Prediction of Electrochemical Performance in Na-NiCl2 Battery Systems Yihan Xu1, Changsoo Kim1* 1Materials Science and Engineering Department, UWM, USA April 8th, UWM Poster Competition 2017, Milwaukee

Sodium Nickel Chloride (Na-NiCl2) Batteries High energy capacity (large cells), Low manufacturing cost, High public safety  developing a lower-temperature Na-beta alumina battery (LT-NBB) can secure the stability/safety and to maintain high performance of Na-NiCl2 batteries. 10 µm Ni, 190oC NaCl, 190oC Ni, 280oC 100 µm NaCl, 280oC SEM images of cathode materials after 200 cycles (Ni/NaCl ratio of 1.8) [1] [1] Li G, et al. Nature Communications, 2016.

Objectives and Methods We introduce a 2D continuum computational model to quantitatively investigate the transport of Na during the charging process (i.e., the usage of Na in NaCl, Na ion concentration distribution and Na ion flux) in the cathode area of a planar Na-NiCl2 battery. [2] M. Salanne, et al. Faraday Discussions, 2012. [3] M. Sudoh, et al. Journal of The Electrochemical Society, 1990.

Usage and Depletion of Na in NaCl in Cathode Usage of Na near BASE Depletion of available Na ions is found during the charging process when the charging current densities are 10 mA/cm2 and 30 mA/cm2  Such prediction is in consistent with experimental observations

Na Ion Concentration Distributions upon Charging Charging current density = 30 mA/cm2 Unit: relative Na concentration w.r.t. initial 200°C 250°C 300°C DOD=0.67 DOD=0.33 DOD=0 Arrow – Flux: Gray – Migration; Black - Diffusion

Usage of Na Near BASE Area Relative contribution from ionic diffusion and migration for the Na ion flux

Impacts of Ni/NaCl Ratio Usage of Na near BASE Ni/NaCl ratio= 1.7 and 1.4 With a lower Ni/NaCl ratio, the Na ion depletion phenomena near BASE at high charging current densities could be mitigated, which will increase the cell capacity

Summary By developing a 2D continuum computational model to predict the Na ion transport of a planar Na-NiCl2 battery: The usage of Na in NaCl, Na ion concentration distribution and Na ion flux in the cathode can be quantitatively investigated. Increasing current density can accelerate the transport process of Na ions significantly. However, charging current density of 30 mA/cm2 can lead to the depletion of the available Na ions near the BASE area during the charging process, which may explain the low measured capacity (compared with ideal theoretical capacity). The diffusion and migration of Na ion are increased with increasing temperature. Na ions are majorly transported through ionic migration. A lower Ni/NaCl ratio will increase the cell capacity. As for the future work, we are in the process of developing a 3D model to elucidate the evolution of Ni and NaCl particles at various temperatures. example of 3D cathode structure (cathode granule)

Acknowledgements This work was supported by POSCO, Korea Institute of Energy Technology Evaluation and Planning (KETEP) of the Republic of Korea, and US Department of Energy (US DoE) (No. 20158510050010). The authors also acknowledge the experimental partners, RIST (Korea) and PNNL.