Borderline Metal Centers on Nonporous Metal-Organic Framework Nanowire Boost Fast Li-Ion Interfacial Transport of Composite Polymer Electrolyte

Enhancing ionic conductivity and expanding the electrochemical window in polymer electrolytes via ferroelectric-metal-organic-frameworks to manipulate charge spatial distribution

Poly (ethylene oxide) (PEO)-based polymer electrolytes have promising applications in all-solid-state lithium metal batteries. However, their wide range of practical applications is severely limited by their relatively low room temperature lithium ion conductivity and narrow electrochemical window. In this paper, based on the ability of spontaneous polarization of ferroelectric materials to generate polarization field under applied electric field and the characteristics of Metal-Organic-Frameworks (MOFs) materials with regular adjustable pore structure, a Nano material combining ferroelectric materials and MOF (NUS-6(Hf)-MOF) was first proposed to be added to PEO polymer electrolyte as a filler. NUS-6(Hf)-MOF can provide rapid transport channel for lithium ion, born under the applied electric field of the latter's consistent dipole polarization field can adjust the interface of the space charge distribution, the composite polymer electrolyte (NUSCPE) modified with NUS-6(Hf)-MOF has high ionic conductivity (1.16 × 10-3 S cm-1) and lithium ion mobility (0.33) at 60 °C. Simultaneously, the phenomenon of self-polarization causes the surface electronization, this results in a higher susceptibility to oxidation compared to PEO, thereby expanding the electrochemical window to 4.7 V, and when paired with commercial LiNi0.8Co0.1Mn0.1O2 cathode, the capacity is maintained at 85 % after 50 stable cycles at 0.2 C at 60 °C. The polymer electrolyte NUSCPE modified by NUS-6(Hf)-MOF provides a novel design idea for all solid-state lithium batteries with excellent electrochemical performance.

Robust MOF-Based Composite Solid-State Electrolyte Membrane for High-Performance Lithium-Metal Batteries

In this work, we construct a robust MOF-based flexible composite membrane based on PVDF-HFP, UIO-66, and ionic liquid (IL). Through their synergistic reinforcement effect, the obtained solid-state electrolytes can simultaneously achieve high ionic conductivity, good mechanical properties, and flame retardance. The abundant pores of the MOF are capable of loading IL, which not only builds continuous ion channels and facilitates the dissociation of Li+ but also balances the mechanical properties and electrochemical performance. Consequently, the as-prepared electrolyte membranes exhibit excellent ionic conductivity (5.55 × 10-4 S cm-1), high Li+ transference number (0.52), moderate electrochemical window (4.3 V), outstanding mechanical properties (tensile strength of 6.63 MPa and elongation of 232%), and good interfacial stability (stable Li plating/stripping behavior). Meanwhile, the assembled LiFePO4//Li battery exhibits an excellent rate capability and long cycle stability. This work demonstrates a realistic strategy for the fabrication of MOF-based composite SSEs toward next generation high-performance lithium metal batteries.

Tailoring Ce-Centered Metal-Organic Frameworks for Fast Li+ Transport in Composite Polymer Electrolyte

Regulating metal nodes to innovate the metal-organic framework (MOF) structure is of great interest to boost the performance of MOFs-incorporated composite solid electrolytes. Herein, Ce4+ with a low-lying 4f orbital is selected as metal center to coordinate with organic ligand to prepare MOF of Ce-UiO-66. The unsaturated open metal sites and defected oxygen vacancies furnish Ce-UiO-66 with strengthened Lewis acidity, which promotes Ce-UiO-66 interacting effectively with both poly(ethylene oxide) (PEO) and Li salt anions. Accordingly, Ce-UiO-66 as additive fillers can be uniformly dispersed in PEO matrix to form an advanced composite solid-state electrolyte (Ce-UiO@PEO) with accelerated Li+ transport. The optimized Ce-UiO@PEO displays a boosted ionic conductivity of 4.20 × 10-4 S cm-1 and an improved Li+ transference number of 0.39 at 60 °C, which are highly comparable to those of other MOFs@PEO electrolytes. Combined with the mechanical and thermal stabilities, such a Ce-UiO@PEO electrolyte enables Li/Li symmetric and Li/LiFePO4 full cells with superior cycling stability and rate performance. The Ce-UiO@PEO electrolytes are of great potential to be applied in high-performance lithium metal batteries.

