Improved Electrochemical Performance of Li-Rich Cathode Materials via Spinel Li₂MoO₄ Coating

1. Introduction

LIBs as a kind of clean and renewable electrochemical energy storage equipment have been widely applied. However, the current energy density of commercial LIBs is insufficient to meet the growing demand, and the development of high-capacity electrode materials is urgent [1,2]. Li-rich manganese-based cathode materials (LRMs) are considered to be one of the most promising cathode materials for the next generation of LIBs due to their high specific capacity (>250 mAh/g), low cost, and excellent energy density (>1000 Wh/kg) [3,4]. The general formula of LRMs is xLi₂MnO₃·(1-x) LiMO₂ (M=Ni, Co, Mn). Based on the oxidation-reduction of transition metal ions (TM) and the additional oxygen redox, more Li can be removed to provide ultra-high capacity [5,6]. However, the practical application of LRMs is impeded by severe voltage and capacity decay, mainly due to continuous migration of transition metals (TM) and the resulting structural evolution from layered to spinel or rock salt phases [7,8].

TM migration or structural evolution begins on the surface of LRMs during the cycling process, so the surface protection of LRMs is crucial [9,10,11]. Surface coating is an effective surface protection method, which can protect electrode materials from electrolyte corrosion, inhibit oxygen release and stabilize crystal structure [12]. So far, many materials, such as oxides Al₂O₃ [13], ZrO₂ [14], fluoride AlF₃ [15], LiF [16], and metal phosphate LiFePO₄ [17], have been used as surface coatings to improve the cyclic stability of LRMs. However, although these efforts have made significant contributions to improving cycle life, most coatings have poor compatibility with LRMs, which is not conducive to uniform protection of LRMs and maintaining overall integrity with LRMs over long cycles [18]. Therefore, it is highly desirable to introduce lithium-reactive coating materials to enhance the compatibility between the coating and the material body.

In this work, we coated a Li₂MoO₄ coating layer with a unique spinel structure on the surface of Li_1.2Mn_0.54Co_0.13Ni_0.13O₂. Through the wet chemistry method and the subsequent calcination process, Li₂MoO₄ can be evenly coated on the surface of LRMs. In addition, when molybdate reacts with the cathode material at high temperature, it can form spinel phase on the surface of the material by removing part of Li. The spinel structure has 3D Li⁺ channels, which are conducive to Li⁺ transport. At the same time, the Li₂MoO₄ coating as a good lithium ion conductor can provide better protection, preventing LRMs from electrolyte corrosion, thus improving structural stability and cycle life.

2. Materials and Methods

2.1. Materials Synthesis

The pristine Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ were synthesized by a high temperature solid state method. In detail, the carbonate precursor (Mn_0.675Co_0.1625Ni_0.1625CO₃) from HaiAnZhiChuan Battery Materials Technology Co., LTD. was uniformly ground and mixed with Li₂CO₃ (an excess of 5% Li₂CO₃ was added to compensate for lithium loss during the elevated calcination step) in an agate mortar. Subsequently, the powder mixture was sintered at 500℃ for 6 h and then 850℃ for 12 h to produce the target Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ (Pristine).

The prepared Li-rich cathode material was mixed with a certain amount of ammonium molybdate. To avoid agglomeration, the mixed materials were dissolved in absolute ethanol after grinding and stirred for 3 h. After evaporation in a water bath at 80℃, the desiccative powder was calcined at 720℃ for 4 h to obtain the Li₂MoO₄@Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ sample. According to different contents of Li₂MoO₄, the prepared samples containing 1,3,5 wt% Li₂MoO₄ coating layer are termed 1wt%, 3wt% and 5wt%, respectively.

2.2. Material Characterization

X-ray diffraction patterns of samples for the crystal structure were obtained by X-ray powder diffractometer (XRD, D8 Advance, German Bruker AXS Co., LTD., Cu Kα radiation), and XRD data were obtained in an angular range of 10-80° with a scanning speed of 1°/min. The test results were refined using GSAS [19]. Scanning electron microscopy (SEM, Thermo Scientific Apreo 2c) was applied to observe the morphology of the material. Transmission electron microscopy (TEM, JEM2200FS) was used to detect the structure and microstructure of the samples. The inductively coupled plasma emission spectrometer (ICP-OES, Agilent 5110) was used to determine the composition of the elements. X-ray photoelectron spectroscopy analyzer (XPS, Thermo ESCALAB 250XI) was used to measure element species and valence state information on the sample surface.

