Origin of Aging of a P2-Na _x Mn_3/4Ni_1/4O₂ Cathode Active Material for Sodium-Ion Batteries

Sodium-ion batteries (SIB) are currently being developed and commercialized as a promising new technology for cost-effective and powerful electrical energy storage. In this study, we investigate the origin of capacity fading in P2-type layered sodium cathode materials for SIBs using a micron-sized single-crystalline P2-Na-x Mn3/4Ni1/4O2 model cathode active material. Using various electrochemical techniques, we identify the following aging effects upon cycling: (i) a state of charge (SOC)-independent increase in polarization, (ii) a SOC-dependent increase in polarization at high voltage, and (iii) a loss of active material due to electronic disconnection after prolonged cycling. With high-resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray (EDX) spectroscopy, we identify surface densification, resulting in 5-10 nm thick surface layers on cycled cathode active materials as the origin for SOC-independent increase of polarization. The corresponding oxygen loss is in accordance with gas evolution in differential electrochemical mass spectrometry (DEMS) measurements. Furthermore, with scanning electron microscopy (SEM) electrode cross sections, we identify (partly) reversible cracking at a high SOC as the cause for increased polarization depending on SOC. Operando X-ray diffraction (XRD) identifies significant anisotropic volume change, which suggests mechanical stress as the cause for cracking at a high SOC and loss of active material after prolonged cycling. We believe that the herein provided understanding on the aging of this highly attractive class of cathode active materials for SIBs will enable the development of future powerful and stable layered oxide cathode materials for SIBs.