The ever-increasing demand for renewable energy storage and electrifying transportation calls for developing high energy density electrode materials for rechargeable alkali metal ion batteries. Cathode materials such as layered oxides play a crucial role in determining the energy density and safety of batteries. Major efforts have been invested to push the cathode capacity approaching its theoretical limit while reducing the cost of raw materials (e.g., Co-free cathode pathways). However, the technical challenges are amplified under aggressive operating conditions, particularly at elevated temperatures or high charging voltages. Understanding the fading mechanisms under these aggressive conditions (e.g., chemomechanical breakdown, interfacial degradation, metal dissolution, charge heterogeneity) will inform the material design principles. With advanced synchrotron X-ray, electron, and electrochemical diagnostics, we will present our recent progress in probing the fading mechanisms in Li ion and Na ion batteries. Our study has treated batteries as ecosystems and thoroughly considered the cathode-anode crosstalk. Finally, we will explain how we have made use of the gained knowledge to guide our syntheses of Co-free Li cathodes, water-processable Na cathodes, and hierarchically doped electrode materials.