Facile Synthesis of Micro CuO Crystals for Li Ion Full Battery

Micro CuO crystals are synthesized by hydrothermal method in one step. The CuO-Li ion full battery is assembled using the porous lithium foil-graphite as a anode and the prepared CuO crystals as a cathode. The micro CuO crystals are characterized by scanning electron micro porousscopy, X-ray powder diffractometer, Fourier Transform infrared spectrometer, thermogravimeter, Brunauer–Emmett–Teller surface area measurements, and differential scanning calorimeter. The full battery is tested by the galvanostatic current charge-discharge technology at higher current densities of 0.25–0.5 mA/cm2. The porous lithium foil-graphite anode can effectively inhibit the powder of anode lithium, and improve the reversibility of the battery.


Introduction
Metal oxides as a cathode, such as CuO [1], Fe 2 O 3 , Nb 2 O 5 [2], MnO 2 [3], Bi 2 O 3 [4], and V 2 O 5 [5], have smaller molecular weight and higher electron gain and loss number, thus can obtain higher specific capacitance (SC) values. CuO is a p-type semiconductor material with low price and abundant resources, which can be used for energy storage. Many cathode materials of Li ion battery, polymorphic CuO [6], CuO nanochains [7], CuO nanoflake arrays [8], CuO-NiO [9], CuO/Cu/TiO 2 NT/Ti [10], and CuO/Cu composite [11], have been investigated, and its carbon and organic polymer composites such as mesocarbon microbead/CuO/Cu [12], C@ SnO 2 /Cu 2 O nanosheet [13], Cu 2 O/CuO/Cu/Carbon-polymer composite fibers [14], and polypyrrole coated Cu/Cu 2 O also showed excellent performance in lithium-ion batteries [15]. However, the discharge capacity of most of CuO-Li half batteries decays rapidly due to the so-called dead lithium (Solid electrolyte interphase (SEI) formed by LiF, LixPFy, LixPOFy, Li 2 CO 3 and ROCO 2 Li, etc. between the lithium and the electrolyte) [16]. When the mass of CuO is large the reduced lithium powder can adsorption the electrolyte, leading to polarization of the battery. Therefore, the CuO-Li half batteries far from commercial battery requirements. The full battery composed of graphite and LiFePO 4 can be recycled more than 2000 times [17], indicated graphite is stable in the charge/discharge progress. In our previous work the simple lithium-rich graphite anode/LiFePO4 battery has been successfully prepared by the electrodeposition of lithium into graphite with a satisfactory full battery, and the lithium-rich graphite anode was very stable [18].
In this paper, the micro CuO crystals were synthesized, and a full cell was assembled with Li-graphite. The charge and discharge performance of the full cell at high density currents was studied. The porous lithium foil-graphite anode could effectively inhibit the powder of lithium and improve the reversibility of the battery.

Materials
Except for CuO crystals the materials were the same as those in references [18]: Commercial acetylene black and natural graphite (purity, > 99.9 wt%, purity, > 99.9 wt %, ~ 500 mesh) were obtained from Shenzhen Tuling Evolution Technology Co. Ltd. (Shenzhen,China), 0.1 mm thick glass fiber filter, and other reagents were purchased from Shanghai Analytical Chemicals Company (Shanghai, China). 60% polyvinylidene fluoride (PVDF) water emulsion was provided by Shanghai San-Ai-Fu new material Ltd (Shanghai, China). All other chemicals were of analytical grade without further purification.

Synthesis of CuO Crystal
The CuO crystal was synthesized as the method: The mixtures of Cu(NO 3 ) 2. 3H 2 O of 10.0 g and 0.1 M HNO 3 of 50 ml in a crucible of 100 ml were stirred with glass rod for 20 min, and put the crucible in a oven of 90 °C for 18 h to evaporate the water. The obtained precipitate in the crucible was put into a muffle furnace, and then the temperature of muffle furnace was raised to 400 °C at 10 °C/Min, then to 500 °C at 3 °C/Min, kept at 500 °C for 240 min, and cooled to room temperature of 20 °C.

Material Characterization
The thermogravimetry (TG) and differential scanning calorimetry (DSC) were performed using a NETZSCH STA 449F3 simultaneous thermal analyzer (German). The Fourier Transform infrared (FT-IR) spectra were measured by a NICOLET NEXUS470 spectrometer (USA) in the frequency range 4000-400 cm −1 . The images of the as-prepared products were examined by scanning electron microscope (SEM) (QUANTA FEG 450, USA), equipped with an EDAX OCTANE PRO energy dispersive spectrometer (EDS) (FEI, USA). X-Ray diffraction (XRD) analysis was performed on the as-prepared products with a Switzerland ARL X'TRA X-ray diffractometer rotating anode with Cu-Кα radiation source (λ = 0.1540562 nm). The nitrogen adsorption and desorption experiments were carried out at 77 K using SA3100 surface area and pore size analyzers (Beckman Coulter, Inc. USA).

