Monitoring rate-limiting processes
Charge photometry offers best-in-class access to explore fast and ultrafast charging. By spatially resolving local variation in charge photometry contrast within individual active particles during battery operation, fundamental rate-limitations can be readily revealed.
Fast-charging batteries are a prerequisite for the mass adoption of electric vehicles. To achieve good high-rate performances, rapid transport of ions through the electrode and the active particles is crucial. However, monitoring the rate-limiting processes is challenging and not routinely achievable with standard characterization tools, due to the short timescales involved during rapid cycling.
Charge photometry can provide insights into these rate-limiting processes, by spatially resolving the development of local Li-ion inhomogeneities both within and between active particles, in real time during cycling.
Figure 1 shows a charge photometry video of a rod-shaped particle of the high-rate anode material, NWO (Nb14W3O44), in a half-cell. During the first 60 s of lithiation, the charge photometry image shows intensity gradients, starting at the ends of the particle and moving in towards the centre. This non-uniform intensity variation indicates Li-ion gradients within the particle, arising as a result of a diffusion-limited lithiation process.
Figure 1. Charge photometry of a NWO anode particle, at the beginning of lithiation at 5C. (top) Half-cell voltage; lithiation begins at 0 seconds. (middle) Optical image of the rod-shaped NWO active particle. (bottom) The charge photometry contrast highlights inhomogeneous scattering intensity changes from the active particle. This reveals ion gradients developing as the Li-ions diffuse along the length of the rod.
As well as impacting fast-charging capabilities, understanding rate-limiting processes can have important implications for some types of capacity loss. For example, Ni-rich NMC cathode materials are known to suffer from significant first-cycle capacity losses. Charge photometry has been used to investigate the mechanism behind this, by spatially resolving local variations in Li-ion concentration.
Figure 2 shows some charge photometry images of a single-crystal Ni-rich NMC particle, near the end of lithiation. The charge photometry contrast varies with Li-ion concentration and, for NMC, higher contrast represents relatively delithiated regions and lower contrast represents relatively lithiated regions. At the end of lithiation, a Li-rich surface develops in the particle, surrounding a core which is still partially delithiated. In the nearly-fully lithiated surface, diffusion of Li-ions is slow, limiting the transport of ions towards the center of the particle. As a result, the cell reaches the cut-off voltage with the particle core still in a Li-deficient state. The rate-limiting slow Li-ion diffusion therefore contributes to the first cycle capacity loss.
Figure 2. Charge photometry to investigate first-cycle capacity loss in Ni-rich NMC cathodes. (top) Half-cell voltage; 1C constant-current charge, 1C constant-current discharge, 2-hour 3.0 V constant-voltage hold, rest. (bottom) Charge photometry images of a Ni-rich NMC cathode particle. Positive contrast (red) indicates regions which are delithiated compared to the pristine state. Zero contrast (white) indicates regions with comparable Li-concentration to the pristine state. Scale bar – 1 µm.
For more examples of how charge photometry can be used to study rate-limiting processes in battery materials, please take a look at our