A metal-free battery working at −80 ​°C

Journal Pre-proof A metal-free battery working at –80 °C Jian Qin, Qing Lan, Ning Liu, Yali Zhao, Zhiping Song, Hui Zhan PII:

S2405-8297(19)31080-3

DOI:

https://doi.org/10.1016/j.ensm.2019.12.002

Reference:

ENSM 1008

To appear in:

Energy Storage Materials

Received Date: 28 June 2019 Revised Date:

15 November 2019

Accepted Date: 3 December 2019

Please cite this article as: J. Qin, Q. Lan, N. Liu, Y. Zhao, Z. Song, H. Zhan, A metal-free battery working at –80 °C, Energy Storage Materials, https://doi.org/10.1016/j.ensm.2019.12.002. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

A metal-free battery working at –80 Jian Qin a, Qing Lan a, Ning Liu a, Yali Zhao a, Zhiping Song a, c, * and Hui Zhan a, b,

a

*

College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei

430072, China b

Hubei Key Lab of Electrochemical Power Sources, Wuhan, Hubei 430072, China

c

Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education),

Nankai University, Tianjin 300071, China

* Corresponding author.

College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei 430072, China

E-mail address: [email protected] (Zhiping Song), [email protected] (Hui Zhan).

A metal-free battery working at –80 Abstract With the rapidly expanded applications of rechargeable batteries, more requirements have been generated, and low-temperature operation of battery has gained increasing concern because of the specified use in extreme weather condition and frequently-occurred extreme weather. However, the battery performance under ultralow temperature of below –40

has so far been

rarely reported because of the electrolyte freezing as well as retarded reaction kinetics. Herein, we propose a novel low-temperature and metal-free battery, in which the pseudocapacitive electrochemical behavior of organic electrode has been fully utilized. With the sophisticatedly optimized ternary electrolyte of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMITFSI) in acetonitrile (AN) and methyl acetate (MA), the high ionic conductivity, sufficient oxidation resistance, and enough-low freezing point is obtained. The PI5/1 M EMITFSI in MA/AN (1/2, v/v)/PTPAn cell is capable of delivering 79% of the theoretical capacity under 1 C-rate when the temperature was down to –80

, and longer than 2000-cycling under 5 C with

slight capacity decay, 65% capacity release at 100 C-rate at –60

. The work sheds new lights

on the unique organic-cation storage behavior of the organics in ionic liquid electrolyte and provides a new avenue for developing efficient and sustainable low-temperature batteries.

Graphical Abstract:

1

A metal-free battery fabricated by PI5 anode, EMITFSI MA/AN electrolyte and PTPAn cathode is proposed, which exhibits a remarkable ultralow temperature performance for all-organic battery.

Keywords: ultra-low temperature; ionic liquid; all-organic; metal-free battery.

1. Introduction In recent years, we have witnessed the great success of electrochemical energy storage and its increasing application in different fields. Among the various electrochemical devices and batteries, metal-based batteries undoubtedly play a dominating role due to the innate advantage of high theoretical capacity, high working voltage, and even light weight, and it now powered the portable electronics, electric tools and most electrochemical vehicles [1,2]. Along with the expanding applications, we have occasionally seen the technology innovation and performance breakthrough [3]. However, we also have noticed that the commercial batteries [4], their use are all confined to mild condition, in another word, room temperature. Nowadays, we have seeing the growing demand for low-temperature (< –40

) battery from specific field, such as high-

altitude aircrafts, polar expedition, some military equipment and so on. Meanwhile, the frequent 2

occurrence of extreme weather, such as the recent polar vortex sweeping across half northern hemisphere, incurred many concerns on reduced range of EV as well as reduced durability of battery in many other electronics or electric tools, and it also promotes the increasing requirement on battery performance under ultralow temperature [5–8]. Unfortunately, the existing battery systems lag far behind in this regard. From the lead-acid [9] to Li-ion battery [10], most aqueous or non-aqueous battery unexceptionally undergoes significant capacity and energy decay when the temperature is approaching the recommended lower limit (~ –20 if lower temperature below –40

