Deciphering the Enigma of Li2CO3 Oxidation Using a Solid-State Li-Air Battery Configuration

Li₂CO₃ is a ubiquitous byproduct in Li-air (O₂) batteries, and its accumulation on the cathode could be detrimental to the devices. As a result, much efforts have been devoted to investigating its formation and decomposition, in particular, upon cycling of Li-O₂ batteries. At high voltages, Li₂CO₃ is expected to decompose into CO₂ and O₂. However, as recognized from the work of many authors, only CO₂, and no O₂, has been identified, and the underlying mechanism remains uncertain so far. Herein, a solid-state Li-O₂ battery (Li|Li_6.4La₃Zr_1.4Ta_0.6O₁₂|Au) has been designed to interrogate the Li₂CO₃ oxidation without interferences from the decomposition of other battery components (organic electrolyte, binder, and carbon cathode) widely applied in conventional Li-O₂ batteries. It is revealed that Li₂CO₃ can indeed be oxidized to CO₂ and O₂ in a more stable solid-state Li-O₂ battery configuration, highlighting the feasibility of reversible operation of Li-O₂ batteries with ambient air as the feeding gas.

Li-air Battery with a Superhydrophobic Li-Protective Layer

Li-air  batteries operated in ambient air are imperative toward real practical applications. However, the passivation of lithium metal anodes induced by attacking air hinders their long-term running, accelerating the degradation of Li-air batteries. Herein, a hydrogel-derived hierarchical porous carbon (HDHPC) layer with superhydrophobicity is proved as an effective Li-protective layer for a Li-air battery that suppresses the H₂O attack and lithium dendrite formation during cycling. Accordingly, the HDHPC protective layer-based Li-air cell exhibits eminent cycling stability in ambient air [relative humidity (RH) of ∼40%], which is far better than that of the Li-air cell without the HDHPC protective layer. It is also demonstrated that the conversion of O₂/Li₂O₂ in Li-air batteries adversely affects the decomposition of the byproduct and electrolyte. The usage of the HDHPC protective layer pioneers a new avenue of developing high-performance Li-air batteries in ambient air.

Cathode Local Curvature Affects Lithium Peroxide Growth in Li-O2 Batteries

The growth of lithium peroxide (Li₂O₂) in cathodes determines the performance of lithium-oxygen batteries (LOBs). The factor affecting the Li₂O₂ growth position is of great importance. Here, three hollow carbon spheres with diameters of 200 nm, 500 nm, and 2 μm, corresponding to different curvatures of 10, 4, and 1, respectively, are prepared as LOB cathodes. It is found that the larger the curvature, the more difficult it is for Li₂O₂ to grow inside the hollow sphere. Increasing the discharge current density can promote the growth of Li₂O₂ onto a highly curved concave substrate. Therefore, to maximize the battery performance, the applied current density and the local curvature of the porous cathode need to match to optimize the pore space utilization and meanwhile to enhance the interface charge transfer between Li₂O₂ and electrode. The revealed relationship among the local curvature of the porous electrode, Li₂O₂ deposition position, and battery performance is valuable to the topography design of the LOB cathode.

Phosphorene as a Catalyst for Highly Efficient Nonaqueous Li-Air Batteries

Li-air batteries can yield exceptionally high predicted energy densities. However, for this technology to become realizable, round-trip efficiency issues and slow kinetics at the cathode require implementation of a catalyst. With design parameters not well understood and limitations on material selection, choosing an ideal catalyst is complex. In Li-air batteries, energy storage is achieved by reactions between Li and O (oxygen reduction reaction for discharge and oxygen evolution reaction for charge). Here, phosphorene is proposed as a solution through simulations of its catalytic behavior toward discharge initiated via either O₂ dissociation or Li adsorption. After obtaining intermediate geometries for both nucleation paths leading to either Li₂O₂ or 2(Li₂O), free-energy diagrams are generated to predict the promoted discharge product of Li₂O₂. Furthermore, considering a final product of Li₂O₂, the overpotentials are predicted to be 1.44 V for discharge and 2.63 V for charge. Activation barriers for the catalytic decomposition of Li₂O₂ (during charge) are found to be 1.01 eV for phosphorene versus 2.06 eV for graphene. This leads to a major difference in reaction rate up to 10¹⁷ times in favor of phosphorene. These results, complemented by electronic analysis, establish phosphorene as a promising catalyst for Li-air batteries.

