Strontium Trifluoroacetate Additive Enables a Fluorine-Rich Interphase for Highly Stable Lithium Metal Anodes

Highly concentrated solutions composed of lithium bis(fluorosulfonyl)imide (LiFSI) and sulfolane (SL) are promising liquid electrolytes for lithium metal batteries because of their high anodic stability, low flammability, and high compatibility with lithium metal anodes. However, it is still challenging to obtain the stable lithium metal anodes in the concentrated electrolytes due to their poor wettability to the conventional polyolefin separators. Here, we report that the highly concentrated 1:2.5 LiFSI/SL electrolyte coupled with a three-dimensionally ordered macroporous polyimide (3DOM PI) separator enables the stable lithium plating/stripping cycling with an average Coulombic efficiency of ca. 98% for over 400 cycles at 1.0 mA cm-2. The 3DOM PI separator shows good electrolyte wettability and large electrolyte uptake due to its high porosity and polar constituent of the imide structure, allowing superior cycling performance in the highly concentrated solution, compared with the polyolefin separators. Electrochemical and spectroscopic analyses reveal that the superior cycling stability in the concentrated electrolyte is attributed to the formation of highly stable and Li+ ion conductive solid electrolyte interphase (SEI) layer derived from FSI- anions, which reduces the side reactions of SL with lithium metal, prevents the growth of lithium dendrites, and suppresses the increase in cell impedance over long-term cycling. Our findings demonstrate that polar and porous separators could effectively improve the affinity to the concentrated electrolytes and allow the formation of the anion-derived SEI layer by increasing the salt concentration of the electrolytes, achieving the long-term stable lithium metal anode.

Stable lithium metal anode enabled by a robust artificial fluorinated hybrid interphase

The uncontrollable growth of dendrites, infinite volume changes, low Coulombic efficiencies, and poor charging/discharging rates in lithium metal anodes have seriously hampered the further development of lithium metal batteries. Trapping lithium (Li) into rationally designed three‐dimensional (3D) structured Li metal anodes in order to construct a 3D‐Li framework is an effective approach to suppress the growth of Li dendrites. However, material inconsistencies and high costs still seriously limit practical applications. In this study, we describe the use of commercial low‐cost graphite fiber (GF) as a suitable conformal scaffold for preparing a lithiophilic SnO2@GF material using facile infiltration method. The lithiophilic 3D porous conductive framework allows homogeneous Li deposition on the surface of a structured electrode and accommodates the volume change during Li plating/stripping, leading to a significant boost in both the charging/discharging rates and cycling stability. This study highlights the significance of interface‐related science and engineering in designing high‐performance Li metal anodes, but also highlights the need for greater dedication to the construction of highly stable lithium anodes and high‐energy density Li metal batteries in a low‐cost manner.

Lithium fluoride additive for inorganic LiAlCl4·3SO2 electrolyte toward stable lithium metal anode

Lithium (Li) metal is among the most promising anode materials in next-generation high-energy-density energy-storage-systems due to its ultrahigh theoretical specific capacity of 3860 mAh g−1 and low negative electrochemical potential (−3.040 V vs. the standard hydrogen electrode). However, Li dendrite growth, “dead Li” formation and unstable solid electrolyte interphase (SEI) have hindered its practical applications.3D Structured lithium metal anodes, which possess customizable conductive framework for electron transfer and designable pore structures for ion transfer, have been widely proposed to settle these issues. We have constructed several carbon material based structured anodes to investigate the mechanisms in dendrite-free plating morphology, “dead Li”-free stripping morphology, and many other issues for achieving stable cycling 3D structured anodes. Based on the unstacked graphene framework, we found that the ultralow local current density induced by the high specific surface area of unstacked graphene can make great progress for stable and high-performance lithium metal anodes. Furthermore, with lithiophilic nitrogen-doped graphene framework, metallic Li nucleation can be regulated, then even plating morphology can be achieved. When apply the similar lithiophilic surface to a structural stable skeleton like carbon fibers, both Li dendrite growth and “dead Li” formation can be inhibited, and ultimately a high Coulombic efficiency can be achieved for high-capacity and high-rate Li metal batteries.However, the mechanisms of stable cycling 3D structured anodes which can guide the design of lithium metal anodes are heavily lacking due to the grand challenges of current trial-and-error investigation based on complex materials innovation. If a quantitative theoretical analysis can be proposed, reliable lithium metal anodes with 3D host is highly expected. Thus, theoretical calculation such as phase field models are also employed to quantitatively describe the lithium plating and stripping process in various conductive structured lithium anodes. We found that structured lithium metal anodes with larger areal surface area and smaller pore-volumetric surface area can be much better for high rate and high capacity battery cycling.Beyond the design and adjustment of lithium metal anodes, further experiments and simulations are required in revealing the mechanisms in lithium metal anodes, such as Li plating and stripping process, dendrite growth, SEI formation and its impact, etc., not only for 3D structured anodes. These mechanism investigations are promising for high-energy-density lithium metal batteries like Li–S and Li–O2 batteries.

