On the current electric vehicle (EV) market, a liquid-cooling battery thermal management system (BTMS) is an effective and efficient thermal management solution for onboard power battery packs and powertrain systems. Its heat transfer efficiency and cooling capacity is theoretically higher than some other mainstream cooling methods such as passive and active air-cooling methods. This review first briefly yet broadly introduced the background of battery thermal management and liquid-cooling BTMS. Then the recent research about the design improvement and optimization for the liquid-cooling BTMSs were comprehensively reviewed. The major design improving approaches include coolant channel, heat transfer jacket, cold plate, coolant, refrigeration cooling system, heat pipe, and liquid cooling based hybrid system improvements. The Pros and cons of these improvement techniques were discussed. The cooling channel, refrigerant cooling, and liquid-PCM hybrid cooling improvements were found to be the most effective approaches to better cooling performance of the liquid-cooling BTMS. Based on the review, this paper highlighted the current gaps and future directions in the research of liquid-cooling BTMS designs for the EV industry.