Nano-Crystallites of P2-Type Layered Transition Metal Oxide High Voltage Cathode for Sodium-Ion Battery
Authors: Sengupta Abhinanda , Kumar Ajit , Ahuja Aakash , Barik Gayatree , Lohani Harshita , Kumari Pratima , Mitra Sagar 
Published: November 2022

In this era of transitioning from conventional sources of energy to non-conventional, sodium-ion battery research has been burgeoning as an indigenous solution to energy storage applications, considering the sustainability, cost effectiveness, high availability and a familiar redox chemistry 1. P2-type Na2/3Ni1/3Mn2/3O2 is one of the preeminent cathode for sodium-ion batteries because of their environmental friendliness, open framework, superior specific capacity, higher operating voltage and air-moisture stability. However, rapid capacity decay on charging it to a higher voltage because of P2 to O2 phase transition and a large volume change leading to exfoliation of the layers have impeded the practicability of this as an electrode material for Na-ion battery 2,3,4.
Here in this work, we report the preparation of hexagonal nanocrystals of P2-type Na2/3Ni1/3Mn1/2Ti1/6O2, which is titanium doped in pristine Na2/3Ni1/3Mn2/3O2 via a novel and quick microwave synthesis technique. This provides sharp and clear facets that allows accelerated sodium-ion migration within the crystal during extraction and insertion of Na-ions, making this material a highly efficient cathode. Unlike conventional heating, which requires around 12-20 hours of synthesis time and high energy consumption, microwave radiation induces rapid solid-state reaction that heats the material on molecular level leading to uniform heating, thus retaining the nanocrystallinity of the structure 5. The distinctive hierarchical nanostructure having large surface area could efficiently facilitate the transportation of Na+ ions, fast utilization of active materials, overcome the effect of internal strain generated inside and reduces the pulverization of active materials, thereby restraining the P2 to O2 phase transition at higher voltage 6. Aiding from the combined effect of titanium doping at manganese site and designing hierarchical nanocrystrals of P2-type Na2/3Ni1/3Mn1/2Ti1/6O2, we obtained a rate capability of 145 mAh g-1 at 0.1 C and a prolonged cycling life (87.3% capacity retention after 500 cycles at 1C) within a voltage range of 2.5 – 4.2 V, restraining the P2 to O2 type phase transition at higher potential. The combined analysis of X-ray diffraction, scanning electron microscopy and transmission electron microscopy along with density functional theory (DFT) calculations demonstrated the optimization of the structure and the physical properties of pristine Na2/3Ni1/3Mn2/3O2 and Ti doped structure along with their Bader charge analysis and electronic properties. Further the mechanical integrity of the nano Na2/3Ni1/3Mn1/2Ti1/6O2 and micro Na2/3Ni1/3Mn1/2Ti1/6O2 were analyzed through micro-compression test of the as prepared pellets. The underlying mechanism for the suppression of phase transition in Na2/3Ni1/3Mn1/2Ti1/6O2 was elucidated by ex-situ X-ray diffraction (XRD) and ex-situ Transmission electron microscopy (TEM). The electrochemical kinetics regarding Na+ diffusion coefficient was further studied through galvanostatic intermittent titration technique (GITT). An analytical model was established to probe deeper into the reason for exfoliation and thus, support our hypothesis. In addition, a sodium-ion full cell was constructed by pairing the as-prepared P2-type Na2/3Ni1/3Mn1/2Ti1/6O2 with a hard carbon anode. This modification of P2-type Na2/3Ni1/3Mn1/2Ti1/6O2 nanocrystallites with comprehensive electrochemical performance can be a path breaking, highly efficient cathode material for large-scale energy storage applications.
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