Bulk-to-interface engineering of solid-state electrolytes toward fast-charging Li/Na-ion batteries

Renewable energy sources such as solar, wind, and tide energy have been implemented to decrease air pollution due to common fossil fuel-generated electricity [1]. However, those systems are intermittent; creating the need for an energy storage system (ESS) that stores over-generated energy for later use and effectively matches the power fluctuation generated because of the sporadic demand throughout the day [2]. A possible solution to this problem is to couple renewable sources with rechargeable batteries. The most widespread electrochemical battery in the market is Lithium-ion, owing to its high energy density and lifetime and capability to resist frequent changes in charging-discharging rates [3]. Nevertheless, the current battery industry already requires 50% of the world's available lithium [4]. Foremost, lithium-ion battery is composed of critical metals such as cobalt, nickel, and manganese. The anticipated growing demand for these metals will lead to their scarcity [5]. Therefore, this study aims to develop strategy to enable a sodium-ion battery based on soluble seawater sodium and address the electrochemical and engineering problems.Seawater batteries have an open cathode compartment that can utilizes Na+ infinite source in the ocean as the active material [6]. There are three main components in this open structure seawater battery design. First is the non-aqueous liquid electrolyte facilitating the sodium ions transfer and deposition on the anode compartment [7-8]. Subsequently, the solid-state electrolyte (SSE) enables the flow of sodium ions from the sweater cathode to the anode which is typically copper current collector [9]. Lastly, a current collector that provides reaction sites for cathode reactions that could be made of carbon-based materials, such as carbon paper, carbon felt, or carbon cloth [10].The Solid-state electrolyte is the component that requires the most attention. It must have high ionic conductivity to increase sodium-ions transfers and maintain good mechanical and physical properties as it represents the interface between cathode and anode, preventing the water from penetrating the anode compartment and short-circuiting the cell.To increase its ionic conductivity, it is necessary to reduce its thickness as much as possible. Through the palletization and sintering process, a ceramic SSE was fabricated with a thickness of ~ 250 µm and ionic conductivity of 0.62 mS/cm. Subsequently, symmetric cells (Na||SSE||Cu) were assembled to further test the pellet's performance. Cells that were tested under continuous charge/discharge cycling for 360 cycles showed stable charge capacity and high Coulombic efficiency (> 95%). Performance of full cells using seawater at the cathode was also demonstrated. Addressing various issues such as water permeation through the SSE, electrode corrosion, Na deactivation in the anode, and catalytic activity of the carbon cathodes are also investigated. Figure 1. Charge/discharge profile of a symmetric Na||SSE||Cu cell at a current density of 0.10 mA/cm2.

The chemical synthesis of MgHf4P6O24 multicomponent nanostructured ceramic oxide electrolyte was achieved using a modified sol-gel chemical process by substituting the tetravalent Zr4+ B-site in MgZr4P6O24 solid-state electrolyte [1, 2] with tetravalent Hf4+ ceramic oxide resulting in a considerably low activation energy (Ea = 0.74 ± 0.04 eV), promising ionic conductivity (4.52 x 10-4 Scm-1 at 747oC), electroceramic MgHf4P6O24 solid-state electrolyte.MgHf4P6O24 solid-state electroceramic oxide electrolyte was calcined at a relatively low temperature range of 800-900oC, the calcined nanopowders were pressed into pellets of 13mm diameter (Ø) and 3.8mm thickness with a uniaxial steel die at 5kN compressive pressure and sintered at 1000 ≤ T/oC ≤ 1550 temperature range. However, 1300oC was adopted as the sintering temperature in this study haven achieved optimum density and stable sample composition at that temperature. X-ray diffraction reveals a good crystallinity of the electroceramic oxide with an average crystallite size of 20±2 nm and 42±2 nm at 800oC and 900oC, respectively, indicating that crystallite size increases as a function of calcination temperature, which is very consistent with the simultaneous TGA-DSC thermal analysis profiles and confirmed using HR-TEM. The refined crystallographic data and ionic conductivity properties of the novel MgHf4P6O24 ceramic oxide solid-state electrolyte was reported for the first time in this study. SEM-EDS characterisation technique was used for determining both structural and compositional homogeneity. The sintered MgHf4P6O24 electroceramic oxide electrolyte was characterised for their electrical and thermodynamic properties thereby identifying reliable ionic and transport properties of the electroceramic oxide; The average transport number for Mg2+-cations in MgHf4P6O24 electroceramic oxide electrolyte measured as a function of Mg concentration in molten Al is 0.84±0.03. The ionic conductivity and thermodynamic analysis of the novel solid-state electroceramic oxide electrolyte in this study was compared with those of MgZr4P6O24 electrolyte [3], showing novel improvement in the trend of the ionic conductivity and thermodynamic properties of both electroceramic oxide electrolytes.Relying on the structural, chemical stability and ionic conductivity data characterised in this study, solid-state electrochemical Mg-sensor was designed and fabricated, then testing of the high-temperature Mg-sensor in molten Al alloys by the electrochemical method was achieved as shown in Figure 1 [3]. The novel high-temperature solid-state Mg-sensor was fabricated using the novel high conducting Mg2+-cation solid-state electroceramic oxide electrolyte characterised in this study by incorporating a biphasic powder mixture of MgCr2O4+Cr2O3 solid-state ceramic reference electrode in air, showing promising trend after successfully sensing Mg dissolved in molten Al alloys at 700±5oC. A linear dependence of sensor voltage on the logarithm of Mg concentration was obtained. The thermodynamic activity of Mg in molten Al alloy shows a rather negative deviation from Raoult's law. Solid-state MgHf4P6O24 electroceramic oxide electrolyte has useful potential applications in solid-state Mg-sensors during refining, virgin metals alloying and scrap metal recycling for the benefit of our environment and depleting climate.

