Electrochemical batteries

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Electrochemical batteries convert chemical energy directly into electrical energy and provide DC current.

A battery consists of electrochemical cells that convert stored chemical energy into electrical energy.

When two dissimilar metals are immersed in an electrolyte (conductive liquid), the breakdown of chemicals into charged particles (ions) results in a flow of electricity when the battery is connected to en external circuit.

The electrochemical battery is powered by the redox reaction. Electrons are added at the cathode during charging, while electrons are removed at the anode. During discharge, the process is the reverse.

The electrodes are electrically connected via the electrolyte. Different electrolytes might be used for each half-cell (+/-). In those cases the electrolytes are not mixed but ions are allowed to flow between the half-cells. The number of voltaic cells (pairs of half-cells) contained in a battery may very pending on the usage and desiered voltage.

The actual (terminal voltage) voltage of a cell is due to internal resistance less than the open circuit voltage (no charge/no discharge taking place) of a cell. In practical terms the terminal voltage of a cell varies over time pending on the discharge, chemistry and internal arrangement of the cell.
 




 
Alkaline and zinc–carbon cells
Alkaline and zinc–carbon cells have different chemistries, but approximately the same emf of 1.5 volts; likewise NiCd and NiMH cells have different chemistries, but approximately the same emf of 1.2 volts.[18] The high electrochemical potential changes in the reactions of lithium compounds give lithium cells emfs of 3 volts or more

The open circuit voltage of a alkaline and a zinc-cell is approximately the same, around 1,5 volts despite different chemistry while the open circuit voltage of NiCd and NiMH cells also with different chemistry is around 1,2 volts. Lithium based cells have a much higher electrochemical potential change and have therefore an open circuit voltage of around 3 volts or higher.

Primary batteries
Primary batteries are used once since the electrode materials are irreversibly changed during discharge. The alkaline battery is a common example of day life disposable electrical battery used in may domestic devices.

Secondary batteries
Secondary batteries are batteries that may be recharged, the composition of the electrodes are restored by reverse current. The lead-acid batteries and lithium ion batteries are common examples of rechargeable electrical battery.

Disposable batteries

ChemistryAnode (−)Cathode

(+)

Nominal voltage

(V)

Specific energy (MJ/kg)
Zinc–carbonZnMnO21.20.13
Alkaline

(zinc–manganese dioxide)

ZnMnO21.150.4–0.59
Lithium

(lithium–iron disulfide)

LiFeS2

LiFeS21.51.07
Zinc–airZnO21.11.59
Silver-oxide (silver–zinc)ZnAg2O1.50.47

Rechargeable batteries

Chemical compoundsCell voltage

(V)

Specific energy

(MJ/kg)

NiCd1.20.14
Lead–acid2.10.14
NiMH1.20.36
NiZn1.60.36
AgZn1.86-1.50.46
Lithium ion4,2-3.60.46

Vehicle batteries (Rechargeable)

Vehicle batteries used for propulsion are rechargeable batteries that have the capability to provide pover continuously over a long period of time.

Relatively high cost, low specific energy (energy /mass) and limited capacity (low range of vehicle) compared to liquid fuels and long charging times are the main limitations related to battery supported electrical road vehicles.

Image of tesla model S
The tesla model S, 85 kWh version is powered by 7104 rechargeable Li-ion battery cells.

 
The types of batteries used in road vehicles are:

  • Lead-acid, nickle cadmium (NiCd), nickel metal hydride (NiMH), lithium ion (Li-ion) Li ion.
  • Conventional Lead-acid batteries
    Floded lead acid batteries mature technology, the cheapest and most common battery type for vehichles.

A lead-acid cell consist of two sets of plates, one set of lead plates (-) (spongy type) and one set of lead dioxide plates (+) that serves as the electrodes, the plates are suspended in diluted sulphuric acid which is the electrolyte. In conventional lead-acid cells, the diluted acid is in liquid form, hence the term “flooded” or “wet” cells.

 

Illustration of lead acid accumulator. (This text is displayed because your browser do not support SVG)

The lead acid battery does not generate a voltage unless it is charged from another source to generate a voltage therefore the lead acid battery function as storage for electrical energy.

When a cell discharges, lead-sulphate and water is produced.
When a cell is subsequently charged, the lead-sulphate and water are turned back into lead and acid.

