Dynamic Changes in Charge Transfer Resistances during Cycling of Aprotic Li–O₂ Batteries

Abstract

Various electrolyte components have been investigated with the aim of improving the cycle life of lithium−oxygen (Li−O 2 ) batteries. A tetraglymebased electrolyte containing dual anions of Br − and NO 3 − is a promising electrolyte system in which the cell voltage during charging is reduced because of the redox-mediator function of the Br − /Br 3 − and NO 2 − /NO 2 couples, while the Li-metal anode is protected by Li 2 O formed via the reaction between Li metal and NO 3− . To maximize the potential of this system, the fundamental factors that limit the cycle life should be clarified. In the present work, we used nondestructive electrochemical impedance spectroscopy to analyze the temporal change of the charge transfer resistances during cycles of Li−O 2 batteries with dual anions. The charge transfer resistance at the cathode was revealed to exhibit good correlation with the reduction of the discharge voltage. These results, combined with the results of electrode surface inspections, revealed that irreversible accumulation of insulating deposits such as Li 2 O 2 and Li 2 CO 3 on the cathode surface was a major cause of the short cycle life. Furthermore, the analyses of the time course of the solution resistance suggested that diminished reactivity between the redox mediators and Li 2 O 2 was a critical factor that led to the irreversible accumulation of the lessreactive Li 2 O 2 on the cathode and eventually to a shortened cycle life. These findings indicated that increasing the reactivity between Br 3 − and Li 2 O 2 is essentially important for improving the cycle stability of Li−O 2 batteries and the reactivity can be nondestructively assessed by tracking the dynamic changes in the solution resistance.

Isotopic Depth Profiling of Discharge Products Identifies Reactive Interfaces in an Aprotic Li–O2 Battery with a Redox Mediator

“…Figure S2a presents representative data obtained from the discharge/charge cycling of m-CP under current-controlled conditions. Notably, although the number of cycles was much lower than could be obtained from a typical porous carbon cathode (Figure S2b) due to the much smaller effective electrode area of the m-CP, both materials showed similar relationships between the state of (dis)­charge and the potential.…”

“…Cell Configuration. This work employed an electrochemical cell previously described in the literature, 14 with a modification such that the cell lid was replaced to make it a flow-type cell to facilitate the DEMS analyses. A section of Li foil (400 μm thick, 25 mm in diameter, Honjo Metal) was used as the anode, while m-CP was used as the cathode material.…”

“…Although NO 2 – thus generated forms a redox pair, NO 2 – /NO 2 , that can also partially function as an RM, the NO 2 – /NO 2 couple is only generated according to the above equation, and its effective concentration as an RM is much lower compared with Br – /Br 3 – . Although the Br – /Br 3 – redox pair shows promise as an RM for these reasons, the addition of this pair does not suppress increases in the charging potential to a sufficient degree . Therefore, an improved understanding of the manner in which Br 3 – promotes the oxidative decomposition of Li 2 O 2 is required to further reduce the charging potential.…”

“…Although the Br − /Br 3 − redox pair shows promise as an RM for these reasons, the addition of this pair does not suppress increases in the charging potential to a sufficient degree. 14 Therefore, an improved understanding of the manner in which Br 3 − promotes the oxidative decomposition of Li 2 O 2 is required to further reduce the charging potential. In particular, it is essential to identify the true reactive interfaces.…”

Interrogating Lithium–Oxygen Battery Reactions and Chemistry with Isotope-Labeling Techniques: A Mini Review

“…Toward unlocking the energy capabilities of LOBs, a wide spectrum of advanced characterization techniques has been employed to shed light on the mechanisms underpinning the operation of LOBs. For instance, differential electrochemical mass spectroscopy (DEMS) that can simultaneously measure the quantity of the gaseous species (e.g., O 2 and CO 2 ) and charge involved in the operation of LOBs has been used to study the reversibility of LOBs; − surface-enhanced Raman spectroscopy (SERS) that can spectroscopically identify the reaction products (e.g., Li 2 O 2 and Li 2 CO 3 ) and intermediates (e.g., O 2 – and LiO 2 ) has been employed to formulate the elementary steps of Li–O 2 electrochemistry; − electrochemical impedance spectroscopy that can obtain the electrical properties (e.g., resistance and capacitance) of the electrode|electrolyte interface has been used to follow the variation of charge transfer resistance and capacitance of the cathode|electrolyte interface during the operation of LOBs. − Interested readers are encouraged to find more details of the advanced characterization techniques in refs − . Among these wide range of the characterization techniques, isotope labeling is a unique and powerful research tool in the mechanistic studies of LOBs; specifically, it is good at tackling two fundamental issues in LOBs: (1) what are the reaction pathways and the associated intermediates and (2) where do these reactions take place?…”