Using a Rotating Ring Disk Electrode to Explore the Effects of Manganese on Solid Electrolyte Interphase

Lithium-ion batteries (LIBs) are currently being used in electric cars due to their high energy density and low self-discharge rate. Although LIBs power electric cars, they are heavy and expensive. Most cathodes in these batteries contain cobalt (such as LiNi_xCo_yAl_zO₂). Manganese oxide based batteries are a promising alternative that provide higher voltage than cobalt oxides. Additionally, manganese is cheap, abundant, and exhibits excellent rate capability. Unfortunately, LIBs with manganese oxide cathodes suffer from accelerated capacity fade [1] due to manganese dissolving from the cathode and disrupting the protective behavior of the solid-electrolyte interphase (SEI). We lack the understanding of the mechanisms by which manganese disrupts the SEI [2].

In order to investigate the impact of manganese on the SEI, we carried out a series of experiments using a rotating ring disk electrode (RRDE) because it allows us to differentiate between kinetic and transport effects. The disk electrode was glassy carbon coated with the cathode material (LiNi_.5Mn_1.5O₄) while the ring was glassy carbon. The SEI was formed on the ring electrode by cycling the ring to low voltage in LP30, a standard battery electrolyte. Meanwhile, the disk was held at high potential to induce electrolyte oxidation and manganese dissolution. The entire assembly was rotated to transport oxidation products, including dissolved manganese, from the disk to ring. After formation of the SEI, ferrocene was added to the electrolyte and oxidized at the disk. Measuring the through-film ferrocenium reduction current at the ring therefore assesses the degree of passivation of the SEI.

Results show that oxidation current on the disk increases at higher potentials. We attribute this current to electrolyte oxidation and manganese dissolution. When the disk is held at high potential, SEI formation on the ring displays a new reduction peak at 1.2V. The peak only appears when the disk is being oxidized at potentials higher than 4.7V, and the height of the peak scales with the potential of the disk electrode. We also observe that electron transfer to ferrocenium through the SEI is less kinetically hindered when the SEI is formed in the presence of electrolyte oxidation products. These observations suggest that dissolved manganese ions are incorporating into the SEI structure and reducing its ability to passivate the electrode. As the voltage on the disk during SEI formation increases, more manganese oxidation occurs. These observations open a window for future work to determine if the effects of manganese on the SEI are predominantly electronic or morphological. Determining the mechanism by which manganese causes the SEI to fail will suggest failure mitigation strategies for materials and system.

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[1] Jung Hyun Kim, Nicholas P W Pieczonka, Zicheng Li, Yan Wu, Stephen Harris, and Bob R. Powell. Understanding the capacity fading mechanism in LiNi0.5Mn 1.5O4/graphite Li-ion batteries. Electrochim. Acta, 90: 556–562, 2013. ISSN 00134686. doi: 10.1016/j.electacta.2012.12.069.

[2] Bhandari, Arihant, and Jishnu Bhattacharya. "Review—Manganese Dissolution from Spinel Cathode: Few Unanswered Questions." Journal of The Electrochemical Society 164.2 (2016): n. pag. Web. 07 July 2017.