Wide-Temperature Electrolytes for Lithium-Ion ... - ACS Publications

May 19, 2017 - Environmental and Molecular Sciences Laboratory, Pacific Northwest ... U.S. Army Research Laboratory, 2800 Powder Mill Road, Adelphi, ...
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Wide-Temperature Electrolytes for Lithium-Ion Batteries Qiuyan Li,† Shuhong Jiao,† Langli Luo,‡ Michael S. Ding,§ Jianming Zheng,† Samuel S. Cartmell,† Chong-Min Wang,‡ Kang Xu,§ Ji-Guang Zhang,† and Wu Xu*,† †

Energy and Environmental Directorate, Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, Washington 99354, United States ‡ Environmental and Molecular Sciences Laboratory, Pacific Northwest National Laboratory, 3335 Innovation Boulevard, Richland, Washington 99354, United States § Sensors and Electron Devices Directorate, U.S. Army Research Laboratory, 2800 Powder Mill Road, Adelphi, Maryland 20783, United States S Supporting Information *

ABSTRACT: Formulating electrolytes with solvents of low freezing points and high dielectric constants is a direct approach to extend the service-temperature range of lithium (Li)-ion batteries (LIBs). In this study, we report such widetemperature electrolyte formulations by optimizing the ethylene carbonate (EC) content in the ternary solvent system of EC, propylene carbonate (PC), and ethyl methyl carbonate (EMC) with LiPF6 salt and CsPF6 additive. An extended service-temperature range from −40 to 60 °C was obtained in LIBs with lithium nickel cobalt aluminum oxide (LiNi0.80Co0.15Al0.05O2, NCA) as cathode and graphite as anode. The discharge capacities at low temperatures and the cycle life at room temperature and elevated temperatures were systematically investigated together with the ionic conductivity and phase-transition behaviors. The most promising electrolyte formulation was identified as 1.0 M LiPF6 in EC−PC−EMC (1:1:8 by wt) with 0.05 M CsPF6, which was demonstrated in both coin cells of graphite∥NCA and 1 Ah pouch cells of graphite∥LiNi1/3Mn1/3Co1/3O2. This optimized electrolyte enables excellent wide-temperature performances, as evidenced by the high capacity retention (68%) at −40 °C and C/5 rate, significantly higher than that (20%) of the conventional LIB electrolyte, and the nearly identical stable cycle life as the conventional LIB electrolyte at room temperature and elevated temperatures up to 60 °C. KEYWORDS: wide-temperature performance, low-temperature discharge, electrolyte, cesium cation, lithium-ion battery

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of 135 °C, and a very high dielectric constant of 64.9.1 Many electrolyte formulations containing PC were investigated in LIBs within wide operating temperature ranges. Zhang et al.11 studied the effect of the addition of PC to electrolytes on the battery performance and suggested that the presence of PC led to a capacity retention of 83% at −20 °C. A formulation of LiBF4−LiBOB salt mixture in the solvent mixture of PC−EC− EMC (ethyl methyl carbonate; 1:1:3 by wt) was also reported over a wide temperature range (−50 to 80 °C),12−14 which shows excellent performances at both low and high temperatures. The beneficial effect of PC on wide range of operating temperature was also shown by Jow et al.15 Therefore, PC as a carbonate solvent component appears to be effective in extending the operating temperature range of LIBs. Its adverse effects on irreversible capacities because of its co-intercalation into graphite and subsequent exfoliation could be mitigated by the addition of electrolyte additives, such as vinylene carbonate16,17 or CsPF6.18,19

