P2-type Structure Research Articles

Investigation of the NaNixMn1-xO2 (0 ≤ x ≤ 1) System for Na-Ion Battery Cathode Materials

Layered NaNixMn1-xO2 (0 ≤ x ≤ 1) oxides were prepared via solid state reactions. Different reaction conditions were required to obtain phase pure samples, depending on the value of x. The 0 ≤ x ≤ 0.1 compositions were prepared in an inert argon atmosphere at 700°C and had a monoclinically distorted O′3 type structure. The 0.25 ≤ x ≤ 0.33 compositions were prepared in air at 850°C and had a P2-type structure. Compositions in the range of 0.5 ≤ x ≤ 0.66 were synthesized in air at 850°C and had an O3-type structure. Lastly, compositions with 0.9 ≤ x ≤ 1 were prepared in an oxygen atmosphere at 700°C and had a monoclinically distorted O′3 type structure. Electrochemical experiments were performed on pure phase samples. All showed reversibility of sodium ions and high capacities. The highest reversible capacity was achieved for x = 0.66, with a capacity of ∼190 mAh/g and an average discharge voltage of 3.07 V, corresponding to a high energy density of 2705 Wh/L. This is among the highest reported volumetric energy densities for Na-ion battery electrodes.

P2-type Na0.66Ni0.33–xZnxMn0.67O2 as new high-voltage cathode materials for sodium-ion batteries

P2-type Na0.66Ni0.33–xZnxMn0.67O2 (x = 0, 0.07, 0.14) are prepared using a conventional solid state method and for the first time developed as promising cathode materials for high-voltage sodium-ion batteries. The XRD patterns show that Zn2+ ions are successfully incorporated into the lattice of the Na–Ni–Mn–O system and the P2-type structure remains unchanged after substitution. The introduction of Zn2+ in the Na–Ni–Mn–O system can effectively overcome the drawback of voltage decay when charged to a higher cutoff voltage (>4.0 V), and significantly improve capacity retention compared to the unsubstituted material during cycling. In addition, a smoother charge/discharge profile can be observed between 3.0 and 4.0 V for Zn-substituted samples, demonstrating that Na+/vacancy ordering can be suppressed during sodium insertion/extraction. Na0.66Ni0.26Zn0.07Mn0.67O2 can deliver an initial capacity of 132 mAh g−1 at 12 mA g−1 with a high average voltage of 3.6 V and a capacity retention of 89% after 30 cycles. EIS measurements demonstrate that Zn-substitution is an effective way to limit the increase of inter-particle contact resistance by suppressing any possible irreversible phase transformation found at low sodium contents.

P-type NaxNi0.22Co0.11Mn0.66O2materials: linking synthesis with structure and electrochemical performance

P-type layered oxides are promising cathode materials for sodium-ion batteries and a wide variety of compounds have been investigated so far. Nevertheless, detailed studies on how to link synthesis temperature, structure and electrochemistry are still rare. Herein, we present a study on P-type NaxNi0.22Co0.11Mn0.66O2 materials, investigating the influence of synthesis temperature on their structure and electrochemical performance. The change of annealing temperature leads to various materials of different morphologies and either P3-type (700 °C), P3/P2-type (750 °C) or P2-type (800–900 °C) structure. Galvanostatic cycling of P3-type materials revealed high initial capacities but also a high capacity fade per cycle leading to a poor long-term cycling performance. In contrast, pure P2-type NaxNi0.22Co0.11Mn0.66O2, synthesized at 800 °C, exhibits lower initial capacities but a stable cycling performance, underlined by a good rate capability, high coulombic efficiencies and high average discharge capacity (117 mA h g−1) and discharge voltage (3.30 V vs. Na/Na+) for 200 cycles.

Synthesis, crystal structure and electrochemical properties of Li0.55Co0.5Mn0.5O2 with the O6-type layered structure

Abstract Lithium cobalt manganese oxide Li0.55Co0.5Mn0.5O2 with the O6-type layered structure was successfully prepared for the first time via Na+/Li+ ion-exchange from the parent Na0.58Co0.5Mn0.5O2 in molten LiNO3 salt at 280 °C. The precursor Na0.58Co0.5Mn0.5O2 sample with the P2-type layered structure was synthesized by solid-state reaction at 900 °C. Rietveld analysis using the powder X-ray diffraction data confirmed that the ion-exchanged Li0.55Co0.5Mn0.5O2 sample was the O6-type single phase with the rhombohedral crystal system, space group R-3m, and the lattice parameters of a = 2.8186(3) A and c = 29.674(4) A. The electrochemical measurements showed the reversible Li-ion insertion and extraction reactions.

