Hydrothermally synthesized nanostructured LiMnxFe1−xPO4 (x = 0–0.3) cathode materials with enhanced properties for lithium-ion batteries

Nanostructured cathode materials based on Mn-doped olivine LiMnxFe1−xPO4 (x = 0, 0.1, 0.2, and 0.3) were successfully synthesized via a hydrothermal route. The field-emission scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analyzed results indicated that the synthesized LiMnxFe1−xPO4 (x = 0, 0.1, 0.2, and 0.3) samples possessed a sphere-like nanostructure and a relatively homogeneous size distribution in the range of 100–200 nm. Electrochemical experiments and analysis showed that the Mn doping increased the redox potential and boosted the capacity. While the undoped olivine (LiFePO4) had a capacity of 169 mAh g−1 with a slight reduction (10%) in the initial capacity after 50 cycles (150 mAh g−1), the Mn-doped olivine samples (LiMnxFe1−xPO4) demonstrated reliable cycling tests with negligible capacity loss, reaching 151, 147, and 157 mAh g−1 for x = 0.1, 0.2, and 0.3, respectively. The results from electrochemical impedance spectroscopy (EIS) accompanied by the galvanostatic intermittent titration technique (GITT) have resulted that the Mn substitution for Fe promoted the charge transfer process and hence the rapid Li transport. These findings indicate that the LiMnxFe1−xPO4 nanostructures are promising cathode materials for lithium ion battery applications.

www.nature.com/scientificreports/ to nanorod, nanoplates or nanorectangular sheets 1,13,20,27,[30][31][32] . In addition, the electrochemical efficiency in olivine phosphate cathode materials also strongly depend on the prepared techniques solid-state reactions 1,33 , sol-gel methods 13,34 , reactions from hydrothermal routes 11,24,31,32,[35][36][37][38] or microwave plasma chemical vapor deposition 19 . Among these methods, the hydrothermal synthesis is a facile water-based precipitation technique that enables the control of the nucleation and development of the crystal. In this work, we improve the chemical properties by partly substation Fe by Mn, nanosizing of particles size and carbon coating of the original olivine. In which, the nanostructured of Mn doped olivine LiMn x Fe 1−x PO 4 (x = 0, 0.1, 0.2, and 0.3) were synthesized by a facile hydrothermal route, and the as-prepared olivines were coated with carbon through pyrolysis. Electrochemical studies showed that the olivines were capable of delivering a reversible capacity around 160 mAh g −1 at a current density of C/10 in the voltage range of 2.5-4.3 V (vs. Li + /Li). The improving on the kinetics of Li transport in these olivines were found and also discussed.
Devices preparation. The synthesized olivine powders were mixed with carbon black as a conductive agent, polyvinylidene fluoride (PVDF) as binder in the ratio of 90:5:5, respectively, in a porcelain mortar and pestle. The prepared mixtures were coated onto a 0.1 mm aluminium sheet, dried at 120 °C in an oven for 12 h, and pressed on a press machine to create a thickness of 20 μm. The electrodes were then formed to circular plates with a diameter of 1.6 cm on a stamping machine. Coin-cells CR-2032 were assembled in an argon-filled glove box with an anode of lithium metal. The electrolyte solutions were 1 M LiPF 6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) at a volumetric ratio of 2:1.

