Low-Temperature Performance of a Ferroelectric Glass Electrolyte

Article Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Low-Temperature Performance of a Ferroelectric Glass Electrolyte Rechargeable Cell M. H. Braga,*,† A. J. Murchison,‡ J. E. Oliveira,† and J. B. Goodenough*,‡ †

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LAETA, Engineering Physics Department, Engineering Faculty, University of Porto, R. Dr. Roberto Frias s/n, 4200-465 Porto, Portugal ‡ Texas Materials Institute and the Materials Science and Engineering Program, The University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *

ABSTRACT: An electrochemical cell that powers all-electric road vehicles will likely have an alkali-metal anode and the ability to operate down to −20 °C. The traditional all-solid-state batteries can only perform well at temperatures above room temperature. We have shown elsewhere that an alkali-metal negative electrode can be plated dendritefree from a ferroelectric amorphous-oxide (glass) Li+ or Na+ electrolyte having a room-temperature Li+ or Na+ conductivity σi ≈ 2.5 × 10−2 S cm−1 which is similar to that of a liquid electrolyte. Here, it is demonstrated that the ionic conductivity of the electrolyte is σi ≈ 10−2 S cm−1 at −20 °C after optimization, and the dielectric constant is ε′r ≈ 6 × 105 at −35 °C. Moreover, it is shown that the remanent polarization of the ferroelectric-electrolyte (polarization at zero potential) adds to the capacity of the cell. The electrochemical cycling performances between −35 and 25 °C of the Li+-glass electrolyte in gold and lithium symmetric cells and in full cells are presented. Furthermore, it is shown that a coin-cell with the ferroelectric Li-glass electrolyte at −35 °C with output current of 56 μA cm−2 can light a red LED at 1.5 V. Finally, it is concluded that the Li+-glass electrolyte performs very well in symmetric cells and performs reasonably well down to −20 °C in asymmetric cells that also rely on the performance of the cathode and on the electrolyte/cathode interface. KEYWORDS: electrochemical cell, low temperature, ferroelectric, Li-glass electrolyte, ionic conductivity, dielectric constant, electrochemical cycling

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INTRODUCTION

In inorganic solid electrolyte materials, glass and crystalline sulfides have high lithium-ion conductivity,7,8 but they may generate hydrogen sulfide upon air exposure. Moreover, metallic lithium could not be used as an anode because of its reactivity with sulfide solid electrolytes. In contrast, oxide solid electrolytes have excellent safety and a wide electrochemical potential window, but the lithium-ion conductivity (10−4 S cm−1) is one order of magnitude lower than the sulfide conductivity.9−13 Recently, it was reported that Li7La3Zr2O12 (garnet-type) solid electrolyte partially replaced with Ga and/ or Sc and Al shows a higher ionic conductivity on the order of 10−3 S cm−1.14−17 However, the garnet-type oxide electrolytes have major problems to be improved. One is a further improvement of the lithium-ion conductivity at room temperature. The other are internal short-circuits when charging due to dendrite growth. Here, we report measurements down to −20 °C of the charge/discharge rates of both symmetric and high-voltage all-

In order to reduce the unsustainable dependence of modern society on the energy stored in a fossil fuel and to eliminate the distributed air pollution coming from the roads and sea lanes of the world largely caused by vehicles powered by an internal combustion engine, a high-priority technical target is a competitive all-electric road vehicle powered by a rechargeable battery. The large-scale batteries needed consist of many identical cells associated in series and in parallel. In addition to the requirements of safety, low cost, and large volumetric energy density at high charge/discharge rates, the U.S. Advanced Battery Consortium (USABC) goals for a battery-powered all-electric road vehicle include the ability to operate over the temperature range of −30 °C1−6 to +52 °C. The all-solid-state lithium batteries using nonflammable inorganic solid electrolytes including oxides and sulfides are superior in safety to the current Li+-ion batteries using flammable organic liquid electrolytes. Nonetheless, traditional inorganic solid electrolytes have not yet been put into practical use because their lithium-ion conductivity is lower than that of organic liquid electrolytes. © XXXX American Chemical Society

Received: March 26, 2019 Accepted: May 15, 2019 Published: May 15, 2019 A

DOI: 10.1021/acsaem.9b00616 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Article

ACS Applied Energy Materials dE p = E dP + T dS

solid-state cells containing the ferroelectric amorphous-oxide (glass) Li+ or Na+ electrolyte of reference.18−20 Moreover, we show we can light a red LED at 1.50 V at temperatures as low as −35 °C with a minimum current of 56 μA cm−2; the data indicate the all-solid-state cells can meet the USABC specification for the operating temperature range of a battery that powers an all-electric road vehicle.

