Role of Fluorinated-Component Positioning in Li Metal Battery Performance

The instability of lithium (Li) metal anodes due to dendritic growth and low Coulombic efficiency (CE) hinders the practical application of high‐energy‐density Li metal batteries. Here, the systematic studies of improving the stability of Li metal anodes and the electrochemical performance of Li metal batteries through the addition of combinational additives and the optimization of solvent compositions in dual‐salt/carbonate electrolytes are reported. A dendrite‐free and high CE of 98.1% for Li metal anode is achieved. The well‐protected Li metal anode and the excellent cyclability and rate capability of the 4‐V Li metal batteries are obtained. This is attributed to the formation of a robust, denser, more polymeric, and higher ionic conductive surface film on the Li metal anode via the electrochemical reductive decompositions of the electrolyte components and the ring‐opening polymerization of additives and cyclic carbonate solvents. The key findings of this work indicate that the optimization of solvent compositions and the manipulation of additives are facile and effective ways to enhance the performances of Li metal batteries.

The demand for higher power and higher energy Li-ion and Li metal batteries continues to increase. In order to meet expectations the utilization of materials and cell designs need to be optimized. The result of the need for high utilization is that small variations within a group of cells can create significant difference in performance and failure. As part of an effort to advance the performance of Li metal batteries and specifically to increase specific energy closer to 500 Wh/kg recent work looking at the ability to more clearly understand and define the role that cell variability has on the performance of Li metal batteries has been undertaken. This work has shown that significant impact to performance arises as a set of cells, which at the beginning of life had low variability, age. Refined analysis of the electrochemical data enables key variation in Li utilization and other fade mechanisms to be clearly identified and quantified across the different cells. Complimentary work looking at the ability to fast charge, Li-ion cells in 10 minutes or less has also begun. This presentation will discuss the analysis methodology, key similarities in identifying the role of variability and how a more refined understanding can significantly impact the technology development pathway.

Electrolyte instability poses a significant challenge in stabilizing the interfacial chemistry of rechargeable batteries. The engineering of electrolyte architecture has emerged as a promising strategy to enhance electrode compatibility and interfacial stability. In this study, we present the synthesis of a rationally designed cyclic sulfonamide electrolyte tailored to improve the electrochemical performance of Anodeless Li metal batteries and Li-S batteries. This electrolyte is meticulously crafted to optimize crucial electrochemical properties, including Li ion transport, stability, and interface compatibility. Through systematic investigation, we illustrate that the incorporation of cyclic sulfonamide moieties leads to enhanced electrolyte stability, thereby mitigating detrimental side reactions and improving the cycling performance of Li metal batteries. Notably, the use of cyclic sulfonamide electrolyte prevents corrosion of the aluminum current collector caused by TFSI anion, which is commonly associated with the use of LiTFSI salt in commercial batteries, thus facilitating safe battery recycling. Additionally, the cyclic sulfonamide-based electrolyte effectively inhibits the polysulfide shuttle in Li-S batteries, achieving a remarkable coulombic efficiency exceeding 99.5% in the absence of LiNO3 additive. Furthermore, these engineered electrolytes demonstrate exceptional compatibility with Li metal anodes, promoting uniform Li deposition and suppressing dendrite formation, thereby enhancing battery safety and longevity. Our findings underscore the importance of rational electrolyte design in driving the advancement of Anodeless Li metal and Li-S batteries, offering a promising avenue for the realization of high-energy-density and durable energy storage solutions across diverse applications.