Influence of the Oxygen Electrode Open Ratio and Electrolyte Evaporation on the Performance of Li-O2 Batteries

This study experimentally investigates and numerically simulates the influence of the cathode electrode open ratio (ratio of oxygen-opening area to the total electrode surface area) on the performance of Li-O₂ batteries at various discharge current densities. At the current density of 0.1 mA/cm², the maximum discharge capacity is achieved at 25% open ratio among the tested open ratios (0-100%). As the open ratio increases from 25% to 100%, the specific discharge capacity decreases from 995 to 397 mA h/g_carbon. A similar trend is observed at 0.3 mA/cm², while the maximum discharge capacity is obtained at 3% open ratio among the tested open ratios. The model that assumes the electrode is always fully saturated by the electrolyte does not obtain similar trends with experimental results, while the model that considers electrolyte loss by evaporation and the volume change of the solid obtains the same trend with experimental observations. The open ratio governs not only availability of oxygen but also the evaporation of the electrolyte and the contact resistance. The faster evaporation of the electrolyte at a higher open ratio can be the main reason for the decrease of the discharge capacity, especially when the open ratio is relatively high (above 25%). Meanwhile, the contact resistance of the battery, measured by the electrochemical impedance spectroscopy (EIS), increases from 3.97 to 7.02 Ω when the open ratio increased from 3% to 95%. The increase of the Ohmic overpotential, however, is negligible (on the order of millivolts) because of the low discharge and charge current rates (on the order of 0.1 mA).

Fluorophore Metal-Organic Complexes: High-Throughput Optical Screening for Aprotic Electrochemical Systems

Combinatorial optical screening of aprotic electrocatalysts has not yet been achieved primarily due to H⁺-associated mechanisms of fluorophore modulation. We have overcome this problem by using fluorophore metal-organic complexes. In particular, eosin Y and quinine can be coordinated with various metallic cations (e.g., Li⁺, Na⁺, Mg²⁺, Zn²⁺, and Al³⁺) in aprotic solvents, triggering changes in their fluorescent properties. These interactions have been used in a reliable screening method to determine oxygen reduction/evolution reaction activities of 100 Mn-based binary catalysts for the aprotic Li-air battery.

N-Co-O Triply Doped Highly Crystalline Porous Carbon: An Acid-Proof Nonprecious Metal Oxygen Evolution Catalyst

In comparison with nonaqueous Li-air batteries, aqueous Li-air batteries are kinetically more facile and there is more variety of non-noble metal catalysts available for oxygen electrocatalysis, especially in alkaline solution. The alkaline battery environment is however vulnerable to electrolyte carbonation by atmospheric CO2 resulting in capacity loss over time. The acid aqueous solution is immune to carbonation but is limited by the lack of effective non-noble metal catalysts for the oxygen evolution reaction (OER). This is contrary to the oxygen reduction reaction (ORR) in acid solution where a few good candidates exist. We report here the development of a N-Co-O triply doped carbon catalyst with substantial OER activity in acid solution by the thermal codecomposition of polyaniline, cobalt salt and cyanamide in nitrogen. Cyanamide and the type of cobalt precursor salt were found to determine the structure, crystallinity, surface area, extent of Co doping and consequently the OER activity of the final carbon catalyst in acid solution. We have also put forward some hypotheses about the active sites that may be useful for guiding further work.

Novel Stable Gel Polymer Electrolyte: Toward a High Safety and Long Life Li-Air Battery

Nonaqueous Li-air battery, as a promising electrochemical energy storage device, has attracted substantial interest, while the safety issues derived from the intrinsic instability of organic liquid electrolytes may become a possible bottleneck for the future application of Li-air battery. Herein, through elaborate design, a novel stable composite gel polymer electrolyte is first proposed and explored for Li-air battery. By use of the composite gel polymer electrolyte, the Li-air polymer batteries composed of a lithium foil anode and Super P cathode are assembled and operated in ambient air and their cycling performance is evaluated. The batteries exhibit enhanced cycling stability and safety, where 100 cycles are achieved in ambient air at room temperature. The feasibility study demonstrates that the gel polymer electrolyte-based polymer Li-air battery is highly advantageous and could be used as a useful alternative strategy for the development of Li-air battery upon further application.

