High Performance of Nickel-rich Layered Cathode Materials for Lithium Secondary Batteries

Abstract

Lithium secondary batteries have become a major power source for portable electronic devices, electric vehicles, and efficient energy storage systems due to their high energy density and outstanding cycle life. In spite of successful application of LiCoO2, cathode in Li-ion batteries (LIBs), the cathode is still considered unsatisfactory due to its cost and toxicity. LiNiO2 with theoretical capacity of 275 mAh g−1 is regarded as an alternative cathode for LIBs and the general trend has been to dope LiCoO2 with increasingly large amount of Ni to move towards LiNiO2. However, a high concentration of unstable Ni4+ in the highly delithated LixNiO2 was easily transformed to more stable and insulting NiO phase, leading to high interfacial impedance and thus resulting in poor electrochemical performance. To address the shortcomings of the Ni-rich layered cathodes, Al-Ti double doping, excess lithiation and two-slope full concentration gradient (TSFCG) structure were examined in this study. In chapter 1, Li[Ni0.90Co0.05Mn0.05]O2, Li[Ni0.95Co0.025Mn0.025]O2, and LiNiO2 were synthesized. The cathode delivered over 220mAh g−1 when charged to 4.3 V. Structure of the host materials was probed using transmission electron microscopy (TEM) and selected area diffraction. Although LiNiO2 exhibited the highest reversible capacity after 100 cycles, the morphology of the as-prepared state was destroyed whereas Co and Mn doping helped to stabilize the mechanical integrity of the particles. In chapter 2, to reduce the inactive NiO phase formation, a tiny amount of Ti is intentionally introduced onto both the surface and grain interfaces of LiNi0.8Co0.15Al0.05O2 (NCA) particles. Interfactial structure and chemistry of Ti-doped LiNi0.8Co0.15Al0.05O2 (NCAT) were investigated using TEM coupled with energy-dispersive X-ray spectroscopy, electron energy loss spectroscopy, and X-ray diffraction (XRD) before and after electrochemical cycling. Formation of NiO inactive phase was inhibited for NCAT, so that the electrode delivered higher capacity upon cycling test. Electrochemical impedance analysis was also performed to understand the interfacial behavior of NCAT. In Chapter 3, Li was incorporated into transition metal layers of Ni-rich Li[Ni0.95Co0.05]O2 by formation of a solid solution with Li2MnO3 (layer notation: Li[Li0.33Mn0.67]O2) to promote the formation of tetravalent Mn ions because tetravalent Mn ions can provide significant structural stability. Structural analysis data obtained by Rietveld refinement of the XRD data indicate that the additional Li ions were found in the transition metal layers, indicating presence of tetravalent Mn ions. The tetravalent Mn results, even in small amounts, helped to stabilize the electrochemical performances and thermal properties in the Ni-rich layer cathodes. In Chapter 4, a Li[Ni0.8Co0.06Mn0.14]O2 positive electrode with TSFCG was synthesized. The TSFCG maximizes the Ni concentration in the particle core and the Mn concentration on the particle surface. The TSFCG Li[Ni0.8Co0.06Mn0.14]O2 cathode showed improved overall electrochemical properties (i.e., reversible capacity, cycle life, and rate capability) and thermal stability compared to a conventional cathode Li[Ni0.8Co0.06Mn0.14]O2. Electrochemical impedance spectroscopy showed that the high stability of the outer surface composition is responsible for reduction in surface resistance and charge transfer resistance by decreasing the parasitic reaction with the electrolyte.