1.   Introduction

Since Waldemar Jungner and Edison patented the first nickel (Ni)-based batteries in 1899–1901 [1], Ni has been widely used as a cathode material in rechargeable batteries for over 120 years. As the first commercialized Ni-based batteries, Ni–Fe batteries were applied to power electric vehicles (EVs) as early as 1910 [2]. However, they were gradually replaced by nickel–cadmium (Ni–Cd) batteries with higher energy density and power density. After about 20 years of technical developments in the electrode and battery design, portable sealed Ni–Cd batteries with a sintered porous cathode were eventually fully developed in the 1970s, delivering superior energy density and rate performance at a reduced cost [3]. Although Ni–Cd batteries remained the preferred battery system in the airline industry for their rugged and forgiving properties, it was withdrawn from the computer, communication, and consumer electronics market owing to the toxicity of Cd [4]. As an environmentally friendly alternative to the Cd anode, a hydrogen-absorbing or metal hydride (MH) alloy was introduced to the Ni-based battery system in 1967 by Battelle–Geneva Research Center. The Ni–MH battery, which provides a 40% higher energy density than standard Ni–Cd batteries, was first used on a large scale in Toyota Prius vehicles in the mid-1990s. Until then, Ni-based batteries, which feature nickel hydroxide (Ni(OH)₂) as the cathode, were the dominant rechargeable batteries for portable electronics and the most promising power sources for cars [5].

Then, lithium-ion batteries (LIBs) were introduced. In 1991, 4 V LIBs were commercialized by Sony initially for camcorders [6]. In the early days of LIBs, Ni could only be found in compartments, such as electrode tags and stainless-steel shells, whereas LiCoO₂ dominated as the cathode of choice in LIBs [7]. LiNiO₂ (LNO), the Ni-based isostructural compound of LiCoO₂, had already been proposed as an alternative cathode in the early 1990s [8–9]. LNO can deliver a practical capacity higher than that of LiCoO₂ owing to the favorable electron structure of Ni [10]. Moreover, Ni is the fifth-most common element on earth, making it cheaper than cobalt (Co). Nevertheless, LNO is prone to Li off-stoichiometry and/or Li–Ni disorder, which severely degrades the thermal stability and cycle performance of the material [11]. Therefore, the commercialization of Ni-based cathode materials had been sluggish for a period of time. As LIBs with high energy density rapidly overtook Ni–MH batteries, the role of Ni in batteries became less important temporarily.

The new opportunity for the Ni-based cathode in LIBs arrived with the global surge of EVs from the 2000s. Batteries with low cost and high energy were on demand, and such requirements can be provided by Ni-containing cathode materials. Starting from ternary layered oxide cathode materials LiNi_1/3Mn_1/3Co_1/3O₂ (NCM 111) [12], Ni has found its way back into high-energy batteries step by step. Ni plays a significant role in delivering high energy density and great capacity at a low raw material cost, with the help of Co to suppress the Li–Ni disorder and Mn to stabilize the crystal structure [13]. Increasing amounts of Ni are employed in cathodes, from NCM111 to NCM523, NCM622, and NCM811 and with NCM900505 under development [14]. As the composition of Ni-rich cathodes was being pushed close to that of LNO, scientists encountered the challenges from the 1990s again, i.e., the thermal instability and poor cycling performance [15]. Material engineering strategies, from atomic scale (cationic doping) to nanometer scale (surface stabilization) and micrometer scale (secondary particles morphology manipulation), are under intensive investigation to solve the issues encountered in Ni-rich cathodes [14]. The usage of Ni in EV batteries will continually grow in the following decade.

The history of Ni in rechargeable batteries featured an S-shaped progression: the importance of Ni gradually increased due to Ni–Cd and Ni–MH batteries before the 1990s, temporally faded because of Co-dominated LIBs in the 1990s, and eventually raised again due to the commercialization of Ni-rich cathode in LIBs in recent years. The uniqueness of Ni in batteries is that it helps deliver high energy density and great storage capacity at a low cost. Therefore, herein, we provide a critical review of the key role of Ni in rechargeable batteries. We first discuss the unique properties of Ni that make it an optimal 3d transition metal to function as a rapid and reversible redox center in the cathode of batteries. Then, we review the Ni(OH)₂ cathode in Ni-based alkaline batteries, including Ni–Fe, Ni–Zn, Ni–Cd, and Ni–MH systems. We then focus on the Ni-based layered oxide cathodes in LIBs. The role of Ni in the electrochemical performance and thermal stability of the Ni-rich cathode is highlighted. This review aims to provide new insights to bridge the “old” Ni-based batteries and the “modern” Ni-rich cathode in LIBs in the hope of shedding new light on the development of Ni-containing batteries with high energy density and long cycle life.

2.   3d transition-metal Ni

Nickel is a naturally occurring metallic element with a silvery-white, shiny appearance. It is the fifth most common element on earth and occurs extensively in the earth’s crust and core. The world’s Ni resources are currently estimated at almost 300 million tons. The supply chain of Ni is very stable, guaranteeing Ni as reliable source material for industrial applications. As shown in Fig. 1(a) [16], nearly 70% of Ni is used as an alloying input in steel production, playing an important role in batteries as the stainless-steel compartment. The usage of Ni in battery chemistry accounts for 7% of the annual Ni production, and this ratio has been increasing since the last decade [17]. In addition to Ni-based alkaline batteries and LIBs, Na–NiCl₂ battery is under commercialization as a low-cost energy storage system operating at high temperatures and with superior thermal stability and cycle performance [18]. The boosted demand for Ni in the battery industry led to shortages of supply, which fueled a rally that resulted in a price of $24435 a ton in January 2022, the highest since August 2011.

Figure  1.  (a) Usage of Ni for different purposes. Modified from ref. [16]. (b) Schematic of the d electronic structure of Ni²⁺ and Ni³⁺. O(p) represents the 2p band of O^2–. [20] Reprinted from J. Power Sources, 340, C.P. Liang, R.C. Longo, F.T. Kong, C.X. Zhang, Y.F. Nie, Y.P. Zheng, J.S. Kim, S. Jeon, S. Choi, and K. Cho, Obstacles toward unity efficiency of LiNi_1−2xCo_xMn_xO₂ (x = 0~1/3) (NCM) cathode materials: Insights from ab initio calculations , 217-228, Copyright 2017, with permission from Elsevier. (c) Schematic band structure of Ni redox energy levels. Reprinted with permission from Z.Q. Deng and A. Manthiram, J. Phys. Chem. C, 115, 7097-7103 (2011) [22]. Copyright 2011 American Chemical Society.