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(1)

where E is the thermo-electric field and T the absolute temperature. In addition to the thermo-electric field, a ferroelectric sample exhibits a dissipative electric field derived from the resistance to the movement 1 of the ions to be proportional to the resistivity, ρ = σ . From Ohm’s i

law EOhm = ρJ = ρ

EXPERIMENTAL SECTION

∂P ∂t

(2)

where EOhm is the ohmic electric field and J the electric flux; the total electric field is E = Ethermal + EOhm, and according to the convention we use below, E can be written as

Materials. The ferroelectric properties and Li-ion transport of a Li+-glass with the nominal composition of Li2.99Ba0.005ClO were characterized via electrochemical impedance spectroscopy (EIS) measurements in the temperature range of 25 °C to −35 °C inside an argon filled glovebox. The synthesis of the lithium glass and sample preparation for EIS measurements have been described in detail previously.18,20 EIS measurements were conducted with a combination Solartron Analytic 1287A potentiostat and a 1260 Impedance/ Gain Analyzer having an applied AC range of 106 to 10 Hz and 10 mV amplitude. Two gold plates were used as blocking electrodes with a cross-sectional area A ≫ d2, where d is the distance between the two inert electrodes (or the thickness of the glass electrolyte film) such that Gauss’s law was valid; here, A = 6.25 cm2 and d = 0.1 cm. Seven cells, all assembled in an argon filled glovebox, were used to evaluate the performance of the Li+-glass electrolyte at different temperatures; in each, the Li+-glass was embedded in a cellulose matrix and all cells were fabricated in a MBraun Ar-filled glovebox. (1) A symmetric Li/Li+-glass/Li cell prepared as described in ref 21 was cycled with a Land CT2001A battery-cycling unit in the MBraun Ar-filled glovebox at an average temperature of −21 °C and then at 25 °C. (2) An asymmetric Li/Li+-glass γ-Mn/O2 cell containing 1.7 mg γ-MnO2 as active material was prepared by coating the surface of an Al current collector with a lithium negative electrode; the cell casing corresponded to a CR2032, but the cell was assembled with a spring and a spacer resulting in an active volume that was around one-third of that of a CR1616 coin-cell. (3) The same cells as in (2) but with an active square surface area of 2.5 × 2.5 cm2 were prepared. The γMnO2 active material in cells 1 and 2 was m1 = 19.6 mg and m2 = 16.2 mg, respectively, and both cells were tested at an average temperature of −20 °C; cell 2 was tested a second time at 24 °C. (4) Three high voltage Li-rich, F-doped, layered-spinel Li[LixNi0.5−yMn1.5−z]O4−x−δFx, with x = y + z ≈ 0.36, δ ≈ 0.36 cathode active material (LNMO) in a Li/Li-glass/succinonitrile-plasticizer/LNMO cell as described in ref 22, and with cathode active mass amounts 1, 2, and 3 corresponding to m1 = 0.25 mg, m2 = 0.33 mg, and m3 = 0.33 mg, were prepared. The pouch cells were never sealed; they remained in the glovebox during assembly and testing. A primary commercial Li-MnO 2 cell with a capacity of approximately 66 mAh (cutoff of 2 V) and theoretical capacity of 308 mAh/g (LiMnO2), corresponding to a minimum amount of active cathode of 216 mg, was also tested as a reference for the discharge method. The γ-MnO2 active material in the coin-cell was dried at 400 °C for 24 h prior to mixing the cathode slurry. The cathode loading for the asymmetric cell (2) was determined to be 80% active material (Alfa Aesar, 99.9%), 10% Carbon Black Super P (Alfa Aesar, 99.9%), and 10% PVDF (MTI), and for (3), it was determined to be 81.6% active material, 6.4% acetylene black, 4% Timcal Super P graphite, and 8% PVDF; both slurries were doctor bladed onto a carbon-coated aluminum foil current collector. The cathode films in (3) were vacuum-dried at 80 °C for 12 h. For more information about the cathode materials, see the Supporting Information, Figures S1−S4. The amount of electrolyte embedded in the 5.7 mg cellulose matrix of the coin-cells was approximately, 24.6 mg in pouch-cell 1, and 55.3 mg in pouch-cell 2. Methods. Equivalent Circuit. The thermodynamic equations of state for a ferroelectric-electrolyte are described by the internal energy per unit volume Ep(P,S)23,24 as a function of polarization P and entropy S per unit volume:

