Evaluation of electrochemical performance and redox activity of Fe in Ti doped layered P2-Na0.67Mn0.5Fe0.5O2 cathode for sodium ion batteries
Graphical abstract
Ti doped NFM increase the Na layer thickness in the crystal structure, improve the rate performances, minimized the lattice volume strain, showed better structural stability and lowered charge transfer resistance in these P2-type cathodes.
Introduction
Sodium ion batteries (SIBs) have been largely viewed as a viable challenger to the dominance of lithium ion batteries (LIBs) primarily due to the abundance of global sodium resources, competitive energy densities and attractive low raw material costs [1], [2], [3], [4]. Though SIBs at present cannot completely overshadow LIBs in terms of cost per unit energy stored ($/kWh), the abundance of natural sodium sources when compared to the depleting lithium sources have directed more research into developing energy dense, stable SIBs. For example, for applications requiring a large array of batteries such as grid storage, research and development efforts are largely directed towards developing SIBs in favor of LIBs owing to these advantages. Research into SIBs are generally focused on two major problems; (i) anode development – aimed at developing a stable high capacity anode material with minimal cycling losses and (ii) cathode development – aimed at developing a stable high capacity cathode material that can house more sodium ions per formula unit. In sodium cathode research, several viable cathode candidates are presently being investigated, of which, the layered oxide cathode materials have garnered great attention owing to their high capacities and ease of handling [5]. The layered transition metal oxide cathodes can be broadly classified as P2, O2, P3 and O3 types based on the location of Na+ ion in their lattice structures [6]. Compared to the other types, the P2-type cathode has been widely investigated owing to the fast charging capabilities, facilitated by the fast Na+ diffusion between the TMO2 slabs due to open prismatic diffusion pathways in the crystal structure [7,8]. Major examples of the widely investigated P2-type layered cathodes include: (i) NaFe0.5Mn0.5O2 [9], (ii) NaxCoO2 [10], (iii) Na0.78Ni0.23Mn0.69O2 [11] and (iv) NaxVO2 [12].
Among the P2-type cathodes, the Na0.66Fe0.5Mn0.5O2 (NFM) has shown immense promise because of high specific capacity of 190mAh/g (resulting in higher energy density of 520 mW/g) on par with mainstream LIB cathodes (LiNi0.8Mn0.1Co0.1O2 [13]). Additionally, the natural abundance of Fe, Mn on earth's crust resulting in lower raw material costs is also an added advantage [9] for these cathode materials. Though, NFM cathode is highly attractive, owing to the higher specific capacity obtained through the redox active of Mn3+and Fe3+ ions in the composition, the cathode material suffers from rapid degradation in capacity due to the detrimental phase transformations from P2 to OP4 to P’2 during Na intercalation/de-intercalation. In this regard, recently, Li et al. demonstrated how Jahn Teller active Fe4+ in Fe containing cathodes facilitate Fe ion migration to Na layer thereby deteriorating the electrochemical performance [14]. To overcome this degradation in specific capacity, various approaches such as designing tailored nanostructures [15], elemental doping [[16], [17], [18]], and cathode surface treatments [19] have been employed. Of these, elemental doping is a simple and effective approach that can easily be adapted towards large scale manufacturing. Ion doping can also improve the structure stability, restrain the detrimental phase transformation and increase the electrochemical performance in these materials.