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Ni is the 28th element in the periodic table, at the first row of d-block metals, and categorized as a 3d transition metal. The outer electron configuration of Ni is 3d⁸4s². The two 4s electrons of Ni atoms can be lost easily to form Ni²⁺ [19]. As shown in Fig. 1(b) [20], the electron configuration of Ni²⁺ is low-spin ${\rm{t}}_{2{\rm{g}}}^6{\rm{e}}_{\rm{g}}^2$, and that of Ni³⁺ is low-spin ${\rm{t}}_{2{\rm{g}}}^6{\rm{e}}_{\rm{g}}^1$. Electrons can be easily removed from or injected into the e_g band of Ni. As a result, Ni²⁺/Ni³⁺ and Ni³⁺/Ni⁴⁺ couples can act as rapid and reversible redox centers to facilitate the repeat charge–discharge electrochemistry in batteries. In addition, Ni ions readily form hexahedral compounds with ligands, which favors the charge transportation during the electrochemical charging and discharging process. In Ni-based and LIBs, the Ni-containing cathodes share similar layered crystal structures established from joint [NiO₆] octahedrons [8]. The strong covalent bond between Ni–O–Ni provides high conductivity, whereas the t_2g orbital remains filled [21]. Meanwhile, a slight overlap of the redox-active Ni^3+/4+ e_g band with the top of the O²⁻ 2p band facilitates the redox reaction without the release of oxygen from the lattice (Fig. 1(c) [22]).

3.   Ni(OH)₂ cathodes in Ni-based alkaline batteries

3.1   Crystal structure and electrochemistry of Ni(OH)₂ cathodes

The first Ni-based battery, a Ni–Cd system, was originally discovered by Dr. Ernst Waldemar Juenger in 1899 [23]. Since then, Ni–Fe, Ni–Cd, Ni–Zn, Ni–H₂, and Ni–MH batteries have been successively developed. Organic anodes, such as phenazine, were introduced to traditional Ni-based batteries and demonstrated advanced energy density and cycle performance [24]. Currently, typical Ni-based alkaline batteries feature a Ni(OH)₂ electrode (including sintered and bubbled Ni) as the cathode and concentrated potassium hydroxide solution or appropriate amount of potassium hydroxide and sodium hydroxide additives as the electrolyte. Despite the variety of metals on the anode side, the reactions on the cathode side in Ni-based batteries are similar. The battery reactions can be expressed as follows [25]:

In the discharge state, nickel(II) hydroxide can crystallize in two different ways: α-Ni (OH)₂ and β-Ni (OH)₂ (Fig. 2) [26]. These types of Ni(OH)₂ share similar brucite structures with [NiO₂] layers stacked along the c-axis [27]. Each [NiO₂] layer is constructed by the hexagonal planer arrangement of NiO₆ octahedrons. β-Ni(OH)₂ exhibits a P-3m symmetry, with the NiO₂ layers stacked in a high order along the c-axis. The interspace of the layers is about 0.46–0.48 nm. By contrast, in α-Ni(OH)₂, the [NiO₂] layers are spirally stacked. The crystal interspace layer spacing is relatively large at up to 0.7–0.8 nm because H₂O molecules or anions, such as ${\rm{CO}}_3^{2-}$ and ${\rm{NO}}_3^{-}$, are embedded between the layers [27]. The larger distance between metal atoms determines that Ni(OH)₂ is a P-type semiconductor material, and its conductivity improves with the increase in Ni³⁺ content. In an alkaline medium, α-Ni(OH)₂ transforms to β-Ni(OH)₂ during aging through dehydration [26].

Figure  2.  (a) Reaction mechanism, structure, and the corresponding challenges of nickel hydroxides. [28] Reprinted from J. Alloys Compd., 834, B. Liu, X.R. Liu, X.Y. Fan, J. Ding, W.B. Hu, and C. Zhong, 120 Years of nickel-based cathodes for alkaline batteries, 155185, Copyright 2020, with permission from Elsevier. (b) Scheme of the electrochemical oxidation of a polymeric hydrous Ni(II) hydroxide [29].

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The oxidation of Ni(OH)₂ to NiOOH is a deprotonation process, which is similar to the delithiation reaction in the layered oxide cathodes of LIBs. α-Ni (OH)₂ and β-Ni (OH)₂ are oxidized to γ-NiOOH and β-NiOOH, respectively. Both types of NiOOH are non-stoichiometric compounds with the oriented stacking of NiO₂ layers. The main difference between the two types of NiOOH is the interspace distance and interlamellar anions. β-NiOOH has an interspace distance of 0.47 nm. The phase transition from β-Ni(OH)₂ to β-NiOOH occurs with a slight structural change, which ensures excellent reversibility. For the conversion from α-Ni (OH)₂ to γ-NiOOH, the change in the interspace distance is relatively larger (from 0.76 to 0.70 nm), and the NiO₂ sheets are re-oriented at the same time. Alkaline cations in the electrolyte, i.e., Na⁺ or K⁺, are intercalated into the γ-NiOOH layers together with water to compensate for the charge change during deprotonation. In addition, γ-NiOOH appears when β-NiOOH is further oxidized. This overcharging leads to the large volume expansion of the cathode and thus should be carefully prevented to ensure the structural stability of the β-Ni (OH)₂/β-NiOOH cathode.

The redox cycling of α-Ni(OH)₂/γ-NiOOH allows for a multi-electron reaction, and the number of electrons transferred can theoretically reach up to 1.67 [30]. In this case, some of the Ni ions in the γ-NiOOH are further oxidized to Ni⁴⁺, forming NiO₂-like sites, as confirmed by the Ni K-edge X-ray adsorption spectroscopy [31] and quartz crystal microbalance tests [32]. α-Ni(OH)₂ has a theoretical energy capacity of 482 mAh·g⁻¹, which is higher than that of β-Ni(OH)₂ (289 mAh·g⁻¹). Therefore, the material design and development of the Ni(OH)₂ cathode follows two routes: (1) enhancement of the β-Ni(OH)₂ cathode toward theoretical limitations to take advantage of its preface reversibility; (2) stabilization of the α-Ni(OH)₂ cathode to exploit its high capacity.

