J. Cent. South Univ. (2019) 26: 2668-2680

DOI: https://doi.org/10.1007/s11771-019-4204-6

Thermodynamic analysis of Li-Ni-Co-Mn-H₂O system and synthesis of LiNi_0.5Co_0.2Mn_0.3O₂ composite oxide via aqueous process

LI Yun-jiao(李运姣)^{1, 2}, LI Ling(李玲)^{1, 2}, SU Qian-ye(苏千叶)^{1, 2}, LU Wei-sheng(卢伟胜)²,HAN Qiang(韩强)^{1, 2}, LI Lin(李林)¹, CHEN Yong-xiang(陈永祥)^{1, 2},DENG Shi-yi(邓诗易)^{1, 2}, LEI Tong-xing(雷同兴)^{1, 2}

1. School of Metallurgy and Environment, Central South University, Changsha 410083, China;

2. Citic Dameng Mining Industries Limited, Nanning 530028, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019

Abstract: The constructed potential-pH diagrams of Li-Ni (Co, Mn)-H₂O system indicate that the LiNiO₂, LiCoO₂ and LiMnO₂ are thermodynamically stable in aqueous solution within the temperature range of 25-200 °C and the activity range of 0.01-1.00. A predominant co-region of LiNiO₂, LiCoO₂ and LiMnO₂ oxides (Li-Ni-Co-Mncomposite oxide) is found in the Li-Ni-Co-Mn-H₂O potential-pH diagrams, in which the co-precipitation region expands towards lower pH with rising temperature, indicating the enhanced possibility of synthesizing Li-Ni-Co-Mn composite oxide in aqueous solution. The experimental results prove that it is feasible to prepare the LiNi_0.5Co_0.2Mn_0.3O₂ cathode materials (NCM523) by an aqueous routine. The as-prepared lithiated precursor and NCM523 both inherit the spherical morphology of the hydroxide precursor and the obtained NCM523 has a hexagonal α-NaFeO₂ structure with good crystallinity. It is reasonable to conclude that the aqueous routine for preparing NCM cathode materials is a promising method with the guidance of the reliable potential-pH diagrams to some extent.

Key words: aqueous process; potential-pH diagrams; thermodynamics; LiNi_0.5Co_0.2Mn_0.3O₂; cathode materials

Cite this article as: LI Yun-jiao, LI Ling, SU Qian-ye, LU Wei-sheng, HAN Qiang, LI Lin, CHEN Yong-xiang, DENG Shi-yi, LEI Tong-xing. Thermodynamic analysis of Li-Ni-Co-Mn-H₂O system and synthesis of LiNi_0.5Co_0.2Mn_0.3O₂ composite oxide via aqueous process [J]. Journal of Central South University, 2019, 26(10): 2668-2680. DOI: https://doi.org/10.1007/s11771-019-4204-6.

1 Introduction

The potential-pH diagrams which derive from thermodynamic data represent the predominant domains of various species in a system. Generally, the potential-pH diagrams are helpful in predicting the aqueous reactions and the stability regions of species in aqueous solutions under different equilibrium conditions, which are not only widely used in the fields of corrosion technology, electro- deposition, but also in the area of various composite oxides synthesis [1-3].

In recent years, LiNiO₂, LiCoO₂ and LiMnO₂ cathode materials have attracted enormous interests from scholars and automobile industries. LiNi_xCo_yMn_1-x-yO₂ (02, LiCoO₂ and LiMnO₂ materials and are considered the most promising power source for new energy vehicles. Nevertheless, the severe poor electrochemical properties of the LiNi_xCo_yMn_1-x-yO₂ materials hinder their widely commercial application.

Many strategies, such as new synthetic methods, ion doping, coating and structure design, have been proposed by the researchers to improve the electrochemical performances [4-9]. Among them, synthetic methods including co-precipitation method [10], sol-gel method [11-13], spray process [14-17] and hydrothermal synthesis [11, 18] have been regarded as effective routines for advanced lithium ion battery and many aqueous reactions are involved in these synthetic methods, especially for wet-chemical synthesis. Thus, it is important for the stability regions of species in aqueous solutions to control the aqueous reactions.