New Na0.7Mn0.6Ni0.3Co0.1O2 Na Layered Oxides As Electrode Materials for Na-Ion Batteries

Introduction Sodium–ion batteries are considered to be one of the favourite for the future hybrid and electric vehicles because of their low price and sodium abundant resources. Sodium layered oxides (NaxMO2) are the expectable candidates as cathode material in respect of potential and sodium–ion diffusion. Especially, O3 type is mainly focused, however, the O3 type structure can be transformed into the spinel type structure that leads to capacity decay in several cycles. From the structural stability point of view, the P2 type structure is able to avoid converting to the spinel type structure due to different oxygen stacking. We focus on synthesizing the new P2 material including three transition metals, such as Mn, Ni, and Co, in order to achieve higher capacity and cyclability. Experimental We adopted a co−precipitation method in order to make the aimed compound. A solution of transition metal nitrates, and another solution of sodium carbonate were dripped into a beaker with distilled water simultaneously. The washed precipitate was dried at 80 ̊C for two days. The obtained powder was mixed with sodium carbonate with an excess amount, and annealed at 900 ̊C in air atmosphere, and finally quenched to room temperature. The crystal structure was studied by XRD. The electrochemical performances were evaluated in two electrodes cell configuration. The positive electrode was a mixture containing 88 wt. % of the active material, 10 wt. % of graphite as the conductor and 2 wt. % of polytetrafluoroethylene (PTFE). The electrolyte was 1 M NaPF6 in propylene carbonate (PC) with fluoroethylene carbonate (FEC) at 2 wt. %. Galvanostatic performances were carried out between 4.0 V and 1.5 V vs. Na+/Na at the current rate of 0.05 C and 25 ̊C. Cycle performance was evaluated between 3.8 V and 1.5 V at same condition. Results Among all composition studied, a pure phase with the P2 structure was obtained for the Na0.7Mn0.6Ni0.3Co0.1O2 composition. Fig. 1 shows the XRD pattern of Na0.7Mn0.6Ni0.3Co0.1O2 phase. It indicates that the sample crystallizes in the hexagonal system (S.G. , P63/mmc) and there were no impurities in the material. The cell parameters are ahex. = 2.8847 Å, chex. = 11.0833 Å. The material has excellent electrochemical performance as shown in Fig. 2. At first, it was discharged until 1.5 V in order to insert sodium ions into the remaining vacancies. The amount of sodium at the end of the first discharge (x = 0.96) shows that almost all sites are occupied between MO2 slabs. In the curves, there were two large potential drops around x = 0.7 and x = 0.5 (close to x = 1/2) identified with the single phase domain. The three sloping curves parts indicate solid solution behaviour. The specific capacity calculated from the sodium content was 153 mAh/g, which was similar to that of the O3 phase [1]. A good cycle retention was observed during 10 cycle test as shown in Fig. 3. In the presentation, we will discuss the crystal structural changes occurring during the sodium extraction deintercalation / intercalation. [1] M. Sathiya, et al., Chem. Mater. 2012, 24, 1846−1853.

Impacts of Mg and Al Substitution on Na2/3Ni1/3Mn2/3O2 As Positive Electrode Materials for Na-Ion Batteries

