Lithium Ion Transference Number Research Articles

Initiator-free in-situ synthesized polymer electrolytes with high ionic conductivity for dendrite-free lithium metal batteries

In-situ preparation of polymer electrolytes (PEs) can enhance electrolyte/electrode interface contact and accommodate the current large-scale production line of lithium-ion batteries (LIBs). However, reactive initiators of in-situ PEs may lead to low capacity, increased impedance and poor cycling performance. Flammable and volatile monomers and plasticizers of in-situ PEs are potential safety risks for the batteries. Herein, we adopt lithium difluoro(oxalate)borate (LiDFOB)-initiated in-situ polymerization of solid-state non-volatile monomer 1,3,5-trioxane (TXE) to fabricate PEs (In-situ PTXE). Fluoroethylene carbonate (FEC) and methyl 2,2,2-trifluoroethyl carbonate (FEMC) with good fire retardance, high flash point, wide electrochemical window and high dielectric constant were introduced as plasticizers to improve ionic conductivity and flame retardant property of In-situ PTXE. Compared with previously reported in-situ PEs, In-situ PTXE exhibits distinct merits, including free of initiators, non-volatile precursors, high ionic conductivity of 3.76 × 10−3 S cm−1, high lithium-ion transference number of 0.76, wide electrochemical stability window (ESW) of 6.06 V, excellent electrolyte/electrode interface stability and effectively inhibition of Li dendrite growth on the lithium metal anode. The fabricated LiFePO4 (LFP)/Li batteries with In-situ PTXE achieve significantly boosted cycle stability (capacity retention rate of 90.4% after 560 cycles) and outstanding rate capability (discharge capacity of 111.7 mAh g−1 at 3C rate).

Novel nanoarchitecture of 3D ion transfer channel containing nanocomposite solid polymer electrolyte membrane based on holey graphene oxide and chitosan biopolymer

In the arena of three-dimensional (3D) nanoarchitecture, holey graphene oxide (HGO)—a class of two-dimensional (2D) graphene oxide porous nanosheet, has emerged as a promising choice of additive material to design advanced 3D nanocomposite for energy and environmental applications. With a facile and cost-effective solution-casting technique, we fabricated here a flexible, wearable, free-standing, and super-thin (∼0.08 mm) solid polymer electrolyte membrane (SPEM) consisting of 2D-HGO, lithium bis(trifluromethanesulfonyl)imide (LiTFSI) salt, polyvinylpyrrolidone (PVP) polymer binder, and chitosan (CH) biopolymer. SPEM exhibited impressive ionic conductivity of 2.76 × 10-3 S⋅cm−1 at room temperature (RT = 23 °C) which is comparable to liquid electrolytes. Robust mechanical property (5.87 MPa) and easy lithium-ion diffusion capability of SPEM were identified by the ultra-low activation energy (Ea) of 0.089 eV, which is one of the best values among the reported solid polymer electrolytes. Good lithium-ion transference number (tLi+= 0.76) and wide electrochemical stability window (ESW = 4.4) indicated single ion conduction and stable battery operation voltage capability of SPEM. Moreover, fast ion transfer mechanism of SPEM was proposed according to the comprehensive characterizations; mostly relating to uniform and strong interconnecting novel 3D ion transferring routes. Temperature and frequency responsive charge carrier mobility trend were also investigated with an in-depth dielectric study. Promising RT galvanostatic Li plating-stripping performance was observed at 5 mA⋅cm−2 with a hybrid symmetric cell. Using Li metal as anode, SPEM, and LiCoO2 as cathode in the full cell, a good RT specific discharge capacity of 142.8 mA⋅h⋅g−1 was achieved at 0.1 C rate.

Interfacial Interaction of Multifunctional GQDs Reinforcing Polymer Electrolytes For All-Solid-State Li Battery.

Solid-state polymer electrolytes are highly anticipated for next generation lithium ion batteries with enhanced safety and energy density. However, a major disadvantage of polymer electrolytes is their low ionic conductivity at room temperature. In order to enhance the ionic conductivity, here, graphene quantum dots (GQDs) are employed to improve the poly (ethylene oxide) (PEO) based electrolyte. Owing to the increased amorphous areas of PEO and mobility of Li+ , GQDs modified composite polymer electrolytes achieved high ionic conductivity and favorable lithium ion transference numbers. Significantly, the abundant hydroxyl groups and amino groups originated from GQDs can serve as Lewis base sites and interact with lithium ions, thus promoting the dissociation of lithium salts and providing more ion pathways. Moreover, lithium dendrite is suppressed, associated with high transference number, enhanced mechanical properties and steady interface stability. It is further observed that all solid-state lithium batteries assembled with GQDs modified composite polymer electrolytes display excellent rate performance and cycling stability.

