Nickel-cadmium accumulator

A nickel-cadmium accumulator (NiCd accumulator) is an accumulator (so-called secondary cell).

In terms of the basic design, a distinction must be made between open and gas-tight cells. Gas-tight cells are often identical to commercially available batteries and can therefore be used as a replacement for these so-called primary cells; open cells are used for stationary applications.

Different types of NiCd batteries

history

Development and dissemination

Electric drive set from PSA Peugeot Citroën with the associated NiCd accumulators in the background. ( Museum Autovision , Altlußheim )

The 1899 by the Swede Jungner Waldemar developed nickel - cadmium - battery is one of the alkaline battery systems, also attended Thomas Edison was working at that time and it u. a. developed the nickel-iron accumulator . By using cadmium instead of iron, Jungner was able to increase the energy and current yield of its battery by around 7% compared to the predecessor by Edison; The NiCd battery also offered numerous advantages over the lead-acid batteries that had prevailed until then , not least the fact that the electrolyte remained unchanged during the charging and discharging of the battery.

In 1910 the industrial production of NiCd batteries began in Sweden. These first examples had so-called pocket electrodes, which are still common today. So-called sintered electrodes were developed in Germany around 1930 . The principle of gas-tight cells was published in 1933 by Adolf Dassler. Gas-tight cells ready for series production were available in the 1950s. In 1983 the Hoppecke batteries company presented a fiber structure nickel cadmium battery with FNC technology (fiber structure technology). In contrast to pocket plates and sintered electrodes, the carrier for the active material is not a heavy, stiff metal, but a light and flexible fleece made of nickel-plated polypropylene fibers . This metallized fiber structure fleece was originally developed for such demanding areas as aerospace, as well as for electric and hybrid vehicles.

By the 1990s, the NiCd battery had become the most widely used rechargeable battery for end users. After that, the market share of nickel-metal hydride (NiMH) and lithium accumulators continued to increase, as they have higher energy densities and do not contain toxic cadmium.

EU-wide ban

In December 2004, the EU Council of Ministers passed a directive to reduce the technical use of cadmium. Subject to the approval of the EU Parliament, it was envisaged that the member states would prohibit the placing on the market of nickel-cadmium batteries through national laws within two years . At the request of some member states, including Germany, cordless power tools , among other things, were initially exempted from the ban because "for power tools, it is not guaranteed that equivalent replacement is currently available". This exemption should be reviewed four years after the entry into force of the directive in order to possibly extend the cadmium ban. See also RoHS .

In 2006 the European Parliament adopted an amended version of the directive, which bans batteries and accumulators with more than 0.002 percent by weight of cadmium.

With the Battery Act (BattG), which came into force on December 1, 2009 , the German legislator implemented the directive into national law. Section 3 (2) BattG prohibits the placing on the market of batteries containing cadmium with the exception of those for emergency or alarm systems, emergency lighting and medical equipment. Cordless power tools were exempt from the ban until December 31, 2016.

properties

Comparison of power and energy density of some energy storage systems

NiCd accumulators have a nominal voltage of 1.2  V , which is 20% below the 1.5 V of normal batteries. However, this is usually not a problem, as most devices are designed for low voltages of 0.9–1.0 V discharged batteries. Due to their low internal resistance , NiCd accumulators can deliver high currents . For this reason, among other things, they are preferably used in model making and other high-current applications. NiCd batteries must be recharged when the residual voltage (end-of- discharge voltage ) is 0.85–0.9 V. Further discharge leads to total discharge with effects similar to those of lead batteries. The memory effect is weak.

A property rarely encountered in other technologies is the excellent low-temperature behavior of NiCd batteries. Even at −40 ° C, a battery with fiber structure plate technology still has over 50% of its nominal capacity at room temperature.

construction

Disassembled NiCd cell

When charged, the electrodes of the NiCd accumulator consist of plates that are charged with finely divided cadmium at the negative pole and nickel (III) oxide hydroxide at the positive pole. A 20% potassium hydroxide solution is used as the electrolyte. This combination delivers a voltage of 1.3 V.

