The present invention relates to formulations, designs and devices that pertain to novel electrolytes with high electrochemical stability and ionic conductivity at low temperature and the low-temperature batteries that they enable. The invention optimizes energy and power delivery of the battery with desirable thermal properties in battery electrolytes.
The advent of lithium-ion batteries (LIBs) has enabled the rapid development of portable electronics and electric vehicles due to their relatively high specific energy and long cycle life under mild operating conditions. Applications of LIBs have been expanding from the already-commercialized mobile phones and laptop computers to electric vehicles (EVs) and energy storage systems (ESSs). In accordance with the diverse applications of LIBs, the cells should be able to maintain their energy and power densities under a wide range of operating temperatures. However, typical LIBs encounter severe performance losses at sub-zero temperatures, primarily due to the carbonate-based liquid electrolytes, which hinders the deployment of high altitude drones and advanced electronics in space. In particular, power density suffers severely, where early studies observed that a commercial LIB retaining 5% of its energy density at −40° C. only retained 1.25% of its power density.
External battery warming systems are typically applied to circumvent this issue, where device charging in harsh environments can be aided by additional components that have recently been demonstrated with minimal impact to total cell mass. However, these heating systems are untenable for discharge as a standalone system due to their non-negligible startup times, requiring a source of high current for initial activation.
Furthermore, many devices requiring low temperature discharge do not require warming systems for charging due to the nature of their operation (e.g., high-altitude drones are charged at low altitudes). For these reasons, the field has traditionally focused on performance retention at low-temperature during discharge after charging under mild conditions.
Approaches to improve low-temperature discharge performance generally focus on the battery electrolyte, which facilitates the transport of ions between electrodes and governs a set of discrete cell impedance contributors. For a typical discharging LIB, these contributions are generally summarized as: 1) migration through the solid-electrolyte-interface (SEI), 2) bulk ionic transport, and 3) charge-transfer, which is dominated by Li+ de-solvation at the positive electrode interface. These resistances are all significantly exacerbated at temperatures <−20 ° C. and at high discharge rate, where Li+ de-solvation in particular has been observed to dominate the capacity fade. In general, these resistances have been minimized by employing low melting-point solvents, and novel salt additives. While progress has been made, it is important to note that these advancements have primarily been applied to conventional LIBs containing transition-metal oxide electrodes. These electrodes operate via the “rocking chair” mechanism, where Li+ diffuses from negative electrode to positive electrode during discharge, undergoing solvation at the negative electrode interface and de-solvation at the positive electrode interface. As a consequence of this mechanism, sluggish Li+ desolvation is unavoidable during both charge and discharge of LIBs. This is not the case for dual-ion batteries (DIBs)—DIBs store both cations and anions during charge, which migrate back into the electrolyte during discharge. This “salt-splitting” mechanism intrinsically decouples solvation and desolvation, where ions undergo desolvation on both electrodes during charge and solvation on both electrodes during discharge, theoretically circumventing the desolvation barrier. Furthermore, the weak solvation of anions suggests that the de-solvation barrier would have little effect on the positive electrode kinetics. These factors indicate that DIBs have the potential to kinetically outperform LIBs at extremely low temperature.
Anion-storing DIB positive electrodes typically fall into the following categories: anion doping in polymers, anion insertion in metal-organic frameworks (MOFs), halogen conversion in graphite, and anion intercalation in graphite. Polymers have recently shown promise for low temperature devices, however the poor volumetric energy density and low electronic conductivity of polymer electrodes hinders their widespread adoption. Furthermore, halogen conversion has yet to be realized in organic electrolytes, and the unclear solvation/desolvation mechanics at the electrode interface of polymers and MOFs overcomplicates a direct comparison between DIB and LIB mechanisms. The well-known intercalation-based graphite positive electrode would be an ideal candidate to make such a comparison, however until now, an electrolyte system with adequate oxidative stability and low temperature ionic conductivity has not been demonstrated.
Improving the low temperature operation of rechargeable batteries is vital to the operation of electronics in extreme environments, where systems capable of high-rate discharge are in short supply. In a first implementation, the inventive approach employs a holistic design of dual-graphite batteries to circumvent the sluggish ion desolvation process found in typical lithium-ion batteries during discharge. These batteries were enabled by the novel electrolyte, which simultaneously provides high electrochemical stability and ionic conductivity at low temperature. The dual-graphite cells, when compared to industry-type graphite∥LiCoO2 full-cells demonstrated an eleven-fold increased capacity retention at −60° C. for a 10C discharge rate, indicative of the superior kinetics of the “dual-ion” storage mechanism. These trends are further supported by GITT and EIS measurements at reduced temperature. The inventive approach provides a new design strategy for extreme low-temperature batteries.
