Improved electrochemical performance of solid-state lithium metal batteries with stable SEI and CEI layers via in situ formation technique†

Abstract

Lithium-metal batteries (LMBs) using sandwich-type hybrid solid electrolytes (SHSEs) have been increasingly popular because of their high safety and improved electrochemical performance. However, the inadequate electrode – SHSE interfacial contact markedly impedes the electrochemical performance, particularly at higher current rates when compared to commercially available LMBs. One of the novel strategies to address such challenges is the addition of electrochemically stable liquid electrolyte (LE). In this work, we added 10 μL of a multifunctional lithium difluoro(oxalato)borate (LiF₂BC₂O₄, LiDFOB)-based LE at the electrode/SHSE interface to simultaneously improve the interfacial wettability, and facilitate and form CEI and SEI protective layers through an in situ technique – improving electrochemical performance. We successfully fabricated a robust SHSE1 composite membrane using poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), LiTFSI, succinonitrile (SN), and 30 wt% polydopamine (PDA)-modified LALZO (PDA@LALZO) suspensions as the middle layer, while the layers facing the anode and cathode comprise the same formulation but only 10 wt% PDA@LALZO, instead of 30 wt% of PDA@LALZO filler. The counterpart SHSE0 membrane was synthesized using the same formulations but without the PDA@LALZO filler from the Li-metal side. The SHSE1 membrane showed greater ionic conductivity (∼5.47 × 10⁻⁴ S cm⁻¹) and a considerably higher t_Li⁺ (∼0.82) than the SHSE0 membrane. Additionally, we modified the surface of the NCM811 cathode material (Li-Nf@NCM811) with Li Nafion and a Li-metal anode (mLi) with the PDA@VGCF filler to suppress particle cracking and Li dendrite growth, respectively. The mLi/SHSE1/mLi symmetric cell can run over 2500 h at a current density of 0.2 mA cm⁻². At 1C and 2C, the Li-Nf@NCM811/SHSE1/mLi CR2032 full cell can run 450 and 400 cycles with improved capacity retention of ∼80.16% and ∼78.01%, respectively, at room temperature between 2.8 and 4.3 V. Thus, our novel SHSE membrane and cell design could be used for commercial LMBs.