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Article Cite This: Acc. Chem. Res. 2018, 51, 258−264
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Can Multielectron Intercalation Reactions Be the Basis of Next Generation Batteries? Published as part of the Accounts of Chemical Research special issue “Energy Storage: Complexities Among Materials and Interfaces at Multiple Length Scales”. M. Stanley Whittingham,* Carrie Siu, and Jia Ding NorthEast Center for Chemical Energy Storage, Binghamton University, Binghamton, New York 13902, United States CONSPECTUS: Intercalation compounds form the basis of essentially all lithium rechargeable batteries. They exhibit a wide range of electronic and crystallographic structures. The former varies from metallic conductors to excellent insulators. The latter often have layer structures or have open tunnel structures that can act as the hosts for the intercalation of a wide range of metal cation and other guest species. They are fascinating materials with almost infinitely variable properties, with the crystal structure controlling the identity and the amount of the guest species that may be intercalated and subsequently removed. The electronic structure controls not only the degree of electron transfer to the host, but also defines the degree of the electrostatic interactions a mobile ion experiences; thus, a metallic host will provide a minimizing of those interactions, whereas in an ionic lattice the interactions will be much greater and the mobile ion will experience a much higher activation energy for motion. This becomes more important for multivalent cations such as Mg2+. Today’s lithium batteries are limited in capacity, because less than one lithium ion is reversibly intercalated per transition metal redox center. There may be an opportunity to increase the storage capacity by utilizing redox centers that can undergo multielectron reactions. This might be accomplished by intercalating multiple monovalent cations or one multivalent cation. In this Account, we review the key theoretical and experimental results on lithium and magnesium reversible intercalation into two prototypical materials: titanium disulfide, TiS2, and vanadyl phosphate, VOPO4. Both of these materials exist in two or more phases, which have different molar volumes and/or dimensionalities and thus are expected to show a range of diffusion opportunities for battery active guest ions such as lithium, sodium, and magnesium. One major conclusion of this Account is that reversibly intercalating two lithium ions into a host lattice while maintaining its crystal structure is possible. A second major conclusion is that theoretical studies are now sufficiently mature that they can be relied upon to predict the key free energy values of simple intercalation reactions, i.e., the energy that might be stored. This could help to focus future choices of battery couples. In hindsight, theory would have predicted that magnesium-based intercalation cells are not a viable electrochemical option, relative to lithium cells, from either power or energy density considerations. However, the fundamental study of such reactions will lead to a better understanding of intercalation reactions in general, and of the critical importance of crystal structure in controlling the rates and degree of chemical reactions.
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INTRODUCTION Essentially all of today’s high energy density rechargeable batteries use intercalation reactions at both electrodes. Such reactions1−3 where the host structure remains essentially unchanged during the intercalation reactions have been in most cases restricted to just one electron reactions per transition metal. The prototypical reaction is that of lithium with titanium disulfide, x Li + TiS2 = LixTiS2 ,
Although the Li/TiS2 system has a theoretical energy density of 480 Wh/kg in a pure lithium cell and experimental cells closely approach that value, it drops markedly when combined with the heavy (91 g vs 7 g/mol for Li metal) and volume intensive C6Li anode. The latter also reduces the cell voltage by about 0.1 V, an additional 5% loss. In contrast, LixCoO2 with its 4 V potential, loses a much lower percentage of its cell voltage when combined with a carbon anode. However, only around 0.5 Li can be cycled resulting in cell capacities of only around 140−180 Ah/kg, compared with the theoretical 280 Ah/kg. Similarly, the LiFePO 4 cathode although it reversibly intercalates close to one lithium for hundreds of cycles, its
where 0 ≤ x ≤ 1
where there is a continuous solid solution of the guest lithium ions in the titanium disulfide host lattice. In this particular case, there is no nucleation of a new phase so no energy is expended unlike most intercalation reactions, where several closely related phases are formed. © 2018 American Chemical Society
Received: October 23, 2017 Published: January 12, 2018 258
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the layered dichalcogenides, such as MxTiS2, and vanadium phosphates/oxides, such as MxVOPO4.
