The continuous optimization of lithium ion batteries regarding the energy density, cycle life and safety has enabled their widespread application various fields, such as for electric vehicles. To satisfy the ever-increasing demands on both high gravimetric and volumetric energy density, high voltage and high capacity materials are being intensively studied. Promising candidates are the Ni-rich layered LiNi_xCo_YMn_1-x-yO₂ materials. High Ni content LiNi_xCo_yMn_1-x-yO₂ materials are attracting more attention in the last years due to their high specific capacity.

Compared to the other commercial cathode materials, e.g. LiFePO₄, LiCoO₂ and Li-rich materials, the high Ni content LiNi_xCo_yMn_1-x-yO₂ materials such as LiNi_0.5Co_0.2Mn_0.3O₂ (NMC-532) possess high specific capacity ca. 200 mAh g^-1 at a cut-off potential of 4.6 V vs. Li/Li⁺.(1)

However, the oxidative decomposition of conventional carbonate-based electrolytes at high voltages ( > 4.5 V vs. Li/Li⁺) and the deterioration of the active material particles result in rapid capacity fading. Different strategies are known to improve the cycle life of the high voltage cathode material NMC-532. One opportunity is to protect the cathode surface by inorganic coatings. Su et al. demonstrated the use of an Al₂0₃ coating which resulted in a significant improvement of the cycle life of NMC-532 by minimizing the structural transformation (2). A further option to improve the cycle life of NMC-532 is to use electrolyte additives (3). On the basis that the added additive decomposed prior to the electrolyte components, a cathode electrolyte interface (CEI) layer is formed on the cathode particles. This CEI layer on the cathode can prevent the electrochemical and chemical oxidation of the electrolyte.

In this work, we present the combination of both an inorganic coating and the use of electrolyte additives to significantly improve the cycle life of NMC-532/graphite LIB full cells at 4.6 V vs. Li/Li⁺.

References:

(1)Yan, G., et al. (2015). "Effects of 1-propylphosphonic acid cyclic anhydride as an electrolyte additive on the high voltage cycling performance of graphite/LiNi0.5Co0.2Mn0.3O2 battery." Electrochimica Acta 166: 190-196.

(2)Su, Y., et al. (2015). "Enhancing the High-Voltage Cycling Performance of LiNi0.5Mn0.3Co0.2O2 by Retarding Its Interfacial Reaction with an Electrolyte by Atomic-Layer-Deposited Al2O3." ACS Applied Materials & Interfaces 7(45): 25105-25112.

(3)Lee, S. H., et al. (2018). "[4,4′-bi(1,3,2-dioxathiolane)] 2,2′-dioxide: A novel cathode additive for high-voltage performance in lithium ion batteries." Journal of Power Sources 378: 112-118.