MaWi SS 2014 Dina Fattakhova-Rohlfing Advanced Materials Science (AMS) Department of Chemistry (LMU) E-mail: Dina.Fattakhova@cup.uni-muenchen.de 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 Voltage Capacity Power Service life Cycle life Charge/discharge cycle Discharge curve Cost Application requirements An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy Most common types: 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
Anode Cathode Batteries: basic components Anode = 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 - + 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 ( ) B ( z) xe A ( ) B ( z x) x Counter ions Electroactive compound Electrons (Charge) 5
Electrochemical processes: basic principles Thermodynamics Charge (amount) Q xfn i Kinetics Energy W Gr Q E +z - Power P I E Potential E G / r xf Electrolyte -z - Current I dq / dt Overpotential IR Cathode 1/ 2F 2 e F Anode Li e Li Resistance I E / R # 6
Energy Energy: amount of electrical work that a battery can produce W Gr Q E Charge Coulomb [C] Potential Volt [V] Joule [J] = [C V] Practical unit: Watt-hour [Wh] Watt = Ampere-Volt [AV] 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 G xf r E cathode E anode J.-M. Tarascon, M. Armand, Nature 2001, 414, 359 12
Voltage Typical voltage of a single cell: 1.5 5 V Galvanic cell - + E Ecell Serial connection Battery I Icell Parallel connection 13
Capacity (charge) 14
Specific (gravimetric) capacity: what is that Capacity: the amount of electric charge that a battery can store Q xf m M w x = electrons transferred per mole F = Faraday s constant (96485 C/mol) m = the mass of the material M w = molecular weight Capacity of one mole of material is Q M xf x = equivalents per mole (0 < x < n) xa B xe A x 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 e M x = 1 M w (Li) = 6.9 g/mol M w (Na) = 23 g/mol M w (K) = 39 g/mol Specific capacity of Li 1 96485 C / mol 6.9 g / mol 13983 C / g 3884 mah / g Specific capacity of Na 1 96485 C / mol 23 g / mol 41985 C / g 1165 mah / g Specific capacity of 1 96485 C / mol K 39 g / mol 2474 C / g 687 mah / g 16
Specific (gravimetric) capacity Maximum (theoretical) capacity ( 4) 0 Ti O2 4 e 4Li Ti 2Li 2O x =4 M w (TiO 2 ) = 80 g/mol 4 96485 C / mol Specific capacity of TiO2 4824 C / g 1340 80 g / mol mah / g Experimental capacity Ti ( 4) O 2 ( 3.5) 0.5 e 0.5Li Li0.5Ti O2 x =0.5 M w (TiO 2 ) = 80 g/mol 0.5 96485 C / mol Specific capacity of TiO2 603 C / g 168 mah / 80 g / mol g 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 + + xe - + BX = A x BX Ionic diffusion 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 (LiFePO 4 ) Layered structure (LiMn x Ni y Co z O 2 ) Spinel structure (LiMn 2 O 4 ) 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 = A x B G xf 1 E1 Reaction 2 A x B + ya = A x+y B G yf 2 E2 Reaction 3 A x+y B + za= A x+y+z B G zf 3 E3 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) Change in composition (activity) leads to a change in potential according to Nernst equation E i E i 0 RT nf ln a i Activity is constant, ln a i 0 0 i E i E 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 M n+ 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 Li x M y X z (X = anion) should have a high Li chemical potential to maximize the cell voltage: Transition metal ion M n+ should have a high oxidation state 27
What determines the voltage? NaSICON framework of Li x M 2 (XO 4 ) 3 : MO 6 octahedra linked by corners to XO 4 tetrahedra 28
Li-ion batteries: positive materials 29
Li-ion batteries: positive materials 1D 2D 3D Olivine structure (LiFePO 4 ) Layered structure (LiMn x Ni y Co z O 2 ) -Transition metal dioxides LiMO 2 (M = V, Cr, Fe, Co, Ni) van der Waals gap between the layers Li insertion between the layers Spinel structure (LiMn 2 O 4 ) -spinels (manganese oxides d-mno 2, LiMn 2 O 4 ) Cross-linked channels allowing Li insertion; sometimes significant volume changes upon Li insertion small degree of lattice expansion/contraction upon Li insertion 30
2D materials: Layered structures Hexagonal/symmetry based on the -NaFeO2 structure Cubic close-packed oxygen array of edge-sharing [MO 6 ] octahedra. Lithium resides In between these layers in octahedral [LiO 6 ] 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. LiCoO 2 : <180 Ah/kg Z. Yang et al, Chem Rev. 2011, 111, 3577 31
3D materials: spinels Spinel-type LiMn 2 O 4 : voltage of > 4.0 V versus Li PO 4 Li + FeO 6 Spinels : 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 Z. Yang et al, Chem Rev. 2011, 111, 3577 32
1D materials: NaSICON or olivine structures Olivine LiFePO4 in projection along [001] direction Oxyanion scaffolded structures corner-sharing MO 6 octahedra (M = Fe, Ti, V or Nb) and XO 4 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 Framework built on FeO 6 octahedra and PO 4 tetrahedra. Tunnel structure, with Li diffusion path along [010] direction 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, Li x M) Positive: dimensionally stable insertion host. Form lithium-rich materials (Li 4.4 Sn and Li 4 Si) 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 LiC 6 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 Positive: 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 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 + C 6 Li x C 6 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 μ A and μ C within the window of the electrolyte An anode with a μ A above the LUMO of electrolyte will reduce the electrolyte. A cathode with a μ C 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 LiPF 6 dissolved in a mixed carbonate solvent. LiPF 6 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 LiPF 6 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/ M w [Ah/kg] = [3.6 C/g] Charge density Q V = Q [Ah/L] = [3.6 C/mL] Specific energy Spec. energy = E Q [Wh/kg] = [3.6 J/g] Energy density Energy density = E Q V [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/ M w [W/kg] Power density P V = P [W/L] Power depends on the kinetics of electrodes, interfaces and electrolyte M = molecular mass = density 49