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.

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.

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.

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.

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.

Tin (Sn)-based anodes for sodium (Na)-ion batteries possess higher Na-storage capacity and better safety aspects compared to hard carbon -based anodes but suffer from poor cyclic stability due to volume expansion/contraction and concomitant loss in mechanical integrity during sodiation/ desodiation. To address this, the usage of nanoscaled electrodeactive particles and nanoscaled-carbon-based buffers has been explored, but with compromises with the tap density, accrued irreversible surface reactions, overall capacity (for "inactive" carbon), and adoption of non-scalable/complex preparation routes. Against this backdrop, anode-active "layered" bismuth (Bi) has been incorporated with Sn via a facile-cum-scalable mechanicalmilling approach, leading to individual electrode-active particles being composed of well-dispersed Sn and Bi phases. The optimized carbon-free Sn−Bi compositions, benefiting from the combined effects of "buffering" action and faster Na transport of Bi, to go with the greater Na-storage capacity and lower operating potential of Sn, exhibit excellent cyclic stability (viz., ∼83−92% capacity retention after 200 cycles at 1C) and rate capability (viz., no capacity drop from C/5 to 2C, with only ∼25% drop at 5C), despite having fairly coarse particles (∼5−10 μm). As proven by operando synchrotron X-ray diffraction and stress measurements, the sequential sodiation/desodiation of the components and, concomitantly, stress build-ups at different potentials provide "buffering" action even for such "active−active" Sn−Bi compositions. Furthermore, the overall stress development upon sodiation of Bi has been found to be significantly lower than that of Sn (by a factor of ∼3.8), which renders Bi promising as a "buffer" material, in general. Dissemination of such complex interplay between electrode-active components during electrochemical cycling also paves the way for the development of high-performance, safe, and scalable "alloyingreaction"-based anode materials for Na-ion batteries and beyond, sans the need for ultrafine/nanoscaled electrode particles or "inactive" nanoscaled-carbon-based "buffer" materials.

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In this work, we report direct/in situ measurements of stress and potential evolution during self-discharge of a fully lithiated silicon electrode. Parasitic reactions, typically attributed to the formation of the solid-electrolyte-interphase (SEI) layer on the surface of the silicon electrode, cause the self-discharge leading to the loss of cyclable lithium ions from the electrode and irreversible capacity loss. These parasitic reactions continuously occur when the electrode potential is below the equilibrium potential (typically 0.8V vs. Li/Li+) for SEI formation, and when the surface is electronically conductive. We previously reported on coupled electrochemical-mechanical measurements in the Li-Si binary system between pure Si and Li15Si4 with the following key observations: the system undergoes cyclic compressive (up to -2GPa) and tensile stresses (up to +2GPa) during electrochemical lithiation and delithiation, respectively, with extensive plastic flow and associated mechanical dissipation1; the biaxial stress and the electrode potential are strongly coupled2; the electrode softens upon lithiation and toughens upon delithiation3; and the energy losses due to mechanical dissipation are comparable to the sum of kinetic/polarization, and ohmic losses4. Using the substrate-curvature measurement technique, we measured the stress and potential evolution during self-discharge (due to parasitic reactions) of a fully lithiated thin-film silicon electrode, and compare it to same measurements made during galvanostatic delithiation. Upon self-discharge, the stress of a fully lithiated electrode evolves from -1.2 GPa (compressive) towards a state of zero stress, and continues to become tensile (+0.5 GPa). The evolution from -1.2 GPa towards zero stress and continual increase in the tensile direction is caused by the removal of lithium ions from the electrode driven by the parasitic reactions5, and tensile stresses typically cause cracking and damage in electrodes. Figure 1 shows (a) the current density, (b) potential vs. Li/Li+, and (c) the electrode stress during a galvanostatic lithiation/delithiation cycle (in red), and during galvanostatic lithiation followed by self-discharge and galvanostatic delithiation (in blue). We also quantified the rates at which the parasitic reactions occur by measuring both potential and stress evolution during self-discharge at various states of charge, SOCs (i.e., by varying the concentration of lithium in the silicon electrode via galvanostatic lithiation, followed by open-circuit measurements). We show that the resulting parameters are useful in predicting changes in electrode stress and its evolution during the self discharge at various SOCs. We will discuss why these measurements are useful in the context of storing fully charged lithium-ion batteries on the shelf for long periods of time. Because of self-discharge-induced mechanical damage, for batteries made with a large-volume-expansion electrodes, it is better to store them at or near a state of zero stress than at a higher SOC. Acknowledgements: This work was supported at the Indian Institute of Science - Bangalore, by XII Plan grant (#12-0509-0457-01), and at Faraday Laboratory LLC by financial support from Unify Inc. (04UNIFY04302018 onward). PG gratefully acknowledges financial support through the Kishore Vaigyanik Protsahan Yojana Scholarship (KVPY, 2012-2017). References: V.A. Sethuraman, M.J. Chon, M. Shimshak, V. Srinivasan, P.R. Guduru, J. Power Sources, 195, 5062 (2010). V.A. Sethuraman, V. Srinivasan, A.F. Bower, P.R. Guduru, J. Electrochem. Soc., 157, A1253 (2010). V.A. Sethuraman, M.J. Chon, N. Van Winkle, PR. Guduru, Electrochem. Comm., 12, 1614 (2010). V.A. Sethuraman, V. Srinivasan, J. Newman, J. Electrochem. Soc., 160, A394 (2013). S.P.V. Nadimpalli, V.A. Sethuraman, S. Dhalavi, B. Lucht, M.J. Chon, V.B. Shenoy, P.R. Guduru, J. Power Sources, 215, 145 (2012). Figure 1