In Situ-Constructed Multifunctional Interface for High-Voltage 4.6 V LiCoO2.

Due to high volumetric energy density, the major market share of cathode materials for lithium-ion batteries is still dominated by LiCoO2 (LCO) at a 3C field. However, a number of challenges will be triggered if the charge voltage is increased from 4.2/4.3 to 4.6 V to further increase energy density, such as a violent interface reaction, Co dissolution, and release of lattice oxygen. Here, LCO is coated with the fast ionic conductor Li1.8Sc0.8Ti1.2(PO4)3 (LSTP) to form LCO@LSTP, while a stable interface of LCO is in situ constructed by the decomposition of LSTP at the LSTP/LCO interface. As decomposition products of LSTP, Ti and Sc elements can be doped into LCO and thus reconstruct the interface from a layered structure to a spinel structure, which improves the stability of the interface. Moreover, Li3PO4 from the decomposition of LSTP and remaining LSTP coating as a fast ionic conductor can improve Li+ transport when compared with bare LCO, and thus boost the specific capacity to 185.3 mAh g-1 at 1C. Benefited from the stable interface and fast ion conducting coating, the LCO@LSTP (1 wt %) cathode delivers a high capacity of 202.3 mAh g-1 at the first cycle (0.5C, 3.0-4.6 V), and shows a higher capacity retention of 89.0% than LCO (50.9%) after 100 cycles. Furthermore, the change of the Fermi level obtained by using a kelvin probe force microscope (KPFM) and the oxygen band structure calculated by using density functional theory further illustrate that LSTP supports the performance of LCO. We anticipate that this study can improve the conversion efficiency of energy-storage devices.

Adjusting Crystal Orientation to Promote Sodium-Ion Transport in V5 S8 @Graphene Anode Materials for High-Performance Sodium-Ion Batteries.

Sodium-ion batteries (SIBs) have inspired the potential for widespread use in energy storage owing to the advantages of abundant resources and low cost. Benefiting from the layered structure, 2D-layered materials enable fast interlayer transport of sodium ions and thus are considered promising candidates as anodes for SIBs. Herein, a strategy of adjusting crystal orientation is proposed via a solvothermal method to improve sodium-ion transport at the edge of the interlayers in 2D-layered materials. By introducing surfactants and templates, the 2D-layered V5 S8 nanosheets are controlled to align the interlayer diffusion channels vertically to the surface, which promotes the fast transport of Na+ at the edge of the interlayers as revealed by experimental methods and ab initio calculations. Benefiting from the aligned crystal orientation and rGO coating, the vertical-V5 S8 @rGO hybrid delivers a high initial discharge capacity of 350.6 mAh g-1 at a high current density of 15 A g-1 . This work provides a strategy for the structural design of 2D-layered anode materials by adjusting crystal orientation, which demonstrates the promise for applications in fast-charging alkaline-ion batteries.

Recent Advances on Heterojunction-Type Anode Materials for Lithium-/Sodium-Ion Batteries.

Rechargeable batteries are key in the field of electrochemical energy storage, and the development of advanced electrode materials is essential to meet the increasing demand of electrochemical energy storage devices with higher density of energy and power. Anode materials are the key components of batteries. However, the anode materials still suffer from several challenges such as low rate capability and poor cycling stability, limiting the development of high-energy and high-power batteries. In recent years, heterojunctions have received increasing attention from researchers as an emerging material, because the constructed heterostructures can significantly improve the rate capability and cycling stability of the materials. Although many research progress has been made in this field, it still lacks review articles that summarize this field in detail. Herein, this review presents the recent research progress of heterojunction-type anode materials, focusing on the application of various types of heterojunctions in lithium/sodium-ion batteries. Finally, the heterojunctions introduced in this review are summarized, and their future development is anticipated.

Strengthened d-p Orbital Hybridization through Asymmetric Coordination Engineering of Single-Atom Catalysts for Durable Lithium-Sulfur Batteries.

Although single-atom catalysts (SACs) have been largely explored in lithium-sulfur (Li-S) batteries, the commonly reported nonpolar transition metal-N4 coordinations only demonstrate inferior adsorption and catalytic activity toward shuttled lithium polysulfides (LiPSs). Herein, single Fe atoms with asymmetric coordination configurations of Fe-N3C2-C were precisely designed and synthesized as efficient immobilizer and catalyst for LiPSs. The experimental and theoretical results elucidate that the asymmetrically coordinated Fe-N3C2-C moieties not only enhance the LiPSs anchoring capability by the formation of extra π-bonds originating from S p orbital and Fe dx2-y2/dxy orbital hybridization but also boost the redox kinetics of LiPSs with reduced Li2S precipitation/decomposition barrier, leading to suppressed shuttle effect. Consequently, the Li-S batteries assembled with Fe-N3C2-C exhibit high areal capacity and cycling stability even under high sulfur loading and lean electrolyte conditions. This work highlights the important role of coordination symmetry of SACs for promoting the practical application of Li-S batteries.

