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Electrochimica Acta 101 (2013) 3– 10

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

jo u r n al hom ep age: www.elsev ier .com/ locate /e lec tac ta

eview

educing Li-ion safety hazards through use of non-flammable solvents andecent work at Sandia National Laboratories

anesan Nagasubramanian ∗, Kyle Fenton546 Advanced Power Sources R & D Department, Sandia National Laboratories, Albuquerque, NM 87185, USA

a r t i c l e i n f o

rticle history:eceived 28 June 2012eceived in revised form7 September 2012ccepted 21 September 2012vailable online 8 October 2012

eywords:

a b s t r a c t

This article will briefly discuss the genesis of the Li-ion chemistry and its meteoric rise to prominence,supplanting aqueous rechargeable batteries such as NiCd and NiMH. The principal intent of this article is todiscuss the issues with thermal instability of common Li-ion electrolytes, which detract from the positiveattributes of this chemistry. The development of an innovative low-cost non-flammable electrolyte willgreatly improve the safety and reliability of lithium batteries, a key technological hurdle that must beovercome for the wider application of this chemistry. This article will also include the advancements madein combating/mitigating solvent flammability through the addition of fire retardants, fluoro-solvents,

ydrofluoro ethersonflammablelash pointhermal abuseccelerated rate calorimetryithium ion battery

ionic liquids etc. The scope of the article will be limited to the flammability of non-aqueous solvents andwill not include the thermal instability issues of anodes and cathodes. We will elaborate using examplesfrom our in-house research aimed at mitigating solvent flammability by using hydrofluoro ethers (HFEs)as a cosolvent. Additionally, we will describe in-house capabilities for prototyping 18,650 cells and in-house thermal abuse test capabilities that allow us to evaluate materials and their thermal responses inactual cell configurations.

Published by Elsevier Ltd.

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32. Fundamentals of combustion and mechanism of fire sustenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43. Strategies to combating solvent flammability in Li-ion cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3.1. Flame retardant additives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43.2. Low flash point electrolytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

4. Methods and evaluation techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65. Results and discussion for thermal testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

5.1. Electrolyte evaluation and thermal ramp studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65.2. ARC evaluation of electrolytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85.3. ARC evaluation of full cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

. Introduction Dr. Goodenough (at Oxford, England). In general the Li-ion chem-istry has an optimal combination of energy and power compared

In 1991 Sony Corporation commercialized the first Li-ion bat-ery by pairing intercalating carbon as the anode [1] and LiCoO2 ashe cathode [2] with a non-aqueous electrolyte. These two mate-ials were invented respectively by Dr. Samar (at Bell Lab) and

∗ Corresponding author at: Sandia National Laboratories, PO Box 5800, MS-0614,lbuquerque, NM 87185-0614, USA. Tel.: +1 505 844 1684; fax: +1 505 844 6972.

E-mail address: gnagasu@sandia.gov (G. Nagasubramanian).

013-4686/$ – see front matter. Published by Elsevier Ltd.ttp://dx.doi.org/10.1016/j.electacta.2012.09.065

to other rechargeable chemistries that enabled it to capture a sub-stantial market share from the other rechargeable contenders. Forhand-held electronic applications, small capacity cells on the orderof 1–3 Ah are primarily used at or near ambient temperature. How-ever, the application space for Li-ion cells is expanding to includetransportation, utility grid storage, as well as space system and mil-

itary power needs. Increasingly demanding mission requirementsin these applications have necessitated high energy and high powerstorage systems capable of operating over a broad temperature

