Influence of Li2O2 morphology on oxygen reduction and evolution kinetics in Li–O2 batteries

Abstract

Understanding the origins of high overpotentials required for Li 2 O 2 oxidation in Li-O 2 batteries is critical for developing practical devices with improved round-trip efficiency. While a number of studies have reported different Li 2 O 2 morphologies formed during discharge, the influence of the morphology and structure of Li 2 O 2 on the oxygen evolution reaction (OER) kinetics and pathways is not known. Here, we show that two characteristic Li 2 O 2 morphologies are formed in carbon nanotube (CNT) electrodes in a 1,2-dimethoxyethane (DME) electrolyte: discs/toroids (50-200 nm) at low rates/overpotentials (10 mA g CÀ1 or E > 2.7 V vs. Li), or small particles (<20 nm) at higher rates/overpotentials. Upon galvanostatic charging, small particles exhibit a sloping profile with low overpotential (<4 V) while discs exhibit a twostage process involving an initially sloping region followed by a voltage plateau. Potentiostatic intermittent titration technique (PITT) measurements reveal that charging in the sloping region corresponds to solid solution-like delithiation, whereas the voltage plateau (E ¼ 3.4 V vs. Li) corresponds to two-phase oxidation. The marked differences in charging profiles are attributed to differences in surface structure, as supported by X-ray absorption near edge structure (XANES) data showing that oxygen anions on disc surfaces have LiO 2 -like electronic features while those on the particle surfaces are more bulk Li 2 O 2 -like with modified electronic structure compared to commercial Li 2 O 2 . Such an integrated structural, chemical, and morphological approach to understanding the OER kinetics provides new insights into the desirable discharge product structure for charging at lower overpotentials. Broader contextLi-O 2 batteries are promising as a next-generation electrochemical energy storage technology due to the potentially several-fold improvement in gravimetric energy compared to today's Li-ion batteries. However, Li-O 2 batteries face substantial challenges that currently limit their practical use such as low rate capability, limited cycle life (typically below 100 cycles) resulting largely from the poor chemical and electrochemical stability of the electrode and electrolyte, and the large voltage polarization ($0.6 to 1 V above thermodynamic) required on charge due to the slow kinetics of oxygen evolution from Li 2 O 2 . While select surfaces or catalysts have been demonstrated to lower the charging voltage, the processes occurring in electrochemically formed Li 2 O 2 during charge are not well understood. Further, the inuence of Li 2 O 2 morphologies (e.g. particle shape and structure) and corresponding surface properties on the charging voltages are not known. Characterization and control over products formed upon discharge, as reported in this study, enable new insights into the governing physical parameters of Li 2 O 2 that inuence the charging behaviour. Combined chemical, electrochemical, morphological and electronic understanding is increasingly important as researchers se...

Effect of the size-selective silver clusters on lithium peroxide morphology in lithium–oxygen batteries

“…The Ag 9 toroids are smaller than the Ag 15 toroids (B500 nm versus B1,000 nm) with the same discharge capacity. Toroids have been found in the discharge product by others in Li-O 2 cells [31][32][33][34] . The X-ray diffraction (XRD) patterns in Fig.…”

Rate-Dependent Morphology of Li2O2 Growth in Li–O2 Batteries

“…4. Our Tafel analysis gives the symmetry factor α = 0.1 on CNTs [12] and α = 0.2 on glassy carbon [49]. The small apparent symmetry factor observed on CNTs could stem from additional overpotentials, e.g., diffuse double layers in the solid due to low electron conductivity in Li 2 O 2 [51,52].…”

“…The exchange current density I 0 is determined via Tafel analysis [12]. We must carefully interpret this measurement because the exchange current density depends on Li 2 O 2 activity and height profile in our model (see Eq.…”

“…Recent experiments on Li-O 2 batteries with etherbased electrolyte have revealed that the electronically isolating discharge product Li 2 O 2 can deposit in complex toroid-like morphologies [11,12] or thin films [12,13]. In contrast, only quad-shaped particles have been observed in sodium-oxygen batteries so far [14].…”

Long life rechargeable Li-O2 batteries enabled by enhanced charge transfer in nanocable-like Fe@N-doped carbon nanotube catalyst

“…Heteroatom doping or hybridization with metal species represents an efficient way to enhance the activity of carbon-based catalysts towards oxygen reduction reaction (ORR)/OER reactions [39][40][41][42]. The presence of heteroatoms (e.g., N) and metal species not only reduces the local work function on carbon surface for better O2 adsorption, but also changes the charge density on the carbon surface via electron transfer effect [16,[43][44][45][46][47][48][49][50][51]. Nevertheless, the understanding on the origin of their activity and the chemical nature of the reaction pathways is still quite limited, which hinders the development of high-performance cathode catalysts.…”