Browsing by Author "Rojo, T"
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- ItemBiphasic P2/O3-Na2/3Li0.18Mn0.8Fe0.2O2: a structural investigation(Royal Society of Chemistry, 2020-12-22) Stansby, JH; Avdeev, M; Brand, HEA; Gonzalo, E; Drewett, NE; Ortiz-Vitoriano, N; Sharma, N; Rojo, TThe P2/O3 layered oxide system is thought to benefit from a synergistic enhancement, resulting from the presence of both phases, which makes it a promising cathode material for Na-ion battery applications. Here, biphasic P2/O3-Na2/3Li0.18Mn0.8Fe0.2O2 is investigated via a combination of neutron and X-ray scattering techniques. Neutron diffraction data indicates that the O3 alkali metal site is fully occupied by Li. Real time operando X-ray diffraction data shows the structural evolution of the composite electrode – at the charged state there is no evidence of O2, OP4 or Z phases. The results presented herein provide new insight into site preference of Li in biphasic materials and highlights the value of utilizing multiple phases to achieve high performance layered cathode materials for sodium battery applications.© The Royal Society of Chemistry 2021
- ItemA comprehensive picture of the current rate dependence of the structural evolution of P2-Na2/3Fe2/3Mn1/3O2(Royal Society of Chemistry, 2015-09-02) Sharma, N; Han, MH; Pramudita, JC; Gonzalo, E; Brand, HEA; Rojo, TCathodes that feature a layered structure are attractive reversible sodium hosts for ambient temperature sodium-ion batteries which may meet the demands for large-scale energy storage devices. However, crystallographic data on these electrodes are limited to equilibrium or quasi-equilibrium information. Here we report the current-dependent structural evolution of the P2-Na2/3Fe2/3Mn1/3O2 electrode during charge/discharge at different current rates. The structural evolution is highly dependent on the current rate used, e.g., there is significant disorder in the layered structure near the charged state at slower rates and following the cessation of high-current rate cycling. At moderate and high rates this disordered structure does not appear. In addition, at the slower rates the disordered structure persists during subsequent discharge. In all rates examined, we show the presence of an additional two-phase region that has not been observed before, where both phases maintain P63/mmc symmetry but with varying sodium contents. Notably, most of the charge at each current rate is transferred via P2 (P63/mmc) phases with varying sodium contents. This illustrates that the high-rate performance of these electrodes is in part due to the preservation of the P2 structure and the disordered phases appear predominantly at lower rates. Such current-dependent structural information is critical to understand how electrodes function in batteries which can be used to develop optimised charge/discharge routines and better materials. © 2015 The Royal Society of Chemistry. This article is Open Access.
- ItemDopant and current rate dependence on the structural evolution of P2-Na2/3Mn0.8Zn0.1M0.1O2 (M=Cu, Ti): an operando study(Wiley, 2021-06-24) Stansby, JH; Sharma, N; Avdeev, M; Brand, HEA; Gonzalo, E; Drewett, NE; Ortiz-Vitoriano, N; Rojo, TVariable current rate operando XRD experiments were performed on the P2- Na2/3Mn0.8Zn0.1Cu0.1O2 composition, which displays promising electrochemical properties. The data reveals the reversible formation of a new and previously undetected ordering reflection upon extraction of Na-ions, and that small compositional alterations may dramatically impact structural evolution and electrochemical properties. For P2- Na2/3Mn0.8Zn0.1Cu0.1O2 at all current rates examined (25, 50 and 100 mA.g−1), comparable structural evolution on charge is observed, but the structural evolution on discharge is shown to be significantly influenced by the current applied during the preceding charge step. For both P2- Na2/3Mn0.8Zn0.1Cu0.1O2 and P2- Na2/3Mn0.8Zn0.1Ti0.1O2 comparable structural evolution is observed only at a slower current rate of 25 mA.g−1. Overall, the structural evolution of these layered materials is shown to be dependent on the cycling history, highlighting the significance of applied current rate during cycling, especially during the initial cycle. © 2021 The Authors.