Many solvents with wide liquid-phase temperature ranges, such as methyl propionate (−88° C. to 80° C.) lack the electrochemical stability to be employed at high volume content in battery electrolytes. The inventive approach solves this issue for multiple low-temperature electrolyte solvents through the addition of a low percentage (˜5% to —30%) additive to the primary ester-based solvent. In the examples described herein, a low percentage, e.g., 10% fluoroethylene carbonate (FEC) allows for superior thermal and electrochemical properties to be maintained by the primary solvent, an ester such as methyl propionate (MP), methyl butyrate (MB), methyl acetate (MA), methyl formate (MF), ethyl propionate (EP), ethyl butyrate (EB), propyl butyrate (PB), etc., or combinations thereof, while benefitting from high electrochemical stability provided by FEC in small amounts. It is believed that, in the prior art, the highest content of similar low-temperature solvents (melting point less than −50° C.) is 80% by volume (reported by Smart, et al (DOI:10.1016/j.jpowsour.2006.10.038). U.S. Pat. No. 9,293,773 of Smart et al. (incorporated herein by reference) discloses ester-based electrolyte systems for use in electrochemical cells, however, the reported results only described contents of up to 70% of the ester. A prototype battery system using these electrolytes was assembled, providing a capacity retention of 50% at −60° C. with a 0.5C discharge rate. In contrast, an analogous battery system employing the inventive electrolyte provided 67.5% retention under the same conditions with significantly higher voltage retention. Furthermore, the inventive approach is not only applicable to ester solvents investigated by Smart, et al. where a similar effect has been observed in other wide temperature solvents such as 3-methoxypropionitrile, which is unstable in mixtures of 100% and stable in mixtures with 10% fluoroethylene carbonate.
A second implementation of the inventive low-temperature electrolyte solvent involves substituting the primary solvent MP with its fluorinated counterpart, methyl 3,3,3-trifluoropionate (MTFP), which has previously been applied to high voltage systems as a low-percentage additive.
In still another implementation of the inventive concept of a low-temperature electrolyte solvent blend, stable charge and discharge cycling of LiNi0.33Co0.33Mn0.33O2 (NMC111)∥graphite pouch-type full cells at temperature as low as −40° C. is achieved using an ester-based electrolyte consisting of MP: FEC (90:10 vol. %) with 1 M LiPF6 (referred to herein as “M9F1”). Use of the novel electrolyte provided significantly enhanced electrochemical cycling performance at −20° C., far superior to the industry standard carbonate electrolytes. It also maintains 60% capacity retention at −40° C. compared with room temperature charge and discharge operation.
In one aspect of the invention, a method for enhancing electrochemical stability of a battery electrolyte in a lithium battery includes adding ethylene carbonate (EC) in an amount of about 5% to about 30% to a primary solvent. The primary solvent may be selected from the group consisting of methyl propionate (MP), methyl butyrate (MB), methyl acetate (MA), methyl formate (MF), ethyl propionate (EP), ethyl butyrate (EB), and propyl butyrate (PB). In some embodiments, the primary solvent is methyl propionate (MP) and a ratio of MP to EC is 90:10 vol. %. The ethylene carbonate may be fluoroethylene carbonate (FEC). The lithium battery may be selected from the group consisting of graphite∥graphite dual-ion batteries (DIB), Li-sulfurized polyacrylonitrile (Li∥SPAN) batteries, graphite∥LiMnxNiyCozO2 (NMC) batteries, graphite∥LiCoO2 batteries, graphite∥LiMn2O4 batteries, and graphite∥LiFePO4 batteries. The lithium battery may be a lithium ion battery, where the lithium ion battery is graphite anode and a cathode selected from LiCO2, NMC, LiMn2O4, LiFePO4 or a combination thereof, or a lithium metal battery comprising a graphite anode and a cathode selected from LiCO2, NMC, LiMn2O4, LiFePO4, SPAN, other transition metal oxides, sulfides, fluorides or combinations thereof. The battery electrolyte may be 1M to 2 M LiPF6 in a solvent comprising MP with 10% (v/v) FEC.
In another aspect of the invention, an electrolyte for a lithium battery includes a lithium salt in a solvent having an ester primary solvent and a low percentage of about 5% to about 30% of fluoroethylene carbonate (FEC). The ester primary solvent may be selected from the group consisting of methyl propionate (MP), methyl butyrate (MB), methyl acetate (MA), methyl formate (MF), ethyl propionate (EP), ethyl butyrate (EB), and propyl butyrate (PB). In some embodiments, the primary solvent is methyl propionate (MP) and a ratio of MP to FEC is 90:10 vol. %. The lithium battery may be selected from the group consisting of graphite∥graphite dual-ion batteries (DIB), Li-sulfurized polyacrylonitrile (Li∥SPAN) batteries, graphite∥LiMnxNiyCozO2 (NMC) batteries, graphite∥LiCoO2 batteries, graphite∥LiMn2O4 batteries, and graphite∥LiFePO4 batteries. The lithium battery may be a lithium ion battery, where the lithium ion battery is graphite anode and a cathode selected from LiCO2, NMC, LiMn2O4, LiFePO4 or a combination thereof, or a lithium metal battery comprising a graphite anode and a cathode selected from LiCO2, NMC, LiMn2O4, LiFePO4, SPAN, other transition metal oxides, sulfides, fluorides or combinations thereof. The battery electrolyte may be 1M to 2 M LiPF6 in a solvent comprising MP with 10% (v/v) FEC.