), and

is applied, decent energy or even appreciable capacity release

has been rarely achieved in lab studies. The origin of the electrochemical performance deterioration at low temperature has been revealed as the following: [8] (1) the electrolyte becomes more viscous or even freezing, worsening the ion mobility and electrode wettability; (2) the charge transfer within the electrode becomes much more difficult due to the intrinsic grainboundary resistance and the slow metal-ion diffusion in inorganic lattice; (3) the solid electrolyte interface (SEI) becomes less permeable to metal-ion; (4) the metal deposition reaction becomes very problematic. These factors collectively lead to the kinetics fading and intertwined impacts make the issue harder to solve. Over the past years, many strategies have been proposed to address this issue including heating accessory, electrode material as well as electrolyte optimization. Firstly, introducing an auxiliary heating unit [11–13] has already been widely adopted in the battery manufacture. Although it can increase the actual battery working temperature to the recommended range, it will also cumber with the overall battery energy and complicate the battery design and management. Secondly, the electrode strategy usually includes two approaches, reducing the particle size for shorter diffusion path and coating the particle surface for smaller inter-grain resistance [14,15]. However, 3

the former brings higher surface area and consequently more parasitic SEI issues. Thirdly, hybrid solvent is popular as the freezing point and conductivity can both be regulated by adding or varying the concentration of co-solvent, usually the ester with low freezing point [16]. Recently, many researches showed that more penetrable SEI at low temperature can be obtained by changing the electrolyte recipe, for example, adding additives or varying the electrolyte salt or its concentration [17,18]. These approaches may help to achieve an acceptable energy retention on conventional Li-ion or Na-ion batteries at low current rate until –40

, but the gap between the

demand and what has been accomplished still remains and further pushing the temperature limit is a great challenge. Different from the above, organic electrode materials provide an alternate strategy because of its non-intercalation redox mechanism, and its successful application in all kinds of rechargeable batteries already triggers the worldwide interests [19]. Conjugated carbonyl compounds, especially the amorphous polymers, exhibits pseudocapacitive behavior and the highly reversible carbonyl/enol couple endows them with much faster redox kinetics than current transition-metal based inorganic materials. For instance, Liang et.al [20] revealed still-low voltage polarization of poly(anthraquinonyl sulfide) (PAQS) at –25

in 10 M KOH electrolyte. Dong et.al [21]

reported an all-organic battery composed of polyimide (PNTCDA) anode, polytriphenylamine (PTPAn) cathode, and 2 mol kg–1 LiTFSI/EA (lithium bis(trifluoromethane sulfonyl)imide in ethyl acetate) electrolyte, realizing 37% capacity or 29% energy release even at –70

. These

results imply the huge potential of organic-electrode based low-temperature battery and inspired us to go further. In our earlier work [22], we proposed a “metal-free battery” concept. Pure ionic liquid electrolyte provided organic cation for n-type organic anode, and organic anion for p-type organic cathode. 4

The battery chemistry was illustrated in Fig. 1 and its construction could be summarized as polyimide/1-ethyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide/polytriphenylamine

(PI5/EMITFSI/PTPAn). Ultrafast charge/discharge was achieved at room temperature and well maintained until –10 12

, very close to the freezing point (–12

) [23] of EMITFSI. Lower than –

unsurprisingly caused the sudden “death” of the battery in Fig. 2(a). The results suggested

that the freezing of electrolyte could be the bottleneck. It further implied that possibly because of the pseudocapacitive behavior of the organic electrodes and the temperature-decided viscosity/ conductivity of electrolyte, rather than electrode played a determining role in low-temperature performance. Fortunately, in comparison with conventional metal salts, ionic liquid is much more readily dissolved in various organic solvents. The high miscibility of ionic liquid then led to an alternative solution of hybrid ionic liquid electrolyte. It can significantly decrease the freezing point and benefit the ionic conductivity. More importantly, even in the hybrid electrolyte, the poor-solvation characteristic of ionic liquid makes the solvation effect barely observed. It facilitates the reaction kinetics, and in some degree, simplifies the issues that should be considered at harsh temperature condition. Therefore, modified “metal-free battery” was proposed in this work. Diluted ionic liquid electrolyte was sophisticatedly designed and investigated by introducing appropriate organic solvents, aiming to lower the freezing point as well as viscosity. We finally selected methyl acetate/acetonitrile (MA/AN) mixture as the co-solvents. With 1 M EMITFSI in MA/AN (1/2, v/v) electrolyte, the working temperature of battery was successfully pushed to –80 capacity utilization at 1 C rate was first fulfilled at –80 200 C was achieved at –60

. 79%

, and ultrafast charge/discharge up to

. The results supplied a reliable and effective solution for the low-

temperature operation of energy storage devices and revealed the potential application of this 5

novel “metal-free low-temperature battery” system in extreme conditions.

