Transformation Plasticity Provides Insights into Concurrent Phase Transformation and Stress Relaxation Observed during Electrochemical Li Alloying of Sn Thin Film

Abstract

Stress development during electrochemical alloying/insertion of “guest species” into electrode materials is known to considerably affect the performance and integrity of the concerned electrode. Monitoring of the average in-plane stress developments in-operando during electrochemical Li alloying/dealloying of electrodes that undergo nucleation/growth-induced phase transformations as a function of Li concentration shows that such stresses nearly cease to build-up (i.e., hit stress “plateaus”) during the phase transformations, in contrast to the nearly monotonic buildup of stresses during the single-phase (i.e., solid solution forming) regimes. To understand such observation and establish correlations between the composition, phase transformation, and stress development (in the case of lithiated Sn and Sn–Li intermetallic phases), comprehensive sets of mathematical modeling have been performed by considering lithiation as a diffusion process and the film to be elastic, perfectly plastic material. The observation of stress “plateaus” in the in-operando experiments has been satisfactorily explained and matched with the computed stress profiles based on the combined effects of Eigen strain during phase transformation and transformation-induced plasticity. Overall, it can now be concluded that the “stress flattening” observed in electrode materials during the electrochemical alloying process is primarily due to localized plasticity arising from the associated nucleation/growth-induced phase transformations.

Operando Investigations toward Changeover of Phase Assemblage and Associated Electrochemical Behavior during Lithiation/Delithiation Cycles of Sn-Based Intermetallic Electrodes

“…An interesting observation is the “inactive” nature of the peaks corresponding to β-Sn, which, in fact, agrees with the potential profiles showing complete dissimilarity with that expected for lithiation/delithiation of Sn per se (compare Figures , S2, and S3 with Figure a). On a similar note, stress profiles recorded in situ during galvanostatic lithiation/delithiation cycles of Sn/Cu–Sn composite electrodes were also dissimilar to those recorded with pure Sn electrodes (as reported in our previously published works ,, ). The “stress profiles” refer to bi-axial stress developments in the concerned electrode-active materials as monitored in real time during electrochemical Li-alloying/dealloying cycles (invoking principles of “substrate curvature”), with further details concerning the mechanistic aspects and technique used being available in a couple of our review articles. , Overall, this new observation concerning the “deactivation” of Sn toward lithiation/delithiation in the presence of the Cu–Sn intermetallic phase will be a subject of further investigation, with a possible reason being complete encapsulation of the remnant Sn with the newly formed Cu 41 Sn 11 phase.…”

“…Distinct potential plateaus at ∼0.7, ∼0.6, and ∼0.46 V (vs Li/Li + ) could be observed during the lithiation half cycle, with reverse plateaus being observed during delithiation. These plateaus are associated with first-order phase transformations between Sn and various Sn–Li intermetallic phases that form during lithiation/delithiation and are known to be particularly detrimental toward the mechanical stability of Sn-based electrodes. ,, No wonder, after an initial reversible Li-storage capacity of ∼800 mA h/g ( viz. , in the first delithiation half cycle), the capacity drops down to <200 mA h/g in the very second cycle and to negligible value after ∼10 cycles, pointing to very poor cyclic stability, which is typical of pure Sn-based film electrodes.…”

“…An island-type morphology can be observed for the as-deposited Sn, which is the usually observed form. 19,20,23,24 By contrast, films of a more uniform thickness/ morphology can be observed for the annealed films on top of the Cu substrate. The slightly brighter appearance of Sn, having higher atomic number with respect to that of Cu, helps distinguish between the Sn-containing film and the Cu substrate in the cross-sectional images.…”

“…These plateaus are associated with first-order phase transformations between Sn and various Sn−Li intermetallic phases that form during lithiation/delithiation and are known to be particularly detrimental toward the mechanical stability of Sn-based electrodes. 19,23,24 No wonder, after an initial reversible Listorage capacity of ∼800 mA h/g (viz., in the first delithiation half cycle), the capacity drops down to <200 mA h/g in the very second cycle and to negligible value after ∼10 cycles, pointing to very poor cyclic stability, which is typical of pure Sn-based film electrodes. Totally different electrochemical behavior and performance can be noted for the annealed films, which consist of primarily ε-Cu 3 Sn as the "active" phase (as shown in Figures 3, S2 and S3 in the Supporting Information).…”

In-Situ Studies toward the Occurrence of “Pseudoelasticity” in Confined Nanostructured NiTi Films and Its Implications toward “Stress Buffering” during Electrochemical Li-Alloying/De-Alloying of Si

“…Notably, NiTi as a buffer “matrix” reportedly led to some improvement in the cyclic stability of Si, possibly due to the associated pseudoelasticity, in the work by Jung et al Subsequently, Loka et al also reported improvement in cyclic stability of a Si-based anode having ∼35% NiTi particles (mixed by high-energy ball-milling), with ex situ nanoindentation measurements providing some evidence toward the occurrence of pseudoelasticity in the NiTi. In the context of Sn-based electrodes (having similar stress-related issues during lithiation/delithiation , ), Hu et al , reported improvements in cyclic stability in the presence of bulk NiTi, as matrix and also as an interlayer between Sn and the current collector. Furthermore, using a “sandwich structured” NiTi/SnO 2 /NiTi electrode, Hu et al also reported the possibility of suppressing the coarsening/agglomeration of Sn particles formed during conversion (lithiation) reaction of SnO 2 , thus alleviating the irreversible capacity loss and cyclic instability.…”