Soundharrajan, V.; Alfaruqi, M.H.; Lee, S.; Sambandam, B.; Kim, S.; Kim, S.; Mathew, V.; Pham, D.T.; Hwang, J.-Y.; Sun, Y.-K.; Kim, J.: Multidimensional Na4VMn0.9Cu0.1(PO4)3/C cotton-candy cathode materials for high energy Na-ion batteries. Journal of Materials Chemistry A 8 (2020), p. 12055-12068

odium-ion batteries (SIBs) have attracted great attention for day-to-day applications as a replacement for lithium-ion batteries (LIBs) that deliver high voltage and high energy because of the low battery-preparation cost and vast availability of sodium resources. The recent exploration of Na+ superionic conductor or NASICON-type Na4VMn(PO4)3 (NVMP) cathodes for SIBs is a pioneering approach because of the high working voltage, high theoretical capacity, and stable three-dimensional framework of the NVMP cathodes. However, the inherently low electronic conductivity results in mediocre rate outputs and poor exploitation of the active material. Herein, we report, for the first time, the preparation of a cotton candy-like carbon-coated Cu-doped NVMP or Na4VMn0.9Cu0.1(PO4)3 (NVMCP/C/CC) cathode by a facile and ultrafast pyro-synthetic method. The robust structure of the NVMCP/C/CC and the highly reversible two-phase reaction upon Na-ion insertion/extraction were systematically revealed by the in situ synchrotron XRD and GITT studies, while the DFT calculations established the crucial reasons behind the enhanced electronic conduction of the NVMCP/C/CC. The superior electrochemical properties of the NVMCP/C/CC cathode at low (79 mA h g-1 after 450 cycles at 1.5C) and high current rates (68 mA h g-1 after 3000 cycles at 30C) demonstrate that the combination of a three-dimensional nanoarchitecture, uniform carbon-coating, and Cu-doping is favorable for improving the electrochemical properties of the NVMP cathodes. This journal is