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The electrochemical activity and stability of Li2MnO3 are affected synchronously by lattice vacancies and local coordination configuration.

Developing the high-capacity anode materials such as conversion-type metal oxides which possess both Li and Na storage activity is very practical for the high-energy LIBs/SIBs. Herein, we use NiCo2O4 anodes as a model to investigate the morphology evolution which accounts for the poor cycling performance and understand the effect of structure optimization on the electrochemical performance. Three NiCo2O4 samples with different morphologies of microspheres, nanospheres and nanosheets are synthesized. Firstly, the serious structural degradation of NiCo2O4 microspheres is observed whether it works as a LIB or SIB anode. In addition, a significant difference between the lithiation and sodiation capacity of NiCo2O4 materials reveals Na+ ions only partially intercalated in NiCo2O4 and the conversion reaction limited by the strain. Next, NiCo2O4 nanosheets on Ni foam as a binder-free anode for LIBs are investigated which suggest the positive effect of 3D nanostructures on the morphology stability. As a result, NiCo2O4 nanosheets deliver a high lithiation capacity of 1092 mAh g− 1 after 100 cycles at 0.5 A g− 1 and an excellent rate capacity of 643 mAh g− 1 at 4 A g− 1. Finally, NiCo2O4 nanospheres are evaluted as a SIB anode which indicate the smaller particle size of active materials is beneficial to the release of stress and structure stability during discharge-charge processes. A rational design of the electrode’ architecture is very important for the conversion-type 3d transition metal oxide anodes for advanced LIBs and SIBs.

Traditional O3-type Li-rich layered materials are attractive with ultra-high specific capacities, but suffering from inherent problems of voltage hysteresis and poor cycle performance. As an alternative, O2-type materials show the potential to improve the oxygen redox reversibility and structural stability. However, their structure-performance relationship is still unclear. Here, we investigate the correlation between the Li component and dynamic chemical reversibility of O2-type Li-rich materials. By exploring the formation mechanism of a series of materials prepared by Na/Li exchange, we reveal that insufficient Li leads to an incomplete replacement, and the residual Na in the Li-layer would hinder the fast diffusion of Li+. Moreover, excessive Li induces the extraction of interlayer Li during the melting chemical reaction stage, resulting in a reduction in the valence of Mn, which leads to a severe Jahn-Teller effect. Structural detection confirms that the regulation of Li can improve the cycle stability of Li-rich materials and suppress the trend of voltage fading. The reversible phase evolution observed in in-situ X-ray diffraction confirms the excellent structural stability of the optimized material, which is conducive to capacity retention. This work highlights the significance of modulating dynamic electrochemical performance through the intrinsic structure.

Various properties of sodium ion batteries deteriorate severely when dropping to subzero temperature. Herein, we reveal an accelerated charge-transfer mechanism for high-voltage Na3V2(PO4)2F3 cathode through constructing weakly-solvating architecture, which endows it with superior temperature adaptability (capacity retention of C − 25 ∘ C / C 25 ∘ C reaches 90.8%). The resulting weak solvation effects synergistically lower the activation energy barrier for charge-transfer reactions, thus accelerating the kinetics at low temperature and increasing the energy density by ∼75 Wh Kg−1. Ab initio molecular dynamics calculations show that a weakly-solvating structure forms spontaneously in a low-concentration electrolyte (merely 0.3 M) and thereby facilitates Na+ desolvation process. Besides, visual TOF-SIMS confirms the construction of a dense and uniform cathode/electrolyte interface layer, which optimizes the interface chemistry and improves the interfacial kinetics. In-situ and ex-situ XRD also evidence a smaller degree of structural evolution of the Na3V2(PO4)2F3 cathode, which contributes to long-term durability (attaining a high capacity retention of 93.4% after 1000 cycles at −25 °C). Furthermore, it is demonstrated that under such extreme conditions the Na3V2(PO4)2F3||hard-carbon full cell functions well for over 300 h. These findings elucidate the roles of weak solvation construction in realizing faster kinetics for high-voltage cathodes and provide a feasible pathway for achieving more practical sodium ion batteries.

As the candidate cathode material of next-generation lithium-ion batteries (LIBs), Li-rich layered oxides (LLOs) suffer from the problems of surface residual lithium and own insufficient performance as an ionic and electronic conductor. Herein, we propose a strategy for in situ utilization of the residual lithium to form a functionalized sulfur-containing surface coating layer for Co-free LLOs by designated consumption of the residual lithium compounds (Li2CO3 and LiOH) with a simple sulfur treatment. The layer significantly improves the lithium-ion diffusion kinetics at high current rates and inhibits the growth of charge transfer resistance. The results of composition and structure characterization prove that the layer mainly containing Li2SO4 guides the favorable evolution of particles’ bulk crystal structure during the cycle. The modified sample exhibits a higher first coulombic efficiency of 84.4%, excellent rate capabilities, and superior cycle capacity retention of 93.3% after 300 cycles at 250 mA g−1.