In recent years, Li secondary batteries have had a profound effect on daily life as the power sources for portable electronics and electric vehicles.[1] However, despite extensive exploration of potential anode materials, the rational design of Li metal anodes that provide high energy densities with a suitable degree of safety and outstanding high-temperature stability remains a challenge.[2] Presently, the majority of studies focus on improving the performance of Li metal batteries (LMBs) at ambient temperature by employing various electrolyte additives,[3, 4] artificial solid electrolyte interfaces[5] and Li metal hosts.[6] Such research has provided detailed insights into the feasibility of increasing the Coulombic efficiency of these devices while inhibiting dendrite growth at ambient temperature. However, the operation of LMBs at high temperatures (100–180 °C, as 180 °C is the melting point of Li metal) and high current densities has rarely been addressed. The rapid formation and growth of Li dendrites decreases the safety of these devices at high temperatures and also leads to low cycling efficiency during charging/discharging.[7] In addition, conventional organic electrolytes suffer from potential issues including leakage, volatilization, flammability and explosion potential,[8] and thus are not suitable for high-temperature LMBs. Herein, we report a novel ionogel (termed ILE@MOF) obtained by mixing imidazolate framework-67 (ZIF-67) particles with an ionic liquid electrolyte (ILE) via simple ball-milling, with the goal of employing these materials to make dendrite-free LMBs operable at high temperatures. The ILE used in this work was a mixture of N-propyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl) imide ([Py13][TFSI]) and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI). When encapsulated in the MOF 3D channels, this ILE does not lose its dynamic mobility and also maintains high ionic conductivity, even though the resulting ionogel has the appearance of a solid. Unlike Li metal at ambient temperature, Li at elevated temperatures is more susceptible to failure due to increased reactivity with electrolytes, which can lead to increases in the cell impedance. Our results demonstrate that the ILE@MOF effectively protects the Li metal anode by forming a particle-rich coating over the anode, and so markedly increases the anode stability at high temperature. When combined with this new electrolyte, the Li metal anode maintains a stable striping/plating voltage over 1200 h at 150 °C and a current density of 0.5 mA·cm-2. To the best of our knowledge, this is the first demonstration of a viable electrolyte that permits stable Li electrochemical striping/plating at 150 °C. Using the ILE@MOF electrolyte, Li/LiFePO4, Li/LiNi0.33Mn0.33Co0.33O2, Li/LiNi0.8Mn0.1Co0.1O2 and Li/Li4Ti5O12 cells were found to exhibit stable cycle performance over the range of 60–150 °C.

A 3D C@AlF3 multifunctional hollow spheres lithium host for lithium metal batteries.

Due to the extremely high energy density of Li metal, Li metal batteries are regarded as one of the most promising candidates for next-generation energy storage systems. However, interfacial issues, particularly the unstable solid electrolyte interphase (SEI) and lithium dendritic growth, hinder practical application. Herein, we induce an anion-rich interface near the Li metal by introducing positively charged self-assembled monolayers (SAMs) on ceramic-coated separators to simultaneously stabilize the SEI and homogenize the Li deposition. The anion-rich interface, originating from the electrostatic attraction of SAMs, promotes the preferential decomposition of salt anions over organic solvent molecules, leading to the formation of a stable anion-derived inorganic component, notably LiF. Furthermore, the positively charged SAMs immobilize anions, significantly mitigating dendritic Li by improving the Li+ transference number (∼0.73) and thereby mitigating dendritic Li growth. Hence, we present SAMs on ceramic-coated separators as an innovative way to improve the long-term cycling performance of Li metal batteries.

Ionic liquid electrolytes (ILEs) have attracted attention as promising electrolyte candidates. ILEs can realize non-flammable rechargeable batteries. In addition, other advantages of ILEs are negligible volatility and high thermal stability. These properties can improve the safety of Li metal batteries. However, conventional polyolefin separators cannot be used due to low affinity with highly viscos electrolytes such as highly concentrated electrolytes and ILEs. So far, glass filters with thicknesses of several hundred micrometers have been used as alternatives. However, it is too thick to realize a real cell. Furthermore, the glass filter shows low reversibility of Li deposition/dissolution due to its non-uniform pore structure. It is known that Li deposition/dissolution sometime results in an internal short circuit which is caused by inhomogeneous Li deposition. The separator has a large effect on the uniformity of Li deposition/dissolution behavior. Therefore, new separators with good properties, such as high affinity with electrolyte, hightypemechanical strength, suitable thickness and excellent reversibility of Li deposition/dissolution are strongly required. So far, we have developed a three-dimensionally ordered macroporous polyimide (3DOM PI) memblane with a thickness of 30 micrometers as a separator having a good affinity with ILEs. In this study, we improved the performance of Li metal batteries by using both ILEs and 3DOM PI separator. Three types of separators were used in this study; glass filter (Whatman, GF/A, 260 μm), surfactant coated polypropylene (PP) (25 μm) and 3DOM PI separator (30 μm). 3DOM PI separator was prepared by using a colloidal template method. Lithium bis(fluorosulfonyl)imide and N-metyl-N-Propylpyrrolidinium bis(fluorosulfonyl)imide mixture with a molar ratio 1:1 was used as the ILE. Li deposition/dissolution was carried out by using Li symmetric cell with three types of separators and the ILE, in order to investigate the Li deposition/dissolution behavior. The Li deposition/dissolution capacities for all tests were 1 mA h cm-2. Fig. 1 shows the voltage profiles during the Li deposition/dissolution using different current densities. In the case of the 3DOM PI separator, the stable Li deposition/dissolution behavior was observed up to the current density of 3.0 mA cm-2. In the case of the surfactant coated PP separator, the applicable maximum current density was 1.5 mA cm-2. In the case of the glass filter, an internal short circuit was observed even at lower current density. The Li deposition/dissolution behavior strongly depends on the types of separators, and the 3DOM PI separator provides most stable Li deposition/dissolution, especially at high current density. Fig. 2 shows the voltage profiles during the Li deposition/dissolution cycles at the current density of 1 mA cm-2. In the case of the 3DOM PI separator, the highly stable Li deposition/dissolution was observed over 150 cycles. In contrast, in the case of the surfactant coated PP separator, unstable Li deposition/dissolution was observed after the 50th cycle, probably due to micro-short circuit. In the case of the glass filter, the internal short circuit was more clearly observed at the initial cycles. From the above results, it is confirmed that the 3DOM PI separator can provide stable Li deposition/dissolution behavior. This is due to the uniform structure of the 3DOM PI separator and high stability of ILE. Figure 1