In still another aspect of the invention, a lithium battery having enhanced electrochemical stability includes an electrolyte having a mixture of a primary solvent and an amount of about 5% to about 30% ethylene carbonate. In some embodiments, the electrolyte further includes a lithium salt, which may be LiPF6. The primary solvent may be selected from the group consisting of methyl propionate (MP), methyl butyrate (MB), methyl acetate (MA), methyl formate (MF), ethyl propionate (EP), ethyl butyrate (EB), and propyl butyrate (PB). In some embodiments, the primary solvent is methyl propionate (MP) and a ratio of MP to FEC is 90:10 vol. %. The lithium battery may be a lithium-ion battery or a lithium metal battery. Lithium ion batteries may be selected from the group consisting of graphite∥graphite dual-ion batteries (DIB), Li-sulfurized polyacrylonitrile (Li∥SPAN) batteries, graphite∥LiMnxNiyCozO2 batteries, graphite∥LiCoO2 batteries, graphite∥LiMn2O4 batteries, and graphite∥LiFePO4 batteries. In some embodiments, the lithium battery is a lithium metal battery comprising a graphite anode and a cathode selected from LiCO2, NMC, LiMn2O4, LiFePO4, SPAN, other transition metal oxides, sulfides, fluorides or combinations thereof. The battery electrolyte may be 1M to 2 M LiPF6 in a solvent comprising MP with 10% (v/v) FEC.
The electrochemical stability provided by fluoroethylene carbonate to electrolytes based on previously unstable solvents likely operates via the formation of stable fluorine-rich interfaces where fluoroethylene carbonate decomposes to form organic fluorides and metallic fluorides, such as Li-F on the surface of the battery electrodes. This mechanism has been outlined in multiple battery publications, however, it is believed that this is the first time it has been applied as a stabilizing agent for low-temperature batteries in order to maximize the content of other previously unstable solvents.
Potential commercial applications of the inventive approach include batteries and super capacitors operating in low-temperature environments. The approaches described herein offer a new avenue for high-performance LIBs capable of ultra-low-temperature charging/discharging operation.
Carboxylate esters have been previously considered a promising additive solvent candidates for low-temperature LIB electrolytes due to their beneficial physiochemical properties. For example, ethyl acetate (EA) was previously chosen for its low freezing point (Tf) and suitable dielectric constant, which produces high ionic conductivity at ultralow temperature (−70° C). when paired with 1M LiTFSI. However, the cathodic stability was constrained to 1.5V vs. Li/Li+, suggesting that this electrolyte may not be capable of utilizing graphite as an anode. While higher-concentration carboxylate ester-based electrolytes were able to expand the reduction stability to ˜0 V (Li/Li+), the compatibility of such systems with full cell LIBs over long term cycles at low temperatures has rarely been reported.
To evaluate the inventive approach of employing an additive to improve the electrolyte's electrochemical stability, methyl propionate (MP), a common carboxylate ester, was chosen as the primary solvent based on its sufficient dielectric constant and low melting point. Table 1 below lists the properties of a number of potential solvents for use in lithium batteries, including, but not limited to, graphite∥graphite dual-ion batteries (DIB s), Li-sulfurized polyacrylonitrile (Li∥SPAN) batteries, graphite∥LiMnxNiyCozO2 batteries, graphite∥LiCoO2 batteries, graphite∥LiMn2O4 batteries, and graphite∥LiFePO4 batteries. While previous ester systems have been demonstrated, the acetate family has generally been associated with poor reductive stability in addition to problematic high-temperature characteristics. MP is particularly attractive for low temperature electrolytes because of its low freezing temperature (−87.5° C.); relatively high boiling point of 79.8° C., and low viscosity (0.43 cP)
Other candidates for uses as a primary solvent include methyl butyrate (MB: Tf=−95° C.; Tb=102° C.), methyl acetate (MA: Tf=−98° C.; Tb=134.8° C.), methyl formate (MF: Tf=−99° C.; Tb=31.8° C.), ethyl propionate (EP: Tf=−73.6° C.; Tb=98.9° C.), ethyl butyrate (EB Tf=−93° C.; Tb=120° C.), and propyl butyrate (PB: Tf=−95.2° C.; Tb=142° C.).
