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Breakthrough in the Electrode Material Design

 Research

How can electronics devices such as mobile phones and laptops, become lighter and thinner? How can electric vehicles have the batteries with a higher energy density in a limited space?With the increasing demand for energy storage, the performance of rechargeable batteries has faced with higher requirements. Nanotechnology can make batteries "lighter" and "faster". However, owing to the low density of nanomaterials, "how to make it smaller" has become major challenge for researchers working in the electrochemical energy storage field.
Professor Yang Quanhong from School of Chemical Engineering and Technology at Tianjin University led his research group and put forward the “Sulfur Template Method” to realize the precise design of carbon cages for anode materials in Li-ion batteries with ultrahigh volumetric energy density, making it promising for lithium-ion batteries to become smaller. The results were published online in "Nature Communications" (2018, 9, 402) on January 26th.
As the most widely used rechargeable lithium batteries, lithium-ion batteries have both high energy density and safety. Non carbon materials, such as tin and silicon, hold promise to replace commercial graphite as novel anode materials that will greatly improve the mass-specific energy density(Wh kg-1)of lithium-ion batteries. However, their huge volume expansion during the charge/discharge process seriously limits their volumetric performance. Carbon-noncarbon hybrid structures are considered as the main strategy to buffer the huge volume expansion, but the process of building carbon network always introduces excess void space which counters the attempt to obtain a high volumetric capacity. Therefore, precisely tuning the carbon cage structure to realize the effective accommodation for volume expansion without introducing useless void is not only an important academic problem, but also crucial for the industrialization of novel electrode materials.
Yang’s team together with the researchers from Tsinghua University, National Center for Nanoscience and Technology and Japan’s National Institute for Materials Science, made a breakthrough in anode materials design for lithium-ion batteries with superior volumetric energy density. Based on the graphene interfacial assembly, they developed a strategy of sulfur-templated shrinkage to prepare graphene-derived carbon cages with a high-density but well-defined void space around noncarbon active materials. During the process of constructing dense graphene network with capillary drying technology, sulfur was introduced as a flowable volume template to make carbon cages for non-carbon active particles. Through matching the amount of sulfur template, three-dimensional carbon cage structure can be accurately controlled to be suitable for the wrapping up of non-carbon active particles. With the effective accommodation of dramatic volume expansion for noncarbon active materials, the carbon-caged anode material showed superior volumetric performance.

Chart: Precise Design of Graphene Cage Structure with Sulfur Template Method
The sulfur template method is making clever use of the characteristics of sulfur, such as flowability, amorphousness and removability in the formation of dense three-dimensional graphene network, to precisely wrap up non-carbon particles (SnO2) with controlled void space. Compared with the traditional "shape" templates, the biggest advantage of sulfur template is the flowable and volumetric template features to fit the shape of active materials, so that the dense carbon cage structure can provide suitable and accurately controllable reserved space to complete the precise customization of active tin oxide. This kind of carbon - noncarbon hybrid electrode material with suitable reserved space and high density can contribute to a high volumetric specific capacity, thereby greatly improving the volumetric energy density of lithium-ion battery and making the battery smaller. The design idea of carbon cage "customization" can be extended to the construction of other electrode materials for next-generation lithium-ion battery, lithium-sulfur battery and lithium-air battery.
In recent years, the Nanoyang Group of Professor Yang Quanhong has made a series of important advances in the field of compact energy storage which emphasizes the volumetric performance of electrodes and devices. They invented the capillary evaporation densification strategy and solved the bottleneck problem of incompatibilities between high density and porosity of carbon materials, yielding porous carbon materials with high density. These achievements have laid the foundation for application of carbon nanomaterials, and effectively promoted the practical use of electrochemical energy storage devices with carbon nanomaterials.
By Bao Yanyan
Editors: Yin Wei & Doris Harrington