2. Experimental Section 2.1 Materials The electrolyte salts of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, 99.9%, J&K), lithium

hexafluorophosphate

(LiPF6,

99.99%,

Aldrich),

1-ethyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide (EMITFSI, 99.0%, J&K), and the ultra-dry solvents of ethylene carbonate (EC, 99.0%, TCI), ethyl methyl carbonate (EMC, 99.0%, J&K), dimethyl carbonate

(DMC,

99.5%,

J&K),

1,3-dioxacyclopentane

(DOL,

99.8%,

J&K),

1,2-

dimethoxyethane (DME, 99.5%, J&K), Tetrahydrofuran (THF, 99.9%, J&K), acetonitrile (AN, 99.9%, J&K) and methyl acetate (MA, 99.0%, J&K) were used as purchased. PI5 was prepared by the polycondensation reaction using 1,4,5,8-naphthalene tetracarboxylic dianhydride (NTCDA, 98.0%, TCI) and hydrazine (98.0%, J&K) as monomers [24]. PTPAn was synthesized by an oxidative polymerization reaction using triphenylamine (TPAn, 99.0%, J&K) as the monomer and FeCl3 (99.99%, Aldrich) as the oxidant [25]. 2.2. Electrodes/electrolytes preparation and characterization The anode and cathode were made by homogeneously mixing 60 wt% active material (PI5 or PTPAn),

30

wt%

conductive

carbon

(Ketjenblack

ECP-600JD)

and

10

wt%

polytetrafluroethylene (PTFE) binder, and then pressing onto the stainless steel mesh (Φ = 8 mm). Typical active material loading was ~0.78 mg cm–2 for PI5 and ~1.3 mg cm–2 for PTPAn. The electrolytes were prepared by quantitatively mixing the electrolyte salt and solvent and stirring for 24 h at room temperature in an argon-filled glove box. Four series of electrolyte was used in the study with the typical recipes of 1 M LiTFSI DOL/DME (1/1, v/v), 1 M LiPF6 6

EC/DMC (1/1, v/v), EMITFSI dissolving in single solvent (THF, DME, EMC, MA, AN), and EMITFSI in binary MA/AN solvent. The freezing behavior of the electrolytes was investigated in term of differential scanning calorimetry (DSC) measurements on DSC 8500 (Perkin Elmer). The hybrid electrolytes were cooled from –30

to –110

at –2

min–1. The surface morphologies of electrodes (PI5 and

PTPAn) before/after cycling in different electrolytes were observed on the field emission scanning electron microscope (FESEM, Zeiss Merlin Compact). 2.3. Electrochemical Measurements CR2016 coin-cell was assembled in an argon-filled glove box and the above anode and cathode was separated by glass fiber separator (Whatman GF/D). During the fabrication of full-cell, the capacity ratio of PI5 anode to PTPAn cathode was fixed at 1.1, namely, the weight ratio of PI5 to PTPAn was around 0.6 (R = 1.1 × 111 mAh g–1PTPAn / 203 mAh g–1PI5). The galvanostatic charge/discharge tests were carried out on LANHE CT2001A battery test system (Wuhan, China). The C rate was defined by the theoretical capacity of PTPAn cathode (1 C = 111 mA g–1) and the capacity retention/utilization was calculated as similar (real capacity/111 mAh g–1). The electrochemical impedance spectroscopy (EIS) of the full-cell was conducted on the Autolab PGSTAT302 electrochemical workstation with frequency range of 105–10–2 Hz and potential amplitude of 10 mV. The ionic conductivity of electrolyte was measured on Leichi DDB 303A conductivity meter (Shanghai, China). The test temperature was controlled by FCI-280 cool incubator (3 – 45

, AS ONE, Japan) or DW-HW50 refrigerator (–86 – 25

and the temperature fluctuation was controlled within ±1

3. Results and Discussion 7

(Fig. S1).

