Introduction

LIBs have been widely used in electric vehicles, energy storage power stations, and portable electronic devices (such as mobile phones, laptop computers, digital cameras, etc.) [1,2,3]. The demands for the next-generation LIBs with higher energy density have increased in terms of capacity, voltage, safety, and cost [4,5,6,7,8,9]. Lithium intercalation compounds based on manganese oxides are cheaper, safer, and less toxic than the layered compound based on cobalt or nickel oxides and, therefore, offer a particularly attractive replacement for the latter compound as a cathode material in LIBs [10,11,12,13]. Among the lithium manganese oxides cathode materials studied, spinel oxides (LiMn2O4), layered oxides (LiMnO2), and Li-rich Mn-based layered compounds (Li2MnO3·LiMO2 (M = Mn, Ni, Co)) cathodes have been proposed and widely investigated [14,15,16,17,18,19,20]. The implementation of the spinel oxide LiMn2O4 has been delayed because of relatively low theoretical capacity (148 mAh g−1), limited storage, and cycling performances at elevated temperatures [21, 22]. Layered LiMnO2 compounds have come to be of interest as cathode material because of their high theoretical capacity (285 mAh g−1), but layered LiMnO2 is not thermodynamically stable, which is easily converted to a spinel-like structure during electrochemical extraction/insertion of Li ions [23]. Recently, Li-rich Mn-based layered compounds have been considered as one of the most promising cathode material for future LIBs because of their advantage of high reversible capacity (>200 mAh g−1) when charged above 4.5 V [24,25,26,27].

A common feature of Li-rich Mn-based layered compound cathode is an irreversible high voltage plateau at around 4.5 V vs Li/Li+ during the first charge. Li2MnO3 is the end member of these materials which exhibits the characteristic first-charge plateau around 4.5 V [28,29,30]. Initial discharge capacity values of this materials are generally high after activation of Li2MnO3 phase, but cycling instability and intrinsically inferior rate capability are observed in all reports [11, 31, 32]. Therefore, it would be beneficial to reinvestigate the properties of Li2MnO3 material to help further understanding of the properties of Li-rich Mn-based layered compounds. In this manuscript, Al-doped sample Li2Mn0.9Al0.1O3 (Al-LMO) and pristine Li2MnO3 (LMO) materials are synthesized by a sol-gel method. The structural and electrochemical properties of the synthesized materials are investigated.

Experimental

The sol-gel method was adopted to prepare the Al-LMO and LMO samples using citric acid as the chelating agent: Stoichiometric amounts of LiCH3COO·2H2O, Mn(CH3COO)2·4H2O, and Al(NO3)3·9H2O (Al-LMO) were dissolved in distilled water to achieve a mixture of 2.0 mol L−1 metal solution. Then, an aqueous solution of citric acid (2.0 mol L−1) was added to the above aqueous solution with constant magnetic stirring. The resultant solution was evaporated at 80 °C under vigorous stirring to get a viscous gel. Next, the resulting gel was dried at 120 °C for 12 h. Finally, the gathered precursor was heated to 450 °C for 6 h and calcined at 950 °C for 12 h under air atmosphere to obtain the target material.

X-ray diffraction (XRD) measurement of material was recorded on a Rigaku 2500 X-ray diffractometer using Cu-Kα radiation. The diffraction data was collected over the range of 10° < 2θ < 80°. The morphology of the powder was investigated by scanning electron microscopy (SEM, TESCAN, MAIA3). For the electrochemical characterization, the positive electrodes for the cells were prepared by mixing active material with polyvinylidene difluoride (PVDF) binder and carbon black in a weight ratio of 8:1:1 in N-methyl-2pyrrolidone (NMP) solvent. Then, the slurry was cast onto an Al foil current collector and subsequently dried at 105 °C for 10 h in air. The laminates were cut into disks (14 mm) and dried under vacuum at 65 °C for 6 h. The electrochemical tests were carried out using coin-type cells which consisted of a cathode and a lithium metal anode separated by a polyethylene/polypropylene film (Celgard 2400). Cells were assembled in an argon-filled glove box with the electrolyte of 1 mol/L LiPF6-EC/DMC/DEC (1:1:1 by volume). The charge-discharge tests were operated on a LAND-CT2001A battery test equipment (Jinnuo Wuhan Co. Ltd., P.R. China) at room temperature. Electrochemical impedance spectroscopy (EIS) was carried out on an electrochemical workstation (CHI660E, Shanghai Chenhua) in the frequency range from 0.1 MHz to 0.01 Hz.

