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.
1) 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
2) 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.
24. Describe the designfeatures of an electrochemical cells an example of the construction of any power source.
An electrochemical battery consists of cathode, anode and electrolyte. When charging the battery, the accumulation of electrons at the anode, which creates a voltage potential between the anode and the cathode. In normal operation, as power supply current flows from the cathode to the anode through the load. When charging the battery the current flows in the opposite direction.
The electrodes of the battery are connected in two different ways, the first is electric circuit through which electrons flow to feed the load and the second through the electrolyte, where ions move between the electrodes through the dielectric separator (separator).
The electrode that releases electrons during a redox reaction is called the anode. The electric potential of the anode of a galvanic cell is negative relative to the cathode. A chemical reaction in the battery is a reversible process, and therefore, the polarity of the electrodes changes depending on the operation mode (charge/discharge), but the terminal marking is always constant. In tables 1, 2 , 3, 4 describes the structure and processes in lithium, lead, Nickel and alkaline batteries.
Тable 1. Composition and processes in Lithium-ion batteries
| Li-ion battery | Cathode | Anode | Electrolyte |
| material composition of the elements | lithium metal oxides | carbon based | lithium salts in an organic solven |
| composition and processes in charged state | a metal oxide with an intercalation structure | migration of lithium ions to the anode | |
| composition and processes in discharged state | lithium ions return to the positive electrode | mostly carbon based |
Тable 2. Composition and processes in lead-acid batteries
| Lead-acid battery | Cathode | Anode | Electrolyte |
| material composition of the elements | lead dioxide | gray spongy lead | hydrochloric acid |
| composition and processes in charged state | lead dioxide, the electrons join | lead, electrons are detached | strong sulphuric acid |
| composition and processes in discharged state | lead is converted to lead sulfide at the anode with the release of electrons, and attached at the cathode | weak sulfuric acid (diluted with water) |
Table 3. Composition and processes in NiMH and NiCd batteries
| NiMH,NiCd | Cathode | Anode | Electrolyte |
| material composition of the elements | Ni | NiMH NiCd | potassium hydroxide |
Table 4. Composition and processes in alkaline batteries
| Alkaline battery | Cathode | Anode | Electrolyte |
| material composition of the elements | manganese dioxide | zinc | an aqueous solution of alkali |
When submerged unpressurized system the design of the battery, a liquid electrolyte flowing between two electrodes. In hermetic designs the electrolyte is usually in the role of impregnation of the separator to provide the movement of ions from anode to cathode and back when charging. Ions are atoms, whic electrons have attached or lost. They acquire the ability to move between the electrodes through the separator.
