In Situ Hard x-Ray Nano-Tomography Study of Lithium Ion Batteries
Authors: Wang Jiajun , Chen-Wiegart Yu-chen Karen , Wang Jun 
Published: February 2020

Lithium-ion batterires (LIBs) are based on insertion reaction chemistry which introduces microstructural changes in host materials via the lithiation and delithation process Particularly, the induced large stain at anode materials (such as widely studied silicon and tin based anodes) can lead to large volume change, fracture, and pulverization, which is considered as a major challenge for anode materials of LIBs (1-2). To address the mechanical degradation requires a fundamental understand of the mechanisms of microstructural change in the electrode as a function of cycling and observing the morphological/structural evolution during the operation of a battery. In-situ measurements have yielded valuable information about the function of electrochemical cells. Since the 3D orientation nature of battery materials in electrodes and possible anisotropy electrochemical reaction, a 3D in-situ visualization technique with nano-scale resolution (battery size requirement) is needed.

Synchrotron X-ray imaging is non-destructive, element sensitive, environmental friendly, and highly penetrating. In particular, hard x-ray with high penetration energy holds larger potentials to image micron sized battery materials with a large sample thickness. Our newly-developed TXM at X8C beamline, National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (BNL), surpasses the TXMs at other synchrotron facilities in performance, providing nano-scale resolution (sub-30 nm at 2D, sub-50 nm at 3D), local tomography, larger field of view, and automated markerless image acquisition/alignment, which allows for detailed 3D quantitative analysis at nano-scale, including volume change, feature size, curvature, chemical information evolution and stress evaluation (3-5). The major challenge is developing a suitable electrochemical cell which works like a real battery but meet the field of view (40x40 µm), allows a 180-degree rotation without blocking x-ray beam, enables to be sensitive enough to record the current density and measure the electrochemical performance, and undergoes long-term cycling (tens of cycles).

Here, we report the implementation of in situ tomography studies of LIBs, aided by a well-designed micron-scaled electrochemical cell consisting of anode particles, a commercial carbonate electrolyte, Li metal as a counter/reference electrode. Different from widely used open nano-battery in in situ TEM study, our designed battery perfectly simulates the true working circumstance and reflect truly the electrochemical reaction in a working LIB. To begin with, our cell allows performing various conventional electrochemical characterizations and analysis. Secondly, replacing carbon paper with conventional Al or Cu foil as the electrode support allows better x-ray penetration, so high-quality tomography with high resolution can be guaranteed. Thirdly, our studying materials are particle shaped (not special nanowire structure) widely applied in battery industries. In addition, our robust battery can undergo considerable stability, and allow long time electrochemical cycling and x-ray characterization.

The 3D morphological evolutions of the Sn particles during the first two electrochemical cycles are shown in Fig. 1. In spite of volume expansion, the first lithiated Sn particles mainly keep the overall structure intact. However, severe pulverization and fracture are observed during the first delithiation, when lithium ion extraction and volume shrinkage occur, and during the lithium ion insertion and the volume expansion that occurs in the second lithiation that ensues. After the second lithiation, the particles reach structural equilibrium and no significant changes develop. The fracture and pulverization, on one hand, favor high electrolyte permeation, contributing to lithium ion insertion and associated volume expansion at the second lithiation step. On the other hand, they lead to collapse during the volume expansion that occurs in the second lithiation, thereby few lithium ions are extracted during the second delithiation process. The statistics information at 3D about true volume change, specific area, particle size and curvature will be discussed. A simulation about the stress distribution and the relation to surface curvatures will also be presented.

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