Li+-Desolvation Dictating Lithium-Ion Battery's Low-Temperature

Lithium (Li) ion battery has penetrated almost every aspect of human life, from portable electronics, vehicles, to grids, and its operation stability ...
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Li+‑Desolvation Dictating Lithium-Ion Battery’s Low-Temperature Performances Qiuyan Li,† Dongping Lu,† Jianming Zheng,† Shuhong Jiao,† Langli Luo,‡ 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 § Electrochemistry Branch, U.S. Army Research Laboratory, 2800 Powder Mill Road, Adelphi, Maryland 20783, United States S Supporting Information *

ABSTRACT: Lithium (Li) ion battery has penetrated almost every aspect of human life, from portable electronics, vehicles, to grids, and its operation stability in extreme environments is becoming increasingly important. Among these, subzero temperature presents a kinetic challenge to the electrochemical reactions required to deliver the stored energy. In this work, we attempted to identify the ratedetermining process for Li+ migration under such low temperatures, so that an optimum electrolyte formulation could be designed to maximize the energy output. Substantial increase in the available capacities from graphite∥LiNi0.80Co0.15Al0.05O2 chemistry down to −40 °C is achieved by reducing the solvent molecule that more tightly binds to Li+ and thus constitutes a high desolvation energy barrier. The fundamental understanding is applicable universally to a wide spectrum of electrochemical devices that have to operate in similar environments. KEYWORDS: low temperature, desolvation, ion transfer, electrolyte, lithium ion battery, cesium cation

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additives like lithium-modified silica nanosalt.14,15 Although improvement was achieved, the fundamental mechanism behind the origin of reduced resistances remains unclear, and controversies often arise. It has been noted early on that bulk ion conductivity of electrolyte may not be the limiting factor determining the low-temperature performance of an LIB;7,12,16 instead, Li+ migration across the SEI might be the most sluggish step.4,17,18 Xu et al.19,20 and Abe et al.21 independently proposed that Li+ ion transfer barrier through the interface of electrolyte/electrode is overwhelmed by the Li+ desolvation process before intercalating into graphite interlayers. Taking the specific LIB chemistry graphite∥LiNi0.8Co0.15Al0.05O2 (NCA) as an example, both an electron and a Li+ leave NCA cathode simultaneously during the charging process and reach to the graphite through the external circuit and the electrolyte, respectively. Compared with the agile electron transfer via an external circuit, the diffusion journey of Li+ from the cathode to the anode is more complicated. At the NCA sides, Li+ will be extracted from the cathode lattice, moves across the interphase

INTRODUCTION A low-temperature capable lithium (Li) ion battery (LIB) would attract interests for applications in subzero environments such as polar areas of the Earth or outer space.1−3 It has been well known that the electrolyte plays an important role in controlling the mass transport across the cell, which includes not only Li+ conduction through the bulk electrolyte but also its migration through the solid−electrolyte interphase (SEI). In particular, the electrolyte formulation dictates the solvation structure of Li+, which consequently influences not only the SEI formation and composition but also the energy barrier associated with the desolvation of Li+ at the electrolyte− electrode interface.4 The resistances and kinetic barrier to Li+ would increase as temperature goes down, and numerous studies have been seeking the reduction of these resistances so as to improve the low-temperature capability of LIBs. Successful strategies include employing low-freezing-point solvents like ethyl methyl carbonate (EMC), propylene carbonate (PC), and diethyl carbonate,5−7 and co-solvents like 2,2,2-trifluoroethyl-N-caproate and carboxylate esters,8,9 utilization of various lithium salts combinations such as lithium oxalyldifluoroborate/lithium tetrafluoroborate (LiBF4),10 lithium bis(oxalato)borate/LiBF4;11−13 and the other additional © XXXX American Chemical Society

Received: September 12, 2017 Accepted: November 17, 2017 Published: November 17, 2017 A

