http://pubs.acs.org/journal/aelccp
Status and Outlook for Magnesium Battery Technologies: A Conversation with Stan Whittingham and Sarbajit Banerjee Downloaded via 91.243.191.197 on January 31, 2019 at 07:51:25 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
L
ithium-ion batteries are very widely used throughout consumer products ranging from small electronics to larger-scale applications such as for powering electric vehicles. The current state-of-the-art for commercial lithiumion batteries is to pair transition metal-based cathodes with anodes composed of graphite rather than of lithium metal, which conceptually would be expected to produce superior performance metrics for the final lithium-ion batteries produced. The use of lithium metal for anodes carries the detrimental limitation of a high propensity for dendrite formation, which can damage the polymer-based separator, rendering the battery nonfunctional with potential for catastrophic consequences such as battery fires. Low dendrite-forming alternatives are being investigated, with magnesium batteries bringing promise since there has been a lack of studies indicating formation of magnesium dendrites during electrodeposition. Furthermore, concerns about the lower natural abundance of lithium as compared to magnesium and other alkali metals has motivated research into diversifying the selection of battery materials available. In their Energy Express article published in the latest issue of ACS Energy Letters (DOI: 10.1021/acsenergylett.8b02470), Sarbajit Banerjee, Partha P. Mukherjee, and co-workers at Texas A&M University and Purdue University contend that, though it has been broadly accepted that magnesium has a lower propensity for dendrite formation as compared to lithium, this understanding has been based on studies that are limited by the use of only moderate electrodeposition conditions. Upon evaluating a wide range of electrodeposition conditions, the authors observed the formation of dendritic magnesium deposits that are harder than their lithium counterparts and therefore potentially more capable of inflicting damage to the polymerbased separators most frequently used in commercial battery production. Therefore, it becomes clear that more research is needed to fully elucidate the potential limitations of magnesium battery technologies and develop feasible workarounds. To delve further into the significance of these findings, we consulted with co-corresponding author Professor Sarbajit Banerjee of Texas A&M University and an independent expert in the field, Professor Stan Whittingham, of SUNY at Binghamton, for their insight. Conversation with Stan Whittingham: Why the interest in magnesium batteries? Lithium-based batteries have been around since the mid-1970s. They now dominate portable energy storage (from phones to electric vehicles) and are even dominating grid storage (over 90% of batteries on the grid are Li-ions, but still tiny compared to pumped hydro). However, there is a perception that there will eventually be a shortage of lithium. Alternative more abundant anode © XXXX American Chemical Society
Figure 1. Dr. Stan Whittingham is currently a Distinguished Professor of Chemistry, and Materials Science & Engineering at Binghamton University (https://www.binghamton.edu/ chemistry/people/whittingham/whittingham.html). Photo courtesy of Binghamton University.
materials include sodium and magnesium. The latter has the advantage that it carries a +2 charge; therefore, only half the number of ions is needed, but also if the transition metal cation has sufficient accessible oxidation states, e.g., Ni4+ to Ni2+ or V5+ to V3+, then only half the amount of cathode material is needed. Thus, such a battery would use less material and in principle cost less and have a higher capacity. Moreover, using half the number of cations reduces the electrostatic interactions between them and allows structures to be used that do not have to undergo a phase transition that might be necessary for the intercalation of two lithium ions. For example, in a layered dichalcogenide, AxM[O,S,Se]2, there is only one octahedral site available per M metal ion; therefore, when x >, 1 all of the A ions must move to the smaller tetrahedral sites (as observed in LixVSe2).1 In contrast, all of the magnesium ions can be situated on just the octahedral sites. Can you provide some history on rechargeable (secondary) magnesium batteries? One of the earliest reports of a Received: January 28, 2019 Accepted: January 28, 2019
572
DOI: 10.1021/acsenergylett.9b00214 ACS Energy Lett. 2019, 4, 572−575
Energy Focus
Cite This: ACS Energy Lett. 2019, 4, 572−575
Energy Focus
ACS Energy Letters
Figure 2. Dendrites formed (left) in Mg cells (from ref 7) and (right) in Li cells (from ref 8). Right: Reprinted from ref 8, Copyright 2018 with permission from Elsevier. Left: Reprinted with permission from ref 7, Copyright 2019 American Chemical Society.
