Yubao Sun

The all‐solid‐state single ion conducting polymer electrolyte has a bottleneck in ionic conductivity even though it can prevent concentration polarization. Here, lithium 3,3′‐(diallylammonio)bis(propane‐1‐sulfonyl(trifluoromethyl sulfonyl)imide) (LiDAA(PSI)₂) with a symmetrical “one positive, two negative” structure and unsaturated double bonds for propagation, is synthesized. LiDAA(PSI)₂ is copolymerized with 1,2‐ethanedithiol and poly(ethylene glycol) diacrylate via photoinitiated thiol‐ene click polymerization and forms a random copolymer, SPZ for short. For comparison, lithium 3‐(diallylamino)propane‐1‐sulfonyl(trifluoromethyl sulfonyl)imide) (LiDAAPSI) and corresponding copolymer SP are synthesized. The ⁷Li resonance peak position of LiDAA(PSI)₂ shifts to a low‐field compared to that of LiDAAPSI, indicating a weaker electrostatic attraction. The symmetrical “one positive, two negative” structure is responsible for the low‐field shift, taking effect of charge conjugation. Unsurprisingly, the ionic conductivity of SPZ is 1.69e‐5 S cm⁻¹ at 60 °C, which is 1.9 times that of SP. Lithium electroplating and stripping at 0.0125 mA cm⁻²@0.05 mAh cm⁻² at 60 °C are performed. An all‐solid‐state single ion conducting lithium metal secondary battery is demonstrated. Zwitterion coupled LiDAA(PSI)₂ possesses a symmetrical “one positive, two negative” structure, charge conjugation to weaken electrostatic interaction, and unsaturated double bonds for propagation, which inspires the design and synthesis of single ion conducting polymer electrolytes with zwitterion effect.

Concentration polarization issues and lithium dendrite formation, which associate inherently with the commercial dual-ion electrolytes, restrict the performance of lithium ion batteries. Single ion conducting polymer electrolytes (SIPEs) with high lithium ion transference numbers (t₊ ≈ 1) are being intensively studied to circumvent these issues. Herein, poly(ethylene-co-vinyl alcohol) (EVOH) is chosen as the backbone and then grafted with lithium 3-chloropropanesulfonyl(trifluoromethanesulfonyl)imide (LiCPSI) via Williamson's reaction, resulting in a side-chain-grafted single ion polymer conductor (EVOH-graft-LiCPSI). The ionomer is further blended with poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) by solution casting for practical use. The SIPE membrane with ethylene carbonate and dimethyl carbonate (EC/DMC = 1 : 1, v/v) as plasticizer (i.e., gel SIPE) exhibits an ionic conductivity of 5.7 × 10⁻⁵ S cm⁻¹, a lithium ion transference number of 0.88, a wide electrochemical window of 4.8 V (vs. Li/Li⁺) and adequate mechanical strength. Finally, the gel SIPE is applied in a lithium ion battery as the electrolyte as well as the separator, delivering an initial discharge capacity of 100 mA h g⁻¹ at 1C which remains at 95 mA h g⁻¹ after 500 cycles.

Li–S secondary batteries use lithium metal as the anode. The safety hazard arising from the Li dendrite formation on the metal surface presents a formidable challenge that has hindered the technology from practical applications for many years. It has been confirmed that tiny and random lithium deposition takes place at the ion depletion layer on the surface of lithium metal. The time required to reach the ion depletion layer can be quantified using Sand's equation, in which the time is inversely proportional to the transference number of anions (t₋). Therefore, restricting the mobility of anions enables avoidance of ion depletion. In this study, lithium 4-aminophenylsulfonyl(trifluoromethylsulfonyl)imide (LiATFSI) is grafted with poly(ethylene-alt-maleic anhydride) (PEMA, M_w = 100 000–500 000) using a cyclic imide to form a single ion conducting polymer electrolyte (PEMA-graft-LiATFSI). The polymer electrolyte membrane made of PEMA-graft-LiATFSI is capable of withstanding a high current density of ±50 mA cm⁻² (normalized to the surface area of the lithium disk) in a lithium symmetric cell. More remarkably, the metallic luster of the lithium foil remains essentially intact even after a galvanostatic cycling test with a current density of ±10 mA cm⁻² for over 1600 hours, suggesting that the membrane can effectively suppress Li dendrite formation and thus pave a way to use lithium metal directly as the anode material with sufficient energy capacity and good safety. The lithium–sulfur battery assembled with the membrane as the electrolyte as well as the separator delivered a stable capacity of 780.8 mA h g⁻¹ after 1000 cycles at 1C. This work demonstrates the necessity and fundamental importance of single ion conducting electrolyte membranes for achieving safe and stable performance with high energy density lithium metal secondary batteries.

Lithium-sulfur batteries are highly promising for electric energy storage with high energy density, abundant resources and low cost. However, the battery technologies have often suffered from a short cycle life and poor rate stability arising from the well-known “polysulfide shuttle” effect. Here, we report a novel cell design by sandwiching a sp³ boron based single ion conducting polymer electrolyte film between two carbon films to fabricate a composite separator for lithium-sulfur batteries. The dense negative charges uniformly distributed in the electrolyte membrane inherently prohibit transport of polysulfide anions formed in the cathode inside the polymer matrix and effectively blocks polysulfide shuttling. A battery assembled with the composite separator exhibits a remarkably long cycle life at high charge/discharge rates.

A hyperbranched conjugated Schiff base polymer network was synthesized by condensation between 4,4′,4′′-nitrilotribenzaldehyde and p-phenylenediamine. The material exhibits excellent rate capability and long cycle life for lithium storage. Coupled with lower electrode potential (0.7 V vs. Li⁺/Li), it may be well suited for fully flexible thin film polymeric batteries as the negative electrode.

We report excellent operability of Li‐ion batteries equipped with selected sp³ boron‐based single‐ion polymer electrolyte membranes (SIPEs) in the temperature range of 25–100 °C. The high performance of the batteries is attributed to the high ionic conductivity of the SIPEs, superior interfacial contacts between the SIPE membranes and the electrodes, and the sizes of the mesopores in the membranes being appropriate for ion transport. It is demonstrated that the use of organic solvents in the SIPE membranes does not lead to apparent molecular decomposition in the batteries even at temperatures as high as 100 °C, which underscores the exceptional performance of the SIPE membranes for safe operation of Li‐ion batteries with excellent thermal and electrochemical stability. The intrinsic flexibility of the membranes enables this class of materials to be used in thin and flexible battery devices.