Practical Energy Density Research Articles

Monodispersed FeS2 Electrocatalyst Anchored to Nitrogen‐Doped Carbon Host for Lithium–Sulfur Batteries

AbstractDespite their high theoretical energy density, lithium–sulfur (Li–S) batteries are hindered by practical challenges including sluggish conversion kinetics and shuttle effect of polysulfides. Here, a nitrogen‐doped continuous porous carbon (CPC) host anchoring monodispersed sub‐10 nm FeS2 nanoclusters (CPC@FeS2) is reported as an efficient catalytic matrix for sulfur cathode. This host shows strong adsorption of polysulfides, promising the inhibition of polysulfide shuttle and the promoted initial stage of catalytic conversion process. Moreover, fast lithium ion (Li‐ion) diffusion and accelerated solid–solid conversion kinetics of Li2S2 to Li2S on CPC@FeS2 host guarantee boosted electrochemical kinetics for conversion process of sulfur species in Li–S cell, which gives a high utilization of sulfur under practical conditions of high loading and low electrolyte/sulfur (E/S) ratio. Therefore, the surfur cathode (S/CPC@FeS2) delivers a high specific capacity of 1459 mAh g−1 at 0.1 C, a stable cycling over 900 cycles with ultralow fading rate of 0.043% per cycle, and an enhanced rate capability compared with cathode only using carbon host. Further demonstration of this cathode in Li–S pouch cell shows a practical energy density of 372 Wh kg−1 with a sulfur loading of 7.1 mg cm−2 and an E/S ratio of 4 µL mg−1.

Understanding slurry formulations to guide solution-processing of solid electrolytes

Scalable processing of thin solid electrolytes is crucial for engineering solid-state batteries with practical energy densities. To leverage existing battery production infrastructure, pathways for solution processing of solid electrolyte films must be investigated. Roll-to-roll compatible film preparation of aluminum-doped lithium lanthanum zirconate oxide was studied. Four slurry configurations accounting for different solvent properties and solid loadings were evaluated to investigate the ternary interactions within the dispersions. Rheological and coating stability analyses were carried out on the processed slurries. The analysis and characterization indicated that improved component interactions within the ethanol/toluene system result in homogenized distribution of the particles, binder, and plasticizer, and enable the extraction of free-standing thin films with thicknesses of 20 μm and large area (>15 in.2). The drawbacks and opportunities of several protocols for high temperature sintering of the dried green films were investigated. This study highlights the importance of engineering dispersions during the development of processing protocols for solid electrolytes and provides guidelines for the best practices that can be leveraged for solution processing–based fabrications for a wide range of solid electrolyte materials.

(Digital Presentation) Pre-Lithiation of Silicon Anodes By Thermal Evaporation of Lithium for Boosting Energy Density of Lithium Ion Cells

Lithium ion batteries (LIBs) do not only dominate the small format battery market for portable electronic devices, but have also been successfully implemented as the technology of choice for electric vehicles. However, for successful consumer acceptance and broad market penetration of electric vehicles, further improvements of LIBs in terms of energy density and cost along are required. The practically usable energy density of LIB cells is reduced by parasitic side reactions including electrolyte decomposition and formation of the “solid electrolyte interphase” (SEI) at the surface of the anode, as this process is related to the consumption of active lithium. Especially high-capacity silicon-based anodes suffer from ongoing lithium loss and rapid capacity fading.Pre-lithiation is considered as a highly appealing technique to compensate for active lithium losses and, therefore, to increase the practical energy density. A critical parameter for a pre-lithiation strategy is to establish a technique for achieving lithiation of the active storage material at the most uniform lateral and in-depth distributions possible. Despite extensive exploration of various pre-lithiation techniques, controlling the lithium amount precisely while keeping an even lithium distribution remains challenging.Here, we report the thermal evaporation of lithium metal as a novel pre-lithiation technique for silicon anodes that allows both, i.e., a precise control of the degree of pre-lithiation and a homogeneous deposition at the electrode surface. Silicon | Lithium metal interface and SEI characterization show that this approach enables the preserved Li to be consumed for the formation of a pre-formed SEI and creates moderate mechanical cracks by volume changes. The highly uniform Li on thin-film Si is beneficial for increasing capacity retention and significantly suppressing active lithium losses. The phase changes are evaluated and the terms dry-state and wet-state pre-lithiation (without/with electrolyte) are revisited. Finally, a series of electrochemical methods are also validated to allow a direct correlation of active lithium losses with the pre-formed SEI formation and improved electrochemical performance of silicon anodes. Figure 1

