Salt Additives for Improving Cyclability of Polymer-Based All-Solid-State Lithium–Sulfur Batteries

Abstract

Lithium−sulfur batteries are attracting extensive attention for energy storage owing to their high theoretical energy density. However, their practical implementation is hindered because of inherent issues of the technology such as the shuttling effect of the polysulfide intermediates and the formation of dendritic lithium metal (Li 0 ) deposits during battery operation leading to the short cycle life of the cell. It is generally accepted that the formation of robust solid electrolyte interphase (SEI) layers on the surface of the Li 0 anode is an effective way to mitigate these issues. Herein, the use of salt additives, lithium (difluoromethanesulfonyl)-LiDFTFSI} and lithium tricyanomethanide [LiC(CN) 3 , LiTCM], added to the classical solid polymer electrolyte (SPE) comprising lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and poly(ethylene oxide) (PEO) is proposed, with the aim to improve the quality of the SEI layer on the Li 0 anode. Through this approach, SEI layers with good mechanical integrity and Li-ion conductivity are formed thanks to the beneficial anion chemistry of these salt additives, allowing the PEO-based all-solid-state lithium−sulfur cells to be cycled for more than 100 cycles with good rate capability and Coulombic efficiency. These results attest to the great importance of electrolyte additives, even at small doses, to improve the battery performance through the selective modification of SEI components.

Understanding Electrochemical Intercalation of Al3+ Cation into the WO3 Electrochromic Electrode from Solid Electrolyte Interphase and Mass Changes

“…This conclusion is further evidenced in the literature for multivalent complex transfer through the electrode/electrolyte interface. 34 The physical properties of the SEI may be significantly affected as the concentration of chelate-like compounds increases, 35,36 in our opinion, namely, its thickness and porosity. 37 In an ideal case, the SEI prevents further degradation of the electrolyte by blocking the electron transport through it while the Al-ions shuttle back and forth in the WO 3 electrode.…”

Molecular-Level Insight into Charge Carrier Transport and Speciation in Solid Polymer Electrolytes by Chemically Tuning Both Polymer and Lithium Salt

“…Overall, despite the slightly higher concentration of lithium salt (∼0.5%) within the PCL-based system compared to PEO-based SPEs, the impact on the thermal properties of these systems for the respective polymer matrices is similar regardless of the Li salt chemistry; however, in the case of PEO, stronger Li + coordination with the EO units allows for higher ionic dissociation and, therefore higher enthalpies of melting (Tables S1 and S2). Additionally, the degree of crystallinity, χ c , significantly reduces upon the introduction of the lithium salts into the respective polymer host matrices, displaying good amorphization effects in both cases. ,,− …”

“…Additionally, the degree of crystallinity, χ c , significantly reduces upon the introduction of the lithium salts into the respective polymer host matrices, displaying good amorphization effects in both cases. 11 , 12 , 23 − 25 …”

“… 8 , 11 , 12 , 20 − 24 Recently, alternative imide lithium salts have been designed, for example, by replacing the F atom with a H atom, on one or both −CF 3 groups, obtaining lithium(difluoromethanesulfonyl)(trifluoromethanesulfonyl)imide (LiDFTFSI) or lithium bis(difluoromethanesulfonyl)imide (LiDFSI), respectively, as we have investigated in recent studies. 11 , 23 − 25 The replacement of electron-withdrawing F atom(s) with electron-donating H atom(s), i.e., weakly acidic −CF 2 H groups, induces a facile decomposition of LiDFTFSI and LiDFSI against a Li 0 anode. This leads to the formation of several favorable solid electrolyte interphase (SEI) species, namely mechanically stable lithium fluoride (LiF) and ionically conductive lithium hydride (LiH), promoting longer cycling performance.…”

“…Among all of the studied lithium salts, LiTFSI has garnered significant interest toward the realization of commercial solid-state batteries because of its thermal and chemical stability, structural flexibility, and amorphization effect when incorporated with PEO as a host matrix. ,,,− Recently, alternative imide lithium salts have been designed, for example, by replacing the F atom with a H atom, on one or both −CF 3 groups, obtaining lithium­(difluoro­methanesulfonyl)­(trifluoro­methanesulfonyl)­imide (LiDFTFSI) or lithium bis­(difluoro­methanesulfonyl)­imide (LiDFSI), respectively, as we have investigated in recent studies. ,− The replacement of electron-withdrawing F atom(s) with electron-donating H atom(s), i.e., weakly acidic −CF 2 H groups, induces a facile decomposition of LiDFTFSI and LiDFSI against a Li 0 anode. This leads to the formation of several favorable solid electrolyte interphase (SEI) species, namely mechanically stable lithium fluoride (LiF) and ionically conductive lithium hydride (LiH), promoting longer cycling performance. , With regard to ionic transport, the presence of acidic H atom(s) enables noncovalent weak hydrogen bond interactions between the Li salt and the Lewis basic O atom from the polymer, resulting in stronger polymer–anion interactions that dwindles anionic mobility and, therefore increases the t Li + and Li + ionic conductivity, in addition to decreasing concentration gradients upon cycling. , …”