Active Separators Featuring PF 6 − Anion‐Regulating Interface for Long‐Term Stable Li‐Based Batteries

Thickness variation of lithium metal anode with cycling

Low Li+ transference number in liquid electrolytes can lead to uneven lithium deposition and dendrite growth, reducing coulombic efficiency and cycle life of lithium metal batteries. In this study, metal‐organic frameworks (MOFs) are used as electrolyte modulator to regulate Li+ transport. The ‐NH2 group introduced into MOFs can form hydrogen bonds with anions in the electrolyte, immobilizing these anions and thereby facilitating the migration of cations. As a result, a uniform and dense deposition morphology of lithium was achieved. The Li+ transference number is increased from 0.26 to 0.57. The assembled Li||Li symmetrical cell can stably cycle for over for 900 h with an average overpotential below 20 mV. In addition, the rate performance of the battery can be enhanced.

Li metal batteries based on solid polymer electrolytes offer the benefits of high energy density and safety, as well as extended cycling life, making them an excellent candidate for the next‐generation battery system. However, current solid polymer electrolytes still suffer from low ion conductivity and Li+ transfer number, which seriously restricts its practical application. Herein, a self‐supporting composite solid polymer electrolyte was prepared, where phenolic resin rich in hydroxyl groups (BR) and polyethylene oxide (PEO) are mixed evenly and poured onto a cellulose membrane in one step. In such an electrolyte, PEO and BR combine to form intermolecular hydrogen bonds, lowering the crystallinity of PEO and increasing the Li+ transfer number. Lastly, the obtained solid electrolytes exhibited a high ion conductivity (1.1×10−4 S cm−1) and Li+ transfer number (0.53), as well as improved electrochemical window. Consequently, Li || Li symmetrical cells can run stably for more than 700 h at 0.1 mA cm−2/0.25 mAh cm−2. And full cells with LiFePO4 cathode can also demonstrate high discharge capacity of 152.12 mAh g−1 and rate performance. We believe that such a design based on supramolecular interaction offer a new avenue to advanced solid polymer electrolytes.

Lithium-metal batteries (LMBs) with high energy densities are highly desirable for energy storage, but generally suffer from dendrite growth and side reactions in liquid electrolytes; thus the need for solid electrolytes with high mechanical strength, ionic conductivity, and compatible interface arises. Herein, a thiol-branched solid polymer electrolyte (SPE) is introduced featuring high Li+ conductivity (2.26 × 10-4 S cm-1 at room temperature) and good mechanical strength (9.4 MPa)/toughness (≈500%), thus unblocking the tradeoff between ionic conductivity and mechanical robustness in polymer electrolytes. The SPE (denoted as M-S-PEGDA) is fabricated by covalently cross-linking metal-organic frameworks (MOFs), tetrakis (3-mercaptopropionic acid) pentaerythritol (PETMP), and poly(ethylene glycol) diacrylate (PEGDA) via multiple CSC bonds. The SPE also exhibits a high electrochemical window (>5.4 V), low interfacial impedance (<550 Ω), and impressive Li+ transference number (tLi+ = 0.44). As a result, Li||Li symmetrical cells with the thiol-branched SPE displayed a high stability in a >1300 h cycling test. Moreover, a Li|M-S-PEGDA|LiFePO4 full cell demonstrates discharge capacity of 143.7 mAh g-1 and maintains 85.6% after 500 cycles at 0.5 C, displaying one of the most outstanding performances for SPEs to date.

Poly(vinylidene fluoride) (PVDF)-based polymer electro-lytes are attracting increasing attention for high-voltage solid-state lithium metal batteries because of their high room temperature ionic conductivity, adequate mechanical strength and good thermal stability. However, the presence of highly reactive residual solvents, such as N, N-dimethylformamide (DMF), severely jeopardizes the long-term cycling stability. Herein, we propose a solvation-tailoring strategy to confine residual solvent molecules by introducing low-cost 3 Å zeolite molecular sieves as fillers. The strong interaction between DMF and the molecular sieve weakens the ability of DMF to participate in the solvation of Li+, leading to more anions being involved in solvation. Benefiting from the tailored anion-rich coordination environment, the interfacial side reactions with the lithium anode and high-voltage NCM811 cathode are effectively suppressed. As a result, the solid-state Li||Li symmetrical cells demonstrates ultra-stable cycling over 5100 h at 0.1 mA cm-2, and the Li||NCM811 full cells achieve excellent cycling stability for more than 1130 and 250 cycles under the charging cut-off voltages of 4.3 V and 4.5 V, respectively. Our work is an innovative exploration to address the negative effects of residual DMF in PVDF-based solid-state electrolytes and highlights the importance of modulating the solvation structures in solid-state polymer electrolytes.