DOI: 10.1021/acsami.7b13887 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

rate of 0.05C at room temperature in the voltage range of 2.5−4.3 V (1C corresponds to a current density of 1.5 mA cm−2). For temperature tests, the cells were, unless specified otherwise, charged at a constant rate of 0.2C and a constant voltage charge to 0.1C at room temperature and then discharged at 0.2C under various temperatures, where all of the cells were kept under each temperature for at least 3 h for thermal equilibration. The temperatures were controlled by a Tenney JR environmental chamber. For specific comparison tests, the NCA and graphite electrodes were harvested from the full cells with various electrolytes after two formation cycles in the argon-filled glovebox, soaked in EMC solvent for 30 min, rinsed three times with fresh EMC to remove the residual electrolytes, dried, and then interactively reassembled back into fresh graphite∥NCA coin cells with different electrolytes. For impedance measurements, the NCA and graphite electrodes retrieved from the full cells after formation cycles as well as the fresh LTO (without experiencing formation process) were assembled into symmetric cells (i.e., NCA∥NCA, graphite∥graphite, and LTO∥LTO) with various electrolytes, followed by EIS measurements under different temperatures. A Solartron 1255B frequency response analyzer coupled with a Solartron 1287 electrochemical workstation were employed to measure EIS with a 10 mV perturbation in the frequency range from 106 to 10−3 Hz. Galvanostatic charge−discharge tests under different temperatures were performed in a Tenney JR environmental chamber. The ionic conductivities of the electrolytes were measured with BioLogic MCS 10 Fully Integrated Multichannel Conductivity Spectroscopy. Highresolution transmission electron microscopy (TEM) was carried out to observe the SEI on graphite particles on an FEI Tian 80-300 microscope at 300 kV.

of cathode (sometimes named cathode−electrolyte interphase, CEI), and then enters the bulk electrolyte and gets solvated with solvents or anions. The solvated Li+, now even clumsier, diffuses through the bulk electrolyte and gets desolvated before it can enter the graphite SEI, followed by Li+ intercalation into the graphitic structure. The reverse process happens during discharge.22−24 Xu et al. proposed that Li+ desolvation is the most sluggish process among those multiple steps; however, understanding of the processes behind low temperatures is still unclear, which is related extensively to the battery’s lowtemperature performance. A dedicated study to identify the key component for low-temperature Li+ transport is still absent. The present work aims to fill the above knowledge gap. By leveraging an interactive design of the experiments and applying the electrochemical impedance spectroscopy (EIS) measurement on symmetric cells including NCA∥NCA, graphite∥graphite, and lithiated spinel titanate (LTO)∥LTO with various electrolytes, the Li+ transport behavior at temperatures lower than −20 °C is examined with the purpose of distinguishing the rate-determining step. The results enabled the formulation of low-temperature electrolytes that can efficiently support LIBs to deliver 68% of their nominal capacities down to −40 °C.

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EXPERIMENTAL SECTION

Materials. LiNi0.80Co0.15Al0.05O2 (NCA, 1.50 mAh cm−2) and graphite (MAG-10, 1.53 mAh cm−2) electrodes were obtained from the Cell Analysis, Modeling and Prototyping (CAMP) Facility at Argonne National Laboratory (ANL) and used as cathode and anode, respectively, in coin cells. Celgard 2500 was used as a separator. Li4Ti5O12 (LTO) electrodes were used for symmetric cell assembly. The electrolyte components such as lithium salt (LiPF6) and solvents (EC, PC, and EMC) of battery grade were purchased from BASF and used as received. Cesium hexafluorophosphate (CsPF6, ≥99.0%) was purchased from SynQuest Laboratories (Alachua, FL) and dried at 65 °C for 4 days under vacuum prior to use. Preparation of electrolytes and coin cell assembly/disassembly were conducted in an argon-filled glovebox, where both oxygen and moisture levels were below 1 ppm. The electrolyte formulations are listed in Table 1. Electrochemical Measurements and Characterization. The electrochemical performances of graphite∥NCA coin cells filled with 100 μL electrolytes were evaluated in 2325 type coin cells (National Research Council Canada) on Land battery testers (Wuhan, China). All of the coin cells were initially subject to two formation cycles at the

