Modulation of Redox Chemistry of Na2Mn3O7 by Selective Boron Doping Prompted by Na Vacancies.

The small energy density and chemomechanical degradation of layered manganese oxide limit practical application to sodium-ion batteries (SIBs). Typically, Na2Mn3O7 shows a low redox plateau at 2.1 V versus Na/Na+, and the oxygen redox reaction at a high voltage causes structural collapse. Herein, a Na vacancy-induced boron doping strategy is demonstrated to improve the properties. Boron is incorporated into selective sites in the lattice in the center of the MnO6 octahedral ring at the O-layer. Bonding of boron in the TM layer enhances the electrochemical activity of low-valence Mn, giving rise to two reversible redox peaks at 2.45 and 2.55 V to enhance the average redox voltage. At the same time, the O 2p chemical state becomes weaker around the Fermi level, thus suppressing oxygen overoxidation for the high charge state and strengthening the layered structure during the redox reactions. The reduced Mn-O covalency and small diffusion barrier energy stemming from bonding of boron in the oxygen layer produce excellent rate characteristics. Modulation of the Mn 3d and O 2p orbital in Na2Mn3O7 by Na vacancies leads to selective doping of boron at different sites, and our results reveal that it is an important strategy for studying transition-metal-oxide-layered electrode materials.

Realization of a High-Voltage and High-Rate Nickel-Rich NCM Cathode Material for LIBs by Co and Ti Dual Modification.

Nickel-rich Li(NixCoyMn1-x-yO2) (x ≥ 0.6) is considered to be a predominant cathode for next-generation lithium-ion batteries (LIBs) due to its towering specific energy density. Unfortunately, serious structural degradation causes rapid capacity degradation with the increase in nickel content. Herein, a Co and Ti co-modified LiNi0.8Co0.1Mn0.1O2 (NCM-811) cathode ameliorates the reversible capacity together with the rate capability by obviously alleviating the lattice structure degradation and microscopic intergranular cracks. Further studies show that the titanium doping effectively reduces the cation mixing and also stabilizes the crystal structure, while the spinel phase formed at the surface by a cobalt oxide coating is much stable than the layered phase at high voltage, which can alleviate the generation of micro-cracks. After 0.5% Co oxide coating and 1% Ti doping (T1Co0.5-NCM), a superior rate capability (121.75 mA h g-1 at 20 C between 2.7 and 4.5 V) and predominant capacity retention (74.2%) are observed compared with the pristine NCM-811 (59.5%) after 400 cycles between 2.7 and 4.7 V. This work supplies an eminent design of high-voltage and high-rate layered cathode materials and has a huge application prospect in the next generation of high-energy LIBs.

In Situ FTIR-Assisted Synthesis of Nickel Hexacyanoferrate Cathodes for Long-Life Sodium-Ion Batteries.

Prussian blue analogs (PBAs) with stable framework structures are ideal cathodes for rechargeable sodium-ion batteries. The chelating agent-assisted coprecipitate method is an effective way to obtain low-defect PBAs that can limit the appearance of too many vacancies and water molecules and achieve an optimized Na-storage performance. However, for this method, the mechanism of chelating agent-assisted synthesis is still unclear. Herein, the synthesis process of nickel hexacyanoferrate (NiHCF) has been investigated by in situ infrared spectroscopy detection. The results show that the chelating agent oxalate slows down the nucleation process and effectively inhibits the formation of the Fe-C≡N-Ni frame in the aging process, producing highly crystallized and low-defect NiHCF samples. High-quality NiHCF presents a high specific capacity of 86.3 mAh g-1 (a theoretical value of ∼85 mAh g-1), an ultrastable cyclic retention of 90% over 800 cycles, and a remarkable high capacity retention of 74.6% at a current density of 4250 mA g-1 (50C). Particularly, the NiHCF//hard carbon full cell presents a high specific energy density of over 210 Wh kg-1 and an outstanding cyclic stability without obvious capacity attenuation over 1000 cycles.

F-Doped NaTi2(PO4)3/C Nanocomposite as a High-Performance Anode for Sodium-Ion Batteries.

We are presenting a sol-gel method for building novel nanostructures made of nanosized F-doped Na1-2 xTi2(PO4)3- xF x (NTP-F x, x = 0, 0.02, 0.05, and 0.10) particles embedded in three-dimensional (3D) carbon matrices (NTP-F x/C). This technique combines advantages of both zero-dimensional materials and 3D-carbon networks. Proper fluorine doping stabilizes the NTP structure and greatly enhances ion/electron transportation, leading to superhigh-rate electrochemical performance and ultralong cycle life. The composite electrode delivers high specific capacities of 121, 115, 112.2, 110.1, 107.7, 103.1, 85.8, and 62.5 mA h g-1 at 0.2, 0.5, 1, 2, 5, 10, 20, and 30 C, respectively. It retains an unbelievable ∼70% capacity after a thousand cycles at a rate as high as 10 C. Electroanalytical results reveal that fluorine doping significantly enhances Na+ diffusion kinetics. Meanwhile, density functional theory calculations demonstrate F-doped NTPs' own outstanding electrochemical properties, which is due to the enhanced intrinsic ionic/electronic conductivity. The results show that anion doping is an efficient way to make high-performance NTP anodes for sodium-ion batteries.

Porous NaTi2(PO4)3/C Hierarchical Nanofibers for Ultrafast Electrochemical Energy Storage.

NaTi2(PO4)3 (NTP) with a sodium superionic conductor three-dimensional (3D) framework is a promising anode material for sodium-ion batteries (SIBs) because of its suitable potential and stable structure. Although its 3D structure enables high Na-ion diffusivity, low electronic conductivity severely limits NTP's practical application in SIBs. Herein, we report porous NTP/C nanofibers (NTP/C-NFs) obtained via an electrospinning method. The NTP/C-NFs exhibit a high reversible capacity (120 mA h g-1 at 0.2 C) and a long cycling stability (a capacity retention of ∼93% after 700 cycles at 2 C). Furthermore, sodium-ion full cells and hybrid sodium-ion capacitors have also been successfully assembled, both of which exhibit high-rate capabilities and remarkable cycling stabilities because of the high electronic/ionic conductivity and impressive structural stability of NTP/C-NFs. The results show that the nanoscale-tailored NTP/C-NFs could deliver new insights into the design of high-performing and highly stable anode materials for room-temperature SIBs.

Enhancing Sodium-Ion Storage Behaviors in TiNb2O7 by Mechanical Ball Milling.

Sodium-ion batteries (SIBs) have shown extensive prospects as alternative rechargeable batteries in large-scale energy storage systems, because of the abundance and low cost of sodium. The development of high-performance cathode and anode materials is a big challenge for SIBs. As is well known, TiNb2O7 (TNO) exhibits a high capacity of ∼250 mAh g-1 with excellent capacity retention as a Li-insertion anode for lithium-ion batteries, but it has rarely been discussed as an anode for SIBs. Here, we demonstrate ball-milled TiNb2O7 (BM-TNO) as an SIB anode, which provides an average voltage of ∼0.6 V and a reversible capacity of ∼180 mAh g-1 at a current density of 15 mA g-1, and presents excellent cyclability with 95% capacity retention after 500 cycles at 500 mA g-1. A possible Na storage mechanism in BM-TNO is also proposed.

Low-Cost and High-Performance Hard Carbon Anode Materials for Sodium-Ion Batteries.

As an anode material for sodium-ion batteries (SIBs), hard carbon (HC) presents high specific capacity and favorable cycling performance. However, high cost and low initial Coulombic efficiency (ICE) of HC seriously limit its future commercialization for SIBs. A typical biowaste, mangosteen shell was selected as a precursor to prepare low-cost and high-performance HC via a facile one-step carbonization method, and the influence of different heat treatments on the morphologies, microstructures, and electrochemical performances was investigated systematically. The microstructure evolution studied using X-ray diffraction, Raman, Brunauer-Emmett-Teller, and high-resolution transmission electron microscopy, along with electrochemical measurements, reveals the optimal carbonization condition of the mangosteen shell: HC carbonized at 1500 °C for 2 h delivers the highest reversible capacity of ∼330 mA h g-1 at a current density of 20 mA g-1, a capacity retention of ∼98% after 100 cycles, and an ICE of ∼83%. Additionally, the sodium-ion storage behavior of HC is deeply analyzed using galvanostatic intermittent titration and cyclic voltammetry technologies.

Graphene-Roll-Wrapped Prussian Blue Nanospheres as a High-Performance Binder-Free Cathode for Sodium-Ion Batteries.

Sodium iron hexacyanoferrate (Fe-HCF) has been proposed as a promising cathode material for sodium-ion batteries (SIBs) because of its desirable advantages, including high theoretical capacity (∼170 mAh g-1), eco-friendliness, and low cost of worldwide rich sodium and iron resources. Nonetheless, its application faces a number of obstacles due to poor electronic conductivity and structural instability. In this work, Fe-HCF nanospheres (NSs) were first synthesized and fabricated by an in situ graphene rolls (GRs) wrapping method, forming a 1D tubular hierarchical structure of Fe-HCF NSs@GRs. GRs not only provide fast electronic conduction path for Fe-HCF NSs but also effectively prevent organic electrolyte from reaching active materials and inhibit the occurrence of side reactions. The Fe-HCF NSs@GRs composite has been used as a binder-free cathode with a capacity of ∼110 mAh g-1 at a current density of 150 mA g-1 (∼1C), the capacity retention of ∼90% after 500 cycles. Moreover, the Fe-HCF NSs@GRs cathode displays a super high rate capability with ∼95 mAh g-1 at 1500 mA g-1 (∼10C). The results suggest that the 1D tubular structure of 2D GRs-wrapped Fe-HCF NSs is promising as a high-performance cathode for SIBs.