magnesium cell was by the Aurbach group,2 who used Mo3S4 as the cathode material. This soft highly electronic conducting lattice allowed the reversible diffusion of the doubly charged Mg2+ as the electrostatic interactions between the diffusing cations and the host lattice are minimized. A more recent study by the Nazar group3 has shown the highest capacity, approaching 200 mAh/g, reported to date using another soft conducting lattice, TiS2. This data was obtained at 60 °C and 0.04 mA/cm2. This shows the double-edged sword of using divalent cations. In principle, they offer the opportunity of higher capacities, but the double charge severely increases the activation energy for diffusion so that high-capacity cycling is only obtained at low rates and elevated temperatures and then in soft conducting lattices. To date, nobody has succeeded in transferring more than 1e in a magnesium cell, and the cell potential of magnesium cells is between 0.5 and 1.0 V lower than that of the corresponding lithium cell.4 Thus, a Mg/TiS2 cell has less than 50% of the energy density of a Li/TiS2 cell. Therefore, the remaining incentive for magnesium batteries, beyond the abundance of magnesium, was the expected increased safety due to the reported lack of formation of dendrites. What is the significance on dendrites in magnesium batteries? There has been a general assumption by researchers5 in the field that magnesium has an advantage over lithium because it does not form dendrites. For example, in a recent review, “The major advantage of multivalent systems such as magnesium is a higher volumetric capacity and the apparent lack of dendrite formation during charging which overcomes major safety and performance challenges encountered with the use of a lithium metal anode.”6 The work by Banerjee et al. in this issue clearly shows that the plating of magnesium can result in the formation of dendrites.7 They show that these dendrites have fractal morphologies, as shown in Figure 2. These are quite similar to the dendrites formed in lithium cells.8 As stated by Banerjee, “the promise of magnesium batteries derives in large measure from claims that they are immune to dendrite formation”. Thus, one now must question whether there is any commercial incentive for further studying magnesium-based batteries as it seems that anything magnesium can do, lithium can do better. Calcium systems, which do not suffer from the same voltage penalty as magnesium, might be a more attractive, but technically very challenging, option. Conversation with Sarbajit Banerjee: What are your insights on the current outlook for widespread advancement of magnesium batteries? I think there is tremendous excitement and opportunities that lie ahead for magnesium batteries as they start to emerge as a viable technology. However, the field has had some false starts such
Figure 3. Dr. Sarbajit Banerjee is currently a Professor and the Davidson Chair of Science in Chemistry, and Materials Science & Engineering at Texas A&M University (http://www.chem.tamu. edu/rgroup/banerjee/). Photo courtesy of Chris Jarvis, Texas A&M University.
as conversion reactions and proton intercalation being confused for magnesium insertion, as well as some unsubstantiated claims about “dendrite-free” plating. Much research has focused on the development of individual components (cathodes, electrolytes, etc.), and there is still much that remains to be done with assembling fuel cells. I am cautiously optimistic about the future of magnesium batteries but also advocate a healthy degree of skepticism and realism. It is now abundantly clear that rigorous characterization is warranted to substantiate claims of new cathode materials and electrolytes. In terms of positive attributes, the potential to use magnesium instead of lithium seems like it will circumvent some of the materials criticality concerns with the latter. The ability to use metal anodes (which remains a formidable challenge with lithium, although there is much exciting research on enabling the dendrite-free plating of lithium) potentially more safely than lithium as well as the promise of obtaining double the capacity is indeed very intriguing. However, the hardness of magnesium ions has been a major challenge and limits its diffusion in many intercalation hosts, particularly transition metal oxides. In addition to the Chevrel phases reported by the Aurbach group9 and the beautiful work by Yan Yao (University of 573
