Lithium battery materials can be advantageously used for the selective sequestration of lithium ions from natural resources, which contain other cations in high excess. However, for practical applications, this new approach for lithium production requires the battery host materials to be stable over many cycles while retaining the high lithium selectivity. Here, a nearly symmetrical cell design was employed to show that LiFePO4 shows good capacity retention with cycling in artificial lithium brines representative of brines from Chile, Bolivia and Argentina. A quantitative correlation was identified between brine viscosity and capacity degradation, and for the first time it was demonstrated that the dilution of viscous brines with water significantly enhanced capacity retention and rate capability. The electrochemical and X‐ray diffraction characterisation of the cycled electrodes also showed that the high lithium selectivity was preserved with cycling. Raman spectra of the cycled electrodes showed no signs of degradation of the carbon coating of LiFePO4, while scanning electron microscopy images showed signs of particle cracking, thus pointing towards interfacial reactions as the cause of capacity degradation. Brine is fine: The electrochemical sequestration of lithium from brines representative of the largest lithium resources in South America is explored, using a battery host material (LiFePO4) as a sustainable approach of lithium production. The brine viscosity is found to critically affect the cycling stability and rate capability, and, surprisingly, significant improvements are achieved by simple brine dilution. Post‐mortem characterisation suggests that interfacial side‐reactions and particle cracking limit the long‐term stability.