Microstructure and High Temperature Charge-Discharge Characteristics of 3D Additive Manufacturing Produced Mg-Ni Anode
2018 Volume 59 Issue 4 Pages 685-689
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2018 Volume 59 Issue 4 Pages 685-689
This study applied a 3D additive manufacturing technique to produce a stable and high performance Mg-Ni alloy anode for lithium ion batteries. This 3D process used Mg-Ni powders to printed two layers on copper foil to obtain the Mg-Ni anodes directly, the 3D printed anodes have not need the traditional stir-slurry, coating and baking processes, and process the performances of high efficiency and low coat. Experiment results show that Mg2Ni, MgNi2 and Mg2Cu intermetallic compounds (IMC) are benefit for 3D printed anode, the anode can slowly form a dense and thick SEI layer at 55°C high temperature charge-discharge; therefore, a better capacity than at 25°C can be achieved. The 85°C charge-discharge environment makes the volume expansion effect serious, which results in capacity decay slightly. The presented high temperature charge-discharge capacities are higher than that of the commercial lithium battery, demonstrate that 3D additive manufacturing can fabricate the novel alloy electrodes simply and quickly, and that 3D Mg-Ni anodes are very suitable for today’s high-end electronic products.
Magnesium is used as an anode in lithium ion batteries due to its high theoretical capacity.1,2) However, pure metal electrodes have a serious volume expansion problem during charge-discharge, which results in a poor cycle life.3,4) So, the performance of batteries reduced with increasing the temperature. Notably, our previous papers reveal that the Mg anodes processed excellent high temperature charge-discharge properties, its capacity increased with increasing temperatures.5) In addition, one solution is to add nickel into the magnesium anodes to reduce the volume expansion effects to raise the cycle life.6) When adding 2 mass% Ni into the Mg matrix, the capacity of Mg-Ni anodes does not decrease in 55°C high-temperature environments.7) High-end digital portable devices have serious heat problems, and often operate at temperatures are more than 55°C. Accordingly, the charge-discharge properties of anodes at high temperature have become an important consideration. The other systems such as Si, Al and graphite decay much faster than Mg system when they work at high temperature.8) Therefore, although Mg-Ni system has lower capacity than Si and other alloy anode systems, we believe the Mg-Ni anode is the potential and suitable system for industry application.
Femtosecond laser sintering is an additive manufacturing (3D printing) technique that uses an ultrashort pulse laser as the power source to sinter powdered materials in a low-oxygen environment.9,10) Samples made with laser sintering have a small heat affected zone and good adhesion with the substrate; as such, this approach can be applied to electrode fabrication. In addition, the additive manufacturing process is quicker and simpler than traditional techniques, and should improve the capacity and electrochemical properties.11,12)
According to reports of thin film batteries,13) we find that intermetallic compounds (IMC) films process high capacity and cycle life. However, the sputter equipment cannot make thicker IMC film. For powder anode process, the intermetallic compound powders are difficult to manufacture.
This study used Mg-Ni metallic powder as a base material and applied 3D additive manufacturing to fabricate a 2-layer IMC anode for lithium ion batteries. The anode was charged and discharged at high temperature (55°C and 85°C) to investigate the electrochemical mechanism. The obtained data could provide references to the lithium-ion battery industry and 3D printing technology.
This study used AM100G2 laser additive manufacturing equipment to fabricate the Mg-Ni anode. The sample manufacturing process is described in the following. First, the Mg-Ni powder (particle size: 30∼50 µm), mixed in a ratio of 98:2 mass%, was coated onto the copper foil, and then processed by femtosecond laser sintering. The printing time was 2 time and printing matrix also contains IMCs. It should be noted that the 3D printed samples did not require extra heat or compaction treatments; rather, they were simply punched into circular electrode samples (Thin film electrodes need heat treatment to form interface IMCs; traditional powder electrodes need the coating and compaction treatments). The atmospheric condition of the 3D printed chamber was 2000 ppm of oxygen. A fiber laser with a wavelength of 1070 nm was used as the laser source, the operating power and spot size of which were set to 50 W and 70 µm, respectively. The surface of printed 2-layer electrodes was flat without any pattern.
After the anode manufacturing, half batteries were assembled within a glovebox filled with argon. The half batteries used lithium foil as the reference electrode, and the electrolyte comprised LiPF6 (1M) as lithium salt, EC+PC+DMC (volume ratio was 3:1:6) as the solvent and VC as the additive.14)
The charge-discharge test of the half batteries was processed by the constant current method, for which the speed was set to 0.2 C. The cut-off voltages of the charging and discharging were 1.5 V and 0.001 V, respectively, the process of which ran for 100 cycles at 25°C, 55°C and 85°C. In addition, we used cyclic voltammetry analysis to study the redox reaction mechanism, the cycle efficiency of the batteries and the microstructure change of the active materials. The scan range was set to between 0 to 1.5 V with a step of 0.05 mV/s.
XRD and SEM imaging were employed to examine the surface of the 3D Mg-Ni anode microstructures before and after charge-discharge. Cross-sectional images and IMC’s structure were acquired using field-emission transmission electron microscopy (FE-TEM).
A picture of the 3D Mg-Ni alloy anode before being punched and a schematic diagram of the additive manufacturing process are presented in Fig. 1(a) and (b), respectively. The macroscopic appearance of the electrode is dense and smooth; similar to that made by the traditional slurry and coating process. Figure 2 shows the XRD of anode and the printing matrix contains Mg2Ni, MgNi2 and Mg2Cu intermetallic compounds (IMC). The traditional electrode manufacturing process uses carbon black to fill the gaps among powder particles, which can lower the resistance and improve the cycling performance. The IMC formed by 3D printing process can instead the carbon black to improve adhesion among each particle and further provide more electron channels, so the IMC has positive effects to cycling performance.
