Commentary pubs.acs.org/accounts
Holy Grails in Chemistry: Investigating and Understanding Fast Electron/Cation Coupled Transport within Inorganic Ionic Matrices Published as part of the Accounts of Chemical Research special issue “Holy Grails in Chemistry”. Paul F. Smith,† Kenneth J. Takeuchi,*,†,‡ Amy C. Marschilok,*,†,‡ and Esther S. Takeuchi*,†,‡,§ †
Department of Chemistry, Stony Brook University, Stony Brook, New York 11794, United States Department of Materials Science and Engineering, Stony Brook University, Stony Brook, New York 11794, United States § Energy Sciences Directorate, Brookhaven National Laboratory, Upton, New York 11793, United States ‡
ABSTRACT: Typically, power and energy are competing concepts in electrochemical energy storage, where one can be optimized only at the expense of the other. However, the specialized and diverse needs of new applications exceed the functional boundaries of existing battery chemistries, where both high power and high energy content are critical. The needed battery paradigms may not be realized by optimization of previous electrochemical energy storage technologies but rather require new basic science breakthroughs involving new materials chemistry. Here we propose that fundamental understanding of electron/cation coupled transport within inorganic ionic matrices is a holy grail that can potentially transform the energy storage landscape.
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A
THE IMPEDIMENT TO FAST ELECTRON/CATION COUPLED TRANSPORT: LOCALIZED RESISTANCE For a battery in operation, thermodynamics dictates a loss between the theoretical potential and usable output. This is quantified via eq 1:
defining challenge worldwide is to provide clean, safe electrical energy to power a growing number of applications. The delivery of electrical energy can range in scale from the grid level down to portable use, and efficient management of both renewable and nonrenewable energy sources can be affected greatly through effective and application-specific electrochemical energy storage (EES) via secondary batteries. A critical issue for batteries has always been the need to match performance to the deliverable power demands of the application. For example, the lead acid battery has remained the industry standard in automobiles due to the high deliverable power associated with electric starter motors. More currently, the lithium-ion battery has remained the industry standard for portable electronics, which use relatively less power but require light weight. Moving forward, there are a number of potential applications that already exist but are awaiting the appropriate battery, such as grid level electrical energy storage, electric vehicles that could also interact with the grid, and intermittent electrical energy storage coupled with renewable sources such as solar and wind. It is important to note that employing batteries inappropriate to the specific application can lead to safety issues such as overheating, which serve only to prevent new applications from widespread deployment. Here we provide a chemistry-focused conceptual framework with representative case studies toward batteries capable of fast charge conduction involving both facile ion transport and rapid electron transfer. While this manuscript uses examples from Libased chemistries, the general concepts should apply analogously to a wide array of battery systems. © 2017 American Chemical Society
E = E 0 − [(ηct)a + (ηc)a ] − [(ηct)c + (ηc)c ] − iR i
(1)
The difference between the operating potential (E) and the thermodynamic potential (E0) is accounted for by fundamental losses due to bulk internal resistance (Ri) and operating current (i), and material-specific polarization (η), broadly defined. Both anode and cathode experience polarization (ηc, ηa) ascribed to ionic concentration gradients (ηc) and activation processes (ηct); for example, reactions with electrolyte components yielding products that impede charge transfer. The goal for EES is to maximize useful work (w) and minimize waste heat (q), known from eq 2. ΔE = q + w
(2)
The combination of eqs 1 and 2 thus infers that higher polarization results in reduced work and increased heat output; hence, it is a major goal to understand and reduce sources of polarization in the battery. A battery may be envisioned as a mesoscale composite comprised of several discrete phases that interact via electrochemical reactions and phase changes, all of which govern its heat−work energy balance.1 When a battery Received: October 28, 2016 Published: March 21, 2017 544
DOI: 10.1021/acs.accounts.6b00540 Acc. Chem. Res. 2017, 50, 544−548
Commentary
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→ yLi2O + xM resulting in significantly higher theoretical capacity.7 However, many conversion reactions show a degradation of delivered capacity with continued electrochemical charge and discharge. The third category of reaction to consider is the formation of alloys (e.g., xM + yLi+ + ye− → LiyMx), which may deliver very high capacity related to the composition of the LiyMx alloy that is formed.8 In general, there is a relationship of capacity and electrode volume change where the materials that can accommodate the largest number of electrons/ions often exhibit the highest amount of volume change over the oxidation−reduction process.