, MEILING, China)

In our earlier work [22], an innovative “metal-free battery” concept was proposed in which the organic cation storage in n-type PI5 anode and anion storage in p-type PTPAn cathode was first simultaneously realized by using pure ionic liquid EMITFSI electrolyte (Fig. 1). Owing to pseudocapacitive electrochemical behavior of PI5 or PTPAn polymer in EMITFSI and the high ionic conductivity and non-solvation feature of the pure ionic liquid electrolyte, the PI5/EMITFSI/PTPAn full cell exhibited reversible capacity and working voltage near the theoretical value as well as extraordinary good rate capability (Fig. 2(a)-2(b)). In addition, in our pioneer work, the PI5/EMITFSI/PTPAn cell seemed to present some temperature-independent electrochemical performance, as being reflected by the unchanged capacity release from 30 –10

and slightly degraded rate performance at –10

to

. The results implied the significant room

for improvement and prompted us to further optimize the electrolyte for better low-temperature endurance. Actually, for the room temperature ionic liquids (RTILs), despite with many merits, their higher freezing point, boiling point, and viscosity [23,26–27] than common organic solvents (Fig. 2(c)), made them a more “functional” electrolyte component in battery with hightemperature requirement and much less appropriate when low-temperature operation was required [10, 28–30]. These unfavorable features also restricted the choice of pure ionic liquid electrolyte in our “metal-free battery”, for example, many ionic liquids with PF6– anion are even solid at room temperature and thus not suitable to be used alone [31]. Obviously, introducing the second or even third ingredient into pure ionic liquid electrolyte should be an effective solution. With the adding of appropriate organic solvent, the concentration of organic cation/anion decreased, and consequently the electrostatic attraction between them weakened, leading to lower viscosity, higher ionic conductivity as well as lower freezing point.

8

In principle, any other solvent adding in the ionic liquid can decrease the freezing point if they are mutually soluble, and the highly miscible characteristic of ionic liquid widens the solvent options. Considering the electrochemical stability and the rule of freezing-point preference, three series of solvent was investigated, and their freezing point was below –40

and their application

in Li+/Na+ battery had been substantially reported (Fig. 2(c)) [10, 26, 32–33]. They were ethertype tetrahydrofuran (THF, –108 methyl carbonate (EMC, –55 (AN, –45

) and 1,2-dimethoxyethane (DME, –58

) and methyl acetate (MA, –98

), ester-type ethyl

), and nitrile-type acetonitrile

). 0.5 M EMITFSI dissolving in the above solvent were used as electrolytes to assess

the conductivity and compatibility. Fig. 2(d)-2(f) compared the galvanostatic charge/discharge of PI5/PTPAn full-cell with different electrolytes under 1 C at 20

. Generally, in the five

electrolytes, PI5/PTPAn cells showed quite similar voltage profiles as in pure EMITFSI electrolyte, with two pairs of plateaus [22]. Only in 0.5 M EMITFSI+THF, obviously reduced discharge capacity and increased irreversible charge capacity was observed (Fig. 2(a)-2(d)), accompany with worst cycling stability (47% capacity retention after 100 cycles, Fig. 2(e)) and lowest coulombic efficiency (CE, 60–80%, Fig. 2(f)). It indicated that THF, though with the lowest freezing point of –108

, was not an appropriate solvent. Meanwhile, the PI5/PTPAn cell

presented the second worst performance in 0.5 M EMITFSI+ DME electrolyte, suggesting the insufficient oxidative stability (usually < 4.0 V vs. Li+/Li) of ether solvent may incur continuous side reaction on PTPAn cathode (redox voltage > 4.2 V vs. Li+/Li) [22] and further harm the cycling stability. Because of the inferior performance and the unfavored electrolyte decomposition, both THF and DME should be removed from the solvent candidate list. Comparing with ether solvent, the two ester-solvent of EMC and MA brought much better battery property, namely, 86% capacity retention after 100 cycles and stable CE of 98% (Fig. 9

2(e)-2(f)). Among the five candidates, AN led to the best performance including the most stable cycling as well as highest CE of above 99%, very possibly due to its excellent oxidation resistance. After carefully screening the solvent, MA and AN was picked because the former had rather low freezing point of –98

and the latter may enhance the stability of cathode electrolyte interface

(CEI). Then using 0.5 M EMITFSI in AN or 0.5 M EMITFSI in MA electrolyte, we further studied the effect of temperature on the ionic conductivity and battery performance (Fig. 2(g)2(h)). Pure EMITFSI showed high ionic conductivity of 8.7 mS cm–1 at 20

, comparable to

most popular electrolytes in Li/Na batteries (~10 mS cm–1) [34,35] and as high as 3.0 mS cm–1 was retained at –10

before being frozen at –12

. Adding MA did not markedly change the

ionic conductivity at room temperature, but brought significant increase at low temperature, such as 7.7 mS cm–1 of EMITFSI in MA versus 4.5 mS cm–1 of EMITFSI at 0

. Further lowering the

temperature, 3.4 and 0.89 mS cm–1 conductivity could be maintained at –60

and –80

respectively. Adding AN dramatically boosted the ionic conductivity to 33 mS cm–1 at 20 it remained high until –40 0 mS cm–1 at –80

, then suddenly plummeted to 0.25 mS cm–1 at –60

, and

and further to

. Obviously, high dielectric constant of AN (38) greatly favored the

electrolyte conductivity, however its relatively higher melting point was a drag when the temperature was down lower; while MA was with low viscosity and much lower melting point, it greatly helped the conductivity at ultralow temperature but the low dielectric constant of 6.7 made it not very good conductivity enhancer.

