Dynamic Nuclear Polarization from Lithium Metal, a New Method to Study Lithium Batteries

The increasing demand for high-specific-energy lithium batteries has stimulated extensive research on the lithium metal anode owing to its high specific capacity and low electrode potential. However, the lithium metal will irreversibly react with the electrolyte during the first cycling process, forming an uneven and unstable solid electrolyte interphase (SEI) layer, which results in the non-uniform deposition of Li ions and thus the formation of lithium dendrites. This could cause a battery short circuit, resulting in safety hazards such as thermal runaway. In addition, the continuous rupture and repair of the SEIs during the repeated charge/discharge processes will constantly consume the active lithium, which leads to a significant decrease in battery capacity. An effective strategy to address these challenges is to design and construct ideal artificial SEIs on the surface of the lithium metal anode. This review analyzes and summarizes the mathematical modeling of SEI, the functional characteristics of SEIs with different components, and finally discusses the challenges faced by artificial SEIs in practical applications of lithium metal batteries and future development directions.

While the idea that fast charging batteries could be utilized in electric vehicles is enticing, current fast charging batteries face significant challenges that must be mitigated before this application is realized. Fast charging batteries currently suffer from capacity loss and degradation over time which not only affects battery life but poses a safety hazard. The current electrolyte, 1.2 M LiPF6 in 3:7 wt.% EC/EMC (Gen2), contains the salt LiPF6 that when exposed to moisture can form dangerous hydrofluoric acid and contains a carbonate-based solvent that when exposed to oxygen can ignite. In addition, batteries experience degradation over many cycles of fast charging which results in capacity fade and poor battery performance. However, previous studies have shown the potential for other electrolyte systems to mitigate these problems, specifically by using highly concentrated electrolytes1. These studies claim that using these electrolytes results in the formation of a solid electrolyte interphase (SEI) that passivates Li ions better than the Gen2 electrolyte. The SEI is a layer that forms on the anode within the battery and serves as a protective layer that allows for Li intercalation into graphite and can therefore protect against degradation. By using different electrolytes, a different SEI is formed, and different degradation mechanisms are observed. In this way, the SEI can be tuned to improve Li passivation and ionic conductivity by changing the electrolyte’s composition and concentration2. This research focuses on such alternative electrolyte systems and aims to characterize the different degradation mechanisms corresponding to each electrolyte. Favorable performance has been observed using 1.2 M LiFSI in 3:7 wt.% EC/EMC as an electrolyte. We are studying LiFSI in acetonitrile (AN) at higher concentrations than traditional Gen2 to quantify the SEI. We expect AN to form an anion derived SEI, a composition of which results in improved passivation of Li ions1. In this study, we characterize degradation of alternative electrolyte systems through x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Through these methods we can observe and quantify both crystalline, amorphous degradation products due to loss of Li inventory and loss of active material. XPS allows quantification of compositional changes in the SEI layer at the surface of the electrode. The SEM provides qualitative information on degradation such as visualizing lithium plating and dendrite formation. We implement quantitative XRD to measure the crystalline phases of the graphite anode, showing the degree of lithium intercalation and further quantifying degradation such as Li plating. Putting the results of these three characterization techniques together in tandem with electrochemical cycling data paints a picture of the degradation that happens on the anode of a fast-charging lithium-ion battery with respect to electrolyte composition.[1] Yamada, Y., Wang, J., Ko, S., Watanabe, E., & Yamada, A. (2019). Advances and issues in developing salt-concentrated battery electrolytes. Nature Energy. [2] Logan, E. R., & Dahn, J. R. (2020). Electrolyte Design for Fast-Charging Li-ion Batteries. Trends in Chemistry.

