Synthesis and Degradation Mechanism of Ni-rich Cathodes for Li-ion Battery


Student thesis: Doctoral Thesis

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Award date23 Sep 2021


Lithium-ion batteries (LIBs) have been widely used in 3C electronic products and electrical vehicles (EVs) in recent years, the annual production of LIBs is expected to increase to 1447 GWh in 2025, tripling from now. At present, the commercial LIBs prefer to select layered Ni-rich cathodes LiNixCoyMn1-x-yO2 (x>0.6) as cathode materials owing to high energy and low cost. However, Ni-rich cathodes suffer from fast capacity fading, which originates from the synthesis approach or intrinsic attributes. In order to promote the application of Ni-rich cathodes, the synthesis of Ni-rich cathodes and the degradation mechanism were studied in this work.

In Chapter 1, the common anode and cathode materials were reviewed, including advantages and disadvantages in the structure or manufacture cost. Then, the synthesis method for Ni-rich cathode materials was introduced, and the drawbacks of the Ni-rich cathodes were concluded. Finally, the research purpose and outline were presented.

In Chapter 2, a new approach was proposed to stabilize Ni-rich cathodes beyond coating and doping. Ni-rich cathodes have high capacity while low stability, searching for new methods to stabilize the structure has never stopped. Herein, Ni-rich cathodes LiNi0.75Co0.14Mn0.11O2, with high capacity and good cycling stability, were successfully obtained by reducing the size of primary particles in the precursor [Ni0.75Co0.14Mn0.11](OH)2. The Ni-rich cathode, synthesized through this approach, delivers a capacity of 206 mAh g-1 and retains 92% and 87.7% for 150 cycles at 0.33 C and 300 cycles at 1 C, respectively. Static and dynamic structural investigations reveal that the modulated precursor enables the Ni-rich cathode to reach a combination of a low Li/Ni mixing ratio and nanosheet primary particles, which realizes the high capacity and high stability in the meantime.

In Chapter 3, the destructive structure transformation of Ni-rich cathodes above 4.4 V was explored and discussed. High energy in situ synchrotron XRD was used to explore the degradation mechanism of Ni-rich cathodes above 4.4 V in this work. The results show that the primary cause for the poor performance is neither the increased volume contraction of the unit cell nor the H3a-H3b phase transformation, it is more likely due to the sharply increased microstrain instead.

In Chapter 4, the long-term cycling capacity fading mechanism was investigated. A Ni-rich cathode (LiNi0.75Co0.14Mn0.11O2) with high capacity (204 mAh g-1) and good rating performance (140 mAh g-1 at 10 C) was synthesized to investigate the capacity degradation mechanism. After long-term cycling, it found that the major capacity fading is due to poor kinetics occurring at the H2-H3 phase transformation region, while there is still an unignorable capacity fading at the H1-M phase transformation region, which may be caused by slight amorphization of the layered structure. This study suggests that besides enhancing the reversibility of H2-H3 phase transformation, suppressing the amorphization process may also be vital to achieving long-term cycling stability for Ni-rich cathodes.

In Chapter 5, the conclusions were presented, and the future outlook was proposed out based on our experimental results.