene oxide (rGO). Such a unique and "near-ideal" micro/nanocomposite architecture has been achieved via a facile/cost-effective route by facilitating heterogeneous nucleation/growth of Sb-oxide particles on/ around dispersed rGO sheets upon incorporation of the same directly into the precursor suspension, followed by the reduction of Sb-oxide to Sb, in intimate contact with the rGO, during the subsequent single heat-treatment step. As a potential anode material for Na-ion batteries, the as-developed Sb/rGO composite exhibits a reversible Na-storage capacity of ∼550 mAh/g (@ 0.2 A/g) and a fairly high first cycle Coulombic efficiency (CE) of ∼79%, with the good reversibility being attributed to the coarse particle size of Sb and encompassing of rGO sheets inside the Sb particles. Furthermore, despite the coarse particle size, the Sb/rGO-based electrode exhibits outstanding cyclic stability, with negligible capacity fade up to 150 cycles (viz., ∼97% capacity retention), and rate capability, with >86% capacity being obtained upon raising the current density from 0.1 to 2 A/g, resulting in a capacity of ∼490 mAh/g, even at 2 A/g.
A current collector, which connects the electrode-active material electronically to the external circuit, is a critical component of an alkali-metal-ion battery. However, it is the least investigated component, especially toward any damage done upon supporting dimensional changes of electrode-active materials during alkali-metal-ion insertion/removal. Against this backdrop, orientation imaging microscopy, together with kernel average misorientation analysis, presents strong evidence toward the occurrence of lattice distortion/deformation, increases of local misorientation, and geometrically necessary dislocations of Cu current collector, in significant terms, upon supporting electrochemical lithiation/delithiation of Si. Creation of such lattice defects also decreases the electronic conductivity of the current collector, thus interfering with its main function. All of the above logically contribute to instability and capacity fade of the electrode. Interestingly, the presence of a graphene-based interlayer between a-Si and Cu suppresses the above damages, thus contributing to the improvement in cyclic stability by protecting not only the active material but also the current collector.
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