Electrochemical Mechanism Investigation and Performance Optimization of Cathode Materials for Alkali Metal Ion Batteries

Abstract

Due to the explosive technology boom and widespread environmental concerns, the clean energy industry has grown extensively over the last few decades, especially concerning electric vehicles (EVs), which place higher demands on energy storage devices. In fact, rechargeable lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) virtually dominate the power battery market, with LIBs being more commercially mature and SIBs being the research focus for the next generation of batteries. As elaborated by previous research, the energy density, life span, and cost of alkali-ion batteries are greatly influenced by the cathode materials, thus, research on cathode materials plays a vital role in improving the performance of alkali metal ion batteries. To improve the electrochemical performance of cathode materials, it is essential to gain insight into the conformational relationship between structure and performance, particularly regarding the dynamic structural evolutions related to the capacity decay mechanism during cycling. In this regard, in-situ high-energy synchrotron X-ray diffraction (XRD) testing is an advanced characterization tool owing to the great advantage of being well-applied to analyze the dynamic coupling between electrochemical properties and anisotropic lattice changes in two-dimensional (2D) materials during charging and discharging process.

Typically, ternary LiNi_xCoyMn_1-x-yO₂ (NCM) material has been considered to dominate the cathode technology of LIBs for the automotive industry due to its high energy density, guaranteeing a long mileage. Despite the excellent prospect, however, severe capacity decay of the NCM cathodes has prevented this promising material from further success. The mechanism underlying this phenomenon is controversial and has been generally unified as arising from the complex structural changes upon Li-(de)intercalation. Yet, deeper insight has not been provided due to the unclear structural kinetics, especially in cycled NCM cathodes. Here, in-situ high-energy synchrotron XRD measurements were conducted on a typical LiNi_0.5Co_0.2Mn_0.3O₂ (NCM523) cathode that has been operated after 90 cycles and compared with the results collected from fresh NCM532 electrode. It is revealed that the H1-H2 phase transition is irreversible and only occurs in the first cycle. Notably, the c-contraction triggered by the H2-H3 transition becomes even more profound after cycling, which is expected to be the primary cause of intergranular cracks on the electrode. Combined structural evolution results with electrochemical testing and microscopic imaging, the interplays between structural dynamics and performance degradation in NCM532 will be detailed.

SIB is a promising next-in-line battery technology, not only because it possesses a similar mechanism to LIBs to be conveniently adapted to the current battery manufacturing systems but also because it has the natural advantage of abundant resources and extensive distribution. Among various cathode materials of SIBs, sodium layered material as the oxygen redox-based material has been regarded as a more favorable choice given the sufficient utilization of the oxidation/reduction process of cationic/anionic reactions. The additional charges generated from the activated oxygen redox reaction enable considerably increased capacity and energy density of the layered cathode materials beyond that of the cationic redox of transition metals (TMs). Nevertheless, sodium layered material usually suffers from the unrecoverable structure change in the charge and discharge process due to the phase transitions associated with TMO₆ distortions, resulting in fast performance attenuation. Here, Mg²⁺ ions doping was carried out to improve the structural stability of Na_0.72Li_0.24Mn_0.76O₂ cathode (NLM), a typical sodium layered material, as well as to promote oxygen redox to obtain the high-capacity and long life-span sodium cathode materials (NLMM). It is demonstrated by scanning transmission electron microscopy (STEM) that the honeycomb superlattice in the TM layers formed by the ordered arrangement of lithium and manganese is partially disrupted after Mg doping, favoring the stability of cation redox. Soft X-ray absorption spectroscopy (sXAS) and neutron pair distribution function (NPDF) results reveal that oxygen redox is enhanced after Mg doping, resulting in an increase in specific capacity from 220 to 272 mAh/g. Moreover, the results obtained from the in-situ XRD show that the variations in lattice parameters of the NLMM material during cycling are substantially reduced, indicating an effective improvement in structural stability, with a capacity retention of 92% for 100 cycles, a significant improvement compared to 81% for pristine NLM cathode.

Date of Award	18 Aug 2023
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Qi LIU (Supervisor)