operates, ions and electrons are transported over multiple size domains. Localized resistance must be addressed not just as a bulk property (heat) but rather over multiple length scales (molecular to mesoscale) to predict and ultimately control the function of EES.2
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DESIGNING THE ACTIVE MATERIAL Conceptually, there are several related electrochemical storage mechanisms to be considered in EES materials: (1) insertion where an ion inserts into a structure on reduction and then is removed from the structural lattice upon oxidation, (2) conversion where there is a chemical reaction leading to a new material or phase, and (3) formation of an alloy, Figure 1.
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VARIABLES TO CONTROL ELECTRON/CATION COUPLED TRANSPORT All these mechanisms have in common the concomitant movement of electrons and positive ions. In cases where electron mobility is rate limiting, reduction−displacement reactions, which generate a conductive matrix in situ, can support higher current with less polarization due to reduced electronic resistance.9 For the remaining majority of cases where ion mobility is rate limiting, it is essential to highlight the quantitative factors that govern speed of ion diffusion. Material designers seeking to minimize diffusion time can in general enact three strategies correlating to the variables that contribute to ion diffusion time (t), eq 3: t=
x2 (qDLi )
(3)
(1) Reduce x, the length of diffusion. This has a parabolic effect on reducing t. (2) Increase q, the dimensionality constant, by increasing the number of directions by which Li may diffuse (e.g., by particle nanostructuring). For 1D, 2D, and 3D diffusion, q is 2, 4, and 6, respectively. (3) Increase DLi. Through a solid, DLi is known by
DLi = D0 e−ΔG /(kBT )
(4)
The most direct way of increasing DLi is by lowering the Li diffusion activation energy ΔG. This has an exponential effect on DLi. In practice, it is easy to envision cases where these variables influence each other and complicate efforts to decouple their effects (vide inf ra), but brief discussion individually is warranted. In principle, strategy 1, reduction of diffusion length, provides a combination of direct, parabolic effect on t and a clear design target independent of active material. An effective approach to increase functional deliverable capacity and decrease charge time is thus to utilize small active material crystallite size ideally achieved through direct synthetic control in the preparation step. There have been significant impacts on battery relevant electrochemistry through synthetic crystallite size control. For example, despite a nearly identical rod morphology and an expected marginal 5% difference in theoretical gravimetric capacity, 10−11 nm crystallites of Ag1.2Mn8O16 deliver nearly 7× greater capacity than 20−25 nm crystallites of Ag1.6Mn8O16.10 This is in qualitative agreement with eq 3, which assuming materials are identical apart from crystal size would predict a ∼3−6× decreased diffusion time. Here it is worth mentioning that experimental verification of strategy success is highly dependent on lithiation rate (C/n, where n = number of hours required to fully (dis)charge the
Figure 1. Depiction of (top) example metal oxide intercalation electrode, (middle) example spinel conversion electrode, and (bottom) example metal alloy electrode.
Insertion materials contain space in the lattice to accommodate Li+ and can be depicted as nLi+ + ne− + MxOy → LinMxOy.3 Functionally, these materials typically exhibit reversible or quasi-reversible electrochemistry with the transfer of one or fewer electron/lithium ion per metal center resulting in only moderate lattice expansion but also in only moderate capacity. At high voltages, delithiation may accompany processes which result in partial decomposition of the active material or oxidation of the electrolyte or both. For instance, many manganese oxides4 and Ni−Co−Mn analogs5 will charge balance the removal of Li+ with evolution of O2− either as O2 or an electrolyte oxidized product. Notably, substitution of small amounts (5%) of titanium into a layered lithium nickel manganese cobalt oxide (Li1(NixMnxCo1−2x−yTiy)O2) shifted the energy of formation, allowing for more functional capacity toward the reversible intercalation of Li, rather than irreversible oxygen evolution and surface reconstruction.6 Conversion materials differ in that they enable multielectron transfers per metal center represented as MxOy + 2yLi+ + 2ye− 545
