In Situ Study of the Gassing Behavior in LiNi0.5Mn1.5O4 Based Full-Cells By Dems and Neutron Imaging

Lithium ion batteries are generally considered to be the technology of choice for hybrid electric and full electric vehicles. However, further enhancement in the energy and power densities of lithium ion batteries is necessary to fulfill the requirements imposed by automotive applications [1, 2].

High voltage LiNi_0.5Mn_1.5O₄ spinel (LNMO) appears to be a very promising cathode-active material in combination with state-of-the-art graphite anodes. This is due, in part, to its low cost, high theoretical specific capacity (147 mAh g^–1) and high nominal operating voltage (4.7 V vs. Li/Li⁺). However, there are severe performance limitations that need to be overcome before commercialization can be contemplated. LNMO/Li half-cells typically show high cycling stability, even at elevated temperatures [3]. Full-cells made of LNMO cathode and graphite anode suffer from capacity fading upon cycling. In recent years, several degradation mechanisms have been identified and correlated with the overall performance. Major issues arise from electrolyte decomposition [4, 5] accompanied by formation of gaseous species [6] and manganese dissolution [7].

In this study, we focus on the in situ gas analysis of LNMO based full-cells during charge and discharge. Two different cell systems, LNMO/graphite and LNMO/LiFePO₄, are investigated. In the LNMO/graphite system both electrodes contribute to the gassing processes. By using delithiated LiFePO₄ as anode, gas evolution on LNMO can be studied separately, because no gaseous byproducts are formed on LiFePO₄. 

The evolving gases were studied using an analysis setup allowing for in situ investigation by means of mass spectrometry (DEMS: differential electrochemical mass spectrometry). A visualization of forming gas bubbles and a quantitative estimation of the formed gas volume in the cell was achieved by in operando neutron imaging. Mechanistic insights into the decomposition processes were gained by correlating the cell potential to the pattern of the evolving gases.

The influence of other electrochemical parameters, like constant voltage steps, charge/discharge current rate and formation processes (i.e., pre-aging of the electrodes at different temperatures), on the gassing behavior and gas composition was also investigated. These effects and their impact on the cycling stability are discussed.

References:

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