Electrochemical performance of Bi-δ-MnO 2 with and without Cu

Birnessite-layered structure is of resurgent interest for battery applications as an ionic intercalation material37,38,39 and with what appears to be some characteristics of a conversion electrode14,15,16,17,27. Bi-δ-MnO 2 in alkaline electrolyte is synthesized either by an in-situ formation step15,16,26,40,41 by discharging a mix of EMD with Bi 2 O 3 and recharging to its charged state to form Bi-δ-MnO 2 (see Methods section) or by ex situ synthesis methods14,15,17,23,26,41,42,43,44. Both methods have been shown to deliver 60–80% of δ-MnO 2 ’s theoretical capacity (617 mAh g−1) at low areal loadings for hundreds of cycles potentiodynamically albeit with constant fade over cycle life15,16,17. Galvanostatically it has been cyclable at low mass loadings with reduced capacity retention15,16,17,26. The best results with higher wt% loading from a Bi-δ-MnO 2 cathode were from Kannan et al.,42 where they showed an ex situ synthesized material could retain ∼310–500 mAh g−1 for ∼200–600 cycles at C/2 with 50–65 wt% loading. However, there was no mention of areal capacity in the paper and a recent report44 that used the same synthesis procedure to make Bi-δ-MnO 2 found that at high areal capacity the cycle life was limited to 60 cycles. To the best of our knowledge, there has been no literature data accessing 80–95% of the 617 mAh g−1 of MnO 2 at high mass loadings and areal capacities for thousands of cycles at C rates of interest.

To indicate the effects of Cu intercalation on cathode performance, cycling data with and without Cu are presented in Fig. 1. Cycle one in Fig. 1a shows the material evolution and in situ formation (see Methods section) of Bi-δ-MnO 2 with high areal capacity (21 mAh cm−2, 45 wt% loading). EMD is mixed with carbon and Bi 2 O 3 and discharged completely to −1 V versus mercury/mercury oxide (Hg/HgO) at C/3 in 37 wt% potassium hydroxide (KOH) solution to form Mn(OH) 2 with Bi. Mn(OH) 2 with Bi is then charged up at C/3 to 0.3 V versus Hg/HgO to form Bi-δ-MnO 2 . Second cycle onwards the flat potential discharge characteristics of the δ-MnO 2 cathode are seen15,16,17; however, it is not able to retain the capacity. The cause of failure appears to be due to Mn 3 O 4 formation as discussed later in the manuscript.

Figure 1: Galvanostatic data for Bi-birnessite cathodes with and without Cu additive. (a) First five C/3 cycles of a 45 wt% (∼21 mAh cm−2) EMD, Bi 2 O 3 and carbon containing cathode fades completely in five cycles. During the first discharge, potentials characteristic of γ-MnO 2 were seen. In the second cycle, potentials characteristic of a δ-MnO 2 layered phase were seen. (b) A comparable electrode with Cu shows negligible capacity fade in five C/3 cycles. (c) Capacity versus cycle number for ∼19 and 60 wt% EMD, Bi 2 O 3 , Cu and carbon containing cathode at 20C and C/3, respectively. Areal capacities of the ∼19 and 60 wt% EMD cells were ∼12 and 29 mAh cm−2, respectively, which are indicative of high active loading electrodes. All cycling was against NiOOH counter electrodes. The 60 wt% cell is still cycling at the time of this writing. Full size image

Figure 1b shows the cycle performance of a Cu2+-intercalated Bi-δ-MnO 2 cathode, which is able to access the theoretical capacity reversibly. The Cu2+-intercalated Bi-δ-MnO 2 formation process is very similar, except with the addition of Cu to the mix of EMD, Bi 2 O 3 and carbon. This mix is discharged completely to −1 V versus Hg/HgO and charged to 0.3 V versus Hg/HgO to form Cu2+-intercalated Bi-δ-MnO 2 . The effect of Cu is seen, and further cycling reveals that Cu addition is necessary for the stability of Bi-δ-MnO 2 . The discharge potentials (−0.406 V versus Hg/HgO) and charge potentials (−0.26 V versus Hg/HgO) for the Cu-containing electrode (blue arrows) were stable, while those of the electrode without Cu (red arrows) were not. Impedance experiments45 were performed at the end of charge (0.3 V versus Hg/HgO) for the first five cycles (Supplementary Fig. 1), showing the charge transfer resistance of the Cu-containing electrode to increase from 0.26 to 0.65 mΩ cm−2, whereas the resistance of the non-Cu electrode increased from 0.78 to 12.7 mΩ cm−2. High cycling rate (20C) and cycle life (>4,000) of the Cu-intercalated material with a loading of ∼19 wt% MnO 2 (∼12 mAh cm−2) is demonstrated in Fig. 1c. The system in general has low charge transfer resistance and can operate at ultrahigh rates. A 60 wt% MnO 2 loading (∼29 mAh cm−2) cell cycled at C/3 is also shown in Fig. 1c, where it is able to access the complete capacity for ∼300 cycles, after which it retains >80% of the 617 mAh g−1 over 1,000 cycles.