4 Electrochimica Acta 101 (2013) 3– 10

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G. Nagasubramanian, K. Fenton /

ange. For example, recent advances are bringing a new generationf hybrid vehicles to the market. However, solvent flammabilityeduces safety hindering broader adoption of Li-ion batteries inhis application. Significant scientific efforts have been made tonderstand the consequences and requirements for runaway [3–6],nderstand policy decisions pertaining to battery safety [7], under-tand safety response in packs [8], and model thermal abuse of cells9]. As energy and power densities increase as in the case of PHEVplug-in hybrid vehicle) and EV (electric vehicle) batteries, so dohe materials safety concerns [10]. The topic of battery safety islso gaining attention rapidly as the number of battery incidentsncreases. Recent incidents such as a cell phone runaway during aegional flight in Australia and a United Parcel Service (UPS) planerash near Dubai reinforce the potential consequence of lithiumon battery runaway events. In the case of anodes and cathodes,he safety issues are mitigated through new materials. In the casef the electrolytes the reactivity and the flammability of the ventedolvents still remains a great challenge. The sheer size of the batterynd the operational demands make it increasingly difficult to findaterials with the necessary properties, especially the required

hermal behavior to ensure fail-proof operation. There are severalossible failure modes for batteries. These are:

A) Field failurea. Manufacturing defectsb. Overheating

B) Abuse failurea. Mechanicalb. Electricalc. Thermal

Any of these failures could increase the internal temperature ofhe cell, leading to electrolyte decomposition, venting, and possi-le fire. While significant strides are being made, major challengesemain in combating solvent flammability. To mitigate the Achilles’eel of thermal instability a number of different approaches haveeen developed with moderate success and varied outcomes.hen overheated due to overcharging, internal shorting, manu-

acturing defects, physical damage, or other failure mechanisms,onventional electrolytes generate highly flammable plumes con-aining toxic chemicals. The prospects of employing Li-ion cellsn applications depend on substantially reducing the flammabil-ty, which requires materials changes (including new lithium salts)o improve the thermal properties. One approach is to use fireetardants (FRs) in the electrolyte as an additive to improve ther-al stability. Most of these additives have a history of use as FRs in

lastics and the investigators chose to study them because of thessumption that the mechanism (this will be discussed later) forame suppression would be similar in batteries. Several differentaterials have been investigated as FR additives. Broadly, these

dditives can be grouped into two categories—those containinghosphorous and that containing fluorine. Different FR additivesill be culled and assembled in a tabular form to get an idea of theifferent materials studied.

. Fundamentals of combustion and mechanism of fireustenance

The combustion process is a complex chemical reaction byhich fuel and an oxidizer in the presence of heat react and burn

s shown in the Fig. 1. Convergence of three things namely heat,

n oxidizer, and fuel must happen to have combustion. The fuels the substance that burns. The oxidizer is the substance thatroduces the oxygen so that the fuel can be burned, and heat ishe energy that drives the combustion process. In the combustion

Fig. 1. Combustion reaction diagram.

process a sequence of chemical reactions occur leading to fire [11].In this melee a variety of oxidizing, hydrogen and fuel radicals areproduced that keep the fire going until at least one of the threeconstituents is exhausted.

The most commonly accepted mechanism for fire propagationrelies on a radical generation mechanism. Ground state O2 (di-oxygen) absorbs heat and produces energetic singlet oxygen (O2

*)as shown in Eq. (1). This singlet oxygen is extremely reactive. Simul-taneously, organics absorb heat producing hydrogen atoms (notprotons) by abstraction (Eq. (2)) which combine with the oxidiz-ing radicals producing hydroperoxide radical (HOO*) (Eq. (3)). Thisbreaks up to give hydroxyl (OH) radicals (Eq. (4)). They are shortlived but very reactive generating more heat/flame. The fuel radi-cal can further react with other radicals including the HO• radicalproducing fire and gaseous products (not shown below). Probablythe O2 generated in step 4 continues the fire propagation by form-ing again singlet oxygen, etc. This mechanism clearly indicates thathydrogen radical and singlet oxygen play a key role in sustainingthe flame [12]. This is the target for many of the materials proposedas FR additives. If a FR material is able to sufficiently bind to the freeradicals during the reaction cascade, then propagation of the flamewill be suppressed.

