Non-Destructive Analysis of Pb-Acid Battery Positive Plates, Based on Neutron Tomography
Authors: Tavola Francesco , Cazzanti Silvia , Hippoliti Raimondo , Rovatti Ludovica , Bozzini Benedetto 
Published: August 2024

Lead-acid batteries (LABs) are one of the first rechargeable batteries. Historically used for backup power during the early public electricity network, LABs now hold a global market share of approximately 50%, mainly in automotive applications and energy reserves for telecommunication and data networks. Their low energy density and high surge current make them suitable for high-current, in-time applications. The LAB electrochemical cell consists of PbO2 as the positive electrode, metallic Pb as the negative electrode, and H2SO4 as the electrolyte.

The composition of the positive electrode active material (PAM) involves mixtures of α- and β-PbO2 and an amorphous gel phase of Pb(IV) oxy-hydroxides, determining the PAM's functional properties. The hydrated Pb(IV) oxy-hydroxide is the electrochemically active phase, conducting ions but not electrons. The mechanical consistency of the PAM relies on α-PbO2, acting as a structural binder, while β-PbO2 provides electronic conductivity. LAB performance is influenced by the proportions of α and β crystalline zones. Jointly, α - and β -PbO2 act as a Pb(IV) reservoir for reduction, occurring through the gel phase during discharge. Together, α- and β-PbO2 function as a reservoir for Pb(IV) reduction, a process that takes place through the gel phase during discharge in lead-acid batteries (LABs). These two crystalline phases, α- and β-PbO2, reform from the gel phase during charging, and their proportions depend on the specifics of the charging process and the material state. Consequently, the relative amounts and distribution of α and β crystalline zones play a crucial role in influencing the overall performance of LABs.

In the present investigation, we propose, for the first time, an ex situ NT investigation of intact PPs, endeavouring to assess the internal structure of the PAM (pore formation) and spine/PAM decohesion of real battery electrodes – fabricated with two alternative metallurgical methods (punching P and gravity casting G) – subjected to harsh electrochemical testing conditions, representative of recharge abuse. Our study is complemented by X-ray radiography - aimed at pinpointing spine and PAM cracking – and cross-sectional SEM imaging aimed at disclosing details with spine/PAM interface: grid shape variation, corrosion depth, mode and mechanism, corrosion layer nature and thickness.

The synergic effect of these measurments enable a complete understanding and tracking of the PPs degradation mechanism : (i) NT offered a 3D quantitative and qualitative assessment of PAM porosity ,spine/PAM decohesions, and highly attenuating regions. Aging led to pore growth, with G electrodes displaying more extensive cracking. Variations in porosity distribution between P and G electrodes is observed. Hydrogenated gel distribution differed between electrode types and with aging. (ii) XR provided a non-destructive grid-scale overview. P electrodes exhibited limited defects, while G electrodes showed extensive spine issues. Corrosion progression differed from local SEM observations (iii) in which corrosion is explored at the spine/PAM interface, revealing no clear correlation between current density and corrosion. Representative micrographs for P and G electrodes were shared.

The combination of these technique enable a complete characterization of positive plate degradation,.The methodical use of NT holds the potential to establish a comprehensive morpho chemical knowledge base for comprehending the manufacturing and aging processes of lead-acid battery electrodes directly visualization of the 3D arrangement of the hydrogen-containing electroactive amorphous phase, is a crucial metric for assessing the state of health (SOH) that is typically challenging to access experimentally This approach, coupled with in operando capabilities, relies on the quantitative examination of functionally significant features and their changes throughout the aging process.

Figure 1