Nano structured electrodes for lithium ion batteries Vivek Krishnan Materials Research and Education Center Auburn University, AL
Presentation Outline • Introduction to Batteries – History and Principles • Solid State Batteries • Lithium Battery Technology • Nano-technology in lithium batteries • Conclusions and future trends
Introduction to BatteriesHistory • Voltaic Cell- In the last years of the 18th century Alessandro Volta (1745-1827) built the first electrochemical cell to generate power. • Leclanche Cell- The Leclanche Cell was described by Georges Leclanche (1839-1882) in 1867. http://www2.kenyon.edu/depts/physics/EarlyApparatus/Electricity/ Electrochemical_Cell/Electrochemical_Cell.html
General Principle and Classification • Principle • Electrons released at the negative electrode. • These flow through the load and are accepted by the positive electrode. • A voltage is measured due to the potential difference between the two electrodes of the cell. http://spacepwr.jpl.nasa.gov/battery.htm#Lithium
Solid State Batteries Classification • Primary and Secondary Batteries. • Solid State Batteries. Properties of solid state devices • natural seal. • resistance to shock and vibrations. • broad stability of electrolyte. • high selectivity of charge carriers. • temperature and pressure resistance. • simpler designs • POTENTIAL TO BE MINIATURIZED. http://www.itnes.com/pages/batteries.html
Advantages of Thin Film Processing • Devices manufactured using the same techniques as the microelectronics industry. Use of silicon. • Deposition in a vacuum chamber avoids moisture problems. • Very good adhesion between layers and large contact areas. • electrode/electrolyte resistance. • battery encapsulation is simple - insulating layer. • microbatteries can be constructed in almost any two dimensional shape. “Lithium batteries: New Materials, Developments and Perspectives”, G. Pistoia, Elsevier 1994
Lithium Batteries • Introduced by Sony in 1991. • Have been widely used to provide power for consumer products. • Offer low safety risks, greater flexibility in battery configuration and energy densities exceeding 120 Wh/kg. • Excellent pressure tolerance and neutral buoyancy. • Make use of intercalant solids as electrodes. • Host atoms or molecules within its lattice with very few structural changes. “Lithium batteries: New Materials, Developments and Perspectives”, G. Pistoia, Elsevier 1994
Working Principles • Electrochemical chain characterized by continued transport of lithium ions from a higher potential ( anode) to a lower potential (cathode). • Electrical energy liberated while discharging is equal to the change in lithium free energy due to the transfer. • Cell reactions in a Li/ Lix(cathode) system: • δx Li δx Li + + δx e- (Li anode) • δx Li + + δx e- + Lix(host)Lix + δx (host cathode) • Overall reaction: • δx Li + Lix(host)Lix + δx(host) “Lithium batteries: New Materials, Developments and Perspectives”, G. Pistoia, Elsevier 1994
Lithium MicroBattery • Miniaturized power supply needed for micro-mechanical devices. • Lithium microbatteries built using thin film technologies. 1 Lithium metal complex 2 Electrolyte 3 Intercalating electrode Substrate Hundreds of microcells on a four-inch diameter silicon wafer. Typical lithium microgenerator http://www.afrlhorizons.com/Briefs/Dec01/PR0104.html
Intercalation Electrodes • Lithiated metal oxides- • VOx, LiCoO2, LixMn2O4, LixNiO2, LixSn • Thin film deposition- CVD, RF Sputtering, Pulsed Laser Deposition. “Lithium batteries: New Materials, Developments and Perspectives”, G. Pistoia, Elsevier 1994
Advantages of Lithium Ion Batteries • Store 2-3 times more energy per unit weight and volume than lead-acid or Ni-Cd batteries. • Long cycle lives (>1000 cycles) • Low self-discharge and long shelf life. • Widespread use in electronic devices. • Potential applications promise in the areas of communications and remote sensing devices too!!! Sides, C.R.; Li, N.; Patrissi, C.J.; Scrosati, B.; Martin, C.R. “Nanoscale Materials for Li-ion Batteries,” MRS Bulletin, 2002, 27, 604-607.
Limitations • Critical area for improvement – rate capability • Rate capability- ability to deliver large capacity when discharged at high C rates. ( rate of C/1 corresponds to the current required to completely discharge an electrode in 1 hour) • Future applications require high-discharge-rate periods. Sides, C.R.; Li, N.; Patrissi, C.J.; Scrosati, B.; Martin, C.R. “Nanoscale Materials for Li-ion Batteries,” MRS Bulletin, 2002, 27, 604-607.
Motivation for use of Nano technology • Limitations in rate capabilities – slow diffusion process • Shorter diffusion distance for Li+ ion • Increased surface area • Promise better rate capabilities. • Smaller effective current density during discharge. • Better cyclability due to smaller particles. • Need for energy sources to power nano devices Martin, C.R.; Li, N.; Scrosati, B. “Nanomaterial-Based Li-Ion Battery Electrodes,” J. Power Sources, 2001, 97-98, 240-243.
