LiF-rich Layer Research Articles

Tuning Reaction Kinetics of Fluorinated Molecules to Construct Robust Solid Electrolyte Interphases on SiOx Anode.

Introducing fluorinated electrolyte additives to construct LiF-rich solid-electrolyte interphase (SEI) on Si-based anodes is proven an effective strategy for coping with its massive volume changes during cycling. However, most current research on fluorine-containing additives focuses on their thermodynamics of decomposition, lacking studies on the correlation between the molecular structure of additives and their decomposition kinetics. Herein, two fluorinated ester additives, diethyl fluoromalonate (F1DEM) and diethyl 2,2-difluoromalonate (F2DEM) were designed and synthesized. Through combining a wealth of characterizations and simulations, it is revealed that despite the similar reduction thermodynamics, the favorable reduction kinetics of single-fluorinated F1DEM facilitate a LiF-rich layer during the early stage of SEI formation, contributing to the formation of a more robust SEI on SiOx anode compared to the difluorinated F2DEM. Consequently, the proposed additive achieves excellent cycling stability (84 % capacity retention after 1000 cycles) for 5 Ah 21700 cylindrical batteries under practical testing conditions. By unveiling the role of reaction kinetics, a long-overlooked aspect for the study of electrolyte additives, this work sheds light on how to construct a stable SEI on Si-based anodes.

Fluoro-Ethylene-Carbonate Plays a Double-Edged Role on the Stability of Si Anode-Based Rechargeable Batteries During Cycling and Calendar Aging.

The energy storage density of Li-ion batteries can be improved by replacing graphite anodes with high-capacity Si-based materials, though instabilities have limited their implementation. Performance degradation mechanisms that occur in Si anodes can be divided into cycling stability (capacity retention after repeated battery cycles) and calendar aging (shelf life). While cycling instabilities and improvement strategies have been researched intensively, there is little known about the underlying mechanisms that cause calendar aging. In this work, multiple electron microscope techniques are used to explore the mechanism that governs calendar aging from the sub-nanometer-to-electrode scale. Plasma focused ion beam tomography is used to create 3D reconstructions of calendar aged electrodes and revealed the growth of a LiF-rich layer at the interface between the copper current collector and the silicon material, which can lead to delamination and increased interfacial impendence. The LiF layer appeared to derive from the fluoro-ethylene-carbonate electrolyte additive, which is commonly used to improve cycling stability in Si-based systems. The results reveal that additives necessary to improve cycling stability can cause performance degradation over the long-term during calendar aging. The results show that high performing, stable systems require careful design to simultaneously mitigate both cycling and calendar aging instabilities.

A functional silicon composite polymer electrolyte with hydrofluoric acid scavenging for quasi-solid-state lithium metal batteries

A functional composite polymer electrolyte (FCPE) containing Si nanoparticles can prevent the growth of lithium dendrites in quasi-solid-state batteries by reacting with HF and H2O to form a protective LiF-rich layer on the lithium metal surface.

A Chemically Bonded Ultraconformal Layer between the Elastic Solid Electrolyte and Lithium Anode for High-performance Lithium Metal Batteries.

Although high ionic conductivities have been achieved in most solid-state electrolytes used in lithium metal batteries (LMBs), rapid and stable lithium-ion transport between solid-state electrolytes and lithium anodes remains a great challenge due to the high interfacial impedances and infinite volume changes of metallic lithium. In this work, a chemical vapor-phase fluorination approach is developed to establish a lithiophilic surface on rubber-derived electrolytes, which results in the formation of a resilient, ultrathin, and mechanically integral LiF-rich layer after electrochemical cycling. The resulting ultraconformal layer chemically connects the electrolyte and lithium anode and maintains dynamic contact during operation, thus facilitating rapid and stable lithium-ion transport across interfaces, as well as promoting uniform lithium deposition and inhibiting side reactions between electrolyte components and metallic lithium. LMBs containing the novel electrolyte have an ultralong cycling life of 2500 h and deliver a high critical current density of 1.1 mA cm-2 in lithium symmetric cells as well as showing good stability over 300 cycles in a full cell.

Double function-layers construction strategy promotes the cycling stability of LiCoO2 under high temperature and high voltage

Lithium cobalt oxide (LCO), as a popular commercial material, can release higher capacity by increasing the charging voltage. However, deep delithiation leads to the dissolution of elements and structural disorder. Meanwhile, high-temperature performance is still in urgent need to be improved in practical application. Here, we use LiNO3, NaF and H3BO3 as raw materials to construct the outer LiF-rich layer and the inner B, F gradient doped hierarchical nanostructures through stirring and calcination. The electrochemically stable LiF-rich surface helps to form a favorable cathode electrolyte interphase (CEI) layer to prevent electrolyte penetration. The gradient doping of B and F in the subsurface region increases the stability of the lattice at high temperature and high potential. The unique structure enables treated LCO to demonstrate stable cycling under high voltage (4.6 V) with a capacity retention of 93.2% after 800 cycles and displayed 95.2% retention for 100 cycles under high temperature (45 °C). The postmortem analyses after long cycles prove the strategy of surface modification helps to protect the structure and reduce adverse reactions. This study provides a simple and extendable method for improving the cycling stability of cathodes at high temperature and high voltage.