∂P ji dEp zyz z +ρ E = jjj j dP zz ∂t k {S

(3)

which is the Landau−Khalatnikov dynamical equation of motion that is equivalent to ij dEp yz zz + R dQ V = jjj j dQ zz dt k {S

(4)

where V is the potential, R the resistance to ionic movement, and Q the charge capacity. If the equivalent capacitance Ceq is approximately dQ Q d linear, then Ceq = dV = V . Furthermore, C = εrε0 A is the eq

capacitance of the parallel plate capacitor formed by the electrode/ electrolyte/electrode, where εr is the dielectric constant of the electrolyte, ε0 the permittivity of the vacuum, d the thickness of the electrolyte, and A the surface area or the equivalent capacitance. Therefore, Ceq is the equivalent capacitance of the electrochemical cell μ −μ in dynamic equilibria V (Ceq , R ) = − e + = ε where μ− is the chemical potential of the anode and μ+ the chemical potential of the cathode; e is the charge of the electron, and R is the internal resistance mostly due to ionic motion (Figure 1).

V (Ceq , R ) = Veq + R

dQ dt

(5)

Figure 1. Equivalent circuit for an electrochemical cell. The potential of the electrochemical cell that is measured by a dQ voltmeter corresponds to V (Ceq , R ) − R dt = ε − R iI = Veq , We highlight that when the capacitor is fully charged, the circuit will be equivalent to the electromotive force, ε, in series with the internal resistance, Ri. The capacitor’s branch will be equivalent to a short-circuit when the electrochemical cell is fully discharged. Moreover, an additional resistive element might be added in series with the equivalent circuit of Figure 1 to represent other losses due to interfacial, electrode, and collector resistance. The equivalent capacitance Ceq and Ri are time dependent, and ε may be substituted −

−

μ −μ

by Cε, with Vε = − e + where μ− is the electrochemical potential of the anode and μ+ of the cathode; Vε is, therefore, time dependent. The equivalent capacitance Ceq can be calculated for the two electrical double-layer capacitors (EDLCs) in series that form at each electrode/electrolyte interface to equalize the electrochemical potentials (or Fermi levels) at the heterojunctions at open circuit potential or as the cell charges or discharges in dynamical equilibrium. B


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ACS Applied Energy Materials μelectrolyte − μ−

The connection between electrode/electrolyte EDLCs is made by the flow of an ionic current. In a ferroelectric electrolyte, the EDLCs are connected through the aligned dipoles and, therefore, the entire cell is a parallel-plate capacitor with a ferroelectric glass-electrolyte with capacitance, C, as in a classical capacitor. In the equivalent circuit in Figure 1, Ri is the resistance to form Ceq when ions and dipoles are exposed to a driving electric-field μ −μ corresponding to the potential V (Ceq , R ) = − e + = ε . The internal resistance Ri is, therefore, dependent on the ionic conductivity σ σi = Ae−Ea / kT = T0 e−Ea / kT , where σ0 is the pre-exponential factor, kT the thermal energy, and Ea the energetic barrier for diffusion, designated as activation energy.25 The resistance Ri affects the formation of both electrolyte/electrode EDLCs, the bulk electrolyte, and the alignment of the dipoles, as shown in Figure 2. It is worth stressing that both the positively and the negatively charged regions in the electrolyte result from the displacement of the cations in the electrolyte.

