Direct Observation of Redox Mediator-Assisted Solution-Phase Discharging of Li–O₂ Battery by Liquid-Phase Transmission Electron Microscopy

Li–O2 battery is one of the important next-generation energy storage systems, as it can potentially offer the highest theoretical energy density among battery chemistries reported thus far. However, realization of its high discharge capacity still remains challenging and is hampered by the nature of how the discharge products are formed, causing premature passivation of the air electrode. Redox mediators are exploited to solve this problem, as they can promote the charge transfer from electrodes to the solution phase. The mechanistic understanding of the fundamental electrochemical reaction involving the redox mediators would aid in the further development of Li–O2 batteries along with rational design of new redox mediators. Herein, we attempt to monitor the discharge reaction of a Li–O2 battery in real time by liquid-phase transmission electron microscopy (TEM). Direct in situ TEM observation reveals the gradual growth of toroidal Li2O2 discharge product in the electrolyte with the redox mediator upon discharge. Moreover, quantitative analyses of the growth profiles elucidate that the growth mechanism involves two steps: dominant lateral growth of Li2O2 into disclike structures in the early stage followed by vertical growth with morphology transformation into a toroidal structure

Direct Observation of Redox Mediator-Assisted Solution-Phase Discharging of Li–O₂ Battery by Liquid-Phase Transmission Electron Microscopy

Li–O2 battery is one of the important next-generation energy storage systems, as it can potentially offer the highest theoretical energy density among battery chemistries reported thus far. However, realization of its high discharge capacity still remains challenging and is hampered by the nature of how the discharge products are formed, causing premature passivation of the air electrode. Redox mediators are exploited to solve this problem, as they can promote the charge transfer from electrodes to the solution phase. The mechanistic understanding of the fundamental electrochemical reaction involving the redox mediators would aid in the further development of Li–O2 batteries along with rational design of new redox mediators. Herein, we attempt to monitor the discharge reaction of a Li–O2 battery in real time by liquid-phase transmission electron microscopy (TEM). Direct in situ TEM observation reveals the gradual growth of toroidal Li2O2 discharge product in the electrolyte with the redox mediator upon discharge. Moreover, quantitative analyses of the growth profiles elucidate that the growth mechanism involves two steps: dominant lateral growth of Li2O2 into disclike structures in the early stage followed by vertical growth with morphology transformation into a toroidal structure

High-Dielectric Polymer Coating for Uniform Lithium Deposition in Anode-Free Lithium Batteries

The use of lithium metal either in an anode or anode-free configuration is envisaged as the most promising way to boost the energy density of the current lithium-ion battery system. Nevertheless, the uncontrolled lithium dendritic growth inhibits practical utilization of lithium metal as an anode due to safety concerns and low Coulombic efficiency. In this work, we show that when a high-dielectric SEI is coated on a current collector, it can effectively promote a uniform lithium deposition by decreasing the overpotential between the surfaces, lowering the local current density and suppressing lithium protrusions. Using a PVDF (polyvinylidene difluoride)-based dielectric medium, it is demonstrated that varying the dielectric properties of PVDF by crystallinity control can regulate the lithium deposition mechanisms. Moreover, when the dielectric properties of PVDF film are tailored by the inclusion of dielectric nanoparticles, a selective formation of high-dielectric β-PVDF phase is induced during its film formation (LiF@PVDF), which synergistically promotes uniform lithium deposition/stripping in an anode-free half-cell setup

A Full Oxide-Based Solid-State Lithium Battery and Its Unexpected Cathode Degradation Mechanism

Fabricating full oxide garnet type Li6.4La3Zr1.4Ta0.6O12 (LLZTO)-based solid-state batteries has posed challenges, particularly in cosintering cathode composites. In this research, we achieve high-performance cathode composites through ultrafast cosintering, facilitated by residual lithium as a sintering agent under an O2 atmosphere. These composites demonstrate compatibility with various cathode materials including LiCoO2 and LiNi1/3Co1/3Mn1/3O2 in an LLZTO-based composite. Significantly, our findings reveal that residual stress on the cathode active material plays a pivotal role in degradation during cycling. The rigid LLZTO framework constrains volume changes in the cathode material during (de)lithiation, leading to mechanical failure. This discovery challenges prior assumptions about the primary susceptibility of the cathode/electrolyte interface to electro-chemo-mechanical failure. Furthermore, stress release mechanisms are found to be influenced by the particle morphology of the cathode material, whether single crystalline LiCoO2 or polycrystalline LiNi1/3Co1/3Mn1/3O2. These insights underscore the importance of managing residual stress and optimizing cathode material morphology for achieving stable performance in full oxide LLZTO-based solid-state batteries

Enhancing Bifunctional Catalytic Activity via a Nanostructured La(Sr)Fe(Co)O_3−δ@Pd Matrix as an Efficient Electrocatalyst for Li–O₂ Batteries

One of the important challenges with a bifunctional electrocatalyst is reducing the large overpotential involved in the slow kinetics of the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) at the air electrode in a metal–air redox battery. Here, we present a nanostructured LSCF@Pd matrix of nanostructured LSCF (Nano-LSCF) with palladium to enhance the bifunctional catalytic activity in Li–O2 battery applications. Pd nanoparticles can be perfectly supported on the surface of the Nano-LSCF, and the ORR catalytic activity was properly improved. When Nano-LSCF@Pd was applied to a cathode catalyst in Li–O2 batteries, the first discharge ability (16912 mA h g–1) was higher than that of Nano-LSCF (6707 mA h g–1) and the cycling property improved. These results demonstrate that the Pd-deposited nanostructured perovskite is a capable catalyst to enhance the ORR activity of LSCF as a promising bifunctional electrocatalyst

High-Energy and Long-Lasting Organic Electrode for a Rechargeable Aqueous Battery

Redox-active organic materials (ROMs) hold great promise as potential electrode materials for eco-friendly, cost-effective, and sustainable batteries; however, the poor cycle stability arising from the chronic dissolution issue of the ROMs in generic battery systems has impeded their practical employment. Herein, we present that a rational selection of electrolytes considering the solubility tendency can unlock the hidden full redox capability of the DMPZ electrode (i.e., 5,10-dihydro-5,10-dimethylphenazine) with unprecedentedly high reversibility. It is demonstrated that a multiredox activity of DMPZ/DMPZ+/DMPZ2+, which has been previously regarded to degrade with repeated cycles, in the newly designed electrolyte can be utilized with surprisingly robust cycle stability over 1000 cycles at 1C. This work signifies that tailoring the electrode–electrolyte compatibility can possibly unleash the hidden potential of many common ROMs, catalyzing the rediscovery of organic electrodes with long-lasting and high energy density

Tuning the Carbon Crystallinity for Highly Stable Li–O₂ Batteries

The Li–O₂ battery is capable of delivering the highest energy density among currently known battery chemistries and is thus regarded as one of the most promising candidates for emerging high-energy-density applications such as electric vehicles. Although much progress has been made in the past decade in understanding the reaction chemistry of this battery system, many issues must be resolved regarding the active components, including the air electrode and electrolyte, to overcome the presently insufficient cycle life. In this work, we demonstrate that the degradation kinetics of both the air electrode and electrolyte during cycles can be significantly retarded through control of the crystallinity of the carbon electrode, the most frequently used air electrode in current Li–O₂ batteries. Using ¹³C-based air electrodes with various degrees of graphitic crystallinity and in situ differential electrochemical mass spectroscopy analysis, it is demonstrated that, as the crystallinity increases in the carbon, the CO₂ evolution from the cell is significantly reduced, which leads to a 3-fold enhancement in the cyclic stability of the cell