1. Introduction

Lithium-ion batteries (LIBs) are widely used in both small electronic devices and transportation system including electric vehicles. As the demand for LIBs with high capacity, energy density, and stability, the advancement of battery performance should be prepared in a timely manner.

With a high capacity (> 200 mAh g^-1) and energy density, lithium- and manganese-rich transition metal oxide (LMRO) is a promising candidate for next-generation energy storage solutions due to the anion redox of Li₂MnO₃ in the 4.5 V region.^1,2) This study explores a coating strategy to enhance the electrochemical properties of cobalt-free LMRO. Co-free LMRO exhibits a composite structure with rhombohedral and monoclinic phases, represented as xLi₂MnO₃･(1-x)LiTMO₂ (TM = Ni, Mn, etc.).³⁾ Despite these advantages, LMRO suffers from several limitations. The irreversible nature of Li₂MnO₃ results in a low initial Coulombic efficiency, while oxygen release from the oxygen redox reaction in the 2.0-4.8 V (vs. Li⁺/Li), Li₂MnO₃ → Li₂O + MnO₂, leads to structural degradation.^1,4,5,6,7) Furthermore, prolonged cycling induces cation migration, disrupting the Li⁺ ion transport pathways and causing structural disorder, which ultimately deteriorates battery performance.⁸⁾ This structural collapse leads to a layered-to-spinel phase transition, resulting in voltage decay, diminished cycle life, and reduced rate capability.⁹⁾ Therefore, further research is required to mitigate these drawbacks and enhance the overall battery performance of the LMRO cathode.

In this study, Nb₂O₅ and Sb₂O₃ nanoparticles were employed to coat Co-free LMRO, aiming to improve its electrochemical properties. Nb shows negligible solubility in the layered structure, while Sb can be easily doped into TM site of the LMRO structure. Cathode active materials often contain residual lithium, which is non-conductive and contributes to low initial Coulombic efficiency. The Nb₂O₅ coating form the conductive LiNbO₃ on the surface, thereby enhancing the initial Coulombic efficiency and capacity of LMRO. Meanwhile, Sb₂O₃ can be doped into the bulk structure and accelerates the spinel phase transition. The spinel phase, characterized by a 3D structure, enables rapid Li⁺-diffusion, significantly improving high-rate performance. Ultimately, our research highlights the fundamental challenges of LMRO and provide the in-depth understanding of the next-generation high-performance lithium-ion battery cathode materials.

2. Experimental Procedure

2.1. Synthesis

To synthesize the precursor, a 2 M transition metal solution with an Mn:Ni ratio of 0.65:0.35, prepared using MnSO₄･H₂O (Samchun Chemical, 98.0 %) and NiSO₄･6H₂O (Samchun Chemical, 98.5-102.0 %), along with a 0.1 M ammonia solution and a 4 M NaOH solution, was used while maintaining the pH at 9.7. Each solution was introduced into a 5 L continuously stirred tank, maintained under N₂ atmosphere, at a temperature of 55 °C and a stirring speed of 1,100 rpm for 10 h. The formed Mn_0.65Ni_0.35(OH)₂ precursor was physically mixed with Li₂CO₃ in a mortar and then calcined in a furnace to synthesize 0.3Li₂MnO₃･0.7LiNi_0.5Mn_0.5O₂. The pre-heating process was carried out at 500 °C for 5 h, followed by heating at 900 °C for 15 h in an air atmosphere, with a ramp rate of 2 °C/min for both steps. For the coating process, the synthesized LMRO was mixed with 0.5 wt% Nb₂O₅ or Sb₂O₃ nano-sized powders (SIGMA-ALDRICH) in a mortar. Each mixture was then heat-treated in an air-atmosphere furnace, with a ramp rate of 2 °C/min and maintained at 600 °C for 4 h. This process resulted in the production of Nb₂O₅-coated LMRO (Nb-LMRO) and Sb₂O₃-coated LMRO (Sb-LMRO).

2.2. Materials characterization

The morphology of the active material and degree of coating were examined using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) (LYRA3 XMU, Tescan). The structural properties of the synthesized material were analyzed using an X-ray diffractometer (XRD, D8 ADVANCE, Bruker) with Cu-Kα radiation over a range of 10-70° at a scanning rate of 2° per minute.

2.3. Electrochemical characterization

To evaluate the electrochemical performance, cathode electrodes were fabricated by the mixture of LMRO materials, carbon black, polyvinylidene difluoride (PVDF) as the active materials, conducting agent, and binder, respectively, in a weight ratio of 90:5:5. The mixture was dispersed in N-methyl-2-pyrrolidone (NMP) to form the electrode slurry and coated onto a carbon-coated aluminum foil by blade casting. The NMP solvent was evaporated by drying the coated foil in an oven at 110 °C for 3 h, followed by a vacuum drying at 120 °C for 12 h. The mean mass loading of fabricated LMRO, Nb-LMRO, and Sb-LMRO electrodes was 7.102 mg, 7.038 mg, and 7.092 mg, respectively. 2032-type coin cells were assembled in an argon gas-filled glovebox, employing lithium metal as the counter and reference electrode. Electrochemical characterization, including capacity assessment, cycling performance, rate capability evaluation, and differential capacity (dQ/dV) measurements, was performed using an electrochemical testing device (CT-4008T-5V50mA-164, Newware). Electrochemical impedance spectroscopy (EIS) analysis was performed with a 10 mV excitation voltage in the frequency range of 1 kHz to 10 mHz to assess internal resistance.