- ItemHigh-performance P2-phase Na2/3Mn0. 8Fe0. 1Ti0. 1O2 cathode material for ambient-temperature sodium-ion batteries(American Chemical Society, 2015-11-25) Han, MH; Gonzalo, E; Sharma, N; López del Amo, JM; Armand, M; Avdeev, M; Saiz Garitaonandia, JJ; Rojo, THigh-performance Mn-rich P2-phase Na2/3Mn0.8Fe0.1Ti0.1O2 is synthesized by a ceramic method, and its stable electrochemical performance is demonstrated. 23Na solid-state NMR confirms the substitution of Ti4+ ions in the transition metal oxide layer and very fast Na+ mobility in the interlayer space. The pristine electrode delivers a second charge/discharge capacity of 146.57/144.16 mA·h·g–1 and retains 95.09% of discharge capacity at the 50th cycle within the voltage range 4.0–2.0 V at C/10. At 1C, the reversible specific capacity still reaches 99.40 mA·h·g–1, and capacity retention of 87.70% is achieved from second to 300th cycle. In addition, the moisture-exposed electrode reaches reversible capacities of more than 130 and 80 mA·h·g–1 for C/10 and 1C, respectively, with excellent capacity retention. The correlation between overall electrochemical performance of both electrodes and crystal structural characteristics are investigated by neutron powder diffraction. The stability of pristine electrode’s crystallographic structure during the charge/discharge process has been investigated by in situ X-ray diffraction, where only a solid solution reaction occurs within the given voltage range except for a small biphasic mechanism occurring at or below 2.2 V during the discharge process. The relatively small substitution (20%) at the transition metal site leads to stable electrochemical performance, which is in part derived from the structural stability during electrochemical cycling. Therefore, the small cosubstitution (e.g., with Ti and Fe) route suggests a possible new scope for the design of sodium-ion battery electrodes that are suitable for long-term cycling. © 2015 American Chemical Society
- ItemInvestigation of K modified P2 Na 0.7 Mn 0.8 Mg 0.2 O 2 as a cathode material for sodium-ion batteries(Royal Society of Chemistry, 2018-11-19) Sehrawat, D; Cheong, S; Rawal, A; Glushenkov, AM; Brand, HEA; Cowie, BCC; Gonzalo, E; Rojo, T; Naeyaert, PJP; Ling, CD; Avdeev, M; Sharma, NSodium-ion batteries (NIBs) are emerging as a potentially cheaper alternative to lithium-ion batteries (LIBs) due to the larger abundance of sodium and in some cases the similar intercalation chemistry to LIBs. Here we report the solid state synthesized K-modified P2 Na0.7Mn0.8Mg0.2O2 which adopts hexagonal P63/mmc symmetry. The second charge/discharge capacity for the as-prepared material is 115/111 mA h g−1 between 1.5–4.2 V at a current density of 15 mA g−1, which reduces to 61/60 mA h g−1 after 100 cycles. Scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy (STEM-EDS) analysis shows a heterogeneous distribution of K and solid state 23Na NMR illustrates that the presence of K perturbs the local environment of Na within the P2 Na0.7Mn0.8Mg0.2O2 crystal structure. Larger scale X-ray absorption near-edge structure (XANES) data on the K L-edge also illustrate that K is present on the surface of electrodes in preference to the bulk. In situ synchrotron X-ray diffraction (XRD) data illustrates that the P2 structural motif is preserved, featuring a solid solution reaction for most of charge–discharge except at the charged and discharged states where multiple phases are present. The K-modified sample of P2 Na0.7Mn0.8Mg0.2O2 is compared with the K-free samples in terms of both structural evolution and electrochemical performance. © The Royal Society of Chemistry 2019
