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Thermal runaway
This failure mode is specific for the valve regulated design and more particularly, it concems the VRLA batteries in which the oxygen cycle is very efficient. In fact, the oxygen production is catalysed by an increase in temperature. In parallel, the oxygen reduction on the negative plate is an exothermic reaction.
That way, in designs where the oxygen transfer between both electrodes is easy and where the temperature is not managed, a process of « catalyse of the oxygen production by the oxygen reduction » happens (see reference [28] ). If the heat exchange with the surrounding medium is insufficient, a dramatic increase of the temperature happens, leading to damages in the battery.
Fai/ure ofthe negative electrode
With the progress made on the positive electrode, negative plate failure is proved to be now the main failllre mode of the VRLA battelY [26]. Just in the same way as for the positive electrode, one can draw a figure of the different premature negative electrode failure modes (Figure 20).
• Top lead corrosion. This type of cOlTosion only concerns the VRLA batteries [16] and can lead to a
rupture of the negative gI’id/top bar connection [26]. The cOlTosion reaction is the discharge reaction of the lead electrode. It stalis in principle at the equilibrium potential of the lead electrode and happens only if the potential is more positive that the equilibrium potential. In flooded batteries, during rest or float charge, concentrate slliphuric acid is always in contact with the lead and the potential of the negative electrode stays low. But an increase of the electrode potential happens in VRLA battelies in areas where oxygen reduction occurs, producing water and leading to a depolarisation of the concerned area. Subsequently the pH of these areas increases. Because the conductivity of the thin electrolyte film leading to the concerned areas is reduced (increased pH), an additional voltage drop occurs that keeps these areas in a discharged state, not allowing for the reduction of the formed sulphates.
• Sulphate passivation by oxygen depolarisation. This sulphation of the negative electrode happens only in the VRLA batteries and is related to the basis principle of the VRLA battelY, namely the oxygen cycle. When oxygen reaches the negative electrode, it is reduced. During charge, the CUITent that flows in the oxygen reduction is lost for the charge reaction. If the oxygen transfer from the positive plate to the negative is too easy, the very efficient oxygen cycle can lead to a chronic insufficient recharge of the negative electrode [38]. Additionally, if the VRLA battery is untight, oxygen from the outside cornes in contact with the negative plate and reacts with the lead to form lead sulphate. This leads also to a sulphate passivation of the negative electrode.
• Compaction. The loss of porosity of the negative electrode with increasing cycling number is a well-known phenomenon both in flooded and valve regulated lead-acid battery designs. This loss of porosity decreases the specific surface of the negative active material and therefore, the number of
available reaction sites is decreasing. Subsequently, the negative material utilisation decreases. There are two processes by which the negative materiallooses its porosity.
The first one is chemica1. It is the leakage of the additives that are adsorbed at the negative plate surface and promote the formation of the spongeous structure of lead by disturbing the growth of the lead crystals. These additives called expander, where discovered as rubber separators supplanted the cellulose separators. One then discovered that this replacement had a bad effect on the cycling life.
The reason for that was the absence of the lignin coming from the cellulose separator. After that, expanders were added to the negative pate. But the expanders progressively dissolve in the solution and leave the negative plate. And since they are mostly of organic nature, in contact with the positive plate, they are oxidised to e.g. carbon dioxide and leave the battery. Thus the restoration of the sponge structure of the negative plate during charge is not possible any more. The second reason for compaction is of mechanical nature (see section 3.6.1). Namely, the fact of applying a mechanical pressure on the positive plate of the battery proved to increase its performance and the battery assembly nowadays is very tight. As a result, a progressive compaction of the soft negative active material occurs.
Different ways for the improvement of the lead-acid battery
One general drawback of the lead-acid system is that the electrolyte is an active materia1. The sulphate ions sot take part to the reactions and must move to the reaction place and their big size hinder a quick movement. By opposition, in the concurring systems like the lithium systems or the nickel systems, the ions that have to move during charge and discharge are very small and thus very mobile. In the alkaline systems, OR and ft are available in a sufficient amount from the water. In the lithium systems, the lithium ions intercalated during discharge in the positive electrode are delivered as the discharge product at the negative electrode (Li => Ln. Therefore, the conductivity of the electrolyte is almost unchanged in alkaline and lithium systems. Their quantity of ions and the number of diffusion path do not have to be oversized. While in the lead-acid system, the conductivity of the electrolyte decreases during discharge and thus, the power capability. Therefore, electrolyte must be present in excess in the lead-acid battery. In order to improve the performance of the lead-acid battery, in particular for assuring to the lead-acid battery a place in the market of electric vehicles, both its life and its energy density have to be improved.
While the life limiting factors were listed in the former section, Figure 21 shows where efforts have to be made in order to increase the specific energy of the lead-acid battery [39].
