Design of Cobalt-Free Ni-Rich Cathodes for High-Performance Sodium-Ion Batteries Using Electrochemical Li⁺/Na⁺ Exchange

Abstract

Sodium-ion batteries are renowned for their abundant reserves, cost-efficiency, safety, and eco-friendliness and are prime candidates for large-scale energy storage applications. The development of cathode materials plays a crucial role in shaping both the commercialization path and the ultimate performance capabilities of SIBs. To overcome the intricate synthesis challenges associated with pure-phase sodium-ion cathode materials, this study introduces an innovative and streamlined electrochemical Li⁺/Na⁺ exchange process, successfully fabricating a high-capacity Ni-rich cathode material. This cathode material boasts a remarkable reversible capacity of 180 mAh g⁻¹ at 0.1 C and retains a high-rate capacity of 115 mAh g⁻¹ even at 5 C. Additionally, it exhibits exceptional cycling stability, retaining about 85% of its capacity at 1 C after 50 cycles and still maintaining a capacity greater than 60% after 100 cycles. The Na-NMA85 full cell preserves a discharge capacity of 110 mAh g⁻¹ after 100 cycles, with a capacity retention rate of 80%. This research underscores innovative strategies for designing ion-intercalation-based cathode materials that enhance battery performance, providing fresh perspectives for advancing high-performance battery technologies.

1. Introduction

Lithium-ion batteries (LIBs) have secured a pivotal role in diverse domains, such as computers, communication devices, electric vehicles, and consumer electronics, owing to their exceptional electrochemical properties [1,2]. However, the limited availability of lithium resources in the Earth’s crust threatens their sustainable use [3]. Conversely, SIBs, which operate on similar principles but boast abundant sodium resources, low costs, and widespread availability, emerge as a promising contender for the next generation of energy storage technologies, presenting a viable alternative to LIBs [4]. However, a significant hurdle remains in developing high-capacity cathode materials for SIBs that can rival LIBs, as current SIB performance falls short of the high energy density demands. Drawing inspiration from layered transition metal oxides in LIBs, sodium-based layered transition metal oxides have garnered extensive attention and research as potential cathode materials for SIBs [5,6]. Among the various candidates, the layered oxide cathode Na_xTMO₂, characterized by its unique two-dimensional layered structure, facilitates rapid and reversible sodium-ion insertion [7,8,9,10,11]. However, in practical applications, the potential of Na⁺ in O3-type layered oxide cathodes remains underutilized, resulting in limited reversible capacity, sluggish diffusion kinetics, and capacity fade due to multiple phase transitions [12,13].

In LIB systems, Ni-rich layered oxide cathodes have achieved a high reversible capacity of up to 200 mAh g⁻¹ due to their expandable layered framework and highly reactive nickel (Ni) ions. Given this, Ni-rich cathodes, serving as effective hosts for Na⁺ insertion, theoretically promise efficient and reversible sodium-ion storage. For example, Jiao et al. [14] prepared a series of O3-type NaNCM433, NaNCM622, and NaNCM712 cathodes using the solid-phase method. Ni-rich NaNCM712 cathodes with high Ni content have a higher initial discharge specific capacity (0.2 C, 165.5 mAh g⁻¹), but the high Ni content and low cobalt (Co) content are detrimental to the stability of the cathode system. In addition, elements such as Mn, Co, Ni, Fe, Cr, Cu, and Ti exhibit electrochemical activity and display a range of properties in SIBs. Therefore, the types and synthesis methods of Ni-rich cathodes for SIBs are also more diverse. For example, Hu et al. [15] synthesized a series of Ni-rich Co-free O3-NaNi_xFe_yMn_1-x-yO₂ (x = 0.6, 0.7, and 0.8) cathodes using the co-precipitation method. They found that Ni ions and iron (Fe) ions participated in charge compensation during the Na⁺ extraction/insertion process, resulting in the high specific capacity and excellent cycling performance of the cathode at 2.0–4.0 V. In addition, Xia et al. [16] used the electrochemical ion-exchange strategy to extract Li⁺ from LiNi_0.82Co_0.12Mn_0.06O₂, and then embedded Na⁺ into the Ni-rich layered oxide framework, successfully synthesizing a Na_0.75Ni_0.82Co_0.12Mn_0.06O₂ cathode with high capacity. However, Co is expensive, which increases the cost.