IntroductionRecently, layered Na x MeO2 (Me = metal element(s), x = ~1) compounds have been extensively explored as positive electrode materials for Na-ion batteries. From a viewpoint of materials abundance, Na-ion batteries are promising candidates as next generation energy storage devices. Average operating potential of Na x MeO2 is usually lower than that of Li x MeO2, resulting in lower energy density in Na-ion cells even though P2-type layered materials are considered to be potential candidates for high capacity positive electrode materials. Among the P2-type layered materials, Na2/3[Ni1/3Mn2/3]O2 performs at a relatively high operating voltage and delivers specific capacity of 150 mAh g−1 in the voltage range of 2.0–4.5 V in Na cells without compensation of initially deficient sodium in the structure.1 However, the reversible capacity of Na2/3[Ni1/3Mn2/3]O2 electrode is known to rapidly fade during cycles in the wide voltage range, probably due to a drastic lattice shrinkage of the fully charged state and electrolyte decomposition. In this study, we significantly improve the capacity retention of Na2/3Ni1/3Mn2/3O2 electrode by partial substitution of Mg and Al for the transition metals.ExperimentalNa2/3[Ni1/3 − x Mn2/3 − y Me x +y ]O2 (x = y = 1/36 for Me = Al, and x = 1/18, y = 0 for Me = Mg) was prepared by the calcination of stoichiometric mixtures of Mn2O3, Ni(OH)2, Na2CO3, Al(OH)3, and MgO with 5% excess sodium.2 The mixtures were pelletized and heated at 900 °C for 24 h in air and then quenched to room temperature. Crystal structures and morphology of the samples were examined by using powder X-ray diffraction (XRD) measurements and scanning electron microscopy (SEM). Electrochemical properties were tested using a coin-type cell. Positive electrodes consisted of the active material, acetylene black, and polyvinylidenefluoride (PVdF) with a gravimetric ratio of 80:10:10. Metallic sodium was used as a counter electrode. The electrolyte used was 1.0 mol dm-3 NaPF6 / PC.Results and discussionIn XRD patterns of synthesized samples, all diffraction lines can be assigned into the P2-type layered structure with space group P63/mmc without any crystalline impurities. Hexagonal platelet morphology for all the samples is found from SEM images. These particles have a diameter from 2 to 4 μm. Single phase P2-type layered samples modified with the Mg and Al substitution with similar particle morphology were successfully synthesized. The cycle stability of the sample electrodes operated at room temperature is shown in Fig. 1. The Mg and Al substitution effectively improves capacity retention. The substituted sample electrodes exhibit excellent cycle capability for 20 cycles. The capacity retention of the Al and Mg substituted samples is approximately 94 and 92%, respectively, after 20 cycles. Moreover, the Mg and Al substitution enhances rate capability. Although, at 2 C rate (about 260 mA g-1), Na2/3[Ni1/3Mn2/3]O2 delivers only 37% specific capacity of that at C / 20, the Al and Mg substituted samples show 78 and 77% of specific capacity of that at C / 20 rate. Influence of the Al and Mg substitution on electrochemical properties and structural changes induced by sodium extraction will be further discussed.

P2-Type Na2/3Ni1/3Mn2/3- x Ti x O2 as a 3.7 V Class Positive Electrode for Na-Ion Batteries

IntroductionLayered Na x MeO2 (Me = transition metal) compounds have been intensively studied as electrode materials for Na-ion batteries. Among them, P2-Na2/3Fe1/2Mn1/2O2 and O3-NaFe0.5Co0.5O2 can deliver 200 and 160 mAh g-1 of reversible capacity with relatively good capacity retention, respectively.1, 2 However, average operating potential of Na x MeO2 is usually lower than that of Li x MeO2, resulting in lower energy density of Na-ion cell. To increase the operating voltage of Na-ion batteries, we focus on P2-Na2/3Ni1/3Mn2/3O2 showing relatively high operating voltage based on a Ni2+/Ni4+ redox couple.3 The discharge capacity of Na//Na2/3Ni1/3Mn2/3O2 cell is, however, limited to only 80 mAh g-1 because of rapid capacity decay during cycles by charge to 4.5 V.4 In this study, we present synthesis and improved electrode performance of titanium substituted Na2/3Ni1/3Mn2/3-x Ti x O2 as positive electrode materials.ExperimentalNa2/3Ni1/3Mn2/3-x Ti x O2 (x = 0, 1/6, 1/3, 2/3) samples were prepared by solid-state reaction from starting materials of Na2CO3, NiO, TiO2, and Mn2O3. The mixture was pelletized and heated at 900-950 °C for 12 h in air. Crystal structures and morphology of the samples were examined by using powder X-ray diffraction (XRD) measurement and scanning electron microscopy (SEM). Electrochemical properties were tested using a coin-type cell. Positive electrodes consisted of the active material, acetylene black and polyvinylidenefluoride(PVdF) with a gravimetric ratio of 80:10:10. Metallic sodium was used as a negative electrode. The electrolyte used was 1.0 mol dm-3 NaPF6 / PC.Results and discussionAll diffraction lines in XRD patterns of synthesized Na2/3Ni1/3Mn2/3-x Ti x O2 (x = 0, 1/6, 1/3, 2/3) samples can be assigned into the P2-type layered structure with space group P63/mmc without any impurities. The calculated lattice parameters linearly increase by the substitution of Ti for Mn, and its increase obeys Vegard’s law with the consideration of larger ionic size of Ti4+ than Mn4+, suggesting that a solid solution is successfully synthesized in the range of 0 ≤ x ≤ 2/3 in Na2/3Ni1/3Mn2/3-x Ti x O2 samples. The substitution of titanium effectively improves capacity retention even though reversible capacity decreases with the titanium substitution. Figure 1 shows charge/discharge curves of a Na//Na2/3Ni1/3Mn1/2Ti1/6O2 cell. Among the substituted samples, the Na2/3Ni1/3Mn1/2Ti1/6O2 particularly delivers 127 mAh g-1 of reversible capacity with ca. 3.7 V of average discharge voltage for the initial cycle with relatively good capacity retention. Ex-situ XRD results reveal that volume shrinkage of the fully charged state is effectively reduced from 23% for Ti-free to 12-13% for Ti-substituted ones, which probably leads to the improved cycle stability. Estimated energy density of a Na//Na2/3Ni1/3Mn1/2Ti1/6O2 cell reaches 470 Wh kg-1 and that of a hard-carbon//Na2/3Ni1/3Mn1/2Ti1/6O2 full cell achieves 300 Wh kg-1, corresponding to approximately 80% of energy density for the graphite//LiCoO2 system. Na2/3Ni1/3Mn1/2Ti1/6O2 is one of the higher energy density materials for Na-ion batteries with good cyclability among layered oxides reported so far. Influence of the titanium substitution on structural changes during charge and electrochemical properties will be further discussed.