Electrical tests on batteries
Cells and batteries were first evaluated concerning their performance under dynamic conditions and under cyclic conditions with different separation systems and electrolyte formulations. All tests were performed at ZSW (Ulm, Germany). They characterise the electric performance of a battery. Delivering electrical energy is finally the actual purpose of a battery, therefore the electrical tests are the ones that unquestionably assess the relevance of an improvement.
Internai resistance, open circuit voltage, peakpower (IROCVP test)
The IROCVP test is taken from the EV battery test procedure adopted by the EUCAR traction battery group [45]. This test was performed on a Digatron bench that can provide 300A at 300V. The software associated with the bench is a Digatron BTS 600.
Within this single test, the internaI resistance, the open circuit voltage and the peak power can be determined versus depth of discharge. First, the open circuit voltage is determined after a rest period of 3 hours (or less if the voltage changes less than 1% over a 30 minutes period). Then, a high discharge CUITent of 300A is applied during 30s followed by a rest period of 3 minutes. The high CUITent internaI resistance is deduced form the ratio between the voltage difference (rest voltage after 3 minutes Di voltage at the end of the high CUITent discharge peak UD and the discharge CUITent. At the end of the 3 minutes rest, the battery is discharged at the C/5 rate until 10% of the rated capacity is removed. The same procedure is then applied again at the next state of charge (SOC). When the discharge voltage limit is reached during the high CUITent discharge or during the following C/5 discharge, the test is completed. A short recapitulation of the procedure is made for one SOC in Figure 24.
Potentiostatic corrosion measurement
In the potentiostatic corrosion measurements, the set-up was similar to the one used for the cyc1ic voltammetry but the variator was replaced by a frequency generator delivering a square signal between the reference electrode and the working electrode. The output between the working electrode and the counter electrode was recorded and the discharge capacity was ca1culated by integrating the discharge current over time. The set-up is schematically represented in Figure 26.
Bibliography of « compression »
On his way to a battery with fast charge abilities, Alzieu [52] was the first to think about applying a mechanical pressure vertically to the plane of the electrodes of a prismatic cell in order to prevent the shedding of the positive active materia1. For this purpose, a multi-layer separator was used, what also was a première. In 1984 [53], Alzieu reports on compressed batteries loaded with a rapid charge regime. In thespecified design, the compression/cycle life curve shows a maximum around 1 bar. At this pressure, the battery which reached 600 cycles uncompressed lasts for over 2000 cycles (cycles: 30 min ru charge, l at 3*C5, U at 2.65V, discharge at (C/5)/1.7 until the cell voltage reaches 1.5V). At higher pressure, the ceUs failed because the separators coUapsed or because the separators design causes damages to the negative grid. In order to increase the cycle life more, Alzieu tried to prevent the shedding of the positive active material from the edges of the plates by melting the separators to form a kind of envelop around the positive e1ectrode. At 0.75 bar compression, the battery performed up to 3000 cycles. Later on, the same team [54] separated the « heat effect » and the « expansion effect ». These two effects lead to opposite expansion and contraction of the active materials during charge and discharge. The work of this team is the reaUy first reference to an active application of mechanical pressure on the waUs of a ceU or battery. E. 1. Ritchie [43] made another early reference to the utilisation of external mechanical pressure. In 1980, he wrote that » A mechanical solution to the problem …[of shedding positive material] … caUs for sealing each positive plate in its own porous envelope, and using a tight assembly. Thus, when active material sheds, it will still be held in place and not be entirely lost from service ».
It is important to differentiate the active mechanical pressure application from the passive contention of the active material that was already performed in sorne types of batteries.
From a wide point of view, one could say that the first « compression battery » was the first battery with tubular plates. Namely in this design, the active material is contained in a gauntlet and is therefore not allowed to expand in any direction. The first who had the idea of gauntlets was Boriolo in 1959 [55], but the first tubular design was produced even earlier (1910) with tubes ofrubber material containing the lead spine and active materia1.
Secondly, another attempt to contain the positive active material was made with the development of the pocketing of the positive plate in a separator. That way, the positive active material is not aUowed to shed easily from the positive grid and cause short circuits. Such a sealing of the electrodes in their porous envelope was made possible by the introduction of polyethylene separators around 1975. They are now the most widely used separators (70% of aU automotive batteries world-wide [56]). But in fact, no real pressure is applied on the active material that way because the separators are not robust enough to constraint the active material expansion, only mudding is hindered.
