3D Mapping of Lithium in Rechargeable Batteries
Authors: Wang Howard , He Yuping , Downing Gregory 
Published: February 2020

Rechargeable Li-ion batteries (LIBs) are an indispensable technology in modern life for efficient energy storage; they are deployed in large numbers and operate close to their electrochemical stability limits. Detrimental reactions at battery electrodes can cause capacity fade and even catastrophic failures, hindering the widespread use of LIBs, particularly in mobile power energy solutions such as all-electric vehicles. It is valuable to visualize the distribution and flow of the active element, lithium, in electrodes for better diagnosis of battery functioning and failure.  Specifically, to address issues of the degradation of electrodes and growth of Li metal dendrites, the neutron depth profiling (NDP) technique was applied with two dimensional (2D) pinhole aperture scans to spatially map lithium in 3D. NDP is a neutron activation analysis method used to quantitatively measure the abundance and depth distribution of several technologically important elements (Li, B, N, He, Na, etc). The technique has been used to measure the Li distribution in modern battery technologies based on the nuclear reaction, 6Li + n → a (2055 keV) + 3H (2727 keV). Charged a and triton (3H) particles lose kinetic energy during transport through the specimen media, which is measured to determine the depth of the activation reaction. The normalized counts of activation events are recorded to determine the abundance of Li at the corresponding depth. 

The method has been applied to measuring the spatial distribution of Li in two LixFePO4 electrode films. Because NDP is sensitive only to the 6Li isotope of Li, which has a natural isotope abundance of 7.5 at%, isotope enrichment of LFP was used to greatly improve mapping efficiency. Two electrodes were prepared, one with a single charge/discharge cycle and the other with 5532 cycles, denoted as LFP1 and LFP5k, respectively. It was shown that the Li concentration is high and homogeneous in LFP1, with x = 0.65 ± 0.06 and Li concentration variation of 9%, where that in LFP5k much lower and spatial distribution much more heterogeneous, with x = 0.38 ± 0.05 and Li concentration variation of 13%.  The significant capacity decay in the LFP5k electrode is discussed in the context of structural changes in electrode materials due to electrochemical processes.

The nature of Li dendritic growth in polymeric electrolytes has also been investigated.  A symmetric sandwich cell of Li / poly(ethyleneoxide) (PEO) : lithium bis(trifluoromethane)sulfonamide (LiTFSI) /  Li was used to as a model system in this study.  In situ NDP measurements has been carried out during directional Li pumping from the bottom Li electrode to the top at a constant electric current of 0.1 mA.  After a period of steady Li plating, dendrites start to grow, and eventually short-circuit the polymer electrolyte.  Li mapping studies reveal rather heterogeneous lateral distribution of Li over length scales from below a millimeter to centimeters.  The lateral mobility of Li appears to be large and the deposited Li layer on top electrode partly deform from its original circular shape.  Most Li in the electrolyte layer resides in dendrites growing from the top electrode, with overall composition decrease linearly from the electrode interface to the bulk of the electrolyte.  It is observed that dendrites also grow from the bottom electrode, where presumably only Li oxidation reaction occurs.  The revelation poses new design and engineering challenges in using Li metal electrode in future batteries.

Figure 1