Effects of nanoparticle addition to poly(ε-caprolactone) electrolytes: Crystallinity, conductivity and ambient temperature battery cycling

Synergistic Enhancement of Mechanical and Electrochemical Properties in Grafted Polymer/Oxide Hybrid Electrolytes

Lithium metal batteries operated with high voltage cathodes are predestined for the realization of high energy storage systems, where solid polymer electrolytes offer a possibility to improve battery safety. Al2O3_PCL is introduced as promising hybrid electrolyte made from polycaprolactone (PCL) and Al2O3 nanoparticles that can be prepared in a one-pot synthesis as a random mixture of linear PCL and PCL-grafted Al2O3. Upon grafting, synergistic effects of mechanical stability and ionic conductivity are achieved. Due to the mechanical stability, manufacture of PCL-based membranes with a thickness of 50 µm is feasible, yielding an ionic conductivity of 5·10-5 S cm-1 at 60 °C. The membrane exhibits an impressive performance of Li deposition in symmetric Li||Li cells, operating for 1200 h at a constant and low overvoltage of 54 mV and a current density of 0.2 mA cm-2. NMC622 | Al2O3_PCL | Li cells are cycled at rates of up to 1 C, achieving 140 cycles at >80% state of health. The straightforward synthesis and opportunity of upscaling as well as solvent-free polymerization render the Al2O3_PCL hybrid material as rather safe, potentially sustainable and affordable alternative to conventional polymer-based electrolytes.

Carbonyl-Containing Solid Polymer Electrolyte Host Materials: Conduction and Coordination in Polyketone, Polyester, and Polycarbonate Systems

Research on solid polymer electrolytes (SPEs) is now moving beyond the realm of polyethers that have dominated the field for several decades. A promising alternative group of candidates for SPE host materials is carbonyl-containing polymers. In this work, SPE properties of three different types of carbonyl-coordinating polymers are compared: polycarbonates, polyesters, and polyketones. The investigated polymers were chosen to be as structurally similar as possible, with only the functional group being different, thereby giving direct insights into the role of the noncoordinating main-chain oxygens. As revealed by experimental measurements as well as molecular dynamics simulations, the polyketone possesses the lowest glass transition temperature, but the ion transport is limited by a high degree of crystallinity. The polycarbonate, on the other hand, displays a relatively low coordination strength but is instead limited by its low molecular flexibility. The polyester performs generally as an intermediate between the other two, which is reasonable when considering its structural relation to the alternatives. This work demonstrates that local changes in the coordinating environment of carbonyl-containing polymers can have a large effect on the overall ion conduction, thereby also showing that desired transport properties can be achieved by fine-tuning the polymer chemistry of carbonyl-containing systems.

Improving the Electrochemical Stability of a Polyester-Polycarbonate Solid Polymer Electrolyte by Zwitterionic Additives

Rechargeable batteries with solid polymer electrolytes (SPEs), Li-metal anodes, and high-voltage cathodes like LiNi _x Mn _y Co _z O₂ (NMC) are promising next-generation high-energy-density storage solutions. However, these types of cells typically experience rapid failure during galvanostatic cycling, visible as an incoherent voltage noise during charging. Herein, two imidazolium-based zwitterions, with varied sulfonate-bearing chain length, are added to a poly(ε-caprolactone-co-trimethylene carbonate):LiTFSI electrolyte as cycling-enhancing additives to study their effect on the electrochemical stability of the electrolyte and the cycling performance of half-cells with NMC cathodes. The oxidative stability is studied with two different voltammetric methods using cells with inert working electrodes: the commonly used cyclic voltammetry and staircase voltammetry. The specific effects of the NMC cathode on the electrolyte stability is moreover investigated with cutoff increase cell cycling (CICC) to study the chemical and electrochemical compatibility between the active material and the SPE. Zwitterionic additives proved to enhance the electrochemical stability of the SPE and to facilitate improved galvanostatic cycling stability in half-cells with NMC by preventing the decomposition of LiTFSI at the polymer-cathode interface, as indicated by X-ray photoelectron spectroscopy (XPS).