Herein, a Na-intercalated Ni-rich Co-free cathode electrode (NaNi_0.85Mn_0.09Al_0.06O₂) employing the electrochemical Li⁺/Na⁺ exchange method, is successfully synthesized. The synthesized cathode electrode demonstrates a high reversible capacity of approximately 180 mAh g⁻¹ at 0.1 C and retains a specific capacity of around 115 mAh g⁻¹ at 5 C. More importantly, its capacity retention rate remains above 85% after 50 cycles and it still maintains a capacity greater than 60% after 100 cycles, showcasing exceptional durability and stability performance for practical energy storage applications.

2. Materials and Methods

2.1. Material Preparation

A Ni_0.85Mn_0.09Al_0.06(OH)₂ precursor (Shaanxi Coal & Chemical Industry, Xi’an, China) and LiOH∙H₂O (Aladdin, analytical grade, 98%) in a molar ratio of 1:1.03 were manually mixed and grounded for 20 min under a drying lamp. This mixture was then calcined in an oxygen-rich environment at 720 °C for 10 h, forming LiNi_0.85Mn_0.09Al_0.06O₂, which was designated as Li-NMA85. To prepare the cathode slurries, the active material was combined with carbon black (super-P) and polyvinylidene fluoride (PVDF) at a precise weight ratio of 8:1:1, respectively, using N-methyl-2-pyrrolidone (NMP) as the solvent. The slurries were uniformly coated onto Al foil and subsequently dried thoroughly in a vacuum oven at 120 °C for 12 h. Furthermore, 1.2 M of LiPF₆ in ethylene carbonate (EC)/ethylmethyl carbonate (EMC) (3:7 by weight) with 2 wt. % vinylene carbonate (VC) was used as an electrolyte. The CR2032 half-cells were assembled in an argon-filled glove box, utilizing Li metal as an anode, Celgard 2325 as the separator, the previously described electrolyte solution, and the cathode electrode. Additionally, a Na-intercalated layered Ni-rich cathode was synthesized using an electrochemical ion exchange method. Initially, Li⁺ was extracted from Li-NMA85 by charging to 4.5 V versus Li/Li⁺ and maintaining this voltage for several hours. Afterward, the delithiated electrode was promptly removed from the half-cell, thoroughly cleaned, and allowed to dry at room temperature inside a glove box. Subsequently, the dried electrode was reassembled into a Na-ion half-cell, utilizing Na metal as the counter electrode. By discharging this setup to 2.0 V versus Na/Na⁺, Na⁺ was introduced into the electrode, resulting in a Na-intercalated electrode, designated as Na-NMA85.

2.2. Material Characterization

The crystalline structure of the cathodes was characterized using an X-ray diffractometer (XRD Bruker D8 Advance, Bruker, Berlin, Germany) equipped with Cu-Kɑ radiation (λ = 1.5406 Å), scanning a 2θ range of 10° to 80° at a step rate of 5° min⁻¹. Surface morphology was characterized under a scanning electron microscope (SEM, HITACHI SU8010, Tokyo, Japan) equipped with an energy-dispersive X-ray spectroscopy (EDS) detector. Additionally, high-resolution transmission electron microscopy (HRTEM, TecnaiG2 F30, FEI Company, Hillsboro, OR, USA) was used to observe the detailed morphology and crystal structure of the cathodes. The surface chemistry of the samples was characterized using X-ray photoelectron spectroscopy (XPS) on an ESCALAB250Xi spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) with an Al Kα radiation source (1486.6 eV).

