Interfacial processes in Li-ion batteries

Tavassol Farahi, Mohammad Hadi

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Description

Title

Interfacial processes in Li-ion batteries

Author(s)

Tavassol Farahi, Mohammad Hadi

Issue Date

2014-09-16

Director of Research (if dissertation) or Advisor (if thesis)

Gewirth, Andrew A.

Doctoral Committee Chair(s)

Gewirth, Andrew A.

Committee Member(s)

• Moore, Jeffrey S.
• Murphy, Catherine J.
• Nuzzo, Ralph G.

Department of Study

Chemistry

Discipline

Chemistry

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Interfacial electrochemistry
• Energy
• Energy storage
• Li-ion Batteries
• Electrochemical interfacial processes

Abstract

This thesis focuses on interfacial processes in Li ion batteries. Rechargeable Li-ion batteries are among the best technologies available for energy storage in both portable electronics and transportation scale applications. Li-ion batteries store and discharge energy by accumulation and transport of Li+ between two electrodes, known as the anode and the cathode. During Li+ transport, a solid electrolyte inter-phase (SEI) composed of breakdown products from the electrode, the solvent, and the electrolyte, forms on both cathode and anode electrode surfaces. The composition and stability of the SEI to a large degree controls performance of the battery. This thesis investigates interfacial processes in Li ion battery electrodes as they relate to SEI, surface coverage, and structural properties. SEI Oligomerization: In this section. we report the results of electrochemical quartz crystal microbalance (EQCM), and Matrix assisted laser desorption ionization (MALDI) time of flight (TOF) mass spectrometry (MS) measurements along with detailed calculations examining the formation of the solid electrolyte interphase (SEI) on battery anode electrodes. EQCM analysis of Au and Sn surfaces in propylene carbonate (PC) and a 1:1 mixture of ethylene carbonate and dimethyl carbonate (EC:DMC) showed major irreversible mass uptake by the electrode surface especially during the first five cycles between +2 and 0.1 V vs. Li/Li+. MALDI-MS on emersed electrodes showed that long chain (m/z = 3000 on PC) oligomerized species were present on Au surfaces in PC and EC:DMC solvents, where oligomerized species formed in PC solutions showed higher mass ratios. The repeating units of the oligomer, visible as oscillations in the MALDI-MS, vary with the type of the solvent and electrode material. Sn surfaces initially showed formation of long chain polymers, but this material was not in evidence on electrode emersed after five cycles, which likely arises as a consequence of the catalytic involvement of Sn in decomposition of initially formed species. Density functional theory (DFT) calculations of cyclic solvent molecules suggested a radical initiated polymerization mechanism and predict oligomer subunits consistent with the experimental results. SEI and Surface Coverage Induced Stress Effects: In this section, we report electrochemical surface stress and potential dependent matrix assisted laser desorption ionization (MALDI)-time of flight (TOF) mass spectrometry (MS) results combined with detailed density functional theory (DFT) analysis of Li deposition on a Au model system for Li ion battery anodes. Deposition of Li on Au surfaces at potentials >0.2 V vs. Li/Li+ occurs through the formation of a Li-Au surface alloy, a result that is predicted by DFT calculations. As the Au surface potential becomes more cathodic, compressive stress develops on the surface, a result again predicted from calculation. The compressive stress is completely removed by cycling the potential back to 2.0 V vs. Li/Li+ through delithiation of the surface alloy. Lithiation of the Au electrode during Li bulk alloy formation at potentials < 0.2 V vs. Li/Li+ results in compressive stress, as expected. However, in this case residual tensile stress is observed following delithiation, the magnitude of which increases with increasing lithiation/delithiation cycles. Potential dependent MALDI-TOF MS analysis shows that solid electrolyte interphase (SEI) oligomers are formed during delithiation following Li bulk alloy formation and that these oligomers are the likely origin of the observed residual tensile stress. This residual tensile stress is not present when the carbonate solvent is replaced with an ionic liquid. These results show that surface stress is determined by Li-host atomistic interactions as well as the nature of the SEI. Oxides Effect on Reversibility and Alloying Reactions in Sn anodes: In this section, we examine the effect of varying the oxygen content in Sn and SnOx films during potential dependent SnOx conversion reactions and LiySn alloying relevant to Li ion battery anodes. The films are analyzed by in-situ stress measurements, voltammetry and imaging. For metallic Sn films, the stresses and stability of the films are controlled by Li alloying reactions. Small, non-contacting separated Sn particles exhibit higher electrochemical stability relative to more continuous polycrystalline films. Metallic Sn particles develop tensile stress during LiySn de-alloying as porous structures are formed. The amount of stress associated with lithiation and delithiation of well-separated metallic particles decreases as a porous, easy to lithiate, material forms with cycling. During the lithiation of oxides, conversion reactions (SnOx → Sn) and the lithiation of the metallic Sn control the stress responses of the films, leading to highly potential-dependent stress developments. In particular, we find evidence for a multi-step electrochemical mechanism, in which partially reversible lithiation of the oxygen-containing phases is conjoined with a fully reversible lithiation of the metallic phases of the Sn. The electrochemical stress analysis provides new insights into these mechanisms and delineates the extent of the reversibility of lithiation and conversion reactions of oxides. Evolution of defects in graphene with cycling: In this section, we use Raman spectroscopy and density functional theory (DFT) to investigate the defect formation as a function of lithiation in a model system of monolayer graphene transferred on a Si(111) substrate. This model system enables the early stages of defect formation to be probed in a manner that could not previously be observed with commonly used reduced graphene oxide or multilayer graphene substrates. Using ex situ and Ar-atmosphere Raman spectroscopy, a rapid increase in graphene defect level is detected for small increments in the number of lithiation/delithiation cycles until the I(D)/I(G) ratio reaches ~1.5-2.0 and the 2D peak intensity drops by ~50%, after which the Raman spectra show minimal changes upon further cycling. Using DFT, the interplay between graphene topological defects and chemical functionalization is explored, thus providing insight into the experimental results. In particular, the DFT results show that defects can act as active sites for species that are present in the electrochemical environment such as Li, O, and F. Furthermore, chemical functionalization with these species lowers subsequent defect formation energies, thus accelerating graphene degradation upon cycling. This positive feedback loop continues until the defect concentration reaches a level where lithium diffusion through the graphene can occur in a relatively unimpeded manner, thus minimizing further degradation upon extended cycling.

Graduation Semester

2014-08

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Copyright 2014 Mohammad Hadi Tavassol Farahi

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