Wet-chemical synthesis exhibits huge potential in synthesizing the LiNi_xCo_yMn_1-x-yO₂ cathode materials and has been studied by many researchers. However, there are few reports on the thermodynamics of the Li-Ni-Co-Mn-H₂O systems. The further progress of the LiNi_xCo_yMn_1-x-yO₂ cathode materials synthesized by wet-chemical method is slow to some extent due to the lack of the aqueous thermodynamics theories. In this paper, the potential-pH diagrams for Li-Ni-H₂O, Li-Co-H₂O, and Li-Mn-H₂O systems as well as Li-Ni-Co- Mn-H₂O complex systems from 25 to 200 °C were constructed. And the influences of the activity and temperature were discussed in detail. The LiNi_0.5Co_0.2Mn_0.3O₂ cathode material, as an example, was prepared by the hydrothermal synthesis routine followed by heat treatment under the guidance of the obtained potential-pH diagrams. The results show that the as-constructed potential-pH diagrams play a significant role in the synthesis of the Li-Ni (Co, Mn) cathode materials and the high value- added products in nickel and cobalt metallurgical processes via aqueous solution.

2 Construction of potential-pH diagrams

2.1 Thermodynamic calculation

The reactions occurring in the system can be summarized as Eq. (1):

                 (1)

So the Gibbs free energy change △_rG_T for the above reaction at a given temperature can be calculated by Eq. (2):

                (2)

where is the standard Gibbs free energy of the reaction at a certain temperature, kJ/mol; R represents the gas constant, 8.314 J/(mol·K); T is the thermodynamic temperature, K; α is the activity for different species in the solution, such as Mn²⁺, Ni²⁺ and Co²⁺.

It is known that △_rG_T will be equal to zero when a reaction reaches equilibrium. So combined with the Nernst equation (Eq. (3)), we can get the relationship (Eqs. (4)-(6)) of pH and E to construct the E-pH diagram:

                           (3)

1) For reactions that are involved with electrons but are independent of pH, we have

                    (4)

2) For reactions that are involved with both electrons and H⁺, we have

       (5)

3) For reactions that are involved H⁺ but are independent of E, we have

           (6)

where E_T denotes the electrode potential of the reaction, V; z is the number of the electrons participating in the reaction; F refers to the Faraday constant, 96485.34 c/mol.

The standard free energy of the reaction at high temperature is calculated by the given equation:

         (7)

where andare the standard Gibbs free energy of formation of species at T and 298.15 K, respectively. is the value of absolute entropy.

The common species in Li-Ni-H₂O, Li-Co-H₂O and Li-Mn-H₂O systems are chosen from the diagrams reported [4, 19-23], which are cited in the construction of most potential-pH diagrams.

The standard Gibbs free energies and the standard entropies of most species in the Li-Me-H₂O (Me=Ni, Co, Mn) systems are taken from Lange's Handbook [24] and Refs. [22, 23], and the others standard entropies are estimated by empirical relations. For example, the entropies of aqueous oxy-anions are estimated by:

               (8)

where Z is the absolute value of the charge of the ions and n is the number of O atoms excluding those in the hydroxyl groups.

The more details of the methods for estimating the entropy and constructing the potential-pH diagrams are described in our previous publication [2]. The obtained relationships between potential and pH of the reactions considered in the systems at different temperatures of 25, 100 and 200 °C are given in the Appendix.

2.2 Potential-pH diagrams of Li-Ni (Co, Mn)- H₂O system

The potential-pH diagrams of Li-Ni(Co, Mn)-H₂O system, are constructed based on the Section 2.1 and corresponding Appendix. Figure 1 shows the potential-pH diagrams of Li-Ni-H₂O system at activity 1.00 in the temperature range from 25 to 200 °C. Figures 2-4 represent the potential-pH diagrams of Li-Me-H₂O system at 200 °C, in which the activities of all the dissolved species are assumed to be 1.00, 0.10 and 0.01, respectively.

As shown in Figure 1, Ni²⁺ shows up at low pH over a wide potential range, and reacts with hydroxide ions to form Ni(OH)₂ with increasing pH. As the pH further increases to higher alkaline level, Ni(OH)₂ transforms into LiNiO₂. More specifically,the predominant region of LiNiO₂ presents in the right, of the middle of the potential-pH diagram. It suggests that LiNiO₂ composite oxide is thermodynamically stable in aqueous solution. The stability regions of different species in the potential-pH diagrams are significantly affected by temperature. It is obviously observed that the equilibrium lines of Ni(OH)₂/LiNiO₂ move towards lower pH and negative potential with rising temperature, resulting in the enlargement of the stability domain of LiNiO₂. It is greatly possible to synthesize LiNiO₂ at relatively higher temperature.

