Enhanced Electrochemical Properties of Zr4+-doped Li1.20[Mn0.52Ni0.20Co0.08]O2 Cathode Material for Lithium-ion Battery at Elevated Temperature

The typical co-precipitation method was adopted to synthesized the Li-excess Li1.20[Mn0.52−xZrxNi0.20Co0.08]O2 (x = 0, 0.01, 0.02, 0.03) series cathode materials. The influences of Zr4+ doping modification on the microstructure and micromorphology of Li1.20[Mn0.52Ni0.20Co0.08]O2 cathode materials were studied intensively by the combinations of XRD, SEM, LPS and XPS. Besides, after the doping modification with zirconium ions, Li1.20[Mn0.52Ni0.20Co0.08]O2 cathode demonstrated the lower cation mixing, superior cycling performance and higher rate capacities. Among the four cathode materials, the Li1.20[Mn0.50Zr0.02Ni0.20Co0.08]O2 exhibited the prime electrochemical properties with a capacity retention of 88.7% (201.0 mAh g−1) after 100 cycles at 45 °C and a discharge capacity of 114.7 mAh g−1 at 2 C rate. The EIS results showed that the Zr4+ doping modification can relieve the thickening of SEI films on the surface of cathode and accelerate the Li+ diffusion rate during the charge and discharge process.


Experimental
The Li-excess Li 1.20 [Mn 0.52−x Zr x Ni 0.20 Co 0.08 ]O 2 (x = 0, 0.01, 0.02, 0.03) series cathode materials were synthesized via using the carbonate co-precipitation method to synthesize the carbonate precursors, followed by sintering with LiOH·H 2 O powder at high temperature to obtain the cathode materials. The typical synthesis route has been shown as follows: (1)  To investigate the influence of the Zr 4+ doping on the crystal structure of Li 1.20 [Mn 0.52 Ni 0.20 Co 0.08 ]O 2 , the XRD measurement were carried out by using Rigaku RINT2400 X-ray diffractometer with Cu Kα radiation in the 10° ≤ 2θ ≤ 80°, accompanied by a step size of 0.02° and a count time of 10.0s. Rietveld refinement of the cathode powder diffraction patterns were performed by using the GSAS/EXPGUI program. The morphologies of Li 1.20 [Mn 0.52−x Zr x Ni 0.20 Co 0.08 ]O 2 (x = 0, 0.01, 0.02, 0.03) were observed by using scanning electron microscopy (SEM, Ultra 55, Zeiss) and high-resolution transmission electron microscopy (TEM, FEI Titan G2 60-300) equipped with energy-dispersive X-ray spectroscopy (EDX, Oxford) to test the elemental distributions of cathode material (x = 0.02). The particle size was measured by using laser particle size Analysis (LPS, TOOLSO, 2005A). The chemical states of the doping element were determined by using X-ray photoelectron spectroscopy (XPS, Perkin Elmer PHI 1600). And the XPS spectra were fitted by using XPSPEAK software. The elemental composition, i.e. Ni, Co, Mn and Yb, was detected by ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometer, iCAP 6000). Phase transformation studies of original and cycled Li 1.20 [Mn 0.52−x Zr x Ni 0.20 Co 0.08 ]O 2 (x = 0, 0.02) were carried out using a micro-Raman spectrometer (LabRAMHREvolution, HORIBA).
The electrochemical properties of Li 1.20 [Mn 0.52−x Zr x Ni 0.20 Co 0.08 ]O 2 (x = 0, 0.01, 0.02, 0.03) samples were measured by using galvanostatic charge and discharge with the coin cell of type CR2025. The coin cells were assembled as follows: (1) The 85 wt.% Li 1.20 [Mn 0.52−x Zr x Ni 0.20 Co 0.08 ]O 2 (x = 0, 0.01, 0.02, 0.03) samples, 10 wt.% carbon black and 5 wt.% polyvinylidene fluoride were evenly mixed to form the cathode slurry; (2) Then the slurry was casted onto Al foil by using a smudge stick and dried at 110 °C for 12 h in vacuum drying oven, followed by squeezed and punched into a circular disc with d = 12 mm; (3) The as-prepared cathode plate, the lithium metal plate as anode, the Celgard 2400 as the separator and 1 M LiPF 6 dissolved in EC/DMC at mass ratio of 1:1 as the electrolyte were assembled in an argon-filled glove box to form the coin cells. The Galvanostatic charge-discharge tests were carried out by on a Land CT2001A (Wuhan, China) tester.
