AL


Table 1; 3) Thermo Scientific IonPac AS11 (250 × 2-mm i.d.) analytical separation col- umn; and 4) Thermo Scientific Dionex AERS 500 (2-mm) suppressor.


High-resolution accurate-mass


spectrometry HRAMS data was generated using a Thermo Scientific Q Exactive Orbitrap mass spectrom- eter with an electrospray interface operating in the negative mode.1


(see Table 1).


Sample preparation Carbon-based anodes extracted from battery


cells aged in different ways were washed with deionized water; the wash solutions were filtered prior to injection into the IC-HRAMS system. The battery cells contained a mixed organic-carbonates electrolyte with lithium hexafluorophosphate. The objectives of the sample preparation were to screen samples to identify compounds and determine any difference among the samples with various aging histories, and to use the features of the anion-exchange separation to help identify analyte properties.


The anode samples, obtained from an automo- bile manufacturer, included a control non-aged sample, a 45 °C calendar-aged sample exhibit- ing 20% loss in capacity and two cycle-aged samples from accelerated operation at 35 o


C


showing 30% loss and 40% loss in capacity. Process blanks were also included.


Surface deposits were observed on the an- odes. All of the anode samples were cut to a known weight, then sonicated and rinsed in weighed amounts of water. The extracts were filtered using Whatman PP 0.45-μm filters (GE Healthcare Life Sciences, Pittsburgh, Penn.), and the weight losses were calculated. The filtered extracts were injected into the IC-CD- HRAMS system.


Results and discussion


Valency in IC and ESI Anions were separated based on anion- exchange selectivity relative to hydroxide using gradient-elution conditions.2


The


sample cations and potassium from the eluent were removed using an inline elec- trolytic suppressor to replace these cations


with hydronium ion, which was simultane- ously neutralized to water with the eluent hydroxide ion, leaving a low-conductance background for detection of the sample an- ions by electrical conductivity. This process also desalted the column effluent before it was mixed with acetonitrile and entered the elec- trospray interface of the mass spectrometer. It is worth noting that anions are separated on the anion-exchange column according to their valency in the high-pH environment of the eluent, but after traveling through the suppressor they usually enter the mass spec- trometer with one charge. Therefore, species identified in the mass spectrometer bear a –1 charge. For example, sulfate is separated as a divalent anion, but is detected in the mass spectrometer as hydrogen sulfate.


Ions confirmed on degradation pathways Vortmann3


proposed a degradation pathway


that included hydrolysis of lithium hexafluoro- phosphate followed by esterification of several hydrolysis and decomposition products, some


requiring small amounts of water as a re- actant. The current authors have been able to confirm the presence of many of the phosphorus-containing species shown in the Vortmann pathway, including methyl hydrogen phosphate, dimethyl phosphate, monofluo- rophosphate methyl ester and phosphate, among others. Understanding a degradation process and which pathway to block to prevent the formation of key breakdown products can inhibit degradation and lead to safer, longer- lasting lithium-ion batteries.


Valency and peak identification Figure 2a and b show the elution of anions detected by the conductivity detector (a) and by the mass spectrometer (b). The detectors are complementary in that small ions such as fluoride (retention time 3.7 minutes) are best detected in the conductivity trace, while ions with higher mass are detected and fragmented for chemical identification by the mass spec- trometer. Since the elution was accomplished


Table 1 – HRAMS data generated using Q Exactive Orbitrap mass spectrometer with electrospray interface operating in negative mode


IC parameters


Column: Thermo Scientific Dionex IonPac Guard (50 × 2 mm i.d.) and AS11 (250 × 2 mm i.d.)


Eluent: 1 mM KOH from 0 to 5 minutes, 1–30 mM KOH from 5 to 25 minutes; 30–65 mM KOH from 25.1 to 45 minutes Eluent source: Thermo Scientific Dionex EGC 500 KOH cartridge


Flow rate: 0.25 mL/min Injection volume: 2.5 µL Temperature: 30 °C


Detection: suppressed conductivity; Thermo Scientific Dionex AERS 500 (2-mm) suppressor, AutoSuppression Post-column solvent: 90/10 acetonitrile/water, 0.25 mL/min


MS parameters Instrument: Thermo Scientific Q Exactive Orbitrap


Mode: ESI (electrospray ionization) negative ion Resolution: high-resolution full-scan MS, 70,000 and data-dependent top 3 MS/MS, 17,500; MS/MS voltages: stepped normalized collision energy (NCE) settings: 30, 45 and 60 Scan range: 50–750 m/z


AMERICAN LABORATORY


35


MAY 2016


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