INTRODUCTION The electrolyte often dictates the temperature range within which lithium (Li)-ion batteries (LIBs) operate.1 The current efforts in extending this operating temperature range mainly resort to minimizing the Li+-ion transport resistance, which includes balancing the bulk ionic conductivity and freezing point of the electrolyte and reducing the charge-transfer resistance at the electrode−electrolyte interface.2−6 Formulating the electrolyte by adjusting the relative composition and species of the solvent, salt, and additive has been regarded as a facile approach to extend the service temperature of LIBs from −20 to 45 °C, and even polymer electrolytes have been studied for this purpose.7,8 However, certain special applications requiring LIBs to work in the range of subzero temperatures to −40 °C or even lower temperatures but still preserving an excellent cycling performance at room temperature as well as elevated temperatures up to 60 °C9,10 present a challenge to the above approach. Propylene carbonate (PC) has been used to partially replace ethylene carbonate (EC) in electrolytes for operation over a wide temperature range because it has a low melting point of −48.8 °C, a high boiling point of 242 °C, a high flashing point © 2017 American Chemical Society

Received: March 22, 2017 Accepted: May 19, 2017 Published: May 19, 2017 18826

DOI: 10.1021/acsami.7b04099 ACS Appl. Mater. Interfaces 2017, 9, 18826−18835

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ACS Applied Materials & Interfaces EC has been regarded as an indispensable component despite its high melting point (36.4 °C).1,11 Xiao et al.20 optimized EC-based electrolytes for low-temperature applications and achieved excellent capacity retention at −40 °C with EC−DMC−EMC (8.3:25.0:66.7 by wt). They also found that the EC-poor (10−20% by wt) and EMC-rich cosolvents could effectively expand the operating temperature range of electrolytes. In addition, a series of electrolyte formulations of quaternary carbonate-based solvent mixtures with varying EC content (15−33% by wt) was reported by Smart et al.9 to enable extremely low-temperature applications (−40 °C) for NASA space missions. These low-EC-content electrolytes show improved capacity and low polarization at low temperatures. Overall, it is generally accepted that although the presence of EC in a carbonate solvent improves the cycling stability and high-temperature performance of LIBs, it has usually detrimental effects on the low-temperature performance because of its high melting point and relative ease with which it precipitates at low temperatures. The EMC solvent, with its low melting point (−55 °C) and low viscosity (0.65 mPa·s), is often used as a low-temperature cosolvent.20−25 Therefore, the combination of EC, PC, and EMC solvent components seems to be a logical step toward formulating a solvent base for both low- and high-temperature operations. In this study, we continue our previous works of using CsPF6 as additive, which prevents PC co-intercalation and graphite exfoliation even at a high PC content by forming an ultrathin solid electrolyte interphase (SEI) layer on the graphite-anode surface. We systematically studied the ionic conductivities and phase changes of EC−PC−EMC ternary solvent electrolytes over a wide temperature range, with the aim of formulating a desired ternary solvent system (EC−PC− EMC) to achieve an optimal balance between capacity retention at −40 °C and a reasonable cycling stability at room temperature and elevated temperatures up to 60 °C.

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Table 1. Electrolyte Formulations Used in This Study electrolyte code E1 E2 E3 E4 E5 E6 E7 E8 E9

electrolyte formulation 1.0 M LiPF6 in EC−PC−EMC (5:2:3 by weight) with 0.05 M CsPF6 1.0 M LiPF6 in EC−PC−EMC (4:2:4 by weight) with 0.05 M CsPF6 1.0 M LiPF6 in EC−PC−EMC (3:2:5 by weight) with 0.05 M CsPF6 1.0 M LiPF6 in EC−PC−EMC (2:2:6 by weight) with 0.05 M CsPF6 1.0 M LiPF6 in EC−PC−EMC (1:2:7 by weight) with 0.05 M CsPF6 1.0 M LiPF6 in EC−PC−EMC (3:1:6 by weight) with 0.05 M CsPF6 1.0 M LiPF6 in EC−PC−EMC (2:1:7 by weight) with 0.05 M CsPF6 1.0 M LiPF6 in EC−PC−EMC (1:1:8 by weight) with 0.05 M CsPF6 1.0 M LiPF6 in EC−EMC (3:7 by volume or 3.67:6.33 by weight)

