Electrochemical refining of silicon in molten salts

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2 Electrochemical refining of silicon in molten salts Geir Martin Haarberg, Ole Edvard Kongstein, Annabelle Laurent, and Shulan Wang 2 Department of Materials Technology Norwegian University of Science and Technology Trondheim, Norway

3 Background Lack of solar grade silicon due to increased production and use of solar cells Solar energy is expected to be important in the future A new process to produce SoG-Si must be developed Electrolysis in molten salts is an attractive alternative 3

4 Background Silicon can be deposited/produced at a cathode in an electrochemical process in molten salts at elevated temperatures Molten salt electrolysis is used to produce 28 million tonnes of aluminium annually Molten salt electrorefining in molten salts is used to produce high purity aluminium Foxy - a new project supported by EU to develop processes for purification and crystallisation of MG-Si 4

5 Background - Industrial Electrolysis PRODUCT ANNUAL PRODUCTION Electrolyte (million tonnes) Sodium hydroxide 55 Aqueous NaCl Chlorine 50 Aqueous NaCl Aluminium 28 Molten fluoride/chloride Copper 15 Aqueous sulfate Refined aluminium 0.05 Molten fluoride 5

6 Electrochemical refining of Al Aluminium is refined commercially in a so-called "three layer" process MG-Al is dissolved anodically in an electrolyte (refining for more noble elements) cathodic deposition (refining for elements less noble than Al) Molten salt ~800 C % purity ~16kWh/kg >90% CE Al % Electrolyte Chamotte lining Graphite cathode Anode alloy Al-Cu MG-Al MG-Al Al 3+ (diss) Al (99.99x pure) - Graphite Magnesite lining + 6

7 Electrorefining of Si in molten salts Background Olson et al (1980s) Molten LiF, KF, NaF + K 2 SiF 6 at 750 o C, Si-Cu anode Elwell and Feigelson (1980s) Molten BaO, BaF 2, SiO 2 at 1420 o C Espen Olsen et al (SINTEF/NTNU, since 1990s) Molten CaCl 2 -CaO-SiO 2 at 850 o C Ito et al (Kyoto) and Fray et al (Cambridge, since 1990s) Molten CaCl 2 at 850 o C - electrodeoxidation of SiO 2 (s) 7

8 Silicon makes up 27.7 percent of the earth's crust IA 1 H 3 Li 11 Na 19 K 37 Rb 55 Cs 87 Fr 2 IIA 4 Be 12 Mg 20 Ca 38 Sr 56 Ba 88 Ra IIIA IVA VA VIA VIIA VIIIA IB IIB 21 Sc 39 Y 71 Lu 103 Lr 22 Ti 40 Zr 72 Hf 104 Rf 23 V 41 Nb 73 Ta 105 Db 24 Cr 42 Mo 74 W 106 Sg 25 Mn 43 Tc 75 Re 107 Bh 26 Fe 44 Ru 76 Os 108 Hs 27 Co 45 Rh 77 Ir 109 Mt 28 Ni 46 Pd 78 Pt 110 Uun 29 Cu 47 Ag 79 Au 111 Uuu 30 Zn 48 Cd 80 Hg 112 Uub 18 VIIIB IIIB IVB VB VIB VIIB He 5 B 13 Al 31 Ga 49 In 81 Tl 113 Uut 6 C 14 Si 32 Ge 50 Sn 82 Pb 7 N 15 P 33 As 51 Sb 83 Bi 8 O 16 S 34 Se 52 Te 84 Po 9 F 17 Cl 35 Br 53 I 85 At 10 Ne 18 Ar 36 Kr 54 Xe 86 Rn La 89 Ac 58 Ce 90 Th 59 Pr 91 Pa 60 Nd 92 U 61 Pm 93 Np 62 Sm 94 Pu 63 Eu 95 Am 64 Gd 96 Cm 65 Tb 97 Bk 66 Dy 98 Cf 67 Ho 99 Es 68 Er 100 Fm 69 Tm 101 Md 70 Yb 102 No 8 Elements in the earths crust (weight %) : oxygen 46.6, silicon 27.7, aluminium 8.1, iron 5.0, calcium 3.6, sodium 2.8, potassium 2.6, magnesium 2.1, all other 1.5

