Search

α-V2O5 has been extensively explored as a Mg2+ intercalation host with potential as a battery cathode, offering high theoretical capacities and potentials vs. Mg2+/Mg. However, large voltage hysteresis is observed with Mg insertion and extraction, introducing significant and unacceptable round-trip energy losses with cycling. Conventional interpretations suggest that bulk ion transport of Mg2+ within the cathode particles is the major source of this hysteresis. Herein, we demonstrate that nanosizing α-V2O5 gives a measurable reduction to voltage hysteresis on the first cycle that substantially raises energy efficiency, indicating that mechanical formatting of the α-V2O5 particles contributes to hysteresis. However, no measurable improvement in hysteresis is found in the nanosized α-V2O5 in latter cycles despite the much shorter diffusion lengths, suggesting that other factors aside from Mg transport, such as Mg transfer between the electrolyte and electrode, contribute to this hysteresis. This observation is in sharp contrast to the conventional interpretation of Mg electrochemistry. Therefore, this study uncovers critical fundamental underpinning limiting factors in Mg battery electrochemistry, and constitutes a pivotal step towards a high-voltage, high-capacity electrode material suitable for Mg batteries with high energy density.

V2O5 is of interest as a Mg intercalation electrode material for Mg batteries, both in its thermodynamically stable layered polymorph (α-V2O5) and in its metastable tunnel structure (ζ-V2O5). However, such oxide cathodes typically display poor Mg insertion/removal kinetics, with large voltage hysteresis. Herein, we report the synthesis and evaluation of nanosized (ca. 100 nm) ζ-V2O5 in Mg-ion cells, which displays significantly enhanced electrochemical kinetics compared to microsized ζ-V2O5. This effect results in a significant boost in stable discharge capacity (130 mA h g-1) compared to bulk ζ-V2O5 (70 mA h g-1), with reduced voltage hysteresis (1.0 V compared to 1.4 V). This study reveals significant advancements in the use of ζ-V2O5 for Mg-based energy storage and yields a better understanding of the kinetic limiting factors for reversible magnesiation reactions into such phases.

Conventionally, we expect the transport of intercalated species in battery active particles is limited by diffusion. MgCr2O4 has been identified as a candidate active material for the Mg batteries based on its theoretical Mg-storing capacity, high open circuit potential, and acceptable diffusion barrier for Mg-ion transport (1,2). However, when operated electrochemically, its performance does not match the promised potential (3). Motivated by these observations, we examine charge transport in dense MgCr2O4 pellets in an electrolyte-free setup to conclusively probe bulk Mg transport. We find that the ion transport in this material cannot be explained by using the conventional understanding. We propose a new theory that extends the conventional picture and accounts for the unusual behavior in MgCr2O4 (4). We further discuss the implications of this understanding for engineering active material particles in MgCr2O4 porous electrodes. References Canepa, P.; Sai Gautam, G.; Hannah, D. C.; Malik, R.; Liu, M.; Gallagher, K. G.; Persson, K. A.; Ceder, G. (2017) "Odyssey of Multivalent Cathode Materials: Open Questions and Future Challenges." Chem. Rev., 117 (5), 4287–4341. DOI: 10.1021/acs.chemrev.6b00614. Bayliss, R. D.; Key, B.; Sai Gautam, G.; Canepa, P.; Kwon, B. J.; Lapidus, S. H.; Dogan, F.; Adil, A. A.; Lipton, A. S.; Baker, P. J.; Ceder, G.; Vaughey, J. T.; Cabana, J. (2020) "Probing Mg Migration in Spinel Oxides." Chem. Mater., 32 (2), 663–670. DOI: 10.1021/acs.chemmater.9b02450. Hu, L.; Johnson, I. D.; Kim, S.; Nolis, G. M.; Freeland, J. W.; Yoo, H. D.; Fister, T. T.; McCafferty, L.; Ashton, T. E.; Darr, J. A.; Cabana, J. (2019) "Tailoring the Electrochemical Activity of Magnesium Chromium Oxide towards Mg Batteries through Control of Size and Crystal Structure." Nanoscale, 11 (2), 639–646. DOI: 10.1039/C8NR08347A. Johnson, I.D.; Mistry, A.; Yin, L.; Murphy, M.; Wolfman, M.; Fister, T. T.; Lapidus, S. H.; Cabana, J.; Srinivasan, V.; Ingram, B. J. (2022) "Investigating Charge Transport in MgCr2O4 Extends the Understanding of Battery Intercalation Hosts.", under review