The electrochemical performances of graphite∥NCA cells filled with 100 μL of various electrolytes were evaluated in 2325-type coin cells (National Research Council Canada) on Land battery testers, where 1C rate corresponded to a current density of 1.5 mA cm−2. 1 Ah graphite∥NMC pouch cells with stacked electrodes were made at the ABF and evaluated on an Arbin BT2000 battery tester, in which 1C corresponded to a current of 1.0 A. All cells were tested under galvanostatic charge−discharge cycles at appropriate temperatures inside TestEquity temperature chambers or Tenney JR environmental chambers. The graphite∥NCA and graphite∥NMC full cells experienced two initial formation cycles at C/20 and room temperature, followed by selected testing protocols. The cutoff voltages of these full cells were set at 2.5 V discharge to 4.3 V charge. For low-temperature discharge tests, the cells were galvanostatically charged to 4.3 V at C/5 and then held potentiostatically at 4.3 V to C/ 10 at room temperature, kept in the temperature chamber at the specified testing temperature for at least 3 h to reach thermal equilibrium, and then discharged at C/5 under the selected temperature. For the cycling test of graphite∥NCA coin cells, the cells were charged to 4.3 V at C/3 rate and at 4.3 V to C/10 and then discharged to 2.5 V at 1C rate at room temperature, 45 °C, and 60 °C. The graphite electrodes after cycling were recovered from the graphite∥NCA full cells, soaked in anhydrous EMC solvent for 30 min, rinsed with fresh EMC three times, and dried under vacuum inside the antechamber of the glovebox. High-resolution transmission electron microscopy (HR TEM) was carried out to observe the SEI layer of graphite particle on an FEI Tian 80−300 microscope at 300 kV.

EXPERIMENTAL SECTION

Materials. Single-side-coated LiNi0.80Co0.15Al0.05O2 (NCA) cathode (1.50 mAh cm−2) and graphite anode (MAG-10, 1.53 mAh cm−2), provided by the Cell Analysis, Modeling, and Prototyping (CAMP) Facility at Argonne National Laboratory (ANL), were used as in coincell tests. Commercial LiNi1/3Mn1/3Co1/3O2 (NMC) cathode material and natural graphite were used to prepare the related electrodes (1.61 and 1.83 mAh cm−2, respectively) in the Advanced Battery Facility (ABF) at Pacific Northwest National Laboratory (PNNL) for pouch cells. Celgard 2500 (polypropylene) was used as separator in coin cells and pouch cells. The ingredients of electrolytes, such as lithium salt (LiPF6) and solvents (EC, PC, and EMC) of battery grade, were obtained from BASF Battery Materials and used as received. Cesium hexafluorophosphate (CsPF6; purity ≥ 99.0%) was purchased from SynQuest Laboratories (Alachua, FL) and dried at 65 °C for 4 days under vacuum before use. Various electrolyte solutions were prepared inside an argon-filled glovebox (MBRAUN), where both oxygen and moisture levels were below 1 ppm. The electrolyte formulations are listed in Table 1. Measurements and Characterizations. The ionic conductivities of the electrolytes were measured by Bio-Logic MCS 10 fully integrated multichannel conductivity spectroscopy. The kinetic lower limit of liquid range and the thermodynamic upper limit of solid range of the electrolytes were determined using a differential scanning calorimeter (model DSC 2920, TA Instruments) cooled with liquid nitrogen. This calorimeter was calibrated for temperature with three phase-transition standards: −87.06 °C of a solid−solid transition, 6.54 °C of melting in cyclohexane, and 75.94 °C of melting in hexatriacontane.

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RESULTS AND DISCUSSION As reported in our previous works,18,19 the presence of Cs+ in LiPF6/EC−PC−EMC electrolytes enables a robust, ultrathin, and compact SEI on the graphite electrode surface due to the “passively” preferential solvation of Cs+ by EC and the higher reduction potentials of Cs+−(EC)1−2 solvates than those of Li+−(PC)a(EC)b (a + b = 3−4) solvates.18 When EC is kept constant at 30% in the ternary solvent mixture, 20% PC gives the best electrochemical performance in graphite∥NCA full cells.19 In this study, we further investigated the effect of EC in the electrolytes of 1.0 M LiPF6 in EC−PC−EMC with 0.05 M CsPF6 on the cell performances of graphite∥NCA cells in a wide temperature range of −40 to 60 °C, including lowtemperature discharge capability and long-term cycling stability at room temperature and elevated temperatures. Figure 1 shows the variations in the ionic conductivity with the EC content in 1.0 M LiPF6/EC−PC−EMC electrolytes 18827

