The Lithium-ion battery is an energy storage device capable of almost continuous charging and discharging. In comparison to traditional battery technologies, lithium-ion batteries provide a significantly higher performance and efficiency In terms of application. The lithium-ion battery market can be segmented into consumer electronics, automotive, storage grid energy, and industrial use.
Improving global economic conditions, rising disposable income, the need for clean energy and surging demand for quality and uninterrupted power are a few of the major factors anticipated to boost demand for lithium-ion battery. The power density, safety, recharge time, cost, and other aspects of its technology are expected to continue to advance.
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.
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.
Separator/ Binder Evaluation
The separator permits ion flow from one electrode to the other while preventing any electron flow, essentially separating the anode from the cathode.
The typical separator is made up of polyolefins, usually polypropylene and/or polyethylene, along with other polymers, ceramics, and ceramic/polymer blends.
Separators are highly porous, typically >40% porosity, approximately 25 μm thick and exhibit low ionic resistivity. Layered or composite separators are used as safety devices to prevent thermal runaway of the cell.
Binder materials are used to hold the active electrode material particles together and in contact with the current collectors, i.e. the Aluminum Foil of the cathode or the Copper Foil of the anode.
Specification of percent porosity is an important parameter in the acceptance criteria for the separator. The separator must have sufficient pore density to hold the liquid electrolyte that supports ionic movement between the anode and cathode. Higher porosity means less heat generated in the cell and greater energy density.
Uniform porosity is essential to avoid variations in ion flow. The more variation in ionic flow within the separator, the greater the effect at the surface of the electrode and the quicker it will fail with a significantly decreased cycle life. Excessive porosity hinders the ability of the pores to close, which is vital to allow the separator to shut down an overheating battery.
TriStar II Plus
Surface Area and Porosity Instrument
AccuPyc II 1340
Gas Displacement Pycnometer System
Mercury Intrusion Porosimetry System
Pore Size, Shape, Distributions, and Tortuosity:
Mercury Intrusion Porosimetry System
The separator pore size must be smaller than the particle size of the electrode components, i.e. the electrode active materials and any conducting additives. Most separator membranes contain submicron pore sizes that block the penetration of particles.
Uniform distribution and a tortuous structure of the pores are also a requirement. Uniform distribution prevents uneven current distribution throughout the separator and tortuosity suppresses the growth of
Liquid electrolyte plays a key role in commercial lithium-ion batteries to allow conduction of the lithium-ions between cathode and anode. The most commonly used electrolyte is comprised of lithium salt, such as LiPF6 in an organic solution.
High purity is required to prevent oxidation at the electrode and to promote good cycle life. In addition to lithium salt, various additives are also included in the final electrolyte solution. These additives are mixed with the LiPF6 solution to prevent lithium dendritic formation and degradation of the solution.
There are electro kinetic phenomena caused by charge separation at the separator-electrolyte interface. Diffusion of charged electrolyte solution through the pores of the separator has to undergo the influence of the zeta potential at the interface.
The zeta potential at that interface may impede or aid the passage of the electrolyte across the separator. The zeta value gives an indication of the potential stability of a system: the larger the value (positive or negative) the more stability of the solution.
NanoPlus HD Zeta Measurement Cell
Manufacturing and Failure Analysis
Materials characterization during and prior to manufacturing is a critical control parameter to ensure the optimal operation of cell components and the final assembled battery.
From raw materials to component manufacture and the assembled battery itself, material characterization plays a vital role in determining the desired electrochemical performance, safety, cell cycling and other important parameters.
Particle Size/Particle Shape - Raw Materials:
Particle size and shape influences packing density which in turn affects electrode thickness and therefore energy density.
It has been shown that the particle size distribution of graphite, as well as particle orientation in the coated foil affects the electrochemical performance of graphite anodes. Purity is also an important issue and low levels of metallic impurities must be maintained in all powders and additives used in electrode manufacture.
Calendering is the most critical step in the production of high performance electrodes. Porosity and thickness of the electrode film will decrease with increasing calendering. Calendering would also be expected to change the pore structure of the electrode, which would thereby impact the wetting behavior of the film by the electrolyte.
Calendering beyond the optimum level
causes reductions in porosity and average pore diameter which can result in irreversible capacity loss, high rate cycling, and poor longevity in cycle performance.
Solid Fraction is a control parameter used
in roller compaction operations. This control parameter assists in determining the optimal setting for speed, compression and nip angle in the roller compactor.
Using the the solid fraction as a critical quality attribute will ensure consistent product batch to batch, along with the end product having the designed and desired electrochemical performance.
Solid Fraction, Envelope and True Density
SF= Solid Fraction (relative density)
Over the life of a cell, physical and electrochemical occurrences contribute to degradation in performance. This drop in performance is most notably recognized through capacity fade during charge and discharge cycling or by reduced shelf life.
Expansion and contraction may cause interfacial stress that adversely effects the electrode performance, to the point that delamination may occur causing a reduction in contact between the electrode material and the current collector. Pore size changes can occur from this mechanical failure resulting in reduction in electrolyte contact and poor cycling behavior.