For sodium-ion batteries (SIBs), the electrochemical process for electrodes involves ion transport in the solid electrolyte interphase (SEI) and active materials. Generally, the large ion radius of Na+ is often considered as the key factor in poor electrode kinetics. However, for conversion-type metal oxide anodes with low redox potentials, unstable SEIs as well as the corresponding kinetic barrier also have significant effects on electrochemical behaviors and deserve more attention. Herein, porous and micro-spherical (Mn0.6Co0.3Ni0.1)3O4 is tailored as a SIB anode using co-precipitation. A high Mn percentage is beneficial to the formation of a micro-spherical morphology during co-precipitation. Due to its lack of electrochemical activity, Mn also contributes to the morphological stability of active materials during cycling. This allows for a clear observation regarding morphological changes of SEI products generated at the electrode surface. It is revealed that branch-like products are gradually converted into a dense interphase layer at electrode surfaces during cycling. The unstable and uneven topography of these electrode surfaces generates kinetic barriers that account for low rate capacities of the as-obtained (Mn0.6Co0.3Ni0.1)3O4 materials. The synthesized metal oxide is able to retain 98.1% of its initial sodiation capacity after 2000 cycles at 0.5 A g−1.

The capability in spatially resolving the interactions between components in lithium (Li)-ion battery cathodes, especially correlating chemistry and electronic structure, is challenging but critical for a better understanding of complex degradation mechanisms for rational developments. X-ray spectro-ptychography and conventional synchrotron-based scanning transmission X-ray microscopy image stacks are the most powerful probes for studying the distribution and chemical state of cations in degraded Li-rich cathodes. Herein, we propose a chemical approach with a spatial resolution of around 5.6 nm to imaging degradation heterogeneities and interplay among components in degraded Li-rich cathodes. Through the chemical imaging reconstruction of the degraded Li-rich cathodes, fluorine (F) ions incorporated into the lattice during charging/discharging processes are proved and strongly correlate with the manganese (Mn) dissolution and oxygen loss within the secondary particles and impact the electronic structure. Otherwise, the electrode-electrolyte interphase component, scattered LiF particles (100-500 nm) along with the MnF2 layer, is also visualized between the primary particles inside the secondary particles of the degraded cathodes. The results provide direct visual evidence for the Li-rich cathode degradation mechanisms and demonstrate that the low-energy ptychography technique offers a superior approach for high-resolution battery material characterization.

Potassium-ion capacitors (KICs) have received a surge of interest because of their higher reserves and lower costs of potassium than lithium. However, the cycle performance and capacity of potassiu...

Sluggish kinetics and limited reversible capacity present two major challenges for layered titanates to achieve satisfactory sodium-ion storage performance at subzero-temperatures (subzero-T). To f...

Layered titanates (LT) have aroused considerable attention as advanced anodes for high-performance Na-ion batteries. However, their intrinsic issues including low electronic conductivity and sluggish sodiation kinetic hinder the implementation of achieving superior rate capability and long cycle stability simultaneously. Herein, a crystallization-induced surface defect engineering to promote the electrochemical activity of LT by electronic structure modulation and diffusion kinetics regulation is proposed. As evidenced by electrochemical characterization, this surface defect modification strategy can effectively reduce the polarization and facilitate fast electronic/ionic diffusion of titanates. Thereby, the targeted low-crystalline layer modified layered titanate (LC-LT) unfolds enhanced rate capability and cycle stability (8000 cycles, 88%). Theoretical calculations reveal that the LC-LT is equipped with narrower bandgap originated from the 3d orbital of oxygen vacancies-induced defective Ti atoms on the surface. Moreover, reduced Na+ migration energies and interconnected Na+ diffusion pathways are predicted in LC-LT by density functional theory (DFT) calculations and bond valence site energy (BVSE) analysis. When applied in Na-ion full cell with NASICON-type Na3V2(PO4)2F3 cathode, the configuration exhibits comparable rate performance and cycle stability (800 cycles, 81.6%).

Phase separation and electronic structure variation of Li0.5FePO4, both in the bulk and surface, under concurrent lithiation, has been tracked by synchrotron X-ray microscopies. Oxygen K-edge XANES along with DFT calculations reveal unusual electronic structure varition which is attributable to the observed lithium gradient and interparticle transport. The new insights will benefit the future design of advanced batteries.

