Supplementary Figure 1. SEM images of LiCoO 2 before (a) and after (b) electrochemical tuning. The size and morphology of synthesized LiCoO 2 and

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1 Supplementary Figure 1. SEM images of LiCoO 2 before (a) and after (b) electrochemical tuning. The size and morphology of synthesized LiCoO 2 and De-LiCoO 2 particles were almost the same, indicating that the improved OER activity of De-LiCoO 2 probably was not caused by the size and morphological changes. The small particles showed in Fig. S1b was conducting additive carbon black introduced during slurry preparation.

2 Supplementary Figure 2. (a) and (b), EDLC curves on De-LiCoO 2 and original LiCoO 2 electrodes with different scan rates; (c) and (d), plots of current densities at 0.25 V versus scan rates of De-LiCoO 2 and LiCoO 2, demonstrating the electrochemical surface area of De-LiCoO 2 is much larger than LiCoO 2. Figure S2 reveals that the De-LiCoO 2 catalyst possesses a high EDLC of ~2000 μf cm -2 (Fig. S2a) while the original LiCoO 2 only shows an EDLC of ~60 μf cm -2 (Fig. S2b). Based on these data, we can conclude that the number of electrochemically effective sites of De-LiCoO 2 catalyst is much higher than that of LiCoO 2 catalyst, which should be a main reason for the enhanced OER activity.

3 Supplementary Figure 3. a, gas chromatography curves of the gaseous products from the OER catalyzed by De-LiCoO 2 catalyst loaded on carbon fiber paper and commercial IrO 2 catalyst (IrO 2 powder from Premetek Co.) loaded onto Nickel foam at the current density of 10 ma cm -2 ; b, zoomed-in gas chromatography curves in oxygen peak region. N 2 and O 2 peak position are assigned by standard gas reference, and the O 2 peaks are normalized by the relative intensity of N 2 peaks. The relative Faradaic efficiency of De-LiCoO 2 to IrO 2 is nearly 100%.

4 Supplementary Figure 4. XPS data of (a) LiCoO 2 and (b) De-LiCoO 2. The black curve was the pristine sample, the red curve was the sample holding at 1.8 V vs. RHE for 2 hours in aqueous solution and the blue curve was the sample tuned back to 1.2 V for another 2 hours. (c) was the XPS results copied from a previous work by L. Dahéron et al. 1 For LiCoO 2, the binding energy of Co 2p 3/2 located at 780 ev and it remained the same at all potentials, indicating a +3 oxidation state of Co. After electrochemical tuning, the Co 2p 3/2 peak shifted to ev. However, after operating OER at a high potential for a period of time, the peak shift back to 780 ev and it remained the same after holding at a low potential. The Co 2p 3/2 peak at ev was attributed to the formation of CoF 2 during the delithiation process 2 and the peak shift back to 780 ev might be caused by washing away the surface contamination during oxygen bubbles releasing. From the results of Ref 2 (Fig. S4c), the XPS difference of the Li x CoO 2 (0<x<1) was only the slight difference of Co 2p 3/2 satellite peak intensity rather than the peak shift of Co 2p 3/2. Thus, it is hard to define whether the oxidation state of Co was changed based on the XPS data in our case. Nevertheless, the crystal structure change observed on the XRD results indicated the formation of Li 0.5 CoO 2. Thus we believe the oxidation state of Co should be higher after delithiation process.

5 Supplementary Figure 5. The OER activities of De-LiCoO 2 produced at different potentials. Since the phase transition from LiCoO 2 to Li 0.5 CoO 2 occurred at V, these samples obtained at greater than 4 V were almost composed of monoclinic Li 0.5 CoO 2. From the above curves, the OER activities of the samples (> 4 V) were enhanced while the sample below 4 V showed negligible improvement, suggesting the formation of Li 0.5 CoO 2 played an important role in enhancing the performance.

6 Supplementary Figure 6. XRD data of LiCo 0.33 Ni 0.33 Fe 0.33 O 2 before (black line) and after delithiation (red line). The above XRD data of LiCo 0.33 Ni 0.33 Fe 0.33 O 2 before and after delithiation indicated that the Li was extracted out from the original lithium transition metal oxides in terms of the different crystal structure and the negative peak shift (2θ 19 o ). However, the XRD data showed that mixed phases were existed in both pristine oxides and delithiated oxides. The detailed characterization and analysis were shown in Table S1 and additional discussion.

