UPTEC K 16013 Examensarbete 30 hp Juli 2016 A study of poly(vinyl alcohol) as a solid polymer electrolyte for lithium ion batteries Gustav Ek Abstract A study of poly(vinyl alcohol) as a solid polymer electrolyte for lithium ion batteries Gustav Ek Teknisk- naturvetenskaplig fakultet UTH-enheten The use of solid polymer electrolytes in lithium-ion batteries has the advantage in terms of safety and processability, however they often lack in terms of performance. Besöksadress: This is of major concern in applications where high current densities or rapidly Ångströmlaboratoriet Lägerhyddsvägen 1 changing currents are important. Such applications include electrical vehicles and Hus 4, Plan 0 energy storage of the electrical grid to accommodate fluctuations when using renewable energy sources such as wind and solar. Postadress: Box 536 751 21 Uppsala In this study, the use of commercial poly(vinyl alcohol) (PVA) as a solid polymer electrolyte for use in lithium-ion batteries has been evaluated. Films were prepared Telefon: using various lithium salts such as lithium bis(trifluoromethane)sulfonimide (LiTFSI) 018 – 471 30 03 and casting techniques. Solvent free films were produced by substituting the solvent Telefax: Dimethyl sulfoxide (DMSO) with water and rigouros drying or by employing a 018 – 471 30 00 hot-pressing technique. The best performing system studied was PVA-LiTFSI-DMSO, which reached ionic conductivities of 4.5E-5 S/cm at room temperature and 0.45 Hemsida: mS/cm at 60 °C. The solvent free films showed a drop of ionic conductivity by http://www.teknat.uu.se/student roughly one order of magnitude compared to films with residual DMSO present. High ionic conductivities in PVA-LiTFSI-DMSO electrolytes are thus ascribed to fast lithium ion transport through the liquid domain of DMSO, or by plasticizing effects of salt and solvent on the polymer. Thermal analysis of the films showed a clear plasticizing effect of DMSO by a decrease in the glass transition temperature. FTIR analysis showed complexation of all the lithium salts investigated with the OH-groups of the polymer by a shift in the characteristic frequencies of both salts and polymer. For the first time, prototype battery cells containing PVA electrolytes were manufactured and evaluated by galvanostatic cycling. PVA-LiTFSI-DMSO showed stable cycling performance for 15 cycles. Solvent free electrolytes were also investigated but did not result in any stable cycling performance. Handledare: Daniel Brandell Ämnesgranskare: Tim Bowden Examinator: Erik Lewin ISSN: 1650-8297, UPTEC K 16013 Popul¨arvetenskaplig sammanfattning Dagens samh¨allest˚arinf¨orstora utmaningar i och med klimatf¨or¨andringarp˚a jorden till f¨oljdav anv¨andandetav fossila br¨anslen.F¨oratt ¨overkomma detta beh¨over vi ist¨alletanv¨andaf¨ornybara energik¨allors˚asomvind- och solkraft, fast dessa har problem med att till exempel vinden inte alltid bl˚aseroch att solen inte skiner p˚anatten. Detta leder till att vi inte alltid kan f˚aut den energi vi beh¨over f¨oratt samh¨alletskall fungera, och vi beh¨over d¨arf¨orspara ¨overskottet fr˚ann¨ardet produceras som mest. En typ av energilagringsteknik som skulle kunna vara en l¨osningp˚adetta ¨ar litium-jon batterier eftersom de kan lagra v¨aldigt mycket energi i j¨amf¨orelse med andra batterier, och har ocks˚av¨aldigtsm˚af¨orluster vid upp- och urladdning. Men eftersom de kan lagra s˚apassmycket energi, s˚auppst˚ardet s¨akerhet- srisker vid storskalig anv¨andning. Detta h¨oroftast samman med anv¨and- ningen av brandfarliga flytande organiska f¨oreningari den s˚akallade elek- trolyten, vars funktion ¨aratt f¨orflyttalitium-joner mellan elektroderna i batteriet. F¨oratt ¨oka s¨akerheten kan man ist¨alletanv¨andasig av en fast elektrolyt best˚aendeav en syntetisk polymer. Problemet med dessa fasta varianter ¨aratt de har s¨amre prestanda ¨ande flytande med avseende p˚a bland annat jonledningsf¨orm˚aga. I detta examensarbete har polymeren poly(vinyl alkohol) studerats f¨oranv¨and- ning som just fast elektrolyt i litium-jon batterier. Polymeren har blandats med olika litiumsalter och l¨osningsmedelf¨oratt kunna se skillnader i pre- standa. Egenskaper s˚asomjonledningsf¨orm˚aga,v¨armeegenskaper och inter- aktioner mellan salt och polymer har studerats. Prototyp-batteri celler in- neh˚allandepoly(vinyl alkohol) som elektrolyt har ocks˚atillverkats och utsatts f¨orrepeterad upp- och urladdning. Resultatet av studien ¨aratt poly(vinyl alkohol) kan fungera i dessa till¨amp- ningar. Men f¨oratt uppn˚atillr¨ackligt h¨ogjonledningsf¨orm˚agas˚akr¨avsatt en viss m¨angdav l¨osningsmedletdimetylsulfoxid finns kvar efter tillverkning av elektrolytfilmerna. Detta pekar p˚aatt polymeren inte leder joner i sin kristallina fas eftersom detta l¨osningsmedel g¨orpolymeren mjukare. III List of abbreviations DMC Dimethyl carbonate ((CH3)2CO) DMSO Dimethyl sulfoxide ((CH3)2OS) DSC Differential scanning calorimetry EC Ethylene