This paper proposes a new bidirectional resonant dc-dc converter suitable for wide voltage gain range applications (e.g., energy storage systems). The proposed converter overcomes the narrow voltage gain range of conventional resonant DC-DC converters, and meanwhile achieves high efficiency. It is realized by configuring a full-bridge mode and a half-bridge mode operation during each switching cycle. The fixed-frequency phase-shift control schemes are applied and the normalized voltage gain can be always from 0.5 to 1, regardless of the load. The transformer root-mean-square (RMS) currents in both the forward and the reverse modes have a negligible variation with respect to the voltage gain, which is beneficial to low conduction losses throughout a wide voltage range. Moreover, the applied MOSFETs are soft-switched, allowing for minimum switching losses. Both the operation principles and characteristics of the proposed converter are analyzed. Finally, a 1-kW converter prototype is built and the experimental results verify the theoretical analyses.