Electronic modification of NaCrO₂via Ni²⁺ substitution as efficient cathode for sodium-ion batteries

INTRODUCTION

The emergence of sodium-ion batteries (SIBs) requires efficient and stable cathodes^[1,2]. Amongst the most investigated cathodic materials, layered transition metal (TM) oxides have attracted significant attention because of their low-cost synthesis and high theoretical specific capacity^[3-6]. For example, NaCrO₂ (coded as NCO) has been intensively studied assignable to its flat discharge curves and strong temperature adaptability^[7,8]. It is also a well-established cathode material with a capacity of ~120 mAh g^-1[9,10]. Nevertheless, the material suffers from a rapid capacity decay at high rate during recycling due to a low Na⁺ diffusivity. Additionally, its crystal structure can be damaged during the repeated (de)intercalation of sodium ions, which leads to an irreversible strain accumulation and the fading of the capacity^[11].

Different methods such as carbon encapsulation^[10,12], morphology control^[13,14] and cation substitution^[15] have been reported to demolish these obstacles for NCO. Among them, cation substitution has been considered one of the most effective and facile ways to enhance cycling stability and rate capability^[16-18]. Cationic substitution refers primarily to replacing parts of TM ions with others. For instance, a delayed phase transition of O3-P3 was observed in the Ti-substituted NCO which achieved a specific discharge capacity of 96.7 mAh g^-1 at a discharge rate of 1 C and a high capacity retention (~80% after 800 cycles)^[19]. Furthermore, Lee et al. proposed a co-substitution strategy to reinforce the structural stability of the NCO, allowing the cathode to deliver a high-rate performance (> 20 C) and a long cycle life (> 1,000 cycles)^[20]. Following this line, it was found that a substitution of Cr with Sb in NCO could expand the Na⁺ transmission channels^[21]. Despite these notable advancements, the impact of low-valent TM substitution on electronic structure of layered oxides for SIBs and its subsequent effect on performance improvement have yet to be thoroughly investigated.

Here, we show an effective method to improve comprehensive performance of NCO by Ni²⁺ substitution. The substitution of Cr³⁺ by low-valent Ni²⁺ to produce NaCr_0.95Ni_0.05O₂ influences the ionic character of the TM-oxygen bonds, which, in turn, increases the interlayer spacing of sodium layers to improve the diffusion kinetics of sodium ions. In addition, Ni²⁺ substitution inhibits the structural degradation of NCO during charge-discharge cycling, which effectively enhances the cycling performance of the material. Consequently, the NaCr_0.95Ni_0.05O₂ exhibits an impressive performance of a remarkable rate performance with 91.2 mAh g^-1 at 50 C and a high retention (80%) of the initial capacity after cycling for 1,000 cycles at 10 C.

RESULTS AND DISCUSSION

The control sample NCO and its derivative NaCr_1-xNi_xO₂ with different Ni contents (x = 0.03, 0.05, 0.07, noted as NCNO-x, respectively, in which x represents the moles of Ni²⁺ substitution to Cr³⁺) were synthesized via the solid-phase method^[22]. The X-ray diffraction (XRD) patterns in Figure 1A and Supplementary Figure 1 confirm the rhombohedral layered phase for NCO, NCNO-0.03, and NCNO-0.05^[14]. An impure phase NiO (marked with asterisk) appears in the NCNO-0.07, which is due to excessive doping of Ni^2+[23]. The XRD peaks at 16.65° and 41.70° can be ascribed to the (003) and (104) crystal faces of Rinem$$\overline{3}$$m structure, respectively. Figure 1A also presents the magnified XRD patterns correlated to the (003) and (104) planes for the control samples before and after ionic substitution. We noticed that the XRD peaks corresponding to the (003) and (104) planes shift to lower angles as the Ni²⁺ concentration increases, indicating an enlargement of spacing of the corresponding slabs. For XRD characterization, these diffraction peaks at lower 2θ angles indicate larger interplanar spacings, and vice versa^[24]. In view of the fact that Na⁺ mostly moves across the (003) planes adjacent to the sodium layers, a larger spacing among the (003) slabs could facilitate the diffusion of cations^[25]. To gain crystallographic information, we further used Rietveld analysis to refine the XRD patterns of the NCO and NCNO-0.05. Supplementary Figure 2 gives the results. Supplementary Table 1 summarizes the corresponding crystallographic parameters for the NCO and the NCNO-0.05 samples. Here, an increase in the lattice parameter a from 2.9697 Å for the NCO to 2.9737 Å for the NCNO-0.05 was observed. Such an increase in the lattice parameter a for the NCNO-0.05 could be ascribed to the comparatively bigger ionic radius of Ni²⁺ (0.69 Å) compared to Cr³⁺ (0.615 Å). A higher lattice parameter a corresponds to an expansion of TM ionic radii on the a-axis, whereas a higher lattice parameter c reflects the effect of the Coulomb repulsion^[26]. Likewise, a strong O-O repulsion between nearby TMO₆ layers is responsible for the rise of lattice parameter c from 15.9746 to 15.9966 Å^[23]. The high accuracy of the Rietveld refinement results is reflected by the low reliability factors for the NCO (R_p: 6.64%, R_wp: 9.11%) and the NCNO-0.05 (R_p: 6.77%, R_wp: 8.46%) samples, where R_p is R-pattern and R_wp is R-weighted pattern.

