Stable Lithium Deposition Generated from Ceramic-Cross-Linked Gel Polymer Electrolytes for Lithium Anode

Long-Cycling Cellulose-Based Gel Polymer Electrolyte Utilizing Nanohydrotalcite as a Li+ Transport Redistributor

The hydroxyl groups on the surface of the cellulose-based gel polymer electrolyte lead to poor interfacial compatibility due to side reactions with lithium sheets. In this paper, a novel cellulose-based gel polymer electrolyte was prepared by uniformly coating the surface of a cellulose membrane with a nanohydrotalcite/PVDF-HFP composite using electrospinning technology. This cellulose-based gel polymer electrolyte exhibits good interfacial compatibility and excellent cycling stability (91.7% specific capacity retention after 500 cycles at 0.5C). Theory and experiments have shown that nanohydrotalcite on the surface of cellulose membrane can effectively prevent the contact of hydroxyl groups with lithium sheets to reduce the side reactions. In addition, nanohydrotalcite can also act as a Li+ transport redistributor to facilitate the uniform deposition of Li+ and reduce the formation of lithium dendrites to extend the cycle life.

In Situ-Initiated Poly-1,3-dioxolane Gel Electrolyte for High-Voltage Lithium Metal Batteries

To realize high-energy-density Li metal batteries at low temperatures, a new electrolyte is needed to solve the high-voltage compatibility and fast lithium-ion de-solvation process. A gel polymer electrolyte with a small-molecular-weight polymer is widely investigated by combining the merits of a solid polymer electrolyte (SPE) and liquid electrolyte (LE). Herein, we present a new gel polymer electrolyte (P-DOL) by the lithium difluoro(oxalate)borate (LiDFOB)-initiated polymerization process using 1,3-dioxolane (DOL) as a monomer solvent. The P-DOL presents excellent ionic conductivity (1.12 × 10-4 S cm-1) at -20 °C, with an oxidation potential of 4.8 V. The Li‖LiCoO2 cell stably cycled at 4.3 V under room temperature, with a discharge capacity of 130 mAh g-1 at 0.5 C and a capacity retention rate of 86.4% after 50 cycles. Moreover, a high-Ni-content LiNi0.8Co0.1Mn0.1O2 (NCM811) cell can steadily run for 120 cycles at -20 °C, with a capacity retention of 88.4%. The underlying mechanism of high-voltage compatibility originates from the dense and robust B- and F-rich cathode interface layer (CEI) formed at the cathode interface. Our report will shed light on the real application of Li metal batteries under all-climate conditions in the future.

Self-Enhancing Gel Polymer Electrolyte by In Situ Construction for Enabling Safe Lithium Metal Battery

Lithium metal battery (LMB) possessing a high theoretical capacity is a promising candidate of advanced energy storage devices. However, its safety and stability are challenged by lithium dendrites and the leakage of liquid electrolyte. Here, a self-enhancing gel polymer electrolyte (GPE) is created by in situ polymerizing 1,3-dioxolane (DOL) in the nanofibrous skeleton for enabling safe LMB. The nanofiber membrane possesses a better affinity with poly-DOL (PDOL) than commercial separator for constructing homogeneous GPE with enhanced ion conductivity. Furthermore, polydopamine is introduced on nanofiber membrane to form hydrogen bonding with PDOL and bis((trifluoromethyl)sulfonyl)imide anion, dramatically improving the mechanical strength, ionic conductivity, and transference number of GPE. Besides, molecular dynamic simulation is used to reveal the intrinsic factors of high ionic conductivity and reinforcing effect in the meantime. Consequently, the LiFePO4 //Li batteries using self-enhancing GPE show extraordinary cyclic stability over 800 cycles under high current density of 2 C, with a capacity decay of 0.021% per cycle, effectively suppressing the growth of lithium dendrites. This ingenious strategy is expected to manufacture advanced performance and high safety LMBs and compatible with the current battery production.

Solid Polymer Electrolytes with High Conductivity and Transference Number of Li Ions for Li-Based Rechargeable Batteries

Smart electronics and wearable devices require batteries with increased energy density, enhanced safety, and improved mechanical flexibility. However, current state-of-the-art Li-based rechargeable batteries (LBRBs) use highly reactive and flowable liquid electrolytes, severely limiting their ability to meet the above requirements. Therefore, solid polymer electrolytes (SPEs) are introduced to tackle the issues of liquid electrolytes. Nevertheless, due to their low Li⁺ conductivity and Li⁺ transference number (LITN) (around 10^-5 S cm^-1 and 0.5, respectively), SPE-based room temperature LBRBs are still in their early stages of development. This paper reviews the principles of Li⁺ conduction inside SPEs and the corresponding strategies to improve the Li⁺ conductivity and LITN of SPEs. Some representative applications of SPEs in high-energy density, safe, and flexible LBRBs are then introduced and prospected.

