First-Principles Study on the Interplay of Strain and State-of-Charge with Li-Ion Diffusion in the Battery Cathode Material LiCoO2.

Cathode degradation of Li-ion batteries (Li+) continues to be a crucial issue for higher energy density. A main cause of this degradation is strain due to stress induced by structural changes according to the state-of-charge (SOC). Moreover, in solid-state batteries, a mismatch between incompatible cathode/electrolyte interfaces also generates a strain effect. In this respect, understanding the effects of the mechanical/elastic phenomena associated with SOC on the cathode performance, such as voltage and Li+ diffusion, is essential. In this work, we focused on LiCoO2 (LCO), a representative LIB cathode material, and investigated the effects of biaxial strain and hydrostatic pressure on its layered structure and Li+ transport properties through first-principles calculations. With the nudged elastic band technique and molecular dynamics, we demonstrated that in Li-deficient LCO, compressive biaxial strain increases the Li+ diffusivity, whereas tensile biaxial strain and hydrostatic pressure tend to suppress it. Structural parameter analysis revealed the key correlation of "Co layer distances" with Li+ diffusion instead of "Li layer distances", as ordinarily expected. Structural analysis further revealed the interplay between the Li-Li Coulomb interaction, SOC, and Li+ diffusion in LCO. The activation volume of LCO under hydrostatic pressure was reported for the first time. Moreover, vacancy formation energy calculations showed that the Li intercalation potential could be decreased under compressive biaxial strain due to the weakening of the Li-O bond interaction. The present findings may serve to improve the control of the energy density performance of layered cathode materials.

Unraveling TM Migration Mechanisms in LiNi1/3Mn1/3Co1/3O2 by Modeling and Experimental Studies.

Electrochemical cycling induces transition-metal (TM) ion migration and oxygen vacancy formation in layered transition-metal oxides, thus causing performance decay. Here, a combination of ab initio calculations and atomic level imaging is used to explore the TM migration mechanisms in LiNi1/3Mn1/3Co1/3O2 (NMC333). For the bulk model, TM/Li exchange is an favorable energy pathway for TM migration. For the surface region with the presence of oxygen vacancies, TM condensation via substitution of Li vacancies (TMsub) deciphers the frequently observed TM segregation phenomena in the surface region. Ni migrates much more easily in both the bulk and surface regions, highlighting the critical role of Ni in stabilizing layered cathodes. Moreover, once TM ions migrate to the Li layer, it is easier for TM ions to diffuse and form a TM-enriched surface layer. The present study provides vital insights into the potential paths to tailor layered cathodes with a high structural stability and superior performance.

Effect of Controlled-Atmosphere Storage and Ethanol Rinsing on NaNi0.5Mn0.5O2 for Sodium-Ion Batteries.

NaNi0.5Mn0.5O2 is a promising sodium-ion battery cathode material that has been extensively studied. However, the air sensitivity of this material limits practical application and is not well understood. Here, we present a detailed study of NaNi0.5Mn0.5O2 powders stored in different atmospheres (oxygen, argon, and carbon dioxide), either dry or wet. X-ray diffraction and Fourier transform infrared measurements were used to characterize the materials and their surface species before and after controlled-atmosphere storage. It was determined that the exposure of untreated NaNi0.5Mn0.5O2 powders to moisture results in desodiation and material degradation, leading to poor cycling. This effect was found to be caused by reactive surface species. From these results, a simple ethanol washing method was found to significantly reduce the air-reactivity and improve the electrochemical performance of NaNi0.5Mn0.5O2 by removing surface impurities formed by air exposure.

Origin of Structural Evolution in Capacity Degradation for Overcharged NMC622 via Operando Coupled Investigation.

The nickel-rich layered oxide materials have been selected as promising cathode materials for the next generation lithium ion batteries because of their large capacity and comparably high operating voltage. However, at high voltage (beyond 4.30 V vs Li/Li+), the members of this family are all suffering from a rapid capacity decay, which was commonly concerned with crystal lattice distortion and related cation disordering. In this work, the quasi-spherical Ni-rich layered LiNi0.6Co0.2Mn0.2O2 (QS-NMC622) material was successfully synthesized through the carbonate co-precipitation method. A coupled measurement, which is a combination of potentiostatic intermittent titration technique (PITT) and in situ X-ray diffraction (XRD), was deployed to simultaneously capture the structural changes and lithium ion diffusion coefficient of QS-NMC622 material during the first cycle. With help of in situ XRD patterns and high-resolution transmission electron microscope (HR-TEM) images, a defective spinel framework of Fd3̅m space group was detected along with a rapid decreasing lattice-parameter c and lattice distortion at deep delithiated state, which causes poor kinetics related to lithium ion mobility. The new-born framework seems to transform and remain as full spinel structure in the parent phase to the end of charge/discharge with high voltage, which could deteriorate both the surface and body structure stability during the subsequent cycles. This established coupled in situ measurement could be applied to simultaneously investigate the structure transformation and kinetics of cathode materials during charge/discharge.