锂离子电池 英文

Electrolyte
• The selection of the electrolyte depends on the potential window. • Potentials are beyond water potential and hence non aqueous electrolyte. • It should not reactive with Li+ ions, since it will decide the transport of the Li+ ions for the intercalation reaction. • The electrolyte solution commonly comprises a lithium salt dissolved in a mixture of organic solvents. • Examples include LiPF6 or LiBOB (the BOB is the anion with the boron coordinated by two oxalate groups) in an ethylene carbonate/dimethyl carbonate solvent.
Li Ion Battery (LIB)
Battery
• Battery: Transducer which converts chemical energy into electrical energy. • Chemical reactions: Oxidation and reduction • Free energy change of the processes appears as electrical energy. • Primary battery-not rechargeable • Secondary battery- rechargeable
J. Mater. Chem., 19 (2009) 5871
Li Ion Battery
• Contains an anode, a cathode, an electrolyte and separator. • Harnesses a set of reversible oxidation/reduction reactions. • Li ion dissolved by a discharge reaction and returns to metallic lithium by a charging reaction • Electrolyte provides for the separation of Li ions and electron transport. • In a perfect battery the Li ion transport number will be unity. • Cell potential is determined by ∆G= -EF • The electrode system must be both a good ionic and an electronic conductor. • It is necessary to add an electronically conductive material such as carbon black. • To hold the electrode together, a binder is also needed.
• Internal resistance of lithium ion battery is high as compared to other batteries. • Due to overcharging and high temperature capacity will diminish.
Why Li-Ion Battery?
Lead acid battery
Appealing to industrial applications due to • Reliable • Inexpensive
Ni metal hydride battery
Appealing to hybrid electric vehicle applications due to • Rechargeable • No memory effect • High power density • High energy density • Currently available battery capacity : 1.5 kWh Disadvantages • Self discharge rates
Advantages
Disadvantages
• Li has greatest electrochemical potential • Lighter than others • Shape and size variation • High open circuit voltage • No memory effect • Low discharge rate 5-ttery
The cathode half reaction (with charging being forwards) is: LiCoO2 ↔ Li 1-x CoO2 + xLi+ + xeThe anode half reaction is: xLi + + xe- + 6C ↔ Lix C6
Graphite
Graphite is commonly selected anode material in LIB due to its • High coulombic efficiency. • However, the specific capacity of graphite is relatively low (theoretical value: 372mA.h/g) since every six carbon atoms can host only one lithium ion by forming an intercalation compound (LiC6). • Sn: 993 mA.h/g, Si: 4200 mA.h/g) than graphite via the formation of alloys with lithium or through the reversible reactions with lithium ions. • Drawback of these substances as anode materials is the huge volume variation during the charge/discharge process which causes the pulverization of the electrode, resulting in poor reversibility.
• Reducing the size of these materials to nanometer scale can partially suppress the expansion and shrinkage of the anode materials. • By designing the structure of these materials as hollow spherical or porous particles can improve the lithium storage capacity and their initial Coulombic efficiency as well to a certain degree. • However, the capacity decay of the electrode along with the charge/discharge cycling still can not be completely avoided.
Disadvantages • Low life cycle • Low energy density (30 ~ 40 Wh/Kg)
Recharge-ability : Basically, when the direction of electron
discharge (negative to positive) is reversed, restoring power.
Intercalation
• Should be completely reversible. • Should be geometrically and electronically fitting to the inter planar spacing. • Should be capable of electron exchange. • Other substrates like mesoporous carbon and other pillared materials can also be considered.
Graphene
2D
Basic building block for all forms of graphitic material made up of monolayer C atoms tightly packed into 2D honeycomb lattice.
Remarkable properties Young’s modulus - 1,100GPa Fracture strength - 125GPa Mobility of charge carriers - 200,000 cm2 V-1 s-1 Thermal conductivity - 5,000 W m-1K-1 Specific surface area - 2630 m2g-1
Memory Effect : When a battery is repeatedly recharged
before it has discharged more than half of its power, it will “forget” its original power capacity. Cadmium crystals have the memory effect (NiCd)
Existing Li-Ion Electrodes
Graphite – 370 mA.h/g LiCoO2– 140 mA.h/g
The development of materials for LIB centres around “intercalation” and suitable electrolyte with appropriate “potential window”.
Intercalation
• Intercalation is an equilibrium process and governed by the free energy considerations. • Entropy factor may predominate over the enthalpy factor. • The lattice in which intercalation takes place should be capable of interacting electronically without affecting the translational（平动 的） entropy of the intercalant. • The value of the equilibrium constant of the intercalation and deintercalation reaction should be near 1.