A bifunctional electrolyte additive for separator wetting and dendrite suppression in lithium metal batteries

Tailoring Lithium Horizontal Deposition for Long-Lasting High-Loading NCA (≥5 mA h cm-2)||Lithium-Metal Full Cells in Carbonate Electrolytes

We report a design for a synergistic lithium (Li) metal hosting layer for high-loading Li(Ni,Co,Al)O2 (NCA) (≥5 mA h cm-2)||Li-metal full cells in carbonate electrolytes. Based on density functional theory calculations, the hosting layer was designed as a three-dimensional silver/carbon composite nanofiber (Ag/CNF) network with high Li affinity and a platinum (Pt)-coated polypropylene separator with low Li affinity. This design enabled the tailoring of horizontal Li deposition on the Ag/CNF hosting layer. The Li deposition behavior modulated by the hosting layer was thoroughly examined based on the initial Li deposition and cycling behaviors of the Li||Li symmetric cell configuration. Cryogenic focused-ion beam cross-sectional images of the cycled Li anodes clearly demonstrated that dense lithium deposition was enabled by the synergistic hosting layer high-loading NCA (≥5 mA h cm-2)||Li-metal full cells. When the hosting layer was used, the average cycling performance improved by 78.27% under various cycling conditions. Our work demonstrates that the synergistic hosting layer design is a fruitful pathway to accelerate the commercialization of high-energy-density Li-metal batteries in carbonate electrolytes.

Separator-Wetted, Acid- and Water-Scavenged Electrolyte with Optimized Li-Ion Solvation to Form Dual Efficient Electrode Electrolyte Interphases via Hexa-Functional Additive

The performance of lithium metal batteries (LMBs) is determined by many factors from the bulk electrolyte to the electrode-electrolyte interphases, which are crucially affected by electrolyte additives. Herein, the authors develop the heptafluorobutyrylimidazole (HFBMZ) as a hexa-functional additive to inhibit the dendrite growth on the surface of lithium (Li) anode, and then improve the cycling performance and rate capabilities of Li||LiNi_0.6 Co_0.2 Mn_0.2 O₂ (NCM622). The HFBMZ can remove the trace H₂ O and HF from the electrolyte, reducing the by-products on the surface of solid electrolyte interphase (SEI) and inhibiting the dissolution of metal ions from NCM622. Also, the HFBMZ can enhance the wettability of the separator to promote uniform Li deposition. HFBMZ can make Li⁺ easy to be desolvated, resulting in the increase of Li⁺ flux on Li anode surface. Moreover, the HFBMZ can optimize the composition and structure of SEI. Therefore, the Li||Li symmetrical cells with 1 wt% HFBMZ-contained electrolyte can achieve stable cycling for more than 1200 h at 0.5 mA cm^-2 . In addition, the capacity retention rate of the Li||NCM622 can reach 92% after 150 cycles at 100 mA g^-1 .

A scalable, ecofriendly, and cost-effective lithium metal protection layer from a Post-it note

Although there have been many studies addressing the dendrite growth issue of lithium (Li)-metal batteries (LMBs), the Li-metal anode has not yet been implemented in today's rechargeable batteries. There is a need to accelerate the practical use of LMBs by considering their cost-effectiveness, ecofriendliness, and scalability. Herein, a cost-effective and uniform protection layer was developed by simple heat treatment of a Post-it note. The carbonized Post-it protection layer, which consisted of electrochemically active carbon fibers and electrochemically inert CaCO3 particles, significantly contributed to stable plating and stripping behaviors. The resulting protected Li anode exhibited excellent electrochemical performance: extremely low polarization during cycling (<40 mV at a current density of 1 mA cm-2) and long lifespan (5000 cycles at 10 mA cm-2) of the symmetric cell, as well as excellent rate performance at 2C (125 mA h g-1) and long cyclability (cycling retention of 62.6% after 200 cycles) of the LiFePO4‖Li full cell. The paper-derived Li protection layer offer a facile and scalable approach to enhance LMB electrochemical performance.

Growth Mechanism of Micro/Nano Metal Dendrites and Cumulative Strategies for Countering Its Impacts in Metal Ion Batteries: A Review

Metal-ion batteries are capable of delivering high energy density with a longer lifespan. However, they are subject to several issues limiting their utilization. One critical impediment is the budding and extension of solid protuberances on the anodic surface, which hinders the cell functionalities. These protuberances expand continuously during the cyclic processes, extending through the separator sheath and leading to electrical shorting. The progression of a protrusion relies on a number of in situ and ex situ factors that can be evaluated theoretically through modeling or via laboratory experimentation. However, it is essential to identify the dynamics and mechanism of protrusion outgrowth. This review article explores recent advances in alleviating metal dendrites in battery systems, specifically alkali metals. In detail, we address the challenges associated with battery breakdown, including the underlying mechanism of dendrite generation and swelling. We discuss the feasible solutions to mitigate the dendrites, as well as their pros and cons, highlighting future research directions. It is of great importance to analyze dendrite suppression within a pragmatic framework with synergy in order to discover a unique solution to ensure the viability of present (Li) and future-generation batteries (Na and K) for commercial use.

