Elucidating the Role of Phase Boundaries in Zero-Strain Behavior Cathode Materials during Long Term-Cycling

Abstract

As energy storage systems (ESS) and the electric vehicle (EV) market continue to expand rapidly, the demand for higher energy density lithium ion batteries has become increasingly critical. Accordingly, the cathode which predominantly determines the cell capacity has become a primary subject of intensive materials investigation. Cathode inevitably undergo severe changes in lattice parameters and volume during charge-discharge, and the resulting mechanical stress induces particle fracture, fundamentally limiting cell performance. To address this challenge, there is a growing need for the development of “zero-strain” cathode material in which lattice and volumetric changes are intrinsically suppressed during electrochemical cycling. To date, most studies on zero-strain behavior have been defined by limiting lattice parameter or unit cell volume variations to ≤~1% during initial cycles. However, as cycling proceeds, repeated Li insertion/extraction unavoidably triggers phase transitions. Such irreversible structural evolution makes it difficult to assume that the initially observed zero-strain state can be preserved over extended cycling. Consequently, a fundamental understanding of whether zero-strain behavior can be sustained in the long term is still lacking. In this work, we confirmed, through in-situ XRD analysis of the lattice parameter changes in Li1.2Ni0.2Mn0.6O2, that zero-strain behavior is preserved both during the initial cycles and under prolonged cycling. By combining PPA, GPA, and BCDI analyses, we verified that the coherent phase boundary existing between adjacent layered domains acts as a structural buffer that efficiently relieves strain. Through this study, we propose that the coherent phase boundary can serve as a structural parameter that not only provides inital structural stability but also effectively buffers strain under long term cycling conditions.