Lead Oxide, Red Lead and Paste: Where a Battery’s Future Capacity Begins

A battery’s final capacity depends not only on assembly or formation. A big part of the result is defined much earlier — when lead oxide is produced and then mixed into paste.

Before formation, the active material is not yet PbO₂ or sponge lead. It is a controlled mixture of lead oxide, metallic lead, sulfates, water, sulfuric acid and additives. The quality of this mixture affects how easily and completely the plates will form later.

Part of this story is common to both plates. Other parts — red lead and the 4BS structure — belong only to the positive plate.

Where the Power Comes From

Battery oxide is mainly produced by two methods: the ball-mill process and the Barton process. Both produce lead oxide, but the resulting particles differ in size, shape, structure and surface properties. These differences affect how the oxide absorbs water and acid, behaves during paste mixing and develops during curing. For this reason, the paste formulation and process conditions must always be matched to the oxide production method.

Battery oxide is extremely fine. In a typical ball-mill oxide, about half of the particles are smaller than 2 µm and around three quarters are smaller than 5 µm — many times thinner than a human hair.

Lead Oxide: The Powder is Half Metal

The “lead oxide” used in battery production is never pure PbO. Industrial oxide from a ball mill or a Barton pot contains roughly 70–85% PbO and 15–30% un-oxidized metallic lead called free lead. This is not impurity. Free lead drives the heat balance of the paste, oxidizes during curing, and helps build the future pore structure of the plate. By the end of curing it must be almost gone: residual free lead in a cured positive plate should fall below about 1%, while for cured negative plates up to about 4% is acceptable.

One Composition, Many Behaviors

Two oxides with identical chemistry can behave completely differently in the paste mixer if their specific surface or particle-size distribution differs. That is why oxide control is a package, not a single number:

Free lead influences heat generation and oxidation during curing. If its level is too high, curing becomes harder to control and requires more oxygen. If it is too low, the desired active-mass structure may not develop fully.
Acid and water absorption determines how the oxide behaves during paste mixing. They affect paste density, plasticity, penetration, 3BS/4BS formation and the risk of cracking. Acid absorption is often used as the most informative single indicator because it reflects both particle size and particle structure.
Bulk density affects oxide dosing, paste porosity and the amount of active material.

Two Pastes, Two Different Logics

Positive and negative pastes perform different functions and therefore require different properties. In the positive plate, the main objective is to form a strong and stable PbO₂ structure that can withstand repeated cycling and corrosion. In the negative plate, the priorities are sufficient porosity and conductivity, good retention of the active mass, and efficient reversibility of the Pb ↔ PbSO₄ reaction. For this reason, positive and negative paste formulations differ in moisture content, density and additive composition.

The clearest illustration is the expander system of the negative paste: lignosulfonates (typically 0.2–0.5%), barium sulfate (0.4–1.2%) and carbon black (0.1–0.3%). Lignosulfonate protects the sponge-lead structure — traction batteries typically use the Indulin type; BaSO₄ acts as a nucleating agent for PbSO₄ crystals thanks to its isomorphism with lead sulfate; carbon black supports conductivity of the active mass at the end of discharge. In production, these additives are blended dry with the lead oxide for several minutes before water and acid are added.

Red Lead: The Orange Accelerator

Red lead (Pb₃O₄) is an important additive for positive active material. It is an orange lead oxide with the composition 2PbO·PbO₂, meaning that part of the lead is already present in the Pb(IV) oxidation state. In industrial paste formulations, red lead may replace up to 25–30% of the total oxide used for the positive mass. Red lead is used exclusively in the positive paste. It is never added to the negative paste — sponge lead needs no PbO₂ seed. When sulfuric acid is added during paste mixing, Pb₃O₄ reacts according to the following equation:

Pb₃O₄ + 2H₂SO₄ → 2PbSO₄ + PbO₂ + 2H₂O

The PbO₂ from Red Lead works as a ready-made “seed” inside the positive mass. The practical payoff: formation runs faster and more uniformly, and the plate reaches its initial capacity sooner.

From Paste to Structure: 3BS and 4BS

Paste mixing is a controlled chemical process, and the order in which the components are added is important. Lead oxide is loaded first, followed by water and then the sulfuric acid solution. Adding water before the acid helps distribute the reaction more evenly and reduces the risk of local overheating when the acid comes into contact with the oxide.

Stage / zone	Reaction or transformation	Technological meaning
Paste mixing	PbO + H₂SO₄ + H₂O → basic lead sulfates + heat	Adding acid is exothermic. Loading order, temperature and mixing time all matter.
Positive paste after curing	3BS: 3PbO·PbSO₄·H₂O 4BS: 4PbO·PbSO₄	3BS gives a finer structure and forms more easily; 4BS builds a stronger skeleton.
Formation, positive	Basic lead sulfates / PbSO₄ → PbO₂	The goal is active PbO₂ with good bonding, structure and a conductive skeleton.

During mixing and curing, the paste develops mainly into two basic lead sulfate structures: 3BS and 4BS. This choice matters above all for the positive paste: phase composition has a much stronger influence on the capacity and cycle life of positive plates than of negative ones. Which structure forms depends primarily on temperature and time. At temperatures below 60 °C, the paste is dominated by 3BS, while 4BS formation is very limited. At temperatures around 80 °C, 3BS crystals formed during the early stage of mixing can gradually convert into 4BS — the transformation is complete within roughly 30 minutes. The two structures give different properties. Fine 3BS crystals are easier to convert during formation and therefore support faster initial capacity development. Larger 4BS crystals create a stronger and more stable structure, but they are more difficult to form completely. For this reason, the paste recipe and curing conditions must balance two objectives: good initial capacity and sufficient mechanical strength for long-term operation.

In the negative paste this question does not even arise. The expander suppresses the formation of orthorhombic PbO — the very compound needed for 4BS nucleation — so 4BS crystals simply do not form in negative-plate pastes. Negative pastes are 3BS-based by design, which suits their job: fine crystals, easy formation, developed surface.

Active Mass that Breathes

During discharge, the active material changes both chemically and physically, with a significant increase in volume:

PbO₂ → PbSO₄: +92%

Pb → PbSO₄: +164%

So the service life of a PzS battery cannot be explained by corrosion or sulfation alone: every deep cycle makes the active mass mechanically “breathe”. In a tubular plate, the gauntlet holds the PAM in place, but internal stresses still depend on porosity, the degree of formation and the quality of the PAM–spine contact.

In a flooded traction battery, this directly affects C₅ capacity. If the pore structure is too dense or uneven, the electrolyte cannot reach all parts of the active material effectively. As a result, some of the active mass remains electrochemically inactive and does not contribute to the measured capacity.