An end-capped poly(ethylene carbonate)-based concentrated electrolyte for stable cyclability of lithium battery

Polymer design for solid-state batteries and wearable electronics

Solid-state batteries are increasingly centre-stage for delivering more energy-dense, safer batteries to follow current lithium-ion rechargeable technologies. At the same time, wearable electronics powered by flexible batteries have experienced rapid technological growth. This perspective discusses the role that polymer design plays in their use as solid polymer electrolytes (SPEs) and as binders, coatings and interlayers to address issues in solid-state batteries with inorganic solid electrolytes (ISEs). We also consider the value of tunable polymer flexibility, added capacity, skin compatibility and end-of-use degradability of polymeric materials in wearable technologies such as smartwatches and health monitoring devices. While many years have been spent on SPE development for batteries, delivering competitive performances to liquid and ISEs requires a deeper understanding of the fundamentals of ion transport in solid polymers. Advanced polymer design, including controlled (de)polymerisation strategies, precision dynamic chemistry and digital learning tools, might help identify these missing fundamental gaps towards faster, more selective ion transport. Regardless of the intended use as an electrolyte, composite electrode binder or bulk component in flexible electrodes, many parallels can be drawn between the various intrinsic polymer properties. These include mechanical performances, namely elasticity and flexibility; electrochemical stability, particularly against higher-voltage electrode materials; durable adhesive/cohesive properties; ionic and/or electronic conductivity; and ultimately, processability and fabrication into the battery. With this, we assess the latest developments, providing our views on the prospects of polymers in batteries and wearables, the challenges they might address, and emerging polymer chemistries that are still relatively under-utilised in this area.

High Performance Ternary Solid Polymer Electrolytes Based on High Dielectric Poly(vinylidene fluoride) Copolymers for Solid State Lithium-Ion Batteries

Renewable energy sources require efficient energy storage systems. Lithium-ion batteries stand out among those systems, but safety and cycling stability problems still need to be improved. This can be achieved by the implementation of solid polymer electrolytes (SPE) instead of the typically used separator/electrolyte system. Thus, ternary SPEs have been developed based on poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene), P(VDF-TrFE-CFE) as host polymers, clinoptilolite (CPT) zeolite added to stabilize the battery cycling performance, and ionic liquids (ILs) (1-butyl-3-methylimidazolium thiocyanate ([BMIM][SCN])), 1-methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([PMPyr][TFSI]) or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), incorporated to increase the ionic conductivity. The samples were processed by doctor blade with solvent evaporation at 160 °C. The nature of the polymer matrix and fillers affect the morphology and mechanical properties of the samples and play an important role in electrochemical parameters such as ionic conductivity value, electrochemical window stability, and lithium-transference number. The best ionic conductivity (4.2 × 10^-5 S cm^-1) and lithium transference number (0.59) were obtained for the PVDF-HFP-CPT-[PMPyr][TFSI] sample. Charge-discharge battery tests at C/10 showed excellent battery performance with values of 150 mAh g^-1 after 50 cycles, regardless of the polymer matrix and IL used. In the rate performance tests, the best SPE was the one based on the P(VDF-TrFE-CFE) host polymer, with a discharge value at C-rate of 98.7 mAh g^-1, as it promoted ionic dissociation. This study proves for the first time the suitability of P(VDF-TrFE-CFE) as SPE in lithium-ion batteries, showing the relevance of the proper selection of the polymer matrix, IL type, and lithium salt in the formulation of the ternary SPE, in order to optimize solid-state battery performance. In particular, the enhancement of the ionic conductivity provided by the IL and the effect of the high dielectric constant polymer P(VDF-TrFE-CFE) in improving battery cyclability in a wide range of discharge rates must be highlighted.