Starting Batteries and Deep-Cycle Batteries
The Two main types in use are the starter battery primarily designed to provide high current over a short period, charged by the vehicles alternator and the deep cycle lead acid battery used for propulsion of various electrical vehichles and other accessories that requires steady levels of power over longer periods of time.

Energy density is 30-40 Wh/Kg and efficieny around 75%.

The valve-regulated lead-acid battery (VRLA), is a verion of the lead acid battery that require limited amount of maintenance and that is more flexibility in respect of orientation.

There are primarily two types of VRLA batteries, the gel cell and absorbed glass mat (ABG). Gel cells add silica dust to the electrolyte, forming a thick putty-like gel.

The VRLA cells have essentially the same lead-acid chemistry as the conventional cell, but the diluted acid electrolyte solution is immobilized, either by soaking a fiberglass mat in it (ABG cell), or by turning the liquid into a paste-like gel by the addition of silica and other gelling agents (gel batteries).

The gases are retained within the VRLA batteries providing the pressure remains within safe levels. The gases may then recombine within the battery itself, therefore no topping-up is needed. If the pressure exceeds the safe limits, valves opens to allow the gases to vent, and thereby regulating the pressure back to the required levels.

The main downside to the VRLA design is that the immobilizing agent also slows down the chemical reactions that generate current. For this reason, The VRLA battery have lower peak power ratings than flooded lead ascid batteries sincde the immobilizing agent also impedes the chemical reaction generating current.

The chemical reaction of a lead acid battery:
When discharged both poles become lead(II) sulfate (PbSO4), and the electrolyte becomes primarily water. Electrons are conducted from the negative plate back into the cell at the positive terminal via the external circuit. When the battery is recharged, the lead sulfate is transformed back to metallic lead and sulfuric acid on the negative terminal or lead dioxide and sulfuric acid on the positive terminal.

Lead(II) sulphate can be often seen as a milky white crystalline substance at the terminals of vehicle batteries, as it is formed when the battery is discharged.

Negative electrode (lead plate reaction):
Pb(s) + HSO4(aq) → PbSO4(s) + H+(aq) + 2e

Positive electrode (lead dioxide) reaction:
PbO2(s) + HSO4(aq) + 3H+(aq) + 2e → PbSO4(s) + 2H2O(l)

The total reaction can be written as:
Pb(s) + PbO2(s) + 2H2SO4(aq) → 2PbSO4(s) + 2H2O(l)

In a typical lead-acid battery, the open circuit voltage is approximately 2,12 volt per cell. Consumer batteries come in 6, 12 and 24 volts versions.

Opencircuit voltage 12V Lead acid battery
Charge:
12,7V – Fully charged100%
12,3V – 50% capacityca. 50%
12,0V – Needs to be chargedca. 30%
11,6V – To be disposedca. 10%
ChargeFreezing temperature
100%ca. -70°C
40%ca. -25°C
10%ca. -25°C

Nickel – Cadmium (NiCd) batteries
The nickel–cadmium battery is a rechargeable battery using nickel oxide hydroxide and metallic cadmium as electrodes.

Ni-Cd batteries have many applications and capacities, from portable sealed types to large ventilated cells used for standby power and motive power.

Compared with other types of rechargeable cells the NiCd cell provides good low temperature erfromance. The main advantage of this type of cell is however the ability to deliver its full capacity at very high discharge rates.

The chemical reaction at the cadmium anode (negative electrode) during discharge is:
Cd + 2OH → Cd(OH)2 + 2e

The chemical reaction at the nickle oxide cathode (positive electrode) is:
2NiO(OH) + 2H2O +2e → 2Ni(OH)2 + 2OH

The net reaction during discharge is:
2NiO(OH) + Cd + 2H2O +2e → 2Ni(OH)2 + Cd(OH)2

During recharge, the reactions go the opposite direction from right to left. The electrolyte is not consumed in this reaction.

A NiCd battery has a terminal voltage during discharge of around 1.2 volts. The terminal voltage remains more or less unchanged during the discharge cyclus.

NiCd batteries are supplied in single cells as well as multiple cell batteries providing up to 360 volts nominally and with the capacity to deliver serveral hundred Ampere hours (AH), such configuration requires a battery of 300 cells. This heavy duty multiple cell configuration is mainly used for automotive and industrial applications.

