Introduction

Rechargeable lithium-ion batteries are widely employed for electronic gadgetry with high capacity and long cycle life [1–6]. Recently, most interest has focused on a new system of LiNi1−x−y Co x Mn y O2 (0 < x < 1, 0 < y < 1) with a-NaFeO2 structure as cathode materials, which have been considered promising to displace LiCoO2 [7–10]. Among them, LiNi0.5Co0.2Mn0.3O2 (NCM-523) has attracted much attention because of its excellent initial discharge capacity [11–13]. However, it has a bad rate capability and obvious capacity degradation at high potential cycle, which indeed limits its extensively practical application [14].

To date, many researches have been done to overcome above problems. One way is to cover the cathode materials with various protective layers. The coating layers can reduce the contact area for electrode in electrolyte [15–18]. The other one is to improve the electrochemical performances with doping to modify the structure of cathode materials. The doping of electrode with alkaline earth metal (Mg, Sr) [19, 20], transition metal (V) [21, 22], main group metal (Al) [20], and halogen element (F) [23] have been explored to improve the property of NCM-523. However, there are hardly any reports about the effects of rare earth element on Li-ion battery.

Among all rare earth elements, Cerium has been much widely used in many fields as addition and catalyst. Wu cong [24] has reported that Cerium doping is effective to improve the electrochemical performance of LiFePO4. Zhong shengkui [25] has successfully synthesized LiNi1/3−x Mn1/3Co1/3Ce x O2 by a co-precipitation method and proved that Ce-doped LiNi1/3Co1/3Mn1/3O2 has much better cycling performance and rate capability. In this work, we explore the influence of Cerium doping on the electrochemical performance of NCM-523 at 4.25 V and a higher potential of 4.5 V. The composition, structure, and morphology of the materials were characterized. The impact of Cerium doping on the electrochemical properties, rate capability, cycle ability, and electrochemical impedance spectroscopy was also investigated.

Experimental

Ni0.5Co0.2Mn0.3(OH)2 precursor was firstly achieved by co-precipitation method. NiSO4·6H2O, CoSO4·7H2O, and MnSO4·H2O were dissolved in distilled water (cationic ratio of Ni:Co:Mn = 5:2:3) to obtain a mixed solution with a concentration of 2.0 mol/L. The prepared solution was dripped into the base solution (pH = 11) in nitrogen atmosphere with continuous rapid stirring. In the meantime, 0.5 mol/L NH3·H2O solution as complexant and 2.0 mol/L NaOH solution as precipitant were added separately. The reaction was conducted for 10 h at 60 °C with pH value of 11. The suspension was washed and filtered repeatedly and dried at 100 °C, then Ni0.5Co0.2Mn0.3(OH)2 precursor was obtained.

The layered Cerium-doped NCM-523 materials were synthesized by solid-state method. Precursor was well mixed with Li2CO3 and CeO2, and sintered at 900 °C in atmosphere for 10 h followed by a nature cooling procedure to achieve layered Cerium-doped NCM-523 cathode materials.

The powder X-ray diffraction (XRD, Dmax-C) with CuKα radiation was chosen to examine the crystalline phase of the prepared powders. The data were collected over 10–80° (2θ) with a scan rate of 5° per minute.

The particle size and morphology of the synthesized materials were analyzed by a scanning electron microscope (SEM, Hitachi, S-4800).

The electrochemical properties were characterized via CR2032 coin-type cell. The mixed slurry of active material, carbon black, and polyvinylidene fluoride (PVDF) binder with a weight ratio of 94.4:2.5:3.1 in N-pyrrolidone (NMP) solvent was prepared and pasted onto an Aluminum current collector. After drying at 90 °C for 2 h, the electrode was rolled and cut. Coin cell was composed of positive electrode, negative electrode (lithium foil), separator (a porous polypropylene film), and electrolyte (1 mol/L LiPF6 in EC: EMC: DMC (1:1:1 in volume)). Argon-filled glove box (UN800S, Dellix) was employed to assemble coin cells. Initial charge–discharge capacities, rate capability, and cycling performance are measured by Neware battery test system (CT-3008 W, Shenzhen, China) in the potential region of 3–4.25 V and 3–4.50 V at room temperature.