Comparing the Ion-Conducting Polymers with Sulfonate and Ether Moieties as Cathode Binders for High-Power Lithium-Ion Batteries

Two types of ion-conducting polyimides with sulfonate or ether functional groups were synthesized as ion-type or coordination-type cathode binders for lithium-ion batteries (LIBs), respectively. Although superior ion transport abilities have been reported for both types of ion-conducting polymers, their electrochemical performances are significantly different and the corresponding transport mechanisms at the electrolyte/electrode interface remain elusive. Here, we combine experimental and computational techniques to investigate the cathode interface in the presence of both functional polymer binders in comparison with the poly(vinylidene fluoride) (PVDF) binder as reference. A broad shoulder in the cyclic voltammogram accompanied by a poor rate performance of battery tests was observed for a LiFePO₄ cathode with coordination-type ether-based polyimide (EPI) binder. In contrast, a LiFePO₄ cathode with ion-type sulfonated polyimide (SPI) binder exhibits smaller concentration polarization, achieving satisfactory capacity at high current density. Simulations show that the ether-based binder strongly coordinates Li ions and thus slows the diffusion of Li ions. This leads to the reduction of the LIB electrochemical performance at a high C-rate. In contrast, the negative moiety of the SPI binder leads to less localization of Li ions, allowing a slightly higher Li-ion mobility. Conventional PVDF shows no affinity to Li ions, leading to less Li-ion accumulation at the electrode/electrolyte interface. Yet, the cathode surface covered with PVDF shows the lowest Li-ion diffusivity compared to those with the two types of Li-ion-conducting binders. Therefore, cathodes with SPI and PVDF binders show less polarization at the electrode interface and allow higher C-rate performance of LIBs. The combined results provide a comprehensive understanding of the mechanism of ion conduction in ion- and coordination-type Li-ion-conducting polymer binders. This gives valuable insight into the design of next-generation polymer materials for high-power LIBs.

Quantification of the Local Topological Variations of Stripped and Plated Lithium Metal by X-ray Tomography

Lithium (Li) metal is the most promising negative electrode to be implemented in batteries for stationary and electric vehicle applications. For years, its use and subsequent industrialization were hampered because of the inhomogeneous Li⁺ ion reduction upon recharge onto Li metal leading to dendrite growth. The use of solid polymer electrolyte is a solution to mitigate dendrite growth. Li reduction leads typically to dense Li deposits, but the Li stripping and plating process remain nonuniform with local current heterogeneities. A precise characterization of the behavior of these heterogeneities during cycling is then essential to move toward an optimized negative electrode. In this work, we have developed a characterization method based on X-ray tomography applied to model Li symmetric cells to quantify and spatially probe the Li stripping/plating processes. Ante- and post-mortem cells are recut in smaller cells to allow a 1 μm voxel size resolution in a conventional laboratory scanner. The reconstructed cell volume is postprocessed to numerically reflatten the Li electrodes, allowing us a subsequent precise measurement of the electrode and electrolyte thicknesses and revealing local interface modifications. This in-depth analysis brings information about the location of heterogeneities and their impact on the electrode microstructure at both the electrode grains and grain boundaries. We show that the plating process (reduction) induces more pronounced heterogeneities compared to the stripping (oxidation) one. The existence of crosstalking between the electrodes is also highlighted. In addition, this simple methodology permits to finely retrieve and then surface map the local current density at both electrodes based on the local thickness change during the redox process.

Key Aspects of Lithium Metal Anodes for Lithium Metal Batteries

Rechargeable batteries are considered promising replacements for environmentally hazardous fossil fuel-based energy technologies. High-energy lithium-metal batteries have received tremendous attention for use in portable electronic devices and electric vehicles. However, the low Coulombic efficiency, short life cycle, huge volume expansion, uncontrolled dendrite growth, and endless interfacial reactions of the metallic lithium anode are major obstacles in their commercialization. Extensive research efforts have been devoted to address these issues and significant progress has been made by tuning electrolyte chemistry, designing electrode frameworks, discovering nanotechnology-based solutions, etc. This Review aims to provide a conceptual understanding of the current issues involved in using a lithium metal anode and to unveil its electrochemistry. The most recent advancements in lithium metal battery technology are outlined and suggestions for future research to develop a safe and stable lithium anode are presented.

Effective Suppression of Lithium Dendrite Growth Using a Flexible Single-Ion Conducting Polymer Electrolyte

A novel single-ion conducting polymer electrolyte (SIPE) membrane with high lithium-ion transference number, good mechanical strength, and excellent ionic conductivity is designed and synthesized by facile coupling of lithium bis(allylmalonato) borate (LiBAMB), pentaerythritol tetrakis (2-mercaptoacetate) (PETMP) and 3,6-dioxa-1,8-octanedithiol (DODT) in an electrospun poly(vinylidienefluoride) (PVDF) supporting membrane via a one-step photoinitiated in situ thiol-ene click reaction. The structure-optimized LiBAMB-PETMP-DODT (LPD)@PVDF SIPE shows an outstanding ionic conductivity of 1.32 × 10-3 S cm-1 at 25 °C, together with a high lithium-ion transference number of 0.92 and wide electrochemical window up to 6.0 V. The SIPE exhibits high tensile strength of 7.2 MPa and elongation at break of 269%. Due to these superior performances, the SIPE can suppress lithium dendrite growth, which is confirmed by galvanostatic Li plating/stripping cycling test and analysis of morphology of Li metal electrode surface after cycling test. Li|LPD@PVDF|Li symmetric cell maintains an extremely stable and low overpotential without short circuiting over the 1050 h cycle. The Li|LPD@PVDF|LiFePO4 cell shows excellent rate capacity and outstanding cycle performance compared to cells based on a conventional liquid electrolyte (LE) with Celgard separator. The facile approach of the SIPE provides an effective and promising electrolyte for safe, long-life, and high-rate lithium metal batteries.