- ItemMoisture exposed layered oxide electrodes as Na-ion battery cathodes(Royal Society of Chemistry, 2016-11-09) Han, MH; Sharma, N; Gonzalo, E; Pramudita, JC; Brand, HEA; López del Amo, JM; Rojo, TMn-rich layered oxides of P2 Na2/3Mn0.8Fe0.1Ti0.1O2 have been shown to exhibit a remarkably stable electrochemical performance even after exposure to moisture for extended periods of time. Here, a detailed investigation of the electrochemical performance of pristine, protonated, and hydrated electrodes is reported. Neutron powder diffraction and 23Na NMR are employed in order to correlate the overall electrochemical performance of each electrode with that of the as-synthesized crystal structure. The effects of proton and water (or OH) moieties on the Na+ layers are discussed based on the electrochemical performance of each phase. The complete structural evolution of the protonated and pristine P2 Na2/3Mn0.8Fe0.1Ti0.1O2 electrodes during charge/discharge is determined via in situ synchrotron X-ray diffraction. The protonated phase at the potential cut-offs (1.5-4.2 and 2-4 V) and the applied currents used shows a predominantly solid-solution reaction with little evidence of a secondary phase while the pristine phase shows the formation of secondary phases and typically better electrochemical capacities. Therefore, the formation of the secondary phase, in part, enhances capacity in this system. Thus moisture exposure (and subsequent treatment) of generally P2 electrodes can lead to significantly different structural evolution during charge/discharge reactions and hence observed capacities. © 2016 The Royal Society of Chemistry.
- ItemP2-Na2/3Mn0.8M0.1M′0.1O2 (M = Zn, Fe and M′ = Cu, Al, Ti): A detailed crystal structure evolution investigation(American Chemical Society, 2021-05-24) Stansby, JH; Sharma, N; Avdeev, M; Brand, HEA; Johannessen, B; Gonzalo, E; Drewett, NE; Ortiz-Vitoriano, N; Rojo, TIncorporation of various transition metals has been shown to improve the electrochemical performance of Mn-rich Na-ion cathode materials. A greater comprehension of the role of dopant ions, particularly with regard to Mn-rich layered oxides as materials for the positive electrode of Na-ion batteries, is required for their continual development. Here two similar series of Mn-rich P2 cathode materials P2-Na2/3Mn0.8M0.1M′0.1O2 (M = Fe, Zn and M′ = Cu, Al, Ti) are explored, focusing on structural analysis using high-resolution operando synchrotron X-ray diffraction. Notably, under the cycling conditions employed, no P2 to O2 phase transitions toward the charged state were identified for any of the materials investigated. Particularly stable solid solution evolution was observed for P2-Na2/3Mn0.8Zn0.1Cu0.1O2 and P2-Na2/3Mn0.8Zn0.1Al0.1O2 when cycled at 40 mA.g–1 which reflects the electrochemical properties of the materials investigated herein and illustrates that Zn is an excellent choice of dopant for Mn-rich cathode materials. Moreover, the better cyclability of P2-Na2/3Mn0.8Zn0.1Al0.1O2 compared with P2-Na2/3Mn0.8Zn0.1Cu0.1O2 is in keeping with the minimal structural changes observed. This demonstrates that although oxidation state predictions to optimize the initial Mn oxidation state are a good way of initially selecting materials, to truly exploit Mn-rich P2-type materials it is necessary to build up an in-depth understanding of both oxidation states and the associated Jahn–Teller distortion as well as the subtle interplay of synergistic and antagonistic interactions between dopants. Overall, this study illustrates the value of structural investigations to assist in the rational design and validation of novel high-performance materials; the results highlight that the interplay between dopants in addition to the average Mn oxidation state are both crucial considerations when designing high-performance Mn-rich layered oxide materials. © 2021 American Chemical Society
