First-Principles Computations of Reactions at the Electrode/Electrolyte Interface Perla B Balbuena Texas A&M University College Station, TX 77843 balbuena@tamu.edu ICTP Cartagena, May 31, 2019 1
Mo Motiv ivati tion on Batte ttery Technology hnology Shift ft in ener ergy gy sour urces ces • Li-ion acceptable for small electronic devices Renewable le Smar art Grids Energies ies • Does not meet long-term performance EVs Near r Futur ture Electri ric Toda day Li-metal Li Li-Air Vehi hicles based Source: memoori.com Increa easing ing de demand and for r ener ergy gy storage $139 B Li Li-ion ion Battery ery Glob obal al Market $30 B droneybee.com Thackeray et al., Energy Environ. Sci., 2012 2012, 5, 7854 2017 2026 TeamViewer.com Research and Markets, Li-ion Battery Outlook (2018) 2
Reactivity: ctivity: Solv lvent ent/LiTFS LiTFSI 1M-LiTFSI/DME LiTFSI/DME 1M-LiTFSI/DOL Li metal reactivity: AIMD simulations show anion reduction due to electron transfer from the surface 5.3 to 6.1 ps Li C O H S F N Camacho-Forero, Smith, Bertolini, Balbuena , JPCC, 2015 3
first principles calculations • Methods of computational surface science/electrochemistry: – DFT, high level ab initio methods – DFT-MD – DFT-MD + free energy calculations • Effect of electrode potential 4
Electrochemical stability eV oc < E g Otherwise: Condition: electron transfer from E electrode to LUMO m a (Li) electrolyte (or m c (Li) vice versa) E g eV oc may occur HOMO Materi rials als desig ign is crucial ial cathode anode Electrolyte/ separator 5
First-principles computational analyses -- understand and predict complex phenomena 6
Solid-Electrolyte-Interphase layer • SEI--surface film formed at anode and cathode surfaces • Due to electrolyte decomposition (reduction or oxidation) • May protect and stabilize anodes (carbon, Li metal); be unstable (metal-oxide cathodes; silicon anodes) 7
SEI “mosaic” composition dense inorganic layer (from salt decomposition) porous organic layer (from solvent decomposition) yes, there are other interfaces in life… Verma, Maire, Novak, EC Acta 2010 any similarity is pure coincidence !! Thickness: from a few Å to tens or hundreds of Å 8
How is the SEI layer formed at the anode/electrolyte interface? Electrolyte: solvent (cyclic and linear carbonates) + salt (e.g. LiPF 6 ) + additives 9
EC reductive dissociation In gas phase: thermodynamically forbidden In solvent: 1 and 2 e - reductions are possible B3PW91/6-311++G(d,p) Wang, Nakamura, Ue, Balbuena, JACS, 123, 11708-11718, (2001) 10
Li + (EC) reductive dissociation Much easier !!! Li ions facilitate the reaction Homolytic ring opening Ion-pair intermediate; e - is transferred to EC Radical anion 11
lithium organic salt Termination with an ester group reactions lithium butylene dicarbonate Li-carbide lithium ethylene dicarbonate + C 2 H 2 insoluble inorganic Another e - transfer Lithium carbonate 12
SEI on carbons for different electrolyte compositions EC Very different SEI composition and EC + DEC + LiPF 6 EC + LiPF 6 product distribution 13 G. Ramos Sanchez, A. Harutyunyan, P . B. Balbuena, JES, 2015
Improving the anode capacity: The Si electrode (capacity: one order of magnitude > carbon) Many Cycles Lithiation Si SEI layer formation Many Cycles Lithiation Si SEI Nano Today, Volume 7, Issue 5, October 2012, 414-429 14
Volta ltage e range e (rel elativ tive e to Li/L /Li + ) 0.8 to~0.4 V ~0.2 V LiSi 15 LiSi 4 LiSi 2 Li Li 13 13 Si Si 4 LiSi (010) • H (100) • O • (101) OH Highly lithiated Early stages of lithiation 15
Extent of lithiation: Effect on EC reduction mechanisms Very low lithiation Li over the surface plane; Si-O E bonds LiSi 15 formed 1 and 2-e - mech. can coexist based on calculated activation energies Ma and Balbuena JES, 2014 C E -O cleavage Intermediate to high lithiation Li 13 Si 4 LiSi 2 LiSi 4 2-e - mech. preferred; at higher lithiation Li on the surface plane or in the 4 e - mech. observed subsurface: Si-C bonds are formed JM Martinez de la Hoz, K Leung and P B Balbuena, ACS Appl. Mat. and Interfaces, 2013 16
VC reduction on lithiated Si anodes Cleavage of C1O2 bond: VC (ads) + 2e - ∙OC 2 H 2 OCO 2- (ads) + 2e - + 2e - a) Li-O interaction All b) Formation of C-Si bond surfaces c) Ring opening (open VC 2- ) d) Cleavage of a 2 nd C1O2 bond Higher lithiated 2- + CO 2 CO 2 formation results from: VC + CO 3 2- ∙OC 2 H 2 OCO 2 2- is a product of EC and oligomers decomposition CO 3 (alternative mechanism to Ushirogata et al, JACS 2013) VC products: open VC 2- , OC 2 H 2 O 2- , OC 2 H 2 OCO 2 2- , CO, CO 2 C=C containing species J. M. Martinez de la Hoz and P. B. Balbuena, PCCP, 16 (32), 17091-17098 (2014) 17
Effect of degree of lithiation on additives very low lithiation FEC: 2 e - mechanism preferred; C C -O E and C c -F bond cleavages: low/moderate barriers ∙OC 2 H 3 - , CO 2 2- , F - Ma and Balbuena JES, 2014 2 e - transfer ring opening multi-electron to FEC reactions ∙OC 2 H 3 O - , CO 2- , F - on highly lithiated surfaces FEC can yield open VC anion C-F bond breaking (path III) and therefore all VC- derived products , in addition to other specific FEC products (paths I, II, and III) ∙OCOC 2 H 2 O 2- , CO 2 2- , H, F - open VC 2- 18 J. M. Martinez de la Hoz and P. B. Balbuena, PCCP, 16, 17091-17098 (2014)
Effects of electrolyte composition AIMD simulations of mixtures of various compositions Salt produces LiF and other fragments interact with solvent products Illustration: mixture 3 15% wt VC 19 JM Martinez de la Hoz, FA Soto and PB Balbuena, JPCC, 2015
How is the ionic/electronic transport through the various SEI components: • LiF (from salt or solvent) Li 2 O (from further reactions among products), Li 2 CO 3 , organic oligomers, polymers?? • How are “good” and “bad” SEI layers characterized? • How does the SEI layer grow beyond the e - tunneling regime?