μ+ − μelectrolyte

V (C − , R ) = = − V (C+ , R ) = − as the elece e trode/electrolyte junctions are heterojunctions. For the EIS measurements, Au/ferroelectric-electrolyte/Au configurations were assembled. At an applied alternating potential, Vapplied = ±10 mV, the current versus potential is linear and no faradic interface reactions are observed. The driving force of the EIS cell is, therefore, the applied potential as shown in Figure 3a; if, however, there is any heterogeneity between electrodes due, for example, to heating or cooling the electrolyte in an electric field, to dissimilar electrodes or to the spontaneous polarization of the ferroelectric electrolyte, a difference in Fermi levels might exist and, therefore, a battery element would exist in the equivalent electrode in series with Ri and would be represented in by a Cε as shown in Figure 3b. It is highlighted that if Cε exists, Ri will also account for the depolarization resistance. A Warburg element, W, must be added to the circuit in series with Ri when the effect of the impedance of the diffusive layer in the vicinity of the EDLCs at the electrode/electrolyte interfaces is observed (the phase angle of a Warburg impedance is 45° in the Nyquist plot). Additional asymmetric resistance, for example due to poor electrode/electrolyte contact, cables, and electrodes, will be reflected in a resistor, RΩ, in series with the electrochemical cell as shown in Figure 3. An additional inductive element, L, must be added in series with RΩ because at high frequencies, in cells with high capacitances, the magnetic flux generated by the potentiostat cables will create a potential in the cell in accordance with Faraday’s law of electromagnetic induction. The impedance spectra were evaluated with the equivalent circuit portrayed in Figure 3b, and the obtained values for resistance (Ri) were used to calculate the ionic conductivity, σLi, according to the relation Ri =

Figure 2. Electrolyte’s polarization at a low applied electrical field. Formation of electrical double-layer capacitors (EDLCs) at the electrode/electrolyte interfaces to equalize the Fermi levels (μ) and variation of the electrodes and electrolyte’s Fermi levels with applied alternate potential ± A mV. ΔφEDLC is the potential difference Au/ electrode or electrode/Au, and e is the current of electrons. The symbols + and − are ions or vacancies, and + − are dipoles.

1 d × σLi A

(6)

where d is the distance between the two inert blocking electrodes (or the thickness of the glass electrolyte film), and A is the electrolyte surface area such that Gauss’s law holds valid (A ≫ d2) as mentioned previously. The calculated ionic conductivity at each temperature was used to construct an Arrhenius plot, the slope of which was used to obtain the activation energy for ionic conduction to occur in the material. The dielectric properties of the ferroelectric Li+-glass were evaluated with the real and imaginary relative permittivities of the glass, ε′r and ε′′r, respectively. The permittivities of the material were calculated directly from the impedance spectra at low frequencies with the equations

At both electrode/electrolyte interfaces of a blocking electrode symmetric cell at low potentials (e.g., 10 mV), there is a movement of the Li+-ions away from each surface, which leaves negatively charged Li+-vacancies behind and equalizes the chemical potentials or Fermi levels, as shown in Figure 2. Since the electrolyte is Li+-rich, each Li+ion at the surfaces or bulk only performs small displacements. Furthermore, even when the two electrodes are at the same Fermi level, there is still the formation of two EDLCs at each interface which mutually cancel,

ε′r (ω) =

d(− Z ′′) Aε0ω(Z′2 + Z ′′2 )

(7)

Figure 3. Equivalent circuits for an EIS cell with gold electrodes and ferroelectric electrolyte. (a) The variation of the Fermi levels of the electrodes is driven by the applied alternate potential. (b) Cε reflects heterogeneous polarization due, for example, to the spontaneous polarization of the electrolyte. RΩ reflects the resistance in cables, interfaces, electrodes, etc. Ceq forms in response to the applied electric field. Ri reflects the formation of the equivalent capacitor; part of the externally available power is dissipated in Ri as the ions diffuse in the electrolyte in order to achieve equilibrium at the two electrolyte/electrode heterojunctions. L is the inductance due to Faraday’s induction law that is unavoidable at high frequencies and high capacitances. W is the Warburg element accounting for a diffusive layer in the vicinity of the EDLCs at the electrode/ electrolyte interfaces. A is the ammeter and ∼ the external AC source. C


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ACS Applied Energy Materials

Figure 4. Ionic conductivity as a function of temperature; (a) Nyquist plots for temperatures between −35 and 25 °C. (b) A detailed representation of (a). (c) Arrhenius plot showing a transition between 10 and 25 °C and activation energies before and after the transition. (d) Discontinuity in the impedance modulus between 37.7 and 60.8 Ω for all temperatures from 25 °C down to −35 °C.

