1. Electronic and Ionic Transport in NCA and NMC Cathodes
Scientific Achievement
First measurements of bulk electronic and ionic transport across the entire state-of-charge range relevant to energy storage in NCA and NMCs, the most widely studied classes of Li-ion cathodes
Significance and Impact
Allows rate-limiting transport species and paths to be understood up to high charge voltage necessary to realize near-theoretical capacities of layered oxide cathodes
Research Details
Prepared pure-phase polycrystals with controlled residual porosity allowing electrochemical titration
Excluded typical electrode additives that interfere with mechanistic determination
Used ion-blocking and electron-blocking cell configurations to isolate electronic and ionic transport
Used both ac and dc-relaxation techniques on same samples for independent corroboration of results
Contacts: Yet-Ming Chiang – Stan Whittingham –
Despite many years of research and emerging widespread commercial use of NCA and NMC cathodes, their fundamental transport properties have never been measured over the lithium concentration (charge voltage) range of use. The few existing measurements disagree by many orders of magnitude. Within NECCES, these transport coefficients must be known in order to understand behavior at many levels.
Here, we have measured the electronic and ionic transport parameters for the most widely studied NCA composition, LiNi0.80Co0.15Al0.05O2, and two NMC compositions of Ni:Mn:Co ratios 3:3:3 and 5:2:3. The sample configuration was designed to avoid effects of composite electrode structure and composition that have plagued previous measurements. Pure-phase samples were prepared that have a controlled amount of residual porosity to allow facile electrochemical titration of the composition, yet are sufficiently dense (~95%) that the measured conductivities are truly representative of the bulk phase. Ion-blocking and electron-blocking cell configurations were used to isolate electronic and ionic conductivity respectively, and both ac and dc-relaxation techniques were used in the case of ionic transport to corroborates results by using independent techniques.
Significance and Impact
The results show that over a wide Li composition range up to x = 0.75, electronic conductivity remains about 104 greater than ionic conductivity, rendering ion transport the rate-limiting step for bulk electrode kinetics over all states of charge of practical interest. The Li concentration dependence of ionic transport furthermore shows a minimum at about x=0.5 which has been traced to the onset of grain boundary microcracking due to electrochemically induced crystalline anisotropy (i.e., “electrochemical shock”). These results further suggested that the most prevalent pathway for accelerated transport in typical polycrystalline cathode particles is stress-induced microcracks. Since the work in the highlight was published, NECCES collaborations between MIT, Univ. of Michigan, and Univ. of Illinois-Chicago have verified this behavior in single particle measurements.
Findings: Across the entire Li concentration range relevant to energy storage, electronic conductivity is ~104 higher than ionic conductivity. Ion transport decreases with increasing Li vacancy concentration, until electrochemical shock creates rapid transport paths through microcracking
R. Amin, D.B. Ravnsbaek, Y.-M. Chiang, J. Electrochem. Soc. 162(7), A1163-A1169 (2015). doi: / jes
R. Amin and Y.-M. Chiang, J. Electrochem. Soc., 163(8) A1512-A1517 (2016). DOI: / jes
Work performed at MIT