Watts New with Clean Energy Materials: Batteries included
Professor Saiful Islam, Department of Chemistry, Bath University
The lecture and slides can be found by clicking the link below:
The challenge for the future is to make it low carbon. What are the green alternatives to fossil fuels?
There is no single solution. We have to make a mix of all the alternatives. There are also political and economic problems. Research into alternative energy sources costs money. The future is uncertain.
CHP may become the norm in the home. SOFC is a bridging technology.
A solid oxide fuel cell (SOFC) is an electrochemical conversion device that produces electricity directly from oxidizing a fuel. Fuel cells are characterized by their electrolyte material; the SOFC has a solid oxide or ceramic, electrolyte. Advantages of this class of fuel cells include high efficiency, long-term stability, fuel flexibility, low emissions, and relatively low cost. The largest disadvantage is the high operating temperature which results in longer start-up times and mechanical and chemical compatibility issues.
Fuel cells and lithium batteries both use crystalline materials.
Lithium batteries are disposable (primary) batteries that have lithium metal or lithium compounds as an anode. They stand apart from other batteries in their high charge density (long life) and high cost per unit. Depending on the design and chemical compounds used, lithium cells can produce voltages from 1.5 V (comparable to a zinc–carbon or alkaline battery) to about 3.7 V.
Upper left photograph is of a CR2032 lithium button cell battery and the upper right photograph shows Lithium 9 volt, AA, & AAA size cells.
The challenge in materials chemistry is that for major advances we need new materials and a greater scientific understanding.
Key questions that need answers
What makes a solid imperfect?
What is a solid oxide fuel cell?
How do lithium ions move in a mobile phone?
A crystal such as sodium chloride has ionic bonding in all directions. Crystals form regular repeating units.
Sodium ion is smaller than the chloride ion.
However most crystals are not perfect.
Silver chloride is an ionic crystal, like sodium chloride.
The major defect in silver halides is the Frenkel defect, where silver ions are located interstitially (Agi+) in high concentration with their corresponding negatively charged silver ion vacancies (Agv−). One of the silver ions moves from its proper place and leaves a vacancy.
Above left is defect free silver chloride structure. Above right is silver chloride structure with two Frenkel defects.
It is often the defects that give crystals their unique properties.
Interstitials are a variety of crystallographic defects, i.e. atoms which occupy a site in the crystal structure at which there is usually not an atom, or two or more atoms sharing one or more lattice sites such that the number of atoms is larger than the number of lattice sites. They are generally high energy configurations. Intrinsic means that are natural to the crystal.
Ionic conduction (denoted by λ-lambda) is the movement of an ion from one site to another through defects in the crystal lattice of a solid (or aqueous solution). The pathway is often not known in new materials.
The properties of a crystal can be changed by adding impurities/dopants. This is what gives some crystals unique colours.
Computer modelling can complement experiments and be predictive in finding out how defects can affect a crystal.
Bonding is the “glue” that keeps the ions together and ionic bonding acts in all directions. The interatomic forces producing there bonds can be mathematically modelled.
Modelling has changed a great deal in the last sixty years.
In the 1950s, three groups made it their goal to determine the structure of DNA. The most famous of these groups consisted of Francis Crick and James D. Watson at Cambridge who built physical models using metal rods and balls. They only really became successful after Maurice Wilkins and Rosalind Franklin, at King’s College London, examined X-ray diffraction patterns of DNA fibres and produced good quality diffraction patterns and thus produced sufficient quantitative data about the structure, which Watson and Crick got to see.
Crick, Watson and Wilkins got the 1962 Nobel prize.
Now scientists just put the information into a computer which then builds the model.
What are the modelling aims?
To give an insight on the atomic-scale and allow predictions to be made;
To allow fundamental science to underpin technology;
The surface of the crystal is important as it can be used for catalysis. A catalyst is a substance that increases the rate of a chemical reaction without being used up. Modelling allows this to be investigated without having to do lot of experiments.
Similarly ion conduction can be modelled and the effects of vacancies and dopants investigated.
Michael Faraday was one of the earliest people to show that certain solids could conduct. The “Godfather” of Ion conduction.
Michael Faraday, FRS (22 September 1791 – 25 August 1867) was an English scientist who contributed to the fields of electromagnetism and electrochemistry. His main discoveries include those of electromagnetic induction, diamagnetism and electrolysis. He said in 1835 “…that materials when sought for would conduct electricity in the solid state”
There several types of fuel cell.
A solid oxide fuel cell (SOFC) is an electrochemical conversion device that produces electricity directly from oxidizing a fuel. Fuel cells are characterized by their electrolyte material; the SOFC has a solid oxide or ceramic, electrolyte. Advantages of this class of fuel cells include high efficiency, long-term stability, fuel flexibility, low emissions, and relatively low cost. The largest disadvantage is the high operating temperature which results in longer start-up times and mechanical and chemical compatibility issues. They also have a high cost. We need to find lower temperature materials.
