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Compared with most cathodes for lithium-ion batteries (LIBs), α-MoO3exhibits high specific capacity and therefore receives widespread attention. However, due to its irreversible structural transformation, the capacity of α-MoO3declines rapidly upon discharge-charge process. Herein, via H2plasma etching, oxygen-deficient α-MoO3-xwith improved electrochemical performance is prepared on the basis of pristine α-MoO3(MoO3(I)). The obtained MoO3(II) (10 min etching) and MoO3(III) (20 min etching) have reduced bandgap and expanded van der Waals gap (vdW gap). Electrochemical test results demonstrate MoO3(II) obtains the largest Li+diffusion coefficient, the lowest charge transfer resistance and the slightest polarization. In situ X-ray diffraction characterization further reveals that during cycling, vdW gap of MoO3(II) changes periodically within a small range, indicating a stabilized crystal structure. This work shows that moderate oxygen vacancies can enhance the capacity and stabilize the structure of molybdenum trioxide cathode for LIBs, which has great potential in other cathode materials.

The application of sodium-ion batteries (SIBs) is largely depended on the high energy density electrode system since it suffers from intrinsic high voltage of Na (0.33V vs.Li). To overcome these limitations, we propose a high-voltage sodium-ion full cell viacathodic polarization suppression. NASICON structured Na3V2(PO4)3(NVP) cathode and poorly-graphitized hard carbon (HC) anode are synthesized as the targeted materials through an easily scalable coprecipitation method and a simple biomass-directed technique, respectively. Through morphologic optimization and carbon decoration, the NVP half cell demonstrates a high output voltage of 3.4V, ultralong cyclability (over 4000 cycles) and distinguished rate capability (77mAhg−1at 150C), which is attributed to the enhanced electrical transport behaviors and carrier transmission dynamics. When extended to NVP//HC, the overpotential has been successfully restrained, and the full cell exhibits a theoretical average voltage of 3.3V, which shows substantial increases in the energy density (>20%) compared to bulk and bare NVP based full cell. The prototype design of high-energy and low-cost sodium-ion full cell in this work opens the door for accelerating the development and application of SIBs.

Three-dimensional (3D) hierarchical heterostructures have been widely studied for energy storage because of their amazing synergistic effect. However, a detailed characterization how the branched structure affects the backbone structure during electrochemical cycling, and the specific relationship between the backbone and the branched heterogeneous structure (namely synergistic effect) have been rarely revealed. In addition, the controllable synthesis of this system still remains a great challenge. Herein, we developed a one-step gradient hydrothermal method to obtain a series of 3D hierarchical heterogeneous nanostructures, including V2O5/NaV6O15, V2O5/ZnV2O6and V2O5/CoV2O6, through controlling the sequence of nucleation and growth processes of different structural units in the same precursor. On the basis of time-resolved in situX-ray diffraction (XRD) characterizations, we clearly elucidated the synergistic effect between the branched and backbone structure. During the synergistic effect, the branched NaV6O15helps to reduce the potential barrier during lithium-ion insertion/extraction, buffers the impact of crystal-system transformations during the charge/discharge process; the backbone V2O5is beneficial to increase the charge/discharge capacity, inhibits the self-aggregation of branched NaV6O15and maintains the stability of 3D structure. Consequently, 3D V2O5/NaV6O15hierarchical heterogeneous microspheres exhibit the best electrochemical performance than pure V2O5and V2O5/NaV6O15physical mixture in lithium-ion batteries (LIBs). When tested at a high rate of 5Ag−1, 92% of the initial capacity can be maintained after 1000 cycles. We believe this method will be in favor of the construction of 3D hierarchical heterostructures and this specific synergistic effect investigated by in situXRD will be significant for the design of better electrodes.

The morphological optimization of Na3V2(PO4)3(NVP) material has a great significance for improving the electrochemical performance since NVP suffers from intrinsic low electronic conductivity. For this purpose, a novel 3D NVP nanofiber network is controllably constructed viaa facile self-sacrificed template method. Based on time-dependent experiments, an outside-in morphological evolution mechanism from microsphere to 3D nanofiber network is proposed. The as-synthesized material exhibits excellent cyclability (95.9% capacity retention over 1000 cycles at 10C) and enhanced high-rate performance (94mAhg−1at 100C) for sodium half cell. Notably, when evaluated as full battery (NaTi2(PO4)3as anode) cathode, it also shows outstanding cycling stability (96.9% capacity retention over 300 cycles at 5C) and superior rate capability (80mAhg−1at 50C). Such remarkable performance is attributed to the 3D nanofiber network structure, which provides multi-channel ionic diffusion pathway, continuous electronic conduction, and improved structural integrity. This self-sacrificed template strategy presented here can inspire new thought in constructing novel nanofiber/nanowire structures and accelerate the development of high-power sodium–ion batteries.

