High-Capacity Ni-Rich Cathode Materials for Lithium-Ion Batteries

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1		Mehdi Ahangariahangarkolaei	--	4110	2024-02-19 03:20:18	\|
2	update references and layout	Rita Xu	Meta information modification	4110	2024-02-19 03:40:27	\|

This entry is adapted from the peer-reviewed paper 10.3390/ma17040801

Lithium-ion batteries are undoubtedly known as the most promising rechargeable batteries. Ternary Ni-rich Li[Ni_xCo_yMn_z]O₂ and Li[Ni_xCo_yAl_z]O₂ cathode materials stand as the ideal candidate for a cathode active material to achieve high capacity and energy density, low manufacturing cost, and high operating voltage.

Ni-rich cathode surface modification elemental doping

1. Introduction

In recent decades, the urgent global issues of ever-increasing fossil fuel demand and the alarming threat of global warming have spurred the development of renewable and green energy sources ^[1]. Within this array of sustainable energy options—for example, hydroelectric, solar, tide, or wind, which all contribute to the global energy needs—rechargeable lithium-ion batteries (LIBs) have prominently emerged as the most sought-after for numerous practical applications, decisively spearheading the electrification of vehicles. This preference arises from their distinctive blend of characteristics, including their lightweight design, impressive power output, substantial energy capacity (ranging from 250 to 693 Wh L⁻¹ and 100 to 265 Wh kg⁻¹), relatively long service life, desirable rate capability, safety features, and minimal environmental impact. Currently, LIBs have achieved widespread success in small-scale portable consumer electronic devices such as cellular phones and laptop computers ^[2]^[3]. However, when it comes to utilization of the current state-of-the-art LIBs in more substantial applications, such as passenger electric vehicles (EVs), hybrid electric vehicles, and large-scale energy storage systems, there are formidable challenges and requirements that must be meticulously addressed to compete with traditional internal combustion engine vehicles and receive wider consumer acceptance. These challenges encompass the imperative to meet demanding criteria, such as fast charging and rating capabilities, elevated energy, power density for starting, accelerating, and uphill driving, cost-effectiveness, battery safety under harsh conditions at elevated temperatures, and prolonged lifespan ^[4]. One way to reduce costs is to utilize high gravimetric and volumetric energy density materials, thereby requiring a smaller number of needed materials. In addition to driving range, charging time is also central to customer experience ^[5]^[6]^[7]^[8]^[9]^[10].

The electrochemical characteristics of LIBs are primarily governed by the properties of their cathode materials. While every component of the battery system plays a vital role in achieving optimal performance and continues to be the focus of extensive research, a notable technical bottleneck in LIBs technology pertains to the limited capacity of cathode materials, typically falling below 270 mAh g⁻¹. This inferior capacity stands in contrast to the more robust capacity of graphite, SnO₂, Sn, and Si anode materials, which can reach up to 350, 782, 994, and 4200 mAh g⁻¹, respectively ^[11]. In order to resolve the capacity shortfall exhibited by cathode materials, immense efforts have been directed on research and development endeavors aimed at creating new materials and refining existing materials that combine high-capacity and high-voltage. Cathode materials are typically categorized into the three most prominent classes: (1) Layered oxide LiMO₂ (M = Mn, Co, and Ni) structure, (2) Spinel-framework LiM₂O₄ (M = Mn, Co, and Ni) structure, and (3) Olivine LiMPO₄ (M = Fe, Mn, Co, and Ni) structure ^[12]. These categories exhibit varying actual capacities, typically falling within the range of 120 mAh g⁻¹ to 210 mAh g⁻¹ ^[13]^[14]^[15]. Among the various materials presently available explored as potential candidates for battery cathodes, layered transition metal oxides (LTMOs) have emerged as particularly promising options. Layered oxide materials (for example LiCoO₂ and LiNi_xCo_yMn_zO₂) with a layered lattice α-NaFeO₂ hexagonal structure and R-3m space group play a crucial role in facilitating the reversible (de)intercalation of lithium ions. In this structure, lithium ions and TM cations are placed within octahedral 3a and 3b sites, respectively, while oxygen anions occupy the octahedral 6c sites, showing a cubic close-packed arrangement (as depicted in Figure 1) ^[16].