DOI: 10.1021/acs.accounts.6b00540 Acc. Chem. Res. 2017, 50, 544−548
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than 0D nanospheres. Analogous work has been performed on TiO2. In a 600 cycle comparison among anatase particles, 1D nanotubes and 2D nanosheets at 2 A/g, the capacity increased from 15 mAh/g (particles) to 45 mAh/g (1D tubes) to ∼80 mAh/g (2D sheets).22 Recent work has shown 3D urchin-like TiO2 particles to deliver 2−3× the capacity of 0D spheres or 1D wires at 50 C rates.23 Collectively the above findings are in qualitative agreement with eq 3, which predicts diffusion times 2−3× faster passing from 1D to 2D and 3D materials, respectively. We finally consider strategy 3, lowering Li diffusion activation energy, ΔG. The magnitude of ΔG is dominated by the highest energy transition state of the Li diffusion pathway, which most commonly are proposed as sites that are sterically crowded or surrounded by repelling, high valent cations. For any material, the (dis)charge process changes the Li content, modifying the net concentration gradient for Li diffusion as well as the number of sites to which Li can diffuse. Further, the decrease in electrical voltage with higher Li ion concentration indicates a constant lowering of driving force relative to the starting material. Hence, the activation energy for Li diffusion may be subject to high variance as a function of depth of discharge. For instance, while stoichiometric Li4Ti5O12 has been described as a rather poor ion conductor with negligible electronic conductivity, insertion of small amounts of Li (∼0.1−0.3) transforms the material into a mixed conducting oxide with significantly improved Li ion diffusivities.24 Additionally, calculations on two diffusion pathways for Li in LiCoO2 have found activation energies to vary from 200 to 1000 meV, correlating with experimental DLi covering nearly 7 orders of magnitude.25 The exponential effects of executing this strategy successfully make it appealing to pursue but require the material designer to have full knowledge of the intermediates, products, and diffusion path.
battery), where a strategy can be viewed as more impactful if a large range of n is covered. As an example that highlights trade-offs of nanosizing, consider Fe3O4, which functions initially as an insertion material, followed by a conversion process upon full reduction generating Fe0. On decreasing from 28 to 8 nm Fe3O4, the delivered energy density during the insertion process (first 100 mAh/g) increases by 12−230% depending on rate (C/8 to C/ 1).11,12 Monitoring the electrode mesostructure both by transmission electron microscopy of thin electrode sections and by transmission X-ray microscopy mapping of entire electrodes revealed intriguing findings regarding aggregation of active material.11 It is important to consider that nanosized materials have higher surface energy and therefore may have an increased tendency to aggregate; as result, design strategy 1 in practice may inadvertently influence electrode mesostructure. Decoupling these effects rationalizes much of the observed electrochemistry: when aggregation of Fe3O4 was controlled, an initial capacity ∼150% higher was observed for highly dispersed 9 nm Fe3O4 relative to aggregated samples,7 consistent with the prediction of a multiscale continuum model (vide inf ra).13 Another possible consequence of nanosizing and material dispersion is the amplification of a common source of activation polarization: side reactions with the electrolyte to form an insulating solid electrolyte interphase (SEI). This can be experimentally identified with Fe3O4 by pulsed discharge− voltage recovery testing, which at some discharge ranges is followed by unexpected voltage decreases.14 Continuum modeling of this behavior indicated that this observation is best described as formation of a passivating surface layer according to a surface area dependent nucleation and growth mechanism.14 Hence, despite the improved initial capacity of well-dispersed Fe3O4 relative to aggregates, over extended cycling, there is poorer capacity retention13 internally consistent with formation of additional SEI in the welldispersed material with more surface area directly exposed to electrolyte. In summary, while strategy 1 is most likely to provide significant improvements in effective ion