Solid-state electrolytes have the great potential to achieve high-voltage and durable zinc-based batteries, but their effectiveness is limited by inferior ionic conductivity and large interfacial voltage polarization. Here, a nonflammable solid-state eutectic electrolyte is prepared in situ by cross-linking polymerization of ternary eutectic electrolyte with ethoxylated trimethylpropane triacrylate. Thanks to the intermolecular interaction among the deep eutectic solvents and polymer skeleton, the solid-state eutectic electrolyte possesses satisfactory room-temperature ionic conductivity of 3.94 × 10-3 S cm-1. It enables the symmetric batteries with 80% Zn utilization operating stably at high current density of 8.0 mA cm-2 for 1700 h, exceeding all non-aqueous and most aqueous zinc batteries. More importantly, due to solvation structure regulation, the solid-state eutectic electrolyte is found to elevate discharge voltage plateau to 2.1 V in Zn full batteries, and presents favorable rate performance and cyclic stability at 25±1 °C.

3D spiny AlF3/Mullite heterostructure nanofiber as solid-state polymer electrolyte fillers with enhanced ionic conductivity and improved interfacial compatibility

Current research on solid-state organic electrolytes mainly focuses on polymer electrolytes where ion transport is facilitated by chain segmental motion. The ionic conductivity of these polymer electrolytes at room temperature in too low for many practical applications. A limited number of prior reports suggest that solid-state electrolytes including organic crystalline charge-transfer complexes can have surprisingly high ionic conductivity. We report1 that processing and environmental conditions drastically impact electron and ion charge transport properties of charge-transfer complex electrolytes based on tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) mixed with lithium bis(trifluoromethylsulfonylimide) (LiTFSI). Thermal annealing and water vapor treatment decrease electronic conductivity and increase ionic conductivity. The electrolyte with 1-1-2-0.45 molar ratio of TTF-TCNQ-LiTFSI-H2O has an ionic conductivity of 2 × 10-3 S/cm at 25 °C with electronic conductivity of order 10-7 S/cm. Thermal annealing helps reduce connectivity of the charge-transfer complexes and expose more surfaces to interact with LiTFSI, thereby decreasing electronic conductivity. Exposure of the sample to water vapor then causes a substantial increase in ionic conductivity, even when the material remains in the solid state. In this presentation, we will also report on additional experimental studies of solid-state electrolytes incorporating TTF-TCNQ.Reference 1. L. Yang and J. L. Schaefer, “Water-Assisted Ion Conduction in Solid-State Charge-Transfer Complex Electrolytes for Lithium Batteries,” ChemRxiv, 2023. https://doi.org/10.26434/chemrxiv-2023-9k024

