Fluorinated-ethers are recognized as promising electrolyte solvents for high-voltage lithium metal batteries (HV-LMBs) due to its exceptional oxidation resistance and high reductive stability on anode. However, their oxidative and chemical stability are strongly influenced by the fluorination degree and α‑hydrogen reactivity which easily be attacked by active oxygen species. Herein, a series of methyl-modified fluorinated-ethers on the acetal structure motif were designed and synthesized, and the fluorinated-ether featuring simultaneous trifluoromethyl and methyl substitution delivered outstanding electrochemical performances. With the designed fluorinated-ether of 1,1,1-trifluoro-2-(1-(2,2,2-trifluoroethoxy) ethoxy) ethane (TFEE), the electrolyte enabled highly reversible Li deposition/stripping, achieving an average Coulombic efficiency of 98.5% over 455 cycles. Li||LiNi_0.8Co_0.1Mn_0.1 cells maintained 79.2% capacity retention after 410 cycles under 4.5 V, exceeding those containing non-methylated or monofluorinated ether analogs. Li||LiNi_0.8Co_0.1Mn_0.1 pouch cell with 1 Ah theoretical capacity could stably cycle for 80 cycles with 79.6% capacity retention. This molecular design strategy provides new insights for developing advanced high-voltage electrolytes for next-generation LMBs.

In traditional in situ polymerization preparation for solid-state electrolytes, initiators are directly added to the liquid precursor. In this article, a novel cellulose paper-based composite separator is fabricated, which employs alumina as the inorganic reinforcing material and is loaded with polymerization initiator aluminum trifluoromethanesulfonate. Based upon this, a separator-induced in situ directional polymerization technique is demonstrated, and the extra addition of initiators into liquid precursors is no longer required. The polymerization starts from the surface and interior of the separator and extends outward with the gradually dissolving of initiators into the precursor. Compared with its traditional counterpart, the separator-induced poly(1,3-dioxolane) electrolyte shows improved interfacial contact as well as appropriately mitigated polymerization rate, which are conducive to practical applications. Electrochemical measurement results show that the prepared poly(1,3-dioxolane) solid electrolyte possesses an oxidation potential up to 4.4 V and a high Li⁺ transference number of 0.72. After 1000 cycles at 2 C rate (340 mA g⁻¹), the assembled Li||LiFePO₄ solid battery possesses a 106.8 mAh g⁻¹ discharge capacity retention and 83.5% capacity retention ratio, with high average Coulombic efficiency of 99.5% achieved. Our work may provide new ideas for the design and application of in situ polymerization technique for solid electrolytes and solid batteries.

Different amounts of LiCoO₂ from waste lithium-ion batteries were dissolved in nitrite acid and the recovered Li and Co were used to modify Na_0.67Fe_0.5Mn_0.5O₂ cathode by a sol–gel method. Based on the results by X-ray diffraction, Li⁺/Co³⁺ ions were doped into Na_0.67Fe_0.5Mn_0.5O₂, with the smaller lattice parameters. The results by X-ray photoelectron spectroscopy show that the doping of Li/Co ions leads to the conversion of Mn³⁺ ions to Mn⁴⁺ ions, resulting in the less Jahn teller effect. The results by electrochemical tests indicate that the cycle stability and rate performance of the modified Na_0.67Fe_0.5Mn_0.5O₂ improve due to LiCoO₂ modification. The sample with x of 0.105 (i.e., Na_0.53Li_0.105Co_0.105(Fe_0.5Mn_0.5)_0.79O₂) possesses a capacity retention of 67% after 150 cycles at a current density of 1 C, and a specific capacity is 30 mA·h/g at 5 C.