Laboratory Products
Battery cells love it dry – how to measure the moisture in the materials Dr Dorit Wilke & Dr Michael Hahn, ECH Elektrochemie Halle GmbH, email:
info@ech.de
1. Introduction. Electromobility is a declared goal both to solve resource problems in fuel production and to reduce environmental pollution.
Battery development, which has made great strides in recent years in particular, is seen as the key to progress in electromobility. A lot has happened since 1981 when the Golf I CitySTROMer from Volkswagen drove a maximum of 60 km with lead-gel batteries and the trunk and other space reserves were completely fi lled with 16 lead-acid batteries [1].
The development towards hybrid and pure electric technology in the automotive industry depends to a large extent on the performance and the costs of the electricity storage media. High energy densities of up to 150 Wh/kg and long cycle lives of >2000 charging cycles are prerequisites for acceptable ranges in motor vehicle use.
In addition to the automotive industry, battery developments are also the key to the extensive use of renewable energies. Stationary, decentralised energy storage systems must have a very high energy content, while being low-maintenance and durable.
Lithium-ion batteries are one of the successful developments. They have signifi cant advantages over conventional batteries such as lead-gel, nickel-cadmium and nickel- metal hydride. They have high rated voltages of 3.4-3.7 V, very high current densities, no memory effect, constant discharge voltage, low self-discharge. This results in a high effi ciency of 95% and a compact, lightweight design. However, lithium-ion systems require special precautions when charging, cannot be used at temperatures above 60°C, and must be secured with built-in protective electronics (PCB) [2]. This results in a correspondingly high price.
Li-ion batteries for motor vehicles today achieve an energy density of over 120 Wh/kg compared to lead-acid batteries with around 30 Wh/kg. A large number of parameters are responsible for the performance of battery systems. Decisive factors for the energy density and performance of lithium-ion batteries are the choice of materials and the purity of the components used.
For example, over the years, e.g. the capacity of the 18650-type accumulators (design comparable to conventional AAA batteries) from 1991 to 2005 tripled to 2550 mAh by changing the electrode materials and the electrolytes used [3]. Today, the energy densities of this type are even 3500 mAh.
Lithium-ion batteries consist of the following basic components:
• 1. Electrode: e. g. LiCoO2 • 2. Electrode: e. g. Li-Graphite
• Electrolyte: water free, e. g. Ethylene carbonate, Dimethyl carbonate, Fluorine ethylene carbonate with conducting salt: e. g. LiPF6 • Separator made of porous membranes (polypropylene, polyethylene, ceramic)
The entire structure of the lithium-ion battery must be such that the water content is very low (H2
reduction in capacity and service life. For production and development, this means constant monitoring of the water content in all the basic materials and assemblies used (electrode materials, separator membranes, basic electrolytes, conducting salt, functional passivation additives, additives for cathode and fl ame retardants [4,5]).
salt LiPF6
Due to the very low water content, the determination can only be carried out with certainty using a coulometric titration.
Investigation of the solids also requires the use of the heating technique with an oven to release the trapped water.
2. Metrology
The coulometric titration is based on the electrochemical generation of the iodine required for the Karl Fischer determination. According to Faraday’s law and the
Figure 1: Karl Fischer titrator with heating technology (AQUA 40.00 Vario Plus).
O content <20 ppm), otherwise the water will react with the conducting to form HF (hydrofl uoric acid). The hydrofl uoric acid leads to a strong
Karl Fischer reaction equation, the amount of water converted can be determined immediately from the amount of electrical charge. The advantage of this coulometric method is that no titre adjustment is required. The titration end point is indicated electrochemically with the aid of two platinum electrodes.
With the coulometric titration, even the smallest amounts of charge can be ‘dosed’ with high precision. For this reason, the coulometric variant of the Karl Fischer technique has established itself in the trace area.
Liquids are injected into the measuring cell through a septum. This direct dosing can be used if no disturbing side reactions occur. A prerequisite for reliable analyses in the trace range is an intact septum to prevent the ingress of atmospheric moisture. In addition, all-glass cells are advantageous.
Direct dosing is only suitable to a limited extent for solids and liquids that are insoluble in the Karl Fischer reagent. It is better, but more labour intensive, to extract the water beforehand with a suitable solvent (e. g. methanol or dioxane). However, other methods of sample pretreatment (homogenisation, crushing, extraction) increase the risk of measurement errors, especially in the ppm range, due to the infl uence of the ambient humidity.
The heating technique represents a very elegant and universal alternative for solids, pasty materials and oils, with which the water can be selectively determined.
The sample to be analysed is dosed into a separate oven. At elevated temperatures, the water is heated out of the sample and transported to the coulometric measuring cell with the help of a dried carrier gas stream. For reliable analysis results in the ppm range, the continuously fl owing carrier gas must be dried very carefully.
It is more expedient to circulate the carrier gas, as implemented in the AQUA 40.00 Vario from ECH Elektrochemie Halle GmbH (Figure 1).
INTERNATIONAL LABMATE - FEBRUARY 2023
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