O2 + heat → O2∗(singletoxygen) (1)

Fuel + heat → H• + fuel (2)

O2∗ + H• → HOO∗•(hydroperoxideradical) (3)

2HOO• → 2HO• + O2(?) (4)

3. Strategies to combating solvent flammability in Li-ioncells

3.1. Flame retardant additives

Researchers have predominantly performed DSC (DifferentialScanning Calorimetry) measurements on electrolytes to determinethermal stability with and without the FR. The ideal outcome is thatthe electrolyte with FR show very little peak in the DSC traces andeven then only at higher temperatures compared to the electrolytewithout the FR [13]. This observation seems to suggest that the FRadditive improves the thermal stability of the electrolyte. Othershave chosen to employ standard test procedures from the AmericanSociety for Testing and Materials (ASTM), Underwriters Laborato-ries (UL), and International Electrotechnical Commission (IEC) such

as ASTM D-5306, ASTM D2863, UL-94VO, and IEC 62133 to computeboth the self-extinguishing time (SET) and the limited oxygen index(LOI) to evaluate the flammability of the electrolytes. The shorterthe SET and higher the LOI (this is the % of oxygen needed in the

G. Nagasubramanian, K. Fenton / Electrochimica Acta 101 (2013) 3– 10 5

Table 1List of fire retardants (FRs) studied in Li-ion batteries.

Name of FR Reference

Phosphate/phosphonateTriphenylphosphate (TPP) [15]Vinyl ethylene carbonate (VEC) + biphenyl (BP) + TPP [16]Dimethyl methylphosphonate (DMMP) [17,18]Polyphosphonate [19]Triphenyl phosphate (TPP), tris(trifluoroethyl)phosphate (TFP) [20]Phosphorus-containing esters [21]Methoxyethoxyethoxyphosphazenes [22]Bis(N,N-diethyl)methoxyethoxymethylphosphonamidate [23]Triphenyl Phosphate (TPP) and tributyl phosphate (TBP) [24]Trimethyl phosphate (TMP) and triethyl phosphate (TEP) [25]Ethylene ethyl phosphate(EEP) + TMP [26]Diphenyloctyl phosphate(DPLP) [27]Cyclic phosphate [28]Fluorinated phosphate/ethersTris(trifluoroethyl)phosphate (TFP), Bis(trifluoroethyl)methyl

phosphate (BMP) and trifluoroethyl phosphate (TDP)[29,30]

Methyl nonafluorobutyl ether (EFE) [31,32]Perfluoro-ether [33]Hydrofluoro ether (HFE) [34,35]PhosphitesTris(2,2,2-trifluoroethyl) phosphite (TTFP) [36,37]Triethyl and tributyl phosphite [38]Trimethyl phosphite (TMP) [39]Ionic liquidsN-Butyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide

(PYR14FSI)[40,41]

N-Butyl-N-methylpyrrolidiniumbis(trifluoromethansulfonyl)imide, PYR14TFSI

[42]

1-Ethyl-3-Methylimidazolium tetrafluoroborate (EMIBF4) [43]Tri-(4-methoxythphenyl) phosphate (TMTP) [44]Miscellaneous compoundsHexamethylphosphoramide (HMPA) [45]Dimethyl methylphosphonate (DMMP) [46]PhosphazenePhoslyte [14]Ethyleneoxy phosphazenes [22,47]

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Table 2Flash pointa for some of the common organic materials.

Chemical Flash point (◦C)

Acetone −17Ethanol 17Gasoline −42DEC 33DMC 18EMC 23EC 145PC 132HFEs (TMMP, TPTP) No flash pointIL (1-ethyl-3-metgyl imadazolium TFSI) 283Canola oil 327

Phosphazene-based flame retardants [48]Hexamethoxycyclotriphosphazene [49,50]

2/N2 mixture to keep the electrolyte burning for at least 60 s) theess flammable the electrolyte is. In general, the electrolyte with thedditives showed shorter SET and higher LOI than the electrolyteithout FR. Descriptions of the thrust of the different ASTM and UL

ests were discussed in depth by Otsuki et al. [14]. Table 1 shows aariety of new and novel FR materials that have been synthesizednd studied as well as low flash point electrolytes that have beeneveloped to fight flame ignition (vide infra).

