Atomistic Conversion Reaction Mechanism of WO3 in Secondary Ion Batteries of Li, Na, and Ca

Intercalation and conversion are two fundamental chemical processes for battery materials in response to ion insertion. The interplay between these two chemical processes has never been directly seen and understood at atomic scale. Here, using in situ HRTEM, we captured the atomistic conversion reaction processes during Li, Na, Ca insertion into a WO3 single crystal model electrode. An intercalation step prior to conversion is explicitly revealed at atomic scale for the first time for Li, Na, Ca. Nanoscale diffraction and ab initio molecular dynamic simulations revealed that after intercalation, the inserted ion–oxygen bond formation destabilizes the transition‐metal framework which gradually shrinks, distorts and finally collapses to an amorphous W and MxO (M=Li, Na, Ca) composite structure. This study provides a full atomistic picture of the transition from intercalation to conversion, which is of essential importance for both secondary ion batteries and electrochromic devices.Das Wechselspiel zwischen Ioneninterkalation und Umwandlung des WO3‐Elektrodenmaterials wurde durch In‐situ‐TEM auf atomarer Ebene untersucht. Die Bildung von Ion‐Sauerstoff‐Bindungen destabilisiert das WO3‐Gerüst: Es schrumpft, wird verzerrt und fällt schließlich zu einer amorphen W‐ und MxO‐Verbundstruktur (M=Li, Na, Ca) zusammen.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/134843/1/ange201601542_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/134843/2/ange201601542.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/134843/3/ange201601542-sup-0001-misc_information.pd

Atomistic Conversion Reaction Mechanism of WO3 in Secondary Ion Batteries of Li, Na, and Ca

Intercalation and conversion are two fundamental chemical processes for battery materials in response to ion insertion. The interplay between these two chemical processes has never been directly seen and understood at atomic scale. Here, using in situ HRTEM, we captured the atomistic conversion reaction processes during Li, Na, Ca insertion into a WO3 single crystal model electrode. An intercalation step prior to conversion is explicitly revealed at atomic scale for the first time for Li, Na, Ca. Nanoscale diffraction and ab initio molecular dynamic simulations revealed that after intercalation, the inserted ion–oxygen bond formation destabilizes the transition‐metal framework which gradually shrinks, distorts and finally collapses to an amorphous W and MxO (M=Li, Na, Ca) composite structure. This study provides a full atomistic picture of the transition from intercalation to conversion, which is of essential importance for both secondary ion batteries and electrochromic devices.The interplay between ion intercalation and WO3 battery electrode conversion was investigated at atomic scale by using in situ HRTEM. The ion–oxygen bond formation destabilizes the WO3 framework which gradually shrinks, distorts and finally collapses to an amorphous W and MxO (M=Li, Na, Ca) composite structure.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/135051/1/anie201601542.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/135051/2/anie201601542-sup-0001-misc_information.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/135051/3/anie201601542_am.pd

Atomic Resolution Study of Reversible Conversion Reaction in Metal Oxide Electrodes for Lithium-Ion Battery

Electrode materials based on conversion reactions with lithium ions have shown much higher energy density than those based on intercalation reactions. Here, nanocubes of a typical metal oxide (Co₃O₄) were grown on few-layer graphene, and their electrochemical lithiation and delithiation were investigated at atomic resolution by <i>in situ</i> transmission electron microscopy to reveal the mechanism of the reversible conversion reaction. During lithiation, a lithium-inserted Co₃O₄ phase and a phase consisting of nanosized Co–Li–O clusters are identified as the intermediate products prior to the subsequent formation of Li₂O crystals. In delithiation, the reduced metal nanoparticles form a network and breakdown into even smaller clusters that act as catalysts to prompt reduction of Li₂O, and CoO nanoparticles are identified as the product of the deconversion reaction. Such direct real-space, real-time atomic-scale observations shed light on the phenomena and mechanisms in reaction-based electrochemical energy conversion and provide impetus for further development in electrochemical charge storage devices

