Nickel-rich materials such as LiNi_1-x-yMn_xCo_yO₂ (NMC) and LiNi_1-x-yCo_xAl_yO₂ (NCA) have become attractive compared to LiCoO₂ due to their lower cost, increased lifetime and increased safety.

Coprecipitation is a common method to synthesize mixed metal hydroxides (M(OH)₂: M = divalent transition metals) as the precursor materials to the lithiated metal oxides. While the syntheses of divalent NMC precursor materials are well understood, the introduction of a trivalent cation, such as Al⁺³, complicates the synthesis and affects the products significantly.¹ In order to balance the charge, an extra anion needs to be incorporated into the layered structure, resulting in the formation of a new layered double hydroxide (LDH) phase.¹ Increases in the Al content of the precursor result in a larger proportion of LDH phase in the material. Most insidious is that the Al atoms are in the LDH phase and not uniformly distributed as a solid solution in the material.

Literature on NCA materials often omit reporting precursor characterization, or obtain commercial precursors that do not have LDH. Even when a group reported precursors with no LDH² and with LDH³ using the same synthesis method, there was only a brief mention about LDH presence, with no discussion as to its formation or removal.

LDH phases have been widely reported in supercapacitor research, even in ­NCA hydroxides.⁴ Unfortunately, there has been little to no work reported on the conversion of LDH phases with trivalent cations to phases with no intercalated molecules. However, there is a large body of work on the 2 known phases of Ni(OH)₂, denoted as α-Ni(OH)₂ and β-Ni(OH)₂.⁵ α-Ni(OH)₂ is analogous to the LDH phase, with water molecules intercalated between layers of Ni(OH)₂; β-Ni(OH)₂ does not have any intercalated molecules. It is known that α-Ni(OH)2, can be converted into β-Ni(OH)2 by a process called chemical ageing.⁵ This is generally performed in concentrated alkaline solutions, usually at higher temperatures.

In this work, [Ni_0.80Co_0.15]_0.95-xAl_0.05+x(OH)₂ (x = 0, 0.05) precursor materials were prepared by the coprecipitation method. The materials with Al contained appreciable amounts of LDH phase. The precursor materials were then washed in a solution of NaOH, filtered and dried. NaOH concentration, initial solution temperature, stirring temperature and stirring time were varied to study their impact on LDH removal. Unwashed and washed samples were characterized by XRD, ICP-OES, and TGA-MS to monitor LDH content, metal ratios and LDH anions. SEM and photographs were also used to monitor morphological and visual changes. Recommended recipes for the production of competitive NCA hydroxide precursors are reported. The competitive hydroxides were reacted with LiOH•H₂O at elevated temperature to create NCA materials with excellent electrochemical behavior.

(1) Zhao, X.; Zhou, F.; Dahn, J. R. J. Electrochem. Soc. 2008, 155, A642–A647.

(2) Duan, J.; Hu, G.; Cao, Y.; Tan, C.; Wu, C.; Du, K.; Peng, Z. J. Power Sources 2016, 326, 322–330.

(3) Duan, J.; Wu, C.; Cao, Y.; Huang, D.; Du, K.; Peng, Z.; Hu, G. J. Alloys Compd. 2017, 695, 91–99.

(4) Wang, X.; Lin, Y.; Su, Y.; Zhang, B.; Li, C.; Wang, H.; Wang, L. Electrochim. Acta 2017, 225, 263–271.

(5) Hall, D. S.; Lockwood, D. J.; Bock, C.; MacDougall, B. R. Proc. R. Soc. A 2014, 471, 20140792(1-65).