Both established lithium-ion and emerging sodium- and magnesium-ion battery chemistries require improved cycle and calendar life in order for electric vehicles to gain widespread market penetration. One of the most serious lifetime problems for these nonaqueous batteries is electrolyte degradation from unwanted side reactions. Electrolyte reduction into insoluble products causes formation and growth of the solid electrolyte interphase (SEI) and has been well-studied. However, both oxidation and reduction can also result in soluble gaseous or liquid products, which then can migrate to the opposite electrode and contribute to electrochemical activity. Such inter-electrode ‘communication’ reactions may be detrimental to battery lifetime; for example, dissolution of transition metals from the cathode and their deposition at the anode is known to accelerate capacity fade [1]. However, communication can also be beneficial when harmful products generated from the cathode are consumed by reduction at the anode[2].

Controlling chemical communication between electrodes for improved battery lifetime requires better understanding of the chemical reactions that generate soluble products. Electrochemical methods are particularly advantageous because they can directly assess the redox activity of reaction products. In particular, four-electrode generator-collector measurements such as the rotation ring-disk electrode (RRDE) are commonly used to quantify reaction selectivity in fuel cell catalysis. Despite their utility, these methods are seldom applied to nonaqueous battery research.

In this work, we apply generator-collector measurements and other electroanalytical techniques in order to understand how chemical communication between electrodes affects cell lifetime. In one application of our approach, the RRDE is used to probe mechanisms of transition metal incorporation into the SEI. Fig. 1(a) shows SEI formation on RRDE. The platinum ring was cycled through SEI formation potentials of 3.0-0.1 V while the gold disk electrode was held at a potential of 4.8 V and the entire assembly rotated at 400 rpm. In contrast to a standard battery, the potentials of the upstream `cathode' (disk) and downstream `anode' (ring) are controlled separately, and current does not necessarily balance between the two. For clarity, only the first of three formation cycles are shown. When the upstream disk electrode was coated with particles of LiNi_0.5Mn_1.5O₄ (red curve), SEI formation showed additional peaks corresponding to reduction of metal cations, or formation of metal-catalyzed reduction products.

After SEI formation, 0.01M ferrocene was added to the electrolyte and the disk was held at 3.5V to oxidize ferrocene while the ring was cycled to low potentials for ferrocenium reduction [3]. The ring current was normalized by the disk oxidation current and is shown in Fig. 1(b). After forming SEI on the ring, the through-film mediator reduction is subject to kinetic and transport limitations (blue), in contrast to the clean Pt ring (grey). These limits are greatly decreased when the SEI is formed in the presence of cathode particles (red), demonstrating that the film is less electronically passivating and ionically resistive. Further experiments show that the through-film diffusivity increases by a factor of eight, while the through-film exchange current density increases by several orders of magnitude. These observations suggest that Mn induces both higher porosity due to mechanical cracking or gas evolution as well as electron conduction through metallic Mn centers. Future work will discuss the mechanism of passivation in greater detail and our development of improved methods for studying these and related reactions.

[1] J. A. Gilbert, I. A. Shkrob, and D. P. Abraham, “Transition Metal Dissolution, Ion Migration, Electrocatalytic Reduction and Capacity Loss in Lithium-Ion Full Cells,” J. Electrochem. Soc., vol. 164, no. 2, pp. A389–A399, 2017.

[2] D. J. Xiong, R. Petibon, M. Nie, L. Ma, J. Xia, and J. R. Dahn, “Interactions between Positive and Negative Electrodes in Li-Ion Cells Operated at High Temperature and High Voltage,” J. Electrochem. Soc., vol. 163, no. 3, pp. 546–551, 2016.

[3] M. H. Tang and J. Newman, “Electrochemical Characterization of SEI-Type Passivating Films Using Redox Shuttles,” J. Electrochem. Soc., vol. 158, no. 5, pp. A530–A536, 2011.