benzene (DBBB

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Improving the Safety of Lithium-ion Battery via a Redox Shuttle Additive 2,5-Di-tert-butyl-1,4-bis(2-methoxyethoxy)benzene (DBBB) Olatz Leonet, Luis Colmenares, Andriy Kvasha, Mikel Oyarbide, Aroa Mainar, Tobias Glossmann, J. Alberto Blazquez, and Zhengcheng Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01298 • Publication Date (Web): 06 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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ACS Applied Materials & Interfaces

Improving the Safety of Lithium-ion Battery via a Redox Shuttle Additive 2,5-Di-tert-butyl1,4-bis(2-methoxyethoxy)benzene (DBBB) Olatz Leonet1, Luis C. Colmenares1, Andriy Kvasha1, Mikel Oyarbide1, Aroa R. Mainar1, Tobias Glossmann2,3, J. Alberto Blázquez1*, and Zhengcheng Zhang4* 1

2

CIDETEC, Pº Miramón, 196, 20014 Donostia-San Sebastián, Spain

Mercedes-Benz Research & Development North America, Inc., 12120 Telegraph Road, Redford, MI 48239, USA 3

4

Department of Chemistry, Oakland University, Rochester, MI 48309, USA

Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL60439, USA

*Corresponding authors: [email protected] (J. Alberto Blázquez); [email protected] (Zhengcheng Zhang) ABSTRACT 2,5-Di-tert-butyl-1,4-bis(2-methoxyethoxy)benzene (DBBB) is studied as a redox shuttle additive for overcharge protection for a 1.5 Ah graphite/C-LFP lithium-ion pouch cell for the first time. The electrochemical performance demonstrated that the protecting additive remains inert during the extended standard cycling for 4000 cycles.

When a 100% overcharge is

introduced in the charging protocol, the baseline cell fails rapidly during the first abusive event whereas the cell containing DBBB additive withstands 700 overcharge cycles with 87% capacity retention and no gas evolution or cell swelling was observed. It is the first time to demonstrate the effectiveness of the DBBB as overcharge protection additive in a large pouch cell format. KEY WORDS Lithium-ion batteries, Redox shuttle additives, Overcharge protection, Pouch cell, Safety 1 ACS Paragon Plus Environment

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

HIGHLIGHTS •

1.5 Ah graphite/C-LFP pouch cell withstands 700 overcharge cycles.

•

Long-lasting overcharged pouch cell with neither swelling nor shape change.

•

Capability of overcharge protection for any type of cell format with C-LFP as cathode material.

2 ACS Paragon Plus Environment

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1. Introduction

Lithium-ion batteries (LIBs) are worldwide implemented in diverse fields including consumer electronics, transportation and stationary energy storage. Among LIB chemistries, a commonly used cathode material is the carbon-coated lithium iron phosphate (C-LiFePO4, henceforth CLFP) with an olivine structure due to its enhanced safety, low toxicity and suitable cost. LIBs using this cathode material have generally shown long-lasting life and improved abuse tolerance in comparison with other high voltage LIBs. These characteristics have made LFP cells suitable for many applications.1

LFP material has been demonstrated to be safer than the layered oxide and spinel material due to its stronger iron-phosphate-oxygen bond. However, recent abuse studies indicated the rapid heat generation could trigger the decomposition and combustion of the flammable organic electrolyte as well as the breakdown of the solid electrolyte interface (SEI) on the graphite surface. Among the abuse conditions, overcharging is a common electrical abuse which activates and promotes self-accelerating chemical and electrochemical reactions between battery components,2 leading to gas evolution,3-4 rapid increase in cell temperature,5-7 and thermal runaway. Under such unintended overcharge conditions, the cathode potential increases beyond the electrochemical stability window of the electrolyte causing electrolyte breakdown and triggers excess Li-deposition on the negative electrode causing dendrite growth and circuit shorting. Battery systems such as Nickel Metal Hydride batteries take advantage of side reactions of aqueous solution at higher charging voltages providing an automatic shutdown mechanism.8 In the case of charge imbalance among these cells, a full charge cycle of the pack will equalize the



state-of-charge (SOC) of each cell. Such an intentional full charge can only be applicable if the impedance rise of the cells is trivial; otherwise cells with a lower SOC connected in series with the fully charged cells will not be able to be fully charged with minimum extra heat generation.

