Highly concentrated polycarbonate-based solid polymer electrolytes having extraordinary electrochemical stability

Graphene Oxide-Grafted Hybrid Diblock Copolymer Brush (GO-graft-PEG6k-block-P(MA-POSS)) as Nanofillers for Enhanced Lithium Ion Conductivity of PEO-Based Nanocomposite Solid Polymer Electrolytes

Nanocomposite solid polymer electrolytes (NSPEs) with PEO as the matrix and (i) GO or (ii) GO-graft-PEG_6k or (iii) GO-graft-PEG_6k-block-P(MA-POSS) as nanofillers have been fabricated to elucidate the impact of the filler morphology on the lithium ion conductivity. GO-graft-PEG_6k was obtained by grafting PEG_6k onto GO via esterification. GO-graft-PEG_6k-block-P(MA-POSS) was prepared via surface-initiated atom transfer radical polymerization. Fourier-transform infrared spectroscopy revealed enhanced salt dissociation and complexation between the filler and PEO host that could be attributed to Lewis acid-base interactions. Electrochemical impedance spectroscopy revealed the improved ion conductivity of the fabricated NSPEs as compared with the pristine PEO-LiClO₄. As an example, at 50 °C, the ion conductivity increased to 4.01 × 10^-5 and 6.31 × 10^-5 S cm^-1 with 0.3% GO and 0.3% GO-graft-PEG_6k, respectively, from 2.36 × 10^-5 S cm^-1 of PEO-LiClO₄, suggesting that the filler with brush-like architecture (GO-graft-PEG_6k) is more efficient in enhancing the ion conductivity. Further increase in filler content resulted in lowering of the ion conductivity that could be ascribed to aggregation of the filler. The most dramatic impact on conductivity was observed with the incorporation of brush-like GO-graft-PEG_6k-block-P(MA-POSS) as a nanofiller (3.0 × 10^-4 S cm^-1 at 50 °C with 1.0 wt % filler content). The increase in ion conductivity in the current systems, as opposed to the conventional view, could not be correlated with the content of the amorphous phase of the matrix. The conduction mechanism is still unclear; nevertheless, it could be assumed that in addition to the ion conduction through the PEO matrix, the filler forms additional low-energy ion conducting channels at its interface with the matrix. The pendent POSS nanocages of GO-graft-PEG_6k-block-P(MAPOSS) might probably increase the free volume at the interface with the matrix that is associated with higher chain and ion mobility, thus further enhancing the ion conductivity as compared with GO and GO-graft-PEG_6k. The faster ion dynamics in 1.0 wt % GO-graft-PEG_6k-block-P(MAPOSS) NSPEs has also been verified by the dielectric relaxation studies. Thus, integration of both the PEG and POSS nanocages into GO-grafted brush-like architecture offers a new tool for tuning the lithium ion conductivity for potential Li ion battery applications.

Polypropylene carbonate-based electrolytes as model for a different approach towards improved ion transport properties for novel electrolytes

Linear poly(alkylene carbonates) such as polyethylene carbonate (PEC) and polypropylene carbonate (PPC) have gained increasing interest due to their remarkable ion transport properties such as high Li⁺ transference numbers. The cause of these properties is not yet fully understood which makes it challenging to replicate them in other polymer electrolytes. Therefore, it is critical to understand the underlying mechanisms in polycarbonate electrolytes such as PPC. In this work we present insights from impedance spectroscopy, transference number measurements, PFG-NMR, IR and Raman spectroscopy as well as molecular dynamics simulations to address this issue. We find that in addition to plasticization, the lithium ion coordination by the carbonate groups of the polymer is weakened upon gelation, leading to a rapid exhange of the lithium ion solvation shell and consequently a strong increase of the conductivity. Moreover, we study the impact of the anions by employing different conducting salts. Interestingly, while the total conductivity decreases with increasing anion size, the reverse trend can be observed for the lithium ion transference numbers. Via our holistic approach, we demonstrate that this behavior can be attributed to differences in the collective ion dynamics.

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.

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.