Synthesis and characterization of a new layered cathode material for sodium ion batteries

Owing to the high abundance of sodium and its low cost compared to lithium, sodium ion batteries have recently attracted a renewed interest as possible candidates for stationary and mobile energy storage devices. Herein, we present a new sodium ion intercalation material, NaxCo2/3Mn2/9Ni1/9O2, which has been synthesized by a sol–gel route in air followed by a heat treatment at 800 °C for 12 h. Its structure has been studied by X-ray diffraction showing that the material crystallized in a P2-type structure (space group P63/mmc). As far as the electrochemical properties of NaxCo2/3Mn2/9Ni1/9O2 as positive electrode are concerned, this compound offers a specific capacity of 110 mAh g−1 when cycled between 2.0 and 4.2 V vs. Na+/Na. The electrodes exhibited a good capacity retention and a coulombic efficiency exceeding 99.4%, as well as a reversible discharge capacity of 140 mAh g−1 when cycled between 2.0 and 4.5 V. These results represent a further step towards the realization of efficient sodium ion batteries, especially considering that the synthesis method proposed here is simple and cost effective and that all the electrochemical measurements were carried out without any use of additives or any optimization for both the materials and the cell components.

New P2 - Na0.70Mn0.60Ni0.30Co0.10O2 Layered Oxide as Electrode Material for Na-Ion Batteries

Sodium layered oxide, Na0.70Mn0.60Ni0.30Co0.10O2, with a P2-type structure was synthesized by a co-precipitation method and characterized as positive electrode of sodium batteries. The P2-NaxMn0.60Ni0.30Co0.10O2 electrode delivered a high specific capacity of 185 mAh/g in the 1.5–4.3 V range at 1st cycle at C/20 rate; however, rapid capacity decay is observed. In the 1.7–4.0 V range the capacity is equal to 125 mAh/g with a fading considerably reduced. The rate capability is reasonably good. According to the electrochemical profile, a potential plateau around 4.2 V vs. Na+/Na, meaning a bi-phasic domain, and several voltage drops that could be related to single phase domains in a narrow sodium content range are observed. Even if the capacity decreases when the battery is cycled above 4.0 V, there is no irreversible structural change.

Combustion-synthesized sodium manganese (cobalt) oxides as cathodes for sodium ion batteries

We report on the electrochemical properties of layered manganese oxides, with and without cobalt substituents, as cathodes in sodium ion batteries. We fabricated sub-micrometre-sized particles of Na0.7MnO2 + z and Na0.7Co0.11Mn0.89O2 + z via combustion synthesis. X-ray diffraction revealed the same layered hexagonal P2-type bronze structure with high crystallinity for both materials. Potentiostatic and galvanostatic charge/discharge cycles in the range 1.5–3.8 V vs. Na | Na+ were performed to identify potential-dependent phase transitions, capacity, and capacity retention. After charging to 3.8 V, both materials had an initial discharge capacity of 138 mA h g−1 at a rate of 0.3 C. For the 20th cycle, those values reduced to 75 and 92 mA h g−1 for Co-free and Co-doped samples, respectively. Our findings indicate that earlier works probably underestimated the potential of (doped) P2-type Na0.7MnO2 + z as cathode material for sodium ion batteries in terms of capacity and cycle stability. Apart from doping, a simple optimization parameter seems to be the particle size of the active material.

