0.65 V

Identifying Redox Active Molecules to Understand Electrochemistry of

Functionalized Silicon Anodes for Lithium-ion BatteriesKate Scholz with Laura Slaymaker and Dr. Robert Hamers

BackgroundSilicon anodes have great promise in Lithium-ion batteries because they have a much greater capacity for lithium intercalation than the more commonly used graphite anodes. However, when cycled silicon anodes expand up to 300% this creates the ability for the greater amount of intercalation, but also causes some problems. With this expansion the SEI (solid electrolyte interface) layer, created when the electrolyte reacts with the anode, can lose adhesion and crumble away leaving new surface area for reaction and eventually depleting the electrolyte.

GoalsPrimary: The goal is to prevent electrolyte from reacting with the surface of the anode by creating a layer of surface attached molecules in an interconnected net, through the silanation of the silicon's surface, that will expand and contract more easily with the anode and thus protecting it from reacting with the electrolyte.

MethodsTo pursue my goals I used a three electrode system (see picture) to do cyclic voltammetry in an inert atmosphere.

Reducing Silanes

Identifying An Indicator MoleculeCriteria: be reversible on Si at a very low potential .5V vs Li to .75V vs Li because Li reduces at -3.04V vs SHE

Lanthanides are a good candidate because of their low reduction potentials around -2.3 V vs SHE

Conclusions• Octadecyltrimethoxysilane does not apparently reduce on silicon• Neither holmium nor samarium are apparently reversible on silicon• Both holmium and samarium are easily reversible on platinum

Intermediate: To test that net, an indicator molecule had to be identified to test the how the “net” effects the electrochemical properties.Also, some of the silanes’ electrochemical properties had to be characterized.

Phenathrenyltriethoxysilane (PATES)

Naphthyltriethoxysilane (NTES)

Styrylethyltrimethoxysilane (STMES)

Octadecyltrimethoxysilane (OTMES)

SEI LayerAnode

Three Electrode SystemElectrodes• Working Electrode (Si wafer or Pt):

takes the measurement• Counter Electrode (Pt): Balances

current during experiment• Reference Electrode (Li): Provides the

baseline for measurementsSolution• 0.01M tetrabutylammonium

perchlorate in dimethyl formamideWorking Electrode

Counter Electrode

Reference Electrode

Functionalization:To functionalize the silicon wafers in silanes, the wafers where cleaned in hydrogen peroxide under UV light and then left over night in a toluene and silane solution.

Understanding the electrochemical properties of these silanes is fundamental to later understanding how the layers of these molecules with effect the anode.

Platinum Working Electrode

Holmium as an Indicator:(holmium(III) trifluoromethansulfonate neat in solution)

P-type Silicon Working Electrode

Samarium as an Indicator:(samarium(III) trifluoromethansulfonate neat in solution)

Platinum Working electrode Silicon Working Electrode

0.55 V

0.58 V

50

Sample CV

Reduction Peaks

Oxidation Peaks

ReferencesSu, X.; Wu, Q.; Li, J.; Xiao, X.; Lott, A.; Lu, W.; Sheldon, B. W.; Wu, J. Adv. Energy Mater. 2014, 4, 1300882. Liu, X. H.; et al. ACS Nano 2012, 6, 1522–1531.P. Silberzan et al., Langmuir, 1991, 7, 1647-1651.