|Capacity change with number of cycles for PS-PVD powders with different C/Si ratios. The battery was charged at a constant current of 0.1 mA for the first three cycles and at 0.5 mA for the rest of the cycles. Homma et al. Click to enlarge.|
Researchers at the University of Tokyo have developed an approach which potentially has industrially compatible high throughputs to produce nano-sized composite silicon-based powders as a strong candidate for the anode of next-generation high density lithium ion batteries. The powders are fundamentally an aggregate of primary ∼20 nm particles, which are composed of a crystalline Si core and SiOx shell structure.
In an open access paper published in the journal Science and Technology of Advanced Materials, they report that half-cell batteries made with their nanocomposite Si/SiOx powders exhibited improved initial efficiency and maintenance of capacity as high as 1000 mAh g−1 after 100 cycles.
The results of much research indicate that the structural control of silicon anodes is an effective approach to exploiting their high theoretical specific capacity while preventing the pulverization and resulting capacity decay caused by the material’s huge volumetric change during the charge-discharge process.
SiO [silicon oxide] is also considered as a potential alternative to Si because batteries using SiO as a negative electrode generally attain a stable cycle capacity retention due mainly to its adaptable amorphous structure and also to the formation of Li2O that may act as buffer for dilation. In addition, this material is also known to transform into Si and SiO2 through disproportionation reaction. This has been promisingly used as a host for the spontaneous composite structure having nanosized Si precipitates within the SiO2 matrix, proving its structural effectiveness, especially with the addition of the secondary element. However, the inevitable formation of Li–O and Li–Si–O phases literally consumes lithium during lithiation as part of irreversible capacity, which reduces the initial efficiency significantly. This is another major issue of this material, along with enhancement of the specific capacity itself.—Homma et al.
The research team earlier had used plasma spraying physical vapor deposition (PS-PVD) to produce nanoparticles from metallurgical-grade Si powders for anode materials. Briefly, the process entails the evaporation of injected raw powders into a thermal plasma jet where the core temperature in general exceeds 10,000 K (9,727 ˚C) to form the vapor phase. This is followed by the consecutive rapid condensation of the high-temperature Si vapors for nanoparticle formation in the low-temperature region of the jet tail. The elemental physical process in the plasma is essentially similar to that for film deposition.
If one uses SiO as feedstock for PS-PVD with an Ar + H2 plasma jet, reduction of SiO is expected to some extent, as has been demonstrated for the reduction of silicon oxide. That is, the decrease in the oxygen content, x, in the raw Si1Ox to less than 1 readily increases the relative number of Si atoms, which could contribute to an increase in the electric capacity by decreasing the irreversible capacity associated with Li–O formation.
In addition, there would be technical advantage for this SiO and PS-PVD combination. According to the literature, SiO(s) sublimates at a temperature around 2200 K, lower than the 3500 K boiling point of Si, and the total energies required for formation of the vapor phase (SiO (g)) from solid SiO is estimated to be approximately 399 kJ mol−1, which is less than the total enthalpy of 523 kJ mol−1 required for evaporation of solid Si. This means that if one considers forming SiO vapor by complete decomposition of SiO powders, compared to the use of Si feedstock, less energy is required to produce the same amount of vapor, or a higher quantity of SiO vapor can be produced at a fixed input power to the plasma.
Furthermore, the vapor pressure of SiO is roughly three orders of magnitude higher than that of Si at least at temperatures below 2000K. It is thus anticipated that if the grain growth after nucleation is effectively suppressed by introducing appropriate quenching, many smaller particles could form preferentially compared to the case of Si.
Introduction of the secondary element together with SiO powder into the plasma leads potentially to various gas phase formation and subsequent co-condensation to certain alloy phases. This suggests that the structure of the nanoparticles can be controlled topologically and compositionally by selection of the overall composition and the quenching protocol.—Homma et al.
The team successfully produced nanocomposite SiO powders by PS-PVD. The core-shell structure is formed in a single-step continuous processing. When methane (CH4) was additionally introduced to the PS-PVD, the volume of the core Si increased while reducing potentially the SiOx shell thickness as a result of the enhanced SiO reduction.
In preparing different PS-PVD powders, the team introduced CH4 to the plasma to maintain overall C/Si molar ratios of 0.25, 1 and 1.5. To understand the effect of the different PS-PVD powders on battery performance, all the PS-PVD powders with different C/Si ratios were mixed identically with a binder and carbon black powders at a fixed amount—Si active material:conducting carbon:binder = 60:15:25.
The slurry was applied to Cu foil to a thickness of 75 μm, which was further reduced to 21μm after being oven-dried under ambient conditions. Using this as anode, half coin cells were assembled in a globe box filled with Ar, together with lithium metal as the counter electrode and 1 M LiPF6 dissolved with ethylene carbonate and diethyl carbonate (1:1 vol.%) as the electrolyte.
The test cell was charged at a constant current of 0.1 mA for the first three cycles and at 0.5 mA for the rest of the cycles, which roughly correspond to 0.02 C and 0.1 C, respectively. All the tests were conducted at a controlled temperature of 23 °C.
Among their findings was that an unfavorable SiC phase emerges when the C/Si molar ratio is greater than 1. C/Si = 0.25 seems to be in the optimal range to produce an enlarged nc-Si core with a reduced SiOx shell while suppressing inactive SiC formation.
As a result of the increased amount of active Si and reduced source for irreversible capacity, this composite structure improved the initial efficiency and maintained high capacity for longer cycles at the same time.
This work was partly supported by the Funding Program for Next Generation World-Leading Researchers (NEXT Program) of Japan.
Keiichiro Homma, Makoto Kambara, Toyonobu Yoshida (2014) “High throughput production of nanocomposite SiOx powders by plasma spray physical vapor deposition for negative electrode of lithium ion batteries,” Sci. Technol. Adv. Mater. 15 025006 doi: 10.1088/1468-6996/15/2/025006