A number of lithium salts are known for use in lithium batteries including LiPF6, LiTSFI, LiFSI, or combinations thereof. LiPF6 was selected as the salt for the present testing of the novel solvent combination due to the established viability of PF6− intercalation in graphite, and 2 M concentration was selected to satisfy the concentration requirements of DIBs without producing a high viscosity. As shown in
The aforementioned electrolyte design principles are perhaps best exemplified in the measured ionic conductivity with decreasing temperature, plotted in
For battery electrode preparation, synthetic graphite (average diameter: 21 μm; surface area: 4.2 m2 g−; >99.5% purity) and LiCoO2 (LCO, average diameter: 9 μm; surface area: 0.2-0.4 m2 g−1; >99% purity) powders were purchased from MTI Corporation. The homogenous positive electrode slurries were prepared by mixing graphite, super-P (TIMCAL, >99%), and sodium carboxymethyl cellulose (CMC, Mw ˜90,999 Sigma Aldrich) (8:1:1 in mass ratio) in deionized water, or LCO, super-P, and polyvinylidene fluoride (PVDF, KYNAR 2800) (8:1:1 in mass ratio) in N-methyl pyrrolidone (NMP, Sigma Aldrich >99.5%) for use in dual-ion and lithium-ion batteries, respectively. Similarly, graphite negative electrode slurries were prepared by mixing the same MTI graphite, carbon black (Alfa Aesar, >99.9%), and PVDF (8:1:1 in mass ratio) in NMP. Positive electrode and negative electrode slurries were spread on Al and Cu foil, respectively and dried overnight under vacuum at 80° C. The same graphite powder was applied in positive and negative electrodes to minimize the effects of particle morphology in an attempt to isolate the effect of the storage mechanisms. For electrolyte preparation, fluoroethylene carbonate (FEC, battery grade) and ethyl methyl carbonate (EMC, battery grade) was provided by CapChem, LiPF6 (99.99%) and ethylene carbonate (EC, >99.9%) was purchased from BASF, and methyl propionate (MP, 99%) and 1 M LiPF6 in ethylene carbonate/diethyl carbonate (EC/DEC 1:1, battery grade) were purchased from Sigma Aldrich. Low-temperature electrolytes were made by dissolving stoichiometric amounts of LiPF6 in the selected solvents under stirring.
To evaluate the electrochemical performance, the prepared electrodes were fabricated into two-electrode CR-2032 type coin cells. Half-cells were evaluated with the electrolyte of interest using a Celgard 2400 poly-propylene membrane as the separator and Li metal (China Energy Lithium Co., 99.9%) as the counter electrode, where the working electrode had a typical mass loading of 3 mg cm−2. For full-cells, the negative: positive electrode active mass ratios of 1:1.75 (N/P=1.5) and 1:2.3 (N/P=2.0) were employed for graphite∥LCO and graphite∥graphite cells, respectively with a typical negative electrode active mass of ·1.6-1.8 mg cm−2. Graphite∥LCO full cells used Celgard 2400 separators (flooded, 75 μL) while graphite∥graphite cells used glass fiber separators flooded with 150 μL of electrolyte. Theoretical capacities of LCO and graphite positive electrodes were taken to be 140 mAh g−1 and 80 mAh g−1, respectively for C-rate determination. In the case of dual-graphite full cells, a pre-lithiation process was applied to the graphite negative electrode, similar to methods found in literature. The negative electrode was first assembled in a half-cell, cycled once at 0.1 C and then discharged to 0.3 V vs. Li/Li+ and held at the potential for 1 hour to stabilize the SEI before disassembly and subsequent reassembly in the full-cell. This method was also applied to graphite∥LCO cells to avoid any difference in SEI formation mechanics for low-temperature tests. All charge/discharge testing was performed on Neware BTS 4000 and Arbin LBT-10V5A systems. Error bars centered on average values for room temperature and low temperature data was based the standard deviation of 3 and 2 cells, respectively.
To evaluate DIBs and LIBs at low temperature we applied the 2 M LiPF6 MP 10% of FEC electrolyte in graphite∥graphite and graphite∥LiCoO2 (LCO) full-cells. Their charge storage mechanisms are shown in
The cycling performance of these cells are shown in
To simulate low temperature device operation, the assembled full-cells were then subjected to testing using a method similar to previous studies. At −40 ° C. with a 1 C discharge rate the graphite∥LCO LIB was found to retain 72.4% of its room temperature capacity, which decreased to 63.2% at −60° C. (
These trends notwithstanding, DIBs typically struggle to match LIBs in specific energy at a cell level due to the higher electrolyte volume demanded. Table 2 provides a summary of calculated average specific energies and standard deviations of the graphite∥LCO full-cells based on methods from Betz, et al. Table 3 summarizes calculated average specific energies and standard deviations of the graphite∥graphite full-cells. Nonetheless, DIBs could be ideal for applications requiring high power output at extremely low temperature due to the sluggish kinetics of LIBs.
To better understand the remarkable kinetic performance of the DIB at extremely low temperature, further electrochemical characterization was performed on the graphite and LCO positive electrodes to isolate the source of the two storage mechanisms.
The electrochemical stability of the electrolytes was investigated in coin cells using Cyclic Voltammetry (CV) of a stainless-steel working electrode vs. a Li metal counter on a Biologic VSP-300 potentiostat, which was also used to perform CV of the half-cells. For low temperature electrochemical tests, the full-cells were first cycled 5 times at room temperature with a rate of 1C. The cells were then charged at 1 C and held at their upper voltage cutoff for 30 minutes. The cells were then placed in the temperature chamber and allowed to rest for 1 hour to reach the set temperature, followed by discharging at various rates. The lower cutoff voltage was set at 1.5 V less than the end of full-cell's room temperature voltage plateaus. The SolidCold C4-76A ultra-low chest freezer with a secondary insulation chamber was used to maintain the cells at a specified temperature.
