(Invited) Tailoring Molecular Interaction in Solid-State Polymer Electrolytes for High-Performance Supercapacitors

Lithium metal batteries (LMBs) have generally become potential candidates for energy storage devices because of their high energy density. However, the practical applications of LMBs have been limited due to the low safety of liquid electrolytes. Compared to liquid electrolytes, solid-state polymer electrolytes (SPEs) have excellent mechanical strength. However, there is room for improvement because traditional PEO-based SPEs need more mechanical properties and electrode contacts for flexible energy device applications. Self-healing solid polymer electrolytes (SHSPEs) have excellent flexibility and healing ability and can also suppress dendrite formation to increase the safety of LMBs. In this study, a novel SHSPE is designed by a combination of the zwitterionic copolymer, SBMA ([2-(methacryloyloxy) ethyl] dimethyl-(3-sulfopropyl) ammonium hydroxide)-co-BA (Butyl Acrylate), and a lithium salt, LiTFSI. We explored their solvation structures and reactivity on the Li metal anode using Density functional theory (DFT) and ab initio molecular dynamics (AIMD) methods. Besides, we also studied the reactivity of the considered SHSPE on the Cu (100) surface to explore the SEI formation mechanisms. The cationic NR4 + and anionic SO3 - groups of SBMA interact electrostatically and form cross-linked network structures within the electrolyte framework, providing the designed SHSPE with prominent self-healing capacity. The decomposition reactions of SHSPE on the Li metal anode are explored, and the resultant decomposition products that contribute to the formation of the solid electrolyte interphase (SEI) are identified. Furthermore, AIMD results indicate that highly concentrated SO3 - groups in the SHSPE promote Li ion diffusion, which increases Li ionic conductivity. The present findings provide design criteria for developing highly conductive self-healing SPEs.

Decarbonizing our energy infrastructure will require implementing battery chemistries beyond lithium-ion that are safe-to-operate and contain non-strategic materials. Silver–zinc, a decades-old battery technology with competitive energy and power density to Li-ion, has been used in both the military and civilian sector, but its widespread adoption is hindered by its poor reversibility and short lifespan. Realizing highly reversible, long cycle life, rechargeable Ag–Zn batteries entails simultaneously addressing two challenges: (i) suppressing shape change at the Zn metal anode that leads to dendrite-driven short-circuits, and (ii) suppressing Ag dissolution and migration out of the cathode. With the issues of shape change at the Zn anode solved by recent advancements in electrode architecture by our group,1 we turn our attention to the design of solid-state polymer electrolytes that suppress Ag dissolution and migration. We report on the synthesis and characterization of a suite of alkaline quaternary ammonium-based single-ion conducting polymer electrolytes suitable for use in Ag–Zn batteries. Specifically, we investigate the link between polymer chemistry and structure on ion transport via systematic tailoring of functional groups within the electrolyte. With these simple structural modifications, we can tune the Ag redox mechanism to shift equilibria away from the formation of Ag complexes responsible for dissolution and migration. By combining solid-state polymer electrolytes that suppress Ag transport with an NRL-patented Zn sponge anode, we demonstrate a high capacity, rechargeable Ag–Zn cell. These results serve as the foundation of a new class of solid-state polymer electrolytes to enable advanced rechargeable Ag–Zn batteries.(1) Parker, Joseph F., et al. "Wiring zinc in three dimensions re-writes battery performance–dendrite-free cycling." Energy & Environmental Science 7.3 (2014): 1117-1124.

Enabling high-areal-capacity all-solid-state lithium-metal batteries by tri-layer electrolyte architectures

Solid‐state lithium metal batteries (SSLMBs) have gained extensive attraction as one kind of next‐generation energy storage device. However, the drawbacks of flammability, low mechanical strength, and low ionic conductivity limit the further development of solid‐state polymer electrolytes (SPEs). In this work, a reactive flame‐retardant unit into the polymer framework via covalent bonding to create a nonflammable and stretchable polyurethane‐based SPEs is introduced. Meanwhile, the mechanical strength of the polymer backbone is increased by grafting a rigid benzene ring unit, which remarkably suppresses the lithium‐dendrite growth. As a result, these inflammable SPEs do not burn after contact with flames for 6 s. Furthermore, the obtained SPE expands the electrochemical stability window up to 5.1 V. Small monomers containing bromine decompose on the surface of lithium metal creating a LiBr riched solid electrolyte interface (SEI). Li|SPEs|Li symmetric battery offers a stable cycling life for more than 2100 h at 0.2 mA cm−2 and 0.2 mAh cm−2. The LiNi0.6Co0.2Mn0.2O2|SPEs|Li cell equipped with the integrated cathode delivers a 142.1 mAh g−1 capacity after 330 cycles at 0.3 C with 85.2% capacity retention.

