Re-evaluating critical current density in solid-state batteries: Challenges and proposing an alternative

Abstract

The critical current density (CCD) test protocols are widely adopted to examine the quality of solid electrolyte (SE), the stability of lithium (Li)/SE interfaces, and solid-solid interfacial kinetics in all solid-state lithium batteries (SSLBs). This work systematically investigated the impact of major experimental parameters on CCD values of Li symmetric cells using argyrodite Li6PS5Cl SE. Four categories of experimental parameters were investigated: (i) SE parameters, (ii) Li metal parameters, (iii) cell die parameters, and (iv) CCD testing protocols. Each parameter uniquely influences the density of the SE or the SE/Li interface, thereby affecting the resulting CCD values. Notably, CCD values are highly dependent on the cell operating histories, particularly the operating period (h) and the applied current density (mA/cm2). Consequently, without standardizing and unifying CCD testing protocols, obtaining reliable and repeatable CCD values remain challenging. To overcome this limitation, we propose the cumulative areal capacity (CAC) test protocol – the total capacity (mAh/cm2) delivered by a cell before its failure – as a more accurate measure of cell performance. CAC analysis of over 30 Li symmetric cells demonstrated statistical convergence in CAC values, highlighting its potential as a supplementary or even alternative testing protocol to conventional CCD methods.