Origin of Structural Degradation in Li-Rich Layered Oxide Cathode

Abstract

The problem Many commercial lithium-ion batteries rely on cathodes that contain alternating layers of transition metal, oxygen, and lithium (e.g. LiCoO2, LiNixMnyCozO2). During charging/discharging, lithium ions diffuse in and out of lithium layers in the cathode to store and release charge. This inevitably changes the material’s crystalline structure, affecting its stability and electrochemical performance. One possible battery failure mechanism is thought to involve the release of oxygen from layered oxide cathodes. This issue is particularly pronounced in lithium- and manganese-rich (LMR) layered cathode materials, which experience rapid voltage fade. Yet these low-cost materials also offer a high energy density that far surpasses commercial cathodes. Although researchers have tried to enhance LMR’s lattice oxygen stability using strategies such as doping or coating, they have had limited success. LMR also contains two distinct types of crystalline regions that undergo different structural changes at the nanoscale during charging/discharging. This could affect the properties of the entire material, but technical limitations have made it difficult to study. Given the ineffectiveness of oxygen stabilizing methods, we suspected that oxygen release might be a symptom, rather than a cause, of voltage fade — and that the problem originates in the material’s unique nanoscale domains. The solution Although the failure mechanism of LMR has been studied extensively with statistical X-ray diffraction and atomic-scale microscopes, less is known about how its structure changes at the mesoscale, spanning nanometers to micrometers. Such observations are technically challenging because they require characterizations at multiple length scales while the cathode is operating. This is particularly difficult for LMR, because its two different nanoscale domains are electrochemically activated at different voltages and involve distinct redox chemistries. This leads to non-equilibrium structural dynamics at the lattice level that contribute to nanoscale lattice strain. These processes were poorly understood, so we conducted a comprehensive investigation of mesoscale lattice evolution in an LMR cathode. In-situ Bragg coherent diffraction imaging (BCDI) results revealed that LMR suffers from severe lattice strain accumulation, which serves as the original driving force to trigger thermodynamic destabilization and oxygen release. By leveraging spinning length X-ray diffraction techniques, we further probed the material from the nanoscale to the macroscale. This showed that the heterogeneous structure dynamics of LMR — in other words, the different responses of its two domains — is the underlying reason that the insertion or deinsertion of lithium ions generates lattice strain. These detrimental reactions play an undeniable role in structure decomposition and electrochemical fade, and we expect they are common in many other cathode materials that host lithium diffusion. The implications We have thoroughly described the origin of lattice strains, lithiation behavior in heterogeneous phases, and their role in oxygen release in LMR. Our discoveries provide solid evidence that the lattice strain we observed in LMR could be affecting the structure, and thus electrochemical performance, of other lithium-based cathodes. This has enabled us to propose potential strategies to effectively suppress strain generation and improve the electrochemical properties of all other layered oxide cathodes, commercial or academic. Previous research into the failure mechanisms of cathode materials have not accounted for lattice strain effects, and these studies should be revisited to properly understand the role of strain. Modifications that have relied on traditional methods like doping or coating to restrict oxygen release — long thought to be the origin of voltage fade in LMR, as well as other cathode types — should also be reevaluated. Our findings suggest that researchers do not completely understand the impact of these modifications on the stability of cathode materials, and they may have actually exacerbated lattice strain behavior. Therefore, our insights into the role of lattice strain have not only opened up a new way to investigate the origin of failure in LMR and lithium-based cathode materials; they have also shown that resolving lattice strain challenges could be a valuable strategy to improve lithium-ion battery performance. Figure 1