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Quasi-Solid Electrolytes for High Temperature Lithium Ion Batteries Kaushik Kalaga, Marco-Tulio F Rodrigues, Hemtej Gullapalli, Ganguli Babu, Leela Mohana Reddy Arava, and Pulickel M Ajayan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b07636 • Publication Date (Web): 04 Nov 2015 Downloaded from http://pubs.acs.org on November 10, 2015

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ACS Applied Materials & Interfaces

Quasi-Solid Electrolytes for High Temperature Lithium Ion Batteries Kaushik Kalaga1, Marco-Tulio F. Rodrigues1, Hemtej Gullapalli1, Ganguli Babu2, Leela Mohana Reddy Arava 2,*, Pulickel M. Ajayan1,* 1

Department of Materials Science and Nano Engineering, Rice University, Houston, TX, USA

2

Department of Mechanical Engineering, Wayne State University, Detroit, MI, USA

Abstract Rechargeable batteries capable of operating at high temperatures have significant use in various targeted applications. Expanding the thermal stability of current Lithium ion batteries require replacing the electrolyte and separators with stable alternatives. As solid-state electrolytes do not have good electrode interface, we report here the development of a new class of quasi-solid-state electrolytes which have the structural stability as a solid and the wettability of a liquid. Micro flakes of clay particles drenched in a solution of lithiated Room Temperature Ionic Liquid (RTIL) forming a quasi-solid system has been demonstrated to have structural stability until 355oC. With an ionic conductivity of ~3.35mS cm-1, the composite electrolyte has been shown to deliver stable electrochemical performance at 120oC and a rechargeable lithium battery with LTO electrodes has been tested to deliver reliable capacity for over several cycles of charge discharge. KEYWORDS: High temperature energy devices, quasi-solid electrolytes, lithium ion battery, ionic liquids, Clay composites, lithium titanate 1 ACS Paragon Plus Environment


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Introduction Rechargeable lithium ion batteries (LIB) have found their place in numerous applications ranging from micro devices to electric grids. Many applications require power sources to operate at extreme environmental conditions, some at sub-zero conditions in cold regions and some at high temperatures in applications such as space, defense, oil and gas exploration, etc. While several technologies have been developed and commercialized to cater to a vast market, high temperature application of LIB still remains largely unexplored. For example, applications such as sensors mounted onto drill bits for oil exploration or gas turbines demand a wide range of temperature stability (between room temperature to 150oC) and require rechargeable energy storage devices operating at temperatures above 100oC. Although, several conventional ceramic and carbon based electrodes can withstand high temperatures, the choice of a suitable electrolyte has been one of the most demanding and challenging problems which limit their usability at high temperatures. Thus, lithium primary cells based on molten salt electrolytes have been developed for specific applications.1–3 However, these are primary cells and could be operated only above 300oC due to compound nature of the electrolytes at lower temperatures preventing Li- ion conduction. On the other hand, LIB technologies based on organic solvents albeit exhibiting excellent electrochemical properties, pose severe safety concerns at temperatures above 60oC. Low thermal stability of organic solvent based conventional electrolytes (b. p. < 80oC) 4 render these technologies unusable upwards of 100oC. Subsequently, solid ceramic electrolytes with good mechanical and thermal properties and large electrochemical window have been developed.5,6 Unlike molten salt electrolytes these systems provide workable Li- ion conduction

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and also eliminate the need for containment of the otherwise used liquid electrolytes. The solid ceramic electrolytes, exhibit ionic conductivities between 10-9 to 10-7 S cm-1 which is still very less compared to organic or polymer gel electrolytes at room temperature (10-2 S cm-1). Inability of solid electrolytes to access interior of the electrodes (poor electrolyte diffusion) limit their use only to thin-film based LIB technology.7 Several attempts to increase areal energy density of such thin-film LIB by modifying the electrode architecture, are not scalable to bulk forms.8–12 Thus, it is necessary for a high temperature electrolyte system to possess good wetting properties with the electrode and high Li- ion conductivity over the entire operating temperature range to realize a working LIB in the desired thermal window. Room temperature ionic liquids (RTILs) have been gaining interest in recent times as a better alternative due to their high thermal stability, low vapor pressure, non-flammability, low toxicity and large electrochemical potential window.13–15 High thermal stability and negligible vapor pressure make these systems safe at high temperature and the liquid state of the electrolyte offers a solution to the issue of electrolyte penetration to the interior of the electrode. RTILs usually have a long chain organic cation combined with an organic or inorganic anion. Cations like alkylpyridinium, alkylpyrrolidinium, alkylpyrazolium and alkylimidazolium combined with large anions such as PF6−, BF4− and TFSI− have already been reported for electrochemical applications.13–25 Among the known RTILs, liquids with imidazolium based cations have been widely used for lithium ion battery applications since they offer a very high Li-ion conductivity (10-2 S cm-1). However, low voltage stability window limit their use with less electronegative anodes such as Li4Ti5O12 (LTO).26,27 Recently, Sakaebe et al.

