Nanomaterial coating for layered lithium rich transition metal oxide cathode for lithium-ion battery

3.1 Minimization of electronic transport length (L _e) with electronic conductive nanolayer coating

To enhance electron transport, the electronic transport length (L _e) should ideally be less than or equal to the particle size of the active material. This enables electrons to move back and forth along the material’s surface, and nanolayer coatings can significantly improve electronic transport. Commercialized batteries currently use micrometer-sized particles for electrodes, which offer a low surface area of less than 10 m² g⁻¹ [61]. However, these larger particles have less contact area for the electrode surface and electrolyte so it exhibit long electronic transport length which is not advantageous. Zhou et al. [62] studied the electrochemical performance of Li₂SnO₃-coated Li_1.2Mn_0.54Ni_0.13Co_0.13O₂. Li₂SnO₃ nanolayer coating decreases R _ct, enhancing Li ion diffusion and electronic conductivity of the material. The coating minimizes electronic transport length, allowing electrons to travel shorter distances with fewer obstacles. This creates a more efficient pathway for electron flow, improving overall electronic conductivity. Enhanced electronic conductivity indicates that electrons can move more freely within the material, which is essential for the efficient flow of electrical current in batteries. Consequently, Coulombic efficiency, rate capability, and capacity retention (CR) are improved after Li₂SnO₃ nanolayer coating.

Nanolayer coatings, with thicknesses ranging from several nanometers to 50 nm, are well-suited for battery application. Figure 3 summarizes the distinction between conventional [63], core/shell [64], and ultrathin nanolayer [65] coatings. Thin nanolayer coatings, thanks to their high surface area, provide more contact area between active material particles and the electrolyte, effectively minimizing the L _e value of the electrode material [66].

Figure 3

Comparison between conventional coatings [63] (Copyright 2015, Elsevier), core/shell coatings [64] (Copyright 2005, American Chemical Society), and ultrathin nanolayer coatings [65] (Copyright 2017, Elsevier).

3.2 Fast Li ion diffusion

Coatings, particularly those utilizing nanostructured materials, play a pivotal role in enhancing the performance of energy storage systems. The inherent properties of nanostructures, such as their increased surface area and shortened ion diffusion pathways, contribute to more efficient lithium ion transport. This results in quicker charge and discharge rates, ultimately improving the overall conductivity of the system. Additionally, nanostructured coatings facilitate improved interaction between the electrode material and electrolyte, promoting rapid lithium ion adsorption and desorption. These coatings also often act as a preventive measure against dendrite formation, enhancing safety and longevity. Altogether, coating methods, especially those incorporating nanostructures, represent a promising strategy for achieving and optimizing fast lithium ion conductivity in advanced energy storage technologies.

Yang et al. [67] conducted in situ characterization of the grain boundary’s impact on Li-ion diffusion in Li-rich cathodes. Conductive-atomic force microscopy revealed that atoms at grain boundaries are more conductive and have lower activation energy than atoms in the grain interior, allowing for faster Li ion diffusion along the grain boundaries. However, grain boundaries are in fact responsible for phase transformations in the material. This is due to the ease with which metal ions diffuse along grain boundaries, forming nuclei that grow into the trigonal phase at the grain boundary. When nanoscale particles are mixed into the cathode material, they form a zigzag structure along the grain boundaries, blocking the diffusion of metal ions and hence preventing the nucleation and growth of the trigonal phase at these grain boundaries.

In addition to the role of grain boundaries, the concept of “Nanoionics” is vital in nanoelectrodes. Nanoionics involves the study and application of phenomena, properties, and mechanisms associated to rapid ion transport within NMs in all-solid-state batteries. In 2013, Zhu et al. discovered that Li _x CoO₂ nanograins have low energy barriers for lithium ion mobility, allowing for rapid lithium ion diffusion. When composed of nanograins with large grain boundary areas, Li _x CoO₂ electrodes operating at optimum voltage demonstrate excellent rate capability and high-power performance in batteries [68]. The comparison between microsized cathode materials and nanolayer-coated cathode materials is shown in Figure 4. Nonetheless, understanding Li-ion transportation in NMs remains incomplete due to a lack of appropriate and effective characterization techniques capable of revealing mechanisms and processes at the molecular level. It is difficult to demonstrate nanoscale mapping of intercalation, charge and ion transport, and dislocation at single grain boundaries.

Figure 4

Comparison between microsized pristine cathode material and nanomaterial-modified microsized cathode material.

3.3 Large contact area between active material and electrolyte from a nanolayer coating

Because of their fine particle size and large surface area, NMs are ideal for promoting Li ion transport in LIBs, and the charge/discharge process is heavily impacted by the Li surface storage capacity. The NM coating has a large surface area for storing Li ions, which reduces the specific current density of the active material and effectively improves the electrode’s stability at high charge/discharge current densities [51]. The increased surface area allows for a greater interface between the active material and electrolyte. This facilitates more effective and rapid interaction between the electrode and electrolyte ions, promoting improved ion diffusion and charge transfer kinetics. Additionally, a larger surface area can enhance the dispersion of particles within the electrode material, contributing to the maintenance of structural integrity during cycling and reducing the risk of mechanical stress and fracture. Improved structural stability contributes to long-term durability and performance of the electrode.

3.4 Significant benefit of nanolayer coating on lithium ions storing capacity

A nanolayer coating on the cathode material significantly impacts lithium ion storage capacity. The increased surface area facilitates efficient lithium ion diffusion and accelerates charge transfer kinetics. Acting as a protective barrier, the coating prevents undesired side reactions, preserving the cathode’s chemical integrity and ensuring sustained lithium storage capacity. Additionally, by mitigating particle agglomeration and providing mechanical support, the nanolayer enhances structural stability, preventing stress and fracture. This improved stability optimizes overall electrochemical performance, resulting in a higher lithium storage capacity. In summary, nanolayer coatings play a crucial role in enhancing the specific capacity of cathode in advanced energy storage systems.

NM coatings exhibit higher lithium storage capacity than pristine electrode, because previously inactive sites become active and react with lithium ions at the nanoscale. As the particle size of the NMs decreases, the discharge capacity increases since more lithium ions may be stored on the surface, substantially shortening the lithium ion transport length [51]. A conformal nanocoating of La₂O₃ can effectively boost Li ion diffusion, resulting in a thin SEI layer that facilitates the Li ion storage capacity of the material [69].

