Abstract
Herein, we reported the composite structure of LiMnO2 and Li2MnO3 as a low-cost and environmentally benign cathode material. This composite with the main phase of LiMnO2 (90%) was synthesized by hydrothermal method at 220°C from LiOH and Mn(CH3COO)2 precursors. The obtained nanosized LiMnO2-LiMnO3 cathode material exhibits a high capacity of 265 mAh g-1 at C/10. The incorporation of Li2MnO3 into the LiMnO2 phase could stabilize the structure, leading to the improved cycle stability of the cathode. The capacity retention of the cathode was 93% after 80 cycles at C/2. Our results facilitate a potential strategy for developing high-performance cathode materials based on the Li-Mn-O system.
1. Introduction
Although lithium-ion batteries (LIBs) have a dominant position as the power source in mobile electronics, they still do not meet the growing demand for these power-consuming devices [1]. In addition, the reduction of product costs, including production costs and treatment costs that affect the environment after disposal, is of considerable concern to manufacturers. Accordingly, the use of inexpensive and environmentally friendly commercial cathode materials such as LiMn2O4 and LiFePO4 takes precedence over LiCoO2 under these conditions [2, 3]. Compared with Fe-based materials, Mn-based materials have a higher range of working potential and conductivity. Accordingly, there are many other Mn-based materials that are studied such as LiMnO2, Li2MnO3, Li4Mn5O12, and Li4Mn2O5 and their composites [4–8]. These materials exhibit superior capacity than available commercial cathode materials and are expected to fulfill the requirement in electric vehicle application. Freire et al. [9] reported that a rock-salt structure of Li4Mn2O5 which is synthesized by a mechanochemical synthesis method could deliver a high capacity of 355 mAh g-1. Liu et al. [10] reported the synthesis of Li4Mn5O12 at low temperature by solid-state reaction, and the cathode could deliver a high capacity of 212 mAh g-1. Li2MnO3 has the highest theoretical capacity among others (485 mAh g-1), and it has been extensively studied in the Li-rich layered oxide system (LLO) as a structural stabilizer and a capacity booster. On the other hand, LiMnO2 also has high theoretical capacity of 285 mAh g-1. It has a lot of metastable states including orthorhombic (o-LiMnO2, space group Pmmn), monoclinic (m-LiMnO2, space group C2/m), and layered LiMnO2 (space group R3m) with α-NaFeO2-like structure. Among these structures, the o-LiMnO2 is the most stable. However, the single phase of Mn-based compounds, such as Li2MnO3, LiMnO2, or Li4Mn5O12, cannot be used as a cathode material for LIBs due to their structural instability. As a way to overcome some of these difficulties, compounds derived from the substitution of Mn by Ni and Co have been studied. The other way is designing a composite structure between them. The LLO, which is xLi2MnO3·(1-x)LiMO2 (), has more than two decades of investigation [11–13]. To promote using standing Mn-based materials, we investigate the stabilization of Li2MnO3 for LiMnO2 as cathode materials for LIBs.
2. Experimental
The integrated structure was synthesized by a hydrothermal method. First, manganese (II) acetate tetrahydrate (4.9 g, Sigma-Aldrich) and lithium hydroxide monohydrate (3.36 g, Sigma-Aldrich) were dissolved in distilled water (40 mL) separately. Then, hydrogen peroxide (H2O2, 30% () in H2O, 1.6 mL) was added to the Li solution before adding Mn solution slowly. The mixture was mixed with methanol (20 mL) and stirred for 0.5 h. Subsequently, it was located into a Teflon-lined autoclave for the hydrothermal reaction at 220°C for 12 h. Finally, the powder was centrifuged and washed with ethanol and distilled water thoroughly.
The phase of the sample was identified by X-ray diffraction (XRD) measurements using Philips X’Pert with Cu-Kα radiation in a range of . The morphology of particles was recorded by scanning electron microscopy (SEM, Nova NanoSEM 450) and high-resolution transmission electron microscopy (HRTEM, JOEL JEM-2100F). The oxidation state of elements was determined by X-ray photon spectroscopy (XPS, K-Alpha+ Thermo Scientific). The chemical composition was analyzed by ICP (Optima 8300 ICP-OES spectrometer).
For cathode fabrication, the active material (70 wt%), Ketjen black (10 wt%), and teflonized acetylene black (binder, 20 wt%) were mixed thoroughly. Then, it was pressed onto a stainless-steel mesh and dried at 120°C under vacuum for 12 h. For coin-cell (2032 coin-type cell) fabrication, the cathode, an electrolyte (1 M LiPF6 solution in a 1 : 1 mixture of ethylene carbonate and dimethyl carbonate), a separator, and Li metal were assembled in an Ar-filled glovebox. The cell was tested of its electrochemical properties by a Neware Battery Tester between 2.0 and 4.8 V vs. Li+/Li.
3. Results and Discussion
3.1. Structural Characterization of Cathode
Figure 1 shows the XRD pattern of the synthesized sample. All peaks can be indexed according to the space group Pmmn of the orthorhombic LiMnO2 structure. There is a slightly weak peak that appears at ~18.3°, which is assignable to the Li2MnO3 phase (space group C2/m). This second phase originated from the oxidation decomposition reaction [14]. All peaks are sharp, indicating the high crystallinity of particles. Note that the ratio of LiMnO2 to Li2MnO3 phase can be changed by using an oxidizing agent [15] or controlling the synthesis temperature [16]. The ICP result shows that the ratio of due to the existence of the Li2MnO3 phase. Rietveld refinement is performed using the model shown in Table 1. The main phase is o-LiMnO2 with space group Pmmn, and the second phase is Li2MnO3 with space group C2/m. In the o-LiMnO2 structure, Mn and Li occupy the 2a Wyckoff site. The oxygen occupies the 2b Wyckoff site. The oxygen array is distorted from idea cubic-close packing due to Jahn-Teller effect on Mn3+. The structure is built up from independent MnO6 and LiO6 octahedra that are arranged in corrugated layers. The Rietveld refinement shows that the degree of substitution of Li/Mn in the octahedra is about 5%. The cation disorder can improve the electrochemical performance of the cathode [17–19]. The lattice parameter of o-LiMnO2 is slightly smaller than that in literature [20]. This might be caused by the effect of the Li2MnO3 phase.