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RESULTS AND DISCUSSION Discharge Capacities at Different Temperatures. Cesium cation (Cs+) was identified to enable PC-rich electrolytes (up to 20% by weight) for stable cycling of graphite electrode by directing EC molecules for surface reduction and forming an EC-rich SEI.25,26 This “disproportionation” between the bulk and the interphasial contents of electrolytes was made possible by the passive “preferential” solvation of Cs+ by EC and the higher reduction potentials of Cs+−(EC)m solvates as compared with the Li+−(PC)n solvates. The high PC content in these electrolytes diluted the effect of high-melting-point EC and provided an alternative path to high-performance low-temperature electrolytes. Figure 1 compares the effect of CsPF6 on the discharge capacity at various temperatures for the graphite∥NCA full cells with two PC contents (20 and 10%) in the solvent mixtures of EC−PC− EMC. With 20% PC (Figure 1a), the electrolyte with CsPF6 additive (E5) dramatically improves the discharge capacity when compared with the electrolyte without CsPF6 additive (E6). The reversible capacity at −40 °C is increased from 9 to 52% of the room temperature capacity with the use of CsPF6. However, using electrolyte with 10% PC (E9 and E10), no obvious difference can be observed in the discharge capacities in the temperature range from 25 to −40 °C, no matter CsPF6 is used or not (Figure 1b). With the presence of CsPF6 additive, a much thinner SEI is formed (see the later section for the TEM morphologies of graphite electrodes),25 but it seems this more compact SEI casts little influence on the delivered capacity. This discrepancy counters the well-accepted belief that Li+-transport through SEI is the rate-determining step. It was found in our previous research that EC/EMC ratio of the electrolyte plays a critical role in a cell’s low-temperature performance (shown in Figure S1).27 However, for fundamental understanding, there are still several questions that need to be answered: (1) how does the bulk electrolyte formulation

Table 1. Electrolyte Formulations Used in This Work electrolyte code E1 E2 E3 E4 E5 E6 E7 E8 E9 E10

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 (1:2:7 by weight) 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−PC−EMC (1:1:8 by weight) B

DOI: 10.1021/acsami.7b13887 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. Discharge profiles of graphite∥NCA full cells at different temperatures (a) with E5 and E6 and (b) with E9 and E10.

Figure 2. Schematic description of Li+ migration during discharging process.

cathode and anode materials were applied for full cells with various electrolytes, which indicates that at the selected temperature, the diffusion of Li+ in the interlayer of the electrode materials (steps 2 and 10) should be considered to be the same, thus they should be excluded. As reported previously,7,12 the ionic conductivity of the electrolyte does not constitute the limiting factor either, which also confirms our results, because electrolyte E2 at −30 °C has a higher ionic conductivity value (shown in Figure S2) compared with the ionic conductivity of electrolyte E9 at −40 °C, whereas the discharge capacity of the full cells with E2 at −30 °C is lower than that of cells with E9 at −40 °C (showed in Figure S3), which cannot well explain their correlation between capacity and ionic conductivity. Therefore, it is reasonable to exclude step 6 as well. Li+ charge transfer on the interface of electrolyte/electrode is directly related to the electrolyte itself and the corresponding SEI formed at the surfaces of NCA and graphite spontaneously become the focus of analyzing the mechanism of discharge process at low temperatures. To observe the effect of the CEI and SEI on the cathode/electrolyte and anode/electrolyte interface in the low-temperature discharge capacity, we selected the cells with electrolytes E1, E2, and E9 as the experimental subjects with two extreme conditions, where both E1 and E2 exhibited the worst discharge capacities at −40 °C, whereas E9 showed the highest discharge capacity at −40 °C according to our previous report.27 We disassembled the cells and harvested the NCA and graphite electrodes after two formation cycles with E1 and E9, which bear completely formed SEIs on their