In Fig. 2(h), the galvanostatic test under

sequentially decreased temperature indicated the collection between specific capacity and temperature. In AN-added electrolyte, the PI5/PTPAn cell showed almost no capacity decay with temperature decreasing in the range of 20 to –40 10

, but the benefit brought by AN disappeared at

–60

, corresponding to the sudden fall in conductivity in Fig. 3(g); while in MA-added

electrolyte, the cell presented much better capacity utilization along with the temperature dropping, and 82% and 70% capacity was maintained at –60 decent capacity release at the ultralow-temperature of –80

and –80

respectively. Now,

was achieved in EMITFSI+MA

electrolyte. However, we still could not ignore the fact that EMITFSI+MA electrolyte led to relatively inferior cycling stability and CE at room temperature comparing with EMITFSI+AN (Fig. 2(e)-2(f)), suggesting that ternary electrolyte may be more appropriate as the EMITFSI+AN+MA combination may take full advantage of the high ionic conductivity of EMITFSI, low freezing point of MA, and high oxidative stability of AN. The optimum electrolyte formulation was determined by orthogonal test, and we first picked the suitable MA/AN ratio in electrolytes with fixed EMITFSI concentration of 0.5 M. As shown in Fig. 3(a)-3(c), different MA/AN volume ratio (with the range from 1/8 to 8/1) varied the ionic conductivity as well as capacity. A general tendency was revealed that higher AN ratio helped to enhance the conductivity and capacity when the temperature was above –40

, while MA was

necessary if the temperature was down below. After balancing the conductivity and capacity within the temperature range of 20 to –80

, MA/AN ratio was determined as 1/2, because lower

MA (MA/AN = 1/8 or 1/4) caused electrolyte freezing and battery failure at –80

(Fig. 3(b),

Fig. S2(a)), while higher MA (MA/AN ≥ 1/1) greatly sacrificed the conductivity at the temperature higher than –40

, and also might be more unfavorable if wide-temperature

application was considered because MA induced less stable cycling (Figure 2e). With 0.5 M EMITFSI MA/AN (1/2, v/v) electrolyte, 78% capacity was reserved at –80 average capacity was obtained at other temperature condition (Fig. 2(c)).

11

and more than

Beside the solvent ratio, the concentration of ionic liquid is another important factor, as it supplies the cation/anion to the electrodes, then we tested the electrolytes with different EMITFSI concentration in the above-optimized solvent of MA/AN (1/2, v/v). In Fig. 3(d), a rule could be summarized that higher EMITFSI concentration led to increased ionic conductivity from 0.1 M to 1.0 M, and the trend reversed when further increasing EMITFSI from 1.0 M to 3.0 M. Among the electrolytes, 1.0 M EMITFSI in MA/AN outperformed the others at all tested temperature. It could be explained by the dual-effect of EMITFSI. Too diluted EMITFSI (e.g., 0.1 M) could not provide enough charge carriers, while too concentrated EMITFSI (e.g., 3.0 M) increased the viscosity and the freezing point (Fig. S2(b)). Similar trade-off was found in battery tests. The effect of EMITFSI content on battery performance was more complicated as EMITFSI was the cation/anion provider to the electrodes. We thus saw that in 0.1 M EMITFSI in MA/AN (1/2, v/v) electrolyte, even with technically lowest viscosity and not-bad ionic conductivity, still long activation process as well as poor capacity far below the theoretical value was observed (Fig. 3(e)), suggesting the insufficient ion-supply from the electrolyte reduced the material utilization. On the other hand, the high viscosity and freezing point as well as low ionic conductivity of 3.0 M EMITFSI resulted in severe capacity drop at –60 80

and battery failure at –

. Comparing with other electrolyte, in 1.0 M EMITFSI in MA/AN (1/2, v/v), moderate