7 Supplementary Figure 7. Polarization curves of LiCo 0.33 Ni 0.33 Mn 0.33 O 2 before (black line) and after (red line) delithiation. The OER activity of De-LiCo 0.33 Ni 0.33 Mn 0.33 O 2 was better than that of LiCo 0.33 Ni 0.33 Mn 0.33 O 2, demonstrating the effectiveness of the delithiation process. However, the boost of activity through delithiation in LiCo 0.33 Ni 0.33 Mn 0.33 O 2 was not as effective as that in LiCoO 2, thereby making De-LiCo 0.33 Ni 0.33 Mn 0.33 O 2 less active than De-LiCoO 2.

8 Supplementary Figure 8. Tafel plots of the mixed lithium transition metal oxides before and after delithiation. (a): LiCo 0.5 Ni 0.5 O 2, (b): LiCo 0.5 Fe 0.5 O 2, (c): LiCo 0.33 Ni 0.33 Fe 0.33 O 2 and (d): LiCo 0.33 Ni 0.33 Mn 0.33 O 2. The black lines stand for the pristine materials and the red lines represent the delithiated materials. For all the couples including LiCoO 2 and De-LiCoO 2 showed in Fig. 1b in main text, the delithiated samples showed better or comparable Tafel slopes to the pristine ones, indicating improved or not influenced OER kinetics after delithiation.

9 Supplementary Figure 9. Cycling stability of the delithiated mixed lithium transition metal oxides. For all the delithiated samples, negligible current loss was observed after 1000 cycles, demonstrating the high stability of the catalysts during oxygen generation.

10 Supplementary Figure 10. Polarization curves of LiMnO 2 before (black line) and after (red line) delithiation. From the above curves, the OER activity of De-LiMnO 2 was not better than LiMnO 2, indicating the electrochemical tuning process may not be applicable for other systems.

11 Supplementary Figure 11: Delithiation process of LiCo 0.33 Ni 0.33 Fe 0.33 O 2.

12 Supplementary Table 1. The contents of the transition metals measured by ICP. Samples Co (ppm) Ni (ppm) Fe (ppm) LiCo 0.5 Ni 0.5 O N/A LiCo 0.5 Fe 0.5 O N/A 0.59 LiCo 0.33 Ni 0.33 Fe 0.33 O

13 Supplementary Note 1: The XRD results (Figure 4a and Figure S6) don t show a pure phase of our lithium mixed transition metal oxides because the theta region contains two main diffraction lines instead of one. Therefore, we calculate the value of the c axis of the cell based on the X-ray diffraction of the (003) plane located at around 19, and after obtaining the c axis parameter, we further calculate the Co:Ni and Co:Fe ratios according to the Vegard's law. The standard c parameters are: LiCoO 2, c Co = Å LiNiO 2, c Ni = Å LiFeO 2, c Fe = Å For LiNi 0.5 Co 0.5 O 2 : c Ni-Co = Å According to the Vegard's law, LiCo x Ni 1-x O 2 : ( ) ( ) Thus, the formula should be LiCo 0.51 Ni 0.49 O 2 For LiCo 0.5 Fe 0.5 O 2 : C Co-Fe = Å According to the Vegard's law, LiCo x Fe 1-x O 2 : ( )

14 ( ) Thus, the formula should be LiCo 0.66 Fe 0.34 O 2 Based on the above analysis, we can roughly conclude that phases of LiCo 0.51 Ni 0.49 O 2 and LiCo 0.66 Fe 0.34 O 2 may be the major phase in the samples of LiCo 0.5 Ni 0.5 O 2 and LiCo 0.5 Fe 0.5 O 2, respectively. However, it is hard to determine the Co:Ni:Fe ratio of LiFe 0.3 Ni 0.3 Co 0.3 O 2, which shows a c axis value of Å because it contains three unknown numbers. We have also measured the atomic ratios of the transition metals by Inductive Coupled Plasma Emission Spectrometer (ICP), and the results are shown in Table S1. The Co:Ni and Co:Fe ratios are relatively consistent with the results calculated from XRD. It should be noted that the measured Co:Ni:Fe ratio of LiFe 0.3 Ni 0.3 Co 0.3 O 2 is close to 3:2.5:2. Therefore, since the major phase of our samples is layered lithium transition metal oxides, the enhanced OER activity observed in this study is originated from tuning the layered lithium transition metal oxides.

15 Supplementary References: 1. Dahéron L, et al. Electron transfer mechanisms upon lithium deintercalation from LiCoO2 to CoO2 investigated by XPS. Chem Mater 20, (2007). 2. Lu Y-C, Mansour AN, Yabuuchi N, Shao-Horn Y. Probing the Origin of Enhanced Stability of AlPO4 Nanoparticle Coated LiCoO2 during Cycling to High Voltages: Combined XRD and XPS Studies. Chem Mater 21, (2009).