Search

Lithium-ion batteries (LIBs) have attracted significant attention as high-energy-density rechargeable power sources for electric vehicles and large-scale energy storage applications. Although LIBs have been successfully commercialized in the world battery market, the expected shortage of lithium resources resulting from the ever-increasing demand for LIBs needs to be addressed. This issue has accelerated the search for alternative power sources such as sodium-ion batteries (SIBs), which are emerging as promising candidates to replace LIBs. Sodium is one of the most plentiful resources on earth; thus, SIBs have been extensively studied because of their low cost as well as their competitiveness in terms of energy density versus LIBs. However, the ionic size of Na+ (1.02 Å) is larger than that of Li+ (0.76 Å), which affects the phase stability, transport properties, and interphase formation. The low standard redox potential of sodium (−2.71 V vs. standard hydrogen electrode (SHE)) compared with that of lithium (−3.04 V vs. SHE) is another demerit of SIBs; hence, a high capacity is necessary to compensate for the low operation voltage to approximate the energy density levels of LIBs. Thus far, various types of electrode materials have been suggested as cathode materials for SIBs, including Na2/3MnO2, Na2/3[MxMn1 −x]O2 (M: transition metals), NaCrO2, α-MnO2, and Na3V2PO3. Vanadium oxides (VxOy) is of interest because of the wide range of oxidation states available from +2 (as in VO) to +5 (as in V2O5), which affects the structural, electrochemical, and magnetic properties. This property has led to the application of vanadium oxides in catalysts, chemical sensors, and energy storage devices such as rechargeable lithium batteries. Herein, we introduce new nanosized hollandite-type VO1.75(OH)0.5 as a novel cathode material for Na-ion batteries. Structural investigation based on X-ray diffraction (XRD), neutron diffraction (ND), and Rietveld refinement suggests the presence of numerous vacant sites for Na+ intercalation and structural information on hydrogen in the VO1.75(OH)0.5. All of the possible Na+ sites and tunnel-type Na+ diffusion pathways along the c-axis are confirmed by bond-valence-sum analyses. The nanosized hollandite-type VO1.75(OH)0.5 delivers an unexpectedly high specific capacity of approximately 351 mAh g−1 at 15.5 mA g-1 in the voltage range of 1.0–3.7 V (vs. Na+/Na), which agrees well with the results predicted by first-principles calculations. In addition, combined studies using first-principles calculations and several experimental techniques including in-situ operando X-ray diffraction and ex situ X-ray absorption spectroscopy confirm that the nanosized hollandite-type VO1.75(OH)0.5 undergoes a single-phase reaction with a capacity retention of 71% over 200 cycles. Furthermore, the open structure and nanosized particles of hollandite-type VO1.75(OH)0.5 contribute to its excellent power capability with 56% of the capacity measured at 0.05C being delivered at 7C. These results indicated the outstanding cyclability of Na x VO1.75(OH)0.5 as a promising cathode material for SIBs. We believe that our novel approach will provide insight for the discovery and understanding of novel electrode materials for not only SIBs but also other rechargeable batteries. Figure 1

Results and Discussion We propose P3-K0.54[Co0.5Mn0.5]O2, which is rationally designed as a promising cathode material for high-performance potassium-ion batteries (KIBs). Its composition adopts the use of the valence state of Mn above 3.5+ to minimize the disruptive effect of Jahn–Teller distortion in the MnO6 octahedra during the electrochemical reaction. Unlike other types of layered materials that suffer from the sluggish diffusion of large potassium ions accompanying multi-step voltage profiles, P3-K0.54[Co0.5Mn0.5]O2 delivers a high specific discharge capacity of 120.4 mAh (g-oxide)-1 with smooth charge and discharge curves. First-principles calculations predict an activation barrier energy of ~260 meV for a large K+ diffusion, which is comparable to those observed in conventional layered cathode materials for lithium-ion batteries. As a result, even at 500 mA g-1, P3-K0.54[Co0.5Mn0.5]O2 is able to deliver a high discharge capacity of 78 mAh g-1, which is a retention of 65% versus the capacity obtained at 20 mA g-1. Combination studies using operando X-ray diffraction, the ex-situ X-ray absorption near-edge structure, and first-principles calculations elucidate the nature of the excellent potassium storage mechanism of K0.54[Co0.5Mn0.5]O2. This work provides a new insight for the development of efficient cathode materials for KIBs. Experimental The P3-K0.54[Co0.5Mn0.5]O2 powder was synthesized using a typical combustion method. An aqueous solution was stoichiometrically prepared by dissolving KNO3, Co(NO3)6H2O, Mn(NO3)26H2O, and citric acid as a chelating agent in distilled water. The starting solution was evaporated overnight on a hot plate at 110 °C under constant stirring. Then, the dried powders were further heated to 200 °C for the auto-combustion of the citric acid. The as-received powders were calcined at 500 °C for 3 h to decompose the nitrates, and then re-calcined at 800 °C for 5 h under an air atmosphere at a heating rate of 2 °C min-1. This was followed by slow cooling at a cooling rate of 1 °C min-1. The X-ray diffraction (XRD) with Cu kα radiation were conducted and structural crystallinity analyzed by Rietveld refinement. The particle morphologies and surface of the powders were observed using SEM and TEM. Galvanostatic charge/discharge test was performed using a R2032-type coin cell. The K-storage mechanisms in the charge and discharge processes were examined by means of ex-situ XRD, in operando XRD, and ex-situ X-ray absorption spectroscopy (XAS). Also, the structural variation of P3-K x [Co0.5Mn0.5]O2 was predicted as a function of the K+ content in the structure based on the first-principles calculation. Figure 1