Dissecting the Solid Polymer Electrolyte-Electrode Interface in the Vicinity of Electrochemical Stability Limits

Proper understanding of solid polymer electrolyte-electrode interfacial layer formation and its implications on cell performance is a vital step toward realizing practical solid-state lithium-ion batteries. At the same time, probing these solid-solid interfaces is extremely challenging as they are buried within the electrochemical system, thereby efficiently evading exposure to surface-sensitive spectroscopic methods. Still, the probing of interfacial degradation layers is essential to render an accurate picture of the behavior of these materials in the vicinity of their electrochemical stability limits and to complement the incomplete picture gained from electrochemical assessments. In this work, we address this issue in conjunction with presenting a thorough evaluation of the electrochemical stability window of the solid polymer electrolyte poly(ε-caprolactone):lithium bis(trifluoromethanesulfonyl)imide (PCL:LiTFSI). According to staircase voltammetry, the electrochemical stability window of the polyester-based electrolyte was found to span from 1.5 to 4 V vs Li⁺/Li. Subsequent decomposition of PCL:LiTFSI outside of the stability window led to a buildup of carbonaceous, lithium oxide and salt-derived species at the electrode-electrolyte interface, identified using postmortem spectroscopic analysis. These species formed highly resistive interphase layers, acting as major bottlenecks in the SPE system. Resistance and thickness values of these layers at different potentials were then estimated based on the impedance response between a lithium iron phosphate reference electrode and carbon-coated working electrodes. Importantly, it is only through the combination of electrochemistry and photoelectron spectroscopy that the full extent of the electrochemical performance at the limits of electrochemical stability can be reliably and accurately determined.

Microwave-assisted in situ ring-opening polymerization of ε-caprolactone in the presence of modified halloysite nanotubes loaded with stannous chloride

Polycaprolactone (PCL) has been widely applied for its excellent physicochemical properties, but it also has common problems with biopolymers. It is important to investigate energy-efficient polymerization crafts and composite catalytic systems in the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) to prepare high-performance PCL matrix composites. In this study, a composite catalytic system of modified halloysite nanotubes loaded with stannous chloride (APTES-P-h-HNTs-SnCl₂) was successfully synthesized via hydroxylation, calcination, silane coupling agent modification and physical loading. It was used to catalyze the microwave-assisted in situ ROP of ε-CL to synthesize PCL matrix nanocomposites with modified halloysite nanotubes (PCL-HNTs). The structure, morphology, polymerization, thermal properties and electrochemical performance of products were subsequently investigated. The results show that PCL-HNTs have been successfully synthesized with connected petal-like and porous structures. Compared with PCL, the film-forming and thermal properties of PCL-HNTs have been significantly improved. Moreover, PCL-HNTs have a potential application value in the field of solid polymer electrolytes (SPEs).

For the composite catalyst, there existed synergetic catalytic effect between the hydroxyl groups and the metal center. All chain growth simultaneously proceeded between the layers or on the surface of HNTs, conducting the in situ ROP.

The role of coordination strength in solid polymer electrolytes: compositional dependence of transference numbers in the poly(ε-caprolactone)-poly(trimethylene carbonate) system

Both polyesters and polycarbonates have been proposed as alternatives to polyethers as host materials for future polymer electrolytes for solid-state lithium-ion batteries. While being comparatively similar functional groups, the electron density on the coordinating carbonyl oxygen is different, thereby rendering different coordinating strength towards lithium ions. In this study, the transport properties of poly(ε-caprolactone) and poly(trimethylene carbonate) as well as random copolymers of systematically varied composition of the two have been investigated, in order to better elucidate the role of the coordination strength. The cationic transference number, a property well-connected with the complexing ability of the polymer, was shown to depend almost linearly on the ester content of the copolymer, increasing from 0.49 for the pure poly(ε-caprolactone) to 0.83 for pure poly(trimethylene carbonate). Contradictory to the transference number measurements that suggest a stronger lithium-to-ester coordination, DFT calculations showed that the carbonyl oxygen in the carbonate coordinates more strongly to the lithium ion than that of the ester. FT-IR measurements showed the coordination number to be higher in the polyester system, resulting in a higher total coordination strength and thereby resolving the paradox. This likely originates in properties that are specific of polymeric solvent systems, e.g. steric properties and chain dynamics, which influence the coordination chemistry. These results highlight the complexity in polymeric systems and their ion transport properties in comparison to low-molecular-weight analogues, and how polymer structure and steric effects together affect the coordination strength and transport properties.

