Solid polymer electrolytes (SPEs) are promising for next-generation solid-state batteries due to their processability and safety. However, their low ionic conductivity at room temperature (10−8 to 10−5 S cm−1) is a limitation. High-dielectric solvents like DMF improve conductivity by facilitating Li+ dissociation and reducing correlated ion motion. The carbonyl oxygen of DMF directly coordinates with Li+. Unfortunately, DMF migrates to the Li metal anode, causing side reactions and instability. Additive engineering attempts to mitigate these issues, but often compromises ionic conductivity. Inorganic additives improve conductivity by reducing polymer crystallinity but may not address stability. This study aims to simultaneously improve ionic conductivity and electrochemical stability by using a rationally designed DMF-containing coordination complex as a dual-functional additive. This approach seeks to create a locally DMF-rich environment to facilitate ion transport while minimizing DMF migration and improving the long-term stability of the Li metal anode.

Previous research has explored various strategies to enhance the performance of SPEs. The use of high-dielectric solvents like DMF as plasticizers has shown promising results in boosting ionic conductivity. However, the inherent instability of DMF at the anode interface, leading to its degradation and subsequent formation of an unstable solid-electrolyte interphase (SEI), remains a major challenge. Additive engineering, involving the incorporation of functional polymers or inorganic materials, has been explored to address this issue. Functional polymers such as poly(acrylic acid) can induce a robust SEI, suppressing interfacial side reactions. However, excessive additive loading can dilute the solvent and compromise ionic conductivity. Incorporating inorganic additives has been shown to enhance conductivity by reducing polymer crystallinity. However, a strategy that effectively addresses both ionic conductivity and electrochemical stability simultaneously has been lacking, limiting the practical applications of SPEs. The current work aims to fill this gap by introducing a novel dual-functional additive approach that leverages the properties of a Hofmann framework material modified with DMF ligands.

The study involved the synthesis of a Hofmann-DMF coordination complex (Ni(DMF)2Ni[CN]4, denoted as Ni-DMF) as a dual-functional additive. A precursor Hofmann-H2O complex (Ni(H2O)2Ni[CN]4·xH2O, denoted as Ni-H2O) was first synthesized using a coprecipitation method. Thermal treatment created Ni-activated with open channels for ligand exchange, subsequently replacing H2O with DMF. Various characterization techniques, including SEM, TEM, N2 adsorption/desorption, PXRD, STXM, XANES, EXAFS, DFT calculations, and ATR-FTIR, were used to confirm the structure and properties of Ni-H2O, Ni-activated, and Ni-DMF. To investigate the ionic conduction behavior, the Ni-DMF was incorporated into a LiFSI/poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) electrolyte (LPE) to yield a composite polymer electrolyte (LPE@Ni-DMF). Ionic conductivity, Li+ transference number (tLi+), and Li+ transport mechanisms were examined using electrochemical impedance spectroscopy, potentiostatic polarization, and molecular dynamics (MD) simulations. Electrochemical tests, including LSV, cycling stability tests in Li||Li symmetric cells, and full-cell tests using sulfurized polyacrylonitrile (SPAN) cathodes, were performed to evaluate the performance of the LPE@Ni-DMF. The interfacial stability between LPE@Ni-DMF and the Li metal anode was investigated through cycling tests, ECD measurements, XPS, cryo-TEM, and finite element method (FEM) simulations. The overall electrochemical performance of the Li||SPAN solid-state batteries using LPE@Ni-DMF was compared to that of batteries using LPE. 2D wide-angle X-ray scattering (WAXS), thermal gravimetric (TG) analysis, and in-situ electrochemical impedance spectroscopy (EIS) coupled with distribution of relaxation time (DRT) analysis were used to assess the change in electrolyte properties and charge-transfer behaviors during cycling. The temperature-dependent performance of the Li||SPAN coin cells and pouch cell were also investigated.

The rationally designed Ni-DMF complex successfully tethered DMF, creating a locally DMF-rich environment at the interface with the polymer electrolyte. This effectively suppressed DMF migration and decomposition, leading to a stable and inorganic-rich SEI layer. The LPE@Ni-DMF exhibited a high room-temperature ionic conductivity of 6.5 × 10−4 S cm−1 and a high Li+ transference number (tLi+) of 0.71. Molecular dynamics simulations revealed a ligand-assisted Li+ transport mechanism, where surface-tethered DMF molecules facilitate Li+ conduction without significant diffusion. The Li||Li symmetric cell using LPE@Ni-DMF cycled stably for over 6000 h at 0.1 mA cm−2 with a low overvoltage of 64 mV, significantly exceeding the performance of the cell with LPE. The Li||SPAN full cell demonstrated prolonged cycle life, achieving 1000 cycles at 1 C with a capacity decay of only 0.04% per cycle. Furthermore, even a pouch cell with high areal sulfur loading (2 mg cm−2) exhibited good cycling performance (47 mAh after 35 cycles). XPS and cryo-TEM analyses confirmed the formation of a LiF-rich inorganic-rich SEI layer in LPE@Ni-DMF, contributing to the enhanced interfacial stability. The FEM simulations supported the experimental observations, illustrating the impact of DMF confinement on SEI formation and Li deposition behavior. The superior performance at low temperatures further solidified the advantage of this system, with the cell exhibiting remarkable performance even at -10°C.

The results demonstrate the successful implementation of a locally solvent-tethered strategy for developing high-performance polymer electrolytes. The key to the success lies in the dual functionality of the Ni-DMF additive: it simultaneously enhances ionic conductivity by providing a locally DMF-rich environment for Li+ transport and improves long-term cycling stability by suppressing DMF migration and decomposition at the anode. The ligand-assisted transport mechanism proposed and supported by MD simulations provides a new understanding of ion transport in SPEs. The formation of a stable, inorganic-rich SEI layer is crucial for ensuring the long-term cycling performance of the Li metal anode. The improved rate capability and low-temperature performance further highlight the practicality of the LPE@Ni-DMF electrolyte. This work challenges conventional additive engineering approaches, offering a pathway to design high-performance SPEs for various battery systems.

This study presents a novel locally solvent-tethered polymer electrolyte design using a Hofmann-DMF coordination complex. This approach effectively addresses the limitations of DMF-containing SPEs, leading to significantly improved ionic conductivity, long-term cycling stability, and overall electrochemical performance in Li||Li symmetric cells and Li||SPAN full cells. The successful fabrication of a high areal capacity pouch cell demonstrates its potential for practical applications. Future research could explore other metal-organic frameworks and solvents to further optimize the electrolyte properties and expand its applicability to various battery chemistries.

While this study demonstrates significant progress in the development of high-performance solid-state electrolytes, several limitations should be considered. The synthesis of the Ni-DMF complex requires careful control of the reaction conditions. The cost-effectiveness and scalability of the synthesis method remain to be further evaluated. The long-term stability and performance of the electrolyte under extreme conditions (e.g., high temperature, high current density) require additional investigations. Further studies are needed to investigate the impact of different Hofmann frameworks and ligands on the electrolyte properties and performance.