The cells were charged and discharged in the voltage range of 2.0 to 4.8 V at the different current densities (1C = 250 mA g −1 ). In addition, the CHI660D workstation was used to perform the electrochemical impedance spectroscopy (a frequency range from 0.01 Hz to 100 kHz and perturbation amplitude of 5 mV) and the cyclic voltammogram (a voltage range from 2.0 V to 4.8 V with a scanning rate of 0.1 mV s −1 ). Figure 1 shows the X-ray diffraction patterns of the Li 1.20 [Mn 0.52−x Zr x Ni 0.20 Co 0.08 ]O 2 (x = 0, 0.01, 0.02, 0.03)samples. The as-prepared samples have mainly demonstrated the typical XRD patterns of the hexagonal α-NaFeO 2 structure with the space group R-3m (the LiMO 2 features), except for the weak super lattice peaks between 20° and 25°, which are related to the Li 2 MnO 3 phase, corresponding to the monocline unit cell C2/m 19,20 . In addition, the distinct splitting of (006)/(102) and (018)/(110) peaks have indicated that the as-prepared cathode materials have formed a well-developed hexagonal layered structure 21 . Besides, to further investigate the cation mixing between the Ni 2+ and Li + in the LiMO 2 main phase, the Rietveld refinement of the diffraction patterns was performed based on the R-3m (used for LiNi 0.50 Co 0.20 Mn 0.30 O 2 phase) and C2/m (used for Li 2 MnO 3 phase) structure, as is shown in Fig. 1 risen owing to the larger radius of Zr 4+ . The larger lattice parameters a and c will contribute to enhancing the Li + diffusion rate during the charge and discharge process 22 . Besides, the c/a ratio is related to the cation mixing and a high ratio represents the well cation ordering has been formed 23 . It can be observed the Zr 4+ -doped samples deliver the higher c/a ratio than that of the un-doped cathode, indicating the cation mixing of the as-prepared samples has been improved after the Zr 4+ doping. Besides, according to the reports of J.R. Dahn 24 has demonstrated the optimal cation ordering. The lower cation mixing will not only suppress formation of spinel-like phase, but also improve the layered structure stability, finally contribute to enhancing the cyclic performance. Besides, the occupancy of Zr cations in 3b-site are respectively 0, 0.012, 0.019 and 0.031 with the Zr doping contents increasing, indicating the molar ratio for Zr doping can be designed experimentally. The Zr 4+ doping can enlarge the lattice parameters, which facilitates Li-ion diffusion and subsequently enhances the high-rate capability.  Fig. 4. The Fig. 4 demonstrates that not only the Ni, Co and Mn atoms have been distributed homogeneously, but also the doping element Zr atom have been evenly distributed in the cathode particles rather than segregated on the oxide surface, indicating the Zr 4+ doping technology has obtained the obvious synthetic efficiency. Based on the above analysis, it has proved that the Zr 4+ has been successfully doped into the Li 1.20 [Mn 0.52 Ni 0.20 Co 0.08 ]O 2 cathode material with uniform dispersion. The uniform dispersion of Zr dopant will make the function of Zr 4+ doping modification more stability, which may be ready to provide a better cycling performance to some extent. Figure 5 shows X-ray photoelectron spectroscopy (XPS) results of Zr, Mn, Ni and Co for the Li 1.20 [Mn 0.52−x Zr x Ni 0.20 Co 0.08 ]O 2 (x = 0, 0.02) samples. In Fig. 5(a), the obvious peaks at the binding energies of 184.9 eV and 182.6 eV are assigned to Zr 3d 5/2 and Zr 3d 3/2 , respectively, which corresponds to the Zr-O bonds at the state of Zr 4+ 28 . In Fig. 5(c), the obvious peaks at the binding energies of 854.2 eV is assigned to Ni 2p3/2 , which corresponds to the oxidation state of Ni 2+ and Ni 3+ after fitting, respectively 29,30 . Besides, it can be calculated that the relative content of Ni 2+ decreased after zirconium doping owing to the reduction of cation mixing degree.     