Different calcination atmospheres of air, 50% oxygen (vs. N2) and pure oxygen have been used to prepare special LiNi0.8Co0.1Mn0.1O2 cathode materials to observe the influence of oxygen composition. To investigate the structure and electrochemical property of the samples using different oxygen compositions, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), cycling performance tests and electrochemical impedance spectroscopy (EIS) were carried out. XRD, SEM, and XPS results show that the sample made using higher oxygen composition has less cation mixing and lower levels of Ni2+. However, both samples have almost the same oxygen environments on their surfaces as well as micro-morphology and size. The sample with a higher oxygen composition shows better electrochemical performance. Interestingly, the electrochemical performance of the sample made using 50% oxygen is similar to that made with pure oxygen and much better than the sample made with air. It has a specific capacity of 202.4 mAh g−1 at 0.1C and a capacity retention of 85.2% after 300 cycles at 1C, which may be meaningful for balancing cost and performance.

Anion doping is considered as an effective method to tap the potential of Li-rich materials to obtain more discharge capacity. Here, we prepare fluorine-doped Li1.2Ni0.2Mn0.6O2 materials by a low-temperature secondary sintering. Rietveld refinements of XRD show an increase in lattice parameters and indicate a wider Li+ diffusion channel after fluorine doping. From TEM and HRTEM images, it is observed that the fluorine-doped sample displays a more pronounced layered appearance, and partial lattice fringes are slightly curved which may be caused by the substitution of F for O to break the symmetry of MnO6 octahedron. The fitting results of XPS show that Mn is partially oxidized and the local electronic environment of O changes. The best one shows a discharge capacity of 288 mAh/g at 0.1 C with a Coulombic efficiency of 95.9% in the first lap. And it performs a capacity retention of 91.0% after 100 cycles at 0.2 C.

Recently, high performance energy storage devices are increasingly required in many new fields such as smartphone, pilotless automobile. Lithium-ion capacitors (LICs) have become the promising energy storage devices because of the higher power density, electrostatic capacity and long cycle life. Nevertheless, the limitation of the battery-type anode electrode and the capacitance-type cathode electrode with slow kinetics and low specific surface area leads to the LICs remaining lower energy density in high current density. In this report, a high performance LIC assembled by Mn3O4-graphene coupled with activated polyaniline-derived carbon (APDC) is firstly presented. Mn3O4-G composite material exhibits an outstanding invertible capacity of 489.8 mAh g−1 (at 1 A g−1) in a wide working window (0.01–3 V vs. Li/Li+) with an excellent coulombic efficiency in half cell, which is the highest capacitance reported for Mn3O4 so far. By utilization of Mn3O4-G composite as anode and APDC with the large surface of 1641.9 m2 g−1 as cathode, the assembled LIC of Mn3O4-G//APDC possesses an energy density of 97.2 Wh kg−1 at power density of 62.5 W kg−1, even at a relatively higher power density of 6250 W kg−1, its energy density can retain 5.0 Wh kg−1.

Li-rich layered oxides, despite their superhigh capacity, still suffer from oxygen release and structural transformation during the first charge process, causing low Coulomb efficiency and continuo...

Li-rich (LR) layered oxide cathode for high-energy-density Li-ion batteries are receiving considerable attention. However, their intrinsic issues hinder the implementation of LR in simultaneously achieving higher energy and power densities. Herein, a dual-conductive surface control strategy is proposed. This surface layer contains an electronic conductive carbon nanotube (CNT) skeleton and an ionic conductive heteroepitaxial spinel structure, which endows the LR with the light-weight and self-standing characteristic. As evidenced by prolonged electrochemical and structural evolution, this surface layer can reduce polarization, restrain structural distortion and facilitate fast electronic/ionic diffusion. Density functional theory (DFT) calculations demonstrate a higher electron conductivity with a narrower band gap across the CNT/LR interface than that of pure LR, and reveal a highly connective Li+ percolation network and reduced Li+ migration energies for the layered-spinel heterogeneous interface. The designed LR cathode presents a high energy density (1077 Wh kg−1 at 0.1 C), excellent rate capability (195 mAh g−1 at 10 C) and superior cycle stability. When utilized as an additive-free cathode for high-voltage full-battery, impressive energy density (645 Wh kg−1 based on the cathode and anode) and ultra-long cycle life (maintaining 87% capacity after 400 cycles) can be achieved. These results and this dual-conductive surface control strategy provide an exciting perspective and avenue for the further development of high-performance electrode material.

