Hanemann, T.

Hofmann, A.

Merklein, L.

Schulz, M.

WInkler, V.

Winkler, V.

KITopen

KIT – Universität des Landes Baden-Württemberg und nationales Forschungszentrum in der Helmholtz-Gemeinschaft Institut für Angewandte Materialien – Werkstoffprozesstechnik – Abteilung Material- und Prozessentwicklung www.kit.edu Anodic dissolution of aluminum in Li-ion cells in terms of the electrolyte  Seminar-Vortrag HIU  Dr. Andreas Hofmann 2 Outline Li-ion battery research at IAM-WPT  Electrolytes in Li-ion cells  Mechanism of aluminum dissolution  Influence of electrolyte composition on aluminum “corrosion” – conducting salt, solvents and additives        Dr. Andreas Hofmann 09.12.2014 3 Energy storage systems: Li-ion batteries   anode   electrolyte electrode preparation    cathode research and development processing cells cell manufacturing, electrolyte filling 09.12.2014 ----- at IAM-WPT ----- technology step batteries cell assembling and battery management Dr. Andreas Hofmann Li-ion battery research -- at KIT -- 4 Electrolyte research at IAM-WPT Dr. Andreas Hofmann 09.12.2014 New materials Additives Liquid electrolytes Improve of Safety Gel polymer electrolytes High voltage electrolytes Ionic liquids and new solvents Structure-activity relationships Current collectors (aluminum) Systems Focus of research   Issues  Interaction of cell parts Interfaces  Structure-property relationships Electrolyte formulation Conducting salts Li-ion battery research Li-ion cells solar cells 5 “Chemical compounds which are able to form ionic crystals in solid state and are composed of ions under liquid or molten conditions or when dissolved in a solvent.                      lithium salt + matrix + additives  Dr. Andreas Hofmann 09.12.2014 ** http://www.itwissen.info Enable the lithium transport through the cell  Basis for the current flow Solid electrolyte interface  Interact with all parts inside the cell Electrolytes in Li-ion cells: basics electrolytes 6 Electrolytes in Li-ion cells: requirements  Nonflammable liquid electrolytes with adequate Li ion conductivity Inherent safety Infinite cycling Low cost Non-toxic salts and solvents Improved low-temperature performance Effective redox shuttles for overcharge protection Electrolyte additives for effective SEI layer formation Stable ionic liquids and solid polymer electrolytes with acceptable conductivity  Dr. Andreas Hofmann 09.12.2014 electrolytes 7 Electrolytes in Li-ion cells: state of the art State of the art: Carbonate-based electrolytes (z.B. EC/DMC + 1M LiPF6) Gel polymer electrolytes with liquids (carbonate based)   Composition: Mixture of organic carbonates LiPF6 (1 mol/l) Additives   Dr. Andreas Hofmann 09.12.2014 Properties: Stable up to 4.2 V vs. Li/Li+  Highly flammable (flash point EC/DMC: 24°C) Temperatures up to 60 °C electrolytes 8 Replacement of LiPF6 Use of electrolytes New electrolytes for new electrode materials Electrolyte uptake Range of temperature Intercalation of solvents (graphite) Electrolytes for high voltage applications New solvents at E > 4.2 – 4.5 V vs. Li/Li+ Electrolyte decomposition, gas formation Safety issues High toxicity of LiPF6 Intrinsic fire safety Reduce of leakage Dr. Andreas Hofmann 09.12.2014 Dimethylcarbonat LiPF6 Electrolytes in Li-ion cells: research Interaction of the electrolyte with all parts inside the cell electrolytes 9 Aluminum dissolution: challenge 09.12.2014 Questions: Dependence of aluminum corrosion on conducting salt Corrosivity of ionic liquids and organic solvents Concentration dependence of aluminum corrosion Additives for protecting the aluminum surface conducting salts solvents additives A. Hofmann et al. Electrochim. Acta, 116, 388–395 (2014).  