(Digital Presentation) Modifying LiNiO2 with W Via a Single Step Synthesis Route

LiNiO2 has been long considered as a promising cathode material owing to its high practical energy density. However, structural and surface instabilities, coupled with complexities in the LiNiO2 synthesis, have thus far prevented its commercialisation. To address issues with the material’s stability during synthesis and cycling, the use of an ammonium tungstate flux is demonstrated here to modify both the LiNiO2 crystal structure and primary particle morphology without introducing additional steps into the synthesis. The successful preparation of LiNiO2 modified with an industrially relevant amount of W (< 5 mol %) was confirmed using a combination of electron microscopy and synchrotron-based X-ray diffraction (XRD). Refinement of structural models against the data suggests tungsten dopant ions occupy the Ni site and concurrently induce migration of Ni2+ to the Li sites. Moreover, W enrichment at grain boundaries has been observed under some of the synthesis conditions. Variable temperature XRD was used to highlight the improved stability of the W-doped materials during the calcination at high temperatures. Electrochemical characterisation shows that W-doped LiNiO2 offers improved cycle life at the expense of little specific capacity. The structural consequences of tungsten doping on the behaviour of the material during electrochemical cycling was also investigated using operando XRD, showing reduced mechanical stress upon cycling. In conclusion, we will show that LiNiO2 modified by W with a simple route and no additional processing steps exhibits structural stability at high temperatures, offering a path towards the reliable synthesis of LiNiO2 with controlled morphology, improved chemomechanics and longer cycling life.

Ultrathin Magnesium Metal Anode – An Essential Component for High‐Energy‐Density Magnesium Battery Materialization

AbstractThe practical loading, i. e., thickness, of metal anodes predominates the practical energy density of batteries that incorporate elemental metals as anode active materials. Despite its significance for achieving high‐energy‐density rechargeable magnesium batteries (RMBs), the application of ultrathin magnesium foils remains a challenge because of the brittleness and unworkability of magnesium. This work provides a critical component of a geometric size applicable to laminate‐type cells of dimensions 42×32 mm2 or larger. Ultrathin magnesium foil without cracked edges can be fabricated by controlling the initial microstructure. Furthermore, the rolling temperature determines the resulting microstructure and thus the electrochemical properties. The optimal sample, a warm‐rolled magnesium foil, exhibited excellent electrochemical characteristics owing to its favorable microstructure, which facilitated a homogeneous distribution of reaction sites. Battery performance using such ultrathin magnesium anodes was investigated with MgMn2O4 and α‐MnO2 cathodes. Considering the weights of both the cathodes and anodes, the gravimetric energy density of primitive [α‐MnO2 || pMg] cells was estimated to be 72 Wh kgelectrode−1.

Towards practical lean-electrolyte Li–S batteries: Highly solvating electrolytes or sparingly solvating electrolytes?

Lithium–sulfur (Li–S) batteries hold great promise to be the next-generation candidate for high-energy-density secondary batteries but in the prerequisite of using low electrolyte-to-sulfur (E/S) ratios. Highly solvating electrolytes (HSEs) and sparingly solvating electrolytes (SSEs), with opposite nature towards the dissolution of polysulfides, have recently emerged as two effective solutions to decrease the E/S ratio and increase the overall practical energy density of Li–S batteries. HSEs featuring with high polysulfide solvation ability have the potential to reduce the E/S ratio by dissolving more polysulfides with less electrolyte, while SSEs alter the sulfur reaction pathway from a dissolution–precipitation mechanism to a quasi-solid mechanism, thereby independent on the use of electrolyte amount. Both HSEs and SSEs show respective effectiveness in lean-electrolyte Li–S batteries, but encounter different challenges to bring Li–S batteries into practical application. This review aims to present a comparative discussion on their unique features and basic electrochemical reaction mechanisms in practical lean-electrolyte Li–S batteries. Emphasis is focused on the current technical challenges and possible solutions for their future development.

