Electrolyte design or functional development is very effective at promoting the performance of sodium-ion batteries, which are attractive for electrochemical energy storage devices due to abundant sodium resources and low cost. The roadmap of the sodium ion batteries based on electrolyte materials was drawn firstly and shows that the electrolyte type decides the electrochemical window and energy density.

A structure optimized Prussian blue analogue Na1.76Ni0.12Mn0.88[Fe(CN)6]0.98 (PBMN) is synthesized and investigated. Coexistence of inactive Ni(2+) (Fe-C≡N-Ni group) with active Mn(2+/3+) (Fe-C≡N-Mn group) balances the structural disturbances caused by the redox reactions. This cathode material exhibits particularly excellent cycle life with high capacity (118.2 mA h g(-1)).

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A micro-cubic Prussian blue (PB) without coordinated water is first developed by electron exchange between graphene oxide and PB. The obtained reduced graphene oxide-PB composite exhibited complete redox reactions of the Fe sites and delivered ultrahigh electrochemical performances as well as excellent cycling stability as a cathode in sodium-ion batteries.

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Crumbled graphene sheet-wrapped nano-Fe2O3 (Fe2O3@GS) composites with a three-dimension (3D) hierarchical structure have been made by a facile and efficient spray drying route with a following mild heat reduction in air. In the as-obtained composites, the crumpled GS around Fe2O3 particles could not only provide a 3D conductive matrix but also buffer the volume change of Fe2O3. Fe2O3 particles which evenly distribute in the crumpled GS could also act as spacers to avoid the close restacking of GS. Compared to the bare Fe2O3, the Fe2O3@GS composites as Li ion battery anodes show dramatically improved electrochemical performance including cyclic stability and rate capability owing to the special encapsulated structure and the excellent synergistic effect between the two components.

Layered transition metal oxides Na x Ni 1/3 Fe 1/3 Mn 1/3 O 2 are promising cathode materials for sodium ion batteries. Here we report the large scale synthesis of NaNi 1/3 Fe 1/3 Mn 1/3 O 2 (NaNFM) by using hydroxide co-precipitation combined with solid-state reaction method. The obtained NaNFM showed good electrochemical performance with 1C rate (130 mA g −1 ) capacities of 125 mAh g −1 (55 • C), 122 mAh g −1 (25 • C), 95 mAh g −1 (−10 • C), and 72 mAh g −1 (−20 • C), respectively. 1 Ah soft-packed batteries using the prepared NaNFM as cathode and hard carbon as anode were prepared and investigated. Electrochemical tests showed that the as prepared 1 Ah soft-packed batteries exhibited excellent cycling performance with capacity retention over 73% after 500 cycles at 1C rate. The ex situ XRD experiments revealed that the charge/discharge process of Na x Ni 1/3 Fe 1/3 Mn 1/3 O 2 cathode showed excellent reversibility and the layered crystallographic structure of the NaNFM electrode remained stable after 500 cycles. This work demonstrates the NaNi 1/3 Fe 1/3 Mn 1/3 O 2 is a promising cathode material for practical application in sodium ion batteries.

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