Ultrathin Bismuth Nanosheets for Stable Na-Ion Batteries: Clarification of Structure and Phase Transition by in Situ Observation

Bismuth has garnered tremendous interest for Na-ion batteries (NIBs) due to potentially high volumetric capacity. Yet, the bismuth upon sodiation/desodiation experiencing structure and phase transitions remains unclear, which sets a challenge for accessing nanotechnology and nanofabrication to achieve its applicability. Here, we use in situ transmission electron microscopy to disclose the structure and phase transitions of layered bismuth (few-layer bismuth nanosheets) during Na+ intercalation and alloying processes. Multistep phase transitions from Bi → NaBi → c-Na3Bi (cubic) → h-Na3Bi (hexagonal) are clearly identified, during which the Na+ migration from interlayer to in-plane evokes the structure transition from ABCABC stacking type of c-Na3Bi to ABABAB stacking type of h-Na3Bi. It is found that the metastable c-Na3Bi devotes to buffer the dramatic structure changes from thermodynamic stable h-Na3Bi, which unveils the origin of volume expansion for bismuth and has important consequences for 2D in-plane structure. As the lateral ductility can efficiently alleviate the in-plane mechanical strain caused by the Na+ migration, the few-layer bismuth nanosheet exhibits a potential cyclability for NIBs. Our findings will encourage more attention to bismuthene as a novel anode material for secondary batteries

Visualizing the Electrochemical Lithiation/Delithiation Behaviors of Black Phosphorus by <i>in Situ</i> Transmission Electron Microscopy

Black phosphorus (BP) has drawn growing attention as the anode material for lithium-ion batteries (LIBs) because of its high theoretical lithium storage capacity. However, its electrochemical processes and fundamental failure mechanisms have not been completely understood due to the lack of direct evidence. Here, we report the direct visualization of the electrochemical lithiation/delithiation behavior of the BP anode in nano-LIBs using the in situ transmission electron microscopy technique. Upon lithiation, the BP anode is found to undergo obvious anisotropic size expansion and phase change from orthorhombic BP to amorphous LixPy compounds. Unexpectedly, the BP anode pulverizes suddenly during discharging, resulting in irreversibility of the lithiated product and thus poor electrochemical cycling performance. This finding discloses that the failure mechanism of the BP anode is mainly correlated with the delithiation process rather than the lithiation one, which subverts the commonly accepted understanding. The new mechanism insights would serve to provide viable solutions for eliminating rapid capacity fading that plagues the bulk BP LIBs

Ultrathin Bismuth Nanosheets for Stable Na-Ion Batteries: Clarification of Structure and Phase Transition by in Situ Observation

Bismuth has garnered tremendous interest for Na-ion batteries (NIBs) due to potentially high volumetric capacity. Yet, the bismuth upon sodiation/desodiation experiencing structure and phase transitions remains unclear, which sets a challenge for accessing nanotechnology and nanofabrication to achieve its applicability. Here, we use in situ transmission electron microscopy to disclose the structure and phase transitions of layered bismuth (few-layer bismuth nanosheets) during Na+ intercalation and alloying processes. Multistep phase transitions from Bi → NaBi → c-Na3Bi (cubic) → h-Na3Bi (hexagonal) are clearly identified, during which the Na+ migration from interlayer to in-plane evokes the structure transition from ABCABC stacking type of c-Na3Bi to ABABAB stacking type of h-Na3Bi. It is found that the metastable c-Na3Bi devotes to buffer the dramatic structure changes from thermodynamic stable h-Na3Bi, which unveils the origin of volume expansion for bismuth and has important consequences for 2D in-plane structure. As the lateral ductility can efficiently alleviate the in-plane mechanical strain caused by the Na+ migration, the few-layer bismuth nanosheet exhibits a potential cyclability for NIBs. Our findings will encourage more attention to bismuthene as a novel anode material for secondary batteries

Visualizing the Electrochemical Lithiation/Delithiation Behaviors of Black Phosphorus by <i>in Situ</i> Transmission Electron Microscopy