ε ′′r (ω) =

dZ′ Aε0ω(Z′2 + Z ′′2 )

The graph in Figure 4d indicates that for each different temperature there is a discontinuity in (Z′2 + Z′′2) from 37.72 to 60.82, which leads to the discontinuity in ε′(ω) and ε′′r(ω) observed in Figure 5a and b and obtained with eqs 7 and 8. The latter discontinuity is possibly due, for example, to a change in resonance from one species to another, from Li2O at a lower frequency to LiO− at a higher frequency,19 which is also observed in Figure 5d for temperatures above 10 °C. The same was observed at T ≥ 28 °C with this glass-electrolyte type tested previously with different instruments and laboratories.19,20,31 Figure 5c shows a ferroelectric maximum at around 40 °C. The relaxation of the glass above the glass transition releases the dipoles to become more mobile to align with the electrical field, allowing the dielectric constant to achieve a maximum as previously observed.19,20,31 Symmetric cells provide the ultimate test for the ionic conductivity of the solid-electrolyte; the bonding performance with the Li is also established with the cycling of a symmetric cell. Figure 6 shows that the conductivity of the electrolyte varies from around 0.1 mS/cm immediately after the cell was introduced in the freezer (corresponding to 523 Ω) and 17 mS/cm for a minimum resistance of 3 Ω obtained after 35 days of optimization, which is of the same order of magnitude as the conductivity at room temperature observed in Figure 6. When the alternating current (AC) EIS experiment is compared with the corresponding DC experiment performed with the Li/Liglass/Li cell, the ionic conductivity drops from 17 to 3.3 mS/ cm at −20 °C. However, the conductivity obtained with a symmetric cell performing charge/discharge cycles is usually

(8)

where ω is the angular frequency, Z′ the real impedance, and Z′′ the imaginary impedance. Equations 7 and 8 are obtained by making ε* = A ε′r(ω) − jε′′r(ω), C0 = ε0 d (C0 is the capacitor with vacuum or air as the dielectric material with permittivity, ε0); the impedance is

Z = Z′ + jZ ′′ ⇔ Z =

1 jωC0ε*

=

d(−ε ′′ +jε ′) ε0ωA(ε′2 + ε ′′2 )

.

The above relations give the relative permittivities’ dependence on frequency and allow for the inverse of the dielectric loss tangent to be calculated. cot δ =

−Im(Z) ε′ = ε ′′ Re(Z)

This inverse of the dielectric loss tangent allows for the identification of different resonant frequencies that correspond to different vibrational processes occurring in the material.19

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RESULTS AND DISCUSSION Electrical impedance spectroscopy (EIS) and an Arrhenius plot of the σLi of the glass-electrolyte are presented in Figure 4a−c. The resistance to the ionic motion was calculated by using the equivalent circuit as shown in Figure 3b. The activation energy is approximately the same as that previously reported for this material at higher temperatures, and it is almost the same before and after the glass transition that seems to occur at ∼14 °C. The activation energy is low compared to that of other competitive candidates for lithium solid-state electrolytes and is retained to a lower temperature range than has ever previously been reported for a solid lithium electrolyte.7,25−30 D


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Figure 5. Real and imaginary permittivities versus frequency for different temperatures: (a) real permittivity or dielectric constant, (b) imaginary permittivity, (c) permittivities vs temperature; and (d) cot δ as a function of the frequency.

lower than that obtained with an EIS experiment since in the latter the contributions of interfaces and electrodes can be identified and then discarded. In these experiments, the maximum conductivity was obtained with a DC experiment; the reason is thought to be the necessity for prolonged optimization that cannot be achieved in a dynamic EIS experiment. Furthermore, as expected the EIS conductivity, σAC/EIS = 3.3 mS/cm, at −20 °C is 0.1 ≤ σAC/EIS ≤ 17 mS/cm, which are the minimum and the maximum values of σDC. Moreover, the Li/Li-glass electrolyte interface has a very low resistance that does not seem to be temperature dependent after optimization. Preliminary experiments with Li/Li-glass/MnO2 full cells were performed with both coin- and 2.5 × 2.5 cm2 square pouch-cells with a Bio-Logic VMP300 potentiostat. The Li/Liglass/MnO2 coin-cell was tested at both room temperature and −20 °C at constant current. All experiments with the coin-cell were performed at about C/50, which was estimated for the theoretical capacity of γ-MnO232 equal to 209 mAh/g (Li5Mn7O16).33 In Figure 7a it is shown that at room temperature the capacity was 743 mAh/g (cut off at 2.0 V) during the first discharge. The latter capacity is higher than the capacity of Li2MnO2 (616 mAh/g) which is a metastable Lirich ternary compound that decomposes into 1 2 Li MnO4 + 3 MnO and that could explain part of the drop 3 6 in capacity from the first to subsequent cycles (616 to 205 mAh/g) shown in Figure 7b. According to the theoretical phase diagram of the ternary Li−Mn−O, MnO2 reacts with Li to form xMnO2 + yLi4Mn5O12 + zLi5Mn7O16 in three-phase equilibrium, and with further addition of lithium, the x′Li4Mn5O12 + y′Li5Mn7O16 + z′Li2MnO3 three-phase

