Mechanically and Thermally Robust Gel Polymer Electrolytes with Dynamic Hydrogen-Bond Networks for Wide-Temperature Lithium Metal Batteries.

Supramolecular polymer cross-linking gel electrolyte for highly stable quasi-solid-state lithium metal batteries

Molecular engineering of a gel polymer electrolyte via in-situ polymerization for high performance lithium metal batteries

High-voltage lithium metal batteries (LMBs) are a promising high-energy-density energy storage system. However, their practical implementations are impeded by short lifespan due to uncontrolled lithium dendrite growth, narrow electrochemical stability window, and safety concerns of liquid electrolytes. Here, a porous composite aerogel is reported as the gel electrolyte (GE) matrix, made of metal-organic framework (MOF)@bacterial cellulose (BC), to enable long-life LMBs under high voltage. The effectiveness of suppressing dendrite growth is achieved by regulating ion deposition and facilitating ion conduction. Specifically, two hierarchical mesoporous Zr-based MOFs with different organic linkers, that is, UiO-66 and NH2 -UiO-66, are embedded into BC aerogel skeletons. The results indicate that NH2 -UiO-66 with anionphilic linkers is more effective in increasing the Li+ transference number; the intermolecular interactions between BC and NH2 -UiO-66 markedly increase the electrochemical stability. The resulting GE shows high ionic conductivity (≈1 mS cm-1 ), high Li+ transference number (0.82), wide electrochemical stability window (4.9V), and excellent thermal stability. Incorporating this GE in a symmetrical Li cell successfully prolongs the cycle life to 1200 h. Paired with the Ni-rich LiNiCoAlO2 (Ni: Co: Al = 8.15:1.5:0.35, NCA) cathode, the NH2 -UiO-66@BC GE significantly improves the capacity, rate performance, and cycle stability, manifesting its feasibility to operate under high voltage.

Gel polymer electrolytes (GPEs) are regarded as a promising alternative to traditional liquid electrolytes due to safety concerns of batteries. However, GPEs encounter challenges primarily related to their low oxidative decomposition potential and inferior interfacial compatibility with electrodes. Poly(methyl methacrylate‐trifluoroethyl methacrylate‐isocyanate ethyl methacrylate) (P(MMA‐TFEMA‐IMA)) fluoropolymer‐based GPE is designed and prepared by a facile in situ thermal polymerization process. Due to the high liquid uptake ability of polymers inherited from MMA monomers, GPE exhibits a high room temperature ionic conductivity. By incorporating the fluorinated functional group with higher bond energy through TFEMA monomer, the GPE achieves an enhanced oxidative decomposition voltage. Due to effective interfacial contact between electrode and electrolyte, the Li||GPE||Li cell demonstrates stable cycling time for over 500 h without short‐circuiting. Furthermore, Li||GPE||LiNi0.6Co0.2Mn0.2O2 cell exhibits an initial discharge capacity of 171.7 mAh g−1 at 0.5 C current rate, maintaining 81.5% of its initial capacity after 200 cycles. The partial IMA segment plays a crucial role in building a high‐quality and stable cathode electrolyte interface film, which contributes to the reduced interfacial resistance and the improved structural stability of high‐voltage cathodes. These findings provide a straightforward method for designing comprehensive electrolytes suitable for high‐energy‐density lithium metal batteries.

Lithium metal batteries (LMBs) have been considered as next-generation energy storages due to their extremely high theoretical specific capacity (3860 mAh g-1). However, current LMBs, using conventional liquid electrolytes, still could not fulfill the demand of soaring expansion of energy era, such as electrical vehicles, because of their safety issues, originated by uncontrollable electrolytic side reaction on the lithium, resulting unstable solid-electrolyte interphase (SEI) and vicious lithium dendritic growth [1]. Also, carbonate-based liquid electrolytes have an intrinsic flammability, and the lithium dendrite, which short-circuits a cell, can lead to severe safety hazard with the unfavorable flammability of current liquid system when they are ignited. Therefore, solid-state electrolytes have been spotlighted recently for a pathway for safe, and high energy and power LMBs, due to their superior thermal stability and low vapor pressure, while maintaining suitable electrolytic performances.In this study, solid-state single-ion conducting polymer electrolytes (SICPEs), utilizing dynamic anion delocalization (DAD), realizing high ionic conductivity and dimensional stability for high-performance LMB, are studied. The SICPEs enable superior lithium transference number, resulting in highly reduced concentration gradient of lithium cation along the electrolyte to suppress the undesirable lithium dendritic growth. However, SICPEs have prominently lower ionic conductivity than dual-ion conducting polymer electrolyte (DICPEs), which is a critical issue to make a slower charge/discharge for SICPEs [2]. Although an approach utilizing gel polymer electrolyte (GPE), using a liquid solvent as a plasticizer, has been exploited to increase the ionic conductivity of SICPEs, GPEs have struggled with lower mechanical stability, compared to solid state, and still existing flammability issue with the plasticizer. The novel plasticizer, which is described here, can interact with bulky anionic polymer matrix, so that the negative charge can be dispersed onto the whole complex by DAD. Once the bulky complex is formed by DAD, the dissociation of lithium cation from anionic matrix can be easier with the decreased activation energy and higher ionic conduction. While increasing the ionic conductivity with DAD, the nature of polymeric plasticizer will highly suppress flammability. DAD allows the membrane endure more tensile strength due to the dynamic structural change in crosslinking state, so that the polymer electrolyte can tolerate dendritic growth of lithium by morphological change on an electrode surface.The obvious advantages of DAD-induced solid polymer electrolytes in this study for a high energy and power, and ultra-safe LMB can present a novel approach of polymer electrolyte design to the astronomical demand of energy storages.[1] F. Ahmed, I. Choi, M.M. Rahman, H. Jang, T. Ryu, S. Yoon, L. Jin, Y. Jin, W. Kim, ACS Appl. Mater. Interfaces 2019, 11, 34930-34938.[2] D.-M. Shin, J.E. Bachman, M.K. Taylor, J. Kamcev, J.G. Park, M.E. Ziebel, E. Velasquez, N.N. Jarenwattananon, G.K. Sethi, Y. Cui, J.R. Long, Adv. Mater. 2020, 32, 1905771.

