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Changing Outlook for Rechargeable Batteries John B. Goodenough* Materials Science Program, Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, United States ef f iciency of a cell is dQ(Idis)/dn%, where n is the nth discharge. A coulomb efficiency greater than 99% is needed for a long cycle life of a cell. Release of an electron to the external circuit from a metallic anode is accompanied by a release of a cation to the electrolyte on discharge; on charge, the cations are plated back on the anode. A reversible reduction of the solid cathode in a rechargeable cell has traditionally been accomplished by insertion of the mobile electrolyte cation as a guest into a host structure, commonly a transition-metal compound. The cathode capacity is limited by the reversible solid-solution range of the guest, and the cell Vdis depends on the energy μC of the cathode transition-metal redox couple relative to the Fermi energy μA of the metallic anode. An alloy or a host for insertion of the mobile electrolyte cation may also be used as an anode. The voltage of a charged cell with a stable shelf life is restricted by the energy gap Eg = LUMO − HOMO of a liquid or EC − EV of a solid electrolyte, where LUMO and HOMO are the lowest unoccupied and highest occupied molecular orbitals; EC and EV are, respectively, the bottom of the conduction band and the top of the valence band. An anode with μA > LUMO of a liquid or μA > EC of a solid electrolyte reduces the electrolyte; a μC < HOMO or EV of the electrolyte oxidizes the electrolyte. However, if reduction of the electrolyte by the anode creates a solid-electrolyte interphase (SEI) that conducts the cation released by the anode and has a LUMO or EC > μA, then reduction at the anode is pacified by the increase in the effective Eg of the composite electrolyte. A suitable additive to a liquid electrolyte or a suitable composition of a solid electrolyte may enable formation of an SEI that stabilizes the anode-electrolyte interface without introducing a large impedance to cation transfer across the anode/electrolyte interface. Similarly, a μC < HOMO or EV requires an SEI with a μC > HOMO or EV, good mobile-cation conductivity, and an ability to accommodate the cathode volume changes on discharge/charge cycling, but the cathode SEI has, to date, only been successfully introduced artificially as a polymer.
1. CONSTRAINTS A rechargeable battery cell stores electric power as chemical energy in two electrodes, an anode and a cathode, that are separated by an electrolyte. The anode is the reductant, the cathode is the oxidant, and the electrolyte is a cation conductor and an electronic insulator. The chemical reaction between the two electrodes has an electronic and a cation component; the electrolyte transfers the cation component inside the cell and forces the electronic component to be conducted in an external circuit where it delivers electric power Pdis = Idis Vdis; Idis is the discharge electronic current and Vdis = VOC − ηdisIdis
(1)
VOC = μA − μC/e
(2)
VOC, the open-circuit voltage, is the difference between the anode and cathode chemical potentials μA and μC; e is the magnitude of the electronic charge. The ηdis is primarily the impedance to cation transfer inside the cell. Because the cation conductivity is much smaller than the electronic conductivity, a cell is fabricated with large-area electrodes separated by a thin electrolyte. If the electrolyte is a liquid, a separator permeable to the liquid is used to keep the two electrodes from contacting one another inside the cell. The capacity of a battery cell is the amount of charge per unit weight (specific capacity) or per unit volume (volumetric capacity) that is transported outside the cell at a constant current Idis until the chemical reaction between the electrodes is completed after a time Δt: Q (Idis) =
∫0
Δt
Idisdt =
∫0
Q (Idis)
dq
(3)
At a constant current Idis = dq/dt, the voltage Vdis changes with the state of charge q if the reaction is single-phase; it is a constant if the reaction is two-phase. The specific or volumetric density of stored electric power is ΔEdis =