Figure 6. Resistance and conductivity in a Li/Li-glass electrolyte/Li symmetric cell. (a) Details of the 19 day optimization of the cell upon introduction of the cell in the freezer. (b) Complete experiment with a first part at approximately (−20 °C) and second at 25 °C.

E


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Figure 7. Asymmetric-full coin-cell Li/Li-glass/γ-MnO2 cycling at room temperature and −20 °C: (a) first discharge at room temperature, (b) room temperature cycling, (c) specific capacity and Coulombic efficiency while cycling at room temperature, and (d) cycling at −20 °C.

experiments in Figure 7 were performed with a coin-cell with one order of magnitude less active material than the pouch-cell in Figure 8 and with a constant current of C/50 instead of an LED. With the green LED, the initial current in Figure 8c achieved about 60 μA, but it is possible that the first potential point(s) was not saved since in Figure 9a and b potentials of about 300 μA were measured at the same temperature (−22 °C) and for the same pouch-cell type. Figure 9a shows the green LED that was connected to the pouch-cell 2 in the glovebox’s freezer. The photograph was taken during the first 5 min. Figures 8c and 9b show a self-cycling feature that was always observed in low-temperature (LT) experiments every time a diode or light emitting diode (LED) was used to rectify the cell’s output current. The period of the self-cycling was around 12 min and it does not depend on the cell format, type of cell, or type of diode; it is not observed at room temperature as shown in Figure 9c. We believe that this self-oscillation is due to the ferroelectric character of the electrolyte and is related to a decrease of thermal energy necessary to overcome the activation energy related with the movement of ions and dipoles in the ferroelectric-glass electrolyte.34 Moreover, other solid-state devices like the ferroelectric field effect transistor (FeFET) also show self-cycling associated with the ferroelectric character of the insulator layer in the gate to channel stack.35 Figures 10−12 show experiments performed with Li/Liglass/SN-plasticizer/LNMO coin-cells22 and comparisons with other cells, including a commercial CR1616 Li-MnO2 coin-cell

equilibrium with corresponding maximum capacity of 520 mAh/g.33 Figure 7 shows the Coulombic efficiency (Qdis/Qch) and the charge and discharge capacities at room temperature. The Coulombic efficiency is higher than 100% for all five cycles, indicating a self-charge as previously observed and discussed in refs 22 and 34. Moreover, the ferroelectric remanent polarization analyzed in (1−5) will have a role in maintaining the cell charged at zero field, after dipole alignment, enhancing self-charge. The low-temperature cycling is shown in Figure 7d; the capacity falls off to 12% at −20 °C. The decrease in capacity is assumed to be due to a deficient interface optimization both at the Li negative electrode side and at the positive electrode. Figure 6 shows that during the first cycles at approximately −20 °C, the resistance is two orders of magnitude higher than when the cell later becomes optimized by 19 days of cycling. Figures 8 and 9 show two different Li/Li-glass/γ-MnO2 pouch-cells discharged at variable potential and current with a green LED. An LED discharges with a univocal correspondence current/potential, and therefore, a cell discharge current and capacity can be easily calculated if the LED’s profile is well-known; we have obtained the profile for each LED that was used. Since our cells have an accentuated capacitance, a certain rectification must be introduced in the circuit to observe the high specific output current in the beginning of discharge. The experiments in Figure 8 with cell 1 at −23 °C agree with the experiments in Figure 7d at −20 °C; nevertheless, the F


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Figure 8. Asymmetric-full 2.5 × 2.5 cm2 squared pouch-cell 1, Li/Li-glass/γ-MnO2 discharged with green LED after one optimization cycle at −23 °C: (a) potential vs time, (b) potential vs specific capacity, and (c) potential and current (inset) vs time.

Figure 9. Asymmetric-full 2.5 × 2.5 cm2 squared pouch-cell 2 Li/Li-glass/γ-MnO2 discharge with green LED after one optimization cycle: (a) green LED lit with I ≈ 0.3 mA at −22 °C, (b) potential vs time at −22 °C, and (c) potential vs time at 24 °C. Insets of (a) and (b) represent the current vs time.

