Ultrahigh-Voltage Lithium Metal Batteries Enabled by Single-Ion and Weakly-Solvating Nanometric Aggregates

The urgent need for high energy density (> 400 Wh kg^-1) has driven advancements in lithium metal batteries (LMBs) with high-voltage cathodes. However, degradation of traditional electrolytes restricts high cut-off voltage < 4.4 V, while low lithium transference numbers (t_{Li +}) lead to polarization and early charge/discharge termination, which typically necessitate use of multiple solvents or salt-concentrated electrolytes to enable high-voltage chemistry. To address this challenge, we developed a single-solvent, single-salt electrolyte with tris(2,2,2-trifluoroethyl)phosphate (TFEP), achieving a high t_{Li +} of 0.78 and enabling ultra-high-voltage LMB operation up to 5.0 V. Large molecular sterics and electron density delocalization of TFEP enabled dominant presence of local aggregates (AGGs), which further populated to form large and ion-rich weakly-solvating nanometric aggregates (n-AGGs), changing redox properties and promoting the interfacial stabilities to a greater extent. As a result, we showed suppressed dendrite formation with stable cycling for over 1500 h, and full-cell operations paired with LiNi_0.8Mn_0.1Co_0.1O₂ (NCM811) at 4.7 V and with LiNi_0.5Mn_1.5O₄ (LNMO) at 5.0 V. The tuning of bulk electrolyte properties from the scale of microscopic electronic structures to mesoscopic solvation structures has effectively enhanced thermodynamic and kinetic stabilities of the electrolyte, paving the way for LMBs with high-voltage tolerance.