Exploring the rate dependence of phase evolution in P2-type Na 2/3 Mn 0.8 Fe 0.1 Ti 0.1 O 2 † Damian Goonetilleke, * a Sunny Wang, a Elena Gonzalo, b Montserrat Galcer ´ an, b Damien Saurel, b Sarah J. Day, c Francois Fauth, d Te ´ ofilo Rojo be and Neeraj Sharma a P2-type Na 2/3 Mn 0.8 Fe 0.1 Ti 0.1 O 2 , a promising high-performance electrode material for use in ambient temperature sodium-ion batteries, is examined using operando and long-term in situ synchrotron X-ray diffraction studies to reveal the structural evolution during battery function. Variable current cycling at current rates as high as 526 mA g 1 (4C) over a wide voltage window (1.5 V to 4.2 V) reveals that the structural transitions of the positive electrode material at higher currents may be suppressed by kinetic limitations which reduce the magnitude of change of the sodium content in the electrode. At low currents, when maximum desodiation is achieved, a collapse in the c lattice parameter is observed as the cell reaches the charged state, however this behaviour is not observed during cycling at higher currents. 1. Introduction Sodium-ion batteries (NIBs) are yet to convincingly match the energy density or specic capacity of lithium-ion batteries (LIBs), however their relatively low cost may prove to be an important factor in driving their adoption. 1,2 Lithium is a rela- tively scarce material and the growth in demand for LIBs means relying on lithium-ion batteries is becoming increasingly impractical and un-economical for large-scale applications. 3–5 Adopters of larger-scale energy storage systems are not as con- cerned with some of the advantages of LIBs, such as their high capacity per unit mass or capacity per unit volume, which may justify the high cost of lithium in compact portable electronics or EVs. 6 However, for large-scale energy storage systems, where saving weight or space is less of a concern, a cheaper NIB system could prove to be a viable alternative if issues with capacity retention, low operating voltages and structural instability of the cathode materials can be overcome. 7,8 The search for commercially feasible NIBs requires nding and optimizing new electrode materials and electrolytes. 9,10 As with LIBs, the capacity of NIBs remains limited by the positive electrode material. A number of positive electrode materials have been reported for NIBs such as layered Na x TMO 2 systems, polyanionic compounds such as sodium phosphates/ uorophosphates, and Prussian blue-type phases. 11–14 More recently organic compounds have also been reported as potential positive electrode materials for NIBs. 15–17 The layered Na x TMO 2 system (where TM is typically a transition metal or combination of transition metals) appears to be the most promising positive electrode and continues to generate a large amount interest from researchers. 18 These sodium metal oxides adopt several polytypes with different stacking arrangements of the metal–oxygen layers, denoted as P-type or O-type depending on whether the Na + co- ordination environment is prismatic or octahedral respectively. The P2-type structure, which exhibits AB-BA oxygen packing, has two trigonal prismatic sites available for Na ions to occupy, labelled Na(1) and Na(2) in Fig. 1. The close proximity of these sites means that they cannot both be occupied simultaneously, and P2-type Na x TMO 2 materials hence tend to be vacancy dominated. 19 Initial investigations into P2-type Na x TMO 2 oxides focused on Na x CoO 2 given the success of its lithium analogue, 11 however more recently iron and manganese oxides have shown better performance and viability. 20–23 Na ions in P2-type phases reside in prismatic sites which share rectangular faces, and allow for relatively easy transport of Na ions through wide passages with a low activation energy, 24,25 compared to O2-type or O3-type structures. 26 The favourable diffusion kinetics have been suggested to account for the superior rate performance observed in P2-type systems. 23,27 The substitution of atoms into the TM layer can also inuence the Na ion mobility and electronic conductivity of the cathode material. 24,28 The diffusion mechanism in P2-type materials is complicated by the presence of a School of Chemistry, UNSW Sydney, Sydney, NSW 2052, Australia. E-mail: d. goonetilleke@unsw.edu.au b CICenergigune, Parque Tecnol´ ogico de ´ Alava, Albert Einstein 48, ED.CIC, 01510, Mi˜ nano, Spain c Diamond Light Source, Harwell Science and Innovation Campus, Didcot OX11 0DE, UK d CELLS – ALBA Synchrotron, E-08290 Cerdanyola del Vall` es, Barcelona, Spain e Departamento de Qu´ ımica Inorg´ anica, Universidad del Pa´ ıs Vasco UPV/EHU, P.O. Box. 644, 48080, Bilbao, Spain † Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ta01366k Cite this: J. Mater. Chem. A, 2019, 7, 12115 Received 4th February 2019 Accepted 16th April 2019 DOI: 10.1039/c9ta01366k rsc.li/materials-a This journal is © The Royal Society of Chemistry 2019 J. Mater. Chem. A, 2019, 7, 12115–12125 | 12115 Journal of Materials Chemistry A PAPER