A novel specimen preparation of porous cathode materials in lithium-ion batteries for high-resolution transmission electron microscopy

Micropores and fissures in cathode materials are associated with deterioration of electrochemical performance of lithium-ion batteries. Phase transformation is responsible for anisotropic volume change and pulverization in conjunction with an expansion of the pores and fissures, occurred dominantly at the outermost cathode particles where lithium ions are repeatedly diffused from parent structure to anode and vice versa. The gallium ion sources, in addition, penetrate into the vacuum and etch the particle in the vicinity of pores and fissures during the cross-sectioning process using focused ion beam. Herein, we propose a noble specimen preparation for high-resolution transmission electron microscopy (TEM) by employing epoxy to infiltrate into micropores. The epoxy can retain the agglomerate of cathode particles and avoid ion-beam-induced damage and contamination as well as suppress the side reactions with moisture and oxygen even after exposure to the air for a few months. The well-designed specimen preparation enables in-depth TEM investigation on the microstructure and phase transformation occurred severely at the outermost regions of porous cathode particles.