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Figure 1. Variations in the ionic conductivities of various electrolytes with EC content in the EC−PC−EMC solvent mixtures at different temperatures. (a) 20% PC content (E1, E2, E3, E4, and E5) and (b) 10% PC content (E6, E7, and E8) in the solvent mixture.

Figure 2. DSC curves of the electrolytes with different EC−PC−EMC solvent mixtures. (a) 20% PC content (E1, E2, E3, E4, and E5) and (b) 10% PC content (E6, E7, and E8) in solvent mixtures.

with two different PC contents at different temperatures. At temperatures below −20 °C, the conductivity obviously increases with decreasing EC content, regardless of whether the PC content is 20 or 10% in the solvent mixtures; nevertheless, the ionic conductivities of 10% PC electrolytes are persistently higher than those of 20% PC electrolytes. This is due to the higher content of EMC resulting in lower freezing point and viscosity of the electrolytes. It appears that less EC and more EMC in the solvent lead to higher ionic conductivities at low temperatures. At temperatures above −10 °C, there is no obvious difference between 10% PC and 20% PC electrolytes. In particular, at room temperature and above, the ionic conductivity slightly decreases when the EC content is lower than 30% in the electrolytes with both PC contents because too low EC content sacrifices the dielectric constant of the electrolyte and decreases the dissociation degree of lithium salt, leading to a lower free Li+ population in the electrolyte.20 To accurately evaluate the service-temperature range of LIBs using these electrolyte formulations, the phase transitions of the electrolytes at different temperatures were measured using a differential scanning calorimeter and the differential scanning calorimetry (DSC) curves are shown in Figures 2 and S1. The measured precipitation points and liquidus points of these electrolytes are summarized in Table 2. The precipitation point of an electrolyte corresponds to the temperature at which the first solid phase precipitates in cooling from a homogeneous liquid electrolyte, and the liquidus point to that at which the

Table 2. Liquidus Temperatures and Freezing Temperatures of Studied Electrolytes electrolyte code

thermodynamic liquidus point (with MCMB at 2 °C/min)

E1 E2 E3 E4 E5 E6 E7 E8 E9

−8.3 °C −16.8 °C −29.4 °C −64.4 °C −61.4 °C −28.2 °C −61.0 °C −58.4 °C −20.4 °C

kinetic precipitation point (with MCMB at −5 °C/min) −37.5 °C cannot be cannot be cannot be −72.4 °C cannot be −70.9 °C −67.2 °C −59.8 °C

measured measured measured measured

last solid phase dissolves during heating from a previously solidified or precipitated electrolyte. As a phase transition rarely occurs under superheated condition but does so often under supercooled condition, the liquidus point obtained in heating provides a lower limit of liquid range above which one can be assured of the absence of any thermodynamically stable solids, whereas above the precipitation point determined in cooling, there can be a solid phase (or phases) that is only kinetically stable due to a sluggish precipitation process at the phasetransition temperature and the lack of a nucleating agent for its precipitation. However, if a precipitation point of an electrolyte is determined under conditions favorable to the precipitation of a solid phase, such as the presence of an effective nucleating agent and a low cooling rate, this point can serve as the 18828

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Figure 3. Discharge voltage profiles of graphite∥NCA coin cells at different temperatures with (a) E1, (b) E2, (c) E3, (d) E4, (e) E5, (f) E6, (g) E7, and (h) E8.