A. Hofmann et al. J. Electrochem. Soc., 161, A431 – A438 (2014). mechanism of Al-dissolution 10 What is aluminum corrosion? Electrochemical reaction of aluminum inside the cell Anodic dissolution of aluminum (oxidation)  Why not „corrosion“? Corrosion towards „DIN EN ISO 8044“:   „Reaktion eines metallischen Werkstoffes mit seiner Umgebung, die eine messbare Veränderung des Werkstoffes bewirkt und zu einer Beeinträchtigung der Funktion eines metallischen Bauteils oder eines ganzen Systems führen kann.“ Corrosion: without a potential difference  Where is aluminum inside the cell? Current collector for cathode material Electrolyte is in contact with aluminum  Dr. Andreas Hofmann 09.12.2014 Mechanism of aluminum dissolution mechanism of Al-dissolution 11 What happens when aluminum dissolves? Aluminum dissolves slowly but completely Cathode material crumbles away Aluminum salts can deposit onto the counter electrode   Why this is not a problem of today’s electrolytes? Inert under the conditions which are used today   (E < 4.3 V vs. Li/Li+, today’s electrolytes) Passivation of aluminum by oxygen (Al2O3) Passivation of aluminum by fluorine (AlF3)  When aluminum corrosion plays a role? Use of new electrolytes (solvents, conducting salts, additives)  Use of higher voltages (E > 4.3 V vs. Li/Li+) Replacement of fluorine (LiPF6)  Dr. Andreas Hofmann 09.12.2014 Mechanism of aluminum dissolution aluminum disc after cycling,  d = 12 mm mechanism of Al-dissolution 12 Multistep process    Mechanism under involvement of corrosive salts*     Dr. Andreas Hofmann 09.12.2014 Mechanism of aluminum dissolution 3.5 V ! * Wang et al., Electrochim. Acta 45 (2000) 2677. mechanism of Al-dissolution 13 Dr. Andreas Hofmann 09.12.2014 Mechanism of aluminum dissolution 3.5 V ! * Wang et al., Electrochim. Acta 45 (2000) 2677. Multistep process    Mechanism under involvement of corrosive salts*     mechanism of Al-dissolution 14 Dr. Andreas Hofmann 09.12.2014 Mechanism of aluminum dissolution 3.5 V ! * Wang et al., Electrochim. Acta 45 (2000) 2677. Multistep process    Mechanism under involvement of corrosive salts* mechanism of Al-dissolution 15 Aluminum dissolution prevention by using solvents with low dielectric constants or ionic liquids Use of conducting salts or additives which enable a formation of a passivation layer (e.g. LiBF4, lithium bis(oxalato)borate, HF) Formation of ﬂuorine containing passivation layer Use of conducting salts with large anions in space which are not able to complex and dissolve the Al (e.g. lithium bis(perﬂuoroethylsulfonyl)imide). Use of solvents with functional groups which are less sensitive to reduction Dr. Andreas Hofmann 09.12.2014 Preventing Al dissolution mechanism of Al-dissolution 16 Cells Swagelok-cells Al | Li  3 – 5 V vs. Li/Li+ X-ray photoelectron spectroscopy Dr. Andreas Hofmann 09.12.2014 Surface layer composition aluminum dissolution Hofmann et al. J. Electrochem. Soc. 161 (2014) A431 LiBF4 Aluminium LiN(SO2CF3)2 Results F and Li can be detected in surface layer O-content is decreased F and Li are important for passivation  17 Potential range: 3 – x V (x = 4 – 5 V vs. Li/Li+) Solvent: ionic liquid + propylene carbonate conducting salt: 1 M LiN(SO2CF3)2  Cells: aluminum | lithium (Swagelok) Critical potential at 4.6 V vs. Li/Li+ However: dependent on solvent! Dr. Andreas Hofmann 09.12.2014 Potential range Hofmann et al. Electrochim. Acta  116 (2014) 388 aluminum dissolution 18 Dr. Andreas Hofmann 09.12.2014   IL-EB 1 IL-EB 2 IL-EB 3 IL-EB 4 IL-EB 5 IL-EB 6 IL-EB 7 IL-EB 8 solvent EMIM-BF4 EMIM- BF4 EMIM- BF4 EMIM- BF4 EMIM- BF4 EMIM- BF4 EMIM- BF4 EMIM- BF4 Conducting salt/ c[mol/kg] LiBF4/ 0,25 LiBF4/  0,5 LiBF4/  0,75 LiBF4/  1,0 LiTFSI/ 0,25 LiTFSI/  0,5 LiTFSI/ 0,75 LiTFSI/  1,0 EMIM-BF4: 1-Ethyl-3-methylimidazolium tetrafluoroborate; LiBF4: Lithium tetrafluoroborate; LiTFSI: Lithium bis(trifluoromethanesulfonyl)imide   Solvents (IL = ionic liquid) aluminum dissolution Swagelok  Cells Al | Li  3 – 5 V vs. Li/Li+ No sign of Al dissolution even with the use of LiTFSI Ionic liquid solvent prevent Al corrosion excellently Explanation:            Unable to dissolve the Al salts in the solvent  19 Dr. Andreas Hofmann 09.12.2014   Solvents (IL = ionic liquid) Hofmann et al. Electrochim. Acta  116 (2014) 388 Cells Swagelok Al | Li  3 – 5 V vs. Li/Li+ Composition S-1 – PC + IL S-2 – SL S-3 – SL + IL LiN(SO2CF3)2 Results Al more stable if sulfolane is used  Better stability of Al, if conducting salt concentration is high   aluminum dissolution 20 Dr. Andreas Hofmann 09.12.2014   Solvents (IL = ionic liquid) aluminum dissolution Composition S-1 – PC + IL S-2 – SL S-3 – SL + IL LiN(SO2CF3)2 Results Chronoamperograms at 4.3 V vs. Li/Li+ Confirmation of the optical results  Better stability of Al, if conducting salt concentration is high   21 Dr. Andreas Hofmann 09.12.2014   Solvents (IL = ionic liquid) aluminum dissolution Composition S-1 – PC + IL S-2 – SL S-3 – SL + IL LiN(SO2CF3)2 Results Chronoamperograms at 5 V vs. Li/Li+ Confirmation of the optical results  Better stability of Al, if conducting salt concentration is high and ionic liquid is present   22 Dr. Andreas Hofmann 09.12.2014 Influence of conducting salt Solvent mixture:   Propylene carbonate and ammonium based ionic liquid L-1 LiBF4 L-2 LiOSO2CF3 L-3 LiClO4 L-4 LiPF6 L-5 LiN(SO2CF3)2  measurement after 20 cycle; U = 3 – 5 V (0.1 mV/s) The conducting salts LiOSO2CF3 and LiN(SO2CF3)2  (= LiTFSI) cause severe corrosion Much less corrosion in case of conducting salts with readily available F- ions     Hofmann et al. J. Electrochem. Soc. 161 (2014) A431 aluminum dissolution 23 Dr. Andreas Hofmann 09.12.2014   C-M 1 C-M 2 C-M 3 C-M 4 Solvent (1:1 wt.) EC- DMC EC- DMC EC- DMC EC- DMC Conducting salt   c [mol/kg] LiBF4  1,0 LiTFSI  1,0 LiPF6  1,0 LiOTf  1,0 EC: ethylene carbonate;  DMC: dimethyl carbonate;  LiBF4: Lithium tetrafluoroborate; LiTFSI: Lithium bis(trifluoromethanesulfonyl)imide; LiPF6: lithium hexafluorophosphat; LiOTf: lithium trifluoromethanesulfonate