Anion Concentration–Regulated Interface Stabilization Enabling High‐Performance Lithium Metal Batteries

Lithium (Li) metal is an ideal anode material for rechargeable batteries due to its extremely high theoretical specific capacity (3860 mAh/g), low density (0.59 g/cm3) and the lowest negative electrochemical potential (-3.04 V vs. the standard hydrogen electrode, SHE). Li battery based on Li metal anode is expected to have an order of magnitude higher energy density than today's Li-ion battery. This will enhance energy and power capability of battery devices used in portable electronics, hybrid electric cars, aerospace, telecommunication, military, etc.). The Li metal battery will fulfill the high energy demand that cannot be met through the current Li-ion batteries (e.g. all electric transportation or many other applications which require light weight, high energy density, high power density batteries with small battery footprint and safest). But, to date, uncontrolled dendritic Li growth and limited Coulombic efficiency (CE) during Li deposition/stripping of a bulk Li anode, are the main reason behind low cycle life, which has limited their practical applications. Furthermore, the rate of dendrite growth gets amplified when the cell is cycled at high charge-discharge current density, high charge voltage, and high areal discharge. Increasing anode surface area would reduce the effective charge/discharge current. Additionally, thin films and high surface area Li anodes exhibit high ionic and electronic conductivity contributing to the high efficiency of the current collection required for high capacity battery. Although low CE can be partially compensated by an excess amount of Li (at the expense of lower useable Li energy), fire and other hazards associated with the dendrite growth have limited the development of rechargeable Li metal batteries. Most approaches to dendrite prevention focus on improving the stability and uniformity of the solid-electrolyte interface (SEI) on the bulk Li surface by adjusting electrolyte components and optimizing SEI formation additives. It is very difficult to achieve sufficient passivation on Li electrode due to the thermodynamic instability of Li with electrolytes [1,2]. As an alternative, various mechanical barriers have been proposed to block dendrite penetration [3]. These approaches rely on a strong mechanical barrier provided by an SEI film or a separator to suppress Li dendrite penetration but do not change the fundamental, self-amplifying behavior of the dendrite growth. Also, it is hard to find a mechanical barrier material with required shear modulus (required > 109 Pa) with desired conductivity. In other words, these methods did not prevent Li dendrites from growing during long-term cycling at high charge-discharge current density, high charge voltage, and high areal discharge, or hardly improved the CE and safety in Li deposition/stripping which is not suitable for practical rechargeable Li batteries [1]. We have developed a novel ionic-electronic-mechanical membrane (an artificial SEI), which allows long-term Li cycleability and improved safety by reducing dendrite formation. The new artificial SEI is comprised of three different materials to fulfill the desired SEI criteria such as: (a) facilitate uniform Li ion dispersion and deposition on entire Li surface, (b) desired SEI conductivity (c) mechanical separation of Li and electrolyte and flexibility to contain anode volume change during Li stripping and deposition. The combined effect of the invented SEI result into more than 300% increase in number of cycles and 3-4 times increase in usable Li energy density (i.e. high Coulombic efficiency (CE)) of a wet battery composed of protected Li metal anode, liquid electrolyte (e.g. lithium hexafluoro arsenate, LiAsF6) and Li-ion cathode (e.g. lithium cobalt oxide, LCO) for the advancement of a battery beyond the state-of-the-art Li-ion battery. The added advantage of the invented SEI is improved safety due to suppression of dendrite growth by mechanically strong/flexible SEI and controlled, uniform deposition/stripping of Li from Li anode surface. The protected Li anode may be incorporated into wet cell batteries or solid state batteries having transition metal oxide cathodes (more specifically the state-of-the-art Li-ion cathodes) or Sulfur cathode (Li-S battery) or Oxygen cathode (Li-O2 battery). Moreover, similar methodologies can be applied to other rechargeable metal anodes (Na, Al, Mg, etc.) used in batteries.