Electronic devices have been advanced rapidly over the last few decades with improved performance through high-level integration. Lithium ion batteries (LIB), which is representative rechargeable energy storage system, is increasingly demanding to high-energy-density storage system for the stable operation of high performance devices like mobile, laptop, etc. In addition, a recent quantum leap in electronic vehicles (EVs) require to energy storage system with long cycling stability incomparable to previous electronic devices. To achieve high performances and long lifetime, the use of lithium metal is one of attractive anode for as LIBs, because Li metal has high theoretical specific capacity of 3860 mAh/g, low redox potential of -3.04V (verse the standard hydrogen electrode) and low gravimetric density of 0.59 g/cm3. Nevertheless, since Li metal anode has the lithiation-delithiation mechanism of the ‘hostless’ electrochemical plating/stripping unlike the conventional graphite anode, it is confronted with serious problems occurred from uncontrollable interfacial reaction between surface of lithium metal anode and electrolyte applying to traditional LIB system; that is (1) the uncontrollable formation of solid electrolyte interphase (SEI) layer limitless volume expansion of Li metal anode (2) the dendritic growth and isolation of Li interface derived from inhomogeneous deposition of Li+ ion flux (3) the Li surface fracture induced by heterogeneously plated Li+ ion. Through these process, repetitive formation of undesirable SEI layer at surface of Li anode make fresh electrolyte dissipation and concurrently the anode surface becomes more distorted. These side-phenomena strongly correlate with low coulombic efficiency and decreasing lifetime of lithium metal battery (LMB) system where lithium metal is used as anode. In this works, in a dimensional aspect, to solve such problem mentioned above through stabilizing of surface of lithium metal anode, we introduce Janus-faced protective layer (JPL) as gel polymer electrolyte. JPL was composed with dual layer of Ni (3~7μm) embedded poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) and bare PVdF-HFP through simple casting method. The process of preparing JPL membrane is so simple to instantly bring in existing roll-to-roll process, and it is also possible to make a low cost process considered relatively low nickel price. In terms of mechanical property, it is based on flexible polymer and shows the merit of being foldable enough to able origami. In addition, JPL play a role of protection of Li metal anode. JPL has two functionalities. The metal part of JPL directly contacted with anode, and this layer was frameworks for preventing irregular SEI layer formation through inward plating of Li ion, the polymer part is stable state enough to be used as a gel polymer electrolyte. When lithium metal anode is subjected to cycle-test while keeping in contact with the nickel side of JPL membrane, Scanning electron microscope (SEM) image showed that the surface of lithium metal anode was very clean without any fluctuation compared to the case where only polypropylene (PP) separator was used. (Figure. 1) Moreover, as shown in Figure 2., the LMB system applied JPL show high stability of tolerable overvoltage within 5mV, 15mV even at high-current of 10mA/cm2, 20mA/cm2, respectively, in Li//JPL/JPL//Li symmetric cell. These results are attributable to effective function of each layer as mentioned earlier. JPL consists of two layer, the tough and adhesive nature of PVdF-HFP prevents the polymer part from being damaged or torn by dendritic growth and simultaneously ensure that metal part functions as robust framework for SEI layer formation without escaped particle from the membrane. This frameworks induce the SEI layer, which can act as a resistive element, to grown in the metal part while surface of Li metal anode become get out of main subject from the SEI layer formation. By virtue of that property, JPL has not allow forming resistive elements on the surface of Li metal anode, preventing the voltage rising up due to the formation of resistive elements at surface, so tolerability of overvoltage with clean surface of anode is maintained. In conclusion, the model study of nickel micro-particle/PVdF-HFP composite suggests the straightforward direction of protective layer to apply lithium metal as anode in LIB system with alleviating the problem caused by the ‘hostless’ property of lithium metal anode. Thus, when JPL is introduced to a LIB system, it shows a fairly positive possibility for high performance Lithium metal battery. Figure 1