Ti-based compounds are considered as attractive anode materials for sodium-ion capacitors (SICs) due to their favorable safety and stability. However, achieving more Na+ intercalated sites and fast sodiation kinetics in Ti-based anodes is still challenging. Herein, a facile strategy to promote the electrochemical properties of H-titanates by regulating their electronic structure and Na+ diffusion kinetics through thermal-induced interlayer defect engineering is developed. The targeted distorted quasi-layered H-titanate (Q-LT) with abundant interlayer defects exhibits superfast and stable cycle performance (97% capacity retention after 10,000 cycles at 25 C) in Na-ion half-cells. Applied in the high-working voltage (1.5–4.5 V) SICs as additive anodes, high energy density (124 Wh kg−1) and competitive cycle stability (88% capacity retained after 5000 fast cycles) are achieved. The thermal-induced structure evolution in layered H-titanate has been probed by in-situ X-ray diffraction. First-principles density functional theory calculations demonstrate that the Q-LT is equipped with lower coordinate Ti-O polyhedral, higher delocalized Ti-O environment, narrowed band gap and reduced Na+ migration energies; bond valence sum maps expose the continuous Na+ diffusion pathways within the interlayer of Q-LT. This work may offer a conceptual advance in the understanding of the structure-function-performance relationship of titanates for energy storage.

High-capacity electrode materials are needed for electrochemical energy storage. Spinel (Ni0.4Co0.4Mn0.2)3O4 nanoparticles have been prepared by simplified and efficient coprecipitation in combination with thermal decomposition. As conversion-type anodes for lithium ion batteries (LIBs), the materials exhibit high capacities of 779 mAh g−1 after 600 cycles at the optimal current density of 0.5 A g−1. The enhanced cycling performance of (Ni0.4Co0.4Mn0.2)3O4 electrodes benefits from multiple metal species providing synergic effects in electrical processes, nanosize favorable for releasing stress and optimal rate lithiation for generating unique solid electrolyte interphase (SEI) with reactivation ability. And, the SEI layer is critical for the cycling properties. Optimal rate lithiation offers in designing long-lived electrodes. Additionally, the spinel materials are evaluated in sodium systems. The different capacity fading behaviors of the (Ni0.4Co0.4Mn0.2)3O4 electrodes between lithium and sodium ion cells are concerned.

Under over-increasing demand of advanced lithium-ion batteries (LIBs) for long-range electric vehicles (EVs), high-capacity transition metal oxide (TMO) negative electrodes for LIBs are thought as potential substitutes of traditional graphite anodes. A major barrier for TMO anodes is volumetric expansion during lithiation processes, leading to active material pulverization and falling off current collectors, which seriously deteriorates capacity retention. Herein, apart from conventional mechanical degradation, another capacity fading mechanism is revealed. Furthermore, the understanding is supported by an interesting cycling property. Firstly, NiCo2O4 with designed morphologies of nanoplates and microspheres are studied for the initial (de)lithiation and prolonged cycles. The morphology changes indicate solid electrolyte interphase (SEI) layer accumulating on the surface of electrodes and impeding the contact and reactions between lithium and active materials with a result of severe capacity loss. It can be understood as SEI layer insulating NiCo2O4, and if SEI film appropriately changes, high capacities can recovery. This conjecture is confirmed by following research of NiCo2O4 nanoparticles with amazing cycling performance prepared by a facile water-bath method. As LIB anodes, NiCo2O4 nanoparticles exhibit capacities of 1144 mAh g−1 at the initial discharging, 230 mAh g−1 at the 100th cycle and 661 mAh g−1 at the 500th cycle. The corresponding coulombic efficiency is of 73.1%, 98.4% and 99.5%, respectively. By TEM characterization in combination with electrochemical analysis, size-dependent SEI layer reactivation (from thick and unstable to thin and stable) is a key role on the dramatic capacity recovery.

Li-rich layered materials, despite their high specific capacity up to 250 mA h g−1, suffer from structural transformation either in the initial activation or after cycling, causing continuous voltage decay and capacity fading. Anion doping has been widely considered as a way to stabilize the intrinsic structure and improve the electrochemical performance of Li-rich materials, though with the pain of process complexity and limitation. Here, we report a simple co-precipitation method with a dual sedimentating agent to realize phosphor doping in both the surface and bulk. X-ray diffraction Rietveld refinement results indicate that the doped sample presents a larger lattice spacing than the normal sample and a Li3PO4 protective layer in situ forms on the surface. Synchrotron scanning transmission X-ray microscopy (STXM) reveals commendable homogeneity in the phase distribution between the surface and bulk in the doped sample. X-ray absorption near edge structure (XANES) shows a more homogeneous local chemical environment of the doped sample by investigating the Mn, Ni, and Co L-edges and O K-edge spectra. The doped sample displays a high discharge capacity of 295 mA h g−1 with an initial coulombic efficiency of 90.5% at 0.1C, showing a high rate performance of 247 mA h g−1 at 1C and a superior capacity retention of 73% after 500 cycles. Moreover, this doping strategy also inhibits the critical voltage decay of Li-rich materials during cycling. The prolonged structural evolution analysis demonstrates that phosphor doping can play a stabilizing role in Li-rich materials to restrain the transformation from layer to spinel.