Overcoming the Obstacle of Polymer-Polymer Resistances in Double Layer Solid Polymer Electrolytes

Double-layer solid polymer electrolytes (DLSPEs) comprising one layer that is stable toward lithium metal and one which is stable against a high-voltage cathode are commonly suggested as a promising strategy to achieve high-energy-density lithium batteries. Through in-depth EIS analysis, it is here concluded that the polymer-polymer interface is the primary contributor to electrolyte resistance in such DLSPEs consisting of polyether-, polyester-, or polycarbonate-bad SPEs. In comparison to the bulk ionic resistance, the polymer-polymer interface resistance is approximately 10-fold higher. Nevertheless, the interfacial resistance was successfully lowered by doubling the salt concentration from 25 to 50 wt % LiTFSI owing to improved miscibility at the interface of the two polymer layers.

Quasi-Solid-State Electrolyte Synthesized Using a Thiol-Ene Click Chemistry for Rechargeable Lithium Metal Batteries with Enhanced Safety

Liquid electrolytes currently used in lithium-ion batteries have critical drawbacks such as high flammability, high reactivity toward electrode materials, and solvent leakage. To overcome these issues, most recent research has focused on synthesis and characterization of highly conductive gel-type polymer electrolytes containing large numbers of organic solvents in the polymer matrix. There are still many hurdles to overcome, however, before they can be applied to commercial-level lithium-ion batteries. Since a large amount of organic solvent is required to achieve high ionic conductivity, battery safety is not significantly enhanced. In our study, we synthesized highly conductive quasi-solid-state electrolytes (QSEs) containing an ionically conductive oligomer (polycaprolactone triacrylate) and a small amount of organic solvent by employing click chemistry. In the QSE, polycaprolactone participates in dissociation of lithium salt and migration of lithium ions, resulting in high ionic conductivity. The Li/LiNi_0.6Co_0.2Mn_0.2O₂ cell that used this QSE exhibited good cycling performance and enhanced thermal stability, and durability; no organic solvent leakage was observed even under high pressure.

Strong Lewis Acid-Base and Weak Hydrogen Bond Synergistically Enhancing Ionic Conductivity of Poly(ethylene oxide)@SiO2 Electrolytes for a High Rate Capability Li-Metal Battery

Solid-state composite polymer electrolytes (CPEs) usually suffer from intrinsic low ionic conductivity and a solid-solid interface, badly inhibiting their widespread commercial application in all-solid-state Li-metal battery (ASSLMB) energy storage. Herein, a synergetic strategy using strong Lewis acid-base and weak hydrogen bonds was employed for self-assembly in situ construction of three-dimensional (3D) network-structured poly(ethylene oxide) (PEO) and SiO₂ CPEs (PEO@SiO₂). Ascribed to this synergistically rigid-flexible coupling dynamic strategy, a harmonious incorporation of monodispersed SiO₂ nanoparticles into PEO could remarkably reduce crystallinity of PEO, significantly enhancing the ionic conductivity (∼1.1 × 10^-4 S cm^-1 at 30 °C) and dramatically facilitating solid electrolyte interface stabilization (electrochemical stability window > 4.8 V at 90 °C). Moreover, the PEO@SiO₂-based ASSLMBs possess excellent rate capability over a wide temperature range (∼105 mA h g^-1 under 2 C at 90 °C), high temperature cycling capacity (retaining 90 mA h g^-1 after 100 cycles at 90 °C), and high specific capacity (146 mA h g^-1 under 0.3 C at 90 °C). Unambiguously, these high ionic conductivity CPEs along with excellent flexibility and safety can be one of the most promising candidates for high-performance ASSLMBs, evidently revealing that this synergistically rigid-flexible coupling dynamic strategy will open up a way to exploit the novel high ionic conductivity solid-state electrolytes.

Studies of air-exposure effects and remediation measures on lithium bis(oxalato)borate

Changes in properties for air-exposure lithium bis(oxalate)borate and reparability study by heating method.