Triggered reversible phase transformation between layered and spinel structure in manganese-based layered compounds

Irreversible phase transformation of layered structure into spinel structure is considered detrimental for most of the layered structure cathode materials. Here we report that this presumably irreversible phase transformation can be rendered to be reversible in sodium birnessite (NaxMnO2·yH2O) as a basic structural unit. This layered structure contains crystal water, which facilitates the formation of a metastable spinel-like phase and the unusual reversal back to layered structure. The mechanism of this phase reversibility was elucidated by combined soft and hard X-ray absorption spectroscopy with X-ray diffraction, corroborated by first-principle calculations and kinetics investigation. These results show that the reversibility, modulated by the crystal water content between the layered and spinel-like phases during the electrochemical reaction, could activate new cation sites, enhance ion diffusion kinetics and improve its structural stability. This work thus provides in-depth insights into the intercalating materials capable of reversible framework changes, thereby setting the precedent for alternative approaches to the development of cathode materials for next-generation rechargeable batteries.


Supplementary Figure 11
Relaxed atomic structure of Mn-migrated Na-bir using firstprinciples calculation.
Supplementary Figure 12 Variations of Raman spectra of hydrated and partially dehydrated Na-birs. Ex situ Raman spectra of (a) hydrated Na-bir and (b) partially dehydrated Na-bir at pristine, 1 st charged, 1 st discharged, 2 nd charged and 2 nd discharged state. Figure 13 Extra structural and chemical analysis of partially dehydrated Na-bir samples. Bright-field (BF) TEM images of (a) charged and (c) discharged samples, respectively. The regions of interest denoted by red boxes in the BF-TEM images were consecutively observed by ADF-STEM imaging at low-magnification and high-resolution. It is clearly observed again that Mn ions (heavier than Na ion thus showing brighter contrast) are migrated into Na layer due to desodiation, while Na ions (lighter than Mn ion thus showing weak contrast) are refilled into the interlayer after discharging. EDX mapping for the regions corresponding to the low-magnification ADF-STEM images was simultaneously carried out as shown in (b) and (d) for Na (red), Mn (green), O (blue), and C (cyan), respectively. (e) From the comparison of EDX spectra for the two respective regions marked by yellow dotted boxes in (b) and (d), it is corroborated that Na ions were almost removed and refilled in the interlayer during charge and discharge processing (see the Na Ka peak in (e)), while the Mn and O were not noticeably altered. Note that strong carbon X-ray signals were attributed to the unremoved binder (undesirably begetting serious hindrance to high-resolution STEM imaging) and small copper ones to Cu TEM grid for sampling. Supplementary Figure 20 Calculations on extraction energy of crystal water from hydrated and partially dehydrated Na-birs. Crystal structure models for calculating the extraction energy of crystal water from (a) hydrated Na-bir and (b) partially dehydrated Nabir.

Supplementary Note 1 Relationship between reversible structural changes and kinetic properties
The relationship between the corresponding reversible phase transformation and kinetic properties of Na-bir were demonstrated by galvanostatic intermittent titration technique (GITT) analysis (Supplementary Figure 4). As shown, it is observed that partially dehydrated Na-bir shows higher Na + ion coefficient than that of hydrated Na-bir. Interestingly, the diffusion coefficient values of both hydrated and partially dehydrated Na-bir were maximized as voltage increased where the layered structure changed into spinel-like structure. This is because the ion transport in spinel or spinel-like structure is usually faster than layered structure due to three dimensional interstitial diffusion pathways.

Supplementary Note 2 Comparison of electrochemical performances of hydrated,
partially dehydrated, and fully hydrated Na-birs.
As shown in Supplementary Figure 5 and 6, 1 st discharge capacity of fully dehydrated Na-bir at 0.1C is 176 mAh g -1 which is similar to that of hydrated Na-bir, but lower than that of partially dehydrated Na-bir. Furthermore, its capacity after 100 cycles just comes to ~30 mAh g -1 which is 17% of its 1 st discharge capacity. Furthermore, its rate capability is much inferior when it is compared to that of hydrated Na-bir and partially dehydrated Na-bir. It only shows 36.26 mAh g -1 at 5C which corresponds to 20% of the discharge capacity at 0.1C, while hydrated Na-bir and partially dehydrated Na-bir maintain 38% and 44%. These results demonstrate that fully dehydrated Na-bir exhibits inferior electrochemical performances to hydrated Na-bir. By conducting these additional experiments, we confirmed that the effect of crystal water contents in birnessite structure is very important to regulate its electrochemical performances.