Nucleation and Growth of Lithium Peroxide in the Li-O2 Battery

We study the relationship between Li2O2 morphology and the electrochemical performance of the Li-O2 battery using a combination of experiment and theory. Experimental Li-O2 battery discharge curves are accurately captured by a theoretical model in which electrode performance is limited by the nucleation and growth of discrete Li2O2 nanostructures in the cathode. We further show that the characteristic sharp voltage drop widely reported at the end of discharge results from the decrease in electrochemical surface area as Li2O2 covers the cathode surface. Preventing surface nucleation is highlighted as a core strategy for increasing Li-O2 battery capacity.

Screening of Novel Li-Air Battery Catalyst Materials by a Thin Film Combinatorial Materials Approach

A combinatorial synthesis and high-throughput screening process was developed for the investigation of potential oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts for use as Li-air battery cathode materials. Libraries of discrete ternary metal alloy compositions were deposited via thin-film sputtering. The samples were electrochemically tested in parallel using cyclic voltammetry in O2-saturated KOH electrolyte. Compositions were ranked by ORR and OER onset potentials with respect to an internal Pt reference. Results from the Pt-Mn-Co, Cr-Mn-Co, Pd-Mn-Co, and Pd-Mn-Ru systems are reported. Many alloy compositions showed marked improvement in catalytic activity compared to pure Pt. Among the systems considered, Pt12Mn44Co44, Pd43Co57 and Pd36Mn28Ru36 in particular exhibited lower overpotentials for oxygen reactions, which occur at the cathode in Li-air batteries.

A Stable, Magnetic, and Metallic Li3O4 Compound as a Discharge Product in a Li-Air Battery

The Li-air battery with the specific energy exceeding that of a Li ion battery has been aimed as the next-generation battery. The improvement of the performance of the Li-air battery needs a full resolution of the actual discharge products. Li2O2 has been long recognized as the main discharge product, with which, however, there are obvious failures on the understanding of various experimental observations (e.g., magnetism, oxygen K-edge spectrum, etc.) on discharge products. There is a possibility of the existence of other Li-O compounds unknown thus far. Here, a hitherto unknown Li3O4 compound as a discharge product of the Li-air battery was predicted through first-principles swarm structure searching calculations. The new compound has a unique structure featuring the mixture of superoxide O2(-) and peroxide O2(2-), the first such example in the Li-O system. The existence of superoxide O2(-) creates magnetism and hole-doped metallicity. Findings of Li3O4 gave rise to direct explanations of the unresolved experimental magnetism, triple peaks of oxygen K-edge spectra, and the Raman peak at 1125 cm(-1) of the discharge products. Our work enables an opportunity for the performance of capacity, charge overpotential, and round-trip efficiency of the Li-air battery.

Combining Accurate O2 and Li2O2 Assays to Separate Discharge and Charge Stability Limitations in Nonaqueous Li-O2 Batteries

Li-air batteries have generated enormous interest as potential high specific energy alternatives to existing energy storage devices. However, Li-air batteries suffer from poor rechargeability caused by the instability of organic electrolytes and carbon cathodes. To understand and address this poor rechargeability, it is essential to elucidate the efficiency in which O2 is converted to Li2O2 (the desired discharge product) during discharge and the efficiency in which Li2O2 is oxidized back to O2 during charge. In this Letter, we combine many quantitative techniques, including a newly developed peroxide titration, to assign and quantify decomposition pathways occurring in cells employing a variety of solvents and cathodes. We find that Li2O2-induced electrolyte solvent and salt instabilities account for nearly all efficiency losses upon discharge, whereas both cathode and electrolyte instabilities are observed upon charge at high potentials.

Twin Problems of Interfacial Carbonate Formation in Nonaqueous Li-O2 Batteries

We use XPS and isotope labeling coupled with differential electrochemical mass spectrometry (DEMS) to show that small amounts of carbonates formed during discharge and charge of Li-O2 cells in ether electrolytes originate from reaction of Li2O2 (or LiO2) both with the electrolyte and with the C cathode. Reaction with the cathode forms approximately a monolayer of Li2CO3 at the C-Li2O2 interface, while reaction with the electrolyte forms approximately a monolayer of carbonate at the Li2O2-electrolyte interface during charge. A simple electrochemical model suggests that the carbonate at the electrolyte-Li2O2 interface is responsible for the large potential increase during charging (and hence indirectly for the poor rechargeability). A theoretical charge-transport model suggests that the carbonate layer at the C-Li2O2 interface causes a 10-100 fold decrease in the exchange current density. These twin "interfacial carbonate problems" are likely general and will ultimately have to be overcome to produce a highly rechargeable Li-air battery.