Figure 1 Potential-pH diagrams of Li-Ni-H₂O system in temperature range from 25 to 200 °C

Figure 2 Potential-pH diagrams of Li-Ni-H₂O system at 200 °C at different activities

Figure 3 Potential-pH diagrams of Li-Co-H₂O system at 200 °C at different activities

Figure 4 Potential-pH diagrams of Li-Mn-H₂O system at 200 °C at different activities

The potential-pH diagram of Li-Ni-H₂O system at 200 °C at different activities is illustrated in Figure 2. The predominant regions of various dissolved species are significantly affected by the activities. Apparently, the stability regions of Ni(OH)₂ and LiNiO₂ enlarge at a high activity, indicating that the higher activities of the dissolved species are beneficial for increasing the stabilities of the Ni(OH)₂ and LiNiO₂ phases in aqueous systems. Noteworthy, the equilibrium line of Ni²⁺/Ni(OH)₂ moves from pH 4.26 to 3.26 with increasing the activity from 0.01 to 1.00, while that of the Ni(OH)₂/HNiO₂^- shifts in an opposite direction (from pH 9.72 to 11.72). Furthermore, all the equilibrium lines of Ni₃O₄/LiNiO₂, Ni(OH)₂/LiNiO₂ and HNiO₂^-/LiNiO₂ move towards lower pH and more negative potential. All of those result in the expanding of the LiNiO₂ stable region. The higher activities of the species greatly facilitate the synthesis of LiNiO₂ composite oxide. GUO et al [23] also confirmed that it is feasible to synthesize the LiNiO₂ in the aqueous solution by soft chemistry method. From the potential-pH diagram, it shows that Ni, Ni²⁺, Ni(OH)₂ and Ni₃O₄ can be chosen as the candidate precursors for the synthesis of LiNiO₂ composite oxide in aqueous solution.

The potential-pH diagrams of the Li-Co-H₂O (Figure 3) and Li-Mn-H₂O (Figure 4) systems are similar to that of the Li-Ni-H₂O system. LiCoO₂ and LiMnO₂ composite oxides are also thermodynamically stable in aqueous solution. It is observed that the LiCoO₂ and LiMnO₂ can be obtained from various Co and Mn compounds at certain pH and potential, respectively. From Figure 3, it shows a large LiCoO₂ stability field, in wide pH and potential ranges. Compared with the stable areas of LiNiO₂ and LiCoO₂, the stability area of LiMnO₂ is a small zone and it locates in the right bottom corner of the diagram (Figure 4). The activities of the dissolved species also play positive roles in the synthesis of LiCoO₂ and LiMnO₂. Above all, the synthesis of LiNiO₂, LiCoO₂ and LiMnO₂ under a low-alkaline environment is thermodynamically possible by enhancing temperature and activities, which are consistent with the previous reports [4, 25-27].

2.3 Potential-pH diagrams of Li-Ni-Co-Mn-H₂O system

Figures 5 and 6 represent the potential-pH diagrams of Li-Ni-Co-Mn-H₂O system at 100 and 200 °C, respectively. The activities of all dissolved species are taken at 1.00.

The predominant co-region of Li-Ni-Co-Mn composite oxide is expressed by the shaded areas. It is specially noted that the co-region of Li-Ni-Co-Mn oxide is determined by the stable areas of LiNiO₂, LiCoO₂, and LiMnO₂ in the given temperature range, especially for the LiNiO₂ and LiMnO₂. Besides, there are also the co-regions of metal ion and metal hydroxide (metal=Ni, Co and Mn). The metal ions (Ni²⁺, Co²⁺ and Mn²⁺) are hydrolyzed to form complex Ni-Co-Mn hydroxide with an increase in pH, which has been confirmed in Ref. [19]. Furthermore, the Ni-Co-Mn hydroxide converts into the Li-Ni-Co-Mn composite oxide with an increase in pH and potential. The predominant co-regions of the Li-Ni-Co-Mn composite oxide (Figures 5 and 6) show a significant expansion as rising temperature, indicating that the synthesis of the Li-Ni-Co-Mn composite oxide is greatly affected by temperature. The higher temperature is obviously favorable to the synthesis of the Li-Ni-Co-Mn composite oxide. Carefully, the predominant co-region of the Li-Ni-Co-Mn composite oxide is a small triangle at 100 °C, which is located at pH above 12.27 and the corresponding potential changes from -0.45 to 0.20 as shown in Figure 5. Interestingly, the predominant co-region enlarges from a triangle to an irregular quadrangle when the temperature increases to 200 °C and the corresponding lowest pH limit also decreases to 9.73 as well as the potential goes down in the range of -0.74-0.19 V. It is suggested that high temperature is much beneficial to synthesizing the Li-Ni-Co-Mn composite oxide.