Li metal is the ultimate anode material for rechargeable Li batteries because of its high capacity and low electrochemical potential. However, Li metal anodes suffer from low Coulombic efficiency and poor cycling stability owing to the growth of Li dendrites. In this study, we report that a localized high-concentration electrolyte comprising lithium bis(fluorosulfonyl)imide (LiFSI), ethylene carbonate (EC), propylene carbonate (PC), and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE) achieves stable Li plating/stripping cycling with a Coulombic efficiency of >98 %. In contrast to LiFSI/EC : PC electrolytes, this electrolyte shows good wettability on a polypropylene separator. Li metal deposited in this electrolyte displays a large, granular, and dense morphology. Spectroscopic analyses confirm strong FSI−–Li+ coordination in this electrolyte, leading to the formation of a solid electrolyte interphase (SEI) layer enriched with LiF and sulfurous compounds derived from FSI−. These results indicate that the SEI layer facilitates the deposition of compact Li and effectively prevents Li loss owing to electrolyte decomposition and dead Li formation, resulting in highly reversible Li plating/stripping cycling. This electrolyte design can be an effective strategy for developing high-energy-density Li metal batteries.

Electrochemical energy conversion and storage is critical for advancing vehicle electrification and sustainability of essential commodity chemicals. In pursuit of these goals, I delved progressively deeper into the foundational research of batteries and beyond: (1) pioneering novel methodologies to fundamentally understand reactive metal deposition, (2) regulating electrolyte to enhance lithium (Li) metal batteries (LMBs) performance, and (3) expanding the technology’s applicability to make impact beyond batteries.I have revealed the intrinsic morphology of electrodeposited Li metal to be a non-dendritic rhombic dodecahedron (Yuan et al. Nature 620, 86-91, 2023), which is surprisingly independent of electrolyte chemistry and current collector substrate. Li deposition is a process in which Li-ions are reduced to metallic form at the electrode, which plays a crucial role for LMBs, because the reversibility of deposition morphology directly determines the cycling performance and safety of the battery. However, the simultaneous formation of a surface corrosion film termed the solid electrolyte interphase (SEI) complicates the deposition process, which underpins our poor understanding of Li metal electrodeposition. I creatively integrated the classical electrochemical method, ultramicroelectrode geometry, and an emerging electron microscopy technique, cryogenic electron microscopy (cryo-EM), to decouple Li deposition from the SEI growth and capture the corresponding nanostructure of Li. This discovery has significant implications for LMBs, as it suggests SEI influence can be effectively mitigated to achieve desired deposition morphologies. Besides enhancing our understanding of Li deposition, I extended this finding to other reactive alkali metal, such as sodium metal that was found to exhibit the same polyhedral structure when outpacing the SEI influence. This emphasizes the applicability of my findings and opens new opportunities to explore how reactive metal deposition fundamentally proceeds without the influence of corrosion film, thereby regulating reversibility of metal deposition to optimize the performance of metal batteries.In addition to deposition morphology, the properties of the SEI also influence the performance of LMBs, as the SEI governs the transportation of Li-ion during cycling. Since the SEI formation results from electrolyte decomposition influenced by electric fields, I systematically examined effects of both electrolyte decomposition and electric fields on SEI formation, progressing from the bulk electrolyte to the electrode surface (Yuan et al. Joule 8, 3038-3053, 2024). I worked with colleagues to quantify reactions driving SEI formation and determine decomposition rates of individual components within the electrolyte, guiding us design and prioritize the decomposition of electrolyte components. Furthermore, I emphasized the importance of electric field to further fine-tune the formation of favorable SEI to improve battery performance.Looking beyond applications of Li metal in batteries, I leveraged the wealth of battery knowledge and established strategies to investigate fundamental aspects of Li metal as electrocatalyst in electrifying ammonia synthesis to help decarbonize the traditional chemical industry (Steinberg, Yuan (co-first) et al. Nature Energy 8, 138-148, 2023). I revealed key driver behind surface phenomena is the rupture of the SEI, enabling nitrogen and electrolyte to penetrate and react with Li metal to make ammonia. The insights from this work expanded our perspective of how Li metal electrodeposition can decarbonize chemical synthesis and inform our future efforts in designing better LMBs as well.