Potassium-ion capacitors (KICs) have received a surge of interest because of their higher reserves and lower costs of potassium than lithium. However, the cycle performance and capacity of potassium devices have been reported to be unsatisfactory. Herein, a unique crystalline MnCo

Sluggish kinetics and limited reversible capacity present two major challenges for layered titanates to achieve satisfactory sodium-ion storage performance at subzero-temperatures (subzero-T). To facilitate sodiation dynamics and improve reversible capacity, we proposed an additive-free anode with Sn(II) located between layers. Sn-5s in interlayer-confining Sn(II), which has a larger negative charge, will hybridize with O-2p to trigger charge redistribution, thereby enhancing electronic conductivity. H-titanates with an open framework are designed to stabilize Sn(II) and restrain subsequent volume expansion, which could potentially surpass the capacity limitation of titanate-based materials via a joint alloying-intercalation reaction with high reversibility. Moreover, the generation of conductive Na

Due to the sluggish interfacial kinetics, the energy density and cycle life of sodium-ion batteries (SIBs) suffer severely at subzero temperatures. Herein, to accelerate the interfacial charge transfer process and improve the low-temperature SIBs performance, a strategy by triggering Na+-solvent co-intercalation is proposed. Using hydrogen titanate nanowires (HT-NW) as a model, we found that the layer structure regulation with oxygen defects could trigger HT-NW presents a unique Na+-solvent co-intercalation behavior in the ether-based electrolyte at −25 °C according to ex-situ FTIR and XRD. By eliminating the Na+ desolvation process, Na+-solvent co-intercalation could effectively accelerate the Na+ diffusion kinetics and reduce the activation energy to 66.0 meV. Benefit from these ameliorations, the defective HT-NW delivers a high capacity of 238 mAh g−1 at −25 °C, which is equivalent to 89% of that at 25 °C. Besides, the defective HT-NW shows great superiority in cycle stability, maintaining capacity retention of 80.6% after 4200 cycles at 1.0 A g−1 at −25 °C. Moreover, at −25 °C, the defective HT-NW//Na3V2(PO4)3 full cell exhibits high energy density (119.1 Wh kg−1) and outstanding stability (94.5% after 1000 cycles at 1.0C). These findings reveal that the ion–solvent co-intercalation is highly feasible to improve the battery performance at low temperatures by accelerating charge transfer kinetics.

Spinel LiNi0.5Mn1.5O4 (LNMO) is widely utilized because of its high-energy-density and high-voltage. Unfortunately, there is still much research to be done for LNMO due to its poor structural stability. Here, a strategy is confirmed to stabilize LNMO via modulating interstitial sites. The interstitial 16c sites of the octahedron are partially occupied by Ni2+ to suppress the migration and dissolution of manganese ions upon electrochemical cycling and stabilize lithium-ion vacancies in the state of charge. Unexpectedly, this protocol not only suppresses the phase separation restraining the phase boundary dislocations and stress but also decreases the magnitude of cell volume change during cycling, which originates from the change in Ni redox couple energy states. This two-pronged modification strategy endows the cathode material with a lower charge transfer barrier and faster Li+ transfer kinetics, revealing superior electrochemical performance. The regulated cathode material remains robust after 900 cycles at 1 C and its capacity retention rate is 29% higher than that of the original sample. Our research is useful for providing a concrete example of how the electrochemical performance of spinel LNMO and other high voltage cathode materials can be enhanced.

LiNi0.5Mn1.5O4(LNMO) material has high theoretical capacity and high operating voltage. Nonetheless, LNMO has still many problems which include oxidative decomposition of electrolyte at high voltage and dissolution of transition metals into electrolyte. In this work, oxides of both La and Al (LAO) were coated on the surface LNMO by a wet chemical method. LAO is estimated to improve cycling performance (both room and elevated temperatures) and rate capability of LNMO due to the synergistic effect of La2O3 and Al2O3. Al2O3 can protect LNMO from the corrosion of HF that derives from decomposition of LiPF6, which results in mitigating the solution of Mn2+. Al2O3 can also prompt electrochemical reversibility of LNMO. La2O3 can enhance electrical conductivity, availing to charge transfer. Furthermore, La2O3 presents splendid thermal stability, which may boost the interfacial stability of LNMO. La2O3 can also alleviate the formation of the passive layer. The effect of coating content on performances of LNMO was also studied in detail. The capacity retention of 94.0% for the optimal coating LNMO with 4.0 wt.% (mLa2O3/mAl2O3 = 3: 1) is shown at 1 C and 25 °C after 200 cycles, and its capacity retention of 95.0% is exhibited at 1 C and 55 °C after 100 cycles. Its remarkable discharge capacities at 10, 15, and 20 C are 107.4, 94.4, and 82.7 mAh g−1, respectively, and its capacity retention of 93.3% is displayed at 5 C at room temperature after 500 cycles.

