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The choice of material within the electrodes of lithium-ion batteries (LIBS) can allow the battery to possess a much higher energy density than alkaline, and other, rechargeable batteries. Meaning that they can store more energy in a smaller space, but only if the choice of material is right.
Today’s consumerist society requires devices which are small, powerful, durable, long-lasting, charge quickly and need charging less frequently. A LIBs performance and manufacturing ability are governed by the choice of materials used within the battery. The ability to control the pore size and shape, particle size and shape, surface area and density of electrode materials is essential for the optimization of LIB performance.
This article discusses the important microstructural properties of electrode materials and offers an insight into the various solutions offered by Micrometrics.
Fig. 2: Schematic diagram of a lithium ion battery.
To facilitate lithium ion transport between the active materials of the electrode using the electrolyte within the cell, electrode materials need to be porous in nature. Being able to control the porosity increases the interactions between the electrode and the conductive diluent and increases the intra-electrode conductivity, with an adequate lithium-ion intercalation.
Electrodes containing tin, iron and cobalt oxides have traditionally been limited in real-world applications due to changes in the volume causing extreme electrode degradation. By introducing nanostructures into the electrode, the porosity of the electrode material becomes increased allowing the pores to act as buffers for the volume changes. This in turn increases the performance of the battery, whilst showcasing the importance of porosity in electrode material development.
Porosity is commonly measured through the determination of the gas/liquid volume which flows into the electrode material and fills the previously empty void. Micrometrics offers a number of instruments to determine the porosity of the electrode, including the AccuPyc 1340, which uses gas displacement, and the Autopore V, which uses mercury displacement to measure porosity.
Learn more about porosity measurement solutions
Particle Size and Shape
The shape and size of the particles that make up LIB electrode have a significant impact on the final performance of the battery. This could be in a number of ways, including influencing the packing density, porosity, ion diffusion and intercalation properties.
It has been stated that a reduction in the particle size can induce a reduction in the volume change upon intercalation, whilst reducing the mechanical stress and risk of fracture. It has also been documented that it is feasible to tune electrode materials in order to achieve a high energy density or high power depending on their particle size distribution.
Computational studies have shown that a polydisperse distribution of particles can provide up to twice the energy density than is possible with a monodisperse particle size distribution. However, a monodisperse particle distribution produces a higher energy and power density at high discharge rates.
The Sub-Sieve AutoSizer and Saturn DigiSizer II from Micrometrics use air-permeability and light scattering analysis techniques, respectively, to provide particle size analysis. Micrometrics also offers the Particle Insight instrument, which is a state-of-the art dynamic image analyzer that offers particle shape analysis.
See Micromeritics' solutions for particle size and shape measurement
A high surface area reduces the diffusion distance within electrodes and helps to facilitate ion exchange between the electrode and the electrolyte, improving the efficiency of the electrochemical reactions.
However, there can be some associated problems a higher surface area due to the electrode being more degraded through increased electrolyte reactions, and in turn causing the electrode to exhibit a lower thermal stability and capacity loss. Lower surface area materials often increase the cycling performance of the electrode and extend the life of the battery. As such, the optimization of the electrode material’s surface area is essential for providing the ideal balance of properties.
Micrometrics offers the Tristar II Plus, which single and multipoint Brunauer–Emmett–Teller (BET) surface area measurements for electrode materials using nitrogen gas physisorption. By using nitrogen physiosorption to measure the surface area of electrode materials, the user can gain vital insights into various performance variables such as capacity, impedance and rate capability.
Learn more about surface area measurement instrumentation
Density and Tap Density
For an insight into the electrochemical performance of electrode materials, measuring the true or absolute density from the internal pore density with the irreversible capacity of the electrode is often the best way.
The tap density gives an understanding of the volumetric energy density of the material and is considered an essential property for LIBs as it allows for a greater amount of energy to be held within a smaller space. Both the GeoPyc 1365 Envelope and Tap Density Analyzer from Micrometrics identify the bulk density, tap density and envelope volume of electrode materials.
Measure absolute density with Micromeritics solutions
Pore Size, Shape and Tortuosity
The size, shape and tortuosity of the pores in the electrode material have a significant effect on the transport properties of the lithium ions through the electrolyte, with a direct influence on the energy density, power, lifetime and reliability of LIBs.
Understanding how the pores connect with other adjacent pores, closed pores and channels within the electrode material helps to ensure that an optimal electrolyte and electrode interaction is achieved. The Micromeritics AutoPore series of mercury intrusion instruments provide pore size measurements and tortuosity. Scanning electron microscopy, such as the Phenom Pro SEM from Micromeritics permits visual inspection of the surface of the electrode with magnifications up to 130,000x.
Learn more about mercury porosimetry
Researchers who are investigating new electrode materials for LIBs require access to instruments that quickly and accurately measure relevant properties including porosity, pore size and shape, particle size and shape, surface area and density. Micrometrics possesses a wide range of instruments to meet these needs, allowing Researchers to efficiently optimize their electrode materials and develop the next generation of LIBs.
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