Figure 1. (a) Schematic view of the bulk structure of NCM111 with R-3m space group, and (b) illustration depicting the ion-ordering in NCM111 TM layers.

The evolution of Ni-rich cathode materials traces its roots to the constrained development of LiCoO₂ electrodes. Despite the excellent performance of LiCoO₂, limitations such as practical delivery of only 50% of the theoretical capacity (140 mAh g⁻¹), high cost, and the material’s toxic property, hinder its application in EVs. As a response, rigorous research endeavors have been undertaken to replace Co with alternative elements such as Ni and Mn in cathode hosts. However, the absence of Co in the layered oxide structure often results in structural instability at low-temperature operations and lower electronic conductivity ^[17]^[18]. N. Zhang et al. ^[19] reported that Co helps Ni and Li ordering, and in Co absence, more Ni and Li ion-exchange sites are possible. Also, Mn provides structural stability, as its oxidation state is +4 throughout electrochemical cycling ^[20]. Consequently, by taking partial advantage of the high capacity of LiNiO₂, the layered character of LiCoO₂, and the low cost and thermal stability of LiMnO₂, LiNi_xCo_yMn_zO₂ (NCM) cathode materials have appeared as viable contenders to substitute the traditional LiCoO₂ cathode, addressing some of the inherent challenges associated with Co-based formulations ^[21]^[22]^[23]. Ni-rich layered oxide cathode materials (x > 0.5) have attracted great interest as the leading candidates in this domain. This enthusiasm is primarily fueled by their exceptional attributes, including a superior actual capacity exceeding 170 mAh g⁻¹, a volume change by less than 2% during Li insertion/extraction, and a higher average operating cutoff potential surpassing 3.4 V ^[24]^[25]^[26]^[27]. While Ni-rich cathode materials have received great attention for their remarkable attributes, such as high capacity and energy density (exceeding 600 Wh kg⁻¹), they are susceptible to a range of significant drawbacks. These issues include premature capacity degradation, severe structural and thermal instability, limited calendar life, electrolyte decomposition, intrinsic low-rate capability, rapid voltage decay leading to sluggish Li⁺ ion transport dynamics, and safety concerns. These intrinsic defects have posed significant obstacles to their widespread commercialization, particularly in the context of EVs ^[28]^[29]^[30]^[31]^[32].

2. Strategies and Countermeasures

Extensive research has been undertaken to enhance Ni-rich cathode materials performance in response to the growing demand for batteries with higher energy density. Common strategies employed for this purpose include elemental substitution, surface treatment, compositional adjustments, as well as the utilization of single crystals and concentration gradient structures. These methods are widely adopted to address the associated challenges and requirements effectively.

