Lithium Ion Battery Dynamics

Lithium Ion Movement During Charging/Discharging

• Lithium ions move to maintain charge neutrality during redox reactions.
• During discharge, the anode oxidizes (Li → Li+ + e-), electrons flow through the external circuit, and lithium ions flow toward the cathode.
• During charge, lithium de-intercalates from the cathode and migrates back to the anode.
• In systems without lithium metal, Li+ ions act as charge carriers, balancing electron flow as zinc oxidizes and Li+ moves to the cathode to neutralize negative charge.

Electrolyte Composition Impact on Gas Evolution and Battery Lifetime

• Electrolyte composition affects solvent stability, SEI formation, and the redox window.
• Solvent stability: Unstable solvents decompose easily, forming gases like CO2, H2, and hydrocarbons.
• SEI formation: Stable SEIs reduce further decomposition, while unstable SEIs constantly regenerate and consume electrolyte.
• Redox window: Electrolytes that degrade outside a narrow voltage range produce unwanted byproducts.
• Non-uniform wetting due to the separator can cause regions of the electrolyte to become electrochemically overdriven, increasing gas production and reducing battery lifetime and safety.

Polymers or Plastics as Separators

• Polymers are widely used in commercial batteries
• Polymers are chemically stable, flexible, and tunable in thickness and porosity.
• Surface energy and wettability of polymers affect battery function, especially gas production.
• Thicker, unconventional plastics are tested to see if their surface properties correlate with electrolyte wetting behavior and electrochemical side effects.

Temperature Effect on Reaction Rates

• Temperature affects ion mobility, reaction kinetics, viscosity, and stability.
• Ion mobility increases with temperature, leading to faster diffusion.
• Reaction kinetics: Higher temperatures result in faster redox rates at electrodes.
• Viscosity decreases as electrolytes become less viscous, improving ion transport.
• Stability: Too much heat can cause thermal decomposition of the electrolyte, separator softening, or runaway gas evolution.
• Small temperature changes can alter electrolyte wetting of the separator and stability of the EC/PC mix, affecting gas production.

Separator Porosity Effect on Battery Efficiency and Stability

• Higher porosity leads to better ion flow and lower resistance, but may allow dendrites to pass through, causing short circuits.
• Lower porosity is safer but leads to higher resistance and slower ion transport, increasing concentration gradients and cell degradation.
• Thick separators likely reduce effective porosity, slowing down lithium-ion flow and leading to localized side reactions.

Voltage and Current Relation to Reactions

• Voltage drives redox reactions, and high voltage can force unwanted decomposition.
• Current reflects the rate of redox reactions, with high current indicating fast electron transfer and high ion demand.
• Local voltages may spike in non-uniform regions, increasing localized reaction rates and electrolyte decomposition.

Reasons for Battery Bulging or Leaking

• Gas buildup from solvent or salt decomposition
• Thermal runaway from overheating or short circuits.
• Increased internal pressure due to trapped gases, causing bulging or rupture.
• Separators that trap bubbles or fail to wet evenly create gas pockets, building pressure

Consequences of Overcharging

• Pushes voltage beyond the electrochemical stability window.
• Results in electrolyte breakdown into gas or solids.
• Damages electrodes via plating, dendrite formation, or excessive SEI growth.
• Leads to capacity fade, gas evolution, and thermal instability, potentially causing fire or explosion.
• Regions of high local resistance can act like mini-overcharged zones, especially near poorly wetted separators.

Separator Materials and Dendrite Formation

• Larger pore size in some separator materials allows dendrites to grow.
• Softer materials with lower mechanical strength are easier to puncture.
• Uneven wetting causes uneven current distribution, encouraging dendrite growth.
• Surface chemistry affects ion adsorption and current distribution.
• Separator behavior, even without lithium metal, relates to ion flux uniformity, affecting gas or degradation.

Battery "Memory Effect"

• The memory effect primarily occurs in Ni-Cd and NiMH batteries.
• Repeated charging/discharging over a limited range forms crystals or irregular deposits, reducing usable capacity.
• It does not typically occur in lithium-ion cells.
• Non-uniform separator performance might cause spatially limited reactions, similar to a "localized memory effect," building gas or resistance in specific areas.

Separator and Electrolyte Material Choices

• Thick plastic separator materials are used to test the relationship between wettability and gas production.
• These plastics are chemically inert, easily available, and have varying surface energies.
• A 7:3 EC:PC mix with 1M lithium salt is the electrolyte, a standard composition for lithium-ion batteries.
• EC helps form a solid electrolyte interphase (SEI). PC lowers viscosity and improves ionic mobility, isolating the impact of the separator on gas evolution.