Versatile solid polymer electrolytes from clickable poly(glycidyl propargyl ether) for lithium metal batteries

We present three new graft copolymers for the application as solid polymer electrolytes (SPEs) in solvent-free, all solid-state lithium metal batteries (LMBs), based on the clickable poly (glycidyl propargyl ether) platform. These fully amorphous polymers consist of an all-ether polymer backbone and different additional Li+-coordinating oxo-functionalities - diester, carbonate and phosphate ester - in the attached sidechain. We aim to address the fundamental question on the influence of Li+-coordination in such doubly functionalized systems having competing Li+-coordination functional groups. We prepared polymer nanocomposite SPEs based on these novel polymers mixed with a highly-dissociating lithium salt (LiTFSI) and TiO2 nanofillers, which improve the electrolyte-electrode compatibility. We perform an extensive and comparative investigation in terms of physicochemical and electrochemical properties including ionic conductivity, lithium transference number, lithium plating and stripping as well as the fabrication and cycling of LMBs utilizing lithium iron phosphate (LFP) cathodes. Thermal characterization is coupled with temperature-dependent ionic conductivity measurements and detailed IR spectroscopy to understand the mechanistic properties and selectivity of the Li+ coordination and conduction. Two of the nanocomposite SPEs show efficient battery cycling with initial capacities of ca. 150 mAh g−1 and a capacity retention of ca. 70–80 % after 100 cycles.

A soft co-crystalline solid electrolyte for lithium-ion batteries.

Alternative solid electrolytes are the next key step in advancing lithium batteries with better thermal and chemical stability. A soft solid electrolyte, (Adpn)2LiPF6 (Adpn, adiponitrile), is synthesized and characterized that exhibits high thermal and electrochemical stability and good ionic conductivity, overcoming several limitations of conventional organic and ceramic materials. The surface of the electrolyte possesses a liquid nano-layer of Adpn that links grains for a facile ionic conduction without high pressure/temperature treatments. Further, the material can quickly self-heal if fractured and provides liquid-like conduction paths via the grain boundaries. A substantially high ion conductivity (~10-4 S cm-1) and lithium-ion transference number (0.54) are obtained due to weak interactions between 'hard' (charge dense) Li+ ions and the 'soft' (electronically polarizable) -C≡N group of Adpn. Molecular simulations predict that Li+ ions migrate at the co-crystal grain boundaries with a (preferentially) lower activation energy Ea and within the interstitial regions between the co-crystals with higher Ea values, where the bulk conductivity is a smaller but extant contribution. These co-crystals establish a special concept of crystal design to increase the thermal stability of LiPF6 by separating ions in the Adpn solvent matrix, and also exhibit a unique mechanism of ion conduction via low-resistance grain boundaries, which contrasts with ceramics or gel electrolytes.

In Situ Thermal Polymerization of a Succinonitrile-Based Gel Polymer Electrolyte for Lithium-Oxygen Batteries.

For lithium-oxygen batteries (LOBs), the leakage and volatilization of a liquid electrolyte and its poor electrochemical performance are the main reasons for the slow industrial advancement. Searching for more stable electrolyte substrates and reducing the use of liquid solvents are crucial to the development of LOBs. In this work, a well-designed succinonitrile-based (SN) gel polymer electrolyte (GPE-SLFE) is prepared by in situ thermal cross-linking of an ethoxylate trimethylolpropane triacrylate (ETPTA) monomer. The continuous Li+ transfer channel, formed by the synergistic effect of an SN-based plastic crystal electrolyte and an ETPTA polymer network, endows the GPE-SLFE with a high room-temperature ionic conductivity (1.61 mS cm-1 at 25 °C), a high lithium-ion transference number (tLi+ = 0.489), and excellent long-term stability of the Li/GPE-SLFE/Li symmetric cell at a current density of 0.1 mA cm-2 for over 220 h. Furthermore, cells with the GPE-SLFE exhibit a high discharge specific capacity of 4629.7 mAh g-1 and achieve 40 cycles.