3.2   Material design and development of Ni(OH)₂ cathodes

In commercial Ni-based batteries, β-Ni(OH)₂ is commonly selected as the cathode because of its structural stability. The main issue of β-Ni(OH)₂ is its poor rate performance and consequent local overcharge. When Edison worked on Ni–Zn batteries in 1908, he observed that adding Co oxide to β-Ni(OH)₂ can improve the high-rate capacity of the cathode. The Co additive had been a routine protocol in the Ni(OH)₂ electrode production since then, whereas the mechanism was unraveled in 1989. Oshitani et al. [33] proposed an in-situ dissolution–precipitation process of the Co additive, which occurred when the battery was standing. A uniform Co(OH)₂ coating layer formed on the surface of the β-Ni(OH)₂ particle, which was subsequently oxidized to CoOOH during charging. Thus, the formed CoOOH provided a good electrical path between the Ni(OH)₂ particles and the current collector. During discharge, CoOOH was not reduced back to Co(OH)₂ at the working potential of Ni(OH)₂. Moreover, CoOOH can prevent the formation of overcharged γ-NiOOH by improving the electrical connection over the cathode particle.

The material design in the above work is simple but sophisticated. Studies should determine the mechanism of the spontaneous growth of the uniform and multifunctional coating layer from oxide additive particles introduced by a simple physical mixing process. In addition, when we study the working mechanism of a certain material modification approach, we should not only focus on the as-obtained materials but also the physical and chemical reactions during cell assembly and operation, which are important for the performance of materials. The old wisdom developed over 30 years ago still inspires battery scientists today. To date, the research on β-Ni(OH)₂ still mainly follows the CoO strategy, aiming to optimize the CoO additive protocols [34–35] or explore alternative additives with a lower cost or better performance [36].

Compared with the traditional β-Ni(OH)₂ cathode, high-capacity α-Ni(OH)₂ has attracted considerable attention from scientists. The main issue of α-Ni(OH)₂ is its phase conversion to β-Ni(OH)₂ at high pH. One milestone in the research of α-Ni(OH)₂ is the discovery of the Ni–Al layered double hydroxide (LDH) in 1994. Inspired by the naturally occurring LDH hydrotalcite of the general formula Mg₆Al₂CO₃(OH)₁₆·4H₂O, Kamath et al. [37] synthesized isostructural Ni–Al LDH Ni_1−xAl_x(CO₃)_x/2(OH)₂·nH₂O (x = 0.1 to 0.25). The key idea was to stabilize the layered structure with a large interlayer distance by the intercalation of anions (such as ${\rm{CO}}_3^{2-}$). By substitution of a part of Ni²⁺ in α-Ni(OH)₂ by Al³⁺, extra anions are intercalated simultaneously for charge neutrality. These anions strongly anchor the positively charged brucite layers and stabilize the α-Ni(OH)₂ structure during alkali corrosion. Moreover, the capacity, cycle performance, and rate performance of Ni–Al LDH are superior to those of Co-modified β-Ni(OH)₂. This result was attributed to the turbostratic structure of Ni–Al LDH.

After this work, enormous efforts have been exerted to further optimize the composition and morphology of Ni-based LDH. Other foreign cations, such as Co, Fe, Mn, Cr, etc., were doped into Ni–Al LDH [38] or applied to replace Al in Ni–Al LDH [39–41]. Compared with cations, anions that are intercalated into the LDH layers are critical because they are the key anchors holding the stacking against high-pH corrosion. As shown in Fig. 3, anions (OH⁻, ${\rm{NO}}_3^-$, ${\rm{SO}}_4^{2-}$, etc.) were applied as the intercalated stabilizer. ${\rm{SO}}_4^{2-}$ failed to improve the cycle performance of Ni–Al LDH, whereas ${\rm{NO}}_3^-$ and OH⁻ succeeded [42]. In addition, the OH⁻ anions intercalated into the layers improved the electrochemical performance more than ${\rm{NO}}_3^-$ [43]. The OH⁻ anions probably prompted the deprotonation of the Ni(OH)₂ layers by the on-site neutralization reaction (H⁺ + OH⁻ = H₂O). Meanwhile, OH⁻ anions can diffuse into the interlayer by a topochemical reaction through proton hopping along the neighboring OH⁻ and H₂O.

Figure  3.  (a) Proposed mechanism for proton transfer in [Ni₄Al(OH)₁₀]OH when it is oxidized. Protons removed from the hydroxyl are trapped by OH⁻ and/or H₂O in between the layers and transported outside the layer. The reduction process occurs in a reverse manner. (b) The process of fast transportation of H⁺ or OH⁻ in aqueous solutions. [43] Reprinted from Electrochim. Acta, 54, L.X. Lei, M. Hu, X.R. Gao, and Y.M. Sun, The effect of the interlayer anions on the electrochemical performance of layered double hydroxide electrode materials, 671-676, Copyright 2008, with permission from Elsevier.

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4.   Ni-based cathodes in LIBs

In comparison with other rechargeable batteries, LIBs, which attract considerable interest owing to their high energy density, are designed and developed through a series of electrode materials. The first commercial LIBs, featuring Goodenough’s layered oxide LiCoO₂, hit the market in the early 1990s. Subsequently, a series of cathode materials (Fig. 4), i.e., layered oxide materials (LiCoO₂ [44], LNO [45], LiNi_xCo_yMn_zO₂ [46], Li₂MnO₃ [47]), spinel-structure materials (LiM₂O₄ [44], M = Mn, Ni, Co), and olivine materials (LiMPO₄ [48], M = Fe, Co, Ni, Mn), has been extensively studied and developed. In particular, Ni-rich layered oxide cathode materials, which attract interest due to their high reversible capacity, low cost, and environmental compatibility, are consolidating their status as cathode materials of choice and enabling remarkable achievements in the portable electronics and EV industry.

Figure  4.  Crystal structures of cathode materials: (a) LiCoO₂, (b) LiMn₂O₄, and (c) LiFePO₄. (d) Comparison of energy densities of typical cathode materials. W. Liu, P. Oh, X.E. Liu, M. J. Lee, W. Cho, S. Chae, Y. Kim, and J. Cho, Angew. Chem. Int. Ed., 54, 4440-4457 (2015) [49]. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.

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With the ever-growing demand for high-energy LIBs, in general, an effective strategy for researchers to improve the energy and power density of LIBs is to increase the Ni content in the cathode material. However, several critical challenges, such as Li⁺/Ni²⁺ cation disorder, severe interfacial side reaction, poor thermal stability and oxygen release, etc., hinder the further commercialization of Ni-rich layered oxide cathodes with the Ni content over 80% (i.e., x > 0.8 in LiNi_xCo_yMn_zO₂). Herein, we focus on understanding the evolution of LIB cathodes from a chemical perspective, thereby inspiring researchers to search for discoveries through basic science research. The role of Ni in LNO and LiNi_1−x−yCo_xMn_yO₂ (NCM) cathode materials is mainly discussed.

