Storage Systems: primary & secondary cells, Li-ion batteries, Pb-storage and Ni-Cd batteries

Batteries: Primary & Secondary Cells, Li-ion, Lead-Acid, Ni-Cd

1. Primary vs secondary

• Primary (non-rechargeable): reaction irreversible; e.g. dry (Leclanché) cell, alkaline cell. Low cost, long shelf life, single use.
• Secondary (rechargeable): reaction reversible on charging; e.g. lead-acid, Ni-Cd, Li-ion. Higher upfront cost, many cycles.

2. Dry / alkaline cell (primary, 1.5 V)

Zinc anode, $MnO_2$ + carbon cathode, $NH_4Cl/ZnCl_2$ paste (dry) or KOH (alkaline):

• Anode: $Zn \to Zn^{2+} + 2e^-$; cathode: $2MnO_2 + 2NH_4^+ + 2e^- \to Mn_2O_3 + 2NH_3 + H_2O$.
• Uses: remotes, clocks, torches. Alkaline version gives higher capacity and leak resistance.

3. Lead-acid (Pb-storage) battery (secondary, 2 V/cell, 12 V pack)

Anode Pb, cathode $PbO_2$, electrolyte dilute $H_2SO_4$:

• Discharge: $Pb + PbO_2 + 2H_2SO_4 \to 2PbSO_4 + 2H_2O$ (both plates coat with $PbSO_4$, acid dilutes).
• Charge reverses it. State of charge read from acid specific gravity (~1.28 charged, ~1.18 discharged).
• Uses: car/SLI batteries, UPS, inverters. Heavy, acid handling needed, but cheap and high surge current.

4. Ni-Cd battery (secondary, 1.2 V/cell)

Anode Cd, cathode $NiO(OH)$, KOH electrolyte:

• Rugged, works in cold, fast charging; suffers memory effect (loses capacity if recharged after partial discharge) and Cd is toxic — largely replaced by Ni-MH/Li-ion.

5. Li-ion battery (secondary, ~3.7 V/cell)

No metallic lithium — $Li^+$ shuttles between graphite anode (intercalated $LiC_6$) and metal-oxide cathode ($LiCoO_2$, NMC, LFP):

• Discharge: $LiC_6 \to C_6 + Li^+ + e^-$ at anode; $Li^+$ inserts into cathode. Charging reverses the shuttle ("rocking-chair").
• Highest energy density, no memory effect, low self-discharge; needs protection circuit (overcharge → thermal runaway).
• Uses: phones, laptops, EVs, solar storage.

Battery	Type	Voltage	Star strength	Star weakness
Dry/alkaline	primary	1.5 V	cheap, long shelf life	single use
Lead-acid	secondary	2 V/cell	cheap, huge surge	heavy, acid
Ni-Cd	secondary	1.2 V	rugged, cold-proof	memory effect, toxic Cd
Li-ion	secondary	3.7 V	top energy density	needs protection circuit

Exam Points

• Primary vs secondary with one example each (2 marks).
• Discharge/charging reaction of lead-acid + voltage + one use (3–5 marks); Li-ion shuttle mechanism and why no memory effect (3 marks favourite).