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Why is sodium-intercalated graphite unstable?

机译：为什么插层石墨不稳定？

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摘要

Na-ion batteries offer an attractive low-cost alternative to Li-ion batteries. Although graphite is used as the negative electrode in conventional Li-ion batteries, attempts to use it in Na-ion batteries have been hampered by the inability of Na to form graphite intercalation compounds (GICs) under moderate conditions. It is generally considered that this is due to the size mismatch between Na and the graphite interlayer spacings, but here we show with detailed first-principles calculations of Li, Na, K, Rb, and Cs GICs that the major reason is the change in chemical bonding between alkali metal (AM) ion and C atoms. van der Waals correction terms are introduced to better reproduce the layered graphite structure, and calculated formation energies of GICs AMC6, where AM = Li, Na, K, Rb, and Cs, are found to become less negative (less stable) as ion size decreases from Cs to Na as a result of weakening ionic bonding, until the formation energy of NaC6 becomes positive. The much smaller Li ion represents an exception to this trend, as its bonds with C atoms contain a covalent component, resulting in a negative formation energy. These subtle differences in staging for the different alkali metals explain why graphite is a good intercalation material for Li and K but not for Na.
机译：钠离子电池是锂离子电池的一种有吸引力的低成本替代品。尽管石墨在传统的锂离子电池中用作负极，但由于钠无法在适度的条件下形成石墨插层化合物（GIC），因此阻碍了将其用于Na离子电池的尝试。通常认为这是由于Na与石墨夹层间距之间的尺寸不匹配所致，但在这里我们通过详细的Li，Na，K，Rb和Cs GIC的第一性原理计算显示出，主要原因是碱金属（AM）离子和C原子之间的化学键。引入范德华校正项以更好地重现层状石墨结构，并计算出GIC AMC 6 的形成能，其中AM = Li，Na，K，Rb和由于离子键的减弱，随着离子尺寸从Cs减小到Na，Cs的负离子变得不太稳定（不稳定），直到NaC 6 的形成能变为正。较小的锂离子代表了这种趋势的一个例外，因为它与C原子的键包含共价成分，导致形成能为负。这些对不同碱金属的分阶段的细微差别解释了为什么石墨对于Li和K来说是一种很好的插层材料，而对于Na来说却不是。

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