Gleb Yushin

Solid polymer electrolytes (SPEs) typically consist of salts with mobile anions that could cause instabilities and parasitic side reactions in solid-state lithium (Li) batteries. To address this challenge, single-Li-ion conducting (SLIC) SPEs, where anions of Li salts are covalently attached to the polymer backbone, have been utilized to reduce the number of mobile anions. This approach improves the cationic transference number but is accompanied by a loss of ionic conductivity. In this work, we investigate a synergetic approach of using both a polymerizable SLIC salt and a conventional Li salt in a polymer matrix by in situ polymerization of the poly(propylene glycol) acrylate (PPGA) monomer. The synthesized hybrid SPEs show a high ionic conductivity of up to ∼2 × 10^-4 S cm^-1 and a relatively high Li-ion transference number of ∼0.4. With a significantly reduced fraction of mobile anions in the combined salt SPE, in situ polymerized SPE cells with a LiFePO₄ (LFP) cathode achieve a stable performance for over 100 cycles at temperatures as high as 100 °C, which is unattainable with conventional Li salts or electrolytes. Furthermore, solid-state nuclear magnetic resonance spectra provide additional insights into differences in Li nucleus environments and emphasize a reduction in activation energy for hybrid SPEs due to their more open structure. This study opens the path for the fabrication of high-performance solid polymer Li batteries capable of operating at high temperatures using commercial battery fabrication equipment, as in situ polymerized acrylate-based polymers provide drop-in compatibility with conventional battery production, ease of acrylate polymerization, and inexpensive, facile SPE chemistry. We expect that further tuning of the acrylate-based SPE composition may allow further increases in its conductivity without sacrificing its electrochemical stability or mechanical properties.

Safety concerns of traditional liquid electrolytes, especially when paired with lithium (Li) metal anodes, have stimulated research of solid polymer electrolytes (SPEs) to exploit the superior thermal and mechanical properties of polymers. Polyphosphazenes are primarily known for their use as flame retardant materials and have demonstrated high Li-ion conductivity owing to their highly flexible P = N backbone which promotes Li-ion conduction via inter- and intrachain hopping along the polymer backbone. While polyphosphazenes are largely unexplored as SPEs in the literature, a few existing examples showed promising ionic conductivity. By anchoring the anion to the polymer backbone, one may primarily allow the movement of Li ions, alleviating the detrimental effects of polarization that are common in conventional dual-ion conducting SPEs. Anion-anchored SPEs, known as single Li-ion conducting solid polymer electrolytes (SLiC-SPEs), exhibit high Li-ion transference numbers (t_Li⁺), which limits Li dendrite growth, thus further increasing the safety of SPEs. However, previously reported SLiC-SPEs suffer from inadequate ionic conductivity, small electrochemical stability windows (ESWs), and limited cycling stability. Herein, we report three polyphosphazene-based SLiC-SPEs comprising lithiated polyphosphazenes. The SLiC polyphosphazenes were prepared through a facile synthesis route, opening the door for enhanced tunability of polymer properties via facile macromolecular nucleophilic substitution and subsequent lithiation. State-of-the-art characterization techniques, such as differential scanning calorimetry (DSC), electrochemical impedance spectroscopy (EIS), and solid-state nuclear magnetic resonance spectroscopy (ssNMR) were employed to probe the effect of the polymer structure on Li-ion dynamics and other electrochemical properties. Produced SPEs showed thermal stability up to ∼208 °C with ionic conductivities comparable to that of the best-reported SLiC-SPEs that definitively comprise no solvents or plasticizers. Among the three lithiated polyphosphazenes, the SPE containing dilithium poly[bis(trifluoroethylamino)phosphazene] (pTFAP2Li) exhibited the most promising electrochemical characteristics with t_Li⁺ of 0.76 and compatibility with both Li metal anodes and LiFePO₄ (LFP) cathodes; through 40 cycles at 100 °C, the PEO-pTFAP2Li blend showed 81.2% capacity utilization and 86.8% capacity retention. This work constitutes one of the first successful demonstrations of the cycling performance of a true all-solid-state Li-metal battery using SLiC polyphosphazene SPEs.