energy density is relatively low because of the added weight of the phosphate group.4 There is a need for batteries with much higher energy densities. One approach is go beyond the one electron redox intercalation reactions of the above systems. This can be accomplished by intercalating multiple lithium ions or for example one magnesium, calcium, or aluminum ion. Several materials have been shown to reversibly intercalate more than one lithium ion, as shown in Figure 1 for LixVSe2,5 LixVOPO46
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THE MxTiS2 SYSTEM There was much interest in the layered dichalcogenides in the 1970s, initially because of their rich and intriguing intercalation chemistry and in particular because their superconducting behavior could be modified and controlled by intercalating a wide range of electron donors.11 These electron donors included simple metals, organic amines and organometallics like metallocenes.12 Many of these guest species could be readily intercalated into the host chalcogenide lattices at room temperature either by direct chemical reaction or by electrointercalation.13 Singly charged species, such as the alkali metals, and divalent species such as Zn, Ca, Hg, etc can be intercalated; however, no more than a one electron transfer has been reported.3 The readiness of these reactions to occur is at least in part related to the host’s soft lattices, in many case metallic, which shields the electrostatic interactions between the diffusing species and the host lattice. As noted in the Introduction, in the 1970s the ability of these materials to store energy was recognized, as was the key role that the intercalation process played in allowing the reactions to be readily reversed many times. Lithium diffusion in these and related materials has recently been the subject of an Account.14 LixTiS2 can be formed in two structures, the HCP layered structure, LixTiS2, and the CCP spinel form, LixTi2S4; these are shown in Figure 3. TiS2 in the layered form can be made in a
Figure 1. Three materials that show the capability of reversibly intercalating two lithium ions.
and LixMO2;7,8 for the layered oxide, LixMO2, two lithium intercalation has been reported for Li2Ni1−yMnyO2, where y = 0, 0.5, and 1. In these oxides, all the lithium ions switch from octahedral sites in LixMO2 to tetrahedral sites in Li2MO2, just as in LixVSe2. As shown in the figure, two voltage steps are to be expected when more than one redox reaction is involved, each of these may be sloped if a single-phase reaction occurs; alternatively, a continuously varying potential may be observed over the entire composition range particularly when much cation disorder is present as in ω-Li3V2O5. There has been much interest in magnesium systems in the past decade,9 following the pioneering work of Aurbach’s group on the chevrel phase, MgxMo3S4.10 They could readily reversibly intercalate 0.65 magnesium ions per formula unit into the lattice, but the overall capacity is less than 80 mAh/g as shown in Figure 2, compared with over 300−500 mAh/g in some of the materials shown in Figure 1. At this time, there is no published work documenting more than a one electron intercalation redox with magnesium; i.e., more than 0.5 Mg ions have not been successfully intercalated. We will use two classes of materials as examples of potential multielectron materials:
Figure 3. Structures of the (a) layered and (b) spinel forms of titanium disulfide.
wide variety of ways, for example by solid state reaction (Ti + S), or from the gas phase (TiCl4 + H2S). In contrast, the spinel LixTi2S4 is not stable at high temperatures, so must be formed by a low temperature exchange reaction from CuTi2S4; this material has been shown to cycle well.15 The thermodynamics and kinetics of alkali metal intercalation into layered TiS2 is well understood. LixTiS2 is a single solid solution for all values of x from 0 to 1. There is electrochemical evidence of some ordering of the Li between the layers, but the ions diffuse too fast for this to be observed in ambient temperature X-ray studies. The chemical diffusion of the Li ions at 21 °C is around 10−8−10−10 cm2/s depending on the composition. The structural changes and lithium diffusion upon intercalation were readily observed by one of the first, if not the first, battery operando studies.16−18 A typical cycling curve for Li/TiS2 is shown in Figure 4. Whereas the experimental data for the Li/TiS2 system was performed long before any theoretical studies of the
Figure 2. Intercalation behavior of Mg into Mo3S4, data from ref 10. Adapted with permission from ref 10. Copyright 2000 Nature. 259