Design Principles of Functional Polymer Separators for High-Energy, Metal-Based Batteries

Next-generation rechargeable batteries that offer high energy density, efficiency, and reversibility rely on cell configurations that enable synergistic operations of individual components. They must also address multiple emerging challenges,which include electrochemical stability, transport efficiency, safety, and active material loss. The perspective of this Review is that rational design of the polymeric separator, which is used widely in rechargeable batteries, provides a rich set of opportunities for new innovations that should enable batteries to meet many of these needs. This perspective is different from the conventional view of the polymer separator as an inert/passive unit in a battery, which has the sole function to prevent direct contact between electrically conductivecomponents that form the battery anode and cathode. Polymer separators, which serve as the core component in a battery, bridge the electrodes and the electrolyte with a large surface contact that can be utilized to apply desirable functions. This Review focuses specifically on recent advances in polymer separator systems, with a detailed analysis of several embedded functional agents that are incorporated to improve mechanical robustness, regulate ion and mass transport, and retard flammability. The discussion is also extended to new composite separator concepts that are designated traditionally as polymer/gel electrolytes.

Recent progress in solid-state electrolytes for alkali-ion batteries

Solid-state electrolytes have a lot of advantages, including the inhibition of alkali metal dendrite growth, the elimination of liquid electrolyte leakage, the improvement of safety, the enhancement of energy density and power density, and the potential application in flexible electronics. Therefore, solid-state electrolytes have become one of the hottest topics in energy-storage research area. An up-to-date review on solid-state electrolytes is of not only scientific significance but also technological imperative. Here, recent progress in solid-state electrolytes for alkali ion batteries is summarized. Through this comprehensive review and the comparison of different solid-state electrolytes, we hope it can give a clear figure of the state-of-art status and the development trend of the future solid-state electrolytes.

A Superior Polymer Electrolyte with Rigid Cyclic Carbonate Backbone for Rechargeable Lithium Ion Batteries

The fabricating process of well-known Bellcore poly(vinylidene fluoride-hexafluoropropylene) (PVdF-HFP)-based polymer electrolytes is very complicated, tedious, and expensive owing to containing a large amount of fluorine substituents. Herein, a novel kind of poly(vinylene carbonate) (PVCA)-based polymer electrolyte is developed via a facile in situ polymerization method, which possesses the merits of good interfacial compatibility with electrodes. In addition, this polymer electrolyte presents a high ionic conductivity of 5.59 × 10^-4 S cm^-1 and a wide electrochemical stability window exceeding 4.8 V vs Li⁺/Li at ambient temperature. In addition, the rigid cyclic carbonate backbone of poly(vinylene carbonate) endows polymer electrolyte a superior mechanical property. The LiFe_0.2Mn_0.8PO₄/graphite lithium ion batteries using this polymer electrolyte deliver good rate capability and excellent cyclability at room temperature. The superior performance demonstrates that the PVCA-based electrolyte via in situ polymerization is a potential alternative polymer electrolyte for high-performance rechargeable lithium ion batteries.

Toward Dendrite-Free Lithium Deposition via Structural and Interfacial Synergistic Effects of 3D Graphene@Ni Scaffold

Owing to its ultrahigh specific capacity and low electrochemical potential, lithium (Li) metal is regarded as one of the most attractive anode materials for next-generation lithium batteries. Nevertheless, the commercialization of Li-metal-based rechargeable batteries (LiMBs) has been retarded by the uncontrollable growth of Li dendrites, as well as the resulting poor cycle stability and safety hazards. In this work, a 3D graphene@Ni scaffold has been proposed to accomplish dendrite-free Li deposition via structural and interfacial synergistic effects. Due to the intrinsic high surface area used to reduce the effective electrode current density and the surface-coated graphene working as an artificial protection layer to provide high cycle stability as well as suppress the growth of Li dendrites, the Coulombic efficiencies of Li deposition on 3D graphene@Ni foam after 100 cycles can be sustained as high as 96, 98, and 92% at the current densities of 0.25, 0.5, and 1.0 mA cm^-2, respectively, which shows more excellent cycle stability than that of its planar Cu foil and bare Ni foam counterparts. The results obtained here demonstrate that the comprehensive consideration of multiaspect factors could be more help to enhance the performance of Li metal anode so as to achieve its real application in next-generation LiMBs.