Preactivation can play a promising role in suppressing oxygen loss during the first charging process in Li-rich oxide cathode materials, relieving a series of problems such as large initial irreversible capacity loss, structure transformation due to ion rearrangement, and oxidation/decomposition of the electrolyte. However, the strategies previous adopted are mainly chemical delithiation, which has violent effect on the crystal structure and deteriorates the cycle performance. Here, we report a facile and effective microwave-assisted treatment method to preactivate Li-rich layered oxides without structural distortion. The microwave-treated sample shows a high discharge capacity of about 281 mAh g–1 with a Coulombic efficiency of 87% at 0.1 C and exhibits unnormal continuous increase of discharge capacity from 203 mAh g–1 to 218 mAh g–1 during 110 cycles at 1 C as well as presents distinctly improved cycling stability. EIS, GITT and XRD studies reveal that the kinetics of electrochemical reaction on electr...

Fabricating high-performance anode materials is of great significance for the realization of advanced Na-ion batteries (SIBs). Poor rate capability and insufficient cycle stability are two main scientific issues urgently needing to be solved for sodium titanate (NaxTiyOz) anodes. In this paper, protonated titanate nanowire arrays are designed rationally as novel additive-free anodes for SIBs. Results reveal that the protonated strategy can controllablly regulate the lattice interlayer spacing of the titanate, which can not only effectively facilitate the Na-ion migration but also suppress the side reaction and inhibit the irreversible trapping of Na-ions in the crystal framework, leading to fast Na-ion diffusion kinetics. Moreover, the protonated titanate material experiences smaller changes in lattice parameters and unit-cell volume during long-term cycling than those of non-protonated material, resulting in less mechanical stresses and capacity loss in an anode. As expected, the protonated titanate material exhibits superior rate performance and ultralong lifespan when utilized as free-standing anode for SIB, remaining 85% capacity retention after 8000 cycles at 5.0 A g−1 (~ 23 C). When assembled as full cell with Na3V2(PO4)3 cathode, high energy density (262.3 Wh kg−1) and power density (1748.9 W kg−1), excellent rate capability and superior cycle stability (260 cycles, 86%) can be achieved.

Ni-rich cathode materials always suffer from serious side reaction and irreversible phase transition leading to capacity fading and thermal instability, which could be improved by surface coating and elemental doping. However, it is difficult and cumbersome to carry on the coating and doping at the same time. Herein, a facile method of bi-functional Ti modification has been employed on LiNi0.8Co0.15Al0.05O2 to enhance surface and structural stability via heterogeneous layer coating and bulk doping. The mechanism and synergistic effect of Ti modification has been investigated by XRD, XPS, SEM, TEM and the half-cell test in details. The existence of Ti occupancy in Ni site of the transition metal layer has been confirmed. Besides, a 22 nm heterogeneous layer has been detected on the particle surface and the composition has been analyzed. Ti bulk doping can reduce the cation mixing degree, and stabilize the lattice due to the pillar effect and charge compensation. Moreover, the heterogeneous coating layer could protect the cathode particles from hydrofluoric acid attack and reduce the decomposition of electrolyte during cycling. With the synergistic effects of heterogeneous layer coating and bulk doping, NCA-T2 exhibits the highest initial capacities of 162.9 and 182.4 mAh·g−1 at 1C and 0.1C, and the discharge capacity retentions of 1C cycling reach 85.0% after 200 cycles.

Samples LixNi0.5Mn1.5O4 (x = 0.8, 0.9, 1.0, 1.1, and 1.2) are synthesized to study the effects of Li contents. When Li contents are not enough, part of the nickel-manganese oxides cannot transfer to LiNi0.5Mn1.5O4 and remain in the product. When Li contents are too much, small amount of lithium-rich oxide is formed. In voltage window of 3.5~4.95 V, Li1.0Ni0.5Mn1.5O4 has the best rate performance due to that the spinel provide a three-dimensional pathway for lithium-ion diffusion. Li0.8Ni0.5Mn1.5O4 and Li0.9Ni0.5Mn1.5O4 show low specific capacities and poor rate performance due to the remained nickel-manganese oxides. Li1.1Ni0.5Mn1.5O4 and Li1.2Ni0.5Mn1.5O4 have lower initial coulombic efficiencies due to the existence of lithium-rich oxide. In voltage window of 1.95~4.95 V, the samples can deliver high specific energy above 800 Wh Kg−1. With the existence of lithium-rich oxide, Li1.2Ni0.5Mn1.5O4 shows good cycling stability with a high specific energy of 645 Wh Kg−1 after 50 cycles.