- ItemRate and composition dependence on the structural–electrochemical relationships in P2–Na2/3Fe1–yMnyO2 positive electrodes for sodium-ion batteries(American Chemical Society, 2018-10-02) Dose, WM; Sharma, N; Pramudita, JC; Avdeev, M; Gonzalo, E; Rojo, TStructural–electrochemical compositional evolution of attractive cathode candidates for sodium-ion batteries is illustrated. Varying the Fe/Mn ratio plays a significant role in phase evolution, which ranges from a simple solid solution or two-phase transitions to more complex combinations and sequences of phase transitions dependent on the Na concentration. Further complexity is added by the kinetic limitations placed on the compositions with applied current and associated material utilization. This work provides a standardized set of electrochemical and structural data for members of the Na2/3Fe1–yMnyO2 series, exploring the phase evolution at a selected rate of 15 mA g–1, comparing this with literature data at various current rates, and focusing on the evolution of the y = 0.9 at higher and lower current rates. The y = 0.8 composition shows the highest capacity, while y = 0.9 shows slightly better capacity retention at 15 mA g–1. Structurally, the y = 0.8 features a solid-solution evolution throughout the charge–discharge process, while the y = 0.9 shows a solid solution and two-phase evolution, yet shows better capacity retention. Such studies illustrate how chemical tuning and electrochemical current influences structural evolution with sodium insertion/extraction and how this in turn influences electrochemical performance. © 2018 American Chemical Society
- ItemRate dependent performance related to crystal structure evolution of Na0.67Mn0.8Mg0.2O2 in a sodium ion battery(American Chemical Society, 2015-09-28) Sharma, N; Tapia-Ruiz, N; Singh, G; Armstrong, AR; Pradumita, JC; Brand, HEA; Billaud, J; Bruce, PG; Rojo, TSodium-ion batteries are considered as a favorable alternative to the widely used lithium-ion batteries for applications such as grid-scale energy storage. However, to meet the energy density and reliability that is necessary, electrodes that are structurally stable and well characterized during electrochemical cycling need to be developed. Here, we report on how the applied discharge current rate influences the structural evolution of Na0.67Mn0.8Mg0.2O2 electrode materials. A combination of ex situ and in situ X-ray diffraction (XRD) data were used to probe the structural transitions at the discharged state and during charge/discharge. Ex situ data shows a two-phase electrode at the discharged state comprised of phases that adopt Cmcm and P63/mmc symmetries at the 100 mA/g rate but a predominantly P63/mmc electrode at 200 and 400 mA/g rates. In situ synchrotron XRD data at 100 mA/g shows a solely P63/mmc electrode when 12 mA/g charge and 100 mA/g discharge is used even though ex situ XRD data shows the presence of both Cmcm and P63/mmc phases. The in situ data allows the Na site occupancy evolution to be determined as well as the rate of lattice expansion and contraction. Electrochemically, lower applied discharge currents, e.g., 100 mA/g, produce better capacity than higher applied currents, e.g., 400 mA/g, and this is related in part to the quantity of the Cmcm phase that is formed near the discharged state during a two-phase reaction (via ex situ measurements), with lower rates producing more of this Cmcm phase. Thus, producing more Cmcm phase allows access to higher capacities while higher rates show a lower utilization of the cathode during discharge as less (if any) Cmcm phase is formed. Therefore, this work shows how structural transitions can depend on the electrochemically applied current which has significant ramifications on how sodium-ion batteries, and batteries in general, are analyzed for performance during operation. © 2015 American Chemical Society.