Improved Cycling Performance and Safety Characteristics of NMC811/G-Si Battery Cells with an Optimised Electrolyte Formulation

An increase in the practical energy density of anodes can be achieved when using 10%–25% silicon in graphite composite anodes. This is sufficient as the practical capacity of lithium-ion batteries (LIBs) is limited by the cathode capacity and applicable current densities (<4 mA cm−2). Using an NMC811 cathode and a graphite–15% silicon (G-Si15%) composite anode, the performance of more than 20 electrolyte formulations have been evaluated. Of these formulations, EC/EMC+1.2 M LiPF6 containing FEC/VC or LiFSI/VC combinations were shown to have superior performance over the parent electrolyte. Safer to operate ionic liquid-based electrolytes (ILELs) were found to be compatible with these electrodes at moderate cycling rates, with discharge capacities 15% lower compared to the conventional electrolytes. DSC data for both electrodes, cycled and uncycled, in contact with three different electrolytes were recorded. For the cathode in contact with the ILEL, temperatures of exothermic events were higher compared to those of carbonate-based electrolyte counterparts, whereas they were significantly lower for the G-Si15% anodes. In both cases when in contact with ILEL, the heat generated is considerably higher not only compared to samples containing carbonate-based electrolytes but also with respect to data reported for common graphite electrodes with the same ILEL.

New Cathode Materials in the Fe-PO4 -F Chemical Space for High-Performance Sodium-Ion Storage.

Sodium and iron make up the perfect combination for the growing demand for sustainable energy storage systems, given the natural abundance and sustainability of the two building block elements. However, most sodium–iron electrode chemistries are plagued by intrinsic low energy densities with continuous ongoing efforts to solve this. Herein, the chemical space of a series of (meta)stable, off‐stoichiometric Fe‐PO4‐F phases is analyzed. Some are found to display markedly improved electrochemical activity for sodium storage, as compared to the amorphous or thermodynamically stable phases of equivalent composition. The metastable crystalline Na1.2Fe1.2PO4F0.6 delivers a reversible capacity of more than 140 mAh g−1 with an average discharge potential of 2.9 V (vs Na+/Na0) resulting in a practical specific energy density of 400 Wh kg−1 (estimated at the material level), outperforming many developed Fe‐PO4 analogs thus far, with further multiple possibilities to be explored toward improved energy storage metrics. Overall, this study unlocks the possibilities of off‐stoichiometric Fe‐PO4‐F cathode materials and reveals the importance to explore the oft‐overlooked metastable or transient state materials for energy storage.

Hollow Ni3Se4 with High Tap Density as a Carbon-Free Sulfur Immobilizer to Realize High Volumetric and Gravimetric Capacity for Lithium-Sulfur Batteries.

Despite that the practical gravimetric energy density of lithium sulfur batteries has exceeded that of the traditional lithium-ion battery, the volumetric energy density still pales due to the low density of carbonaceous materials. Herein, hollow polar nickel selenide (Ni3Se4) with various architectures was designed and employed as a carbon-free sulfur immobilizer. Among them, hollow sea urchins like Ni3Se4 with high porosity (0.39 cm3 g-1) and large specific surface area (82.7 m2 g-1) exhibit abundant adsorptive and electrocatalytic sites, which pledge excellent electrochemical performances of the Li-S battery. Correspondingly, the Ni3Se4-based sulfur electrode presents excellent rate endurability (581 mAh g-1-composite at 2.0 C) and superior cycle stability (ultralow fading rate of 0.042% per cycle during the 1000 cycles at 1.0 C). More importantly, thanks to the higher tap density (Ni3Se4/S: 1.57 g cm-3 vs super P/S: 0.7 g cm-3), the volumetric specific capacity of Ni3Se4-based cathodes is as high as 1699 mAh cm-3-composite at 0.1 C, which is almost 2.8 times that of the carbonaceous electrode. Hence, rational transition metal selenide architecture design with synergistic function of good conductivity, well-defined catalyst and adsorption, as well as high tap density provide a promising route toward high gravimetric and volumetric energy density of Li-S batteries.

Ultra-high-rate lithium-sulfur batteries with high sulfur loading enabled by Mn2O3-carbonized bacterial cellulose composite as a cathode host

