Upgrades In Sight for the Lithium Ion Battery

Sibani Lisa Biswal, an assistant professor of chemical and biomolecular engineering at Rice, and Madhuri Thakur, a Rice research scientist, devised a process by which Swiss cheese-like silicon “sponges” that store more than four times their weight in lithium, can be electrochemically lifted off of silicon wafers.

With Lockheed Martin researcher Mark Isaacson and Rice postdoctoral researcher Roderick Pernites, and alumnus Naoki Nitta the team reported in the American Chemical Society journal Chemistry of Materials, they’ve found a way to make multiple high-performance anodes from a single silicon wafer. The process uses common silicon to replace graphite as an element in rechargeable lithium-ion batteries, laying the groundwork for longer-lasting, more powerful batteries for such applications as commercial electronics and electric vehicles.

The new anode is a tough sponge like film that can be attached to a current collector (in their lab case, a thin layer of titanium on copper) and placed in a battery configuration. The result is a working lithium-ion battery with a discharge capacity of 1,260 milliamp-hours per gram, a capability that should lead to batteries that last longer between charges.

Silicon is one of the most common elements on Earth and is a highly sought after material to replace graphite as the anode in batteries. In a previous advance by Biswal and her team, porous silicon was found to soak up 10 times more lithium than graphite.  That property makes silicon the material of choice if it can be made to work.

The holdup is silicon expands as it absorbs lithium ions and soon breaks apart over the charge and discharge cycles.

Biswal’s sponge-like silicon configuration gives it room to grow internally without degrading the battery’s performance, the team reports in the new paper. The hope that the silicon sponges, with pores a micron wide and 12 microns deep, held for batteries was first discussed in 2010 at Rice’s Buckyball Discovery Conference by Thakur, Biswal, their Rice colleague Michael Wong, a professor of chemical and biomolecular engineering and of chemistry, and Steven Sinsabaugh, a Lockheed Martin Fellow.

But even then Thakur saw room for improvement, as the solid silicon substrate served no purpose in absorbing lithium, limiting capacity and adding weight and complicating the means to attach a current collector.

In the latest paper the team has worked out the electrochemical etching process used to create the pores and they can also separate the sponge from the substrate, which is then reused to make more sponges. The team noted that at least four films can be drawn from a standard 250-micron-thick wafer.

Removing the sponge from the silicon substrate also eliminates a limiting factor to the amount of lithium that can be stored.
The team has also progressed on finding a way make the pores over four times deeper, now 50 microns deep. Once lifted from the wafer, the sponges, now open at the top and bottom, were enhanced for conductivity by soaking them in a conductive polymer binder, pyrolyzed polyacrylonitrile (PAN).

That gets the team to 1,260 milliamp-hours per gram.

Then the team compared batteries using their film before and after the PAN soaking and a bake treatment. Before, the batteries started with a discharge capacity of 757 milliamp-hours per gram, dropped rapidly after the second charge-discharge cycle and failed completely by cycle 15. The treated film increased in discharge capacity over the first four cycles – typical for porous silicon, the researchers said – and the capacity remained consistent through 20 cycles.

That’s where the research stood as the paper was accepted for publication sometime in May or June.

Meanwhile the team is investigating techniques that promise to vastly increase the number of charge-discharge cycles, the critical feature for commercial applications where rechargeable batteries are expected to last for years.

Etching silicon is a common industrial process so this research holds out good prospects for commercial scale if the charge and discharge cycles can be extended. We’re not told how many steps are involved, but computer processors and memory chips are quite complex with many layers and different treatments to reach completion.

An anode should be a much simpler challenge when the chemistry and process is worked out.  If the team can get the silicon to stay together for cycles in the thousands they will have hit the home run for batteries.

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