Na ion batteries (NIBs) have attracted much attention in the field of electrochemical energy storage, due to the natural abundance of Na resources. The practical applicability of NIBs relies on the development of electrode materials with high performance and low cost. Presently the need for high performing anode materials is the main bottleneck in the full cell performance. Due to the large ionic radius (1.02 Å for Na compared to 0.76 Å for Li) and relatively high ionization potential (5.139 eV), Na ions cannot intercalate into graphite regions (interlayer spacing of 0.335 nm) in carbonate electrolyte. This limitation is partially lifted in nongraphitizable hard carbon, which is currently the most promising anode material available showing specific capacity over 300 mAh g-1 and average potential of 0.15 V vs. Na/Na+. Hard carbon has a “house of cards” structure containing graphite-like microcrystallites and amorphous region. The microcrystallites consist of few approximately parallel graphene sheets stacked together with large d-spacing (0.36-0.4 nm). Besides, these materials have numerous active sites such as edges, defects and functional groups containing O, N, P, S, etc. Three different Na storage environments and three corresponding modes have been reported in the literature for Na interactions with hard carbon: adsorption on the surface active sites; nanopore filling analogous to adsorption and intercalation between the graphene layers with suitable d-spacing. Experimentally two distinct voltage regions have been observed: a slope above 0.1 V and a plateau below 0.1 V. Although a number of studies have been conducted aiming to elucidate Na interactions with hard carbon, the assignment of Na storage mechanisms to different voltage regions is still debated. In order to resolve some of the differences in the literature data and develop a comprehensive understanding of Na storage, we synthesized a series of nanostructured hard carbon materials with controlled architectures. Using a combination of in-situ XRD mapping, ex-situ NMR, EPR, electrochemical techniques and simulations, an “adsorption-intercalation” (A-I) mechanism is established for Na ion storage. During the initial stages of Na insertion, Na ions adsorb on the defect sites of hard carbon with a wide adsorption energy distribution, producing a sloping voltage profile. In the second stage, Na ions intercalate into graphitic layers with suitable spacing to form NaCx compounds similar to the Li ion intercalation process in graphite, producing a flat low voltage plateau as illustrated in Scheme 1.[1-3] The clarification of Na storage mechanism provides new insights critical for knowledge-based design and synthesis of carbonaceous materials with improved Na ion storage capacity and reduced coulombic efficiency, that is, design of highly ordered layered structures with suitable layer spacing (0.37~0.38 nm) to improve Na storage capacity in the low potential plateau region, and low defective carbon to improve coulombic efficiency to a great extend to meet the practical application.
Scheme 1. Schematic illustration of the mechanisms for Na-ion storage in hard carbon. “adsorption-intercalation” mechanism.
1. Y. Cao, L. Xiao, M. L. Sushko, W. Wang, B. Schwenzer, J. Xiao, Z. Nie, L. V. Saraf, Z. Yang, J. Liu, Nano Lett., 2012, 12, 3783。 2. L. Xiao, Y. Cao, W. A. Henderson, M. L. Sushko, Y. Shao, J. Xiao, W. Wang, M. H. Engelhard, Z. Nie, J. Liu, Nano Energy, 2016, 19, 279. 3. Shen Qiu, Lifen Xiao, Maria L. Sushko, Kee Sung Han, Yuyan Shao, Mengyu Yan, Xinmiao Liang, Liqiang Mai, Jiwen Feng, Yuliang Cao, Xinping Ai, Hanxi Yang and Jun Liu, Advanced Energy Materials, Accepted.