Chromium oxides with the spinel structure have been predicted to be promising high voltage cathode materials in magnesium batteries. Perennial challenges involving the mobility of Mg2+ and reaction kinetics can be circumvented by nano-sizing the materials in order to reduce diffusion distances, and by using elevated temperatures to overcome activation energy barriers. Herein, ordered 7 nm crystals of spinel-type MgCr2O4 were synthesized by a conventional batch hydrothermal method. In comparison, the relatively underexplored Continuous Hydrothermal Flow Synthesis (CHFS) method was used to make highly defective sub-5 nm MgCr2O4 crystals. When these materials were made into electrodes, they were shown to possess markedly different electrochemical behavior in a Mg2+ ionic liquid electrolyte, at moderate temperature (110 °C). The anodic activity of the ordered nanocrystals was attributed to surface reactions, most likely involving the electrolyte. In contrast, evidence was gathered regarding the reversible bulk deintercalation of Mg2+ from the nanocrystals made by CHFS. This work highlights the impact on electrochemical behavior of a precise control of size and crystal structure of MgCr2O4. It advances the understanding and design of new cathode materials for Mg-based batteries.

The Li+ ion diffusion characteristics of V- and Nb-doped LiFePO4 were examined with respect to undoped LiFePO4 using muon spectroscopy (µSR) as a local probe. As little difference in diffusion coefficient between the pure and doped samples was observed, offering DLi values in the range 1.8–2.3 × 10−10 cm2 s−1, this implied the improvement in electrochemical performance observed within doped LiFePO4 was not a result of increased local Li+ diffusion. This unexpected observation was made possible with the µSR technique, which can measure Li+ self-diffusion within LiFePO4, and therefore negated the effect of the LiFePO4 two-phase delithiation mechanism, which has previously prevented accurate Li+ diffusion comparison between the doped and undoped materials. Therefore, the authors suggest that µSR is an excellent technique for analysing materials on a local scale to elucidate the effects of dopants on solid-state diffusion behaviour.

Vanadium dioxide–carbon nanotube (VO2–CNT) nanocomposite materials were produced via a continuous hydrothermal flow synthesis (CHFS) method. The composites were made in a single step from CHFS using dispersions of commercially available single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes (MWCNTs) in a metal salt solution (aq.). The room temperature gas sensing characteristics of the VO2-CNT nanocomposites were investigated and compared with sensors of CHFS-made VO2 without added carbon. The VO2-CNT nanocomposites were found to display high sensitivity to H2O vapour, showing excellent potential as humidity sensors. Furthermore, p-type responses to ammonia gas were observed, with the VO2 (no carbon) sensors showing the largest response. Overall, surface composition and microstructure were found to greatly influence sensor responses to H2O vapour and NH3 gas.

