Safe, fast, and energy efficient cycling of lithium ion batteries is desired in many practical applications. However, modeling studies predict steep Li⁺ ion gradients in the electrodes during cycling at the higher currents. Such gradients introduce heterogeneities in the electrodes, which make it difficult to predict cell lifetimes as different portions of the cell age at different rates. There is a dearth of experimental methods to probe these concentration gradients across the depth of the electrode. Here we use spatially resolved energy dispersive X-ray diffraction to obtain a “movie” of lithiation and delithiation in different sections of the cell and quantify lithium gradients that develop in a porous graphite electrode during cycling at a 1C rate. Inhomogeneity in the total Li content, and in the individual ordered Li_xC₆ phases formed during lithium insertion into (and extraction from) the graphite, has been observed in an operando fashion. The complex dynamics of lithium-staging in graphite with the distinct front propagation of phase changes have been characterized and new features of these dynamics are highlighted here. As large Li⁺ ion gradients contribute to cell polarization, our results suggest that Li plating conditions can be met near the graphite electrode surface, even when the cell is charged at a moderate (1C) rate.

中文翻译：

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使用操作能量色散X射线衍射对锂离子电池石墨电极中的锂浓度梯度进行量化†

在许多实际应用中，需要安全，快速且节能的锂离子电池循环。但是，建模研究预测，Li ⁺在较高电流下循环期间，电极中的离子梯度变大。这样的梯度在电极中引入异质性，这使得难以预测细胞寿命，因为细胞年龄的不同部分以不同的速率发生。缺乏实验方法来探测整个电极深度上的这些浓度梯度。在这里，我们使用空间分辨的能量色散X射线衍射来获得电池不同部分的锂化和脱锂的“电影”，并量化在以1C速率循环期间在多孔石墨电极中形成的锂梯度。在锂中插入锂（或从中提取）过程中，观察到了总Li含量以及在各个有序Li _x C ₆相中的不均匀性。操作时尚。表征了锂在石墨中的复杂动力学，具有相变的明显前传播，并在此突出了这些动力学的新特征。由于较大的Li ⁺离子梯度会导致电池极化，因此我们的结果表明，即使以中等（1C）速率对电池充电，也可以在石墨电极表面附近满足Li镀层条件。