<p>LiFe_<i>x</i>Mn₁_−<i>x</i>PO₄ positive electrode materials show significant potential for enhancing battery safety, power density and cost-effectiveness. However, gas evolution shortens the cyclability and raises additional safety concerns, presenting a major challenge. The gas evolution mechanisms in LiFe_<i>x</i>Mn₁_−<i>x</i>PO₄ batteries are poorly understood, impeding the material improvement efforts. Here we examine a LiFe_<i>x</i>Mn₁_−<i>x</i>PO₄-graphite full cell, simultaneously quantifying and probing gas evolution from positive and negative electrodes. We found over 90% of the evolved gas was composed of CO₂ and H₂. CO₂ originated from side reactions at LiFe_<i>x</i>Mn₁_−<i>x</i>PO₄, with almost equal contributions from electrochemical and chemical pathways. H₂ resulted from chemical side reactions at the graphite’s solid–electrolyte interface and was closely associated with the dissolution of Mn/Fe ions from the LiFe_<i>x</i>Mn₁_−<i>x</i>PO₄. We developed a LiFe_<i>x</i>Mn₁_−<i>x</i>PO₄ with a dense carbon layer coating, which inhibited metal ion dissolution by an order of magnitude and minimized side reactions at both electrodes. A 4.1-Ah pouch cell exhibited stable performance over 540 cycles with over 90% capacity retention.</p><p></p>