P. Lavela, M. Aranda, A. Medina, R. Alcántara, J.L. Tirado
Universidad de Córdoba,
Spain
Keywords: sodium-ion batteries, layered oxide cathodes, carbon anodes, waste-derived carbons
Summary:
Recently, the fast development of sodium-ion batteries (SIB) makes them plausible candidates to partly replace lithium analogs (LIB). The major concerns commonly associated with LIB are related to lithium abundance and geographic distribution, which could compromise their sustainable use. Also, other critical metals involved in LIB technologies, such as cobalt, suffer from serious environmental and sustainability issues. The quest for electrode materials for SIB should learn from the LIB case by harmonizing a high electrochemical performance with a long and sustainable future. In this communication, we discuss several strategies to achieve both goals in full SIB cells, dealing with both cathode and anode materials. Among the different families of SIB cathodes, layered oxides have shown a success story since their early knowledge in the last century, facilitated by their optimization in different companies worldwide. One of the most successful systems, P2-Na2/3Ni1/3Mn2/3O2 (NNM) with sodium in trigonal prismatic coordination and two sodium layers per unit cell, contains a critical metal: nickel. Attempts to replace Ni with more abundant and sustainable transition metal elements such as iron, usually lead to poorer performances. On the other hand, the introduction of small amounts of magnesium in the transition metal layers results in improved performance by getting rid of the deleterious ion-ordering processes and irreversible structural transformation to an O2 phase at high voltage. Here, we show the effects of partially replacing Ni with Fe in Mg-doped NNM, prepared by an easily scalable sol-gel route. This process finally allows reaching a P2-Na0.67Mg0.05Fe0.1Mn0.85O2 material that outstands in capacity retention in Na half-cells, and rate performance (94 mAh/g at 5C). The improvement is associated with low charge-discharge voltage hysteresis, high sodium-ion diffusion coefficient, and low cell impedance. Regarding the anode of SIB, carbon materials, particularly hard carbons, provide numerous examples. Carbon sources can be pure organic compounds that pyrolise to apt carbon forms. For a large-scale application, these materials can be considered a high-cost option. Replacing these organic source compounds with waste-derived organic residua could result in different advantages: cheaper production, lower accumulation of unwanted residua, higher sustainability, and added value to the waste products. However, one of the major concerns of this source is related to potential electrochemically inactive impurities that could result in lower specific capacities and loss of electric contacts between carbon particles. To avoid these drawbacks we describe the application of a simple and inexpensive acid treatment of carbons produced from several food-waste residua derived from coffee, olive stone, and acorn. The initial carbonous materials were obtained by annealing the residua 800°C under an Ar flow. After a sulfuric acid treatment, galvanostatic experiments revealed increases in specific capacities around 33 %, and improved capacity retention. The origin of these changes is discussed by using X-ray diffraction data, and different spectroscopic techniques. Finally, the optimized cathode and anode materials are joined together in an SIB full cell that shows outstanding energy density.