Solvated pyromellitic acid-modified separator for stable lithium metal anodes and high-performance Li–S batteries

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

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

Unraveling the mechanical origin of stable solid electrolyte 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.

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

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

Conventional Li-ion electrolytes consist of carbonate solvents and Li-ion salts that were made for carbonaceous negative electrodes. When the electrolytes are used with metallic lithium anodes, carbonate electrolytes lead to low charge-discharge cycle stability due to adverse reactive species at the anode surface.1-4 This reactive species are called the solid electrolyte interface (SEI).1 SEI instabilities/inhomogeneities result in its continuous thickening and formation of electronically non-active dead lithium.2 SEI composition and structure are affected by both solvent and electrolyte chemistry in bulk solution and, more importantly, at the electrode-electrolyte interface.4 Current research studies suggest that X-rich (where X is a halogen) SEI yields superior performance compared to halogen-free SEI.1,5 Electrolytes that have large volume fractions of halogenated species have statistically higher probability to be reduced at the electrode surface and yield X-rich SEI layers.6 Therefore, the use of halogenated solvent and/or high concentrated (> 1 M) lithium salt with halogenated anions has been actively explored for LiMBs. However, the high cost of Li salts and high viscosity of the electrolyte at high salt concentration make the concentrated electrolytes unrealistic for commercial battery applications. In search of a cost-effective solution for high performance LiMBs, halogenated electrolyte additives could serve as an ideal approach since only a small amount of the additive would be potentially required to induce similar X-rich interface that can be seen in the case of the halogenated ether electrolyte. In this meeting abstract, we present a new concept that leverages favorable electrostatic interactions with the electrolyte additive to drive the formation of robust SEI layers even at low additive content. Specifically, custom-designed additives can be electrostatically attracted to the negatively charged electrode, creating a high population of halogenated species at the anode surface even at a dilute Li salt concentration. Effective SEI formation with a low-concentration additive circumvents the challenges associated with the current state-of-the-art approach of using halogenated species at high concentration (i.e., unfavorably high solution viscosity and high cost). Reference 1 Louli, A. J. et al. Diagnosing and correcting anode-free cell failure via electrolyte and morphological analysis. Nature Energy 5, 693-702, (2020).2 Chen, K.-H. et al. Dead lithium: mass transport effects on voltage, capacity, and failure of lithium metal anodes. Journal of Materials Chemistry A 5, 11671-11681, (2017).3 Lin, D. et al. Three-dimensional stable lithium metal anode with nanoscale lithium islands embedded in ionically conductive solid matrix. Proc Natl Acad Sci U S A 114, 4613-4618, (2017).4 Lin, D., Liu, Y. & Cui, Y. Reviving the lithium metal anode for high-energy batteries. Nat Nanotechnol 12, 194-206, (2017).5 Yu, Z. et al. Molecular design for electrolyte solvents enabling energy-dense and long-cycling lithium metal batteries. Nature Energy 5, 526-533, (2020).6 Yamada, Y., Wang, J., Ko, S., Watanabe, E. & Yamada, A. Advances and issues in developing salt-concentrated battery electrolytes. Nature Energy 4, 269-280, (2019).

Stable lithium metal anode achieved by shortening diffusion path on solid electrolyte interface derived from Cu2O lithiophilic layer

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.

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

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.

Over-lithiated organosulfides, such as sulfurized polyacrylonitrile (SPAN), are promising candidates of lithium metal anode (LMA) protection since they could form robust solid electrolyte interphases (SEIs), which is the key toward stable lithium metal batteries. So far, the mechanism of over-lithiation and evolution of the electrode surface is poorly understood. Herein, several in situ techniques were employed to study the over-lithiation process in SPAN, including in situ Raman spectroscopy to reveal the chemical transformation and in situ electrochemical atomic force microscopy (EC-AFM) to visualize interfacial evolution. The results undoubtedly prove the breaking of the C-S bond and formation of the C-Li bond during the over-lithiation process. The nucleophilic C-Li could further trigger the decomposition of the electrolyte to form an inorganic-organic hybrid SEI on the surface of SPAN, which allows uniform Li deposition and significantly improves the cycle stability of LMAs, as supported by the in situ EC-AFM characterization as well as a series of full cell tests. New insights into the over-lithiation mechanism of SPAN should facilitate the design of organosulfides to construct stable lithium metal anodes.

Lithium-metal batteries (LMBs) are promising electrochemical energy storage devices with high energy densities. However, the extreme reactivity of metallic lithium, the large volumetric change of the electrode during cycling, and the notorious dendrite formation issues lead to low cyclic stability and safety concerns, hindering the practical application of LMBs. In particular, the intrinsic tendency of uneven lithium deposition and the large internal electrode stress lead to the piecing of solid electrolyte interphases (SEIs), thereby resulting in fast decay of the anode. We develop a facile laser processing technique to fabricate laser-structured copper foils (LSCFs) that are able to regulate the lithium deposition kinetics and increase the cycle life of LMBs. By simply scribing commercial foils using a 355 nm laser, microstructural features with fish-scale patterns are obtained. The lithium deposition follows a drastically different mode on the LSCF compared with commercial planar copper foils which relieves the internal stress of lithium and prohibits the piecing of SEI. A high Coulombic efficiency of >96% of the lithium metal anode is maintained for over 100 cycles on the LSCF at a current density of 1 mA cm-2 and an areal capacity of 1 mAh cm-2 while the benchmark decayed to below 80% after 50 cycles. Full cells based on LiFePO4 cathodes display a reasonable specific capacity of 125 mAh g-1 over 300 cycles at a rate of 1 C. This work provides a fast yet effective laser-based approach to construct highly stable lithium metal anodes.

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.

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.