Currently, the overcharge is typically controlled by a battery management system (BMS) embedded in each single cell. Nonetheless, in large, high voltage battery packs consisting of hundreds of thousands individual cells, this method will be costly and compromises the overall energy density of the pack. Also, malfunction of the BMS could lead to a catastrophic event, a condition that may trigger the need of additional monitoring measures that will drive up system complexity and cost. An alternative solution is the development of redox shuttle additives9-12 to provide repeated overcharge protection of the individual cell. During the overcharge process, the additive converts the overcharge electricity mainly into heat to avoid the side reaction between the electrodes and the electrolyte at high voltages.13 These additives are recognized as suitable approach for high-energy and high-power lithium-ion batteries with the benefit of reduced cost, weight and volume of the battery pack.

Zhang et al.7,12,14 at Argonne National Laboratory has reported the overcharge abuse tolerance of LFP-based cells by a redox shuttle additive 2,5-di-tert-butyl-1,4-bis(2-methoxyethoxy)benzene (DBBB). The assessment of this additive has been performed and reported for hard packaging configurations such as coin cell or cylindrical cells,7 while its use in soft-packing cells has not been demonstrated so far. Soft-packing makes the most efficient use of space up to 90 ~ 95%, the highest among the entire cell formats. This advantage makes soft packaged cells, also known as pouch cells, more suitable for electrical vehicle application. Although elimination of the metal


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enclosure reduces the overall weight of the cell, the soft-packing battery is highly sensitive to any internal pressure change. Therefore, swelling triggered by overcharging becomes a major concern for soft-packaging cells. Although swelling issue is usually blamed for improper manufacturing process, the instability of electrolyte and side reactions at elevated temperatures triggered by the overcharge is one of the major sources. In this context, we evaluate the effectiveness of the overcharge abuse tolerance of graphite/CLiFePO4 pouch cells with a nominal capacity of 1.5 Ah with and without DBBB redox shuttle. With DBBB additive, no gassing or swelling was observed over long-term overcharging cycling. The data shown in this report confirm the capability of DBBB to be implemented in any type of LFP cell format. 2. Experimental

Lithium-ion pouch format cells with a stacked design (95x140 mm2) were manufactured at CIDETEC for this study. The cells were fabricated with a nominal capacity of 1.5 Ah using CLFP cathode and graphite anode. The pouch cell capacity was limited by the positive electrode with 10% excess of the negative electrode. Detailed description about electrode fabrication and cell assembly was previously reported.1 Graphite anode was prepared using artificial graphite powder with average particles size of 8 µm. C-NERGY Super C45 (IMERYS Carbon & Graphite) was used as conductive additive and battery grade styrene-butadiene rubber (SBR) latex was employed as binder. LFP cathode was fabricated using a micro-sized carbon-coated LiFePO4 powder with average particles size of 3 µm. C-NERGY Super C45 and graphite KS6L (both from IMERYS Carbon & Graphite) were utilized as conductive additives. Battery grade sodium carboxymethyl cellulose (Na-CMC) was used as disperser and thickener to regulate the 5 ACS Paragon Plus Environment


rheological behavior of the anode and cathode slurries. The cathode and anode slurries were then casted on aluminum and copper foil, respectively. Afterwards, the coated electrodes were dried in a convection oven at 60ºC until complete water evaporation. Finally, the dried electrodes were calendered to achieve 1.3 and 2.0 g/cc electrode density, respectively. The N/P ratio is 1.1. 1.2 M LiPF6 in ethylene carbonate (EC):ethyl methyl carbonate (EMC) (3:7 wt.%) was used as the baseline electrolyte. 0.4 M redox shuttle additive DBBB was dissolved in the baseline electrolyte and used as the overcharge protection electrolyte.12