2.1. Elemental Doping

The incorporation of cation and anion doping has emerged as a prominent strategy to enhance the structural stability of Ni-enriched layered oxides ^[33]^[34]. This approach proves pivotal in improving the cycling performance of cathode materials by substituting unstable elements with inactive materials. It effectively hinders microcrack propagation within primary particles through the pillaring effect of cationic species, while stabilizing the oxygen octahedral site through anionic doping ^[35]^[36]. As an example, Mg emerged as a promising cation element with the potential to serve as an inert stabilizer, leading to enhanced performance in NCM622 cathode materials by mitigating structural deterioration. The introduction of Mg at concentrations of 1%, 3%, and 5% resulted in a reduction of Li/Ni cation intermixing by 1.58%, 1.77%, and 3.20%, respectively, compared to the 5.18% observed in undoped NCM. When Mg ions partially substituted for Ni in Mg-doped NCM, the unit cell parameters a and c decreased in magnitude, while the c/a ratios increased. This change typically signified the development of a layered material structure. The reduction in unit cell dimensions a and c was attributed to the incorporation of Mg²⁺ ions into both the Li layer and the transition metal layer. This incorporation led to a decrease in the overall content of Ni²⁺ ions residing within the Li layer. This phenomenon is comprehensible because the diminished presence of Ni²⁺ ions in the structure of Mg-doped powders resulted in a slight reduction in lattice parameters. Furthermore, it is noteworthy that the bond dissociation energy of Mg-O (∆H_f298 = 394 kJ mol⁻¹) surpasses that of Ni-O (∆H_f298 = 391 kJ mol⁻¹). Therefore, the introduction of Mg²⁺ ions into the host structure contributed to the enhancement of structural stability ^[37]. Anionic dopants, like F, have the capacity to occupy oxygen sites without compromising the capacity and energy density of the active materials. Among these anionic dopants, fluorine exhibits the highest electronegativity and establishes a remarkably stable structure due to the formidable bonds formed between transition metals and fluorine. Furthermore, the enhanced electrochemical performance of cathode materials doped with fluorine may be attributed to their shielding effect against HF production, which can arise during the decomposition of the liquid electrolyte ^[38]^[39]^[40]. Sung-Beom Kim et al. ^[41] employed a solid-state reaction method to synthesize F-doped NCM811. The pristine material exhibited a discharge capacity of 97.9 mAh g⁻¹ after 100 cycles at a rate of 100 mA g⁻¹, within the voltage range of 2.8–4.3 V. In contrast, the discharge capacities of LiNi_0.8Co_0.1Mn_0.1O_2−xF_x, with x values of 0.02, 0.04, 0.06, and 0.08, were measured at 107.8, 133.2, 169.6, and 154.5 mAh g⁻¹, respectively. Furthermore, assessing the capacity retention of various samples, including NCM-bare, NCMF-2, NCMF-4, NCMF-6, and NCMF-8 after 100 cycles, revealed retention percentages of 60.7%, 66.2%, 78.8%, 96.8%, and 89.5%, respectively. A diverse array of elements has been successfully employed as cation and anion dopants for transition metals and oxygen sites, aiming to enhance the electrochemical performance of Ni-rich cathode materials. Notable examples include Mo ^[42]^[43], Al ^[44]^[45], B ^[46], Nb ^[47]^[48]^[49], K ^[50], Na ^[51], Mg ^[37], Ti ^[52]^[53]^[54]^[55]^[56]^[57]^[58], PO₄^3- ^[59], and F ^[41]^[60]^[61]. It is worth mentioning that doping Ni-rich cathode materials with cations of varying valence states engenders distinct mechanisms that contribute to performance enhancement. Figure 2a–d provides a visual representation of the electrochemical effects resulting from a 1% molar elemental doping with diverse valence states on LiNi_0.91Co_0.09O₂ active material ^[62]. Notably, all doped materials, irrespective of low valence (Mg²⁺ and Al³⁺) or high valence (Ti⁴⁺, Ta⁵⁺, and Mo⁶⁺) states, exhibited superior electrochemical performance compared to the undoped counterpart in initial cycles, regardless of cycling rate and temperature. While samples doped with low valence states displayed satisfactory performance over extended cycles, such as 1000 cycles, capacity retention significantly decreased to 54.2% and 46.9% for Al and Mg-doped samples, respectively. In contrast, Ta- and Mo-doped samples maintained capacity retention even after 3000 cycles. Structural evaluations of samples, depicted in cross-sectional SEM images (Figure 2e) of cathodes cycled up to 1000 cycles, revealed that undoped NC90 exhibited severe intergranular cracks and disintegration into individual grains. Conversely, Mg and Al-doped samples showed incipient microcrack propagation, while Ta and Mo-doped samples retained particle coherency with minimal signs of microcrack nucleation and propagation, aligning with their exceptional cycling stabilities. These observations suggest diverse mechanisms contributing to the enhancement of doped samples.

