Manganese Vacancies Explain Battery Fade in Potential Grid-Scale Energy Storage

Scientists are actively investigating the capacity fade observed in manganese dioxide (MnO₂) cathodes, a significant limitation for rechargeable aqueous zinc-ion batteries intended for large-scale energy storage. Caio Miranda Miliante, Kevin J. Sanders, and Liam J. McGoldrick, working with colleagues from the Department of Materials Science and Engineering, Department of Chemistry & Chemical Biology at McMaster University, the Condensed Matter and Statistical Physics Section at The Abdus Salam ICTP, and Salient Energy Inc., have now elucidated the origin of this degradation through detailed analysis of manganese vacancy formation. Their research addresses conflicting theories surrounding manganese dissolution, identifying an energetically unfavourable zinc coordination environment during battery discharge as the key driver. By combining density functional theory calculations with operando hydrogen nuclear magnetic resonance experiments, the team demonstrates that Mn dissolution is thermodynamically feasible under operating conditions, providing crucial insight into cathode degradation not only for zinc-ion batteries but also for other intercalating ion battery chemistries.

These batteries, offering advantages in safety and cost over lithium-ion alternatives, currently suffer from a significant limitation: a rapid decline in performance during extended cycling at rates relevant for practical grid applications.

The degradation stems from the dissolution of manganese atoms from the cathode material into the electrolyte, a process previously linked to complex chemical reactions but lacking a definitive explanation. This work establishes that the instability of zinc coordination within the cathode material during battery discharge directly promotes manganese dissolution.

The study focused on alpha-manganese dioxide (α-MnO2), a widely investigated cathode material, and utilised density functional theory to probe the energetic favourability of manganese vacancy formation in both charged and discharged states. Calculations revealed that forming a manganese vacancy, and subsequent dissolution as Mn2+ ions, is thermodynamically feasible specifically within the discharged α-ZnMn2O4 phase.

This is due to the energetically unfavourable bent coordination formed around zinc ions during the intercalation process, creating an unstable environment that encourages manganese to detach from the structure. These theoretical findings were validated through operando hydrogen nuclear magnetic resonance experiments, a technique that allowed researchers to directly observe manganese dissolution occurring throughout the discharge cycle.

Subsequent electrochemical deposition of manganese onto the electrode was also confirmed during charging, demonstrating a dynamic process of dissolution and re-deposition. The combined computational and experimental approach highlights the critical role of defect energetics and the local coordination environment in dictating active material dissolution, and consequently, the capacity fade observed in these batteries.

This discovery extends beyond zinc-ion batteries, offering insights into cathode degradation mechanisms applicable to other battery chemistries that rely on the intercalation of ions. By pinpointing the root cause of manganese dissolution, this research provides a foundation for developing strategies to mitigate capacity fade and design more stable cathode materials, paving the way for the widespread adoption of aqueous zinc-ion batteries in large-scale energy storage systems.

The work offers a fundamental understanding of active material dissolution, potentially leading to prolonged stable battery operation and improved performance. Theoretical calculations reveal that the formation of a manganese vacancy, and subsequent dissolution of manganese as Mn2+, is thermodynamically feasible within the α-ZnMn2O4 phase. This finding stems from the energetically unfavourable bent coordination formed around zinc during the zinc intercalation process, demonstrating that manganese dissolution is actively promoted by an unstable zinc coordination environment.

Specifically, the calculations determined that Mn dissolution is energetically favourable even for a partially discharged α-ZnxMn2O4 phase where x is less than 0.5, due to this unstable coordination. Formation energies were calculated for neutral and charged vacancy defects, capturing the thermodynamic susceptibility of each solid phase to manganese dissolution.

These calculations established a direct link between defect energetics and the coordination environment driving active material dissolution, a phenomenon with implications extending beyond zinc-ion batteries. Further corroboration came from operando 1H nuclear magnetic resonance experiments, which detected continuous manganese dissolution from the α-MnO2 electrode throughout the discharge process.

These experiments quantified the concentration of Mn2+ in the electrolyte, confirming the theoretical predictions of manganese dissolution occurring during RAZIB cycling. Subsequent electrochemical deposition of the dissolved manganese onto the electrode was also observed during the charge phase. The combined computational and experimental analysis highlights the critical role of the unstable zinc coordination in promoting capacity fade.

Plane-wave density functional theory (DFT) calculations underpinned the investigation into manganese (Mn) dissolution within manganese dioxide (MnO₂) cathode materials used in rechargeable aqueous zinc-ion batteries. These calculations, performed using the VASP software package (version 6.4.3), employed the Perdew, Burke-Ernzerhof (PBE) exchange-correlation functional to accurately describe the electronic interactions within the materials.

Projector augmented wave pseudopotentials were utilised to represent the valence electrons of manganese (3p⁶4s²3d⁵), oxygen (2s²2p⁴), and zinc (4s²3d¹⁰), enabling precise modelling of their behaviour. A plane-wave cutoff energy of 400 eV was established, representing the highest energy threshold for describing the electronic wavefunctions. Spin polarization with ferromagnetic ordering was considered throughout, although antiferromagnetic ordering was also investigated and found to yield slightly higher energies, a difference unlikely to alter the study’s conclusions.

To accurately sample the electronic properties of the materials, a Γ-centred k-mesh with a density of 20 subdivisions per Å⁻¹ was used. The convergence of the calculations was ensured by relaxing atomic positions and cell parameters until the energy change and ionic forces reached tolerances of 10⁻⁷ eV and 10⁻² eV Å⁻¹, respectively. Gaussian smearing with a width of 0.01 eV was applied to facilitate convergence.

The study focused on the α-MnO₂ and λ-MnO₂ polymorphs, chosen as representative materials due to their differing structural characteristics and relevance to zinc-ion battery operation. Supercells were constructed from unit cells obtained from the Materials Project database to minimise the impact of periodic boundary conditions on the defect formation energy calculations.

The persistent challenge of energy storage has driven intense scrutiny of battery cathode materials, yet many promising candidates succumb to degradation over repeated charge-discharge cycles. This work offers a crucial insight into a specific failure mode, the dissolution of manganese in zinc-ion batteries, by pinpointing the underlying energetic instability.

For years, researchers have debated the precise mechanism of this dissolution, oscillating between proposed chemical reactions and structural defects. This study moves beyond speculation, demonstrating that the problem isn’t necessarily what manganese is doing, but where it’s doing it. The team’s calculations reveal that the formation of manganese vacancies is surprisingly easy within the discharged state of the battery, specifically due to the distorted coordination environment around zinc ions.

This isn’t a simple case of manganese wanting to dissolve; it’s being actively encouraged to do so by the very process of ion intercalation. The corroborating experimental evidence, capturing manganese dissolving during discharge and redepositing during charge, solidifies this picture. This is significant because it shifts the focus from mitigating manganese dissolution as a chemical problem to engineering more stable structural environments within the cathode itself.

However, the study’s scope is limited to specific manganese oxide polymorphs and aqueous electrolytes. The relevance to other cathode chemistries, while suggested, requires further investigation. Moreover, while the energetic calculations are compelling, they don’t fully capture the complex interplay of factors at play within a functioning battery, such as electrolyte composition or the influence of current density. Future work should explore strategies to reinforce these unstable zinc coordination sites, perhaps through doping or surface coatings, and assess the broader applicability of these findings to solid-state electrolytes, where degradation mechanisms may differ.