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1. Computational Modeling of Heterogeneity of Stress, Charge, and Cyclic Damage in Composite Electrodes of Li-Ion Batteries

   https://doi.org/10.1149/1945-7111/ab78fa  

   Liu, Pengfei; Xu, Rong; Liu, Yijin; Lin, Feng; Zhao, Kejie (March 2020, Journal of The Electrochemical Society)

   Charge heterogeneity is a prevalent feature in many electrochemical systems. In a commercial cathode of Li-ion batteries, the composite is hierarchically structured across multiple length scales including the sub-micron single-crystal primary-particle domains up to the macroscopic particle ensembles. The redox kinetics of charge transfer and mass transport strongly couples with mechanical stresses. This interplay catalyzes substantial heterogeneity in the charge (re)distribution, stresses, and mechanical damage in the composite electrode during charging and discharging. We assess the heterogeneous electrochemistry and mechanics in a LiNi_xMn_yCo_zO₂(NMC) cathode using a fully coupled electro-chemo-mechanics model at the cell level. A microstructure-resolved model is constructed based on the synchrotron X-ray tomography data. We calculate the stress field in the composite and then quantitatively evaluate the kinetics of surface charge transfer and Li transport biased by mechanical stresses. We further model the cyclic behavior of the cell. The repetitive deformation of the active particles and the weakening of the interfacial strength cause gradual increase of the interfacial debonding. The mechanical damage impedes electron transfer, incurs more charge heterogeneity, and results in the capacity degradation in batteries over cycles. 

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2. Bending good beats breaking bad: phase separation patterns in individual cathode particles upon lithiation and delithiation

   https://doi.org/10.1039/d0mh01240h  

   Santos, David A.; Andrews, Justin L.; Bai, Yang; Stein, Peter; Luo, Yuting; Zhang, Yuwei; Pharr, Matt; Xu, Bai-Xiang; Banerjee, Sarbajit (December 2020, Materials Horizons)

   null (Ed.)

   The operation of a Li-ion battery involves a concerted sequence of mass and charge transport processes, which are underpinned by alternating dilation/contraction of the active electrode materials. Several Li-ion battery failure mechanisms can be directly traced to lattice-mismatch strain arising from local compositional heterogeneities. The mechanisms of chemo-mechanical coupling that effect phase separation and the resulting complex evolution of internal stress fields remain inadequately understood. This work employs X-ray microscopy techniques to image the evolution of composition and stress across individual bent V 2 O 5 particles. Experimental findings show that lattice strain imposed by the deformation of an individual cathode particle profoundly modifies phase separation patterns, yielding striated Li-rich domains ensconced within a Li-poor matrix. Particle-level inhomogeneities compound across scales resulting in fracture and capacity fade. Coupled phase field modeling of the evolution of domains reveals that the observed patterns minimize the energetic costs incurred by the geometrically imposed strain gradients during lithiation of the material and illustrate that phase separation motifs depend sensitively on the particle geometry, dimensions, interfacial energetics, and lattice incommensurability. Sharp differences in phase separation patterns are observed between lithiation and delithiation. This work demonstrates the promise of strain-engineering and particle geometry to deterministically control phase separation motifs such as to minimize accumulated stresses and mitigate important degradation mechanisms. 

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3. Concentration-gradient Prussian blue cathodes for Na-ion batteries

   Hu P.P, Peng W.P (January 2020, ACS energy letters)

   A concentration-gradient composition is proposed as an effective approach to solve the mechanical degradation and improve the electrochemical cyclability for cathodes of sodium-ion batteries. Concentration-gradient shell NaxNiyMn1-yFe(CN)6·nH2O, in which the Ni content gradually increases from the interior to the particle surface, is synthesized by a facile co-precipitation process. The as-obtained cathode exhibits an improved electrochemical performance compared to homogeneous NaxMnFe(CN)6·nH2O, delivering a high reversible specific capacity of 110 mA h g-1 at 0.2 C and outstanding cycling stability (93% retention after 1000 cycles at 5 C). The improvement of electrochemical performance can be attributed to its robust microstructure that effectively alleviates the electrochemically induced stresses and accumulated damage during sodiation/desodiation and thus prevents the initiation of fracture in the particles upon long term cycling. These findings render a prospective strategy to develop high-performance electrode materials for sodium-ion batteries. 

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4. Concentration-Gradient Prussian Blue Cathodes for Na-Ion Batteries

   Hu P., Peng W. (January 2020, ACS energy letters)

   A concentration-gradient composition is proposed as an effective approach to solve the mechanical degradation and improve the electrochemical cyclability for cathodes of sodium-ion batteries. Concentration-gradient shell NaxNiyMn1-yFe(CN)6·nH2O, in which the Ni content gradually increases from the interior to the particle surface, is synthesized by a facile co-precipitation process. The as-obtained cathode exhibits an improved electrochemical performance compared to homogeneous NaxMnFe(CN)6·nH2O, delivering a high reversible specific capacity of 110 mA h g-1 at 0.2 C and outstanding cycling stability (93% retention after 1000 cycles at 5 C). The improvement of electrochemical performance can be attributed to its robust microstructure that effectively alleviates the electrochemically induced stresses and accumulated damage during sodiation/desodiation and thus prevents the initiation of fracture in the particles upon long term cycling. These findings render a prospective strategy to develop high-performance electrode materials for sodium-ion batteries. 

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5. Reversible planar gliding and microcracking in a single-crystalline Ni-rich cathode

   https://doi.org/10.1126/science.abc3167  

   Bi, Yujing; Tao, Jinhui; Wu, Yuqin; Li, Linze; Xu, Yaobin; Hu, Enyuan; Wu, Bingbin; Hu, Jiangtao; Wang, Chongmin; Zhang, Ji-Guang; et al (December 2020, Science)

   High-energy nickel (Ni)–rich cathode will play a key role in advanced lithium (Li)–ion batteries, but it suffers from moisture sensitivity, side reactions, and gas generation. Single-crystalline Ni-rich cathode has a great potential to address the challenges present in its polycrystalline counterpart by reducing phase boundaries and materials surfaces. However, synthesis of high-performance single-crystalline Ni-rich cathode is very challenging, notwithstanding a fundamental linkage between overpotential, microstructure, and electrochemical behaviors in single-crystalline Ni-rich cathodes. We observe reversible planar gliding and microcracking along the (003) plane in a single-crystalline Ni-rich cathode. The reversible formation of microstructure defects is correlated with the localized stresses induced by a concentration gradient of Li atoms in the lattice, providing clues to mitigate particle fracture from synthesis modifications. 

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