In situ neutron diffraction study of lithiation gradients in graphite anodes during discharge and relaxation

Reduced capacity and rate capability at low temperature operation is a limiting factor for application of lithium-ion batteries in electro mobility. The visualization of electrode inhomogeneity resulting from low-temperature lithium transport limitations can illustrate the shortcomings of current electrode design. To study anode lithiation gradients and relaxation processes, we performed time-resolved in situ neutron diffraction measurements of a commercial high power LiCoO₂/Graphite pouch bag battery operated in the temperature range of −20^◦C to 40^◦C. With decreasing temperature, strong anode polarization is observed with phase coexistence of LiC₁₂ and lower lithiated phases LiC₁₈ and Li_1-xC₅₄ during and after discharge. Phase coexistence is maintained only briefly at 25^◦C but lasts over 6 hours at −10^◦C before equilibration. The long-term relaxation phenomena can be attributed to slow lithium transport within particles at low temperature. Analysis of the relaxation times and particle size distribution yields effective diffusion constants for lithium transport in graphite of 1.7 × 10−¹⁰ cm² s⁻¹ at room temperature and 0.7 × 10−¹¹ cm²s⁻¹ at −10^◦C. We find that a moderate discharge rate is sufficient to cause considerable anode polarization at low temperature, while at higher discharge rates up to 5C, self-heating reduces polarization and relaxation time.