28

has studied piperidinium cation

and reported better electrochemical properties compared to RTILs with other cations.



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However, RTILs exhibit decrease in viscosity with increase in temperature making it difficult to confine them in a cell assembly at all temperatures. This fluidic nature of RTILs mandates the use of a suitable material to constraint them. Use of polymer matrices for this purpose has been proposed and resulted in the development of gel polymer electrolytes (GPE). Polymers such as (poly) ethylene oxide (PEO), (poly) ethylene glycol (PEG), (poly) Vinyledene fluoride (pVdF) have been widely used to develop several GPE compositions

6,29–32

with high voltage stability

and moderate ionic conductivity at room temperature (10-5 to 10-4 S cm-1). Variations such as incorporation of small amounts (up to 7 wt%) of ceramic fillers such as alumina32, silica33–36, titania37, BaTiO3 38, etc. into the GPEs was also explored to tailor the porosity and to enhance mechanical properties of the gel electrolyte. Such ceramic fillers have also been used as stabilizing agents in RTIL electrolytes for other battery chemistries such as lithium-sulfur cells.39,40 In spite of good electrochemical properties of GPEs, significant expansion and shrinkage during operation causing buildup of internal stresses and structural instability and melting of most of the polymer hosts at temperatures above 100oC, results in cell failure. Recently, attempts have been made by our group to address this electrolyte-separator bottleneck at high temperature by eliminating the low melting polymer component and formulating

an

RTIL-infused

clay

composite

electrolyte

for

supercapacitor

application.41Bentonites are naturally occurring class of clays known to exhibit high thermal and mechanical stability by undergoing structural expansion and contraction while maintaining crystallographic integrity.42,43 Pinnavaia et al. and many others have performed extensive research in understanding the catalytic and electrochemical properties of different types of clays.44–49 They have shown that the layered structure of clays facilitates the intercalation of various cations and promotes ionic conduction. However, presence of trapped moisture in clay


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and limited knowledge on clay-RTIL interfacial properties present a challenging task of employing these electrolyte composites for LIB applications. Li- ion exhibits high reactivity with the water molecules leading to the decomposition of the electrolyte. By overcoming the above challenges, we report here a versatile electrolyte composition for lithium ion batteries which is a hybrid-electrolyte-separator in itself and can operate at both ambient and high temperatures. The composite consisting of Lithiated-RTIL impregnated into Clay structure, eliminates the need of a secondary separator or a polymer host. In this paper, we have adopted a systematic approach to quantify various properties such as thermal stability, ionic conductivity and electrochemical voltage stability window of this Clay-RTIL system. The composite electrolyte was demonstrated to be stable up to 355oC with a stable voltage window of 3V at 120oC. Further, to test its workability, a lithium half-cell using this quasi-solid-state electrolyte was fabricated and tested for several cycles of charge discharge at various current rates at 120oC. Results and Discussion A schematic representation of the components employed for clay-RTIL quasi-solid-state electrolyte preparation and the envisioned structure of LIB is depicted in Figure 1. Bentonite clay, commonly used as an absorbent material in numerous applications, has good affinity towards RTILs and is found to form a stable composition by simple mechanical grinding. RTIL’s are known to exfoliate layered structures and previous reports have shown such a phenomenon in Clay.50,51 In our case also, the clay platelets have been found to be exfoliated at their edges as a result of minimal mechanical agitation as seen from the Scanning electron microscopy images shown in Supporting Information, Figure S1. Lithium salt dissolved in the RTIL provides the required lithium ions for use of this composite as an electrolyte in a LIB. The choice of these



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individual components used would eventually define the structural, electrochemical and thermal properties of the quasi-solid system being developed. First, as the moisture trapped in the structure of clay is a hindrance to the stability of lithium, it has been heat treated at 650oC under vacuum to remove both absorbed and lattice water molecules.42 1-methyl-1-propylpiperidinium bis(trifluoromethylsulfonyl)imide (PPMI) was chosen as the RTIL due to its good lithium ion conductivity as

a

result

of localized

electrons

on the

N-atom

of its

cation.52

Bis(trifluoromethane)sulfonimide lithium salt (LiTFSI)) was chosen as the lithium salt for the presence of the same sulfonylimide anion as that of PPMI.