A copolyether with pendant cyclic carbonate segment for PEO-based solid polymer electrolyte

The application of PEO-based solid polymer electrolyte (SPE) is limited by the low ionic conductivity and lithium-ion transference number near room temperature. In this paper, a pendant polyether poly (EO-co-PCGE) (PECP) with low molecular weight is prepared by copolymerization of ethylene oxide (EO) monomer with typical lithium-ion conduction group and propylene carbonate glycidyl ether (PCGE) monomer with high dielectric group. By blended with PECP, the PEO-based SPE is obtained and its ionic conductivity can be increased to 3.54 × 10−5 S cm−1 at room temperature and 1.32 × 10−4 S cm−1 at 40 °C. In one aspect, the pendant structure and low molecular weight are beneficial to the reduction of the crystallinity of PEO matrix; in a further aspect, the cyclic carbonate pendant offers a competitive effect on segmental motion and the lithium-ion complexation of PEO, thus affecting the conduction mechanism of lithium-ions in the conventional PEO-based SPE and significantly increasing the lithium-ion transference number. The synergistic effect of these two aspects makes the prepared PEO-based SPE have excellent properties of ionic conduction especially at room temperature, and the LiFePO4/Li batteries with the SPE present excellent cycling performance at the temperature of 40 °C with a discharge capacity of about 100 mA h g−1 after 150 battery cycles.

Simultaneously Suppressing Shuttle Effect and Dendrite Growth in Lithium-Sulfur Batteries via Building Dual-Functional Asymmetric-Cellulose Gel Electrolyte.

Polysulfides huttling and interfacial instability of Lithium-anode are the main technical issues hindering commercialization of high-energy-density lithium-sulfur batteries. Simply addressing the problem of polysulfide shuttling or lithium dendrite growth can result in safety hazards or short lifespan. To synchronously tackle the aforementioned issues,the authorshave designed an asymmetric cellulose gel electrolyte, a defective and ionized UiO66/black phosphorus heterostructure coating layer (Di-UiO66/BP) and a cationic cellulose gelelectrolyte (QACA). Defective and ionized engineered UiO66 particles significantly enhances performance of UiO66/BP layer in anchoring free polysulfides, promoting smooth and effective polysulfide conversion and expediting the redox kinetics of sulfur cathode, therefore suppressing polysulfide shuttling. QACA electrolyte with numerous cationic groups can interact with anions via electrostatic adsorption, thus enhancing lithium-ion transference number and contributing to formation of stable solid electrolyte interface to suppress lithium dendrite growth. Owing to the superior performance of QACA/Di-UiO66/BP, the final cells exhibit outstanding electrochemical performance, presenting high sulfur utilization (1420.1 mAh g-1 at 0.1 C), high-rate capacity (665.4 mAh g-1 at 4 C) and long cycle lifespan. This work proposes a strategy of designing asymmetric electrolytes to simultaneously address the challenges in both S-cathode and Li-anode, which contributes to advanced Li-S batteries and their practical application.

Polybenzimidazole-reinforced polyethylene oxide-based polymer-in-salt electrolytes enabling excellent structural stability and superior electrochemical performance for lithium metal batteries

Polyethylene oxide (PEO)-based solid polymer electrolyte (SPEs) is full of attraction due to its exceptional lithium ion dissolubility and strong resistance toward lithium metal reduction. Nevertheless, it still suffers from the unfulfilling room temperature ionic conductivity and poor mechanical properties. Herein, a flexible and robust polymer-in-salt electrolyte based on composite of polybenzimidazole (PBI) and PEO via a facile preparation approach has been developed for room-temperature lithium metal batteries (LMBs). The rigid framework structure of PBI and dense hydrogen bonds formed between PBI and PEO jointly reinforce the structural stability of SPEs at high lithium salt concentration, which makes the SPEs possess sufficient mechanical properties to resist the lithium dendrites growth. The thermal stability and fire resistance of SPEs have also been improved relying on the distinctive flame retardancy and incombustibility of PBI. Besides, PBI is prone to forming hydrogen bonds with lithium salt anions to limit their mobility, while the rich N atoms on PBI chains can promote the dissociation of lithium salt through electrostatic attraction interaction. Their synergistic effects make the PEO-based polymer-in-salt electrolytes achieve the high room temperature ionic conductivity of 5.7 × 10−4 S cm−1, wide electrochemical window of 4.45 V, and large lithium transference number of 0.639. When applied to LMBs with LiFePO4 or LiNi0.88Co0.06Mn0.06O2 cathode, both coin cells exhibit improved cycling performance and rate capability. Furthermore, the LiNi0.88Co0.06Mn0.06O2/SiOx-C pouch cells using the SPEs demonstrate remarkable flexibility and safety, clarifying that the PEO-based polymer-in-salt electrolytes possess a promising application prospect for safe and high-performance LMBs.

