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Global reserves and supply risk for critical elements The share of global reserves for main metal elements in the world is shown in Fig. 2c.4 These values are presented in percent of the world total and are reported for the countries with above 3% global shares. The unbalanced geographical distribution of some critical metals highlights the limitation of the supply chain around the world. For example, the Demo- cratic Republic of Congo solely retains more than 60% of global reserves for cobalt while Russia, Australia, and Cuba are the following countries with less than 5% of the global share in each country. Another critical element is vanadium, which is distributed mainly in China, Russia, and South Africa with about 56%, 25%, and 11% of the global share, respectively, leading to above 92% of the total global share. On the other hand, manganese is distributed in Australia, Asia, Africa, and South America, yet there is a lack of resources in North Amer- ica. Overall, the unequal geographical distribution of critical elements leads to long-term economic, ecological, and political challenges over the world especially for countries with no suf- ficient share reserves. The limited share of global reserves and the increasing demand for critical metals have a great impact on their economic values around the world.121–123 Therefore, ensuring a sustain- able supply of these metals is the essential key for industrial and large-scale manufacturing. The supply risk index, defined as the ratio between demand over supply, for some critical metals from 2015 to the forecast of 2030 is presented in Fig. 2d. Co, V, and Ni are predicted to suffer high supply risks with a rapid demand in the coming years. This risk will be substantial by 2030 and the demand can hit the supply need. In order to ensure a sustainable supply of critical metals for future applications, recycling could be an essential solution. Direct, pyrometallurgy, and hydromet- allurgy recycling methods have been extensively developed for LIBs,3,124,125 which can thus be translated to Na-ion technol- ogy. Even though several encouraging achievements have been obtained in the field of NIBs,126–129 further studies are required to develop more sustainable recycling methods that can be applied to different types of materials.130 Furthermore, battery recyclability and planning for batteries’ end of life (EOL) must be considered in the design step to minimize environmental and economic effects.131,132 Main sodium cathode categories: Oxides, polyanions, and PBAs Over the past 20 years, research for positive electrode mate- rials in NIBs has been mainly centered around layered oxides, polyanions, and PBAs. The representative crystal structure of these materials and their general formulae are given in Fig. 3a. In this figure, M is representative of the transition metals (TMs) in these structures and the most common TMs for each cat- egory are listed as well. In LIBs, there is a great interest focus- ing on high-Ni NMC-based layered oxides (LiNixMnyCo1−x−yO2); nevertheless, there is no clear trend on which class of sodium cathode materials should be the main target for successful commercialization. To gain insight on the most promising candidates for the next generation of sustainable NIBs cathode structures, one needs to hold a full picture of the current experimental results in the liter- ature. Here, we summarized the performance of the 295 Na-ion half-cells using oxide, PBA, or polyanion as cathode materials. The upper cut-off voltage (V) versus the 1st discharge specific capacity (mAh g−1) and the capacity retention (%) versus energy density (Wh kg−1) are shown in Fig. 3b and c, respectively. The size of the circle diameter represents the number of cycles. Over- all, we can observe that high energy densities do not lead to high capacity-retentions and long-term cycling. On average, layered oxides exhibit a higher specific capacity and energy density com- pared to polyanions and PBAs, owing in part to their lower molar mass, but they usually suffer from shorter lifetimes. It should be noted that most layered oxides possess a high electronic con- ductivity in the pristine or desodiated states, allowing them to exhibit excellent electrochemical performance even without any coatings. On the other hand, polyanions are usually electronic insulator due to the strong covalent bonds in the structure and thus carbon coating is widely used to help them achieving good electrochemical behaviors, especially at high current rates. In layered oxides, the layer exfoliation and high-volume expansion occur during cycling, leading to capacity loss and short lifetime. The strong covalent bonds in polyanions and PBAs result in a robust network that can support long-term cycling (Fig. 3c). It is important to note that the operating voltage, stability, and energy density of the oxides strongly depend on the structure, Na-content, and the nature of the transition metals present in the composition. Furthermore, most electrolytes reported in the literature utilize organic carbonate- or ether-based solvents, exhibiting an upper stable voltage at ~ 4.2 to 4.5 V vs. Na+/Na, which also limits the performance of cathode materials. To achieve a compositional design of a sustainable cathode material, implementation of advanced tools such as machine learning on predictive models with descriptors such as crystal structure of materials, their surface characteristics, and their electrochemical performance is vital.133–137 Figure 4 summa- rizes the energy density and capacity retention of different sodium cathode materials depending on their crystal type, space group, and TM-content in the composition. Layered oxides (NaxMO2) are generally classified by the Na crystallographic site and the number of the metal oxide sheets (MO2) in the stacking sequence. In layered oxides, Na+ can reside in the prismatic (P) or octahedral (O) sites between MO2 sheets, and Delmas et al.18 suggested that the resulted structure should be called as P- or O-type. These characters are followed by an index indicating the number of MO2 slabs required to generate a repeating unit. While O-type structure is exclusively encountered in LixMO2, the large ionic radius of Na+ allows the stability of both O- and P-types in NaxMO2 with P2 and O3 are the two common ones. In P2-NaxMO2 layered oxides, Mn, Fe, Ti, Ni, and Co are the most used TMs. Mn is the most frequent TM used in P2 oxides in the range of (0.5–1) thanks to its low cost, high abundance, 6 MRS ENERGY & SUSTAINABILITY // VOLUME XX // www.mrs.org/energy-sustainability-journalPDF Image | cathode materials for sustainable sodium‐ion batteries
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