The galvanostatic intermittent titration technique (GITT) was applied to the positive electrode half-cells at a 1 C rate for current pulses of 60 seconds followed by resting periods of 30 minutes. For spherical particles and a short pulse time (r), the ion diffusion coefficient (D) was calculated based on:
where r is the particle radius of the positive electrode material, ΔEs is the change in steady-state voltage, and ΔEt is the change in transient voltage. The half-cells were cycled 5 times at room-temperature before GITT tests. For EIS testing the electrodes of interest were first cycled in Li half-cells 5 times and then brought to 50% state of charge
(SOC) before disassembly and subsequent reassembly into the symmetric arrangement. EIS was performed on half-cells using a Biologic VSP-300 potentiostat from 1 MHz to 100 mHz with an AC amplitude of 5 mV. The RCT values were obtained with the Z-Fit module in EC-Lab.
The theoretical specific energies of the full-cells were calculated in a method similar to the route suggested by Betz et al. (Adv. Energy Mater. 2019, 9, 1803170) excluding inactive materials found at the cell or stack levels. For the graphite∥LCO LIB system, the minimum electrolyte mass required to fill the positive and negative electrode and pores with an assumed porosity of 30% and 10 μm thickness. Because of the nature of the graphite∥graphite DIB mechanism, the electrolyte mass was determined based on the minimum electrolyte concentration required to satisfy the positive electrode capacity (80 mAh g−1 for a 2 M electrolyte at 1.2 g/mL), with a 15% excess in order to maintain viable ionic conductivity. These theoretical electrolyte mass requirements were found to be
for the DIB and LIB systems, respectively.
The galvanostatic intermittent titration technique (GITT) was applied to the two positive electrodes at room temperature and −60 ° C. The results of these tests are shown in
It should be noted that there are a few limitations to electrochemical characterization techniques. Specifically, the EIS equivalent circuit and GITT diffusion coefficient model may be insufficient to quantitatively differentiate the effects of solid-state and desolvation effects during charge-transfer. Furthermore, the assumptions inherent in GITT such as negligible volume change, and dominant solid-state diffusion are problematic. While these factors may undermine the quantitative precision of these techniques, the qualitative comparison between the graphite and LCO positive electrodes clearly supports the performance data presented in this work, demonstrating the superiority of the DIB mechanism for low temperature applications.
To summarize, the foregoing demonstrates the kinetic superiority of the salt-splitting charge-transfer mechanism found in DIBs for low temperature operation. Using MP as the primary solvent with a low percentage of FEC provides a novel electrolyte with exceptional ionic conductivity at low temperature and high oxidative stability without compromising graphite negative electrode performance. This electrolyte used in practical dual-graphite and graphite∥LCO full-cells exhibited consistently greater capacity retention compared to the LIBs at the extremely low temperature of −60 ° C., even when their respective performances were comparable at room temperature. The preceding description illustrates the basic principles of the inventive approach of employing an MP-based ester electrolyte to confer high electrochemical stability and ionic conductivity at low temperature for use in low-temperature batteries. The following examples further illustrate the benefits of the MP/FEC electrolyte with variations of the materials.
This example focuses on stabilization of LiNi0.8Co0.1Mn0.1O2, or NMC 811, cathodes, which has been of interest due to its high operating window (≤4.5V) and high capacity (>200mAh g−1), but which exhibits poor long-term cycling stability compared to cathodes with less Ni, e.g., NMC 523. This is generally believed to be a result of an increased level of parasitic electrolyte reactions at high voltages, an increase level of gas generation, and interfacial phase transformation. Some of these issues have been mitigated using advanced electrolytes. Traditional electrolyte chemistries have not been shown to support high-voltage cathodes for LMBs at extremely low temperatures. The poor energy retention of LIBs at low temperatures has generally been attributed to the internal resistances associated with Li+ conduction through the bulk electrolyte, interfacial migration of the Li+ at the electrode interfaces, and Li+ desolvation. These resistances have been primarily addressed through improvements in the battery electrolyte employing novel salt additive, and employing low-melting point and/or low-polarizability solvents with low viscosity. In spite of this progress, a system providing high energy retention at low temperatures and stable performance for high-voltage cathodes and Li metal anodes has been elusive.
Expanding on the general principles described above relating to use of MP as the primary solvent with a 10% (v/v) FEC additive and a 1 M LiPF6 salt to form an electrolyte system (referred to as “M9F1”) capable of maintaining electrochemical stability with improved low-temperature performance, an all-fluorinated carboxylate ester-based electrolyte was employed by substituting MP with its fluorinated counterpart, methyl 3,3,3-trifluoropionate (MTFP), which has previously been applied to high-voltage systems as a low-percentage additive. The substitution of fluorine groups is known to decrease the HOMO energy of molecules due to the high ionic potential, high electronegativity, and low polarizability of the fluorine atom, resulting in an increased resistance to oxidation. Applying a 1 M LiPF6 in a MTFP/FEC (9:1) electrolyte, i.e., M9F1, simultaneously provided high-voltage cathode and Li metal anode reversibility at room temperature, which can be attributed to the production of fluorine-rich interphases formed in the MTFP-based system. Furthermore, the all fluorinated electrolyte provided 161, 149, and 133 mAh g−1 when discharged at −40° C., −50° C., and −60° C., respectively, far exceeding the performance of the commercial electrolyte, and demonstrating the capability of high-voltage operation at ultra-low temperatures.
