Mechanisms of Electronic and Ionic Transport during Mg Intercalation in Mg–S Cathode Materials and Their Decomposition Products

10.1021/acs.chemmater.2c03784Chemistry of Material

Ab Initio Study of the Combined Effects of Alloying Elements and H on Grain Boundary Cohesion in Ferritic Steels

Hydrogen enhanced decohesion is expected to play a major role in ferritic steels, especially at grain boundaries. Here, we address the effects of some common alloying elements C, V, Cr, and Mn on the H segregation behaviour and the decohesion mechanism at a Σ 5 ( 310 ) [ 001 ] 36.9 ∘ grain boundary in bcc Fe using spin polarized density functional theory calculations. We find that V, Cr, and Mn enhance grain boundary cohesion. Furthermore, all elements have an influence on the segregation energies of the interstitial elements as well as on these elements’ impact on grain boundary cohesion. V slightly promotes segregation of the cohesion enhancing element C. However, none of the elements increase the cohesion enhancing effect of C and reduce the detrimental effect of H on interfacial cohesion at the same time. At an interface which is co-segregated with C, H, and a substitutional element, C and H show only weak interaction, and the highest work of separation is obtained when the substitute is Mn

Prussian Blue Analogues as Positive Electrodes for Mg Batteries: Insights into Mg2+ Intercalation

Potassium manganese hexacianoferrate has been prepared by co-precipitation from manganese (II) chloride and potassium citrate, with chemical analysis yielding the formula K1.72 Mn[Fe(CN)6 ]0.92 □0.08  ⋅ 1.1H2 O (KMnHCF). Its X-ray diffraction pattern is consistent with a monoclinic structure (space group P 21 /n, no. 14) with cell parameters a=10.1202(6)Å, b=7.2890(5)Å, c=7.0193(4)Å, and β=89.90(1)°. Its redox behavior has been studied in magnesium containing electrolytes. Both K+ ions deintercalated from the structure upon oxidation and contamination with Na+ ions coming from the separator were found to interfere in the electrochemical response. In the absence of alkaline ions, pre-oxidized manganese hexacianoferrate showed reversible magnesium intercalation, and the process has been studied by operando synchrotron X-ray diffraction. The location of Mg2+ ions in the crystal structure was not possible with the available experimental data. Still, density functional theory simulations indicated that the most favorable position for Mg2+ intercalation is at 32f sites (considering a pseudo cubic F m-3m phase), which are located between 8c and Mn sites.The authors are grateful to Ashley Black (ICMAB-CSIC) and Jean-Frederic Martin (CEA) for helpful discussions and to François Fauth (ALBA synchrotron beamline scientist) for assistance during SXRD measurements (beamtime awarded under proposal 2021075225). ICMAB-CSIC authors are grateful to the Spanish Agencia Estatal de Investigación for Severo Ochoa FUNFUTURE (CEX2019-000917-S) distinction. Funding through the European Union's Horizon 2020 research and innovation programme under grant agreement No 824066 (E-MAGIC), Junta de Andalucía (EMERGIA_00153, ProyExcel_00330 PAIDI 2020), and project TED2021-129314A-100 funded by MCIN/AEI/10.13039/501100011033 and European Union NextGenerationEU/PRTR is gratefully acknowledged. Authors acknowledge support of the publication fee by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI).With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe

Prussian Blue Analogues as Positive Electrodes for Mg batteries: Insights into Mg²⁺ Intercalation

Potassium manganese hexacianoferrate has been prepared by co-precipitation from manganese (II) chloride and potassium citrate, with chemical analysis yielding the formula K1.72Mn[Fe(CN)6]0.92□0.08·1.1H2O (KMnHCF). Its X-ray diffraction pattern is consistent with a monoclinic structure (space group P 21/n, no. 14) with cell parameters a= 10.1202(6)Å, b= 7.2890(5)Å, c= 7.0193(4)Å, and β= 89.90(1)°. Its redox behavior has been studied in magnesium containing electrolytes. Both K+ ions deintercalated from the structure upon oxidation and contamination with Na+ ions coming from the separator were found to interfere in the electrochemical response. In the absence of alkaline ions, pre-oxidized manganese hexacianoferrate showed reversible magnesium intercalation, and the process has been studied by operando synchrotron X-ray diffraction. The location of Mg2+ ions in the crystal structure was not possible with the available experimental data. Still, density functional theory simulations indicated that the most favorable position for Mg2+ intercalation is at 32f sites ((considering a pseudo cubic F m-3m phase), which are located between 8c and Mn sites

Mechanisms of Electronic and Ionic Transport during Mg Intercalation in Mg–S Cathode Materials and Their Decomposition Products

Rechargeable Mg–S batteries are attractive for next-generation energy storage devices due to their high theoretical energy density (1684 W h kg–1 and 3286 W h L–1) and low costs. The poor cycling performance of Mg–S batteries is linked to the formation of the solid discharge products, i.e., MgS2 and MgS, which are electronic and ionic insulators. However, the formation of MgS itself contradicts such a premise because it requires further oxidation of MgS2. Indeed, the insulating nature of MgS2 should inhibit such an oxidation process in the first place. Using first-principles calculations and ab initio molecular dynamics simulations, we evaluate the charge transport associated with point defects in MgS2 and MgS. In MgS2, the single-electron polaron is the most abundant type of defect that emerges from our model, which appears at a low concentration at thermodynamic equilibrium but displays high mobility. However, under conditions far from thermodynamic equilibrium, mimicking those for battery operations, the concentration of electron polarons increases, enhancing the electronic conductivity in MgS2. We demonstrate that in regimes far from thermodynamic equilibrium, the single-electron polarons coalesce to form double-electron polarons, whose mobilities are similar to that of a single-electron polaron. MgS2 holds electronic conduction through a polaron migration mechanism for ≤3 μm thick deposits, enabling further oxidation to form MgS. For MgS, our model suggests that the doubly positive Mg interstitial and doubly negative Mg vacancy are identified as the prevalent defects with high concentrations. However, due to the low mobility of these defects, their contribution to charge transport is negligible, which stops the oxidation process and severely hinders battery cyclability. Our results indicate that rechargeable Mg–S batteries can be developed if we ensure that the battery discharge does not push the oxidation process beyond the formation of MgS2

Addressing the Sluggish Kinetics of Sulfur Redox for High‐Energy Mg–S Batteries

A key challenge for practical magnesium–sulfur (Mg–S) batteries is to overcome the sluggish conversion kinetics of sulfur cathodes, achieving a high energy density and long-lasting battery life. To address this issue, a doping strategy is demonstrated in a model Ketjenblack sulfur (KBS) cathode by introducing selenium with a high electronic conductivity. This leads to a significantly enhanced charge transfer in the resultant KBS1−xSex cathodes, giving rise to a higher S utilization and less polysulfide dissolution. Compared to the bare S cathode, the S-Se composite cathodes exhibit a higher capacity, smaller overpotentials, and improved efficiency, serving as better benchmark compounds for high-performance Mg–S batteries. First principles calculations reveal a charge transport mechanism via electron polaron diffusion in the redox end-products, that enhances the reaction kinetics. By suppressing polysulfide dissolution in the electrolyte, the use of the KBS1−xSex cathodes also enables a more uniform anode reaction, and thereby significantly extends the cyclability of the cells. To improve the performance, further efforts are made by implementing a Mo6S8 modified separator into the cell. With an optimized cathode composition of KBS0.86Se0.14, the cell applying modified separator shows an improvement of capacity retention by >50% after 200 cycles