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Figure 3. The three main types of cathode materials for sodium-ion batteries: oxide, polyanions with NASICON as a representative, and PBAs. (a) A representative crystal structure of oxide, polyanion (NASICON), and PBA as the three main types of cathode materials for sodium-ion batteries. The general formula for each type of material is listed below the schematic. M is the representative of TM. The performance of the 295 NIB system using oxide, polyanion, and PBA cathodes is presented as (b) Upper cut off voltage versus capacity, and (c) Capacity retention versus energy density. The circle diameter is representative of cycle numbers. and good electrochemical performance that can ensure high energy densities. Many Mn-rich layered oxides (Mn-content of 0.7–0.8) show high capacity-retention and high cycle life as the electroactivity of Mn can be activated in the average potential range with minimal degradation of the active material. There- fore, Mn-containing layered oxides are usually considered as the main cathode material138,139 for NIBs while other substitutions can be implemented to further enhance their performance. The presence of Ni can increase the operating voltage and thus the energy density; however, high voltages always lead to severe deg- radation processes. This leads to a limited Ni usage in the layer oxides (usually in the range of 0.1–0.3).140–149 Co can increase the intrinsic electronic conductivity of the materials and its usage in a higher amount is desired to optimize the cycling rate and capacity retention. However, our dataset shows that it will be at the cost of energy density.150–152 Ti- and Fe-substitutions are also widely used. The presence of Ti usually lowers the voltage and energy density of the active materials,153,154 but its usage at low concentration helps to shift down the operating volt- age to the electrochemical window of most currently available electrolytes and to lead to higher energy density and capacity retention.155–158 Fe-substitution in NaxMO2 usually increases the material’s theoretical capacity thanks to the reversible activity of Fe4+/Fe3+ redox couple at high voltages in sodium layered oxides.159,160 Furthermore, a recent study has shown that the presence of Fe3+ also helps to obtain reversible anionic activ- ity.140 However, a closer look showed us that higher Fe content can lead to lower capacity retention in a wide range of energy MRS ENERGY & SUSTAINABILITY // VOLUME XX // www.mrs.org/energy-sustainability-journal 7PDF Image | cathode materials for sustainable sodium‐ion batteries
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