The use of ultrathin lithium (Li) metal anode in Li metal batteries (LMBs) has the potential to significantly improve the energy density in comparison to the conventional LMBs. However, they possess several challenges such as intrinsic dendrite growth and dead Li, leading to poor cyclability and coulombic efficiency (CE). In addition, the ultrathin Li metal can cause much faster degradation of performances than thicker one owing to the exhaustion of Li resource with less compensation. To address these problems, silver trifluoromethanesulfonate (AgCF 3 SO 3 , AgTFMS) is proposed as a functional electrolyte additive in carbonate‐based electrolyte to buffer the dendritic Li growth and to provide enhanced cyclability. Interestingly, Ag metal derived from the AgTFMS exhibits lithiophilic properties through an alloying reaction with Li. Furthermore, the CF 3 functional group of AgTFMS generates a physically stable LiF‐rich solid‐electrolyte interphase (SEI), which further suppresses the Li dendrite growth. An LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) full‐cell comprising the ultrathin Li metal anode (20 µm) with AgTFMS additive reveals an excellent capacity retention of up to 88.2% over 200 cycles, as well as outstanding rate capability under harsh practical condition. As a result, the AgTFMS additive can pave a new dimension for the design of high energy density LMBs using the ultrathin Li metal anode.

(Oxalato)borate: The key ingredient for polyethylene oxide based composite electrolyte to achieve ultra-stable performance of high voltage solid-state LiNi0.8Co0.1Mn0.1O2/lithium metal battery

Lithium (Li) metal batteries (LMBs) are among the most promising candidates for future battery technology due to their high theoretical capacity and energy density. However, the formation of dendritic Li, characterized by needle-like structures, poses serious safety issues. To address this, numerous methods are developed to prevent Li dendrite formation. Another significant challenge in LMBs is the formation of inactive Li, known as dead Li, which significantly impacts their Coulombic efficiency and overall performance. This review explores the issues surrounding dead Li in LMBs, specifically focusing on electrically isolated Li metal and the repeatedly generated solid electrolyte interphase (SEI). Advanced techniques for characterizing inactive Li are discussed, alongside various strategies designed to activate or suppress dead Li, thus restoring battery capacity. The review summarizes recent advancements in research related to the activation, reuse, and prevention of dead Li, offering valuable insights for enhancing the efficiency and safety of LMBs. This comprehensive overview provides fundamental guidance for the practical application of Li metal anodes and similar metal batteries.

Lithium (Li) metal is an ultra‐high specific capacity and low potential ‘‘holy grail’’ anode for developing high energy density rechargeable Li metal batteries (LMBs), but its practical application suffers from the low Coulombic efficiency (CE) and dendritic lithium growth due to its thermodynamic instability to organic solvents. Here, we report a new diluted high concentration electrolyte (DHCE) containing lithium bis(fluorosulfonyl)imide, triethyl phosphate and fluorobenzene cosolvent, which not only guarantees lithium metal anode (LMA) with an 98.4 % high average CE over 250 cycles but greatly improves Li||NMC622 cells property (70 % capacity retention after 200 cycles at C/3 current density). Therefore, this new electrolyte can provide more possibilities for developing high‐energy‐density LMBs.

Ambiently fostering solid electrolyte interphase for low-temperature lithium metal batteries