Current lithium-ion battery separators made from polyolefins such as polypropylene and polyethylene generally suffer from low porosity, low wettability, and slow ionic conductivity and tend to perform poorly against heat-triggering reactions that may cause potentially catastrophic issues, such as fire. To overcome these limitations, here we report that a porous composite membrane consisting of poly(vinylidene fluoride-co-hexafluoropropylene) nanofibers functionalized with nanodiamonds (NDs) can realize a thermally resistant, mechanically robust, and ionically conductive separator. We critically reveal the role of NDs in the polymer matrix of the membrane to improve the thermal, mechanical, crystalline, and electrochemical properties of the composites. Taking advantages of these characteristics, the ND-functionalized nanofiber separator enables high-capacity and stable cycling of lithium cells with LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) as the cathode, much superior to those using conventional polyolefin separators in otherwise identical cells.

Lithium-ion batteries (LIBs) dominate the market of rechargeable power sources. To meet the increasing market demands, technology updates focus on advanced battery materials, especially cathodes, the most important component in LIBs. In this review, we provide an overview of the development of materials and processing technologies for cathodes from both academic and industrial perspectives. We briefly compared the fundamentals of cathode materials based on intercalation and conversion chemistries. We then discussed the processing of cathodes, with specific focuses on the mechanisms of a drying process and the role of the binders. Several key parameters for the development of thick electrodes were critically assessed, which may offer insights into the design of next-generation batteries.

Iron trifluoride (FeF₃), a conversion-type cathode for sodium-ion batteries (SIBs), is based on cheap and abundant Fe and provides high theoretical capacity. However, the applications of FeF₃-based SIBs have been hindered by their low-capacity utilization and poor cycling stability. Herein, we report greatly enhanced performance of FeF₃ in multiple types of ionic liquid (IL) electrolytes at both room temperature (RT) and elevated temperatures. The Pyr_1,4FSI electrolyte demonstrated the best cycling stability with an unprecedented decay rate of only ∼0.023% per cycle after the initial stabilization and an average coulombic efficiency of ∼99.5% for over 1000 cycles at RT. The Pyr_1,3FSI electrolyte demonstrated the best cycling stability with a capacity decay rate of only ∼0.25% per cycle at 60 °C. Cells using Pyr_1,3FSI and EMIMFSI electrolytes also showed promising cycling stability with capacity decay rates of ∼0.039% and ∼0.030% per cycle over 1000 cycles, respectively. A protective and ionically conductive cathode electrolyte interphase (CEI) layer is formed during cycling in ILs, diminishing side reactions that commonly lead to gassing and excessive CEI growth in organic electrolytes, especially at elevated temperatures. Furthermore, the increased ionic conductivity and decreased viscosity of ILs at elevated temperatures help attain higher accessible capacity. The application of ILs sheds light on designing a protective CEI for its use in stable SIBs.

We report on a new synthesis pathway for Mg n-propoxide nanowires (NWs) from Mg ethoxide nanoparticles using a simple alkoxy ligand exchange reaction followed by condensation polymerization in n-propanol. In order to uncover the morphology–structure correlation in the metal alkoxide family, we employed a powerful range of state-of-the-art characterization techniques. The morphology transformation from nanoparticles to nanowires was demonstrated by time-lapse SEM micrographs. Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance spectroscopy (such as ¹H NMR and solid-state ¹³C cross-polarization (CP)-MAS NMR) illustrated the replacement of ethyl by n-propyl and metal alkoxide condensation polymerization. We identified chemical formulas of the products also using NMR spectroscopy. The crystal structure simulation of Mg ethoxide particles and Mg n-propoxide NWs provided insights on how the ligand exchange and the associated increase in the fraction of OH groups greatly enhanced Mg alkoxide bonding and enabled a higher degree of coordination polymerization to facilitate the formation and growth of the Mg n-propoxide NWs. The discovered synthesis method could be extended for the fabrication of other metal alkoxide (nano) structures with various morphologies.