Yiling Dai

In order to accurately predict the behavior or electrochemical devices, it is necessary to develop sophisticated models that take into consideration transport processes, electrochemical phenomena, mechanical stresses, and structural deformations on the operation of an electrochemical system. Many different models exist that can predict the electrochemical performance of these devices under a variety of operating and design conditions.[1-7] However, many of these models assume constant porosity and do not take into consideration the effects of a variable porosity on the porous electrode during cycling and what effects that may have on performance. In recent years, electrodes have been developed that show a significant volume change during intercalation and deintercalation, which are unable to be accurately predicted using these constant porosity models. Porosity and dimensional changes in an electrode can significantly affect the resistance of the battery during cycling and cause premature failure of the battery due to generated stresses. Previously, we have shown the ability to incorporate dimensional and porosity changes in a porous electrode into a model to predict volume changes in the active material during intercalation through the coupling of porous rock mechanics and porous electrode theory.[6] However, that model incorporated many assumptions in order to obtain an analytical solution, including the assumption of bulk stress, strain, and porosity. Here, we present a model that removes those assumptions and illustrates the coupling of porous electrode theory and rock mechanics in order to predict stress, strain, and porosity gradients across a porous electrode during cycling. 1. M. Jain and J. W. Weidner, J Electrochem Soc, 146, 1370 (1999). 2. M. Jain, G. Nagasubramanian, R. G. Jungst and J. W. Weidner, J Electrochem Soc, 146, 4023 (1999). 3. P. M. Gomadam and J. W. Weidner, J Electrochem Soc, 153, A179 (2006). 4. X. C. Zhang, A. M. Sastry and W. Shyy, J Electrochem Soc, 155, A542 (2008). 5. J. Christensen and J. Newman, J Solid State Electr, 10, 293 (2006). 6. T. R. Garrick, K. Kanneganti, X. Y. Huang and J. W. Weidner, J Electrochem Soc, 161, E3297 (2014). 7. K. Higa and V. Srinivasan, J Electrochem Soc, 162, A1111 (2015).

Li₃V₂(PO₄)₃ (LVP) particles dispersed in different inorganic carbons ([email protected]) have been successfully synthesized via an in situ synthesis method. The inorganic carbon materials with different dimensions including zero-dimensional Super P (SP) nanospheres, one-dimensional carbon nanotubes (CNTs), two-dimensional graphene nanosheets, and three-dimensional graphite particles. The effects of carbon dimensions on the structure, morphology, and electrochemical performance of [email protected] composites have been systematically investigated. The carbon materials can maintain their original morphology even after oxidation (by NH₄VO₃) and high-temperature sintering (850 °C). [email protected] exhibits the best electrochemical performances among all of the samples. At an ultrahigh discharge rate of 100C, it presents a discharge capacity of 91.94 mAh g^–1 (69.13% of its theoretical capacity) and maintains 79.82% of its original capacity even after 382 cycles. Its excellent electrochemical performance makes [email protected] a promising cathode candidate for lithium-ion batteries.

Degradation phenomena are not distributed uniformly in a large-format cell. To better understand the inhomogeneous degradation in large-format cells, a two-dimensional model was developed for a LiMn₂O₄ (LMO)/Carbon cell. The model includes both the non-uniform porous electrode properties and the electrode mismatch. The simulation results show that when the anode edge is extended over the cathode edge, the LMO particles near the edge suffer larger potential difference, larger charge/discharge depth, and higher insertion-induced stress. Therefore, the predicted loss of LMO is more pronounced near the edge as has been seen experimentally. The influence of different design adjustable parameters (such as: the anode extension length, the capacity ratio, the porosity, and the electrode thickness) and electrolyte properties (such as: the diffusion coefficient and the ionic conductivity) on the cathode performance. Among them, the over-potential behavior of the cathode is most sensitive to the extent of the electrode mismatch. Larger anode extensions will increase the possibility of the LMO degradation at the cathode edge. This suggests that a longer extension of the anode is not better for improvement of cell life. An optimal design of the anode extension length should be carried out.

A mathematical model for the capacity fade of a LiMn₂O₄ (LMO) electrode is developed in this paper by including the acid attack on the active material and the solid electrolyte interphase (SEI) film formation on the LMO particle surface. The acid generated by the LiPF₆ and the solvent decompositions are coupled to the manganese (Mn) dissolution. The decrease of the Li ion diffusion coefficient is involved as another contribution to the capacity fade, which is caused by the passive film formation on the active material surface. The effects of cell practical operation/fabrication conditions and kinetics of side reactions on battery life are also investigated by utilizing the developed mathematical model.