The surface termination of MXenes greatly determines the electrochemical properties and ion kinetics on their surfaces. So far, hydroxyl-, oxygen-, and fluorine-terminated MXenes have been widely studied for energy storage applications. Recently, sulfur-functionalized MXene structures, which possess low diffusion barriers, have been proposed as candidate materials to enhance battery performance. We performed first-principles calculations on the structural, stability, electrochemical, and ion dynamic properties of Li-adsorbed sulfur-functionalized groups 3B, 4B, 5B, and 6B transition-metal (M)-based MXenes (i.e., M₂CS₂ with M = Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W). We performed phonon calculations, which indicated that all of the above M₂CS₂ MXenes, except for Sc, are dynamically stable at T = 0 K. The ground-state structure of each M₂CS₂ monolayer depends on the type of M atom. For instance, while sulfur prefers to sit at the FCC site on Ti₂CS₂, it occupies the HCP site of Cr-based MXene. We determined the Li adsorption configurations at different concentrations using the cluster expansion method. The highest maximum open-circuit voltages were computed for the group 4B element (i.e., Ti, Zr, and Hf)-based M₂CS₂, which are larger than 2.1 V, while their average voltages are approximately 1 V. The maximum voltage for the group 6B element (i.e., Cr, Mo, W)-based M₂CS₂ is less than 1 V, and the average voltage is less than 0.71 V. We found that S functionalization is helpful for capacity improvements over the O-terminated MXenes. In this respect, the computed storage gravimetric capacity may reach up to 417.4 mAh/g for Ti₂CS₂ and 404.5 mAh/g for V₂CS₂. Ta-, Cr-, Mo-, and W-based M₂CS₂ MXenes show very low capacities, which are less than 100 mAh/g. The Li surface diffusion energy barriers for all of the considered MXenes are less than 0.22 eV, which is favorable for high charging and discharging rates. Finally, ab initio molecular dynamic simulations performed at 400 K and bond-length analysis with respect to Li concentration verify that selected promising systems are robust against thermally induced perturbations that may induce structural transformations or distortions and undesirable Li release.

中文翻译：

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锂离子电池应用中的硫官能化MXene的评估

MXene的表面终止极大地决定了其表面的电化学性质和离子动力学。迄今为止，已广泛研究了羟基，氧和氟封端的MXene，用于储能应用。近来，已经提出具有低扩散阻挡层的硫官能化的MXene结构作为增强电池性能的候选材料。我们对基于锂的硫官能团3B，4B，5B和6B过渡金属（M）的MXenes（即M ₂ CS ₂）的结构，稳定性，电化学和离子动力学性质进行了第一性原理计算M = Sc，Ti，Zr，Hf，V，Nb，Ta，Cr，Mo和W）。我们进行了声子计算，表明上述所有M ₂ CS ₂除Sc外，MXene在T = 0 K时是动态稳定的。每个M ₂ CS ₂单层的基态结构取决于M原子的类型。例如，尽管硫更倾向于位于Ti ₂ CS ₂的FCC位置，但它占据了Cr基MXene的HCP位置。我们使用簇扩展方法确定了不同浓度下的Li吸附构型。对于基于4B组元素（即Ti，Zr和Hf）的M ₂ CS ₂计算出最大的最大开路电压，该电压大于2.1 V，而它们的平均电压约为1V。最大电压用于基于6B族元素（即Cr，Mo，W）的M ₂ CS₂小于1 V，平均电压小于0.71V。我们发现S官能化有助于提高O末端MXene的容量。在这方面，对于Ti ₂ CS ₂，计算出的存储重量容量可以达到417.4 mAh / g，对于V ₂ CS ₂可以达到404.5 mAh / g 。基于Ta，Cr，Mo和W的M ₂ CS ₂MXene的容量非常低，低于100 mAh / g。所有考虑的MXene的Li表面扩散能垒均小于0.22 eV，这有利于高充电和放电速率。最后，在400 K下进行的从头算起的分子动力学模拟和有关Li浓度的键长分析证明，所选的有前途的系统对于抵抗可能引起结构转变或变形以及不希望的Li释放的热诱导扰动具有鲁棒性。