In this report, we state a facile synthesis approach to dope Na0.66Fe0.5Mn0.5O2 cathode material with Ti4+ ions (5 at.%) equally at Fe and Mn sites. We performed detailed material and crystallographic investigations and comprehensively investigated the resulting electrochemical performance of the cathode material. Using Rietveld refinement technique on the obtained X-Ray Diffraction (XRD) patterns, we observed an increase in the Na layer thickness in the resulting Na0.67Fe0.475Mn0.475Ti0.05O2 (Ti-NFM) cathode because of incorporation of smaller ionic radius Ti4+ and the higher bonding energy of Ti-O (ΔHf298K = 662 kJ/mol) in the crystal structure. The particle morphologies and elemental distributions were evaluated using Scanning Electron Microscope (SEM) and Electron Dispersive Spectroscopy (EDS). To study the chemical environment in transition metal slab, ex-situ Raman spectroscopy was performed on NFM and Ti doped NFM cathode materials. Ex-situ Mössbauer spectroscopy was used to determine the oxidation states and investigate the migration behavior of Fe ions to Na layer in charged cathodes. Ex-situ XRD analysis was also performed on the cathode material after continuous extraction and insertion of Na+ions to understand the structural stability of the cathode material. Electrochemical Impedance spectroscopy (EIS) was performed before and after cycling to investigate the impedance changes resulting from the electrochemical cycling in the doped cathodes. Overall, the findings reported here highlights the feasibility and effectiveness of Ti4+ doping in the electrochemical performance and advances scientific understanding through mechanistic insights into the cycling behavior of NFM cathodes towards enabling next generation SIBs.
Section snippets
Material synthesis
P2-type Na0.67Fe0.5-x/2Mn0.5-x/2TixO2 (x = 0, 0.01, 0.05, 0.10) was synthesized using the sol-gel technique. CH3COONa (10% excess), Fe(NO3)3•9H2O, (CH3COO)2Mn•4H2O, TiO2 (0.67:0.5:0.5:0 molar ratio for x = 0, 0.67:0.495:0.495:0.01 molar ratio for x = 0.01, 0.67:0.475:0.475:0.05 molar ratio for x = 0.05, 0.67:0.45:0.45:0.10 molar ratio for x = 0.10) and citric acid as chelating agent was dissolved in deionized water. The mixed solution was heated at 80 °C and stirred until the deionized water
Crystallographic and morphological evaluations of undoped and doped Na0.67Fe0.5Mn0.5O2 (NFM)
Crystallographic evaluations of the sol-gel synthesized P2-type Na0.67Fe0.5-x/2Mn0.5-x/2TixO2 (x = 0, 0.01, 0.05, 0.10) were performed using the obtained XRD patterns shown in Fig. S1. The patterns indicate that undoped and doped NFM powder samples had a hexagonal layered structure with P63/mmc space group and shows small unknown impurities at ~14.5° Further, Na0.67Fe0.45Mn0.45Ti0.10O2 shows an additional impurity at ~41.5° which might be attributed to excess Ti dopant. Thus, Rietveld
Conclusion
In conclusion, cobalt-free Na0.66Fe0.5Mn0.5O2 cathode material was doped with Ti4+using a sol gel synthesis process followed by detailed structural analysis and electrochemical performance evaluations. The Ti-NFM cathode increased Na-layer thickness due to smaller ionic size of Ti4+ and higher bonding energy of Ti-O(ΔHf298K = 662 kJ/mol). Consequently, the Ti-NFM showed better cycling stability and rate performances demonstrating better structural stability, good reversibility and low charge
Supporting information
The supplemental information includes Rietveld refinement data, SEM image, Galvanostatic charge discharge curve and Ex-situ Raman Spectroscopy.
CRediT authorship contribution statement
Devendrasinh Darbar: Formal analysis, Writing - original draft. Nitin Muralidharan: Formal analysis, Writing - review & editing. Raphaël P. Hermann: Formal analysis. Jagjit Nanda: Formal analysis, Writing - review & editing. Indranil Bhattacharya: Formal analysis, Writing - original draft.
Declaration of Competing Interest
None.
Acknowledgment
The material synthesis is been carried out at SOLBAT-TTU Energy Research Laboratory, Tennessee Technological University. The major physical and electrochemical characterization were performed at Oak Ridge National Laboratory, Oak Ridge Tennessee. Mössbauer characterization work was sponsored by funding from the US Department of Energy - Energy Efficiency and Renewable Energy (EERE), Vehicle Technologies Office (VTO). Jagjit Nanda is supported by Energy Storage Program, Office of
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