4.1 Agglomeration

The uncontrolled growth of nanoparticles is a disadvantage of NMs in a variety of applications. Nanoparticle agglomeration can result in the formation of nanoparticle clusters, which results in a non-uniform NM coating layer and has an impact on the morphological stability of NMs. It is preferable to use a capping agent during the NM synthesis to control nanoparticle growth. To reduce agglomeration, various capping agents, such as surfactants, polymers, and ligands have been utilized in the NM synthesis [70].

4.2 Undesired surface reactions

NMs have the characteristic of high surface area and low density which makes them prone to static charges and rapid dispersion in the air. An unfavourable reaction between active material particles and the electrolyte can lead to the formation of a thick SEI layer, which hinders the transport of lithium ions and electrons. As a result, careful handling of bulk NMs is required, and a glove box can be used for this purpose.

4.3 High cost of production

The production cost can be doubled when using NM-modified cathodes because it involves two steps: (i) preparing the pristine cathode and (ii) modifying NMs onto the pristine cathode. Implementing a one-step synchronous coating process is an effective way to reduce production costs.

4.4 Complex synthesis routes

Highly scalable preparation methods such as co-precipitation and solid-state synthesis are suitable for mass production of the micrometer-sized pristine cathodes because they are simple and effective. However, methods applicable for NM coatings, such as hydrothermal synthesis, sol–gel synthesis, and atomic layer deposition (ALD), are not scalable. Therefore, the scalability of NM-modified cathode synthesis should be carefully analysed.

5.1 Synchronous coating process

In the synchronous route, the coating material (shell) is directly mixed during the synthesis process of the core cathode material. Synchronous coatings are preferred because, in post-synthesis coatings, the core cathode material and the coating material shell are mixed in the presence of water, which increases the mixture’s alkalinity. This can lead to the formation of Li₂CO₃ and LiOH impurities due to the reaction of dissolved Li species with water, subsequently reducing the cathode material’s performance [71]. During the synchronous coatings, each particle of the cathode material can be uniformly covered by the coating material, resulting in consistent coating thicknesses and particle sizes. This approach also reduces manufacturing costs and time.

Zhang et al. [72] proposed a synchronous Li₂ZrO₃ coating strategy on LLR (indicated as LMNO in the original work). Figure 5(a) illustrates two strategies: the synchronous modification route (material named syn-Li₂ZrO₃@LMNO) and the post-coating method (material named post-Li₂ZrO₃@LMNO). In synchronous coating, hydrolysis of Zr(OC₄H₉)₄ occurs on the Ni_0.25Mn_0.75O₂ surface. Syn-Li₂ZrO₃@LMNO comprises bulk Zr⁴⁺ doping integrated with Li₂ZrO₃ coating, whereas bulk Zr⁴⁺ doping is absent in post-Li₂ZrO₃@LMNO. Li₂ZrO₃ coating serves as a Li-ion conductor, while Zr⁴⁺ doping enhances the stability of the layered structure and reduces the voltage fade. The Syn-Li₂ZrO₃@LMNO showed a capacity of 239.8 mAh/g, surpassing the bare LMNO, which exhibited a capacity of 207.8 mAh/g at 0.1 C. However, the electrochemical performance of bare LMNO and post-Li₂ZrO₃@LMNO material shows a striking similarity. The modified LLR cathode exhibits superior rate capability and cycle stability compared to the pristine cathode.

Figure 5

(a) Schematic illustration of the preparation process for syn-Li₂ZrO₃@LMNO and post-Li₂ZrO₃@LMNO [72] (Copyright 2016, Royal Society of Chemistry), (b) chemical deposition method for preparation of F-doped TiO₂ coating on LLR [73] (Copyright 2019, Elsevier), (c) sol–gel preparation of LiAl₅O₈ coating on LLR [76] (Copyright 2019, Elsevier), (d) hydrolysis-hydrothermal synthesis procedure for the preparation of the LiAlO₂-coated LiNi_0.8Co_0.1Mn_0.1O₂ composite [79] (Copyright 2018, Springer), and (e) deposition of Al₂O₃ by atomic layer deposition technique [91].

5.2 Post-synthesis process

Various post-synthesis coating processes have been developed, which can be categorized into four groups.

5.3 Dry physical synthesis

Dry physical synthesis is a cost-effective and scalable coating process that requires minimal chemicals. It typically involves the use of dry milling equipment such as planetary ball mills and Nobilta mini bowls [81,82]. In dry chemical synthesis, a coating precursor is mixed with the pristine cathode using ball milling or similar equipment. Subsequently, the powder is sintered at an optimal high temperature, a process referred to as a “solid state reaction.” For instance, Nisar et al. [81] employed this method to coat ZrO₂ on Li_1.2Ni_0.16Mn_0.56Co_0.08O₂ through ball milling and solid-state reaction, resulting in the formation of a thin, uniform coating layer with a thickness less than 10 nm.

Another dry chemical synthesis approach is chemical etching. In this method, the precursor of the desired coating material is mixed with the LLR cathode material to etch the cathode’s surface. Cui et al. employed a similar strategy to modify LLR [83]. They mixed LLR material with ceric ammonium nitrate (CAN) using a microtube homogenizer, resulting in an increase in Mn and Co content and a decrease in Ni content.

5.4 Gas phase chemical synthesis

Gas phase chemical synthesis includes various coating methods such as low-pressure vaporization and ALD [65,84,85].

5.5 Advanced physical synthesis

Advanced physical synthesis is an emerging method for preparing the ultrathin and uniform coating layer on the cathode’s surface, which is explained in detail as follows.

6.1 Oxides

Metal oxides act as effective barriers, inhibiting the formation of SEI by physically separating the electrode and electrolyte. Commonly used metal oxides for coating LLR surfaces include ZnO [65], ZrO₂ [81], TiO₂ [95], MoO₃ [96], and more. The electrochemical performance varies with the choice of metal oxide due to differences in their structures and properties. Coating with metal oxides significantly extends the cycle life of LLR cathodes. These coating materials are cost-effective and readily available. Various methods, such as chemical deposition, sol–gel, ALD, ball milling, and precipitation, have been employed for the modification of oxide coatings on LLR surfaces [65,73,97,98].