Figure 1. XRD patterns of (A) synthesized samples and the magnified diffraction peaks for the (003) and the (104) planes. SEM images of (B) the NCO and (C) the NCNO-0.05 samples. HRTEM images of (D) the NCO and (E) NCNO-0.05 samples.

Figure 1B and C presents scanning electron microscopy (SEM) images of the corresponding samples featuring micro-particles with an inconsistent configuration. We used the Nano Measurer software to statistically analyze the size distribution of particles. Particle size distribution analysis [Supplementary Figure 3] indicates that the NCNO-0.05 sample (d₅₀~0.40 μm) is averagely smaller than the NCO sample (d₅₀~0.57 μm). The energy dispersive spectroscopy mapping image characterization further verifies uniformly dispersed elements in NCNO-0.05 [Supplementary Figure 4], from which the ratio of Cr:Ni was evaluated to be 0.952:0.047 [Supplementary Table 2]. This is further confirmed with the inductively coupled plasma spectroscopy measurement (ratio = 0.946:0.048, Supplementary Table 3).

Figure 1D and E presents the high-resolution transmission electron microscopy (HRTEM) characterization on the NCO and NCNO-0.05 samples, respectively. The original images have been included in Supplementary Figure 5. After Ni²⁺-substitution, the fringe spacing corresponding to the (003) plane increases from 0.53 to 0.55 nm for the NCNO-0.05 sample, which belongs to the Rinm phase of O3-type layered metal oxide. The wider interlayer distance is beneficial to the faster kinetic^[19,27]. Therefore, introduction of Ni²⁺ leads to local adjustment and optimization of the lattice structure, making the lattice more favorable for the accommodation and diffusion of sodium ions.

We further carried out electronic localization function (ELF) calculations to rationalize the influence of Ni substitution onto the electronic structures of samples. Figure 2A illustrates the ELF map of valence electrons at the slices along the (001) plane for the NCO and NCNO-0.05 samples. The value in the two-dimensional ELF slice along (001) surface is in the range between 0 and 1. The value of 1 in red region indicates a complete localization, while that of 0.5 in green region corresponds to a delocalized electron gas. In short, a big ELF value indicates a strong localization of electrons. Therefore, in the ELF profile, compared with those in the NCO sample, the NCNO-0.05 sample shows an easily distinguished electron localization in the O-Cr coordination. This result indicates this bond has a more ionic character within the NCNO-0.05 sample than within NCO^[28]. This could be ascribed to the fact that the Cr³⁺ ion has a higher coordination capacity than the Ni²⁺ due to the existence of empty 3d orbitals. In addition, enrichment of electrons between Cr-O could reduce the electrostatic interaction between the electrons and the sodium ions in the interlayer, which favors the diffusion of Na^+[29,30]. This deduction can be explored by the accumulation of electrons on the O atoms to enhance the repulsion between O-O bonds. Previous reports have indicated that an enhancement of the ionic character of the TM-O bond can destabilize the P3 phase by sharing faces between NaO₆ and TMO₆ polyhedra in the layered crystal, thus promoting a reversible transition from the O3 to P3 phase^[31,32].

