In situ observation of the electrochemical lithiation of a single MnO@C nanorod electrode with core/shell structure.

Transition metal oxides (TMOs) play a crucial role in lithium-ion batteries (LIBs) due to their high theoretical capacity, natural abundance, and benign environmental impact, but they suffer from limitations such as cyclability and high-rate discharge ability. One leading cause is the lithiation-induced volume expansion (LIVE) for "conversion"-type TMOs, which can result in high stress, fracture and pulverization. Using carbon layers is an effective strategy to provide effective volumetric accommodation for lithium-ion (Li+) insertion; however, the detailed mechanism is unknown. In order to clarify the working mechanism of nanoscale LIBs, herein, the discharge reactions in a nanoscale LIB were investigated through in situ environmental transmission electron microscopy (ETEM). Visualization of the Li+ insertion process of MnO@C nanorods (NRs) with core/shell structure (CSS) and internal void space (IVS) was achieved. The LIVE occurred in a consecutive two-step mode, i.e., a LIVE of the carbon layer followed by a co-LIVE of the carbon layer and MnO. No volume contraction of the IVS was observed. The IVS acted as a buffer relieving the stress of the carbon layer. The carbon layer with IVS simultaneously improved the cyclability and the high-rate discharge ability of the electrode, pointing to a promising route for building better TMO electrode materials.

In situ observation of cracking and self-healing of solid electrolyte interphases during lithium deposition.

The growth of lithium (Li) whiskers is detrimental to Li batteries. However, it remains a challenge to directly track Li whisker growth. Here we report in situ observations of electrochemically induced Li deposition under a CO2 atmosphere inside an environmental transmission electron microscope. We find that the morphology of individual Li deposits is strongly influenced by the competing processes of cracking and self-healing of the solid electrolyte interphase (SEI). When cracking overwhelms self-healing, the directional growth of Li whiskers predominates. In contrast, when self-healing dominates over cracking, the isotropic growth of round Li particles prevails. The Li deposition rate and SEI constituent can be tuned to control the Li morphologies. We reveal a new "weak-spot" mode of Li dendrite growth, which is attributed to the operation of the Bardeen-Herring growth mechanism in the whisker's cross section. This work has implications for the control of Li dendrite growth in Li batteries.

A 2.9 GPa Strength Nano-Grained and Nano-Precipitated 304L-Type Austenitic Stainless Steel.

Austenitic stainless steel has high potential as nuclear and engineering materials, but it is often coarse grained and has relatively low yield strength, typically 200-400 MPa. We prepared a bulk nanocrystalline lanthanum-doped 304L austenitic stainless steel alloy by a novel technique that combines mechanical alloying and high-pressure sintering. The achieved alloy has an average grain size of 30 ± 12 nm and contains a high density (~1024 m-3) of lanthanum-enriched nanoprecipitates with an average particle size of approx. 4 nm, leading to strong grain boundary strengthening and dispersion strengthening effects, respectively. The yield strength of nano-grained and nano-precipitated stainless steel reaches 2.9 GPa, which well exceeds that of ultrafine-grained (100-1000 nm) and nano-grained (<100 nm) stainless steels prepared by other techniques developed in recent decades. The strategy to combine nano-grain strengthening and nanoprecipitation strengthening should be generally applicable to developing other ultra-strong metallic alloys.

In situ imaging electrocatalytic CO2 reduction and evolution reactions in all-solid-state Li-CO2 nanobatteries.