9 Silicon History Silicon was first isolated and described as an element in 1824 by Jöns Jacob Berzelius, a Swedish chemist. Silicon does not occur uncombined in nature; but it is found in practically all rocks as well as in sand, clays, and soils, combined either with oxygen as silica (SiO 2, silicon dioxide) or with oxygen and other elements (e.g., aluminum, magnesium, calcium, sodium, potassium, or iron) as silicates. Jöns Jacob Berzelius,

10 The Foxy Project (EU) WP2: Cleaning & Refining HDN Deutsche Solar: Highly doped n- type waste WP1: Cleaning & Refining DMR ScanA/SUN: SINTEF/Fesil: SOLSILC feedstock Recycled Si WP3: Electrochemical refining ScanA/SUN: Fesil: SOLSILC MG-Si feedstock production 10 Deutsche Solar: NTNU, SINTEF: SINTEF: Small scale purification SINTEF: n-type purification Electrochemical Fesil: Pilot scale purification Modelling in pilot equipment refining Deutsche Solar: Bridgman crystallisation (large scale) Pillar: Cz crystallisation SINTEF: Bridgman crystallisation (small scale) SINTEF: Bridgman crystallisation (small scale) WP4: Material Characterisation WP5: Cell optimisation WP6:Modules& Recycling SINTEF: SIMS, LECO analysis UKON: lifetime NTNU: GD-MS, PVScan (particle analysis) UMIB: PL, EBIC ECN: ICP-AES, IR, lifetime analysis P-type cell process: N-type cell process: UKON: high efficiency baseline, ECN: industrial UKON: high efficiency baseline, ECN: industrial baseline, Isofoton: industrial pilot baseline, Isofoton: industrial pilot Characterisation: UKON: lifetime, IV/SPR, IR Increased yield: thermography, UMIB: PL, EBIC, ECN: lifetime, ECN: RPECVD, belt furnace gett., UKON: IV/SPR, FTIR, CoRe mechanical stability, MIRHP, tube furnace gett, Isofoton: demo module, n-type module recycling, ECN: LCA WP7: Integration & exploitation

11 Electrorefining principles E ref O 2 /O 2- [V] at 850 C 0.0 O 2 /2O 2- Cu 2+ /Cu -0.5 Reduction potential of various elements at 850 C Fe 3+ /Fe C 4+ /C P 2+ /P B 3+ /B Ti 2+ /Ti Si 4+ /Si Al 3+ /Al Ca 2+ /Ca

12 Electrochemical refining of Si - principles Source: MG-Si Potentially low cost Chloride/fluoride electrolyte anode + cathode - alloy substrate Solid deposit, 800 C Efficient removal of elements less noble than Si (B,P,Ca) will not deposit at the cathode MG -Si Si 4+ CaCl 2 Si SoG- Si Efficient removal of elements more noble than Si will not dissolve anodically Si (with impurities) Si e - (anode) Si 4+ +4e - Si (without impurities) (cathode) 12

13 Target impurity concentrations (ppm w ) in SoG silicon Impurity MG-Si UMG-Si SoG-Si B <30 <1 P <15 <5 O 3000 <2000 <10 C <250 <10 Fe 2000 <150 <10 Al Ca <50 <500 <2 <2 Sarti&Einhaus (2001) Ti 200 <5 1 Cr 50 <

14 Solar grade silicon - Experimental Electrolyte: CaCl 2 + NaCl + CaO at 850 C Gold film furnace W working electrode W reference electrode Si counter electrode Glassy carbon crucible 14

15 Silicon dissolution I /A E /V W Before 4 min. 12 min. 22 min. 42 min. 64 min. Silicon powder is dissolving in the melt quite rapidly Easy to dissolve silicon, but difficult to avoid uncontrolled corrosion of silicon in the melt 15

