At the NorthEast Center for Chemical Energy Storage (NECCES), researchers are exploring new avenues for rechargeable lithium-ion battery chemistry, and the results point the way to more efficient energy storage. Using a novel computational approach that examined reactions at multiple time and length scales, the group identified fundamental factors that determine the reaction pathways taken during battery charging and discharging. The disparity between the charging and discharging reactions is thought to be a major cause of a phenomenon, called hysteresis, which ultimately results in unacceptable energy loss.
As the demand for higher capacity lithium-ion batteries for applications such as mobile electronics or electric vehicles continually grows, new battery chemistries are being sought out. Current industrial batteries rely on intercalation processes in the cathode, which require a host material that readily incorporates lithium with minimal changes to its structural framework. To overcome the capacity limitations inherent to these cathodes, other reaction mechanisms are being considered, including conversion and displacement reactions. In the displacement reaction process, the host framework is retained just as in intercalation reactions, although as the lithium is inserted, there is a concomitant extrusion and/or precipitation of another metal from the cathode. This opens up more possible lithium sites, meaning more energy storage.
The team chose a material made from copper, titanium, and sulfur atoms, Cu0.5TiS2, as their model system because it is known to display the undesirable hysteresis in the voltage curve when used in a battery. Computer modeling of the material started with a view at the atomic level. The team investigated which sites the lithium and copper ions prefer and how each would migrate from site to site within the titanium and sulfur, TiS2, framework. Using computer simulations, specifically kinetic Monte Carlo simulations that provided key input to phase field simulations, the team scaled up their results to length and time scales relevant for the charging and discharging of a cathode particle as in a rechargeable battery.
The results of these simulations provide a comprehensive description of the disparate charging and discharging reaction pathways that underlie the observed hysteresis. For the discharge simulation, the lithium insertion required a simultaneous expulsion of the copper to the surface of the electrode particle. The charging simulations, however, showed that the rate of the copper returning into the particle greatly lagged that of the lithium moving out. The lag is a result of the copper having a much lower mobility than the lithium and a lack of "drive" for the reinsertion reaction. Because of this, much more electrical work must be put into the battery than can later be used to power a device.
Through their calculations, the team points out that this problem can be substantially reduced by carefully choosing future cathode materials. The research could lead to streamlining the process of searching for new materials by identifying the atomic-scale metrics that will minimize the energy wasting hysteresis. This pioneering study across multiple scales is relevant to any displacement reaction and has pointed the way toward energy-efficient high-capacity rechargeable batteries.