The realization of all-solid-state lithium-ion batteries (LIBs) is often considered as the final challenge in the development of LIBs. Replacing Li-ion conductive liquid electrolytes with high-performance solid-state electrolytes is indispensable for the development of all-solid-state LIBs. Solid-state electrolytes under development fall into three classes: polymers, oxides, and sulfides. Polymer-based electrolytes have advantages over oxides and sulfides, such as formation of low-resistance electrolyte/electrode interfaces, good processability, and high energy-density owing to low density. Therefore, polymer-based solid-state electrolytes are being developed in both industry and academia as a practical route for realizing high-capacity LIBs.[1]For application in commercial LIBs, the electrolyte should have an ionic conductivity higher than 10-4 S/cm at room temperature. Conventional solid polymer electrolytes, such as polyethylene oxide (PEO)-based electrolytes, do not meet the performance requirements due to insufficient ionic conductivity in the range of 10-6 to 10-5 S/cm. Recently, polyphenylene sulfide (PPS)-based polymer electrolytes have been reported to yield ion conductivities as high as those of liquid electrolytes over a wide temperature range (> 1.0 × 10-4 S/cm at 25 °C, > 1.0 × 10-3 S/cm at 80 °C, and > 1.0 × 10-3 S/cm at −40 °C).[2] These electrolytes consist of base polymer chains containing PPS, Li salts that can dissociate into cations and anions, and neutral agent molecules. However, the detailed Li-ion transport mechanism in terms of the respective roles of the molecular components of PPS electrolytes is yet to be determined. This limited understanding hinders the further improvement of PPS-based electrolytes.In this study, we perform a series of first-principle calculations and demonstrate that certain types of neutral molecules (so-called agent molecules) accelerate solid-state lithium-ion migration when mixed with lithium salts.[3] We find that the intermolecular interaction in a selected agent-molecule/lithium-salt binary system is governed by the strong coupling between lithium and oxygen atoms. Upon the addition of agent molecules, the anionic species surrounding the lithium of lithium salts is replaced by the agent molecules. The resulting weakened Coulomb energy coupling between lithium and oxygen atoms is determined to be a key factor in enabling fast lithium-ion migration via facile dissociation of lithium salts and subsequent formation of ion-hopping sites in the form of lithium-free oxygen-cages. The structure-based interpretation of agent molecules suggests that neutral molecules with functional groups which enhance chemical resonance can be selected as potential agent molecules. We believe that the results obtained in this study serve as a theoretical basis for the future development of solid-state polymer electrolytes, particularly toward mitigating the dependence of lithium-ion transport on the movement of polymer chains.

Pseudo-solid State Batteries See the Light

A development nanocrystalline TiO2 based on dye sensitized solar cells with solid state electrolyte

Poly(ethylene oxide) (PEO)-based solid-state electrolytes (SSEs) have attracted significant attention owing to their unique flexibility, great surface affinity to electrodes, and ease of processing. Nevertheless, the poor ionic conductivity and mechanical strength hinder their practical applications. In this work, we prepared PEO-based SSEs by blending bismuth oxide formate (BiOCOOH) nanowires with poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP) fibers and PEO-containing lithium bis (trifluoromethane sulfonyl) imide (LiTFSI) through coaxial electrospinning, followed by heat treatment. The well-dispersed BiOCOOH nanowires immobilize TFSI– through positively charged BiO+ groups, thereby improving Li+ conductivity. The unique morphology of BiOCOOH nanowires also reduces the degree of crystallinity in the PEO, boosting the ionic conductivity of the SSEs. The interconnected PVDF-HFP fibers as hosts can provide the mechanical strength of the SSEs. Moreover, these fibers can accelerate the dissociation of LiTFSI. The as-fabricated electrolyte shows an excellent ionic conductivity (1.56 × 10–4 S cm–1) and a high Li+ transference number (0.51). The LiFePO4||SSEs||Li cells with the as-prepared electrolyte exhibit high specific capacity after more than 600 charge/discharging cycles at 25 °C.

Li metal anode has regained intensive interest in recent years in order to develop high energy next-generation battery technologies. Unfortunately, Li metal suffers from poor cycling stability and low efficiency as well as the formation of dangerous Li dendrites raising safety concerns. The employment of solid state electrolyte (SSE) to prevent Li dendrite growth provides a promising approach to address the dendrite issue. However, recent studies indicate that Li dendrites easily form at elevated current densities, while the fundamental mechanisms of forming Li dendrites within SSE is still unknown. This calls for further investigation to understand, and control the detrimental observation. The origin and evolution of Li dendrite growth through SSE will be discussed in this talk by using two different SSE membranes, garnet and phosphate-type, as the separators. During cycling, Li is repeatedly deposited on SSE. Although a good SSE should not react with the deposited Li, the reactions between Li deposits and SSE generate a self-terminating interface, which stops the fast propagation of dendritic Li through the SSE membrane. New insights are provided on the further modification and/or incorporation of interfacial layers between SSE and Li metal to enable future solid state batteries.