We report a monoclinic Na2.4V2(PO4)3/poly(3,4-ethylenedioxythiophene) (M-NVP/PEDOT) composite as a novel cathode material for sodium-ion batteries. The active materials were homogenously encapsulated by thin layers of the electro-conductive polymer PEDOT, and X-ray diffraction analysis indicated that the final products were crystallized into a monoclinic structure. The composite exhibited much more outstanding power capability and cyclability than pristine M-NVP. The discharge capacity of the M-NVP/PEDOT electrode was maintained up to ~109.8 mA h g-1 at 10C (1C = ~178 mA g-1), which was ~9 times higher than that of pristine M-NVP under the same conditions. Moreover, whereas severe capacity degradation was observed for pristine M-NVP, the M-NVP/PEDOT composite delivered ~81% capacity retention compared with the initial capacity over ~500 cycles at 1C. In particular, the available capacity of M-NVP/PEDOT was ~143.3 mA h g-1, corresponding to ~2.4 mol Na+ (de)intercalation, which is larger than the reversible capacity of rhombohedral Na3V2(PO4)3, at which only 2 mol Na+ can be (de)intercalated. This unexpectedly high capacity of the M-NVP/PEDOT composite was clearly confirmed through combined studies using several experiments and first-principles calculation. Figure 1

In this work, a novel cathode material for Na-ion batteries, Na0.97KFe(SO4)2, was successfully prepared via slow evaporation and a low-temperature process, and its outstanding electrochemical performance was demonstrated. Based on the structural information of KFe(SO4)2 obtained from Rietveld refinement of X-ray diffraction data, the possible atomic sites for Na ions entering the structure were verified using bond-valence sum energy maps. Electrochemical measurements and X-ray absorption near-edge structure analyses revealed that ~0.97 mol Na ions can be reversibly (de)intercalated into the structure via the Fe3+/Fe2+ redox reaction. The average redox potential of Na0.97KFe(SO4)2 was shown to be ~3.27 V (vs. Na+/Na), which is higher than that of other Fe-based phosphates owing to the inductive effect of (SO4)2-. It was verified that the specific capacity of Na0.97KFe(SO4)2 at C/10 was ~85 mA h g-1. At 10C (full charge/discharge in 12 min), ~53% of its capacity measured at C/10 was retained. In addition, up to ~99% of its initial capacity was retained over 200 cycles at 1C with a high coulombic efficiency of 99.3097%, which was attributed to the negligible volume change (~1.8%) during charge/discharge. Figure 1

The structural evolution of multi-component olivines upon delithiation and lithiation was investigated by ex-situ X-ray diffraction. We found that one-phase de/lithiation occurs for multi-component olivines for a fairly large compositional range of transition metal species given that two-phase de/lithiation is generally accepted for olivine cathodes. We discuss the driving force that determines the phase behavior (two-phase reaction vs. one-phase reaction) of olivines in terms of enthalpy of mixing presumably dominated from the electrostatic interaction among cations and the elastic strain stored in the crystal due to the volume change of redox active element. We believe that this fundamental study on the phase behavior of olivines will provide insight into the intrinsic electrochemical properties of olivine electrodes for rechargeable lithium batteries.

We report Na2Fe1.96V0.96(PO4)3 (NFVP) as a new cathode material for Na ion batteries with outstanding cycle life and high power density for the first time. Structural characterization of NFVP was performed using Rietveld refinement of X-ray diffraction and neutron diffraction patterns, which revealed large Na diffusion paths in the structure that enabled rapid Na (de)intercalation without a large volume change. Using X-ray diffraction and X-ray photoelectron spectroscopy with charge/discharge measurements, it was confirmed that based on redox reactions of Fe and V ions, ~2 Na ions are reversibly (de)intercalated from the structure with an average voltage of ~3.1 V (vs. Na+/Na). At 30C, which denotes the rapid charge/discharge, ~74% of the theoretical capacity was delivered. Furthermore, NFVP exhibited an outstanding cycle life of over 2500 cycles (70% retention of the initial capacity), which can be attributed to the low volume change (~1%) during charge/discharge.