∫0
Δt
VdisIdisdt = ·Q (Idis)
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(4)
HISTORY Before 1970, rechargeable batteries used an aqueous acidic or alkaline electrolyte with H+ as the mobile electrolyte cation. The Eg of water is 1.23 eV, which restricts the cell discharge voltage to Vdis ≤ 1.5 V for a charged battery cell having a stable shelf life; the charged lead-acid battery having a cell Vdis ≈ 2 V is not long-term stable. The layered NiOOH cathode host of the long-term stable Ni−Cd cell has its Ni3+/Ni2+ redox energy well-matched to the oxygen-evolution HOMO of an alkaline KOH electrolyte and can accept an additional H+ per formula
To reduce ηdis of eq 1, each large-area electrode contacts a large-area current collector of high electronic conductivity that collects electrons from the anode and distributes them to the cathode via the external circuit on discharge and vice versa on charge. This geometry limits the electron conduction within an electrode to the thickness of the electrode. A rechargeable battery must have a chemical reaction that is reversible on the application of a charging power Pch = IchVch, where
Vch = VOC + ηchIch
(5)
The storage eff iciency of the electric power Pdis/Pch%, depends on the impedance ηdis + ηch and can be over 95%; the coulomb © 2016 American Chemical Society
Received: November 1, 2016 Published: December 20, 2016 1132
DOI: 10.1021/acscatal.6b03110 ACS Catal. 2017, 7, 1132−1135
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ACS Catalysis unit in the H+ gallery to form, reversibly, Ni(OH)2; the Fermi energy of metallic Cd is well-matched to the H2-evolution LUMO, and a metal hydride host may substitute for metallic Cd as an anode. In 1967, Kummer and Webber of the Ford Motor Co. had discovered fast Na+ conduction in the ceramic Na β-alumina, and they invented a Na−S cell with the solid ceramic substituting for water as the electrolyte with molten sodium as the anode, molten sulfur in carbon felt as the cathode. This geometry promised a large cell capacity for stationary power storage, but the cell operates at 350 °C. In South Africa, the sulfur cathode was replaced by NaCl in what is referred to as the Zebra cell. Although a commercial Na−S cell has been operating in Japan, its maintenance cost has proven too high to make the battery commercially competitive. Nevertheless, this development stimulated extensive interest in cation transport in ceramic electrolytes, and interest in alkali-ion batteries was enhanced by the decision of the Arabs in about 1970 to use oil as a weapon against those who supported Israel, a decision that alerted modern society to its vulnerability by dependence on imported oil. In the 1960s, Rouxel in France and Schöllhorn in Germany were exploring the chemistry of reversible insertion of lithium into the layered sulfides MS2 containing M as a transition-metal atom. The interest in the possibility of using an alkali-metal ion as the mobile electrolyte cation led Steele of Imperial College, London, to suggest at a conference the use of TiS2 as the cathode of a lithium battery in analogy of the insertion of H+ into layered NiOH with an aqueous electrolyte; a nonrechargeable lithium battery of long shelf life that used an organic-carbonate liquid electrolyte was in the market. At Stanford University, Gamble was a Ph.D. student with Geballe exploring 2D superconductivity in TiS2 and Whittingham was an electrochemist postdoctoral fellow with Huggins; Whittingham used Geballe’s TiS2 to demonstrate a good capacity and rate in a Li-TiS2 cell, which led Exxon-Mobile to hire Whittingham and Geballe to develop a commercial Li-TiS2 battery. With Whittingham’s publication of fast and reversible Li insertion into LixTiS2 (0 ≤ x ≤ 1) with a ≈ 2.5 V, the Bell-Telephone Laboratories also became active; however, it soon became evident that a lithium anode with a flammable liquid electrolyte is not safe. Lithium dendrites form and grow during charge; after a few discharge/charge