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Anhydrous ferrous (II) oxalate (AFO) outperforms its hydrated form when used as an anode material in Li-ion batteries (LIBs). With the increasing interest in Na-ion batteries (NIBs) in mind, we examine the potential of AFO as the anode in NIBs through first principles calculations involving both periodic and non-periodic structures. Our analysis based on periodic (non-periodic) modeling scheme shows that the AFO anode generates a low reaction potential of 1.22 V (1.45 V) in the NIBs, and 1.34 V (1.24 V) in the LIBs, which is much lower than the potential of NIBs with mixed oxalates. The conversion mechanism in the underlying electrochemical process involves the reduction of Fe<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow></mrow><mrow><mn>2</mn><mo>+</mo></mrow></msup></semantics></math></inline-formula> with the addition of Na or Li. Such conversion electrodes can achieve high capacities through the Fe<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow></mrow><mrow><mn>2</mn><mo>+</mo></mrow></msup></semantics></math></inline-formula> valence states of iron.