## 1 Introduction
studied the effect of a PCM on LIB, and found that it could make the temperature distribution uniform and keep the battery in a safe temperature range [26]. [PERSON] et al. systematically investigated the cooling behavior and the influence of several detailed factors on the performance of a PCM. The experimental results showed that a PCM module with a thickness of \(\sim\)10 mm presented the optimal cooling performance [27]. [PERSON] et al. investigated the heat transfer with a PCM in passive thermal management of electric and hybrid electric vehicles. The results showed that the temperature distribution became about 10% more uniform when a PCM was applied in a 3 mm thickness around the cell. The PCM with 12 mm thickness decreased the maximum temperature about 3.04 \({}^{\circ}\)C [28]. However, it is found that PCM is easy to get heat saturation when absorbing a large amount of heat in the practical application. BTMS based on pure phase-varying materials cannot work for a long time in high-power batteries and effectively control the battery temperature. Commonly used PCM is paraffin wax whose thermal conductivity is very low. The thermal conductivity of PCM is improved by adding high thermal conductivity materials such as expanded graphite, carbon fiber, graphene, aluminum foam, copper foam and so on [29; 30; 31; 32]. On the other hand, many researchers combined other cooling methods with PCM-based BTMS in order to improve the cycling stability and thermal management ability [33; 34]. The complex PCM based BTMS can effectively reduce temperature rise and temperature difference, and maintain battery performance, particularly in extreme environments [35; 36]. [PERSON] et al. proposed a PCM combined with heat-dissipating fins to enhance the heat transfer. The results showed that the performance of PCM-fin system was better than that of pure PCM system [37]. [PERSON] et al. used a PCM and composite materials of aluminum mesh plates to carry on the thermal management to the LiFePO4 battery pack under the high temperature environment. The results showed that the usage of PCM and aluminum wire mesh could significantly reduce the surface temperature of the batteries and improve the performance of the battery pack [38]. Although the use of PCM either alone or combined with heat dissipation fins had been extensively studied, there is no optimal solution for the best cooling effect, quantity, and price of PCMs used in LIBs at present. The combination of a PCM and heat dissipation fin will inevitably increase the weight of the BTMS device. As far as we know, a PCM with high thermal conductivity in the battery charging and discharging cycles and the optimization of the coexistence system of PCM and heat dissipation fin also needs further research. In this paper, we first tested the cooling effect of a PCM with higher thermal conductivity whose phase change process is a solid-solid phase change, and then designed a new type of heat dissipation fins. The main purpose of this paper is to study the cooling effect of a PCM with high thermal conductivity on the battery and the effect of a PCM combined with a new type of heat dissipation fins with a relatively small volume. The authors conducted the experiments to investigate the temperature and capacity changing of two kinds of batteries under different conditions.