Accurate Determination of Coulombic Efficiency for Lithium Metal Anodes and Lithium Metal Batteries

Lithium (Li) metal is an ideal anode material for high energy density batteries. However, the low Coulombic efficiency (CE) and the formation of dendrites during repeated plating and stripping processes have hindered its applications in rechargeable Li metal batteries. The accurate measurement of Li CE is a critical factor to predict the cycle life of Li metal batteries, but the measurement of Li CE is affected by various factors that often lead to conflicting values reported in the literature. Here, several parameters that affect the measurement of Li CE are investigated and a more accurate method of determining Li CE is proposed. It is also found that the capacity used for cycling greatly affects the stabilization cycles and the average CE. A higher cycling capacity leads to faster stabilization of Li anode and a higher average CE. With a proper operating protocol, the average Li CE can be increased from 99.0% to 99.5% at a high capacity of 6 mA h cm^?2 (which is suitable for practical applications) when a high‐concentration ether‐based electrolyte is used.     

Plating/Stripping Behavior of Actual Lithium Metal Anode

Lithium (Li) metal anodes exhibits the potential to enable rechargeable Li batteries with a high energy density. However, the irreversible plating and stripping behaviors of Li metal anodes with high reactivity and dendrite growth when matching different cathodes in working cells are not fully understood yet. Herein the working manner of very thin Li metal anodes (50 µm, 10 mAh cm^?2) is probed with different sequences of Li plating and stripping at 3.0 mA cm^?2 and 3.0 mAh cm^?2. Dendrite growth and dead Li forms on the surface of the initially plated Li electrode (P‐Li), while Li dendrites form in the pit of the initially stripped Li electrode (S‐Li). This induces the differences in reactive sites, distribution of dead Li, and voltage polarization of Li metal anodes. There is a gap of 15–20 and 13–16 mV for the end voltages between S‐Li and P‐Li during stripping and plating, respectively. When matching LiFePO₄ and FePO₄ cathodes, P‐Li | LiFePO₄ cells exhibit a 30‐cycle longer lifespan with smaller end polarization due to differences in the sequences of Li plating and stripping. This contribution affords emerging working principles for actual Li metal anodes when matching lithium‐containing and lithium‐free cathodes.     

A Coaxial‐Interweaved Hybrid Lithium Metal Anode for Long‐Lifespan Lithium Metal Batteries

Lithium (Li) metal is one of the most promising anode materials to construct next‐generation rechargeable batteries owing to its ultrahigh theoretical capacity and the lowest electrochemical potential. Unfortunately, practical application of Li metal batteries is severely hindered by short lifespan and safety concerns caused by Li dendrite growth during cycling. Herein, a coaxial‐interweaved hybrid Li metal anode is proposed for dendrite inhibition that significantly improves the cycling stability of Li metal batteries. The hybrid Li metal anode is fabricated by Li composition into a 3D interweaved scaffold, where each fiber of the interwoven scaffold is composed of a conductive skeleton and a coaxial lithiophilic layer modified on the surface. The coaxial‐interweaved structure endows the hybrid anode with favored Li affinity to guide uniform Li deposition, sufficient channels for ion transportation and electron conduction, and enhanced stability during Li plating and stripping. Consequently, the hybrid Li metal anode affords high Coulombic efficiency over 98.5% for 750 cycles with dendrite‐free morphologies in half cells and improved capacity retention of 80.1% after 100 cycles in LiFePO₄ full cells. The innovative coaxial‐interweaved hybrid Li metal anode demonstrates multiscale design strategy from lithiophilic modification to scaffold construction and promises the prospect of Li metal batteries for future applications.     

Cross Talk between Transition Metal Cathode and Li Metal Anode: Unraveling Its Influence on the Deposition/Dissolution Behavior and Morphology of Lithium

Lithium metal batteries (LMBs) combining a Li metal anode with a transition metal (TM) cathode can achieve higher practical energy densities (Wh L^?1) than Li/S or Li/O₂ cells. Research for improving the electrochemical behavior of the Li metal anode by, for example, modifying the liquid electrolyte is often conducted in symmetrical Li/Li or Li/Cu cells. This study now demonstrates the influence of the TM cathode on the Li metal anode, thus full cell behavior is analyzed in a way not considered so far in research with LMBs. Therefore, the deposition/dissolution behavior of Li metal and the resulting morphology is investigated with three different cathode materials (LiNi_0.5Mn_1.5O₄, LiNi_0.6Mn_0.2Co_0.2O₂, and LiFePO₄) by post mortem analysis with a scanning electron microscope. The observed large differences of the Li metal morphology are ascribed to the dissolution and crossover of TMs found deposited on Li metal and in the electrolyte by X‐ray photoelectron spectroscopy, energy‐dispersive X‐ray spectroscopy, and total reflection X‐ray fluorescence analysis. To support this correlation, the TM dissolution is simulated by adding Mn salt to the electrolyte. This study offers new insights into the cross talk between the Li metal anodes and TM cathodes, which is essential, when investigating Li metal electrodes for LMB full cells.     

