Charge Transport Properties of Lithium Superoxide in Li–O₂Batteries

Abstract

The theoretical energy density of lithium–oxygen (Li–O2) batteries is extremely high, although there are many challenges that must be overcome to achieve high energy density in a manufactured cell. For example, little is known about the properties of one of the key intermediates, lithium superoxide (LiO2), which until recently had not been stabilized in bulk form. In this work, lithium superoxide was deposited onto iridium–reduced graphene oxide (Ir–rGO) cathodes in a Li–O2 system under a flow of O2. Lithium peroxide (Li2O2) was subsequently produced on the cathode surface in an inert Ar atmosphere. Based on a detailed analysis of electrochemical impedance spectroscopy data, it was demonstrated experimentally for the first time that the charge transport resistance through LiO2 was much lower than for Li2O2 and correlated with lower LiO2 charge overpotentials. This result indicates that LiO2 has good electronic conductivity and confirms previous theoretical predictions that bulk LiO2 has better charge transport properties than Li2O2. In addition, impedance and other characterization of Li2O2 formation from LiO2 in an Ar atmosphere revealed that when surface-mediated Li2O2 formation occurs, it has a significantly lower discharge potential than when it forms through a solution-phase-mediated process. These significant findings will contribute to the development of Li–O2 batteries through better understanding of LiO2 properties and formation mechanisms.

Lithium superoxide encapsulated in a benzoquinone anion matrix

“…In the core-shell structure of 1, there are two components determining electron and ion conduction throughout the material: the LiO2 core and a Li[p-C6H4O2] shell. With regard to the core, recent experimental (72) and first-principles studies (73)(74)(75) have indicated that LiO2 possesses remarkable ionic and electronic conductivities, greatly exceeding that of other alkali superoxides and peroxides (75), presumably crucial properties necessary to form the structure shown in Fig. 10.…”

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

“…Then, an oxidative linear potential scan was applied to the discharged LOBs, and the evolved gaseous species were monitored with DEMS. It can be seen from panels a and b of Figure 1 that essentially no 16,18 O 2 is evolved for the two LOBs during the oxidative scan, which means that the O−O bond does not break during ORR and OER.…”

“…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?…”

“…At the end of this review, we prospect that more isotope-labeling techniques, e.g., kinetic isotope effect (KIE), can be introduced into the research of LOBs and the application of isotope labeling can be extended to other controversial issues in LOBs, e.g., the reactions and process at the anode|electrolyte interface. To answer this question, McCloskey et al 26 discharged the LOBs under a 50:50 mixture of 16 O 2 and 18 O 2 and under the atmosphere of 18 O 2 first and then 16 O 2 , separately. Then, an oxidative linear potential scan was applied to the discharged LOBs, and the evolved gaseous species were monitored with DEMS.…”

Aprotic Alkali Metal–O2 Batteries: Role of Cathode Surface-Mediated Processes and Heterogeneous Electrocatalysis

“…A synthesis approach that has been successful toward addressing this challenge is reverse microemulsion, which has been shown to lead to control over the nanostructure of complex oxide electrocatalysts (i.e., La 2 NiO 4 with a Ruddlesden–Popper (R-P) crystal structure) with well-defined oxide surface termination . Such electrocatalysts have been reported to possess selective catalytic properties toward the growth of non-traditional, electronically conductive Li-deficient discharge products (Li x O 2 ; 1 ≤ x < 2) in Li–O 2 cathodes with electrolytes containing low DN (<20) solvents. ,,− Figure a clearly shows that the electrocatalyst nanostructure, and consequently the underlying (001)-NiO surface facets of La 2 NiO 4 nanorods, played a pivotal role in controlling the discharge product selectivity and the overall cell performance . Li–O 2 batteries containing (001)-NiO-terminated La 2 NiO 4 nanorod electrocatalysts exhibited a ∼0.7 V reduction in charge overpotentials, as compared to cathodes composed of only carbon or irregularly shaped La 2 NiO 4 nanostructures.…”