Long-term cycling test using standard and overcharge cycling protocol has been performed using a BaSyTec cell test system at 24±1 °C. The cell was first charged at C/4 with a cut-off current of C/20, and then cycled at 1C for repeated charge and discharge. The cut-off voltage was 3.65-2.00 V. To evaluate the aging of the cell, the capacity and DC internal resistance (Rint) were evaluated periodically. To obtain Rint, the cell was charged at C/4 with a constant current/constant voltage (CC-CV) mode. Then, the cell was discharged (at C/4) to 50% of state of charge (SOC). After the cell rested for 10 minutes, a current pulse (discharged 10 s at 1C) was applied. The voltage drop was used to determine the value of the Rint. 3. Result and Discussions

The first prerequisite to serve as a redox shuttle additive is that it should not affect the normal cell operation cycled at regular cut-off voltage, therefore, LFP pouch cells with 0.4 M DBBB (DBBB Cell) and without (STD Cell) were first evaluated under standard cycling conditions. Figure 1 shows the capacity retention of the LFP cells with and without DBBB additive for 4000 cycles. The discharge capacity profiles for both cells are identical, indicating that DBBB additive


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does not exert any negative impact on the cell normal cycling performance. STD cell and DBBB cell have shown a capacity retention of ca. 87.4% for 4000 extended cycles with high Coulombic efficiency. Both cells also showed similar internal resistance trends (inset in Figure 1, normalized by Rint at 1st cycle – ca. 50 mΩ for both cells).

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Figure 1. Capacity retention and Coulombic efficiency of graphite/C-LFP pouch cells with (DBB Cell) and without 0.4 M DBBB redox shuttle additive (STD Cell). Inset: normalized Rint with cycling. Cycling condition: charge CC-CV - 1st cycle at C/4, following cycles at 1C charge and 1C discharge, charge current cut-off C/20, cut-off voltage: 3.65-2.00 V.

DBBB has its own redox potential dictated by its molecular structure. As reported by Zhang et al.,12 DBBB shows a reversible oxidation/reduction peak averaged at 3.95 V vs Li+/Li from the cyclic voltammetry experiment. To verify its positive response with the cell voltage, LFP cells with and without DBBB additive were intentionally charged to 200% SOC or 5.0 V whichever comes early with a current of C/4. Figure 2 illustrates the voltage evolution of the STD Cell and DBBB Cell during the overcharging process. For the STD cell, the voltage increases sharply 7 ACS Paragon Plus Environment


after the cell is fully charged or reaches the 100% SOC state and the charging was terminated by reaching the safety cut-off voltage of 5.0 V. The cell temperature remains stable at the normal charge process and rises up to 31 °C at 110% SOC, and then increases rapidly to 102°C at even 113% SOC, a slightly overcharged state. In contrast, the voltage of the DBBB cell is locked at 3.85 V with a flat plateau after the cell is fully charged and this plateau remains flat through the whole overcharge process. The potential of the LFP cathode at overcharge state is the same as the redox potential of DBBB, which was measured to be 3.95 V vs. Li+/Li.12 DBBB was oxidized by losing one electron on the LFP surface and the radical cation DBBB•+ diffuses to the graphite anode and gets reduced by accepting one electron from the anode. This continuous oxidationdiffusion-reduction process helps shunt the overcharge electrons between two electrodes and effectively prevents the overcharging of the cathode and over-lithiation of the anode. Without DBBB, as illustrated with the photograph in the inset of Figure 2, the STD cell suffered from a severe swelling due to the increase of the internal pressure caused by gas generation from the electrochemical oxidation/decomposition of the electrolyte and the solvent evaporation by overheating.(15 and references therein) In contrast, no gas generation or obvious change in physical shape for the DBBB cell was detected as illustrated in the inset of Figure 2.


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Figure 2. Charging voltage profile as a function of the SOC for graphite/C-LFP pouch cells without (STD Cell) and with 0.4 M DBBB additive (DBBB Cell). Test condition: C/4 charge to 200% SOC (100% overcharge), safety cut-off voltage 5.0 V. (Inset: pouch cell photographs at the end of overcharging).

