Development of a Core Void Shell Silicon Nanoparticle for Use in High Capacity Anodes
Authors: Verheijen Maarten , Moeremans Boaz , Vranken Thomas , Hardy An , Van Bael Marlies K 
Published: February 2020

In the field of energy there is a continuous drive to increase available capacity. To satisfy this growing need for energy storage, the capacity available for Li-ion batteries should increase appropriately. For the cathode of LIB's a continuous exploration of new materials is in effect, while for the construction of the anode, carbonaceous materials have been in use since the first LIB’s commercialized by Sony in 1991. This material was not chosen because of its high capacity (approx. 372 mAh/g), but because of its stability against degradation due to a low volume expansion (10%). [1,2]
In the differentiation towards new elements as to replace graphitic carbon as the anode in LIB's, silicon stands out due to its high volumetric (8330 mAh/cm³) and gravimetric (3579 mAh/g) capacity. This large increase in capacity being a full order of magnitude can be understood by looking at the reaction mechanism as Si undergoes an alloying reaction with Li rather than an intercalation reaction such as with carbonaceous materials. Unfortunately there is also a negative side to this alloying reaction which has prohibited its use in commercial batteries up until this point. Namely the anode material expands widely in volume (up to 275%). A bulk layer of Si would crack up when cycling the battery. A method to circumvent this volume-expansion problem exists, consisting of nanosizing the Si to spherical particles of a few 100 nanometres. These particles survive cycling during charge and discharge cycles, yet reveal a new problem. Due to the large volume changes no stable SEI can be formed on the particles. As with each volume expansion the SEI would crack and reveal new exposed silicon surface to the electrolyte. Thus each cycle more SEI is formed and lithium is irreversibly consumed. Using this silicon in a full cell would quickly deplete the available lithium. In order for silicon to be considered an alternative for commercial batteries a solution to this problem must be found. [3-6]
In this work the authors demonstrate a novel way to combine the high capacity of silicon and the stable surface of carbon. The route of preference for this goal is the synthesis of a core/shell particle. However conventional core/shell particles would have the same problem as the SEI formation. Namely the shell would crack with each volume expansion rendering the protective surface ineffectual. To alleviate this problem free space must be introduced in the core/shell particle into which the silicon can expand freely to allow for the large volume expansion. This has been accomplished by the conformal coating of a sacrificial alumina layer on top of the silicon particles using a surfactant controlled synthesis step via the thermal decomposition route of a suitable aluminium precursor. Controlling the reaction time and precursor concentration allows for optimal control of the resulting coating so this can be matched to accommodate the volume expansion.
On top of these silicon alumina core/shell particles a carbon layer is deposited by firing an aliphatic precursor. Conformal coating is achieved by dispersion in solution using the precursor as a surfactant. In a final step the alumina layer can be removed by a mild acidification. This technique results in a core/void/shell particle which can absorb the volume expansion internally and present a stable SEI forming carbon surface to the electrolyte. Electrochemical characterizations were carried out on the coin cell level showing a decrease in the irreversible capacity on the first cycles as well as enhanced capacity retention during lifetime testing. Total capacity was less than for pure silicon based anodes due to the need to incorporate inactive material in the formation of the core/shell particles. The novelty of this research lies in the use of alumina as the sacrificial layer, thus requiring only a mild acid treatment. As previous core/shell designs have made extensive use of TEOS based silica growth, thus requiring an aggressive hydrogen fluoride. In contrast the here proposed route requires only hydrogen chloride which does not corrode the underlying silicon particles. [7,8]
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