Recently, Lithium(Li) metal has been considered as a “ideal” anode material due to the extra-high capacity (3,860 mAh g-1), the lowest electrochemical equilibrium potential (-3.040 VSHE) and low density (0.534 g cm-3). Despite its ideal characteristics, Li metal anode(LMA) has a critical drawback that Li deposits grow in the form of dendrite due to its ununiform plating/stripping behaviors. The undesired dendritic growth of Li results in the detrimental effects on the cell performance, such as cell short-circuit, low coulombic efficiency(CE), and dead lithium, which have hampered widespread exploits of lithium metal-based batteries. Therefore, in recent years, numerous strategies to prevent or mitigate the problematic dendrite growth, including construction of artificial solid electrolyte interphase (SEI), lithiophilicity/morphology control of current collector, modification of electrolyte, and so forth [1,2].Meanwhile, by virtue of quite successful improvement in performance of LMAs, the battery system using in-situ formed LMAs, called anode-free lithium metal batteries (AFLMBs) have gained much interests [3]. In AFLMBs, the anode compartment has only a current collector at the initial state and then lithium metal anode is formed by electroplating with lithium ions provided from the cathode through initial charging step. The absence of corrosive/reactive Li metal in assembly process decreases the production cost and the energy density of a full-cell can be increased due to additional loading of cathode active materials. However, the dendritic growth of lithium could become more problematic because the cyclable lithium ions are fully limited to those provided from the cathode active materials. For solving notorious dendrite problem in AFLMBs, many strategies such as a use of highly concentrated salt electrolytes [3], the addition of functional electrolyte additives [4] and the introduction of lithiophilic elements on the current collector have been reported [5]. Unfortunately, however, it still seems difficult to nurture all strengths of AFLMBs related to the cost-effectiveness, simple cell design and high-energy density.One of the effective ways to regulate electroplating behavior of lithium is to introduce the surface-roughed metal current collector. Using such the structures, the charge accumulation sites on the microscopic structure of current collector can be randomly distributed, resulting in uniform electric field on the surface of current collector. Additionally, the localized current density can be reduced, and problematic volume changes of lithium can be buffered [2].In this context, we introduced the surface-roughed metal current collector as a substrate of AFLMBs to improve their stability. The surface roughness was controlled by an electrochemical method using a pulse-reverse polarization [6]. In particular, we focused on studying the effect of various pulse parameters on the morphological evolution of a metal current collector. To investigate their effects on the stability of AFLMBs, the CR 2032-type coin cells composed of the roughly surface-treated metal foil as a current collector and high-loading Li-Ni0.5Co0.2Mn0.3 (≈ 12.1 mg/cm2) as a cathode were assembled in an Ar-filled glove box. Figure 1-(a) presents an example of pulse signal used to modify roughness of metal surfaces. First, to investigate effect of pulse current density(iox=ire=i), we fixed the amount of metals, which were electrochemically oxidized and reduced by giving different pulse duration time (tox = tre = t,). Figure 1-(b) presents the effect of pulse current on the morphology of metal foils. After pulse with 0.5 mA/cm2, the surface of metal foil is covered by the clusters of metal/metal oxide particles. With increasing current density, the diameter of the clusters decreased and the number of clusters increased. Figure 2 shows the effect of surface-roughed metal surface on the electroplating behavior of lithium metals. As can be seen in Figure 2-(a), the morphology of electrodeposited lithium on the surface-treated metal foil has a denser and flatter Li deposit than that on pristine metal foil, where a needle-like dendritic structure is shown. Figure 2-(b) presents the CE of the repeated plating/stripping process of lithium metal. All the cells adopting the surface-treated foils showed a higher cycling stability compared to a pristine metal foil.Additionally, in this presentation, the correlation between pulse-reverse parameters and morphological evolution of a metal current collector, and the effect on the electrochemical long-term stability of AFLMBs will be discussed in more detail References [1] X.-B. Cheng et al., Chem. Rev, 117, 10403, 2017.[2] J. Zheng et al., Chem. Soc. Rev., 49, 2701, 2020.[3] J. Qian et al., Adv. Funct. Mater., 26, 7094, 2016.[4] N. A. Sahalie et al., J. Power Sources, 437, 226912, 2019.[5] S. S. Zhang et al., Electrochimica Acta, 258, 1201, 2017.[6]M. S. Chandrasekar and M. Pushpavanam, Eletrochimica Acta, 53, 3313, 2008 Figure 1

