Novel Material Design of Co-/Ni-rich Cathodes for Advanced Lithium-ion Batteries

Abstract

In our modern society, the demands on lithium-ion batteries (LIBs) with higher energy density are ever increasing to apply this system currently prevailing in portable electronics to large-scale energy storage systems and electric vehicles. Cathode material, the major components of LIBs, could enable smaller and lighter rechargeable batteries theoretically but accomplishing this mission would be tough because it acts as a bottleneck of total energy density of a battery in practical. The feasible ways to boost the energy density of cathode materials could be achieved by widening the operational voltage range of conventional material. However, at higher voltages where the O 2p orbitals gradually hybridizes with the TM 3d orbitals, the peroxide ion O1‒ with high ionic mobility near the surface are especially prone to leaving the cathode particle, disrupting the cathode-electrolyte interface, and the effluent oxygen will react with liquid electrolyte and burn up this scarce resource, leaving voids and reduced transition metals (TM) and resistive cathode-electrolyte interphase behind within cathode electrode. For now, what then happen afterwards inside cathode material are not very clear, but there are theories and practices about mitigating the ill effects, by either (i) suppressing irreversible phase transformations in the bulk by bulk doping or (ii) suppressing surface instabilities, including formation of spinel-phase cathode-electrolyte interphase (CEI) by engineering cathode surface via various coating process. Moreover, the thorough studies on this field is still scarce due to the needs on interdisciplinary research on degradation origin of cathode materials and comprehensive deterioration phenomena in cell system and limitation of evaluation tool. Therefore, in this thesis, fundamental studies on interfacial behavior between cathode and electrolyte have been conducted to contribute to this unattended field. In Chapter 2, we have investigated and unveil the role of dopant within high-voltage LiCoO2. It has turned out bulk doping and surface coating both help, which seems to diverge the degradation mechanism of high-voltage LiCoO2—is it a bulk (otherwise bulk doping should not work) or surface (otherwise surface coating should not work) dominating process or of equal importance from both sides? This is the question we seek to address in the present work, by a systematical study of how a uniform nickel bulk doping helps. The surprising conclusion is this “thought” bulk doping has minimal bulk effect but very profound surface effect to increase the stability of 4.45 V LiCoO2, and all the previous studies were “fooled” by the apparent suppression of reversible high-voltage phase transitions, which we argued has (almost) nothing to do with the degradation. In this sense, while in the present work we do provide one of best practical solution to stabilize 4.45 V LiCoO2 in a simple cost-effective way, we consider our work more to be a conceptual paper, which calls for better understandings of surface chemistry that may be hidden behind robotic practice of bulk doping in battery industry. In Chapter 3, new method to modify both outer surface and interconnected grain boundaries of Ni-rich with high-voltage cycling stability was suggested. This work applies the materials theory of reactive wetting to innovate the state-of-the-art electrode coating techniques for lithium-ion batteries (LIBs). We report a new “coating-plus-infusion” strategy of a refractory boride compound that (i) not only fully covers the surface of the micron-sized secondary particles of Ni-rich layered cathodes (proved by X-ray photoelectron spectroscopy over a large sample area on the order of mm2, offering much better statistics than typical “proof” in the literature by transmission electron microscope over a local area on the order of 100 nm2), (ii) but also infuses into the grain boundaries between the nano-sized primary particles with zero wetting angle during room-temperature synthesis without subsequent heat treatment. Such facile kinetics and resultant irrigated surface-grain-boundary network are enabled by the huge chemical driving force from the strongly hybridized interfacial bonding. It not only ensures ultra-uniform conformal coating/infusion, but also de-activates the labile surface oxygen of the cathodes. As a result, our strategy offers superior electrochemical performance of high-loading (10.5 mg cm−2, ~2.05 mAh cm−2) and high-electrode-density (3.20 g cm−3) cathodes under high-rate (up to 1540 mA g−1, ~7 C) and high-temperature (45°C) conditions, by mitigating the correlated microstructural degradation (stress-corrosion cracking, SCC problem) and side reactions of Ni-rich layered cathodes as well as the cross-over effect on lithium metal anodes (all of them are fully supported by extensive, direct experimental evidences). Beyond technical advances, a consistent unified theory is provided by analyzing the electronic structure of local structural motifs using first-principles calculations, which explains the reactive wetting and suppressed oxygen activity observed experimentally, and the scientific approach can be applied to understand other surface/interface problems in energy materials.