Examining the Impact of Polyzwitterion Chemistry on Lithium Ion Transport in Ionogel Electrolytes

Abstract

A series of polyzwitterion-supported gels featuring two classes of lithium-containing ionic liquid (IL) electrolytes have been created to examine the impact of different zwitterionic (ZI) group chemistries on lithium ion conductivity in these nonvolatile electrolytes. ZI homopolymer-supported gels containing poly­(carboxybetaine methacrylate) (pCBMA), poly­(2-methacryl­oyloxyethyl­phosphorylcholine) (pMPC), poly­(sulfobetaine vinylimidazole) (pSBVI), and poly­(sulfobetaine 2-vinylpyridine) (pSB2VP) were realized by rapid, in situ UV photopolymerization. Within a 1 M solution of lithium bis­(trifluoro­methyl­sulfonyl)­imide (LiTFSI) in a conventional IL, strong Coulombic interactions between ZI moieties and Li+ cations promoted higher ion self-diffusivities for all zwitterion types and generated improved Li+ conductivities. In particular, the pCBMA and pMPC gels exhibited improved lithium transference numbers of 0.37 and 0.38, respectively, compared to 0.23 for the IL solution. In the solvate ionic liquid (SIL) prepared from an equimolar mixture of LiTFSI and tetraglyme, the pCBMA scaffold resulted in the largest room temperature Li+ conductivity achieved, 0.44 mS cm–1 (vs 0.23 mS cm–1 in the neat SIL). The carboxybetaine ZI motif yielded the largest boost in Li+ conductivity in both IL electrolyte types, which was found to be correlated to this monomer generating the largest downfield 7Li NMR chemical shift in solution. This study illustrates the great potential of polyzwitterions for future application in lithium ion batteries and reveals the importance of zwitterion chemistry when selecting materials for nonaqueous ionogel electrolytes.

Polymer Zwitterion-Based Artificial Interphase Layers for Stable Lithium Metal Anodes

“…The above results confirm that the strong zwitterion effect within the polymer zwitterion-based AILs could effectively enhance the Li + ion transport. Moreover, the obvious chemical shift in 7 Li NMR spectra (Figure b) reveal notable Coulombic interactions between polymer zwitterions and Li + ions, indicating the increased dissociation of Li salt and the boosted ion transport . The largest downfield chemical shift occurs in PDMAPS, which is consistent with its highest Li + transport.…”

“…Moreover, the obvious chemical shift in 7 Li NMR spectra (Figure 3b) reveal notable Coulombic interactions between polymer zwitterions and Li + ions, indicating the increased dissociation of Li salt and the boosted ion transport. 37 The largest downfield chemical shift occurs in PDMAPS, which is consistent with its highest Li + transport. Meanwhile, the chemical shift of the (PDMAPS + PMPC) film falls in between that of the PDMAPS layer and the PMPC layer, which also agrees with their conductivity.…”

“…31,33,35 Therefore, polymer zwitterions with both cation and anion modulations have been considered as potential additives in gel electrolytes to realize favorable ionic conductivity and a high Li + ion transference number. 31,37 Nevertheless, the anion group regulation within polymer zwitterions is rarely reported and the long-life cycling of an LMA in a zwitterionic gel electrolyte is still a challenge. 31,38 Therefore, the design of a robust AIL layer based on a zwitterionic polymer with an advisable anion group is promising for sustaining prolonged cycling of LMAs.…”

“…Zwitterionic polymers consisting of cationic and anionic charges immobilized on the same molecule have been considered as dissociation enhancers for Li salts, which could improve the transport of Li + ions . As a result, zwitterion-based gel electrolytes have proven to be effective cation regulators for stable LMAs. , Apart from the cation control, the prevention of anion depletion is rather critical to maintain the space-charge region near the anode surface for inhibiting dendritic Li growth. , Specially, the immobilization of anionic functional groups (such as sulfonates and phosphates) via strong binding and physical confinement could realize this aim in electrolytes. ,, Therefore, polymer zwitterions with both cation and anion modulations have been considered as potential additives in gel electrolytes to realize favorable ionic conductivity and a high Li + ion transference number. , Nevertheless, the anion group regulation within polymer zwitterions is rarely reported and the long-life cycling of an LMA in a zwitterionic gel electrolyte is still a challenge. , Therefore, the design of a robust AIL layer based on a zwitterionic polymer with an advisable anion group is promising for sustaining prolonged cycling of LMAs.…”

Monolithic Task‐Specific Ionogel Electrolyte Membrane Enables High‐Performance Solid‐State Lithium‐Metal Batteries in Wide Temperature Range

“…[ 7 ] Engineering function groups based on ion coordination could effectively stimulate ion‐pair dissociation and anchor anions to polymer chains, increasing the Li + transference number () of ionogels. [ 7,19,20 ] Despite these advances, tailored design of ion environment in ionogels without sacrificing their attractive intrinsic attributes and enabling a preferable shift from vehicular to structural Li + transport for practical, safe, and high‐power batteries still remain a significant challenge.…”