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Chem Soc Rev Review Article hard carbon is thought to be the best candidate in practical applications due to its low operation voltage, cycling stability, and high Coulombic efficiency at the first cycle. Reviewing many kinds of cathode and anode materials in the present review, in summary, there is no doubt about the practical use of the O3 type cathode and hard carbon anode assembly for successful full cell configuration. At present, leading companies are promoting several proto- types of SIBs for commercialization. Sumitomo Chemical Co. Ltd demonstrated a pouch type full cell using the O3-type NaNi0.3Fe0.4Mn0.3O2 cathode in 2013.638 Recently, CNRS and RS2E have launched the first commercial cylindrical 18650 SIBs, which guaranty 2000 cycles with an energy density of 90 W h kg1.639 In 2015, FRADION developed prototype SIBs and successfully mounted SIBs into E-bikes.640 Sharp Laboratory also promotes SIBs that adopt O3 type or Prussian white cathodes, hard carbon anodes, and conventional carbonate- based electrolytes for high energy density SIBs.641,642 Recent a report by Choi et al. and Doron et al.643 proposed the impor- tance of minimization of energy consumption ($ W h1) rather than lowering of material costs to successful spread of SIBs towards energy storage applications. Also, SIBs are faced with intrinsic low energy density relative to LIBs, and this further lowers volumetric energy density in the limited space in battery pack. One option proposed is to improve the tap density of cathode materials. Our recent work on the radially aligned hierarchical columnar (RAHC) structure87 and spoke-like nanorod assembly (SNA) in a spherical secondary particle566 can be a good example to have high capacity and, more importantly, to maximize the electrode density achieved from its unique feature of intrinsic robustness under high compres- sion pressure in the electrode pressing process. To date, specific energy density of SIBs was only estimated based on the weight of active materials by calculation with the assump- tion of 300 mA h g1 for hard carbon as a negative electrode material for full cells. Such an estimated specific energy density of SIBs with some of the cathode materials reaches and/or exceeds 300 W h kg1.18,643 Unfortunately, in consideration of not only the active materials but all components that compose SIBs, the actual value of energy density in the first commer- cial cylindrical 18650 SIBs is dramatically lowered below 100 W h kg1. Therefore, it is very important to keep in our mind how we can improve the energy density of a SIB system when developing high capacity cathode and anode materials and stable electrolytes including additives that operate in a high voltage region, in particular, to maximize the energy density. Furthermore, more systematic studies on surface modification of active materials to minimize the side reactions with electro- lytes, binders, current collectors, and the other components should be intensively progressed so as to advance the SIBs in practical use towards energy storage applications. Conflict of interest The authors declare that there is no conflict of interest for publishing this review in Chemical Society Reviews. close to 100%. Provided that the abnormal behavior is resolved commercialization of high capacity SIBs can be readily advanced, because many kinds of dopants are known to be effective to retain the capacity for long term cycling. For this purpose, pioneering works were conducted using Na3N60,635,636 and Na3P637 as the sacrificing agents to minimize the abnormally high Coulombic efficiency of over 130% at the first cycle and to compensate the sodium deficiency. Although such works could improve the first irreversible capacity of P2-type layer cathodes, the resulting Coulombic efficiency at the first cycle is still not sufficient to fabricate the practical SIBs in comparison with O3-type layer cathodes. Therefore, to compete with LIBs, academic community should revisit O3-type layer cathode materials such as Co-free Na[Ni0.5Mn0.5]O283 and carbon-coated NaCrO2131 which show high Coulombic efficiency at initial cycle and excellent cyclability. The know-hows accumulated from LIBs were further used to synthesize O3-type full concentration gradient (FCG) cathode materials that demonstrate high capacity and good cycling performances.87,160,566 Full cells adopting hard carbon anodes confirmed cycling stability of the materials for a long time. Spoke-like assembly of nanorods greatly improved the strength of the particles, and this enabled unprecedentedly good cycling stability and rate performances. Although the chemistries are very interesting for P2 type layer cathodes for investigation of the structural change during Na+ insertion and extraction, elaboration is required in order to stabilize the Coulombic efficiency at the first cycle. Polyanion materials are attractive because of high operation voltage and cycling stability, whereas low capacities below 100 mA h g1 and more seriously moisture uptake in air are the critical issues to be resolved for those compounds. These suggest suitability of O3 type cathode materials in practical uses for future EVs and energy storage applications. Anode materials are activated by insertion, conversion, and alloying reactions as summarized in Fig. 1. Due to the low operation voltage and extraordinary cycling stability, hard carbon proposed by Dahn et al.22 in 2000 is regarded as the better candidate than any other electrode material for anode materials. Since the first LIBs adopted hard carbon as the anode materials, hard carbon can be the first commercial anode materials even in SIBs as well. Most of all, most urgent is the sodium metal deposition onto the surface of hard carbon at low voltage, in which safety issues should be considered because of the high reactivity of sodium metal. In addition, capacity below 300 mA h g1, which is lower than graphite anodes for LIBs, is not advantageous to improve the energy density of SIBs. Thus, conversion and alloying reaction based materials are intensively being studied to find alternative anode materials that deliver high capacity and do not have sodium metal deposition on discharge. It is evident that these conversion and alloying materials have superiority in capacity, while volume expansion of electrodes in the sodiate state arising from the large ionic size of Na+ is the intrinsic problem for these electrodes. Low Coulombic efficiency at the first cycle is another critical issue to be studied further. Provided that the above-mentioned issues are cleared, phosphorous materials that deliver over 2000 mA h g1 are expected to be some of the candidate materials for anodes to improve energy density of SIBs. So far, various anode materials are under investigation, it is believed that View Article Online Thisjournalis©TheRoyalSocietyofChemistry2017 Chem.Soc.Rev.,2017,46,3529--3614 | 3601 Open Access Article. Published on 28 March 2017. Downloaded on 7/1/2019 3:41:21 AM. 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