Stable Lithium Metal Plating/Stripping in a Localized High-Concentration Cyclic Carbonate-Based Electrolyte

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

Lithium (Li) metal batteries (LMBs) are enjoying a renaissance due to the high energy densities. However, they still suffer from the problem of uncontrollable Li dendrite and pulverization caused by continuous cracking of solid electrolyte interphase (SEI) layers. To address these issues, developing spontaneously built robust polymer-reinforced SEI layers during electrochemical conditioning can be a simple yet effective solution. Herein, a robust homopolymer of cyclic carbonate urethane methacrylate is presented as the polymer matrix through an in situ polymerization method, in which cyclic carbonate units can participate in building a stable polymer-integrated SEI layer during cycling. The as-investigated gel polymer electrolyte (GPE) assembled LiCoO2 /Li metal batteries exhibit a fantastic cyclability with a capacity retention of 92% after 200 cycles at 0.5 C (1 C = 180 mAh g-1 ), evidently exceeding that of the counterpart using liquid electrolytes. It is noted that the anionic ring-opening polymerization of the cyclic carbonate units on the polymer close to the Li metal anodes enables a mechanically reinforced SEI layer, thus rendering excellent compatibility with Li anodes. The in situ formed polymer-reinforced SEI layers afford a splendid strategy for developing high voltage resistant GPEs compatible with Li metal anodes toward high energy LMBs.

Li metal is an ideal anode material for next-generation energy storage devices due to its extremely high theoretical specific capacity and the lowest negative electrochemical potential [1,2]. However, uncontrolled Li dendrite growth and the side reaction between Li metal and electrolytes have prevented its practical application in rechargeable Li metal batteries. In our contributions, artificial solid electrolyte interphase (SEI) layer and 3D current collector have been developed to address these issues [3,4]. In this presentation, we will introduce an artificial Li3PO4 SEI layer [3] fabricated by in situ reaction of polyphosphoric acid (PPA) with Li metal and its native film (Li2CO3, LiOH, and Li2O). The partial dissolution of the Li native film and formation of the SEI layer lead to a porous structure of the Li surface, thereby promoting Li dendrite formation and growth. In the ingenious design, the native film of Li metal is replaced by the stable and uniform Li3PO4 SEI layer. The artificial Li3PO4 SEI layer with thickness of 50 nm exhibits a smooth surface and a high Young’s modulus (10-11 GPa). Furthermore, the artificial Li3PO4 SEI layer is demonstrated to be stable in the electrolyte and during cycling in a Li|LiFePO4 battery system. The Li-conducting Li3PO4SEI layer can achieve uniform Li deposition, and reduce the side reaction between the Li metal and the electrolyte. Thus, the artificial SEI strategy we described is a promising route to tackle the intrinsic problems of Li metal anodes and other metal based anodes. Furthermore, we will show that Li metal anode can be accommodated in the reserved pores of the 3D current collector [4] with a submicron skeleton and high surface area, thereby suppressing the Li dendrite growth and solving the associated problems of Li metal anodes. Because of the high electroactive area of the submicron 3D structure, the 3D current collector can improve the Li plating/stripping efficiency and reduce the resistance of the Li metal anode. The utilization of the 3D architecture to accommodate the Li metal anode will highlight the importance of rational design of current collectors and reveal a new avenue for developing Li metal anodes with a long lifespan. References Y.-X. Yin, S. Xin, Y.-G. Guo, L.-J. Wan, Angew. Chem. Int. Ed. 2013, 52, 13186.W. Xu, J. L. Wang, F. Ding, X. L. Chen, E. Nasybutin, Y. H. Zhang, J. G. Zhang, Energ. Environ. Sci. 2014, 7, 513.N.-W. Li, Y.-X. Yin, C.-P. Yang, Y.-G. Guo, Adv. Mater. 2015, DOI: 10.1002/adma.201504526.C.-P. Yang, Y.-X. Yin, S.-F. Zhang, N.-W. Li, Y.-G. Guo, Nat. Commun. 2015, 6, 8058

To meet the rapidly growing market in electrical vehicles (EVs) and portable electronic devices, a battery with high energy density is required. In order to reach this goal, Li metal, is considered as the most promising candidate as anode. However, the dendrite forming, low Coulombic efficiency (CE), and unstable solid electrolyte interphase (SEI) pose big challenges in applying Li metal in batteries. Here we introduce a dual-layer coated Li metal surface, which consists a lithium carboxylate organic layer and a doped MXene layer. The porous organic layer increases the wetting of electrolyte and forms a thin solid electrolyte interface (SEI). While the doped MXene leads to about 50% reduced thickness of inorganic-rich components among the SEI layer. A low overpotential of less than 30 mV over 1000 h cycling in symmetric cells was received. The functional groups along with the good electronic conductivity and smooth surface of the MXene greatly reduce the lithium plating/stripping energy barrier, enabling a dendrite-free lithium metal anode. The battery paired with NMC811 exhibited a high initial CE of 82.1%. A high capacity-retention of 175.4 mAh/g at 1.0 C was achieved after 350 cycles. After long 500 cycles, a reliable capacity was still remained while the cell with un-modified Li metal quickly dropped to below 74.2 mAh/g. In a 475 mAh pouch cell, the battery still exhibited a high discharge capacity of 165.6 mAh/g with a capacity retention of 90.2% after 200 cycles, while the cell with pure Li metal decreased to 80.5%. Keywords: Lithium metal; Battery; Anode; MXene; Doping

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.

