Layered lithium transition-metal oxides have a common chemical formula LiTMO₂ (TM = Ni, Co, Mn, Al, etc.), in which TM representatives one or more of the transition metals. LiCoO₂ is the most widely used cathode material in electronic devices because of its easy synthesis process, stable electrochemical performance, and highest compaction density [20,21]. However, the high cost and toxicity of cobalt make it unsuitable for electric vehicles and other large applications. LiNiO₂ is another promising layered cathode material with a similar theoretical specific capacity of 274 mAh g⁻¹ vs. LiCoO₂ (275 mAh g⁻¹), while the practical specific capacity of LiNiO₂ (>220 mAh g⁻¹) is much higher than that of LiCoO₂ (~160 mAh g⁻¹) below 4.3 V [22]. Unfortunately, it’s very difficult to synthesize a pure LiNiO₂ since lithium vacancies, cation disordering, and spinel phase are easier to form in the sintering process [23].

As mentioned before, single transition metal oxide (such as LiCoO₂ and LiNiO₂) cannot meet the increasing requirements of the battery market due to various deficiencies. Ternary cathode materials, especially LiNi_xCo_yMn_zO₂ (x + y + z = 1), have attracted great attention since 1999 when Liu first proposed a co-substitution of Ni by Co and Mn in LiNiO₂ [24]. Nickel, Cobalt, and Manganese are randomly and uniformly distributed in the transition metal layer with performing their own functions, i.e., Ni for capacity, Co for layered characteristics, and Mn for stability. The synergistical effect of the three elements significantly improves the electrochemical performance of NCM and makes it a very successful commercial cathode material. In recent years, nickel fraction in commercial NCM materials has exceeded 60% in order to pursue higher energy density, which is called Ni-rich materials, such as NCA, NCM811 and LiNi_0.89Co_0.05Mn_0.05Al_0.01O₂ (NCMA) [15,25,26]. Besides, the rapidly rising cost of cobalt also promotes the development of cobalt-free Ni-rich cathode materials (LiNi_0.883Mn_0.056Al_0.061O₂ for example).

However, the biggest challenge for Ni-rich materials is how to balance the contradiction between high energy density and reduced thermal stability. It’s well known that the thermal stability of layered LiTMO₂ materials strongly depends on their composition, especially the Ni content, i.e., the higher the Ni content, the lower the stability. As shown in Figure 1a, Noh [27] compared the thermal stability of delithiated NCM materials with different nickel contents and found that the exothermic peak temperature drastically decreased from 306 °C to 225 °C for Li_0.37Ni_1/3Co_1/3Mn_1/3O₂ and Li_0.21Ni_0.85Co_0.075Mn_0.075O₂ respectively, while the heat generation significantly increased from 512.5 J g⁻¹ to 971.5 J g⁻¹. These differences can be attributed to the varied valence with different Ni content. According to the results of in situ time-resolved X-ray diffraction and mass spectroscopy (TR-XRD/MS), Bak et al. schematically depicted the phase stability map of the charged NMC cathode materials during heating (Figure 1b), which also clearly showed that the thermal stability of the charged NCM samples decreases with increasing Ni content [28]. Further study shows that the average valence of Ni increases from +2 to +3 with the increasing Ni content, while Ni³⁺-TM bond strengths decrease and Ni³⁺-O bonds become increasingly covalent, leading to the intrinsic thermodynamical instability of the layered structure [29].

All of the layered LiTMO₂ compounds correspond to a hexagonal α-NaFeO₂ structure with the $R\overline{3}m$ space group. Specifically, oxygen atoms form a cubic close-packed lattice with rhombohedral distortion along the c-direction, resulting in layers formed by edge-sharing octahedra sites which are occupied by transition metals (3a) and lithium (3b) alternately [30]. But there is a common problem in layered nickel-based cathode materials named Li⁺/Ni²⁺ anti-site defects or cation disordering which mainly result from their similar ionic radii (Li⁺: 0.072 nm, Ni²⁺: 0.069 nm) [31]. These disordering occupations hinder the Li⁺ diffusion in the two-dimensional channels and greatly affect the electrochemical performance. Besides that [32], the distribution of TM ions inside the layer is no longer uniform, but the minority of TM ions form clusters at the atomic level when Ni content exceed 80%. The clusters make Mn⁴⁺ reduce to Mn³⁺, leading to aggravating Jahn–Teller distortions. These atomic-level structure deficiencies weaken the TM-TM interactions and TM-O bonds, thus significantly affecting the thermal stability of nickel-based materials.