to the temperature values in Table 2, electrolyte E1 has a freezing temperature of −37.5 °C and a liquidus temperature of −8.3 °C, which are in agreement with the visual findings that E1 keeps liquid phase at −20 °C, liquid−solid dual phase at −30 °C, and solid phase at −40 °C, as shown in Figure S2a−c. With the decrease of the EC amount in the electrolytes, the endothermic points are either not obvious for electrolytes E2,

practical lower limit of liquid range, above which the electrolyte will remain free of any solid phase under normal operating conditions of the battery using the electrolyte. The addition of MCMB as an effective nucleating agent and the low cooling rate of −5 °C/min in the determination of precipitation points in this study were measures we took to create such favorable conditions for the precipitation of the electrolytes.26 According 18829

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Figure 4. Variations of discharge capacity (a, b) and capacity retention (c, d) of graphite∥NCA cells with EC content in EC−PC−EMC solvent mixtures at different temperatures. (a, c) 20% PC content (E1, E2, E3, E4, and E5) and (b) 10% PC content (E6, E7, and E8) in the solvent mixtures.

temperature is at and below −20 °C, the discharge capacity and the capacity retention drop sharply and monotonously when the EC content is increased from 10 to 50%. For instance, the discharge capacity and the capacity retention of electrolyte E5 with EC−PC−EMC of 10:20:70 by weight are 134.2 mAh g−1 and 87.5% at −30 °C, respectively, and 94.5 mAh g−1 and 61.6% at −40 °C, respectively, but those of electrolyte E1 with EC−PC−EMC of 50:20:30 by weight are only 16.7 mAh g−1 and 11.1% at −30 °C, respectively, and 0.6 mAh g−1 and 0.4% at −40 °C, respectively. For the electrolytes containing 10% PC (Figure 4b,d), the discharge capacity at 25 °C shows a slight increase from 10% EC to 20% EC and then stabilizes up to 30%. At 0 and −10 °C, the discharge capacities and the capacity retention show minor decrease with increasing EC content from 10 to 20% and then stabilize from 20 to 30% EC. For discharge at temperature from −20 to −40 °C, the discharge capacity and the capacity retention monotonously decrease with increasing EC from 10 to 30% EC and the decrease pace accelerates when the temperature is decreased. For instance, when the EC contents in the solvent mixture are 10, 20, and 30%, the discharge capacities and capacity retentions at −20 °C are 150.0/91.0, 146.4/86.5, and 140.3 mAh g−1/83.4%, respectively; at −30 °C are 138.7/84.2, 130.4/77.1, and 117.3 mAh g−1/69.7%, respectively; and at −40 °C are 112.3/68.1, 83.6/49.4, and 36.3 mAh g−1/21.6%, respectively. The above results confirm that the EC content in the electrolyte significantly affects the discharge performance of LIBs at low temperatures, especially below −20 °C. A higher EC content will result in a relatively higher solidification temperature of the electrolyte, a higher viscosity, and a lower ionic conductivity and thus a lower discharge capacity.

E3, E4, and E6 or are measureable at −72.4 °C for E5, −70.9 °C for E7, and −67.2 °C for E8. That is the reason why electrolyte E2 stays in liquid state at −40 °C, as shown in Figure S2d. Even the control electrolyte E9 has a kinetic precipitation point of −59.8 °C, that is, a wide liquidtemperature range. However, because of the high thermodynamic liquidus points for electrolytes with EC content at and above 30% in the EC−PC−EMC carbonate solvent mixture, normally higher than −30 °C (see E1, E2, E3, E7, and E9), these electrolytes may have narrower effective liquid-phase ranges. The working temperature range of an LIB is mostly dictated by the electrolyte composition, which not only affects the Li+ion transport through the bulk electrolyte but also determines the properties of the formed SEI. Thus, the discharge capacities at different temperatures with the increase of EC in the EC− PC−EMC solvent mixtures with PC contents of 20 and 10% are compared in Figure 3, which also shows the discharge voltage profiles of the graphite∥NCA cells containing the eight electrolyte formulations at various temperatures. Their corresponding charge curves at room temperature are shown in Figure S3. The specific discharge capacity and the capacity retention of these electrolytes at different temperatures as well as their variations with EC content are plotted in Figure 4a−d. As shown in Figure 4a, for the electrolytes with 20% PC in the EC−PC−EMC solvent mixtures, when the temperature is at and above −10 °C, the increase of EC content from 10 to 50% in the solvent mixture leads to a slight change in the discharge capacity, which reaches the maximum value at 20% EC in the 20% PC-containing EC−PC−EMC solvent mixture; however, there is nearly no difference in the capacity retention with the change of the EC content (Figure 4c). When the 18830

