Kim Seong Shik, Agyeman-Budu David N, Zak Joshua J, Andrews Jessica L, Li Jonathan, Melot Brent C, Nelson Weker Johanna, See Kimberly A
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States.
Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States.
Chem Mater. 2024 Jun 21;36(13):6454-6463. doi: 10.1021/acs.chemmater.4c00490. eCollection 2024 Jul 9.
New energy storage methods are emerging to increase the energy density of state-of-the-art battery systems beyond conventional intercalation electrode materials. For instance, employing anion redox can yield higher capacities compared with transition metal redox alone. Anion redox in sulfides has been recognized since the early days of rechargeable battery research. Here, we study the effect of overlap in controlling anion redox by shifting the metal band position relative to the S band. We aim to determine the effect of shifting the band position on the electronic structure and, ultimately, on charge compensation. Two isostructural sulfides LiNaFeS and LiNaCoS are directly compared to the hypothesis that the Co material should yield more covalent metal-anion bonds. LiNaCoS exhibits a multielectron capacity of ≥1.7 electrons per formula unit, but despite the lowered Co band, the voltage of anion redox is close to that of LiNaFeS. Interestingly, the material suffers from rapid capacity fade. Through a combination of solid-state nuclear magnetic resonance spectroscopy, Co and S X-ray absorption spectroscopy, X-ray diffraction, and partial density of states calculations, we demonstrate that oxidation of S nonbonding states to S occurs in early states of charge, which leads to an irreversible phase transition. We conclude that the lower energy of Co bands increases their overlap with S bands while maintaining S nonbonding states at the same higher energy level, thus causing no alteration in the oxidation potential. Further, the higher crystal field stabilization energy for octahedral coordination over tetrahedral coordination is proposed to cause the irreversible phase transition in LiNaCoS.
新型储能方法不断涌现，以提高现有电池系统的能量密度，超越传统的嵌入电极材料。例如，与单独的过渡金属氧化还原相比，采用阴离子氧化还原可产生更高的容量。自可充电电池研究早期以来，硫化物中的阴离子氧化还原就已得到认可。在此，我们通过相对于S能带移动金属能带位置来研究重叠在控制阴离子氧化还原方面的作用。我们旨在确定能带位置移动对电子结构的影响，并最终对电荷补偿的影响。将两种同构硫化物LiNaFeS和LiNaCoS直接进行比较，以验证Co材料应产生更多共价金属 - 阴离子键的假设。LiNaCoS表现出每分子式单位≥1.7个电子的多电子容量，但尽管Co能带降低，阴离子氧化还原的电压却与LiNaFeS相近。有趣的是，该材料的容量迅速衰减。通过结合固态核磁共振光谱、Co和S的X射线吸收光谱、X射线衍射以及态密度计算，我们证明在充电早期S非键态会氧化为S，这会导致不可逆的相变。我们得出结论，Co能带较低的能量增加了它们与S能带的重叠，同时将S非键态维持在相同的较高能量水平，从而不会改变氧化电位。此外，八面体配位相对于四面体配位具有更高的晶体场稳定能，这被认为是导致LiNaCoS中不可逆相变的原因。