Optimizing Electrode/Electrolyte Interphases and Li-Ion Flux/Solvation for Lithium-Metal Batteries with Qua-Functional Heptafluorobutyric Anhydride

The safety and electrochemical performance of rechargeable lithium-metal batteries (LMBs) are primarily influenced by the additives in the organic liquid electrolytes. However, multi-functional additives are still rarely reported. Herein, we proposed heptafluorobutyric anhydride (HFA) as a qua-functional additive to optimize the composition and structure of the solid electrolyte interphase (SEI) at the electrode/electrolyte interface. The reduction/oxidation decomposition of the fluorine-rich HFA facilitate uniform inorganic-rich SEI and compact cathode electrolyte interphase (CEI) formation, which enables stable lithium plating during charge and suppresses the dissolution of transition-metal ions. Moreover, HFA optimizes the Li-ion solvation for stable Li plating/stripping and serves as the surfactant to enhance the wettability of the separator by the electrolyte to increase Li-ion flux. The symmetric Li∥Li cell with 1.0 wt % HFA electrolyte had an excellent cycling performance over 340 h at 1.0 mA cm^-2 with a capacity of 0.5 mAh cm^-2 while the Li∥NCM622 cell maintained high capacity retention after 250 cycles and outstanding rate performance even at 15 C.

Dendrite-Suppressing Polymer Materials for Safe Rechargeable Metal Battery Applications: From the Electro-Chemo-Mechanical Viewpoint of Macromolecular Design

Metal batteries have been emerging as next-generation battery systems by virtue of ultrahigh theoretical specific capacities and low reduction potentials of metallic anodes. However, significant concerns regarding the uncontrolled metallic dendrite growth accompanied by safety hazards and short lifespan have impeded practical applications of metal batteries. Although a great deal of effort has been pursued to highlight the thermodynamic origin of dendrite growth and a variety of experimental methodologies for dendrite suppression, the roles of polymer materials in suppressing the dendrite growth have been underestimated. This review aims to give a state-of-the-art overview of contemporary dendrite-suppressing polymer materials from the electro-chemo-mechanical viewpoint of macromolecular design, including i) homogeneous distribution of metal ion flux, ii) mechanical blocking of metal dendrites, iii) tailoring polymer structures, and iv) modulating the physical configuration of polymer membranes. Judiciously tailoring electro-chemo-mechanical properties of polymer materials provides virtually unlimited opportunities to afford safe and high-performance metal battery systems by resolving problematic dendrite issues. Transforming these rational design strategies into building dendrite-suppressing polymer materials and exploiting them towards polymer electrolytes, separators, and coating materials hold the key to realizing safe, dendrite-free, and long-lasting metal battery systems.

Stable Lithium Deposition Enabled by an Acid-Treated g-C3N4 Interface Layer for a Lithium Metal Anode

Li metal has been regarded as one of the most promising anode candidates for high-energy rechargeable lithium batteries. Nevertheless, the practical applications of the Li anode have been hampered because of its low Coulombic efficiency and safety hazards. Here, acid-treated g-C₃N₄ with O- and N-containing groups are coated on Li foil through a facile physical pressing method. The O- and N-containing groups cooperate to rearrange the concentration of Li ions and enhance the Li ion transfer. Hence, the cycle and rate performances of acid-treated g-C₃N₄-coated Li electrodes are greatly improved in symmetric cells, which show cycling stability over 400 h at 1 mA cm^-2 in ester-based electrolytes and over 2100 h in ether-based electrolytes. As for the Li//LiFePO₄ full cells, there is a high capacity retention of 80% over 400 cycles at 1 C. The full cells of Li//S in ether-based electrolytes also exhibit a capacity of 520 mA h g^-1 after 400 cycles at 1 C.