Results and discussion

The XRD patterns of the LMO and Al-LMO samples are shown in Fig. 1. All the major peaks in each pattern can be indexed to the layered structure with a C2/m space group [33, 34]. The superlattice peaks between 20° and 30° in the XRD profiles, which are characteristic of Li-rich Mn-based layered compounds, are due to Li/Mn cation arrangement in the transitional metal (TM) layers [35]. No impurity peak is detected in XRD patterns of Al-LMO sample due to the low quantity of the doping. In addition, the intensity of superlattice peak between 20° and 30° is reduced by Al-doping, indicating that the Li/Mn ordering in the TM layer is disrupted by Al-doping.

Fig. 1
figure 1

X-ray diffraction patterns of the pristine Li2MnO3 (LMO) and Al-doped Li2MnO3 (Al-LMO) samples

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SEM micrographs of the LMO and Al-LMO samples are presented in Fig. 2a. The morphology of samples has not changed clearly with Al doping. As can be seen, the primary particles (300–400 nm) of the LMO and Al-LMO samples agglomerate with each other and form secondary particles. The EDS images of Al-LMO sample (Fig. 2b) confirm that the Al element is uniformly distributed in the sample.

Fig. 2
figure 2

a The SEM images of the pristine Li2MnO3 (LMO) and Al-doped Li2MnO3 (Al-LMO) samples. b The EDS images of the Al-LMO sample

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The Li2MnO3 phase is originally considered to be electrochemically inactive because the oxidation state of Mn ion is +4 and it is not expected to be oxidized to higher oxidation state. However, it is now well established that Li can be electrochemically extracted and reinserted. Many researchers attributed the electrochemical activity of Li2MnO3 phase to the removal of Li2O from the active material [30, 36], and another researchers found that proton exchange (exchange Li+ by H+) might have contributed to such an anomalous phenomenon [37]. Figure 3 shows the first charge/discharge curves of the LMO and Al-LMO cathodes. The charge/discharge process is operated in the voltage range from 2.0 to 4.8 V at 0.05 C (12.5 mA g−1), which is low enough to ensure a quasistatic process. A classic platform feature of Li-rich Mn-based cathode materials at 4.5 V is observed for all samples’ initial charge curves. The initial discharge capacities of LMO and Al-LMO samples are 122 and 99 mAh g−1, and the first cycle efficiency (FCE) is 50 and 58.9% for LMO and Al-LMO samples, respectively. It can be obviously observed that the Al-LMO sample delivered a lower charge/discharge capacity but a higher FCE than the LMO sample. The decrease of capacities can be attributed to the stronger Al-O bond (comparing to Li-O bond) which can suppress the exaction of Li2O from Li2MnO3 component during the first activation [38].

Fig. 3
figure 3

Charge/discharge profiles of the pristine Li2MnO3 (LMO) and Al-doped Li2MnO3 (Al-LMO) samples

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The differential capacity versus voltage (dQ/dV) plots corresponding to the 1st, 2nd, 3rd, and 10th cycles of samples are collected and shown in Fig. 4. The oxidation reaction of LMO and Al-LMO cathodes can be characterized by two main peaks around 4.6 and 4.7 V, which are related with the removal of Li2O from the active material Li2MnO3 component or proton exchange [28, 30]. It can be found from Fig. 4a that there are two oxidation peaks at 3.3 and 3.7 V in the initial charge process for the LMO sample. These phenomena indicate that the layered phase transformed to the cubic spinel-like phase with the LMO sample in the initial charge process [39]. Compared to the LMO sample, these two oxidation peaks at 3.3 and 3.7 V are not observed in the initial charge process for the Al-LMO sample, which implied that Al doping would prevent the first charge phase transformation. A main broad reduction peak at 3.4 V, which can be attributed to the lithiation of the layered active MnO2 into layered LiMnO2 [40], is observed in the initial discharge process for LMO and Al-LMO samples. During subsequent discharges (Fig. 4b, c), the reduction of Mn4+ in the layered active MnO2 component that occurs initially at ∼3.4 V also shifts to lower potentials but increases in magnitude toward ∼2.8 V, characteristic of a lithium manganese oxide spinel-like phase [40]. Additionally, the oxidation peaks around 3.3 V appeared slowly upon cycling by the Al-LMO sample from Fig. 4c, which indicates that Al doping would slowdown the rate of cubic spinel-like phase transformation of layered phase and thus implies excellent cycle stability.