As shown in
As previously stated, designing an electrolyte for ultra-low-temperature batteries requires the use of solvents with a low melting point and low viscosity, yielding a high ionic conductivity at low temperatures.
For battery electrode preparation, LiNi0.8Co0.1Mn0.1O2 (NMC 811) powder was purchased from Targray Technology International, Inc. The cathode slurries were prepared by mixing NMC 811, super-P, and polyvinylidene fluoride (PVDF, KYNAR 2800) (8:1:1 mass ratio) in N-methyl pyrrolidone (NMP, Sigma Aldrich >99.5%), which was then cast on aluminum foil and dried overnight under vacuum at 80° C. Blocking electrodes for electrolyte stability determination were prepared by mixing super-P and PVDF (9:1 mass ratio) in NMP, which was then cast on stainless steel and dried under vacuum at 80° C. overnight.
For electrolyte preparation, fluoroethylene carbonate (FEC, >98%) was purchased from Alfa Aesar. Methyl 3,3,3-trifluoroproprionate (MTFP, >98%) was purchased from TCI. LiPF6 was obtained from BASF. Methyl propionate (MP, >99%) and 1 M LiPF6 in ethylene carbonate/diethyl carbonate (EC/DEC 1:1, battery grade) were purchased from Sigma Aldrich. MP and MTFP solvents were dried over molecular sieves for at least 24 hours before use in electrolytes. Low-temperature electrolytes were made by dissolving stoichiometric amounts of LiPF6 in selected solvents under stirring.
A customized two-electrode coin cell was formed using two stainless steel electrodes spaced symmetrically between a PTFE (TEFLON™) washer with a thickness of 0.030 inc. A glass-fiber separator soaked with electrolyte was housed on the inside of the washer to constrain its surface area to a known dimension. To evaluate electrochemical performance, the prepared electrodes were fabricated into two-electrode CR-2032-type coin cells. Half-cells were evaluated with the electrolyte of interest using a Celgard 2400 poly-propylene membrane as the separator and Li metal (99.9%) as the counter electrode, where the working electrode had a typical mass loading of 0.84 mAh cm−2. Full-cells were evaluated in a similar fashion, where the limited Li metal anode was prepared via electrodeposition in in Li∥Cu coin cells prior to application in full cells, where the plated Li anode was paired with a NMC811 cathode of 1.3 mAh cm−2 loading. The theoretical capacity of NMC 811 was set to 200 mAh g−1 for the C-rate determination.
To test the practical stability of MP- and MTFP-based electrolytes, cycling tests of NMC811∥Li half-cells were conducted with an aggressive cut-off voltage of 4.5 V to exploit the high capacity of the nickel-rich cathode. As shown in
To demonstrate the advantage of the designed ester electrolytes in low temperatures, NMC 811∥Li half-cells were tested using a method in which the cells were charged at room temperature followed by discharge at low temperatures to simulate practice device applications. Without intending to be bound by theory, the primary factors in low-temperature performance degradation are believed to be the reduced bulk ionic conductivity of the electrolyte and a severe reduction in charge transfer kinetics, which has been suggested to be dominated by Li+ desolvation. In terms of these metrics, the inventive ester electrolytes appear to be superior in every respect based on the high retention of ionic conductivity at ultralow temperatures and the introduction of FEC, which has been noted to have a Li+ solvation energy significantly lower than that of EC, allowing for facile desolvation. The voltage profiles for tests in NMC 811∥Li half-cells are displayed in
This example evaluates the effect of the inventive ester electrolyte chemistry on low-temperature charge and discharge behavior in NMC∥graphite type full cells.
As described above, the ionic conductivities (σi) of the ester-based electrolyte consisting of MP:FEC (90:10 vol. %) with 1 M LiPF6 (“M9F1”) are significantly improved compared with that of the conventional ethylene carbonate (EC) electrolytes, here identified as LP40 and LP30 (LP40=1 M LiPF6 in EC: DEC (5:5 vol. %); LP30=1 M LiPF6 in EC: DMC (5:5 vol. %)), especially at temperatures below −20° C. In
LP30 displayed a freezing point at −21.9° C., whereas LP40 was found to exhibit two noticeable endothermic peaks at ˜0.5° C. as well as −18.1° C. These phase transitions have been previously observed in the EC/DEC systems, where the initial peak corresponds to the initial solidification of EC, while we believe the secondary peak at −18.1° C. could be attributed to further internal ordering of the solid similar to a crystallization. The single peak observed in the EC/DMC system is likely due to the significantly closer freezing points of the two solvents as compared to the large difference between EC and DEC (see Table 1 above). The significantly steeper ionic conductivity degradation observed in LP30 is likely indicative of this trend, as liquid DEC is likely still present in the LP40 system below 0° C. These DSC results are also supported through observation of the electrolytes after storing at −20 ° C. and −80° C. for 12 hours. 1 mL of electrolyte solution was added to each glass vial. As shown in
NMC111∥graphite pouch-type full cells were assembled for electrochemical performance assessment. Briefly, as is known in the art, pouch cells are layered structures in which the electrodes and separators are retained within flexible foil enclosures. These pouch-type full-cells were used to investigate the effects of our designed electrolyte and conventional electrolytes on battery cell performances. Cathode composition=LiNi1/3Mn1/3Co1/3O2/PVdF (Kynar® HSV 1800)/carbon black (Super-P) at 80:10:10 (wt. %); cathode areal capacity=1.5 mAh cm−2, anode composition=graphite (GCP-80)/PVdF (Kynar® HSV 1800)/carbon black (Super-P) at 90:5:5 (wt. %); anode areal capacity=1.7 mAh cm−2. Slurries of positive and negative electrodes were coated on aluminum and copper foils, respectively and dried overnight under vacuum at 80° C. The sizes of the cathode and anode were 44 mm×57 mm and 45 mm×58 mm, respectively. A microporous polyethylene separator (PE) (Celgard 2400) was placed between a cathode and an anode.
