Date of Award

December 2016

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Engineering

First Advisor

Benjamin C. Church

Committee Members

Nidal Abu-Zahra, Pradeep Rohatgi, Junhong Chen, Ilya Avdeev

Keywords

Aluminum, Anions, Corrosion, Electrochemistry, Intermetallic, Lithium-I on Battery

Abstract

Rechargeable lithium ion batteries (LIB) have been widely used as commercial energy storage systems for portable equipment, electronic devices and high power applications (e.g. electronic vehicles). One issue with the commercialized LIB is that expensive, highly toxic and flammable organic solvents are used in the electrolyte and the fabrication process of electrodes. The toxic organic based solvents increase the production cost and lead to significant safety concerns in the event of a battery overcharge or short circuit. The recent development of “green manufacturing” technology allows manufacturers to replace the organic solvents used in the cathode coating process by aqueous based slurries. In addition, the further transition from organic based LIB system to completely aqueous based lithium ion battery (ARLB) has attracted a lot of attention recently because of its potential to significantly reduce manufacturing cost and eliminate the risks and environmental issues associated with the commercialized, organic based lithium ion batteries. Such new aqueous-based technologies often use basic aqueous solutions with high pH value, which brings concerns on the possible occurrence of aluminum current collector corrosion. The corrosion of aluminum current collector in lithium ion batteries is one of the possible factors that affect the long-term performance and safety of lithium-ion batteries. In this work, the corrosion phenomenon of aluminum current collector in lithium ion batteries that use aqueous-based chemistries is explored experimentally and theoretically. Here, the corrosive aqueous media defined in lithium-ion battery systems includes the aqueous based slurry used in the fabrication of cathode coating, aqueous lithium nitrate electrolyte and aqueous lithium sulfate electrolyte. This research aims to reveal the corrosion behavior, corrosion mechanisms and corrosion kinetics of aluminum in exposure to aqueous environment during the fabrication and service life of aqueous-based lithium-ion battery systems, and shed light on the management of corrosion in the design of cost effective lithium ion batteries.

Corrosion of aluminum can occur during the manufacturing of lithium ion batteries when aqueous-based cathode slurries is used during cathode coating process. The corrosion mechanism of AA1085 in exposure to aqueous based cathode slurry was investigated by surface characterization on aluminum after exposure tests and measuring electrochemical characteristics. In exposure tests, the alkaline pH value of aqueous-based cathode slurries and immersing time were revealed as the principle factors that control the corrosion of aluminum during the cathode manufacturing process. The nickel manganese cobalt oxide active material used in the slurry does not have a direct impact on corrosion of the aluminum current collector. The initiation and evolution of localized corrosion on aluminum are closely related to the formation of galvanic cells between aluminum matrix and intermetallic particles. X-ray photoelectron spectroscopy confirmed that the pH of cathode slurry was the only factor that influence the surface composition of aluminum. The oxide passive film gradually degraded into hydroxide with the elapsing exposure time. Electrochemical characterizations showed that aluminum electrodes gave remarkably different response to the different pH of test solutions. The time-pH-variant electrochemical response was ascribed to the change of passive film and electric double layer properties.

The electrochemical stability of high-purity aluminum in 2 M Li2SO4 and 5 M LiNO3 ARLB electrolytes was evaluated over a range of pH conditions by cyclic voltammetry, linear sweep voltammetry and chronoamperometry. Aluminum presented high corrosion resistance at pH 5, pH 7 and pH 9 within the stability windows of both electrolytes. At the pH 11 condition, 2 M Li2SO4 is capable of inhibiting aluminum from pitting, although the inhibiting effect is not sustainable and crystallographic pitting occurs under a continuously applied anodic potential. Aluminum was well passivated against pitting in 5 M LiNO3 electrolyte at pH 11 due to the formation of a thick corrosion product barrier layer. Raman spectra showed the presence of sulfate and nitrate anions on aluminum surface after cyclic voltammetry at pH 11. The chemical adsorption mechanisms of sulfate and nitrate anions on aluminum were proposed to explain the dependency of electrochemical stability of aluminum on pH, anodic potential and type of anions. The applicability of aluminum as current collector in ARLB using the 2 M Li2SO4 and 5 M LiNO3 electrolytes was discussed.

The corrosion kinetics of AA1085 in Li2SO4 and LiNO3 aqueous rechargeable lithium-ion battery electrolytes at pH 11 under the influence of various experimental variables was studied using chromoamperometry. AA1085 is susceptible to crystallographic pitting corrosion in Li2SO4 electrolytes. The rate of pit nucleation and the rate of pitting growth on AA1085 both decreased at higher Li2SO4 concentrations or at lower anodic potentials. In LiNO3 electrolytes, AA1085 was passivated against pitting corrosion due to the formation of a thick, uniform corrosion product layer. The repassivation rate was slightly enhanced by increasing the electrolyte concentration and anodic potentials. X-ray photon electron spectroscopy spectra showed the formation of a thin sulfate-incorporated passive film, which comprises Al2(SO)418H2O, Al(OH)SO4 and Al(OH)3 on electrode before the occurrence of pitting growth in 2 M Li2SO4 electrolyte. The thick corrosion product layer formed in 5 M LiNO3 electrolyte is composed of Al(OH)3 and AlOOH. Raman spectroscopy on deionized water, LiOH solution, Li2SO4 and LiNO3 depicted changes of solution structure with increasing electrolyte concentrations. The influence of extrinsic factors, including the alkaline solution and the anodic potential, and intrinsic factors, such as the surface chemical adsorption of anions, chemical state of passive films and dissolubility of electrolytes, on the corrosion kinetics of AA1085 in slightly alkaline Li2SO4 and LiNO3 electrolytes are revealed.

The intermetallic particles containing Fe and Si in aluminum alloys have electrochemical potentials that differ from that of aluminum matrix, resulting in the formation of galvanic couples and detrimental pitting corrosion. The electrochemical characteristics of AA1100, surface treated AA1100 with “intermetallic-free” surface, home-synthesized Al2Fe and Al2FeSi0.67 alloy were measured by potentiodynamic polarization in alkaline solutions with the addition of Li2SO4 and LiNO3. In general, intermetallic alloys presented noble corrosion potentials compared to AA1100 specimens. The addition of sulfate anions in the solution does not suppress the selective dissolution of aluminum on intermetallic alloys in 0.001 M and 1 M LiOH solutions, which increases the cathodic efficiency of intermetallic alloys and promotes the galvanic corrosion. The corrosion potential difference is significantly reduced when 2 M LiNO3 is added into the alkaline solution. Meanwhile the anodic dissolution rate that corresponds to the preferable dissolution of Al also decreases. Raman spectra revealed that the inhibiting effect of LiNO3 on selective dissolution of aluminum is due to the formation of Fe3O4 passive film above the corrosion potential. the cathodic polarization curves showed that the intermetallic alloys sustain higher cathodic current than AA1100 and surface-treated Al. The magnitude of cathodic current density measured on the electrodes follows the following order: Al2Fe>Al2FeSi0.67>AA1100>surface-treated AA1100. The change of composition and structure on the intermetallic surface during anodic polarization influences the selective dissolution process, the passivity status and in turn affects the cathodic efficiency of the intermetallic.

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