Rechargeable lithium metal batteries (LMBs) are considered the "holy grail" of energy storage systems. Unfortunately, uncontrollable dendritic lithium growth inherent in these batteries has prevented their practical applications. The benefits of solid-state electrolyte for LMBs are limited due to the common compromise between ionic conductivity and mechanical property. This work proposes a mechanism for simultaneous improvement in ionic conductivity and mechanical strength of gel polymer electrolyte (GPE) which is based on tunable cross-linked polymer network through adjusting monomer ratios. With increasing bisphenol A ethoxylate dimethacrylate (E2BADMA) and poly(ethylene glycol) diacrylate (PEGDA) mass ratios in GPE precursors, the formed polymer network experienced a composition evolution from a 3D cross-linked mono PEGDA network to triple PEGDA, E2BADMA, and PEGDA/E2BADMA networks and then to dual E2BADMA and PEGDA/E2BADMA networks, accompanied by the increase in both storage modulus (from 6 to 37 MPa) and ionic conductivity (from 0.06 to 0.44 mS cm-1). As a result, the E2BADMA/PEGDA mass ratio of 2:1 facilitates the successful fabrication of a dual-network-supported GPE (PEEPL-12) with a mechanical strength of 37 MPa and superior electrochemical properties (a high ionic conductivity of 0.44 mS cm-1 and a wide electrochemical stability window of 4.85 V vs Li/Li+). Such polymer electrolyte-based symmetric lithium metal batteries delivered a long cycle life (2000 h at 0.1 mA cm-2 and 0.1 mAh cm-2), and the Li|PEEPL-12|LiFePO4 cell delivered a high capacity of 140 mAh g-1 at the 100th cycle at the current density of 0.1 C (1 C = 170 mAh g-1). A more thorough investigation indicated the formation of a stable solid electrolyte interphase layer on a lithium metal anode. These extraordinary features open up a venue for fabrication of advanced polymer electrolyte for long-cycle-life lithium metal batteries.

Lithium metal batteries (LMBs) are considered among the most promising energy storage technologies due to their very high energy density and lightweight. While liquid electrolytes (LEs) offer high ionic conductivities (10−3−10−2 S cm−1) and low interfacial resistance with electrodes, yet the risks of leakage and combustion of organic solvents, lithium (Li) dendrite formation, thermal runaway, and environmental concerns regarding using organic solvents hinder their full potential applications. Li dendrites are mainly induced by nonuniform distribution of charge or ions and formation of a fragile, unstable solid electrolyte interphase (SEI) layer on Li metal anode during electrochemical Li deposition. The dendritic morphology of electrochemically deposited Li results in inefficient lithiation/delithiation and the formation of inactive or dead Li and constant degradation of organic solvents during Li plating/stripping, leading to capacity decade and decline in cycle life. Furthermore, lithium dendrites can pose a safety issue by penetrating through a porous or mechanically soft separator and making a short-circuit. High voltage instability of conventional electrolytes is another issue restricting application of high potential cathodes to enhance the energy density of LMBs. Gel polymer electrolytes (GPEs) are considered a promising alternative for LEs because of their high ionic conductivities (10−3−10−2 S cm−1), prevention of liquid leakage, low flammability, chemical and electrochemical stability, high flexibility for portable and wearable electronics, suppression of dendrite formation, and ample chemistry and design [1-4].In this work, we have developed a novel, highly efficient PEO-based GPE by a simple, facile chemical method and using UV light-induced cross-linking. Cross-linking is essential to retain the structure of the polymer electrolyte and it is also shown to suppress Li dendrite growth by offering a high stiffness and rigidity (high shear modulus) toward Li dendrites. The designed GPE owns a very high ionic conductivity similar to commercial LEs, electrochemical stability up to 5 V, long-term cycle life and stability. While Li|Li symmetric cells made with 1 M LiPF6 in EC/DEC and 1 M LiTFSI in DOL/DME LEs showed an increase in overpotential during cycling (mainly due to the formation of dead Li and evolution of SEI) followed by a short-circuit (probably due to penetration of Li dendrites through Celgard separator), Li|Li cells made with GPE exhibit a long-term stability with a low voltage polarization of less than 0.1 V at a high current density of 0.5 mAh cm−2 for 500 cycles. Furthermore, Li|LFP full cells made with GPE display a long-term cycle life with discharge capacity of ~120 mAh g−1 at 0.5 C in ambient temperature with around 80% capacity retention after 500 cycles.In situ light microscopy which is a simple, versatile technique to study the nucleation and growth of Li particles during electrochemical platting/stripping was also used to further illustrate uniform deposition of Li by the developed GPEs. Hence, we have also performed light microscopy analysis to investigate Li deposition in Li|Li cells made of LEs and GPEs during electrochemical deposition process. It was observed that unlike LEs made of 1 M LiPF6 in EC/DEC and 1 M LiTFSI in DOL/DME which results in non-uniform, dendritic growth of Li, a uniform and non-dendritic deposition of Li was observed in case of GPE, which can be due to uniform regulation of Li through complexation with oxygen atoms of the PEO-based GPE.