Poly(vinylidene fluoride) (PVDF)-based solid-state electrolytes (SSEs) have been considered promising candidates for advanced Li metal batteries due to their adequate mechanical strength and acceptable thermal stability. However, the poor compatibility between residual solvent and Li metal inevitably leads to fast capacity decay. Herein, we propose a multifunctional cation-anchor strategy to regulate solvation chemistry in PVDF-based SSEs to boost the electrochemical performance of Li metal batteries. The strong interaction between N,N-dimethylformamide (DMF) and Zn2+ decreases the participation of DMF in the inner solvation sheath of Li+, inducing an anion-reinforced solvation structure. The unique solvation structure facilitates the formation of a robust LiF-rich solid electrolyte interphase layer to eliminate interfacial side reactions. In addition, a continuous ion-conducting network is constructed by introducing extra TFSI- anions, enabling accelerated Li+ transport. As a result, the corresponding Li‖Li symmetrical cells achieve stable lithium plating/stripping over 780 h, and the rate performance and cycling stability of Li‖LiFePO4 cells are significantly improved. This work highlights the key role of regulation of solvation chemistry in PVDF-based SSEs for Li metal batteries.

Polyolefin separators are the most common separators used in rechargeable lithium (Li)-ion batteries. However, the influence of different polyolefin separators on the performance of Li metal batteries (LMBs) has not been well studied. By performing particle injection simulations on the reconstructed three-dimensionalpores of different polyethylene separators, it is revealed that the pore structure of the separator has a significant impact on the ion flux distribution, the Li deposition behavior, and consequently, the cycle life of LMBs. It is also discovered that the homogeneity factor of Li-ion toward Li metal electrode is positively correlated to the longevity and reproducibility of LMBs. This work not only emphasizes the importance of the pore structure of polyolefin separators but also provides an economic and effective method to screen favorable separators for LMBs.

Boosted Li+ transport ensured by high-entropy Prussian blue analogues with tuned PF6− adsorption for stable Li metal anode

Inorganic-rich solid-electrolyte interphases (SEIs) on Li metal anodes improve the electrochemical performance of Li metal batteries (LMBs). Therefore, a fundamental understanding of the roles played by essential inorganic compounds in SEIs is critical to realizing and developing high-performance LMBs. Among the prevalent SEI inorganic compounds observed for Li metal anodes, Li3N is often found in the SEIs of high-performance LMBs. Herein, we elucidate new features of Li3N by utilizing a suspension electrolyte design that contributes to the improved electrochemical performance of the Li metal anode. Through empirical and computational studies, we show that Li3N guides Li electrodeposition along its surface, creates a weakly solvating environment by decreasing Li+-solvent coordination, induces organic-poor SEI on the Li metal anode, and facilitates Li+ transport in the electrolyte. Importantly, recognizing specific roles of SEI inorganics for Li metal anodes can serve as one of the rational guidelines to design and optimize SEIs through electrolyte engineering for LMBs.

