Intermolecular Chemistry in Solid Polymer Electrolytes for High-Energy-Density Lithium Batteries

Electrolytes for Lithium- and Sodium-Metal Batteries

High-energy-density batteries have attracted significant attention due to the huge demand in electric transportation in future. Metal-based batteries, especially lithium metal batteries (LMBs) and sodium metal batteries (SMBs), have been hot research topics nowadays. The uncontrolled growth of metal dendrites has retarded the development of LMBs and SMBs. Various electrolytes have been explored to meet the demand of high-performance metal-based batteries, such as additives-contained electrolytes, polymer electrolytes, and solid-state electrolytes. To guide the development of electrolytes in LMBs and SMBs, we organize this roadmap to give out the status of present research and future challenges in this field. We also hope that the readers can get the knowledge and ideas from this roadmap.

Study of a composite solid electrolyte made from a new pyrrolidone-containing polymer and LLZTO

Improving the safety and performance of lithium ion batteries (LIB) sparked the idea of using a solid electrolyte to construct all-solid-state ones. In this study, a composite solid polymer electrolyte based on Li_6.40La₃Zr_1.40Ta_0.60O₁₂ (LLZTO) nanoparticles and a random copolymer, poly(vinyl pyrrolidone-co-poly(oligo(ethylene oxide) methyl ether methacrylate) (PPO), was successfully prepared and investigated in detail. The copolymer PPO is mixed with LiTFSI and LLZTO at different ratios and the Li conductivity and other electrochemical properties were studied. The copolymer matrix shows the highest ionic conductivity, 2.43 × 10^-5 S/cm at 60 °C, at the content of 20 wt% LiTFSI, the highest lithium ion transference number is determined to be 0.33 at room temperature, and the electrochemical stability reaches 4.3 V vs. Li⁺/Li. Interestingly, when compounded with LLZTO nanoparticles, the ionic conductivity is not improved much. For example, the highest ionic conductivity increases a little to 2.74 × 10^-5 S/cm at 60 °C when 5 wt% LLZTO is added. However, a large increase in electrochemical stability to 5.0 V is obtained for the sample of PPO-20%-10LLZTO. Both PPO and the composite electrolyte show good cycling performance during a plating/stripping experiment at a current density of 0.01 mA/cm². The limited improvement of properties is possibly due to the poor interface contact between PPO and LLZTO nanoparticles. The result may shed light on the complexity of fabricating composite electrolytes using mixtures of polymer and lithium-conducting ceramics.

Recent Progress in Designing Stable Composite Lithium Anodes with Improved Wettability

Lithium (Li) is a promising battery anode because of its high theoretical capacity and low reduction potential, but safety hazards that arise from its continuous dendrite growth and huge volume changes limit its practical applications. Li can be hosted in a framework material to address these key issues, but methods to encage Li inside scaffolds remain challenging. The melt infusion of molten Li into substrates has attracted enormous attention in both academia and industry because it provides an industrially adoptable technology capable of fabricating composite Li anodes. In this review, the wetting mechanism driving the spread of liquefied Li toward a substrate is discussed. Following this, various strategies are proposed to engineer stable Li metal composite anodes that are suitable for liquid and solid-state electrolytes. A general conclusion and a perspective on the current limitations and possible future research directions for constructing composite Li anodes for high-energy lithium metal batteries are presented.

A pH-Neutral, Aqueous Redox Flow Battery with a 3600-Cycle Lifetime: Micellization-Enabled High Stability and Crossover Suppression

Redox-flow batteries (RFBs) are a highly promising large-scale energy storage technology for mitigating the intermittent nature of renewable energy sources. Here, the design and implementation of a micellization strategy in an anthraquinone-based, pH-neutral, nontoxic, and metal-free aqueous RFB is reported. The micellization strategy (1) improves stability by protecting the redox-active anthraquinone core with a hydrophilic poly(ethylene glycol) shell and (2) increases the overall size to mitigate the crossover issue through a physical blocking mechanism. Paired with a well-established potassium ferrocyanide catholyte, the micelle-based RFB displayed an excellent capacity retention of 90.7 % after 3600 charge/discharge cycles (28.3 days), corresponding to a capacity retention of 99.67 % per day and 99.998 % per cycle. The mechanistic studies of redox-active materials were also conducted and indicated the absence of side reactions commonly observed in other anthraquinone-based RFBs. The outstanding performance of the RFB demonstrates the effectiveness of the micellization strategy for enhancing the performance of organic material-based aqueous RFBs.