2.3. Electrochemical Characterization

The active cathode material, acetylene black, and PVDF binder (mass ratio of 8:1:1) were mixed and dispersed in N-methyl-2-pyrrolidone (NMP) solvent to prepare the cathode slurry, which was then coated on Al foil and dried at 60℃ for 12 h in a oven. The dried electrode was cut into a round film with a diameter of 12 mm and the weight loading of the active materials was about 1.5 mg/cm. The test cells were assembled as CR2025-type coin cells in an Ar glove box (H₂O and O₂ content below 0.1 ppm), and a high voltage electrolyte (LB-111, Duoduo Company) was used as the electrolyte. Electrochemical measurements of the assembled cells were carried out on a LAND at a temperature of 30 °C. The batteries were tested in the voltage range of 2.0-4.8 V. For 1 C (1 C=250 mA/g) cycling tests, the batteries were first cycled at 0.1 C three times and then cycled to 100 cycles at 1 C. The rate performance of the batteries was tested at 0.1, 0.2, 0.5, 1, 2, and 5 C, and then returned to 0.1 C. CV was conducted on a VSP electrochemical workstation at a voltage between 2.0 and 4.8 V at a scan rate of 0.1 mV/s. Electrochemical impedance spectroscopy (EIS) was performed on a VSP electrochemical workstation at a frequency range of 0.01 Hz-100 Hz and an AC amplitude of 5 mV.

3. Results and Discussion

3.1. Material Characterizations

The inductively coupled plasma optical emission spectrometer (ICP-OES) analysis was used to investigate the cation ratio of as-obtained cathode materials, and the results are listed in Table S1. The results indicate that the experimental results are in good agreement with the design values.

Figure 1 shows the X-ray powder diffraction (XRD) patterns of the samples and their Rietveld refinement results. Figure 1 indicates the presence of a lamellar α-NaFeO₂ structure with the R-3m space group in the four samples, and the characteristic peak in the range of 20-25° also demonstrates the presence of the Li₂MnO₃ phase with the C2/m space group in the samples. The clear split of the (006)/(102) and (108)/(110) peaks implies a highly ordered layered structure of the samples [20]. In addition, other peaks can be clearly observed near 18, 30, and 35°, and the peak intensity increases with the increase of Li₂MoO₄ coating amount. After analysis and comparison, it can be determined that the weak peak is the spinel structure of Li₂MoO₄ (Figure S1 [21]).

we performed a Rietveld refinement on the XRD curves of four samples using a two-phase structure model consisting of rhomboidal R-3m and monoclinic C2/m phases, and the results are shown in Figure 1b-e. According to reports, the diffraction peak intensity ratio between the planes (003) and (104) can be used to estimate the degree of Li/Ni mixing. The smaller the ratio, the greater the degree of Li/Ni mixing [22]. As shown in Table 1, the ratios I(003)/I(104) of all samples are greater than 1.2, indicating a lower Li/Ni miscibility [23]. Compared to the original sample, the ratio I(003)/I(104) of the sample coated with Li₂MoO₄ decreased slightly, possibly due to the intensification of Li/Ni mixing during high-temperature calcination during the preparation process. Among them, the ratio I(003)/I(104) of the 3wt% sample is closest to the original sample, and the content of Ni²⁺in the Li layer in the refined results of Table 1 also corresponds to the ratio I(003)/I(104). Additionally, the spacing between layers of the sample coated with Li₂MoO₄ increased slightly, possibly due to partial doping of Mo in the TM layer on the surface. The subsequent X-ray photoelectron spectroscopy (XPS) results provide evidence for this result. However, the ratio between the lattice parameters c and a of the four samples is greater than 4.99, indicating that the samples have a fine-layered structure [24]. At the same time, the content of Li₂MoO₄ in the coated samples was refined and the results are shown in Table 1. Except for the low peak intensity of Li₂MoO₄ in the 1wt% sample, which cannot obtain the content of Li₂MoO₄, the content of Li₂MoO₄ in the 3wt% and 5wt% samples is in good agreement with the design values and ICP results.

