4′H)-diyl)bis(hexan-1-ol) (TSBZ6D), was successfully developed as a semi-interpenetrating polymer network (s-IPN) matrix for solid polymer electrolytes (SPEs). The designed spiro-twisted molecular structure of TSBZ6D constitutes an SPE matrix with a high fractional free volume that facilitates the transport of lithium ions. In the meantime, its cross-linking network structure enables a remarkable enhancement in the mechanical properties of SPEs. When combined with poly-(ethylene oxide) (PEO) electrolytes, the TSBZ6D s-IPN imparts an enhancement of >300 and 1100% in tensile stress and in tensile strain, respectively, to the PEO-TSBZ6D (PT) composite SPEs over the PEO/lithium salt sample. Electrochemical testing of symmetric Li/PT/Li cells indicates the effectiveness of PT SPEs in mitigating the short-circuiting problem caused by Li dendrite formation. We demonstrated that a specific capacity of 166 mAh g −1 (0.1 C) at 80 °C and a promising cycle stability have been achieved by an all-solid-state LiFePO 4 /PT/Li.

A high-performance solid polymer electrolyte (SPE) membrane that simultaneously addresses the issues of enhanced toughness and Li-dendrite mitigation for all-solid-state Li-ion batteries (ASSLIBs) is demonstrated. The membrane has a sandwiched structure consisting of a center poly(ethylene oxide)/ lithium bis(trifluoromethanesulfonyl) imide (PEO/LiTFSI) electrolyte matrix laminated on both sides with electrospun high-polarity β-phase poly(vinylidene fluoride-co-hexafluoropropylene) (β-PVDF-HFP) nanofibers. The nanofiber layers impart remarkable enhancement in both mechanical and electrochemical properties of the SPE, including a 20-fold increase in tensile strength and a 48-fold increase in toughness, along with up to 4-fold enhancement in Li-ionic conductivity. Moreover, the highly polar fluorinated nanofiber cladding layers enable a stable Li-plating/ stripping interface on the Li anode to efficiently mitigate dendrite formation, while improving the electrochemical interfacial stability with the cathode. In a Li|SPE|Li symmetric cell, the use of the sandwiched SPE is demonstrated to improve the cycle stability from short-circuiting at 144 h for a pristine PEO/LiTFSI membrane to no short-circuiting even up to 3600 h (1800 Liplating-stripping cycles). In an example of LiFePO 4 |SPE|Li ASSLIBs, using the sandwiched membrane enables substantial reduction irreversible capacity upon charging to the high-voltage end and more than 80% capacity retention for over 1600 h. This work presents a feasible and facile design for an SPE for high-performance ASSLIBs.

In this study, a series of crosslinked membranes were prepared as solid polymer electrolytes (SPEs) for all-solid-state lithium ion batteries (ASSLIBs). An epoxy-containing copolymer (glycidyl methacrylate-co-poly(ethylene glycol) methyl ether methacrylate, PGA) and two amine curing agents, linear Jeffamine ED2003 and hyperbranched polyethyleneimine (PEI), were utilized to prepare SPEs with various crosslinking degrees. The PGA/polyethylene oxide (PEO) blends were cured by ED2003 and PEI to obtain slightly and heavily crosslinked structures, respectively. For further optimizing the interfacial and the electrochemical properties, an interlocking bilayer membrane based on overlapping and subsequent curing of PGA/PEO/ED2003 and PEO/PEI layers was developed. The presence of this amino/epoxy network can inhibit PEO crystallinity and maintain the dimensional stability of membranes. For the slightly crosslinked PGA/PEO/ED2003 membrane, an ionic conductivity of 5.61 × 10−4 S cm−1 and a lithium ion transference number (tLi+) of 0.43 were obtained, along with a specific capacity of 156 mAh g−1 (0.05 C) acquired from an assembled half-cell battery. However, the capacity retention retained only 54% after 100 cycles (0.2 C, 80 °C), possibly because the PEO-based electrolyte was inclined to recrystallize after long term thermal treatment. On the other hand, the highly crosslinked PGA/PEO/PEI membrane exhibited a similar ionic conductivity of 3.44 × 10−4 S cm−1 and a tLi+ of 0.52. Yet, poor interfacial adhesion between the membrane and the cathode brought about a low specific capacity of 48 mAh g−1. For the reinforced interlocking bilayer membrane, an ionic conductivity of 3.24 × 10−4 S cm−1 and a tLi+ of 0.42 could be achieved. Moreover, the capacity retention reached as high as 80% after 100 cycles (0.2 C, 80 °C). This is because the presence of the epoxy-based interlocking bilayer structure can block the pathway of lithium dendrite puncture effectively. We demonstrate that the unique interlocking bilayer structure is capable of offering a new approach to fabricate a robust SPE for ASSLIBs.

In this study, two nitrile-functionalized spiro-twisted benzoxazine monomers, namely 2,2′-((6,6,6′,6′-tetramethyl-6,6′,7,7′-tetrahydro-2H,2′H-8,8′-spirobi[indeno[5,6-e][1,3]oxazin]-3,3′(4H,4′H)-diyl)bis(4,1-phenylene))diacetonitrile (TSBZBC) and 4,4′-(6,6,6′,6′-tetramethyl-6,6′,7,7′-tetrahydro-2H,2′H-8,8′-spirobi[indeno[5,6-e][1,3]oxazin]-3,3′(4H,4′H)-diyl)dibenzonitrile (TSBZBN) were successfully developed as cross-linkable precursors. In addition, the incorporation of the nitrile group by covalent bonding onto the crosslinked spiro-twisted molecular chains improve the miscibility of SPE membranes with lithium salts while maintaining good mechanical properties. Owing to the presence of a high fractional free volume of spiro-twisted matrix, the –CN groups would have more space for rotation and vibration to assist lithium migration, especially for the benzyl cyanide-containing SPE. When combined with poly (ethylene oxide) (PEO) electrolytes, a new type of CN-containing semi-interpenetrating polymer networks for solid polymer electrolytes (SPEs) were prepared. The PEO-TSBZBC and PEO-TSBZBN composite SPEs (with 20 wt% crosslinked structure in the polymer) are denoted as the BC20 and BN20, respectively. The BC20 sample exhibited an ionic conductivity (σ) of 3.23 × 10−4 S cm−1 at 80 °C and a Li+ ion transference number of 0.187. The LiFePO4 (LFP)|BC20|Li sample exhibited a satisfactory charge–discharge capacity of 163.6 mAh g−1 at 0.1 C (with approximately 100% coulombic efficiency). Furthermore, the Li|BC20|Li cell was more stable during the Li plating/stripping process than the Li|BN20|Li and Li|PEO|Li samples. The Li|BC20|Li symmetric cell could be cycled continuously for more than 2700 h without short-circuiting. In addition, the specific capacity of the LFP|BC20|Li cell retained 87% of the original value after 50 cycles.

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