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An Experimental and Theoretical Study of the Mass Transport in Lithium-Ion Battery Electrolytes
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Applied Electrochemistry.
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Lithium‐ion batteries are particularly suitable as energy storage solutions in high power applications, such as hybrid electric vehicles. It is generally considered that one of the processes that limit the power density for lithium‐ion batteries is the mass transport in the electrolyte. Yet, it is still difficult to find a set of properties that fully describe the mass transport for the most common electrolytes. In this work, characterization studies of the mass transport were undertaken for two technically important lithium‐ion battery electrolytes: (1) a liquid electrolyte which consist of LiPF6 dissolved in ethyl methyl carbonate (EMC) and ethylene carbonate (EC) and, (2) a gel electrolyte which consists of LiPF6 dissolved in ethylene carbonate, propylene carbonate (PC) and poly(vinylidenefluoride‐hexafluoropropylene) (P(VdFHFP)).The mass transport in the electrolytes was characterized by combining several experiments. The Maxwell‐Stefan equation was used as basis for the characterization. Models of the transport were formulated from the equation and the apparent transport properties were identified. The characterization methods were first analyzed mathematically in order to establish at which conditions the characterization experiments should be performed. The values of the apparent transport properties were then obtained by optimizing the models to the experimental responses. In order to give the characterization results a comprehensible interpretation and to allow benchmarking of electrolytes, the concept of a normalized potential gradient was introduced.The characterization results of the liquid electrolyte were used in a full cell model of a LiNi0.8Co0.15Al0.05O2 | LiPF6 EC:EMC (3:7) | MAG‐10 cell. The model was developed to analyze the mass transport during a hybrid pulse power characterization (HPPC) test. The analysis was made with a method where the polarization was split up into parts each associated with a process within the cell. The optimum composition in terms of mass transport was found to lie between 0.5 and 1.2 mol/dm3 LiPF6 for the liquid electrolyte and between 5 and 7 wt. % LiPF6 for the gel electrolyte. Less amount of polymer in the gel electrolyte gave a faster mass transport. It was also found that the mass transport in the liquid electrolyte contributed to a major part of the polarization during HPPC tests.

Abstract [sv]

Litiumjonbatterier är speciellt lämpliga som ackumulatorer i högeffektsapplikationer som elhybridfordon. Det är idag allmänt accepterat att en av processerna som begränsar effekttätheten för litiumjonbatterier är masstransporten i elektrolyten. Trots detta är det fortfarande svårt att få tag på data som fullständigt beskriver masstransporten i de vanligaste elektrolyterna.

I det här arbetet har masstransportkarakteriseringar gjorts för två tekniskt viktiga elektrolyter: (1) en vätskeelektrolyt som består av LiPF6 upplöst i etylenkarbonat (EC) och etylmetylkarbonat (EMC), och (2) en gel elektrolyt som består av LiPF6 upplöst i EC, propylenkarbonat (PC) och poly(vinylidene fluoride‐hexafluoro propylene) (P(VdFHFP)).

Masstransporten i elektrolyterna karakteriserades genom att kombinera ett antal karakteriseringsexperiment. Maxwell‐Stefans ekvation användes som utgångspunkt i karakteriseringarna. Modeller av transporten formulerades från ekvationen och de effektiva transportegenskaperna identifierades. En matematisk analys gjordes först av karakteriseringstekniken, så att det kunde fastslås för vilka förhållanden experimenten skulle utföras. Värderna av transportegenskaperna erhölls genom att optimera modellerna till det experimentella beteendet. För att ge karakteriseringsresultaten en begriplig tolkning och för att kunna mäta prestandan av elektrolyter, infördes konceptet normaliserad potentialgradient.

Resultatet från karakteriseringen av vätskeelektrolyten användes i en model av en LiNi0.8Co0.15Al0.05O2 | LiPF6 EC:EMC (3:7) | MAG‐10 cell. Modellen utvecklades för att analysera masstransporten i cellen under ett hybridpulstest (HPPC). Analysen gjordes med en metod där polarisationen delades upp i delar som var och en var kopplad till en process i batteriet.