Temperature-Mediated Dynamic Lithium Loss and its Implications for High-Efficiency Lithium Metal Anodes

Lithium (Li) metal has been strongly regarded as the ultimate anode option for next-generation high-energy-density batteries. Nevertheless, the insufficient Coulombic efficiency induced by the extensive active Li loss largely hinders the practical operation of Li metal batteries under wide temperature range. Herein, the temperature-mediated dynamic growth of inactive Li from −20 to 60°C via titration gas chromatograph measurements is quantitatively decoupled. Combined X-ray photoelectronic spectroscopy, cryo-transmission electronic microscopy, and scanning electronic microscopy methods depicted that both solid electrolyte interphase (SEI) characteristics and Li deposition compactness can be profoundly manipulated by working temperature. The elevation of temperature is found to fundamentally aggravate the parasitic reactions and deteriorate the spatial uniformity of SEI, yet promote the lateral growth of Li by kinetic reason. The opposite effects of temperature on SEI properties and Li deposition compactness can properly explain the intricate temperature-dependent growth rates of SEI-Li⁺ and dead Li⁰ capacity loss observed under titration gas chromatograph measurements. Design implications towards more stable Li metal anodes with higher reversibility can thus be yielded.     

In Situ NMR Verification for Stacking Pressure-Induced Lithium Deposition and Dead Lithium in Anode-Free Lithium Metal Batteries

Anode-free lithium metal batteries have emerged as strong contenders for next-generation rechargeable batteries due to their ultra-high energy density. However, their safety and life span are insufficient because of the easy generation of dendrites and dead lithium during lithium plating and stripping. Understanding the formation mechanism for lithium dendrites and dead lithium is essential to further improve battery performance. By employing in situ solid-state nuclear magnetic resonance (NMR) spectroscopy, the influence of stacking pressure on dendritic behavior and dead lithium is systematically investigated. At 0.1 MPa, lithium dendrite is rapidly formed, followed by a linear increase of dead lithium. High stacking pressure not only causes lithium metal to fracture but also leads to form dendrites and dead lithium at the fracture site. At 0.5 MPa stacking pressure, the least quantity of dead lithium is attained, and the growth pattern of dead lithium is exponential growth. The exponential growth pattern is distinguished by the high growth of dead lithium early in the battery cycle and essentially no growth later in the cycle. As a result, it is believed that efficient suppression of dead lithium generation early in battery cycling can play a critical role in improving battery performance.     

Uniform High Ionic Conducting Lithium Sulfide Protection Layer for Stable Lithium Metal Anode

Artificial solid‐electrolyte interphase (SEI) is one of the key approaches in addressing the low reversibility and dendritic growth problems of lithium metal anode, yet its current effect is still insufficient due to insufficient stability. Here, a new principle of “simultaneous high ionic conductivity and homogeneity” is proposed for stabilizing SEI and lithium metal anodes. Fabricated by a facile, environmentally friendly, and low‐cost lithium solid‐sulfur vapor reaction at elevated temperature, a designed lithium sulfide protective layer successfully maintains its protection function during cycling, which is confirmed by both simulations and experiments. Stable dendrite‐free cycling of lithium metal anode is realized even at a high areal capacity of 5 mAh cm^?2, and prototype Li–Li₄Ti₅O₁₂ cell with limited lithium also achieves 900 stable cycles. These findings give new insight into the ideal SEI composition and structure and provide new design strategies for stable lithium metal batteries.     