Compared with Li-ions, it is difficult to achieve fast dynamics for Na/K-ion storage. H-titanates as potential candidates suffer from low theoretical capacity, limiting the energy density of full cells. Herein, we report an Sn(Ⅱ)/Sn(Ⅳ) gradient doping strategy to simultaneously realizes the construction of a disordered interface with abundant defects and well-organized interlayered structure with larger interlayer spacing in H-titanate. The interfacial-located Sn2+ with a larger ionic radius triggers surface structural distortion to realize a favorable electronic structure with a narrower bandgap. Meanwhile, the interlayer-located Sn4+ plays a pillar effect to enhance the structural stability and modify the interlayer ion transport channel. Based on the analysis of ex-situ XRD, time of flight secondary ion mass spectrometry and temperature-dependent EIS, it is found that Sn(Ⅱ)/Sn(Ⅳ) gradient doping can effectively enhance structural stability, lower charge transfers barrier and boost ion diffusion kinetics, then resulting in 2.4- and 1.5-fold improvement for Li-ion and Na-ion storage, high capacity of 223 mAh g−1 and long cycle stability over 1800 cycles for K-ion storage. When used as additive-free anodes for hybrid capacitors, impressive energy densities in Li/Na-ion systems and superior cycle stability in Na/K-ion configurations have been achieved. These findings indicating that Sn(Ⅱ)/Sn(Ⅳ) gradient doping strategy has significant advantages for fast and durable alkali-ion storage.

Hybrid capacitors, especially sodium-ion capacitors (SICs), which combine the complementary merits of high-energy batteries and high-power capacitors, have received increasing research interest and have been expected to bridge the gap between the rechargeable batteries and EDLCs. The biggest challenge for SICs is to overcome the kinetics discrepancy between the sluggish faradaic anode and the rapid nonfaradaic capacitive cathode. To boost the Na+ reaction kinetics, robust and conductive Na2Ti2O5–x nanowire arrays have been constructed as an accessible and affordable SIC anode. It is found that the utilization of oxygen vacancies (OVs) can endow Na2Ti2O5–x high electrical conductivity, introduce intercalation pseudocapacitance, and maintain the crystal structure integrity. It exhibits high reversible discharge capacity (225 mAh g–1 at 0.5C), superior rate capability, and ultralong lifespan when utilized as self-supported and additive-free anode for SIB, remaining almost 100% capacity retention after 20 000...

Long-life property is one of the key factors for wide applications of lithium-ion batteries. Here, Mo-modified Ni-rich cathode material LiNi 0.5 Co 0.2 Mn 0.3 O 2 (NCM) is synthesized successfully via a solvent evaporating way followed with a calcination method. This strategy delivers two kinds of effects including Mo-doping and Mo-coating. Mo not only intercalates into the crystal lattice of NCM, but also forms a film-like coating layer on the surface to impede side reactions between electrode and electrolyte. Thus, its specific capacity, rate capability and cycle performance are improved simultaneously, especially in terms of long cycling life property. A series of physical and electrochemical characterizations are used to study the modified performance, and the sample with 1.0 wt% Mo modifying presents the best property with an approximate 3.5 nm coating layer surrounding the surface. Besides, the capacity retention ratio reaches to 89.7% even after 500 cycles between 3.0 and 4.3 V. However, Mo-modified samples have an obvious attenuation in the later period after charging to a higher voltage of 4.6 V although they have preferable cycle performance at the preliminary stage. The results indicate that the reaction mechanisms are diverse at different voltage ranges, which may guide subsequent researches.

LiNi0.5Mn1.5O4(LNMO) is synthesized using MnCO3 as manganese source in this paper. MnCO3 is prepared by precipitation reaction between MnSO4 and NH4HCO3. Two different adding processes of precipitant, slow dropping and rapid addition, are compared. MnCO3 particles have core-shell structure. When the precipitant is added slowly, the shell occupies a larger proportion in MnCO3 particles. Compared to slow dropping process, rapid addition of precipitant may reduce particle size and improve dispersity of MnCO3 and LNMO particles. LNMO prepared by large MnCO3 particles (>8 μm) and small MnCO3 particles (∼1 μm) have core-shell structure and hollow structure, respectively. LNMO with hollow structure are not stable and break easily during cycling. Slow addition process for precipitant can improve the rate performance of LNMO. When the concentration of MnSO4 solution is 0.04 mol L−1 and precipitant is added slowly, LNMO exhibits the best electrochemical performances. Its particle size is 8 μm and its discharge capacities at 0.2C and 20 C are 123 and 62 mAh g−1, respectively. After 500 cycles at 1 C, its discharge capacity is still as high as 114.1mAh g−1, which maintains 92.4 % of its initial capacity.

