Functional Batteries with Apple Bio-Waste
Towards Sodium-Ion Batteries
- Figure 1: Bio-mass derived anodes can open new possibilities for the implementation of Na-ion batteries into the grid.
- Figure 2: Schematic synthesis route of hard carbon, derived from bio-mass and structural differences between hard carbon, soft carbon and graphite.
- Figure 3: Electrochemical behavior of apple biomass derived hard carbon in a Na-ion cell.
Sodium-ion batteries (SIBs) have seen increasing interest during recent years as they are regarded as a potential substitution or complement to conventional lithium-ion batteries (LIBs). The batteries are composed of cathodes, synthesized from low-cost and rather environmentally friendly elements like Na, Mn, Fe and Mg, and carbonaceous anode materials, preferably hard carbons. These latter materials are synthesized from various precursors and scientific interest has been set on the search for renewable and possibly waste-derived precursors. Herein, we will present the general characteristics of hard carbon materials, the schematic synthesis via renewable waste precursors and the electrochemical performance in SIBs.
Introduction to Na-Ion Batteries
An increased public awareness about environmental concerns and energy independence has resulted in a new focus on the domestic production of renewable “green” energy. Since renewable sources tend to deliver unsteady power, buffering technologies are necessary to pave the way for the next generation grid and future energy supply needs. Driven by portable electronics, stationary energy storage and electric vehicles, enormous efforts and resources were and are invested for the development of advanced materials to increase batteries’ energy and power. Additionally, raw materials for commercial batteries are frequently rare, pricy, toxic and not environmentally friendly. Thus scientists are looking for materials oriented approaches to overcome these problems.
One approach is the development of SIBs, presenting some advantages versus the most common LIBs in terms of materials costs and availability. On the other hand, SIBs share the same working principle and cell structure with LIBs and thus their production is easily implementable. Both technologies were developed in parallel from the 1970s on, but the focus shifted mostly to LIBs after the commercialization of the first Li-ion cell in 1991. From about 2010, researchers intensified work on SIBs as a complement to LIBs. Fast progress was made due to the transfer of knowledge from LIB research. For example, five years ago, SIB materials were predominantly tested in sodium-metal half cells and mostly cycled for few cycles while nowadays several hundred to thousand cycles are demonstrated, often also in complete SIBs.
The differences between SIBs and LIBs are due to the different properties of the ionic carrier shuttling the electric charge between the two cathode and anode (Na+ and Li+, respectively). It should be noted that the use of Na+ (higher atomic weight and ionic radius than Li+) generally leads to decreased gravimetric and volumetric energies of SIBs versus LIBs. However, SIBs offer the advantage of lower cost and more abundant (Na2CO3 is available worldwide) raw materials. In addition, aluminum can be used as anodic current collector instead of the expensive and heavy copper needed for LIBs (Li alloys with aluminum at low potentials). Finally, SIBs make use of disordered carbonaceous materials, especially hard carbons, which can be synthesized by pyrolysis from many sources (including bio-waste, see Figure 1) at relatively low temperatures.
Structure and Synthesis of Hard Carbons
The unique electrochemical characteristics of hard carbons are correlated with the disordered structure. In greater detail, these materials are composed of distorted graphitic sheets interlinked by sp3-hybridized carbon and heteroatoms, e.g., O, H, N, and S, which prevent the growth of parallel graphitic sheets and the transformation into graphite. Therefore hard carbon has a 3D-disordered structure and the presence of cavities.
Multiple factors influence the storage capacity of hard carbons, such as the raw materials properties, pre-treatment conditions (e.g., drying, grinding, mixing, chemical activation) and annealing temperature (usually between 1000 and 1500 °C) in an inert atmosphere (e.g., N2, Ar and their flow rate), with the latter two being the most costly steps.
However, a large variety of carbon-containing raw materials could, in principle, be used as a precursor. In fact, hard carbon materials have been synthesized from various materials, such as sucrose and petroleum coke in 1993, glucose (2000), carbon black (2001), saccharose coke/PAN based nanofibres/ pyrolyzed cellulose fibers (2002) and resorcinol-formaldehyde (2005). Research efforts began to significantly increase around 2010 when scientists began to investigate a variety of other raw materials derived from bio-mass like pomelo peel, banana peels, cellulose, leafs, peanut shell, peat mossa wild plant that covers 3% of the earth‘s surface, and apple bio- waste. These could serve as ideal precursors for the negative electrode in SIBs.
Electrochemical Behavior of Hard Carbons
Figure 3 illustrates the working principle for the reversible storage of Na+ ions in hard carbon. It is interesting to mention that the mechanism is found to be similar for sodium and lithium by the majority of research works. In fact, the shape of the voltage profile of hard carbon materials upon charge and discharge is the same for both cations, which supports a similar storage mechanism. The potential profile can roughly be divided into a sloping region extending from high potentials to about 0.2 V, followed by a constant potential region (at low potentials). The sloping region originates from the reversible adsorption of cations on the hard carbon surface (and defects) as well as the reversible intercalation between the disordered graphitic sheets. Instead the plateau is related to the so-called “pore filling” mechanism, i.e., the adsorption of cations into the pores/cavities and microstructures. It should be noted that this characteristic shape of the potential profile is independent of the precursor.
The electrochemical performance of the apple bio-waste derived hard carbon in a SIB is exemplary, shown in Figure 3 (for details, please refer to ). The material showed a good reversible capacity of 235 mAh g-1 at low current rates of 20 mA g-1. Interestingly, the material exhibited an extraordinary cycling stability with a capacity retention of 85% after more than 1000 cycles at 1000 mA g-1. The material also showed a good electrochemical performance over 100 cycles in a SIB with a layered transition metal oxide as positive electrode. This surely indicates that the use of bio-waste derived hard carbons in batteries is feasible and worth to be investigated in greater detail in future.
In summary, SIBs have the potential to complement LIBs in all applications in which the final weight and volume of the battery are not the most important criteria (e.g., stationary energy storage). This is possible due to good electrochemical performance and potential cost reduction (€/kWh), resulting from low investment costs (SIBs can be regarded as “drop-in” technology), use of low cost and abundant raw materials, such as Na, Mn, Fe, Mg and Al (instead of Cu) and hard carbons from renewable resources (e.g., bio-waste). Especially this latter underlines the low-cost and environmentally friendly philosophy of SIBs. The synthesis of bio-waste derived hard carbon is facile and, indeed, feasible with a broad variety of precursors (like apple bio-waste, cellulose, banana peels, etc.). Good electrochemical performance has been demonstrated, which is attracting increasing attention in academia and industry. Nevertheless, synthesis, structural design and electrochemical performance of hard carbons and positive electrodes (e.g. layered transition metal oxides) for SIBs require still optimization to be an alternative to LIBs in large-scale applications such as stationary energy storage.
Christoph Vaalma1,2, Daniel Buchholz1,2,*, Stefano Passerini1,2,*
1Helmholtz Institute Ulm (HIU), Electrochemistry I, Ulm, Germany
2Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany
Prof. Dr. Stefano Passerini
Helmholtz Institute Ulm (HIU)
Dr. Daniel Buchholz
Helmholtz Institute Ulm (HIU)
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