Li_1.035Mn_1.965O₄ and Al-doped Li_1.035Mn_1.965O₄ cathodic materials were prepared by a simple wet-chemical process combined with heat treatment. EMD (Mn≥59.36%, Xiangtan Chemical Industry), LiOH·H₂O (purity≥98.9%, Sichuan Tianqi Lithium Industries, Inc., China) and Al(OH)₃ (purity≥98.9%, Tianjin Kermel, Inc., China) were used as raw materials. The precursors were prepared from the reaction of EMD with LiOH·H₂O and Al(OH)₃ in aqueous solution at a required molar ratio of Li:Al:Mn. The precursors were preheated at 430℃ for 12 h and then heated at 800℃ for 12 h to obtain the final cathodic materials. A detailed description of the synthetic process has been published in Reference^{[ 14]}.

The XRD patterns were recorded between 10° and 80° of 2 θ at a scanning rate of 1º/s using a Rigaku D/max-2500 X-ray powder diffractometer with Cu Kα radiation. The particle morphologies of the products were carried out by SEM using JEOL JSM-6360LV operated at 20 kV. The TEM images were obtained on a Tecnai G12 microscope.

The working electrode was a mixture of 84wt% active material, 8wt% acetylene black and 8wt% PVDF binder and then the mixture was pressed into a film. A disk was cut off from the film and used as the cathode (13 mm diameter, ca. 200 mm). A typical weight of the cathode was 4-5 mg. The electrolyte was based on 1 mol/L LiPF₆ in a 1:1 (volume ratio) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) and a thin polypropylene film was used as the separator. The 2016 cells were assembled in an argon-filled glove box and were charged and discharged at different rates (1 C=mAh/g)with cut-off voltage 4.3 and 3.0 V at room temperature using Land (CT2001A) cell systems.

The XRD patterns of the as-prepared Li_1.035Mn_1.965O₄ and Li_1.035Al_0.035Mn_1.930O₄ samples are shown in Fig. 1. Both of the two samples are identified as a single-phase spinel (JCPDS 35-0782) with a space group Fd3m where Li⁺ ions locate on the 8a sites, Mn³⁺, Mn⁴⁺ and Al³⁺ ions reside at the octahedral 16(d) sites and O^2- ions occupy the 32(e) sites^{[ 15]}. The shifting peak to larger 2 θ angle can be attributed to the difference in ionic radius between Al³⁺ (0.053 nm) and Mn³⁺ (0.065 nm)^{[ 16]}. The lattice constant obtained from the Rietvelt refinement on the XRD data decreases by doping Al in the host structure of Li_1.035Mn_1.965O₄. For example, the undoped sample delivers the spinel lattice constant of 0.8237 nm, while the Al-doped sample lattice constant decreases to 0.8231 nm. The small substitution of Al³⁺ for Mn³⁺ reduces the lattice parameters and increases the bonding covalency of Mn-O as well as the strength of the framework of the intercalation compound.

Fig. 1

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	Fig. 1 XRD patterns of (a) Li1.035Mn1.965O4 and (b) Li1.035Al0.035Mn1.930O4 powders

Typical powder morphologies of the Li_1.035Mn_1.965O₄ and Al-doped Li_1.035Al_0.035Mn_1.930O₄ powders are shown in Fig. 2. Both samples have an average size of about 300 nm and show uniform particles.

Fig. 2

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	Fig. 2 SEM images of (a) Li1.035Mn1.965O4 and (b) Al-doped Li1.035Al0.035Mn1.930O4 powders

To further understand the morphology and microstructure details of the Li_1.035Al_0.035Mn_1.930O₄ powders, the TEM image, selected area electron diffraction (SAED) pattern and high-resolution transmission electron microscopy (HRTEM), are performed. From the TEM image (Fig. 3(a)), interference fringes are observed on fine particles. The SAED pattern taken from Li_1.035Al_0.035Mn_1.930O₄powder is shown in Fig. 3(b). The diffraction spots, from inner to outer, correspond to the (111), (311), (400), (440) diffraction planes of spinel LiMn₂O₄, respectively, which indicate that Al ions are distributed homogeneously in the spinel structure without impurity phases existing in the doped sample. A layer separation of 0.245 nm, which corresponds to (400) plane in bulk LiMn₂O₄, is observed in the HRTEM image (Fig. 3(c)).

