An Inorganic‐Dominate Molecular Diluent Enables Safe Localized High Concentration Electrolyte for High‐Voltage Lithium‐Metal Batteries

Abstract

The development of high-voltage Lithium-metal batteries (LMBs) is hindered by suitable electrolytes that are simultaneously compatible with both highvoltage cathodes and Li anodes. Herein, a novel localized high-concentration electrolyte with ethoxy(pentafluoro)cyclotriphosphazene (PFPN) as a nonflammable diluent is developed. The inorganic-dominate diluent improves the safety of the organic electrolyte, and helps to construct robust passivation interphases on both electrodes. Specifically, PFPN accelerates the complete reduction of anions, leading to a stable anion-derived interphase layer on Li anode. Meanwhile, PFPN and anions co-participate in the formation of cathode-electrolyte interphase, suppressing side reactions and the structural damage of high-voltage Ni-rich cathodes. As a result, PFPN-based electrolyte prolongs the cycling life of LMBs based on high-voltage LiNi 0.6 Co 0.2 Mn 0.2 (NCM622) and LiNi 0.8 Co 0.1 Mn 0.1 (NCM811) cathodes. Specially, 25-µm-thick Li paired with NCM622 with a N/P ratio of 1.3 (4.4 V) exhibits excellent capacity retention of above 90% after 200 cycles. This study highlights the important role of diluent in tailoring electrode/electrolyte interphases and provides a new strategy for designing high-safety electrolytes toward highvoltage LMBs.

Localized High‐Concentration Electrolytes with Low‐Cost Diluents Compatible with Both Cobalt‐Free LiNiO2 Cathode and Lithium‐Metal Anode

“…[39,40] Ethoxy(pentafluoro)cyclotriphosphazene was reported both by Liu et al and Zhang et al as a nonflammable diluent for LHCE. [41,42] 2,2,2-trifluoroethyl trifluoromethanesulfonate was also reported by Zhu et al to improve the oxidative stability of the electrolyte. [43] Moreover, it is also critical to understand the effect of the diluents on the chemistry of EEI.…”

Tuning the Li+ Solvation Structure by a “Bulky Coordinating” Strategy Enables Nonflammable Electrolyte for Ultrahigh Voltage Lithium Metal Batteries

“…Plentiful works on electrolyte engineering emphasized the pivotal role of Li + solvation structure and its significant influence on interfacial chemistry. − In the traditional electrolyte, Li + ions are mostly coordinated with solvent molecules to form a solvent-separated ion pairs (SSIP) structure, which induces an organics-rich interphase derived from solvents and leads to poor performances for LMBs. − Increasing the salt concentrations can force the anions enter into solvation to create contact ion pairs (CIP) and aggregated ions (AGG) structures in the electrolyte, which can generate an anion-derived interphase to improve Li metal stability. Nevertheless, the high viscosity and high cost of the high concentration electrolyte (HCE) hinder its practical application. − By adding inert diluents, a localized high concentration electrolyte (LHCE) is proposed to reduce the viscosity while retaining the solvation structure of HCE. − Using weak solvents to weaken the Li + –solvent coordination is another efficient tuning strategy, as this weakly solvating electrolyte (WSE) contains abundant ion pairs or aggregates under low salt concentrations. But WSE is rarely investigated because of its extremely limited salt solubility and terrible oxidation stability. − Moreover, functional fluorination was implemented to create the fluorinated weakly solvating electrolyte (FWSE).…”

Armor‐like Inorganic‐rich Cathode Electrolyte Interphase Enabled by the Pentafluorophenylboronic Acid Additive for High‐voltage Li||NCM622 Batteries

“…Compared with recent other electrolyte research, the cycling stability achieved by our electrolyte strategy shows that the armor-like inorganic-rich CEI with high mechanical strength and high ionic conductivity can more effectively stabilize the high-voltage cycle performance (Table S1). [5,34,[37][38][39][40][41]…”