G


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Figure 10. Asymmetric Li/Li-glass/SN-plasticizer/LNMO coin-cell 1 connected to a red LED at different temperatures. The cell was not charged before the tests.

Figure 11. Asymmetric Li/Li-glass/SN-plasticizer/LNMO coin-cell 1 after being optimized at room temperature for more than 1000 cycles: (a) cycles 21 to 23 at −20 °C, with a slow charge and discharge at −20 °C with a constant resistor load of 560 kΩ in series with a rectifying diode and (b) comparison between the discharge profile of the cell at −20 and 25 °C.

corresponding to the layered-spinel phase shown in Figure 11 seems to be approximately equal at both room and low temperatures; nonetheless, the discharge features at LT occur at a potential 1.3 V lower than at RT, which seems to be due to an increase in internal resistance of about one order of magnitude as shown in Figure 12a and b. A Li/Li-glass/SNplasticizer/LNMO coin-cell possesses three EDLCs; therefore, it is possible to observe what seems to be three overlapped semicircles in Figure 12a and b. Figure 12c shows the discharge of the cell with a green LED, showing an initial current of around 27 μA or 108 mA/g, which is approximately 1C of the active cathode. Self-cycling with a period similar to that of the period shown in cells in Figures 8 and 9 for a γ-MnO2 cathode

with liquid electrolyte and theoretical capacity, 308 mAh/g (LiMnO2). Figure 10 shows how a red LED can be lit with the coin-cell 1 containing 0.25 mg of active Li-rich, F-doped LNMO at −35 °C. A red LED, even when emitting a very dim light, shows an input current of about 23 μA for a voltage V ≈ 1.48 V; it is, therefore, very encouraging that a red LED can be lit at −35 °C. The luminescence of the LED increases with temperature from −35 °C up to 20 °C, as expected. Figure 11 shows cycles 21−23 that were obtained after subjecting cell 1 to more than 1000 cycles at room temperature; cell 1 was charged and discharged at −20 °C in the freezer of the glovebox as all the other LT cells. The RT pouch-cells were also discharged in the argon filled glovebox. The capacity H


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Figure 12. Asymmetric Li/Li-glass/plasticizer/LNMO coin-cell 1 after being optimized at room temperature for more than 1000 cycles and after more than 35 cycles at low temperature: (a) EIS at room temperature; (b) EIS at −22 °C; (c) discharge with a green LED at −20 °C; (d) discharge of coin-cells 2 and 3 and comparison with commercial cell (with liquid electrolyte) referencing the active material in the cathode; (e) discharge of coin-cells 2 and 3 referencing the capacity of the anode; and (f) discharging times for a commercial Li-MnO2 cell with liquid electrolyte, Li/Li-glass/MnO2 and Li/Li-glass/plasticizer/LNMO cells at room temperature (RT) and at −20 and −23 °C.

is observed with these cells at low temperature and current. The discharge with a rectifying diode and a resistor for the same Li/Li-glass/SN-plasticizer/LNMO coin-cell in Figure 11 shows the same self-cycling phenomenon. Figures 12d−f show comparisons between the commercial cell, Li/Li-glass/SN-plasticizer/LNMO coin-cells, and Li/Liglass/γ-MnO2 pouch-cells. Although it is the LED that determines the potential profile, the potential is a response to the input current. Therefore, the variable discharge current and potential profiles are useful to determine the maximum current output of the battery cell, the standard discharge current, and the cell profile. As observed in Figure S5, the commercial cell’s maximum current is 13 mA, and the standard discharge current plateaus below 100 μA when the cell is almost fully discharged. Most of the capacity of the cell is obtained in the first 30 h of discharge. The pouch-cell with 16.2 mg of active material discharged at room temperature shows a specific capacity of 374 mAh/g (for a theoretical capacity of 209 mAh/g), which is higher than the 308 mAh/g

of the commercial cell, obtained after discharging it for 96 days with the LED lit (Figure S6). In Figure S5c, a much different profile for the current in the commercial and in the pouch-cell is observed; while the commercial cell varies its output current from 13 mA to 95 μA in 2 days and finally to 0.16 μA in 5 days (the LED was not lit from day six with 14 h to day seven); the Li-glass pouch-cell varies its output current from 1 mA to 17 μA in 1 day and then to 2 μA in 59 days, and the LED remains lit for at least 96 days (see green LED vs time photographs in Figure S6). The initial maximum output current of the Li-glass cells can achieve 1 mA at 2.4 V; these cells contain at least 11 to about 650 times less active material than the commercial CR1616. The discharge rates for the Li-glass cells are, therefore, expected to be substantially lower than for the CR1616 commercial cell. Furthermore, the profile of the current in the Li-glass cells resembles that of a capacitor but one in which the current does not quickly approach to zero. There is always a residual current I ≤ 5 μA, which is enough to keep the LED lit I