Figure 2. Performance evaluation of cathodes (NC90, Mg-NC90, Al-NC90, Ti-NC90, Ta-NC90, and Mo-NC90) in pouch-type full cells, (a) showcasing first charge–discharge cycle curves at 0.1C and 30 °C, (b,c) cycling at 0.5C over 100 cycles at both 30 °C and 60 °C, and (d) extended cycling at 1C and 25 °C within the voltage range of 3.0–4.2 V vs. graphite; (e) presents cross-sectional images of recovered cathodes after 1000 cycles at 100% DOD, discharged state of 2.7 V. (f) Distinctions in the mechanisms and impacts of low-valence (Al) and high-valence (Nb, Ta, and Mo) dopants, encompassing their incorporation into the bulk material and the coating of grain boundaries. Reprinted from Ref. ^[63] with permission from John Wiley and Sons.

Park et al. ^[63] have proposed mechanisms for improvement through cation doping with different valence states (Figure 2f). The authors investigated the effects of a 1% molar doping of Al³⁺, Nb⁵⁺, Ta⁵⁺, and Mo⁶⁺ in LiNiO₂. They suggested that cations with low valence states, such as Al³⁺, participated in the bulk and acted as pillars to improve structural stability and electrochemical performance. Regarding high valence cations, such as Mo⁶⁺, Nb⁵⁺, and Ta⁵⁺, these elements not only participated in the bulk structure, but also reacted with Li resources and Li-X-O compounds forming and segregating at grain boundaries, inhibiting grain boundary migration, and preserving the size of primary particles. For instance, Al 1% molar doping in LiNiO₂ resulted in LiAlO₂ formation at 650 °C, disappearing at 680 °C. In contrast, compounds such as LiNbO₃ appeared at low temperatures, and at higher calcination temperatures, the insoluble Li₃NbO₄ phase were segregated in the grain boundaries. Consequently, if doping elements persisted as Li-X-O compounds during high-temperature calcination, they coated the grain boundary, inhibiting boundary migration and suppressing primary particle coarsening. For elements like Al and Mg with low valence states, it was suggested that higher doping molar concentrations (3%) could be beneficial for grain boundary reinforcement. In a study by Hüger et al. ^[64], it was revealed that compounds like LiNbO₃ and LiTaO₃ exhibit higher Li diffusivity, or in other words, had a higher Li insertion/extraction rate (about 1 × 10⁻¹⁸ and 8 × 10⁻¹⁹ m² s⁻¹, respectively) compared to other compounds such as LiAlO₂ and LiGaO₂ (about 4 × 10⁻²¹ and 1 × 10⁻²¹ m² s⁻¹, respectively). Furthermore, the bond dissociation energy of Nb-O and Ta-O is nearly twice (∆H_f298 = 753 and 805 kJ mol⁻¹, respectively) that of Mg-O and Ni-O, thereby enhancing structural stability.