4.1   LiNiO₂ (LNO)

Nickel exists in Ni–MH and Ni–Cd batteries in the form of NiO(OH), but Ni is used in the form of oxide in rechargeable lithium batteries, in which Li⁺ are intercalated into its crystal structure at ~4 V, thus forming layered nickel oxide, namely, LNO [21]. Given the cost and environmental concerns of LiCoO₂ and the capacity degradation of LiMn₂O₄ during cycling, an alternative to LiCoO₂ and LiMn₂O₄ has been identified in the 1990s, that is, the isostructural compound LNO [50], which attracts considerable interest owing to its high theoretical energy density, excellent cycle life, and low cost with a higher natural abundance of Ni relative to that of Co. However, LNO has not been commercialized because of its major drawbacks. Two critical issues emerge with LNO: (1) stoichiometric LNO is difficult to synthesize because of its strong tendency for Li off-stoichiometry (z > 0 in Li_1−zNi_1+zO₂) [15]; (2) the stability of LNO is threatened at high potentials. We discuss the above in detail in the following sections.

LNO is a layered oxide with an α-NaFeO₂-type structure (R-3m space group) [45]. As shown in Fig. 5(a), Li and Ni cations are located in Wyckoff 3a and octahedral 3b sites on alternate (111) planes, respectively, whereas oxygen anions are in a cubic and closely packed, occupying the 6c sites. LNO can reversibly offer a considerably higher charge capacity (200–250 mAh·g⁻¹) than LiCoO₂ (150 mAh·g⁻¹) within the same operating voltage range. This finding is mainly attributed to the lower average redox potential of LNO (~3.7 V) than that of LiCoO₂ (~3.9 V). From the perspective of electronic structure (Fig. 5(b)), the t_2g orbitals of Ni are fully filled during the lithiation/delithiation process, and the strong Ni–O–Ni covalent bonds result in a reasonably high conductivity with the semiconducting behavior [21]. Moreover, the two-dimensional channels for Li⁺ diffusion provide an enhanced lithium-ion conductivity. More importantly, given that the 3d e_g orbital energy level of the outermost layer in Ni³⁺ is almost just above the 2p orbital of O²⁻, Ni-based materials can achieve a deeper delithiation before the precipitation of lattice oxygen; that is, they can deliver a higher capacity [51].

Figure  5.  (a) Structure of LNO, with the red ball representing O and the yellow ball representing Li; (b) comparison of the energy diagrams of LiCoO₂, LNO, and LiMnO₂. [51] Reprinted from Energy Storage Mater., 6, A. Manthiram, B.H. Song, and W.D. Li, A perspective on nickel-rich layered oxide cathodes for lithium-ion batteries, 125-139, Copyright 2017, with permission from Elsevier.

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4.2   A major challenge for commercialization of the successful synthesis of LNO

LNO is synthesized via the simultaneous lithiation and oxidation of NiO. If Li₂O is used, the synthesis equation [15] is as follows:

The chemical formula of LNO can be written as ${\mathrm{L}\mathrm{i}}_{z'}{\mathrm{N}\mathrm{i}}_{1-z'}\mathrm{O}$ or ${\mathrm{L}\mathrm{i}}_{1-z}{\mathrm{N}\mathrm{i}}_{1+z}{\mathrm{O}}_{2}$(${z}'=(1-z)/2)$ [52]. When the lithium content is low (z > 0.38), Li_1−xNi_1+xO₂ is the rock-salt structure, and Li and Ni cations statistically occupy the 4a sites. As the ratio of Li⁺/Ni²⁺ increases, and most of Ni²⁺ is oxidized to Ni³⁺, the structure transforms into a layered one, accompanied by alternating Li and Ni planes along the [111]_cubic direction. Fig. 6(a) [15] demonstrates the structural changes during the synthesis of LNO. The transformation from cubic to hexagonal is driven by steric effects [53], because the oxidation of Ni from divalent to trivalent reduces the radii (r) of the nickel ion (r(Ni²⁺) = 0.069 nm, r(Li⁺) = 0.076 nm, and low-spin r(Ni³⁺) = 0.056 nm) [54]. The layered structure helps to contain the trivalent Ni ions. However, the R-$\stackrel{-}{3}$m structure is not the most stable phase among those extracted on the basis of theoretical calculation (Fig. 6(b) and (c)) [55]. In-depth computational studies also demonstrated that the disordered spinel phase is more stable than the layered LNO [30]. In general, the metastable phases in the Li–Ni–O phase diagram are represented by the non-stoichiometric formula Li_1−xNi_1+xO₂. In particular, when x is about 0, we can obtain the desired layered phase [10]. In addition, Li–Ni cation disorder easily occurs during the charge–discharge process, and therefore, the adaptability of Li_1−xNi_1+xO₂-layered crystals to the changes in x suggests that LNO is extremely susceptible to off-stoichiometry [56–57]. Thus, this condition leads to the need for the precise control of the synthesis process for the preparation of high-quality LNO, for instance, the use of precursors, annealing temperature settings, delicate control of annealing atmosphere, etc.

Figure  6.  (a) Changes in crystal structure during synthesis. These changes include those of Li_0.3Ni_0.7O₂ with a rock-salt structure, LNO with a layered structure, and LiNi₂O₄ with a spinel structure. M. Bianchini, M. Roca-Ayats, P. Hartmann, T. Brezesinski, and J. Janek, Angew. Chem. Int. Ed., 58, 10434-10458 (2019) [15]. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission. The green, gray, and red balls represent Li, Ni, and O, respectively. The red double arrows represent the movement of Ni and Li along the [001] hexagonal direction to a narrower Ni layer and a wider Li layer. (b) Formation energy of Li–Ni–O compounds along the binary lines. (c) Magnification of (b) demonstrating the formation energy of various phases of LNO. [55] Reprinted from Energy Storage Mater., 34, Y. Kim, W.M. Seong, and A. Manthiram, Cobalt-free, high-nickel layered oxide cathodes for lithium-ion batteries: Progress, challenges, and perspectives, 250-259, Copyright 2021, with permission from Elsevier.