Table of contents :
1 The lead-acid battery
1.1 Electric energy accumulation
1.2 IIistory of the lead-acid battery
1.3 Principle
1.4 The electrochemical system « lead-acid battery »
1.4.1 General considerations
1.4.2 Secondary reactions
1.5 Different designs of the lead-acid battery
1.5.1 Flooded battery .
1.5.2 VRLA battery
1.5.3 Bipolar lead-acid battery
1.5.4 Different designs of lead-acid batteries
1.6 Life limiting factors
1.6.1 Drying out
1.6.2 Short circuits .
1.6.3 Sulphation
1.6.4 Thennal runaway
1.6.5 Stratification
1.6.6 Failure ofthe positive electrode .
1.6.7 Failure of the negative electrode .
1.7 Different ways for the improvement ofthe lead-acid battery
1.7.1 Design
1.7.2 Composition and characteristics of the active materials .
1.8 Conclusion about possible improvements ofthe lead-acid battery
2 Experimental techniques
2.1 Electrical tests on batteries
2.1.1 InternaI resistance, open circuit voltage, peak power (IROCVP test) ..
2.1.2 ECE 15 test
2.1.3 Cycling test.
2.1.4 Dependence of the discharge capacity on the discharge current
2.2 Preparation of polished sections
2.3 Microscopy
2.3.1 Optical microscopy
2.3.2 Scanning Electron Microscopy
2.3.3 Microprobe Analysis
2.4 Chemical analysis and electrolyte concentration
2.5 Porosimetry
2.6 X-ray diffraction
2.7 IR pictures
2.8 Electrochemical techniques
2.8.1 Cyclic voltammetry
2.8.2 Potentiostatic corrosion measurement
2.8.3 Galvanostatic corrosion measurement
3 Mechanical pressure application
3.1 Bibliography of « compression »
3.2 Electrolyte immobilisation/separation systems
3.2.1 The complex influence of a passive part: the separator
3.2.2 Ge1.
3.2.3 AGM
3.2.4 The new Acid Jellying Separator (AJS)
3.2.5 More than compression
3.3 Performance improvement through EMPA
3.3.1 Experimental conditions
3.3.2 Parameter test
3.3.3 Cycling life
3.3.4 Conclusion about the electrical tests
3.4 Mechanical pressure development during one cycle
3.4.1 Typical evolution of the mechanical pressure
3.4.2 Increase of the mechanical pressure during discharge
3.4.3 Evolution of the mechanical pressure during charge
3.4.4 Influence of the separation system
3.4.5 Influence of the initial mechanical pressure
3.4.6 Behaviour for a battery
3.4.7 Conclusion about the mechanical pressure evolution during one cycle
3.5 Evolution ofthe mechanical pressure over cycling life
3.5.1 Variation ofmechanical pressure between beginning and end of discharge
3.5.2 Increase ofmechanical pressure between the beginning and the end of one cycle
3.6 How does EMPA affect the performance of the lead-acid battery
3.6.1 Changes in the structure ofthe negative electrode
3.6.2 Post mortem analysis ofthe positive electrode
3.6.3 Models describing the positive active materia1
3.6.4 Why mechanical pressure application increases the life ofthe positive active material.
3.7 Conclusion about mechanical pressure application on the lead-acid cell l0l
3.7. 1 Mechanical pressure variation on the cell walls
3.7.2 Structural changes in the negative electrode
3.7.3 Structural changes in the positive electrode
3.7.4 How does mechanical pressure application improve the life of the lead-acid battery
3.7.5 Importance ofthe separator « .. , ,~j.:,< (, r’I.’. »’L’,~-~’ t, .(~ »~ ·(‘if\~’4c:\(1
3.7.6 Perspectives
4 Corrosion
4.1 Generais about corrosion oflead and lead aUoys
4.1.1 Definition
4.1.2 When does corrosion occur
4.1.3 The corrosion layer on the positive lead grid
4.1.4 Effect of the active material on the corrosion
4.1.5 Conclusion of the literature study on corrosion
4.1.6 Effect of antimony
4.2 Effect of mechanical pressure on corrosion
4.2.1 Corrosion measurement under potentiostatic conditions
4.2.2 Mechanical stabilisation of the plate stack with the AJS separator
4.2.3 Observation of grids after cycling
4.2.4 Conclusion about the effect ofmechanical pressure on the positive grid corrosion
5 Phosphoric acid
5.1 Bibliography of the phosphoric acid
5.2 Results concerning the influence of phosphoric acid
5.2.1 Cyclic voltammetry on pure lead
5.2.2 Phosphoric acid and electrical performance
5.2.3 Mechanical pressure and the effect ofphosphoric acid
5.2.4 Effect ofphosphoric acid on the potential ofpasted electrodes
5.2.5 Phosphoric acid and gas formation
5.2.6 Oxidative effect ofphosphoric acid
5.2.7 Phosphoric acid and corrosion
5.2.8 Structural changes in the positive active material due to phosphoric acid
5.3 Conclusion about the effect of phosphoric acid
6 Oxygen cycle
6.1 Observation ofthe gas effects trough mechanical pressure recording
6.1.1 Mechanical pressure during one charge
6.1.2 Evolution over life of the gas effects
6.2 Recombination efficiency
6.2.1 Current repartition and recombination efficiency determination
6.2.2 Recombination efficiency ofan AJS cell.
6.3 Transfer of oxygen from the positive electrode to the negative electrode
6.3.1 IR pictures for the determination of the recombination sites
6.3.2 Oxygen transfer trough the AJS separator
6.4 Conclusion about the oxygen cycle
General conclusion
Achievements of this Ph.D. work
Perspectives
Towards sustainable development
References
