In your cell phone's lithium-ion battery, the materials that make up the negative electrode, or cathode, lose structural integrity after repeated charging and discharging. Over time, the battery both stores less energy and provides less energy for use. A study shed new light on the process of cathode degradation. Scientists revealed that high-voltage cycling is the driving force leading to cracks inside grains that make up the cathode, causing the cathode to fail.
By identifying how electrodes degrade in lithium-ion batteries, the study could lead to the design of superior cathode materials. These materials could improve battery performance. Better batteries could mean more time between charges for electric vehicles and cell phones. Further, batteries could store renewable energy for later use on the grid.
Rechargeable lithium-ion batteries are common in portable electronics and in today's plug-in hybrid electric vehicles. One of the most attractive cathode materials in these batteries is layer-structured lithium transition metal oxide. However, this material suffers from capacity and voltage fade during battery cycling, limiting its performance. Further, the underlying mechanisms for this failure are not fully understood. To address this challenge, researchers from Pacific Northwest National Laboratory used electron microscopy to directly monitor the structural evolution of the electrode at the atomic level and in unprecedented detail. The researchers used the aberration-corrected scanning transmission electron microscope at the Environmental Molecular Sciences Laboratory, a U.S. Department of Energy Office of Science user facility. They discovered that intragranular cracks form inside the cathode of lithium-ion batteries. Moreover, they found that high-voltage cycling is the direct driving force for the generation of these cracks, which limit the battery's cycle life. When the cycle voltage exceeded a certain threshold, significant cracks formed inside the grains making up the cathode, compromising structural stability. This finding is in sharp contrast to theoretical models predicting that these cracks form at grain boundaries or on particle surfaces. Taken together, the results suggest mitigation of intragranular cracking requires a stable structural framework of the cathode material and carefully controlled cycling conditions. For high-voltage applications, the material's chemistry and structure must be adjusted to minimize internal grain strain during charge and discharge cycling. In the end, these findings could generate new strategies to design stable, safe lithium-ion batteries with high energy density and a long cycle life.
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Environmental Molecular Sciences Laboratory
This work was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Biological and Environmental Research, including support of the Environmental Molecular Sciences Laboratory, an Office of Science User Facility; and DOE's Office of Energy Efficiency and Renewable Energy, Office of Vehicle Technologies, Advanced Battery Materials Research program.
P. Yan, J. Zheng, M. Gu, J. Xiao, J.G. Zhang, and C.M. Wang, "Intragranular cracking as a critical barrier for high-voltage usage of layer-structured cathode for lithium-ion batteries." Nature Communications 8, article 14101 (2016). [DOI: 10.1038/ncomms14101]
Environmental Molecular Sciences Laboratory science highlight: Extending the life of Li-ion batteries