Insights into the Performance Enhancement of Co and Mn Doped LiNiO2 Cathodes Using Synchrotron Techniques
Authors: Espinosa Villatoro Erick , Marques Otavio , Cui Zehao , Manthiram Arumugam , Preefer Molleigh , O'Sullivan Daniel , Willis Peyton , Williams Jeffrey , Weker Johanna Nelson 
Published: December 2024

High nickel materials, particularly LiNiO2 (LNO) with energy densities >200 mAh/g, are promising candidates and have attracted considerable interest as cathode materials for a new generation of energy storage devices [1]. However, LNO faces challenges related to low thermal stability and capacity degradation during cycling. To address these issues, several strategies have been employed, including bulk morphology modifications, electrolyte engineering, and the incorporation of metals such as Mn, Co, and Mg [2]. Doping has been identified as an effective approach to mitigate anisotropic variations in lattice parameters during cycling, thereby reducing capacity fading, improving electrochemical performance, structural and thermal stability [3]. Previous research has concluded that certain dopants are necessary to stabilize LNO cathodes over long-term cycling. Cobalt, manganese and aluminum can improve surface stability and reduce oxidative decomposition of the electrolyte. Furthermore, manganese can suppress phase transitions and contribute to higher structural stability [4]. Nonetheless, there is a significant gap in our understanding of many factors that affect the electrochemical performance of LNO cathodes.

We are exploring Co and Mn doped LNO cathodes using advanced synchrotron radiation-based techniques. Using X-ray absorption spectroscopy, the X-ray absorption near edge structure (XANES) reveals the change of oxidation states as a function of the state of charge (SoC) and demonstrates stable electrochemical reversibility for LiNi0.95Co0.05O2 (NC). Notably, the edge energy positions remain stable in discharged states for NC and LiNi0.95Mn0.05O2 (NM). The extended X-ray absorption fine structure (EXAFS) provides insights into the local atomic bonding and coordination of the transition metal of interest. EXAFS reveals that Co stabilizes the local structure even with long-term cycling. Furthermore, combining X-ray transmission with spectroscopy (TXM-XANES) enables detailed chemical and morphological studies of LNO across several SoCs through both operando and post-mortem characterization. LNO, NC and NM cathodes show spherical-shape morphology, and LNO and NC cathodes show crack formation after the first charge. The edge energy positions remain consistent within the electrode with no local (interparticle) heterogeneity, but there is noticeable variation between pristine and charged cathodes. Complementary to this, we are studying the cathode-electrolyte interface (CEI) using laser ablation mass spectroscopy. By analyzing the Li content and other CEI species during charging, we observe significant electrolyte decomposition resulting in the formation of a dense CEI layer [5,6]. Although this layer is initially unstable, it stabilizes after several charge/discharge cycles. Our work provides insight into the challenges of high-energy density, high-nickel cathode design and enhanced cathode performance attributed to the dopants influence.

References

[1] Minkyung Kim, et al., J. Mater. Chem. A, 10 (2022) 12890.

[2] Juho Välikangas, et al., Appl. Sci., 10 (2020) 8988.

[3] Ravindranadh Koutavarapu, et al., Ionics, 26 (2020) 5991-6002.

[4] Zehao Cui, et al. Adv. Energy Mater. 13 (2023) 2203853.

[5] Taehoon Kim, et al., J. Mater. Chem. A, 11 (2023) 221-231.

[6] Patrick Harte, Anal. Bioanal. Chem., 411 (2019) 581–589.