锂离子电池的失效模式
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On Coated Cathode On Bare Al
OK, self discharge
Fail
• This test is not suitable for a standard because it is too dependant on particle shape, operator and technique. • E.g. shorts are at corners, shape is not reproducible.
IEEE 1625 Sept 25
Power to short (PR)
z
R V
Note: In real situations both z and 0 0 R are variable
z
Short Impedance (R)
IEEE 1625 Sept 25
Beating the radial nail test
‘Standard’ cell design: Nail first shorts to active coated cathode Easily fails Cell Can Can
Rc Al
Rs z Ps ~ 0 * I2 Cu
• When Al melts back Rs ~ 0, so these are usually benign • This can be used to beat the radial nail test
IEEE 1625 Sept 25
Impedance Matching
IEEE 1625 Sept 25
Exception: Large shorts on cathode coating
Rc Al + Cathode Rs z Cu - Anode
Ra
• Particles large enough to substantially penetrate the cathode coating are dangerous
Bare outer Al design: Nail first shorts bare foils
Can Can
EasiΒιβλιοθήκη Baiduy passes
• Cells with bare outer Al wrap appear to be safer
IEEE 1625 Sept 25
How beneficial is bare Al outer wrap?
IEEE 1625 Sept 25
Experimentally verify the theory
• Place Al lathe turnings on positive electrode during assembly • Approx. 2mm x 0.2mm x 0.1mm • Charge and cycle cells to increase the stack pressure until the metal particle punctures the separator.
• Pmax = V2 / 4z where z = cell internal impedance V = cell open circuit voltage R = internal short impedance Hi/Lo impedance shorts are OK Worst case is R ~ z (impedance match condition) Bare Al to anode coating is nearly impedance matched Impedance Matching
• The only thing bare outer Al is good for is to pass radial nail tests (including other sharp object penetration) • It does not provide protection against ‘blunt’ mechanical abuse (e.g. crush, impact) which cause interior shorts • Takes up space that must then be compensated for (e.g. with thinner separator or foils) • **Gave the appearance of safety, causing complacency**
IEEE 1625 Sept 25
Small shorts become large as bare Al melts
Even if initial short ‘burns out’ molten Al keeps shorting
Example: produce small short in the bare Al section of Mn power cells -I’ve seen Al foil melt back to edge of cathode and the Al tab
IEEE 1625 Sept 25
Solution: bare Al foil with protective layer
Al Protective Layer Rs z Cu - Anode
Ra
IEEE 1625 Sept 25
Shorts between bare Al and bare Cu
IEEE 1625 Sept 25
Shorts from bare Al to coated anode
Rc Al
Rs z Cu - Anode
Ra
• Cathode coating does not limit the power dissipation, local heating can easily melt the Al
Li ion failures Cause & Prevention
IEEE 1625 Sept 25
Shorts on cathode coating generally benign
Rc Al + Cathode Rs Z Cu - Anode
Ra
• Z ~ 15 mohm, Ra << Rc ~ 100 ohm for sub mm size short • Rc limits current, localized heating • Normal self-discharge and Li dendrites fall in this category
OK, self discharge
Fail
• This test is not suitable for a standard because it is too dependant on particle shape, operator and technique. • E.g. shorts are at corners, shape is not reproducible.
IEEE 1625 Sept 25
Power to short (PR)
z
R V
Note: In real situations both z and 0 0 R are variable
z
Short Impedance (R)
IEEE 1625 Sept 25
Beating the radial nail test
‘Standard’ cell design: Nail first shorts to active coated cathode Easily fails Cell Can Can
Rc Al
Rs z Ps ~ 0 * I2 Cu
• When Al melts back Rs ~ 0, so these are usually benign • This can be used to beat the radial nail test
IEEE 1625 Sept 25
Impedance Matching
IEEE 1625 Sept 25
Exception: Large shorts on cathode coating
Rc Al + Cathode Rs z Cu - Anode
Ra
• Particles large enough to substantially penetrate the cathode coating are dangerous
Bare outer Al design: Nail first shorts bare foils
Can Can
EasiΒιβλιοθήκη Baiduy passes
• Cells with bare outer Al wrap appear to be safer
IEEE 1625 Sept 25
How beneficial is bare Al outer wrap?
IEEE 1625 Sept 25
Experimentally verify the theory
• Place Al lathe turnings on positive electrode during assembly • Approx. 2mm x 0.2mm x 0.1mm • Charge and cycle cells to increase the stack pressure until the metal particle punctures the separator.
• Pmax = V2 / 4z where z = cell internal impedance V = cell open circuit voltage R = internal short impedance Hi/Lo impedance shorts are OK Worst case is R ~ z (impedance match condition) Bare Al to anode coating is nearly impedance matched Impedance Matching
• The only thing bare outer Al is good for is to pass radial nail tests (including other sharp object penetration) • It does not provide protection against ‘blunt’ mechanical abuse (e.g. crush, impact) which cause interior shorts • Takes up space that must then be compensated for (e.g. with thinner separator or foils) • **Gave the appearance of safety, causing complacency**
IEEE 1625 Sept 25
Small shorts become large as bare Al melts
Even if initial short ‘burns out’ molten Al keeps shorting
Example: produce small short in the bare Al section of Mn power cells -I’ve seen Al foil melt back to edge of cathode and the Al tab
IEEE 1625 Sept 25
Solution: bare Al foil with protective layer
Al Protective Layer Rs z Cu - Anode
Ra
IEEE 1625 Sept 25
Shorts between bare Al and bare Cu
IEEE 1625 Sept 25
Shorts from bare Al to coated anode
Rc Al
Rs z Cu - Anode
Ra
• Cathode coating does not limit the power dissipation, local heating can easily melt the Al
Li ion failures Cause & Prevention
IEEE 1625 Sept 25
Shorts on cathode coating generally benign
Rc Al + Cathode Rs Z Cu - Anode
Ra
• Z ~ 15 mohm, Ra << Rc ~ 100 ohm for sub mm size short • Rc limits current, localized heating • Normal self-discharge and Li dendrites fall in this category