(a) The macroscopic picture of the Mg-Ni anode fabricated by additive manufacturing before being punched. (b) A schematic diagram of the additive manufacturing process for fabricating the Mg-Ni anode.
XRD of additive manufactured Mg-Ni anode.
Figure 3(a)–(d) show the surface morphology of the Mg-Ni anode before and after charging-discharging at different temperatures. It was noted that some spherical structures were present on the surface of the anode before the charge-discharge cycles. In addition, there were nano-scale particles spread around the surface as well. These nano-particles formed due to the instant cooling after the laser process, and could significantly increase the surface area. After charging-discharging, the surface was covered with a large amount of solid electrolyte interphase (SEI) compounds. The SEI surface compounds of the 55°C and 85°C samples were more obvious than those of the 25°C sample. Further, the 85°C sample also had some surface cracks due to the volume expansion effect, which decreases the cycle life of anodes.
Surface SEM images of the Mg-Ni anodes: (a) before charging-discharging; (b) after 100 cycles of charging-discharging at 25°C; (c) after 100 cycles charging-discharging at 55°C; and, (d) after 100 cycles charging-discharging at 85°C.
Figure 4 shows the cross-sectional and interface image of the Mg-Ni anode before the charge-discharge cycles. The thickness of the Mg-Ni alloy layer was about 1.66 µm, and was composed of two different phases defined as α-Mg and Mg2Ni, as supported by the diffraction patterns in Figs. 4(a) and (b). The laser heat made the Cu and Mg atoms diffuse and form the Mg2Cu phase. It was found that a thin intermetallic compound (IMC) layer of about 100 nm formed between the α-Mg and Mg2Cu phases. According to phase diagram and diffraction patterns in Fig. 4(c)(d), this IMC layer was determined to be the Mg2Cu and Cu1.1MgNi0.9 phase. Apparently, the IMC filled the gaps between the alloy particles and matrix, improved the adhesion, and further lowered the resistance of the electrode. As such, this IMC layer was the reason why the 3D anodes produced by additive manufacturing did not require the addition of carbon black powders. All of these phases are the active phase during charge-discharge process, but the alloy phases as Mg2Ni, MgNi2, Mg2Cu, and Cu1.1MgNi0.9 provide lesser insertion/extraction activity of lithium ion. However, these alloy phases has lower expansion ratio, so the cycle life of electrode can enhance relatively.
The cross-sectional TEM bright-field image of the Mg-Ni anode and the diffraction patterns of: (a) α-Mg, (b) Mg2Ni, (c) Cu1.1MgNi0.9, and (d) Mg2Cu.
Figure 5 was the first three time CV curves of the Mg-Ni anodes under different temperatures. It is clear that the capacity of Mg-Ni anodes increased with increasing temperatures. In general, the capacity of batteries is deterioration in higher temperature, that of present Mg-Ni anode are excellent.
The first-three time CV curves of the Mg-Ni anodes under different temperature: (a) 25°C (b) 55°C (c) 85°C.
Figure 6(a) shows the charge-discharge results of the Mg-Ni anode at 25°C, 55°C and 85°C. As can be seen, each sample stabilized after the 10th cycle. In addition, the high temperature environment promoted the formation of the SEI layer and enhanced the insertion and extraction of lithium ions.15) However, the volume expansion during charging-discharging became serious at high temperature and shortened the cycle life of the battery.16) The 85°C sample showed the best capacity due to the thicker SEI layer in initial stage. However, the volume expansion effects became progressively more seriously with increasing cycle times, which greatly reduced the capacity performance. After 10 cycles, the 85°C sample showed the lowest capacity, worse than even the 25°C sample; then after 100 cycles, it decreased to only 88.6 mAh/g. By comparison, the 55°C sample formed a finer SEI layer than the 25°C sample, and the expansion problem was not as serious as that of 85°C; accordingly, it had the best capacity after several cycles. To further investigate these phenomena, cyclic voltammetry analyses were conducted to study the electrochemical reaction at high temperature, the results of which are shown in Fig. 6(b). The Mg-Ni anode has a main reduction potential at 0.004 V and an oxidation potential at 0.135 V. Although the 55°C sample had reduction peaks near 0.3 V and 0.5 V due to the formation of SEI layer, the 85°C sample with the completely formed SEI layer did not have such peaks. This indicates that the formation speed of the SEI layer at 55°C was much slower than at 85°C, and that the slow-forming SEI layer was denser and had lower resistance. Notably, the 85°C sample had an oxidation peak near 0.3 V due to the thermal decomposition reaction of the electrolyte;17) as such, the electrode became unstable and decayed much faster. Moreover, the electrochemical experiments showed that the SEI layer of Mg-Ni anodes fabricated by additive manufacturing had the best formation environment at 55°C. Based on the above, if we change the electrolyte into which hard to thermal decomposition, the cycle performance should be improved at extra high temperature as 85°C; however, the volume expansion still remains a challenging problem. Nevertheless, the 55°C temperature is sufficient for most applied situations, and so the 3D printed Mg-Ni alloy anode produced by additive manufacturing is suitable for application at a high working temperature. In the future, this 3D technique could be applied to improve the structural design of collectors or produce full solid-state batteries that include solid electrolytes.
(a) Cycle life diagram of the Mg-Ni anode at different charge-discharge temperatures (b) Cyclic voltammetry analysis of the Mg-Ni anode between 55°C and 85°C at the 11th cycle.
The authors are grateful to the Instrument Center of National Cheng Kung University and the the Ministry of Science and Technology of the Republic of China (MOST 105-2628-E-006-001-MY2) for their financial support.