The choices of FR for suppressing flame in Li-ion cells centersn the mechanism described above. In the vapor phase, the tra-itionally accepted mechanism is that FR decomposes producinghosphorous or halogen radicals that act as scavengers and reactith hydrogen radicals, thereby terminating the free radical reac-

ion. Despite wide availability for FR materials, two primary classesf materials have been investigated extensively for use in Li-ionatteries, which are phosphorus containing materials and fluorineontaining materials. The phosphorous containing materials pri-arily rely on the free radical scavenging mechanism, but on rare

ccasion inhibit reaction through char formation on the reactiveurface. The fluorinate materials have a couple of attractive prop-rties over the hydrocarbons, especially for fighting flammability.hese are:

1) The C-F bond (105.4 kcal mol−1) is stronger than the C H bond(98.8 kcal mol−1) [51], meaning more heat energy is required to

cleave the bond.

2) By replacing the hydrogen with fluorine the compound shouldbe more thermally stable. Remember H• is one of the radicals

a As per OSHA (Occupational Safety & Health Administration) classification anyliquid with a flash point below 38.7 ◦C is flammable.

that sustains combustion and by eliminating the generation ofH• there could be minimal or no fire in the electrolyte.

3.2. Low flash point electrolytes

Despite several studies on the issue of flammability, completeelimination of fire in Li-ion cells has yet to be achieved. One possi-ble reason for the failure could be linked to lower flash point (FP)(<38.7 ◦C) of the solvents (defined as “the lowest temperature atwhich it can vaporize to form an ignitable mixture in air”) [52]. Theflammable nature of the low FP solvents overwhelms the FR, makingit ineffectual. Table 2 compares the FP for a few chemicals, showingthe lower the Fp is the more ignitable the material is. For exam-ple: dimethyl carbonate is a commonly used combustible solventin Li-ion cells and has an FP of 18 ◦C, whereas Canola oil is non-combustible and has a FP of 327 ◦C. Obviously worse is gasoline,which has an Fp of −42 ◦C.

Then the next logical question is “can high FP solvents elimi-nate combustion and thus avoid the consequences of a catastrophicfire?” To test this idea higher molecular weight oligomers or poly-mers and ionic liquids which usually exhibit higher FP (refer toTable 2) are being studied as electrolytes [53]. The data published inthe literature show that polyphosphazene polymers and ionic liq-uids used as electrolytes are nonflammable. However, the high flashpoint of these chemicals is generally accompanied by increasedviscosity, thus limiting low temperature operation and degrad-ing cell performance at sub-ambient temperatures. These materialsmay not be suitable for wide temperature operation. These mate-rials may also have other problems such as poor wetting of theelectrodes and separator materials, excluding them from use incells despite being nonflammable. In our laboratory we tested theflammability of numerous electrolytes (see Table 3). We comparedthe effect of the amount of EC on the flammability and freezingpoint of the electrolyte. For example, EC:DEC (5:95 vol.%)-1 M LiPF6was flammable whereas EC:DEC (55:45 vol.%)-1 M LiPF6 electrolytedid not burn. This is the good news. The bad news is that EC:DEC(55:45 vol.%)-1 M LiPF6 begins to freeze at −5 ◦C while EC:DEC(5:95 vol.%)-1 M LiPF6 remained liquid even at −50 ◦C.

Ideally, solvents would be used that have no flash point whilesimultaneously exhibiting ideal electrolyte behavior (see below fora number of critical properties that the electrolytes need to meet)and would remain liquid at low temperatures down to −50 ◦C orbelow for use in Li-ion cells. There are materials such as fluoroethers that exhibit no FP. We will discuss later the performanceof hydrofluoro ethers (HFEs) that exhibit no FP in Li-ion cells.

A number of critical electrochemical and thermal properties are

given below that FRs have to meet simultaneously. The tradeoffsbetween properties are possible but when it comes to safety therecannot be tradeoffs.

6 G. Nagasubramanian, K. Fenton / Electrochimica Acta 101 (2013) 3– 10

Table 3Electrolyte composition.

