Variation in surface energy and reduction drive of a metal oxide lithium-ion anode with stoichiometry: a DFT study of lithium titanate spinel surfaces
Variation in surface energy and reduction drive of a metal oxide lithium-ion anode with stoichiometry: a DFT study of lithium titanate spinel surfaces
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DOI：
10.1039/c6ta05980e
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发表时间：
2016-11
影响因子：
--
通讯作者：
B. Morgan;J. Carrasco;G. Teobaldi
中科院分区：
文献类型：
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作者：
B. Morgan;J. Carrasco;G. Teobaldi
Li4Ti5O12是一种“零应变”锂离子负极材料，在重复的锂插拔循环中表现出极好的稳定性。尽管锂(脱)插层在块体材料中的特征已经得到很好的描述，但我们对表面原子尺度结构及其与电化学行为的关系的理解是不完整的。为了解决这个问题，我们用哈伯德修正的密度泛函理论对Li4Ti5O12(111)，Li7Ti5O12(111)和α-Li2TiO3(100)，(110)和(111)表面进行了模拟，筛选了600多个化学计量比的Li4Ti5O12和Li7Ti5O12(111)表面。对于Li4Ti5O12和Li7Ti5O12，我们发现Li端面比Li/Ti端面更稳定，后者通常会重构。对于α-Li2TiO_3，(100)面的表面能明显低于(110)面和(111)面，与原始的Li_7Ti5O_(12)(111)面竞争。以这些化学计量比表面为参照，我们还模拟了锂表面覆盖率随锂化学势的变化。对于Li4Ti5O12，化学计量比表面在热力学稳定性的全化学势范围内最稳定，而对于Li7Ti5O12，缺Li表面在低Li化学势下稳定。Li7Ti5O12(111)的最高占据电子态比真空能量低2.56 eV。这比金属锂的功函数小0.3 eV，这表明还原的热力学驱动是极端的。相反，α-Li2TiO_3(100)表面的最高占有态比真空度低4.71 eV，这表明还原驱动大大降低。这一结果表明，化学计量比可以强烈地影响金属氧化物电极表面还原的热力学驱动。在此背景下，我们讨论了高度还原的金属氧化物电极涂层的设计，该涂层具有通过平衡化学控制固体-电解液-界面形成的潜力，通过在没有任何外加偏压的情况下通过电极润湿来实现。
Li4Ti5O12 is a “zero-strain” lithium-ion anode material that shows excellent stability over repeated lithium insertion–extraction cycles. Although lithium (de)intercalation in the bulk material has been well characterised, our understanding of surface atomic-scale-structure and the relationship with electrochemical behaviour is incomplete. To address this, we have modelled the Li4Ti5O12 (111), Li7Ti5O12 (111) and α-Li2TiO3 (100), (110), and (111) surfaces using Hubbard-corrected density-functional theory (GGA+U), screening more than 600 stoichiometric Li4Ti5O12 and Li7Ti5O12 (111) surfaces. For Li4Ti5O12 and Li7Ti5O12 we find Li-terminated surfaces are more stable than mixed Li/Ti-terminated surfaces, which typically reconstruct. For α-Li2TiO3, the (100) surface energy is significantly lower than for the (110) and (111) surfaces, and is competitive with the pristine Li7Ti5O12 (111) surface. Using these stoichiometric surfaces as reference, we also model variation in Li surface coverage as a function of lithium chemical potential. For Li4Ti5O12, the stoichiometric surface is most stable across the full chemical potential range of thermodynamic stability, whereas for Li7Ti5O12, Li deficient surfaces are stabilised at low Li chemical potentials. The highest occupied electronic state for Li7Ti5O12 (111) is 2.56 eV below the vacuum energy. This is 0.3 eV smaller than the work function for metallic lithium, indicating an extreme thermodynamic drive for reduction. In contrast, the highest occupied state for the α-Li2TiO3 (100) surface is 4.71 eV below the vacuum level, indicating a substantially lower reduction drive. This result demonstrates how stoichiometry can strongly affect the thermodynamic drive for reduction at metal-oxide-electrode surfaces. In this context, we conclude by discussing the design of highly-reducible metal-oxide electrode coatings, with the potential for controlled solid-electrolyte-interphase formation via equilibrium chemistry, by electrode wetting in the absence of any applied bias.