Download Battery Materials

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  QE
Power
+z℮-
Potential
P  IE
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  QE
Joule [J] = [CV]
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 eM
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