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4.2.1   Over-lithiated Li_1−zNi_1+zO₂ (−1/3$\le z\le 0$)

When z = −1/3, the chemical formula is Li[Li_1/3Ni_2/3]_3aO₂, and the Li⁺/Ni²⁺ in the metal layer is arranged in an orderly manner, which reduces the symmetry of the compound to monoclinic C2/m [15]. As z → −1/3, the lengths of Li–O and Ni–O bonds shorten, resulting in a decreased battery volume. When the oxygen partial pressure is increased, the samples can be stabilized in the −1/3 ≤ z ≤ 0 region. For sample synthesis, either a partial pressure of oxygen p(O₂) up to 15 MPa or high-pressure up to 3–4 GPa (at almost 700°C) is employed. The compound still displays a layered structure, in which z lithium ions are in the Ni layer. Consequently, the whole series of solid solutions for −1/3 ≤ z ≤ 0 has been successfully prepared.

4.2.2   Li_1−zNi_1+zO₂ (0 ≤ z ≤ 1) and Li off-stoichiometry

The cell volume decreases as z decreases due to the size reduction of metallic Ni after lithiation and oxidation. A linear relationship exists between 0 ≤ z ≤ 1 and −1/3 ≤ z ≤ 0 (Vegard’s law), and the slope change occurs at z = 0 (LNO) because the extra Ni occupies the Li layer. Ideally, LNO is stable when z = 0; however, despite being under strictly controlled conditions, at least 1%–2% of Ni²⁺ ions remain in the Li layer [58]. The stability range of LNO is narrow [59], partly due to the strong oxidation conditions required to stabilize Ni³⁺. Moreover, the control of the actual stoichiometry is a critical challenge due to the loss of Li (accurately Li₂O) at high temperatures, and this issue can be attributed to the unstable material at high calcination temperatures or extremely long calcination time. Eventually, the stoichiometric LNO exhibits partial decomposition, which affects the diffusion of lithium ions.

4.2.3   Li_xNiO₂ (0 < x < 2)

When LNO is delithified, Li_xNiO₂ (0 < x < 1) can be obtained. For instance, the most typical Li_0.5NiO₂ is the layered structure, and the stoichiometry is the same as that of LiNi₂O₄ with a typical spinel stoichiometry. Numerous studies [60] further proved that the layered structure irreversibly converts into the spinel structure above 150°C, and it is stable below 300°C. Li_xNi₂O₄ with a spinel structure (1 < x < 2) can be reversibly delithiated around 2 V vs. Li⁺/Li [61]. The precise stoichiometric Li₂NiO₂ can be synthesized [62], which is isostructural with Li₂MnO₂ and Ni(OH)₂. This compound has an hcp oxygen sublattice, in which Ni occupies octahedral sites, and Li occupies tetrahedral sites.

4.3   Issues of LNO materials

4.3.1   Lattice distortion

When lithium is deintercalated from LNO, that is, Li_xNi_1+zO₂ (0 < x < 1−z), a series of phase transitions will occur. The phase transitions process is as follows: H1 (original hexagonal phase) → M (monoclinic phase) → H2 (second hexagonal phase) → H3 (third hexagonal phase). When x > 0.85, the original hexagonal phase remains unchanged. At x ≈ 0.8, the monoclinic phase crystallizes until the system becomes a single phase at x ≈ 0.75. The M phase remains stable up to x ≈ 0.5, and the system forms the second hexagonal phase H2, which is stable until deep delithiation (x ≈ 0.25), that is, when the third hexagonal phase H3 forms. In particular, in the process of H2 and H3 phase transition, the crystal changes along a and c axis, respectively, which decreases the volume of LNO (9%); the particles are prone to cracks at working voltages above 4.2 V (x < 0.25) [63]. Therefore, the safe use range of LNO material is 0–0.75, and the maximum discharge capacity is about 200 mAh·g⁻¹.

LNO suffers from a series of phase transitions during the delithiation process, which will cause cracks. In the H2 → H3 phase transition process, a volume change of 4% will cause lattice cracks over a narrow composition range [64]. The volume change indicates the significant strain at the interface between them. During cycling, strain is released through microcrack formation. The electrolyte will enter through the cracks, reacting with the newly generated LNO surfaces, which aggravates the generation and propagation of microcracks and eventually causes the pulverization of secondary particles [65]. This lattice distortion is related to the upper limit voltage. Fig. 7 shows the variation in cracks with the number of cycles under different upper limit voltages. When the upper limit voltage is low (less than 4.1 V), the cracking is minimized [63]. Therefore, the maximum voltage should be controlled at less than 4.1 V to effectively avoid cracks in the H2 → H3 phase transition.

Figure  7.  Relationship between capacitance retention and scanning electron microscopy (SEM) cracks and upper limit voltage. Reprinted with permission from C.S. Yoon, D.W. Jun, S.T. Myung, and Y.K. Sun, ACS Energy Lett., 2, 1150-1155 (2017) [66] Copyright 2017 American Chemical Society.

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4.3.2   Thermal instability

The poor thermal stability of LNO is one of the main drawbacks limiting its commercialization as cathode materials. The instability of cathodes may lead to the release of O₂, with the presence of flammable electrolytes for reactions. Moreover, stoichiometric LNO is thermally stable, but it is very unstable in the delithiated state [67]. First, the layered structure forms a defective spinel phase with the precipitation of Li and the migration of Ni to the Li layer. With the release of oxygen [68], the spinel phase converts into a disordered layered structure, and the Li–Ni mixture is finally formed into the disordered rock-salt structure.

4.3.3   Electrochemical instability

The poor electrochemical stability of LNO is also one of the main factors inhibiting the commercialization of LNO cathode. First, Ni⁴⁺ is formed in the highly delithiated LNO, which then reacts with the electrolyte to produce a thick solid–electrolyte film on the surface of cathode materials, thereby increasing the resistance of the reaction [69]. In addition, spinel LiNi₂O₄ is one of the products during the delithiation process. The disordered rock-salt structure formed on the surface of LNO affects the progress of the electrochemical reaction, further increasing the impedance of cathode materials. Finally, the gases (CO and CO₂) in the electrolyte are released, which increases the impedance and attenuates the capacity.

