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Electrode Analysis


The development of cathode and anode materials for lithium-ion batteries is based
on improvement to power and energy density
as well as the thermal/chemical stability for enhancements in battery life and
charge cycling.

The theoretical capacity of a lithium-ion battery is determined by the materials used.
In electrode processing, knowledge of particle morphology—including particle size, shape, powder density, porosity and surface area—have critical affect to manufacturability and the desired performance characteristics of
the electrode.

Porosity Measurements

The electrodes porosity structure has a direct influence on particle to particle contact between the active material and the conductive diluent. Porosity is essential for the electrolyte to transport lithium-ions to and from the active materials of the electrode.

By controlling porosity, higher intra-electrode conductivity can be achieved to ensure adequate electron exchange as well as sufficient void space for electrolyte access/transport of lithium-ions for intercalation of the cathode. Porosity blocking/clogging during intercalation
can lead to capacity fade.

Particle size influences capacity, cycling, and coulomb efficiency. Particle size will impact the amount of solid-state diffusion of lithium-ions that intercalate at the electrode. Smaller particles, especially nanoparticles, will lead to smaller volume changes upon cycling. This contributes to less mechanical stress, increased hardness, and greater resistance
to fracture.

It has been reported that a broad particle size distribution may increase the energy density more than a mono-dispersed distribution.

Controlling and customizing particle size distribution can result in the ability to make available custom tuning that will result in high power (mono-disperse) or high energy density (poly-disperse).

Shape will affect packing density. Spherical shaped particles will pack more densely than fibrous or flake shaped particles. The average strain energy density stored in a particle increases with the increasing sphericality. Fibrous and flake shaped particles are expected to have lower tendency for mechanical degradation than the
spherical shaped particles.

Increasing the surface area of the electrode will result in improvement in the efficiency of the electrochemical reaction and facilitates
the ion exchange between electrode and electrolyte, especially within the anode as higher surface area permits short diffusion paths to the lithium-ions between the graphite particles. Lower surface area materials are better suited for improved cycling performance of the cell resulting in
longer battery life.

Greater surface area does present some limitations due to the degradation interaction of the electrolyte at the surface and resultant capacity loss along with thermal stability. Nanoparticles hold much promise to increase surface area without capacity loss. For the anode, higher surface area permits shorter diffusion paths for lithium-ions between the graphite particles This facilitates fast charge and more efficient discharge rates and improves the capacity of the battery.

The density of the graphite anode has an effect on its ability to withstand degradation under challenging load and discharge operations. A higher anode electrode particle density decreases the porosity resulting in a lower active surface area of the electrode. This reduces the electrode/electrolyte
contact area.

True/absolute density and envelope density can help by determining electrochemical performance attributed to the electrodes available porosity for intercalation.

A clear correlation has been foundbetween irreversible capacity and internal pore volume.

T.A.P. density measurement is an important indicator of volumetric energy density. A low T.A.P. density translates into a low volumetric energy density where the converse indicates a high volumetric density. Higher T.A.P density permits denser electrode films (more active material per unit volume) to be made for coating the electrode.

The size, shape, and tortuosity of the electrodes pores will significantly affect lithium ion transport rates through the electrolyte retained within this porous structure. Electrode microstructure resulting from the manufacturing process has direct influence on energy density, power, lifetime, and reliability of the lithium-ion cell.

A better understanding of the interconnectivity of adjacent pores, closed pores, and channels that may be created during the manufacturing process helps to ensure optimal electrolyte and electrode interaction. Knowing the tortuosity of a porous electrode and electrolyte interface makes it possible to determine if cell performance limitations are due to its microstructure.