Further, for a series of cells, the wt% loading of EMD was varied from 5 to 45, and the cells were cycled galvanostatically from 1 C to 40 C against nickel oxyhydroxide (NiOOH) counter electrode (Supplementary Fig. 2). The cell performance was stable, delivering a large fraction of the two-electron theoretical capacity. Even at high cycling rates, a high loading cell (45 wt%,∼19 mAh cm−2) delivered ∼60–65% of the theoretical capacity in galvanostatic cycling at 4C for >1,700 cycles (Supplementary Fig. 2b). In several such experiments, much of the capacity was obtained for voltages above −0.6 V, where the effect of the Bi and Cu additives would be expected to contribute little as indicated by the potentiodynamic data in Supplementary Fig. 4. In some experiments, such as that shown in Supplementary Fig. 2c, the role of the additives on the capacity is more difficult to interpret because some portion of the discharge capacity is extracted at lower voltages. Nonetheless, potentiodynamic data indicate that even in this low voltage region a majority of the capacity is still derived from the MnO 2 (Supplementary Figs 4 and 5). The potentials at which these capacities were obtained appeared to be dependent on wt% loadings and cycling rates. In practical applications, these conditions would need to be optimized. Statistical reliability of the data was also ensured by gathering the average discharge capacity of three similar cells with 50 wt% MnO 2 cycled at 1C, which is shown in Supplementary Fig. 3, where the standard deviation of the discharged capacity is <2% of the mean.

Regeneration mechanism and electrode characterization

Figure 2a shows performance of the second cycle of a Cu2+-intercalated Bi-δ-MnO 2 electrode under galvanostatic or potentiodynamic cycling. The slow potentiodynamic cycle (0.1 mV s−1) shows the potentials of electrochemical reactions from which the oxidation state of the reactions shown in Fig. 2a were derived. Detailed combinatorial cyclic voltammograms and the reactions are presented in Supplementary Figs 4–6 and Supplementary Table 1, where each component was methodically added and the electrode potentials were slowly swept to observe the faradaic reactions. The reaction mechanism of Cu2+-intercalated Bi-δ-MnO 2 is illustrated in Fig. 2a, wherein three regions of reaction are observered during both charge and discharge. In the discharge region labelled 1, MnIV and CuII partially reduce to form MnIII and Cu0. The partial reduction of MnIV is perhaps from surface sites. Discharge region 2 is where Bi-δ-MnO 2 without Cu is previously known to undergo MnIV to MnIII to MnII reactions, and here we find that Bi-δ-MnO 2 with Cu also has its significant capacity. This reaction pathway includes a dissolved MnIII state followed by formation of solid Mn(OH) 2 . In the discharge region 3, the remaining CuI is reduced to Cu0 and BiIII to Bi0. These regions occur for short times and hence the capacity in those regions is small as supported by the low capacity below −0.6 V versus Hg/HgO in Fig. 1b. In the charge region labelled as 4, Bi0 oxidizes to BiIII with partial oxidation of Cu0 to CuI, then substantial charging occurs in region 5 where MnII is oxidized to MnIII. Just above the dissolution potential of MnII, Cu0 also oxidizes to form CuII, which is present as a dissolved species. In the charge region 6, the formation of Bi-δ-MnO 2 (MnIII to MnIV) takes place, and CuII ions intercalate within the interlayer regions to completely reform Cu2+-intercalated Bi-δ-MnO 2 . The mechanism to the improved performance of the Cu2+-intercalated Bi-δ-MnO 2 material is that Cu decreases the charge transfer resistance of the δ-MnO 2 , allowing complete regeneration of the materials from the dissolved state during each discharge and charge.

