Delineating Adsorption-Insertion Mechanisms in Hard Carbon Materials for Sodium Ion Storage
College of Chemistry and Molecular Sciences, Wuhan University
Abstract: 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.[1,2] 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.
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.
Biography: Prof. Cao is an expert on electrochemical energy storage materials and systems in Wuhan University. In the past ten years, he has published more than 130 papers with 18 highly cited papers selected by ESI in Web of Science. The sum of Times Cited is 5419. The personal h-index is 42. He received Second Class Prizes of National Award for Technological Invention at 2013. In recent years, his research focuses on new electrode materials and electrolytes for Li-Ion (Na-ion) batteries.