The structural character and mechanical stability of solid-electrolyte interphase (SEI) play a critical role in the formation of dendrites in lithium metal batteries (LMBs). However, due to the complex structures of SEI, the mechanisms by which dendrites nucleate at the interface of Li metal and SEI layer are not well understood. In this work, we employed first-principles calculations using density functional theory (DFT) to study the stability of the innermost layer of the grain-structured SEI on the Li metal surface. The grain structures of LiF/LiF, Li2O/Li2O, and LiF/Li2O and their interaction with the Li surface are considered. The stability of different SEI grain structures is analyzed based on the overpotential due to Li addition. Also, the excess energies required for SEI components to crack along their grain boundary (GB) defects are investigated. The initial observation of these structures suggests that there is a significant rearrangement in the interfacial layers (first few layers) of Li. The system reaches a metastable state when more Li is added to the GB and/or to the triple-phase boundary (TPB) between the GB and Li surface. The energetics from the DFT calculations vary significantly depending upon the grain structures, with LiF/LiF grain structures being the most stable and LiF/Li2O being the least stable. The calculated energies, with and without vacancies, at the TPB region of Li/SEI interfaces, show that the interface between Li2O/Li2O on Li is the least favorable energetically and more susceptible to stress accumulation and cracking followed by LiF/Li2O on Li and LiF/LiF on Li, respectively.

Alkali and alkaline earth metal anodes are gaining increasing research attention because they could provide high energy densities and theoretical specific capacities. Among them, lithium metal is the most investigated anode candidate for next generation batteries. However, due to some challenges, the commercialization of lithium metal anodes is still delayed. Reactions between lithium metal and the electrolyte lead to the formation of an inhomogeneous solid electrolyte interphase (SEI). Consequently, electrodissolution/-deposition of lithium is favored where the SEI is less resistive or cracked, resulting in high surface area lithium and dendrite growth. This does not only lower the coulombic efficiency (CE) and cell specific capacity but also causes safety issues due to an increased risk of short-circuits and thermal runaway.1,2 To enable increased safety in high energy batteries with lithium metal anodes an effective SEI is required to limit lithium dendrite growth. There is a variety of approaches to grow an effective SEI, such as the use of electrolyte additives, mechanical methods and chemical modification.3-5 Recently, alkaline earth metal anodes, such as magnesium or calcium, are also of considerable interest for next generation anodes. Their main advantage is the significantly lower susceptibility to dendrite formation, while still offering high theoretical specific capacities. However, their oxide passivation layer does not allow significant mobility for their divalent ions, leading to large overvoltage and slow reaction kinetics which hinders the cycling. Therefore, electrolytes which enable a non-passivating SEI have to be applied and additives are investigated on their ability to remove the oxide layers.6,7 Herein, we present a novel mechanochemical approach to form an effective SEI on the lithium metal surface by combining mechanical and chemical modification utilizing ionic liquids (ILs) prior to cell assembly. This method suppresses dendrite growth even at high current densities of 10 mA cm-2 in lithium metal batteries with liquid electrolytes. To further highlight the importance of surface treatment and SEI formation, we investigate electrolytes for calcium metal batteries as well as mechanical modification of calcium metal anodes. Acknowledgements: The authors would like to acknowledge financial support from the European Union through the Horizon 2020 framework program for research and innovation within the project “VIDICAT” (829145).

The performance of lithium metal batteries can be significantly enhanced by incorporating fluorinated ether-based electrolytes, yet the solid electrolyte interphase (SEI) formation mechanism on lithium metal surfaces remains elusive. This study employs classical and ab initio molecular dynamics simulations to investigate the decomposition mechanisms of lithium bis(fluoromethanesulfonyl)imide (LiFSI) in 1,2-diethoxyethane (DEE) and its fluorinated analogues, F5DEE and F2DEE, when in contact with lithium metal. Our findings indicate that F5DEE-based electrolytes favor the formation of a FSI-rich primary solvation shell around Li+, while F2DEE-based electrolytes yield a solvent-rich environment. The normalized number density at the Li/electrolyte/Li interface shows a depletion of FSI anions in the electrochemical double layer (EDL) structure near the Li anode upon charging, with the distance between the first main peak of the FSI anion and Li anode following the order F5DEE < DEE < F2DEE. Analysis of the electronic projected density of states and charge transfer dynamics unveils the reductive dissociation pathways of FSI anions and fluorinated DEE solvents on the lithium metal surface, taking into account the influence of the EDL structure. DEE is identified as the most reduction-stable solvent, leading to the selective dissociation of FSI anions and the formation of an entirely inorganic SEI. In contrast, F2DEE displays a pronounced reduction tendency, forming an organic-rich SEI due to the solvent-dominated lowest unoccupied molecular orbital at the interface. F5DEE, competing with FSI anions for reduction, results in the formation of an inorganic-rich hybrid SEI with the highest LiF content. The simulation results correlate well with experimental observations and underscore the pivotal role of various fluorinated functional groups in the formation of EDL and SEI near the lithium metal surface.