EMITFSI concentration ensured the adequate ion supply as well as ion mobility, and PI5/PTPAn cell gave top capacity at all tested temperature (Fig. 3(e)). Only 8% capacity decrease happened at –60

and 79% capacity was retained at –80

(Fig. 3(f)). Therefore, the optimum electrolyte

recipe was given as 1 M EMITFSI MA/AN (1/2, v/v). The above results clearly proved the dramatically improved low-temperature performance of PI5/PTPAn cell in optimized 1 M EMITFSI in MA/AN (1/2, v/v) electrolyte, and we further 12

compared it with conventional ester (1 M LiPF6 in EC/DMC, 1/1, v/v) or ether (1 M LiTFSI in DOL/DME, 1/1, v/v) electrolyte in Fig. 4. If using 1 M EMITFSI MA/AN (1/2, v/v) electrolyte, significant capacity reduction and increased voltage polarization was only observed when the temperature was as low as –80

(Fig. 4(a)), and slight capacity decrease was found at –60

,

while performance degradation was barely seen at other temperature. Actually, even at –80

,

the 88 mAh g–1 capacity (far more than 79% of the theoretical value) and the average discharge voltage of 1.17 V made the battery much more functionable than that being previously reported. On the other side, in the ester electrolyte of 1 M LiPF6 in EC/DMC (1/1, v/v), the battery worked quite properly at 0

though with insignificantly decreased capacity and increased voltage

hysteresis between charging and discharging plateau, but battery failure occurred at –20

,

where the capacity rapidly fell to 24 mAh g–1 along with serious IR drop at the initial stage of charge/discharge (Fig. 4(b)). The result well corresponded to the near-freezing phenomenon of 1 M LiPF6 EC/DMC (1/1, v/v) at –20

in Fig. 4(d). When using 1 M LiTFSI DOL/DME (1/1, v/v)

electrolyte (Fig. 4(c)), so poor cycling stability was observed at 20

and the capacity release

could not last longer than 10 cycles. The result was very similar to that in 0.5 M EMITFSI in THF or DME electrolyte (Fig. 2(e)-2(f)) but even worse. The co-solvent of DOL+DME should be blamed, as they were both anodically instable at voltage higher than 3.8-4V and DOL even might bring unwanted anodic polymerization [36–38]. SEM images in Fig. S3 showed the obvious deposit on PTPAn electrode after being cycled in 1 M LiTFSI DOL/DME (1/1, v/v) electrolyte which further evidenced the electrolyte decomposition on it. By contrast, the PTPAn cathode cycled in 1 M EMITFSI MA/AN (1/2, v/v) electrolyte mostly maintained the original block-shaped morphology. The results again prove that ether solvent is not compatible with ptype cathode materials with charging cutoff potential above 4.0 V vs. Li+/Li [10]. In Fig. 4(e), 13

the operation of PI5/PTPAn cells were monitored by LED indicator. At 20

, either in 1 M

EMITFSI MA/AN (1/2, v/v) or 1 M LiPF6 EC/DMC (1/1, v/v) electrolyte, the cell ran well; while at –60

, in the latter electrolyte, the cell failed to drive the LED and the cell using the

former electrolyte still could light the LED. Hence, the advantage of ionic liquid electrolyte over conventional electrolytes in battery with low-temperature or wide-range operation requirement is confirmed by the all-organic battery model. We next investigated the effect of temperature on the long-term cycling stability. In Fig. 5(a), at 20

, the cycling is rather stable that after 2000 cycles the specific capacity still remains 93 mAh

g–1, 86% of the initial value of 108 mAh g–1. When the temperature was down to –60

,

improved cycling stability was observed, and after initial activation cycles, maximum capacity of 103 mAh g–1, approaching the theoretical value of 111 mAh g–1, was achieved and well maintained in the subsequent cycles. Meanwhile, the CE increased from 99.51% at room temperature to 99.99% at –60

