Mechanism of Formation of Metal Acetylacetonates at the LixNi0.5Mn1.5O4-S/Carbonate Ester Electrolyte Interface

Detailed insight into the mechanism of interaction between organic substances, such as the electrolyte solvents, and electrodes in electrochemical systems is a fundamental requirement for development of improved electrical energy storage (EES) devices. For instance Ni and Mn based high voltage cathode are the most promising candidates for positive electrode. However their surface chemical reactivity is enhanced by their high operating potential, above the upper limit of the thermodynamic stability window of standard organic carbonate-based electrolytes.^[1]

This results in electrolyte oxidation accompanied by transition metal dissolution, responsible for the gradual degradation of electrochemical performance in Li-ion batteries.^[2] Manganese and nickel with a 2+ oxidation state has been observed in the solid electrolyte interphase (SEI) layer on the negative electrode.^[3,4] As the impedance rise is directly proportional to the concentration of the unknown Mn^II species in the SEI^[4] originating from LiMn₂O₄, no doubt exists concerning their crucial role in the observed failure modes in Li-ion batteries. However the mechanism of Mn and Ni dissolution from the Li_xNi_0.5Mn_1.5O_4-δ positive electrode materials, their transport across the electrolyte and the effect on function of the cell remain unclear.

Interestingly, in situ Raman measurements at the  Li_xNi_0.5Mn_1.5O_4-δ spinel electrode/electrolyte interface revealed soluble fluorescent species.^[5] The observed fluorescence increase upon electrolyte oxidation, at potentials above 4.2, which coincides with the transition metal oxidation threshold (Mn³⁺ and Ni²⁺) and corresponding removal of Li⁺ from the LiNi_0.5Mn_1.5O_4-δ host lattice. This strong fluorescence emission associated with manganese and nickel dissolution suggests that the soluble fluorescent compounds are metal ion based. Interestingly, the majority of these electrolyte oxidation products diffuse away into the electrolyte and post mortem diagnostic evaluations of numerous Li-ion cells aged negative electrode reveal strong fluorescence.^[6]

The identification of those species involved in degradation phenomena is a requirement to design rational strategies to enable durable high energy storage devices. For this purpose, we used two spectroscopic techniques: Fluorescence associated with Raman, which allows the detection of a single molecule at a nanosecond timescale and is dependent upon the molecular environment, and X-ray absorption, which highlights structural and oxidation state changes. A detailed description of possible reaction mechanisms leading to the formation of fluorescent metal complexes at a positively charge Li_xNi_0.5Mn_1.5O_4-δ/organic carbonate electrolyte interfaces in a chemical energy storage device will be presented and discussed.

Acknowledgement

This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 under the Batteries for Advanced Transportation Technologies (BATT) Program.

References

[1]  Ellis, B. L.; Lee, K. T.; Nazar L. F., Chemistry of Materials 2010, 22, 691-714.

[2]  Pieczonka, N. P. W.; Liu, Z.; Lu, P.; Olson, K. L.; Moote, J.; Powell, B. R.; Kim, J.-H., The Journal of Physical Chemistry C 2013, 117, 15947-15957.

[3]  Zhan, C.; Lu, J.; Jeremy Kropf, A.; Wu, T.; Jansen, A. N.; Sun, Y.-K.; Qiu, X.; Amine, K., Nat Commun. 2013, 4, 2437.

[4]  Park, M.; Zhang, X.; Chung, M.; Less, G. B.; Sastry, A. M., Chemistry of Materials 2014, 26,3128-3134.

[5]  Norberg, N. S.; Lux, S. F.; Kostecki, R., Electrochemistry Communications 2013, 34, 29-32.

[6]  Kostecki, R.; Norin, L.; Song, X.; McLarnon, F., J. Electrochem. Soc. 2004, 151, A522-A526.