4.4   Nickel-rich layered oxide cathode materials NCM and LiNi_1−x−yCo_xAl_yO₂ (NCA) and Co-free Ni-rich cathode materials

With the excessive emission of global carbon dioxide, worldwide environmental concerns have been addressed seriously, and EVs are considered as one promising solution. However, the universal acceptance of low-cost EVs is highly dependent on the development of rechargeable batteries with high energy density, long calendar life, and low price. Compared with other groups of conventional cathode materials, such as layered LiCoO₂, spinel LiMn₂O₄, and olivine LiFePO₄, Ni-based cathode materials offer a combination of high reversible capacity (200–250 mAh·g⁻¹), high operating voltage (~3.8 V vs. Li/Li⁺), and improved chemical stability without oxygen loss due to the lack of significant overlap of the Ni^3+/4+ redox energy with top of the O²⁻ 2p band. Li_1−xCoO₂ suffers from serious chemical instability when x > 0.5, whereas LiMn₂O₄ has a limited reversible capacity (~120 mAh·g⁻¹), with the critical issue of Mn³⁺ dissolution. On the other hand, LiFePO₄ delivers ~160 mAh·g⁻¹ at a low operating voltage of ~3.4 V, which cannot meet the increasing demand for high energy density. Therefore, Ni-based cathode materials, which can be classified into unary and multiple layered oxides, are of immense research interest owing to their low cost and high discharge capacities. In general, LiMO₂ is doped to form binary or ternary transition-metal oxides to optimize its performance [55,70–77]. One of the most successful approaches is the simultaneous introduction of Ni, Co, and Mn/Al into the layered structure to synthesize ternary layered materials (NCM and NCA) [13,78–79] within a large compositional range. In NCMs, Mn remains in a +4 oxidation state throughout cycling and stabilizes the structure. Co suppresses structural defects by reducing the Li⁺/Ni²⁺ cation disorder in ternary layered compositions and achieves a well-crystallized layered structure. In addition, Ni^2+/3+ and/or Ni^3+/4+ redox couples provide the majority of the reversible capacity [80], and Mn is used to stabilize the local structure [81]. Based on their different molar ratios, Ni, Co, and Mn can be divided into various systems. The Ni-rich NCM (1−x−y ≥ 0.5) has attracted wide interest due to its high discharge capacity. Fig. 8(a) shows the phase diagram of LiCoO₂–LNO–LiMnO₂ ternary materials.

Figure  8.  (a) Phase diagram of LiCoO₂–LNO–LiMnO₂ layer oxides. Reprinted with permission from [97]. Copyright 2012 The Royal Society of Chemistry. (b–d) Typical electrochemical performance curves of Ni-rich cathode materials. Reprinted with permission from Y.K. Sun, D.J. Lee, Y.J. Lee, Z.H. Chen, and S.T. Myung, ACS Appl. Mater. Interfaces, 5, 11434-11440 (2013) [98] Copyright 2013 American Chemical Society.

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NCA has also been commercialized by Panasonic for Tesla EVs. NCA cathode materials have higher capacity (~200 mAh·g⁻¹) and specific energy (680–760 Wh/kg) and lower cost than NCM. With the current technology, the energy density of commercial single-cell NCM lithium batteries is around 230–250 Wh/kg, whereas that of Panasonic NCA battery is 322 Wh/kg. The presence of Al improves the thermal stability of NCA by preventing phase transitions, which are believed to be responsible for the instability of layered materials at high temperatures. The prevention of phase transitions in NCA is due to Al³⁺ ions occupying stable tetrahedral sites, which inhibits cation migrations [82–85]. In NCA materials, Ni ions are in a 3+ oxidation state and Jahn–Teller (JT) active, and this JT behavior strains the Ni octahedra. However, the presence of Al reduces this strain due to the preferential ordering of long JT Ni³⁺–O bonds near the Al³⁺ ions, instead of a preference for Al³⁺ to be coordinated by Ni³⁺; as a result, the ordering of atoms in the TM layers is improved [86]. The local JT ordering increases the number of long JT Ni³⁺–O bonds directed toward Al, thereby accommodating the strain of the dynamic JT distortion. The presence of Al in NCA also reduces the variation in the c lattice parameter during cycling [87]. In addition, the presence of Al in NCA reduces the oxygen released by reducing the probability of an exothermic reaction with the cathode and electrolyte [87–88].

The most potential cathode materials for high-energy-density LIBs have been widely studied and developed, and they include LiNi_0.5Co_0.3Mn_0.2O₂ (532) [89–90], LiNi_0.6Co_0.2Mn_0.2O₂ (622) [91], LiNi_0.7Co_0.15Mn_0.15O₂ (71515) [92–93], LiNi_0.8Co_0.1Mn_0.1O₂ (811) [94–95], LiNi_0.9Co_0.05Mn_0.05O₂ (90505) [96], LiNi_0.33Co_0.33Al_0.33O₂ (333), LiNi_0.8Co_0.15Al_0.05O₂ (8155) [84], etc. The above ternary cathode materials have been widely used in practical applications, and the mainstream EVs currently running on the road have high-Ni ternary batteries. As a critical review of the role of Ni-based cathode in batteries, we will focus on the Ni-rich cathode materials with Ni content higher than 0.6. Fig. 8(b)–(d) show the typical electrochemical performance curves of Ni-rich cathode materials within an electrochemical window of up to 4.3 V vs. Li/Li⁺.

The current bottlenecks in Co supply have negatively affected the commercial battery production and inspired the development of Co-free high-Ni cathode materials. Thus, Co-free high-Ni layered cathodes will enable the achievement of price parity between EVs and internal combustion engine vehicles. Co-free high-Ni cathode materials, which are of substantial interest due to their high energy density, low cost, and environmental benignity, are pursued as an alternative to the recently developed commercial cathode materials. However, they still face critical challenges for further commercialization, such as off-stoichiometry [55], Li/Ni cation mixing [99], instability of Ni³⁺ [100], low coulombic efficiency [101–103], thermal instability [104–105], and multiple phase transitions during cycling [106–107]. Thus far, to mitigate these issues, scholars have been studying Co-free Ni-based layered oxides using a multielement substitution strategy to improve their structural stability and electrochemical performance. Surface coating has also been a promising approach to prevent the undesired side reactions and phase transformation of Co-free high-Ni layered oxides. Ultimately, under optimized synthesis conditions and with the bulk/surface modification of cathode materials, Co-free cathode materials with excellent performance will be successfully synthesized in the near future. We believe that through the cooperation of academia and industries, Co-free cathode materials will be widely integrated into rechargeable batteries.