Fabrication Technique • Template method- • General method to synthesize nanomaterials. • Synthesis entails deposition of material of interest/ precursor, within cylindrical and monodisperse pores of a microporous template membrane. • Cylindrical nanostructures with monodisperse diameters and lengths obtained. • May be solid nanofibers or hollow nanotubes depending on membrane used. C.R.Martin, Science, 266, 1961 (1994)
Electrode Fabrication Precursor deposition Gel formation Template removal Thermal processing • 50 nm pores filled with triisopropoxyvanadium oxide. • Gel formation after 12 hrs. • Template removed with oxygen plasma(100mTorr O2, 2hrs) • Processed at 400C for 10hrs in 150psi O2 Patrissi, C.J.; Martin, C.R. J. Electrochem. Soc. 1999, 146, 3176-3180.
SEM micrographs SEM images of the componenents of a nanostructured electrode: (A) low-magnification image of the V2O5 nanofibrils, (B) high-magnification image of the nanofibrils, and (C) the underlying V2O5 surface layer. Patrissi, C.J.; Martin, C.R. J. Electrochem. Soc., 2001, 148, A1247-A1253.
Sn-based anodes • Can store twice as much lithium compared to carbon anodes. • 4Li+ + 4e- + SnO2 2Li2O + Sn • xLi + + xe - + Sn LixSn • Can store upto 4.4 Li atoms per atom of Sn. • Volume changes during alloying/dealloying cause internal damage to electrode – lower cyclability. • Nanostructure based designs can better accommodate for volume changes Li, Naichao; Martin, C.R. J. Electrochem. Soc. 2001, 148, A164-A170.
Sn- based anodes • Fabricated using template synthesis with a SnCl2 based precursor. • Nanofibers heated at 440C to convert them to crystalline SnO2. • Fiber dia. – 110nm • Thin film electrode fabricated without template membrane.(550nm) Li, Naichao; Martin, C.R. J. Electrochem. Soc. 2001, 148, A164-A170.
Template based electrodes • Observed improved rate capabilities • Loss of volumetric energy density due to extremely low porosity of polycarbonate membranes(1.2%) • Low number density of nanofibers protruding from current collector surface. • Problem addressed by using alumina membranes (highly porous) • Dissolution of membrane in aqueous acid or base; but this also dissolves the electrode materials. • Chemical etching used to increase porosity of polycarbonate membranes. Sides, C.R.; Li, N.; Patrissi, C.J.; Scrosati, B.; Martin, C.R. MRS Bulletin, 2002, 27, 604-607.
Conclusions and Future Trends • Nano materials are useful to fabricate lithium-ion batteries with improved performances. • Current research is on to develop improved anodes, cathodes and electrolytes. • Work is needed to integrate these components and build devices. • Prototype device predicted within 3 years. (An AAAAAAAAA battery???!!!) http://www.napa.ufl.edu/2002news/nanobattery.htm
Questions 1. Why is intercalation important in secondary lithium batteries? 2. What is the template method for fabricating nanomaterials?
Glossary • Cycle LifeHow many charge/discharge cycles the battery can endure before it loses its ability to hold a useful charge. • Current density Electric flux per unit area. It is generally defined in terms of the geometric or projected electrode area and is measured in Am-2 or ma cm-2: cut-off voltage: Final voltage of a discharge or charge operation. • Capacity, rated: The value of the output capability of a battery, expressed in Ah, at a given discharge rate before the voltage falls below a given cut-off value • Depth of Discharge The amount of energy that has been removed from a battery (or battery pack). • ElectrodeA conductor by which electrical current enters or leaves a non-metallic medium, such as the electrolyte in a battery (as well as vacuum tubes and lots of other devices). • ElectrolyteAn elctrically conductive medium, in which current flow is due to the movement of ions. In a lead-acid battery, the electrolyte is a solution of sulfuric acid. In other batteries, the electrolyte may be very different. • Energy Density The amount of energy that can be contained in a specific quantity of the fuel source. Typically quoted in watt-hours per pound, wh/lb, or watt-hours per kilogram, wh/kg. http://www.rtpnet.org/~teaa/battery.html
Glossary contd. • Inhibitor - A substance added to the electrolyte which prevents an electrochemical process, generally by modifying the surface state of an electrode. • Load - Electrical power being consumed at any given moment. • Open circuit voltage (OCV): The voltage of a cell or battery under no-load condition, measured with a high impedance voltmeter or potentiometer. • Overpotential, overvoltage: Difference between the actual electrode voltage when a current is passing and the equilibrium (zero current) potential. • Polarization:Deviation from equilibrium conditions in an electrode or galvanic cell caused by the passage of current. It is related to the irreversible phenomena at the electrodes (electrode polarization) or in the electrolytic phase (concentration polarization). • Power density: The power output of a battery per unit volume, usually expressed in W dm-3 and quoted at 80 per cent depth of discharge. • Shelf-life:Period of time a cell can be kept idle after manufacture without significant deterioration. • Working electrode:the test or specimen electrode in an electrochemical cell http://voltaicpower.com/Principles/Glossary.htm
Feedback sheet Information given by the presenter: Date: 06/23/03 Presenter’s name: Vivek Krishnan Name of student turning in this form _______________________ From 1 to 10 (ten being the best), how do you grade the materials presented? ______ From 1 to 10 (ten being the best), were complete references given for each side? ______ From 1 to 10 (ten being the best), how well is the presentation understandable? _______ From 1 to 10 (ten being the best), how are the glossary, questions and problems presented? ______ Suggestion: