Self-Assembled Framework Formed During Lithiation of SnS₂ Nanoplates Revealed by in Situ Electron Microscopy

ConspectusLithium-ion batteries (LIBs) commercially dominate portable energy storage and have been extended to hybrid/electric vehicles by utilizing electrode materials with enhanced energy density. However, the energy density and cycling life of LIBs must extend beyond the current reach of commercial electrodes to meet the performance requirements for transportation applications. Carbon-based anodes, serving as the main negative electrodes in LIBs, have an intrinsic capacity limitation due to the intercalation mechanism. Some nanostructured carbon materials offer very interesting reversible capacities and can be considered as future anode materials. However, their fabrication processes are often complicated and expensive. Theoretically, using a lithium metal anode is the best way of delivering high energy density due to its largest theoretical capacity of more than 3800 mAh g^–1; however, lithium metal is highly reactive with liquid electrolytes. Alternative anodes are being explored, including other lithium-reactive metals, such as Si, Ge, Zn, V, and so forth. These metals react reversibly with a large amount of Li per formula unit to form lithium–metal alloys, rendering these materials promising candidates for next-generation LIBs with high energy density. Though, most of these pure metallic anodes experience large volume changes during lithiation and delithiation processes that often results in cracking of the anode material and a loss electrical contact between the particles.Nanosized metal sulfides were recently found to possess better cycling stability and larger reversible capacities over pure metals. Further improvements and developments of metal sulfide-based anodes rely on a fundamental understanding of their electrochemical cycling mechanisms. Not only must the specific electrochemical reactions be correctly identified, but also the microstructural evolution upon electrochemical cycling, which often dictates the cyclability and stability of nanomaterials in batteries, must be clearly understood. Probing these dynamic evolution processes, i.e. the lithiation reactions and morphology evolutions, are often challenging. It requires both high-resolution chemical analysis and microstructural identification. In situ transmission electron microscopy (TEM) coupled with electron energy loss spectroscopy (EELS) has recently been raised as one of the most powerful techniques for monitoring electrochemical processes in anode materials for LIBs.In this work, we focus on elucidating the origin of the structural stability of SnS₂ during electrochemical cycling by revealing the microstructural evolution of SnS₂ upon lithiation using in situ TEM. Crystalline SnS₂ was observed to undergo a two-step reaction after the initial lithium intercalation: (1) irreversible formation of metallic tin and amorphous lithium sulfide and (2) reversible transformation of metallic tin to Li–Sn alloys, which is determined to be the rate-determining step. More interestingly, it was discovered that a self-assembled composite framework formed during the irreversible conversion reaction, which has not been previously reported. Crystalline Sn nanoparticles are well arranged within an amorphous Li₂S “matrix” in this self-assembled framework. This nanoscale framework confines the locations of individual Sn nanoparticles and prevents particle agglomeration during the subsequent cycling processes, therefore providing desired structural tolerance and warranting a sufficientelectron pathway. Our results not only explain the outstanding cycling stability of SnS₂ over metallic tin anodes, but also provide important mechanistic insights into the design of high-performance electrodes for next-generation LIBs through the integration of a unique nanoframework

Electron Beam Etching of CaO Crystals Observed Atom by Atom

With the rapid development of nanoscale structuring technology, the precision in the etching reaches the sub-10 nm scale today. However, with the ongoing development of nanofabrication the etching mechanisms with atomic precision still have to be understood in detail and improved. Here we observe, atom by atom, how preferential facets form in CaO crystals that are etched by an electron beam in an in situ high-resolution transmission electron microscope (HRTEM). An etching mechanism under electron beam irradiation is observed that is surprisingly similar to chemical etching and results in the formation of nanofacets. The observations also explain the dynamics of surface roughening. Our findings show how electron beam etching technology can be developed to ultimately realize tailoring of the facets of various crystalline materials with atomic precision

Interfacial Stability of Li Metal–Solid Electrolyte Elucidated via in Situ Electron Microscopy

Despite their different chemistries, novel energy-storage systems, e.g., Li–air, Li–S, all-solid-state Li batteries, etc., face one critical challenge of forming a conductive and stable interface between Li metal and a solid electrolyte. An accurate understanding of the formation mechanism and the exact structure and chemistry of the rarely existing benign interfaces, such as the Li–cubic-Li_{7–3<i>x</i>}Al_<i>x</i>La₃Zr₂O₁₂ (c-LLZO) interface, is crucial for enabling the use of Li metal anodes. Due to spatial confinement and structural and chemical complications, current investigations are largely limited to theoretical calculations. Here, through an in situ formation of Li–c-LLZO interfaces inside an aberration-corrected scanning transmission electron microscope, we successfully reveal the interfacial chemical and structural progression. Upon contact with Li metal, the LLZO surface is reduced, which is accompanied by the simultaneous implantation of Li⁺, resulting in a tetragonal-like LLZO interphase that stabilizes at an extremely small thickness of around five unit cells. This interphase effectively prevented further interfacial reactions without compromising the ionic conductivity. Although the cubic-to-tetragonal transition is typically undesired during LLZO synthesis, the similar structural change was found to be the likely key to the observed benign interface. These insights provide a new perspective for designing Li–solid electrolyte interfaces that can enable the use of Li metal anodes in next-generation batteries