If the battery is overcharged, hydrogen is produced on the negative electrode and oxygen on the positive electrode; the battery is said to be “gast”. In closed, i.e. gas-tight cells, this must be prevented because of the risk of explosion. For this reason, the negative cadmium electrode is oversized and serves as a negative discharge reserve. The positive nickel electrode contains some cadmium hydroxide as an "antipolar mass". In the event of overcharging at lower charging rates (approx. 0.1 C), an equilibrium is established between the release and consumption of oxygen; no hydrogen is developed.

In gas-tight fiber structure NiCd cells, the resulting oxygen is recombined so quickly on a catalytically active surface of the fiber structure recombination electrode that even a slight negative pressure is created during operation.

charge

→ Main article : charging method

Electrochemistry

NiCd accumulators consist of the following components:

• an electrolyte , usually a 20 percent potassium hydroxide solution
• a negative electrode: Cd
• a positive electrode: NiO (OH)
• a separator

Discharge process: Cadmium is oxidized to cadmium hydroxide (Cd (OH) ₂ ) at the anode / negative electrode . The released electrons then flow via the consumer to the cathode / positive electrode. There the nickel (III) oxide hydroxide NiOOH is reduced to nickel (II) hydroxide Ni (OH) ₂ .

Standard electrode potentials : E (Cd / Cd ²⁺ ) = −0.81 V (in basic solution); E (NiO (OH) / Ni (OH) ₂ ) = +0.49 V

Reactions:

Negative electrode:	${\displaystyle \mathrm {Cd+2OH^{-}\leftrightarrow Cd(OH)_{2}+2e^{-}} }$
Positive electrode:	${\displaystyle \mathrm {2\,NiO(OH)+2\,H_{2}O+2e^{-}\leftrightarrow 2\,Ni(OH)_{2}+2OH^{-}} }$
Overall reaction:	${\displaystyle \mathrm {2\,NiO(OH)+Cd+2\,H_{2}O\leftrightarrow 2\,Ni(OH)_{2}+Cd(OH)_{2}} }$

${\displaystyle \rightarrow }$: Discharge

${\displaystyle \leftarrow }$: Charge

Charging process: The reactions take place in the opposite direction, the cadmium electrode is then also the negative pole, but the cathode, as there is reduction here, the nickel electrode is the positive pole / anode at which oxidation takes place.

Overcharging: Towards the end of the charging cycle, the cell voltage increases, from around 1.55 to 1.6 V the decomposition voltage of the water is exceeded under the conditions of the cell, resulting in gassing:

Negative electrode:	${\displaystyle \mathrm {4\,H_{2}O+4\,e^{-}\ \rightarrow 2\,H_{2}+4\,OH^{-}} }$
Positive electrode:	${\displaystyle \mathrm {4\,OH^{-}\ \rightarrow 2\,H_{2}O+O_{2}+4\,e^{-}} }$
Overall reaction:	${\displaystyle \mathrm {2\,H_{2}O\ \rightarrow 2\,H_{2}+O_{2}} }$

An excess of cadmium (II) hydroxide is used in gas-tight NiCd batteries. When overcharging, oxygen is generated at the positive pole, while Cd ^{2+ is} reduced at the negative pole . The oxygen then reacts with cadmium to form cadmium (II) hydroxide and is thus used up again immediately.

Problems

NiCd batteries contain the poisonous heavy metal cadmium and must therefore be disposed of separately using a special collection system (see section Disposal ).

Overcharging NiCd batteries can be damaged:

• Outgassing through overheating / overloading (irreversible)
• Formation of γ-NiOOH and thus voltage drop (44–50 mV)
• Formation of intermetallic compound Ni ₅ Cd ₂₁ and thus voltage drop (120 mV)

Incorrect charging (polarity reversal) also damages a cell by outgassing at the anode . This also creates extremely flammable hydrogen . The wrong polarity of a cell within a battery pack occurs when it is deeply discharged . The cells are connected in series. When the weakest cell is discharged, the positive pole is on its negative electrode and the negative pole of the neighboring cells on the positive electrode.