DOI: 10.1021/acsenergylett.9b00214 ACS Energy Lett. 2019, 4, 572−575
Energy Focus
ACS Energy Letters Houston) on expanded transition metal dichalcogenides,10 early last year we reported a high-voltage, highly cyclable, highcapacity cathode material, zeta-V2O5, and were unambiguously able to evidence Mg-ion insertion within tunnel sites of this metastable framework.11 We remain excited about the possibilities presented by this material as a highly reversible oxide intercalation host for Mg-ions and have an article coming soon where we have used nanostructuring to mitigate some of the diffusion limitations. In general, our approach illustrated the idea that compounds with frustrated coordination environments for Mg-ions hold promise as intercalation hosts.12 This notion seems to be applicable to several other restructured V2O5 phases. There has been some truly elegant work from David Prendergast at LBNL and Yan Yao emphasizing the need to ensure (catalytic?) desolvation of Mg-ions at electrode interfaces.10,13,14 Unlike in the case of Liions, breaking apart the interactions between Mg-ions and electrolyte anions is quite challenging and can be a bottleneck to insertion of Mg-ions within intercalation hosts. I see this as a major challenge for the continued development of magnesium batteries. What is the significance of and motivation for your ACS Energy Letters article? The primary purpose of our Energy Express article was to dispel the notion that somehow magnesium is “dendrite-free”. Some beautiful density functional theory calculations from Groβ’s group in Ulm/KIT have established that the lower predilection of magnesium for dendrite formation likely has to do with its easier self-diffusion.15,16 While this is excellent news, it only buys a somewhat expanded operational window and does not render the system immune to dendrite formation. We have now heard from several groups from across the world that they too have seen magnesium dendrites with different electrolytes. There were a couple of comments that we did not have room to expand upon given the two page format of Energy Express articles: (a) Much too often, studies of magnesium plating and electrolytes examine narrow current/voltage windows. The work proves that it is imperative to examine expanded operational windows and/or operational extremes in order to identify viable operational windows where magnesium can be reversibly plated/stripped without dendrite formation; note that high current densities, such as those reported by us to lead to magnesium dendrite formation, are imperative for some of the intended applications such as fast charge, low-temperature operation. In other words, instead of stating that magnesium is dendrite-free, let us start accurately defining windows where magnesium can be plated/ stripped without dendrite formation. (b) Second, several studies17−19 have shown that local current densities (rather than the global current density or voltage) govern to a large extent the spatiotemporal propagation of ions and the state of charge/discharge. As such, dendrite formation can be a concern even when the global current density is low as there are local inhomogeneities (or local impedance buildup). One of our important findings that should cause some concern is that when they are formed the magnesium dendrites are much harder than lithium dendrites and therefore have potential to inflict greater damage unto separators. As with every Energy Express article, expanded studies are warranted and ongoing with the help of several collaborators. For our part, we are performing a detailed evaluation of deposition morphology as a function of deposition conditions and indeed have identified some approaches that promote the deposition of nanowires instead of extended dendrites. Finally, the motivation for our
article is not to negate opportunities in alternative battery materials research but to call for cautionwe have seen the “dendrites don’t matter” movie before, and it has not ended well. I think we ignore dendrite formation at our own considerable peril.
