* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Battery Materials
Chemical thermodynamics wikipedia , lookup
Photoelectric effect wikipedia , lookup
Spinodal decomposition wikipedia , lookup
Electrical resistivity and conductivity wikipedia , lookup
Transition state theory wikipedia , lookup
Thermoelectric materials wikipedia , lookup
Bioorthogonal chemistry wikipedia , lookup
X-ray photoelectron spectroscopy wikipedia , lookup
Nanofluidic circuitry wikipedia , lookup
Resonance (chemistry) wikipedia , lookup
Double layer forces wikipedia , lookup
Rutherford backscattering spectrometry wikipedia , lookup
Scanning electrochemical microscopy wikipedia , lookup
History of electrochemistry wikipedia , lookup
Nanogenerator wikipedia , lookup
Lead–acid battery wikipedia , lookup
Electrolysis of water wikipedia , lookup
Gaseous detection device wikipedia , lookup
MaWi SS 2014 Dina Fattakhova-Rohlfing Advanced Materials Science (AMS) Department of Chemistry (LMU) E-mail: [email protected] Tel: 2180 77604, Room E3.002 Battery Materials 1 Batteries Lithium coin battery Car battery Alkaline – manganese dioxide battery Cylindrical lithium - iron disulfide battery 2 Electrochemical energy storage Type Energy An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy Voltage Capacity Most common types: Power Service life Cycle life Charge/discharge cycle Discharge curve Cost Application requirements • Primary galvanic cells „one way cells“, disposable, non-rechargeable • Secondary cells accumulators (rechargeable batteries) • Capacitors - Double layer capacitors (“Supercapacitors“) - Double layer capacitors with near-surface redox processes (“Ultracapacitors“) • Fuel cells 3 Batteries: basic components - Cathode negative electrode during discharge: gives up electrons to the external circuit and is oxidised during the discharge Cathode = positive electrode during discharge: accepts electrons from the external circuit and is reduced during the discharge Anode Anode = + A practical battery has a number of passive components that are not involved in the chemical reaction acting as the energy storage mechanism: - Electrolyte - Separator - Current connectors - Container - Regulating electronics (optional) Minimum set of battery components - anode Active materials producing energy - cathode - electrolyte Passive components - separator 4 Redox reaction xA Counter ions () B ( z) Electroactive compound () x xe A B ( z x) Electrons (Charge) 5 Electrochemical processes: basic principles Thermodynamics Charge (amount) Kinetics Q xFN i Energy W Gr QE Power +z℮- Potential P IE Electrolyte E Gr / xF Overpotential IR Current -z℮- I dQ / dt Cathode 1 / 2 F2 e F Anode Li e Li Resistance I E/R #6 Energy Energy: amount of electrical work that a battery can produce W Gr QE Joule [J] = [CV] Practical unit: Watt-hour [Wh] Watt = Ampere-Volt [AV] Charge Coulomb [C] Potential Volt [V] Specific energy: energy per unit weight Unit: joule per kg [J/kg] Practical unit: watt-hour per kg [Wh/kg] Energy density: energy per unit volume Unit: joule per liter [J/L] or [kJ/L] Practical unit: watt-hour per liter [Wh/L] 7 Energy density 8 Potential (voltage) 9 Potential Anodes (Electropositive) Cathodes (Electronegative) Electrochemical potentials Pa rt Potential of Li batteries E Gr Ecathode Eanode xF J.-M. Tarascon, M. Armand, Nature 2001, 414, 359 12 Voltage Typical voltage of a single cell: 1.5 – 5 V Galvanic cell - + E E cell Serial connection Battery I I cell Parallel connection 13 Capacity (charge) 14 Specific (gravimetric) capacity: what is that • Capacity: the amount of electric charge that a battery can store m Q xF Mw • x = electrons transferred per mole F = Faraday’s constant (96485 C/mol) m = the mass of the material Mw = molecular weight Capacity of one mole of material is QM xF x = equivalents per mole (0 < x < n) xA B xe Ax B Dimension: amount of charge Unit: coulomb (C/kg) Practical unit: Amper-hour per kg [Ah/kg or mAh/g] 15 Specific (gravimetric) capacity : how to calculate it Alkali metals: M = Li, Na, K M eM x=1 Mw(Li) = 6.9 g/mol Mw(Na) = 23 g/mol Mw(K) = 39 g/mol Specific capacity of Li 1 96485 C / mol 13983 C / g 3884 mAh / g 6.9 g / mol Specific capacity of Na Specific capacity of K 1 96485 C / mol 41985 C / g 1165 mAh / g 23 g / mol 1 96485 C / mol 2474 C / g 687 mAh / g 39 g / mol 16 Specific (gravimetric) capacity Maximum (theoretical) capacity Ti ( 4 )O2 4 e 4 Li Ti 0 2 Li 2 O x =4 Mw(TiO2) = 80 g/mol Specific capacity of TiO 2 4 96485 C / mol 4824 C / g 1340 mAh / g 80 g / mol Experimental capacity Ti ( 4 )O2 0.5 e 0.5 Li Li0.5Ti ( 3.5)O2 x =0.5 Mw(TiO2) = 80 g/mol Specific capacity of TiO 2 0.5 96485 C / mol 603 C / g 168 mAh / g 80 g / mol 17 What determines the capacity of different materials? -A A A -A A A + + A A A+ - ℮ The electroneutrality principle: all pure substances carry a net charge of zero • Addition of cations into structure requires the concurrent addition of electrons Electrode xA+ Ionic diffusion + xe- + BX = AxBX Electron diffusion (conductivity) Electrode materials have to be ionic and electronic conductors The capacity depends on: • the amount of electroactive atoms (concentration of the redox species) • the number of electrons which can be transferred per atom The capacity is restricted by the: • presence of vacancies in the crystalline structure for incorporation of guest ions • structure transformations accompanying the addition of guest ions 18 What determines the capacity of different materials? • Insertion of the guest species requires the presence of vacancies (unoccupied states) in the crystalline structure of the host material • Amount of the guest which can be inserted (specific capacity) depends on the concentration of the vacant sites in the crystal lattice (together with the concentration of the redox species) Maximum cell capacity: • High number of available Li sites • Accessibility for multiple valences for M in the insertion host Long cycle life: • As small as possible structural modifications during insertion and extraction • Good structural stability without breaking any M-X bonds 19 Types of host structures: arrangement of vacancies Olivine structure (LiFePO4) Layered structure (LiMnxNiyCozO2) Spinel structure (LiMn2O4) 20 Coulometric titration (Galvanostatic charge-discharge) Galvanostatic technique: • • Applied signal is constant current during some period of time Measured response is potential Q it And at the same time: Q = xFN Thus, one can measure E vs.Q, which means changes in potential of the system with the change in the composition and the amount of phase ≡ Phase diagram 21 Galvanostatic charge-discharge In case when the electrochemical reaction proceeds via a series of sequential reactions: Discharge curve with a series of constant (or sloped) voltage steps at the potentials corresponding to the Gibbs free energy of each reaction step Reaction 1 B + xA = AxB E1 G1 xF Reaction 2 AxB + yA = Ax+yB E2 G2 yF Reaction 3 Ax+yB + zA= Ax+y+zB E3 G3 zF 22 Phase diagrams One-phase region: Potential varies continuously as a function of composition (or state of charge) Two-phase region: Potential does not vary with composition (or state of charge) Ei Ei0 Change in composition (activity) leads to a change in potential according to Nernst equation RT ln ai nF Activity is constant, ln ai 0 Ei Ei0 23 Sequential reactions Lattice parameters of the electrode material: • • Remain constant within the two-phase regions Vary with the composition within the single-phase solution reactions 24 Lithium batteries: Cathode (positive) materials Requirements for the positive materials 1. High Li chemical potential to maximize the cell voltage: Transition metal ion Mn+ should have a high oxidation state 2. Insertion of a large amount of Li to maximize the cell capacity: Depends on the number of available Li sites and the accessibility for multiple valences for electroactive metal atoms. 