Figure 2. (A) The electron localization function of valence electrons at the slices along the (001) plane of NCO and NCNO-0.05 samples. (B) The Cr 2p XPS spectra of the as-obtained NCO and NCNO-0.05 samples. (C) Ni 2p XPS spectra of NCNO-0.05.

We further carried out X-ray photoelectron spectroscopy (XPS) measurements to examine the chemical states of interested elements in samples. Figure 2B presents the Cr 2p_3/2 XPS signal in which the peaks at binding energies of about 575.1 and 576.2 eV can be ascribed to the Cr²⁺ and Cr³⁺ species^[33]. Compared to the NCO sample, the content of Cr²⁺ in the NCNO sample increases from 15.8% to 18.4% for the NCNO-0.05 by integrating the corresponding XPS peaks. Figure 2C illustrates Ni 2p XPS spectra of the NCNO-0.05 electrode, from which the Ni 2p_3/2 peaks at binding energies of about 855.2 eV can be deconvoluted to the Ni^2+[34,35]. This result indicates a constant valence of Ni²⁺ after synthesis. This further informs that the change in valence from Cr³⁺ to Cr²⁺ is not caused by electron transfer from the Ni²⁺. The O 1s spectrum can be divided into three peaks according to different oxygen species [Supplementary Figure 6]: lattice oxygen (O^2-) at 528.9 eV, oxygen vacancies (O_V) at 530.6 eV, and surface adsorbed oxygen (surface OH^-/O₂) at 531.8 eV^[36,37]. Though it is difficult to evaluate a precise concentration for these species, a relative comparison of weight ratio can give a hint on the change of O_V in samples. By calculating the percentage of each species to the total O 1s XPS, we noticed the ratio of O_V to O^2- in the NCNO-0.05 (1.7) is higher than that of NCO samples (1.3), suggesting a higher contribution from O_V upon Ni²⁺ substitution. Moreover, the ratios of O_V to O^2- in NCNO-0.03 and NCNO-0.07 are approximately 1.4 and 0.9, respectively. The Ni-substitution-induced augment of O_V concentration in samples could be attributed to two mechanisms. The first involves the reduction of Cr³⁺ cations, where two Cr³⁺ ions are converted into two Cr²⁺ ions, resulting in the creation of one O_V. Alternatively, O_V can be introduced by substituting Ni²⁺ cations for Cr³⁺ cations in the lattice^[38-42]. Supplementary Figure 7 shows ex-situ XPS of Cr 2p and Ni 2p in NCNO-0.05, revealing that only Cr³⁺ is involved in charge compensation when charged to 3.6 V. We also attempted to charge the batteries to 3.8 V. Supplementary Figure 8 exhibits the charge-discharge curves and the capacity differential curves of the NCO and NCNO-0.05 samples. Charging the batteries to 3.8 V enables the Ni²⁺/Ni³⁺ redox couple to become active and participate in the redox reaction, but unfortunately, it also compromises the structural stability of the material. Consequently, subsequent electrochemical testing was conducted within the voltage range of 2.0-3.6 V.

Several coin-type cells (CR2032) were assembled with the NaCr_xNi_1-xO₂ samples of various Ni-contents in order to study their performance in a voltage range from 2.0 to 3.6 V (vs. Na⁺/Na). Figure 3A and Supplementary Figure 9 compare the first curves of charging and discharging processes at 0.2 C for the NCO, NCNO-0.03, NCNO-0.05, and NCNO-0.07 samples. We noticed that the initial discharge capacity increases as the Ni²⁺ content in the samples rises up to 5%. Considering that the NCNO-0.05 sample has the best performance in terms of specific capacity, we then chose this composition for further detailed investigations. The NCNO-0.05 sample achieved a discharge capacity of 120.3 mAh g^-1 at a rate of 0.2 C, which is higher than the NCO electrode (116.0 mAh g^-1). The coulombic efficiency (CE) of the first charge-discharge procedure for the NCNO-0.05 electrode (97.7%) is also higher than that of the NCO electrode (95.6%). The enhancement in initial charge-discharge capacity of NCNO-0.05 is closely associated with the reversible insertion and extraction of sodium ions, as evidenced by the higher Coulombic efficiency. The air stability of NCO and NCNO-0.05 was compared by exposing them to air for 12 h. As shown in the Supplementary Figure 10, NCO displayed a capacity degradation of 2.9 mAh g^-1, while NCNO-0.05 showed a slightly higher degradation of 3.3 mAh g^-1 due to its larger interlayer spacing that facilitates H₂O molecule insertion^[43].

