Investigation of K modified P2 Na0.7Mn0.8Mg0.2O2 as a cathode material for sodium-ion batteries

• Published in: CrystEngComm Vol. 21; no. 1; pp. 172 - 181
• Main Authors: Sehrawat, Divya, Cheong, Soshan, Rawal, Aditya, Glushenkov, Alexey M, Brand, Helen E A, Cowie, Bruce, Gonzalo, Elena, Rojo, Teófilo, Naeyaert, Pierre J P, Ling, Chris D, Avdeev, Maxim, Sharma, Neeraj
• Format: Journal Article
• Language: English
• Published: Cambridge Royal Society of Chemistry 01.01.2019
• Subjects: Chemical synthesis; Crystal structure; Discharge; Electrochemical analysis; Electrode materials; Energy dispersive X ray spectroscopy; Energy transmission; Lithium-ion batteries; NMR; Nuclear magnetic resonance; Organic chemistry; Rechargeable batteries; Scanning electron microscopy; Scanning transmission electron microscopy; Sodium-ion batteries; Solid solutions; Solid state; Structural analysis; Synchrotron radiation; Transmission electron microscopy; X ray absorption; X-ray diffraction
• ISSN: 1466-8033
• DOI: 10.1039/c8ce01532e

Sodium-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.