Proton exchange membrane fuel cells, also known as polymer electrolyte membrane (PEM) fuel cells (PEMFC), are a type of fuel cell being developed for transport applications as well as for stationary fuel cell applications and portable fuel cell applications. Their distinguishing features include lower temperature/pressure ranges (50 to 100 °C) and a special polymer electrolyte membrane. PEMFCs operate on a similar principle to their younger sister technology PEM electrolysis. They are a leading candidate to replace the aging alkaline fuel cell technology, which was used in the Space Shuttle.
Both fuel cells and batteries produce electrical energy from a chemical reaction (electrochemistry).
Electrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor (the electrode: a metal or a semiconductor) and an ionic conductor (the electrolyte). These reactions involve electron transfer between the electrode and the electrolyte or species in solution.
A fuel cell is an electrochemical sandwich
Above is scheme of a solid-oxide fuel cell which is fuel flexible. The air contains oxygen, which is an oxidant.
2o– + 4H+ –> 2H2O This is a high temperature process (900 – 1000oC)
Si- and Ge-apatite compounds are attracting considerable interest as new fast oxide-ion conductors for use in solid oxide fuel cells (SOFCs).
Apatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite and chlorapatite, named for high concentrations of OH−, F− and Cl− ions, respectively, in the crystal. The formula of the admixture of the four most common endmembers is written as Ca10(PO4)6(OH,F,Cl)2, and the crystal unit cell formulae of the individual minerals are written as Ca10(PO4)6(OH)2, Ca10(PO4)6(F)2 and Ca10(PO4)6(Cl)2.. The calcium formsare found in bones and teeth. New silicate forms are good conductors. What are there atomic structures like?
Silicon dioxide, also known as silica (from the Latin silex), is a chemical compound that is an oxide of silicon with the chemical formula SiO2. It has been known since ancient times. Silica is most commonly found in nature as sand or quartz, as well as in the cell walls of diatoms (frustule).
Tetrahedral coordination of silica (SiO2), the basic building block
Each silicon-oxygen tetrahedral is connected at the corners.
Apatite ionic conductors (rare earth silicates) were first found by Nakayama et al. in 1995. Early studies have revealed that there are many interesting characteristics in these materials. The most important one, however, is their ionic conductivity. They show fairly high ionic conductivity in an intermediate temperature range of 500-800° C. In addition, they have ionic transport numbers which are very close to 1.0 with a wide oxygen partial pressure range, thus being considered as candidates for intermediate temperature solid oxide fuel cell (IT-SOFC) electrolytes. Another interesting characteristic is their unique conduction mechanism. They show an interstitial conduction mechanism with their ionic conductivity enhanced by excess oxide ions. The ionic conductivity of lanthanum silicate reported by Nakayama et al., however, was still low (10-20 mS cm-1) for the SOFC application, thus research has been started to enhance the conductivity.
Silicon allows the use of cheap materials.
Do they have ionic defects? How do the ions move?
The path is curved near the tetrahedral. NMR confirmed it. Ion conduction is negative and there is a charge inbalance.
Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a magnetic field absorb and re-emit electromagnetic radiation.
Apatite-type oxides of general formula La9.33+x(SiO4)6O2 + 3x/2 have been attracting considerable interest recently because of their observed high oxide-ion conductivity and potential use in solid oxide fuel cells (SOFCs), oxygen sensors, and ceramic membranes. Computer modelling techniques are used to investigate, at the atomic level, the energetics of defect formation, oxide-ion migration, and cation migration in the oxygen-excess apatite silicate, La9.67(SiO4)6O2.5. Recent research has suggested that oxide-ion conduction in these apatite systems proceeds by an interstitial mechanism.
In the future doping of choice could be Boron oxynitride (BON) forming Bon apatite.
Application of lithium batteries
Size matters. Electrical equipment needs to be portable so the batteries need to be small and light. The energy density of lithium is three times the energy density of other cells. The industry is worth £1 billion. That works out as 3 billion cells per year)
The fundamental research was done in the 1980s and the SONY cell was produced in 1991.
Low carbon transport
One method of reducing carbon emissions is to change the technology for running our transport. At the moment cars, trucks, buses, and trains represent nearly 30% of global greenhouse gas emissions. With increased urbanisation and development, this sector represents one of the fastest growing sources of emissions, as more people become dependent on motorised transport. 80% of car journeys are less than 40 miles.
One method for reducing carbon emissions is to use hybrid vehicles.
A hybrid vehicle is a vehicle that uses two or more distinct power sources to move the vehicle. The term most commonly refers to hybrid electric vehicles (HEVs), which combine an internal combustion engine and one or more electric motors. However other mechanisms to capture and utilise energy are included.
The Toyota Prius is the world’s top selling hybrid electric vehicle, with cumulative global sales of over 3 million units by June 2013.
Electric cars are becoming more popular.
An electric car is a car that is propelled by one electric motor or more, using electrical energy stored in batteries or another energy storage device. Electric motors give electric cars instant torque, creating strong and smooth acceleration.
The above left picture shows a smart electric car charging at an on-street station. This method isn’t particularly green as the electricity has probably been generated at a fossil fuel power station.