Li metal batteries promise the next-generation rechargeable batteries for electric vehicles applications due to the highest specific capacity (3840 mA h g-1) and lowest reduction potential (−3.04 V vs. S.H.E.) of Li among all alkali and alkaline earth metals. However, a non-uniform solid electrolyte interphase (SEI) layer formed via parasite reactions of Li with the electrolytes leads to locally site-specific Li deposition, i.e., dendritic plating.1 Dynamic breaking and forming of this layer, in turn, leads to the accumulative loss of Li, build-up of the SEI and consumption of the electrolyte causing cell failure. Upon prolonged cycling, penetration of dendrites through the separator causes hazardous cell short-circuits. Modified electrolytes including concentrated electrolytes,2 LiF additive3 and artificial SEI layer4 have shown promise in suppressing dendrites. In this presentation, we will demonstrate an in-vivo formed sulfide-based ionic conductive layer — realized by using a low concentration complexed electrolyte additive — that not only generates a kinetically favorable interface, but also reduces the corrosive reactions of the electrolyte with Li. Upon Li plating, the high ionic conductivity of the solid electrolyte layer allows the Li+flux to be evenly distributed over the conductive surface. Furthermore, even at extremely high current densities when incipient dendritic Li start to form, the reservoir additives in the electrolyte facilitate local formation of the conductive layer. Upon resting at OCV, the Li|Li symmetric cell in the presence of the electrolyte additive shows much lower interfacial charge transfer resistance than a cell with a “blank” electrolyte (20 vs. 320 Ω cm-2). Furthermore, the impedance is stable over 2 days of rest, in contrast with significantly increasing impedance exhibited by the blank cell. Greatly improved long-term cycling of the symmetric cells is observed. The cell with additive exhibits extremely stable voltage evolution over 400 hours, whereas the blank electrolyte cell experiences voltage fluctuation followed by short-circuit after 270 hours (Figure 1a). Notably, stable cycling of Li|Li symmetric cells over 2500 hours at 1 mA cm-2 with 1 mA h cm-2 capacity was achieved using 100 mM additive (Figure 1b), and a significant decrease in polymerized solvent is observed compared to a cell with no additive. In a full Li metal cell using Li4Ti5O12 as the positive electrode, the cell with the electrolyte additive exhibits very stable capacity retention (88%) over 400 cycles. Reference 1. Y. S. Cohen, Y. Cohen, D. Aurbach, J. Phys. Chem. B, 2000, 104, 12282-12291. 2. J. Qian, W. A. Henderson, W. Xu, P. Bhattacharya, M. Engelhard, O. Borodin, J.-G. Zhang, Nat. Commun., 2015, 6, 6362. 3. Y. Lu, Z. Tu, L. A. Archer, Nat. Mater., 2014, 13, 961–969. 4. G. Zheng, S. W. Lee, Z. Liang, H.-W. Lee, K. Yan, H. Yao, H. Wang, W. Li, S. Chu, Y. Cui, Nat. Nanotech., 2014, 9, 618–623. Figure 1

Solid state lithium metal batteries are the most promising next‐generation power sources owing to their high energy density and safety. Solid polymer electrolytes (SPE) have gained wide attention due to the excellent flexibility, manufacturability, lightweight, and low‐cost processing. However, fatal drawbacks of the SPE such as the insufficient ionic conductivity and Li+ transference number at room temperature restrict their practical application. Here vertically aligned 2D sheets are demonstrated as an advanced filler for SPE with enhanced ionic conductivity, Li+ transference number, mechanical modulus, and electrochemical stability, using vermiculite nanosheets as an example. The vertically aligned vermiculite sheets (VAVS), prepared by the temperature gradient freezing, provide aligned, continuous, run‐through polymer‐filler interfaces after infiltrating with polyethylene oxide (PEO)‐based SPE. As a result, ionic conductivity as high as 1.89 × 10−4 S cm−1 at 25 °C is achieved with Li+ transference number close to 0.5. Along with their enhanced mechanical strength, Li|Li symmetric cells using VAVS–CSPE are stable over 1300 h with a low overpotential. LiFePO4 in all‐solid‐state lithium metal batteries with VAVS–CSPE could deliver a specific capacity of 167 mAh g−1 at 0.1 C at 35 °C and 82% capacity retention after 200 cycles at 0.5 C.