3. Result and Discussion

3.1. Material characterization

To confirm the formation of the coating layer on the surface of Co-free LMRO cathode particles, SEM analysis was performed. As illustrated in Fig. 1, the secondary particle surface of the coated sample displayed a denser morphology compared to the uncoated sample, which confirms the successful formation of the coating layer.

Fig. 1.

SEM images of uncoated and coated samples. (a) LMRO, (b) Nb-LMRO, (c) Sb-LMRO.

To investigate the structural effects of each coating element, XRD analysis was conducted. As shown in Fig. 2, no significant changes in peak intensity were observed among the samples. However, in the case of the Sb-coated LMRO sample, a slight leftward shift of the (003) peak was observed. This shift is attributed to the diffusion of Sb into the bulk crystal structure by coating process, into the crystal structure.^10,11) In contrast, peak shift was not observed in the Nb-coated LMRO sample due to the negligible diffusion of Nb. However, the formation of LiNbO₃ (ICDD 01-084-3000) was confirmed by the presence of a characteristic peak at 23.7° in the XRD analysis. This can be attributed to the reaction between surface lithium and Nb⁵⁺ ions, forming LiNbO₃.¹²⁾ Given that LiNbO₃ exhibits electrical conductivity, it may influence the electrochemical properties of the material.¹³⁾

Fig. 2.

XRD patterns of uncoated and coated samples. (a) 10°~70° peak, (b) (003) peak, (c) 20°~25° Li₂MnO₃ superlattice peak.

The distribution of the elements was identified by EDS analysis, as shown in Fig. 3. The elemental mapping reveals that Sb is uniformly distributed throughout the entire particle due to the doping into the bulk structure. In contrast, Nb is presumed to form LiNbO₃, which is segregated on the particle surface.

Fig. 3.

EDS images of coated samples. (a) Nb-LMRO, (b) Sb- LMRO.

3.2. Electrochemical characterization

To investigate the effect of coating on the electrochemical performance of LMRO, electrochemical properties were studied using the prepared LMRO cathode materials. The capacity, cycle stability, and rate capability of each sample are presented in Fig. 4 and Tables 1 and 2. Fig. 4(a) displays the first charge-discharge curves at 0.1 C (1 C = 200 mA/g). The initial discharge capacities of Nb-LMRO and Sb-LMRO were 245.44 mAh/g and 250.01 mAh/g, respectively, both exceeding the 240.52 mAh/g capacity of the bare LMRO, which can result from less irreversible oxygen evolution in the coated LMRO cathodes. Moreover, Nb-LMRO exhibited a slight increase in the initial Coulombic efficiency due to the presence of conductive LiNbO₃ on the particle surface.^12,14)

Fig. 4.

Electrochemical properties of uncoated and coated samples. (a) first charge/discharge profiles in a voltage range of 2.0 V-4.8 V at 0.1 C-rate (20 mA/g), (b) cycle performance at 1 C-rate (200 mA/g), (c) rate capability.

Consequently, Nb-coated LMRO exhibited the highest energy density of 727.22 Wh/kg at a 1 C rate. However, Nb-coating layer cannot retard a gradual layered-to-spinel phase transformation, which ultimately led to capacity fading and energy density decrease.¹⁵⁾ This spinel transformation is corroborated by Fig. S2(b), where the Mn reduction peak shifts leftward by 0.05 V compared to LRMO (Fig. S2(a)).¹⁶⁾

As shown in Fig. 4(b), although Nb-LMRO exhibited higher capacity than LMRO at a 1C rate in the initial cycle, it suffered significant capacity loss after 100 cycles, resulting in a low capacity retention of 78.67 % and an energy density retention of 71.23 % (Table 2). Moreover, in Fig. 4(c), rate capability measurements revealed that while Nb-LMRO outperformed LMRO at low C-rates, its capacity retention deteriorated sharply at high C-rates (5 C). It seems that bulk diffusion of Li⁺ is the kinetically limiting step at the accelerated current rate. A small amount of Nb in the bulk structure can impede facile Li⁺ diffusion.

