Aging investigations of a lithium-ion battery electrolyte from a field-tested hybrid electric vehicle

doi:10.1016/j.jpowsour.2014.09.064

Journal of Power Sources

Volume 273, 1 January 2015, Pages 83-88

https://doi.org/10.1016/j.jpowsour.2014.09.064 Get rights and content

Highlights

•
Characterization of electrolyte from a field-tested hybrid electric vehicle.
•
Quantification of the main compounds and additives.
•
Proposed reaction scheme including the identified fluoro- and alkyl phosphates.

Abstract

The electrolyte of a used lithium-ion battery from a hybrid electric vehicle (HEV) was investigated. The liquid electrolyte was collected through the pressure valve of these 5 Ah cells. It consists of (29.8 ± 0.2) wt.% dimethyl carbonate (DMC), (21.7 ± 0.1) wt.% ethyl methyl carbonate (EMC), (30.3 ± 0.3) wt.% ethylene carbonate (EC) and (2.2 ± 0.1) wt.% cyclohexyl benzene (CHB) which were identified with GC–MS and quantified with GC–FID. Li⁺ (1.29 ± 0.04) mol L⁻¹ and PF₆⁻ were determined with IC as the main ionic species in the solution. Furthermore, BF₄⁻ was clearly identified with IC–ESI–MS, IC–ICP–MS and ¹¹B NMR and quantified to a concentration of (120.8 ± 8.3) mg L⁻¹ with ICP–OES. The presence of POF₃ (detected with GC–MS), F⁻, PO₂F₂⁻, HPO₃F⁻ and H₂PO₄⁻ (determined with IC–ESI–MS) can be attributed to the reaction of the conducting salt LiPF₆ via PF₅ with traces of water. HPO₃F⁻ and H₂PO₄⁻ could only be observed in cells which were opened in a laboratory hood under exposure of air humidity. This experiment was done to simulate escaping electrolyte from an HEV battery pack. Furthermore, several alkyl phosphates (identified with GC–MS and IC–ESI–MS) are present in the solution due to further reaction of the different fluorinated phosphates with organic carbonates.

Introduction

Due to a high energy density and an excellent cycling stability, lithium-ion batteries (LIBs) are a promising battery technology for electric vehicles (EVs) and hybrid electric vehicles (HEVs) [1], [2]. LIBs usually consist of a graphitic anode and a transition metal oxide cathode coated on current collectors of copper and aluminum foil, respectively. A separator consisting for example of a micro porous polymeric foil or a polymeric film coated with ceramics is located between these two electrodes and soaked with an electrolyte. This ion conducting liquid typically contains LiPF₆ as conducting salt in a concentration of 0.8–1.2 mol L⁻¹ dissolved in a mixture of linear (e.g. DMC or EMC) and cyclic (e.g. EC or propylene carbonate, PC) organic carbonates [3], [4]. Apart from these basic components, additives can be used to enhance safety and performance of the cells [5], [6].

One limiting factor for the lifetime of a LIB is the decomposition of the electrolyte, which can be chemically, thermally and electrochemically induced. LiPF₆ is in equilibrium with LiF and PF₅. Due to the high hygroscopicity of LiPF₆, traces of water are always present in the electrolyte which react with PF₅ to HF and POF₃ [7]. POF₃ reacts stepwise with H₂O via different fluorinated phosphates to phosphoric acid under further release of HF [8]. The presence of phosphoric and especially hydrofluoric acid leads to the degradation of other battery components [7]. Moreover, it could also be a problem of safety in case of an EV or HEV accident due to escaping acid containing electrolyte and the resulting risks for human beings and the environment. So far, not all of the decomposition reactions, especially not the reactions of the electrolyte, are fully understood. It is necessary to gain more information for a better understanding of the whole decomposition process of LIBs to create for example additives which suppress this progression. Furthermore, it is advisable to do experiments simulating the cell opening under field conditions, e.g. an HEV accident.

To the author's best knowledge, investigations of a field-tested electrolyte from an EV or HEV have never been published before. Electrolyte decomposition or aging was only investigated on the laboratory scale, but not with industrially applied battery cells. Obviously, the choice of analytical methods for the investigation of an electrolyte solution of unknown composition is challenging. Usually, gas chromatography (GC) and ion chromatography (IC) are applied to get basic information about the solvent mixture and the conducting salt of a LIB electrolyte. Suitable methods have been recently developed [9], [10], [11], [12]. Since it will not be possible to identify every compound with these devices, further analysis with hyphenated methods should be taken into account. With ion chromatography–electrospray ionization–mass spectrometry (IC–ESI–MS) it is for example possible to separate ionic compounds from the complex mixture and obtain the mass to charge ratio of the molecular mass of each compound. [13], [14] In a second run, individual mass to charge ratios can be chosen for ESI–MS/MS fragmentation experiments for the identification of every compound.

Herein, we report the characterization of a battery electrolyte of unknown composition from a field-tested HEV by application of several analytical methods.

Section snippets

Sample preparation

The discharged battery pack of an HEV was disassembled into single cells. These field-tested prismatic 5 Ah cells were based on NMC-cathodes. Field-tested means that the cells were used in an HEV driven on the street and neither any information about the electrochemical cell performance, nor information about the used battery materials were available from the car manufacturer. Three of the cells were opened by crushing the pressure valve of the aluminum housing and the electrolyte was directly

Results and discussion

Electrolytes for lithium-ion batteries commonly consist of different linear and cyclic organic carbonates as solvent and a Li⁺ containing conducting salt. GC–MS is the method of choice to gain information about the composition of the organic carbonates and other volatile constituents of the electrolyte. [9], [11] Thus, the electrolytes were first analyzed with GC–MS (Fig. 1) and quantified with GC–FID (Table 1). The ion conducting liquid of all three cells consists of (29.8 ± 0.2) wt.% dimethyl

Conclusions

The electrolyte of a LIB cell from an HEV has been investigated. The solution consists of LiPF₆ diluted in DMC, EMC and EC (3:2:3) with 2 wt.% CHB as additive for overcharge protection. It is remarkable that besides LiPF₆, an additional conducting salt namely LiBF₄ could be identified in low a concentration of (120.8 ± 8.3) mg L⁻¹. We suppose that it has either been applied as an additive, or was formed during cell lifetime due to the reaction of hydrofluoric acid with other boron containing

Acknowledgment

We kindly thank the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety for funding of the project LithoRec II (project grant number: 16EM1025) whereby this work could be realized and also the project partners for support and cooperation. Verena Naber and Constantin Lürenbaum are acknowledged for their help during sample preparation and investigations with IC and GC.

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Aging investigations of a lithium-ion battery electrolyte from a field-tested hybrid electric vehicle

Highlights

Abstract

Introduction

Section snippets

Sample preparation

Results and discussion

Conclusions

Acknowledgment

Electrochim. Acta

J. Power Sources

J. Power Sources

J. Power Sources

J. Fluor. Chem.

Anal. Chim. Acta

J. Power Sources

J. Supercrit. Fluids

J. Power Sources

J. Chromatogr. A

J. Power Sources

J. Power Sources

Chem. Unserer Zeit