Structural and Dynamic Insights into the Conduction of Lithium-Ionic-Liquid Mixtures in Nanoporous Metal–Organic Frameworks as Solid-State Electrolytes

Abstract

Metal–organic framework (MOF)-based separators in Li-ion batteries (LIBs) have the potential to improve the battery performance. The mobility and conduction of lithium and organic ionic liquids (ILs) in these materials acting as (quasi) solid-state electrolytes are crucial for the battery power output. Here, we investigate the mobility of a Li-based IL in MOF nanopores and unveil the details of the conduction mechanism by molecular dynamics (MD) simulations. A complex conductivity depending on the Li-IL loading and on the IL composition is observed. Most importantly, the presence of Li prevents the collapse of the conductivity at high IL loadings. The fully atomistic MD simulations including guest–guest and guest–host interactions elucidate the competing mechanisms: Li follows a Grotthuss-like conduction mechanism with large mobility. While at small pore fillings, the Li conduction is limited by the large distance between the anions facilitating the Grotthuss-like conduction; the conduction at high pore fillings is governed by field-induced concentration inhomogeneities. Because of the small MOF pore windows, which hinders the simultaneous passage of the large IL cations and anions in opposite directions, the IL shows field-induced MOF pore blocking and ion bunching. The regions of low anion concentration and high cation concentration represent barriers for Li, decreasing its mobility. In comparison to Li-free IL, the IL bunching effect is attenuated by the formation of charge-neutral Li-anion complexes, resulting in a tremendously increased conductivity at maximum pore filling. The exploitation of this mechanism may enhance the development of advanced batteries based on IL and nanoporous separators.

A Quasi‐Double‐Layer Solid Electrolyte with Adjustable Interphases Enabling High‐Voltage Solid‐State Batteries

“…The addition of plasticizers could boost the intermolecular interaction, thus changing the highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO) of SPEs, as well as adjusting the cathode electrolyte interphase (CEI) and solid electrolyte interphase (SEI) components to achieve highperformance solid-state batteries. [18,19] However, traditional single CPEs cannot offer a large enough energy gap to achieve high-voltage compatibility on the cathode side and low anodic reactivity simultaneously in a high-voltage cell. Therefore, enlarging the redox voltage window of electrolytes remains a challenge.…”

A Hollow Porous Metal‐Organic Framework Enabled Polyethylene Oxide Based Composite Polymer Electrolytes for All‐Solid‐State Lithium Batteries

“…The analysis above was consistent with the MD simulation of the Li-IL conduction mechanism in MOF nanopores, which illustrated the LiTFSI was formed due to the interaction of Li-IL and MOF to restrain anion-bunching. [59] Especially, as shown in Figure 3 (c), compared to 30 H-Z-CPE before cycle, the content of LiF was essentially unchanged after cycle, which illustrated the novel core-shell structure filler could suppress the decomposition of LiTFSI to keep the stability of LiF on the interface between electrolyte and electrode.…”

“…[70] The great improvement of the new hollow ZIF-8/IL core-shell filler on the lithium ion migration number was attributed to the special Grotthuss-like conduction mechanism of lithium ion in the pores of ZIF-8 shell. [59] The interaction of ZIF-8 and Li-IL restrained the anion-bunching and lead Li + to follow the Grotthuss-like conduction mechanism with large mobility. On the above analysis, the galvanostatic charge-discharge tests were conducted by assembling the Li/CPE/Li symmetric batteries to evaluate the interfacial stability between CPEs and Li anode.…”

Controlling the Mobility of Ionic Liquids in the Nanopores of MOFs by Adjusting the Pore Size: From Conduction Collapse by Mutual Pore Blocking to Unhindered Ion Transport

“…[11a] In the presence of lithium ions, the IL-pore-blockage and conduction collapse is attenuated by the formation of charge-neutral Li-anion complexes. [10] So far, the impact of the pore size on the IL conduction in nanoporous material has not yet been explored.…”