Diamond Annual Review 2019/20

64 65 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 1 9 / 2 0 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 1 9 / 2 0 Investigating new electrode materials for sodium-ion batteries Related publication: Wheeler S., Capone I., Day S., Tang C. & Pasta M. Low-Potential Prussian Blue Analogues for Sodium-Ion Batteries: Manganese Hexacyanochromate. Chem. Mater. 31 , 2619 (2019). DOI: 10.1021/acs.chemmater.9b00471 Publication keywords: Prussian blue analogues; Sodium-ion batteries T he lithium-ion (Li-ion) battery powers the modern world - our smartphones and other mobile devices, and even electric vehicles. As we transition to low-carbon transport and energy generation, we will rely more heavily on efficient battery technology. However, lithium is a costly and limited resource. In contrast, sodium is cheap and widely available. Sodium-ion batteries offer the prospect of cost-effective, environmentally-benign batteries, but their development will require newmaterials. Prussian blue analogues (PBAs) have been identified as potential new battery materials. A team of researchers are investigating a new application for the PBA compound, manganese hexacyanochromate, that can be used as an anode material for sodium-ion batteries. They used theHighResolutionPowder Diffractionbeamline (I11) to runpowder X-ray diffraction (XRD) on severalmanganesehexacyanochromate samples. XRD can be used to study the crystal structure and composition of materials. Manganese hexacyanochromate is suited as electrode material in an aqueous electrolyte battery. In these experiments, the scientists dehydrated the samples at several different temperatures, to observe changes in structure andwater content. They were able to distinguish water in two bonding environments clearly. They could selectively remove one and not the other by drying at a specified temperature. These results will help us to understand the structure and composition of the candidate materials, an essential step in developing the efficient batteries of the future. Prussian blue analogues (PBAs) are a family of materials that have an analogous structure but differ in composition. Their crystal structure (Fig. 1) is cubic and has two distinct transition metal sites, R and P, which are bridged by cyanide ligands. R is bonded to 6 carbons in an octahedral arrangement whereas P is bonded to 6 nitrogens in an octahedral arrangement. P and R can be Fe, Mn, Cr, Co among others, and when substituted the structure remains approximately the same.The PBA open-framework structure can host a number of alkali cations on interstitial sites including sodium and potassium, making them interesting as battery materials 1 . To store charge as an electrode material the transition metals change valence state (e.g. Fe 3+ +e - ⇌ Fe 2+ ) and alkali cations insert or are withdrawn from the structure to maintain charge neutrality. The PBA structure is particularly complex because, in addition to the range of composition, there are vacancies and water present within the material (Fig. 1b). These vacancies are the absence of the hexacyanometallate [R(CN) 6 ] n- complex and the fraction of vacancies can be as high as 1/3. Water molecules take two distinct bonding environments; ‘coordinated’ bonded to the P site within vacancies and ‘interstitial’occupying interstitial sites. Most PBA compounds researched for application in batteries have Fe in the R site and have reduction potentials (voltage at which thematerial reacts) that are high and suitable as cathode materials for sodium or potassium-ion batteries. They show excellent performance as cathode materials including exceptional cycle life (number of charge/discharge cycles) and rate capability (time to fully charge or discharge the battery) 2 . However, in the full battery they are coupled to an anode with inferior performance. In this project we are aiming to develop and characterise PBA compounds that have reduction potentials at low voltages that can be used as an anodematerial in combinationwith a PBA cathode 3,4 .The full cell voltage is the difference in voltage between the cathode and the anode. Manganese hexacyanochromate (P = Mn, R = Cr) is a candidate low reduction potential PBA compound. It is easily synthesised in water at room Crystallography Group Beamline I11 temperature from manganese chloride and potassium hexacyanochromate. X-ray diffraction (XRD) is an essential technique to study the structure and composition of the material. Synchrotron powder XRD (I11) was performed on as-synthesised manganese hexacyanochromate and the model refined against the data using the whole-pattern fitting Reitveld method (Fig. 2). The material had a lattice parameter of 10.803920(13) Å. For the purpose of refinement, water molecules are treated as oxygen atoms only as hydrogen has negligible X-ray scattering. Positions of oxygen in the two bonding environments, coordinated and interstitial, could be distinguished and quantified. The occupancy