Improvement of Lithium Metal Polymer Batteries through a Small Dose of Fluorinated Salt

Abstract

Introducing a small dose of an electrolyte additive into solid polymer electrolytes (SPEs) is an appealing strategy for improving the quality of the solid–electrolyte–interphase (SEI) layer formed on the lithium metal (Li°) anode, thereby extending the cycling life of solid-state lithium metal batteries (SSLMBs). In this work, we report a new type of SPEs comprising a low-cost, fluorine-free salt, lithium tricyanomethanide, as the main conducting salt and a fluorinated salt, lithium bis­(fluorosulfonyl)­imide (LiFSI), as the electrolyte additive for enhancing the performance of SPE-based SSLMBs. Our results demonstrate that a homogeneous and stable SEI layer is readily formed on the surface of the Li° electrode through the preferential reductive decomposition of LiFSI, and consequently, the cycle stabilities of Li°||Li° and Li°||LiFePO4 cells are significantly improved after the incorporation of LiFSI as an additive. The intriguing chemistry of the salt anion revealed in this work may expedite the large-scale implementation of SSLMBs in the near future.

Enhancing the polymer electrolyte–Li metal interface on high-voltage solid-state batteries with Li-based additives inspired by the surface chemistry of Li7La3Zr2O12

“…Both spectra reveal two peak contributions at 688 eV and 685 eV, corresponding to the residual salt and LiF, respectively, which is formed as a result of the reduction/nucleophilic attack of LiFSI salt. 48,49 Notably, the electrode cycled with the electrolyte containing LiOH shows more F (Fig. 5b) than the pristine electrolyte at all depths, which is consistent with the formation of a LiF rich SEI layer.…”

Electrolyte and anode‐electrolyte interphase in solid‐state lithium metal polymer batteries: A perspective

“…Since LiF is a sword with two sides, stability of novel Ffree salts in SPE is also investigated in SSLMPBs seeking for alternative anion reduction products which could facilitate a superior SEI film. 53,56,57 Lithium tricyanomethanide [LiC(CN) 3 , LiTCM] has a fairly stable structure of a triple C≡N group; however, it can be reduced by Li • electrode in PEO-based SPE at 70 • C forming a C = N network and Li 3 N-rich SEI layer. Due to the high Li + ionic conductivity of Li 3 N, Li stripping and plating of such cell has an extremely low overpotential of 10 mV at 0.2 mAh/cm 2 at 70 • C ( Figure 4B).…”

“…53 It is worth mentioning that conjugated C = N network can be electronically conductive at some extend leading to continuous decompositions of the salt. 56 Another interesting study is to combine LiTFSI and lithium bis(oxalate)borate (LiBOB) in presence of glutaronitrile (GN), and poly(ethyleneglycol)diacrylate (PEGDA) as prepolymer host. 29 A free-standing SPE was obtained through a crosslinking reaction.…”

“…Due to the high Li + ionic conductivity of Li 3 N, Li stripping and plating of such cell has an extremely low overpotential of 10 mV at 0.2 mAh/cm 2 at 70°C (Figure 4B). 53 It is worth mentioning that conjugated C = N network can be electronically conductive at some extend leading to continuous decompositions of the salt 56 . Another interesting study is to combine LiTFSI and lithium bis(oxalate)borate (LiBOB) in presence of glutaronitrile (GN), and poly(ethyleneglycol)diacrylate (PEGDA) as prepolymer host 29 .…”

“…Since LiF is a sword with two sides, stability of novel Ffree salts in SPE is also investigated in SSLMPBs seeking for alternative anion reduction products which could facilitate a superior SEI film. 53,56,57 4B). 53 It is worth mentioning that conjugated C = N network can be electronically conductive at some extend leading to continuous decompositions of the salt.…”