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J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al 2.3. Alloy and conversion materials Lorenzo Stievano1,2, Moulay Tahar Sougrati1,2 and Laure Monconduit1,2 1 ICGM, Univ. Montpellier, CNRS, Montpellier, France 2 RS2E, CNRS, Amiens, France Status The development of alloy and conversion negative electrode materials for NIBs (which characteristics are resumed in figure 13) represent a substantial fraction of the work carried out to develop a suitable anode for application in commercial systems. While, on the one hand, the high reactivity of sodium and its low melting point (98 ◦C), as well as the formation of dendrites during its electrodeposition, make it significantly less safe than Li, on the other hand, in contrast to the case of LIBs, graphite does not intercalate Na+ in conventional electrolytes, and thus alternative anodes have to be found. Several families of materials capable of reacting electrochemically with sodium have been studied and can be classified, as for LIBs, according to their reaction mechanism, as either insertion, alloy, or conversion materials [123]. However, the mechanism and performance of such materials in NIBs cannot be directly extrapolated from those observed in LIBs. The alloying reaction is usually observed with p-block elements (Ge, Sn, Pb, P, Sb, Bi, etc) and some transition metals (e.g., Ag, Zn and Au) which form stable alloys with Na [124]. Alloy materials (AM) usually involve a multiple-electron exchange, thus leading to very high capacities. However, the formation of the sodiated alloy causes a strong expansion of volume, even worse than that of LIBs, which represents a real limitation on their application. The most interesting AMs are surely phosphorus and antimony, which lead to the formation of Na3P (2596 mAh g−1) and Na3Sb (660 mAh g−1), respectively. Interestingly, silicon, while very promising in LIBs, is practically inactive in NIBs. While phosphorus delivers a very high theoretical capacity, antimony has shown the greatest affinity for sodium [125]. The family of conversion materials (CM) mainly includes metal oxides and sulphides, even though other families, such as metal selenides, carbodiimides, halides, and phosphides have been explored [126], all of which underwent the so-called conversion reaction [127] with Na. CMs can deliver high specific and gravimetric capacities with excellent cycling stability, but they also face at least one critical issue related to their low initial coulombic efficiency or high volume expansion. The study of the electrochemical and aging mechanisms in both AMs and CMs has been rather challenging, given that multi-phase systems are often obtained during cycling, and many of the species formed are amorphous, nanosized, and especially metastable. The development of specific operando tools is therefore mandatory for the study of these materials [128, 129]. Current and future challenges The main challenges for AMs and CMs in NIBs are related to volume expansion and both the ionic and electronic conductivities of these materials. The large reversible volume exchange during charge/discharge cycles produces mechanical stress, which, on the one hand, rapidly leads to the electrochemical pulverisation of the electrode material, and, on the other hand, exposes new portions of the metal surface to the electrolyte at each cycle, leading to continuous electrolyte degradation and thus difficult stabilisation of the SEI. Moreover, in some cases, such as that of phosphorus, the active material is intrinsically insulating, hindering electronic percolation in the electrode. Control of these phenomena is necessary for practical applications, and several strategies have been proposed to reduce the consequences of drastic volume changes and to boost electronic conduction. The simplest approach is to optimise the electrode formulation using conducting additives, electrolytes, and binders. In the case of phosphorus, for instance, the addition of 1D (carbon nanotubes) or 2D (graphene) nanomaterials to the electrode formulation has been shown to improve the conductivity of C/P composites, leading to an outstanding capacity retention of 80% over 2000 cycles [130], despite noticeably impacting capacity and energy performance. Nanostructuring is also a common approach to accommodate the stress and strain in AMs and CMs without pulverising the electrode, and to improve the ionic and electronic transport pathways. For instance, tin particles smaller than 10 nm allow a decrease in the strain, effectively mitigate pulverisation, and also limit their aggregation during cycling. Such nanoparticles, homogeneously embedded in a carbon matrix, deliver a stable capacity of 415 mAh g−1 after 500 cycles at 1 A g−1 [131]. Even tuning the morphology and porosity of the materials may allow an improvement in the performance. An interesting way to do so is to transform bulk materials into 2D materials which can buffer volume expansion during alkali insertion [132]. For instance, phosphorene synthesised by exfoliating phosphorus shows a rapid charge transfer between less-conductive phosphorene interlayers. In the same spirit, layered graphene/phosphorene electrodes achieved a reversible capacity of 2440 mAh g−1 (per gram of P) over 100 cycles [133]. 28PDF Image | 2021 roadmap for sodium-ion batteries
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