4.4.1   Structure of NCM

The layered NCM ternary materials, which fully utilize the advantages of three oxides, have become one of the preferred cathode materials for mainstream power batteries due to their high energy density and low cost (Fig. 9(a)) [108]. Typically, an oxygen stacking sequence with a specific Li-vacancy order is used to describe the composition of the structure [20], which is the result of the sliding between adjacent TMO₆ octahedra. Fig. 9(b) shows three oxygen stacking sequences: ABC oxygen-ion stacking (O3) structure, AAA oxygen-ion stacking (O1) structure, and a mixed O3 and O1 structure with ABCAAB stacking.

Figure  9.  (a) Triangle showing the layered ternary materials and the structure model of NCM; (b) top and side views of the structure model of (532). The inset shows the LCSU of oxygen in NCM materials. Reprinted with permission from J.X. Zheng, T.C. Liu, Z.X. Hu, Y. Wei, X.H. Song, Y. Ren, W.D. Wang, M.M. Rao, Y. Lin, Z.H. Chen, J. Lu, C.M. Wang, K. Amine, and F. Pan, J. Am. Chem. Soc., 138, 13326-13334 (2016) [108] Copyright 2016 American Chemical Society. (c) Schematic of layered structures, O3, C2/m, H1–H3, O1, and Li-vacancy ordering of Li_yNiO₂ ground states. (d) Li-vacancy ordering of Li_yNiO₂ ground states. The Li-vacancy orderings hold for other Li_yNi_1−2xCo_xMn_xO₂ phases at the same Li concentration (y). [20] Reprinted from J. Power Sources, 340, C.P. Liang, R.C. Longo, F.T. Kong, C.X. Zhang, Y.F. Nie, Y.P. Zheng, J.S. Kim, S. Jeon, S. Choi, and K. Cho, Obstacles toward unity efficiency of LiNi_1−2xCo_xMn_xO₂ (x = 0~1/3) (NCM) cathode materials: Insights from ab initio calculations, 217-228, Copyright 2017, with permission from Elsevier.

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4.4.2   Issues of NCM materials

As the Ni content increases, the energy density of the layered oxide NCM can meet the ever-growing demands of consumers. However, their intrinsic and practical issues similar to those of LNO have become a critical challenge for further commercialization. The charging of NCM materials to their upper voltage limit results in serious performance degradation, including capacity loss and thermal runaway after several cycles [49,97]. NCM materials undergo complex redox reactions and phase transitions during charging and discharging [109–110]; thus, insights into the capacity-fading mechanisms associated with complex surface chemistry must be obtained.

(1) Li⁺/Ni²⁺ cation disorder.

The Li⁺/Ni²⁺ cation disorder is related to intrinsic issues in the NCM, which result in a high activation energy barrier for lithium diffusion and a low lithium diffusivity due to the obstacles caused by transition metals in the lithium layer, thereby decreasing the rate performance. This condition can be attributed to the inherently unstable Ni in the transition metal layers, which is caused by the relatively strong magnetic moment by itself. Three Ni²⁺ cations that are arranged in a triangle always have two opposite magnetic moments, which in turn create a “magnetic frustration” [111]. Then, given their lack of magnetic moment, lithium ions preferentially exchange with some of the Ni ions. The spin loss in one site relieves the magnetic frustration. The strong interlayer anti-ferromagnetic coupling between Ni in the transition-metal layer and the migrated Ni in the Li layer generates super-exchange interactions, thus further stabilizing the lithium ions, which leads to the irreversible transformation of the crystal structure from the original R-3m structure to the NiO-type Fm-3m (Fig. 10). In conclusion, Li⁺/Ni²⁺ cation disorder degrades the electrochemical performance of the cell due to the reduced thickness of the Li-deficient LiO₂ inter-slab layer, which severely inhibits the transport of lithium ions, eventually leading to rapid degradation of the layered structure of NCM [112–113].

Figure  10.  (a) Illustrations of the instability of Ni from the perspective of magnetic frustration. Nickel as a replacement for Co in LIB cathodes suffers from magnetic frustration. Discharging mixes Li ions into the Ni layer versus their storage between oxide layers. Reprinted with permission from [114]. Copyright 2020 Science. (b) Phase transitions of highly delithiated Ni-rich oxide. Reprinted with permission from [115]. Copyright 2019 The Royal Society of Chemistry.

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(2) Mechanical instability.

The mechanical degradation of Ni-based ternary materials mainly occurs at the grain boundaries of secondary particles and within the grains of primary particles, that is, the so-called intergranular and intragranular cracks, respectively [116–117]. The intrinsic origin of cracks is the anisotropic volume change in crystal lattices, eventually leading to the pulverization of particles. Micro-voids of individual grains are often readily abundant along grain boundaries for the as-prepared materials. With the change in Ni content during the charge/discharge process, the material will undergo a series of phase transitions, which cause the volume change [118]. The formation and expansion of the resulting microcracks along grain boundaries continually occur, which leads to mechanical fracture. During cycling, strain is released via the formation of cracks. Fig. 11 shows the mechanical fracture of NCM materials.

Figure  11.  (a, c) SEM image of NMC811 cathode cross-section after a single charge to 100% SoC (SoC = state of charge, defined as the ratio between the amount of charge extractable from the cell at a specific point in time and the total capacity) at C/3 and high-resolution focused ion beam (FIB) image of the lower half of larger particle shown in (a), respectively. (b, d) Illustration of Mode I and II (in-plane shear) fractures between two primary NMC particles. (e) Intergranular void evolution vs. SoC in NMC811 material observed by FIB–SEM cross-sectional imaging. Reprinted with permission from [119]. Copyright 2020 The Royal Society of Chemistry. (f) Schematic showing the evolution of intergranular cracks on the secondary particle. Reprinted with permission from L.F. Wang, R. Wang, J.Y. Wang, R. Xu, X.D. Wang, and C. Zhan, ACS Appl. Mater. Interfaces, 13, 8324-8336 (2021) [120]. Copyright 2021 American Chemical Society.

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(3) Thermal runaway.