Functional Ionic Liquid Modified Core-Shell Structured Fibrous Gel Polymer Electrolyte for Safe and Efficient Fast Charging Lithium-Ion Batteries

Fast charging is of enormous concerns in the development of power batteries, while the low conductivity and lithium ion transference number in current electrolytes degraded the charge balance, limited the rate performance, and even cause safety issues for dendrite growth. Combine inorganic fillers and ionic liquid plasticizer, here in this paper we prepared a core-shell structured nanofibrous membrane, by incorporating with carbonate based electrolyte, a gel polymer electrolyte (GPE) with high conductivity, outstanding Li+ transference number was obtained. Notably, the Li/electrolyte/LiNi0.6Co0.2Mn0.2O2 (NCM622) half-cell with this composite electrolyte delivers a reversible capacity of 65 mAh/g at 20C, which is 13 times higher than that of with Celgard 2325 membrane. It also shows enhanced long-term cycle stability at both 3C and 5C for the suppression of lithium dendrite. This organic-inorganic co-modified GPE guarantees the fast charging ability and safety of LIBs, thus provides a promising method in high performance electrolyte design.

Graphene Oxide Induced Surface Modification for Functional Separators in Lithium Secondary Batteries

Functional separators, which have additional functions apart from the ionic conduction and electronic insulation of conventional separators, are highly in demand to realize the development of advanced lithium ion secondary batteries with high safety, high power density, and so on. Their fabrication is simply performed by additional deposition of diverse functional materials on conventional separators. However, the hydrophobic wetting nature of conventional separators induces the polarity-dependent wetting feature of slurries. Thus, an eco-friendly coating process of water-based slurry that is highly polar is hard to realize, which restricts the use of various functional materials dispersible in the polar solvent. This paper presents a surface modification of conventional separators that uses a solution-based coating of graphene oxide with a hydrophilic group. The simple method enables the large-scale tuning of surface wetting properties by altering the morphology and the surface polarity of conventional separators, without significant degradation of lithium ion transport. On the surface modified separator, superior wetting properties are realized and a functional separator, applicable to lithium metal secondary batteries, is demonstrated as an example. We believe that this simple surface modification using graphene oxide contributes to successful fabrication of various functional separators that are suitable for advanced secondary batteries.

Vinyl Ethylene Carbonate as an Effective SEI-Forming Additive in Carbonate-Based Electrolyte for Lithium-Metal Anodes

We report the use of vinyl ethylene carbonate as a new solid electrolyte interface (SEI)-forming additive for Li-metal anodes in carbonate-based electrolyte, which has the advantages of both good storage performance and low price. Compared to the SEI formed in vinyl ethylene carbonate-free electrolyte, the SEI film formed in 10% vinyl ethylene carbonate electrolyte contains a higher relative content of polycarbonate species and a greater amount of decomposition products of LiPF₆ salt. Both components are expected to have positive effects on the passivation of Li-metal surface and the accommodation of volume changes of anode during cycling. Scanning electron microscopy images and COMSOL numerical simulation results further confirm that uniform Li deposition morphology can be achieved in the presence of vinyl ethylene carbonate additive. When cycling at the current density of 0.25 mA cm^-2 with a cycling capacity of 1.0 mAh cm^-2, the vinyl ethylene carbonate-contained Li-Cu cell exhibits a long life span of 816 h (100 cycles) and a relatively high Coulombic efficiency of 93.2%.

In Situ Dendrite Suppression Study of Nanolayer Encapsulated Li Metal Enabled by Zirconia Atomic Layer Deposition

Progressing toward the emerging era of high-energy-density batteries, stable and safe employment of lithium (Li) metal anodes is highly desired. The primary concern with Li metal anodes is their uncontrollable dendrites growth and extreme sensitivity to parasitic degradation reactions, raising the alarms for battery safety and shelf life. Nanolayer protection encapsulation, which is conformal and ionically conductive with a high-κ dielectric property, can suppress the degradation and empower stabilization of Li metal. In this work, engineering of a zirconia (ZrO₂) encapsulation layer on Li metal enabled by atomic layer deposition (ALD) was employed and investigated for surface-enhanced dendrite suppression properties using in situ optical observations and electrochemical cycling. The ALD process involved a combination of plasma subcycle activation and thermal subcycle activation in increasing the surface functionalization and chemisorption sites for precursors to obtain highly dense and conformal deposition. The encapsulation of Li with ZrO₂ ALD nanolayer further demonstrated excellent tolerance to atmospheric exposure for at least 1-5 h because of a conformal physical barrier, and excellent heat tolerance up-to 170-180 °C (close to Li melting point) and high rate capability due to thermal resistive property and high ionic transport property, respectively, of the ZrO₂ ceramic. The results establish a technology transferable to other metal anode chemistries and offer a potential insight into carrying out solid-state electrolyte multilayer coatings with high processing temperature flexibility and thereby providing a leap in the advancing of a range of high energy density all-solid-state batteries.