2.2. Surface Coating

Surface treatment represents a foremost strategy to mitigate interfacial side reactions between cathode materials surface and electrolytes, with the aim of enhancing the retention of oxide ion vacancies within the crystal lattice post-initial charge. This approach concurrently addresses challenges such as the suppression of electrolyte decomposition, the mitigation of cathode-electrolyte interphase formation, the preservation of low microstrain for improved structural integrity and crystallinity during cycling, and the sequestration of HF from the electrolyte ^[65]^[66]^[67]^[68]^[69]. In the context of Ni-rich materials, a coating which is generally composed of nanoparticles and typically ranging from 5–20 nm can be broadly classified into three categories: oxides, phosphates, and fluorides. Among these, metal oxide coatings, including Al₂O₃ ^[70], Li₄Ti₅O₁₂ ^[71], ZrO₂ ^[72]^[73], Li₂WO₄ ^[74], and Li₂ZrO₃ ^[75] have emerged as widely adopted choices to address the limitations of Ni-rich materials. Shim et al. ^[76] specifically applied an acidic WO₃ coating to NCM811 particles, leveraging its intercalation host properties attributed to its well-defined cubic ReO₃-type structure and high resistance against HF attack. As depicted in Figure 3a,b, the WO₃ coating significantly enhanced the cycling performance of NCM811 within the 3–4.3 V range, and notably, under challenging conditions spanning a voltage range of 3–4.6 V. The capacity retention of the pristine active material at upper cutoff voltages of 4.3 V and 4.6V was 82% and 64.44%, respectively, while for the WO₃-coated NCM811, the corresponding values were 85.8% and 76.46%, respectively. In terms of rate capability performance (Figure 3c), the coated sample exhibited superior performance attributed to expedited Li ion and electron kinetics, stemming from enhanced structural stability and reduced resistance. This improvement was further attributed to the suppression of side reactions between the cathode material and electrolyte. Moreover, Figure 3d,e illustrates the absence of discernible microcracks along grain boundaries in NCM protected by a 2.14 nm thick WO₃ coating. In contrast, the untreated NCM, subjected to cycling-induced expansion and shrinkage, exhibited electrolyte infiltration into the interior, leading to structural collapse. This observation underscored the effectiveness of the WO₃ coating in preserving the structural integrity of NCM during cycling. The incorporation of a La₂O₃ coating as a protective layer has proven highly effective in augmenting the electrochemical performance of the Ni_0.91Co_0.06Mn_0.03 electrode in LIB ^[77]. Notably, the capacity retention of the cathode after 100 cycles at 0.5C demonstrated a substantial improvement, with a retention rate of 68.1% for the pristine NCM, whereas the La₂O₃-coated material exhibited superior retention at 87.2%. The cyclic voltammetry plots (Figure 3f) of both pristine and coated samples revealed significant improvements by coating deposition on NCM material. First, the intensity peak related to phase transformation H2 → H3 that is in charge of particle pulverization and capacity fading decreased significantly. Secondly, the oxidation-reduction gap (ΔE = E_oxidation − E_reduction), which serves as a determinant of electrode polarization, reduced from 0.111 V to 0.094 V ^[76]. A TiO₂ coating was employed to provide comprehensive protection for NCM811 cathode active materials, capitalizing on its structural stability and chemical inertness against the electrolyte at high voltages ^[78]. Q. Fan et al. ^[78] reported that the application of a continuous TiO₂ nano-coating layer on NCM811 not only resulted in a higher specific discharge capacity, but also significantly improved cycling performance, with 72.2% capacity retention for the coated sample and 45.7% for the bare sample. This improvement was attributed to the reduction in side reactions at the electrode and electrolyte interface, particularly at high upper cut-off voltages. The exceptional barrier properties of the TiO₂ coating led to a reduction in electrolyte consumption and the formation of a thick passivation solid electrolyte film. This reduction helped alleviate polarization at the material interface, preventing the inevitable damage resulting from direct contact between the active material and the electrolyte. Figure 3g illustrated a schematic representation of the protective mechanism of TiO₂, emphasizing the significance of an intact coating for active materials. This depiction underscores that the development of a partial coating may result in the isolation failure of particles, potentially leading to the collapse of secondary particles. Phosphate coatings have emerged as highly protective coatings for matrix materials, acclaimed for their exceptional thermal and electrochemical stability. This stability arises from the robust P=O bond, with a bond energy of 5.64 eV and the formation of strong covalent bonds between polyanions and metal ions, rendering them resistant to chemical attacks ^[79]^[80]^[81]^[82]. Among phosphate coatings, the Li₃PO₄ coating stands out as a prominent choice for Ni-rich active materials due to its rapid Li⁺ conductivity and high chemical stability ^[83]. The wet-coating method, utilizing an aqueous solution of NH₄H₂PO₄ or phosphoric acid, has been widely employed to extensively create Li₃PO₄ coatings on active materials, enabling a substantial reduction of lithium-based surface impurities (LiOH and Li₂CO₃) and the suppression of detrimental side reactions during both storage and cycling ^[84]^[85]. T. Sattar et al. ^[86] coated Li₃PO₄ onto layered oxide Ni_0.91Co_0.06Mn_0.03 active material by transforming Li residual compounds into the coating (Figure 3h). The superior electrochemical rating and cycling performance of the coated samples were attributed to the removal of lithium residual compounds and the isolation of particles, preventing direct contact with the electrolyte and excessive growth of the solid electrolyte interface layer. Additionally, the higher dissociation energy bond of P=O compared to transition metal–oxygen bonds contributed to better electrochemical performance, with capacity retention rates of 68.1% for pristine NCM and 82.4% for the surface-modified sample. In the context of EV industries, where thermal stability is crucial, Ni_0.8Co_0.15Al_0.05 cathode materials, employed in Tesla’s Model S and X, have gained prominence ^[87]. A Li₃PO₄ coating developed on Ni_0.815Co_0.15Al_0.035 active material has been shown to decrease heat generation during cycling. A study conducted by Tang et al. ^[88] confirmed through DSC profiles that the Li₃PO₄-modified samples generated less heat (525 J g⁻¹) compared to the pristine sample (757 J g⁻¹), highlighting the efficacy of the coating in suppressing surface side reactions with the organic electrolyte. The entire process of reaction between Li residual compounds and phosphoric acid can be expressed through the following Reactions (10)–(12) ^[89]^[90]:

3Li₂O + 2H₃PO₄ = 2Li₃PO₄ + 3H₂O, (R10)

3LiOH + H₃PO₄ = Li₃PO₄ + 3H₂O, (R11)

3Li₂CO₃ + 2H₃PO₄ = 2Li₃PO₄ + 3H₂O + 3CO₂(R12)

Figure 3. Cyclabilities of pristine and WO₃-coated NCM at a voltage of (a) 3.0–4.3 V and (b) 3.0–4.6 V at 0.5C, (c) rate capabilities of pristine and WO₃-coated NCM. FE-SEM images after 100 cycling for (d) WO₃ coated NCM and (e) pristine NCM. (f) Cyclic voltammetry curves for pristine and LaO-0.2. (g) Mechanism for the protective action of TiO₂ nano-coating on Ni-rich cathode materials. (h) Schematic illustration depicting the transformation of residual lithium into a Li₃PO₄ coating.

Fluorides exhibit lower Gibbs energy compared to oxides, indicating their heightened stability in comparison to oxide counterparts. The utilization of metal fluorides as protective coatings has been extensively investigated to address challenges associated with electrode side parasitic reactions with the electrolyte and HF etching. Among the various fluorine coatings, LiF is frequently employed for Ni-rich active materials. A thermodynamically stable lithium fluoride coating, characterized by superior Li ion conductivity compared to other fluorides, has been typically achieved through the mixing of NH₄F and NH₄HF₂ with the original cathode materials ^[91]^[92]. H. Kim et al. ^[93] treated Ni_0.85Co_0.12Al_0.03 cathode material with NH₄HF₂ and reported that this treatment helped to remove Li residual compounds from the particles’ surface and additionally, both cycling performance and gas evolution behavior were enhanced significantly in the modified sample.