DOI: 10.1021/acsami.7b04099 ACS Appl. Mater. Interfaces 2017, 9, 18826−18835

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Figure 5. Cycling performance of graphite∥NCA coin cells with various electrolytes at 30 °C (a, b) and 60 °C (c, d). (a, c) 20% PC content (E1, E2, E3, E4, and E5) and (b, d) 10% PC content (E6, E7, and E8) in the solvent mixtures.

compositions of the EC−PC−EMC ternary solvent mixture investigated in this study do not show obvious differences in cycling stability at room temperature. As shown in the TEM images (Figure 6), there is no detectable graphite exfoliation after 500 cycles. It should be pointed out that the SEI on the cycled graphite electrodes with different electrolytes remains ultrathin and clear even after 500 cycles. This should be attributed to the function of Cs+ with EC in the presence of a small amount of PC reported previously.18,19 The cycling stabilities of electrolytes E1−E5 (with 20% PC) and E6−E8 (with 10% PC) at 60 °C are also compared in Figure 5c,d. The graphite∥NCA cells with E1−E4 and E6−E8 show very similar high-temperature cycling performance to each other. However, the cell with E5 exhibits lower capacity and faster capacity decay than other electrolytes. This is probably because the SEI layer is easily decomposed at high temperatures and E5 has a lower EC content and a higher PC content so that there is not enough EC to form a good SEI to suppress PC co-intercalation at the elevated temperature. The above results indicate that optimizing the EC−PC− EMC ternary solvent mixture in the Cs+-containing LiPF6 electrolytes can achieve excellent discharge capacity at temperature as low as −40 °C, with only a slight compromise in roomtemperature and high-temperature cycling stabilities, resulting in a wide working temperature range for LIBs. In this study, E8 with EC−PC−EMC of 1:1:8 by weight turns out to be the best formulation because of its high initial capacity (164.8 mAh g−1), high discharge capacity (112.3 mAh g−1), and capacity retention (68.1%) at −40 °C as well as stable long cycle life at room temperature and high temperature. Then, the performances of graphite∥NCA cells containing electrolyte E8 and baseline electrolyte E9 are compared in Figure 7. It is seen that the optimized formulation E8 shows

Comparing the electrolyte pairs of E3 with EC/PC at 30:20 and E6 with EC/PC at 30:10, E4 with EC/PC at 20:20 and E7 with EC/PC at 20:10, E5 with EC/PC at 10:20 and E8 with EC/PC at 10:10, where each pair has the same EC content but different PC contents, it is easily found that the electrolytes with 10% PC in the solvent mixture have a higher discharge capacity than those with 20% PC. This is probably because PC has a higher viscosity than EMC (2.53 cP for PC vs 0.65 cP for EMC, at 25 °C).1 The discharge capacities of electrolyte E8 in graphite∥NCA coin cells at −30 and −40 °C are, respectively, 138.7 and 112.3 mAh g−1, corresponding to 84.2 and 68.1% capacity retentions, compared to the discharge capacity at room temperature (25 °C). Therefore, to achieve excellent lowtemperature discharge capacity of LIBs, it is necessary to decrease the high-freezing or high-viscosity EC and PC and increase the low-freezing and low-viscosity EMC in the electrolytes. The effects of EC on the long-term cycling performance at 30 and 60 °C are shown in Figure 5. Figure 5a,b compares the room-temperature (30 °C) cycling stabilities of electrolytes E1−E5 (with 20% PC) and E6−E8 (with 10% PC) in graphite∥NCA coin cells. The electrolytes containing 20% PC show rather similar initial discharge capacity and long-term cycling fading tendency, with the exception of E1 containing 50% EC that shows slightly faster capacity fade than the other four electrolytes (Figure 5a). The capacities of these cells with E1, E2, E3, E4, and E5 after 500 cycles are 118.0, 121.4, 119.0, 131.9, and 125.2 mAh g−1, respectively, indicating a slight capacity decay during long-term cycling. For the electrolytes containing 10% PC (Figure 5b), with decreasing EC content from 30 to 10%, the specific capacity fading trend is very similar although there were slight differences in the capacity after 500 cycles: 137.0, 126.1, and 121.0 mAh g−1. Apparently, the eight 18831