Toward High-Energy-Density Lithium Metal Batteries: Opportunities and Challenges for Solid Organic Electrolytes

With increasing demands for safe, high capacity energy storage to support personal electronics, newer devices such as unmanned aerial vehicles, as well as the commercialization of electric vehicles, current energy storage technologies are facing increased challenges. Although alternative batteries have been intensively investigated, lithium (Li) batteries are still recognized as the preferred energy storage solution for the consumer electronics markets and next generation automobiles. However, the commercialized Li batteries still have disadvantages, such as low capacities, potential safety issues, and unfavorable cycling life. Therefore, the design and development of electromaterials toward high-energy-density, long-life-span Li batteries with improved safety is a focus for researchers in the field of energy materials. Herein, recent advances in the development of novel organic electrolytes are summarized toward solid-state Li batteries with higher energy density and improved safety. On the basis of new insights into ionic conduction and design principles of organic-based solid-state electrolytes, specific strategies toward developing these electrolytes for Li metal anodes, high-energy-density cathode materials (e.g., high voltage materials), as well as the optimization of cathode formulations are outlined. Finally, prospects for next generation solid-state electrolytes are also proposed.

Dioxolanone-Anchored Poly(allyl ether)-Based Cross-Linked Dual-Salt Polymer Electrolytes for High-Voltage Lithium Metal Batteries

Novel cross-linked polymer electrolytes (XPEs) are synthesized by free-radical copolymerization induced by ultraviolet (UV)-light irradiation of a reactive solution, which is composed of a difunctional poly(ethylene glycol) diallyl ether oligomer (PEGDAE), a monofunctional reactive diluent 4-vinyl-1,3-dioxolan-2-one (VEC), and a stock solution containing lithium salt (lithium bis(trifluoromethanesulfonyl)imide, LiTFSI) in a carbonate-free nonvolatile plasticizer, poly(ethylene glycol) dimethyl ether (PEGDME). The resulting polymer matrix can be represented as a linear polyethylene chain functionalized with cyclic carbonate (dioxolanone) moieties and cross-linked by ethylene oxide units. A series of XPEs are prepared by varying the [O]/[Li] ratio (24 to 3) of the stock solution and thoroughly characterized using physicochemical (thermogravimetric analysis-mass spectrometry, differential scanning calorimetry, NMR, etc.) and electrochemical techniques. In addition, quantum chemical calculations are performed to elucidate the correlation between the electrochemical oxidation potential and the lithium ion-ethylene oxide coordination in the stock solution. Later, lithium bis(fluorosulfonyl)imide (LiFSI) salt is incorporated into the electrolyte system to produce a dual-salt XPE that exhibits improved electrochemical performance, a stable interface against lithium metal, and enhanced physical and chemical characteristics to be employed against high-voltage cathodes. The XPE membranes demonstrated excellent resistance against lithium dendrite growth even after reversibly plating and stripping lithium ions for more than 1000 h with a total capacity of 0.5 mAh cm^-2. Finally, the XPE films are assembled in a lab-scale lithium metal battery configuration by using carbon-coated LiFePO₄ (LFP) or LiNi_0.8Co_0.15Al_0.05O₂ (NCA) as a cathode and galvanostatically cycled at 20, 40, and 60 °C. Remarkably, at 20 °C, the NCA-based lithium metal cells displayed excellent cycling stability and good capacity retention (>50%) even after 1000 cycles.

Computational Investigation of Mixed Anion Effect on Lithium Coordination and Transport in Salt Concentrated Ionic Liquid Electrolytes

The use of high concentrations of alkali metal ion salts in ionic liquids (ILs) has been demonstrated to significantly improve electrolyte performance, increase alkali metal ion transference numbers, and promote the formation of favorable SEI structures enabling long-term stable cycling. One challenge in using this material is the overall low ionic conductivity, which is a common effect of increased salt concentration. This simulation work first investigated the strategy of using mixed anions to tune the ionic conductivity in a concentrated IL (or "ionic liquid-in-salt") system having 50 mol % lithium salt. The effects of binding strength, size, and mobility of selected anions on coordination and dynamics of lithium ions were discussed. The results confirm its feasibility and provide general guidance for the selection of anions to improve the ionic conductivity of salt-concentrated electrolyte systems based on ionic liquids and other solvent systems.