The magnesium battery (utilizing the reversible reactivity of Mg2+) offers the potential to surpass the Li-ion battery in terms of energy-density and cost, thereby enabling next-generation devices such as lower-cost and higher-range electric vehicles.1 A crucial component defining the energy density of the Mg battery is the cathode, which must display reversible Mg intercalation and redox chemistry at sufficiently high capacities and potentials. Of all known cathode materials, the Mg spinels MgM2O4 (where M = Cr, Mn, V) have conclusively demonstrated high capacity and high potentials vs Mg2+/Mg.2–5 The spinel structure possesses 3D networks of diffusion channels, with a combination of theoretical and experimental studies suggesting relatively low barriers for Mg2+ diffusion.5,6 However, reversible Mg2+ intercalation kinetics have proven to be extremely inhibitive in oxide cathode materials in general, with large overpotentials (and voltage hysteresis) observed on charge and discharge. While the origin of this effect is still a matter of debate, it has been suggested that sluggish bulk transport of Mg2+ can partially account for the hysteresis observed, although surprisingly little is known about the nature of ion transport (both electrons and Mg2+) in these systems. In this presentation, we discuss a combination of experimental and theoretical tools used to extract key ion and electron transport properties across a range of Mg-containing spinels, Mg(Cr1−x M)2O4, where M = Mn, V, Ti. We reveal the structure-composition-property relationships that emerge across this range of compounds and discuss the relative and cooperative role that ion and electron motion play in limiting conduction kinetics in these materials. This study represents a detailed treatment of bulk ion and electron transport in these systems, and provides analytical and theoretical tools to further study electrode materials for such next-generation energy storage devices. References: (1) Canepa, P.; Sai Gautam, G.; Hannah, D. C.; Malik, R.; Liu, M.; Gallagher, K. G.; Persson, K. A.; Ceder, G. Odyssey of Multivalent Cathode Materials: Open Questions and Future Challenges. Chem. Rev. 2017, 117 (5), 4287–4341. (2) Hu, L.; Jokisaari, J. R.; Kwon, B. J.; Yin, L.; Kim, S.; Park, H.; Lapidus, S. H.; Klie, R. F.; Key, B.; Zapol, P.; Ingram, B. J.; Vaughey, J. T.; Cabana, J. High Capacity for Mg2+ Deintercalation in Spinel Vanadium Oxide Nanocrystals. ACS Energy Lett. 2020, 5 (8), 2721–2727. (3) Kwon, B. J.; Yin, L.; Park, H.; Parajuli, P.; Kumar, K.; Kim, S.; Yang, M.; Murphy, M.; Zapol, P.; Liao, C.; Fister, T. T.; Klie, R. F.; Cabana, J.; Vaughey, J. T.; Lapidus, S. H.; Key, B. High Voltage Mg-Ion Battery Cathode via a Solid Solution Cr–Mn Spinel Oxide. Chem. Mater. 2020, 32 (15), 6577–6587. (4) Kwon, B. J.; Lau, K. C.; Park, H.; Wu, Y. A.; Hawthorne, K. L.; Li, H.; Kim, S.; Bolotin, I. L.; Fister, T. T.; Zapol, P.; Klie, R. F.; Cabana, J.; Liao, C.; Lapidus, S. H.; Key, B.; Vaughey, J. T. Probing Electrochemical Mg-Ion Activity in MgCr2 − x V x O4 Spinel Oxides. Chem. Mater. 2020, 32 (3), 1162–1171. (5) Bayliss, R. D.; Key, B.; Sai Gautam, G.; Canepa, P.; Kwon, B. J.; Lapidus, S. H.; Dogan, F.; Adil, A. A.; Lipton, A. S.; Baker, P. J.; Ceder, G.; Vaughey, J. T.; Cabana, J. Probing Mg Migration in Spinel Oxides. Chem. Mater. 2020, 32 (15), 663–670. (6) Liu, M.; Rong, Z.; Malik, R.; Canepa, P.; Jain, A.; Ceder, G.; Persson, K. A.; Liu, M. Spinel Compounds as Multivalent Battery Cathodes: A Systematic Evaluation Based on Ab Initio Calculations. Energy Environ. Sci 2015, 8 (3), 964–974. Acknowledgements: The authors would like to acknowledge funding support from the Joint Center for Energy Storage Research (JCESR, a U.S. Department of Energy, Energy Innovation Hub).