Enhanced Conductivity via Extraction of Hydrocarbon Templates from Nanophase-Separated PEO-LiOTf Polymer Electrolyte Films

A series of poly(ethylene oxide)-LiOTf electrolyte films were prepared using a variety of hydrocarbon templates as nanofillers, resulting in observable nanophase separation in the polymer electrolyte. Upon partial extraction of the nanofiller template, an enhanced conductivity over 2 orders of magnitude was measured using ac impedance. Scanning electron microscopy, differential scanning calorimetry, and thermogravimetric analysis were employed to characterize the porosity, composition, and mass loss of template-extracted and nonextracted film samples.

In Situ Construction of a LiF-Enriched Interface for Stable All-Solid-State Batteries and its Origin Revealed by Cryo-TEM

The application of solid polymer electrolytes (SPEs) is still inherently limited by the unstable lithium (Li)/electrolyte interface, despite the advantages of security, flexibility, and workability of SPEs. Herein, the Li/electrolyte interface is modified by introducing Li₂ S additive to harvest stable all-solid-state lithium metal batteries (LMBs). Cryo-transmission electron microscopy (cryo-TEM) results demonstrate a mosaic interface between poly(ethylene oxide) (PEO) electrolytes and Li metal anodes, in which abundant crystalline grains of Li, Li₂ O, LiOH, and Li₂ CO₃ are randomly distributed. Besides, cryo-TEM visualization, combined with molecular dynamics simulations, reveals that the introduction of Li₂ S accelerates the decomposition of N(CF₃ SO₂ )₂ ^- and consequently promotes the formation of abundant LiF nanocrystals in the Li/PEO interface. The generated LiF is further verified to inhibit the breakage of CO bonds in the polymer chains and prevents the continuous interface reaction between Li and PEO. Therefore, the all-solid-state LMBs with the LiF-enriched interface exhibit improved cycling capability and stability in a cell configuration with an ultralong lifespan over 1800 h. This work is believed to open up a new avenue for rational design of high-performance all-solid-state LMBs.

Quantitative Mapping of Molecular Substituents to Macroscopic Properties Enables Predictive Design of Oligoethylene Glycol-Based Lithium Electrolytes

Molecular details often dictate the macroscopic properties of materials, yet due to their vastly different length scales, relationships between molecular structure and bulk properties can be difficult to predict a priori, requiring Edisonian optimizations and preventing rational design. Here, we introduce an easy-to-execute strategy based on linear free energy relationships (LFERs) that enables quantitative correlation and prediction of how molecular modifications, i.e., substituents, impact the ensemble properties of materials. First, we developed substituent parameters based on inexpensive, DFT-computed energetics of elementary pairwise interactions between a given substituent and other constant components of the material. These substituent parameters were then used as inputs to regression analyses of experimentally measured bulk properties, generating a predictive statistical model. We applied this approach to a widely studied class of electrolyte materials: oligo-ethylene glycol (OEG)-LiTFSI mixtures; the resulting model enables elucidation of fundamental physical principles that govern the properties of these electrolytes and also enables prediction of the properties of novel, improved OEG-LiTFSI-based electrolytes. The framework presented here for using context-specific substituent parameters will potentially enhance the throughput of screening new molecular designs for next-generation energy storage devices and other materials-oriented contexts where classical substituent parameters (e.g., Hammett parameters) may not be available or effective.

Emerging Potassium Metal Anodes: Perspectives on Control of the Electrochemical Interfaces