The formation of parasitic impurities at the active material–electrolyte interface can negatively impact the electrode performance. This issue is primarily caused by the loss of oxygen from the LLR structure due to irreversible anionic redox activity [99]. To prevent parasitic impurities formation, it is essential to reduce this irreversible anionic redox activity. Some theories suggest that a small amount of ZnO can suppress hydrogen fluoride (HF) formation and improve the electrochemical coupling coefficient, chemical stability, and cycle performance of Mn-based cathode material [100,101]. Yu et al. conducted a post-annealing treatment on ZnO-coated material in an Ar atmosphere [63]. Initially, they prepared Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ using the sol–gel method and then modified it with ZnO using a wet chemical method. The results showed that the ZnO-coated sample annealed in the Ar atmosphere exhibited more oxygen vacancies compared to the unannealed ZnO-coated sample and the ZnO-coated sample annealed in an O₂ atmosphere. These oxygen vacancies played a crucial role in improving electrochemical activities by enhancing the electronic conductivity, reducing resistance to Li-ion intercalation, and suppressing the formation of highly reactive oxygen radicals during charge and discharge processes [102].

Kong et al. [65] recently explored the advantages of achieving a uniform, homogeneous coating of ZnO on pristine LLR material. They opted for the ALD process for this modification. When comparing the findings of Yu et al. [63] and Kong et al. [65], it becomes evident that ALD has the potential to form uniform, ultrathin, and homogeneous nanocoatings when compared to the wet chemical method. In the TEM images, it is clear that ALD can create uniform, ultrathin, and homogeneous nanocoatings (Figure 6(a)–(c) vs (d)–(f)). Figure 6(e), shows a minor change in the voltage plateau at 2.5 V during discharge for ZnO-coated LMNCO material, is attributed to the formation of an ultrathin spinel layer [103]. This suggests that ALD holds significant promise for preparing an ultrathin NM layer on cathode materials.

Figure 6

(a) TEM image of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂@ZnO particles. (b) The initial charge and discharge curves of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ and Li_1.2Ni_0.13Co_0.13Mn_0.54O₂@ZnO at 0.1 C. (c) Rate capability of bare and ZnO-coated Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ composite [63] (Copyright 2015, Elsevier). (d) LMNCO coated with 20 ZnO layers. (e) Initial charge–discharge curves of the pristine LMNCO and ZnO-coated LMNCO electrodes at 0.12 C. (f) Rate capability of the pristine and ZnO-coated LMNCO electrodes [65] (Copyright 2017, Elsevier). HRTEM images of (g) and (h) LNCM electrode and (i) and (j) NFO₃ electrodes, respectively. All electrodes were operated in a charge/discharge process after 200 cycles at 1 C rate. (k) Schematic of the heterostructural O-sharing bonding of LNCM (104 plane) and NFO (400 plane) [104] (Copyright 2021, American Chemical Society). Cycling performance of P-LLMO, S-LLMO-0.5 wt%, and S-LLMO-1.0 wt% at a current density of 0.5 C between 2.5 and 4.6 V at (l) 25°C and (m) 60°C [105] (Copyright 2019, Springer).

The release of active oxygen from pristine LLR can disrupt redox reactions during charge and discharge processes, leading to decreased capacity and increased ICL. Additionally, oxygen release and cation migration in LLR can result in the transformation of the layered structure to spinel in the pristine electrode. To address these challenges, Hu et al. developed a perovskite-type hexagonal LSM material for modifying Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ (LM) [74]. They observed the formation of strong Mn‒O‒M bonding at the LSM/LM interface, which reduced oxygen release and inhibited the dissolution of Mn. This innovative approach significantly improved CR, with a 48% increase for 2 wt% LSM@LM and 82% ICE for 3 wt% LSM coating. The LSM coating helped to mitigate defects and nanovoid formation during cycling and reduced oxygen release during charging.

Subsequently, the same research group developed an inverse spinel NiFe₂O₄ (NFO) coating on Li_1.2Ni_0.13Co_0.13Mn_0.54O₂, offering a cost savings of 36.9% compared to LSM coating [104]. Unlike LSM, NFO contains NO₆ octahedra. The M‒O‒N (M = TM ions from LNCM, N = TM ions from NiFe₂O₄) bonding is established after coating. As seen in Figure 6(g) and (h), the layered to spinel transformation occurred in the LNCM cathode along with structural defects and nanovoids, resulting in the degradation of electrochemical performance. NFO-coated LNCM increased the structural stability, and therefore the electrochemical performance is enhanced (Figure 6(i) and (j)). The mechanism of O-sharing between M and N is illustrated in Figure 6(k). The M‒O‒N bonding mechanism prevented lattice oxygen near the surface from participating in anionic redox reactions and resisted structural collapse during Li ion extraction. As a result, NFO-coated LNCM exhibited enhanced structural stability, improved ICE, and reduced voltage decay.

SiO₂ NM coating can show good cycle stability at both room temperature and high temperature. Meng et al. [105] modified Li_1.2Mn_0.56Ni_0.16Co_0.08O₂ by single shell SiO₂, demonstrating enhanced electrochemical performance. The cycle performance for P-LLMO, 0.5 wt% S-LLMO, and 1 wt% S-LLMO is assessed at 0.5 C in a potential range of 2.5–4.6 V at both 25 and 60°C (Figure 6(l) and (m)). The results showed that the SiO₂ NM coating improves the thermal stability of the LLR cathode but decreases discharge capacity and rate capability, suggesting it does not reduce oxygen release. α-MoO₃/Li₂MoO₄ modification achieves a superior ICE of 89.6% due to enhanced Li insertion channels and improved electrical conductivity of the interface. Rate capability is also enhanced, though cycle stability remains similar to other oxide coatings. La–Co–O surface modification can suppress the voltage decay [97]. The voltage intervals (ΔV) between La–Co–O coated LLR and pristine LLR increase with higher C-rates, indicating greater polarization for pristine LLR. But the specific capacity and cycle stability of La–Co–O coated LLR are not adequate when compared to surface coatings discussed earlier in this section. From the graphs, it is clear that ICE decreased after La–Co–O coating, implying inefficient Li-ion intercalation.

Metal oxides act as HF scavengers or HF barrier agent. It is investigated that HF barrier agents are more effective than HF scavengers.

