4. Give examples of the secondary electrochemical cells.
Depending on the operational characteristics and the electrochemical system (a set of electrodes and electrolyte) used, chemical current sources (CCS) are divided into primary (not rechargeable, galvanic cells), which usually become out-of-use after they are fully discharged, and secondary (rechargeable, batteries), where the reagents are recovered when charging by passing current from an external source. Currently, there are a lot of different types of secondary CCSs: lithium-ion (Li-ion), nickel metal hydride (Ni-MH), nickel-cadmium (Ni-Cd), etc.
Lithium ion batteries usually use lithium cobalt oxide (LiCoO2) for the positive electrode and graphite for the negative electrode. When we charge the battery, lithium ions and electrons move from the positive electrode to the negative electrode. When the battery discharging, the opposite happens and the follow of electrons powers the device. Lithium has the most negative electrode potential of all metals: -3.055V in water. In non-aqueous solvents on the surface of the lithium forms a protective film of insoluble products of the interaction of -Li2O; -Li2CO3; -lithium halides; -other salts of lithium. Film of nanometer thickness has a noticeable ionic conductivity:
the layers of the passive film.
Simple lithium salts and base (LiOH, LiNO3, etc.) do not dissolve in non-aqueous solvents. Тhe solution to this problem is the use of complex salts (LiBH4, LiPF6, LiAsF6, LiClAl4).
Internal short circuit.
The
negative electrode – carbon matrix in which lithium ions are
incorporated in the charge and is pulled back in the discharge. Тhe
half-reaction:
.
(–) LixC | неводный электролит | Li1-xMO2 (+)
.
Current production reaction:
.
Cathode:
-LiCoO2,
80-90% of market;
-LiCo1-xMxO2,
5-7%
of market; M=Ni,Mn,Al…
-LiMn2O4,
5-7% of market;
-LiFePO4.
.
A liquid solution of a complex lithium salt in nonaqueous solvent: -The ethylene carbonate; -Propylenecarbonate; -Dimethyl carbonate; -Diethylcarbonat; -Ethylmethylketone; -Dimethoxyethane; Polymer; -Dry; -Gel polymer; -Microporous. The advantages of Li-ion batteries: -high voltage range of 2.5-4.2 V; -resource 500-1000 cycles, and more; -high specific energy and power; -low self-discharge; -no memory effect (*); -the ability to operate in a wide temperature range; -the charge at temp from 20 to 60°C; -discharge with temperature from -40 to +65 °C.
Components
of NiCd and NiFe battery:
Positive
electrode – NiOOH with the addition of graphite;
The
negative electrode – Cd or Fe, unlike cadmium, iron rust;
The
electrolyte - 20-22% aqueous solution of KOH.
Electrochemical
system: (–) Cd or Fe | KOH | NiOOH (+).
The
electrode reactions:
on
the positive oxide-Nickel electrode:
Ni(OH)2
+ OH-
→ NiOOH + H2O
+ e-
(charge);
NiOOH
+ H2O
+ e-
→ Ni(OH)2
+ OH-
(discharge) (E0
= 0.49 B).
On
the negative cadmium electrode:
Cd(OH)2
+ 2e-
→ Cd + 2OH-
(charge).
Cd
+ 2OH-
→ Cd(OH)2
+ 2e-
(discharge) (ECd0
= -0.81 B EFe0
= -0.88 B).
Current
production reaction:
2NiOOH
+ 2H2O
+ Me = 2Ni(OH)2
+ Me(OH)2.
Me=Cd
or Fe.
Discharge
characteristics of Nickel-cadmium battery (Ni-Cd) at different
discharge currents
.
Nominal voltage of sealed Ni-Cd battery 1.2 V. Nominal (standard) mode of charging the Nickel-cadmium battery, a current of 0.1 C for 16 h nominal discharge Nickel-cadmium battery with current of 0.2 C to a voltage of 1 V.
characteristic |
Ni-Cd |
Ni-MH |
Rated voltage, V |
1,2 |
1,2 |
The discharge current, the maximum |
10С |
4С |
specific energy: Wh/kg Wh/l |
20-40 |
50-80 |
60-120 |
100-270 |
|
durability: years cycles |
1-5 |
1-5 |
500-1000 |
500-2000 |
|
Selfdischarge, % |
20-30 (for 28 day.) |
20-40 (for 28 day.) |
Work temperature, °С |
-50 - +60 |
-40 - +60 |
Design
of Ni-MH-battery:
Positive
electrod – NiOOH,
Negative
electrod: Metal alloy (M), which can reversibly absorb hydrogen
(forming hydride MH) and be conducted in order to him. LaNi5;
TiFe; Mg2Ni.
Electrolyte
- 26-31 % aqueous solution of KOH.
Electrochemical
system: (–) MH| KOH | NiOOH (+). The
electrode reactions:
on
the positive oxide-Nickel electrode: Ni(OH)2
+ OH-
→ NiOOH + H2O
+ e-
(charge).
NiOOH
+ H2O
+ e-
→ Ni(OH)2
+ OH-
(discharge) (E0
= 0.49 B).
At
the negative electrode metal with absorbed hydrogen turns into a
metal hydride:
M
+ H2O + e-
→ MH + OH-
(заряд);
MH
+ OH-
→ M + H2O + e-
(разряд) (E0
≈ -0.9 B).
Current
production reaction:
NiOOH
+ MH = Ni(OH)2
+ M.
Rules of operation of NiCd/NiMh batteries: Try to use only a standard charger. When using non-automatic chargers do not charge the battery more than the time specified in the instructions. Overcharging greatly accelerates the aging process of the battery. Do not leave a discharged battery in the included equipment. Further, the uncontrolled discharge* fully displays the battery failure. Avoid charging a completely discharged battery. Every 3-4 weeks produce complete discharge* the battery in the instrument. Observe the operating temperature range. Before storing for more than 1 month NiCd battery must be unloaded*. NiMh battery stored at 30-50% charge level. Keep at temperature +5°C...+20°C. shelf Life - up to 4 years. Every 6 months for NiMh and 12 months for NiCd storage it is recommended to do at least 3 cycles of charge-discharge in standard mode. *Note: the Battery is fully discharged when its voltage drops to 83% of nominal. For example, a battery with nominal value of 1.2 V will be fully discharged when operating the apparatus the voltage across it becomes equal to 1. Typically this level of tension coincides with the disabling threshold of the instrument.