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intercalated in these phases above 0 V.22 A comparison of Figures 4 and 5 shows that the Mg/MgxTiS2 couples are predicted to have much lower voltages than the corresponding Li/LixTiS2 couples, almost one volt lower (40% lower). In 2016, the Nazar group reported an experimental study of these same titanium sulfide layered23 and spinel24 phases in magnesium cells, and their cell potentials were in essentially total agreement with the calculated values, as shown in Figure 6. This may be one of the first examples of DFT calculations accurately predicting cell voltages, prior to the experimental studies. This data was collected at 60 °C, and the layered phase shows strong evidence for ordering of the magnesium ions. So even in the presence of a soft lattice host, there is a marked repulsion between the individual magnesium ions. In contrast, the experimental data on the spinel MgxTi2S4 shows single phase behavior with no evidence for ordering of the magnesium ions. Moreover, this phase shows excellent cycling behavior, with minimum polarization and very good capacity retention over 40 cycles. This is the highest capacity yet obtained for a reversible intercalation-based magnesium cell, more than doubling the capacity of the chevrel MgxMo3S4.10 It is all the more remarkable as the spinel phase still contains some copper ions.24 However, if we take the two experimental curves for the layered LixTiS2 at 21 °C and 5 mA/cm2 and the spinel MgxTi2S4 at 60 °C and 0.04 mA/cm2,25 then the energy density of the lithium cell is around double that of the magnesium cell; 460 Wh/kg vs 230 Wh/kg. In neither case has more than one electron been cycled. The energy density difference is essentially the same on a volumetric basis. The key finding of these studies is that theory is robust enough to accurately predict experimental results, that magnesium can indeed be reversibly intercalated into these soft host lattices, but magnesium cells are not a viable electrical storage medium relative to lithium. The major reason is the voltage penalty magnesium takes, and secondarily the lower diffusion coefficient that may require the use of higher temperatures. Switching to oxides will increase the voltage, but at the penalty of lower diffusion because of the harder lattice. Even so, it is unlikely that an oxide magnesium based cell will be able to approach the energy or power capabilities of the corresponding oxide lithium cell. An even tougher technical challenge would be to use calcium-based cells, where the voltage penalty is much reduced. To date, no calcium-base extended reversible intercalation cells have been reported. The Nazar group also reports that attempts to insert (or remove) Mg2+ ions into the thiospinels of the other first row transition metal thiospinels were unsuccessful. However, they found that Mg2+ ions can be reversibly intercalated in the second row thiospinel Zr2S4, but at an even lower potential than that in Ti2S4.26 The layered VSe2, shown in Figure 1, might be a viable candidate but there are no published reports. Other sulfides, such as MoS2, have also been proposed by several authors. Liang et al.27 were able to cycle a cell comprising an anode of nanosized Mg particles against “exfoliated MoS2” and found a stable capacity of around 170 mAh/g over 50 cycles at a current density of 26 μA/cm2 at room temperature; they reported an average discharge voltage of 1.8 V. This data is shown in Figure 7. They explain the good behavior as due to “the adequately separated single layers, the inorganic graphene-like MoS2 can facilitate Mg2+ intercalation and migration.”28 Their average cycling voltages are higher than those of the Mg/TiS2 cells described above. Here again the
Figure 4. The 76th cycle of a LixTiS2 cell at 21 °C and at 10 mA/cm2.
thermodynamics were made, in the case of magnesium intercalation the opposite was true. Van der Ven and Emly in 2015 calculated the potentials of the MgxTiS2 system19 for both TiS2 structures. These are compared with the corresponding Li cells in Figure 5. There appears to be some ordering of the
Figure 5. Calculated potentials of (a) the layered TiS2 couples Li/ LixTiS2 and Mg/Mgx/2TiS2, and (b) the spinel TiS2 couples Li/LixTiS2 and Mg/Mgx/2TiS2; data from refs 19−21.
lithium ions in the layered structure just as for layered LixTiS2;20,21 this ordering occurs when around 1/3rd of the octahedral sites are occupied, so around 0.33 for Li and 0.67 for Mg. However, for the spinel material a single-phase reaction is predicted19 for 0 ≤ x ≤ 0.5, just as calculated for LixTiS2.20 Ongoing theoretical studies indicate that 0.8 Mg ions can be 260
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Figure 6. (a) Electrochemical cycling curves of the layered Mg/TiS2 couple at 60 °C, and (b) the first cycle of the spinel phase also at 60 °C. (a) Adapted with permission from ref 23. Copyright 2016 American Chemical Society. (b) Courtesy of L. Nazar.
cathode. On applying a potential of around 4 V most of the protons could be removed from the lattice at room temperature; on reversing the current flow, lithium ions were intercalated into the lattice in place of the original protons reducing the vanadium back to the +3 oxidation state. We have shown recently using a range of characterization tools that the lithium reactions are totally reversible, but that the electrochemical behavior is very dependent on the synthesis approach, and that the reaction rate for the 4 V intercalation is slower than that of the 2.5 V intercalation. Two other phases of VOPO4, the α1 and β, are also capable of intercalating two lithium ions.32 Using a combination of experimental and theoretical studies33 to understand the behavior of LixVOPO4, we have now been able to reversibly intercalate fully both Li ions, as shown in Figure 9a. The theoretical capacity is 308 mAh/g. As is to be expected there are two voltage plateaus corresponding to the 5+/4+ and 4+/3+ redox couples, but in addition there is lithium ordering at around 1.5 and 1.75 Li/VOPO4. These
Figure 7. Cycling curve of a nano-magnesium anode against a graphene-like MoS2 cathode at 26 μA/cm2. Data from ref 27.