correlate to the low-temperature discharge capacity? (2) How does the SEI composition affect the low-temperature properties? and (3) what is the determining factor of low-temperature performance in full cells? Aiming to answer these questions, the Li+ migration process during discharging at −40 °C was studied. Predominant Factor during Low-Temperature Discharge. It has been well established that the energy barrier or resistance determines how fast and how much Li+ can be extracted out or intercalated back into the electrodes.20,22 A complete pathway of Li+ transfer during a discharge process in a graphite∥NCA full cell can be broken down into the following steps (as shown in Figure 2). All of these steps will possibly affect the discharge capacity at low temperatures. (1) Li+ extraction from LixC6 anode; (2) Li+ diffusion in the graphite interlayer; (3) Li+ across the solid−solid interface between the graphite electrode and the graphite SEI; (4) Li+ diffusion through the SEI layer of graphite; (5) Li+ solvation in electrolyte; (6) Li+ migration in electrolyte toward NCA cathode surface; (7) Li+ desolvation from electrolyte at the SEI layer on NCA; (8) Li+ diffusion through the CEI layer on NCA; (9) Li+ across the solid−solid interface between the NCA CEI and the NCA electrode; (10) Li+ diffusion in the interlayer of NCA; (11) Li+ reaches appropriate site of the NCA lattice. At low-temperature which step constitutes the limiting factor to discharge is the subject of interest. In this work, the same C

DOI: 10.1021/acsami.7b13887 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 3. Discharge profiles of new graphite∥NCA full cells: (a) E2A + E9C + E2, (b) E9A + E2C + E2, (c) E9A + E9C + E2, (d) E9A + E2C + E9, (e) E2A + E9C + E9, and (f) E2A + E2C + E9, where A and C denote the anode and cathode, respectively, and E2 and E9 are the electrolytes. E2A and E2C are the graphite anode and the NCA cathode, respectively, from the cell with E2 after two formation cycles, and E9A and E9C are the graphite anode and the NCA cathode, respectively, from the cell with E9 after two formation cycles. Cells were charged at 0.2C rate to 4.3 V and then at 4.3 V to 0.1C at room temperature.

surfaces with each electrolyte, and subsequently, interactively reassembled them back into new full cells by exchanging electrolytes. All of these new cells can operate normally at room temperature. At low temperatures, however, their different behaviors reveal the key factor that dictates the low-temperature performance. The new graphite∥NCA cells with E9, with exchanged NCA electrode, or graphite electrode, or both harvested from the cells containing E1, maintained decent lowtemperature performances at −40 °C (Figure S4). On the contrary, the new graphite∥NCA cells containing E1, no matter the NCA or graphite electrode or both were recovered from the cells containing E9, did not deliver any capacity at −40 °C. Considering that E1 is frozen at −40 °C resulting in large interface resistance between the frozen E1 and the surface of electrode,27 we repeated the above interactive experiments using E2 and E9 (Figure 3), with the same results obtained. To eliminate the interference factor of recharge process at room temperature prior to discharging at −40 °C, we also directly switched the 100% states of charge (SOC) graphite electrodes from the fully charged cells with E9 and E1, followed by discharging the reassembled cells at −40 °C (Figure S5). All of the discharge capacities at −40 °C are summarized in Figure 4, demonstrating beyond any doubt that the low-temperature performance is correlated to the bulk electrolyte rather to the SEI formed, and Li+ is free to migrate through the intrinsic SEI on either cathode or anode at −40 °C, despite their formation history. TEM (Figure 5a−d) also reveals that the morphology and thickness of the SEIs show no obvious difference despite different electrolyte formulations. On the contrary, similar discharge capacity at −40 °C is obtained regardless of the apparently different SEIs (Figure 5e,f versus g,h). Therefore, the above results remove steps 3, 4 and 8, 9 as the limiting

Figure 4. Summary of the discharge capacity at −40 °C for the graphite∥NCA cells with electrodes that were removed from the original full cells with E1, E2, and E9 electrolytes after two formation cycles (at the fully discharged state), and then interactively reassembled back into new full cells. EnA and EnC denote the graphite anode and the NCA cathode, respectively, retrieved from the full cell with the electrolyte En, where n = 1, 2, and 9. 100% SOC means the anode and the cathodes were disassembled from the fully charged full cells with the related electrolyte. Cells were charged at 0.2C rate to 4.3 V and then at 4.3 V to 0.1C at room temperature.

factors of low-temperature kinetics. In other words, the properties of bulk electrolyte itself dictate the low-temperature performance. The overall cell resistance consists of the bulk electrolyte (Rb), SEI (RSEI), and charge transfer (Rct),22 among which Rct is considered as the most significant factor affecting the lowtemperature performance of the batteries,17,18 and Xu et al. suggested that the desolvation resistance of Li+ before it enters D

DOI: 10.1021/acsami.7b13887 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 5. TEM images of graphite electrodes from full cells with (a) E1, (b) E3, (c) E5, and (d) E9; (e) E5, (f) E6, (g) E9, and (h) E10 after two formation cycles.