Organic ionic plastic crystals (OIPCs) are a class of solid-state electrolytes with good thermal stability, non-flammability, non-volatility, and good electrochemical stability. When prepared in a composite with electrospun polyvinylidene fluoride (PVdF) nanofibers, a 1:1 mixture of the OIPC N-ethyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide ([C2 mpyr][FSI]) and lithium bis(fluorosulfonyl)imide (LiFSI) produced a free-standing, robust solid-state electrolyte. These high-concentration Li-containing electrolyte membranes had a transference number of 0.37(±0.02) and supported stable lithium symmetric-cell cycling at a current density of 0.13 mA cm-2 . The effect of incorporating PVdF in the Li-containing plastic crystal was investigated for different ratios of PVdF and [Li][FSI]/[C2 mpyr][FSI]. In addition, Li|LiNi1/3 Co1/3 Mn1/3 O2 cells were prepared and cycled at ambient temperature and displayed a good rate performance and stability.

LiNi00.8Co0.1Mn0.1O2 (NCM811) is perceived as a promising cathode material in lithium-ion batteries for its high specific capacity and low cost. However, the catalytic surface between the cathode and electrolyte leads to intensive interfacial reactions and gas generation, ultimately causing rapid capacity fading and a great threat to the safety performance of the battery, which hinders its applications at high voltages. Herein, ethoxy(pentafluoro)cyclotriphosphazene (PFPN) as the electrolyte additive is applied to alleviate the above problems by constructing a solid electrolyte interphase on the cathode and anode. PFPN can be decomposed preferentially over basic electrolytes to shape the thin and stable interface film between the electrolyte and NCM811 cathode for restraining the production of gas and the corrosion of NCM811. The wettability of the separator can be enhanced by PFPN to promote uniform Li deposition. Also, PFPN has a flame-retardant effect due to its phosphoronitrile functional group. Moreover, the Li||Li symmetrical cells can achieve long-cycling stability with the PFPN-electrolyte (cycling performance of 800 h at 0.5 mA cm–2). Last, the capacity retention of NCM811||Li reaches 89.5% after 200 cycles at 1 C and 4.5 V.

Sulfide solid‐state electrolytes (SEs) face severe interfacial instability against lithium metal anodes, leading to detrimental interfacial reactions and rapid degradation. Existing artificial interphases often lack graded functionality to simultaneously block side reactions and guide uniform Li deposition. Herein, guided by theoretical screening of adsorption competition mechanism and interfacial reaction kinetics, we propose a dynamically tailored gradient self‐assembled interphase (SAI) via synergistic reaction of heptafluorobutyric acid (HFA) and fluoroethylene carbonate (FEC) on Li metal surface. The SAI layer features an organic‐rich outer layer, a LiF‐rich intermediate barrier, and a lithiophilic lithium heptafluorobutyrate (LiHFA) inner layer, achieving high interfacial energy, smooth morphology ( R a = 296 nm), and enhanced ion transport. The design enables an ultrahigh critical current density (CCD) of 2.7 mA cm −2 and ultra‐stable cycle over 2000 h at 0.5 mA cm −2 in Li||Li symmetric cells. Moreover, the LiCoO 2 |Li 6 PS 5 Cl|Li full cells with the SAI layer demonstrate remarkable capacity retention of 89.81% after 200 cycles at 0.3 C. This work provides a fundamental thermodynamics‐guided framework for the targeted design of high‐performance interphases in all‐solid‐state Li metal batteries (ASSLMBs).