, suggesting the inhibiting effect of low temperature on the side

reaction. We further evaluated the influence of temperature on the rate performance of the PI5/1 M EMITFSI MA/AN (1/2, v/v)/PTPAn cell (Fig. 5(b)). In our earlier work, the PI5/PTPAn cell with pure EMITFSI electrolyte already demonstrated excellent rate discharge property of 70% capacity utillization up to 200 C at room temperature. After electrolyte optimization, the 200C capacity utilization was promoted to 87%, and 71% capacity was reserved at an ultra-high rate of 500 C, which had been seldom reported before. We ascribed it to two aspects. Firstly, the inherent pseudocapacitive behavior of PI5 anode and PTPAn cathode enabled ultrafast charge storage, and secondly, the 1 M EMITFSI MA/AN (1/2, v/v) electrolyte provides very high ionic conductivity and ion-mobility because of the poor-solvation or even non-solvation feature of EMITFSI and conductivity enhancing effect of AN. All these combinedly helped to realize the 14

ultrafast reaction kinetics at room temperature as well as ultralow temperature. The wellmaintained fast reaction kinetics was verified by the impedance measurements. In Fig. 5(c), we could see that, for the PI5/1 M EMITFSI MA/AN (1/2, v/v) /PTPAn cell, the Rs value (solution resistance) was kept within 7 Ω even at –60 resistance) rose only by twice from 20 evidenced the decent rate capability at –60

, and in addition, the Rct value (charge transfer to –60

. The charge/discharge curves further

. For instance, 83% and 65% of the theoretical was

released at 20 C and 100 C, accompany with still-high average discharging voltage of 1.1 V and 0.7 V (1.3 V at 1 C in Figure 5d), respectively. Such fast charging/discharging property at –60 was rarely achieved by other reported battery. The above results showed distinguished electrochemical performance of PI5/1 M EMITFSI MA/AN (1/2, v/v) /PTPAn cell. In Fig. 6, we tried to illustrate the progress of low-temperature battery research and our result was also put in. Here, we used the energy density retention (low temperature energy density/room temperature energy density at low current rate) as the indicator because low temperature usually caused not only capacity fading but also voltage decay which both damaged the energy output. A tendency could be revealed that the battery with organic pseudocapacitive materials had better energy retention than that using inorganic intercalation materials when the comparison was made with similar metal-salt electrolyte, and in our result, using ionic liquid electrolyte could much increase the advantages. For example, the temperature limit achieved by the intercalation material was about –40 accomplish charge/discharge at –60

, while the all-organic battery could

, and in this work, 68% energy retention at –80

was

fulfilled. The intrinsic fast redox kinetics of the organic electrodes certainly provided the base of the low-temperature battery, however, it was the ionic liquid electrolyte that eventually led to the realization of almost temperature-independent performance of the battery, and in which, low 15

freezing point, poor- or non-solvation feature as well as well-maintained conductivity all played indispensable role. Certainly, if the practical application is concerned, the total energy density of the full-cell must be considered, which imposes more requirement on cell design and electrode optimization. We have to say the energy density (Eq. S2) of our current full cell (below 10 Wh/kg) is not very competitive comparing with the mainstreamed battery system because excess electrolyte and much inactive carbon has been added in the cell fabrication. However, the potential of this dual-ion battery cannot be ignored, especially in the low-temperature application. In our recent attempt, we tried to optimize the cell manufacture and enhanced the energy density by about 3 times without significant performance degradation. Actually, the further improvement seemed not far. For example, electrolyte strategy, such as introducing solvent-in-salt electrolyte design might reduce the actual electrolyte consumption and using graphene-like conductive carbon could lowered its ratio in the electrode (many examples have been reported in the references) [43–45], and both could enhance the actual capacity. On the other hand, because of structure diversity of organic electrode materials, especially n-type anode, there was large room to boost the working volatge (for example, terephthalate [46] showed redox potential below 1.0 V vs. Li+/Li) and thus double or triple the energy density. More importantly, the ionic liquid and organic solvent electrolyte-design proposed in this work actually greatly widen the electrolyte choice because it well resolved the viscosity, freezing point and conductivity issues of ionic liquid and made it much more applicable. In brief, there is a big margin of performance improvement of the metal-free battery design, including energy density, power density, and wide-temperature performance.

4. Conclusion

16

In summary, ultra-low temperature operation was realized by metal-free battery design and ionic liquid-based hybrid electrolyte. The working temperature was first pushed to –80

. With

optimized electrolyte of 1 M EMITFSI MA/AN (1/2, v/v), oxidative stability was ensured and decent ionic conductivity of 0.36 mS cm–1 was obtained at –80

. The PI5/1 M

EMITFSI/MA+AN/PTPAn cell exhibited excellent rate capability a room temperature and well maintained the capacity as the temperature was down to –80

