Hydrothermal Synthesis of Mn-doped VO2 (B) as Cathode Material for Lithium-ion Battery

Mn-doped VO2(B) was synthesized via a hydrothermal reaction from manganese sulfate (MnSO4) and Vanadyl oxalate (VOC2O4·5H2O). The effects of the content of Mn on the electrochemical performance of material were studied by means of galvanostatic charge-discharge, cyclic voltammetry and electrochemical impedance spectroscopy (EIS). Compared with pure material VO2(B), the MnxVO2(B) which was doped with little Mn(x=0.01,0.02,0.05) had higher specific capacity and more preferable cycle performance. When galvanostatic charged-discharged with 0. 1 C in 1.5-4.0 V. the initial discharge specific capacity of sample Mn1, Mn2 and Mn3 were 204 mAh g−1, 242 mAh g−1 and 249 mAh g−1 respectively larger than pure VO2 sample (181mAh g−1). In particular, the sample of Mn1 exhibited best electrochemical performance. The initial discharge capacity maintained 164 mAh g−1 after 50 cycles. The retention rate of capacity was 80%.


Introduction
In recent years, new energy vehicles, communications, energy storage and other emerging fields have been rapidly developed, which greatly promoted the development of large capacity , high energy density Rechargeable lithium-ion batteries (LIBs) [1][2][3]. For the positive electrode active material for energy storage, improve energy density is to improve the discharge voltage and discharge capacity. At present, in order to improve the energy density as the main development goals of the third generation of lithium-ion batteries, the cathode material is at the stage of upgrading.
However, due to inferior electronic conductivity and poor cycle stability of VO 2 (B), it seriously affected its practical application. In recent years, in order to solve these problems effectively, the electrochemical properties of vanadium oxides have been improved by doping [12][13][14],cladding [15][16][17], nanocrystallization [18][19][20][21][22][23],and othermethods [24]. CHENG H, et al [12]synthesized Cu x VO 2 ,with Cu 2+ doping, the Fermi level of Li in VO 2 (B) and the conductivity have promoted, the ability of the Lithium-ion insertion and extraction and the stabilityof the charging and discharging were improved as well. When the amount of doping Cu 2+ is 1.03at%, the charge and discharge performance of VO 2 (B) had improved at low and high rate, the discharge specific capacity for the first time is 317.1 mAh g −1 at 0.132C, along with after 51 cycles measurement, the specific capacity was still kept 234.3mAh g −1 . In this article, we synthesized pure VO 2 (B) and Mn-doped VO 2 (B) through the hydrothermal reaction combining with the calcination process and investigated the structural and electrochemical characteristics of Mn x VO 2 (B) electrodes during cycling tests in lithium-ion batteries.

Synthesis of Mn doping VO 2 (B)
All chemical reagents were analytical grade and implemented without any further purification. Mn-doped VO 2 were ready according to the following procedure. 0.4g Vanadium pentoxide and 1.25g Oxalic acid dihydrate were dissolved in 20mL of deionized water. The mixed solution was kept under stirring at 70℃ in a water bath until a blue colored solution formed. And then naturally cooled to room-temperature.Then the obtained precursor solution filtration. The suitable amount of manganese sulfate was dissolved in 30mL of deionized water when it completely dissolves, pour it into as-synthesized Vanadyl oxalate solution, then 3ml 30%(H 2 O 2 ) was added to the mixed solution, after stirring it without come up to bubbles and transferred into a 100mL Teflon-lined autoclave. The autoclave was sealed and maintained at 160℃ for 24 h and then cooled to room temperature naturally. The precipitate was washed three times with deionized water and alcohol, respectively and dried in the freeze dryer for 24 h. Finally, the obtained precursor was sintered at 350℃ at 3℃/min for 1 h in argon. Molar ratios of Mn to V were increased gradually from 0 to 1/20. The samples were identified as Mn 0 , Mn 1 , Mn 2 and Mn 3 , corresponding to different molar ratios of Mn to V (0, 1/100, 1/50, 1/20).

Characterization
The X-ray powder diffractometer (XRD) with a Cu Kα radiation source (λ = 0.154 nm, the scanning rate of 6 °/min) was used to analyze the phase of the products. The Hitachi S-4800 field emission scanning electron microscopy (FESEM) was carried out to observe the morphology of the products.