Christina M. MacLaughlin, Development Editor
■
ACS Publications
AUTHOR INFORMATION
Notes
Views expressed in this Energy Focus are those of the authors and not necessarily the views of the ACS. The author declares no competing financial interest.
■
REFERENCES
(1) Whittingham, M. S. The Electrochemical Characteristics of VSe2 in Lithium Cells. Mater. Res. Bull. 1978, 13, 959−965. (2) Aurbach, D.; Lu, Z.; Schechter, A.; Gofer, Y.; Gizbar, H.; Turgeman, R.; Cohen, Y.; Moshkovich, M.; Levi, E. Prototype systems for rechargeable magnesium batteries. Nature 2000, 407, 724−727. (3) Sun, X.; Bonnick, P.; Duffort, V.; Liu, M.; Rong, Z.; Persson, K. A.; Ceder, G.; Nazar, L. F. A high capacity thiospinel cathode for Mg batteries. Energy Environ. Sci. 2016, 9, 2273−2277. (4) Emly, A.; Van der Ven, A. Mg Intercalation in Layered and Spinel Host Crystal Structures for Mg Batteries. Inorg. Chem. 2015, 54, 4394−4402. (5) Matsui, M. Study on electrochemically deposited Mg metal. J. Power Sources 2011, 196, 7048−7055. (6) Muldoon, J.; Bucur, C. B.; Gregory, T. Quest for Nonaqueous Multivalent Secondary Batteries: Magnesium and Beyond. Chem. Rev. 2014, 114, 11683−11720. (7) Davidson, R.; Verma, A.; Santos, D.; Hao, F.; Fincher, C.; Xiang, S.; van Buskirk, J.; Xie, K.; Pharr, M.; Mukherjee, P.; Banerjee, S. On the Formation of Magnesium Dendrites. ACS Energy Letters 2019, 375. (8) Whittingham, M. S.; Omenya, F.; Siu, C. Solid State Ionics - the key to the discovery, introduction and domination of lithium batteries for portable energy storage. Solid State Ionics 2018, 317, 60−68. (9) Aurbach, D.; Lu, Z.; Schechter, A.; Gofer, Y.; Gizbar, H.; Turgeman, R.; Cohen, Y.; Moshkovich, M.; Levi, E. Prototype systems for rechargeable magnesium batteries. Nature 2000, 407, 724. (10) Yoo, H. D.; Liang, Y.; Dong, H.; Lin, J.; Wang, H.; Liu, Y.; Ma, L.; Wu, T.; Li, Y.; Ru, Q.; Jing, Y.; An, Q.; Zhou, W.; Guo, J.; Lu, J.; Pantelides, S. T.; Qian, X.; Yao, Y. Fast kinetics of magnesium monochloride cations in interlayer-expanded titanium disulfide for magnesium rechargeable batteries. Nat. Commun. 2017, 8, 339. (11) Andrews, J. L.; et al. Reversible Mg-Ion Insertion in a Metastable One-Dimensional Polymorph of V2O5. Chem 2018, 4, 564−585. (12) De Jesus, L. R.; Andrews, J. L.; Parija, A.; Banerjee, S. Defining Diffusion Pathways in Intercalation Cathode Materials: Some Lessons from V2O5 on Directing Cation Traffic. ACS Energy Letters 2018, 3, 915−931. (13) Wan, L. F.; Perdue, B. R.; Apblett, C. A.; Prendergast, D. Mg Desolvation and Intercalation Mechanism at the Mo6S8 Chevrel Phase Surface. Chem. Mater. 2015, 27, 5932−5940. (14) Wan, L. F.; Prendergast, D. Ion-Pair Dissociation on α-MoO3 Surfaces: Focus on the Electrolyte−Cathode Compatibility Issue in Mg Batteries. J. Phys. Chem. C 2018, 122, 398−405. (15) Jäckle, M.; Groß, A. Microscopic Properties of Lithium, Sodium, and Magnesium Battery Anode Materials Related to Possible Dendrite Growth. J. Chem. Phys. 2014, 141, 174710. (16) Jäckle, M.; Helmbrecht, K.; Smits, M.; Stottmeister, D.; Groß, A. Self-diffusion barriers: possible descriptors for dendrite growth in batteries? Energy Environ. Sci. 2018, 11, 3400−3407. 574
DOI: 10.1021/acsenergylett.9b00214 ACS Energy Lett. 2019, 4, 572−575
Energy Focus
ACS Energy Letters (17) Lim, J.; Li, Y.; Alsem, D. H.; So, H.; Lee, S. C.; Bai, P.; Cogswell, D. A.; Liu, X.; Jin, N.; Yu, Y.-s.; Salmon, N. J.; Shapiro, D. A.; Bazant, M. Z.; Tyliszczak, T.; Chueh, W. C. Origin and hysteresis of lithium compositional spatiodynamics within battery primary particles. Science 2016, 353, 566. (18) Yu, Y.-S.; Farmand, M.; Kim, C.; Liu, Y.; Grey, C. P.; Strobridge, F. C.; Tyliszczak, T.; Celestre, R.; Denes, P.; Joseph, J.; Krishnan, H.; Maia, F. R. N. C.; Kilcoyne, A. L. D.; Marchesini, S.; Leite, T. P. C.; Warwick, T.; Padmore, H.; Cabana, J.; Shapiro, D. A. Three-dimensional localization of nanoscale battery reactions using soft X-ray tomography. Nat. Commun. 2018, 9, 921. (19) De Jesus, L. R.; Stein, P.; Andrews, J. L.; Luo, Y.; Xu, B.-X.; Banerjee, S. Striping modulations and strain gradients within individual particles of a cathode material upon lithiation. Mater. Horiz. 2018, 5, 486−498.
575
DOI: 10.1021/acsenergylett.9b00214 ACS Energy Lett. 2019, 4, 572−575