Fig. 4
figure 4

Differential capacity versus voltage (dQ/dV) for cells of the pristine Li2MnO3 (LMO) and Al-doped Li2MnO3 (Al-LMO) samples in the voltage range of 2.0–4.8 V. a The initial dQ/dV curves for LMO and Al-LMO samples. b The dQ/dV curves for LMO sample. c The dQ/dV curves for Al-LMO sample

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To further investigate the Al-doped effects on the electrochemical properties of the sample, the electrodes are cycled at different rates (0.05, 0.1, 0.2, 0.5, 1.0, 2.0 C) between 2 and 4.8 V. Figure 5 shows the rate capabilities of the LMO and Al-LMO samples. When the electrodes are cycled at high rates up to 2.0 C and then returned to 0.1 C, the low rate capacities are returned to the initial value, which revealed that cycling at high rates (0.2, 0.5, 1.0, 2.0 C) did not have any adverse effect on the low rate (0.1 C) capacity. It can be obviously observed that the rate capacities of Al-LMO sample are higher than LMO sample. The electrode of Al-LMO sample cycled at 2.0 C rate exhibited about 31.2% capacity retention as compared to 1.8% for LMO sample with regard to the capacity obtained at 0.05 C rate, which indicates that Al-LMO sample has perfect electrical conductivity than LMO sample.

Fig. 5
figure 5

Rate capability of the pristine Li2MnO3 (LMO) and Al-doped Li2MnO3 (Al-LMO) samples at 0.05, 0.1, 0.2, 0.5, 1.0, and 2.0 C rate

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Figure 6 shows the cycle performances of the LMO and Al-LMO samples at 0.1 C between 2.0 and 4.8 V at room temperature. It can be obviously observed that Al-LMO sample behaves better than LMO sample: an initial discharge capacity of 99.4 mAh g−1 with capacity retention of 97.1% after 40 cycles for the former but only 84 mAh g−1 with 84.8% for the latter.

Fig. 6
figure 6

Cycling performance of the pristine Li2MnO3 (LMO) and Al-doped Li2MnO3 (Al-LMO) samples at 0.1 C between 2.0 and 4.8 V at room temperature

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It is expected that the conducting Li ions can move to the neighboring positions more easily when the Mn4+ ions are replaced by low-valence cations [29]. To verify the improvement in electrochemical performances of the Al-LMO sample shown above, EIS of LMO and Al-LMO samples are collected after 40 cycles. Figure 7 shows the Nyquist plots and the corresponding equivalent circuit. The plots of the two electrodes are well fitted with the equivalent circuit. In the equivalent circuit, R e, R f, R ct, and W1 represent the resistance of liquid electrolyte, the resistance of the SEI film, the charge-transfer resistance, and the Warburg impedance of lithium ion diffusion, respectively [41,42,43]. It is found that Al-LMO sample has a relatively lower R ct (5185 Ω) compared with LMO sample (10,037 Ω). The decrease in R ct demonstrated that Al-doping can improve both the electronic conductivity and the Li+ diffusion during insertion/extraction [44, 45], which is also supported by the rate capability and charge/discharge cycling data.

Fig. 7
figure 7

Nyquist plots of the pristine Li2MnO3 (LMO) and Al-doped Li2MnO3 (Al-LMO) samples after 40 cycles

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Conclusions

The pristine LMO and Al-LMO cathode materials for lithium-ion battery are synthesized through the conventional sol-gel method. Its structure, morphology, and electrochemical properties are investigated in this work. All the major peaks in each XRD pattern can be indexed to the layered structure with a C2/m space group. SEM-EDS (SEM-EDS) analyses revealed that the Al element is distributed in the sample homogenously. The electrochemical behavior of cycled LMO and Al-LMO samples indicates the phase transformation from a layered to a spinel. The Al-LMO sample exhibited a great improvement on cycle and rate performances compared to the LMO sample. The dQ/dV results show that Al doping would prevent the phase transformation in the first charge process and slowdown the rate of spinel phase transformation of layered phase in the following cycles. EIS results confirm that Al doping decreases the charge-transfer resistance and improves the electrochemical reaction kinetics.