Building Better Batteries in the Solid State: A Review

Most of the current commercialized lithium batteries employ liquid electrolytes, despite their vulnerability to battery fire hazards, because they avoid the formation of dendrites on the anode side, which is commonly encountered in solid-state batteries. In a review two years ago, we focused on the challenges and issues facing lithium metal for solid-state rechargeable batteries, pointed to the progress made in addressing this drawback, and concluded that a situation could be envisioned where solid-state batteries would again win over liquid batteries for different applications in the near future. However, an additional drawback of solid-state batteries is the lower ionic conductivity of the electrolyte. Therefore, extensive research efforts have been invested in the last few years to overcome this problem, the reward of which has been significant progress. It is the purpose of this review to report these recent works and the state of the art on solid electrolytes. In addition to solid electrolytes stricto sensu, there are other electrolytes that are mainly solids, but with some added liquid. In some cases, the amount of liquid added is only on the microliter scale; the addition of liquid is aimed at only improving the contact between a solid-state electrolyte and an electrode, for instance. In some other cases, the amount of liquid is larger, as in the case of gel polymers. It is also an acceptable solution if the amount of liquid is small enough to maintain the safety of the cell; such cases are also considered in this review. Different chemistries are examined, including not only Li-air, Li-O2, and Li-S, but also sodium-ion batteries, which are also subject to intensive research. The challenges toward commercialization are also considered.

High-Charge Density Polymerized Ionic Networks Boosting High Ionic Conductivity as Quasi-Solid Electrolytes for High-Voltage Batteries

Solid-state electrolytes are actively sought for their potential application in energy storage devices, especially lithium metal rechargeable batteries. However, one of the key challenges in the development of solid-state electrolytes is their lower ionic conductivity compared with that of liquid electrolytes (10^-2 S cm^-1 at room temperature), where a large gap still exists. Therefore, the pursuit of high ionic conductivity equal to that of liquid electrolytes remains the main objective for the design of solid-state electrolytes. Here, we show a series of high-charge density polymerized ionic networks as solid-state electrolytes that take inspiration from poly(ionic liquid)s. The obtained quasi-solid electrolyte slice displays an astonishingly high ionic conductivity of 5.89 × 10^-3 S cm^-1 at 25 °C (the highest conductivity among those of the state-of-art polymer gel electrolytes and polymer solid electrolytes) and ultrahigh decomposition potential, >5.2 V versus Li/Li⁺, which are attributed to the continuous ion transport channel formed by an ultrahigh ion density and an enhanced chemical stability endowed by highly cross-linked networks. The Li/LiFePO₄ and Li/LiCoO₂ batteries (3.0-4.4 V) assembled with the solid electrolytes show high stable capacities of around 155 and 130 mAh g^-1, respectively. In principle, our work breaks new ground for the design and fabrication of the solid-state electrolytes in various energy conversion devices.

Aliphatic Polycarbonate-Based Solid-State Polymer Electrolytes for Advanced Lithium Batteries: Advances and Perspective

Conventional liquid electrolytes based lithium-ion batteries (LIBs) might suffer from serious safety hazards. Solid-state polymer electrolytes (SPEs) are very promising candidate with high security for advanced LIBs. However, the quintessential frailties of pristine polyethylene oxide/lithium salts SPEs are poor ionic conductivity (≈10^-8 S cm^-1 ) at 25 °C and narrow electrochemical window (<4 V). Many innovative researches are carried out to enhance their lithium-ion conductivity (10^-4 S cm^-1 at 25 °C), which is still far from meeting the needs of high-performance power LIBs at ambient temperature. Therefore, it is a pressing urgency of exploring novel polymer host materials for advanced SPEs aimed to develop high-performance solid lithium batteries. Aliphatic polycarbonate, an emerging and promising solid polymer electrolyte, has attracted much attention of academia and industry. The amorphous structure, flexible chain segments, and high dielectric constant endow this class of polymer electrolyte excellent comprehensive performance especially in ionic conductivity, electrochemical stability, and thermally dimensional stability. To date, many types of aliphatic polycarbonate solid polymer electrolyte are discovered. Herein, the latest developments on aliphatic polycarbonate SPEs for solid-state lithium batteries are summarized. Finally, main challenges and perspective of aliphatic polycarbonate solid polymer electrolytes are illustrated at the end of this review.

ε-Caprolactone-based solid polymer electrolytes for lithium-ion batteries: synthesis, electrochemical characterization and mechanical stabilization by block copolymerization

Three different polymers were synthesized and evaluated as solid polymer electrolytes: poly(ε-caprolactone) (PCL), polystyrene-poly(ε-caprolactone) (SC), and polystyrene-poly(ε-caprolactone-r-trimethylene carbonate) (SCT).