Short-Range Order and Unusual Modes of Nickel Redox in a Fluorine-Substituted Disordered Rocksalt Oxide Lithium-Ion Cathode
Short-Range Order and Unusual Modes of Nickel Redox in a Fluorine-Substituted Disordered Rocksalt Oxide Lithium-Ion Cathode
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DOI：
10.1021/acs.chemmater.8b03794
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发表时间：
2018-10-09
影响因子：
8.6
通讯作者：
Ceder, Gerbrand
中科院分区：
文献类型：
--
作者：
Clement, Raphaele J.;Kitchaev, Daniil;Ceder, Gerbrand
最近证明了氟取代用作锂离子阴极的阳离子无序锂过量过渡金属氧化物（Li 1 + xTM 1-xO 2）中的氧，通过减少充电时的氧损失量和防止高电压下的主要结构重排来改善充电和放电过程的可逆性。然而，人们对氟如何结合氧化物结构并影响其电化学性能知之甚少。在这里，我们使用实验性的（固态核磁共振（NMR）光谱）和理论技术（密度泛函理论（DFT）计算和蒙特卡罗模拟）来研究在Li1.15Ni0.45Ti0.3Mo0.1O1.85F0.15（LNF 15）的充电和放电期间氟和锂周围的局部结构的演变以及氧化还原活性镍的氧化态。阴极我们发现，氟掺杂引入短程有序合成LNF 1 S通过将在富锂网站与五个或六个锂最近的邻居。我们观察到出现新的信号在非原位F-19 NMR光谱在高的电荷状态，我们暂时分配到欠协调，抗磁性氟环境中看到我们的计算模型。我们的理论研究结果还表明，八面体镍离子直接键合到氟遵循不同的氧化机制比那些被六个氧包围，形成Ni 3+中间体，而不是直接从Ni 2+氧化到Ni 4+。虽然Ni 2+向Ni 4+的氧化在氧化物中是不完全的，但由于氧和镍价态之间的重叠，该结果表明，Ni 2 +/Ni 4+氧化还原储层可能是更大程度上利用Ni 2 +/Ni 4+氧化还原储层的有效策略。
Fluorine substitution for oxygen in cation-disordered lithium-excess transition metal oxides (Li1+xTM1-xO2) used as lithium-ion cathodes was recently demonstrated to improve the reversibility of the processes taking place on charge and discharge by reducing the amount of oxygen loss on charge and preventing major structural rearrangements at high voltage. Yet, little is understood about how fluorine incorporates the oxide structure and impacts its electrochemical properties. Here, we use a combination of experimental (solid-state nuclear magnetic resonance (NMR) spectroscopy) and theoretical techniques (density functional theory (DFT) calculations and Monte Carlo simulations) to investigate the evolution of the local structure around fluorine and lithium and the oxidation state of redox-active nickel during charge and discharge of the Li1.15Ni0.45Ti0.3Mo0.1O1.85F0.15 (LNF15) cathode. We show that fluorine doping introduces short-range order in as-synthesized LNF1S by incorporating in lithium-rich sites with five or six lithium nearest neighbors. We observe the emergence of new signals in the ex situ F-19 NMR spectra taken at high states of charge, which we tentatively assign to undercoordinated, diamagnetic fluorine environments seen in our computed models. Our theoretical results also suggest that octahedral nickel ions directly bonded to fluorine follow a different oxidation mechanism than those surrounded by six oxygens, forming Ni3+ intermediates instead of oxidizing from Ni2+ directly to Ni4+. While the oxidation of Ni2+ toward Ni4+ is incomplete in oxides, due to overlap between the oxygen and the nickel valence states, this result suggests that fluorination may be an efficient strategy to utilize the Ni2+/Ni4+ redox reservoir to a greater extent.