Figure 2: Regeneration mechanism and supporting XANES spectra. (a) Electrochemical reactions for the regeneration cycle of Cu2+-intercalated Bi-birnessite. The reactions taking place in each region are colour-coded and numbered corresponding to the galvanostatic and potentiodynamic curves. (b) XANES spectra of a cycled electrode in the charged state for the Mn K edge indicating match with birnessite and (c) for the Cu K-edge of the same material indicating that Cu exists as Cu2+ intercalated into birnessite matching ref. 37. Full size image

Reversible formation of Cu2+-intercalated Bi-δ-MnO 2 is confirmed by XANES analysis of an electrode that was cycled 60 times and stopped on charge (Fig. 2b,c). The cycled electrode showed characteristic Mn K-edges of a birnessite phase, and also the Cu K-edge matches that of Cu2+ ions in birnessite37,46,47. The presence of Cu2+ ions reduces the oxidation state of Mn in Bi-birnessite as seen in Supplementary Fig. 7a.

Figure 3 shows further characterization to confirm the intercalation of Cu2+ within the layers of birnessite on cycled electrodes (also see Supplementary Figs 7–11). Figure 3a shows a X-ray diffraction (XRD) comparison of three materials: a Cu Bi-birnessite electrode cycled 60 times, a control Bi-birnessite electrode with no Cu (failed after four cycles), and non-cycled initial electrode mix. The cycled and control (no-Cu) patterns are both composed predominantly of δ-MnO 2 (indexed to hexagonal space group), indicating the successful Bi-δ-MnO 2 formation process, distinct from the ‘no cyling’ pattern. The c-direction peaks, corresponding to the birnessite interlayer spacing, are shown magnified on the right-side plot. With Cu present, the birnessite interlayer is expanded compared to the control electrode. This expansion is consistent with other reports where metal ions have been intercalated in the birnessite interlayers31,48. Other peaks are also seen in the XRD of the control electrode, which are indexed to Mn 3 O 4 . To further confirm the fingerprints of these respective phases, Raman spectra was taken for the control and cycled electrode at its charged state. The Raman spectra in Fig. 3b show peaks from the control material match to Mn 3 O 4 (ref. 49), whereas different peaks dominate the cycled Cu Bi-δ-MnO 2 material. Raman spectra for synthesized birnessites have the strongest intensity ∼575 cm−1 due to the Mn-O stretching along the chains in the MnO 6 sheets followed by a band at ∼640–645 cm−1 due to the out of plane stretching of Mn-O in the MnO 6 groups. However, in the Raman spectra of the cycled electrode a significant broadening feature of the band at ∼640–645 cm−1 is seen compared to the control electrode. This broadening feature could mean a disruption of the Mn-O caused by the presence of Cu2+ ions50. Ex situ synthetic birnessite was hydrothermally made51 and soaked in a Cu2+ solution (copper sulfate (CuSO 4 )) for 48 h, and Raman and XRD were performed. The XRD (Supplementary Fig. 7c) for the synthetic Cu-soaked birnessite showed the same peak shifts that were seen in the cycled Cu2+-intercalated Bi-birnessite material. The Raman spectra in Fig. 3c also show a match between synthetic birnessite soaked in Cu2+ and battery-cycled Cu2+ Bi-δ-MnO 2 , indicating expansion of the birnessite interlayer and pointing the Cu2+ ions as the origin of the broadening feature36,50,52. This shows that Cu2+ is intercalated in the Bi-δ-MnO 2 -layered structure during battery cycling, and that this structure is continually reformed upon charging the electrode.

Figure 3: Characterization of cycled (with Cu) and control (no Cu) electrodes. (a) XRD of cycled Cu Bi-birnessite, cycled Bi-birnessite without Cu and uncycled initial electrode mix. Magnified panels show interlayer expansion in the case with Cu. (b) Raman spectra of cycled and control electrodes. (c) Raman spectra of cycled electrode, Cu2+-intercalated birnessite produced ex situ and synthetic birnessite. (d) HRTEM image of the cycled electrode. The darker region and lighter region is indexed to δ-MnO 2 . (e) TEM EDX mapping of a region of the cycled electrode. Scale bars in d,e are 10 and 500 nm, respectively. Scale bar for maps in e is 250 nm. Full size image

