Developing High-Energy Long-Life Ni-Rich Layered Cathode Materials for Lithium Batteries via Microstructure Control

Abstract

The electrification of road transport is progressing at a steady pace to reduce greenhouse gas emissions and mitigate climate change. Since Sony introduced the rechargeable LiCoO2-graphite cell in 1991, Li-ion batteries (LIBs) have become a main power source for EVs as well as portable electronic devices. However, LIBs are beset with significant challenges for replacing gasoline-powered automobiles with fully EVs, because LIBs energy density is still less than a few percent of gasoline. The overall performance and cost of LIBs are largely determined by the cathode material because of its relatively low capacity and poor cycling stability. Therefore, it is not too much to say that the success of EVs depends on the development of high-energy and long-life cathode materials. This dissertation covers a comprehensive study of the capacity fading mechanisms of layered Li[Ni1-x-yCox(Mn and/or Al)y]O2 (NCM, NCA, and NCMA, respectively) cathodes for a wide range of Ni-rich compositions and proposes the optimal designs of Ni-rich layered oxide cathode materials to address the intrinsic poor cycling performance. Herein, these researches are organized into five chapters and perspectives for development advanced Ni-rich cathodes are also discussed in the last sixth chapter. Specifically, Chapter 1 first reviews the current status of lithium-ion battery market and challenges of layered oxide cathode materials for next lithium-ion batteries. Increasing importance of high-performance layered-type cathode materials is highlighted. Briefly, technical challenges for developing Ni-rich layered cathodes are reviewed, which provides justification for the research directions in this dissertation. In Chapter 2, we first investigate a series of Ni-rich NCM and NCA cathodes to characterize fundamental properties and capacity fading mechanism of extremely rich Ni composition. The incorporation of more Ni in the NCM and NCA cathodes has significant benefits in LIB energy density and cost, while cycling and thermal stability are sacrificed due to the high reactivity of Ni. Such a trade-off between energy density and stability becomes increasingly severe with increasing Ni content. The relatively inferior cycling stability of layered oxide cathodes with Ni content above 80% is attributed to the phase transition near the charge-end, causing an abrupt anisotropic shrinkage or expansion, which is suppressed for compositions of below Ni 80%. Residual stress stemming from the phase transition precipitates the internal microcracks and allows the microcracks to propagate to the surface, providing channels for electrolyte penetration and subsequent degradation of the exposed internal surfaces formed by the microcracks. These studies were published in Chemistry of Materials, 30, 1155–1163 (2018); ACS Energy Letters, 4, 1394–1400 (2019); Small, 14, 1803179 (2018). In Chapter 3, we propose novel cathode materials by doping tungsten into a layered structure Firstly, substituting W for Al in the Ni-rich cathode Li[Ni0.885Co0.10Al0.015]O2 produces Li[Ni0.9Co0.09W0.01]O2 (NCW90) with markedly reduced primary particle size. Particle size refinement considerably improves the cathode’s cycling stability. Thus, the proposed NCW90 can deliver high energy density and a long battery lifetime simultaneously, unlike other Ni-rich layered oxide cathodes. This superior cycling stability is mainly attributed to a series of interparticular microfractures that absorb the anisotropic lattice strain caused by a deleterious phase transition near the charge end, thereby improving the cathode’s resistance to fracture. Microcrack suppression preserves the mechanical integrity of the cathode particles during cycling and protects the particle interior from detrimental electrolyte attacks. Secondly, a series of W-doped (1.0, 1.5, and 2.0 mol%) LiNiO2 cathodes was synthesized to systematically investigate the stabilization effect of W doping. In situ X-ray diffraction analysis of the cathodes during charging showed that the W doping protracted the deleterious phase transition to the extent that the two-phase reaction (H2 → H3) merged into a single phase; thus, the phase transition proceeded through a solid-solution-like reaction. The significantly enhanced cycling stability due to W doping largely originated from the reduction of the structural stress associated with the repetitive phase transition caused by the reduction of the abrupt lattice collapse/expansion. The effect of the reduced lattice distortion together with the W-rich surface phase and cation ordering greatly stabilized the LiNiO2 structure during cycling, making W-doped LiNiO2 a candidate material for practical high-energy density cathodes. These proposed cathodes can be developed for next-generation electric vehicles. These studies were published in Advanced Energy Materials, 9, 1902695 (2019); Journal of Materials Chemistry A, 7, 18580–18588 (2019). Chapter 4 demonstrates microstructure modification of Ni-rich layered cathodes induced by boron doping. We have demonstrated that boron doping of Ni-rich Li[NixCoyAl1-x-y]O2 dramatically alters the microstructure of the material. Li[Ni0.885Co0.1Al0.015]O2 is composed of large equiaxed primary particles, whereas a boron-doped Li[Ni0.878Co0.097Al0.015B0.01]O2 cathode consists of elongated particles that are highly oriented to produce a strong, crystallographic texture. Boron reduces the surface energy of the (003) planes, resulting in a preferential growth mode that maximizes the (003) facet. This microstructure modification greatly improves the cycling stability. In addition, a new class of layered cathodes, Li[NixCoyB1-x-y]O2 (NCB), is synthesized. The proposed NCB cathodes have a unique microstructure in which elongated primary particles are tightly packed into spherical secondary particles. The cathodes also exhibit a strong crystallographic texture in which the a–b layer planes are aligned along the radial direction, facilitating Li migration. The microstructure, which effectively suppresses the formation of microcracks, improves the cycling stability of the NCB cathodes. The superior cycling stability of B-doped cathodes clearly indicates the importance of the particle microstructure in mitigating the abrupt internal strain caused by phase transitions in the deeply charged state, which occurs in all Ni-rich layered cathodes. These studies were published in Materials Today, 36, 73–82 (2020); Advanced Energy Materials, 10, 2000495 (2020). In Chapter 5, we follow a more generic and developed a concentration gradient (CG) approach to develop advanced Ni-rich layered cathodes. A multi-compositional particulate cathode in which Ni-rich composition at the particle center is encapsulated by a 1.5-μm-thick CG shell with the outermost surface composition of Ni-poor is synthesized using a differential coprecipitation process. The microscale compositional partitioning at the particle level combined with the radial texturing of the refined primary particles in the CG shell layer protracts the detrimental H2 → H3 phase transition, giving rise to sharp changes in the unit cell dimensions. This protraction, confirmed by in-situ X-ray diffraction and transmission electron microscopy, allows effective dissipation of the internal strain generated upon the H2 → H3 phase transition, markedly improving cycling performance and thermochemical stability as compared to those of the conventional single-composition cathode. In addition, the high-valence doping of a CSG cathode with a trace amount substantially improves its cycling stability while providing manufacturing flexibility. High-valence doping allows precise tailoring of the cathode microstructure through the retardation of cation migration and the inhibition of coarsening by pinning particle boundaries. Thus, the proposed cathode material provides an opportunity for the rational design and development of a wide range of multi-functional cathodes, especially Ni-rich NCM cathodes, by compositionally partitioning the cathode particles and thus optimizing the microstructural response to the internal strain produced in the deeply charged state. These studies were published in Advanced Energy Materials, 9, 1803902 (2019); Advanced Functional Materials, 28, 1802090 (2018); ACS Energy Letters, 6, 216–223 (2021); ACS Energy Letters, 6, 4195–4202 (2021). Lastly, Chapter 6 summarizes all the research accomplishments in this dissertation, and brief perspectives are suggested for further development in Ni-rich layered oxide cathodes, which published in Energy & Environmental Science, 14, 844–852 (2021).