Hydrothermal Synthesis of Li2MnO3-Stabilized LiMnO2 as a Cathode Material for Li-Ion Battery

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.


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 LiMn 2 O 4 and LiFePO 4 takes precedence over LiCoO 2 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 LiMnO 2 , Li 2 MnO 3 , Li 4 Mn 5 O 12 , and Li 4 Mn 2 O 5 and their composites [4][5][6][7][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 Li 4 Mn 2 O 5 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 Li 4 Mn 5 O 12 at low temperature by solid-state reaction, and the cathode could deliver a high capacity of 212 mAh g -1 . Li 2 MnO 3 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, LiMnO 2 also has high theoretical capacity of 285 mAh g -1 .
It has a lot of metastable states including orthorhombic (o-LiMnO 2 , space group Pmmn), monoclinic (m-LiMnO 2 , space group C2/m), and layered LiMnO 2 (space group R3m) with α-NaFeO 2 -like structure. Among these structures, the o-LiMnO 2 is the most stable. However, the single phase of Mn-based compounds, such as Li 2 MnO 3 , LiMnO 2 , or Li 4 Mn 5 O 12 , 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 xLi 2 MnO 3 ·(1-x)LiMO 2 (M = Mn, Ni, Co), has more than two decades of investigation [11][12][13]. To promote using standing Mn-based materials, we investigate the stabilization of Li 2 MnO 3 for LiMnO 2 as cathode materials for LIBs.

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 (H 2 O 2 , 30% (w/w) in H 2 O, 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 10°≤ 2θ ≤ 100°. 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 LiPF 6 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.

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 LiMnO 2 structure. There is a slightly weak peak that appears at~18.3°, which is assignable to the Li 2 MnO 3 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 LiMnO 2 to Li 2 MnO 3 phase can be changed by using an oxidizing agent [15] or controlling the synthesis temperature [16]. The ICP result shows that the ratio of Li : Mn = 1:095 : 1 due to the existence of the Li 2 MnO 3 phase. Rietveld refinement is performed using the model shown in Table 1. The main phase is o-LiMnO 2 with space group Pmmn, and the second phase is Li 2 MnO 3 with space group C2/m. In the o-LiMnO 2 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 Mn 3+ . The structure is built up from independent MnO 6 and LiO 6 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][18][19]. The lattice parameter of o-LiMnO 2 is slightly smaller than that in literature [20]. This might be caused by the effect of the Li 2 MnO 3 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 d-spacing is calculated as 0.588 nm, which corresponds to the (010) plane at 2θ = 15°. These results further confirm the predominance of the o-LiMnO 2 phase with good crystallinity of the particles.
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)   2 Journal of Nanomaterials [21][22][23]. Figure 3(b) shows the Mn 2p core-level spectrum, which exhibits two peaks, namely, Mn 2p 3/2 and Mn 2p 1/2 . 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 2p 3/2 of the sample is in between binding energy of those in Mn 2 O 3 (641.6 eV) and MnO 2 (642.6 eV) [24]. This result indicates the coexistence of both Mn 3+ and Mn 4+ 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. Figure 4 shows the charge-discharge curve and the corresponding dQ/dV plot, measured between 2.0 and 4.8 V at C/10 rate (1C = 280 mA g −1 ) at the 1 st , 2 nd , and 5 th 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   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][33][34][35][36][37]. The dQ/dV 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 LiMnO 4 , indicating the transformation of o-LiMnO 2 to a spinel-like phase during cycling [38].
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 Crate, 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-LiMnO 2 to a new spinellike phase [4,39]. However, the transformation is slow in this compound due to the stabilization of the Li 2 MnO 3 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.

Conclusion
o-LiMnO 2 was successfully synthesized by the hydrothermal method. The XRD and XPS results show the existence of the Li 2 MnO 3 phase. SEM and TEM analyses confirmed the presence of a dominant o-LiMnO 2 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 Li 2 MnO 3 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.