Lithium-sulfur batteries have received significant attention due to their high specific capacity and natural abundance of the sulfur cathode. However, the notorious issue of higher-order lithium-polysulfides dissolution causes inferior cycling and poor electrochemical performance. To overcome the challenge, here, biodegradable and environmentally friendly carbonized bacterial cellulose (CBC) is used as the sulfur host. Further, Mn2O3 is incorporated in bacterial cellulose using hydrothermal synthesis and investigated as a sulfur host. The composite is prepared by a facile encapsulation “melt-diffusion” strategy. The high electronic conductivity of the CBC provides an efficient pathway to carry out redox reactions, and Mn2O3 helps to restrain and convert higher-order lithium-polysulfides to lower-order lithium-polysulfides. Further, the first-principle density functional theory calculation is performed to investigate and understand the role of carbon (CBC) and Mn2O3 in the host material. The lithium-sulfur cell with Mn2O3@CBC cathode host delivers a significantly high initial reversible capacity of 1150 mAh g−1(cathode) (i.e., 1450 mAh g−1(sulfur)) at 0.1 C. It retains a reversible capacity of 254 mAh g−1(cathode) (i.e., 554 mAh g−1(sulfur)) even at the ultra-high rate of 15.0 C. Furthermore, to achieve a practical high energy density and its commercial adoption, we tested lithium-sulfur cells with high areal sulfur loading of 4.2 mg cm−2. The cell shows promising electrochemical performance for long 500 cycles with a capacity retention of 70%. The cell performance is competitive and superior to the current state-of-the-art lithium-sulfur cells reported in the literature.

Pushing Lithium Cobalt Oxides to 4.7 V by Lattice‐Matched Interfacial Engineering

AbstractThe utilization of high‐voltage LiCoO2 is imperative to break the bottleneck of the practical energy density of lithium‐ion batteries. However, LiCoO2 suffers from severe structural and interfacial degradation at &gt;4.55 V. Herein, a novel lattice‐matched LiCoPO4 coating is rationally designed for LiCoO2 which works at 4.6 V (vs Li/Li+) or above. This LiCoPO4 coating, derived by an in situ chemical reaction, grows epitaxially on LiCoO2 crystallite with strong bonding and complete coverage to LiCoO2, ensuring a stable cathode–electrolyte interface with fewer side reactions and alleviated intergranular cracking and phase collapse during repeated high‐voltage lithiation/delithiation processes. In addition, the formed strong covalent P–O tetrahedron configuration at the interface effectively decreases the surface oxygen activity of LiCoO2, further suppressing oxygen release and irreversible phase transition. Therefore, the LiCoPO4‐LiCoO2ǁLi cells display excellent capacity retention of 87% after 300 cycles at 4.6 V and stable operation at 4.6 V/55 °C or 4.7 V/30 °C. The strategy of lattice‐matching growth affords a new way to impact the development of high‐voltage LiCoO2 and beyond.

Synthesis analysis of electric propulsion implement in large civil aircraft

Abstract With more and more concerns of carbon emission reduction in civil aviation, this article aims to illustrate the current development and product of electric propulsion in civil aviation, also to investigate the feasibility and benefit of electric propulsion implement on large civil aircraft. In order to achieve this objective, a single aisle passenger airliner concept with electric propulsion was designed and trade-studied. There are both advantages and disadvantages of different energy architecture such as Turbine generator, APU generator, Lithium battery and Hydrogen fuel cell system. As the practical energy density of lithium battery is much lower than needed, considering the current situation and development potential, fuel cell with liquid hydrogen are most possibly used as the main power source in future large civil aircraft.

A novel construction of Ti3C2@complete edge-nitrogen doped carbon spheres with excellent K-ion storage performance

Edge nitrogen doping, including pyridinic and pyrrolic nitrogen, is an effective strategy to enhance the electrochemical performance of carbons for K-ion batteries owing to the abundant defective sites and lower K-ion adsorption energy than graphitic nitrogen. Nevertheless, due to the singleness of precursor and imprecise carbonization conditions, designing complete edge-nitrogen doped carbonaceous materials is still a grand challenge. Herein, we design a precise controlled pyrolysis strategy of the self-assembled Ti3C2 @melamine-E7 microcapsules (E7: one of the liquid crystals). This method leads to a complete edge nitrogen (100 at% edge nitrogen out of 15.07 at% nitrogen) doped Ti3C2-carbon double-shell hollow microsphere (NTC-HM). To date, such complete edge nitrogen doping level is the highest among the reported nitrogen doped carbon materials. We find that the NTC-HM, as anode for K-ion batteries, demonstrates an outstanding reversible capacity of 436.6 mAh/g of 0.1 A/g over 150 cycles and high capacity retention of 81.3% (Cycle 500/Cycle 3) under 1.0 A/g, which is among the best in all the excellent carbonaceous electrodes for K-ion batteries. Also, a full cell assembled perylenetetracarboxylic dianhydride (PTCDA) cathode and NTC-HM anode delivers a practical energy density of 126.9 Wh/kg under 0.2 A/g.