Figure 3. (A) The charge/discharge profiles of CR2032 type coin cells assembled with the prepared the NCO and NCNO-0.05 samples as the working electrode and Na metal as the counter electrode. The charge/discharge rate is 0.2 C. (B) The rate behaviors of CR2032 type coin cells. (C) The cyclic voltammogram curves of the NCO and the NCNO-0.05 electrodes at the first cycle at a scan rate of 0.1 mV^-1. (D) The diffusion coefficient of Na⁺.

Figure 3B compares the rate behavior of the NCO and NCNO-0.05 electrodes. At a discharge rate of 50 C, the NCNO-0.05 electrode retained a capacity of 91.2 mAh g^-1, while the NCO electrode presented a much lower capacity (45.3 mAh g^-1). After the discharge rate was set back to 0.2 C, the capacities of the NCNO-0.05 and the NCO electrodes were retained to 112.4 and 98.8 mAh g^-1, corresponding to 94% and 84% retention of their original capacity, respectively.

Figure 3C shows the cyclic voltammetry (CV) curves for the first charging cycle at a sweep rate of 0.1 mV s^-1. The two major redox peaks at 3.04/2.95 V correspond to the redox reaction between Cr³⁺/Cr^4+[13]. The other smaller peaks are likely related to phase transitions in the layered metal oxide structure, as suggested by previous studies^[36,44,45]. The NCO cathode material principally operates with the compensation mechanism in terms of deintercalation of Na⁺ during the charging process and its re-intercalation during the discharging process^[46-49]. In the whole procedure, the charge equivalence is sustained by the redox reaction of Cr ions. The separation of the redox peaks of the NCNO-0.05 sample (ΔE = 87 mV) is smaller than that of the NCO sample (ΔE = 145 mV), suggesting that Ni substitution enhances the kinetics dynamics^[20]. The diffusion coefficient of Na⁺ ($$D_{N a^{+}}$$) was further explored with Galvanostatic Intermittent Titration Technique (GITT) characterization^[50]. The electrodes were charged at a current density of 20 mA g^-1 for 15 min, followed by relaxation at open circuit conditions for 1 h [Supplementary Figure 11]. Figure 3D presents the calculated $$D_{N a^{+}}$$ from the GITT characterization of both samples. Accordingly, the NCNO-0.05 sample (4.25 × 10^-11 cm² s^-1) has a higher calculated average value of $$D_{N a^{+}}$$ than the NCO sample (1.84 × 10^-11 cm² s^-1), indicating the faster kinetic of Na⁺ diffusion in the NCNO-0.05 sample. As a result, sodium storage can be facilitated by the facile transfer of sodium ions, leading to an excellent rate performance^[51].

Figure 4A shows the cycling stability for the prepared samples. The discharge capacities of the NCO and NCNO-0.05 electrodes at a rate of 10 C after 500 cycles were retained to 50.4 and 89.4 mAh g^-1, with the capacity retentions of 52% and 88%, respectively. After 1,000 cycles at a rate of 10 C, the NCNO-0.05 sample showed a discharge capacity of 81 mAh g^-1, higher than the NCO electrode of a corresponding capacity of 39.1 mAh g^-1. The capacity retention was 80% for the NCNO-0.05 and 40% for the NCO electrode. The capacity loss ratio at a rate of 10 C corresponding to each cycle was 0.060% and 0.021% for the NCO and NCNO-0.05 electrodes, respectively. The rate capability and cycle performance of NaCr_0.95Ni_0.05O₂ are higher than those of Na_0.88Cr_0.88Ru_0.12O₂ (83.6 mAh g^-1 at 50 C) and Na_0.9Ca_0.05CrO₂ (50 mAh g^-1 at 20 C) (see Supplementary Table 4). Supplementary Table 4 compares the electrochemical properties of NCO-based cathodes.