Spinel materials offer structural stability and 3D channels for Li-ion diffusion, making them suitable for improving the electrochemical performance of LLR. Researchers have developed a technique where a spinel layer is uniformly coated onto host layered material, resulting in a “heterostructured material” [106–109]. The concept of Mn-based integrated layered spinel composites is introduced by Thackeray’s group in 2005 [110], and since then, many researchers have explored this approach. Both post-synthesis modification and one-step modification (synchronous route) techniques have been employed to enhance rate performance [108,111–113]. However, a high content of spinel material can reduce the overall capacity, making it essential to modify the host material with an ultrathin layer of the spinel phase to achieve good electrochemical properties. Spinel materials are known for their high ionic and electronic conductivity [114], high voltage window, cost-effectiveness, and lower toxicity. Two different research groups reported Al₂O₃ nanocoatings on LLR material [85,115]. Comparing these two studies, it is found that 1 wt% Al₂O₃ coated LLR via the ALD process exhibited ∼147 mAh/g at 1 C, with 84% CR after 200 cycles [85]. No spinel layer is formed in this material. In contrast, LLR nanotubes synthesized via the electrospinning method and coated with 4 wt% Al₂O₃ using the freeze-drying method exhibited 210 mAh/g at 1 C with 97.6% CR after 90 discharge cycles [115]. An ultrathin spinel layer (2 nm thickness) formed between the LLR nanotubes and the Al₂O₃ layer, promoting Li ion diffusion and reducing ICL during initial cycling. Therefore, the ultrathin spinel layer has been proven beneficial for electrochemical cycling.

While oxide coatings have shown promise, their specific capacity has room for improvement. This improvement can be achieved by reducing the charge transfer resistance (R _ct) and balancing the ratio between cationic and anionic redox reactions. Porous electrode materials have the potential to improve R _ct, and enhancing the rate capability of oxide coatings is crucial as it relies on the electronic and ionic conductivity of the material [99].

6.2 Phosphates

Phosphate coatings applied to cathode materials serve to enhance thermal stability by introducing PO₄ polyanions [116]. These coatings not only safeguard the cathode surface from HF electrolyte species but also effectively reduce ICL, thereby improving electrochemical properties [117]. The methods used for LLR modification include chemical deposition and sol–gel [118–121].

To improve the cycle performance of Li₃PO₄ coatings, a thin spinel layer can be incorporated between the coating and the LLR surface, as demonstrated by Yang et al. [119]. They treated Li_1.2Ni_0.2Mn_0.6O₂ with H₃PO₄, resulting in the in situ formation of a Li₃PO₄ layer. Four samples with varying levels of H₃PO₄ pre-treatment were prepared, namely HPO-0, HPO-2, HPO-4, and HPO-6 (sample pre-treated with 0, 0.02, 0.04, and 0.06 mol L⁻¹ H₃PO₄ solution). The spinel phase is formed in HPO-4 and HPO-6 samples. Voltage decay, a significant issue in LLR, is primarily caused by oxygen release and irreversible cation migration. Li₃PO₄ coatings have the potential to mitigate voltage fade in LLR. The voltage decay of four samples HPO-0, HPO-2, HPO-4, and HPO-6 is shown in Figure 7(a)–(d). The graph shows that the voltage decay is effectively reduced for HPO-4. The voltage decay is hindered for HPO-6 because of the production of a high amount of spinel coating. Cycle performance at 1 C for 100 cycles is shown in Figure 7(e). The CR after 100 cycles for HPO-4 reached 92.2%, whereas the pristine sample only retained about 70.9% of its capacity. The transition from layered to spinel structure in Li₃PO₄-coated LLR is reduced due to the presence of a thin spinel phase in the Li₃PO₄ coating. However, H₃PO₄ is acidic and can potentially damage the crystal structure. In contrast, the aqueous solution of the alkaline (NH₄)₂HPO₃ helps maintain the crystallinity of the modified electrode.

Figure 7

Discharge curves of (a) HPO-0, (b) HPO-2, (c) HPO-4, and (d) HPO-6 at 1 C between 2 and 4.8 V, over the course of 100 cycles. (e) Cycling performance for HPO-treated electrode at 1 C in the voltage range 2–4.8 V [119] (Copyright 2019, Springer). (f) Cycling performance at 1 C rate of the pristine and LLO-LP sample, SEM images of electrode sheet, TEM images of active material particle for the pristine (g), (i), and (k) and for LLO-LP sample (h), (j), and (l), (m) SAED patterns from (k), and (n) SAED patterns from (l). Both samples were analysed after 200 cycles at 1 C rate [122] (Copyright 2020, Royal Society of Chemistry).

Liu et al. [122] employed a synchronous route with a (NH₄)₂HPO₃ precursor to prepare Li₃PO₄-modified Li_1.18Mn_0.52Ni_0.15Co_0.15O₂ (LLO). In this process, the primary particles of LLO were covered by a Li₃PO₄ layer, referred to as LLO-LP, with a thickness of 2 nm. This Li₃PO₄ coating significantly improved the material’s cycle performance (Figure 7(f)). After 200 cycles, the CR for 2 wt% Li₃PO₄-LLO reached 81.3%, compared to just 47.2% for the uncoated LLO. A significant increase in discharge capacity is observed in the (NH₄)₂HPO₃-treated sample compared to the H₃PO₄-treated sample, relative to their respective discharge capacities of pristine materials. The first cycle discharge capacity also improved from 260.9 to 280.1 mAh/g for (NH₄)₂HPO₃-treated sample (LLO-LP), outperforming the discharge capacity of 262.6 mAh/g for the H₃PO₄-treated sample. The initial Coulombic efficiencies for H₃PO₄-treated sample and (NH₄)₂HPO₃-treated sample are 84.4 and 86.3%, respectively. This is because alkaline conditions may influence the electrode/electrolyte interface, promoting enhanced ion mobility and facilitating more efficient electrochemical reactions during discharge. However, the rate capability of H₃PO₄-treated sample is better than that of (NH₄)₂HPO₃-treated sample. This suggests that the acidic medium treatment may provide superior performance under certain conditions, possibly due to faster charge and discharge kinetics. In summary, the choice between acidic and basic treatments significantly influences the electrochemical performance of the samples. The alkaline treatment appears to enhance discharge capacity and initial Coulombic efficiencies, while the acidic treatment demonstrates superior rate capability. The specific mechanisms behind these effects would require more detailed analysis of the electrochemical processes occurring at the electrode–electrolyte interface under each treatment condition.

