Overcoming Incomplete Digestion of Ti-Doped LiFePO4 Cathode Materials: Microwave-Assisted Sample Preparation and Single-Laboratory Validated ICP-OES Determination of Li, Fe, P, and Ti

Reliable compositional quality control of Ti-doped LiFePO4 (LFP) requires quantitative release of the low-level Ti dopant from a carbon-coated phosphate matrix. We evaluated aqua regia hot-plate digestion and developed a closed-vessel microwave digestion–inductively coupled plasma optical emission spectrometry (ICP-OES) method for Li, Fe, P, and Ti. Aqua regia left a carbon-rich residue containing Ti; mass balance on seven digestions showed that 5.86 ± 0.37% of total Ti was retained, compared with 0.39 ± 0.12%, 0.53 ± 0.11%, and 0.81 ± 0.13% for Li, Fe, and P. The mean Ti recovery across the 14 aqua regia digestions was 92.8% of the assigned value, compared with 100.0% for the microwave procedure. Digestion with 6 mL HNO3 and 2 mL HCl at 240 °C gave quantitative Ti release: between 220 and 240 °C the Ti result rose by 14.0% to a plateau, while Li, Fe, and P changed by less than 2%. Calibration R2 values exceeded 0.999. Recoveries were 98.6–99.8%, repeatability RSDs 0.49–1.51%, and relative expanded uncertainties 0.64–2.20%; the lower calibration limit for Ti was 250 mg kg−1. Method means agreed with values assigned by 14–16 laboratories (compatibility indices ≤ 0.49). Under routine conditions, the method would support Ti specifications of ±13.4% relative or wider.