Air-Stable Lithium Spheres Produced by Electrochemical Plating

Engineering Triple-Phase Interfaces around the Anode toward Practical Alkali Metal-Air Batteries

Alkali metal-air batteries (AMABs) promise ultrahigh gravimetric energy densities, while the inherent poor cycle stability hinders their practical application. To address this challenge, most previous efforts are devoted to advancing the air cathodes with high electrocatalytic activity. Recent studies have underlined the solid-liquid-gas triple-phase interface around the anode can play far more significant roles than previously acknowledged by the scientific community. Besides the bottlenecks of uncontrollable dendrite growth and gas evolution in conventional alkali metal batteries, the corrosive gases, intermediate oxygen species, and redox mediators in AMABs cause more severe anode corrosion and structural collapse, posing greater challenges to the stabilization of the anode triple-phase interface. This work aims to provide a timely perspective on the anode interface engineering for durable AMABs. Taking the Li-air battery as a typical example, this critical review shows the latest developed anode stabilization strategies, including formulating electrolytes to build protective interphases, fabricating advanced anodes to improve their anti-corrosion capability, and designing functional separator to shield the corrosive species. Finally, the remaining scientific and technical issues from the prospects of anode interface engineering are highlighted, particularly materials system engineering, for the practical use of AMABs.

Recent Progress on the Air-Stable Battery Materials for Solid-State Lithium Metal Batteries

Solid-state lithium metal batteries (SSLMBs) offer numerous advantages in terms of safety and theoretical specific energy density. However, their main components namely lithium metal anode, solid-state electrolyte, and cathode, show chemical instability when exposed to humid air, which results in low capacities and poor cycling stability. Recent studies have shown that bioinspired hydrophobic materials with low specific surface energies can protect battery components from corrosion caused by humid air. Air-stable inorganic materials that densely cover the surface of battery components can also provide protection, which improves the storage stability of the battery components, broadens their processing conditions, and ultimately decreases their processing costs while enhancing their safety. In this review, the mechanism behind the surface structural degradation of battery components and the resulting consequences are discussed. Subsequently, recent strategies are reviewed to address this issue from the perspectives of lithium metal anodes, solid-state electrolytes, and cathodes. Finally, a brief conclusion is provided on the current strategies and fabrication suggestions for future safe air-stable SSLMBs.

In Situ Measurements of the Mechanical Properties of Electrochemically Deposited Li2CO3 and Li2O Nanorods

Solid-electrolyte interface (SEI) is "the most important but least understood (component) in rechargeable Li-ion batteries". The ideal SEI requires high elastic strength and can resist the penetration of a Li dendrite mechanically, which is vital for inhibiting the dendrite growth in lithium batteries. Even though Li₂CO₃ and Li₂O are identified as the major components of SEI, their mechanical properties are not well understood. Herein, SEI-related materials such as Li₂CO₃ and Li₂O were electrochemically deposited using an environmental transmission electron microscopy (ETEM), and their mechanical properties were assessed by in situ atomic force microscopy (AFM) and inverse finite element simulations. Both Li₂CO₃ and Li₂O exhibit nanocrystalline structures and good plasticity. The ultimate strength of Li₂CO₃ ranges from 192 to 330 MPa, while that of Li₂O is less than 100 MPa. These results provide a new understanding of the SEI and its related dendritic problems in lithium batteries.

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.

Lithium Deposition-Induced Fracture of Carbon Nanotubes and Its Implication to Solid-State Batteries

The increasing demand for safe and dense energy storage has shifted research focus from liquid electrolyte-based Li-ion batteries toward solid-state batteries (SSBs). However, the application of SSBs is impeded by uncontrollable Li dendrite growth and short circuiting, the mechanism of which remains elusive. Herein, we conceptualize a scheme to visualize Li deposition in the confined space inside carbon nanotubes (CNTs) to mimic Li deposition dynamics inside solid electrolyte (SE) cracks, where the high-strength CNT walls mimic the mechanically strong SEs. We observed that the deposited Li propagates as a creeping solid in the CNTs, presenting an effective pathway for stress relaxation. When the stress-relaxation pathway is blocked, the Li deposition-induced stress reaches the gigapascal level and causes CNT fracture. Mechanics analysis suggests that interfacial lithiophilicity critically governs Li deposition dynamics and stress relaxation. Our study offers critical strategies for suppressing Li dendritic growth and constructing high-energy-density, electrochemically and mechanically robust SSBs.

Highly Lithiophilic Copper-Reinforced Scaffold Enables Stable Li Metal Anode

Lithium (Li) metal is regarded as one of the most prospective electrodes for next-generation rechargeable batteries. However, its widespread usage has been fettered by low coulombic efficiency (CE), poor cycling stability, and serious safety concerns, mainly arising from huge volumetric variation, inhomogeneous Li deposition, and dendrite growth during repeated Li plating/stripping cycles. Herein, we propose a facile one-pot electrospinning-derived highly lithiophilic nanocopper-reinforced three-dimensional-structured carbon nanofiber (Cu-CNF) as functional scaffold to stabilize the Li metal. The Cu-CNF scaffolded Li metal demonstrates homogeneous nanoplate-like Li deposition, enhanced CE, and ultrastable long lifespan cycling. As coupled with LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811), the cell possesses a remarkably stable high capacity retention of 93% over 300 cycles at 0.2 C. Furthermore, the cells paired with a thick LiFePO₄ (LFP) electrode (∼12 mg cm^-2) still can deliver a superior cycling performance even under the harsh conditions of an extremely low negative/positive electrode capacity (N/P) ratio (∼1.5) and lean electrolyte. Density functional theory calculations are performed to disclose the mechanism of the enhanced electrochemical performance of Cu-CNF scaffolded Li. This work provides a handy and cost-effective method to design superior performance Li metal anodes for practical applications.

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 batteries^1,2, but the development of Li metal-based rechargeable batteries has achieved only limited success due to uncontrollable Li dendrite growth^3-7. 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 electrolytes^8,9. However, Li dendrites still grow through them^10,11. 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.