Solid-state dye-sensitized solar cells were fabricated using a polymer matrix in polymer electrolyte in the purpose of the improvement of the durability in the dye-sensitized solar cells. In these dye-sensitized solar cells, the polymer electrolyte consisting of I2, LiI, ionic liquid, ethylene carbonate/propylene carbonate (EC/PC) and polymer matrix was casted onto TiO2 electrode impregnated Ruthenium dye as a photosensitizer. Photovoltaic properties of dye-sensitized solar cells using polymer matrix (PMMA, PEG, or PAN) in solid-state polymer electrolytes were investigated. Comparing photovoltaic effects of cells using hole conducting polymers (BE or 6P) instead of polymer matrix, we have investigated the availability of the solid-state polymer electrolyte in dye-sensitized solar cells.

The Li-metal interphases formed in contact with solid-state (polymer) electrolytes (SSE) during electrochemical plating and stripping influences remarkably the lifespan and safety of solid-state batteries (SSBs). Effective interphases play a crucial role in impeding the continuous decomposition of electrolytes, the growth of dendrites, and consequently, enhancing the overall battery performance. However, the investigation of interphases of solid electrolytes remains a challenge, necessitating development of advanced techniques and methodologies. Moreover, highly sensitive instruments are demanded to detect the extremely thin interphases. Furthermore, real-time monitoring experimentations are needed to capture the interfacial dynamics. Addressing the interphase in solid-state battery under real working conditions is another major complication regarding the signal acquisition of the buried interfacial products between non-transparent electrodes. As a result, there are very limited studies of such a kind, which make our state-of-the-art understanding of interfacial process in SSBs very poor.Towards addressing this, we have developed and operando spectroscopy technique based-on Attenuated Total Reflection Fourier-Transform Infrared (ATR-FTIR) for real time monitoring of interphase composition between Li-metal and solid polymer electrolytes. ATR-FTIR spectroscopy is a powerful analytical technique that offers several significant advantages, including rapid data acquisition, applicability for various sample forms, and high sensitivity to trace even small amounts of substances non-destructively. The preservation of the original interphase composition is crucial for the characterization under real working conditions. Moreover, it is rather straightforward to combine the ATR-FTIR measurements with the electrochemical system by utilizing specifically designed spectro-electrochemical cells.Our team has been developing the advanced ATR-FTIR set-up for operando interphase analysis, which has already worked successfully for characterizing the interphases of liquid electrolyte system on Si and Li-metal electrode1. Based on the successful implementation of this technique, we further developed the setup for the analysis the interphases on solid-state electrolytes with lithium metal in anode-free configuration. In our investigation, a well-known system comprising polyethylene oxide (PEO) polymer, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and benzophenone as photoinitiated crosslinker, was chosen for use as a proof of concept. PEO is one of the most extensively used polymer for SSE, its conductivity at room temperature is however relatively low, as its optimated usage temperature is above 60oC. To ensure that our experimental setup remains comparable to contemporary research on PEO electrolytes, a temperature control system was implemented in the design of our spectro-electrochemical cell, besides, pressure controlling is also incorporated to enhance internal material adhesion. Furthermore, the ATR-FTIR instrument is equipped with adjustable incident angles for the IR beam, enabling the characterization of the interphases at different penetration depths. In terms of cell internal configuration, an anode-free setup was opted, which not only facilitates penetration of infrared beam, but is also regarded as a promising configuration for next-generation SSBs.Following the acquisition of IR spectrum during the Li plating-stripping process, it was observed that certain bands exhibited intensity variations, as a result of dynamicity in interfacial composition. Notably, some prominent variations were observed in the IR spectrum around 1500-1750 cm-1, primarily originated from bands associated with benzophenone. The reactions of the crosslinker may undergo within the electrolyte system during electrochemical process are often overlooked, which is due to the lack of techniques with the ability to monitor these reactions. The presented study outlines the need for advanced operando methods to increase the understanding of interphases formed by solid-electrolytes.In future perspectives, efforts will be directed towards further optimizing the cell setups and with the aim of applying this technique to investigate interphases and aging mechanisms in various solid electrolyte systems.Reference: Weiling, Matthias, et al. "Mechanistic Understanding of Additive Reductive Degradation and SEI Formation in High‐Voltage NMC811|| SiOx‐Containing Cells via Operando ATR‐FTIR Spectroscopy." Advanced Energy Materials5 (2024): 2303568.

