Dynamic delocalization of stress in brittle battery positive electrode active materials by shape-memory polymer nanocoating

Abstract

Stress-induced fractures are recognized as a primary cause of degradation in a wide range of positive electrode active materials during battery operation. However, the state-of-the-art mechanistic understanding and strategy development often overlook the brittle nature of these materials, as well as the dynamic and localized characteristics of mechanical stress during the charge and discharge cycles of the cell. Here we present a shape-memory polymer nanocoating method using initiated chemical vapour deposition to dynamically delocalize concentrated stresses in various positive electrode active materials, including Ni-rich layered oxides with different Ni contents and LiFePO4. Fracture simulations and surface-to-bulk physicochemical characterizations collectively show that the balanced stiffness and deformability of the shape-memory polymer nanocoating on the positive electrode material effectively mitigate stress gradients and the consequent surface reconstruction, chemical heterogeneity and intergranular cracking during battery operation. In particular, when a polymeric nanocoated nickel-rich layered oxide positive electrode active material (90 at% of Ni) is tested in non-aqueous lithium metal coin cell configuration at 25 °C, the cells can be consistently charged and discharged over long cycles at moderate (for example, 1,000 cycles at 400 mA g−1) and high (for example, 500 cycles at 1 A g−1) specific currents. Nanocoating of brittle positive electrode active materials with an ionically conductive polymer enables efficient operation of non-aqueous lithium metal coin cells at specific currents up to 2 A g−1.