Constructing static two-electron lithium-bromide battery

Abstract

Despite their potential as conversion-type energy storage technologies, the performance of static lithium-bromide (SLB) batteries has remained stagnant for decades. Progress has been hindered by the intrinsic liquid-liquid redox mode and single-electron transfer of these batteries. Here, we developed a high-performance SLB battery based on the active bromine salt cathode and the two-electron transfer chemistry with a Br^-/Br⁺ redox couple by electrolyte tailoring. The introduction of NO₃^- improved the reversible single-electron transition of Br^-, and more impressively, the coordinated Cl^- anions activated the Br⁺ conversion to provide an additional electron transfer. A voltage plateau was observed at 3.8 V, and the discharge capacity and energy density were increased by 142 and 159% compared to the one-electron reaction benchmark. This two-step conversion mechanism exhibited excellent stability, with the battery functioning for 1000 cycles. These performances already approach the state of the art of currently established Li-halogen batteries. We consider the established two-electron redox mechanism highly exemplary for diversified halogen batteries.

Fig. 1.. TBABr₃ cathode and one-electron transfer chemistry.

(A) Molecular model of TBABr₃. (B) SEM image with EDX mapping data. (C) Cyclic voltammetry (CV) curves of the Li||TBABr₃ battery with different electrolytes at 2.0 to 3.8 V and 2 mV s⁻¹. (D) Galvanostatic charge-discharge (GCD) curves of the Li||TBABr₃ battery with different electrolytes. (E) CV curves of the Li||TBABr₃ battery with E2 electrolyte at various scanning rates. (F) GCD curves of the Li||TBABr₃ battery with E2 electrolyte at different current densities. wt %, weight %.

Fig. 2.. Two-electron transfer chemistry.

(A) CV curves of the Li||TBABr₃ battery with different electrolytes in the 2.0 to 4.0 V range. (B) Corresponding GCD curves. (C) CV curves of the Li||TBABr₃ battery with CLE2 electrolyte in different scanning ranges. (D) GCD curves of the Li||TBABr₃ battery with CLE2 electrolyte in different scanning ranges. (E) Capacity and energy density enhancement by the two-electron transfer with one-electron transfer reaction as the benchmark. (F) CV curves of the Li||TBABr₃ battery with CLE2 at different scanning rates with calculated b values of redox peaks.

Fig. 3.. Li metal anode compatibility.

(A) LSV curve of the CLE2 at 10 mV s⁻¹. (B) CV curve of asymmetric Li||stainless steel battery at 10 mV s⁻¹. (C) GCD curves of the asymmetric Li||stainless steel battery at 1 mA, 1 mAh cm⁻² in specific cycles. (D) Prolonged cyclic performance of a symmetric Li||Li battery at 1 mA, 1 mAh cm⁻².

Fig. 4.. Electrochemical performance.

(A) Prolonged cyclic performance at 1.5 A g⁻¹. (B) Corresponding GCD curves in cycles 2 and 1000. (C) Rate performance in the current densities of 1 to 4 A g⁻¹. (D) Corresponding discharge curves. (E) dQ/dV curves. (F) Galvanostatic intermittent titration technique (GITT) curve. (G) Performance comparison between this work and reported Li-ion batteries.

Fig. 5.. Redox mechanism characterization.

(A) Ex situ high-resolution I 3d XPS spectra at selected SOC points. (B) High-resolution Cl 2p XPS spectra at selected SOC points. (C) In situ Raman spectra at selected SOC points. (D) In situ EIS patterns at selected SOC points. (E) Profiles of R_o and R_ct values as a function of SOC. The yellow rectangular identifies the R_ct mutation. a.u., arbitrary units.

Fig. 6.. DFT calculations.

(A) Proposed reaction route. (B) Cohesive energy during the charging process for the system with and without Cl⁻. (C) ELF for BrCl, BrCl₂, and BrCl₃. (D) Atomic charge for BrCl, BrCl₂, and BrCl₃.