Compensatory measures to overcome performance limitations of recycled Li-ion battery materials

The increasing demand of lithium-ion batteries for electric vehicles, combined with the lack of critical raw materials and the necessity to dispose of spent batteries, is driving towards the use of recycled cathode active materials in the next generation of batteries. However, closed loop recycling solutions, such as direct recycling methods and co-precipitation, result in cathode materials which have a lower quality than those prepared from virgin precursors, which translate into smaller volumetric capacity and solid-state diffusivity. In order to meet the stringent requirements of the automotive sector, changes in the electrode design are thus required when recycled cathode active materials are used. In this contribution we use a pseudo-2D thermo-electrochemical model to provide design guidelines to overcome the performance losses of recycled cathode active materials (namely, LiNixMnyCozO2, NMC). The model is first validated with discharge/charge data of a commercial cell (Samsung SDI 94 Ah used in the BMW i3 2016 model) by using a parametrization coming from a comprehensive survey of material properties [1], showing deviations smaller than 1 % in the prediction of capacity (Figure 1). The model is used to quantify the performance loss due to lower-quality NMC particles: a reduction in accessible capacity of 10 % is predicted by a 10 % reduction in maximum Li concentration in NMC, while a 1-4% capacity loss results from a halved solid-state diffusivity. In order to compensate for such losses, design guidelines are conceived and discussed (Figure 2). In particular, increasing the electrode thicknesses by 11 % as a maximum compensates the capacity loss given by recycled NMC, resulting in a minor decrease in specific energy density (from 141 Wh kg-1 to 136 Wh kg-1 at the battery pack level) without impacting the roundtrip efficiency and the fast charge capability (i.e., recovering 50 % of capacity in 15 min). A decrease in solid-state diffusivity can be compensated by reducing the NMC diameter from 8 mm to 6 mm, although a better strategy consists in increasing the electrode thicknesses by ca. 1 mm. In summary, results indicate that lower-quality recycled materials can be effectively used without compromising the performance of a Li-ion battery for electric vehicle applications.