Black phosphorus (BP) has drawn growing attention as the anode material for lithium-ion batteries (LIBs) because of its high theoretical lithium storage capacity. However, its electrochemical processes and fundamental failure mechanisms have not been completely understood due to the lack of direct evidence. Here, we report the direct visualization of the electrochemical lithiation/delithiation behavior of the BP anode in nano-LIBs using the <i>in situ</i> transmission electron microscopy technique. Upon lithiation, the BP anode is found to undergo obvious anisotropic size expansion and phase change from orthorhombic BP to amorphous Li_<i>x</i>P_<i>y</i> compounds. Unexpectedly, the BP anode pulverizes suddenly during discharging, resulting in irreversibility of the lithiated product and thus poor electrochemical cycling performance. This finding discloses that the failure mechanism of the BP anode is mainly correlated with the delithiation process rather than the lithiation one, which subverts the commonly accepted understanding. The new mechanism insights would serve to provide viable solutions for eliminating rapid capacity fading that plagues the bulk BP LIBs

Construction of air-stable pre-lithiated SiOx anodes for next-generation high-energy-density lithium-ion batteries

Due to the high energy density and low production cost, silicon oxide (SiOx) is recognized as one of the most promising anode materials for lithium-ion batteries. However, the low initial coulombic efficiency and rapid capacity attenuation of SiOx anodes severely restricts its commercial application. Here, we propose a scalable strategy of pre-lithiation followed by a thermal passivation to improve the initial coulombic efficiency of SiOx anodes, boosting the large-scale commercial applications of the pre-lithiation strategy. First, the hollow porous SiOx@C spheres (Hp-SiOx@C) with adjustable shell thickness are designed using a self-transformation method, and then an air-stable pre-lithiated Hp-SiOx@C (ASP-Hp-SiOx@C) anode is prepared through an electrochemical pre-lithiation followed by a thermal passivation strategy. The ASP-Hp-SiOx@C anode delivers high initial coulombic efficiency of 99.2% and stable cycling performance after being exposed to the atmosphere with 10%–20% relative humidity for 48 h

In Situ Visualization of Structural Evolution and Fissure Breathing in (De)lithiated H₂V₃O₈ Nanorods

Layered H2V3O8 material consisting of V3O8 layers features the elastic space for buffering volume change upon repeated ion (de)­intercalations. However, its ion transport and phase transformations still remain largely unknown due to lack of direct evidence. Here we employ in situ transmission electron microscopy to revisit this material carefully. Upon lithiation, the localized phase transformation from H2V3O8 to V2O3 via an intermediate VO2 phase was observed, and large structural fissures gradually formed. Unexpectedly, the large fissures were able to self-heal during delithiation with the VO2 phase as the delithiated product. The fissures could appear and disappear alternately upon subsequent (de)­lithiation, in which a stable and reversible phase transformation between V2O3 and VO2 phases was established. These unreported findings are expected to call for renewed attention to this electrode material for a more comprehensive understanding in rechargeable metal-ion batteries

In Situ Visualization of Structural Evolution and Fissure Breathing in (De)lithiated H₂V₃O₈ Nanorods

Layered H2V3O8 material consisting of V3O8 layers features the elastic space for buffering volume change upon repeated ion (de)­intercalations. However, its ion transport and phase transformations still remain largely unknown due to lack of direct evidence. Here we employ in situ transmission electron microscopy to revisit this material carefully. Upon lithiation, the localized phase transformation from H2V3O8 to V2O3 via an intermediate VO2 phase was observed, and large structural fissures gradually formed. Unexpectedly, the large fissures were able to self-heal during delithiation with the VO2 phase as the delithiated product. The fissures could appear and disappear alternately upon subsequent (de)­lithiation, in which a stable and reversible phase transformation between V2O3 and VO2 phases was established. These unreported findings are expected to call for renewed attention to this electrode material for a more comprehensive understanding in rechargeable metal-ion batteries

In Situ Visualization of Structural Evolution and Fissure Breathing in (De)lithiated H₂V₃O₈ Nanorods

Layered H2V3O8 material consisting of V3O8 layers features the elastic space for buffering volume change upon repeated ion (de)­intercalations. However, its ion transport and phase transformations still remain largely unknown due to lack of direct evidence. Here we employ in situ transmission electron microscopy to revisit this material carefully. Upon lithiation, the localized phase transformation from H2V3O8 to V2O3 via an intermediate VO2 phase was observed, and large structural fissures gradually formed. Unexpectedly, the large fissures were able to self-heal during delithiation with the VO2 phase as the delithiated product. The fissures could appear and disappear alternately upon subsequent (de)­lithiation, in which a stable and reversible phase transformation between V2O3 and VO2 phases was established. These unreported findings are expected to call for renewed attention to this electrode material for a more comprehensive understanding in rechargeable metal-ion batteries