In order to increase the energy density of lithium-ion batteries (LIB) one promising approach is rooted in the increase of the nickel content in layered oxide cathode materials (LiNi_xMn_yCo_1-x-yO₂, x ≥ 0.6, LiNMC) for an application as positive electrode due to the high specific discharge capacity at moderate upper cutoff voltages below 4.4 V vs. Li/Li⁺.[1, 2] However, the insufficient cycling stability as well as significant safety concerns restricted this material class from industrial application so far. For that reason, new electrolyte compositions including specifically designed additives are of great importance to enable a sufficient electrochemical performance of Ni-rich NCM materials.[3]

In this work, we introduce a low molecular weight, low cost and non-toxic chemical compound as electrolyte additive for the application in graphite/LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) cells.[4] The addition of only 0.5 wt.% of the additive into the control electrolyte (1M LiPF₆ in EC / EMC 3:7, by weight) significantly increases the reversible capacity as well as the 1^st cycle Coulombic efficiency (CE) improving the capacity retention of the standard full cell system between 2.8 - 4.3 V. Irreversible lithium loss in the negative electrode was found to be the predominant factor for the full cell capacity fade and the poor cycling performance of the control electrolyte. Inductively coupled plasma-mass spectrometry (ICP-MS) and X-ray photoelectron spectroscopy (XPS) analysis reveals the contribution of the electrolyte additive in the solid electrolyte interface (SEI) formation on the negative electrode as well as in the cathode electrolyte interface (CEI) formation on the positive electrode. As a result, less active lithium loss during cycling is present in the additive containing electrolyte which leads to an impressive increase in capacity retention of 77%. In addition, a comprehensive performance study of state of the art electrolyte additives such as triphenylphosphine (TPP), vinylene carbonate (VC) and diphenyl carbonate (DPC) is presented and compared to the invented electrolyte additive, demonstrating the outstanding working ability of this compound in graphite/NMC811 cells. To the best of our knowledge the presented cycling results outperform the so far reported electrolyte additives in the field.

Reference:

[1] J. Kasnatscheew, M. Evertz, R. Kloepsch, B. Streipert, R. Wagner, I. Cekic Laskovic, M. Winter, Energy Technology, 5 (2017) 1670-1679.

[2] H.-J. Noh, S. Youn, C.S. Yoon, Y.-K. Sun, Journal of Power Sources, 233 (2013) 121-130.

[3] A.M. Haregewoin, A.S. Wotango, B.J. Hwang, Energy & Environmental Science, 9 (2016) 1955-1988.

[4] K. Beltrop, S. Klein, R. Nölle, A. Wilken, J.J. Lee, J. Reiter, L. Tao, C. Liang, M. Winter, X. Qi, T. Placke, Chemistry of Materials, (2018), submitted.