- ItemSodium distribution and reaction mechanisms of a Na3V2O2(PO4)2F electrode during use in a sodium-ion battery(American Chemical Society, 2014-05-13) Sharma, N; Serras, P; Palomares, V; Brand, HEA; Alonso, J; Kubiak, P; Fdez-Gubieda, ML; Rojo, TAmbient temperature sodium-ion batteries are emerging as an exciting alternative to commercially dominant lithium-ion batteries for larger scale stationary applications. In order to realize such a sodium-ion battery, electrodes need to be developed, understood, and improved. Here, Na 3V2O2(PO4)2F is investigated from the perspective of sodium. Reaction mechanisms for this cathode during battery function include the following: a region comprising at least three phases with subtly varying sodium compositions that transform via two two-phase reaction mechanisms, which appears at the lower potential plateau-like region during both charge and discharge; an extended solid solution region for majority of the cycling process, including most of the higher potential plateau; and a second two-phase region near the highest charge state during charge and between the first and second plateau-like regions during discharge. Notably, the distinct asymmetry in the reaction mechanism, lattice, and volume evolution on charge relative to discharge manifests an interesting question: Is such an asymmetry beneficial for this cathode? These reaction mechanisms are inherently related to sodium evolution, which shows complex behavior between the two sodium crystallographic sites in this compound that in turn mediate the lattice and reaction evolution. Thus, this work relates atomic-level sodium perturbations directly with electrochemical cycling. © 2014 American Chemical Society.
- ItemStructural evolution of high energy density V3+/V4+ mixed valent Na3V2O2x(PO4)2F3−2x (x = 0.8) sodium vanadium fluorophosphate using in situ synchrotron X-ray powder diffraction(Royal Society of Chemistry, 2014-01-31) Serras, P; Palomares, V; Rojo, T; Brand, HEA; Sharma, NSodium-ion batteries have become good candidates for energy storage technology. For this purpose it is crucial to search for and optimize new electrode and electrolyte materials. Sodium vanadium fluorophosphates are considered promising cathodes but further studies are required to elucidate their electrochemical and structural behavior. Therefore, this work focuses on the time-resolved in situ synchrotron X-ray powder diffraction study of Na 3V2O2x(PO4)2F 3-2x (x = 0.8) while electrochemically cycling. Reaction mechanism evolution, lattice parameters and sodium evolution, and the maximum possible sodium extraction under the applied electrochemical constraints, are some of the features that have been determined for both a fresh and an offline pre-cycled cell. The reaction mechanism evolution undergoes a solid solution reaction with a two-phase region for the first lower-potential plateau while a predominantly solid solution behavior is observed for the second higher-potential plateau. Lattice and volume evolution is clearly dependent on the Na insertion/extraction mechanism, the sodium occupancy and distribution amongst the two crystallographic sites, and the electrochemical cycling history. The comparison between the fresh and the pre-cycled cell shows that there is a Na site preference depending on the cell and history and that Na swaps from one site to the other during cycling. This suggests sodium site occupancy and mobility in the tunnels is interchangeable and fluid, a favorable characteristic for a cathode in a sodium-ion battery. © 2014 The Royal Society of Chemistry. This article is Open Access.
- ItemStructural evolution of mixed valent (V3+/V4+) and V4+ sodium vanadium fluorophosphates as cathodes in sodium-ion batteries: comparisons, overcharging and mid-term cycling(Royal Society of Chemistry, 2015-09-28) Palomares, V; Serras, P; Brand, HEA; Rojo, T; Sharma, NSodium vanadium fluorophosphates belonging to the Na3V2O2x(PO4)2F3−2x family of compounds have recently shown very good electrochemical performance versus Na/Na+ providing high working voltages (3.6 and 4.1 V) and good specific capacity values. In this work the electrochemical behaviour and structural evolution of two compositions, Na3V2O1.6(PO4)2F1.4 (V3.8+) and Na3V2O2(PO4)2F (V4+), are detailed using time-resolved in situ synchrotron X-ray powder diffraction. For the first time in sodium-ion batteries the effects of overcharging and mid-term cycling are analyzed using this technique. Differences in the composition of both materials lead to different combinations of biphasic and single-phase reaction mechanisms while charging up to 4.3 V and overcharging up to 4.8 V. Moreover, the analysis of particle size broadening of both samples reveals the higher stress suffered by the V4+ compared to the more disordered V3.8+ sample. The more “flexible” structure of the V3.8+ sample allows for maximum sodium extraction when overcharging up to 4.8 V while in the case of the V4+ sample no evidence is shown of more sodium extraction between 4.3 V and 4.8 V. Furthermore, the analysis of both materials after 10 cycles shows the appearance of secondary phases due to the degradation of the material or the battery itself (e.g. electrolyte degradation). This study shows examples of the possible degradation mechanisms (and phases) while overcharging and mid-term cycling which is in turn crucial to making better electrodes, either based on these materials or generally in cathodes for sodium-ion batteries. © The Royal Society of Chemistry 2015. This article is Open Access.
