Simulation and Optimization of the Dual Lithium Ion Insertion Cell

The galvanostatic charge and discharge of a dual lithium ion insertion (rocking-chair) cell are modeled. Transport in the electrolyte is described with concentrated solution theory. Insertion of lithium into and out of the active electrode material is simulated using superposition, greatly simplifying the numerical calculations. Simulation results are presented for the Li=C~lpropylene carbonate +IM LiC1OJLiyMn204 cell, and these results are compared with experimental data from the literature. Criteria are established to assess the importance of diffusion in the solid matrix and of transport in the electrolytic solution. Various procedures to optimize the utilization of active material are considered. Simulation results for the dual lithium ion insertion cell are compared with those for a cell with a solid lithium negative electrode. The storage and conversion of energy continues to be important to society. Batteries, which interconvert chemical and electrical energy, are widely used in industry and for consumer applications (e.g., appliances and laptop computers). At the same time, environmental concerns are reshaping many industries. The ecological hazards of batteries through their operation and disposal is a primary consideration for battery manufacturers. In addition, stricter emission standards on automobiles are spurring interest in batteries for electric-vehicle applications. The energy and power requirements for vehicle propulsion are rigorous. 1 Consequently, research on rechargeable battery systems is receiving renewed attention. Lithium batteries are attractive for energy storage because of their high theoretical energy densities. Furthermore, they are less toxic than nickel cadmium or lead acid cells, and their disposal poses fewer environmental problems. Although primary lithium batteries have been massproduced for years, 2 the secondary (rechargeable) lithium cell has been commercialized only recently. 3-~ The typical lithium cell is made up of a lithium metal negative electrode, an electrolyte which serves as an ionic path between electrodes and separates the two materials, and a positive electrode, such as Mn204. In general, a highly reactive material is desired for the negative electrode to give a higher cell potential, and hence a higher theoretical energy density. Unfortunately, the more reactive the electrode material the more likely it reacts irreversibly with the electrolyte. The high reactivity of lithium metal is a significant problem for lithium batteries. Successful lithium battery systems operate due to a protective film that forms on the electrode surface. ~ This protective film retards further reaction with the electrolyte but impacts the capacity and cycle life of the cell through increased resistance and material isolation. The highly reactive lithium metal is a safety issue as well, becoming especially important in larger cells. One alternative scheme has been to replace the lithium metal negative electrode with a lithium alloy or compound, such as: LiA1, 7 LiFe203, s LiW02, 8 or LiC6. 9 Although these materials stabilize the lithium, this reduces the energy density of the cell, since the added material is not used in the