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This thesis focuses broadly on characterizing and understanding the Li intercalation mechanism in phospho-olivines, namely LiFePO 4 and Li(Fe,Mn)P0 4, using first-principles calculations. Currently Li-ion battery technology is critically relied upon for the operation of electrified vehicles, but further improvements mainly in cathode performance are required to ensure widespread adoption, which in itself requires learning from existing commercial cathode chemistries. LiFePO 4 is presently used in commercial Li-ion batteries, known for its rapid charge and discharge capability but with underwhelming energy density. This motivates the three central research efforts presented herein. First, we investigate the modified phase diagram and electrochemical properties of mixed olivines, such as Li(Fe,Mn)P04, which offer improved theoretical energy density over LiFePO4 (due to the higher redox voltage associated with Mn 2+/Mn 3+). The Lix(FelyMny)PO 4 phase diagram is constructed by Monte Carlo simulation on a cluster expansion Hamiltonian parametrized by first-principles determine