The development of low-temperature lithium metal batteries (LMBs) encounters significant challenges because of severe dendritic lithium growth during the charging/discharging processes. To date, the precise origin of lithium dendrite formation still remains elusive due to the intricate interplay between the highly reactive lithium metal anode and organic electrolytes. Herein, we unveil the critical role of interfacial defluorination kinetics of localized high-concentration electrolytes (LHCEs) in regulating lithium dendrite formation, thereby determining the performance of low-temperature LMBs. We investigate the impact of solvation structures of LHCEs on low-temperature LMBs by employing tetrahydrofuran (THF) and 2-methyltetrahydrofuran (2-MeTHF) as comparative solvents. The combination of comprehensive characterizations and theoretical simulations reveals that the THF-based LHCE featured with a strong solvation strength exhibits fast interfacial defluorination reaction kinetics, thus leading to the formation of an amorphous and inorganic-rich solid-electrolyte interphase (SEI) that can effectively suppress the growth of lithium dendrites. As a result, the highly reversible Li metal anode achieves an exceptional Coulombic efficiency (CE) of up to ∼99.63% at a low temperature of -30 °C, thereby enabling stable cycling of low-temperature LMB full cells. These findings underscore the crucial role of electrolyte interfacial reaction kinetics in shaping SEI formation and provide valuable insights into the fundamental understanding of electrolyte chemistry in LMBs.