SEM and TEM analyses were performed to study the particle’s morphology and are shown in Figure 2. Accordingly, the particles have a well-defined shape (Figure 2(a)). The particles are elongated, parallelogram-shaped grains. The particle size ranges from 100 to 400 nm. The -spacing is calculated as 0.588 nm, which corresponds to the (010) plane at . These results further confirm the predominance of the o-LiMnO2 phase with good crystallinity of the particles.
(a)
(b)
To examine the oxidation state of Mn in the compound, XPS measurement was carried out and shown in Figure 3. The survey XPS profiles (Figure 3(a)) contain peaks at ~641, 530, 285, and 55 eV, which are allocated for Mn 2p, O 1s, C 1s, and Li 1s, respectively. This indicates the presence of Li, Mn, C, and O in the sample. The presence of C is due to the absorption of CO2 from the air onto the sample’s surface [21–23]. Figure 3(b) shows the Mn 2p core-level spectrum, which exhibits two peaks, namely, Mn 2p3/2 and Mn 2p1/2. These peaks locate at 641.88 and 653.58 eV, respectively, with the spin-orbital splitting value of 11.7 eV. The binding energy of Mn 2p3/2 of the sample is in between binding energy of those in Mn2O3 (641.6 eV) and MnO2 (642.6 eV) [24]. This result indicates the coexistence of both Mn3+ and Mn4+ in the sample. Figure 3(c) shows the Mn 3s spectrum to further evaluate the oxidation state of Mn. The splitting in Mn 3s spectrum is caused by the coupling of the nonionized 3s electron with 3d valence-band electrons, and its value indicates the oxidation state of Mn. Here, it is 5.3 eV so the oxidation state of Mn in the compound is +3.
(a)
(b)
(c)
Figure 4 shows the charge-discharge curve and the corresponding plot, measured between 2.0 and 4.8 V at C/10 rate () at the 1st, 2nd, and 5th cycles. The cell exhibits two charging voltage plateaus at 3.45 and 4.3 V. The plateau at 4.5 V which is typical for the activation of Li2MnO3 does not appear. It might be caused by the small fraction of the Li2MnO3 phase. The plateaus at 3.45 V are irreversible due to the structural change. For the first discharge, there is a plateau at 4 V and a long plateau ~3.0 V. The first charge and discharge capacity is 297 and 216 mAh g-1, respectively. The low first Coulombic efficiency (72.7%) is due to the irreversible structural change during the first cycle of the LiMnO2 and Li2MnO3 phase [4, 6, 25–31]. During cycling, the capacity contribution in the 4 V region increases, resulting in an increase in the overall capacity. After 5 cycles, the cathode could deliver a capacity of 265 mAh g-1, which is higher than those reported in the literature [20, 32–37]. The plot (Figure 4(b)) shows peaks that correspond to the plateaus observed in Figure 4(a). For the first cycle, the peak at 3.5 V is irreversible while the peak at 4.3 V is reversible and there is a strong peak appearing at 2.9 V. For the subsequent cycles, the peak at 4.3 V shifts to lower voltage and induces two peaks at 3.8 and 4.0 V. These are typical peaks of spinel LiMnO4, indicating the transformation of o-LiMnO2 to a spinel-like phase during cycling [38].
(a)
(b)
A fresh cell was cycled 5 times at each C-rate including C/10, C/5, C/2, 1C, 2C, and 5C between 2.0 and 4.8 V to check its C-rate performance. Figure 5(a) shows the resulting discharge capacities. At C/10 rate, the highest capacity of 265 mAh g-1 is obtained after 5 cycles. With increasing C-rate, the capacities decrease as expected. The capacity is 249, 226, 208, 180, and 103 mAh g-1 for C/5, C/2, 1C, 2C, and 5C, respectively. The capacity is still as high as 263 mAh g-1 at C/10 after a severe test at 5C. Figure 5(b) shows the cycling stability and Coulombic efficiency of the sample at C/2. The capacity increases gradually over the first 12 cycles due to the transition of o-LiMnO2 to a new spinel-like phase [4, 39]. However, the transformation is slow in this compound due to the stabilization of the Li2MnO3 phase. After 80 cycles, the capacity retention is 93%. The Coulombic efficiency is close to 100%, indicating less energy loss during the charge-discharge process.
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(b)
4. Conclusion
o-LiMnO2 was successfully synthesized by the hydrothermal method. The XRD and XPS results show the existence of the Li2MnO3 phase. SEM and TEM analyses confirmed the presence of a dominant o-LiMnO2 phase with particle sizes in the range of 100-400 nm. The galvanostatic cycling demonstrates that a high capacity of 265 mAh g-1 and 93% capacity retention after 80 cycles at C/2 could be achieved with this cathode. The structural change from the initial phase to the spinel-like phase is retarded due to the stabilization of the Li2MnO3 phase. Lastly, this work promotes environmentally friendly, low-cost, and high-capacity cathode materials for LIBs.
Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that there is no conflict of interest regarding the publication.
Acknowledgments
This research is funded by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.02-2019.26 and the Ministry of Science and Technology of Vietnam under the project code NĐT.75.CHN/19.