The Effect of Annealing on the Structure, Composition and Electrochemistry of NMC811 Coated with Al₂O₃ Using an Alkoxide Precursor

Increasing capacities and lowering costs of the cathode material is a key challenge in lithium ion-battery research. Towards this end, nickel-rich layered oxides of the formula Li[NixMnyCoy]O2 (NMC) with x ≥ 0.8 were developed. Compared to previously used cathodes such as LiCoO2 , nickel-rich NMCs reach lower costs by replacing most of the cobalt with nickel and they enable higher discharge capacities.1 Despite the presence of small amounts of cobalt and manganese as dopants to improve stability and rate-capability, these materials still show fast capacity fade during electrochemical cycling which cannot be overcome by modifying the ratio of their elements.2 New strategies to mitigate this degradation are therefore urgently needed for the use of these materials in practical applications such as electric vehicles.3 Most of the degradation that occurs in nickel-rich NMCs upon cycling starts at the cathode-electrolyte interface via surface reactions,oxygen evolution followed by rock-salt formationand transition metal dissolution.4–6 One way to slow down or stop these processes is by changing the nature of this interface through coatings. Although there is a large number of studies showing the benefits of using them, knowledge on the design of coatings with specific properties is still lacking.7,8 In this work, we develop a new solution-based deposition method for the synthesis of aluminium oxide coatings onto LiNi0.8Mn0.1Co0.1O2 (NMC811) secondary particles (Figure 1) and study the effect of annealing on their structure and electrochemical lifetime as new-generation cathode for lithium-ion batteries. Using energy dispersive X-ray spectroscopy (EDS) and X-ray fluorescence spectroscopy (XRF) we quantify the amount and distribution of aluminium oxide on the cathode particles. By using solid-state nuclear magnetic resonance (SS-NMR) and X-ray photoelectron spectroscopy (XPS), we track changes in the coating phase and composition as a function of annealing temperature. 27Al NMR spectroscopy provides direct evidence of the diffusion of the coating into the bulk of the particles leading to surface-layer doping. Finally, we evaluate the electrochemical performance of the coated materials in half cells using long-term galvanostatic cycling. This work provides insight on the effects of surface coating and doping on battery degradation and shows how, by carefully selecting synthetic conditions, coatings of cathode particles with tailored properties can be prepared. (1) Myung, S.-T.; Maglia, F.; Park, K.-J.; Yoon, C. S.; Lamp, P.; Kim, S.-J.; Sun, Y.-K. Nickel-Rich Layered Cathode Materials for Automotive Lithium-Ion Batteries: Achievements and Perspectives. ACS Energy Letters 2017, 2 (1), 196–223. https://doi.org/10.1021/acsenergylett.6b00594. (2) Noh, H.-J.; Youn, S.; Yoon, C. S.; Sun, Y.-K. Comparison of the Structural and Electrochemical Properties of Layered Li[NixCoyMnz]O2 (x = 1/3, 0.5, 0.6, 0.7, 0.8 and 0.85) Cathode Material for Lithium-Ion Batteries. Journal of Power Sources 2013, 233, 121–130. https://doi.org/10.1016/j.jpowsour.2013.01.063. (3) Kim, J.; Lee, H.; Cha, H.; Yoon, M.; Park, M.; Cho, J. Prospect and Reality of Ni-Rich Cathode for Commercialization. Advanced Energy Materials 2018, 8 (6), 1702028. https://doi.org/10.1002/aenm.201702028. (4) Rinkel, B. L. D.; Hall, D. S.; Temprano, I.; Grey, C. P. Electrolyte Oxidation Pathways in Lithium-Ion Batteries. J. Am. Chem. Soc. 2020, 142 (35), 15058–15074. https://doi.org/10.1021/jacs.0c06363. (5) Wandt, J.; Freiberg, A.; Thomas, R.; Gorlin, Y.; Siebel, A.; Jung, R.; Gasteiger, H. A.; Tromp, M. Transition Metal Dissolution and Deposition in Li-Ion Batteries Investigated by Operando X-Ray Absorption Spectroscopy. J. Mater. Chem. A 2016, 4 (47), 18300–18305. https://doi.org/10.1039/C6TA08865A. (6) Xu, C.; Märker, K.; Lee, J.; Mahadevegowda, A.; Reeves, P. J.; Day, S. J.; Groh, M. F.; Emge, S. P.; Ducati, C.; Layla Mehdi, B.; Tang, C. C.; Grey, C. P. Bulk Fatigue Induced by Surface Reconstruction in Layered Ni-Rich Cathodes for Li-Ion Batteries. Nat. Mater. 2020. https://doi.org/10.1038/s41563-020-0767-8. (7) Shi, Y.; Zhang, M.; Qian, D.; Meng, Y. S. Ultrathin Al2O3 Coatings for Improved Cycling Performance and Thermal Stability of LiNi0.5Co0.2Mn0.3O2 Cathode Material. Electrochimica Acta 2016, 203, 154–161. https://doi.org/10.1016/j.electacta.2016.03.185. (8) Neudeck, S.; Strauss, F.; Garcia, G.; Wolf, H.; Janek, J.; Hartmann, P.; Brezesinski, T. Room Temperature, Liquid-Phase Al2O3 Surface Coating Approach for Ni-Rich Layered Oxide Cathode Material. Chemical Communications 2019, 55 (15), 2174–2177. https://doi.org/10.1039/C8CC09618J. Figure 1