A novel phosphorus-containing pyridinium porous organic framework with flame retardancy for enhancing efficiency and safety of solid-state polymer electrolyte

In the development of the next-generation safe solid-state supercapacitors with high energy density, durability, and flexibility, the synthesis of high ion conducting solid-state polymer electrolyte (SSPE) with electrochemical and mechanical stabilities is a great challenge. This study shows the promising potential of epoxy-based SSPEs containing oligoether, ionic liquid, and Li salt, where a microscopic fast ion-diffusing channel is bicontinuously interlaced with the macroscopic mechanical supporting cross-linked matrix via polymerization-induced microphase separation. Their ionic conductivities, mechanical, and dielectric properties are finely tuned through varying the Li salt concentration, thermodynamically leading to a change of the relative interactions of the two conducting and insulating phases, thereby creating variant morphologies (such as microscale, nanoscale, or less phase separation). The optimized SSPE, exhibiting an ionic conductivity of ∼10–3 S/cm and elongation at break of 153% at 25 °C, is assembled with activated carbon electrodes, allowing us to fabricate a flexible all-solid-state supercapacitor. The device shows a broad potential window (3 V), high specific capacitance (178 F/g at 0.1 A/g), large energy and power density (56 Wh/kg at 99 W/kg and 3.5 kW/kg at 13 Wh/kg), and remarkable electrochemical and mechanical stabilities under mechanical deformation (80% capacitance retention under rolling condition for 2000 cycles).

An ultrathin polymer-based solid-state electrolyte membrane with high conductivity for lithium metal batteries operable at -25°C.

Replacing liquid electrolytes with solid‐state polymer electrolytes (SPEs) can solve the safety hazards of Li metal batteries (LMBs) while increasing their energy density. However, there has been limited success so far in preparing advanced SPEs with controllable molecular structure and chemical composition, posing great obstacles to further promoting its application in LMBs. Recently, ring‐opening polymerization (ROP), including cationic ROP, anionic ROP, and ring‐opening metathesis polymerization, has become a dazzling new star in achieving SPEs due to its mild polymerization conditions and controllable chemical composition (molecular structure, functional group), etc. Besides, there is no small molecule released during the polymerization process, which means reduced interfacial side reaction. Hence, in this review, the merits of ROP in preparing SPEs and its mechanism as well as interfering factors, etc are evaluated from the perspective of synthetic chemistry. Furthermore, the review focuses on outlining the existing cases related to ROP as much as possible and summarize them from different ring structures (from triple ring to multivariate ring) and polymerization methods, hoping to provide a comprehensive understanding and serve as strategic guidance for designing high‐performance SPEs. Challenges and opportunities regarding this burgeoning field are also discussed at the end.

Solid-state batteries enabled by solid-state polymer electrolytes (SPEs) are under active consideration for their promise as cost-effective platforms that simultaneously support high-energy and safe electrochemical energy storage. The limited oxidative stability and poor interfacial charge transport in conventional polymer electrolytes are well known, but difficult challenges must be addressed if high-voltage intercalating cathodes are to be used in such batteries. Here, ether-based electrolytes are in situ polymerized by a ring-opening reaction in the presence of aluminum fluoride (AlF3 ) to create SPEs inside LiNi0.6 Co0.2 Mn0.2 O2 (NCM) || Li batteries that are able to overcome both challenges. AlF3 plays a dual role as a Lewis acid catalyst and for the building of fluoridized cathode-electrolyte interphases, protecting both the electrolyte and aluminum current collector from degradation reactions. The solid-state NCM || Li metal batteries exhibit enhanced specific capacity of 153 mAh g-1 under high areal capacity of 3.0 mAh cm-2 . This work offers an important pathway toward solid-state polymer electrolytes for high-voltage solid-state batteries.

