Rice University, Lockheed Martin researchers developing silicon thin films for cost-effective high-performance Li-ion battery anodes

Comparison of discharge capacity and coulombic efficiency vs. cycle number for the freestanding MPSF and MPSF with pyrolyzed PAN composite during galvanostatic charge/discharge tested between 0.07-1.5V at 200 μA cm-2. Credit: ACS, Thankur et al. Click to enlarge.

Researchers at Rice University and Lockheed Martin have developed a method for creating macroporous silicon thin films (MPSF) for use as cost-effective and high-performance anode materials in Li-ion batteries. The researchers infiltrate a polymer binder, polyacrylonitrile (PAN), to these thin films. When PAN is pyrolyzed, it forms a conjugated-chain chemical structure with a specific capacity for lithium.
Experimental results reported in a paper in the ACS journal Chemistry of Materials indicate that a composite formed from freestanding MPSF and pyrolyzed PAN can deliver a specific capacity of 1,200 mAhg^-1 and a better cycle life and coulombic efficiency compared to bare MPSF.

...considerable effort have been made to improve the performance of Lithium Ion Batteries (LIBs). Silicon has become an attractive material because it can react with lithium to form binary alloys with a maximum uptake of 4.4 lithium atoms per silicon atom, Li₂₂Si₅. At room temperature, the highest achievable specific capacity for silicon is 3,579 mAhg^-1, far greater than the theoretical capacity of 372 mAhg^-1 for graphite, which is the most commonly used anode material. However, lithium alloying with silicon results in a large volume change (~280%), which results in severe cracking in the silicon and eventual electrode failure.

Several strategies have been developed to accommodate this severe volume expansion, including silicon sub micrometer pillars, silicon nanowires, silicon carbon composite, crystalline- amorphous silicon nanowires, porous thin films and silicon nanowire arrays attached to silicon substrates, which have all been considered as promising candidates. Among these designs, the film structures remain one of the most cost-effective, since it is compatible with common microfabrication techniques for easy scale up. The challenge with silicon films is that oftentimes, the adherence of these films to the current collector, one of the key factors in electrochemical performance, is poor. Many groups have included additional surface treatments, such as metal coatings, but this adds costly processing steps. Another limitation with silicon films has been that they oftentimes include a bulk silicon substrate that does not contribute to the specific capacity, leading to an increase in the weight of the anode. Some groups have removed this bulk silicon substrate through backside chemical etching processes, but at the expense of possibly useful silicon material.

—Thankur et al.

Unlike structured porous silicon or silicon nanowire arrays attached to a substrate, the Rice/Lockheed Martin thin films are electrochemically removed from the bulk silicon substrate. The researchers fabricate this freestanding MPSF by the electrochemical etching of silicon wafer in hydrofluoric acid (HF) solution using a multi-step lift-off procedure. The team noted that at least four films can be drawn from a standard 250-micron-thick wafer.

A macroporous silicon film (MPSF) is electrochemically detached from an underlying bulk silicon and combined with pyrolyzed polyacrylonitrile (PAN) to form a composite anode for lithium ion batteries. Credit: ACS, Thankur et al. Click to enlarge.

This leads to materials that are significantly lighter. Another benefit is that multiple porous silicon liftoff procedures can be performed using the same wafer, leading to little silicon waste. For their anodes, the team typically fabricated MPSFs having an average port diameter of 1.5 µm and a thickness of 50 µm.
After soaking in a PAN solution for 24 hours, the MPSF/PAN film was heated to 550 °C for 1 hour to pyrolyze the PAN. They then compared the performance of bare freestanding MPSF and MPSF with pyrolyzed PAN.
The discharge capacity for first cycle of MPSF/pyrolyzed PAN was 850 mAhg^-1, whereas that for the bare MPSF was 757 mAhg^-1. The bare MPSF rapidly dropped in capacity after the second cycle and is at 200 mAhg^-1 by cycle 10 and completely failed by cycle 15. For MPSF with pyrolyzed PAN, the discharge capacity increased for the first four cycles to a discharge capacity of 1,260 mAhg^-1 and remained constant through cycle 20. This initial increase in capacity is typical for porous silicon films, the researchers noted. Pyrolyzed PAN alone has a capacity of 55 mAhg^-1 after the first cycle, indicating that the PAN contribution to the capacity is small, they said.
They also found that the average coulombic efficiency for MPSF with pyrolyzed PAN is 95% after the first cycle, but dropped to 90% after 17 cycles. They concluded that there is an irreversible capacity fade correlating with the coulombic efficiency, most likely related to electrolyte decomposition and an increase in thickness of the solid electrolyte interface (SEI) layer. Further work is underway to determine if direct polymerization of PAN into MPSF instead of solution infiltration would further improve the electrochemical performance.
The work was supported by the Lockheed Martin Advanced Nanotechnology Center of Excellence at Rice.



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