Thermal stability is one of the most important considerations for large-scale power applications of LIBs in the automotive industry. In the deep delithiated state, the oxidation state of Ni is extremely unstable. In general, the reduction of Ni⁴⁺ to Ni²⁺ is considered to be the fastest in the thermal decomposition process. Thus, the stabilization of highly active Ni is an important factor affecting the thermal stability of NCM cathode materials. Moreover, Ni-rich cathode materials undergo an irreversible phase transition from the original layered structure to the rock-salt phase during cycling, accompanied by the release of oxygen and heat (Fig. 12(a)) [121]. When heat continually accumulates to a certain extent, O₂ and Ni⁴⁺ at the interface may undergo more violent chemical reactions with the electrolyte, thereby generating more heat and leading to thermal runaway. Additionally, the reduction of Ni ions triggers the formation of oxygen vacancies. The presence of oxygen vacancies lowers the barrier to cation migration and promotes the irreversible phase transition of the charged cathode materials during heating (Fig. 12(b)). Therefore, layered oxide cathodes generally result in the release of molecular oxygen from the host lattice during overcharging or heating. The above reactions are usually exothermic, and when the heat generated is extremely high to dissipate on its own, a series of catastrophic chain reactions proceeds aggressively, eventually causing the entire battery system to completely catch fire or explode. This outcome can be related to factors, such as interfacial side reactions, irreversible phase transitions, etc. Moreover, as the Ni content increases, the thermal stability deteriorates (Fig. 12(c–d)) [122] . Therefore, improving the safety of Ni-rich layered oxide cathodes involves increasing the onset temperature and reducing the exothermal reaction during oxygen generation [51].

Figure  12.  (a) Time-resolved X-ray diffraction patterns and simultaneously measured mass spectra of O₂ and CO₂, and m/z is the mass-to-charge ratio in the mass spectroscopy. Reprinted with permission from S.M. Bak, E.Y. Hu, Y.N. Zhou, X.Q. Yu, S.D. Senanayake, S.J. Cho, K.B. Kim, K.Y. Chung, X.Q. Yang, and K.W. Nam, ACS Appl. Mater. Interfaces, 6, 22594-22601 (2014) [121]. Copyright 2014 American Chemical Society. (b) Schematic summarizing the crystallographic and electronic structure changes that occur in Li_xNi_0.8Co_0.15Al_0.05O₂ cathode materials during electrochemical charging. Reprinted with permission from J.K. Ngala, N.A. Chernova, M.M. Ma, M. Mamak, P.Y. Zavalij, and M.S. Whittingham, J. Mater. Chem., 14, 214-220(2004) [78]. Copyright 2004 American Chemical Society. (c, d) Thermal stability of as-prepared NCM samples and a relationship map between discharge capacity, thermal stability, and capacity retention. [122] Reprinted with permission from J. Power Sources, 233, H.J. Noh, S. Youn, C.S. Yoon, and Y.K. Sun, Comparison of the structural and electrochemical properties of layered Li[Ni_xCo_yMn_z]O₂ (x = 1/3, 0.5, 0.6, 0.7, 0.8 and 0.85) cathode material for lithium-ion batteries, 121-130, Copyright 2013, with permission from Elsevier.

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Regarding these issues, element doping, surface coating, core-shell structure, and gradient materials have been widely adopted to improve the structural stability and maintain the stable solid–electrolyte interface and bulk structural and thermal stabilities of Ni-rich ternary materials [123].

5.   Challenges and perspectives

Ni-rich layered oxides have become the principal candidate for rechargeable EV batteries owing to their high energy, high power, and low material cost. The main challenge for Ni-rich cathodes is their instability during prolonged cycling and/or under thermal abuse. Global efforts have been exerted for the stabilization of Ni-rich cathodes, and most of the work focuses on material modification and electrode optimization. Surficial modification prevents the cathode/electrolyte side reaction, whereas cationic doping suppresses the Li–Ni disorder. However, the fundamental understanding of the chemistry of Ni is insufficient to push forward technical breakthroughs in material design. The correlation between the unique prosperities of the 3d electron configuration of Ni ions, advanced energy and power density, and the poor stability of Ni-rich cathodes is unclear. Moreover, the interaction between the Ni 3d and O 2p bands, which has not been well understood, is critical to oxygen evolution in Ni-rich cathodes at high voltages and temperatures. More attention should be paid to the basics of Ni chemistry to overcome the instability challenges.

Ni-based alkaline batteries are not the dominant battery anymore. The Ni(OH)₂ cathode reviewed here is a Ni-based cathode with a long history to provide perspectives on the modern Ni-rich cathode. α- and β-Ni(OH)₂ share similar [NiO₂] layered structures to Ni-rich layered oxides. Their structural similarity makes them optimal precursors for the one-step solid-state synthesis of Ni-rich cathodes, especially when the Ni content x approaches one in the LiNi_xCo_yM_1–x–yO₂ (M = Mn, Al, et al.). Hence, other cations, i.e., Co, Mn, or Al, can be considered as dopants. The long-time wisdom developed on the material modification of Ni(OH)₂ for improved electrochemical performance can be adopted to prepare advanced precursors for Ni-rich cathodes. For instance, the turbostratic structure and anionic and cationic intercalations of Ni–Al LDH may offer novel discoveries on typical Ni-rich cathodes.

6.   Conclusions

Nickel has been used in the cathode materials of rechargeable batteries for over 120 years. This element was introduced into Ni-based alkaline batteries in the early 1900s and remains the most important metal element in state-of-the-art LIBs powering EVs. Herein, we provide a critical review of the key role of Ni in rechargeable batteries. We first discuss the unique properties of Ni that make it an optimal 3d transition metal to function as a rapid and reversible redox center in the cathode of batteries. Then, we review the structure, redox mechanism, and material design and development of Ni(OH)₂ cathodes in Ni-based alkaline batteries. We then focus on Ni-based layered oxide cathodes in LIBs. The role of Ni in the electrochemical performance and thermal stability of Ni-rich cathodes is highlighted. At present, the research focus of cathode materials of LIBs gradually transfers from LiCoO₂ to other cheaper and environmentally friendly layered materials, such as LNO and Ni-rich ternary materials. However, the commercial development of stable and low-cost Ni-based oxides still faces numerous challenges. From this perspective, we discuss related issues from the fundamental aspects of LNO and high-Ni ternary materials (NCM) to advance the commercialization of high-Ni layered oxides. Moreover, we specifically emphasize the effect of Ni on mechanical instability, thermal runaway, and Li⁺/Ni²⁺ cation disorder in layered oxides (LNO and NCM). This review aims to provide new insights to bridge the “old” Ni-based batteries and the “modern” Ni-rich cathode in LIBs, with the hope of shedding new light on the development of Ni-containing batteries with high energy density and long cycle life.

Acknowledgements

This work was financially supported by the China Postdoctoral Science Foundation (No. 2021M700396) and the National Natural Science Foundation of China (No. 52102206).

Conflict of Interest

The authors declare no conflict of interest.