Uniform Lithium Deposition Assisted by Single‐Atom Doping toward High‐Performance Lithium Metal Anodes

For a long time lithium (Li) metal has been considered one of the most promising anodes for next‐generation rechargeable batteries. Despite decades of concentrated research, its practical application is still hindered by dendritic Li deposition and infinite volume change of Li metal anodes. Here, atomically dispersed metals doped graphene is synthesized to regulate Li metal nucleation and guide Li metal deposition. The single‐atom (SA) metals, supported on the nitrogen‐doped graphene can not only increase the Li adsorption energy of the localized area around the metal atomic sites with a moderate adsorption energy gradient but also improve the atomic structural stability of the overall materials by constructing a coordination mode of M‐N_x‐C (M, N, and C denoted as metal, nitrogen, and carbon atoms, respectively). As a result, the as‐obtained electrode exhibits an ultralow voltage hysteresis of 19 mV, a high average Coulombic efficiency of 98.45% over 250 cycles, and a stable Li plating/stripping performance even at a high current density of 4.0 mA cm^?2. This work demonstrates the application of SA metal doping in the rational design of Li metal anodes and provides a new concept for further development of Li metal batteries.     

Microscale Lithium Metal Stored inside Cellular Graphene Scaffold toward Advanced Metallic Lithium Anodes

The volume expansion and dendrite growth of metallic Li anode during charge/discharge processes hinder its practical application in energy storage. Seeking an appropriate host for distributing bulk Li in a 3D manner is an effective way to solve these problems. Here, a novel porous graphene scaffold with cellular chambers for incorporating Li metal is presented. Using such a unique host, ultrathin Li layers of 3 µm in thickness are anchored on graphene to form porous microstructures, which provides much more reaction sites for Li ions compared with that of bulk Li, significantly promoting the reversibility of Li stripping and plating. Also the high current density can be effectively dissipated by the graphene scaffold to remarkably improve the rate capability of Li anode. The symmetrical Li cell using such a Li anode can run stably for 200 cycles at 5 mA cm^?2 and even 70 cycles at 10 mA cm^?2 in an unmodified carbonate‐based electrolyte, which has rarely been achieved in such aggressive working conditions. Lithium‐ion capacitor cells using this anode also show outstanding rate capability and cycling stability, which can work at an ultrahigh current density of 30 A g^?1 and keep steady for over 4000 cycles at 3.75 A g^?1.     

Interface Engineering for Lithium Metal Anodes in Liquid Electrolyte

Interfacial chemistry between lithium metal anodes and electrolytes plays a vital role in regulating the Li plating/stripping behavior and improving the cycling performance of Li metal batteries. Constructing a stable solid electrolyte interphase (SEI) on Li metal anodes is now understood to be a requirement for progress in achieving feasible Li‐metal batteries. Recently, the application of novel analytical tools has led to a clearer understanding of composition and the fine structure of the SEI. This further promoted the development of interface engineering for stable Li metal anodes. In this review, the SEI formation mechanism, conceptual models, and the nature of the SEI are briefly summarized. Recent progress in probing the atomic structure of the SEI and elucidating the fundamental effect of interfacial stability on battery performance are emphasized. Multiple factors including current density, mechanical strength, operating temperature, and structure/composition homogeneity that affect the interfacial properties are comprehensively discussed. Moreover, strategies for designing stable Li‐metal/electrolyte interfaces are also reviewed. Finally, new insights and future directions associated with Li‐metal anode interfaces are proposed to inspire more revolutionary solutions toward commercialization of Li metal batteries.     

Vertically Aligned Lithiophilic CuO Nanosheets on a Cu Collector to Stabilize Lithium Deposition for Lithium Metal Batteries

Uncontrollable dendrite growth hinders the direct use of a lithium metal anode in batteries, even though it has the highest energy density of all anode materials. Achieving uniform lithium deposition is the key to solving this problem, but it is hard to be realized on a planar electrode surface. In this study, a thin lithiophilic layer consisting of vertically aligned CuO nanosheets directly grown on a planar Cu current collector is prepared by a simple wet chemical reaction. The lithiophilic nature of the CuO nanosheets reduces the polarization of the electrode, ensuring uniform Li nucleation and continuous smooth Li plating, which is difficult to realize on the normally used lithiophobic Cu current collector surface. The integration of the grown CuO arrays and the Cu current collector guarantees good electron transfer, and moreover, the vertically aligned channels between the CuO nanosheets guarantee fast ion diffusion and reduce the local current density. As a result, a high Columbic efficiency of 94% for 180 cycles at a current density of 1 mA cm^?2 and a prolonged lifespan of a symmetrical cell (700 h at 0.5 mA cm^?2) can be easily achieved, showing a simple but effective way to realize Li metal‐based anode stabilization.