Figure 6. Comparison of EIS results for symmetric cells of (a) LTO∥LTO, NCA∥NCA, and graphite∥graphite with E9 at −40 °C; (b) LTO∥LTO with various electrolytes at −40 °C; and (c) LTO∥LTO with E2 and E9 at various temperatures. Both insets in (b) and (c) show an enlarged part in the high-frequency regions.

the interlayer of materials (Rdesolvation) is a main section of Rct.20 Having excluded the Li+ migration across SEI and its solid− solid diffusion as the limiting factors that affects the lowtemperature performance, the stripping of a solvated Li+ (step 7) becomes the only possible main energy-consuming step at low temperature. Given its moderate intercalation potential (1.50 V vs Li), LTO is an ideal candidate to further eliminate the complication of Li+ migration due to its nearly interphase-free surface, whose semicircle of EIS should reasonably represent the resistance of pure Li+ desolvation.20 The symmetric cells with configurations of graphite∥graphite, cathode∥cathode, and LTO∥LTO, respectively, were assembled and the temperature dependence of their cell impedances was compared. Among these, the impedance semicircles at high- and medium-frequency ranges of the symmetric NCA∥NCA and graphite∥graphite cells, respectively, reflect the sum of all of the interphasial components

Although for LTO∥LTO, given the absence of SEI, the cell impedance should be represented by RLTO/LTO = R ct + R desolvation + R b

Given the same Rb and Rdesolvation, each cell should possess rather distinct Rct and RSEI values. However, despite these different components, similar impedance spectra with almost identical resistances were obtained for all of the three symmetric cells at −40 °C (Figure 6a). The only one possible explanation to the EIS spectra shown in Figure 6a is that the resistance from Li+ desolvation is the predominant factor that overwhelms contributions from all of the other components. By changing the electrolytes in LTO∥LTO symmetric cells, obvious difference in impedance spectra arose at −40 °C (Figure 6b), where the semicircle (Rdesolvation) gradually becomes larger with increasing EC/EMC ratio, but the initial interception (Rb) sees no significant change except E1. This is because E1 freezes at −40 °C, where the bulk electrolyte resistance increases sharply. Apparently, the different resistance behavior induced by bulk electrolyte composition change can be attributed to the change in the Li+ solvation structure, which is then reflected in the change of energy barrier for Li+

R cathode/cathode = R ct + R SEI of cathode + R desolvation + R b andR anode/anode = R ct + R SEI of anode + R desolvation + R b E

DOI: 10.1021/acsami.7b13887 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 7. (a) Cycling performance and efficiency of full cells with various electrolytes; (b) charge and discharge profiles of E5 at −20 °C and selected cycles; (c) charge profiles at the 101st cycle; (d) summary of discharge capacity of graphite∥NCA cells with various electrolytes at −20 °C but at different charging condition; (e) optical images of graphite anodes at the end of the charged state after 100 cycles in (i) E1; (ii) E2; (iii) E3; (iv) E4; (v) E5; and (vi) E9. Test protocol: charge at 0.2C rate to 4.3 V and at 4.3 V to 0.1C, and then discharge at 0.2C to 2.5 V after two formation cycles; efficiency is equal to the discharge capacity of the previous cycle divided by the charge capacity of the next cycle.