As renewable energy and battery-powered technologies become more widespread, high energy-density and high-performing batteries are required to meet society’s needs. While lithium-ion batteries have been the staple, their energy density development has been slow, and at gravimetric energy densities of around 250 Wh/kg, they are approaching their theoretical limit.1 The lithium-metal battery (LMB) is considered a next-generation battery due to high theoretical energy densities (ex: Lithium-Sulfur, 2600 Wh/kg), and lithium metal’s high gravimetric capacity (3860 mAh/g) and low reduction potential (-3.04 V vs S.H.E.). However, commercialization of LMBs has been limited due to various issues.Two properties that dictate the performance of the LMBs are: 1. The composition of the solid electrolyte interphase (SEI), an electronically conducting, yet passivating, layer at the anode-electrolyte interface, and 2. The morphology of the lithium deposited at the anode. Morphological studies focus on creating dense, low surface area, deposits that limit SEI fracturing and the formation of electronically disconnected lithium. SEI quality is improved by incorporating inorganic or anion-rich SEIs, formed from the electrolyte salt, to promote SEIs that are mechanically robust, ionically conducting, and homogeneous. Our group’s previous work aimed to improve LMB performance via both routes by growing resistive Al2O3 coatings on the copper (Cu) current collector using atomic layer deposition (ALD). Defects in the thin film create areas of low resistance that behave like ultramicroelectrodes which encourage radial diffusion of lithium ions to the nuclei promoting lateral growth and producing low surface area, dense, planar lithium deposits. Simultaneously, the Al2O3 coating’s Lewis acidic properties promote the decomposition and subsequent incorporation of inorganic electrolyte species into the SEI.We present a study of how the different interfaces present at the anode in LMBs impact SEI composition and battery performance. To hold the Li nucleation morphology at the anode constant, we deposit metal oxide films of a fixed resistance onto the Cu current collector, generating planar, low surface area, lithium deposits. This experimental design allows us to decouple the effect of the metal oxide thin film-electrolyte interface on SEI quality and battery performance from that of the lithium morphology. First, as shown in Fig. 1a, different metal oxide films of equal resistance, but different thicknesses, are grown via ALD, confirmed by measuring the first-cycle nucleation overpotential. The resistive metal oxides investigated are Al2O3, HfO2, AlHfxOy, and ZrO2. Scanning electron microscopy results indicate that all cells modified with equal resistance metal oxide films exhibit the same low surface area, dense, planar Li nucleation morphology. However, long-term cycling tests show differences in cell performance and life cycle. Subsequently, the composition of the SEI is investigated using X-ray photoelectron spectroscopy (XPS). Through XPS, an elemental atomic ratio analysis is conducted to investigate the relative number of species derived from the anion of the electrolyte versus the solvent of the electrolyte to reflect the anion-derived nature of the SEI. Additionally, high-resolution peaks are used to determine the relative abundance of high-quality SEI species such as lithium fluoride. This analysis confirms that the metal oxide thin film tunes the SEI composition. Additionally, we identify trends present between the SEI quality of the ALD-modified cell and its performance.To further investigate the thin film-electrolyte interface, a set of cells modified with binary stacked metal oxide thin films with two different stack orders were created, as shown in Fig. 1b. Investigating SEI composition and LMB performance, with the morphology fixed by the nucleation overpotential, allowed for deconvolution of the impact of the Cu-thin film interface and thin film-electrolyte interface. Both interfaces can in principle impact the battery’s function because 1. The Cu-thin film interface controls the electron transport to form Lio from Li+ ion during electrodeposition, and 2. The thin film-electrolyte interface can modulate the SEI. Our results indicated that the SEI and performance were both primarily regulated by the thin film-electrolyte interface. Based on our fixed-resistance and binary stack experimental design, we propose that modulation of the SEI composition by the metal oxide coating controls LMB performance when the Li nucleation morphology is fixed. This result suggests the potential for tuning LMB performance by careful selection of the metal oxide interfacial coating. References (1) Cheng, X.-B., et al. Toward Safe Lithium Metal Anode in Rechargeable Batteries: A Review. Chem. Rev. 2017, 117 (15), 10403–10473. https://doi.org/10.1021/acs.chemrev.7b00115.(2) Oyakhire, S. T., et al. Electrical Resistance of the Current Collector Controls Lithium Morphology. Nature Communications 2022, 13 (1), 3986. https://doi.org/10.1038/s41467-022-31507-w. Figure 1

Stabilizing interface between Li10SnP2S12 and Li metal by molecular layer deposition

Solvent-derived inorganic F and N-rich solid electrolyte interface for stable lithium metal batteries

Lithium dendritic growth inhibitor enabling high capacity, dendrite-free, and high current operation for rechargeable lithium batteries

Lithium metal anode is a promising candidate anode in lithium‐sulfurized polyacrylonitrile (SPAN) batteries. Nevertheless, the formation of lithium dendrites is recognized as the worst problem for lithium metal batteries. Herein, a hybrid solid interphase is synthesized on lithium with a facile method. This modified lithium anode can not only suppress lithium dendrites, but also gives rise to a fast interfacial transport mechanism. When used in a lithium‐SPAN battery, a reversible capacity of 450 mAh g−1 (based on SPAN) is delivered at 10 A g−1, which is a promising candidate for high specific power batteries.

Morphology and chemical compositions of surface films of lithium deposited on a Ni substrate in nonaqueous electrolytes

The practical application of lithium metal batteries (LMBs) is hindered by the imbalanced periodic oscillatory distribution of cations/anions in liquid electrolytes (LEs) and thus the formed mechanically vulnerable solid electrolyte interphase (SEI), which collectively exacerbate lithium (Li) dendrite formation and degrade electrochemical stability. To overcome these issues, a polyphosphoester electrolyte (PPUM-PE) is designed through a dual-ion regulation strategy. The ‒NH‒ moieties in PPUM polymer effectively anchor anions, while its P═O/C═O functional groups reconstruct Li+ solvation architecture, collectively enabling an exceptional Li+ transference number (0.82) and improved reductive stability of the solvation sheath. A bilayer SEI layer formed on Li anodes-composed of an outer lithium-containing alkyl phosphate polymer and an inner LiF-enriched inorganic phase-exhibits high Young's modulus, effectively suppressing Li dendrite propagation and continuous electrolyte decomposition. Impressively, the as-assembled LMBs employing LiFePO4 cathodes retain 91.28% capacity retention after 1000 cycles at 1C. The electrolyte also demonstrates good compatibility with high-voltage cathodes (LiCoO2, LiNi0.8Co0.1Mn0.1O2) and substantially improves battery thermal safety. This dual-ion synergistic regulation provides a scalable pathway toward high-energy-density LMBs.

