Evolution of Dynamic Heterogeneities in Phase-Separating Electrodes during Ion Intercalation
Authors: Agrawal Shubham , Bai Peng 
Published: February 2020

Lithium-ion batteries (LIBs) have undergone fast development during the past decades and will continue to be the dominant energy storage. Meanwhile, sodium-ion and potassium-ion batteries are gaining momentum due to the abundance and lower costs of the respective alkali metals. Many ion-intercalation electrodes used in these alkali-ion batteries exhibit strong reaction heterogeneities, but the underlying mechanisms are yet to be fully understood. Graphite, the negative electrode in commercial LIBs, is known to form ordered stages upon intercalation of lithium ion[4], which are optically visible as blue, red and gold colors[5
,
6], making it one of the best candidates to observe the reaction heterogeneity. In this study, we present ex-situ optical imaging of thin-film graphite electrodes at different state-of-charges (SOCs) during constant-current lithiation process. The thin-film, with the loading of active material <0.5 mg/cm2, ensures monolayer of active particles which can all be assumed connected in parallel, subjected to the same electrical and electrolytic environment. The stable Li-poor and Li-rich phases are blue and gold respectively, while the current carrying active particles are red in color. The analysis shows that the empty graphite particles turn into stable Li-poor phase (blue) homogeneously by ~20% SOC. On increasing the SOC, the blue particles undergo selective activation and become red one by one. At ~60% SOC, maximum population of red particles is observed followed by their rapid and selective transformation into fully-filled Li-rich (gold) phase due to phase separation. Figure 1 shows representative ex-situ optical images of graphite electrodes at a few SOCs along with the fraction of observed colored areas. The conservation and inter-particle exchange of lithium ions between different ordered stages can be clearly explained through a stochastic Markov chain model[7]. A multi-particle phase-field model is also to be developed to further aid in the understanding of the dynamics. Our work provides a platform for determining the competition between activation and transformation rates in graphite particles along with the exact measurement of actual area of evolving active interface/particles available for the electrochemical reaction, which will facilitate the rational design of heterogeneous particulate porous electrodes.

References

Li, Yiyang, et al. "Current-induced transition from particle-by-particle to concurrent intercalation in phase-separating battery electrodes." Nature materials 13.12 (2014): 1149.

Gent, William E., et al. "Persistent State‐of‐Charge Heterogeneity in Relaxed, Partially Charged Li1−xNi1/3Co1/3Mn1/3O2 Secondary Particles." Advanced Materials 28.31 (2016): 6631-6638.

Tian, Chixia, et al. "Charge heterogeneity and surface chemistry in polycrystalline cathode materials." Joule 2.3 (2018): 464-477.

Dresselhaus, Mildred S., and G. Dresselhaus. "Intercalation compounds of graphite." Advances in physics 51.1 (2002): 1-186.

Thomas-Alyea, Karen E., et al. "In situ observation and mathematical modeling of lithium distribution within graphite." Journal of The Electrochemical Society 164.11 (2017): E3063-E3072.

Guo, Yinsheng, et al. "Li intercalation into graphite: direct optical imaging and Cahn–Hilliard reaction dynamics." The journal of physical chemistry letters 7.11 (2016): 2151-2156.

Bai, Peng, and Guangyu Tian. "Statistical kinetics of phase-transforming nanoparticles in LiFePO4 porous electrodes." Electrochimica Acta 89 (2013): 644-651.

Figure 1