Interface Design Enabling Stable Polymer/Thiophosphate Electrolyte Separators for Dendrite-Free Lithium Metal Batteries.

Organic/inorganic interfaces greatly affect Li+ transport in composite solid electrolytes (SEs), while SE/electrode interfacial stability plays a critical role in the cycling performance of solid-state batteries (SSBs). However, incomplete understanding of interfacial (in)stability hinders the practical application of composite SEs in SSBs. Herein, chemical degradation between Li6 PS5 Cl (LPSCl) and poly(ethylene glycol) (PEG) is revealed. The high polarity of PEG changes the electronic state and structural bonding of the PS4 3- tetrahedra, thus triggering a series of side reactions. A substituted terminal group of PEG not only stabilizes the inner interfaces but also extends the electrochemical window of the composite SE. Moreover, a LiF-rich layer can effectively prevent side reactions at the Li/SE interface. The results provide insights into the chemical stability of polymer/sulfide composites and demonstrate an interface design to achieve dendrite-free lithium metal batteries.

In-situ formation of LiF-rich solid-electrolyte interphases on 3D lithiophilic skeleton for stable lithium metal anode

Lithium (Li) metal with high theoretical capacity is considered as one of the important anode materials for the next-generation high-energy density batteries. However, the further commercial application of Li metal anode is hindered by the severe growth of uncontrollable Li dendrites and the destructive regrowth of unstable solid-electrolyte interphases (SEI) layer. Herein, through the combination of thin film deposition coating and in-situ fluoridation, a composite layer with the lithophilic layer and the LiF-rich SEI layer is realized on Cu foam current collector. Specifically, the lithiophilic layer on the surface of current collector helps to regulate the homogeneous Li plating/stripping behavior. More importantly, after in-situ fluoridation, the LiF-rich layer as a robustly artificial SEI can be in-situ generated on the surface of current collector during initial activation process to stabilize cycling performance with dendrite-free. Consequently, the well-designed 3D current collector enables dendrite-free Li deposition behavior, high average coulombic efficiency (97.74% for 500 cycles at 1 mA cm−2 & 1 mAh cm−2), stable Li plating/stripping behavior (over 2400 h with an ultra-low overpotential of 10 mV at 2 mA cm−2 & 2 mAh cm−2) and remarkable cycling stability/rate performance in both coin-type and pouch full cells. These encouraging results provide new insights into the design of high-performance Li metal anodes for the development of Li metal batteries.

In-situ forming lithiophilic-lithiophobic gradient interphases for dendrite-free all-solid-state Li metal batteries

Solid polymer electrolytes offer a promise for all-solid-state Li batteries due to their low cost and good processability. However, dendrites and the associated contact loss occurring at the undesirable Li/electrolyte interface during repeated plating and stripping remain a challenge. To address the issue, here, we propose to coat a thin layer containing Al/Li dual-salt onto the polyethylene oxide (PEO) electrolyte. When cycled with the Li metal anode, the salts are sequentially reduced, in-situ forming a lithiophilic Li-Al alloy-rich layer near the anode and a lithiophobic LiF-rich layer close to the electrolyte. The former improves the interfacial adhesion and regulates the Li nucleation, while the latter contributes to dendrite suppression due to its high interface energy against Li. As a result, the gradient interphase enables a Li/Li symmetrical cell to be stably cycled for over 1000 h without short circuits. Moreover, the full cell paired with the LiFePO4 cathode shows enhanced cyclability, retaining 89.1% capacity after 350 cycles at 0.5 C. A pouch cell using the dual-salt coated electrolyte demonstrates good performance and safety. This work provides a facile yet effective approach to construct functional interphase for achieving stable batteries using solid polymer electrolytes.

A self-regulated gradient interphase for dendrite-free solid-state Li batteries

A functional gradient lithium anode (FGLA), where a LiF-rich layer faces garnet side and gradually changes to a LiAl-rich layer facing Li side, effectively prevents solid-state electrolytes from dendrite penetration at high current densities.