A series of LiMn1–x–yFexVyPO4 (LMFVP) nanomaterials have been synthesized using a pilot-scale continuous hydrothermal synthesis process (CHFS) and evaluated as high voltage cathodes in Li-ion batteries at a production rate of 0.25 kg h–1. The rapid synthesis and screening approach has allowed the specific capacity of the high Mn content olivines to be optimized, particularly at high discharge rates. Consistent and gradual changes in the structure and performance are observed across the compositional region under investigation; the doping of Fe at 20 at% (with respect to Mn) into lithium manganese phosphate, rather than V or indeed codoping of Fe and V, gives the best balance of high capacity and high rate performance.

Multivalent-ion batteries, based on the reversible reactivity of metal cations Mn+ (where n ≥ 2), offer great promise to supersede the current state-of-the-art Li-ion battery and provide a step change in energy storage capability. Oxide materials based on the spinel structure, AB2O4 (where A is the diffusing ion, M2+) have previously demonstrated potential to be high capacity, high voltage cathode materials for such batteries, with a 3D network of M2+ diffusion channels to facilitate ion transport.1–3 In particular, the MgCr2O4 spinel has been of interest due the predicted high electrochemical potential (~3.5 V) of Cr3+/Cr4+ vs. Mg/Mg2+.2 However, reversible Mg2+ intercalation kinetics has often proven to be poor in such systems, with large overpotentials observed on charge and discharge.3,4 The origin of these kinetic barriers is still poorly understood, where it is postulated that poor Mg2+ diffusion through the oxide lattice, sluggish ion transfer between the electrode and electrolyte, or resistive surface layers from electrolyte decomposition products may be responsible. In this presentation, we discuss the application of solid-state impedance spectroscopy to a library of doped spinel compounds, M1−x Cr2−2x Ti2x O4, where M = Mg or Zn, to elucidate the diffusion behavior of Mg2+ and Zn2+ in these systems. We reveal that incorporation of Ti had significant impact on the impedance behavior of the materials. As both M2+ and e− were found to diffuse in these materials, they displayed mixed ionic and electronic conductor (MIEC) behavior.5,6 This presentation includes the development and interpretation of a new physical model to describe the motion of M2+ and e− in the M1−x Cr2−2x Ti2x O4 spinel, which was necessary to accurately describe their diffusive behavior and extract their conductivities. This study therefore represents a breakthrough in the understanding of M2+ motion in these systems, and provides a methodological framework for further studies of other electrode chemistries. References: (1) Canepa, P.; Sai Gautam, G.; Hannah, D. C.; Malik, R.; Liu, M.; Gallagher, K. G.; Persson, K. A.; Ceder, G. Odyssey of Multivalent Cathode Materials: Open Questions and Future Challenges. Chem. Rev. 2017, 117 (5), 4287–4341. (2) Liu, M.; Rong, Z.; Malik, R.; Canepa, P.; Jain, A.; Ceder, G.; Persson, K. A.; Liu, M. Spinel Compounds as Multivalent Battery Cathodes: A Systematic Evaluation Based on Ab Initio Calculations. Energy Environ. Sci 2015, 8 (3), 964–974. (3) Hu, L.; Johnson, I. D.; Kim, S.; Nolis, G. M.; Freeland, J.; Yoo, H. D.; Fister, T. T.; McCafferty, L.; Ashton, T. E.; Darr, J. A.; et al. Tailoring the Electrochemical Activity of Magnesium Chromium Oxide Towards Mg Batteries Through Control of Size and Crystal Structure. Nanoscale 2019, 11, 639–646. (4) Shimokawa, K.; Ichitsubo, T. Spinel–Rocksalt Transition as a Key Cathode Reaction toward High-Energy-Density Magnesium Rechargeable Batteries. Current Opinion in Electrochemistry. Elsevier B.V. 2020, pp 93–99. (5) Lai, W.; Haile, S. M. Impedance Spectroscopy as a Tool for Chemical and Electrochemical Analysis of Mixed Conductors: A Case Study of Ceria. J. Am. Ceram. Soc. 2005, 88 (11), 2979–2997. (6) Jamnik, J.; Maier, J. Treatment of the Impedance of Mixed Conductors. Equivalent Circuit Model and Explicit Approximate Solutions. J. Electrochem. Soc. 1999, 146 (11), 4183–4188.