Fig. 3

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	Fig. 3 Images and of the as-prepared Li1.035Al0.035Mn1.930O4 powder (a)TEM, SAED (b) and HRTEM (c)

Figure 4(a) presents the discharge profiles of the pristine and Al-doped samples at 0.1 C rate. Both of these two samples deliver two typical characteristic plateaus at 4 V. It is reported that the first voltage plateau is induced by λ-MnO₂ inlaid into Li⁺ to form Li_0.5Mn₂O₄ and the second voltage plateau is induced by Li_0.5Mn₂O₄ inlaid into Li⁺ to form LiMn₂O₄^{[ 17]}. The initial capacity of the Al-doped sample (116.2 mAh/g) is lower than the undoped one (117.5 mAh/g), which is due to the substitution of Al³⁺ for Mn³⁺. But the small Al substitution results in the sample exhibiting a better rate performance. Figure 4(b) shows that at 4 C rate, Al-doped sample gives a discharge capacity of over 90.6 mAh/g, which is higher than that of the pristine sample (74.3 mAh/g). Figure 4(c) shows the cycling performances of the Li_1.035Mn_1.965O₄ and the Li_1.035Al_0.035Mn_1.930O₄ powders at 0.5 C rate. The Li_1.035Al_0.035Mn_1.930O₄ sample displays a capacity retention ratio of 96.4% after 100 cycles, which is higher than that of the undoped sample (88.3%). The improvement of the cycling performance can be accounted for the high structural stability during the electrochemical cycling, because of the stronger Al-O bond (512 kJ/mol)compared to the Mn-O bond (402 kJ/mol) in the octahedron^{[ 18]}. What’s more, the reduction of Mn³⁺ ions by doping Al can suppress the Jahn-Teller effect and cause less Mn dissolution, which leads to a higher capacity and a better cycling performance.

Fig. 4

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	Fig. 4 (a) Discharge profiles at 0.1 C rate, (b) discharge capacity with cycling number at different current rates and (c) cycling performances at 0.5 C rate of Li1.035Mn1.965O4 and Al-doped Li1.035Al0.035Mn1.930O4 powders

To investigate changes of the electrode/electrolyte interface, which may contribute to the different electrochemical performances between the Li_1.035Mn_1.965O₄ and Li_1.035Al_0.035Mn_1.930O₄ electrodes, the electrochemical impedance spectra (EIS) are carried out in the discharged state after three cycles. The Nyquist plots are presented in Fig. 5. It can be seen that each spectrum contains one semicircle in the high frequency region and a straight sloping line in the low frequency region. The high-frequency semicircle is from the charge-transfer process, and the straight line is attributed to the diffusion of lithium ions in the spinel^{[ 19]}. The EIS is fitted by the ZView 2.70 software. The corresponding equivalent circuit is inserted in Fig. 5. Here R_s is the resistance of the ohmic electrolyte, R_ct is the charge transfer resistance at the particle/electrolyte interface, and CPE1 represents the double layer capacitance and Z_w the Warburg impedance.

Fig. 5

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	Fig. 5 Electrochemical impedance spectra (EIS) of the two samples in the discharged state (about 3.0 V, vs Li+/Li) after 3 cycles with an equivalent circuit diagram is inserted

lists the results calculated from ac impedance spectra based on the equivalent circuit shown in Fig. 5. It can be seen that both R_s and R_ct of the Al-doped sample are smaller than those of the undoped one. For example, the value of the charge transfer resistance R_ct for Li_1.035Al_0.035Mn_1.930O₄ sample is 83.49 Ω/cm², which is much lower than the undoped sample (129.4 Ω/cm²). This indicates that the small Al ions doping can effectively enhance the electrical conductivity and decrease electrochemical polarization which contributes to improved rate capability.

Table 1

Table 1

Table 1 The impedance parameters of Li1.035Mn1.965O4 and Li1.035Al0.035Mn1.930O4samples Samples Rs/Ω Rct/Ω Li1.035Mn1.965O4 4.32 129.4 Li1.035Al0.035Mn1.930O4 3.79 83.49

Table 1 The impedance parameters of Li1.035Mn1.965O4 and Li1.035Al0.035Mn1.930O4samples