capacity of the cells is less than that of the corresponding Li/ MoS2 cells. The authors did not report whether the coordination of the Mo converted from trigonal prismatic to octahedral (1T phase) on deep cycling as observed for the LixMoS2 materials.29 This 1T phase shows over 96% utilization, i.e., Li0.96MoS2, as shown in Figure 8. Chang et al. showed data
Figure 8. Initial conversion of MoS2 from the trigonal prismatic phase to the octahedral phase (1T) at 1 V, followed by cycling of the 1T phase (β). Reproduced with permission from ref 29. Copyright 1983 NRC Research Press.
for a lithium cell with a MoS2/graphene cathode of capacity greater than 500 mAh/g, whose cycling curve is more reminiscent of a surface capacitor-like behavior rather than an intercalation reaction;30 Such a deep reaction has not been reported for magnesium.
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VANADIUM PHOSPHATES Vanadyl phosphate, VOPO4, has been reported to form at least seven different phases.31 We showed6 in 2005 in an electrointercalation reaction that both protons and lithium ions are mobile in the three-dimensional structure, made up of VO6 distorted octahedra and VO4 tetrahedra. In this reaction, a cell was constructed comprising a lithium anode and an H2VOPO4
Figure 9. (a) Initial eight cycles of the ε-LixVOPOV vs Li metal at 21 °C and (b) cyclic voltammetry curve of the Li/ε-LixVOPO4 cell. 261
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Figure 10. Structures of the ε-VOPO4 (a), ε-LiVOPO4 (b), and KVOPO4 (c) materials.
lattice. At this time, neither magnesium nor calcium intercalation has been successful in any of the VOPO4 phases; the magnesium appears to abstract oxygen from the lattice to form MgO.
become very apparent in cyclic voltammograms, as shown in Figure 9b. These results clearly show that this 3-dimensional lattice can tolerate the insertion and removal of two lithium ions without structural damage. However, a more ideal situation would be a material in which the two redox potentials are closer to each other. Replacing the vanadyl oxygen by the more electronegative fluoride ion, as in LixVPO4F, drives the redox potentials even further apart to 4.2 and 1.8 V.34 What is needed is a substitution ion that is less electronegative than oxygen. Studies are ongoing so that the redox potentials can be controlled. Vanadyl phosphate just like WO3 and TiO2 forms phases with quite different molar volumes. It would be expected that the larger the molar volume and/or the existence of large tunnels would allow the reversible intercalation of the larger sodium and potassium ions. The ε-VOPO4 phase can also reversibly intercalate a sodium ion.35 The vanadyl phosphate, KVOPO4 has a different structure with larger cavities, as shown in Figure 10. Some reversibility of the potassium ion has been reported, around 0.55 K ions can be cycled.36 The expansion of the volume of that structure from the 85.51 Å3 of ε-LiVOPO4 to the 106.8 Å3 of KVOPO4 should perhaps allow for the enhanced insertion and mobility of sodium ions vs the εVOPO4 lattice. Indeed, this is the case. Just as for the H2VOPO4 material discussed above, the potassium ions can be electrochemically removed from the lattice, and sodium ions reversibly intercalated. Figure 11 shows that around 1.6 Na ions can be cycled, with about 36% of the K ions remaining in the
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VANADIUM OXIDES
In contrast to the structurally stable phosphates, the structures of many vanadium oxides are not stable for lithium insertion, as the lithium and vanadium ions are of comparable size. Hence the ions tend to become randomized in the lattice, as in the layered V2O5 itself where on insertion of three lithium ions the layer structure converts into a rock salt structure of formula [Li3V2]O5. In this case the vanadium is only partially reduced to the 3+ state, leaving an average oxidation state of 3.5. This structural change at 21 °C suggests high mobility of all the ions, not just the lithium ions contrary to popular believe that highly charged ions are immobile under ambient conditions. This is similar to the conversion of the layered LixMnO2 to the spinel phase, LixMn2O4, on lithium removal in a cell at room temperature. Magnesium intercalation into oxide based lattices, which are much harder than the conducting soft sulfide lattices, have not been particularly successful. Where magnesium intercalation has been reported, it is enabled by the cointercalation of other species. For example, magnesium intercalation has been achieved in V2O5 but proton insertion plays a key role. The protons shift the stacking of the V2O5 sheets leading to channels into which the magnesium ions can be reversibly intercalated forming materials of the general formula