Den optimala sammansättningen med avseende på masstransporten låg i regionen 0.5–1.2 mol/dm3 LiPF6 för vätskeelektrolyten och 5‐7 vikt% LiPF6 för gelelektrolyten. Mindre mängd polymer i gelelektrolyten gav en snabbare masstransport. Det konstaterades också att masstransporten i vätskeelektrolyten bidrog med en av de största delarna till polarisationen i HPPC testen.

Place, publisher, year, edition, pages
Stockholm: KTH , 2011. , 64 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2011:6
Keyword [en]
Lithium‐ion batteries, Electrolytes, Transport properties, Conductivity, Diffusion coefficients, Transport number, Maxwell-Stefan equation, Simulations, Mathematical analysis, Polarization, Hybrid electric vehicles
Keyword [sv]
Litiumjonbatterier, Aprotiska elektrolyter, Transport egenskaper, Konduktivitet, Diffusion koefficienter, Transporttal, Maxwell-Stefans ekvation, Simuleringar, Matematisk analys, Polarisation, Elhybridfordon
National Category
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-29121ISBN: 978-91-7415-852-6 (print)OAI: oai:DiVA.org:kth-29121DiVA: diva2:392067
Public defence
2011-02-28, K2, Teknikringen 28, Entréplan, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20110128Available from: 2011-01-28 Created: 2011-01-25 Last updated: 2011-05-20Bibliographically approved
List of papers
1. Electrochemical characterisation and modelling of the mass transport phenomena in LiPF6-EC-EMC electrolyte
Open this publication in new window or tab >>Electrochemical characterisation and modelling of the mass transport phenomena in LiPF6-EC-EMC electrolyte
2008 (English)In: Electrochimica Acta, ISSN 0013-4686, E-ISSN 1873-3859, Vol. 53, no 22, 6356-6365 p.Article in journal (Refereed) Published
Abstract [en]

The conductivity, the salt diffusion coefficient, the lithium-ion transport number and the thermodynamic factor of the salt and the solvent were reported for LiPF6 in EC:EMC (3:7) at 25 IC and for concentrations between 0.2 and 2.0 mol/d M3. The mass transport in the electrolyte was fully characterised by combining three types of electrochemical experiments; concentration cells, galvanostatic polarisation experiments and electrochemical impedance measurements with a mathematical description of the mass transport in the electrolyte. The apparent salt diffusion coefficient had a local maximum in the concentration range, while the viscosity-dependent salt diffusion coefficient decreased from 4.1 X 10-10 M2/s at 0.2 mol/d M3 to 4.4 x 10-11 M2/s at 2.0 mol/dM3. Both the thermodynamic factor and the conductivity varied strongly with the concentration. The conductivity had a maximum of 9.5 mS/cm at 1.0 mol/dm 3. The lithium-ion transport numberwith respect to the room decreased with increasing salt concentration, with a maximum of 0.37 at 0.2 molldm 3 in the concentration range. The Maxwell-Stefan diffusivities and the electrolyte potential drop in a lithium-ion battery at steady state were lastly calculated from the obtained transport properties. An analysis of the characterisation method was also done on the basis of the characterisation results.

Keyword
Li-ion battery, lithium hexafluorophosphate, Maxwell-Stefan equation, transport properties, characterisation, li-ion battery, polymer electrolyte, diffusion, polarization, carbonate
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-17731 (URN)10.1016/j.electacta.2008.04.023 (DOI)000258009800010 ()2-s2.0-54249111513 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
2. Analysis of the Polarization in a Li-Ion Battery Cell by Numerical Simulations
Open this publication in new window or tab >>Analysis of the Polarization in a Li-Ion Battery Cell by Numerical Simulations
Show others...
2010 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 157, no 11, A1236-A1246 p.Article in journal (Refereed) Published
Abstract [en]

An experimentally validated model was developed to analyze the polarization of a LiNi0.8Co0.15Al0.05O2 vertical bar 1.2 M LiPF6 in ethylene carbonate (EC):ethyl methyl carbonate (EMC) (3:7)vertical bar MAG-10 battery cell during a hybrid pulse power characterization (HPPC) cycle. The analysis was made with a method where the polarization was split up into parts associated with activation of the electrochemical reactions, mass transport of species in the electrolyte and in the solid phase, and inadequate contact between the materials in the electrodes. Each contribution to the polarization was quantified as a snapshot in time and as an average over the HPPC cycle. The polarization during a cycle according to EUCAR was analyzed in detail for state of charge (SOC) 40 and 80. It arose mainly due to the mass transport in the electrolyte, e. g., at SOC 40 it contributed to 43% of the total polarization. In an ISO (International Organization for Standardization)-energy cycle where the current loads are higher and applied for longer times than the EUCAR cycle, the mass transport by diffusion in the electrolyte and in the solid phase of the negative electrode became more significant. The presented method offers the possibility to find a battery cell's optimal operational condition and design.

