Iron phosphates are widely used as polyanion-based positive electrode materials for lithium-ion batteries. In light of the considerable academic and industrial interest in the development of solid-state 3D-structured thin-film lithium-ion batteries, where the conformal deposition of very thin and pinhole-free electrode films on complex structures is an essential requirement, we have developed a plasma-enhanced ALD process to deposit amorphous iron phosphate at an unusually high growth rate of 1.1 nm/cycle. Our deposition process is based on alternating exposures to trimethyl phosphate (TMP, Me3PO4) plasma, O2 plasma, and tert-Butylferrocene (TBF) vapor. The main novelty of the process lies in the use of a TMP plasma as the phosphorus source, according to a mechanism which we recently reported for the deposition of aluminum phosphate (Dobbelaere et al., Chem. Mater. 2014, 26, 6863−6871). Firstly, we performed a detailed characterization of the deposition process of iron phosphate, including in-situ growth monitoring by spectroscopic ellipsometry and an investigation of temperature and pulse time dependency. Linear growth was observed without nucleation delay, proceeding optimally at 300°C. Saturation behavior could be demonstrated for all exposure steps, confirming a PE-ALD process. Secondly, we studied the properties of the deposited films. The as-deposited amorphous films had a density of 3.1 g/cm3 and ERD analysis revealed an empirical stoichiometry of FeP1.5O4.7 with 0.9% hydrogen and no detectable carbon residue (fig. 1a). Post-deposition annealing in air resulted in the formation of trigonal FePO4, while the same anneal in helium resulted in the formation of elemental phosphorus. Finally, we investigated the electrochemical behavior of the as-deposited iron phosphate in lithium-ion test cells. Cyclic voltammetry measurements showed lithiation and delithiation around 3V vs. Li/Li+, albeit reaching only 8% of the theoretical capacity. However, by reduction below 0.5V and subsequent oxidation back to 3V, we observed a drastic capacity increase to 60% of the theoretical capacity, resulting in an active 3V cathode material. Long-term charge/discharge cycling of a 33 nm planar film revealed that the capacity decreases over the first 10 cycles, and then stabilizes to 0.7 μAh/cm2. On a 3D micropillar substrate, the capacity increased from 7 μAh/cm2 to 22 μAh/cm2 over the first 150 cycles, and then remained constant at 22 μAh/cm2. Rate testing (fig. 1b) showed that both electrodes can be used at high C-rates, proving that the capacity is greatly enhanced by 3D structuring without sacrificing kinetics. Figure 1: (a) depth profile of a 55 nm iron phosphate film on a silicon substrate; (b) comparison of charge/discharge capacity and rate capability between planar and 3D-structured electrodes. Figure 1