Yoongu Kim

Nanoscale mapping of ion diffusion in a lithium-ion battery cathode

The movement of lithium ions into and out of electrodes is central to the operation of lithium-ion batteries. Although this process has been extensively studied at the device level, it remains insufficiently characterized at the nanoscale level of grain clusters, single grains and defects. Here, we probe the spatial variation of lithium-ion diffusion times in the battery-cathode material LiCoO(2) at a resolution of ∼100 nm by using an atomic force microscope to both redistribute lithium ions and measure the resulting cathode deformation. The relationship between diffusion and single grains and grain boundaries is observed, revealing that the diffusion coefficient increases for certain grain orientations and single-grain boundaries. This knowledge provides feedback to improve understanding of the nanoscale mechanisms underpinning lithium-ion battery operation.

Real Space Mapping of Li-Ion Transport in Amorphous Si Anodes with Nanometer Resolution

The electrical bias driven Li-ion motion in silicon anode materials in thin film battery heterostructures is investigated using electrochemical strain microscopy (ESM), which is a newly developed scanning probe microscopy based characterization method. ESM utilizes the intrinsic link between bias-controlled Li-ion concentration and molar volume of electrode materials, providing the capability for studies on the sub-20 nm scale, and allows the relationship between Li-ion flow and microstructure to be established. The evolution of Li-ion transport during the battery charging is directly observed.

Decoupling electrochemical reaction and diffusion processes in ionically-conductive solids on the nanometer scale.

We have developed a scanning probe microscopy approach to explore voltage-controlled ion dynamics in ionically conductive solids and decouple transport and local electrochemical reactivity on the nanometer scale. Electrochemical strain microscopy allows detection of bias-induced ionic motion through the dynamic (0.1-1 MHz) local strain. Spectroscopic modes based on low-frequency (∼1 Hz) voltage sweeps allow local ion dynamics to be probed locally. The bias dependence of the hysteretic strain response accessed through first-order reversal curve (FORC) measurements demonstrates that the process is activated at a certain critical voltage and is linear above this voltage everywhere on the surface. This suggests that FORC spectroscopic ESM data separates local electrochemical reaction and transport processes. The relevant parameters such as critical voltage and effective mobility can be extracted for each location and correlated with the microstructure. The evolution of these behaviors with the charging of the amorphous Si anode in a thin-film Li-ion battery is explored. A broad applicability of this method to other ionically conductive systems is predicted.

Properties of Lithium Phosphorus Oxynitride (Lipon) for 3D Solid-State Lithium Batteries

Abstract The thin film electrolyte known as Lipon has proven successful for planar thin film battery applications. Here the sputter deposition of the amorphous lithium phosphorous oxynitride electrolyte onto more complex 3D structures is examined. The 3D structures include off-axis alignment of planar substrates and also 10-100 m arrays of pores, columns and grooves. For magnetron sputtering in N2 gas at 2.6 Pa, the Lipon film deposition is not restricted by the line-of-sight to the target, but forms conformal and dense films over the 3D and off-axis substrates. The deposition rate decreases for areas and grooves that are less accessible for the sputtered flux. The composition varies, but remains within the range that gives sufficient Li+ ionic conductivity, 2 1 S/cm