Figure 4. (A) The cycling performances of CR2032 type coin cells at a rate of 10 C. Charge-discharge profiles of (B) NCO and (C) NCNO-0.05 at 5 C. XRD patterns for the (003) of the (D) NCO and (E) NCNO after 0, 10th, and 100th cycles.

We further reduced the current density to 5 C in order to observe in more detail the performance of the electrode materials [Figure 4B and C]. The cycling performance comparison underscores the superiority of the NCNO-0.05 electrode, which maintains the capacity retention of 91.6% after 100 cycles at 5 C [Figure 4C], far surpassing the 75.4% retention demonstrated by the NCO electrode [Figure 4B]. In addition, the NCO electrode experienced severe voltage decay during cycling as indicated by the slanted arrows in Figure 4B, which could be related to the structural degradation of the material. In contrast, the capacity of NCNO-0.05 is well preserved after cycles.

To verify the structural stability of the NCO and NCNO-0.05 electrodes after different cycles (with a state of charge, SOC = 0%), we further characterized them using XRD. Figure 4D and E presents the results of XRD characterization of the (003) phase. The whole XRD pattern is shown in Supplementary Figure 12. Along with a decrease in diffraction peak intensity, the XRD signal peak for the NCO sample at the (003) phase showed a distinct shift to a lower angle after 100 cycles of sodium-ion intercalation/deintercalation. It suggests that the layered structure has introduced more stacking faults due to repeated volume changes of lattice^[47,51]. On the contrary, the XRD signal for the (003) phases of the NCNO-0.05 electrode is well preserved even after 100 cycles. This result verifies the improved stabilization of the O3 structure by Ni²⁺ substitution, which correlates to a highly reversible sodium-ion intercalation/deintercalation for this sample. In short, the highly stable material structure and reversible electrochemical reaction guarantee a long cycle life of Ni²⁺-substituted NCO.

CONCLUSIONS

In summary, we propose NaCr_0.95Ni_0.05O₂ as a promising cathode for high-efficiency SIBs. The chemical substitution of Cr³⁺ by low-valent Ni²⁺ in NCO can be an attractive method to simultaneously expand the sodium-ion diffusion channel and stabilize the structure during Na⁺ (de)intercalation. The NaCr_0.95Ni_0.05O₂ has achieved a high capacity (120.3 mA h g^-1 at 0.2 C), rate performance (91.2 mA h g^-1 at 50 C), and outstanding long cyclic life (80% after 1,000 cycles at 10 C). Moreover, our research suggests that Ni²⁺ substitution strategies can be extended to other Na-layered oxides. By optimizing the ionic of the TM-O bonds, the properties of layered oxide cathodes can be tailored for improved electrochemical performance.

EXPERIMENTAL

A solid phase reaction was used to synthesize NaCr_1-xNi_xO₂ (x = 0, 0.03, 0.05, 0.07). A slight excess of 5 mol% Na₂CO₃ was used to compensate for the loss of sodium volatility. A mixture of Na₂CO₃ (99.8%, Shanghai Macklin Biochemical Co., Ltd.), Cr₂O₃ (99%, Sinopharm Chemical Reagent CO., Ltd.), and NiO (99%, Shanghai Aladdin Biochemical Technology CO., Ltd.) was prepared in a stoichiometric ratio and thoroughly mixed by ball-milling with ethanol as a dispersing agent for 10 h. The powder was then heated to 900 °C for 5 h in an alumina crucible in an argon atmosphere at a rate of 5 °C per min with a gas flow rate of 100 mL min^-1. After cooling to room temperature, the samples were immediately put into a vacuum glove box.

The details about material characterization and electrochemical characterization of the samples are given in the supporting information.