16 Anode passivation 0.6 I /A V 2 V Si SiO 2 (Insulator) time/ s The current is decreasing with time, probably caused by formation of SiO 2 at the anode Undesired high cell potential or short deposition time 16

17 Electrodeposition of silicon from a room temperature ionic liquid 1-Butyl-1-methylpyrrolidiniumbis(trifluoromethylsulfonyl)imide ([BMP]Tf 2 N) Cation: 1-Butyl-1-methylpyrrolidinium Anion: bis(trifluoromethylsulfonyl)imide At 25 C: Viscosity: 85 mpa s Conductivity: 2.0 ms cm Silicon chloride SiCl 4 is a liquid at room temperature (Boiling point: 57.6 C) SiCl 4 reacts easily with water to form hydrochloric acid (SiCl 4 + 2H 2 O 4HCl +SiO 2 )

18 Experimental Working electrode: Aluminium Reference electrode: Platinum Counter electrode: Platinum Aluminium was first electrodeposited on stainless steel, and the experiments was then performed on the electrodeposited aluminium 18

19 Electrochemical stability of [BMP]Tf 2 N i/ macm Cyclic Sweep voltammetry on aluminium in pure [BMP]Tf 2 N. The sweep rate was 100 mvs E /V Pt The electrochemical window is approximately 4 V using aluminium 19

20 Cyclic voltammetry on aluminium i /ma cm Cyclic sweep voltammetry on aluminium in [BMP]Tf 2 N saturated in SiCl 4 ( ~ 1 M). The sweep rate was 10 mvs E/ V Pt Silicon is probably deposited during the cathodic scan 20

21 Electrodeposition of silicon I / ma cm Silicon was electrodeposited at -2.2 V Pt for two hours. Electrolyte: [BMP]Tf 2 N + 1 M SiCl time/ s The results indicate deposition of silicon. 21

22 SEM and EDAX Element Point 1 Atom % Point 2 Atom % Point 3 Atom % 22 The silicon was probably oxidized, when it was taken out in open atmosphere. The deposition is preferred at the Al grain boundaries Si 13,66 1,12 0,79 O 34,71 9,55 6,87 Al 47,22 29,70 46,30 Fe 2,36 41,99 32,11 Cl 1,05 0,35 0,48 Cr 0,84 13,01 10,25 Ni 0,16 4,28 3,20

23 Conclusion Silicon can be electrodeposited from SiCl 4 in [BMP]Tf 2 N. The electrodeposited silicon is oxidized when exposed to air. 23

24 Si Electrorefining Experimental recent studies Melt composition: 80 mol % CaCl 2 10 mol % CaO 5 mol % NaCl 5 mol % Si Temperature 850 C Argon atmosphere 24

25 Phase diagram for copper silicon The liquid alloy: 31 atomic % Si 69 atomic % Cu 25

26 Results - Electrochemical studies Cyclic voltammograms in molten 81 CaCl 2-6 NaCl - 9 CaO - 4 Si (mol%) on molybdenum electrode, 1223 K 1.5 I/A.cm V/s 0.3V/s 0.4V/s 0.5V/s 0.6V/s 0.7V/s 0.8V/s 1223K E/V Vs Mo 26

27 Deposition at 400 mv (cell voltage) tungsten cathode I / A t / s 27

28 Deposition at -400 mv Element Weight% Atomic% Si, K O, K Totals

29 Deposition at -200 mv tungsten cathode I / A 0,00-0,01-0,02-0,03-0,04-0, t / s 29

30 Deposition at cell voltage -200 mv Element Weight % Atomic % Si, K W, M Totals

31 Deposition at 200 mv Element Weight % Atomic % Si, K Totals Pure silicon was detected by EDS 31

32 Conclusions Electrorefining of Si in Chloride Melts Silicon can dissolve anodically and deposit cathodically Promising results when using a Cu-Si liquid alloy anode and high purity silicon as cathode substrate Apparently good refining results (but better analyses are needed) Larger cell to produce larger quantities must be designed 32

33 33 Thank you