value for the coordinated water was in good agreement with the hexacyanochromate vacancy fraction of 1/3. For the interstitial water position the refined atomic displacement value was relatively large at 0.19 Å 2 . This is the case as there is not a single water position within the subcube but instead multiple positions that are indistinguishable by Reitveld refinement. These positions can be water molecules shifted off the centre position due to the presence of hexacyanochromate vacancies at one or more of the subcube corners, and also from hydrogen bonding within the material. Efforts to further refine these positions was unsuccessful. There is a significant difference in bonding strength between the two types of water. As the material is heated, the interstitial water, more loosely bound, leaves the material first and the coordinated water, more strongly bound, leaves the material at higher temperature. This was studied using thermal gravimetric analysis differential scanning calorimetry (TGA-DSC) coupled to a mass spectrometer (MS). Drying the material at 80 o C was found to remove most of the interstitial water but left most of the coordinatedwater. Synchrotron powder XRD found that the lattice parameter decreased to 10.660688(19) Å, a substantial change. Refined values for the quantity of water present show a decrease of 76% in interstitial water and a decrease of 41% in coordinated water. Drying thematerial at a higher temperature, 150 o C, resulted in amajority of both types of water being removed from the structure. The electrochemical properties of manganese hexacyanochromate was subsequently characterised (Fig. 3). The material had a reduction potential of -0.86 V vs. SHE (standard hydrogen electrode), the lowest reported for any PBA material, and a specific capacity of 63 mAh g -1 . In an aqueous electrolyte the material exhibited extremely fast bulk diffusion of sodium ions, with a Figure 1: The cubic crystal structure of Prussian blue analogues (PBAs). (a) The crystal structure is composed of two transition metal sites (R and P) bridged by cyanide ligands. Within the open- framework structure are interstitial sites that can host alkali cations. (b) The structure contains hexacyanometallate (R(CN) 6 ) vacancies and type bonding environments for water, bonded to P site metals in vacancies and in interstitial sites. calculated diffusion coefficient of 1.9 x10 -6 cm 2 s -1 . Characterisation of the charge storage mechanism showed that the crystalline structure was maintained and there was reversible sodium insertion and chromium redox behaviour (Na 2/3 Mn II [Cr II (CN) 6 ] 2/3 + 2/3Na + + 2/3e - ⇌ Mn II [Cr III (CN) 6 ] 2/3 ). This study, for the first time, characterises a metal hexacyanochromate as an anode material for batteries. This is important as it identifies metal hexacyanochromates as a subset of PBAs for further investigation and improvement. PBA materials are extremely diverse due to their wide composition space and number of tuneable parameters. They are also highly crystalline which makes synchrotron XRD an essential characterisation tool. References: 1. Hurlbutt K. et al. Prussian Blue Analogs as Battery Materials. Joule 2 , 1950 (2018). DOI: 10.1016/j.joule.2018.07.017 2. Jiang L. et al. Building aqueous K-ion batteries for energy storage. Nat. Energy 4 , 495 (2019). DOI: 10.1038/s41560-019-0388-0 3. Pasta M. et al. Full open-framework batteries for stationary energy storage. Nat. Commun. 5 , 3007 (2014). DOI: 10.1038/ncomms4007 4. Firouzi A. et al. Monovalent manganese-based anodes and co-solvent electrolyte for stable low-cost high-rate sodium-ion batteries, Nat. Commun. 9 , 861 (2018). DOI: 10.1038/s41467-018-03257-1 Funding acknowledgement: This publication arises from research funded by the John Fell Oxford University Press Research Fund. We acknowledge financial support from the Henry Royce Institute (through UK Engineering and Physical Science Research Council grant no. EP/ R010145/1) for capital equipment. I.C. S.W. acknowledges financial support from EPSRC. We thank Diamond Light Source for access to beamline I11 (proposal number EE14809-1) Corresponding authors: Dr Samuel Wheeler, Department of Materials, University of Oxford, samuel. wheeler@materials.ox.ac.uk and Prof Mauro Pasta, Department of Materials, University of Oxford, mauro.pasta@materials.ox.ac.uk Figure 2: Physiochemical characterisation of manganese hexacyanochromate (P =Mn, R = Cr). (a) Scanning electron micrograph (SEM) and (b) synchrotron powder X-ray diffraction (XRD) of as-synthesised manganese hexacyanochromate (λ = 0.824525 A). Experimental data (black dots), fitted (red curve) and difference (lower trace) are shown. Figure 3: Electrochemical characterisation of manganese hexacyanochromate in an aqueous sodium-ion electrolyte. (a) Differential capacity plot of manganese hexacyanochromate and manganese hexacyanoferrate (for comparison), (b) and (c) cyclic voltammetry at a range of scan rates and peak current against root of scan rate.