Nickel metal hydride (NiMH) batteries
The nickel–metal hydride battery is a rechargeable battery.

The chemical reaction at the positive electrode is similar to that of the nickel–cadmium cell (NiCd), with both using nickel oxyhydroxide (NiOOH). However, the negative electrodes use a hydrogen-absorbing alloy instead of cadmium.

A NiMH battery can have two to three times the capacity of an equivalent size NiCd, and its energy density can approach that of a lithium-ion battery.

The chemical reaction at the negative electrode is:
H2O + M + e →OH + MH
The chemical reaction at the positive electrode is:
Ni(OH)2 + OH→NiO(OH) + H2O + e

During discharge the reactions go the opposite direction from right to left.

The metal (M) used in the negative electrode mayvary but will normally fall into one of two categories AB5 and AB2.

AB5; A= A mixture of cerium, lanthanum, neodymium, praseodymium and B= A mixture of alumknium or manganese, cobalt and nickel.

AB2; A= Titanium or vanadium and B= cobalt, manganese, nickel or aluminium

The NiMH cell has an alkaline electrolyte, usually potassium hydroxide. The positive electrode is nickel hydroxide and the negative electrode is hydrogen ions or protons. The hydrogen ions are stored in a metal hydride structure that is the electrode

A NiMH battery has a terminal voltage during discharge of around 1.2 volts.

Lithium-ion (Li-ion) batteries (LIB)
The lithium-ion batteries belong to a group of battery types where lithium ions move from the negative electrode to the positive electrode during discharge and back when charging.

Conventional Li – ion cells have a carbon based (often graphite) negative eletrode and a metal oxide based positive electrode, the electrolyte would be a lithium salt in some form of organic solvent.

The positive electrode would normally be made from compounds like; iron phosphate (LiFePO4 cell), lithium manganese oxide (LMO cell), lithium nickel manganese cobalt oxide (NMC cell), lithium nickel cobalt aluminum oxide (NCA) or lithium titanate (LTO).

The non-aqueous electrolyte would be a mixture of organic carbonates (ethylene or diethyl) mixed with a lithium salt (ex. LiPF- lithium hexafluoroarsenate monohydrate, LiAsF6;-lithium perchlorate, LiClO4– lithium tetrafluoroborate, LiBF4– or lithium triflate LiCF3SO3).

Typical cylindrical Li-ion cell

 

Illustration of Li-ion battery cell. (This text is displayed because your browser do not support SVG)

 

Since lithium is very reactive with water it is important that the electrolyte is non aqueos and that sealed containers are used to avoid moister within the battery pack.

The voltage and energy density will vary based on the materials used; nanotechnology is often used to improve the performance of the cells.

The chemical reaction at the cathode (positive electrode) during discharge is:
LiCoO2 → LiCoO2 + Li+ +e

The chemical reaction at the anode (negative electrode) during discharge is:
Li+ + e + C6 → LiC6

Lithium ion batteries is today the most common battery technology used in modern electrical road vehicles.

To obtain the desired voltage and capacity a number of cells would be stacked.

Typical Li-ion battery pack for road vehicles.
Typical a 56 kWh, 375 volt battery pack for a road vehicle capable of delivering a maximum of 215 kW would consist of 6831 cells, each cell, 3,6 Volt and 2,9 Ah
(ampere hours). Such a Li-ion battery pack would have a weight of around 450 kg.

Nickel-iron (NiFe) batteries
The nickel-iron battery (NiFe) uses an oxide-hydroxide cathode and an iron anode with potassium hydroxide electrolyte to produce a nominal cell voltage of 1.2V. NiFe is resilient to overcharge and over-discharge and can last for more than 20 years in standby applications. NiFe has a low specific energy of about 50Wh/kg, has poor low-temperature performance and exhibits high self-discharge of 20 to 40 percent a month

Nickel-iron Battery Specifications

Energy/weight            30-50Wh/kg
Energy/size                30 Wh/l
Power/weight             100W/kg
Charge/discharge efficiency           65% – 85%
Time durability           30– 100 years
Cycle durability          Repeated deep discharge does not reduce life significantly.
Nominal cell voltage            1.2 V
Charge temperature interval          min.-40°C

The chemical reaction at the cathode (positive electrode) during discharge is:
2 NiOOH + 2 H2O + 2 e → 2 Ni(OH)2 + 2 OH

The chemical reaction at the anode (negative electrode) during discharge is:
Fe + 2 OH → Fe(OH)2 + 2 e

During re-charge the reactions go the opposite direction from right to left

The open-circuit voltage is 1.4 volts, dropping to 1.2 volts during discharge.

