The generalization of the Local Density Approximation (LDA) method for the systems with strong Coulomb correlations is presented which gives a correct description of the Mott insulators. The LDA+U method is based on the model hamiltonian approach and allows to take into account the non-sphericity of the Coulomb and exchange interactions. parameters. Orbital-dependent LDA+U potential gives correct orbital polarization and corresponding Jahn-Teller distortion. To calculate the spectra of the strongly correlated systems the impurity Anderson model should be solved with a many-electron trial wave function. All parameters of the many-electron hamiltonian are taken from LDA+U calculations. The method was applied to NiO and has shown good agreement with experimental photoemission spectra and with the oxygen Kα X-ray emission spectrum.

http://absimage.aps.org/image/MAR06/MWS_MAR06-2005-007233.pdf

First-principles calculations of electronic structure and spectra of strongly correlated systems: the LDA+U method

at an average storage voltage of 3.2 V with long cycle life. When coupled with hard carbon anode, a prototype rechargeable sodium-ion battery offers an energy density of 210 Wh kg −1 , a round-trip energy effi ciency of 90%, high rate capability, and excellent cycling stability. These desired performances make this system to be closer to the level of practical applications. The Na 0.9 [Cu 0.22 Fe 0.30 Mn 0.48 ]O 2 (Na 0.9 [Cu II 0.22 Fe III 0.30 Mn III 0.16 Mn IV 0.32 ]O 2 ) material was synthesized by a simple solid-state reaction at 850 °C in air atmosphere using precursors of Na 2 CO 3 , CuO, Fe 2 O 3 , and Mn 2 O 3 . The crystal structure of the as-synthesized material was determined by X-ray diffraction (XRD) as shown in Figure 1 a together with its refi nement results by the Rietveld method (see Table S1, Supporting Information). It can be seen that all the Bragg diffraction peaks are in excellent agreement with the JC DS No. 01-0821495 (O3-type α-NaFeO 2 ) and can be indexed to a hexagonal layered structure with a space group of 3 R m − , indicative of a typical O3-type layered structure (note that the letter “O” refers to the Na coordination environment of octahedral site whereas the number “3” refers to the number of MO 2 slab according to Delmas’ notation. [ 13 ] A schematic illustration of the O3-type structure is also shown in Figure 1 b. The structure refi nement gives the lattice parameters a = 2.9587(7) A, c = 16.3742(6) A. The lattice parameter of c -axis is slightly larger than that of other O3-type materials [ 5f–h , 6b , 8a ] because the Na content is less than 1. The inductively coupled plasma (ICP) result confi rms the composition of Na 0.89 [Cu 0.22 Fe 0.30 Mn 0.48 ]O 2 (see Table S2, Supporting Information). The morphology of the resulting sample is shown in Figure 1 c. The distribution of the particle size is in the range of 10–30 μm with about 3 μm sized primary particle agglomerations together (Figure 1 d). Most importantly, unlike other O3-type materials, [ 4–9,12 ] this material is very stable against water. In order to confi rm this, we intentionally design an accelerated aging experiment as described in the Experimental Section which was verifi ed by LiMO 2 as shown in Figure S1 (Supporting Information). We placed the as-synthesized material in deionized water for 3 d and then dried the material at 100 °C for overnight. The obtained material was checked by XRD again. It can be seen that the XRD pattern is nearly identical to that of the as-synthesized material, which is very different from other O3-type materials as shown in Figure S2 (Supporting Information). These results suggest that the O3-Na 0.9 [Cu 0.22 Fe 0.30 Mn 0.48 ]O 2 is very stable against water. Furthermore, after the material was stored in air for one month Large-scale electrical energy storage systems are one of the core technologies in renewable energies and smart grid, among which sodium-ion batteries show great promise due to the abundant sodium resources. Layered metal oxides (of general formula: A x MO 2 , where A = Li, Na; M = Co, Ni, Mn, Cr, Fe, etc.) with alternating alkali metal layer and transition metal layer have long been of particular interest since the early 1980s as an important class of cathode materials for rechargeable batteries due to their easy synthesis and high energy density. [ 1 ] One of the most successful examples is LiCoO 2 , [ 1a ] which is commonly used as a cathode in lithium-ion batteries with the highest volumetric energy density for portable electronic devices. Its metal substituted materials (LiCo 1− x − y − z Ni x Mn y Al z O 2 ) are being used in power batteries for electric vehicles. In the case of sodiumion batteries operated at room temperature which are proposed for large-scale electrical energy storage owing to the naturally abundant sodium resources in recent years. [ 2,3 ] Na x CoO 2 that can electrochemically and reversibly intercalate Na is the fi rst example, [ 1b ] then a large number of layered metal oxides have been extensively exploited. [ 4–12 ] However, the practical applications have been hindered by two major challenges. First, unlike LiMO 2 , almost all the Na x MO 2 are not stable against moisture (either they can be oxidized by water or water/carbon dioxide molecules can be intercalated into alkali metal layer). [ 4–9,11,12 ]

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Prototype Sodium-Ion Batteries Using Air-Stable and Co/Ni-Free O3-Layered Metal Oxide Cathode

Flexible power sources featuring high-performance, prominent flexibility and raised safety have received mounting attention in the area of wearable electronic devices. However, many great challenges remain to be overcome, notably the design and fabrication of flexible electrodes with excellent electrochemical performance and matching them with safe and reliable electrolytes. Herein, a facile approach for preparing flexible electrodes, which employs carbon cloth derived from commercial cotton cloth as the substrate of cathode and a flexible anode, is proposed and investigated. The promising cathode (NVPOF@FCC) with high conductivity and outstanding flexibility is prepared by efficiently coating Na3V2(PO4)2O2F (NVPOF) on flexible carbon cloth (FCC), which exhibits remarkable electrochemical performance and the significantly improved reaction kinetics. More importantly, a novel flexible quasi-solid-state sodium-ion full battery (QSFB) is feasibly assembled by sandwiching a P(VDF-HFP)-NaClO4 gel-polymer electrolyte film between the advanced NVPOF@FCC cathode and FCC anode. And the QSFBs are further evaluated in flexible pouch cells, which not only demonstrates excellent energy-storage performance in aspect of great cycling stability and high-rate capability, but also impressive flexibility and safety. This work offers a feasible and effective strategy for the design of flexible electrodes, paving the way for the progression of practical and sustainable flexible batteries.

