Li-ion batteries (LIBs) are found in many rechargeable battery-powered devices, including mobile phones, laptops, and digital cameras. Researchers are continually searching for alternative electrode materials to improve the cost, performance and energy density of lithium ion batteries. To achieve this, accurate, reliable and rapid techniques are required to measure the materials under development.
The push for new materials
In LIBs, the lithium ions migrate from the anode to the cathode during discharge, and vice versa during charging. The electrolyte medium in LiBs is generally a mixtures of lithium salts and organic solvent, producing a voltage which is at least twice greater than that of standard AA alkaline batteries. LIBs also possess a greater energy per volume ratio (energy density) than both alkaline batteries and other rechargeable batteries and allows LIBs to store more energy and run for longer.
Increasing the energy density of LIBs is one of the major challenges facing LIB research today. Recent efforts to improve LIBs have focused around developing electrode materials with higher energy densities, through holding more charge in a fixed volume. One potential solution is to make an anode composed of silicon (instead of graphite), with the potential of storing up to ten times the current capacity.
In electrode research, a lot of the focus is on the cathode as it plays a significant role in the electrochemical performance of a battery. All three main states of matter are involved in the electrode reactions during charge and discharge. As such, the cathode is required to be porous so that oxygen and electrolytes can be rapidly transported and the cathode can accommodate solid reaction products. These are commonly carbon-based materials.
Another developing area is separators, which is the thin material that separate the anode and the cathode and is crucial for safety. LIBs commonly use separators composed of polymeric materials, sometimes with a ceramic coating, which are permeable and have a uniform pore size (30-100nm) and a porosity of 30-50%. If overheating occurs, the polymer melts and blocks the pores, shutting the battery down.
The importance of porosity for energy density
The energy density of an LIB can be influenced by the size, volume and distribution of the pores and the materials surface area. Cathodes with a higher porosity accommodate more ions and a larger surface area allows for ion formation and decomposition processes to occur. Achieving these simultaneously is difficult, as both properties require differing pore sizes. Porosity is also key for separators.
During R&D and testing phases, battery manufacturers require key data on porosity and other properties, with accurate measurements for porosity, pore size, surface area, particle size and other parameters. This is where Micrometrics Analytical Services comes in, as they offer tailored materials characterisation techniques based around porosity and surface area measurements. These include the TriStar, GeoPyc, Particle Insight, NanoPlus and Autopore series.
Request more information about material characterization solutions for lithium-ion battery research and development
A range of instruments
The TriStar II Plus is an automated surface area and porosity analyzer that delivers excellent performance and speed of analysis. It possesses three analysis ports for a high sample throughput and the specific surface area and pore size distribution are determined using advanced data analysis features.
The GeoPyc 1365 Envelope Density Analyzer is an excellent tool for obtaining solid fraction information. It uses a unique displacement technique and Dry Flo, a quasi-fluid composed of small, rigid spheres with a high flow-ability, to deduce the volume and density of a solid material.
The Particle Insight is a dynamic image analyser to determine the shape and size of particles. There are three models available: 1-150 μm, 3-300μm and 10-800 μm. In these machines, the optic camera enables the analysis of tens of thousands of particles; particle size and the zeta potential are measured by NanoPlus dynamic light scattering (DLS) and are important considerations for the determination of a separators efficiency. This instrument can measure suspended particles ranging from 0.1nm to 12.30μm, with concentrations between 0.00001% to 40%; and zeta potentials of suspensions from -200 mV to +200 mV.
For porosity characterisation, look no further than the Micrometrics AutoPore IV Series of Mercury Porosimeters. These work by applying variable pressures to a mercury-suspended sample, where the pressure to infiltrate the sample’s pores is inversely proportional to the pore size. These instruments determine a broader pore size distribution (0.003 to 1100 µm) quicker and more accurately than other approaches. In addition, mercury porosimetry also calculates numerous sample properties including pore size distributions, total pore volume, total pore surface area, median pore diameter, and sample densities.
The challenge is maximising the energy density of LIBs, without losing other characteristics, and by developing new electrode and separator materials, a range of data on various properties is required. Micrometrics offers a wide range of instruments to provide this data, allowing researchers to fine-tune their individual material and overall battery structures.
"Hierarchical nanoporosity enhanced reversible capacity of bicontinuous nanoporous metal based Li-O2 battery", Guo, X. W. et al, Sci. Rep. 2016, DOI:10.1038/srep33466
"What is the function of the separator?" - Battery University
Lithium-Ion Batteries - Physics Central
Image credit: Shutterstock/