Open Access

Lithium-ion batteries (LIBs) have been one of the greatest achievements in the field of energy storage of the last century. However, with the increasing demand for energy storage, battery technologies must be diversified according to their specific requirements in different applications. In terms of stationary energy storage and electric vehicles, sodium-ion batteries (SIBs) are considered to be the most attractive alternative to LIBs due to the uniformly high abundance and cost-effectiveness of raw materials, enabling large-scale and low-cost energy storage. Unlike LIBs where crystalline graphite is commonly used as the anode material, disordered carbons are regarded as more promising for SIBs. However, further advances towards better reversibility and higher specific capacity are still needed to match or exceed that of graphite in LIBs. The main challenge is the complex and unpredictable Na+ storage mechanism in disordered carbons, as they may have different properties depending on the precursors and carbonization conditions. [1] Recently, Matsukawa et al. have reported that the reversibility of (de)sodiation processes of disordered carbons is better for ultramicroporous carbons.[2] These pores are accessible only to Na+ ions and not to solvent molecules. Therefore, the (de)sodiation processes are protected from the side reactions caused by the solvent, which reduces the related irreversible capacity loss. With this idea, the origin of the specific capacity and irreversible losses will be further investigated by tailoring chemical composition (e.g., heteroatom doping) as well as pore structure of hard carbons used as anode materials in SIBs. For the synthesis scalable and industrially relevant processes will be used. The structure and morphology of the materials will be characterized by electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy. In addition, different gas sorption techniques will be carried out to analyze the porosity, pore sizes and specific surface areas. The electrochemical performance of the prepared materials will be evaluated based on coin cells, three-electrode Swagelok cells, and pouch cells. Moreover, solid-state nuclear magnetic resonance spectroscopy will be performed to help understand the mechanism of Na+ storage.

Hard carbons (HCs) are currently one of the most promising anode materials for sodium-ion batteries (SIBs). However, the Na storage mechanism remains controversial, leaving the theoretical limits of carbon anodes unclear. To deconvolute the specific capacity from irreversible capacity losses, resulting from the formation of solid electrolyte interphase (SEI), core-shell carbon materials were synthesized. Based on these core-shell materials, an investigation of the storage mechanism of sodium is possible.

The current strong interest in electromotive mobility and the need to transition to an energy grid with sustainable energy storage has led to a renewed interest in sodium ion batteries (SIBs). Hard carbons are promising candidates for high-capacity negative electrode materials in SIBs. Their high capacities, however, are often accompanied with high irreversible capacity losses during the initial cycles.[1] The goal of this project is to use analytical techniques to establish a correlation between the structure and the capacities of hard carbons. This has previously been difficult, in part because the sodium storage mechanism is not stoichiometric and due to the disordered structure of hard carbons. Large irreversible capacities associated with hard carbons are often in contradiction to the experimentally determined low surface area of the sample material.[1] A better understanding of the structure-property relationship should enable quantification and understanding of the potential of hard carbon materials for SIBs. Our approach is to explore whether a core-shell structure can separate sodium storage and solid electrolyte interphase formation so that storage capacity and irreversible losses can be investigated separately. The synthesis of a selection of porous carbon structures serving as the core material, will be attempted. Simultaneously, sodium-conducting shell structures will be developed to allow for separation of sodium ions and electrolyte molecules. Subsequently the combination of core and shell materials will be undertaken. These anodes should enable high capacities accompanied with low irreversible capacity due to optimized solid electrolyte interphase formation.

The current strong interest in electromotive mobility and the need to transition to an energy grid with sustainable energy storage has led to a renewed interest in sodium ion batteries (SIBs). Hard carbons are promising candidates for high-capacity negative electrode materials in SIBs. Their high capacities, however, are often accompanied with high irreversible capacity losses during the initial cycles.[1] The goal of this project is to use analytical techniques to establish a correlation between the structure and the capacities of hard carbons. This has previously been difficult, in part because the sodium storage mechanism is not stoichiometric and due to the disordered structure of hard carbons. Large irreversible capacities associated with hard carbons are often in contradiction to the experimentally determined low surface area of the sample material.[1] A better understanding of the structure-property relationship should enable quantification and understanding of the potential of hard carbon materials for SIBs. Our approach is to explore whether a core-shell structure can separate sodium storage and solid electrolyte interphase formation so that storage capacity and irreversible losses can be investigated separately. The synthesis of a selection of porous carbon structures serving as the core material, will be attempted. Simultaneously, sodium-conducting shell structures will be developed to allow for separation of sodium ions and electrolyte molecules. Subsequently the combination of core and shell materials will be undertaken. These anodes should enable high capacities accompanied with low irreversible capacity due to optimized solid electrolyte interphase formation.

A core-shell strategy is introduced to overcome the dilemma of common non-graphitic hard carbon anodes, linking high reversible storage capacity to practically unacceptable irreversible losses in the first cycle(s). Just as Graphite homogeneously combines effective lithium storage with an electrolyte solvent-sieving function, we show that both of these functions could be strategically integrated into non-graphitic carbons in a heterogeneous structure. Highly porous activated carbons are sealed by kinetically tuned gas-phase deposition of non-graphitic carbon to form a functional core-shell structure. Gas sorption porosimetry on core, shell, core–shell, and cracked core-shell particles confirms preserved core porosity and a semi-permeable shell. Diethyl carbonate sorption analysis is introduced as a more suitable probe than N2 or CO2 sorption, linking first-cycle losses to the liquid–solid interface of carbon anodes. The functional core-shell particles with much reduced diethyl carbonate uptake allow for high storage capacity and reduced first cycle losses. Delivering 400 ± 24 mAh g−1 with 82 ± 2% first-cycle reversibility, it is shown that three-stage Na storage in designed core-shell anodes can compensate for the larger size of sodium compared to lithium stored in graphite anodes (372 mAh g−1). The designed core-shell anodes show state-of-the-art performance with commercial promise.