Sandwich-Structured Quasi-Solid Polymer Electrolyte Enables High-Capacity, Long-Cycling, and Dendrite-Free Lithium Metal Battery at Room Temperature.

The insufficient ionic conductivity, limited lithium-ion transference number (tLi +), and high interfacial impedance severely hinder the practical application of quasi-solid polymer electrolytes (QSPEs). Here, a sandwich-structured polyacrylonitrile (PAN) based QSPE is constructedin which MXene-SiO2 nanosheets act as a functional filler to facilitate the rapid transfer of lithium-ion in the QSPE, and a polymer and plastic crystalline electrolyte (PPCE) interface modification layer is coated on the surface of the PAN-based QSPE of 3wt.% MXene-SiO2 (SS-PPCE/PAN-3%) to reduce interfacial impedance. Consequently, the synthesized SS-PPCE/PAN-3% QSPE delivers a promising ionic conductivity of ≈1.7mScm-1 at 30°C, a satisfactory tLi + of 0.51, and a low interfacial impedance. As expected, the assembled Li symmetric battery with SS-PPCE/PAN-3% QSPE can stably cycle more than 1550h at 0.2 mA cm-2 . The Li||LiFePO4 quasi-solid-state lithium metal battery (QSSLMB) of this QSPE exhibits a high capacity retention of 81.5% after 300 cycles at 1.0C and at RT. Even under the high-loading cathode (LiFePO4 ≈10.0mgcm-2 ) and RT, the QSSLMB achieves a superior area capacity and good cycling performance. Besides, the assembled high voltage Li||NMC811(loading≈7.1 mgcm-2 ) QSSLMB has potential applications in high-energy fields.

The effects of functionalization of imidazolium-based ionic liquid electrolytes on the lithium-ion transport

The application of salt-in-ionic liquid electrolytes (SILEs) in lithium-ion batteries is restricted by the negative transference number of lithium ion (tLi+) from strong ion-ion correlations. In this study, we applied molecular dynamics (MD) simulations on three imidazolium-based functionalized SILEs, composed of 1-hexyl-3-methylimidazolium (HMIM+), 1-(2-methoxyethoxy)methyl-3-methylimidazolium (MOEOMMIM+), or 1-[2-(methoxycarbonyloxy)ethyl]-3-methylimidazolium (EMCMIM+), and bis(trifluoromethanesulfonyl)imide (TFSI−), doped with 0.25 molar fraction of LiTFSI. MD simulations show that tLi+ is improved in the SILE composed of [EMCMIMTFSI]0.75[LiTFSI]0.25. Detailed analyses show that Li+ can coordinate with the carbonyl oxygen on the side chain of EMCMIM+, which competes with TFSI−, and thus weakens Li+–TFSI− coordination. On the other hand, one of ether oxygens on the side chain of MOEOMMIM+ tends to form intra-ionic hydrogen bonds (HBs) with the hydrogen atoms on the imidazolium ring, which even hinders the coordination between MOEOMMIM+ and TFSI−, and thus reinforces Li+–TFSI− coordination. The net result is that tLi+ increases in the order [MOEOMMIM] < [HMIM] < [EMCMIM]. These findings may help on the further design of functionalized SILEs toward a desired tLi+.