Swagelok  Cells Al | Li  3 – 5 V vs. Li/Li+ State of the art solvent mixture (EC/DMC) Use of LiTFSI and LiOTf yields in pitting corrosion No sign of Al dissolution if LiBF4 or LiPF6 is used up to 5 V vs. Li/Li+ Explanation:            Destruction of the AlF3 and/or Al2O3 layer  Influence of conducting salt aluminum dissolution 24 Dr. Andreas Hofmann 09.12.2014 Improve by additives (e.g. LiBOB) increase of LiBOB-concentration Hofmann et al. J. Electrochem. Soc. 161 (2014) A431 aluminum dissolution Suppress of pitting corrosion using lithium bis(oxalato) borate Cells Swagelok Al | Li  3 – 5 V vs. Li/Li+ Composition Solvent: ammonium based ionic liquid + propylene carbonate L-8 – LiTFSI + LiBOB L-7 – LiPF6 + LiBOB L-6 – LiOTf + LiBOB Results Better stability of Al, if LiBOB concentration is high   25 Dr. Andreas Hofmann  09.12.2014 Improve by additives (e.g. LiBOB) aluminum dissolution Suppress of pitting corrosion using lithium bis(oxalato) borate  Microscopical analysis of the aluminum foil after 20 cycles at 3–5 V  Composition Solvent: ammonium based ionic liquid + propylene carbonate L-8 – LiTFSI + LiBOB L-7 – LiPF6 + LiBOB L-6 – LiOTf + LiBOB Results Less Al dissolution at high concentrations of LiBOB Al stability: LiPF6 > LiTFSI > LiOTf  26 Potential range Non-corrosive conducting salts Additives for additional aluminum protecting layer Use of ionic liquids Solubility of Al salts in electrolyte Conducting salt concentration should be high    Small changes in the electrolyte composition                                     can cause severe effects in cell chemistry! Dr. Andreas Hofmann 09.12.2014 Conclusion:  Important factors to suppress Al-dissolution: Einflussfaktoren auf Al-Korrosion 27 Publications A. Hofmann, M. Schulz, T. Hanemann   “Gel Electrolytes based on Ionic Liquids for Advanced Lithium Polymer Batteries”  Electrochimica Acta, 89, 823-831 (2013). A. Hofmann, M. Schulz, T. Hanemann  „Effect of Conducting Salts in Ionic Liquid based Electrolytes: Viscosity, Conductivity, and Li-Ion Cell Studies”  International Journal of the Electrochemical Science, 8, 10170 - 10189 (2013).  N. Schweikert, A. Hofmann, M. Schulz, M. Scheuermann, S. T Boles, T. Hanemann, H. Hahn, S. Indris  “Suppressed Lithium Dendrite Growth in Lithium Batteries Using Ionic Liquid Electrolytes: Investigation by Electrochemical Impedance Spectroscopy, Scanning Electron Microscopy, and In Situ 7Li Nuclear Magnetic Resonance Spectroscopy”  J. Power Sources, 228, 237-243 (2013). A. Hofmann, L. Merklein, M. Schulz, T. Hanemann  “Anodic Aluminum Dissolution of LiTFSA Containing Electrolytes for Li-Ion-Batteries”  Electrochimica Acta, 116, 388–395 (2014).  A. Hofmann, V. Winkler, M. Schulz, T. Hanemann  „Anodic Aluminum Dissolution in Conducting Salt Containing Electrolytes for Lithium-Ion Batteries”  Journal of the Electrochemical Society, 161, A431 – A438 (2014). Dr. Andreas Hofmann 09.12.2014 28 Acknowledgement Dr. Andreas Hofmann 09.12.2014 Prof. Dr. Thomas Hanemann Lisa Merklein Volker Winkler Dr. Michael Schulz Dr. Martin Tosoni Bing Li Oliver Schwindt Matthias Migeot   29 Dr. Andreas Hofmann 09.12.2014 Thank you very much  for your attention! Further information:  Andreas Hofmann, andreas.hofmann2@kit.edu 