- ItemStructure–electrochemical evolution of a Mn-rich P2 Na 2/3 Fe 0.2 Mn 0.8 O2 Na-ion battery cathode(American Chemical Society, 2017-08-04) Dose, WM; Sharma, N; Pramudita, JC; Brand, HEA; Gonzalo, E; Rojo, TThe structural evolution of electrode materials directly influences the performance of sodium-ion batteries. In this work, in situ synchrotron X-ray diffraction is used to investigate the evolution of the crystal structure of a Mn-rich P2-phase Na2/3Fe0.2Mn0.8O2 cathode. A single-phase reaction takes place for the majority of the discharge-charge cycle at ∼C/10, with only a short, subtle hexagonal P2 to hexagonal P2 two-phase region early in the first charge. Thus, a higher fraction of Mn compared to previous studies is demonstrated to stabilize the P2 structure at high and low potentials, with neither "Z"/OP4 phases in the charged state nor significant quantities of the P′2 phase in the discharged state between 1.5 and 4.2 V. Notably, sodium ions inserted during discharge are located on both available crystallographic sites, albeit with a preference for the site sharing edges with the MO6 octahedral unit. The composition Na∼0.70Fe0.2Mn0.8O2 prompts a reversible single-phase sodium redistribution between the two sites. Sodium ions vacate the site sharing faces (Naf), favoring the site sharing edges (Nae) to give a Nae/Naf site occupation of 4:1 in the discharged state. This site preference could be an intermediate state prior to the formation of the P′2 phase. Thus, this work shows how the Mn-rich Na2/3Fe0.2Mn0.8O2 composition and its sodium-ion distribution can minimize phase transitions during battery function, especially in the discharged state. © 2017 American Chemical Society.
- ItemThe unique structural evolution of the O3-phase Na2/3Fe2/3Mn1/3O2 during high rate charge/discharge: a sodium-centred perspective(John Wiley & Sons, Inc, 2015-08-17) Sharma, N; Gonzalo, E; Pramudita, JC; Han, MH; Brand, HEA; Hart, JN; Pang, WK; Guo, ZP; Rojo, TThe development of new insertion electrodes in sodium-ion batteries requires an in-depth understanding of the relationship between electrochemical performance and the structural evolution during cycling. To date in situ synchrotron X-ray and neutron diffraction methods appear to be the only probes of in situ electrode evolution at high rates, a critical condition for battery development. Here, the structural evolution of the recently synthesized O3-phase of Na2/3Fe2/3Mn1/3O2 is reported under relatively high current rates. The evolution of the phases, their lattice parameters, and phase fractions, and the sodium content in the crystal structure as a function of the charge/discharge process are shown. It is found that the O3-phase persists throughout the charge/discharge cycle but undergoes a series of two-phase and solid-solution transitions subtly modifying the sodium content and atomic positions but keeping the overall space-group symmetry (structural motif). In addition, for the first time, evidence of a structurally characterized region is shown that undergoes two-phase and solid-solution phase transitions simultaneously. The Mn/Fe-O bond lengths, c lattice parameter evolution, and the distance between the Mn/FeO6 layers are shown to concertedly change in a favorable manner for Na+ insertion/extraction. The exceptional electrochemical performance of this electrode can be related in part to the electrode maintaining the O3-phase throughout the charge/discharge process. © 2015 Wiley-VCH Verlag GmbH & Co.