Bifunctional Solid-State Copolymer Electrolyte with Stabilized Interphase for High-Performance Lithium Metal Battery in a Wide Temperature Range

Solid-state polymer electrolytes (SPEs) are expected to guarantee safe and durable operations of lithium metal batteries (LMBs). Herein, inspired by the salutary poly(vinyl ethylene carbonate) (PVEC) component in the solid electrolyte interphase, cross-linking vinyl ethylene carbonate and ionic liquid copolymers were synthesized by in-situ polymerization to serve as polymer electrolyte for LMBs. On one hand, due to rich ester bonds of PVEC, Li⁺ could transfer by coupling/decoupling with oxygen atoms. On the other hand, the imidazole ring of ionic liquid could facilitate the dissociation of lithium salt to promote the free movement of Li⁺ . The bifunctional component synergistically increased the ionic conductivity of the SPE to 1.97×10^-4  S cm^-1 at 25 °C. Meanwhile, it also showed a wide electrochemical window, superior mechanical properties, outstanding non-combustibility, and excellent interfacial compatibility. The bifunctional copolymer-based LiFePO₄ batteries could normally operate at 0 to 60 °C, making them a promising candidate for wide-temperature-rang LMBs.

Antioxidation Mechanism of Highly Concentrated Electrolytes at High Voltage

It has been researched that highly concentrated electrolytes (HCEs) can solve the problem of the excessive decomposition of dilute electrolytes at a high voltage, but the mechanism is not clear. In this work, the antioxidation mechanism of HCE at a high voltage was investigated by in situ electrochemical tests and theoretical calculations from the perspective of the solvation structure and physicochemical property. The results indicate that compared with the dilute electrolyte, the change of solvation structures in HCE makes more PF₆^- anions easier to be oxidized prior to the dimethyl carbonate solvents, resulting in a more stable cathode-electrolyte interphase (CEI) film. First, the lower oxidation potential of the solvation structure with more PF₆^- anions inhibits the side effects of the electrolyte effectively. Second, the CEI film, consisted of LiF and Li_xPO_yF_z generated from the oxidation of PF₆^- and Li₃PO₄ generated from the hydrolysis of LiPF₆ via the soluble PO₂F₂^- intermediate, can reduce the interface impedance and improve the conductivity. Intriguingly, the high viscosity of HCEs and the hydrolysis of LiPF₆ are proven to play a positive role in enhancing the interfacial stability of the electrolyte/electrode at a high voltage. This study builds a deep understanding of the bulk and interface properties of HCEs and provides theoretical support for their large-scale application in high-voltage battery materials.

High-Performance Room Temperature Lithium-Ion Battery Solid Polymer Electrolytes Based on Poly(vinylidene fluoride-co-hexafluoropropylene) Combining Ionic Liquid and Zeolite

The demand for more efficient energy storage devices has led to the exponential growth of lithium-ion batteries. To overcome the limitations of these systems in terms of safety and to reduce environmental impact, solid-state technology emerges as a suitable approach. This work reports on a three-component solid polymer electrolyte system based on poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), the ionic liquid 1-butyl-3-methylimidazolium thiocyanate ([BMIM][SCN]), and clinoptilolite zeolite (CPT). The influences of the preparation method and of the dopants on the electrolyte stability, ionic conductivity, and battery performance were studied. The developed electrolytes show an improved room temperature ionic conductivity (1.9 × 10^-4 S cm^-1), thermal stability (up to 300 °C), and mechanical stability. The corresponding batteries exhibit an outstanding room temperature performance of 160.3 mAh g^-1 at a C/15-rate, with a capacity retention of 76% after 50 cycles. These results represent a step forward in a promising technology aiming the widespread implementation of solid-state batteries.

Macromolecular Design of Lithium Conductive Polymer as Electrolyte for Solid-State Lithium Batteries

In the development of solid-state lithium batteries, solid polymer electrolyte (SPE) has drawn extensive concerns for its thermal and chemical stability, low density, and good processability. Especially SPE efficiently suppresses the formation of lithium dendrite and promotes battery safety. However, most of SPE is derived from the matrix with simple functional group, which suffers from low ionic conductivity, reduced mechanical properties after conductivity modification, bad electrochemical stability, and low lithium-ion transference number. Appling macromolecular design with multiple functional groups to polymer matrix is accepted as a strategy to solve the problems of SPE fundamentally. In this review, macromolecular design based on lithium conducting groups is summarized including copolymerization, network construction, and grafting. Meanwhile, the construction of single-ion conductor polymer is also focused herein. Moreover, synergistic effects between the designed matrix, lithium salt, and fillers are reviewed with the objective to further improve the performance of SPE. At last, future studies on macromolecular design are proposed in the development of SPE for solid-state batteries with high energy density and durability.