Counter electrodes were prepared by dipping carbon felt in a slurry of 8:1:1 LiFePO4 (MTI):Super P: PVDF in NMP solvent and drying at 100 °C overnight similar to previous work . The mass loading of LiFePO4 was about 80 mg per electrode. For layered oxide intercalation experiments, the LiFePO4 was used as counter to maintain close to 1:1000 1M Li:Na ratio in the solution. For layered oxide deintercalation experiments, the counter was first prepared by the deintercalation of LiFePO4 in 1 M LiCl (aq) solution at C/5 (based on 160 mAh/g theoretical capacity) to 0.4 V vs Ag/AgCl to make FePO4.