Yuanyuan Xie

The impedance behavior of a commercial Nickel-cobalt-aluminum(NCA) cylindrical cell was studied in this work. A series of impedance experiments was designed to explore the impact of battery state of charge (SOC) and other operating conditions. Our results indicated the impedance response of this cylindrical battery varies with the state of charge in both charge and discharge process. Experimental results also showed the SOC dependent impedance existed in different operating temperatures and C-rates. An impedance model was developed to explore the potential mechanisms behind experimental observations. Model results showed that the operating temperature and charge-discharge rate could significantly influence the impedance response of the battery but could not directly cause the variation of the impedance response at different SOC conditions. Results also showed that the contact resistance variation, resulted from the expansion and contraction of cell electrodes during charge and discharge, could lead to a SOC dependent impedance behavior of the commercial cell and influence battery output performance.

High voltage Li_xNi_0.5Mn_1.5O₄ (LNMO) spinel is an attractive cathode material for Li-ion battery to meet today’s need of high energy density. One of issues that challenges the implementation of LNMO is the invariant voltage with state of charge in the charge-discharge operations of a battery. This is believed to be originated from two consecutive phase transition processes of LNMO in the cycling. In this study, a mathematical model is developed to understand the impact of multiple characteristic particles on the voltage behavior of LNMO electrode in the charge-discharge process, which couples local phase change with lithium transport process. Effect of varied particle size on the electrode voltage behavior and charge-discharge capacity is simulated and analyzed by the developed model. Obtained results indicate that varied size range of LNMO particles can significantly influence the electrode performance and discharge capacity at different operating conditions. Further, mixed particles can influence the path-dependent voltage behavior of the LNMO electrode due to the phase transition. The comparison between this developed model and current existing models will be discussed at last, especially on the prediction of voltage behavior of multiple particle size LNMO electrode.

PH distribution and non-equilibrium state of water in hydrogen evolution reaction are studied using continuum models. In the first model, we analyze the pH distribution in a rotating ring disk electrode system, where the hydrogen evolution occurs on the disk electrode. The model predicts a pH distribution comparable to the experimental data and the nonequilibrium state of water (cH^*cOH>1.0 × 10⁻¹⁴) in a small portion of the diffusion layer (ca. 5 μm in thickness) adjacent to the bulk electrolyte under forced convection. The second model explores the pH distribution on an electrode with nanovoids in hydrogen evolution reaction in an acidic media. The value of cH^*cOH shifts significantly when close to the electrode surface, e.g., ≤ 2 μm, indicating pH is not viable to assess its impact on an electrochemical reaction involving hydroxide ions. Modeling results also prove that, for an electrode with nanovoids, the concentration gradient of hydroxide between the plain field and the bottom of the nanovoid is minimal. Therefore it should not be the root cause for the differential kinetics of metal electrodeposition inside/outside the nanovoids.

The electrochemical performance of cathode materials for Li-ion batteries depends on the morphology of the material, which, in turn, depends on the synthesis conditions. Very few studies have focused on the impact of process conditions on the final morphology of the cathode particles and analyzed the growth during synthesis. In this paper, the evolution of nickel, manganese, and cobalt hydroxide precursor, Ni_1/3Mn_1/3Co_1/3(OH)₂, is investigated using a combination of in situ and ex situ techniques during the commonly-used coprecipitation process. These include in situ wide angle X-ray scattering, in-situ ultra-small angle X-ray scattering and ex situ particle size analysis. The growth rate of crystalline Ni_1/3Mn_1/3Co_1/3(OH)₂ primary particle is found to be almost constant, consistent with a mathematical analysis of process. The growth of the Ni_1/3Mn_1/3Co_1/3(OH)₂ secondary particle and its particle size distribution revealed different growth stages for samples prepared at different pH. These techniques are complimented with scanning electron microscopy and electrochemical testing to track the morphology and performance of the hydroxide particle and the subsequent calcined LiNi_1/3Mn_1/3Co_1/3O₂ cathode active material. This study presents insights into the synthesis process and provides a deeper understanding to aid in process optimization.