To enhance the CR of an electrode, it is essential to minimize charge transfer impedance R _ct (Ω), electrolyte decomposition, Mn dissolution, and internal resistance R _s (Ω) caused by the electrolyte and other resistive components. SEM and TEM images of pristine and LLO-LP coated electrodes cycled at 1 C for 200 cycles were examined. The SEM and TEM images (Figure 7(g), (i), and (k) for the pristine sample and Figure 7(h), (j), and (l) for the (NH₄)₂HPO₃-treated sample (LLO-LP)) revealed that the pristine electrode, cycled for 200 cycles, exhibited side reactions, and corrosion pits on the electrode surface, and a thick SEI layer. In contrast, the LLO-LP sample featured a clean surface without side reactions or corrosion pits, with visible fringes. Selected area electron diffraction (SAED) patterns demonstrated that the pristine electrode underwent the transition from a layered to a spinel and then to a rock-salt structure, as shown in Figure 7(m) and (n). While Li₃PO₄ modification did not prevent the formation of a spinel structure, it effectively stabilized the spinel structure and preserved high capacity.

In Table 2, several critical electrochemical parameters are provided, including CR, resistance in series R _s (Ω), charge transfer resistance R _ct (Ω), and Li⁺ diffusion coefficient (D _Li+). D _Li+ can be calculated using the following equation:

where R is the molar gas constant, T is the absolute temperature, A is the active electrode area, n is the number of electrons per molecule during the redox process, F is Faraday’s constant, C is the Li⁺ concentration in moles, and σ is the Warburg impedance coefficient. The Warburg impedance coefficient σ can be calculated using the following equation:

Here, the values are determined from the slope drawn on Z′ versus the square root of frequency ( w − 1 / 2 ) ; whereas, k = R s + R ct .

LiTi₂(PO₄)₃ (LTP)-modified LLR does not effectively reduce the transition from a layered to a spinel structure, resulting in similar capacity and voltage decay compared to unmodified LLR [123]. Researchers have introduced a novel material, Li_1+y Ti_2−y Al _y (PO₄)₃ (LTA-0.3), to enhance Li-ion conductivity in LLR [120]. LTA-0.3 exhibits a discharge capacity of 200 mAh/g after 170 cycles, maintaining 140 mAh/g after 500 cycles at 1 C. The voltage fading decreases with increasing cycle number, and the Li⁺ diffusion coefficient is calculated as 1.641 × 10⁻¹¹ cm² s⁻¹. Another research group worked on the Li_1.4Al_0.4Ti_1.6(PO₄)₃/polyanilline (LATP@PANI) modification of LLR [121]. While this material showed high ICE (ICE >80% at 0.1 C), its specific capacity, cycle stability, and rate capability were not ideal compared to the work by Yang et al. Vanadium coatings offer excellent chemical and thermal stability, and electrochemical activity at a very low cost [124]. Shi et al. [118] modified LLR with 5 wt% Li₃V_1.5Al_0.5(PO₄)₃ (LVAP) using a chemical deposition method. LVAP-modified LLMO demonstrated a discharge capacity of 228 mAh/g at 1 C with 96% CR after 100 cycles, outperforming Li₃PO₄ modification.

Overall, the study of phosphate coatings reveals several insights: (1) controlling the spinel phase layer is crucial, (2) the thickness of NMs should be appropriate, (3) vanadium-containing phosphates can enhance structural and chemical stability while reducing voltage fade, and (4) a small amount of Al³⁺ doping in LiTi₂(PO₄)₃ structures can strengthen the Ti–O bond in the lattice.

6.3 Fluorides

The reversible extraction of Li⁺ from lithium oxide (Li₂O) offers high capacity and increased ICE. However, irreversibility in Li⁺ extraction and the formation of Li vacancies are key issues leading to capacity loss and low ICE. Fluorine (F) is highly electronegative and can form strong M‒F bonds (e.g., Li‒F = 684.8 eV), which reduces the irreversibility of Li⁺ extraction and Li vacancy formation. Various fluoride coatings, including ZrF₄, YF₃, SmF₃, and MnF₂ composites, have been demonstrated to enhance LIB performance. Chemical deposition methods are commonly used for depositing fluorides on LLR surfaces [125–127].

Niu et al. modified LLR with YF₃ and conducted secondary calcination to investigate the effect of F⁻ diffusion into the bulk region of LLR [126]. YF₃ modification activated the electrochemically inactive component Li₂MnO₃, while secondary calcination led to the formation of the spinel phase with 3D Li⁺ channels and F⁻ diffusion into bulk regions. Figure 8(a) shows the voltage decay for each sample after 100 cycles. The voltage decay for 0.05-YF₃ at 600°C is 331.9 mV, which is comparatively less than pristine material. This indicates that secondary calcination stabilized the material’s structure and reduced voltage decay during cycling. Samarium fluoride (SmF₃)-coated LLR showed improved electrochemical performance compared to pristine LLR but it did not outperform coating materials [127]. Zhu et al. [128] studied the performance of an electrochemically active MnF₂ coating on LLR. The highest discharge capacity of about ∼300 mAh/g is achieved for the 1.5 wt% MnF₂ coating, with good rate capability (150 mAh/g at 1 C rate). However, the cycle performance of this material is unsatisfactory.

Figure 8

(a) Voltage decay of each material after 100 cycles [126], charge–discharge characteristics of pristine LRNCM and LRNCM @CF sample: the (b) first and (c) second cycle curves. The electrochemical property of pristine LRNCM and LRNCM @CF samples. (d) The cycling performance. (e) Discharge capacity above and below 3.5 V vs Li/Li⁺ during cycling at 1 C [129] (Copyright 2019, Elsevier). (f) Long-term stability test of LNMO, LNMO@AlF₃, and LNMO@LiAlF₄ electrodes at 5 C for 3,000 cycles [130] (Copyright 2018, American Chemical Society).

Coating Li_1.14Ni_0.133Co_0.133Mn_0.544O₂ (LRNCM) with a 10 nm thick layer of graphite fluoride resulted in a significant increase in discharge capacity, reaching 325 mAh/g at 0.1 C, which is 22% higher than the pristine electrode within the 2.0‒4.8 V voltage range [129]. During the first discharge, graphite fluoride produced LiF and carbon, improving ICE and capacity by suppressing interfacial side reactions. Figure 8(b) and (c) shows the first and second charge–discharge cycles, respectively. The voltage plateau at 2.4 V during the first discharge cycle is attributed to the reaction between (CF) _n and Li⁺ ions to form LiF. The ICE for pristine LRNCM is 77%, and after modification with 10% CF (LRNMC@10%CF), it improved to 99%. The cycle performance showed that for LRNCM, the discharge capacity decreased from 240.2 to 180 mAh/g after 100 cycles, while for LRNMC@10%CF, it decreased from 273.5 to 203 mAh/g (Figure 8(d)). Figure 8(e) displays the discharge capacities for 100 cycles, plotted separately for low potential (below 3.5 V) and high potential (above 3.5 V). The discharge capacity for all samples in high-potential regions is almost similar, while it is improved in low-potential regions. The rate capability test is conducted at different rates (0.1, 0.2, 0.5, 1, 2, 5, 10 C). LRNMC@10%CF exhibited high rate capability below 2 C, with some reduction in specific capacities at 5 and 10 C due to polarization. The study did not investigate voltage decay for graphite fluoride-modified electrodes, and the reported capacity loss after 100 cycles may be exaggerated. To further enhance the performance, efforts should focus on minimizing internal resistance, electrolyte decomposition, and Mn dissolution.