Metal-organic frameworks (MOFs) with tunable ion transport pathways are considered promising solid-state electrolyte (SSE) candidates for developing lithium or sodium metal batteries. However, their low ionic conductivity and inferior stability with metal anodes limit practical applications. Herein we synthesized a high-stability tris-benzotriazolate-based MOFCu-TTBTwith ordered pore channels for SSE applications via a network-directed approach. Cu-TTBT, overcoming the synthetic challenge of tritopic benzotriazolate-based linkers, greatly advances the field of azolate-based MOFs. The resultant framework displays fast ion transport pathways with a high ionic conductivity of 1.83 × 10-4 S cm-1 and 1.1 × 10-4 S cm-1 at 298 K for Cu-TTBT-Li and Cu-TTBT-Na, respectively, among the highest in azolate-based MOFs. The Li|SSE|LiFePO4 and Na|SSE|Na3V2(PO4)3 coin cells exhibit stable cycling performances over 200 cycles at 1.0 C and 298 K. This research advances the synthetic chemistry of azolate-based MOFs and paves the way for the development of robust frameworks with high-efficiency SSE performances.

Lithium–sulfur (Li–S) batteries are one of the most promising next‐generation battery types for their high energy density and low cost. On the other hand, safety issues and poor cyclability strongly limit practical application. Solid‐state electrolytes (SSEs) can present as high ionic conductivity as aprotic electrolytes and eventually avoid the shuttle effect, which provides an ultimate solution for safe Li–S batteries with good cyclability. In this review, the recent achievements in all‐solid‐state Li–S batteries based on inorganic SSEs are summarized. Furthermore, the main attentions are paid to the interfaces, including metallic lithium|SSEs, SSEs|SSEs, and composite sulfur cathode|SSEs. The potential approaches to deal with these interfacial issues are proposed as well, such as composite SSEs with an asymmetric structure to enhance their compatibility with lithium anodes and sulfur cathodes, adding Li2O or LiF and increasing the densification to reduce the grain boundary resistance, and nanomaterials used to improve the kinetic process in cathode.

Coupling high-capacity cathode and Li-anode with solid-state electrolyte has been demonstrated as an effective strategy for increasing the energy densities and safety of rechargeable batteries. However, the limited ion conductivity, the large interfacial resistance, and unconstrained Li-dendrite growth hinder the application of solid-state Li-metal batteries. Here, a poly(ether-urethane)-based solid-state polymer electrolyte with self-healing capability is designed to reduce the interfacial resistance and provides a high-performance solid-state Li-metal battery. With its dynamic covalent disulfide bonds and hydrogen bonds, the proposed solid-state polymer electrolyte exhibits excellent interfacial self-healing ability and maintains good interfacial contact. Full cells are assembled with the two integrated electrodes/electrolytes. As a result, the Li||Li symmetric cells exhibit stable long-term cycling for more than 6000 h, and the solid-state Li-S battery shows a prolonged cycling life of 700 cycles at 0.3 C. The use of ultrasound imaging technology shows that the interfacial contact of the integrated structure is much better than those of traditional laminated structure. This work provides an interesting interfacial dual-integrated strategy for designing high-performance solid-state Li-metal batteries.

Flexible ZIBs are gaining significant attention as a cost-effective and inherently safe energy storage technology with promising applications in next-generation flexible and wearable devices. The rising demand for flexible electronics has spurred the advancement of flexible batteries. However, the widespread adoption of liquid electrolytes in zinc-ion batteries has been hindered by persistent challenges, including liquid leakage, water evaporation, and parasitic water-splitting reactions, which pose significant obstacles to commercialization. Free-standing flexible substrates and solid-state polymer electrolytes are key to enhancing the energy density, ionic conductivity, power density, mechanical strength, and flexibility of ZIBs. Herein, this review highlights recent progress and strategies for developing high-efficiency flexible ZIBs as energy storage systems, focusing on advancements in flexibility (transitioning from rigid to flexible), electrolytes (shifting from liquid to solid), adaptability (from non-portable to portable designs), and the transition from laboratory research to practical industrial applications. Critical assessments of advanced modification approaches for flexible substrates and solid-state electrolytes are presented, emphasizing their role in achieving safe, flexible, stretchable, wearable, and self-healing ZIBs. Finally, future research directions and development strategies for designing effective solid-state polymer electrolytes and flexible substrates for next-generation flexible ZIBs are discussed.

Enhanced ionic conductivity and interface stability of hybrid solid-state polymer electrolyte for rechargeable lithium metal batteries