A Study of the Reactivity of De-Intercalated P2-NaxCoO2 with Non-Aqueous Solvent and Electrolyte by Accelerating Rate Calorimetry

P2-type layered structure NaxCoO2 (x∼0.65) was successfully synthesized by solid state reaction. The electrochemical performance of Na0.65CoO2 as a positive electrode material for sodium-ion batteries was tested at room temperature using two-electrode coin cells with NaClO4/PC electrolyte. Na0.65CoO2 delivered a reversible capacity of 70 mAh/g during the first charge to 4.3 V, which is related to the de-intercalation of about 0.3 mol Na per mole Co. The reactivity of de-intercalated NaxCoO2 (∼Na0.35CoO2) with ethylene carbonate/diethyl carbonate (EC:DEC) solvent and with NaPF6/EC:DEC electrolyte was studied using Accelerating Rate Calorimetry (ARC). X-ray diffraction was used to study the products of the reactions of Na0.35CoO2 with solvent or electrolyte after the ARC experiments. De-intercalated NaxCoO2 shows high reactivity in both solvent and electrolyte. Na0.35CoO2 reacts with solvent at elevated temperature in a manner which is similar to de-intercalated LixCoO2, leading to the products Na0.7CoO2 and Co3O4. Then Na0.7CoO2 reacts to form Co3O4 and NaHCO3. Then Co3O4 reacts to form CoO or Co metal. In NaPF6-based electrolyte, the reaction mechanism changes completely as the major reaction products are NaCoF3 and CoO or Co.

Synthesis and characterization of high-temperature hexagonal P2-Na0.6 MnO2 and its electrochemical behaviour as cathode in sodium cells

A layered sodium manganese oxide, Na0.6MnO2, stable at temperatures above 800 °C, was synthesized by using a sol–gel method that employs Mn(acac)3 (ac = acetylacetonate), Na2CO3 and propionic acid to form the resin framework. This layered bronze possesses a hexagonal, P2-type structure, in which the distortion associated with Mn3+ is hardly perceptible. It reacts slowly, though reversibly, with atmospheric moisture, which causes the interlayer spacing in the structure to increase by ca. 2.5 A, through intercalation of water molecules into the interlayer gap occupied by Na+ ions. The anhydrous material was tested as a cathode in sodium cells. Although the electrochemical intercalation of Na+ occurs in two steps, the host retains its main structural features, with a slight tendency in the interlayer spacing to contract as the sodium content increases. The similarity between the discharge and charge profiles of the first cycles reveals a quasi-reversible nature in the intercalation process and that the cell can deliver a constant specific capacity of ca. 140 A h kg−1 at 0.1 mA cm−2 when cycled in a voltage window of 3.8–2.0 V. However, the continuous strains and distortions resulting from the insertion and extraction of Na+ ions cause the host structure to gradually collapse and yield an amorphous material after the first eight cycles. This leads to a progressive reduction of the cell capacity, irrespective of the specific voltage window used.

Preparation of Layered MnO2 via Thermal Decomposition of KMnO4 and Its Electrochemical Characterizations

We report here the preparation of layered MnO2 and the preliminary results on its cathodic performance in Li secondary batteries. The thermal decomposition of KMnO4 powder at 250−1000 °C in air produces KxMnO2+δ·yH2O (x = 0.27−0.31, δ = 0.07−0.13, and y = 0.47−0.89) with a product yield of 67−79% based on the Mn molar quantity. It can be judged from the Rietveld refinement on the X-ray diffraction pattern that the 800 °C-prepared sample has a layered structure (hexagonal unit cell, space group = P63/mmc, a = 2.84 Å, and c = 14.16 Å), where the K+ ions and H2O molecules reside at the interlayer trigonal prismatic sites (P2-type structure). Contrary to the previous findings whereby the layered MnO2 transforms to α-/γ-MnO2 phases or manganese suboxides at >450 °C, such impurities are negligible in this synthesis even at higher temperatures. The success of synthesis is ascribed to the high population of K+ ions in the pyrolyzing media that act as pillaring cations to stabilize the layered framework. In addition, the absence of a suboxide transition is indebted to the highly oxidizing species such as O2, MnO42- and MnO43-, which are produced during the pyrolyzing process. The materials show a powder density as high as 1.36 g cm-3 and the Mn4+ fraction of >85%, which gives a theoretical capacity of 210−230 mA h g-1 based on a one-electron charge/discharge reaction. A higher product yield up to >98% is achieved by pyrolyzing KMnO4 with an addition of manganese suboxides (Mn2O3, Mn3O4, or MnO). Finally, the preliminary cell tests show that the materials give some promising features as the cathode materials for Li secondary batteries.