Design Rules of a Sulfur Redox Electrocatalyst for Lithium-Sulfur Batteries.

Seeking an electrochemical catalyst to accelerate the liquid-to-solid conversion of soluble lithium polysulfides to insoluble products is crucial to inhibit the shuttle effect in lithium-sulfur (Li-S) batteries and thus increase their practical energy density. Mn-based mullite (SmMn2 O5 ) is used as a model catalyst for the sulfur redox reaction to show how the design rules involving lattice matching and 3d-orbital selection improve catalyst performance. Theoretical simulation shows that the positions of Mn and O active sites on the (001) surface are a good match with those of Li and S atoms in polysulfides, resulting in their tight anchoring to each other. Fundamentally, dz2 and dx2 -y2 around the Fermi level are found to be crucial for strongly coupling with the p-orbitals of the polysulfides and thus decreasing the redox overpotential. Following the theoretical calculation, SmMn2 O5 catalyst is synthesized and used as an interlayer in a Li-S battery. The resulted battery has a high cycling stability over 1500 cycles at 0.5 C and more promisingly a high areal capacity of 7.5 mAh cm-2 is achieved with a sulfur loading of ≈5.6mg cm-2 under the condition of a low electrolyte/sulfur (E/S) value ≈4.6µL mg-1 .

The Role of Electrolyte Composition in Enabling Li Metal-Iron Fluoride Full-Cell Batteries.

FeF3 conversion cathodes, paired with Li metal, are promising for use in next‐generation secondary batteries and offer a remarkable theoretical energy density of 1947 Wh kg−1 compared to 690 Wh kg−1 for LiNi0.5Mn1.5O4; however, many successful studies on FeF3 cathodes are performed in cells with a large (>90‐fold) excess of Li that disguises the effects of tested variables on the anode and decreases the practical energy density of the battery. Herein, it is demonstrated that for full‐cell compatibility, the electrolyte must produce both a protective solid‐electrolyte interphase and cathode‐electrolyte interphase and that an electrolyte composed of 1:1.3:3 (m/m) LiFSI, 1,2‐dimethoxyethane, and 1,1,2,2‐tetrafluoroethyl‐2,2,3,3‐tetrafluoropropyl ether fulfills both these requirements. This work demonstrates the importance of verifying electrode level solutions on the full‐cell level when developing new battery chemistries and represents the first full cell demonstration of a Li/FeF3 cell, with both limited Li and high capacity FeF3 utilization.

Unraveling the Dynamic Interfacial Behavior of LiCoO2 at Various Voltages with Lithium Bis(oxalato)borate for Lithium-Ion Batteries.

The electrochemical dynamic behavior of the solid electrolyte interface (SEI) formed on LiCoO2 (LCO) by lithium bis(oxalato)borate (LiBOB) is investigated at various cutoff voltages. Particularly, for layered cathode active materials, various cutoff voltages are used to control the delithiation states; however, systematic investigations of the voltage and SEI are lacking. To increase the practical energy density of the LCO, a high cutoff voltage is pursued to utilize a state of high delithiation. However, this high cutoff voltage causes the electrolyte to undergo side reactions and the crystalline structure changes irreversibly, limiting the cycle life. In a low-voltage environment (<4.7 V), LiBOB improves the initial Coulombic efficiency and cycling performance by forming an effective SEI, which suppresses side reactions. At higher voltage levels (4.7-4.9 V), LiBOB no longer effectively protects the surface, causing the electrochemical performance to decrease rapidly. The main cause of this phenomenon is the decomposition of LiBOB-SEI at a high voltage, as shown by systematic surface and electrochemical analyses comprising linear sweep voltammetry, cyclic voltammetry, and electrochemical impedance spectroscopy. In conclusion, LiBOB can suppress side reactions of the electrolyte by SEI formation, but the SEI decomposes at voltage levels higher than 4.7 V.

A Fiber-Based 3D Lithium Host for Lean Electrolyte Lithium Metal Batteries.

