Tyler Evans

We are currently in the midst of a race to discover and develop new battery materials capable of providing high energy-density at low cost. By combining a high-performance Si electrode architecture with a room temperature ionic liquid electrolyte, here we demonstrate a highly energy-dense lithium-ion cell with an impressively long cycling life, maintaining over 75% capacity after 500 cycles. Such high performance is enabled by a stable half-cell coulombic efficiency of 99.97%, averaged over the first 200 cycles. Equally as significant, our detailed characterization elucidates the previously convoluted mechanisms of the solid-electrolyte interphase on Si electrodes. We provide a theoretical simulation to model the interface and microstructural-compositional analyses that confirm our theoretical predictions and allow us to visualize the precise location and constitution of various interfacial components. This work provides new science related to the interfacial stability of Si-based materials while granting positive exposure to ionic liquid electrochemistry. Silicon is a promising anode material for lithium ion batteries, but suffers from structural degradation during operations. Here, the authors combine silicon with a room temperature ionic liquid to stabilize the electrode-electrolyte interface and achieve long-term cyclability.

High-energy-density FeS2 cathodes en-abled by a bis(trifluoromethanesulfonyl)imide (TFSI-) anion-based room temperature ionic liquid (RTIL) electrolyte are demonstrated. A TFSI-based ionic liquid (IL) significantly mitigates polysulfide dissolution, and therefore the parasitic redox shuttle mechanism, that plagues sulfur-based electrode chemistries. FeS2 stabilization with a TFSI(-) -based IL results in one of the highest energy density cathodes, 542 W h kg(-1) (normalized to cathode composite mass), reported to date.

A two-dimensional thermal model for spirally wound batteries has been developed. The governing equation of the model is the energy balance. Convective and insulated boundary conditions are used, and the equations are solved using a finite element code called TOPAZ2D. The finite element mesh is generated using a preprocessor to TOPAZ2D called MAZE. The model is used to estimate temperature profiles within a spirally wound D-size cell. The model is applied to the lithium/thionyl chloride cell because of the thermal management problems that this cell exhibits. Simplified one-dimensional models are presented that can be used to predict best and worst temperature profiles. The two-dimensional model is used to predict the regions of maximum temperature within the spirally wound cell. Normal discharge as well as thermal runaway conditions are investigated.

Mathematical models which have been developed to study various aspects of the zinc/bromine cell and stack of cells are reviewed. Development of these macroscopic models begins with a material balance, a transport equation which includes a migration term for charged species in an electric field, and an electrode kinetic expression. Various types of models are discussed: partial differential equation models that can be used to predict current and potential distributions, an algebraic model that includes shunt currents and associated energy losses and can be used to determine the optimum resistivity of an electrolyte, and ordinary differential equation models that can be used to predict the energy efficiency of the cell as a function of the state of charge. These models have allowed researchers to better understand the physical phenomena occurring within parallel plate electrochemical flow reactors and have been instrumental in the improvement of the zinc/bromine cell design. Suggestions are made for future modeling work.