Mg0.17HyV2O5, where 0.66 ≤ y ≤ 1.16.37 In the absence of moisture, there is minimal, 30−40 mAh/g, reversible magnesium intercalation.38 The voltage penalty Mg reactions suffer relative to Li reactions almost demands that higher voltage systems are required to be viable for energy storage. Then the lattice might not be soft enough to allow for the ready diffusion of the magnesium ions. This is the catch-22 for multivalent guest ions in intercalation reactions. The Ceder group has recently proposed in a theoretical study that MgCr2O4 might be that material, with a calculated potential of 3.5 V. They have successfully synthesized the compound, but to-date have not been able to remove the magnesium ions, because the electrolyte is unstable at the high voltages required, >3.5 V.39
Figure 11. Cycling curves of the Na/KxVOPO4 cell at 21 °C. 262
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CONCLUSION Lithium based intercalation reactions can achieve extended reversibility of two lithium ions per transition metal redox center. The crystalline lattice, certainly in the case of VOP4, is stable to the extended cycling of lithium ions. One of the remaining challenges is to close the gap between the two redox potentials. There are the beginnings of the possibility of intercalating up to two sodium ions per transition metal in the same lattice type. The present status of Mg-based intercalation reactions is that (1) these reactions can occur in soft lattices, particularly when the host is a metallic or close to metallic conductor, where the ionic charges are shielded from the highly charged Mg2+ ion; (2) the spinel-type lattice appears to be preferred over layered structures, where in the latter there is less shielding between Mg2+−Mg2+ interactions which appears to cause ordering of the magnesium ions; (3) there is no experimental evidence yet that more than a one electron reaction can be achieved in a magnesium intercalation reaction: and (4) there is a deficit of around 0.5 V versus lithium cells. Thus, the question remains unanswered as to whether any magnesium based intercalation chemistry with a high free energy of reaction, average of 3 V for a two-electron reaction, can be made to be reversible at an acceptable rate of reaction while maintaining its structure. Such systems will remain a scientific curiosity unless the overall energy stored can exceed around 800 Wh/kg, where the weight is based on active material alone.
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ACKNOWLEDGMENTS
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REFERENCES
This work was supported as part of NECCES, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0012583. C.S. was partially supported as part of the Multidisciplinary GAANN in Smart Energy Materials, a Graduate Areas of National Need, funded by the U.S. Department of Education, under Award # P200A150135. We thank Linda Nazar, Anton Van der Ven, and Natasha Chernova for helpful discussions and for providing figures.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
M. Stanley Whittingham: 0000-0002-5039-9334 Notes
The authors declare no competing financial interest. Biographies M. Stanley Whittingham received a D. Phil. Degree from Oxford University in 1968, following a M.A. there in 1965. He joined Binghamton University in 1988, following a research associate position at Stanford University from 1968 and 1972 and various research and management positions at Exxon and Schlumberger. He is currently a distinguished professor of Chemistry and Materials Science and Engineering. He is also Director of the DOE NECCES Energy Frontier Research Center. His interests are in materials chemistry, and he studies both the fundamental and applied aspects of materials that might find application in energy storage devices. Carrie Siu is a Ph.D. graduate student in Materials Science and Engineering at Binghamton University. She received her Bachelors in Physics from University of the Sciences in Philadelphia in 2013. Her current research is focused on the synthesis and characterization of cathode materials in lithium-ion batteries. Jia Ding has been a postdoctoral researcher in NECCES at Binghamton University (SUNY) since 2015 after he received his Ph.D. from University of Alberta, Canada. He also holds a M.S. degree in Materials Science from Chinese Academy of Science (2012) and a B.S. degree from Huazhong University of Sciences and Technology, China. Jia is passionate in design and fabrication of advanced materials for various energy storage systems including Li/Na ion batteries and capacitors. 263
DOI: 10.1021/acs.accounts.7b00527 Acc. Chem. Res. 2018, 51, 258−264
Article
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DOI: 10.1021/acs.accounts.7b00527 Acc. Chem. Res. 2018, 51, 258−264