Li-CO2 batteries are promising energy storage devices owing to their high energy density and possible applications for CO2 capture. However, still some critical issues, such as high charging overpotential and poor cycling stability caused by the sluggish decomposition of Li2CO3 discharge products, need to be addressed before the practical applications of Li-CO2 batteries. Exploring highly efficient catalysts and understanding their catalytic mechanisms for the CO2 reduction reaction (CORR) and evolution reaction (COER) are critical for the application of Li-CO2 batteries. However, the direct imaging of electrocatalysis during CORR and COER is still elusive. Herein, we report the in situ imaging of electrocatalysis during CORR and COER in a Li-CO2 nanobattery using a Ni-Ru-coated α-MnO2 nanowire (Ni-Ru/MnO2) cathode in an advanced aberration corrected environmental transmission electron microscope. During the CORR, a thick Li2CO3 and carbon mixture layer was formed on the surface of the Ni-Ru/MnO2 nanowires via 4Li+ + 3CO2 + 4e-→ 2Li2CO3 + C. During the COER, the as-formed Li2CO3 decomposed via 2Li2CO3→ 2CO2 + O2 + 4Li+ + 4e-, while the as-formed amorphous carbon remained. In contrast, the decomposition of Li2CO3 on bare MnO2 nanowires was difficult, underscoring the important Ni-Ru bimetal electrocatalytic role in facilitating the COER. Our results provide an important understanding of the CO2 chemistry in Li-CO2 batteries, possibly helping in the designing of Li-CO2 batteries for energy storage applications.

In Situ Electrochemical Study of Na-O2/CO2 Batteries in an Environmental Transmission Electron Microscope.

Metal-air batteries are potential candidates for post-lithium energy storage devices due to their high theoretical energy densities. However, our understanding of the electrochemistry of metal-air batteries is still in its infancy. Herein we report in situ studies of Na-O2/CO2 (O2 and CO2 mixture) and Na-O2 batteries with either carbon nanotubes (CNTs) or Ag nanowires as the air cathode medium in an advanced aberration corrected environmental transmission electron microscope. In the Na-O2/CO2-CNT nanobattery, the discharge reactions occurred in two steps: (1) 2Na+ + 2e- + O2 → Na2O2; (2) Na2O2+ CO2 → Na2CO3 + O2; concurrently a parasitic Na plating reaction took place. The charge reaction proceeded via (3) 2Na2CO3 + C → 4Na+ + 3CO2 + 4e-. In the Na-O2/CO2-Ag nanobattery, the discharge reactions were essentially the same as those for the Na-O2/CO2-CNT nanobattery; however, the charge reaction in the former was very sluggish, suggesting that direct decomposition of Na2CO3 is difficult. In the Na-O2 battery, the discharge reaction occurred via reaction 1, but the reverse reaction was very difficult, indicating the sluggish decomposition of Na2O2. Overall the Na-O2/CO2-CNT nanobattery exhibited much better cyclability and performance than the Na-O2/CO2-Ag and the Na-O2-CNT nanobatteries, underscoring the importance of carbon and CO2 in facilitating the Na-O2 nanobatteries. Our study provides important understanding of the electrochemistry of the Na-O2/CO2 and Na-O2 nanobatteries, which may aid the development of high performance Na-O2/CO2 and Na-O2 batteries for energy storage applications.

Lithium whisker growth and stress generation in an in situ atomic force microscope-environmental transmission electron microscope set-up.

Lithium metal is considered the ultimate anode material for future rechargeable batteries1,2, but the development of Li metal-based rechargeable batteries has achieved only limited success due to uncontrollable Li dendrite growth3-7. In a broad class of all-solid-state Li batteries, one approach to suppress Li dendrite growth has been the use of mechanically stiff solid electrolytes8,9. However, Li dendrites still grow through them10,11. Resolving this issue requires a fundamental understanding of the growth and associated electro-chemo-mechanical behaviour of Li dendrites. Here, we report in situ growth observation and stress measurement of individual Li whiskers, the primary Li dendrite morphologies12. We combine an atomic force microscope with an environmental transmission electron microscope in a novel experimental set-up. At room temperature, a submicrometre whisker grows under an applied voltage (overpotential) against the atomic force microscope tip, generating a growth stress up to 130 MPa; this value is substantially higher than the stresses previously reported for bulk13 and micrometre-sized Li14. The measured yield strength of Li whiskers under pure mechanical loading reaches as high as 244 MPa. Our results provide quantitative benchmarks for the design of Li dendrite growth suppression strategies in all-solid-state batteries.