Electrochemical measurements
The positive electrode was prepared by mixing active material, acetylene black carbon powder and polyvinylidene fluoride (PVDF) binder in a weight ratio of 7/2/1, grinding the mixture with a certain amount of N-methy1-2pyrrolidone (NMP), then the slurry was coated uniformly on Mn foil. The Mn foil was dried in vacuum at 90℃ for 12 h. 1 mol/L LiPF 6 in a mixture of dimethyl carbonate (DMC)， ethylene carbonate (EC) and diethyl carbonate (DEC) were used as an electrolyte (DMC/EC/ DEC=2:2:1 in volume). The cells were assembled in a glove box filled with pure dry argon gas. Galvanostatic charge/discharge measurements were carried out in the voltage region between 1.5 V and 4.0 V. Both the electrochemical impedance spectroscopy (EIS) and the cyclic voltammetry (CV) were tested through a CHI 760D electrochemical workstation, and the CV measurement was performed in the potential range from 4.0 to 1.5 V at a scan rate of 0.1 mv.s −1 Figure 1(a) showed the XRD patterns of Mn x VO 2 (x=0.00,0.01,0.02,0.05)synthesized at 350℃. All peaks were basically consistent with the standard diffraction peaks of VO 2 (B) (JCPDS no. 81-2392). It became apparent that compounds are well-crystallized. As can be observed figure 1(b), with the doping content increasing, the diffraction peaks broaden and the intensity weakens, which was attributed to the crystallinity decrease after doping with Mn 2+ .   Figure 3a displayed the cycling performance of Mn-doped samples. The charge and discharge curves at the current density of 32.4 mAh g −1 in 1.5−4.0 V. it can be seen from Figure 3a that all doping samples VO 2 discharge performance were better than pure VO 2 . The initial and after 50 cycles discharge capacities were 181 mAh g −1 and 124 mAh g −1 (Mn 0 ), 204 mAh g −1 and 164 mAh g −1 (Mn 1 ), 242 mAh g −1 and132mAh g −1 (Mn 2 ), 249 mAh g −1 and 129 mAh g −1 (Mn 3 ). The retention rates of capacities were 69%, 80%, 55%, and 51.8%, respectively. Obviously, the first discharge specific capacity increases with the increase of Mn doping. However, the cycle performance did not augment with the increase in manganese doping. Among which, Mn1 showed the best cycling performance of 80% after 50 cycles at 0.1C. The result suggested that doping appropriate Mn 2+ could stabilize the crystal structure of VO 2 (B) and reduce the irreversible destruction of cathode materials. Combined with scanned images, we see that the addition of manganese make better dispersion of samples, simultaneously the sample becomes more subtle. It can be drawn from the structure of the sample that slender flower samples are not conducive to the stability of the battery. samples at the current density of 32.4 mA g −1 in the voltage ranging from 1.5 V to 4.0 V at room temperature. All of them showed a distinct charge platform at around 2.7 V and a distinct discharge platform at around 2.5V , the result was agreement with that of the reference [25,26].  Figure 4 showed the Nyquist plots of the samples, the Nyquist plots were measured by EIS(electrochemical impedance spectra) after cycling for 2 times, It can be seen from figure 4 that the charge transfer impedance (Rct) value of Mn 0 , Mn 1 , Mn 2 and Mn 3 in the high-frequency region's semicircles were corresponding to 1184Ω, 1430 Ω，1980 Ω and 1060 Ω respectively. Obviously, the charge transfer impedance of Mn 1, Mn 2 is larger than Mn 0. Attributed to because the flake structure of samples became finer long after doping,consequently the contact area between the sheet and the sheet is diminutive. Therefore, the lithium ion has a longer diffusion distance between the particles, thereby the charge transfer resistance between the electrode and electrolyte aggrandized significantly. Meanwhile, Rct value of Mn 3 was found to be the smallest, which indicates that the addition of more manganese ions to replace the vanadium position, will make the structure conducive to the transmission of lithium ions. Figure 4 the Nyquist plots of the samples with different stirring rates Figure 5 showed the cyclic voltammetry curves of Mn 0 , Mn 1 , Mn 2 and Mn 3 with the voltage range of 1.5-4.0 V in the scanning speed of 0.2 mV/s after cycling for 2 times. A pair of oxidation and reduction peaks could be observed in the figure, which represents the extraction and insertion of Li + during the electrochemical processes. The peak shape of the sample has good symmetry, indicating that the electrochemical performance before and after doping is relatively stable. With the increase of manganese content, the oxidation peak shifts to low potential and then the high potential, whilst the reduction peak shifts to the high potential and then to the low potential, which indicates that doping can affect the polarization of the material. In addition, the area of the curve in the sample of Mn 3 was the biggest, which represented the sample capacity of Mn 3 was the largest.

Conclusion
Pure VO 2 (B) and Mn-doped VO 2 (B) were synthesized via a completely aqueous solution based synthesis method from manganese sulfate (MnSO 4 ) and Vanadyl oxalate (VOC 2 O 4 · 5H 2 O). It was noted that the incorporation of Mn 2+ significantly affected the morphology and structure of VO 2 (B). With the increase of manganese content, the nanobelts became thin and long, the morphology changed from irregular state to flower-like structures. Electrochemical properties demonstrated that the charge/discharge capacity of the samples have been enhanced greatly after doping with Mn 2+ . In particular, the sample of Mn 1 exhibited the best electrochemical performance. The initial discharge capacity was 204mAh g −1 and maintained 164mAh g −1 after 50 cycles. The retention rate of capacity was 80%. Far more than samples of Mn 0 , Mn 2 and Mn 3.