The designed electrolyte of this work is 1M LiPF6 in a mixture of methyl propionate (MP, 99%; Sigma Aldrich) and fluoroethylene carbonate (FEC, >98.0%; TCI Chemical) at 90:10 (vol. %), i.e., M9F1. Both LP40 and LP30 were used as control electrolytes. LP40=1M LiPF6 in EC/DEC at 50:50 (vol. %) (Sigma Aldrich); LP30=1M LiPF6 in EC/DMC at 50:50 (vol. %) (Sigma Aldrich).
For the rate capability test at room temperature, the pouch cells were galvanostatically charged to 4.3 V and then held potentiostatically until the current reached 5% of the charge current. Subsequently, the cells were galvanostatically discharged to 2.7 V at different C-rates. For low-temperature cycles, the pouch cells were galvanostatically charged to 4.3V and then potentiostatically charged at the same voltage under room temperature, and then galvanostatically discharged to 2.7 V after switching the temperature to either −20° C. or −40° C. After discharge, the cells were repeatedly charged and discharged at same temperature condition at either −20° C. or −40° C. Battery testers (Neware BTS-4000 and Arbin LBT-10V5A) were used for electrochemical cycling.
The discharge capacity of the cell was found to be 157.9 mAh g−1 (based on the cathode mass) for M9F1 with clear SEI formation peaks in the initial formation cycle, which was similar to the cells using LP30 and LP40. Next, the discharge rate capability test of the cells was assessed after charging all the cells at 0.1C to observe any variance in kinetic behavior between the electrolytes at room temperature. It was found that M9F1 delivered a discharge capacity of 80 mAh g−1 at the discharge rate of 10C, as shown in
This phenomenon was further investigated by electrochemical impedance spectroscopy (EIS) measurement of the three discharged cells between R.T. and −20° C., respectively, at 0% stage of charge (SOC). The impedance spectra were obtained between 1 mHz to 1 MHz with an AC amplitude of 10 mV at both room temperature and −20° C. For the EIS of three-electrode pouch cells, the cells were pre-cycled for 5 times and then brought to 50% state of charge (SOC). A small, thin lithium metal strip was used as a reference electrode and placed next to the cathode and the anode. The impedance spectra were obtained from 5 mHz to 200 MHz, and from 1 mHz to 1 MHz for room temperature and −20° C. operation, respectively. All impedance spectra were collected by EC-Lab, and component values were obtained by fitting the spectra with a Z-Fit module in EC-Lab. Three-electrode pouch cells subject to low temperature discharge were assembled in small pouch cells, composed of 12-mm diameter anode and cathode electrodes separated by 2 Celgard separators in between which a Li metal reference electrode was placed. These cells were charged at C/5 at their temperatures of interest until the graphite was fully lithiated (0V) before discharge measurements. Average polarization was calculated by subtracting the room temperature voltage of each electrode from their −20° C. voltage at equivalent capacities. The EIS results are shown in
With the assessment of any prospective electrolyte for use in LIBs, it is also vital to address concerns regarding operational reversibility under standard conditions. As such, long cycling tests were conducted in NMC111|graphite pouch-type full cells at room temperature. The cycling test was conducted on 0.1C for 5 cycles, 0.2C for 10 cycles, 0.5C for 10 cycles, and 0.2C for 100 cycles. The first closed circle at each C-rate is the first discharge capacity after charge at room temperature. Following open circles are the repeated charge and discharge cycling after discharge at the same rate at −20° C. As shown in
NMC111∥graphite pouch-type full cells were then discharged at −20° C. after they were fully charged at R.T. and a 0.1C rate. Referring to
Additionally, the LP30 and LP40 electrolytes did not exhibit any significant capacity at the aforementioned higher rates. To the contrary, highly stable capacity retention was recorded at the same operating conditions in the cells employing M9F1. Assessing the last 100 cycles at 0.2C, 92% of the cell capacity was retained (i.e., 88 mAh g−1 of 100th, 95 mAh g−1 of 1st cycle). The retained capacities at −20° C. and each C rate is summarized in
To further confirm the improved low-temperature performance of the cells containing M9F1 during operation, EIS measurements of three-electrode cells was carried out to record the resistances without influence from the counter electrode (CE). In our system, a strip of lithium metal was used as the reference electrode (RE) in the pouch cells, and the EIS was measured at 50% SOC after 5 cycles at 0.1C to ensure the stable formation of SEI and CEI layers. Referring to
The tables shown in
Three-electrode cells were also investigated for low temperature discharge capability. It was found that the graphite's average polarization between room temperature and −20° C. was 171 and 122 mV for LP30 and M9F1, respectively. The NMC 111, on the other hand showed substantially increased polarizations of 677 and 241 mV for LP30 and M9F1, respectively, which supports the trends revealed by EIS. However, it is important to note that while the polarization of the graphite anode is indeed lower than the NMC 111 cathode, its thermodynamic proximity to Li metal indicates that even small changes in the anode overpotential may result in Li metal plating.