Turbostratic carbon-localised FeS2 nanocrystals as anodes for high-performance sodium-ion batteries.

Low-cost metal sulfides are promising anode materials for sodium-ion batteries (SIBs); however, they suffer from sluggish kinetics and large volume expansion upon cycling. Here, a strategy to grow FeS2 on turbostratic carbon (t-carbon) assisted by chemical interactions between Fe and C electrons was realized via a simple and scalable mechanical alloying (MA) approach with a trace amount of CNTs. The structural change in CNTs synchronized with the in situ growth of FeS2 on the transformed t-carbon during the MA process, forming localised FeS2 nanocrystals wrapped in the frameworks of t-carbon. This intertwined structure within a grain consisted of chemical bonding by electronic hybridization between FeS2 and its adjacent carbon layer, resulting in enhanced structural stability upon cycling. Moreover, the localised FeS2 nanocrystals with an ultrasmall diameter of 10 nm encapsulated in the nanoframeworks of t-carbon could effectively shorten the diffusion paths of electrons/ions and withstand the volume expansion. The as-synthesized FeS2-C hybrid composite electrode exhibited a pseudocapacitive diffusion behavior with high specific capacity, good cycling stability, and remarkable rate capability. This strategy is a facile, scalable, and low-cost route toward high-performance metal sulfide anode materials for the commercial utilization of SIBs.

Ultrastrong nanocrystalline steel with exceptional thermal stability and radiation tolerance.

Nanocrystalline (NC) metals are stronger and more radiation-tolerant than their coarse-grained (CG) counterparts, but they often suffer from poor thermal stability as nanograins coarsen significantly when heated to 0.3 to 0.5 of their melting temperature (Tm). Here, we report an NC austenitic stainless steel (NC-SS) containing 1 at% lanthanum with an average grain size of 45 nm and an ultrahigh yield strength of ~2.5 GPa that exhibits exceptional thermal stability up to 1000 °C (0.75 Tm). In-situ irradiation to 40 dpa at 450 °C and ex-situ irradiation to 108 dpa at 600 °C produce neither significant grain growth nor void swelling, in contrast to significant void swelling of CG-SS at similar doses. This thermal stability is due to segregation of elemental lanthanum and (La, O, Si)-rich nanoprecipitates at grain boundaries. Microstructure dependent cluster dynamics show grain boundary sinks effectively reduce steady-state vacancy concentrations to suppress void swelling upon irradiation.

Radiation tolerance of La-doped nanocrystalline steel under heavy-ion irradiation at different temperatures.

Nanostructured materials have great potential for use as structural materials in advanced nuclear reactors due to the high density of grain boundaries that can serve as sinks to absorb irradiation-induced defects. In the present study, the irradiation tolerance of a La-doped nanocrystalline 304 austenitic stainless steel (NC-La) with a grain size of about 40 nm was investigated under an irradiation of 6 MeV Au ions to 1.5 × 1016 ions cm-2 at 600 °C and room temperature. Compared to its coarse-grained counterpart, in La-doped nanocrystalline steel no visible voids were observed at high-temperature irradiation, and no significant difference in extended defects, such as irradiation-induced dislocation loops or clusters, were found between irradiated and unirradiated areas at room temperature irradiation. Furthermore, the nano grain remains stable under irradiation, and no significant grain growth occurs at both irradiation temperatures. The excellent irradiation tolerance of the La-doped nanocrystalline alloys is attributed to the abundant grain boundaries and enhanced stability of nano grains induced by the Zener pinning effect and La segregation on grain boundaries. This study therefore demonstrates the superior irradiation tolerance of the La-doped nanocrystalline steel.

Air-Stable Lithium Spheres Produced by Electrochemical Plating.