doping, the oxygen release of the Zr 4+ -doped samples will face more resistance than the un-doped sample, subsequently the irreversible capacity loss has been suppressed 17 . Figure 7 shows have mainly been attributed to the fast Li + migration speed during the charge and discharge process. One reason is that with the Zr 4+ doping, the larger lattice parameters of the Zr 4+ -doped samples have contributed to enhancing the Li + diffusion speed. Besides, the better crystallization property of the Zr 4+ -doped samples will also help to strengthen the conductivity ability of ions and electrons during the charge-discharge process. Figure 8 shows the cycling performance of the Li 1 cathode, owing to the lower cation mixing and faster Li + migration speed for the Zr 4+ -doped samples. Besides, the discharge voltage plateau will gradually decrease during the cyclic process, owing to the enlargement of polarization and the formation of spinel-like phase for cathode materials 35 . It can be observed that the discharge voltage drops to lower plateau for the all cathodes after different cycles, as the arrows pointed in Fig. 9. Table 5      The poor cycling performance at high temperature for the Li-excess Li 1.20 [Mn 0.52 Ni 0.20 Co 0.08 ]O 2 has become one of the main drawbacks for the commercial application owing to the enhancement of the side reaction between cathode and electrolyte. Figure 10 shows the cycling performance of the Li 1.20 [Mn 0.52−x Zr x Ni 0.20 Co 0.08 ] O 2 (x = 0, 0.01, 0.02, 0.03) at 0.5 C rate in the voltage range of 2.0~4.8 V at 45 °C. In comparison with the cycling performance at room temperature, the bare Li 1.20 [Mn 0.52 Ni 0.20 Co 0.08 ]O 2 demonstrates the more severe capacity fading. During the early cycle period, the fast capacity attenuation can be observed owing to the bare cathode particles surface. After several cycles, the side reaction between the cathode and electrolyte can generate some by-product, which will deposit at the electrode/electrolyte interface to form the Solid Electrolyte Interface (SEI) film. And the SEI film will protect the cathode materials from erosion by the electrolyte, making the capacity attenuation slightly slow 36,37 17 .

Results and Discussion
To further understand the influence of Zr 4+ doping on the electrochemical properties of Zr 4+ -doped Li 1.20 [ Mn 0.52 Ni 0.20 Co 0.08 ]O 2 , the electrochemical impedance spectroscopy (EIS) for the four samples have been carried out after charging to 4.5 V in the 1st, 30th cycles. Figure 11 shows the Nyquist curves of the four cathodes and all the Nyquist curves demonstrate the similar characteristics, containing a small semicircle in the high frequency, a large semicircle in the high to medium frequency and a quasi-straight line in the low frequency, which respectively correspond to the impedance of Li + migration across the SEI film (R sf and CPE sf ), the impedance of charge transfer (R ct and CPE dl ) and the impedance of Li-ion migration in the cathode (Z W ) 39,40 . The corresponding equivalent circuit in Fig. 10(e) is used to simulate the Nyquist curves and the corresponding R s , R sf and R ct values can be acquired, as is shown in where, F, n, A, C R is gas constant, T is the absolute temperature, F represents the Faraday constant, n is the number of electrons per molecule during oxidation, A corresponds to the area of the electrode-electrolyte interface, i.e. 1.13 cm 2 and C is the concentration of lithium ion, respectively. Besides, τ W is the Warburg coefficient of the bulk cathode, which is can be calculated by the Eqs (2). Thereinto, the Z re is the real part of impedance, ω is the angular frequency 42 and Fig. 12

Conclusions
In order to enhance the electrochemical properties of Li-excess Li 1 The stronger total metal-oxygen bonding for the Zr 4+ -doped samples has mainly contributed to stabilizing the structure of cathode and improving the cycling performance. The Zr 4+ doping modification has provided a potential approach to enhance the electrochemical properties of the Li-excess cathodes for Li-ion battery.