With the increasing environmental problems caused by conventional energy sources and the gradual depletion of oil resources, clean energy is becoming an important topic for the whole world. As an electrochemical energy storage device, the lithium ion battery, which has the highest energy density among secondary batteries, has been widely used in portable electronic devices, and has also been proposed for use in electric vehicles and large-scale energy storage. [1][2][3] However, the performance of current lithium ion batteries cannot meet the requirements in these areas in terms of high power density, long cycle life, and safety. Graphite is widely used as the anode material for Li-ion batteries. The lithiation potential is below 0.2 V versus Li/Li + . This voltage is close to the lithium stripping voltage, especially at high rate, which may cause a safety issue. In addition, a layer of electronically insulating solid-electrolyte interphase (SEI) is inevitably formed on the surface of graphite below 1.0 V versus Li/Li + . Also the graphite anode undergoes a 9% volume variation during full lithium insertion and extraction. Spinel Li 4 Ti 5 O 12 has a relatively high lithiation voltage plateau at 1.54 V versus Li/Li + , which can avoid the formation of the SEI and is very safe. [ 2 , 3 ] In particular, as a zero-strain insertion material, [ 4 ] it has excellent cycling performance. These features make it a promising anode material for large-scale long-life energy storage batteries. However, Li 4 Ti 5 O 12 has pretty low electronic conductivity (ca. 10 − 13 S cm − 1 ) and moderate Li + diffusion coeffi cient (10 − 9 -10 − 13 cm 2 s − 1 ); [ 5 ] thus the high rate performance is not satifi ed for such applications.The most commonly used strategies to solve this problem are to reduce the particle size [ 6 , 7 ] and to coat conductive materials on the Li 4 Ti 5 O 12 surface. [8][9][10][11] Reducing the particle size decreases the lithium diffusion length; therefore the electroactivity and/or rate capability of electrode materials can be improved. Coating conductive materials on the surface enhances the surface conductivity and the electrical contact in the electrode. Several methods of surface modifi cation on Li 4 Ti 5 O 12 have been developed to increase its electrical conductivity and electrical contact, such as using highly conductive carbon, metal, or metal nitrides. [10][11][12][13] These methods signifi cantly improved the electrochemical performance at high rate, however, most of the processes are either complex or have to be performed at high temperature ( > 600 ° C).Recently, porous electrode materials have attracted much attention because of their large contact surface area with the electrolyte and the possibility of forming a 3D mixed conducting network in which metallized mesopores allow both Li + and e − to migrate rapidly, leading to a superior rate performance. [ 14 ] In the work reported here, an ionic liquid was used as a carbon precursor to form a 3D mixed conducting network in porous Li 4 Ti 5 O...

In the fi eld of energy storage, lithium (Li)-ion batteries dominate the portable consumer electronic market because of their high energy density. Recently, however, the sodium (Na)-ion battery has again aroused interest for large-scale energy storage due to Na abundance. [1][2][3][4][5][6] Conventionally, the chemistry behind Li-ion and Na-ion batteries involves transition metal elements, [ 4 , 6 ] thus giving rise to problems of cost and environmental concern. Therefore, intensive efforts have been aimed at the development of new Li storage materials shifting from inorganic to organic compounds. [ 7 ] Numerous advantages exist in using organic materials as electrodes for energy storage, such as their tremendous chemical compounds, the tuning of the redox potential in a wide range, possible multi-electron reactions, the abundant resources from biomass, and ease in recycling. [7][8][9][10][11][12] A number of Li-containing organic compounds have been demonstrated to have high lithium storage capacity, good cycleability, and moderate rate capability, making them promising applications in Li-ion batteries. [ 7 , 10-12 ] However, there is no report on the use of such organic compounds in Na-ion batteries.In this contribution, a carboxylate-based organic material, disodium terephthalate (Na 2 C 8 H 4 O 4 ), is introduced as a novel anode material for low-cost room-temperature Na-ion batteries. To the best of our knowledge, this is the fi rst time that an organic compound is reported for the use as an anode material for Na-ion batteries. This material exhibits a low Na insertion voltage at 0.29 V vs. Na + /Na and a high reversible capacity of 250 mAh/g with excellent cycleability. It is found that Na storage performance can be further improved by a thin layer of Al 2 O 3 coating on the Na 2 C 8 H 4 O 4 electrode surface.Scheme 1 shows the molecular structure of Na 2 C 8 H 4 O 4 . The existence of two carbonyl groups allows for the insertion and deinsertion of two Na ions, corresponding to a theoretical capacity of 255 mAh/g. In order to confi rm the purity of the as-received Na 2 C 8 H 4 O 4 , nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) measurements were performed. Both NMR and FTIR results as shown in Figures S1 and S2 reveal the high purity of the sample. The Na 2 C 8 H 4 O 4 has an orthorhombic structure and can be indexed in space group Pbc21, according to JCPDS card No. 00-052-2146 ( Figure S3). [ 13 ] The lattice constants of Na 2 C 8 H 4 O 4 are a = 3.5480 Å, b = 10.8160 Å, and c = 18.9943 Å, and its lattice volume is V = 728.92 Å 3 . The XRD pattern of as-received Na 2 C 8 H 4 O 4 showed preferentially oriented (006) and (008) planes. Thus, refi ning the structure is diffi cult. This can be evidenced by the scanning electron microscopy (SEM) image in Figure 1 b, where the as-received sample is shown to have a fl ake-like structure with very large size near 100 μ m. To reduce the particle size and increase electronic conductivity, the sample wa...

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