TiH2 Nanodots Exfoliated via Facile Sonication as Bifunctional Electrocatalysts for Li-S Batteries.

Mediating the redox kinetics of polysulfides is a promising strategy to mitigate the shuttling and sluggish conversion of polysulfides for practical application of lithium-sulfur (Li-S) batteries. Herein, novel TiH2 nanodots (THNDs) fabricated by sonication-assisted liquid-phase exfoliation are used as bifunctional electrocatalysts for Li-S batteries. Both experimental and theoretical results reveal that THNDs can not only provide a strong chemical affinity to polysulfides but also bidirectionally promote the precipitation/decomposition of Li2S from/to polysulfides during the discharge/charge process, thus effectively suppressing the shuttle effect and improving the redox kinetics of polysulfides. Owing to these advantages accompanied by the abundant catalytically active sites of THNDs, the assembled Li-S batteries deliver a low capacity fading rate of 0.055% per cycle over 1000 cycles at 1C and a high areal capacity of 5.38 mAh cm-2 after 50 cycles with a high sulfur loading of 8.5 mg cm-2. This work demonstrates the great potential of utilizing functional metal hydrides as effective electrocatalysts for Li-S batteries, which will incite more investigation into the specific selection of metal compounds to boost the redox kinetics of polysulfides.

Enhancing Cell Performance of Lithium-Rich Manganese-Based Materials via Tailoring Crystalline States of a Coating Layer.

Li-rich Mn-based-layered oxides are considered to be the most felicitous cathode material candidates for commercial application of lithium-ion batteries on account of high energy density. Nevertheless, defects containing an unsatisfactory initial Coulombic efficiency and rapid voltage decay seriously impede their practical utilization. Herein, a coating layer with three distinct crystalline states are employed as a coating layer to modify Li[Li0.2Mn0.54Ni0.13Co0.13]O2, respectively, and the effects of coating layers with distinct crystalline states on the crystal structure, diffusion kinetics, and cell performance of host materials are further explored. A coating layer with high crystallinity enables mitigatory voltage decay and better cyclic stability of materials, while a coating layer with planar defects facilitates Li+ transfer and enhances the rate performance of materials. Consequently, optimizing the crystalline state of coating substances is critical for preferable surface modification.

Novel Hoberman Sphere Design for Interlaced Mn3O4@CNT Architecture with Atomic Layer Deposition-Coated TiO2 Overlayer as Advanced Anodes in Li-Ion Battery.

The Hoberman sphere is a stable and stretchable spatial structure with a unique design concept, which can be taken as the ideal prototype of the internal mechanical/conductive skeleton for the anode with large volume change. Herein, Mn3O4 nanoparticles are interlaced with a Hoberman sphere-like interconnected carbon nanotube (CNT) network via a facile self-assembly strategy in which Mn3O4 can "locally expand" in the CNT network, limit the volume expansion to the interior space, and maintain a stable outer surface of the hybrid particle. Furthermore, an ultrathin uniform ALD-coated TiO2 shell is adopted to stabilize the solid electrolyte interphase (SEI), provide high electron conductivity and lithium ion (Li+) diffusivity with lithiated LixTiO2, and enhance the reaction kinetics of the Mn3O4 by an "electron-density enhancement effect". With this design, the Mn3O4@CNT/TiO2 exhibits a high capacity of 1064 mAh g-1 at 0.1 A g-1, a stable cycling stability over 200 cycles, a superior rate capability, and a commercial-level areal capacity of 4.9 mAh cm-2. In this way, a novel electrode design strategy is achieved by the Hoberman sphere-like CNT design along with the in situ porous formation, which can not only achieve a high-performance anode for LIBs but also can be widely adapted in a variety of advanced electrode materials for alkali metal ion batteries.

Entropy Change Characteristics of the LiNi0.5Mn1.5O4 Cathode Material for Lithium-Ion Batteries.

