Charge Dynamics Induced by Lithiation Heterogeneity in Silicon‐Graphite Composite Anodes

Abstract

The reaction processes in Li‐ion batteries can be highly heterogeneous at the electrode scale, leading to local deviations in the lithium content or local degradation phenomena. To access the distribution of lithiated phases throughout a high energy density silicon‐graphite composite anode, correlative operando SAXS and WAXS tomography are applied. In‐plane and out‐of‐plane inhomogeneities are resolved during cycling at moderate rates, as well as during relaxation steps performed at open circuit voltage at given states of charge. Lithium concentration gradients in the silicon phase are formed during cycling, with regions close to the current collector being less lithiated when charging. In relaxing conditions, the multi‐phase and multi‐scale heterogeneities vanish to equilibrate the chemical potential. In particular, Li‐poor silicon regions pump lithium ions from both lithiated graphite and Li‐rich silicon regions. This charge redistribution between active materials is governed by distinct potential homogenization throughout the electrode and hysteretic behaviors. Such intrinsic concentration gradients and out‐of‐equilibrium charge dynamics, which depend on electrode and cell state of charge, must be considered to model the durability of high capacity Li‐ion batteries.

Lithium Redistribution Mechanism within Silicon-Graphite Electrodes: Multi-Method Approach and Method Validation

“…Since a similar phenomenon is observed in the pure graphite cell as well, it confirms the simultaneous occurrence of two relaxation processes within the SiG20.8 electrode by our simulations: (1) Equilibration throughout the electrode due to local differences in the lithiation state, as previously reported by others, 43,62 and (2) equilibration between graphite and Si particles, as previously observed in experiments conducted by others. 39,[41][42][43] Figure 10 provides a summary of the observed Li redistribution processes within SiG composite electrodes. Even after charging at a low C-rate of 0.1 C, Li is moving from graphite to Si at the selected SOC values (25%, 50%, 75%).…”

“…Even after charging at a low C-rate of 0.1 C, Li is moving from graphite to Si at the selected SOC values (25%, 50%, 75%). At 0.5 C, an additional equilibration process is assumed between particles close to the surface and those closer to the current collector, as it was observed in the pure graphite electrodes and by Berhaut et al 43 in SiG electrodes. Richter et al 41 reported a similar redistribution of Li from graphite to the Si compound in a commercial 18650 cell during low temperature operando neutron-scattering measurements following Li metal deposition and re-intercalation.…”

“…42 It was observed that during relaxation periods following charging pulses, Li is redistributed from the graphite portion of their cell to the Si portion, even at low C-rates, which is intensified at higher C-rates. 42 Recently, Berhaut et al 43 discovered in-plane and out-of-plane concentration gradients in SiG for both the graphite and Si component. Li redistribution was observed from graphite to Si in-plane and within the different electrode sections.…”

“…Li redistribution was observed from graphite to Si in-plane and within the different electrode sections. 43 Despite these existing studies, there is still a lack of understanding on the relaxation processes in SiG anodes, especially with complimentary methods, which are validated against each other. In this study, the Li redistribution within SiG electrodes in full cells during relaxation was investigated.…”

Fast-charging anodes for lithium ion batteries: progress and challenges

“…44 Silicon-based materials and metal halides have impressive theoretical capacities, but their severe volume effect causes them to experience significant capacity decay during cycling. 45–47 Therefore, graphite remains the most widely used anode due to its low cost and excellent comprehensive electrochemical performance. 48 Benefitting from the low redox potential of graphite (0.1–0.2 V vs. Li + /Li), most cathodes can match well with it and exhibit a satisfactory high voltage platform in a full-battery.…”