Flexible quasi-solid-state sodium-ion full battery with ultralong cycle life, high energy density and high-rate capability

Layered transition metal oxide P2‐Na2/3Ni1/3Mn2/3O2 usually suffers from large‐volume phase transitions and different Na‐vacancy ordering during sodium (de)intercalation, incurring rapid capacity decline and poor rate capability. Herein, an effective strategy based on synergetic effect of selected multiple metal ions is designed for P2‐type cathodes with improved performance. The role of tetravalent titanium provides high redox potential, inactive divalent magnesium stabilizes the structure, and the monovalent lithium smooths the electrochemical curves. The combined analysis of in operando X‐ray diffraction, in operando X‐ray absorption spectroscopy and density functional theory calculations demonstrates the contribution of multi‐metal ions converts the unfavorable and large‐volume P2 to O2 transition into a moderate “Z”‐intergrowth structure by increasing the energy barrier of transition metal slab gliding. As a consequence, the resultant P2‐Na0.7Li0.03Mg0.03Ni0.27Mn0.6Ti0.07O2 electrode delivers a reversible capacity of 134 mAh g−1, a working voltage of 3.57 V, excellent cycling stability (82% of capacity retention after 200 cycles), and superior rate performance (110 mAh g−1 at 4 C). Full cells fabricated with a hard carbon anode achieve an energy density of 296 Wh kg−1. This study presents a route to rationally design cathode materials with this functionalization to improve the cell performance for sodium‐ion batteries.

Mitigating the Large‐Volume Phase Transition of P2‐Type Cathodes by Synergetic Effect of Multiple Ions for Improved Sodium‐Ion Batteries

As one of the fascinating high capacity cathodes, O3‐type layered oxides usually suffer from their intrinsic air sensitivity and sluggish kinetics originating from the spontaneous lattice Na extraction during air exposure and high tetrahedral site energy of Na+ diffusion transition state. What is worse, the improvement on the two handicaps is hard to simultaneously realize because of the contradiction between Na containment suggested in air stability mechanism and enhanced Na diffusion mentioned in kinetics strategy. Herein, it is shown that a simple strategy of introducing proper Na vacancies into lattice can simultaneously realize a dual performance improvement. Na vacancies decrease the charge density on transitional metal ions and enhance the antioxidative capability of material, ensuring a stable lattice Na containment for Na0.93Li0.12Ni0.25Fe0.15Mn0.48O2 when exposed to air. Additionally, more Na+ diffusional sites and enlarged Na layer spacing are obtained and result in a significantly decreased energy barrier from ≈1000 to 300 meV and a high rate capability of 70.8% retention at 2000 mA g−1. Remarkably, such a strategy can be easily realized by either pre‐ or post‐treating, which exhibits excellent universality for various O3 materials, implying its enormous potential to promote the commercial application of O3‐type cathodes.

A Universal Strategy toward Air‐Stable and High‐Rate O3 Layered Oxide Cathodes for Na‐Ion Batteries

Understanding the Capacity Fading Mechanisms of O3-Type Na[Ni0.5Mn0.5]O2 Cathode for Sodium-Ion Batteries

To facilitate the practical realization of sodium-ion batteries, the energy density, determined by the output operating voltage and/or capacity, needs to be improved to the level of commercial Li-ion batteries Herein, O3-type Na098Ca001[Ni05Mn05]O2 is synthesized by incorporating Ca2+ into the NaO6 octahedron of Na[Ni05Mn05]O2 and its potential use as a cathode material for high energy density SIBs is demonstrated The ionic radius of calcium (≈100 A) is similar to that of sodium (≈102 A); hence, it is energetically favorable for calcium to occupy sites in the sodium layers Within a wide operating voltage range of 20–43 V, O3-type Na098Ca001[Ni05Mn05]O2 exhibits a reversible O3–P3–O3 phase transition with small volume changes compared to Ca-free Na[Ni05Mn05]O2 because of the strong interaction between Ca2+ and O2− and delivers a high reversible capacity of 209 mA h g−1 at 15 mA g−1 with improved cycling stability Moreover, Ca substitution improves the practically useful aspects such as thermal and air stability A prototype pouch full cell with a hard carbon anode shows an excellent capacity retention of 67% over 300 cycles Thus, this study provides an efficient and simple method to boost the performance and applicability of layered oxide cathode materials for practical applications

High-energy O3-Na1−2xCax[Ni0.5Mn0.5]O2 cathodes for long-life sodium-ion batteries

Despite their high discharge capacity, O3-type layered cathodes for sodium-ion batteries (SIBs) suffer from rapid capacity fading caused by detrimental phase transitions. A series of O3-type layere...

Geumjae Han

Papers

Understanding the Capacity Fading Mechanisms of O3-Type Na[Ni0.5Mn0.5]O2 Cathode for Sodium-Ion Batteries

High-energy O3-Na1−2xCax[Ni0.5Mn0.5]O2 cathodes for long-life sodium-ion batteries

Enhanced Cycling Stability of O3-Type Na[Ni0.5Mn0.5]O2 Cathode through Sn Addition for Sodium-Ion Batteries