Fluorine grafted gel polymer electrolyte by in situ construction for high-voltage lithium metal batteries

Ether electrolytes are considered to be one of the most promising candidates for lithium metal batteries (LMBs) since they help to form a stable solid electrolyte interface (SEI) on lithium anodes. However, conventional liquid ether electrolytes are not suitable for the high‐voltage LMBs over 4.5 V due to their narrow potential windows. Herein, a gel electrolyte strategy is demonstrated to improve the high voltage stability of ether electrolytes in LMBs with Li‐rich layered oxide (LLO) cathodes that usually operate at 4.6 V and above. Compared with a liquid electrolyte, the electrochemical window of the gel electrolyte with cross‐linked amide framework is increased by 1 V. The strong cross‐linked framework not only regulates the uniform growth of cathode electrolyte interface (CEI)/SEI on the cathode/anode sides, but also greatly increases the Li+ transference number and inhibits the generation of harmful substances by anchoring PF6− anions, thus providing a long cycle for LMBs at a high voltage up to 4.7 V. Simultaneously, the flame retardancy and flexibility of the gel electrolyte ensures the safe operation of high‐voltage LMBs. Therefore, the LLO||Li pouch cell using gel electrolyte realizes stable cycling at a high voltage 4.6 V, and can pass the acupuncture test.

A fluorinated gel polymer electrolyte (FGPE) was synthesized via in situ copolymerization of acrylamide (AM) and 1, 1, 1, 3, 3, 3-hexafluoroisopropyl acrylate (HFA). The synergistic interaction between –CF3 and CCreated by potrace 1.16, written by Peter Selinger 2001-2019]]>O groups endows the electrolyte with high ionic conductivity (1.21 × 10−3 S cm−1), a lithium-ion transference number of 0.68, and an electrochemical stability window up to 4.75 V. Symmetric Li‖FGPE‖Li cells exhibit stable cycling for over 1000 hours with a polarization voltage of 25 mV. Meanwhile, LFP‖Li full cells retain 87% of their initial capacity after 200 cycles, confirming the effectiveness of synergistic interactions between –CF3 and CO in enhancing the performance of high-energy lithium metal batteries. This study establishes a design paradigm for high-conductivity functional gel polymer electrolytes, providing a viable pathway toward lithium metal batteries with integrated high stability and high conductivity capabilities.

Self-assembly formation of solid-electrolyte interphase in gel polymer electrolytes for high performance lithium metal batteries

In-situ construction of cellulose acetate propionate-based hybrid gel polymer electrolyte for high-performance lithium metal batteries

Herein, we present the synthesis and electrochemical performance of a comb-like polycaprolactone-based gel electrolyte from acrylate terminated polycaprolactone oligomers and liquid electrolyte for high-voltage lithium metal batteries. The ionic conductivity of this gel electrolyte at room temperature was measured to be 8.8 × 10-3 S cm-1, which is an exceptionally high value that is more than sufficient for the stable cycling of solid-state lithium metal batteries. The Li+ transference number was detected to be 0.45, facilitating the prohibition of concentration gradients and polarization, thereby prohibiting lithium dendrite formation. In addition, the gel electrolyte exhibits high oxidation voltage up to 5.0 V vs. Li+/Li and perfect compatibility against metallic lithium electrodes. The superior electrochemical properties provide the LiFePO4-based solid-state lithium metal batteries with excellent cycling stability, displaying a high initial discharge capacity of 141 mAh g-1 and an extraordinary capacity retention exceeding 74% of its initial specific capacity after being cycled for 280 cycles at 0.5C at room temperature. This paper presents a simple and effective in situ preparation process yielding an excellent gel electrolyte for high-performance lithium metal battery applications.

Multilayered amidated polymer of intrinsic microporosity for gel polymer electrolyte in lithium metal batteries

Polymer/ceramic gel electrolyte with in-situ interface forming enhances the performance of lithium metal batteries