Due to the costly short-term transients, frequency regulation, and load balancing, the electrical power grid faces an urgent need for largescale energy storage. The long durability, high power and energy density, and low cost needed for stationary energy storage posing constant challenges for conventional battery technology inspire people to explore new kinds of energy storage technologies. Here, we assembled an aqueous rechargeable sodium ion battery by using NaMnO 2 as a cathode material and NaTi 2 (PO 4 ) 3 /C composites as anode materials in 2 M CH 3 COONa aqueous electrolyte. This battery system could work in a wide voltage range from 0.5 V to 1.8 V, giving an energy density of 30 W h kg À1 (based on the total mass of active materials) and could retain 75% of the initial capacity after 500 cycles at the 5 C rate. What is more, the earth-abundant precursors, environmental friendliness and inherent safety made this battery system particularly attractive for stationary energy storage applications. † Electronic supplementary information (ESI) available: SEM images of the as-prepared fresh cathode and anode electrodes; cycle performance of the full cell using 1 M Na 2 SO 4 as an electrolyte at a rate of 5 C; XRD pattern and SEM images of the anode material aer 500 cycles; XRD pattern and SEM images of the cathode material aer 500 cycles; the calculation procedure for the energy density/power density of a full cell; and energy densities and average operating voltages of different aqueous rechargeable sodium ion batteries. See

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Honeycomb porous silicon (hp-Si) has been synthesized by a low temperature (200 °C) magnesiothermic reduction of Na2SiO3·9H2O. This process can be regarded as a general synthesis method for other silicide materials. Significantly, hp-Si features excellent electrochemical properties after graphene coating.

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