Benefits of Fast Battery Formation in a Model System

Abstract

Lithium-ion battery formation affects battery cost, energy density, and lifetime. An improved understanding of the first cycle of solid-electrolyte interphase (SEI) growth on carbonaceous negative electrodes could aid in the design of optimized formation protocols. In this work, we systematically study SEI growth during the formation of carbon black negative electrodes in a standard carbonate electrolyte. We show that the initial ethylene carbonate (EC) reduction reaction occurs at ∼0.5-1.2 V during the first lithiation, except under fast lithiation rates (⩾10C). The products of this EC reduction reaction do not passivate the electrode; only the SEI formed at lower potentials affects the second-cycle Coulombic efficiency. Thus, cycling quickly through the voltage regime of this reaction can decrease both formation time and first-cycle capacity loss, without an increase in subsequent-cycle capacity loss. We also show that the capacity consumed by this reaction is minimized at low temperatures and low salt concentrations. Finally, we discuss the mechanism behind our experimental results. This work reveals the fundamental processes underlying initial SEI growth on carbonaceous negative electrodes and provides insights for both optimizing the battery formation process and enabling novel electrolytes.

Predicting the impact of formation protocols on battery lifetime immediately after manufacturing

“…Fast formation experimental design Two formation protocols have been implemented in this work: a fast formation protocol previously reported by Wood et al 15,16 that completes within 14 h (Figure S1B) and a baseline formation protocol (Figure S1C) that completes in 56 h. The fast formation protocol maximizes the time spent at low negative electrode potentials to promote the creation of a more passivating SEI. 15,[38][39][40] Forty nickel manganese cobalt (NMC)/graphite pouch cells with a nominal capacity of 2.36 Ah were built for this study (Table S1). Half of the cells underwent fast formation, and the remaining cells underwent baseline formation.…”

“…So far, we have explored the sensitivity of R LS to lithium lost during formation for an NMC/graphite system. By understanding the benefits of fast formation, 38 we rationalized why R LS was predictive of cycle life for our system. Here, we discuss the application of R LS toward understanding other degradation modes, chemistries, and use cases.…”

“…Together, these results support the growing body of evidence that well-designed fast formation protocols can improve cycle life. 15,22,38 Finding diagnostic signals at the beginning of life Given the demonstrated impact of formation protocol on battery cycle life, we next investigate methods to quantify the impact of fast formation on the initial cell state. Differences in the initial cell state (e.g., the amount of lithium consumed during formation) may offer clues as to how fast formation could have improved cycle life.…”

“…12,14,40,47,48 Conversely, lithium intercalation at negative electrode potentials below 0.25-0.5 V have been found to promote the formation of a more conductive and passivating SEI film. 38,40 Attia et al 38 showed that the reduction of ethylene carbonate (EC) at negative electrode potentials above 0.5 V versus Li/Li + is non-passivating. This negative electrode potential corresponds to a full cell voltage of below 3.5 V, neglecting overpotential contributions.…”

“…Fast formation spent more time above 3.5 V, creating a higher quantity of SEI that is more passivating. 38 The passivating SEI improved cycle life by protecting the negative electrode against side reactions over life. R LS provided an estimate of the amount of lithium consumption during formation, Q LLI , where more lithium consumed implied a higher amount of passivating SEI formed leading to improved cycle life..…”