High states of charge when storing NiCd batteries lead to crystal growth on the Cd electrode. Crystals can pierce the separating layers and cause an internal short circuit in the cell. NiCd batteries are best stored at a charge level of 40% in order to avoid deep discharge and to reduce crystal growth.

advantages

NiCd cells are more robust than NiMH and lithium-based batteries when they are deeply discharged or overcharged. In this way, they can be stored in a discharged state for several years without being damaged. A uniform state of charge can also be achieved in a battery made up of NiCd cells connected in series by specifically overcharging them with low current (1/10 of the capacity per hour). Cells with an already high charge level convert the excess energy into heat without suffering irreversible damage. This procedure is not possible or only possible to a limited extent with other battery types. Furthermore, NiCd cells have favorable properties at temperatures below 0 ° C.

Both properties are important in safety-critical applications, so that in many cases lithium or NiMH batteries are not suitable here.

disposal

Pictograms on a NiCd battery.

→ Main article : Battery recycling

Due to the toxicity of cadmium , NiCd batteries must not be disposed of with household waste. There are special take-back systems for them, for example operated in Germany by the Foundation for the Joint Take-Back System for Batteries .

NiCd batteries can be easily recycled if disposed of properly. The cadmium can be recovered by distillation because it has a much lower boiling point than the other components of the accumulator (usually nickel and steel ).

application

Open cells

• Starter batteries for internal combustion engines and traction batteries for electric vehicles
• Uninterruptible power supply (UPS) ( emergency power supply )
• Miner's lamps
• Central power supply systems for emergency lighting

Gastight cells

• Consumer area in general (toys, cameras, electrical tools, remote controls, etc.)
• Emergency lighting (self-contained lights )

See also

• Electrochemical series

literature

• Lucien F. Trueb, Paul Rüetschi: Batteries and accumulators - Mobile energy sources for today and tomorrow . Springer, Berlin 1998 ISBN 3-540-62997-1
• Thomas B. Reddy (Ed.): Linden's Handbook of Batteries . 4th edition. McGraw-Hill, New York 2011, ISBN 978-0-07-162421-3 :
  • Chapter 19: John K. Erbacher: Industrial and Aerospace Nickel-Cadmium Batteries
  • Chapter 20: R. David Lucero: Vented Sintered-Plate Nickel-Cadmium Batteries
  • Chapter 21: Joseph A. Carcone: Portable Sealed Nickel-Cadmium Batteries
• Claus Daniel, Jürgen O. Besenhard: Handbook of Battery Materials. Wiley-VCH, Weinheim 2011, ISBN 3-527-32695-2 .
• Chapter 2.2. The nickel-cadmium accumulator. In: P. Birke, M. Schiemann: Accumulators: Past, Present and Future of Electrochemical Energy Storage , H. Utz Verlag, Munich 2013, ISBN 978-3-8316-0958-1 , pp. 119–146

Web links

• Foundation for a joint take-back system for batteries
• BMZ: Not all chargers are equivalent - higher battery performance through correct charging (Nov. 28, 2012)

Individual evidence

1. ↑ Ernst Grimsehl, Walter Schallreuter, Rolf Gradewald: Textbook of Physics . tape 2 : Electricity. Teubner, 1954, ISBN 3-322-00756-1 , p. 217 .
2. ↑ Patent DE602702 : Galvanic element, in particular an electric accumulator. Published September 14, 1934 . ‌
3. ↑ Press release of the European Parliament
4. ↑ VARTA: Gas-tight Ni-Cd accumulators. VARTA Batterie AG (Ed.), Hanover.
5. ↑ Foundation for a common battery take-back system: The concept of batteries ( Memento from September 28, 2007 in the Internet Archive ) (with an explanation of the recycling of batteries, as of September 17, 2007)