Multipolar Conjugated Polymer Framework Derived Ionic Sieves via Electronic Modulation for Long-Life All-Solid-State Li Batteries

Here, we build a tunable multipolar conjugated polymer framework platform via pore wall chemistry to probe the role of electronic structure engineering in improving the Li+ conduction by theoretical studies. Guided by theoretical prediction, we develop a new cyano-vinylene-linked multipolar polymer framework namely CNF-COF, which can act as efficient ion sieves to modify solid polymer electrolytes to simultaneously tune Li+ migration and stable Li anodes for long-lifespan all-solid-state (ASS) Li metal batteries at high rate. The dual-decoration of cyano and fluorine groups in CNF-COF favorably regulates electronic structure via multipolar donor-acceptor electronic effects to afford proper energy band structure and abundant electron-rich sites for enhanced oxidative stability, facilitated ion-pair dissociation and suppressed anion movements. Thus, the CNF-COF incorporation into poly (ethylene oxide) (PEO) electrolytes not only renders fast selective Li+ transport but also facilitates the Li dendrite suppression. Specifically, the constructed PEO composite electrolyte with an ultra-low CNF-COF content of only 0.5 wt % is endowed with a wide electrochemical window, a high ionic conductivity of 0.634 mS cm-1 at 60 °C and a large Li+ transference number of 0.81-remarkably outperforming CNF-COF-free counterparts (0.183 mS cm-1 and 0.22). As such, the Li symmetric cell delivers stable Li plating/stripping over 1400 h at 0.1 mA cm-2. Impressively, by coupling with LiFePO4 (LFP) cathodes, the assembled ASS Li battery under 60 °C allows for stable cycling over 2000 cycles at 1 C and over 1000 cycles even at 2 C with a large capacity retention of ~75 %, surpassing most reported ASS Li batteries using PEO-based electrolytes.

Composite poly(ethylene oxide)-based solid electrolyte with consecutive and fast ion transport channels constructed by upper-dimensional MIL-53(Al) nanofibers

Novel structural designs for metal organic frameworks (MOFs) are expected to improve ion-transport behavior in composite solid electrolytes. Herein, upper-dimensional MIL-53(Al) nanofibers (MNFs, MIL-53 belongs to the MIL (Material Institute Lavoisier) group) with flower-like nanoflake structures have been designed and constructed via modified hydrothermal coordination. The optimized MNFs with high surface area and porosity can form abundant interfaces with poly(ethylene oxide) (PEO) matrix. The plasticization of MNFs to the PEO matrix will facilitate segmental movement of PEO chains to facilitate Li+ conduction. The unsaturated open metal centers of MNFs can effectively capture bis(trifluoromethanesulfonyl)imide anions (TFSI-) to deliver more free lithium ions for transfer. Moreover, the upper-dimensional nanofiber structure endows lithium ions with a long-range and consecutive transport pathway. The obtained composite solid electrolyte (MNFs@PEO) presents a high ionic conductivity of 4.1 × 10-4 S cm-1 and a great Li+ transference number of 0.4 at 60 °C. The electrolyte also exhibits a stable Li plating/stripping behavior over 1000 h at 0.1 mA cm-1 with inhibited Li dendrite growth. Furthermore, the Li/LiFePO4 and Li/LiNi0.8Mn0.1Co0.1O2 batteries with MNFs@PEO as electrolytes both display great cycling stabilities with high-capacity retention, indicating their potential applications in lithium metal batteries. The study will put forward new inspirations for designing advanced MOF-based composite solid electrolytes.

Polydopamine-Induced Metal-Organic Framework Network-Enhanced High-Performance Composite Solid-State Electrolytes for Dendrite-Free Lithium Metal Batteries

Due to the high safety, flexibility, and excellent compatibility with lithium metals, composite solid-state electrolytes (CSEs) are the best candidates for next-generation lithium metal batteries, and the construction of fast and uniform Li+ transport is a critical part of the development of CSEs. In this paper, a stable three-dimensional metal-organic framework (MOF) network was obtained using polydopamine as a medium, and a high-performance CSE reinforced by the three-dimensional MOF network was constructed, which not only provides a continuous channel for Li+ transport but also restricts large anions and releases more mobile Li+ through a Lewis acid-base interaction. This strategy endows our CSEs with an ionic conductivity (7.1 × 10-4 S cm-1 at 60 °C), a wide electrochemical window (5.0 V), and a higher Li+ transfer number (0.54). At the same time, the lithium symmetric batteries can be stably cycled for 2000 h at 0.1 mA cm-2, exhibiting excellent electrochemical stability. The LiFePO4/Li cells have a high initial discharge specific capacity of 153.9 mAh g-1 at 1C, with a capacity retention of 80% after 915 cycles. This paper proposes an approach for constructing three-dimensional MOF network-enhanced CSEs, which provides insights into the design and development of MOFs for the positive effects of high-performance CSEs.