Furthermore, incomplete capacity recovery at a 0.1 C rate recharge indicated that the degradation of the crystal structure in Nb-LMRO occurred during prolonged repeated cycles. Based on our observation, the gradual transition to the spinel phase can degrade the structural stability of the material followed by less capacity retention.¹⁷⁾

In the meantime, Fig. 4(a) and (b) demonstrate that Sb-LMRO delivered the highest initial capacity at both 0.1 C and 1 C. This capacity enhancement is primarily attributed to the improved Li⁺ conductivity induced by Sb ions.¹⁰⁾ As discussed earlier, antimony is readily doped into the bulk structure of LMRO during the Sb₂O₃ coating process. The antimony in the structure accelerates spinel phase transition, which can be observed even during the second cycle. The spinel transformation enables a more contribution of Mn to the total capacity, as evidenced in Fig. S2(c), where the Mn⁴⁺/Mn³⁺ reduction occurs at a lower potential (~3.1 V) compared to other LMRO cathodes (~3.3 V).^18,19)

However, this accelerated phase transition, as shown in Fig. S1(c), induces voltage decay. Therefore, energy density retention of 71.27 % could be achieved despite a capacity retention of 77.36 %. However, the rapid spinel phase transformation can be advantageous to the Li⁺ diffusion kinetics. Since the spinel structure facilitates 3D Li⁺-ion transport, Sb-LMRO exhibits superior capacity retention at a 5 C rate and excellent capacity recovery at 0.1 C,^20,21,22) as presented in Fig. 4(c). Therefore, the fast transformation combined with Sb doping can enhance the rate capability. In contrast, uncoated LMRO exhibited the lowest initial capacity due to its relatively slow electrochemical activation.¹⁸⁾ However, its delayed spinel transformation suppressed voltage decay, as observed in Fig. S1, leading to the highest capacity retention (83.61 %) and the best energy density retention (77.46 %).

The EIS results of the LMRO, Nb-LMRO, and Sb-LMRO samples before and after cycling are summarized in Fig. 5 and Table 3, allowing a comparative evaluation of their charge transfer characteristics and Li⁺ diffusion behavior. The uncoated LMRO sample exhibited the lowest Li⁺ diffusion coefficients after cycling (4.12 × 10^-15 → 1.76 × 10^-14 cm²/s), along with relatively high R_ct values and the poorest capacity performance among the three samples. In contrast, Nb-LMRO showed a significant reduction in charge transfer resistance from 64.26 Ω to 5.62 Ω after cycling, accompanied by a substantial increase in diffusion coefficient from 6.73 × 10^-16 to 7.89 × 10^-14 cm²/s. These results can be attributed to the formation of a conductive LiNbO₃ interfacial phase, which enhances interfacial stability and electronic conductivity, and this effect is also reflected in the improved capacity compared to LMRO. Notably, Sb-LMRO exhibited the lowest charge transfer resistance of 1.718 Ω and the highest Li⁺ coefficient of 5.02 × 10^-13 cm²/s, indicating the most favorable electrochemical reactivity. This behavior is likely due to the Sb-induced enhancement of interfacial reactions and the accelerated transformation to the spinel phase, which enables the development of a three-dimensional Li⁺-ion migration pathway. As a result, Sb-LMRO delivered the highest discharge capacity and superior rate capability among all samples, suggesting that the low charge transfer resistance and high diffusion coefficient directly contributed to its enhanced electrochemical performance.

Fig. 5.

EIS results for each sample. Nyquist plot before (a) and after (b) 0.1 C (200 mAh/g) first cycling. Warburg plot (c) corresponding to (a), (d) corresponding to (b).

Our findings suggest that understanding of the nature of the coating layer is crucial for optimizing the electrochemical performance of LMRO. In the case of Nb-LMRO, the formation of LiNbO₃ enhanced initial conductivity and reactivity but also promoted a gradual spinel phase transition, ultimately compromising long-term structural stability. In contrast, Sb₂O₃ coating induced a rapid spinel phase transition, which improved the rate capability. Meanwhile, uncoated LMRO exhibited relatively slower electrochemical reactivity and lower initial capacity. However, it demonstrated the highest capacity and energy density retention. Therefore, for optimal electrochemical performance, a well-balanced design strategy that considers the trade-offs among spinel phase transformation kinetics, rate capability, capacity, and lifespan is of importance.

4. Conclusion

This study demonstrates that Nb₂O₅ and Sb₂O₃ coatings tune the electrochemical performance of Co-free LMRO.

(1) Nb₂O₅ promotes the formation of conductive LiNbO₃, improving initial Coulombic efficiency and energy density. However, it allows a gradual layered-to-spinel phase transition.

(2) Sb₂O₃ induces a rapid spinel phase transition, which causes voltage decay but enhances high-rate capability.

(3) The bare LMRO exhibits the highest long-term stability due to its slow electrochemical reactivity; however, its lower reaction rate results in reduced initial capacity.

Our findings highlight the trade-off among electrochemical reactivity, lifespan, spinel phase transition kinetics, and rate capability. Our studies provide the insight that enables optimization of LMRO-based cathodes for next-generation lithium-ion batteries.