Ion-transport visualization points toward a better battery

Rechargeable lithium-ion batteries power devices large and small, from electric vehicles to smartphones. Lithium ions from one electrode are donated to another across a liquid; the stream of ions reverses course during the recharge cycle. Using pure lithium metal for one electrode can improve efficiency, but it comes with a cost: The nonuniform flux of ions near that electrode promotes nucleation and the growth of needle-like lithium crystals that sap efficiency and can even trigger explosions. Measuring the growth of those crystals requires imaging the movement of ions, a difficult task due to low ionic concentrations and fast reaction times. Qian Cheng (Columbia University), Lu Wei (now at Caltech), and colleagues have overcome those obstacles with a technique already widely used in the biomedical community: stimulated Raman scattering microscopy.

Standard Raman scattering determines a molecule’s characteristic vibrational frequency by measuring the energy from spontaneous photon scattering. Stimulated Raman scattering amplifies the photon signal by adding a Stokes laser to the pump laser source, as illustrated here, and matching the energy difference between them to the molecules’ vibrational transition. The researchers applied the technique to measure the wavenumber intensity of an organic anion molecule. With a similar concentration and scale to the lithium cation it bonds with, the anion can be used as an analogue for lithium-ion concentrations. When the researchers analyzed the environment surrounding the lithium electrodes, they discovered a feedback mechanism: Regions of higher ion concentration experienced a greater growth rate, which in turn further increased ion concentration and accelerated crystal growth. Spurred by that insight, the researchers devised a lithium phosphate coating strong enough to suppress protrusions on the electrodes. Once installed, the coating acted as a buffer that equalized the local lithium-ion concentration and decreased the crystal growth rate without altering the voltage profile. Now that materials researchers know what stimulated Raman scattering microscopy is capable of, they can use the imaging technique to investigate ion transport and catalytic processes in many applications. (Q. Cheng et al., Nat. Commun. 9, 2942, 2018 ; illustration courtesy of Qian Cheng/Columbia Engineering.)

Editor’s note, 9 August: The second and third sentences of the article have been updated to correct the composition of the electrodes of a lithium-ion battery.