Structure modulation strategies for improved Lithium-rich manganese-based positrode materials

Abstract

The perpetual pursuing for convenient life flourishes the development of electric and electronic products (EEPs). As an indispensable part of portable EEPs, particularly electric vehicles in recent years, rechargeable lithium-related batteries (Li-batteries) have been intensively investigated, developed, and employed in research, commercial processes, and daily life. However, electric vehicles have rigorous requirements on power supply for their regular performance, including considerably high specific energy for onboard storage. Under this circumstance, developing high specific capacity positive electrode (positrode) materials with considerably high (positive) working potential is essential for meeting the urgent demands of high specific energy Li-batteries. As compared to traditional layered positrode materials (for example, LiCoO2, LiNi0.33Co0.33Mn0.33O2, etc.), lithium-rich manganese-based positrode materials, lithium-rich layered oxides (LLOs), have promising specific capacity (exceeds 250 mAh g-1) at sufficiently positive potentials up to 4.5 V vs Li/Li+, satisfying the requirement of next generation high specific energy batteries. The participation of anionic redox species, especially the oxygen redox in the LLOs, completely regulates the extra capacity. However, it should be noted that triggering the oxygen-related activity is followed by irreversible degradation, such as lattice oxygen loss and structural transformation, which are detrimental for maintaining the electrochemical performance of LLOs. Thus, in order to derive high performance LLOs, various modification methods have been employed to suppress those undesired deteriorations, which are reviewed and summarised in this thesis. Based on the revealed mechanisms rooted in the methods, this thesis provides two advanced structure modulation strategies for designing unique LLOs for distinct purposes. The details are listed below:

(1) In order to optimise the electrochemical performance of LLOs, it is important to figure out the connection between various modifications and electrochemical data. Several kinds of modifications like doping, coating, and pore regulation with various conditions are employed to make a comprehensive comparison. In particular, the doping effect of TM ions and main-group ions are separately investigated, which involves the associative effects of both ions. Based on obtained results through systemic characterisations, it is clearly that the structural detail of modified LLO particles exhibits strong correlation with the corresponding electrochemical performance. According to the Rietveld refinement, the variation of structural parameters, especially the phase ratio of C2/m space group, confirms the significant impact on structural insights from external modifications. Except pore regulation, doping and coating can effectively facilitate the uniform distribution of C2/m phase through either heating treatment or surface reconstruction. SEM figures provide complementary evidence on the structural evolution after different modifications. Especially, though pore regulation hardly interrupts the segregation of C2/m phase inside lattice structures, it can effectively reduce the side reaction such as electrolyte decomposition at the interface by shrinking the contact area with electrolytes, which is verified through DEMS measurement. Through aforementioned investigations, a promoted electrochemical performance of LLOs can be realised through three alternative ways, such as trace doping, uniform coating and shrinking pore width, respectively.

(2) To reduce gas evolution and alleviate the accompanied surface degradation, the surface of LLOs is covered with a composite of a Nb/Al co-doping layer, an oxygen vacancy at interlayer and an outermost amorphous Al2O3 layer. Electrochemical measurements combined with corresponding structural characterisations provide persuasive evidence that the surface modification has successfully alleviated detrimental side reactions in association with LLOs, such as oxygen release and surficial phase transformation. Besides, the Li+ behaviour can also be regulated after the surface modification. Benefited from these merits, the structural stability of the modified sample was accordingly much improved, leading to an unprecedented long cycling performance at 1 C from 2.0 V to 4.6 V. To be specific, the modified sample achieved 57% capacity retention after 1000 cycles without a sudden capacity drop. Furthermore, a 20 Ah pouch cell was fabricated to verify the large-scale production and practical application of this surface modified sample. With the assistance of a suitable negative electrode (negatrode), the full pouch cell delivered a high specific energy of 345 Wh kg-1. The promoted structural stability of the positrode was repeated at the practical level, whilst the cell preserved 77.9% of its specific energy after 350 cycles at 0.2 C.

(3) In order to reduce the amount of redundant electrolyte used for LLOs, a facile structural strategy of reconstructing the pore structure is proposed towards small pore widths and hence large cavity to throat ratios, which exerts a bottleneck effect on regulating electrolyte infiltration. Through mathematical simulation and experimental characterisations, the relationship between pore structure evolution, electrode/electrolyte interface (EEI) formation, and generated by-products are comprehensively elucidated. A novel pore-structure-determined mechanism is proposed to account for the distinct electrochemical performance at different electrolyte/capacity ratios, from 3 g/Ah to 1.5 g/Ah. Because of the pore structure with bottleneck effects, the LLOs can deliver unreduced capacity in a lean electrolyte whose electrolyte/capacity ratio was as low as 1.5 g/Ah. Combined with untreated Li foils, the LLOs successfully realised an unmatched discharge specific energy of 606.5 Wh/kg for Li-metal batteries with a 1.6 g/Ah electrolyte/capacity ratio, which further retained 80% capacity and 75.3% energy of this pouch cell upon 70 cycles of discharging at 2.4 A.

In summary, this thesis has introduced two structure modulation strategies for synthesising LLOs with appealing electrochemical performance. The structure-related redox behaviour of oxygen, especially at the EEI is discussed with the assistance of advanced structural characterisations and gas detection devices. With meticulously designed surface and pore structures, LLOs can successfully avoid most, if not all, irreversible oxygen redox and other side reactions at the EEI, subsequently, much improved performances have been achieved, showing long cycling stability and extremely high specific energy, at both research and practical levels.

Date of Award	15 Oct 2023
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Di Hu (Supervisor), George Zheng Chen (Supervisor) & Zhaoping Liu (Supervisor)