Layered Li-rich oxide (LLRO) is an attractive candidate for high-energy-density and high-voltage cathode material for next generation lithium ion batteries because of its high specific capacity and low cost. There still remain challenges in simultaneously achieving a multi-functional structure and composition in a LLRO, to achieve better electrochemical performance. Here we report a controllable co-precipitation and calcination method to synthesize LLRO by tuning the crystal nucleation, growth and heterogeneous contraction processes. The resultant LLRO adopts a hierarchical ball-in-ball hollow structure consisting of uniform multi-elemental (Mn–Ni–Co) primary nanocrystals, and exhibits high reversible capacity, remarkable cycle stability and superior rate performance. As a result, the resultant LLRO presents a high capacity of 193 mA h g−1 at 3C (a current density of 750 mA g−1) with a capacity retention of 87.6% after 400 cycles, and exhibits a capacity of 132 mA h g−1 at a high rate of 10C; moreover, it displays a quite slow voltage decay of ∼240 mV and a high energy density of 668 W h kg−1 after 200 cycles at 1C. The excellent electrochemical performance can be attributed to the combined merits of the multi-functional structure and composition, wherein the hierarchical hollow architecture facilitates efficient electron/ion transport and high structural stability, while multi-elemental components offer high reversible capacity.

Well-aligned hierarchical heterostructures composed of hydrogen titanate (H2Ti2O5·H2O, labeled as HTO) nanowire stems and anatase TiO2 nanoparticle branches (HTO/TiO2 NWAs) have been synthesized successfully on Ti-foil substrate as additive-free electrodes for hybrid Li-ion capacitors (LICs). The inner 3D HTO conductive scaffold consisted of vertically aligned nanowires and porous rooftop network is beneficial to large capacitance. Moreover, the countless anatase TiO2 particles on the surface of HTO nanowires can enrich electro-active sites, reduce the ion and electron resistance, shorten Li-ion transport pathways and enhance structural stability. Owning to the synergistic effects of HTO and anatase TiO2, the electrodes deliver large capacitance, superior rate capability and excellent cycle stability. The HTO/TiO2//AC (activated carbon) hybrid system presents fascinating energy density (84.7 Wh kg−1 at 0.5 A g1) and ultralong lifespan (78.5% capacity retention after 10000 cycles at 10.0 A g−1 within 0.0-4.0 V). Even at high power density of 20 kW kg−1, an excellent energy density of 44.4 Wh kg−1 can be retained. Thus, the LIC assembled with HTO/TiO2 NWAs anode and AC cathode can be a potential candidate as high-performance energy storage application.

Na3V2(PO4)2F3 is a promising cathode candidate for sodium ion batteries due to its high working voltage. However, an “unknown” low-voltage plateau around ~3.3 V lowers its operating voltage and puzzles researchers. Herein, the reasons for its appearance are explored. It is found that the formation of the low-voltage plateau originates from the extra-generated [VO6] induced by the fluorine loss during the synthesis. To enable better performance, a feasible strategy of stabilizing fluorine is proposed to eliminate the low-voltage plateau. It is demonstrated that this strategy can effectively guarantee the existence of fluorine and modulate the local electronic structure of V to maintain [VO4F2] in Na3V2(PO4)2F3 phase, thus eliminating the undesirable plateau and achieving an operating-voltage increase of ~100 mV. Moreover, the enhanced structural stability and kinetics of the optimized Na3V2(PO4)2F3 cathode are confirmed by in-situ XRD. As expected, the Na3V2(PO4)2F3 cathode delivers higher energy density (446.4 Wh kg−1), better rate capability and longer cycling performance (89.2%@30 C after 1000 cycles). Furthermore, its excellent adaptability to wide temperature range (−25 °C–55 °C) and strong competence in Na3V2(PO4)2F3||hard carbon full-cell are verified. These excellent properties further qualify Na3V2(PO4)2F3 as a competitive cathode for sodium ion batteries.

The critical challenges hindering the commercialization of Li-rich cathodes are their rapid-decaying capacity and voltage during cycling, originating from the degradation of lattice structure and interface side reaction between electrode and electrolyte. Surface engineering is considered to be an effective strategy to mitigate these disadvantages. Herein, an in-situ self-reconstruction strategy is proposed and developed to simultaneously optimize surface chemical composition and local structure of Li-rich cathodes. Specifically, the multifunction protective layer consisting of cation disorder phase and LiTMPO4-like (TM: Ni, Co, Mn) phase is produced by a simple PH3 gas treatment. LiTMPO4 featuring the ability against high potential is responsible for preventing interface side reaction and further reduce the dissolution of Mn. Both LiTMPO4-like phase and surface cation disorder phase contribute to stabilizing surface oxygen structure and limiting surface O2 release. Compared to the pristine one, better integrity of chemical phases and higher oxidation state of TM cations after long-term cycling are confirmed in the modified sample by synchrotron-based scanning transmission X-ray microscopy, highlighting the key roles of the multifunction protective layer in stabilizing the capacity and voltage during cycling. This surface self-reconstruction strategy provides a new path for guiding the interface design of high energy density cathodes.