Herein, we measured the spatial distribution of the Li concentration and its mass transport processes in a porous graphite electrode in-situ. Insertion of Li into graphite changes the color of graphite by altering the visible light absorption; hence, we designed an optical electrochemical analog half-cell to enable direct real-time observations of the color change occurring during lithiation and delithiation. Using the relationship between the electrode color and Li concentration, we determined the distribution of the Li concentration in the electrode during intercalation. Our experimental results show that the distribution of the Li concentration was approximatively linear at the early stage of intercalation. As the Li content in the electrode increased, the distribution of the concentration became nonlinear and exhibited a ladder-like distribution owing to the effects of phase transformations. In addition, the time evolution of the Li concentration distribution curves was unsteady. On the basis of the distribution of the experimental concentration curves at different lithiation times, we determined the normalized overall lithium content within the electrode and its derivative over time (i.e., the Li mass transport rate). These results show that the overall lithium content increased nonlinearly over time. The Li transport rate decreased as the lithium content increased and fluctuated during phase transformation. On the basis of our experimental results, the effects of the Li concentration and the structural phase transformation on the Li transport rate are discussed.

本文中，我们在多孔石墨电极中原位测量了Li浓度的空间分布及其传质过程。将Li插入石墨可以通过改变可见光吸收率来改变石墨的颜色。因此，我们设计了一种光学电化学模拟半电池，可以直接实时观察锂化和脱锂过程中发生的颜色变化。利用电极颜色和Li浓度之间的关系，我们确定了插层过程中Li浓度在电极中的分布。我们的实验结果表明，在嵌入的初期，Li的浓度分布近似线性。随着电极中Li含量的增加，由于相变的影响，浓度的分布变为非线性，并表现出阶梯状分布。另外，Li浓度分布曲线的时间演化不稳定。基于在不同锂化时间的实验浓度曲线的分布，我们确定了电极内的归一化总锂含量及其随时间的导数（即Li的质量传输速率）。这些结果表明，总锂含量随时间非线性增加。随着相变过程中锂含量的增加和波动，Li的传输速率降低。根据我们的实验结果，讨论了锂浓度和结构相变对锂传输速率的影响。