Abstract: As renewable energy penetration increases and coal-fired power generation transitions toward low-carbon operation, coal-fired units are increasingly required to provide deep peak-shaving and flexible regulation, while also facilitating the repurposing of retired assets. Carnot batteries shift electricity over time through an “electricity-to-heat–heat storage–heat-to-electricity” process. They can reuse existing steam turbines, steam–water systems, and other infrastructure in coal-fired power plants, providing a new pathway to enhance the flexibility of operating units and convert retired units into energy storage systems. The research progress on converting coal-fired power units into Carnot batteries is systematically reviewed, with a focus on technical pathways, system performance, and techno-economic characteristics. Firstly, the research boundary between coal-fired power Carnot batteries and related technologies, such as extraction-steam thermal storage, electric heating, and waste heat utilization, is clarified. A two-level classification system based on “boiler participation status + system coupling mode” is then established. Within this framework, existing schemes are classified into two main categories: boiler-participating systems and boiler-out-of-service systems, covering seven typical technical pathways. Secondly, the two categories are compared in terms of system configuration, equipment reuse, round-trip efficiency, peak-shaving capability, and key influencing factors. The results show that the matching between thermal storage temperature and steam parameters, heat-source quality, charging and discharging pathways, and system boundaries are key factors affecting system performance evaluation. Finally, research progress on economic performance under electricity spot market conditions is summarized in terms of initial investment, electricity purchase cost for charging, energy market revenue, ancillary service revenue, and capacity compensation. Reusing existing assets can reduce the initial investment, while project profitability largely depends on time-varying electricity prices, operating strategies, and market mechanisms. For future engineering applications, further efforts should focus on comparing different technical pathways under unified evaluation boundaries, off-design and dynamic operation modeling, reliability validation of key equipment, and coordinated optimization of capacity configuration, operation scheduling, and life-cycle economics, so as to improve the accuracy of system design and operational decision-making.