The electrolyte mixture of potassium hydroxide and lithium hydroxide is not consumed in charging or discharging, so unlike a lead-acid battery the electrolyte specific gravity does not indicate state of charge.

Nickel-zinc (NiZn) batteries
Nickel-zinc (NiZn) batteries are similar to nickel-cadmium in that they use an alkaline electrolyte and a nickel electrode, but differ in voltage; NiZn provides 1.6V/cell rather than 1.2V, which NiCd delivers.

Low cost, high power output and good temperature operating range make this chemistry attractive.

The specific energy is similar to other nickel-based systems.Water is consumed and generated on the discharge and charge cycles respectively.

Discharge Reaction is left to right:
2H2O + Zn + 2NiOOH → Zn(OH)2 +2Ni(OH)2.

Electrochemical open circuit voltage potential: ~1.73 V

Nickel-hydrogen (NiH) batteries
NiH uses a steel canister to store the hydrogen gases at a pressure of 1,200psi (8,270kPa/ 82,7 bar). The cell includes solid nickel electrodes, hydrogen electrodes, gas screens and electrolyte that are encapsulated in the pressurized vessel.

NiH has a nominal cell voltage of 1.25V and the specific energy is 40–75Wh/kg

Zinc – air batteries
Zinc–air batteries (non-rechargeable), and zinc–air fuel cells (mechanically rechargeable) are metal-air batteries powered by oxidizing zinc with oxygen from the air.

Zinc–air batteries have some properties of fuel cells as well as batteries: the zinc is the fuel, the reaction rate can be controlled by varying the air flow, and oxidized zinc/electrolyte paste can be replaced with fresh paste.

Zinc–air batteries cannot be used in a sealed battery holder since some air must come in; the oxygen in 1 liter of air is required for every ampere-hour of capacity used.

The chemical equations for the zinc–air cell are:

The chemical reaction at the anode (negative electrode) during discharge is:

Zn + 4OH− → Zn(OH)42− + 2e

The chemical reaction in the Fluid is:
Zn(OH)42− → ZnO + H2O + 2OH

The chemical reaction at the cathode (positive electrode) during discharge is
1/2 O2 + H2O + 2e → 2OH

The overallchemical reaction is:
2Zn + O2 → 2ZnO

The reactions produce a theoretical 1.65 volts, but this is reduced to 1.35–1.4 V in available cells.

Silver-zinc batteries
Delivers one of the highest specific energies of all presently known electrochemical power sources.

The electrolyte used is a potassium hydroxide / water solution.

During the charging process, silver is first oxidised to silver(I) oxide and then to silver(II) oxide:
2Ag(s) + 2OH → Ag2O + H2O + 2e

and then to silver(II) oxide:
Ag2O + 2OH → 2AgO + H2O + 2e,

while the zinc oxide is reduced to metallic zinc:
2Zn(OH)2 + 4e = 2Zn + 4OH.

The process is continued until the cell potential reaches a level where the decomposition of the electrolyte is possible at about 1.55 Volts.

Sodium-sulfur (NaS)(or molten salt or thermal) batteries
The batteries uses molten salts as an electrolyte constructed from liquid sodium and sulphur, operates at a temperature of 2250–350°C temperature.

The electrolyte of the molten salt batteries is inactive when cold and can be stored for lengthy periods of time. Once activated with a heat source, the battery can provide a high power burst for a fraction of a second or deliver energy over several hours. High power is made possible with good ionic conductivity of the molten salt.

The modern rechargeable sodium-sulfur is known as sodium-nickel-chloride battery.

Molten sulphur – positive electrode

Molten sodioum negative electrode

The battery has a nominal cell voltage of 2.58 volts and a specific energy of 90–120Wh/Kg

As the cell discharges, the sodium level drops. During the charging phase the reverse process takes place.