The work presented in this thesis concerns the reactions of phosphites when replacing phosphates in synthesis by atomic layer deposition (ALD). One goal of this research is keeping the oxidation state of phosphorous at +3 to gain new products and using these in Li-ion batteries. This could also expand the possibilities of ALD synthesis. Two phosphite precursors have been tested, trimethyl phosphite (Me3PO3) and triethyl phosphite (Et3PO3). Me3PO3 has been successfully combined with the known precursors Lithium tert-butoxide (LiO(CH3)3) and trimethyl aluminium (Al(CH3)3). The products formed using M33PO3 are amorphous Li3PO4 and AlP0,5O4 thin films with growth rates of 0.11 and 0.09 nm/cycle, respectively, at a deposition temperature of 225 °C. ALD synthesis with the Et3PO3 precursor resulted in films without phosphorous and are deemed incompatible for ALD synthesis due to low reactivity. The ternary films have been characterised using X-ray photoelectron spectroscopy to define oxidation states of phosphorous as well as quantification of composition. The oxidation state of phosphorous in the deposited thin films was +5 for all the deposited films, proving that the phosphorous is oxidised during deposition. Li3PO4 is a compound that has been previously used in Li-ion batteries as solid electrolytes. Building on that work, the LiPO films in this work have been analysed for use as a barrier layer between a LiFePO4 cathode and a liquid electrolyte (LiPF6). This was done by depositing thin films directly onto the cathode material using ALD and assembling the cathode material into a coin cell with a lithium metal anode. The results show increased stability and capacity retention at low specific discharge currents as measured using galvanostatic cycling. They also reduced the over-potential required for charge and discharge of the electrochemical cells at film thicknesses less than 5 nm as measured by cyclic voltammetry. 30 nm barrier layers reduced performance, but LiPO might still be viable as a solid electrolyte if LiPO is used as the only electrolyte. This work can facilitate the future deposition of ALD batteries in one single experiment.