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.

Despite the extremely high ionic conductivity, the commercialization of Li10GeP2S12-type materials is hindered by the poor stability against Li metal. Herein, to address that issue, a simple strategy is proposed and demonstrated for the first time, i.e., in situ modification of the interface between Li metal and Li10SnP2S12 (LSPS) by pretreatment with specific ionic liquid and salts. X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy results reveal that a stable solid electrolyte interphase (SEI) layer instead of a mixed conducting layer is formed on Li metal by adding 1.5 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)/ N-propyl- N-methyl pyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr13TFSI) ionic liquid, where ionic liquid not only acts as a wetting agent but also improves the stability at the Li/LSPS interface. This stable SEI layer can prevent LSPS from directly contacting the Li metal and further decomposition, and the Li/LSPS/Li symmetric cell with 1.5 M LiTFSI/Pyr13TFSI attains a stable cycle life of over 1000 h with both the charge and discharge voltages reaching about 50 mV at 0.038 mA cm-2. Furthermore, the effects of different Li salts on the interfacial modification is also compared and investigated. It is shown that lithium bis(fluorosulfonyl) imide (LiFSI) salt causes the enrichment of LiF in the SEI layer and results in a higher resistance of the cell upon a long cycling life.

Lithium (Li) metal has attracted much attention as an anode material for high energy density batteries due to the high theoretical specific capacity and low negative electrochemical potential of Li. However, the growth of Li dendrite on the metal electrode surface can lead to short circuits during the repeated electrochemical discharge-charge cycles. The control of lithium dendrite is one of the greatest challenges hindering the successful development of Li metal based batteries. In particular, Li metal is generally unstable against most chemical species and reacts with organic solvents as well as salts in liquid electrolyte. Furthermore, repeated stripping and deposition of Li may induce the uncontrolled dendritic growth of Li. To overcome aforementioned obstacles and difficulties in Li metal anode, we propose the use of an organic chemical as an electrolyte additive for controlling Li dendrite. The base electrolyte of LiTFSI/TEGDME was used for tests. With the addition of the additive, the Li grains formed, while the fiber-like Li dendrite formed in the Li/Li symmetric cell without the additive in electrolyte. Furthermore, the uniform solid electrolyte interphase (SEI) layer formed by the addition of the organic additive, leading to the suppression of Li dendritic growth and electrolyte decomposition. Accordingly, the Li/Li symmetric cell containing the additive operated for longer than 700 cycles, which was four time longer than that of symmetric cell without additive in the electrolyte, under the current density of 0.1 mA cm-2 when the areal capacity was limited to 0.2 mAh cm-2. Under the limited areal capacity of 1 mAh cm-2 at the current density of 0.5 mA cm-2, the cell containing the additive maintained the cycle life longer than 350 h. Further development of this electrolyte additive may enable the practical applications in Li metal based batteries such as Li-air battery.

