In Situ Neutron Reflectometry Study of a Tungsten Oxide/Li-Ion Battery Electrolyte Interface

The solid electrolyte interface/interphase (SEI) is of great importance to the viable operation of lithium-ion batteries. In the present work, the interface between a tungsten oxide electrode and an electrolyte solution consisting of LiPF6 in a deuterated ethylene carbonate/diethyl carbonate solvent was characterized with in situ neutron reflectometry (NR) at a series of applied electrochemical potentials. NR data were fit to yield neutron scattering length density (SLD) depth profiles in the surface normal direction, from which composition depth profiles were inferred. The goals of this work were to characterize SEI formation on a model transition-metal oxide, an example of a conversion electrode, to characterize the lithiation of WO3, and to help interpret the results of an earlier study of tungsten electrodes without an intentionally grown surface oxide. The WO3 electrode was produced by thermal oxidation of a W thin film. Co-analysis of NR and X-ray reflectivity data indicated that the stoichiometry of the thermal oxide was WO3. As the electrode was polarized to progressively more reducing potentials, starting from open circuit and down to +0.25 V versus Li/Li+, the layer that was originally WO3 expanded and increased in lithium content. The reduced electrode consisted of two to three layers: an inner layer (the evolving conversion electrode) which may have been mixed W and Li2O and unreacted WO3 or LixWO3, a layer rich in protons and/or lithium, possibly corresponding to LiOH or LiH (the inner SEI), and an outermost layer adjacent to the solution with an SLD close to that of the solution, possibly consisting of lower SLD species with solution-filled porosity or deuteron-rich species derived from the solvents (the outer SEI), though the presence of this layer was tenuous. For the steps in the direction of more oxidizing potentials, the evolution of the layer structure was qualitatively the reverse of that seen when stepping toward more negative potentials, though with hysteresis. The SLD gradient suggested that the reaction was not limited by diffusion within the film. No clear phase boundary was evident in the evolving conversion electrode.固体电解质界面/相间层（SEI）对锂离子电池的可靠运行至关重要。本研究采用原位中子反射法（NR），在一系列施加的电化学电位下，对氧化钨电极与以氘代碳酸乙烯酯/碳酸二乙酯为溶剂、六氟磷酸锂（LiPF6）为溶质的电解液之间的界面进行了表征。研究中对中子反射法数据进行拟合，得到了表面法线方向的中子散射长度密度（SLD）深度分布，并由此推导出成分深度分布。本研究的目标包括：对作为转化型电极代表的模型过渡金属氧化物表面的SEI形成过程进行表征、对三氧化钨（WO3）的锂化过程进行表征，以及辅助解读早期一项未经过刻意生长表面氧化层的钨电极研究的结果。本研究使用的三氧化钨电极通过钨薄膜的热氧化制备得到。对中子反射法与X射线反射法数据的联合分析表明，该热氧化层的化学计量比为WO3。当电极从开路电位开始，逐步被极化至相对于Li/Li+参比电极更负的还原电位直至+0.25 V时，初始的WO3层发生膨胀，锂含量也随之升高。还原后的电极由2~3层结构组成：内层为不断演化的转化型电极层，可能为钨与氧化锂（Li2O）的混合物，同时残留未反应的WO3或LixWO3；一层富含质子和/或锂离子的层，可能对应氢氧化锂（LiOH）或氢化锂（LiH），即内层SEI；以及紧邻电解液的最外层，其SLD与电解液相近，可能由低SLD物质（伴随电解液填充的孔隙）或源自溶剂的富氘物质组成，即外层SEI，但该层的存在性尚不明确。当极化方向转向更氧化的电位时，层结构的演化过程与向负电位极化时恰好相反，但存在滞后现象。中子散射长度密度梯度表明，该反应并未受到膜内扩散的限制。在不断演化的转化型电极层中，未观察到明确的相界。