Delta Differential Capacity Analysis

Abstract

Even the best lithium-ion cells degrade slightly from one charge-discharge cycle to the next. This degradation, and its origin, can be studied using “delta differential capacity analysis”. Constant-current chronopoteniometry is used to collect voltage (V) versus charge (Q), data as cells are charged and discharged during cycles n, n + 1, n + 2, etc. as V(Q, n) This data is then differentiated, using finite differences, to create differential capacity, dQ/dV(V, n), versus V for the nth measured cycle. “Delta dQ/dV” is calculated as the difference between the differential capacities of the nth and mth cycles, i.e. ΔdQ/dV(V, n, m) = dQ/dV(V, n) – dQ/dV(V, m). Three different battery testers were used to measure ΔdQ/dV(V, n, m) for LiCoO2/graphite commercial Li-ion cells where n and m differed only by a few cycles (2 < n – m < 20). When precision test equipment was used, noise-free ΔdQ/dV(V, n, m) was measured, even when adjacent cycles were used for the calculation (i.e. n − m = 1) and even when very stable cell chemistries were studied. Unfortunately, typical battery test equipment, availably commercially, cannot make such measurements, even when n – m > 20. The best Li-ion cell, that does not degrade from cycle to cycle should have ΔdQ/dV(V, n, mo) = 0 for all V and n, where mo is the number of formation cycles required for a particular cell chemistry. Thus, monitoring ΔdQ/dV(V, n, mo) over just a few cycles can be used as a quality assurance tool for Li-ion cells destined for long lifetime applications, such as in electric vehicles.