Ageing and Post-Mortem Study of LNMO-Based Lithium Ion Battery Pouch Cells

Abstract

Lithium ion batteries are nowadays widely employed in a variety of applications from consumer electronics to electric vehicles. Due to the rising demand for batteries, more affordable and sustainable batteries are needed that exhibit at the same time high energy densities, high power capabilities, long cycle life, and increased safety. One concept to meet these requirements is to develop cobalt-free cathode materials of which the high-voltage spinel LNMO (LiNi0.5Mn1.5O4) is a promising material due to its high energy density and high operating voltage of 4.7 V. However, the high operating voltage has also its drawbacks. Commonly used carbonate-based electrolytes oxidize at high voltages, leading to the continuous formation of a cathode-electrolyte interface (CEI). Additionally, LNMO is prone to transition metal dissolution which not only leads to a reduced stability of the cathode active material but due to migration and deposition of the transition metals on the anode surface also to an altered solid electrolyte interface (SEI) formation and significant impedance rise of the anode.1 Both ageing mechanisms provoke increased lithium loss and high resistances and thus a poor cycling performance.2, 3 In the Horizon 2020 LC-BAT5 project HYDRA, LNMO-based prototype pouch cells are developed and built. Herein, we want to present the results from the comprehensive analysis of their performance and cycling behavior. The influence of upper cut-off voltage, charge and discharge current rate as well as temperature on the degradation are thoroughly investigated by electrochemical methods such as differential voltage analysis and impedance spectroscopy. After cycling, selected cells are opened under argon atmosphere followed by electrochemical characterization as well as physical analysis such as SEM-EDX and XPS in order to get further insights into the underlying ageing mechanisms. We thereby reveal the main degradation mechanisms as a function of the cycling conditions and derive solutions on how to increase the cycle life.1. C. Zhan, T. Wu, J. Lu and K. Amine, Energy Environ. Sci.,11 (2), 243-257 (2018).2. J. Ma, P. Hu, G. Cui and L. Chen, Chem. Mater.,28 (11), 3578-3606 (2016).3. W. Li, Y.-G. Cho, W. Yao, Y. Li, A. Cronk, R. Shimizu, M. A. Schroeder, Y. Fu, F. Zou, V. Battaglia, A. Manthiram, M. Zhang and Y. S. Meng, J. Power Sources,473, (2020).Pseudo-OCV (a) and differential voltage analysis curves (b) of LNMO-graphite pouch cell cycled with 0.3C CCCV charge and 0.1C discharge at 25°C Figure 1