Figure 1. A schematic representation of the components employed for synthesis of clay-RTIL quasi-solid-state electrolyte, along with the envisioned structure of lithium-ion battery. The concentrations of these individual components in the quasi-solid system dictate the performance and stability and an ideal composition is sought for. While an equal weight ratio of RTIL and clay has been identified as the optimum consistency in our previous reported work, the addition of lithium salt further affects ion mobility and thermal stability. Varying molar concentrations of LiTFSI in clay-RTIL composite was tested for ionic conductivity, across different temperatures, and the corresponding Arrhenius plots are shown in Figure 2 (a). Three 6 ACS Paragon Plus Environment

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concentrations of lithium salt, 0.2M, 0.6M and 1M, have been probed and their conductivity is observed to be increasing with increase in both concentration of the salt and the operating temperature. A value of 3.35 mS cm-1 was observed at 120oC for a composition with 1M. Though higher concentrations of lithium salt increases viscosity of the RTIL, it contributes more ions for conduction. Also, faster ionic diffusion is observed at high temperatures due to a significant decrease in viscosity. The 1M system was hence observed to have higher net conductivity at the targeted temperature of 120oC and is employed further in this study. The ionic conductivity of RTIL systems follow Vogel-Tamman-Fulcher (VTF) behavior with temperature. Reports predict the reason for the nonlinear behavior to be the increased interactions between the components in the system.53,54 In case of the RTIL-Clay composite, there exists a sizeable interaction between the ions and the clay platelets. Particularly, a notable change in the binding of TFSI- has been observed, deduced using Raman spectroscopy (Supporting information, Figure S2). As suggested by previously proposed models55, the increase in high-frequency component of the band at 745 cm

-1

indicates an increase in interaction of

TFSI- anions. This results in larger concentration of solvated ions. The lower viscosity of RTIL at high temperatures promote these interactions. However, presence of several movable cations in clay adds to the overall conductivity and is not an accurate measure of the mobility of Li+ ions. Effective charge transport was quantitatively evaluated by measuring the Li-ion transference number (TLi+) of the clay-based electrolyte. The Li- ion transference number of a 1M solution of LiTFSI in PPMI quasi-solid electrolyte was measured to be 0.06 at 25oC and 0.08 ±0.02 at 120oC as compared to a TLi+ of 0.10 ± 0.03 at room temperature for a pure 1M solution of LiTFSI in PPMI (Supporting Information, Figure S3). This measured transport numbers have been verified to be comparable with several other



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ionic liquids from literature.56–58 Low values of TLi+ in the quasi- solid electrolytes as compared to that of a pure RTIL solution could be attributed to the exfoliation of ceramic particles liberating metal cations in the composite electrolyte, consequently trapping piperidinium cations to balance the negative charges of the clay platelets. However, it is unclear if such a process would lead to a measurable change in the transport properties of Li+. The thermal stability of the composite was further confirmed using thermo-gravimetric analysis. RTILs are typically known to have high thermal stability24,52 and as expected, PPMI with 1M LiTFSI salt had no thermal decomposition until 370oC as seen in Figure 2 (b). Also, in case of the quasi-solid electrolyte with Clay/PPMI/LiTFSI, no apparent weight loss has been observed until 355oC and on further heating a 50% weight loss is observed, owing to the decomposition of RTIL while Clay being stable. The structural decomposition of the RTIL beyond 350oC can be attributed to the decomposition of the TFSI- anion which has the same decomposition temperature of 350oC.59 On confirming the ionic conductivity and thermal stability of the composite system, in order to use it as an electrolyte for an LIB, its electrochemical stability has been probed. Cyclic voltammetry measurements have been conducted using a two electrode setup to estimate the voltage stability window of the electrolyte with respect to Li+/Li and stainless steel as the working electrode. The plot shown in Figure 2(c) is a potential sweep between 0.01V to 4V performed at 120oC. A stable current was observed until 3.1V, after which, an anomaly in the profile was observed. However, no degradation was observed in the cell at 25oC (Supporting Information, Figure S4). These results suggest a thermally activated interaction between clay and RTIL triggering an irreversible reaction at 3.1V. This is assumed to be a result of the TFSI--clay interactions as understood from the Raman spectra of the composite (Supporting Information, Figure S2). The choice of electrodes to be