Electronic devices have been advanced rapidly over the last few decades with improved performance through high-level integration. Lithium ion batteries (LIB), which is representative rechargeable energy storage system, is increasingly demanding to high-energy-density storage system for the stable operation of high performance devices like mobile, laptop, etc. In addition, a recent quantum leap in electronic vehicles (EVs) require to energy storage system with long cycling stability incomparable to previous electronic devices. To achieve high performances and long lifetime, the use of lithium metal is one of attractive anode for as LIBs, because Li metal has high theoretical specific capacity of 3860 mAh/g, low redox potential of -3.04V (verse the standard hydrogen electrode) and low gravimetric density of 0.59 g/cm3. Nevertheless, since Li metal anode has the lithiation-delithiation mechanism of the ‘hostless’ electrochemical plating/stripping unlike the conventional graphite anode, it is confronted with serious problems occurred from uncontrollable interfacial reaction between surface of lithium metal anode and electrolyte applying to traditional LIB system; that is (1) the uncontrollable formation of solid electrolyte interphase (SEI) layer limitless volume expansion of Li metal anode (2) the dendritic growth and isolation of Li interface derived from inhomogeneous deposition of Li+ ion flux (3) the Li surface fracture induced by heterogeneously plated Li+ ion. Through these process, repetitive formation of undesirable SEI layer at surface of Li anode make fresh electrolyte dissipation and concurrently the anode surface becomes more distorted. These side-phenomena strongly correlate with low coulombic efficiency and decreasing lifetime of lithium metal battery (LMB) system where lithium metal is used as anode. In this works, in a dimensional aspect, to solve such problem mentioned above through stabilizing of surface of lithium metal anode, we introduce Janus-faced protective layer (JPL) as gel polymer electrolyte. JPL was composed with dual layer of Ni (3~7μm) embedded poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) and bare PVdF-HFP through simple casting method. The process of preparing JPL membrane is so simple to instantly bring in existing roll-to-roll process, and it is also possible to make a low cost process considered relatively low nickel price. In terms of mechanical property, it is based on flexible polymer and shows the merit of being foldable enough to able origami. In addition, JPL play a role of protection of Li metal anode. JPL has two functionalities. The metal part of JPL directly contacted with anode, and this layer was frameworks for preventing irregular SEI layer formation through inward plating of Li ion, the polymer part is stable state enough to be used as a gel polymer electrolyte. When lithium metal anode is subjected to cycle-test while keeping in contact with the nickel side of JPL membrane, Scanning electron microscope (SEM) image showed that the surface of lithium metal anode was very clean without any fluctuation compared to the case where only polypropylene (PP) separator was used. (Figure. 1) Moreover, as shown in Figure 2., the LMB system applied JPL show high stability of tolerable overvoltage within 5mV, 15mV even at high-current of 10mA/cm2, 20mA/cm2, respectively, in Li//JPL/JPL//Li symmetric cell. These results are attributable to effective function of each layer as mentioned earlier. JPL consists of two layer, the tough and adhesive nature of PVdF-HFP prevents the polymer part from being damaged or torn by dendritic growth and simultaneously ensure that metal part functions as robust framework for SEI layer formation without escaped particle from the membrane. This frameworks induce the SEI layer, which can act as a resistive element, to grown in the metal part while surface of Li metal anode become get out of main subject from the SEI layer formation. By virtue of that property, JPL has not allow forming resistive elements on the surface of Li metal anode, preventing the voltage rising up due to the formation of resistive elements at surface, so tolerability of overvoltage with clean surface of anode is maintained. In conclusion, the model study of nickel micro-particle/PVdF-HFP composite suggests the straightforward direction of protective layer to apply lithium metal as anode in LIB system with alleviating the problem caused by the ‘hostless’ property of lithium metal anode. Thus, when JPL is introduced to a LIB system, it shows a fairly positive possibility for high performance Lithium metal battery. Figure 1

Anion modification for stable solid electrolyte interphase in anode-free lithium metal batteries