Electrochemical impedance spectroscopy (EIS, CHI660D) tests were performed by an electrochemical work station, within 5 mV voltage amplitude between 10 kHz and 10 MHz. Cyclic voltammogram (CV) was tested in a three-electrode system, which worked within a scanning speed of 0.2 mV/s between a potential range of 2.5–4.6 V.

Results and discussion

The XRD patterns of prepared pristine NCM-523 and Cerium-doped NCM-523 samples are showed in Fig. 1. All peaks are highly indexed to the α-NaFeO2 structure (R-3m) [26] and no extra phase peaks of impurities exist. Obvious splitting of the (006)/(102) and (108)/(110) peaks means the formation of layered structure. Cerium doping does not change the basic NCM-523 crystal structure. The lattice parameters of the samples and the values of I (003)/I (104) are shown in Table 1. The values of lattice constant a and cell volume V increase and become larger as doping amount of Cerium increased, which can be attributed to the larger size of Ce4+ than Co3+/Mn4+. What is more, the (003) peaks of XRD shift to the right with the addition of Ce. The more Ce doped, the more the peaks shifted. Above changes indicate that the doping Ce4+ ions might be incorporated into the crystal lattice. As is known, the integrated intensity ratio of I (003)/I (104) is regarded as an important sign to determine the cation mixing degree in the lattice [27]. The values of I (003)/I (104) are 1.45574, 1.45792, 1.46025 and 1.46051 for the pristine and doped samples, respectively. Both ratios are larger than 1.2, which states a highly ordered a-NaFeO2 structure with limited cation mixing.

Fig. 1
figure 1

X-ray diffraction patterns of pristine and Cerium-doped NCM-523 samples

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Table 1 Lattice parameters of samples and the values of I (003)/I (104)
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Figure 2 shows the SEM data of prepared pristine NCM-523 and Cerium-doped NCM-523 samples. As shown, there is no great difference between them. They both consist of primary particles and secondary particles. The secondary particles exhibit a uniform near spherical microstructure with a size of 5 µm, which is made up of numerous primary grains. Thus, Cerium doping did not change the morphology of material.

Fig. 2
figure 2

SEM images of pristine and Cerium-doped NCM-523 samples (a pristine, b Cerium-doped)

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The initial charge–discharge profiles of the pristine and Cerium-doped NCM-523 samples at a rate of 0.2 C in the potential range of 3–4.25 and 3–4.5 V at room temperature are shown in Fig. 3. As shown, all of the stable and smooth curves display a typical electrochemical behavior without an apparent voltage plateau, which indicates that no related phases are formed during charging and discharging. As seen in Fig. 3, the initial discharge capacity of pristine and Cerium-doped samples are 154.59 mAh/g and 150.86 mAh/g at 4.25 V, respectively, with coulomb efficiency of 81.97 and 82.92 %, while the corresponding initial discharge capacity at 4.5 V are 188.81 mAh/g and 184.12 mAh/g with coulomb efficiency of 83.41 and 83.29 %. It is seen that the initial discharge capacity of the Cerium-doped sample is lower than that of NCM-523 both at 4.25 and 4.5 V, which should be ascribed to the decrease in the electrochemically active ions [28] in the prepared products doped by a small amount of Cerium. The coulomb efficiency of Cerium-doped NCM-523 is better than that of pristine one. The likely explanation is Cerium doping effectively suppressed the Li+/Ni2+ mixing degree so that limited Li+ is blocked by Ni2+.

