Effects of Molecular Weight on the Electrochemical Properties of Poly(vinylidene difluoride)-Based Polymer Electrolytes.

Polymer-based electrolytes have attracted ever-increasing attention for solid-state batteries due to their excellent flexibility and processability. Among them, poly(vinylidene difluoride) (PVDF)-based electrolytes with high ionic conductivity, wide electrochemical stability window, and good mechanical properties show great potential and have been widely investigated by using different Li salts, solvents, and inorganic fillers. Here, we report the influence of the molecular weight of PVDF itself on the electrochemical properties of the electrolytes by using two kinds of common PVDF polymers, i.e., PVDF 761 and 5130. Our results demonstrate that the electrolyte with a larger molecular weight (PVDF 5130) has a denser structure and lower crystallinity, and thus much better electrochemical performance, than one with a smaller molecular weight (PVDF 761). With PVDF 5130, the LiFePO4-based solid-state cells present a steady cycling performance with a capacity retention of 85% after 1000 cycles at 1 C and 30 °C. The cycle life of the LiCoO2-based solid-state cells is also extended by using PVDF 5130.

Ion-Dipole Interaction Regulation Enables High-Performance Single-Ion Polymer Conductors for Solid-State Batteries.

Solid polymer electrolytes with large ionic conductivity, high ionic transference number, and good interfacial compatibility with electrodes are highly desired for solid-state batteries. However, unwanted polarizations and side reactions occurring in traditional dual-ion polymer conductors hinder their practical applications. Here, single-ion polymer conductors (SIPCs) with exceptional selectivity for Li-ion conduction (Li-ion transference number up to 0.93), high room-temperature ionic conductivity of about 10-4 S cm-1 , and a wide electrochemical stability window (>4.5 V, vs Li/Li+ ) are prepared by precisely regulating the ion-dipole interactions between Li+ and carbonyl/cyano groups. The resulting SIPCs show an excellent electrochemical stability with Li metal during long-term cycling at room temperature and 60 °C. LiFePO4 -based solid-state cells containing the SIPCs exhibit good rate and cycling performance in a wide temperature range from -20 to 90 °C. By the same way of ion-dipole interaction regulation, sodium- and potassium-based SIPCs with both high ionic conductivity and high cationic transference numbers are also prepared. The findings in this work provide guidance for the development of high-performance SIPCs and other metal-ion systems beyond Li+ .

Organic-Organic Composite Electrolyte Enables Ultralong Cycle Life in Solid-State Lithium Metal Batteries.

Ionic conducting polymer electrolytes for solid-state lithium-ion batteries have attracted ever-increasing attention because of their decent ionic conductivity, flexibility, no liquid leakage, and good processability. Poly(vinylidene fluoride) (PVDF)-based polymer electrolytes have recently stood out among the polymer electrolytes due to their high room temperature ionic conductivity. However, the interface between PVDF-based polymer electrolytes and lithium metal decays over time until the batteries break down. Here, we introduce a small amount of poly(acrylic acid) (PAA) into a PVDF-based polymer electrolyte and synthesize an organic-organic composite electrolyte that alleviates the interfacial reaction with lithium metal, which shows great superiority over other modification methods such as coating. The cycle life of lithium symmetric cells is prolonged from 130 to 850 h at 0.44 mA cm-2 due to the effective suppression of interfacial reaction. The much more stable interface also enables excellent cycle performance in a solid-state LiCoO2||Li cell at 30 °C with a capacity decay of 0.03% per cycle for 1000 cycles, which is much lower than that of a cell without blending PAA (0.13% per cycle for only 450 cycles). The results would shed light on the applications of PVDF-based polymer electrolytes in solid-state lithium metal batteries.

Self-Suppression of Lithium Dendrite in All-Solid-State Lithium Metal Batteries with Poly(vinylidene difluoride)-Based Solid Electrolytes.

Polymer-based electrolytes have attracted ever-increasing attention for all-solid-state lithium (Li) metal batteries due to their ionic conductivity, flexibility, and easy assembling into batteries, and are expected to overcome safety issues by replacing flammable liquid electrolytes. However, it is still a critical challenge to effectively block Li dendrite growth and improve the long-term cycling stability of all-solid-state batteries with polymer electrolytes. Here, the interface between novel poly(vinylidene difluoride) (PVDF)-based solid electrolytes and the Li anode is explored via systematical experiments in combination with first-principles calculations, and it is found that an in situ formed nanoscale interface layer with a stable and uniform mosaic structure can suppress Li dendrite growth. Unlike the typical short-circuiting that often occurs in most studied poly(ethylene oxide) systems, this interface layer in the PVDF-based system causes an open-circuiting feature at high current density and thus avoids the risk of over-current. The effective self-suppression of the Li dendrite observed in the PVDF-LiN(SO2 F)2 (LiFSI) system enables over 2000 h cycling of repeated Li plating-stripping at 0.1 mA cm-2 and excellent cycling performance in an all-solid-state LiCoO2 ||Li cell with almost no capacity fade after 200 cycles at 0.15 mA cm-2 at 25 °C. These findings will promote the development of safe all-solid-state Li metal batteries.