Challenges with Material Compatibility

• Overcoming separator thickness and geometry. Commercial separators are thin and mine are thick and non-porous, which raised questions about wetting the EC:PC mix electrolyte without significant ionic transport
• Ensuring electrolyte seepage and retention since the plastics aren't designed to absorb or wick electrolyte

Separator Thickness Effects on Ion Mobility

• A thicker separator raises internal resistance and can limit ion transport. Ions would likely take longer to move between electrodes, particularly if the material is not porous
• This increase helped highlight separator-induced effects by making poor wettability result in localized ion accumulation or solvent decomposition

Advanced Equipment for Further Testing

• Electrochemical Impedance Spectroscopy (EIS) to quantify internal resistance and model how separators affect charge transfer and diffusion resistance.
• Gas chromatography or mass spectrometry to identify specific gases produced during operation.
• Porosity and surface energy measurements via BET analysis or goniometers for high-precision wettability metrics.
• Infrared or Raman spectroscopy to analyze potential chemical changes in the electrolyte or separator after operation.

Real World Applications

• Understanding how separator materials influence battery stability and gas production is relevant to separator material selection, battery safety, and more
• Battery safety as in gas buildup that leads to swelling, leakage, or even fires
• Alternative separator materials may be cheaper or more sustainable in Recyclable or low-cost battery design that would lead to cheaper, more green batteries
• Separator engineering as a lever for improving battery performance in custom or nonstandard battery designs

Ion Migration with Thick or Non-Porous Separators

• Ions that don't make it through can still move through the bulk electrolyte
• Ions can diffuse around the edges or along the surface, especially if the separator wets well
• Separators with better wettability enable enabling some surface conduction.

Driving Forces for Lithium-Ion Migration

• The main driving force is the electrochemical potential gradient (the difference in electric potential between the electrodes).
• Secondary driving forces include concentration gradients (Fick's Law) and solvation shell dynamics.
• Lithium movement happens even with Cu and Zn because the salt dissociates and balances charge flow initiated by zinc oxidation.

Dielectric Constant Influence

• A high dielectric constant helps reduce electrostatic attraction between lithium ions and their counterions, allowing the salt to fully dissociate into free-moving ions.
• EC (~90) promotes strong ion dissociation.
• PC (~65) improves viscosity and wetting.
• The EC:PC mix ensures sufficient ion separation, while PC improves fluidity.

Solvated Lithium-Ion Complexes Role

• When lithium ions dissolve in the electrolyte, they become solvated and are the mobile charge carriers.
• The EC:PC ratio affects solvation structure, viscosity, and stability.

Ionic vs. Electronic Conductivity

• Ionic conductivity is the ability of the electrolyte and separator to let ions (Li+) move.
• Electronic conductivity is how well electrons move through electrodes and external circuits.
• High ionic conductivity in the electrolyte + separator.
• High electronic conductivity in the electrodes.
• No electronic conductivity in the electrolyte/separator.
• Low ionic conductivity leads to voltage drop, heat generation, and gas production.

Concentration Polarization & Diffusion Limitations

• Concentration polarization occurs when ions are consumed faster at the electrode than they can be replenished, causing local concentration gradients.
• Diffusion limitations happen when the physical movement of ions is hindered (e.g., thick separator, low porosity, high viscosity electrolyte).
• A non-wetting separator might prevent electrolyte from spreading evenly, causing lithium-ion buildup.
• Thick, non-porous separators limit diffusion paths.
• Local zones of high or low ion concentration cause overpotential, electrolyte decomposition, and gas formation.

Solid Electrolyte Interphase (SEI) Formation

• The SEI forms when the electrolyte (especially EC) decomposes slightly at low voltages during the first few cycles, primarily at the anode.
• Typical reactions include EC + e- + Li+ forming Li2CO3 + organic compounds and LiPF6 forming LiF + PF5.
• The SEI passivates the electrode, allows Li+ transport but blocks electron flow, and prevents further electrolyte breakdown.
• Undergo reduction reactions, forming thin SEI-like layers on the zinc or copper surfaces, influencing gas generation and surface resistance.

Factors Affecting SEI Stability

• Electrolyte composition, with solvents like EC and additives like vinylene carbonate stabilizing the SEI.
• Temperature which degrades the SEI at high temps.
• Current density which causes uneven SEI growth or cracking at high current density.
• Mechanical stress from volume changes can fracture the SEI.
• Unstable SEI leads to breaking and reforming, consuming more electrolyte, producing more gas, and increasing resistance and heat.

Metallic Passivation Layers

• Zinc and copper can form native oxide or salt layers (native oxide or salt layers) when exposed to electrolyte components
• Formation of surface films on zinc might lead to local hot spots for electrolyte reduction, amplifying gas reduction, leads to less wetting of electrolyte
• May initially reduce current flow but cracks and reforms like SEI analogs

Electrolyte Decomposition Differences

• Electrolyte decomposition depends on electrode potential
• Under anode (low potential), there's reduction of solvant (EC) and LiPF6 breaks down to form SEI
• Under cathode (high potential), there's oxidation of solvent or salt anioins

Common SEI Components

• The SEI is formed by inorganic compounds, organic compounds, and salt residues to function in Li-ion batteries.
• By using EC as a solvant, the surface generates partial SEI formation on zinc with a unstable/incomplete with high reduction affinity
• An unstable or incomplete SEI might amplify gas formation due to poor ionic transport

Consequences of Separator Failure

• Electrodes may touch causing a short circuit if the separator fails physically
• Ionic resistance may initially drop under an over-saturated separator.
• Excess electrolyte in the separator can enable continuous side reactions.
• Nonuniform cuurrent paths trigger decomposition reactions because of higher contact angles reducing electrolytes

Wetted vs. Not Wetted Separator

• A wetted separator has uniform ionic transport, lower resistance, stable redox conditions
• As opposed to a not wetted separator that has fluctuating ionic transport, higher resistance, electro breakdown from hotspots, and localized gas formation being formed

Impact of Tortuosity

• High tortuosity causes low diffusion and high resistance
• Pore shape and alignment, or thickness can affect tourtuosity in the electrode

Separator Thickness & Ion Diffusion

• Diffusion time scales with the square of the thickness

Separator Engineering with reaction uniformity and stability

• Correlating contact angle with the surface to test for high surface energy materials increases the rate of more uniform batteries

Thermal Gradient Influencer

• Influences ion mobility, viscosity, pressure, and reactivity to disrupt ion uniformity and gas pressure dynamics

Thermal Gradient Reactions

• Overheating or overcharging has a risk of forming ethylene gas and CO2
• High temps degrade the SEI, increasing gas production, electrolyte concentration and more heat
• Driving the electrodes beyond the redox window while promoting electrolyte oxidation and can cause corrosion

Internal Gas Pressure Relations

• Leads to separator deformation and delamination in electrochemical batteries
• The bubbles get blocked around electrolytes, causing pressure buildup due to disrupting content and reducing electrode area
• Results to gas evoluation and effects conductivity which leads to driving solvents out of solution in batteries operation
• Creating a feedback loop because batteries are at risked of failure due to the small defects

How stress impacts wettability in electrochemical

• Affects bending/compressive, distort surface roughtness and creating microcracking in the seperator
• Internal/Thermal stress can change depending on the way electronytes they bsorb.
• mechanical deformation introduces nonuniformity and is electrochemical. Mechanical deformation introduces non uniformity which makes a portion behave unlike unlike another

Mechanical Rigidity Influencer

• Mechanical Rigidity protects battery safety by preventing internal short circuts

High/Low Rigidity Influencer

• Resistent due to cycling/assembly and is less likley to collapse
• May not form well due to electorlyte poling reducing ion starcation. Can not conform well to electrilyte surfaces.

Primary Isolate Impactin Gas Production

• To isolate electrode/electrolyte by the materials used, its fixed
• Standard duration is 8 hours at ambient temperature/ current

Causes of Impurities

• Can arise from numerous sources as a method to exclude variabes.
• Known well-wetting separators (cellulose paper) to esablish a reference

Consistency of electrodes

• Electrodes cleaned with sanding/ethanol rinse with same dimensions

What would happen if contacted angles is extremely high

• Indicate super hydrophic behaivor where it cannot wet the separator.
• Leads to gas evoluation/corrosion and the battery will not react initially

Quantify the ionic resistance introduced

• Data can be obtained from electrchemical impediance spectroscopy
• Factors to test it includes: geometry/thickness and contact angle

Feedback Mechanisms over time

• Is one of the most important to understand batteries are nonlinear system with loop that can spiral out
• Poor separation, high resistance, decompensation, gas accumulation, conduct and more