A Highly Salt-Soluble Ketone-Based All-Solid-State Polymer Electrolyte with Superior Performances for Lithium-Ion Batteries

Solid polymer electrolytes (SPEs) have great potential to be used in high-safety lithium-ion batteries (LIBs). However, it is still a significant challenge for SPEs to develop high ionic conductivity, high mechanical strength, and good interior interfacial compatibility. In this work, a ketone-based all-solid-state electrolyte (PAD) resulting from allyl acetoacetate (AAA), diacetone acrylamide (DAAM), and poly(ethylene glycol) diacrylate (PEGDA) was prepared by UV-inducing photopolymerization. The abundant ketone groups endow the prepared PAD all-solid-state electrolyte with strong dissociation of lithium salts and weak coordination interactions between ketone groups and Li+. Depending on the unique properties of the ketone groups in the electrolyte system, the prepared polymer electrolytes show a high lithium-ion transference number of 0.87 and a wide electrochemical window of 4.95 V. Furthermore, the PAD electrolyte also exhibits superior viscoelasticity, which is beneficial for good contact with electrodes. As a result, the assembled LFP/PAD/Li cells with PAD electrolytes show good cycle performance and rate performance. Concretely, the all-solid-state symmetric lithium cells with the PAD electrolyte can achieve stable lithium plating and stripping at 0.05 mA cm-2 for over 1000 h at 60 °C. This work highlights the advantages of ketone-based electrolyte as a polymer electrolyte and provides a design method for advanced polymer electrolytes applied in high-performance solid lithium batteries.

Ultrathin and Robust Composite Electrolyte for Stable Solid-State Lithium Metal Batteries

Solid-state polymer electrolytes (SPEs) are considered as one of the most promising candidates for the next-generation lithium metal batteries (LMBs). However, the large thickness and severe interfacial side reactions with electrodes seriously restrict the application of SPEs. Herein, we developed an ultrathin and robust poly(vinylidene fluoride) (PVDF)-based composite polymer electrolyte (PPSE) by introducing polyethylene (PE) separators and SiO2 nanoparticles with rich silicon hydroxyl (Si-OH) groups (nano-SiO2). The thickness of the PPSE is only 20 μm but possesses a quite high mechanical strength of 64 MPa. The introduction of nano-SiO2 fillers can tightly anchor the essential N,N-dimethylformamide (DMF) to reinforce the ion-transport ability of PVDF and suppress the side reactions of DMF with Li metal, which can significantly enhance the electrochemical stability of the PPSE. Meanwhile, the Si-OH groups on the surface of nano-SiO2 as a Lewis acid promote the dissociation of the lithium bis(fluorosulfonyl)imide (LiFSI) and immobilize the FSI- anions, achieving a high lithium transference number (0.59) and an ideal ionic conductivity (4.81 × 10-4 S cm-1) for the PPSE. The assembled Li/PPSE/Li battery can stably cycle for a record of 11,000 h, and the LiNi0.8Co0.1Mn0.1O2/PPSE/Li battery presents an initial specific capacity of 173.3 mA h g-1 at 0.5 C, which can stably cycle 300 times. This work provides a new strategy for designing composite solid-state electrolytes with high mechanical strength and ionic conductivity by modulating their framework.

UV-photopolymerized cellulose acetate-acrylate membranes for lithium-ion battery separator

Cellulose acetate (CA) is attractive for use as a separator in lithium-ion batteries (LIBs). However, the dissolubility in the liquid electrolyte limits its applications. Herein, we would report a cellulose acetate (CA) cross-linked film for the separator of lithium-ion battery. First, methacrylate modified cellulose acetate (M-CA) was prepared by esterification between CA and methylacryloyl chloride. Subsequently, an insoluble polymer membrane was obtained through the UV-induced polymerization of M-CA, poly(ethylene glycol) methyl ether methacrylate (PEGMEMA) and trihydroxymethylpropyl trimethylacrylaye (TMPTMA). The results show that these polymer membranes have good thermal and chemical stability due to the cross-linked polymer matrix. When the amount of TMPTMA is 20% of the molar amount of PEGMEMA, not only has the obtained gel polymer electrolyte (GPE) a suitable ionic conductivity of 4.17 mS cm−1 and an excellent lithium ion transfer number of 0.80, but also it has discharge capacity of the half battery composed of LiFePO4/GPE/Li 120 mAh g−1 at 2 C. This study demonstrated a great possibility that the high-performance and biodegradable GPE could be applied in LIBs. In addition, there are many other methacrylate monomers that can be polymerized with M-CA to prepare various GPEs, which will open a new avenue for the application of CA in GPEs.

