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into graphite due to the instability of the Na+ − graphite inter- calation (Na-GIC) and the low potentials of the reaction, which hinder the application of graphite in NIBs.50–54 Na+ can only be inserted into graphite as a solvated form in diglyme (or bis(2- methoxyethyl) ether) solvent, but the capacity of the reaction is too low, which is not suitable for practical applications.55,56 Hard carbon, also known as non-graphitizable carbon, was initially introduced by Dahn et al.57 as anodes for NIBs in 2000. Hard car- bon (with a gravimetric theoretical capacity of 300 mAh g−1 and volumetric theoretical capacity of 420 mAh cm−3)58 can be syn- thesized by a wide range of methods such as chemical, thermal, or biomass-derived processes from various organic compounds at elevated temperatures (700–2000 °C).58 Low-cost precursors and easy synthesis procedures have made hard carbon a promis- ing anode material for NIBs. However, there are still some fun- damental challenges for the implantation of hard carbon in NIBs: (i) its limited practical storage capacity (around 300 mAh g−1), (ii) the electrochemical performance of hard carbon depends strongly on the nature of the precursors as well as processing temperature, which requires a careful optimization, and (iii) the Na+ storage mechanism needs to be better understood by employ- ing advanced characterization techniques.59–61 Besides hard carbon, non-carbonaceous anodes for NIBs were also developed in the last few years.62 Depending on the reaction mechanism with Na+, they can be classified as con- version (metal oxides,63,64 sulfides,65,66 and selenides67,68), alloying [tin (Sn),69–71 bismuth (Bi),72 phosphorus (P),73,74 and antimony (Sb)75,76], or insertion materials (titanium- based oxides,77–79 transition metal chalcogenides,80,81 and MXenes82). Among them, Sn and Sb have shown the greatest promises83–87 due to their high theoretical capacities (with a theoretical gravimetric capacity of 847 mAh g−1 for Sn and 660 mAh g−1 for Sb),88,89 good electrical conductivity (8.7 × 106 S m−1 for Sn and 2.55×106 S m−1 for Sb at 20 °C), low reac- tion potentials vs. Na+/Na (0.2–0.4 V for Sn and 0.4–0.8 V for Sb),89 and less safety concerns associated with them. However, both Sn and Sb suffer significant volume expansions during sodiation process (~ 420% for Sn and ~ 390% for Sb),7,72 which might lead to contact loss and substantial irreversible capaci- ties upon long-term cycling. Furthermore, the high price and the low abundance in the earth’s crust of these two elements (0.00023% for Sn and 0.00002% for Sb)12 have prevented their wide implementation in NIBs. Beyond research level prototypes, the advances in this field have enabled some commercialized energy storage sys- tems based on NIB technology in the past few decades. In the 1980s, prior to the commercialization of LIBs, Elsenbaumer et al.90–92 from Allied Corp. (USA) and Takeuchi et al.93,94 from Hitachi Ltd. (Japan) have introduced the first full-cell model for NIBs with P2-NaxCoO2 as cathode and sodium-lead alloy as anode. The cell could operate up to 300 cycles with no failure. However, the low energy density due to the low operational voltage (< 3 V) did not help these systems compete with the relatively high energy density LIBs available at that time (3.7 V for a Graphite||LiCoO2 cell). Many years later, the first non-aqueous NIB system was introduced by Faradion Limited in 2015. The company, estab- lished in 2011 in the United Kingdom (UK), showcased their first product as an e-bike, powered by NIBs with an energy greater 400Wh per pack. Faradion batteries utilized O3/ P2-type NaaNi1−x−y−zMnxMgyTizO2 layered oxides (130 mAh g−1) as the cathode and hard carbon anode (230 mAh g−1). The cells utilized non-aqueous electrolyte and could oper- ate from − 20 to 60 °C.95–97 In the next few years, several start-ups and companies aiming at commercializing NIBs were created all around the world with a great diversity in the choice of chemistry in the commercialized products. In 2017, the start-up Tiamat98–100 (France) introduced their first Hard carbon||Na3V2(PO4)2F3 cylindrical 18,650 cells, which could deliver an energy density of 100–120 Wh kg−1. At the same time, HiNa company101 (China) developed power banks (120 Wh kg−1) using O3-type Nax[Cu,Fe,Mn]O2 layered oxide cathode. Altris AB (Sweden) and Natron Energy102,103 (USA) have also developed prototypes using PBA cathode materi- als. Recently, Contemporary Amperex Technology Co., Ltd. (CATL)104 (China) announced a NIB prototype (160 Wh kg−1) for electric vehicles (EVs) with a plan for supply chain (target to 200 Wh kg−1) with a cost $40 per kWh in 2023. So far, we provided a historical timeline on the advances in the field of NIBs with examples on efforts on both cathode and anode materials as well as some commercialization attempts. A roadmap of NIBs over the years is shown in Fig. 1. While all these efforts have pushed the performance boundaries of NIBs and highlighted their promise as a complement to LIBs, most of the current reports in the literature are rather scattered on different electrode materials with different cycling condi- tions. Therefore, there remains the need to a clearer roadmap on the design and required performance metrics for NIBs to meet the current market goals. Amongst various components of NIBs, cathode materials are widely considered as the pri- mary limiting factor in part because of their restraint energy density and substantial structural complexity; leading to chal- lenging stability over long cycle life.7 Given the critical role of cathode material in the cost efficacy and performance of NIB systems,5,105 we first discuss the important metrics in the key metals in cathode materials that will impact the supply risk and market price in near future. Next, we provide a perspective on design metrics of cathode materials based on an extensive survey on 295 reports in the past 10 years with the focus on the implemented chemistry and composition, and its correla- tion to battery performance. An outlook on supply risk factor for NIB technology along with performance insights based on a large experimental data set as shown in this work, can set clear directions for future research efforts and pave the path for design of next generations of NIB cathode materials from sustainable and abundance resources. MRS ENERGY & SUSTAINABILITY // VOLUME XX // www.mrs.org/energy-sustainability-journal 3PDF Image | cathode materials for sustainable sodium‐ion batteries
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