Lithium ion batteries (LIBs), as the state-of-the-art energy storage technique, possess high energy and power density. The target to decrease the emission of carbon dioxide, released by the combustion of fossil fuels promotes the market of electric vehicles (EVs) or hybrid electric vehicles (HEVs), resulting in a high demand for LIBs. LIBs are expected to be the most promising candidate to replace the fossil fuels in conventional means of transportation [1].

LiNi_xMn_yCo_1-x-yO₂ (x ≤ 0.5) has been widely applied as the state-of-the-art cathode material for lithium ion batteries, with the application in E-bikes or xEVs. However, the application in competitive xEVs requires high specific capacity as well as high working potential (> 4.5 V vs. Li/Li⁺). Therefore, Ni-rich or high-voltage NMC cathode materials will be established as future cathode materials, offering higher discharge capacities at equivalent cut-off potentials. Certainly, as the operation voltage or Ni content increases, not only the intrinsic stability of the layered oxides decreases due to the higher delithiation degree, but also the oxidative decomposition of the electrolyte by e.g. chemical reaction with highly reactive Ni⁴⁺ species on the cathode surface becomes more severe. These side reactions can promote the loss of active material (Ni⁴⁺ → Ni²⁺) and the formation of a thick layer of decomposition products on the electrode surface, resulting in an overall impedance increase. Furthermore, Ni-rich layered oxides particles tend to form micro-cracks, revealing the pristine active material and further accelerating capacity fading, due to ongoing electrolyte decomposition.

The use of electrolyte additives to prevent these cathode fading mechanisms is one promising approach to improve the capacity retention and cell performance [2,3]. A variety of electrolyte additives to act as a film forming agent to hinder the fading of different cathode materials have been reported in literature so far. These additives are oxidized prior to the blank electrolyte components and in situ form a protective layer on the surface of the electrode [4].

Within this work, several new compounds were synthesized and evaluated as possible electrolyte additives for NMC/graphite cells, to address the previously mentioned fading mechanisms. The novel compounds were characterized towards their reductive and oxidative stability on active electrode materials, as opposed to commonly used inactive materials (e.g. Pt, or glassy carbon).The addition of these nitrogen-based electrolyte additives lead to an increased Coulombic efficiency and enhanced capacity retention during long-term cycling in comparison to the baseline electrolyte. The working mechanism was tried to elucidate using different ex situ analytical techniques. Post-mortem investigations of the extracted electrolyte and the cathode surface were performed to study the cathode electrolyte interphase (CEI) layer formed by the addition of these additives. The improved cycling performance of these additives in LIB full cells can be correlated to the formation of a passivation film on the cathode surface.

References

[1] J.M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359–367.

[2] R. Jung, M. Metzger, F. Maglia, C. Stinner, H.A. Gasteiger, Oxygen Release and Its Effect on the Cycling Stability of LiNi_xMn_yCo_zO₂ (NMC) Cathode Materials for Li-Ion Batteries, J. Electrochem. Soc. 164 (2017) A1361-A1377.

[3] J. Kasnatscheew, M. Evertz, B. Streipert, R. Wagner, S. Nowak, I. Cekic Laskovic, M. Winter, Changing Established Belief on Capacity Fade Mechanisms, J. Phys. Chem. C 121 (2017) 1521–1529.

[4] Y. Dong, B.T. Young, Y. Zhang, T. Yoon, D.R. Heskett, Y. Hu, B.L. Lucht, Effect of Lithium Borate Additives on Cathode Film Formation in LiNi_0.5Mn_1.5O₄/Li Cells, ACS Appl. Mater. Interfaces 9 (2017) 20467–20475.