3D hosts are promising to extend the cycle life of lithium metal anodes but have rarely been implemented with lean electrolytes thus impacting the practical cell energy density. To overcome this challenge, a 3D host that is lightweight and easy to fabricate with optimum pore size that enables full utilization of its pore volume, essential for lean electrolyte operations, is reported. The host is fabricated by casting a VGCF (vapor‐grown carbon fiber)‐based slurry loaded with a sparingly soluble rubidium nitrate salt as an additive. The network of fibers generates uniform pores of ≈3 µm in diameter with a porosity of 80%, while the nitrate additive enhances lithiophilicity. This 3D host delivers an average coulombic efficiency of 99.36% at 1 mA cm−2 and 1 mAh cm−2 for over 860 cycles in half‐cell tests. Full cells containing an anode with 1.35‐fold excess lithium paired with LiNi0.8Mn0.1Co0.1O2 (NMC811) cathodes exhibit capacity retention of 80% over 176 cycles at C/2 under a lean electrolyte condition of 3 g Ah−1. This work provides a facile and scalable method to advance 3D lithium hosts closer to practical lithium‐metal batteries.

Diphenyl diselenide as a bifunctional electrolyte additive in a high-voltage LiNi0.8Mn0.1Co0.1O2/graphite battery

To increase the practical energy density and capacity of high-nickel cathode materials, it is crucial to develop a novel electrolyte additive that can enhance the unstable electrolyte/electrode interface over a wide operating voltage range. In this study, diphenyl diselenide (DPDS) is selected as a bifunctional electrolyte additive for high-voltage LiNi0.8Mn0.1Co0.1O2 (NMC811)/graphite batteries, considering the energy levels of its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). DPDS is oxidized and reduced prior to the decomposition of ethylene carbonate (EC) and diethyl carbonate (DEC), forming protective layers on both electrode surfaces. The DPDS-driven layers prevent parasitic reactions, maintain the structural stability of electrodes, and reduce active lithium loss. As a result, in the high voltage window of 2.8–4.5 V, DPDS enhances the capacity retention of NMC811/graphite batteries from 68.0% to 82.0% after 200 cycles at 1C. This paper provides an understanding of the DPDS operating principle in high voltage NMC811/graphite full cells to achieve high energy density.

Stabilized Li metal anode with robust C-Li3N interphase for high energy density batteries

The combination of commercial high-voltage cathode and a thin lithium metal anode has emerged as a promising approach to realize rechargeable high energy density lithium batteries. However, abundant challenges, including huge volume change of Li metal and its severe side reactions with electrolyte for wide electrochemical potential window, hamper the practical application of lithium metal anode in high energy density batteries. In this work, we reported a Li/graphene composite electrode with in situ formed robust C/Li3N interphase (Li/graphene-C/Li3N electrode). Stabilized by the graphene framework, the electrode can endure large volume change during lithium stripping/plating cycles and realize stable cycling. The Li3N protection layer can not only help the composite electrode to avoid the direct physical contact between active metallic lithium and electrolyte, and suppress the side reactions between them, but also help to homogenize Li+ diffusion and plating/stripping behaviors due to its high ionic conductivity. As a result, the composite structure and electrochemical interphase stability enable superior electrochemical performance of the Li/graphene-C/Li3N electrodes. As a demonstration, the Li/graphene-C/Li3N symmetric cell displayed a low and stable overpotential for more than 1000 hours at 1 mA cm−2 and 1 mAh cm−2. LCoO2||Li/graphene-C/Li3N cell delivered a high energy density of 694.05 Wh kg−1 in the working potential range between 2.8 V and 4.6V and displayed high Coulombic efficiency of over 99% for more than 150 cycles, showing promising for practical high energy density lithium metal batteries.

Multiple roles of graphene in electrocatalysts for metal-air batteries

Metal-air batteries are the most promising next-generation energy conversion and storage devices due to their high theoretical energy densities. However, their practical energy densities and voltages are limited by the sluggish kinetics of the oxygen reduction and evolution reactions due to the multi-step electron transfer and the triple-phase mass transfer. To address this issue, a number of research efforts have been devoted to materials science and interface chemistry, in which graphene-based electrocatalysts have attracted much attention due to their tunable catalytic properties, high electrical conductivities, and large surface areas. In this review, we reveal the principles of oxygen electrocatalytic reactions and the requirements for efficient oxygen electrocatalysts with regards to the structural feature, intrinsic activity, and electrical conductivity. Afterwards, the morphological design of the electrode is clearly elucidated, followed by the ample discussion on the main research progresses and challenges of graphene-based materials as either electrocatalysts with intrinsic active sites or supports for other traditional metal or metal oxide catalysts. Furthermore, we outline the research directions for developing highly-efficient and low-cost graphene-based electrocatalysts.