Ultrathin poly(cyclocarbonate-ether)-based composite electrolyte reinforced with high-strength functional skeleton

Composite polymer electrolytes (CPEs) are considered to be the most promising to break through the performance and safety limitations of traditional lithium-ion batteries because of their excellent electrochemical and mechanical properties. Aiming at the performance limitations of the most common polyether matrix such as poly(ethylene oxide) (PEO), a novel poly(cyclocarbonate-ether) polymer matrix was prepared by in-situ thermal curing, the weaker interaction between its C = O bond and Li+ can promote the rapid transport of Li+. Adding ionic liquid and active filler LLZTO to the matrix can synergistically reduce the crystallinity of matrix and promote the dissociation of lithium salts. In addition, a 3D functional skeleton made of polyacrylonitrile (PAN) and lithium fluoride (LiF) can greatly improve the mechanical strength of polymer matrix after cold pressing, and LiF is also conducive to interface stability. The thickness of the optimal sample (VP6L/CPL) was only 25 μm, and its ionic conductivity, lithium ion transference number, and electrochemical stability window were as high as 7.17 × 10−4 S cm−1 (25 °C), 0.54 and 5.4 V, respectively, while the mechanical strength reaches 6.1 MPa, which can fully inhibit the growth of lithium dendrites. The excellent electrochemical performance and mechanical strength enable the assembled Li|VP6L/CPL|Li battery to be continuously charged for over 200 h and cycled stably for more than 2300 h, and Li|VP6L/CPL|LFP battery can be stably cycled for more than 400 and 550 cycles at 1 C (40 °C) and 0.5 C (25 °C), respectively.

Effect of LiTFSI Electrolyte Salt Composition on Characteristics of PVDF-PEO-LiTFSI-Based Solid Polymer Electrolyte (SPE) for Lithium-Ion Battery

A lithium-ion battery with PVDF-PEO synthetic polymer sheet added by LiTFSI electrolyte salt has been made by assembling method. This study aims to determine the effect of LiTFSI salt concentration on the performance of lithium-ion batteries. The composition of LiTFSI electrolyte salts was varied into 5%; 10%; 15%; and 20%. Several characterizations were carried out to determine battery performance, including Electrochemical Impedance Spectrometry (ElS), Cyclic Voltammetry (CV), Charge/Discharge (CD), and Lithium Transference Number (LTN). The results showed that the synthesized separator sheet with a LiTFSI salt composition of 20% producing voltage, ionic conductivity, and lithium-ion transfer number of 0.72 V; 3.94 x 10-8 SCm-1; and 0.895, respectively is potential for lithium-ion batteries application. These results indicate the use of LiTFSI electrolyte salts with a concentration of 20% shows the best performance for PVDF-PEO-LiTFSI-based lithium-ion batteries.

A dielectric electrolyte composite with high lithium-ion conductivity for high-voltage solid-state lithium metal batteries.

The ionic conductivity of composite solid-state electrolytes does not meet the application requirements of solid-state lithium (Li) metal batteries owing to the harsh space charge layer of different phases and low concentration of movable Li+. Herein, we propose a robust strategy for creating high-throughput Li+ transport pathways by coupling the ceramic dielectric and electrolyte to overcome the low ionic conductivity challenge of composite solid-state electrolytes. A highly conductive and dielectric composite solid-state electrolyte is constructed by compositing the poly(vinylidene difluoride) matrix and the BaTiO3-Li0.33La0.56TiO3-x nanowires with a side-by-side heterojunction structure (PVBL). The polarized dielectric BaTiO3 greatly promotes the dissociation of Li salt to produce more movable Li+, which locally and spontaneously transfers across the interface to coupled Li0.33La0.56TiO3-x for highly efficient transport. The BaTiO3-Li0.33La0.56TiO3-x effectively restrains the formation of the space charge layer with poly(vinylidene difluoride). These coupling effects contribute to a quite high ionic conductivity (8.2 × 10-4 S cm-1) and lithium transference number (0.57) of the PVBL at 25 °C. The PVBL also homogenizes the interfacial electric field with electrodes. The LiNi0.8Co0.1Mn0.1O2/PVBL/Li solid-state batteries stably cycle 1,500 times at a current density of 180 mA g-1, and pouch batteries also exhibit an excellent electrochemical and safety performance.

