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Choi, Ji Ung,Park, Yun Ji,Jo, Jae Hyeon,Kuo, Liang-Yin,Kaghazchi, Payam,Myung, Seung-Taek American Chemical Society 2018 ACS APPLIED MATERIALS & INTERFACES Vol.10 No.48
<P>Layered Na<SUB>2/3</SUB>MnO<SUB>2</SUB> suffers from capacity loss due to Jahn-Teller (J-T) distortion by Mn<SUP>3+</SUP> ions. Herein, density functional theory calculations suggest Na<SUB>2/3</SUB>[Fe<SUB><I>x</I></SUB>Mn<SUB>1-<I>x</I></SUB>]O<SUB>2</SUB> suppresses the J-T effect. The Fe substitution results in a decreased oxygen-metal-oxygen length, leading to decreases in the <I>b</I> and <I>c</I> lattice parameters but an increase in the <I>a</I> lattice constant. As a result, the capacity retention and rate capability are enhanced with an additional redox pair associated with Fe<SUP>4+/3+</SUP>. Finally, the thermal properties are improved, with the Fe substitution delaying the exothermic reaction and reducing exothermic heat.</P> [FIG OMISSION]</BR>
Choi, Ji Ung,Yoon, Chong Seung,Zhang, Qian,Kaghazchi, Payam,Jung, Young Hwa,Lee, Kug-Seung,Ahn, Do-Cheon,Sun, Yang-Kook,Myung, Seung-Taek The Royal Society of Chemistry 2019 Journal of materials chemistry. A, Materials for e Vol.7 No.1
<P>We investigate the orthorhombic Na0.67[NixMn1−x]O2 (<I>x</I> = 0 and 0.05) cathode materials that provide high capacity for prolonged cycles. X-ray absorption studies revealed that the redox activity of the Mn<SUP>3+/4+</SUP> and Ni<SUP>2+/3+</SUP> pairs is effective in suppressing the Jahn-Teller effect of Mn<SUP>3+</SUP> ions because of the network with Ni<SUP>2+</SUP> ions. This effect influenced the smooth voltage variations in the voltage profile for Na0.67[Ni0.05Mn0.95]O2, whereas several complicated voltage plateaus associated with the first-order phase transition were noticed in Na0.67MnO2. <I>Operando</I> synchrotron X-ray diffraction and transmission microscopy studies confirmed the simplicity of the phase transition for Na0.67[Ni0.05Mn0.95]O2 due to suppression of the Jahn-Teller effect of Mn<SUP>3+</SUP> in the oxide lattice. These findings, along with the capacity retention during prolonged cycling and the acceptable thermal properties, make high-capacity sodium-ion batteries feasible, inexpensive, and safe for energy storage application.</P>