Electrochemical Experimental
To reduce the errors of measurement the loading mass of CuO was 0.15 g. The CuO (graphite of 0.2000 g), acetylene black and PVDF were taken in the weight ratios of 80: 20: 10. The glassy fiber was used as the separator due to its strong structure and good adsorption of electrolyte. 1 M LiPF 6 dissolved in the solution of dimethyl carbonate (DMC), ethylene carbonate (EC) and ethyl methyl carbonate (EMC) with a 1:1:1 volume ratio, which was used as the electrolyte. The electroactive mixture was firstly formed to slurry by ethyl alcohol, the CuO mixture was coated on the aluminum foil in diameter of 1.6 cm, and the graphite mixture was also coated on the copper foil in diameter 1.6 cm, finally all battery materials were dried in vacuum at 80 °C for 24 h. The electrochemical performances of battery were investigated using a CR2025 coin-type cell with diameter of 20 mm and thickness of 2.5 mm (Yajun Battery Materials Co., Ltd, Taizhou, China). The full battery were constructed as the method [18] in the order of CuO cathode/ battery separator/porous lithium sheet/graphite. The lithium foil of 18 mg in diameter of 1.6 cm with 2 mm 10 holes was prepared using a multi hole punch in an argon-filled glovebox for the transmission of Li ions. The amount of electrolyte injected into CR2025 battery was about 0.15 ml. The full battery was galvanostatically charged and discharged between 1.0 and 4.0 V using a Land-CT2001A battery analyzer (Wuhan, China). All electrochemical measurements were carried out at room temperature. Figure 1 shows the SEM images of micro CuO crystals, the micro honeycomb-shaped irregular spherical CuO crystals were found in the sample, and the size of micro CuO crystals was ~ 570 × 560 nm.

EDS Spectra and Element Analysis of Micro CuO Crystals
The EDS spectra and element analysis of micro CuO crystals are shown in Fig. 2. Only O and Cu elements in the sample were found, and the ratio of O: Cu was 1.15: 1, which was close to the theoretical value of 1: 1 in stoichiometry, revealing that the micro CuO crystals were pure.

XRD Pattern of CuO Crystals
The XRD pattern of micro CuO crystals shown in Fig. 3

FT-IR of Micro CuO Crystals
The FT-IR spectra of CuO crystal are shown in Fig. 4. For micro CuO crystals, the typical peaks of deformation vibration of Cu-O were found at 503 and 586 cm −1 , and the peaks at 808 and 1086 cm −1 was attributed to Cu-O stretching vibrations [19]. The bond at 3437 cm −1 and 1646 cm −1 were assigned to the O-H stretching vibration and the O-H deformation vibration, which were related with the adsorbed water in the sample.

TG and DSC Curves of Micro CuO Crystals
The decomposition temperature of battery material has a great influence on the safety of battery. The TG and DSC curves of CuO crystals are shown in Fig. 5. Within 816 °C, CuO crystals showed higher stability, and when the temperature raised to 1000 °C, the CuO crystals of 89.7% remained, indicated that CuO crystals were a higher safe battery material. From DSC curves in Fig. 5 the endothermic temperature were found at 142 and 919 °C, which were assigned to the volatilization of bound water and the decomposition of CuO (2CuO = Cu 2 O + 0.5 O 2 ) [20], respectively.

Specific Surface Area of Micro CuO Crystals
As shown in Fig. 6, the isotherm of micro CuO crystals was classified as type IV with an H3 hysteresis loop. The specific surface area (SSA) was calculated using the Brunauer Emmett-Teller (BET) method. The pore-size distributions (PSDs) were computed by the Barrett Joyner Halenda (BJH) plots. The peak in PSDs (shown in Fig. 6 inset) was centered at 467 nm for the micro CuO crystals. The SSA values of the micro CuO crystals calculated by BET method were 2.54 m 2 g −1 . The mesoporous structure was conducive to infiltration of electrolysis.

Charging and Discharging of Full Battery
The Charge-discharge curves of batteries are shown in Fig. 7a and d. From the first discharge in Fig. 7a and b, when the voltage decreased from 3.45 to 0 V, a flat discharge curve at about 1.7 V was found, and the first discharge capacitances at 0.25 mA.cm −2 were 59.2 mAh (SC values were 394.7 mAh/g), which were close to the SC values of half battery at 2 C [11] and 0.50 mA [8]. However, the mass of cathode materials in the full battery was 75 mg/cm 2 , which was more 37 times that of half battery (Mass of cathode materials are usually 1-3 mg/cm 2 for half battery), and the Li-graphite anode could effectively inhibit the powder of lithium and improve the reversibility of the battery. When the voltage was set between 4-1.0 V, the capacity decreases greatly due to the formation of SEI and the stored Li in the micro CuO crystals. The first charge capacities at 0.25 mA. cm −1 was 78.9 mAh.g −1 , and the discharge capacity at 0.50 mA.cm −1 was 47.1 mAh.g −1 , but the second charging/ discharging capacities at 0.50 mA.cm −1 decays to 39.2 and 37.6 mAh.g −1 , respectively, indicated that the fast charging led to the decrease of capacity due to the increase of polarization and the higher currents (i) (E = i R). Figure 7c shows the charge/discharge curves at 0.5 mA.cm −2 between cycles 420 and 440 approximately, and the discharging capacities and efficiency (cycle 1-440) were shown in Fig. 7d. The discharge efficiency in cycle of 1-5 increased with the continuous dissolution of lithium metal, while the discharge efficiency in cycle of 3-420 fluctuated within ± 5%. From Fig. 7d the discharge capacities in the voltage range of 1-4 V decayed from 37.6 to about 9.9 mAh.g −1 in the cycle of 3-200.0 due to the continuous SEI formation, and then to from 9.9 to about 8.6 mAh.g −1 in the cycle 200 to 420, indicting the discharge capacities of full battery was stable.

Conclusions
In this paper, synthesis of micro CuO crystals was carried out by hydrothermal method. The porous lithium foil/Ligraphite was used as the anode for CuO full battery. The performance of micro CuO crystal battery was studied. The porous lithium foil-graphite anode could effectively inhibit the powder of lithium, and improve the reversibility of the battery.