Ion Transport Regulated Lithium Metal Batteries Achieved by Electrospun ZIF/PAN Composite Separator with Suitable Electrolyte Wettability

Lithium metal battery (LMB) is a topic receiving growing attention due to the high theoretical capacity, while its practical application is seriously hindered by the lithium dendrites issue. As the physical barrier between two electrodes, the separator can achieve dendrite suppression by means of providing higher mechanical strength, regulating ion transport and facilitating homogeneous lithium deposition. Based on this, a composite separator is fabricated with zeolitic imidazolate framework (ZIF-8) and polyacrylonitrile (PAN) via electrospinning techniques, and its physical properties and electrochemical performances, together with its dendrite suppression mechanism, are investigated. The ZIF8-PAN separator possesses a unique 3D interconnected porous skeleton, displaying higher electrolyte uptake, preferable electrolyte wettability, and lower thermal shrinkage compared with the commercial polypropylene separator. In addition, a battery assembled with the ZIF8-PAN separator can effectively regulate ion transport and suppress dendrites growth, which exhibits an enhanced ionic conductivity (1.176 mS/cm), an increased lithium-ion transference number (0.306), a wider electrochemical stability window (5.04 V), and superior cycling stability (over 600 h with voltage hysteresis of 30 mV). This work offers a promising strategy to realize safe separator for dendrite suppression in LMB.

Click chemistry-initiated highly uniform semi-interpenetrating polymer electrolyte with dual salts for high-performance lithium metal batteries

Solid-state lithium metal batteries built with polymer electrolytes are attractive owing to their high energy density, easy manufacturing, and high safety. However, polymer electrolytes' low ionic conductivity and poor interfacial stability seriously restrict their practical application. Herein, we designed a highly uniform semi-interpenetrating polymer network electrolyte (SIPE) based on thiol–ene click chemistry. This SIPE embedded with linear poly(ethylene glycol) dimethyl (PEGDME) short chains achieves good ionic conductivity (1.78 × 10−4 S cm−1 at 30 °C), low activation energy (0.21 eV), and high lithium-ion transference number (0.53). The thiol–ene click reaction-initiated network effectively promotes homogenous distribution of PEGDME nanodomain in polymer matrix. Therefore, short PEG chains can activate the segmental motion of polymer matrix, provide strong Li+-coordination for fast diffusion, and thus facilitate the dissociation of more lithium salts. We demonstrate a dual-salt of LiTFSI and LiDFOB salt in SIPE (D-SIPE), which contributes to a rigid and stable LiF-rich solid electrolyte interphase towards electrodes. The D-SIPE-based cell achieves improved specific capacity and cyclability when tested in solid-state Li metal batteries with LiFePO4 cathode. This work provides a strategy for rationally designing polymer electrolytes and broadening the application of click chemistry in solid-state lithium metal batteries.

Polymers with Cyanoethyl Ether and Propanesulfonate Ether Side Chains for Solid-State Li-Ion Battery Applications

Two key challenges of solid-state polymer electrolytes (SPEs) are low ionic conductivity and low lithium transference number, which must be resolved in order to achieve satisfactory cycling performance for solid-state lithium batteries. Herein, we successfully substituted β-cyanoethyl ether [−O–(CH2)2–CN] and propanesulfonate ether [−O–(CH2)3–SO3–] groups for the −OH group in polyvinyl alcohol. SPEs were then prepared by mixing the resultant graft-polymer with various levels of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). The SPE with an optimal concentration of LiTFSI (50 wt %) showed a maximum ionic conductivity of 5.4 × 10–4 S cm–1 with an activation energy of 31.6 kJ mol–1, an electrochemical stability window of 5.3 V (vs Li/Li+), and a lithium-ion transference number of 0.48 at room temperature (∼25 °C). The plating/stripping tests for a Li|SPE|Li cell demonstrated a steady voltage profile without short-circuiting issues for 500 cycles (i.e., 500 h). Furthermore, a solid-state Li|SPE|LiFePO4 battery was assembled with exceptional cycling stability at 25 °C. A discharge (DC) capacity of 159.1 mA h g–1 was attained at a 0.1 C-rate. In addition, the DC capacity at a 0.5 C-rate and capacity retention after 576 cycles are 138.0 mA h g–1 and 70%, respectively.