DBBB additive could effectively prevent the potential increase of the cathode and eliminate the overcharge safety concern. Nevertheless, since the electrical charging energy was converted to thermal energy during the shuttling process, it is expected that thermal abuse might be triggered due to the heat generation during extended overcharge cycling especially for the large format cells or high power cells.6,7 To examine the side impact of the heat generation, external cell temperature was monitored during the 200% overcharge process and the results were shown in Figure 3. Similar to the findings in the coin or prismatic cell reported by Leising et al.,6 our results indicated that the external cell temperature did not increase dramatically even at a relatively high overcharging rate. Figure 3 provides the simultaneous temperature variation at the 1st and the 100th overcharge cycle, as well as the change of the internal resistance Rint. Both parameters were normalized against initial value after the 1st charge (Tinit = 23.6 °C and Rint = 50 mΩ). As noticed, the external temperature maintains unchanged during the regular charging process and abruptly goes up to a slightly higher level. With extended overcharge, the temperature rises slightly higher than that for the initial overcharge, but generally between 1.10 and 1.15 normalized temperature corresponding to 26 to 27oC. The slightly higher temperature at 100th overcharging cycle could be attributed to the cell internal resistance build-up, which will be discussed in the next section.



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1.00

200

SoC (100 % = 1.5 Ah)

Figure 3. The 1st and 100th overcharge voltage profiles and the corresponding normalized temperature at the external cell package. Overcharge condition: Constant current with charge rate C/4 until 200% of SOC.

To investigate the overcharge protection efficiency of the DBBB additive, C-LFP/graphite pouch cells were repeated overcharge cycling for 700 cycles. Figure 4a shows the representative voltage profiles at the 1st, the 400th and the 700th cycle with 100% overcharging. The charge and discharge capacity retention with overcharge cycling is shown in Figure 4b. As expected, the charging capacity for each cycle keeps constant of 3.0 Ah due to the 100% overcharging protocol, however, the discharge capacity delivered after the 100% overcharge is very stable. The discharge capacity still remains 87.7% even after 700 overcharge cycles, the exact the same results as the normally cycled DBBB cells without overcharge shown in Figure 1. This data confirm the sufficient function of the DBBB as redox shuttle additive for LFP based cells. This is the first research that demonstrates the advantage of using a redox shuttle additive in the large format soft-packaging Li-ion cells.


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Figure 4. (a) The 1st, the 400th and the 700th cycle voltage profiles and (b) charge capacity, discharge capacity retention and Coulombic efficiency for graphite/C-LFP pouch cells with 0.4 M DBBB additive cycled with 100% overcharge. 4. Conclusions

A redox shuttle additive DBBB was evaluated in 1.5 Ah LiFePO4/graphite pouch cells under standard and abusive overcharging condition. DBBB additive did not affect the performance of the cell at normal charging condition, whereas provides repeated overcharge protection during a 700-cycle test period demonstrated for the first time in a large pouch cell format under constant overcharge conditions (100%). It is the first time that effectiveness of the DBBB redox shuttle has 11 ACS Paragon Plus Environment


been demonstrated with a pouch cell, a format that is very sensitive to any internal gas evolution. No swelling of the cell was observed over long-lasting overcharging cycles. This result confirms the capability of DBBB to be implemented in any type of cell format using C-LFP cathode with non-aqueous liquid electrolyte. Acknowledgement The redox shuttle additive research was supported by the Vehicle Technology Office, Office of Energy of Efficiency and Renewable Energy, U.S. Department of Energy (DOE). The fabrication of electrodes and pouch cells, the electrochemical testing of the redox shuttle additive was performed at CIDETEC Energy Storage. DBBB sample was provided by the Material Engineering Research Facility at Argonne National Laboratory. Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357.

AUTHORS INFORMATION

Corresponding Authors Emails: [email protected] (J. Alberto Blázquez); Address: CIDETEC Energy Storage, Pº Miramón, 196, 20014 Donostia-San Sebastián, Spain. URL: http://www.cidetec.es/en/energystorage-4 Email: [email protected] (Zhengcheng Zhang). Address: Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL60439, USA. Notes The authors declare no competing financial interest.


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benzene (DBBB

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