By Kent J. Griffith
September 16, 2019 | The use of multivalent-ion battery chemistry is among the myriad approaches to supersede lithium and lead to higher energy, more sustainable batteries. Large global research projects are underway on systems including Mg2+, Ca2+, Zn2+, and Al3+ batteries. Any battery chemistry that could lead to cheaper or higher energy batteries is of intense interest to the commercial sector, but the relatively early stage of multivalent chemistries means that this technology is still largely in the research stage at universities and national laboratories around the world.
Recently, a group of physics researchers from the Indian Institute of Technology Madras in Chennai, India, have proposed and filed a patent on a new multivalent-ion battery based on iron. The basic idea is that Fe2+ ions, rather than the Li+ ions in a convention lithium-ion battery, are inserted into and extracted from host electrodes. The new Fe-ion battery is composed of a vanadium oxide (V2O5) cathode, mild steel anode, and iron perchlorate (Fe(ClO4)2) electrolyte dissolved in an ether electrolyte (tetraethylene glycol dimethylether, TEGDME). The results were published last month in the journal Chemical Communications (DOI: 10.1039/C9CC04610K).
Vanadium pentoxide has been widely used in proof-of-concept studies for the intercalation of monovalent and multivalent ions due to its large interlayer spacing that is able to accommodate a wide variety of ions. Mild steel is an extremely low-cost material that is widely available. The authors described the main advantage of an iron-ion battery as low cost, enabling large-scale energy storage.
On the other hand, iron-ion batteries come with numerous inherent challenges. Iron is much less electropositive than lithium, leading to low-voltage batteries. The reaction between V2O5 and Fe presented in this initial study occurs at an average voltage of about 0.6 V; high energy lithium-ion batteries provide about 3.7 V. In addition, iron is heavier than lithium by a factor of eight. The maximum gravimetric capacity of the reported Fe cathode is about 250 mAh/g, which slightly exceeds state-of-the-art commercial lithium cathodes; however, the low voltage means that the energy density of Fe-ion would be only 20% of Li-ion (cathode basis).
Professor Kimberly See, California Institute of Technology, is an emerging leader in the field of multivalent ionics and notes the inherent challenges with multivalent ion diffusion in both the electrode and electrolyte. “Development of electrolytes with high conductivity and stability at interfaces that solubilize and transport the working cation [Fe2+] but allow for desolvation at electrode interfaces is a significant challenge for any multivalent battery chemistry. Ideally, the bare ion is reversibly incorporated into the active material, but some multivalent ions, such as Al3+, hold on to their solution-phase structures strongly and intercalate with their solvation shell making them much larger and more massive than anticipated.” The intercalation of solvated Li-ions into graphite was an early stumbling block with propylene carbonate solvent that led to continuous electrolyte decomposition and particle delamination, which required the development of new electrolyte mixtures. Professor See goes on to say that, “Solid-state ion diffusion is an additional challenge as multivalent cations are conventionally suggested to incur higher energy barriers to diffusion compared to Li+.”
From these initial results, the Fe2+-ion batteries would require extreme improvements in stability and efficiency. At present, the capacity retention over 173 cycles is only 43.7%, and there is a large overpotential of nearly 1.0 V between discharge and charge. A greater concern for the reported iron battery is the coulombic efficiency of 30% on the first cycle, which increases to 60–70% over long-term cycling. This implies that a significant amount of electrolyte is being consumed in side reactions.
According to Professor M. Rosa Palacín of the Institut de Ciència de Materials de Barcelona, a world-leading chemist in the field of multivalent batteries, Fe is interesting because it is “is cheap and abundant and non-toxic… besides, an aqueous electrolyte could be used”. However, the authors here used an organic liquid electrolyte that is nonflammable but relatively expensive. A shift toward water-based electrolytes, as Professor Palacín explains, would offer low cost and high safety. On the other hand, she notes that the energy density of Fe chemistry is limited due to the low voltage, and thus Fe-ion batteries would only make sense where “energy density is not the main priority”.
Taking a lesson from the other multivalent ion communities and the fickle stability of their electrolytes, great care must be taken to identify that Fe2+-ion intercalation is truly occurring. The observed capacity could be attributed solely or predominantly to side reactions; electrochemical data alone are not enough to prove a new battery chemistry.
Professor See strongly advocates for careful structural analysis: “With any new working ion or new material, extensive characterization must be shown to prove the working ion is indeed reversibly incorporated during charge and discharge. Structural characterization, quantitative elemental analysis, and spectroscopic characterization in addition to the electrochemistry are all necessary pieces of data. The electrochemistry can be convoluted by unintended side reactions at the electrode-electrolyte interface or by the activity of spurious compounds in the electrolyte, such as protons from water… The chemistry of the new working ion must be considered; it’s very difficult to impose conventional materials on unconventional working ions.”
Toward this end, the authors present routine diffraction data, X-ray photoelectron spectroscopy (XPS) on the cathode, and scanning electron microscope images with energy-dispersive X-ray spectroscopy (SEM/EDX) of the anode as evidence for the proposed Fe-ion battery. Diffraction measures the bulk crystal structure, while XPS and SEM/EDX measure the surface chemistry and particle appearance.
It is necessary to examine the technical data presented to evaluate the status of the proposed Fe2+-ion battery. The diffraction patterns show a change from the V2O5 structure, but there is no clear FexV2O5 phase as would be expected from Fe-ion intercalation. Numerous secondary phases are proposed, mostly iron vanadium oxides, but the basis for these is not strong given that the predicted and observed peaks do not match consistently. The vanadium XPS data show a reduction from V5+ to V4+, consistent with ion intercalation; however, the charged sample does not show any reversal to V5+, so the discharge process may not be highly reversible. The asymmetry of the discharge and charge electrochemical curves would also suggest that the process may not be reversible.
The iron XPS spectra show an increased amount of iron with the cathode after cycling. XPS is a surface-sensitive technique and would not be able to distinguish from Fe-ions intercalated into the cathode and iron-containing surface species. Again, there is no change in the iron XPS from discharge to charge, indicating that the observed iron is not removed or changing oxidation states on charge, which is surprising because it should be oxidatively removed. Finally, the SEM/EDX data show a change in morphology on the mild steel anode from a flat surface before reaction to a particle-coated surface after cycling. It is suggested that the particles are iron oxides, which is consistent with the battery undergoing side reactions.
“[Cycling of the] Fe+2/Fe0 redox couple is uncertain (see Figure S1, the reduction wave starts above 0 V), and no proof of iron metal formation is given [only oxides by SEM], nor is there conclusive proof of Fe+2 intercalation in V2O5, just some evidence for formation of mixed oxides,” according to Professor Palacín.
A multivalent iron-ion battery could be a future option for cheap and sustainable energy storage, albeit with moderate performance characteristics. More work is required to understand which reactions are occurring in the reported V2O5/Fe system to determine whether there is a reversible electrochemical redox process that could be leveraged for rechargeable energy storage applications.