Results and discussion
Structural and morphological characterizations. The XRD spectra of the synthesized olivine samples ( Fig. 1A) show a pure and very well crystallized single olivine phase without impurity (e.g., MnO 2 , Li 2 O, and Fe 2 O 3 ). All the diffraction peaks were identified in an orthorhombic structure with a space group of P nma (JCPDS 81-1173) 14,27 . In the olivine phase, the atoms are located at 4a for lithium, 4c for iron, 4c for phosphorous, and 4c and 8a for oxygen. The Mn substitute of Fe is at the 4c site. The lattice parameters are calculated by the Celref program and detailed in Table 1. The lattice expansion of the Mn-doped olivine samples depended on the degree of Mn substitution as well as the larger ionic radius of Mn 2+ (r Mn2+ = 0.08 nm > r Fe2+ = 0.074 nm) 14,27 .
The broadening of the diffraction peaks was attributed to the nanocrystallinity of the samples. The average crystalline size is calculated from the full width at half maximum (FWHM) through the Debye-Scherrer equation 32 : where d hkl is the average particle size, k is the constant depending on the crystallite shape (0.9), λ is the wavelength of the copper K α X-ray radiation, β is the FWHM of the most intense peak (in rad), and θ is the diffraction angle. The crystallite size average can be estimated around 20 nm for all samples. This sub-micron size could indicate fast kinetics of lithium insertion due to a shortening of the lithium pathway for diffusion. The Raman spectra of three samples LiMn 0.1 Fe 0.9 PO 4 , LiMn 0.2 Fe 0.8 PO 4 and LiMn 0.3 Fe 0.7 PO 4 at high frequency 1000-2000 cm −1 (Fig. 1B) confirms the successful carbon coating on surface olivines from the pyrolysis process. The Raman spectrum showed the finger-print band of coated carbon through two characteristic peaks at 1351 cm −1 (D-band) and 1570 cm −1 (G-band), respectively. This result confirm the successfully coated carbon onto olivine surface 22,39 . The SEM image in Fig. 2 shows that the synthesized olivine LiMn x Fe 1−x PO 4 samples crystallized with uniform size. The SEM of the original olivine LiFePO 4 sample (Fig. 2a) shows LiFePO 4 as consisting of spherical-like particles with a narrow particle size distribution in range from 200 to 400 nm. When Mn is doped into LiMn x Fe 1−x PO 4 , the www.nature.com/scientificreports/ particle size strongly reduces and the samples became more porous see Fig. 2b-d. It can be seen that particle size decreased with increasing Mn. In particular, the particle size of LiMn 0.3 Fe 0 . 7 PO 4 is around 50 nm (Fig. 2d). The EDX spectra of the three Mn-doped samples (Fig. 3) showed the indicators of Mn, Fe, P, and O and confirmed the stoichiometric relationship between Mn:Fe ( Table 2) was similar to the desired composition.   www.nature.com/scientificreports/ very low frequencies (f < 10 -3 Hz) the phase angle is increases due to the finite diffusion process 5,40 . It is observed that Mn doping is helpful for decreasing charge transfer resistance in olivine, which suggests fast electron transfer as well as stability in cycling performance. The galvanostatic cycling tests at a constant current of 0.1 C in the potential range of 2.5-4.3 V are shown in Fig. 5A. It is well known that the LiFePO 4 profile drops rapidly to plateau at a voltage of 3.39 V when discharging and leaps to 3.45 V when charging. This profile can be considered as a two-phase mechanism between the LiFePO 4 phase and FePO 4 phase (Fig. 5A, curve a). However, the Mn substitution in olivine leads a significant change in the cycling profile (Fig. 5A, curve b to curve d). The discharge curve of the lowest Mn-doped sample falls slowly with a light bend over 3.7 V and reaches the main plateau at 3.41 V, while the reverse plateau appears at 3.47 V. It was reasonable that the Mn-redox signal of LiMn 0.1 Fe 0.9 PO 4 was hardly seen in the obtained voltammogram. In the other Mn-doped samples, the cycling profiles are demonstrated by a well-defined plateau at over 3.45 V and a short one at around 3.90 V. It was recognized that increasing the degree of Mn doping prolongs the capacity in the high voltage region and shortens it in the lower region. Indeed, the capacity was boosted from 20 mAh g −1 for LiMn 0.2 Fe 0.8 PO 4 to nearly 40 mAh g −1 for LiMn 0.3 Fe 0.7 PO 4 27,31 . On the other hand, it was observed that Mn doping also lead to shifts in the redox potential as well as an expansion of the polarization. Notably, at mid-capacity (~ 75 mAh g −1 ), the discharge potential shifted towards 50 mV for the Fe-redox potential (from 3.40 to 3.45 V) with an increase in the degree of Mn doping and the polarization considerably increased from 50 mV (for LiFePO 4 ) to 63 mV (for LiMn 0.3 Fe 0.7 PO 4 ). This finding is consistent with the obtained cyclic voltammograms and can be interpreted as (i) the Mn is more electropostitive than Fe and (ii) the Mn substitution of Fe can be expected to strengthen the Fe-O covalence, which raises the Fe 2+ /Fe 3+ redox energy and shifts the redox voltage of the Fe 2+ /Fe 3+ couple higher 25 .
The cycling performance of the olivines are presented in Fig. 5B. The initial capacity of LiFePO 4 reached 169 mAh g −1 , which was close to the theoretical capacity, however 10% of the initial capacity was lost after 50 cycles with a final capacity of 152 mAh g −1 . For the Mn-doped olivines, LiMn x Fe 1−x PO 4 (x = 0.1, 0.2, and 0.3), there was negligible capacity loss and their final capacity reached 151, 147, and 157 mAh g −1 , respectively. The capacity loss can be interpreted by the release of Mn during the charge-discharge process 41 . The Coulombic retentions were over 95% during the cycling test, which indicated a reversible Li-intercalation into the olivine hosts.
The apparent chemical coefficient of diffusion of lithium (D Li ) is a key parameter that evaluates the lithium transport into the intercalation hosts, which can be determined by the galvanostatic intermittent titration technique (GITT). This method imposes a constant current through the cell for a certain time interval 42,43 . The open circuit voltage (OCV) curve was measured with a constant discharge rate of C/50 for 30 min followed by an OCV relaxation period of 5 h to the equilibrium voltage. The diffusion process within the host was assumed to obey Fick's second law of diffusion, and under galvanostatic conditions, it obeys the following Eq. (2):  www.nature.com/scientificreports/ function of time during the constant current flux. The galvanostatic curve described the phase transition between LiFePO 4 -FePO 4 can be divided in three segments: (i) a quickly dropping voltage as a solid solution segment for Li content below 0.1, (ii) a phase transition segment with Li content ranging from 0.1 to 0.9, and (iii) other solid solution segments for Li content below 1 44,45 . The D Li in the solid solution segment is usually more rapid than those in the phase transition region due to the slope dE/dx. Indeed, the phase transition is characterized by a flat voltage that leads to a small dE/dx. Figure 6      www.nature.com/scientificreports/ synthesized Mn-doped olivines, the LiMn 0.3 Fe 0.7 PO 4 exhibits the superior cycling stability with the retention of 100% initial capacity (157 mAh g −1 ) upon 50 cycles. Following the EIS and GITT results, the Mn-substitutions benefits the electron-transfer process as well as the Li-transport in 1D channel (010) plan due to the enlargement of lattice parameter. The olivine LiMn x Fe 1−x PO 4 , therefore, can lead to the development of high-performance lithium-ion batteries.