Rationally Engineering Interfaces at Cathodes and Anodes in Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries have received increasing attention due to the high theoretical capacity and low cost of the naturally abundant sulfur cathode.However, its practical application suffers formidable challenges due to instable solid electrolyte interphases (SEIs) on Li metal anode, shuttle caused by the dissolution of intermediate polysulfides into the electrolyte and sluggish kinetics of the sulfur redox reaction, etc. In order to address the above issues, in our presentation, we will propose the novel approaches to rationally engineer the interfaces at both cathode and anode sides to improve Li-S performances. On the one hand, a modified Pechini method is developed to synthesize nanoporous carbon host decorated with Ni3S2@Ni particles. Such cathode delivers enhanced specific capacities with extended cycling life in lean electrolytes, due to the dual functions of Ni3S2 shell, which can both facilitate reaction kinetics and promote electrolyte wetting. On the other hand, we demonstrated a bi-layer structure, constructed of an interconnected, tortuous, porous LiF-rich layer in contact with lithium and a dense in-situ formed inorganic-rich layer on top of the porous structure, showing enhanced anode stability. The porous layer increases the number of Li/LiF interfaces, which reduces local volume fluctuations and improves Li+ diffusion along these interfaces. Additionally, the tortuous porous structure guides uniform Li+ flux distribution and mechanically suppresses dendrite propagation. The dense upper layer of the SEI accomplishes a closed-host design, preventing continuous consumption of active materials. The duality of a dense top layer with porous bottom layer led to extended cycle life and improved rate performance, evidenced with symmetric cell testing, as well as full cell testing paired with sulfur cathodes.

Two-dimensional lithiophilic YFδ enabled lithium dendrite removal for quasi-solid-state lithium batteries

Lithium metal batteries are regarded as promising alternatives to lithium ion batteries due to their high specific capacity. However, lithium dendrite growth during cycling causes safety problem and rapid capacity loss. Here, we report a composite Li anode composed (LYF) of metallic Li and trace amounts (1–2 wt%) of two-dimensional YFδ. The lithiophilic nature of YFδ enables its homogeneous dispersion in metallic lithium. The LYF electrode exhibits lower resistance, higher chemical and mechanical stability, and longer cycle life compared to bare Li electrode due to uniform Li stripping and plating with YFδ incorporation, which was confirmed by in-situ optical microscope observation. X-ray photoelectron spectroscopy reveals that LiF can in-situ form on the LYF electrode with reactions between Li and YFδ during cycling. The spontaneous reactions are clarified by density functional theory calculations. A quasi-solid-state cell with LYF anode, LiFePO4 cathode and cathode-supported solid electrolyte layer has been constructed with a soft interface constructed between Li anode and solid electrolyte by in-situ thermal polymerization. The cell shows a high initial discharge capacity of 147 mAh g−1 at 0.5 C at 60 °C and sustains a stable cycling over 50 cycles with the in-situ formed LiF-rich layer and soft interface.

Probing the Li4 Ti5 O12 Interface Upon Lithium Uptake by Operando Small Angle Neutron Scattering.

The formation of a solid-electrolyte interphase (SEI) on the surface of Li4 Ti5 O12 (LTO) has become a highly controversial topic, with arguments for it and against it. However, prior studies supporting the formation of an SEI layer have typically suggested that a layer forms upon cycling of a cell, although the layer is probed after disassembling. In this study, cubic mesostructured LTO is synthesized with crystallite domain sizes between 3 and 4 nm and uniform pores with diameters ≤8 nm. The mean pore size is controlled between 4-8 nm through the use of a triblock amphipathic copolymer with a tunable hydrophobic block as template and by thermal treatment. The LTO morphology obtained is spherical and evolves upon heat treatment. These materials show excellent electrochemical performance, including high rate capability and capacity retention. The LTO material is subjected to operando small-angle neutron scattering and X-ray photoelectron spectroscopy experiments, which reveal that the highly debated SEI forms at potentials as high as 2.2 V, first as a LiF-rich layer and subsequently by the growth of a carbonaceous layer. These SEI products form first on the smaller pores before forming on the mesopores.

In situ fluorinated solid electrolyte interphase towards long-life lithium metal anodes

The urgent demands for high-energy-density rechargeable batteries promote a flourishing development of Li metal anode. However, the uncontrollable dendrites growth and serious side reactions severely limit its commercial application. Herein, an artificial LiF-rich solid electrolyte interphase (SEI) is constructed at molecular-level using one-step photopolymerization of hexafluorobutyl acrylate based solution, where the LiF is in situ generated during photopolymerization process (denoted as PHALF). The LiF-rich layer comprised flexible polymer matrix and inorganic LiF filler not only ensures intimate contact with Li anode and adapts volume fluctuations during cycling but also regulates Li deposition behavior, enabling it to suppress the dendrite growth and block side reactions between the electrolyte and Li metal. Accordingly, the PHALF-Li anode presents superior stable cycling performance over 500 h at 1 mA·cm−2 for 1 mA·h·cm−2 without dendrites growth in carbonate electrolyte. The work provides a novel approach to design and build in situ artificial SEI layer for high-safety and stable Li metal anodes.