El Elect ectron ron tr transfer ansfer th thro rough ugh gro rowing wing SE SEI We examined oligomers: CO bond breaking Li Li 2 EDC EDC, , Li 2 VDC VDC energy values in Kcal/mol new radicals are formed; electron transfer shown by blue regions Attack of Li 2 EDC by radical species debilitates its bonds causing fast decomposition. Same for Li 2 VD C 21
SEI from VC/FEC (“good”) vs. EC (“bad”) EC VC/FEC oligomers (formed from Li 2 EDC, Li 2 VDC and others) decompose by radical attack; generate more radicals SEI uncontrolled growth bes est t ad additiv tives es co contr trol ol exce cessive e rad adical cal forma mation tion Soto, Martinez, Ma, Seminario, Balbuena, Chem. Mater. 2015 22
surface structure and electrolyte chemistry play important roles; how does the surface chemistry matter? -native oxides -artificial coating
SiO 2 : lithiation and reactivity hydroxylated amorphous Si surface 24
Lithiation formation energy gy per Si [eV] -0.25 -0.75 tion energy -1.25 Saturation point rmation Forma -1.75 -2.25 0 0.5 1 1.5 2 2.5 3 3.5 4 x in Li Li x SiO iO 2.48 2.48 H 0.9 0.963 63 Δ E(x) = [E(Li x Surface) – x E(Li metallic ) – E(Surface)] / N 25
Structural evolution x = 0 0.37 0.74 1.11 1.48 1.85 2.22 2.59 2.96 3.33 x in Li x SiO 2.48 H 0.963 S. Perez Beltran, G. E. Ramirez-Caballero, and PB Balbuena, JPCC, 2015 26
Lithiation mechanisms in native oxides Li 1 Si 2 O 1 Li 1 Si 1 Li 2 Li 2 Si-O broken, Si-Si formed, Li 6 O complexes formed hydroxylated amorphous film Li x SiO 2.48 H 0.97 x = 0.37 x = 1.48 x = 3.33 Perez-Beltran Ramirez-Caballero & Balbuena, JPCC 2015 27
Lower reactivity of the hydroxylated surface 2 e - reduction of EC decomposition of OH groups and formation of SiH bonds EC (ac) + 2e - → O(C 2 H 4 )OCO 2- 28 (ads)
Aluminum alcoxide (alucone) coating • Collaboration with Chunmei Ban (NREL) film formation, lithiation, reactivity 29
Film lithiation -2.63 eV; -3.15 eV; -3.3 eV binding to agreement with the film stronger experiment: than to Si fast film lithiation 30
Electronic conductivity in alucone film film saturation once the film is saturated with Li, Collaboration it becomes electronically conductive; with C. Ban (NREL) SEI reactions observed Balbuena, Seminario, C.H. Ban et al, ACS Appl. Mater. Inter., 7, 11948, (2015) 31
what type of SEI layer could be formed over alucone-covered Si? 32
Si Simulation tion set etti ting 33
film res estr tructuring; ucturing; Al co coord. . # 5 5 34
EC molecu ecule le bonding nding to AlOx Ox groups ups inside the film or at the interface 35
2 e - red 2 e educ uction tion of EC EC at t th the e aluco al cone/el /electr ectrol olyte te inte terf rface ace Gomez-Ballesteros and Balbuena, JPC Lett 2017 36
Artif ific icia ial l layer er modif ifie ied d & cover ered ed by y a n natu tural al SEI - Solvent and its decomposition products able to penetrate the film -The alucone film is modified because SEI decomposition products form complexes with AlOx groups inside the film or at the interfaces -Reactions may take place at the film/electrolyte interface or at the anode/film interface 37
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