Supplementary Note 3 Calculations of projected density of states and Bader charge.
We calculated the projected density of states and Bader charge for the both cases as shown in

Supplementary Note 4 Variations of Raman spectra of hydrated and partially dehydrated Na-birs.
In Raman spectra, similarity of spectral signal suggests that the materials have a similar structure. Basically, Raman spectra of layered birnessite shows bands at ~575 cm -1 and ~650 cm -1 arising from the Mn-O bond stretching vibration from the basal plane of [MnO6] laminates while spinel phase shows a sharp band at ~650 cm -1 which can be assigned to A1g mode of tetrahedrally coordinated Mn-O. The Raman spectra of hydrated Na-bir at pristine state exhibit bands at ~575 cm -1 and ~650 cm -1 which demonstrates that the pristine state has layered structure. After it charged, band at ~575 cm -1 disappeared and only sharp band at ~650 cm -1 remained. Thus, it is observed that hydrated Na-bir also go through phase transformation from layered to spinel-like phase when it is charged up to 4.3V as similar to partially dehydrated Na-bir. However, although the band at ~575 cm -1 appears again after discharged, the intensity of the band at ~650 cm -1 is still stronger than that of band at ~575 cm -1 in hydrated Na-bir. This means that the layered structure evolved again during discharge, but there is still spinel-like structure in hydrated Na-bir. This change is also maintained in the 2 nd cycle, indicating that the hydrated Na-bir exhibits lower structural reversibility during electrochemical reaction.

Supplementary Note 6 XPS analysis of pristine and after 100 cycles of hydrated and partially dehydrated Na-bir electrodes.
To study the CEI layer of hydrated Na-bir and partially dehydrated Na-bir, O1s spectra at pristine state and after 100 cycles of both samples were analyzed. As for pristine electrode, both O1s spectra shows the most intense lattice oxygen peak with several oxygen moieties like O-H, C=O from Na2CO3 and C-O which may be generated due to the reaction with atmospheric moisture and CO2. After 100 cycles, lattice oxygen peak intensity is decreased while the peaks corresponding to O-H, C=O and C-O become intense. The change in the O1s spectra is indicative of CEI layer formation during cycling. However, it is observed that relative peak intensity of lattice oxygen from partially dehydrated Na-bir is much higher than that of hydrated Na-bir. Considering that the limited depth of XPS analysis, the difference in lattice oxygen peak intensity is associated with different CEI thickness. Therefore, it is clear that hydrated Na-bir is much more sensitive toward electrolyte decomposition compared to partially dehydrated Na-bir.

Supplementary Note 7 Mn ion dissolution in hydrated and partially dehydrated Na-birs after 100 cycles.
Dissolution and migration of Mn species from Mn-containing cathodes to anode have been considered to be an important issue. This phenomenon is irreversible and is related to a

Supplementary Note 8 XRD analysis of hydrated and partially dehydrated Na-bir electrodes at pristine and after 100 cycles.
To investigate the structural stability of both samples, ex situ XRD analyses were conducted at pristine state and after 100 cycles. As observed in Supplementary Figure 16, pristine electrodes of hydrated Na-bir and partially dehydrated Na-bir show intense (002) peaks at 12.5 o and 16.3 o , respectively. The difference in peak position between pristine hydrated and partially dehydrated Na-bir electrodes well corresponds to that observed in the powder XRD patterns, which stems from the different crystal water contents. For hydrated Na-bir after 100 cycles, the (002) peak almost diminished and a broad peak around 17.06 o appeared. However, the characteristic (002) peak is well preserved in partially dehydrated Na-bir electrodes even after 100 cycles. This observation demonstrates that hydrated Na-bir undergoes a drastic structure collapse. Thus, controlling crystal water contents in birnessite structure can be regarded as a key to facilitate the reversible phase transformation and stabilize the structure.
Supplementary Note 9 TGA analysis of hydrated and partially dehydrated Na-bir electrodes after 100 cycles.
To verify the remaining crystal water contents in hydrated and partially dehydrated Na-bir after electrochemical cycling, TGA analyses were conducted for the electrode after 100 cycles. As shown in Supplementary Figure 17, the weight loss up to 100 o C is not significant from both electrodes, which demonstrates that loss of adsorbed moisture from the electrode is negligible. From 100 o C to 170 o C, the weight loss of both samples is nearly 1 wt% which comes from crystal water loss. This result demonstrates that remaining crystal water contents in both samples is similar after 100 cycles.

Supplementary Note 10 Calculations on extraction energy of crystal water from hydrated and partially dehydrated Na-birs.
In order to investigate the stability of crystal water in birnessite structure, we calculated the extraction energy of crystal water considering the environment (Supplementary Figure 20).
The stabilized model structure for first-principles calculations tells us that one crystal water prefers to interact with another rather than the Mn ions migrated to tetrahedral sites and any other constituting element of framework if the amount of crystal water is above that in partially dehydrated Na-bir. So, we could assume the crystal water between Mn layers for hydrated Na-bir, while the crystal water bonded to migrated Mn was used for the calculation on partially dehydrated Na-bir.