It is difficult to achieve higher energy density with the existing system of lithium (Li)-ion batteries. As a powerful candidate, Li metal batteries are in the renaissance. Unfortunately, the uncontrolled growth process of Li dendrites has limited their actual application. Hence, inhibiting the formation and spread of Li dendrites has become an enormous challenge. Herein, a novel composite separator is developed with functionalized boron nitride nanosheet modification layer as a Li-ion regulator to regulate Li-ion fluxes. The composite separator contains abundant polar groups and nanoscale channels and could achieve uniform electrochemical deposition via the lithiophilic effect and shunting action. Under the synergy influence of the lithiophilic effect and shunting action, Li dendrites are effectively suppressed. As proof, the Li||Li symmetrical cells with composite separators can circulate steadily for a long time under high current densities (10 mA cm-2, 800 h). Moreover, the LiFePO4||Li full cells display excellent long cycling performance (82% retention after 800 cycles).

ABSTRACTComposite quasi‐solid‐state electrolytes are pivotal for enabling high‐energy‐density lithium metal batteries (LMBs), yet their practical application is hindered by discontinuous ion transport, poor interfacial stability, and limited high‐voltage endurance. Here, a universal in situ growth strategy is developed to construct a metal‐organic framework (MOF)/polymer composite electrolyte (ZCPSE) with hierarchically ordered ion‐conducting networks. The ultra‐uniform MOF nanoparticles (e.g., ZIF‐8) are anchored onto polymer nanofibers, creating abundant nanopores and Lewis acid sites that synergistically enhance Li⁺ transport and oxidative stability. The resulting ZCPSE exhibits unprecedented ionic conductivity (0.46 mS cm−1 at 25°C), a wide electrochemical window (5.15 V vs. Li/Li+), and exceptional mechanical strength (151.2 MPa, 4× higher than pristine polymer membrane). Theoretical simulations reveal that the 3D continuous MOF/polymer interface facilitates rapid Li+ dissociation and uniform flux distribution, endowing ZCPSE with a high Li+ transference number (0.74) and dendrite‐free Li plating/stripping (2000 h in Li|Li symmetric cells). Practical applicability is demonstrated in Li|LiFePO4 cells (stable cycling at 25°C–100°C) and high‐voltage Li|LiNi0.8Co0.1Mn0.1O2 full cells (4.5 V, 100 cycles with 99.2% capacity retention). This study provides a paradigm for designing MOF‐based hybrid electrolytes with simultaneous ionic, mechanical, and interfacial optimization, paving the way for safe and high‐energy LMBs.

ETPTA polymer network confined amide-based eutectic electrolyte for safe and long- life lithium metal battery

Solid-state lithium-metal (Li0) batteries are gaining traction for electric vehicle applications because they replace flammable liquid electrolytes with a safer, solid-form electrolyte that also offers higher energy density and better resistance against Li dendrite formation. Solid polymer electrolytes (SPEs) are highly promising candidates because of their tuneable mechanical properties and easy manufacturability; however, their electrochemical instability against lithium-metal (Li0), mediocre conductivity and poorly understood Li0/SPE interphases have prevented extensive application in real batteries. In particular, the origin of the low Coulombic efficiency (CE) associated with SPEs remains elusive, as the debate continues as to whether it originates from unfavoured interfacial reactions or lithium dendritic growth and dead lithium formation. In this work, we use state-of-the-art cryo-EM imaging and spectroscopic techniques to characterize the structure and chemistry of the interface between Li0 and a polyacrylate-based SPE. Contradicting the conventional knowledge, we find that no protective interphase forms, owing to the sustained reactions between deposited Li dendrites and polyacrylic backbones and succinonitrile plasticizer. Due to the reaction-induced volume change, large amounts of cracks form inside the Li dendrites with a stress-corrosion-cracking behaviour, indicating that Li0 cannot be passivated in this SPE system. On the basis of this observation, we then introduce additive engineering, leveraging from knowledge of liquid electrolytes, and demonstrate that the Li0 surface can be effectively protected against corrosion using fluoroethylene carbonate, leading to densely packed Li0 domes with conformal and stable solid-electrolyte interphase films. Owing to the high room-temperature ionic conductivity of 1.01 mS cm-1, the high transference number of 0.57 and the stabilized lithium-electrolyte interface, this improved SPE delivers an excellent lithium plating/stripping CE of 99% and 1,800 hours of stable cycling in Li||Li symmetric cells (0.2 mA cm-2, 1 mAh cm-2). This improved cathodic stability, along with the high anodic stability, enables a record high cycle life of >2,000 cycles for Li||LiFePO4 and >400 cycles for Li||LiCoO2 full cells.