2.3. Electrochemical Measurements

The Na-ion half-cells were assembled with Na-NMA85 as the cathode, sodium metal as the anode, 1 M of NaClO₄ EC/EMC (1:1 in volume) as the electrolyte, and glass microfiber as the separator in a box filled with argon (H₂O and O₂ < 0.01 ppm). Assembled button-type batteries were cycled galvanostatically with Land (CT3001A, Wuhan) between 2.0 V and 4.0 V versus Na/Na⁺ at 25 °C. The rate performance was evaluated by cycling at various rates (C/5, C/2, 1 C, 2 C and 5 C) for 5 cycles each. Cyclic voltammetry (CV) profiles were recorded on an electrochemical workstation (CHI760E) at a scanning rate of 0.2 mV s⁻¹ within a voltage range of 2–4 V.

3. Results and Discussion

The Li-NMA85 cathode material was successfully fabricated through a straightforward and efficient one-step sintering approach. As illustrated in Figures S1 and S2, spherical Ni_0.85Mn_0.09Al_0.06(OH)₂ particles with an average diameter of ~11 μm were employed as the precursor. The spherical Ni_0.85Mn_0.09Al_0.06(OH)₂ particles have a specific surface area of 15.03 m²g⁻¹. Following the synthesis process, the Li-NMA85 cathode retained the secondary particle morphology of the precursor, as confirmed by the SEM images presented in Figure 1a-b and Figures S3 and S4. This morphology preservation indicates the stability and robustness of the synthesis method. Furthermore, EDS analysis revealed a highly uniform and well-dispersed distribution of Ni, Mn, Al, and O elements within the primary particles of the Li-NMA85 cathode, as shown in Figure 1c–f. This uniform elemental distribution underscores the exceptional compositional homogeneity of the synthesized cathode material, which is crucial for achieving consistent electrochemical performance. The combination of morphological stability and compositional uniformity highlights the effectiveness of the one-step sintering strategy in producing high-quality Li-NMA85 cathode materials.

As illustrated in Figure 2a, the initial layered Li-NMA85 cathode undergoes a process of Li⁺ extraction during charging to 4.5 V versus Li/Li⁺. Through calculations, we determined that the theoretical capacity of Li-NMA85 is 280 mAh g⁻¹. Upon charging to 4.5 V (delithiation), the observed specific capacity is 244 mAh g⁻¹, indicating a residual Li⁺ content of approximately 12.8% (the specific formula is presented in the Supporting Material). During the discharging process to 2.0 V versus Na/Na⁺, this transformation results in an electrode exhibiting a remarkable Na-intercalation capacity of 180 mAh g⁻¹, accompanied by a Na⁺ content of 65 mol% within. Ultimately, the anticipated cathode material (Na_0.65Ni_0.85Mn_0.09Al_0.06O₂) was successfully obtained and designated as Na-NMA85 (Figure 2b). Furthermore, an in-depth analysis of the crystal lattice configuration of the electrodes during delithiation and sodiation was conducted using XRD, as depicted in Figure 2c. The pristine Li-NMA85 cathode, identified as spot-A, exhibits an identical crystal structure to LiNiO₂, conforming to a layered hexagonal α-NaFeO₂ structure within the R3m space group [17,18]. Upon charging to 4.5 V versus Li/Li⁺, the Li-NMA85 transforms into a highly delithiated oxide phase, designated as spot-B, which retains a similar layered structure to Li_0.06NiO₂ [19]. Subsequently, when Na⁺ ions are intercalated into the interlayer spaces of this Ni-rich oxide, the delithiated cathode evolves into a Na-intercalated cathode (designated as spot-C), which closely resembles the structural configuration of O3-type NaNi_0.5Mn_0.5O₂. The Al impurity peak observed in the XRD pattern is attributed to the current collector Al foil. Remarkably, the (003) diffraction peak of Na-NMA85 shifts towards a smaller angle than Li-NMA85, unequivocally signifying an expansion of the interlayer spacing. This expansion can be directly attributed to the larger ionic radius of the Na⁺ ion [20,21]. The SEM images in Figure 3a–c illustrate that the Na-NMA85 cathodes comprise spherical secondary particles, with average diameters of ~11 μm. Each of these secondary particles is further composed of primary particles measuring less than 1 μm. Additionally, EDS revealed a uniform and well-dispersed distribution of Na, Ni, Mn, Al, and O elements within the primary particles of Na-NMA85, underscoring the exceptional compositional homogeneity of the material. The deconvoluted XPS spectra of Na 1s, O1s, Ni 2p, and Mn 2p shown in Figure 4 provide more information about the surface chemistry of NaNMA85. The peaks at 1072 eV correspond to Na species (Figure 4a). The O 1s spectra in Figure 4b include peaks corresponding to O_impurity (532.2 eV) and O_lattice (529.7 eV) [22]. O_impurity mainly relates to the reactive oxygen species namely, O⁻, O²⁻, and CO₃²⁻ in NaOH/Na₂CO₃. O_lattice is derived from lattice oxygen O²⁻ in an M-O (M = Ni, Mn, and Al) bond [23]. The Ni 2p spectra in Figure 4c include peaks corresponding to Ni²⁺ (854.2 eV and 872.17 eV) and Ni³⁺ (855.9 and 874.4 eV), implying the coexistence of Ni²⁺ and Ni³⁺ in all samples [24]. The Mn 2p spectra in Figure 4d include peaks corresponding to Mn2p3/2 (642.55 eV) and Mn2p1/2 (656.6 eV) [24].

