Key UHPC Metrics — CE, Fade, and Slippage
Three quantities form a decoding chain: coulombic efficiency is the summary, and capacity fade and charge-endpoint slippage are the two components it splits into.
Coulombic efficiency (CE) — summary signal
├─ Capacity fade — irreversible loss of lithium (LLI) / active material (LAM)
└─ Endpoint slippage — accumulated self-discharge-like side reactionsCapacity fade
The cyclable capacity that is lost, step by step, as cycling proceeds. Two families of mechanism dominate: loss of lithium inventory (LLI), where active lithium is fixed irreversibly (for example by SEI growth), and loss of active material (LAM), where material deactivates or impedance rises. Because fade accumulates, taking differences between neighbouring cycles cancels part of a fixed instrument offset, which is why ordinary equipment can still track fade approximately.
Charge-endpoint slippage
On a cumulative-capacity axis, the charge endpoint of each cycle drifts forward. That drift is a proxy for self-discharge-like side reactions — electrolyte oxidation, transition-metal dissolution, redox shuttle, micro-shorts — that add counted charge without necessarily removing cyclable capacity. Slippage lowers CE while capacity may look unchanged, which is exactly why it hides from ordinary cycling.
Slippage is harder on the instrument than fade: it depends on the absolute accuracy of both charge and discharge, so channel-to-channel accuracy has to be much tighter.
Reading a low CE
Decomposing CE into a fade-dominated case versus a slippage-dominated case points at the change to make — electrode versus electrolyte versus interface. CE, fade, and slippage tell you whether a class of mechanism is present; assigning a specific reaction needs supporting experiments (for example dV/dQ or EIS). Treating the summary as if it named a mechanism is the most common misread.