mobility, the potential impacts of aggregation and amplified side reactions must also be considered. In considering strategy 2, active material dimensionality, improved discharge/charge time can be expected for materials that allow Li diffusion via multiple directions. At the crystallographic level, this strategy was highlighted recently via in situ microscopy including electron diffraction, imaging, and spectroscopy, coupled with density functional theory and phase field calculations for silver hollandite, AgyMn8O16.15 Despite the one-dimensional crystallographic structure of the hollandite material, multiple Li+ transport pathways were revealed upon electrochemical lithiation, including the first evidence of two-dimensional (ab plane) diffusion. Active material dimensionality can also be considered at the particle level, where morphology can be controlled to expose numerous facets facilitating multidirectional ion diffusion. Consider from the titanium oxide class16−18 the appealing cathode Li4Ti5O12, which exhibits Li diffusivity of ∼10−11 cm2 s−1, has a theoretical capacity of 175 mAh/g, and commonly delivers ∼80% of this value at 10 C rates.16,19 Several studies have compared spherical Li4Ti5O12 particles to 3D architectures including hollow spheres,20 mesoporous particles,21 and flowers,19 with improvement universally seen by nanostructuring. For some of the highest rates tested (50 and 100 C), 3D nanoflowers deliver 1.5× and 3× higher capacity, respectively,
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DESIGNING THE ACTIVE MATERIAL ENVIRONMENT In addition to the fundamental materials properties, it is important to note that the local environment of the active material and mesoscale structure of the electrode play significant roles. Passive components such as binder can lead to higher rate capability and improved capacity retention with cycling upon consideration of both ion and electron transport in design of the binder composition and the binder−active material interfacial structure.26,27 Further, intimacy of contact of the active materials with the conductive matrix significantly influences the electrochemistry of the active material,28 where one-pot simultaneous synthesis or structural incorporation of the conducting and electroactive material components29,30 may show significant energy density improvements relative to simple mechanical mixing.30
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FUTURE OUTLOOK Since batteries need to match performance to the demands of a wide range of applications, we argue that there is unlikely to be any one major solution to the whole of EES research. Rather, an understanding of the tools that most directly tailor deliverable battery capacity is warranted and are briefly summarized here. Much current EES research focuses on pushing the upper limits of battery power, from both basic and applied perspectives. The majority of this research has a focus on battery output or discharge normalized to mass. However, 546
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(4) Armstrong, A. R.; Holzapfel, M.; Novák, P.; Johnson, C. S.; Kang, S.-H.; Thackeray, M. M.; Bruce, P. G. Demonstrating Oxygen Loss and Associated Structural Reorganization in the Lithium Battery Cathode Li[Ni0.2Li0.2Mn0.6]O2. J. Am. Chem. Soc. 2006, 128, 8694− 8698. (5) Arunkumar, T. A.; Wu, Y.; Manthiram, A. Factors Influencing the Irreversible Oxygen Loss and Reversible Capacity in Layered Li[Li1/ 3Mn2/3]O2−Li[M]O2 (M = Mn0.5-yNi0.5-yCo2y and Ni1-yCoy) Solid Solutions. Chem. Mater. 2007, 19, 3067−3073. (6) Markus, I. M.; Lin, F.; Kam, K. C.; Asta, M.; Doeff, M. M. Computational and Experimental Investigation of Ti Substitution in Li1(NixMnxCo1−2x−yTiy)O2 for Lithium Ion Batteries. J. Phys. Chem. Lett. 2014, 5, 3649−3655. (7) Zhang, W.; Bock, D. C.; Pelliccione, C. J.; Li, Y.; Wu, L.; Zhu, Y.; Marschilok, A. C.; Takeuchi, E. S.; Takeuchi, K. J.; Wang, F. Insights into Ionic Transport and Structural Changes in Magnetite during Multiple-Electron Transfer Reactions. Adv. Energy Mater. 2016, 6, 1502471. (8) DiLeo, R. A.; Zhang, Q.; Marschilok, A. C.; Takeuchi, K. J.; Takeuchi, E. S. Composite Anodes for Secondary Magnesium Ion Batteries Prepared via Electrodeposition of Nanostructured Bismuth on Carbon Nanotube Substrates. ECS Electrochem. Lett. 2015, 4, A10− A14. (9) Kirshenbaum, K.; Bock, D. C.; Lee, C.