Solvent EMC*** Salt – 1 M Freezing point (◦C)

TMMP TPTP EC* DEC**

Composition vol.%

50 5 45 LiBETIa −5550 5 45 LiBETIa <−5550 5 45 LiPF6 −5550 5 45 LiTFSIb

5 95 LiPF6 −5555 45 LiPF6 −5

Composition wt.% 30 70 1.2 M LiPF6

Flash point No No Yes Yes Yes

* Ethylene carbonate.** Diethyl carbonate.

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*** Ethyl methyl carbonate.a Lithium bis (perfluoroethyl sulfonyl)imide.b Lithium trifluoro sulfonamide.

High voltage stability.Comparable:electrochemical performance with baseline;conductivity similar to baseline.Lower flame propagation rate or no fire at all.Lower self-heating rate (accelerated rate calorimetry (ARC) mea-surement) depending on the amount of the additive.Stable against both the electrodes.Able to wet the electrodes and separator materials.Higher onset temperature for exothermic peaks with reducedoverall heat production.No miscibility problems with baseline electrolytes.

Electrolyte non-flammability is essential for cell safety. Sev-ral of the FRs showed promising performance in small cells.nhanced safety of electrolytes containing FR additives is mostlyccompanied by performance degradation including low capacitytilization, high rate of capacity fade, or poor low temperature per-ormance. Additionally, some are electrochemically unstable andre consumed during cycling, becoming unavailable for cell protec-ion. Furthermore, no information is available in the open literaturen the performance of these additives in large capacity cells underctual use conditions. This prompted us to try a different approachuse of solvents with no FP) to improve the electrolyte thermalroperties. Before we discuss our approach we will describe the

n-house facilities which we used extensively for prototyping andhermal abuse studies.

. Methods and evaluation techniques

We and others have taken a slightly different approach where aR with no FP is used as a cosolvent to the electrolyte and is an activeomponent, not an incipient onlooker which only comes to lifender thermal abuse conditions. In this approach the FR can be used

n large enough quantity to ensure non-flammability of the elec-rolyte. Initially, Arai studied methyl nonafluorobutyl ether [31,32]s a cosolvent and showed that even an overcharged Li-ion cellid not produce thermal runaway under nail penetration testing.ollowing this, Chen et al. [54] used Allyl tris(2,2,2-trifluoroethyl)arbonate (ATFEC) as cosolvent, but it is not clear if the solventhowed no FP. However, the results seem to indicate that theTFEC reduces flammability of the electrolyte. Several years lateraoi et al. [34] have shown in small capacity cells that Hydroflu-ro ethers (HFE) were thermally stable. Recently we evaluated

FEs in large capacity cells (18,650-cell configuration) under actual

hermal abuse conditions [35] and have shown that they exhibitonflammable behavior. We performed flammability test on thelectrolytes and ARC studies on electrolyte and full cells. We

studied the conventional electrolytes without HFEs for compari-son. The results of our measurements will be described in detaillater. The structure of HFEs, electrolytes investigated are shown inFig. 2. These liquids are “Engineered” fluids developed by 3 M as areplacement for perfluorocarbons (PFCs) and perfluoropolyethers(PFPEs) and for use in heat transfer applications.

In addition we studied the conventional electrolytes containingeither EC:EMC (3:7 wt.%) and 1.2 M LiPF6 or EC:DEC (5:95 vol.%) and1 M LiPF6 as electrolytes. These are designated as “baseline” elec-trolytes. Remember the amount of HFE in the electrolytes is 50% byvolume which is significant compared to the percent used for FRadditives (<20 vol.%) in the electrolytes.

All testing was done using the 18,650 cell size as a standard. Cellswere fabricated in two dry rooms (1000 sq. ft.) using our primaryelectrode coater and winder units (as seen in Fig. 3), while the othersupporting equipment has been described elsewhere [55]. The elec-trochemical performance of the cells used for flammability tests hasalso been published elsewhere [35] and this article will primarilyfocus on the flammability results.