ConspectusPotassium metal serves as the anode in emerging potassium metal batteries (KMBs). It also serves as the counter electrode for potassium ion battery (KIB) half-cells, with its reliable performance being critical for assessing the working electrode material. This first-of-its-kind critical Account focuses on the dual challenge of controlling the potassium metal-substrate and the potassium metal-electrolyte interface so as to prevent dendrites. The discussion begins with a comparison of the physical and chemical properties of K metal anodes versus the much oft studied Li and Na metal anodes. Based on established descriptions for root causes of dendrites, the problem should be less severe for K than for Li or Na, while in fact the opposite is observed. The key reason that the K metal surface rapidly becomes dendritic in common electrolytes is its unstable solid electrolyte interphase (SEI). An unstable SEI layer is defined as being non-self-passivating. No SEI is perfectly stable during cycling, and all SEI structures are heterogeneous both vertically and horizontally relative to the electrolyte interface. The difference between a "stable" and an "unstable" SEI may be viewed as the relative degree to which during cycling it thickens and becomes further heterogeneous. The unstable SEI on K metal leads to a number of interrelated problems, such as low cycling Coulombic efficiency (CE), a severe impedance rise, large overpotentials, and possibly electrical shorting, all of which have been reported to occur as early as in the first 10 plating/stripping cycles. Many of the traditional "interface fixes" employed for Li and Na metal anodes, such as various artificial SEIs, surface membranes, barrier layers, secondary separators, etc., have not been attempted or optimized for the case of K. This is an important area for further exploration, with an understanding that success may come harder with K than with Li due to K-based SEI reactivity with both ether and ester solvents.The second critical problem with K metal anodes is that they do not thermally or electrochemically wet a standard (untreated) Cu foil current collector. Published experimental and modeling research directly highlights the weak bonding between the K atoms and a Cu surface. Existing surface treatment approaches that achieve improved K wetting are discussed, along with the general design rules for future studies. Also discussed are geometry-based methods to tune nucleation as well dual approaches where nucleation and SEI structure are tuned through complementary schemes to achieve extended half-cell and full battery stability. We hypothesize that K metal never achieves a planar wetting morphology even at cycle one, making the dendrites "baked-in". We propose that classical thin film growth models, Frank van der Merwe (F-M), Volmer-Weber (V-W), and Stranski-Krastanov (S-K), can be employed to describe early stage plating behavior. It is demonstrated that islandlike V-W growth is the applicable description for the natural plating behavior of K on pristine Cu. Moving forward, there are three inter-related thrusts to be pursued: First, K salt-based electrolyte formulations have to mature and become further tailored to handle the increased reactivity of a metal rather than an ion anode. Second, the K-based SEI structure needs to be further understood and ultimately tuned to be less reactive. Third, the energetics of the K metal-current collector interface must be controlled to promote planar wetting/dewetting throughout cycling.

Template-assisted loading of Fe3O4 nanoparticles inside hollow carbon "rooms" to achieve high volumetric lithium storage

The design of electrodes with simultaneously high compaction density, developed porosity, and structural stability has always been a challenge so as to meet the demand of high volumetric performance lithium ion storage devices. In this paper, we demonstrate a new compositing method for hollow carbon "room" loading of Fe₃O₄ nanoparticles (HCR@Fe₃O₄) with the assistance of Na₂CO₃ salt crystal templates. The as-obtained HCR@Fe₃O₄ composites have a massive compaction density (1.79 g cm^-3), abundant multimodal pores (1.26 cm³ g^-1), and a large content of Fe₃O₄ (64.2 wt%), which leads to excellent volumetric capacitive performance. More importantly, the unique compositing model not only provides a fast transmission channel for Li⁺ but also alleviates the mechanical strain efficiently through the cavity between the Fe₃O₄ nanoparticles and the carbon wall. When evaluated as an anode of lithium ion batteries, the resultant HCR@Fe₃O₄ electrode exhibits a remarkable volumetric capacity of 2044 mA h cm^-3 at 0.2 A g^-1 and a stable cycle life of 828 mA h cm^-3 after 1000 cycles at 5 A g^-1. The assembled HCR@Fe₃O₄//AC lithium ion hybrid capacitor device exhibits a high energy density of 173 W h L^-1 at a power density of 190 W L^-1, demonstrating its high-level integrated volumetric density/power density.

PEGylation-Enabled Extended Cyclability of a Non-aqueous Redox Flow Battery

Non-aqueous redox flow batteries (RFBs) are promising energy storage devices owing to the broad electrochemical window of organic solvents. Nonetheless, the wide application of these batteries has been limited by the low stability and limited solubility of organic materials, as well as the insufficient ion conductivity of the cell separators in non-aqueous electrolytes. In this study, two viologen analogues with poly(ethylene glycol) (PEG) tails are designed as anolytes for non-aqueous RFBs. The PEGylation of viologen not only enhances the solubility in acetonitrile but also increases the overall molecular size for alleviated crossover. In addition, a composite nanoporous aramid nanofiber separator, which allows the permeation of supporting ions while inhibiting the crossover of the designer viologens, is developed using a scalable doctor-blading method. Paired with ferrocene, the full organic material-based RFB presents excellent cyclability (500 cycles) with a retention capacity per cycle of 99.93% and an average Coulombic efficiency of 99.3% at a current density of 2.0 mA/cm². The high performance of the PEGylated viologen validates the potential of the PEGylation strategy for enhanced organic material-based non-aqueous RFBs.

Incorporating trifunctional 1,6-heptadiyne moiety into polyacetylene ionomer for improving its physical and conductive properties

A trifunctional diyne comonomer can regulate the structure and optimize the physical state of polyacetylene ionomers, which exhibit a high ionic conductivity of 2.6 × 10⁻⁵–1.0 × 10⁻³ S cm⁻¹ at 30 °C.

Covalently cross-linked polymer stabilized electrolytes with self-healing performance via boronic ester bonds

This article reported a facile fabrication of self-healing solid polymer electrolytes via boronic ester bonds.