The deployment of safe and high‐energy density lithium metal polymer batteries (LMPBs) still requires further advances in the quest for new solid polymer electrolytes (SPEs). In this regard, salt anions have a decisive role in the overall SPE performance. While lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was chosen earlier to have a highly flexible sulfonimide center and an extensively delocalized negative charge, it still suffers from several drawbacks ascribed to its poor interfacial compatibility with the lithium metal (LiM) anode and the fact that it is a PFAS. In this work, a novel lithium salt is cunningly designed, aiming to combine the advantages of previously reported lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(difluoromethanesulfonyl)imide (LiDFSI) to overcome the limitations of the state‐of‐the‐art SPE based on LiTFSI/poly(ethylene oxide) (PEO). The SPE containing the developed (difluoromethanesulfonyl)(fluorosulfonyl)imide (LiDFFSI) salt presented reduced interfacial resistance and improved compatibility with the lithium metal (LiM) anode compared with LiTFSI/PEO, enabled by the formation of a stable, uniform, and ionically conductive solid–electrolyte interphase (SEI). In addition, LiDFFSI‐based SPEs demonstrated a prolonged cycling stability, achieving over 125 cycles at C/10 with minimal capacity fading in LiM||LiFePO4 cell configuration. These findings evidence how a rational design of the lithium salt chemistry allows tuning the formed SEI, directly impacting the overall SPE performance. Thus, LiDFFSI is presented as a promising alternative lithium salt to improve electrochemical performance and interfacial stability in next‐generation LiM batteries.

Carbonate-based electrolytes show distinct advantages in high-voltage cathodes but generate nonuniform and mechanically fragile solid-electrolyte interphase (SEI) in lithium (Li) metal batteries. Herein, we propose a LiF-rich SEI incorporating an in situ polymerized poly(hexamethylene diisocyanate)-based gel polymer electrolyte (GPE) to improve the homogeneity and mechanical stability of SEI. Fluoroethylene carbonate (FEC) as a fluorine-based additive for building LiF-rich SEI on Li metal electrodes. With this strategy, the assembled Li symmetric batteries cycled stably for 700 h, and the formation of byproducts on the Li electrode surface was significantly inhibited. The Li/LiFePO4 battery delivered significant capacity retention (91% retention after 800 cycles) at 1 C. With high-voltage LiNi0.8Co0.1Mn0.1O2 (NCM811) as cathode, the Li/GPE-FEC/NCM811 cell delivered a discharge capacity of 168.9 mAh g-1 with a capacity retention of 82% after 300 cycles at 0.5 C. From the above, the work could assist the rapid development of high-energy-density rechargeable Li metal batteries toward remarkable performance.

The practical application of lithium (Li) metal battery is impeded by the Li dendrite growth and unstable solid electrolyte interphase (SEI) layer. Herein, an ultra‐stretchable and ionic conducting chemically crosslinked pressure‐sensitive adhesive (cPSA) synthesized via the copolymerization of 2‐ethylhexyl acrylate and acrylic acid with poly(ethyleneglycol)dimethacrylate as crosslinker (short for 70cPSA), is developed as both artificial SEI layer and solid polymer electrolyte (SPE) for stable Li‐metal electrode, enabling all‐solid‐state Li metal batteries with excellent cycling performance. As an artificial SEI layer, the 70cPSA‐modified electrodes exhibit excellent electrochemical performance in Li|70cPSA@Cu half cells and 70cPSA@Li|70cPSA@Li symmetric cells. In full cells with LiFePO4 (LFP) as cathode, the 70cPSA@Li|LFP cell exhibits stable cycling performance over 250 cycles. Utilized as SPE, the all‐solid‐state Li|SPE|LFP cell delivers excellent cycling stability with a capacity retention of 86% over 500 cycles. With high‐voltage LiNi0.8Mn0.1Co0.1O2 (NMC811) as cathode, the Li|SPE|NMC811 cell exhibits a discharge capacity of 124.3 mAh g−1 with a capacity retention of 71% after 200 cycles. The rational design of PSAs and investigation of their dual role for stable and safe Li‐metal batteries may shed a light on adhesive polymers for battery applications.

Suppressing dendrite growth by a functional electrolyte additive for robust Li metal anodes