A path to better lithium-ion battery performance

The amount of energy that a lithium-ion battery can store depends on how many Li⁺ ions can be inserted into and extracted from the anode and cathode. (See the article by Héctor Abruña, Yasuyuki Kiya, and Jay Henderson, Physics Today, December 2008, page 43 .) Electrodes typically consist of spherical nanoparticles and a labyrinthine network of electrolyte-filled pores in between. Thicker electrodes can accommodate more Li⁺, but slow ion transport through the pores limits electrode thickness to less than 0.1 mm.

To make the pores less tortuous and thus enable thicker electrodes, MIT’s Yet-Ming Chiang and his colleagues have turned to magnetic alignment. In one approach, they emulsified a ferrofluid in a suspension of lithium cobalt oxide, a common cathode material, and then applied an external magnetic field. The fine emulsion droplets aligned themselves into chains; drying the suspension removed the ferrofluid and left behind a solid cathode full of straight pores. In another approach, nylon microrods coated with magnetic nanoparticles served as the templates. Heating the dried cathode burned away the microrods to, again, leave behind straight pores. The scanning electron microscopy images show cathodes made with the microrod (top) and emulsion (bottom) techniques.

Independently, a collaboration led by Claire Villevieille at the Paul Scherrer Institute and André Studart at ETH Zürich, both in Switzerland, took a different tack on magnetic alignment. Rather than trying to line up the pores, the researchers opted to align the electrode material itself. They attached magnetic nanoparticles to flakes of graphite, the most common anode material for lithium-ion batteries, and lined them up with a rotating magnetic field. At sufficiently large rotation rates, the flakes fell into alignment with their faces parallel to the plane of the field’s rotation. The pores between them thus present an easy path for the Li⁺ ions to get to the graphite.

Both groups produced 0.2 mm or thicker electrodes that more than doubled the storage capacity of similarly made but unaligned reference electrodes. (J. S. Sander et al., Nat. Energy 1, 16099, 2016 ; J. Billaud et al., Nat. Energy 1, 16097, 2016 .)