Synthesis and Modifications of Lithium-rich Manganese-based Layered Oxide Cathodes for High Energy Density Lithium-ion Batteries

Abstract

Driven by the rapid development of the electric vehicle (EV) industry, there is an increasing global demand for high energy density and low-cost rechargeable lithium-ion batteries (LIBs). However, developing next-generation LIBs with high energy storage capacity at low cost still remains a formidable challenge. Normally, in a LIB, the anode materials typically offer much higher capacity and lower cost than the cathode materials. Thus, the cathode materials are considered a performance limiting factor in increasing the energy density of the batteries. Among all cathode materials, Li- and Mn-rich layered oxides (LMRs) are particularly notable as one of the most promising candidates. This is due to their utilization of both cation and anion redox reactions, which can lead to substantial increases in energy density. Unfortunately, several issues urgently need to be solved. In detail, at high voltage, the irreversible oxygen redox (OR) process brings about irreversible structural changes and oxygen release, exacerbating the transition metal (TM) migration and phase transition. These factors contribute to a continuous decrease in the capacity and voltage during cycling. In this thesis, we mainly regulate the structure of LMRs to enhance its capacity and voltage stability.

Firstly, we propose a strategy to design the LMR cathode using a one-step precursor hydrothermal treatment to achieve the Mo-doping in the bulk phase and the enrichment of TMs on the surface simultaneously. On the surface, the highly active lone-pair O 2p electrons will be coupled with the d electrons of extra TM atoms. This is beneficial to alleviate the irreversible vigorous OR process and O₂ release. Meanwhile, in the interior of the modified LMR (M-LMR), the electrons between Mn and O are delocalized by surrounding doping Mo atoms which enhances the Mn and O electron cloud overlapping, and forms short, strongly covalent Mn-O bonds at high voltages. Compared to the pristine sample (i.e., P-LMR), the shorter TM-O bonds with a stronger bond energy of the charged M-LMR sample can effectively stabilize the O atom and inhibit the formation of O-O dimers. By employing this electron regulation strategy for both surface and internal structures, reversible structural evolution and suppressed oxygen release of the LMR are observed through in-situ synchrotron X-ray diffraction (XRD) and differential electrochemical mass spectrometry (DEMS). Consequently, the modified LMR shows better capacity stability (95.10%) and lower voltage drop (224 mV) after 100 cycles.

Secondly, we designed and successfully prepared an LMR cathode material with TM ions occupying the interlayer Li sites of Li₂MnO₃. These TM ions, taking advantage of the special O2-stacked atomic arrangement, are located just above or below the Li ions in the honeycomb structure, serving as a “cap” to pin the oxygen ions around the honeycomb Li. This capped-honeycomb structure is persistent after high-voltage cycling. At atomic-scale level, the root cause of the voltage decay is related to the instability of the honeycomb structure in Li₂MnO₃ at high voltage. In high-voltage conditions, the oxygen atoms in the honeycomb structure are destabilized through O-O dimers, leading to irreversible oxygen release and structure degradation. This atomic-level design strategy effectively suppresses irreversible TM migration under high voltage by pinning the honeycomb structure internally. Demonstrated by microscopic examination, scattering characterizations, and first-principles calculations, the cap stabilizes the honeycomb structure and suppresses oxygen release, cation migration, and structure degradation. As a result, our LMR material exhibits negligible voltage decay upon cycling. Additionally, this work involves attacking the root cause of the voltage decay and demonstrated a route to directly stabilize the honeycomb structure in the LMRs.

Thirdly, we systematically investigated the cost-effective and scalable synthesis methods for LMR materials suitable for industrial production—coprecipitation and solid-state sintering synthesis. First, we explored the coprecipitation method to synthesize carbonate microsphere precursors. By adjusting parameters such as ammonia concentration (0.5M) and pH value (8-8.2), we obtained precursors with suitable morphology and composition, which showed uniform particle size, good sphericity, and tightly packed primary particles. Based on this, we optimized the lithium content during calcination to synthesize high-performance LMR materials, achieving a capacity retention rate of 94.3% over 100 cycles and a voltage drop of 1.4 mV/cycle. Additionally, we attempted to streamline the precursor synthesis process by employing a solid-state sintering method to directly synthesize LMR materials. By optimizing the calcination temperature and lithium content, the LMR materials obtained through this method exhibited even better voltage stability, with a drop as low as 0.77 mV/cycle. This advancement reduces obstacles for the battery management system of LMR cathodes, thus facilitating the industrialization process of LMR cathodes.

In conclusion, we systematically explored the synthesis and structural regulation strategies for lithium-rich manganese-based layered oxide cathodes to stabilize its oxygen reaction and inhibit irreversible TM migration. This thesis has guiding significance for the structural design and the development of next-generation high-energy density cathode materials.

Date of Award	1 Aug 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Qi LIU (Supervisor)