A Li-rich cathode modified with LiAlF₄ showed remarkable electrochemical performance, maintaining a high capacity over many charge–discharge cycles [130]. The presence of a high percentage of unbounded oxygen species and a Li-ion conductive coating layer (LiAlF₄) is advantageous for the rapid kinetics of LIBs. The unbound oxygen within the crystal lattice is also attributed to increased charge–discharge efficiency [131]. The discharge capacity of LNMO is 187 mAh/g after 100 cycles. The sample showed 6.4% capacity loss after 3,000 cycles, while LNMO exhibited 14.4% loss (Figure 8(f)). The conversion from layered to spinel structure after 100 cycles, typically observed in LNMO cathode due to TM ion relocation to vacant lithium ions, is diminished for LNMO@LiAlF₄ [132].

While fluoride coating still faces challenges in electrochemical performance, it does moderately suppress voltage fading and slightly increase rate capability. The formation of an ultrathin spinel phase beneath the fluoride NM coating can enhance electrode’s discharge capacity. Significant improvements in specific capacity and ICE were observed for graphite fluoride and LiAlF₄ coatings, demonstrating exceptional and outstanding performance.

6.4 Carbon compounds

In the electrochemical process of xLi₂MnO₃ (1−x)LiMO₂, the Li₂MnO₃ component typically acts as an insulator, and the presence of the SEI layer, along with its low electronic conductivity, hinders electrochemical activity [133]. To address practical application requirements, efforts have been made to enhance electrical conductivity and Li-ion diffusion in these cathodes. Carbon, known for its high electrical conductivity and Li diffusion properties, is utilized for cathode modification. Moreover, it can induce HF attack on the LLR cathode’s surface during cycling, significantly improving CR [112]. Various methods are employed for carbon nanolayer modification on LLR, including low-pressure vaporization, solvothermal, and polymerization techniques [84,134,135].

A novel structure involving MOF modification on Li-rich cathode has been developed. Through low-pressure vapour super-assembly, NiCo nanodots are embedded inside a thin carbon shell on LLR [84]. At 0.2 C, LLO@C and NiCo exhibited a CR of 95% with good stability, whereas LLO delivered only 75% retention after 100 cycles. This improvement in rate capability, stability, and ICE is attributed to the suppression of spinel phase formation by LLO@C and NiCo during cycling. The presence of an amorphous carbon layer facilitates fast ion diffusion and acts as a barrier to undesired reactions at the electrode–electrolyte interface. This strategy introduces a new direction where carbon serves as a conducting network and NiCo dots contribute to a stable and robust structure. Recent work has focused on the development of a dopamine-derived carbon spinel layer. Ku et al. [135] prepared a dopamine coating on LLR, followed by carbothermal reduction, resulting in a carbon layer. This layered–spinel material exhibited enhanced electrochemical activity, with a CR of 90.9% after 100 cycles at 0.2 C. At 5 C, the discharge capacity reached 128 mAh/g.

Cai et al. [103] executed surface engineering on hierarchical LLR by carbonizing a dopamine layer. This process introduced oxygen vacancies to the modified LLR surface, leading to structural rearrangement and the formation of a thin spinel layer in situ on the surface. After 200 cycles, the CR reached 85.2% for L@S (spinel layer coating on LLR material) and 77.15% for LMNC at 5 C. The L@S sample exhibited a discharge capacity of 221 mAh/g at 10 C, while the pristine electrode had only 170 mAh/g. The presence of oxygen vacancies activated the electrochemically inactive Li₂MnO₃ phase and enhanced Li-ion diffusion rates. Additionally, the spinel layer encapsulated on LLR created a 3D Li⁺ diffusion channel. These findings suggest that both the spinel layer and oxygen vacancies introduced on the surface have the potential to improve the rate capability, ICE, and cycle performance of LLR. Lu et al. also recently developed a glucose-derived spinel layer on the LLR surface [134]. The modified sample, LR@S@C, significantly enhanced the electrochemical activity of pristine LLR (LR). In terms of rate capability, LR@S@C exhibited a discharge capacity of 186.1 mAh/g at 5 C, while LLR achieved only about 100 mAh/g. Both dopamine and glucose-derived spinel nanolayers contributed significantly to the improvement in LLR cathode performance.

In a study by Zhao et al. [136], a novel bi-functional strategy is proposed to achieve fast charging and high volumetric density in LLR cathodes. They synthesized Li-rich MNC semi-hollow microspheres through ultrasonic treatment of porous MnO₂ microspheres and metal acetates. This material is then surface treated with lithium and cobalt sources, resulting in ST-MNC. To further enhance its properties, the Li-rich MNC is modified with 2D graphene oxide and 1D carbon nanotubes (CNT), creating GCNT@ST-MNC. Figure 9(a) shows the conventional and bi-functional approaches and their advantages. Both half-cell and full-cell electrochemical measurements were conducted. The Li metal reference electrode is used for half-cell testing, while a pre-lithiated nanographite electrode served as the anode in full-cell tests. Figure 9(b)–(e) displays electrochemical performance for MNC and GCNT@ST-MNC half cells. Figure 9(b) shows the cyclic voltammetry (CV) curve for GCNT@ST-MNC from the first to fourth cycle at 0.1 mV/s. Figure 9(c) shows the charge/discharge curve at 0.1 C. The ICE for MNC and GCNT@ST-MNC is 77.6 and 87.8%, respectively. The rate performance of both electrodes at different current rates are displayed in Figure 9(d). GCNT@ST-MNC demonstrated good CR of 65.5% even at 10 C. This indicates that the dual surface modification strategy provides a fast ion/electron pathway for Li transport. After 1,000 cycles, the CR for the GCNT@ST-MNC electrode is approximately 94.5%. The electrochemical performance for GCNT@ST-MNC/nanographite full-cell is shown in Figure 9(f)–(k). As seen from Figure 9(h), the capacity retained at 0.1 and 2 C is 91 and 82%, respectively, for 1,000 cycles. At 20 C, the capacity for GCNT@ST-MNC is 164 mAh/g (Figure 9(j)). An inset shows a comparison between works carried out by Zhao et al. and other works. Therefore, we conclude that the combination of semi-hollow Li-rich MNC microspheres, surface treatment, and the integration of 1D and 2D NMs created more exposed sites for electrochemical reactions and enhanced Li-ion transport.