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

Introduction The solid electrolyte interphase (SEI) on the negative electrode is known to play a key role on cell safety. Namely, if the SEI layer is damaged at high temperatures, electrode/electrolyte decomposition is induced along with cell heating, which can trigger thermal runaway. Lithium hexafluorophosphate (LiPF6) is known be thermally decomposed (LiPF6 → LiF + PF5) over 70oC. Here, the as-generated phosphorus pentafluoride (PF5) is known to react with organic solvents such as ethylene carbonate (EC) at high temperatures.1 It is thus very likely that PF5 can attack the organic and/or inorganic components of SEI layer to deteriorate the passivating ability of SEI layer. In this work, to confirm the SEI damage by PF5 attack, the open-circuit voltage (OCV) was monitored while the Li/SiO cells, wherein the concentration of PF5 precursor (LiPF6) was varied, were stored at 85oC. A more serious SEI damage is expected with a larger LiPF6 loading. As a PF5 scavenger, tris(2,2,2-trifluoroethyl) phosphite (TTFP) was added into the electrolyte solution. The former (PF5) is a strong Lewis acid, while the latter is Lewis base, such that a strong complex formation is expected from the two.2 Experimental Two different experiments were performed. (1) The 2032-type coin cells with SiO working electrode, Li counter electrode and electrolyte (1.3 M LiPF6 EC:DEC (3:7, v/v)) were assembled, and cycled 5 times to obtain a stable SEI layer on the SiO electrode. The cells were then dismantled and re-assembled by adding different electrolyte (0.0 M, 0.7 M and 1.3 M LiPF6 in EC:DEC (3:7, (v/v)). The re-assembled cells were stored at 85oC for 1 day, while the OCV was monitored. (2) The 2032-type coin cells with SiO working electrode, Li counter electrode and two different electrolyte solution (1.3 M LiPF6 in EC:DEC (3:7, v/v) and 1.3 M LiPF6 in EC:DEC (3:7, v/v) + 5% TTFP)) were assembled, and cycled 5 times to obtain a stable SEI layer on the SiO electrode. The same storage experiment was performed. The passivation ability of SEI layer was assessed by monitoring the OCV values and the chemical composition of SEI layer was analyzed by using XPS. Results and Discussion Fig. 1 (a) shows the OCV profiles obtained from the Li/SiO cells during the 85oC storage. As is seen, the OCV values increase at a faster rate with an increase in LiPF6 concentration. The OCV increase signifies the oxidation (self-discharge) of SiO electrode, which in turn indicates the damage and loss of passivating ability of SEI layer.3 The SEI damage is resulted from PF5 attack, which is generated by thermal decomposition of LiPF6. Fig. 1 (b) compares the OCV change for two Li/SiO cells. The cell containing TTFP shows a much slower OCV increase, illustrating that the SEI layer is rather intact even upon 85oC exposure. The SEI damage by PF5 attack was marginal due to the scavenging action of TTFP for PF5. Reference 1. S. E. Sloop, J. B. Kerr, and K. Kinoshita, J. Power Sources, 119-121, 330 (2003). 2. S. S. Zhang, K. Xu, and T. R. Jow, Electrochemical and Solid-state Letters, 5, A206 (2002). 3. H. Park, T. Yoon, J. Mun, J. H. Ryu, J. J. Kim, and S. M. Oh, J. Electrochem. Soc., 160, A1539 (2013). Fig ure 1 . The OCV profiles obtained from the Li/SiO cells that were stored at 85oC. (a) The concentration of LiPF6 was varied in the electrolyte solutions, (b) TTFP was added into the electrolyte solution in one cell. Figure 1

In situ measurements of the growth of solid electrolyte interphase (SEI) layer on silicon and the lithiation‐induced volume changes in silicon in lithium ion half‐cells are reported. Thin film amorphous silicon electrodes are fabricated in a configuration that allows unambiguous separation of the total thickness change into contribution from SEI thickness and silicon volume change. Electrodes are assembled into a custom‐designed electrochemical cell, which is integrated with an atomic force microscope. The electrodes are subjected to constant potential lithiation/delithiation at a sequence of potential values and the thickness measurements are made at each potential after equilibrium is reached. Experiments are carried out with two electrolytes—1.2 m lithium hexafluoro‐phosphate (LiPF6) in ethylene carbonate (EC) and 1.2 m LiPF6 in propylene carbonate (PC)—to investigate the influence of electrolyte composition on SEI evolution. It is observed that SEI formation occurs predominantly during the first lithiation and the maximum SEI thickness is ≈17 and 10 nm respectively for EC and PC electrolytes. This study also presents the measured Si expansion ratio versus equilibrium potential and charge capacity versus equilibrium potential; both relationships display hysteresis, which is explained in terms of the stress–potential coupling in silicon.

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

The roles of MXenes in developing advanced lithium metal anodes

Wildcat Discovery Technologies is developing novel electrolyte additives and noncarbonate solvent formulations that yield improved solid electrolyte interphase (SEI) layers on silicon alloy anodes. The large volumetric changes in silicon during charge/discharge cycles result in mechanical disruption of conventional SEI's leading to rapid capacity fade as the cell cycles. Novel additives that could help to form more mechanically robust SEI passivation layers, hence improving cycle life of silicon based materials are identified through the screening in propylene carbonate (PC) based electrolyte since PC is not expected to form an effective SEI layer. The additives are tested with PC to determine their efficacy. Wildcat's high throughput electrolyte formulation, cell assembly, and test capabilities enable the evaluation of hundreds of additives in a very short timeframe. Once effective SEI additives are identified in a PC-based formulation, other non-SEI forming solvents can be evaluated. Since the additives are effective without solvent participation in SEI formation, solvents that may not be reductively stable on the anode can also be considered, opening the possibility to more oxidatively stable or less flammable solvents. Wildcat has evaluated hundreds of combinations of noncarbonate solvents with its initial silicon anode SEI additives, and identified a few very unique combinations that showed further improved cycle life over the state-of-the-art carbonate based electrolyte. Finally, with the most promising noncarbonate solvents in hand, new electrolyte additives are screened for further improved cycle life. The new additives selection is based on the earlier additive screening in PC electrolyte. A thorough Structure Activity Relationship (SAR) study for those promising additives is carried out. This presentation will focus on the results of the additive improvement to cycle life in NMC//Si full cells, and illustrate the opportunities for novel solvent combinations. We will demonstrate the power of high throughput development of lithium ion battery advanced electrolytes within this presentation.

Bifunctional composite separator with a solid-state-battery strategy for dendrite-free lithium metal batteries