Lithium-ion batteries are widely used in the field of new energy vehicles and energy storage. Understanding the electrode reaction of lithium-ion batteries is the key to improve their cycle life and safety. Direct measurement of thermodynamic data of the electrode reaction is a practical, economical, and nondestructive method for electrode characterization. In this paper, the open-circuit voltage of the LiNi0.5Mn1.5O4/Li half-cell is measured at different discharge states and different temperatures. The dE/dT-SOD (state of discharge) relation curves are fitted linearly by the least square method, and the entropy change values of different SODs are calculated. Finally, the Gibbs free energy and enthalpy change of different SODs are obtained. The electrode reaction of LiNi0.5Mn1.5O4 in different SODs was discussed by the entropy change in different SODs. According to the evolution trend of ΔS, the lithium intercalation reaction of LiNi0.5Mn1.5O4 may be a single-phase solid solution reaction rather than a two-phase reaction. Finally, the reversible heat generation at different current values and SODs are calculated.

Dissociate lattice oxygen redox reactions from capacity and voltage drops of battery electrodes.

The oxygen redox (OR) activity is conventionally considered detrimental to the stability and kinetics of batteries. However, OR reactions are often confused by irreversible oxygen oxidation. Here, based on high-efficiency mapping of resonant inelastic x-ray scattering of both the transition metal and oxygen, we distinguish the lattice OR in Na0.6[Li0.2Mn0.8]O2 and compare it with Na2/3[Mg1/3Mn2/3]O2. Both systems display strong lattice OR activities but with distinct electrochemical stability. The comparison shows that the substantial capacity drop in Na0.6[Li0.2Mn0.8]O2 stems from non-lattice oxygen oxidations, and its voltage decay from an increasing Mn redox contribution upon cycling, contrasting those in Na2/3[Mg1/3Mn2/3]O2. We conclude that lattice OR is not the ringleader of the stability issue. Instead, irreversible oxygen oxidation and the changing cationic reactions lead to the capacity and voltage fade. We argue that lattice OR and other oxygen activities should/could be studied and treated separately to achieve viable OR-based electrodes.

Investigation of the Nanocrystal CoS2 Embedded in 3D Honeycomb-like Graphitic Carbon with a Synergistic Effect for High-Performance Lithium Sulfur Batteries.

Lithium sulfur (Li-S) batteries can offer great opportunities for the next-generation energy storage systems with tremendous energy density. However, challenges still exist in practical Li-S batteries including low sulfur utilization, and poor cycling stability and rate capability. Herein, we propose a novel hybrid catalyst structure by in situ implanting nanocrystal CoS2 in three-dimensional honeycomb-like hierarchical porous graphitic carbon (HPGC) for high-performance Li-S batteries. A unique synergistic absorption-catalysis-functional effect is demonstrated by comprehensive experimental and theoretical analysis: strong physical and chemical co-absorption effects are originated from the large quantity of microporous HPGC and the polar surface of metallic CoS2; the introduced nanocrystal CoS2 with a large specific area can impose an exceptional catalytic effect on the liquid-liquid, solid-liquid, and solid-solid phase redox reactions in Li-S batteries; the reaction dynamics are further guaranteed by the multifunctional properties of the HPGC backbone, including the capabilities in polysulfide sustention, reaction product transportation, electrolyte compensation, and efficiency in assisting diverse electrochemical reaction dynamics. In this way, our results not only develop a novel CoS2@HPGC structure, but also provide fundamental understanding on the catalytic dynamics during each reaction process. Moreover, we further propose the necessity and philosophy of the rational design of catalysts' special structure, which can fulfill the functional dynamics requirements of Li-S batteries, and can be promoted to other Li-S-related cathode design and composite catalytic structure design.

Spectroscopic Signature of Oxidized Oxygen States in Peroxides.

Recent debates on the oxygen redox behaviors in battery electrodes have triggered a pressing demand for the reliable detection and understanding of nondivalent oxygen states beyond conventional absorption spectroscopy. Here, enabled by high-efficiency mapping of resonant inelastic X-ray scattering (mRIXS) coupled with first-principles calculations, we report distinct mRIXS features of the oxygen states in Li2O, Li2CO3, and especially, Li2O2, which are successfully reproduced and interpreted theoretically. mRIXS signals are dominated by valence-band decays in Li2O and Li2CO3. However, the oxidized oxygen in Li2O2 leads to partially unoccupied O-2p states that yield a specific intraband excitonic feature in mRIXS. Such a feature displays a specific emission energy in mRIXS, which disentangles the oxidized oxygen states from the dominating transition-metal/oxygen hybridization features in absorption spectroscopy, thus providing critical hints for both detecting and understanding the oxygen redox reactions in transition-metal oxide based battery materials.