In the delithiation process, the structure phase converts from hexagonal (H1) over monoclinic (M) to hexagonal (H2 and H3) phases, and the phase transitions reverse in the discharging process [33,34,35]. With the increase of lithium vacancy in the charging process, the distortion of TM-O octahedra gradually increases along the c-axis direction. As shown in Figure 2, when the amount of lithium removal exceeds the threshold (depending on the nickel content and charging cutoff voltage), parts of the region (H3 phase) cannot remain the layered characteristic anymore and convert to spinel Ni₃O₄ with active oxygen release [35]. The H2 to H3 phase transition usually occurs at a high delithiated state and results in oxygen evolution, which is also one of the main reasons for thermal runaway caused by overcharge [18]. With regard to the thermal decomposition under abusive conditions, the delithiated layered LiTMO₂ materials go through phase transitions from layered structure to the disordered spinel structure, and finally to the rock-salt structure, accompanied by a large amount of oxygen gas release and CO₂ formation [35,36]. Noted that the oxygen gas release occurs around 200 °C, at which the electrolyte has been decomposed or oxidized into combustible gas. Oxygen is the most common combustion promoter among the three elements of combustion. Therefore, batteries using layered LiTMO₂ materials always catch fire or even explode when thermal runaway happens. Similarly, layered oxides like LiCoO₂ and Mn-based Li-rich cathode materials also have the common problem of oxygen release. Samsung’s Galaxy Note 7 explosion strongly indicates that LiCoO₂ battery with design defects may cause serious accidents. In contrast, the dilithiated LiFePO₄ remains quite stable even above 350 °C and with little oxygen release, thus the weaknesses of the LiFePO₄ battery are the separator and electrolyte rather than the cathode material.

The particle microstructures and morphologies also play an important role in influencing the electrochemical properties and thermal stability of electrode materials. For commercial Ni-based materials, the spherical precursors are firstly agglomerated by nano-sized primary crystals with random orientations and a large number of defects in the coprecipitation process. Then the subsequent high-temperature treatment makes nanocrystals grow up and poly-crystallization, which means many crystal defects, such as grain boundaries, still exist inside the particle [23]. As shown in Figure 3a, the resulting unbalanced mechanical stress inside the particles during the charge-discharge process can cause microcracks that weaken the structural stability of particles and aggravate the undesired cathode-electrolyte side reactions [37,38,39,40]. In particular, the particles in at highly delithiated state may smash at high temperatures and accelerate side reactions and oxygen release (Figure 3b–d) [36].

The chemical activity of electrode materials is directly related to the gas production inside the battery. Because of the different out shell electron configurations for Ni and Co, the Ni²⁺/Ni³⁺/Ni⁴⁺ redox contributes to most of the capacity of Ni-based materials below 4.3 V (vs. Li/Li⁺) while Co³⁺/Co⁴⁺ redox occurs at around 4.3 V in the charging process [41]. However, the highly active Ni⁴⁺ on the particle surface can easily be reactive with liquid electrolytes accompanied by the gas generation (mainly including CO₂ and CO) [42,43]. Secondly, the lithium residues (such as Li₂CO₃ and LiOH) introduced in the synthesis process or by-products of the slow reduction of Ni³⁺ to Ni²⁺ for Ni-rich materials when exposed to moisture, can also induce gas generation inside the battery [44,45]. Finally, transition metals (especially Mn) can slowly dissolve into the electrolyte, then migrate and incorporate into solid electrolyte interphase (SEI) film of the graphite-based negative electrode, resulting in the damage of SEI and continues alkane gas evolution [46,47]. Actually, the gas evolution related to the chemical activity of electrode materials is a common phenomenon of battery failure, which makes the battery bulge and leakage, and causes safety problems.

In summary, the thermal instability of layered LiTMO₂ materials attributes to: (1) compositions, specifically, the higher the Ni content, the worse the thermal stability; (2) oxygen release from delithiated materials due to the phase transitions under overcharge or heated conditions; (3) gas evolution caused by the highly oxidative Ni⁴⁺, lithium residues, and transition metals dissolution. Nevertheless, the core reason is the oxygen release due to the astable layered structure of delithiated LiTMO₂ materials under abusive conditions.