Nanosized Na4Fe(CN)6/C Composite as a Low‐Cost and High‐Rate Cathode Material for Sodium‐Ion Batteries

Large-scale electric energy storage (EES) requires battery systems not only to have suffi cient storage capacity but also to be cost-effective and environmentally friendly. [ 1 , 2 ] Conventional rechargeable batteries are mainly comprised of Cd, Pb, and Ni compounds, all of which are severely pollutive with neither high energy density nor environmental compatibility. [ 3 ] Currently advanced Li-ion batteries [ 4–6 ] are considered the technology of choice for the development of renewable energy technologies and electric vehicles, however, it is now debatable as to whether or not the world’s lithium reserves can match the demand of upcoming electric or hybrid electric cars. [ 7 ] For larger-scale EES applications, it is vital to develop new batteries where the electrode-active materials must be made from environmentally benign and abundant natural resources. Sodium is one of the most abundant and nontoxic elements in the earth with a very similar redox potential ( − 2.71 V vs. SHE, Standard Hydrogen Electrode) to lithium ( − 3.02 V vs. SHE). In principle, one can build a Na-ion battery in a very similar way to Li-ion batteries by use of the two Na host electrodes to store and release Na + ions reversibly at different potential intervals. However, attempts to realize effective Na-ion batteries have been made with little success up to date, because of the diffi culties in fi nding appropriate cathodic and anodic materials that have an adequate electrochemical capacity and reversibility for Na + insertion reaction. Previous pioneering works have demonstrated reversible Na + intercalation in a variety of carbonaceous materials such as mesocarbon microbeads, [ 8 ] carbon fi ber, [ 9 ] acetylene black, [ 10 ] and pyrolytic carbon. [ 11–14 ] However, suitable cathode materials for Na-ion batteries are diffi cult to fi nd because Na + ions have a 45% larger radius than Li + ions, which restricts insertion and extraction of the Na + ions from the host materials commonly explored for Li-ion batteries. Recently, a number of transition metal oxides, [ 15–17 ] fl uorophosphates, [ 18 , 19 ] and fl uorosulphate [ 20 ] were reported as positive cathodes with a certain capacity and suffi cient cycleability for reversible Na + insertion, however, these materials are kinetically sluggish and energy-consuming during synthesis. Very recently, Goodenough and co-workers [ 21 ] described a half-redox fl ow battery using lithium metal as an anode and a soluble Fe(CN) 6 − 3/–4 redox couple as a liquid cathode, which could only give a theoretical volumetric energy density of 9.8 mAh cm − 3