Electron microscopy was also performed to understand the morphological effects of Cu2+ intercalation. The d-spacing expansion ((0 0 6) direction) from the high-resolution transmission electron microscopy image of the cycled electrode, as shown in Fig. 3d, corroborated the expansion seen in the XRD in Fig. 3a. The energy dispersive X-ray spectroscopy (EDX) mapping image in Fig. 3e and line scan (Supplementary Fig. 11) clearly show the colocation of Cu with Mn throughout the Bi-birnessite nanoparticle, while no Cu was seen in the electrode’s surrounding carbon material. Scanning electron microscopy (SEM) and EDX analysis (Supplementary Fig. 8a,c) of the control electrode show an amorphous nature and a Mn to O ratio of ∼0.68, indicating Mn 3 O 4 presence. SEM and EDX analysis of the cycled electrodes show sheet-type structures with Cu colocated with Mn (Supplementary Fig. 8b,d). EDX of the synthetic, Cu2+-soaked birnessite samples showed that Cu2+ had almost completely replaced the K+ ions present within the layers (Supplementary Fig. 9e–h).

Large-format rechargeable energy-dense cell

The in situ synthesis of the δ-MnO 2 using inexpensive, abundant raw materials and the ease of manufacturability make the Cu2+-intercalated Bi-δ-MnO 2 an attractive cathode. Ex situ synthesized δ-MnO 2 can also be used with no difference in performance as shown in Supplementary Fig. 12. From a device standpoint, addition of Cu is not a significant cost contributor because low areal loadings of Cu are sufficient to result in the enhanced capacity and performance (Supplementary Fig. 13). This also indicates that the capacity obtained is predominantly associated with the δ-MnO 2 as the effect of different Cu loadings on capacity is insignificant. We also cycled a Cu cathode against NiOOH counter electrode in a cell of similar design and found that the capacity was <5 mAh g−1 (Supplementary Fig. 14), which supported the finding that Cu added little to the overall capacity of the birnessite cells we tested. To demonstrate the practicality of a Cu2+-intercalated Bi-δ-MnO 2 as a battery electrode, in situ synthesized materials were prefered as considerable time and money are saved in comparison to making ex situ synthesized birnessites. The Cu2+-intercalated Bi-δ-MnO 2 electrode was cycled against a Zn anode in a large-format prismatic cell with industrial-strength loading of Mn per volume (60 wt%). The battery delivered an energy density of ∼160 Wh l−1 for the first cycle and then cycled ∼120 Wh l−1 for 90 cycles, as shown in Fig. 4a. This rechargeable aqueous battery possesses a high energy density relative to existing technology, as shown in Fig. 4b. Energy density is currently limited by the Zn DOD (15% in this cell). Zn anodes face cycle life challenges when cycled at more than 10% of theoretical capacity53. Calculation shows that when Cu2+-intercalated Bi-δ-MnO 2 is paired with a Zn anode that cycles at ∼35% of its theoretical capacity, the resulting cell would deliver >250 Wh l−1, equal to some varieties of Li-ion battery. A comparison of the best MnO 2 cathodes reported in literature is shown in Fig. 4c (see Supplementary Table 2), where a striking advancement in cycle life and capacity is noted for Cu2+-intercalated Bi-δ-MnO 2 electrodes. An electrode with high areal capacity is preferred as more active mass is used meaning higher energy density and lower cost. Figure 4d illustrates the best areal capacity and cycle life from references in Fig. 4c. Obtaining high cycle life is easy when areal capacities are very low. We show for the first time the attainment of 80–100% of the second electron capacity of MnO 2 with higher areal capacities for thousands of cycles, which also results in high volumetric capacity (mAh ml−1, see Supplementary Table 2).

Figure 4: Performance characteristics of a Zn-birnessite cell. (a) The discharge energy density (Wh l−1) and capacity (mAh g−1) for a Zn-birnessite cell using the Cu2+-intercalated Bi-birnessite electrode with 60 wt% EMD in the initial mix (continues to cycle at the time of this writing). The voltage-time characteristics for the first 5 cycles are shown in the inset. The electrode sizes are 5.08 cm × 7.62 cm. (b) Energy density comparison of energy storage systems for grid applications59 indicating competitive performance of the Cu2+-intercalated Bi-birnessite. (c) Comparison of the best MnO 2 cathodes reported in literature for aqueous Zn-MnO 2 batteries. (d) Comparison of the areal capacities and cycle life obtained gathered from reported results in literature60,61,62,63. CAES, compressed air energy storage; PHS, pumped hydro storage; SMES, superconducting magnetic energy storage. Full size image