The morphological characteristics of the prepared samples were investigated by scanning electron microscope (SEM), and the obtained images are shown in Figure S2. The morphology of all the sample particles is spherical and the diameter of the secondary particles ranges from 10 to 15 μm. Primary particles can be clearly seen on the surface of the original sample particles, but the surface of the coated Li-rich material becomes smooth.

Transmission electron microscope (TEM) and fast Fourier transform (FFT) were performed to display the crystal structure near the surface areas of the 3wt% sample. The Li₂MoO₄ coating layers on the surface of the material can be clearly observed in Figure 2a. Figure 2b shows clear lattice stripes of the layered structure. Combining with the FFT image (Figure 2e) of the corresponding position in Figure 2d, it can be determined that the spacing of lattice stripes is 0.474 nm, which is very consistent with the plane (003) of the R-3m space group of the li-rich layered structure. Through the analysis of the FFT image of the blue region in Figure 2c, it can be found that a spinel structure coating is formed on the surface of the material coated with Li₂MoO₄. The thickness of the coating can reach over 20 nm due to not only the unique spinel structure of Li₂MoO₄ formed by coating, but also the spinel phase formed by the detachment of some Li from the material surface during the formation of Li₂MoO₄ coating [21]. To further investigate the existence of the Li₂MoO₄ coating layer on the surface of 3wt% sample, EDS was carried out and the corresponding elemental mapping is shown in Figure 2f-k. The elements of Mn, Co, Ni, O, and Mo are evenly distributed on the particle, illustrating that the Li₂MoO₄ layer was successfully coated on the bulk material.

X-ray photoelectron spectroscopy (XPS) measurements were performed to confirm the composition of the surface elements and the chemical state of the Pristine and 3wt% samples (Figure 3 and Figure S2). Figure 3a and b present the XPS full spectra of the Pristine and 3wt% sample, respectively. Figure 3c shows that the 3wt% sample had a Mo 3d photoelectron emission intensity peak near 232 eV and235 eV, which corresponded to Mo 3d_5/2 and Mo 3d_3/2, respectively, indicating that Mo⁶⁺ on the surface of 3wt% sample may be in the form of Li₂MoO₄, and the presence of Mo⁴⁺ indicates that some Mo⁶⁺ is doped in the lattice interior to convert to Mo⁴⁺ to maintain electroneutrality [25]. Figure S2 presents the XPS spectra of Mn, Co, and Ni elements for Pristine and 3wt% sample, respectively. The valence states of Mn, Co, and Ni do not change greatly, which indicates that the coating maintains the original system status of the Li-rich materials.

3.2. Electrochemical Performances

Figure 4a shows the initial charge-discharge curves of the four samples (2.0-4.8 V, 0.1 C). During the first discharge, the Pristine, 1wt%, 3wt%, and 5wt% sample release 259.65, 250.29, 257.31 and 248.74 mAh/g, respectively, corresponds to Coulombic efficiencies of 56.66, 76.92, 77.79 and 75.16%. All samples show the typical initial charge process of Mn-based lithium-rich cathode materials. In detail, the charge curve can be divided into two parts, a sloping part below 4.5 V and a long plane part at 4.5 V. The sloping region belongs to the oxidation reaction of Ni²⁺/Ni⁴⁺ and Co³⁺/Co⁴⁺, while the long plateau is related to the oxygen activation of Li₂MnO₃ [26]. Compared to the original sample, the platform of the sample coated with Li₂MoO₄ becomes shorter near 4.5 V. This is because the formation of the Li₂MoO₄ coating layer removes some Li from the material and pre-activates the Li₂MnO₃ component [27,28]. Therefore, the sample coated with Li2MoO₄ has a higher initial Coulombic efficiency. The cyclic performance of four samples at 1 C is shown in Figure S4b. For comparison purposes, only the cyclic performance of the Pristine and 3wt% samples is shown in Figure 4b. The results showed that the Li₂MoO₄ coating improved the cycling performance of the material, with only 69.25% capacity retention rate of the original sample after 100 cycles, while the 3wt% sample still had 81.85% capacity retention rate after 100 cycles.

