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Three mononuclear metal complexes [M II (L-N 3 O 2 )(MeCN) 2 ][BPh 4 ] 2 (M = Fe, 1 ; Co, 2 ; Ni, 3 ) were isolated and structurally characterized. Magnetic studies revealed uniaxial magnetic anisotropy for 1 ( D = −17.1 cm −1 ) and 3 ( D = −14.3 cm −1 ) and easy-plane magnetic anisotropy for 2 ( D = +36.9 cm −1 ). Slow magnetic relaxation was observed for complexes 1 and 2 under an applied magnetic field, both of which are dominated by a Raman process. 

NASICON‐type sodium vanadium phosphate (Na₃V₂(PO₄)₃, or NVP) cathode materials have great potential for fast charging and long cycling sodium‐ion batteries (SIBs) similar to lithium iron phosphate (LiFePO₄, or LFP) cathode materials used in lithium‐ion batteries (LIBs). However, the cycle life and energy density in the full cell using NVP materials need to be significantly improved. This paper investigates the degradation mechanisms of NVP‐based SIBs and identifies the Na loss from the cathode to the anode solid electrolyte interphase (SEI) reactions as the main cause of capacity degradation. A new Na‐rich NVP cathode (e.g., Na₄V₂(PO₄)₃, or Na₄VP) is developed to address the Na loss problem. Conventional NVP can be easily transformed into the Na₄VP by a facile and fast chemical solution treatment (30 s). Na‐free‐anode Na₄VP and hard carbon‐Na₄VP full cells are assembled to evaluate the electrochemical properties of the Na‐rich NVP cathode. The Na₄VP cathode provides excess Na to compensate for the Na loss, resulting much longer cycle life in the full cells (>400 cycles) and a high specific energy and power density. Good low‐temperature performance is also observed.