Negative plate mechanism

Positive and negative active-mass formation. The cured pastes of both positive and negative plates comprise identical mixtures of bivalent lead compounds (3BS, 4BS, PbO), which cannot create electromotive forces when the pasted plates are assembled into cells. The purpose of the formation step is to convert the cured pastes into electrochemically active porous materials — Pb02 in the positive plates and Pb in the negative plates — which are connected mechanically and electrically to the grids. The process of formation can be conducted via two basic schemes, as shown in Fig. 3.1. 

The eoneern over the performance of negative plates in VRLA batteries has resulted in renewed interest in the influence and mechanisms of organic additives and extensive research programmes have been carried out under the auspices of the ALABC. This work has included an assessment of 34 materials, five of which were synthetie organie compounds that were identified to have the potential to act as effective expander components in lead-acid batteries [32]. Preliminary screening tests for stability in acid, impurities and thermal stability, followed by studies of potentiostatic transients, impedance plots, and cyclic voltammograms [33], have... 

In a different battery test with a simulated EV load pattern, a SWP-7 cell with an assembly pressure of 60 kPa achieved 450 cycles versus 270 cycles for an AGM cell with 73 kPa. The failure mode was found not to be the expansion of positive plate but, rather, sulfation of the negative plate. This led to the conclusion that the favourable mechanical properties of SWP-type separators suppress degradation of the positive active-material. 

Recent investigations conducted by CSIRO [7,8] have provided the following explanation for the mechanism of lead sulfate accumulation in negative plates during HRPSoC duty. 

Mechanism of the self-discharge processes on the negative plate at open circuit. 

Parallel mechanism of the charge reactions on negative plates containing carbons or graphites [36]... 

Diagrammatic representation of the charge transfer through the Pb/solution and AC/solution interfaces according to the parallel mechanism of charge of negative plates. 

The cycle life within one cycle-set depends strongly on the nature and properties of the carbon or graphite additives used. These materials differ in particle size, structure and affinity to lead and to the expander. Of special importance is the interface between carbon and lead particles, and its area as it determines the resistance that electrons have to overcome when transferred between these two phases and thus affects the potential and the rate of the electrochemical reactions at the carbon/solution interface. Only a limited number of carbon and graphite materials have optimum structural characteristics and may improve substantially the cycle life performance of the cells. It is of crucial importance to identify the most effective carbon (graphite) additives, i.e. with most beneficial effect on the parallel mechanism of charge of the negative plates. 

Fibres of the above types (3 mm in length) and polypropylene fibres are introduced into the paste with the aim to provide mechanical stability to the structure of NAM. Because of the poor adhesion between lead and fibre particles, the role of mechanical strengthener of the NAM structure is not always very pronounced. Beyond any douht, however, fibre additions reduce substantially the waste during the pasting process and throughout the technological process of negative plate manufacture, which provides very important economical effect. 

Schematic representation of the electrochemical mechanism of oxygen reduction in active centres at the phase boundary metal/thin liquid film (see point (a) in Section Mechanism of the oxygen reduction at the negative plates chapter) [37].

Figure 9.21 Nickel-cadmium rechargeable battery, (a) Cover, (b) positive contact, (c) resealable vent mechanism (in the event of a pressure buUd up), (d) positive tab welded to positive contact, (e) insulating seal ring, (f) negative plate, (g) separator, (h) positive plate, (i) negative tab, and (j) nickel-plated steel case. (Reproduced with permission from Ref. [25], 1985, WUey-VCH.)...

One important market is the battery sector and polymer use here can be categorised in three distinct ways. For example the polymer may be used in the manufacture of the battery separators used in traditional cells to provide physical separation of the positive and negative plates whilst permitting electron flow through the electrolyte. Polyester and polypropylene (PP) fibres may also be used to reinforce the battery plates themselves in traditional cells. The second function is as a battery container material which must resist chemical attack by the electrolyte and give the container mechanical strength. 

Lead-Acid Battery The basic operation of a lead-add (Pb — H SO ) battery is based on groups of positive and negative plates immersed in an electrolyte that consists of diluted sulfuric(fl2 S 04) acid and water. Hence, the mechanism of this t5T)e of battery is based on the electron-balanced anodic (-) and cathodic (+) reactions. Hence, the ideal electrode reactions are reversed... 

D. Pavlov, Mechanism of the Processes of Formation of Lead-Acid Batteries Positive and Negative Plates, in Proc. Symp. on Batteries for Traction and Propulsion, 1972, Columbus Section of the Electrochemical Society, 135. 

The pony mixer is the traditional unit. A preweighed amount of leady oxide is placed into the mixing tub, and this is wetted first with water and then with sulfuric acid solution. Dry paste additives, if any, are premixed into the leady oxide before water addition. These additives can be plastic fibers to enhance the mechanical strength of the dried paste, expanders to maintain negative-plate porosity in operation, and various other proprietary additives which ease processing or are believed to improve battery performance. Muher mixers are usually filled first with the water component, then the oxide, then the acid. 

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