Scientists found the cause of voltage lag in the cathode material of sodium-ion batteries

The manganese-rich sodium superionic conductor (NASICON) type cathode material in sodium-ion batteries has potential application prospects due to high voltage and abundant raw materials. However, due to the significant voltage lag in the charge/discharge curve, the reversible capacity is low, which hinders its application.

Zhao Junmei, researcher at the Institute of Process Engineering, Chinese Academy of Sciences, and Hu Yongsheng, a researcher at the Institute of Physics, Chinese Academy of Sciences, explained the voltage hysteresis of manganese-rich NASICON cathodes from the crystal structure, and explored practical strategies to overcome this phenomenon. The findings were published July 13 in Nature Energy.

The cathode material determines not only the energy density of the battery, but also its cost. Manganese-rich NASICON cathode materials such as Na3MnZr(PO4)3 and Na3MnTi(PO4)3 have attracted widespread attention to advanced polyanionic cathode materials. However, due to kinetics, manganese-based NASICON cathode diffusion shows limited electrochemical activity in the available electrochemical window, and its essential reasons are not yet known.

Based on the difference in charge-discharge behavior, the researchers defined two types of defects in polyanionic materials, namely intrinsic anti-occupancy defects (IASD) generated during material preparation and derived anti-occupancy defects (DASD) generated by the accompanying charge-discharge process. Combining spectroscopy, structural characterization and theoretical calculations, the IASD in which Mn occupies the vacancy (Mn/Na2_v) of Na2 (Wyckoff position 18e) is captured in the synthesized Na3MnTi(PO4)3 cathode material, which is completely different from the DASD phenomenon exhibited by the Na3VCr(PO4)3 cathode. Based on this, the essential cause of Na3MnTi(PO4)3 voltage hysteresis is revealed: the IASD of Mn/Na2_v blocks the diffusion channel of Na+ ions, resulting in slow diffusion kinetics of Na ions during Mn2+/3+/4+ redox reactions, so voltage polarization and capacity loss occur within the available electrochemical window.

Subsequently, the research team explored practical strategies to overcome this voltage hysteresis. The formation energy of IASD is increased by doping Mo at the transition metal site, thereby reducing the Na2 vacancy occupied by Mn, i.e., reducing the defect concentration. Finally, the reversible specific capacity of Mo-doped Na3MnTi(PO4)3 increased from 82.1 mAh·g-1 to 103.7 mAh·g-1 at 0.1C, and retained 78.7% of the initial capacity (in the voltage range of 2.5-4.2 V) after 600 cycles at 0.5C.


Voltage hysteresis of Na3MnTi(PO4)3 and atomic resolution HADDF-STEM images before and after Mo doping (Photo courtesy of the research team)

The researchers believe that a series of studies is of great significance for a broader understanding of the attenuation mechanism of NASICON-type cathodes, which provides an effective way to develop low-cost and high-energy density sodium battery cathode materials, and will also promote the practical application of manganese-based NASICON-type cathodes.

Liu Yuan, a doctoral student at the Institute of Physics, Chinese Academy of Sciences, and Rong Xiaohui, a distinguished researcher, are the first authors of this paper, and Hu Yongsheng and Zhao Junmei are the corresponding authors. (Source: China Science News Gan Xiao)

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