Revealing the role of hydrogen bond coupling structure for enhanced performance of the solid-state electrolyte

Achieving practical applications of PEO-based composite solid electrolyte (CPE) batteries requires the precise design of filler structures at the molecular level to form stable composite interfacial phases, which in turn improve the conductivity of Li+ and inhibit the nucleation growth of lithium dendrites. Some functional fillers suffer from severe agglomeration due to poor compatibility with the polymer base or grain boundary migration, resulting in limited improvement in cell performance. In this paper, ILs@KAP1 is reported as a filler to enhance the performance of PEO-based batteries. Thereinto, the hypercrosslinked phosphorus ligand polymer-containing KAP1, designed at the molecular level, has an abundant porous structure, hydrogen bonding network, and a rigid skeleton structure of benzene rings. These can be used both to improve the flammability with PEO-based and to reduce the crystallinity of the polymer electrolyte. Ionic liquids (ILs) are encapsulated in the nanochannels of KAP1, and thus a 3D Li+ conducting framework could be formed. In this case, it could not only facilitate the wettability of the contact interface with the electrode, significantly promoting its compatibility and providing a fast Li+ transport path, but also facilitate the formation of LiF, Li3N and Li2O rich SEI components, further fostering the uniform deposition/exfoliation of lithium. The LFP||CPE||Li battery assembled with ILs@KAP1-PEO-CPE has a high initial discharge specific capacity about 156 mAh/g at 1C and a remaining capacity about 121.8 mAh/g after 300 cycles (capacity retention of 78.07%).

The Critical Role of Fillers in Composite Polymer Electrolytes for Lithium Battery

With excellent energy densities and highly safe performance, solid-state lithium batteries (SSLBs) have been hailed as promising energy storage devices. Solid-state electrolyte is the core component of SSLBs and plays an essential role in the safety and electrochemical performance of the cells. Composite polymer electrolytes (CPEs) are considered as one of the most promising candidates among all solid-state electrolytes due to their excellent comprehensive performance. In this review, we briefly introduce the components of CPEs, such as the polymer matrix and the species of fillers, as well as the integration of fillers in the polymers. In particular, we focus on the two major obstacles that affect the development of CPEs: the low ionic conductivity of the electrolyte and high interfacial impedance. We provide insight into the factors influencing ionic conductivity, in terms of macroscopic and microscopic aspects, including the aggregated structure of the polymer, ion migration rate and carrier concentration. In addition, we also discuss the electrode-electrolyte interface and summarize methods for improving this interface. It is expected that this review will provide feasible solutions for modifying CPEs through further understanding of the ion conduction mechanism in CPEs and for improving the compatibility of the electrode-electrolyte interface.

CoSe2 nanoparticles anchored on porous carbon network structure for efficient Na-ion storage

Cobalt selenide, as a star material in battery industry, has attracted much attention. However, when it is applied solely in sodium ion batteries, it will cause large volume expansion and material agglomeration, which will seriously affect the overall performance of batteries. In this work, we use ice bath impregnation to combine CoSe₂ nanoparticles with porous nitrogen-doped carbon networks (NC) as advanced anodes for ultra-long cycle life sodium ion batteries (SIBs). CoSe₂ nanoparticles are evenly attached to NC with strong interfacial contacts in CoSe₂@NC. The strong contact of CoSe₂ on the porous carbon network, along with the carbon network's unique network cross-linking structure, results in rapid electron transfer and Na ion diffusion kinetics of CoSe₂@NC, resulting in superior electrochemical performance. Besides, we have synthesized CoSe₂@NC with different loading by changing Co²⁺ concentration. The results show that CoSe₂@NC anode thus provides a high reversible capacity of 406 mAh/g. In addition, at high current 5 A/g, it can keep a reversible capacity of 300 mAh/g after 4500 cycles with an average capacity loss of less than 0.01 % per cycle. The excellent anchoring structure enables it to form stable solid electrolyte film (SEI) and reduce the amount of dead sodium in the first charge-discharge process, showing high Initial Coulombic Efficiency (ICE) (89.2 %). Finally, CoSe₂@NC and Na₃V₂(PO₄)₃ (NVP) are assembled into a full cell and the results shows an ultra-long cycle stability at 0.1 A/g. This strategy will facilitate the application of transition metal selenides in next-generation energy storage systems.