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used in conjunction with the electrolyte in an LIB would therefore need to have lithiation/delithiation potentials within this stable voltage window of 0.2V to 3.1V.

a

b

-1

1

0.1

Clay/PPMI + 0.2 M LTFSI Clay/PPMI + 0.6 M LTFSI Clay/PPMI + 1.0 M LTFSI

370oC

100

Weight (%)

10

σ (mS cm )

o

80

355 C Clay/PPMI 1M LiTFSI PPMI 1M LiTFSI

60 40 20 0

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6

0

-1

100

c

200

300

400 o

Temperature ( C)

[1000/T] K

500

0.08 -2

Current (mA.cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.06 0.04

3.1 V

0.02 0.00

-0.02

0.2 V

-0.04 0

1

2

3

4

+

Voltage (V vs Li /Li) Figure 2. (a) Arrhenius plots of varying molar concentrations of LiTFSI in Clay/PPMI composite (b) Thermo-gravimetric analysis curves of PPMI- (1M) LiTFSI solution and Clay/PPMI- (1M) (c) Linear sweep voltammetry measurements of clay/PPMI‐ (1M) LiTFSI composites between 0.02V to 4V. Unlike conventional LIB electrodes such as graphite or Si which have lithium intercalation voltages under 0.2V and exhibit formation of SEI, LTO has been known to have high thermal stability and a lithium intercalation potential of 1.5 V,

60–62

and thus expected to be compatible

for use with the composite electrolyte. With an intention to test the applicability of the composite 9 ACS Paragon Plus Environment


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as an electrolyte for Li-ion secondary battery at high temperatures, cells were assembled with LTO as the working electrode and metallic Lithium as counter electrode. To understand the compatibility of the electrolyte and electrodes at extreme temperatures, cyclic voltammetry has been carried out on the cells from RT to 120oC. Cyclic Voltammetry (CV) profiles measured with a scan rate of 0.1 mV s-1 at different temperatures, and with different scan rates from 0.1 to 1 mV s-1 at 120oC are compared in Figure 3 (a) and (b) respectively. The representative CV recorded at RT (in Figure 3 (a)) exhibits broad red-ox peaks at 1.74/1.28 V corresponding to deintercalation/ intercalation of Li-ion into LTO. Such a large separation (polarization) of red-ox peaks are attributed to the sluggish kinetics (low Li-ion conductivity) at RT. As temperature increases, the peak separation is narrowed and an increase in peak current was observed, owing to the drastically improved conductivity of Li-ions in Clay/PPMI matrix. The red-ox peak potentials of LTO at 120oC were observed to be at 1.64/1.45 V, which are comparable to that of LTO with organic electrolytes and negate any additional reactions in the system.62 Further, the feasibility of using this quasi-solid state electrolyte for high rate LIB applications has been understood through high scan rate CV studies (Figure 3 (b)). The variation of peak currents as a function of scan rate and operating temperatures has been depicted in Figure 3 (c). With increasing scan rate, the positions of anodic and cathodic peaks shift, the reactions can be deduced to be quasi-reversible as the cathodic shift observed is more than that of the anode (Figure 3, Table 1). From these cyclic voltammetry tests and its derived parameters, it can be concluded that the new class of quasi-solid-state electrolyte system exhibits thermal and electrochemical stability even at 120oC.