Fig. 3
figure 3

Initial charge–discharge curves of pristine and Cerium-doped NCM-523 samples at different cut-off potentials of 4.25 and 4.5 V

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Rate capability of the pristine and Cerium-doped NCM-523 samples is shown in Fig. 4. The cells are charged-discharged at different rates of 0.2, 0.5, 1, 2, and 5 C in the potential region of 3–4.25 and 3–4.5 V for every five cycles. It is clear that Cerium-doped NCM-523 exhibits a superior rate capability than the pristine one. From Fig. 4, we can clearly see that although the discharged capacity of pristine sample is higher than that of the Cerium-doped sample at 0.2 C, they all drop off dramatically with increasing C-rates due to polarization [18]. The pristine material displays a higher discharge capacity at low C-rates. However, Cerium-doped one shows better rate capability and capacity retention than the pristine material above 0.5 C. Cerium doping is favorable to improve the rate capacity of NCM-523, especially at high potential of 4.5 V. When charged to 4.5 V, Cerium-doped NCM-523 discharges a capacity of 109.75 mAh/g at 5 C, which is 2.56 times higher than 42.72 mAh/g of pristine one. When charged to 4.25 V, doped sample presents 71.87 mAh/g at 5 C, which is only 1.94 times higher than 36.92 mAh/g of pristine one. The results reveal that Cerium doped-NCM523 expresses better rate capacity at 4.5 V than at 4.25 V. The increasing stability of the layered crystal structure can be attributed to the doping of inactive Cerium which is more effective especially at high rate and high potential. Namely, Cerium doping is powerful to enhance the capacity at high rate and high potential charge–discharge.

Fig. 4
figure 4

Rate capability of pristine and Cerium-doped NCM-523 samples at different cut-off potentials of a 4.25 V and b 4.5 V

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The impact of Cerium doping on the cycling performance of NCM-523 is also investigated in Fig. 5. As seen, although the Cerium-doped material has lower initial discharge capacities, it shows excellent cycle stability and better capacity retention than pristine one at 1 C after 30 cycles. The capacity retention of Cerium-doped NCM-523 is increased to 96.77 % from 93.28 % of pristine sample at 4.25 V and to 97.35 % from 90.90 % at 4.5 V. What is more, the discharge capacity of Cerium-doped material is higher than that of pristine one after cycles. The profiles obviously show that the increased capacity degree for Cerium-doped NCM-523 to pristine sample after 30 cycles at 4.5 V is higher than that at 4.25 V. One possible reason is the stability of inactive ion Cerium. There is no oxidation and reduction during charge–discharge process, so a small amount Cerium doped into layered crystal structure could be a “backbone” role in the layered structure. This “backbone” function works effectively in the cycle process, especially at high potential. What is more, Less Ni2+ occupies the sites of Li+, which could also enhance the diffuser efficiency of Li+ and improve cyclic performance.

Fig. 5
figure 5

Cyclic ability of pristine and Cerium-doped NCM-523 samples at different cut-off potentials of 4.25 and 4.5 V

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The EIS profiles of pristine and Cerium-doped LiNi0.5Co0.2Mn0.3O2 samples after 10 cycles and 30 cycles are exhibited in Fig. 6. The equivalent circuit is shown in Fig. 6c to further understand the effect of Cerium doping. Rs is the solution resistance, corresponding to the small break in the high frequency. Rsei is the impedance for a solid electrolyte interface layer, related to the arc in the medium frequency region. Rct is a charge-transfer at the electrolyte/electrode interface, corresponding to the arc in the low frequency region. Zw is the diffusion of Li ions in the crystal lattice, related to the quasi-straight line in the low frequency [15, 29]. All characteristic parameters were obtained via zview-2soft. The Rs values of both samples changed slightly with cycle times increased. The Rsei values of pristine NCM-523 are almost equal to that of the Cerium-doped sample after 10 or 30 cycles, which indicate that the solid electrolyte interface layer formed on the surface of the electrode has no great distinction. But there is obvious difference between their charge-transfer impedances. The Rct values of the pristine LiNi0.5Co0.2Mn0.3O2 is 228.71 Ω after 10 cycles, and then enlarges to 387.7 Ω after 30 cycles. By contrast, the Rct of the Cerium-doped sample only increases from 108.67 Ω after 10 cycles to 308.43 Ω after 30 cycles. The lower Rct of doped material indicates that the Cerium doping may cause some defects in the system, which increases the electronic conductivity and improves the Li+ kinetic behavior. Therefore, it could be claimed that Cerium doping has a significant function on reducing the increasing of charge-transfer impedance during cycling.