-Y.; Zhong, Z.; Takeuchi, K. J.; Marschilok, A. C.; Takeuchi, E. S. In situ visualization of Li/ Ag2VP2O8 batteries revealing rate-dependent discharge mechanism. Science 2015, 347, 149. (10) Wu, L.; Xu, F.; Zhu, Y.; Brady, A. B.; Huang, J.; Durham, J. L.; Dooryhee, E.; Marschilok, A. C.; Takeuchi, E. S.; Takeuchi, K. J. Structural Defects of Silver Hollandite, AgxMn8Oy, Nanorods: Dramatic Impact on Electrochemistry. ACS Nano 2015, 9, 8430− 8439. (11) Bock, D. C.; Kirshenbaum, K. C.; Wang, J.; Zhang, W.; Wang, F.; Wang, J.; Marschilok, A. C.; Takeuchi, K. J.; Takeuchi, E. S. 2D Cross Sectional Analysis and Associated Electrochemistry of Composite Electrodes Containing Dispersed Agglomerates of Nanocrystalline Magnetite, Fe3O4. ACS Appl. Mater. Interfaces 2015, 7, 13457−13466. (12) Zhu, S.; Marschilok, A. C.; Takeuchi, E. S.; Takeuchi, K. J. Crystallite size control and resulting electrochemistry of magnetite, Fe3O4. Electrochem. Solid-State Lett. 2009, 12, A91−A94. (13) Knehr, K. W.; Brady, N. W.; Cama, C. A.; Bock, D. C.; Lin, Z.; Lininger, C. N.; Marschilok, A. C.; Takeuchi, K. J.; Takeuchi, E. S.; West, A. C. Modeling the Mesoscale Transport of Lithium-Magnetite Electrodes Using Insight from Discharge and Voltage Recovery Experiments. J. Electrochem. Soc. 2015, 162, A2817−A2826. (14) Brady, N. W.; Knehr, K. W.; Cama, C. A.; Lininger, C. N.; Lin, Z.; Marschilok, A. C.; Takeuchi, K. J.; Takeuchi, E. S.; West, A. C. Galvanostatic interruption of lithium insertion into magnetite: Evidence of surface layer formation. J. Power Sources 2016, 321, 106−111. (15) Xu, F.; Wu, L.; Meng, Q.; Kaltak, M.; Huang, J.; Durham, J. L.; Fernandez-Serra, M.; Sun, L.; Marschilok, A. C.; Takeuchi, E. S.; Takeuchi, K. J.; Hybertsen, M. S.; Zhu, Y. Visualization of Lithium-Ion Transport and Phase Evolution Within and Between Manganese Oxide Nanorods. 2017, Nat. Commun., under review. (16) Zhu, G.-N.; Wang, Y.-G.; Xia, Y.-Y. Ti-based compounds as anode materials for Li-ion batteries. Energy Environ. Sci. 2012, 5, 6652−6667. (17) Zhang, Y.; Jiang, Z.; Huang, J.; Lim, L. Y.; Li, W.; Deng, J.; Gong, D.; Tang, Y.; Lai, Y.; Chen, Z. Titanate and titania nanostructured materials for environmental and energy applications: a review. RSC Adv. 2015, 5, 79479−79510. (18) Yi, T.-F.; Yang, S.-Y.; Xie, Y. Recent advances of Li4Ti5O12 as a promising next generation anode material for high power lithium-ion batteries. J. Mater. Chem. A 2015, 3, 5750−5777. (19) Wang, L.; Zhang, Y.; Scofield, M. E.; Yue, S.; McBean, C.; Marschilok, A. C.; Takeuchi, K. J.; Takeuchi, E. S.; Wong, S. S. Enhanced Performance of “Flower-like” Li4Ti5O12 Motifs as Anode
we propose that an associated issue to deliverable power by battery discharge is an understanding of the basic science associated with fast charge. The basic science of battery charging is conceptually (but not in practice) related to a simple inverse of battery discharge, due largely to the kinetics of the complex processes that occur within inorganic ionic matrices. Enabling rapid battery charging would address a number of important issues associated with tethering batteries to applications such as intermittent electrical energy generation or long-term power usage associated with electric vehicles. For the former, rapid charging of batteries associated with intermittent electrical energy generation will increase the penetration of renewables such as solar and wind. Similarly, rapid charge and discharge capabilities of batteries tethered to the grid would enable more agile management of electricity by communities. For the latter, acceptance by society will be facilitated if charging of an electric vehicle occurs in similar time required to fill the gas tank of an automobile. The knowledge gained about fast battery charging might lend additional insight into the phenomena associated with fast battery discharge, a key issue associated with deliverable battery power output. It is our hope that the concepts presented here serve as inspiration for material designers seeking to achieve fast electron/cation coupled transport within inorganic ionic matrices in response to these challenges.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Esther S. Takeuchi: 0000-0001-8518-1047 Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge funding by the Center for Mesoscale Transport Properties, an Energy Frontier Research Center supported by the U.S. Department of Energy, Office of Basic Energy Sciences, No. DE-SC0012673.
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