3. The structural modifications during intercalation and deintercalation should be as small as possible (long cycle life). 4. Mixed conduction: Good electronic conductivity and Li-ion conductivity. What determines the voltage? Positive materials (cathodes) for Li-ion batteries: The compound LixMyXz (X = anion) should have a high Li chemical potential to maximize the cell voltage: Transition metal ion Mn+ should have a high oxidation state 27 What determines the voltage? NaSICON framework of LixM2(XO4)3: MO6 octahedra linked by corners to XO4 tetrahedra 28 Li-ion batteries: positive materials 29 Li-ion batteries: positive materials 1D Olivine structure (LiFePO4) 2D Layered structure (LiMnxNiyCozO2) 3D Spinel structure (LiMn2O4) -Transition metal dioxides LiMO2 (M = V, Cr, Fe, Co, Ni) -spinels (manganese oxides d-MnO2, LiMn2O4) • van der Waals gap between the layers • Li insertion between the layers • Cross-linked channels allowing Li insertion; sometimes significant volume changes upon Li insertion small degree of lattice expansion/contraction upon 30 Li insertion 2D materials: Layered structures Hexagonal/symmetry based on the -NaFeO2 structure • Cubic close-packed oxygen array of edge-sharing [MO6] octahedra. • Lithium resides In between these layers in octahedral [LiO6] coordination, leading to alternating (111) planes of the cubic rock-salt structure. The (111) ordering induces a slight distortion of the lattice to hexagonal symmetry. LiCoO2: <180 Ah/kg Z. Yang et al, Chem Rev. 2011, 111, 3577 31 3D materials: spinels Spinel-type LiMn2O4: voltage of > 4.0 V versus Li FeO6 Spinels : PO4 • lithium ions occupy tetrahedral sites (8a), transition metal ions reside at octahedral sites (16d) • contains empty tetrahedral (8b, 48f) and octahedra (16c) sites • Three-dimensional lattice for the insertion of lithium ions because of their cubic structure Li+ Z. Yang et al, Chem Rev. 2011, 111, 3577 32 1D materials: NaSICON or olivine structures Olivine LiFePO4 in projection along [001] direction Framework built on FeO6 octahedra and PO4 tetrahedra. Tunnel structure, with Li diffusion path along [010] direction • Oxyanion scaffolded structures • corner-sharing MO6 octahedra (M = Fe, Ti, V or Nb) and XO4 tetrahedral anions (X = S, P, As, Mo or W) • Potential: 3 - 3.5 V vs. Li • Tuning potential via altering the nature of X in the M–O–X bonds Olivine LiFePO4: Capacity ca. 170 mAh/g Voltage 3.45 V vs. Li Z. Yang et al, Chem Rev. 2011, 111, 3577 33 Lithium batteries: Anode (negative) materials 34 Anode (negative) materials Lithium-metal Specific capacity: ca. 3900 A·h/kg Very negative potential But: Formation of dendrites upon charging Short circuiting of the cell. Negative: metallic Li Positive: insertion host. M. Winter, Adv. Mater. 1998, 10, 725 35 Li-ion batteries: negative materials Formation of dendrites upon re-deposition of Li: can penetrate the separator, short the battery, lead to thermal runaway, and eventually cause a fire. K. Xu, Chem. Rev. 2004, 104, 4303 36 Negative materials Alloys • • Large specific capacity (780 A·h/kg) Very negative potential But: Large volume changes upon Li insertion: • electrode crumbling, • loss of electrical contact between particles, • rapid capacity loss. Very promising: tin and silicon anodes Negative: dimensionally unstable insertion host (Li alloy, LixM) Positive: dimensionally stable insertion host. • • Form lithium-rich materials (Li4.4Sn and Li4Si) Great energy-storage capability, very large capacities (up to 4 electrons/atom) But: Very large volume changes severely limiting extended deep cycling. M. Winter, Adv. Mater. 