To further elucidate the distribution patterns, HRTEM investigations were conducted on both Li-NMA85 and Na-NMA85. As illustrated in Figure 5, the HRTEM images of Li-NMA85 exhibit an interplanar spacing of 0.480 nm (Figure 5a1,a2), corresponding to the (003) planes of its layered structure [25,26]. By contrast, Na-NMA85 (Figure 5b1,b2) displays two distinct sets of interplanar spacings, both measuring 0.533 nm, aligning with the (003) planes of its layered structure. These findings provide further evidence of the structural changes and expansion of interlayer spacing upon Na⁺ intercalation.

Moreover, the electrochemical performance of the synthesized Na-NMA85 cathode was rigorously assessed in a sodium half-cell configuration. As illustrated in Figure 6a, the cycling stability demonstrates that reversible capacity slowly decays with cycles and it maintains a high discharge capacity of 80 mAh g⁻¹ with 60% capacity retention at 1 C after 100 cycles. Figure 6b displays the voltage profiles of the Na-NMA85 cathode over 100 cycles at 1 C, highlighting that despite the specific capacities progressively decreasing during cycling, the overall electrochemical performance remains stable and consistent. The SEM image of the cyclic electrode reveals that some particles of the Na-NMA85 electrode have broken after undergoing the cycling process. This is likely due to the penetration of electrolytes through the interior and between microcracks, which causes damage to the bulk and surface structures of the particles, crushing the cathode particles and accelerating the decline in battery capacity (Figure S5). The CV curve offers a direct visualization of the phase transition process that accompanies the redox reaction of active metallic ions. Figure 6c illustrates the CV curve of Na-NMA85, showcasing series redox peaks at potentials of 2.78/2.39 V, 3.13/2.61 V, 3.51/2.89 V, and 3.51/3.10 V. The rapid scanning speed of CV testing results in the merging of two high-voltage peaks into a single broad peak, causing a disparity in the number of reduction and oxidation peaks within the CV curve. As evident from Figure 6c, the highly overlapping curves during the initial two cycles of Na-NMA85 suggest good reversibility in the phase transition process. Notably, the presence of a pair of sharp redox peaks at 2.88 V (oxidation) and 2.31 V (reduction) indicates the transition between the O3 and P3 phases. As the voltage increases, the cathode undergoes a solid-state process within the range of 2.8–3.7 V and a phase transition between P3 and P″3 above 3.7 V. In solid-state processes, the layered structure typically exhibits Na⁺/vacancy ordering, leading to distinct voltage plateaus. Intriguingly, the redox peaks in the high-voltage region (2.8–3.7 V) are broad and weak, suggesting suppressed phase transitions in the Na-NMA85 cathode. This may be attributed to trace amounts of Li-O3 phase in Na-NMA85, which acts as a stabilizing pillar for the layered structure and an effective dopant to disrupt Na⁺/vacancy ordering during Na⁺ extraction and insertion [27]. Consequently, this suppression of phase transitions is anticipated to enhance the cycling performance of the cathode material. Furthermore, the cathode’s rate capability was evaluated across a broad range of current rates, from 0.2 C to 5 C. As depicted in Figure 6d, while the discharge capacity experiences