carbonate ((CH2O)2CO) EDLC Electric double layer capacitor EIS Electrochemical impedance spectroscopy FTIR Fourier transform infrared spectroscopy LFP Lithium iron phosphate (LiFePO4) { TFSI Bis(trifluoromethane)sulfonimide (N(CF3SO2)2 ) { Tf Trifluoromethanesulfonate (CF3SO3 ) Ni-MH Nickel-metal hydride NMR Nuclear magnetic resonance PVA Poly(vinyl alcohol) PEO Poly(ethylene oxide) PTFE Poly(tetrafluoroethylene) Tg Glass transition temperature TGA Thermograviometric analysis SPE Solid polymer electrolyte VTF Vogel-Tammann-Fulcher IV Contents 1 Introduction 1 1.1 Background . .1 1.1.1 Li-ion batteries . .2 1.1.2 Solid polymer electrolytes (SPE) . .3 1.1.3 Poly(vinyl alcohol) . .5 1.2 PVA-based polymer electrolytes . .6 2 Scope of this thesis 9 3 Theory 10 3.1 Ionic conductivity . 10 3.2 Ion-transport mechanisms in solid polymers . 11 4 Methodology 14 4.1 Electrode and electrolyte preparation . 14 4.1.1 Polymer electrolyte films . 14 4.1.2 Electrodes . 14 4.1.3 Battery cell assembly . 15 V 4.2 Analysis . 15 4.2.1 Thermograviometric Analysis (TGA) . 15 4.2.2 Differential Scanning Calorimetry (DSC) . 16 4.2.3 Electrochemical Impedance Spectroscopy (EIS) . 17 4.2.4 Galvanostatic cell cycling . 20 4.2.5 Fourier Transform Infrared Spectroscopy (FTIR) . 20 5 Experimental 21 5.1 Electrode and electrolyte preparation . 21 5.1.1 Polymer electrolyte films . 21 5.1.2 Electrodes . 23 5.1.3 Battery cell assembly . 23 5.2 Analysis . 24 5.2.1 Thermograviometric Analysis (TGA) . 24 5.2.2 Differential Scanning Caliometry (DSC) . 24 5.2.3 Electrochemical Impedance Spectroscopy (EIS) . 24 5.2.4 Galvanostatic cell cycling . 25 5.2.5 Fourier Transform Infrared Spectroscopy (FTIR) . 25 6 Results and discussion 26 6.1 Ionic conductivity . 26 6.1.1 Variation of salt content in the PVA-LiTFSI-DMSO system . 26 6.1.2 Effects of different salts and solvents . 27 VI 6.2 Thermal properties . 30 6.2.1 TGA . 30 6.2.2 DSC . 32 6.3 Fourier Transform Infrared Spectroscopy (FTIR) . 35 6.3.1 Polymer-salt interactions . 35 6.3.2 Interactions between lithium salt and functional groups 39 6.4 Cycling . 41 7 Conclusions 44 8 Acknowledgements 46 VII 1 Introduction 1.1 Background Today's society faces a number of major challenges in the process of switching from fossil to renewable energy resources. One of these challenges is the nature of renewable energy sources (e.g wind and solar). Energy storage systems need to be put in place in order to accomendate fluctuations in the electrical grid, for instance such as at peak hours [1, 2]. Today, pumped- storage hydroelectric is most used for this purpose, although the technique lacks in term of efficiency seeing at least 15 % of the energy lost [3]. Another challenge is converting the vehicle fleet from gasoline into electric powered or fuel cell based/hydrogen powered. This also calls for not only high energy storage, but also for light weight solutions. Lithium-ion batteries are very promising solutions to these problems because of their superior energy and power density in comparison to other battery systems such as lead-acid and Ni-MH (figure 1.1). The system is also superior in terms of energy conversion efficiency compared to pumped-storage hydro- electricity; the charge-discharge efficiency of commercial Li-ion batteries is around 90 % [4]. As mentioned above, the storage most used today is pumped-hydro which has an energy efficiency of 70-80 % compared to the 90 % of the lithium- ion battery. Hydrogen storage and fuel cells are also alternative solutions to grid stabilisation. These systems however presents problems such as the risk of explosion with compressed hydrogen gas and considerably lower energy efficiencies of around 40 % [5]. 1 Fossil fuels Lithium-ion Energy density [Wh/kg]density Energy NiCd, NiMH Lead Acid Power density [W/kg] Figure 1.1: Ragone chart of different energy storage technologies (redrawn from ref [6]) 1.1.1 Li-ion batteries The state-of-the-art Li-ion battery is made up of two electrodes (a negative anode and a positive cathode), an electrolyte and a separator. It is often referred to as the "rocking chair battery", and functions by shuttling Li+ between the electrodes through the electrolyte (figure 1.2), thus generating a current during discharge. The electrodes are made out of compounds that have a layered or channeled structure, so that they can store Li+ in be- tween the layers/channels. This process is called intercalation, and common materials used today are LiCoO2 as the positive electrode ("cathode"), and graphite as the negative electrode ("anode"). The electrolyte is usually a mixture of organic solvents such as ethylene carbonate (EC) and a variety of linear alkyl carbonates, such as dimethyl carbonate (DMC) and a lithium salt, typically LiPF6 for sufficient ionic transport at high concentrations. To 2 keep the cell from short-circuiting a porous separator is soaked in the liquid electrolyte and placed between the electrodes.