Anodic aluminum dissolution in electrolytes for lithium-ion batteries

KIT – University of the State of Baden-Wuerttemberg andNational Research Center of the Helmholtz AssociationAnodic Aluminum Dissolution in Electrolytes for Lithium-Ion BatteriesAndreas Hofmann*, Lisa Merklein*, Volker Winkler*, Michael Schulz*, Thomas Hanemann*,**Institut für Angewandte MaterialienWerkstoffprozesstechnik*  Karlsruher Institut für Technologie, Institut für Angewandte Materialien - Werkstoffprozesstechnik, Karlsruhe, Germany Andreas Hofmann, e-mail: andreas.hofmann2@kit.edu, Tel. +49 (0)721-608-25920** Universität Freiburg, Professur für Werkstoffprozesstechnik, Institut für Mikrosystemtechnik (IMTEK), Freiburg, Germany Summary MotivationMechanism*Potential range Use of less-toxic conducting salts in Li-ion cells Understanding of the passivation layer on aluminum Specific formation of an Al protective layer Enabling the use of high voltage materials Pre-formation of a sufficient protective layer Investigation of the compatibility of aluminum in Li-ion cells for high voltage applications  Strong dependence of Al dissolution on solvents and conducting salts  Additives are able to prevent anodic aluminum dissolution considerably  Composition of the aluminum surfaceEffect of conducting salt concentration and LiBOB as additiveLiBOB as additiveConclusionsAcknowledgements Anodic aluminum dissolution is critical in high voltage applications Additives can improve the oxidative stability significantly Conducting salts affect the formation of the Al surface layer greatly Ionic liquid based solvents are able to reduce the aluminum dissolution Solvation of Al salts as prerequisite for Al dissolutionBatterietag NRW & Kraftwerk Batterie - No. P1/6We acknowledge IoLiTec Ionic Liquids Technologies GmbH for kindly providing ionic liquids andDr. Martin Tosoni for helpful discussions.Critical potential is highly dependent from solvent, conducting salt and additives SC1: propylene carbonat + ammonium based ionic liquid SC2: sulfolane SC3: sulfolane + ammonium based ionic liquid Conducting salt: LiTFSA(0.5 – 1.0 – 1.5 mol kg-1) E = 3 – 5 V vs. Li/Li+ Al|Li Swagelok cells 20 cycles at 0.1 mV s-1• * Wang et al., Electrochim. Acta 45 (2000) 2677.• Hofmann et al. Electrochim. Acta 116 (2014) 388; Hofmann et al. J. Electrochem. Soc. 161 (2014) A431. Conducting salts are able to destroy the passivation layer Unprotected Al can react readily at common potentials applied in a Li-ion cell (< 4.2 V) In common electrolytes: formation of a stable lithiumoxyfluoride layer Al salts have to be soluble for continual dissolutionsample conducting salt Ecrit / V vs. Li/Li+(first cycle)L-6a LiOTf / LiBOB 4.0L-6b LiOTf / LiBOB 4.4L-6c LiOTf / LiBOB 4.6L-6d LiOTf / LiBOB 4.5L-6e LiOTf / LiBOB > 4.7L-7a LiPF6 / LiBOB 4.1L-7b LiPF6 / LiBOB 4.6LiTFSA Strong dependence of pitting corrosion on potential range Study of Al|Li Swagelok® cells Potential range = 3 – x V (x = 4 – 5 V) Solvent: ammonium-based ionic liquidpropylene carbonate (1:1)  Conducting salt: lithium bis(trifluoro-methanesulfonyl)azanide (= LiTFSA) Critical potential at 4.6 V vs. Li/Li+ 30 cycles at 1 mV s-1LiBF4(after cycling)LiN(SO2CF3)2(after cycling)aluminum reference(without cycling)0.5 mol/kg         1.0 mol/kg          1.5 mol/kg    increase of c (LiBOB)Strong dependence of corrosion pits from solvent mixture and salt concentrationEnhance of LiBOB concentration improves the oxidative Al stability Cycling Swagelok cells (Al|Li, 3 – 5 V vs. Li/Li+) at 0.1 mV s-1 Analysis of the aluminum surface via XPS (depth profiling) F and Li rich species can be detected in the surface layer substituting O containing species by using conducting salts with “available” fluorine ionsSurface modification of Al by conducting salts under cycling 

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Anodic aluminum dissolution in electrolytes for lithium-ion batteries

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