cycles, they can grow across the electrolyte to the cathode to create an internal short-circuit with incendiary consequences. This situation led to two developments: Nishi in Japan and Basu at the Bell-Telephone laboratories started to explore the intercalation of Li+ into graphite, a layered carbon, for an anode with no metallic lithium; in Oxford, I decided to explore extraction of lithium from a discharged layered oxide. The oxide would give a larger voltage and would allow use of a cell containing a discharged anode having no metallic lithium because assembly of a discharged rechargeable cell could be charged. Although fast removal of Li+ from Li1−xCoO2 and Li1−xNiO2 was shown to be reversible over 0 ≤ x ≤ 0.55, or 0.8, only a little over half the range of Li1−xTiS2 (0 ≤ x ≤ 1), these oxides gave a Vdis = 4.0 V versus Li+/Li0. The battery industries of Europe, England, and the U.S. remained skeptical of fabrication of a discharged cell without specification of the discharged anode. However, Yoshino of Japan was aware of the Li-intercalated graphite, so he demonstrated the first Li-ion cell with Li1−xCoO2 as cathode and graphitic carbon as anode assembled in the discharged state. This cell was commercialized
by the SONY Corporation to power a camcorder and a cell telephone; thus the wireless revolution was born. The LiCoO2/carbon or Li1−x(Co1−y−zNiyAlz)O2/carbon cell remains the principal Li-ion battery that powers today’s handheld electronic devices and power tools. This application has been successful because it does not compete with the energy stored in a fossil fuel. Large-scale rechargeable batteries containing stacks of many cells for powering electric road vehicles or for storing electric power generated by wind or solar energy must compete in cost, safety, and convenience with the energy density stored in a fossil fuel, but the Li-ion batteries with a flammable liquid electrolyte have been stalled at incremental improvements. Moreover, a carbon anode has a μA − LUMO ≈ 1.2 V, and the SEI formed on the anode takes its Li+ from the cathode on the initial charge, which reduces further the cathode capacity unless the anode SEI is preformed before cell assembly. Electrode volume changes on cycling create fresh graphitic surfaces requiring additional SEI formation, which contributes to a capacity fade on cycling that reduces the cell cycle life. Only with a spinel Li4Ti5O12 insertion-host anode having a μA < LUMO and a LiFePO4 olivine cathode having a μC > HOMO has Hydro Quebec of Canada achieved a cell cycle life over 2500 cycles, but the voltage is reduced to about Vdis = 2.2 V. Moreover, if a graphite/Li1−xCoO2 cell is charged too rapidly, lithium is plated on the graphite; and if the cathode is overcharged, it evolves O2 gas. As a result, a Li-ion battery cell needs to be monitored to restrict the rate of charge and to prevent a cathode overcharge. Large-scale energy storage requires electronic management of all the individual cells of the stack of battery cells, which doubles the cost of the battery. The battery that powers a Tesla road vehicle contains about 7000 individual cells, which is why cost is prohibitive and the safety problem remains to haunt development. With the realization that the flammable liquid electrolyte creates a safety issue, particularly with the large-scale batteries, attention has turned to the development of a solid electrolyte with a room-temperature ionic conductivity σi > 10−4 S cm−1, but finding a solid oxide or polymer electrolyte with a large Eg and σi has been difficult. The lower the σi, the thinner must be the electrolyte, and fabrication of a thin, mechanically robust, and flexible ceramic electrolyte of large area is a challenge. The development of a suitable polymer electrolyte remains unsolved. It was originally assumed that a solid electrolyte would block dendrite growth across it, but blocking dendrites is not an adequate strategy. The Weppener ceramic garnet electrolyte with a σLi ≈ 10−3 S cm−1, for example, is penetrated at the grain boundaries by lithium-anode dendrites.