Elemental-sensitive Detection of the Chemistry in Batteries through Soft X-ray Absorption Spectroscopy and Resonant Inelastic X-ray Scattering.

Energy storage has become more and more a limiting factor of today's sustainable energy applications, including electric vehicles and green electric grid based on volatile solar and wind sources. The pressing demand of developing high-performance electrochemical energy storage solutions, i.e., batteries, relies on both fundamental understanding and practical developments from both the academy and industry. The formidable challenge of developing successful battery technology stems from the different requirements for different energy-storage applications. Energy density, power, stability, safety, and cost parameters all have to be balanced in batteries to meet the requirements of different applications. Therefore, multiple battery technologies based on different materials and mechanisms need to be developed and optimized. Incisive tools that could directly probe the chemical reactions in various battery materials are becoming critical to advance the field beyond its conventional trial-and-error approach. Here, we present detailed protocols for soft X-ray absorption spectroscopy (sXAS), soft X-ray emission spectroscopy (sXES), and resonant inelastic X-ray scattering (RIXS) experiments, which are inherently elemental-sensitive probes of the transition-metal 3d and anion 2p states in battery compounds. We provide the details on the experimental techniques and demonstrations revealing the key chemical states in battery materials through these soft X-ray spectroscopy techniques.

Modification of Transition-Metal Redox by Interstitial Water in Hexacyanometalate Electrodes for Sodium-Ion Batteries.

A sodium-ion battery (SIB) solution is attractive for grid-scale electrical energy storage. Low-cost hexacyanometalate is a promising electrode material for SIBs because of its easy synthesis and open framework. Most hexacyanometalate-based SIBs work with aqueous electrolyte, and interstitial water in the material has been found to strongly affect the electrochemical profile, but the mechanism remains elusive. Here we provide a comparative study of the transition-metal redox in hexacyanometalate electrodes with and without interstitial water based on soft X-ray absorption spectroscopy and theoretical calculations. We found distinct transition-metal redox sequences in hydrated and anhydrated NaxMnFe(CN)6·zH2O. The Fe and Mn redox in hydrated electrodes are separated and are at different potentials, leading to two voltage plateaus. On the contrary, mixed Fe and Mn redox in the same potential range is found in the anhydrated system. This work reveals for the first time how transition-metal redox in batteries is strongly affected by interstitial molecules that are seemingly spectators. The results suggest a fundamental mechanism based on three competing factors that determine the transition-metal redox potentials. Because most hexacyanometalate electrodes contain water, this work directly reveals the mechanism of how interstitial molecules could define the electrochemical profile, especially for electrodes based on transition-metal redox with well-defined spin states.

Effect of Chromium and Niobium Doping on the Morphology and Electrochemical Performance of High-Voltage Spinel LiNi(0.5)Mn(1.5)O4 Cathode Material.

Undoped, Cr-doped, and Nb-doped LiMn(1.5)Ni(0.5)O4 (LNMO) is synthesized via a PVP (polyvinylpyrrolidone)-combustion method by calcinating at 1000 °C for 6 h. SEM images show that the morphology of LNMO particles is affected by Cr and Nb doping. Cr doping results in sharper edges and corners and smaller particle size, and Nb doping leads to smoother edges and corners and more rounded and larger particles. The crystal and electron structure is investigated by XRD- and synchrotron-based soft X-ray absorption spectroscopy (sXAS). Cr doping and light Nb doping (LiNb(0.02)Ni(0.49)Mn(1.49)O4) improve the rate performance of LNMO. To explore the reason for rate-performance improvement, we conducted potential intermittent titration technique (PITT) and electrochemical impedance spectroscopy (EIS) tests. The Li(+) chemical diffusion coefficient at different state of charge (SOC) is calculated and suggests that both Cr and light Nb doping speeds up Li(+) diffusion in LNMO particles. The impedance spectra show that both R(SEI) and R(ct) are reduced by Cr and light Nb doping. The cycling performance is improved by Cr or Nb doping, and Cr doping increases both Coulombic efficiency and energy efficiency of LNMO at 1 C cycling. The LiCr(0.1)Ni(0.45)Mn(1.45)O4 remains at 94.1% capacity after 500 cycles at 1 C, and during the cycling, the Coulombic efficiency and energy efficiency remain at over 99.7% and 97.5%, respectively.