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good cycle ability and rate capability [150], but their very small energy density has hindered their development. In particular, among P1 structure based cathode materials, their electrochemical performance is outperformed by manganese-based oxide with copper doping, i.e., Na2.3Cu1.1Mn2O7-δ that surpassed most of the copper-doped cathode materials, with its energy density of 383 Wh kg−1 (capacity of 106.6 mA h g−1, capacity retention 95.8% after 1000 cycles at 20C) and its average voltage Materials 2020, 13, 3453 13 of 58 of 3.6 V (Figure 6) [151]. Figure 6. Electrochemical performance of P1-Na2.3Cu1.1Mn2O7-δ (P1-NCM) in half cells and full Figure 6. Electrochemical performance of P1-Na2.3Cu1.1Mn2O7-δ (P1-NCM) in half cells and full cells. cells. (a) Typical potential profiles of P1-NCM//Na at 0.1C rate. (b) Capability and (c) potential (a) Typical potential profiles of P1-NCM//Na at 0.1C rate. (b) Capability and (c) potential profiles of profiles of P1-NCM//Na at various current rates. (d) Cycling performance and Coulombic efficiency of P1-NCM//Na at various current rates. (d) Cycling performance and Coulombic efficiency of P1- P1-NCM//Na at 20C rate. (e) Summary of cycling stability for conventional layered metal oxide materials. NCM//Na at 20C rate. (e) Summary of cycling stability for conventional layered metal oxide materials. (f) Typical potential profiles of P1-NCM//hard carbon at 0.1C rate. (g) Capability of P1-NCM//hard (f) Typical potential profiles of P1-NCM//hard carbon at 0.1C rate. (g) Capability of P1-NCM//hard carbon at various current rates. (h) The cycling performance and Coulombic efficiency of P1-NCM//hard carbon at various current rates. (h) The cycling performance and Coulombic efficiency of P1- carbon at 2C rate. Reproduced with permission from [151]. Copyright 2018 Elsevier. NCM//hard carbon at 2C rate. Reproduced with permission from [151]. Copyright 2018 Elsevier. 2.7. Mixed Polyanionic Compounds V-based pyrophosphate compounds look more promising than Fe-based ones, even though their Synergetic effects resulting from the mixing of phosphates and polyphosphates have been specific capacity is almost the same (80 mA h g−1), because their operating voltage is higher, whic−h1 explored. Na4Mn3(PO4)2(P2O7) as a cathode demonstrated a reversible capacity of 109 mA·h·g at a rate of C/20 at the Mn2+/Mn3+ redox potential of 3.84 V vs. Na+/Na, demonstrating an energy density of 416 Wh·kg−1. [153]. Contrary to other Mn-based compounds with small cycle ability due to the JT distortion due to Mn3+, the cycle stability and the rate capability were good. The first-principle calculations showed that these features result from the low-activation-energy barriers of the three-dimensional Na diffusion pathways. Moreover, the JT distortion opens up sodium diffusion channels, contrary to the situation met in most manganese-based electrodes [154,155]. Na4Fe3(PO4)2(P2O7) (NFPP) integrates the advantages of both iron-based phosphates (NaFePO4) and pyrophosphates (Na2FeP2O7) but both ionic and electronic conductivities are very low. Recently, however, a facile spray-drying route was used to synthesize a NFPP@rGO composite in which NFPP particles with an average size of about 60 nm are homogeneously enwrapped by three-dimension (3D) interconnected rGO networks [156]. This material demonstrated a capacity of 128 mA·h·g−1 at 0.1C, an outstanding rate capability (35.1 mA·h·g−1 at 200C) and long cycling life (62.3% of capacity retention over 6000 cycles at 10C rate). Na7V4(P2O7)4(PO4) operates at 3.88 V vs. Na+/Na, with a good cycle ability (capacity retention of 78% over 1000 cycles) [157]. When nano-structured, it can deliver 80% of the theoretical capacity at 10C rate and 95% of the initial capacity after 200 cycles [158]. The problem, however, is the small capacity (73 mA·h·g−1) which limits its practical application. In Na3M(CO3)(PO4) with M = Mn [159], or M = Fe [160], the capacity is higher (120–125 mA·h·g−1); but now, it is the poor rate capability that hinders their application.PDF Image | Electrode Materials for Sodium-Ion Batteries
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