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OVERCOMING PREVAILING ASSUMPTIONS In the past few years, shattering of widely held assumptions has opened up the possibility of a transformation of the safety, cost, volumetric energy density, and cycle life of a rechargeable battery cell. Assumption (1): Cation Conductivity σi of a Solid Oxide Electrolyte Cannot Compete with That of the Flammable Liquid Electrolyte. To date, the best oxide ceramic or dry-polymer alkali-ion electrolytes have a σi ≲ 10−3 S cm−1 at room temperature. Although a sulfide glass electrolyte1 has a σi > 10−2 S cm−1 at 100 °C, the sulfides need to be stabilized against high-voltage cathodes. Solid glass alkali-ion electrolytes with an alkali-ion conductivity 10−2 < σi < 10−1 S cm−1 at room temperature have been identified;2 these 1133
DOI: 10.1021/acscatal.6b03110 ACS Catal. 2017, 7, 1132−1135
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ACS Catalysis
dendrite-free plating of an alkali-metal anode with a ceramic electrolyte.11 The glass electrolyte developed by M. Helena Braga provides a competitive approach that is more stable than the Japanese sulfide glass. Wetting of the surface of the electrolyte by interfacial bonding constrains the volume changes of the anode during cycling to be perpendicular to the interface, which preserves the interface over a long cycle life. Moreover, a glass EC > μA of lithium eliminates any reduction of the electrolyte beyond an interface bonding. This development solves the anode and solid-electrolyte problems, but the cathode problem remains. Assumption (3): Chemical Discharge Reaction of a Rechargeable Battery Cell Requires a Reversible Reduction of a Compound on Contacting a Working Ion from the Electrolyte and an Electron from the External Circuit. Given a solid alkali-ion glass electrolyte from/to which a dendrite-free alkali-metal can be plated/ stripped reversibly and is stable on contact with an organicliquid or polymer catholyte, a conventional insertion-host cathode can be used; however, other cathode strategies can also be explored. One such strategy that has been extensively studied is a sulfur cathode, but intermediate Li2Sx (1 < x ≤ 4) species of the 16Li + S8 = 8Li2S reaction are soluble in the liquid electrolytes, which leads to capacity fade unless the soluble species are trapped at an electronically conductive surface that is in contact with the cathode current collector. To date, the resulting cathode geometries do not provide the volumetric energy density needed for a portable battery, but this strategy may prove to be competitive for stationary energy storage. An alternative strategy is to take advantage of the large dielectric constant and the large Eg window of the glass electrolyte that allows plating/stripping of dendrite-free alkalimetal anodes onto a current collector that it wets. The glass/ alkali-metal interface has been shown, with symmetric cells, to allow fast cation transfer with an impedance under 10 ohms. If a cathode current collector that is wet by the alkali-metal anode has a Fermi energy well below that of the alkali metal, the anode can be plated onto the cathode during discharge and, during charge, can be stripped from the cathode current collector to be plated back on the anode during charge. This strategy gives a cell capacity that is only determined by the amount of alkali metal that can be stripped from the anode and plated on the cathode reversibly; the cell discharge voltage is only limited by the differences in the Fermi energies of the alkali-metal anode and the cathode current collector. To date, this strategy has been demonstrated with a copper cathode current-collector with both a lithium and a sodium anode. With a copper current collector, the discharge voltage is Vdis ≲ 3.5 V versus a lithium-anode, but a Vdis < 3.5 V can be tuned by the introduction of a catalytic relay. With a lithium anode, a copper current collector, and a γ-MnO2 relay, a coin cell with a Vdis ≈ 3.08 V has shown a long cycle life at commercial rates without fading of a large volumetric energy density. However, plating directly on a copper current collector is problematic, so the cathode impedance has yet to be optimized. Nevertheless, this observation shows that new strategies hold promise for the development soon of safe rechargeable batteries that can power an all-electric road vehicle with a performance and convenience competitive with automobiles powered by gasoline in an internal combustion engine.