Figure 9

(a) Schematic illustration of highlighting the high-conformal structure of the LMR particle. Compared with nanostructured or densely assembled LMR particle that may still suffer structure degradation, integrated advantages enabled by bifunctional strategy can be achieved. Electrochemical performance of MNC and GCNT@ST-MNC half cells. (b) CV curves of GCNT@ST-MNC from first and fourth cycle at 0.1 mV s⁻¹. (c) Galvanostatic charge/discharge curves at 0.1 C. (d) Rate performance at various rates from 0.1 to 10 C (the inset is EIS measurement). (e) Cycling performance at 0.1 C (Coulombic efficiency of GCNT@ST-MNC). Electrochemical performance of GCNT@ST-MNC//nanographite full cell. (f) Schematic of the assembled full cell. (g) Galvanostatic charge/discharge curves with 1st, 2nd, 50th, and 100th cycle at 0.1 C. (h) Cycling performance at 0.1 and 2 C (Coulombic efficiency at 0.1 C). (i) Comparison of specific capacity and cycling performance of our work with other reported full cells. (j) Rate performance at various rates from 0.1 to 20 C (the inset is the comparison of specific capacity at high rate). (k) Ragone plots based on total mass of cathode and anode (the inset is the comparison of energy density) [136] (Copyright 2021, Springer).

Carbon NM coatings have shown promise in improving the electrical conductivity of the LLR cathodes. However, there are challenges associated with their use, such as incomplete combustion of gases such as CO and H₂ during high-temperature calcination in inert conditions, which can lead to Mn dissolution [137]. To enhance the electrochemical performance of LLR with carbon NM coatings, several strategies have been proposed: (1) incorporating oxygen vacancies within a thin spinel layer of carbon NM can activate the Li₂MnO₃ phase in LLR, providing 3D pathways for Li⁺ diffusion; (2) semi-hollow microspheres can help mitigate volume changes that occur during cycling, contributing to long-term stability; (3) combining different dimensional NMs, such as 1D and 2D materials, offers various properties and benefits that strengthen electrochemical performance; and (4) in the context of full-cell LIBs, pre-lithiation may be required to compensate for the loss of Li ions in irreversible side reactions during initial cycling [138]. This ensures that the cell starts with sufficient capacity and maintains its performance over time.

6.5 Polymer

Certain polymer composites or conductive polymer materials have gained attention for their high conductivity, which is crucial for efficient lithium-ion and electron transportation during charge–discharge cycles. Additionally, they offer the potential to reduce the dissolution of active electrode materials into the electrolyte during battery operation. Common methods for applying polymer coatings include sol–gel and solid-state reaction techniques [139–141].

Polymer coating of carboxyl-containing benzene diazonium salt on LLR provides excellent stability under high voltage during cycling. The polymer coating suppresses the electrolyte decomposition on electrode’s surface and TM dissolution. The CR after 350 cycles and 500 cycles is 84.2 and 76.3%, respectively. The X-ray photoelectron spectroscopy results also confirmed that the oxygen vacancies are increased after modification [142]. However, it is worth noting that polymer-modified LLR cathodes tend to exhibit lower Coulombic efficiencies compared to other coating materials. Additionally, their rate capability at high current densities can be relatively poor. Recent work by Liu et al. introduced a double-layer coating of Li₂ZrO₃ (LZO) and poly(3,4-ethylene dioxythiophene) (PEDOT) on LLR material. The LZO/PEDOT electrode demonstrated an initial discharge capacity of 282.9 mAh/g and an ICE of 79.1% at 0.1 C rate. Furthermore, the LZO/PEDOT-coated LLR exhibited excellent cycle stability, with CR reaching 90.8 and 91.7% after 300 cycles at 0.5 and 2 C rates, respectively. Voltage fading is also reduced with this modification [139]. Another study explored the surface modification of LLR with a germanium-based polymer, conducted by Becker et al. [140]. Unfortunately, the rate capability of this particular coating is found to be extremely poor compared to other coatings. However, the specific thickness of the germanium-based coating is not provided in the discussed literature, which limits a detailed analysis.

Recently, 6–8 nm thin layer of polyaniline is formed on Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ (LMO) microrod by in situ polymerization of aniline. It provides excellent cycle stability by reducing the electrolyte corrosion. The polymerization process is conducted in acidic media using HCl or (CF₃SO₂)₂NH (HTFSI). The initial discharge capacity of LMO is 235.5 mAh/g, and this capacity is significantly improved after polyaniline modification. The polymerization of aniline in HTFSI (PANI-HTFSI@LMO) exhibited 270.1 mAh/g discharge capacity. However, the polymerization of aniline in HCl (PANI-HCl@LMO) exhibited 251.1 mAh/g discharge capacity. bis(trifluorosulfonyl)imide group (TFSI) positively affects the conductivity and electric stability of PANI [143].

Table 3 summarizes various types of NM coatings (shell), their thickness, the pristine LLR material (core), and their electrochemical performance. Scientists are actively researching polymer coatings for LLR cathodes to improve LIB performance.

6.6 Coupling of doping and nanoscale coating

Although nanoscale coatings have shown potential in enhancing LLR cathode performance, it is essential to recognize their limitations in dealing with LLR issues like oxygen evolution, layered to spinel transformation, and Mn dissolution. These issues are rooted in the lattice structure of LLR, making it necessary to consider additional strategies, such as partial doping, to preserve the lattice structure and further enhance the LLR cathode. Despite the improvements achieved through coatings, there is still room for enhancing specific capacity, rate performance, and ICE. The degree of polarization in LLR electrodes remains relatively high and has not been satisfactorily improved by coating alone. As discussed earlier, both doping and coating have their advantages, and combining these strategies synergistically enhances cathode performance by addressing various aspects of LLR electrode behaviour.