Dendrite-Free Fluorinated Graphene/Lithium Anodes Enabling in Situ LiF Formation for High-Performance Lithium-Oxygen Cells.

Metallic Li is considered as the ultimate choice of negative electrodes for Li batteries because of its largest theoretical specific capacity. However, formidable issues such as poor safety and cyclability caused by lithium dendrite growth and tremendous interfacial side reactions have strictly hindered its practical applications. Here, we report a fluorinated graphene (FG)-modified Li negative electrode (LFG) for high-performance lithium-oxygen (Li-O2) cells. The results show that only 3 wt % FG introduction leads to a significant enhancement on rate capability and cycling life of Li electrodes. Compared with the half cells with bare Li, the cells with LFG exhibit much more stable voltage profiles even at a large areal capacity up to 5 mA h cm-2 or a large current density up to 5 mA cm-2. Li-O2 cells with the LFG anode show a longer cycle life than the cell with the pristine lithium anode. It was found that a LiF-rich layer could be in situ built upon cycling when FG was used, which ensures uniform Li stripping/plating and effectively suppresses Li dendrite growth. Density functional theory calculations confirm the possibility of conversion from FG to graphene and LiF after Li intercalation into LFG during cycling. In situ optical microscopy observation vividly exhibits the obvious inhibition effect of FG for Li dendrite growth.

Porous LiF layer fabricated by a facile chemical method toward dendrite-free lithium metal anode

Abstract Lithium metal is supposed to be critical material for constructing next-generation batteries due to extremely high capacity and ultralow redox potential. However, the perplexing issue of lithium dendrite growth impedes the commercial application. The initial nucleation and low Li ions diffusion rate in the electrolyte/electrode interface dominate the deposition behavior. Therefore, a uniform and flexible interface is urgently needed. Here, a facile method is proposed to prepare a thin and porous LiF-rich layer (TPL) by the in-situ reaction of small amount of ammonium hydrogen difluoride (NH4HF2) and Li metal. The deposition morphology on Li metal anode with LiF layer is significantly flat and homogeneous owning to low lateral diffusion barrier on LiF crystals and the porous structure of TPL film. Additionally, the symmetrical cells made with such TPL Li anodes show significantly stable cycling over 100 cycles at high current density of 6 mA/cm2. The TPL Li | LiFePO4 full cells keep over 99% capacity retention after 100 cycles at 2.0 C. This approach serves as a facile and controllable way of adjusting the protective layer on Li metal.

Stabilizing Li/electrolyte interface with a transplantable protective layer based on nanoscale LiF domains

Although Li metal has been regarded as one of the most promising anode materials, an unstable Li/electrolyte interface during the cycling process seriously limits its practical application in rechargeable batteries. Herein, we report a transplantable LiF-rich layer (TLL) that can suppress the side reactions between electrolyte and lithium metal. This peelable layer cross-linked by nanoscale LiF domains is obtained by electrochemically reducing NiF2 electrodes and could be used to protect Li metal anodes. Cu-Li cells using the TLL protection can operate for more than 300 cycles with a Coulombic efficiency as high as ~ 98% in carbonate-based electrolytes. In Li-LiFePO4 cells, lithium metal with a TLL still looks shiny after 1000 cycles (~ 6 months) in contrast to the black surface of bare lithium foil after ~ 500 cycles (~ 3 months). These results clearly demonstrate that the TLL could greatly limit the side reactions between lithium metal and the carbonate-based electrolytes, and may enable long-term stable operation of Li metal batteries.

1,1-Difluoro-1-alkenes as new electrolyte additives for lithium ion batteries

Five 1,1-difluoro-1-alkenes bearing aryl groups are investigated as electrolyte additives for high-charge-voltage lithium ion batteries with an aim to induce favorable solid electrolyte interphase (SEI) formation. Among them, 1,1-difluoro-4-phenylbut-1-ene (DF-1) forms appropriate SEIs on the cathode and anode by its reactions under oxidative and reductive conditions, respectively, which was suggested by linear sweep and cyclic voltammetry. The SEIs formed with the DF-1 containing electrolyte on the cathode and anode were analyzed by X-ray photoelectron spectroscopy and time of flight-secondary ion mass spectrometry, indicating the following: (i) the cathode surface has a LiF-rich layer with unsaturated hydrocarbons, and (ii) the anode surface also has a LiF-rich layer, which probably includes fluoromethylene (CHF) compounds. The formation of the SEIs on both electrodes with DF-1 results in low impedance being maintained, and thus, particularly good cycle performance even at high-charge-voltage.