1998, 10, 725 37 Negative materials Lithium-ion (“rocking chair”) Dimensionally stable anode materials: carbon • • • Reacts with lithium to form the intercalation compound LiC6 very readily at room temperature Only a 5% increase in volume Used in essentially all lithium batteries since 1990 But: Negative: dimensionally stable insertion host Low gravimetric capacity: ca. 340 A·h/kg Low volumetric capacity: 740 A·h/l Low rate of intercalation of lithium into the carbon Positive: dimensionally stable insertion host M. Winter, Adv. Mater. 1998, 10, 725 38 Lithium insertion in graphite During intercalation: • Change of stacking order of the carbon layers from AB to AA • Stepwise formation of a periodic array of unoccupied layer gaps (stage formation) • In the fully intercalated state, the lithium is distributed in-plane in such a manner that it avoids the occupation of the nearest neighbor sites. Staging is related to: • The energy required to expand the van der Waals gap between the layers • The repulsive interactions between guest species Few but highly occupied van der Waals gaps are energetically favored over a more random distribution of guests. M. Winter, Adv. Mater. 1998, 10, 725 39 Lithium insertion in graphite Stage index s: Number of graphene layers between two nearest guest layers xLi+ +xe + C6 LixC6 0<x≤1 M. Winter, Adv. Mater. 1998, 10, 725 40 Li-insertion batteries Li-ion batteries made from “zero” straining electrodes Z. Yang, Chem. Rev. 2011, 111, 3577 41 Li-insertion batteries Combinations of Negative and Positive Electrodes in Li-Ion Batteries Z. Yang, Chem. Rev. 2011, 111, 3577 42 Electrolyte 43 Negative materials: stability of electrolytes Thermodynamic stability requires locating the electrode electrochemical potentials μA and μC within the window of the electrolyte An anode with a μA above the LUMO of electrolyte will reduce the electrolyte. A cathode with a μC below the HOMO will oxidize the electrolyte Potential window of water: 1.23 V To achieve the higher voltages, the non-aqueous electrolytes have to be used 44 Electrolyte Requirements: • High ionic conductivity • Enabling high cycling rates over a wide range of temperatures • High chemical and electrochemical stability to allow for higher-voltage systems • Compatibility with other cell components, low corrosion/reaction; • Low cost, environmental friendliness. Typical solvents: polar ethers and esters Ethylenecarbonate Dimethoxyethane Lithium batteries use electrolytes containing the salt LiPF6 dissolved in a mixed carbonate solvent. LiPF6 salt can produce HF in even traces of moisture, which can cause dissolution of the cathode materials. Research opportunity: ionic liquids (salts that are liquid under ambient conditions). Low vapor pressures, nonflammable Can be too reactive to be used with lithium, can form complexes with some cathode materials 45 Electrolyte stability window 1M LiPF6 in ethylene carbonate/dimethoxyethane (EC/DEC 1:1) 46 Solid electrolyte interface (SEI) Solid electrolyte interphase (SEI): • Passivating layer at the electrode/electrolyte boundary • Permeable for Li+, non-permeable for other electrolyte components • • The structure of SEI depends on the electrode material and the electrolyte used A mix of organic and inorganic components, at present ill characterized 47 Microstructure of the electrodes Implementation of the active materials into a 3D-conducting matrix (carbon) 48 Kinetic properties Thermodynamic properties Battery characteristics Capacity Q = xF [mAh] = [3.6 C] Specific capacity Q = xF/Mw [Ah/kg] = [3.6 C/g] Charge density QV = Q∙ [Ah/L] = [3.6 C/mL] Specific energy Spec. energy = E∙Q [Wh/kg] = [3.6 J/g] Energy density Energy density = E∙QV [Wh/L] = [3.6 J/mL] • • • Energy density depends on capacity and voltage Inactive components of the battery reduce practical energy density Practical energy density is typically only 20 – 25 % of the theoretical value Specific power P = E∙I/Mw [W/kg] Power density PV = P∙ [W/L] • Power depends on the kinetics of electrodes, interfaces and electrolyte M = molecular mass = density 49