A Redox-Active Organic Cation for Safer Metallic Lithium-Based Batteries

Abstract

Safety concerns have severely impeded the practical application of high-energy-density lithium-based batteries. Dendrite growth and overcharging can lead to particularly catastrophic thermal failure. Here we report an organic cation, trisaminocyclopropenium (TAC), as a bi-functional electrolyte additive to suppress dendrite growth and offer reversible overcharge protection for metallic lithium-based batteries. During the Li plating process, TAC cations with aliphatic chains can form a positively charged electrostatic shield around Li protrusions, repelling the approaching Li⁺ and thereby attaining a more uniform plating. A two times longer cycle life of 300 h at 1 mA cm^-2 is achieved in a Li|Li symmetric cell in comparison with the control. During the overcharging process, the redox-active TAC can repeatedly shuttle between two electrodes, maintaining the cell voltage within a safe value. A solid protection of 117 cycles (~1640 h) at 0.2 C with a 100% overcharge is achieved in a LiFePO₄/Li₄Ti₅O₁₂ cell. This study sheds fresh light on the ability of organic cations to build safer batteries.

Keywords: dendrite suppression; electrolyte additive; metallic lithium-based batteries; overcharge protection; trisaminocyclopropenium cation.

Fig. 1.

a) A schematic illustration of cation-based electrolyte additives for dendrite suppression; b) metal cations; c) quaternary ammonium cations and d) TAC cation.

Fig. 2.

a) Li plating/stripping behaviour in electrolyte containing 50 mM TAC, the inset is a Li disk stored in the same electrolyte after six months; b) cyclic voltammograms at various scan rates; c) plots of peak current vs square root of the scan rate and linear fits; d) cyclic voltammograms scanned at 100 mV s⁻¹; e) photographs of the as-synthesized TAC cation salt and dication salt, photographs of holding a electrochemical device contained TAC cation electrolyte at a constant-potential of 4.1 V (vs. Li⁺/Li); f) UV-vis spectra of TAC cation and dication solutions.

Fig. 3.

Electrochemical performance of Li plating/stripping in TAC electrolyte. a) Voltage profiles of Li|Li symmetric cells cycling at 1 mA cm⁻² 1 mAh cm⁻²; b) Coulombic efficiencies of Li|Cu asymmetric cells cycling at 0.5 mA cm⁻² 1 mAh cm⁻² and c) the voltage profiles of Li|Cu cell at 1st and 120th; d) in-situ optical microscopic images during Li plating process at 4 mA cm⁻², scale bar 200 μm; e) schematic illustration of a cation-shield mechanism. Note: the TAC cation is not drawn to scale.

Fig. 4.

a) Comparison of cyclic voltammograms between LFP and TAC at a scan rate of 0.1 mV s⁻¹ and 20 mV s⁻¹, respectively; b) cycling performance of LFP/Li cell with 100% overcharge; LFP/LTO cell: c) voltage profiles with 100% overcharge at different current rates; d) continuously charging voltage profile at 0.5 C; e) cycling performance with 100% overcharge at 0.2 C; f) normal cycling performance at 0.5 C between 1.5–2.2 V, inset is the voltage profiles of the first two cycles; g) normal rate performance between 1.5–2.2 V.