maintained at 97.7 and 97.5% after 100 cycles, respectively, with cycling efficiency of 99.96 and 99.99% (shown in Figure S6). The charge and discharge profiles of E5 at −20 °C (Figure 7b) indicate the low cycle efficiency in the 1st cycle, but the capacity stabilizes after the 2nd cycle and the electrochemical system also remains stable after the Li-dendrite is gradually consumed or reacts with the electrolyte. It should be noted that the first discharge capacity delivered after first charging at −20 °C is a little lower than that after charging at room temperature, which is caused by a little Li-plating in the first charging process at −20 °C, even E1 shows a higher capacity due to the reversible plating Li, shown in Figure 7d; however, the irreversible capacity becomes larger after 100 cycles, indicating more Li-plating due to the increase in resistance during the charging process with the increasing ratio of EC/EMC. When the cells were fully charged to 4.3 V after 100 cycles (shown in Figure 7c), the high polarization and the low capacity are attributed to the high ratio of EC/EMC at −20 °C. We also found the different states of charge (SOC) of graphite anodes when the cells were opened (shown in Figure 7e), rather than plenty of Li-dendrite on the surface of graphite. Comparing E5 and E6 as well as E9 and E10 shown in Figure S7, when the EC content is reduced to 10% by weight in the total solvent, the cells exhibit good cycling stability despite the electrolytes with or without CsPF6 additive. For those different properties of graphite∥NCA full cells with various electrolytes at −20 °C, we elaborate the following three

desolvation. Not surprisingly, this desolvation effect becomes especially pronounced at −40 °C and starts to dictate the entire Li+ migration process during the discharge of the Li-ion cell. The temperature dependence of cell impedances for E2 and E9 is plotted in Figure 6c, which demonstrates that the resistance from Li+ desolvation (Rdesolvation) in E2 increases at a much faster pace than that in E9 below −20 °C due to the higher EC content in the former and the increasing difficulty to strip off the EC molecules from Li+ at low temperatures. Although Li+ desolvation has been identified as the limiting step for Li+ migration at low temperatures, it should be pointed out that this step may be determined or at least affected by factors other than the electrolyte compositions; the particle size of the electrode (active reaction site, interface condition) or the driving force from interior of the cell may also contribute to the Li+ desolvation process, which needs to be further investigated. Cycling Stability at −20 °C. To further verify the desolvation effect of various electrolytes on low-temperature performances, the above electrolyte compositions were further tested under more rigorous conditions, where the graphite∥NCA full cells with these electrolytes were both charged and discharged at −20 °C (Figure 7a). The obvious improvements, no matter in initial discharge capacity or cycling stability, were achieved with the electrolyte compositions that contain reduced ratio of EC/EMC. When the EC content is 10%, the cells with E5 and E9 deliver the initial discharge capacity of 139 and 142 mAh g−1, respectively, which were F

DOI: 10.1021/acsami.7b13887 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

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 the 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.

aspects for the understanding about the Li-ion battery performances at low temperatures. (1) For reversible discharge capacity delivery, Li-plating occurs during the charging process at low temperature and gets consumed during the cycling process, resulting in the irreversible capacity loss, whereas the discharge capacity is mainly determined by the Li+ desolvation resistance especially for low ratio of EC/EMC; (2) for cycling stability, the low resistance is responsible for less irreversible Li loss during charge processes. After the deposited Li is consumed, the cathode and anode will be operated at nonfull charge/discharge state, which is helpful for cycling at low temperatures; (3) for efficiency, the high efficiency of cells with E5 and E9 during cycling indicates that the electrochemical system is stable and the energy barrier of charging and discharging approaches an equilibrium, further suggesting that the dominant resistance and energy-consuming step of Li+ migration during charging and discharging processes should be similar at low temperature.

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CONCLUSIONS In this work, using combined techniques of analysis and cellcomponents switching, we successfully identified that Li+ desolvation process is the major kinetic barrier to Li+ transport at low temperatures rather than Li+ migration through the SEI or diffusion in solid electrodes. The study confirms the earlier hypothesis that the stripping of solvent sheath of Li+ is the most energy-consuming step for Li+ to enter the electrode interior, and this energy barrier becomes ever more pronounced as the temperature decreases below −20 °C. This fundamental understanding sheds light on a series of complicated interfacial processes that could universally happen in a variety of electrochemical devices. The efforts led to the development of a number of low-temperature electrolyte formulations that can support cell reactions down to −40 °C.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b13887. Comparison of discharge capacity of various electrolytes at different temperatures; ionic conductivities of E2 and E9; discharge curves; efficiency; cycling stability at −20 °C (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Dongping Lu: 0000-0001-9597-8500 Langli Luo: 0000-0002-6311-051X 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 work 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 G

DOI: 10.1021/acsami.7b13887 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.7b13887 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX