Tuning Li2MnO3-Like Domain Size and Surface Structure Enables Highly Stabilized Li-Rich Layered Oxide Cathodes.

Severe capacity/voltage fading still poses substantial obstacles in the commercial applications of Li-rich layered oxides, which stems from the aggregation of Li2MnO3-like domains and unstable surface structure. Here, we report highly stabilized Co-free Li1.2Ni0.2Mn0.6O2 with uniformly dispersed Li2MnO3-like domains and a protective rock-salt structure shell by reducing the oxygen partial pressure during high-temperature calcination. Experimental characterizations and DFT calculations reveal that the uniformly dispersed and small-sized Li2MnO3-like domains suppress the peroxidation of lattice oxygen, enabling highly reversible oxygen redox and excellent structural stability. Moreover, the induced rock-salt structure shell significantly restrains lattice oxygen release, TM dissolution, and interfacial side reactions, thereby improving the interfacial stability and facilitating Li+ diffusion. Consequently, the obtained Li1.2Ni0.2Mn0.6O2 which was calcinated under an oxygen partial pressure of 0.1% (LNMO-0.1) delivers a high reversible capacity of 276.5 mAh g-1 at 0.1 C with superior cycling performance (a capacity retention rate of 85.4% after 300 cycles with a small voltage fading rate of 0.76 mV cycle-1) and excellent thermal stability. This work links the synthesis conditions with the domain structure and electrochemical performance of Li-rich cathode materials, providing some insights for designing high-performance Li-rich cathodes.

Structural transformation and electrochemical properties of a nanosized flower-like R-MnO2 cathode in a sodium battery.

High-energy density and low-cost sodium-ion batteries are being sought to meet increasing energy demand. Here, R-MnO2 is chosen as a cathode material of sodium-ion batteries owing to its low cost and high energy density. The structural transformation from the tunnel R-MnO2 to the layered NaMnO2 and electrochemical properties during the charge/discharge are investigated at the atomic level by combining XRD and related electrochemical experiments. Na≤0.04MnO2 has a tunnel R-MnO2 phase structure, Na≥0.42MnO2 has a layered NaMnO2 phase structure, and Na0.04-0.42MnO2 is their mixed phase. Mn3+ 3d4[t2gß3dz2(1)3dx2-y2(0)] in NaMnO2 loses one 3dz2 electron and the redox couple Mn3+/Mn4+ delivers 206 mA h g-1 during the initial charge. The case that the Fermi energy level difference between R-MnO2 and NaMnO2 is lower than that between the layered Na(12-x)/12MnO2 and NaMnO2 makes the potential plateau of R-MnO2 turning into NaMnO2 lower than that of the layered Na(12-x)/12MnO2 to NaMnO2. This can be confirmed by our experiment from the 1st-2nd voltage capacity profile of R-MnO2 in EC/PC (ethylene carbonate/propylene carbonate) electrolyte. The study would give a new view of the production of sustainable sodium battery cathode materials.

Cr-Doped Fe1-xCrxF3·0.33H2O Nanomaterials as Cathode Materials for Sodium-Ion Batteries.

Due to the high theoretical specific capacity and low cost, FeF3·0.33H2O has become one of the potential choices of cathode materials for sodium-ion batteries. However, the poor intrinsic conductivity limits its practical applications. Herein, the atomic substitution is used to improve its intrinsic conductivity. The first-principles calculation results show that Cr3+ doping can reduce the band gap of FeF3·0.33H2O to improve its intrinsic conductivity. The discharge specific capacity of Fe0.95Cr0.05F3·0.33H2O with a narrowest band gap is 194.02 mA h/g at 0.1 C within the range of 1.4-4.0 V, which is higher than that of FeF3·0.33H2O (136.47 mA h/g). Using the electrochemical impedance spectroscopy and galvanostatic intermittent titration technique tests, it is found that Rct of Fe0.95Cr0.05F3·0.33H2O is reduced and DNa+ is almost unchanged, as compared to FeF3·0.33H2O.

FeP/C Composites as an Anode Material for K-Ion Batteries.

Owing to their natural abundance, the low potential, and the low cost of potassium, potassium-ion batteries are regarded as one of the alternatives to lithium-ion batteries. In this work, we successfully fabricated a FeP/C composite, a novel electrode material for PIBs, through a simple and productive high-energy ball-milling method. The electrode delivers a reversible capacity of 288.9 mA·h·g-1 (2nd) at a discharge rate of 50 mA g-1, which can meet the future energy storage requirements. Density functional theory calculations suggest a lower diffusion barrier energy of K+ than Na+, which allows faster K+ diffusion in FeP.

A Cobalt-Free Li(Li0.17 Ni0.17 Fe0.17 Mn0.49 )O2 Cathode with More Oxygen-Involving Charge Compensation for Lithium-Ion Batteries.

High-energy-density and low-cost lithium-ion batteries are sought to meet increasing demand for portable electronics. In this study, a cobalt-free Li(Li0.17 Ni0.17 Fe0.17 Mn0.49 )O2 (LNFMO) cathode material is chosen, owing to the reversible anionic redox couple O2- /O- . The aim is to elucidate the Fe-substitution function and oxygen redox mechanism of experimentally synthesized Li(Li0.16 Ni0.19 Fe0.18 Mn0.46 )O2 by DFT. The redox processes of cobalt-containing Li(Li0.17 Ni0.17 Co0.17 Mn0.49 )O2 (LNCMO) are compared with those of LNFMO. Redox couples including Ni2+ /Ni3+ /Ni4+ , Fe3+ /Fe4+ or Co3+ /Co4+ , and O2- /O- are found, confirmed by a X-ray photoelectron spectroscopy, and explained by redox competition between O and transition metals. In LNFMO and LNCMO, O ions with an Li-O-Li configuration readily participate in oxidation, and the most active O ions are coordinated to Mn4+ and Li+ . Oxidation of O in LNCMO is triggered earlier, along with that of Co. Fe substitution activates O ions, contributes additional oxygen redox charge compensation of 0.44 e per formula unit, avoids concentrated accumulation of oxygen oxidation, and improves structural stability. This work provides new scope for designing cobalt-free, low-cost, and higher-energy-density cathode materials for Li-ion batteries.

A Cobalt-Free Li(Li0.16 Ni0.19 Fe0.18 Mn0.46 )O2 Cathode for Lithium-Ion Batteries with Anionic Redox Reactions.

Lithium-rich, Mn-based layered oxides Li2 MnO3 -LiMO2 (M=Ni, Co) have been considered as promising cathode candidates owing to their high capacity. However, the resources shortage and high price of cobalt make it imperious to substitute cobalt with other high-abundance elements. Here, we synthesized a low-cost, cobalt-free, Fe-substituted oxide material, Li(Li0.16 Ni0.19 Fe0.18 Mn0.46 )O2 . It exhibited a high reversible capacity of 169.2 mAh g-1 after 100 cycles and maintained an extraordinarily high discharge potential during cycling. X-ray photoelectron spectroscopy and DFT calculations revealed that super iron FeIV exists in the delithiated state, and oxygen participates in the redox reaction in addition to the Ni2+ /Ni4+ and Fe3+ /Fe4+ redox couples. The anionic oxidation preferentially occurred on oxygen with a Li-O-Li configuration and with oxidized Fe and Ni coordination.

Chemical anisotropies of carbon nanotubes and fullerenes caused by the curvature directivity.

The directional-curvature theory is developed as a rational basis for the strain energy and the chemical reactivity in single-walled carbon nanotubes (SWCNTs) and fullerenes. The directional curvature KD and its mean KM, derived from this theory, cover the overall curvatures of their bonds and atoms and break through the limitations of the pyramidalized-angle thetap approach, which is only available to atomic curvature. The directional-curvature theory demonstrates that KD and KM depend directly on the strain or reactive binding energies of the bonds and atoms and that there is approximate curvature conservation in SWCNTs and fullerenes. Application of this theory to addition reactions of various SWCNTs and fullerenes shows that the slope of the straight line between the strain or binding energies and KD is close to a constant, which helps clarify the puzzle as to why some functionalizations of C70 occur at the relatively flat midsection.