Ni-cd Batteries

Transcript

Ni-Cd batteries (+): 2NiOOH + 2H2O + 2e- 2Ni(OH)2 + 2OH- (0.5 V) (-): Cd + 2OH- Cd(OH)2 + 2e- (0.8 V) -----------------------------------------------------------Cd + 2NiOOH + 2H2O Cd(OH)2 + 2Ni(OH)2 (1.3 V) In KOH 6~8M electrolyte The KOH electrolyte The KOH electrolyte Ni-Cd batteries (+): 2NiOOH + 2H2O + 2e- 2Ni(OH)2 + 2OH- (0.5 V) (-): Cd + 2OH- Cd(OH)2 + 2e- (0.8 V) -----------------------------------------------------------Cd + 2NiOOH + 2H2O Cd(OH)2 + 2Ni(OH)2 (1.3 V) The solids are insoluble in KOH electrolyte. --Longer life than Pb-acid batteries (for UPS: no acid vapor, but more expensive!!) --But shorter life than Ni-MH batteries The electrolyte • KOH + LiOH: ↑ high rate discharge performance ↑ cycle life ↑ high temperature performance The current collectors and can are typically Ni or Ni-plated steel! The discharge performance The discharge performance Ni plated steel case The drawbacks • Cd pollution • Lower capacity vs. that of Ni-MH batteries • Poor charge retention • Memory effects Poor charge retention Voltage depression (memory effect) --a reversible failure Especially at high temperature Two-step discharge (lower capacity) • Cd may undergo a physical change, increasing the cell resistance. • Cutoff voltage is important. • Practically, performing discharge before charging. • The charger (to match the batteries) is critical. Ni-Cd batteries (+): 2NiOOH + 2H2O + 2e- 2Ni(OH)2 + 2OH- (0.5 V) (-): Cd + 2OH- Cd(OH)2 + 2e- (0.8 V) -----------------------------------------------------------Cd + 2NiOOH + 2H2O Cd(OH)2 + 2Ni(OH)2 (1.3 V) In KOH 6~8M electrolyte Ni-Cd batteries (+): 2NiOOH + 2H2O + 2e- 2Ni(OH)2 + 2OH- (0.5 V) (-): Cd + 2OH- Cd(OH)2 + 2e- (0.8 V) -----------------------------------------------------------Cd + 2NiOOH + 2H2O Cd(OH)2 + 2Ni(OH)2 (1.3 V) The solids are insoluble in KOH electrolyte. --Longer life than Pb-acid batteries (for UPS: no acid vapor, but more expensive!!) --But shorter life than Ni-MH batteries The drawbacks • Cd pollution • Lower capacity vs. that of Ni-MH batteries • Poor charge retention • Memory effects Poor charge retention Voltage depression (memory effect) --a reversible failure Especially at high temperature Two-step discharge (lower capacity) Ni-Metal Hydride (Ni-MH) batteries Ni-metal hydride (Ni-MH) batteries • Using hydrogen instead of Cd. • The hydrogen is in metal hydrides (the hydrogen storage materials). The energy storage mechanism It is rechargeable! No H2O is involved in the reaction, so the cycle life is better. Also using 6~8 M aqueous KOH as the electrolyte. Ni-metal hydride (Ni-MH) batteries Advantages： • higher energy density than Ni-Cd • free of Cd • memory effect ↓ • longer cycle than Ni-Cd • ~1.3 V • high safety • simple, convenient, low cost than LIB Ni-metal hydride (Ni-MH) batteries Disadvantages ： • inferior rate performance than Ni-Cd • less tolerant of overcharge • higher cost than Ni-Cd (due to MH, e.g. LaNi5) • heavy 於充放電過程中氫在鎳氫電池電解液中的 移動及電荷轉移情況 Expensive & heavy 儲氫合金材料（hydrogen storage alloy） M＋H2 MHX＋△Η 系列 備註 LaNi5、 MmNi5 以荷蘭飛利浦公 司之專利為代表 AB2 TiCr2、TiMn2 以美國Ovonics 公司之專利為代 表 AB TiFe、TiNi AB5 A2 B 代表合金 Mg2Ni、Ti2Ni The important properties of hydrogen storage alloy 儲氫合金材料吸氫機構示意圖 LaNi5型儲氫合金材料之結晶結構與氫化物形 成時氫原子所佔據的位置 鎳氫電池的內部結構包括 •正極材料：氫氧化鎳 •負極材料：儲氫合金 •電解質：氫氧化鉀為主的水溶液(濃度約為7 M) •正、負極片之間的隔離板 • 附註：一般來說，適當的隔離板材料須有以下性能：(1) 在強鹼電解質中要有抗腐蝕性；(2)多孔性，便於離子 之傳導；(3)吸附力強，減少電解液的使用量；(4)電絕 緣性，防止正、負極間漏電或短路 The safety concern -electrode design The safety concern -electrode design The larger anode: prevent H2 evolution and avoid high pressure. Polarity reversal during over-discharge The charge retention Charging the Ni-MH betteries Charging the Ni-MH betteries Techniques for charge control (1) Timed charge: inexpensive, suitable for applications where the batteries are usually fully discharged (but at low rates). (2) -ΔV(10~20 mV): preferred for Ni-Cd O2 recombination → 放熱 →T↑ → resistance ↓ →V↓ (3) Temperature (T) detection: usually set at 45 ℃, but is influenced by ambient temperature, cell design, charge rate… (4)ΔT: the influence of ambient temperature is minimized. (5)ΔT/Δt: preferred for Ni-MH batteries Moreover, charge methods should match the types of batteries from different manufacturers. Cost: Li-ion > Ni-MH > Ni-Cd Cell voltages are different Li-ion batteries • Commercialization by Sony in 1990, employing Li intercalation compounds as the positive and negative materials. • The rocking-chair batteries. Li-ion batteries • Lithium-ion (Li-ion) batteries are comprised of cells that employ lithium intercalation compounds as the positive and negative materials. As a battery is cycled, lithium ions (Li) exchange between the positive and negative electrodes. They are also referred to as rocking-chair batteries as the lithium ions ‘‘rock’’ back and forth between the positive and negative electrodes as the cell is charged and discharged. • The positive electrode material is typically a metal oxide with a layered structure, such as lithium cobalt oxide (LiCoO2), or a material with a tunneled structure, such as lithium manganese oxide (LiMn2O4), on a current collector of Al foil. The negative electrode material is typically a graphitic carbon, also a layered material, on a Cu current collector. In the charge/ discharge process, lithium ions are inserted or extracted from interstitial space between atomic layers within the active materials. (Al) (Cu) (~4 V) Li4Ti5O12