Concerns over global energy supply have spurred intensive research on energy storage technologies to enable electric vehicles and an electric grid that can utilize renewable energy sources such as wind and solar. [1][2][3][4][5][6][7][8][9][10][11] Recently, several emerging aqueous energy storage technologies have been demonstrated that feature low cost, high rate capability, and durability. The potential for use in large-scale grid application has revived focus on these systems. [12][13][14][15][16][17] Here, we demonstrate ultrafast rate capability ( > 100C current rates) and superior high-rate cycling stability ( > 1500 cycles) in aqueous NaTi 2 (PO 4 ) 3 /Na 0.44 MnO 2 (NTP/NMO) cells, an electrochemical couple which has the highest specifi c and volumetric energy amongst reported aqueous Na-ion systems to date.Aqueous rechargeable Li-ion batteries previously garnered interest as a possible substitute for conventional aqueous rechargeable systems such as Pb-acid and NiMH. These were fi rst proposed and demonstrated by the Dahn group using VO 2 /LiMn 2 O 4 electrodes and LiNO 3 aqueous solution as the electrolyte. [18][19][20] Additional electrode couples were later studied, most of which exhibited limited cycle life. [21][22][23] However, the Xia group was able to raise the cycling stability of a LiTi 2 (PO 4 ) 3 /LiFePO 4 aqueous rechargeable Li-ion batteries to a commercially acceptable level. [ 15 ] Of equal or greater interest, due to the high natural abundance and low cost of Na, are aqueous rechargeable Na-ion batteries. A variety of Na intercalation electrode compounds have been identifi ed. [ 6 ] The fi rst working demonstration of an aqueous Na-ion full cell was presented by Okada using a NMO cathode and NTP anode. [ 24 ] However, only two low rate cycles were shown for these cells. The reported properties of NMO and NTP suggest that this electrochemical couple may be capable of stable and high rate electrochemical performance. [ 16 , 25 ] In the present work, we fi rst show that without modifi cation, the low electronic conductivity of the NTP anode causes it to be rate-limiting. Secondly, upon optimizing the synthesis of NTPcarbon composite anodes to remove the electronic conductivity limitation, ultrafast rate capability and superior high-rate cycling stability are obtained. Finally, careful measurement of the NTP crystallite size in the modifi ed and unmodifi ed electrodes allow a lower limit for the Na diffusivity in NTP to be established.NMO was synthesized by solid-state reaction as described in the Experimental Section. The crystal structure derived from Reitveld refi nement, and an SEM image of the as-prepared powder are shown in Figure 1 . The structure of the as-prepared compound was refi ned using the atomic coordinates of isostructural Na 4 Mn 4 Ti 5 O 18 ( Pbam space group). The orthorhombic lattice cell parameters thus obtained are a = 9.09355(26) Å, b = 26.4628(5) Å and c = 2.82683(12) Å with wRp = 3.72%, in good agreement with literature. [26][27][28][29][30][31] The inset in Figur...