We synthesized mesoporous/crystalline composite with a mass ratio of 1:2 ͑mesoporous: crystalline͒, and the resulting mesoporous tin phosphate has different pore size depending on the alkyl chain length ͑n = 11, 13, 15, and 17͒ of the surfactant CH 3 ͑CH 2 ͒ n N͑CH 3 ͒ 3 Br. Because the crystalline phase ͑Sn 2 P 2 O 7 ͒ to mesoporous phase mass ratio is fixed, a decrease in the capacity retention is mainly attributed to the deterioration of the mesophase. The mesoporous phase with smaller pore

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... ize causes higher capacity retention, with n = 11 showing the highest capacity retention corresponding to 82%. This decreased to 67, 63, and 59% with increasing pore size, n = 13, 15, and 17, respectively.