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1.5 1.64V

a

1.69V

0.1

-1

0.2

Current (mA.g )

-1

Current (mA.g )

0.3

1.74V

0.0 -0.1 1.28V 1.37V -0.2 1.45V -0.3 1.0 1.5

RT o 60 C o 120 C

2.0

2.5

3.0

b

1.0 0.5

o

120 C

0.0 0.1 mV/s 0.5 mV/s 1 mV/s

-0.5 -1.0 1.0

1.5

-1

1.0

c

0.5

-1.0

2.5

3.0

Table 1: Data obtained from CV curves Scan rate (mV s-1)

0.0 -0.5

2.0

Voltage (V)

Voltage (V) Peak Current (mA.g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


RT, Charge RT, Discharge o 60 C, Charge o 60 C, Discharge o 120 C, Charge o 120 C, Discharge

Epa (V)

Epc (V)

E1/2 (V)

?E (V)

Ipa/Ipc

0.1

1.65 1.45 1.55

0.2

1.0

0.5

1.73 1.36 1.54 0.37

1.56

1.0

1.81 1.28 1.54 0.53

1.57

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 -1

Scan rate (mV.s )

Figure 3. (a) Cyclic voltammetry (CV) measurements of LTO half-cell using Clay/PPMI-(1M) LiTFSI composite electrolyte at (a) various temperatures at a scan rate of 0.1 mV s-1 and (b) various scan rates at 120oC (c) Plot of peak currents vs scan rates and different temperatures during charge and discharge cycles. Galvanostatic charge-discharge measurements were conducted on the same battery configuration to test its capacity and cyclability at elevated temperatures. Figure 4 (a) presents the charge-discharge profile of the cell, obtained with a current density of 50 mA g-1 corresponding to a C- rate of C/3, operated at a stabilized temperature of 120oC. A stable voltage plateau region with small polarization has been observed in the profile and interestingly, the polarization decreases with cycling due to an increase in the accessed area for charge transfer



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under electrochemical potential. Such smooth charge-discharge plateaus observed even after numerous cycles indicate absence of any side reactions in the cell. The charge-discharge capacities realized over the cycles is plotted in Figure 4 (b) and demonstrates good capacity retention even after 120 cycles, with a stable capacity of 65 mAh g-1 delivered. Measurements at higher (1C) and lower (C/8) C- rates also demonstrate good cycling properties (Supporting Information, Figure S5). Further, rate capability tests were performed to understand the efficiency and stability of the electrolyte towards high currents. Figure 4 (c) plots the capacities observed over cycling the cell with varying current rates, increased in steps from C/16 to 1C for every 10 cycles. Though a nominal drop in capacity is observed as the current is changed, stable capacity is observed for each step. Finally, when the current rate was decreased to C/3 from 1C, the nominal capacity is regained, confirming that the high current had caused no structural damage to the cell. Hence, the present study demonstrated that the electrolyte composite consisting of Clay/PPMI (1M LiTFSI) is extremely efficient and could be stable upto 120oC for desired safe and high temperature LIB applications.


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+

Voltage (V vs. Li /Li)

a

2.0 1.8

120oC

1.6

C/3

1.4 Cycle 1 Cycle 10 Cycle 25 Cycle 50 Cycle 75 Cycle 100

1.2 1.0 0

10

20

30

40

50

60

70

80

-1

Capacity (mAh.g ) 100

120 100

80

C/3

80

60 60 40 40

120oC

Charge Discharge

20 0 0

20

c

40

60

80

20 0 120

100

Coulombic efficiency (%)

-1

Capacity (mAh.g )

b

Cycle number

-1

)

140

Capacity (mAh.g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


120 100

C/16

C/8

80 C/3

60

C/3

120oC

40

1C

20 0

0

10

20

30

40

50

Cycle number

Figure 4. (a) Charge-discharge profiles of and (b) Cyclic stability of LTO half cells cycled at 120oC at C/3 (c) Rate capability of LTO half cells at 120oC cycled at different C-rates using Clay/PPMI- (1M) LiTFSI composite electrolyte.



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Experimental Details SYNTHESIS OF QUASI-SOLID-STATE ELECTROLYTE A purified form of naturally occurring bentonite clay has been obtained from Southern Clay Products Inc. To remove the moisture intrinsically absorbed in the structure, the clay was heated in a vacuum furnace at 650oC for 1 hour and stored in an argon filled glove box. 1-methyl-1propylpiperidinium bis(trifluoromethylsulfonyl)imide (PPMI, from IoLiTec) was taken in a vial and

heated

at

120oC

for

12

hours

while

stirring

inside

the

glove

box.