Designing Li-protective layer via SOCl2 additive for stabilizing lithium-sulfur battery

The rechargeable lithium-air battery is promising for the power source of electrical vehicles, because of its high specific energy density. Now two types of the lithium-air battery have been developed, namely aqueous and non-aqueous. The aqueous system has larger gravimetric energy density of 3505 Wh kg-1 than that of the non-aqueous system of 1917 Wh kg-1. However, the none-aqueous system has severe problems to be solved such as electrolyte decomposition. The gravimetric specific energy density of the aqueous system is comparable to that of internal conversion engines. The severe problem of the electrolyte decomposition would be removed in the aqueous system. The aqueous lithium-air battery consists of a lithium metal anode, a water stable lithium conducting solid electrolyte, an aqueous electrolyte and an air electrode. The concept was proposed by Visco et al. in 20041. The key material for the aqueous system is the water stable solid electrolyte. A few types of the lithium conducting solid electrolytes such as NASICON-type Li1+xAlxTi2-x(PO4)3 (LATP), perovskite-type Li2/3-xLi3xTiO3 (LLTO) and garnet-type Li7La3Zr2O12 (LLZ) are known to be stable in aqueous solutions with LiCl. LLZ is stable in contact with lithium metal, but it has serious problem of a lithium dendrite formation on lithium at a high current density. LATP and LLTO are unstable in contact with lithium metal and an interlayer should be used between lithium and LATP (or LLTO). As the interlayer, polyethylene oxide (PEO) based polymer electrolytes have been successfully used, but the electrical conductivity of the PEO based electrolyte is low at room temperature. The Li/PEO18Li(CF3SO2)2N/LATP/ aqueous 1M LiCl/Pt, air cell was successfully discharge and charge at 60 0C and 0. 5 mA cm-2 for 24 h.2 Imanishi and co-workers3 have proposed a lithium dendrite formation free liquid electrolyte of a solvate ionic liquid of tetraethylene glycol dimethyl ether (TEGDME) and Li(FSO2)2N (LiFSI). A metallic lithium anode could deliver a high area capacity of 12 mAh cm-2 for 40 cycles in TEGDME-2LiFSI with a current density of as high as 6 mA cm-2 at 60 oC. The novel electrolyte as the interlayer between lithium metal and LATP allowed favorable cyclability for the aqueous lithium-oxygen at 0.64 mA cm-2 and 60 0C. To operate the lithium dendrite free electrolyte at room temperature, 1,3-dioxolane (DOL) was added into TEGDME-2LiFSI. TEGDME-2LiFSI-50 vol.% DOL showed a low interface resistance between lithium metal and the electrolyte. No short circuit was observed in the Li/TEGDME-2LiFSI-50 vol% DOL/Li cell at 1.0 mA cm-2 and 25 oC for 40 h polarization. The TEGDME-2FSI-50 vol% DOL electrolyte is quite attractive interlayer between the lithium metal electrode and LATP for the lithium-air battery.References S.J. Visco, E. Nimon, B. Katz, L.C. De Jonghe and M.Y. Chu, 12th International Meeting on Lithium Batteries, Abst. #53, Nara, Japan (2004) T. Zhang, N. Imanishi, S. Hasegawa, A. Hirano, J. Xie, Y. Takeda, O. Yamamoto, and N. Sammes, Electrochem. and Solid State Lett., 12, A132 (2009) H. Wang, S. Sunahiro, M. Matsui, P. Zhang, Y. Takeda, O. Yamamoto, and N. Imanishi, ChemElectroChem, 2, 1144 (2015)