Adv. Energy Mater. 2020, 10, 2001726 DOI: 10.1002/aenm.202001726 The polarization P(z) shown in Figure 1c in the original paper and reproduced here as Figure 1A was calculated by moving the Cu-atom sheets in a synchronous (concerted) way. The modern theory of polarization (MTP)[1] however, yields the two curves shown in Figure 1B, defined by P(z) = Pcalc(z) − Pref, where Pref are the polarizations for the Cu sheets at the mid-layer and mid-gap planes. In constructing Figure 1A, the red curve was mistakenly plotted as P(z) = Pref − Pcalc(z), which reversed its slope. Here it shown that a form of asynchronous Cu migration (ACM) is more appropriate to describe the experimental observations and results in the P(z) curve shown in Figure 1C. The markedly different behavior of the calculated synchronous Cu migration (SCM) and ACM P(z)'s can be traced to differences in the evolution of the Cu ions relative to the InP2S6 cages in the pertinent sequences of periodic unit cells. Midgap-to-midgap unit cells are adopted and the black curves in Figure 1B are compared with the ACM curve in Figure 1C. In the SCM, the Cu sheets always move from left to right relative to the stationary InP2S6 cages. Thus, P(z) rises monotonically in the entire unit cell and is then repeated periodically both on the z axis, as shown in Figure 1B, featuring a discontinuity from +25 to -25 μC/cm−2, and on the P axis without discontinuities (the MTP's polarization quanta,[1] not shown). In the ACM, as viewed in each unit cell, the average position of the Cu ions is identical to the actual positions of the SCM Cu sheets only while transiting within the layers from −HP to +HP. Thus, the SCM and ACM P(z)'s are identical only from −HP to +HP. From +HP on, however, the Cu ions transit across the vdW-gaps a fraction at a time, into the adjacent unit cells on the right. By periodicity, the same fractions of Cu ions enter each unit cell on the left. Thus, within each unit cell, the average position of the Cu ions appears to move from right to left, which leads to the calculated negative-slope P(z) in Figure 1C. Note that the negative-slope P(z) in effect replaces the discontinuity in the black curve in Figure 1B at the midgap planes with a gradual transition from +HP to −HP, in accord with the experimental data. Like the SCM curves, the ACM curve is also repeated periodically on the z and P axes. For a more detailed discussion of the asynchronous scheme and insights into the results, see Ref. [6]. The contents of this correction do not affect the analysis of the experimental data, and the conclusions in the original paper. The authors apologize for any inconvenience caused, they would also like to thank David Vanderbilt for inquiring about the slope of the red curve in Figure 1A and Raffaele Resta for valuable discussions.