One example of successful doping involves the introduction of a small amount of chromium (Cr) into the LLR structure. Cr doping can improve the electrochemical performance by facilitating charge transfer from Cr to Mn, which can reduce Mn dissolution. Additionally, Cr doping can mitigate thermodynamic instability between phases during the delithiation process [144]. Tai et al. [145] explored the effects of Cr doping and Li₃PO₄ coating on the LLR cathode. While the CR is not optimal, they observed minor voltage decay. Notably, the specific capacity of Cr-doped LLR is higher than that of Cr-doped Li₃PO₄-coated LLR, highlighting the potential of Cr doping to reduce charge transfer resistance and enhance specific capacity. Li₃PO₄ coating, on the other hand, contributed to reducing voltage decay. In another study, Lu et al. [125] investigated molybdenum (Mo) doping in ZrF₄-modified LLR. Mo, known for its high electrical conductivity, is introduced into the LLR structure. However, this material displayed good discharge capacity while suffering from poor rate capability, cycle stability, and relatively low ICE of less than 78%. While Mo doping enhanced electrical conductivity, the combination with ZrF₄ did not satisfactorily address the resistivity of the material, as ZrF₄ may counteract the electrical conductivity improvement by Mo.

Researchers have been working on boosting the electrical conductivity of TiO₂ by adding the anionic element fluorine, which creates Ti³⁺ and oxygen vacancies on the coating surface. Rastgoo-Deylami et al. [73] introduced F-doped anatase TiO₂ (FATO) as a nanoscale coating layer (4–6 nm thickness) on Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ (LMNCO) cathodes using a wet chemical method. They compared the electrochemical performance of pristine (LMNCO), 1 wt% (LMNCO-ATO1), 2 wt% anatase TiO₂ (LMNCO-ATO2), and 2 wt% F-doped anatase TiO₂ (LMNCO-FATO 02) coating. As seen from Figure 10(a) and (d), the initial charge/discharge profiles at 0.1 C for LMNCO-ATO2 and LMNCO-FATO 02 revealed two voltage plateaus in the range of 4.0–4.3 V and above 4.5 V. The first plateau corresponds to the extraction of Li⁺ ions from the LiNi_1/3Co_1/3Mn_1/3O₂ component, accompanied by the oxidation of Ni²⁺ and Co³⁺ to form Ni⁴⁺ and Co⁴⁺, respectively. The second plateau signifies the activation of the Li₂MnO₃ component, during which Li₂O is removed. The ICE values calculated from the data were 83.6% for LMNCO-ATO2 and 85.6% for LMNCO-FATO 02. This suggests that oxygen generated during the initial charging process can be captured by the oxygen vacancies formed in the F-doped anatase TiO₂ coating. As seen from Figure 10(b) and (e), LMNCO-FATO 02 exhibited an outstanding rate performance as compared to LMNCO-ATO2, indicating that fluorine doping reduced the polarization at high current densities. The cycle performance over 500 cycles for both samples demonstrated that F-doping in the TiO₂ coating enhances both ionic and electrical conductivity, resulting in excellent cycle stability (Figure 10(c) and (f)). The mechanism of oxygen vacancies is displayed in Figure 10(g). During the initial cycling, oxygen vacancies within the FATO nanolayer removed the lattice oxygen. This prevents the formation of thick SEI and contributes to the enhanced electrochemical performance of the LMNCO cathode. Recently, Wu et al. [146] modified Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ material with Cd doping and CdO surface coating. The Cd doping improves the electrical conductivity and CdO reduces the direct contact between electrolyte and the material and improves structural stability.

Figure 10

(a) Initial cycle charge/discharge curves of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ with 2 wt% anatase TiO₂ coating in the voltage range between 2.0 and 4.8 V at 0.1 C. (b) Rate performances of 2 wt% anatase TiO₂ coating between 2.0 and 4.8 V from 0.1 to 10 C. (c) Cycling performance of all anatase TiO₂-coated samples at 1 C. (d) Initial charge and discharge profiles of LMNCO-FATO2. (e) Rate capacities of pristine LMNCO, LMNCO-FATO1, LMNCO-FATO2, and LMNCO-FATO4 electrodes at 0.1 to 10 C. (f) Cycling rate performance of LMNCO-FTO2. (g) Schematic of oxygen vacancies mechanism in FATO coated-Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ particle [73] (Copyright 2019, Elsevier).

Ding et al. [147] discovered a novel three-in-one surface treatment approach through the decomposition of urea. This method involved the creation of a carbon nanolayer doped with nitrogen (N) to modify LMNO (Li_1.2Mn_0.6Ni_0.2O₂), facilitating the integration of a spinel phase between the N-doped carbon layer and LMNO. The N-doped carbon layer crucially reduces electrolyte corrosion and initiates the formation of oxygen vacancies on the modified surface layer. The CV curve displayed oxidation and reduction peaks at 3.02 and 2.85 V, respectively, attributed to the formation of the spinel phase. At 10 C, the CR for the LMNO electrode (P-LMNO) and the modified electrode (M-LMNO) is 143.5 and 173.6 mAh/g, respectively. Impressively, the CR at 1 C for 500 cycles is significantly improved for M-LMNO at about 89.9%; P-LMNO exhibited a retention of only 55.31%. This suggests that the innovative structure effectively reduces the evolution of oxygen from the electrode structure.

In another study, an electrode is prepared by doping Nb⁵⁺ into the LLR structure and coating it with poly(3, 4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS). This surface modification led to a slight increase in rate performance compared to an electrode modified with Li₂ZrO₃ (LZO)/PEDOT. Additionally, the Nb⁵⁺-doped PEDOT:PSS-coated electrode exhibited higher cycle stability than the pristine electrode. For various electrode configurations, without Nb⁵⁺-doped LLR (pristine), Nb⁵⁺-doped LLR (2 wt%), PEDOT:PSS coated, and Nb⁵⁺-doped PEDOT:PSS coated (2 wt%) electrodes, the CR values were 84.3, 86.7, 92.2, and 90.8%, respectively. The Nb⁵⁺-doped PEDOT:PSS coating not only prevented direct contact between the electrode and the electrolyte but also improved Li⁺ ion diffusion kinetics [141].

In summary, the three-in-one surface treatment, as well as Nb⁵⁺ doping and PEDOT:PSS coating, offer promising strategies to address issues related to oxygen evolution, corrosion, and cycling stability in LLR-based LIBs. The summarized research in this area, including coupling of doping and coating on the LLR surface (shell), the amount of doping and coating, the thickness of the coating layer, pristine LLR material (core), and their electrochemical performance, is presented in Table 4.