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
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
• 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
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
Rc Al
Rs z Ps ~ 0 * I2 Cu
ห้องสมุดไป่ตู้
• 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
• 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
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.
Bare outer Al design: Nail first shorts bare foils
Can Can
Easily 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.
IEEE 1625 Sept 25
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
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