Spring-Like Pseudoelectroelasticity of Monocrystalline Cu₂S Nanowire

Prediction from the dual-phase nature of superionic conductorsboth solid and liquid-likeis that mobile ions in the material may experience reversible extraction–reinsertion by an external electric field. However, this type of pseudoelectroelasticity has not been confirmed <i>in situ</i>, and no details on the microscopic mechanism are known. Here, we <i>in situ</i> monitor the pseudoelectroelasticity of monocrystalline Cu₂S nanowires (NWs) using transmission electron microscopy (TEM). Specifically, we reveal the atomic scale details including phase transformation, migration and redox reactions of Cu⁺ ions, nucleation, growth, as well as spontaneous shrinking of Cu protrusion. Caterpillar-diffusion-dominated deformation is confirmed by the high-resolution transmission electron microscopy (HRTEM) observation and <i>ab initio</i> calculation, which can be driven by either an external electric field or chemical potential difference. The observed spring-like behavior was creatively adopted for electric nanoactuators. Our findings are crucial to elucidate the mechanism of pseudoelectroelasticity and could potentially stimulate in-depth research into electrochemical and nanoelectromechanical systems

Spring-Like Pseudoelectroelasticity of Monocrystalline Cu₂S Nanowire

Prediction from the dual-phase nature of superionic conductorsboth solid and liquid-likeis that mobile ions in the material may experience reversible extraction–reinsertion by an external electric field. However, this type of pseudoelectroelasticity has not been confirmed <i>in situ</i>, and no details on the microscopic mechanism are known. Here, we <i>in situ</i> monitor the pseudoelectroelasticity of monocrystalline Cu₂S nanowires (NWs) using transmission electron microscopy (TEM). Specifically, we reveal the atomic scale details including phase transformation, migration and redox reactions of Cu⁺ ions, nucleation, growth, as well as spontaneous shrinking of Cu protrusion. Caterpillar-diffusion-dominated deformation is confirmed by the high-resolution transmission electron microscopy (HRTEM) observation and <i>ab initio</i> calculation, which can be driven by either an external electric field or chemical potential difference. The observed spring-like behavior was creatively adopted for electric nanoactuators. Our findings are crucial to elucidate the mechanism of pseudoelectroelasticity and could potentially stimulate in-depth research into electrochemical and nanoelectromechanical systems

Spring-Like Pseudoelectroelasticity of Monocrystalline Cu₂S Nanowire

Prediction from the dual-phase nature of superionic conductorsboth solid and liquid-likeis that mobile ions in the material may experience reversible extraction–reinsertion by an external electric field. However, this type of pseudoelectroelasticity has not been confirmed <i>in situ</i>, and no details on the microscopic mechanism are known. Here, we <i>in situ</i> monitor the pseudoelectroelasticity of monocrystalline Cu₂S nanowires (NWs) using transmission electron microscopy (TEM). Specifically, we reveal the atomic scale details including phase transformation, migration and redox reactions of Cu⁺ ions, nucleation, growth, as well as spontaneous shrinking of Cu protrusion. Caterpillar-diffusion-dominated deformation is confirmed by the high-resolution transmission electron microscopy (HRTEM) observation and <i>ab initio</i> calculation, which can be driven by either an external electric field or chemical potential difference. The observed spring-like behavior was creatively adopted for electric nanoactuators. Our findings are crucial to elucidate the mechanism of pseudoelectroelasticity and could potentially stimulate in-depth research into electrochemical and nanoelectromechanical systems

New Nanoconfined Galvanic Replacement Synthesis of Hollow Sb@C Yolk–Shell Spheres Constituting a Stable Anode for High-Rate Li/Na-Ion Batteries

In the current research project, we have prepared a novel Sb@C nanosphere anode with biomimetic yolk–shell structure for Li/Na-ion batteries via a nanoconfined galvanic replacement route. The yolk–shell microstructure consists of Sb hollow yolk completely protected by a well-conductive carbon thin shell. The substantial void space in the these hollow Sb@C yolk–shell particles allows for the full volume expansion of inner Sb while maintaining the framework of the Sb@C anode and developing a stable SEI film on the outside carbon shell. As for Li-ion battery anode, they displayed a large specific capacity (634 mAh g^–1), high rate capability (specific capabilities of 622, 557, 496, 439, and 384 mAh g^–1 at 100, 200, 500, 1000, and 2000 mA g^–1, respectively) and stable cycling performance (a specific capacity of 405 mAh g^–1 after long 300 cycles at 1000 mA g^–1). As for Na-ion storage, these yolk–shell Sb@C particles also maintained a reversible capacity of approximate 280 mAh g^–1 at 1000 mA g^–1 after 200 cycles

New Nanoconfined Galvanic Replacement Synthesis of Hollow Sb@C Yolk–Shell Spheres Constituting a Stable Anode for High-Rate Li/Na-Ion Batteries

In the current research project, we have prepared a novel Sb@C nanosphere anode with biomimetic yolk–shell structure for Li/Na-ion batteries via a nanoconfined galvanic replacement route. The yolk–shell microstructure consists of Sb hollow yolk completely protected by a well-conductive carbon thin shell. The substantial void space in the these hollow Sb@C yolk–shell particles allows for the full volume expansion of inner Sb while maintaining the framework of the Sb@C anode and developing a stable SEI film on the outside carbon shell. As for Li-ion battery anode, they displayed a large specific capacity (634 mAh g^–1), high rate capability (specific capabilities of 622, 557, 496, 439, and 384 mAh g^–1 at 100, 200, 500, 1000, and 2000 mA g^–1, respectively) and stable cycling performance (a specific capacity of 405 mAh g^–1 after long 300 cycles at 1000 mA g^–1). As for Na-ion storage, these yolk–shell Sb@C particles also maintained a reversible capacity of approximate 280 mAh g^–1 at 1000 mA g^–1 after 200 cycles