A high performance vanadium-doped LiFePO4 (LFP) electrode is synthesized using a continuous hydrothermal method at a production rate of 6 kg per day. The supercritical water reagent rapidly generates core/shell nanoparticles with a thin, continuous carbon coating on the surface of LFP, which aids electron transport dynamics across the particle surface. Vanadium dopant concentration has a profound effect on the performance of LFP, where the composition LiFe0.95V0.05PO4, achieves a specific discharge capacity which is among the highest in the comparable literature (119 mA h g−1 at a discharge rate of 1500 mA g−1). Additionally, a combination of X-ray absorption spectroscopy analysis and hybrid-exchange density functional theory, suggest that vanadium ions replace both phosphorous and iron in the structure, thereby facilitating Li+ diffusion due to Li+ vacancy generation and changes in the crystal structure.

The Li+ ion diffusion characteristics of V- and Nb-doped LiFePO4 were examined with respect to undoped LiFePO4 using muon spectroscopy (µSR) as a local probe. As little difference in diffusion coefficient between the pure and doped samples was observed, offering DLi values in the range 1.8–2.3 × 10−10 cm2 s−1, this implied the improvement in electrochemical performance observed within doped LiFePO4 was not a result of increased local Li+ diffusion. This unexpected observation was made possible with the µSR technique, which can measure Li+ self-diffusion within LiFePO4, and therefore negated the effect of the LiFePO4 two-phase delithiation mechanism, which has previously prevented accurate Li+ diffusion comparison between the doped and undoped materials. Therefore, the authors suggest that µSR is an excellent technique for analysing materials on a local scale to elucidate the effects of dopants on solid-state diffusion behaviour.

The lithium storage properties of the distorted metal-organic framework (MOF) derived nanosized ZnO@C are significantly improved by the introduction of Ag2S quantum dots (QDs) during the processing of the material. In the thermal treatment, the Ag2S QDs react to produce Ag nanoparticles and ZnS. The metal nanoparticles act to shorten electron pathways and improve the connectivity of the matrix and the partial sulfidation of the ZnO surface improves the cycling stability of the material. The electrochemical properties of ZnO@C, Ag2S QDs treated ZnO@C and the amorphous carbon in ZnO@C have been compared. The small weight ratio of Ag2S QDs to ZnO@C at 1:180 shows the best performance in lithium storage. The exhibited specific capacities are improved and retained remarkably in the cycling at high current rates. At low current densi-ties (200 mA g-1) treatment of ZnO@C with Ag2S QDs results in a 38% increase in the specific capacity.

Lithium iron phosphate, LiFePO4 (LFP) has demonstrated promising performance as a cathode material in lithium ion batteries (LIBs), by overcoming the rate performance issues from limited electronic conductivity. Nano-sized vanadium-doped LFP (V-LFP) was synthesized using a continuous hydrothermal process using supercritical water as a reagent. The atomic % of dopant determined the particle shape. 5 at. % gave mixed plate and rod-like morphology, showing optimal electrochemical performance and good rate properties vs. Li. Specific capacities of >160 mAh g−1 were achieved. In order to increase the capacity of a full cell, V-LFP was cycled against an inexpensive micron-sized metallurgical grade Si-containing anode. This electrode was capable of reversible capacities of approximately 2000 mAh g−1 for over 150 cycles vs. Li, with improved performance resulting from the incorporation of few layer graphene (FLG) to enhance conductivity, tensile behaviour and thus, the composite stability. The cathode material synthesis and electrode formulation are scalable, inexpensive and are suitable for the fabrication of larger format cells suited to grid and transport applications.