Detailed insight into the mechanism of interaction between electrolyte solvents and Li-ion high voltage electrodes as well as subsequent chemical cross talk is a fundamental requirement for development of improved electrical energy storage (EES) devices. At potentials > 4.2 V, surface reactivity of Li-ion positive electrode materials toward the electrolyte results in electrolyte oxidation accompanied by transition metal dissolution.^[1-4] There is no doubt concerning the crucial role of the transition metal products formed in the gradual degradation of electrochemical performance in Li-ion batteries. Indeed, an impedance rise directly proportional to the concentration of the unknown Mn^II species in the SEI originating from LiMn₂O₄ was observed.^[4]

However, the mechanism of transition metal dissolution from Li[Ni_xMn_yCo_z]O₂positive electrode materials, their transport across the electrolyte, and the effect on function of the cell remain unclear.

 We  have reported that contrary to the disproportionation mechanism proposed 30 years ago, transition metal dissolution occurs via the formation of Ni(II), Co (II/III) and Mn(II/III) coordination complexes at the Li[Ni_xMn_yCo_z]O₂ surface via oxidation of DEC and EC at potentials > 4.2 V. This result in the deposition of a mixture of M(II)(acac)_2, M(II) oxalate, and M(II) carbonate at the anode electrode surface in the SEI.^[1]

In this work, using model systems, we have demonstrated that Li⁺ transport across the solid electrolyte interphases in Li-ion anodes is not only strongly dependent on the concentration but also on the nature of the transition metal species present in the SEI. While only a slight impedance rise is observed in the presence of Mn(II) carbonate and Mn(II) oxalate, the presence of  Mn(II)(acac)₂  and Mn(III)(acac)₃ results in a drastic increase of resistance during cycling (Figure 1). This clearly highlights the peculiar interaction of β-diketone complexes with SEI components pinpointing these species as primary responsible for the observed impedance increase of the graphite anode. A detailed description of possible reaction mechanisms leading to the formation of metal complexes at the Li[Ni_xMn_yCo_z]O₂/organic carbonate electrolyte interfaces together with their effect on the the Li-ion transport across the SEI layer will be presented and discussed.

References

[1] Jarry, A.; Gottis, S.; Yu, Y.-S.; Roque-Rosell, J.; Kim, C.; Cabana, J.; Kerr, J.; Kostecki, R. J. Am. Chem. Soc. 2015, 137(10), 3533–3539.

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

[3]  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.

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

 

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 and by the Nanostructures for Electrical Energy Storage (NEES), an Energy Frontier Research Center (EFRC) funded by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences under award DESC0001160.