, where 79% capacity utilization

and 68% energy retention at 1 C was achieved. The pseudocapacitive behavior of organic anode and cathode enabled the fast charging/discharging, but more importantly, the optimized electrolyte with ionic liquid as the major component played the determining role in the capacity/energy release at ultra-low temperature. The all organic electrodes and electrolyte (nonmetal-salt) in turn led to well-guaranteed cycling stability, such as slight capacity decay within 2000 cycles at 5 C, and extremely good rate performance of 65% capacity utilization at 100 C under –60

, much more superior than being previously reported. Our unusual attempts indicate

that the combination of all-organic electrodes and ionic liquid electrolyte not just accomplished the breakthrough on the working temperature of battery, it revealed a win-win strategy for organic electrode as well as ionic liquid because the non-intercalation redox characteristic of organics and poor or non-solvation feature of ionic liquid could be best utilized. Benefitting from the structure diversity and property tunability of organic electrode materials and ionic liquid, we believe our work will inspire more effort on this topic, towards higher energy density and better low-temperature performance for extreme-condition applications.

Acknowledgements

17

This work was supported by the National Key Research and Development Program (2016YFB0100400), the National Natural Science Foundation of China (No. 21875172, 21603167 and 21975189), the National “Thousand Young Talents Program”, the Fundamental Research Funds for the Central Universities (No. 2042017kf0028), 111 Project (B12015), and Wuhan University.

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Fig. 1. Schematic of the metal-free low-temperature battery and electrochemical reaction of anode and cathode.

21

Fig. 2. Typical voltage profiles of PI5/PTPAn full-cell with pure EMITFSI electrolyte (a) under 1 C at different temperatures of 30

, 10

current rate from 1 C to 200 C at –10

, –10

, and –20

, and (b) with step-wise increasing

. (c) Physical parameters of EMITFSI and other five

solvent of THF, DME, EMC, MA and AN [31–34], (d) the 20th voltage profiles, (e) Cycling performance and (f) coulombic efficiency evolvement of PI5/PTPAn cell with the electrolyte of 0.5 M EMITFSI dissolving in different solvent under 1 C at 20

. (g) The conductivity

dependence on temperature of pure EMITFSI [22], 0.5 M EMITFSI in MA and 0.5 M EMITFSI 22

in AN. (h) Discharge capacity and coulombic efficiency vs. cycle number curves of PI5/PTPAn cell with 0.5 M EMITFSI in MA or AN electrolyte under 1 C at different temperatures.

Fig. 3. Variation of conductivity (a), capacity evolvement with temperature (b) and capacity utilization with temperature(c) of 0.5M EMITFSI in MA/AN (volume ratio of MA/AN ranged from 1/8 to 8/1) electrolytes; Variation of conductivity (d), capacity evolvement with temperature (e) and capacity utilization with temperature (f) of EMITFSI in MA/AN (1/2, v/v) electrolytes, the EMITFSI concertation was 0.1 M ~3 M. The capacity utilization was defined by real discharging capacity of PTPAn dividing by theoretic capacity of PTPAn (111 mAh g–1).

23

Fig. 4. Comparison of PI5/PTPAn cells in the optimum low-temperature electrolyte (1 M EMITFSI in MA/AN, 1/2, v/v), conventional ester (1 M LiPF6 in EC/DMC, 1/1, v/v) and ether (1 M LiTFSI in DOL/DME, 1/1, v/v) electrolyte. (a–c) Typical voltage profiles under 1 C at different temperature (only 20

-cycling was shown when using 1 M LiTFSI in DOL/DME, as

the temperature was step-wise decreased after finishing 10 cycles at each test temperature and the cell almost could not give capacity after 10 cycles at 20 temperature electrolyte and ester electrolyte at –20

). (d) Pictures of the low-

, and (e) LED indicator light of PI5/PTPAn

full-cell using 1 M EMITFSI in MA/AN (1/2, v/v) or 1 M LiPF6 in EC/DMC (1/1, v/v) electrolyte at 20

and –60

.

24

Fig. 5. Effect of temperature on the electrochemical property of PI5/1 M EMITFSI MA/AN (1/2, v/v) /PTPAn cell. Comparison of 5 C-rate long-term cycling (a), rate capability (b), and impedance diagrams (c) at 20 performance test at –60

or –60

, and typical voltage profiles in step-wise rate

(d).

Fig. 6. Illustration of the progress of low-temperature battery research. The energy retention is evaluated by the equation of Er %= (Cs×Vad)Tl ×100%/(Cs×Vad)Tr, where Er % is energy retention, Cs is the specific discharge capacity, Vad is the average discharge voltage, and the Tl and Tr represent the low temperature and room temperature, respectively. 25