Lithium metal is an ideal anode for next-generation lithium batteries owing to its very high theoretical specific capacity of 3860 mAh g-1 but very reactive upon exposure to ambient air, rendering it difficult to handle and transport. Air-stable lithium spheres (ASLSs) were produced by electrochemical plating under CO2 atmosphere inside an advanced aberration-corrected environmental transmission electron microscope. The ASLSs exhibit a core-shell structure with a Li core and a Li2 CO3 shell. In ambient air, the ASLSs do not react with moisture and maintain their core-shell structures. Furthermore, the ASLSs can be used as anodes in lithium-ion batteries, and they exhibit similar electrochemical behavior to metallic Li, indicating that the surface Li2 CO3 layer is a good Li+ ion conductor. The air stability of the ASLSs is attributed to the surface Li2 CO3 layer, which is barely soluble in water and does not react with oxygen and nitrogen in air at room temperature, thus passivating the Li core.

In Situ Imaging the Oxygen Reduction Reactions of Solid State Na-O2 Batteries with CuO Nanowires as the Air Cathode.

We report real time imaging of the oxygen reduction reactions (ORRs) in all solid state sodium oxygen batteries (SOBs) with CuO nanowires (NWs) as the air cathode in an aberration-corrected environmental transmission electron microscope under an oxygen environment. The ORR occurred in a distinct two-step reaction, namely, a first conversion reaction followed by a second multiple ORR. In the former, CuO was first converted to Cu2O and then to Cu; in the latter, NaO2 formed first, followed by its disproportionation to Na2O2 and O2. Concurrent with the two distinct electrochemical reactions, the CuO NWs experienced multiple consecutive large volume expansions. It is evident that the freshly formed ultrafine-grained Cu in the conversion reaction catalyzed the latter one-electron-transfer ORR, leading to the formation of NaO2. Remarkably, no carbonate formation was detected in the oxygen cathode after cycling due to the absence of carbon source in the whole battery setup. These results provide fundamental understanding into the oxygen chemistry in the carbonless air cathode in all solid state Na-O2 batteries.

Ceramic nanowelding.

Ceramics possess high temperature resistance, extreme hardness, high chemical inertness and a lower density compared to metals, but there is currently no technology that can produce satisfactory joints in ceramic parts and preserve the excellent properties of the material. The lack of suitable joining techniques for ceramics is thus a major road block for their wider applications. Herein we report a technology to weld ceramic nanowires, with the mechanical strength of the weld stronger than that of the pristine nanowires. Using an advanced aberration-corrected environmental transmission electron microscope (ETEM) under a CO2 environment, we achieved ceramic nanowelding through the chemical reaction MgO + CO2 → MgCO3 by using porous MgO as the solder. We conducted not only nanowelding on MgO, CuO, and V2O5 nanowires and successfully tested them in tension, but also macroscopic welding on a ceramic material such as SiO2, indicating the application potential of this technology in bottom-up ceramic tools and devices.

A quenchable superhard carbon phase synthesized by cold compression of carbon nanotubes.

A quenchable superhard high-pressure carbon phase was synthesized by cold compression of carbon nanotubes. Carbon nanotubes were placed in a diamond anvil cell, and x-ray diffraction measurements were conducted to pressures of approximately 100 GPa. A hexagonal carbon phase was formed at approximately 75 GPa and preserved at room conditions. X-ray and transmission electron microscopy electron diffraction, as well as Raman spectroscopy at ambient conditions, explicitly indicate that this phase is a sp(3)-rich hexagonal carbon polymorph, rather than hexagonal diamond. The cell parameters were refined to a(0) = 2.496(4) A, c(0) = 4.123(8) A, and V(0) = 22.24(7) A (3). There is a significant ratio of defects in this nonhomogeneous sample that contains regions with different stacking faults. In addition to the possibly existing amorphous carbon, an average density was estimated to be 3.6 +/- 0.2 g/cm(3), which is at least compatible to that of diamond (3.52 g/cm(3)). The bulk modulus was determined to be 447 GPa at fixed K' identical with 4, slightly greater than the reported value for diamond of approximately 440-442 GPa. An indented mark, along with radial cracks on the diamond anvils, demonstrates that this hexagonal carbon is a superhard material, at least comparable in hardness to cubic diamond.