Science - USA (2020-08-21)

SCIENCE sciencemag.org

PHOTOS: MILENA ZORKO/CENTER FOR NANOSCALE MATERIALS AT ARGONNE

By Pietro P. Lopes and

Vojislav R. Stamenkovic

W

hen Gaston Planté invented the

lead–acid battery more than 160

years ago, he could not have fore-

seen it spurring a multibillion-dol-

lar industry. Despite an apparently

low energy density—30 to 40% of

the theoretical limit versus 90% for lithium-

ion batteries (LIBs)—lead–acid batteries are

made from abundant low-cost materials and

nonflammable water-based electrolyte, while

manufacturing practices that operate at 99%

recycling rates substantially minimize envi-

ronmental impact ( 1 ). Nevertheless, forecasts

of the demise of lead–acid batteries ( 2 ) have

focused on the health effects of lead and the

rise of LIBs ( 2 ). A large gap in technologi-

cal advancements should be

seen as an opportunity for

scientific engagement to ex-

pand the scope of lead–acid

batteries into power grid ap-

plications, which currently

lack a single energy stor-

age technology with opti-

mal technical and economic

performance.

In principle, lead–acid

rechargeable batteries are

relatively simple energy stor-

age devices based on the lead

electrodes that operate in aqueous electro-

lytes with sulfuric acid, while the details of

the charging and discharging processes are

complex and pose a number of challenges to

efforts to improve their performance. This

technology accounts for 70% of the global

energy storage market, with a revenue of 80

billion USD and about 600 gigawatt-hours

(GWh) of total production in 2018 ( 3 ). Lead–

acid batteries are currently used in uninter-

rupted power modules, electric grid, and

automotive applications ( 4 , 5 ), including all

hybrid and LIB-powered vehicles, as an in-

dependent 12-V supply to support starting,

lighting, and ignition modules, as well as crit-

ical systems, under cold conditions and in the

event of a high-voltage battery disconnect ( 3 ).

Although the principle of operation has not

changed, manufacturers have improved this

technology by optimizing performance of the

electrodes and active components mainly for

application in vehicles. Future performance

goals include enhanced material utilization

through more effective access of the active

materials, achieving faster recharging rates

to further extend both the cycle life and cal-

endar life and to reduce their overall life cycle

cost with a direct impact on the implementa-

tion of grid storage systems.

The constant dissolution and redeposi-

tion of the cell’s active materials, over each

charge–discharge cycle, creates a situation

where both positive and negative electrode

morphology and microstructure are con-

stantly changing (see first the figure). These

structural changes enable the corrosion of

electrode grids typically made of pure lead

or of lead-calcium or lead-antimony alloys

and affect the battery cycle life and mate-

rial utilization efficiency. Because such mor-

phological evolution is integral to lead–acid

battery operation, discovering its governing

principles at the atomic scale may open ex-

citing new directions in science in the areas

of materials design, surface electrochemistry,

high-precision synthesis, and dynamic man-

agement of energy materials at electrochemi-

cal interfaces. This understanding could have

a direct impact on battery life, as preserving

the overall electrode surface area ensures ef-

fective charge–discharge processes.

These efforts must take into account the

complex interplay of electrochemical and

chemical processes that occur at multiple

length scales with particles from 10 nm to 10

μm (see the second figure) ( 5 ). The active ma-

terials, Pb and PbO 2 , are traditionally packed

as a self-structured porous electrode. When

discharged, Pb2+ ions quickly react with the

available sulfuric acid in the electrolyte and

nucleate insoluble PbSO 4 crystals. During

charging, PbSO 4 must be converted back to

Pb and PbO 2 , which is a thermodynamically

and kinetically more demanding process

given the poor solubility of the PbSO 4 crys-

tals. The intricate relationship between acid

concentration gradients within the electrode

pores and lead sulfate dissolution rates un-

derscores the challenge of improving the bat-

tery’s ability to recharge at fast rates.

All of these processes occur in competition

with the thermodynamically favored but un-

desired water-splitting reactions that evolve

O 2 and H 2 gases. Lead and lead dioxide are

poor catalysts for these reactions and have

high overpotentials that kinetically limit

these processes unless fast charging occurs

with high voltages. However, metal and ionic

impurities in electrodes and electrolyte fa-

cilitate electrolysis of water and its loss ( 5 ).

The requirement for a small yet constant

charging of idling batter-

ies to ensure full charging

(trickle charging) mitigates

water losses by promoting

the oxygen reduction reac-

tion, a key process present

in valve-regulated lead–acid

batteries that do not require

adding water to the battery,

which was a common prac-

tice in the past.

Some of the issues fac-

ing lead–acid batteries dis-

cussed here are being ad-

dressed by introduction of new component

and cell designs ( 6 ) and alternative flow

chemistries ( 7 ), but mainly by using car-

bon additives and scaffolds at the negative

electrode of the battery ( 4 ), which enables

different complementary modes of charge

storage (supercapacitor plus faradaic Pb

charge–discharge). These electrodes also of-

fer a rigid, unreactive, and conductive elec-

trode backbone that prolongs cycle life.

At the positive electrode, identification of a

material that can withstand the high electrode

potentials and harsh acidic environment re-

mains a problem to be solved. Utilization of

bipolar electrodes can reduce the amount of

lead used for structural components (elec-

trode grid), immediately improving material

utilization, but challenges with corrosion and

cost-effective manufacturing are still a limit-

ing factor. Implementation of battery man-

agement systems, a key component of every

LIB system, could improve lead–acid battery

operation, efficiency, and cycle life.

BATTERIES

Past, present, and future of lead–acid batteries

Improvements could increase energy density and enable power-grid storage applications

Materials Science Division, Argonne National Laboratory,

Lemont, IL 60439, USA. Email: [email protected]

A charged Pb electrode First discharge at a slow rate First discharge at a faster rate

2 mm 2 mm 2 mm

Morphological changes

Both electrodes form surface PbSO 4 during discharging. Scanning electron microscopy

images of Pb/PbSO 4 electrodes show marked surface morphology changes for distinct

charge and discharge protocols.

21 AUGUST 2020 • VOL 369 ISSUE 6506 923

Published by AAAS