Figure 4c shows the rate performance of the samples. The discharge capacity of all four samples decreases with the increase of current density, with a 3wt% sample exhibiting the best rate performance. The spinel has a 3D Li⁺ channel compared to the 2D Li⁺ channel of the LRMs, which favors an improvement in the rate capability [29]. Meanwhile, Li₂MoO₄ belongs to the fast-ion conductor, which is conducive to Li⁺ transport. Therefore, the material coated with Li₂MoO₄ has higher rate performance [30].

In addition, according to Figure 4d and e, the Pristine sample exhibits faster voltage and capacity decay, which is also one of the main problems of Li-rich materials. Figure 4f shows that the average voltage decay of the 3wt% sample (voltage drop of 4.98 mV per cycle, 100 cycles) is less than that of the Pristine sample (voltage drop of 7.06 mV per cycle, 100 cycles) at high cutoff voltage (4.8 V). The increase in capacity and discharge voltage retention of the 3wt% sample is due to the effective reduction of direct contact between the electrode and the electrolyte by the Li₂MoO₄ coating, which inhibits the erosion of Li-rich materials during discharge, prevents the oxidation of the electrolyte on the surface of the cathode material during charging, and improves its structural stability.

Figure 4. (a) AC impedance spectra of four samples, and (b) equivalent circuit diagram and impedance parameters of the samples. (c) CV curves of 3wt% sample.

Figure 4. (a) AC impedance spectra of four samples, and (b) equivalent circuit diagram and impedance parameters of the samples. (c) CV curves of 3wt% sample.

In order to investigate the kinetic properties of the prepared samples, the electrochemical impedance spectroscopy (EIS) measurement was conducted. As shown in Figure 5a, the impedance curve consists of a semicircle whose diameter represents the charge transfer resistance (Rct) and the oblique line represents the Warburg impedance. The equivalent circuit and the impedance parameters fitted to four samples are shown in Figure 5b. As expected, Rct decreased after coating with Li2MoO4, which was attributed to the spinel structure of the Li2MoO4 coating that is capable of providing 3D channels for Li+ diffusion, which is favorable for Li+ diffusion.

Figure 5c shows the CV curve of the 3wt% sample between 2.0-4.8 V. During the initial charge and discharge cycle, the CV curve exhibits two different oxidation processes, corresponding to the oxidation reaction of Ni2+/Ni4+ and Co3+/Co4+, and the activation of the Li2MnO3 phase, respectively. When the voltage is higher than 4.5 V, the deintercalation of Li would be accompanied by the release of oxygen, but there is no oxidation peak at 4.68 V in the subsequent cycles [31]. The activation of the Li2MnO3 phase occurs only during the first charge discharge cycle, which is also the reason for the low initial Coulombic efficiency of Li-rich materials.

4. Conclusions

In summary, Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ coated with Li₂MoO₄ was successfully prepared. XRD results indicate that the samples coated with Li₂MoO₄ exhibit a good layered structure, while the Li₂MoO₄ coating exhibits a unique spinel structure. The SEM and TEM results indicate that Li₂MoO₄ is perfectly coated on the surface of the material. To determine the optimal amount of coating, a series of Li-rich materials coated with Li₂MoO₄ were prepared, among which the 3wt% Li₂MoO₄ sample exhibited the best electrochemical performance. Compared to the Pristine sample, the initial Coulomb efficiency of the sample coated with Li₂MoO₄ has greatly improved, from 56.66% to over 75%, effectively alleviating the problem of low initial Coulomb efficiency in Li-rich materials. After 100 cycles at 1 C, the original sample only had a capacity retention rate of 69.25%, while the sample coated with 3wt% Li₂MoO₄ had a capacity retention rate of 81.85%, greatly improving cycling performance. Meanwhile, the magnification performance curve also indicates that the sample coated with 3wt% Li₂MoO₄ has higher capacity at high magnification. Furthermore, the voltage attenuation of Li-rich materials coated with Li₂MoO₄ was also suppressed. Compared to the original sample, the voltage attenuation of the sample coated with 3wt% Li₂MoO₄ slowed by an average of 2.08 mV per 100 cycles. The EIS results showed that the R_ct of the sample coated with Li₂MoO₄ decreased significantly and the diffusion of Li⁺ was enhanced. This work indicates that the Li₂MoO₄ coating can enhance the cyclic stability of Li-rich materials and improve their electrochemical performance.