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Figure 6. HR TEM images of (a) pristine graphite electrode and (b−i) cycled graphite electrodes from graphite∥NCA coin cells with electrolytes (b) E1, (c) E2, (d) E3, (e) E4, (f) E5, (g) E6, (h) E7, and (i) E8 after 500 cycles at room temperature.

retention at −40 °C, whereas E8 cells can still maintain 67.7 mAh g−1 capacity and 42% capacity retention at −40 °C. This is probably because of the large difference in the thermodynamic liquidus temperature, −58.4 °C for E8 and −20.4 °C for E9, although their kinetic precipitation points are similar, −67.2 °C for E8 and −59.8 °C for E9, as shown in Table 2. To further verify the potential practical application of E8, we employed 1 Ah pouch cells using NMC as cathode and natural graphite as anode and evaluated the discharge capacity retention at room temperature to −40 °C, different rate capabilities at −40 °C, long-term cycling performance at room temperature, and cycling stability at −30 °C. As displayed in Figure 8a, the pouch cells at −40 °C can still deliver 67% retention of the room-temperature capacity. Figure 8b shows

nearly the same cycling stability as the conventional electrolyte at 30, 45, and 60 °C, as shown in Figure 7a−c, and both electrolytes have similar discharge capacities and voltage plateaus in the temperature range of 20 to −20 °C. However, when the temperature drops to −30 and −40 °C, the difference between E8 and E9 becomes pronounced. The cells with E9 can only deliver capacities of 108.9 and 32.0 mAh g−1 at −30 and −40 °C, respectively, with corresponding discharge capacity retentions of 66.9 and 19.6%; however, the cells with E8 can deliver capacities of 138.7 and 112.3 mAh g−1 at −30 and −40 °C, respectively, with corresponding discharge capacity retentions of 84.2 and 68.1%. If the discharge cutoff voltage is elevated to the practical level of 3.0 V, E9 cells can only deliver 8.9 mAh g−1 capacity corresponding to 5% capacity 18832

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Figure 7. (a−c) Comparison of cycling performance of graphite∥NCA coin cells with E8 and E9 at (a) 30 °C, (b) 45 °C, and (c) 60 °C. (d) Comparison of discharge capacities and voltage profiles of graphite∥NCA full cells with E8 and E9 at C/5 rate and different temperatures, from 25 to −40 °C.

Figure 8. Electrochemical performances of 1 Ah graphite∥NMC pouch cells with E8 electrolyte. (a) Discharge voltage profiles and discharge capacity retention compared with room-temperature capacity. The inset shows a 1 Ah pouch cell. (b) Discharge curves at −40 °C and three different discharge rates. (c) Long-term cycling performance and CE at room temperature. (d) Discharge capacity retention and CE of the pouch cells cycled at −30 °C, where the cells after two formation cycles were cycled at −30 °C by charging at C/5 rate to 4.3 V and at 4.3 V to C/10 and then discharging at C/5 rate to 2.5 V.

