2.4 Types of charge carriers in solid electrolytes
4 min read•july 30, 2024
Solid electrolytes contain various charge carriers, primarily ionic and electronic. are the most common mobile carriers in battery electrolytes, while serve in some systems. Electronic carriers include and , contributing to conductivity in certain electrolytes.
Charge carrier behavior is influenced by , , , , and . These factors directly impact the overall conductivity of solid electrolytes. Understanding and controlling these aspects is crucial for optimizing performance in applications like batteries and fuel cells.
Charge carriers in solid electrolytes
Types of charge carriers
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Solid electrolytes contain various charge carriers primarily ionic and electronic in nature
Cations act as the most common mobile charge carriers in solid electrolytes used for batteries (lithium ions in lithium-ion batteries)
Anions serve as charge carriers in certain solid electrolyte systems (oxide ions in solid oxide fuel cells)
Electronic charge carriers include electrons and holes which contribute to conductivity in some solid electrolytes
contain both ionic and electronic charge carriers
Allow simultaneous transport of ions and electrons
Find applications in fuel cells and gas sensors
Factors affecting charge carrier behavior
Concentration and mobility of charge carriers directly influence the overall conductivity of the solid electrolyte
Higher concentration generally leads to increased conductivity
Mobility depends on factors like ion size, charge, and crystal structure
Dopants and defects in the crystal structure introduce additional charge carriers or enhance mobility of existing ones
Aliovalent doping creates vacancies or interstitials
Isovalent doping alters lattice parameters
Temperature impacts charge carrier behavior
Higher temperatures generally increase mobility and conductivity
Follow Arrhenius-type relationship: σ=Aexp(−Ea/kT)
where σ is conductivity, A is a pre-exponential factor, E_a is , k is Boltzmann's constant, and T is temperature
Properties of mobile ions
Ion transport mechanisms
Mobile ions in solid electrolytes move through crystal lattice via hopping mechanisms between vacant sites or interstitial positions
involves ions jumping into neighboring vacant lattice sites
involves ions moving between interstitial positions
Activation energy for ion migration determines ease with which ions move through solid electrolyte structure
Lower activation energy results in higher
Typical values range from 0.1 to 1 eV
Size and charge of mobile ions influence their mobility and overall ionic conductivity of solid electrolyte
Smaller ions generally have higher mobility
Higher charge leads to stronger interactions with the lattice, reducing mobility
Structural influences on ion mobility
in polycrystalline solid electrolytes act as barriers or pathways for ionic conduction depending on composition and structure
Can block ion movement if poorly connected or aligned
Can provide fast conduction pathways if well-designed
Structural features such as or facilitate movement of mobile ions in certain solid electrolyte materials
provide 3D pathways for ion conduction
Layered structures like β-alumina allow 2D ion movement
Concentration of mobile ions affects ionic conductivity with higher concentrations generally leading to increased conductivity
Follows a non-linear relationship due to ion-ion interactions
Optimal concentration depends on specific material and application
Electronic charge carriers' role
Electronic conductivity mechanisms
Electronic charge carriers (electrons and holes) contribute to overall conductivity in some solid electrolytes
and in solid electrolytes determine ease of electronic charge carrier movement
Smaller bandgap materials tend to have higher electronic conductivity
Larger bandgap materials (>4 eV) typically have negligible electronic conductivity
or impurities in solid electrolyte introduce electronic charge carriers
Transition metal ions with multiple oxidation states (Fe2+/Fe3+)
in oxide-based electrolytes
Impact on electrolyte performance
Presence of electronic charge carriers leads to undesired self-discharge in battery systems reducing overall efficiency
Causes capacity loss over time
Shortens battery shelf life
Electronic conductivity in solid electrolytes often undesirable for battery applications as it can lead to internal short-circuiting
Compromises the electrolyte's ability to separate anode and cathode
Can result in safety issues such as thermal runaway
Ratio of ionic to electronic conductivity () serves as crucial parameter in evaluating solid electrolyte performance
Ideal electrolytes have a transference number close to 1 (purely ionic conduction)
Calculated as t_ion = σ_ion / (σ_ion + σ_electron)
Some solid electrolytes such as those used in fuel cells intentionally utilize electronic conductivity for specific electrochemical processes
Mixed ionic-electronic conductors in solid oxide fuel cell electrodes
Allow for expanded reaction zones and improved performance
Mixed ionic-electronic conductivity
Fundamentals of MIECs
Mixed ionic-electronic conductors (MIECs) simultaneously transport both ionic and electronic charge carriers
Total conductivity in MIECs equals the sum of ionic and electronic conductivities
σ_total = σ_ion + σ_electron
MIECs classified based on dominant charge carrier type (ionic or electronic) and relative magnitudes of each conductivity
Predominantly ionic MIECs (t_ion > 0.5)
Predominantly electronic MIECs (t_electron > 0.5)
Balanced MIECs (t_ion ≈ t_electron)
Applications and optimization
describes relationship between chemical potential gradients and charge carrier fluxes in MIECs
Relates oxygen chemical potential gradient to ionic and electronic conductivities
Used to analyze oxygen transport in MIEC membranes
Applications of MIECs include solid oxide fuel cells gas sensors and electrochemical membranes
Cathode materials in solid oxide fuel cells (La0.6Sr0.4Co0.2Fe0.8O3-δ)
Oxygen transport membranes for gas separation (Ba0.5Sr0.5Co0.8Fe0.2O3-δ)
Properties of MIECs tuned by adjusting composition microstructure and environmental conditions
Doping to alter defect concentrations
Controlling grain size and porosity
Modifying temperature and atmosphere
Understanding and controlling balance between ionic and electronic conductivity crucial for optimizing MIEC performance in specific applications
Tailoring conductivity ratios for desired functionality
Balancing transport properties with stability and compatibility requirements