Size-Controlled Intercalation-to-Conversion Transition in Lithiation of Transition-Metal ChalcogenidesNbSe₃

Transition-metal chalcogenides (TMCs) can be used either as intercalation cathodes or as conversion-type anodes for lithium ion batteries, for which two distinctively different lithiation reaction mechanisms govern the electrochemical performance of TMCs. However, the factors that control the transition of lithiation mechanisms remain elusive. In this work, we investigated the lithiation process of NbSe₃ ribbons using <i>in situ</i> transmission electron microscopy and observed a size-dependent transition from intercalation to the conversion reaction. Large NbSe₃ ribbons can accommodate high concentrations of Li⁺ through intercalation by relaxing their internal spacing, while lithiation of small NbSe₃ ribbons proceeds readily to full conversion. We found that the size-dependent variation of the lithiation mechanism is associated with both Li⁺ diffusion in NbSe₃ and the accommodation of newly formed phases. For large NbSe₃ ribbons, the intercalation-to-conversion transition is impeded by both long-range Li⁺ diffusion and large-scale accommodation of volume expansion induced by the formation of new phases. These results demonstrate the inherent structural instability of NbSe₃ as an intercalation cathode and its high lithiation rate as a promising conversion-type anode

Revealing the Dynamics of Platinum Nanoparticle Catalysts on Carbon in Oxygen and Water Using Environmental TEM

Deactivation of supported metal nanoparticle catalysts, especially under relevant gas conditions, is a critical challenge for many technological applications, including heterogeneous catalysis, electrocatalysis, and fuel cells. It has been commonly realized that deactivation of catalysts stems from surface area loss due to particle coarsening; however, the mechanism for this remains largely unclear. Herein, we use aberration-corrected environmental transmission electron microscopy, at an atomic level, to observe in situ the dynamics of Pt catalysts under fuel cell relevant gas and temperature conditions. Particle migration and coalescence is observed to be the dominant coarsening process. In comparison with the case of H₂O, O₂ promotes Pt nanoparticle migration on the carbon surface. Surprisingly, coating Pt/carbon with a nanofilm of electrolyte (Nafion ionomer) leads to a faster migration of Pt in H₂O than in O₂, a consequence of a Nafion–carbon interface water “lubrication” effect. Atomically, the particle coalescence features reorientation of particles toward lattice matching, a process driven by orientation-dependent van der Waals forces. These results provide direct observations of the dynamics of metal nanoparticles at the critical surface/interface under relevant conditions and yield significant insights into the multiphase interaction in related technological processes

High Selenium Loading in Vertically Aligned Porous Carbon Framework with Visualized Fast Kinetics for Enhanced Lithium/Sodium Storage

Lithium/sodium–selenium (Li/Na–Se) batteries with high volumetric specific capacity are considered promising as next-generation battery technologies. However, their practical application is hindered by challenges such as low Se loading in cathodes and the polyselenides shuttle effect. To address these challenges, a new Se host is introduced in the form of a free-standing N, O co-doped vertically aligned porous carbon framework decorated with a carbon nanotube forest (VCF-CNTs), allowing for high mass loading of up to 16 mg cm−2. The low-tortuosity Se@VCF-CNTs architecture facilitates rapid lithiation/sodiation kinetics, while the CNT forests in vertical microchannels enhance efficient Se loading and serve as a multi-layer fence to prevent undesired polyselenide shuttling. Consequently, the Se@VCF-CNTs cathode displays a significant areal capacity of 10.3 mAh cm−2 at 0.1 C with a Se loading of 16 mg cm−2 for Li–Se batteries, exceeding that of commercial lithium ion batteries (4.0 mAh cm−2). In Na–Se batteries, the Se@VCF-CNTs electrode with a Se loading of 5 mg cm−2 exhibits a discharge capacity of 436 mAh g−1 after 200 cycles, proving its consistent cycling performance. This study enriches the field of knowledge concerning high-loading Se-based battery systems, offering a promising avenue for enhancing energy density in the field