Storing the energy from renewable energy sources has become a critical issue in recent years. In order to develop a large-scale energy storage system (ESS) connected to green and sustainable energy beyond the conventional size of batteries, cost effectiveness must be the foremost requirement. For a large-scale ESS, battery electrode materials that are based on earth-abundant, readily available, and low-cost elements are highly desired. In this regard, using electrochemistry that utilizes an iron-based redox reaction combined with Na guest ions would be an optimal choice for such batteries because of the unlimited availability of Na from seawater and the ready availability of iron. However, only a limited number of such materials have been reported to date (NaFePO4, Na2FePO4F, NaFeSO4F, and Na2FeP2O7). 1–6 Recently, we reported a new iron-based mixed-phosphate cathode, Na4Fe3(PO4)2(P2O7), which shows promising electrochemical properties in both Na- and Li-ion cells (H. Kim et al., J. Am. Chem. Soc. 2012, 134, 10369−103722). 7 However, a fundamental understanding of the electrochemical reaction mechanism and structural stability under various conditions, which is essential for further development of this promising cathode, had not yet been achieved. In this report, we clarify the electrochemical reaction of Na4Fe3(PO4)2(P2O7) in Na-ion cells. We discovered that the electrode reaction is governed mainly by a one-phase reaction with a small volumetric change of less than 4%, despite significant distortion of pyrophosphate (P2O7) occurring upon electrochemical cycling. Furthermore, the partially desodiated phases are thermally stable at temperatures up to 530 °C at all states of charge. With its open framework, high voltage (~3.2 V vs. Na), low volumetric change (~4%) and safety characteristics, Na4Fe3(PO4)2(P2O7) should be an outstanding candidate for a Na rechargeable battery electrode. References 1. Moreau, P.; Guyomard, D.; Gaubicher, J.; Boucher. F. Chem. Mater. 2010, 22, 4126–4128. 2. Lee, K. T; Ramesh, T. N.; Nan, F.; Botton, G.; Nazar, L. F. Chem. Mater. 2011, 23, 3593–3600. 3. Ellis, B. L.; Makahnouk, W. R. M.; Makimura, Y.; Toghill, K.; Nazar, L. F. Nat. Mater. 2007, 6, 749–753. 4. Barpanda, P.; Chotard, J.; Recham, N.; Delacourt, C.; Ati, M.; Dupont, L.; Armand, M.; Tarascon, J. –M. Inorg. Chem. 2010, 49, 7401–7413. 5. Barpanda, P.; Ye, T.; Nishimura, S.; Chung, S–C.; Yamada, Y.; Okubo, M.; Zhou, H.; Yamada, A. Electrochem. Commun. 2012, 24, 116–119. 6. Kim, H.; Shakoor, R. A.; Park, C.; Lim, S–Y.; Kim, J–S.; Jo, Y–N.; Cho, W.; Miyasaka, K.; Kahraman, R.; Jung, Y.; Choi, J. –W. Adv. Funct. Mater. 2012, 23, 1147–1155. 7. Kim, H.; Park, I.; Seo, D. –H.; Lee, S.; Kim, S. –W.; Kwon, W. J.; Park, Y. –U.; Kim, C. S.; Jeon, S.; Kang, K. J. Am. Chem. Soc. 2012, 134, 10369−103722.

not Available.

not Available.

not Available.

not Available.

Here we demonstrate that the electrochemical properties of Mn based olivine cathode materials can be significantly improved by small amount of co-dopants, Fe and Mg. While nucleation and growth are important in determining the kinetics of a two-phase reaction based olivine electrode, the presence of Fe and Mg in LiMnPO 4 framework notably enhances the power capability of a LiMnP0 4 electrode providing multiple nucleation sites. The electrochemical activity of an Fe―Mg co-doped Mn olivine cathode material measured by galvanostatic charge/discharge indicates that a discharge capacity of about 140 mAh g ―1 can be obtained at a C/5 rate and this high capacity is retained at even higher current rates (110 mAh g ―1 at 3C), which has yet to be achieved in LiMnPO 4 without nucleation enhancers.

not Available.

The phase stability of Li 1-x MnPO 4 (0≤x≤ 1) is investigated in this study for different Li compositions and temperatures by high temperature X-ray diffraction and electron microscopy. The map of stable phases is determined at temperature ranges between room temperature and 410°C. While pure LiMnPO 4 phase is stable at high temperature, partial phase transformation of MnPO 4 into Mn 2 P 2 O 7 is observed in delithiated phases above 210°C. Electron microscopy study also indicates the instability of the delithiated phase. The morphology of LiMnPO 4 is severely damaged upon delithiation. The instability of the delithiated phase and the phase transformation into Mn 2 P 2 O 7 may imply that safety concerns can be raised regarding the LiMnPO 4 cathode, unlike its Fe counterpart.