National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-27674 (URN)10.1149/1.3486161 (DOI)000283857900021 ()2-s2.0-77957695680 (Scopus ID)
Funder
Swedish Research CouncilStandUp
Note

QC 20150629

Available from: 2010-12-21 Created: 2010-12-20 Last updated: 2017-12-11Bibliographically approved
3. A Theoretical and Experimental Study of the Mass Transport in Gel Electrolytes: I. Mathematical Analysis of Characterization Method
Open this publication in new window or tab >>A Theoretical and Experimental Study of the Mass Transport in Gel Electrolytes: I. Mathematical Analysis of Characterization Method
2011 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 158, no 6, A628-A635 p.Article in journal (Refereed) Published
Abstract [en]

Mass transport of lithium ions is one of the major limitations to the performance of high-rate lithium-ion batteries. This paper presents an analysis of a mass transport characterization method for gel electrolytes. The method is based on a Maxwell-Stefan transport model, which takes into account the polymer as an active specie in the transport. Nine apparent transport properties are defined from the model and their dependence on the Maxwell-Stefan diffusivities and the thermodynamic enhancement factors are presented. The characterization method is analyzed by finding analytical expressions that describe the characterization experiments. From the expressions it can be seen how the nine apparent transport properties influence the experimental response. The conclusions of the analysis will be used later in a characterization of the gel electrolyte: LiPF6-ethylene carbonate (EC)-propylene carbonate (PC)-poly(vinylidenefluoride-hexafluoropropylene) P(VdF-HFP).

Keyword
TRANSFERENCE NUMBER MEASUREMENTS, POLYMER ELECTROLYTES, ANION, POLARIZATION, CATION
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-29119 (URN)10.1149/1.3560025 (DOI)000289854700002 ()2-s2.0-79955528667 (Scopus ID)
Funder
Swedish Research CouncilStandUp
Note

QC 20150716 Uppdaterad från manuskript till published (20110520).

Available from: 2011-01-25 Created: 2011-01-25 Last updated: 2017-12-11Bibliographically approved
4. A Theoretical and Experimental Study of the Mass Transport in Gel Electrolytes: II. Experimental Characterization of LiPF6-EC-PC-P(VdF-HFP)
Open this publication in new window or tab >>A Theoretical and Experimental Study of the Mass Transport in Gel Electrolytes: II. Experimental Characterization of LiPF6-EC-PC-P(VdF-HFP)
2011 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 158, no 6, A636-A643 p.Article in journal (Refereed) Published
Abstract [en]

The mass transport in a gel consisting of LiPF6 dissolved in ethylene carbonate (EC), propylene carbonate (PC) and poly(vinylide-nefluoride-hexafluoropropylene) (P(VdF-HFP)) was characterized at 25 degrees C. Four diffusion coefficients, two transport numbers, one conductivity and two parameters describing the relationship between a concentration and a potential gradient were obtained in the characterization. The transport properties were obtained by optimizing a Maxwell-Stefan based transport model to data from four types of experiments. The transport model and the characterization method were analyzed in Part I. In order to give the results a comprehensible interpretation and to present a way of benchmarking electrolytes, we introduce the concept of a normalized potential gradient for gel electrolytes. The optimum composition range of the gel in terms of mass transport was found to lie between 5 and 7 wt % LiPF6 and as little polymer as possible without compromising the mechanical stability. It is suggested that measuring the normalized potential gradient can be used as a screening method when benchmarking electrolytes.

Keyword
BATTERY ELECTROLYTES, POLARIZATION
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:kth:diva-29120 (URN)10.1149/1.3565023 (DOI)000289854700003 ()2-s2.0-79955547805 (Scopus ID)
Funder
Swedish Research CouncilStandUp
Note

QC 20150716

Available from: 2011-01-25 Created: 2011-01-25 Last updated: 2017-12-11Bibliographically approved

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