Figure 5 Potential-pH diagrams of Li-Ni-Co-Mn-H₂O system at 100 °C

Figure 6 Potential-pH diagrams of Li-Ni-Co-Mn-H₂O system at 200 °C

Above all, Li-Ni-Co-Mn compound oxide is thermodynamically stable at a certain pH and potential range in an aqueous solution (i.e., pH>12.27 and potential -0.45 to 0.20 V at 100 °C). Therefore, it is speculated that an aqueous method may be adopted to synthesize the composite oxide LiNi_xCo_yMn_1-x-yO₂ under an alkaline aqueous environment.

3 Experimental confirmation

The predominant region of the Li-Ni-Co-Mn composite oxide is successfully predicted by the constructed potential-pH diagrams in aqueous solution. In order to verify the results, the LiNi_0.5Co_0.2Mn_0.3O₂ cathode material, as an example, has been obtained in aqueous solution by a hydrothermal process followed by heat treatment.

3.1 Sample preparation

The Ni_0.5Co_0.2Mn_0.3(OH)₂ (NCM hydroxide, Xiamen Tungsten Co., Ltd, China) and LiOH·H₂O (analytical grade, Tianjin chemical reagent Co., Ltd, China) were used as the raw materials. 200 g of the (Ni_0.5Co_0.2Mn_0.3)(OH)₂ (NCM hydroxide) and 93.4 g of LiOH·H₂O were mixed and added into an autoclave (Parr 4568 reactor, USA) together with the required amount of deionized water. The autoclave was sealed and heated up to 250 °C and then maintained for 4 h. Then the slurry was discharged and subjected to solid-liquid separation. The solid was dried at 90 °C overnight to obtain the lithiated precursor (LNCM precursor). The precursor was heat-treated at 500 °C for 6 h and then at 900 °C for 10 h in air to obtain the LiNi_0.5Co_0.2Mn_0.3O₂ (NCM523 cathode material).

The crystal structure of the powders was characterized by the Rigaku D/max-2500 X-ray powder diffraction (XRD) using K_α radiation. The morphology of the particles was examined with the scanning electron microscope (SEM) using JEOL JSM-6360LV operated at 20 kV.

3.2 Results and discussion

XRD patterns of the NCM hydroxide, the LNCM precursor and the as-prepared NCM523 cathode material are depicted in Figure 7. The XRD patterns of the NCM hydroxide (Figure 7(a)) is indexed with the typical fingerprint of Me(OH)₂ (Me=Ni, Co, Mn) with a hexagonal α-NaFeO₂ structure and the same as that reported by ZHU [27] and LEE et al [10]. It is evidently observed that the peaks of the LNCM precursor (Figure 7(b)) is quite different from that of the NCM hydroxide, but similar to that of the as-prepared NCM523 cathode material. In addition, there are no LiOH peaks detected. So it is reasonable to deduce that the NCM hydroxide precursor has reacted with LiOH·H₂O in the aqueous solution and transformed into the LNCM composite oxide precursor with a hexagonal α-NaFeO₂ structure. The LNCM oxide precursor shows a broader and low-intensity diffraction peaks, which demonstrates that the material has a low crystallinity and an imperfect crystal structure. Compared to the LNCM oxide precursor (Figure 7(b)), the diffraction peaks of the NCM523 cathode material become sharper and higher, indicating a good crystallinity with a hexagonal α-NaFeO₂ structure. Furthermore, no diffraction peaks of impurity or other phases are detected, suggesting a pure phase structure. In conclusion, the LNCM oxide precursor with α-NaFeO₂ hexagonal structure but a low crystallinity has been synthesized via an aqueous process with the guidance of the constructed potential-pH diagrams, indicating the reliability of the constructed potential-pH diagrams.