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for approximately 2307 h as shown in Figure 12f. This residual current is similar to the standard discharge current of commercial battery cells with a capacity of 1 mAh, which have active cathodes with masses that are more in the range of the active material in our coin-cell cathodes and even our pouch-cells. The reason for a lack of an abrupt discharge like the one observed with the CR1616 in Figure 12e and f and Figure S5 seems to be associated with the ferroelectric character of the electrolyte (see Figure 2); while the cells are formed and optimized, the electrolyte’s dipoles align in the direction of the electric field. When the electric field becomes zero, the polarization is maximum as shown previously in ref 20, and unless the electric field reverses, the cell remains polarized even if energy is lost in the LED’s resistance. This process is activated and sustained by the thermal energy at room temperature, and the discharge proceeds provided it is not limited by the capacity of the anode; Figure 12e shows that the lithium anode is far from reaching its maximum capacity. Figure 12f shows the potential profile of Li-glass cells in comparison with a commercial cell. Although the output currents and active weights are very different and lead to very different final capacities, it is important to highlight that all the Li-glass cells have a very similar profile at RT that is very different from the profile of a liquid electrolyte CR1616, which demonstrates a marked battery profile with an exponential increase of the internal resistance as discharge approaches 100%. In a liquid electrolyte cell like the CR1616, the electrolyte is soaked in the cathode and, therefore, a higher cathode mass corresponds a higher surface area; the liquid electrolyte cell is a three-dimensional (3D) cell at the cathode side. In a solid electrolyte cell, the EDLCs are closer to the layered capacitor model and the cell changes in one dimension, 1D. Hence, a certain amount of cathode active material mass in a liquidelectrolyte cell should be compared with a similar amount of mass applied as a thin-layer of the correspondent surface area. The commercial CR1616 validates the LED discharge method, since the capacity obtained at 2 V matches the technical specifications of the cell.

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Article

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00616. Cathode characterization by XRD, SEM, and EDX; activation energy calculations method; and current and voltage profiles for the discharge of a green LED connected to a commercial CR1616 and to a Li-glass pouch-cell at room temperature (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

J. B. Goodenough: 0000-0001-9350-3034 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support from the Robert A. Welch Foundation, Houston, Texas (F-1066) and the FCT Portuguese project PTDC/CTM-ENE/2391/2014. We would also like to acknowledge the X-ray Scattering and Electron Microscopy facilities within the Texas Materials Institute at the University of Texas at Austin.

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REFERENCES

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CONCLUSIONS

We have demonstrated that it is possible for a ferroelectric glass-electrolyte to maintain an ionic conductivity of about 10−2 S/cm, a dielectric constant of 6 × 105, and the ability to reversibly plate/strip onto a lithium metal electrode at −20 °C. It was demonstrated that the ferroelectric character of the electrolyte in a Li−Li glass-MnO2 cell not only maintains the polarization at room temperature, sustaining a green LED lit for 96 h and overcoming the theoretical capacity, but it also allows for lighting the same LED down to −20 °C. Another type of cell with the same Li-glass electrolyte, Li/Liglass/SN-plasticizer/LNMO, shows a similar LED discharge profile as the Li/Li-glass/MnO2 cell at room temperature down to −35 °C, indicating that the profile is not as dependent on the cathode material as it is on the type of electrolyte. Although the ability to use an asymmetric cell at low temperatures was demonstrated, further work needs to be developed to improve the cathode/solid-electrolyte interface and allow for a greater contribution of the electrochemical storage component to cell capacity and potential. J


Article


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NOTE ADDED AFTER ASAP PUBLICATION This paper was originally published ASAP on June 4, 2019. Additional corrections were received, and the paper was reposted on June 4, 2019.

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Low-Temperature Performance of a Ferroelectric Glass Electrolyte

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