The Nissan Leaf (also formatted “LEAF” as a backronym for Leading, Environmentally friendly, Affordable, Family car) is a five-door hatchback electric car manufactured by Nissan and introduced in Japan and the United States in December 2010. The US Environmental Protection Agency official range for the 2013 model year Leaf is 121 km and rated the Leaf’s combined fuel economy at 115 miles per US gallon gasoline equivalent (2.0 L/100 km). The 2013 Leaf has a range of 200 km on the New European Driving Cycle.
Materials for lithium batteries
A lithium-ion battery (sometimes Li-ion battery or LIB) is a member of a family of rechargeable battery types in which lithium ions move from the anode to the cathode during discharge and back when charging. Li-ion batteries use an intercalated lithium compound as the electrode material, compared to the metallic lithium used in non-rechargeable lithium battery.
The positive electrode is made of Lithium cobalt oxide, or LiCoO2. The negative electrode is made of carbon graphite. When the battery charges, ions of lithium move through the electrolyte from the positive electrode to the negative electrode and attach to the carbon. During discharge, the lithium ions move back to the LiCoO2 from the carbon.
There needs to be a move away from cobalt as it is toxic.
The above picture shows the diffusion path of lithium ions in the layered structure of the cobalt oxide sheets of the LiCoO2 electrode.
The lithium migrates from one octahedral site to another by passing through an intermediate tetrahedral site.
If we want to make new battery materials does the shape of the crystal particle have an effect? If we stick to lithium what path will mobile lithium ions take?
We need to use new iron or manganese based materials.
SUPERGEN brings together some of the UK’s leading academics to investigate and overcome the hurdles involved in transforming energy storage.
The Scottish Fuel Cell Consortium (SFCC) has developed Scotland’s first fuel cell battery hybrid powered electric car. The vehicle is equipped with an alkaline fuel cell range extender, compressed hydrogen gas storage, a lead acid battery pack, and a water-cooled induction motor drive system. The prototype fuel cell vehicle is a Mark 1 drivetrain demonstration unit with the lowest possible cost configuration achievable with standard production items. This hybrid drivetrain and system configuration is also being applied to a small delivery van, retrofitted to take a fuel cell/battery electric drive, and an 18-seat battery-powered bus with a fuel cell range extender for inner city transport use. Other units for transport fleet application customers are in development.
The fuel cell battery hybrid drivetrain has been packaged into the space frame of an AC Cobra sports car. The range which the vehicle can achieve is a function of the amount of hydrogen stored onboard which, in this case, doubles the range available from the lead acid battery pack.
Lithium iron phosphate: LiFePO4 (A new direction)
The lithium iron phosphate (LiFePO4) battery, also called LFP battery (with “LFP” standing for “lithium ferrophosphate”), is a type of rechargeable battery, specifically a lithium-ion battery, which uses LiFePO4 as a cathode material. LiFePO4 batteries have somewhat lower energy density than the more common LiCoO2 design found in consumer electronics (such as power tools), but offers longer lifetimes, better power density (the rate that energy can be drawn from them), cheapness and are inherently safer (they are non-toxic and the P-O bonds are stronger). LiFePO4 is finding a number of roles in vehicle use and backup power.
The above left picture shows the structure of the lithium iron phosphate electrode. The blue parts are the iron-oxygen octahedral, the red parts are the phosphate tetrahedral and the grey parts are the lithium ions.
The above picture is a model of the path of lithium ions. The circles represent the iron ocatahedra and the phosphate tetrahedra. The ions will take the lowest energy path available.
The images above are from the research work of Prof. Yamada, Mr. Nishimura and Prof. Kanno and was published in Nature Materials, Vol.7, pp.707-711 (2008) ”Experimental visualization of lithium diffusion in LixFePO4” Diffusion paths of lithium ions in the olivine-type iron phosphate for next-generation lithium ion battery was visualized experimentally for the first time. It shows that the predictions were right and that the lithium ions do follow a curved path.
Modelling predicted that Lithium iron phosphate (LiFePO4) would have a thin platelet shape. Guoying Chen and Thomas Richardson of Lawrence Berkeley National Laboratory’s Environmental Energy Technologies Division showed this to be true.
Below is an image showing that as lithium ions move out of a hexagonal lithium iron phosphate crystal, the material undergoes a phase shift to iron phosphate. Since ions flow only in the b direction, the ideal particle shape is a plate of lithium iron phosphate as thin as possible.
The advantage of the thin platelet shape is that lithium ions move easily in and out which is important for battery performance. Movement is maximised with many channels.
The real purpose of Chen and Richardson’s work is to make lithium batteries safer. They wanted to create an electrolyte that wouldn’t burst into flames if the device overheated and electrodes that were less prone to overcharge. Lithium iron phosphate, LiFePO4, for use as the cathode material seems to be the answer. In lithium iron phosphate, strong chemical bonds between the phosphorus and oxygen, known as covalent bonds, reduce the tendency of the cathode to release oxygen gas and batteries made with this material show longer shelf-life, a larger number of charging cycles, and greater stability. They also have a high charge capacity, ideal for the plug-in hybrid application.
Materials chemistry is important for the development of clean energy applications. At the moment charging is not fully reversible and research is looking in to overcome this.