Flammability testing was performed at the Sandia National Lab-oratories Battery Abuse Testing Laboratory, which is capable oftesting power sources ranging in size from single cells to batterypacks. The 18,650 size cells were chosen to ensure that abuse eventswould be similar to field failures. A partial list of standard abusetesting is shown in Fig. 4. This capability allows for thermal abusetesting on a range of batteries from a few Wh cell capacity to EVsized batteries (cells, modules, and packs).

5. Results and discussion for thermal testing

5.1. Electrolyte evaluation and thermal ramp studies

The electrolytes were characterized for conductivity and elec-trochemical voltage stability using a commercial 2-electrode cellwith a cell constant (K) of one [35]. Before measuring conductiv-ity of the organic electrolytes, the conductivity of a KCl standardsolution was measured to verify the cell constant was unity. Theconductivity of the KCl standard at 25 ◦C computed from the mea-sured resistance, using a value of 1 for K, was identical to thatcertified by the National Institute of Standard and Technology forthat concentration of the KCl solution. After measurement, the cellwas cleaned, rinsed with deionized water and dried in an ovenovernight before using it for measuring conductivity of the organic

electrolytes. Electrolyte resistance was measured in the frequencyrange 106–0.1 Hz using a Solartron SI 1287 Electrochemical Inter-face coupled with a Solartron SI 1260 Impedance Phase Analyzerand controlled with Zplot software. The x-intercept was used to

G. Nagasubramanian, K. Fenton / Electrochimica Acta 101 (2013) 3– 10 7

Fig. 2. Hydrofluoro ether used for evaluation.

Fig. 3. Primary electrode coater (A) and 18,650 cell winding equipment (B).

Fig. 4. Tests routinely performed at SNL in our abuse testing facility.

8 G. Nagasubramanian, K. Fenton / Electrochimica Acta 101 (2013) 3– 10

Fig. 5. Experimental setup showing (a) picture of the copper heater block used and (b) line drawing of the experimental setup.

dg

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electrolytes. Additionally, the onset temperature for the HFE elec-trolytes is shifted from ∼160 ◦C for the baseline electrolytes to∼240 ◦C.

Fig. 6. EC:EMC(3:7 wt.%)-1.2 M LiPF6 is mildly flammable.

etermine the electrolyte resistance and the reciprocal of this valueives conductivity.

Subsequently, flammability of the electrolyte was tested on theet-up shown below in Fig. 5a. Fig. 5b shows a line drawing of theetup indicating the location of the thermocouples, spark source,nd heater cartridges. For flammability evaluation we use 5 mlf electrolyte in an 18,650 cell (approximately the same quan-ity normally used for full cells). After crimping, we load the celln the thermal ramp test setup. Using two heater cartridges, theemperature of the copper block is raised at a programmed rate∼5 ◦C/min) while recording the temperatures of the Cu block andhe cell. The spark source is also kept on for the whole time.imultaneously, we record video and audio of the event. As theell temperature increases, the electrolyte begins to evaporate andressure builds up inside the cell, eventually resulting in cell vent-

ng. If the vapor is combustible and has mixed with the air in theroper fuel/air ratio it starts to burn when it comes in contactith the spark source. Fig. 6 shows a snap shot of the flame as a

esult of the vented vapor catching fire for EC:EMC(3:7 wt.%)-1.2 MiPF6. EC:DEC(5:95 wt.%)-1 M LiPF6 also showed similar behaviorFig. 7). Similar measurements were performed on TMMP and TPTPontaining electrolytes. These electrolytes did not burn althoughhey vented as shown in Figs. 8 and 9. These data clearly showhat the carbonate EC:DEC-1 M LiPF6 and EC:EMC-1.2 M LiPF6 areammable and the HFE containing electrolytes are nonflammable.rom the intensity and height of the flames it also appears that aigher concentration of linear carbonate produces more flame (thisgrees with the flammability of low FP solvents). After flammability

ests ARC bomb studies were performed on electrolytes to comparehe onset of gas evolution and volume of gas generated as a functionf temperature.