One dimensional (1D) nanomaterials, which show tremendous potential in constructing high performance energy storage device, have gained considerable research interests. However the electrochemical reaction mechanism is still elusive. The most challenging issue in energy storage is developing insightful operandoprobes for the electrochemical processes. Notably, the in situcharacterization of 1D nanomaterials is crucial to investigate the structural changes and uncover the intrinsic reasons for the capacity fading. Therefore numerous in situcharacterization methods have been developed, such as in situelectron microscopy, X-ray detection techniques, spectroscopic techniques, nuclear magnetic resonance techniques, etc. Recent developments on in situcharacterization technologies are summarized in this review. Different energy storage devices are involved, including Li-ion batteries, Na-ion batteries, supercapacitors, Li–air batteries and Li–S batteries. A new type of energy storage device, single nanowire device, has been also emphasized in this article. The reported highlights and developments are further discussed in details according to their reaction mechanisms. Through in situcharacterization, no completely reversible volume expansion and phase transformation were observed during the lithiation and delithiation process. More structural/morphological damage and different electrochemical mechanisms were caused by the replace of lithium ions with sodium ions. The electron/hole doping of electric double layer capacitive materials and oxidation state of pseudocapacitive materials were focused. The growth and decomposition process of Li2O2and polysulfide were also observed. The optimization mechanisms, including coating and doping of high conductivity materials, were proved to increase the electric/ion conductivity and reduce the cracks. The above mentioned results indicated that 1D nanomaterials with continuous ion/electron channels and short diffusion distance for electrolyte ions showed superior structural stability. Finally, the challenges and perspectives of the in situcharacterization of 1D nanomaterials during electrochemical processes are emphasized as the conclusion.

LiFePO4is one of the most outstanding cathodes for the high performance lithium-ion battery, while it is restricted by its unsatisfactory low temperature performance. Here we detect the structural dynamics and reaction routes of LiFePO4via operando condition with high rates, well reproducibility over cycles and low temperature in common laboratory X-ray without the synchrotron light source. The intermediate phases between LiFePO4and FePO4, driven by the overpotential and limited ion transfer rate along the bdirection at low temperature, are captured. Our results demonstrate that the existence of intermediate can greatly improve the diffusion kinetics of LiFePO4. The deep understanding of reaction routes of LiFePO4at low temperature will guide the further material optimization design. Besides LiFePO4, such high time resolution in-situX-ray diffraction testing method with laboratory source is available to understand the reaction mechanisms of other electrochemical reaction system.

High-speed electron transfer channels and short Li ion transport distance are beneficial to improvement of Li ion battery properties. Here, a two-step solution phase synthesis method is developed to construct the V2O5quantum dots/graphene hybrid nanocomposite by controlling nucleation and growth processes. It is demonstrated that the V2O5quantum dots with an average size of 2-3 nm are uniformly anchored on the graphene sheets. The specific capacity can achieve 212mAhg−1at 100mAg−1after 100 cycles. Significantly, the novel V2O5quantum dots/graphene shows a stable cycling performance with 89% capacity retention after 300 cycles. The improvement in electrochemical properties could be attributed to the short Li ion transfer distance, two-dimensional electron channels, homogeneous dispersion and immobilization of V2O5quantum dots. Meanwhile, it indicates that V2O5quantum dots/graphene is promising cathode material for use in long-life rechargeable lithium batteries. This design conception and synthesis strategy for V2O5could also be extended to other electrode material systems.

Transition metal phosphides or partially phosphatized oxides usually suffer from surface reconstruction during oxygen evolution reaction (OER), but still possess enhanced catalytic activity than directly synthesized oxides, which has aroused great interest in exploring the causes of such high catalytic activity. To monitor electronic property of catalyst during the OER can provide crucial insights into catalytic ability. Here we design a planar electrochemical microdevice based on individual thin-film catalyst, and for the first time explore the continuous electric conductance evolution of lattice P-doped oxides during the electrochemical activation process. Moreover, combining on-chip electrochemical impedance spectra measurements, in situ I-Vmeasurements, and theoretical simulations of reconstructed lattice P-doped oxides, the effect of P–O groups on new-formed oxides is clarified. The induced electronic coupling between new-formed oxides and P-O groups has been studied and demonstrated. The coupled P–O groups effectively promote the metal–oxygen covalency of new-formed oxides, which accelerates electron transfer between active metallic center and oxygen adsorbates, thus leading to the enhanced electrocatalytic activity. Our study highlights the role of surface P–O groups in Co3O4during the OER, and such unique on-chip electrochemical microdevice platform can also be applied in other related fields to understand the dynamic behavior of energy materials at nanoscale.