High-concentration electrolytes (HCEs) can effectively enhance interface stability and cycle performance of Li metal batteries (LMBs). However, HCEs suffer from low ionic conductivity, high viscosity, high cost, and high density. Herein, fluorobenzene (FB) diluted localized high-concentration electrolytes (LHCEs) consisting of lithium bis(fluorosulfonyl)imide (LiFSI)/triethyl phosphate (TEP)/FB are developed. 2.3 M LHCE can reserve concentrated Li+-FSI--TEP solvation structures. Diluent FB possesses low density, low viscosity, low cost, low dielectric constant, low LUMO, and a good fluorine-donating property, which can significantly reduce viscosity, improve ionic conductivity, promote the formation of LiF-rich SEI, and enhance interaction of Li+-TEP and Li+-FSI- ion-pairs of the electrolytes. 2.3 M LHCE is a highly safe nonflammable electrolyte. 2.3 M LHCE can effectively inhibit dendrite growth on Li metal anode. 2.3 M LHCE endows LiFePO4 cells with good rate capability (discharge capacity of 112.7 mAh g-1 at 5 C rate) and excellent cycling performance (capacity retention of 95.4% after 1000 cycles). 2.3 M LHCE also shows good compatibility with LiNi0.8Co0.1Mn0.1O2 and exhibits outstanding cycle stability (capacity retention of 86.4% after 500 cycles). Therefore, 2.3 M LHCE is a promising electrolyte for practical applications in LMBs.

Replacement of flammable organic liquid electrolytes with solid Li+ conductors is a promising approach to realize excellent performance of Li metal batteries. However, ceramic electrolytes are either easily reduced by Li metal or penetrated by Li dendrites through their grain boundaries, and polymer electrolytes are also faced with instability on the electrode/electrolyte interface and weak mechanical property. Here, we report a three-dimensional fiber-network-reinforced bicontinuous solid composite electrolyte with flexible Li+-conductive network (lithium aluminum titanium phosphate (LATP)/polyacrylonitrile), which helps to enhance electrochemical stability on the electrode/electrolyte interface by isolating Li and LATP and suppress Li dendrites growth by mechanical reinforcement of fiber network for the composite solid electrolyte. The composite electrolyte shows an excellent electrochemical stability after 15 days of contact with Li metal and has an enlarged tensile strength (10.72 MPa) compared to the pure poly(ethylene oxide)-bistrifluoromethanesulfonimide lithium salt electrolyte, leading to a long-term stability and safety of the Li symmetric battery with a current density of 0.3 mA cm-2 for 400 h. In addition, the composite electrolyte also shows good electrochemical and thermal stability. These results provide such fiber-reinforced membranes that present stable electrode/electrolyte interface and suppress lithium dendrite growth for high-safety all-solid-state Li metal batteries.

Uncontrollable dendrites growth during electrochemical cycles leads to low Coulombic efficiency and critical safety issues in Li metal batteries. Hence, a comprehensive understanding of the dendrite formation mechanism is essential for further enhancing the performance of Li metal batteries. Machine learning accelerated molecular dynamics simulations can provide atomic-scale resolution for various key processes at an ab-initio level accuracy. However, traditional molecular dynamics simulation tools hardly capture Li electrochemical depositions, due to lack of an electrochemical constant potential condition. In this work, we propose a constant potential approach that combines a machine learning force field with the charge equilibration method to reveal the dynamic process of dendrites nucleation at Li metal anode surfaces. Our simulations show that inhomogeneous Li depositions, following Li aggregations in amorphous inorganic components of solid electrolyte interphases, can initiate dendrites nucleation. Our study provides microscopic insights for Li dendrites formations in Li metal anodes. More importantly, we present an efficient and accurate simulation method for modeling realistic constant potential conditions, which holds considerable potential for broader applications in modeling complex electrochemical interfaces.

The separator is an essential inactive component in liquid-electrolyte based batteries, with the function of separating the positive electrode from the negative electrode to avoid the electronic flow but enable the free ionic transport. The major commercial separators are thin, porous polymeric membranes with single layer or multilayers, and the polymer materials are typically polyolefins, polyesters, and so on. In recent years, separators with surface modifications have also been reported and produced for lithium (Li)-ion batteries, with the purposes of improving the mechanical strength, wettability, cell performance and safety tolerance under various abuse conditions. A lot of positive results have been achieved in Li-ion batteries. In recent years, with the demand for higher energy density than that from the state-of-the-art Li-ion batteries, the research and development of rechargeable Li metal batteries has been revived. However, the effects of the separators on the stability of Li metal anode and the performance of Li metal batteries have seldom reported. In this work, we have comprehensively studied the chemical and electrochemical stabilities of commercially available separators with Li metal anode in two kinds of liquid electrolytes, and found that the separators do have different stabilities with Li metal anode especially in conventional LiPF6-based electrolytes. The details of the investigations will be reported at the presentation.