Understanding the Lithiation of the Sn Anode for High-Performance Li-Ion Batteries with Exploration of Novel Li–Sn Compounds at Ambient and Moderately High Pressure

Volume expansion and elastic softening of the Sn anode on lithiation result in mechanical degradation and pulverization of Sn, affecting the overall performance of Li–Sn batteries. It can, however, be overcome with the help of void space engineering by using a Li<i>_x</i>Sn phase as the prelithiated anode, where an optimal value for <i>x</i> is desired. Currently, Li_4.25Sn is known as the most lithiated Li–Sn compound, but recent studies have shown that at high pressure, several exotic and unusual stoichiometries can be obtained that may even survive decompression from high-to-ambient pressure with improved mechanical properties. With a belief that hydrostatic pressure may help in realizing Li-richer (<i>x</i> > 4.25) Li–Sn compounds as well, we performed extensive calculations using an evolutionary algorithm and density functional theory to explore all stable and low-energy metastable Li–Sn compositions at pressures ranging from 1 atm to 20 GPa. This not only helped us in enriching the chemistry of a Li–Sn system, in general, but also in improving our understanding of the reaction mechanism in Li–Sn batteries, in particular, and guiding a route to improve the performance of Li-ion batteries through synthesis of Li-rich phases. Besides the experimentally known Li–Sn compounds, our study reveals the existence of five unreported stoichiometries (Li₈Sn₃, Li₃Sn₁, Li₄Sn₁, Li₅Sn₁, and Li₇Sn₁) and their associated crystal structures at ambient and high pressure. Although Li₈Sn₃ has been identified as one of the most stable Li–Sn compound in the entire pressure range (1 atm–20 GPa) with <i>R</i>3̅<i>m</i> symmetry, the Li-rich compounds like Li₃Sn₁-<i>P</i>2/<i>m</i>, Li₄Sn₁-<i>R</i>3̅<i>m</i>, Li₅Sn₁-<i>C</i>2/<i>m</i>, and Li₇Sn₁-<i>C</i>2/<i>m</i> are predicted to be metastable at ambient pressure and found to get thermodynamically stable at high pressure. Here, the discovery of Li₅Sn₁ and Li₇Sn₁ opens up the possibility to integrate them as a prelithiated anode for efficiently preventing electrochemical pulverization, as compared to the experimentally known highest lithiated compound, Li₁₇Sn₄

Dependence of the Structure and Electronic Properties of D–A–D Based Molecules on the D/A Ratio and the Strength of the Acceptor Moiety

A series of donor–acceptor–donor (D–A–D) scheme based organic molecules was studied to examine the dependence of molecular structure and electronic properties on the D/A ratio and the strength of the acceptor moiety, using first-principles density-functional-theory based calculations. Thiophenes were taken as the donor moiety and a series of benzo-X-diazoles and benzobis-X-diazoles (X = O, S, Se, and Te) were considered to account the strength of the acceptor moieties. The role of different exchange–correlation functionals was also investigated to search for the functional that best describes the properties of such D–A–D based molecules. Our systematic calculations reveal that both the D/A ratio and the strength of the acceptor moiety largely affect the energy gap between energies of the highest occupied molecular orbital (H) and the lowest unoccupied molecular orbital (L). In thiophene–benzo-X-diazole molecules, the H–L gap varies from 7% to 25%, whereas in thiophene–benzobis-X-diazoles, it can be tuned from 40% to 80%, by changing the D/A ratio from 0.5 to 4.0. In the latter case, higher steric hindrance (>50°) between A–A units disrupts the conjugation length with the increase in acceptor units. This leads to a monotonic decrease of the H–L gap with the increase in the D/A ratio, and a larger variation as compared to the case for thiophene–benzo-X-diazoles. On accounting for the effect of strength of the acceptor moiety, we observed that the H–L gap of the bis molecule was roughly 1 eV smaller than its respective non-bis configuration. A decrease in the H–L gap was also found on moving from S to Se to Te. Quantitatively, the H–L gap of the investigated molecules was found within a wide range of 0.2–2.4 eV, which not only is smaller than the H–L gap of isolated thiophene or the benzo-(bis)­X-diazole molecules but also lies in the desired range for the applications in optoelectronic devices, including solar cells. Thus, our study affirms that by choosing a suitable acceptor moiety and the D/A ratio, the structural and electronic properties of D–A–D based materials can be widely tuned. Through this work we emphasize the need to understand the tuning of molecular properties by examining the structure–property correlation, which is essential for rational design of high performing novel organic materials

Strategical Designing of Donor–Acceptor–Donor Based Organic Molecules for Tuning Their Linear Optical Properties

Low-energy linear absorption spectrum of a series of 48 donor–acceptor–donor (D–A–D) scheme based thiophone–benzo­(bis-)­X-diazole molecules with X = O, S, Se, or Te are calculated using time dependent density functional theory in order to propose strategical design of molecules that can efficiently absorb light in the infrared and visible region of the solar spectrum. Our study establishes that optical properties of the D–A–D based organic molecules significantly depend on the donor-to-acceptor (D/A) ratio and the strength of the acceptor moiety. Thus, by choice of a suitable D/A ratio and type of the acceptor moiety, the linear absorption spectrum can be largely shifted, in general, while the optical gap can be engineered over a wide energy range of ∼0.2–2.3 eV, in particular. It is also noticed that the increase in acceptor units (i.e., when D/A ≤ 1) leads to increase in steric hindrance in between them. This, in turn, disrupts the effective conjugation length and increases the optical gap. However, this effect is found to dominate strongly in the bis-configurations of the molecules as compared to the nonbis compositions. In order to reduce this effect for rational designing of effective D–A–D type chromophores with less steric hindrance, the role of π-conjugated ethylene (−CHCH−) linkage/spacer between the A–A units is explored further. Here, it is found that introduction of such linkage substantially decreases the steric hindrance and, thereby, the optical gap as well. Besides this, our study also highlights and explains the impact of the acceptor moiety in improving the absorption capabilities of these molecules in the low-energy region

Li Segregation Induces Structure and Strength Changes at the Amorphous Si/Cu Interface

The study of interfacial properties, especially of their change upon lithiation, is a fundamentally significant and challenging topic in designing heterogeneous nanostructured electrodes for lithium ion batteries. This issue becomes more intriguing for Si electrodes, whose ultrahigh capacity is accompanied by large volume expansion and mechanical stress, threatening with delamination of silicon from the metal current collector and failure of the electrode. Instead of inferring interfacial properties from experiments, in this work, we have combined density functional theory (DFT) and ab initio molecular dynamics (AIMD) calculations with time-of-flight secondary ion mass spectrometry (TOF-SIMS) measurements of the lithium depth profile, to study the effect of lithiation on the a-Si/Cu interface. Our results clearly demonstrate Li segregation at the lithiated a-Si/Cu interface (more than 20% compared to the bulk concentration). The segregation of Li is responsible for a small decrease (up to 16%) of the adhesion strength and a dramatic reduction (by one order of magnitude) of the sliding resistance of the fully lithiated a-Si/Cu interface. Our results suggest that this almost frictionless sliding stems from the change of the bonding nature at the interface with increasing lithium content, from directional covalent bonding to uniform metallic. These findings are an essential first step toward an in-depth understanding of the role of lithiation on the a-Si/Cu interface, which may contribute in the development of quantitative electrochemical mechanical models and the design of nonfracture-and-always-connected heterogeneous nanostructured Si electrodes