Structural water and disordered structure promote aqueous sodium-ion energy storage in sodium-birnessite

Birnessite is a low-cost and environmentally friendly layered material for aqueous electrochemical energy storage; however, its storage capacity is poor due to its narrow potential window in aqueous electrolyte and low redox activity. Herein we report a sodium rich disordered birnessite (Na0.27MnO2) for aqueous sodium-ion electrochemical storage with a much-enhanced capacity and cycling life (83 mAh g−1 after 5000 cycles in full-cell). Neutron total scattering and in situ X-ray diffraction measurements show that both structural water and the Na-rich disordered structure contribute to the improved electrochemical performance of current cathode material. Particularly, the co-deintercalation of the hydrated water and sodium-ion during the high potential charging process results in the shrinkage of interlayer distance and thus stabilizes the layered structure. Our results provide a genuine insight into how structural disordering and structural water improve sodium-ion storage in a layered electrode and open up an exciting direction for improving aqueous batteries.

Supplementary Figure 11. CVs of sodium˗manganese oxides at the scan rate of 50 mV s −1 , showing the anodic peak of Na 0.27 MnO 2 shifted to a lower potential and the cathodic peak shifed to a higher potential compared with those of other materials as the Na concentration increased. (a,b) CP tests of the charge and discharge cycles at a current density of 0.3 A g −1 , where capacity around 144 mAh g −1 could be attributed to Na-ion storage, while addition charge transfer could be attributed to hydrogen evolution reaction (HER); (c,d) Four charge and discharge cycles of CP tests conducted at a current density of 0.6 A g −1 .
Supplementary Figure 14. The charge and discharge potential differences at the midpoint of the capacity of Na 0.27 MnO 2 at the current densities from 0.6 A g −1 to 2.0 A g −1 . The initial four charge and discharge curves of hydrated Na 0.27 MnO 2 and less hydrated Na 0.19 MnO 2 birnessite, and the summarized specific capacities vs. cycle number; (d,e,f) the charge and discharge curves (2 nd cycle) of hydrated Na 0.27 MnO 2 and less hydrated Na 0.19 MnO 2 birnessite at current densities from 0.6 to 2.0 A g -1 , and the summarized specific capacities vs. current density; (g,h,i) the charge and discharge curves of hydrated Na 0.27 MnO 2 and less hydrated Na 0.19 MnO 2 birnessite at 1 A g −1 with cycle 1 st , 2 nd , 3 rd 5 th , 10 th , 25 th , 50 th and 100 th , and the specific capacities and coulombic efficiencies vs. cycle number.

Supplementary Tables
Supplementary Table 1. ICP-MS results of sodium-manganese oxides (Na  MnO x ) materials obtained by thermal solid-state reaction of NaOH and Mn 3 O 4 with the molar ratios of 0.5:1, 1:1, 2:1, and other synthesized methods.

Samples
Chemical formula determined from ICP-MS

Supplementary Notes
Supplementary Note 1.
The half-cell mass specific capacitance C MS (F g −1 ) and electrode capacity C electrode (mAh g −1 ) were calculated with the third CV scan.
Electrode mass specific capacitance: Eq. 1 Electrode capacity: Eq. 2 where i (A) is the measured current at certain time of t (s), m (g) is the mass of active material loaded on working electrode, V (V) is potential window, t 0 (s) and t F (s) are respective times at the initial potential and the final potential.

Supplementary Note 2.
In the manuscript, we conducted the two-electrode symmetric full-cell measurements using Na 0.27 MnO 2 materials as both anode and cathode in 1M Na 2 SO 4 aqueous electrolyte. During the full cell CP tests, the initial cell voltage is 0 V (namely, the potential difference between two electrodes is 0 V, and thus no redox reactions happen). When the full-cell started to discharge, one electrode (designated as M1) discharged (Na-ion insertion), while the other electrode (designated as M2) charged (Na-ion extraction) simultaneously. Reversely, when the full-cell started to charge, M1 is charged along with Na-ion extraction and M2 is discharged along with Na-ion insertion. In this context, the flux of Na-ion insertion into one electrode is always identical to the flux of Na-ion extraction from another electrode. Thus, fullcell maintained its charge neutrality as the Na-ions flowed between two electrodes, whereas the difference of the Na-ion concentration between two electrodes resulted in the difference of overall cell potential. More examples of the symmetric cell operation can also be found in recent publications. 1,2,3 Supplementary Note 3.

Eq. 11
Cell resistance: Eq. 12 where i (A) is the applied constant current, t (s) is discharge time of the cell device, U (V) is potential window, ∆V (V) is the i-R drop during discharge curve, M (g) is the total mass of active materials on both electrodes and m (g) is the mass of active materials on one electrode.

Supplementary Note 4.
The current contributed from surface-controlled capacitive process can be represented by: Eq. 13 where i is the current (A) and v is the scan rate (mV s −1 ).
While the current contributed from diffusion-limited redox process can be represented by: Eq. 14 Therefore, the overall current can be represented by: Eq. 15 After rearrangement, it can be written as: Eq. 16 By plotting i/ v ½ vs. v ½ curves at a given potential, k 1 and k 2 values can be determined, and hence the current response per 0.1 V is calculated and plotted in CV and the contribution of capacitive charge and diffusion-limited redox charge during the CV measurements in half-cell can be analyzed quantitatively.

→ ⏟
Eq. 21 In Supplementary Figure 30, structural study including XRD and X-ray PDF analysis was conducted. Xray PDF analysis of Na 0.27 MnO 2 confirms its disordered properties with a triclinic (C-1) birnessite structure (R wp =12.97%), better than a monoclinic model (R wp =19.61%) with a more ordered lattice structure (Supplementary Table 8), which is also supported by our previous neutron PDF analysis. Though disordered Na 0.27 MnO 2 and ordered Na 0.19 MnO 2 both show the layered birnessite structure, disordered Na 0.27 MnO 2 has a smaller crystalline size with shorter coherence (Supplementary Figure 29). Further X-ray PDF analysis were conducted for ordered Na 0.19 MnO 2 birnessite and commercial MnO 2 as shown in Supplementary Figure 30 with strutural parameters listed in Supplementary Table 9-10, Na 0.19 MnO 2 synthesized via decomposition of NaMnO 4 at high temperature shows a more ordered lattice structure monoclinic (C 2/m) while commercial MnO 2 is a highly crystalline -MnO 2 . The difference of local structures of disordered Na 0.27 MnO 2 and ordered Na 0.19 MnO 2 birnessite can be clearly observed in Supplementary Figure 30. Therefore, the smaller crystalline size with shorter coherence of Na 0.27 MnO 2 as well as more disordered lattice structure is showed compared to those of Na 0.19 MnO 2 birnessite and commercial MnO 2 .