a gradual decline with increasing current density, the electrode retains an impressive reversible capacity of 115 mAh g⁻¹ even at an elevated rate of 5 C, corresponding to a retention of 63% of its initial capacity at 0.1 C. Additionally, Figure 6e showcases the discharge profiles of Na-NMA85 at different rates, further reinforcing the material’s remarkable rate capability and discharge voltage stability, highlighting its potential for high-performance energy storage applications. Long-cycle testing shows that it preserves a discharge capacity of 110 mAh g⁻¹ after 100 cycles at 25 °C, with a capacity retention rate of 80% (Figure 6f). This robust performance underscores the significant potential of Na-NMA85 for large-scale energy storage applications. The high-capacity retention indicates that Na-NMA85 can sustain stable performance over numerous charge–discharge cycles, which is crucial for ensuring the reliability and durability of energy storage systems. Consequently, Na-NMA85 stands out as a promising cathode material for sodium-ion batteries, set to play a pivotal role in the future of large-scale energy storage.

The SEM image of the cyclic electrode reveals that some particles of the Na-NMA85 electrode have broken after undergoing the cycling process. This is likely due to the penetration of electrolytes through the interior and between microcracks, which damages the bulk and surface structures of the particles, crushing the cathode particles and accelerating the decline in battery capacity. The fragmentation of Ni-rich cathode electrode particles provides a channel for electrolytes to penetrate the interior of the particles. As a result, the fresh interface is continuously exposed to the electrolyte. Highly active Ni⁴⁺ reacts with the electrolyte to form an inert rock salt phase, consuming active Na and blocking ion transport. Meanwhile, side reaction products accumulate on the crack surface, hindering the diffusion of sodium ions. Additionally, particle breakage can cause the conductive network to break, increasing the internal contact resistance of the electrode. Therefore, we have provided EIS data before and after cycling (Figure S6). The test results showed that the ohmic impedance of the system increased from 2.1 Ω (0) and 3.2 Ω (1st) at the beginning to 8.6 Ω (80th) after cycling, and the CEI interface impedance also increased significantly. This is related to the side reactions between the electrolyte and the interface, indirectly indicating the decomposition of the electrolyte after cycling [28].

4. Conclusions

This research synthesized a Na-intercalated Ni-rich layered cathode through an innovative electrochemical Li⁺/Na⁺ exchange methodology. The cathode exhibits exceptional electrochemical performance, as evidenced by its high reversible capacity of approximately 180 mAh g⁻¹ at a rate of 0.1 C, and an equally impressive capacity of roughly 115 mAh g⁻¹ even at a high rate of 5 C. Notably, the capacity of the as-prepared Na-NMA85 is maintained at 85% at 1 C after 50 cycles, and it still retains a capacity greater than 60% after 100 cycles, showcasing its remarkable durability and stability. Therefore, this study provides a new reference and valuable guidance for the preparation of pure-phase nickel-rich layered sodium-positive electrodes, positively contributing to the research and development of these electrodes.