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the discharge capacity retention at different rates at −40 °C. The pouch cells exhibit capacity retentions of 67.0, 56.0, and 48.0% at C/5, C/2, and 1C rates on the basis of their roomtemperature capacity, respectively, demonstrating that E8 exhibits significant improvement in the low-temperature performance even at relatively high discharge rates. Their corresponding charge curves at room temperature and discharge curves at different temperatures are shown in Figure S4. The long-term cycling performance of the pouch cells at room temperature (30 °C), shown in Figure 8c, demonstrates that the cells with E8 exhibit a reasonable fading rate corresponding to 99.98% Coulombic efficiency (CE) and 82% retention of the initial capacity even after 400 cycles. More interestingly, from Figure 8d, the pouch cells with E8 can also be continuously charged and discharged at −30 °C for at least 40 cycles (where the cells were stopped to avoid serious Li dendrite growth for the safety consideration). The CE of the first cycle is only about 72% at −30 °C, which means that charging the graphite∥NMC cells at such a low temperature induces Li plating on the graphite anode, but it quickly increases to 99.67% at the second cycle and approaches 100% from the third cycle. The discharge capacity retention is about 60% of the room-temperature capacity, and 94.5% of the initial capacity is maintained after 40 cycles even at temperatures as low as −30 °C. These results further confirm that the electrolyte formulation of E8 identified in this study is indeed an electrolyte with excellent low-temperature performances and good room-temperature and high-temperature cycling stabilities comparable to those of the baseline electrolyte; therefore, it is applicable in large LIBs with practically usable level for wide-temperature-range applications.

Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b04099. DSC curve of the control electrolyte E9, pictures of the states of electrolytes E1 and E2 at different temperatures, charge voltage profiles at room temperature for different discharges, and charge/discharge curves of 1 Ah pouch cells (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Langli Luo: 0000-0002-6311-051X Michael S. Ding: 0000-0002-9302-1032 Chong-Min Wang: 0000-0003-3327-0958 Ji-Guang Zhang: 0000-0001-7343-4609 Wu Xu: 0000-0002-2685-8684 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the Laboratory Directed Research and Development (LDRD) Project under Technology Investment Program (TIP) at PNNL as well as the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy (DOE) under Contract No. DE-AC02-05CH11231, Subcontract No. 6951379 under the Batteries for Advanced Battery Materials Research (BMR). The authors thank Bryant Polzin of ANL for supplying NCA and graphite electrodes that were produced at U.S. DOE’s CAMP Facility, ANL. The CAMP Facility is fully supported by the DOE Vehicle Technologies Program (VTP) within the core funding of the Applied Battery Research (ABR) for Transportation Program. The TEM characterizations were conducted in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL)a national scientific user facility located at PNNL, which is sponsored by DOE’s Office of Biological and Environmental Research. PNNL is operated by Battelle for the Department of Energy under Contract DE-AC05-76RLO1830.

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CONCLUSIONS In this study, we attempted to achieve an optimum balance between low-temperature discharge capacity, room-temperature cycle life, and high-temperature (up to 60 °C) stability through the composition optimization of the ternary EC−PC−EMC solvent mixtures with LiPF6 as conductive salt and CsPF6 as additive. The ionic conductive behaviors and the phasetransition behaviors of these electrolytes were investigated to understand the performance over a wide temperature range, with varying EC content in the solvent mixtures. The most promising electrolyte formulation of 1.0 M LiPF6 in EC−PC− EMC (1:1:8 by wt) with 0.05 M CsPF6 was identified from the comparative studies in graphite∥NCA coin cells in a wide temperature range through the optimization of the EC content in electrolytes. Compared to that of the baseline 1.0 M LiPF6 in EC−EMC (3:7 by vol), the optimized electrolyte identified in this study shows a significantly superior discharge capacity at −40 °C and nearly identical long-term cycling stabilities at 30, 45, and 60 °C. The 1 Ah pouch cells of the graphite∥NMC system using this optimized electrolyte also exhibit excellent low-temperature discharge performance, that is, retaining 68% of the room-temperature capacity at −40 °C, and preserve 82% capacity retention after 400 cycles at room temperature. Although the battery performances of the CsPF6-containing LiPF6/EC−PC−EMC electrolytes have been extensively studied via balancing the ionic conductivity and phase transition, a comprehensive and quantitative understanding for the tailoring strategy of wide-temperature-range properties has not been reported. The investigations on the mechanism of extending the operating temperature range in these electrolytes will be conducted in further work.

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