Figure 7 XRD patterns of NCM hydroxide precursor (a), LNCM precursor (b) and NCM523 cathode materials (c)

Figure 8 represents the SEM images of the NCM hydroxide, the LNCM precursor and the as-prepared NCM523 cathode material. The NCM hydroxide precursor (Figure 8(a)) is composed of the spherical powders with a smooth and clean surface. The LNCM oxide precursor (Figure 8(b)) inherits the primary morphology of the NCM hydroxide at treatment. The obtained LiNi_0.5Co_0.2Mn_0.3O₂ cathode material (Figure 8(c)) exhibits a much rougher surface and the second particles are clearly observed, demonstrating the good crystallinity of the NCM523 cathode material. Above all, the constructed potential-pH diagrams of Li-Ni (Co, Mn)-H₂O systems provide a good interpretation for the occurrence of the Li-Ni (Co, Mn) oxides.

4 Conclusions

The potential-pH diagrams of Li-Ni (Co, Mn)-H₂O systems at different activities and temperatures are constructed. The Li-Ni-Co-Mn composite oxides are thermodynamically stable in an aqueous solution, which indicates the great possibility of preparing the composite oxides via an aqueous process. The constructed potential-pH diagrams demonstrate that temperature and activity have significant influences on the stable fields of the composite oxides. As a result of the movement of the equilibrium lines in the potential-pH diagrams, the stability regions of the composite oxides expand towards lower pH with an increase in temperature and activity. Under the guidance of the potential-pH diagrams, the LiNi_0.5Co_0.2Mn_0.3O₂ cathode material has been successfully prepared via the hydrothermal process followed by heat treatment. The good crystallinity and well layered structure as well as perfect morphology of the as-prepared material have confirmed the reliability of the potential-pH diagrams. It implies a most promising future to develop an aqueous routine for the preparation of the NCM cathode materials for lithium ion batteries.

Figure 8 SEM images of NCM hydroxide (a), LNCM precursor (b) and as-prepared NCM523 cathode materials (c)

Appendix

Reactions in the Li-Me-H₂O systems (Me=Ni, Co, Mn) and relationships between potential and pH of these reactions at different temperatures

to be continued

Continued

to be continued

Continued

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Continued

to be continued

Continued

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(Edited by ZHENG Yu-tong)

中文导读

Li-Ni-Co-Mn-H₂O系热力学分析及LiNi_0.5 Co_0.2Mn_0.3O₂复合氧化物的湿法合成

摘要：本文构建的Li-Ni(Co, Mn)-H₂O系电位-pH图从热力学角度表明，在25~250 °C温度范围和0.01~1.00活度范围内，LiNiO₂, LiCoO₂和LiMnO₂可以稳定存在于水溶液中。在Li-Ni-Co-Mn-H₂O系电位-pH图中，同时出现了LiNiO₂、LiCoO₂和LiMnO₂氧化物（即Li-Ni-Co-Mn复合氧化物）的优势区，并且该共沉淀优势区随温度的升高向低pH方向扩大，表明在水溶液中合成Li-Ni-Co-Mn复合氧化物是可能的。在此基础上，本文通过实验采用湿法过程合成了LiNi_0.5 Co_0.2Mn_0.3O₂ (NCM523)正极材料，证明了上述理论的可行性。合成的锂化前驱体和NCM523材料均继承了氢氧化物前驱体的球形形貌，同时所得NCM523材料具有良好的六方α-NaFeO₂型晶体结构。在电位-pH图指导下，湿法合成工艺将一定程度上成为三元正极材料极具前景的制备方法之一。

关键词：溶液过程；电位-pH图；热力学；LiNi_0.5 Co_0.2Mn_0.3O₂；正极材料

Foundation item: Project(FA2019015) supported by the Government of Chongzuo, Guangxi Zhuang Autonomous Region, China; Project(AD18281073) supported by Science and Technology Department of Guangxi Zhuang Autonomous Region, China

Received date: 2018-01-08; Accepted date: 2019-04-01

Corresponding author: LI Yun-jiao, PhD, Professor; Tel: +86-731-88830872; E-mail: yunjiao_li@csu.edu.cn; ORCID: 0000-0001- 8882-7747