Science - USA (2022-04-29)

GRAPHIC: KELLIE HOLOSKI/

SCIENCE

SCIENCE science.org

By Jie Xiao

T

ailoring the intrinsic properties of

battery materials and designing

their multiscale structures are very

important to maximize their elec-

trochemical performance. When

electrochemically active materials—

often in powder form— are used in bat-

teries, they are usually dispersed within

porous composite electrodes immersed in

a liquid electrolyte and provide ionic and

electronic conduction pathways for the

electrochemical reactions. Therefore, the

full utilization of electrochemically active

particles in the electrodes (i.e., cathodes

and anodes) depends not only on their

own properties but also on those of the

conductive network for the particles, as

well as the interactions between the par-

ticles and the network. On page 517 of this

issue, Li et al. ( 1 ) report on the dynamics

of particle networks in composite cathode

for lithium-ion batteries and highlight the

importance of size consistency for the elec-

trode particles.

The functionality of an electrode de-

pends on the integration of the respective

contributions from every active particle in

the electrode network. A typical battery

electrode consists of electrochemically ac-

tive particles embedded in a porous com-

posite whose porosity can be tailored for

specific applications ( 2 ). Ideally, the elec-

trolyte infiltrates into every pore of the

electrode to ensure good ionic conductiv-

ity. In the perfect scenario, ions are ac-

cessible everywhere, and the conductivity

within the electrodes is good because of

the interconnected carbon in the electrode

structure ( 3 ). However, in reality, the lo-

cal conductive network surrounding the

active cathode particles, which facilitates

the interactions of ions and electrons for

the redox reactions, is not homogeneous,

and thus the reaction rate of the active par-

ticles is also not homogeneous.

The electrode heterogeneity creates a

dynamic evolution of the electrochemical

activities of individual particles inside the

electrode, which affects battery perfor-

mance. The heterogeneous and dynamic

electrochemical activities of electrode par-

ticles cannot be measured by the electro-

chemical output, such as the voltage or the

battery capacity, because these bulk mea-

surements do not give information about

the movement and interactions among the

individual particles. During charge-dis-

charge cycles, when lithium ions go in and

out of the host particles in the cathode, a

dynamic equilibrium of the particle system

is reached through the coevolution of all

particles driven by spontaneously yielded

internal pressure. Tracking the interac-

tions of individual active particles within

the electrode is needed to understand the

dynamic equilibrium within the electrode.

Li et al. investigated the dynamic evo-

lution of the particles in a battery cath-

ode (see the figure). Using synchrotron

nanoresolution holo-tomography, they

tracked the morphological and chemical

characteristics of thousands of individual

active particles. Inside the cathode, the

morphological defects and the electro-

chemical activities of individual particles

coevolved and transitioned through three

distinct stages: single-particle activation,

where electrode particles begin partici-

pating in the electrochemical reactions;

followed by segregation of groups of par-

ticles, where the utilization of particles

and degree of damage to the particles

vary; and finally, global homogenization,

where the modulation between the electro-

chemical activity and mechanical damage

reduces those variations. The authors also

observed a correlation between the collec-

tive electrochemical activity of, and the

damage to, the particles that occur during

charge-discharge cycles. According to their

model, particles with higher electrochemi-

cal activity are more likely to experience

mechanical damage, such as particle frac-

ture or detachment from the electrodes.

This mechanical damage in return would

suppress the electrochemical reaction by

increasing the resistance.

The findings of Li et al. highlight the

need for high-throughput analysis and the

importance of morphological and chemi-

cal consistencies of electrochemically active

particles for electrode robustness against

charge-discharge cycles. The relevance of

The consistency of cathode particles plays a pivotal role in battery performance

BATTERIES

A granular approach to electrode design

Capacity contribution

Dynamic

equilibrium

50% 100%

Conductive carbon network

Particles with fewer points of contact with

the conductive carbon have less cracking

and a lower capacity contribution.

Particles with more points of contact with

the conductive carbon have more cracking

and a higher capacity contribution.

Particles with a higher capacity

contribution clump together, affecting

the global homogenization transition.

Pacific Northwest National Laboratory, Richland, WA

99352, USA. Email: [email protected]

The evolution of electrochemically active particles in composite cathodes

Each active cathode particle evolves and shifts inside composite cathodes differently during charge-discharge cycles. This is caused by the heterogeneity of the

conductive structure (shown here as carbon) and various inconsistencies among the particles, such as in size and degree of cracking. Over time, the particles settle into

clusters and stabilize the electrochemical property of the battery.

29 APRIL 2022 • VOL 376 ISSUE 6592 455