2.3. Structural Engineering

The innovative approach to developing functional layered oxide materials involves creating a comprehensive concentration gradient structure. This strategy utilizes a high-capacity Ni-rich core in conjunction with a chemically stable Co, Mn-rich surface to prevent undesired Ni⁴⁺ reduction on the material’s surface. In a typical cathode with a full concentration gradient, the concentration of Ni gradually decreases from the particle’s center to its outer surface, while the concentrations of Co and Mn increase ^[94]. Hou et al. ^[95] synthesized a full concentration gradient (FCG) NCM622 with LiNi_0.8Co_0.2O₂ for the inner composition, and LiNi_0.4Co_0.2Mn_0.4O₂ for the outer composition. Reports indicated that the FCG-NCM622 cathode exhibited superior capacity retention at both room temperature and 55 °C. Moreover, DSC analysis confirmed the enhanced thermal stability of the structurally modified NCM622 compared to the normal sample. The exothermic peak observed at 282.7 °C for FCG-NCM622 and 266.2 °C for NCM622 indicated better thermal stability for the former. The generated heat was 692 J g⁻¹ for FCG-NCM622 and 969 J g⁻¹ for NCM622, attributed to the presence of Mn on the outer surface of the active material. Recently, a combination of FCG and coating-doping modifications has been employed to improve the thermal and electrochemical performance of Ni-enriched cathode materials ^[96]^[97]. Liao et al. ^[98] utilized dual modification for NCM material, incorporating both a concentration gradient and a protective Al₂O₃ coating. Figure 4a–c demonstrates that this dual modification significantly enhanced electrochemical performance, resulting in lower voltage decay over cycling compared to a normal cathode. This improvement was attributed to the reduced reaction of the cathode material with the electrolyte, and higher structural stability. The capacity retention for coated CG-NCM after 100 cycles was approximately 88%, outperforming CG (80%) and normal NCM (62.6%) as shown in Figure 4d. The impact of dual modification became more evident in cycling at higher temperatures (55 °C), as illustrated in Figure 4e. In terms of thermal stability, DSC profiles (Figure 4f) confirmed that both coated CG and CG samples exhibit a higher onset temperature for the exothermic reaction compared to the normal sample, indicating the effectiveness of both modifications. Park et al. ^[99] explored the electrochemical and thermal characteristics of a concentration gradient Ni_0.865Co_0.120Al_0.015 cathode material. Although conventional NCA demonstrated a slightly higher initial discharge capacity at 0.1C in the operating potential range of 2.7–4.3 V compared to CG NCA (225 and 222 mAh g⁻¹, respectively), the capacity retention of the CG electrode at 0.5C, both at room temperature and 60 °C, was notably superior to that of the standard sample. Moreover, differential scanning calorimetry (DSC) measurements on charged samples at 4.3 V revealed an exothermic peak at 192 °C with a heat generation of 1789 J g⁻¹ for conventionally synthesized NCA, whereas for CG NCA, it was observed at 202 °C with a lower heat generation of 1409 J g⁻¹, indicating enhanced thermal stability of the structurally modified NCA. Xu et al. ^[100] successfully synthesized a concentration gradient cathode material comprising Ni_0.7Co_0.13Mn_0.17 with NCM811 composition at the core and NCM523 at the outer surface (Figure 5a–e). The initial charge–discharge profile (Figure 5f) of NCM523, NCM811, and the CG NCM indicated that the CG sample exhibited a lower discharge capacity compared to NCM811, attributed to its lower Ni content on the surface. Additionally, the plateau at around 4.2 V was shorter for NCM523 and CG compared to NCM811. Moreover, the H2 → H3 phase transition, recognized as the primary reason for volume change and subsequent capacity fading in Ni-rich electrodes, was pronounced for NCM811 compared to the CG cathode material (Figure 5g). The rate capability of the CG electrode outperformed that of NCM811 and NCM523 counterparts, delivering much higher capacity at high discharge rates (Figure 5h). Cycling profiles of the samples at 1C (Figure 5i) revealed that NCM811 experienced catastrophic capacity retention, while the performance of CG was highly similar to that of NCM523. These results suggest that utilizing the CG cathode offers benefits for both NCM523 in terms of capacity retention, and for NCM811 in terms of high capacity delivery.

Figure 4. Charge discharge curves for (a) normal, (b) CG, and (c) CG-Al₂O₃ electrodes within the voltage range of 2.7 to 4.5 V at room temperature, comparative cycling performances (C/5) for the three cathodes at (d) room temperature and (e) 55 °C, and (f) DSC traces illustrating the heat flow during the reaction of the electrolyte with CC, CG, and CG-Al₂O₃ electrodes when charged to 4 V.

Figure 5. (a) Illustration depicting the distribution of transition metals within the concentration CG NCM particle, (b) profiles showing the calculated transition metal compositions over time, and (c) from the particle’s core to its surface. (d) EDS line scanning analysis conducted across the precursor particle (the yellow line denotes the scanning path), (e) comparative analysis of the measured composition profiles with the calculated outcomes. Electrochemical performance of CG NCM, NCM523, and NCM811 cells within the voltage range of 3.0–4.3 V: (f) initial charge–discharge profiles at a 0.1C rate, (g) differential capacity curves for the initial charge–discharge cycle, (h) rate capability assessed at different charge-discharge current densities, and (i) cycling performance evaluated at a 1C rate.

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