Fig. 7. EC:DEC(5:95 vol.%)-1 M LiPF6 is violently flammable.

5.2. ARC evaluation of electrolytes

A detailed description of the ARC measurement has been pub-lished elsewhere [35]. In this study we measured the total gasvolume generated due to electrolyte decomposition for the elec-trolytes. Fig. 10 shows volume of gas evolved vs. temperature andthe onset of gas evolution. The top two curves are for the baselineelectrolytes and the bottom three are for electrolytes contain-ing HFEs. The volume of gas generated for the HFE electrolytesis approximately half of the volume generated by the baseline

Fig. 8. EC:DEC:TMMP(5:45:50 vol.%)-1 M LiBETI is nonflammable.

G. Nagasubramanian, K. Fenton / Electr

Fig. 9. EC:DEC:TPTP(5:45:50 vol.%)-1 M LiBETI is nonflammable.

5

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Fig. 10. Volume of gas generated vs. temperature.

.3. ARC evaluation of full cell

Encouraged by the superior performance of the HFE electrolytes,everal 18,650 full cells were assembled for ARC studies. The cath-de is made up of 94 wt.% LiNi0.4Mn0.3Co0.3O2, 3 wt.% PvdF and

wt.% conductive carbon. The anode contains 92 wt.% anode, 6%vdF and 2 wt.% conductive carbon. The electrodes were coated atur in-house facility. After formation and cycling we performedRC measurements to compute the volume of gas evolved in the

ull cells. Fig. 11 compares the total gas volume generated as a func-ion of temperature for cells with and without the HFE. The topwo plots are for cells containing the baseline and the bottom twore for HFE electrolytes. The data clearly show that for the cells

Fig. 11. HFE electrolytes show lower gas volume than the baseline.

ochimica Acta 101 (2013) 3– 10 9

containing electrolytes, the generated gas volume is ∼60% less thanfor cells containing baseline electrolytes. These observations areencouraging and we are in the process of testing the electrolytesin large capacity cells. Additionally, we are testing the electrolytesat the Combustion Research Facility (CRF) located in Sandia, Liv-ermore, California to gain a comprehensive understanding of themechanism of the nonflammable nature of our HFE electrolytes.Efforts are focused on understanding the mechanism that leads tothe significant decrease in gas generated for the HFE electrolytes.

6. Conclusions

Li-ion cells are dominating the field of hand-held devices suchas cell phones and computers. Due to their widespread success,the application space is expanding to include transportation, utilitygrid scale energy storage, space system, and military applications.Although Li-ion cells have tremendous potential, the flammabil-ity of the organic electrolyte hinders its consideration for largescale applications where the battery is expected to operate overa wide temperature range and the ramifications of a safety eventare of major consequence. Since safety cannot be compromised,well thought out and judicious approaches have been implementedto combat flammability with varied success. FR additives seem toimprove the thermal stability of the electrolytes but none seem toperform adequately. Ionic liquids have been tried as electrolytessince they are nonflammable but their higher viscosity and lowerionic conductivity is detrimental to low temperature performance.While performing adequately under thermal abuse conditions, thejury is still out on the ability of these electrolytes to remain non-flammable as the cell ages. We have shown that the electrolytescontaining HFE are nonflammable, produce less gas and the onset ofgas evolution is pushed out in temperature by ∼80 ◦C compared tothe baseline. Encouraged by these observations, we are in the pro-cess of testing electrolytes in large capacity cells to more preciselyemulate real use conditions.

Acknowledgments

Sandia National Laboratories is a multi-program laboratorymanaged and operated by Sandia Corporation, a wholly owned sub-sidiary of Lockheed Martin Corporation, for the U.S. Department ofEnergy’s National Nuclear Security Administration under contractDE-AC04-94AL85000. The authors would like to thank the Labo-ratory Directed Research and Development for funding and LorieDavis and Jill Langendorf for cell fabrication and testing. This workwas performed partially under the auspices of DOE Vehicle Tech-nologies Office and the authors would like to thank David Howelland Peter Faguy.

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