Digging deeper into the structures at the heart of the lead battery

The physical properties of solids are, of course, dependent on their chemical composition and the spatial distribution of the constituent atoms. Diffraction methods, using X-rays or neutrons, can provide not only the 3-dimensional coordinates of atomic positions but also the percentage occupancy of each such site.

Further, the structural model of the crystal is generally subjected to a least-squares refinement process to minimize the difference between observed and calculated diffraction patterns.

This type of calculation provides standard deviations for each of the positional and site-occupancy parameters and an overall indication of the confidence with which the model structure can be viewed. Such data are invaluable in the process of unravelling the chemistry of charge and discharge reactions in batteries.

In this article, we highlight two cases where neutron diffraction has shed new light on aspects of lead–acid battery chemistry.

Whereas X-ray scattering factors for heavy atoms are much greater than those for light atoms there is not the same disparity between the neutron scattering lengths for heavy and light atoms. Therefore, neutrons are particularly useful for the study of structures of lead compounds that contain both hydrogen and oxygen.

The first, and perhaps a more significant topic, is the recognition of the full range of stoichiometry that can be sustained by the crystal structure of beta lead dioxide and the amorphous material that occurs alongside the oxide in the active mass of the positive plate (PAM) of the battery.

The second topic relates to the formula of the compound heretofore known as ‘tribasic lead sulfate’.

The Positive Active Mass Lead dioxide

The different neutron studies of lead dioxide were initially published in many different Journals. Nevertheless, it was not until recently that they were assessed in a coordinated manner.

Once the results of these studies are drawn together [1], the nature of the discharge reaction in the positive active mass is as shown in Figure 1.

The equation as written above the schematic requires that all the reactants converge in the reaction zone. In other words, there must be an adequate flux of electrons to (and from) the interface where the electrolyte meets the surface of the oxide.

The conductivity of the oxide is crucial in maintaining discharge (and charge) reactions.

Nevertheless, the neutron diffraction studies have also shown that the stoichiometry (and, most likely, the conductivity) of lead dioxide can differ over a substantial range. More than that, the type of defect that is responsible for the non-stoichiometry varies depending on the way the oxide is prepared [1].

If the oxide is produced by chemical methods, such as the reaction between red lead and nitric acid then the product is oxygen-deficient (PbO2-x) and has been described as an n-type semiconductor.

If, on the other hand, the oxide is formed during an electrochemical process (as in the battery) then it is found to be lead-deficient with the charge-balance seemingly guaranteed by the incorporation of hydrogen (Pb1-yO2Hz).

For much of its operational life, the stoichiometry of the battery oxide appears to be close to Pb0.96O2 which would imply a hydrogen content of 0.16. This is in good agreement with the hydrogen content (0.16) indicated by neutron transmission.

Between the oxygen-deficient and the lead-deficient compositions there is expected to be a composition with a minimum of conductivity corresponding to the perfect stoichiometry, Pb1.0O2.0.

It is also anticipated that the conductivity of the oxide relates directly to the number of charge-carriers and this, in turn, is dictated by the degree of non-stoichiometry.

Neutron diffraction studies have also shown that the stoichiometry — and, most likely, the conductivity — of lead dioxide can differ over a substantial range. More than that, the type of defect that is responsible for the non-stoichiometry varies depending on the way the oxide is prepared

The extent of the non-stoichiometry is clearly an important parameter. Ian Steele and his team measured the way whereby the non-stoichiometry evolved with the cycle-life of cells provided by Cominco.

The research was funded by the Advanced Lead–acid Battery Consortium from its project on fast-charging (Project RMC-002A). Early in cycle-life, it was found [2] that the degree of lead-deficiency of the crystalline oxide when cycled with a normal charge rate was greater than when cycled with a high charge rate.

However, the cycle-life of the fast-charged cells was four times greater than that of the cells charged normally. Further research is required on factors that affect the performance of the PAM, particularly, in the following areas.

• What is the conduction mechanism of the electrochemically prepared variant of lead dioxide?

• Is the greater non-stoichiometry associated with a higher hydrogen content?

• Is there an optimum hydrogen content?

• Can performance (capacity, cycle-life) be enhanced through identification of optimum charging parameters (rate, temperature, acid strength)?

Amorphous material

Hard information on the composition and the function of the amorphous component of the PAM has been difficult to obtain.

Although one of the diffraction studies [3] has indicated that as much as 33 wt.% of the PAM can be amorphous in a fresh battery, high resolution scanning electron micrographs (SEMs) have provided no firm confirmation [2]. It appears, however, that the amorphous matter may be present as a somewhat thin (~30 Å) layer covering the surface of the dioxide crystals, as shown in Figure 1. SEM would find it difficult to detect a layer this thin, and it could account for the high local hydrogen content (30% per surface oxygen) that has been observed by X-ray photoelectron spectroscopy.

A layer overlaying the electron-conducting solid in the PAM could arise because the oxidation of lead sulfate to lead dioxide is not a single reaction but takes place through a sequence of steps such as those shown in equations 1, 2 and 3 below.

These are the reverse of the discharge reactions proposed by Detchko Pavlov [4]. The steps involve the production and the consumption of intermediate species that are accommodated in the amorphous layer.

PbSO4 + 2H2O → Pb(OH)2 + H2SO4 (1)

Pb(OH)2 + H2O → PbO(OH)2 + 2H+ + 2e- (2)

PbO(OH)2 → PbO2 + H2O (3)

For reaction (2) to continue, there must be both electronic and protonic conductivity to (from) the point of reaction. Any change that disrupts either conductivity would result in a decrease in discharge capacity.

This model of the amorphous material is redolent of the solid | electrolyte interphase (SEI, see Figure1) that forms in lithium-ion batteries. The SEI must allow the charge and discharge reactions to take place but, at open circuit, it must also prevent the self-discharge reaction between the oxide and sulfuric acid.

References

[1]. P. T. Moseley, I. M. Steele, J. Energy Storage, 61 (2023) 106754.

[2]. I. M. Steele, J. J. Pluth, J. W. Richardson Jr, J. Power Sources, 95 (2001) 79–84.

[3]. R. J. Hill, Mater. Res. Bull., 17 (1982) 769-7.

[4]. D. Pavlov, Lead–Acid Batteries: Science and Technology. A Handbook of Lead–Acid Battery Technology and Its Influence on the Product, 2nd Edition, Elsevier, 2017.

[5]. P.T. Moseley, D.A.J. Rand, J. Energy Storage, 2023, in press.

A neutron diffraction study of the crystal structure of tribasic lead sulfate shows that the two hydrogen atoms in the formula – 3PbO•PbSO4•H2O – are not bonded to the same oxygen atom, as they would be if the structure included a water molecule.

Rather, they are bonded to two different oxygen atoms and therefore constitute a pair of hydroxyl entities. There are no discrete PbO units and neither a discrete PbSO4 unit. The crystal structure is thus seen [5] as an assembly of lead ions, oxygen ions, hydroxyl ions, and sulfate ions in the proportions 4:2:2:1. Consequently, it makes sense to record the formula as – Pb4O2(OH)2SO4 – and refer to it by the following name:

'Tetra lead dioxy dihydroxy monosulfate’