Bis(trifluoromethane)sulfonimide lithium salt (LiTFSI, from Sigma Aldrich Inc.) was then added to PPMI in the required quantity and further stirred for 12 hours to obtain a homogeneous solution of desired molarity. Dried clay and the LiTFSI-PPMI solution were mixed in 1:1 weight ratio and mechanically ground in a mortar to obtain the quasi-solid slurry. ELECTROCHEMICAL CELL ASSEMBLY For conductivity measurements, the quasi-solid electrolyte slurry was spread on and sandwiched between two stainless steel foil discs and packed in a CR2032 type coin cell. For the voltage stability studies, the electrolyte was instead sandwiched between a stainless steel foil disc and a lithium metal foil. For the assembly of LIB, LTO electrodes were prepared by spray casting a slurry consisting of LTO (80 % w/w), Poly-vinyledenefluoride (10 % w/w) binder, carbon black (10 % w/w) in a 1-methyl- 2-pyrrolidone (NMP) solvent on to copper current collectors. The electrodes were then hot rolled to a thickness of 40µm. The electrolyte slurry was spread onto the lithium foil and sandwiched with the LTO electrode, and packed into a coin cell. LITHIUM TRANSPORT NUMBER MEASUREMENT Lithium ion transference numbers were calculated following the method proposed by Vincent et al. 63, based on the current relaxation after application of a constant bias corrected by changes


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in the interfacial resistance of the passivation layer formed on top of lithium electrodes. Measurements were made using symmetric cells containing two lithium metal electrodes and the electrolyte, with the interfacial resistance evaluated by electrochemical impedance spectroscopy (100 kHz to 100 Hz, 5 mV amplitude with respect to the open-circuit potential). The current relaxation was performed by applying and holding a constant bias, with magnitude chosen to assure that the ohmic drop in the passivation layer is not much larger than the effective potential within the electrolyte. Lithium ion transference numbers were calculated correcting for changes in the electrolyte conductivity, as described by Abraham et al.64 ELECTROCHEMICAL MEASUREMENT Ionic conductivity and CV measurements were performed using a Potentiostat/Galvanostat with a frequency response analysis capabilities (AUTOLAB PGSTAT 302 N ECOCHEMIE). Ionic conductivities have been deduced from the electrochemical impedance spectra (EIS) measured. The Galvanostatic charge-discharge measurements were conducted using a battery test station (Arbin Instruments). Conclusions A stable quasi-solid electrolyte system that can facilitate operating a LIB at high temperatures has been developed and reported for the first time. The synthesized electrolyte composites have been tested using LTO as the active electrode at various temperatures up to 120oC. The use of lithiated composite of clay infused with RTIL as an electrolyte/separator demonstrated good cyclic stability implying their structural integrity at high temperatures even at long exposures and constant diffusion of lithium ions. A high reversible capacity of 65 mAh g-1 could be obtained at a cycling rate of C/3 for 120 cycles due to the increased wetting of the interior surfaces of the electrode by the infused RTIL. Thus, we have demonstrated a new class of quasi- solid



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electrolyte composition as an apt electrolyte for LIBs operating at high temperatures. These electrolyte systems offer two fold advantage of both thermal stability of solid state electrolytes and good wetting properties of liquid electrolytes with high electrochemical stability window enabling the application of these electrolytes over a wide temperature range unlike conventional solid electrolyte systems operable only at high temperatures. The ability to tune the electrolytes facilitates catering to any design of the battery from thin films to commercial scale batteries. Acknowledgements The authors acknowledge funding support from Advanced Energy Consortium (AEC) under the project number BEG10-02. Supporting Information Microstructural characterization using scanning electron microscopy (SEM), analysis of interaction between ion species and Clay using Raman spectroscopy, measurement or lithium transference number, additional data for voltage stability of electrolyte and additional cyclic stability results of LTO/Li battery. Author information Corresponding Authors: P.M. Ajayan : E-mail: [email protected]; Leela Arava: E-mail: [email protected] References (1)

Vissers, D. R.; Tomczuk, Z.; Steunenberg, R. K. A Preliminary Investigation of High Temperature Lithium/Iron Sulfide Secondary Cells. J. Electrochem. Soc. 1974, 121, 665– 665.

(2)

Gay, E. C. Performance Characteristics of Solid Lithium-Aluminum Alloy Electrodes. J. Electrochem. Soc. 1976, 123, 1591–1591.

(3)

Kaun, T. D.; Nelson, P. A.; Redey, L.; Vissers, D. R.; Henriksen, G. L. High Temperature Lithium/sulfide Batteries. Electrochimica Acta 1993, 38, 1269–1287.

(4)

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