glasses also have a huge dielectric constant. The large dielectric constant is caused by the presence of electric dipoles that form over time dipole-rich regions containing some negatively charged ferroelectric molecules; alignment of the dipoles in an electric field enhances the ionic conductivity, and the aging time can be reduced from days at 25 °C to minutes at 100 °C.3 The glass electrolytes have a large window; they are not reduced on contact with metallic lithium, sodium, or potassium and are not oxidized by cathodes giving voltages greater than 6 V versus lithium. Moreover, the glass electrolytes are stable in aprotic liquids and make good contact with polymers; they can be applied to large curved or flat areas as a slurry embedded in a flexible, porous membrane, and the slurry reforms as a solid glass without grain boundaries on evaporation below 200 °C of the aprotic liquid of the slurry. An activation energy for cation mobility of ΔHm = 0.06 eV has shown4 the glass is still operational down to −25 °C. Assumption (2): Plating of a Metallic Lithium or Sodium Anode during Charge Inevitably Results in Anode-Dendrite Formation and Growth. Anode dendrites can penetrate a liquid electrolyte, a polymer gel, and even a solid with permeable grain boundaries; growth of an anode dendrite across the electrolyte to the cathode creates an internal short-circuit with incendiary consequences if the electrolyte is flammable. Safety concerns have led to the investigation of solid electrolytes, but blocking of electrolytes by a solid electrolyte is not a viable strategy; dendrites open up an anode/solidelectrolyte interface to create a resistance to alkali-ion transport that increases with cycling to give a short cycle life of the cell. It has recently been demonstrated that a dendrite-free alkalimetal anode can be plated from liquid,5 polymer,6 and solid7 alkali-ion electrolytes provided the alkali-metal wets a solid surface. The term “wetting” refers to a bonding of a material to a solid surface that is stronger than the surface bonding to itself, whether the bonding occurs at a liquid−solid or a solid−solid interface. The glass electrolytes are wet by an alkali-metal. Symmetric Li/Li-glass/Li and Na/Na-glass/Na cells have been cycled dendrite-free at 3 mA cm−2 over 1000 times without degradation;8 the interface impedances of these cells was under 10 ohms. Wachsman’s group9 has been exploring a ceramic electrolyte with the garnet structure; the garnet composition allows fabrication of a more dense ceramic than that penetrated at grain boundaries by lithium-anode dendrites. The garnet surface is coated with an ultrathin oxide layer; this artificial layer is presumably transformed to a Li+-conducting layer that allows a low-impedance Li+ transfer between a lithium anode and the solid electrolyte. Importantly, the artificial layer bonds to the solid electrolyte and is wet by the metallic-lithium anode, which suppresses dendrite formation and growth. However, the Li+ conductivity of 10−3 S cm−1 is not high enough to provide a high Idis unless a large-area, mechanically robust thin garnet electrolyte can be fabricated. A different example is the development of a thin intrinsic interfacial (SEI) layer; this approach has been demonstrated by Yutao Li et al.10 with a sodium anode wetting the frameworkstructured A1+3xZr2(P1−xSixO4)3 Na+ ceramic electrolyte developed in 1975. Reduction of the (PO4)3− polyanions by the alkali-metal anode creates a thin anode/electrolyte interfacial layer containing Li3P that is wet by the anode and reduces the Na+-transfer impedance. A thin polymer artificial layer that is wet by an alkali-metal anode and conducts the working cation has also been demonstrated to provide 1134
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
John B. Goodenough: 0000-0001-9350-3034 Notes
The author declares no competing financial interest.
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REFERENCES
(1) Tatsumisago, M.; Hayashi, A. Solid State Ionics 2012, 225, 342− 345. (2) Braga, M. H.; Stockhausen, V.; Ferreira, J. A.; Oliveira, J. C. A.; El-Azab, A. J. Mater. Chem. A 2014, 2, 5470−5180. (3) Braga, M. H.; Ferreira, J. A.; Murchison, A. J.; Goodenough, J. B. J. Electrochem. Soc., in press. (4) Unpublished. (5) Xue, L.; Gao, H.; Zhou, W.; Xin, S.; Park, K.; Li, Y.; Goodenough, J. B. Adv. Mater. 2016, 28, 9608−9612. (6) Zhou, W.; Wang, S.; Li, Y. T.; Xin, S.; Manthiram, A.; Goodenough, J. B. J. Am. Chem. Soc. 2016, 138, 9385−9388. (7) Braga, M. H.; Murchison, A. J.; Ferreira, J. A.; Singh, P.; Goodenough, J. B. Energy Environ. Sci. 2016, 9, 948−954. (8) Braga, M. H.; Grundish, N. S.; Murchison, A. J.; Goodenough, J. B. Energy Environ. Sci., in press. (9) Wachsman, E. Personal Communication. (10) Li, Y. T.; Zhou, W.; Chen, X.; Lü, X.; Cui, Z.; Xin, S.; Xue, L.; Jia, Q.; Goodenough, J. B. Proc. Natl. Acad. Sci. U. S. A. 2016, 113, 13313−13317. (11) Zhou, W.; Li, Y. T.; Xin, S.; Goodenough, J. B. ACS Central Science, in press.
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