Fig. 6
figure 6

EIS plots of a pristine and b Cerium-doped NCM-523 samples after 10th and 30th cycles, c equivalent circuit performed to fit the Nyquist plots in a and b and d the relationship plot between inverse square root of angular frequency ω −1/2 and Z′

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The diffusion coefficient of lithium ions for cells can be calculated according to the following equation: [30]

$$ D = 0.5 \times R^{2} T^{2} /(nAF^{2} \sigma C)^{2} $$
(1)

and [31]

$$ Z^{\prime} = Rs + Rct + \sigma \omega^{ - 1/2}, $$
(2)

where R is the gas constant (8.314 J/(mol K)), T is the absolute temperature (298.15 K), n is the number of electrons per molecule during oxidization, A is the area of interface between cathode material and electrolyte (here A is 2.00 cm−2), F is the Faraday constant, σis the Warburg factor, and C is the concentration of lithium ion. From the plot of the relationship between inverse square root of angular frequency ω −1/2 and Z′, σ corresponding to the slop in Fig. 6d can be obtained. So, according to the Eq. (1), Cerium-doped NCM-523 after 30 cycles has a better DLi+ of 1.84 × 10−13 cm2/s, compared to 5.47 × 10−14 cm2/s for pristine material. A possible reason is that Li+ is easier to intercalate/deintercalate with lattice parameters a increased by Cerium doping. Thus, lower charge-transfer impendence and better diffusion coefficient of lithium ions confirm the effect of Cerium-doped layered NCM-523 again, and support the excellent rate capability and cycling stability shown in Figs. 4 and 5.

Cyclic voltammogram curves of pristine and Cerium-doped LiNi0.5Co0.2Mn0.3O2 cells are exhibited in Fig. 7. As seen, both curves had a similar profile with a couple of redox peaks. The anodic peak for pristine NCM-523 appears at 3.98 V, accompanied with the cathodic peak at 3.55 V. For doped sample, the anodic peak is observed at 3.96 V, while the corresponding reduction peak shifts up to 3.58 V. It is obvious that the potential difference of redox couple is greatly reduced, when compared with that of pristine sample. Namely, Cerium doping reduces the potential difference between anodic and cathodic peaks. The smaller difference indicates limited electrode polarization and better reversibility of Li ions during electrode reaction, which also means better capacity retention during cycling.

Fig. 7
figure 7

Cyclic voltammogram curves of pristine and Cerium-doped NCM-523 cells

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Conclusion

The composition, structure, morphology, and electrochemical performance of pristine and Cerium-doped NCM-523 materials synthesized by co-precipitation and solid-state methods were studied via XRD, SEM, EIS, and CV tests. The electrochemical properties of the pristine and Cerium-doped LiNi0.5Co0.2Mn0.3O2 cathode materials were compared at conventional potential of 4.25 V and higher potential of 4.5 V. XRD results showed that Cerium did not change the layer structure and might be incorporated into the crystal lattice. Electrochemical test indicated that Cerium-doped samples had outstanding high rate capability and cycling stability, especially at high potential of 4.5 V, when compared to the pristine one. EIS exhibited that Cerium doping contributed to the decreasing of charge-transfer impendence and increasing of diffusion coefficient of lithium ions. Thus, Cerium doping was meaningful to LiNi0.5Co0.2Mn0.3O2, especially at high potential.