X-ray photoelectron spectroscopy (XPS) was then conducted on the NMC111 cathodes and graphite anodes after cycling at −20° C. to probe the differences in interphase composition. X-ray photoelectron spectroscopy (XPS) was performed with an AXIS Supra DLD XPS by Kratos Analytical, and XPS spectra were corrected using a monochromatized Al Kα radiation (λ=0.83 nm and hv=1,486.7 eV) under a base pressure at 10−9 Pa. The samples were directly transferred into the XPS chamber from a nitrogen-filled glovebox to avoid moisture and air exposure. Argon plasma of 5 keV was used for etching the electrode surface with a pre-etching for 5 s, etching for 120 s.
Morphologies on the electrodes were examined by a scanning electron microscopy (SEM) on a Bruker X8-ApexII CCD diffractometer equipped with Mo Kα radiation at kV and 50 mA. Before investigating, all electrode samples were carefully washed by dimethyl carbonate (DMC) to remove the residual electrolyte solvents and salts from the electrodes, which was conducted in an argon-filled glovebox (Mbraun; <0.1 ppm O2 and H2O). The XPS results are shown in
As observed in the comparison of O 1s spectra (
From the viewpoint of cathode surface (
Furthermore, it was found that the resistive nature of the graphite anode encouraged lithium metal plating on the surface during low temperature charging due to its increased polarization. The amount of deposited lithium and the speed of its growth can be controlled by (1) Li+ transport limitations, and (2) the interphasial kinetic resistance due to its inhomogeneity, which are exacerbated at sub-zero temperatures. Through the application of scanning electron microscopy (SEM), it was observed that the graphite anodes cycled at −20° C. in LP30 and LP40 presented a significant amount of mossy and dendritic Li (
Cell gassing testing was performed through three-layer pouch-type full cells (cell capacity ˜120 mAh) and the gas volume was measured using the Arrhenius method according to Dahn et al. Cells were weighed while submerged in deionized water (18MΩ cm) before and after the experiment. The difference in weight of the pouch cell while submerged in water is then directly proportional to the volume of gas produced. The volume of the gas generated after cycling is then calculated by the following equations:
It was found that after 8 cycles at 0.1C, the cell employing M9F1 ester electrolyte generated only 0.042 mL of gas compared to 0.1532 mL generated by the carbonate system. See Table 6 below.
The importance of the low-percentage FEC content was also observed via the gassing and cycling comparison between the M9F1 system, and a M95F5 formulation (MP:FEC=95:5 in vol.), where the capacity was shown to sharply decay after only 5 cycles (
While the advantage of the M9F1 electrolyte is clear at −20° C., its ionic conductivity of 2.48 mS cm−1 of M9F1 at −60° C. indicates that this electrolyte may enable stable performance at even lower temperatures (
The pouch cells employing M9F1 were also cycled at −40° C., which produced a discharge capacity of 109 mAh g−1 at a 0.1C rate. Furthermore, when cycled, the M9F1 cell was able to display a 94.5% and 96.3% capacity retention under 0.05C/0.05C, and of charge and discharge, respectively, as well as maintaining stable coulombic efficiencies. These results are shown in
The capacity retention as well as Coulombic efficiency of NMC111∥graphite pouch-type full-cells can be significantly improved in the presence of an MP-based ester electrolyte, M9F1, at the sub-zero temperatures of −20° C. and −40° C. This electrolyte enables ionic conductivity at low-temperatures with an extremely low freezing point and LiF-rich interface layers on the cathode and anode, which facilities charge transfer at both charge and discharge cycling. The 1M LiPF6 in MP: FEC electrolyte was effective in: (1) enhancing rate capability at room-temperature and outstanding discharge properties at low-temperatures even at −40° C., and (2) protecting the cathode and anode by suppressing thick SEI layer formation and metallic Li deposition. Based on the above-described testing of the ester-based electrolyte in different battery types, the inventive electrolyte formula can be used to improve the low-temperature performance of a variety of electrode chemistries.
This application claims the benefit of the priority of each of U.S. Provisional Application No. 63/093,043, filed Oct. 16, 2020, and U.S. Provisional Application No. 63/183,030, filed May 2, 2021, each of which is incorporated herein by reference in its entirety.
This invention was made with government support under Space Technology Research Grant No. 80NSSC18K1512 awarded by the National Aeronautics and Space Administration. The government has certain rights in the invention.
Filing Document | Filing Date | Country | Kind |
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PCT/US2021/055478 | 10/18/2021 | WO |
Number | Date | Country | |
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63183030 | May 2021 | US | |
63093043 | Oct 2020 | US |