Unraveling the mechanical origin of stable solid electrolyte interphase

All‐solid‐state batteries (ASSB) require stable and safe lithium (Li) metal anode, which needs surface preparation to increase lithium diffusion and impede the formation of dendrites. In this work, the formation of a thin LiZn layer on lithium metal using sputter deposition is reported. This method was selected due to the absence of solvents and by‐products generated during the modification, for its rapidity and because the formation of the alloy is performed in a clean and controlled atmosphere. Zinc has been chosen for its low cost and high Li+ ion diffusion coefficient of the corresponding LiZn alloy that is 1000 times higher than Li. Different parameters for the Zn deposition were investigated such as the distance between the Zn target and Li foil, the effect of substrate tilt and the direct current applied to the target. Electrochemical performance of LiFePO4/solid polymer electrolyte/Li ASSB demonstrated the superiority of the LiZn anodes and the clear influence of deposition parameters on the durability and performance at high C‐rates. Scanning electron microscopy images of the cross‐sectional view of LFP/SPE/Li stackings extracted from pouch cells after cycling showed an evident migration of Zn into the bulk Li metal anode as well as the formation of AlZn nanoparticles.

In situ formation of lithiophilic Li22Sn5 alloy and high Li-ion conductive Li2S/Li2Se via metal chalcogenide SnSSe for dendrite-free Li metal anodes

Lithiophilic V2O5 nanobelt arrays decorated 3D framework hosts for highly stable composite lithium metal anodes

Inhibition of lithium dendrites and dead lithium by an ionic liquid additive toward safe and stable lithium metal anodes

Evaluating Solid-Electrolyte Interphases for Lithium and Lithium-free Anodes from Nanoindentation Features

Practical applications of lithium metal anodes are gravely impeded by inhomogeneous lithium deposition, which results in dendrite growth. Electrolyte additives are proven to be effective in improving performance but usually serve only a single function. Herein, nitrofullerene is introduced as a bifunctional additive with a smoothing effect and forms a protective solid electrolyte interphase (SEI) layer on stable lithium metal anodes. By design, nitro-C60 can gather on electrode protuberances via electrostatic interactions and then be reduced to NO2 − and insoluble C60. Next, the C60 anchors on the uneven groove of the lithium surface, resulting in a homogeneous distribution of Li ions. Finally, NO2 – anions can react with metallic Li to build a compact and stable SEI with high ion transport. With a 5 mM nitro-C60 additive, Li−Li symmetric cells show superior cycle stability in both carbonate and ether electrolytes, Li−sulfur batteries with a high cathode loading (10.6 mg cm− 2 , 6 mAh cm− 2) can achieve improved cycle retention of 63.2% over 100 cycles in a carbonate electrolyte, and full cells paired with a high-areal-capacity LiNi0.6Co0.2Mn0.2O2 cathode (3.5 mAh cm− 2) exhibit a significantly enhanced cycle lifespan even under lean electrolyte conditions. This work not only develops a new insight into the application of fullerene but also provides a novel design strategy for electrolyte additives with multiple functions for stable lithium metal anodes. Reference: Jiang, Z.; Zeng, Z.; Yang, C.; Han, Z.; Hu, W.; Lu, J.; Xie, J. Nano Lett. 2019, 19, 8780−8786. Figure 1

Construction of copper oxynitride nanoarrays with enhanced lithiophilicity toward stable lithium metal anodes

Lithium metal is the most ideal anode for next‐generation lithium‐ion batteries. However, the formation of lithium dendrites and the continuous consumption of electrolyte during cycling lead to a serious safety problems. Developing stable lithium metal anode with uniform lithium deposition is highly desirable. Herein, a nitrogen plasma strengthening strategy is proposed for copper oxide nanosheet–decorated Cu foil as an advanced current collector, and deep insights into the plasma regulating mechanism are elaborated. The plasma‐treated electrode can maintain a high coulombic efficiency of 99.6% for 500 cycles. The symmetric cell using the lithium‐plated electrode can be cycled for more than 600 h with a low‐voltage hysteresis (23.1 mV), which is much better than those of electrodes without plasma treatment. It is well confirmed that this plasma‐induced nitrogen doping method can provide sufficient active sites for lithium nucleation to enhance the stability of lithium deposition on copper oxide nanosheets decorated on Cu foil and improve the electrical conductivity to greatly reduce the overpotential of the lithium nucleation, which can be extended to other modified current collectors for stable lithium metal anode.

Stable metal anode cycling for high energy density batteries can be realized through modification of electrolyte composition and optimization of formation protocols, i.e., electrode interphase preconditioning conditions. However, the relationship between these and the electrochemical performance is still unclear due to a lack of molecular level understanding of electric double layer (EDL) changes with modification of these two parameters. Herein, we examine the impact of ionic liquid (IL) electrolyte composition (salt concentration and cosolvent) and preconditioning cycling conditions on Li anode performance through EDL changes affecting both the solid–electrolyte interphase (SEI) and deposition morphology. Each electrolyte composition results in a particular interfacial Li-ion solvation environment, which controls the reductive stability, Li deposition potential, and ultimately the composition of properties of the SEI. The latter is dependent on the EDL composition such as the IL cation/Li-anion ratio or the presence of other surface active additives. It is found that in a superconcentrated electrolyte, a high current density (≥10.0 mA cm–2/1.0 mAh cm–2) is beneficial during the metal anode preconditioning step, compared with the case of low Li salt-containing IL. This correlates with a predominance of Lix(anion)y (x > y) at a highly negatively charged interface, which is present when higher current densities are used for preconditioning, as suggested by molecular dynamics simulations. In contrast, for the lower viscosity superconcentrated electrolyte containing 20 wt % of ether cosolvent, a more moderate preconditioning step current density (6.0 mA cm–2/1.0 mAh cm–2) leads to an optimized deposition morphology and improved cycling performance. This is a consequence of the competing processes of ion transport at the interface, which controls the Li+ ion flux and the intrinsic reduction kinetics occurring at the more negative electrode.