Developing an O3 type layered oxide cathode and its application in 18650 commercial type Na-ion batteries

A novel, water-stable and high energy density cathode material Na0.9Cu0.12Ni0.10Fe0.30Mn0.43Ti0.05O2 (NCNFMT) is reported here along with a thorough understanding of structural events during battery operation. Systematic substitutions are carried out, which lead to increase in specific energy densities of this family of cathodes from 274.6 W h kg(cathode)(-1) (NCFM - Na0.9Cu0.22Fe0.30Mn0.48O2) to 304.2 W h kg(cathode)(-1) (NCFMT - Na0.9Cu0.22Fe0.30Mn0.43Ti0.05O2) and finally to 350.7 W h kg(cathode)(-1) (NCNFMT - Na0.9Cu0.12Ni0.10Fe0.30Mn0.43Ti0.05O2). Operando X-ray diffraction reveals phase transformations and ex situ EXAFS shows the evolution of local environments around transition metals during charge/discharge. Monoclinic distortions in the NCFM material during O3-P3 phase transformations are suppressed by Ti4+ substitution leading to improvements in the cycling performance of NCFMT. Cu-O octahedral sites exhibit huge Jahn-Teller distortion: Ni2+ substitution in place of Cu2+ not only leads to more ordered Ni-O, but it also helps extract more Na ions from the O3 cathode structure, thus boosting the capacity while also showing good cycling stability due to the highly reversible bond-length and local environmental changes as revealed by EXAFS analyses. X-ray photoelectron spectroscopy shows a titanium-rich surface for NCFMT and NCNFMT which helps improve water-stability. The capacity retention after 200 cycles at 0.2C is 84%, 96% and 90% for NCFM, NCFMT and NCNFMT respectively. The delivered storage capacities of NCFM, NCFMT and NCNFMT are 21 mA h g(-1), 47 mA h g(-1) and 60 mA h g(-1) respectively at 3C. 18650 type Na-ion batteries using the NCNFMT cathode material against a hard carbon anode are also reported to demonstrate potential scalability of the NCNFMT cathode and efficacy of a 1 M NaBF4 tetraglyme electrolyte system for the first time. 18650 cells deliver a specific energy density of 62 W h kg(total_18650_weight)(-1) with 90% energy efficiency, thus being suitable for large scale energy storage applications.