Development of a PEO-based lithium ion conductive epoxy resin polymer electrolyte

The unsatisfactory ionic conductivity of solid polymer electrolytes hinders their practical use as substitutes for liquid electrolytes to address safety concerns. Although various plasticizers have been introduced to improve lithium-ion conduction kinetics, the lack of microenvironment understanding impedes the rational design of high-performance polymer electrolytes. Here, we design a class of Hofmann complexes that offer continuous two-dimensional lithium-ion conduction channels with functional ligands, creating highly conductive electrolytes. Assisting with unsupervised learning, we use Climbing Image-Nudged Elastic Band simulations to screen lithium-ion conductors and screen out five potential candidates that elucidate the impact of lithium coordination environment on conduction behavior. By adjusting the covalency competition between Metal−O and Li−O bonds within Hofmann complexes, we can manipulate weak coordination environment of lithium-ion for rapid conduction kinetics. Li | |sulfurized polyacrylonitrile (SPAN) cell using solid-state polymer electrolytes with predicted Co(dimethylformamide)2Ni(CN)4 delivers an initial discharge capacity of 1264 mAh g−1 with a capacity retention of 65% after 500 cycles at 0.2 C (335 mA g−1), at 30 °C ± 3 °C. The assembled 0.6 Ah Li | |SPAN pouch cell delivers an areal discharge capacity of 3.8 mAh cm−2 at the second cycle with a solid electrolyte areal mass loading of 18.6 mg cm−2 (mass-to-capacity ratio of 4.9).

Restricted by the poor ability of polymers to dissociate lithium salts and transport ions, solid‐state polymer electrolytes (SPEs) show extremely low ionic conductivities (≈10−7–10−5 S cm−1) and transference number of lithium ions (tLi+ ≈0.2–0.4) at 25 °C. Here, a novel polymer matrix of SPEs that simultaneously promotes lithium salt dissociation and ion transportation based on a high dielectric poly(vinylidene fluoride‐trifluoroethylene‐chlorotrifluoroethylene) (TerP) and an all‐trans conformational poly(vinylidene fluoride‐trifluoroethylene) (CoP), is developed. The high dielectric constant increases the polarity of CH2CF2 polar groups; then, brings a strong electronegative end that dissociates Li+ from lithium salts. The all‐trans conformation assures all fluorine atoms locate on one side of the chain, constructing ion hopping highways. As a result, the TerP/CoP (TC) SPE exhibits a high ionic conductivity (2.37 × 10−4 S cm−1) and a quite large tLi+ of 0.61 at 25 °C. The Li/TC SPE/Li symmetric cells cycle stably for more than half a year (>4500 h) and the LiNi0.8Co0.1Mn0.1O2/TC SPE/Li cell cycles steadily for 1000 and 600 cycles at 1 C and 2 C at 25 °C, respectively. This work paves a new way to prepare high‐performance SPEs by simultaneously modulating dielectric constants and conformation of polymers.

Poly(vinylidene fluoride) (PVDF)-based polymer electro-lytes are attracting increasing attention for high-voltage solid-state lithium metal batteries because of their high room temperature ionic conductivity, adequate mechanical strength and good thermal stability. However, the presence of highly reactive residual solvents, such as N, N-dimethylformamide (DMF), severely jeopardizes the long-term cycling stability. Herein, we propose a solvation-tailoring strategy to confine residual solvent molecules by introducing low-cost 3 Å zeolite molecular sieves as fillers. The strong interaction between DMF and the molecular sieve weakens the ability of DMF to participate in the solvation of Li+, leading to more anions being involved in solvation. Benefiting from the tailored anion-rich coordination environment, the interfacial side reactions with the lithium anode and high-voltage NCM811 cathode are effectively suppressed. As a result, the solid-state Li||Li symmetrical cells demonstrates ultra-stable cycling over 5100 h at 0.1 mA cm-2, and the Li||NCM811 full cells achieve excellent cycling stability for more than 1130 and 250 cycles under the charging cut-off voltages of 4.3 V and 4.5 V, respectively. Our work is an innovative exploration to address the negative effects of residual DMF in PVDF-based solid-state electrolytes and highlights the importance of modulating the solvation structures in solid-state polymer electrolytes.