自放电SelfdischargecausesBWII.ppt

Causes of self-discharge Brian Way December 12, 2007 Dendrites caused by metal contamination in cathode This type is responsible for most self-discharging cells Metal dissolves from cathode, plates on anode and grows dendrite through separator See example on next page Causes of Self-discharge Example: Fe contamination in cathode Fe Anode 1) Fe2++2e- → Fe 2) yLixC6 ? yLix- 2/yC6 + 2Li++2e- 1. Uncharged cell: Fe particle in cathode 3. Self-discharging cell: Fe dendrite grows through separator, touches cathode Fe ↓ Fe2+↓ Fe Li ↑ Li+ ↑ Li Cathode 1) Fe → Fe2++2e- 2) yLi1-xCoO2+2Li++2e-?yLi1-x+2/yCoO2 2. Charged cell: Fe particle dissolves And plates on anode Total reaction: Fe moves from cathode to anode, growing dendrite on anode surface, into separator. Li moves from anode to cathode, discharging cell slightly. Li ↑ Li+ ↑ Li ↑ e- Electrons flow from anode to cathode through dendrite, allowing cell to self-discharge. Rate of self-discharge is controlled by resistance of electron path  Facts about metal contamination Metal must dissolve at cathode potential (4.3V) Fe, Ni, Cu, CuZn (Brass), Steel (most), FeS, many low oxidation state metal compounds Very few metals are stable: Al (passivated), Au, Pt(?), some stainless steels Some materials take longer than others to form dendrites that conduct Particles must touch the cathode to dissolve Metal contamination in anode does not cause self-discharge Particles must be large enough to form dendrite that penetrates separator Particles diameter should be approx. separator thickness or greater Self-discharge rate depends on the resistance of the electron path. Cu, Ni, CuZn form better conducting dendrites than stainless steel Cathode is high resistance, so self-discharge is slower if particle is in coated region compared to on bare Al foil Sources of metal contamination Raw materials in cathode Active materials (LiCoO2), Cond. Diluents All processes which may produce metal contamination Especially: Powder handlin